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This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Nanocellulose is one of the studies that have attracted a lot of attention because of its biodegradability, unique chemical and physical properties, diverse precursors, and wide applications. From cellulose derived from natural cellulose fibers, nanocellulose can be synthesized. Cellulose itself is a particle that can be in the form of fibers or crystals. Cellulose has dimensions of nanometers with a diameter of less than 100 nm [1]. Several previous studies have stated that nanocellulose has a surface that is rich in reactive hydroxyl groups so that it can facilitate the modification of nanocellulose and increase its application value [2–4].

In general, nanocellulose can be distinguished based on size, morphology, and mode of synthesis. Cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) are two types of nanocellulose that can be distinguished based on these differences [5–7]. CNCs particles were synthesized using chemical methods such as the hydrolysis method to produce short rod-shaped particles with a diameter about 3–10 nm [8–11]. CNF particles are generally synthesized by mechanical methods such as ball-milling, grinding, and homogenization to produce particles with a fibril shape and that are flexible [12–14]. The dimensions of both CNCs and CNFs particles can still be reduced using an additional process in the synthesis process, using the ultrasonication process [15–17].

Several studies mention the advantages and disadvantages of CNCs and CNFs. In general, CNCs and CNFs have advantages in terms of production in that both can be synthesized from abundant types of precursors, such as biomass and plants. CNCs and CNFs are rich in hydroxyl groups on their surfaces, which make it easy for them to be modified [18, 19]. Both can be made in various forms, such as foams, thin films, membranes, and others, depending on the application needs [20–23]. When they have been produced, both have a large surface area and the surface area is needed in the application of nanocellulose as an adsorbent [24–26].

Both are known to have differences in morphology and dimensions. CNCs have smaller dimensions than CNFs, so they will produce a higher crystallinity index and can be used as reinforcing agents in various polymer matrices [15, 17, 27]. On the other hand, CNFs form a network of long and flexible fibers with a larger diameter compared to the size of CNCs. Fibrils with a smaller diameter and a longer size show a stronger strengthening effect [27]. It is difficult for CNFs to avoid the agglomeration process, so modification of CNFs with chemicals requires more attention [28].

CNCs and CNFs were characterized to determine their morphology, thermal properties, crystallinity index, and particle size. Lignocellulosic components can be analyzed from the characterization results using the Fourier transform infrared spectroscopy (FTIR) instrument. The morphology of the CNCs and CNFs surfaces can be analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) instruments. The crystallinity index and crystal size can be known from the analysis using X-ray diffraction (XRD). TEM, dynamic light scattering (DLS), and thermogravimetry (TG) can be used to examine the particle size and thermal properties of each CNCs and CNFs [29–32].

The application of CNCs and CNFs as heavy metal ion adsorbents will significantly increase the specific surface area with a short adsorption site and intraparticle diffusion distance so that the adsorption kinetics are faster [33–35]. Many applications of CNCs and CNFs as heavy metal ion adsorbents have been done before to reduce water pollution, especially that caused by human activities. Some of the heavy metal ions that have been studied are mercury [36, 37], arsenic [38, 39], selenium [40], cadmium [41, 42], chromium [43], lead [44], and others [45–47].

To maximize their adsorption ability, CNCs and CNFs must be modified. Modifications of CNCs and CNFs were carried out to increase their surface area and active functional groups [46–48]. In addition, the selectivity of the adsorbent to adsorbate (heavy metal ions) is influenced by the type of functional group used during the modification of CNCs and CNFs and is also influenced by pH [39, 49, 50]. An increase in pH will cause precipitation of metal species, which can reduce the adsorption capacity [36, 50]. As a result, choosing the appropriate pH setting will have a significant impact on the selectivity of CNCs and CNFs for heavy metal ions.

Several previous reviewers provided very detailed feedback on CNCs and CNFs. Randhawa et al. [51] explored CNC, CNF, and bacterial nanocellulose (BNC) in terms of classification, synthesis, modification, and applications in the biomedical field. Functionalization, copolymerization, and the addition of additives were among the modifications reviewed in the manufacturing of nanocellulose composites. Phuong et al. [52] reviewed cellulose nanomaterials from definition to preparation method, surface modification, and application. Nanocellulose has been explored as an adsorbent in water treatment (especially Cu(II) metal) and as an antimicrobial material. Aravind et al. [53] conducted a review on CNCs, which is described starting with definitions, various properties of CNCs, extraction methods, modification methods, and applications of CNC as nanocomposites in the biomedical field.

Norizan et al. [54] provided a thorough review of nanocellulose obtained from plant fibers, including discussions about how to isolate it, modifications, nanocellulose-based nanocomposite production techniques, and several manufacturers that could support nanocomposite applications. Nagarajan et al. [55] reviewed lignocellulosic extraction methods from a wide range of sources, which include plants and cellulose waste. It then goes over energy-efficient raw material pretreating methods, various CNC and CNF preparation methods, and CNC and CNF characterization techniques. Essa et al. [56] discussed the use of polyethylene terephthalate nanofiber multiwalled carbon nanotube (PET NF-MWCNTs) composite for methylene blue removal. The PET NF-MWCNT composites were characterized using SEM, and the optimization of adsorption parameters was studied using the Taguchi approach. Mokhena and John [57] go through the history, evolution, definition, and types of cellulose nanomaterials, as well as isolation methods and their wide range of applications. Several applications of cellulose nanomaterials discussed in the journal Mokhena and John [57] are catalysts, drug delivery, composites, membrane filtration, and others. Zhang, et al. [58] discussed the morphology and chemical composition of nanocellulose, which affect the properties of polyhydroxy butyrate (PHB/CNF) composite films. The structure and properties of the PHB/CNF film composite are also discussed in [58]. Rana et al. [59] reviewed CNC definitions, plant extraction methods, preparation methods, and surface modification. Some of the CNCs preparation methods discussed by Rana et al. [59] include acid hydrolysis, enzyme hydrolysis, preparation using deep eutectic solvents, and others. Adil et al. [60] discussed two strategies for producing metal–organic frameworks (MOFs) nanofiber composites. In situ MOF on the nanofiber surface and presynthesized MOF/CNF composites are the two strategies. The review then goes on to describe the mechanisms and kinetics of adsorption.

Based on previous reviews, we can conclude that CNCs and CNFs have numerous advantages and can be used in a variety of fields. However, based on the review journals we reviewed above, we revealed that no one had thoroughly studied the properties of CNCs and CNFs synthesized from various types of plants. Furthermore, the explanation of CNC and CNF modification is limited to a few modifiers, such as functionalization, grafting, surfactant, and others. The use of CNCs and CNFs as adsorbents is also limited to a few heavy metal ions, such as Cu(II) and Pb(II), as well as their isotherms and adsorption kinetics. There was also no discussion of the impact of pH on the adsorption process of various types of heavy metal ions using CNCs and CNFs adsorbents in the previous literature review. As a result, this review article will provide a more comprehensive description of the methods for simultaneously synthesizing and characterizing CNCs and CNFs derived from various plant species. Moreover, various types of modifiers that can be used to modify the surface of CNCs and CNFs will be discussed. Regarding that the application of CNCs and CNFs as adsorbents for various types of heavy metal ions, including their isotherms and adsorption kinetics based on the pH factor of the solution, will be explored. Figure 1 summarizes and illustrates some of the main points that will be discussed in this review.

[figure(s) omitted; refer to PDF]

2. The Cellulose Isolation

Biomass comprises cellulose, lignin, hemicellulose, pectin, and other components. The presence of cellulose is greater than that of other constituent components [61–63]. The cellulose substance is around 20–93% depending on the cellulose segregation strategy and the raw materials (Table 1). The chemical structure of cellulose (Figure 2(a)) reveals that the polymer is composed of monomers linked together by glycosidic oxygen bridges. Cellulose is a linear homopolymer made up of β-1,4-linked glucopyranose units that are corkscrewed to one another. This natural polymer’s repeating unit is a dimer of glucose known as cellobiose [76, 78, 79]. The anhydroglucose unit within the chair adaptation agrees with the nuclear numbering and glycosidic affiliation and the two closes of the polymer chain [80, 81]. As Figure 2(b) shows, the intramolecular hydrogen bonds between natural cellulose (Cellulose I) and regenerated cellulose (Cellulose II) are different because of the hydroxymethyl group differences [77]. Cellulose is stable because of its structure with hydrogen bond chains [24, 76, 82, 83].

Table 1

Cellulose, hemicellulose, and lignin content from various biomasses.

BiomassMethod/TreatmentCellulose (%)Hemicellulose (%)Lignin (%)Reference
Calotropis procera fibers2% NaOH (3 hrs), 93% CH3COOH, and 0.3% HCl (90°C, 3 hrs), then bleached using 5% H2O2 and 3.8% NaOH (3 hrs)91.38.20.4[15]
BambooMicrowave liquefaction process (1200 W, 100 bar, 300°C); bleaching by 0.1% NaClO2 (75°C, 1 h), 0.4% NaOH (75°C, 30 min)83.6713.970.13[17]
Rice plant0.125% NaOH and 1.3% H2SO4 (90°C; 3 hrs) and soaked for 10 min in hot water (200°C)61.94.851.6[64]
Pineapple wasteIsolated using acetone (90°C, 2 hrs), 0.5 M NaOH (80°C, 3.5 hrs), and 98% H2SO4303722[65]
Cassava peelTreated using alkaline treatment of 4% NaOH (90°C, 2 hrs), bleached using 4% NaOCl (80°C, 1 h)86.41.32.6[66]
Jackfruit peelBleached using 1.5% NaClO2 (70°C, 2 hrs), 5% NaOH (2 hrs), and hydrolyzed using H2SO420.0824.041.85[67]
Siam weed0.5 M NaOH (3.5 hrs) and 72% H2SO4 (2 hrs)40.229.923.2[68]
Shea tree45.920.329.9
BagasseBleached using 0.735% sodium hypochlorite (45°C, 6 hrs) and a reflux process with 17.5% NaOH (3 hrs, 45°C)402518[69]
Acid hydrolysis using 65% H2SO4 (45°C, 45 min), centrifuged at 4500 rpm and dialyzed against distilled water for 3 days70.719.37.6[70]
Carrot1 M HCl (30 min, 85°C), 1 M NaOH (30 min, 85°C). Bleaching treatment by 1-2% NaClO2 (1 h, 95-96°C)10.015.732.50[71]
Tomato8.605.335.85
Cucumber16.134.334.51
Apple8.815.442.98
OPEFBThe first treatment was done by a bleaching process using 0.7% NaClO2 and 17.5% NaOH (2 hrs, 70–80°C). The second treatment was with 5% Na2CO3 (2 hrs, 70–80°C)93.015.001.84[72]
Isolated using 10% NaOH (48 hrs) at room temperature, neutralized with CH3COOH (pH 7), and refluxed with 2% H2O2 (60–70°C, 3 hrs)62.4019.909.00[73]
60% H2SO4 at 35°C for 6 hrs36.6713.531.16[74]
85.7% phosphoric acid (H3PO4) (90 rpm, 50°C, 5 hrs)53.819.0737.22[75]

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Cellulose can be isolated by destroying lignocellulosic components such as lignin and hemicellulose [72, 73, 84]. Hemicellulose is unstable when compared to cellulose and lignin, making it more difficult to hydrolyze [85, 86]. Hemicellulose is a compound with an amorphous structure that makes hemicellulose easy to convert into simple sugars through enzymatic and hydrothermal methods [87, 88]. Meanwhile, lignin is a complex polymer compound that acts as a barrier that coats cellulose and hemicellulose so that lignin is more difficult to remove. Several previous studies have shown that lignin can be removed using a delignification method based on sodium chlorite (NaClO2) and combining it with an alkaline solution [72, 89, 90].

The main purpose of alkaline treatment or delignification is to destroy the lignin structure in the biomass so that only polysaccharides remain to be continued with other treatments. This mechanism includes saponification of intermolecular ester bonds, making cellulose easier to extract [90, 91]. Table 1 shows some previous studies on cellulose isolation, which showed that cellulose isolation conducted by Ngadi and Lani [72] could produce 93.01% cellulose from oil palm empty fruit bunches (OPEFB).

3. Nanocellulose and the Processing Methods

Over the years, scientists have explored numerous ways to incorporate cellulose into such a wide range of interdisciplinary fields as environmental remediation, biomedical, the food industry, and more. Therefore, researchers are currently developing a lot about nanocellulose. Nanocellulose is a type of cellulose developed through chemical modifications and mechanical processes. In general, nanocellulose is divided into the following three types: cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs), and bacterial cellulose (BC). CNCs are nanocelluloses with a rod-like shape that are formed through chemical synthesis [10]. CNFs are cellulose nanofibers developed through a mechanical process [14]. Then, the BC is a combination of nanocellulose and bacteria. The BC is included in the type of cellulose with a monoclinic structure [92]. In this review we focus on CNCs and CNFs.

3.1. The Processing Method: Nanocellulose-Based Foams

Foams can be formed from nanocelluloses such as CNCs and CNFs. Formation of wet foams using the gas dispersion method in liquid or gel. This method is used to avoid or delay the formation of coalescence bubbles. Desorption or decomposition of added compounds, defined as foaming agents, can also create the gas bubbles. When the films between the bubbles burst, the foam drains, coarsens, and finally, collapses. The chances of film breaking and bubble coalescence increase as the foam thins due to drainage and evaporation. The stability of the foam is strongly influenced by particle size and contact angle. Smaller particles, as they refer to larger particles and/or flocculated particles, delay or remove coalescence more efficiently [20].

3.2. The Processing Method: Nanocellulose-Based Membrane

Membranes are a form of cellulose that have been extensively studied and applied, especially in water purification. The method that can be used to produce membranes is the interfacial polymerization method. The membranes were immersed in a solution containing sodium dodecyl sulfate (SDS) for 12 hours. SDS solution is used to increase the wettability of the substrate so that the monomer will easily move from one phase to another. The membranes were then washed, moistened with deionized water, and dried. The membrane is then placed on a glass plate with double-sided tape. CNCs were added to the aqueous phase containing m-phenylene diamine (MPDA) and trimesoyl chloride (TMC). Both will act as monomers. The polymerization reaction between the two can reduce the reactivity of the monomer reactants in the aqueous phase. The membrane produced by this method has high hydrophilicity. This causes a high-water flux when compared to ordinary membranes. In addition, the membrane CNCs have good thermal stability and stable [93].

3.3. The Processing Method: Nanocellulose-Based Thin Films

Thin films (less than 100 nm in thickness) are a major type of modern materials. Continuous developments in deposition techniques, and micro- and nano-scale patterning, have greatly contributed to the emergence of thin film functionality. The thin films, particularly in materials technology, play an essential part in products such as solar cells, sensors, and displays. Current research prospects are substantial. Individual nanocellulose crystals have the highest elastic modulus and breaking strength properties per unit mass of any common material. The tensile strength of nanocellulose-based thin films can be comparable to metals and advanced synthetic polymer materials due to their extensive hydrogen bonding and high density [22, 23].

The CNCs and CNFs in the form of thin films can be made using the spin coating method. The spin coating method is stabilized with a mild heat treatment. This treatment can remove the water content from CNCs so that it will prevent redispersion when exposed to water. Giving the CNCs a mild heat treatment does not affect the clusters on the surface of the CNCs. The resulting thin film has a large electrostatic component. This makes the spin coating method widely used to produce nanocellulose in the form of thin films. In addition to the spin coating method, there is also a layer-by-layer (LBL) deposition method. The LBL method is a method for making thin films that utilizes the charge on the surface of CNCs. Films with CNCs produced by this method exhibit antireflective properties due to a suitable pore size distribution in the elongated network of CNCs [22].

4. The Synthesis of Cellulose Nanocrystals (CNCs)

A CNC is a rod-shaped particle with a diameter of 2–20 nm and a length of more than 100 nm [10–13]. Table 2 shows the CNC synthesis method. The acid solution was effective in removing irregular (amorphous) areas [99, 100]. The acid hydrolysis method involves hydrogen ions in the reaction mechanism so that it can break the glycosidic bond in the amorphous region of cellulose. The process of breaking the glycosidic bond causes the collapse of the amorphous region and maintains the crystalline region of the cellulose. The acid hydrolysis method can be carried out using acids such as sulfuric acid [101–103], hydrochloric acid [104, 105], phosphoric acid [106–108], and other strong acids [109–111].

Table 2

Summary of CNCs synthesis methods from various biomasses.

BiomassMethods/TreatmentResultsReference
Calotropis procera fibersDelignification (93% CH3COOH, 0.3% HCl (90°C, 3 hrs)), bleached using 5% H2O2 and 3.8% NaOH (3 hrs), and hydrolyzed 63% H2SO4 (1 h) with ultrasonication treatmentThe CNC has a shape like a needle with an average length of 250 nm and a diameter of 12 nm. The crystallinity index was 68.7%[15]
Corchorus olitorius dried stalkTreated using 2.5 M NaOH (90°C, 45 min) with microwave irradiation 350 W and hydrolyzed using 1 M 1-ethyl-3-methylimidazolium chloride and 1 M H2SO4 followed by ultrasonication treatment (90°C, 35 min; 90 W)The crystallinity index was 88.32% with a diameter and length of 10–5 nm and 92–105 nm, respectively[16]
Jackfruit peel65% H2SO4 (37°C, 1 h) and centrifugation at 3000 rpm (30 min), then ultrasonicated for 15 minThe crystallinity index of CNCs was 83.42%. The crystal size obtained was 2.80 nm, with an interplanar distance of 0.21 nm[67]
BagasseCNCs ware obtained by the hydrolysis acid method using 64% H2SO4 (45°C, 1 h)It has a needle shape, has a crystallinity index (73%) and an average particle size of 100 nm[91]
Bleaching procedure (4% NaOH, 24% H2O2, 70°C). Hydrolysis with 65% H2SO4 (45°C, 45 min) and centrifuged at 6500 rpm (30 min)The average length of CNCs is 413 ± 52 nm, the width is 10 ± 2.5 nm, and the crystallinity index (56%)[70]
OPEFB60% H2SO4 (45°C, 2 hrs), centrifugation at 7000 rpm (15 min) and ultrasonication treatment (53 kHz, 15 min)The CNCs have a particle diameter and length of 34.40 nm and 236.6 nm[41]
H2SO4 hydrolysis (1–3 hrs, 45°C) and alkaline method (30% NaOH)The highest yield of CNCs was 40% using 22% H2SO4. The particle size of CNCs was 264.9 nm[94]
2% NaOH (3 hrs, 70°C), bleached with acetate buffer and NaClO2 (2 hrs, 80°C), hydrolysis using 64% H2SO4 (45 min, 40°C) and centrifugation at 7000 rpm (10 min)The CNC had a particle size of 499.2 nm[95]
H2SO4 (15 min, 45°C), centrifugation at 4000 rpm (30°C, 10 min) and dialysis using a membrane of regenerated cellulose with distilled water 72 hrsCrystallinity index, particle size, and diameter (78.5%, 62.27 ± 4.17 nm, 7.44 ± 0.17 nm)[96]
65–85% phosphotungstic acid (H3PW12O40) (80–100°C, 25–35 hrs), and centrifuged at 4000 rpm (15 min)The CNCs yield was obtained at 44.8% and the crystallinity index of 73.3% with an average diameter of 140 nm. The optimum condition of phosphotungstic acid: reaction time: temperature (85%: 35 hrs: 80°C)[97]
Pineapple peel1.7% NaClO2 (80°C, 6 hrs), 64% H2SO4 (50°C, 30 min), centrifugation at 7000 rpm (10 min), and sonicated (5 min)The best results were achieved at the optimum condition of bleaching (6 hrs) and hydrolysis (30 min) producing a particle size of 657.8 nm[98]

4.1. Synthesis of CNCs Using Hydrolysis Method

Acid hydrolysis using sulfuric acid can produce a transparent suspension that is stabilized by negatively charged sulfate groups [101]. The morphology, particle size, and thermal stability of the produced CNCs will be impacted by the hydrolysis process. In particular, the CNC particle size and thermal stability will reduce as the concentration, temperature, and hydrolysis reaction time increase [94, 95, 112]. This shows that the selection of an acid to be used in the thermal hydrolysis process is an important factor. Figure 3 shows the synthesis of CNCs using the hydrolysis method with different types of acids that affect the CNCs’ morphology. Based on these results, it is known that CNCs hydrolyzed using H3PO4 have much higher thermal stability than CNCs in H2SO4. This implies a connection between charge density and the onset of thermal degradation [113].

[figure(s) omitted; refer to PDF]

4.2. The Ultrasonication Process

The ultrasonication process is widely used as an additional procedure in the CNC synthesis process. The purpose of ultrasonication is to reduce the particle size of the CNCs with the help of ultrasonic waves [15–17, 74]. The ultrasonication process helps improve the accessibility and reactivity of chemical compounds (such as acids) that can easily hydrolyze the amorphous part of cellulose [16, 17]. Furthermore, the optimal crystal structure of the nanocellulose can be increased by extracting it with solvents under optimal conditions [99]. The mechanical strength of ultrasonication causes the formation of microbubbles that move at high speeds and are aided by the defragmentation of the microfiber to nanosize [96, 114]. Based on Table 2, the synthesis of CNCs using NaClO2 1.5% along with 1 M NaOH and 65% H2SO4 from jackfruit peel was carried out by Trilokesh and Uppuluri [67], who were able to produce CNCs with the best characteristics.

5. The Characterization of Cellulose Nanocrystals (CNCs)

5.1. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

The FTIR characterization of CNCs particles aims to determine the functional groups indicated by the infrared adsorption wavenumber. Functional groups can indicate the presence of lignocellulosic compounds (such as cellulose, hemicellulose, and lignin) contained in CNCs particles. Each lignocellulosic compound is indicated by a different wavenumber [91, 95, 96, 115, 116]. The FTIR spectra of groundnut shell (GNS), chemically purified cellulose (CPC-GNS), and cellulose nanocrystals (CNC-GNS) are shown in Figure 4 [11]. Some of the adsorption peaks (1735 cm−1, 1512 cm−1, and 1264 cm−1) in the GNS particles were lost in the CPC-GNS and CNC-GNS. The loss of the adsorption peak indicates that the extraction process for GNS has succeeded in lignin, hemicellulose, and other compounds removal. This also shows that the CPC-GNS particle synthesis process using NaOH can remove noncellulose compounds. Then the adsorption peak of 3410 cm−1 on the CNC-GNS (spectrum c) has a high intensity, which proves that the noncellulose component in the CNCS-GNS particles has disappeared [72, 117, 118].

[figure(s) omitted; refer to PDF]

5.2. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Analysis

The purpose of SEM and TEM characterization is to determine the morphology of CNCs particles. The data that can be obtained from the morphological characterization results are the size and porosity of the CNCs particles [98]. Several previous studies have shown that the larger the diameter of the fiber particles, the greater the strength [96, 98]. Zhao et al. [119] analysis results are used as examples of CNC characterization using a field emission scanning electron microscope (FESEM). The study showed FESEM imaging of rice straw CNCs (CNCS-RS) and poplar wood CNCs (CNCs-PS) that had been freeze-dried at −75°C for 48 hours (Figure 5). The average diameter of CNCs-RS and CNCs-PS has been obtained at 11.6 nm and 13.7 nm, respectively. The freeze-dried treatment of CNCs is affecting the morphological structure. Therefore, the CNCs crystallites form a more cohesive geometry with many layers, as shown in Figure 5. The function of TEM analysis is to identify the pore morphology, phase, and particle size of the CNCs. An example of depicting the analysis results using TEM on CNCs OPEFB using sulfuric acid was carried out by Burhani and Septevani [120]. As a result, CNCs-OPEFB had a morphology rod with a length and diameter of 147 and 43 nm, respectively.

[figure(s) omitted; refer to PDF]

5.3. X-Ray Diffraction (XRD) Analysis

XRD is a good nondestructive technique for the characterization of CNCs. Structure, phase, texture, grain size, crystallinity, and crystal defects are all obtained from the XRD analysis [121]. Bano and Negi’s [11] research results provided an example of XRD analysis. This study used a groundnut shell (GNS) precursor that had been dried and subjected to various treatments. All diffractograms (Figure 6) had three peaks at 2θ = 16.4°, 22.5°, and 34.4°. It showed that the crystal structure of cellulose did not change significantly during the alkaline and ultrasonication treatments [122, 123]. The 2θ = 16° peaks could be classified as secondary peaks for amorphous regions of cellulose, whereas 22° represented the crystalline area of cellulose [123]. The crystallinity index obtained from CNC-GNS was 74%, which is higher than GNS itself (58%) and CPC-GNS (65%) [11].

[figure(s) omitted; refer to PDF]

5.4. Thermogravimetric (TG) Analysis

TG analysis is used to determine CNCs’ thermal stability [67, 124]. Trilokesh and Uppuluri [67] research investigated the TG CNCs analysis of jackfruit skin. In the research results, there are three levels of thermal degradation observed in the form of mass loss. Stable conditions were obtained at 150°C, cellulose thermal properties were observed at 312–350°C, and mass loss occurred at 200–380°C due to cellulose depolymerization. The mass loss was approximately 85% because of drying at a temperature of 300–350°C. At this temperature, 85% of the mass is lost during the drying process. Akinjokun et al. [125] used cocoa pods with three types of treatment, which were raw material (CPH), chemical treated (CPC), and acid hydrolysis (CNCs). The result shows that the cocoa pods’ biomass started to degrade at 219–265°C, with the lignin pyrolysis being at or above 400°C. The maximum peaks were found at CPH (339°C), CPC (350°C), and CNCs (332°C). Lignin pyrolysis activities are much higher in CPH because of the lignin content in the sample based on the maximum peaks. On the other hand, rice straw CNCs (CNCs-RS) and poplar wood CNCs (CNCs-PS) were compared in Zhao et al. [119] research. The TG analysis shows CNCs-PS has a higher peak than CNCs-RS. Poplar wood’s cellulose has a higher degree of polymerization and hydrogen bond energy, making the CNCs have good thermal stability.

5.5. Dynamic Light Scattering (DLS) Analysis

The DLS analysis method was used to assess the particle size distribution of CNCs. The basic idea behind DLS analysis was to use dynamic light scattering to measure the size distribution of particles in Brownian motion. Laser light scattering from the pinhole (small needle) was transmitted to the particles in the sample [91]. Leszczynska et al. [126] research yielded an example of a DLS characterization analysis. The sample used in the study was CNCs succinic anhydride (SA) or CNCs-SA. The result shows that all samples have small particle sizes with nanometric fractions of hydrodynamic diameter of 50–100 nm, except for those modified at 90°C. It indicated the presence of little agglomerates or microparticles in the sample [127].

6. Synthesis of Cellulose Nanofibers (CNFs)

Homogenization, grinding, and ultrasonication methods are several types of CNF synthesis methods that have been widely used in previous studies [50, 97, 98, 116, 117]. According to some research, the pretreatment process came first, followed by the mechanical synthesis process. Due to differences in precursors and methods used, chemical pretreatment is required before carrying out the process using a machine [128–134].

Table 3 shows several previous studies that used ultrasonic processing as an additional process to reduce the particle size of cellulose [17, 135–137]. The ultrasonic process is used to separate the nanofibrils from the remaining purified nanofibers. Xie et al. [17] show that using the ultrasonic method with a frequency of 25 kHz and a power of 750 W for 30 minutes will produce CNFs in an alkaline process with a higher crystallinity than nanofibers. The low crystallinity is caused by the breaking of hydrogen bonds in the cellulose fibers during the ultrasonic process. Research by Rosazley et al. [135] and Lu et al. [141] also showed that the ultrasonic process can destroy the crystalline regions of cellulose, which causes a decrease in the CNF crystallinity index. Research by Berglund et al. [138] is recommended in the CNFs synthesis process because it produces CNFs with good characteristics and because it was produced from the pretreatment process and the grinding process.

Table 3

The summary of CNFs synthesis methods from various biomasses.

BiomassMethods/TreatmentResultsReference
BambooMicrowave liquefaction treatment were then bleached in 0.1% NaClO2 (75°C, 1 h), 0.4% NaOH (75°C, 30 min), and ultrasonication treatment (25 kHz, 750 W, 30 min)The CNFs have a diameter size (2–30 nm) and a crystallinity index (74.2%)[17]
BagasseBall-milling (Spex 8000 M shaker mill for 1 h)The crystallinity index was 68%, with the CNFs particle size distribution obtained 1–10 µm[91]
Cassava peelHomogenization (10000 rpm, 10 min)The crystallinity index (62.1%) and the average diameter particle size (8.2 nm)[66]
OPEFBWet grounded using nanogrinder (722 rpm, 30 min) with 5% fiber to water ratioThe CNFs’ diameter (17.85 nm) and the crystallinity index (85.09%)[115]
Grinded using a rotary digester (1 h, 170°C), dispersed in 500 ml of distilled water, and ultrasonicated (20 kHz, 700 w, 1 h)The average diameter of CNFs (4–23 nm), crystallinity index (70.50%), and the order of thermal stability and crystallinity (CNFS > Pulp > cellulose)[135]
CorncobSamples were treated by steam explosion and bleaching procedure. Bleached samples were blended in a high-speed blender (48000 rpm, 1 min) and ultrasonicated (1000 W, 30 min)The CNFs’ diameter (5–10 nm) and the crystallinity index (40.08%)[136]
Banana peelBleached three times with 1% NaClO2 and 10% acetic acid (pH 5, 70°C, 1 h), then treated with 10% KOH (80°C, 2 hrs), hydrolyzed with 1% H2SO4 (80°C, 1 h), and ultrasonicated at high intensityThe CNFs’ particle size obtained 90–150 nm[137]
Carrot fibersFirstly, bleached in 2% NaOH (80°C, 2 hrs) and then bleached again in an acetic buffer with 1.7% NaClO2 (pH 4.5, 80°C, 2 hrs). Bleached pulps were ground using a supermass collider (1500 rpm, 20 min)The crystallinity index (77%), length (5–30 nm), and width (12 ± 8 nm)[138]
Aspen woodThe treated celluloses were diluted and disintegrated in a high shear fluid homogenizer at 1000 barThe average particle size was 2–100 nm[139]
Banana leaf stalkThe ground fibers were treated using 1 M NaOH (80°C, 4 hrs) and bleached using 10% NaOH (80°C, 5 min), then 30% H2SO4 was added (80°C, 30 min), centrifuged (13000 rpm, 5°C, 15 min), and ultrasonicated for 5 minThe crystallinity index of CNFs is 48.83%, the diameter (14.02 ± 2.10 nm), and length (619.6 ± 90.7 nm)[140]

6.1. Pretreatment

Pretreatment is a widely known additional method in the production of CNFs. Pretreatment and mechanical methods combined can improve hydration and swelling capacity [133, 134]. Common CNFs are commonly synthesized through oxidation with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), cationization, carboxymethylation, and other methods. Negatively charged groups, such as carboxyl and carboxymethyl, can be included in cellulose, and electrostatic repulsion between negatively charged cellulose nanofibrils increases nanofibril delamination [142, 143].

Ionic solutions are another popular method because of their chemical and thermal stability and their low vapor pressure [144, 145]. Ionic solutions have the capacity to either dissolve and hydrolyze cellulose or to disrupt its crystal structure prior to hydrolysis. Na2CO3 and NaOH treatments produced sodium carbonate (CO32−), which reduced hydrogen bonds between cellulose fibers and influenced nanocellulose dispersion [146–148]. Another positive impact of combining pretreatment as well as mechanical methods for CNFs synthesis is that it can reduce energy consumption [149, 150]. Overall, some pretreatment methods are environmentally friendly technologies that influence CNF characteristics [151–153].

6.2. Homogenization

The homogenization method is used to convert microfibers into nanofibers using a homogenizing machine and a microfluidizer machine. Under high-pressure and high-speed conditions, the cellulose pulp is forced through a small gap between the impacting and homogenizing valves. It will destroy the fiber and cause cellulose fibrillation [14]. There are several drawbacks to using this method, including high-energy consumption, clogging, and the use of long fibers as the raw material [150, 154]. Chemical pretreatment can be used to reduce the high energy consumption [155, 156].

6.3. Grinding

The grinding method is a common grinding process used in CNF synthesis to produce smooth cellulose. The cellulosic material is fed into the grinding area that consists of moveable and fixed discs [14, 157]. The grinding method has more advantages compared with the homogenization method, such as high efficiency, large capacity, low-energy consumption, and less-clogging susceptibility. The grinding machine is so powerful that it damages the fiber structure. As a result, the CNF properties produced are poor in physical strength, crystallinity, and thermal stability [158, 159].

6.4. Ball Milling

Ball milling is another method for producing CNFs. This method of cellulose processing involves crushing it with a high-energy ball. Ceramic, metal, or zirconia are the most common spherical materials used [14, 160, 161]. Cellulose will be crushed between the balls as it enters the hollow cylinder container. For example, Kekalainen et al. [162] used this method to produce CNFs from undried wood. CNFs have a diameter of 3.2 nm and nanofibrils have a diameter of 10–150 nm. Sofla et al. [91] used the ball milling method for 1 hour to produce CNFs from bagasse.

7. The Characterization of Cellulose Nanofibers (CNFs)

7.1. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

The results of CNF characterization using FTIR are derived from Soni et al. [163] study. The cotton stalk CNFs are compared in this study with untreated bleached pulp, sulfuric acid neutralized treated, sulfuric acid dialyzed treated, and TEMPO-oxidized treated CNFs. Figure 7 shows that the lignocellulosic material has peak adsorption characteristics at 3350, 2900, 1740, 1430, 1166, and 896 cm−1 [13, 164]. The missing peak at wavenumber 1734 cm−1 indicates that lignin and hemicellulose have completely disappeared from the CNFs [163]. Several missing peaks indicate the cellulose content in the sample has increased [10, 66, 165].

[figure(s) omitted; refer to PDF]

7.2. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Analysis

As Johar et al. [112] revealed, the synthesized CNCs can undergo morphological changes because of the synthesis process, as shown in Figure 8, which is the result of characterization using SEM. Figure 9 depicts the residual state prior to and following bleaching and treatment. The untreated rice husk fiber has a smooth structure (a). The fiber structure is rougher after alkali treatment (b), indicating that the impurity components (such as pectin content) were partially removed. This causes the diameter of the CNFs to shrink after the bleaching process. Atiqah et al. [166] used a TEM image of the kenaf plant (Figure 9(a)) in their study. As shown in Figure 9(b), the CNFs were synthesized using supercritical carbon dioxide (SC-CO2) and acid hydrolysis. The SC-CO2 procedure results in the removal of lignocelluloses, as shown in Figure 9(a). The diameter of SC-CO2 CNFs was in the 10–15 nm range on a 200 nm scale. The ultrasonicator nanofibers are strong enough to defibrillate the cellulosic fibers obtained through microwave liquefaction and chemical treatment to reach the raw fiber. This is due to an ultrasonic treatment that can break hydrogen bonds and turn microfibers into nanofibers [17, 135].

[figure(s) omitted; refer to PDF]

7.3. X-Ray Diffraction (XRD) Analysis

Tibolla et al. [167] illustrated the XRD spectrum (Figure 10) showing the diffractogram pattern of chemically and enzymatically synthesized banana peel nanofibers. The peaks are located at 2θ = 16°, 17°, and 22°. The banana peel bran XRD analysis revealed a peak at 2θ = 17°. This peak indicates that the bran contains many amorphous regions. At the same time, the treated one has sharp peaks in 2θ = 16° and 22° bands. The sharp diffraction peaks at 2θ = 16° and 2θ = 22° were typical of cellulose I, which has a crystal structure in which it forms β(1 ⟶ 4)-D-Glucopyranose units and a parallel block of glucan chains. This significant increase was caused by these treatments removing half of the hemicellulose and most of the lignin [17, 66, 167].

[figure(s) omitted; refer to PDF]

7.4. Thermogravimetric (TG) Analysis

The results of the analysis using TG in the research of Widiarto et al. [66] showed that cellulose, lignin, and hemicellulose have different composite points. Cellulose begins to decompose at a temperature of 315°C and continues up to 400°C. Hemicellulose begins to decompose at a temperature of 220°C, at a temperature of 315°C, a broader temperature range for lignin. The percentages of solid residues after pyrolysis at 700°C for cellulose, hemicellulose, and lignin were 6.5; 20; 46 wt% [66]. The mass of lignocellulose has decreased significantly, caused by the decomposition of lignocellulose at a temperature of 200–600°C.

Another example is the Radakisnin et al. [168] study, which varies sulfuric acid molarities (1 M, 3.8 M, and 5.6 M). In the second thermal degradation, the weight loss of the CNFs modified by sulfuric acids (CNFs-PP) ranged between 40.94 and 50.46%. Due to the thermal peak (277.08°C) and weight loss, CNFs-PP5.6 M has the most outstanding thermal stability compared to the other acid molarities. In addition to the ball-milling methods, it is linked to another research. Sulfuric acid and ball-milling methods were used to reduce the weight of bagasse CNFs at temperatures ranging from 299°C to 364°C, with 50% weight loss achieved at 331°C. Ball-milling treatment methods for sulfuric acid-treated CNFs have high thermal stability [91].

8. Modification of CNCs and CNFs

CNCs and CNFs were modified to improve or increase their physical and chemical properties [36, 169]. Modifications were carried out by adding chemicals or combining specific methods to achieve the desired property change [169–171]. Surface and functional group modifications are two of the most common CNCs and CNFs modifications. Since the nanocellulose has many hydroxyl groups on its surface, it is easy to modify. The adsorbent surface was modified to increase the number of ionic, ionizable, or complex sites, thereby, increasing the adsorption capacity [172–174].

8.1. CNCs and CNFs Modification with Various Materials

Table 4 shows several CNC and CNF modifications that were successfully completed. Some articles have demonstrated the effect of pH on the CNCs and CNFs modification processes. Abitbol et al. [1] explained the effect of pH on CNC synthesis using an alkalinization process that deprotonated COOH anions on the CNC surface. The effect of this reaction appeared at pH 4 and pH 10, which was the minimum pH of the surface charge density. The addition of NaOH at pH 10 increased the ionic strength. It shows that the ionic strength at pH 4 has doubled at pH 10 [1, 182]. Figure 11 depicts the ionic exchange reaction caused by modifying CNCs with four different types of ammonium salts in a pH 10 NaOH solution. CNCs were treated with NaOH to maximize carboxyl group dissociation to the nanocrystal surface area [183].

Table 4

The modification of CNCs and CNFs with various materials.

CNCs/CNFs precursorModifier and methods/treatmentResultsReference
Filter paperModification of CNCs has been done using L-Cysteine. CNCs is reacted with NaIO4 (40°C, 12 hrs, without light) and produces dialdehyde cellulose (DAC)The surface area of CNCs (10.81 m2/g) and Lys-CNCs (72.10 m2/g)[36]
CottonThe grafting procedure of polyethyleneimine (PEI) onto TOCNF was conducted using the glutaraldehyde crosslinking method. The PEI was added to TOCNF, mixed for 24 hrs, and then centrifuged at 8000 rpm. After that, 25% glutaraldehyde was added and 0.4 M NaOH for adjusting the pH of the solution. TOCNF is then freeze-dried and ground to produce TOCNF-PEIAfter the addition of PEI, the TOCNF becomes covered by the PEI layer and forms small-sized pores between the TOCNF branches[47]
Centrifuged at 10000 rpm for 10 min after being dispersed in 64% H2SO4 (45°C, 45 min). Dried CNCs are then mixed with 15 g of succinic anhydride to produce SCNCs (120°C, 12 hrs)SCNCs have a smaller size than the CNCs, which have rod-like particles with a 30 nm diameter[103]
The CNCs were mixed with polyhydric alcohol and then modified them with polyurethane foam (PUF)The rod-shaped CNCs-PUF has a diameter of 10 nm[107]
CelluloseThe CNCs powder was modified by crosslinking with 3-chloro propyl trimethoxysilane (CPTMS) to produce CPTMS/CNCsCPTMS/CNCs have good thermal stability and are smaller and have less porosity[48]
CNCs were treated by acid hydrolysis (3 hrs) and modified by polypyrrole to produce nanocellulose polypyrrole (NCPPY)The NCPPY has a spherical shape (10–100 nm). Pristine CNCs have a surface area of 197 g/m2 and a NCPPY of 488 g/m2[175]
OPEFBThe CNCs are modified with 3-(2-aminoethylamino) propyl dimethoxymethylsilane (AEAPDMS) 1, 2, 3, 4%) at pH 4 and reflux (60°C, 3 hrs) to produce the CNC-AEAPDMS (1 : 1, 1 : 2, 1 : 3, 1 : 4)The crystallinity index of CNCs without modification was 76.8%. The average length and width of CNCs particle size were obtained at 173 ± 0.6 nm and 9 ± 0.5 nm, respectively. The average length and width of CNC-AEAPDMS (1 : 3) were obtained as 193 ± 0.9 nm and 10 ± 0.4 nm[80]
BagasseCNCs were treated by H2SO4 65% (45°C, 45 min). The CNCs were modified by adipic acid (C6H10O4) and H2SO4 as a catalyst. The mixture was stirred (80°C, 2 hrs) and centrifuged (14000 rpm, 10 min) to produce an MCNCThe crystallinity index of MCNCs was 56%. The average length of unmodified CNCs and MCNCs was 413 ± 52 nm and 242 ± 57 nm, respectively. The average width of CNCs and MCNCs was 10 ± 2.5 mm and 6.8 ± 1.7 nm, respectively[70]
Banana peelCNFs were modified using acetylation (acetate acid and acetic anhydride mixture) and pyridine (reflux 100°C, 4 hrs) were used to create a MCNF from CNFsCNFs and banana CNFs (BCNFs) particle sizes were obtained at 90–150 nm and 30–60 nm. The acetylation process causes a decrease in CNF crystallinity[137]
Aspen woodCNFs treated by TEMPO oxidation and soaked in deep eutectic solvent (DES)Produced 73% cellulose[139]
Microcrystalline cellulose (MCC)Succinic anhydride (SA) (C4 H4 O3 ) (70, 80, 90, 100, and 110°C; SA : OH = 2 : 1, 3 : 1, 5 : 1, and 10 : 1 for 60, 120, 180, and 240 minutes). Centrifugal by centrifuge (5200 rpm, 15 min) and sonication (15 min)The crystallinity index of CNCs-SA (SA : OH) 5 : 1 was 78%. The CNCs-SA has a particle size of 60 nm. The unmodified CNC particle size was 100 nm[126]
CNFs modified by acid hydrolysis treatment. The suspension was strongly stirred (45°C, 2 h). Modified by in situ polymerization of polyaniline (PANI) and produces PANI-CNFs in film formThe average diameter is 30–45 nm[169]
Modification of CNCs using 3-amino propyltrimethoxysilane (APTMS) and 3-mercapto propyltrimethoxysilane (MPTMS). APTMS/MPTMS for 2 hrs, followed by drying (110°C for 2 hrs) and centrifugation (1400 rpm for 2 min)The CNCs-APTMS and CNCs MPTMS yields were obtained at 60–70% and 82%, respectively[173]
Raw kenaf bast fiberThe CNFs are modified by polylactic acid (PLA) to produce acetylated nanofibers (ACNFs)The crystallinity and thermal stability of ACNFs were lower than unmodified PLA. The crystallinity of PLA and ACNFs decreased (81% to 74%)[170]
Cellulose sludgeCNFs were treated by mechanical treatment and modified by 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) to produce TOCNFThe average diameter size of TOCNF was 18.6–29.8 nm and the average fiber diameters of TOCNF were 15–40 nm[176]
Commercial CNFsCNFs were synthesized by rosin (C20H30O2) and stirred (130°C, 1 day), followed by being washed by ethanol and centrifuged to produce a CNFRThe crystallinity index of CNFs and CNFRs was 59.91 ± 0.11% and 63.42 ± 0.27%. The particle diameters of CNFs and CNFRs were 10–60 nm and less than 10 nm, respectively[177]
BambooCellulose was treated by mechanical treatment and modified by acrylic acid and sodium humate to produce bamboo cellulose nanofibers/Acrylic acid grafted (BCN-g-PAA) and bamboo cellulose nanofibers/Acrylic acid grafted/Sodium humate (BCN-g-PAA/SH)The unmodified BCN had a large specific surface area of 79 m2/g[178]
Rice strawCNFs were treated by alkaline treatment using NaOH 15% (150°C, 2 hrs). Followed by bleaching (NaClO, 80°C, 1 h) and an oxidation process using a mixture of TEMPO. 4′-Chloro [2,2′] terpyridine (Cu-Tpy) is used to modify CNFs to create CNFS-Cu-TpyThe crystallinity index of CNFs and CNF-Cu-Tpy was obtained at 69.5% and 69%, respectively. The CNFs width was obtained at 3–5 nm[179]
WoodCNFs were treated by oxidation with TEMPO (pH 6.8; 60°C, 2 days). CNFs modified with NaIO4 (at room temperature for 20 hrs) yield dialdehyde cellulose (DAC)The surface area of CNFs and DACs was obtained at 185.1 m2/g and 134.4 m2/g, respectively[174]
CNCs treated with 64% H2SO4 (45°C, 30 min). CNCs were modified by NaIO4 (1.5, 3, 6, and 9 mmol/g) and stirred for 2 days at 40°C to produce dialdehyde nanocrystalline cellulose (DANC). DANC was reacted with NaBH4 (30°C, 6 hrs) and produced amino-functionalized nanocrystalline cellulose (ANCC)The crystallinity index of CNCs, DANC, and ANCC was obtained at 63%, 56.3%, and 59.8%, respectively. The crystal size of CNCs, DANC, and ANCC was 3.68 nm, 3.35 nm, and 3.26 nm, respectively[180]
The softwood is bleached and modified by an oxidation process using TEMPO. Then CNCs/TEMPO were sonicated for 8 min for mechanical fibrillation treatmentThe TOCN has a 4 nm width, and the average lengths of TOCN were 1 µm to 500 nm[181]

[figure(s) omitted; refer to PDF]

8.2. Cellulose Derivatives

Cellulose can be synthesized by the oxidation method using TEMPO, which is carried out as a pre-treatment process before the mechanical synthesis process is carried out. The result of this synthesis is usually referred to as TOCNF [92]. The TEMPO oxidation method has several advantages, including its efficiency, selectivity, and convenience [165, 180]. The TEMPO radicals are used to convert monosaccharide primary alcohol groups to carboxyl groups. Nitrosonium ion (+N=O) is produced in situ during the reaction between TEMPO radicals and oxidants. Both will occur in the oxidation of cellulose fibers. As a result, before being converted to carboxyls, primary alcohols are converted to aldehydes. At the same time, the phenomenon of cellulose depolymerization appears [181, 184]. The TEMPO/NaClO/NaClO2 study revealed that the reaction occurred in neutral or weak acid conditions. According to the findings, the degree of polymerization remained essentially constant [181, 185]. The TEMPO oxidation method could reduce the energy needed for further cellulose disintegration [139, 184, 186, 187]. The hydroxyl groups on cellulose can react with chloroacetic acid to produce carboxymethylation products in alkaline conditions [19]. Electrostatic repulsion between the quaternary ammonium cations causes the cations on the cellulose surface to form during the cationization process.

In situ polymerization of polyaniline (PANI) onto the nanocrystal surfaces were synthesized from this cellulose crystal. The average diameter is calculated to be 30–45 nm [169]. CNFs are made from aspen wood that has been TEMPO-oxidized and soaked in deep eutectic solvent (DES), which produces a 73% cellulose yield [139]. Mechanical treatments were used to create CNFs from raw kenaf bast fibers and modified by polylactic acid (PLA). The articles then compared PLA with acetylated nanofibers (ACNFs). The decrease in crystallinity is expected due to cellulose crystalline structure degradation during the acetylation process. The results showed that acetylated nanofibers had lower crystallinity and thermal stability than non-acetylated nanofibers [170]. Cotton nanocrystalline cellulose was synthesized in anhydrous phosphoric acid and used to modify the CNCs-PUF, according to Li et al. [107]. The modifier has a significant impact on cell structure. The CNCs-PUF is shaped like a rod and has a diameter of 10 nm. The ammonia neutralization phosphates remained in the PUF products.

The cross-linked method was used to synthesize commercial CNC powders with 3-chloropropyl trimethoxysilane (CPTMS/CNCs). CNCs pores were larger and had more porosity than CPTMS/CNCs pores [48]. The acid hydrolysis of cellulose was used to create nanocellulose and modify it with polypyrrole (NCPPY). According to FESEM analysis, the sulfuric acid hydrolyzed NC has a spherical shape and is approximately 10–100 nm in size. The surface area of pristine CNCs is 197 g/m2, whereas NCPPY has a larger surface area (488 g/m2) [175]. The mechanically prepared TEMPO-oxidized cellulose nanofibers from cellulose sludge show good morphological homogeneity for TOCNFs with higher oxidation or carboxylation. Another study found that sonication produced individual nanofibrils of TOCNFs that ranged in length from 1 µm–500 nm on average. Another study found that TOCNFs were grafted with polyethyleneimine (PEI). Following the addition of PEI, the TOCNFs are covered by the PEI layer, which forms small pores between the TOCNFs branches [47, 176, 181].

Cotton was hydrolyzed to obtain cellulose nanocrystals (CNCs) with the freeze-dried method. CNCs have rod-like particles with a 30 nm diameter and a hundred nanometers in length. The SCNCs (succinic CNCs) have smaller sized particles than the CNCs. These results indicate the surface hydroxyl groups reacted with succinic anhydride, leading to slight destruction of the aggregates [103]. Cellulose from bamboo was modified by acrylic acid to form bamboo cellulose nanofibers/acrylic acid grafted (BCN-g-PAA) and bamboo cellulose nanofibers/acrylic acid grafted/sodium humate (BCN-g-PAA/SH) using sodium humate. Bamboo fibers were treated by a high shear homogenizer [178]. Li et al. [36] conducted research that revealed CNC modification using L-Cysteine (Lys). The CNCs and Lys-CNCs have surface areas of 10.81 m2/g and 72.10 m2/g, respectively. The mechanism for the formation of CNCs-Lys has the effect of acid hydrolysis, which leaves a crystal area while losing the amorphous part. Meanwhile, sodium periodate selectively oxidizes CNCs.

Based on the research of Singh et al. [173], they have synthesized CNCs by using 3-aminopropyltrimethoxy silane (APTMS) (C6H17NO3Si) and 3-mercaptopropyltrimethoxy silane (MPTMS) (C6H16O3SSi) to produce CNCs-APTMS and CNCs-MPTMS. CNCs-APTMS and CNCs-MPTMS are both silane compound reactions. Silene, with amino and thiol groups, was chosen because it had reactivity. The silanol groups easily formed Si-O-Si as the final fabrication resulted in a CNC surface [48, 80, 173]. MPTMS was also used in the research of Rong et al. as a CNF modifier. A schematic of the reaction between CNFs and MPTMS to form a sponge is shown in Figure 12. According to Rong et al. [37], CNFs with a carboxylic group can form additional complexation sites for Hg(II) ions and increase the material’s hydrophilicity. MPTMS has a thiol group that acts as a cross-linker for CNFs, which helps to improve the mechanics of the resulting sponge CNFs. Hg(II) ions can be selectively absorbed by porous materials such as honeycomb [188].

[figure(s) omitted; refer to PDF]

Some articles, such as those by Leszczyńska et al. [126] and Mohd et al. [80], used amine groups to modify CNCs and CNFs and revealed that the modification could increase the crystallinity index and affect the particle size of the resulting CNCs and CNFs (Table 5). Jin et al. [180] discovered a change in the crystallinity index and particle size because of modification. This study discovered an alteration in the isoelectric point of each type of modified CNC, namely, amino-functionalized nanocrystalline cellulose, in addition to the crystallinity index and particle size (ANCC). This isoelectric point influences the particle attraction in a specific pH atmosphere, which influences the adsorption capacity [179, 204]. The maximum adsorption capacity of ANCC was 516.1 mg/g at the optimum isoelectric point of pH 8. Ferreira et al. [70] developed MCNCs (Modified CNCs) by modifying CNCs with adipic acid. The results revealed that the particle size was also altered in the CNCs crystallinity index. Yao et al. [174] synthesized CNCs with TEMPO and modified them with NaIO4. This change influenced the pore diameter and specific surface area. The pore diameters of CNFs and dialdehyde cellulose (DAC) were 3–30 nm and 2–30 nm, respectively. CNFs and DACs had specific surface areas of 185.1 m2/g and 134.4 m2/g, respectively.

Table 5

The heavy metal ions adsorption capacity and effectiveness of various modified CNCs and CNFs at optimum pH.

CNCs/CNFs typeHeavy metal ionsEffectivity of adsorption (%)Qmax (mg/g)Optimum pHReference
CNC-LysHg(II)—5875[36]
CNC50
CNF-CMCAg(I)—106.07—[46]
Cu(II)74.79
Pb(II)111.46
Hg(II)131.38
CNF-TEMPO-PEICu(II)—52.325[47]
SCNCPb(II)—367.65.5[103]
Cd(II)259.76.0
NaSCNCPb(II) 465.15.5
Cd(II) 344.86.0
CNC-APTMSAs(V)85.70—2.5[173]
CNC-MPTMSAs(III)94.47.5
CNFCu(II)58.60——[174]
Pb(III)75.09
CNC-P (MB-IA)Co(II)90——[189]
Cd(II)—240[190]
CNC-TSCHg(II)—331.1—[191]
Diethylenetriamine-CNCCu(II)—63.094.5[192]
Pb(II)87.41
Carboxylated-CNCCu(II)631854[193]
CNF-nitric acidCd(II)98.234.716.5[194]
Pb(II)96.126.89
Ni(II)90.312.98
CNCAg(I)—34.356.39[195]
CNF15.455.45
CNC-phosphatetCu(II)99——[196]
CNC-succinateZn(II)—0.0106—[197]
Ni(II)0.0118
Co(II)0.0122
Cd(II)0.0067
CNC-citric acidPb(II)86.4721.7046[198]
Cu(II)77.4021.85
Zn(II)70.0411.29
Cd(II)85.2023.47
CNF-TEMPOCu(II)—60—[199]
CNFPb(II)96.3—5[200]
CNC253
CNCF-PEICu(II)85616.486[201]
CNCCr(VI)96.97652[202]
CNF-CysHg(II)—410.56.8[203]

Krishnan and Ramesh [137] produced MCNFs via the acetylation process. The CNFs and Banana CNFs (BCNFs) particle sizes ranged from 90 to 150 nm and 30 to 60 nm, respectively. The acetylation process influenced the crystallinity index of CNFs. Niu et al. [177] reported rosin-based CNFs modifications. The crystallinity indexes of CNFs and R-CNFs were 59.91 ± 0.11% and 63.42 ± 0.27%, respectively, with CNFs and R-CNFs particle diameters ranging from 10 to 60 nm, influenced by the formation of esters on rosin resins [177]. CNF-4′-Chloro (2,2′ : 6′,2″) terpyridine (Cu-tpy) modification produced CNFs with lower crystallinity indexes (69% and 69.5%) and particle sizes ranging from 3 to 5 nm, according to Hassan et al. [179]. It is possible to conclude that changes to CNCs and CNFs can alter the physical properties of the resulting CNCs and CNFs. Pore size, crystallinity index, surface area, and other physical properties are affected by the modification.

9. Application of CNCs and CNFs as Heavy Metal Ions Adsorbent

9.1. The Challenges and Key Parameters for Heavy Metal Ions Adsorption

Based on several journals that we collected and reviewed, the adsorption process, especially for water treatment, is influenced by a variety of factors. This makes the adsorption process challenging in and of itself. According to Dotto and McKay [205], the overall challenges in the adsorption process emerge from economic and environmental factors. This is because adsorbents that are generally considered great have low production costs, are environmentally friendly, and can be reused. According to the journal, providing the adsorbent costs 70% of the total cost of the adsorption treatment plant. This also encourages many adsorption studies that use and develop adsorbents from biomass-based precursors because they are less costly.

Dotto and McKay [205] mention in their journal that environmentally friendly adsorbents have been extensively developed extensively to develop adsorbent manufacturing industries with better waste management and lower pollutant concentrations. However, this discharge should be reduced further, necessitating additional research into adsorbents with a focus on adsorption capacity and isotherm equilibrium analysis.

Several other factors that influence the adsorption process are pH, surface area of the adsorbent, active sites, selectivity ability of the adsorbent for certain pollutants, an adsorption model (including isotherms and adsorption kinetics), adsorption reaction time, and adsorbent dosage [206]. These factors will be discussed briefly in the next subpoint, and this review journal will focus more on discussing the effect of pH on the adsorption process of heavy metal ions using CNCs and CNFs.

9.2. The Effect of pH on Heavy Metal Ions Adsorption Selectivity

One of the factors influencing the adsorption process is pH regulation. Heavy metal ions in water can combine to form anionic and cationic species. The efficient adsorption of heavy metal ion species is typically determined by the type of adsorbent and the species used in the study [207–209]. Proper pH adjustment will affect the adsorbent’s selectivity toward pollutants, maximizing adsorption capacity, partition coefficient, and selectivity [189, 210, 211]. In some research, nanocellulose modification altered the adsorption performance of materials [49, 175, 178, 212]. Surface modification by anchoring functional groups is the most used CNCs and CNFs modification. Several previous studies have shown in Table 5 that CNCs and CNFs modifications containing amino groups have selectivity to certain metal ions at low pH.

Table 5 shows the adsorption capacity and efficiency of several heavy metal ions by various modified CNCs and CNFs at optimum pH. Table 5 will be discussed further in Sub-Section9.2.1 and so on.

9.2.1. Hg(II) Ion Adsorbents

Other articles have modified CNCs and CNFs with L-Cysteine as a Hg(II) ion adsorbent [36, 203]. L-cysteine contained an amino group, it could form covalent bonds with CNCs and CNFs. CNC-Lcys in the study of Li et al. [36] has an optimum adsorption capacity at pH 5. When the solution atmosphere is in the pH range of 2–5, it will affect the adsorption capacity of CNC-Lcys to Hg(II) ions. This can happen because the amine group of CNC-Lcys is protonated. At the same time, protons will compete with Hg(II) ions to enter the active site of the CNC-Lcys adsorbent. Increased pH causes deprotonation of the amine and carboxyl groups, which increases the amount of OH on the surface of the CNC-Lcys adsorbent, causing the OH group to react with Hg(II) ions to form Hg(OH)+, Hg(OH)2, and Hg(OH)3, reducing adsorption effectiveness [208, 209]. CNC-Lcys are able to adsorb Hg(II) ions with an adsorption capacity of 587 mg/g, and unmodified CNCs have a lower adsorption capacity of less than 50 mg/g [36]. Bansal et al. [203] synthesized CNF-Cys from bagasse, and CNF-Cys has free amine and sulfhydryl groups. These groups will bind with Hg(II) ions to form mercaptides. At low pH, protonation can occur at the active site of the adsorbent. At a higher pH, the free amine and sulfide groups are active, so that the deprotonation process occurs in the adsorbent with a negatively charged adsorbent surface [212]. This affects the efficiency of adsorption and increases the value of adsorption capacity. The increasing the pH value will cause Hg(II) ions to precipitate into Hg(OH)2, which causes the adsorption capacity of CNF-Cys to decrease. The optimum adsorption capacity of Hg(II) ions by CNF-Cys was obtained at pH 6 of 410.5 mg/g. Jiang and Wang [191] also discussed Hg(II) metal adsorption using a CNCs modified thiosemicarbazide (CNC-TSC). When CNC-TSC adsorbed the Hg(II) metal, the decrease in the concentration of Hg(II) ions in the system encouraged ionization to produce HgCl−, making precipitation almost impossible.

9.2.2. Cu(II) Ion Adsorbents

Sheikhi et al. [193] discovered that electrosterically stabilized nanocrystalline cellulose (ENCC) synthesized from wood fibers via chloride oxidation had high charge and colloidal stability. ENCC has a carboxyl group that serves to neutralize Cu(II) ions to be adsorbed. Based on the results of the study, it was found that ENCC has the capability of adsorption of Cu(II) ions with a capacity of 185 mg/g at pH 4, which is achieved at a concentration of Cu (II) of 300 ppm and has a high efficiency of Cu(II) ion removal of 63%. At higher Cu(II) concentrations, 300 or 400 ppm will cause the ENCC aggregate to grow gradually over time. This indicates the need for a higher concentration of Cu(II) ions to neutralize ENCC particles. The pH conditions of less than pH 4 will facilitate the breakdown of aggregates by replacing some of the adsorbed Cu(II) ions with protons. Larger particle size can shrink, which is the effect of polymer chain contraction on ENCC due to a decrease in pH.

Si et al. [201] and Mohamed et al. [202] have done recent studies on CNFs and CNCs. The carboxymethyl cellulose nanofibrils (CMCNFs) were prepared by varying the ratio of monochloroacetic acid (MCA) and sodium hydroxide to eucalyptus bleached pulp (EBP), and the CMCNFs were modified using a PEI crosslinking reaction to produce CMCF-PEI aerogels. The study by Si et al. [201] showed that CMCNFs-PEI has an amino group, so that the surface charge of CMCNFs-PEI changes with changes in pH, which will affect the adsorption capacity. The adsorption capacity of Cu(II) ions will increase along with the pH value caused by the protonation process of the amino group. In other words, the adsorption capacity of Cu(II) ions by CMCNFs-PEI was low at low pH conditions. In this study, the best Cu(II) ion adsorption capacity was obtained at pH 6 conditions. Because at pH 6, the carboxyl group content of the airgel also increased. Cu(II) ion adsorption could be attributed to the active sites on the surface of CMCNF30-PEI (30 being the amount of MCA). The carboxyl and amino groups act as electrostatic attractors and chelators to Cu(II). The results show that the adsorption capacity of Cu(II) ions increased as the carboxyl group content increased, with the maximum adsorption capacity of CMCF-PEI being 616.48 mg/g.

In addition, Mautner et al. [199] used CNF-TEMPO to adsorb Cu(II) ions with a capacity of 60 mg/g. According to Zhang et al. [47], the functionalization of TOCNF (CNF-TEMPO) with polyethyleneimine (PEI) and the crosslinking method resulted in an adsorption capacity of 52.32 m/g at pH 5. When the pH increased, the TOCNF surface became negatively charged due to a decrease in zeta potential, which affected increasing the adsorption capacity of Cu(II). These findings suggest that Cu(II) and the adsorbent interact depending on the functional groups present on the TOCNF surface [47].

9.2.3. Ag(I) Ion Adsorbents

Liu et al. [195] investigated the pH selectivity on CNCs and CNFs as Ag(I) metal adsorbents. The outcomes showed that CNCs had a greater adsorption capacity than CNFs. CNCs had an adsorption capacity of 34.35 mg/g, whereas CNFs had an adsorption capacity of only 15.45 mg/g. It was caused by the sulfate functional group on the CNCs surface. The adsorption properties were pH dependent, with the best adsorption occurring in the neutral pH range. Adsorption was low at low pH because H+ ions competed with Ag+ for adsorption on the SO3− functional groups on the CNCs surface.

9.2.4. Cr(VI) Ion Adsorbents

Mohamed et al. [202] investigated the use of supercritical carbon dioxide (scCO2) as a waterless pulping agent for separating CNCs from waste cotton cloth (WCC) for Cr(VI) ion adsorption. It was discovered that the CNC of WCC was a biosorbent for Cr(VI) ion removal, with the maximum Cr(VI) ion removal determined to be 96.97% (pH 2; 60°C). Because of the large number of H+ protons, the percentage of Cr(VI) ions removal is high at pH 1-2, and CrO42− and Cr2O72− will be protonated and adsorbed on the CNCs. However, when the pH increases, the percentage of Cr(VI) ions removed will decrease because there will be fewer H+ protons in the solution that can protonate Cr(VI) ions. The percentage of Cr(VI) ions removal becomes lower at higher pH conditions because the abundance of OH groups causes a mutual repulsion during the adsorption process.

9.2.5. Other Heavy Metal Ion Adsorbents

Chen et al. [46] produced CNF-CMC by modifying CNFs with carboxymethyl cellulose (CMC) and testing it for adsorption of several heavy metal ions. The adsorption capacities of Ag(I), Cu(II), Pb(II), and Hg(II) metal ions were 106.07, 74.70, 111.46, and 131.38, mg/g, respectively. Modifying CNCs with amino groups was also effective for arsenic metal species such as As(III) and As(V), as examined by Singh et al. [173]. The covalent bonds between silane and CNCs connect the silane hydrocarbon chains to form a network. The resulting CNCs silane had a porous structure that was interconnected to form a cavity between the silane and the CNCs, allowing easy access to arsenic species and increasing the decontamination efficiency [48, 80]. On CNC-MPTMS, As(III) had a strong affinity for sulfur and sulphydryl groups, whereas As(V) had a strong affinity for reduced nitrogen groups. CNC-APTMS was more effective for As(V) adsorption, with 95.6% effectiveness, and CNC-MPTMS was more effective for As(III), with a 94.4% effectiveness [173].

Anirudhan et al. [210] also modified CNCs by using a hybrid material that has excellent properties and can be used as a Co(II) metal adsorbent. The modification was made by improving the carboxyl functional group in a mixture of cellulose and nanobentonite used as an adsorbent to remove Co(II). When the adsorbent dose was low, 0.25 g/L, this adsorbent effectively removed Co(II) up to 90%. Liu et al. [196] discovered that phosphate groups enzymatically tethered to the CNCs surface could improve the adsorption efficiency of Cu(II) and Fe(III) metal ions. The CNC-phosphate modification produced 99% efficiency for Cu(II) and Fe(III) metal ions.

Anirudhan and Shainy [190] utilized CNCs to produce 2-mercaptobenzamide (P (MB-IA)). Itaconic acid copolymerization with various vinyl monomers could be used as an adsorber for wastewater containing Cd(II) and Hg(II). The maximum adsorption capacity for Cd(II) and Hg(II) at a dose of 2.0 g/L or lower was found to be 240 mg/g. Kardam et al. [194] showed the percentage of adsorption of CNF cations to metal ions (Cd, Ni, and Pb) increased at pH 2–4 and remained constant at pH conditions above 6.5. Based on the results of these studies, it appears that pH 7–8.5 causes metal ions to precipitate. Precipitated metal ions will become a nuisance and cannot be distinguished from the adsorption phenomenon at pH conditions of more than pH 7. CNFs and metal ions will bind to the van der Waals attraction and most of the ion exchange process between the OH and COO groups, which have a charge negative on the surface of CNFs and metal cations. The adsorption capacities of modified CNFs with nitric acid for Cd(II), Pb(II), and Ni(II) ions were obtained at pH 6.5 conditions of 98.23, 96.12, and 90.31%, respectively. Hokkanen et al. [197] used succinic acid (SA) to modify CNCs. CNC-succinate was used for the adsorption of Zn(II), Ni(II), Co(II), and Cd(II) metal ions with adsorption capacities of 0.0106, 0.0118, 0.122, and 0.0067 mg/g, respectively.

The adsorption capacities of Pb(II) and Cd(II) on succinic CNC (SCNCs) and sodic nanoadsorbent (NaSCNCs) in the Yu et al. [103] study were found to increase with increasing pH. When the pH is lower, the concentration of protons competing with metal ions for active sites rises. On the other hand, the adsorbent surface is positively charged and metal ions with positive charges are due to electrostatic repulsion when approaching functional groups. The concentration of protons decreases as pH increases, and the adsorbent surface charge becomes negative. As a result, the electrostatic attraction between the metal ions and the adsorbent increases, resulting in a higher adsorption capacity. The optimal pH values, defined as the maximum adsorption capacity of Pb(II) and Cd(II), were found to be 5.5 and 6.5, respectively. Madivoli et al. [198] used a CNCs modification based on citric acid as an adsorbent for Pb(II), Cu(II), Cd(II), and Zn(II). It had a minimum adsorption capacity at pH 3 and a maximum adsorption capacity at pH 6. Metal ion adsorption decreased at low pH as proton competition at the active site increased. If the solution’s pH increased by 4, the carboxylate groups were deprotonated, the adsorbent surface became negatively charged, and the adsorption capacity increased [198, 213]. Ramos-Vargas et al. [200] discovered that the adsorption capacity of CNCs and CNFs increases with increasing pH value when used to remove Pb(II). The proton concentration will fall and the surface charge will become negative. The electrostatic attraction between the adsorbate and the adsorbent increases the adsorption capacity of CNFs at pH 5 to 96.3%. Due to the generally increased interaction between Pb(II) ions and the adsorbent in CNCs, the adsorption capacity of Pb(II) ions increases to 25% at pH 3. Furthermore, the effect of pH on Cr(VI) ion adsorption was investigated using a pH range of 2 to 9. Cr(VI) ions form primarily in anionic form as the pH decreases, with affinity for the protonated NCPPY adsorbent. The rate of adsorption decreases as pH rises due to a decrease in protonation on the adsorbent surface [175]. The CNC, which was modified with amino groups, was used by Shen et al. [192] to adsorb Cu(II) and Pb(II) metal ions. At pH 4.5, CNCs could adsorb the most Cu(II) and Pb (II), and as the pH rises, the adsorption capacity decreases. At pH greater than 4.5, complex compounds are volatile between metal ions and N atoms of amino groups.

Based on those articles, we can assume that the modification of CNCs and CNFs significantly affects the surface area and adsorption capacity of heavy metal ions, but CNFs has a lower adsorption capacity than the adsorption capacity of CNCs. The results of the research showed that CNCs modification with L-cysteine provided a tremendous increase in adsorption capacity.

9.3. Other Heavy Metal Adsorption Factors

According to some research the success and effectiveness of the heavy metal adsorption process are influenced by a few factors. These factors include adsorbent chemical structure changes, adsorbent surface area, the availability of active sites on the adsorbent’s surface, adsorption constants, heavy metal ionic size differences, pH, temperature, adsorbent dosage, and contact time during the adsorption process [214, 215]. Ojembarrena et al. [215] and Al-Senani and Al-Fawzan [216] research, increasing the dose of adsorbent used raises the percentage of adsorption. The increased availability of surface area or exchange sites at higher adsorbent concentrations ended up causing this increase.

In general, the adsorption process moves so fast during the first few minutes because the active site of the adsorbent is still empty. As an outcome, the adsorbate will have an easier time inhabiting the active site. As the adsorption contact time increases, the active site of the adsorbent tends to fill up, resulting in a concentration difference between the adsorbent and the adsorbate and slowing the adsorption rate. In brief, as contact time increases, so does the adsorption capacity and numbers of other factors that affect the adsorption process [214].

9.4. The Mechanism of Adsorption

Based on a review conducted by Akter et al. [217], the adsorption mechanism for heavy metal ions utilizes nanocellulose material, which contains electrostatic interactions, ion exchange, hydrogen bonding, hydrophobic interactions and coordination. According to Syeda and Yap [214], the adsorption mechanism in general involves electrostatic interactions between the metal ion to be adsorbed and the protonated functional groups from CNCs and CNFs. For example, CNCs or CNFs could be modified to contain amino groups. Amino and hydroxyl groups on the adsorbent’s surface will act as electron donors and participate in redox reactions. The metal ions thus formed will then be immobilized on the adsorbent’s surface through an ion exchange process or by forming a surface complex. The adsorbent surface was modified to increase the number of ionic, ionizable, or complex sites. Li et al. [36] research is the adsorption mechanism of heavy metal ions by CNCs surfaces. The mechanism between CNC-Lcys and mercury metal ion (Hg(II)) is depicted in Figure 13. Figure 13 depicts the addition of CNC-Lcys to the Hg(II) ion solution. The high specific surface area of CNC-Lcys causes more Hg(II) ions to interact with the CNCs surface. The faster the electrostatic interaction between Hg(II) and the adsorbent surface, the more negative the CNC-Lcys charge. The CNC-Lcys surface’s lone S, N, and O pairs can quickly form the Hg(II) ion complex. The active groups (thiol and amino) indicated excellent adsorption capacity. As a result of the complex formation and electrostatic interactions, CNC-Lcys can adsorb Hg(II) ions effectively and quickly [36, 204].

[figure(s) omitted; refer to PDF]

10. Isotherm and Kinetic Adsorption of Heavy Metal Ions

The most important calculation for predicting and analyzing the various possible mechanisms that occur during the adsorption process is isotherm adsorption. The adsorption isotherm describes the entire adsorption data set. The adsorption isotherm can also be used to understand the relationship between the adsorbate in the liquid phase and the adsorbate adsorbed on the adsorbent’s surface at constant temperature equilibrium [218, 219]. There are various kinds of adsorption isotherms (the most used are Langmuir and Freundlich Models) [220–223]. This review discusses some studies on heavy metal ion isotherms of CNCs and CNFs from various precursors. Table 6 shows the isotherm and kinetics of the adsorption of various heavy metal ions by various modified CNCs and CNFs.

Table 6

The adsorption isotherm models of various modified CNCs and CNFs.

CNCs/CNFs typesPrecursors/ModifierHeavy metal ionsIsotherm modelsParamatersAdsorption capacities (mg/g)Reference
Lys-CNCL-CysteineHg(II)LangmuirR2 = 0.98181116[36]
b = 3.11 × 10−2 L/mg
CNF-2MPTMSWood pulpHg(II)LangmuirR2 = 0.99766700[37]
CCN-PEIPolyethylenemineCr(VI)LangmuirR2 = 0.9934358.42[43]
b = 0.03 L/mg
CPCNCCassava peelPb(II)LangmuirR2 = 16.4[44]
b = 0.5 L/mg
TOCN-PEICottonCu(II)LangmuirR2 = 0.98552.32[47]
b = 0.17 L/mg
NaSCNCCottonPb(II)LangmuirR2 = 0.999458.3[103]
b = 4.13 L/mg
Cd(II)R2 = 0.999335
b = 41.88 L/mg
CNFAldehyde functional groupsCu(II)LangmuirR2 = 0.99837.21[174]
Pb(II)R2 = 0.996155.58
BCN-g-PAA/SHBambooCu(II)FreundlichR2 = 0.9915—[178]
n = 1.508
Kf = 0.114 mmol/g
LangmuirR2 = 0.9696
b = 0.175
qm = 1.016 mmol/g
Cys-CNCCysteamineHg(II)LangmuirR2 = 0.9971847[204]
b = 1.66 × 10−1 L/mg
NCFRice strawCd(II)LangmuirR2 = 0.959.7[194]
b = 0.63 L/mg
Pb(II)R2 = 0.979.42
b = 1.81 L/mg
Ni(II)R2 = 0.988.55
b = 0.33 L/mg
CNCHyacinthPb(II)LangmuirR2 = 0.981434.47[200]
FreundlichR2 = 0.9932—
CNFLangmuirR2 = 0.973587.518
FreundlichR2 = 0.9171—
Cys-CNFBagasseHg(II)LangmuirR2 = 0.9726116.822[203]
Fe/Cu-CNCProduced CNCsPb(II)LangmuirR2 = 0.9185.8[224]
CNC-g-P (AA/AM)BambooPb(II)LangmuirR2 = 0.998366.3[225]
CNCHyachinthCd(II)LangmuirR2 = 0.97775.76[226]
MA-CNCR2 = 0.977215.52
MCNCSawdustCu(II)LangmuirR2 = 0.999111.1[227]
b = 0.72 L/mg
Pb(II)R2 = 0.9992.82
b = 0.47 L/mg
Fe(II)FreundlichR2 = 0.999—
Kf = 8.99 L/g
NOCNFMoringaHg(II)LangmuirR2 = 0.9946257.07[228]
b = 0.00086
FreundlichR2 = 0.9951—
n = 1.095
Kf = 3.673 mg/g
IL-CNC/DPDate pits and ionic liquidLi(I)Dubinin-RadushkevichR2 = 0.981098[229]
CNMErythrina bruceiPb(II)FreundlichR2 = 0.989—[230]
Kf = 0.899 mg/g
n = 1.91
LangmuirR2 = 0.947384.62
b = 0.011 L/mg
Azeh-TDIPhosphorylatedCd(II)LangmuirR2 = 0.9990.1563[231]
b = 0.219 L/mg
CNCSawdustVLangmuirR2 = 0.99647.2[232]
b = 0.41 L/mg
MDAC-cysWood pulpAs(III)LangmuirR2 = 0.943344.82[233]
NDAC-cysR2 = 0.815357.14
HEMA-CNFHEMACd(II)LangmuirR2 = 0.998840[234]
KL = 0.5 L/mg
Pb(II)R2 = 0.999926
KL = 0.75
M-OPEFB-CNFEmpty fruit bunchCu(II)FreundlichR2 = 0.974—[235]
Kf = 0.0162 L/mg
n = 1.0586
Cr(VI)R2 = 0.9825
Kf = 0.0282 L/mg
n = 1.4405
Cu(II)LangmuirR2 = 0.9646
b = 8.1633 mg/mg
Cr(VI)R2 = 0.9413
b = 8.1622 mg/mg
MgS/CNFBagasseCd(II)LangmuirR2 = 0.99333.3[236]
b = 0.1071 L/mg
FeNP/CNFMoringa plantCd(II)LangmuirR2 = 0.992934.99[237]
b = 0.0040 (L/mg)
Pb(II)Langmuir-FreundlichR2 = 0.9934—
a = 3.1486 mg/L

10.1. Adsorption Isotherms

The presence of a monolayer of the adsorbate surface causes the maximum adsorbent capacity, according to the Langmuir isotherm. The Langmuir equation is shown in equation (1), where qe (mmol/g) is the equilibrium concentration of heavy metal ions or adsorbate per mass unit of adsorbent and Ce (mmol/L) is the equilibrium concentration of the remaining adsorbate in solution. The mass of the adsorbent at complete monolayer coverage is given by qmax (mmol/g), and the Langmuir constant (L/mmol) is related to the heat of adsorption. The slopes (1/qmax) were used to calculate the qmax and b values [63]. The Langmuir isotherm model makes four assumptions. First, the molecules are adsorbed by a fixed site. Each site can only hold one adsorbate molecule. All sites have the same energy, and the adsorbed molecules have no interaction with the surrounding sites. According to the Langmuir isotherm theory, adsorption occurs at specific homogeneous sites within the adsorbent and that interactions between adsorbed substances occur [178].(1)Langmuir equation:Ceqe=1bqmax+Ceqmax.

The Freundlich isotherm model describes a physical type of adsorption in which adsorption occurs in several layers with weak or multilayer bonds. The Freundlich model can also assume heterogeneous adsorption sites. The Freundlich isotherm equation is given in equation (2), where Kf is the Freundlich constant and Ce is the adsorbate concentration under equilibrium conditions (mg/L). While Qe denotes the amount of adsorbent (mg/g), n denotes the degree of linearity of the adsorbate solution and the adsorption process [238].(2)Freundl ich equation:lnQe=lnKf1nlnCe.

10.2. Adsorption Kinetics

Adsorption kinetics is a curve or line that describes the rate of solute uptake and the time required for the adsorption process. Furthermore, at a given adsorbent dose, flow rate, temperature, and pH, the rate of retention of a solute from an aqueous environment to a solid-phase interface is determined. Table 7 contains several studies on adsorption kinetic models. Adsorption can be either physical or chemical. Physical adsorption is caused by a weak force of attraction, whereas chemisorption is caused by a strong bond formed between the solute and the adsorbent, which involves electron transfer. The most commonly used adsorption kinetic models are pseudo-first order and pseudo-second order [240–242].

Table 7

The adsorption kinetic models of various modified CNCs and CNFs.

CNCs/CNFs typesPrecursors/ModifierHeavy metal ionsKinetic modelsParametersReference
Lys-CNCHyachinthHg(II)Pseudo-second-orderR2 = 0.999[36]
K2 = 1.08 × 10−2 g/mg.min
Q = 209 mg/g
CNF-2MPTMSWood pulpHg(II)Pseudo-second-orderR2 = 0.99483[37]
CCN-PEIPolyethylenemineCr(VI)Pseudo-second-orderR2 = 0.9994[43]
K2 = 0.0014 min−1
Q = 56.75 mg/g
TOCN-PEICottonCu(II)Pseudo-second-orderR2 = 0.999[47]
K2 = 0.053 g/mg/h
NaSCNCCottonPb(II)Pseudo-second-orderR2 = 1[103]
K2 = 52 × 10−2 g/mg.min
Q = 300.3 mg/g
Cd(II)R2 = 0.999
K2 = 4.17 × 10−2 g/mg.min
Q = 166.50 mg/g
CNFAldehyde functional groupsCu(II)Pseudo-second-orderR2 = 0.99[174]
Q = 38.36 mg/g
Pb(II)R2 = 0.99
Q = 157.73 mg/g
BCN-g-PAA/SHBambooCu(II)Pseudo-second-orderR2 = 0.9994[178]
K2 = 0.723 g/mmol.min
Q = 0.709 mmol/g
Cys-CNCCysteamineHg(II)Pseudo-second-orderR2 = 0.999[204]
K2 = 1.085 × 10−2 g/mg.min
Q = 132 mg/g
CNCHyachinthPb(II)Pseudo-second-orderR2 = 0.9939[200]
K2 = 2.496 g/mg.min
Q = 0.303 mg/g
CNFR2 = 0.9911
K2 = 0.354 g/mg.min
Q = 1.3345 mg/g
Cys-CNFBagasseHg(II)Pseudo-second-orderR2 = 0.9826[203]
Q = 87.41 mg/g
K2 = 1.5 × 10−3 g/mg.min
Fe/Cu-CNCProduced CNCsPb(II)Pseudo-second-orderR2 = 0.99954[224]
K2 = 0.00913 min−1
Q = 54.259 mg/g
CNCS-g-P (AA/AM)BambooPb(II)Pseudo-second-orderR2 = 0.999[225]
K2 = 1.35 × 10−4 g/mg.min
Q = 207.5 mg/g
CNCHyachinthCd(II)Pseudo-second-orderR2 = 0.998[226]
K2 = 0.074 g/mg.min
Q = 9.37 mg/g
MA-CNCErythrina bruceiPb(II)Pseudo-second-orderR2 = 0.995[230]
K2 = 0.03 g/mg.min
Q = 14.96 mg/g
CNMR2 = 0.965
K2 = 0.003 g/mg.min
Q = 89.78 mg/g
Azeh-TDIPhosphorylatedCd(II)Pseudo-second-orderR2 = 0.99[231]
K2 = 6.675 × 10−4 g/mg.min
Q = 232.56 mg/g
CNCSawdustVPseudo-second-orderR2 = 0.999[232]
K2 = 0.009 g/mg.min
Q = 19.8 mg/g
HEMA-CNFHEMACd(II)Pseudo-second-orderR2 = 0.99[234]
Q = 893 mg/g
Pb(II)R2 = 1
Q = 497 mg/g
M-OPEFB/CNFEmpty fruit bunchCu(III)Pseudo-second-orderR2 = 0.9996[235]
K2 = 0.1901 mg/mg.min
Q = 0.7349 mg/mg
Cr(VI)R2 = 0.9998
K2 = 1.2573 mg/mg.min
Q = 0.3785 mg/mg
MgS/CNFBagasseCd(II)Pseudo-first-orderR2 = 0.98[236]
K1 = 0.0875 h−1
Q = 17.02 mg/g
FeNP/CNFMoringa plantCd(II)ElovichR2 = 0.9895[237]
b = 59.077 mg/g
a = 0.8112/min
Pb(II)Pseudo-first-orderR2 = 0.9858
KL = 0.022/min
Q = 0.3401 mg/g
TEMPO-CNFWood pulpCu(II)Bohart–AdamR2 = 0.9781[239]
KBA = 0.244 cm3/mg.min
No = 3.028 mg/cm3

10.2.1. Pseudo-First Order

Equation (3) integrating for the condition of qo = 0 yields equation (5). Equation 5 used to fit the kinetic data and to calculate the parameters qe and k1, by plotting ln (qe − qt) vs. t. The linearized form may cause inaccurate estimations of the parameters and the nonlinear method can provide accurate estimations of the parameters. The pseudo-first-order equation (3)–(6) is shown as follows, where K1 is the pseudo-first-order rate constant and qe and qt (mg/g) are the equilibrium and time-dependent adsorption capacities. Lagergren’s constant was K1 (min−1) [243].(3)dqtdt=k1qe−qt,(4)The linearized form:qt=qe1−e−k1t,(5)ln qe−qt=lnqe−k1t.

10.2.2. Pseudo-Second Order

To calculate the model parameters, the nonlinear form of pseudo-second order (equation (7)) is always transformed to the linear form (equation (8)). Where K2 is the pseudo-second-order rate constant and qe and qt (mg/g) are the equilibrium and time-dependent adsorption capacities. Lagergren’s constant was k2 (g−1min−1) is denoted by a pseudo-second-order rate constant. The t is the adsorption time (h). The pseudo-second-order model is described as follows [243]:(6)dqtdt=k2qe−qt2,(7)qt=qe2k2t1+qek2t,(8)tqt=1k2qe2+1qe.

10.3. Hg(II) Ion Adsorbents

Several previous studies (Tables 6 and 7) have investigated the adsorption ability of Hg(II) metal ions by various types of CNCs and CNFs adsorbents. Li et al. [36] prepared Lcys-CNC as an adsorbent of Hg(II) ions. Lcys-CNC has a high specific surface area and many active sites. It can quickly and efficiently adsorb Hg(II) ions. As a result, the adsorption rate gradually slowed before reaching adsorption equilibrium in 5 minutes. A pseudo-second-order model accurately described the adsorption kinetics [244]. The Hg(II) ions adsorption on Lcys-CNC was primarily accomplished through chemical reactive adsorption between the active sites of Lcys-CNC and Hg(II) ions. The Langmuir model was the best fit to describe Hg(II) ion adsorption with a monolayer coverage. Rong et al. [37] developed CNFs by modifying it using thiol and labeling it as CNF-1MPTMS and CNF-2MPTMS. The CNF-1MPTMS and CNF-2MPTMS adsorption mechanisms for Hg(II) ions were compared. These findings indicate that the pseudo-second-order kinetic model better represents the adsorption kinetics. The Hg(II) ions adsorption on CNF-MPTMS appears to be a chemisorption process involving covalent binding and ion exchange. Furthermore, the adsorption capacity on the CNF-2MPTMS is 700 mg/g and 480 mg/g on the CNF-1MPTMS. These results indicate that Hg(II) ion adsorption on CNF-MPTMS should be described as a surface monolayer sorption process.

Another experiment was done by Li et al. [204] using CNCs modified by cystamine (Cys). The Langmuir isotherm model is an appropriate adsorption isotherm model for this study. When compared with the pseudo-first-order model, the pseudo-second-order model is more accurate. This means that Hg(II) ions adsorption on Cys-CNC is primarily accomplished through chemical reactive adsorption between Cys-CNC and Hg(II) ions. Chen et al. [228] prepared moringa plant nitro-oxidized carboxy cellulose nanofibers (NOCNF). The R2 values for the Langmuir model were lower than those for the Freundlich model. The adsorption capacity for the Langmuir model is 257.07 mg/g for Hg(II). These results suggest that the adsorption mechanism cannot be truly differentiated based on the various model analyses. Bansal et al. [203] synthesized CNF from bagasse and modified it with L-cystine. A pseudo-second-order kinetic model revealed that the interaction between adsorbent and adsorbate involved chemical adsorption in the Hg(II) ion adsorption reaction using Cys-CNF. While the isotherm model for the adsorption of Hg(II) by Cys-CNF is suitable for the Langmuir isotherm.

10.4. Cu(II) Ion Adsorbents

Zhang et al. [47] produced a cellulose nanofibril with carboxyl/amino groups. This cellulose nanofibril was polyethyleneimine (TOCN-PEI) modified and used in Cu(II) removal. The adsorption capacity results reveal the impact of the initial Cu(II) concentration. The qe of TOCN-PEI gradually increased as the Cu(II) concentration increased. They discovered that Cu(II) adsorption on TOCN-PEI was a typical pseudo-second-order process. The results of the Cu(II) adsorption isotherm demonstrate that the Langmuir isotherm model was well fitted, with higher R2 values. This indicates that the homogeneous monolayer of TOCN-PEI adsorbed Cu(II).

Zhang et al. [178] synthesized cellulose nanofibers cross-linked polyacrylic acid/sodium humate (BCN-g-PAA/SH) from bamboo. In this study, the pseudo-second-order model for Cu(II) adsorption provided a good correlation. The chemical modification of BCN resulted in a significant improvement in adsorption capability. Furthermore, the R2 of Langmuir was low and could be fitted to the Freundlich. A high intercept value indicates a high adsorption capacity. Fiol et al. [239] produced CNFs from bleached kraft eucalyptus pulp. The CNF was modified using the TEMPO-oxidized method to produce TEMPO-CNF, which was then used to remove Cu(II). The results showed that the kinetics of Cu(II) adsorption onto the TEMPO-CNF surface could be quantified using the Bohart–Adams model. The tested adsorbent fitted the Bohart–Adam model well, with R2 values ranging from 0.9158 to 0.9781.

10.5. Pb(II) and Cd(II) Ions Adsorbents

Abiaziem et al. [44] created CNC from cassava peel called CPCNC. The CPCNC is appropriate for using the Langmuir isotherm model according to the adsorption equation (R2) of Pb(II) metal ions. The formation of Pb(II) ions in contact with the surface area of CPCNC caused adsorption, according to the Langmuir isotherm. The Freundlich isotherm reflects the good surface heterogeneity of active sites [223, 245]. According to the results of this experiment, the optimal adsorption capacity at pH 6 was 6.4 mg/g. Ramos-Vargas et al. [200] used water hyacinth to obtain CNFs and CNCs. The results show that the adsorption of Pb(II) ions on CNCs and CNFs was significantly faster. This is due to the surface adsorption phenomenon, in which the transport process and adsorbate diffusion occur between the film and the adsorbent. The best kinetic model for the adsorption process using CNCs and CNFs based on the data obtained is pseudo-second order. The adsorption isotherm was fitted to the Langmuir–Freundlich isotherm model so that the adsorption process of CNFs and CNCs can run chemically or physically with adsorption capacities of 87.518 mg/g for CNFs and 34.47 mg/g for CNCs. Chen et al. [224] created Fe-Cu/CNC by modifying a CNC with Fe-Cu alloy. The Fe-Cu/CNC-2 achieved a 70.76% removal in 5 minutes and a 93.98% Pb(II) removal ratio after an hour. After an hour of adsorption, this Pb(II) removal ratio was higher than 78.79% for the Fe-Cu/CNC-1. Furthermore, the adsorption behavior of Fe-Cu/CNC can be described by pseudo-second-order equations [246, 247]. Chen et al. [225] synthesized cellulose nanocrystal-g-poly (acrylic acid-co-acrylamide) or CNCS-g-P (AA/AM) from bamboo powder. The Pb(II) removal study showed that the Langmuir isotherm model best fits this adsorption process and that the adsorption process involves Langmuir monolayer adsorption with a maximum Pb(II) adsorption capacity of 366.3 mg/g. Electrostatic interactions between carboxyl groups and Pb(II) [248]. The best adsorption kinetic of CNCS-g-P (AA/AM) describes Pb(II) ions removal fitted to pseudo-second order [222, 245]. Tsade et al. [230] synthesized pristine cellulose nanomaterial and sodium periodate modified cellulose nanomaterial (NaIO4- CNM) from the Erythrina brucei plant. Based on the value of R2, the isotherm models of Langmuir and Freundlich were the best fitting for Pb(II) adsorption. This indicated that the CNM and NaIO4− CNM surface adsorbents were heterogeneous and physically compatible with monolayer adsorption, as well as that the adsorption process was appropriate. The CNM and NaIO4− CNM had maximum uptake capacities of 91.74 and 384.62 mg/g, respectively. While the adsorption kinetics of CNM and NaIO4− CNM data show the highest R2 values for pseudo-second order, it was determined that pseudo-second order was the best fit for the kinetic model.

The MA-CNC of hyacinth weed by Kara et al. [226] was used in the adsorption of Cd(II). This adsorption follows the Langmuir and Freundlich isotherm models and pseudo-second-order kinetics, indicating that Cd(II) adsorption on the CNCs surface can happen homogeneously and chemically [249, 250]. Yakubu et al. [231] investigated Cd(II) adsorption using toluene diisocyanate (Azeh-TDI). According to the findings, the Langmuir isotherm and pseudo-second-order kinetic model were the best-fitting isotherm models for Azeh-TDI. Sankararamakrishnan et al. [236] produced cellulose nanofibers doped with MgS (MgS/CNF). Both adsorbents produce straight lines with high R2 values and intercepts for Cd(II) adsorption. The pseudo-first-order and web-morris results have high R2 values. Simultaneously, the adsorption isotherm analysis revealed that the Langmuir isotherm model had a higher R2 value for both adsorbents. This demonstrates that the adsorption reaction occurs chemically. MgS/CNF had a greater Langmuir monolayer adsorption capacity towards Cd(II) than CNF.

Yu et al. [103] created CNCs from cotton and modified it with succinic anhydride to produce SCNC and modified the CNCs with NaHCO3 to produce NaSCNC. Based on the findings, a pseudo-second-order kinetic model of Pb(II) and Cd(II) can adequately explain the adsorption process and the process may be a chemical adsorption process involving the sharing or exchange of electrons between the adsorbent and adsorbate. Furthermore, the initial adsorption rate of NaSCNC was faster than that of SCNC and CNC and, based on the experimental data, could be well fitted by the Langmuir model. Gouda and Aljaafari [234] investigated the removal of Cd(II) and Pb(II) using hydroxyethyl methacrylate (HEMA) modified CNFs. Langmuir and Freundlich isotherm models were fitted to the adsorption isotherm results for Cd(II) and Pb(II) adsorption. Simultaneously, the results show that the pseudo-first-order response kinetic model was insufficient for describing the adsorption of Cd(II) and Pb(II) onto the HEMA/CNF. The amount of adsorbate adsorbed on the adsorbent surface and the amount adsorbed at equilibrium are used in the pseudo-second-order kinetic model. It implies that chemisorption is the rate-controlling step. The K2 values decreased as Cd(II) and Pb(II) concentrations increased, which could be attributed to increased competition for adsorption sites at high concentrations compared to low concentrations. Vázquez-Guerrero et al. [237] produced a composite using moringa plant cellulose nanofibers impregnated with iron nanoparticles (FeNP/CNF). The Pb(II) and Cd(II) adsorption kinetics of FeNP/CNF were significantly different. The Cd(II) adsorption was very fast in the initial stage, with 22% of total Cd(II) elimination. Then it gradually slowed down, eventually reaching equilibrium with a maximum efficiency of 32%. As adsorption progresses, the concentration of metal ions and active sites decreases, resulting in a lower adsorption rate when the systems reach equilibrium. Cd(II) adsorption kinetic data were fitted to pseudo-first-order, pseudo-second-order, and Elovich models. A pseudo-first-order model that shows that the adsorption reaction occurs physically is appropriate in the case of Pb(II) adsorption. According to this model, adsorption is superficial, with one solute molecule adsorbed at a single defined specific site.

10.6. Other Heavy Metal Ions Adsorbents

Research on the adsorption of other heavy metal ions such as Cr(IV), Fe(II), Ni(II), and others has also been carried out (Tables 6 and 7). In the research of Liu et al. [43], CNC-PEI was used to adsorb Cr(VI). This adsorption matched the pseudo-second-order kinetics, indicating that the adsorption was chemically occurring. The Langmuir isotherm is suitable for this adsorption model because it is monolayer and spontaneous adsorption. Yao et al. [174] prepared CNFs with aldehyde functional groups. The kinetic analysis revealed that the entire adsorption process of Pb(II) and Cu(II) was followed by pseudo-second-order reaction kinetics. For both metal ions, the adsorption equilibrium study discovered that the experimental data at equilibrium fit well to the Langmuir isotherm model. These results show that the chemisorption process was monolayer and that all aldehyde adsorption sites were equivalent. Kardam et al. [194] prepared the CNFs through the physicochemical treatment of rice straw. Both the Freundlich and Langmuir isotherm models fit the Cd(II), Pb(II), and Ni(II) adsorption processes well. The maximum adsorption capacity and intensity were obtained based on the higher efficiency of CNFs for divalent ions according to the Freundlich model. The Langmuir model was used to calculate the adsorption capacity (Qo) and energy (b) values for Cd(II), Pb(II), and Ni (II).

Oyewo et al. [227] prepared CNCs from sawdust (MCNC). The data obtained from Cu(II) and Pb(II) adsorption fitted well with the Langmuir adsorption model from the values of the R2, which was indicative of the chemical adsorption of Cu(II) and Pb(II) onto the MCNCs. The data obtained from Fe was fitted to the Freundlich isotherm model and indicated that the adsorption sites were uneven and nonspecific. Wahib et al. [229] aims to produce an adsorbent using date pits (DP) impregnated with CNCs and ionic liquid (IL) called IL-CNC/DP. According to general observations, the Dubinin-Radushkevich isotherm is the best-fitting isotherm model for Li(I) adsorption based on R2. Zulu et al. [232] prepared CNCs from sawdust with hexadecyltrimethylammonium bromide (HDTMA-Br) for vanadium (V) adsorption. The adsorption of metal ions V followed the Langmuir isotherm and pseudo-second-order kinetic, which indicated that the adsorption occurred on a homogeneous CNCs surface and favorable. Chen et al. [233] prepared CNFs at various scales, labeling it with dialdehyde cellulose (MDAC) for the microscale and dialdehyde cellulose (NDAC) for the nanoscale and then modifying it with L-Cysteine. Both adsorbents are suitable for using the Langmuir isotherm model for the As(III) adsorption reaction because they have a multilayer adsorption system and a complex and heterogeneous system. Khalid et al. [235] developed magnetic oil palm empty fruit bunch cellulose nanofiber (M-OPEFB-CNF). The Freundlich isotherm R2 values indicate that the Freundlich model is a better fitted framework for understanding the adsorption behavior of Cu(II) and Cr(VI) using M-OPEFB-CNF. Furthermore, when compared to the pseudo-first order, the experimental values of Cu(II) and Cr(VI) were more closely matched with the theoretical values (qe) of the pseudo-second order. Finally, the pseudo-second-order kinetic model better described the adsorption mechanism of M-OPEFB-CNF for removing Cu(II) and Cr(VI).

According to the studies above, mostly all adsorption processes of CNCs and CNFs are fitted to the Langmuir isotherm model and the pseudo-second-order kinetic model. Based on the adsorption data, the isotherm and kinetic model of adsorption can be determined (the R2 values). According to the Langmuir isotherm model, the presence of an adsorbate monolayer on the adsorbent surface causes the maximum adsorption capacity. Then, according to some of the studies, several adsorbents followed a pseudo-second-order kinetic model, indicating that the adsorption capacity was proportional to the number of active adsorbent sites.

11. Conclusions and Areas of Future Research

Cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) are two types of nanocellulose with different morphology and particle sizes. CNCs and CNFs could be synthesized using a variety of methods, including chemical (acid hydrolysis) and mechanical methods (homogenization, grinding, and ball milling). Ultrasonication may be utilized to reduce the particle size of the completed CNCs and CNFs. FTIR, SEM, TEM, XRD, DLS, and TG could be used to analyze the characteristics of the derived CNCs and CNFs. The CNCs and CNFs had to be modified to improve the performance and efficiency of heavy metal ions adsorption. By adding functional groups to CNCs and CNFs surfaces, these surfaces can be modified. The CNCs and CNFs modifications increased the active group and adsorption capacity while decreasing the particle size. In adsorption heavy metal ions, CNCs and CNFs had a large surface area and a high adsorption capacity. Amino and thiol groups are frequently used in CNCs and CNFs modifications. Heavy metal ion selectivity was significant in modified CNCs and CNFs. The pH setting had a significant influence on the adsorption capacity because it could reduce the adsorption of ions with the same charge. To obtain an overview of the adsorbate distribution process, the isotherm and kinetic pattern of adsorption can be determined from the value of the regression coefficient (R2). The adsorption isotherm describes the equilibrium and kinetics relationship between the adsorbed particles and their adsorption. The Langmuir isotherm states that the maximum adsorbent capacity occurs due to the presence of a single layer (monolayer) and chemisorption of adsorbate on the surface of the adsorbent, while the Freundlich isotherm includes multilayer adsorption and physisorption. The rate of adsorption of the adsorbent on the adsorbate can be seen from the adsorption kinetics.

Based on the previous studies that we reviewed, it seems that research in general is still mostly carried out on a laboratory scale to evaluate the activity, mechanism and adsorption capacity of heavy metal ions using CNCs and CNFs. Based on this, research into the adsorption activity of CNCs and CNFs on industrial wastes with complex levels of heavy metal ions pollution is necessary. Complex level of contamination contains multiple heavy metal ions as well as interfering ions. This enables the adsorption selectivity of CNCs and CNFs to be studied. Besides that, more study is required to find the best synthesis method and modifier to produce modified CNCs and CNFs with the best characteristics. It is hoped that these CNCs and CNFs can be utilized as nanoadsorbents with the maximum adsorption capacity and that the CNCs and CNFs with the best properties can be produced and applied on a larger scale. CNCs and CNFs with high adsorption capabilities and characteristics will provide improved and more efficient adsorption applications for water treatment, allowing everyone in the society to have equal access to clean water.

Acknowledgments

The authors of this work acknowledge every author of the works cited in this review.

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Copyright © 2023 Tjhang Winny Kurniawan et al. This is an open access article distributed under the Creative Commons Attribution License (the “License”), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License. https://creativecommons.org/licenses/by/4.0/

Abstract

The isolation of nanocellulose has been extensively investigated due to the growing demand for sustainable green materials. Cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), which have the same chemical composition but have different morphology, particle size, crystallinity, and other properties depending on the precursor and the synthesis method used. In comparison, CNC particles have a short rod-like shape and have smaller particle dimensions when compared to CNF particles in the form of fibers. CNC synthesis was carried out chemically (hydrolysis method), and CNF synthesis was carried out mechanically (homogenization, ball milling, and grinding), and both can be modified because they have a large surface area and are rich in hydroxyl groups. Modifications were made to increase the adsorption ability of heavy metal ions. The Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric (TG), and dynamic light scattering (DLS) can reveal the characteristics and morphology of CNCs and CNFs. The success and effectiveness of the heavy metal adsorption process are influenced by a few factors. These factors include adsorbent chemical structure changes, adsorbent surface area, the availability of active sites on the adsorbent’s surface, adsorption constants, heavy metal ionic size differences, pH, temperature, adsorbent dosage, and contact time during the adsorption process. In this review, we will discuss the characteristics of CNCs and CNFs synthesized from various precursors and methods, the modification methods, and the application of CNCs and CNFs as heavy metal ion adsorbents, which includes suitable isotherm and kinetics models and the effect of pH on the selectivity of various types of heavy metal ions.

Details

Title
Cellulose Nanocrystals (CNCs) and Cellulose Nanofibers (CNFs) as Adsorbents of Heavy Metal Ions
Author
Tjhang Winny Kurniawan 1   VIAFID ORCID Logo  ; Sulistyarti, Hermin 2   VIAFID ORCID Logo  ; Rumhayati, Barlah 3   VIAFID ORCID Logo  ; Sabarudin, Akhmad 4   VIAFID ORCID Logo 

1  Department of Chemistry, Faculty of Science, Brawijaya University, Jl Veteran, Malang 65145, Indonesia; Gembala Baik Catholic Senior High School, Jl Achmad Yani, Pontianak 78124, Indonesia 
2  Department of Chemistry, Faculty of Science, Brawijaya University, Jl Veteran, Malang 65145, Indonesia; Research Centre for Low Cost and Automated Method & Instrumentation Analysis (LCAMIA), Brawijaya University, Jl. Veteran, Malang 65145, Indonesia 
3  Department of Chemistry, Faculty of Science, Brawijaya University, Jl Veteran, Malang 65145, Indonesia 
4  Department of Chemistry, Faculty of Science, Brawijaya University, Jl Veteran, Malang 65145, Indonesia; Research Center for Advanced System and Material Technology (ASMAT), Brawijaya University, Jl. Veteran, Malang 65145, Indonesia 
Editor
Gomaa A M Ali
Publication year
2023
Publication date
2023
Publisher
John Wiley & Sons, Inc.
ISSN
20909063
e-ISSN
20909071
Source type
Scholarly Journal
Language of publication
English
DOI
https://doi.org/10.1155/2023/5037027
ProQuest document ID
2798352363
Copyright
Copyright © 2023 Tjhang Winny Kurniawan et al. This is an open access article distributed under the Creative Commons Attribution License (the “License”), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License. https://creativecommons.org/licenses/by/4.0/