Fabrication of magnetic lignin-based adsorbent for removal of methyl orange dye from aqueous solution

Abstract

Magnetic lignin-based adsorbent (MLA) was successfully fabricated to remove methyl orange dye from aqueous solution. The synthesized MLA was characterized by means of Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), N2 adsorption-desorption, scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). In the process of adsorption, influence factors and recycling performance were considered, and the adsorption mechanisms such as isotherm and kinetics were investigated. The result showed that the equilibrium data was consisted with the Langmuir model with a maximum adsorption capacity of 85.0 mg/g. The adsorption kinetics followed a pseudo-second-order model. Based the adsorption performance, MLA showed good recyclability. Therefore, these results demonstrate that MLA could offer a great potential as an efficient and reusable adsorbent in the wastewater treatments.

INTRODUCTION

Dyes are widely used in textile, paper, plastics, rubber, cosmetics and other industries, resulting in the discharge of wastewater that still contains some residual dyes (Carneiro et al. 2010). Dyes generally have some complex organic compounds that are toxic (Yagub et al. 2014; Yang et al. 2018). Due to the negative effect of dyes in water on the photosynthetic rate of hydrophytes and on the health of human beings, the waste streams must be treated prior to discharge into environment (Nair et al. 2014; Banerjee et al. 2019). A variety of processing techniques for removal of dyes from wastewater have been adopted, such as biodegradation, flocculation, membrane filtration, adsorption, and ion exchange, etc. (Liu et al. 2017; Song et al. 2017). Generally, adsorption technology is regarded as an environmentally friendly and feasible method for the advantages of high efficiency, easy operation, low cost, and not producing secondary pollution (Kaykhaii et al. 2018). According to reports, the adsorbents used in dye wastewater treatment mainly include bentonite and clay, activated carbon, synthetic polymer, and nanocomposites (Essandoh and Garcia 2018). In recent years, biomass-based adsorbents derived from low-cost, abundant, and renewable biomass have received considerable attention for the removal of dyes (Liu et al. 2015; Wang et al. 2018).

Lignin, the most abundant natural aromatic polymer on earth, is primarily composed of three phenylpropane units, which are syringyl (S) units, guaiacyl (G) units, and p-hydroxyphenol (H) units (Shuai et al. 2016). Annually, more than 70 million tons of commercial lignin is generated from pulping industries and biorefineries. The majority of commercial lignin is burned for power and heat generation, whereas less than 10% of commercial lignin is used for high value-added applications (Xu et al. 2014; Shao et al. 2018). The development of lignin-based materials with high performance has been gaining more and more attention, which not only diversifies the products, but also encourages the utilization of waste as part of the development of circular economies (Kazzaz et al. 2019; Supanchaiyamat et al. 2019). Especially, preparation of lignin-based adsorbent opens up a new application area for dye wastewater treatment (Wang et al. 2018; Li et al. 2019).

A large number of studies have been dedicated to the investigation of removal of dyes from wastewater using lignin or its derivatives as adsorbents (Zhai et al. 2020). Studies suggest that adsorption capacity of lignin-based adsorbent relies on the pore structure and oxygen functional groups, such as carboxyl and phenolic hydroxyl groups, which can absorb dyes by physicochemical interactions (Guo et al. 2008; Aro and Fatehi 2017; González-López et al. 2020). However, for most lignin-based adsorbents, the saturated adsorption capacity for dyes was not high enough. Moreover, separation and recycling of adsorbents after adsorption is an additional burden, which would limit its application (Zhang et al. 2012). As a consequence, further investigation of recyclable lignin-based adsorbents with high adsorption capacity for wastewater treatment is developed. Magnetic separation technology is considered a proven method to address this issue for the reason that magnetic adsorbent can be easily separated from the aqueous solutions in magnetic field (Chen et al. 2015; Zhang et al. 2019). Therefore, more researchers have focused on the preparation of magnetic lignin-based adsorbents. Geng et al. (2019) prepared magnetic lignosulfonate for wastewater treatment, and the desorption efficiencies of Cr(VI) and Rhodamine B could reach more than 70% and 85% after five regeneration cycles. Li et al. (2018) prepared responsive lignin-coated Fe₃O₄ nanoparticles to gain a good adsorption capacity for cationic and anionic dyes. The result showed that the dye-loaded adsorbents could be regenerated by simply adjusting the pH.

In this study, magnetic lignin-based adsorbent (MLA) was prepared for adsorption of methyl orange. During the preparation process, lignin was first coated onto the surface of iron oxide through precipitation, creating a lignin-iron oxide complex. Then the complex was calcined in the tube furnace to produce magnetic lignin particles. The adsorbent was characterized by FTIR, XRD, N₂ adsorption desorption, SEM and VSM. The mechanisms of the adsorption and desorption behavior were also investigated. This research would provide a new way to prepare magnetic materials and increase the added value of lignin.

EXPERIMENTAL

Materials

Biorefinery lignin from corncob, provided by Shandong Longlive Bio-Technology Co., Ltd, China, was used as the feedstock. Chemicals, including anhydrous lithium chloride (LiCl), methyl orange, ferric chloride hexahydrate (FeCl₃·6H₂O), ferrous sulfate heptahydrate (FeSO₄·7H₂O), ammonia water, and dimethyl sulfoxide (DMSO), were purchased from Aladdin Biochemical Technology Co., LTD. (Shanghai, China). All chemicals were analytical grade and used without further purification.

Preparation of Magnetic Lignin-based Adsorbent (MLA)

Two grams of lignin was dissolved in 80 mL of LiCl/DMSO solution (6%, w/w%). Then deionized water was gradually added to the above mixed solution under stirring conditions until the deionized water accounted for 80%. The suspension was transferred to the dialysis bag (2000 Da) for dialysis in deionized water. Finally, concentrated solution was obtained after rotary evaporation.

5.5 g of FeCl₃·6H₂O and 3.4 g FeSO₄·7H₂O were added together into 100 mL deionized water in four-neck flask; 30 mL of 10 wt% ammonia solution was injected into the solution under vigorous stirring when the temperature of solution rose to 90 °C. The color of the solution immediately changed from orange to black. The solution was heated at 90 °C in pH of 10 for 1.5 h with stirring. Concentrated solution was added into the reaction system, followed by 20 mL of 10 wt% ammonia solution. The mixture was stirred for 2 h at 90 °C. When the reaction finished, magnetic particles were separated by a magnet and thoroughly washed with ethanol and deionized water in turn. The final products were freeze-dried.

The magnetic particles were carbonized in the tube furnace for 3 h at 500 °C under a N₂ atmosphere. The final MLA was collected for further use.

Characterization of MLA

Different technologies were adopted to characterize the physical properties of MLA. FTIR spectra were collected in a VERTEX70 instrument (Bruker, Germany) using the KBr pellet technique. The molecular weight of MLA was evaluated using Agilent 1200 gel permeation chromatography (GPC) with a refraction index detector (RID). X-ray diffraction (XRD) analysis was recorded on an X-ray diffractometer (D8 Venture, Bruker, Germany) with a scanning rate of 2°/min from 10° to 80°. N₂ gas adsorption and desorption analysis was performed using BET analyzer (ASAP2020, Micromeritics, USA), and the specific surface area of the samples was calculated by the BET equation. The morphology of MLA was analyzed using a scanning electron microscopy (SEM, Regulus 8200, Hitachi, Japan). The magnetic property of MLA was explored by VSM equipment (7404 series, Lakeshore, USA). In order to explore the thermal stability of MLA, thermogravimetric analysis (TGA) experiments were performed in a TA Q50 thermogravimetric analyzer (TA Instrument, USA). The zeta potential of MLA was measured by Zetasizer Nano-ZS90 (Malven Instruments, UK).

Adsorption Experiment

In a typical experiment, 20 mg of MLA was added to the methyl orange solution, and the flasks was oscillated in a water bath. MLA was removed from the solution by magnetic separation after the adsorption was completed. The concentration of methyl orange in the solution was measured with an UV spectrophotometer (Cary 5000, Agilent, USA).

RESULTS AND DISCUSSION

Structural Characterization of MLA

The FTIR spectra of lignin and MLA are shown in Fig. 1a. The peak at 1610 cm^-1 is attributable to the skeletal vibration of the aromatic ring (Shao et al. 2017). The band at 830 cm^-1 is assigned to the syringyl structure in lignin. An obvious absorption peak at 573 cm^-1 occurred in MLA, which belonged to the characteristic adsorption peak of Fe-O in Fe₃O₄ (You et al. 2012). This demonstrated the formation of lignin-coated oxide particles. The FTIR result indicated that most of aromatic ring disappeared in MLA, but the Fe₃O₄ structure was retained as well as some O-H groups after carbonization.

Fig. 1. (parts a to e). Characterization of lignin and MLA. (a) FTIR spectra, (b) XRD patterns, (c) N2 adsorption desorption isotherm, (d) Pore size distribution, (e) SEM images, (f) Magnetic hysteresis curves, (g) TG curves of lignin and MLA, (h) zeta potential

Fig. 1. cont. Characterization of lignin and MLA. (a) FTIR spectra, (b) XRD patterns, (c) N2 adsorption desorption isotherm, (d) Pore size distribution, (e) SEM images, (f) Magnetic hysteresis curves, (g) TG curves of lignin and MLA, (h) zeta potential

The weight-average molecular weight (M_w), number-average molecular weight (M_n), and polydispersity (M_w/M_n) of lignin and MLA are shown in Table 1. It could be seen that there were no obvious changes in the molecular weight and polydispersity. This indicated that magnetic Fe₃O₄ loading had little effect on the molecular weight and polydispersity of lignin.

The XRD spectra of lignin and MLA are shown in Fig. 1b. Five diffraction characteristic peaks at 30.3, 35.5, 43.1, 57.3, and 62.3 were attributed to (220), (311), (401), (501), and (441) crystal planes respectively, which indexed to a Fe₃O₄ cubic spinel structure (Zheng et al. 2013; An et al. 2017). This demonstrated that Fe₃O₄ was contained in MLA.

The N₂ adsorption-desorption isotherms and pore distribution of MLA are shown in Figs. 1c and 1d. The isotherms maintained a type IV shape with an obvious hysteresis loop, indicating that there were rich mesoporous in the structure of MLA (Sun et al. 2006). It could be calculated that the specific surface area of MLA was 42.1 m²/g. The average pore size was 12.3 nm, which belonged to mesoporous.

The SEM images are shown in Fig. 1e. A rough irregular surface and large surface area occurred in MLA, which improved the adsorption capacity.

Figure 1f shows the magnetic hysteresis loop of Fe₃O₄ and MLA. The saturation magnetization intensity of MLA was about 51.3 emu/g. Almost no hysteresis loops were found in the magnetization, certifying the superparamagnetism of MLA (Xie et al. 2014). It can be magnetically separated from the solution due to the high magnetization values and superparamagnetic characteristics. This feature allows the reusable application of MLA as adsorbents, which is economical and sustainable.

Thermogravimetric (TG) curves of lignin and MLA, obtained at a heating rate of 10 °C/min, are plotted in Fig. 1g. For lignin, a much more significant weight loss was detected when temperature exceeded 200 °C. Meanwhile, for MLA, a similar trend of weight loss occurred when temperature exceeded 350 °C. Furthermore, the weight of MLA remained constant after 650 °C. The result suggested that MLA had better thermal stability than lignin.

The zeta potential of MLA is displayed in Fig. 2h. A zero potential point occurred at pH 6.0. It is clearly observed that zeta potential increased rapidly when the pH was decreased below 6.0, indicating the functional groups and surface of MLA were easily protonated. In addition, the zeta potential was negative under basic condition, for the reason that the ferric ions on the surface of MLA could attract OH^-1.

adsorbate and the adsorbent. 20 mg of MLA was added into 20 mL of methyl orange solution (100 mg/L) and stirred for 2.5 h at 25 °C. The pH of the solution was set as 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0. The adsorption results at different pH are displayed in Fig. 2.

Fig. 2. The effect of pH on adsorption of methyl orange

The data showed that adsorption capacity decreased with the increase of pH value. The presence of H⁺ promotes the adsorbent’s binding with methyl orange. The surface of MLA was positively charged under acidic conditions, which could attract the sulfonic acid group of methyl orange (Nair et al. 2014; Ma et al. 2018). Under neutral condition, MLA still had relative large adsorption capacity, and the surface of MLA was negatively charged. This indicated that there was a pi-pi effect between MLA surface and methyl orange, which contributed to adsorption. Based on the results and operation conditions, pH 5.0 was selected as an optimized parameter for following experiments, though the adsorption capacity was larger under strong acidic conditions.

Effect of Concentration of Methyl Orange on Adsorption

MLA (20 mg) was added into 20 mL of methyl orange solution with different concentrations (20 mg/L to 140 mg/L). The pH of solution was adjusted to 5 and oscillated for 2.5 h at 25 °C. The result of adsorption capacity is displayed in Fig. 3. As the concentration of methyl orange increased, the adsorption capacity increased from 18.72 to 92.14 mg/g, showing a positive correlation. The reason was that the contact chance between the adsorption sites on the surface of MLA and methyl orange increased with the increase of the concentration of methyl orange (Zhang et al. 2011). Finally, the adsorption capacity tended to be stable beyond a certain concentration, while percent adsorption decreased from 93.6% to 64.2%. It could be seen that methyl orange was removed more thoroughly by MLA at lower concentration. Based on actual operation, the concentration of methyl orange in further study was set as 100 mg/L.

Fig. 3. Adsorption isotherm for the adsorption of methyl orange onto MLA at 25 °C

In general, the Langmuir and Freundlich models have been adopted to describe the equilibrium adsorption isotherms (Brdar et al. 2012; Feng et al. 2014).

The Langmuir adsorption equation:

(1)

The Freundlich adsorption equation:

the Freundlich constants, correlated to the adsorption capacity and adsorption intensity, respectively.

Fig. 4. Line of best fit of adsorption isotherm by (a) Langmuir equation, (b) Freundlich equation

The isotherm adsorption parameters and linear fitting curves are shown in Table 2 and Fig. 4. The Freundlich constant (1/n) represents the adsorption intensity of the adsorbent. When its value is in the range 0.1<1/n≤0.5, the adsorbate was easy to adsorb (Luo and Zhang 2009). The value of 1/n in this study was 0.3522, manifesting that methyl orange was easily adsorbed by MLA. The coefficient of determination (R²) of the Langmuir model was 0.9872, which was higher than that of the Freundlich model (R²=0.9422). Based on the results, the adsorption behavior of MLA on methyl orange could be better represented by Langmuir adsorption isotherm equation. In addition, based on the calculation data, the maximum adsorption capacity was 85.0 mg/g when the concentration of methyl orange was 100 mg/L.

Adsorption Kinetics

The effect of contact time on the adsorption of MLA for methyl orange was investigated. The results are displayed in Fig. 5. It can be observed that the adsorption capacity increased quickly as adsorption time increased. The adsorption mainly happened on the surface of MLA in this stage. The equilibrium time for methyl orange adsorption was about 2.5 h. After that, the increased rate of adsorption capacity slowed down. To investigated the kinetics further, two kinetic models (pseudo-first-order and pseudo-second-order) were used to describe the adsorption process (Asthana et al. 2016).

Pseudo-first-order kinetics model:

(3)

Pseudo-second-order kinetics model:

(4)

In Eqs. 3 and 4, and are the adsorption capacity at experimental equilibrium and at time t (mg/g), respectively; is the pseudo-first-order rate constant (min^-1); and is the pseudo-second-order rate constant (g/(mg•min)).

The kinetic model parameters as well as correlation coefficients were calculated and listed in Table 3.

Fig. 5. Effect of contact time on the adsorption capacity of MLA for methyl orange (T=25℃, pH=5, methyl orange concentration=100mg/L)

Fig. 6. The line of best fit of adsorption kinetics using (a) pseudo-first-order-model, (b) pseudo-second-order-model

The adsorption capacity at experimental equilibrium (q_e,exp=72.2 mg/g) was closer to the theoretically calculated one (q_e,cal=73.0 mg/g), indicating that adsorption kinetics could be fit to the pseudo-second-order model well. In addition, in Fig. 6, pseudo-second-order model provided the extremely high coefficients of determination (R²=0.9962), which was higher than that in pseudo-first-order model (R²=0.9572). These results supported the conclusion that the adsorption of methyl orange by MLA obeyed a pseudo-second-order model (Zhang et al. 2016). The pseudo-second-order relationship might be used to fit data in which some sites of adsorption take a lot longer to be filled. In the adsorption process, adsorption sites far from a pellet’s outer surface were more difficult to reach, which led to a diffusion-controlled rate of adsorption (Hubbe et al. 2019).

Regeneration Study

The recycling of adsorbents is considered to be a key parameter for industrial application. Ethanol was adopted as the eluent to wash MLA which was separated in magnetic field (Fig. 7). Regenerative functions of MLA are shown in Fig 8. It could be seen that almost no decrease of adsorption ability happened after the second cycle. After four cycles of regeneration, the adsorption capacity of MLA for methyl orange was 64.0 mg/g (82% of initial capacity), indicating that the great mass of adsorption sites on the adsorbent surface could be renewed. The data supported the evidence that MLA could be a sustainable adsorbent.

Fig. 7. The separation of MLA in magnetic field

Fig. 8. The effect of cycles on the adsorption of MLA for methyl orange (T=25 °C, methyl orange concentration=100 mg/L, contact time=2.5 h)

CONCLUSIONS

1. The magnetic lignin-based adsorbent (MLA) was found to be very efficient in removing methyl orange from aqueous solutions. The adsorption kinetics agreed well with the pseudo-second-order model.

2. The adsorption isotherm fitted the Langmuir model with a maximum adsorption capacity for methyl orange of 85.0 mg/g (based on the methyl orange concentration of 100 mg/g).

3. MLA exhibited good recycling performance. After four cycles of regeneration, the adsorption capacity of MLA for methyl orange was 64.0 mg/g (82% of initial capacity)

ACKNOWLEDGMENTS

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 31770626), the Shandong Provincial Natural Science Foundation (Grant No. ZR2019BC074), the Foundation of the State Key Laboratory of Biobased Material and Green Papermaking of the Qilu University of Technology and the Shandong Academy of Science (Grant No. ZZ20190103), and the Doctoral Cooperation Foundation of Qilu University of Technology and the Shandong Academy of Science (Grant No. 2018BSHZ0025) for financial support of this research.

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