Rapid Adsorption of Ammonium on Coffee Husk and Chicken Manure-Derived Biochars: Mechanisms Unveiled by Chemical Speciation, Physical, and Spectroscopic Approaches

Abstract

Biochars have emerged as a sustainable technology for converting waste into high-value, stable carbon products. Depending on its properties, biochar can retain various elements, including nitrogen (N) as ammonium (N-NH₄⁺). This study aimed to evaluate the rapid retention of N-NH₄⁺ in biochars produced from coffee husk (CH) and chicken manure (CM) at different pyrolysis temperatures (PTs) (300 °C, 400 °C, and 900 °C) and investigate the mechanisms involved. A rapid N-NH₄⁺ adsorption experiment was conducted, in which an NH₄Cl solution was passed through the biochars. The following analyses were performed: cation exchange capacity (CEC), surface area, pore volume and size, total N content, N retention, infrared analysis (ATR-FTIR), and leachate solution analysis, followed by chemical speciation using Visual MINTEQ software. The results indicated that different mechanisms were involved in rapid N-NH₄⁺ retention. In CH-derived biochars produced at 300 °C, N-NH₄⁺ retention occurred primarily through electrostatic interactions with negative charges (CEC), as confirmed by ATR-FTIR analysis. In CM-derived biochars produced at 400 °C, N-NH₄⁺ retention was mainly through the formation of struvite (NH₄MgPO₄·6H₂O), as confirmed by chemical speciation of leachate solution in Visual MINTEQ. In CH-derived biochars produced at 900 °C, N-NH₄⁺ ions were trapped in the pores of the charred matrix due to the increased biochar surface area, pore volume, and decreased pore size. The biochars studied proved effective in retaining N-NH₄⁺ through different mechanisms, suggesting that biochars can enhance rapid N retention and reduce N leaching, potentially serving as a source of N for crops.

1. Introduction

Biochars are derived from the pyrolysis of various residues, including agricultural, animal, and industrial organic waste [1,2,3]. Biochar represents a sustainable method to convert organic wastes into environmentally friendly products [4,5,6,7]. Its effects depend on the applied dose, frequency, interaction with soil and plants, and the properties of the biochar [2,3,4,5,6,7]. The properties of biochars and their effects are strongly influenced by the feedstock used and the pyrolysis conditions, such as pyrolysis temperature (PT), heating rate, and residence time at the target temperature [7,8,9,10].

Increasing the PT leads to losing functional groups, reducing the biochar cation exchange capacity (CEC) and pore size while improving its specific surface area, ash content, and total porosity, primarily micropores [11,12,13,14,15]. At the same PT, biochar properties differ depending on the feedstock used [3,8]. For instance, lignocellulose-based biochars have higher porosity and surface area than manure-based biochars [16]. Manures, on the other hand, generally contain more mineral elements than lignin-rich raw materials. This condition typically results in higher ash content, promoting the precipitation of different elements when manure-derived biochars are used [17]. Regarding CEC, materials vary widely, whether they are lignocellulose-based or manure-based. For example, biochars derived from coffee husk (CH) and chicken manure (CM) have demonstrated higher CEC than those derived from other agricultural residues [3,8].

Due to these properties, biochars can retain elements through various mechanisms such as precipitation, electrostatic interactions (CEC), and/or porosity, as it occurs with ammonium (N-NH₄⁺) ions trapped in biochar pores [1,16,17,18]. Biochars produced from CM (high CEC and ash content) and CH (high CEC and surface area) demonstrate significant potential for retaining cations due to their properties [3,8,19,20]. Brazil, a major producer of poultry and coffee, consequently generates large quantities of CM and CH, with an estimated production in 2020 of 33.7 million tons of CM and 3.0 million tons of CH [21,22]. This reason contributes to its potential for use in biochar synthesis.

Nitrogen (N) can be retained in the form of N-NH₄⁺, which is particularly important as various wastewater types can have high concentrations of it, such as wastewater from dairy, meat, and fish processing, activated sludge, chemical industry residues, biofuels, and mining waste [23,24]. Biochar can retain N-NH₄⁺ in the soil, and when N-NH₄⁺ is retained, there is also a reduction in nitrate (N-NO₃⁻) formation, contributing to decreased N leaching in the soil [25]. When N-NH₄⁺ is passed through adsorbent materials, it can be retained and later used as an N-containing fertilizer [23,24]. Some studies highlight the potential of biochar to retain N through chemical adsorption processes [10,18]. However, these studies require a specific contact time between the solute solution and the adsorbent material. This result indicates that even if the material has a high N-NH₄⁺ retention capacity, its adsorption velocity may be slow. Rapid N-NH₄⁺ adsorption studies provide essential insights, particularly for large-scale applications where a solution containing N-NH₄⁺ is directly passed through the adsorbent material. If rapid adsorption occurs, the outcomes of these trials could help elucidate the mechanisms of N-NH₄⁺ retention [10,25,26].

The amount of N-NH₄⁺ adsorbed and N retention highly depends on the adsorbent material’s (biochar) properties and the pyrolysis conditions, such as PT, residence time, and feedstock type [10,18,26,27]. Therefore, studies are required to determine these relationships and establish the main mechanisms involved in the rapid adsorption of N-NH₄⁺. Various techniques can be employed to investigate the mechanism driving the interaction between N and organic materials, such as biochar. These include chemical and physicochemical characterization and spectroscopy analysis of the material and chemical speciation of the eluted extracts [22,26,28,29]. Furthermore, physical characterization techniques, such as surface area determination, pore volume, and pore size analysis, help elucidate the physical mechanisms involved in N-NH₄⁺ retention [1,26,27]. Tools such as classical chemical methods, the Brunner–Emmett–Teller method, infrared analysis, and chemical speciation of pre- and post-leached matrices, as well as their ions and organic compounds, can infer the ions retained [19,26,28,29,30].

Previous studies using adsorption isotherms have been employed to assess N-NH₄⁺ adsorption. This process is time-consuming and does not fully elucidate the mechanisms underlying the rapid retention of N-NH₄⁺ in biochars. The novelty of the present study lies in demonstrating the mechanisms of rapid N-NH₄⁺ retention in biochars and identifying the physical and chemical processes involved. Biochars with contrasting properties were utilized to examine the effects of such retention, providing a deeper understanding of the mechanisms at play. Thus, this study hypothesized that biochars derived from CH and CM, produced at different PTs, promote rapid N-NH₄⁺ retention through distinct mechanisms, chemical adsorption, physical retention, and formation of precipitated compounds. Therefore, this study aimed to (i) characterize the main properties of CH and CM-derived biochars produced at 300, 400, and 900 °C; (ii) determine the N-NH₄⁺ retention in CH and CM-derived biochars; (iii) evaluate the main mechanisms involved in N-NH₄⁺ adsorption in CH and CM-derived biochars through infrared spectroscopy, the BET method, and leachate solution speciation; and (iv) determine, after rapid adsorption studies, which charred matrix is more effective in retaining N for potential use as an N fertilizer.

2. Materials and Methods

2.1. Biochar Synthesis and Characterization

Coffee husk (CH) and chicken manure (CM) are highly available organic residues in Brazil and are used in pyrolysis and biochar synthesis. These organic residues were collected from farms near Lavras, State of Minas Gerais, Brazil (21°14′43″ S and 44°59′59″ W). After collection, the materials were dried in a forced-air circulation oven at 60 °C until a constant weight was achieved. The main properties of the feedstocks are shown in Table 1. The feedstock was then pyrolyzed in a muffle furnace with a stainless-steel chamber at temperatures 300, 400, and 900 °C. The choice of pyrolysis temperatures (PTs) aimed to produce biochars with different properties, as already highlighted in the literature [2,21,31]. Stainless steel cylinders were used to compact the feedstocks and limit the entry of oxygen into the system, which were then pyrolyzed in a Jung (N1200) muffle furnace. Pyrolysis was conducted by maintaining the feedstocks at the target temperature for 60 min, with a heating rate of 10 °C min⁻¹. After pyrolysis, the biochars were slowly cooled to room temperature before opening the chamber. The produced biochars are shown in Table 2.

The total carbon (C) content in the produced biochars was determined using a dry combustion C analyzer (Elementar, model Vario TOC Cube). The materials’ pH and electrical conductivity (EC) were determined in water at a 1:10 ratio [3,8]. The cation exchange capacity (CEC) was determined using the modified ammonium acetate compulsory displacement method, adapted for biochar [3]. The ash content was determined by heating the biochar samples at 750 °C for 6 h, with the residual mass representing the ash content [8].

The biochar samples’ surface area, pore volume, and pore size mode were determined using the Brunauer–Emmett–Teller (BET) method. Approximately 0.150 g of each sample was subjected to N gas adsorption at 77.35 K using a Quantachrome NOVA 1200e instrument (Quantachrome Instruments, Boynton Beach, FL, USA). The BET method determined the surface area, while the pore diameters were obtained using the BJH method. Scanning electron microscope (SEM) analysis was conducted on all biochar samples using a JSM-7610F Schottky field emission scanning electron microscope (JEOL, Tokyo, Japan). The samples were first prepared by oven-drying non-conductive pieces measuring 0.5 × 0.5 × 0.5 cm, which were then mounted on stubs using double-sided carbon tape and coated with carbon.

2.2. Ammonium Adsorption in Biochars

For the ammonium (N-NH₄⁺) retention study, a solution containing N-NH₄⁺ was passed through biochar samples (six types of biochar described in Table 2). The study followed a completely randomized design with three replicates, maintaining a temperature close to 25 °C at the study site. An ammonium solution containing 500 mg N as N-NH₄⁺ per liter was prepared using ammonium chloride (NH₄Cl) (reagent grade, Synth). Six samples were prepared for each biochar: three were designated for water washing, while the other three underwent water washing followed by the passage of the N-NH₄⁺-containing solution. The procedure steps are described as follows:

Phase 1—Biochar Preparation: Six samples were prepared for each biochar. Three were used for water wash only (as the control), and three for N-NH₄⁺ adsorption. Initially, 1.0 g of each sample was weighed into a 40 mL polypropylene tube.

Phase 2—Water Washing Step: To each 40 mL polypropylene tube containing the biochar, 20 mL of deionized (DI) water was added. The mixture was shaken on a horizontal shaker at 100–120 oscillations per minute for 4 h. Afterward, the suspension was poured into a vacuum filtration system with a glass membrane filter (Unifil^®, Calibre Scientific Brasil LTDA, Porto Alegre, Brazil), and the leachate was collected in a 125 mL Erlenmeyer flask. After filtration, each sample was then leached four more times, with 20 mL of DI water per wash (totaling 100 mL) to remove excess salts. A portion of the sample was saved for further analysis. For three of the six samples for each biochar, the biochar on the filter was transferred into a small plastic container, placed in a freezer, freeze-dried, and stored in a desiccator to maintain dryness.

Phase 3—N Adsorption Step: After the water washing, 25 mL of N-NH₄⁺ solution (500 mg N-NH₄⁺ L⁻¹) was added to each of the remaining three samples for each biochar. The samples were left to sit for approximately 10 min. The high N concentration was used to achieve maximum rapid adsorption. Following this, the vacuum filtration was initiated. This procedure was repeated thrice, with 25 mL of 500 mg N-NH₄⁺ L⁻¹ each time. An aliquot of 100 mL of filtrate was collected and treated the same way as the water-washing step for analysis. As in the previous step, the biochar samples remaining on the filter were removed. However, the adsorption procedure did not perform well for the CM biochar samples with a PT above 300 °C. The filter paper became continuously clogged during the water-washing step, and the vacuum took more than four hours to pull the liquid through. Consequently, a modified procedure was applied to all CM biochar samples for the N-NH₄⁺ adsorption step. The biochar and 25 mL of 500 mg N-NH₄⁺ L⁻¹ solution were transferred to a 40 mL polypropylene centrifuge tube for these samples. The mixture was shaken for 10 min at 100–120 oscillations per minute in a horizontal shaker, followed by centrifugation at 3100× g for 10 min. This condition allowed the same equilibration time to be used for all samples. The supernatant was primarily clear and carefully decanted into a clean 125 mL Erlenmeyer flask. The process was repeated thrice, with 100 mL of filtrate collected. Any unclear supernatant was filtered in the same way as the CH biochar samples. The biochar samples were processed similarly to the CH biochar samples.

Phase 4—Sample storage: Before (during the washing phase) and after the adsorption process, the biochar samples were freeze-dried at −35 °C for 48 h and then stored in desiccators before total N and spectroscopic analyses. The filtrate samples were immediately measured for pH and EC. A portion of the filtrate samples was acidified with approximately 1 M HCl (by adding 5 mL of 1 N HCl to 40 mL of filtrate). These acidified samples were then split into three portions to analyze water-soluble C, N-NH₄⁺, and N-NO₃⁻, and all other element analyses (see below).

2.3. Analysis of Leachate Samples

The pH and EC of the leachate were determined using a pH meter (Hanna Instruments, HI2221, Smithfield, VA, USA) and a conductivity meter (Mettler 175 Toledo, FE30, Columbus, OH, USA). Based on the EC, the ionic strength of the solution was estimated as described in Sparks [32]. In these samples, the content of water-soluble C was also determined using a dry combustion C analyzer (Elementar, model Vario TOC Cube—Liquid mode, Langenselbold, Germany). The concentrations of N-NH₄⁺ and N-NO₃⁻ in the leachate were determined by the distillation method, followed by the titration of N evolved from the samples as N-NH₃, using MgO to convert N-NH₄⁺ into N-NH₃ and Devarda’s alloy to convert N-NO₃⁻ into N-NH₃ [33]. Chlorine (Cl) concentration in leachate was estimated based on the concentration of NH₄Cl used, considering that almost all Cl was not retained due to the high CEC of the material. The concentration of aluminum (Al), boron (B), calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), phosphorus (P), sulfur (S), and zinc (Zn) in the leachate were determined through Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES).

All chemical data were processed using the Visual MINTEQ 3.0 software [34], applying the Davies activity correction method (b parameter: 0.3) and using the setting “oversaturated solids are allowed to precipitate, but only after the final answer is reached”. The formation of solids was estimated to assess whether N could be retained in the biochars through precipitation.

2.4. Analysis of Biochar Samples Before and After Ammonium Adsorption

The dried biochar samples from the adsorption study (before and after adsorption) were scanned using the Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy technique using an Agilent^® Cary 630 spectrometer (Agilent, Santa Clara, CA, USA). The samples were scanned in the 4000–650 cm⁻¹ region using a resolution of 4 cm⁻¹. After that, the generated infrared spectrum was pre-processed by normalizing the spectrum to allow the comparison [35]. The samples were compared within the same biochar before and after adsorption to identify possible changes in peaks and intensities at the same peak. The peaks of infrared spectra were identified according to studies, libraries, and bands published [3,8,22].

Subsequently, the total N content in the samples was determined using an automatic Nitrogen Analyzer (LECO Series 828, St. Joseph, MI, USA). Based on the amount of N before the adsorption study and after the adsorption step, the retention of N in the form of N-NH₄⁺ in each biochar studied was calculated according to Equation (1).

N retention (g kg⁻¹) = N after adsorption (g kg⁻¹) − N before adsorption (g kg⁻¹) (1)

The data regarding the N content in the materials before and after adsorption, as well as the amount of N retained in the materials, were compared using the Tukey test (p < 0.05) after confirming that the basic assumptions of analysis of variance (normality, homoscedasticity, additivity, and independence of residuals) were satisfied. The F-test was significant (p < 0.05). All statistical analyses were conducted using the following packages of the R software version 4.3.1: stats, base, tidyverse, and agricolae [36,37,38].

3. Results

3.1. Biochar Properties

Regarding C content, an increase in PT led to a rise in C content in CHB and a decrease in CMB, with higher values observed for CHB than CMB (Table 3). The pH of the biochars was alkaline, and both pH and EC increased as PT rose (Table 3). At temperatures up to 400 °C, CHB exhibited higher CEC compared with CMB (Table 3). However, at the highest temperature (900 °C), CMB showed a higher CEC than CHB. This result indicates a significant reduction in CEC for CHB at higher PT. The ash content was greater in CMB than in CHB at the same PT, with both biochars showing an increase in ash content as PT increased (Table 3). The surface area and pore volume were higher for biochars produced from CH, and among CHB, the increase in PT increased the surface area from 6.10 m² g⁻¹ (CHB300) to 22.27 m² g⁻¹ (CHB900) (Table 3). Pore size decreased for both biochars at 400 and 900 °C temperatures compared with 300 °C. This effect observed in the analysis performed by the BET method was also confirmed by the SEM images, demonstrating that the higher pyrolysis temperature (900 °C) significantly impacted CHB (Figure 1). SEM images showed that higher PT promoted increased porosity and surface area by reducing the particle size of the biochar produced (Figure 1).

3.2. Leaching Solution and Chemical Speciation in Visual MINTEQ

The pH, ionic strength (IS) calculated from EC, and concentrations of soluble C, N-NH₄⁺, N-NO₃⁻, Cl, Al, B, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, and Zn in the leached solution are shown in Table 4. The data show that the pH of the leached solutions was in the alkaline range, and after N-NH₄⁺ adsorption, the pH of the leachate was always lower than before adsorption. The ionic strength also increased after N-NH₄⁺ adsorption. Regarding soluble C concentration, the values after adsorption were lower than before, and the biochars produced at 900 °C had lower soluble C. Concentration of NH₄⁺ was always higher after than before the adsorption study. The lower value indicated higher retention of N-NH₄⁺ when N-NH₄⁺ was passed through biochar samples. The behavior of the other elements varied with the elemental composition of biochar. Still, generally, the elements found in higher concentrations in the leachate were Ca, K, Mg, Na, P, and S, in addition to C, Cl, and not-adsorbed N-NH₄⁺.

According to the dataset shown in Table 4 and through chemical speciation in Visual MINTEQ, it was verified that several solids were found in the biochar before or after the elution with N-NH₄⁺ solution (Table 5). The solids formed were MnHPO₄(s), hercynite (Fe₂⁺Al₂O₄), vivianite (Fe₃(PO₄)₂·8H₂O), diaspore (AlO(OH)), hydroxyapatite (Ca₅(PO₄)₃(OH)), ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O), and Fe(OH)₂(c). However, it was observed that struvite (NH₄MgPO₄·6H₂O), a compound containing N, was only formed or precipitated when the NH₄⁺ solution was passed in the CMB400. In all other conditions, there were no conditions (pH, proportion of reagents, presence of Mg, N-NH₄⁺, and phosphate in the leachates) to form the precipitates containing N in the cationic form.

3.3. Infrared Spectroscopy Analysis

In the infrared analysis of CH-derived biochars produced at up to 400 °C (CHB300 and CHB400), peaks were observed at 3300 cm⁻¹ (hydroxyl (OH) groups assigned for compounds such as alcohols, phenols, and carboxylic acids), 2915 and 2815 cm⁻¹ (C-H stretching of aliphatic C-H groups), 1560 cm⁻¹ (stretching of the C=C, vibrations of phenolic groups), 1375 cm⁻¹ (deformation vibrations of methyl (–CH₃) or carboxylic groups), 1250 cm⁻¹ (deformation vibrations of C–O groups in esters, alcohol and phenols), and 1025 cm⁻¹ (stretching of C–O groups in esters and alcohols) (Figure 2). When comparing the spectra before and after the N-NH₄⁺ adsorption experiment, only CHB300 showed differences, with a decrease in the intensity of the peak at 3300 cm⁻¹ and an increase in the intensity of the peak at 1375–1250 cm⁻¹ after the previous N-NH₄⁺ adsorption, compared with before adsorption. The increase or reduction in intensity at the same peak due to adding new substances to an organic matrix signals the reaction of this substance with the organic matrix, forming new compounds or modifying the pattern of chemical bonds. For CH900 FTIR-ATR, no peak could be identified, signaling the thermal degradation of polar oxygenated groups in the biochar matrix.

Analyzing the infrared spectra of CM-derived biochars produced up to 400 °C, bands were observed at 3300 cm⁻¹ and 2915 and 2815 cm⁻¹ (groups similar to CH-derived biochars) (Figure 2). In all CM-derived biochars, observed peaks at 1765 cm⁻¹ (C=O bond in esters), 1680 cm⁻¹ (stretching of the C=O bond of carbonyl groups or stretching vibrations of amide groups (–CONH₂)), 1030 cm⁻¹ (stretching of C–O groups in ethers, alcohols or polysaccharides) and 870 cm⁻¹ (deformation of C–H groups in aromatic or heterocyclic compounds) (Figure 2). No significant differences were observed before and after the N-NH₄⁺ adsorption experiment for CM-derived biochars.

3.4. Content and Retention of Nitrogen in Biochar

The highest total N content in biochar was observed for CMB300 (29.7 g kg⁻¹) and the lowest for CMB900 (5.8 g kg⁻¹) compared with the other biochars, regardless of NH₄⁺ adsorption (before or after) (Figure 3a). At PT 900 °C, significant N loss (>50%) occurred for both biochar products based on the total N concentration. After N-NH₄⁺ adsorption, the highest total N content values were observed in the following order: CMB300 > CMB400 = CHB300 > CHB400 > CHB900 > CMB900 (Figure 3b). Among the materials used, CHB900 had the highest N retention, with an average value of 3.74 g kg⁻¹, compared with the other biochars, representing a 78% increase compared with CHB300 and CMB400 and a 520% increase compared with the other biochars (CHB400, CMB300, and CMB900). Following CHB900, CHB300, and CMB400 exhibited the highest retentions, with an average retention of 2.10 g kg⁻¹, representing a 248% increase compared with the other treatments. The CHB400, CMB300, and CMB900 biochars showed an average N-NH₄⁺ retention of 0.60 g kg⁻¹.

4. Discussion

There was a rapid N-NH₄⁺ retention by biochars produced from CH at 300 °C and 900 °C (up to 3.74 g kg⁻¹) and by biochar produced from CM at 400 °C (2.10 g kg⁻¹) (Figure 3b). These biochars promoted rapid N-NH₄⁺ retention through both chemical and physical retention mechanisms. Lower PT tends to produce biochars with higher CEC compared with higher PT [2,8]. This effect is mainly due to preserving polar organic functional groups when lower PT is used in biochar synthesis [2,8]. As a result, depending on the feedstock used in biochar synthesis, the material can have high CEC values, as observed in our study with CHB300 (89.3 cmol_c kg⁻¹) and CHB400 (70.0 cmol_c kg⁻¹). These values align with those reported in other studies, where CH pyrolyzed at 300 °C for 30 min yielded biochars with a CEC of 121.0 cmol_c kg⁻¹ [3]. When N in the form of N-NH₄⁺ is passed through biochars with high CEC values, N-NH₄⁺ can be retained by the negative charges on the biochar [39,40,41], as observed in our study using CHB300 (Figure 2 and Figure 3).

The interaction between CEC and N-NH₄⁺ is primarily due to electrostatic bonds between NH₄⁺ and the negative charges resulting from the dissociation of hydroxyl groups (OH) from compounds such as alcohols, phenols, and carboxylic acids [39,40,41]. This effect of the interaction between hydroxyl groups and N-NH₄⁺ was confirmed in our study by ATR-FTIR analysis, where a reduction in the intensity of the peak associated with hydroxyl groups at 3300 cm⁻¹ was observed, as well as an increase in the intensity of peaks at 1375 cm⁻¹ (associated with carboxylic groups) and 1250 cm⁻¹ (associated with C–O groups) (Figure 2). This change in the infrared spectra fingerprint occurs because the vibrational movement of the OH groups is restricted, leading to a reduction in the corresponding peak, as noted in our previous study [39]. However, our study found that the retention of N-NH₄⁺ in negative charges occurred only for CHB300 and not for CHB400 (Figure 3b). For CHB300, with an added amount of 50 mg of N-NH₄⁺, the biochar retained 3.8% of the total N.

For CHB900, the increase in PT resulted in significant losses of functional groups, as indicated by the low CEC values and the absence of peaks in the ATR-FTIR analysis (Table 3 and Figure 2). Similar CEC and functional group loss with increased PT were observed in a study comparing CH pyrolyzed at 750 °C to those at 350 °C [8]. This condition occurs because, during pyrolysis, functional groups such as carboxyls and hydroxyls are volatile and thermally unstable. Higher PT leads to decarboxylation, dehydration, and dehydrogenation, resulting in the loss of these compounds as CO₂, H₂O, and H₂, which promotes a greater aromatization of the pyrolyzed matrix, with concomitant losses of negative charges and CEC [42,43,44]. Despite this effect, CHB900 exhibited the highest rapid N-NH₄⁺ retention compared to the other biochars, retaining about 7.5% of the added total N. The observed higher N-NH₄⁺ retention in CHB900 is more related to physical adsorption than chemical interactions of N-NH₄⁺ with the biochar matrix. This effect can be followed by an increase in surface area and pore volume and a reduction in pore size due to the higher PT (CHB900) compared with the lower PT, as shown in Table 3. The SEM image also visually confirmed this effect (Figure 1). A study with coffee residue-derived biochar showed that smaller pore size consequently leads to an increase in surface area due to higher pore volume, contributing to the enhancement of N-NH₄⁺ physical retention [45], similar to our study.

With increased PT, there is a significant rise in the pore size, surface area, and total porosity of biochars produced from lignified materials like CH (Table 3 and Figure 1), mainly due to the increase in surface area in response to a greater pore volume and microporosity of the biochars [2,19,26,46,47]. As a result, N-NH₄⁺, due to its small hydrated size (0.331 nm) [48], can access these biochar micropores, defined as pores smaller than 20 Å, where it becomes physically sorbed or trapped [2,16,47]. This relationship between increased surface area and rapid N-NH₄⁺ retention was also reported for CH pyrolyzed at 700 °C, compared with those biochar counterparts generated at 400 °C [26]. However, our study shows that this relationship also occurs for biochar charred at 900 °C due to the higher surface area, pore volume, lower pore size, and rapid N-NH₄⁺ retention, demonstrating that the primary mechanism of N-NH₄⁺ retention in CHB900 is physical adsorption (Table 3 and Figure 1, Figure 2 and Figure 3).

When comparing CH and CM pyrolyzed at the same temperature of 900 °C, it was found that there was no significant N-NH₄⁺ retention in the biochar produced from CM, CMB900 (Figure 3b). This low N-NH₄⁺ retention occurs because, unlike lignin-rich materials where increasing the PT enhances the surface area, in manures, raising the PT leads to the clogging of existing pores due to the high mineral content in manures [16,19,27], as indicated by the ash content, the low values of porosity and surface area (Table 3), and the lack of structure in the biochar. Therefore, compared to lignified materials, the physical retention in biochars derived from manures is insignificant, as demonstrated by our results.

Materials such as manures, which are rich in ash, contain high mineral elements [8,21,49]. Biochars derived from CM are rich in chemical elements such as Mg and P [50,51], a fact verified by the levels of these elements in the leachates from our study (Table 4). Being materials rich in Mg and P, depending on the pyrolysis condition and the total and soluble contents of these elements in the biochar, adding an N-NH₄⁺ solution may create the chemical conditions for the precipitation of N-NH₄⁺ in the form of struvite [40,52]. This effect was observed in our study, where CMB400 created favorable conditions for the formation and precipitation of N-NH₄⁺ in the form of struvite (NH₄MgPO₄·6H₂O), according to the soluble contents of the collected extracts and chemical speciation outcomes of the Visual MINTEQ software (Table 4 and Table 5).

Several studies show that N-NH₄⁺ can be retained in biochar by co-precipitation in the form of struvite, with these materials subsequently serving as a slow-release source of N and P for plants [52,53,54]. In our study, it was possible to retain 4.6% of the total N added by CMB400 through precipitation in the form of struvite (Table 5). In the biochars CHB300 and CHB900, struvite formation conditions were absent. In CHB300, the main limiting factor for struvite formation was the pH, which is required in the range of 8–9.5 for struvite synthesis. In contrast, in CMB900, despite pH not being a limiting factor, the low concentrations of Mg and P in the extracted solution were the limiting factors for struvite synthesis (Table 4 and Table 5).

Thus, in our study, it was possible to observe that depending on the feedstock to be pyrolyzed, as well as the conditions during pyrolysis, the resulting biochars, when interacting with N-NH₄⁺-containing solutions, can rapidly retain N-NH₄⁺ through different mechanisms. These include chemical interactions such as electrostatic bonds or the formation of precipitates or physical retention (trapping) of N-NH₄⁺ in biochar pores due to a biochar larger surface area, as well as the increased microporosity of the lignified-derived biochars generated at high PT (600–900 °C) [2,16,26]. Our findings have practical significance for other cations that biochar can retain through physical adsorption and/or adsorption on electrical charges.

Additionally, when these materials are used as a matrix for N-NH₄⁺ retention when adequately processed, they can be employed as N organomineral fertilizers, gradually releasing N, which better synchronizes N release with plant nutritional requirements, thereby increasing N use efficiency [52,54,55]. Although CHB900 retained more N-NH₄⁺, its implications for use as an organomineral fertilizer are limited due to the high loss of N during pyrolysis (Figure 3a). However, using CHM300 and CMB400 is desirable for biochar products that retain N, as they have higher levels of N that enable their use as organomineral fertilizers.

Compared with other studies, the adsorbed N-NH₄⁺ amount in this study used a short equilibration time, i.e., a total of 40 min. The adsorbed amount was likely lower than most other adsorption studies (usually overnight or 24 h), and since contact time influences the N-NH₄⁺ adsorption process, our study focused on explaining the mechanism behind the rapid adsorption of N-NH₄⁺ in biochars. This result suggests that increasing the equilibration time would enhance the adsorption capacity, thereby improving the potential value of biochar as an organomineral fertilizer. Additionally, the stability and availability of the adsorbed N-NH₄⁺ in the biochar are expected to vary, necessitating further testing to provide recommendations on optimizing biochar synthesis for effective N fertilizer development.

The interaction of N with biochar to produce biochar-based fertilizers has proven to be an effective alternative for improving nitrogen use efficiency in cropping systems [40,55], mainly due to the significant losses that nitrogen can experience when applied to soil. Furthermore, in the leachates, it was observed that the tested materials may release other elements, such as Ca, K, Mg, P, and S, which can serve as plant nutrient sources. Thus, new studies with pyrolyzed matrices + retained N should be conducted to assess the effectiveness of these materials in providing N in synchrony with plant demand and evaluate their potential to release other nutrients and elements to plants. Initial evaluations should be carried out with these materials under greenhouse conditions, followed by field cultivation trials.

5. Conclusions

The pyrolysis of the coffee husk (CH) at a lower pyrolysis temperature (PT) (300 °C) produces biochar with higher cation exchange capacity (CEC), which is able to promote ammonium (N-NH₄⁺) retention by electrostatic interactions between N-NH₄⁺ ions and hydroxyl groups, associated with higher CEC values. In contrast, higher PTs (900 °C) result in the loss of functional groups, reducing the CEC. However, the surface area pore volume increases. Pore size decreases, and this condition promotes retention of N-NH₄⁺ through physical interaction by trapping N-NH₄⁺ in biochar pores, with the N-NH₄⁺ retention greater than 300 °C. In the case of CM-derived biochars, significant N-NH₄⁺ retention was observed only in material pyrolyzed at 400 °C. This condition is due to a reaction between the added N-NH₄⁺ in solution and phosphorus (P) and magnesium (Mg) in the biochar. Under suitable biochar’s pH conditions, these interactions create favorable conditions for N-NH₄⁺ precipitation as struvite (NH₄MgPO₄·6H₂O). Due to the higher N losses during pyrolysis, even with more excellent N retention in CH-derived biochar produced at 900 °C, its use as an organomineral fertilizer is limited. The biochars produced from CH at 300 °C and CM at 400 °C are more promising for use as organomineral fertilizers, as they experienced lower N losses during pyrolysis, followed by greater capacities to retain N from the leachate solution, resulting in organomineral fertilizers with higher value-added capacity. This research revealed that the contrasting biochar matrices exhibit different mechanisms for rapid N-NH₄⁺ adsorption. The results imply the ability or capacity of biochar to retain N quickly, and these materials can potentially be utilized to produce organomineral N fertilizers.

Author Contributions

Conceptualization, E.G.d.M., C.A.S., S.G., L.C.A.M. and L.R.G.G.; methodology, E.G.d.M., C.A.S., S.G., B.C.L., J.C.T. and P.A.N.B.; software, E.G.d.M.; validation, E.G.d.M., C.A.S. and S.G.; formal analysis, E.G.d.M., C.A.S., S.G. and P.A.N.B.; investigation, E.G.d.M., C.A.S., and S.G.; resources, C.A.S., L.C.A.M. and L.R.G.G.; data curation, E.G.d.M.; writing—original draft preparation, E.G.d.M. and P.A.N.B.; writing—review and editing, E.G.d.M., C.A.S., S.G., L.C.A.M., P.A.N.B. and L.R.G.G.; visualization, E.G.d.M.; supervision, C.A.S., S.G., L.C.A.M. and L.R.G.G.; project administration, C.A.S., S.G., L.C.A.M. and L.R.G.G.; funding acquisition, C.A.S., S.G., L.C.A.M. and L.R.G.G. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting this study’s findings are available on request from the corresponding author.

Acknowledgments

Special thanks to Flavio Vieira Martins de Almeida and Stefania Priscilla de Souza for helping us determine the biochar N contents using a LECO automatic analyzer at the Department of Animal Science. We also thank Eduardo Alves for helping us determine the SEM images at the Laboratory of Electron Microscopy and Ultrastructural Analysis (LME) in the Department of Phytopathology. Many thanks also to Lívia Botelho de Abreu and Mariene Helena Duarte for helping us determine the leachate’s nitrogen and elemental content at the Department of Soil Science.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the study’s design, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Footnotes

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Figures and Tables

Figure 1. Scanning microscopy obtained for biochars derived from the coffee husk and chicken manure synthesized at different pyrolysis temperatures.

Figure 2. Normalized ATR-FTIR spectrum of biochars before and after ammonium adsorption.

Figure 3. Total nitrogen content (a) and retention (b) according to different biochars. Bars with standard error (n = 3) followed by different uppercase and lowercase letters indicate, respectively, significant differences between biochars within each retention condition (before and after) and between adsorption conditions within each biochar according to the Tukey test (p [less than] 0.05). Bars with standard error (n = 3) followed by different uppercase italics and bold indicate significant differences between biochars in nitrogen retention according to the Tukey test (p [less than] 0.05).

Table 1

Main properties of coffee husk and chicken manure used in biochar synthesis (adapted from Morais et al. [21]).

Feedstock	Carbon Content(g kg−1) *	pH *	ElectricalConductivity (dS m−1) *	CECcmolc kg−1	Ash *% (w/w)
Coffee husk	407	4.7 ± 0.03	6.93 ± 0.04	71.9 ± 0.6	6.93 ± 0.73
Chicken manure	247	7.7 ± 0.12	8.00 ± 0.04	42.4 ± 1.9	34.2 ± 0.75

* The methodologies employed were similar to those described for the characterization of the biochars.

Table 2

Description of the biochars produced.

Feedstock	Temperature of Pyrolysis (°C)	Biochar
Coffee husk	300	CHB300
	400	CHB400
	900	CHB900
Chicken manure	300	CMB300
	400	CMB400
	900	CMB900

Table 3

Main properties of coffee husk and chicken manure-derived biochars.

Feedstock	Temperatureof Pyrolysis(°C)	CarbonContent(g kg−1)	pH	ElectricalConductivity(dS m−1)	CECcmolc kg−1	Ash% (w/w)	SurfaceAream2 g−1	PoreVolume(cm3 g−1)	PoreSize(Å)
Coffeehusk	300	562 *	9.8 ± 0.1 *	9.4 ± 0.2 *	89.3 ± 5.7	13.1 ± 2.0 *	6.10	0.0167	61.49
	400	587 *	10.2 ± 0.1 *	12.9 ± 0.2 *	70.0 ± 7.3	15.1 ± 1.4 *	10.06	0.0242	17.11
	900	626	10.4 ± 0.1	15.7 ± 0.2	9.7 ± 2.3	21.3 ± 0.8	22.27	0.0431	17.24
Chickenmanure	300	274 *	8.7 ± 0.1 *	7.1 ± 0.1 *	38.8 ± 5.0	44.9 ± 0.1 *	0.11	0.0004	62.90
	400	258 *	10.5 ± 0.1 *	7.5 ± 0.1 *	31.9 ± 20.5	50.9 ± 0.1 *	0.53	0.0017	90.60
	900	185	12.9 ± 0.1	19.5 ± 0.4	20.2 ± 16.9	65.5 ± 0.3	0.90	0.0024	17.09

*: Adapted from Morais et al. [21].

Table 4

Value of pH, ionic strength (IS), and element concentration in leached solution before and after performing the ammonium adsorption experiment.

Biochar	Addition of N-NH4+	pH	IS (mol L−1)	C (mg L−1)	Cl (mg L−1)	N-NH4+ (mg L−1)	N-NO₃− (mg L−1)	Al (mg L−1)	B (mg L−1)	Ca (mg L−1)
CHB300	Before	8.84 ± 0.03	0.016 ± 0.001	290 ± 9.6	0 ± 0	2.3 ± 0.2	<LOD	0.43 ± 0.02	0.19 ± 0.01	0.0 ± 0.1
	After	7.92 ± 0.02	0.06 ± 0.001	46.6 ± 2.4	1272 ± 0	426.4 ± 3.5	<LOD	0.31 ± 0.1	0.16 ± 0.02	0.7 ± 0.1
CHB400	Before	10.24 ± 0.06	0.021 ± 0.001	142.3 ± 4.8	0 ± 0	1.4 ± 0.5	<LOD	0.54 ± 0.1	0.24 ± 0.02	0.6 ± 0.1
	After	8.2 ± 0.02	0.06 ± 0.001	42.6 ± 0.2	1272 ± 0	457.8 ± 6.5	<LOD	0.03 ± 0.01	0.13 ± 0.01	4.3 ± 0.4
CHB900	Before	9.72 ± 0.01	0.028 ± 0.001	33.9 ± 1.3	0 ± 0	0.5 ± 0.1	<LOD	0.06 ± 0.01	0.11 ± 0.01	3.3 ± 0.1
	After	7.99 ± 0.01	0.061 ± 0.001	37.3 ± 1.4	1272 ± 0	477.6 ± 1.1	<LOD	0.06 ± 0.01	0.11 ± 0.01	34.5 ± 0.3
CMB300	Before	8.81 ± 0.03	0.02 ± 0.001	271.5 ± 2.8	0 ± 0	2.4 ± 0.4	<LOD	0.32 ± 0.02	0.26 ± 0.01	11.2 ± 0.9
	After	7.63 ± 0.03	0.062 ± 0.001	52.3 ± 0.3	1272 ± 0	479.8 ± 9.4	<LOD	0.12 ± 0.04	0.13 ± 0.01	34.2 ± 5.4
CMB400	Before	8.51 ± 0.09	0.016 ± 0.001	144.1 ± 1.3	0 ± 0	0.9 ± 0.1	<LOD	0.64 ± 0.05	0.27 ± 0.01	6.1 ± 0.9
	After	8.06 ± 0.03	0.061 ± 0.001	45.9 ± 0.5	1272 ± 0	468.9 ± 4.5	<LOD	0.18 ± 0.08	0.06 ± 0.01	29.1 ± 0.2
CMB900	Before	12.39 ± 0.02	0.059 ± 0.005	38.3 ± 0.5	0 ± 0	0.8 ± 0.4	<LOD	0.72 ± 0.04	0.02 ± 0.01	466.4 ± 40.1
	After	9.15 ± 0.05	0.059 ± 0.001	26.1 ± 1	1272 ± 0	478.8 ± 8.6	<LOD	0.45 ± 0.11	0.06 ± 0.01	320.6 ± 30.2
Biochar	Addition of N-NH4+	Cu (mg L−1)	Fe (mg L−1)	K (mg L−1)	Mg (mg L−1)	Mn (mg L−1)	Na (mg L−1)	P (mg L−1)	S (mg L−1)	Zn (mg L−1)
CHB300	Before	0.009 ± 0.003	0.64 ± 0.12	409.0 ± 4.6	0.9 ± 0.1	0.024 ± 0.001	3.1 ± 0.4	17.6 ± 0.2	12.8 ± 0.1	0.34 ± 0.02
	After	0.000 ± 0.001	0.14 ± 0.01	205.7 ± 2.6	4.3 ± 0.1	0.019 ± 0.001	0.9 ± 0.1	2.1 ± 0.1	0.3 ± 0.1	0.14 ± 0.01
CHB400	Before	0.003 ± 0.002	0.28 ± 0.03	466.9 ± 6.0	0.0 ± 0.1	0.021 ± 0.002	3.3 ± 0.3	9.6 ± 0.4	12.4 ± 0.2	0.21 ± 0.07
	After	0.000 ± 0.001	0.07 ± 0.01	201.6 ± 3.9	12.9 ± 1.0	0.024 ± 0.002	0.9 ± 0.1	3.1 ± 0.3	0.5 ± 0.1	0.10 ± 0.03
CHB900	Before	0.000 ± 0.001	0.22 ± 0.01	629.5 ± 9.2	8.7 ± 0.3	0.019 ± 0.001	2.3 ± 0.1	2.7 ± 0.1	13.5 ± 0.3	0.03 ± 0.01
	After	0.000 ± 0.001	0.04 ± 0.01	57.1 ± 0.6	1.4 ± 0.1	0.047 ± 0.001	0.6 ± 0.1	0.4 ± 0.1	0.5 ± 0.1	0.07 ± 0.01
CMB300	Before	0.033 ± 0.001	0.63 ± 0.17	362.9 ± 3.7	12.1 ± 0.2	0.131 ± 0.007	24.0 ± 0.4	50.8 ± 0.6	34.2 ± 0.2	0.26 ± 0.01
	After	0.000 ± 0.001	0.13 ± 0.01	26.3 ± 1.6	26.4 ± 0.8	0.097 ± 0.006	2.4 ± 0.1	21.9 ± 1.2	1.8 ± 0.6	0.12 ± 0.02
CMB400	Before	0.030 ± 0.003	0.4 ± 0.04	325.6 ± 12.1	9.4 ± 0.3	0.082 ± 0.008	16.9 ± 0.6	32.6 ± 1.0	29.3 ± 1.8	0.16 ± 0.01
	After	0.000 ± 0.001	0.11 ± 0.03	70.0 ± 0.8	28.2 ± 0.7	0.102 ± 0.012	5.8 ± 0.1	30.2 ± 0.1	2.4 ± 0.1	0.02 ± 0.01
CMB900	Before	0.007 ± 0.007	0.29 ± 0.01	355.3 ± 5.0	0.0 ± 0.1	0.078 ± 0.002	18.8 ± 0.3	0.0 ± 0.1	47.2 ± 8.8	0.08 ± 0.07
	After	0.008 ± 0.005	0.23 ± 0.01	47.0 ± 1.9	11.7 ± 1.0	0.084 ± 0.003	4.8 ± 0.2	0.5 ± 0.4	33.7 ± 2.1	0.05 ± 0.05

Each value represents the means of three replicates. <LOD: Lower limit of detection of method (<0.2 mg L⁻¹).

Table 5

Amount of finite solids formed according to the analysis of elements carried out in leached solution.

Biochar	Additionof N-NH4+	MnHPO4(s)(µmol L−1)	Hercynite(µmol L−1)	Vivianite(µmol L−1)	Diaspore(µmol L−1)	Hydroxyapatite(µmol L−1)	Struvite(µmol L−1)	Ettringite(µmol L−1)	Fe(OH)2(c)(µmol L−1)
CHB300	No	0.428	8.017	1.004	-	-	-	-	-
	Yes	0.335	1.816	-	8.006	-	-	-	-
CHB400	No	0.367	5.094	-	8.379	2.448	-	-	-
	Yes	0.439	0.527	-	-	17.470	-	-	-
CHB900	No	0.330	1.0197	0.907	-	15.938	-	-	-
	Yes	-	0.315	-	1.485	4.726	-	-	-
CMB300	No	2.388	5.828	1.779	-	55.330	-	-	-
	Yes	1.769	-	0.524	4.408	165.590	-	-	-
CMB400	No	1.487	7.045	-	9.753	29.659	-	-	-
	Yes	1.855	-	0.543	6.711	143.700	190.01	-	-
CMB900	No	-	-	-	-	0.180	-	13.201	2.851
	Yes	-	4.043	-	8.286	5.385	-	-	-

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