Glyphosate Removal from Water Using Biochar Based Coffee Husk Loaded Fe₃O₄

Abstract

Glyphosate is an herbicide that is usually used by farmers and is considered harmful to the environment in excess amounts. To address these issues, coffee-husk-biochar-loaded Fe₃O₄ (CHB- Fe₃O₄) was used as an adsorbent to remove glyphosate from water. CHB-Fe₃O₄ characteristics such as pH_pzc, FTIR, and SEM were measured to understand the properties of this adsorbent. The best conditions for glyphosate removal by CHB-Fe₃O₄ were obtained at pH 2.0, where the adsorption capacity and percentage removal are 22.44 mg/g and 99.64%, respectively, after 4 h of adsorption. The Freundlich model provided the best fit for the adsorption isotherm, demonstrating multilayer sorption. The most effective model for characterizing the adsorption kinetics was the pseudo-second-order model with a chemical adsorption mechanism. The desorption studies found that the use of 0.1 M NaOH was the best concentration to effectively desorb glyphosate with a desorption percentage of 69.4%. This indicates that CHB- Fe₃O₄ is a feasible adsorbent for glyphosate removal from water.

1. Introduction

Glyphosate (C₃H₈NO₅P), also known as N-phosphonomethyl glycine, is one of the active ingredients in herbicides that is commonly used by farmers to eradicate postharvest weeds. In the last few years, glyphosate usage has increased annually, by about 8.6 million kilograms from 1974 to 2014 [1]. The uncontrolled application of herbicides containing this active ingredient (glyphosate) without adhering to the recommended levels can cause serious damage to environmental ecosystems, especially soil and water. Glyphosate enters the water system through several sources such as agricultural runoff from rainfall or irrigation and groundwater leakage from contaminated crop residues [2]. The current detection of glyphosate, and its transforming products amino methyl phosphonic acid, in food, human bodily fluids, waterways, and soil demonstrates an urgent need for further research on how this herbicide affects the environment [3]. The usage of glyphosate exceeding the threshold level can have adverse effects on the environment, human health, and aquatic organisms, and can also impart ecological threats to sustainable agriculture [4]. This herbicide can penetrate into water systems and threaten and cause damage to ecosystems as it generally remains in the environment for a long time and is carcinogenic. The presence of herbicides in drinking water can cause major health problems to humans, therefore it is necessary to monitor the concentration of this herbicide in tap water [5]. A previous study has reported that tap water and river water have the slowest degradation rate of glyphosate. This is because the organic carbon content contained in these waters absorbed most of the photons, therefore slowing down the degradation rate of the herbicide [6]. Thus, an effective solution is needed to treat glyphosate-contaminated wastewaters and streams to prevent excess concentration of this toxin and to avoid the glyphosate from interfering with the human food chain [5].

Glyphosate contamination in the environment has driven scientists to develop efficient and reliable methods to purify water resources. As highlighted by previous researchers, the urgent need to remove this substance has led to the development of various appropriate and practical alternatives [7,8,9,10,11]. There are many conventional methods that have been conducted by several researchers to remove contaminants from water, such as membrane separation [12], microwave UV [13], UV irradiation [14], biodegradation [15], and the most popular method of adsorption [16,17]. Adsorption is frequently preferred as the alternative removal method for various reasons, including the fact that the process may be personalized to specific applications (selectable for dissolved glyphosate), and the adsorbent material is considered to be relatively safe to handle [18,19].

There are various types of adsorbents that have been synthesized for glyphosate adsorption, such as magnetic-coated silica trimethyl chitosan hybrid shells [20], zinc oxide nanoparticles [21], Mg-Al-layered double oxides [22], and clay biochar [23]. However, certain aspects, such as accessibility of the adsorbent’s materials and the abundant waste produced from agricultural industries, including coffee husks, need to be considered in order to fully utilize the source and minimize the waste. In 2021–2022, coffee husk production reached 0.3 billion tons [24], and this waste is discarded near the factory, which can pollute the environment [25]. Therefore, instead of discarding the coffee husk to the environment, this research intends to utilize it as an effective adsorbent material.

In conjunction with this issue, coffee husks that undergo heat treatment to produce biochar (a pyrolysis process of numerous types of biomasses under low-concentrated oxygen conditions) have been proven to hold great potential to create adsorbent material [26]. Several studies have used coffee husk biochar to remove pollutants, such as dye and heavy metals [27,28]. However, there is no reported study on the use of magnetite coffee husk biochar to remove glyphosate from water. In this study, we synthesized coffee husk biochar with magnetite properties to remove glyphosate. The raw coffee husk was burned at 600 °C for 2 h. Subsequently, Fe₃O₄ is coated to the biochar to increase the number of surface-active sites. The adsorption process and mechanism of CHB-Fe₃O₄ adsorbing glyphosate were investigated by analyzing its isotherm and through kinetic studies.

2. Materials and Methods

2.1. Materials and Chemicals

Glyphosate with purity of 95% was purchased from Anhui Fengle Agrochemical Co, Ltd. Hefei, China. Sodium oxide (NaOH), sodium chloride (NaCl), and sodium molybdate (Na₂MoO₄) were bought from Kanto Chemical CO., INC, Tokyo, Japan. Ninhydrin (C₉H₆O₄) was obtained from Sigma Aldrich chemicals, MA, USA.

2.2. Preparation of Coffee Husk Biochar (CHB)

Coffee husk was obtained from Solok Radjo Cooperative, West Sumatera province, Indonesia. Coffee husk (CH) was ground and sieved into a fine particle size < 2 mm, and then burned using a Muffle furnace (FO100, Yamato, Japan) at temperature 600 °C for 2 h with a heating rate of 10 °C min ⁻¹ under nitrogen atmosphere (Figure 1). The coffee husk biochar (CHB) was then stored in a desiccator before further treatment.

2.3. Preparation of Coffee-Husk-Biochar-Loaded Fe₃O₄ (CHB-Fe₃O₄)

CHB (3 g) was mixed with FeSO₄.7H₂O (0.5 M) and FeCl₃ (1 M). Then, the mixed solution was shaken for 1 h while heated at 60 °C. Then, the mixture was stirred for 24 h at 25–30 °C by using a magnetic stirrer (Mag-mixer MG600) after adjusting the pH to 10 by using 3 M NaOH. Following that, the filtration process by filtering the mixture using filter paper (qualitative filter paper no. 5C) was performed, and the filtrate was dried for 72 h at 60 °C. This adsorbent is called CHB-Fe₃O₄.

2.4. Characterization of Adsorbent

The pH zero-point charge (pH_zpc) was also defined in this study for better understanding of the characteristics of this adsorbent. Samples were subjected to different initial pHs (2.0, 4.0, 6.0, 8.0, and 10.0) of NaCl (0.01 M). Mixture of NaCl (50 mL) with 0.1 g adsorbent was shaken for 24 h using a shaker before being centrifuged for 15 min at 1500 rpm. Then, the pH was measured for calculation of pH_zpc. Analysis of SEM images and functional groups CHB- Fe₃O₄ before and after adsorption were carried out by using SEM (JIED-2300, Shimadzu, Kyoto, Japan) and FTIR (Thermo Scientific Nicolet iS10, Waltham, MA, USA), respectively.

2.5. Detection of Glyphosate and Calibration Curve

The technique for detecting glyphosate was conducted by using the method discovered by [29]. The standard solution of 1000 mg/L glyphosate was used to prepare different concentrations of glyphosate (10, 20, 30, 40, and 50 mg/L). For detection of glyphosate, 0.5 mL of each standard glyphosate/sample were added into test tubes containing 0.5 mL of 5% (m/v) C₉H₆O₄ and 0.5 mL of 5% (m/v) Na₂MoO₄. The solution mixture was then heated in a water bath at a temperature ranging from 80 to 90 °C for 12 min until the sample solution turned a purple color. The sample was cooled down to room temperature, placed in a volumetric flask, and added 1 mL of distilled water. A spectrophotometer from Kyoritsu Chemical-Check (Lab., Corp, Yokohama, Japan) with a wavelength of 570 nm was used to measure the absorbance. From the measurement of the standard solution data, linear absorbance of the standard curve was obtained as the concentration of glyphosate increased, as presented in Figure 2.

2.6. Batch Adsorption and Desorption Experiments

CHB- Fe₃O₄ (0.05 g) was added into the 25 mL glyphosate solution and shaken at 30 °C with a bioshaker (V-BR-36). The influence of different experimental parameters (pH: 2.0, 4.0, 6.0, 8.0, and 10; initial glyphosate concentration: 10, 20, 30, 40, and 50 mg/L; contact time: 30, 60, 90, 150, 180, 240, 300, and 1440 min) were investigated. An external magnet was used to separate the adsorbent from the adsorbate. Quantification of adsorbed glyphosate was calculated using Equation (1), while Equation (2) was applied to determine the removal percentage. All procedures were conducted twice [30,31].

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{C}}_{\mathrm{a}}-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{W}}\times \mathrm{V}$$

(1)

$$\%\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}=\frac{{\mathrm{C}}_{\mathrm{a}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{a}}}\times 100$$

(2)

where C_a and C_e are the initial and after concentration of glyphosate (mg/L), respectively. q_e is the adsorption quantity (mg/g), W is the mass of adsorbent (g), and V is the glyphosate volume (L).

2.7. Desorption Studies

Desorption studies were performed using the equilibrium data. NaOH was used as the desorption solvent. Different concentrations of NaOH (0.1, 0.5, and 1 M), contact times (30–1440 min), and temperatures (30, 40, and 50 °C) were conducted to determine the effect of these parameters on desorption efficiency. The calculation for desorption percentage is presented in Equation (3) below [16].

$$\%\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{{\mathrm{D}}_{\mathrm{e}}}{{\mathrm{A}}_{\mathrm{e}}}\times 100$$

(3)

where % desorption is the percent desorption (%), De and Ae are the desorption equilibrium and adsorption equilibrium (mg/g), respectively.

3. Results and Discussion

3.1. SEM Results of CHB-Fe₃O₄

Figure 3 shows SEM images of CHB-Fe₃O₄ before and after adsorption of glyphosate. It can be observed that the surface of CHB-Fe₃O₄ before adsorption is rougher and contains many available pores compared to the surface of the adsorbent after glyphosate adsorption. This could be due to the interactions between the adsorbate and adsorbent, where the adsorbate (glyphosate) diffuses into the available pores on the surface of the adsorbent (CHB-Fe₃O₄), and the binding of glyphosate to the active sites of the adsorbent containing Fe₃O₄. Based on the results of a study by [32], the porosity structure of magnetite coffee husk, obtained from SEM results, is in accordance with the BET results of coffee husk biochar and magnetic coffee husk biochar samples, which are 48.84 m²/g and 142.32 m²/g, respectively. This information signifies that the adsorption ability of magnetite coffee husk biochar is better than coffee husk biochar.

3.2. FTIR Spectra of Adsorbent before and after Glyphosate Adsorption

The FTIR data before and after the adsorption are shown in Figure 4. The band at 3369 cm⁻¹ changed to 3341 cm⁻¹ after adsorption, indicating a strong interaction between O-H surface groups and glyphosate [33,34]. The band at 2119 cm⁻¹ changed to 2117 cm⁻¹ after adsorption of glyphosate which was specific to the functional group of C≡C [35]. The peak band at 1630 cm⁻¹‘s move to 1629 cm⁻¹ after adsorption is attributed to the C=O group [36].The peak at the 1118 cm⁻¹ and 1124 cm⁻¹ bands demonstrates the (C-O) polysaccharide carbohydrate region [37]. In contrast, bands at 883 and 888 cm⁻¹ were assigned to C–H vibrations in the aromatic structures [36]. The bands at 632 cm⁻¹ and 598 cm⁻¹ are the result of the stretching vibrational interaction which connected to the metal–oxygen bonds in the Fe-O bond of the Fe₃O₄ crystal lattice [38]. Based on previous research conducted by [32], comparison of FITR results showed that the OH, C-H, and C=C functional groups in raw coffee husk become more obvious after turned into coffee husk biochar, because of hydrothermal carbonization process making the intensity of the FTIR spectrum of the coffee husk biochar become higher and more obvious. The thermal process causes the raw coffee husk to have more OH and C=C functional groups. Furthermore, the chemical treatment by adding magnetite (Fe₃O₄) into coffee husk biochar has resulted in more oxygen-containing functional groups in the adsorbent.

3.3. Interaction Glyphosate with CHB-Fe₃O₄

After adsorption of glyphosate using CHB-Fe₃O₄, it is assumed that there will be several interactions occurring between the adsorbent and the adsorbate as shown in Figure 5. Previous studies explained that there are several interactions defined in glyphosate adsorption such as pore diffusion, electrostatic interactions, and H-bonding [39,40]. Pore diffusion is the interaction that is most likely to occur, because the glyphosate will move into the adsorbent through the available pore sites.

3.4. Initial pH Effects

The pH is one of the most important parameters that affect the surface charge of the adsorbate (glyphosate) and adsorbent (CHB-Fe₃O₄). As shown in Figure 6a the highest adsorption was obtained at pH 2, with an adsorption capacity of 3.07 mg/g after 30 min of adsorption. Figure 6b shows that the pH_zpc was at pH 2. If the pH < pH_zpc, the adsorbent surface will be positively charged, while if the pH > pH_zpc, the surface of the adsorbent will be negatively charged. At pH 2, the surface of the adsorbent is positively charged, while glyphosate is negatively charged because the pKa of glyphosate is 2.23 (<2.23 is negative charge). Thus, electrostatic interactions between the net charge of the adsorbent and the negative charge of glyphosate occurred. The lower pH caused the deprotonation of amine (NH⁻), phosphate (PO₃⁻), and carboxyl (COOH⁻) groups in glyphosate molecules [10,11,15,41].

3.5. Initial Concentration Effects

The effect of the initial concentration is shown in Figure 7. The result displays an increase in adsorption capacity from 2.77 to 17.13 mg/g when the glyphosate concentration increases. This is because the quantity of glyphosate molecules increases with increasing concentration, resulting in a greater tendency for attachment of the adsorbate onto the adsorbent surface [42].

3.6. Adsorption Isotherm Studies

Isotherm models are a crucial concept for observing or predicting the processes that occur during adsorption equilibrium [43]. The Langmuir and Freundlich models are the most commonly used models for glyphosate adsorption and are used for isotherm analysis in this study. The Langmuir isotherm shows that active sites of the adsorbent have equal energy and that the adsorption process primarily occurs with regards to monolayers [39,40,44]. In contrast, the Freundlich isotherm shows that energy declines logarithmically as the number of molecules adsorbed on the adsorbent sites increases [7,9,40,45]. The experiment was conducted by adding 0.05 g of adsorbent into 25 mL of glyphosate at different initial concentrations ranging from 10 to 50 mg/L. The linear equations for the Langmuir and Freundlich models are listed in

Table 1. Linear equations of isotherm models.

Model	Equation	Supporting Equation
Langmuir	${\mathrm{C}}_{\mathrm{e}}/{\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\max }}+1/({\mathrm{K}}_1{\mathrm{q}}_{\max })$	${\mathrm{R}}_{\mathrm{L}}=\frac{1}{(1+{\mathrm{K}}_{\mathrm{L}}\times {\mathrm{C}}_{\mathrm{a}})}$
Freundlich	$\mathrm{Ln} {\mathrm{q}}_{\mathrm{e}}={{\mathrm{L}}_{\mathrm{n}}\mathrm{k}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}\times {\mathrm{L}}_{\mathrm{n}}{\mathrm{C}}_{\mathrm{b}}$

Note: q_e = adsorption capacity (mg/g), K_l = equilibrium constant of adsorption (L/mg), K_f = equilibrium constant of adsorption, q_max = maximal adsorption capacity (mg/g), C_e and C_a (mg/L) = equilibrium concentration and initial concentration, respectively.

.

q_max in the Langmuir equation indicates that the maximum adsorption capacity obtained in this study was 18.315. R_L is the separation factor, where 0 < (favorable), > 1 (unfavorable) and = 1 (linear).

Table 2. Isotherm Constants for Glyphosate Adsorption.

Adsorption Isotherm	Isotherm Constant	Glyphosate
Langmuir	qmax	18.315
	KL	0.6727
	RL	0.0146
	R2	0.2178
Freundlich	KF	4.1783
	1/n	1.3711
	R2	0.8456

shows that the R_L value of glyphosate adsorption was 0.0146, indicating that the adsorption was favorable [16]. The graph in Figure 8a shows that the regression coefficient R² in the Freundlich equation was close to one, making it more suitable than the Langmuir model (Figure 8b).

3.7. Adsorption Kinetic Studies

Figure 9 shows the effect of contact time on glyphosate adsorption. The adsorption capacity increased rapidly from 30 to 60 min, and then steadily increased to 240 min with an adsorption capacity of 22.29 mg/g, then decreased and increased again until 1440 min. This is because the adsorbent’s surface site was saturated with adsorbate, and this condition could lead to adsorption or desorption of the glyphosate. Finally, adsorption capacity equilibrium was attained at 1440 min with an adsorption capacity of 22.44 mg/g.

Kinetic analysis can be helpful in determining the mechanism and pace of the adsorption process [18]. In this study, pseudo-first-order (Equation (4)) and pseudo-second-order (Equation (5)) models were used to predict the adsorption kinetic of this study.

Log (q_e − q_t) = Log q_e − k₁t

(4)

t/q_t = (1/k₂ q² qe) + t/qe

(5)

where k₁ (min⁻¹) is the constant of the pseudo-first order, and k₂ (g/mg min⁻¹) is the constant of the pseudo-second order. t (time) (min), and q_t (mg/g) is the adsorption capacity at time.

The results show that the pseudo-second-order model (R = 0.999) (Figure 10) is more appropriate for describing the kinetic study data in this research, with an adsorption capacity of 22.57 mg/g (

Table 3. Kinetic model parameters for adsorption of glyphosate with CHB-Fe₃O₄.

Pseudo-First-Order Model			Pseudo-Second-Order Model
qe (mg/g)	K1 (min −1)	R2	qe (mg/g)	K2 (g mg−1 min−1)	R2
1.1904	1.38889−7	0.0686	22.5733	0.0050	0.9999

). These results are in line with those reported by a previous study [46], which showed that glyphosate adsorption using nanosized copper hydroxide-modified resin fits the pseudo-second-order model, and chemisorption could be the rate-determining process.

3.8. Desorption Study

The desorption percentages of glyphosate are shown in Figure 11. The results showed that the NaOH concentration significantly affects the release of glyphosate from the adsorbent. Increases in the concentrations of NaOH have the contrary effect on the desorption percentage, as the concentration of glyphosate released decreases from 69.40 to 5.99% (Figure 11a). Subsequently, different temperatures were also tested for desorption effectivity, and the results showed that 30 °C is the best temperature for desorption of glyphosate (Figure 11b). Interestingly, the desorption of glyphosate varied as the time increased. The desorption percentage increased and then decreased for up to 1440 min, indicating the occurrence of adsorp/desorp during these processes (Figure 11c). The best results for desorption of glyphosate were obtained at a temperature of 30 °C by using NaOH (0.1 M) for 30 min, with a desorption percentage of 69.4%.

Various materials have been reported in the literature for the adsorption-based removal of glyphosate, as shown in

Table 4. Materials with capacity for adsorption of glyphosate reported in the literature.

Adsorbents	qe (mg/g)	Reference
Woody Biochar Zinc oxide nanoparticles	44 82.97	[9] [21]
Synthesis of clay-biochar	37.06	[23]
Multi-walled carbon nanotubes (MWCNTs)	21.17	[47]
Cu-zeolite 4A	112.7	[48]
Nanosized Copper Hydroxide	140	[46]
CHB-Fe3O4	22.44	This study

. The results indicate that CHB-Fe₃O₄ is a feasible adsorbent for the removal of glyphosate from water.

4. Conclusions

In this study, biochar-based coffee-husk-loaded Fe₃O₄ (CHB- Fe₃O₄) was used as an adsorbent to remove glyphosate from water. The results show that the best glyphosate adsorption was achieved at pH 2 solution after a contact time of 1440 min, with maximum adsorption capacity of 22.44 mg/g. The Freundlich model provided the best fit to the isotherm analysis, demonstrating multilayer sorption. The most effective model for characterizing the adsorption kinetics was the pseudo-second-order model, with chemical adsorption as the rate-limiting phase, possibly including valence forces by sharing or exchanging electrons between the adsorbent and the adsorbate. The desorption studies showed that the use of 0.1 M NaOH is the best concentration to achieve a desorption percentage of 69.4% in 30 min.