L-Glutathione-Functionalized Silica Adsorbent for the Removal of Pesticide Malathion from Aqueous Solutions

Abstract

An L-glutathione-functionalized silica adsorbent was applied in this study to remove malathion from aqueous media. This adsorbent has demonstrated an improved adsorption efficiency of malathion. The maximum uptake achieved was 130 mg g⁻¹ at pH 8. Equilibrium was reached after about 90 min. A pseudo-second-order model best described the adsorption kinetics. The adsorption isotherms were best simulated by the Freundlich model. The functional groups are thermally stable up to about 150 °C. The elemental analysis results indicated high glutathione ligand densities. The results of this study show that the environmentally friendly L-glutathione functionalized silica is a promising candidate for the removal of malathion from water at the industrial level.

1. Introduction

The application of pesticides in agriculture is employed to regulate plant growth and to hinder the development of damaging organisms in crops. It has been reported that about one-third of the crop production worldwide would be wasted if pesticides were not used to eliminate pests [1]. Other uses of pesticides include disinfection, wood protection, and domestic applications [2]. However, the presence of pesticides and their analogs in aquatic natural resources disrupts the ecosystems and can affect human health. The toxic effects of pesticides on human health include kidney disease, a high incidence of cancer, infertility in both men and women, suppression of the immune system, neurological disorders, especially in children [3,4], and they are associated with various mechanisms of action [5]. The longstanding effects of pesticides compromise potential affections on the endocrine system in mammals, fowl, and fish [6]. The capabilities of pesticides in causing endocrine breakdowns by interacting with stretch receptors of androgen and estrogen, as well as thyroid functionality, are yet to be understood [7,8,9]. Issues such as high toxicity to humans and fauna and release into the environment have prompted concern of many environmental agencies. The European Community Drinking Water Directive has set a limit of 0.1 µg L⁻¹ for pesticides level in drinking water [10]. As a result of the above issues, the presence and management of pesticides in the ground, surface, and drinking water indicate that there is an actual need for studies dealing with the removal/prevention of these chemicals from aquatic sources.

Among organophosphorous pesticides (OPP), the most widely used is malathion (O,O-dimethyl S-(1,2-dicarbethoxyethyl) dithiophosphate) (Figure 1), which is a non-systemic OPP and has the most comprehensive spectrum of utilization. Over 14 million tons of malathion are used annually [11]. It is employed to kill a variety of insects in the open in varied agricultural and residential habitats [12,13,14]. The most common uses of malathion include the control of mosquitoes that attack fruits, vegetables, landscaping plants, shrubs, and lice proliferation. Analogous to many organophosphorus pesticides, malathion can be hindered to tie up to neurotransmitter acetylcholinesterase (AChE), and the enzyme activity is then inhibited, causing hyper-excitation of nerve extremes [15] that phosphorylate across the active site [16]. It has been reported [17] that high amounts of malathion in the human body obtained through exposure to air, water, or food may cause trouble with breathing, vomiting, diarrhea, headaches, loss of consciousness, and death. The World Health Organization (WHO) sets a limit for this OPP at 1 µg L⁻¹ in drinking water [18]. Consequently, an increasing interest exists in creating water purification technologies for waters polluted with OPPs such as malathion and their degradation products to reduce them to a minimum and/or eliminate their life-threatening effects on humans and the detrimental effects on the environment [19].

It is an extremely difficult task to produce a universal technology to treat pesticide-contaminated waters due to the broad spectra of used pesticides. As a result of this, independent or joint technologies have been applied in the elimination/removal of pesticides, and these include ozone chemical oxidation [20], separation with membranes [21], biological degradation [22], photodegradation [23,24], Fenton degradation [25], electrochemical decomposition [26], coagulation [27], ozone and UV irradiation [28], and adsorption [29,30]. Regarding the adsorption method, this is contemplated as an exceptional treatment technology because of the low cost of implementation and simplicity of application; it is an environmentally friendly technology and possesses superb removal efficiencies. Furthermore, adsorption presents the advantage of reusability of the adsorbent, high performance on selectivity for mixtures of contaminants and the capacity of handling and treating concentrated waste solutions [31,32]. A great variety of pesticide removal has been studied using low-cost and commercially available adsorbents [33,34]. For example, many solid adsorbents have been applied in the removal of malathion from aqueous media, and these are activated carbons [35,36], nanoparticles such as multi-walled carbon nanotubes, MWCNTs [37] or Ni-doped TiO2 nanoparticles [38], rice shells [39], clays, and organoclays [40]. There are few works in the literature on the use of sorbents based on silicon dioxide for the removal of malathion [41]. However, to the best of our knowledge, the possibility of using silica gel functionalized with amino acids (L-glutathione) to remove malathion from aqueous solutions has not been studied before. Herein, we report on the application of an L-glutathione-functionalized silica adsorbent with improved malathion removal capacities, fast kinetics, and acceptable regeneration capacities. The reason for using L-glutathione was that we advocated in synthesizing a nontoxic and environmentally friendly adsorbent that could be tested directly in drinking water treatment plants without the risks of possible contamination of the treated waters. L-glutathione possesses the feature of being harmless to humans in case it detaches from the surface of the adsorbent. Furthermore, L-glutathione was selected among other amino acids due to the ease of coupling with the silica matrices under mild synthesis conditions due to the zwitterionic properties possessed by this amino acid.

In view of our interest in industrial implementations of adsorptive solids that can be prepared under mild reaction conditions and used in the removal of toxic pesticides such as malathion, an adsorbent previously synthesized in our research group has been applied here in the removal of malathion from aqueous media. Hence, the aim of this study is to report on the structural properties of an L-glutathione-functionalized silica environmentally friendly adsorbent (SG-LGPS). The maximum malathion capacities and insights about the adsorption mechanism were gained from adsorption equilibrium isotherms; the feasibility of industrial application was obtained from the kinetics studies and regeneration reports. The adsorbent was characterized by SEM analysis, elemental analysis, and thermogravimetric analysis (TGA).

2. Materials and Methods

2.1. Materials

All the reagents used in this study were of analytical grade. Deionized water with a resistivity of 18 MΩ (Barnstead, Chicago, IL, USA) was employed to prepare all solutions. A Dragon Malathion 1000E^® commercial grade malathion solution of 84% purity was acquired from the CEDIS Company (Mexico City, Mexico) and used without further purification. All glassware was washed with a phosphate-free detergent, followed by 50% v/v HNO₃ acid washings, and finally rinsed several times with deionized water.

2.2. Synthesis of Adsorbent SG-LGPS

The synthesis of an L-glutathione-functionalized silica adsorbent was carried out recently by our research group using the sol-gel technique, and details can be found elsewhere [42]. Briefly, the synthesis procedure involved the formation of a coupled functional precursor L-glutathione-propyltriethoxysilane. This functional precursor was then hydrolyzed and homocondensed with distilled water, anhydrous EtOH, and NaCl. In a separate way, the silica gel matrix (SG) was prepared by hydrolysis and homocondensation of tetraethoxysilane (TEOS) in a polypropylene beaker with TEOS, distilled water, anhydrous EtOH, and NaCl. When hydrolysis and homocondensation of the functional precursor and TEOS were carried out, the two solutions were mixed to co-condense using triethylamine (TEA). The product thus obtained was a gel. The gel was left to dry for 24 h at 60 °C, and the product obtained was now a xerogel, which was ground and sieved to a particle size of 125–180 µm. The adsorbent was identified as SG-LGPS.

2.3. Malathion Analysis

Commercial malathion solution was analyzed using a UNICO UV-2150 UV/Vis instrument. A series of scans at varying malathion concentrations (5–100 mg L⁻¹) was performed to determine the optimum wavelength in the range of 200–350 nm, which turned out to be 220 nm (Figure 2). For this wavelength, calibration curves were constructed for malathion solutions of various concentrations (5–100 mg L⁻¹); all curves presented linearities with R² > 0.99.

2.4. Characterization

2.4.1. Malathion Adsorption as a Function of Solution pH

With the aim of finding the optimum pH value for malathion adsorption, 200 mg L⁻¹ malathion solutions with varying initial pH (3–9) adjusted with 0.1 M NaOH or 0.1 M HNO₃ were contacted in amber flasks with about 0.1 g of SG-LGPS. Malathion-loaded samples were identified as SG-LGPS-ML. The solutions were put in a temperature-controlled bath using an orbital shaker incubator NB-T205, N-BIOTEK operated at 120 rpm and 298 °K for 48 h. Then, the suspensions were filtered, and the residual malathion concentration was measured. If solution volume remained constant, the amount of malathion adsorbed was determined as:

$$q=\frac{V\left(C_{i, ML}-C_{f, ML}\right)}{m}$$

(1)

where:

q = amount of malathion adsorbed (mg g⁻¹);

V = solution volume (L);

C_{i, ML} = initial malathion concentration (mg L⁻¹);

C_{f, ML} = final malathion concentration (mg L⁻¹);

m = mass of adsorbent (g).

2.4.2. Malathion Adsorption Kinetics

Malathion adsorption kinetics on SG-LGPS was studied in a batch reactor. The effect of initial concentration on the adsorption rate was studied at three concentrations (100, 200, and 300 mg L⁻¹) and at an initial pH = 7. Although the investigated concentration range exceeds the expected ranges of environmental concern, at lower concentrations, it would not be possible to assess the full adsorption capacity of the adsorbent. All solutions were prepared using amber flasks to avoid contact with sunlight. About 0.1 g of SG-LGPS was contacted with 25 mL of each initial solution in the controlled-temperature bath using an orbital shaker incubator operated at 120 rpm and 298 °K. Aliquot samples were taken at time intervals (5–240 min). Then, the suspensions were filtered, and malathion concentrations of initial and final solutions were measured. The amount of malathion adsorbed was determined from Equation (1). The reported values correspond to the average over three experiments on selected points of the curve.

2.4.3. Malathion Adsorption Isotherm Measurements

The malathion adsorption isotherm experiments onto SG-LGPS were performed in the batch mode at a pH range of 5.0–8.0 to study the effect of solution pH on the extent of malathion adsorption. Initial malathion solutions were prepared at varying concentrations (50–800 mg L⁻¹) by dilution of a 1000 mg L⁻¹ stock solution at a fixed initial pH. An amount of 0.1 g of adsorbent was contacted with 25 mL of initial solution in the orbital shaker incubator at 120 rpm and 298 °K for 90 min (determined from the kinetic measurements). The suspensions were then filtered; the pH of the solution and the final concentration of malathion were measured. The amount of malathion adsorbed was determined from Equation (1). The reported values correspond to the average over three experiments on selected points of the curve.

2.4.4. SEM Measurements

The morphology and surface characteristics of SG-LGPS before and after malathion uptake were studied with Scanning Electron Microscopy (SEM) using a Tescan Mira 3 LMU high-resolution microscope operated at 15 kV with the detector operating at 13.3–13.4 mm distance. The textural properties of SG-LGPS are reported elsewhere [42]: S_BET = 466.8 m² g⁻¹, mean pore diameter D_p = 31.3 Å; average pore volume, V_p = 0.47 cm³ g⁻¹, and C_BET = 217.9.

2.4.5. TGA Measurements

For SG-LGPS and SG-LGPS-ML, thermogravimetric measurements (TGA) were carried out to study changes in the chemical structure of these samples during their thermal decomposition. The analyses were performed using dried and sieved samples. The TGA analysis was performed using a Discovery model analyzer from TA Instruments (Amarillo, TX, USA). About 10 mg of sample was placed in a platinum pan; a temperature scanning in the range of 30–850 °C was used; the heating rate was 10 °C min⁻¹ under a nitrogen atmosphere of 25 mL min⁻¹.

2.4.6. Elemental Analysis Measurements

Percentage compositions of carbon, nitrogen, sulfur, and hydrogen in SG-LGPS and SG-LGPS-ML were determined by elemental analysis using a LECO TruSpec CHNS instrument (St. Joseph, MI, USA). About 2 mg of a dried sample was heated to 1100 °C and analyzed for C, N, H, and S. The oxygen percentage was obtained from the difference. The reported values correspond to the average over three experiments.

3. Results and Discussion

3.1. Characterization

3.1.1. Effect of Solution pH on Malathion Uptake

One of the most important parameters in the removal of malathion is the pH of the initial solution, as it impacts its solubility and the uptake capacity of the adsorbent [2]. The effect of pH has been studied in the pH range 3 to 9, as malathion degrades under both strongly acidic and alkaline conditions [43]. Furthermore, since malathion is unstable with increasing temperatures, a fixed temperature of 25 ± 0.1 °C was used. The results are shown in Figure 3. Although the general trend is an increase in uptake with an increase in pH, the dependence is weakly expressed. The maximum malathion uptake was 17.22 mg g⁻¹ at pH = 8.

3.1.2. Malathion Adsorption Kinetic Results

Three malathion initial concentrations (100, 200, and 300 mg L⁻¹) were used to explore the reliance of the adsorption rate by SG-LGPS on the initial malathion concentration. This range of concentrations ensured the saturation of the adsorbent to come up with a surplus of glutathione groups on the adsorbent. The results are shown in Figure 4a. Obviously, the initial concentration of malathion is a defining feature of the adsorption kinetics on SG-LGPS. About 90 min at low concentrations (100 and 200 mg L⁻¹) and 180 min at 300 mg L⁻¹ are required to reach equilibrium. These differences can be correlated to the non-linearity of the isotherms. Furthermore, the malathion initial concentration is a decisive feature of the adsorption phenomenon and not the reactivity of the malathion molecule. On this subject, the extent of malathion uptake and the reactive character of the glutathione active sites on the adsorbent is driven by the reactiveness of either amine, thiol, or carboxylic groups to promote substitution reactions with malathion molecules. Thus, SG-LGPS demonstrates relatively fast kinetics, which is a desirable property for materials used in industrial applications such as wastewater treatment and water purification.

There are two main processes that control the rate of adsorption, namely, chemical reaction and diffusion (including film diffusion and intraparticle diffusion). The kinetic data were analyzed to ascertain which mechanism controls the adsorption rate. Three models of adsorption kinetics have been tested in this study.

Pseudo-first order model. Lagergren’s pseudo-first order model [44] considers the process of achieving dynamic equilibrium in the presence of a reversible reaction occurring between adsorption sites of the solid and molecules in solution. The accumulation of the adsorbate on the surface of the adsorbent is assumed to occur as time elapses [45]. The following linear expression represents this model:

$$log\left(q_e-q\right)=\log q_e-\frac{k_1 t}{2.303}$$

(2)

where k₁ is the pseudo-first-order-rate constant (min⁻¹), q_e and q (mg g⁻¹) are the adsorption capacity at equilibrium and at time t, respectively. k₁ and q_e are obtained from the slope and the intercept of the linear plot of log (q_e − q) vs. t, respectively.

Pseudo-second-order model. The process of achieving dynamic equilibrium may turn out to be a second-order process. This model considers the maximum adsorption capacity at equilibrium and the dependence of the uptake extent on the number of occupied active sites on the superficial side of the adsorbent. Ho’s pseudo-second-order [46] model is described by the following equation:

$$\frac{t}{q}=\frac{1}{k_2{q}_e{}^2}+\frac{1}{q_e}t$$

(3)

where k₂ is the second-order-rate constant (g mg⁻¹ min⁻¹). k₂ and q_e are obtained from the slope and the intercept of the linear plot of t/q vs. t, respectively.

Intraparticle diffusion model. The process of achieving dynamic equilibrium can be limited by the diffusion of malathion over and through SG-LGPS. This effect can be considered using the intraparticle diffusion model [47,48]:

$$\frac{q}{t^{1/2}}=A-B{t}^{1/2}$$

(4)

where A (mg g⁻¹ min^−1/2) and B (mg g⁻¹ min⁻¹) are fitting constants and are estimated from the linear plot of q/t^1/2 vs. t.

The results of fitting the three models described above to the experimental data are shown in Figure 4b–d.

Table 1. Results of fitting parameters to experimental data of three kinetic models onto SG-LGPS.

Model Parameters	Malathion Initial Concentration, mg L−1
	100	200	300
qe (mg g−1) experimental	12.8	46.3	74.4
Pseudo-first-order
qe (mg g−1)	8.34	23.27	67.70
k1 (min−1)	−0.0152	−0.0191	−0.0379
R2	0.883	0.914	0.933
Pseudo-second-order
qe (mg g−1)	12.72	46.72	75.75
k2 (g mg min−1)	0.0065	0.0026	0.0017
R2	0.990	0.995	0.997
Intraparticle diffusion
A (mg g−1 min−1/2)	2.366	11.432	16.220
B (mg g−1 min−1)	−0.1169	−0.6597	−0.8531
R2	0.826	0.727	0.833

reports the values of the fitting parameters for each of the models. The pseudo-first-order model (Figure 4b) gives an unsatisfactory approximation, as evidenced by the small value of the coefficient of determination R². Furthermore, this fitting leads to negative values of k₁. The coefficient of determination R² is also small for fitting using the intraparticle diffusion model (Figure 4d). The best fitting has been achieved in the case of the pseudo-second-order model (Figure 4c). The values of the adsorption capacity at equilibrium, q_e, obtained for this model are close to the experimental values for all three concentrations, Table 1. Thus, the kinetics of malathion adsorption on SG-LGPS obey a pseudo-second-order mechanism, as can be seen in Figure 4a.

3.1.3. Malathion Adsorption Isotherm Results

The adsorption capacity of SG-LGPS for malathion was determined for four pH values (5–8). Although pH 5 is not within the common pH range of drinking waters [49], this value was included with the purpose of observing the maximum adsorption capacity. Figure 5a shows experimental adsorption isotherms at 298 K. In the first approximation, at pH ≤ 6, the amount of adsorbed malathion depends approximately linearly on its concentration in solution. At pH ≥ 7, at low concentrations, a similar dependence is observed. On the contrary, at a concentration above 100 mg L⁻¹, the adsorption capacity increases abruptly, reaching a value of 130 mg g⁻¹. The adsorbent was regenerated with a 0.1 M NaOH solution, but it failed to give positive results in posterior cycles, possibly due to the dissolution of the silica matrix. It can be observed that successful malathion adsorption (q = 3.89 mg g⁻¹) begins at about pH 7 for an equilibrium malathion concentration of about 45.2 mg L⁻¹, and the maximum malathion capacity of 130.12 mg g⁻¹ is achieved at pH 8. Figure 5a shows four different shapes of isotherms. To explain the possible adsorption mechanism and the shapes of the isotherms involved in malathion uptake by the adsorbent, Giles’s classification of isotherms for liquid–solid systems is followed in this discussion [50]. According to this classification, the isotherms at pH 5, 7, and 8 are categorized as types S f, S c, and S a, respectively. In general, the S shapes of isotherms are generated when the adsorption of solute becomes easier as the liquid concentration increases. Furthermore, several conceptual situations are involved in the generation of these types of isotherms, namely: (a) the presence of monofunctional adsorbate species; by monofunctional is meant that the attraction forces for the adsorbate are pronouncedly localized over a short section on the adsorbent that is there are specific binding sites for malathion on the surface; (b) the presence of modest intermolecular interactions of malathion causing vertical arrangement of packed molecules in a systematic array on the adsorbed layer; and (c) there is a powerful competition for the adsorption active sites from the solvent (water molecules). It has been reported in [51] that even though malathion is not monofunctional, its molecular structure comprises two parts: one is hydrophobic (hydrocarbon terminals), and the other is partially hydrophilic (double bonds and oxygen atoms). This arrangement produces a monofunctional interaction between malathion and the silanol surface groups in kaolinites through H-bonding [51]. This situation also applies in the present study with glutathione functional groups of the adsorbent. In the case of the isotherm at pH 6, it corresponds to the L 4 type. In this case, it is observed that at the initial stages of the adsorption curvature, it is difficult for malathion to find empty glutathione sites available, as more of the adsorbent sites are occupied. This means that either the malathion molecules are not vertically adsorbed, or these sites are strongly solvated by water molecules (solvent).

For a more detailed analysis of the possible mechanisms of malathion adsorption, the isotherms have been analyzed using different isotherm models.

The Langmuir isotherm model assumes that a single layer of solute is present on a homogeneous solid surface and that the adsorption activation energy is the same for the entire surface. The Freundlich isotherm model involves a heterogeneous surface with randomly distributed heats of adsorption on the solid surface. Multilayer adsorption is permitted during the adsorption process. The Temkin isotherm model assumes that the heat of adsorption decreases linearly with increasing coverage and that there is a uniform distribution of adsorption energies up to some maximum value. These three models are presented as follows:

The Langmuir isotherm can be determined as:

$$q=\frac{K_L{q}_{max}C}{1+K_{L}C}$$

(5)

where q (mmol g⁻¹) and C (mg L⁻¹) stand for the amount of malathion adsorbed and its concentration in solution at equilibrium, respectively. q_max (mmol g⁻¹) and K_L (L mg⁻¹) stand for the maximum adsorption capacity and Langmuir’s constant. The latter can be obtained from the slope and the intercept of the plot of C/q vs. C, respectively.

The Freundlich isotherm can be determined as:

$$q=K_F{C}^{1/n}$$

(6)

where n stands for the intensity of adsorption, and K_F (mg^1−1/n L^1/n g⁻¹) is the equilibrium constant related to the bonding enthalpy between adsorbate and adsorbent. These parameters can be obtained from the slope and the intercept of the plot of ln q vs. ln C, respectively.

The Temkin isotherm can be determined as:

$$q=\frac{RT}{b}\ln \left(A C\right)$$

(7)

where b (kJ mol⁻¹) is related to the heat of adsorption, and A (L mg⁻¹) is the equilibrium binding constant corresponding to the maximum binding energy. b and A can be obtained from the slope and the intercept of the plot of q vs. ln C, respectively.

The values of the parameters of the considered models that best describe the experimental isotherms are given in

Table 2. Fitting parameters of the adsorption isotherms.

Initial pH	Langmuir				Freundlich			Temkin
	KL (L mg−1)	qmax exp (mg g−1)	qmax calc (mg g−1)	R2	n	KF (mg1−1/n L1/n g−1)	R2	b (J mol−1)/(mg g−1)	A (L mg−1)	R2
5.00	−615.41	94.66	0.082	0.991	0.681	0.012	0.963	62.21	0.014	0.778
6.00	126.05	71.02	0.777	0.988	1.600	2.123	0.957	130.45	0.107	0.991
7.00	−233.56	126.45	0.077	0.910	a	0.004	0.969	41.84	0.017	0.738
8.00	−204.75	130.12	0.047	0.990	0.386	9.6 × 10−5	0.942	31.35	0.014	0.665

. The values of the q_max parameter in the case of the Langmuir model deviate critically from the experimental values. This model should be rejected. The Temkin model gives an unsatisfactory approximation, as evidenced by the small value of the coefficient of determination R². In the case of the Freundlich model, all values of the coefficient of determination R² are above 0.9, which indicates a satisfactory agreement with the experimental data. For comparison with the experimental data, the fitting results of the Freundlich model are shown in Figure 5b.

3.1.4. Efficiency of Adsorption of Malathion by Various Adsorbents

The maximum adsorption capacity and the removal efficiency of malathion by different adsorbents are compared in

Table 3. The maximum adsorption capacity and the removal efficiency of malathion by different adsorbents.

Adsorbent	OPP Removed	qmax, mg g−1	Removal Efficiency, %	Time, min	Reference
Chitosan-alginate	Malathion	98.0	82.35	20	[52]
MWCNTs	Malathion	---	~100	30	[37]
De-Acidite FF-IP resin	Malathion	16.39	96	40	[17]
Amberlyst-15 resin	Malathion	---	96	30	[2]
Fe3O4@SiO2@GO-PEA	Chlorpyrifos, malathion, and parathion	32.6	---	15	[53]
Activated carbon from waste	Malathion	32.1	---	120	[54]
SG-LGPS material	Malathion	130.12	80	90	This study

. All adsorbents show a high removal efficiency. However, SG-LGPS shows the highest adsorption capacity (q_max). This result proves the advantage of using SG-LGPS in industrial applications.

3.1.5. SEM Results

The surface morphology of SG-LGPS and textural changes caused by contact with an aqueous solution of malathion were studied by SEM analysis, Figure 6. The surface of SG-LGPS is smooth and clean (Figure 6a,b). After the adsorption of malathion, its structure changed to wrinkled-rough, SG-LGPS-ML (Figure 6c).

3.1.6. Elemental Analysis Results

The results of elemental analysis on SG-LGPS and SG-LGPS-ML samples are reported in

Table 4. Elemental analysis on SG-LGPS and SG-LGPS-ML.

Sample	N%	C%	H%	S%	C/N	Si%
SG-LGPS	2.91	18.60	3.51	11.73	6.39	17.2 a,b
SG-LGPS-ML	2.34	19.10	3.57	13.23	8.16	---

^a Determined by EDX, ^b from reference [42].

. For SG-LGPS. The nitrogen content is 2.91%, which is equivalent to 2.07 mmol of glutathione per g of silica. The carbon and sulfur contents increase after the malathion adsorption, indicating the uptake of malathion. The C/N ratio is 6.39 in SG-LGPS and 8.16 in SG-LGPS-ML, which confirms the adsorption of malathion.

3.1.7. TGA Results

Chemical and structural changes in SG-LGPS and SG-LGPS-ML when subjected to thermal decomposition were studied by TGA analysis before and after malathion adsorption. Weight loss of water for each sample was obtained from the thermograms using the derivatives of the data. Figure 7a depicts the thermogram of SG-LGPS. Changes in the range of 35–100 °C correspond to the detachment of adsorbed water molecules and amount to ~5% of the mass loss. It has been repeatedly shown that physisorbed water leaves the silica surface without any changes in the surface structure [55,56]. This is not hindered even by the functionalization of the surface by weakly acidic groups [57]. To retain water, the presence of such functional groups as sulfonic and phosphonic acid moieties is necessary. [58]. Changes observed in the ranges of 100–198 °C (~6.5% weight loss) and ~210–524 °C (~28.5% weight loss) can be attributed to the thermal decomposition of L-glutathione in two stages. It has been reported [59] that first, the CO₂ and NH₃ units of glutathione are released, and then the rest of the molecule decomposes. The effect of malathion on the thermal stability of SG-LGPS-ML is shown in Figure 7b. This thermogram shows no significant change compared to SG-LGPS, indicating that the presence of malathion does not change the thermal stability of the surface functional groups and that SG-LGPS is thermally stable up to about 150 °C.

4. Conclusions

An L-glutathione-functionalized hybrid silica adsorbent was successfully applied in the efficient removal of organophosphorus pesticide malathion. The presence of amino acid glutathione makes this adsorbent ecofriendly due to the null toxicity of the functional groups and the silica matrix. A neutral pH between 6 and 8 is the optimum condition for the maximum removal of malathion. The maximum uptake was 130 mg g⁻¹, which is significantly higher than that obtained for other previously studied adsorbents and is only two times less than the maximum uptake of lead cations [60]. Although this adsorbent cannot be regenerated with an aqueous solution of NaOH, this disadvantage is expected for any silica adsorbent. The adsorption kinetic results indicated that equilibrium was reached in about 90 min of contact time. The kinetic data were best fitted to a pseudo-second-order model, as evidenced by the high values of the determination coefficient (R² = 0.990–0.997) indicating the chemical reaction between malathion and glutathione functional groups. The Freundlich model best fitted the adsorption equilibrium data of malathion. The thermal stability of the SG-LGPS adsorbent was investigated through the TGA technique, and the results indicated that the adsorbent is stable up to ~150 °C, at which point glutathione starts decomposing. Elemental analysis results proved the formation of the glutathione-TEOS copolymer. The results obtained in this study confirm the high potential value of L-glutathione-functionalized silica adsorbents in industrial applications for the removal of pesticides such as malathion, which is often found in drinking and groundwater.