Decontamination of fenitrothion from aqueous solutions using rGO/MoS₂/Fe₃O₄ magnetic nanosorbent: synthesis, characterization and removal application

Abstract

In the present work, rGO/MoS₂/Fe₃O₄ nanocomposite was synthesized and after confirmation of the structure by FTIR, XRD, and FESEM techniques, its performance as nanosorbent was investigated for the removal of fenitrothion pesticide from the aqueous media. The parameters affecting the removal process including agitation time, pH of the reaction medium, adsorbent content, initial analyte concentration as well as desorption parameters were investigated and optimized. Under optimum conditions (pH = 7, adsorbent amount: 30 mg, adsorption and desorption time: 5 min, eluent type and volume: 0.01 M ethanol-acetic acid and 4 mL), the synthesized adsorbent was able to remove fenitrothion pesticide up to 98% with an adsorption capacity of 33.4 mg/g. By investigation of the line equation and the correlation coefficient value, it was found that the adsorption process, in this study, follows the Langmuir model.

Introduction

One of the challenges and concerns of the modern world is environmental pollution, especially surface and groundwater resources [1, 2]. Increasing industrial activity and increasing population growth led to large amounts of industrial and agricultural wastewater and pollutants. Their improper disposal causes irreparable damage to human health and the environment [3]. Various chemicals are used in agriculture and the chemical industry, which introduce dangerous, non-destructive toxic compounds into the environment [4]. Pesticides, one of these types of contaminants, have been used extensively in agriculture to ensure the production of high-quality crops [5, 6]. Organophosphorus pesticides (OPs) are the most common type of pesticide, accounting for about 34% of global sales. OPs are widely used in agricultural activities for the control of pests [7, 8]. Many of these compounds are not easily digestible, and often, their complete removal is expensive, or incomplete removal is a hazard for health [9].

Fenitrothion (O, O-dimethyl O-(4-nitro-m-tolyl) phosphorothioate), is a potent organophosphorus insecticide that is important for preventing acetylcholinesterase activity and is used to control pests and insects in rice, cotton, cereals, fruits, and vegetables. Fenitrothion, although a moderately toxic pesticide, the use of it may lead to long-term adverse effects on the environment and can cause water pollution and transfer to aquatic organisms [10–12]. Various methods are used to eliminate, including photocatalysis, biochemical degradation, electrochemical decomposition, separation with various membranes, oxidation, and solid-phase extraction using an adsorbent. Adsorption is a very common method of removing pollutants because is simple, flexible, and insensitive to toxic pollutants [13, 14]. The toxic pollutants removal using adsorption on metal oxides is very interesting. The metal oxides have good performance for the removal of contaminants due to low toxicity, large surface area, porous structures, thermal stability, and easy recovery [15].

2D transition metal dichalcogenides (TMDs) have achieved significant interest as in the atomically thin nature of 2D-TMDs the catalytically active sites are exposed. Among intermediate metal dichalcogenides, molybdenum disulfide (MoS₂) is known as an efficient and active structure in different fields [16]. The performance of covalently cross-linked MoS₂ with other 2D-layered materials has been proposed as a new strategy to generate novel materials with enhanced properties. Graphene-based materials have attracted attention because of their various specific properties such as high surface area, chemical stability, and good affinity for the organophosphorus pesticides and can be considered good candidates for the wastewater treatment application [17].

This work reports a graphene-based nanocomposite containing synthesized oxide nanoparticles and its function as a nanosorbent. The proposed adsorbent combines the advantages of the high capability of modified rGO and magnetic separability for solid-phase extraction of fenitrothion. The main experimental parameters affecting the adsorption and desorption recovery of fenitrothion including adsorbent amount, pH, and time were investigated and optimized.

Experimental

Reagents and materials

Graphite (99.99%), sodium molybdate dihydrate (Na₂MoO₄, 99%),. FeCl₃.6H₂O (99%), FeCl₂.4H₂O (98%), were purchased from Merck (Darmstadt, Germany). polyethylene glycol (PEG, 95%), thiourea (99.99%), sodium nitrate (99.99%), potassium permanganate (99%), methanol (99.8%), ethanol (96%), sodium hydroxide, acetic acid, sulfuric acid, hydrochloric acid, dimethyl sulfoxide, and dichloromethane were purchased from Sigma-Aldrich.

Instrumentation

The phase structure of the prepared nanostructures was determined by a Rigaku D-max C III X-ray diffractometer equipped with Ni filtered Cu K_α. For investigation of the composition and morphology of nanostructures, a CARL ZEISS-AURIGA 60 microscope (Jena, Germany) was used. The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded with a Bruker IFS-66 spectrometer (Karlsruhe, Germany). The measurements of fenitrothion in aqueous solutions were carried with a two-beam UV-Vis Spectrophotometer made by Unico, USA.

Synthesis of GO

Graphene oxide (GO) was prepared by the Hummers method [18]. First, 0.3 g (0.25 mol) of graphite was added to a solution of 1.5 g of sodium nitrate (0.017 mol) and 70 mL of sulfuric acid and kept in the ice bath for 30 min. Then 0.9 g of potassium permanganate (0.056 mol) was added slowly to the mixture for 1 h at 20 °C. 138 mL of distilled water was gently added to it. Then 7 mL of hydrogen peroxide was added to the mixture for stopping the reaction. The mixture was subjected to ultrasonic waves with 300 w. The obtained precipitation was washed three times with dilute hydrochloric acid and then, with distilled water until the pH reached 6–7. The synthesized graphene oxide was transferred to the freeze-dryer for 24 h and dried.

Synthesis of rGO/MoS₂

0.54 g sodium molybdate and 0.51 g thiourea were added in 70 mL GO solution. Afterward, the suspension was transformed to a 100 mL teflon-lined steel autoclave and placed at 150 °C for 24 h. During this process, the mildly oxidized GO was transformed to rGO by thiourea reduction. Then, the solution was naturally cooled down to room temperature. Finally, the sediment was collected by centrifugation, washed several times with ethanol and distilled water, and dried in an oven at 70 °C overnight.

Synthesis of rGO/MoS₂/Fe₃O₄

0.2 g rGO/MoS₂ magnetic nanocomposite was dispersed in 80 mL distilled water under ultrasonic conditions for 20 min. Afterward, 0.11 g FeCl₂.4H₂O and 0. 14 g FeCl₃.6H₂O were added in this suspension, and pH was adjusted to 11, while the stirring at 80 °C for 3 h. The precipitate was collected using a magnet and washed several times with distilled water and ethanol until reaching a neutral pH; it was dried at 60 °C.

SPE procedure

The fenitrothion extraction procedure was as follows: 4 mL of fenitrothion solution containing (10 μg mL⁻¹) was treated with 30 mg of rGO/MoS₂/Fe₃O₄ nanosorbent at pH 7.0 and the mixture stirred using a shaker (200 rpm) for 5 min at 25 °C. After that, the nanosorbent was separated and the adsorbed analyte ions were desorbed with 4.0 mL ethanol- acetic acid (0.01 M) under shaking for 5 min. The nanocomposite was magnetically separated and the pesticide concentrations of the diluted solution were determined by UV-Vis.

Results and discussion

Characterization of the adsorbent

The chemical structure of rGO/MoS₂/Fe₃O₄ was further investigated using FT-IR spectroscopy. In the GO spectrum, adsorption bands of 1627 cm⁻¹, 1058 cm⁻¹, and 3402 cm⁻¹ are related to C=C, C-O, and OH functional groups, respectively (Fig. 1a). The FTIR of rGO/MoS₂ shows adsorption bands of 532 cm⁻¹ and 471 cm⁻¹ are related to vibrations S-H and strong connection S-S in MoS₂ structure (Fig. 1b). The spectrum of rGO/MoS₂/Fe₃O₄ in Fig. 1c shows a band at 570 cm⁻¹ assigned to Fe-O bond confirms the presence of Fe₃O₄ structure is not visible due to overlap with the MoS₂ peak. Furthermore, the bands at 3419 cm⁻¹ related to adsorbed water and OH functional group.

Fig. 1.

The FT-IR spectra of a GO, b rGO/MoS₂, c rGO/MoS₂/Fe₃O₄

The crystalline properties of the nanostructures were investigated by XRD measurements, and the XRD patterns are shown in Fig. 2. The diffraction peak at 2θ = 10.86° is observed for GO nanoparticles, which correspond to the (002) plane (Fig. 2a). In Fig. 2b, the diffraction peaks at 2θ = 15°, 33°, 39°, and 58° are observed for MoS₂, which correspond to the (002), (100), (103), and (110) planes. The diffraction peaks at 2θ = 30°, 37°, 43°, 53°, 57°, and 63° are observed for Fe₃O₄ nanoparticles, which correspond to the (220), (222), (400), (422), (511) and (440) planes. All recorded patterns confirm the formation of nanoparticles with crystal structure in the nanocomposite.

Fig. 2.

The XRD patterns of a GO and \b rGO/MoS₂/Fe₃O₄

FESEM images of GO (Fig. 3a) and rGO/MoS₂/Fe₃O₄ (Fig. 3b) were recorded to detect morphology and estimate particle size. The observed point in the background of graphene oxide represents nanoparticles synthesized with uniform morphology and mean particle size of 40 nm.

Fig. 3.

FESEM images of a GO and b rGO/MoS₂/Fe₃O₄

Optimization of effective parameters

The pH is one of the effective parameters for investigating the efficiency of the removal of fenitrothion from an aqueous solution. To evaluate the effect of pH on the measurement of fenitrothion absorption, solution pHs were adjusted in the range of 3–9 by HCl or NaOH. Finally, the pesticide removal efficiency was measured. According to Fig. 4a, pH = 7 was selected as the optimal pH.

Fig. 4.

Effect of experimental parameters on the extraction efficiency: a pH, b nanosorbent amount, c adsorption time, d type of eluent, e eluent volume, f desorption time

Another effective parameter in increasing adsorption efficiency is the amount of nanosorbent. To select it, the different amounts of rGO/MoS₂/Fe₃O₄ were investigated in the range of 5–70 mg. According to Fig. 4b, the adsorption efficiency increased from 5 to 30 mg and then remained constant. This is due to the increase in the number of active sites for pesticide removal.

The sorption time was investigated from 2 to 30 min. According to Fig. 4c, maximum removal was reached in the time of 5 min for the fenitrothion. This is likely due to the high level of contact, and afterward, the rapid transfer of the mass of the analytes from the aqueous solution to the nanosorbent.

It is necessary to select a suitable eluent that separates the analyte from the adsorbent. Also, the eluent should not destroy or inactive the adsorbent and be suitable for further measurement. To achieve a desirable eluent to fully desorption of the pesticide from the adsorbent surface, various eluent solvents including water, methanol, ethanol, dimethyl sulfoxide, acetic acid, ethanol- sodium hydroxide (0.01 M), ethanol- acetic acid (0.01 M), methanol- acetic acid (0.01 M), methanol- sodium hydroxide (0.01 M) and sodium hydroxide, were examined. According to the results in Fig. 4d, ethanol- acetic acid (0.01 M) was selected as the elution solvent.

To optimize the eluent volume, different amounts of 2 to 6 mL of eluent were examined. However, the use of eluent in higher amounts does not cause any problems in the washing and measuring process. However, to achieve the maximum preconcentration factor, the minimum volume of eluent must be used. Therefore, the optimal volume of 4 mL was selected (Fig. 4e).

To optimize the desorption time, the time of 2 to 60 min was examed, and then the recovery was investigated (Fig. 4f). According to the obtained result after 5 min, the recovery efficiency was over 98%. Therefore, 5 min was chosen as the optimal time for the desorption.

Reusability

From a practical point of view, recovery and reuse of adsorbent are important factors. The efficiency of fenitrothion pesticide removal by adsorbent after 10 uses is about 95%. Therefore, the synthesized adsorbent can be used at least 10 times without significantly reusing its efficiency. (Fig. 5).

Fig. 5.

The reusability cycles of prepared rGO /MoS₂/Fe₃O₄

Adsorption isotherms

Langmuir and Freundlich’s isotherms are widely used to describe the absorption process at the solid-liquid joint level. Fenitrothion adsorption isotherms and parameters were shown in Fig. 6 and Table 1. With the investigation of the line equation and the correlation coefficient, it was found that this process follows the Langmuir model.

Fig. 6.

The linear fitting curves of a Langmuir and b Freundlich model

Table 1.

Parameters and correlation coefficient of isotherm equilibrium

Langmuier isotherm			Freundlich isotherm
K1 (L mg−1)	qm (mg g−1)	R2	Kf (mg g−1 (L mg−1)1/n)	n	R2
−200.401	0.757	0.9999	0.709	−11.668	0.9891

Adsorption kinetics

The pseudo-first and pseudo-second-order models are the two most common kinetic models, which were used to study the kinetic behavior of pesticide attraction on the adsorbent. According to the obtained results in Fig. 7 and Table 2, the best time for the fenitrothion pesticide sorption is 5 min. The correlation coefficient (R²) of the pseudo-first and pseudo-second is 0.999 and 0.6399, respectively which suggests that the experimental data can fit very well using the pseudo-second model. This study follows a pseudo-first-order model that shows that the rate of adsorption depends on chemical adsorption. In this case, the molecules were connected and absorbed by the active sites of the adsorbent surface.

Fig. 7.

a Pseudo-first and b Pseudo-second kinetics models of fenitrothion pesticides

Table 2.

Parameters and correlation coefficient of adsorption kinetics

Pseudo-second-order				Pseudo-first-order
qe,cal (mg g−1)	qe,exp (mg g−1)	K2 (g mg−1 min−1)	R2	qe,cal (mg g−1)	K1 (min−1)	R2
0.788	0.780	4.619	0.9999	1.467	−0.2692	0.6399

Comparison with other adsorbents

The proposed method was compared with other methods used in articles to remove fenitrothion pesticides. As can be seen, the present work was higher removal efficiency than most methods, and it is easier to separate from the reaction medium due to the absorbent magnetic property. The distinctive feature of the proposed adsorbent was reported in Table 3.

Table 3.

Comparison of the developed method with other methods

Type of Adsorbent	Time (min)	Capacity (mg/g)	RSD (%)	Removal (%)	Ref.
β-Cyclodextrin/ Fe3O4@SiO2	5	30	4 ± 0.03	>99%	[19]
Magnetite octadecylsilane NPs	10	–	5.4	92%	[20]
CoFe2O4 nanoparticles	10	–	3.5	87.5%	[21]
MMIP-DSPE-HPLC–UV	11	31.5	1.6–3.1	>99%	[22]
Fe3O4/CNTs	30	–	4.5	92%	[23]
rGO/MoS2/Fe3O4	5	33.4	3.2	>98%	This work

Conclusions

This work reports the preparation of a magnetic graphene-based nanocomposite containing oxide nanoparticles of molybdenum disulfide. The synthesized nanostructure was characterized by FTIR, XRD, and FESEM-EDX. This nanostructure was investigated as an adsorbent in a magnetic solid-phase extraction (MSPE) process for the removal of organophosphorus pesticide (OPPs) of fenitrothion from aqueous media. The studies were performed with batch mode adsorption to evaluate the adsorption kinetics and adsorption isotherms. The proposed adsorbent merges the advantages of good adsorption capacity and easy separation from sample solutions. Under the optimum conditions, fast magnetic separation of nanoparticles from sample solution (< 1 min), neutral pH, and low extraction time (about 5 min) are the merits of the prepared adsorbent. These results indicated that the proposed nanostructures have the great adsorptive ability and can be applied in a fast, simple, and efficient MSPE technique for OPPs extraction in different media.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.