Adsorption and Photocatalytic Degradation of Pesticides into Nanocomposites: A Review

The extensive use of pesticides in agriculture has significantly impacted the environment and human health, as these pollutants are inadequately disposed of into water bodies. In addition, pesticides can cause adverse effects on humans and aquatic animals due to their incomplete removal from the aqueous medium by conventional wastewater treatments. Therefore, processes such as heterogeneous photocatalysis and adsorption by nanocomposites have received special attention in the scientific community due to their unique properties and ability to degrade and remove several organic pollutants, including pesticides. This report reviews the use of nanocomposites in pesticide adsorption and photocatalytic degradation from aqueous solutions. A bibliographic search was performed using the ScienceDirect, American Chemical Society (ACS), and Royal Society of Chemistry (RSC) indexes, using Boolean logic and the following descriptors: “pesticide degradation” AND “photocatalysis” AND “nanocomposites”; “nanocomposites” AND “pesticides” AND “adsorption”. The search was limited to research article documents in the last ten years (from January 2012 to June 2022). The results made it possible to verify that the most dangerous pesticides are not the most commonly degraded/removed from wastewater. At the same time, the potential of the supported nanocatalysts and nanoadsorbents in the decontamination of wastewater-containing pesticides is confirmed once they present reduced bandgap energy, which occurs over a wide range of wavelengths. Moreover, due to the great affinity of the supported nanocatalysts with pesticides, better charge separation, high removal, and degradation values are reported for these organic compounds. Thus, the class of the nanocomposites investigated in this work, magnetic or not, can be characterized as suitable nanomaterials with potential and unique properties useful in heterogeneous photocatalysts and the adsorption of pesticides.


Introduction
The intense populational growth and industrial expansion in the most diverse segments of society have led to a substantial increase in the demand for drinking water supply and large-scale food production [1]. Thus, to increase productivity at an economically profitable level, the employment of agrochemicals has been widely used to combat pests Table 1. Classification of pesticides in terms of hazardousness and chemical structure.

Pesticide Class Toxic Effects Classification * References
Trifluoromethyl urea

Degeneration of renal tubule epithelial cells and hemorrhage in sheep
Unlikely to have acute problems with normal use [43] Triazine Endocrine disruptor, carcinogen, and degeneration of renal tubule epithelial cells of Cyprinus carpio species Unlikely to present a hazard in normal use [44,45] Triazine Hematological abnormalities, degenerative and hormonal changes, cardiotoxicity, and acute microalgae toxicity Slightly hazardous [46,47] Arylurea Acute toxicity in zebrafish embryos and abnormalities in embryonic development, reduced cell viability, and production of reactive species in the HepG2 lineage Slightly hazardous [48,49] Anilinopyrimidine Amphibian endocrine disruptor, cardiotoxicity, induction of reactive species, and apoptotic gene induction in zebrafish Slightly hazardous [50,51] Organophosphate Cytotoxic, carcinogenic, and genotoxic to human lymphocytes Slightly hazardous [52,53] Triazine Change in enzymatic activity and decrease in chlorophyll production in wheat crops, delay in larvae growth and carp development Slightly hazardous [54,55] Trifluoromethyl urea Degeneration of renal tubule epithelial cells and hemorrhage in sheep Unlikely to have acute problems with normal use [43] Molecules 2022, 27, x FOR PEER REVIEW 4 of 29 and endocrine-disrupting potential. This scenario shows that the removal of these compounds from waters and wastewater is fundamental.

Pesticide Class Toxic Effects Classification * References
Trifluoromethyl urea

Degeneration of renal tubule epithelial cells and hemorrhage in sheep
Unlikely to have acute problems with normal use [43] Triazine Endocrine disruptor, carcinogen, and degeneration of renal tubule epithelial cells of Cyprinus carpio species Unlikely to present a hazard in normal use [44,45] Triazine Hematological abnormalities, degenerative and hormonal changes, cardiotoxicity, and acute microalgae toxicity Slightly hazardous [46,47] Arylurea Acute toxicity in zebrafish embryos and abnormalities in embryonic development, reduced cell viability, and production of reactive species in the HepG2 lineage Slightly hazardous [48,49] Anilinopyrimidine Amphibian endocrine disruptor, cardiotoxicity, induction of reactive species, and apoptotic gene induction in zebrafish Slightly hazardous [50,51] Organophosphate Cytotoxic, carcinogenic, and genotoxic to human lymphocytes Slightly hazardous [52,53] Triazine Change in enzymatic activity and decrease in chlorophyll production in wheat crops, delay in larvae growth and carp development Slightly hazardous [54,55] Triazine Endocrine disruptor, carcinogen, and degeneration of renal tubule epithelial cells of Cyprinus carpio species Unlikely to present a hazard in normal use [44,45] Molecules 2022, 27, x FOR PEER REVIEW 4 of 29 and endocrine-disrupting potential. This scenario shows that the removal of these compounds from waters and wastewater is fundamental.

Pesticide Class Toxic Effects Classification * References
Trifluoromethyl urea

Degeneration of renal tubule epithelial cells and hemorrhage in sheep
Unlikely to have acute problems with normal use [43] Triazine Endocrine disruptor, carcinogen, and degeneration of renal tubule epithelial cells of Cyprinus carpio species Unlikely to present a hazard in normal use [44,45] Triazine Hematological abnormalities, degenerative and hormonal changes, cardiotoxicity, and acute microalgae toxicity Slightly hazardous [46,47] Arylurea Acute toxicity in zebrafish embryos and abnormalities in embryonic development, reduced cell viability, and production of reactive species in the HepG2 lineage Slightly hazardous [48,49] Anilinopyrimidine Amphibian endocrine disruptor, cardiotoxicity, induction of reactive species, and apoptotic gene induction in zebrafish Slightly hazardous [50,51] Organophosphate Cytotoxic, carcinogenic, and genotoxic to human lymphocytes Slightly hazardous [52,53] Triazine Change in enzymatic activity and decrease in chlorophyll production in wheat crops, delay in larvae growth and carp development Slightly hazardous [54,55] Triazine Hematological abnormalities, degenerative and hormonal changes, cardiotoxicity, and acute microalgae toxicity Slightly hazardous [46,47] Molecules 2022, 27, x FOR PEER REVIEW 4 of 29 and endocrine-disrupting potential. This scenario shows that the removal of these compounds from waters and wastewater is fundamental.

Pesticide Class Toxic Effects Classification * References
Trifluoromethyl urea

Degeneration of renal tubule epithelial cells and hemorrhage in sheep
Unlikely to have acute problems with normal use [43] Triazine Endocrine disruptor, carcinogen, and degeneration of renal tubule epithelial cells of Cyprinus carpio species Unlikely to present a hazard in normal use [44,45] Triazine Hematological abnormalities, degenerative and hormonal changes, cardiotoxicity, and acute microalgae toxicity Slightly hazardous [46,47] Arylurea Acute toxicity in zebrafish embryos and abnormalities in embryonic development, reduced cell viability, and production of reactive species in the HepG2 lineage Slightly hazardous [48,49] Anilinopyrimidine Amphibian endocrine disruptor, cardiotoxicity, induction of reactive species, and apoptotic gene induction in zebrafish Slightly hazardous [50,51] Organophosphate Cytotoxic, carcinogenic, and genotoxic to human lymphocytes Slightly hazardous [52,53] Triazine Change in enzymatic activity and decrease in chlorophyll production in wheat crops, delay in larvae growth and carp development Slightly hazardous [54,55] Arylurea Acute toxicity in zebrafish embryos and abnormalities in embryonic development, reduced cell viability, and production of reactive species in the HepG2 lineage Slightly hazardous [48,49] and endocrine-disrupting potential. This scenario shows that the removal of these compounds from waters and wastewater is fundamental.

Pesticide Class Toxic Effects Classification * References
Trifluoromethyl urea

Degeneration of renal tubule epithelial cells and hemorrhage in sheep
Unlikely to have acute problems with normal use [43] Triazine Endocrine disruptor, carcinogen, and degeneration of renal tubule epithelial cells of Cyprinus carpio species Unlikely to present a hazard in normal use [44,45] Triazine Hematological abnormalities, degenerative and hormonal changes, cardiotoxicity, and acute microalgae toxicity Slightly hazardous [46,47] Arylurea Acute toxicity in zebrafish embryos and abnormalities in embryonic development, reduced cell viability, and production of reactive species in the HepG2 lineage Slightly hazardous [48,49] Anilinopyrimidine Amphibian endocrine disruptor, cardiotoxicity, induction of reactive species, and apoptotic gene induction in zebrafish Slightly hazardous [50,51] Organophosphate Cytotoxic, carcinogenic, and genotoxic to human lymphocytes Slightly hazardous [52,53] Triazine Change in enzymatic activity and decrease in chlorophyll production in wheat crops, delay in larvae growth and carp development Slightly hazardous [54,55] Anilinopyrimidine Amphibian endocrine disruptor, cardiotoxicity, induction of reactive species, and apoptotic gene induction in zebrafish Slightly hazardous [50,51] pounds from waters and wastewater is fundamental.  [52,53] Triazine Change in enzymatic activity and decrease in chlorophyll production in wheat crops, delay in larvae growth and carp development Slightly hazardous [54,55] Organophosphate Cytotoxic, carcinogenic, and genotoxic to human lymphocytes Slightly hazardous [52,53] and endocrine-disrupting potential. This scenario shows that the removal of these compounds from waters and wastewater is fundamental.

Nanocomposites
Nanocomposites are solid materials composed of two phases, the matrix and the reinforcement phase, with one of the constituents having dimensions in nanometers [22,107]. The matrix phase can be ceramic, polymeric, or metallic in origin and the reinforcement phase can be any class of nanomaterial.
From this perspective, metallic nanoparticles, graphene oxide, chitosan, and reduced graphene oxide can be used as reinforcement, which are dispersed on the matrix to generate materials with high mechanical resistance, good optical properties, and high surface area [28,108,109]. Thus, nanocomposites can be easily applied to water and wastewater treatment, catalysis, and structural applications. Furthermore, the synthesis of nanocomposites can reduce surface energy and the tendency for magnetic nanoparticles to agglomerate, increasing the physicochemical stability [110,111].
Due to cost-effectiveness and ease of application, polymeric and ceramic nanocomposites are commonly used in various applications. Polymeric-based matrix nanocomposites can be prepared by either in situ intercalation polymerization, melt intercalation of the pre-polymer solution, or sol-gel synthesis [112], while ceramic-based matrix nanocomposites can be synthesized by either sol-gel synthesis, powder process or polymer precursor process, for example [113].

Heterogeneous Supported Nanocatalysts
Supported nanocatalysts consist of an active nanomaterial dispersed on a less active material called a catalytic support. They are widely used in heterogeneous photocatalysis due to their high surface area, considerable photocatalytic activity, and chemical/thermal stability [114].
The application of supported nanocatalysts (nanocomposites) helps overcome some drawbacks commonly encountered when applying isolated nanocatalysts, including nanoparticle agglomeration due to their high surface energy and poor dispersion in the aqueous solution [115]. Additionally, these catalytic supports increase the contact between organic pollutants and the catalytic surface [116].
Among the supported nanocatalysts used for organic pollutant degradation, TiO 2 , CuO, Fe 2 O 3 , and ZnO, supported on zeolites or silica, can be highlighted due to their significant photocatalytic activity, high surface area, and thermo-chemical stability [117]. Additionally, supported catalysts show reduced bandgap energy due to better charge separation by the presence of the support. Thus, the possibility of using this supported nanocatalyst even in the visible light range of spectrum compliments their potential applicability in light-driven processes, in particular, heterogenous photocatalysis [118]. Moreover, these nanocatalysts can be easily prepared by alternative synthetic methods, such as those based on leaves or plant extracts, microorganism strains, and industrial waste [119]. Plant-based biosynthesis of nanocatalysts is based on the reduction and stabilization of the metallic precursor. In contrast, biogenic synthesis using microorganism strains is based on a biocatalytic reduction reaction that transforms the precursor into nanoparticle suspensions, followed by nucleation and stabilization [120]. Figure 1 shows a schematic representation of the heterogeneous photocatalysis process for application in the degradation of organic pollutants.

Photocatalytic Degradation of Pesticides
All scientific works discussed were found in indexed platforms, and research articles were selected as the basis for investigation. Table 2 shows the main results observed in this work. les 2022, 27, x FOR PEER REVIEW 9 of 29

Photocatalytic Degradation of Pesticides
All scientific works discussed were found in indexed platforms, and research articles were selected as the basis for investigation. Table 2 shows the main results observed in this work.     The nanocomposite showed higher photocatalytic activity than the isolated compounds. [135] The ternary semiconductor is used for organophosphate degradation.
The nanocatalyst containing 3% silver resulted in greater photocatalytic activity. [137] From Table 2, it was possible to observe a tendency for specific pesticide degradation, such as chlorpyrifos, atrazine, and imidacloprid. This is due to the wide use of these pesticides in agriculture, generating residues that can be released into wastewater [138][139][140].
It is worth mentioning that the utilization of magnetic nanocatalysts has been increasing once they have been characterized by improved photocatalytic activity compared to non-magnetic ones. In addition, magnetic nanocatalysts can solve operational drawbacks faced in heterogeneous photocatalysis by applying isolated nanocatalysts such as nanoparticle agglomeration and difficult recovery of the solid material after treatment [141]. Thus, in the following sections, the mechanisms involved in the pesticide degradation on support nanocatalysts will be described, as well as the enhancement of these nanocatalysts by adding a dopant material, which can be magnetic or non-magnetic. Table 2 shows that pesticide degradation percentages are reported in the range of 80 to 90% after 30 to 180 min, with initial pesticide and nanocatalyst concentrations ranging from 15 to 50 mol L −1 and 0.1 to 0.5 g L −1 , respectively. In addition, the selected scientific works report the good dispersibility of the nanocatalysts on the catalytic support, resulting in a nanocomposite with high chemical stability and preserved photocatalytic activity after 4 to 6 cycles of heterogeneous photocatalysis.

Effect of Initial Pesticide Concentration
Regarding the initial pesticide concentration, it is important to notice that an increase in this parameter leads to a decrease in the degradation percentage; at a high pollutant concentration, the number of pesticides molecules and active sites of the nanocomposite are altered, which results in an insufficient number of sites available to receive pesticide molecules and degrade them [142]. Therefore, an ideal initial concentration that favors photocatalytic pesticide degradation can be assumed, yielding higher oxidation percentages. In most scientific works, the concentration of 15 mol L −1 seems to be favorable for pesticide degradation (considering 0.1-0.3 g L −1 ) due the abundance of active sites available for pesticide molecules on the catalytic surfaces at this concentration [124,129]. In contrast, at higher concentrations (about 20 mg L −1 or more), pesticide molecules hindered light absorption by the nanocatalyst, which reduces hydroxyl radical generation and consequently degradation of the target pollutant [143].

Effect of Nanocatalyst Concentration and Dopant Incorporation
Based on the investigated scientific papers presented in Table 2, it is noticeable that an increase in the supported nanocatalyst concentration (metallic nanoparticles dispersed on catalytic support) favors the photocatalytic degradation of pesticides. This is due to the imbalance between the number of molecules of the pollutant in an aqueous solution and the number of active sites on the nanocatalyst [109]. Therefore, the constant rate values (k 1 , considering the Langmuir-Hinshelwood equation for the pseudo-first-order kinetic model) increase considerably, which results in greater pesticide degradation [132]. However, at extremely high nanocatalyst concentrations, pesticide degradation is reduced due to the tendency for nanocatalyst particles to agglomerate and increase the opacity of the aqueous solution, which reduces the photocatalytic activity of the supported nanocatalyst [144].
It is worth pointing out that there is a limiting value responsible for considerable pesticide degradation percentages in aqueous solution regarding dopant material. The reason for this is the same as for the increase in nanocatalyst concentration.
At the same time, it can be noticed that the incorporation of two materials (nanocomposite) can yield greater nanocatalyst surface area and greater light absorption in the electromagnetic spectrum, allowing the application of this nanocatalyst under UV and visible light radiation for heterogeneous photocatalysis.

Effect of pH
The solution pH has a considerable effect on pesticide degradation in heterogeneous photocatalysis. According to the scientific articles investigated (Table 2), it was possible to verify that the solution pH affects the surface charge of the nanocatalyst. The net charge of the nanocatalyst surface is measured by the point of zero charge (pH PZC ), at which the nanocatalyst has a net charge of zero [145]. At a pH lower than pH PZC , the surface of the nanocatalyst is protonated and, therefore, positive. In contrast, at a pH higher than pH PZC , the surface of the nanocatalyst is deprotonated and, therefore, negative.
The pH interferes with pesticide degradation as it changes the charges associated with the surface of the nanocomposites used in heterogeneous photocatalysis. Thus, the surface charge is strongly related to the adsorption of pesticide molecules before their photocatalytic oxidation [29]. When the pesticide molecule has anionic character (imidacloprid), the adsorption on the surface of the nanocatalyst is favored at acidic pH. On the other hand, when the pesticide molecule has more acidic character, i.e., chlorpyrifos, the adsorption onto the catalyst surface is favored at alkaline pH, in which pH PZC is greater than the solution pH.
Additionally, the solution pH can affect the reaction between hydroxyl ions and positive holes of the supported nanocatalyst and, therefore, determine a greater or lesser extent of pesticide degradation [128]. The above-mentioned reaction seems to be favored at higher solution pH (alkaline pH), resulting in more hydroxyl radicals, which are responsible for pesticide photocatalytic degradation.

Effect of Temperature
In some scientific research involving heterogeneous photocatalysis and pesticide degradation (Table 2), the effect of temperature on pesticide photocatalytic degradation was investigated. The effects of temperature on heterogeneous photocatalysis using supported nanocatalysts, magnetic or non-magnetic, are associated with the energy required for the adsorption and desorption of pesticide molecules. Some studies found that the adsorption of imidacloprid was facilitated by increasing the temperature from 20 to 80 • C. Due to the endothermic nature of the pesticide adsorption-degradation process, which is the majority of cases studied, pesticide absorption on the catalytic surface and adsorption rate increase with temperature [131]. In addition, the degradation rate seems to be increased at temperatures above the atmospheric temperature (i.e., 25 ± 5 • C).

Effect of Nanocatalyst Reuse
In most scientific research involving heterogeneous photocatalysis and pesticide degradation (Table 2), the effect of nanocomposite reuse is evaluated. According to the results presented in the selected studies, the combination of two or more nanomaterials in the nanocatalyst composition yields greater photocatalytic activity, even after five cycles of reuse. However, it is worth pointing out that when one of the nanocomposite phases has magnetic properties, its efficiency in pesticide degradation improves considerably after various photocatalytic cycles. Therefore, magnetic nanomaterials have been highlighted due to their versatility, ease of operation (magnetic separation), and reuse capacity. In this view, about 72 to 80% of pesticide degradation is reported even after six to seven cycles of heterogeneous photocatalysis [128,129].

Adsorption
Adsorption is a physicochemical process and surface phenomenon wherein a fluid (liquid or gas) interacts with a solid surface (adsorbent), resulting in the mass transfer of a solute from a fluid phase to the solid surface [146]. In this system, the degree of interaction of ions and molecules depends on the concentration, pH, temperature, and available specific surface are [147]. The attractive forces between the adsorbent and the adsorbate can be divided into physical and chemical adsorption [148,149].
Chemical adsorption (chemisorption) involves the transfer of electrons and chemical bond formation. On the other hand, physical adsorption results from weak intermolecular interactions called van der Waals forces, electrostatic interactions, H-bonds, and π-π bonds [150].
Regarding the nature of adsorption, some parameters must be observed, such as adsorbent selectivity, homogeneity, and heterogeneity of the solid nanomaterial and the adsorption rate, which can be fast or slow. Furthermore, depending on the process, the adsorbate can accumulate at the adsorbent interface in a monolayer or multilayer [151]. For a further explanation, Figure 2 illustrates the adsorption mechanisms in mono and multilayer models. adsorbent selectivity, homogeneity, and heterogeneity of the solid nanomaterial and the adsorption rate, which can be fast or slow. Furthermore, depending on the process, the adsorbate can accumulate at the adsorbent interface in a monolayer or multilayer [151]. For a further explanation, Figure 2 illustrates the adsorption mechanisms in mono and multilayer models. Adsorption is a relatively simple technology with low cost and energy demand and can effectively remove a range of chemical compounds from aqueous medium. However, the effectiveness of the process depends on the intrinsic properties of adsorbents, such as particle diameter, pore volume, and chemical/thermal stability [152]. In this context, diverse strategies have been explored, such as the use of thermal treatments, modification, and surface doping that aim not only to increase the adsorptive capacity but also to re- Adsorption is a relatively simple technology with low cost and energy demand and can effectively remove a range of chemical compounds from aqueous medium. However, the effectiveness of the process depends on the intrinsic properties of adsorbents, such as particle diameter, pore volume, and chemical/thermal stability [152]. In this context, diverse strategies have been explored, such as the use of thermal treatments, modification, and surface doping that aim not only to increase the adsorptive capacity but also to reduce costs associated with adsorbent synthesis [153,154]. For instance, 70% of the cost of an adsorption operation is relative to the adsorbent [155].

Adsorption of Pesticides into Nanocomposites
The research was performed (Table 3) to verify the efficiency of nanocomposites used as nanoadsorbents.
Based on the results presented in Table 3, it was possible to verify that the pesticide adsorption studies using nanocomposites were efficient for the removal of organic compounds from aqueous solution, especially in the class of organophosphates neonicotinoids and triazines.
Moreover, the studies were heterogeneous enough and contemplated different compounds belonging to the same chemical class. However, in most articles, the nanoadsorbent showed selectivity and higher adsorption capacity when compared to isolated materials, i.e., applied individually. Hybrid materials were usually synthesized to combine individual properties and make them multifunctional [112].
In this study, it was found that nanocomposites have good performance in the adsorption process. In addition, the efficiency and stability of the adsorbent were maintained after several adsorption/desorption cycles. Furthermore, because some materials exhibit magnetic behavior, the adsorbent can be easily separated from the solution by the application of an external magnetic field. Figure 3 shows a simple approach to magnetic nanocomposites used for pesticide adsorption, magnetic retention, removal, and reuse. Table 3. Pesticide adsorption by magnetic and non-magnetic nanocomposites.
The nanocomposite has an excellent adsorptive capacity (466.7 mg g −1 ). The adsorption occurs by chemical mechanisms, in which the π-π interactions and hydrogen interactions are contemplated. [156] Magnetic nanocomposite with a metal-organic structure for the removal of organophosphate pesticides Fenitrothion Initial concentration of adsorbate (5-−50 mg L −1 ), solution pH (3-10), adsorbent concentration (5-40 mg L −1 ). An ideal condition for the pesticide removal was pH 7.0, using 30 mg L −1 of adsorbent, and 10 mg L −1 of the adsorbate.
The nanocomposite exhibits greater pesticide adsorption capacity when compared to non-hybrid compounds. The q max ranged between 37.18-76.34 mg g −1 , and the adsorbent showed a high affinity for the pesticide diazinon. [158] Magnetic reduced graphene oxide nanocomposite for triazine removal Ametrine, promethrin, simazine and atrazine Study the influence of pH, temperature, adsorbent dosage, and contact time.
The magnetic nanocomposite is effective in removing pesticides. Maximum capacity was reached at pH 5.0. The ions present in the solution, increase in temperature, and initial adsorbate concentration caused a significant increase in the adsorptive capacity. Due to its magnetic behavior, the absorbent can be easily recovered and proved effective after 7 cycles of the process. [159] Hybrid nanocomposite intercalated with bentonite as adsorbent for the pesticide methyl-parathion Methyl-parathion pH effect (2-9), initial concentration of adsorbate (200-1400 mg L −1 ), adsorbent mass (0.4-1.6 g L 1 ), and thermodynamic study (25-50 • The adsorption capacity is pH-dependent and increases proportionally with the pH of the system, contact time with the adsorbent, initial adsorbate concentration, and temperature. The pollutant adsorption is an endothermic process, spontaneous, with experimental data represented by the Freundlich isotherm and pseudo-second-order model. The nanoadsorbent proved to be efficient in removing about 97% of the pesticide after five adsorption cycles. [160] The initial concentration of insecticides (0.5-100 mg L −1 ) and adsorbent dosage (5.0 g L −1 ).
The adsorbent showed a greater affinity for the insecticide imidacloprid, but the q max values are similar. The mechanisms involved in adsorption include hydrophobic interactions, hydrogen bonds, and the stacking of π-π bonds between the adsorbent/adsorbate complex. [161] Application of chitosan-graphene oxide nanocomposite as rotenone adsorbent
The adsorption capacity decreased at alkaline pH. However, a considerable increase in the q max was observed with nanocomposites containing a greater amount of GO, corroborating the hypothesis with more adsorption sites available. The adsorbent demonstrated a greater affinity for anionic compounds. Removal percentages ranged from 54 to 75%. [163] Montmorillonite/structured carbon nanocomposite for pesticide removal

Chlorpyrifos
The initial concentration of adsorbate and adsorbent dosage (8.0 g L −1 ), contact time (24 h), no pH adjustment Pristine montmorillonite showed greater pesticide adsorption capacity. The possible mechanism involved in the adsorption of the organic compound is the physical interaction with the surface of the adsorbent. [164] Removal of permethrin from aqueous solutions using chitosan/zinc oxide nanocomposite Permethrin Initial concentration of adsorbate (0.05-2.5 mg L −1 ), adsorbent amount (0.01-1.5 g L −1 ), contact time (45 min), solution pH (3)(4)(5)(6)(7)(8)(9)(10)(11), and temperature at 25 • C The CS-ZnO nanocomposite was able to remove about 96% of the pesticide. Adsorbent concentration, initial concentration of adsorbate, contact time, and solution pH influence the removal percentage. The removal percentage increases proportionally with the amount of adsorbent and adsorbate. However, it decreases with increasing pH, mainly due to electrostatic repulsion. Adsorption of chlorpyrifos occurs in monolayers and with uniform energy distribution. [166] Synthesis of SiO 2 @Fe 3 O 4 @GOphenylethylamine nanocomposite for removal of organophosphate pesticides Chlorpyrifos, malathion, and parathion pH effect (3)(4)(5)(6)(7)(8)(9)(10)(11), adsorbent dosage (2-40 mg L −1 ), initial concentration of adsorbate (3-50 mg L −1 ) The nanoadsorbent showed good performance after 10 adsorption cycles. [145] tained after several adsorption/desorption cycles. Furthermore, because some materials exhibit magnetic behavior, the adsorbent can be easily separated from the solution by the application of an external magnetic field. Figure 3 shows a simple approach to magnetic nanocomposites used for pesticide adsorption, magnetic retention, removal, and reuse.

Effect of Initial Adsorbate Concentration and Adsorbent Dosage
Studying the effect of experimental parameters such as initial adsorbate concentration, adsorbent dosage, and adsorption contact time is essential to determine the most favorable conditions for contaminant removal and to investigate the phenomenon of adsorption.
Muda et al. [162] demonstrated that the maximum adsorption capacity was directly proportional to the increase in the initial concentration of adsorbate and adsorbent mass, mainly due to the high probability of adsorbate molecules binding to the adsorption sites [160]. Likewise, the gradual increase in the initial concentration of adsorbate and the amount of adsorbent can reduce the contaminant removal percentage due to the saturation of higher energy sites and the interaction between the adsorbent molecules [165].
Another important factor for the adsorption efficiency is the contact time between adsorbent and adsorbate. In most of the studies, adsorption showed distinct behavior over time. In the initial stages, a fast adsorption rate is observed (mass transfer of liquid phase to the solid phase) and a tendency to plateau in the subsequent steps because of saturation of the adsorption sites (equilibrium between vacancy sites and adsorbed molecules) [145]. At the same time, the nanocomposite content (ratio mass: mass) of the of constituent materials can, in some cases, affect the efficiency of the process, either positively or negatively. For example, Gupta et al. [166] reported that the combination of cellulose nanofibers and cadmium sulfide resulted in more efficient nanomaterials for removing chlorpyrifos, mainly due to the increase in the specific area and, consequently, the greater number of adsorption sites available. However, the increase in biopolymer charge (>10%) significantly reduced the adsorbent performance due to the aggregation of particles and the steric hindrance promoted by the functional groups present in the polymeric chains.

Effect of pH
The pH of the solution is an important parameter that influences the adsorption

Effect of Initial Adsorbate Concentration and Adsorbent Dosage
Studying the effect of experimental parameters such as initial adsorbate concentration, adsorbent dosage, and adsorption contact time is essential to determine the most favorable conditions for contaminant removal and to investigate the phenomenon of adsorption.
Muda et al. [162] demonstrated that the maximum adsorption capacity was directly proportional to the increase in the initial concentration of adsorbate and adsorbent mass, mainly due to the high probability of adsorbate molecules binding to the adsorption sites [160]. Likewise, the gradual increase in the initial concentration of adsorbate and the amount of adsorbent can reduce the contaminant removal percentage due to the saturation of higher energy sites and the interaction between the adsorbent molecules [165].
Another important factor for the adsorption efficiency is the contact time between adsorbent and adsorbate. In most of the studies, adsorption showed distinct behavior over time. In the initial stages, a fast adsorption rate is observed (mass transfer of liquid phase to the solid phase) and a tendency to plateau in the subsequent steps because of saturation of the adsorption sites (equilibrium between vacancy sites and adsorbed molecules) [145]. At the same time, the nanocomposite content (ratio mass: mass) of the of constituent materials can, in some cases, affect the efficiency of the process, either positively or negatively. For example, Gupta et al. [166] reported that the combination of cellulose nanofibers and cadmium sulfide resulted in more efficient nanomaterials for removing chlorpyrifos, mainly due to the increase in the specific area and, consequently, the greater number of adsorption sites available. However, the increase in biopolymer charge (>10%) significantly reduced the adsorbent performance due to the aggregation of particles and the steric hindrance promoted by the functional groups present in the polymeric chains.

Effect of pH
The pH of the solution is an important parameter that influences the adsorption capacity since it is related to the surface properties of the adsorbent, degree of speciation of chemical compounds, and competition of ions in solution for adsorbent sites [28,161]. Nevertheless, the ideal pH for each process is quite specific, as it depends on the adsorbent's surface charge and the adsorbate's intrinsic characteristics, such as the acid dissociation constant (pK a ) and chemical stability to pH changes. In addition, the study of pH effects can also help to understand the possible chemical or physical mechanisms involved in the removal of the adsorbate [167,168].
Recently, Abukhadra et al. [160] developed a polymeric nanocomposite containing bentonite for methyl-parathion removal. The study of the effect of pH verified a considerable increase in the adsorption capacity in alkaline medium (pH = 9.0). This adsorption behavior is related to the point of zero charge (pH PZC ) of the adsorbent and deprotonation at basic pH, which results in strong electrostatic attraction with the cationic species of methyl-parathion.
However, van der Waals forces are not the only interactions existing between the adsorbent and the adsorbate since the pH of the solution may not influence the process [145]. According to the chemical structure of compounds and functional groups, the interaction between adsorbent/adsorbate systems can occur through stacking bonds π-π, hydrogen bonds, and ionic exchange [156,161].

Effect of Temperature on Adsorption
The efficiency of the adsorption process is highly dependent on experimental conditions, such as temperature. Evaluating the effect of temperature on adsorption behavior is an important step considering a future application in wastewater treatment systems [111].
In an adsorption system, the temperature can affect the adsorption velocity and kinetic energy, resulting in an increase or decrease in the removal percentage and adsorption capacity [156,169]. The effects of temperature on adsorption occur due to viscosity, rate diffusion, and equilibrium state [170].
Adsorption is a spontaneous process that can be endothermic or exothermic [171,172]. Observations concerning the temperature dependence of adsorption of pesticides were reported. All studies that evaluated the effect of temperature on adsorption assumed an endothermic reaction, i.e., the reaction became more favorable at higher temperatures [156,157,159,160].

Equilibrium and Adsorption Kinetics
Adsorption isotherms are helpful mathematical equations to describe the equilibrium of adsorption; that is, the amount of solute adsorbed on the surface of the adsorbent as a function of concentration in the liquid phase at a constant temperature [173]. Furthermore, they are relevant tools for understanding the interaction between the adsorbent and the adsorbate, which can help process design [174]. Several isothermal models can describe the adsorption phenomenon. However, the most commonly reported in the literature are the Langmuir, Freundlich, and Temkin isotherms.
The Langmuir isotherm is a theoretical model used to describe processes on a uniform, non-porous surface. The model is based on the hypothesis that adsorption occurs in monolayers with uniform energy distribution, in which the molecules of the adsorbed solutes do not interact with each other [175]. The expression of the Langmuir isotherm is represented by Equation (1): where q e is the amount of adsorbate adsorbed per unit of mass of adsorbent in equilibrium (mg g −1 ), q m is the maximum adsorbed amount (mg g −1 ), K L is the adsorption equilibrium constant or Langmuir constant (L mg −1 ), and C e is the concentration of adsorbate in the solution at equilibrium (mg L −1 ). The Freundlich isotherm is an empirical mathematical model that represents the adsorption equilibrium of a fluid (liquid or gas) on the surface of a solid material. This equation assumes that the adsorption occurs on heterogeneous surfaces and adsorption sites of different energy levels [176]. The Freundlich isotherm is expressed by Equation (2): where K F is the adsorption equilibrium constant or Freundlich constant ((mg g −1 ) (L mg −1 ) −1/n ), and 1/n is the heterogeneity factor.
The Temkin isotherm is an empirical model used to describe the indirect interactions between the adsorbent and the adsorbate and assumes that the heat of adsorption decreases as the surface of the adsorbent is covered. Unlike the Freundlich isothermal model, the system energy decreases in a non-logarithmic form (uniform energy distribution) [177]. Temkin's model is given by Equation (3) and is applied in studies to measure the energetic contributions of the adsorption process. Thus, the process temperature must be considered.
where B is the constant associated with adsorption heat (kJ mol −1 ), T is the temperature (K), R is the universal constant of ideal gases (8.314 J mol −1 K −1 ), b is the constant associated with the interaction between adsorbate and adsorbent, and K T is the Temkin constant (L mg −1 ). Furthermore, the Dubinin-Radushkevich (DR) model is often reported as a good fit for the experimental data to the adsorption isotherms [178]. However, this empirical model assumes active sites with Gaussian energy distribution and is widely applied only to intermediate ranges of adsorbate concentration, as it presents an unrealistic behavior [179].
Kinetic models have been widely used to assess adsorbent performance, construct the kinetic profile, and investigate the possible mechanisms involved in mass transfer between liquid and solid phases [180]. Among the proposed models, the most common are pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich kinetics [181,182].
The PFO model was initially proposed by Lagergren and describes that the adsorption process is independent of the initial concentration of adsorbate and which adsorbent has some active sites, with the external/internal diffusion being the step determining the adsorption rate. Moreover, the model suggests that the interactions between the molecules of the adsorbent and the adsorbed substance are relatively weak attractive forces, indicating a physisorption process. The PFO kinetics is based on the non-competition of molecules for an active site (1:1 ratio) and is expressed by Equation (4): where q t is the amount of solute adsorbed on the solid surface at time t (mg g −1 ), and k 1 is the pseudo-first-order kinetic constant (min −1 ). The PSO model assumes that adsorption on active sites is the control step of the adsorption process, where the adsorption rate increases proportionally with the available adsorption sites. Thus, adsorption occurs in specific sites, and adsorbate molecules do not interact with each other. In terms of adsorption mechanisms, if this model presents the best adjustment of the experimental data (higher values of R 2 ), the electrons transfer is the limiting rate of the process, indicating a phenomenon of a chemical nature. The pseudo-second-order kinetics, however, consider the competition between the adsorbate molecules for an active site (2:1 ratio) and are expressed by Equation (5): where k 2 is the pseudo-second-order kinetic constant (g mg −1 min −1 ), and t is the contact time (min) Elovich model is largely used to describe the chemisorption of solutes onto solid sorbents and can be expressed by Equation (6): where a is the constant that describes the initial sorption rate (mg g −1 min −1 ) and b is the constant related to surface coverage and activated energy for chemisorption (g mg −1 ). From the scientific articles shown in Table 3, it was possible to verify that the kinetic models of PPO and PSO are the most used to measure the adsorption rates of pesticides on nanoadsorbents (magnetic and non-magnetic). Regarding the adsorption equilibrium models, the Langmuir and Freundlich isotherms are applied in adsorption studies in the aqueous phase to represent a good adjustment of the experimental data.
The Langmuir isotherm was reported as the better model for adjusting the equilibrium of the pesticide's adsorption, suggesting adsorption in monolayers with uniform distribution of active sites of nanocomposites. Furthermore, it is assumed that there is no competition between the adsorbent molecules for a single active site of the adsorbent.
Concerning the kinetic models, the PSO model demonstrated the best adjustment of experimental data, indicating that mass transfer as a function of a concentration gradient is the limiting step of adsorption. Furthermore, the number adsorbate and adsorbent sites had a 2:1 relationship [183]. Furthermore, this model suggests that the transfer of pesticides to the solid surface occurs predominantly by chemical mechanisms, i.e., chemisorption.

Conclusions and Future Perspectives
The present work made it possible to verify the potential and extensive application of magnetic and conventional nanomaterials in either degradation or removal of pesticides from the aqueous solution. Thus, it is observed that the degradation of these pollutants occurs mainly by heterogeneous photocatalysis, whereas the removal of wastewater is mainly by adsorption processes.
The pesticides methyl-parathion, dinoseb, and monocrotophos are the most dangerous. However, photodegradation and adsorption are effective for moderately toxic pollutants, such as chlorpyrifos and atrazine. Values of up to 90 and 99% are reported for degradation and the removal of pesticides from wastewater. This result demonstrates a certain lack of research involving the adsorption or photodegradation of highly hazardous pesticides.
At the same time, the wide-ranging use of magnetic nanocomposites is justified by their operational ease, reuse, and chemical or thermal stability. In addition, the percentage of materials used in the synthesis of nanoadsorbents can affect the interaction between the adsorbent and the adsorbate. The pH of the solution, initial concentration of adsorbate, contact time, and surface area are factors determining the efficiency and optimization of the process.
Photocatalytic degradation is reported under visible light and UV radiation due to the addition of different nanomaterials that make up the reinforcement layer, and the nanocomposite matrix is combined. Further, the catalytic efficiency is considerably enhanced by the incorporation two materials in the nanocatalyst structure. Therefore, it was possible to show the potential of nanocomposites in the degradation/removal of pesticides in aqueous solutions.
Studies involving the scale-up of adsorption and photocatalytic processes should be encouraged, considering the higher reuse capabilities and efficiency of nanocomposites, especially the magnetic ones. In this view, heterogeneous photocatalysis for the degradation of pesticides at higher concentrations and adsorption of these pollutants in fixed-bed adsorbers should be conducted to expand the breakthrough performed by the research discussed in this review. In addition, the search for alternative sources to produce nanocomposite matrixes, such as residual or abundant materials (biomass), can intensify the use of these nanocomposites in heterogeneous photocatalysis and the adsorption of pesticides. At the same time, ecotoxicity and cytotoxicity studies should be performed to assess the safety of the use of these materials and their implementation at a large scale.