Photocatalytic Degradation and Adsorptive Removal of Emerging Organic Pesticides Using Metal Oxide and Their Composites: Recent Trends and Future Perspectives

Abstract

For applications involving water cleanup, metal oxide nanoparticles are exceptionally successful. They are useful for the adsorption and photocatalytic destruction of organic pollutants due to their distinctive qualities, which include their wide surface/volume area, high number of active sites, porous structure, stability, recovery, and low toxicity. Metal oxide nanomaterials have drawn a lot of attention from researchers in the past ten years because of their various production pathways, simplicity in surface modification, abundance, and inexpensive cost. A wide range of metal oxides, such as iron oxides, MgO, TiO₂, ZnO, WO₃, CuO, Cu₂O, metal oxides composites, and graphene–metal oxides composites, with variable structural, crystalline, and morphological features, are reviewed, emphasizing the recent development, challenges, and opportunities for adsorptive removal and photocatalytic degradation of organic pollutants such as dyes, pesticides, phenolic compounds, and so on. In-depth study of the photocatalytic mechanism of metal oxides, their composites, and photocatalytically important characteristics is also covered in this paper. Metal oxides are particularly effective photocatalysts for the degradation of organic pollutants due to their high photodegradation efficiency, economically sound methods for producing photo-catalytic materials, and precise band-gap engineering. Due to their detrimental effects on human health, pesticides—one of the highly hazardous organic pollutants—play a significant part in environmental contamination. Depending on where they come from and who they are targeting, they are categorized in various ways. Researchers focusing on metal oxides and their composites for the adsorptive and photocatalytic degradation of pesticides would find the review to be a beneficial resource. Detailed information on many pesticides, difficulties associated with pesticides, environmental concentration, and the necessity of degradation has been presented.

1. Introduction

Persistent organic chemicals (POPs) are highly hazardous to the ecosystem and living organisms [1]. Their non-biodegradability allows them to accumulate easily in the food chain, affecting both humans and wildlife [2]. Pesticides are one class of POPs with half-lives that can extend to years [3]. They have been used abundantly to control the growth of the crops by exterminating pests including insects, fungi, and microorganisms in agricultural farms [4]. However, the highly toxic nature of these material has become an alarming concern to humans and the environment since they can readily contaminate soil, air, and water through sewage water and industrial and domestic wastes [5]. Additionally, pesticides present in water can reach groundwater through leakages and leaching, surface water, and drinkable water [6,7,8].

The adverse effect of different pesticides on human health have been extensively reported and investigated [9]. For example, organochlorines (OCs) and organophosphates (OPs) accumulate acetylcholine in the central nervous system of humans, leading to serious brain disorder [10]. Carbamates are another class of pesticides that induce apoptosis for human cells and raise the risk of cancer [11]. Other pesticides such as simazine affect humans in trace amounts, leading to kidney failure and heart and lung diseases [12]. Accordingly, numerous attempts have been made to remediate these harmful toxicants from the environmental matrices, utilizing various removal techniques including chemical precipitation, membrane separation, electrocatalysis, advanced oxidation processes, and adsorption [13,14].

The adsorption process has become one of the most researched practices due to its ease of applicability and low cost, depending on the adsorbent [15]. Although adsorption is an easily accessible method for remediation, it generates secondary products and requires further treatment. Accordingly, photocatalytic degradation has attracted the attention of researchers as an alternative technique for the mineralization of toxicants. Photocatalytic degradation employs solar energy, UV, or/and visible light to irradiate a semiconducting material resulting, in the generation of a series of radicals, including hydroxide and superoxide radicals, which are responsible for photodegradation [16]. Studies have shown that photocatalytic activity can be enhanced when the material is adsorbed on the surface of the photocatalyst [17]. Photoactivity, photostability, chemical stability, and the band gap of the photocatalyst should be considered when choosing an adequate material for photocatalytic degradation [18,19]. Photocatalytic degradation can be regarded as a better approach towards the disposing of toxicants; however, it is a more complex process and requires the presence of oxygen as an oxidant [20]. Several studies have corroborated that the higher adsorption capacity of the photocatalyst leads to higher efficiency in degradation, where adsorption facilitates the contact between the pollutant and the photocatalyst. Therefore, a high photodegradation rate requires an effective adsorption process at optimum conditions [21].

Recently, various materials have been investigated for the effective removal of pesticides using adsorption and photocatalytic degradation. Metal oxide nanoparticles, including ferric oxides, cobalt oxides, copper oxides, zinc oxides, titanium oxides, magnesium oxides, cerium oxide, aluminum oxides, and other metal oxides have shown promising removal and degradation results with respect to pollutants such as heavy metals and POPs [22,23,24]. Likewise, they have been widely implemented as nano-adsorbents and photocatalysts for the remediation of pesticides [17]. Their exquisite physical and chemical properties, influenced by their size (1–100 nm), permit them to differ from their corresponding bulk material [25]. Their eminent properties, including large surface/volume area, high number of active sites, porous structure, stability, recovery, and low toxicity, make them valuable nano-adsorbents and photocatalysts [22,26,27].

Developing the structural properties of nano-adsorbents and photocatalysts has become a demand. Therefore, different strategies have been applied to achieve superior performance in physisorption, in chemisorption, and in reducing the band gaps of the photocatalysts to attain the slow recombination of the charge carriers and enhance photo degradation [28,29]. Doping photocatalysts with different metals [30,31], the multi-functionalization of the surface [32], developing heterostructures [33], developing nanocomposites such as metal oxides/metal-organic frameworks [34], metal oxides/polymers [35], and metal/metal oxides [36], are the main strategies followed to manipulate the characteristics of nano-adsorbents and photocatalysts. Many in vitro and in vivo studies have been conducted recently to better understand the toxicological effects and potential risks of various nanoparticle exposures on people and the environment. There is still a major gap in knowledge about the toxic effects of nanoparticle exposure.

Finding an effective remediation process for the harmful effects of pesticides requires immediate. This review provides a detailed collection of recent references to demonstrate the importance of metal oxides and their functionalized nanocomposites in the removal of different types of pesticides using adsorption and photocatalytic degradation.

2. Nanoarchitectures of Metal Oxides and Oxide Perovskites

2.1. Cobalt Oxide

Cobalt oxides are a type of inorganic metal oxides that exists abundantly in nature. Cobalt oxides have many properties that are favorable to environmental applications; they are highly stable, exert no toxicity, exhibit a magnetic behavior, highly resistant to corrosion and oxidation, and have high mechanical strength [37,38,39]. They are p-type semiconducting materials at room temperature that show good conductivity [40]. Cobalt oxides have differences in oxygen vacancies; thus, cobalt oxide exists in many oxidation states [41]. Some of the most used oxidation states include cobalt (II) oxide and cobalt (III) oxide [42]. The structure of Co₃O₄ NPs consists of a cubic spinel structure with Co (II) at the tetrahedral sites and Co (III) at the octahedral sites [38,43]. Figure 1 shows the structure of Co₃O₄ spinel structure, with Co (II) surrounded by four oxygen atoms and Co (III), surrounded by six oxygen atoms [44]. Several synthesis methods of cobalt oxide have been reported, including sol–gel, hydrothermal, and microwave-assisted methods. The variable oxidation states of cobalt have made the particles applicable to many fields [45]. Cobalt oxide plays a vital role in many applications, including pollutants sensing, degradation of harmful materials, drug delivery systems, supercapacitors, and storage devices [46,47,48,49,50,51,52,53,54].

2.2. Copper Oxide

Copper oxides exert various properties that make them easily appliable. Numerous structures of copper oxides can be synthesized, such as nanowires, nanorods, nanotubes, and nanoparticles [55]. They are abundant, inexpensive, and highly stable, with high catalytic and antibacterial activity [56]. Different oxidation states of copper oxides are available; however, cuprous oxide (Cu₂O) and cupric oxide (CuO) are the most stable, while paramelaconite (Cu₄O₃) is metastable. Figure 2 shows the three crystal structures for copper oxides [57]. Cuprous oxide (Cu₂O) crystallizes in a cubic structure and is a p-type semiconductor. On the other hand, cupric oxide (CuO) exists in a monoclinic crystal structure and is believed to be both a p- and an n-type semiconductor [58,59]. Figure 3 illustrates the difference between a p-type and an n-type semiconductors.

Copper oxide semiconductors are recognized for their role in the remediation of environmental contaminants due to their strong oxidation and reduction ability and environmental compatibility [60]. A difference between cobalt oxides and copper oxides is that cobalt oxides have higher stability at high temperatures. However, the precursor salts of cobalt oxides are more expensive than the precursor salts of copper oxide, making copper oxides more easily obtained and applied in many fields [61]. Copper oxides have many prospective applications in the fields of sensors, antimicrobial activity, catalysis, coatings, polymers, and electronics [62,63,64,65,66]. Moreover, cupric oxide shows promising results for the decontamination of water since it is relatively cheap, with high catalytic activity [67,68]. Many routes for the preparation of copper oxides are available. Chemical precipitation, the sonochemical method, hydrothermal synthesis, and synthesis via plant extracts and micro-organisms, are all examples for chemical and biological methods for the synthesis of copper oxide NPs [69,70].

2.3. Zinc Oxide

Zinc oxide is the second most abundant metal with different morphological shapes and sizes [71]. It is an inorganic multifunctional material that has favorable properties and characteristics [72]. Zinc oxide exhibits optical and piezoelectric properties that are attractive to important fields such as optoelectronics and transparent electronics [73]. It also offers a high surface area with a wide band gap of 3.37 eV, making it appropriate for photocatalysis [74]. Zinc oxides are widely used for medicinal applications since zinc is considered a dietary supplement, in addition to their antibacterial activity [75,76,77]. It has also been reported that the antibacterial activity of zinc oxides depends on their size and shape, which can be controlled through the synthesis route [78]. Zinc oxides can be synthesized by chemical, physical, and biological methods. These methods include microemulsion, precipitation, plasma and ultrasonic techniques, the sol–gel method, combustion, hydrothermal synthesis, and green synthesis from plants extracts [79,80,81]. Moreover, zinc oxides have gained attention for water purification and the removal of hazardous materials since they are biocompatible [82]. Polymorphs of zinc oxide consist of three phases shown in Figure 4: hexagonal wurtzite, cubic zinc blende, and cubic rock salt [83]. The wurtzite phase is the most thermodynamically stable, with every zinc atom tetrahedrally coordinated with four oxygen atoms. Zinc blende and rock salt are metastable [84,85]. More zinc oxides applications include solar cells [86,87,88], sensors [89,90], drug delivery [91,92], and the cosmetic industry [93].

2.4. Iron Oxide

Iron oxides have several forms which consist mainly of iron and oxygen. These forms include iron (II) oxide (wüstite, FeO), iron (II,III) oxide (magnetite, Fe₃O₄), and iron (III) oxide (ferric oxide, Fe₂O₃) [95], and are shown in Figure 5 [96]. Ferric oxide is the most common form and it has four polymorphs [97]: alpha phase hematite (α-Fe₂O₃), beta phase (β-Fe₂O₃), gamma phase maghemite (γ-Fe₂O₃), and epsilon phase (ε-Fe₂O₃) [98].

The magnetic properties of iron oxide NPs are highly affected by their size, dispersion, and surface [99]. Magnetite and maghemite are intrinsic ferrimagnetic materials, while hematite’s magnetic properties are thermally induced [100]. The main properties that allow IONPs to be of such interest in many fields is their superparamagnetic behavior, their high surface/volume ratio, non-toxicity, reusability, biocompatibility, high stability, and resistance to change [101,102].

IONPs are also widely utilized in biomedical applications such as drug delivery systems, where the particles can be carried to a specific site with high accuracy due to the use of an external magnetic field to direct the particles [103,104]. Iron oxide NPs are usefully applied in many fields including sensing [105,106,107], catalysis [108,109,110], photodegradation [111,112], and adsorption of pollutants [113,114,115]. The only disadvantage in IONPs is the aggregation of particles in aqueous media, which is unfavorable in water remediation applications. Therefore, IONPs can be further stabilized by surface modification, including coating with surfactants and polymers [116,117]. For example, a study coated iron oxide NPs with chitosan (made from chitin), which stabilized the particles and further functionalized them with amine groups, thus increasing the number of binding sites available [104].

2.5. Titanium Oxide

Titania or titanium dioxide (TiO₂) is abundant in nature and has favorable advantages with respect to energy and environmental applications [118,119]. It has been shown to be promising for these applications due to its chemical stability, biological and chemical inertness, and non-toxicity [120]. TiO₂ has long durability and transparency to visible light. It is active under UV light and functions as a semiconductor with a band gap around 3.2 eV [121]. Furthermore, titanium oxide exists in three crystalline forms, which are tetragonal anatase, tetragonal rutile, and orthorhombic brookite, with rutile being the most thermodynamically stable form [122]. Figure 6 shows the three polymorphs of titania [123]. Even though rutile is the most stable, anatase is more efficient when it comes to photocatalysis. However, some studies showed that rutile can possess good photocatalytic activity [124,125]. Anatase and rutile have tetragonal symmetry, while brookite has an orthorhombic crystalline structure [126]. The method of preparation for TiO₂ nanomaterials controls their morphology; therefore, their performance in applications can be enhanced [127]. Tailoring particle size and crystal surfaces determines which facets are exposed in TiO₂, affecting its photocatalytic activity tremendously. One of the most common methods for morphology control is the use of organic surfactants [128]. Moreover, a study synthesized different morphologies of titania by controlling the temperature in the solvothermal method [129]. Rose-like, chrysanthemum-like, and sea-urchin-like TiO₂ nanostructures (shown in Figure 7) were successfully prepared and applied for the photocatalytic degradation of Rhodamine B, where each nanostructure had a different photocatalytic performance. The applications for titania are wide, including the removal of organic pollutants [130], medical applications [131], energy storage, and sustainable energy production [132,133].

2.6. Magnesium Oxide

Magnesium oxide (MgO) is a multifunctional inorganic material that holds great technological importance [134]. Magnesium oxide or magnesia exhibits a rock salt-type structure similar to simple NaCl [135], which is shown in Figure 8. It is known for its excellent optical, thermal, electrical, mechanical, and chemical properties [136]. Additionally, MgO has high thermal stability, with a melting point around (2852 °C) [137], and low heat capacity, making it a good insulator [138]. The particles are non-poisonous since MgO is considered an essential nutrient for plants and humans [139]. Magnesium oxides are known for their biocompatibility and stability; thus, they are frequently used in drug delivery systems and biomedical applications [140,141,142,143,144]. The method of synthesis of MgO highly influences the morphology and the physical structure of the nanoparticles, as in TiO₂ [145]. A study synthesized MgO nanostructures with the microwave-assisted process using two different capping agents [139]. The structures obtained were MgO nanospheres and MgO nano-cubes. The nanostructures were then used for the remediation of ciprofloxacin from aqueous solutions, and MgO nanospheres exhibited higher adsorption capacity. Moreover, MgO NPs can be fabricated by several other methods such as biosynthesis [146], ultrasound-assisted methods [147,148], sol–gel methods [149,150], pyrolysis [151], hydrothermal [152], solution combustion [153], and the co-precipitation method [154,155]. Further properties of MgO include a wide bandgap of 7.8 eV, along with high porosity and high surface area, making MgO widely applicable in many technologies [156]. The technological fields involve optoelectronics [157], enhancement of energy conversion efficiency in perovskite solar cells [158], sensors [159], superconductors [160], and toxic waste remediation [161,162,163]. For remediation applications, MgO nanostructures are preferred due to their nano size, which allows them to have high surface area and high surface charge [164]. Lastly, MgO nanomaterials are broadly used in the field of catalysis [165,166,167,168,169].

2.7. Cerium Oxide

Cerium oxide (ceria) is a lanthanide rare earth metal oxide that attracted tremendous interest for its many applications [170]. Different types of cerium oxide nanomaterials are applied in various fields, such as biomedical fields [171,172,173,174,175], sensors [176,177], supercapacitors [178], fuel cells [179,180], adsorbents [181,182], photoprotective coating [183], solar cells [184], and the photodegradation of toxic pollutants [185,186,187]. Moreover, ceria differs from alkaline earth metals and post-transition metals due to its shielded 4f orbital electrons that affect its remarkable properties [188]. Cerium oxide has excellent chemical stability, is inexpensive, and is environmentally friendly [189]. It is highly conductive, with a large magnetic moment [190]. Cerium oxide has two oxidation states, Ce⁺³ and Ce⁺⁴, where cerium (IV) is more thermally stable than the reduced version of cerium. However, cerium can switch to its other oxidation state depending on its surrounding environment [191]. Ceria’s structure consists of a cubic fluorite-type oxide with many oxygen vacancies [192]. The cubic fluorite-type structure of ceria is shown in Figure 9. The surface of cerium oxide has Ce⁺³ ions as well [193]. Consequently, when cerium oxide nanoparticles (nanoceria) are synthesized, the reactivity of nanoceria increases with the increase of Ce⁺³ ions concentration, making the NPs good for catalysis applications [194,195]. Several methods are available for the synthesis of nanoceria including the sol–gel [196], biosynthesis [197], hydrothermal [198], sonochemical [199], microwave-assisted [200], and co-precipitation [201] methods.

2.8. Aluminum Oxide

Aluminum oxide (Alumina) is a promising candidate for many applications, it is used in catalysis [202,203], insulators [204], microelectronics [205,206,207], sensors [208], and remediation processes [209,210]. Alumina has unique acid/base characteristics, good mechanical strength, chemical inertness towards oxidation and reduction, excellent electrical insulation, sufficient thermal stability, high surface area, and a high melting point [211,212]. Although alumina is a good insulator, F. Tzompantzi proposed that it might also be effective for photocatalytic degradation [213]. Yanet Pina-Perez proposed that the hydroxyl groups on Al₂O₃’s surface might be the reason behind its photoactivity [214]. The photocatalytic activity of alumina can be increased by doping the metal ions with other metal oxides [215]. Alumina can be fabricated by various methods such as thermal decomposition [216], hydrothermal [217], combustion [218], co-precipitation [219], and sol–gel methods [220]. However, unlike other metal oxides, alumina needs high calcination temperatures (>1000 °C), which makes the fabrication costly process [221]. Alumina has many crystal structures. The most common one is α- Al₂O₃ since it is the most thermodynamically stable [222], and the structure is shown in Figure 10 [223]. Alumina has other polymorphs, including η, δ, κ, θ, γ, and ρ phases, which are metastable [224]. The type of phase produced depends highly on the method of synthesis followed. Some factors that affect the phase of alumina include the temperature, pH, pressure, and speed of stirring [225].

2.9. Other Metal Oxides

Besides the former metal oxides discussed, several metal oxides, including MnO₂, WO₃, and NiO, have been reported for application in the remediation of pesticides from water. Manganese dioxide nanoparticles are prepared using various synthetic routes, including hydrothermal, sol–gel, homogeneous hydrolysis, and sono-chemical methods [11,226,227,228]. They exist in different crystalline structures, such α-MnO₂ (hollandite), β-MnO₂ (pyrolusite), δ-MnO₂ (birnessite), ε-MnO₂ (akhtenskite), γ-MnO₂ (ramsdellite), λ-MnO₂ (defect spinels), and amorphous MnO₂ [229,230,231]. The structures of MnO₂ polymorphs are shown in Figure 11 [232,233]. They are widely applied as adsorbents and photocatalyst for the removal of different heavy metals and organic pollutants because they are cost effective, their structures are flexible, and they exert no toxicity [234]. Few studies were reported for the removal of pesticides using MnO₂ NPs. A removal percentage of 66% within 2 h was found for the photodegradation of 2,4-dichlorophenoxyacetic acid using manganese-doped zinc oxide/graphene nanocomposite under LED light [235].

Various publications were considered for the fabrication of semi-conductor tungsten oxide nanoparticles using the hydrothermal method. For the remediation purposes, most of the time, tungsten oxide nanoparticles are coupled with zinc oxide and used as photocatalyst for the removal of different contaminants. This is attributed to the fact that tungsten oxide NPs have a narrow bandgap of 2.8 eV; therefore, they are weak as photocatalysts, and enhancements can be conducted by doping the material with other metals for high removal efficiency. The degradation efficiency increases with the increase in dopant percentage. At pH = 7, WO₃-doped ZnO NPs immobilized on glass plates show 80% removal of 2,4-dichlorophenoxyacetic acid pesticide using UV light within 2 h. Additionally, they show 99% removal of diazinon within 3 h at pH = 7 [236,237]. Figure 12 represents the effect of doping percentage on the degradation efficiency of diazinon [237].

Nickel oxide nanoparticles are hierarchical porous structures, thermally stable and with large surface area [238]. Hence, they are efficient adsorbents for different pollutants [239,240]. Nickel oxide nanoparticles are fabricated using different methods, including sol–gel [241], thermal decomposition [242], biosynthesis [243], and laser ablation [244]. A study synthesized different nanostructures of NiO using the hydrothermal method, varying the reaction temperature, time, and the molar ratios of the precursors [245]. SEM images of the synthesized nanorods, nanoplates, and nanoparticles of NiO are shown in Figure 13 [245]. Furthermore, they are p-type semiconductors with a band gap of 3.6–4.0 eV, which is sufficient for photocatalytic degradation of pesticides [246]. A layered and flower-like structure of S-doped Ni–Co LDH with uniformly dispersed spherical Fe₃O₄ NPs has shown 92% degradation of chlorpyrifos using visible light at pH = 10 within 150 min [247].

2.10. Effect of Metal Oxide’s Crystalline Structure on the Photocatalytic/Sorption Performance

Transition Metal oxide nanoparticles (NPs) displayed remarkable surface properties, structural characteristics, and a significant specific surface area, which made them desirable candidates for adsorption processes [22]. When the size of the molecules reduces from bulk to nanoscale, it creates an exponential increment in surface-to-volume ratio. By decreasing the size and adding active edges on organic molecules’ surfaces for interaction, surface energy or adsorbent composites are improved. In contrast to their bulkier cousins, nanoparticles are far better at adsorbing organic pollutants from water. Moreover, MO NPs have lately shown a distinct potential as highly selective adsorbents intended for fast and effective removal of organic pollutants, whether used alone or in nanocomposites.

The metal oxides-based nanocomposites serve as large bandgap energy (Eg) semiconductors and have beneficial properties such non-toxicity and stability in water for the breakdown of organic contaminants. They also have correct structure, crystalline, and surface features. The fundamental process for the photocatalytic destruction of impurities from the surface of semiconducting materials is produced by oxygen. Oxygen vaccinations can benefit from semiconducting nanoparticles absorbing photons. By transforming organic pollutants into low/intermediate harmful yields, resulting in substances such as carbon dioxide, water, and inorganic ions, it supports environmental restoration. Once upon a time, photocatalytic treatment was thought to be the most environmentally friendly method of removing organic contaminants from wastewater. Using a short-range solar spectrum is thus a considerable obstacle to photocatalytic activity. The flaw could be fixed, for instance, by fabricating nanomaterials, doping hetero-atoms, and designing metal oxide nanocomposites through chemical and structural alterations. Worthwhile photocatalysts effectively delay electron-hole (e—h+) pair recombination, efficiently absorb the solar spectrum in the visible range, and function well as photocatalysts [248]. As photocatalysts and adsorbents, several metal oxide nanomaterials, such as Al₂O₃, CuO, CeO₂, ZnO, and TiO₂, have attracted a lot of interest [248]. To increase effectiveness and selectivity, several MO-based composites, including porous materials–reinforced metal oxides, magnetic metal oxides, metal–metal oxides, graphene–metal oxides, etc., were being studied. Adsorption events are controlled by these nanocomposites’ surface properties, size, and textural characteristics. Several NP morphologies provide flexible crystal defects, such as active edges on material surfaces for photocatalysis and adsorption applications.

3. Classification of Pesticides

The demand for categorizing pesticides has been raised significantly because of the increased number of pesticides, along with the variation in physical and chemical properties [249]. A considerable volume of literature has been published in this field. Recently, scientists classified pesticides based on origin and on target. Pesticides generally originate from organic, inorganic, and biological sources [250].

Table 1. Classification of organic pesticides based on origin.

Origin	Source	Class	Example	Features	Refs.
Organic	Natural	Plants Phytochemicals	Essential oil, plant extracts, and leftover oilseed cakes.	Low Toxicity, limited persistence in the environment, and complicated structures that prevent resistance in pests.	[251,252]
	Synthetic	Pyrethroids	Phenthion, Diazinon, Cypermethrin, Deltamethrin, Cyfluthrin, and Cypermethrin	Effect the sodium channel in insects, resulting in paralysis of the organism; highly toxic to insects and fish but less to mammals; unstable upon the exposure of light; and commonly used in food.	[253,254,255,256]
		Organophosphates	Aldrin, Dieldrin, Glyphosate, and Chlorpyrifos.	Cause paralysis, resulting in death, and dominant for variety of pests.	[257,258]
		Carbamates	Fenvalerate, Permethrin, Cyhalothrin, and Carbofuran.	Effect the nerve system of the pests, resulting in poisoning and death, and low pollution is caused upon degradation.	[259,260,261,262]
		Organochlorine	Chlorothalonil and Endrin Aldehyde.	Used for insects, long persistent in environment, affecting the nerve system and causing paralysis and death of the pests.

elaborates on the organic class of pesticides, while

Table 2. Classification of pesticides based on target.

Class	Target Pests	Example	Chemical Structures	Ref.
Acaricides	Mites	Bifonazole		[250]
Avicides	Birds	Avitrol
Fungicides	Fungi	Azoxystrobin
Herbicides	Weeds	Atrazine
Insecticides	Insects	Aldicarb
Larvicides	Larvae	Methoprene
Molluscicides	Snail	Metaldehyde
Nematicides	Nematodes	Aldicarb
Ovicides	Egg (prevents hatching of eggs in insects and mites)	Benzoxazine
Piscicides	Fishes	Rotenone
Repellents	Insects	Methiocarb
Rodenticides	Rodents	Warfarin
Termiticides	Kills termites	Fipronil
Viricides	Viruses	Scytovirin

shows the classification of pesticides based on target. The pesticides’ chemical structures are shown in Figure 14.

4. Removal of Pesticides Using Functionalized Metal Oxide Nanomaterials by Adsorption

The hazards and consequences resulting from the massive use of pesticides raised the demand for efficient techniques to be employed for the removal of these contaminants. The adsorption technique has gained popularity as a simple, effective, insensitive, and flexible method [263]. It is a physiochemical method that occurs mostly in the solid–liquid form, though liquid–liquid and liquid–gas forms are also known [264,265,266].

In adsorption, the molecules of liquid or gases are bound to the surface of the solid. The material that provides the surface is called the adsorbent. The contaminants in the liquid or the gaseous phase are called adsorbates. Among the adsorbents reported in the literature, metal oxides have been proven as excellent adsorbents for the remediation of pesticides because of the large surface area provided for the adsorption of the pollutant [267]. The active sites and the functional groups, such as -OH, -COOH, and -C=OH, have a great impact on the efficiency of the adsorption process [268,269]. Moreover, metal oxides, having porous structures, thermal stability, low toxicity, and easy recovery, are all important for a good adsorbent. Two types of interaction between the adsorbent and the adsorbate are present: chemisorption and physisorption. Chemisorption is basically a chemical reaction between the adsorbent and the adsorbate, and it is an irreversible process. It is controlled by chemical bonds such as covalent, chelation, complex formation, proton displacement, and redox-reactions. On the other hand, physisorption, which is more dominant, is a reversible process controlled by Van der Wal’s bonds, dipole–dipole attraction, and London force, etc. [270].

Table 3. The advantages and disadvantages of chemisorption and physisorption.

	Physisorption	Chemisorption
Advantages	Reversible in nature Low adsorption enthalpy Favors low temperature Low activation energy	Strong interaction between the adsorbent and the adsorbate by chemical bonds Higher selectivity
Disadvantages	Weak interaction between the adsorbate and the adsorbent The extent of adsorption is inversely proportional to temperature. Low selectivity	Irreversible in nature High adsorption enthalpy Favors high temperatures High activation energy

provides a comparison between the types of adsorption process [271].

The adsorption process depends on various parameters that need to be optimized, including pH, temperature, time, concentration of contaminant, and sorbent dosage. Table 4 represents the adsorption capacity Q_max (mg/g) and the percentage removal of targeted pesticides using metal oxide nanoparticles at different parameters. The adsorption capacity is calculated in (mg/g) using the formula in Equation (1):

$$Q_{max}=\frac{C_{°}-C_e}{m}\times V$$

(1)

where C_o is the initial concentration of the pesticide (mg/L), C_e is the pesticide concentration at equilibrium (mg/L), m is the mass of adsorbent (g), and V is the volume of the solution (L).

The adsorption isotherm and the adsorption kinetics are used to elucidate the adsorption process and to indicate the type of mechanism. The adsorption isotherm is expressed by Langmuir, Freundlich, Sips, Temkin, Redlich Peterson, Henry, and Dubinin–Astakhov (DA) models. Langmuir, Freundlich, and Dubinin–Astakhov models are most frequently used. Langmuir isotherm investigates a monolayer adsorption onto a homogeneous adsorbent, whereas Freundlich illustrates a multilayer adsorption onto a heterogeneous adsorbent. The Dubinin–Astakhov model is used to calculate the mean free adsorption energy E (J/mol). The physisorption mechanism gives an E value smaller than 8 J/mol. However, values of E from 16 J/mol to 40 J/mol indicate a chemisorption mechanism. The adsorption kinetics are equations that indicate the type of interactions between the adsorbent and the adsorbate (contaminant). Chemisorption interaction is described by a pseudo-second-order equation. The pseudo-first-order equation is applied for the physisorption interaction [272,273].

Despite the advantages of adsorption, there is one certain drawback associated with the use of this technique: it produces secondary pollutants which require highly advanced procedures for recycling and decomposing for them to be used in the industrial field [22].

Table 4. Adsorptive remediation of pesticides using metal oxides NPs.

Adsorbent a	Targeted Pesticides b	Target Operation Parameters					Adsorption Modelling				Ref.
		Pesticide Conc.	Adsorbent Dosage (g) or g/L	pH	Temp. (K)	Time (min)	Kinetics c	Isotherm d	Mechanism e	Qmax (mg/g) or Percentage Removal (%)/Percentage Recovery
Co3O4/G-MCM-41	Methyl parathion	-	-	-	-	-	PFO, PSO	L, F, DA	-	175.2	[274]
NiO/Co@C	Chlorothalonil	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	62.2	[275]
	Tebuconazole	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	40.5
	Chlorpyrifos	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	60.3
	Butralin	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	50.2
	Deltamethrin	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	54.1
	Pyridaben	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	51.3
CeO2	2,4-Dichlorophenoxyacetic acid	0.01 g/L	0.025 g	-	308	120	PSO	L, F, S	π–π, e−	95.78	[276]
Fe3O4@ZnAl-LDH@MIL-53(Al)	Triadimefon	5.0–600 mg kg−1	30 g/L	6	308.15	5	PSO	L	π–π, H, C, (π-CM), P	46.08	[277]
MgFe2O4	Chlorpyrifos	20 mg/L	0.01 g/L	10	295	360	PSO	L	-	4461	[278]
Fe3O4	Atrazine	50 mg/L	0.1 g	2	298	55	PFO	L	-	77.5	[279]
	Methoxychlor	50 mg/L	0.1 g	2	298	55	PFO	L	-	163.9
ZnO	Naphthalene	25 mg/L	0.012 g	4	298	40	PSO	L, F, T	-	66.8	[280]
CTAB-ZnO	Naphthalene	25 mg/L	0.08 g	4	298	40	PSO	L, F, T	-	89.96
BMTF-IL-ZnO	Naphthalene	25 mg/L	0.06 g	4	298	40	PSO	L, F, T	-	148.3
ZnO/ZnFe2O4	Atrazine	50 mL aq. solution	0.4 g/L	7	298	4320	-	D.A	π–π, H, h, e-	-	[281]
Fe3O4@SiO2@GO-2- phenylethylamine	Chlorpyrifos	10 mL aq. Solution	0.015 g	7	298	15	PSO	S	π–π, H	88%	[32]
	Malathion	10 mL aq. Solution	0.015 g	7	298	15	PSO	S	H	76%
	Parathion	10 mL aq. Solution	0.015 g	7	298	15	PSO	S	π–π, H	85%
Fe3O4/MOF-99	Dinotefuran	0.3–1.5 ng/mL	0.015 g	-	-	20	-	-	π–π	88–107%	[282]
	Thiamethoxam	0.3–1.5 ng/mL	0.015 g	-	-	20	-	-	π–π	88–107%
Fe3O4@SiO2@MOF/TiO2	Triadimenol	0.001 g/L	0.04 g	7	298–313.15	1–60	PSO	-	π–π	90.2–104%	[283]
	Hexaconazole	0.001 g/L	0.04 g	7	298–313.15	1–60	PSO	-	π–π	90.2–104%
	Diniconazole	0.001 g/L	0.04 g	7	298–313.15	1–60	PSO	-	π–π	90.2–104%
Fe3O4-GO@MOF-199.	Flusilazole	0.002 g/L	0.02 g	-	-	15	-	-	h, π–π, H, e−	0.0356	[284]
	Fenbuconazole	0.002 g/L	0.02 g	-	-	15	-	-	h, π–π, H, e−	0.0342
	Myclobutanil	0.002 g/L	0.02 g	-	-	15	-	-	h, π–π, H, e−	0.0324
Fe3O4–MWCNTs-ZIF-8	Triazophos	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3.12	[285]
	Diazinon	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.59
	Phosalone	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3.80
	Profenofos	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3.89
	Methidathion	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.34
	Ethoprop	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.18
	Sulfotep	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.84
	Isazofos	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3
Chitosan–CuO	Thiophanate-methyl	0.1 g/L	0.1 g	7	RT	25	-	L, F	h	250	[286]
	Methomyl	0.1 g/L	0.1 g	7	RT	25	-	L.F	-	20
	Malathion	0.02 g/L	1 g/L	2	303	960	PSO	L, F	-	322.6
Chitosan-ZnO	Thiophanate-methyl	0.1 g/L	0.1 g	7	RT	25	-	L, F	h	100
	Methomyl	0.1 g/L	0.1 g	7	RT	25	-	L, F	-	10
	Permethrin	0.0001 g/L	0.5 g	7	298	90	-	-	-	99%	[287]
Fe3O4/CuO/Activa-ted carbon	Imidacloprid	0.01 g/L	0.02 g	7	298	10	PSO	F	C	99%	[288]
ZnO-IPPs	Chlorpyrifos	0.01–0.6 g/L	0.03 g	2	303–323	30	PSO	L, F, T, D. A	-	47.846	[289]
ZnO-CP	Metribuzin	0.033–0.155	0.08 g	3	303–363	80	PSO	F	-	200	[290]
MOM-Fe3O4	Triclosan	0.005–0.2 g/L	0.01–0.05 g/L	4, 7, 10	293, 303, 313	600	PFO	L	-	103.45	[291]
N-NiO@N-Fe3O4@N-ZnO	Atrazine	0.04 g/L	0.1 g	5	-	80	PSO	L	-	92%	[292]
MgAl2O4	Dimethomorph	-	0.5–2 g	5.5	-	10	-	-	-	% Recovery = 90–94%	[293]
Fe3O4 @PS	Lindane	2, 10, 50, 200 µg/L	0.02 g/L	-	RT	<20	PSO	L	-	10.2	[294]
	Aldrin	2, 10, 50, 200 µg/L	0.02 g/L	-	RT	<20	PSO	L	-	24.7
	Dieldrin	2, 10, 50, 200 µg/L	2 × 10−5 g/L	-	RT	<20	PSO	L	-	21.3
	Endrin	2, 10, 50, 200 µg/L	2 × 10−5 g/L	-	RT	<20	PSO	L	-	33.5
MgO	Diazinon	0.30 g/L	0.05 or 0.10 g	-	-	<5	-	-	-	21–37%	[295]
	Fenitrothion	0.28 g/L	0.05 or 0.10 g	-	-	5–60	-	-		27–47%
Fe3O4@nSiO2@mSiO2	DDT	0.0015 g/L	0.05 g	-	-	15	PSO	-	-	94%	[296]

RT = room temperature; ^a Adsorbent: ZnONPs-IPPs = zinc oxide nanoparticles-impregnated pea peels; MOM-Fe₃O₄ = functionalized iron oxide nanoparticles with Moringa oleifera Lam. seeds; Fe₃O₄ @PS = magnetic nanosphere coated by polystyrene; ZnO-CP = zinc oxide with cucumber peel; CTAB-ZnO = cetyltrimethylammonium bromide functionalized zinc oxide; BMTF-IL-ZnO = 1-Butyl-3-methylimidazolium tetrafluoroborate functionalized zinc oxide; Hr-MgO = hierarchical magnesium oxide; ^b targeted pesticides fenitrothion = dimethoxy-(3-methyl-4-nitrophenoxy)-thioxophosphorane; DDT = dichloro-diphenyl-trichloroethane; Diazinone = diethoxy-[(2-isopropyl-6- methyl-4-pyrimidinyl)oxy]-thioxophosphorane; ^c Kinetic equation; PSO = pseudo-second order; PFO = pseudo-first order; ^d Isotherm equation; L = Langmuir; F = Freundlich; S = Sips; T = Temkin; DA = Dubinin–Astakhov; ^e Mechanisms: electrostatic interaction (e⁻), hydrophobic interaction (h), π–π interaction (π–π), π-complex formation with cations (including metal or positive ion charge groups) (π-CM), hydrogen bond interaction (H), coordination or covalent bond (C).

Table 4. Adsorptive remediation of pesticides using metal oxides NPs.

Adsorbent a	Targeted Pesticides b	Target Operation Parameters					Adsorption Modelling				Ref.
		Pesticide Conc.	Adsorbent Dosage (g) or g/L	pH	Temp. (K)	Time (min)	Kinetics c	Isotherm d	Mechanism e	Qmax (mg/g) or Percentage Removal (%)/Percentage Recovery
Co3O4/G-MCM-41	Methyl parathion	-	-	-	-	-	PFO, PSO	L, F, DA	-	175.2	[274]
NiO/Co@C	Chlorothalonil	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	62.2	[275]
	Tebuconazole	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	40.5
	Chlorpyrifos	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	60.3
	Butralin	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	50.2
	Deltamethrin	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	54.1
	Pyridaben	0.045 g/L	0.01 g	-	-	15	PSO	L	π-CM, H	51.3
CeO2	2,4-Dichlorophenoxyacetic acid	0.01 g/L	0.025 g	-	308	120	PSO	L, F, S	π–π, e−	95.78	[276]
Fe3O4@ZnAl-LDH@MIL-53(Al)	Triadimefon	5.0–600 mg kg−1	30 g/L	6	308.15	5	PSO	L	π–π, H, C, (π-CM), P	46.08	[277]
MgFe2O4	Chlorpyrifos	20 mg/L	0.01 g/L	10	295	360	PSO	L	-	4461	[278]
Fe3O4	Atrazine	50 mg/L	0.1 g	2	298	55	PFO	L	-	77.5	[279]
	Methoxychlor	50 mg/L	0.1 g	2	298	55	PFO	L	-	163.9
ZnO	Naphthalene	25 mg/L	0.012 g	4	298	40	PSO	L, F, T	-	66.8	[280]
CTAB-ZnO	Naphthalene	25 mg/L	0.08 g	4	298	40	PSO	L, F, T	-	89.96
BMTF-IL-ZnO	Naphthalene	25 mg/L	0.06 g	4	298	40	PSO	L, F, T	-	148.3
ZnO/ZnFe2O4	Atrazine	50 mL aq. solution	0.4 g/L	7	298	4320	-	D.A	π–π, H, h, e-	-	[281]
Fe3O4@SiO2@GO-2- phenylethylamine	Chlorpyrifos	10 mL aq. Solution	0.015 g	7	298	15	PSO	S	π–π, H	88%	[32]
	Malathion	10 mL aq. Solution	0.015 g	7	298	15	PSO	S	H	76%
	Parathion	10 mL aq. Solution	0.015 g	7	298	15	PSO	S	π–π, H	85%
Fe3O4/MOF-99	Dinotefuran	0.3–1.5 ng/mL	0.015 g	-	-	20	-	-	π–π	88–107%	[282]
	Thiamethoxam	0.3–1.5 ng/mL	0.015 g	-	-	20	-	-	π–π	88–107%
Fe3O4@SiO2@MOF/TiO2	Triadimenol	0.001 g/L	0.04 g	7	298–313.15	1–60	PSO	-	π–π	90.2–104%	[283]
	Hexaconazole	0.001 g/L	0.04 g	7	298–313.15	1–60	PSO	-	π–π	90.2–104%
	Diniconazole	0.001 g/L	0.04 g	7	298–313.15	1–60	PSO	-	π–π	90.2–104%
Fe3O4-GO@MOF-199.	Flusilazole	0.002 g/L	0.02 g	-	-	15	-	-	h, π–π, H, e−	0.0356	[284]
	Fenbuconazole	0.002 g/L	0.02 g	-	-	15	-	-	h, π–π, H, e−	0.0342
	Myclobutanil	0.002 g/L	0.02 g	-	-	15	-	-	h, π–π, H, e−	0.0324
Fe3O4–MWCNTs-ZIF-8	Triazophos	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3.12	[285]
	Diazinon	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.59
	Phosalone	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3.80
	Profenofos	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3.89
	Methidathion	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.34
	Ethoprop	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.18
	Sulfotep	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	2.84
	Isazofos	0.015 g	0.002–0.08 g/L	4	RT	15	-	F	-	3
Chitosan–CuO	Thiophanate-methyl	0.1 g/L	0.1 g	7	RT	25	-	L, F	h	250	[286]
	Methomyl	0.1 g/L	0.1 g	7	RT	25	-	L.F	-	20
	Malathion	0.02 g/L	1 g/L	2	303	960	PSO	L, F	-	322.6
Chitosan-ZnO	Thiophanate-methyl	0.1 g/L	0.1 g	7	RT	25	-	L, F	h	100
	Methomyl	0.1 g/L	0.1 g	7	RT	25	-	L, F	-	10
	Permethrin	0.0001 g/L	0.5 g	7	298	90	-	-	-	99%	[287]
Fe3O4/CuO/Activa-ted carbon	Imidacloprid	0.01 g/L	0.02 g	7	298	10	PSO	F	C	99%	[288]
ZnO-IPPs	Chlorpyrifos	0.01–0.6 g/L	0.03 g	2	303–323	30	PSO	L, F, T, D. A	-	47.846	[289]
ZnO-CP	Metribuzin	0.033–0.155	0.08 g	3	303–363	80	PSO	F	-	200	[290]
MOM-Fe3O4	Triclosan	0.005–0.2 g/L	0.01–0.05 g/L	4, 7, 10	293, 303, 313	600	PFO	L	-	103.45	[291]
N-NiO@N-Fe3O4@N-ZnO	Atrazine	0.04 g/L	0.1 g	5	-	80	PSO	L	-	92%	[292]
MgAl2O4	Dimethomorph	-	0.5–2 g	5.5	-	10	-	-	-	% Recovery = 90–94%	[293]
Fe3O4 @PS	Lindane	2, 10, 50, 200 µg/L	0.02 g/L	-	RT	<20	PSO	L	-	10.2	[294]
	Aldrin	2, 10, 50, 200 µg/L	0.02 g/L	-	RT	<20	PSO	L	-	24.7
	Dieldrin	2, 10, 50, 200 µg/L	2 × 10−5 g/L	-	RT	<20	PSO	L	-	21.3
	Endrin	2, 10, 50, 200 µg/L	2 × 10−5 g/L	-	RT	<20	PSO	L	-	33.5
MgO	Diazinon	0.30 g/L	0.05 or 0.10 g	-	-	<5	-	-	-	21–37%	[295]
	Fenitrothion	0.28 g/L	0.05 or 0.10 g	-	-	5–60	-	-		27–47%
Fe3O4@nSiO2@mSiO2	DDT	0.0015 g/L	0.05 g	-	-	15	PSO	-	-	94%	[296]

RT = room temperature; ^a Adsorbent: ZnONPs-IPPs = zinc oxide nanoparticles-impregnated pea peels; MOM-Fe₃O₄ = functionalized iron oxide nanoparticles with Moringa oleifera Lam. seeds; Fe₃O₄ @PS = magnetic nanosphere coated by polystyrene; ZnO-CP = zinc oxide with cucumber peel; CTAB-ZnO = cetyltrimethylammonium bromide functionalized zinc oxide; BMTF-IL-ZnO = 1-Butyl-3-methylimidazolium tetrafluoroborate functionalized zinc oxide; Hr-MgO = hierarchical magnesium oxide; ^b targeted pesticides fenitrothion = dimethoxy-(3-methyl-4-nitrophenoxy)-thioxophosphorane; DDT = dichloro-diphenyl-trichloroethane; Diazinone = diethoxy-[(2-isopropyl-6- methyl-4-pyrimidinyl)oxy]-thioxophosphorane; ^c Kinetic equation; PSO = pseudo-second order; PFO = pseudo-first order; ^d Isotherm equation; L = Langmuir; F = Freundlich; S = Sips; T = Temkin; DA = Dubinin–Astakhov; ^e Mechanisms: electrostatic interaction (e⁻), hydrophobic interaction (h), π–π interaction (π–π), π-complex formation with cations (including metal or positive ion charge groups) (π-CM), hydrogen bond interaction (H), coordination or covalent bond (C).

5. Removal of Pesticides Using Functionalized Metal Oxide Nanomaterials by Photocatalytic Degradation

Photocatalytic degradation is an advanced oxidation process that destroys toxic substances into other harmless products. Unlike other remediation techniques, photocatalytic degradation completely mineralizes the toxicant, without the production of secondary waste [36]. The mechanism of photocatalytic degradation starts when the photocatalyst is irradiated under UV or visible light that has energy equal to or greater than its band gap [297]. The detailed mechanism of the reaction is shown in Equation (2) to Equation (8). Notably, photocatalytic degradation of organic molecules is carried out in a similar manner [21]. When the photocatalyst is irradiated, electrons are excited from the valence band of the photocatalyst to the conduction band generating electron/hole pairs (e⁻/h⁺), as seen in Equation (2).

Oxygen in water becomes attracted to the positive holes generated by the radiation, and a proton leaves the water molecule, leaving hydroxyl ions adsorbed on the surface, which is shown in Equation (3). It is noted that *X resembles a species absorbed into the hole. Electrons act as reducing agents while positive holes act as oxidizing agents. Electrons reduce the oxygen adsorbed on the surface of the photocatalyst, generating a superoxide radical in Equation (4). Then, a superoxide and a proton react to produce a peroxide radical that is still adsorbed on the surface, and a hydrogen transfer from two peroxides occurs to produce hydrogen peroxide and oxygen (Equations (5) and (6)). Finally, hydrogen peroxide is irradiated to produce hydroxyl radicals in Equation (7), and hydroxyl radicals degrade the organic pesticide to water, carbon dioxide, and other products, depending on the type of pesticide (Equation (8)). Figure 15 illustrates a schematic mechanism for the photodegradation of a pesticide [298].

photocatalyst + hv → h⁺ + e⁻

(2)

h⁺ + H₂O → *OH + H⁺

(3)

*O₂ + e⁻ → *O₂⁻

(4)

*O₂⁻ + H⁺ → *OOH

(5)

2*OOH → *O₂ + H₂O₂

(6)

H₂O₂ + hv → 2 ^.OH

(7)

Pesticide + ^.OH → intermediates → H₂O + CO₂

(8)

Finding the optimum conditions for photocatalysis is extremely important to achieve maximum efficiency of degradation. The recent studies reporting on the photodegradation of different types of pesticides by metal oxide nanomaterials and their composites under UV or visible light have been cited in Table 5. The conditions that correspond to the maximum efficiency of degradation in the studies have been reported.

Several parameters should be considered when carrying out photocatalytic degradation [248]. The nature and type of the photocatalyst, concentration of the photocatalyst, concentration of the pesticide, pH, and irradiation time. Surface morphology, agglomeration, and size affect the behavior of the photocatalyst during the process. Moreover, the higher the concentration of the photocatalyst, the more efficient the degradation [299]. This is a result of having more active sites on the surface of the photocatalyst, thus generating more electron/hole pairs and, consequently, more hydroxyl radicals. However, it is worth mentioning that after very high dosages of the photocatalyst, the efficiency of the reaction decreases due to the blockage of light penetration [300]. Concerning the concentration of the pesticide, at high dosages of the pollutant, most studies reported a decrease in the efficiency of degradation, as reported in Table 4. Increasing the dosage of the pesticide allows for the adsorption of the pesticide on the active sites of the catalyst, preventing the generation of hydroxyl radicals [301]. Depending on the structure of both pesticide and the nano-photocatalyst, the pH can affect the reaction behavior between them. The reaction will be favorable in the pH that allows for the attraction of the photocatalyst and the pesticide, as well as the accelerated production of hydroxyl radicals [302]. The effect of irradiation time is directly proportional to the efficiency of degradation. The increase of irradiation time permits more excitation to occur, and consequently, more radicals are formed [303].

Metal oxide semiconductors, such as ZnO and TiO₂ nanomaterials, are the most appropriate for photocatalytic degradation (Table 4) [298]. This is attributed to the fact that they can produce electron/hole pairs (e⁻/h⁺) more when irradiated with light. Most photocatalysis research focuses on TiO₂ nanomaterials [304,305,306]. The problem with ZnO NPs is the fast recombination of the generated electron/holes [301]. However, recently, it has been discovered that doping the semiconductors with other metals, or further functionalizing them, leads to better separation of charges [307].

Table 5. Reported studies for the photodegradation of pesticides by metal oxide nanomaterials and their composites.

Photocatalyst	Structure	Target Pesticide	Light Source	Conc. of Pollutant	Conc. of Photocatalyst	Irradiation Time (min)	pH	Degradation Efficiency (%)	Ref.
Co3O4/MCM-41 NPs	MCM-41 nanospheres decorated Co3O4.	methyl parathion	visible	100 mg/L	0.25 g	90	8	100	[274]
MCM-41/Co3O4 nanocomposite	Spherical shape.	acephate	visible	100 mg/L	0.25 g	70	8	100	[308]
Co3O4/MCM-41 nanocomposite	MCM-41 spherical grains decorated by Co3O4 NPs.	omethoate	visible	50 mg/L	0.25 g	30	>6.5	100	[309]
Cu/ZnO nanocomposite	Spherical and elliptical.	monocrotophos	visible	0.5 L	0.5 g	180	7	~90	[310]
CuO/TiO2/PANI nanocomposite	CuO/TiO2 spherical NPs embedded in tubular PANI.	chlorpyrifos	visible	5 mg/L	45 mg	90	7	95	[35]
ZnO/CuO nanocomposites	Shape depends on the synthesis conditions.	triclopyr	UV	10 mg/L	0.10 g/150 mL	100	4	100	[311]
CuO NPs	Spherical and flower-like shape.	lambda-cyhalothrin	UV	10 mg/L	3 mg/L	180	7	99	[312]
NiO NPs	Spherical and flower-like shape.	lambda-cyhalothrin	UV	10 mg/L	4 mg/L	180	7	89	[312]
Cu2O/BiVO4 composites	Shape depends on the synthesis conditions.	4-chlorophenol	visible	50 mg/L	5 g/L	240	-	44	[313]
Mn-doped zinc oxide/graphene nanocomposite	Spherical particles distributed onto graphene sheets.	2,4-dichlorophenoxyacetic acid	LED	25 mg/L	2 g/L	120	5	66.2	[235]
WO3 doped ZnO NPs immobilized on glass plates	Heterogenous surface.	2,4-dichlorophenoxyacetic acid	UV	25 mg/L	-	120	7	80	[236]
Nano hydroxyapatite modified CFGO/ZnO nanorod composite	A complex porous surface.	chlorpyrifos	visible	5 mg/L	0.1 g	30	3	100	[302]
WO3 doped ZnO NPs immobilized on glass plates	Heterogenous surface.	diazinon	UV	10 mg/L	10 mg/cm2	180	7	99	[237]
ZnO/rGO nanocomposite	rGO film with agglomerations of ZnO nanosheets.	dimethoate	UV	5 mg/L	50 mg	180	-	~99	[301]
ZnO NPs	Spherical.	monocrotophos	UV	500 mL aq. solution	2 g	120	4	88	[314]
ZnO NPs	-	methyl parathion	UV	-	85 mg/L	100	>9	~70	[315]
ZnO NPs	-	parathion	UV	-	85 mg/L	100	>9	~65	[315]
Cu-doped ZnO nanorods	Nanorods.	diazinon	UV	20 mg/L	0.2 g/L	120	7	96.97	[36]
ZnO nanorods nanorod incorporated carboxylic GR/PANI composite	A complex porous surface.	diuron	visible	5 mg/L	0.1 g	40	3.0	100	[316]
Fe-ZnO nanocomposite	Rough surface due to Fe ions doped in ZnO.	chlorpyrifos	UV	10 mg/L	25 mg/L	60	-	93.5	[317]
Ag-ZnO nanocomposite	Uniform distribution of Ag onto ZnO surfaces.	chlorpyrifos	Sunlight	50 mg/L	20 mg	40	-	~90	[318]
TiO2 NPs	Aggregated semi-spherical.	imidacloprid	UV	100 mg	100 mg/L	20	7.5	88.15	[319]
ZnO NPs	Aggregated semi-spherical.	imidacloprid	UV	100 mg	100 mg/L	20	7.5	~80	[319]
rGO/Fe3O4/ZnO ternary nanohybrid	A complex layered surface.	metalaxyl	visible	10 mg/L	0.5 g/L	120	7	92.11	[320]
La-ZnO-PAN fibers	La and ZnO embedded on PAN nanofibers.	methyl parathion	UV	10 mg/L	50 mg/L	150	<3	100	[321]
La-ZnO-PAN fibers	La and ZnO embedded on PAN nanofibers.	atrazine	UV	10 mg/L	30 mg/L	60	7	98	[299]
rGO/ZnO nanocatalyst	ZnO NPs uniformly distributed on rGO nanosheets.	metalaxyl	UV	10 mg/L	0.75 g/L	120	7	90.25	[322]
Cu-ZnO nanocomposite	Cu loaded on ZnO nanorods.	methyl parathion	UV	500 mg/L	20 mg/L	80	-	99	[323]
ZnO/CeO2 nanocomposite	CeO2 NPs loaded onto ZnO hexagonal nano-carrots.	triclopyr	UV	150 mL aq. solution	100 mg	70	7	83.24	[324]
ZnO nanofilms	Nanoflowers.	temephos	Sunlight simulator	10 mg/L	-	12	-	100	[325]
Fe/Ag@ZnO nanostructures	Nanoflowers.	2,4-dichlorophenoxyacetic acid	UV/visible	62 mg/L	0.078 g/L	63	5	80	[326]
ZnO/TiO2-Fe3O4 nanocomposite	Fe3O4 and TiO2 uniformly distributed on the porous nanostructure of ZnO.	chlorpyrifos	visible	8 mg/L	60 mg	50	10	94.8	[327]
PANI/ZnO-CoMoO4 nanocomposite	Spherical CoMoO4 and ZnO NPs distributed on PANI.	imidacloprid	visible	4.5 mg/L	163.5 mg	180	4	97	[328]
Ag@ZnO nano-stars	Star-like shape.	methyl parathion	visible	0.01 mg/L	25 mg	200	7	-	[329]
Pd@ZnO nano-stars	Star-like shape.	methyl parathion	visible	0.01 mg/L	25 mg	200	7	-	[329]
Cu-ZnO nano heterojunction particles	Cu NPs embedded onto ZnO surface.	chlorpyrifos	sunlight	200 mg/L	250 mg	240	6	91	[330]
Li dope ZnO nanostructures	Aggregated spherical NPs.	triclopyr	UV	100 mL aq. solution	1 g/L	120	7	~50	[331]
ZnO@CdS nanostructures	CdS aggregated spherical NPs and ZnO nanoflowers.	chlorpyrifos	sunlight	2 mg/L	25 mg/L	360	7	91	[300]
ZnO@CdS nanostructures	CdS aggregated spherical NPs and ZnO nanoflowers.	atrazine	sunlight	50 mg/L	20 mg/L	360	7	89	[300]
MgO NPs immobilized on concrete	MgO NPs immobilized on concrete surface.	diazinon	UV	5 mg/L	-	120	7	99.46	[332]
CeO2/TiO2/SiO2 nano-catalyst	Nearly spherical	chlorpyrifos	UV	2 mg/L	0.21 g/L	90	5.4	81.1	[333]
CeO2-SiO2 NPs	-	chlorpyrifos	UV	10 mL aq. solution	7 mg	150	9	~90	[334]
Fe doped CeO2-SiO2 nanocomposite	Spherical NPs	chlorpyrifos	UV	20 mg/L	7 mg	~230	-	81.31	[335]
GO/Fe3O4/TiO2-NiO nanocomposite	Spherical Fe3O4, TiO2, NiO dispersed on GO nanosheets.	imidacloprid	visible	5 mg/L	0.08 g	45	9	97.47	[303]
Au/Fe3O4 core/shell NPs	Spherical	malathion	UV	10 mg/L	10−4 mol/L	90	-	76	[336]
S-doped Ni–Co LDH/Fe3O4 nanocomposite	A layered and flower-like structure with uniformly dispersed spherical Fe3O4 NPs.	chlorpyrifos	visible	2.5 mg/L	60 mg	150	10	92.5	[247]
KIT-5/Bi2S3-Fe3O4 nanocomposite	Spherical Bi2S3 and Fe3O4 NPs uniformly distributed on 3-D mesoporous cubic KIT-5 surface.	parathion	visible	4.5 mg/L	55 mg	55	8	98.7	[337]
GO- Fe3O4/TiO2 nanocomposite	Fe3O4 NPs and mesoporous TiO2 dispersed uniformly on GO nanosheets.	chlorpyrifos	visible	5 mg/L	100 mg	60	~8	97	[304]
KIT-6/WS2-Fe3O4 nanocomposite	Spherical WS2 and Fe3O4 NPs uniformly distributed on 3-D mesoporous cubic KIT-6 surface.	chlorpyrifos	visible	7.2 mg/L	50 mg	52	6	92.1	[338]
Fe3O4/CdS–ZnS nanocomposite	Spherical CdS, Fe3O4 and ZnS NPs.	chlorpyrifos	visible	10 mg/L	0.01 g	60	7	94.55	[339]
Fe3O4@WO3/SBA-15 nanocomposite	Agglomerates of WO3 nanoplates on Fe3O4 NPs and uniform rods of hexagonal SBA-15.	2,4-dichlorophenoxyacetic acid	UV	10−6 mol/L	40 mg	240	-	90.73	[340]
TNP-Pd-Fe3O4/GO photocatalyst	Fe3O4 NPs, Pd, and TiO2 nanoplates were dispersed uniformly on GO sheets.	parathion	visible	10 mg/L	80 mg	40	10	98.5	[341]
BiOBr/Fe3O4 photocatalyst	Agglomerated Fe3O4 NPs deposited on BiOBr microspheres.	glyphosate	visible	100 mg/L	0.08 g	60	-	97	[342]
Ag2S doped nanostructures of Fe3O4 @Ag3PO4 ultrathin films	Ag2S and Fe3O4 NPs doped on Ag3PO4 ultrathin film.	imidacloprid	visible	2 mg/L	30 mg	90	4.3–9	98.9	[343]
Ag2S doped nanostructures of Fe3O4 @Ag3PO4 ultrathin films	Ag2S and Fe3O4 NPs doped on Ag3PO4 ultrathin film.	thiacloprid	visible	2 mg/L	30 mg	60	-	90	[343]
g-C3N4/Cu/TiO2 nanocomposite	Cu and TiO2 NPs dispersed on the irregular layered structure of graphitic-C3N4.	endosulfan	visible	5 mg/L	40 mg	80	6.8	60	[344]
SBA-15/TiO2 nanocomposite	TiO2 NPs dispersed on the hexagonal array of SBA-15.	trifluralin	UV	60 mg/L	0.2 g/L	30	10	90	[345]
SBA-15/TiO2 nanocomposite	TiO2 NPs dispersed on the hexagonal array of SBA-15.	pendimethalin	UV	60 mg/L	0.2 g/L	30	10	82.5	[345]
TiO2 NPs	Irregular agglomerated NPs.	imidacloprid	UV	5 mg/L	0.6 g/L	300	6.35	99	[305]
TiO2 nanostructures modified with Cu	Homogenous nano-porous structure of TiO2 with Cu dispersed on the surface.	imidacloprid	UV/vis	25 mg/L	-	60	-	-	[306]
TiO2/CNT/Pd photocatalyst	Heterostructure spherical Pd-doped TiO2 nanoparticles on carbon nanotubes.	neonicotinoids thiacloprid	sunlight	5 mg/L	0.1 g/L	180	7	100	[346]
TiO2/CNT/Pd photocatalyst	Heterostructure spherical Pd-doped TiO2 nanoparticles on carbon nanotubes.	imidacloprid	sunlight	5 mg/L	0.1 g/L	180	7	99.8	[346]
TiO2/CNT/Pd photocatalyst	Heterostructure spherical Pd-doped TiO2 nanoparticles on carbon nanotubes.	clothianidin	sunlight	5 mg/L	0.1 g/L	180	7	100	[346]
TiO2 nanoparticles	Spherical with only a small quantity of hexagonal diameters.	dimethoate	UV	5 mg/L	300 mg/L	320	-	100	[347]
TiO2 nanoparticles	Spherical with only a small quantity of hexagonal diameters.	methomyl	UV	5 mg/L	300 mg/L	320	-	100	[347]
CuS/TiO2 (CuST) nanoparticles	Coalesced and form a textured/porous nanostructure.	4-chlorophenol	UV	20 mg/L	100 mg	150	-	87	[348]
Pt@TiO2/rGO nanocomposite	Monodisperse quasi-spherical Pt@TiO2 NPs deposited on the rGO nanosheets.	diuron	UV	0.03 mmol/L	7 mg	-	7	100	[349]
(CMC/Tryp/TiO2).	Platelet-like crystallites.	2,4-dichlorophenol	UV	200 mg/L	0.5 g/L	-	-	-	[350]
SBA-16/TiO2 nanocomposites	Rutile phase.	commercial paraquat (PQ) herbicide	UV	50 mg/L	100 mg in 250 mL aq. Solution	1440	-	70	[351]
Ce-TiO2@RGO nanocomposite	Non-uniform deposition of Ce-TiO2 with spherical crystalline TiO2 on a reduced graphene oxide sheet.	quinalphos	Visible	-	20 mg/L	240	-	92	[352]
Ce-TiO2@RGO nanocomposite	Non-uniform deposition of Ce-TiO2 with spherical crystalline TiO2 on a reduced graphene oxide sheet.	imidacloprid	Visible	-	20 mg/L	240	-	85	[352]
Ag3PO4/TiO2 NPs	Crystallized structure with cubed shape Ag3PO4 and anatase TiO2	2,4-dichlorophenoxyacetic acid	Visible	10 mg/L	1 g/L	60	3	98.4	[353]
2D/2D TiO2/MIL88(Fe) (TCS@MOF) nanocomposite	Stacked layer thin MIL-88(Fe) nanosheet with micro-sized TiO2 nano-granular spherical shape.	monocrotophos	visible	20 mg/L	0.05 g/L	30	5	~98.79%	[354]
TiO2 nanotubes	Nanotubes	Simazine	UV	1 mg/L	-	54	-	48	[355]
TiO2 NPs	Agglomerated spherical shape.	Acetamiprid	UV	4.5 mg/L	2000 mg/L	240	-	100	[356]
TiO2 NPs	-	Imidaclopride	UV	25 mg/L	200 mg/L	48	-	90	[357]
TiO2 NPs	-	1,2-dichloroethane	UV	100–200 mg/L	100 mg/L	360	7	95	[358]
N-doped TiO2 nanoparticles	Agglomerated small particles.	dichlorodiphenyltrichloroethane	UV	10,000 mg/L	1000	48	7	70	[359]
lanthanide-doped TiO2 photocatalysts	Solely anatase.	metazachlor	UV	10 mg/L	1000 mg/L	300	-	85	[360]

Table 5. Reported studies for the photodegradation of pesticides by metal oxide nanomaterials and their composites.

Photocatalyst	Structure	Target Pesticide	Light Source	Conc. of Pollutant	Conc. of Photocatalyst	Irradiation Time (min)	pH	Degradation Efficiency (%)	Ref.
Co3O4/MCM-41 NPs	MCM-41 nanospheres decorated Co3O4.	methyl parathion	visible	100 mg/L	0.25 g	90	8	100	[274]
MCM-41/Co3O4 nanocomposite	Spherical shape.	acephate	visible	100 mg/L	0.25 g	70	8	100	[308]
Co3O4/MCM-41 nanocomposite	MCM-41 spherical grains decorated by Co3O4 NPs.	omethoate	visible	50 mg/L	0.25 g	30	>6.5	100	[309]
Cu/ZnO nanocomposite	Spherical and elliptical.	monocrotophos	visible	0.5 L	0.5 g	180	7	~90	[310]
CuO/TiO2/PANI nanocomposite	CuO/TiO2 spherical NPs embedded in tubular PANI.	chlorpyrifos	visible	5 mg/L	45 mg	90	7	95	[35]
ZnO/CuO nanocomposites	Shape depends on the synthesis conditions.	triclopyr	UV	10 mg/L	0.10 g/150 mL	100	4	100	[311]
CuO NPs	Spherical and flower-like shape.	lambda-cyhalothrin	UV	10 mg/L	3 mg/L	180	7	99	[312]
NiO NPs	Spherical and flower-like shape.	lambda-cyhalothrin	UV	10 mg/L	4 mg/L	180	7	89	[312]
Cu2O/BiVO4 composites	Shape depends on the synthesis conditions.	4-chlorophenol	visible	50 mg/L	5 g/L	240	-	44	[313]
Mn-doped zinc oxide/graphene nanocomposite	Spherical particles distributed onto graphene sheets.	2,4-dichlorophenoxyacetic acid	LED	25 mg/L	2 g/L	120	5	66.2	[235]
WO3 doped ZnO NPs immobilized on glass plates	Heterogenous surface.	2,4-dichlorophenoxyacetic acid	UV	25 mg/L	-	120	7	80	[236]
Nano hydroxyapatite modified CFGO/ZnO nanorod composite	A complex porous surface.	chlorpyrifos	visible	5 mg/L	0.1 g	30	3	100	[302]
WO3 doped ZnO NPs immobilized on glass plates	Heterogenous surface.	diazinon	UV	10 mg/L	10 mg/cm2	180	7	99	[237]
ZnO/rGO nanocomposite	rGO film with agglomerations of ZnO nanosheets.	dimethoate	UV	5 mg/L	50 mg	180	-	~99	[301]
ZnO NPs	Spherical.	monocrotophos	UV	500 mL aq. solution	2 g	120	4	88	[314]
ZnO NPs	-	methyl parathion	UV	-	85 mg/L	100	>9	~70	[315]
ZnO NPs	-	parathion	UV	-	85 mg/L	100	>9	~65	[315]
Cu-doped ZnO nanorods	Nanorods.	diazinon	UV	20 mg/L	0.2 g/L	120	7	96.97	[36]
ZnO nanorods nanorod incorporated carboxylic GR/PANI composite	A complex porous surface.	diuron	visible	5 mg/L	0.1 g	40	3.0	100	[316]
Fe-ZnO nanocomposite	Rough surface due to Fe ions doped in ZnO.	chlorpyrifos	UV	10 mg/L	25 mg/L	60	-	93.5	[317]
Ag-ZnO nanocomposite	Uniform distribution of Ag onto ZnO surfaces.	chlorpyrifos	Sunlight	50 mg/L	20 mg	40	-	~90	[318]
TiO2 NPs	Aggregated semi-spherical.	imidacloprid	UV	100 mg	100 mg/L	20	7.5	88.15	[319]
ZnO NPs	Aggregated semi-spherical.	imidacloprid	UV	100 mg	100 mg/L	20	7.5	~80	[319]
rGO/Fe3O4/ZnO ternary nanohybrid	A complex layered surface.	metalaxyl	visible	10 mg/L	0.5 g/L	120	7	92.11	[320]
La-ZnO-PAN fibers	La and ZnO embedded on PAN nanofibers.	methyl parathion	UV	10 mg/L	50 mg/L	150	<3	100	[321]
La-ZnO-PAN fibers	La and ZnO embedded on PAN nanofibers.	atrazine	UV	10 mg/L	30 mg/L	60	7	98	[299]
rGO/ZnO nanocatalyst	ZnO NPs uniformly distributed on rGO nanosheets.	metalaxyl	UV	10 mg/L	0.75 g/L	120	7	90.25	[322]
Cu-ZnO nanocomposite	Cu loaded on ZnO nanorods.	methyl parathion	UV	500 mg/L	20 mg/L	80	-	99	[323]
ZnO/CeO2 nanocomposite	CeO2 NPs loaded onto ZnO hexagonal nano-carrots.	triclopyr	UV	150 mL aq. solution	100 mg	70	7	83.24	[324]
ZnO nanofilms	Nanoflowers.	temephos	Sunlight simulator	10 mg/L	-	12	-	100	[325]
Fe/Ag@ZnO nanostructures	Nanoflowers.	2,4-dichlorophenoxyacetic acid	UV/visible	62 mg/L	0.078 g/L	63	5	80	[326]
ZnO/TiO2-Fe3O4 nanocomposite	Fe3O4 and TiO2 uniformly distributed on the porous nanostructure of ZnO.	chlorpyrifos	visible	8 mg/L	60 mg	50	10	94.8	[327]
PANI/ZnO-CoMoO4 nanocomposite	Spherical CoMoO4 and ZnO NPs distributed on PANI.	imidacloprid	visible	4.5 mg/L	163.5 mg	180	4	97	[328]
Ag@ZnO nano-stars	Star-like shape.	methyl parathion	visible	0.01 mg/L	25 mg	200	7	-	[329]
Pd@ZnO nano-stars	Star-like shape.	methyl parathion	visible	0.01 mg/L	25 mg	200	7	-	[329]
Cu-ZnO nano heterojunction particles	Cu NPs embedded onto ZnO surface.	chlorpyrifos	sunlight	200 mg/L	250 mg	240	6	91	[330]
Li dope ZnO nanostructures	Aggregated spherical NPs.	triclopyr	UV	100 mL aq. solution	1 g/L	120	7	~50	[331]
ZnO@CdS nanostructures	CdS aggregated spherical NPs and ZnO nanoflowers.	chlorpyrifos	sunlight	2 mg/L	25 mg/L	360	7	91	[300]
ZnO@CdS nanostructures	CdS aggregated spherical NPs and ZnO nanoflowers.	atrazine	sunlight	50 mg/L	20 mg/L	360	7	89	[300]
MgO NPs immobilized on concrete	MgO NPs immobilized on concrete surface.	diazinon	UV	5 mg/L	-	120	7	99.46	[332]
CeO2/TiO2/SiO2 nano-catalyst	Nearly spherical	chlorpyrifos	UV	2 mg/L	0.21 g/L	90	5.4	81.1	[333]
CeO2-SiO2 NPs	-	chlorpyrifos	UV	10 mL aq. solution	7 mg	150	9	~90	[334]
Fe doped CeO2-SiO2 nanocomposite	Spherical NPs	chlorpyrifos	UV	20 mg/L	7 mg	~230	-	81.31	[335]
GO/Fe3O4/TiO2-NiO nanocomposite	Spherical Fe3O4, TiO2, NiO dispersed on GO nanosheets.	imidacloprid	visible	5 mg/L	0.08 g	45	9	97.47	[303]
Au/Fe3O4 core/shell NPs	Spherical	malathion	UV	10 mg/L	10−4 mol/L	90	-	76	[336]
S-doped Ni–Co LDH/Fe3O4 nanocomposite	A layered and flower-like structure with uniformly dispersed spherical Fe3O4 NPs.	chlorpyrifos	visible	2.5 mg/L	60 mg	150	10	92.5	[247]
KIT-5/Bi2S3-Fe3O4 nanocomposite	Spherical Bi2S3 and Fe3O4 NPs uniformly distributed on 3-D mesoporous cubic KIT-5 surface.	parathion	visible	4.5 mg/L	55 mg	55	8	98.7	[337]
GO- Fe3O4/TiO2 nanocomposite	Fe3O4 NPs and mesoporous TiO2 dispersed uniformly on GO nanosheets.	chlorpyrifos	visible	5 mg/L	100 mg	60	~8	97	[304]
KIT-6/WS2-Fe3O4 nanocomposite	Spherical WS2 and Fe3O4 NPs uniformly distributed on 3-D mesoporous cubic KIT-6 surface.	chlorpyrifos	visible	7.2 mg/L	50 mg	52	6	92.1	[338]
Fe3O4/CdS–ZnS nanocomposite	Spherical CdS, Fe3O4 and ZnS NPs.	chlorpyrifos	visible	10 mg/L	0.01 g	60	7	94.55	[339]
Fe3O4@WO3/SBA-15 nanocomposite	Agglomerates of WO3 nanoplates on Fe3O4 NPs and uniform rods of hexagonal SBA-15.	2,4-dichlorophenoxyacetic acid	UV	10−6 mol/L	40 mg	240	-	90.73	[340]
TNP-Pd-Fe3O4/GO photocatalyst	Fe3O4 NPs, Pd, and TiO2 nanoplates were dispersed uniformly on GO sheets.	parathion	visible	10 mg/L	80 mg	40	10	98.5	[341]
BiOBr/Fe3O4 photocatalyst	Agglomerated Fe3O4 NPs deposited on BiOBr microspheres.	glyphosate	visible	100 mg/L	0.08 g	60	-	97	[342]
Ag2S doped nanostructures of Fe3O4 @Ag3PO4 ultrathin films	Ag2S and Fe3O4 NPs doped on Ag3PO4 ultrathin film.	imidacloprid	visible	2 mg/L	30 mg	90	4.3–9	98.9	[343]
Ag2S doped nanostructures of Fe3O4 @Ag3PO4 ultrathin films	Ag2S and Fe3O4 NPs doped on Ag3PO4 ultrathin film.	thiacloprid	visible	2 mg/L	30 mg	60	-	90	[343]
g-C3N4/Cu/TiO2 nanocomposite	Cu and TiO2 NPs dispersed on the irregular layered structure of graphitic-C3N4.	endosulfan	visible	5 mg/L	40 mg	80	6.8	60	[344]
SBA-15/TiO2 nanocomposite	TiO2 NPs dispersed on the hexagonal array of SBA-15.	trifluralin	UV	60 mg/L	0.2 g/L	30	10	90	[345]
SBA-15/TiO2 nanocomposite	TiO2 NPs dispersed on the hexagonal array of SBA-15.	pendimethalin	UV	60 mg/L	0.2 g/L	30	10	82.5	[345]
TiO2 NPs	Irregular agglomerated NPs.	imidacloprid	UV	5 mg/L	0.6 g/L	300	6.35	99	[305]
TiO2 nanostructures modified with Cu	Homogenous nano-porous structure of TiO2 with Cu dispersed on the surface.	imidacloprid	UV/vis	25 mg/L	-	60	-	-	[306]
TiO2/CNT/Pd photocatalyst	Heterostructure spherical Pd-doped TiO2 nanoparticles on carbon nanotubes.	neonicotinoids thiacloprid	sunlight	5 mg/L	0.1 g/L	180	7	100	[346]
TiO2/CNT/Pd photocatalyst	Heterostructure spherical Pd-doped TiO2 nanoparticles on carbon nanotubes.	imidacloprid	sunlight	5 mg/L	0.1 g/L	180	7	99.8	[346]
TiO2/CNT/Pd photocatalyst	Heterostructure spherical Pd-doped TiO2 nanoparticles on carbon nanotubes.	clothianidin	sunlight	5 mg/L	0.1 g/L	180	7	100	[346]
TiO2 nanoparticles	Spherical with only a small quantity of hexagonal diameters.	dimethoate	UV	5 mg/L	300 mg/L	320	-	100	[347]
TiO2 nanoparticles	Spherical with only a small quantity of hexagonal diameters.	methomyl	UV	5 mg/L	300 mg/L	320	-	100	[347]
CuS/TiO2 (CuST) nanoparticles	Coalesced and form a textured/porous nanostructure.	4-chlorophenol	UV	20 mg/L	100 mg	150	-	87	[348]
Pt@TiO2/rGO nanocomposite	Monodisperse quasi-spherical Pt@TiO2 NPs deposited on the rGO nanosheets.	diuron	UV	0.03 mmol/L	7 mg	-	7	100	[349]
(CMC/Tryp/TiO2).	Platelet-like crystallites.	2,4-dichlorophenol	UV	200 mg/L	0.5 g/L	-	-	-	[350]
SBA-16/TiO2 nanocomposites	Rutile phase.	commercial paraquat (PQ) herbicide	UV	50 mg/L	100 mg in 250 mL aq. Solution	1440	-	70	[351]
Ce-TiO2@RGO nanocomposite	Non-uniform deposition of Ce-TiO2 with spherical crystalline TiO2 on a reduced graphene oxide sheet.	quinalphos	Visible	-	20 mg/L	240	-	92	[352]
Ce-TiO2@RGO nanocomposite	Non-uniform deposition of Ce-TiO2 with spherical crystalline TiO2 on a reduced graphene oxide sheet.	imidacloprid	Visible	-	20 mg/L	240	-	85	[352]
Ag3PO4/TiO2 NPs	Crystallized structure with cubed shape Ag3PO4 and anatase TiO2	2,4-dichlorophenoxyacetic acid	Visible	10 mg/L	1 g/L	60	3	98.4	[353]
2D/2D TiO2/MIL88(Fe) (TCS@MOF) nanocomposite	Stacked layer thin MIL-88(Fe) nanosheet with micro-sized TiO2 nano-granular spherical shape.	monocrotophos	visible	20 mg/L	0.05 g/L	30	5	~98.79%	[354]
TiO2 nanotubes	Nanotubes	Simazine	UV	1 mg/L	-	54	-	48	[355]
TiO2 NPs	Agglomerated spherical shape.	Acetamiprid	UV	4.5 mg/L	2000 mg/L	240	-	100	[356]
TiO2 NPs	-	Imidaclopride	UV	25 mg/L	200 mg/L	48	-	90	[357]
TiO2 NPs	-	1,2-dichloroethane	UV	100–200 mg/L	100 mg/L	360	7	95	[358]
N-doped TiO2 nanoparticles	Agglomerated small particles.	dichlorodiphenyltrichloroethane	UV	10,000 mg/L	1000	48	7	70	[359]
lanthanide-doped TiO2 photocatalysts	Solely anatase.	metazachlor	UV	10 mg/L	1000 mg/L	300	-	85	[360]

6. Challenges and Outlook

Despite the exquisite properties and the versatile applications of metal oxide nanomaterials and their composites, there are still inadequacies that cannot be ignored as the prepared material should be cost-effective, eco-friendly, and non-toxic. Recently, the use of functionalized metal oxides as adsorbents for the removal of pesticides has been riddled with many challenges. One challenge is the secondary waste produced from the adsorption process, which has not yet been addressed for the use of these materials as recycled materials in industries. Therefore, further studies on the implementation of the recycled metal oxides in the industrial field should be considered. Additionally, most of the published research neglects the fact that the water bodies are contaminated with multi-contaminants. Therefore, investigations should be conducted to assess the efficiency of metal oxides in the presence of multi-pollutants and a real representative matrix. Although the synthesized metal oxides have wide application, much of the research published does not include assessment on the toxicity of the material itself. It is very important to address and examine the toxicity of these materials and their composites, and to employ metal oxides as adsorbents and photocatalytic materials in commercial applications for the treatment of real samples.

7. Conclusions

Metal oxide nanomaterials and their composites have received considerable attention in recent years owing to their wide applications and eminent properties. Their porous structure, thermal stability, low toxicity, easy recovery, and large surface area make them extensively efficient for remediation applications as adsorbents and photocatalytic materials. Many publications have been collected on the removal of organic pesticides such as algaecides, fungicides, herbicides, insecticides, etc., using metal oxide nanomaterials and their nanocomposites, including metal oxides/metal-organic frameworks, metal oxides/polymers, metal/metal oxides other hybridized composites.

From the research reviewed, it can clearly be concluded that the prominent adsorptive interaction between metal oxides and pesticides is chemisorption. This finding is further supported by the type of mechanism and the type of kinetics, as the pseudo-second-order kinetic equation is used to express the chemisorption interaction. Additionally, the π–π interaction, π-complex interaction, and coordination or covalent bond are all types of chemical bonds. The adsorptive removal of pesticides using metal oxides has gained prominence due to its simplicity, effectivity, insensitivity, and flexibility. It has one limitation, in that it produces secondary products which need further recycling, decomposing, and management to be utilized in industries. Accordingly, photocatalytic degradation has emerged alternatively, which results in the complete mineralization of the pollutant to intermediates and H₂O and CO₂. Assessment of material toxicity should be focused on more, along with the by-products of adsorption. To scale up the material on an industrial scale, the investigated materials should be tested in real representative matrices that resemble the contaminated water.