Advanced oxidation processes for chlorpyrifos removal from aqueous solution: a systematic review

Abstract

Chlorpyrifos (CPF), an organophosphate insecticide, due to its high efficiency and low cost is widely used in the agricultural industry. CPF may lead to lung deficiency, central nervous system damage, developmental and autoimmune disorders. In recent decades, the advanced oxidation processes (AOPs) have been considered in water and wastewater treatment due to their high efficiency in decomposition of organic and inorganic compounds, specially hardly biodegradable or non-biodegradable compounds. In the present review study, the most common AOPs (such as Fenton and Photo-Fenton processes, UV/H₂O₂ photolysis, UV/TiO₂ heterogeneous photo catalysis, electrochemical processes, sonolysis technology, gamma irradiation technology and sulfate-based AOPs) applied for CPF removal from aqueous matrices has been investigated. It can be concluded that the use of AOPs are effective for CPF removal from aqueous media. In addition, Fenton and photocatalytic processes appear to be the most common techniques for CPF degradation.

Introduction

In order to increase the agricultural productivity, pesticides (a group of insecticide compounds) are widely used for pest control [1, 2]. Pesticides enter the water bodies through different ways, including agricultural runoff, leaching via the soil, and in some cases through directly addition into the surface water such as dam or ponds [3]. The organophosphate (OP) pesticides are one of the most commonly agricultural pesticides that introduced in the 1970s [4, 5]. OPs, as extremely toxic compounds, are not readily biodegradable [1, 6], and may enter the food chain through accumulation in water resources and agricultural crops [7, 8]. Chlorpyrifos (CPF), an organophosphorus compound and one of the top five commercial insecticides, is widely used in developing countries [9, 10]. CPF is used to control a wide range of insects and crop pests [4]; however, it can be toxic and harmful to different types of arthropods such as bees, ladybird beetles and parasitic wasps [6]. Furthermore, CPF presence in water bodies can harmfully affect aquatic life such as fish, invertebrates, and plants [11, 12]. CPF pesticide has been detected in the aquatic environments because of its widespread use in agriculture. Related studies have reported varying amounts of CPF in water resources [13–15]. According to the environmental conditions, CPF has a half-life varying from 16 h to 77 days in freshwater bodies [16]. Furthermore, CPF is strongly absorbed by the soil and can remain between 60 and 120 days [17]. Accordingly, the degradation of CPF in the environment is becoming a public concern. A recent study in China estimated 10% annual growth in CPF usage worldwide [18]. It has been reported that exposure to CPF can cause lung system damage, embryonic malformations and DNA damage, influence on the mental development in children, immunological disorders and effect on male reproductive system [19, 20]. It has also been shown that exposure to CPF can enhance the growth of breast tumors and be a risk factor for developing breast cancer [21]. Headache, nausea, muscle twitching and convulsions and in some extreme cases even death, have been reported as the main outcomes of acute poisoning with this pesticide [6, 22–24].

Different technologies such as flocculation, filtration, precipitation, adsorption, ozonation, or biological treatment have been offered for pesticides removal from aqueous solutions. These techniques, in addition to high cost and the problem of sludge production, are ineffective against the extent of pesticide contamination [25]. Advanced oxidation processes (AOPs), as innovative water and wastewater treatment technologies, are considered for the removal of low concentrations of toxic and refractory pollutants such as pesticides [26–28]. In fact, AOPs are the techniques that degrade organic pollutants through in situ production of the hydroxyl or sulfate radicals [29, 30]. Sulfate based AOPs are widely accepted due to the higher redox potential (2.5–3.1 V) and longer half-life (30–40 μs) of sulfate radicals than hydroxyl radicals (20 ns) [31]. Also, the activity of sulfate radicals does not depend on pH. Hence, sulfate based AOPs are very effective in organic pollutant degradation. Sulfate radicals are generally produced from peroximonosulfate (PMS) or persulfate (PS) [32, 33].

Figure 1 shows the classification of various AOPs including chemical, photochemical, sonochemical and electrochemical reactions [34]. In addition, AOPs can be classified as homogeneous or heterogeneous. Homogeneous AOPs are divided into two categories; the processes that use different forms of energy such as ultraviolet radiation, ultrasonic or electrical energy; and the processes that do not use energy to degrade pollutants, such as ozonation in alkaline medium or ozonation with H₂O₂ [35]. Heterogeneous AOPs use catalysts such as metallic species, metal oxide or organometal species to remove contaminants [36, 37]. This paper, presents a systematic review of the literature to appraise the most common AOPs used for the removal of CPF pesticide from aqueous matrices.

Fig. 1.

The classification of advanced oxidation processes (AOPs)

Methods

Search strategy

A comprehensive literature search pertaining to advanced oxidation processes for the removal of CPF pesticide in aqueous medium was conducted. In this regard, relevant studies were retrieved from international databases of Google Scholar, Scopus, PubMed, ProQuest, Springer, and Science Direct. In this review, we searched articles that were published until September 2020. The search strategy was developed using key words such as advanced oxidation process, CPF pesticide, water, wastewater, CPF removal, degradation, and treatment. Besides, to increase the search sensitivity and selection of greater number of studies, additional records were identified by hand searching and review of the referenced list of retrieved papers. In the first step 2578 articles were retrieved from international databases and by limiting to CPF pesticide, the number of 826 documents was selected for evaluation. In the next step, by reviewing titles and abstracts many articles were omitted. Finally, 19 articles were selected for detailed consideration in this study. Figure 2 shows the search and analyzing procedures.

Fig. 2.

Flow diagram of the literature review process

Chemical structure of the CPF

CPF (O, O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate) contains an aryl ring and a pentavalent phosphorus atom with a double bond to sulphur (P=S). Chemical structure of CPF is shown in Fig. 3. Also, the key physicochemical properties of CPF are listed in Table 1 [2, 9, 38, 39]. According to this Table, CPF has low solubility in water, while its octanol water partitioning is high that tend to centralize on lipid portion of the membrane [40].

Fig. 3.

Three-dimensional space (a) and two-dimensional figure (b) representations of the chlorpyrifos molecule

Table 1.

Physicochemical properties of CPF

Property	Value
Empirical formula	C9H11Cl3NO3PS
Melting point	42 °C
Boiling point	160 °C (decomposes)
Molecular mass	350.6 g/mol
Vapor pressure	2.49 × 10−3 Pa at 25 °C
Solubility in water	1.4 mg/L at 25 °C
Log KOW	5
Density	1.398 g/cm3
Odor	Mercaptan-like

Degradation of CPF pesticide by AOPs

CPF is known as an organic pollutant with low biodegradability, so conventional biological treatment processes are not able to degrade it efficiently. On the other hand, CPF is soluble in organic solvents which are high miscibility with water, so coagulation-sedimentation processes also are not efficient for its degradation [41]. In recent years, AOPs have shown high potential for removal of non-biodegradable pollutants and convert them into harmless by-products; therefore, these processes can be considered as an effective and promising method to eliminate pollutants, including pesticides. AOPs work based on the generation of hydroxyl radicals (oxidation potential of 2.72 V) that are accountable for the degradation of most organic pollutants in aqueous media [42]. However, sulfate radical-based oxidation processes, as an innovative option, have attracted more attention than conventional AOPs [42]. In this paper, a review of the literature to evaluate the most common AOPs used to degrade CPF pesticide in aqueous media is presented.

Fenton (Fe²⁺/H₂O₂) process

Fenton reaction was introduced for the first time by Fenton in 1894 [43]. Fenton oxidation is a combination of H₂O₂ and Fe(II)/Fe(III) that produces highly reactive hydroxyl radicals to degrade organic pollutants into more degradable compounds [44, 45]. The Fenton process is more effective at acidic pH; thus, mainly due to the presence of iron (as a catalyst) and hydrogen peroxide (as an oxidizing agent), the process is strongly depends on the pH of the solution [46]. This process has also been proved to be effective in mineralizing organic contaminants and converting them to non-toxic CO₂ [47]. Gandhi et al., reported that under optimum conditions of 100 mg/L H₂O₂ and 5 mg/L Fe²⁺, the removal rate of CPF by Fenton process was 15.1% after 240 min of reaction time. CPF degradation rate constant in this study was reported as 6.8 × 10⁻⁴ min⁻¹ [25]. The results of another study indicated that complete degradation of CPF under optimum operating conditions (initial pH 3, H₂O₂/Fe²⁺ molar ratio 10, H₂O₂/COD molar ratio 3) occurred in 1 min. The CPF spectrum demonstrated absorption bands from 1549 to 968 cm⁻¹ due to the stretch of C=N, stretch of pyridine, ring vibration, ring breathing, stretch of Cl = C, trigonal ring breathing and stretch of P=S [48]. In a similar study, Saini and Kumar examined the optimization of CPF degradation by Fenton oxidation using CCD and ANFIS computing techniques. One of the important parameters that affected the Fenton performance was the initial pH. The optimum conditions for the best reduction of CPF and COD were achieved at pH = 3, H₂O₂ = 0.571 mol/L, and Fe²⁺ = 3 g/L [49]. Table 2 summarizes the findings regarding CPF degradation using the Fenton process.

Table 2.

Summary of the Fenton processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 1 mg/L	H2O2 = 100 mg/L	Removal efficiency (240 min)	GC-MS	[25]
	Fe2+ = 5 mg/L	CPF = 15.1%
	pH = 3.5–6.5	k = 6.8 × 10−4 min−1
CPF = 100 mg/L	H2O2: COD = (3:1)	Removal efficiency (1 min)	HPLC and FTIR	[48]
COD = 1130 mg/L	H2O2:Fe2+ = (10:1)	CPF = 100%	spectra
	pH = 3	Removal efficiency (60 min)	HPLC	[49]
	T = 40 °C	COD = 69.03%
CPF = 30 mg/L	BOD5/COD ratio ~ 0	TOC = 55.61%
COD = 385 mg/L	pH = 3	Reduction efficiency (60 min)
	T = 25 ± 2 °C	CPF = 94%
	H2O2 = 0.571 mol/L	COD = 83.51%
	Fe2+ = 3 g/L

Photo-Fenton (UV/Fe²⁺/H₂O₂) process

In the Photo-Fenton process, one of the effective methods for oxidation of pollutants, hydroxyl radicals formed from the combination of H₂O₂ and UV radiation with Fe²⁺ or Fe³⁺ ions increase the rate of decomposition. This process performs better at acidic conditions (about pH 3). The use of sunlight instead of UV irradiation in the Photo-Fenton process, due to the lack of need for an energy source, is very cost-effective, however, the degradation rate is slower compared to the former. It is noteworthy that the Photo-Fenton process is more efficient than the Fenton process [50]. Table 3 reports the degradation of CPF by Photo-Fenton process in aqueous solutions. Gandhi et al., [25] reported that the Photo-Fenton reaction is efficient for CPF degradation (∼50% CPF removal). In their study, a UV lamp (radiation at ranging from 200 to 400 nm) with an average pressure of 400 W was used as a light source. Several degradation byproducts of CPF were detected using GC-MS as follows: 2,4 Diamino 6 (3,4-chlorobenzyl] 5-methylthieno [2,3-d] pyrimidine, Diethyl, 3,5,6, Trichloro 2 pyridinyl ester, Chlorpyriphos, Phosphorothioic acid o o Diethyl, o [3,5,6, Trichloropyridinyl] ester, diethyl [3,5,6, Trichloro 2 pyridinyl] ester, 7,9 di-tert-butyl-1-oxaspiro (4,5) and deca- 6,9-diene-2,8-dione. The rate constant (k) for CPF degradation was reported as 3.3 × 10⁻⁴ min⁻¹ [25]. Affam et al., studied the removal efficiency of combined CPF, cypermethrin and chlorothalonil pesticides in aqueous solution by the UV Photo-Fenton process. The results showed that complete destruction of the pesticides occured after 1 min, under optimum conditions (H₂O₂/COD molar ratio: 2, H₂O₂/Fe²⁺ molar ratio: 25 and pH = 3). Furthermore, the release and mineralization of organic carbon and nitrogen was observed using this treatment process. In addition to a high degree of oxidative degradation of CPF (∼64% TOC reduction), the biodegradability (BOD₅/COD) was increased from 0 to 0.38 [51]. In a similar study, complete removal of CPF was obtained after 15 min by a solar chamber as light source under the optimum conditions (H₂O₂ dose 0.01 M, Fe³⁺ dose 10 mg/l and initial pH of 3.5). Also, it was found that the principal by-product identified from degradation of CPF was chlorpyrifos-oxon at a concentration of less than 80 μg/L, which is more toxic than CPF [52].

Table 3.

Summary of the Photo-Fenton processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 1 mg/L	UV Photo-Fenton	Removal efficiency (30 min UV exposure) CPF = 50.3%	GC-MS	[25]
	Fe2+:H2O2 ratio = 0.05	k = 3.3 × 10−4 min−1
CPF = 100 mg/L		complete degradation of the pesticides in 1 min	HPLC	[51]
COD = 1130 mg/L	UV Photo-Fenton
	H2O2: COD = (2:1)	Removal efficiency (60 min)
	H2O2:Fe2+ = (25:1)	COD = 78.56%
	pH = 3	TOC = 63.76%
CPF = 30 mg/L	solar Photo-Fenton	complete degradation of CPF in 15 min	GC-MS	[52]
	H2O2 = 0.01 M
	Fe3+ = 10 mg/L
	pH = 3.5

UV/H₂O₂ photolysis process

In the UV/H₂O₂ photolysis, the target compounds absorb UV photons and the released energy causes the light-induced oxidation process [53]. In this process, hydrogen peroxide helps for hydroxyl radicals formation and leads to the oxidation of organic compounds [53]. Oliveira et al., investigated the effect of anions interference on CPF removal from aqueous solution using UV/H₂O₂. In this study, a tubular germicidal fluorescent lamp (UV-C, 8 W, 253.7 nm) was used as light source. The presence of chloride, nitrate, phosphate and bicarbonate anions in the aqueous solution had no effect on CPF removal. In other words, the presence of these anions did not interfere with CPF degradation. Solution pH and temperature also did not significantly affect the process. In addition, the results showed that by increasing the CPF concentration from 50 to 150 mg/L at pH 8 and 1.5 g/L H₂O₂, the pseudo first order rate constant (k) decreased by approximately 40% (from 0.408 to 0.242 min⁻¹) [54]. In another study, the role of hydroxyl and carbonate radicals was investigated in the UV/H₂O₂ process during the degradation of parathion and CPF from aqueous solution. The results showed that addition of H₂O₂ at lower concentrations increased the degradation rate of both pesticides; but at higher concentrations, it could not affect the overall reaction rates. On the other hand, carbonate and bicarbonate ions in aqueous solution reduced the reaction rate by inhibiting hydroxyl radicals. It was also reported that the second order rate constant between hydroxyl radicals and CPF (k_OP) and the second order rate constant between carbonate radicals and CPF (k_OP,_CO3) were 4.9 ± 0.1 × 10⁹ M⁻¹ s⁻¹ and 8.8 ± 0.4 × 10⁶ M⁻¹ s⁻¹, respectively [23]. Gandhi et al., [25] reported that in the UV/H₂O₂ process in a solution containing 100 mg/L H₂O₂, complete degradation of CPF was achieved after approximately 240 min. The rate constant (k) of CPF degradation was 7.8 × 10⁻³ min⁻¹. The results of GC-MS analysis also showed that less toxic substances and byproducts were formed compared to other degradation processes, which include: 7,9 di-tertbutyl-1-oxaspiro (4,5) deca- 6,9-diene-2,8-dione, Diethyl, 3,5,6, Trichloro 2 pyridinyl ester (after 30 min of exposure) and 7,9 di-tert-butyl-1-oxaspiro (4,5) deca- 6,9-diene-2,8-dione, phosphorothioic acid [25]. In a similar study, Femia et al., reported that CPF was completely eliminated in 60 min under optimum conditions (450 mg/L of H₂O₂, 15 mg/L of CPF and natural pH). In this study, the photodegradation of CPF was performed in a cyclic reactor with UV radiation (20 W, 253.7 nm). TOC analysis indicated significant mineralization of the contaminant (achieving to 70% conversion after 4 h) [55]. Table 4 shows the results of studies related to CPF degradation by this process.

Table 4.

Summary of the UV/H₂O₂ photolysis processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 50–150 mg/L	H2O2 = 1.5 g/L	pseudo first-order rate	HPLC	[54]
	pH = 4–10
	T = 30–45 °C
CPF = 2.1 μm	H2O2 = 0–50 mg/L	second-order rate constants	HPLC	[23]
	pH = 7	kOP = 4.9 ± 0.1 × 109 M−1 s−1
	UV light (254 nm)	kOP, CO3 = 8.8 ± 0.4 × 106 M−1 s−1	GC-MS	[25]
CPF = 1 mg/L	H2O2 = 100 mg/L	Removal efficiency (120 min)
		CPF = 79.8%
		k = 7.8 × 10−3 min−1
CPF = 15 mg/L	H2O2 = 450 mg/L	complete degradation of CPF before 60 min
TOC = 20 mg/L	pH = natural	Removal efficiency (240 min) TOC = 70%
		k = 0.127 min−1	HPLC	[55]

UV/TiO₂ heterogeneous photo catalysis process

The heterogeneous photo catalysis, due to its particular characteristics (non-toxicity, easy operational conditions and low operational cost), has been considered as an effective and executable treatment technology [8]. Titanium dioxide (TiO₂) is a strong oxidizer and an efficient semiconductor that because of its advantages such as low cost, low toxicity, and chemical stability has been employed as a photo catalyst. Some limitations of TiO₂ include non-porosity, low adsorption ability especially for non-polar organic compounds, and restrictions on the use of TiO₂ in bulk [56]. In the UV/TiO₂ process, excited high-energy states of electrons and hole pairs produced in the reaction. These holes, through the production of hydroxyl radicals, can destruct organic compounds. The hydroxyl radicals produced in this process convert toxic and non-biodegradable organic pollutants into relatively harmless compounds such as CO₂, H₂O and mineral acids. Therefore, TiO₂ under UV light irradiation is an excellent photo catalyst for the degradation of CPF [57, 58]. Table 5 reports the degradation of CPF by UV/TiO₂ process in aqueous solutions. Photodegradation studies by Amalraj et al., [6] for monocrotophos and CPF pesticides were performed with a photo reactor using TiO₂ and UV irradiation (16 W, 251 nm). The results showed that with increasing the illumination time, the reaction efficiency increased. It was also found that the maximum degradation of both pesticides occurred at pH = 5 (due to the electrostatic interaction between the positive charges of TiO₂ surface and the negative charges of monocrotophos and CPF) and 100 mg of TiO₂. In this study, the photodegradation of both pesticides was consistent with pseudo-first order kinetics [6]. In another study, the degradation of CPF in agriculture runoff was examined using TiO₂ solar photocatalytic at circumneutral pH. The efficiency of CPF removal under the optimum condition (dosage of TiO₂ 15.72 mg/L, concentration of CPF 2.74 ppm and time of reaction 62.5 min) was 84.01%. The results showed that increasing the dose of TiO₂ (up to the concentration of 40 mg/L TiO₂) increased the removal efficiency of CPF, which attributed to the increases of UV light photons and the number of CPF molecules absorbed on the catalyst surface. It was also reported that by increasing time, the percentage of pesticide degradation improves [58].

Table 5.

Summary of the UV/TiO₂ processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 2–10 mg/L	UV/TiO2	pseudo first-order	UV–vis	[6]
	Catalyst dosage = 1 g/L	Removal efficiency (∼60 min) CPF ≥ 98%	spectrometer
	pH = 5	k = 2.2761 mg/L/min
		KOP = 0.0277 L/mg
CPF = 2.74 mg/L	TiO2 + solar radiation	Removal efficiency (62.5 min) CPF = 84.01%	GC-ECD	[58]
	pH = natural
	TiO2 = 15.72 mg/L
	UV solar energy = 697 ± 5.33 lx

Electrochemical processes

Electrochemical processes can be defined as the conversion of toxic compounds into biodegradable products or the complete combustion of organic compounds into CO₂. These technologies include electrochemical, sonoelectrochemical, photoelectrochemical processes and surface and bulk oxidation processes that can produce hydroxyl radicals using direct or indirect electrochemical oxidation. The main features of these techniques are being environmentally friendly and no production of new toxic wastes [59]. Samet et al., examined the electrochemical process under galvanostatic polarization mode using niobium Nb/PbO₂ anodes and graphite carbon bar as cathode for oxidation of CPF in aqueous solution. The results showed that increasing the apparent current density from 10 to 50 mA/cm², produced more hydroxyl radicals, resulting further elimination of COD. As shown in Table 6, in the optimum conditions for the PbO₂ to degrade CPF, 76% of COD was removed after 10 h electrolysis. Temperature had a significant effect on COD removal, where increasing temperature from 20 °C to 70 °C, COD removal increased from 41% to 76%. COD removal was performed following a pseudo second order kinetics. It was also observed that the values of instantaneous current efficiency (ICE) decreased when electrolysis or oxidation of CPF in dilute solutions prolonged [59]. In another study, the novel Ti/IrO₂-SnO₂ anode was used for electrochemical degradation of CPF in chloride free water. In this study, cyclic voltammetry and open circuit potential (OCP) were accomplished to evaluate the electrochemical active surface and anodes stability. Under the optimal conditions (0.3 M [Ir], 7.5 mM [Sn] and 6 h reaction time), OCP showed the highest stability of −0.64 V (Table 6). Relatively high mineralization (55.55%) was also reported. Therefore, Ti/ IrO₂-SnO₂ anode could be used as a cost-effective method for CPF degradation [60].

Table 6.

Summary of the electrochemical processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
COD = 450 mg O2/L	Nb/PbO2 electrodes	pseudo second order	Spectrophotometer	[59]
	pH = 2	Removal efficiency (10 h)
	T = 70 °C	COD ∼76%
	current density = 50 mA/cm
CPF = 274 mg/L	Ti/IrO2/SnO2 anode	Removal efficiency (6 h)	TOC analyzer	[60]
	[Ir] = 0.3 M	CPF = 55.56%
	[Sn] = 7.5 mM
	T = 25 °C
	current density = 20 mA/cm

Sonchemical processes

The sonochemical process, as a developed treatment technology, can form hydroxyl radicals, and oxygen and hydrogen atoms to eliminate organic pollutants [61]. The ultrasonic action has high efficiency of water purification and can effectively remove recalcitrant organic pollutants. The ultrasound frequencies are between 20 kHz and 1 MHz which is higher than the realms of human hearing. [62, 63]. Some of the advantages of ultrasound technology include: Operation at ambient temperature and pressure conditions, no production of any secondary pollutants, no need for additional chemicals, suitable from the economic viewpoint, less safety problems and faster remediation rate, not affected by the biodegradability and toxicity of compounds and compatible with other biological or physical processes [63]. In the ultrasound process, hydroxyl radicals are produced through the acoustic cavitation, so that both temperature (~ 5000 K) and pressure (~ 500 bar) increase in the bubbles (because of transient collapse of cavitation bubbles) and lead to the formation of hydroxyl radicals [64, 65]. Table 7 shows the results of studies related to CPF degradation by this process. Agarwal et al., reported that under optimum conditions (initial concentration of CPF 1 mg/L, frequency 130 kHz, electric power 500 W and initial pH = 9), 98.96% of CPF was degraded within 20 min by ultrasound technique. In addition, the results of the study demonstrated that CPF degradation decreases with increasing the initial concentration of contaminant and decreasing the contact time and frequency. However, the rate of pesticide removal was not affected by pH [66]. Zhang and et al., investigated CPF and diazinon degradation by ultrasonic irradiation in aqueous solution [14]. They reported that all three parameters (ultrasonic power, temperature and pH) examined, affected the degradation of CPF and diazinon pesticides. The highest percentage of degradation of both pesticides was observed at pH 7 and 25 °C. Pesticides degradation also improved with increasing electric power. CPF degradation products that identified by GC–MS included chlorpyrifos oxon and TCP (3,5,6-trichloro-2-pyridinol). The oxidation of P=O and hydrolysis were proposed as the predominant degradation pathways. Toxicity assessment revealed that in the treated effluent the toxicity of solution containing CPF was increased, while it was decreased in case of diazinon [14].

Table 7.

Summary of the sonchemical processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 1 mg/L	pH = 9	Removal efficiency (20 min) CPF = 98.96%	GC–FID	[66]
	Frequency = 130 kHz
	Electric power = 500 W
CPF = 1.4 mg/L	T = 25.0 ± 1.0 °C	Removal efficiency (60 min) CPF = 85%	GC–MS	[14]
	pH = 7
	Frequency = 20 kHz
	Electric power = 900 W
	T = 25 °C

Gamma irradiation technology

The radiation process is known as a cost-effective and eco-friendly process in water and wastewater treatment [67]. Hence, this process is considered as an efficient technology for degradation of organic compounds [68]. Gamma irradiation technology uses reactive species (according to the eq. 1) to degrade toxic or resistant organic pollutants [69, 70].

$${\mathrm{H}}_2\mathrm{O}\stackrel{\gamma }{\to }{{\mathrm{e}}_{\mathrm{aq}}}^{-}+{\mathrm{OH}}^{•}+{\mathrm{H}}^{•}+{\mathrm{H}}^{+}+{\mathrm{H}}_2+{\mathrm{H}}_2{\mathrm{O}}_2+{{\mathrm{OH}}_{\mathrm{aq}}}^{-}$$ 1

In the irradiation process, there are two pathways to destroy the organic pollutants, which includes (A) oxidation by OH^• and (B) reduction by e⁻(aq) (hydrated electron). The species of oxidative OH^• and the species of reductive e⁻(aq) are produced in situ by water radiolysis [71]. The advantages of the gamma radiation process include the following: suitable for the degradation of organophosphorus pesticides in low concentrations [1], used at room temperature, no need to add harmful chemicals [72], high efficiency, proper radiation penetration and no residuals generation [71]. Hossain et al., investigated the degradation of CPF in aqueous solution using ⁶⁰Co γ-rays and natural sunlight. By comparing the efficiency of gamma-ray irradiation and natural sunlight in CPF degradation, it was found that CPF removal using sunlight was more efficient than gamma radiation. Gamma-ray induced degradation showed that the degradation process occurs faster at low concentrations of CPF than at higher concentrations. Also, the absorbed dose and the percentage of CPF removal were directly related. On the other hand, the CPF degradation under direct sunlight in distilled water and lake water showed that under the same duration and light intensity, the percentage of CPF removal in the former (39.5%) was less than the latter (51.95%) [73]. In other study, which was conducted on a laboratory scale, Ismail et al., [2] investigated the removal of CPF from aqueous solution by irradiation using ⁶⁰Co γ-rays. According to their findings (Table 8) complete degradation of 500 μg/L CPF by gamma irradiation was observed at an absorbed dose of 575 Gy. It was also reported that CPF degradation increased with the addition of 4 mM H₂O₂, but decreased with increasing the H₂O₂ concentration to 50 mM. In this study, the main pathway of CPF degradation was reported through the oxidative hydroxyl radicals [2].

Table 8.

Summary of the Gamma irradiation technology for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 5–20 mg/L	60Co γ-rays and natural sunlight T = 30 °C	Highest degradation rate: using sunlight in distilled water = 4.2% per day, in lake water = 7.4% per day 12 kGy using gamma (CPF = 5 mg/L) (∼100%)	HPLC	[73]
CPF = 500 μg/L	60Co γ-rays	pseudo-first order	SPME–GC– ECD	[2]
	Dose rate = 300 Gy/h	Absorbed dose of 575 Gy
		Removal efficiency CPF = 100%

Sulfate-based AOPs

AOPs are based on free radicals formation. Among these processes, sulfate radical based AOPs, due to their higher oxidation potential, longer half-life, and more selectivity than hydroxyl radicals, have significantly been considered in recent years. In addition, after oxidation, sulfate radicals are converted to non-toxic sulfate that do not require any further treatment [74, 75]. As shown in Table 9, Zhou et al., evaluated the degradation efficiency of CPF by heat activated persulfate oxidation process. The results showed that CPF degradation increased with increasing temperature and initial concentration of persulfate, but not affected by pH. In addition, the presence of HCO₃⁻ had a destructive role in CPF oxidation, while Cl⁻ ions had no effect on the process. The oxidation products of CPF identified by LC-MS are as follows: chlorpyrifod oxon; O, O-diethyl O-5,6-dichloropyridin-2-yl phosphate; O-5,6-dichloropyridin-2-yl phosphate; and O-5- hydroxyl-6-chloropyridin-2-yl phosphate. The oxidation of P=S to P=O bond, dechlorination, dealkylation, and dechlorination-hydroxylation were proposed as the predominant degradation pathways. It was also reported that the P=S bond is the best site for oxidation of the molecules and the formation of products with P=O structure increases the toxicity of the solution. In this study, heat activated persulfate was introduced as an efficient method for remove of CPF from aqueous solutions [76]. The study of nano zerovalent zinc (nZVZn) catalyzed peroxymonosulfate (PMS) based advanced oxidation technologies for treatment of CPF in aqueous solution showed that under optimum conditions ([CPF]₀ = 10 mg/L, [nZVZn]₀ = 1 g/L, and flow rate = 0.2 L/min), nZVZn removed 55% of CPF at 90 min; while under the same conditions, the PMS catalyzed with nZVZn removed 99.5% of CPF. It was also found that electron transfer, replacement, hydroxylation, bond cleavage, and oxidation reactions led to the production of degradation products. In addition, the generation of non-toxic final DPs indicated that this process is an effective and environmentally friendly method for remove of CPF (Table 9) [77].

Table 10.

Summary of the different AOPs processes for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 1 μM	T = 25 °C pH = 7 [Fe(VI)]0: [CPF]0 (100:1)	Second-order kinetic Removal efficiency (300 s) CPF = 100%	HPLC	[10]

Table 9.

Summary of the sulfate-based reactions for the removal/degradation of CPF in aqueous solution

Initial concentration	AOP features	Kinetic data	Detection Method	Ref
CPF = 2 mg/L	T = 70 °C	pseudo-first-order	UPLC	[76]
Persulfate = 90 mM	reaction time = 180 min pH = 3–11
CPF = 10 mg/L	flow rate = 0.2 L/min	pseudo-first order	GC–MS	[77]
[PMS]0 = 40 mg/L	[nZVZn]0 = 1 g/L pH = 2.5	Removal efficiency CPF = 99.5%

Other AOPs

Iron-based AOPs can be considered because of the lower toxicity of iron than other metals [78]. Liu et al., investigated the oxidation potential of ferrate(VI) in the removal of CPF from water and wastewater samples. CPF was completely degraded under optimum conditions (CPF initial concentration of 1 μM, [Fe(VI)]₀:[CPF]₀ ratio = 100:1, temperature 25 °C and pH = 7) within 5 min. The results showed that Ca²⁺ and Mg²⁺ cations as well as humic acids prevented the degradation of CPF, while other cations and anions including Fe³⁺, Cu²⁺, NH₄⁺, Cl⁻, SO₄²⁻, NO₃⁻, and HCO₃⁻ had no effect on the removal efficiency. The most important mechanisms of reaction between CPF and ferrate included hydroxyl substitution reaction, cleavage of C–O bonds, and the oxidation of P=S bonds. Liu et al., concluded that Fe(VI) with redox potential of 0.7–2.2 V is an efficient technology for degrading CPF from aqueous solutions [10].

Comparison of advanced oxidation processes

AOPs are defined as aqueous phase oxidation processes, which usually by the production of hydroxyl or sulfate radicals, leads to degradation of the target pollutants. When the biological treatments are unfeasible or inefficient, these processes are the best option for destroying pollutants [79]. In recent years, researchers have used different AOPs in a defined experimental situation to remove a variety of pollutants, including pesticides. Fenton and Fenton-like reactions have two main limitations, including high production of iron sludge and slow reduction of ferric ions (Fe³⁺) by H₂O₂. These processes perform better at acidic conditions. In the Fenton and Photo-Fenton processes, hydrogen peroxide is oxidized to produce hydroxyl radicals. The factors affecting the production of these radicals are: (1) pH, (2) Fe²⁺ dose, (3) H₂O₂ initial concentration, and (4) the ratio between organic loads and H₂O₂ [80]. In general, the Photo-Fenton process is more efficient than the Fenton process. The H₂O₂/UV process is more suitable for groundwater and drinking water treatment, because unlike ozone-based AOPs, it does not produce carcinogenic bromate ions. The rate of H₂O₂ photolysis is pH dependent and increases under alkaline conditions. The limited formation of hydroxyl radicals due to the relatively small molar adsorption of H₂O₂ at 254 nm, is the main disadvantage of this method. Another point is that direct UV photolysis of pollutants should also be considered depending on the chemical structure of the compounds [43]. Heterogeneous TiO₂ photocatalytic degradation is generally very efficient for treating a substantial range of inorganic and organic pollutants, such as pesticides. In heterogeneous photocatalysis, TiO₂ is irradiated with 300–400 nm of ultraviolet light and the produced hydroxyl radicals destroy the pollutants. Easier operating conditions, non-toxicity and lower operating costs are the advantages of this process. In addition, pH is an important factor in the efficiency of photocatalysis and according to the type of pollutant being treated should be optimized before the process [8, 81]. Electrochemistry is an effective method for the production of hydroxyl radicals in situ without the addition of chemical reagents or high proportions of catalysts. Some of other advantages of this method include rapid destruction of organic contaminants, prevention of new toxic species, complete mineralization of organic contaminants and low energy costs [81]. Ultrasounds have been widely used for the oxidation and degradation of organic pollutants in aqueous environment. There is no need to add chemicals in this technique, therefore it is very cost-effective. The point to consider about sonochemical AOPs, is that the number of hydroxyl radicals produced during the process is usually insufficient [63, 81]. Hence, the efficiency of the process can be enhanced by adding external oxidants such as H₂O₂ [82]. The irradiation process is also a cost-effective process for water treatment that has a high efficiency for pollutant removal [67, 72]. Sulfate radical based advanced oxidation processes have high chemical oxidation potential. Therefore, these processes can be used to remove resistant and non-biodegradable organic matter, including pesticides [83, 84].

Table 11 shows the advanced oxidation processes covered in this study for CPF pesticide degradation. Table 12 also provides a comparison of different AOPs for CPF removal from aqueous solutions. In recent years, various advanced oxidation processes have been used to remove the organophosphorus pesticides. It seems that photocatalytic and Fenton processes appear to be the most common techniques for destroying CPF. However, Sulfate radical based AOPs can be considered as future technologies due to their efficiency and environmentally friendly properties. It is noteworthy that different AOPs may be combined in various manners to maximize the efficiency in removing organic pesticides.

Table 11.

Advanced oxidation processes covered in this review for CPF degradation

AOPs	Reactions	Oxidant(s)	Other chemical(s)
Fenton	Fe2+ + H2O2 → Fe3+ + OH− + OH	H2O2	Fe2+
Photo-Fenton	Fe2+ + H2O2 + hν → Fe3+ + OH− + OH• Fe(OH)2+ + hν → Fe2+ + OH Fe(OOCR)2+ + hν → Fe2+ + CO2 + R	H2O2	Fe2+ or Fe3+
UV/H2O2	H2O2 + hν → 2OH	H2O2	None
UV/TiO2	TiO2 + hν → TiO2(e− + h+) h+ + H2O → OH• + H+ e− + O2 → O2−•	TiO2	None
Electrochemical processes (anodic oxidation)	H2O → HO• + H+ + e−	electrons	None
Sonchemical processes (ultrasonic)	H2O → OH• + H• H2O → 2OH	Ultrasonic waves	None
Gamma irradiation	H2O → eaq−(0.27) + H• (0.06) + OH• (0.28) + H2(0.05) + H2O2(0.07) + H3O+(0.27) H• + O2 → HO2• eaq− + O2 → O2•	Gamma irradiation	None

Table 12.

Comparison of different AOPs for the removal of CPF in aqueous solution

Characteristics	Fenton	Photo Fenton	UV/ H2O2	UV/ TiO2	Electrochemical processes	Sonchemicl processes	γ-ray	Sulfate based AOPs
Cost-effective				✔	✔	✔
High efficiency							✔	✔
Ease of implementation			✔	✔
High adaptability	✔	✔		✔				✔
Good flexibility			✔
No need to additional chemicals						✔	✔
High modification potential			✔
Proper performance at ambient temperature						✔		✔
Environmental friendly					✔			✔
High energy efficiency	✔	✔

Conclusion

This study provides a review of different AOPs for CPF removal from aqueous solutions. In a general conclusion, AOPs are a powerful treatment method for degradation of refractory and/or toxic pollutants in aqueous matrix, although their performance depends on the various parameters of each process. It should be noted that with increasing the initial concentration of pollutants the efficiency of AOPs decreases. The Photo-Fenton process is much more efficient than the Fenton process. In addition, using sunlight instead of UV irradiation in the Photo-Fenton process is very cost-effective, but it takes longer to completely eliminate CPF. The average pooled percentage of electrochemical processes for CPF pesticide degradation was 65.77%. The percentage of CPF degradation is improved by increasing the catalyst dose in the heterogeneous photo catalysis process. Sonochemical and gamma irradiation processes are very efficient in eliminating CPF pesticide under optimal conditions. However, sulfate-based AOPs, as an innovative technology, have been considered by researchers in recent years.

Authors contribution

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by [Samira Sheikhi], [Reza Dehghanzadeh] and [Hassan Aslani]. The first draft of the manuscript was written by [Samira Sheikhi] and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

This study was supported by Tabriz University of Medical Sciences.