Integrated electrodialysis/electro-oxidation process for the treatment of chlorpyrifos wastewater with high energy efficiency

High-salt and difficult-to-degrade wastewater in surface water, groundwater, and soil and other environmental media with a long residual cycle and biological toxicity will not only aggravate the deterioration of the water environment but also accelerate the process of desertification and salinization of the land. Not only that, the high complexity of pollutants in wastewater, including nutrients, microplastics, and pharmaceuticals, means that the study of wastewater in the process of desalination of organic pollutants and salt migration laws is proposed to optimize the effect of wastewater treatment programs. This is not only conducive to alleviating the status quo of surface water and groundwater pollution in China, but also improving the aquatic ecological environment. The prevention and control of pollution have a significant positive significance on the smooth implementation of the war (Cui et al. 2024; Guo et al. 2024; Tian et al. 2024a, b). As modern industry develops, various industrial waste streams containing toxic and bio-refractory pollutants cause serious environmental problems. Chlorpyrifos is an organic pollutant representative for the pesticide effluents, which are characterized by high ecotoxicity and low biodegradability (Kennedy & Mackie 2018; Abdi et al. 2020). Moreover, the manufacturing wastewater of chlorpyrifos contains numerous compounds such as organophosphate, chlorpyrifos, sodium trichloropyridinol, and sodium chloride. The chlorpyrifos wastewater with high concentrations of salts and organic compounds causes serious wastewater treatment issues all around the world (Robles-Molina 2012; Mir-Tutusaus et al. 2018; Sharma & Kakkar 2018). In general, the conventional biological degradation process is unsuitable and inefficient for the treatment of wastewater in the presence of high concentrations of salts and organic compounds (Chen et al. 2011a, b; Vilar et al. 2012; Yang et al. 2013). Technologies, such as evaporation crystallization, have been used as a conventional technology in the treatment of wastewater with high salinity. However, the evaporation process is not effective in crystallizing salt for organic wastewater because of the presence of a high concentration of organic pollutants (Fernandez-Torres et al. 2012; Lu et al. 2017). Therefore, an appropriate pretreatment process is urgently required for the effective degradation of chlorpyrifos manufacturing wastewater.

Based on the two points of the footprint of wastewater treatment (Borzooei et al. 2020a, b; Goodarzi et al. 2022) and the necessity of natural solutions, there are many novel processes in wastewater treatment methods. For example, biological nests made from modified basalt fiber (MBF bio-nests) effectively enhance wastewater treatment (Wang et al. 2024a, b). In addition, as a new type of ‘green and clean’ wastewater treatment process, the unique advantages of the electrochemical process for treating complex, high-salt, and difficult-to-biodegrade industrial wastewater will receive wider attention as people pay more and more attention to environmental issues and the government's requirements for environmental standards are constantly improved. Electrochemical oxidation is one of the most promising technologies, especially for the treatment of wastewater containing bio-refractory pollutants (Martínez-Huitle et al. 2015; Feng et al. 2016; Moreira et al. 2017; Garcia-Segura et al. 2018). The ^·OH radicals generated on the electroactive sites of the electrode assist in the oxidation of organic pollutants (Comninellis 1994). The effectiveness of electrochemical degradation mainly relies on the nature of the electrodes, the construction of the electrochemical reactor, and the electrolysis conditions (Chen et al. 2011a, b; Zhao et al. 2014; Sopaj et al. 2015). Different types of anodes including RuO₂ (Fukunaga et al. 2008; Bagastyo et al. 2011), SnO₂ (Chen et al. 2018), PbO₂ (Lin et al. 2012; Chen et al. 2014), and boron-doped diamond have received considerable attention for the electro-oxidation (EO) of organic compounds (Pereira et al. 2012; Fernandes et al. 2014). For the enhancement of the electrochemical oxidation processes, the microwave (Gao et al. 2009), ultrasound (Srivastava 2022), and other physical methods have been explored to reach a higher oxidation efficiency (Wei et al. 2011; He et al. 2018). However, the additional energy consumption of microwave or ultrasound systems is high, and the electrochemical reactor is not easy to scale up.

Electrodialysis requires high pretreatment of raw water, and scaling problems may arise during the water treatment process, which can affect the efficiency of electrodialysis (Wang et al. 2024a, b). The combination of electrodialysis and EO technology enables the electrical energy generated during EO to be captured and used for electrodialysis, which can effectively remove electrolyte ions and organic pollutants from the solution, reduce energy consumption, and improve overall energy efficiency. Electrodialysis is an electro-membrane process that is used for the separation and concentration of inorganics in wastewater (Chen et al. 2020; Vineyard et al. 2020; Zhou et al. 2020). A combination of electrodialysis and EO processes has been designed to treat wastewater containing low concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) (Llanos et al. 2018). The ionic 2,4-D was concentrated at the electrode by the combination process, enhancing the performance of EO (Raschitor et al. 2020). Nevertheless, a high concentration of salts can be adsorbed at the electrode surface, competing with the organics for active sites on the electrode. This competition has a negative effect on the EO process (Scialdone et al. 2009; García-Espinoza et al. 2018).

In this work, an integrated electrodialysis/electro-oxidation (IEDEO) system was created for the effective pretreatment of manufacturing wastewater of chlorpyrifos, providing a new idea for the simultaneous removal of bio-refractory organics and inorganics, which has important significance for the research and application on the treatment of pesticide wastewater with high concentrations of salts and organic compounds.

The chlorpyrifos wastewater was obtained from the manufacturing process of a pesticide factory, which is located in Jiangsu, China. The properties of the manufacturing wastewater are presented in Table 1. The membranes were provided by Yanrun Membrane Tech. Corp. in Beijing. The cation-exchange membrane (CEM) has a thickness of 0.17 mm, an electric resistance of 4.0 Ω cm², and an ion-exchange capacity between 1.6 and 2.0 mol kg⁻¹. The anion-exchange membrane (AEM) has a thickness of 0.18 mm, an electrical resistance of 5.0 Ω cm², and an ion-exchange capacity between 1.5 and 1.8 mol kg⁻¹. The type of membrane we chose is an electrodialysis special membrane, which is characterized by strong resistance to contamination, high densification, and selective permeability. In addition, this special membrane is also resistant to oxidation, acid and alkali corrosion, and organic contamination, which is very suitable for chlorpyrifos wastewater experiments.

For comparison, the single EO process for the treatment of chlorpyrifos wastewater (5.0 L) was carried out by the electrodes (0.05 m²) in the electrolysis cell connected with an external tank (i.e., no membrane stack was packed). The wastewater was pumped into the electrolysis cell at a flow rate of 80 L h⁻¹, allowing continuous recirculation.

The evaporation system consists of an electric heating evaporator and a water-cooled condenser. About 2.0 L of the chlorpyrifos wastewater pretreatment by the EO process and the concentrated solution from the IEDEO process were added into the evaporator, respectively. The evaporation temperature was maintained at 85 °C for both tests. The wastewater evaporates and then passes through the condenser to form desalted water. Reproducibility validation experiments were carried out 24 times, which is the same as the above. The stepwise method schematic and flowchart of IEDEO are presented in Figure S1.

Preliminary experiments were performed for the IEDEO process in order to characterize the effect of operating parameters including initial pH and constant voltage on the chlorpyrifos wastewater treatment.

As shown in Figure 2(b), a lower pH value gives a lower COD concentration in the concentrate compartment during the IEDEO process. This can be explained by the fact that ionic organic pollutants can undergo protonation reactions at low pH values, resulting in the formation of non-charged species and consequently decreasing the conductivity of the system (Martí-Calatayud et al. 2020). Hence, the transfer of ionic organics from the chlorpyrifos wastewater to the concentrate by electro-migration is decreased at low pH. Similarly, the transfer of TS from the chlorpyrifos wastewater to the concentrate was reduced at low pH (as observed in Figure 3(a) and Figure 3(b)), which means the slower electro-migration of pollutants at initial pH value of 5.0 when compared to initial pH values of 7.0 and 9.0.

As shown in Figures 4(a) and 5(a), increasing cell voltage had a positive influence on the COD and TS removal efficiency in the chlorpyrifos wastewater treatment by the IEDEO process. The increase in voltage would increase the applied current due to the ohmic relationship, which leads to higher production of oxidizing species (e.g. ·OH, ClO⁻, H₂O₂) that originated from water electrolysis. As the voltage potential increases (Figures 4(b) and 5(b)), the driving force of electromigration also increases, enhancing the transport of the ionic pollutants across the ion-exchange membrane from the chlorpyrifos wastewater to the concentrate. This finding aligns with reports from many previous works that use electrodialysis for the transfer of pollutants (Zhang et al. 2011; Han et al. 2017).

In addition, the longevity of the electrode decreases as the voltage increases. When the voltage is too low, the current is small, the electrochemical reaction is insufficient, and the catalytic activity of the electrode is reduced, leading to a decrease in the treatment efficiency. Although the electrode loss may be relatively small, the overall treatment effect is poor. When the voltage is too high, the current is high, and the electrochemical reaction on the surface of the electrode becomes more violent, leading to the rapid loss of electrode materials. In particular, anode materials are prone to dissolve and corrode under high voltage, thus shortening the service life of the electrode, which is consistent with literature reports (Li et al. 2024). Based on the above studies, we chose the voltage of 13 V for subsequent degradation experiments

Before IEDEO tests, membrane adsorption was carried out in the electrodialysis cell without applied current. The dilute and electrolysis compartments were filled with the chlorpyrifos wastewater for 48 h. The COD removal was only 1.1 ± 0.5% in membrane adsorption. That is to say only adsorption by exchange membranes has little effect on the removal of COD under the experimental condition.

Moreover, Figure 9(c) describes the variations of the biodegradability index (BOD₅/COD) in the chlorpyrifos wastewater during different processes. The BOD₅/COD could be used to analyze the biodegradability of industrial wastewater (Parama Kalyani et al. 2009). The initial BOD₅/COD is very low (0.08 ± 0.01), which means that the original chlorpyrifos wastewater is resistant to biodegradation, resulting in poor biodegradability. It is clear that the BOD₅/COD increases with electrolysis time, indicating that the biodegradability of chlorpyrifos wastewater can be improved by the electrochemical oxidation of bio-refractory organics. After 2 h of electrolysis, the BOD₅/COD of the chlorpyrifos wastewater in the EO process only increased to 0.13 ± 0.02 from the initial value of 0.08 ± 0.01. It has been reported that high salinity could also inhibit the activity of microorganisms, resulting in a limited biodegradation of the wastewater (Singlande et al. 2006). Thanks to the excellent performance of the IEDEO process in simultaneously removing bio-refractory organics and inorganics, the BOD₅/COD ratio of the chlorpyrifos wastewater in the IEDEO process reached 0.28 ± 0.03, which is beneficial for subsequent biological treatment.

GC–MS was used to identify the organic pollutants in the chlorpyrifos wastewater during different electrolysis processes, and the results are shown in Table 2. It is observed that the original chlorpyrifos wastewater contained many organic compounds including chlorpyrifos, N,N-Dimethyl-4-aminopyridine, N,N-Diethylbenzylamine, 2-Hydroxy-3,5,6-trichloropyridine, chlormephos, and O,O-diethyl phosphoramidothioic acid. Almost all of them are bio-refractory organics. The original chlorpyrifos wastewater showed a low biodegradability index. After 2 h of electrolysis, some of the above-mentioned organics in the chlorpyrifos wastewater were removed and not detected by GC–MS analysis, and some degradation intermediates were formed during the electrolysis processes. It is seen that the organic pollutants and degradation intermediates in the EO process are seriously accumulated. However, refractory organics in chlorpyrifos wastewater were reduced by the IEDEO process after 2 h. Some ionic organics could be transferred to the concentrate solution. Hence, the electrochemical treatment of chlorpyrifos wastewater was carried out more efficiently by the IEDEO process in contrast with the EO process.

Table 2

Main organic compounds in the chlorpyrifos wastewater analyzed by GC–MS after 2 h of electrolysis

Compounds .	Molecular structure .	Influent .	EO .	IEDEO .	IEDEO concentrate .
Chlorpyrifos		√	√	√	–
N,N-Dimethyl-4-aminopyridine		√	√	√	–
N,N-Diethylbenzylamine		√	√	√	–
2-Hydroxy-3,5,6-trichloropyridine		√	√	–	√
Phosphoramidothioic acid, O,O-diethyl ester		√	√	–	√
Benzaldehyde, 4-benzyloxy-3methoxy-2-nitro-		√	√	–	–
Chlormephos		√	√	–	–
O,O,S-triethyl phosphorothioate		–	√	––	––
Benzyl chloride		–	√	––	––
9-Octadecenamide		–	√	√	––
3-Ethyl-3-methylheptane		–	√	√	––
2,4-Di-tert-butylphenol		–	√	√	√
Maleamic acid		–	–	√	√
(Z)-Butenedioic acid		–	–	√	√
Propionic acid		–	–	–	√

Compounds .	Molecular structure .	Influent .	EO .	IEDEO .	IEDEO concentrate .
Chlorpyrifos		√	√	√	–
N,N-Dimethyl-4-aminopyridine		√	√	√	–
N,N-Diethylbenzylamine		√	√	√	–
2-Hydroxy-3,5,6-trichloropyridine		√	√	–	√
Phosphoramidothioic acid, O,O-diethyl ester		√	√	–	√
Benzaldehyde, 4-benzyloxy-3methoxy-2-nitro-		√	√	–	–
Chlormephos		√	√	–	–
O,O,S-triethyl phosphorothioate		–	√	––	––
Benzyl chloride		–	√	––	––
9-Octadecenamide		–	√	√	––
3-Ethyl-3-methylheptane		–	√	√	––
2,4-Di-tert-butylphenol		–	√	√	√
Maleamic acid		–	–	√	√
(Z)-Butenedioic acid		–	–	√	√
Propionic acid		–	–	–	√

‘√’, detected; ‘–’, not detected.

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Besides, the TS and COD of the concentrate solution during the IEDEO process were, respectively, 83.5 ± 2.1 g/L and 4,280 ± 150 mg/L after 2 h. The TS and COD of the chlorpyrifos wastewater in the EO process were, respectively, 48.5 ± 0.9 g/L and 16,500 ± 160 mg/L after 2 h. Evaporation crystallization is one of the most common methods to desalt wastewater with high salinity. About 2.0 L of the chlorpyrifos wastewater pretreatment by the EO process and concentrated solution from the IEDEO process were added into the evaporator, respectively. As evaporation increased, the concentration of dissolved salts in the wastewater, the scale formation accelerated. The wastewater evaporates and then passes through the condenser to form desalted water. After evaporation, the scale contained dissolvable salts in crystals that were residual in the evaporator. SEM and XRD analysis were performed to detect the structural features and composition of the scale, and the results are shown in Figures S2 and S3. The scales evaporated from the concentrated solution of the IEDEO process were more regular and dense (ss shown in Figure S2a). By contrast, the scales generated from the evaporated wastewater by the EO pre-treatment process were irregular and their particulates were loosely connected with obvious impurities, which were caused by high concentrations of organic pollutants (Figure S2b). According to the XRD analysis (Figure S3), both types of morphology were indicative of sodium chloride (NaCl) crystals. There are also many impurity peaks of the scales evaporated from the wastewater pretreated by the EO process, pointing out again that the pre-treatment performance of the EO process for the chlorpyrifos wastewater is poor. Furthermore, the desalted water by evaporation from the IEDEO concentrate is better than that from the wastewater pretreated by the EO process (as shown in Table 3). Therefore, the IEDEO process also provides us with a valuable pretreatment technique for evaporative desalination treatment.

The integrated electrodialysis/EO system was successfully developed for the electrochemical pretreatment of chlorpyrifos wastewater. The IEDEO process provides new ideas for the simultaneous removal of bio-refractory organics and salts. The initial pH 5.0 favors electrochemical degradation of the wastewater when compared to initial pH values of 7.0 and 9.0. Increasing cell voltage had a positive influence on COD and TS removal efficiency during the IEDEO process in our case. Both the COD removal and TS removal of the chlorpyrifos wastewater obtained by the IEDEO process were significantly higher than those reached with the EO process. The total mass of COD removed during the IEDEO process at 2 h was about two times higher than that of the EO process, and the EC of the IEDEO process decreased by 60–65% when compared to the EO process. The BOD₅/COD of the chlorpyrifos wastewater in the IEDEO process reached 0.28 ± 0.03, which is beneficial for subsequent biological treatment. However, it is still in the small-scale pilot stage. The next step will be to carry out scaled-up experiments and pilot studies for IEDEO treatment of actual chlorpyrifos wastewater based on the optimized operating conditions and process parameters established in this work. The synergistic effect of IEDEO, evaporation, and biochemical degradation on practical wastewater with high salt and organic concentrations will be further investigated in the future.

This work was supported by the National Key Research and Development Program of China (No. 2023YFE0100900) and the Science and Technology Planning Project of Jiangsu Provincial Environmental Protection Group (No. JSEP-GJ20220009-RE-ZL and JSEP-TZ-20221001-RE).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.