Introduction

Agrochemicals (pesticides) has become an essential aspect of modern agriculture, helping to increase yields and crop growth while also ensuring agricultural output stability1. Pests, diseases, and weeds are estimated to account for roughly 25% of global crop output losses that traditional pesticides could control. However, the pesticide contamination of water sources has recently become a serious environmental issue2.

Organochlorines are among the most widely used pesticides to control pests and diseases carriers. In particular, atrazine stands out as high production and worldwide distribution3. Atrazine was first registered in the United States in 1959 and was used as herbicides to control broadleaf and grassy weeds pre-and post-emergence phases. Therefore, possibly these compounds can be found in the environment. In a previous study, the atrazine concentration detected in the surface water is up to 2.1 µg/L that is higher than the limited standard of 0.1 µg/L 4. Phyu, Warne5 found that atrazine was moderately toxic to tropical freshwater daphnia species (48-h-LC 24.6 mg/L). Lerro, Beane Freeman6 also reported that atrazine could be the main responsible for human disease in Iowa, the USA, which required a lot of time for recovery. Therefore, research on appropriate techniques is necessary to preserve the environment from contaminated water.

To degrade pesticides, diverse techniques, i.e., adsorption7, membrane filtration8, 9, coagulation10, biological methods11, and other methods 12, have been successfully used to eliminate pesticide contaminants from wastewater. However, the majority of these techniques are constrained by costs or operational difficulties. As a result, they could not be used for true agrochemical wastewater treatment for real agrochemical wastewater treatment. Recently, the Advanced Oxidation Processes (AOPs) studied focuses on its organic pollutant removal.

AOPs have successfully been applied to remove persistent organic pollutants through reaction with hydroxyl radicals (OH). Among the AOPs, Fenton technology is broadly used in practice because of its cheap and high efficiency with the low toxicity of its reagent (Fe ions and hydrogen peroxide-HO)13.

In the Fenton process, iron (Fe) catalyzes HO into OH, a powerful oxidant capable of decomposing organic pollutants14. The Fenton reagent effectively mineralizes contaminants using cheap materials. Moreover, both Fe and Fe can act as flocculants that further remove the organic pollutants. Because of its high efficiency and simplicity, the Fenton process is regarded as one of the most appealing AOPs used in wastewater treatment.

However, several challenges remain in conventional Fenton-based technologies, including materials for storing corrosive chemicals, i.e., HO, HSO, NaOH, etc. and massive production of sludge formation. As a result, many efforts have been made to develop new technologies that address these issues while still exploiting the Fenton process's powerful oxidation efficiency.

The electrochemical Fenton technique developed lately from the Fenton process is highly attracted among wastewater treatment to remove toxic organic pollutants. In the electrochemical Fenton process, pollutants are eliminated with the Fenton reagent and cathode oxidation across the anode surface. Anode oxidation is unsuccessful at mineralizing most organic contaminants due to the persistent production of organic acids. On the other hand, the electrochemical Fenton might accomplish a spectacular degrading effect of organic pollutants by producing •OH, as shown in equation 115.

Compared to the traditional Fenton, the electrochemical Fenton process could in-situ HO generation that decreased the chemical usage. It has successfully been applied to eliminating organic contamination from wastewaters like dyeing and textile16, 17, pharmaceutical18, coke19, and pesticides20, 21, 22. Even though EF can reduce the organic compounds in aqueous solutions, studies on pesticide contaminants removal are limited.

Aside from deciding on removal strategies, experimental design plays a vital role in reducing both the time and cost associated with wastewater treatment. Taguchi method, a type of experimental design that based on orthogonal arrays and signal to noise ratio (S/N) qualification23, 24, has succeeded in improving, optimizing, and interpreting the factor effects in many treatment processes from wastewater of textile 17, pulp and paper mill 25, oily23, etc. However, the research literature of Taguchi design on the elimination of pesticides using EF method is still insufficient.

Thus, this study aims to investigate the atrazine elimination of EF technology from aqueous solutions. This EF process study was investigated using a variety of parameters, including electrolysis time, current density, initial pH, sodium sulfate (NaSO) and Fe concentration.

Materials and methods

Experimental setup and analysis

All experiments were conducted in 4 L electro-Fenton batch reactor, as illustrated in Figure 1. The reactor was involved two iron electrodes anode (Fe: 99.25% - CT) and carbon plated iron cathode, with 146 × 150 × 4 mm dimensions linked in parallel. At the same time, the DC power supply (QJ3003XE, 30V - 3A) was linked to the outer electrodes.

The L27 Taguchi design was conducted with signal-to-noise ratio (S/N) to determine the efficiency of the EF treatment. Five independent parameters, e.g., Fe concentration, NaSO concentration, pH, current density and reaction time with three levels, were evaluated as in Table 1. S/N ratio plots have been used to explore the effect of variables on the response by describing the relationship between the response and the variables. The EF process was subjected to multiple response adjustments to establish the best parameters for maximum atrazine removal efficiency. Minitab 18 was used as a statistical program to conduct the study.

Independent factors	Level 1	Level 2	Level 3
pH	3	3.5	4
Current density (mA/cm2)	1.11	2.22	3.33
Fe2+ concentration (mM)	0.05	0.2	1
Sodium sulfate (mg/L)	330	660	990
Reaction time (min)	60	120	180

Before experimenting, compressed air was forced into the cathode at a rate of 2.5 L/min for 15 min and maintained throughout the electrolysis. In each run, atrazine solutions were delivered to the feed tank and reaction reactor. The feed pump controlled the appropriate flow rate while HSO changed the initial pH values.

At various time intervals, samples were obtained from the reactor and instantly examined with a syringe equipped with a 0.2 µm filter. The atrazine concentration was determined using High-Performance Liquid Chromatography (HPLC) with the following parameters: wavelength 224 nm, C18 column, length and diameter of column 4.6 × 250 mm, and injection volume of 20 µL. The percentage of atrazine removal was calculated as follows:

Herein, C is the initial atrazine concentration and C is the atrazine concentration at t reaction time.

Results and discussion

Minitab analysis of the Taguchi design

The effect of the independent factors, i.e., pH, current density, Fe concentration, sodium sulfate, reaction time on atrazine removal, had been investigated. Minitab used the signal-to-noise ratios and means in each level to rank process factors for effective atrazine removal (Table 3).

Level	pH	Current density	Fe2+ concentration	Sodium sulfate	Reaction time
1	33.57	30.16	29.59	29.52	28.71
2	30.12	30.22	31.53	30.43	30.24
3	27.04	30.35	29.62	30.78	31.79
Delta	6.53	0.19	1.94	1.26	3.09
Rank	1	5	3	4	2

Besides, the S/N ratio figure also shows how important the influence parameters are in determining the response. Figure 2 shows the mean of S/N ratios for each parameter category corresponding to its’ level. The response with the highest S/N ratio consistently produces the best results. As a result, pH level 1 (pH 3), current density level 3 (2.22 mA/cm), Fe concentration level 2 (0.2 mM), sodium sulfate level 3 (990 mg/L), and reaction time level 3 (180 min) was the optimal combination of parameters for obtaining the maximum value for S/N for pesticide removal effectiveness during EF process. In a previous study of Zhang, Zhang26 EF process mainly influence by pH and reaction time, who stated that the decomposition of 5000 mg/L of COD from landfill leachate to around 2500 mg/L only within 30 min and obtained 1000 mg/L around 75 min at pH approximately 3.5.

Experiments confirm optimal atrazine treatment efficiency

According to the S/N ratio and predicted regression model, the optimal operation treatment using EF to atrazine removal efficiency could be selected as 1st level of pH value (3), 2nd level of current density (2.22 mA/cm), 2nd level of Fe concentration (0.2 mM), 3rd level of sodium sulfate (990 mg/L) and 3rd level of reaction time (180 min). The serial confirmation experiments of the four factors with these values on various reaction times (30 to 180 min) were also conducted. The results of the confirmation experiments in Figure 3 proved to be consistent with predicted optimal results with the highest atrazine removal efficiency around 76.52% and minimal error variation (1.21-3.54%). The obtained results again demonstrated the excellent accuracy of the proposed model or Taguchi approach.

Conclusion

In this study, the oxidation of atrazine was investigated by electro-Fenton (EF) under various operating conditions with Taguchi design. An atrazine degradation efficiency of 76.52% was obtained when the current density of 2.22 mA/cm, Fe dosage of 0.2 mM, sodium sulfate of 990 mg/L with initial atrazine concentration of 10 mg/L during 180 min reaction. An increase in reaction time and sodium sulfate increase the degradation efficiency, whereas excessive Fe concentration and current density can hamper the atrazine elimination. The initial pH is also an essential factor. A pH of 3 was shown to be the ideal pH for the EF process, resulting in a rapid degradation rate. This study found that the EF method can be utilized to pre-treat atrazine-contaminated wastewater before biological treatment.

ACKNOWLEDGMENTS

This research is funded by Vietnam National University HoChiMinh City (VNU-HCM) under grant number C2019-24-01.

Abbreviation

AOPs: Advanced Oxidation Processes

COD: Chemical Oxygen Demand

DC: Direct current

EF: Electro-Fenton

S/N: Signal/Noise

Conflict of interest

The authors declare that there is no conflict of interest regarding the publication of this article

Authors’ contributions

Huynh Ngoc Loan, Hoan Dinh Duong and Bui Manh Ha have made substantial contributions to the work reported in the manuscript.