Degradation of atrazine in river sediment by dielectric barrier discharge plasma (DBDP) combined with a persulfate (PS) oxidation system: response surface methodology, degradation mechanisms, and pathways

Single degradation systems based on dielectric barrier discharge plasma (DBDP) or persulfate (PS) oxidation cannot achieve the desired goals (high degradation efficiency, high mineralization rate, and low product toxicity) of degrading atrazine (ATZ) in river sediment. In this study, DBDP was combined with a PS oxidation system (DBDP/PS synergistic system) to degrade ATZ in river sediment. A Box–Behnken design (BBD) including five factors (discharge voltage, air flow, initial concentration, oxidizer dose, and activator dose) and three levels (− 1, 0, and 1) was established to test a mathematical model by response surface methodology (RSM). The results confirmed that the degradation efficiency of ATZ in river sediment was 96.5% in the DBDP/PS synergistic system after 10 min of degradation. The experimental total organic carbon (TOC) removal efficiency results indicated that 85.3% of ATZ is mineralized into CO2, H2O, and NH4+, which effectively reduces the possible biological toxicity of the intermediate products. Active species (sulfate (SO4•−), hydroxy (•OH), and superoxide (•O2−) radicals) were found to exert positive effects in the DBDP/PS synergistic system and illustrated the degradation mechanism of ATZ. The ATZ degradation pathway, composed of 7 main intermediates, was clarified by Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC–MS). This study indicates that the DBDP/PS synergistic system is a highly efficient, environmentally friendly, novel method for the remediation of river sediment containing ATZ pollution.


Introduction
Atrazine (ATZ) is a persistent dryland broad-spectrum herbicide that effectively inhibits the photosynthesis of grasses (Vieira et al. 2021). In recent years, ATZ has been extensively used in agriculture, forestry, fruit cultivation, and other economic cultivation areas (Gaffar et al. 2021;Yang et al. 2021;Rastegar-Moghaddam et al. 2019). ATZ enters river sediment from the water due to its lipophilic and hydrophobic characteristics, causing pollution to the river sediment as well as potential risks to humans (Qu et al. 2018). Considering the strong biological toxicity, stability and degradation resistance of ATZ in river sediment make a lot of treatment methods limited Labianca et al. 2022;Zhang et al. 2021a;Chao et al. 2022). Hence, a highly efficient and environmentally friendly treatment method needs to be developed to treat ATZ in river sediment. In view of efficient and environmentally friendly considerations, advanced oxidation processes (AOPs), including hydroxy radicals (•OH), superoxide radicals (•O 2 − ), sulfate radical (SO 4 −• ) based advanced Responsible Editor: Ricardo A. Torres-Palma oxidation processes, and photocatalysis, are often used for ATZ removal (Ding et al. 2022;Roy et al. 2022;Wang et al. 2022;Xu et al. 2022).
In recent years, dielectric barrier discharge plasma (DBDP) technology as a novel AOP has been widely considered for treating water (Wang et al. 2021b), air (Dong et al. 2022a, b), and soil pollutants (Jiang et al. 2022). DBDP produces many physical agents and radicals, such as ultraviolet (UV), shock waves, heat, hydroxy radicals (•OH), and superoxide radicals (•O 2 − ) Mai-Prochnow et al. 2021). The principle is to use the introduction of high pressure to achieve strong degradation of pollutants (Hatzisymeon et al. 2021). However, the pollutant removal capacity of DBDP is limited in a short period of time, which is one of the important problems in river sediment treatment by DBDP. The discharge voltage continues to increase; it can cause the waste of energy and increase the safety risk (Zhang et al. 2021c). Hence, a synergistic treatment system based on DBDP should be adopted to achieve excellent degradation results. On the basis of realizing energy saving through the collaborative system, the degradation efficiency of ATZ in river sediment is improved.
The persulfate (PS) oxidation system is widely used as an effective remediation method for organic pollutants and is an advanced oxidation technology (Ike et al. 2018). PS mainly includes ammonium persulfate, potassium persulfate, and sodium persulfate (SPS). SPS was selected as the oxidizer in the PS oxidation system in this study in consideration of its high solubility and low cost (Wu et al. 2021), which trigger a lot of scholars to study (Hayat et al. 2022;). Analogously, due to low consumption and high availability, ferrous sulfate (Fesulf) is a commonly used activator for SPS (Xiao et al. 2022). Moreover, UV and heat can be used as PS activators to further increase the production rate of radicals without the addition of further chemicals, which has attracted the attention of an increasing number of scholars (Shang et al. 2019;Tang et al. 2018). Coincidentally, the DBD generator is accompanied by high temperature and UV generation during the discharge process, which contribute to produce a large number of strongly oxidizing radicals during degradation in the PS oxidation system, such as sulfate (SO 4 • − , E 0 = 2.5-3.1 V) and hydroxy (•OH, E 0 = 2.8 V) radicals (Pan et al. 2018;Zuo et al. 2020). These radicals are promising and environmentally friendly candidates (Soltani et al. 2018). In some cases, SO 4 •− has better selectivity and a longer half-life than •OH (Qiu et al. 2019).
DBDP can activate PS to achieve highly efficient and environmentally friendly degradation of organic pollutants in river sediment, which has not been fully investigated. Based on the above context, this study proposes the use of a DBDP/PS synergistic system to degrade ATZ in river sediment. The optimal degradation efficiency of ATZ was investigated, and the results revealed the degradation mechanism, mineralization rates, possible degradation pathways, and biotoxicity of ATZ intermediates in the DBDP/PS synergistic system. The ultimate objective is to develop a reference for the industrial application of DBDP/PS synergistic systems for the highly efficient remediation of organic pollutants in river sediments.

Materials
Atrazine (ATZ) was purchased at a purity of 98% from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). HPLC-grade methanol (MeOH) was purchased from the United States Tedia Co., Ltd. (Anqing, China). All other chemicals, including sodium persulfate (SPS) and ferrous sulfate (Fesulf), were of analytical grade and were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

ATZ degradation experiments
The details of pretreatment and physicochemical property analysis of the river sediment samples are given in supporting information (SI)-1, Tables S1 and S2. The simulated ATZ sediment pollution process is presented in (SI)-2. Figure 1 shows that the DBDP/PS synergistic system as a whole is composed of eight parts, including a plasma discharge power source and dielectric barrier discharge (DBD) generator. The internal structure of the DBD generator is shown in Fig. S1. River sediment (5 g), oxidizer (SPS), and activator (Fesulf) are put into the DBD generator together, and the discharge voltage, carrier gas (air) flow, and initial concentration of ATZ in the river sediment are adjusted. The method of DBDP/PS synergistic degradation of ATZ is shown in (SI)-3.

PS oxidation system
The PS oxidation test was mainly divided into five processes: river sediment sampling, sediment pretreatment, simulation of ATZ pollution, river sediment preparation, and oxidation degradation. The detailed experimental procedures are presented in (SI)-4.

Analysis of ATZ degradation efficiency
The ATZ degradation effects of the DBDP system, PS oxidation system, and DBDP/PS synergistic treatment system were evaluated by the organic pollutant degradation efficiency η. The residual concentration of ATZ in river sediment was analyzed by high-performance liquid chromatography (HPLC) (Chromaster, Hitachi, Japan), and the degradation efficiency of ATZ was calculated. The ATZ degradation efficiency was calculated by Eq. (1): where C 0 (mg/L) is the initial concentration of ATZ and C t (mg/L) is the concentration of ATZ after treatment. The method of quantitative analysis of ATZ in river sediment is shown in (SI)-5.

Analysis of ATZ degradation mechanisms
In this work, the major species of radicals in the DBDP/PS synergistic system were assessed using electron paramagnetic resonance (EPR) (Bruker EMX PLUS, Bruker, Germany). The degradation mechanism of ATZ was analyzed from the viewpoint of radical action.

Analysis of ATZ degradation pathways
A Fourier transform infrared spectrometry (FTIR) instrument (Nicolet IS 50 + Continuum Fourier, Thermo Fisher, USA) was used to analyze and compare the changes in functional groups and chemical bonds in the contaminated river sediment before and after DBDP/PS synergistic treatment. Gas chromatography-mass spectrometry (GC-MS) (7890A, Agilent, USA) was used for qualitative analysis of substances in the synergistic degradation system and to determine the possible degradation intermediates of ATZ.

Analysis of TOC removal and evaluation of degradation toxicity
A total organic carbon (TOC) analyzer (Multi N/C 3100, Analytik Jena, Germany) was used to determine the variation in the TOC content in river sediment in different systems after different degradation times, which indicated the mineralization ability of the different systems for organic pollutants. In addition, the biotoxicity of the precursor (ATZ) and degradation intermediates in the DBDP/PS synergistic system were evaluated by quantitative structure-activity relationship (QSAR) prediction by applying the program Toxicity Estimation Software Tool (T.E.S.T.).

Analysis of the DBDP system
The effect of discharge voltage on ATZ degradation in the DBDP system Discharge voltage is usually considered the primary factor affecting the degradation efficiency of pollutants in DBDP systems Liu et al. 2019). Figure 2A shows that the ATZ degradation efficiency increased with applied voltage. The discharge voltage was controlled at 5, 7, and 9 kV, and the degradation efficiency of ATZ in the DBDP system reached 40.1, 55.8, and 67.3%, respectively. The results further illustrate that a better degradation effect can be achieved at high voltages. Moreover, higher voltages can effectively increase the number of radicals and electrons, and various physical effects (UV and heat) are produced in the DBDP system (Guo et al. 2022;Machala et al. 2018). Therefore, in the DBDP system, ATZ is more likely to be

The effect of air flow on ATZ degradation in the DBDP system
Considering the safety of the experiment, air was selected as the carrier gas for DBDP system operation in this study. The air flow was controlled at three levels: 0, 4, and 8 L/min. Under different air flows, the ATZ degradation efficiency showed a trend of first increasing and then decreasing. The highest degradation efficiency of ATZ was 76.6% at 4 L/min when the other conditions remained unchanged (Fig. 2B). This phenomenon indicates that air flow is an important reference factor in DBDP systems. Low air flow results in an insufficient amount of the discharge medium (water or oxygen) to produce the expected number of radicals ). However, when the air flow is too high in the DBDP system, a large number of radicals will be lost via the carrier gas (Wang et al. 2010). Therefore, 4 L/min was selected as the air flow in subsequent experiments.

The effect of the initial concentration on ATZ degradation in the DBDP system
The initial concentration of ATZ has a certain influence on its degradation efficiency in the DBDP system (Hatzisymeon et al. 2021;Zhang et al. 2022). In this study, the initial concentration of ATZ was set to 100, 200, and 300 mg/kg, with a discharge voltage of 9 kV and an air flow of 4 L/min. At 100 mg/kg, the ATZ degradation efficiency reached 76.6% at 10 min (Fig. 2C). To achieve the same degradation efficiency of contaminated river sediment, more radicals are required when the ATZ concentration is high than when it is low ). Therefore, due to the high concentration of intermediates produced during degradation (Jiang et al. 2022), a higher initial concentration decreased the action effect of radicals and hindered the degradation of ATZ. In subsequent experiments, 100 mg/kg was selected as the initial concentration of ATZ.

Analysis of the PS oxidation system
The effect of oxidizer dose on ATZ degradation in the PS oxidation system The dose of oxidizer (SPS) during oxidation plays a major role in oxidative degradation (Ganhão et al. 2010). Figure 3A shows that the degradation efficiency of ATZ varied significantly with the dose (0.01, 0.03, and 0.05 g) of SPS. The optimal degradation efficiency of ATZ was 50% with 0.05 g SPS. These results indicate that an increased dose of oxidizer can promote the decomposition of ATZ. An increased dose of SPS further stimulates the production of radicals in the system, which in turn attack ATZ on a large scale (Ge et al. 2022). Therefore, to achieve a good degradation effect, an SPS dose of 0.05 g was selected in subsequent studies of ATZ degradation in river sediment.
The effect of activator dose on ATZ degradation in the PS oxidation system Figure 4B reveals a significant positive correlation between activator (Fesulf) dose (0.025 and 0.05 g) and ATZ degradation efficiency. As an important part of the PS oxidation system, Fesulf can activate SPS to produce radicals and improve the collision probability between radicals and ATZ. When other conditions were kept constant, the degradation efficiency of ATZ was only 51.4% with a Fesulf dose of 0.05 g at 10 min, which was 1.4% higher than that obtained with the addition of 0.025 g Fesulf (Fig. 3B). The reason is that Fe 2+ may be oxidized by the radicals produced in the oxidation system during the reaction process (Salari et al. 2009;Wang et al. 2014). The forward oxidation reaction is hindered, and the degradation efficiency of the target pollutant changes slowly. Therefore, in subsequent experiments, the activator dose for ATZ degradation was set as 0.025 g

The effect of the initial concentration on ATZ degradation in the PS oxidation system
During the experiment, the oxidizer dose, activator dose, and other conditions remained unchanged. A lower ATZ concentration in river sediment is more conducive to oxidative degradation (Fig. 3C). This phenomenon demonstrates that when the other conditions in the system remain unchanged, the number of radicals in the system remains unchanged. The degradation of ATZ with a high initial concentration in river sediment was further hindered in view of the large number of intermediate products produced in the process of oxidation and decomposition of ATZ (Farshid et al. 2016). Therefore, the lower the concentration of pollutants in river sediment is, the more favorable it is to use the PS oxidation system for degradation.

Response surface methodology assessment of the degradation of ATZ in the DBDP/PS synergistic system
The degradation efficiency of ATZ by the DBDP or PS oxidation system alone was not optimal, and the ATZ degradation efficiency at 10 min was 76.6% and 51.4%, respectively. Therefore, in this study, a DBDP/PS synergistic system was proposed to synergistically degrade ATZ in river sediment. The DBDP/PS synergistic system was analyzed by response surface methodology (RSM). According to the results of preliminary test analysis, five factors (discharge voltage, air flow, initial concentration, oxidizer dose, and activator dose) and three levels (− 1, 0, and 1) (Table S3) were designed considering practical application problems to conduct RSM analysis. Except for these five factors, all other factors remained unchanged in the degradation process of ATZ in river sediment. A Box-Behnken design (BBD) was used in the RSM to establish a mathematical model, and the degradation efficiency was used as an index to optimize the experimental design. The best test parameters were obtained. The test results are shown in Table S4. Statistical analysis system (SAS) software was adopted for regression analysis of the results in Table S4 to obtain the quadratic regression equation (Eq. (S1)).
After quadratic regression fitting, the ATZ removal model was subjected to analysis of variance (ANOVA), and the results are shown in Table S5. The results showed that the model had a high degree of significance (P < 0.001). In addition, P = 0.3903 > 0.05 was obtained from the angle of the lack of fit error value, indicating that the lack of fit was not significant, and the regression equation fits well with

Optimal degradation of ATZ in the DBDP/PS synergistic system
As displayed in Fig. S2, the model shows that the ATZ degradation efficiency in river sediment has a maximum stable point. The optimal theoretical conditions in the synergistic system predicted by the regression model are as follows: discharge voltage 8.85 kV, air flow 2.47 L/min, initial concentration 105.75 mg/kg, oxidation dose 0.03 g, activator dose 0.05 g, and degradation efficiency 97.4%. Three parallel verification tests (under the same conditions) were carried out, and the average degradation efficiency was 96.5%. The relative error was 0.9%. The relative error between the theoretical predicted value and the RSM experimental value meets the error deviation requirement (< 5%), indicating that the factor levels and the selected model are accurate and appropriate. The quadratic equation obtained from the regression analysis fits the actual situation well, and the feasibility of applying the model in practice is high. Moreover, the degradation efficiency of ATZ was 19.9% and 45.1% higher with the DBDP/PS synergistic system than with the DBDP-or PS oxidation-alone systems, respectively (Fig. 4). Compared with DBDP (Feng et al. 2016) and PS oxidation (Dong et al. 2022a, b) technology, DBDP/PS synergistic technology increased the generation of radicals. This further verifies that the synergistic treatment technology used in river sediment remediation can realize promising treatment efficiency.

Degradation mechanism of ATZ by radicals in the DBDP/PS synergistic system
Considering the large amount of SPS in the DBDP/PS synergistic system, electron paramagnetic resonance (EPR) was applied to verify the presence of sulfate radicals (SO 4 • − ) and associated radicals . The EPR spectrum of the persulfate system was obtained at 2 min ( Fig. 5A and B). The characteristic signal peaks of dimethylpyridine nitrogen oxide (DMPO), DMPO-SO 4 • − (six lines, 1: 1: 1: 1: 1: 1, diamonds), DMPO-•OH (four lines, 1: 2: 2: 1, triangles), and DMPO-•O 2 − (four lines, 1: 1: 1: 1, plums) all appeared in the EPR spectrum Zhang et al. 2018), indicating that SO 4 • − , •OH and •O 2 − were the existing radicals during ATZ degradation in the DBDP/PS synergistic system. Figure 5C-E show the absolute quantitative values of SO 4 • − , •OH, and •O 2 − produced by SPS and SPS + Fesulf, respectively. Compared with the results for SPS, the SO 4 • − and •OH total number of spins induced by SPS + Fesulf consistently remained the largest. Moreover, the amount of •O 2 − produced remains constant different systems (SPS or SPS + Fesulf), and the yield is relatively low. In particular, the SO 4 • − and •OH total number of spins produced by SPS + Fesulf peaked at 6 and 2 min, respectively. The Combining the degradation efficiency analysis shown in  Fig. 4, it was reasonable to infer that SO 4 • − and •OH play a leading role in the DBDP/PS synergistic system. According to the EPR analysis results, the free radicals in the DBDP/PS synergistic system are SO 4 • − , •OH, and •O 2 − . Therefore, the degradation mechanism of ATZ in the DBDP/ PS system is free radical action (Fig. S3). Some O 2 is ionized to produce O and O ions and free electrons (e − ) (Eq. (2)). Under a high discharge voltage, O then forms O 3 with the remaining portion of O 2 (Eq. (3)). However, the O 3 structure is unstable (Bandara et al. 2022) and is decomposed into •O 2 − and O under the action of free electrons (Eq. (4)). In the continuous discharge environment, the energy inside the system is constantly accumulating, and H 2 O is further transformed into hydroxyl radicals (•OH) and hydrogen ions (H + ) by electron gain under ionization conditions (Eqs. (5) and (6)). Usually, Fesulf is a powerful approach to activate SPS in the PS oxidation system (Ioannidi et al. 2020). However, UV radiation and heat are produced by DBDP, and SPS can be activated to produce high levels of radicals with stronger oxidation ability under such conditions (Ferkous et al. 2017;Huang et al. 2021) in the DBDP/PS synergistic system. Therefore, based on the presence of UV, heat and Fe 2+ S 2 O 8 2− generates SO 4 • − (2.6-3.1 eV) (Eq. (7) and (8)) , judging by the characteristics of radicals production. Subsequently, a cyclic reaction occurs between Fe 2+ and Fe 3+ . Fe 3+ oxidizes S 2 O 8 2− to generate Fe 2+ , and S 2 O 8 2− is continuously consumed in this process (Eq. (9)). Moreover, SO 4 • − can react with H 2 O in the system and break it down to form •OH (E 0 = 2.8 V) (Eq. (10)) . Finally, under the joint action of SO 4 • − , •OH, and •O 2 − , ATZ is degraded to produce a series of degradation products in the DBDP/PS synergistic system (Eq. (11)). (

Total organic carbon removal
In ATZ-polluted river sediment, the change in TOC removal efficiency can be used as a reference index to explore the degree of organic mineralization and can reflect the effect of pollutant treatment. After 10 min of continuous degradation in the DBDP/PS synergistic system, the TOC removal efficiency gradually increased to 47.8% (Fig. S4). However, the TOC removal efficiencies in the separate DBDP and PS oxidation systems were only 10.7% and 8.5%, respectively, demonstrating that the separate systems could not effectively further mineralize intermediates into H 2 O and CO 2 . Moreover, with a reaction time of 10 min to 60 min, the TOC removal efficiency of the river sediment sample was the highest in the DBDP/PS synergistic system. The maximum TOC removal efficiency reached 85.3% at 60 min in the DBDP/PS synergistic system, which illustrates that ATZ in polluted river sediment can be completely mineralized in the DBDP/PS synergistic system with an adequate degradation time. These results lay a foundation for ATZ degradation path analysis and biotoxicity evaluation of intermediate products.

Analysis of ATZ degradation pathways
Fourier transform infrared spectrometry (FTIR) was used for comparative spectral analysis of ATZ-contaminated river sediment before and after DBDP/PS synergistic treatment to determine the changes in functional groups and chemical bonds. The possible structural characteristics of the intermediate products were inferred by analyzing functional groups and chemical bonds, as shown in Fig. 6A. There were four absorption peaks at 1637, 1032, 779, and 693 cm −1 before the degradation of ATZ in river sediment. The spectral band at 1637 cm −1 is due to C = C stretching vibrations in ATZ (Qu et al. 2013). The absorption peak region at 1032 cm −1 is related to C-H stretching vibrations, and the absorption peaks at 779 cm −1 and 693 cm −1 are related to C-N out-ofplane bending vibrations (Zhan et al. 2018). After DBDP/ PS synergistic treatment, the absorption peak intensity at 1637 cm −1 (corresponding to the bending vibration of C = C) decreased significantly, indicating that the homogeneous triazobenzene ring of ATZ was destroyed and fractured into NH 2 (Czaplicka and Kaczmarczyk 2006). The decrease in peak intensity at 1032 cm −1 indicated that C-H substitution might take place in the ATZ aromatic ring. There was no significant change in the intensity of the absorption peaks at Semiquantitative analysis of the ATZ concentrations in river sediment before and after oxidative degradation for 10 min was performed using gas chromatography-mass spectrometry (GC-MS) ( Fig. 6B and C). The absorption peak of ATZ decreased significantly at 10 min. The intermediates produced in the degradation process of ATZ were determined. The detected intermediates mainly included 3-(3-carboxy-4-hydroxyphenyl)-D-alanine (a) and other substances, which was consistent with the changes in functional groups and chemical bonds determined by FTIR before and after ATZ degradation in river sediment. According to the identified intermediates, it can be inferred that the DBDP/PS synergistic system degrades ATZ via two pathways (Fig. 6D).
In the first pathway, the homogeneous triazobenzene ring and C-Cl of ATZ are attacked by SO 4 • − , •OH, and •O 2 − . On this basis, a series of substitution reactions occurs. Nitrogen atoms and chlorine atoms in the ring are replaced by carbon atoms (Latosińska 2003). After substitution reactions, 3-(3-carboxy-4-hydroxyphenyl)-Dalanine (m/z = 151) (a) with a side chain is formed. Then, the side chain is oxidized to break the chain and generate 3-methoxy-2-methylphenol (m/z = 140) (c). Phenol compounds are further attacked by active species, and the double bond is broken and oxidized to p-dioxane-2,3-diol (m/z = 120) (e). The oxidized products are completely oxidized to 4-hydroxy-butanoic acid (m/z = 96) (g) in a strong oxidation environment and finally further mineralized into CO 2 , H 2 O, and NH 4 + ). In another probable degradation pathway, the triazine ring of ATZ may be attacked by radicals such as SO 4 • − . First, the hemiacetal reaction results in the transformation of the six-ring structure into pilocarpine (m/z = 208) (b) with a pentacyclic structure. Then, pilocarpine (b) itself is acidic and easily undergoes dehydration, causing an intramolecular rearrangement reaction that produces 4-pyridinol-1-oxide (m/z = 111) (d). 4-Pyridinol-1-oxide is further oxidized and decomposed under continuous radicals attack to form acetamide (m/z = 59) (f), which is eventually mineralized into CO 2 , H 2 O, and NH 4 + .

Evaluation of degradation toxicity
According to FTIR and GCMS analysis, 7 intermediates were generated in the degradation of ATZ by the DBDP/ After synergistic treatment C D Fig. 6 FTIR spectra of ATZ before and after DBDP + PS synergistic treatment (A); GC-MS spectra of ATZ before degradation (B) and after degradation (C); degradation pathway of ATZ (D) PS synergistic system. T.E.S.T. version 4.2 (United States Environmental Protection Agency) was used to evaluate ATZ degradation toxicity. The acute toxic semilethal concentration (oral rat LD50) values of the precursor (ATZ) and its intermediates were predicted and evaluated. The oral rat LD50 values of 3-(3-carboxy-4-hydroxyphenyl)-D-alanine, 3-methoxy-2-methyl, pilocarpine, and 4-pyridinol-1-oxide were higher than that of the precursor. The toxicities of the other 3 intermediates (p-dioxane-2,3-diol, 4-hydroxy-butanoic acid, and acetamide) were lower than that of the precursor. 4-Pyridinol-1-oxide had the highest toxicity (Fig. 7). The results showed that the acute toxicity of most intermediates was higher than that of the precursor. Therefore, ATZ could not be completely degraded, which would cause great biotoxicity and harm. However, based on the results of the TOC removal efficiency test and ATZ synergistic degradation path correlation analysis, in this study, the DBDP/PS synergistic degradation method successfully achieved complete pollutant mineralization and further effective removal of the biotoxicity of ATZ and its intermediates. Finally, the remediation of organic pollutants in river sediment was realized.

Conclusion
This study made full use of the high efficiency and environmental friendliness of strongly oxidizing radicals and proposed a new combined degradation method for ATZ in polluted river sediment using a synergistic DBDP/PS system based on conventional treatment methods. EPR analyses showed that SO 4 • − and •OH played a leading role in the degradation process of ATZ in the river sediment. The experimental parameters and results were optimized by RSM. The FTIR and GCMS results demonstrated that ATZ was decomposed into 7 intermediates, such as 3-(3-carboxy-4-hydroxyphenyl)-D-alanine, in the DBDP/PS synergistic system. Substitution and hemiacetal reactions were the two dominant degradation paths. The novel combined degradation method of the DBDP/PS synergistic system not only improves the mineralization rate of ATZ but also effectively reduces the biological toxicity of intermediate products. The above conclusions fully indicate that the DBDP/PS synergistic system can be used to degrade refractory organic pollutants represented by ATZ in river sediment with environmental friendliness and high efficiency. Moreover, the DBDP/ PS synergistic treatment method discussed in this study can provide a reference for the future treatment of pollutants in river sediments.