Analysis of influencing parameters and reactive substance for enrofloxacin degradation in a dielectric barrier discharge plasma/peroxydisulfate system

Shilin Song; Huijuan Wang; Yuyue Huang; Yixing Ma

doi:10.4491/eer.2023.717

Environ Eng Res > Volume 29(4); 2024 > Article

Song, Wang, Huang, and Ma: Analysis of influencing parameters and reactive substance for enrofloxacin degradation in a dielectric barrier discharge plasma/peroxydisulfate system

Research

Environmental Engineering Research 2024; 29(4): 230717.

Published online: December 16, 2023

DOI: https://doi.org/10.4491/eer.2023.717

Analysis of influencing parameters and reactive substance for enrofloxacin degradation in a dielectric barrier discharge plasma/peroxydisulfate system

Shilin Song^1,², Huijuan Wang^1,^†

, Yuyue Huang¹, Yixing Ma^2,^†

¹School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China

²Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China

^†Corresponding author: E-mail: wendyjuaner@126.com (H.W.), mayixing99@kust.edu.cn (Y.M.), Tel: +86-511-88790931 (H.W.), +86-0871-65920508 (Y.M.), Fax: +86-511-88790955 (H.W.), +86-0871-65920492 (Y.M.)

Received November 22, 2023 Revised December 01, 2023 Accepted December 15, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this paper, the chemical and physical effects generated during the dielectric barrier discharge plasma (DBDP) process, including O₃, H₂O₂, light, e*, etc., were utilized to activate the peroxydisulfate (PDS) to form SO₄⁻•. Then, the original reactive oxygen species (ROS), such as •OH, formed in the discharge system was combined to degrade the enrofloxacin (ENR) in water, and the corresponding influencing parameters and reactive substance in the DBDP/PDS system were analyzed. From the investigation, it was found that the neutral (pH =6.5) solution was more conducive to the ENR decomposition than the acidic and alkaline solution conditions. The presence of Fe²⁺ and Cu²⁺ in the reaction solution could hasten the ENR degradation, whereas the addition of Cl⁻ and the HCO₃⁻ in the solution had a negative effect. Analysis of the reactive species and quenching tests were carried out to explore the generation of H₂O₂, O₃, •OH, and SO₄⁻• in the DBDP/PDS system and their effects on the ENR degradation. The UV-Vis and 3D fluorescence spectra analysis were applied to demonstrate the cooperative effects of the DBDP and the PDS. The TOC and COD removals of the ENR solutions in the DBDP and the DBDP/PDS systems were also compared. Based on the intermediates analysis of the ENR degradation, three possible pathways of ENR decomposition in the synergistic system have been inferred.

Keywords: DBDP, Enrofloxacin degradation, Influencing parameters, Persulfate, Reactive substances

Graphical Abstract

Keywords: DBDP, Enrofloxacin degradation, Influencing parameters, Persulfate, Reactive substances

1. Introduction

As a third-generation fluoroquinolone antibiotic, enrofloxacin (ENR) can persist in the form of an archetype or product of metabolism for a long time after entering the aquatic environment. Thus, ENR can often be detected in various surface waters. To resolve the probable environmental crisis induced by the ENR, developing promising technologies to remove the ENR from water has been necessary.

Advanced oxidation technologies (AOTs) have been demonstrated as efficient wastewater treatment technologies (including ozone oxidation [1], photocatalytic oxidation [2], electrochemical oxidation [3], Fenton oxidation [4], and non-thermal plasma, etc.) to oxidize the nondegradable pollutants. Among AOTs, non-thermal plasma technology has drawn significant attention because of its high efficiency and non-selectivity for recalcitrant degradation and organic wastewater treatment. During the discharge process, the physical effects of high-energy electrons and ultraviolet light could be formed, and the chemical effects of the reactive oxygen species (ROS) generation could also be obtained [5–7]. These physical and chemical phenomena induced by plasma could oxidize refractory organic pollutants directly or indirectly, potentially leading to mineralization. However, some physical (light) and chemical effects (O₃ and H₂O₂) in the system have not been fully utilized, resulting in the relatively weak oxidation performance of pollutants in the discharge system, which limits the further development of the non-thermal plasma technology. In order to fully utilize the various physical and chemical effects and improve its treatment efficiency, establishing collaborative wastewater treatment systems combined with different technologies have become the good research direction.

Due to the stronger oxidation ability (E₀=2.5~3.1 V) and longer half-life of the sulfate radical (SO₄⁻•) [8], the AOT based on the SO₄⁻• is becoming increasingly popular in the field of refractory organic wastewater treatment [8]. Multiple methods could activate peroxydisulfate (PDS) to produce SO₄⁻•, including heat [9], light [10], ozone (O₃) [11], hydrogen peroxide (H₂O₂) [12], electrochemistry [13], transition metals [14], and alkali [15], etc. As the physical effects (heat and light) and chemical effects (O₃ and H₂O₂) could be formed simultaneously in the non-thermal plasma system, research on the PDS activation by non-thermal plasma has been attracted attention recently. Furthermore, the generation of •OH in the system could also be promoted by PDS activation [16]. The generation of the new kind of active substance (SO₄⁻•) as well as the enhancement of the critical reactive species (•OH) could lead to the efficient decomposition and mineralization of the organic pollutants. Though researchers have combined the dielectric barrier discharge plasma (DBDP) and PDS to apply in the different organic compounds’ degradation, the detailed analysis is still insufficient. To figure out the influence of more parameters on the oxidation in the collaborative process of the DBDP/PDS, there is still much work to be done.

In this study, the DBDP was chosen to produce the non-thermal plasma and the DBDP/PDS cooperative system was established to treat the ENR wastewater. Different influencing factors, including the initial pH value and the different inorganic ions’ (Fe²⁺, Cu²⁺, Cl⁻ and HCO₃⁻) addition on the ENR degradation in the DBDP/PDS system were studied. The cooperative mechanism was also studied by analyzing the critical reactive substances (O₃, H₂O₂, •OH, and SO₄⁻•) through concentration detection or electron paramagnetic resonance (ESR) analysis. The degradation of the ENR solution was characterized by UV-Vis spectroscopy and 3D fluorescence spectrum analysis, and the removals of COD and TOC of the ENR solutions were studied to illustrate the mineralization of the ENR. The intermediates of ENR degradation were analyzed by HPLC-MS, and then possible pathways of the ENR degradation in the DBDP/PDS system were speculated.

2. Experimental

2.1. Material

The acetonitrile (≥99.9%, Macklin Biochemical Technology Co., Ltd. Shanghai) used in the research was HPLC-pure. All other reagents were analytical pure. ENR (≥98%), DMPO (≥97%), ferrous sulfate (≥98%), copper sulfate (≥99%) were purchased from Aladdin Biochemical Technology Co., Ltd. Peroxydisulfate (≥98%), sodium chloride (≥99.5%), ethanol (≥99.8%), tert-butyl alcohol (≥99%), and sodium hydroxide (≥96%) were purchased from the China National Pharmaceutical Group Corporation. All solutions were prepared with deionized water.

2.2. DBDP Reaction System

Fig. S1 shows the experimental system of the study. A cyclic water-falling film DBDP reactor was used as the source for the non-thermal plasma generation (purchased from Nanjing Suman Plasma Technology Co., Ltd., PG-3000). In the DBDP reactor, the stainless-steel mesh covered on the surface of the outer quartz tube served as the high-voltage electrode and the solution flowing between the outer and inner quartz tubes served as the grounding electrode. The detailed parameters of the DBDP reactor were given in our former research [17, 18]. The target solution entered the reactor from the bottom of the inner quartz tube and then flew out through the uniform flow port above it. Finally, the solution flew along the outer wall of the inner quartz tube in the form of a water film. The higher gas-liquid ratio between the discharge electrode and the ground electrode was favorable for the O₃ formation of the water-falling film DBDP reactor. The voltage and current waveforms were recorded by an oscilloscope (Tektronix DPO 2014b) and probes (Tektronix, P6015A, and TPP0200). During the experiments, the injection power was 0.8 kW and was kept as a constant parameter for each experiment.

2.3. Experimental Method

The whole reaction solution was circulated with a peristaltic pump (Baoding Lead Fluid, BT600/YZ15, China) and the ENR solution (3 L, 20 mg L⁻¹) was pumped into the reactor at the 1.5 L min⁻¹ flow rate was. 3 mM PDS was added to the ENR solution. A magnetic stirrer (China, Xinrui, DJ-1) was used to mix the solution evenly. For the reactive substances analysis, deionized water was applied as the reaction solution. In all experiments, the total reaction time was controlled at 25 min and the samples were taken at 5 min intervals. The reaction temperature was controlled below 35°C throughout the experiments.

2.4. Analytical Method

The HPLC (Lunan Ruihong, P1010, China) was used for analyzing the ENR concentration. 0.1 M NaOH and 0.1 M HCl solutions were used to adjust the pH value of the solution. A pH meter (INESA, PHS-3E, Shanghai) and a conductivity analyzer (INESA, DDS-307A, Shanghai) were applied to measure the pH and conductivity of the solutions. An UV-Vis full band scanner (Shimadzu, UV-2600, Japan) was used for the full-wave band scanning of the solutions during the treatment. The TOC and COD values of the ENR solution were measured using TOC (German element, Vario TOC select, German) and COD (Lianhua, 5B-3A, China) instruments, respectively. A 3D fluorescence spectrophotometer (Hitachi, FL4500) was used to measure the molecular fluorescence properties of the different samples. Analysis of the sulfate and the hydroxyl radicals was performed by an ESR (Bruker, Micro ESR, Germany). The O₃ concentration of the solution was detected using an ozone analyzer (Shanghai Haiheng, CY-1A, China). The TiOSO₄ method was used to identify H₂O₂ concentrations. A LC-MS (America, Thermo LXQ) was used to detect the byproducts formed during the ENR degradation. The degradation efficiency (removal percentage) of the ENR was calculated by comparing the change of the concentration during the treatment. The corresponding kinetics analysis was fit based on the pseudo first-order equation. The energy efficiency (G₅₀, mg (k Wh)⁻¹) was also investigated. The corresponding calculation methods are given in Table S1. The experimental results are presented by the average value of the three parallel experiments with an error bar.

3. Results and Discussion

3.1. Influence of the Initial pH Value

Firstly, the influence of the solution initial pH (3, 5, 6.5, 9 and 11) on the ENR degradation in the DBDP/PDS system was studied. The degradation efficiency, k value and G₅₀ were calculated, and the results are shown in Fig. 1. The figure showed that the degradation efficiency, k value, and G₅₀ were the highest at the initial pH of 6.5. The ENR degradation under the acidic and alkaline solution conditions was relatively lower. For the reason of the results, it can be ascribed as follows: the ENR is an amphoteric compound, it has a positively charged piperazine moiety and a negatively charged carboxyl group. The two pKa values of the ENR are pK_a1=6.1 and pK_a2=7.7 [19]. Therefore, when the ENR solution was at its initial pH value of 6.5 (the value was between pK_a1 and pK_a2), the smaller charge on the ENR molecules would be favorable for the mass transfer of the oxidation reaction, which was conducive to the ENR degradation. Furthermore, •OH could be captured by the CO₃²⁻ (formed by the cleavage of ENR molecules) in the alkaline solution [20], which would weaken the effectiveness of the formed •OH for the ENR degradation.

3.2. Influence of the Inorganic Ion Addition

As multiple ions often coexist in the practical wastewater, the influence of different ions’ addition on the ENR degradation in the DBDP/PDS was investigated. The four chemicals, FeSO₄, CuSO₄, NaCl and NaHCO₃ were dissolved in the solution to simulate the effect of the Fe²⁺, Cu²⁺, Cl⁻ and HCO₃⁻ existence in the practical water environment, respectively. The ENR degradation under different conditions of inorganic ion addition is displayed in Fig. 2.

Fig. 2a shows that the addition of Fe²⁺ was favorable for enhancing the ENR degradation, and the effect of the 2 mM Fe²⁺ addition was more evident than the 1 mM and 3 mM Fe²⁺ additions. The positive effect of the Fe²⁺ addition can be explained for the following two reasons: one was the promotion of the Fe²⁺ for the SO₄⁻• production, and the other was the production of •OH stimulated by the Fenton reaction between Fe²⁺ and H₂O₂ (Eqs. (1) and (2)) [21]. However, Fe²⁺ could also consume the SO₄⁻• and the •OH (Eqs. (3) and (4)) [22], and the processes might result in a gradual change of the ENR degradation with the extension of the reaction. Fig. 2b shows that the ENR removal was enhanced by increasing the Cu²⁺ concentration in the reaction solution. When 2 mM Cu²⁺ was added, the degradation efficiency of ENR could reach 100%. The results were owing to the formation of oxidative free radicals by reactions of Cu²⁺/Cu⁺ with the PDS and the H₂O₂ (Eqs. (2), (5)) [23]. Fig. 2c shows that the addition of Cl⁻ could inhibit the ENR degradation because the Cl⁻ could react with the SO₄⁻• or the •OH to produce Cl•, which had less oxidizing capacity (Eqs. (6)) [24]. The results shown in Fig. 2d revealed that there was an increase of the ENR degradation in the first 10 min when there was the HCO₃⁻ in the reaction system, while the degradation rates decreased in the following reaction period. The reductions were 10.79% (1 mM) and 40.73% (2 mM) at the 25 min. The initial increase might be due to the extra production of •O²⁻ and •CO₃⁻ (Eq. (7)) [25], which have certain oxidation capacities. Furthermore, the following formation of the ¹O₂ and the •OH by the •O²⁻ could also oxidize the ENR (Eq. (8)). However, the inhibition reactions of HCO₃⁻ with SO₄⁻• and •OH would result in a decrease of the ENR removal in the following reaction stages (Eqs. (9) and (10)) [26].

(1)

S_{2} {O_{8}}^{2 -} + M^{n +} ({Fe}^{2 +} / {Cu}^{2 +}) \to {SO}_{4}^{-} • + {SO}_{4}^{2 -} + M^{n + 1} ({Fe}^{3 +} / {Cu}^{3 +})

(2)

H_{2} O_{2} + M^{n +} ({Fe}^{2 +} / {Cu}^{+}) \to {OH}^{-} + • OH + M^{n + 1} ({Fe}^{3 +} / {Cu}^{2 +})

(3)

{SO}_{4}^{-} • + {Fe}^{2 +} \to {Fe}^{3 +} + {SO}_{4}^{2 -}

(4)

• OH + {Fe}^{2 +} \to {Fe}^{3 +} + {OH}^{-}

(5)

{Cu}^{2 +} + H_{2} O_{2} \to {Cu}^{+} + {HO}_{2} • + H^{+}

(6)

{Cl}^{-} + {SO}_{4}^{-} • (• OH) \to Cl • + {SO}_{4}^{2 -} ({OH}^{-})

(7)

{HCO}_{3}^{-} + H_{2} O_{2} \to • {O_{2}}^{-} + • {CO}_{3}^{-} + H^{+}

(8)

• {O_{2}}^{-} + H_{2} O_{2} \to {{}^{1}O}_{2} + • OH + {OH}^{-}

(9)

{SO}_{4}^{-} • + {HCO}_{3}^{-} \to {CO}_{3}^{-} • + {SO}_{4}^{2 -} + H^{+}

(10)

• OH + {HCO}_{3}^{-} \to {CO}_{3}^{-} • + H_{2} O

3.3. Reactive Species Analysis

3.3.1. O₃ and H₂O₂

The cooperative effect of the DBDP and the PDS could be indicated by comparing the changes of O₃ and H₂O₂ concentrations in demonized water in the DBDP system with and without the PDS addition. Fig. 3a shows that the O₃ concentration in the two reaction systems increased first and then decreased during the whole reaction time. The reason for the increase might be attributed to the increased solubility of ozone at lower pH values, which was induced by the formation of organic acid byproducts [27]. The following decrease was because of the increase in solution temperature during the reaction, which led to the decomposition of O₃. The changes of H₂O₂ concentration are shown in Fig. 3b. As shown in the figure, the H₂O₂ concentration decreased when the PDS was present in the reaction system. The results could be explained as the PDS promoted the decomposition of H₂O₂ into HO₂• (Eqs. (11)–(13)) [28, 29], which in turn induced the generation of SO₄⁻•. Furthermore, H₂O₂ could be generated by the reaction of HO₂• and O₂⁻•, which could also react with SO₄⁻• when PDS was added (Eqs. (14)–(15)) [30]. All the above reactions led to the reduction of the formed H₂O₂.

(11)

H_{2} O_{2} \leftrightarrow {HO}_{2}^{-} + H^{+}

(12)

{HO}_{2}^{-} + O_{3} \to {HO}_{2} • + {O_{3}}^{-} •

(13)

S_{2} {O_{8}}^{2 +} + {HO}_{2} • \to {SO}_{4}^{2 -} + {SO}_{4}^{-} • + O_{2} + H^{+}

(14)

{SO}_{4}^{-} • + {HO}_{2} • \to {SO}_{4}^{2 -} + O_{2} + H^{+}

(15)

{SO}_{4}^{-} • + {O_{2}}^{-} • \to {SO}_{4}^{2 -} + O_{2}

3.3.2.•OH and SO₄⁻•

The ESR analyzer could identify the active substances using the different characteristic signals exhibited by the compounds formed by the reaction of DMPO with •OH and SO₄⁻• [31, 32]. The analysis results are displayed in Fig. 4. In the two reaction systems, the linear relationship between the characteristic peak intensities of DMPO-•OH was shown as 1:2:2:1. Meanwhile, the linear relationship of DMPO-SO₄⁻• peak intensity would also exhibit an approximate relationship of 1:1:1:1:1:1. From the characteristic peaks shown in Fig. 4a, it can be observed that the •OH signal was able to be detected in the DBDP-alone system, but the intensity was relatively low. Fig. 4b shows that both SO₄⁻• and •OH could be formed in the system of DBDP/PDS, and the intensity of •OH was higher in the cooperation system. The results indicate that the activation of the PDS in the DBDP system and the chain reactions could induce the formation of SO₄⁻•. It was found that a stronger DMPO-•OH peak was generated in the DBDP/PDS system compared to the DBDP-alone system. The results verified the cooperative effect of the DBDP and the PDS on the reactive species and accordingly the ENR degradation.

3.3.3. Quenching test

The quenching test was carried out in the research to determine the contribution of different active substances to the ENR degradation in the DBDP/PDS system. In this part, TBA was used as a scavenger for •OH, and EtOH was applied to quench SO₄⁻• [33]. The results are displayed in Fig. 5. As displayed in Fig. 5a, the addition of TBA and EtOH reduced the degradation rates of ENR by 39.33% and 18.15%, respectively. The corresponding k values also performed the same trend of change (Fig. 5b). The different oxidation results induced by Et-OH and TBA in the DBDP/PDS system also indicated that the •OH and SO₄⁻• played an important role in the ENR degradation.

3.4. ENR Degradation

3.4.1. UV-Vis analysis

The UV-Vis scanning of ENR solutions in both DBDP-alone and DBDP/PDS systems at different reaction periods is shown in Fig. 6. It can be observed that the characteristic peak of ENR at 276 nm was progressively decreasing, and the reduction was more pronounced in the DBDP/PDS system than that in the DBDP-alone system. Moreover, the characteristic peaks at 185–250 nm were increasing due to the accumulation of many byproducts in this region as the ENR degradation [34]. The results also confirmed the cooperative effect of DBDP and PDS on the ENR degradation.

3.4.2. 3D fluorescence spectra analysis

The fluorescence intensity of organic compounds detected by 3D fluorescence spectroscopy was used to characterize the change of the ENR solution during the degradation process. The weaker the fluorescence intensity presents the lower the concentration of organic substances [35, 36]. The analysis methods were also used to analyze the composition and concentration changes of the ENR solution during the degradation in the DBDP/PDS system, and the results are displayed in Fig. 7. It can be observed from Fig. 7 that the fluorescence intensity at Ex/Em = (400–558 nm)/(281–392 nm) was gradually disappearing when the ENR solution was treated in the DBDP/PDS cooperative process, which demonstrated that the chemical bond of ENR was decomposed and the functional group was replaced. The target chemicals decomposed continuously into the small molecular compounds by the oxidation of •OH, SO₄⁻•, O₃, and H₂O₂ generated in the discharge process. It was noteworthy that after 25 min, all fluorescence belonging to the ENR region disappeared, indicating that ENR was effectively degraded in the DBDP/PDS system.

3.4.3. Change of solution pH and conductivity

In the DBDP/PDS system, a series of chain reactions could lead to the formation of different intermediates and then the change of solution properties. To illustrate the change, the pH and conductivity values of the ENR solution in the DBDP and DBDP/PDS reaction systems at different reaction stages were measured. The changing rules are displayed in Fig. 8. The figure shows that the pH value of the ENR solution in the two systems decreased, while the conductivity value showed the opposite trend. In addition, when PDS was introduced into the DBDP system, the changing trend of pH and conductivity values was more obvious. As ENR molecules could dissociate to form a series of intermediates, including organic acids and CO₃²⁻, HCO₃²⁻ during the reaction process [37, 38], it would affect the pH and conductivity of the solution. The addition of PDS could further accelerate the generation of these substances and ultimately affect the solution's properties. Besides that, ionization of air induced by the plasma could also generate HNO₃ and HNO₂ and lead to the change of solution properties [39].

3.4.4. TOC and COD

The removal of solution COD and TOC of the ENR solutions after treating in the DBDP-alone and DBDP/PDS systems are shown in Fig. S2. It can be seen from the figure that the removal rates of TOC and COD in the DBDP/PDS system were 14.34% and 43.53% after 25 min of treatment, which were 7.66% and 19.58% higher than the DBDP-alone system. The results evidenced that the addition of PDS could reduce improve the mineralization of the ENR solution, which also further proved the cooperative effect of the DBDP and the PDS.

3.5. Degradation Pathway Analysis

The byproducts formed in the DBDP/PDS system were analyzed and identified through LC-MS analysis. Based on the 14 identified products and related references [40–42], three degradation pathways were speculated, as shown in Fig. S3.

Pathway I: Once the F atom of the ENR molecule was substituted by •OH, the piperazine ring was oxidized and opened to form the E1 and the following E2, which were further transformed to the E3 by losing the piperazine ring. Finally, E4 was formed through the decarboxylation reaction.

Pathway II: The E5 was first formed through the decarboxylation and the •OH substitution reactions that took place on the quinolone ring. Then the E6 was produced through the oxidation and ring-opening of the piperazine ring. Under the continuous attack of reactive oxygen species, the piperazine ring disappeared and the E6 transformed into the E7, then the F and NH₂ groups were dropped to form the E8.

Pathway III: Firstly, the E9 was formed after the F atoms were substituted by the •OH. Then, the quinolone ring cleavage reaction and decarboxylation occurred, and the E10 and the E11 were generated. In addition, the N atom between the carbon group and the piperazine ring and the Quinolone ring were attacked, and the E12 was formed by losing the aldehyde groups and the piperazine ring branches. Finally, the E13 and the E14 were generated through the cleavage of the quinolone rings and the loss of N atoms.

When there are sufficient reactive oxygen species in the DBD/PDS system, the intermediate products will further oxidize to CO₂, H₂O, and inorganic ions after sustained attack, ultimately achieving complete removal of ENR.

4. Conclusions

In the study, a DBDP/PDS water treatment system was constructed and applied to degrade ENR. In the cooperation system, neutral (pH=6.5) solution conditions were more advantageous for the decomposition of ENR compared to the acidic and alkaline solutions; Fe²⁺ and Cu²⁺ ions’ addition could promote the formation of active substance in the system and then accelerate the ENR degradation, while Cl⁻ and HCO₃⁻ would hinder the active substances formation. The analysis and quenching experiments of active substances also confirmed that the addition of PDS was conducive to the generation of active substances. The results of the UV-vis analysis verified that there was a better degradation of the ENR in the DBDP/PDS system. 3D fluorescence spectroscopy analysis indicated that the ENR was effectively degraded in the DBDP/PDS system. Three pathways of ENR degradation in the DBDP/PDS cooperative system were finally speculated. In summary, the addition of PDS could improve the removal rate and mineralization degree of ENR in the DBDP system. The results achieved from the study will provide some references for the possible application of the DBDP/PDS cooperative process in practical wastewater treatment in the future.

Supplementary Information

eer-2023-717-Supplementary.pdf

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21876070) and Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment.

Notes

Author Contributions

S. S. (Master student) made the experiment plan, data analysis and Writing-Original Draft the experiment in the whole process. H. W. (Professor) conducted all the experiments and wrote the manuscript. Y. H. (Master student) was responsible for water quality analysis. Y. M. (Associate Professor) wrote and revised the manuscript.

Conflict of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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