Electrochemical activation of peroxymonosulfate (PMS) by carbon cloth anode for sulfamethoxazole degradation

Highlights

• PMS could be activated by E (Carbon cloth Anode)/PMS system.

• •SO₄·^-, ·OH and ¹O₂ contributed to SMX removal in E (Carbon cloth Anode)/PMS system.

• •SMX degraded via hydroxylation, S–N bond cleavage and aniline oxidation.

• •E (Carbon Cloth Anode)/PMS system could reduce the genetic toxicity of SMX.

Abstract

Electrochemical activation of peroxymonosulfate (PMS) at carbon cloth anode (E (Carbon cloth Anode)/PMS system) was investigated for sulfamethoxazole (SMX) degradation. The results indicated that PMS could be activated at carbon cloth anode during electrolysis, resulting in the improvement of SMX degradation. The degradation efficiency of SMX was facilitated with the higher PMS concentration and current density, respectively. The degradation rate constant of SMX increased with the rising pH from 3.6 to 6.0, and reached the highest value at pH 6.0, and then decreased with further increasing pH to 8.0. The presence of chloride ion (Cl⁻, 5–100 mM) significantly enhanced SMX degradation, while addition of humic acid (HA, 1–5 mgC L⁻¹) inhibited SMX degradation. Addition of carbonate (HCO₃⁻, 5–20 mM) had a negligible impact on SMX degradation. Small amounts of phosphate (PO₄³⁻, 0–5 mM) could promote degradation, while a large amount of PO₄³⁻ (10–20 mM) inhibited the degradation. Moreover, the quenching experiments demonstrated that sulfate radical (SO₄·⁻), hydroxyl radical (·OH) and singlet oxygen (¹O₂) contributed to SMX degradation in E (Carbon cloth Anode)/PMS system. The degradation intermediates of SMX were identified by LC-MS/MS and the degradation pathways were deduced to be hydroxylation, the cleavage of S–N bond, and oxidation of aniline group. Moreover, the micronucleus test of Vicia faba root tips indicated that the E (Carbon Cloth Anode)/PMS system could reduce the genetic toxicity of SMX contaminated water to some extent.

Introduction

Sulfamethoxazole (SMX) is a typical sulfonamide, which has been used extensively in previous decades for human and animal antimicrobial treatment as one of the broad-spectrum antibiotics (Santos et al., 2010; Qin et al., 2020). Due to incomplete metabolism, SMX and their metabolites are discharged along with the wastewater effluent and have been frequently detected in surface water and groundwater (Chen et al., 2018a). The residue antibiotics in water environment would transform and accumulate in aquatic organisms, which lead to the proliferation of drug-resistant organisms and disrupt the ecological environment (Cui et al., 2018; Fang et al., 2021).

In recent years, in order to effectively control and remove these toxic and harmful organic pollutants, a variety of advanced water treatment technologies have been developed, including adsorption, oxidation, membrane treatment, etc. (Hussain et al., 2017; Iakovides et al., 2019; Tian et al., 2019). Among these treatment processes, advanced oxidation processes (AOPs) can effectively convert toxic and harmful organic matter to biodegradable non-toxic organic matter (Oturan and Aaron, 2014; Brillas and Martínez-Huitle, 2015; Moreira et al., 2017). AOPs based on persulfate (PS, including peroxymonosulfates (PMS) and peroxydisulfate (PDS)) activation generate several reactive species such as radicals (hydroxyl radicals (·OH) and sulfate radicals (SO₄·^-)) and non-radicals (singlet oxygen (¹O₂)), which could effectively remove antibiotics from water and wastewater (Zou et al., 2014; Fan et al., 2015; Ji et al., 2015; Zhang et al., 2016; Liu et al., 2018). Traditional persulfate activation methods include metal ions activation, ultraviolet radiation, alkali and thermal activation (Zou et al., 2014; Fan et al., 2015; Bu et al., 2017), which have drawbacks such as high energy consumption and the secondary pollution caused by metal ions leaching.

Lately, electrochemical activation of PMS has been developed, which is an effective method with low energy consumption, environmental friendliness and good recycling performance. In the electrolysis process, both cathode and anode could activate PS to produce active species. Compared with cathodic activation, anodic activation could produce both radicals and non-radicals (Dirany et al., 2012; Bu et al., 2019), which are more advantageous to degradation of organic matters in real water. Several anode materials such as Blue-TiO₂ nanotubes, boron-doped diamond (BDD), multi-walled carbon nanotubes and graphite have been used for electrochemical activation of PS to degrade organic contaminants such as diatrizoate, sulfamethoxazole and so on(Farhat et al., 2015; Song et al., 2017; Cai et al., 2021). However, these anodes are generally expensive, complicated to prepare, difficult to apply in the real water treatment. Carbon cloth is made of carbon fiber through textile with high strength, low density, thin thickness, high-temperature resistance, widely used in sports equipment, industry, fire protection, construction, fuel cell and other fields. As a cheap, stable and readily available industrial material, carbon cloth has a potential application for electrochemical activation of PS, which has not been investigated yet.

Therefore, this study evaluated the performance of electrochemical activation PMS to degrade SMX using industrial carbon cloth as anode. Firstly, we evaluated the effects of PMS dosage, current density and SMX concentration on the SMX degradation, as well as the recycling usability of carbon cloth anode. Secondly, the effects of real water matrixes such as chlorine (Cl⁻), carbonate (HCO₃⁻), phosphate (PO₄³⁻) and humic acid (HA) on the degradation of SMX were also investigated. The reactive species in the electrochemical activation process were identified by the quenching methods using tert-butanol, methanol and sodium azide as scavengers of ·OH, SO₄·⁻ and ¹O₂, respectively. Moreover, the mechanism of electrochemical activation PMS was clarified using cyclic voltammetry (CV). Finally, the degradation pathway and the genotoxicity variation of SMX degradation were deduced.