Degradation of benzotriazole by sulfate radical-based advanced oxidation process

Unique effect of bromide ion on intensification of advanced oxidation processes for pollutants removal: A systematic review

Advanced oxidation processes (AOPs) have been developed to decompose toxic pollutants to protect the aquatic environment. AOP has been considered an alternative treatment method for wastewater treatment. Bromine is present in natural waters posing toxic effects on human health and hence, its removal from drinking water sources is necessary. Of the many techniques advanced oxidation is covered in this review. This review systematically examines literature published from 1997 to April 2024, sourced from Scopus, PubMed, Science Direct, and Web of Science databases, focusing on the efficacy of AOPs for pollutant removal from aqueous solutions containing bromide ions to investigate the impact of bromide ions on AOPs. Data and information extracted from each article eligible for inclusion in the review include the type of AOP, type of pollutants, and removal efficiency of AOP under the presence and absence of bromide ion. Of the 1784 documents screened, 90 studies met inclusion criteria, providing insights into various AOPs, including UV/chlorine, UV/PS, UV/H2O2, UV/catalyst, and visible light/catalyst processes. The observed impact of bromide ion presence on the efficacy of AOP processes, alongside the AOP method under scrutiny, is contingent upon various factors such as the nature of the target pollutant, catalyst type, and bromide ion concentration. These considerations are crucial in selecting the best method for removing specific pollutants under defined conditions. Challenges were encountered during result analysis included variations in experimental setups, disparities in pollutant types and concentrations, and inconsistencies in reporting AOP performance metrics. Addressing these parameters in research reports will enhance the coherence and utility of subsequent systematic reviews.

A review on recent advancements in the treatment of polyaromatic hydrocarbons (PAHs) using sulfate radicals based advanced oxidation process

Polyaromatic hydrocarbons (PAHs) are the most persistent compounds that get contaminated in the soil and water. Nearly 16 PAHs was considered to be a very toxic according US protection Agency. Though its concentration level is low in the environments but the effects due to it, is enormous. Advanced Oxidation Process (AOP) is an emergent methodology towards treating such pollutants with low and high molecular weight of complex substances. In this study, sulfate radical (SO4‾•) based AOP is emphasized for purging PAH from different sources. This review essentially concentrated on the mechanism of SO4‾• for the remediation of pollutants from different sources and the effects caused due to these pollutants in the environment was reduced by this mechanism is revealed in this review. It also talks about the SO4‾• precursors like Peroxymonosulfate (PMS) and Persulfate (PS) and their active participation in treating the different sources of toxic pollutants. Though PS and PMS is used for removing different contaminants, the degradation of PAH due to SO4‾• was presented particularly. The hydroxyl radical (•OH) mechanism-based methods are also emphasized in this review along with their limitations. In addition to that, different activation methods of PS and PMS were discussed which highlighted the performance of transition metals in activation. Also this review opened up about the degradation efficiency of contaminants, which was mostly higher than 90% where transition metals were used for activation. Especially, on usage of nanoparticles even 100% of degradation could be able to achieve was clearly showed in this literature study. This study mainly proposed the treatment of PAH present in the soil and water using SO4‾• with different activation methodologies. Particularly, it emphasized about the importance of treating the PAH to overcome the risk associated with the environment and humans due to its contamination.

Orange G degradation by heterogeneous peroxymonosulfate activation based on magnetic MnFe2O4/α-MnO2 hybrid

Wastewater containing an azo dye Orange G (OG) causes massive environmental pollution, thus it is critical to develop a highly effective, environmental-friendly, and reusable catalyst in peroxymonosulfate (PMS) activation for OG degradation. In this work, we successfully applied a magnetic MnFe₂O₄/α-MnO₂ hybrid fabricated by a simple hydrothermal method for OG removal in water. The characteristics of the hybrid were investigated by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller method, vibrating sample magnetometry, electron paramagnetic resonance, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The effects of operational parameters (i.e., catalytic system, catalytic dose, solution pH, and temperature) were investigated. The results exhibited that 96.8% of OG degradation was obtained with MnFe₂O₄/α-MnO₂(1:9)/PMS system in 30 min regardless of solution pH changes. Furthermore, the possible reaction mechanism of the coupling system was proposed, and the degradation intermediates of OG were identified by mass spectroscopy. The radical quenching experiments and EPR tests demonstrated that SO₄^•̶, O₂^•̶, and ¹O₂ were the primary reactive oxygen species responsible for the OG degradation. The hybrid also displayed unusual stability with less than 30% loss in the OG removal after four sequential cycles. Overall, magnetic MnFe₂O₄/α-MnO₂ hybrid could be used as a high potential activator of PMS to remove orange G and maybe other dyes from wastewater.

Wastewater treatment by anodic oxidation in electrochemical advanced oxidation process: Advance in mechanism, direct and indirect oxidation detection methods

Electrochemical Advanced Oxidation Process (EAOP) has been applied to the degradation of refractory pollutants in wastewater due to its strong oxidation capacity, high degradation efficiency, simple operation, and mild reaction. Among electrochemical processes, anodic oxidation (AO) is the most widely used and its mechanism is mainly divided into direct oxidation and indirect oxidation. Direct oxidation means that pollutants are oxidized at the anode by direct electron transfer. Indirect oxidation refers to the generation of active species during the electrolytic reaction, which acts on pollutants. The mechanism of AO process is controlled by many factors, including electrode type, electrocatalyst material, wastewater composition, pH, applied current and voltage levels. It is very important to explore the reaction mechanism of electrochemical treatment, which determines the efficiency of the reaction, the products of the reaction, and the extent of reaction. This paper firstly reviews the current research progress on the mechanism of AO process, and summarizes in detail the different mechanisms caused by influencing factors under common AO process. Then, strategies and methods to distinguish direct oxidation and indirect oxidation mechanisms are reviewed, such as intermediate product analysis, electrochemical test analysis, active species detection, theoretical calculation, and the limitations of these methods are analyzed. Finally some suggestions are put forward for the study of the mechanism of electrochemical advanced oxidation.

Effect of electrolyte composition on electrochemical oxidation: Active sulfate formation, benzotriazole degradation, and chlorinated by-products distribution

Electrochemical oxidation is an effective technique for treating persistent organic pollutants, which are hardly removed in conventional wastewater treatment plants. Sulfate and chloride salts commonly used and present in natural wastewater influence the electrochemical degradation process. In this study, the effect of electrolyte composition on the active sulfate species (SO₄^●⁻ and S₂O₈²⁻) formation, benzotriazole degradation-a model organic compound, and chlorinated by-products distribution have been investigated while using a boron-doped diamond (BDD) anode. Different Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratios with constant conductivity of 10 mS/cm were used in the experiments and applied anode potential was kept constant at 4.3 V vs. Ag/AgCl. The electrogenerated SO₄^●⁻ and S₂O₈²⁻ formation were faster in 10:1 and 2:1 Na₂SO₄:NaNO₃ ratios than in the 1:0 ratio. The ^●OH-mediated SO₄^●⁻ production has prevailed in 10:1 and 2:1 ratios. However, ^●OH-mediated SO₄^●⁻ production has hindered the 1:0 ratio due to excess chemisorption of SO₄²⁻ on the BDD anode. Similarly, the faster benzotriazole degradation, mineralization, and lowest energy consumption were achieved in the 10:1 Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratio. Besides, chlorinated organic by-product concentration (AOX) was lower in the 10:1 Na₂SO₄:NaCl ratio but increased with the increasing chloride ratio in the electrolyte. LC-MS analysis shows that several chlorinated organic transformation products were produced in 0:1 to 2:1 ratio, which was not found in the 10:1 Na₂SO₄:NaCl ratio. A comparatively higher amount of ClO₄⁻ was formed in the 10:1 ratio than in 2:1 to 0:1 ratio. This ClO₄⁻ formation train evidence the effective ^●OH generation in a sulfate-enriched condition because the ClO₄⁻ formation is positively correlated to ^●OH concentration. Overall results show that sulfate-enriched electrolyte compositions are beneficial for electrochemical oxidation of biorecalcitrant organic pollutants.

Degradation of benzothiazole pollutant by sulfate radical-based advanced oxidation process

Benzothiazole (BTH) is an aromatic heterocyclic compound with wide industrial applications. In view of its toxicity and wide environmental presence, previous efforts have been made to decompose BTH via different degradation pathways. However, due to its recalcitrant nature, conventional biological treatment methods cannot completely degrade BTH in the wastewater. In this study, sulfate radical-based advanced oxidation process (AOP) technique has been adopted to degrade BTH in aqueous phase. Persulfate (PS) was employed as radical promotor to generate sulfate radical via heat activation. Degradation of BTH by thermally activated persulfate via AOP has been experimentally evaluated in a systematic manner. Laboratory efforts have been made to examine the impact of a number of physiochemical parameters including the type of oxidants, reaction temperature, initial concentrations of PS and BTH, solution pH, and the presence of anionic species. It shows that a higher BTH degradation rate can be achieved by lowering BTH initial concentration or increasing PS concentration. Increasing solution pH or the presence of 10 mM of Cl^-, Br^-, CO32-, or HCO3- species can decrease BTH degradation rate. Furthermore, the primary radical(s) responsible for BTH degradation have been identified as sulfate radical at an acidic aqueous condition, and hydroxyl radical and sulfate radical combined at a basic condition. This study provides the necessary theoretical and technical foundations for BTH degradation via sulfate radical-based AOP technique. The conclusions from this study can substantially promote the field application of AOP, especially sulfate radical-based AOP technique, for BTH degradation in wastewater treatment process.

Transformation of amino acids and formation of nitrophenolic byproducts in sulfate radical oxidation processes

In this study, the transformation of five amino acids (AAs), i.e., glycine (GLY), alanine (ALA), serine (SER), aspartic acid (ASP), and methionine (MET), in a heat activated peroxydisulfate (PDS) oxidation process was investigated. Experimental data showed that the nitrogen in the AA molecules was oxidized to NH₄⁺ and nitrate (NO₃^-) sequentially. However, in the presence of natural organic matter (NOM), nitrophenolic byproducts including 4-nitrophenol, 2,4-dinitrophenol, 5-nitrosalicylic acid, 3,5-dinitrosalicylic acid were formed. The nitrogen dioxide radical (NO₂^•) generated during the transformation of NH₄⁺ to NO₃^- was presumed to be the key nitrating agent. It reacted with phenolic moieties in NOM and was transformed to nitrophenolic byproducts. Among the selected AAs, SER showed the highest nitrophenolic byproducts formation potential. A total yield of 0.258 μM was observed at the condition of 0.1 mM SER, 5 mg/L (as TOC) NOM, 2 mM PDS, and pH 7.0. The formation from GLY and ALA was lowest, only 0.009 μM detected under the same conditions. The nitrophenolic byproducts formation potential of the AAs was positively related to their reactivity toward SO₄^•- and can be explained by the local nucleophilicity index (N_k). These findings underline the potential risks in the application of SO₄^•- based oxidation technology.