A combined radical and non-radical oxidation processes for efficient degradation of Acid Orange 7 in the homogeneous Cu(II)/PMS system: important role of chloride

Degradation of refractory organic matter in MBR effluent from treating landfill leachate by UV/PMS and UV/H2O2: a comparative study

This study applied ultraviolet/peroxymonosulfate (UV/PMS) and UV/hydrogen peroxide (UV/H2O2) processes to the advanced treatment of membrane bioreactor (MBR) effluent. The degradation efficiency of refractory organic matter and the reaction mechanisms of the two processes were systematically investigated. The results showed that the degradation efficiency of the UV/PMS processes was significantly lower than that of the UV/H2O2 process when the PMS concentration was significantly lower than the H2O2 concentration, e.g. the UV254 removals under optimal conditions were 72.92% and 82.21%, respectively. Additionally, the UV/PMS process could operate over a broader pH range. The degradation efficiency of the UV/PMS process was slightly increased by HCO3- and Cl- due to the activation of PMS, while in the UV/H2O2 process, HCO3- and Cl- depressed the degradation efficiency by competing with organic matter to react with reactive oxygen species (ROS). After the two processes, the aromaticity, humification, condensation degree, and molecular weight of refractory organic matter in the MBR effluent were considerably decreased. Fulvic- (HA) and humic-like substances (FA) were greatly degraded by the two processes. The UV/PMS had a superior degradation efficiency for macromolecular HA in the early stage of the reaction, and the UV/H2O2 could degrade HA to protein-like substances in the latter stage of the reaction. These differences between the two processes could be attributed to the dominance of different ROS, with SO4•- and HO• dominating in the UV/PMS, and HO• dominating in the UV/H2O2. The results of this study provide theoretical support for the application of MBR effluent treatment.Highlights Comparison on the MBR effluent treatment of UV/PMS and UV/H2O2 is studied.UV/PMS process can better destroy humic-like substances in the early reaction stage.Humic-like substances are transformed into protein-like compounds in UV/H2O2 process.UV/PMS and UV/PMS performs differently due to their different dominant ROS.

Cu(II) assisted peroxymonosulfate for antibiotic resistant bacteria inactivation: A potential disinfection technology in swimming pool

Alternative disinfection technology to chlorination is required to control the risk of antibiotic resistance in swimming pools. In this study, copper ions (Cu(II)), which often exist in swimming pools as algicides, were used to activate peroxymonosulfate (PMS) for the inactivation of ampicillin-resistant E. coli. Cu(II) and PMS showed synergistic effects on E. coli inactivation in weak alkaline conditions, obtaining 3.4 log inactivation in 20 min with 10 μM Cu(II) and 100 μM PMS at pH 8.0. Quenching experiments indicated that radicals (i.e., OH and SO₄^-) were not the main disinfectors for E. coli inactivation. Based on the structure of Cu(II) and density functional theory calculations, the Cu(II)-PMS complex (Cu(H₂O)₅SO₅) was recommended as the active species for E. coli inactivation. Under the experimental conditions, the PMS concentration had a greater influence on E. coli inactivation than the Cu(II) concentration, possibly because increasing PMS concentration accelerates ligand exchange reaction and facilitates active species generation. By forming hypohalous acids, halogen ions could improve the disinfection efficiency of Cu(II)/PMS. The addition of HCO₃^- concentration (from 0 to 1.0 mM) and humic acid (0.5 and 1.5 mg/l) did not significantly inhibit the E. coli inactivation. The feasibility of adding PMS to waters containing Cu(II) for the inactivation of antibiotic-resistant bacteria was validated in actual swimming pool waters, where 4.7 log inactivation of E. coli was achieved in 60 min.

Phosphate enhanced Cu(II)/peracetic acid process for diclofenac removal: Performance and mechanism

Since limitedly existing researches suggested Cu(II) had deficiently catalytic ability to PAA, in this work, we tested the oxidation performance of Cu(II)/PAA system on diclofenac (DCF) degradation under neutral conditions. It was found that overwhelming DCF removal could be obtained in Cu(II)/PAA system at pH 7.4 using phosphate buffer solution (PBS) compared to poor loss of DCF without PBS, and the apparent rate constant of DCF removal in PBS/Cu(II)/PAA system was 0.0359 min^-1, 6.53 times of that in Cu(II)/PAA system. Organic radicals (i.e., CH₃C(O)O• and CH₃C(O)OO•) were evidenced as the dominant contributors to DCF destruction in PBS/Cu(II)/PAA system. PBS motivated the reduction of Cu(II) to Cu(I) through chelation effect, and then the activation of PAA by Cu(I) was facilitated. Besides, due to the steric hindrance of Cu(II)-PBS complex (CuHPO₄), PAA activation was mediated from non-radical-generating pathway to radical-generating pathway, leading to desirably effective DCF removal by radicals. The transformation of DCF mainly experienced hydroxylation, decarboxylation, formylation and dehydrogenation in PBS/Cu(II)/PAA system. This work proposes the potential of coupling of phosphate and Cu(II) in optimizing PAA activation for organic pollutants elimination.

Thermal effect on sulfamethoxazole degradation in a trivalent copper involved peroxymonosulfate system

Persulfate (PS) activated by thermal or homogeneous metals can generate reactive oxygen species (ROS) and high-valence-state metals for contaminants degradation, showing great potential for applications. However, thermal effect in peroxymonosulfate (PMS) system with high-valence-state metal is still ambiguous. In this study, divalent copper (Cu(II)) catalysis was taken to explore thermal effect on PMS performance. Results showed that the Sulfamethoxazole (SMX) removal efficiency in the Cu(II)/PMS system at 60 min increased by only 5.9% with temperature increase from 30 °C to 60 °C. Moreover, SMX removal efficiency was excellent at neutral or basic pH, best with PMS concentration of 2.4 mM, and slightly affected by Cu(II) concentration. The singlet oxygen (¹O₂) was identified as main active species at low temperature while sulfate radicals (SO₄^-) was more effective at high temperature with Cu(II) co-activation. Also, trivalent copper (Cu(III)) was an important active species. The higher Cu(III) content, the better SMX removal efficiency, but the stronger intermediates toxicity. In combination with removal efficiency and intermediates toxicity at different temperatures, 30 °C was the optimal reaction temperature. Overall, this study provides new perspective on utilization of waste heat and high-valence-state metal for organic wastewater treatment in PMS systems.

Hydroxylamine-enhanced Fe(II)-peroxymonosulfate activation for efficient degradation of organic pollutants: optimization by response surface methodology

In this work, the response surface methodology (RSM) was used to model and optimize the hydroxylamine (HA) enhanced Fe(II)/peroxymonosulfate (PMS) process. The enhanced effect of HA on the degradation efficiency of Orange II (AO7) in the Fe(II)/PMS system was quantitatively analyzed. Pareto analysis showed that the individual and interactive effects of HA, Fe(II) and PMS were of the following order: HA > Fe(II) > PMS and Fe(II)/PMS > HA/PMS > Fe(II)/HA. The optimal conditions of HA/Fe(II)/PMS system were as follows: Fe(II) concentration was 34.0 μM, HA concentration was 0.4 mM, and PMS concentration was 0.9 mM. When the initial pH was 4.0-6.0, the degradation efficiency of AO7 in the HA/Fe(II)/PMS system was significantly higher than that in the Fe(II)/PMS system (P < 0.05). Hydroxylamine enhances the degradation of AO7 in the Fe(II)/PMS system by reducing Fe(III) to Fe(II). The results of quenching experiment showed that SO₄^•- was the dominating reactive oxygen species (ROS) in the HA/Fe(II)/PMS system. In the HA/Fe(II)/PMS system, CO₃^2- and humic acid inhibited the degradation efficiency of AO7. This work provides a novel mathematical model for the degradation of AO7 in the HA/Fe(II)/PMS system, which can be popularized and applied in similar experiments.

EPR Evidence for Mechanistic Diversity of Cu(II)/Peroxygen Oxidation Systems by Tracing the Origin of DMPO Spin Adducts

Electron paramagnetic resonance (EPR) has been extensively used for the identification of free radicals that are generated from advanced oxidation processes (AOPs) so as to establish the reaction mechanism. However, some misinterpretations or controversies on the identity of detected EPR signals remain in the literature. This study, with Cu(II)-based AOPs as examples, comprehensively investigated the origin of 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adducts in Cu(II) alone, Cu(II)/H₂O₂, Cu(II)/peroxymonosulfate (PMS), and Cu(II)/peroxydisulfate (PDS) systems. In most Cu(II) systems, DMPO-OH signals can be detected even without any peroxygens, indicating the presence of other origins of this adduct in addition to the genuine spin trapping of ^•OH by DMPO. According to the formed secondary radical adducts (DMPO-OCH₃ from a nonradical process or DMPO-CH₂OH from a radical oxidation) derived from methanol quenching, we propose that CuO⁺, instead of free radicals, is involved in the Cu(II)/PMS system, while ^•OH is indeed generated in the Cu(II)/H₂O₂ and Cu(II)/PDS systems under neutral conditions. Notably, ¹⁷O-incorporation experiments demonstrate that -OH in the detected DMPO-OH adduct originates 100% from water in the Cu(II) alone system but the amount of -OH is over 99.8% from the oxidant while peroxygens are added. In addition, DMPO-O₂^- appears only in the Cu(II)/PDS system under highly alkaline conditions and H₂O is not involved in superoxide formation.