Unraveling the role of Mn(V)/Mn(III) in the enhanced permanganate oxidation under Vis-LED radiation.

A novel approach of visible light-emitting diode (Vis-LED) radiation was employed to activate permanganate (Mn(VII)) for efficient organic micropollutant (OMP) removal. The degradation rates of OMPs by Vis-LED/Mn(VII) were 2-5.29 times higher than those by Mn(VII) except for benzoic acid and atrazine. Increasing wavelengths (445-525 nm) suppressed the degradation of diclofenac (DCF) and 4-chlorophenol (4-CP) owing to the decreased quantum yields of Mn(VII). Comparatively, light intensity and Mn(VII) dosage had a positive effect on the degradation of DCF and 4-CP. Experimental data revealed that Mn(V) dominated the DCF degradation whereas Mn(III) was the active oxidant in the 4-CP degradation. Mn(V) and Mn(III) formed from the photo-decomposition of Mn(VII), meanwhile, Mn(III) also formed from the Mn(V) photo-decomposition. The increase in solution pH inhibited DCF degradation but had a positive impact on 4-CP degradation, mainly due to the changing speciation of DCF and 4-CP. Inorganic anions (Cl- and HCO3-) had little impact on DCF and 4-CP degradation, while humic acid (HA) showed a positive impact because of the π-π interaction between HA and DCF/4-CP. The transformation products of DCF and 4-CP were identified and transformation pathways were proposed. Finally, the Vis-LED/Mn(VII) exhibited great degradation performance in various authentic waters. Overall, this study boosts the development of Mn(VII)-based oxidation processes.

Role of bipyridyl in enhancing ferrate oxidation toward micropollutants.

Enhancing Fe(VI) oxidation ability by generating high-valent iron-oxo species (Fe(IV)/Fe(V)) has attracted continuous interest. This work for the first time reports the efficient activation of Fe(VI) by a well-known aza-aromatic chelating agent 2,2'-bipyridyl (BPY) for micropollutant degradation. The presence of BPY increased the degradation constants of six model compounds (i.e., sulfamethoxazole (SMX), diclofenac (DCF), atenolol (ATL), flumequine (FLU), 4-chlorophenol (4-CP), carbamazepine (CBZ)) with Fe(VI) by 2 - 6 folds compared to those by Fe(VI) alone at pH 8.0. Lines of evidence indicated the dominant role of Fe(IV)/Fe(V) intermediates. Density functional theory calculations suggested that the binding of Fe(III) to one or two BPY molecules initiated the oxidation of Fe(III) to Fe(IV) by Fe(VI), while Fe(VI) was reduced to Fe(V). The increased exposures of Fe(IV)/Fe(V) were experimentally verified by the pre-generated Fe(III) complex with BPY and using methyl phenyl sulfoxide as the probe compound. The presence of chloride and bicarbonate slightly affected model compound degradation by Fe(VI) in the presence of BPY, while a negative effect of humic acid was obtained under the same conditions. This work demonstrates the potential of N-donor heterocyclic ligand to activate Fe(VI) for micropollutant degradation, which is instructive for the Fe(VI)-based oxidation processes.

Enhanced hydroxyl radical generation for micropollutant degradation in the In2O3/Vis-LED process through the addition of periodate.

Periodate (PI) as an oxidant has been extensively studied for organic foulants removal in advanced oxidation processes. Here PI was introduced into In2O3/Vis-LED process to enhance the formation of ·OH for promoting the degradation of organic foulants. Results showed that the addition of PI would significantly promote the removal of sulfamethoxazole (SMX) in the In2O3/Vis-LED process (from 9.26% to 100%), and ·OH was proved to be the dominant species in the system. Besides, the process exhibited non-selectivity in the removal of different organic foulants. Comparatively, various oxidants (e.g., peroxymonosulfate, peroxydisulfate, and hydrogen peroxide) did not markedly promote the removal of SMX in the In2O3/Vis-LED process. Electrochemical analyses demonstrated that PI could effectively receive photoelectrons, thus inhibiting the recombination of photogenerated electron-hole (e-/h+) pairs. The holes then oxidized the adsorbed H2O to generate ·OH, and the PI converted to iodate at the same time. Additionally, the removal rate of SMX reduced from 100% to 17.2% as Vis-LED wavelengths increased from 440 to 560 nm, because of the low energy of photons produced at longer wavelengths. Notably, the species of PI do not affect its ability to accept electrons, resulting in the degradation efficiency of SMX irrespective of pH (4.0-10.0). The coexistence of inorganic cations and anions (such as Cl-, CO32-/HCO3-, SO42-, Ca2+, and Mg2+) also had an insignificant effect on SMX degradation. Furthermore, the process also showed excellent degradation potential in real water. The proposed strategy provides a new insight for visible light-catalyzed activation of PI and guidance to explore green catalytic processes for high-efficiency removal of various organic foulants.

Generation of hydroxyl radicals via activation of Cr(VI) by UVA-LED for rapid decontamination: The important role of Cr(V).

Hexavalent chromium (Cr(VI)) was activated by ultraviolet-A light-emitting diode (UVA-LED), resulting in efficient removal of various pollutants, including dye, pharmaceuticals, and pesticides, with pseudo-first-order rate constants of 0.0610-0.159 min-1. Comparatively, UVA-LED or Cr(VI) alone barely degraded selected pollutants. Both HO• and Cr(V) were produced in the UVA-LED/Cr(VI) system based on scavenging and probing experiments, UV-visible and electron spin resonance spectra analysis. HO• was demonstrated to be the dominant reactive species via stepwise regeneration of Cr(V) to Cr(VI). The quantum yield of HO• was determined to be 7.79 × 10-4 mol Es-1 at a Cr(VI) dosage of 0.5 mM and pH of 6.0. Additionally, the degradation efficiency of sulfamethoxazole (SMX) as a model compound decreased linearly as UVA-LED wavelengths increased from 365 to 405 nm, while SMX was barely degraded at visible light irradiation wavelength ranges (449-505 nm). SMX degradation efficiency increased from 71.0 % to 97.5 % as Cr(VI) dosage increased from 0.05 to 0.7 mM. pH displayed a negative impact on SMX degradation with its removal efficiency decreasing from 99.4 % to 13.3 % as pH increased from 3.0 to 9.0. This study first reported that HO• was generated via activation of Cr(VI) by UVA-LED, which is instructive for the removal of pollutants co-existed in chromium-containing wastewater.

Enhanced Permanganate Activation under UVA-LED Irradiation: Unraveled Mechanism Involving Manganese Species and Hydroxyl Radical.

Permanganate [Mn(VII)] has gained broad attention in water treatment. However, its limited reactivity toward some refractory micropollutants hinders its application for micropollutant degradation. Herein, we introduced UVA-LED photolysis of Mn(VII) (UVA-LED/Mn(VII)) to degrade micropollutants (diclofenac (DCF), 4-chlorophenol (4-CP), atrazine, and nitrobenzene) by selecting DCF and 4-CP as target micropollutants. The effects of operating conditions (e.g., light intensity, radiation wavelengths, pH, and water constituents) on DCF and 4-CP degradation as well as the underlying mechanisms were systematically studied. The degradation rates of DCF and 4-CP linearly decreased with increasing radiation wavelengths (from 365 to 405 nm), likely due to the decreased molar absorption coefficients and quantum yields of Mn(VII). Reactive manganese species (RMnS), including Mn(V), Mn(III), and HO•, were generated in the UVA-LED/Mn(VII) process. Mn(V) and HO• were responsible for DCF degradation, while Mn(III), HO•, and likely Mn(V) accounted for 4-CP degradation. Competitive kinetic results revealed that contributions of RMnS and HO• decreased with increasing radiation wavelengths, wherein RMnS played the dominant role. Increasing pH displayed opposite effects on DCF and 4-CP degradation with higher degradation efficiency obtained at acidic pH for the former one but alkaline pH for the latter one. The presence of water background ions (e.g., Cl-, HCO3-, and Ca2+) barely influenced DCF and 4-CP degradation. Finally, in comparison with Mn(VII) alone, enhanced degradation of DCF and 4-CP by UVA-LED/Mn(VII) was observed in real waters. This work advances the understanding of the photochemistry of manganese species in micropollutant degradation and facilitates Mn(VII) oxidation in practical application.

Novel solar-driven ferrate(VI) activation system for micropollutant degradation: Elucidating the role of Fe(IV) and Fe(V).

This paper presents a novel process of solar-ferrate(VI) [Fe(VI)] for micropollutant degradation. The solar-Fe(VI) process promoted micropollutant degradation compared with Fe(VI) alone and solar. The radical scavenging and probing experiment results suggested that Fe(V) and Fe(IV) but not reactive oxygen species were most likely involved in the solar-Fe(VI) process. Through building a kinetic model, Fe(IV) and Fe(V) were observed to play an equally significant role in the solar-Fe(VI) process. Afterward, the reaction mechanism of the photochemistry of Fe(VI) was elaborated. Fe(IV) formed from Fe(VI) photolysis and then decomposed to Fe(II) which reacted with Fe(VI) to form Fe(V). Furthermore, the effect of pH on carbamazepine (CBZ) degradation was studied and the quantum yields of Fe(VI) were determined, with (1.98 ± 0.16)× 10-3 mol∙einstein-1, (5.90 ± 0.27)× 10-4 mol∙einstein-1, and (1.66 ± 0.14)× 10-4 mol∙einstein-1 at pH 7.0, 8.0, and 9.0, respectively. Inorganic ions, including Cl-, HCO3-, and Br- displayed negligible influence on the CBZ degradation, whereas humic acid inhibited the CBZ degradation. Finally, the solar-Fe(VI) process exhibited good applicability in authentic waters and under different irradiation (natural sunlight, ultraviolet light, and visible light from solar cut-off emission). Overall, this study provides a new routine for efficient micropollutant elimination and reveals the photochemistry of Fe(VI).

Enhanced removal of organoarsenic by chlorination: Kinetics, effect of humic acid, and adsorbable chlorinated organoarsenic.

In this study, the effective removal of organoarsenic by the combined process of "chlorination + Fe(II)" was achieved. Chlorine could effectively degrade roxarsone (ROX) over pH from 5 to 10. The fitting results of acid-base protonation model proved that the degradation of ROX was mainly attributed to the reaction of HOCl and deprotonated ROX. The transformation of arsenic species conformed to the fitting results of two-channel kinetic model, in which 32.4% of ROX was oxidized to As(V) via electron transfer pathway (ii) and the rest was converted into monochloro-ROX via electrophilic substitution pathway (i). Humic acid inhibited the degradation of ROX due to the competitive consumption of chlorine and the restraint on the pathway ii. Subsequently, an enhanced removal of total arsenic achieved after chlorination, due to that the generating As(V) and monochloro-ROX were easier adsorbed compared with ROX, over 97.8% of total arsenic was removed by ferric (oxyhydr)oxides which in-situ formed from the oxidation of Fe(II). Additionally, toxicity studies indicated that the acute toxicity was significantly eliminated by adding Fe(II) after chlorination, likely due to the removal of As(V) and chlorinated products. Furthermore, organoarsenic was also effectively removed by the combined process of "chlorination + Fe(II)" in real water.

UVA-LED-Assisted Activation of the Ferrate(VI) Process for Enhanced Micropollutant Degradation: Important Role of Ferrate(IV) and Ferrate(V).

This paper investigated ultraviolet A light-emitting diode (UVA-LED) irradiation to activate Fe(VI) for the degradation of micropollutants (e.g., sulfamethoxazole (SMX), enrofloxacin, and trimethoprim). UVA-LED/Fe(VI) could significantly promote the degradation of micropollutants, with rates that were 2.6-7.2-fold faster than for Fe(VI) alone. Comparatively, UVA-LED alone hardly degraded selected micropollutants. The degradation performance was further evaluated in SMX degradation via different wavelengths (365-405 nm), light intensity, and pH. Increased wavelengths led to linearly decreased SMX degradation rates because Fe(VI) has a lower molar absorption coefficient at higher wavelengths. Higher light intensity caused faster SMX degradation, owing to the enhanced level of reactive species by stronger photolysis of Fe(VI). Significantly, SMX degradation was gradually suppressed from pH 7.0 to 9.0 due to the changing speciation of Fe(VI). Scavenging and probing experiments for identifying oxidative species indicated that high-valent iron species (Fe(V)/Fe(IV)) were responsible for the enhanced degradation. A kinetic model involving target compound (TC) degradation by Fe(VI), Fe(V), and Fe(IV) was employed to fit the TC degradation kinetics by UVA-LED/Fe(VI). The fitted results revealed that Fe(IV) and Fe(V) primarily contributed to TC degradation in this system. In addition, transformation products of SMX degradation by Fe(VI) and UVA-LED/Fe(VI) were identified and the possible pathways included hydroxylation, self-coupling, bond cleavage, and oxidation reactions. Removal of SMX in real water also showed remarkable promotion by UVA-LED/Fe(VI). Overall, these findings could shed light on the understanding and application of UVA-LED/Fe(VI) for eliminating micropollutants in water treatments.

Insight into enhanced activation of permanganate under simulated solar irradiation: Rapid formation of manganese species.

Herein, permanganate [Mn(VII)] was activated by simulated solar (SS) (SS/Mn(VII)), resulting in rapid degradation of micropollutants in several minutes, with rates of target micropollutants outnumbered those in the Mn(VII) alone and SS. To explore the mechanism in this process, 4-cholorphenol (4-CP), p-hydroxybenzoic acid (p-HBA), and enrofloxacin (ENR) were selected as model compounds. Lines of evidence indicated that reactive manganese species (RMnS) (i.e., Mn(III) and Mn(V)) rather than radicals from Mn(VII) photolysis participated in the conversion of model compounds. Interestingly, roles of RMnS differed among three model compounds, suggesting their selectivity toward micropollutants. Increasing Mn(VII) dosage proved greater micropollutant degradation, while impacts of pH on SS/Mn(VII) performance varied among model compounds. P-HBA and ENR showed the lowest degradation efficiency at alkaline, whereas 4-CP demonstrated the best performance at alkaline, indicating the reactivity of RMnS varied toward micropollutants at different pH values. The quantum yield of Mn(VII) was 8.36 ± 0.03 X 10-6 mol Einstein-1 at pH 7.0. Effects of common co-existing constituents (Cl-, HCO3-, and humic acid (HA)) on micropollutant degradation by SS/Mn(VII) were examined. Specifically, HCO3- positively influenced the 4-CP and p-HBA degradation, whereas ENR was not affected, likely owing to the selectivity of RMnS-HCO3- complexes. HA was conducive to degrade p-HBA due to the production of RMnS-HA complexes, but unfavorable for ENR and 4-CP degradation because of the competitive light absorption and Mn(VII). Furthermore, a number of degradation products of 4-CP, p-HBA, and ENR were identified and possible pathways were proposed accordingly. The effectiveness of this process for micropollutant degradation in real waters, natural sunlight, ultraviolet and visible light via cut-off filtering SS emission was confirmed. This work revealed a great potential of applying SS/Mn(VII) for the marked degradation of micropollutants and facilitated the understandings of Mn(III)/Mn(V) behaviors.

UV/chlorine process for degradation of benzothiazole and benzotriazole in water: Efficiency, mechanism and toxicity evaluation.

Benzothiazole (BZA) and benzotriazole (BTZ) as emerging contaminants were found persistent in aquatic environments and toxic to aquatic organisms. The degradation of BZA and BTZ by UV/chlorine was systematically investigated in this study, and the results showed that BZA and BTZ can be remarkably removed by UV/chlorine compared with UV alone and dark chlorination. The radical quenching tests showed that degradation of BZA and BTZ by UV/chlorine involved the participation of reactive chlorine species (RCS), hydroxyl radical (HO·), and UV photolysis. HO· dominated BZA degradation at neutral and alkalinity, while RCS dominated BTZ degradation. The second-rate order constants for ClO· and BZA and BTZ were 2.22 × 108 M-1 s-1, and 2.40 × 108 M-1 s-1, respectively. Besides, the second-order rate constants for HO· and BZA and BTZ were also determined at pH 5.0, 7.0, and 9.0, respectively. The degradation efficiency of BZA by UV/chlorine was substantially promoted at acidic conditions, while the degradation efficiency of BTZ was promoted at both acidic and specific alkaline range mainly due to the reactivity of radical species and deprotonated form. The influence of Cl- was negligible, but the suppression effect of humic acid was slight during the BZA and BZT degradation by UV/chlorine. The transformation products were detected and the possible pathways were proposed. Seven disinfection by-products (DBPs) were identified both in BZA and BTZ degradation and trichloromethane was the main DBP. The toxicity assessment performed by luminescent bacteria and ECOSAR analysis indicated that the detoxification of BZA could be achieved by UV/chlorine, whereas the toxicity of BTZ was increased mainly due to the formation of intermediates. The findings from this study demonstrated UV/chlorine is likewise efficient for BZA and BTZ removal but the toxicity should be considered in the BTZ degradation.