Novel sunlight-induced monochloramine activation system for efficient microcontaminant abatement.

As an eco-friendly and sustainable energy, solar energy has great application potential in water treatment. Herein, simulated sunlight was for the first time utilized to activate monochloramine for the degradation of environmental organic microcontaminants. Various microcontaminants could be efficiently degraded in the simulated sunlight/monochloramine system. The average innate quantum yield of monochloramine over the wavelength range of simulated sunlight was determined to be 0.068 mol/Einstein. With the determined quantum yield, a kinetic model was established. Based on the good agreement between the simulated and measured photolysis and radical contributions to the degradation of ibuprofen and carbamazepine, the major mechanism of monochloramine activation by simulated sunlight was proposed. Chlorine radical (Cl∙) and hydroxyl radical (HO∙) were major radicals responsible for microcontaminant degradation in the system. Moreover, the model facilitated a deep investigation into the effects of different reaction conditions (pH, monochloramine concentration, and water matrix components) on the degradation of ibuprofen and carbamazepine, as well as the roles of the involved radicals. The differences between simulated and measured degradation data of each microcontaminant under all conditions were less than 10 %, indicating the strong reliability of the model. The model could also make good prediction for microcontaminant degradation in the natural sunlight/monochloramine system. Furthermore, the formation of disinfection byproducts (DBPs) was evaluated at different oxidation time in simulated sunlight/monochloramine with and without post-chloramination treatment. In real waters, organic components showed more pronounced suppression on microcontaminant degradation efficiency than inorganic ions. This study provided a systematic investigation into the novel sunlight-induced monochloramine activation system for efficient microcontaminant degradation, and demonstrated the potential of the system in practical applications.

Photo-assisted natural chalcopyrite activated peracetic acid for efficient micropollutant degradation.

The effective activation of natural chalcopyrite (CuFeS2) on peracetic acid (PAA) to remove organic micropollutants was studied under visible light irradiation. Results showed than an effective sulfamethoxazole (SMX) degradation (95.0 %) was achieved under visible light irradiation for 30 min at pH 7.0. Quenching experiments, electron spin resonance analysis, and LC/MS spectrum demonstrated that HO• and CH3C(O)OO• were the main reactive species for SMX degradation, accounting for 43.3 % and 56.7 % of the contributions, respectively. Combined with X-ray photoelectron spectroscopy analysis, the photoelectrons generated on CuFeS2 activated by visible light enhanced the Fe3+/Fe2+ and Cu2+/Cu+ cycles on the surface, thereby activating PAA to generate HO•/CH3C(O)OO•. The removal rate of SMX decreased with the increase in wavelengths, due to the formation of low energy photons at longer wavelengths. Besides, the optimal pH for degradation of SMX by CuFeS2/PAA/Vis-LED process was neutral, which was attributed to the increasing easily activated anionic form of PAA during the increase in pH and the depletion of Fe species at alkaline conditions. Cl-, HCO3-, and HA slightly inhibited SMX degradation because of reactive species being quenched and/or shielding effect. Furthermore, the degradation efficiency of different pollutants by CuFeS2/PAA/Vis-LED was also measured, and the removal efficiency was different owing to the selectivity of CH3C(O)OO•. Finally, the process exhibited good applicability in real waters. Overall, this study provides new insight into visible light-catalyzed activation of PAA and suggests on further exploration of the intrinsic activation mechanism of PAA.

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.

Refinement of kinetic model and understanding the role of dichloride radical (Cl2•-) in radical transformation in the UV/NH2Cl process.

The ultraviolet/monochloramine (UV/NH2Cl) process is an emerging advanced oxidation process with promising prospects in water treatment. Previous studies developed kinetic models of UV/NH2Cl for simulating radical concentrations and pollutant degradation. However, the reaction rate constants of Cl2•- with bicarbonate and carbonate (kCl2•-, HCO3- and kCl2•-, CO32-) were overestimated in literature. Consequently, when dosing 1 mM chloride and 1 mM bicarbonate, the current models of UV/NH2Cl severely under-predicted the experimental concentrations of three important radicals (i.e., hydroxyl radical (HO•), chlorine radical (Cl•), and dichloride radical (Cl2•-)) with great deviations (> 90 %). To investigate this issue, the transformation reactions among these three radicals in UV/NH2Cl were systematically studied. For the first time, it was found that in addition to Cl•, Cl2•- was also an important parent radical of HO• in the presence of chloride, and chloride could effectively compensate the inhibitory effect of bicarbonate on HO• generation in the system. Moreover, reactions and rate constants in current models were scrutinized from corresponding literature, and the reaction rate constants of Cl2•- with bicarbonate and carbonate (kCl2•-, HCO3- and kCl2•-, CO32-) were reevaluated to be 1.47 × 105 and 3.78 × 106 M-1s-1, respectively, by laser flash photolysis. With the newly obtained rate constants, the refined model could accurately simulate concentrations of all three radicals under different chloride and bicarbonate dosages with satisfactory deviations (< 30 %). Meanwhile, the refined model performed much better in predicting pollutant degradation and radical contribution compared with the unrefined model (with the previously estimated kCl2•-, HCO3- and kCl2•-, CO32-). The results of this study enhanced the accuracy and applicability of the kinetic model of UV/NH2Cl, and deepened the understanding of radical transformation in the process.

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.

The overlooked role of Cr(VI) in micropollutant degradation under solar light irradiation.

Hexavalent chromium (Cr(VI)) is ubiquitous in natural environments, whereas its role in the transformation of coexisting contaminants may have been overlooked. In this work, it was reported for the first time that the irradiation of Cr(VI) by solar light (solar light/Cr(VI) system) could effectively degrade various micropollutants with different structures. The removal efficiency of selected micropollutants was increased by 13.3-64.8% by the solar light/Cr(VI) system compared to that by direct solar photolysis. Meanwhile, the oxidation rates were enhanced by 2.2-21.5 folds, while they were negligible by Cr(VI) oxidation alone. Experiments by specific scavengers, probe compounds, fluorescence absorbance, and electron spin resonance analysis demonstrated that hydroxyl radical (•OH) was the major reactive species in the solar light/Cr(VI) system. Further experiments showed that the generation of •OH was closely related to the intermediate Cr(V) generated from Cr(VI) reduction, and Cr(V) could be re-oxidized back to Cr(VI). Increasing solution pH negatively affected model micropollutant (carbamazepine (CBZ)) degradation by the solar light/Cr(VI) system, mainly due to the decreased quantum yield of •OH at higher pH. Coexisting sulfate ions showed negligible effect on CBZ degradation in the solar light/Cr(VI) system, while the presence of bicarbonate, chloride, and humic acid inhibited CBZ degradation to varying degrees, owing to their diverse scavenging effects on •OH. Furthermore, moderate CBZ degradation was also achieved by natural solar light photolysis of Cr(VI). This study demonstrated the pivotal role of Cr(VI) in the transformation of micropollutants under solar irradiation, which advances the understanding of the fate of micropollutants in natural environments.

Efficient removal of p-arsanilic acid and arsenite by Fe(II)/peracetic acid (Fe(II)/PAA) and PAA processes.

The widespread occurrence of p-arsanilic acid (p-ASA) in natural environments poses big threats to the biosphere due to the generation of toxic inorganic arsenic (i.e., As(III) and As(V), especially As(III) with higher toxicity and mobility). Oxidation of p-ASA or As(III) to As(V) followed by precipitation of total arsenic using Fe-based advanced oxidation processes demonstrated to be a promising approach for the treatment of arsenic contamination. This study for the first time investigated the efficiency and inherent mechanism of p-ASA and As(III) oxidation by Fe(II)/peracetic acid (Fe(II)/PAA) and PAA processes. p-ASA was rapidly degraded by the Fe(II)/PAA process within 20 s at neutral to acidic pHs under different conditions, while it was insignificantly degraded by PAA oxidation alone. Lines of evidence suggested that hydroxyl radicals and organic radicals generated from the homolytic OO bond cleavage of PAA contributed to the degradation of p-ASA in the Fe(II)/PAA process. p-ASA was mainly oxidized to As (V), NH4+, and p-aminophenol by the Fe(II)/PAA process, wherein the aniline group and its para position were the most vulnerable sites. As(III) of concern was likely generated as an intermediate during p-ASA oxidation and it could be readily oxidized to As(V) by the Fe(II)/PAA process as well as PAA alone. The in-depth investigation demonstrated that PAA alone was effective in the oxidation of As(III) under varied conditions with a stoichiometric molar ratio of 1:1. Efficient removal (> 80%) of total arsenic during p-ASA oxidation by Fe(II)/PAA process or during As(III) oxidation by PAA process with additional Fe(III) in synthetic or real waters were observed, mainly due to the adsorptive interactions of amorphous ferric (oxy)hydroxide precipitates. This study systematically investigates the oxidation of p-ASA and As(III) by the Fe(II)/PAA and PAA processes, which is instructive for the future development of arsenic remediation technology.

Photolysis of chlorite by solar light: An overlooked mitigation pathway for chlorite and micropollutants.

Chlorite (ClO2-) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO2- but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO2- by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO2- could be efficiently decomposed to chloride (Cl-) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl- yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O3), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO2- system with the steady-state concentrations following the order of O3 (≈ 0.8 µΜ) > ClO• (≈ 4.4 × 10-6 µΜ)> •OH (≈ 1.1 × 10-7 µΜ)> Cl• (≈ 6.8 × 10-8 µΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO2- system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min-1 at pH 7.0, while most of them were negligibly degraded by SSL or ClO2- treatment alone. Kinetic modeling of BZF degradation by SSL/ClO2- at pHs 6.0 - 8.0 suggested that •OH contributed the most, followed by Cl•, O3, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO2- system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO2- and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO2- and micropollutants, which has significant implications for understanding their fate in natural environments.

Ozone- and Hydroxyl Radical-Induced Degradation of Micropollutants in a Novel UVA-LED-Activated Periodate Advanced Oxidation Process.

In this study, novel light emitting diode (LED)-activated periodate (PI) advanced oxidation process (AOP) at an irradiation wavelength in the ultraviolet A range (UVA, UVA-LED/PI AOP) was developed and investigated using naproxen (NPX) as a model micropollutant. The UVA-LED/PI AOP remarkably enhanced the degradation of NPX and seven other selected micropollutants with the observed pseudo-first-order rate constants ranging from 0.069 ± 0.001 to 4.50 ± 0.145 min-1 at pH 7.0, demonstrating a broad-spectrum micropollutant degradation ability. Lines of evidence from experimental analysis and kinetic modeling confirmed that hydroxyl radical (•OH) and ozone (O3) were the dominant species generated in UVA-LED/PI AOP, and they contributed evenly to NPX degradation. Increasing the pH and irradiation wavelength negatively affected NPX degradation, and this could be well explained by the decreased quantum yield (ΦPI) of PI. The degradation kinetics of NPX by the UVA-LED/PI AOP in the presence of water matrices (i.e., chloride, bicarbonate, and humic acid) and in real waters were examined, and the underlying mechanisms were illustrated. A total of nine transformation products were identified from NPX oxidation by the UVA-LED/PI AOP, mainly via hydroxylation, dealkylation, and oxidation pathways. The UVA-LED/PI AOP proposed might be a promising technology for the treatment of micropollutants in aqueous solutions. The pivotal role of ΦPI during light photolysis of PI may guide the future design of light-assisted PI AOPs.