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The photolysis of chlorine by UV light leads to the formation of the hydroxyl radicals (OH) as well as reactive chlorine species (RCS) that can be effective as advanced oxidation processes (AOPs) for water treatment. Much of the research to date has been done at laboratory- or bench-scale. This study reports results from a model that demonstrates that the relative effectiveness of the UV/Cl 2 AOP compared to the more traditional UV/H 2 O 2 AOP is a function of optical path length. As such, the relative effectiveness of the two treatment options evaluated at small scale may not reflect the relative performance at full-scale, making results previously obtained at small-scale potentially less scalable. This study therefore compares the performance of UV/Cl 2 to UV/H 2 O 2 at a full-scale water treatment plant, using sucralose and caffeine as spiked surrogates for contaminants that are reactive solely to OH radicals, and to both OH and RCS, respectively. pH was varied between 6.5 and 8.0. The results demonstrated that when using a medium pressure UV lamp, UV/Cl 2 might lead to approximately twice the production of OH radicals as UV/H 2 O 2 at pH 6.5 when using the same molar oxidant concentration, but adding chlorine to the UV reactor at pH 8.0 had a negligible impact on OH radical concentration in comparison to UV alone. The study also confirmed previous small-scale results that RCS can be a major contributor to UV/Cl 2 treatment for compounds such as caffeine that are susceptible to RCS, with UV/Cl 2 effective at both pH 6.5 and 8.0 for such compounds. Disinfection byproducts were monitored, with adsorbable organohalide (AOX) formation increasing by approximately 10 μg-Cl/L due to chlorine photolysis, but only at pH 6.5 and not at pH 8.0. This implies that UV/Cl 2 might increase AOX mostly due to reaction between OH and organic precursors to make them more reactive with chlorine, and not due to RCS. The formation of specific DBPs of current or emerging regulatory interest was minimal under all conditions, except for chlorate. Chlorate yields were in the order of 6–18% of the photolysed chlorine. Image 1 • The relative effectiveness of MP UV/H 2 O 2 versus UV/Cl 2 is light path length dependent. • UV/Cl 2 produced twice as much OH radical as UV/H 2 O 2 at pH 6.5. • The reactive chlorine species generated in UV/Cl 2 at pH 8.0 did not contribute to AOX formation. • Chlorate yields were in the order of 6–18% of the photolysed chlorine in UV/Cl 2. [ABSTRACT FROM AUTHOR]

The efficiency of three different oxidation processes, UV/H 2 O 2 oxidation, ferrate(VI) oxidation, and ozonation with and without hydroxyl radical ( OH) scavenger tert-butyl alcohol (TBA) on the removal of organic compounds from oil sands process-affected water (OSPW) was investigated and compared. The removal of aromatics and naphthenic acids (NAs) was explored by synchronous fluorescence spectra (SFS), ion mobility spectra (IMS), proton and carbon nuclear magnetic resonance ( 1 H and 13 C NMR), and ultra-performance liquid chromatography coupled with time-of-flight mass spectrometry (UPLC TOF-MS). UV/H 2 O 2 oxidation occurred through radical reaction and photolysis, transforming one-ring, two-ring, and three-ring fluorescing aromatics simultaneously and achieving 42.4% of classical NAs removal at 2.0 mM H 2 O 2 and 950 mJ/cm 2 UV dose provided with medium pressure mercury lamp. Ferrate(VI) oxidation exhibited high selectivity, preferentially removing two-ring and three-ring fluorescing aromatics, sulfur-containing NAs (NAs + S), and NAs with high carbon and high hydrogen deficiency. At 2.0 mM Fe(VI), 46.7% of classical NAs was removed. Ozonation achieved almost complete removal of fluorescing aromatics, NAs + S, and classical NAs (NAs with two oxygen atoms) at the dose of 2.0 mM O 3 . Both molecular ozone reaction and OH reaction were important pathways in transforming the organics in OSPW as supported by ozonation performance with and without TBA. 1 H NMR analyses further confirmed the removal of aromatics and NAs both qualitatively and quantitatively. All the three oxidation processes reduced the acute toxicity towards Vibrio fischeri and on goldfish primary kidney macrophages (PKMs), with ozonation being the most efficient. [ABSTRACT FROM AUTHOR]

• CHA degradation was mainly attributed to OH radical attack. • Dividing the chlorine dose into five separated doses increased the removal rate. • Different by-products were identified including two chlorine-containing by-products. • The reaction pathway for CHA degradation during UV/Chlorine treatment was proposed. Degradation kinetics, by-products identification and pathways of a model naphthenic acid, cyclohexanecarboxylic acid (CHA), by the UV/Chlorine process were investigated in this study. Mathematical modeling indicated that the initial CHA decay rate increased rapidly with the chlorine dose when the chlorine dose was lower than 45 mg/L and decreased with further chlorine dose increases. Increasing the chlorine dose from 400 to 800 mg/L resulted in a steady increase in the total removal of CHA after 60 min of UV photolysis. By dividing the 700 mg/L chlorine dose into five separated doses (140 mg/L each) added at 10 min intervals, the total CHA removal increased from 72% to 91%. This implies that the ideal condition of the UV/Chlorine process in degrading CHA is to add chlorine continuously at a constant rate to compensate any chlorine consumption to reduce the radical scavenging effect. It was found that the CHA decay was mainly attributed to the hydroxyl radical (OH) attack and the reactive chlorine species (RCS) contribution was relatively small. Various by-products, including the mono-chlorinated and di-chlorinated by-products, were identified and the reaction pathway for CHA degradation during UV/Chlorine treatment was proposed. [ABSTRACT FROM AUTHOR]

• SO 4 •– acted as the dominant radical while •OH and 1O 2 were minor contributors. • SO 4 •– could be generated via DO-dependent or DO-independent way. • Crucial roles of •O 2 – in the sulfate radical-generating process were explored. • Vulnerable sites of BPAF molecule were evaluated by DFT analysis using Fukui index. • The removal pathway of BPAF was systematically investigated using MS and DFT. Though natural reducing agents have been demonstrated as desirable catalysts for environmental remediation, the mechanism of catalytic activation of persulfate (PS) by bisulfite (S(IV)) remains unclear. In this study, an emerging contaminant bisphenol AF (BPAF) was employed as the target compound to examine the activation and degradation mechanism in PS/S(IV) system. Sulfate radical (SO 4 •–) was evidenced as the dominant radical accounting for BPAF degradation via quantitative analysis, while hydroxyl radical (•OH) and singlet oxygen (1O 2) were minor contributors. Superoxide radical (O 2 •–) was identified as an intermediate radical in promoting BPAF removal through quenching experiments and electron paramagnetic resonance analysis. Tests in oxygen-rich and oxygen-deficient systems were conducted and the results were contrasted to elucidate the important role of oxygen in BPAF degradation and SO 4 •–-formation. In addition, the effect of Dissolved Oxygen (DO) was simulated using two separate kinetic models. Decomposition mechanism of BPAF was afterwards clarified via the density-functional theory calculations using Fukui index to predict the vulnerable sites and the intermediate products. This study provides a mechanistic understanding of the activation of PS/S(IV) system on the BPAF removal, especially the critical role of DO and O 2 •– in SO 4 •– generation. [ABSTRACT FROM AUTHOR]

• The second-order rate constants of PCMX with SO 4 − and HO were determined; • Degradation of PCMX was predicted by the steady-state kinetic model under various conditions; • Density functional theory was applied to identify the radical reactive sites of PCMX; • Six main elimination pathways were proposed based on HPLC/MS/MS analysis; • Accurate toxicity of forty-seven byproducts was calculated via the QSAR method. 4-Chloro-3,5-dimethylphenol (PCMX), as a typical antimicrobial compound, has been widely used in medical products and cosmetics and has been detected in municipal wastewater. Significant concerns about its persistence and potential toxicity has been paid to in the aquatic environment. In this study, we explored UV and UV/persulfate (UV/PS) advanced oxidation process as potential options to remove PCMX for the first time. A steady-state kinetic model was established to simulate the experimental results and to predict the concentration of reactive oxidant species (ROS) under different reaction conditions. The second-order rate constants for PCMX reacting with SO 4 − and HO were determined to be (1.75 ± 0.04) × 109 M−1 s−1 and (2.74 ± 0.14) × 109 M−1 s−1, respectively, with SO 4 − as the dominant species in PCMX decay by UV/PS. Interestingly, the pseudo-first-order rate constant of PCMX changed slightly in a broad pH range of 3.0 – 11.0, but was significantly increased when pH > 14.0. Natural organic matter (up to 5 mg/L as C) hindered the degradation rate of PCMX from 3.85 × 10−4 s−1 to 1.43 × 10−4 s−1 due to the effects of radical scavenging and the "inner filter" effect. Furthermore, the potential reactive sites (i.e., certain carbons in the benzene ring and phenolic hydroxyl group), in PCMX were identified based on the electronic structure of the molecule, using frontier molecular orbital theory, natural population analysis (NPA), and density functional theory (DFT) calculation. A variety of byproducts were identified using HPLC/MS/MS, which, in combination with the reactive site analysis, led to six main degradation pathways. Quantitative Structure Activity Relationship (QSAR) method of Toxicity Estimation Software Tool was applied to estimate the developmental toxicity of the byproducts. [ABSTRACT FROM AUTHOR]