Liang Wu

Fluorotelomer sulfonates (FTSAs; CnF2n+1–(CH2)2–SO3H) are PFAS precursors that can transform into persistent perfluorocarboxylates (PFCAs), yet their −CH2– moieties hinder reductive degradation. This study developed a two-stage, redox-selective vacuum UV (VUV) system to destroy three FTSAs (CnF2n+1–(CH2)2–SO3H; n = 4, 6, 8). In the first oxidative stage (VUV/Oxd), air-saturated conditions generated hydroxyl radicals (HO•) to convert FTSAs into PFCAs. In the second reductive stage (VUV/Red), an H2-tuned environment generated hydrated electrons (eaq–) that further degrade PFCAs to fluoride, achieving up to 100% defluorination across pH 4–12 and in the presence of up to1000 mM chloride or 200 mM sulfate. In contrast, the one-stage VUV/Red system produced H-substituted and unsaturated transformation products that were subsequently degraded and achieved 15–36% lower defluorination. The effects of chloride and sulfate were pH-dependent: alkaline pH promoted the VUV/Oxd defluorination percentage 6-fold at ≥100 mM chloride by enhancing HO• generation via OH– photolysis, whereas at <100 mM chloride or with sulfate, neutral and acidic pH enhanced the defluorination kinetics by 3–10 times through greater HO• generation from water photolysis. This study demonstrates an effective treatment platform to achieve near-complete defluorination of FTSAs in impacted water with diverse levels of salinity.

The 185-nm ultraviolet (UV) direct photolysis of PFAS is unknown due to interference from radical-driven degradation and photon competition with coexisting aqueous species. This study established an N₂O-saturated condition to evaluate the kinetics and mechanisms of 185-nm UV direct photolysis and defluorination of three PFAS subclasses: perfluorocarboxylates (PFCAs; C_nF_2n+1-COOH, n = 1-9), perfluorosulfonic acids (PFSAs), and per- and polyfluoroalkyl ether carboxylic acids (PFECAs). PFCAs and PFECAs underwent significant direct photolysis, whereas PFSAs remain resistant. First-order direct photolysis rate constants strongly correlated with molar absorption coefficients (68-3396 M^-1cm^-1), exhibiting enhanced kinetics for longer-chain PFCAs (n > 7), PFECAs (n > 6), and multiether PFECAs. 185-nm quantum yields ranged from 0.12 to 0.28 mol/Einstein, resulting in 30-84% defluorination. Products analysis identified neutral gaseous transformation products (e.g., C_nF_2n+1H and C_nF_2n+2), indicating dominant C-C cleavage for PFCAs and combined C-O and C-C cleavage for PFECAs. Kinetic modeling revealed that the contribution of direct photolysis to overall PFAS degradation in 185-nm water photolysis increased with decreasing pH, accounting for 87% at pH 7 and 10% at pH 11, respectively. This study discovered the existence and extent of direct PFAS photolysis at 185 nm and provided mechanistic insights for future UV PFAS water treatment.

In membrane-UV potable reuse, dichloramine (NHCl ₂ ) hydrolysis forms peroxynitrite (ONOO ⁻ ) in RO permeate. UV photolysis of ONOO ⁻ generates hydroxyl radicals, enabling rapid micropollutant degradation and enhancing UV/AOP treatment performance.

2,6-Dichloro-1,4-benzoquinone (2,6-DCBQ) is an emerging chlorinated disinfection byproduct (DBP) in bodies of water. However, this compound poses an unknown toxic effect on cyanobacteria. In this study, the toxicological mechanisms of 2,6-DCBQ in Microcystis aeruginosa (M. aeruginosa) were investigated through physiological and nontargeted metabolomic assessments. The results show that 2,6-DCBQ inhibited the growth of M. aeruginosa, reduced its photosynthetic pigment and protein contents, increased the levels of reactive oxygen species, damaged the antioxidant defense system, and aggravated the cytomembrane. Meanwhile, 2,6-DCBQ stimulated the production and release of microcystin-LR (MC-LR) and altered the transcripts of genes associated with its synthesis (mcyA, mcyD) and transport (mcyH). In addition, nontargeted metabolomics of M. aeruginosa cells exposed to 0.1 mg/L 2,6-DCBQ identified 208 differential metabolites belonging to 10 metabolic pathways and revealed the considerable interference caused by 2,6-DCBQ among ABC transporters, the two-component system, and folate biosynthesis. This study deepens the understanding of the physiological and nontargeted metabolomic responses of M. aeruginosa exposed to 2,6-DCBQ, offers insights into the toxic effect of 2,6-DCBQ on M. aeruginosa, and provides a theoretical basis for the ecological risk assessment of emerging DBPs in accordance with water quality criteria.

Dichloramine (NHCl2) naturally exists in reverse osmosis (RO) permeate due to its application as an antifouling chemical in membrane-based potable reuse treatment. This study investigated mechanisms of background NHCl2 hydrolysis associated with the generation of oxidative radical species in RO permeate, established a kinetic model to predict the oxidative capacity, and examined its removal efficiency on trace organic contaminants in potable reuse. Results showed that NHCl2 hydrolysis generated transient peroxynitrite (ONOO-) and subsequently dissociated into hydroxyl radical (HO center dot). The maximal HO center dot exposure was observed at an RO permeate pH of 8.4, higher than that from typical ultraviolet (UV)-based advanced oxidation processes. The HO center dot exposure during NHCl2 hydrolysis also peaked at a NH2Cl-to-NHCl2 molar ratio of 1:1. The oxidative capacity rapidly degraded 1,4-dioxane, carbamazepine, atenolol, and sulfamethoxazole in RO permeate. Furthermore, background elevated carbonate in fresh RO permeate can convert HO center dot to carbonate radical (CO3 center dot-). Aeration of the RO permeate removed total carbonate, significantly increased HO center dot exposure, and enhanced the degradation kinetics of trace organic contaminants. The kinetic model of NHCl2 hydrolysis predicted well the degradation of contaminants in RO permeate. This study provides new mechanistic insights into NHCl2 hydrolysis that contributes to the oxidative degradation of trace organic contaminants in potable reuse systems.

Tapping municipal wastewater for potable reuse significantly enhances drinking water supply in drought-stricken regions worldwide. Membrane-based potable reuse treatment trains commonly employ ultraviolet-based advanced oxidation processes (UV-AOPs) to degrade trace organic contaminants in water to produce high-quality recycled water. Hydrogen peroxide (H2O2) is used as the default photo-oxidant. Meanwhile, chloramines, which are added to prevent biofouling, pass through the membranes and impact the treatment efficiency of UV-AOP. Water reuse facilities therefore face the dilemma of optimizing H2O2 (an added photo-oxidant) and chloramines (a carry-over photo-oxidant) doses. Utilizing a uniquely designed pilot-scale reactor and real-time recycled water, we evaluated treatment efficiencies of UV-AOP on six important indicator contaminants, with monochloramine (NH2Cl) and H2O2 as photo-oxidants. Hydroxyl radical (HO•), and reactive chlorine species, such as the chlorine atom (Cl•) and chlorine dimer (Cl2•¬-), were the major reactive species. Overall, radicals generated from photolysis of NH2Cl alone achieved removal of indicator compounds, which can be further improved by optimizing UV fluence, i.e., the UV dose. Furthermore, the addition of H2O2 enhanced HO• formation and improved contaminant removal. However, the addition of H2O2, when the background NH2Cl level was above 2 mg L-1 (as Cl2) provided limited improvement in treatment efficiency. These trade-offs between chloramine and H2O2 as oxidants, and the recommended optimization of the associated effective UV fluence, are critical for energy-efficient and cost-effective potable reuse to address the challenges of global water scarcity.