Facile ammonium oxidation to nitrogen gas in acid wastewater by in situ photogenerated chlorine radicals

The treatment of low-concentration ammonium (e.g., <50 mg L^-1) in highly acidic wastewaters through traditional biological nitrification, physical separation, or chemical stripping remains a huge challenge. Herein, we report that photocatalytic ammonium oxidation using bismuth oxychloride (BiOCl) can successfully occur in Cl^--laden solutions within a pH range of 1.0-6.0. All reactions follow pseudo-zero-order kinetics (with rate constants of 0.27-0.32 mg L^-1 min^-1 at pH 2.0-6.0 and 0.14 mg L^-1 min^-1 at pH 1.0), indicating the saturation of reactive species by the reactants. The interlayer is self-oxidized by the valence band holes (VB h⁺), resulting in the formation of Cl^• and subsequently HClO, which is excited upon UV irradiation to provoke consecutive photoreactions for chlorine radical generation. Compared to the free chlorine, HO^•, Cl^•, and Cl₂^•-, the ClO^• produced using the UV/BiOCl system plays a predominant role in oxidizing ammonium under acidic conditions. BiOCl exhibits good stability because of the compensation of Cl^- from solution and maintains high activity under different conditions (i.e., different cations and co-existing anions, temperatures, and initial substrate concentrations). The successful removal of ammonium from real wastewater using the UV/BiOCl system suggests that this is a promising method for treating diluted ammonium under highly acidic conditions.

Chemical behaviors and toxic effects of ametryn during the UV/chlorine process

Ametryn (AMT), one of the most widely used herbicides in agriculture, has been frequently detected as a micropollutant in many aquatic environments. AMT residue not only pollutes water but also acts as a precursor for the production of disinfection by-products (DBPs). This study systematically investigated the fate of AMT during the UV/chlorine process. It was observed that the combination of UV irradiation and chlorination degraded AMT synergistically. The results of the radical quenching experiments suggested that AMT degradation by the UV/chlorine process involved the participation of UV photolysis, hydroxyl radical (OH) reactions, and reactive chlorine species (RCS) reactions, which accounted for 45.4%, 36.4%, and 14.5% of the degradation, respectively. Moreover, we found that Cl- 2 was an important reactive radical for AMT degradation. The chlorine dose, pH, coexisting anions (Cl^- and HCO₃^-), and natural organic matter (NOM) were found to affect AMT degradation during the UV/chlorine process. Nineteen predominant intermediates/products of AMT degradation during UV/chlorine process were identified, including atrazine. Moreover, the corresponding transformation pathways were proposed, including electron transfer, bond cleavage (C-S, C-N), radical (OH, Cl and Cl- 2) reactions, and subsequent hydroxylation. The toxicity tests with Vibrio fischeri on AMT degradation suggested that more DBPs were generated by UV/chlorine-treated AMT, which possessed higher acute toxicity than AMT did. Although the UV/chlorine process evidently promoted the AMT degradation, optimization of process parameters may reduce the DBP production and merits further investigation.

Organic matter removal and membrane fouling mitigation during algae-rich surface water treatment by powdered activated carbon adsorption pretreatment: Enhanced by UV and UV/chlorine oxidation

In this work, UV and UV/chlorine (UV/Cl) were employed to enhance powdered activated carbon (PAC) adsorption pretreatment prior to ultrafiltration process for algae-contaminated surface water treatment. Their performance on membrane fouling mitigation and organic pollutant rejection was systematically evaluated. A comparative experiment was conducted under varying pollution degrees of algal extracellular organic matter (EOM) contamination in surface river water. The results indicated that UV/PAC and UV/Cl/PAC pretreatment effectively enhanced the removal of dissolved organic carbon (DOC) and UV-absorbing at 254 nm (UV₂₅₄). The characteristics of feed water after pretreatments were investigated through apparent molecular-weight (MW) distribution and fluorescence parallel factor analysis (PARAFAC). In regard to membrane fouling mitigation, UV/Cl/PAC noticeably decreased reversible and irreversible fouling resistance simultaneously and UV/PAC preferred reducing reversible membrane fouling. Combined fouling modeling was operated to scrutinize the fouling mitigation mechanisms and standard pore blocking was proved to be dominant during the filtration process. Moreover, the UV/Cl and UV/Cl/PAC pretreatments were proved positive for emerging micropollutants degradation and disinfection by-products formation potential reduction. The results suggested that UV and UV/Cl are likely strategies to enhance the efficiency of PAC adsorption pretreatments prior to ultrafiltration during algae-contaminated water treatment.