Elevated risk of haloacetonitrile formation during post-chlorination when applying sulfite/UV advanced reduction technology to eliminate bromate.

The formation of haloacetonitriles (HANs) during chlorination after sulfite/ultraviolet (UV) treatment of bromate (BrO3-) in the presence of amino acids (AAs) was investigated. During sulfite/UV treatment, the primary species hydrated electrons (eaq-) and hydrogen atom radicals (H) dominated the reduction of BrO3- to bromide (Br-), whereas the sulfite anion radicals (SO3-) and H degraded AAs to produce the intermediates HN=C(CH3)-COOH, CH3-CH=NH, and CH3-C≡N via α­hydrogen abstraction and NH2-hydrogen abstraction mechanisms. During post-chlorination, Br- was converted to HBrO/BrO-, and the HN=C(CH3)-COOH, CH3-CH=NH, and CH3-C≡N groups featured higher bromine utilization factor (BUF) and chlorine utilization factor (CUF) values than AAs, enhancing the formation of dibromoacetonitrile (DBAN) and dichloroacetonitrile (DCAN). The energetic feasibility of the transformation pathway, that is, HN=C(CH3)-COOH, CH3-CH=NH, and CH3-C ≡ N formation via hydrogen abstraction by SO3- and H and their further conversion to HANs, was proved by density functional theory calculations, which showed stepwise negative Gibbs free energy changes (ΔG < 0). The effects of pH and water matrices (e.g., HCO3-, Cl-, Fe3+, and natural organic matter) were comprehensively evaluated. Although 72% of BrO3- was removed by sulfite/UV treatment in the presence of AAs, the cytotoxicity index (CTI) and genotoxicity index (GTI) during post-chlorination increased by 213% and 125%, respectively, due to the formation of 24 CX3R-type disinfection by-products (DBPs), especially brominated DBPs. Accordingly, more attention should be given to the formation of brominated DBPs during post-chlorination when using sulfite/UV processes to remove BrO3- in the presence of AAs. As a solution, using monochloramine instead of chlorine as a disinfectant after the sulfite/UV process could significantly lower the CTI and GTI values by alleviating the formation of brominated DBPs.

Advanced reduction process to achieve efficient degradation of pyridine.

Pyridine and its derivatives are widely consumed and detected in the environment persistently, which can cause potential adverse impacts on environment and human health. Considering the fact that pyridine could absorb UV light at 254 nm to generate excited one, which could react with reductive radicals, promoting its structural changes, we proposed that one typical efficient advanced reduction process (ARP) which combines UV irradiation with sulfite could be used to eliminate pyridine quickly. Sulfite/UV process showed a higher pyridine removal rate with a pseudo-first-order reaction rate constant of 0.1439 min-1, which was 3 times of that in UV irradiation and 1.3 times in UV/H2O2 process. This was primarily due to reductive radicals (eaq-, H• and SO3•-) produced by UV irradiation. The removal rate of pyridine was highest in slightly alkaline environment. And the presence of oxygen, as well as certain concentration of humid acid just showed slight inhibition, indicating the possibility of application in practical environment. A positive impact was observed with increasing sulfite dosage, but it was gradually inhabited when the dosage was over 5 mM. The present study may provide an alternative efficient technology for the degradation of pyridine ring-containing substances.

Efficiency and mechanism of diclofenac degradation by sulfite/UV advanced reduction processes (ARPs).

Diclofenac (DCF) is a non-steroidal anti-inflammatory drug which is frequently detected in the aqueous environment. The synergistic treatment using sulfite and UV irradiation is proposed to be one of the most effective advanced reduction processes (ARPs) to degrade refractory contaminants. This paper systematically investigated the performance and mechanism of DCF degradation by sulfite/UV ARP under various conditions. A significant enhancement in degradation efficiency of DCF was exhibited via sulfite/UV ARP compared with direct UV photolysis, which is primarily due to the generation of reductive radicals (eaq- and H). This process was well described by a pseudo first-order kinetic model with a rate constant of 0.154 min-1. The influence of solution pH, sulfite dosage, initial DCF concentration and UV intensity were evaluated. Results revealed that DCF more favorably reacted with H in an acidic environment than with eaq- under alkaline conditions. A positive impact on the DCF decomposition was observed with increasing sulfite dosage, but with an inhibiting trend at high sulfite concentrations. The degradation rate constant was accelerated by increasing the UV intensity, while decreased by promoting the initial DCF concentration. Degradation mechanisms at different pH levels revealed that the reduction reactions were induced by eaq- at pH 9.2, and dominated by H at pH 6.0. Complete dechlorination was readily achieved with all chlorine atoms in DCF released as chloride ions under sulfite/UV ARP, which may lead to a decreased toxicity of the degradation products. This observation emphasized the advantages of sulfite/UV ARP on DCF degradation, in comparison with that under direct UV photolysis.