Removal of haloacetonitriles (HANS) and dissolved organic nitrogen (DON) in water by vacuum ultraviolet (VUV)

Abstract

The main objective of this dissertation was to investigate the feasibility of using vacuum ultraviolet (VUV, 185+254 nm) and ultraviolet (UV, 254 nm) for removal of haloacetronitriles (HANs) and dissolved organic nitrogen (DON). The first part of the work aimed to study the degradation of four HANs species (monochloroacetonitrile (MCAN), dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), and dibromoacetonitrile (DBAN)) directly by VUV and UV. The order of degradation rate was DBAN>TCAN>DCAN>MCAN for both systems. Degradation rate constants of HANs under VUV were 2-7 times greater than UV. HANs removal under nitrogen purging was much higher than under air purging. The removal efficiencies of mixed HANs were lower than that of single HANs. The major degradation mechanism of DBAN and MCAN was 254 nm (direct photolysis) and hydroxyl radical, respectively. DCAN and TCAN were more susceptible to degradation by 185 nm (direct photolysis). The intermediates from HANs removal by VUV were produced from substitution, addition, and polymerization reactions.

The second part focused on DON removal by VUV, VUV/H2O2, UV and UV/H2O2 to reduce HANs formation potential (HANFP). This part was conducted using not only surface water (SW) but also treated wastewater effluent (WW) for water reclamation application. The results showed that the reduction of dissolved organic carbon (DOC), DON, hydrophobicity, absorbance at 254 nm (UV254), and fluorescence excitation-emission matrix (FEEM) of both water samples by VUV was higher than UV. Addition of H2O2 improved the performances of VUV and UV. VUV/H2O2 exhibited the highest removal efficiency for all parameters studied. Even though HANFP increased at the early stage, its concentration decreased at the end of treatment (60 min). Decreases in DON, DOC, hydrophobicity, and UV254 led to HANFP reduction. Moreover, FEEM revealed that substantial reduction in tryptophan (nitrogen-rich organic) had strong correlation with HANFP reduction, implying this group of compounds act as a precursor of HANs.