Transformation of gemfibrozil by the interaction of chloride with sulfate radicals: Radical chemistry, transient intermediates and pathways

Abstract

The radical chemistry of SO4·– is strongly affected by its interaction with chloride in natural waters, during which SO4·– can be converted to HO· and reactive chlorine species (RCS). This study investigated the effects of chloride on gemfibrozil (GFRZ) transformation via the UV/peroxydisulfate (PDS) process, elucidating the kinetics, degradation pathways and solution toxicity. The pseudo-first-order rate constants (k′) of GFRZ by UV/PDS changed slightly from 1.0 × 10−3 s−1 to 9.3 × 10−4 s−1 as the chloride content increased from 0 to 10 mM because the increase in HO· and RCS levels compensated for the decrease in SO4·– concentration. However, the transformation pathways in the presence of chloride changed significantly. From the transient absorption spectra, we inferred that RCS and SO4·– attacked GFRZ mainly through hydrogen abstraction and/or electron transfer, while HO· interacted with the GFRZ aromatic ring by addition. Hydroxylation, carboxylation and cleavage products were enhanced in UV/PDS/Cl– compared to UV/PDS through the addition of HO· and the cleavage of CO bonds by RCS, and total organic chlorine (TOCl) was undetectable. Interestingly, the acute toxicity was lowest in UV/PDS/Cl–, with an inhibition percentage of 1% at 30 min. The higher inhibition percentages in UV/PDS (13%) and UV alone (53%) at 30 min likely resulted from the stronger capacity of HO· and RCS to oxidize aldehydes to carboxylic groups and cleave CO bonds, respectively, than that of SO4·–. This study provides a better understanding of contaminant transformation mechanisms under UV/PDS treatment at chloride levels present in natural waters.