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

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 x 10-3 s- 1 to 9.3 x 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 C-O 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 C-O 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.