Impact of Halide Ions on Natural Organic Matter-Sensitized Photolysis of 17β-Estradiol in Saline Waters

Abstract

Indirect (sensitized) photolysis by natural organic matter (NOM), mainly from terrestrial sources, can be an important mechanism for attenuation of organic contaminants in estuarine waters, but the effect of salt gradients has been poorly investigated. We studied Suwannee River NOM-sensitized photolysis of 17β-estradiol (E2) in freshwater and saline media. Indirect photolysis by 4 mg-C/L SRNOM was much faster than direct photolysis, and quenching by sorbic acid verified the importance of triplet-excited NOM chromophores. Increasing halide concentrations up to seawater levels decreased the photolysis rate by 90%, with approximately 70% of this decrease associated with ionic strength effects, and the remainder due to halide-specific effects. Bromide (0.8 mM in seawater) accounted for 70% of the halide-specific effect. Halide promotion of NOM chromophore photobleaching was shown to play a major role in the halide-specific effect. Compared to chromophore bleaching, indirect photolysis of E2 was 230% faster in freshwater, but 63% slower in seawater. The involvement of hydroxyl radical (HO(•)) in indirect photolysis of E2 was ruled out by the lack of suppression by tert-butanol. Experiments in D(2)O-H(2)O demonstrated that (1)O(2) was unimportant in freshwater, but accounted for 42% of NOM-sensitized photolysis of E2 in seawater. We project that, as a parcel of water containing E2 moves through the gradient from freshwater to seawater, overall photolysis will decline due to ionic strength, indirect photolysis will decrease due to specific halide effects on NOM photobleaching, and indirect photolysis will decline relative to direct photolysis. Estuarine contaminant fate models may need to account for halide impacts on indirect photolysis of contaminants.