Previous research indicates that radical and non-radical reactive halogen species (RHS) can be generated via sunlight-driven oxidation of Br- and I- in surface seawater, suggesting the possibility that photochemical pathways could lead to non-specific halogenation of dissolved organic matter (DOM), particularly in nearshore seawaters rich in photoreactive terrestrial DOM. However, non-specific halogenation of DOM in seawater has not been explored in depth, due in part to difficulties in quantifying organohalogen content in the presence of high inorganic halide concentrations. In this work, two novel analytical methods were developed to enable highly-sensitive mass-spectrometric quantification of (a) non-specific organohalogen content of DOM, and (b) specific halogenation of a chemical probe for RHS. These methods were employed to quantify native organobromine and organoiodine contents of marine DOM � ranging from 3.2-6.4 � 10-4 mol Br/mol C and 1.1-3.8 � 10-4 mol I/mol C (or 19-160 nmol Br L-1 and 6-36 nmol I L-1), with decreasing organohalogen content versus depth � in a wide variety of natural seawater samples, and to demonstrate that nanomolar levels of organobromine and organoiodine are generated via photochemical halogenation of DOM during solar irradiation of simulated and natural estuarine seawater. The highest organobromine and organoiodine contents measured after extended solar irradiation of simulated estuarine seawater (artificial seawater amended with freshwater NOM isolates) were 25-50% and 10-45%, respectively, of levels measured in natural surface seawaters. Representative levels of iodide promoted photochemical organoiodine formation at the expense of organobromine. On the basis of these findings, it is estimated that 30 and 70 Gg yr-1 of organobromine and organoiodine may be generated by photochemical halogenation of riverine terrestrial DOM in estuaries alone, suggesting that sunlight-driven reactions of RHS with DOM may be an important natural source of organohalogen compounds in the ocean and may play a significant role in biogeochemical halogen and carbon cycling.

Sunlight-driven photochemical bromination and iodination of dissolved organic matter in seawater

248th National Meeting (2014)

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