Carbonyl sulfide (OCS) and carbon monoxide (CO) in natural waters: evidence of a coupled production pathway

Abstract

The mechanisms for the photoproduction of carbonyl sulfide (OCS) and carbon monoxide (CO) in natural waters were studied by evaluating experimental results from different aqueous systems. A coupled photoproduction mechanism was observed operating on CO and OCS. For CO photoproduction, the presence of a carbonyl group is necessary, while for OCS, a source of reduced sulfur in addition to the carbonyl is required. An acyl radical is postulated to be the key intermediary for OCS and CO photoproduction while a sulfur-centered radical (thiyl or sulfhydryl radical) is likely to be the key species that reacts with acyl radicals to produce OCS. Laboratory experiments indicated that addition of reduced sulfur to seawater and subsequent irradiation leads to a decrease in CO and an increase in OCS photoproduction rates relative to original water. Furthermore, treatment of Suwannee fulvic acid (SFA) or Aldrich humic (AH) aqueous solutions with sodium borohydride (NaBH4) decreased photoproduction of CO compared to untreated samples. A metal redox system (Ce(IV)/Ce(III)) was also used to generate radicals in solution and demonstrate radical participation in the production processes of OCS and CO. Based on these results, potential pathways are proposed for the photoproduction of both gases in natural waters involving the formation of free radicals. In natural waters, the anti-correlation CO/OCS is likely to be seen in areas with high biological productivity in which reduced sulfur compounds and dissolved organic matter are abundant. This study furthers our understanding of sulfur chemistry in aqueous systems and provides another demonstration of the complex link of the biogeochemical cycles of carbon and sulfur.