CARBONYL SULFIDE (OCS) AND CARBON-MONOXIDE (CO) IN NATURAL-WATERS - EVIDENCE OF A COUPLED PRODUCTION PATHWAY

The mechanisms for the photoproduction of carbonyl sulfide (OCS) and c arbon monoxide (CO) in natural waters were studied by evaluating exper imental results from different aqueous systems. A coupled photoproduct ion mechanism was observed operating on CO and OCS. For CO photoproduc tion, the presence of a carbonyl group is necessary, while for OCS, a source of reduced sulfur in addition to the carbonyl is required. An a cyl radical is postulated to be the key intermediary for OCS and CO ph otoproduction while a sulfur-centered radical (thiyl or sulfhydryl rad ical) is likely to be the key species that reacts with acyl radicals t o produce OCS. Laboratory experiments indicated that addition of reduc ed sulfur to seawater and subsequent irradiation leads to a decrease i n CO and an increase in OCS photoproduction rates relative to original water. Furthermore, treatment of Suwannee fulvic acid (SFA) or Aldric h humic (AH) aqueous solutions with sodium borohydride (NaBH4) decreas ed 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 fo r the photoproduction of both gases in natural waters involving the fo rmation of free radicals. In natural waters, the anti-correlation CO/O CS is likely to be seen in areas with high biological productivity in which reduced sulfur compounds and dissolved organic matter are abunda nt. This study furthers our understanding of sulfur chemistry in aqueo us systems and provides another demonstration of the complex link of t he biogeochemical cycles of carbon and sulfur. (C) 1998 Elsevier Scien ce B.V. All rights reserved.