Voltammetric Evidence for Soluble FeS Complexes in Anoxic

Voltammetric methods using direct insertion of a gold-amalgam microelectrode with a sensitive, computer- controlled voltammeter detected soluble iron(II) sulfide, (FeS)q, in the porewaters of anoxic, sulfidic, fine-grained sediments from the Loughor Estuary, Wales. The voltammetric results are reproducible. Studies of cores stored in sealed, refrigerated containers for up to 21 d reveal no measurable oxidation. (FeS)aq forms in this estuarine environment as a result of the dissolution of amorphous FeS, and appears to be involved in the formation of pyrite. (FeS)q makes no significant contribution to the total sulfide and iron contents of the sediment but could constitute an important com- ponent of the dissolved Fe(II) and S(-II) contents of the porewater. Mass balance calculations show pyrite forms in this system by the addition of sulfur to FeS rather than by the loss of iron from FeS. The overall process appears to involve (FeS),q as an intermediary. Although the porewaters of the Loughor Estuary sediments are iron-rich relative to seawater, the iron sulfide-forming process is iron-limited rather than sulfide-limited. Reactive iron is bound to sulfide rapidly in the sediment. After the reactive iron is bound to sulfide, additional sulfide produced is fixed as pyrite.