Voltammetric evidence for soluble FeS complexes in anoxic estuarine muds

Voltammetric methods using direct insertion of a gold-amalgam microelectrod e with a sensitive, computer-controlled voltammeter detected soluble iron(I I) sulfide, [FeS](aq), in the porewaters of anoxic, sulfidic, fine-grained sediments from the Loughor Estuary, Wales. The voltammetric results are rep roducible. Studies of cores stored in sealed, refrigerated containers for u p 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 t o be involved in the formation of pyrite. [FeS](aq) makes no significant co ntribution to the total sulfide and iron contents of the sediment but could constitute an important component of the dissolved Fe(II) and S(-II) conte nts of the porewater. Mass balance calculations show pyrite forms in this s ystem by the addition of sulfur to FeS rather than by the loss of iron from FeS. The overall process appears to involve [FeS](aq) as an intermediary. Although the porewaters of the Loughor Estuary sediments are iron-rich rela tive to seawater, the iron sulfide-forming process is iron-limited rather t han sulfide-limited. Reactive iron is bound to sulfide rapidly in the sedim ent. After the reactive iron is bound to sulfide, additional sulfide produc ed is fixed as pyrite.