Mediated Electrochemical Reduction: A Control for Electron Transfer Reactions and Implications for Redox Cycling of Iron and Phosphorus Between Sediment and Water

Abstract

Eutrophication of terrestrial lacustrine systems is an ongoing environmental issue that has grown more troublesome in recent years due to continuous historic loading of phosphorus in these systems. A major hurdle for estimating phosphorus loading in lakes arises from the difficulties in determining internal loading mechanisms that release phosphorus from iron-rich sediments during reducing conditions, driven by stable hydrodynamic conditions that occur typically during colder months. Using mediated electrochemical reduction techniques to drive reduction in experimental sediment systems representing those near the sediment water interface, we found that both iron and phosphorus are released from sediments, however subsequent aqueous geochemical analysis of the system does not account for all the iron and phosphorus in the initial sediment sample. With natural sediment samples obtained from Missisquoi Bay, Lake Champlain, Vermont, iron and phosphorus released from sediments after reduction continued to increase with increasing sediment samples. Further experimentation with phosphate-sorbed ferrihydrite and goethite samples resulted in aqueous solution saturation of these components, and loss of these components in the aqueous phase likely as a result of reprecipitation of iron-phosphate mineral precipitation. Increasing phosphate in solution also significantly decreased the reduction extents and rates of phosphate-sorbed ferrihydrite minerals, indicating that phosphate sorption may create a shielding effect of these minerals and prevent electron transfer at surface sites.