ACS Symposium Series 811 Environmental Electrochemistry: Analyses of Trace element Biogeochemistry Marital Taillefert, Editor Georgia Institute of Technology Tim F. Rozan, Editor University of Delaware 2002 American Chemical Society 371 Chapter 19 Voltammetric evidence suggesting Ag speciation is dominated by sulfide complexation in river water Tim F. Rozan and George W. Luther III College of Marine Studies, University of Delaware, Lewes, DE 19958 Using voltammetric methods, free HS - /H 2 S, acid volatile sulfide (AVS), and only Cr(II) reducible sulfide (OCRS) fractions were measured in seven rivers spanning the mid- Atlantic region of the United States to determine the effect of sulfide complexation on dissolved Ag (Ag diss ) speciation in freshwaters. In all waters, the AVS: Ag diss ratio was observed to be greater than 600. This increased to a range of 1000 – 5000 for rivers that received sewage treatment plant (STP) effluent. In laboratory experiments, Ag was observed to quickly (< 1 min.) replace both Cu and Zn in metal sulfide solutions, indicating that the measured AVS concentrations in natural waters provide an excess pool of available sulfide for complete Ag complexation. Introduction In natural waters determining the speciation of a trace metal, such as Ag, is an important, if not daunting task. Yet knowledge of the exact speciation is critical for understanding the geochemical behavior and bioavailabilty of trace metals in aquatic environments. Unfortunately, most current analytical