Major advances have been made in our understanding of the environmental chemistry and toxicology of silver, Ag(I), over the past few years. The scientific community has learned a great deal about sources, concentration levels in natural waters and biota, physicochemical forms, adsorption/desorption reactions, toxicology, bioaccumulation, and the transport and fate characteristics of silver. These advances have in large measure been stimulated by research funding from a coalition of companies in the photographic industry.
Effluent from public wastewater treatment plants (POTWs) introduces silver into the environment. Prior to recent work, available data suggested that effluent-receiving waters might be near the provisional silver criteria (based on ionic silver: Ag ฯฉ ) for protection of aquatic life. Had this assessment been correct, then POTWs might have refused to accept effluents from photo processors and other commercial users of silver, even though most of these companies had already implemented advanced recovery processes. However, information became available that indicated errors in previously reported data on the silver concentration in natural waters. Because of these uncertainties, questions also arose regarding the interpretation of bioassay data for silver. Consequently, many researchers and government regulators began to reevaluate the basis on which federal and state effluent criteria were established. Because of these and other challenges, research was implemented to improve our understanding of silver.
As reported previously at five Argentum conferences (Argentum I-V; see www.seagrant.wisc.edu/argentum/ index.html), and in the papers published in Environmental Toxicology and Chemistry, Volume 17:537-649, implemen- tation of clean, sensitive field and laboratory analytical methods has shown that silver levels in natural waters are typically very low (in the sub-nanogram per liter or the picomolar range). Public wastewater treatment plants can provide efficient removal of silver: In effluents studied to date, the levels of silver concentration are typically 10 nM or lower, even in effluents from plants receiving high loadings of silver. Subsequently, levels in effluent-receiving streams rapidly fall to subnanomolar and then to picomolar concentration. The low levels reflect, in part, the reactivity of Ag ฯฉ with particles and colloids. This reactivity results in efficient particle scavenging, although complexation by chloride reduces scavenging efficiency in seawater as compared to freshwater. Significantly, the Ag ฯฉ ion's ability to bind to dissolved organic carbon (DOC) and to other ligands (reduced sulfur compounds in particular) suggests that only a small fraction of aqueous silver is likely to exist as the Ag ฯฉ ion.
Research on fish and other aquatic organisms has provided substantial information on Ag ฯฉ toxicity, especially acute toxicity and the mechanisms of toxicity. This research has shown that binding to DOC and other ligands reduces the toxicity of Ag ฯฉ ion to fish. Also, the Ag ฯฉ ion concentrations required to produce toxicity in fish are typically several orders of magnitude above the total silver levels found in natural waters.