The environmental chemistry and toxicology of silver

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) 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, implementation 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 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 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 ion. Research on fish and other aquatic organisms has provided substantial information on Ag 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 ion to fish. Also, the Ag ion concentrations required to produce toxicity in fish are typically several orders of magnitude above the total silver levels found in natural waters. However, preliminary data suggests subacute effects in lower organisms, such as planktonic herbivores, may occur at considerably lower levels. In this issue, recent advances are reported. The papers are organized around major themes. Purcell and Peters [1] review the history of regulations of silver in the environment. The next two papers focus on the environmental chemistry of silver, including a review of silver–sulfur compounds [2] and the importance of colloidal and acid-volatile sulfide fractions of aqueous silver at an old mining site [3]. Call et al. [4] and Berry et al. [5] report experiments on the toxicity of silver in sediments. The toxicity of silver to benthic organisms is controlled in part by sedimentary sulfides. Using Ag to spike sediments that have an excess of acid-volatile sulfide generally renders the sediments nontoxic [5]. Five papers [6–10] focus on the toxicity of silver to aquatic organisms, especially to fish, while the final manuscript [11] reviews the bioaccumulation and toxicity of silver compounds. Recent research has resolved several important questions, but additional challenges remain before we can reach a full understanding of the environmental chemistry and toxicology of silver. Environmental risk assessment for aquatic silver should be based on the exposure of aquatic organisms to Ag ion. Thus, we need to be able to measure or model the speciation of silver, including association with all important ligands. In particular, the factors and mechanisms controlling the binding of Ag to natural organic ligands, colloids, and particles are not well understood. Thermodynamic predictions of the physicochemical forms of silver in natural waters do not work well because of the existence of nonequilibrium conditions, noncharacterized ligands and colloids, and noncharacterized binding groups on particles. The effect of redox conditions on Ag(I)/Ag(0) speciation conversions should also be studied. Similarly, further dose-response information for aquatic organisms is needed, especially with respect to possible chronic or subacute effects. Though important linkages between the speciation and toxicology of silver have been identified, information needed for quantitative risk assessments remains incomplete. We expect this to be a major area for future advances in the environmental chemistry and toxicology of silver and other metals.