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April 2009 Williams and Denholm 77 Most importantly, as Ratte (1999) has noted inter a/ia, "...the perception of high silver toxicity has long been due to the fact that most laboratory toxicity trials used AgN03i which readily dia- solves releasing the highly toxic free silver ion". Since water solubility generaily controls bioavail- ability, silver compounds that are not readily solu- bie or insoluble are of less environmental con- cem, a point emphaslzed by Karen et al. (1999). The concentratlon of Ag' derived from an insolu- ble silver salt is detennined by the dissociation constant ICsP which (in the case of silver iodide limits the concentration of silver ion to ca. 9x10 M. This means that the concentration of Ag' in a solution containing silver iodide cannot be greater than this value. In contrast, the Ag` con- centration derived from silver nitrate (as shown fn Table 1), is many orders of magnitude greater than that of silver lodide. For the insoluble silver salts, the concentration of silver ion ln equilibrium with the solid silver sait can be determined from the solubility product constant. The solubility product canstants for the silver salts of interest are shown in Table 2 be- low. The caiculated silver lon concentrations that would exist in aqueous solution (at equilibrium) are aiso shown. The signfflcance of the data in Table 2 is that for silver iodlde, the maximum concentration of silver ion in an aqueous solution in equilibrium with solid silver ladide is 9.2x10'9 M(9.84x10'' g/L). in comparison, the maximum silver concenVation that can be reached for silver sulfide is 2.56x10'" M (2.73x106 g/L). This means that salid silver iodide is an ex- tremely poor source of silver ions in solution. 3. TYPICAL LEVELS OF SILVER !N THE ENVIRONMENT Silver is a widely distributed element, and unttl relatively recent times used for many applicabons and extensively in the photographic industry. Waste water from that industry is known to have accounted for significant silver fluxes into the en- vironment (Htrsch, 1998). Although that use is declining, silver is being used more widely in me- dicinal applicatlons. Silver ions have also been reported in waters adjacent to silver mine sites and municipal waste waster treatment plants (Kramer et al. 1999). Silver is a normal trace constltuent of many or- ganisms. Terrestrial plants for example usually contaln silver at less than 0.1 mg/kg dry-weight, with seeds, nuts, and fruits containing higher concentrations than other plant parts (USEPA (1980), cited in The Concise Intematlonal Chemi- cal Assessment Document 44 (2002) ("CICAD 44") and Irwin (1997). 3.1 Soils. Sediments and Water The CICAD 44 monograph reports silver levels for various environmental matrices. For prlstine, unpolluted areas such as rivers, lakes and estu- aries, levels of about 0.01 Ng/L were found, while for urban and industrialised areas the leveis were typically 0.01 to 0.1 Ng/L. Estuarine waters in San Francisco Bay were found to range from 6 to 250 pM (0.65 to 27 ng/ L), while in a number of Wiscansin rivers, silver concentr atfons ranged from 1.2 to 72 ng/L. Wen et al. (2002) note rapid removal rates in freshwa- ter environments of the Ag`, with a one to two weeks half-removal time even for prisbne envi- ronments. TabN 2: Solubility Product Constants for Some Silver Salta (Handbook of Chemistry and Physics) Salt Solubility product SiivK ion concentration _ constant ICu at 25°C (M) Sliver chloride AgCi I 1.77x10"10 I 1.33x10'6 Siiver iodide Agl I 8.51x10-"? 9.2x10-0 ?-- SiNer sulfide AgzS ? 6.69x10"50 2.56x10-" M - 1 - Scientific Papers -