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Volume 41 84 )oumal of Weather Modification it should be noted that this work involved sedi- ments, which are very likely to be anoxic, whereas the work reported by Kramer et a!. (1999) involved oxic waters which were shown to contain AVS. Other research groups have found that in more oxic sediments, amorphous Fe oxides or manga- nese oxides or coltoids are important sinks for binding silver and other metals. Because of the large stabi4ity constants found for metals such as silver sulfide, the importance of a pool of sulfides including zinc and iron sulfides (AVS) in detoxifying metals such as silver in natural waters cannot be overstated. In a recent study of multinuclear sulfide clusters in natural waters, Rezan et al. (2000), showed that the most abundant metal sulfides were iron su{fides and that they were composed mainly of a soluble FeS cluster. They found that FeS pre- dominated in rivers that drained less-urbanised watersheds. Metal sulfide clusters were kineti- cally stable and as a result persisted in oxic wa- ters. On the basis of their observations it was sug- gested (Rezan et al. 2000) that sulfur complexa- tion might dramatically lower the acute toxicity of 'b"-class metals including silver. 5.5 Hardness Water hardness, principat{y in the form of cal- cium, was recognised by the USEPA (1980) (cited in CICAD 44) as having a critical role in reducing ha?m to aquatic organisms from acute sitver toxicity. The current view however is that hardness due to calcium ion is now thought to be less effective in modifying the toxicity of silver ion than the other factors discussed, unless it is the only significant ameliorating factor present. Recent work by Bianchini and Wood (2008) iden- tified both hardness and sulfides as being impor- tant in protecting against lethal acute effects as well as chronic silver toxicity in terms of mortality, whereas sulfide alone showed a protective effect against the sub-lethal chronic silver effects on growth and reproduction. 5.6 Colloids The strong affinity of silver for suspended particu- lates in river and estuarine water was demon- strated in a study repoRed by Wen et al. (1997). In experiments to determine phase speciation, they used cross-flow ultrafiltration to separate water samples into particulate >0.45 Nm or >0.1 Nm, colloidal (0.1 - 0.45 Nm), or truly dissolved (<0.1 Nm) fractions. They were able to show that between 33-89% of the silver was bound to the particulate fraction. The high affinity of silver for suspended particu- lates was reflected by a high mean particle/water partition coefficient of 1og KD = 5.0. They also noted that the ratio of colloidal silver to filter- passing silver was similar to the ratio of colloidal organic carbon to total dissolved organic carbon. They further concluded that silver is complexed by organic macromolecules and that the func- tional groups with affinity for silver are evenly distributed across the different molecular weight fractions. The maximum total recoverable silver in water was related to hardness by the equation: il 72[Ir hartlnessi 6 52 Max totaf recoverabAe Ag Ivg,1-f = e .r MOrP 'PrB??tly hc;WE've' `' ?85 hetUr•F ie?d' ,l???, this expression ?s under pro?tectwe at n,yh harJ ness levels, and is overly conservabve wherP waters have low hardness (Galvez and Wood, 1997). In the study reported by Gatvez and Wood (1997), they found that the protective ef- fect of chloride is much more significant than that due to ca4cium CICAD 44 notes that silver is ?PSS !„x. tt, ?,?,r•,f-?:, .,, ...?wS ,Nnt,r' J???r'? r,,?"J In their detailed study, the particulate silver was found to be associated mainly with an iron - manganese oxyhydroxide/sulfide phase Be- cause of the close reiationship between silver bU1h !t;(1 (C>IIC?.di3i enfj n»ftiCU+atP pnases a._,R:m()n surfac e? or-;Vlex I bel!eved tc; be sulphydryt yroupsi was proposed Of particu lar reievance to this prU}ect was their finding that partlculate silver from riverine inputs was rapidly removed from water. Further support for the effectiveness of colloids in S'',afE', , i • 4 +" 4- 4.: H •