Silver interactions with dissolved organic matter: Importance of reduced sulfur and implications for aquatic toxicity mitigation

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Silver is potentially input to the environment from personal care products (soap, shampoo, deodorant, ...) such as those manufactured and distributed globally by Unilever and Unilever Canada. It is important to be able to understand and predict impacts of silver discharged into the environment. Just knowing total silver is not enough as different forms of a metal have different potential toxicity. Bioavailability approaches have been useful in setting appropriate and protective criteria for metals in aquatic environments. These approaches use calculations to predict what fraction of the total metal is available to bind to organisms and potentially be toxic. To properly accomplish bioavailability modelling for silver, there is a data gap that needs to be filled first. This data gap is a detailed understanding of the role of reduced sulfide in silver binding and ultimately toxicity mitigation. It is well understood that silver binds strongly to reduced sulfide ligands but measuring sulfide is not part of routine analysis when characterizing samples for risk assessment purposes. Even with sulfide data, there do not currently exist validated computer models to estimate bioavailabilty from measured water chemistry data. Thus regulators and dischargers, such as Unilever Canada, do not yet have sufficient information or tools to make appropriate management decisions for silver in the environment. This research will provide data necessary to develop predictive tools for silver in aquatic systems. To accomplish this real world samples will have silver added to them and the fraction of added silver in the free form will be deterimined by ion selective electrode. Using this experimental approach so-called binding isotherms will be generated for a variety of samples of variable sufide content. These isotherms will be mathematically fit to models for silver binding to compounds occuring in surface as well as waste waters. The resultant speciation model can be coupled with existing bioavailability experiments and used to predict safe levels for silver in water depending on characteristics of the specific receiving water.