Chemistry of Ionic Silver and Implications for Design of Potable Water Systems

Ionic silver (Ag+) at approximately 400 µg/L is proposed as an ingestible biocide for potable water systems in future human spacecraft. This paper investigates the underlying chemistry of the silver ion at potable concentrations and its interactions with other solutes and surfaces. The goal of the chemistry evaluation is to understand and predict the behavior of ionic silver in potable water systems. This will allow design optimization to minimize losses of ionic silver during long duration crewed and uncrewed segments of missions. Ionic silver can be lost from the water phase to the water-solid interface by way of adsorption as the cation and by way of oxidation-reduction reactions with unoxidized sites (defects) on passivated surfaces. The adsorption and oxidation-reduction reactions of heritage piping materials used on spacecraft – stainless steel 316L, titanium, Inconel 718, and Teflon flex hoses – are considered. Conceptual models of adsorption isotherms are applied to understand the role of pH and surface-to-volume ratios of pipes and tanks on the partitioning of ionic silver between the aqueous and the solid phase. Traditional models of ionic silver and metal oxide adsorption are modified to incorporate the effect of the unpassivated alloy underlying the metal oxide film. The effect of repeatedly exposure-aging two passivated alloys is evaluated to determine if an equilibrium concentration of ionic silver in the potable range is attainable for a given alloy and surface-to-volume ratio. Optimal aging techniques and water quality parameters, to both maintain potable ionic silver concentrations during storage and maximize the operational lifetime of tank and piping systems, are evaluated and ranked.