Adsorption-desorption behavior of silver ions on stainless steel as a proxy for disinfection of domestic hot water

The water quality of domestic hot water can be controlled by disinfection with silver ions (Ag+). In order to maximize the antibacterial effect of Ag+and at the same time ensure the water is safe for human use, the adsorption and desorption properties of Ag+onto 304 stainless steel (SS) was studied. The effects of the initial Ag+concentration, the contact time, and the water temperature on the adsorption process were examined. The antibacterial properties of SS adsorbed with Ag+(SS-Ag) were also investigated. The results showed that with an increase of the Ag+concentration, water temperature or adsorption time, the equilibrium adsorption capacity increased gradually, and the adsorption process reached equilibrium after 72 h. Adsorption kinetic studies showed that the adsorption of Ag+on SS was best described by the pseudo-second order model. The adsorption isotherm data fitted well in the Langmuir isotherm model at 45°C (R2= 0.9894), which indicated that the adsorbed silver forms a monolayer on the SS. The thermodynamic modelling indicated that adsorption occurred by an endothermic reaction that occurred spontaneously under high temperature conditions. Elution was impossible with ultra-pure water and tap water was inefficient to desorb the Ag+from SS. It can be inferred that the shear action of a water flow inside a water pipe would not remove large amounts of adsorbed Ag+, so that adsorbed Ag+can maintain its antibacterial properties for extended periods. Solutions of sodium thiosulfate or thiourea could effectively desorb Ag+from the SS surface, and near to complete desorption (98.35%) was obtained with a 2 mg/L sodium thiosulfate solution. Antibacterial experiments indicated that the SS-Ag could inhibit the biofilm formation and effectively reduce the risk of biofilm pollution. The results presented here provide a theoretical basis for the use of silver in hot water supplies using SS pipes for disinfection purposes.