Silver ion desorption kinetics from iron oxides and soil organic matter: effect of adsorption period

Abstract

In order to have a better understanding of the mobility and bioavailability of silver in the environment, it is important to investigate the desorption behaviour of silver from some environmentally relevant soil components. Four single soil constituents (charcoal, humic acid, ferrihydrite, goethite) were chosen to investigate the rate of desorption of silver ions. The effect of reaction period between silver and the soil constituents on subsequent desorption was also investigated. A significant proportion of Ag⁺ sorbed by Fe oxides, humic acid, and charcoal cannot readily be desorbed back into solution. For goethite and humic acid, a longer contact period between Ag⁺ and the adsorbing phase caused subsequent slower rates of desorption, and a smaller proportion of adsorbed Ag⁺ was desorbed back into solution. Two-site and log-normal equations, assuming the desorption reaction was first-order, gave excellent fits to experimental data, except for Ag⁺ desorption from charcoal, in which case the kinetic experimental data fitted to a 1-site model better. The parameters obtained from the modelling provide information regarding the possible changes in metal–oxide binding and metal–humic acid complexing mechanisms, and these changes can be interpreted as a re-arrangement of Ag ions to sites with slower desorption reaction rates. All the reaction sites on charcoal have very similar desorption rates (rate constants), and these sites are relatively slow to desorb Ag. Fe oxides, and humic acid and charcoal could be very important sinks for Ag. The longer the Ag interacts with soils in environment, the less mobile Ag becomes, and the less toxic and bioavailable it will be to living organisms.