PNNL

Abstract

The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3-10), Fe2+ concentration (0.01-50 mM), equilibration time (1 to 35 days), and ionic strength (0.01 to 0.5 M NaClO4). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site properties for the interpretation of Fe2+ sorption. Fe2+ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe2+ sorption through ion exchange and surface complexation reactions. Fe2+ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, reached the first plateau from pH 4.5 to 7.0, increased again with increasing pH from pH 7.0 to 8.5, and reached the maximum (the second plateau) above pH 8.5. The Fe2+ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe2+ sorption at different ionic strength, however, diminished with increasing equilibration time. The Fe2+ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. Increasing Fe2+ concentration nonlinearly decreased Fe2+ sorption, resulting from the mass action and site saturation. An equilibrium model that integrated ion exchange, surface complexation and aqueous speciation reactions reasonably well described the Fe2+ sorption data as a function of pH, ionic strength, and Fe2+ concentration measured at 24 hours of equilibration. Model calculations showed that species Fe(OH)+ was required to describe Fe2+ sorption above pH 8.0. Model calculation also implied that the ion exchange reactions was responsible for the kinetic behavior of Fe2+ sorption. Overall, this study demonstrated that Fe2+ sorption to NAu-2 was complexly affected by equilibrium and kinetic processes.