Fe 2+ sorption onto nontronite (NAu-2)

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), Fe 2+ concentration (0.01–50 mM), equilibration time (1–35 days), and ionic strength (0.01–0.5 M NaClO 4). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe 2+ sorption data. Fe 2+ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe 2+ sorption through ion exchange and surface complexation reactions. Fe 2+ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe 2+ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe 2+ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe 2+ 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. The kinetic uptake of Fe 2+ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe 2+ sorption data as a function of pH, ionic strength, and Fe 2+ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH) + was required to describe Fe 2+ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe 2+ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions.