Extent of As(III) versus As(V) adsorption on iron (oxyhydr) oxides depends on the presence of vacancy cluster-like micropore sites: Insights into a seesaw effect.

Iron (oxyhydr)oxides are ubiquitous in terrestrial environments and play a crucial role in controling the fate of arsenic in sediments and groundwater. Although there is evidence that different iron (oxyhydr)oxides have different affinities towards As(III) and As(V), it is still unclear why As(V) adsorption on some iron (oxyhydr)oxides is larger than As(III) adsorption, while it is opposite for other ones. In this study, six typical iron (oxyhydr)oxides are selected to evaluate their adsorption capacities for As(III) and As(V). The characteristics of these iron minerals such as morphology, arsenic adsorption species, and pore size distribution are carefully examined using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), positron annihilation lifetime (PAL) spectroscopy, and X-ray absorption spectroscopy (XAS). We confirm a seesaw effect occurred in different iron minerals for As(III) and As(V) immobilization, i.e., at pH 6.0, adsorption of As(V) on hematite (0.73 μmol m) and magnetite (0.33 μmol m) is higher than for As(III) (0.61 μmol m and 0.27 μmol m, respectively), for goethite and lepidocrocite it is almost equal, while As(III) sorption on ferrihydrite (5.77 μmol m) and schwertmannite (28.41 μmol m) showed higher sorption than As(V) (1.53 μmol m and 12.99 μmol m, respectively). PAL analysis demonstrates that ferrihydrite and schwertmannite have a large concentration of vacancy cluster-like micropores, significantly more than goethite and lepidocrocite, followed by hematite and magnetite. The difference of adsorption of As(III) and As(V) to different iron (oxyhydr)oxides is due to differences in the abundance of vacancy cluster-like micropore sites, which are conducive for smaller size As(III) immobilization but not for larger size of As(V). The findings of this study provide novel insights into a seesaw effect for As(III) and As(V) immobilization on naturally occurring iron mineral.