co-precipitation of Sb(III) and Sb(V) with ferrihydrite

Elevated antimony concentrations in aqueous environments from anthropogenic sources 9 is becoming of global concern, here iron oxides are known to strongly adsorb aqueous antimony 10 species with different oxidation states, but the effect of silica on the removal characteristics is not 11 well understood despite being a common component in the environment. In this study, ferrihydrite 12 was synthesized at various Si/Fe molar ratios to investigate its adsorption and co-precipitation 13 behaviors with aqueous antimony anionic species, Sb(III) and Sb(V). The XRD analyses of the 14 precipitates showed two broad diffraction features at approximately 35° and 62° 2θ, which are 15 characteristic of 2-line ferrihydrite, no significant shifts in peak positions in the ferrihydrite 16 regardless of the Si/Fe ratios. The infrared spectra showed a sharp band at ~990 cm-1, corresponding 17 to asymmetric stretching vibrations of Si-O-Fe bonds which increased in intensity with increasing 18 Si/Fe molar ratios. Further, the surface charge on the precipitates became more negative with 19 increasing Si/Fe molar ratios. The adsorption experiments indicated that Sb(V) was preferentially 20 adsorbed at acidic conditions and decreased dramatically with increasing pH while the adsorption 21 rate of Sb(III) ions was independent of pH, however, the presence of silica suppressed the 22 adsorption of both Sb(III) and Sb(V) ions. The results showed that Sb(III) and Sb(V) ions were 23 significantly inhibited by co-precipitation with ferrihydrite even in the presence of silica by 24 isomorphous substitution in the ferrihydrite crystal structure. 25