Dissolved Silica Effects on Adsorption and Co-Precipitation of Sb(III) and Sb(V) with Ferrihydrite

Shuang Zhou, Tsutomu Sato, Tsubasa Otake
2018 Minerals  
Elevated antimony concentrations in aqueous environments from anthropogenic sources are becoming of global concern. In this respect iron oxides are known to strongly adsorb aqueous antimony species with different oxidation states, but the effect of silica on the removal characteristics is not well understood despite being a common component in the environment. In this study, ferrihydrite was synthesized at various Si/Fe molar ratios to investigate its adsorption and co-precipitation behaviors
more » ... th aqueous antimony anionic species, Sb(III) and Sb(V). The X-ray diffraction analyses of the precipitates showed two broad diffraction features at approximately 35 • and 62 • 2θ, which are characteristics of 2-line ferrihydrite, but no significant shifts in peak positions in the ferrihydrite regardless of the Si/Fe ratios. The infrared spectra showed a sharp band at~930 cm −1 , corresponding to asymmetric stretching vibrations of Si-O-Fe bonds which increased in intensity with increasing Si/Fe molar ratios. Further, the surface charge on the precipitates became more negative with increasing Si/Fe molar ratios. The adsorption experiments indicated that Sb(V) was preferentially adsorbed under acidic conditions which decreased dramatically with increasing pH while the adsorption rate of Sb(III) ions was independent of pH. However, the presence of silica suppressed the adsorption of both Sb(III) and Sb(V) ions. The results showed that Sb(III) and Sb(V) ions were significantly inhibited by co-precipitation with ferrihydrite even in the presence of silica by isomorphous substitution in the ferrihydrite crystal structure. on its oxidation state, and reduced Sb(III) species are ten times more poisonous than oxidized Sb(V), similar to the case of arsenic [16] . In spite of the widespread usage and the substantial toxicity, the geochemical behaviors of antimony in soil and aquatic systems are poorly understood [17, 18] . Recent studies have shown that both Sb(III) and Sb(V) appear to adsorb strongly onto iron oxides [10, [18] [19] [20] , which thereby strongly influence the speciation, mobility, and final states of Sb in the environment. Sb is preferentially associated with iron(III) (oxyhydr)oxide in soils and sediments on the basis of direct evidence using extended X-ray absorption fine structure spectroscopy (EXAFS) [6, 10, 21] . This would suggest that adsorption and incorporation processes into the iron(III) (oxyhydr)oxide phases would be able to control the mobility of Sb in natural environments. Several experimental studies have investigated the Sb adsorption mechanism on iron(III) oxyhydroxides, focusing on Sb speciation at the solid-liquid (water) interface using EXAFS [10, 22] . However, the surface structure of Sb(III) and Sb(V) binding with iron(III) oxyhydroxides is still unclear. A further important process, co-precipitation with iron(III) (oxyhydr)oxides, does not appear to have been reported. In natural systems, iron(III)(hydro)oxides are often identified in precipitates from the oxidation of iron(II) in the presence of the relevant anions. Thus, the precipitation process may be as important as adsorption, as a sequestration process of Sb species by iron (III) (oxyhdr)oxides when groundwater with natural or added Fe comes in contact with air or oxygenated water takes place. Further, as iron minerals are closely associated with silica, one of the most common ligands present in natural environments, pure ferrihydrite is not, strictly speaking, present in nature. Silica always associates with ferrihydrite in the structure or on the ferrihydrite surface. This could mean that silica may affect the crystallization behavior as well as the capacity of ferrihydrite to regulate hazardous element recycling. The effect of silica on arsenic adsorption has been reported for ferrihydrite [23] , but no reports of the effect of silica on Sb adsorption or co-precipitation have been reported for other oxides. Based on the above, the objectives of this study were (1) to compare the relative adsorption capabilities of Sb(III) and Sb(V) onto ferrihydrite; (2) to evaluate the effect of dissolved silica during the adsorption and co-precipitation of Sb(III) and Sb(V) on ferrihydrite; and also (3) to better understand the differences between the adsorption and co-precipitation processes of Sb.
doi:10.3390/min8030101 fatcat:gkndi4jnmvar5jshhuvfdzqhre