Antimony(V) incorporation into synthetic ferrihydrite, goethite, and natural iron oxyhydroxides

In this study, we investigated local structures of Sb species in synthetic Sb(V)-coprecipitated and -adsorbed ferrihydrite and goethite, which are common iron(III) oxyhydroxides in environment, at various Sb/Fe molar ratios by extended X-ray absorption fine structure (EXAFS) analyses. The EXAFS analyses showed that Sb(V) is adsorbed on ferrihydrite and goethite by the formation of an inner-sphere surface complex at pH 7.5. In the EXAFS spectra of the coprecipitated ferrihydrite and goethite, some features of the spectra significantly differed from those in the adsorbed samples. The EXAFS simulation indicated that the difference is due to the larger coordination number of the Fe atom to the Sb atom in the coprecipitation samples, indicating a structural incorporation (heterovalent substitution) of Sb(V) into ferrihydrite and goethite. The incorporation of Sb(V) into the structure was also confirmed in natural iron(III) oxyhydroxides in contaminated soil near an Sb mine tailing using mu-EXAFS. This study directly provided the first evidence for the structural incorporation of Sb(V) into the iron(III) oxide structure. Our findings are important for understanding the fate of Sb in the aquatic environment because the behavior of the elements incorporated into solids by such a substitution is not greatly influenced by aquatic factors such as the pH and ionic strength because of isolation of the incorporated metal(loid) ions from the aqueous phase.