Preferential phosphate sorption and Al substitution on goethite

Abstract

Iron (hydr)oxides, which are ubiquitously distributed in the environment, often contain impurities such as Al. Aluminum-substituted goethite (AlG) is a typical assemblage of Al and Fe (hydr)oxides. In this study, the molecular-level mechanisms of PO₄ sorption in relation to structural changes in AlG with Al/(Al + Fe) molar ratios up to 17.4% and the PO₄ distribution between Al and Fe were determined. While the XRD results showed that Al preferred to substitute for Fe on relatively low-index planes of goethite, the Fe-XAS and XPS data indicated the particular Al substitution in edge-shared FeO₆ octahedral linkages and a tendency of Al segregation near the surface of AlG, respectively. The maximum PO₄ sorption capacity increased from 135 to 584 mmol kg⁻¹ as Al/(Al + Fe) mol% increased from 0 to 17.4%. Phosphorus-XANES data of PO₄ sorbed on AlG showed either preferential PO₄ bonding for Al or no preference for Al or Fe. Compared to goethite with adsorbed PO₄, the density functional theory (DFT) result of AlG containing 12.5 Al mol% showed more relaxed Al atoms relative to the topmost atomic layers of the supercell upon PO₄ adsorption, a smaller Al–O–P angle than the corresponding Fe–O–P angle, and a relatively stable PO₄ complex formed on the AlG surface. New insights into the PO₄ sorption mechanisms and related structural changes in Al/Fe assemblages could improve the assessment of the P dynamics and mass balance in agricultural and PO₄-induced eutrophication systems.