Preferential phosphate sorption and Al substitution on goethite†
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.