Preferential phosphate sorption and Al substitution on goethite

• Published in: Environmental science. Nano 2020-11, Vol.7 (11), p.3497-358
• Main Authors: Hsu, Liang-Ching, Tzou, Yu-Min, Ho, Mon-Shu, Sivakumar, Chandrasekar, Cho, Yen-Lin, Li, Wen-Hui, Chiang, Po-Neng, Teah, Heng Yi, Liu, Yu-Ting
• Format: Article
• Language: English
• Subjects: Aluminium; Aluminum; Density functional theory; Eutrophication; Goethite; Impurities; Iron; Mass balance; Oxides; Phosphates; Phosphorus; Segregation; Sorption; Substitutes
• ISSN: 2051-8153; 2051-8161
• DOI: 10.1039/c9en01435g

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 PO 4 sorption in relation to structural changes in AlG with Al/(Al + Fe) molar ratios up to 17.4% and the PO 4 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 FeO 6 octahedral linkages and a tendency of Al segregation near the surface of AlG, respectively. The maximum PO 4 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 PO 4 sorbed on AlG showed either preferential PO 4 bonding for Al or no preference for Al or Fe. Compared to goethite with adsorbed PO 4 , 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 PO 4 adsorption, a smaller Al-O-P angle than the corresponding Fe-O-P angle, and a relatively stable PO 4 complex formed on the AlG surface. New insights into the PO 4 sorption mechanisms and related structural changes in Al/Fe assemblages could improve the assessment of the P dynamics and mass balance in agricultural and PO 4 -induced eutrophication systems. The Al-substitution affected not only the basal spacing between adjacent lattice planes but also the structural arrangement in goethite which results in the Al-preferential PO 4 sorption on Al-substituted goethite.