XANES spectroscopy proofs pH-dependent P sorption partitioning to Fe oxyhydroxides versus montmorillonite in acidic soils

Background: Fe and Al oxyhydroxides are well-known phosphorus (P)-retaining minerals in soils. Little information is available regarding the relevance of clay minerals for the sorption of P in mixed oxyhydroxide–clay mineral systems and pH effects on P sorption partitioning. Aims and Methods: We wanted to investigate pH effects on P sorption partitioning between Fe oxyhydroxides and high-activity clay minerals in mixed-mineral systems. We quantified the relative contribution of ferrihydrite or goethite versus Al-saturated montmorillonite (Al-MT) to the retention of orthophosphate (oPO₄) and inositol hexakisphosphate (IHP) at different pH values (3–6). We combined the analysis of P solution concentration changes with the quantification of P bound to Fe(III) versus Al in the Fe oxyhydroxide/Al-MT mixtures by X-ray absorption near-edge structure (XANES) spectroscopy. Results: Orthophosphate was preferentially retained by ferrihydrite, compared to Al-MT at pH 3–6. The opposite was observed for goethite at low P solution concentrations. The contribution of Al-MT versus Fe oxyhydroxides to oPO₄ retention increased with pH. This is attributed to a speciation change of clay-adsorbed Al from monomeric Al³⁺ to polynuclear Al hydroxy species. IHP was predominantly retained by Al-MT rather than ferrihydrite at pH 3–6, probably by the formation of adsorbed and surface-precipitated Al phytate complexes. Conclusions: Synchrotron-based XANES spectroscopy allows for quantifying P adsorbed to co-existing pedogenic Fe(III) oxyhydroxides versus Al-MT in mixed-mineral systems. Soil P retention partitioning to these minerals depends on (1) dominating P form (oPO₄, IHP), (2) relative abundance of high-activity clay minerals, short-range ordered and crystalline Fe oxyhydroxides, and (3) solution pH.