Sulfate affinity controls phosphate sorption and the proto-transformation of schwertmannite

Schwertmannite is a metastable sulfate-rich ferric iron Fe(III) (oxyhydr)oxide and a common mineral in acid mine drainage sites and acid sulfate soils. Schwertmannite is also used as a sorbent in various industrial applications, including phosphate removal in water treatment and environmental remediation. Phosphate sorption to schwertmannite, however, is complex and likely involves ligand exchange for inner- and outer-spherically coordinated sulfate groups, both on the surface and in the tunnel structure of the mineral. Here, we investigated phosphate sorption, concomitant sulfate release and their impact on the structure of schwertmannite as a function of pH and phosphate concentration. Kinetic and equilibrium batch experiments with synthetic schwertmannite were carried out at pH 3, 6, and 8, and the solid-phase was analyzed using a combination of microscopic, spectroscopic, and X-ray diffraction techniques. We found a strong correlation between phosphate sorption and sulfate release, with both following a two-step sorption model. The kinetics of phosphate sorption and sulfate release were faster at higher pH levels. Maximum phosphate sorption was found at pH 6 (1.7 mmol PO43− g−1), which decreased to 1.5 and 1.2 mmol PO43− g−1 at pH 3 and 8, respectively. Fourier transform infrared spectroscopy revealed a shift from inner- to outer-spherical coordination of sulfate with increasing pH. 57Fe-Mössbauer analyses of schwertmannite demonstrated an initial transformation of schwertmannite at neutral to alkaline pH, characterized by a rise in a partially ordered sextet area. This change was interpreted as an increase in crystallinity resulting from the transition from Fe-SO4 to FeO domains. The emerging phase differed from the original schwertmannite, but did not represent a complete change to a new crystalline phase, thus indicating a proto-transformation of schwertmannite. This pH-induced proto-transformation was inhibited in the presence of phosphate. We concluded that the phosphate sorption rate and maximum as well as the proto-transformation of schwertmannite were strongly affected by the mineral's affinity for sulfate. Sorption to schwertmannite should primarily be regarded as a competitive exchange reaction between the sorbing oxyanion and the bound sulfate. As a result, the highest phosphate sorption occurred at circumneutral pH, in stark contrast to non-sulfate-containing Fe(III) (oxyhydr)oxides, where phosphate sorption is highest at acidic pH. Our findings are important for a fundamental understanding of the sorption properties of schwertmannite in phosphate-rich environments as they point towards the central role of sulfate coordination for phosphate immobilization.