Iron-Tuned Depth-Dependent P Transformation on Calcium Carbonate Coprecipitates: The Impact of Fe and P Loadings

The incorporation of iron (Fe) into calcium carbonate coprecipitates (CCCs) to form Fe-incorporated CCC (CCCFe) widely occurred in CCC-enriched soils and sediments. However, the molecular mechanism of phosphate (P) immobilization on CCCFe remains largely unknown. In this study, batch experiments were conducted to investigate the immobilization mechanisms of P on CCCFe with low and high Fe loadings (CCCFe_l vs CCCFe_h) using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy with surface-sensitive total electron yield (P-XANES_TEY) and bulk-sensitive fluorescence yield (P-XANES_FLY) modes. Results indicated that the high Fe loading inhibited calcite formation but favored vaterite production in the CCC and enhanced its P retention capacity. Hydroxyapatite (HAP) dominated over ferrihydrite-associated P (Fe-P) in the bulk of P sorbed on the CCCFe_h, but amorphous calcium phosphate (ACP) dominated over HAP for the CCC, suggesting Fe-induced transformation of the ACP to the HAP. Furthermore, the P-XANES_TEY analysis indicated brushite primarily formed on the surface of the P sorbed on the CCCFe_h at high P loading. Consistently, the spherical aberration-corrected scanning transmission electron microscopy analysis directly revealed ferrihydrite coating on the CCC and the presence of Ca-P and Fe-P associations. This study provides new molecular-level insights into Fe-tuned depth-dependent transformation of P on the CCC, thus benefiting P management in the CCC-enriched environmental settings.