Interception of sedimentary phosphorus release by iron-modified calcite capping

Purpose The main aim of this work was to determine the efficiency and mechanism of iron (Fe)-modified calcite (FMCA) capping for preventing the release of phosphorus (P) from sediments. Methods The performance of phosphate removal by FMCAs with different iron loading amounts was investigated using batch experiments, the mechanism for the phosphate removal by FMCA was explored by X-ray photoelectron spectroscopy, and the effect of FMCA capping on P mobilization in sediments at various depths was studied using high-resolution dialysis and diffusive gradient in thin films (DGT). Results FMCA possessed better phosphate adsorption ability for aqueous phosphate than raw calcite, and the phosphate removal efficiency of FMCA increased with its loading amount of Fe. The replacement of Fe-bound hydroxyl groups with phosphate anions to the inner-sphere surface complexes and the precipitation of calcium phosphate were vital to the sorption of phosphate onto FMCA. FMCA capping could effectively inhibit the release of soluble reactive P (SRP) and labile P monitored by DGT (DGT-P) from sediment to the overlying water. The decrease of DGT-P and pore water SRP in the top sediment layer is very important for the control of sediment-P release into the overlying water. FMCA capping caused reductions of the concentrations of Fe 2+ and labile Fe monitored by DGT in the overlying water and the top sediment, suggesting a low risk of Fe releasing from the FMCA capping layer. Conclusion Iron-modified calcite has the potential to be an active capping material to suppress sedimentary P release.