[Adsorption of Phosphate on Mg/Fe Layered Double Hydroxides (Mg/Fe-LDH) and Use of Mg/Fe-LDH as an Amendment for Controlling Phosphorus Release from Sediments].

Abstract

We determine the efficiency and mechanism of Mg/Fe layered double hydroxides (Mg/Fe-LDH) addition for the control of phosphorus (P) release from sediments by studying the adsorption behavior and mechanism of phosphate from an aqueous solution on Mg/Fe-LDH. The impact of Mg/Fe-LDH addition on the mobilization of P in sediments as well as the adsorptive removal of phosphate by sediments is investigated, and the stabilization of P bound by Mg/Fe-LDH is also evaluated. Results showed that the kinetics data of phosphate adsorption onto Mg/Fe-LDH fitted better with the Elovich kinetics model than with the pseudo-first-order and pseudo-second-order kinetics models, and that the Freundlich and Dubinin-Radushkevich models were more suitable for describing the adsorption isotherm behavior of phosphate on Mg/Fe-LDH than the Langmuir model. Phosphate adsorption possessed a wide effective pH range of 4-10. Coexisting Ca2+ and Mg2+ enhanced phosphate adsorption onto Mg/Fe-LDH, while coexisting Na+, K+, and Cl- had negligible impacts on the phosphate adsorption. The presence of SO42- and HCO3- in aqueous solution inhibited the adsorption of phosphate on Mg/Fe-LDH. The phosphate adsorption mechanisms were deduced to be anion exchange, electrostatic attraction, ligand exchange and inner-sphere complex formation. The addition of Mg/Fe-LDH into sediments not only greatly reduced the concentration of reactive soluble P (SRP) in the overlying water, but also significantly decreased the level of SRP in the pore water. In addition, Mg/Fe-LDH addition also increased the adsorption capacity for the sediments, and the phosphate adsorption ability for the Mg/Fe-LDH-amended sediments increased with increased amendment dosage. Almost half of the phosphate bound by Mg/Fe-LDH existed in the form of relatively stable P, i.e., metal oxide-bound P (NaOH-rP), which was difficult to release back into the water column under normal pH and anoxic conditions. Nearly half of the phosphate bound by Mg/Fe-LDH existed in the form of easily released P, i.e., NH4Cl extractable P (NH4Cl-P) and redox-sensitive P (BD-P), which had a high risk of re-releasing into the water column. We conclude that it is very necessary for Mg/Fe-LDH to be recycled from the sediments after the application of Mg/Fe-LDH as an amendment to control sedimentary P liberation.