[Evaluation of in situ capping with lanthanum-modified zeolite to control phosphate and ammonium release from sediments in heavily polluted river].

The efficiency and mechanism of sediment capping with an active barrier system (ABS) using lanthanum-modified zeolite (LaMZ) to simultaneously prevent phosphate and ammonium release from sediments in heavily polluted river under low dissolved oxygen conditions were investigated using batch and sediment incubation experiments. The adsorption kinetics of phosphate and ammonium on LaMZ followed a pseudo-second-order model. The equilibrium adsorption data of phosphate on LaMZ could be described by the Langmuir isotherm model. The equilibrium adsorption data of ammonium on LaMZ could be described by the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The mechanisms for phosphate adsorption onto LaMZ at pH 7 included ligand exchange, Lewis acid-base interaction and electrostatic attraction. The mechanism for ammonium adsorption onto LaMZ at pH 7 was cation exchange. The fluxes of phosphate and ammonium from the sediment to the overlying water were significantly reduced by the ABS using LaMZ under low dissolved oxygen conditions. Higher LaMZ dosage was found to be favorable for the prevention of ammonium release from the sediments using the ABS. Sequential extraction of phosphorus from phosphate-adsorbed LaMZ indicated that most of phosphate immobilized by the ABS using LaMZ was stable and unlikely to be released under low dissolved oxygen conditions. Results of this work indicate that LaMZ is a suitable capping material for preventing phosphate and ammonium release from sediments in heavily polluted river under low dissolved oxygen conditions.