Contrasting effect of lanthanum hydroxide and lanthanum carbonate treatments on phosphorus mobilization in sediment

Abstract

A comparative study on the effects of lanthanum carbonate (LC) and lanthanum hydroxide (LH) treatments (capping and amendment) on the migration of phosphorus (P) from sediment into overlying water (OVER-water) was investigated. Results showed that LC outperformed LH for phosphate adsorption in pure aqueous solution at pH 5–7, but the latter possessed higher adsorption ability for phosphate in pure aqueous solution at pH 8–11 than the former. Under the existence of chloride, bicarbonate, sulfate, sodium and potassium ions, the adsorption ability of LC for phosphate was higher than that of LH, but the latter exhibited stronger phosphate adsorption ability than the former in the presence of calcium and magnesium ions. Most of phosphate ions bound by LC and LH were relatively stable, and hard to be liberated again under the conditions of normal pH and anaerobic environment. The main mechanism of phosphate absorption onto LC was the ligand replacement of carbonate group with phosphate ion forming the inner-sphere complexes of lanthanum-phosphate, while the adsorption of phosphate on LH was mainly ascribed to the ligand replacement of hydroxyl group with phosphate ion and subsequently the formation of inner-sphere La-O-P complex and LaPO4. LC and LH capping/amendment all could greatly reduce the risk of P release from sediment. As an active capping or amendment material, LH had a better controlling performance of sediment-P release into OVER-water than LC. The fabric-wrapped LH capping could also more effectively inhibit the liberation of P from sediment to OVER-water than the fabric-wrapped LC capping. LC had a lower risk of La release into OVER-water than LH. The amendment of sediment with LC had a small impact on the microbial diversity of the sediment, while the amendment of sediment with LH had a great negative influence on the microbial diversity of the sediment. Viewed from the controlling efficiency, application convenience, and environmental friendliness, LC capping is a promising way for reducing P release from sediment to OVER-water in surface water bodies.