Interception of phosphorus release from sediments using Mg/Fe-based layered double hydroxide (MF-LDH) and MF-LDH coated magnetite as geo-engineering tools

Abstract

A magnesium/iron-based layered double hydroxide (MF-LDH) and a composite of MF-LDH and magnetite (MF-LDH@Fe3O4) were synthesized, characterized and used as solid-phase phosphorus (P)-sorbents (SPPSs) to control the release of sedimentary P. The behavior and mechanism of phosphate adsorption onto MF-LDH and MF-LDH@Fe3O4 were studied. The effect of MF-LDH capping and amendment on the migration of P in sediments were comparatively investigated, and the impact of fabric-wrapped and unwrapped MF-LDH@Fe3O4 capping on P mobilization in sediments were also comparatively investigated. Results showed that both MF-LDH and MF-LDH@Fe3O4 had good phosphate adsorption performance, and the adsorption mechanisms included cation exchange, electrostatic attraction, ligand exchange and inner-sphere complex formation. Sediment capping and amendment using MF-LDH both could dramatically reduce the risk of the release of soluble reactive P (SRP) and diffusive gradient in thin-films-labile P (P-DGT) from sediments into overlying waters (OLY-Ws), and the MF-LDH capping had a better suppressing efficiency of sediment-P release into OLY-W than the MF-LDH amendment. Sediment capping with the fabric-wrapped and unwrapped MF-LDH@Fe3O4 both greatly decreased the risk of SRP and P-DGT released from sediment into OLY-W, and the efficiency of the prevention of SRP released from sediment into OLY-W by the fabric-wrapped MF-LDH@Fe3O4 capping layer (about 81–90%) was slightly lower than that by the unwrapped MF-LDH@Fe3O4 capping layer (about 94–99%). The reduction of P-DGT in the top sediment and the direct interception of the soluble P from pore water (POR-W) to OLY-W by the MF-LDH@Fe3O4 capping layer were the keys to the management of P released from sediment by the MF-LDH@Fe3O4 capping. From the standpoint of the efficiency of sedimentary P suppression, the convenience of application and the sustainability of sediment remediation, sediment capping with the fabric-wrapped MF-LDH@Fe3O4 is a promising approach to manage the release of sedimentary P into OLY-W.