Control of phosphorus release from sediment by humic acid-loaded ferrihydrite and magnetite

This study investigated the efficacy and mechanisms of humic acid (HA)-loaded ferrihydrite (Fh) and magnetite (Mag) in suppressing phosphorus (P) release from sediment into the overlying water (OW) via direct capping, fabric-wrapped capping, and sediment amendment. The results showed that the direct capping, fabric-wrapped capping, and sediment amendment with HA-loaded ferrihydrite (HA-Fh) and magnetite (HA-Mag) effectively mitigated the sedimentary P migration into OW. Specifically, the average reduction efficiencies of soluble reactive phosphorus (SRP) in OW were 94.6%, 74.6%, and 45.0% for HA-Fh under direct capping, fabric-wrapped capping, and amendment conditions, respectively. For HA-Mag, the corresponding efficiencies were 83.9%, 71.1%, and 47.1%. Under HA-Fh and HA-Mag capping, P in the sediment continued to mobilise into the pore water through the microbial and sulphide-mediated chemical reduction of iron (III) (hydr)oxides. However, the capping layers significantly reduced the concentrations of pore water SRP and labile P measured by diffusive gradients in thin-films (DGT) in the upper sediment. This reduction was crucial for effectively intercepting the sedimentary P migration into OW by HA-Fh and HA-Mag. Our study demonstrates that HA-Fh and HA-Mag are effective capping materials for managing the sediment-derived internal P loading. In other words, even after HA aging treatment, Fh and Mag still remain effective sediment phosphorus release control materials. In addition, the direct capping methods using HA-Fh and HA-Mag are promising due to their high control efficiency and ease of application in preventing P release from sediment into OW.