Magnetite-modified activated carbon based capping and mixing technology for sedimentary phosphorus release control

In this study, magnetite-modified activated carbon (MAC) was synthesized, characterized and used as capping and amendment materials to control sedimentary phosphorus (P) release. Batch experiments were applied to determine the behavior of phosphate adsorption and desorption on/from MAC. Sediment incubation experiments were utilized to evaluate the impact of MAC capping and addition on the mobilization of P in sediments. Sediment capping and amendment with MAC both can greatly reduce the amount of reactive soluble P (RS-P) in the overlying water (OLY-water), with a reduction efficiency of higher than 83%. MAC capping and amendment both can significantly reduce the concentrations of labile P measured by diffusive gradient in thin-films (DGT) in the upper sediment, which gives rise to in the formation of the static layer of P (P–S-Layer) in the upper sediment. The forms of P bound by MAC were mainly redox-sensitive P (PRS), NaOH extractable inorganic P (IPNaOH) and HCl extractable P (PHCl), which accounted for 47.2, 18.5 and 32.9% of the total adsorbed P, respectively. Almost half of P adsorbed by MAC existed in the form of PRS, which is easy to be released under anoxic condition, and the retrieval of MAC from the waterbody after its application is very necessary. The concentrations of RS-P in OLY-water and mean DGT-labile P in P–S-Layer under capping condition were much less than those under amendment condition. The reduction of the apparent diffusion efflux of P across the interface between OLY-water and sediment by the MAC capping was much larger than that by the MAC amendment. Results of this work suggest that MAC capping and amendment are very promising methods for blocking the liberation of P from sediments into OLY-water, and MAC capping can achieve a higher efficiency of sedimentary P release control compared to MAC amendment.