Controlled release of phosphorus using lanthanum-modified hydrochar synthesized from water treatment sludge: Adsorption behavior and immobilization mechanism

For the proper disposal of water treatment sludge, novel Lanthanum (La)-modified water treatment sludge composites were synthesized using simple one-step hydrothermal carbonization. The composite La-modified hydrochar (WTSLa170–3) prepared at 170 °C for 3 h exhibited a phosphate adsorption capacity of up to 72.69 mg/g. The pore volume, pore size and specific surface area of the prepared material significantly increased, and La loaded successfully, which improved the phosphate adsorption performance compared with that of raw sludge. The adsorption kinetics and isotherms fitiing results revealed that the adsorption process was in agreement with the Freundlich model and pseudo-second-order kinetic model, and was multimolecular layer chemisorption and insensitive to the coexisting anions. The thermodynamic parameters revealed that the adsorption process had an endothermic and spontaneous nature. The adsorption mechanism was through electrostatic attraction and ligand exchange to form an inner-sphere complex, which was explored by using zero point charge analysis, Fourier infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Adding La-modified hydrochar into sediment effectively inhibited the release of internal P in the sediment. The addition of this hydrochar promoted the transformation of active NH4Cl-P, Org-P and BD-P into the very stable HCl-P in the sediment, which also reduced the biologically available P content significantly. These results indicate that WTSLa170–3 can effectively adsorb and remove P in water and immobilize P in sediment, and provide a new way for the resource utilization of water treatment sludge.