Efficient removal of phosphorus in bioretention system by sludge pyrolysis residue

Phosphorus (P) removal in a bioretention system has been highly variable. In this study, we investigated the potential application of sludge pyrolysis residue, which was prepared from sludge pyrolysis, for adsorption of P from a bioretention system. Kinetics result suggested that the P adsorbed on sludge pyrolysis residue was faster than that of two soils, and both the surface and intraparticle diffusion were the rate-controlling steps for the adsorption. All adsorption isotherms were highly nonlinear and fitted well by the Langmuir model in comparison to the Freundlich model. The maximum adsorption capacity of P by sludge pyrolysis residue reaches 1,250 mg/kg at an initial concentration of 100 mg/L. Batch adsorption experiments and post-adsorption characterization results indicated that the adsorption was mainly controlled by both the precipitation and surface deposition mechanisms. The presence of Al, Fe, and Ca in sludge pyrolysis residue can facilitate the P adsorption. The release of P adsorbed on sludge pyrolysis residue is negligible because of a strong interaction between P and Al, Fe, and Ca in sludge pyrolysis residue. The findings of this study suggested that sludge pyrolysis residue could be used as an optimal filter medium to improve the removal performance of P in a bioretention system.