[Effect of Biomass Particle Size on the Adsorption of Phosphorus from Aqueous Solution by MgO-loaded Biochar]

To study the effect of biomass particle size on the rate and ability of phosphorus removal from aqueous solution by MgO-loaded Phragmites australis biochar (MBC), MBC was prepared using 0.0-0.5, 1.0-2.0, and 6.0-8.0 mm Phragmites australis particles as the feedstock and MgCl₂ as the modification material. The MBC was characterized using FTIR, XRD, and SEM techniques. Kinetic and isotherm experiments of phosphate (PO₄^3--P) adsorption from aqueous solution by the MBC were conducted, and the experimental data were fitted with various kinetic and isotherm models. The results showed that the adsorption rate of PO₄^3--P by the MBC increased with the increase in biomass particle size. The amount of PO₄^3--P adsorbed by the MBC prepared from 0.0-0.5, 1.0-2.0, and 6.0-8.0 mm particles reached 15.4%, 25.8%, and 80.8%, respectively, within 2 h. The biomass particle size did not affect the maximum PO₄^3--P adsorption capacity (249.0-254.7 mg·g^-1) of the MBC. MBC prepared from the 6-8 mm particles retained the complete cell wall structure of the Phragmites australis, and a large number of micropores and mesopores were generated during pyrolysis, thereby forming a hierarchical, regular, and well-connected pore structure. MBC prepared from the 0.0-0.5 mm and 1.0-2.0 mm particles had inferior pore structures with inferior pore connectivity, which affected the diffusion rate of PO₄^3- ions inside the MBC and limited the PO₄^3--P adsorption rate. Therefore, when using waste Phragmites australis harvested from a constructed wetland to produce MBC and remove phosphorus from water, the Phragmites australis should be crushed into 6-8 mm particles. Over-crushing deteriorates the pore structure of the produced MBC and reduces the removal rate of phosphorus by the MBC.