Phosphate and ammonium adsorption of the modified biochar based on Phragmites australis after phytoremediation

To effectively remove N and P from eutrophic water, the Phragmites australis after phytoremediation was harvested for preparation of modified biochar. The MgCl₂-modified biochar (MPB) was successfully synthesized at 600 °C under N₂ circumstance. The physiochemical characteristics, the adsorption capacity for N and P in the simulated solution, and their adsorption mechanism of MPB were then determined, followed by the treatment of eutrophic water of Tai lake and its inflow river from agricultural source. The results demonstrated that the MPB presented high adsorption capacity to both simulated NH₄-N and PO₄-P with the maximum adsorption capacity exceeding 30 and 100 mg g^-1, respectively. The entire ammonium adsorption process could be described by a pseudo-second-order kinetic model whereas the phosphate adsorption process could be divided into three phases, as described by both intra-particle diffusion model and the pseudo-first-order kinetic. It was further found that the dominant mechanism for ammonium adsorption was Mg²⁺ exchange instead of functional groups and surface areas and the Mg-P precipitation was the main mechanism for phosphate adsorption. The MPB also showed high removal ratio of practical TP which reached nearly 90% for both the water in Tai lake and its agricultural source. It suggested that MPB based on harvested P. australis was a promising composite for eutrophic water treatment and it could deliver multiple benefits. Graphic abstract.