Characterization and mechanism of lanthanum-modified biochar for efficient phosphate removal from water

Biochar (BC) can be applied for the removal of phosphorus in water bodies. The eutrophication of water bodies can also be alleviated in the disposal of solid wastes. However, the low adsorption capability has seriously limited the application of biochar in phosphate removal from water bodies. In this study, a novel La-modified biochar (LBC) adsorbent was successfully synthesized by a pyrolysis-hydrothermal method, in order to efficiently remove the phosphate from water. A systematic investigation was implemented to explore the effects of preparation parameters (including La to BC mass ratio, hydrothermal time and temperature), adsorption time, initial phosphate concentration, solution pH, and coexisting ions on the phosphate adsorption performance of LBC. The adsorption behavior of LBC on the phosphate was characterized by X-ray diffraction (XRD), Fourier infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) techniques. In addition, the slow-release performance of LBC in water was also evaluated after the saturated adsorption of phosphate. Scanning electron microscopy (SEM) demonstrated that the lanthanum species on the surface of LBC exhibited a spherical porous structure, providing for the abundant adsorption sites of phosphate. Meanwhile, the adsorption equilibrium of LBC rapidly reached within 1 h, indicating the superior adsorption rate for phosphate, compared with the as-reported ones. The pseudo-secondary kinetic model was more suitable to represent the adsorption of phosphate by LBC, compared with the pseudo-first ones. In addition, there were no fitting lines of the intraparticle diffusion model that passed through the origin. It infers that the adsorption rate of phosphate by LBC was controlled by both surface chemisorption and intraparticle diffusion. The maximum phosphate adsorption of LBC fitted by the Langmuir model reached 136.4 mg/g. LBC also exhibited a favorable adsorption capacity for phosphate. The rapid adsorption rate and high adsorption capacity of LBC were attributed to the spherical porous structure of La species on the surface of LBC, indicating the abundant adsorption sites of phosphate. In addition, the value of 1/n in the Freundlich model was below 0.5, indicating that the adsorption of phosphate by LBC was easy to occur, owing to the strong affinity of LBC for phosphate. The pH experiments showed that the LBC maintained the stable performance of phosphate adsorption in the pH range of 3.0-7.0, whereas the phosphate adsorption of LBC was inhibited at pH above 7.0. The reduced capacity of phosphate adsorption under alkaline conditions was attributed mainly to the variation in the phosphate ion species, competitive adsorption of hydroxide ions, and the declined electrostatic attraction. There were negligible effects of Cl^- and NO^3- on the phosphate adsorption by LBC. The presence of SO₄^2- slightly inhibited the phosphate adsorption, whereas HCO^3-, Mg²⁺, and Ca²⁺ promoted the phosphate adsorption by LBC. The ion coexistence experiment demonstrated that the high selectivity of LBC was achieved in the phosphate. Therefore, the LBC shared the promising potential for the treatment of actual aqueous environments. The main mechanisms of phosphate adsorption on the LBC included electrostatic attraction, LaPO₄ precipitation, and La-O-P inner-sphere complexation. In addition, the LBC that saturated with adsorbed phosphate was proceeded with the continuous phosphate release over 15 d. There was a negligible amount of La released from LBC into the water in the pH range of 3.0-11.0, indicating a low risk of La leaching. As such, the phosphate-adsorbed LBC can be expected to serve as a slow-release phosphate fertilizer for hydroponic crops. This finding can provide new ideas for the resource utilization of forestry wastes and the synthetic design of efficient lanthanum-modified biochar adsorbent materials for phosphate removal.