Elimination of zinc ions from aqueous solution by a hydroxylapatite-biochar composite material with the hierarchical porous microstructures of sugarcane waste

A hydroxylapatite-biochar composite material with the hierarchical porous microstructures of sugarcane top internodes (HAP/C-SC) was produced by carbonizing sugarcane top stalks, an agricultural waste residue, and then soaking them in limewater and (NH4)(2)HPO4 solution in turns. The HAP/C-SC with the surface area of 8.52-28.44 m(2)/g inherited various macropores from parallel vessels, thick-walled cells of sclerenchyma, thin-walled cells of storage parenchyma, and pits in cell and vessel walls of sugarcane top stalks. When the initial Zn2+ concentration is 5-400 mg/L, the Zn2+ absorption capacity of HAP/C-SC is 19.91, 22.54, and 25.37 mg/g at 25 degrees C, 35 degrees C, and 45 degrees C, respectively, which is equivalent to the Zn2+ absorption capacity of synthetic nano-hydroxyapatite. The HAP/C-SC showed a good removal performance for Zn cations, and the adsorption followed Langmuir model and pseudo-second-order kinetic model well. The Zn-containing hydroxylapatite solid solutions [(ZnxCa1?x)(5)(PO4)(3)(OH)] were considered to be the key mechanism for Zn2+ elimination, which formed through the co-action processes including adsorption, ion exchange (x = 0.06-0.12) and dissolution-precipitation (x = similar to 0.27). The M(2) vacancy sites are believed to be energetically more favorable for the Zn ion occupation. The substitution and occupation of M(2) sites by Zn ions may be affected by spatial constraints.