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Removal of Copper (II) from Aqueous Solution by a Hierarchical Porous Hydroxylapatite-Biochar Composite Prepared with Sugarcane Top Internode Biotemplate

Porous hydroxyapatite-biochar composites with layered microstructures (SC–HA/C) were prepared by carbonizing sugarcane stem nodes and then soaking them in lime water and (NH4)2HPO4 solutions in rotation. The surface area of SC–HA/C ranges from 8.52 to 28.44 m2/g, and its microstructure inherits various macro-, meso-, and micro-pores in the cell walls of sugarcane and in the pits of the vessel walls. The maximum removal capacities were 11.50, 14.65, and 19.81 mg/g for the Cu (II) immobilization at 25 °C, 35 °C, and 45 °C with the solution Cu (II) concentration of 10~320 mg/L, respectively, which were in accordance with the copper sorption capacities of synthesized nano-hydroxylapatites. The Cu (II)-removal kinetics and isotherm followed the pseudo-second-order equation and the Langmuir equation very well. The formation of the Cu-containing hydroxylapatite solid solutions ((CuxCa1−x)5(PO4)3(OH)) through adsorption, ion exchange (x = 0.01~0.04), and dissolution-coprecipitation (x = 0.13~0.35) was the dominant process for the Cu (II) removal by the SC–HA/C composite.