Adsorption recovery of phosphate from aqueous solution by CaO-biochar composites prepared from eggshell and rice straw

CaO-biochar composites were prepared by mixed ball milling and pyrolysis of agricultural wastes eggshell and rice straw. The resulting CaO-biochar composites (E-C) showed excellent performance for phosphate adsorption from aqueous solution in a wide range of solution pH (5–11), and a maximum adsorption capacity of 231 mg/g could be obtained by E-C sample that was prepared from the eggshell and rice straw with a mass ratio of 1:1 (E-C 1:1). The adsorption of phosphate onto the E-C samples could be well described by pseudo-second-order (R2 > 0.975) and Langmuir models (R2 > 0.979). Thermodynamic analysis revealed that the adsorption process was spontaneous (ΔG0 < 0) and endothermic (ΔH0 > 0). This work provides a promising method to prepare functionalized biochar adsorbents from agricultural wastes for the recovery of phosphate from aqueous solution, and the phosphate adsorbed CaO-biochar composites can be directly applied as a slow-release fertilizer to farmland soil, which have the functions of improving soil physical structure, increasing soil fertility, and regulating soil pH.