Enhanced recovery of phosphate and ammonium from aqueous solution by wheat straw biochar modified by oyster shell

In this study, the effect of oyster shell powder as a substitute for calcium chemical reagents to modify straw biochar for enhancing removal of phosphate and ammonium from aqueous solution was investigated. The results showed that the biochar composite (OSB3:1) with the maximum capacity (151.37 mg center dot P/g and 0.8 mg center dot N/g) was synthesized at optimum mass ratio of oyster shell-to-straw (3:1) and pyrolysis temperature (800 degrees C). The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic models, and it achieved excellent performance at pH 2.0-11.0. According to the thermodynamic analysis, the adsorption reaction was an endothermic and spontaneous process, and increasing the temperature increased the adsorption capacity. The removal mechanisms for phosphate included the formation of hydroxyapatite precipitation, ligand exchange, surface complexation and electrostatic attraction, while for ammonium only electrostatic attrac-tion. The modified biochar can effectively remove phosphate (over 85%) from real domestic sew-age within 3 h. Reuse of P-laden biochar as heavy metal adsorbent was proposed, as it exhibited a good removal efficiency for Cd, Zn and Cu, especially application for acidic wastewater.