Efficient recovery of phosphate from aqueous solution using biochar derived from co-pyrolysis of sewage sludge with eggshell

Abstract A novel Ca-modified biochar were prepared via co-pyrolysis of eggshell and sewage sludge (mass ratio 2:1) for recovering phosphorus from wastewater. The maximum adsorption capacity reached 106.99 mg P/g, and it exhibited good ability for phosphate adsorption from solution in a wide range of pH (2–11) with removal efficiency more than 96% for 50 mg P/L with adsorbent dosage 2 g/L. The co-existing NO3-, Cl- and SO42- did not significantly influence adsorption performance because of the reduction of adsorption capacity less than 5%, except for HCO3-. Adsorption thermodynamic parameters indicated that adsorption process was spontaneous and endothermic, suggesting that increasing temperature is favorable for the adsorption. Based on the comparison analysis of physico-chemical properties (FTIR, XPS, XRD and SEM-EDS) for biochars before and after phosphate adsorption, electrostatic attraction and precipitate with hydroxyapatite (Ca5(PO4)3(OH)) were main removal mechanisms. The P-laden biochar exhibited excellent ability for phosphorus recovery and application: (1) the P-laden biochar can be used as adsorbent for heavy metal removal from solution. The removal efficiency of Pb2+, Cu2+, Cd2+ and Zn2+ (5–10 mg/L) reached more than 99% with the absorbent dosage of 2 g/L; (2) the P-laden biochar can be used as slow-release P fertilizer for soil. Pot experiment results showed that it significantly promoted Brassica chinensis germination (increasing by 18.89%) and growth (increasing by 50.62%) compared with control treatment. The utilization of biochar increased soil pH and salt content, so it is essential to determine rational usage amount and long-term monitoring for soil health.