Calcium carbonate-enriched rice straw biochar can reclaim phosphate along with glyphosate, arsenic, cadmium, and lead contamination from wastewater

This research presents the synthesis and characterization of a novel calcium carbonate-enriched rice straw biochar (CRSB-3), which was utilized to remove phosphate from contaminated wastewater. The optimized CRSB dose for phosphate removal was further used for the removal of glyphosate and heavy metals, namely lead, arsenic, and cadmium from wastewater. The CRSB-3 was prepared using 15 g of rice straw and 3 g of CaCO₃ at 700 °C for 2 h, and had maximum phosphate adsorption capacity of 70.79 mg g⁻¹. Rice straw biochar and CRSB-3 characteristics were ascertained and compared using FTIR, BET, and FESEM-EDAX. Adsorption of phosphate on CRSB-3 followed non-linear pseudo-second-order model and non-linear Freundlich model, with primary mechanisms of adsorption being chemical sorption and precipitation. The key sorption variables, namely, initial phosphate concentration of 71.44 mg L⁻¹, 24.07 mg of CRSB-3, contact time of 2.11 h, and 8.28 pH, were optimized using response surface methodology’s central composite design leading to a phosphate removal of 73.71% from contaminated water. Additionally, CRSB-3 could adsorb 47.19, 63.51, 100, and 84% of applied Pb, As, Cd, and glyphosate, respectively. The CRSB-3 could also be effectively recycled for at least three times with less than 10% reduction in sorption capacity for phosphate after the third cycle. CSRB-3 combines the high surface area and porous structure of rice straw biochar with the reactive properties of calcium carbonate to form an efficient adsorbent capable of removing multiple contaminants, ensuring a cost-effective and sustainable solution for wastewater treatment.