Efficient removal and stepwise recovery of various heavy metals from water by using calcium carbonate with different activity

Heavy metals in wastewater have caused serious environmental contamination in the world. However, a key challenge with the most effective solution through chemical precipitation is the high pH of in the effluent by the use of lime. Herein, we propose a strategy using highly active calcium carbonate (HACC) for enhancing its slow-release activity, which efficiently removes heavy metal contamination while avoiding high pH of the effluent. The HACC demonstrate a removal of more than 99 % for Cd(II), Cu(II), Zn(II) and Pb(II) from the composite contaminated water within 50 min, while maintaining the pH of the effluent close to neutral. In addition, mechanistic analyses unveil that the free heavy metals in different solutions combine with OH^– and CO₃^2– released from HACC in different precipitation behaviors to form their respective precipitates, thereby discovering the mechanism of regulating the precipitation rates of different heavy metals by controlling the activity of calcium carbonate. Based on it, the stepwise recovery of heavy metals was realized by adjusting the calcium carbonates with different activity. The present work provides guidance for gentle and efficient removal of heavy metal pollution in water bodies and inspires the future development of recovery of heavy metals.