Immobilization of Cd in contaminated soil by mechanically activated calcite: Sustained release activity-depended performance and mechanisms

Large scale of Cd-contaminated soil has become a serious threat to environmental safety and human health, whereas field applicability of existing remediation technologies has encountered numerous obstacles, such as poor stability, secondary pollution, long operation time, and high chemical cost. Herein, a novel activated calcite with significantly enhanced immobilization stability of Cd in soil has been synthesized based on mechanochemical technology. Our results demonstrate that the passivation rate of bioavailable Cd in Cd-contaminated soil (initial concentration 3 mg/kg) could be maintained at 65 % after 50 days of passivation cultured with activated calcite. Mechanistic studies have revealed that the outstanding performance is attributed to the enhancement of the CO32- and Ca2+ release properties of calcite through mechanical activation, leading to a 550 % increase in Ca2+ release. The CO32- fraction binds in situ to the free Cd in the soil, forming a stable carbonatebound state. Simultaneously, the OH- released from the continuous hydrolysis of CO32- effectively regulates and stabilizes the soil pH within the neutral range. Further, SEM-EDS and sequential extraction procedures (SEP) analyses implied that the specific adsorption of Cd by in-situ formed iron oxides in soil was visibly related to the release of OH- from activated calcite and immobilized the free state Cd in the more stable iron oxide-bound state. The current work shows the high sustainability and stability of Cd-contaminated soil immobilization by activated calcite materials.