Enhanced simultaneous CO2 mineralization and cadmium immobilization in a wide pH range by using ball-milled serpentine

Utilization of widely-available serpentine for direct capture of carbon dioxide from air is desirable due to unlimited application sites, but possesses quite limited efficiency especially under acidic conditions. Cadmium, as one of the most toxic heavy metals demanding for removal from contaminated soil or water, can be precipitated as carbonate for stable fixation. In this study, the feasibility of coupling CO2 sequestration with cadmium immobilization under ambient conditions in a wide pH range by using ball-milled serpentine was investigated. It was found that, slight direct CO2 capture by serpentine during pretreatment provided CO32- active sites on serpentine surface for the heterogeneous nucleation of initial CdCO3 seed crystals, which promoted the precipitation of rhombohedral otavite initially. More importantly, co-precipitation of Cd and Mg to form Mg-CdCO3 became possible under increased Mg/Cd ratio in the later stage, therefore to improve the performance of Mg for CO2 sequestration, finally ensuring simultaneous enhanced sequestration of Cd and CO2 in a wide initial pH range. A cadmium uptake capacity of 161.6 mg/g along with CO2 mineralization >63 mg/g (calculated from CdCO3 precipitate) within 24 h was achieved in this study. These observed synergistic interactions on the ballmilled serpentine may offer an effective approach to simultaneous sequestration of atmospheric CO2 and hazardous metals, potentially applicable to comprehensive treatment/remediation of heavy metal-polluted wastewater and soils.