A new method of alkalinity remediation for Cd-contaminated groundwater by PAAS-modified MgCO₃/Mg(OH)₂ colloid

Mg(OH)₂ dissolves slowly and can provide a long-term source of alkalinity, thus a promising alternative reagent for the in situ remediation of heavy metal polluted groundwater. Unfortunately, it exhibits a relatively poor stabilization effect on heavy metal Cd due to the higher solubility of the resulting stabilized product, Cd(OH)₂. To overcome this limitation, we investigated the use of MgCO₃/Mg(OH)₂ colloid modified by sodium polyacrylate (PAAS) to remove Cd from groundwater. Through ultrasonic dispersion, the molecular chains of PAAS are broken, causing a transformation from flocculation to surface modification, resulting in the production of a stable colloid. The colloidal particles of MgCO₃/Mg(OH)₂ have a smaller size and a negatively charged surface, which significantly enhances their migration ability in aquifers. The combination of MgCO₃ and Mg(OH)₂ provides a complementary effect, where MgCO₃ effectively precipitates Cd in the aquifer while Mg(OH)₂ maintains the required pH level for stabilization. The optimal compounding ratio of MgCO₃ to Mg(OH)₂ for achieving the best stabilization effect on Cd is found to be 1:1. Column experiments demonstrate that the injection of MgCO₃/Mg(OH)₂ colloid substantially enhances Cd stability, reducing the exchangeable fraction of Cd in aquifer media from 88.61% to a range of 22.50-34.38%. Based on these results, the MgCO₃/Mg(OH)₂ colloid shows great potential as a reactive medium for remediating Cd-contaminated groundwater.