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 MgCO 3 /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 MgCO 3 /Mg(OH) 2 have a smaller size and a negatively charged surface, which significantly enhances their migration ability in aquifers. The combination of MgCO 3 and Mg(OH) 2 provides a complementary effect, where MgCO 3 effectively precipitates Cd in the aquifer while Mg(OH) 2 maintains the required pH level for stabilization. The optimal compounding ratio of MgCO 3 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 MgCO 3 /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 MgCO 3 /Mg(OH) 2 colloid shows great potential as a reactive medium for remediating Cd-contaminated groundwater. [Display omitted] • MgCO 3 /Mg(OH) 2 colloid was proposed as a novel reagent for Cd-contaminated groundwater remediation. • Ultrasound can transform the flocculation effect of PAAS on MgCO 3 /Mg(OH) 2 particles into dispersion. • MgCO 3 effectively precipitates Cd and Mg(OH) 2 maintains the necessary pH level for stabilization. • A remarkable decrease in the exchangeable fraction of Cd in aquifer media after the injection of the MgCO 3 /Mg(OH) 2 colloid. [ABSTRACT FROM AUTHOR]