Heavy metals immobilization of LDH@biochar-containing cementitious materials: Effectiveness and mechanisms

The utilization of tailings for producing solid waste-based concrete can effectively mitigate the environmental consequences stemming from tailings accumulation. However, the presence of high concentrations of heavy metal ions in certain tailings can deleteriously affect the microstructure of the cement matrix. This study investigated the in-situ immobilization performance of Pb2+ and Mn2+ by calcium-aluminum layered double hydroxides (CaAl-LDH) and LDH@biochar blended in cement-based materials. The equilibrium adsorption capacity of CaAl-LDH/LDH@biochar for heavy metals in aqueous solution was assessed by adsorption kinetics, while the effect of pH value on the removal rate of heavy metals was also studied. The workability and mechanical properties of cement mortar are significantly improved with the addition of 4 % LDH@biochar. LDHs@biochar can effectively reduce the deteriorating effects of heavy metal ions on the fundamental properties of cement materials. Compared with CaAl-LDH, the leaching concentrations for Pb2+ and Mn2+ in LDH@biochar at 28 d is decreased by 59 % and 39 %, respectively. The stable adsorption of Pb2+ and Mn2+ by LDH@biochar is attributed to multiple mechanisms, including interlayer anion exchange effect, chemical adsorption of surface functional groups and electrostatic attraction. This study provide valuable insights for enhancing the efficiency of heavy metal immobilization in cement-based materials.