Green synthesis of layered double hydroxides (LDH) for the remediation of As and Cd in water and soil

Layered double hydroxides (LDH) show great promise for the remediation of heavy metals and metalloids in aqueous media and soils. In this study, we propose a green synthesis approach for LDH utilizing industrial waste granulated ground blast furnace slag (GGBS) and readily available magnesium oxide (MgO), and applied these green synthesized LDH for As and Cd remediation in both water and soil. The synthesized LDH were characterized via multiple approaches including SEM, HRTEM-EDS, TGA, XRD, and FTIR, demonstrating that the addition of 20% MgO in the raw materials (LDHb) substantially improved the stability of LDH compared to LDH synthesized solely from pure GGBS (LDHa). Batch adsorption experiments were conducted to evaluate the adsorption efficiency of LDHa and LDHb for Cd(II) and As(V) under varying initial solution pH conditions. Results showed that as the pH increased, calcium leaching remained high while aluminum leaching gradually increased. The adsorption isotherms and kinetic studies further revealed that LDHb displayed better adsorption performance for As(V) across different pH conditions, while both LDH showed a 100% removal rate of Cd(II) at pH ranging from 6 to 9. Both LDHa and LDHb exhibited promising immobilization effects on As and Cd in contaminated soils, with LDHa showing superior stabilization performance toward As, immobilizing 83.8% of bioavailable As in the farmland soil and 78.7% in the industrial site soil, compared to LDHb's 54.8% and 50.6%, respectively. The findings of this study hold promise for the practical application of LDH as an efficient and environmentally friendly remediation material for contaminated water and soils containing toxic metal(loid)s.