Removal of Mn and Cd contained in mine wastewater by Mg–Al-layered double hydroxides

Abstract

The fate of heavy metals in the environment is significantly influenced by their adsorption onto clay minerals. Herein, we investigate the mechanism of MnO4− and Cd2+ removal by nitrate-intercalated Mg–Al-layered double hydroxides (LDHs) in aqueous solution and optimize the corresponding conditions, showing that these adsorbents can effectively remove Zn, Mn, and Cd from mine wastewater and revealing that adsorbent structure is retained after wastewater treatment. The removal of MnO4− ions from aqueous solution is demonstrated to proceed via their exchange for NO3− ions intercalated within LDHs and is well described by the Freundlich adsorption model, following pseudo-second-order kinetics and featuring chemisorption as the rate-determining step. In contrast, Cd2+ is shown to be removed via the formation of Cd5(OH)8(NO3)2·2H2O (at LDH2) and Cd(NO3)2·Cd(OH)2 (at LDH1).