Mechanochemical synthesis of carbon-loading layered double hydroxides by waste magnesia-carbon bricks for removal heavy metals

Waste magnesia-carbon bricks (WMCBs) with rich magnesium oxide and graphite resources are buried directly resulting in wasting Mg/C resources and its ultimate disposal urgently needs a reasonable solution. This study recycled WMCBs as the main raw materials to prepare carbon-loading layered double hydroxides (WM-LDHs) by the mechanochemical method, which was used as a low-cost adsorbent for removing heavy metals form soil. The results indicated that LDHs was completely synthesized under the condition of dry milling for 3 h and wet milling for 2 h, and meanwhile ball-milling could lead to the oxidation of functional groups on the surface of graphite. The data of kinetic experiment was well fitted with the pseudo-second-order model, and the results of isotherm experiment was well followed by the Generalized-Langmuir model. In addition, the results of site energy distribution theory and molecular stimulation were applied to further explore the removal of Cr(VI)/As(V)/Pb(II)/ Cu(II) which suggested heavy metals were preferentially adsorbed onto LDHs (Delta Eads= -6.38 to -1.65 eV) and then fixed onto graphite (Delta Eads= -0.95 to -0.11 eV) after high-energy sites on LDHs were occupied. The removal mechanism for LDHs was ion exchange, chemical precipitation and surface complexation, redox, electrostatic interaction for graphite. Therefore, it could be concluded that the preparation of WM-DLHs simultaneously reached the recycling of WMCBs and the treatment of heavy metal-contaminated soil.