Distribution of OH bond to metal-oxide in Mg3−xCaxFe-layered double hydroxide (x = 0–1.5): Its role in adsorption of selenate and chromate

Abstract

This work investigated the distribution of OH bond to various metal oxides in MgCaFe layered double hydroxide (LDH), which led to different removal processes of selenate and chromate on LDH. The removal of selenite and chromate on Mg3−xCaxFe-LDH (LDHx, x=0–1.5) showed that adsorption capacities of the MgCaFe LDH were lower than those of MgFe-LDH, as the ternary LDH collapsed after Ca dissolution. Moreover, LDH1.5 removed similar amount of anions to LDHx (x<1.0), independent of the different Ca leaching amounts. Analysis of thermal released H2O (m/z=18) before and after anion removal exhibited the thermal stability of hydroxyl as follows: OH-(MgCaFe)(∼360°C)>OH-(Mg2Fe)(∼300–340°C)>OH-(Mg3)(∼270–290°C)≈OH-(Ca2Fe)(∼260°C). Thus, the bond strength of OH to metal oxide increased with increasing x in Mg3−xCaxFe-LDH. Since anion is adsorbed via the exchange to OH bond in LDH, the weak affinity of OH bond to MgCaFe after Ca dissolution at x>1 is responsible for the bigger adsorption capacities on MgCaFe-O structure, compared to the case at x<1. Therefore, our result provides mechanism of anion removal by Ca contained ternary LDH.