Multi-Metal Modified Layered Double Hydroxides for Cr(VI) and Fluoride Ion Adsorption in Single and Competitive Systems: Experimental and Mechanism Studies

Different metal cation (Co2+, Fe2+, La3+, and Zn2+)-doped layered double hydroxides (LDHs), yielded CoMgAl-LDH, FeMgAl-LDH, MgAlLa-LDH, and ZnMgAl-LDH, respectively. The four LDHs synthesized from same molar ratio of M-II:M(III)had different adsorption capacities toward Cr(VI) and fluoride (F-). Single and competitive adsorption kinetic and isotherms of Cr (VI) and F(-)were studied. For each LDH, the adsorption capacity of Cr(VI) was higher compared with F(-)in single adsorption, which confirmed that the LDHs were more selective for Cr(VI) adsorption. In single adsorption, CoMgAl-LDH achieved the best sorption capacity for F(-)and Cr(VI) removal, reaching 38.01 and 59.27 mg/g, respectively. The maximum adsorption capacity of CoMgAl-LDH to Cr(VI) and F(-)was 39.33 and 13.46 mg/g in competitive adsorption. Various experimental parameters, such as pH, contact time, concentration of coexisting anions, and anion (F(-)and Cr(VI)) addition order were investigated in competitive adsorption system. Cr(VI) and F(-)competed for active sites during adsorption and the affinity of Cr(VI) to LDHs was stronger compared with F-. Co-doped method is a valid method to improve adsorption performance of the LDHs.