Adsorption of Cd(II) from Aqueous Solutions by a Novel Layered Double Hydroxide FeMnMg-LDH
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The layered double hydroxides (LDHs) with a hydrotalcite-like structure are believed to possess great potentials as environmental remediation materials including removal of heavy metals from aqueous solutions by adsorption. A new LDH was synthesized with Mg2+ as the structure-stabilizing ion (FeMnMg-LDH) based on a co-precipitation method, which showed promised adsorption capacity for Pb. Its adsorption characteristics for Cd2+, an environmental active element relative to Pb, were examined in this paper. The results showed that adsorption equilibria were well described by Langmuir isotherm and the adsorption kinetics well followed a pseudo-second-order kinetic model. The maximum Cd2+ adsorption capacity of FeMnMg-LDH was about 59.99 mg/g at 25 °C, which is significantly higher than that of other similar kinds of absorbents. The high Cd2+ removal efficiency could maintain at a wide pH range due to its buffering capacity. The coexisting cations competed with Cd2+ adsorption on the FeMnMg-LDH with a sequence of Cu2+ > Pb2+ > Mg2+ > Ca2+ when coexisting ions were added in the adsorption system separately. The positive value of ΔH° (14.016 kJ/mol) suggested that the adsorption process is endothermic while the positive ΔS° value (0.08 kJ/mol K) revealed that the randomness increased at the solid-solution interface during the adsorption process. FeMnMg-LDH removes Cd2+ from aqueous solution mainly by surface adsorption, surface-induced precipitation, and ion exchange. The FeMnMg-LDH has been further proved to be a good absorbent for the removal of heavy metals from aqueous solution.
KeywordsHydrotalcite FeMnMg-LDH Cadmium Adsorption Heavy metals
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