Effect of metal composition in lanthanum-doped ferric-based layered double hydroxides and their calcined products on adsorption of arsenate

Abstract

A series of lanthanum-doped ferric-based layered double hydroxides with the carbonate intercalation (Mg–Fe–La–LDHs) of different M²⁺/M³⁺ molar ratio and their corresponding calcined products were successfully synthesized and characterized. In order to understand the effect of metal compositions in these materials on their adsorption performances for arsenate, various factors such as solution pH, contact time and initial arsenate concentrations were investigated. The results showed that the maximum adsorption capacity for Mg–Fe–La–LDHs decreased with the increment of the M²⁺/M³⁺ molar ratio, but the reversed trend occurred for the calcined products of Mg–Fe–La–CLDHs. This difference is closely related to the different adsorption mechanisms for layered double hydroxides (LDHs) and calcined layered double hydroxide (CLDHs). It was found that the adsorption isotherms can be well described by the Langmuir equation, and the adsorption kinetics followed the pseudo-second-order kinetic model. The results suggested that the obtained Mg–Fe–La–LDHs with lower molar ratio of M²⁺/M³⁺ in the materials were more suitable for removal of arsenate, while their calcined products Mg–Fe–La–CLDHs with higher molar ratio of M²⁺/M³⁺ were efficient adsorbents for arsenate. When the M²⁺/M³⁺ molar ratio in the material was 4.38, the maximum adsorption capacity of CLDH-3 was as high as 47.4 mg g⁻¹.