Sorption of arsenate on MgAl and MgFe layered double hydroxides derived from calcined dolomite

Layered double hydroxides (LDHs) have the property to remove different anions due to their high anion exchange ability. Synthesis of LDHs using natural earth resources is highly desired in environmental concern. Single phases of Mg_2.3Al-, Mg_2.9Al- and Mg₄Fe-LDHs were successfully synthesized from calcined dolomite (CaO·MgO) as Mg source as well as precipitant through optimization of microwave-assisted hydrothermal aging. Sorption data of arsenate over these LDHs were well fitted to non-linear Langmuir adsorption isotherm. Among the materials studied Mg_2.3Al-LDH showed not only superior arsenate sorption capacity but also superior coverage of arsenate. This suggests there are some additional factors other than charge density to influence the sorption density. Mechanism of arsenate sorption was probed by different characterization techniques of solid residues after sorption such as X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transformed infrared, extended X-ray absorption fine structure analysis. Sorption of arsenate over MgAl-LDHs mediated through ion-exchange as well as surface adsorption, depending on charge density of metallic layers. However, sorption of arsenate over MgFe-LDH occurred through ion-exchange along with the formation of inner surface complexes, which leads disturbed layer stacking.