Mechanism of complexation of toxic arsenate, selenate, and molybdate with hydrotalcites

Wastewater from the uranium mining industry contains toxic arsenate (AsO43–), selenate (SeO42–), and molybdate (MoO42–) that can be removed by complexation with MgAlFe(III)CO3SO4 hydrotalcites, yet underlying mechanisms are poorly known. Here, we studied complexation by K–edge Extended X-ray Absorption Fine Structure Spectroscopy, Density Functional Theory, Powder X-ray Diffraction, and Infrared and Raman Spectroscopy. We found that single shell paths with a coordination number of 4 and bond lengths of As–O = 1.69 Å, Se–O = 1.65 Å and Mo–O = 1.76 Å were best fitted for As–MgAlFe, Se–MgAlFe, and Mo–MgAlFe hydrotalcites. We observed a single antisymmetric υ3 band with no splitting at 821 cm−1 for AsO43–, 856 cm−1 for SeO42–, and 852 cm−1 for MoO42–. In all cases, basal (d003) and interlayer (d006) spacings before and after anion adsorption varied by less than 0.1 Å. Therefore, our findings demonstrate that arsenate, selenate, molybdate form outer-sphere-like complexes with MgAlFe hydrotalcite, and that these complexes are surface-bound and not intercalated.