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

Abstract

Wastewater from the uranium mining industry contains toxic arsenate (AsO₄^3–), selenate (SeO₄^2–)_, and molybdate (MoO₄^2–) 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 AsO₄^3–, 856 cm⁻¹ for SeO₄^2–_, and 852 cm⁻¹ for MoO₄^2–. 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.