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

Wastewater from the uranium mining industry contains toxic arsenate (AsO 4 3– ), selenate (SeO 4 2– ) , and molybdate (MoO 4 2– ) that can be removed by complexation with MgAlFe(III)CO 3 SO 4 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 υ 3 band with no splitting at 821 cm −1 for AsO 4 3– , 856 cm −1 for SeO 4 2– , and 852 cm −1 for MoO 4 2– . In all cases, basal ( d 003 ) and interlayer ( d 006 ) 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.