Extraction of polyoxotantalate by Mg-Fe layered double hydroxides: elucidation of sorption mechanisms

The extraction of Ta( v ) as polyoxometallate species (H x Ta 6 O 19 (8− x )− ) using Mg-Fe based Layered Double Hydroxide (LDH) was evaluated using pristine material or after different pre-treatments. Thus, the uptake increased from 100 ± 5 mg g −1 to 604 ± 30 mg g −1 , for respectively the carbonated LDH and after calcination at 400 °C. The uptake with calcined solid after its reconstruction with Cl − or NO 3 − anions has also been studied. However, the expected exchange mechanism was not found by X-ray Diffraction analysis. On the contrary, an adsorption mechanism of Ta( v ) on LDH was consistent with measurements of zeta potential, characterized by very negative values for a wide pH range. Moreover, another mechanism was identified as the main contributor to the uptake by calcinated LDH, even after its reconstruction with Cl − or NO 3 − : the precipitation of Ta( v ) with magnesium cations released from MgO formed by calcination of the LDH. This latter reaction has been confirmed by the comparison of the uptake of Ta( v ) in dedicated experiments with solids characterized by a higher magnesium solubility (MgO and MgCl 2 ). The obtained precipitate has been analyzed by X-ray diffraction (XRD) and would correspond to a magnesium (polyoxo)tantalate phase not yet referenced in the powder diffraction databases. Reaction of polyoxotantalate ions and MgFe Layered Double Hydroxide leads to magnesium polyoxotantalate precipitate.