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.