Synthesis, characterization, and evaluation of thiocarbazide-functionalized maleic-based polymer for thorium (IV) removal from aqueous solutions

The copolymerization of maleic anhydride with N,N'-methylenebisacrylamide allowed synthesizing a sorbent (MaMb) that can be functionalized by amidoacidification (for grafting thiocarbazide) (synthesis of TcMaMb). The two sorbents are tested in batch systems for the removal of thorium from aqueous solutions. The characterization of the materials shows similar textural properties and the involvement of amine, carboxylic groups (for MaMb), completed with thiocarbonyl groups (for TcMaMb), in the binding of Th(IV). The maximum sorption occurs at pH close to 4 and the functionalization doubles the maximum sorption capacity (up to 1.36 mmol Th g-1). The sorption isotherm for TcMaMb is fitted by the Langmuir equation (in the case of pristine sorbent, the saturation of the sorbent is not achieved in the tested concentration range making the Freundlich equation more appropriate). The functionalization improves the uptake kinetics (equilibrium time decreased from 120-180 min to 30 min), and kinetic profiles are fitted by the pseudo-first order rate equation. The selectivity of the sorbent for Th (IV) (and to a certain extent U(VI)) is improved by thiocarbazide grafting, especially at pH above 4 (in the case of equimolar multi-component solutions). The functionalization of MaMb also enhances the selectivity of TcMaMB for thorium when applied to metal removal from pre-treated mining leachate (consistently with synthetic solutions). Bound metal can be readily desorbed (complete desorption achieved in 30 min) using 0.3 M HCl solutions for both MaMb and TcMaMb sorbents. Another advantage brought by the functionalization concerns the stability of the sorbent at recycling: after five cycles of re-use the sorption efficiency decreases by less than 2 % (meaning less than for MaMb, at about 8 %).