Effect of mono- vs. bi-functionality of aminophosphonate derivatives on the enhancement of U(VI) sorption: physicochemical properties and sorption performance

Abstract

Aminophosphonates are excellent complexing agents for numerous metal ions (including uranyl). Based on this property, two sorbents have been synthesized using simple one-pot reaction: thiocarbazide (amine source), p-phthaldehyde (di-aldehyde), and trimethylphosphite (phosphorous precursor) are mixed with different molar ratios to produce bis- (2:1:2) and mono (1:1:1) α-aminophosphonate-based derivatives (B-AmPh and M-AmPh, respectively). Materials are characterized using XRD, BET, SEM, titration, elemental, FTIR, and XPS analyses to evaluate the effect of substitution rate on the physicochemical properties. FTIR and XPS are also used for elucidating binding mechanisms. Sorption properties of the materials have been tested for U(VI): the pH effect is completed by investigating uptake kinetics and sorption isotherms. At pH 4.5 (and T: 55 °C), sorption capacity increases from 0.89 to 1.22 mmol/g with the increase in the substitution degree (M-AmPh < B-AmPh). This increase is not correlated to the fraction of grafted reactive groups, meaning that steric hindrance probably contributes to limiting the accessibility and availability of aminophosphonate moieties. Pseudo-second order and Crank equation preferentially fit kinetic profiles. Langmuir equation successfully describes sorption isotherms. Sorption process is systematically endothermic and spontaneous: enthalpy and entropy changes decrease with the substitution rate. Uranium successfully desorbed using HCl (0.2 M); allows six cycles of re-use with limited loss in sorption/desorption efficiencies. The sorbents were used for U(VI) recovery from acidic leachate of uranium ore. This test shows remarkable affinity and selectivity of B-AmPh for uranium; however, after elution and precipitation the U concentrate contains about 10 % of impurities.