Aminophosphonate CuO nanocomposites for uranium(VI) removal: Sorption performance and mechanistic study

Abstract

Two aminophosphonate matrices (mono-aminophopshonate, MAP, and bi-aminophosphonate, BAP) are synthesized, varying the molar proportions of three precursors (thiocarbazide, p-phthalaldehyde, and triphenylphosphite). Nano-composites (CuO-MAP and CuO-BAP) are successfully elaborated by impregnation of the matrices with copper sulfate solution before processing to in situ precipitation of copper in alkaline solution (simultaneous partial Cu reduction). The sorbents are characterized using: TEM, XRD, elemental analysis, SEM-EDX, BET, titration (pHPZC), XPS and FTIR analyses. Uranium sorption properties are compared for the different sorbents through the study of pH effect (optimum close to 4.5), the uptake kinetics (equilibrium reached in 60 min) and sorption isotherms. Uptake kinetics are finely fitted by the pseudo-second order rate equation (and the resistance to intraparticle diffusion). The sorption isotherms are modeled by the Langmuir equation with maximum sorption capacity (mmol U g−1) decreasing according to CuO-BAP(1.15) ≫ CuO-MAP(0.80) > BAP(0.74) ≫ MAP(0.50). The enhanced textural properties and the increase in the density of sorption sites (probably completed by a better steric arrangement for improved reactivity) may explain this ranking. The sorption is endothermic: sorption capacity and affinity coefficient increase with temperature. Uranium is readily desorbed from loaded-sorbents (efficiency > 98 %) using either HCl (0.2 M) solution or acidified urea (0.25 M) solution (pH 2.5) for aminophosphonate matrices and nano-composites, respectively. The type of sorbent does not affect the stability at recycling: at the sixth cycle, the loss in sorption and desorption performances varies by 6.2–7 % and 4.2–4.9 %, respectively. CuO-BAP sorbent shows high potential for uranium recovery from complex acid ore leachate. The broad spectrum of bound metals does not allow producing a pure yellow cake after ammonia precipitation of eluate (containing up to 15–22 % impurities).