Synthesis of polyamide 6/nano-hydroxyapatite hybrid (PA6/n-HAp) for the sorption of rare earth elements and uranium

Abstract

Polyamide (PA6) and hydroxyapatite (n-HAp) have intrinsic sorption properties for metal ions. Their association by melt compounding allows manufacturing a composite material (PA6/n-HAp) efficient for the binding of uranyl and rare earth metal ions (REEs) in acidic solution (in the pH range 2–2.5). The composite shows enhanced sorption capacities (5–7 times) compared with single PA6 material. The structuration of the material slightly improves textural properties but contributes to improve the accessibility and availability of reactive groups (including by size distribution). Metal sorption proceeds mainly through complexation of amide reactive groups as shown by FTIR characterization (modification of the environment of CO and NH groups) rather than ion-exchange/electrostatic attraction. Maximum sorption capacities approach 0.34 mmol U g−1, 0.49 mmol Er g−1 and 0.70 mmol Nd g−1: the preference may be correlated to the covalent rather than ionic character of these metal ions. Uptake kinetics are relatively slow (requiring up to 6−8 h, under selected experimental conditions): the textural properties of the composite (pore size: 2.6 nm) limit the mass transfer properties (though slightly enhanced compared with PA6 precursor). The resistance to intraparticle diffusion constitutes the major controlling step for uptake kinetics. Nitric acid is the most efficient eluent for the desorption of loaded metals (efficiency exceeds 94 %); noticeably U(VI) elution is optimal at 1 M HNO3 concentration, contrary to REEs that require lower concentration (i.e., 0.1 M). Preliminary tests on sulfuric acid leachates of Egyptian ores demonstrate that the sorbent maintains good sorption properties for REEs and U despite the complexity of the solution. The sorbent has a marked preference for REEs, U and Th against base and alkali metals.