Evidence of the Impact of Substitutions in Goethite on Rare Earth Element Adsorption
Résumé
Since the 1980's the uses, and thus mining, of Rare EarthElements (REE) have grown exponentially. This has led toquestions about the impact of REE on the environment, and REEbeing designated emerging pollutants. Currently, knowledgeabout the behaviour of REE in the environment is stillincomplete. For example, there are few studies on REEadsorption on iron (Fe) (oxy)hydroxides, which are consideredkey carriers of metal pollution, and on the other factorscontrolling this adsorption [1-4]. Knowing that in natural(oxy)hydroxides, Fe is frequently substituted by other elementsin the environment in which they form, we were interested in theeffect of these substitutions on the adsorption capacity of(oxy)hydroxides towards REE.This study focusses on aluminium (Al, common in naturalenvironments and easily substituted into Fe-oxyhydroxides) andgallium (Ga, chemical analogue to Al) substitutions in goethite(Goe), representative of commonly found Fe-(oxy)hydroxideswhose well-defined structure allows for modelling of sorptionand surface interactions. For this purpose, experiments of REEwere conducted on pure and substituted goethites (Goe) withvarying Al and Ga content.A characterisation of these goethites, shows that thesubstitution does not affect the shape, size, and specific surfacearea significantly. Nevertheless, the REE adsorption patterns areaffected by substitutions: 5% substitution decreases the sorptioncapacity of the goethite, and at 10% substitution the preferredadsorption of the mid and heavy REE diminishes, resulting in a'flatter' pattern. Meanwhile, the Neodymium (Nd) L3-edgeEXAFS experiments show no modification of its binding modeto the Goe surfaces, forming corner sharing bidentate-binuclearcomplexes regardless of the substitution rate. However, TEMobservations and Cerium (Ce) L 3-edge XANES experimentsdemonstrate that the Ga substitution favours a higher Ce(IV)O2adsorption, while the Al substitution favours Ce(III) adsorptioncompared to the pure Goe.These results reveal the large impact of substitutions on thereactivity of goethite towards REE.