Fate of Lu(III) sorbed on 2-line ferrihydrite at pH 5.7 and aged for 12 years at room temperature. I: insights from ICP-OES, XRD, ESEM, AsFlFFF/ICP-MS, and EXAFS spectroscopy

Abstract

Two-line ferrihydrite (2LFh) was aged for 12 years under ambient conditions and sheltered from light in the presence of Lu(III) used as surrogate for trivalent actinides. 2LFh aging produced hematite rhombohedra with overgrown acicular goethite particles. Analysis of the homogeneous suspension by asymmetrical flow field-flow fractionation (AsFlFFF) coupled to ICP-MS indicated that particles have a mean hydrodynamic diameter of about 140 nm and the strong correlation of the Fe and Lu fractograms hinted at a structural association of the lanthanide with the solid phase(s). Unfortunately, recoveries were low and thus results cannot be considered representative of the whole sample. The suspension was centrifuged and X-ray absorption spectroscopy (XAS) at the Lu L -edge on the settled particles indicated that Lu(III) is sixfold coordinated by oxygen atoms, pointing to a retention by structural incorporation within particles. This result is consistent with AsFlFFF results on the same suspension without centrifugation. The detection of next nearest Fe and O atoms were consistent with the structure of goethite, ruling out incorporation within hematite. After centrifugation of the suspension, only nanoparticulate needle-like particles, very likely goethite, could be detected in the supernatant by ESEM. AsFlFFF data of the supernatant were comparable to that obtained for the homogeneous suspension, whereas XAS indicated that Lu(III) is predominantly present as dissolved species in the supernatant. Results from both techniques can be interpreted as a major fraction of Lu present as aqueous ions and a minor fraction as structurally incorporated. Findings from this study are corroborated by STEM-HAADF data and results from DFT calculations in a companion paper.

Ferrihydrite Transformation Impacted by Adsorption and Structural Incorporation of Rare Earth Elements

“…Finck et al 27 reported that Lu, one of HREEs, incorporation in hematite and goethite after Lu coprecipitated with ferrihydrite was aged for 12 years at room temperature. 27 LREEs and HREEs behaved incoherently due to the subtle difference of their effective radii and affinities to iron oxides.…”

“…Finck et al 27 reported that Lu, one of HREEs, incorporation in hematite and goethite after Lu coprecipitated with ferrihydrite was aged for 12 years at room temperature. 27 LREEs and HREEs behaved incoherently due to the subtle difference of their effective radii and affinities to iron oxides. The ionic radii of REE(III) decrease from La 3+ (1.032 Å, 6coordination) to Lu 3+ (0.861 Å, 6-coordination), which are larger than those of Fe 3+ (0.645 Å, 6-coordination).…”

“…Along with phase transformation, various metal ions are often associated with ferrihydrite, leading to the variation of stability and mobility of metals in terrestrial environments. Aging studies of ferrihydrite with metals (e.g., Cd­(II), Mn­(II), Ni­(II), and Pb­(II)) showed that the transformation reactions were influenced by the structural incorporation of metals into recrystallized oxides. , A few studies showed that the phase transformation of ferrihydrite was also affected by adsorbed REEs (e.g., Lu­(III)). , However, the mechanisms responsible for the suppressed ferrihydrite transformation by REEs are not clearly understood.…”

“…25,26 A few studies showed that the phase transformation of ferrihydrite was also affected by adsorbed REEs (e.g., Lu(III)). 27,28 However, the mechanisms responsible for the suppressed ferrihydrite transformation by REEs are not clearly understood.…”

Phase Transformation of Cr(VI)-Adsorbed Ferrihydrite at Different pH in The Presence of Mn(II): Fate of Mn(II) and Cr(VI)

“…Although ferrihydrite exhibits a high adsorption capacity for various metal(loid) ions, it is metastable and gradually transforms into secondary minerals, including hematite and goethite (Finck et al, 2019). Hematite is formed by dehydration and reorganization among ferrihydrite particles (Wang et al, 2015a), whereas goethite is formed by ferrihydrite dissolution and reprecipitation (Shen et al, 2012).…”