Scandium immobilization by goethite: Surface adsorption versus structural incorporation

Abstract

Several recent studies have reported a strong association between Sc and goethite (α-FeOOH) in synthetic analogs and natural samples. However, the mechanism of Sc immobilization by goethite and controlling factors remain unclear. This study investigated the adsorption behavior and molecular-scale immobilization mechanisms of Sc at water/goethite interfaces through a combination of batch adsorption and desorption experiments, X-ray absorption fine structure (XAFS) analyses, and density functional theory (DFT) calculations. Results indicate that Sc is preferentially adsorbed on goethite with the formation of bidentate-binuclear inner-sphere complexes at the corner-sharing sites. Bulk Sc K-edge XAFS analyses suggest that Sc is incorporated into the goethite structure by substituting for Fe(III) within the crystal in synthetic Sc-substituted goethite, which is further confirmed in natural goethite particles in the laterite by using micro-focused XAFS (μ-XAFS). Furthermore, we demonstrate that the adsorbed Sc on the goethite surface can be structurally incorporated into the goethite lattice in the presence of aqueous Fe(II) possibly through goethite recrystallization induced by aqueous Fe(II). This process may affect the (re)partitioning of Sc between the goethite surface and the mineral bulk, which could be used to rationally explain disparate Sc speciation in laterites from different regions. Our study elucidates the molecular-scale mechanisms underlying Sc adsorption on and structural incorporation into goethite, providing critical insights into the understanding of geochemical behavior and environmental fates of Sc.

Introduction

Scandium (Sc) is a critical metal with wide industrial applications, such as in Al-Sc alloys for the aerospace and automotive industries, in solid oxide fuel cells (SOFC), and in the electronic industry (US Geological Survey, 2020). The growing demands have recently attracted an increasing interest with respect to Sc resources. In particular, the laterite shows a great potential as a new (by-product) resource of Sc (e.g., Wang et al., 2011, Chassé et al., 2017, Williams-Jones and Vasyukova, 2018, Orberger and van der Ent, 2019, Ulrich et al., 2019). Nonetheless, little is known about the geochemical behavior and environmental fates of Sc because of its scarce accumulation in nature. The identification of Sc speciation in complex natural systems remains challenging due to the scarcity of direct Sc structural data and the poor understanding of Sc K-edge X-ray absorption near-edge structure (XANES) reference spectra (Chassé et al., 2018).

Iron (oxyhydr)oxides, such as goethite (α-FeOOH), hematite, and ferrihydrite, are ubiquitous in soils, ore deposits (e.g., laterites), and continental and marine sediments (e.g., ferromanganese nodules). The uptake of trace elements (e.g., Ni, Cd, Se, Te, As, and Sb) by these Fe(III) (oxyhydr)oxide minerals plays an important role in the control of geochemical behavior and environmental fates of the metal(loid)s (Takahashi et al., 2007, Takahashi et al., 2015, Harada and Takahashi, 2008, Brown and Calas, 2013, Qin et al., 2017a, Qin et al., 2019). In the case of Sc, a strong association with goethite has been recently demonstrated in synthetic analogs and natural samples (e.g., Chassé et al., 2017, Chassé et al., 2019, Vind et al., 2018, Ulrich et al., 2019). Nevertheless, the interaction between Sc and goethite is still poorly understood and remains debated. By using XANES analysis, the pioneering study by Chassé et al. (2017) showed that Sc speciation in the lateritic duricrusts in eastern Australia is dominated by species adsorbed on goethite along with a small amount of substituted species in the hematite structure. By contrast, several studies have demonstrated that the structural incorporation of Sc into goethite could be the critical Sc species in laterites (Muñoz et al., 2017, Ulrich et al., 2019, Qin et al., 2020). Indeed, a continuous Fe_xSc_(1-x)OOH solid solution has been synthesized in the laboratory (Levard et al., 2018). Thus, the adsorption and incorporation processes of Sc by goethite could play critical roles in the control of speciation and distribution of Sc in the environment. However, the mechanisms of Sc immobilization by goethite and controlling factors remain unclear.

Goethite recrystallization has been proposed as a fundamental geochemical process affecting the cycling of various trace elements (e.g., Catalano et al., 2011, Frierdich et al., 2011, Frierdich et al., 2019a, Hinkle and Catalano, 2015, Burton et al., 2020), although this process occurs slowly under oxidizing conditions where the solubility of goethite is low. Nonetheless, aqueous Fe(II) can catalyze goethite recrystallization rapidly via Fe atom exchange under reducing conditions, as clearly demonstrated by an Fe isotope tracer and 3D atom probe tomography (APT) approach (Handler et al., 2009, Handler et al., 2014, Frierdich et al., 2014, Frierdich et al., 2019b, Taylor et al., 2019). During aqueous Fe(II)-activated recrystallization, some trace elements (e.g., Ni and Sb) adsorbed on the goethite surface can be progressively incorporated into the crystal structure (e.g., Frierdich et al., 2011, Burton et al., 2020). Previous studies on Fe geochemical cycles in laterites indicated that Fe is mobile in the forms of Fe(II)-rich groundwater and soil water during lateritization, although aqueous Fe(II) can be oxidized to Fe(III)-(oxyhydr)oxides (e.g., goethite) by molecular oxygen during dry seasons (Beukes et al., 2002, Yamaguchi et al., 2007). Given the presence of aqueous Fe(II) in the lateritic environment (e.g., Beukes et al., 2002, Yamaguchi et al., 2007, Wu et al., 2019) and the ubiquity of Sc substitution in goethite in laterites (Ulrich et al., 2019, Qin et al., 2020), it is critical to assess whether or not the adsorbed Sc becomes structurally incorporated into the goethite lattice during Fe(II)-driven goethite recrystallization.

In this study, batch adsorption experiments were performed to investigate the macroscopic adsorption behavior of Sc onto goethite. The effect of aqueous Fe(II) on the adsorption behavior of Sc was also examined. The local coordination environments of Sc in synthetic Sc-adsorbed and Sc-substituted goethite were determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Quantum chemical calculations were performed to complement EXAFS data interpretation for clarifying the mechanisms of Sc immobilization by goethite at the molecular level. Moreover, the in situ speciation and crystal chemistry of Sc in natural goethite particles in the laterite were determined by using micro-focused X-ray fluorescence (μ-XRF), X-ray diffraction (μ-XRD), and μ-XAFS techniques. The aims of this study were to elucidate the molecular-scale mechanisms for Sc adsorption on and structural incorporation into goethite and to provide insights into the geochemical behavior and environmental fates of Sc.