Title: Rare earth elements, yttrium and scandium retention in fe- and al-hydroxysulfates (basaluminite and schwertmannite) under the hydrogeochemical conditions of the Tinto river estuary (sw Spain)
The PhD student Joan Gutiérrez León, from the Groundwater and Hydrogeochemistry group, will defend his thesis on 19th July at 10:30h in Aula Carmina Virgili, Science Earth Faculty (University of Barcelona)
Title: Rare earth elements, yttrium and scandium retention in fe- and al-hydroxysulfates (basaluminite and schwertmannite) under the hydrogeochemical conditions of the Tinto river estuary (sw Spain)
Directors: Jordi Cama and Josep M. Soler
Thesis Committee: Mercè Corbella, Oriol Gibert and Jordi Palau
Abstract:
Rare-earth elements (REEs: lanthanides, yttrium and scandium) are scarce in mining concentrations in the Earth’s crust, freshwater and seawater. In recent decades, REEs have become crucial to new technologies for computer chips (gadolinium), missile guidance systems (neodymium), nuclear reactor control rods (samarium), microwave emitters (yttrium), long-live missile batteries (promethium), electric vehicles (dysprosium) or in the glass industry (lanthanum). The need of REEs forces European manufacturing companies to invest in the exploration of alternative sources. An alternative source is the areas impacted by acid mine drainage (AMD), which display REE concentrations that are several orders of magnitude higher than those of freshwater and seawater. AMD neutralization as a result of mixing with seawater in estuaries prompts a spontaneous precipitation of Fe- and Al-oxyhydroxysulfate nanominerals (i.e., schwertmannite (Fe8O8(OH)6SO4) and basaluminite (Al4(SO4)(OH)10·5H2O), respectively), which play a significant role in the fate of Rare Earth Elements (REEs). The conditions for precipitating schwertmannite and basaluminite (pH 2.5-3.5 and 4.5, respectively) are not suitable to adsorb REEs, but when AMD mixes with the water of the estuary of Ría de Huelva (Spain), the pH increases up to between 4.5 and 8, yielding optimal conditions for REE adsorption on schwertmannite.
A geochemical study of this estuarine system is needed to shed light on the REE distribution. Earlier studies focused on the adsorption of REEs onto these Fe- and Al-oxyhydroxysulfates. However, two important aspects related to the affinity of REEs for these minerals remained unsolved and are necessary to comprehensible understand the distribution of REEs in the estuarine system. The first one is to know the capacity of schwertmannite to retain adsorbed REEs at a pH between 4.5 and 7. Desorption batch experiments in the presence of sulfate were performed to study this capacity at this pH range and room temperature. The solution chemistry was analysed to obtain the REE surface constants for the surface complexation reactions. Desorption of Lu-enriched schwertmannite at different pH values was investigated with High Energy X-Ray Diffraction (HEXD) and Extended X-ray Adsorption Fine Structure (EXAFS) to characterize the molecular structure of the surface complexes involved in the desorption reaction. The results indicate (1) that REE adsorption/desorption on schwertmannite is pH dependent and, as a consequence, schwertmannite retains adsorbed REE at pH > 6, and (2) both monodentate and bidentate surface complexes are involved in the Lu-desorption reaction.
The second aspect concerns the effects of an increase in pH and in ionic strength on the REE adsorption onto both nanominerals. In this study, REE adsorption onto schwertmannite and basaluminite has been studied in the pH range of 4.5-7 and ionic strength range of 0.25-0.5 M using batch experiments and EXAFS analysis to elucidate the behaviour of REEs under AMD impacted-estuarine conditions. The log KREE values calculated for the adsorption of REEs onto schwertmannite and basaluminite were implemented in a non-electrostatic surface complexation model (NEM), which indicated that REE adsorption is both pH dependent and ionic strength independent. For schwertmannite, low pH resulted in a low retention (up to 10% at pH 4.5) in a monodentate coordination whereas high pH increased the adsorbed fraction (up to 99% at pH 6.5) in a bidentate coordination. For basaluminite, the REE affinity was affected by the REE atomic number, enhancing the adsorption of heavy REEs (HREEs, up to 90% at pH 6.5) with respect to light REEs (LREEs, up to 20% at pH 6.5). The agreement between the NEM calculations and the EXAFS analysis in Gd-basaluminite suggests that monodentate binuclear and mononuclear are probable REE coordination in basaluminite, although the presence of outer sphere complexes is also likely for LREE. Thus, the thermodynamic parameters provided in this study prove useful to predict the geochemical behaviour of REEs in AMD-impacted estuarine areas.
To shed light on the complex geochemistry of the overall estuarine system, a field campaign was carried out to collect samples of the estuarine sediment and water and to measure concentrations of relevant trace elements ((REEs) and metal(loid)s). This enabled to (1) better understand the elemental distribution in the sediments and water in the water-mixing area, (2) discern the elemental behaviour via mixing modeling, (3) reproduce the aqueous behaviour of REEs related to REE adsorption onto schwertmannite and basaluminite, and (4) evaluate the effect of the adjacent phosphogypsum stacks on the estuarine geochemistry.
