Abstract
This paper provides new insights into the geochemical cycling of rare earth elements (REEs) in acid sulfate soils developed on salt marsh sediments of the Huelva estuary (Spain) as a result of sulfide mineral oxidation in abandoned ore stockpiles. The study was aimed at determining the REE abundance, fractionation pattern and mineralogical control of the dispersal and retention of REEs in the soil system. Forty-one samples were collected at 13 core sampling sites along two transects extending across the degraded marshland, and they were subjected to XRD, ESEM-EDS and ICP-MS analyses. Measurements revealed that the soil receiving acid discharges has relatively high concentrations of ΣREEs (174.77 ± 19.77 mg kg−1) compared to local baseline concentrations. Shale-normalized REE patterns are generally flat, but a slight middle REE (MREE) enrichment is consistently apparent in all soil samples, involving relatively low LaN/GdN ratios (0.83 ± 0.08) and GdN/LuN ratios up to 1.42. The convex-upward REE pattern supports the possibility that iron oxy-hydroxide minerals play an important role in MREE retention through adsorption and co-precipitation mechanisms. Efflorescent sulfate salts left on the topsoil by the evaporation of acid waters show a strong depletion of light REEs (LaN/GdN = 0.16 ± 0.10) and act as a temporary reservoir of labile MREEs and heavy REEs during dry periods.
Highlights
Rare earth elements (REEs) are a group of transition metals with similar atomic structure and chemical properties, and are comprised of the lanthanide series (La to Lu) plus scandium (Sc) as well as yttrium (Y) [1], in most geochemical discussions the term is restricted to the lanthanides
REEs are arbitrarily segregated into light REEs (LREEs: La to Nd), middle REEs (MREEs: Sm to Dy), and heavy REEs (HREEs: Ho to Lu) based on their atomic mass and the gradual reduction in ionic radius across the series
The pyrite concentrates stockpiled on the banks of the estuarine salt marshes are a legacy source of leachable REEs in the acid mine drainage (AMD)-impacted marshland
Summary
Rare earth elements (REEs) are a group of transition metals with similar atomic structure and chemical properties, and are comprised of the lanthanide series (La to Lu) plus scandium (Sc) as well as yttrium (Y) [1], in most geochemical discussions the term is restricted to the lanthanides. REEs are lithophile refractory elements that display similar behavior through many geological processes [3]. During Earth surface processes, such as weathering and pedogenesis, REEs undergo significant fractionation [4], which reflects the depletion or enrichment of individual REEs, or subgroups of REEs, relative to a standard reference material. In soil science the reference commonly used to normalize lanthanide concentrations is either internal (parent material or geological substratum) or external (upper continental crust or shale composites used as proxies) to the surface environment under investigation [4]. Anomalies can occur due to redox lability for Ce and Eu within the Eh-pH regime of water stability [5]
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