The rare earth elements (REEs) are commonly defined as lanthanum (La) and the 14 elements comprising the Lanthanide series. The REE’s typically exhibit trivalent oxidation states; however, Europium may also occur as Eu2+ and Cerium may occur as Ce4+. The REE’s ionic radii decrease on progression from La to Lu, which results in a slight but predictable change in their chemical affinity. Typically, the light REE (La to Sm) reside in trace minerals such as apatite, epidote and allanite, whereas the heavy REE (Gd to Lu) are associated with minerals such as zircon. Investigations typically show that the REE are depleted in near-surface horizons and accumulate in deeper horizons or the regolith as clay-oxyhydroxide adsorbates or REE-phosphate precipitates. Numerous studies show the heavy REE accumulating in the deeper soil regions to a greater extent than the light REE, whereas other studies show the light REE’s preferentially accumulating at greater soil depths. The degree of interhorizon transport has great potential to become an index of weather intensity. The various REE soil migration pathways have been isolated, including lessivage, soil organic matter complexation, leaching in percolating water, adsorption by inorganic colloids, and precipitated by phosphate-bearing minerals.
The soils of the St. Francois Mountains in Missouri are developed in loess and rhyolite residuum. Loess contributions may be expected to modify the texture, clay mineralogy, and nutrient availabilities, awarding characteristics significantly different from soils developed entirely in rhyolite residuum. The objective of this investigation is to assess the validity of using the rare‐earth elements (REEs) as a geochemical indicator to discriminate between two contrasting parent materials. Elemental analysis of the whole soil, the rhyolite residuum, and a representative loess deposit were analyzed by instrumental neutron activation analysis (INAA) and X‐ray fluorescence (XRF). Whole soil Ti contents are consistent with the local loess and incompatible with rhyolite residuum as the only parent material. The REE signatures of the whole soil are consistent with a mixture of local loess and rhyolite residuum. The clay fractions show a preferential REE accumulation, suggesting the REE may co‐illuviate with clay across soil profile boundaries. Neodymium, and to a smaller extent Eu, show dramatic associations with illuviated clay, altering the REE signatures and their interpretation. However, interhorizon transfers of the REE because of lessivage are not sufficient to render the use of the REE signatures ineffective for recognizing loess as a parent material. The migration potential of the REE must be further investigated before the method may be accepted as a diagnostic tool for recognizing lithologic discontinuities, especially for extremely weathered soils.
The lanthanide elements or rare earth elements (REEs) are an active soil science research area, given their usage as micro-fertilizers, documented cases of environmental impact attributed to industry/mining, and their ability to identify lithologic discontinuities and reveal active soil processes. To fully understand REEs requires an understanding of their chemical reactivity, both for the individual elements and their behavior as a group of elements. The elements of the lanthanide series, including La and Y, may have subtle to very perceptible chemical differences that when viewed collectively reveal information that gives emphasis to soil processes that clarify soil behavior or soil genesis. This chapter concentrates on lanthanide soil chemistry and shows how the soil chemistry of REEs may support soil science investigations. Lee [2], Henderson [3].CN6 is coordination number six and CN8 is coordination number eight. Table 1. Chemical properties of the trivalent rare earth elements, including La and Y. Lanthanides 50(nodules of Fe-and Mn-oxyhydroxides) and an abrupt increase in pH from an acidic to alkaline regime. Thus, oxidation-reduction and pH appear to be the controlling variables. Future research needsFuture research needs include; (i) understanding of the REE-microbiological interactions, especially in the rhizosphere, (ii) are the REE elements plant essential elements or growth promoting entities, (iii) more complex models (along with thermodynamic data) to better simulate the soil environment, and (iv) anticipate REE impacts to the soil environment because of increasing industrial REE utilization.
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