Abstract
Exploring the distributions of rare earth elements (REEs) in soil profiles is essential to understanding how natural and anthropogenic factors influence the geochemical behaviors of REEs. This study aimed to learn about the distribution characteristics of REEs in soils, including their fractionation and enrichment, and to explore the influence of soil pH and soil organic carbon (SOC) on REEs. One hundred and three samples were collected from six soil profiles under different land uses (paddy field: T1, T3; forest land: T2, T6; wasteland: T4; building site: T5) in the Mun River Basin, Northeast Thailand. The average total REE contents (∑REE) are much lower (<80 mg kg−1) than that of Earth’s crust (153.80 mg kg−1) in soil profiles T2, T3, T4, and T6. The contents of REEs tend to increase slightly with depth in all soil profiles. The ratios of (La/Yb)N range from 0.35 to 0.96 in most samples, indicating that the enrichment of heavy REEs (HREEs) relative to light REEs (LREEs) is the main fractionation pattern. Samples from profile T2 show relatively obvious negative Ce anomalies (0.55–0.78) and positive Eu anomalies (1.41–1.56), but there are almost no anomalies of Ce and Eu in other soil profiles. Enrichment factors of LREEs (EFLREEs) range from 0.23 to 1.54 and EFHREEs range from 0.34 to 2.27, which demonstrates that all soil samples show no LREE enrichment and only parts of samples show minor HREE enrichment. Soil organic carbon (SOC) contents positively correlate with the enrichment factors of REEs (EFREE) in soil profiles T1 (R = 0.56, p < 0.01) and T6 (R = 0.71), while soil pH values correlate well with EFREE in soil profiles T2 (R = 0.75) and T4 (R = −0.66, p < 0.01), indicating the important influence of soil pH and SOC on the mobility of REEs in some soil profiles.
Highlights
Rare earth elements (REEs) comprise lanthanides from lanthanum (La) to lutetium (Lu), of which fourteen are relatively abundant in the Earth’s crust [1,2]
It is obvious that average REE contents of samples from soil profile T1 are higher than those from other profiles
The light REEs (LREEs) contents of all soil samples account for a large part (>87%) of total REE contents, which is consistent with the distribution characteristics of REEs in the Earth’s crust [35]
Summary
Rare earth elements (REEs) comprise lanthanides from lanthanum (La) to lutetium (Lu), of which fourteen (except promethium) are relatively abundant in the Earth’s crust [1,2]. Many studies showed that REEs were fractionated during weathering, always characterized by the occurrence of Ce and Eu anomalies as well as the fractionation between light REEs (LREEs) and heavy REEs (HREEs) [1,7]. Based on these characteristics, the influence of weathering on REEs can be evaluated by the relative enrichment of REEs. On the other hand, REEs are highly sensitive to redox potential, soil pH, and adsorption/desorption reactions related to organic matter (OM) and iron–manganese oxides [8,9].
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