Distribution, fractionation and sources of rare earth elements in suspended particulate matter in a tropical agricultural catchment, northeast Thailand

Abstract

Forty-eight suspended particulate matter (SPM) samples were collected from the Mun River, northeast Thailand and its junction with the Mekong River, to investigate the relationship between the distribution of rare earth elements (REE) in SPM and the soils in the watershed. The total REE contents (∑REE) in SPM in the Mun River ranged from 78.5 to 377.8 mg/kg with the average of 189.3 mg/kg, which was lower than ∑REE of 222.3 mg/kg at the Mekong River (one sample at junction). The Post Archean Australia Shale (PAAS)-normalized ratios of light REE (LREE), middle REE (MREE) and heavy REE (HREE) were averaged to 1.0, 1.3 and 1.0, which showed a clear enrichment in MREE. In short, along the Mun River, the REE contents in SPM were decreasing, and the PAAS-normalized patterns of REE showed gradually flat. The REE content in SPM and soils are highest in the upper catchment, indicating that soil/bedrock is the most important source of REE in SPM. Additionally, the positive Eu anomaly was enhanced by the higher Ca content in SPM (R = 0.45), which may be caused by more feldspars or carbonates with Ca and Eu substituting Ca. The results present the REE behaviors of SPM in the Mun River and relationship between REE in SPM and soil/bedrock, the findings may support the other studies in catchment weathering.

Tracing Fe Sources in Suspended Particulate Matter (SPM) in the Mun River: Application of Fe-Stable Isotopes Based on a Binary Mixing Model

“…On the other hand, rare earth elements (REE, lanthanides La to Lu, except Pm) have similar chemical properties and behave similarly in geochemical processes, and have been widely employed for tracing source materials and variations of environmental parameters like redox conditions. , Generally speaking, REE are divided into light REE (La to Eu) and heavy REE (Gd to Lu) according to their atomic radii and weights . As shown in our previous studies, the average PAAS-normalized fractionation patterns of REE are relatively flat without marked fractionation between LREE and HREE in SPM, and most SPM samples are characterized by obviously positive Eu anomaly.…”

“…With respect to the river system, general hydrogeochemistry and sampling procedures and detailed description has been given in the previous literature. , Out of 54 samples, 17 (W1–W17) were sampled by UMR, 24 (W18–W41) by MMR, and 13 (W42–W54) by LMR. Detailed digestion processes and pretreatments of SPM have been documented in previous studies. , …”

“…The Mun River and its tributaries stretch across a tropical agricultural catchment in Northeast Thailand, where a large amount of research in various fields has been conducted to reveal the cycles of materials and elements. − Intensive agricultural activities and urbanization could potentially cause pollution of the local region . There is an urgency of detecting potential environmental contamination and tracing the sources of the pollutants, through using stable Fe isotopes.…”

Distribution and fractionation of rare earth elements in suspended particulate matter in a coastal river, Southeast China

“…Compared to the wide forest upstream of the Beixi River, the Xixi River and Nanxi River mainly pass through farmlands and residential areas ( Liu & Han, 2020 ). Human activities can directly cause the increment of REE into the river and disturb REE fractionation by affecting physicochemical parameters of river water ( Da Silva et al, 2018 ; Yang et al, 2021 ).…”

“…According to the geochemical behaviors of REEs, the three groups including light REE (LREE, from La to Nd), middle REE (MREE, from Sm to Ho), and heavy REE (HREE, from Er to Lu) are commonly classified. The weathering of rocks is the primary source of REEs in the earth’s surface environments ( Yang et al, 2021 ). However, as the important annexing agents of modern materials, REEs have been extensively applied in agriculture, military, aviation, and medical industries ( Altomare, Young & Beazley, 2020 ; Bau & Dulski, 1996 ; Dushyantha et al, 2020 ; Louis et al, 2020 ; Volokh et al, 1990 ).…”