Organic-mineral colloids regulate the migration and fractionation of rare earth elements in groundwater systems impacted by ion-adsorption deposits mining in South China

• Published in: Water research (Oxford) 2024-06, Vol.256, p.121582-121582, Article 121582
• Main Authors: Liu, Xiao-Rui, Liu, Wen-Shen, Zhang, Miaoyue, Jin, Chao, Ding, Keng-Bo, Baker, Alan J.M., Qiu, Rong-Liang, Tang, Ye-Tao, Wang, Shi-Zhong
• Format: Article
• Language: English
• Subjects: Asymmetric flow field-flow fractionation (AF4); Groundwater; Ion-adsorption deposit; Organic-mineral colloids; Rare earth element (REE) fractionation
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2024.121582

•Leachate was affected by in situ leaching and contained dissolved REE(SO4)+.•The main vectors of transport for REEs away from the mines were organic-mineral fine colloids.•The organic-rich population of the fine colloids controlled the REEs in downstream regions.•Organic matter in composite colloids was the binding fraction of REEs. Ion-adsorption rare earth element (REE) deposits distributed in the subtropics provide a rich global source of REEs, but in situ injection of REEs extractant into the mine can result in leachate being leaked into the surrounding groundwater systems. Due to the lack of understanding of REE speciation distribution, particularly colloidal characteristics in a mining area, the risks of REEs migration caused by in situ leaching of ion-adsorption REE deposits has not been concerned. Here, ultrafiltration and asymmetric flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4-ICP-MS) were integrated to characterize the size and composition of REEs in leachate and groundwater from mining catchments in South China. Results show that REEs were associated with four fractions: 1) the 220 nm coarse colloids and acid soluble particles (ASPs) comprising minerals. Influenced by the ion exchange effect of in situ leaching, REEs in leachate were mostly dissolved (79 %). The pH of the groundwater far from the mine site was increased (5.8 – 7.3), the fine organic-mineral colloids (46 % – 80 %) were the main vectors of transport for REEs. Further analysis by AF4 revealed that the fine colloids can be divided into mineral-rich (F1, 100 kDa – 120 nm) and organic matter-rich (F2, 120 – 220 nm) populations. The main colloids associated with REEs shifted from F1 (64 % ∼ 76 %) to F2 (50 % ∼ 52 %) away from the mining area. For F1 and F2, the metal/C molar ratio decreased away from the mining area and middle to heavy REE enrichment was presented. According to the REE fractionation, organic matter was the predominant component capable of binding REEs in fine colloids. Overall, our results indicate that REEs in the groundwater system shifted from the dissolved to the colloidal phase in a catchment affected by in situ leaching, and organic-mineral colloids play an important role in facili