Inorganic speciation of deep groundwater and its effect on the characteristics of rare earth elements: with aquifers in coal bearing masures in Renlou coal mine in the north of Anhui Province as example

Based on analysis of major ions and rare earth element (REEs) concentrations of groundwater in an aquifer of coal bearing measures in Renlou coal mine in the north of Anhui Province, the inorganic speciation of dissolved REEs have been modeled by using the program-Visual Minteq. The results show that the groundwater can be subdivided into SO₄^2- enriched and alkaline (CO₃^2- or HCO₃^-) enriched water with pH values lower and higher than 8.20/8.36 respectively. The SO₄^2- enriched water has relative lower total REE concentration than HCO₃^-enriched water, although they all show light REEs depletion after PAAS (Post Archean Average Shale) normalization. The inorganic species of SO₄^2-enriched waters include Ln³⁺, LnCO₃⁺, LnSO₄⁺, Ln(CO₃)₂^- and Ln(SO₄)₂^- (Ln-REE), whereas Ln(CO₃)₂^- and LnCO₃⁺ are predominant in HCO₃^-enriched water, and the relative proportions of species are closely related to pH values and element types (e.g. LREEs and HREEs). The concentration of HCO₃^-is well correlated with ∑REE and Nd_SN/Yb_SN ratio, indicating that the total concentration and fractionation of REEs of groundwater probably has been affected by different proportions of different species (especially Ln(CO₃)₂^-).