Geochemical Characteristics of Soil Rare Earth Elements within Spontaneous Combustion Coalfields of Rujigou Coal Mine

Abstract

(1) Background: The spontaneous combustion of coal (SCC) not only consumes huge amounts of coal resources but also causes environmental degradation. Rare earth elements (REE) can be taken as an effective indicator to evaluate the environmental effects of SCC. Coal in the Rujigou Mine has been spontaneously combusting for hundreds of years. (2) Methods: The geochemical properties of REE and major elements in the soil of the Rujigou coal mine are methodically examined to reveal the environmental effects of SCC. (3) Results: Soil REE concentration in the Rujigou mine is 216.09 mg/kg, and there is an enrichment of light rare earth elements (LREE) and a depletion of heavy rare earth elements (HREE), LREE/HREE in Rujigou mine was 5.52. The spontaneous combustion of coal could change the vertical distribution of REE, which is conducive to the enrichment of LREE. According to the Eu anomaly and δCe/δEu, the source of material in this mine may be derived from the terrigenous clastic rock controlled by weak reduction. Aluminum and titanium have similar geochemical behavior to REE, especially LREE. The concentration of sulfur is negatively correlated with REE, especially HREE. Calcium, sodium, and magnesium all had a negative correlation with LREE. (4) Conclusions: The spontaneous combustion of coal can lead to the fractionation of light and heavy rare earth elements, resulting in the enrichment of LREE and depletion of HREE.