环境地球化学国家重点实验室知识库

Due to the unique chemical properties that are similar but still progressively change, the rare earth elements (REEs) are useful tracers of various geochemical processes in the lithosphere and hydrosphere. However, despite many studies of REE geochemistry in the ocean, the aqueous geochemistry of REEs in lake waters has been poorly documented. In the present study, two special karst lakes are chosen as case studies to investigate the distributions of dissolved REEs in lake water. Although the two lakes, Hongfeng and Aha, are both alkaline and have high pH from 7.9 to 8.7 and high carbonate concentrations, the Aha Lake has been more severely affected by acidic mining drainage with high Fe, Mn and SO42 concentrations. In the present study, the concentrations of dissolved rare-earth elements in lake waters were determined by inductively coupled plasma mass spectrometry. The result shows that the concentrations of dissolved REEs in the studied alkaline karst lakes, as compared to the concentrations of REEs in seawater, are much lower than the other investigated terrestrial surface waters in previous studies. The key factor controlling dissolved REE distributions is pH value which is negatively correlated with REE concentrations. Due to high concentration of carbonate ion and alkaline character of water chemistry, the shale (PAAS) normalized patterns of dissolved REEs show marked HREE enrichment in all water samples. This is primarily the result of the preferential formation of stronger carbonate complexes with the HREEs. In alkaline or intermediate waters, REE-carbonate complexes are the dominant and typical species, which account for about more than 90% of the total dissolved REEs. The shale-normalized REEs patterns of the dissolved phase also show very remarkable Ce anomaly in all lake water samples with the Ce/Ce* values ranging from 0.1 to 0.8, mainly from 0.2 to 0.4. Moreover, the vertical distributions of Ce anomaly in the studied lakes are similar to the distribution of dissolved Mn, implying redox reaction is the main process for controlling the Ce anomaly in the lakes. This also shows Ce anomaly and Mn are two sensitive tracers of redox process in surface waters.