Determination of rare earth elements in rainwater collected in suburban Tokyo

Abstract

Rare earth element (REE) concentrations were determined in 32 rainfall events collected in suburban Tokyo. Two or more samples were collected for 13 events out of 32 events. A pre-concentration method was applied using a chelate resin solid phase extraction column. Good and uniform recoveries for REEs (Y to Lu, 99~101% at pH = 3.7~4.0) were obtained. Detection limits reached as low as 0.001 ng L^–1 (for Tb) with a 30 fold pre-concentration using a conventional Q-pole ICP-MS. Clear La and Gd excesses (definition is in the text) were evident in many cases. Y, Eu, Lu and Yb excesses may also exist in some events. In some events where multiple samplings were performed, crust normalized REE patterns changed during the rainfall events. These are the first evidences of clear REE pattern variations during the same rainfall events. The initial and final stage of some rainfall events may have different origins for the REEs. The correlations among the REEs with respect to logarithmic concentrations indicate that most of the REEs are of natural origin while La and Gd may have common anthropogenic components. We propose that materials for high refractive index and low dispersion optical glass production processes could be potential sources of La and Gd excesses because 1) La and Gd excesses are correlated, 2) other sources such as a cracking catalyst for petroleum refining or a contrast agent of magnetic resonance imaging cannot explain the excesses.