Fractionation of rare earth elements (REEs) and actinides (U and Th) originating from acid thermal water during artificial and natural neutralization processes of surface waters

Abstract

Abstract In the Tamagawa geothermal area of Akita Prefecture, northern Japan, Obuki spring discharges a large amount of thermal water (∼9000\u202fL/min), which is chloride-rich and acidic (pH 1.2). We have investigated changes in the physico-chemical nature and fractionation mechanisms of rare earth elements (REEs) including Y and actinides (Th and U) in the Shibukuro and Tama rivers into which acid thermal water from the Obuki spring discharges. The geochemical behavior of these elements is shown to be mainly controlled by pH-dependent sorption onto ferric and/or aluminum oxy-hydroxides (HFO and HAO). HFO is formed around pH\u202f>\u202f3. In the upstream region, where pH is less than 4, dissolved Th is removed due to precipitation of Th(SO4)2(s). Whereas, under pH condition greater than 4, remaining dissolved Th is nearly completely sorbed onto suspended HFO, where hydroxyl species of Th are thermodynamically significant. Then, the Th(SO4)2(s) and HFO-sorbed Th are nearly completely removed in an upstream man-made lake. Dissolved U is also removed mainly by sorption onto HFO, where hydroxyl species of UO2 are predominant. A considerable portion of U is also trapped in the sediments of this lake. On the other hand, removal of REEs is nearly negligible until the lake. This is the first fractionation among REEs and actinides caused by pH-dependent sorption, and the order of removal from river water is Th\u202f>\u202fU\u202f≫\u202fREEs. The remaining dissolved U and REEs in the river waters are transported farther downstream. At pH values of >6, suspended HAO is effectively formed, and the second fractionation among REEs and U occurs. During the second fractionation, REEs are removed by HAO, whereas U largely remains as a dissolved species in river water under higher pH conditions. Lighter REEs, such as La and Ce, tend to remain in the river waters compared to the other REEs including Y. The predominant chemical species for UO2 during the second fractionation are carbonate complex species, and relative proportions of hydroxyl species drastically decreases, resulting in prevention of U sorption onto HAO.