Geochemical mobility of rare earth elements (REEs) and actinides (U and Th) originating from Kusatsu acid thermal waters during neutralization and river transport: Effect of aqueous speciation on sorption onto suspended materials and fractionation among REEs and actinides

Abstract

Acidic thermal water originating from the Kusatsu geothermal area of Gunma Prefecture, Japan is introduced into rivers, where there are many anthropogenicobjects such as a neutralization plant, a dam and water channels for hydroelectric power plants. We have investigated changes in the physico-chemical nature and fractionation mechanisms of rare earth elements (REEs) including Y and actinides (Th and U) by artificial and natural neutralization processes during river transport.The geochemical behavior of these elements and fractionation among them are mainly controlled by pH-dependent sorption and/or precipitation, once the pH increases up to 5.5. Dissolved Th is nearly completely removed, and hydrous Fe and Al oxides (HFO and HAO) sorbing Th or Th(SO4)2s settle onto the reservoir's bed. On the other hand, REEs and U remain as dissolved species. The effect of temperature on removal of these metals is limited.At pH values of ~5.5, REEs and U are removed by HFO and/or HAO, resulting in less fractionation among them. As pH increases to greater than 6.5, dissolved REEs continue to be removed by HFO and/or HAO, whereas U largely remains as a dissolved species in river water. Formation of UO2‑carbonate complex species prevents removal of dissolved U. When pH approaches 8, U is desorbed from suspended materials.Fractionation of REEs including Y occurs during sorption onto suspended materials and is strongly pH-dependent. REEs are less fractionated, when sorption occurs under pH less than ~6.2. As pH increases, heavier REEs tend to be removed by sorption onto suspended materials, and lighter REEs tend to remain in river water. Compared to fractionation between MREE and HREE, fractionation between LREE and MREE is larger. Yttrium is sorbed more effectively than LREE but less effectively than HREE. REE fractionation may occur significantly under pH 6.2–7.5.