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
Introduction
In the Tamagawa geothermal area of Akita Prefecture in northeastern Japan, several types of thermal waters emerge (Satoh et al., 2010) as a result of Quaternary volcanic activity. The most representative of the hot springs is Obuki spring (sometimes pronounced “Obuke” by local residents). Thermal waters of Obuki spring are characterized by a high discharge rate (9000 L/min) of chloride-rich acidic water (pH 1.2).
Several authors discuss the behavior of the REE in acidic thermal waters, including the Obuki spring, in Japan (Honda et al., 1989, Kikawada et al., 1993, Sanada et al., 2006, Satoh et al., 2008, Satoh, 2010), New Zealand (Wood, 2003, Wood, 2006), the USA (Lewis et al., 1997), and Kamchatka, Russia (Chudaev et al., 2017). Actinides are also occasionally concentrated in acidic thermal waters such as those from Obuki spring (Satoh, 2010) and the Peito hot spring in Taiwan (Lin et al., 2011). In both of these springs, the radioactive mineral, hokutolite (Pb and Ra-bearing barite), is being formed.
Many studies have investigated the geochemical mobility of REEs (e.g., Gimeno et al., 2000, Gammons et al., 2005a, Gammons et al., 2005b, Gammons et al., 2005c, Gammons et al., 2005c, Wood et al., 2006, Parker et al., 2008, McCleskey et al., 2009) and U in acidic aqueous environments (Bernhard et al., 1998, Gammons et al., 2003). However, combined studies of REEs and actinides from the same aquatic system are limited, excluding a study of alkaline thermal springs (van Middlesworth and Wood, 1998). Hydrous Al or Fe oxy-hydroxides (HAO or HFO) act as transporting and precipitating phases of many kinds of elements in aqueous systems (e.g., Gammons et al., 2003, Gammons et al., 2005a, Gammons et al., 2005b, Gammons et al., 2005c, Wood et al., 2006, McCleskey et al., 2009, Ogawa et al., 2014). The sorption onto HAO or HFO is largely pH-dependent.
Recent studies of the mobility of Ga and In in rivers acidified by thermal waters reveal that physico-chemical speciation changes (chemical state of target elements such as dissolved species and sorbate on HAO and HFO) are strongly influenced by aqueous speciation changes (such as free ions or complex species with some anions) (Ogawa et al., 2013), and thermodynamic calculation can accurately predict the sites (or times) at which sorption reactions onto HFO begin, as well as the semi-quantitative amounts sorbed onto HFOs (Ogawa et al., 2018). However, although there are many reports of REE and U sorption experiments involving aqueous REE species (e.g., Walter et al., 2003, Davranche et al., 2004), research on clarifying the relationship between physico-chemical and aqueous speciation of REEs and actinides in natural aquatic systems is very rare. The speciation changes of both groups of elements strongly depend on several geochemical factors such as pH, Eh and water temperature.
The Obuki thermal water is neutralized by lime, and then introduced into the Shibukuro River. Nevertheless, the upstream reach of the river system comprising the Shibukuro River and the Tama River is still significantly acidic due to the influence of Tamagawa thermal waters. On the other hand, this river water is used for electrical generation and agriculture. For these purposes, there are dams and reservoirs on this river system. These artifacts provide an opportunity to obtain and study river water samples with a wide range of pH over short distances. The air temperatures in the Tamagawa area in summer and winter seasons are close to 30 °C and less than 0 °C, respectively, meaning that the annual air temperature range in these areas is greater than approximately 30 °C; consequently, seasonal observation proved useful in understanding of the effect of water temperature on geochemistry of target metals. So, this unique region is expected to emphasize pH and water temperature dependent geochemical mobility of REEs and actinides.
We have conducted a long-term investigation of the Shibukuro-Tama river system from May 2008 to November 2010. We first consider the geochemical behaviors from the viewpoint of physico-chemical speciation (chemical state of target elements such as dissolved species and sorbate on HFO and HAO). Then, we predict the aqueous species change during the river transport on the basis of thermodynamic calculation using the thermodynamic calculation code PHREEQC (Parkhurst and Appelo, 2013), and clarify the relationship between physico-chemical and aqueous speciation. Finally, we determine the most important geochemical factors controlling the partitioning of metals between HFO and HAO and river water and then, elucidate the mechanism of fractionation among REEs and actinides during the river transport and seasonal variation of their mobility.
In this study, we clarify REE and actinide mobility under wider pH condition in association with their aqueous speciation changes. If fractionation mechanisms among REEs and actinides or among REEs become clearer, these elements can be used as more helpful geochemical indicators. Recently, both health effects and toxicity of REE are reviewed (Pagano et al., 2015a, Pagano et al., 2015b), and with the increase in REE uses in high-tech industry, elevated concentration of anthropogenic gadolinium (Gd) in the river waters flowing through a highly populated and industrialized area was reported (e.g., Nozaki et al. 2000). From the environmental viewpoint, the REE mobility in connection with their aqueous speciation change may become important. Furthermore, trivalent actinides such as Am3+ and Cm3+ have chemical properties similar to trivalent REEs (e.g., Choppin, 1983, Choppin, 1986). We hope that our new findings about REE and actinide mobility clarified in this study may also contribute to problems concerning nuclear waste disposal.
Section snippets
Shibukuro–Tama river system
The locations of sampling sites are presented in Fig. 1. Thermal waters of Obuki spring drain into the Shibukuro River after artificial lime input, where the pH of the thermal water is raised to >3 (site T-1). Other thermal springs besides the Obuki in the Tamagawa area, most of which are characterized by high sulfate concentrations (Satoh et al., 2010), flow directly into the Shibukuro River via the Hiyamizu Stream. The headwaters of the Shibukuro River are also acidified due to the input of
Sampling and chemical analysis
Sampling of river and lake waters was carried out in May and October 2008, March, June, October, and November 2009, and July and November 2010. Measurements of pH, Eh, and water temperature were performed in the field. River water samples were taken from sites T-1–T-15, as well as from Tama River (Tm-1) and Obonai River (Ob-1) in Fig. 1.
To investigate the physico-chemical speciation of elements (Al, REEs including Y, Th, and U), a successive filtration technique was used (See Fig. S1-1 in
Results
All field and analytical data are presented in tables in Supplementary Data 2 (SD2). The pH, Eh, and water temperature measurements are shown in Table S2-2, and the chemical compositions of the river water samples are summarized in Table S2-3.
In this study, we take data from June 2009 as being representative of the snowmelt runoff season, and data from July and November 2010 as being representative of the low-flow season.
Differences in the distribution of Fe and Al solid particulates
Small amounts of HFO existed in the thermal waters after lime input (site T-1), where pH values were greater than 3 (Fig. S5-1 in SD5). Mixing with the Tama River, where pH values increased to about 3.2 in the low flow seasons and 4.0 in the snowmelt runoff season, induced obvious formation of HFO coatings on river boulders and suspended HFO particulates from sites T-5 to T-6. Most suspended HFO and remaining dissolved Fe then tended to settle out as lakebed sediments in Lake Hosen.
HAO
Conclusions
- (1)
In spite of Th(SO4)2(s) precipitation in the upper stream regions, the physico-chemical state changes in REEs and actinides are essentially pH-dependent. During the neutralization processes, the order of removal by HFO and HAO up to pH 4.5–5.5 is Th > U ≫ REEs. As a result of the first fractionation step, HFO removes most of the Th and considerable amounts of U. Because of sedimentation of Th(SO4)2(s) and HFO-sorbing Th and U at the reservoir, subsequent river water has high REE/actinide ratios.
Acknowledgements
The authors wish to thank Prof. Chihiro Inoue and Dr. Shin-ichi Yamasaki of Tohoku University and Prof. Kazuhiro Kimura of Miyagi University for advice on various aspects of this study. This research was supported, in part, by the River Fund in charge of the Foundation of River and Watershed Environment Management (FOREM; Japan), and by the Water Resources Environment Technology Center (WEC; Japan). This manuscript was greatly improved by the valuable comments from two anonymous reviewers.
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