Characterization of saline groundwater at Horonobe, Hokkaido, Japan by SEC-UV-ICP-MS: Speciation of uranium and iodine

The saline groundwater collected at a depth of about 500 m in Horonobe, Japan, where an underground research laboratory (URL) has been built, is rich in saline (Na 4900 ppm, Cl 7600 ppm), iodine (42 ppm), and methane gas. We analyzed the colloids and ions of this groundwater mainly by employing a size exclusion chromatography (SEC) coupled on-line to ultraviolet–visible (UV–Vis) detection and inductively coupled plasma mass spectrometry (ICP-MS) technique and focused on the speciation of uranium and iodine, both of which are of particular importance for radioactive waste disposal. For this purpose, the groundwater sample was introduced to SEC columns after being passed through a 0.45 μm filter but without further pretreatment, such as isolation of colloids. The chromatographic profiles obtained with two different SEC columns were compared. This study revealed that uranium present in the groundwater at several tens of ppt was associated with low molecular weight silica species with neutral charge. The silica species were virtually free of metal elements such as Na, K, Mg, Ca, and Al. This study also found that almost all of the iodine in the groundwater was iodide (I−). The groundwater contained an unidentified organic colloid that was not a carrier for the radioactive waste-relevant elements Se, Sr, I, Cs, Th, and U.

► Uranium was present in low molecular weight silicic acid species with neutral charge. ► Almost all of the iodine in the groundwater was iodide. ► Organic colloid did not contain radioactive waste-relevant elements such as Se and Cs.