The halogens belong to Group 17 of the periodic table consisting of fluorine, chlorine, bromine, iodine, and astatine. The halogens play an important role in geochemistry because their incompatible nature in most mineral phases can be used for geofluid tracers. Among the halogens, especially iodine has unique features. Iodine is an essential trace element for humans and animals as a component of thyroid hormone. In contrast, radioactive iodine released from nuclear fuel reprocessing plants and nuclear accidents to natural environments increases the risk for thyroid cancer. Here, we would like to introduce the special issue on the geochemistry of radioiodine and halogens. This special issue is dedicated to the late professor Yasuyuki Muramatsu.

Iodine is strongly partitioned to aqueous fluid and highly biophilic, resulting in enrichment in seawater and marine sediments, but depletion in mantle. This element can be a useful geochemical tracer for water and material recycling into the mantle at subduction zones. In environmental chemistry, iodine is an essential trace element in human diet which plays an important role in controlling the metabolism. Once radioiodine is ingested to human body, it is accumulated in the thyroid gland, which causes internal exposure. For safe management of radioactive materials in the environment, it is also crucial to understand the geochemical cycles of iodine. In this paper, various analytical methods of iodine in geochemical samples are introduced. Especially, detailed experimental conditions of the Pyrohydrolysis method, which is widely used for sample pretreatment method are described, and the analytical results of geochemical standard samples by this method are compared with those by other methods.

We collected 14 saline groundwater samples from coastal land areas of Hiroshima and Hyogo Prefectures, and Shodoshima of Kagawa Prefecture. Data on δD, δ¹⁸O, Li/Cl, Br/Cl, I/Cl, and ¹²⁹I/¹²⁷I for the samples reveal that the groundwater collected from Hiroshima Pref. are mainly derived from relatively young seawater, whereas samples from Hyogo Pref. and Shodoshima clearly contain slab-derived fluid characterized by>0.001 Li/Cl weight ratios, and show 1) <1.25×10^–3 Br/Cl mole ratios, 2) >50×10^–6 I/Cl mole ratios, and 3) low ¹²⁹I/¹²⁷I ratios (i.e., old iodine age).In addition, the latter sampling sites were characterized by shallow cutoff depth (D90: the depth above which 90% of earthquakes occur), suggesting that D90 becomes shallow where slab-derived fluids ascend to shallow depths. Halogen composition and ¹²⁹I/¹²⁷I ratio are concluded to be good indicators of slab-derived fluid contained in saline groundwater.

Iodine (¹²⁷I) is an essential trace element for humans and animals as a component of thyroid hormone, while radioactive iodine-129 (¹²⁹I) released from nuclear fuel reprocessing plants and nuclear accidents is a significant risk factor. Together with its volatilization and oxidation, iodine reduction is one of important processes in the global iodine cycling, whose details are still unknown. Iodine exists mainly as inorganic iodate (IO₃⁻) and iodide (I⁻),although iodate is thermodynamically more stable than iodide in seawater. Nevertheless, up to 50% of total iodine are observed as iodide in surface seawater. This apparent thermodynamic disequilibrium is thought to be due to the activity of iodate reduction by algae and marine bacteria in ocean surface. In particular, a hypothesis that iodate is reduced by nitrate reductase of marine organisms has been paid attention. This article summarizes the research history of biological iodate reduction, and also introduces recent findings on enzymes potentially involved in iodate reduction in nature.

The accident at the Fukushima Daiichi Nuclear Power Plant resulted in a substantial release of radionuclides including Iodine-131, Cesium-134, and Cesium-137 to the atmosphere, and caused significant contamination of the environment. Most parts of the eastern Japan were subjected to the radioactive contamination, especially in Fukushima Prefecture. An effective dose estimation of released ¹³¹I is important but difficult due to lack of data on the deposition of ¹³¹I. The long-lived radioiodine isotope ¹²⁹I (half-life: 1.57×10⁷ y) is one of the important radionuclides released from nuclear fuel reprocessing plants and nuclear accidents into the environment. This radionuclide has provided useful information on the behavior of radioiodines, whose half-lives are very short, in the environment. In particular, ¹²⁹I has been used as a tool to reconstruct the initial distribution of ¹³¹I (half-life: 8 days) at nuclear accidents. Therefore the determination of ¹²⁹I in soils in Fukushima is needed to reconstruct the distribution of ¹³¹I in the environment. In this article, I review recent studies on radioactive iodine in Fukushima.