Sorption and transport of iodine species in sediments from the Savannah River and Hanford Sites

Abstract

Iodine is an important element in studies of environmental protection and human health, global-scale hydrologic processes and nuclear nonproliferation. Biogeochemical cycling of iodine is complex, because iodine occurs in multiple oxidation states and as inorganic and organic species that may be hydrophilic, atmophilic, and biophilic. In this study, we applied new analytical techniques to study the sorption and transport behavior of iodine species (iodide, iodate, and 4-iodoaniline) in sediments collected at the Savannah River and Hanford Sites, where anthropogenic (129)I from prior nuclear fuel processing activities poses an environmental risk. We conducted integrated column and batch experiments to investigate the interconversion, sorption and transport of iodine species, and the sediments we examined exhibit a wide range in organic matter, clay mineralogy, soil pH, and texture. The results of our experiments illustrate complex behavior with various processes occurring, including iodate reduction, irreversible retention or mass loss of iodide, and rate-limited and nonlinear sorption. There was an appreciable iodate reduction to iodide, presumably mediated by the structural Fe(II) in some clay minerals; therefore, careful attention must be given to potential interconversion among species when interpreting the biogeochemical behavior of iodine in the environment. The different iodine species exhibited dramatically different sorption and transport behavior in three sediment samples, possessing different physico-chemical properties, collected from different depths at the Savannah River Site. Our study yielded additional insight into processes and mechanisms affecting the geochemical cycling of iodine in the environment, and provided quantitative estimates of key parameters (e.g., extent and rate of sorption) for risk assessment at these sites.

Surficial redistribution of fallout131iodine in a small temperate catchment

“…2A), when 131 I activity in precipitation exceeded 2.5 Bq L −1 . A secondary peak in 131 I deposition during early April coincided with a spike in natural 7 Be deposition ( Table S1). Given that the cosmogenic production of 7 Be in the atmosphere is approximately constant (15), we attribute the second 131 I peak to more efficient removal of radionuclides from the atmosphere rather than to a secondary pulse of 131 I from Japan.…”

“…4A; adj R 2 ¼ 0.92, p < 10 −4 ) demonstrates that the particle-reactivity of both 131 I and 7 Be remains stable during transport to the fluvial system and subsequent deposition. In this regard, despite radically different elemental chemistry, 7 Be is an effective tracer of 131 I redistribution and fate.…”

“…The relative activities of 131 I and 7 Be in sediment are dependent on the differential decay rates of 131 I and 7 Be during particle transport, deposition, and storage, as well as on inheritance of 7 Be from previous or extended atmospheric exposure predating the FDNPP disaster. The strong correlation of 131 I and 7 Be in channel margin deposits along the entire length of Mink Brook suggests uniform inheritance and residence time of 131 I in this depositional environment.…”

“…Mobilization of historic (pre-Fukushima) 137 cesium observed concurrently in these soils suggests that the focusing of iodine to surface soils may be biologically mediated. Atmospherically deposited iodine is subsequently redistributed from the soil system via fluvial processes in a manner analogous to that of the particle-reactive tracer 7 beryllium, a consequence of the radionuclides' shared sorption affinity for fine, particulate organic matter. These processes of surficial redistribution create iodine hotspots in the terrestrial environment where fine, particulate organic matter accumulates, and in this manner regulate the delivery of iodine nutrients and toxins alike from small catchments to larger river systems, lakes and estuaries.…”

“…Here, we use gamma spectroscopic measurements of 131 I contamination, in conjunction with measurements of 7 Be and 137 Cs, to develop a broad perspective of iodine transport and fate in a terrestrial ecosystem. Our observations of 131 I, summarized in Fig.…”