Iodine and plutonium association with natural organic matter: A review of recent advances

Radionuclides reach the environment from natural or anthropogenic sources and are equilibrating over time with different phases through sorption and precipitation reactions onto inorganic phases and macromolecular natural organic matter (NOM). Strong binding to NOM can occur by chelation of clustered binding sites (i.e., binding sites from different branches in the macromolecule) in the absence of conventional chelating sites. Despite many years of research and strong evidence of its significance, transport of many radionuclides is still modeled without taking into consideration NOM as a redox regulator and a sorbent or chelating agent. Microbially mediated chelation and incorporation reactions can control a number of radionuclides, e.g., plutonium (Pu) and iodine (I) isotopes, leading to retardation or mobilization, depending on whether the carrier compound is in solution or particle-bound. The presence of NOM in contaminated soils complicates conventional remediation techniques for I, where base has been added to either increase the cation exchange capacity of soils or to promote direct co-precipitation of the cationic radionuclide in the waste stream. Even though Pu at waste sites did not have to be remediated, base addition would likely also bring surprises. This addition may then have unexpected consequences; while promoting the immobilization of inorganic Pu, it has been shown to also remobilize inorganic-I and low-molecular weight organic compounds that are bound to I and Pu. Iodine occurs in multiple oxidation states in aquatic systems, existing not only as inorganic species (iodide (I⁻) and iodate (IO₃⁻)), but also as organic species where I is covalently bound to aromatic moieties. Thus, stable iodine, ¹²⁷I, and its long-lived isotope, ¹²⁹I, a major by-product of nuclear fission, undergo complex biogeochemical cycling in the environment, which renders them less mobile than when assuming that all I is in the form of the highly mobile form of iodide. In the laboratory and the field, plutonium strongly associates with NOM, when present, and is strongly chelated by specific moieties such as hydroxamate siderophores and other N-containing compounds. As a consequence, its mobility is controlled by the transport behavior of the anionic organic forms rather than the much more strongly sorbing cationic form of Pu(IV). NOM, even at trace levels, can play a significant role in controlling the fate and transport of radionuclides. - Highlights: • Iodine in the environment occurs in multiple oxidation states in aquatic systems, existing not only as inorganic species (iodide (I⁻) and iodate (IO³⁻)), but also as organic species where I is covalently bound to aromatic moieties. • Thus, stable iodine, ¹²⁷I, and its longlived isotope, ¹²⁹I, a major by-product of nuclear fission, undergo complex biogeochemical cycling in the environment, which renders them less mobile than when assuming that all I is in the form of the highly mobile form of iodide. • Plutonium also strongly associates with natural organic matter whereby it is strongly chelated by specific moieties such as hydroxamate siderophores and other nitrogen containing compounds. • As a consequence, its mobility is controlled by the transport behavior of the anionic organic forms rather than the much more strongly sorbing cationic form of Pu(IV).