Iodate interactions with calcite: implications for natural attenuation

Solid-phase interactions and speciation are important to radioiodine transport in groundwater. At the Hanford Site in Southeastern Washington State, iodate (IO3−) is the main aqueous species in dilute radioiodine groundwater plumes. Like other oxyanions, IO3− may be incorporated into and/or adsorbed onto calcite, a common mineral at Hanford, decreasing its mobility in the environment. A series of macroscale batch experiments combined with solid-phase characterization were conducted to identify variables impacting time-dependent aqueous IO3− removal via calcite precipitation and determine the location of IO3− within the calcite crystal structure. Results demonstrated 11.5–97% aqueous IO3− removal during initial rapid calcite precipitation. Incorporation was apparently the main removal mechanism, although later slower precipitation and/or adsorption may have also contributed to IO3− removal. Using a higher concentration of the calcite-forming solutions (i.e., using 1 M vs. 0.1 M concentrations) resulted in an increase in the amount of precipitated calcite and a greater percentage of IO3− removed; however, calcite formed with lower molarity solutions resulted in higher IO3− mass (µg/g) removal. Solubility testing of laboratory-produced calcites showed only small differences in solubility for calcite with and without IO3− incorporated into the structure. Evidence collected from SEM/FIB and TEM/SAED suggested that the IO3− incorporated into calcite was present in regions close to the surface (implying potential easy release upon calcite dissolution).