Chromate Effect on Iodate Incorporation into Calcite

Incorporation of iodate into calcite (CaCO3) may be used as an in situ treatment strategy for radioiodine in contaminated soils and groundwater, but the presence of other contaminants may inhibit its efficiency. To this end, the potential for chromate to interfere with iodate incorporation into CaCO3 was investigated as an example of how co-located contaminants may impact in situ remediation efficacy. Here, batch precipitation experiments were periodically sub-sampled over 21 days to determine the kinetic effects of chromate on iodate removal and incorporation into calcite. From these experiments, a decrease in iodate removal from >60 to <40% was observed upon chromate addition (1-112 ppm of chromate) and <= 11% of chromate was removed with a minor dependence upon the initial chromate concentration. Analysis of the solid phase using extended X-ray absorption fine structure (EXAFS) spectroscopy informed by ab initio molecular dynamics simulations revealed that the iodate incorporation mode remains unchanged by the presence of chromate. Iodate readily substitutes for carbonate (CO32-), and the calcite structure is charge-balanced primarily by substituting H+ for Ca2+. Furthermore, chromate incorporated as a nearest neighbor to iodate did not contribute to the EXAFS fit; therefore, iodate and chromate clustering is unlikely when co-incorporated into calcite.