CALCITE AS A HOST FOR CHROMATE AND IODATE OXYANIONS

Two contaminants of concern in the Hanford site located in southeastern Washington are chromate and iodate. A conceptual model was developed and revealed research gaps related to 1) evaluating the persistent release of chromate from some portions of the aquifer and 2) better understanding the fate and transport of iodate and chromate. One phenomenon being examined as part of this effort is the potential of these oxyanions to be incorporated into the calcite structure during calcite formation. This incorporation may be a potentially significant attenuation mechanism during transport of these oxyanions. However, dissolution of existing chromate-bearing calcite precipitates near former source areas may be a continuing secondary source to groundwater.

A series of batch experiments were conducted using different methods [i.e., variations of mixing CaCl₂ and either (NH₄)₂CO₃ or NaCO₃] to form calcite in the presence of chromate or iodate. Variables included the concentration of the calcite forming solutions (0.1M or 1M), chromate and iodate concentrations (0 ppb to 500 ppb), timing of contaminant addition (during or after calcite formation) and pH. Results of these batch experiments have shown incorporation of these contaminants in calcite may be occurring at the Hanford site. In the chromate tests, post-experiment XRD analysis determined that the presence of chromate delayed the transformation of vaterite to calcite; the vaterite structure may be a better fit for incorporation of chromate oxyanions. In the iodate experiments, the iodate removed from solution was correlated with the initial iodate concentration and the molarity of the starting solutions. Both experiments show incorporation of these contaminants in calcite, despite the unsuited match between the structure of the oxyanions and the calcite structure. These results will help to better characterize, conceptualize, and ultimately remediate the contaminated subsurface at the Hanford site.