Silicon concentration and pH controls over competitive or simultaneous incorporation of iodate and chromate into calcium carbonate phases

Abstract

Co-precipitation of iodate and chromate with calcium carbonate (CaCO3) minerals is a potential approach that can significantly decrease their mobility in the subsurface. There is a need to identify the impact of pH and elements of natural abundance such as Si on co-precipitation process to understand potential for natural attenuation or targeted removal via injection of CaCO3 forming solutions in the subsurface. Batch experiments were combined with solid phase characterization studies to examine the potential for co-precipitation/incorporation of iodate and chromate into CaCO3 minerals. These experiments considered uptake during and after precipitation of CaCO3 phases and measured the release of contaminants during dissolution. Experiments conducted at variable pH (6.5, 8.0 and 9.0) and Si concentrations (0 mM, 0.5 mM and 20 mM) showed that the incorporation of iodate was pH dependent and was favored at lower pH and higher Si concentration within 7 days. The presence of chromate had little effect on the removal of iodate except at pH 6.5 where iodate uptake increased 6–10% due to the formation of dietzeite, Ca2H2O(IO3)2(CrO4). More aqueous iodate was removed in all experiments compared to chromate except some of the investigated conditions (Si 20 mM, pH 9) where the removal of CrO42− was found similar to IO3−. The release of iodate and chromate from dissolving CaCO3 was also pH and Si concentration dependent. The I fraction released from CaCO3 doped with IO3− was 1.5–2.3%, which reached a steady state after 24h; however, 2.6–4.6% of I was released from CaCO3 doped with IO3− and CrO42− for all tested conditions. More chromate (up to 36.5 ± 4.6%) than iodate (3.2 ± 0.07%) was released during dissolution at pH 6.5 and 20 mM Si. Although less uptake of Cr occurred, it may have led to increased release of I when both were co-precipitated simultaneously.