Retention and Transport of Hexavalent Chromium in Calcareous Karst Soils

Hexavalent chromium, which is a highly toxic and soluble form of the chromium, is a common soil and groundwater contaminant found in many contaminated sites throughout the world and poses a serious risk to human health when released to environment. The objective of this study was to determine transport and retention behavior of aqueous Cr(VI) species in a highly calcareous and alkaline top soil of a karst system that is vulnerable to groundwater contamination. Batch sorption studies and column experiments conducted at both saturated and unsaturated steady-state flow conditions were used to evaluate the sorption/desorption and the transport of Cr(VI) in the alkaline soil conditions. The results of batch experiments indicated that Cr(VI) sorption was linear up to 200 mg L-1 Cr(VI) initial concentration. Kd values obtained from both batch and column studies showed that Cr(VI) sorption was relatively low. Kd value obtained from batch studies (0.4263 L Kg-1) was about five times higher than the average Kd value (0.09 L Kg-1) obtained from column studies. The results of the column experiments showed that transport of Cr(VI) was slightly retarded compared to non-reactive tracer at both saturated and unsaturated flow conditions. Extraction studies conducted on the soil columns revealed that Cr(VI) oxyanions (dominantly CrO4-2, HCrO4-) were only temporarily retained by adsorption to easily exchangeable sorption sites in the soil and the sorption was reversible. In addition, both geochemical modeling and extraction studies indicated that mineral precipitation and redox processes did not have any effect on the fate of Cr(VI) in the highly calcareous soil. Cr(VI) transport under variably saturated conditions was also modeled using both deterministic equilibrium and two-region physical nonequilibrium 1-D transport models coupling linear sorption. The models used in simulating Cr(VI) breakthrough curves generally provided a good match to the measured data (r-squared 0.97- 0.99), suggesting that the processes included in the models reasonably well represented the transport of Cr(VI) in the calcareous soil. Low retardation factors and Kd values obtained from column experiments suggest that Cr(VI) species are very mobile and bioavailable in highly calcareous soil conditions. High mobility of Cr(VI) in calcareous soils increases the rate of contaminant spreading and the potential risk of groundwater contamination. However, increased bioavailability of Cr(VI) for plant uptake in such soils will enhance removal of Cr(VI).