Retention and Transport of Hexavalent Chromium in Calcareous Karst Soils

Authors: İRFAN YOLCUBAL, NİHAT HAKAN AKYOL

Abstract: Hexavalent chromium, which is a highly toxic and soluble form of the element 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 into environment. The objective of this study was to determine transport and retention behaviour of aqueous Cr(VI) species in a highly calcareous and alkaline top soil of a karst system that is vulnerable to 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} of Cr(VI) initial concentration. K_d values obtained from both batch and column studies showed that Cr(VI) sorption was relatively low. The average K_d value obtained from batch studies (0.53 L Kg^{-1}) was about six times higher than the average K_d 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 also revealed that Cr(VI) oxyanions (dominantly CrO_4^{-2}, HcrO_4^-) were only temporarily retained by adsorption to easily exchangeable sorption sites in the soil and the sorption was reversible. Low retardation factors and K_d values obtained from column experiments suggest that Cr(VI) species are very mobile and bioavailable in highly calcareous soil conditions. This increases the spreading of contamination in such contaminated systems with Cr(VI) and contamination risk of groundwater resources. However, if the bioavailability is a key factor for remediating Cr(VI) contaminated soil, as in phytoremediation, increased availability of Cr(VI) for plant uptake in calcareous soil will enhance its removal.

Keywords: Hexavalent chromium, calcareous soil, sorption, desorption, solute transport, heavy metal, contamination, column experiment

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