Remobilization of trivalent chromium and the regeneration of in situ permeable reactive barriers during operation

Abstract

Chromium exists largely in two oxidation states, namely hexavalent chromium (Cr(VI)) which is carcinogenic, mutagenic to living organisms including humans and trivalent chromium (Cr(III)) which is known to be 1000 times less toxic than Cr(VI). It is therefore desirable in most cases to reduce Cr(VI) to Cr(III). Various studies have been conducted on the Cr(VI) reduction process either in situ or ex situ. However in situ bioremediation using permeable reactive barrier system appears as a potential and attractive technology compared to other in situ technologies. This study was conducted to evaluate the reduction of Cr(VI) to Cr(III) in the short term and regeneration of the biological reactive barrier to achieve continuous long term operation. It was observed from the study that the chromium hydroxide Cr(OH)3(s) precipitated and thus affected the porosity and hydraulic conductivity of the barrier system. It was therefore proposed to implement a regeneration process involving remobilization of precipitated Cr(OH)3 using a dilute acid (0.1% HCl) and recover Cr(III) by electrokinetics. Lowering the pH in the reactor introduced harsh conditions which necessitated the evaluation of a possible culture shift during the regeneration phase. Microbial culture composition during bioremediation and after soil washing was evaluated using a 16S rRNA finger printing method. The microbial barrier was initially inoculated with indigenous bacterial species from dried sludge. The results presented in the phylogenic tree diagrams confirm that, after microbial barrier system operation, the well-known Cr(VI) reducers Bacillus mycoides, Lysinibacillus fusiformis and Micrococcus lylae were the predominant species in the microbial community of the barrier. The microbial barrier system successfully achieved near complete removal of Cr(VI), whereby approximately 75% Cr(VI) removal was achieved within 63 days of operation. The formation of Cr(OH)3(s) was observed in the second week of operation. After 4 weeks of operating the mesocosm under soil washing with 0.1% HCl and electrokinetics remediation with a DC voltage of 50-150 V an increase in total chromium (73%) was observed suggesting that the trapped chromium species in the mesocosm was effectively remobilized with the assumption that Cr(III) had attached to the cathode forming a white-yellow precipitate layer around the cathode. Additionally more than 95% Cr(VI) was transformed to lower toxicity Cr(III) during electrokinetics and soil washing remediation. However, one of the limitations of electrokinetics is near anode focusing effect whereby a layer of precipitate is formed around the anode that lead to the reduction of efficiency of the technology.