Efficiency of remobilisation of chromium precipitates in an aquifer permeable reactive barrier

Abstract

Chromium exists largely in two oxidation states, namely Cr(VI) which is carcinogenic and mutagenic to living organisms including humans, and Cr(III) which is about 1,000 times less toxic than Cr(VI). It is therefore desirable in most cases to reduce Cr(VI) to Cr(III) as the first step towards complete treatment of Cr(VI). 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 systems offers an attractive option 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. The precipitate could be washed using a dilute acid solution. However, 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 with dilute acid was evaluated using the 16S rRNA genotype fingerprinting. The microbial barrier was initially inoculated with indigenous bacterial species from dried sludge. The results showed the presence of well-known Cr(VI) reducers such as Bacillus mycoides, Lysinibacillus fusiformis and Micrococcus lylae 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 microcosm 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 microcosm 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 during electrokinetic and soil washing remediation. However, one of the limitations of electrokinetic remediation is the near anode focusing effect whereby a layer of precipitate is formed around the anode that leads to the reduction in mobility of the species through the aquifer medium.