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 and mutagenic to living organisms including humans, and tetravalent chromium (Cr(III)) which is known to be 1,000 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. Among the suggested treatment technologies, in situ bioremediation using permeable reactive barrier as seen as the most attractive option because it can be implemented with the least impact to the environment. In this study, we evaluated the short-term operation of a Cr(VI) reducing barrier 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) produced 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 M HCl). Lowering the pH of these introduced harsh conditions which necessitated the evaluation of a possible culture shift during the regeneration phase. Microbial culture composition was evaluated using a 16S RNA finger printing method. The microbial barrier was initially inoculated with indigenous bacterial species from dried sludge. The possible microbial culture community shift in the system was evaluated using 16S rRNA fingerprinting of colonies observed from samples collected after operating the system for seven days. Phylogenic results confirmed that, after the microbial barrier system operation for seven days, the well-known Cr(VI) reducers Pseudomonas plecoglossiccida, Acinetobacter haemolyticus and Comamonas testosterone remained predominant in the culture community. The microbial barrier system successfully immobilized Cr(VI) at feed concentrations as high as 50 mg/l Cr(VI). Continuous operation of a barrier based on this technology will depend on a development of a pumping system which will ensure successful remobilisation of the metal precipitate for recovery during the regeneration process.