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.
