Simulation of in situ bioremediation of CR(VI) in groundwater aquifer environments using a microbial culture barrier

Abstract

The feasibility of in situ bioremediation of Cr(VI) in groundwater and aquifer media was investigated using microcosm and mesocosm reactors inoculated with indigenous species of bacteria from dry sludge. Microcosm cores were used to simulate contaminant movement in the vadose and aquifer zones of the aquifer system. Cr(VI) breakthrough analysis through the experimental cores demonstrated successful Cr(VI) immobilisation in simulated barrier systems. Cr(VI) reduction was continuously monitored and microbial culture dynamics were evaluated using 16S rRNA genomic fingerprinting. A culture shift was observed in the microcosm cores with the emerging predominance of known Cr(VI) reducers - Enterococci from soil and Lysinibacilli from sludge - after operation for 45 days. The Cr(VI) reduction process in the columns was determined to be enzyme mediated and non-competitively inhibited by Cr(VI). The microbial cultures under microaerobic conditions depicted a threshold Cr(VI) concentration (Cr) of approximately 100 mg/L which was much higher than the target operation concentration of 40 mg/L at the proposed remediation sites. Using the Computer Program for the Identification and Simulation of Aquatic Systems (Aquasim), it was possible to predict Cr(VI) removal efficiency and the impact of Cr(VI) toxicity on culture dynamics in the barrier. The study demonstrates the potential of applying selected Cr(VI) reducing bacteria in biological permeable reactive barrier systems in preventing the spread of the pollutant into adjacent water supply aquifers. The impact of the presence of natural carbon sources was also evaluated by filtering the feed water through a saw dust bed. Reactors without added carbon source removed up to 70% Cr(VI), and no removal was observed in sterile controls. In the packed mesocosm reactor, the areas before the reactive barrier had no chromium reduction whereas most of the areas after the barrier achieved near 100% reduction. The microbial dynamics were monitored by the 16S rRNA fingerprinting after exposure to Cr(VI). After operating the microcosm reactors under oxygen stressed conditions in the presence of other soil bacteria, a community shift was expected. The soil from inoculated reactors contained a wide range of soil dwelling species of bacteria as well as the newly introduced bacteria from the dried sludge. There was a noted presence of Cr(VI) reducing bacteria, Microbacterium, Acinetobacter, Arthrobacter, Brevibacterium, Rumen bacteria, and several Enterococci in the sludge culture and Arthrobacter spp., Clostridium spp., and Klebsella spp. were amongst the evident among identified species. A non-competitive inhibition model was used for the evaluation of aerobic performances in batch experimental studies, whereas the inhibition threshold term C0-Cr/C0, was introduced for the anaerobic model performance for the reduction of chromium in batch studies. In sterile packed soil columns a model for saturated soil column with dispersion was adopted from AQUASIM 2.0. This model was used in combination with the chromium reduction rate adopted from the anaerobic batch modelling for most non sterile reactors in the microcosm performance. The study demonstrates the potential of applying selected Cr(VI) reducing bacteria in biological permeable reactive barrier systems in restraining the spread of the pollutant into adjacent water supply aquifers. The outcome of this exercise could be useful in the formulation of biological permeable barriers for protection against the spread of the pollutant from hot spots in the area. This is serves as a significant step towards a pilot study.