Kinetic studies of Cr(VI) reduction in an indigenous mixed culture of bacteria in the presence of As(III)

Abstract

An indigenous mixed culture of bacteria collected from a Wastewater Treatment Plant (Brits, North West Province, South Africa), biocatalytically reduced Cr(VI) in the presence of As(III). Both the reduced chromium (Cr(III)) and the oxidised arsenic (As(V)) readily form amorphous hydroxides that can be easily separated or precipitated from the aqueous phase as part of the treatment process. Treatment of Cr(VI) and As(III) before disposal of wastewater is critical since both compounds are known to be carcinogenic and mutagenic at very low concentrations, and acutely toxic at high concentrations. Batch experiments were conducted to evaluate the rate of Cr(VI) reduction under anaerobic condition in the presence of its co-contaminant As(III) typically found in the groundwater and mining effluent. Results showed near complete Cr(VI) reduction under initial Cr(VI) concentrations up to 70 mg/L in a batch amended with 20 mg/L As(III). However, increasing Cr(VI) concentrations up to 100 mg/L resulted in the inhibition of Cr(VI) reduction activity. Further investigation was conducted in a batch reactor amended with 70 mg/L Cr(VI) concentration at different As(III) concentrations ranging from 5-70 mg/L to evaluate the effect of varying As(III) concentration on Cr(VI) reduction efficiency. Results showed that Cr(VI) reduction efficiency increased as As(III) concentrations increased from 5-40 mg/L. However, further increase in As(III) concentration up to 50 mg/L resulted in incomplete Cr(VI) reduction and decrease in Cr(VI) reduction efficiency. These results suggest that the rate of Cr(VI) reduction depends on the redox reaction of As(III) and As(V) with Cr(VI). Moreover, the inhibitory effect observed at high Cr(VI) and As(III) concentration may also be attributed to the dual toxicity effect of Cr(VI) and As(III) on microbial cell. From the above batch kinetic studies lethal concentration of Cr(VI) and As(III) for these strains was evaluated and established. Initial evaluation of the bacteria using 16S rRNA partial sequence method showed that cells in the mixed culture comprised predominantly of the Gram-positive species: Staphylococcus sp., Enterobacter sp., and Bacillus sp. The biokinetic parameters of these strains were estimated using a non-competitive inhibition model with a computer programme for simulation of the Aquatic System “AQUASIM 2.0”. Microbial reduction of Cr(VI) in the presence of As(III) was further investigated in continuous-flow bioreactors (biofilm reactor) under varying Cr(VI) loading rates. The reactor achieved Cr(VI) removal efficiency of more than 96 % in the first three phases of continuous operation at lower Cr(VI) concentration ranging from 20-50 mg/L. However, 20 % decrease in Cr(VI) removal efficiency was observed as Cr(VI) concentration increase up to 100 mg/L. The reactor was able to recover from Cr(VI) and As(III) overloading phase after establishing the resilient nature of the microorganism. Similarly to the batch reactor studies the overall performance of the reactor also demonstrated that the presence of As(III) greatly enhance Cr(VI) reduction in a bioreactor. This was evident by near complete removal of Cr(VI) concentration up to 50 mg/L. The basic mass balance expressions on Cr(VI) along with the non-competitive inhibition model were used to estimate the biokinetic parameters in the continuous flow bioreactor system. Cr(VI) reduction efficiency along the longitudinal column was also evaluated in this study. Results showed that Cr(VI) efficiency increased as Cr(VI) concentration travels along the longitudinal column. Other important factors such as oxygen and pH during biological Cr(VI) reduction in the presence of As(III) oxidation were also evaluated.