The aim of this study was to investigate the reduction of oxidised metallic (CrO42-, VO3-, MoO42-) and non-metallic anions (ClO3-) by selected bacteria. Anaerobic cultures were incubated at 28°C for up to 96 hrs in a glucose minimal medium or meat broth containing 0 to 1000 mg/l oxyanion. Medium pH and Eh were monitored and oxyanion reduction followed throughout culture growth. It was found that a number of aerobic and facultatively anaerobic bacteria e.g. Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli and Clostridium lituseburense, were able to reduce VO3-, MoO42- at concentrations between 100 and 1000 mg/l. Over a period of 24 hrs growth 70 - 90% of the anions were reduced to V(IV) and Mo(V). The reduction of these oxyanions correlated with bacterial activity. The metals remained in soluble form for the first 18 to 48 hrs of growth but during the next 24 hrs growth insoluble metal precipitates, probably Mo(V) oxide and V(IV) oxide, formed. The reduction of the anions was linked to a decrease in Eh from -120 to -380 mV. The pH of the cultures decreased from 6.8 to 7.5. The ability of Aeromonas dechromatica KC-11 (a newly isolated species) to reduce CrO42- and other oxyanions was also studied. The Cr(VI) (initial concentration 35 mg/l) content of the medium began to decline within 1-2 hrs of inoculation and was complete after 20 to 24 hrs. Optimum reduction occurred between pH 6.0 and 7.8 with pH remaining constant throughout the incubation. Outside this pH range Cr(VI) reduction was 1.3-2.5 times slower. The rate of Cr(VI) reduction was higher at low Eh. During culture growth Eh declined with increasing Cr(VI) reduction. A.dechromatica KC-11 was also capable of reducing ClO3-, NO3- and SO42-. Again the Eh and pH of the medium were the main factors determining the rate of reduction. During the reduction of ClO3- and NO3- the Eh decreased from +300 to -100 mV while during SO42- reduction Eh decreased from 0 to -400 mV. A.dechromatica KC-11 reduced chromate, nitrate and chlorate in the first 1-2 hrs of growth. A period of adaptation to SO42- appeared necessary since reduction only started after 10-15 hrs growth. The results suggest that the oxyanions are reduced through a dissimilative process with the anions acting as a terminal electron acceptor in anaerobic respiration. It appears that a variety of facultatively anaerobic heterotrophic bacteria are capable of reducing oxyanions such as CrO42-, VO3-, MoO42-, and ClO3-, with an efficiency that may make them suitable for utilization in the removal of such anions from waste water.
ASJC Scopus subject areas
- Geotechnical Engineering and Engineering Geology
- Industrial and Manufacturing Engineering