|The removal of toxic and heavy metal contaminants from aqueous waste streams and industrial effluents is one of the most important environmental issues being faced the world over. In order to combat this problem, the commonly used procedures for removing metal ions from dilute aqueous streams include chemical precipitation, ion exchange, reverse osmosis and solvent extraction. However, these techniques have certain disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal. The hazardous wastes generated from metal mining and smelting operations also need to be decontaminated before entering the ecosystem. Chromium contamination of soil and ground water is a significant problem worldwide. The extensive distribution of this pollutant is due to its numerous industrial applications such as metal plating, alloying, leather tanning and wood industry. Cr (VI) is toxic and carcinogenic in nature while Cr (III) is innocuous. Conventional chromium removal techniques involve reduction to the Cr (III) form and subsequent precipitation as its hydroxide. However, disposal of the solid sludge remains a problem. The search for alternative and innovative treatment techniques has focussed attention on the metal uptake capacities of various microorganisms such as yeast, algae, fungi and bacteria. It is well documented that microbial biomass is capable of adsorbing metal ions from aqueous solution even when the cells have been killed.
In the present investigation, the potential of utilising a gram positive, neutrophilic, facultative anaerobe like Bacillus polymyxa, in the bioremoval of Cr (VI) and Cr (III), has been assessed under different conditions. The growth of Bacillus polymyxa has been studied in the presence of varying concentrations of chromium ions. Subsequently, adaptation of the bacteria to Cr (VI) and Cr (III) has also been carried out. The biological reduction of Cr (VI) and its biosorption have been monitored during the growth of the unadapted and 2 ppm Cr (VI) adapted strains. The bioremoval of Cr (VI) and Cr (III) has also been assessed using the metabolic products obtained during bacterial growth. Detailed investigations have been carried out to determine the bioremoval efficiencies of both living and non-living cells of Bacillus polymyxa, with respect to Cr (III) and Cr (VI). The various parameters influencing the bioremoval of chromium by the cells, such as time, pH, wet biomass loading and initial metal concentration, have been studied. Electrokinetic studies on the bacterial cells, before and after interaction with Cr (VI) and Cr(III)have been carried out. The morphological changes induced in the bacterial strains consequent to interaction with Cr (III) and Cr (VI) have been examined by scanning electron microscopy.
The results of the present investigation revealed that bioreduction of Cr (VI) was feasible during the growth of both adapted and unadapted bacteria. The time taken for 90% bioremoval was 72 h in the case of the unadapted strain, whereas with the adapted strain only around 48 h were required to achieve comparable results. The metabolic products obtained by enzymatic bacterial action were also found to be efficient in bringing about the bioremoval of Cr (VI). The bioremoval efficiency was marginally better when a lower concentration of Cr (VI) was used. Over 80% bioremoval was achieved in about 10 h using 2 ppm Cr (VI) while almost 48 h were necessary for a similar amount of removal to be effected using 5 ppm Cr (VI). In the case of the metabolite obtained from the adapted strain, complete removal of 2 ppm Cr (VI) was possible in 24 h. The living cells of Bacillus polymyxa were not only able to accumulate Cr (VI) but were also capable of bioreduction to the Cr (III) form, when the pH was in the range of 1.5 to 4. The maximum bioremoval of about 75% of Cr (VI) was observed at pH 2, with 45% being attributed to bioreduction, with an equilibration time of 48 h. In the case of Cr (III) nearly 90% uptake could be achieved at a natural pH of 5.5, equilibration time of 24 h and using 1 g of wet biomass. Biosorption was the only method of removal present in the non-living system. In the case of nonliving biomass, the optimum conditions for maximum Cr (VI) removal (65%) were pH 2, equilibration time of 12 h and a biomass loading of 1 g, whereas for Cr (III), the maximum uptake of about 97% occurred at an initial pH of 5, equilibration time of 12 h and 0.4 g wet biomass. The non-living cells showed a better efficiency in removing Cr (III), while the living cells exhibited a greater tendency towards the bioremoval of Cr (VI) than the non-living ones.
Electrokinetic measurements revealed that consequent to interaction with Cr (VI) or Cr (III), significant surface modification was brought about on the cells of Bacillus polymyxa. Further, the isoelectric point was found to be shifted towards less acidic values after interaction with Cr (III) or Cr (VI). The probable mechanisms of the bioremoval processes are highlighted.