Bioreduction And Biosorption Of Cr (Ⅵ) By A Chromium-Resistant Bacterium Isolated From A Sewage Treatment Plant

Bioreduction of toxic hexavalent chromium to less toxic and less mobile trivalent form is considered to be an alternative and a viable clean-up treatment method for chromium remediation as the knowledge and control of microbial activities involved lately significant advanced, compared to the conventional physicochemical treatment. Large volumes of metal-contaminated water can be economically purified. Cr (Ⅵ) biotransformation also produces Cr (Ⅲ)-organic complexes with insoluble Cr (Ⅲ) compounds. It was reported that once Cr (Ⅲ)-organic complexes are formed, they are relatively stable and recalcitrant in short-term biodegradation. In this way, Cr (Ⅲ)-organic compounds are known as stable chromium species in the environment and taking part in natural biogeochemical cycle of the chromium. The mutagenicity and carcinogenicity effects of Cr (Ⅵ) in plants and animals including humans have been proven by several scientific papers. So, detoxification of our ecosystem polluted with this metal is a challenge for the whole scientific community.In other hand, biosorption commonly refers to the passive binding of metal ions or radioactive elements by dead or metabolically inactive biomass. It has to be distinguished from bioaccumulation which is usually understood to be an active, metabolically mediated metal-accumulation process occurring specifically in living organisms. Biosorption removal of toxic heavy metals is especially suited as a polishing wastewater treatment step because it can produce close to drinking water quality. Removal of toxic heavy metals by biosorption phenomenon is also considered as a cost-effective option compared to the traditional treatment such as ion exchange.The goal of the present study was to assess the potentiality of bioreduction and biosorption phenomena in hexavalent chromium remediation by a newly isolated Bacillus cereus in aqueous solution.To fulfill the goal of the current study a series of experiments were carried out such as DNA isolation and PCR purification for the identification of the bacterium as well as potent biochemical tests, the determination of minimal inhibitory concentration of Cr (VI) as potassium dichromate (K2Cr2O7) for the current strain, the effect of Cr (VI) concentration on the cell growth, the effect of various environmental factors such as temperature and pH on the cell growth and Cr (VI) reduction as well as the kinetic of Cr (VI) reduction. Diphenylcarbazide methods (DPC) were used to determine the remaining Cr (VI) concentration in the solution measuring the absorbance at540nm with UV/VIS Spectrophotometer. Additionally, the permeabilized assays as well as the sub-cellular fractionation of the cell for the localization of the chromate reductase activity were performed. Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-ray analysis were also performed to measure the potentiality of biosorption by the current biomass.It can be clearly observed that our newly isolated strain was closely related to Bacillus cereus, namely Pf-1and can effectively reduce toxic and soluble Cr (VI) to the less toxic and insoluble Cr (III). The optimum performance of Cr (VI) reduction was observed at pH7.0±0.3and temperature30±2℃. The strain can tolerate60mg Cr (VI)/L in nutrient broth agar and was able to reduce65%of Cr (VI) at the concentration of10mg/L within24h incubation time. Moreover, addition of1%glucose as external electron donor significantly enhances Cr (VI) reduction. Even though the reduction rate gradually decreased as the initial Cr (VI) concentration increased, the reduction capacity increased with Cr (Ⅵ) concentrations. At the same time, the average volumetric rate of Cr (Ⅵ) reduction increased as the initial Cr (Ⅵ) concentration until it reaches the maximum of approximately2.58mg h-1. Additionally, it was also observed that the Cr (Ⅵ) reductase generated by this strain of Bacillus cereus was primarily intracellular. Meanwhile, the cell permeabilization with Toluene and Triton X-100at different concentrations statistically increased Cr (Ⅵ) reduction compared to the untreated or resting cells. The biosorption process was found to be highly pH dependent and the optimum pH was observed at2±0.3. The EDX images revealed the presence of different elements in the surface of the native and loaded biomass, and the amount of these elements was drastically changed in the loaded biomass. Moreover, the Cr peak was observed only in the EDX image when the biomass was exposed to Cr (Ⅵ). Additionally, the FT-IR spectrum of the native biomass showed the presence of characteristic bacterial signatures as expected and a significant shift in frequency to lower range was observed in the metal loaded biomass. Both Langmuir and Freundlich models fitted our experimental data. However, the pseudo-second order kinetic model was better described our sorption system compared to the pseudo-first order kinetic model.Based on the results obtained in the present dissertation, it can be concluded that Bacillus cereus strain Pf-1can remediate Cr (Ⅵ) pollution. Furthermore, the development of biosorption and bioreduction may provide a basic for a technology aimed at the removal of heavy metal species, particularly for hexavalent chromium from industrial effluents.