Study On The Removal Of Cd(Ⅱ) From Aqueous Solutions By Bacillus Cereus RC-1: Biosorption Characteristics And Mechanism

Microbiological biosorption because of its security and economic advantages is one ofthe cadmium pollution restoration potential techniques, and it also causes widespread concernin the domestic and foreign. In this thesis, on the basis of the pollution evaluation of heavymetals for Dabaoshan mine water, suspended particles and sediment, the main object of thisstudy is research on the bisoroption characteristics and mechanism of the cadmium-resistantbacteria in water, which is screened by our team preliminary. The main conclusions are asfollows:1. The heavy metals pollution evaluation of mine tailing from Dabaoshan mine wascarried out. The tailings dam suffered severe water pollution of heavy metals by measuringthe water, suspended particles and sediments, such as Cu, Zn, Cd and Pb, especially Cd.Moreover, the maximum exceeded multiples of the standard reached6,20,35and3timesrespectively. Meanwhile, the Cd（II） concentration in water is not only have a negativecorrelation with pH, but also has a high correlation coefficient with the redox potential of thewater. In addition, based on the discussion of the pollution distribution of heavy metals insediments by using the methods of land accumulation index and potential ecological riskindex, it was found that the degree of pollution of heavy metals in sediments was very high,showing the combination contamination of heavy metals in sediments, such as Cu, Zn, Cd andPb. Furthermore, the order of pollution of heavy metals in sediments was Cu> Cd>Zn> Pb.2. The study on the biological characteristics and the degree of Cd-tolerance was carriedout with a certain cadmium-resistant bacteria existing in our research group. The strainbelongs to Bacillus cereus and its GenBank accession number is JF683581from the analysisof16S rDNA sequence homology and the comparison of NCBI database. The growth rate ofthis strain in liquid medium containing20mg/L Cd²⁺was quite well by comparison with thecontrol and the minimum inhibitory concentration of strain was200mg/L, which indicatedthat Bacillus cereus RC-1had a great potential application in the cadmium pollutionrestoration. In addition, it was found that the Cd（II）-resistance of this strain was insignificantand showed instability by applying the method of high-temperature and SDS plasmid elimination test, while the test of plasmid extraction and detection didn’t find any plasmidbands. Therefore, we presented a preliminary view of the strain Cd（II）-resistance gene may belocated on the chromosome.3. The biosorption characteristics for the growing cells of B.cereus RC-1wereinvestigated. The optimum conditions of the biosorption was pH=7.0, temperature=28±2℃and rotate speed=150rpm. As for the growing cells, the Freundlich isotherm model was betterin the two kinds of biosorption model, showing the biosorption process of B.cereus RC-1wasa heterogeneous surface. The dynamics simulations of growing cells showed that thefirst-order kinetics could simulate the initial stages of biosorption process and predicatedvalues were very close to the experimental data. Besides, the second-order kinetics had morecapable of simulating the entire biological biosorption process with higher correlationcoefficient.4. The comparative study of biosorption characteristics and mechanisms for the live anddead cells of this strain was carried out. For the both types of cells, the optimum biosorptionconditions was pH=5.0, biosorbent dose=1.5g/L and rotate speed=150rpm. Among the threekinds of biosorption model, such as Langmuir, Freundlich and Redlich-Peterson, all the threemodels were suitable for the simulation of the Cd（II） biosorption by live cells, whichsuggested the biosorption process was a heterogeneous surface, and some other biosorptionmechanisms such as ion exchange may be involved in the Cd（II） biosorpiton. As for the deadcells, Langmuir isotherm was the best model for simulating the biosorption process, and theRLvalue is far less than the live cells, suggesting that the dead cells were more favorable forthe Cd （II） biosorption. In addition, the bisorption capacity of dead cells reached the93%ofthe maximum biosorption capacity within30min, while it required approximately50min forthe live cells. Moreover, the second-order kinetic model was more suitable to describe thebiosorption process for both types of cells, and the biosorption was dominated by chemicalbiosorption process. Finally, it could be concluded that the biosorption capacity of dead cellswas higher than that of live cells, which was mainly due to the more biosorption sites on thedead cells surface displayed by heat treatment and the lower intracellular accumulation for the live cells by means of several analysis instruments, such as Zeta potential measurement, TEM,SEM-EDS, FTIR, the test of intracellular accumulation and desorption.5. The study on the biosorption mechanisms of the growing cells was carried out througha variety of analytical methods. Firstly, the biosorption process of the growing cells was aprocess of acid production for the initial biosorption, followed by alkaline production.Therefore, we presented that the growing cells have different biosorption mechanisms fordifferent growth periods, but the overall change in pH of the culture medium is not large. Atthe same time, the electrostatic adsorption played an important role in the biosorption forlower concentration Cd²⁺（≤20mg/L） and had different impact on the biosorption with differentgrowth periods. However, as for the higher concentration Cd²⁺（≥50mg/L）, the electrostaticadsorption is not significant, suggesting some other mechanisms involved in the removal ofCd（II）. Secondly, it had been concluded that the cell surface biosorpiton played a veryimportant role in the Cd（II） biosorption for the growing cells by the measurement ofSEM-EDS, TEM and AFM. Moreover, the RMS and Ra of the growing cells were increasedwith the increase of the Cd²⁺concentration. When the initial Cd²⁺concentration was greaterthan or equal to50mg/L, some cells were deformed or even cracked. At this time, the growingcells had resistance to the higher concentration of Cd²⁺by both extracellular biosorption andintracellular accumulation. Finally, the functional groups on the surface of the growing cells,such as-OH, C=O,-NH and-CH, were involved in the biosorption process, of which-OHand C=O were the preferred adsorption sites. Furthermore, the amides I, II and III bandspectrum peak changed significantly, which indicated that amide group of the protein playedan important role in the biosroption.6. The bisorption mechanisms of the growing cells were studied deeply. The growingcells have some different mechanisms in different growth period under different initial Cd²⁺concentrations. These results showed that intracellular accumulation was higher thanextracellular biosorption for the lower Cd²⁺concentration （<20mg/L）, while the extracellularbiosorption is far greater than the intracellular accumulation for the higher Cd²⁺concentration（>20mg/L）. In addition, the extracellular biosorption and intracellular accumulation have a great relationship with the growth periods through the test of accumulation for the growingcells. Moreover, the growing cells of logarithmic growth phase showed greater bindingcapacity for Cd（II） than the growing cells of stationary phase. Finally, when the liquidmedium contained DCC inhibiting ATP activities, both types of the growing cells not onlyimproved the removal efficiency, but also prevented intracellular Cd²⁺re-released into theenvironment, avoiding secondary contamination.