| The heavy metal ions can not be degraded by bacteria and other microorganisms like organic pollutants, and usually get into the food-chain directly threatening human health when entering the environment. Recently it has been found that some natural microorganisms have adsorption effect on heavy metals, thus removing heavy metals efficiently by selected microorganisms. However, the adsorption has some disadvantages, such as death of active microorganism due to the toxicity of heavy metal, shortened lifespan of non-active microorganism and second pollution caused by expansion or dissociation, a long domestication period, and difficulty in application to abrupt accidents. So immobilized technique has been adopted to eliminate the defects of microorganism adsorption, which opens up broad prospects for treatment of heavy metal wastewater of low concentration.In present thesis, immobilization materials and methods were applied to the adsorption of low concentration Pb2+ and Cd2+ by bacteria. Firstly, dominant bacteria with good adsorption capacity to low concentration Pb2+ and Cd2+ were screened from the active sludge of sewage treatment plant, and were identified as Gram positive bacteria.Then when PVA and sea alginic acid sodium used as carrier materials;charcoal, diatomaceous earth and silicon dioxide used as sorbent; hexamethylene diamine and glutaraldehyde used as crosslinker,the immobilized bacteria small ball can be made.The experiment of adsorption-desorption to Pb2+ and Cd2+ were done.By examining the capability of adsorption heavy metal, combining remarkable influence factors, such as the mechanical strength, the mass transfer and acid resistance etc., the optimized immobilization parameters were achieved by the orthogonal experiment. The result is that when mass fraction of PVA, sea alginic acid sodium, charcoal, diatomaceous earth, bacteria, silicon dioxide is 3% and calcium chloride in the saturated boric acid were 10%,0.5%,1.5%,1%,0.8%,3% and 4% respectively, time of the rubber association were 24h, and the concentration of hexamethylene diamine were 0.03mol/L, the adsorption capacity to Pb2+ and Cd2+ would get to 4.7440μmol/g and 4.1696μmol/g and the removal rate of Pb2+ and Cd2+ achieved 54.00% and 30.10%. Meanwhile, the adsorption capacity, the mechanical strength, the mass transfer and the acid resistance were moderate.The adsorption regularity was studied from the aspects, namely heavy metals initial concentration, adsorption time, initial pH value, temperature, initial NaNO3 ionic concentration and coexistence ion.The results indicated that the adsorption capacity of immobilized bacteria all gradually increased with the increase of Pb2+ and Cd2+ initial concentration and the increase of adsorption time from 1 min to 240 min; the adsorption equilibrium was basically achieved at 240 min; The adsorption capacity of Pb2+ achieved maximum value when the pH value was 6.0, while the adsorption capacity of Cd2+ achieved maximum value when pH=7.0. In the experiment, temperature ranged from 5℃to 40℃and didn't show apparent influence on the adsorption capacity of Pb2+ and Cd2+; when NaNO3 ionic concentration ranged from 1μmol/L to1000μmol/L, the NaNO3 ionic concentration didn't show apparent influence on the adsorption capacity of Pb2+ and Cd2+, and the best adsorption condition is 1μmol/L. Because of the competitive ability of Pb2+ bigger than Cd2+, the existence of Pb2+ would have more tremendous influence on the immobilized bacteria adsorption ability of Cd2+, while the influence of Cd2+would be smaller on that of Pb2+.In order to better describe the immobilized bacteria small ball activity, repeated adsorption experiment were conducted. The result indicated that immobilized bacteria had strong repeated adsorption ability to Pb2+ and Cd2+; with adsorption time passing, the repeated adsorption ability of immobilized bacteria to Pb2+ and Cd2+ gradually enlarges,but not the longer the better.The thermodynamic study indicates that both Langmuir and Frendlich equation can describe the equilibrium of immobilized bacteria thermodynamic adsorption to Pb2+ and Cd2+,of which Langmuir equation has the best fitted effect. The dynamic experiment indicates that Elovich model and the rate model with two constan both can describe the procedure of immobilized bacteria dynamic adsorption to Pb2+ and Cd2+.The desorption regularity was also studied based on the above-noted adsorption study. The result indicates that in different adsorption condition, the desorption rate of Cd2+ after adsorption is higher than that of Pb2+.No matter that when two kinds of heavy metal were coexisted, the desorption rate were all descended,but the effct of Pb2+ is remarkably not better than Cd2+.
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