| Environmental cadmium pollution is of increasing concern. Application of microorganisms in treatment of cadmium pollution has great prospects, in particular using of cadmium resistance strains for removal of cadmium in sewage and sludge, which is more advantage than traditional physical and chemical treatment methods. There have been a number of studies for cadmium resistance strains which were screened from land-based source of water, contaminated soil, plant roots. However, there have been only a few reports on cadmium resistance strains screened from deep sea sludge. In this study, strains with high ability of cadmium resistance and effective cadmium removal were screened and characteristics and mechanisms of cadmium removal by selected strain were studied.Sixty-one, twenty and three strains exhibited cadmium resistance100.200and250mg/L. respectively, were screened out of193marine bacteria. Their Cd(Ⅱ) removal ability in growth medium containing50mg/L Cd(Ⅱ) were examined. The results showed that there was no correlation between Cd(Ⅱ) resistance and removal ability. After repeated screening, one strain with the most stable and highest Cd(Ⅱ) removal of94.3%at15℃of100mg/L Cd(Ⅱ) resistance level was selected for further research. It was identified as Pseudoalteromonas sp. SCSE709-6based on the16S rRNA gene sequence analysis. Marine bacteria with high Cd(Ⅱ) resistance and low removal could protect cells by forming a barrier by secreting more soluble, small molecules extracellular carbohydrate. Marine bacteria Pseudoalleromonas sp. SCSE709-6could remove Cd(Ⅱ) by functional biosorption of COO-,-NH2,-OH and there was no observed sulfide precipitation or alkali precipitation in cadmium removal.The bioremoval of cadmium by Pseudoalteromonas sp. SCSE709-6was studied under different values of cadmium concentration, temperature, initial pH and salinity. The results showed that when cadmium concentrations ranged from0to50mg/L, incubation temperature15℃～30℃, initial pH7～8, culture salinity2%～5%, the removal efficiency all can reach up to90%. The excellent capability of cadmium removal was expected to solve the application restrictions of other growing cells sensitivity of high metal and salinity concentration, extremes of pH and temperature. In addition, the patterns of cellular distribution of cadmium under various cadmium concentrations, the competitive removal of zinc, cadmium and copper were studied to explore removal mechanism. The results showed that with the cadmium concentration increasing, the intracellular accumulation decreased and the external adsorption increased gradually. The competitive removal between cadmium and zinc was related to the ration of cadmium to zinc in the growth medium. The strains preferentially removed cadmium at the molar ratio of1:1, otherwise removed the more molar one. The strain preferentially removed cadmium in the binary system of cadmium and copper.Bacterialcultures including the viable cells and macromolecules of polysaccharide or protein with the ability of adsorbed and chelate or precipitate cadmium ions respectively can be effective to remove cadmium ions from wastewater theoretically. Therefore, the stationary phase bacterialcultures of cadmium resistant strain Pseudoalteromonas sp. SCSE709-6was used as adsorbents to remove cadmium ions. Effects of pH, salinity, adsorption time were studied by batch experiments. The results showed that the maximum cadmium removal was occurred at pH of6, and salinity of1%, and the adsorption equilibrium was reached at20h. Effects of nutrient substance content on removal capacity per unit mass and removal capacity per unit cost were also studied. Results showed that both of the removal capacity achieved the maximal at the nutrient content of1%of peptone and0.5%of yeast extract. The thermodynamic isotherm of cadmium adsorption on bacterialcultures could be best described by the Langmuir model, with a regression coefficient of0.999and a theoretical maximum adsorption capacity of101.52mg/g. The pseudo-second-order model well fitted the kinetic process of cadmium removal by bacterialcultures. |
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