Study On The Removal And Transformation Of Cu²⁺?Cd²⁺ And Cr⁶⁺ From Aqueous Solutions By Pseudomonas Sp.DC-B3:Adsorption Characteristics And Preliminary Machanism

The removal and repair of heavy metals in polluted environment has been one of the most important contents in the field of environmental protection.The bioremediation method has been paid more attention because of its low cost,environmentally friendliness and simple operation.Among them,the application of microorganisms for absorption and transformation of heavy metals is one of the feasible biological methods.In this study,a bacterium Pseudomonas sp.Resistant to copper,cadmium and chromium was screened from the soil of heavy metal-contaminated areas Dongchuan,Yunnan province,China.The strain named Pseudomonas sp.DC-B3,was used to adsorb Cu²⁺and Cd²⁺in water,and to reduce and adsorb Cr⁶⁺,and to the characteristics and mechanism of bioadsorption and transformation were investigated.Through 16s rRNA gene sequence analysis,this strain was found to belong to Pseudomonas sp.The minimum inhibitory concentrations of Cu²⁺,Cd²⁺and Cr⁶⁺were 60,45 and 90 mg/L respectively.As for the adsorption characteristics,the adsorption rate of strain for Cu²⁺and Cd²⁺increased with the increase of temperature and pH.While the highest reduction of hexavalent chromium was achieved at pH=2.The higher the initial heavy metal concentrations were,the greater the adsorption capacity and Cr?VI?reduction were,and the lower the removal rate was.With the increase of bacteria dosage,the removal rate of heavy metals and the reduction rate of Cr?VI?gradually increased,and the removal capacity decreased gradually.The adsorption of heavy metals by the strain was gradually increased to reach the equilibrium after rapid adsorption,while Cr?VI?reduction was relatively slow.The simultaneous adsorption of Cu²⁺,Cd²⁺and Cr⁶⁺by Pseudomonas sp.DC-B3 indicated that the strain was preferable to absorb Cu²⁺when three ions coexisted and the pH was not regulated?pH=5.2?.When pH was adjusted to 2,Pseudomonas sp.DC-B3 still showed a strong reduction ability to Cr?VI?,and there was a certain amount of total chromium adsorbed,and the coexisted ions did not affect its reduction effect.The adsorption of Cu²⁺and Cd²⁺by the bacteria was in accordance with the Freundlich adsorption isothermal equation and pseudo-second-order kinetic equation,while the adsorption Cr⁶⁺was more consistent with the isothermal equation of Langmuir adsorption and pseudo-second-order kinetic equation.The adsorption capacity of Cu²⁺,Cd²⁺and total chromium were obtained by the adsorption isotherm equation,which was 9.48mg/g,18.24mg/g and 4.0mg/g respectively,and was close to the experimental values.The morphological changes of the strain cells were observed after the metal adsorption by SEM.The adsorption effect of bacteria for Cu²⁺,Cd²⁺and Cr⁶⁺was verified by energy spectrum analysis.Fourier transform infrared spectroscopy?FTIR?analysis showed that hydroxyl group,carboxyl and amino groups played a role in the adsorption,but their contributions to the adsorption of different heavy metal ions were different.H₂O has almost no desorption effect for three metal ions,while the desorption rate of H₂SO₄ and EDTA for Cu²⁺and Cd²⁺were 91%,84%and 89%,77%,which suggested that complexation and ion exchange might play important roles in the adsorption process.NaOH has the strongest desorption ability for total Cr and Cr?VI?,and the desorption experiment showed that Cr?VI?and Cr?III?co-existed on the bacterial body,which confirmed the reduction-adsorption of Cr?VI?by the strain DC-B3.