Biosorption Efficiency And Mechanism Of Cr（Ⅵ） In Aquous Solution By CTAB/Fe₃O₄-Aspergillus Niger Spores

Low concentration chromium plating wastewater from industrial parks is usually treated by reduction precipitation method,which is difficult to meet the discharge standard due to its limitation by solubility product constant.However,it is difficult to meet the discharge standard because it is limited by the solubility product constant.In contrast,adsorption method is more suitable for treating low concentration chromium plating wastewater.Therefore,Aspergillus niger spores（AS）,a solid-state fermentation product of the fungus Aspergillus niger,was studied and modified by quaternization and magnetization,which can achieve magnetic separation while improving the adsorption efficiency and is beneficial to promote its application in actual industry.In order to improve the adsorption capacity of AS,the freeze-thawing pretreatment AS and chemical modified AS were prepared by physical and chemical methods.Magnetic ferric oxide modified AS（Fe₃O₄-AS and CTAB/Fe₃O₄-AS）was prepared to improve the adsorption efficiency and to make the adsorbent separable after adsorption.The properties of these spore-based adsorbents were characterized,and the removal rules of Cr（Ⅵ）with different modification methods were compared.In this study,experimental characterization analysis and theoretical calculation were used to explore the removal mechanism of Cr（Ⅵ）by Aspergillus niger sporium-based adsorbent.The composition,structure and physicochemical properties of AS were analyzed by qualitative and quantitative methods.The results showed that the main components of AS were polysaccharide（91.68%）and protein（1.77%）,belonging to the polysaccharide biological adsorbent.Abundant functional groups on the surface provided favorable conditions for the removal of heavy metals.The adsorption capacity of six types of heavy metal ions by AS was found significantly different.Cr（Ⅵ）was considered as a typical toxic heavy metal ion in water,and AS were treated by freeze-thaw treatment and chemically modified by physical and chemical methods.The removal rule of Cr（Ⅵ）under the same adsorption conditions was investigated to explore effective means to improve the adsorption capacity.Freeze-thaw method increased the porosity of AS and the adsorption capacity.The adsorption capacity was 48.6 mg/g,which was slightly higher than that of spores without treatment.Cetyltrimethyl ammonium bromide（CTAB）can effectively improve the surface charge of AS,and the adsorption efficiency was significantly improved.The adsorption capacity of CTAB-AS was 77.6 mg/g,which was nearly 80%higher.Magnetic oxide and chemical surface modification were applied to prepare CTAB/Fe₃O₄-AS adsorbent,which improved the adsorption efficiency and achieved the recovery of adsorbent.The magnetic material used in this thesis is magnetic Fe₃O₄nanoparticles.In order to reduce the agglomeration of nanoparticles and improve their hydrophilicity,the nanoparticles are dispersed into surfactants to prepare water-based magnetic fluids.The uniform coating of magnetic nanomaterials on AS surface was conducted.Considering the adsorption efficiency as the most important index,when the mass ratio of Fe₃O₄ and AS was 1:1,the adsorption efficiency of the adsorbent prepared by composite preparation was better than that of the magnetic composite biological adsorbent Fe₃O₄-AS prepared by other ratios.Therefore,on the basis of determining this ratio,Fe₃O₄-AS was further chemically modified to obtain CTAB/Fe₃O₄-AS.Also,the structural characteristics of magnetic adsorbents Fe₃O₄-AS and CTAB/Fe₃O₄-AS were analyzed.Using Cr（Ⅵ）AS probe,the adsorption processes of AS,CTAB-AS,Fe₃O₄-AS and CTAB/Fe₃O₄-AS were compared under different influencing factors.The adsorption capacity from high to low is CTAB/Fe₃O₄-AS>CTAB-AS>AS>Fe₃O₄-AS.Meanwhile,the total chromium and Cr（Ⅵ）on the surface of the solution and adsorbent were investigated.The results showed that Cr（Ⅵ）and Cr（III）existed in the solution and on the surface of the adsorbent.The adsorption capacity of CTAB/Fe₃O₄-AS is more than 2 times that of AS,also with great separation efficiency.The response surface method was used to establish the CTAB/Fe₃O₄-AS adsorption optimization equation and optimize the reaction parameters.The practical electroplating wastewater was treated,Cr（Ⅵ）66.3 mg/L practical wastewater was treated,and the removal rate was over 98%.In this thesis,the reductive adsorption mechanism of AS-based adsorbent and Cr（Ⅵ）was studied by combining experimental characterization and theoretical calculation.The results of X-ray absorption near edge structure（XANES）of synchrotron radiation indicate that chromium and O/N atoms in AS and CTAB/Fe₃O₄-AS samples formed Cr（III）tri-coordination compounds mainly.Both Cr（Ⅵ）and Cr（III）are present on the CTAB/Fe₃O₄-AS sample surface.The results of XPS analysis are consistent with the results of XANES.The proportion of Cr（III）on the surface of AS is about 93%,that is,Cr（Ⅵ）on the surface of AS is basically completely reduced.The Cr（III）on CTAB/Fe₃O₄-AS surface accounts for 60%,part of Cr（Ⅵ）is reduced,and part of Cr（Ⅵ）is adsorbed on CTAB/Fe₃O₄-AS surface by electrostatic action,without reduction.To clarify the effective components of the interaction between biological adsorbents and Cr（Ⅵ）,and the reduction adsorption mechanism,density functional theory and molecular dynamics quantitative calculation were applied for further analysis.The results show that the binding energy,Fukui function and electrostatic potential absolute values of AS components contain electron-donating groups（amide groups）.According to the DFT calculation results,chitin in AS plays a key role in the reduction adsorption of Cr（Ⅵ）.Based on these,chitin was used as the active component of AS for molecular dynamics simulation.The biological adsorbent model was constructed at the molecular level and the binding energy with Cr（Ⅵ）was calculated.Through the method of combining experimental verification and theoretical calculation,this thesis clarified the important components of the interaction between biological adsorbents and heavy metals,which has important theoretical research significance for the removal of heavy metal pollutants in water by biological adsorbents.