Study On The Synergistic Removal Of Nitrosamines And Hexavalent Chromium From Aqueous Solution By Coupled Immobilization Of Ac＿Fe（0）＿Degrading Bacteria

Aim at the problems of the human health and safety risk from combined pollutions in aquatic environments,it is an urgent issue to develop eco-friendly,cost effective,and efficient remediation technology.In this study,activated carbon（AC）＿Fe（0）＿degrading bacteria immobilized beads were prepared by integrating physical adsorption of AC,chemical reduction of Fe（0）,and biodegradation of nitrosamine-reducing bacteria,and used to explore the simultaneous removal of nitrosamines and Cr（VI）.The degradation pathway and mechanism of nitrosamines and Cr（VI）were studied.The effects of the initial concentrations of nitrosamine and Cr（VI）,temperature,p H,humic acid,and coexisting anions on the removal of nitrosamines and Cr（VI）were also explored.The operating parameters of the immobilized beads were optimized by eval uating the removal efficiency of nitrosamines and Cr（VI）.Emission scanning electron microscope（SEM-EDS）and X-ray photoelectron spectroscopy（XPS）were used to characterize the changes of the immobilized beads,and further explore the reaction mechanism and pathways.The contents were listed as following:（1）Preparation and characterization of AC＿Fe（0）＿degrading bacteria immobilized beads:Fe（0）,activated carbon and nitrosamine degrading bacteria were cross-linked with sodium alginate and agar to prepare AC＿Fe（0）＿bacteria immobilized beads.The SEM results showed that Fe（0）,AC and nitrosamine-degrading bacteria were distributed uniformly in the immobilized beads,and the communities of degradation bacteria was found in the pores of AC.Mechanical properties test showed that the AC＿Fe（0）＿bacteria immobilized beads had good repeatability and mechanical strength.（2）Removal effeciency and kinetics:The highest removal ratios of nitrosamines by AC＿Fe（0）＿degrading bacteria immobilized beads were 84.8%,99.2%,93.4%,99.2%,94.3%,and 98.0%at a nitrosamine concentration of 200 ng/L,（500 ng/L）,respectively.The reaction kinetics agreed with a pseudo-second-order model（R₂~2>0.945）.The removal ratios and rate constant k₂ of the five linear nitrosamines with high concentration（500 ng/L）are correlated with their physicochemical properties（R~2=0.499-0.745）.Cr（VI）promoted the removal of nitrosamines,while there had no significant effect on Cr（VI）removal in the presence of nitrosamines.The XPS results showed that peaks of Fe（0）2p3/2,Fe（0）2p1/2（712.7 e V and 720.4 e V）and Fe（III）2p3/2（717 e V and 730.5 e V）were detected in AC＿Fe（0）＿degrading bacteria immobilized beads before reaction;however,only oxidized iron Fe2P3/2（712.7 e V and 726 e V）was detected following the reaction.The SEM-EDS results indentified that Fe（0）in the immobilized beads decreased significantly after the reaction and N element increased,which indicated that nitrosamines were adsorbed or degraded by the immobilized beads.（3）Influencing factors:The removal effeciencies of nitrosamines and Cr（VI）decreased with the increase of p H（p H:4.0,6.75,and 9.0）,and the removal ratios was the highest under acidic conditions.Coexisting anions(SO₄^2-,CO₃^2-,NO₃^-,and NO₂^-)inhibited the removal of other five nitrosamines and Cr（VI）,except of NMor.It had an obvious inhibition effect on NDEA in the presence of anion（10 mmol/L）,and the removal ratios decreased by 48.4%.The removal of NDMA,NDEA and Cr（VI）were inhibited by CO₃^2-and NO₂^-,while the removal of NDMA,NDEA and Cr（VI）were promoted by NO₃^-and SO₄^2-.It had a negative effect on the removal of nitrosamines and Cr（VI）in the presence of HA in the AC＿Fe（0）＿microorganism coupling immobilized system,the removal ratios of nitrosamines decreased by10%～20%,yet there was no change on Cr（VI）removal.During the removal of nitrosamine and Cr（VI）combined pollution,nitrosamine promoted Cr（VI）removal from aqueous solution.（4）The degradation products and reaction mechanism showed that nitrosamines were reduced by Fe（0）and biodegraded by nitrosamine-reducing bacteria,and AC has a certain adsorption ability on nitrosamine.Cr（VI）is mainly reduced to Cr（III）by Fe（0）.The degradation products were mainly,secondary amines,nitrate,Cr（III）and ferro-chromium complex materials.