Adsorption Of Trivalent Chromium In Leather Wastewater By Magnetic Carrier Immobilization Engineered Strain

In the process of leather processing,tanning is extremely important in which chromium tanning is very common tanning method.The wastewater from this process contains high Cr（III）which is harmful to biodiversity and environment.Traditional treatment such as chemical precipitation is used to add alkaline precipitant to environment removal of Cr（III）.The addition of these precipitants not only cause secondary pollution but also increases the cost and makes the wastewater treatment process economically infeasible.In recent years,the adsorption of heavy metals by microorganisms has appealed much attention for research.However,use of microorganisms only,face many challenges in wastewater treatment,such as low wastewater treatment efficiency and unstable microbial activity.In the current study,the removal of trivalent chromium from leather wastewater was achieved by magnetic carrier immobilized engineered strain.Merp protein has been reported to absorb Cr（III）,which appear on the surface of E.coli.This engineered E.coli strain was immobilized in a magnetic carrier to prepare magnetic adsorbent.The doping of magnetic nanoparticles into immobilized materials is helpful for the recovery of magnetic pellet and trivalent chromium.In addition,magnetic nanoparticles can increase the surface area and mechanical strength of the biosorption agent.Compositional analysis was done for each material in magnetic pellets.The results showed the optimal content of each material prepared by the magnetic pellets（alginate 2.5%,polyvinyl alcohol 8%,Fe₃O₄ nanoparticles 3.5%and M-BL21 3 g/L）.When the concentration of trivalent chromium was 210 mg/L and dosage of magnetic pellets was 40 g/L,the optimum adsorption efficiency was recorded as 91.29%.The adsorption of trivalent chromium on magnetic pellet was characterized by adsorption kinetics and isothermal adsorption model.The adsorption process of magnetic pellet was following quasi second-order kinetics and freundlich isotherm.Scanning electron microscopy（SEM）was used to illustrate the internal and external structure of pellet and M-BL21.Energy-dispersive X-ray spectrometer（EDX）was used to detect elements before and after adsorption of Cr（III）by magnetic pellet.The result shows that magnetic adsorbent after adsorption contained chromium element.In order to study re-usability of magnetic adsorbent,the adsorption process of magnetic pellet was repeated five times,and the degree of adsorption was measured.Vibration probe magnetometer（VSM）was used to detect the magnetic properties of magnetic adsorbent and adsorbent after adsorption tests.Finally,the magnetic adsorbent applied to actual leather wastewater treatment,which successfully achieved 88.2%Cr（III）removal efficiency.Therefore,it is feasible and recommended to use the magnetic adsorbent for removal and recovery of Cr（III）in leather wastewater.