Interaction Between Extracellular Polymeric Substances And Cr(?) In Wastewater And Its Influences On Cr(?) Reduction By Anaerobic Activated Sludge

In 2014,the total chromium(Cr)emission from industrial wastewater was 131.8 tons and Cr became the largest heavy metal pollutant.In particular,Cr(?)is approximately 100 times more toxic than Cr(?)and has been listed as a priority toxic chemical.Several methods to remove Cr(?)have been developed,such as chemical reduction,physical-chemical treatments and biological methods.Bioremediation was considered as a promising and cost-effective alternative to chemical process,due to its low cost and ecological compatibility.Although microorganism could reduce Cr(?)to Cr(?),Cr(?)has toxic effect on microbial activity and inhibited coexisting organic and NO3--N removal.In addition,microbial extracellular polymer substances(EPS)could promote Cr(?)reduction;generated Cr(?)could form precipitates or form organo-Cr complexes with EPS.Soluble organo-Cr complexes could be re-oxidized into Cr(?)in oxidative environments and cause second-pollution.Therefore,this study was conducted to explore the interaction between Cr(?)and EPS,characteristic of Cr in sludge and Cr(?)anaerobic reduction mechanism.Initially,effects of different Cr(?)concentrations on the CODCr and NO3--N removal efficiencies,NO3--N reductase activity and electron transfer activity were investigated.With the increase of Cr(?)concentration,Cr(?)inhibition on sludge activity increased and Cr(?)exerted sustained toxicity on activated sludge.Linear and equation equations were applied to fit inhibition rates of Cr(?)on CODCr and NO3--N removal efficiencies,and the half inhibition concentration of Cr(?)on sludge was 11.45 mg/L.Results showed that Cr(?)reduction mainly occurred in intercellular and most reduction Cr(?)(>88.5%)was intercellular.To further study the removal performance of Cr(?)in anaerobic reactor.Influent Cr(?)concentration was increased slowly from 0 to 120 mg/L to acclimate anaerobic sludge.Results showed that low Cr(?)concentration(0-80 mg/L)had no significant effects on CODCr,NO3--N and Cr(?)removal.However,CODCr,NO3--N and Cr(?)efficiencies were inhibited by high Cr(?)concentration(?80 mg/L).When influent Cr(?)concentration was 120 mg/L,CODCr removal efficiency decreased from 81.5% to 73.9%,and NO3--N removal efficiency decreased from 89.2% to 76.5%.Besides,NO2--N concentration of effluent increased to 6.1 mg/L.Cr(?)removal efficiency also decreased from 96.5 to 74.8%,meanwhile Cr(?)and Cr(?)concentration of effluent increased to 30.2 and 5.1 mg/L,respectively.Efficient immobilization of Cr(?)were achieved and more than 97.7% of generated Cr(?)was located in sludge.Microbial community analysis showed that variety and abundance of the major microbial community decreased with the increase of Cr(?)concentration.Morphology analysis showed that Cr in activated sludge was mainly in the form of Cr(?)and generated Cr(?)formed Cr precipitates adhered to the sludge cells.In addition,Cr(?)also promoted the production of EPS.With Cr(?)concentration increasing from 0 to 50 mg/L,the amount of EPS increased from 231.42 to 1262.90 mg/gVSS.When Cr(?)concentration increased to 120 mg/L,the amount of EPS decreased to 452.85 mg/gVSS.Under different Cr(?)concentration,the amount of PN was greater than that of PS.With the increase of Cr(?)concentration,peaks in three-dimensional fluorescence spectra of EPS shifted,indicating structure of the EPS component changed.Additionally,increasing Cr(?)concentration caused shifts of peak regions around 3400 cm-1,1640 cm-1,1400 cm-1 and 1100 cm-1,which showed that functional groups such as hydroxyl,carboxyl and amino groups changed to accommodate higher Cr(?)concentration.Results revealed that sludge could reduce Cr(?),but EPS and nutrient solution could not reduce Cr(?)directly.Heat denaturalization and protein denaturants could restrain Cr(?)reduction,proving that Cr(?)reduction process was enzyme-mediated.Membrane-associated chromate reductase was responsible for Cr(?)reduction by anaerobic sludge.Interaction between the EPS and Cr(?)led to red shifts of Cr(?)absorption peaks.Proteins-like substances and humic-like substances in EPS were responsible for Cr(?)and EPS interaction.Values of ksv for protein-like and humic-like substances were calculated to be 2.817×104(R2=0.9594)and 1.25×103(R2=0.9538)mol/L,respectively.Proteins-like substances had a stronger complex ability with Cr(?)than humic-like substances in EPS.Thermodynamics analysis showed that the binding reaction between EPS and Cr(?)was an exothermic reaction.The change of Gibbs' energy was-23.18 kJ/mol,and the process was a thermodynamically favorable binding reaction.