| Nano-zero-valentiron(nFe~0)couldbeemployedfortheremediation of chromium-contaminated soil because of its advantages of small particle size,large specific surface area and strong reducing power.However,nFe~0 tend to either agglomerate rapidly or oxidation due to its magnetic properties and high surface energy.Besides,with the progress of the reaction,a passivated layer formed on the surface of nFe~0 would reduce its reactivity.Therefore,it is an urgent need to find effective ways to to improve the stability of nFe~0 and solve the problem of passivated layer.In this study,carboxymethyl cellulose was used to modify nFe~0(CMC-nFe~0)and a synergistic system of CMC-nFe~0 and anaerobic microorganisms were investigated to study the remediation of chromium-contaminated soil.The experimental results showed that CMC-nFe~0has a higher dispersibility and antioxidant than nFe~0.CMC-nFe~0 maintained a good suspension state within 14 days,and it was not oxidized within 56 days.In the CMC-nFe~0+cell system,the removal efficiency of aqueous-bound Cr(?)was 95.3%,which was 4 times higher than that in the nFe~0 system(18.9%).Among them,0.9 times higher Cr(?)removal efficiency was due to the improved stability of nFe~0 after the modification of CMC.And 0.5 times higher Cr(?)removal efficiency was due to microorganisms used CMC as a carbon source to maintain its growth and enhanced the ability to remove Cr(?).Others 2.6times higher Cr(?)removal efficiency was due to the synergistic effect between nFe~0 and microorganisms.The solid phase characterization by scanning electron microscopy and X-ray diffraction revealed that microorganisms scavenged the passivated layer on the surface of nFe~0,promoting iron corrosion and induced the formation of iron minerals with high specific surface area and strong reactivity,such as magnetite,lepidocrocite and green rust,which provided a large number of reaction sites for the removal of Cr(?).In addition,microorganisms could released the reaction sites of nFe~0 by consuming CMC,promoting the mass transfer and reduction reactions between nFe~0 and Cr(?).After remediation,64%of soluble Cr(?)and total chromium in the soil were converted to chromium-iron-bonded precipitates,and then embedded into the crystal phase during the iron mineral transformation process.Eventually,the chromium was stabilized in the solid phase with the low-toxicity and hard-bioavailable state,greatly reducing the harm of chromium in the soil.Soil common environmental factors have different effects on the removal of Cr(?)in the CMC-nFe~0+cell system.In a certain range,low pH can accelerate the corrosion of iron and the renewal of surface reaction sites,which is conducive to the removal of Cr(?)in the synergistic system.Sodium acetate and humic acid can be used as a carbon source by microorganisms to enhance the reactivity of microorganisms,thus promotes the removal of Cr(?)from soil in the synergistic system.The presence of oxygen can inhibits the activity of anaerobic microorganisms,and at the same time acts as an electron acceptor to compete with Cr(?),resulting in the repression of Cr(?)removal in the system.Nitrate is easy to compete with Cr(?)for the electrons produced by iron corrosion in the denitrification process,resulting in a decrease of Cr(?)removal in the synergistic system.Sulfate via accelerate iron corrosion and renewal of active sites to counteract the negative effects of compete with Cr(?).At high sulfate concentrations,the formation of reactive minerals such as tetragonal pyrite were strengthened due to the interactive effect between microorganisms and nFe~0,which further enhances the role of synergistic systems to remove Cr(?).These results indicate that the inoculation of CMC-nFe~0 with anaerobic microorganisms can enhance the ability to remove Cr(?)from soil by abiotic and biological processes.It is expected to be a superior method for the remediation of chromium-contaminated soil. |
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