Reduction Of Cr(Ⅵ) In The Groundwater By Micro Current-Fe~0-PRB

Zero-Valent Iron Permeable Reactive Barrier is a typically in-situ groundwater remediation technique,which was successful applied to remediation of various groundwater contaminated sites.However,the active surface of Fe⁰ is easily oxidized to form oxide film or covered by hydroxide,which limits the application of this technology.Therefore,how to reduce the passivation of Fe⁰ and improve the efficiency of Fe⁰-PRB treatment of pollutants become more and more important in the current research.The effect of micro-current on the process when Cr（Ⅵ）was treated by Fe⁰-PRB was researched through the batch and column experiment with 0.15⁰.35 mm industrial iron of the reaction medium.The batch experiments showed that the removal efficiency of Cr（Ⅵ）improved with increase of temperature and dosage of Fe⁰ and low voltage DC.Furthermore,the lower initial pH and the more favorable of the reaction proceeded.The reaction between Cr（Ⅵ）and Fe⁰ was not only oxidation-reduction but also existed coprecipitation because of the low concentration of Cr（III）in sample water.When the temperature was around10^oC and the initial pH was 5.6,the first order reaction kinetics showed that the rate of k was 0.0779 h^-1 without loading voltage,and it raised to 0.0907 h^-1 with micro-current.The thermodynamic results showed the Ea of chemical reaction between Fe⁰ and CrⅥ)decreased under the microelectric current（4⁶ mA）and the Gibbs Free Energy of reaction decreased from-2.358 kJ/mol to-8.588 kJ/mol in the temperature of 20 ^oC.The column experiments presented that the operation time of Fe⁰-PRB depressed with the augment of the influent concentration of Cr（Ⅵ）,and the passivation of Fe⁰ and the plate junction are the main factors affecting the removal of Cr（Ⅵ）.The application of micro-current could promote the reduction of Cr（Ⅵ）by Fe0-PRB,and the removal efficiency aggrandized with the increase of micro-current,The amount of water,which can reach removal rate of 60%,produced by micro current test group II（1³ mA）,III（5⁸ mA）and IV（12¹7 mA）was 1.42 1.88 and 2.75 of control group I（0 mA）,respectively.The pH of the water decreased from 7.2⁷.6（tap water configuration）to6.5⁷.0 after the reaction in the micro-current test group（II^IV）,and the total iron in the effluent increased with the increase of the micro-current.The low concentration of Cr（III）in the effluent indicated that the microcurrent didn’t affect the co-precipitation of Cr（III）and Fe（III）.The results of XRD indicated that iron oxides reduced under the condition of micro-current,which may be due to the reduction of Cr（Ⅵ）by the dissolution of iron oxides and the formation of reductive iron,The mechanism of micro-current-Fe⁰-PRB enhanced the removal rate of Cr（Ⅵ）was inferred from two aspects.Firstly,the micro current could accelerate the transfer of electrons between Fe⁰ and Cr（Ⅵ）,which quickened the reaction rate of k and reduced the activation energy required for the reaction.On another side,H⁺was electrolyzed from the H₂O under the micro current,which urged the shedding of passivation film from the surface of zero-valent iron and it increased the active point of zero valent iron.