ZVI Reducing The TCE And Cr(Ⅵ) In Groundwater

Although groundwater is a kind of ecological resource accounting for a small fraction of the global water, the role of the ground water is crucial. Problem of groundwater pollution becomes worse and worse in China in recent years, the mixed pollution of volatile chlorinated hydrocarbons and heavy metals caused widespread concern.PRBs(Permeable Reactive Barriers) are widely used in various regions to reduce organic and inorganic chemicals pollution, especially in the water body containing chlorinated solvents. Zero-Valent Iron PRBs technology can be use to dispose mixed polluted plume. Hexavalent chromium(Cr(VI)) and trichloroethylene(TCE) were selected as representative research object, and column experinment was adopted to study removal of Cr(VI) and TCE in the system of zero-valent iron(ZVI). Besides column experinment, iron surface film were detected by Raman spectrum, Scanning Electron Microscope(SEM) and Energy Dispwrsive Spcetrometer( EDS).Five experiment columns were respectively filled in different combination forms of TCE(5 mg/L), Cr(VI)(10 mg/L) and dissolve Ca CO3(300 mg/L), running continuously for 12 months.ZVI could effectively remove the Cr(VI) in the polluted groundwater, the main reaction mechanism was based on the redox reaction and coprecipitation, which mainly included by-products of Fe(OH) 3, Cr(OH) 3 and(Crx Fe1- x)(OH) 3; Removal efficiency was affected by pollutants initial concentration, reaction time, and so on. ZVI had obvious dechlorination effect to TCE, and the TCE degradation continued to follow pseudo-first-order kinetics. Different dynamic models were studied for the effect of ZVI on single pollutant removal and mixed pollutants removal, the experimental results showed that whether a single pollutant or compound pollutants, the degradation of Cr(VI) and TCE complied with first order kinetics equation, and as the initial concentration and initial p H decreased, the reaction rate constant increased.The results from the column receiving only Cr(VI) showed that Cr(VI) was reduced rapidly by ZVI and was not detected beyond 10 cm from the influent end of the column at the end of the experiment. For the column receiving Cr(VI)+TCE, the co-existence of TCE do not affected Cr(VI) reduction kinetics. However, the presence of Cr(VI) affected TCE degradation significantly. When dissolved Ca CO3 was added to the column with Cr(VI) and TCE, either as single contaminant, or as co-contaminants, the presence of dissolved Ca CO3 resulted in a stable corrosion potential and faster degradation rates of TCE and Cr(VI). With the time increasing, through the pictures of Raman spectroscopy showed in the three columns containing Ca CO3, the concentration of Cr(VI) reduced and the speed became slow. Iron corrosion products OH- and bicarbonate arrived balance, which lead to carbonate mineral secondary precipitation, and secondary precipitation adhered on the surface of the iron content decreased activity of iron accumulation.