| Chromium is widely used in various industries such as metallurgy,leather tanning and electroplating.Due to inappropriate disposal and accidental leakages,a great deal of chromium has been discharged into the environment,causing severe pollution in groundwater.Since Cr(?)is more hazardous because of its stable,toxic and mobile properties among various forms of Chromium,the removal of Cr(?)form groundwater has drawn great attention of the environmental scholars.To overcome the technical defects of the Fe0-based PRB and enhance its long-term performance,Fe0 and mixed anaerobic culture were integrated in one column to investigate the coupled abiotic and biotic effects on Cr(?)removal in groundwater.According to the breakthrough study,slower Cr(?)breakthrough rate of 0.19 cm/PV was observed in the biotic Fe0 column whereas the value in the abiotic Fe0 column was 0.30 cm/PV at initial pH 7,Cr(?)10 mg/L,569 m/a and 25 oC,resulting in 64% longer life-span and 62% higher Cr(?)removal capacity in the biotic Fe0 column than the abiotic one.The solid phase characterization by scanning electron microscopy(SEM),energy dispersive X-ray(EDX)and X-ray diffraction(XRD)confirmed that this enhancement was attributed to the higher consumption of iron and greater production of diverse reactive minerals(e.g.,green rust,magnetite and lepidocrocite)induced by the synergistic interaction of Fe0 and anaerobic culture,providing more reactive sites for Cr(?)adsorption,reduction and co-precipitation.Furthermore,due to the inhibitory effect of relatively high concentration of Cr(?)and the residual dissolved oxygen in the influent section,inhomogeneous reactive zones distributed along the biotic Fe0 column and the effluent section of the column was considered a more reactive zone where Cr(?)was removed via the synergistic effects with higher efficiency,whereas the physical removal by Fe0 played the major role in the influent section with lower Cr removal.Based on the aforementioned results,the effects of hydraulic condition such as the change of seepage velocity and initial Cr(?)concentration were evaluated.By increasing seepage velocity solely from 189 m/y to 1138 m/y,there were approximately 67% and 61% decrease in the longevity of the abiotic Fe0 column and biotic Fe0 column respectively,and there were approximately 70% and 64% decrease in the corresponding Cr(?)removal capacities.By increasing initial Cr(?)concentration solely from 5 mg/L to 40 mg/L,there were approximately 88% and 84% decrease in the longevity of the abiotic Fe0 column and the biotic Fe0 column respectively,whereas no obvious decrease of the Cr(?)removal capacities were found in both columns.Nevertheless,the biotic Fe0 columns maintained over 40% higher removal capacities of Cr(?)and 60% longer life-span than the abiotic.The electron donors and electron acceptors existed in groundwater may exert different effects on the removal of Cr in the biotic Fe0 columns.The addition of substrates such as the sodium acetate and humic acid can improve the removal of Cr(?)by serving as electron donors for microorganisms to metabolize.However,as nitrate can be reduced by microorganisms or Fe0 directly,the existent of nitrate exerted serious inhibition in Cr(?)removal by shunting the electron flow from Cr(?)to nitrate,and the inhibitory effect was more serious with increasing nitrate concentration.The presence of sulfate can promote the corrosion of Fe0 and accelerated the regeneration of iron surface,additionally,larger numbers of reactive minerals such as green rusts and iron sulfide were generated on the iron surface due to the interactive effect between microorganisms and Fe0,which could significantly enhance the Cr(?)removal for their stronger adsorptive and reductive capacity..These results indicate that the inoculation of microorganisms in Fe0-based permeable reactive barriers will enable this technology a higher removal capacity and longer life-span for the remediation of Cr(?)-contaminated groundwater. |
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