Iron Catalyzed Percarbonate Technologies For The Remediation Of Benzene Contaminated Groundwater

BTEX(benzene,toluene,ethylbenzene and xylenes),essential chemical materials and important constituents of petroleum,are widely used in petroleum,print,pharmaceuticals and chemical industries.BTEX have already been found to be the major contaminants of soil and groundwater due to their leakage during the process of production and usage.In view of their extensive contamination and potential threat to human health,benzene,especially toxic one in BTEX components,was selected as the target pollutant.Sodium percarbonate(SPC),a novel oxidant,was applied for the remediation of benzene-contaminated groundwater in this study.The degradation performance of benzene in Fe2+/Fe3+-catalyzed SPC systems was investigated,and various chelating agents and reductants were tested to enhance the benzene removal efficiency by alleviating the drawbacks of the Fe2+-catalyzed SPC system.Effects of chemical agent concentrations and groundwater matrix-constituents on benzene degradation,and benzene removal efficiency in actual groundwater were also investigated.Free radical probe tests,scavengers tests as well as electron paramagnetic resonance technique(EPR)were conducted to confirm the genetation of reactive oxygen species and their roles played in benzene degradation.In addition,benzene degradation products were identified to deduce its degradation pathways in the Fe3+-catalyzed SPC system.The main conclusions of this study are listed below:1)Effective and rapid degradation of benzene was observed in the Fe2+-catalyzed SPC system,and benzene was degraded completely in the initial 2 min at SPC and Fe2+concentrations of 10 mM.Benzene removal rate increased with the increasing of initial concentrations of SPC and Fe2+,while excess dosage of SPC and Fe2+ adversely degraded benzene.Benzene degradation was obviously affected by the order of SPC and Fe2+ addition that prior addition of Fe2+ favored benzene removal.The major reactive oxygen species in the Fee+-catalyzed SPC system were hydroxyl radical(HO·)and superoxide anion radical(O2·-),and HO· was the dominant species for benzene degradation,while the role of O2·-in benzene degradation could be ignored.Higher intensity of HO· was sustained for a longer time and the highest benzene degradation was observed at the initial solution pH of 3.NO3-and SO42-have less influence on benzene degradation,while HCO3-,high concentrations of Cl-and humic acid(HA)significantly inhibited benzene degradation.Benzene degradation rate in the Fe2+-catalyzed SPC system decreased in actual groundwater compared to ultrapure water due to the complicated constituents,while increasing dosages of SPC and Fe2+ suitably could eliminate this adverse effect caused by actual groundwater.2)The effects of various chelating agents on Fe2+-catalyzed SPC system for benzene degradation were significantly different.The addition of citrate acid(CIA),oxalic acid(OXA)and glutamic acid(GA)effectively decreased Fe precipitation which improved the catalytic ability of Fe2+-catalyzed SPC system,and futher remarkably enhanced the degradation of benzene.Compared to the Fe2+-catalyzed SPC system,the generation of HO· was enhanced with the addition of CIA,OXA or GA,and a higher intensity and stability of HO· was observed in CIA-Fe2+,OXA-Fe2+ and GA-Fe2+ catalyzed SPC systems.HO· was identified as the predominant radicals for benzene degradation in these three systems,while O2·-also participated in benzene degradation and its contribution was enhanced with the addition of CIA,OXA or GA.Though extremely alkaline pH,high concentrations of HCO3-and HA inhibited benzene degradation in CIA-Fe2+,OXA-Fe2+ and GA-Fe2+ catalyzed SPC systems,CIA,OXA or GA weakened the adverse effect of above mentioned factors and favored benzene degradation in actual groundwater.3)The moderate addition of hydroxylamine hydrochloride(HYH),ascorbic acid(ASA),sodium ascorbate(SA)to the CIA-Fe2+ or OXA-Fe2+ catalyzed SPC system enhanced Fe2+generation from Fe3+ leading to Fe2+ concentration improvement and accelerated decomposition of H2O2 that obviously improved benzene degradation efficiency.Excessive HYH,SAS or SA went against further enhancement of benzene degradation.Results of free radical probe tests and EPR showed that HYH,ASA or SA enhanced the generation of both HO· and O2·-in the OXA-Fe2+ catalyzed SPC system.The enhanced effect of HYH on HO·generation in the CIA-Fe2+ catalyzed SPC system was more superior than that of ASA and SA,while ASA and SA promoted O2-· generation more obviously.HO· was the predominant radicals for benzene degradation in all systems,while the contribution of O2·-to benzene degradation was enhanced with the addition of HYH,SAS and SA.HYH,SAS and SA modified CIA-Fe2+ or OXA-Fe2+ catalyzed SPC system could be well applied to acidic and neutral conditions and slightly affected by Cl-,NO3-and SO42-.While alkaline condition,HCO3-and high concentration of HA were not conducive to benzene degradation in each system.Compared to CIA-Fe2+ and OXA-Fe2+ catalyzed SPC systems,the reducing agent modified systems favored benzene degradation in actual groundwater.4)Benzene in ultrapure water was degraded effectively in Fe3+-catalyzed SPC system which was significantly affected by the initial concentration of SPC and Fe3+.HO· was the dominant species for benzene degradation,but its formation strongly depended on the generation of O2·-and organic compound radicals(R·).Thus O2·-and R· were indirectly involved in benzene degradation.Benzene degradation products include hydroxylated derivatives and short chain fatty acids.Two different degradation pathways were proposed for benzene degradation.In addition to the high concentration of Cl-and HCO3-which inhibited benzene degradation obviously,the rest of tested matrix constituents have only slight effect on benzene degradation.Effective degradation of benzene in actual groundwater was also observed in the Fe3+-catalyzed SPC system,demonstrating the applicability of this technique.