| 英文摘要 |
Contamination of soil/groundwater supplies by gasoline and other petroleum-derived hydrocarbons released from underground storage tanks (USTs) is a serious and widespread environmental problem. Corrosion, ground movement, and poor sealing can cause leaks in tanks and associated piping. Petroleum hydrocarbons contain methyl tertiary-butyl ether (MTBE) (a fuel oxygenate), benzene, toluene, ethylbenzene, and xylene isomers (BTEX), the major components of gasoline, which are hazardous substances regulated by many nations. MTBE possesses all the characteristics of a persistent compound in the subsurface: high solubility, low volatility, low sediment sorption, and resistance to biodegradation. Among BTEX, benzene is a carcinogenic compound and is more recalcitrant under anaerobic conditions. Thus, MTBE and benzene are selected as the target compounds in this study. One cost-effective approach for the remediation of the chlorinated-solvent and petroleum products contaminated aquifers is the installation of permeable reactive zones or barriers within aquifers. As contaminated groundwater moves through the emplaced reactive zones, the contaminants are removed, and uncontaminated groundwater emerges from the downgradient side of the reactive zones. The objective of this proposed study is to assess the potential of using a passive in situ oxidation barrier system. This passive active barrier system has advantages over conventional systems including less maintenance, cost-effectiveness, no above-ground facilities, no groundwater pumping and reinjection, and groundwater remediation in situ. The oxidation barrier system included a persulfate-releasing barrier, which contains persulfate-releasing materials. The slow-released persulfate would oxidize MTBE and benzene in aquifer. The persulfate-releasing materials would release persulfate when contacts with groundwater, thus oxidizes the MTBE and benzene. In the first part of this study, bench scale experiment was also performed to produce the persulfate-releasing materials high persulfate-releasing rate. The components of the persulfate-releasing materials and optimal concentrations of those components were determined in this study. Results indicate that the highest persulfate releasing rate can be obtained when the mass ratio of cement/sand/water was 1/0.16/0.5. Batch experiments were also performed to test the feasibility of using persulfate as the oxidants for MTBE and benzene oxidation. Several oxidation conditions including the concentrations of contaminants and oxidants, appearance of ferric and ferrous irons, appearance of hydrocarbon peroxide, various ambient pH values, and sulfate concentrations were evaluated. Results indicate that the most effective MTBE and benzene removal rates were observed when the molar ratio of MTBE/Na2S2O8/Fe2+ and benzene/Na2S2O8/Fe2+ were in the range of 1/50/31 to 1/500/31 and 1/50/31 to 1/100/31, respectively. Moreover, higher degradation rates of MTBE and benzene can be obtained with higher persulfate concentrations. Results also show that degradation rates of MTBE and benzene correlated with the amount of Fe(II) addition. However, only small amount of Fe(II) was required to activate the oxidation. Extra Fe(II) would cause the decrease the oxidation rates due to the reaction of sulfate and Fe(II). Results also reveal that the produced oxidation byproducts of MTBE, tert-butyl formate (TBF) and tert-butyl alcohol (TBA), can also be degraded completely. Result obtained from the persulfate-releasing materials test and bench-scale were used for the design and operation of the following column experiments. Results from the column experiment indicate that approximately 87% of MTBE and 99% of benzene could be removed during the early persulfate-releasing stage. However, the removal efficiencies for MTBE and benzene dropped to approximately 38% and 54%, respectively, during the latter part of the releasing period. Results reveal that TBF and TBA, byproducts of MTBE, were observed. Complete degradation of TBF and TBA were also observed in this study. Results from this study suggest that extra Fe(II) would cause the decrease in oxidation rates due to the reaction of sulfate with Fe(II). Results show that the parameters, which would affect the oxidation rate include persulfate concentration, oxidant reduction potential (ORP), conductivity, sulfate concentration, and contaminant concentration. The proposed treatment scheme would be expected to provide a more cost-effective alternative to remediate MTBE and other petroleum-hydrocarbon contaminated aquifers. Knowledge obtained from this study will aid in designing a persulfate oxidation system for site remediation.
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