Oxidative and Reductive Degradation of Mixed Contaminants
by Bifunctional Aluminum
Hsing-Lung Lien
1Abstract: Transformation of various contaminants including carbon tetrachloride共CT兲, methyl tert-butyl ether 共MTBE兲, trichloroethyl-ene共TCE兲, and bis共2-chloroethyl兲 ether 共BCEE兲 using bifunctional aluminum was examined in batch reactors. Reductive degradation was observed only in reactions with CT while MTBE, TCE, and BCEE underwent oxidative pathways. In a batch reactor containing both CT and MTBE, oxidation of MTBE and reduction of CT by bifunctional aluminum took place simultaneously in the presence of oxygen. The MTBE was degraded to tert-butyl formate, tert-butyl alcohol, acetone, methyl acetate, and isobutene while the reduction of CT produced chloroform and dichloromethane. This indicates that bifunctional aluminum has a dual functionality of decomposing both oxidatively and reductively degradable contaminants together. Aluminum metal serves as a reductant while oxygen acts as an oxidant. Oxidizing capacity of bifunctional aluminum, resulted from reductive activation of dioxygen共O2兲, is dependent on both oxygen level and effectiveness of reductants. It was found that the redox potential of reaction systems can function as a simple indicator to determine the oxidizing capacity of bifunctional aluminum.
DOI: 10.1061/共ASCE兲1090-025X共2006兲10:1共41兲
CE Database subject headings: Gasoline; Halogen organic compounds; Ground-water pollution; Aluminum; Oxidation; Degradation.
Introduction
The use of zero-valent metals共e.g., iron, zinc, and aluminum兲 has shown much success in treatments of a wide array contaminants 共e.g., Gillham and O’Hannesin 1994; Matheson and Tratnyek 1994; Arnold and Roberts 1998; Lien and Zhang 2002a兲. In par-ticular, implementation of full or pilot scale in situ permeable reactive barriers 共PRBs兲 using zero-valent iron as a reactive media has been demonstrated to effectively remediate groundwa-ter contaminated with chlorinated organic solvents 共e.g., Puls et al. 1999; Wilkin et al. 2003兲. The fact that chlorinated organic solvents such as trichloroethylene共TCE兲, vinyl chloride, and car-bon tetrachloride共CT兲 are readily degraded by zero-valent iron is not surprising because iron serves as an effective reductant 共Matheson and Tratnyek 1994兲. Iron releases electrons through iron corrosion while contaminants such as CT undergo reduction reactions gaining electrons to form less chlorinated intermediates such as chloroform
Fe0→ Fe2++ 2e− 共1兲 CCl4+ 2e−+ H+→ CHCl3+ Cl− 共2兲 While most attention was attracted by the powerful reducing
capacity of zero-valent iron, its oxidizing capacity seemed to be overlooked long ago. Recently, it has been found that zero-valent iron is capable of oxidizing benzoic acid, carbothioate herbicide, and molinate 共Joo et al. 2004, 2005兲. The oxidizing capacity is attributed to the generation of hydroxyl radicals through the for-mation of hydrogen peroxide in the presence of iron and oxygen. Fenton’s reaction that involves hydrogen peroxide reacting with Fe共II兲 may occur in the zero-valent iron system.
Bifunctional aluminum, prepared by sulfating zero-valent alu-minum with sulfuric acid, has been shown to oxidatively degrade fuel oxygenates such as methyl tert-butyl ether 共MTBE兲 in the presence of oxygen 共Lien and Wilkin 2002; Lien and Zhang 2002b兲. Sulfation of aluminum resulted in the formation of cata-lytic sulfur-containing species at the surface. Oxidizing capacity of bifunctional aluminum was attributed to the reductive activa-tion of dioxygen共O2兲. The reductive activation of dioxygen is a process that catalytically converts oxygen into reactive radical oxygen species in the presence of electron donors such as NaBH4, reduced nicotinamide adenine dinucleotide phosphate共NADPH兲, and zinc共Otsuka et al. 1990; Sheldon 1994; Akita and Moro-oka 1998兲. The formation of radical oxygen species can generally be expressed as the following equation where关O兴 represents a reac-tive reduced oxygen species共Akita and Moro-oka 1998兲:
O2+ 2H++ 2e−→ 关O兴 + H2O 共3兲 The reduced oxygen species are strongly electrophilic and there-fore can serve as strong oxidants for oxidative reactions. On the other hand, electron donors creating reducing conditions facilitate reductive reactions. In other words, Eq. 共3兲 pointed out a possi-bility for zero-valent metals to simultaneously support oxidative and reductive degradation of contaminants. Therefore, it is pos-sible to engineer a system where gasoline oxygenates can be oxi-dized through the reductive activation of dioxygen, while chlori-1
Dept. of Civil and Environmental Engineering, National Univ. of Kaohsiung, Kaohsiung 811, Taiwan, Republic of China. E-mail: [email protected]
Note. Discussion open until June 1, 2006. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on August 9, 2005; approved on August 9, 2005. This paper is part of the Practice Periodical of Hazardous, Toxic, and Radioactive
Waste Management, Vol. 10, No. 1, January 1, 2006. ©ASCE, ISSN
nated organic solvents can be reduced by taking advantage of reducing conditions.
This work, a proof-of-concept study, was aimed at providing fundamental understanding and evidence of how bifunctional alu-minum can simultaneously degrade oxidized and reduced con-taminants in the same reaction system. Methyl tert-butyl ether and CT were chosen as major model compounds in reactions with bifunctional aluminum in the presence of oxygen. Carbon tetra-chloride with a saturated carbon oxidation state of +4 was used as a probe for reductive reactions and MTBE was used as a probe for oxidative reactions. Chlorinated organic solvents and gasoline oxygenates are two common classes of synthetic organic com-pounds that are often detected in contaminated groundwater 共Squillace et al. 1999; Johnson et al. 2000兲. In addition, contami-nants including TCE and bis共2-chloroethyl兲 ether 共BCEE兲 were also tested. The TCE has been known to undergo both oxidative degradation共e.g., Fenton’s reaction兲 共Glaze et al. 1993; Pignatello et al. 1999兲 and reductive degradation 共e.g., zero-valent iron re-duction兲 共Arnold and Roberts 2000兲. Bis共2-chloroethyl兲 ether is part of the large class of chloroalkyl ethers used as solvents in several industrial processes and as an intermediate in the manu-facture of chemicals 共WHO 1998兲. Elevated concentrations of BCEE共200 ppm兲 have been found in contaminated groundwater in Southeast Texas, caused by the leachate from a landfill共Huang et al. 1999兲.
The results from batch experiments on the rate and extent of contaminant degradation, and the identification of reaction prod-ucts are presented. Analysis of product distributions and reaction rates provides insight into degradation processes, and the effect of environmental factors on degradation rates. In addition, insights gained from this study offer a better understanding of potential applications and limitations of this novel material for further research.
Materials and Methods Materials
Methyl tert-butyl ether共MTBE兲, butyl formate 共TBF兲, tert-butyl alcohol 共TBA兲, methyl acetate 共MA兲, isobutene, acetone, CT, chloroform共CF兲, dichloromethane 共DCM兲, TCE, and BCEE with reagent grade or better共⬎99%兲 were purchased from Ald-rich. The materials 1.0 N H2SO4and concentrated HCl were ob-tained from Fisher and EM Science, respectively. Aluminum powder共 +99%, ⬃20 m兲 was purchased from Aldrich. Preparation of Bifunctional Aluminum
Bifunctional aluminum was prepared in a fume hood under am-bient temperature and pressure. Ten milliliters of concentrated HCl were slowly added to a 500 mL glass beaker containing 5.0 g of aluminum powder and the suspension was mixed with a magnetic stirrer. Immediate fume evolution was observed. Ten milliliters of distilled water were added quickly to dissipate heat for 30 s. A 0.5 mL of 1.0 N H2SO4was added into the suspension and it was mixed again for 30 s. Five milliliters of concentrated HCl were added in the suspension followed by the addition of 1.0 mL of 1.0 N H2SO4. After the suspension was stirred for 30 s, 5 mL of concentrated HCl was added again and then the suspension was quenched with 15 mL of distilled water. Finally, the suspen-sion was stirred for 20 min before bifunctional aluminum was harvested via vacuum filtration.
Batch Experiments
Batch experiments were carried out in 150 mL serum bottles con-taining about 1.0 g of bifunctional aluminum. For each batch bottle, a predetermined volume of stock solutions of organics was spiked into a 50 mL aqueous solution to achieve a desired initial concentration. Typical concentrations were 0.4 mM共35.2 mg/L兲 for MTBE, 0.33 mM 共50.8 mg/L兲 for CT, 0.23 mM 共30 mg/L兲 for TCE, and 0.21 mM共30 mg/L兲 for BCEE. Batch bottles were mixed on a wrist-action shaker 共100 oscillations/min兲 at room temperature共22±1°C兲. All experiments were associated with the control tests where identical experimental conditions and initial concentration of reactants were employed in the absence of bi-functional aluminum. Analyses of organic mass in the controls indicated that the mass varied by less than 5% over the course of a typical experiment. Experiments were conducted under acidic conditions共initial pH of 3.5–4.0兲.
Methods of Analyses
At selected time intervals, 1 mL of aqueous aliquot withdrawn by a gas-tight syringe was diluted with 4 mL of distilled water for gas chromatography/mass spectrometer 共GC/MS兲 analysis. A Shimadzu QP5000 GC/MS coupled to a Tekmar 3000 purge and trap concentrator was used for qualitative identification and quan-titative analysis of MTBE, CT, and their reaction products. A VOCARB 3000 trap column共Supelco兲 was installed in the purge and trap concentrator to remove excessive water. A DB-624 col-umn 共J&W, 0.25 mm⫻30 m兲 was equipped with GC/MS. The oven temperature was programmed as follows: hold at 50°C for 5 min and ramp at 5°C / min to 100°C. Injection and detector tem-peratures were set at 150 and 230°C, respectively. A quadrupole mass spectrometer was set to scan from 20 to 150 m / z with data collection every 0.1 s. Identification of reaction products was con-ducted by matching the resultant mass spectral patterns with those in the National Testing and Information Service Spectral Library and further verified with the standard chemicals purchased from Aldrich.
Kinetic Analysis
Reaction rates of contaminant degradation were determined with a pseudo-first-order equation
dC
dt = − kaC 共4兲
where C⫽concentration of contaminants in the aqueous phase 共mg/L兲; ka⫽observed first-order rate constant 共h−1兲; and t⫽time 共h兲. Plots of the natural logarithm of contaminant concentration versus time through linear regression analysis gave straight-line results. Observed first-order rate constants then can be calculated by linear regression analyses.
Results and Discussion
Transformation of Contaminants: Carbon Tetrachloride, Methyl Tert-Butyl Ether,
Trichloroethylene, and Bis„2-Chloroethyl… Ether
The results for the transformation of CT by bifunctional alumi-num are shown in Fig. 1. Rapid and complete degradation of CT was observed within a few hours. Chloroform 共CF兲 and
dichlo-romethane 共DCM兲 accounting for about 60.6 and 15.2% of the CT lost, respectively, were primary products. Methane was found in minor amounts 共⬍1%兲. The carbon mass balance was about 75.8% at the end of the experiment. The observed rate constant of CT degradation was determined to be about 1.4 h−1.
Oxidative transformation of MTBE by bifunctional aluminum has previously been reported 共Lien and Wilkin 2002; Lien and Zhang 2002b兲. In this study, experiments were carried out under similar conditions. Results of repeated experiments were consis-tent with those in previous reports. Reaction products included TBF 共11%兲, TBA 共15%兲, MA 共13%兲, acetone 共30%兲, and isobutene 共trace兲. It should be noted that there was 1 order of magnitude difference in reaction rates of MTBE degradation between two previous reports. This is because the experiments were conducted under different mixing conditions. The use of an orbital shaker providing only mild horizontal mixing conditions resulted in a lower rate constant共0.1 h−1兲 共Lien and Wilkin 2002兲, whereas a wrist-action shaker mimicking side-to-side action of hand mixing led to a higher rate constant 共1.0 h−1兲 共Lien and Zhang 2002b兲. From the viewpoint of PRB applications, the mild horizontal mixing alone is much more appropriate than the wrist-action mixing to simulate the flow conditions of groundwater. This is because groundwater primarily moves in horizontal or lateral directions and flow rates are usually quite slow 共1–500 m/year in general兲 共Bouwer 1978兲. Nevertheless, this difference did not alter conclusions as it, in fact, reflected the importance of mass transfer effect. In this study, a wrist-action shaker was used and the rate constant was determined to be approximately 1.0 h−1, which is in good agreement with the previous study共Lien and Zhang 2002b兲.
Reaction of TCE with bifunctional aluminum was tested in the presence of oxygen. The TCE was rapidly degraded within a pe-riod of 6 h and surprisingly there were no by-products observed. Reductive degradation of TCE by zero-valent iron led to the for-mation of less chlorinated ehtylenes and hydrocarbons 关e.g., ethane in Eq. 共5兲兴 共Arnold and Roberts 2000兲 while oxidative degradation of TCE through Fenton’s reaction resulted in the for-mation of chlorinated organic acids 关e.g., dichloroacetic acid in Eq.共6兲兴 共Glaze et al. 1993; Pignatello et al. 1999兲
C2HCl3+ 5H++ 4Fe0→ C2H6+ 3Cl−+ 4Fe2+ 共5兲
共6兲
Accordingly, it is likely that TCE underwent oxidative pathways to form chlorinated organic acids that are unable to be detected under the experimental conditions using GC analysis in this study. Because half-reaction reduction potential of CT to CF 共0.77 V兲 was greater than that of TCE to dichloroethylenes 共0.5–0.54 V兲 共Vogel et al. 1987兲, CT should be more favorable than TCE to undergo reductive reactions. Oxidative rather than reductive deg-radation occurring in reactions of bifunctional aluminum with TCE suggests that bifunctional aluminum may be capable of oxi-dizing certain chlorinated organic compounds such as dichlo-romethane and dichloroethanes that were not degraded via metal-mediated reduction 共Matheson and Tratnyek 1994; Lien and Zhang 2005兲.
Oxidative degradation of BCEE using UV peroxidation has been evaluated共Li et al. 1995兲. In the presence of hydrogen per-oxide, BCEE was reduced to undetectable levels after 30 min of UV irradiation and produced intermediates including 2-chloroethyl 1-hydroxyethenyl ether and 2-chloroethyl acetate. In the presence of oxygen, bifunctional aluminum effectively transformed BCEE to 2-chloroethyl 1-hydroxyethenyl ether and 2-chloroethyl acetate within 1 h. The formation of 2-chloroethyl 1-hydroxyethenyl ether was determined by GC/MS spectrum. Its
m / z values are 15 共13%兲, 31 共100%兲, 43 共9%兲, and 62 共23%兲
where m / z 31 is the base peak. This is consistent with results reported by Li et al. 共1995兲 and indicates BCEE underwent an oxidative reaction pathway.
Simultaneous Degradation of Both Methyl Tert-Butyl Ether and Carbon Tetrachloride
Fig. 2 illustrates a GC/MS chromatogram indicating the simulta-neous degradation of both MTBE and CT occurred in the pres-ence of bifunctional aluminum and oxygen. In this study, reac-tions of a mixture of MTBE共0.4 mM兲 and CT 共0.33 mM兲 with bifunctional aluminum were performed in the same batch system.
tert-Butyl formate, TBA, methyl acetate共MA兲, and acetone were
found from the MTBE degradation while the degradation of CT led to the formation of CF and DCM. Product distributions and reaction rates were similar to those observed from the single com-pound tested under identical conditions as mentioned above共e.g., Fig. 1兲. This indicates that the oxidation of MTBE and the reduc-tion of CT can simultaneously proceed in the presence of bifunc-Fig. 1. Transformation of carbon tetrachloride by bifunctional
aluminum
Fig. 2. Gas chromatography/mass spectrometer chromatogram
measured from the transformation of a mixture of methyl tert-butyl ether and carbon tetrachloride by bifunctional aluminum at 1 h
tional aluminum. It should be pointed out that a very close reten-tion time for the appearance of DCM and TBA was observed in GC analysis共Fig. 2兲. Both appeared at about 5.8 min. The TBA peak appeared initially but it was gradually overwhelmed by the DCM peak as the production of DCM increased in the mixed solvent system.
Impact of Environmental Factors: Oxygen and Redox Potential
Transformation of both CT and MTBE together was further con-ducted in the presence and absence of oxygen to compare the oxygen effect on different types of contaminant degradation共Fig. 3兲. To create an oxygen-free condition, batch reactors were purged with pure nitrogen for 2 h to remove dissolved oxygen before MTBE was added while experiments were carried out under ambient conditions for the oxygen-present study. As shown in Fig. 3, reduction of CT by bifunctional aluminum proceeded under both oxygen-free and oxygen-present conditions. Similar amounts of CF 共⬃61%兲 and DCM 共⬃15%兲 were observed in both systems. Because the aluminum is the only electron donor in the system, the reduction of CT occurring under both conditions indicates that the role of aluminum metal is the reductant.
However, degradation of MTBE with bifunctional aluminum took place only in the presence of oxygen共Fig. 3, Al+O2兲. In the absence of oxygen, no reaction of MTBE with bifunctional alu-minum was observed and no reaction by-products were detected 共Fig. 3, Al+N2兲. This indicated that bifunctional aluminum alone did not degrade MTBE. The oxygen-dependent reaction indicates that oxygen serves as a primary oxidant. Due to the basic charac-ter of etheric oxygen on MTBE, bifunctional aluminum with acidic surface should adsorb MTBE to a certain degree, which reflected to be about 25% of the MTBE lost in the absence of oxygen. Nevertheless, when the batch bottle was purged with pure oxygen gas共99.9%兲 in the headspace for 1 min after a con-tact period of 8 h, the degradation of MTBE resumed 共Fig. 4兲. Approximately 5% of MTBE escaping from the batch bottle dur-ing the process of oxygen addition was determined. The loss of MTBE is insignificant and should largely be attributed to the high water solubility of MTBE. The transformation of MTBE to TBF, TBA, MA, and acetone was observed after the oxygen addition 共Fig. 4兲. Although attempts were not made to measure the residual dissolved oxygen in solutions after nitrogen purging, it is clear that the purge of nitrogen should effectively lower the dissolved oxygen to a trace level where bifunctional aluminum was unable to oxidize MTBE according to the result shown in Fig. 4.
Oxidation reduction potential共ORP兲 of reaction systems was
measured when a multispiking test of MTBE was conducted in a batch reactor during a period of 24 h. An Orion pH/ mv meter equipped with a combination redox electrode was used for ORP measurement. The batch reactor containing 20 g / L of bifunc-tional aluminum was spiked with 0.4 mM of MTBE three times. As shown in Fig. 5, fast MTBE degradation occurred initially but reactions slowed down at the second time of MTBE spiking and eventually stopped at the third time. On the other hand, ORP measurement indicated that the system quickly reached reducing conditions 共⬃−300 mV兲 within 6 h and then ORP increased slowly to about +360 mV at the end of the experiments. The changes of redox potential from reducing to oxidizing conditions indicated that aluminum is no longer serving as the effective elec-tron donor. This suggests that no reaction of MTBE with bifunc-tional aluminum at the third spiking time is caused by lack of an effective reductant in the system. The failure of aluminum for serving as a reductant may be attributed to the consequence of inhibition of aluminum corrosion at higher pH because it was found that the pH solution increased to about 5.6 at the end of the experiment. Aluminum is resistant to corrosion in aqueous solu-tions with a pH in the neutral range from 4.5 to 8.5共Davis 1999兲. The oxidative degradation of MTBE by bifunctional aluminum proceeded via the reductive activation of dioxygen that occurred only under reducing conditions.
Fig. 3. Comparison of oxygen effects on degradation of methyl tert-butyl ether and carbon tetrachloride together 共Al represents
bifunctional aluminum兲
Fig. 4. Oxygen dependence of methyl tert-butyl ether transformation
by bifunctional aluminum
Fig. 5. Effect of oxidation reduction potential on methyl tert-butyl
Conclusions
Technologies for the treatment of specific organic contaminants are being developed and proven; however, there are few technolo-gies capable of treating both reductively degradable and oxida-tively degradable contaminants together. In this work, oxidative and reductive degradation of contaminants with bifunctional aluminum in the presence of oxygen was found. Zero-valent alu-minum served as a reductant while oxygen acted as an oxidant. This study demonstrates that bifunctional aluminum could be a promising reactive reagent for simultaneous treatments of a wide array of mixed contaminants. It could be used for both in situ and ex situ remediation of water contaminated with gasoline oxygen-ates and chlorinated solvents. Based on the dual functionality, a custom-designed bifunctional aluminum might become a poten-tial strategy for the development of a contaminant-specific reme-dial system. For example, bifunctional aluminum can be modified by adding hydrogenation catalysts共e.g., Ni, Co, or Cu兲 onto the aluminum surface to promote its catalytic capability of the reduc-tion. The aluminum-based bimetals have been found for a better treatment of chlorinated solvents共Lien and Zhang 2002a兲. On the other hand, to better treat the contamination of gasoline oxygen-ates, bifunctional aluminum may be optimized toward the enhancement of the oxidizing capability. By combining the ad-vantages of zero-valent metal technology with the reductive acti-vation of dioxygen process, bifunctional aluminum opens a new avenue for environmental remediation where chemical treatments of oxidatively and reductively degradable contaminants together in the same system are possible.
Acknowledgment
Funding for this research was provided by the National Science Council, Republic of China 共Taiwan兲 through Contract No. 93-2211-E-390-006-.
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