Sequential Chemical Oxidation and Aerobic Biodegradation of

Sequential Chemical Oxidation and Aerobic Biodegradation of Equivalent Carbon Number-Based Hydrocarbon Fractions in Jet Fuel. Guibo Xie*, and Michael ...
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Environ. Sci. Technol. 2003, 37, 4751-4760

Sequential Chemical Oxidation and Aerobic Biodegradation of Equivalent Carbon Number-Based Hydrocarbon Fractions in Jet Fuel GUIBO XIE* AND MICHAEL J. BARCELONA‡ Division of Environmental Health Engineering, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland 21205

Remediation of petroleum mixtures is complicated by the differing environmental degradabilities of hundreds of individual hydrocarbons in the mixtures. By grouping the individual hydrocarbons into a few fractions based on equivalent carbon number (EC), the present study examined the chemical and biological degradation of the fractions. With or without prechemical oxidation (25 days) by three oxidants (KMnO4, H2O2, MgO2), sterile and live microcosms were constituted with aquifer samples for aerobic biodegradation (134 days) of JP-4 jet fuel. Eighty-seven hydrocarbons were recovered and grouped into nine EC fractions. The apparent removal and actual transformation rate constants were estimated for both chemical and biological degradations. The data show that prechemical oxidations facilitated removal of total petroleum hydrocarbons (TPH) (up to 80%) within shorter times (8-10 8.34 0.14 46 cis-1-methyl-2-ethylcyclohexane 8.34 0.08 47 isopropylcyclohexane 8.38 0.71 48 propylcyclohexane 8.45 0.62 49 3,6-dimethyloctane 8.47 0.28 50 2,3-dimethyloctane 8.47 0.09 51 4-methylnonane 8.55 0.50 52 2-methylnonane 8.64 0.61 53 3-methylnonane 8.78 0.75 54 decane 9.00 5.69

Fraction: Aliphatic EC >6-8 0.08 22 ethylcyclopentane 1.89 23 2,4-dimethylhexane 0.16 24 ctc-1,2,4-trimethylcyclopentane 0.06 25 ctc-1,2,3-trimethylcyclopentane 1.37 26 2,3-dimethylhexane 1.51 27 2-methylheptane 1.96 28 4-methylheptane 0.39 29 3-methylheptane 1.02 30 3-ethylhexane 0.66 31 cis-1,4-dimethylcyclohexane 0.99 32 trans-1,4-dimethylcyclohexane 3.54 33 trans-1,2-dimethylcyclohexane 4.22 34 trans-1,3-dimethylcyclohexane 0.04 35 octane

Fraction: Aliphatic EC >10-12 10.27 0.52 57 pentylcyclohexane 11.00 5.76 58 dodecane

55 4-methyldecane 56 undecane

11 11

59 benzene

6

6.50

Fraction: Aromatic EC 6.5 0.19

60 toluene

7

7.58

Fraction: Aromatic EC 7.58 1.22

61 62 63 64 65

ethylbenzene m/p-xylene o-xylene isopropylbenzene propylbenzene

8 8 8 9 9

Fraction: Aromatic EC >8-10 8.50 0.33 66 1-methyl-3-ethylbenzene 8.61 1.43 67 1,3,5-trimethylbenzene 8.81 0.42 68 1-methyl-2-ethylbenzene 9.13 0.06 69 1,2,4-trimethylbenzene 9.47 0.10 70 (1-methylpropyl)benzene

71 72 73 74 75 76 77 78

1-methyl-3-isopropylbenzene 1-methyl-4-isopropylbenzene 1-methyl-3-propylbenzene 1,3-dimethyl-5-ethylbenzene 1-methyl-2-propylbenzene 1,4-dimethyl-2-ethylbenzene 2,4-dimethyl-1-ethylbenzene 1,2-dimethyl-4-ethylbenzene

10 10 10 10 10 10 10 10

Fraction; Aromatic EC >10-12 10.09 0.07 79 1,3-dimethyl-2-ethylbenzene 10.13 0.04 80 1,2-dimethyl-3-ethylbenzene 10.44 0.16 81 1,2,4,5-tetramethylbenzene 10.50 0.11 82 1-methyl-2-isopropylbenzene 10.62 0.05 83 1,2,3,4-tetramethylbenzene 10.68 0.05 84 (1,1-dimethylpropyl)benzene 10.72 0.08 85 naphthalene 10.75 0.09

11

Fraction: Aromatic EC >12-1 12.84 0.11 87 1-methylnaphthalene

86 2-methylnaphthalene

Eq 7 can be easily solved with the initial condition of C ) Co at t ) 0.

LnC ) - kappt + LnCo or C ) Coexp(- kappt)

(8)

Based on eq 8, if the plot of LnC vs t yields a straight line, the estimated value of the apparent degradation rate constant will be given by the slope. The actual reaction rate constant, k, can be obtained via the relationship between k and kapp, i.e., kapp ) k/(1 + θswKd + θgwKH), where Kd was experimentally determined and roughly a constant for a specific EC fraction 4754

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carbon no.

EC

weight percent (%)

6 6 6

5.72 5.85 6.00

1.93 1.27 2.40

7 8 8 8 8 8 8 8 8 8 8 8 8 8

7.34 7.38 7.40 7.51 7.65 7.71 7.72 7.78 7.79 7.79 7.80 7.94 7.99 8.00

0.20 0.24 0.50 0.46 0.23 2.21 0.49 1.06 0.15 1.27 0.54 0.72 0.29 4.44

9 9 9 10 10 10 10 10 10

9.10 9.24 9.31 9.41 9.63 9.67 9.72 9.78 10.00

0.28 0.08 0.74 0.66 0.29 0.44 0.36 0.34 5.76

11 12

11.19 12.00

0.15 6.71

9 9 9 9 10

9.55 9.62 9.71 9.84 9.96

0.38 0.26 0.10 0.76 0.03

10 10 10 10 10 11 10

10.81 10.93 11.05 11.11 11.57 11.59 11.69

0.04 0.03 0.05 0.09 0.14 0.03 0.10

11

12.99

0.06

(15); KH was cited from ref 1; θsw was 0.25 g/mL and 0.36 g/mL for the chemical oxidation reactors and biological microcosms, respectively; and θgw was 0 mL/mL and 0.43 mL/mL for the chemical oxidation reactors and biological microcosms, respectively.

Results and Discussion Hydrocarbon Components and Fractions in JP-4 Jet Fuel. The EC fractions and their weight percentages in the JP-4 standard are presented in Table 3. JP-4 jet fuel is a mixture of over 100 hydrocarbons, and 87 of them were identified in

FIGURE 2. Apparent chemical oxidation of EC fractions of JP-4. Treatment A: without chemical oxidation. Treatment B: with prechemical oxidation by KMnO4. Treatment C: with prechemical oxidation by H2O2. Treatment D: with prechemical oxidation by MgO2. EC ) equivalent carbon number, Al ) aliphatic, Ar ) aromatic.

FIGURE 3. Apparent degradation rate constants (a) and actual transformation rate constants (b) of the EC fractions in JP-4 for chemical oxidations. Treatment B: KMnO4. Treatment C: H2O2. Treatment D: MgO2. EC ) equivalent carbon number, Al ) aliphatic, Ar ) aromatic. our study by comparing both retention indices and mass spectra with those of standards as well as by matching the mass spectra with those in a database. The total recovery was 76.6 wt % with alkanes 51.7%, cycloalkanes 18.3%, and aromatics 6.6%. According to the criteria used by the TPHCWG, the hydrocarbons in the JP-4 jet fuel fell into nine of the 13 EC fractions. Apparent Sequential Chemical and Microbial Oxidations of Total Petroleum Hydrocarbons (TPH) in JP-4 Jet Fuel (Day 0 to Day 159). Figure 1 shows that, during the period of prechemical oxidation (day 0 to day 25), chemical oxidation controls (treatments N or A) proceeded with negligible removal of TPH. KMnO4 removed TPH up to 70% (treatment B); H2O2 removed 46% (treatment C), and ORC (MgO2)

removed approximately 25% (treatment D). KMnO4 was the most effective oxidant, H2O2 the second, and ORC (MgO2) the least with respect to TPH mass reduction. The accepted hypothesis on a redox reaction states that the rate-limiting step is the transfer of the first electron (24, 25). According to data compiled from refs 26 and 27, the first-electron transfer of MnO4- is more favorable compared to that of H2O2, given the E° for MnO4-/HMnO4- and H2O2/ •OH at pH 7 are 0.49 v and 0.38 v, respectively. The oxidation by KMnO4 is thus expected to be more favorable than H2O2 oxidation. Oxidation by ORC (MgO2) is a more complicated process. The mechanism of the oxidation by MgO2 may be postulated to be similar to that by H2O2, i.e., via production of •OH (28-30). The significant difference is that the ORC VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Selective biological oxidation of EC fractions of JP-4 jet fuel. Treatment A: without chemical oxidation, with biological oxidation. Treatment B: with prechemical oxidation by KMnO4, with biological oxidation. Treatment C: with prechemical oxidation by H2O2, with biological oxidation. Treatment D: with prechemical oxidation by MgO2, with biological oxidation. Treatment N: without chemical oxidation, without biological oxidation. EC ) equivalent carbon number, Al ) aliphatic, and Ar ) aromatic. solid matrix releases H2O2 or radicals via diffusion and may trap produced radicals in a cemented structure formed by Mg(OH)2 (31). The cemented structure was observed in our experiment as well as other studies (10, 30, 32, 33). The slow diffusion processes are likely responsible for less TPH oxidation by MgO2. In the course of microbial oxidation from day 25 to day 159, the first subsampling and datum point was obtained at day 33. Thus, day 33 is taken as the zeroth day in this paper for the purpose of kinetic modeling and estimation of biodegradation. It can be seen in Figure 1 that a steep decline in TPH levels occurred for most treatments from day 25 to day 33. This sharp change of TPH was primarily due to the loss caused by the displacement of JP-4 solution with redox titrant solution and oxygen. A comparison between treatment N (without biological oxidation) and treatment A (with biological oxidation) suggests that microbial oxidation played a role as well in live microcosms. From day 33 to day 159, the sterile control (treatment N) showed negligible losses of TPH, while the live microcosm with prechemical oxidation (treatment A) showed notable removal of TPH. There was only slight biological removal of TPH in the live microcosms with prechemical oxidations by KMnO4 and H2O2 (treatments B and C, respectively). More removal was achieved in the live microcosm with prechemical oxidation by ORC (MgO2) (treatment D). The reasons may be that the toxicity of partial chemical oxidation products inhibited microbial activities and that favorable substrates for microbial metabolism were removed by prechemical oxidation. More details are discussed below in the sections of chemical and biological oxidations of EC fractions. Therefore, prechemical oxidation facilitated an overall removal of more TPH within a shorter time than biological alone. The stronger the oxidizing strength of an oxidant was, the more TPH was removed by prechemical oxidation; without prechemical oxidation (e.g., in treatments N and A), the major part of TPH was removed by the physical displacement of JP-4 solutions. KMnO4 and H2O2 were better oxidants in terms of more TPH reduction within shorter times but to some extent slowed subsequent microbial activities. ORC (MgO2) was a moderate oxidant but exerted little inhibition of microbial activity. Chemical Oxidation of the EC Fractions in JP-4 Jet Fuel (Day 0 to Day 25). Apparent Degradation. Figure 2 shows the apparent chemical degradation of the EC fractions by the three oxidants within 25 days in the defined microcosms. The chemical oxidation control (treatment A) exhibited little

destruction of any fraction because there was no oxidant added. Oxidation by KMnO4 (treatment B) achieved removals of more than 80% of each aromatic fraction and 40-60% of aliphatic fractions. The bulk of the oxidation by KMnO4 was finished within 1 day for both aromatic and aliphatic fractions. Oxidation by H2O2 (treatment C) removed 60-80% of each aromatic fraction, ca. 15% of aliphatic EC 5-6 and aliphatic EC >6-8, and 50-70% of aliphatic EC >8-10 and aliphatic EC >10-12. Most of the oxidation by H2O2 was completed within 8 days for both aromatic and aliphatic fractions. Oxidation by ORC (MgO2) (treatment D) destroyed 50-70% of toluene, aromatic EC >10-12, and aromatic EC >12-16, and 8-10. Approximately 50% of aliphatic EC >8-10 and aliphatic EC >10-12, and negligible aliphatic EC 5-6 and aliphatic EC >6-8 were oxidized. Figure 3(a) shows the corresponding apparent chemical degradation rate constants (kapp) in the defined microcosms. Generally speaking, 2-3 half-lives (i.e., 75-85% of degradation) are desired to obtain reliable first-order rate constants. In our experiment, however, degradation of EC fractions did not typically reach that level. Therefore, the rate constants derived in this study are better used as relative estimates, primarily for comparison of the degradation trends between EC fractions. The apparent chemical oxidation rate constants ranged from (0.026 ( 0.006) d-1 to (0.184 ( 0.063) d-1 for KMnO4 (treatment B), from (0.006 ( 0.002) d-1 to (0.086 ( 0.010) d-1 for H2O2 (treatment C), and from nondetectable to (0.040 ( 0.014) d-1 for ORC (MgO2) (treatment D), where ( denotes intervals of one standard deviation with sample size of 5. It is worthy to note that, aromatic fractions, the more toxic fractions, were more easily removed from the system by all the treatments with KMnO4, H2O2, and ORC (MgO2). As already explained in the Materials and Methods section, apparent removal rate of a compound rises in accordance with its increasing susceptibility to chemical transformation, higher aqueous solubility, and lower volatility. For example, toluene was relatively easily transformed (see the Actual Transformation paragraph below), more soluble than the aromatic fractions of EC >8, and less volatile than benzene. A relatively high removal rate was observed for toluene. The differing removal rates of other EC fractions can be qualitatively interpreted in a similar fashion. In summary, selective chemical removal of the EC fractions were observed. For example, aromatic fractions were more likely removed from the microcosms than aliphatic fractions. The most likely removable fractions were toluene VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Apparent degradation rate constants and actual transformation rate constants of EC fractions for aerobic biological oxidation. Treatment A: without chemical oxidation, with biological oxidation. Treatment B: with prechemical oxidation by KMnO4, with biological oxidation. Treatment C: with prechemical oxidation by H2O2, with biological oxidation. Treatment D: with prechemical oxidation by MgO2, with biological oxidation. EC ) equivalent carbon number, Al ) aliphatic, Ar ) aromatic. and aromatic EC >8-10 by KMnO4, benzene and toluene by H2O2, and aromatic EC >12-16 and toluene by ORC (MgO2). Actual Transformation. Figure 3(b) exhibits the actual pseudo-first-order chemical transformation rate constants (k) for the EC fractions. It can be clearly seen that the actual transformation of the EC fractions was different from the apparent degradation shown in Figure 3(a) in the selectivity and magnitude of rate constants. The actual pseudo-firstorder transformation rate constants ranged from (0.027 ( 0.006) d-1 to (0.380 ( 0.172) d-1 for KMnO4 (treatment B), from (0.010 ( 0.003) d-1 to (0.708 ( 0.378) d-1 for H2O2 (treatment C), and from nondetectable to (0.350 ( 0.187) d-1 for ORC (MgO2) (treatment D). The most easily transformed fraction was aliphatic EC >10-12 and the transformation rates of aliphatic EC fractions rose with increasing EC number for all the treatments with KMnO4, H2O2, and ORC (MgO2). The reason for this is not clear. Despite the use of KMnO4 as an oxidant for over 100 years, the mechanism of its oxidation of organics has remained unclear (34, 35), especially for the oxidation of alkanes. However, some general concepts may help in understanding the results of the present study. For instance, with increasing carbon chain length of hydrocarbons, the electron-donating effect is more significant and thus their susceptibility to electrophilic attack by oxidants is promoted. The oxidation rate differences for methine (CH), methylene (CH2), and methyl (CH3) are 1: 35-70:3000-3500, respectively (36, 37). Figure 3(b) also shows that for the oxidations by KMnO4 and H2O2, the transformation rates of aromatic EC fractions slowed with increasing EC number. The reasons may be that, with increasing number of multisubstitutions and size of the substituents on aromatic rings, steric effects become significant (22) and hinder electrophilic attack by oxidants. It is noted in Figure 3(b) that oxidation of benzene by KMnO4 was quite slow and that, in contrast with the oxidation by KMnO4, benzene was easily oxidized by H2O2 and the sidechain effect was not as significant as that by KMnO4. This is probably because the initial attack of alkylbenzene by MnO4occurs predominantly at the R-carbon of the side alkyl chain (38, 39), which further enhances the electrophilic attack on the aromatic rings, and yet there is no side chain for benzene. The oxidation by H2O2, however, more likely attacked the ring rather than the side chain (27) with hydroxyl radical (•OH), produced by H2O2 through a Fenton-like reagent mechanism (40, 29). The transformation rates of aromatic EC fractions by ORC (MgO2) slightly increased from aromatic EC >8-10 to aromatic EC >12-16. The reason for this is 4758

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unknown. The increasing number of aromatic rings may increase nucleophilicity, thereby facilitating the electrophilic attack. To conclude the actual transformation discussion, the most readily transformed fraction was around aliphatic EC >10-12 for all the treatments with KMnO4, H2O2, and ORC (MgO2), especially for the treatment with H2O2. The selective actual transformation of the EC fractions was observed and would affect their apparent removal from the microcosms as well as the subsequent microbial oxidation. Biological Oxidation of the EC Fractions in JP-4 Jet Fuel (Day 25 to Day 159). Apparent Biological Degradation. Figure 4 shows the difference of apparent aerobic microbial degradation of the EC fractions from day 33 to day 159 in all the defined treatments. The sterile control (treatment N) proceeded with negligible change for all fractions as expected. The live control without prechemical oxidation (treatment A) underwent apparent degradation to some extent of 4075% of both aliphatic and aromatic fractions. In general, the live microcosms with prechemical oxidation by (treatments B, C, and D) removed less aliphatic and aromatic fractions than biological oxidation alone (treatment A); the reason for this observation is discussed below in the Actual Biological Transformation section. Figure 5(a) shows the apparent biological removal rate constants (kapp) of the EC fractions. The apparent biological removal rate constants ranged from (0.001 ( 0.000) d-1 to (0.013 ( 0.001) d-1 for the live microcosm without prechemical oxidation (treatment A), from (0.001 ( 0.000) d-1 to (0.006 ( 0.002) d-1 for the live microcosm with prechemical oxidation by KMnO4 (treatment B), from nondetectable to (0.006 ( 0.001) d-1 for the microcosm with prechemical oxidation by H2O2 (treatment C), and from (0.001 ( 0.000) d-1 to (0.009 ( 0.004) d-1 for the microcosm with prechemical oxidation by ORC (MgO2) (treatment D), where the standard deviations were determined on a sample size of 9. It can be seen that the aerobic biological removal rates of all fractions were slower than chemical oxidation. Prechemical oxidation may accelerate the whole remediation process. Similar to the selective removal of the EC fractions by the prechemical oxidations, the differing biological removal rates rise with increasing susceptibility to biological transformation, higher aqueous solubility, and lower volatility. In summary, selective biological oxidations of the EC fractions were observed. For the microcosms constituted in the present study, the most likely removable fractions were aliphatic EC >10-12, benzene, toluene, and aromatic EC

>8-10 for all live treatments. Aerobic microbial removal rates of all fractions were slower than chemical oxidation. Actual Biological Transformation. Figure 5(b) exhibits the actual pseudo-first-order transformation rate constants (k) of the EC fractions for the microbial processes. The rate constants ranged from (0.002 ( 0.000) d-1 to (0.629 ( 0.119) d-1 for the live microcosm without prechemical oxidation (treatment A), from (0.003 ( 0.000) d-1 to (0.364 ( 0.130) d-1 for the live microcosm with prechemical oxidation by KMnO4 (treatment B), from nondetectable to (0.017 ( 0.000) d-1 for the live microcosm with prechemical oxidation by H2O2 (treatment C), and from (0.003 ( 0.001) d-1 to (0.264 ( 0.080) d-1 for the live microcosm with prechemical oxidation by ORC (MgO2) (treatment D). It can be seen in Figure 5(b) that aliphatic fractions were transformed at higher rates than aromatic fractions for all the treatments. This may be caused by the stabilizing effect of the ring resonance structures. In addition, short-chained aliphatic fractions were less degradable than relative long-chained aliphatic fractions. Previous studies reported as well that short-chained hydrocarbons except methane were more difficult to degrade than longchained hydrocarbons, the reason for which is still not clear (11, 41). For aromatic fractions, the degradation rates increased from benzene to toluene and EC >8-10. Barker et al. (42) documented a similar trend in aerobic degradation (i.e., xylene > toluene > benzene). This order is expected if one recognizes that aerobic degradation of aromatics starts with an electrophilic attack on the ring π-electrons via the mediation of oxygenases (39) and that the side alkyl groups exert electron-donating effects on the ring. However, the transformation rates of aromatic fractions with more numerous rings and substitutions such as EC >10-12 and EC >12-16 may be slowed by increasing steric hindrance. Figure 5b also exhibits the impact of the different prechemical oxidation treatments on the biodegradation rates of the EC fractions. The three oxidants exerted differing effects and, compared to treatment A (without prechemical oxidation), slowed to some extent the subsequent biodegradation of EC fractions. The preferred substrates for microorganisms were aliphatic EC >10-12 and aliphatic EC >8-10. These fractions were, however, also the substrates most vulnerable to chemical oxidation (Figure 3(b)). After the prechemical oxidations, these fractions probably did not remain sufficient to support microbial growth. For example, the biodegradation for these fractions was negligible after the strong H2O2 prechemical oxidation of these fractions shown in Figure 3(b) (treatment C). It is noted that while aromatic fractions, the more toxic fractions, were not readily transformed by microbial processes (Figure 5(b)), they were well transformed by the prechemical oxidations (Figure 3(b)). Thus, the favorable EC fractions (i.e., aliphatic EC >1012 and aliphatic EC >8-10) for microbial growth were also the fractions most subject to chemical oxidation. This may partially explain why the prechemical oxidations did not enhance the biological removal of the EC fractions. The biological processes were not effective in transforming the more toxic aromatic fractions. The prechemical oxidations, however, may be so. Overall, prechemical oxidation can enhance the removal of both petroleum in general and toxic aromatic fractions, especially in highly contaminated, toxic, and anaerobic environments; combined chemical-biological oxidation may be superior to biological alone.

Acknowledgments This work has been supported by a student fellowship from the American Petroleum Institute and the National Ground Water Association, an international travel grant from the University of Michigan, and a fund from the Department of Defense-Strategic Environmental Research and Development Program (through cooperative agreement #CR86441-01 with

USEPA-NRMRL). The authors thank Dr. Kim Hayes, Dr. Stuart Batterman, and Dr. Jeremy Semrau for their precious thoughts on this work. Thanks go to Dr. Donald Kampbell, Dr. Robert Menzer, Nitin Barad, Tim Baker, Hong-Ming Chen, Lee Major, and David Mioduch for their laboratory and field assistance. The authors appreciate the careful review and constructive comments of the reviewers and editors.

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Received for review October 22, 2002. Revised manuscript received June 18, 2003. Accepted June 24, 2003. ES026260T