|Home | About | Journals | Submit | Contact Us | Français|
Photochemical degradation of 1-nitropyrene, 2-nitrofluorene, 2,7-dinitrofluorene, 6-nitrochrysene, 3-nitrofluoranthene, 5-nitroacenaphthene, and 9-nitroanthracene were examined in CHCl3, CH2Cl2, DMF, DMF/H2O (80/20), CH3CN, or CH3CN/H2O (80/20). The degradation follows mostly the 1st order kinetics; but a few follow 2nd order kinetics or undergo self-catalysis. The photodegradation rates follow the order: CHCl3 > CH2Cl2 > DMF > DMF/H2O > CH3CN > CH3CN/H2O. DMF is an exceptional solvent because 3 of the 7 compounds undergo self-catalytic reaction. 9-Nitroanthracene, which has a perpendicular nitro group, is the fastest, while the more compact 1-nitropyrene and 3-nitrofluoranthene, are the slowest degrading compounds.
Nitro-polycyclic aromatic hydrocarbons (Nitro-PAHs) are generated as a result of incomplete combustion of organic materials and petrochemicals, or formed by the reaction of PAHs with NO2 or NO3− in the atmosphere . Nitro-PAHs are of primary concern because many of them are much more carcinogenic than their parent PAHs, and display a greater degree of mutagenicity although they appear in the environment one to two orders of magnitude lower than their parent PAHs . They are found in the environment in the vapor and aqueous phase, and are absorbed onto particulate matters. Nitro-PAHs undergo various chemical reactions in the environment including photochemical reactions when exposed to light. It has been proposed that the photochemical reaction rate and mechanism of Nitro-PAHs depends on the orientation of the nitro group, whether it is parallel (co-planar) or perpendicular to the aromatic rings . The nitro group is forced out of the plane into a perpendicular position by the peri hydrogens to minimize energy due to steric hindrance. Co-planar Nitro-PAHs usually undergo photochemical oxidation of the aromatic rings, while the perpendicular Nitro-PAHs are proposed to undergo rearrangement of the nitro group first to a nitrite and then to a nitroso substituted ketone. The nitroso substituted ketone is not stable, and is easily oxidized further to quinones [4, 5].
There have been a number of studies performed on the degradation of Nitro-PAHs, and numerous techniques to determine their concentration in the environment or their photoproducts. Warner et al examined the relationship between nitro group orientation and photochemical degradation of Nitro-PAHs . Fourteen different Nitro-PAH samples were dissolved in CH3CN or absorbed onto a surface (silica, alumina, carbon, and cellulose) and irradiated. Their results showed that not all Nitro-PAHs in solution demonstrated a relationship between nitro group orientations. The compounds absorbed onto a surface showed that the nature of the particle had more of an influence on degradation than the structure of the Nitro-PAHs.
It is common knowledge that solvents have different effects on the rate of degradation, and affect the formation of photoproducts [7, 8]. Moeini-Nombel and Matsuzawa investigated the effect of solvents and substituent groups on the photo-oxidation of fluorene . This research compared the rate constants, degradation half-lives, and quantum yield of fluorene and its derivatives when irradiated with UV light. The photodegradation varied based on the solvent used with CH2Cl2 giving a faster rate, and 50/50 CH3CN/H2O giving a slower rate. Fluorene and 1-methylflourene displayed similar photostabilities, while 2-nitroflourene was more resistant to photodegradation. In this paper, we will present the photoreaction kinetics and the effect by solvents and position/orientation of the nitro group on the photodegradation rate of seven Nitro-PAHs.
1-Nitropyrene (1-NP), 2-nitrofluorene (2-NF), 3-nitrofluoranthene (3-NFA), 5-nitroacenaphthene (5-NAN), 6-nitrochrysene (6-NC), 9-nitroanthracene (9-NA), and 2,7-dinitrofluorene (2,7-DNF) were purchased from Sigma-Aldrich Chemical Company (Milwaukee, WI) and used without further purification. The solvents used were: CH3CN, CH3CN/H2O (80/20), CH2Cl2, CHCl3, N,N-dimethyl formamide (DMF), DMF/H2O (80/20), and MeOH. All solvents were HPLC grade purchased from Fisher Scientific (Fairlawn, NJ, USA) and used without further purification. Water used (18 Ω) in this experiment was deionized with a Barnstead Nanopure Infinity water deionization system (Dubuque, IA). All of the selected Nitro-PAHs were prepared in each solvent at 100 µM concentration, and were irradiated with a UVA Lamp (Spectraline SB-100P, 365±20 nm, Spectronics Corporation, Westbury, NY, USA) at 8 cm distance with an output energy of 144 J/cm2•h.
A Nitro-PAHs solution in a quartz cuvett (1 cm light path) was irradiated and the degradation was monitored using a CARY 300E UV-Vis absorption spectrophotometer from Varian Inc. (Houston, TX). The absorption at a certain wavelength, [A]0 and [A]t at the start and at time t, respectively, were recorded. The wavelength selected to record the absorption (relative concentration of the starting compound) was carefully chosen so that there is no absorption from the photoproducts to interfere with the measurement. Specifically, the wavelength used for each compound was: 1-NP: 405 nm; 2-NF: 336 nm; 2,7-DNF: 346 nm; 3-NFA: 377 nm; 5-NA: 374 nm; 6-NC: 373 nm; 9-NAN: 364 nm. A plot of Ln([A]0/[A]t) vs. irradiation time t was used for first order reactions and a plot of 1/[A] versus t for second order reactions. For first order reactions, Ln([A]0/[A]t) = kt was used to determine the rate constant (k). Once k was calculated, the degradation half-life (t1/2) was determined by t1/2 = 0.693/k. For second order reactions, 1/[A] = 1/[A]0 + kt was used to determine k. Once k was calculated, the degradation half-life was determined by t1/2 = 1/(k[A]0).
Irradiation by UVA light of the solutions of Nitro-PAHs in various solvents was carried out. The absorption spectra at each irradiation time interval and the absorption value at the peak were recorded. The progress of the photodegradation was analyzed by either fitting to a 1st or a 2nd order reaction. Their degradation half lives were calculated. Some did not fit well with either the 1st or the 2nd order reaction and the Ln([A]0/[A]] versus t plot curved upward. We classified these as self-catalytic reactions. Due to solubility problem, some of the degradation could not be accomplished. There were also a few cases that did not appear to have any degradation during the period irradiated. The results are summarized in Table 1.
1-NP degraded in all of the selected solvents; however, there was only 60% degradation after 75 min of UV irradiation in CH3CN, 37% after 180 min in CH3CN/H2O, and 60% after 60 min in CH2Cl2, not enough degradation to calculate the degradation half-lives. 1-NP has a t1/2 of 37 min in DMF, 69 min in DMF/H2O, and 12 min in CHCl3. The rate of degradation of 1-NP is CHCl3 > DMF > DMF/H2O. The nitro group in 1-NP has one peri-hydrogen and should be co-planar to the pyrene rings.
2-NF degraded in all solvent systems used for this study. It showed 21% degradation after 160 min of irradiation in CH3CN, and 15% degradation after 60 min in CH3CN/H2O, not enough to determine a half-life. 2-NF in CH2Cl2 followed a zero order reaction and a half-life was determined to be 90 min. The degradation of 2-NF in CHCl3 is a 2nd order reaction with a half-life of 1.6 min calculated by t1/2 = (1/k[A]0). 2-NF underwent self-catalysis in DMF and DMF/H2O, and a half-life could not be determined. The plot of Ln([A]0/[A]t] vs. t curved upward, indicating the reaction speeded up during the irradiation. It is known that photolysis of 2-NF produces 2-nitrofluorenones , which can be the photosensitizer to catalyze the further reaction of 2-NF.
2,7-DNF was insoluble in nearly all solvents except in DMF, although Warner et al used CH3CN for 2,7-DNF . 2,7-DNF in DMF and DMF/H2O became intensely yellow after 1 min of irradiation while it was clear before irradiation. The solution became darker as time lapsed. The t1/2 = 7 min when 2,7-DNF is irradiated in DMF/H2O. The half-life of 2,7-DNF in DMF could not be determined due to not enough degradation after 60 min of irradiation.
3-NFA was one of the most resistant to photodegradation in that it showed little or no degradation in CH3CN, CH3CN/H2O, or CH2Cl2. It did show 85% degradation in CHCl3 (Figure 2) or DMF. However, it only degrades 52% after 150 min of irradiation in DMF/H2O. 3-NFA was sparingly soluble in MeOH. The half-life of 3-NFA in CHCl3 was 24 min and was a 1st order reaction. However, 3-NFA underwent self-catalysis in DMF and the half-life could not be determined. The orientation of the nitro group of 3-NFA is parallel to the ring, and is more stable under light irradiation [3, 6].
5-NAN degraded in all selected solvents. 5-NAN has a λmax between 372 – 376 nm. 5-NAN in CHCl3 was a 2nd order reaction with t1/2 = 8.5 min. The degradation in CH3CN, CH3CN/H2O, and DMF/H2O are 1st order reactions. The half-life of 5-NAN in these solvents are: CH3CN = 63 min, CH3CN/H2O = 88 min, DMF/H2O = 37 min. The half-life could not be determined in CH2Cl2 due to slow degradation. 5-NAN in DMF underwent self-catalysis and a half-life could not be determined (Figure 3). 5-NAN has one peri-hydrogen and the nitro group is co-planar. The rate of degradation of 5-NAN from fastest to slowest was: CHCl3 > DMF > DMF/H2O > CH3CN > CH3CN/H2O. As an example for the three types of photoreaction kenetics, 1st order, 2nd order and self-catalysis, figure 3 plots the Ln([A]0/[A]t] vs. t for 5-NAN in all of its solvents. As can be seen, the upward curvature in DMF signifies self-catalysis.
The plot of Ln([A]0/[A]t] versus t for 6-NC in CHCl3, CH2Cl2, and DMF/H2O are straight lines while it curves upward in DMF, signifying self-catalysis (not shown). 6-NC showed a minimum of 80% degradation in DMF, DMF/H2O, and CH2Cl2. 6-NC degraded rapidly in CHCl3, and showed 87% degradation within 8 min. 6-NC was sparingly soluble in MeOH, and it did not generate enough data in CH3CN, and CH3CN/H2O to plot. The half-lives of 6-NC are 2 min in CHCl3, 18 min in CH2Cl2, 17 min in DMF, and 35 min in DMF/H2O. The rate of degradation of 6-NC from fastest to slowest is: CHCl3 > DMF ≈ CH2Cl2 > DMF/H2O.
The plot of Ln([A]0/[A]t] versus t for 9-NA yielded straight lines in the initial stages of the irradiation and became curved after a significant of degradation had occurred. 9-NA degraded very rapidly in all selected solvents, and was the fastest of all Nitro-PAHs studied here. The majority of the starting material had totally degraded within 10 min of irradiation in most solvents. 9-NA in MeOH was also soluble. The formation of photoproducts of 9-NA in DMF/H2O solvent were observed and they are known as 9,10-anthraquinone and bianthrone [4, 10–12]. The half-lives for 9-NA are as follows: CHCl3 = 1 min, CH2Cl2 = 2 min, CH3CN = 6 min, DMF = 3 min, DMF/H2O = 4 min, CH3CN/H2O = 6 min.
From the data in Table 1, we found that the photoreaction rates of Nitro-PAHs are governed by several factors related to solvent and their intrinsic structure. Solubility and polarity of a solvent affect the photochemical reaction rate. Structure, both the PAH ring arrangement and position or orientation of the nitro-group, play important roles in the photoreaction of Nitro-PAHs. In addition, photoreactions of Nitro-PAHs follow different kinetics, 1st and 2nd order or self-catalytic reaction.
Nitro-PAHs are relatively insoluble in water, but soluble in organic solvents. Six of our selected Nitro-PAHs were soluble in DMF and DMF/H2O, but were only sparingly soluble in MeOH. 2,7-DNF was soluble only in DMF and DMF/H2O although Warner et al was able to carry out the photolysis in CH3CN. 6-NC was soluble in majority of the solvents, but was sparingly soluble in CH3CN and DMF/H2O. 1-NP, 2-NF, 3-NFA, 5-NAN, and 9-NA were all soluble in CH3CN, CH3CN /H2O, CH2Cl2, CHCl3, DMF, and DMF/H2O. Solvent rank based on solubility is as follows: DMF > DMF/H2O > CHCl3 > CH2Cl2 >CH3CN > CH3CN /H2O MeOH. Although DMF seems to be the best solvent to dissolve the Nitro-PAHs, chloroform is the best solvent for kinetics study.
If the degradation half-lives are compared for a chosen Nitro-PAH in all the solvents, it degrades fastest in CHCl3, followed by CH2Cl2 and DMF. For 9-NA, which has a degradation half-life in all solvents, the degradation rate rank is: CHCl3 > CH2Cl2 > DMF > DMF/H2O > CH3CN > CH3CN/H2O. The other Nitro-PAHs have a similar pattern. This seems that the degradation rate is anti-parallel to the polarity of the solvents, which is CHCl3 < CH2Cl2 < CH3CN ≈ CH3CN/H2O, except for DMF and DMF/H2O. The rate of degradation of our selected Nitro-PAHs in the various solvents may be attributed to the solubility of dissolved oxygen (O2). In general, the solubility of O2 increases as the solvent polarity decreases [7, 13]. CHCl3 is the least polar solvent while water is the most polar. The results indicate that the oxygen content decreases with increasing water content which explains why the rate of degradation is slower with the addition of water to CH3CN or DMF. The presence of oxygen oxidizes the parent compound leading to photodegradation.
In DMF, three Nitro-PAHs, 2-NF, 3-NFA and 5-NAN, follow a self-catalytic kinetics, indicating that DMF is different from other solvents. The degradation reaction starts slow, but picks up speed as more and more starting compound is converted to photoproducts, resulting in an upward curvature for the ln[A0]/[A] versus irradiation time t plot. The logical explanation is that one or more of the photoproducts are photosensitizers. Possible photosensitizers are quinones since they are known to catalyze PAH photodegradation . 2-NF may produce 2-nitrofluorenone . 5-NAN may produce acenaphthene-5,6-dione and 3-NFA may produce of fluoranthene-2,3-dione. However, these photoproducts have not been isolated and characterized.
Using the degradation rates (half-lives) of Nitro-PAHs in CHCl3 in Table 1, the rank of the degradation rates is: 9-NA > 2-NF ≈ 6-NC > 5-NAN > 1-NP > 3-NFA. Similar ranking can be achieved in other solvents, although the data are not as complete as in CHCl3 due to solubility or slow degradation. It is known that the nitro group in 9-NA is in the perpendicular conformation to the anthracene ring and undergoes rearrangement to nitrite very quickly due to photolysis. This is followed by transformation to a nitroso ketone intermediate, which converts to anthraquinone [4, 10]. None of the other Nitro-PAHs has two peri-hydrogens as 9-NA and would not likely follow this reaction mechanism. The likely photoreaction is oxidation of the aromatic rings. These are slower reactions than the nitro to nitrite rearrangement reaction [3, 15]. The relatively faster rate for 2-NF and 6-NC may be due to the quick conversion to the respective quinones . Finally, 1-NF and 3-NFA are the slowest degrading nitro-PAHs due to the more compact ring systems that are difficult to be oxidized.
The photoreaction of the seven selected Nitro-PAHs in six solvent systems follows three kinetics models: 1st order, 2nd order, or self-catalysis. The complexity of the photoreaction kinetics of Nitro-PAHs is reflective of the variety of reaction pathways the Nitro-PAHs can take: rearrangement reaction, oxidation of the ring, reaction with molecular oxygen or solvent molecules, and bi-molecular reactions . The fact that most photoreactions are 1st order indicates that the rate determining step for the photoreaction involves one Nitro-PAH molecule. The 2nd order reaction is reflective of the bi-molecular reaction or complex parallel reactions. The self-catalytic reaction is reflective of the photosensitizing effect or the reactive nature of the photoproducts that can facilitate degradation of the original Nitro-PAHs. Another issue is the concentration of molecular oxygen in the solvent. Since most of the degradation of the Nitro-PAHs in solution requires molecular oxygen , the concentration differences of molecular oxygen in the solvents will contribute to the complexity of the photoreaction kinetics.
In conclusion, the photoreaction of Nitro-PAHs is complex with various pathways, kinetics models, and photoproducts. Solvent polarity, concentration of oxygen in the solvent, orientation of the nitro group, and the ring structure of the PAH are main contributors to the complexity. Degradation rate ranks as CHCl3 > CH2Cl2 ≈ DMF > DMF/H2O (80/20) > CH3CN > CH3CN/H2O (80/20). 5-NAN degradation starts slow in DMF, but becomes faster after 30 min due to self-catalysis. The rate of degradation of the seven selected Nitro-PAHs in various solvents is attributed to the solubility of dissolved oxygen and how solvents facilitate the photoreaction. In general, the solubility of O2 increases as the solvent polarity decreases [7, 13]. CHCl3 is the least polar solvent while water is the most polar. This explains why the rate of degradation is slower with the addition of water to CH3CN or DMF. The presence of oxygen oxidizes the parent compound leading to photodegradation.
This research was in part supported by the National Institutes of Health (NIH-SCORE S06 GM08047). We thank NIH-RCMI for Analytical Chemistry Facility established at JSU. Gernerique Stewart would like to thank the U.S. Department of Education Title III for funding (Grant P031B040101).