Environ. Sci. Technol. 2000, 34, 505-508
Effects of Surfactants on Reduction and Photolysis (>290 nm) of Nitroaromatic Compounds
CHART 1
R I C H A R D A . L A R S O N , * ,† C H A D T . J A F V E R T , ‡ F R A N C I S C O B O S C AÄ , † KAREN A. MARLEY,† AND PENNEY L. MILLER† Department of Natural Resources and Environmental Sciences, University of Illinois, 1101 West Peabody Drive, Urbana, Illinois 61801, and School of Civil Engineering, Purdue University, 1284 CE Building, West Lafayette, Indiana 47907
The rates of disappearance of some nitroaromatic compounds, namely, dinitrotoluene, dinitrobenzene, and dinitrobenzenesulfonic acid from alkaline borohydride solution were greatly enhanced in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), both in the presence and in the absence of light. A neutral surfactant (Tween 80) had little or no effect on the photoreactions, and an anionic surfactant (sodium dodecyl sulfate) slowed the photoreactions slightly.
Introduction Nitroaromatic compounds are among the most abundant of environmental pollutants because of their uses as military ordnance compounds and as precursors for monomer synthesis. Their occurrence in the environment is of great concern because of their toxicity, stability, and role as precursors in the formation of compounds of possibly greater toxicity (i.e., azo, azoxy, and nitroso compounds). Although nitroaromatic compounds are relatively stable, at least in an aerobic environment, they become more reactive when photochemically excited. In such an excited state, nitroaromatic compounds may undergo photoisomerization reactions or, in the presence of nucleophiles and/or electron donors, undergo facile substitution and reduction reactions. The reduction pathway may be initiated by electron transfer to or hydrogen atom abstraction by the nitro excited state. We demonstrated (1) that alkaline solutions of sodium borohydride accelerated the photodecomposition of several nitroaromatic compounds in the presence of solar wavelengths of ultraviolet light (>290 nm). The disappearance of the nitro compounds was facilitated if the illumination reaction was conducted in the absence of oxygen. A variety of reaction products were identified, including denitration, desulfonation, and condensation products. Surface-active agents (surfactants) are well-known to have major influences on photochemical and photophysical processes. Such phenomena as fluorescence, redox reactions, and singlet and triplet quenching may either be enhanced or inhibited by the presence of surfactants, particularly when they are present at high enough concentrations to form micelles (2). Micelles, especially when they consist of cationic * Corresponding author e-mail:
[email protected]; phone: (217)333-7269; fax: (217)333-8046. † University of Illinois. ‡ Purdue University. 10.1021/es990891e CCC: $19.00 Published on Web 12/28/1999
2000 American Chemical Society
or anionic species, are also recognized to promote reactions between molecules that involve charged reaction partners or intermediates. For example, the reduction of ketones by sodium borohydride is greatly enhanced by the presence of cationic surfactants (3). Several groups have reported that the photodecomposition of environmental pollutants such as PCBs (4-6) and chlorinated benzenes (7) was promoted by the presence of neutral or anionic surfactants. We report that the rates of disappearance of some nitroaromatic compounds, namely, dinitrotoluene, dinitrobenzene, and dinitrobenzenesulfonic acid (1-3, see Chart 1) from alkaline borohydride solution were greatly enhanced in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), both in the presence and in the absence of light.
Experimental Methods Reagents. HPLC-grade methanol and acetonitrile were purchased from Baxter Healthcare (Muskegon, MI) and Fisher Chemical (Fair Lawn, NJ), respectively. 1,3-Dinitrobenzene (DNB, 1), 2,4-dinitrotoluene (DNT, 2), 2,4-dinitrobenzenesulfonic acid sodium salt (DNBSA, 3), cetyltrimethylammonium bromide (CTAB), dodecyl sulfate sodium salt (SDS), Tween 80 (TW), and Dowex 50X8-100 ion-exchange resin were purchased from Aldrich Chemical Co. (Milwaukee, WI). Anhydrous magnesium sulfate, hydrochloric acid, sodium bicarbonate, and sodium hydroxide were supplied by Fisher Scientific (Pittsburgh, PA). High-purity methylene chloride was obtained from Baxter, Burdick and Jackson (Muskegon, MI). Sodium borohydride was purchased from Alfa Products (Danvers, MA). Laboratory water was deionized and glass distilled from a solution of KMnO4/KOH before use. Instrumentation. The HPLC system consisted of a Beckman 110B pump (San Ramon, CA) fitted with a manual injector from Rheodyne (Cotati, CA) and a Kratos Spectroflow 757 detector with both tungsten and deuterium lamps (Ramsey, NJ). Hamilton polymeric reversed-phase (PRP-1) analytical columns (Reno, NV: 10 µm packing, 25 cm × 4.1 mm) were used. VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The combined GC/MS instrument used was a 5890 Hewlett-Packard gas chromatograph (Avondale, PA) coupled with a Finnigan MAT ion trap detector (Sunnyvale, CA) and a DB-1 capillary column, 30 m × 0.32 mm i.d. (J&W Scientific, Folsom, CA). UV-visible spectra were obtained using a Hitachi model U-2000 double-beam spectrophotometer (Hitachi Instruments Inc., San Jose, CA). Samples were pipetted into 1-cm quartz cuvettes, and a wavelength scan was performed with a scan speed of 200 nm/min. Photoreactions were performed using a medium-pressure Pyrex-filtered mercury arc lamp (λ > 290 nm) with power supplied by a 200-W source (Ace Glass, Vineland, NJ). The lamp was contained in borosilicate immersion well, which was water-cooled. Kinetic studies were carried out using a “merry-go-round” reactor (Ace Glass). Photoreaction Conditions. To stabilize the highly reactive solutions of sodium borohydride, it was necessary to add sodium hydroxide to all solutions of this compound before addition to the solutions of the nitroaromatic compounds. The sodium borohydride solutions were typically prepared by first dissolving 0.8 g of NaOH in 100 mL of water (0.2 M) and then adding 1.25 g of sodium borohydride (final concentration, 0.33 M). Kinetic reactions were performed by pipetting aliquots totaling 3 mL into Pyrex disposable screw-cap culture tubes (13 × 100 mm). Half of the samples were in the dark, and the others were placed at a distance of 5 cm from the mediumpressure Pyrex-filtered mercury arc lamp and illuminated. To obtain enough material to analyze the reaction products by GC/MS, we performed larger scale reactions employing 20 tubes, each one containing 5 mL of reaction mixture. Both dark and light reactions lasted 1 h. DNT Photoreactions in Micelles. Stock solutions of each surfactant (CTAB, SDS, TW) were combined with 0.4 mM DNT solutions and water at a 1:1:2 volume ratio to produce the following concentrations: 8 mM ionic surfactants (CTAB, SDS), 1 mM TW, 0.1 mM DNT. These reactions were studied at the same concentrations in the presence of 12.5 mM NaOH. For CTAB-DNT photoreactions, additional reaction mixtures at 1.25 and 125 mM concentrations of NaOH were used. DNT photoreactions with NaBH4 in the presence of SDS or TW were studied combining each surfactant stock solution with DNT (0.4 mM), NaBH4-NaOH (0.33-0.2 M), and water to produce the following concentrations: 8 mM SDS, 1 mM TW, 0.1 mM DNT, and 16.5-12.5 mM NaBH4-NaOH. DNT-NaBH4 photoreactions in the presence of CTAB were studied changing the concentrations of DNT, NaBH4, and CTAB. For this, stock solutions described above were used at different relative ratios to give the following reaction mixture concentrations: (a) 8 mM CTAB, 0.1 mM DNT, and 16.5-12.5 mM NaBH4-NaOH; (b) 8 mM CTAB, 1 mM DNT, and 16.5-12.5 mM NaBH4-NaOH; (c) 8 mM CTAB, 0.1 mM DNT, and 1.65-12.5 mM NaBH4-NaOH; (d) 0.8 mM CTAB, 0.1 mM DNT, and 16.5-12.5 mM NaBH4-NaOH; (e) 0.08 mM CTAB, 0.1 mM DNT, and 16.5-12.5 mM NaBH4-NaOH. DNB and DNBSA Photoreactions in Micelles. DNB photoreactions in micellar aqueous solutions (CTAB) were studied in the presence and in the absence of NaBH4 in reaction mixtures with the following concentrations: 16.512.5 mM NaBH4-NaOH or 12.5 mM NaOH, with 8 mM CTAB and 0.1 mM DNB. DNBSA photoreactions in micelles were prepared in the same way as DNB photoreactions using a stock solution of 0.4 M DNBSA to give a final [DNBSA] of 0.1 mM. Kinetics. The loss of the nitrobenzene derivatives was followed by HPLC using a monitoring wavelength of 254 nm and a flow rate of 1.0 mL/min. Aliquots (200 µL) of samples of the reaction mixtures were taken at various time intervals and analyzed by HPLC using a mobile phase of 80:20 MeCN/ H2O (v/v). Under these conditions, DNT eluted at 4.2 min 506
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TABLE 1. Half-Lives of Nitroaromatic Compounds (Concentration, 10-4 M) Exposed to UV (>290 nm) compound
[CTAB], mM
pH
[NaBH4], mM
t1/2, min
DNT DNT DNT DNT DNT DNT DNT DNB DNB DNB DNB DNBSA DNBSA DNBSA DNBSA
0 0 0 8 8 8 8 0 0 8 8 0 0 8 8
8.0 12.0 12.0 8.0 12.1 11.7 11.7 12.0 12.0 11.8 11.8 12.0 12.0 11.8 11.8
0 0 16.5 0 0 16.5 1.7 0 16.5 0 16.5 0 16.5 0 16.5
>1000 85 46 >1000 9 1000 55 173 1 720 55 347 1.4
and DNB eluted at 3.5 min. For the analysis of DNBSA, a mobile phase comprised by volume 55/3/42 of MeCN/ MeOH/(0.3% CTAB/H2O [w/v]) was used. Under these conditions, DNBSA eluted at 6.1 min. All concentrations were determined by reference to standard curves. The half-lives (t1/2) were calculated from first-order plots of the loss of the starting material. Product Analysis. In preparative reactions (from a series of 20 tubes) after 1 h of reaction (dark or light), 100 mL of the reaction mixture (from the 20 tubes) was placed into a 250-mL Erlenmeyer flask and neutralized with HCl (0.2 M) to approximately pH 7. After passing the reaction mixture (100 mL) through a column of cation-exchange resin (Dowex50, Na+ form) to remove the CTAB, 200 mL of deionizeddistilled water was added to the column and collected together with the eluate from the reaction mixture. This solution (300 mL) was extracted with CH2Cl2 (3 × 20 mL) and collected in a 100-mL flask. After removing water with magnesium sulfate, the organic solution was evaporated under reduced pressure at 35 °C to dryness and redissolved in 2 mL of CH2Cl2. One portion (2 µL) of this solution was injected onto the GC (FID) for quantification, and another portion (1 µL) was injected onto the GC/MS for identification of products. Identification was performed, where possible, by matching the spectra to those published for reference compounds or by comparing spectra and retention times to those of authentic standard compounds.
Results and Discussion Effects of Structure. The pseudo-first-order half-lives of the nitro compounds exposed to >290 nm UV illumination under various pH values and CTAB and borohydride concentrations are summarized in Table 1. [Regression analyses for these experiments (r 2 ) 0.89-0.99) supported the pseudo-firstorder kinetic assumption.] All the nitro compounds tested were relatively stable to illumination in distilled water or buffer (without borohydride or surfactant) at >290 nm (1). However, dinitrotoluene, for reasons that are not entirely clear, was susceptible to direct photolysis at pH 12 but not at pH 8. 2,4-Dinitrotoluene (DNT). This compound was reduced with borohydride in the presence of one of the three surfactants SDS, Tween 80, or CTAB. The results are presented in Figure 1. The anionic surfactant, SDS, slowed the photoreaction (t1/2 ) 63 min) relative to the measured half-life in the absence of SDS (46 min; ref 1). A plausible explanation is that DNT is largely incorporated into the SDS micelles, but their surface negative charge causes electrostatic repulsion of the reagent, BH4-, from the micelle environment. In the presence of the neutral surfactant, Tween 80, the loss rate
FIGURE 1. Photodegradation of 2,4-dinitrotoluene (DNT) in the presence of NaBH4 and Tween 80 (closed triangles), SDS (open circles), and CTAB (solid circles).
FIGURE 2. Half-life for photolytic loss of DNT as a function of pH (at pH 8, t1/2 was >1000 min). for DNT was almost unaffected (t1/2 ) 43 min). In both cases, low yields of the nitro reduction products, 2-methyl-5nitroaniline (4) and 4-methyl-3-nitroaniline (5) were observed. However, CTAB, a cationic surfactant, strongly promoted the loss of DNT even in the absence of light. When an aliquot of DNT was added to a solution containing CTAB, base (pH 12), and borohydride, the solution became purple. This color faded rapidly ( 1000 min) in the absence of borohydride or surfactant. Addition of CTAB alone increased its loss rate (t1/2 ) 170 min). Photodecomposition of this compound also occurred with borohydride alone (t1/2 ) 55 min), but CTAB greatly promoted its loss in the presence of borohydride, with a half-life of approximately 1 min. Low yields of 3-nitrophenol and 3-nitroaniline were observed by HPLC and GC/MS of the photoreaction mixtures. DNB reacted rapidly with CTAB and borohydride in the dark (t1/2 ) 3 min), but there was virtually no reaction with CTAB alone or borohydride alone. Again, this would be consistent with micellar enhancement of the aromatic nucleophilic substitution process. 2,4-Dinitrobenzenesulfonic Acid (DNBSA). This compound was examined because it is an analogue of several similar acids formed during the sulfonation of TNT impurities (10). At alkaline pH values in the dark, it was stable (t1/2 > 1000 min) in the absence of surfactant, whether borohydride was present or absent. At 8 mM CTAB, however, its half-life was only 5.5 min. As in the reaction with DNT, a colored (pink) intermediate was observed, but in this case the reaction darkened with time, eventually becoming a deep orange. In the light, when illuminated in the presence of borohydride, it photodecomposed with a half-life of 55 min (1). A neutral surfactant (Tween 80) inhibited the photoreaction (t1/2 ) 100 min), but CTAB greatly promoted it (t1/2 ) 1.4 min). The products of the photoreaction of DNBSA included the desulfonation product, 1,3-dinitrobenzene (2); a mononitrosulfonic acid (denitration product); a further reduction product of 2, m-nitroaniline; and several unidentified polar products. Effects of Surfactant Type. The results indicate that, in general, product formation was driven by electrostatic interactions among the constituents of the system. Experiments with DNT and DNBSA demonstrated that when these interactions were favorable, as in the case of CTAB micelles, the reactants accumulated in the micellar environment and the rate of reaction increased. If electrostatic interactions were unfavorable, as in SDS micelles, a reduction in degradation rate was observed because, although the organic compound might partition into the micelle, borohydride would be excluded. In the case of a neutral surfactant, the rate of the reaction would not be expected to be greatly affected, although other factors could operate to either accelerate or inhibit it. Products and Mechanisms. Reductions and photoreductions of nitro substituents to amino groups predominated in the alkaline CTAB-BH4- systems, suggesting that electronor hydride-transfer reactions were occurring. Because in our previous experiments without surfactants (1), we observed similar rates of reaction when either BH4- or the analogous tetraphenylborate were used, it seems likely that at least the first step of the photoreaction is a photoinduced one-electron transfer to the excited state of the nitro compound. The VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Dinitrobenzene (2) was isolated from a dark reaction of DNT. This suggests that side-chain degradation, presumably via oxidation, must have been occurring even in the presence of excess reductant (BH4-). The most probable explanation for such a process is given in Figure 3; an intermediate free radical having some odd-electron contributions from the side chain methyl group, reacting with molecular oxygen to produce dinitrobenzaldehyde and dinitrobenzoic acid intermediates that could decarbonylate, decarboxylate, or be displaced by hydride to form the observed product.
Acknowledgments The research was partly supported by a contract from the U.S. Army Construction Engineering Research Laboratory (DACA88-93-D-0011). The Spanish Ministerio de Educacion y Ciencia provided a postdoctoral fellowship for F.B. We thank Gary Epling and Gary Peyton for helpful discussions.
Literature Cited
FIGURE 3. Possible mechanism of conversion of DNT to DNB. conversion of a nitro group to an amino group requires the equivalent of six electrons, and without further evidence of intermediates, little further can be said concerning the mechanistic details of further aspects of the process. Neither nitroso nor hydroxylamino compounds were detected in the reaction mixtures, unlike reactions using iron metal as a reductant in our laboratory, where the sequential reduction products were present (11). A number of products could be formed only by displacement reactions, most likely entailing either nucleophilic or photonucleophilic substitution processes. These products can be explained by Meisenheimer intermediates in which either H- or HO- were added to the ring and SO3- or NO2were subsequently lost. When HO- was the nucleophile, phenols were observed among the products.
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Received for review August 2, 1999. Revised manuscript received November 1, 1999. Accepted November 12, 1999. ES990891E
