1772
Ind. Eng. Chem. Res. 1993,32, 1772-1779
Nitrotoluenesulfonic Acid: UV, IR, and NMR Properties and Rate Studies of Wet Air Oxidation Kotu K. Phullt and Oliver J. Hao' Department of Civil Engineering, University of Maryland, College Park, Maryland 20742
Environmental fate of aromatic sulfonates is relatively unknown. This study reports the findings of wet air oxidation (WAO) of several sulfonic compounds, specifically 5-nitro-o-toluenesulfonic acid (NTSA). WAO is effective in the oxidation of NTSA, a compound similar in structure to dinitrotoluenesulfonates which are major components of T N T red water. The WAO reaction is first-order with respect to NTSA; the order with respect to oxygen pressure is 0.6. The activation energy is approximately 21 kcal/mol. The reaction rate is significantly enhanced with the addition of 5 mg/L Cu(I1) catalyst. The effect of initial pH on rate of NTSA oxidation is complex. Higher reaction rates are observed both at low and high p H s , with a better overall rate being achieved at low pH. Some properties of NTSA (e.g., UV, IR, and NMR) were obtained to provide additional information.
Introduction Sulfonation, the introduction of a sulfo (SO3) group into an organic molecule to yield a product with so3- linkages, is a common industrial process. On the basis of the large production of sulfonated compounds [annual 1.6 million tons in the US. (Kirk-Othmer, 1983)1, it is envisioned that their release into the environment may pose potential problems. For example, dinitrotoluenesulfonates are the major products formed in the trinitrotoluene (TNT) purification stage, which generates the so-called TNT red water (Hao et al., 1993a). Treatment and disposal of red water presents a significant problem for the U.S. Department of Defense. Aromatic sulfonates are not volatile, and their environmental fate is relatively unknown, with the notable exception of LBS (linear alkylbenzenesulfonate)for which the toxicity (Lewis, 1992), biodegradation (Federle and Schwab, 1992), sorption (Hinrichs and Snoeyink, 19761, and fate in wastewater plants (Brunner et al., 1988) have been extensively investigated. Unlike the insoluble parent compound, the sulfonated aromatics are highly soluble. Relatively high solubility and the consequential lower octanol-water partition coefficients of these compounds will render them less prone to soil sorption. Also, the toxicity of some sulfonated compounds is more severe than that of the parent compound. For example,the oral LDm for rats is 890 mg/kg for benzenesulfonic acid while the corresponding value for the parent compound, benzene, is 4894 mg/kg (RTECS,1983). Some sulfonates themselves may not be toxic even at relatively high concentrations, but chemical and/or biological transformations in the environment to other intermediates may pose significant problems. Consequently, identification of different methods for treatment of these sulfonic compounds present in industrial wastes is of great interest and importance. This study reports the findings of WAO (wet air oxidation) of several aromatic sulfonates. WAO is the oxidation of organics in an aqueous phase under high-pressure (6-17 MPa) and -temperature (150-350 "C) conditions. WAO has been shown to be an effective treatment technology for a variety of hazardous wastes (Baillod et al., 1979;
* To whom correspondence should be addressed. E-mail: ghuaain@I eng.umd.edu. + Current address: U.S. Army Environmental Center, Aberdeen Proving Ground, MD. 0888-5~85/93/2632-1772$o4.00/0
Miller et al., 1980;Heimbuch and Wilhelmi, 1985;Kalman et al., 1989). Sodium benzenesulfonate and five other substituted benzenesulfonic compounds containing the chloro, nitro, and methyl functional groups or combinations thereof (Table I) were subjected to WAO. After preliminary screening of the six compounds, one compound, 5-nitroo-toluenesulfonic acid (NTSA) was selected for detailed WAO studies; the criteria for selection of NTSA are discussed below. The specific objective of this study was to perform WAO experiments on NTSA to study the effects of temperature, oxygen pressure, NTSA concentration, pH, and catalyst addition on the rate of WAO. These factors are important as the oxygen dosage and the operating pressure determine both the capital and operational costs. Corrosion under low pH conditions may also limit practical application of WAO systems. Additionally, because available chemical data for NTSA are limited, some chemical properties (i.e., UV, IR, and NMR spectra) were obtained to provide useful information. Identification of some intermediates and suggestion of possible pathways are shown elsewhere (Hao and Phull, 1993).
Materials and Methods Aromatic sulfonic compounds were obtained from Aldrich Chemicals, with the exception of NTSA, which was required from Eastman Kodak. All chemicals were used without any further purification. The kinetic experiments were performed using procedures described elsewhere (Phull, 1992). Briefly, the 2-L reactor (Parr Instrument) was filled with 910 mL of distilled water and charged with a prescribed amount of oxygen (Figure 1). After reaching the desired temperature, NTSA and distilled water were injected sequentially into the reactor using helium backpressure. Samples were collected periodically via a sample loop immersed in an ice bath. WAO Experiments. A. Screening Tests. WAO experiments were performed under the following experimental conditions to select one of the six model compounds for detailed studies: T = 170-320 "C, initial compound concentration = 2 g/L, Po, = 0.48 MPa (at 25 "C), and mixing speed = 250 rpm. Samples were periodically withdrawn and analyzed for pH, TOC (total organic carbon), UV absorbance, and compound concentration. 0 1993 American Chemical Society
c
Ind. Eng. Chem. Res., Vol. 32,No. 8,1993 1773
a-OuZkmph
n*-
t
1
n.
C.nrnl*r)
t
.
--
To controlkr
mple
4
ZL-4 J
\
Ice Bath
2-L REACTOR Figure 1. Schematic of WAO setup.
Table I. Model Compounds Used compound sodium
sodium p -to1uena sulfonate
4chlorobenzene- sodium 3-nitro- 5-nitrosulfonic acid benzeneo-toluenesulfonate sulfonic acid
CdHaOsNa 98-11-3
CHsCst4SOsNa 657-84-1
98-66-8
la0
194
193
benzene sulfonate formula CAS Registry N0.O molecular weight structure
6
CAClSOaH
C&&IO#OsNa 127-ea4
2
c,WQH
CHaC&NOaOsH 121-03-9
sodium 2,4-dinitrobenzenesulfonate Cas(NO&SOaNa 885-62-1
253
270
8
&cH3
&
NO2
Na0S-S NOz
Ofi NO2
W absorbance bu,nm
258,264,270 224 linear range, M 5.5 X 1-5.5 X lo-' 5 X 1-1 molar absorptivity, 435 at 264 nm 11545 L/(molan) WAO fmt-order 1x 10-1 5.1 X lo-' rate const, min-1 a
X
226 lo-' l e - 2 13720
X
lo-'
7.54 x 10-9
210,264 l@-2 X lo-' 9040 at 278 nm
210,278 4 X 1 0 9 2 X lo-' 11365
254 4 X 1 0 9 7 X 1O-a
5.8 X 10-9
2.3
6.6 X
X
Supplied by the authors.
Table 11. Conditions Used in WAO Experimentsa parameter variable parameter values mixing speed, rpm 100,300,500,and 650 tempe, OC 280,290,300,320, and 335 0.2,0.5,1.0,2.0, and 3.0 initial NTSA concn, g/L 0.&,0.93,1.38, and 1.90 initial PO,, MPa PH 4.6,6.8, and 10.2 buffered 1.4,1.7,7.0, and 11.0 unbuffered initiatorlcatal, mg/L 6 and 60 Ha02 5andM) Cu(I1) inert gas press., MPa 0.83,1.21, and 1.72 He 1.31 Na a Experimenta were performed at 300 OC and 300 rpm unless otherwise noted.
B. Kinetic Experiments. A series of experiments was performed under different conditions to study the effect of the following factors on WAO of NTSA: mixing speed, temperature, initial NTSA concentration, oxygen pressure, initial pH (with and without buffers), total pressure, and
constant parameter values
CO= 2 g/L, PO,= 0.48 MPa CO= 2 g/L, PO,= 0.48 MPa
PO,= 0.97 MPa
co = 2 g/L
CO= 0.2 g/L, PO,= 0.97 MPa CO= 0 . 2 4 5 g/L, PO,= 0.79 MPa CO= 0.5 g/L, PO,= 0.72 MPa CO= 0.5 g/L, PO,= 0.72 MPa CO= 0.5 g/L, PO,= 0.79 MPa CO= 0.5 gIL, PO,= 0.79 MPa
w e of CatalyWinitiator (Table 11). With the exception of the experiments concerning the effect of mixing, all kinetic experiments were performed at a mixing speed of 300rpm. All experiments were conducted at 300 "C except those for evaluating the temperature effect.
1774 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
0
-K;fJfq Mo
Wavelength, nm
200
Wavelength, nm
400
".
? 3
ui
4 0
0
0
200
Wavntnnath. nm
3!50
350
200
Wavelength. nm
350
Figure 2. UV spectra of model compounds in water: (a) sodium benzenesulfonate (10 mg/L), (b)sodium p-toluenesulfonate(10 mg/L), (c) sodium 3-nitrobenzenesulfonate (40 mg/L), (d) 5-nitro-o-toluenesulfonicacid (25 mg/L), (e) p-chlorobenzenesulfonic acid (50 mg/L), and (0 sodium 2,4-dinitrobenzenesulfonate(20 mg/L).
Slightly modified procedures were used in the experiments to study the effect of initial pH by adjusting the solution pH with NaOH or H2S04. In these experiments, the reactor was initially charged with 0.5 g/L NTSA and heated. The first sample (time = 0 min) was collected when the desired temperature (300 "C) was reached. Analytical Met hods. RPIPC (reverse-phase ionpairing chromatography) was used for the analysis of all untreated as well as WAO-treated sulfonated compounds. A Dionex Model 2020i with a Hewlett Packard 3396A integrator was used. The eluant consisted of 28% acetonitrile and 2 mM tetrabutylammonium hydroxide, and the regenerant used was 0.025 N H2S04. A TOC analyzer (0.1.Model 700) with an IR detector was used for TOC analysis. Potassium hydrogen phthalate standard solution was used. A Hewlett Packard spectrophotometer (Model 8451A) was used for measuring UV absorbance. Metal analyses were performed using a Perkin-Elmer (Model 6500) inductively coupled plasma (ICP) spectrophometer. Soluble metal concentrations were determined by first digesting a sample with concentrated sulfuric acid, filtering through a 0.2-pm filter, and analyzing the filtrate for metals. A. NMR, IR, and X-ray Spectroscopy. NTSA was dissolved in 1 mL of D2O (Norell Chemicals), and the sample was transferred to a 5-mm Pyrex NMR tube. The 'H and 13C NMR spectra were obtained using a Varian VXR-400s FT NMR system. The "-1H COSY (correlated spectroscopy) spectra were obtained using the standard Varian software. Spectra were recorded a t probe temperature (22 "C), and TSP (sodium 3-trimethylsilylpropionate-2,2,3,3-&) was used as the external standard. NTSA was mixed with potassium bromide to obtain a KBr disk for IR analysis. A Perkin-Elmer FTIR spectroscope (Model 1760)equipped with a mercury cadmium telluride detector and a data station (Model 7700) was used. An X-ray elemental spectrometer (EDAX Model PV 9900) with a solid state silicon detector was used for elemental analysis of settleable solids observed in one of the WAO experiments. A transmission electron microscope (Philips Model CM/ 12 Stem Electron Microscope PW 6030) provided the desired sample magnification. The output is a spectral analysis of the sample showing percentages of the elements present.
Results and Discussion
U V Absorbance. It was initially intended to use UV spectroscopy to analyze the samples for all six model compounds screened. The UV spectra in solvents other than water (e.g., methanol/KOH) were available in the published literature (Sadtler, 1972) for some sulfonic compounds (e.g., benzenesulfonic acid, p-toluenesulfonic acid), but little information could be located for the UV spectra in water. Consequently, UV spectra for aqueous samples were obtained for all six compounds (Figure 2). The molar absorptivities (or molar extinction coefficients) were calculated as A = ccb, where A = absorbance (AU), = molar absorptivity (L/(mol.cm)), c = concentration (mol/L), and b = sample path length (cm). Results of the maximum absorption wavelengths, linear ranges of concentration, and molar absorptivity are summarized in Table I. Hinrichs and Snoeyink (1976) reported the maximum absorbance wavelength and molar absorptivity of 220nm and 10 530L/(mol-cm),respectively, for sodium p-toluenesulfonate, which are similar to those determined in the present study. The UV spectra for the sodium salts of sulfonic acids in water are similar to those of the corresponding acids in organic solvents. For example, the spectra for sodium benzenesulfonate (Figure 2a) in water showed maximum absorbances a t 258, 264, and 270 nm; the maximum absorbances for benzenesulfonic acid in methanol were reported to be at 252,259,263,265, and 269 nm (Sadtler, 1972). The maximum absorbances at 210 and 264 nm for aqueous solution of sodium 3-nitrobenzenesulfonate were slightly different compared to the maximum absorbances at 209 and 258 nm for 3-nitrobenzenesulfonic acid in methanol. It appears that the solvent and/or chemical structure (i.e., salt versus acid) had little effect on the maximum absorbance characteristics of the sulfonated compounds examined. Comparison of spectra for the sulfonated compounds with the parent compound showed some influence of the sulfonic group. For example, nitrobenzene in hexane shows maximum absorbance at 252 nm; sodium 3-nitrobenzenesulfonate in water absorbed most at 264 nm. The observed wavelength shift is perhaps primarily due to the substitution effect, but may also partially be due to the differences in the polarity of the solvents. In the case of NTSA in water, the maximum UV
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1775 SO,H
I (22.8)
CH, 2.70
-t 800
-
1" -8
TOC
T1
_I.
0
200 -
H 8.23, J = 2.6,8.4
0
Figure 3. Chemical ehifta and coupling constante for 1H NMR and chemical shifta for 1qNMR spectra.
Table 111. IR S m t r a for NTSA freq, cm-1 841 1078 1188 1353 1630
Io
I
1
1
I
I
I
I
1
20
40
BO
Bo
100
120
140
160
180
Time, minutes
Figure 4. WAO of sodium benzeneaulfonata(2 g/L) at 260 "C, 0.48 MPa Po,.
stretch C-N s ~ SOa m
=Ym sos s ~ NOz m
=P
NOz
absorbance occurred at 278 nm compared to 274 nm for the parent compoundp-nitrotoluenein methanol (Sadtler, 1972). UV spectroscopy was found to be adequate, easy, and quick to use. However, due to absorption of the intermediates formed during WAO in the same wavelength range as the starting material, UV analytical technique could not be used for quantitative analysisof WAO-treated NTSA. Instead, RPIPC was used for the sulfonate analyses. IR and NMR Spectra. The IR spectra for NTSA are summarized in Table 111, which coincides with those frequency regions exhibited by -NO2 and -SO3 groups (Bellamy, 1975; Silverstein et al., 1991). The values for chemical shifts (6) and coupling constant (J)for the 'H and 13C NMR for NTSA are annotated in Figure 3. Screening Tests. At Pol = 0.48 MPa (at 25 OC) and temperatures up to 260 "C, induction periods of slow or little oxidation were observed for all six compounds in terms of the reduction in TOC and UV absorption at wavelengths characteristic of the six compounds. The induction period is presumably due to the lack of free radicals which are responsible for the WAO reaction. Sodium benzenesulfonate showed no change in UV absorption at 264 and 270 nm for WAO experiments at 170, 200, and 230 O C . At the higher temperature of 260 OC, the induction period was approximately 80 min (Figure 4). The TOC reduction a t the end of the 3-h experiment was about 60%,and UV absorption at 264 nm was reduced by nearly 50%. At T = 290 OC, only NTSA and sodium 2,4dinitrobenzenesulfonate exhibited notable reduction in TOC. Dimensionlessconcentration profiles (C/Co) of the six model compounds with time at the higher temperature of 320 OC are shown in Fiugre 5. The semilog plots of Figure 5 indicate that the disappearance of the compound followed a fist-order rate for all six compounds. The approximate firsborder rate constants are listed in Table I. The pH values shown in parentheses (Figure 5) indicate the initial pH values without any pH adjustment. Sodium benzenesulfonate again showed a significantly long induction period; thereafter, its oxidation rate was the highest among
1
p', ,
(pH;8 ,)
,
,\\
~ : ' " ,(
~
1
,
,
(pH, = 2 2)
0.01
0
20
40
W
80
100
120
140
1W
180
200
Figure 5. WAO of model cornpounds (2 g/L) at 320 "C, 0.48 MPa
Po,. the six compounds studied. Induction periods are considered to be characteristic of free-radical reactions (Emanuel et al., 1967);they are inversely proportional to WAO temperature and Pol (Sadana and Katzer, 1974) and may range from a few minutes to several hours for different compounds, such as phenol and xylene (Sadana andKatzer, 197%Willmsetal., 1985;Joglekaretal., 1991). The results of these six compounds evaluated in this study further indicate that the induction period is compound specific,and it can be eliminated or shortened under harsh WAO conditions. The extent of the disappearance of the six compounds and the rates of oxidation are supposedly affected by the substituents. According to the Hammet equation (Lowry and Richardson, 1987)and as reported for WAO of several substituted phenols (Joglekar et al., 19911, the electrondonating substituent (e.g., 4 H 3 ) should enhance oxidation. Conversely,electron-withdrawingsubstituents, such as the chloro ( 4 1 ) and nitro (-Nod groups, should retard the oxidation rate. This was, however, not the case in the present study. Sodiump-toluenesulfonatewith the methyl group yielded a lower oxidation rate than the baseline compound, sodium benzenesulfonate. Sodium 2,4-dinitrobenaenesulfonatewith two nitro groups showed amuch higher oxidationrate thansodium 3-nitrobenzenesulfonate (one nitro group). The observed oxidation rates showed the following order for the effect of substituents: 2 NO2 groups > NO2 + CHa > C1> NO2 >C&. The exact reasons for the observed WAO rates for the six compounds &e., effect of functional group, sulfonic acid versus salt, initial
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
.
2500h
3 g/L
2 g/L
a
2000
1wo
' 100 rpm 500
0 3W rpm
A 500 rpm 0 650 rpm
0 20
0
40
80
80
100
140
120
180
180
Time, min
0
10
20
30
40
50
60
lb
Time, rnin
Figure 6. Effect of mixing on WAO of NTSA (2 g/L), 300 "C,0.48 MPa PO,.
pH) are, however, unclear. The initial pH (Figure 5)for sodium p-toluenesulfonate was higher than that for any other compound. As discussed later, the initial pH does affect the WAO rate of NTSA. In the absence of detailed reaction mechanisms for WAO of the six model compounds screened, it is not possible to provide a more meaningful analysis of their relative oxidative behavior. The compound, NTSA, was selected for detailed investigation primarily for two reasons. First, it showed reasonable oxidation rates in the desired temperature range between 280and 335 OC. Second,with the three functional groups (methyl, nitro, and sulfonic) on the benzene ring, NTSA most closely resembles one of the major components of the TNT red water, dinitrotoluenesulfonates. On the basis of an estimated pKavalue of -3.6 for NTSA (Guthrie, 1978),it can be assumed that NTSA was present mostly in the ionic form in the aqueous solutions used in WAO experiments (8X 1V to 1.2 X le2M). This assumption is supported by the measured pH values (=2.2),which are close to what would be expected in case of complete ionization of NTSA (pH 2.1). Effect of Mixing. Considering the low concentrations of NTSA used (2g/L) and the greatly enhanced solubility of oxygen at elevated temperatures and pressures (Himmelblau, 1960),it is fair to say that oxygen transfer may not be limiting for the range of Pop wed. In order to demonstrate the absence of mass-transfer limitations, WAO experiments were run at four different mixing speeds. As shown in Figure 6, varying the mixing speed between 100and 650rpm did not show a significant change in the oxidation rate. Willms et al. (1985)found no masstransfer limitations at similar mixing speeds for WAO of phenol and xylene. Effect of Initial NTSA Concentration. Five experimenta under different initial NTSA concentrations(0.2-3 g/L) were conducted at 300 O C and 0.97 MPa Pol. The amount of oxygen was considered as excess relative to even the highest NTSA concentration. A regression analysis of the initial rate data (first three data pointa, Figure 7a) showed the WAO of NTSA to be first-order with respect to NTSA concentration, A similar concentration dependence was also observed for TOC (Figure 7b). The finding of the reaction rate being first-order in substrate concentration is in agreement with the typical first-order reaction rates reported for other compounds by many investigators (Phull, 1992). It should be noted that the same initial Pop (0.97MPa) was used for all five experiments in which the NTSA
I"
0
20
40
60
80
100
120
140
160
180
200
Time, rnin
Figure 7. Effect of initial NTSA concentration on rata of oxidation, 300 OC,0.97 MPa PO,: (a) linear plote; (b) TOC data.
concentration was varied by a factor of 15. As overall reaction rates were first-order with respect to NTSA, it is fair to say that 02 transfer at 300 rpm was not limiting. Effect of Temperature. The resulta of five experimenta (initial NTSA = 2 g/L, Pop = 0.48 MPa, and T = 280-335 "C) show a clear dependence of oxidation rate on temperature (Figure 8). At 280 OC, there was very little reduction in NTSA, but at 335 OC the removal approached 99% after 70 min. An increase in temperature from 280 to 290 "C resulted in a significant reduction in the extent and rate of NTSA disappearance, thus suggesting 290 OC to be the temperature at which desulfonation, among other possible transformations, of NTSA becomes more perceptible. This temperature is over 100 OC higher than the desulfonation temperature of 2-toluenesulfonic acid (Gilbert, 1965),indicting that the nitro group in NTSA may impede desulfonation. This fiiding is in agreement with the observationsmade by Gilbert (1965)that the electronwithdrawing substituents generally hinder desulfonation. The observed first-order rate constants were calculated to be 1.7 X le2min-l at 290 OC and 6.1 X le2min-l at 335 "C. On the basis of the Arrhenius plot (not shown), the activation energieswere approximately21 and 19 kc& mol, respectively, from NTSA and TOC data. The similarity in the two activation energies may indicate a common rate-limiting step for WAO of NTSA, perhaps the formation of low molecular weight organic acids and other stable intermediates. The observed activation energies are similar in magnitude to those reported by other investigators for WAO of a variety of organic compounds including TNT red water (Hao et al., 1993b). Typical TOC removal for Pop = 0.48MPa ranged from a mere 4% at 280 "C to 80% at 320 "C at t = 3 h (Figure
Ind. Eng. Chem. Res., Vol. 32, NO. 8, 1993 1777
l a
335
10
*c
\
A 1.38 MPa 1.90 MPa
A
1
1 0
50
100
\
200
150
Time, min
o
1000
I_
\
TOC. = 664 mgiL 280 *C
20
40
60
a0
100
120
140
160
180
Time, min
1 b
An 1 280 ' C
300 .C
\
100 0
TOC, = 654 mg/L 0.48 MPa 0.93 MPa
,
I
1
50
100
150
200
Time, min
Figure 8. Effect of temperature on WAO of NTSA (2 g/L), 0.48 MPa PO,: (a) NTSA; (b) TOC.
8b). Complete TOC removal was not achieved under the conditions studied. Although harsher conditions and longer contact time may provide a more reasonable effluent TOC concentration (e.g., TOC removal of 95% in 4 h at 300 "C and Poz = 0.93 MPa), economic considerations may limit such applications. The relatively high effluent TOC (e.g., 100 mg/L) at reasonable WAO temperature, PG, and contact time may be partially due to the accumulation and slow oxidation of the low molecular weight organic acids. The accumulated low molecular weight organics provide excellent substrate for microbes. Assuming that the WAO-treated effluent would not be otherwise inhibitory to microbial growth, integration of partial WAO with suitable biological treatment might provide a practical and economic alternative for the treatment of aromatic sulfonates. Effect of Initial OxygenPressure. At constant initial concentration of 2 g/L and a temperature of 300 "C, five experiments were conducted at Pozranging from 0 to 1.90 MPa. In the experiment without oxygen, the reactor contents were purged with Nz for about 1 h before the heater was turned on. Several interesting observations were made from the results obtained. The extent and rate of disappearance of NTSA were highest when no oxygen was used; NTSA was not detected in the samples collected 40 min after sample injection. In contrast, the experiment with the highest oxygen pressure showed detectable amounts of NTSA up to 80 min after sample injection. The TOC efficiency was, however, lowest in the case without oxygen. The semilog plots of NTSA and TOC versus Poz show an effect of initial oxygen pressure up to a certain level, beyond which the effect diminishes (Figure 9). On the basis of experimental evidence, Emanuel et al. (1967)have
e
1.38 MPa t.90 MPa l
I
1
I
,
I
I
I
I
20
40
60
80
100
120
140
160
180
Time, min
Figure 9. Effect of initial PO,on WAO of NTSA (2 g/L), 300 O C : (a) NTSA; (b) TOC.
concluded that the oxidation rate increases with a rise in the partial pressure oxygen, reaches some limiting value, and then remains constant. Regression analysis of the data indicates that the order of NTSA oxidation with respect to Pozis 0.6. This agrees with the fractional orders of 0.4-0.5 previously cited by other investigators for WAO of propionic acid (Day et al., 1973) and phenol (Sadana and Katzer, 19741, although others have reported values ranging from 0 to 1 (Phull, 1992). Effect of Initial pH. Two sets of experiments were conducted to study the effect of pH from 1.4 to 11 on NTSA oxidation. The first set consisted of three experiments; appropriate buffering systems were used to achieve system pH values of 4.6 (0.05 M acetate buffer), 6.8 (0.06 M phosphate buffer), and 10.2 (0.05 M carbonate buffer). Large quantities of salts were present in the samples from those experiments in which buffers were used. Samples from the experiments at initial pH 10.2 exhibited dark yellow color. The ion-pairing chromatograms for these samples were differnt from those for the control without pH adjustment. No NTSA was detected in any of the samples from this run. In order to check whether the carbonate buffer interfered with NTSA measurements, two samples from this experiment were spiked with known amounts of NTSA. Nearly 100% of the added amount was recovered, and interference from the sample matrix was, therefore, ruled out. Samples from the experiment with pH 6.8 had a light yellow color. A large peak due to phosphate made the quantification of NTSA difficult; NTSA seemed to be present in all samples collected from this experiment.
1778 Ind. Eng. Chem. Res., Vol. 32,No. 8, 1993
1
I N o NTSA 0
I
was detected at pH = 1 7
20
40
Bo
80
10
1
* CONTROL 6 mg/L H,O,
A 60 mg/L H,O, 0Fenton s Reagent ,
20
Figure 10. Effect of initial pH on WAO of NTSA (0.5 g/L), 300 "C, 0.79 MPa Pol.
Samples from the experiments at pH 4.6 also provided different chromatograms (Phull, 1992) and yielded the lowest NTSA oxidation rate. The pH in this case was adjusted using an acetic acid/acetate buffer. It is wellknown that acetic acid can be oxidized with WAO, although the rate is relatively slow at 300 O C (Joglekar et al., 1991). Since the concentration of acetic acid was many-foldhigher compared to that of NTSA, the rate of NTSA conversion would be expected to be lower than that observed for the control sample. Additionally, 02may be the limiting factor with additional organic concentration. The second set of experiments involved only pH adjustment using NaOH or HzS04 for pH 1.7, 7.0, and 11.0;another experiment was performed at pH 1.4 with HC1 addition. Slightly modified procedures as described in the Materials and Methods section were used for these experiments. Adjustment of pH to 7 resulted in an overall lower oxidation rate (Figure 10). In the case of initial pH of 11, there was an increase in the oxidation rate in the initial start-up stage resulting in a significantly lower concentration of NTSA (110 mg/L) at t = 0 min. The subsequentoxidation showed two distinct phases with each oxidation rate below that for the control (no pH adjustment). Samples exhibited a yellow color, possibly due to the presence of nitroaromatic anions (e.g., nitrophenolate ion). Clearly, the best NTSA removal was achieved with low initial pH of 1.7. No NTSA was detected in any of the samples collected from this experiment. This could be due to the accelerated acidic hydrolysis of NTSA to the parent compound, nitrotoluene (Gilbert, 1965). The lower TOC value (82mg/L) at t = 0 also indicates a significant (50%) conversion of the initial organic carbon to COZ during the heat-up period. A large amount of charred solids was present in the samples from this experiment. Infrared spectroscopic analysis of the dark residue suggested the presence of iron and other oxides and the absence of organics (Phull, 1992). An X-ray elemental spectral analysis showed the presence of large amounts of iron in the residue in addition to copper, chromium, zinc, and phosphorus. Corrosion products (visual observation) and high chlorides (RPIPC results) were apparent in the samples from the experiment in which HCl was used for pH adjustment to 1.4. Samples had a large amount of dark purple to dark brown settleable solids. The brown color was probably due to iron oxide. An inductively coupled plasma spectrophotometric analysis of the 60-min sample from this experiment yielded the following high concentrations of metals: Cr, 90 mg/L; Cu, 150mg/L; Ni, 620 mg/L; Mn, 90
,
,
eo
80
,
,
,
1W
120
140
Time, rnin
100
Time, rnin
, 40
Figure 11. Effect of initiator on WAO of NTSA (0.5 g/L), 300 "e,
0.72 MPa PO%.
mg/L; and Fe, 1500 mg/L. This is not unexpected considering the fact that significant corrosion of the T-316 stainless steel reactor can occur at a low pH of 1.4. The results of experiments at different initial pH's with and without the use of buffers, as well as the qualitative observations, show the effects of pH on NTSA itself and on WAO of NTSA to be complex. The initial removal of NTSA is accelerated at both very low and very high pH's. For practical application, however, the potential corrosion of the reactor at low pH's needs to be further investigated and quantified. Effect of Catalyst/Initiator Addition. Two experiments were performed (300'C, 0.72MPaPo,, and 0.5g/L NTSA) to evaluate the effects of HzOz (6and 60 mg/L) on NTSA oxidation rates. As shown in the semilog plots of Figure 11,the addition of 6 mg/L HzOz did not show any noticeable rate enhancement. On the other hand, the addition of 60 mg/L HZOZ increased thereduction of NTSA concentration to 37% as compared to 25 % for the control within the first 5 min. The subsequent overall oxidation rate was also approximately 30% higher. One would expect to see the effects of initiators more clearly in the case of WAO of compoundslike phenol and xylene, which exhibit a significant induction period. Although NTSA did not show appreciable induction periods under the conditions studied, the observed initial increase in the oxidation rate may be due to the momentary presence of excess radicals. Since Fenton's reagent (Fez++ HzOZ)has been often cited as an effective catalyst in autoxidation, one experiment was performed to observe its effect on WAO rate of NTSA. The effect of Fenton's reagent (17mg/L Fe and 60 mg/L HzOZ)is similar to that of the addition of 60 mg/L HzOz (Figure 11). In addition to the action of metal catalyst, Fenton's reagent may also enhance the reaction rate by readily producing additional OH' radicals. AB a result, the NTSA concentration of the first sample (5min after sample injection) was the lowest (230 mg/L, a reduction of 54% 1. With the addition of Fenton's reagent, the radical concentration in the reactor may exceed the critical concentration of the radicals (Emanuel et al., 1974; SadanaandKatzer,1974;Willmset al., 19861,thus causing the increased NTSA oxidation rate within the first 5 min. As the free radical chain reactions proceed and the radicals are consumed in the propagation and termination stages, an equilibrium concentration of radicals is reached and the oxidation rate reverts back to the normal, unenhanced rate. In fact, the subsequent oxidation rate was slightly lower than that for the control in the present study. Two experiments were conducted to evaluate the catalytic effect of copper on NTSA oxidation rate (300 OC, 0.72MPa Po2,and 0.5 g/L NTSA) by using CuClz (5
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1779 and to Linda L. Szafrariec of the CRDEC for assisting in NMR analysis. Literature Cited
CONTROL
s mg/L CU(II)
A SO mg/L CU(II) 10
20
40
60
80
100
120
140
Time, min
Figure 12. Catalytic effect of copper(I1) on WAO of NTSA (0.5 g/L), 300 "C, 0.72 MPa P+
and 50 mg/L as Cu). As shown in Figure 12, addition of 5 mg/L Cu(I1)followed the fmt-order kinetics;the reaction rate constant was approximately 3 times the rate for the uncatalyzed reaction. No NTSA was detected after the 45-min samples. At a higher dosage of 50 mg/L Cu(II), the reaction rate was even higher initially, but later decreased (after about 30 min) to slightly less than the overall rate for the uncatalyzed WAO of NTSA under the same experimental conditions. Additionally, the higher catalyst concentration appeared to have affected the reaction pathway somewhat, as evidenced by the presence of two reaction regimes and the deep yellow color of the treated samples, not seen a t the lower concentration of the catalyst or in the case of the control. The inhibitory effect of the high catalyst concentration on the oxidation of tetralin (1,2,3,4-tetrahydronaphthalene)was also reported (Kamiya and Ingold, 1964). Conclusions The maximum UV absorbances for NTSA were 210 and 278 nm, and maximum molar absorptivity was 11370 L/(mol*cm).The NMR And IR data obtained in this study should provide references for subsequent investigators. WAO of several aromatic sulfonates indicates the effects of functional group and pH on oxidation rates. WAO is effective in the oxidation of NTSA, a compound similar in structure to dinitrotoluenesulfonates which are major components of TNT red water. It appears that oxygen transfer at 300 rpm used for the present study was not limiting. The WAO reaction is first-order with respect to NTSA; the order with respect to oxygen pressure is 0.6. The extent and rate of NTSA disappearance were highest in the absence of oxygen; different reaction mechanisms occur for experiments with or without oxygen. The effect of initial pH on rate of NTSA oxidation is complex. Higher reaction rates are observed both a t low and high pH's, with a better overall rate being achieved at low pH. Because of the presence of a high concentration of metals, the corrosive nature of the reactor at low pH must be quantified. The reaction rate is significantly enhanced with the addition of 5 mg/L (Cu(11) catalyst. Acknowledgment Funding for this research was partially provided by the US. Army Construction EngineeringResearch Laboratory under Contract Number DACA88-91-M-0216. Dr. A. W. Maloney was the project officer. The authors are grateful to Colonel W.Kavanagh and his staff at the U.S.Army Environmental Hygiene Agency, Aberdeen Proving Ground, MD, for the use of their laboratory and support
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Received for review December 1, 1992 Revised manuscript received April 6, 1993 Accepted April 23, 1993
