Ind. Eng. Chem. Res. 2000, 39, 3659-3665
3659
Wet Air Oxidation of Aqueous Solutions of Linear Alkylbenzene Sulfonates Dionissios Mantzavinos,† David M. P. Burrows,‡ Roy Willey,‡ Giuseppe Lo Biundo,§ Sheng F. Zhang,§ Andrew G. Livingston,§ and Ian S. Metcalfe*,§ Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London SW7 2BY, United Kingdom, Unilever Research Port Sunlight, Quarry Road East, Bebington, Wirral L63 3JW, United Kingdom, and Department of Chemical Engineering, University of Leeds, Clarendon Road, Leeds LS2 9JT, United Kingdom
The semibatch wet air oxidation of aqueous solutions of linear alkylbenzene sulfonate (LAS), an anionic surfactant, has been investigated at temperatures of 453 and 473 K, total pressures of 2.8 and 3.3 MPa, and reaction times varying from 40 to 390 min. The concentration of total organic carbon, chemical oxygen demand, and active detergent were followed throughout the reaction, the main intermediates formed were identified by means of electrospray-MS and highperformance liquid chromatography, and a mechanism describing LAS oxidation was proposed. It was found that LAS could easily be oxidized at 473 K to yield a group of molecules with short alkyl chains that do not behave as active detergents. The segments of alkyl chains broken off the reaction intermediates appear primarily as short-chain organic acids that are resistant to total oxidation. Oxidation experiments were also performed at 473 K with solutions of 4-hydroxybenzene sulfonic acid, an intermediate formed during the oxidation of LAS. It was found that 4-hydroxybenzene sulfonic acid could easily be oxidized. Introduction Wet air oxidation (WAO) is an emerging technology for treating wastewaters containing organic pollutants that may not be readily biodegradable. Although the main application of WAO is still the conditioning and/ or destruction of waste-activated sludge1, over the last several years, increased interest has been shown in the potential capability of WAO for treating wastewaters containing organic compounds. WAO provides an efficient method for either partial or total destruction of organic compounds such as phenol and substituted phenols, nitrogenous compounds, and carboxylic acids, whereas other classes of organics such as polyphenols, alcohols, ketones, and polymers have been given less attention2. Considerable attention has also been given to catalytic WAO using various heterogeneous3 and homogeneous4 catalysts as catalysts can potentially promote oxidation at shorter reaction times and under milder operating conditions. However, potential drawbacks associated with the use of catalysts, such as catalyst stability and deactivation, toxicity of leached metals, and need for additional process steps to recycle or remove the catalysts, cannot be disregarded. WAO studies cover a broad spectrum of aqueous solutions of organics ranging from single-compound model solutions to more complex synthetic mixtures to actual wastewaters. However, most of the studies reported in the literature suffer from a lack of detailed information on the properties of the reaction intermedi* Author whom all correspondence should be addressed. Present and corresponding address: School of Chemical Engineering, University of Edinburgh, Mayfield Rd, Edinburgh EH9 3JL, United Kingdom. Telephone: +44 131 6508553. Fax: +44 131 6506551. E-mail:
[email protected]. † University of Leeds. ‡ Unilever Research Port Sunlight. § Imperial College of Science, Technology and Medicine.
ates formed and mechanisms involved during the chemical oxidation, because they mainly focus on the determination of lumped parameters such as total organic carbon content and chemical oxygen demand. This is probably so because characterization of an oxidized reaction mixture requires state-of-the-art analytical techniques and is generally much more complex and time-consuming than the use of lumped parameters. However, information regarding intermediates is necessary in deciding the degree of oxidation required to remove a specific pollutant. Our previous work has shown that WAO can be employed effectively as either a single process or part of a process combination to remove various organic compounds. Polyphenols can be completely removed through a WAO process,5-6 while high molecular weight water soluble polymers can be effectively pretreated by WAO to undergo further physical and biological treatment processes.7-10 Linear alkylbenzene sulfonates (LAS) are anionic surfactants widely used in the production of household and industrial detergents, and their presence in waters and wastewaters may cause environmental concerns.11 However, the treatment of aqueous solutions of anionic surfactants (and particularly of LAS) by means of chemical oxidation has received little attention. Ozonation has been employed to oxidize various anionic surfactants,12 while photocatalytic processes have been employed to degrade dodecylbenzene sulfonate, sodium salt.13-14 In a recent study, the electrochemical oxidation (with and without hydrogen peroxide) of LAS has also been reported.15 The purpose of the work described in this paper is to study the noncatalytic WAO of LAS with respect to the effects of operating conditions (temperature, pressure, reaction time) on the reaction kinetics, the identification of the main intermediate compounds formed and their stability toward further oxidation, and the elucidation of reaction pathways.
10.1021/ie000385u CCC: $19.00 © 2000 American Chemical Society Published on Web 09/09/2000
3660
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000
Figure 1. Schematic diagram of the WAO reactor system.
Experimental and Analytical Material. A commercially available LAS paste (Petrelab 550, manufactured by Shell) was used to prepare aqueous LAS solutions. The LAS paste was characterized with respect to its inorganic sulfate, active detergent (AD), and moisture content; these were found to be 0.52, 50.5, and 52 ( 2% wt/wt, respectively. The LAS used in this study has the formula (CH2)nCH3C6H4SO3with n taking values between 9 and 12. Determination of the alkyl chain length distribution of the LAS present in the paste was performed using fast atom bombardment (FAB-MS), and it was found that the paste contained 8, 34, 35, and 23% wt/wt for n equal to 9, 10, 11, and 12 alkyl chains, respectively. A standard aqueous LAS solution was used in all of the experiments and was prepared by adding 2 g of the LAS paste per liter of distilled water. This resulted in an LAS solution with the following properties: LAS concentration, 1000 mg L-1; TOC, 710 mg L-1; COD, 2672 mg L-1; pH, 5.65.8. All of the experiments were performed with an intial LAS concentration of 1000 mg L-1. Higher concentrations were avoided to comply with safety regulations and to minimize any hazards associated with the exothermic nature of liquid-phase oxidation reactions. Wet Air Oxidation Reactor. A 300-mL Hastelloy high-pressure reactor (Baskervilles Ltd., U.K.) capable of performing semibatch or continuous experiments at pressures up to 10 MPa and temperatures up to 573 K was used. A schematic diagram of this reactor system is shown in Figure 1. In a typical semibatch run, 300 mL of the LAS solution were loaded into the reaction vessel, which was then rapidly pressurized to the operating pressure with nitrogen through a bypass valve (V4). The vessel was then heated to the operating temperature (473 K in most experiments) under nitrogen, while being stirred at 1 000 rpm. When the set temperature was reached, oxygen was fed continuously to the reactor at a flowrate of 1 L min-1 (STP) (through M/EV2) to start the reaction. Gas left the reactor through a pressure relief valve (RV1), which was manually set to the desired operating pressure. An airactuated relief valve (AV2) was set to a pressure that exceeded the operating pressure by 1 MPa and was used for safety reasons. The system was also equipped with a bursting disk to vent the reactor contents in the case
of reactor overpressure. The total pressure was kept at 2.8 MPa (in most experiments) so as to give an oxygen partial pressure of 1.3 MPa for the experiments performed at 473 K. For those experiments in which relatively small liquid volumes were required for further analysis (e.g., only for TOC or COD measurements), liquid samples of approximately 5-10 mL were withdrawn from the gas sparge tube located at the bottom of the reactor during a brief shut-off of the gas feed (V2). The use of a sample cylinder (between V6 and V7) directly connected to the gas line through a special quick-connect coupling assembly minimized contamination of the samples as well as loss of reaction liquid. Any liquid remaining in the sample port was blown back into the reactor after the gas feed was turned on again. For those experiments in which a large volume of liquid sample was needed for further analysis (e.g., AD determination, chromatographic analysis), the reaction vessel contents were collected at the end of each run. To minimize the extent of any reactions that would occur during the cooling time, the following procedure was utilized: At the end of the reaction time, oxygen flow was shut off, and the vessel was immediately depressurized (through V5) to a pressure that exceeded the vapor pressure at the conditions under consideration by 0.5 MPa, so as to remove most of the oxygen from gas phase, while ensuring that no evaporation of liquid phase would occur. The vessel was then repressurized with nitrogen (through V4) and sparged continuously with nitrogen. To decrease the length of cooling time, the vessel was immersed in a water bath. As soon as the temperature dropped to ambient conditions, the vessel was opened, and its contents were collected. Total Organic Carbon (TOC). TOC was measured with a Shimadzu 5050 TOC Analyzer that is based on combustion and subsequent nondispersive infrared (NDIR) gas analysis. Total carbon (TC) was first measured, and then the inorganic carbon (IC) was measured. Total organic carbon (TOC) was determined by subtracting IC from TC. The uncertainty in this assay, quoted as the deviation of three separate measurements, was never larger than 1% for the range of TOC concentrations measured. Chemical Oxygen Demand (COD). COD was determined by the dichromate microdigester method.16
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3661
The average value of three separate measurements per vial was taken, and the maximum deviation between three different sample vials did not exceed 1.5%. HighPerformanceLiquidChromatography(HPLC). HPLC was used for the determination of short-chain organic acids by comparing them with external standards of the supposed compounds. Separation was achieved using an AHINEX-HPX874 300 × 7.6 mm column with a 0.01 N solution of H2SO4 used as the mobile phase (pH ) 2.3) at a flowrate of 0.5 cm3 min-1 and a temperature of 333 K. Detection was through a UV detector that was set at a wavelength of 210 nm. The identity of organic acids was also verified through a refractive index detector. Quantification was based on the comparison of UV chromatograms with those of external standards that were prepared with known concentrations (200, 500, and 1 000 mg L-1) of the identified compounds. The linearity between absorbance and concentration was also tested, and the response was found to be linear over the entire range of concentrations under consideration. Blanks were run between two consecutive HPLC runs to ensure that no residuals from the previous run were carried over to the next run. The analysis of the chromatograms obtained was performed with the SUMMIT chromatography data handling system. Electrospray-MS. Electrospray-MS analysis was performed with a Micromass Platform mass spectrometer (Fisons Scientific, U.K.). Ionization mode was electrospray with samples diluted at 1% v/v in a 50/50 methanol/water solution. The injection volume was 100 µL, and sodium lauryl sulfate and gramicidin S were added to the samples and used as calibration standards for negative and positive ion mode, respectively. Calibration was between ion masses of 22 and 500 with nominal mass accuracy. Preliminary tests with original LAS solutions showed that LAS was not detected during positive ion mode operation. Therefore, further analysis was performed with negative ion mode operation. Active Detergent (AD) and Inorganic Bisulfate Content. These measurements were made using appropriate titration methods. For AD determination, an aqueous solution of the sample was titrated in a stirred titration vessel with benzethonium chloride solution in a two-phase chloroform-water system using dimidium bromide and disulfine blue V as indicators. For inorganic bisulfate determination, a weak acid solution of the sample in a water/acetone/2-propanol mixture was titrated with lead nitrate solution using dithizone as indicator. The sulfonate present remains in solution, whereas the bisulfate is nearly quantitatively precipitated as lead bisulfate. Results and Discussion Effect of Stirring. There was a concern that stirring of the reactor contents could result in foam formation in the reactor and that this foam might contain an excess of organic surfactant. Sampling from the bulk of the liquid phase would then give spurious results, indicating a much greater degree of oxidation than had actually taken place. Therefore, to investigate the effect of stirring on foam formation 1 L of LAS solution was stirred for 3 h at room temperature and ambient pressure in a conical flask, and the TOC of the liquid was measured before and after stirring. Moreover, the same solution was re-stirred after 1 day for 6 h, and
Table 1. Effect of Temperature and Pressure on TOC Removal (1-TOC/TOCo)
after 45 min of heating after 60 min of oxidation after 120 min of oxidation
T ) 453 K, Pt ) 2.8 MPa, PO2 ) 1.8 MPa
T ) 473 K, Pt ) 2.8 MPa, PO2 ) 1.3 MPa
T ) 473 K, Pt ) 3.3 MPa, PO2 ) 1.8 MPa
0.11
0.07
0.13
0.17
0.30
0.24
0.15
0.34
0.33
again TOC measurements were recorded before and after stirring, as was the liquid volume. It was found that there was no significant effect in terms of TOC reduction during foam formation for the aforementioned conditions, although there might be a slight decrease (of about 3%) in bulk liquid TOC during and after stirring. Furthermore, active detergent analysis was performed for the LAS solution before and after stirring. This confirmed that the active detergent concentration was unchanged. Organic Volatility and Loss. Although the reactor was operated in batch mode with respect to the liquid phase, there was continuous feed of oxygen and hence continuous venting of off-gas. There was, therefore, a need to investigate the loss of organic carbon volatilized and carried from the reactor in the vapor phase. Therefore, LAS was oxidized for 40 min at a temperature of 473 K and a total pressure of 2.8 MPa, and a material balance on the liquid phase was performed. Ten mL of the initial liquid volume (this was 300 mL) left the reactor, thus leading to a 3% liquid loss after 40 min of oxidation. Of these, 6 mL were collected in the off-gas after the pressure relief valve. However, as cooling was performed at room temperature, significant amounts of vapor may have been left in the off-gas. Nevertheless, the TOC value of this “lost” liquid was less than that of the final liquor. This implies that, although organic carbon will be lost from the reactor in the vapor phase, there is no significant reduction in the concentration of the organic carbon that remains within the reactor. (It should be noted that the TOC lost from the reactor may well have been in the form of shortchain organic acids, and therefore, it would have been reasonable to analyze the composition of the liquid lost from the reactor. However, because of the small volumes of liquid lost over reasonable times of operation, it was not possible to perform this analysis.) Effect of Temperature and Pressure. Preliminary oxidation runs were performed at temperatures of 453 and 473 K and total pressures of 2.8 and 3.3 MPa to obtain an indication of the effect of temperature and pressure on the oxidation of LAS. Samples were withdrawn from the reactor and analyzed with respect to their TOC content. Table 1 shows the TOC removal during the oxidation of LAS. It can be seen that the TOC decrease is greater for the two runs at 473 K than for the run at 453 K, as would be expected. Conversely, an increase in the total pressure of operation appears to have little effect. As a result of this significant decrease in TOC, it was decided that 473 K would be a reasonable temperature at which to perform further oxidation experiments. It should be noted that the TOC of the solution decreased during the heating period even though, for all runs, the heating was performed under nitrogen. This could have been due to some form of reaction, e.g., thermal degradation or polymerization.
3662
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000
Table 2. TOC and Liquid Volume Change after the Oxidation of LAS at 473 K andVarious Reaction Times reaction time (min)
initial TOC (mg L-1)
TOC after heating (mg L-1)
final TOC (mg L-1)
liquid volume left (mL)
liquid volume from off-gas (mL)
sample volume (mL)
liquid volume lost (mL)
40 80 120 160 200 390
702 713 714 715 710 710
NDa 676 618 636 657 630
574 555 522 487 466 348
290 255 240 235 230 195
6 14 25 30 36 73
0 8 10 9 7 9
4 23 25 26 27 23
a
ND ) not determined.
Figure 3. Electrospray mass spectrum (negative ion mode) after 40 min of LAS oxidation at 473 K. Figure 2. COD, active detergent removal, and pH change after the oxidation of LAS at 473 K and various reaction times. b, COD; O, active detergent; 4, pH (shown in secondary axis).
(This TOC decrease cannot be explained by foam formation in the reactor.) Effect of Reaction Time. Additional experiments were performed at a temperature of 473 K, a total pressure of 2.8 MPa, an oxygen partial pressure of 1.3 MPa, an initial liquid volume of 300 mL, and with reaction times varying from 40 to 200 min. At the end of each experiment, the reactor contents were collected and analyzed. Table 2 shows the change of TOC and liquid volume, while Figure 2 shows the change of COD, AD content, and pH as a function of reaction time. (The liquid volume in the off-gas was collected; however, as cooling was performed at room temperature, significant amounts of vapor might have been left in the off-gas. Furthermore, there is inevitably loss of vapor during sampling. During each experiment, a sample of approximately 10 mL was withdrawn after the heating period and analyzed with respect to its TOC content. The liquid volume lost was calculated by closing the material balance.) TOC decreases of only about 18%, 22%, and 34% were recorded after 40, 80, and 200 min of oxidation respectively, while the COD decreases were 32%, 37%, and 50%, respectively (these values take into account any TOC and COD decrease that had occurred during the heating period). However, there is a signifcant decrease in the concentration of active detergent with 68%, 76%, and 89% reduction recorded after 40, 80, and 200 min of oxidation, respectively. One additional experiment was performed at a temperature of 473 K and a reaction time of 390 min in order to investigate the maximum oxidation potential of the LAS solution, and the results are also shown in Table 2 and Figure 2. These results show that not all of the intermediates are completely oxidized on this time scale, as decrease of only about 51% and 58% TOC and COD,
Figure 4. Electrospray mass spectrum (negative ion mode) after 160 min of LAS oxidation at 473 K.
respectively, were recorded in the liquid left in the reactor. However, 98% of the total anionic active detergent was removed; this implies that the remaining organic compounds do not behave as detergents. The liquid exiting the reactor in the off-gas was also condensed, collected, and analyzed with respect to its TOC, COD, and active detergent content. For the liquid collected from the off-gas, decreases of 65% and 70% TOC and COD, respectively, were recorded, while the AD removal was more than 99%, thus suggesting that the more volatile organics are not active detergents. Oxidation Intermediates and Reaction Pathways. By means of electrospray-MS and HPLC analysis, various intermediate compounds formed during the oxidation of LAS were identified. Figures 3 and 4 show the mass spectra for the oxidized solutions after 40 and 160 min of oxidation, respectively, at 473 K in which peaks at 297, 311, 325, and 339 correspond to n9-n12 alkyl chains, respectively, of the original LAS solution. It can be seen that, after 40 and 160 min of oxidation at 473 K, the original LAS peaks can all still be clearly seen, while peaks of lower ion masses also appear. Peaks at 185, 199, 213, 227, 241, 255, 269, and 283, as well as
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3663
Figure 5. Concentration-time profile for TOC and organic carbon contained in short-chain organic acids during the oxidation of LAS at 473 K. O, TOC; b, organic carbon contained in short-chain organic acids.
peaks that appear in the series 201, 215, 229, 243, and so on can be clearly seen and are part of clear homologous series as the mass difference of 14 corresponds to a CH2 group in the side chain. A peak at 97 can also clearly be seen and corresponds to inorganic bisulfate. As the reaction time increases from 40 to 80 to 160 min, the concentration of the higher-molecular-weight aromatics decreases, and the concentration of the lowmolecular-weight aromatics increases (peaks at 173, 185, 199, 201) (see also Support Material 1). However, after 200 min of oxidation, the concentration of the lowmolecular-weight aromatics also decreases, presumably because of their further oxidation (see Support Material 2). By means of HPLC analysis, it was found that the oxidation of LAS was also accompanied by the formation of various short-chain organic acids. Of these, formic, acetic, propionic, and butyric acids were successfully identified. HPLC analysis also showed that, as the reaction time increases, the concentration of short-chain organic acids also increases, and this result is consistent with the decreasing pH of the solution. Even after 390 min of oxidation at 473 K, the concentration of shortchain organic acids in the reaction mixture remains relatively high. Figure 5 shows the concentration time profile for TOC and the sum of the organic carbon contributions calculated from the four identified shortchain organic acids during the oxidation at 473 K. (Samples were withdrawn from the reactor at various time intervals during a 200-min oxidation experiment). It can be seen that the organic carbon contained in the identified short-chain organic acids comprises between 15 and 25% of TOC throughout the course of the experiment, while the discrepancy between TOC and the carbon contained in short-chain organic acids is presumably due to the rest of the intermediates (both shortchain organic acids and aromatics) that were not identified and/or quantified. Figure 6 shows the proposed reaction pathways for the oxidation of LAS. Oxidation of LAS (e.g., XnR1) could consecutively lead to the formation of the corresponding alcohol (XnR2), aldehyde (XnR3), and acid (XnR4) with the same number of carbon atoms in the alkyl chain. These compounds could then undergo random scission, resulting in molecules that contain fewer carbon atoms in the alkyl side chain (e.g., Xn-yRi). Alternatively, acids (e.g., XnR4) could also undergo a decarboxylation reaction to form LAS molecules that have the number of
Figure 6. Proposed reaction pathways for the oxidation of LAS. Xn ) (CH2)nC6H4SO3-. For Ri, i ) 1-4 with R1 ) CH3, R2 ) CH2OH, R3 ) CHO, and R4 ) COOH. Use of brackets indicates reactive species or species not identified.
carbon atoms in the alkyl chain reduced by one (e.g., Xn-1R1). Peaks that appear in the series 185, 199, 213, 227, and so on would then correspond to the presence of LAS molecules (XnR1) or aldehydes (Xn-1R3) or a combination of both. However, if this series is due to the formation of LASs, there appears to be no X0R1 formed (peak at 171). Peaks that appear in the series 201, 215, 229, and so on would then correspond to the presence of alcohols (XnR2) or acids (Xn-1R4) or a combination of both. However, if this series is due to the formation of alcohols, there appears to be no X0R2 alcohol formed (peak at 187). These results suggest that the oxidation of LAS most likely occurs through the formation of aldehydes and acids rather than LASs and alcohols. If LASs and alcohols are formed, they must be very reactive and rapidly oxidized to form aldehydes and acids. Therefore, a peak that can be seen at 185 would correspond to X0R3 (sulfobenzaldehyde) rather than X1R1, while a peak that is clearly observed at 201 most likely corresponds to X0R4 (sulfobenzoic acid) rather than X1R2. A peak at 173 can also be observed, and it is most likely that this corresponds to 4-hydroxybenzene sulfonic acid (X0OH). 4-Hydroxybenzene sulfonic acid (X0OH) could possibly be formed through the oxidative decarboxylation of sulfobenzaldehyde (X0R3) and/or sulfobenzoic acid (X0R4). Decarboxylation of sulfobenzoic acid (X0R4) could also lead to the formation of benzene sulfonic acid (X0H). However, there appears to be no benzene sulfonic acid (peak at 157) formed in the reaction mixture; this suggests that benzene sulfonic acid is either not formed or, if formed, is very reactive. A peak at 97 can also clearly be seen and corresponds to inorganic bisulfate; this implies that attack of the aromatic ring accompanied by removal of the sulfoxy group has occurred. The segments of alkyl chains broken off the reaction intermediates appear primarily as shortchain organic acids that might undergo total oxidation to carbon dioxide and water. Ring-cleavage compounds could also be oxidized to form short-chain organic acids. It is also interesting to note that peaks of ion mass greater than that of the original LAS are formed during the oxidation of LAS. These may be due to the presence
3664
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000
of aldehydes and acids formed from the oxidation of the original LAS molecules (e.g., peaks at 355 and 369 would correspond to X11R4 and X12R4, respectively, while a peak at 353 would correspond to X12R3. Peaks that can be seen at 383 and 397 are probably due to the acids (e.g., X13R4 and X14R4, respectively) formed from the oxidation of higher-MW original LAS molecules that are present as impurities in the original solution.) Mechanistically, it can be hypothesized that there are two parallel routes for active detergent removal. First, the alkyl side chain can undergo random scission, resulting in a chain that is too short to have the ability to act as a detergent. In parallel with this is the attack of the aromatic ring and removal of the sulfoxy group, which would also destroy the detergent nature of the original molecule. Furthermore, some of the low-molecular-weight (173, 185, 199, 201) intermediates might be relatively stable as they are still present in the reaction mixture after 200 min. This suggests that the aromatic ring itself is difficult to attack and that, therefore, much of the early removal of active detergency is due to attack on the alkyl side chain. In a previous work,7 the wet air oxidation of aqueous solutions of high-molecular-weight poly(ethylene glycol)s was studied. It was found that a free-radical autoxidation mechanism is capable of converting macromolecules to lower-molecular-weight molecules that are mainly oligomers and carboxylic acids at short reaction times and mild operating conditions. Further oxidation of these intermediates to carbon dioxide proved to be extremely difficult as compounds such as acetic acid are very resistant to chemical oxidation even under more severe conditions. Several studies concerning the oxidation of lower carboxylic acids such as acetic acid,17-19 propionic acid,20 and valeric (pentanoic) acid21 have also shown that such compounds are very resistant to total oxidation and that their oxidation to carbon dioxide is usually the rate-limiting step for TOC removal in a wet air oxidation process. Oxidation of Intermediates. After identifying possible intermediates of the wet oxidation of the LAS solution, batch runs were performed on one intermediate. The intermediate chosen was 4-hydroxybenzene sulfonic acid (X0OH). This intermediate was selected because of its accumulation as a function of reaction time, suggesting that it might be relatively stable and could, in some way, be rate-determining in terms of TOC removal. Aqueous solutions of 4-hydroxybenzene sulfonic acid (in the form of sodium salt dihydrate) with an initial concentration of 500 mg L-1 were subjected to wet oxidation experiments at a temperature of 473 K, total pressure of 2.8 MPa, and reaction times varying between 40 and 120 min. At the end of each experiment, the reactor contents were collected and analyzed with respect to their TOC content and pH. A TOC decrease of only about 6% was recorded after 40 min of oxidation, while a more pronounced decrease of approximately 50% was recorded after 120 min of oxidation. The electrospray analysis showed that, after 40 min of oxidation, 4-hydroxybenzene sulfonic acid still remained in the reaction mixture at significant concentrations, while there were relatively few other compounds present. However, after 120 min of oxidation (see Support Material 3), a significant amount of the original molecule had been attacked, and a large number of ring cleavage compounds were present. A peak at 161 would most likely correspond to CHOCHdCHCHdCHSO3-,
while a peak at 95 would correspond to CH2dCHCHd CHCHdCHO-. High concentrations of inorganic sulfate (peak at 81) and bisulfate (peak at 97) in the reaction mixture suggest that removal of the sulfoxy group had occurred. These results suggest that the 4-hydroxybenzene sulfonic acid was, in fact, relatively easy to further chemically degrade. HPLC analysis was also performed on the final reactor contents associated with these experiments. It was found that the oxidation of 4-hydroxybenzene sulfonic acid was also accompanied by the formation of formic, acetic, propionic, and butyric acids, as well as other organic acids that were not identified. The concentration of these acids was found to increase with increasing reaction time, and this result was consistent with a pH decrease from 5.2 (for the original solution) to 3.2 after 120 min of oxidation. Additional experiments were performed to investigate the reactivity of benzene sulfonic acid (X0H). Although benzene sulfonic acid was not identified as an intermediate, it could possibly be formed through the decarboxylation of sulfobenzoic acid (X0R4) and then rapidly be converted to other compounds. Aqueous solutions of benzene sulfonic acid (in the form of potassium salt) with an initial concentration of 500 mg L-1 were subjected to wet oxidation experiments at a temperature of 473 K, total pressure of 2.8 MPa, and reaction times varying between 40 and 120 min. At the end of each experiment, the reactor contents were collected and analyzed with respect to their TOC content and pH. It was found that TOC remained nearly unchanged, thus indicating that benzene sulfonic acid was very stable toward wet oxidation. (It should be noted that electrospray analysis was not performed on the solutions associated with these experiments.) HPLC analysis showed that short-chain organic acids formed during the oxidation of benzene sulfonic acid are present in concentrations that are significantly lower than those for the short-chain organic acids formed during the oxidation of 4-hydroxybenzene sulfonic acid; this is consistent with the benzene sulfonic acid being a relatively stable intermediate. Therefore, it is most likely that benzene sulfonic acid is not formed as an intermediate during the oxidation of LAS rather than that it is formed and rapidly converted to other compounds. These results also suggest that the oxidation of LAS might not occur through the decarboxylation of XnR4 acid to form LAS molecules that have the number of carbon atoms in the alkyl chain reduced by one (Xn-1R1). Figure 7 shows the refined principal reaction pathways for the oxidation of LAS. Conclusions The wet air oxidation of LAS was studied in detail with respect to the effect of operating conditions on the reaction kinetics and the identification of the main intermediates formed during the oxidation. It was found that LAS is readily oxidized under relatively mild conditions (473 K) to yield a group of molecules with short alkyl chains as well as short-chain organic acids. 4-hydroxybenzene sulfonic acid, which was identified as an aromatic intermediate formed during the wet oxidation of LAS, was relatively easy to oxidize at 473 K. Analysis of active detergent (AD) showed far greater proportional decreases in AD than in TOC or COD during oxidation, thus implying that the remaining organic compounds do not act as active detergents. Even at relatively long reaction times (e.g., 390 min), the TOC
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3665
Figure 7. Principal reaction pathways for the oxidation of LAS. Xn ) (CH2)nC6H4SO3-. For Ri, i ) 1-4 with R1 ) CH3, R2 ) CH2OH, R3 ) CHO, and R4 ) COOH.
removal remained as low as about 50%. This is assumed to be attributable to the resistance of short-chain organic acids and some aromatic compounds to further oxidation under these conditions. Future work will focus on the use of suitable catalysts to promote the oxidation of LAS under milder conditions. Supporting Information Available: The available Supporting Information includes electrospray-MS mass spectra of LAS and 4-hydroxybenzene sulfonic acid after oxidation. This material is available free of charge via the Internet at http://pubs.acs.org. Nomenclature AD: Active detergent (% wt/wt) COD: Chemical oxygen demand (mg L-1) C: Concentration (mg L-1) LAS: Linear alkylbenzene sulfonate Pt: Total pressure (MPa) PO2: Oxygen partial pressure (MPa) T: Temperature (K) TOC: Total organic carbon (mg L-1) WAO: Wet Air Oxidation
Literature Cited (1) Dietrich, M. J.; Randall, T. L.; Canney, P. J. Wet air oxidation of hazardous organics in wastewater. Environ. Prog. 1985, 4, 171-177. (2) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34, 2-48. (3) Levec, J.; Pintar, A. Catalytic oxidation of aqueous solutions of organics. An effective method for removal of toxic pollutants from wastewaters. Catal. Today 1995, 24, 51-58.
(4) Luck, F. A review of industrial catalytic wet air oxidation processes. Catal. Today 1996, 27, 195-202. (5) Mantzavinos, D.; Hellenbrand, R.; Metcalfe I. S.; Livingston, A. G. Partial wet oxidation of p-coumaric acid: oxidation intermediates, reaction pathways and implications for wastewater treatment. Water Res. 1996, 30, 2969-2976. (6) Mantzavinos, D.; Hellenbrand, R.; Livingston, A. G.; Metcalfe, I. S. Catalytic wet oxidation of p-coumaric acid: partial oxidation intermediates, reaction pathways and catalyst leaching. Appl. Catal. B: Environ. 1996, 7, 379-396. (7) Mantzavinos, D.; Livingston, A. G.; Hellenbrand, R.; Metcalfe, I. S. Wet air oxidation of poly(ethylene glycol)s; mechanisms, intermediates and implications for integrated chemical-biological wastewater treatment. Chem. Eng. Sci. 1996, 51, 4219-4235. (8) Mantzavinos, D.; Hellenbrand, R.; Livingston, A. G.; Metcalfe, I. S. Beneficial combination of wet oxidation, membrane separation and biodegradation processes for treatment of polymer processing wastewaters. Can. J. Chem. Eng. 2000, 78, 418-422. (9) Hellenbrand, R.; Mantzavinos, D.; Metcalfe, I. S.; Livingston, A. G. Integration of wet oxidation and nanofiltration for treatment of recalcitrant organics in wastewater. Ind. Eng. Chem. Res. 1997, 36, 5054-5062. (10) Otal, E.; Mantzavinos, D.; Delgado, M. V.; Hellenbrand, R.; Lebrato, J.; Metcalfe, I. S.; Livingston, A. G. Integrated wet air oxidation and biological treatment of poly(ethylene glycol)containing wastewaters. J. Chem. Technol. Biotechnol. 1997, 70, 147-156. (11) Huber, L. Conclusions for an ecological evaluation of LAS. Soap Cosmet. Chem. Spec. 1989, 65, 44. (12) Delanghe, B.; Mekras, C. I.; Craham, N. J. D. Aqueous ozonation of surfactantssA review. Ozone Sci. Eng. 1991, 13, 639673. (13) Hidaka, H.; Zhao, J.; Pelizzetti, E.; Serpone, N. Photodegradation of surfactants: Comparison of photocatalytic processes between anionic sodium dodecylbenzenesulfonate and cationic benzyldodecyldimethylammonium chloride on the TiO2 surface. J. Phys. Chem. 1992, 96, 2226-2230. (14) Rao, N. N.; Dube, S. Photocatalytic degradation of mixed surfactants and some commercial soap detergent products using suspended TiO2 catalysts. J. Mol. Catal. 1996, A104, L197-L199. (15) Leu, H. G.; Lin, S. H.; Lin, T. M. Enhanced electrochemical oxidation of anionic surfactants. J. Environ. Sci. Health. 1998, A33, 681-699. (16) Eaton, A. D.; Clesceri, L. S.; Greenberg, A. E. Standard Methods for the Examination of Water and Wastewater; American Public Health Association: Washington, D.C., 1995; pp 5/15-5/ 16. (17) Imamura, S. Catalytic and noncatalytic wet oxidation. Ind. Eng. Chem. Res. 1999, 38, 1743-1753. (18) Levec, J.; Smith, J. M. Oxidation of acetic acid solutions in a trickle-bed reactor. AIChE J. 1976, 22, 159-168. (19) Imamura, S.; Hirano, A.; Kawabata, N. Wet oxidation of acetic acid catalyzed by Co-Bi complex oxides. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 570-575. (20) Shende, R. V.; Levec, J. Kinetics of wet oxidation of propionic and 3-hydroxypropionic acids. Ind. Eng. Chem. Res. 1999, 38, 2557-2563. (21) Nikolaou, N.; Abatzoglou, N.; Gasso, S.; Chornet, E. The oxidation of valeric acid in aqueous solution. Can. J. Chem. Eng. 1994, 72, 522-533.
Received for review April 5, 2000 Revised manuscript received July 17, 2000 Accepted July 20, 2000 IE000385U
