Wet Air Oxidation of Linear Alkylbenzene Sulfonate ... - ACS Publications

EH 3JL, U.K., and Air Products PLC, European Technology Group, Basingstoke RG24 8FE, U.K.. The semibatch wet air oxidation (WAO) of 1600 mg L-1 of a ...
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Ind. Eng. Chem. Res. 2001, 40, 5507-5516

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Wet Air Oxidation of Linear Alkylbenzene Sulfonate 1. Effect of Temperature and Pressure Darrell A. Patterson,† Ian S. Metcalfe,‡ Feng Xiong,§ and Andrew G. Livingston*,† Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology & Medicine, London SW7 2BY, U.K., Department of Chemical Engineering, University of Edinburgh, Edinburgh EH 3JL, U.K., and Air Products PLC, European Technology Group, Basingstoke RG24 8FE, U.K.

The semibatch wet air oxidation (WAO) of 1600 mg L-1 of a linear alkylbenzene sulfonate (LAS) known as sodium dodecylbenzene sulfonate (SDBS) was investigated, for varying oxidation temperature from 180 to 240 °C and pressure from 3.05 to 6.55 MPa. The major reaction products identified were low-molecular-weight volatile fatty acids (VFAs) such as formic and acetic acids, sulfonated aromatics, and sulfate. For 120-min WAO at 1.5 MPa oxygen partial pressure, a temperature increase from 180 to 240 °C led to increases in the liquid-phase LAS removal from 79 to 100%, in the COD removal from 23 to 70%, and in the desulfonation of the LAS molecule from 7.4 to 57% of the total theoretical sulfate. Increasing the overall oxidation pressure from 3.05 to 6.55 MPa had little effect on the overall TOC and COD degradation, but increased the desulfonation. On the basis of these results, a reaction mechanism is proposed. Sulfonated aromatic WAO intermediates accumulated under the conditions used, indicating that, to achieve further organic degradation, more effective desulfonation conditions will be required. Introduction Linear alkylbenzene sulfonates (LASs) are the predominant surfactant in both industrial and household cleaning agents, after soaps.1 In 1999, the global LAS production was estimated at 2 305 000 metric tons [based on more than 99% conversion of the world production of linear alkylbenzene (LAB) to LAS], which was expected to rise to 2 365 000 metric tons in 2000.2 Consumption of LAS in western Europe was estimated to be 320 000 metric tons.2 The surfactant properties of LAS (Figure 1) derive from its hydrophobic linear alkyl group and benzene ring and hydrophilic sulfonate anion and cation. The linear alkyl chain is thought to be readily biodegradable, which is why LAS is so widely used.3 To justify this claim, the biodegradation of LAS has been thoroughly studied.3-5 It has been found that the linear alkyl chain is indeed biodegradable;3-5 however, the biodegradation of many surfactants, including LAS, is inhibited at concentrations above 20-50 mg L-1.6,7 Even below these concentrations, LAS is still difficult to biodegrade, because as much as 30-35% of the LAS can be adsorbed and left untreated in the sludge in biological reactors.8 At higher concentrations, such as those of detergent manufacturing wastewater streams (which can have a COD of up to 50 000 mg L-1), LAS is biologically recalcitrant. Consequently, these wastewaters are treated by flocculating the surfactant with lime and metal salts and landfilling the resulting sludge. This treatment technique is not desirable as LAS is not being remediated, but rather is transferred * Author to whom all correspondence should be addressed. Present and corresponding address: Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2BY, United Kingdom. Telephone: +44 20 75945582. Fax: +44 20 75945629. E-mail: [email protected]. † Imperial College of Science, Technology & Medicine. ‡ University of Edinburgh. § Air Products PLC.

Figure 1. Sodium linear alkylbenzene sulfonate, m + n ) 7-11 (adapted from Hons42).

from being a liquid contaminant to a sludge waste, with the additional salts increasing the quantity of waste. A new method that destroys LAS in aqueous solution is therefore required. Advanced oxidation and chemical oxidation wastewater treatment methods have been used to destroy LAS. Ozonation,9 TiO2-catalyzed photolytic treatment,10 Fenton’s reagent treatment,11 and electrochemical treatment12 have all been assessed. In all cases, however, the oxidation of high-concentration LAS (greater than 100 mg L-1) has not been studied. A suitable technology for the treatment of high-concentration LAS is wet air oxidation (WAO), as LAS is both water-soluble and soluble at appropriate concentrations in the industrial waste streams of interest. WAO has been applied to a wide variety of waste streams.13-15 However, intensive studies of the mechanisms, kinetics, and reaction conditionsssuch as the effects of pH and catalystsshave been performed mainly for phenolic compounds13,16,17 and volatile fatty acids (VFAs).13,18-20 There is a lack of knowledge for many important classes of pollutants, including sulfonated organics13 such as LAS. A few studies have addressed sulfonated organics, including one focused on dinitrotoluene sulfonates (TNT red water)21-23 and, in particular, an extensive study of the intermediates, products, and kinetics of the WAO of nitrotoluenesulfonic acid (NTSA).24,25 Additionally, p-toluenesulfonic acid26 and 5-sulfo salicyclic acid27 have both been briefly

10.1021/ie010293k CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001

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Figure 2. Schematic of the wet air oxidation reactor.

studied. The WAO treatment of alkylbenzene sulfonates (ABSs) such as LAS has not yet been fully assessed; in fact, only one study of the WAO of LAS has been undertaken.28 The aims of this previous work were to study the effect of operating conditions on the noncatalytic WAO of LAS, to identify the main reaction intermediates, to determine the stability of these intermediates to further oxidation, and to elucidate a reaction pathway. Semibatch WAO experiments were performed at 180 and 200 °C, using a small number of oxygen partial pressures (1.3 and 1.8 MPa), with reaction times up to 390 min. It was shown that aqueous LAS, at a concentration of 1000 mg L-1, could be degraded at 200 °C with TOC and COD reductions of 22 and 37%, respectively, after 80 min and with up to 13% of this TOC reduction occurring during the anoxic heating of the reactor. Reaction intermediates were identified by HPLC and electrospray mass spectrometry (EMS), and a preliminary reaction pathway was proposed. Additional work is required, first, to determine the effect of operating conditions over a wider range of temperatures and pressures, and, second, to fully elucidate the reaction mechanism and any limiting factors. One disadvantage with WAO treatment is that total oxidation to carbon dioxide and water, using WAO alone, is often not possible. However, many compounds that are recalcitrant, inhibitory, or toxic to the usually more cost-effective biological treatment, such as highconcentration aqueous LAS, can be pretreated by an oxidation process such as WAO to make them more biogenic.29 This is because the main reaction products of WAO are usually volatile fatty acids (VFAs) such as acetic acid, which are readily biodegradable. Hence, WAO is suitable as a pretreatment to biological remediation, and as such, it has already been successfully integrated with different biotreatment processes.15,22,30 This work presents the results of the first step toward a treatment process for high-concentration LAS wastewater: integrated WAO and biodegradation of LAS. This study concentrates on the first stage of this process, the WAO of LAS. To design such integrated chemicalbiological systems, a rational approach must be adopted. Scott and Ollis29 state that the key to designing these integrated systems is to know the physical, chemical,

and biological properties of the major reaction intermediates and the extent of degradation of these compounds by each process, so that a synergy between the two integrated systems can be engineered. Such an approach is adopted in analyzing this WAO process. Consequently, this paper presents an investigation of the WAO reaction pathway of LAS at temperatures between 180 and 240 °C and total pressures of 3.55-6.55 MPa. The aims are to determine the effects of these operating conditions on the WAO of LAS, to further elucidate the previously proposed reaction mechanism,28 and to determine the limiting factors. A second paper (the following paper in this issue) investigates the effect of pH on this reaction mechanism, as well as a method of improving the WAO reaction rates. Methods and Materials Apparatus and Procedure. A 400-mL working volume WAO autoclave (Baskerville Ltd., Manchester, U.K.) was used for all experiments. The reactor is schematically illustrated in Figure 2. The autoclave was heated with an external 2-kW electric heater, the temperature was monitored using two thermocouples in the same reactor well (TI), and the heating was controlled by a PID controller. The electric heating jacket, reactor pressure control valve, magnetically coupled impeller, gas flowmeters, and liquid feed pump were all monitored and controlled. A pressure-relief valve throttled the off-gas pipe to minimize the vent gas pressure drop. In a typical semibatch experiment, 300 mL of reactant solution was first loaded into the autoclave. The autoclave was rapidly pressurized to the set pressure in the presence of oxygen-free nitrogen. The reactant solution was then sparged with oxygen-free nitrogen at 1 L min-1 for 10 min at ambient temperature. Afterward, the stirrer was started. The stirrer speed was maintained at 1000 rpm for all experiments, as previous studies have shown that this speed ensures that the reaction is not mass-transfer-limited.15 Thereafter, the vessel was heated to the reaction temperature, still under nitrogen. When the reaction temperature was reached, defined as time zero, a sample was taken, the

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nitrogen flow was shut off, and oxygen was then continuously fed at 1 L min-1 to start oxidation. During the experiment, liquid samples with volumes of approximately 10 mL were rapidly transferred from the reactor via the gas sparge tube during a brief shut-off of the gas feed. The gas pipes were initially flushed with a small amount of sample, and then, the liquid sample was pushed from the pressurized reactor into a sample cylinder directly connected to the gas line. The sample was then cooled, to minimize loss of volatiles and to prevent flashing of the liquid out of the sample cylinder, and then transferred to a sample storage vial. After sampling, any liquid in the gas line was blown back into the reactor, thereby minimizing loss of liquid and sample contamination. After the chosen reaction time, the oxygen was shut off, and the vessel was depressurized to 1.0 MPa to remove most of the oxygen from the reactor, so as to rapidly stop the reaction. The vessel was then repressurized and sparged continuously with 1 L min-1 of nitrogen to remove the rest of the oxygen. Thereafter, the autoclave was cooled to at least 30 °C in a water bath and then opened, and the reactor contents collected. To avoid cross contamination between experiments, the reactor and all connecting pipes were thoroughly washed with detergent and then deionized water before each experiment. Analytical Techniques. For all experiments, the samples and initial solutions were analyzed for total organic carbon (TOC) using a Shimadzu 5050 TOC analyzer. Chemical oxidation demand (COD) was measured by reacting samples in a Hach COD reactor by the dichromate closed-reflux method.31 Absorbances were measured in a Shimadzu UV-2101PC UV-vis scanning spectrophotometer. For each sample, three measurements were made of both TOC and COD, and the percentage variation was never greater than 4%. Both TOC and COD were assessed, because sulfonated aromatics such as LAS are not oxidized under COD test conditions.21 Therefore, to study the effectiveness of the WAO treatment of LAS or other sulfonated waste streams, additional parameters were needed. Residual LAS, aromatic reaction intermediates, and products were determined by reverse-phase high-performance liquid chromatography (HPLC) on a Unicam Crystal 200 HPLC with a Crystal 250 diode array detector using a reverse-phase C18 column (Jones Chromatography, Hengoed, Mid Glamorgan, U.K.). A specially developed gradient method utilizing an acetonitrile-watersodium perchlorate mobile phase was used to elute and separate the LAS and the reaction products. The method was a modification of the Matthijs and De Henau32 method for eluting LAS. Two mobile phases were used: solution A containing 70% acetonitrile (Aldrich, Dorset, U.K.), deionized water, and 0.15 M sodium perchlorate (Acros, Leicestershire, U.K.) and solution B containing 0.15 M sodium perchlorate in deionized water. The gradient used to separate the WAO reaction products from the LAS starting isomers is detailed in Table 1. A 20-µL sample loop was used with a flow rate of 1 mL min-1 for the mobile phase. The average coefficient of variation (standard deviation divided by the mean) for the HPLC quantification of LAS peaks was 0.062. Volatile fatty acid reaction intermediates and products were determined on a Polyspher OAHY reversephase column (Merck, Dorset, U.K.) at 35 °C with 0.01 N sulfuric acid (Merck) as the mobile phase, at a flow

Table 1. HPLC Gradient Method for Separating LAS and Its WAO Reaction Products time period (min)

gradient segment

3.0 10.0 25.0 5.0 5.0 5.0

equilibration 1 2 3 4 reset to initial

percentage solution (%) solution A solution B 100.0 30.0 10.0 10.0 0.0 100.0

0.0 70.0 90.0 90.0 100.0 0.0

rate of 0.5 mL min-1. A Shimadzu LC-10AT liquid chromatograph with an SCL-10A system controller, CTO-10AC column oven, and SIL-10AD autoinjector was used for this analysis. Separated VFAs were detected with an SPD-10A UV-vis detector at 210 nm. The sulfate concentration was quantified by ion chromatography on a Dionex DX120 ion chromatogram with an Ion PAC AS14 (4 × 250 mm) column at ambient temperature. The mobile phase was a mixture of 3.5 mM Na2CO3 and 1.0 mM NaHCO3 at 1.1 mL min-1. An AS40 automated sampler was used. For all analyses, all concentrations of reactants and identified reaction products were determined using external standards. pH was determined at room temperature using a Corning 240 pH meter. Materials. All WAO experiments were performed on 1600 mg L-1 solutions of sodium dodecylbenzene sulfonate (SDBS), a commercially available LAS, used as received from Aldrich, U.K. Biodegradation studies by the authors have shown that LAS is poorly biodegraded at such concentrations. Note that the alkyl chain of SDBS, like commercial LAS, is not strictly linear as is often assumed.33 Commercial LAS is a “blend” of LAS homologues and isomers, varying not only in the length of the alkyl chain but also in the position of the alkyl chain on the benzene ring.34 As a result of the manufacturing process, commercial LAS is also always contaminated with small concentrations of coproducts such as dialkyltetrin sulfonates and iso-LAS.34 All water used was deionized using a Purite HP700 purifier. Results and Discussion Effect of Temperature. LAS with an initial concentration of 1600 mg L-1 (TOC ≈ 800 mg L-1, COD ≈ 3000 mg L-1) was reacted at 180, 200, 220, 230, and 240 °C for 120 min at a partial oxygen pressure of 1.5 MPa (total pressures of 2.50, 3.05, 4.30, and 4.85, respectively). Experiments at 200 °C were used as a reference, as at this temperature, significant TOC, COD, and LAS reductions occurred. The formation and destruction of reaction intermediates and products were also readily observable at this temperature. Control Trials and Reproducibility. Control trials in which only nitrogen was fed to 1600 mg L-1 LAS at 200 and 220 °C were conducted to determine the effect of thermal degradation. No degradation of LAS, no significant formation of sulfate, and no significant drop in TOC or COD were found during these experiments, indicating that, at 200 and 220 °C, thermal degradation has little effect on the WAO of LAS. Furthermore, in all experiments, no increase in sulfate occurred during the initial anoxic heating period. An increase in sulfate concentration and organic degradation was recorded only when oxygen was introduced to the reactants. This result differs from that of Mantzavinos et al.,28 who recorded a significant (13%) TOC reduction after 45 min

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Figure 3. Effect of reaction time and temperature on the normalized COD reduction during the WAO of LAS (total pressure in parentheses). 0, 180 °C (2.50 MPa); [, 200 °C (3.05 MPa); b, 220 °C (3.80 MPa); 9, 230 °C (4.30 MPa); 2, 240 °C (4.85 MPa).

Figure 4. Effect of reaction time and temperature on sulfate formed after WAO at PO2 ) 1.5 MPa (total pressure in parentheses). 0, 180 °C (2.50 MPa); [, 200 °C (3.05 MPa); b, 220 °C (3.80 MPa); 9, 230 °C (4.30 MPa); 2, 240 °C (4.85 MPa).

of heating to 200 °C under nitrogen. However, other researchers have found that aqueous LAS is stable at high temperatures,35 supporting the findings of this paper. To determine whether the trends in the data were a result of uncertainties or anomalies, experiments at 200 and 240 °C were repeated under identical conditions. All figures show that the spread of the data at each temperature (measured by the coefficient of variation) was less than the difference between the results at different temperatures. The coefficients of variation for the three trials at 200 °C for 60 min, in terms of the reduced TOC, COD, and LAS; pH; and formic acid, acetic acid, and sulfate increases were 0.018, 0.0052, 0.24, 0.010, 0.21, 0.18, and 0.15, respectively. Consequently, there are actual differences between the presented data sets, which are the result of neither experimental scatter nor anomalies. TOC and COD Removal. WAO at higher temperatures produced greater removals of COD (Figure 3) and TOC. The TOC results show the same trends as the COD results, so they are not presented. An increase of 60 °C in the reaction temperature yields a 47% greater oxidation to CO2 and water (and other losses from the reactor such as volatiles) and a 44% decrease in the relative amount of chemically oxidizable intermediates in the reactor liquid. However, TOC and COD are not completely destroyed under the conditions studied, indicating that refractory organics are present. These results are therefore consistent with those of Mantzavinos et al.,28 and with those for oxidations of LAS by ozonation and TiO2-catalyzed photolytic treatment.9,10 Desulfonation. The LAS molecule (Figure 1) has three points of attack during WAO: (A) the sulfonate anion, (B) the benzene ring, and (C) the alkyl group. In theory, oxidative attack and cleavage of the sulfonate anion should form sulfite (SO3-), which should rapidly oxidize to sulfate (SO42-), removing the hydrophilic end of the molecule. Thus, sulfate production indicates both the degradation and loss of surfactancy of LAS. An increase in sulfate concentration was detected in all LAS solutions subjected to WAO. Figure 4 shows the concentration of sulfate expressed as a percentage of the sulfate that would be produced if all of the LAS initially present were desulfonated (henceforth referred to as theoretical sulfate). For reaction temperatures of 180-200 °C, the desulfonation of LAS and its sulfonated WAO interme-

diates was not a major reaction compared to LAS removal (see below). After 120 min of WAO at 180 °C, only 7.4% of the theoretical sulfate was generated, increasing to only 19% at 200 °C. Attack and cleavage of the sulfonate moiety, however, increased at 220 °C (50% of theoretical sulfate after 120 min). For 230 and 240 °C, after 120 min, approximately 58% of the theoretical sulfate was formed. It can therefore be concluded that, as complete desulfonation did not occur, much of the TOC and COD removal at each temperature must have be due to an alternate oxidation pathway, the most likely being alkyl chain degradation. An increase in sulfate concentration with increasing temperature is consistent with the WAO of two other sulfonated organics: TNT red water21 and NTSA.24 Even under the mildest WAO conditions (200 °C, 0.13 MPa oxygen partial pressure), TNT red water generated considerable sulfate concentrations. For the WAO of NTSA, the increase in sulfate concentration did not stoichiometrically match the amount predicted on the basis of the NTSA decrease. Thus, as for LAS WAO, sulfonated intermediates were formed throughout the reaction. Unlike LAS, however, complete desulfonation of NTSA was observed in experiments with high temperatures and reaction times (T ) 300 °C, t ) 240 min, PO2 ) 0.93 MPa), and high oxygen partial pressures (PO2 ) 1.90 MPa, T ) 188 °C, t ) 188 min).24 This indicates that longer reaction times and higher temperatures could be used to desulfonate LAS completely. Reaction Intermediates and LAS Removal. The initial HPLC chromatogram in Figure 5 is representative of those obtained for the LAS used in this study. It consists of multiple peaks, typical of the mixed isomer blend of commercial LAS. These peaks define what is referred to as LAS in this work. Removal of LAS implies that these peaks have reduced areas. Thus, once LAS has been subjected to WAO at 200 °C, these peaks mostly disappear, and a new set of peaks appear at lower retention times. Because the HPLC mobile phase gradient is initially polar and gradually becomes less polar, this indicates that the WAO reaction intermediates and products are more polar than the LAS reactant molecules. Thus, these intermediates have most likely lost part, or all, of the nonpolar alkyl group on the LAS molecule. Most of the non-LAS peaks in Figure 5 are yet to be identified. If the reaction consists of WAO attack on the LAS alkyl chain to form more polar

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Figure 5. HPLC chromatograms of LAS as it is subjected to WAO at 200 °C, PO2 ) 1.5 MPa, after 0, 20, 40, 60, 80, and 120 min.

organics with limited desulfonation, then the most likely reaction products are either sulfophenylcarboxylic acids [SPCs, HSO3C6H5(CH2)nCOOH] and sulfophenylaldehydes [SPAs, HSO3C6H5(CH2)nCHO]. However, pure solutions of these compounds were not readily available, so identification using external standards could not be used. One aromatic product has been tentatively identified: 4-hydroxybenzenesulfonic acid (HOC6H5SO3H). Additionally, 4-sulfobenzenecarboxylic acid (HSO3C6H5COOH) might be a component of the initial (unseparated) peak, but as such, it cannot be positively confirmed by this HPLC technique. Note that these are sulfonated aromatic intermediates, further substantiating the premise that complete desulfonation did not occur under the oxidation time and conditions studied. These molecules are considered to be reaction intermediates, because for the purposes of this work, oxidized LAS ceases to be considered as LAS when it no longer acts as a surfactant. Thus, if it is desulfonated or if the alkyl chain is oxidized (formation of lower-molecularweight linear alkyl benzene sulfonates during WAO is unlikely28), then the molecule is no longer LAS. In the study of LAS WAO by Mantzavinos et al.,28 the main aromatic intermediates and products were also speculated to be sulfonated phenyl organics, on the basis of the masses from an electrospray mass spectrometry (EMS) study. Four possible sulfonated intermediates were proposed: SPCs and SPAs (as speculated above), as well as sulfophenyl alcohols (SPLs) and shorter-chain LAS. Although the homologous series of the latter two reaction products can be assigned to masses in the MS spectrum, they are unlikely to be stable intermediates in the typical WAO oxidizing reaction pathways. Furthermore, both 4-hydroxybenzenesulfonic acid and 4-sul-

fobenzenecarboxylic acid were identified as WAO products. Note that several other possible reaction species can account for these results. Because the aromatic ring of LAS is attached to the alkyl chain at every position but the terminal end,1 both ends of the alkyl chain can undergo oxidative attack. Thus, in theory, dialdehyde, dicarboxyl, or mixed aldehyde-acid species can form. Dicarboxylic acid species form in the aerobic biodegradation of LAS1 and, so, could potentially form during its WAO. Therefore, in the results of Mantzavinos et al.,28 EMS peaks with mass differences of 14 (corresponding to CH2) forming a homologous series of 227, 241, 255, etc., could also be sulfophenyldialdehydes (SPDAs) of the form (OHC)(CH2)mCH(CH2)n(CHO)C6H4SO3-. EMS peaks forming the series 243, 257, 271, etc., might also be carboxylsulfophenylaldehydes (CSPAs) of the form (OHC)(CH2)mCH(CH2)n(COOH)C6H5SO3-. No peaks can be assigned to sulfophenyldicarboxylic acids, perhaps indicating that such species either are short-lived reaction intermediates or are not formed. From chromatograms such as Figure 5, the disappearance of LAS as a complete entity over time was quantified. The peak areas at the retention times of the initial LAS molecule were assumed to be attributable to LAS. Using LAS external standards, the concentrations attributable to each peak were calculated and summed for the bulk LAS concentration in each sample. From this analysis, the typical LAS disappearance over time at the various reaction temperatures was obtained (Figure 6). Figures 5 and 6 indicate that LAS is effectively removed by WAO. Consequently, it can be concluded that the WAO reaction rapidly oxidizes the

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Figure 6. Effect of reaction time and temperature on LAS in the liquid phase after WAO at PO2 ) 1.5 MPa (total pressure in parentheses). 0, 180 °C (2.50 MPa); [, 200 °C (3.05 MPa); b, 220 °C (3.80 MPa); 9, 230 °C (4.30 MPa); 2, 240 °C (4.85 MPa).

Figure 8. Effect of reaction time and temperature on acetic acid in the liquid phase after WAO at PO2 ) 1.5 MPa (total pressure in parentheses). 0, 180 °C (2.50 MPa); [, 200 °C (3.05 MPa); b, 220 °C (3.80 MPa); 9, 230 °C (4.30 MPa); 2, 240 °C (4.85 MPa).

the measured liquid-phase TOC for every sample (eq 1).

equivalent acetic acid TOC ) TOC TOC calculated from acetic acid concentration × total TOC

(

)

100%

Figure 7. Effect of reaction time and temperature on the liquidphase pH during the WAO of LAS (total pressure in parentheses). 0, 180 °C (2.50 MPa); [, 200 °C (3.05 MPa); b, 220 °C (3.80 MPa); 9, 230 °C (4.30 MPa); 2, 240 °C (4.85 MPa).

alkyl chain to (speculatively) form SPCs, SPAs, SPDAs, and CSPAs and/or that it desulfonates the aromatic ring. pH and Organic Acids. pH was left uncontrolled during the reaction, so once the initially neutral LAS was subjected to WAO, its pH dropped significantly (Figure 7). The pH decreased to 3.5 at 180 °C and to as low as 2.9 at 240 °C. Such a drop in pH might be responsible for an acceleration in reaction rates and thus overall degradation. This matter is further investigated in part 2 of this work (following paper, this issue). A drop in pH over reaction time is also consistent with the formation of volatile fatty acids (VFAs) and other acids, which are typical products of the WAO of organic compounds.13,19 HPLC analysis showed that multiple short-chain VFAs formed during the course of the WAO. The major identified products were formic and acetic acids. Typically, during WAO, formic acid oxidizes to carbon dioxide and water at temperatures as low as 150 °C, but acetic acid is stable in WAO environments at temperatures as high as 275 °C.19 Therefore, unsurprisingly, acetic acid accumulated in the reactor at the conditions studied, as shown in Figure 8. These data represent the TOC calculated from the measured acetic acid concentration as a percentage of

t

(1)

During oxidation, a greater concentration of acetic acid was observed at higher reaction temperatures. The concentrations of formic acid observed were more variable, but after 120 min of WAO, less formic acid appeared at the higher temperatures studied, indicating that it was not being formed at these temperatures, it was reacting with oxidation intermediates, or it was undergoing further oxidative breakdown. Other organic acids identified in much smaller concentrations were succinic, propanoic, n-butanoic, and n-pentanoic acids. Numerous other peaks were present in the HPLC chromatograms; however, they are yet to be identified. These results are consistent those of with Mantzavinos et al.,28 who found that the main LAS WAO reaction intermediates they could positively identify were formic, acetic, propanoic, and butanoic acids. Taken as a lump sum TOC, these intermediates also accumulated during 120 min of WAO at 200 °C. Effect of Pressure. Semibatch WAO of LAS was performed at total pressures of 3.05, 4.55, and 6.55 MPa, using a reaction temperature of 200 °C, to determine the effect of pressure. TOC, COD, and Reaction Products. The effect of higher pressures on the TOC reduction was negligible (Figure 9) compared to the effect of temperature. The COD reduction shows similar patterns to the TOC, so it is not detailed. Additionally, increased pressure did not change the normalized COD/TOC ratio of each reaction (results not presented). At a molecular level, pressure also had little effect on the formation and destruction of the species in the LAS WAO reaction. LAS destruction was similar at all three pressures. The pH of the liquid phase throughout the reaction was also the same for all three pressures, decreasing from 3.6 after 20 min to 3.2 after 120 min. There was also no statistically significant increase in the concentrations of acetic and formic acids that accumulated in the liquid

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Figure 9. Effect of reaction time and total pressure on TOC reduction of LAS under WAO at 200 °C. [, 3.05 MPa; 9, 4.55 MPa; 2, 6.55 MPa.

Figure 10. Effect of reaction time and total pressure on the sulfate generated in the liquid phase during the WAO of LAS at 200 °C. [, 3.05 MPa; 9, 4.55 MPa; 2, 6.55 MPa.

Table 2. pKa of the Identified Carboxylic Acids at Temperatures Used in the Laboratory

pressure. Further experiments are required to test this hypothesis. Thus, this result shows that greater desulfonation does not necessarily increase the TOC and COD removal during LAS WAO. This is most likely because only desulfonation occurred, without further organic degradation. This indicates that ring cleavages the only other mechanism that can greatly reduce TOC and COD other than alkyl chain degradationsis a separate reaction step from desulfonation. The two reactions are linked however, because the previous results have shown that ring cleavage probably does not occur without prior desulfonation. Thus, although the higher dissolved oxygen in the liquid phase at higher pressure possibly catalyzed or reacted with LAS and its sulfonated aromatic WAO intermediates to cause this desulfonation, it was probably neither sufficiently reactive nor sufficiently catalytic to cause aromatic ring cleavage to remove further TOC and COD. Volatiles and the Gas Phase. In all WAO experiments, carbon and organic losses with the reactor effluent gas occurred. There were two types of losses: (1) losses of a condensable fraction and (2) losses of a speculated noncondensable fraction. It is hypothesized that both fractions were lost from the reactor because the overall reactor pressure was not high enough to keep them in the liquid phase. Previous work by the authors37 has found that loss of these volatiles from the WAO reactor is likely to be responsible for a major portion of the TOC reduction during the WAO of LAS. In all experiments, the noncondensable loss fraction was not quantified because it flowed through the separator and was lost to ventilation. The condensable fraction was collected in the gas-liquid separator, attached in-line with the exit gas pipe from the WAO reactor (refer to Figure 2), and the volume and HPLC peak characteristics of this liquid were quantified at the end of each experiment. When the overall pressure was increased, the amount of liquid collected in the gas-liquid separator in the gas line dropped significantly. At 3.05 MPa, 20.2 mL of liquid was collected, which was significantly more than the 9.9 and 4.2 mL from the experiments at 200 °C and 4.55 and 6.55 MPa, respectively. Thus, increasing the overall pressure decreased the amount of volatiles and other liquids lost from the reactor through the gas pipe. The major components of this liquid were generally found to be LAS and various volatile fatty acids, including formic and acetic acid. The LAS in this liquid was a result of foaming of the surfactant in the reactor.

pKa aqueous carboxylic acid solution formic acetic propanoic n-butanoic n-pentanoic

25

temperature (°C) 200

240

3.752 4.756 4.874 4.820 4.842

4.741 5.649 5.828 5.872 5.817

5.126 5.992 6.191 6.235 6.150

phase at all three pressures (accounting for the errors in the data). Table 2 gives the pKa of the identified carboxylic acids at the temperature at which the pH was measured (approximately 25 °C) and at 200 and 240 °C, the two temperatures at which repeat experiments were performed. The high-temperature pKa’s were calculated using the method in Robinson and Stokes (1968).36 Table 2 shows that some species other than the identified VFAs are responsible for the pH of the reaction mixture. This species is most likely WAO-generated acid: the overall oxidation reaction of SDBS (the major constituent of the LAS studied) shows that 1 mol of acid is produced for every mole of sulfate.

C12H25C6H4SO3-Na+ + 51/2O2 f 18 CO2 + 14 H2O + H+ + SO42- + Na+ (2) A pH of 2.8 can be calculated using eq 2 and the observed sulfate concentration after 120 min at 200 °C. This is close to the pH value of 3 measured, indicating that this acid is the major cause of the low observed pH. This hypothesis fits with the observation that higher WAO temperatures result in lower liquid pH’s. Higher temperatures also lead to greater desulfonation, which would therefore form more acid to give a lower liquid pH. This is also consistent with the WAO of NTSA, where the pH decrease was attributed to desulfonation and the consequent formation of sulfuric acid.24 Figure 10 shows that a small yet significantly greater concentration of sulfate was consistently generated at higher reactor pressures, despite negligible TOC and COD removal under the same conditions. A possible reason for this is that the desulfonation reaction is either catalyzed by or perpetrated by dissolved oxygen, which is more concentrated in the liquid phase at higher

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Figure 11. Proposed reaction pathway for the WAO of LAS.

It is hypothesized that this foam was entrained in the gas stream, thereby enabling it to exit the reactor with the waste gas. Of the other components, Table 2 illustrates that all of the identified VFAs were most likely in an associated state once oxidation started, because the pH dropped to 3.6 or lower. Thus, if the overpressure were not high enough, a fraction of these volatile acids would be lost, and this fraction would be lower with increasing overpressure. Also note that, because the vaporization and evaporation of volatiles is suppressed at higher pressures, the liquid-phase concentrations of these volatiles, such as the VFAs, would be higher. As WAO degradation has been monitored in terms of organic removal from the liquid phase, this effect might have masked a higher organic degradation rate, which is sometimes found at higher pressures.13 However, the fact that the degradation of nonvolatile LAS was the same at all three pressures confirms that this did not occur. Mantzavinos et al.28 noted that volatiles exited the reactor during the 200 °C WAO of LAS and also collected the condensable portion. This liquid, however, contained virtually no active detergent (99% removal). Lin and Ho38 also observed loss of organic liquid during the WAO of a high-concentration petroleum wastewater.

They also found that increasing the pressure tended to suppress the vaporization of volatiles in their reactor, thereby decreasing the volume of liquid collected in a cold trap in their off-gas pipe. They speculated that the liquid collected at low pressures was also primarily acidic in nature. These cases show that liquid and organic loss in the off-gas is a problem during WAO. Therefore, although pressure is not as significant a factor as temperature for the LAS WAO reaction kinetics, it is a significant factor in overall WAO process and reactor engineering. This therefore poses a rarely mentioned optimization problem in the design of WAO reactors: minimization of the cost of downstream gas treatment compared with the cost of increasing reaction pressure (which increases because of the need for more expensive reactors and compressors) required to keep the volatiles in the liquid phase. To solve this problem, we suggest that future WAO studies should attempt to quantify both the gas and the liquid product streams. Proposed Reaction Pathway. On the basis of the above observations and results, the reaction pathway in Figure 11 has been proposed. The WAO of LAS initially proceeds via two parallel reaction pathways. The primary mechanism of WAO LAS degradation is

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via alkyl chain scission (reaction A1). Here, the alkyl chain of the isomer mixture of LAS (predominantly SDBS) undergoes random scission to form a range of VFA products with different chain lengths (reactions A1 and A2), including formic, acetic, and propanoic acids. Short-chain acids are speculated to be the major products because the longer-chain acids would further degrade to these smaller acids.39 The aromatic nucleus of LAS is converted to refractory sulfonated aromatics, which are suspected to be SPAs, SPCs, SPDAs, and CSPAs (reaction A1). The alkyl chains of these aromatics can further degrade via reaction A2. The organics shown are the terminal products of alkyl chain degradation. All sulfonated aromatics are refractory (the C-S bond of the sulfonate moiety is thermodynamically stable40), and so, they are only cleaved to a small extent at temperatures below 220 °C. Consequently, reactions A1 and A2 predominate under these conditions. The secondary reaction pathway for LAS degradation is desulfonation (reactions B1-B3), which only occurs to a small extent below 220 °C. Above 220 °C, reactions B1-B3 become significant, and at least 50% of the theoretical sulfate is liberated. Because sulfonated aromatics accumulate in the reactor in the absence of desulfonation, it follows that desulfonation allows aromatic ring cleavage (reactions C1-C3) and is therefore the key reaction step to further degradation beyond that observed in this work. This is why reactions B and C are grouped together: ring cleavage (reactions C1-C3) does not occur without prior desulfonation (reactions B1-B3). Subsequently, the fractured carbon chain would oxidize to VFAs and other short-chain refractory organics. This reaction sequence is also the prevalent pathway for the biological mineralization of LAS.41 VFAs are the most common refractory TOC contributors in WAO, so for complete TOC destruction of LAS, VFA degradation will need to occur (reactions D1-D3). For the major VFA, acetic acid, this will only occur catalytically or at temperatures higher than those used in this study. Reactions D1-D3 are well-documented by previous researchers.18,19 Under the conditions studied in this work, reactions B1-B3, C1-C3, and D1-D3 were never taken to completion, giving residual LAS, VFAs, TOC, and COD in the final effluents. Implications for Integrated Treatment. To further degrade LAS by WAO beyond the refractory sulfonated aromatics (Figure 11), the extent of desulfonation must be increased. Methods of increasing the extent and rate of desulfonation are explored in part 2 of this work (following paper, this issue). The present results indicate that this could be achieved by employing more extreme temperatures. However, because WAO is to be used as a pretreatment to biological remediation, the main concern is whether desulfonation will make LAS more biodegradable. To determine this, the biodegradability of the end products of the WAO of LAS needs to be quantified. However, the available literature provides some clues. For example, WAO produces more biodegradable products from LAS; it is well-known that formic and acetic acids are biodegradable. Consequently, the WAO effluent should be more biodegradable as they constitute up to 28% of the final TOC. Additionally, conclusions can be drawn from the known mechanism of LAS biological degradation. It is well-accepted that LAS has three points at which microbial attack can proceed3s the same three places at which WAO attack also takes

place. As in WAO, desulfonation is the key step in the biological degradation of LAS. Research has shown that the sulfonate group is almost entirely responsible for the biological recalcitrance of the sulfonated aromatic nuclei formed during the biological oxidation of LAS.3 Consequently, the extent of LAS degradation via reactions A1, A2, B1-B3, and C1-C3 (Figure 11), the combined pathway to complete desulphonationswhich is the reaction pathway to biodegradable partial oxidation productssneeds to be increased. Because greater desulfonation appears to be possible at temperatures of at least 220 °C, WAO could be effective as a pretreatment to biological oxidation of high-concentration aqueous LAS waste streams. Conclusions The major reaction products identified in the WAO of LAS were low-molecular-weight VFAs such as formic and acetic acid, sulfonated aromatics, and sulfate. A greater temperature increases the degradation of LAS: for the 120-min uncatalyzed WAO of LAS at 1.5 MPa oxygen partial pressure, increasing the temperature from 180 to 240 °C increased the liquid-phase LAS removal from 79 to 100%, the COD removal from 23 to 70%, and the sulfate concentration from 7.4 to 57% of the theoretical maximum. The concentrations of the other reaction intermediates and products were also increased by the same increase in temperature. Increasing the total pressure had little effect on the overall TOC, COD, and VFA degradation, but an increase in total pressure from 3.05 to 6.55 MPa increased desulfonation. This is thought to be because the higher dissolved oxygen in the liquid phase at higher pressure possibly catalyzed or reacted with LAS to cause this desulfonation but was not sufficiently reactive or catalytic to cause aromatic ring cleavage to remove further TOC and COD. Increasing the reactor pressure suppressed the loss of LAS and volatiles through the reactor off-gas pipe. Therefore, the cost of a higher pressure must be minimized against the cost of waste gas treatment when designing a WAO system for the treatment of LAS. Desulfonation is the key reaction step for further TOC and COD removal. If LAS is desulfonated, literature indicates that the resulting reaction products would be more biologically oxidizable. As LAS can be significantly desulfonated by uncatalyzed WAO at temperatures above 220 °C, WAO is promising as a pretreatment to biological oxidation of high-concentration LAS waste streams. Nomenclature COD ) chemical oxygen demand (mg L-1) CSPA ) carboxylsulfophenylaldehyde HPLC ) high-performance liquid chromatography LAS ) linear alkylbenzene sulfonate PO2 ) oxygen partial pressure (MPa) SDBS ) sodium dodecylbenzene sulfonate SPA ) sulfophenylaldehyde SPC ) sulfophenylcarboxylic acid SPDA: sulfophenyldialdehyde T ) temperature (°C) t ) reaction time (min) TOC ) total organic carbon (mg L-1) VFA ) volatile fatty acid WAO ) wet air oxidation

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Subscript 0 ) initial

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Received for review March 30, 2001 Revised manuscript received August 2, 2001 Accepted August 15, 2001 IE010293K