Wet Air Oxidation of Linear Alkylbenzene Sulfonate 2. Effect of pH

Biodegradability of linear alkylbenzene sulfonates subjected to wet air oxidation. Darrell A Patterson , Ian S Metcalfe , Feng Xiong , Andrew G Living...
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Ind. Eng. Chem. Res. 2001, 40, 5517-5525

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Wet Air Oxidation of Linear Alkylbenzene Sulfonate 2. Effect of pH 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.

Semibatch wet air oxidation (WAO) experiments were conducted on aqueous solutions of linear alkylbenzene sulfonates (LAS) at various pH’s, to determine whether acidic and/or basic pH affects the rate and extent of the degradation and desulfonation of LAS and its refractory sulfonated intermediates. It was found that acidic pH increases the removal of TOC, COD, and LAS, as well as increasing desulfonation. Acid catalysis of the desulfonation and aromatic ring cleavage reactions is thought to be responsible. Conversely, the WAO of LAS and its intermediates was less effective at high pH. It is hypothesised that this is due to radical scavengers and the dominance of the superoxide radical in the alkaline environment. Desulfonation might also be autocatalyzed in unmodified LAS WAO reactions by reaction products, as acid is a product of LAS WAO reactions. Experiments were conducted at 240 °C at neutral (unmodified) and alkaline pH’s to compare WAO in the presence and “absence” of this acid. Desulfonation occurred to the same extent at both pH’s, indicating that it was not significantly autocatalyzed by the acid generated during WAO. Introduction Wet air oxidation (WAO) is a well-established technology for the oxidative treatment of a wide variety of waste streams.1-3 Because WAO cannot fully degrade organics to carbon dioxide and water without employing either high temperatures (>275 °C 4) and/or a catalyst, it is frequently used as a pretreatment to biological treatment processes.3,5,6 Unfortunately, the range of detailed studies on specific compounds is limited to mainly phenolic organics1,7,8 and volatile fatty acids (VFAs).1,4,9,10 High-priority, toxic, or biologically recalcitrant waste streams, such as those containing sulfonated organics have not been adequately studied.1 This paper details the second part of an investigation into the WAO pretreatment of the most prolific class of sulfonated organics in wastewaters, the linear alkylbenzene sulfonates (LASs). In part 1 of this work (previous paper, this issue), we outlined the basics of LAS WAO oxidation under various temperatures and pressures. It was demonstrated that, to increase the effectiveness of the WAO of LAS, the rate and extent of the desulfonation of LAS and its sulfonated reaction intermediates (Figure 1, reactions B1-B3) must be increased to enable further oxidation to volatile fatty acids (VFAs) and other typical WAO refractory (yet biodegradable) products. Literature indicates that, if LAS and its WAO intermediates are desulfonated, they will become more biodegradable. Therefore, in theory, WAO must be able to completely desulfonate LAS if it is to be an effective pretreatment to biological degradation in the proposed integrated process. A promising method of increasing desulfonation is by running the WAO at different pH’s: acidic or basic. * 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.

Past WAO studies have shown that pH has a significant effect on the WAO reaction pathway8 and is one of the most important parameters in WAO reactor design.11 Acidic conditions might significantly enhance the WAO of LAS, as aromatic sulfonates can be hydrolytically desulfonated in the presence of high concentrations of mineral acids via reaction 112

RSO3H + H2O f RH + H2SO4

(1)

Thus, if a mineral acid is added, reaction 1 is catalytically accelerated, increasing in rate 2.5-3 times for every 1 M rise in acid in the solution or for every 10 °C rise in temperature.12 Phosphoric acid is the most widely used desulfonation catalyst, despite being the least active. This is because it leaves the compounds unchanged if used between 190 and 220 °C12 and causes less charring than some of the other mineral acids.13 The potential for using phosphoric acid to enhance the WAO of LAS is encouraged by past studies,14-16 which used phosphoric acid to desulfonate LAS and other phenyl sulfonates for analysis by gas chromatography (GC). However, much work is still to be done on the large-scale applicability of these desulfonations, as microdesulfonation techniques are typically used.14,17 Moreover, these studies further confirm that desulfonation is the most difficult reaction step in the WAO of LASseven in hot phosphoric acid, detergent range sulfonates such as tetrapropylenebenzene sulfonate show very little desulfonation.13 Furthermore, all previous reactions have been performed in the absence of an oxidant. Consequently, the effect of acid-catalyzed desulfonation of LAS in an oxidizing medium is yet to be demonstrated. Alkaline pH might also enhance the WAO of LAS. For example, the WAO destruction of phenol is most effective at pH’s greater than 12.8 The potential of alkaline pH for the desulfonation of LAS is suggested by a study of the thermal stability of LAS and other alkylaryl sulfonates in alkaline media, where no oxygen was fed to the reactors.18 At 204 °C, minor degradation occurred via two primary pathways, namely, (1) desulfonation and (2) alkyl chain scission, as in the WAO of LAS. The

10.1021/ie010294c CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001

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Figure 1. Reaction mechanism for the WAO of LAS, as it is presently understood.

latter pathway produced hydrogen sulfide and a range of low-molecular-weight hydrocarbons as products, including gaseous hydrogen, methane, ethane, ethylene, and a range of butenes. No degradation of LAS occurs at such temperatures in the absence of oxygen at neutral pH (see previous paper, this issue). Thus, the desulfonation of LAS might be more effective at alkaline pH. This work presents the second step toward developing a new treatment process for high-concentration LAS wastewater: the integrated WAO and biodegradation of LAS. The following study concentrates on increasing the effectiveness of the first stage of the process, the WAO of LAS. This paper aims to determine whether pH adjustment can be used to increase the amount of desulfonation during the WAO treatment of LAS (compared to unmodified LAS WAO) and to determine the effect of pH on the LAS WAO reaction pathway, reaction intermediates, and products. To achieve this aim, the work was split into three parts. First, the effect of acidic, neutral, and basic initial pH’s was assessed to determine the best conditions for the desulfonation and overall degradation of LAS by WAO. To elucidate the effect of acid concentration on the desulfonation and overall WAO degradation of LAS, the acid concentration was

increased in further experiments. These experiments also served as a means of determining whether acid catalysis of LAS was occurring. Finally, in part 1 of this work (previous paper, this issue), it was shown that, if the WAO temperature was increased, the resultant effluent became more acidic, and the TOC and COD degradation increased. This suggests that the acid generated during the reaction might have affected the reaction mechanism, intermediates, and products. By performing WAO experiments at 240 °C and alkaline pH, the effect of temperature and acidic pH were divorced to determine whether the acid produced by the WAO reaction autocatalyzed the desulfonation reaction. Materials and Methods Apparatus and WAO Reaction Method. The reactor and semibatch WAO experimental methods are described elsewhere (previous paper, this issue). All analytical techniques are also identical to those of the previous work. Unless otherwise stated, all solutions were reacted at 200 °C with 1.5 MPa oxygen overpressure (3.05 MPa total pressure). The pH during the reaction was left uncontrolled. To determine the variation and error in the data, experiments at temperatures of 200 and 240 °C fed with

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LAS at neutral pH and experiments fed with acidified LAS (initial pH of 1.9) at a temperature of 200 °C were repeated several times. The experimental error was calculated and reported as the coefficient of variation (the standard deviation divided by the mean). To determine the contribution of anoxic thermal degradation to the WAO degradation of LAS, several control experiments in which only nitrogen was fed to the reactor were performed. Materials. All WAO experiments were performed on 1600 mg L-1 (0.046 mol L-1) solutions of sodium dodecylbenzene sulfonate (SDBS), used as received from Aldrich, Dorset, U.K. The pH was adjusted by batch loading the required amount of orthophosphoric acid (BDH AnalaR, 85%) or sodium hydroxide (BDH AnalaR) to SDBS (referred to as LAS throughout) solutions, which were preconcentrated so that the final mixed concentration of LAS was 1600 mg L-1. Orthophosphoric acid rather than hydrochloric or sulfuric acid was used to adjust LAS to acidic pH, because hydrochloric acid can cause corrosion at high temperatures and sulfuric acid masks the measurement of sulfate in the reacted liquid. Furthermore, phosphoric acid is the most commonly used aromatic desulfonation catalyst.12,14-16 Sodium hydroxide was chosen as the alkaline adjuster, as it is used to neutralize LAS in most production processes.19 All water used was deionized using a Purite HP 700 purifier.

Figure 2. Effect of initial pH on the pH during the WAO of LAS at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). 2, pH0 1.9; [, pH0 6.8; /, pH0 8.3; 0 pH0 12.1.

Results and Discussion Effect of Initial pH. Semibatch WAO experiments loaded with LAS at initial pH’s (pH0’s) of 1.9, 6.8, 8.3, and 12.1 were used to determine the effects of acidic, neutral, and alkaline conditions on the WAO reaction mechanism, intermediates, and products. The experiments that were repeated for solutions with pH0 6.8 and 1.9 showed that the data were repeatable and that the differences observed and described between data sets were not due to anomalous or erroneous experiments. pH Change during WAO. WAO reactions are typically run without pH control. Consequently, the evolution of different chemical species throughout the course of the WAO reaction changes the liquid-phase pH with time. For most WAO reactions, more acidic species form during the oxidation, making the pH of the reacting liquid acidic.1 However, during the course of the WAO reaction in this study, where four different initial pH’s (pH0’s) were used, there was a distinct difference in the pattern of liquid-phase pH between the alkaline and the neutral and acidic pH experiments (Figure 2). The pH for the WAO experiment with pH0 12.1 initially dropped as expected, but remained alkaline throughout the experiment. In contrast, the liquid pH in the other experiments decreased and remained acidic throughout. Note that the pH at time zero in Figure 2 (and all other figures) is the liquid pH taken from the reactor after it was at reaction temperature. TOC, COD, and Organic Reaction Intermediates. All results illustrate that the WAO of LAS is more effective when the pH is initially acidic. At pH0 1.9, the TOC and COD reduction was larger than either the neutral or the alkaline pH0 experiments (Figure 3). In contrast, these experiments had similar TOC and COD removals. Also, a more rapid LAS removal occurred under acidic conditions (Figure 4). Increasingly less LAS was removed at higher pH’s, indicating that one or more species present under alkaline conditions inhibit the

Figure 3. Effect of initial pH on the TOC and COD for LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). 2, pH0 1.9; [, pH0 6.8; /, pH0 8.3; 0 pH0 12.1.

WAO reaction. The experiments at pH 8.3 are not an exception to this trend, as the pH during the course of the WAO reaction (Figure 2) was as acidic as that for the experiments at pH0 6.8. Furthermore, the greatest amounts of formic and acetic acid accumulated during the most acidic experiment (Figure 5). Note that the data for formic and acetic acids are presented as the ratio of TOC calculated from the VFA concentration to the total TOC at that time in the reaction. Unusually, the alkaline experiment accumulated greater concentrations of formic acid than neutral pH0 experiments,

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Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 Table 1. pKa Values for the Dissociation of Various Acids in the WAO of LASa pKa temperature (°C)

a

acid

25

200

240

HCOOH H3CCOOH H2CO3 HCO3-

3.75 4.76 6.35 10.33

4.74 5.65 7.87 10.89

5.12 5.99 8.62 11.37

Estimated using the method in Robinson and Stokes.29

Figure 4. Effect of initial pH on the LAS concentration for LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). 2, pH0 1.9; [, pH0 6.8; /, pH0 8.3; 0 pH0 12.1.

Figure 6. Effect of initial pH on the sulfate formed for LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). 2, pH0 1.9; [, pH0 6.8; /, pH0 8.3; 0 pH0 12.1.

Figure 5. Effect of initial pH on the formic and acetic acid concentrations for LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). 2, pH0 1.9; [, pH0 6.8; /, pH0 8.3; 0 pH0 12.1.

despite having similar overall WAO degradations (at 120 min, the ratios of formic acid TOC to total TOC were 7.2, 2.8, and 1.7% for pH0 values of 12.1, 8.3, and 6.8, respectively). In contrast, the net acetic acid accumulations were equivalent in these experiments (Figure 5). High concentrations of formic acid were not expected, as it oxidizes to carbon dioxide and water at temperatures as low as 150 °C.4 In this case, the greater formic acid concentration is a consequence of the pH of the alkaline experiment (pH 8-10): in this range, formic and acetic acid are present in their dissociated forms,

i.e., as formate and acetate anions (refer to Table 1 for pKas). Dissociation completely changes the WAO kinetics of formic acid. Formate oxidation has been found to be six times slower than formic acid oxidation.20 Additionally, formic acid degrades through both thermal nonoxidative and oxidative reaction pathways, whereas formate only decomposes in oxidative environments.20 This is because formate is stabilized by resonance between two identical structures, whereas formic acid has an unstable resonance.20 Therefore, formate is more stable than formic acid and, so, will accumulate in the alkaline experiments, whereas formic acid will be degraded in the experiments when the pH is acidic during the course of the WAO reaction. Acetic acid and acetate are both stable in WAO environments at temperatures as high as 275 °C,4 so they are refractory intermediates in the WAO of LAS at 200 °C and accumulate in the reactor at all pH’s. Desulfonation. The desulfonation of LAS and its WAO intermediates increased at lower pH’s (Figure 6). However, for all pH’s, the extent of desulfonation was far below the theoretical maximum. The greatest desulfonation occurred during the experiment in which LAS was fed at pH 1.9, where 26% of the total possible sulfate was formed. Literature indicates that acid catalysis is likely to be the cause,12 and this possibility was investigated in further experiments. Similar results were obtained in studies of the WAO of the sulfonated organic nitrotoluenesulfonic acid (NTSA) 21 at initial pH’s of 1.7, 2.8, 7, and 11. The lowest pH (pH 1.7), adjusted with sulfuric acid, also gave the best NTSA removal. The researchers speculated that this could have been due to the acidic hydrolytic desulfonation of NTSA to nitrotoluene, but did not confirm this hypothesis.

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Figure 7. Effect of increasing the concentration of orthophosphoric acid on the sulfate formed during LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). [, 0 mol L-1; 2, 0.093 mol L-1; ×, 0.21 mol L-1.

Control Experiments and Experiments at Neutral pH0. Control experiments performed at initial pH’s of 1.9 and 8.1, with a feed of only nitrogen, showed that no sulfate was formed. Furthermore, in all experiments, no sulfate formed during the initial anoxic heating period. An increase in sulfate concentration and organic degradation was detected only when oxygen was introduced to the reactants. Thus, desulfonation was oxygen-initiated under these conditions. No TOC, COD, or LAS degradation was apparent either, showing that thermal degradation of LAS was not significant. Furthermore, orthophosphoric acid did not act as an oxidant during the WAO, as degradation was not observed in the acidified control experiment. Hence, orthophosphoric acid most likely behaved as a catalyst when oxygen was present. In all of the previous data sets, the experiments with initial pH’s close to neutral (pH0 6.8 and 8.3) had similar results. The coefficients of variation in the data at pH0 6.8 at 120 min for the normalized TOC, COD, and LAS, and the formic acid, acetic acid, and sulfate formed were 0.023, 0.023, 0.77, 0.34, 0.33, and 0.082, respectively. The data at pH 8.3 fit within this variation. Therefore, data from experiments at pH0 6.8 and 8.3 will henceforth be treated as originating from the same initial pH: neutral or 7. The combined coefficients of variation in these data for the above parameters are 0.017, 0.019, 1.1, 0.35, 0.25, and 0.076, respectively. This is mostly less than the uncombined data set, verifying that the data are approximately the same. It is accepted that the LAS data are more variable, as the process of measuring and calculating LAS concentrations (HPLC peak quantification) requires more data-processing steps to obtain a single data point than the other measurements. Orthophosphoric Acid Catalyzed Desulfonation of LAS. To substantiate the hypothesis that orthophosphoric acid was catalyzing the desulfonation reaction during LAS WAO, the concentration of orthophosphoric acid was increased from 0.093 to 0.21 to 1.0 mol L-1. All experiments were conducted at a WAO temperature of 200 °C, a total pressure of 3.05 MPa, and a LAS concentration of 1600 mg L-1 (0.046 mol L-1). Desulfonation, TOC, and COD Results. Results indicate that acid catalysis was occurring. Greater desulfonation was observed with greater addition of acid (Figure 7). The amounts of sulfate formed compared to the theoretical total were 18, 26, and 38% for ortho-

Figure 8. Effect of increasing the concentration of orthophosphoric acid on the TOC and COD for LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). [, 0 mol L-1; 2, 0.093 mol L-1; ×, 0.21 mol L-1; O, 1.0 mol L-1.

phosphoric concentrations of 0, 0.093, and 0.21 mol L-1, respectively. Thus, orthophosphoric acid did catalyze the desulfonation reaction. Moreover, these experiments showed that a greater concentration of orthophosphoric acid in an oxidizing environment increases not only the rate of desulfonation but also the rate and extent of total organic degradation. Consequently, the TOC and COD reductions increased with greater additions of acid (Figure 8). The relative increase in degradation in going from 0.21 to 1.0 mol L-1 acid was small when compared to the relative increase achieved in going from uncatalyzed to 0.21 mol L-1 acid. Thus, a small addition of acid greatly increased the TOC and COD destruction compared to uncatalyzed LAS WAO, but increasing the quantity of acid past this threshold had comparatively little effect. The small gain in degradation with greater amounts of acid was highlighted by an experiment initially loaded with 6.1 mol L-1 orthophosphoric acid, which had a TOC reduction of only 41.7% after 60 min. Compared to the experiment with only 1.0 mol L-1 acid, this is only a 6.9% increase in TOC destruction for a 610% increase in acid concentration. The samples from the 6.1 mol L-1 acid trial could not be analyzed further, however, because the analytical techniques were incompatible with the high acid concentration. Reaction Intermediates. The accumulation of formic and acetic acids (Figure 9) also increased with higher orthophosphoric acid concentrations. This indicates that orthophosphoric acid does not appreciably catalyze VFA degradation. In addition, more LAS was degraded in the

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Figure 10. Effect of increasing the pH on the liquid pH during the WAO of LAS. O, 200 °C and pH0 7 (Ptotal ) 3.05 MPa); 2, 200 °C and pH0 12.1 (Ptotal ) 3.05 MPa); [, 240 °C and pH0 8.5 (Ptotal ) 4.85 MPa); 9, 240 °C and pH0 11.9 (Ptotal ) 4.85 MPa).

atures might be unnecessary. Consequently, the role of this acid in the desulfonation of LAS must be elucidated. Autocatalysis by Acid at High Temperature. In part 1 (previous paper, this issue), significant desulfonation of LAS was observed above a temperature of 220 °C. For example, WAO of initially neutral (pH0 ) 8.5) LAS at 240 °C gave 56.8% desulfonation. The pH for these same experiments dropped to 2.86 after 120 min of WAO. This is much lower than the equivalent pH’s for LAS WAO at temperatures of 180, 200, and 220 °C (3.5, 3.0, and 3.0 respectively) and is a result of the acid generated during desulfonation Figure 9. Effect of increasing the concentration of orthophosphoric acid on the formic and acetic acid concentrations for LAS WAO at T ) 200 °C, PO2 ) 1.5 MPa (Ptotal ) 3.05 MPa). [, 0 mol L-1; 2, 0.093 mol L-1; ×, 0.21 mol L-1; O, 1.0 mol L-1.

acid-catalyzed experiments than in the experiments with no acid. However, the LAS degradation rate did not substantially increase with greater added acid concentration (results not presented). This is partly because LAS degrades rapidly (compared to TOC and COD). Additionally, this could also indicate that orthophosphoric acid does not catalyze the oxidation of the LAS alkyl chain, which is the main LAS degradation reaction (previous paper, this issue). Because a greater amount of TOC and COD was removed at higher concentrations of orthophosphoric acid (Figure 8), the orthophosphoric acid must therefore also catalyze the aromatic ring cleavage reaction (reactions C1-C3 in Figure 1). This is the only other mechanism that can greatly reduce TOC and COD other than alkyl chain degradation. Figure 1 shows that this is feasible: if reactions B1-B3 and C1-C3 are driven further toward completion, they yield a product mixture with lower overall TOC and COD. The maximum desulfonation was only 38% using a orthophosphoric acid concentration of 0.21 mol L-1. This is lower than the 56.8% desulfonation measured during WAO at 240 °C without added acid (previous paper, this issue). An obvious method of achieving greater desulfonation is therefore to acid catalyze higher-temperature WAO reactions. However, acid is already generated by the WAO reaction at these conditions, and the above results show that greater concentrations of acid give little increase in the LAS degradation. Therefore, adding acid to catalyze the reaction at higher temper-

C12H25C6H4SO3- Na+ + 51/2O2 f 18CO2 + 14H2O + H+ + SO42- + Na+ (2) A pH of 2.6 can be calculated using eq 2 and the observed sulfate concentration after 120 min at 240 °C. This is in close agreement with the measured pH of 2.86, indicating that this acid is the cause of the low pH. Hypothetically, autocatalysis of the desulfonation reactions could occur, as a greater desulfonation at higher temperatures would produce a greater concentration of acid during the WAO, autocatalyzing desulfonation (reactions B1-B3 in Figure 1). To determine whether desulfonation at high WAO temperature is induced by this autocatalysis, an experiment was performed at 240 °C and 1.5 MPa oxygen overpressure with an initial pH of 11.9, adjusted using sodium hydroxide (Sigma, Dorset, U.K.). This was compared to experiments at 240 °C with initial pH’s of 8.5 and 12.1 and neutral pH experiments at 200 °C. pH change during WAO. The liquid pH in the alkaline experiments did not decrease during the course of the reaction. In the 240 °C alkaline pH experiment, the pH initially dropped as is expected during WAO because of the generation of acidic species.1 However, unlike all of the other LAS WAO experiments, after 20 min, the pH increased (Figure 10). This is most likely due to the formation of carbonates from the destruction of formate (reaction 3) and from the carbon dioxide generated by TOC destruction (reactions 4-6).

HCOO- + 1/2O2 f HCO3-

(3)

CO2 + H2O S H2CO3

(4)

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H2CO3 + H2O S HCO3- + H3O+

(5)

HCO3- + H2O S CO32- + H3O+

(6)

The pKa values of these reactions (Table 1) indicate that the pH could be increased to the values in Figure 10, despite the formation of formic, acetic, and other organic acids. Higher than usual inorganic carbon was measured in the samples from these experiments, which is consistent with the formation of bicarbonate and carbonate ions in solution (reactions 4-6). TOC, COD, and Reaction Intermediates. For LAS WAO degradation at 240 °C, the TOC and COD reductions (Figure 11) were much greater at neutral pH0 than in the pH0 11.9 experiment. These TOC and COD trends are not as distinct at 200 °C, where the TOC and COD reductions for both alkaline and neutral experiments were the same (accounting for the experimental uncertainties and variation in the data). The amounts of sulfate formed (Figure 12) were approximately the same for both pH0 neutral and alkaline experiments. LAS (Figure 13) was degraded at a slightly faster rate in the pH0 neutral experiments. This difference could only be distinguished for the 200 °C experiments at 120 min, where the average LAS reductions were 96.6 and 75.7% for the pH0 neutral and alkaline experiments, respectively, as the LAS destruction is too rapid at 240 °C. At 240 °C, formic acid appears to be mineralized in the pH0 neutral experiment (Figure 14) (0.9% of the TOC at 120 min), but it accumulates in the pH0 alkaline experiment as previously observed (5.8% of the TOC at 120 min). Acetic acid again accumulates at both pH’s (Figure 14), for reasons discussed above. Discussion. These results show that, at high temperature (240 °C), despite the clear absence of acid catalysis at alkaline pH, the extents of desulfonation are approximately the same during the experiments with initial pH’s of 8.5 and 11.9. This demonstrates that the same amount of desulfonation can occur despite the additional acid generated by the WAO reaction. Thus, desulfonation reactions for experiments without initial additions of acid are not significantly autocatalyzed by the acid generated during LAS WAO. Because there were no other significant desulfonation activators present during these experiments, other than the energy supplied to and generated by the reaction, it can be concluded that the desulfonation reaction during LAS WAO is mainly temperature-activated. At 240 °C, the same amounts of desulfonation occurred in both the acidic and alkaline experiments, because, at alkaline pH, alternative desulfonation mechanisms (other than reaction 1) can be activated at high temperature. For example, at the higher pH conditions, desulfonation could occur via a hydroxydesulfonation mechanism. Hydroxydesulfonation of aromatic sulfonates could occur by the following SN2 mechanism22

All of the experiments without initial additions of base had greater TOC, COD, and LAS degradations and formed more LAS WAO intermediates and products at

Figure 11. Effect of increasing the pH on the TOC and COD for LAS WAO. O, 200 °C and pH0 7 (Ptotal ) 3.05 MPa); 2, 200 °C and pH0 12.1 (Ptotal ) 3.05 MPa); [, 240 °C and pH0 8.5 (Ptotal ) 4.85 MPa); 9, 240 °C and pH0 11.9 (Ptotal ) 4.85 MPa).

Figure 12. Effect of increasing the pH on the sulfate formed for the WAO of LAS. O, 200 °C and pH0 7 (Ptotal ) 3.05 MPa); 2, 200 °C and pH0 12.1 (Ptotal ) 3.05 MPa); [, 240 °C and pH0 8.5 (Ptotal ) 4.85 MPa); 9, 240 °C and pH0 11.9 (Ptotal ) 4.85 MPa).

higher temperatures. This indicates that organic degradation is inhibited at alkaline pH. Aromatic ring cleavage would most likely be inhibited, as it is catalyzed by acid. However, other typical WAO reactions, such as the degradation of organic carbon to VFAs, were also inhibited. The reason for this might be two-fold. First, bicarbonate and carbonate, which are most likely present in the alkaline experiments (reactions 3-6), act

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proposed radical mechanism of WAO, which is well summarized by numerous authors.24-26 It is thought that two of the major radical initiation species are the hydroxyl radical (HO•) and the perhydroxyl radical (HO2•). However, at alkaline pH’s, the perhydroxyl radical would have dissociated to the superoxide anion (O2- •)

HO2• S O2- • + H+ pKa (25 °C) ) 4.827

(9)

The superoxide anion is thought to have little activity in aqueous solution.28 This is because it is understood to have strong solvation in water and rapidly disproportionates under alkaline conditions28 Figure 13. Effect of increasing the pH on the LAS concentration for the WAO of LAS. O, 200 °C and pH0 7 (Ptotal ) 3.05 MPa); 2, 200 °C and pH0 12.1 (Ptotal ) 3.05 MPa); [, 240 °C and pH0 8.5 (Ptotal ) 4.85 MPa); 9, 240 °C and pH0 11.9 (Ptotal ) 4.85 MPa).

2O2- • + H2O f HOO- + HO- + O2

(10)

Thus, without the perhydroxyl radical species at the alkaline pH, the WAO degradation of LAS and its WAO intermediates would be less effective, as observed. However, to render LAS biodegradable, a greater desulfonation beyond that achieved in this study is still required. In theory, this requires complete desulfonation. Studies on the biodegradability of the WAO intermediates and products are necessary to substantiate this premise. The above results indicate that acid catalysis at higher temperatures could increase this desulfonation. Further experiments were not performed using this system, because increasing the WAO temperature increases the total oxidation of LAS and creates a more expensive process (because thicker reactor walls and larger compressors are required). Although sufficiently high temperatures would give complete desulfonation, they would also inevitably lead to total oxidation of LAS and higher capital costs, negating the benefits of an integrated process. Future work should therefore concentrate on two fronts: (1) determining whether the above treatments yield biodegradable effluents and the factors controlling their biodegradability and (2) if the effluents prove to be insufficiently biodegradable, developing a catalyst that makes LAS more biodegradable under mild conditions (200 °C, 3.05 MPa total pressure). Conclusions

Figure 14. Effect of increasing the pH on the formic and acetic acid concentrations for the WAO of LAS. O, 200 °C and pH0 7 (Ptotal ) 3.05 MPa); 2, 200 °C and pH0 12.1 (Ptotal ) 3.05 MPa); [, 240 °C and pH0 8.5 (Ptotal ) 4.85 MPa); 9, 240 °C and pH0 11.9 (Ptotal ) 4.85 MPa).

as radical scavengers,23 lowering the effectiveness of the WAO

HO• + CO32- f OH- + •CO3-

(8)

Second, the radicals that propagate the WAO reaction under alkaline conditions have lower oxidation potentials at alkaline pH. This hypothesis is based on the

The increased desulfonation at acidic pH increases the degradation of TOC and COD during the WAO of LAS. Thus, at 200 °C and 1.5 MPa oxygen overpressure with initial pH’s of 1.9, 6.8, 8.3, and 12.1, the greatest TOC and COD decrease was at pH 1.9. With initially neutral pH’s and pH 12.1, these reductions were approximately the same. The LAS removal decreased as the initial pH increased. Sulfate formation was significantly enhanced as the pH became more acidic (26, 18, 17, and 10% of theoretical sulfate, respectively, after 120 min). Formic acid accumulated in WAO experiments initially at alkaline pH. Desulfonation of LAS is catalyzed by orthophosphoric acid. As the molar concentration of acid was increased from 0 to 0.093 to 0.21 mol L-1, the amount of sulfate formed after 120 min increased from 18 to 26 to 38%. The TOC, COD, and LAS reductions and the amounts of WAO intermediates and products such as formic and acetic acid also increased. This indicates that small amounts of orthophosphoric acid also catalyze aromatic ring cleavage during the WAO of LAS.

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5525

The acid generated during the unmodified WAO of LAS does not appreciably autocatalyze the desulfonation reaction. Consequently, it is hypothesized that the desulfonation reaction during WAO is mainly temperature-activated. Although the desulfonation for experiments at 240 °C with an initial pH of 11.9 were the same as those initially at pH 8.5, the overall WAO of LAS and its intermediates was less effective at alkaline pH’s. It is hypothesised that this is due to the inhibition of ring cleavage, the presence of radical scavengers, and the dominance of the superoxide radical in the alkaline environment. Nomenclature COD ) chemical oxygen demand (mg L-1) HPLC ) high-performance liquid chromatography LAS ) linear alkylbenzene sulfonate pH0 ) initial pH PO2 ) oxygen partial pressure (MPa) Ptotal ) total pressure (MPa) SDBS ) sodium dodecylbenzene sulfonate T ) temperature (°C) TOC ) total organic carbon (mg L-1) VFA ) volatile fatty acid WAO ) wet air oxidation Subscript 0 ) initial

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Received for review March 30, 2001 Revised manuscript received August 2, 2001 Accepted August 15, 2001 IE010294C