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Kinetics of the Autoxidation of Sodium Dodecyl Sulfate Catalyzed by Alumina-Supported Co-Zn Composite K. Usman,† A. A. Adesina,*,† F. P. Lucien,† and T. D. Waite‡ Reactor Engineering and Technology Group, School of Chemical Engineering and Industrial Chemistry, and School of Civil and Environmental Engineering, University of New South Wales, Sydney 2052, Australia
Sodium dodecyl (lauryl) sulfate (SDS), an important anionic surfactant used in a variety of textile and biotechnology operations, frequently ends up in stationary water bodies where it promotes the formation of blue-green algae, an environmentally offensive species. The catalytic wet oxidation of SDS has been studied over alumina-supported cobalt-zinc oxide catalysts under relatively mild conditions of pressure (900 kPa). A temperature-dependent expression for the degree (extent) of mineralization, R, in terms of the activation energy for SDS degradation and TOC oxidation was derived as R ) 1.43 × 10-2 e1503.8/T, valid between 403 and 473 K. A mechanism was also proposed to explain the oxidative degradation of SDS. 1. Introduction The most widely used surfactants are those containing anionic groups such as sodium dodecyl sulfate, SDS. As a result, SDS is a frequent toxic pollutant in aqueous effluent from textile, electronics, polymer, and bioprocessing industries.1 Although SDS can be degraded by epilithic and planktonic bacteria in river water,2 it is biologically resistant in stationary water bodies because of microbial acclimatization and can accentuate growth of blue-green algae. Consequently, activated sludge, trickling filter, or anaerobic digester processes cannot be effectively utilized to remove this biorecalcitrant organic compound. Alternative technologies such as chemical oxidation pretreatment can, however, be used to initially convert nonbiodegradable organics to smaller molecules.3 Because wet air oxidation requires elevated pressures and temperatures (>3 MPa and >573 K), recent studies have focused on the application of catalysts to bring about mild operating conditions.4-6 Additionally, catalytic autoxidation on supported transition metal oxides such as ZnO and CuO reduces corrosion problems and hence diminishes equipment cost. Even so, bimetallic catalysts have reportedly shown superior activity as a result of synergistic effects.4,7 Composite oxide catalysts have been used to effect the aqueous-phase oxidation of phenolic compounds. The exceptionally good activity * Author for communication. Ph.: (61-2) 93855268. Fax: (61-2) 93855966. E-mail:
[email protected]. † School of Chemical Engineering and Industrial Chemistry. ‡ School of Civil and Environmental Engineering
of the Co-Zn catalyst was attributed to the presence of unique active sites in a new zinc aluminate phase formed via the solid reaction between ZnO and the alumina support. These new sites are presumably energetically superior to the sites on either the cobalt or zinc oxide phases. In a previous investigation, we also observed8 that the sum of the individual activities of the single-component Co and Zn alumina-supported catalysts is about 30-40% lower than the activity recorded over the bimetallic Co-Zn system. Therefore, the purpose of the present work is to examine the kinetics of SDS aqueous-phase oxidation using this catalyst under relatively mild operating conditions. 2. Materials and Methods Analytical reagent grade SDS and metal nitrates were obtained from Aldrich Chemicals and used without further purification. Catalysts were prepared using the incipient wetness technique via the addition of quantitative amounts of the nitrates of cobalt and zinc to appropriate masses of ground and sieved γ-alumina (support). The mixing was done at a constant isoelectric point (pH ) 6.8) using Milli-Q deionized water in an ultrasonic agitator for 1 h at 333 K. The resulting slurry was degassed and dried at 383 K overnight and then calcined in air for 3 h at 923 K. Five catalyst compositions were prepared, namely, 10 Co/10 Zn/80 alumina, 15 Co/5 Zn/80 alumina, 5 Co/15 Zn/80 alumina, 0 Co/20 Zn/80 alumina, and 20 Co/0 Zn/80 alumina. Autoxidation experiments were performed in a 1-L stainless steel reactor at pressures of up to 1.3 MPa and temperatures up to 448 K. Oxidizing gas (O2/N2 mix-
10.1021/ie001007s CCC: $20.00 © 2001 American Chemical Society Published on Web 07/24/2001
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Table 1. BET Surface Area for the Catalysts no.
catalyst
BET (m2 g-1)
1 2 3 4 5 6
20 Co/80 alumina 15 Co/5 Zn/80 alumina 10 Co/10 Zn/80 alumina 5 Co/15 Zn/80 alumina 20 Zn/80 alumina δ-alumina
145 144 142 120 64 150
ture) was introduced through a porous glass bubbler at the bottom of the reactor and supplemented with agitation by a 3-cm-diameter circular magnetic stirrer. The reactor was heated using an external electric jacket (tape) wound around the entire length of the cylindrical stainless steel vessel and wrapped with insulation. The vessel was fitted with a top flanged lid equipped with a condenser, a pressure control and relief valve, a temperature port, and a sampling port. The reaction mixture was pressurized with N2 to the required operating pressure (0.6-1.3 MPa) and temperature (403448 K). O2 was later introduced, and the N2 flow was appropriately adjusted to maintain constant total pressure. Gas flow rates were regulated by electronic mass flow controllers. At specified time intervals, 5-mL aliquots were withdrawn from the reactor via a 1/16-in. tubing, microfiltered, and analyzed. The exit gas stream was passed through a condenser and subsequently analyzed on a TCD gas chromatograph for CO2. The SDS concentration was analyzed (from the filtered aliquot) on a Shimadzu UV-1601 spectrophotometer (set at 658 nm) using a modified Mukerjee’s method.9 The total organic carbon concentration was determined on a TOC analyzer (Shimadzu TOC-5000A), and the sulfate, SO42-, ionic concentration was measured on a computer-controlled HPLC equipped with a conductivity detector(Shimadzu model CDD-6A). Separation was effected over an Allsep anion 100 mm × 4.6 mm Alltech column using as the mobile phase, 4 mM p-hydroxybenzoic acid adjusted to pH 7.5 by LiOH and pumped isocratically at 1 mL min-1 at 308 K. 3. Results and Discussion 3.1. Catalyst Characterization. The total surface area (BET area) of the catalyst samples was determined using N2 adsorption at 77 K on a Phlosorb apparatus. As can be seen from Table 1, the surface area of the alumina-supported catalysts varied between 64 and 145 m2 g-1 depending on composition. Calcination at 923 K converted the original γ-alumina support (BET area ≈ 250 m2 g-1) to δ-alumina, whose total surface area is about 150 m2 g-1. From the table, is seems that addition of ZnO to the catalyst caused a steady decrease in BET area. This can be attributed to the formation of a new zinc aluminate phase occasioned by the solid-solid reaction between ZnO and Al2O3 during calcination. In particular, it appears that cobalt oxide species did not react with the support under similar conditions, as, at 20% Co loading, the difference in surface area between the pure support and the 20 Co/80 alumina catalyst was only 5 m2 g-1 (ca. 3% drop). Such a small change is probably only due to pore blockage by cobalt oxide crystallites. In fact, the metal dispersion was relatively high under these conditions, as the XRD data indicated that the catalyst was essentially amorphous. Figure 1 shows a plot of the results for CO2 production over a 1-h period at 438 K and 1.1 MPa for all five
Figure 1. Effect of catalyst composition on CO2 evolution during oxidative SDS degradation using pure oxygen. Catalyst loading ) 8 g L-1, Ptot ) 1.1 MPa, T ) 438 K, gas flow rate ) 10 mL s-1.
Figure 2. Influence of gas flow rate on reaction rate during SDS oxidation over 10 Co/10 Zn/80 alumina. CSDS ) 0.014 mol L-1, T ) 438 K, catalyst loading ) 8 g L-1, Ptot ) 1.1 MPa.
catalysts using pure O2 as the oxidizing gas. It is evident from these runs that the pure (single) ZnO catalyst was the least active for SDS degradation. The initial depression in the CO2 profiles might be due to absorption of CO2 in the reaction medium (pH ) 7) before equilibrium was attained with the gas-phase composition. The data also suggest that CO2 evolution increased with Co content in the catalyst, with the highest CO2 production corresponding to the 20% Co catalyst. In general, the bimetallic catalysts, 15 Co/5 Zn/80 alumina and 10 Co/ 10 Zn/80 alumina, showed superior activities for both SDS degradation and TOC oxidation. Thus, subsequent experiments were performed with the 15 Co/5 Zn/80 alumina catalyst. 3.2. Mass Transport Analysis. Detailed analysis of the transport resistances prevalent in a slurry reactor is necessary for a meaningful kinetic evaluation of the rate data. Figure 2 shows the results of preliminary runs carried out to determine the effect of gas flow rate on SDS degradation rate. It is apparent that, beyond a gas flow of about 7 mL s-1, the degradation rate was practically constant with increased flow rate, indicating that gas-liquid transport resistance was negligible. Indeed, the plot of (1/-rA) versus 1/m (catalyst loading) shown in Figure 3 indicates that the gas absorption resistance (intercept on the 1/-rA axis) was only about 10% of the combined resistances due to diffusion and chemical reaction at 10 mL s-1 and 8 g L-1 catalyst loading. Moreover, separate runs showed that the reaction rate is invariant with particle size below about 70 µm. The
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oxidizing species in the aqueous-phase oxidation of organics. Surface hydroxyl (or peroxy) radicals can be generated in situ by the protonation of the adsorbed superoxide anion O2- or the chemisorption of OH- ions in solution onto conduction-band (positive) holes. Thus, the reactions for surface peroxy or hydroxyl radical generation are
Figure 3. Effect of catalyst loading. Ptot ) 1.1 MPa, T ) 438 K, CSDS ) 0.014 mol L-1, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
O2(aq) + X- ) O2-(ads)
(2a)
H +(aq) + O2-(ads) ) HO2•(ads)
(2b)
OH-(aq) + X+ ) OH•(ads)
(2c)
where X+ and X- are conduction-band holes and valenceband electrons, respectively. In a separate set of runs, we actually observed that the SDS degradation rate was improved with a highly alkaline slurry (feed), implying that eq 2c might be the more favorable pathway. However, because the kinetic runs here were carried out at the natural pH of the SDS solution (no adjustment), the aqueous dissociation of the sodium salt would yield the dodecyl sulfate anions and Na+ species as the first step in the catalytic process, prior to adsorption of the alkyl sulfate on catalyst surface. Hence
C12H25OSO3Na(aq) T C12H25OSO3-(aq) + Na+(aq) (3) H2O T OH-(aq) + H+(aq) Figure 4. Dependency of SDS degradation rate on initial SDS concentration. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina. Table 2. Values of Parameters Used to Estimate Thiele Modulus variable
value
dp Deff CSDS (-rrxn)
30 µm 0.1DABa 0.0139 mol L-1 4.8 × 10-6 mol L-1 s-1 at 448 K
experimental Thiele modulus, Φ of 30-70 µm was obtained from 2
Φ exp
exp,
for particle sizes
(r′exp)Fpdp2 ) 4DeffCAo
(1)
Application of our data (see Table 2) to eq 1 gave Φ exp ) 0.006, which suggests an effectiveness factor of nearly 1, irrespective of the intrinsic kinetics and particle size. As a result of these considerations, further kinetic investigation was done using a total gas flow rate in excess of 10 mL s-1 with catalyst particles in the range of 30-50 µm at a loading of 8 g L-1. 3.3. Effect of SDS Concentration. As can be seen from Figure 4, the SDS degradation rate increased monotonically with SDS concentration and the data could be described by a pseudo-first-order rate law
-rSDS ) kSDSCSDS
Because SDS is highly soluble in water, the degradation rate would be directly proportional to the surface concentration of SDS, in agreement with the linear dependency observed. The adsorption of the dodecyl sulfate anion occurs via
C12H25OSO3-(aq) + X+ T ROSO3•(ads)
a D -5 AB was obtained from the Nernst equation as 1.849 × 10 cm2 s-1.
(2)
to give kSDS ) 3.82 × 10-4 s-1. It is generally acknowledged10 that hydroxyl or peroxy radicals are the active
(4)
(5)
Interestingly, the fact that the production of SO42(or HSO4- as the ion chromatograph column used could not discriminate between sulfate or bisulfate species) was also parallel to the degradation of SDS supports the view that the initial breakdown of the adsorbed organic molecule is accompanied by a concomitant release of the bisulfate anion
ROSO3•(ads) + OH•(ads) f RO•(ads) + HSO4•(ads) (6) followed by
HSO4•(ads) T HSO4-(aq) + X+
(7)
Indeed, the rate of sulfate production was also described by a pseudo-first-order kinetics with respect to SDS concentration, namely
rSO4 ) kSO4CSDS
(8)
where linear regression of the data of Figure 5 gave kSO4 ) 8.33 × 10-5 mol of SO42- (mol of SDS)-1 s-1 with a correlation coefficient of 0.92. Because 1 mol of SO42is formed from 1 mol of SDS, this simply becomes, kSO4 ) 8.33 × 10-5 s-1 . The slight scatter in Figure 5 was
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Figure 5. Dependency of sulfate formation rate on initial SDS concentration. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 7. Effect of oxygen partial pressure on SDS degradation rate. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 6. Effect of initial SDS concentration on TOC oxidation rate. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 8. Effect of oxygen partial pressure on TOC oxidation rate. Ptot) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
probably due to the fact that the concentrations of SO42used in computing the initial sulfate production rate were small (at low SDS conversions) and close to the lower detection limit of the ion chromatograph. However, a linear kinetic rate law is still more acceptable than a complex kinetic expression to describe this behavior. We remark that independent pH monitoring during the course of the reaction revealed a continuous increase in solution acidity consistent with the production of SO42- (or HSO4-) from the degradation of the dodecyl sulfate anion. Although SDS can be degraded, complete mineralization to smaller molecules (such as CO2 and H2O) might be hindered because of the relative stability of the reaction intermediates (partially oxygenated compounds) such as alcohols. Thus, the concentration of total organic carbon (TOC) in solution is a direct indicator of the degree of organic mineralization. In particular, TOC is a useful parameter for wastewater quality assessment. Figure 6 shows that the TOC oxidation rate is also favored by high SDS concentrations. Regression analysis of the TOC data to a first-order kinetic law (correlation coefficient of 0.997) gave the estimate of the rate constant kTOC as 1.71 × 10-3 mol of C (mol of SDS)-1 s-1 or 1.43 × 10-4 s-1, assuming that 1mol of SDS yields 12 mol of C. It would seem that, on the basis of kinetics, SDS oxidative degradation is about 2.5 times faster than the oxidation of TOC at 448 K, suggesting that partially
oxygenated intermediates such as alcohols are present in significant amounts in solution. Evidence for the accumulation of intermediate alcohols, ketones, or carboxylic acids has been reported10-12 during aqueousphase phenol oxidation. 3.4. Influence of Oxygen Partial Pressure. Figure 7 reveals that the oxidative degradation of SDS increased almost linearly with O2 partial pressure in the feed gas. This monotonic increase in rate is consistent with the relatively high molecular weight and long chain of the dodecyl backbone. The pseudo-first-order rate constant implicated by the data on Figure 7 can easily be obtained as kO2 ) 1.005 × 10-5 mol L-1s-1MPa-1. The intercept on the O2 axis arose simply because the starting solution was not presaturated with oxygen at 0.36 MPa. Presaturation with oxygen would have had the effect of shifting the origin to the 0.36 MPa mark. This observation is consistent with the fact that SDS would not normally degrade in aqueous media under atmospheric pressure (0.1 MPa). Interestingly, the effect of O2 partial pressure on TOC (Figure 8) reveals that, although an increase in the O2 partial pressure will cause a higher concentration of dissolved O2, the TOC oxidation rate leveled off after an O2 partial pressure of about 0.8 MPa, indicating the resilience of the organic carbon (intermediates) to further oxidation, even with increased aqueous O2 availability. The change of kinetic order, from first order at pressures below 0.8 MPa to nearly zero order at higher pressures signals a possible change in the rate-controlling step. In agreement with the data for SDS degradation, the production of HSO4from the organic dodecyl sulfate anion also increased
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Figure 9. Effect of oxygen partial pressure on sulfate formation rate. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 12. Arrhenius plots for SDS degradation rate and TOC oxidation rate. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 10. Effect of oxygen partial pressure on CO2 production rate. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 13. Arrhenius plots for SO42- and CO2 production rates. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
Figure 11. SDS and TOC concentration profiles during reaction. Ptot ) 1.1 MPa, T ) 448 K, gas flow rate ) 10 mL s-1, dp ) 30 µm, catalyst ) 15 Co/5 Zn/80 alumina.
3.5. Effect of Temperature. Temperature has a strong influence on the reaction system. Figure 12 illustrates the Arrhenius behavior of SDS degradation and TOC oxidation over the 15 Co/5 Zn/80 alumina catalyst. Parameter estimation from the data yields ESDS ) 89.70 kJ mol-1 and ETOC ) 77.20 kJ mol-1, with associated frequency factors ASDS ) 1.144 × 107 s-1 and ATOC ) 1.65 × 105 s-1. The degree of mineralization of the organic substrate, R, for the catalyst can be defined as the ratio of the TOC oxidation rate to the actual rate of degradation of the organic compound. Obviously, complete mineralization, R ) 1, is the ideal for wastewater treatment. Because the TOC oxidation and SDS degradation rates are both first-order with respect to the concentration of SDS
R) monotonically with O2 partial pressure (cf. Figure 9), as HSO4- ions are directly produced from the primary cleavage of the dodecyl sulfate anion by hydroxyl attack (cf. eq 6). It can be seen from Figure 10 that the CO2 evolution profiles passed through a maximum between 20 and 25 min after reaction had begun, irrespective of the O2 partial pressure used. Because CO2 does not undergo further oxidation, this drop might be due to the continuous flow of gas through the reactor and, hence, a purging of CO2 from the system with time-on-stream. As Figure 11 shows, both TOC oxidation (responsible primarily for CO2 production) and SDS degradation increased almost exponentially with time-on-stream.
ATOC (ESDS-ETOC)/RT e ASDS
(9)
which yields R ) 1.43 × 10-2 e1503.8/T for the catalyst used in this investigation. Intriguingly, whereas reaction rate is generally favored by increased temperature, the degree of mineralization appears to be higher at lower temperatures . As a result, a compromise temperature in the range of 403-473 K is desired to achieve both a reasonable degradation rate and a higher degree of mineralization. Figure 13 also shows the Arrhenius lines for the CO2 and SO42- production rates. Interestingly, the activation energy values, ECO2 ) 76.10 kJ mol-1 and ESO4 ) 75.96 kJ mol-1, are very similar to the value for TOC oxidation (ETOC ) 77.20 kJ mol-1).
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This further substantiates the earlier discussion that CO2 is primarily produced from TOC oxidation (because the energy barriers for both TOC oxidation and CO2 formation are almost identical). By the same token, the nearly identical activation energy estimates for sulfate production and TOC oxidation is additional proof that SO42- (or HSO4-) ions are released concurrently with the initial oxidation of the dodecyl sulfate species. Subsequent oxidation of the alkyl group to lowermolecular-weight alcohols or aldehydes is probably due to a nonkinetic effect such as the nature of the entities in question. 3.6. Mechanistic Proposition. In view of the preceding discussion, it seems reasonable that the oxidative degradation of SDS is first initiated by aqueous-phase dissociation of the organic salt, followed by a series of surface reaction steps as outlined below
tion of mineralization) was also characterized by similar linear kinetics. However, Arrhenius treatment of the data revealed that SDS degradation had a higher activation energy (89.70 kJ mol-1) than the TOC oxidation (77.20 kJ mol-1), even though the former has a much higher rate constant. Indeed, the degree of mineralization over the catalyst is temperature-dependent and is favored at lower temperature. CO2 and SO42- (or HSO4-) production rates also exhibited activation energy values that were nearly identical to that for TOC oxidation. On the strength of the data obtained, a mechanism was proposed to describe the various phenomena associated with the reaction system. Acknowledgment
Dissociation: C12H25OSO3Na(aq) T C12H25OSO3-(aq) + Na+(aq)
The authors are grateful to the Australian Research Council for provision of a major equipment grant. K.U. appreciates the study leave and scholarship provided by the Bandung Institute of Technology, Indonesia.
Hydroxyl ion generation: Na+(aq) + H2O(l) T Na+(aq) + OH-(aq) + H+(aq)
Nomenclature
Adsorption: C12H25OSO3-(aq) + X+ T C12H25OSO3•(ads) Adsorption: O2(aq) + X- T O2-(ads) H+(aq) + O2-(ads) T HO2•(ads) C12H25OSO3•(ads) + OH•(ads) f C12H25O•(ads) + HSO4•(ads) •
-
+
HSO4 (ads) T HSO4 (aq) + X OH-(aq) + X+ T OH•(ads)
ROO•(ads) f R1O•(ads) + R2OH(aq)
C ) concentration, mol cm-3 DABo ) binary diffusion coefficient of A in B, m2 s-1 d ) diameter, cm E ) activation energy of carbon dioxide, kJ mol-1 F ) volumetric flow rate, mL s-1 k ) specific reaction rate m ) catalyst loading, g L-1 P ) pressure, MPa R ) ideal gas constant, J mol-1 K-1 r ) rate of reaction, mol s-1 L-1 T ) absolute temperature, K Greek Symbols η ) effectiveness factor Φ ) Thiele modulus F ) density, g cm-3 Subscripts
R1O•(ads) + OH•(ads) f R3O•(ads) + R4OH(aq)
A ) species A eff ) effective exp ) experiment tot ) total
R3O•(ads) + HO2•(ads) f ... CO2 + H2O
Literature Cited
HSO4-(aq) + H+(aq) T 2H+(aq) + SO42-(aq)
NaOH(aq) + CO2 f Na+HCO3-(aq) Although it was difficult to detect all possible intermediates with the HPLC-ion chromatograph analytical unit employed, the bulk of our qualitative data (TOC, sulfate production, and SDS concentration measurements) agrees with the sequence of elementary steps illustrated in this mechanism. 4. Conclusions This investigation reports the catalytic autoxidation of sodium dodecyl sulfate in the aqueous phase using alumina-supported catalysts. Preliminary catalyst evaluation revealed that the dual metal oxide component catalyst, 15 Co/5 Zn, was the best among the five options tested. Kinetic analysis of the oxidative degradation of SDS over this catalyst shows essentially first-order dependency on both the SDS concentration and the O2 partial pressure. The rate of TOC oxidation (an indica-
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Received for review November 30, 2000 Revised manuscript received April 4, 2001 Accepted June 6, 2001 IE001007S
