Kinetics of 1,3,6-naphthalenetrisulphonic acid ozonation in presence of activated carbon

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1 Carbon 43 (2005) Kinetics of 1,3,6-naphthalenetrisulphonic acid ozonation in presence of activated carbon M. Sánchez-Polo a, R. Leyva-Ramos b, J. Rivera-Utrilla c, * a Swiss Federal Institute for Environmental Science and Technology (EAWAG), Ueberlandstrasse, 133, CH-8600 Dübendorf, Switzerland b Centro de Investigación y Estudios de Postgrado, Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, México c Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Avd. Fuentenueva, Granada, Spain Received 19 March 2004; accepted 15 November 2004 Available online 30 December 2004 Abstract Processes based on the simultaneous use of ozone and activated carbon have proven very effective for removing contaminants of high toxicity and low biodegradability. The present study is aimed to determine the kinetic constants involved in this purification process and their relationship with the surface chemistry of the activated carbon. For this purpose, the ozonation of 1,3,6-naphthalenetrisulphonic acid (NTS), selected as model compound, was carried out in the presence of different activated carbons. Determination of the Weisz Prater parameter (C WP ) revealed that intraparticular diffusion limitations exist in the system for particles >500 lm. The degradation kinetics of NTS in the presence of activated carbon depends on the concentrations of both, the contaminant and the dissolved ozone, with a global reaction order of 2. The heterogeneous reaction constants were determined using a model that allowed quantification of the capacity of the activated carbon to increase the NTS degradation rate and of the chemical surface properties responsible for this increase. The basicity of the activated carbon is mainly responsible for the catalytic activity of the carbon in NTS ozonation, even though, mineral matter contributes positively to the catalytic activity. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Activated carbon; B. Oxidation; D. Catalytic properties 1. Introduction The decontamination of waters is of growing interest to society, as reflected by the increasingly strict regulations being applied. This has led to the development of novel treatment technologies over the past decade. Waters polluted by anthropogenic sources can generally be efficiently processed by biological treatment plants, by adsorption on activated carbon or other adsorbents, or by conventional chemical treatments such as thermal oxidation, chlorination, and ozonation, among others * Corresponding author. Tel.: ; fax: address: jrivera@ugr.es (J. Rivera-Utrilla). [1 3]. However, in some cases these procedures cannot achieve the degree of removal required by law or necessary for subsequent use of the treated effluent. The ozonation of contaminants in the presence of activated carbon has recently attracted attention as a very promising purification process [4,5]. The combination of the high oxidant power of ozone and the high adsorptive capacity of activated carbon has been demonstrated to remove organic contaminants of high toxicity and low biodegradability more effectively [4,5]. In a previous study, it was observed that activated carbon can enhance the decomposition of ozone into highly oxidative species, thereby increasing even further the removal efficiency of this process [6,7]. In this study, it has been proposed that the metallic centers of mineral /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 M. Sánchez-Polo et al. / Carbon 43 (2005) matter present in activated carbon, the electrons of the carbon basal planes, and the oxygenated basic groups present on the surface of activated carbon (chromene and pyrone) are mainly responsible for ozone decomposition in aqueous medium. Thus, the reduction of ozone on the activated carbon surface generates OH ions and H 2 O 2 (reactions 1 and 2) that initiate the ozone decomposition into OH radicals, which are eventually responsible for the increase in the oxidation rate of the contaminants. Moreover, these radicals transform dissolved organic matter into CO 2, thereby reducing the concentration of total organic carbon present in the system [6,7]. O 3 þ H 2 O þ 2e O 2 þ 2OH O H C R or O C H R O 2/ O 3 HCl/H 2 O O CR + ð1þ + Cl - + H 2 O 2 ð2þ Nevertheless, a kinetic study of this process is necessary if it is to be improved, optimized, and implemented on a large scale and in an economic and efficient manner. Thus, the present study was aimed to analyze the kinetics of ozonation of organic contaminants in the presence of activated carbon with the following specific objectives: (i) to determine possible diffusion limitations, (ii) to propose a mathematical model that represents reasonably well the experimental results, and (iii) to determine the heterogeneous rate constants and their relationship with the surface chemical properties of the activated carbon. The ozonation of 1,3,6-naphthalenetrisulphonic acid (NTS), selected as the model compound, was carried out in the absence and presence of different commercial activated carbons. According to previous studies, this contaminant is characterized by a low reactivity to ozone [8] and a low and slow adsorption on activated carbon [9]. 2. Experimental 2.1. Activated carbons Because of their different textural and chemical properties the commercial activated carbons Filtrasorb 400, Merck, Norit, Ceca AC-40, Ceca GAC, Sorbo, and Witco were used as catalysts of the ozonation process. All activated carbons used were texturally and chemically characterized as described in detail elsewhere [6]. Tables 1 and 2 summarize the results of this characterization Methods The ozone was produced from oxygen using an OZO- KAV ozone generator with a maximum capacity of 76 mg/min. The reactor used was equipped with gas inlet and outlet, reactive alimentation, and sampling accessories. A more detailed description of the experimental setup used was previously reported [8]. The method used for the ozonation of NTS in presence and absence of activated carbon was also previously detailed elsewhere [6]. The reaction order was assessed by the method of initial rates of reaction. The reaction order with respect to NTS was determined by adding variable volumes of a concentrated NTS solution to the reactor, obtaining concentrations of NTS ranging from to mol/l. Simultaneously, 0.5 g of activated carbon was added. After 1 min of treatment, the concentrations of NTS and ozone present in the system were measured. The concentration of dissolved ozone in the system was kept constant at mol/l throughout the experiment, this was carried out by bubbling ozone gas continuously in the solution. The reaction order with respect to ozone was determined by a similar procedure. In this case, the NTS concentration in the system remained constant and the dissolved ozone concentration ranged from to mol/l. Table 1 Chemical and textural characterization of the activated carbons Activated carbon S N2 (m 2 /g) V a 2 (cm 3 /g) V b 3 (cm 3 /g) ph PZC Acid groups c (leq/g) Basic groups d (leq/g) Ash (%) Filtrasorb Sorbo-Norit 3 (A-7472) Merck (K ) Ceca GAC (1240) Ceca AC Norit R 2030 (A6122) Witco a Volume of pores with diameter of nm. b Volume of pores with diameter above 50 nm. c Determined by NaOH (0.1 N) neutralization. d Determined by HCl (2 N) neutralization.

3 964 M. Sánchez-Polo et al. / Carbon 43 (2005) Table 2 Some elements present in the ash of the activated carbons (wt.%) Activated carbon Si Ti Al Fe Mn Mg Ca Na K P Filtrasorb Sorbo Merck Ceca GAC Ceca AC Norit Witco Analytical methods The gas phase concentration of ozone was determined using a Spectronic Genesis 5 spectrophotometer. The dissolved ozone concentration in aqueous solutions was determined colorimetrically by the Karman Indigo method [10]. The NTS (Fluka) concentration was determined by means of a Merck-Hitachi HPLC apparatus with UV detector and using a RP-18 (5 lm) LiChrosphere 100 column, 250 mm long. A methanol water solution (35/65) was used as the mobile phase, containing mol/l of tetrabutylammonium bromide (Merck), TBABr, as ion exchanger and 10 2 mol/l NaH 2 PO 4 as ph regulator, at a flow rate of 1.3 ml/min. 3. Results and discussion 3.1. Influence of carbon particle size on NTS ozonation rate Fig. 1 depicts, as an example, the results of the NTS ozonation in presence of Filtrasorb 400 activated carbon for different particle sizes. The NTS degradation was greater in the presence of this carbon than in its absence, t (s) Fig. 1. Influence of the particle size of Filtrasorb 400 carbon on the NTS ozonation. ph = 2.3, T = 298 K. () Without carbon; (s) lm; (h) lm; (n) lm. and the NTS ozonation rate increased with decreasing particle size. Moreover, at short contact times the ozonation rate of NTS was slightly decreased by the presence of the activated carbon with particle sizes ranging lm; however, the ozonation rate was substantially raised when adding activated carbon with particle sizes ranging and lm. These results indicate that the degradation rate of NTS in the presence of activated carbon is influenced by diffusional processes. Since, as discussed later, the adsorption kinetics of NTS on activated carbon surface is very slow, the greater NTS elimination rate in presence of activated carbon results from an increase in the concentration of free radicals present in solution due to the ozone-activated carbon interaction, as previously reported [6,7]. In order to determine whether the increase in the ozonation rate by decreasing particle size was due to intraparticular diffusion limitations in the carbon, the Weisz Prater parameter [11] was calculated (Eq. (1)) for each particle size studied: C WP ¼ ð r MÞq p R 2 P ð1þ D E C M where ( r M ) is the NTS degradation rate (mol/l s), q P is the particle density (g/cm 3 ), (q P = 7 g/cm 3 for Filtrasorb 400), R p is the particle radius (cm), D E is the effective diffusivity (cm 2 /s), and C M is the concentration of NTS (mol/l). The Weisz Prater criterion indicates that the NTS ozonation rate in presence of activated carbon would not be influenced by pore diffusion when C WP 1 [11]. The effective diffusivity of NTS was estimated using Eq. (2) [12]: D E ¼ e PD AB ð2þ s P where e P is the void fraction of the carbon particle, D AB is the molecular diffusivity, and s P is the tortuosity factor of the activated carbon. The void fraction was calculated from the experimental values of pore volume and particle density (e P = 3 for Filtrasorb 400). The tortuosity factor for activated carbon can be considered approximately 3 [12] and the molecular diffusivity of the NTS was calculated by using the Wilke Chang equation (Eq. (3)).

4 M. Sánchez-Polo et al. / Carbon 43 (2005) Table 3 Values of the Weisz Prater parameter (C WP ) for each particle size of activated carbon Filtrasorb 400 Particle size (lm) C WP D AB ¼ 7: ð/m B Þ 1=2 T ð3þ g BV V 0:6 A In this equation, / is the association parameter of water and has a value of 2.6, M B is the molecular weight of water (18 g/mol), T is the temperature (298 K), g BV is viscosity of water (904 cp), and V A is the molar volume of the solute at its boiling temperature (196 cm 3 /mol). The value of V A was calculated using SchroederÕs method [13]. The value of the Weisz Prater (C WP ) parameter for each particle size of activated carbon Filtrasorb 400 is shown, as an example, in Table 3. The C WP value was smaller than 1 for particle sizes ranging lm and became greater than 1 for larger particle sizes. These results reveal that for particle sizes above lm, the NTS degradation rate is dependent on the intraparticular diffusion, whereas for particle sizes of lm, the rate is only slightly influenced by this phenomenon. Similar experiments to those presented for carbon Filtrasorb 400 were carried out for the rest of activated carbons. In all cases the values of C WP were lower than 1 when using particle sizes of lm. From the above results, a particle size of lm was selected to carry out the rest of the ozonation experiments in presence of the different activated carbons NTS ozonation in presence of different commercial activated carbons t (s) Fig. 2. NTS ozonation in presence of different activated carbons. ph = 2.3, T = 298 K. () Without carbon; (s) Filtrasorb 400; (n) Merck; (h) Ceca GAC; (*) Ceca AC40; ( ) Norit; (+) Sorbo; ( ) Witco. Fig. 2 depicts the results of NTS ozonation in presence of different commercial activated carbons. The Filtrasorb 400, Norit, Sorbo, and Ceca GAC carbons enhanced more the ozonation reaction of the NTS, and these carbons had the highest values of ph PZC, basic surface groups concentration, and/or ash concentration (Table 1). However, no clear relationship was found between the reaction rate and the surface area (S N2 )of the activated carbons. More detailed studies were previously published [6,7] on the influence of the chemical and textural properties of activated carbon on NTS ozonation. The kinetic study of the NTS adsorption on all the activated carbons showed that after 60 min of contact, none of this compound was adsorbed on any of the activated carbons studied. Therefore, the total NTS elimination is due to the ozonation process. Thus, the total NTS degradation rate in presence of activated carbon can be defined as the sum of the homogeneous reaction rate, ( r M ) homo, calculated in absence of activated carbon, and the heterogeneous reaction rate, ( r M ) hetero, due to the presence of activated carbon. As discussed earlier the activated carbon enhanced the oxidation rate of NTS by promoting the reduction of dissolved ozone into hydroxyl radicals that cause the NTS degradation. The total NTS degradation rate can be mathematically expressed as: ð r total Þ¼ð r homo Þþð r hetero Þ ¼ dc M homo þ dc M hetero ð4þ In accordance with Hoigné et al. [14], the homogeneous reaction rate can be represented by Eq. (5): dc M ¼ð r D Þþð r OH Þ homo ¼ k D C O3 C M þ k OH C OH C M ð5þ where r D represents the contribution of the direct ozonation reaction and r OH the contribution of the radical oxidation, and k D and k OH are the corresponding reaction constants. In a previous study [8,15], the value of the constants were k D = 6.72 (mol/l) 1 s 1 and k OH = (mol/l) 1 s 1 and a good fit was observed between the proposed model and the experimental results. The NTS concentration decay data shown in Fig. 2 were obtained under the same experimental conditions except that the type of activated carbon was different for each run. In all these experiments the concentration of dissolved ozone was kept constant at mol/ L. Fig. 2 indicates that the NTS degradation rate decreased considerably with increased treatment time.

5 966 M. Sánchez-Polo et al. / Carbon 43 (2005) t(s) Fig. 3. Influence of the ozone concentration on the NTS degradation in presence of Filtrasorb 400 carbon. (h) NTS concentration without interruption of ozone supply to reactor; (j) NTS concentration with interruption of ozone supply to reactor; (s) ozone concentration without interruption of ozone supply to reactor; (d) ozone concentration with interruption of ozone supply to reactor. The NTS ozonation rate in presence of Filtrasorb 400 carbon, as an example, was approximately mol/ls in the first 5 min of treatment, whereas this rate was reduced by 84% in the next 5 min. This behavior can be explained by considering that the heterogeneous degradation rate is dependent on the concentration of NTS. Likewise, experiments were conducted to determine whether the dissolved ozone concentration affects the heterogeneous reaction rate of NTS. For this purpose, NTS ozonation was carried out in presence of Filtrasorb 400 and the supply of ozone to the reactor was interrupted after 60 s of treatment (Fig. 3). Fig. 3 shows that the NTS degradation rate was reduced when the dissolved ozone concentration in the system diminished. These results indicate that, as in the case of the kinetic equation of the homogeneous reaction rate, the concentration of dissolved ozone is a factor to be included in the kinetic equation that represents the heterogeneous degradation rate of NTS. According to the results depicted in Figs. 2 and 3, it may be inferred that the heterogeneous reaction rate should be of the following type: dc M ¼ k hetero f ðc M ; C O3 Þ ð6þ hetero where k hetero represents the heterogeneous reaction constant when activated carbon catalyzes the reaction. The order of this reaction with respect to the NTS and ozone were determined using the method of initial rates of reaction. For this purpose, NTS ozonation experiments were conducted in presence of Filtrasorb 400 activated carbon, varying the NTS concentration and keeping the ozone concentration constant and vice versa. The initial concentration of the NTS was varied from to mol/l and that of the ozone from to mol/l. The results 1.2 [O 3 ] x10 4 (mol/l) indicate that the reaction order with respect to both NTS and ozone is 1, therefore the overall reaction order is 2. Eq. (6) can be written as: ð r hetero Þ¼ dc M ¼ k hetero C M C O3 ð7þ hetero The k hetero value for each activated carbon was determined by using Eq. (7), which is expressed as follows at a constant concentration of dissolved ozone: ð r hetero Þ¼ dc M ¼ k obs C M ð8þ hetero The ( r hetero ) values were calculated as the difference between ( r total ) and ( r homo ) (Eq. (4)). These reaction rates were estimated by measuring the slope of the NTS concentration decay curve against time (Fig. 2). The experimental data of ( r hetero ) vs. C M were interpreted with Eq. (8), yielding the value of k obs. Fig. 4 depicts, as an example, the experimental data for Filtrasorb 400 carbon and it can be observed that Eq. (8) fits the data relatively well since a correlation coefficient of around 0.99 was achieved. The values of k obs are given in Table 4. The Thiele modulus, u, and effectiveness factor, g, were applied in order to confirm that diffusion limitations were not important to determine k obs under the conditions used. For a pseudo-first order reaction (Eq. (8)) and spherical particles, these parameters are defined as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðk obs Þ u ¼ R real p ð9þ D E g ¼ 3 ðu coth u 1Þ u ð10þ 2 where the (k obs ) real is the k obs without diffusion effects. Intraparticle diffusion effects are not important when the effectiveness factor equals 1. Assuming that k obs had -r hetero x 10 6 (mol/ L s) [NTS] x10 4 (mol/l) Fig. 4. Representation of calculated values of r hetero vs. the concentration of NTS for Filtrasorb 400 activated carbon. ph = 2.3, T = 298 K. Particle size = lm.

6 M. Sánchez-Polo et al. / Carbon 43 (2005) Table 4 Values of the heterogeneous reaction constants for the original and demineralized activated carbons evaluated according to the proposed model Activated carbon k obs (s 1 ) (k obs ) real (s 1 ðkheteroþ ) k hetero (k hetero ) demi khetero ðkheteroþ demi 100 demi ((mol/l) 1 s 1 ) ((mol/l) 1 s 1 khetero %ash ((mol/l) 1 s 1 ) ) Filtrasorb Sorbo Merck Ceca GAC Ceca AC Norit Witco no diffusion effects ((k obs ) real = k obs ) and using the numerical values for F-400 carbon in these equations (Table 4), it was calculated that u = 1.73 and g = 4, indicating that the k obs value included diffusion effects of little importance. This result was expected since the C WP parameter was less than 1 but not much less than 1(C WP 1), in line with the Weisz Prater criterion [11]. The k obs must be corrected to eliminate the diffusion effects, and this can be done on the basis of the definition of the effectiveness factor [11]. ð r hetero Þ¼gð r hetero Þ real ð11þ where ( r hetero ) real represents the heterogeneous reaction rate without diffusion limitations, which is defined as Eq. (12): ð r hetero Þ real ¼ðk obs Þ real C M ð12þ Replacing Eqs. (8) and (12) in Eq. (11), the following equation is obtained: k obs ¼ gðk obs Þ real ð13þ The (k obs ) real value was estimated by an iterative method using the Eqs. (9), (10) and (13) and the k obs value. The final results for F-400 carbon were u = 1.91, g = 18 and (k obs ) real = 140 s 1. Table 4 lists the (k obs ) real values for the different carbons. The relationship between k hetero and (k obs ) real is (k obs ) real = k hetero C O3. The k hetero value for each carbon was obtained by substituting the dissolved ozone concentration value ( mol/l), and the results are shown in Table 4. The k hetero values were higher than the k D value but much lower than the k OH value. Therefore, the global ozonation rate in presence of activated carbon can be expressed as follows: dc M ¼ k D C O3 þ k OH C OH þ gk hetero C O3 CM ð14þ This is a general kinetic model because it includes the homogeneous and heterogeneous reaction rates as well as the diffusion effects. In the ozonation catalyzed by activated carbon the NTS concentration decay data for different particle diameters can be predicted by using the above equation and calculating the effectiveness factor with Eqs. (9) and (10). The results are depicted in Fig. 5, demonstrating that the proposed model, shown t (s) Fig. 5. Prediction of the proposed general kinetic model (discontinuous line) for the NTS ozonation in presence of Filtrasorb 400 activated carbon as a function of the particle size. ph = 2.3, T = 298 K. (s) lm, g = 18 and u = 1.91; (h) lm, g = 4 and u = 3.26; (n) lm, g = 0.58 and u = by the discontinuous line, adequately represented the experimental results. The k hetero value can be used to quantify the catalytic activity of activated carbon in the NTS ozonation Effect of surface characteristics of activated carbon on the k hetero constant The relationship between the k hetero value (Table 4) and the chemical characteristics (Table 1) of each activated carbon was further studied. It was noticed that the NTS degradation was especially enhanced in the presence of carbons with a high content of ashes and a high concentration of basic groups. Both are catalytic centers that can decompose the ozone into highly reactive species. Consequently, the Norit, Sorbo, and Ceca GAC activated carbons presented the highest k hetero values. More detailed studies were previously published on the mechanism by which activated carbon surface basic groups and the metals in the mineral matter of carbon act in the decomposition of ozone into highly reactive species [6,7]. In order to determine the contribution of the ash to the heterogeneous ozonation rate of NTS, ozonation

7 968 M. Sánchez-Polo et al. / Carbon 43 (2005) experiments were conducted in presence of demineralized carbons. Demineralization of activated carbons with HCl and HF was carried out using the procedure described in detail elsewhere [6]. Fig. 6 depicts, as an example, the results obtained for the original and demineralized Ceca GAC carbon. In all of the carbons except for Witco, due to its very low ash content (Table 1), the degradation rate was reduced when the activated carbon was demineralized. These results corroborate that the mineral matter present in activated carbon contributes positively to the catalytic effect of activated carbon in the NTS ozonation. All the metals that have been found to show catalytic activity in the ozonation of organic compounds [16 18] are present in the mineral matter of the activated carbons used (Table 2). However, it is difficult to ascertain the role of each metal in the catalyzed ozonation of NTS. Previous studies demonstrated that the demineralization of carbons has no major effect on the concentration of surface basic groups [19]. In addition, the ph PZC values of the demineralized carbons were very similar to those of the original carbons. Therefore, in the case of demineralized carbons, the NTS degradation rate was enhanced due to the presence of basic groups on the activated carbon surface. A similar method (Eqs. (6) (10) and (13)) was applied to determine the heterogeneous reaction constant in presence of the demineralized carbons, (k hetero ) demi (Table 4). Once the (k hetero ) demi value is known, the contribution of the basic groups to the catalytic activity of the activated carbons in the NTS ozonation process can be determined by dividing the (k hetero ) demi value by the k hetero value. Except in the case of Ceca GAC carbon, the (k hetero ) demi contributed more than 50% of the k hetero value (Table 4). These results indicate that, although the mineral matter contributed positively to the catalytic activity of the activated carbon, especially in the case of Ceca GAC carbon, the basicity of the carbon was t (s) Fig. 6. Effect of demineralization on the catalytic capacity of Ceca GAC carbon in the NTS ozonation. ph = 2.3, T = 298 K. (h) Untreated; (j) demineralized. mainly responsible for the enhancement of the NTS degradation rate. Furthermore, the heterogeneous reaction constant obtained for the demineralized carbons (Table 4) can be related to the concentration of basic groups in each carbon (Fig. 7a) or to the surface concentration of these groups (Fig. 7b), a linear relationship between them was observed. On the other hand, the fact that the ordinate in the origin was not zero indicates that other aspects of the surface chemistry of the activated carbon contributed positively to the catalytic effect of the carbon in the NTS ozonation process. Another aspect of major interest was observed when the difference between the k hetero and (k hetero ) demi values for each carbon was divided by its ash content (Table 4). The results indicate that the contribution of the mineral matter of the activated carbon to its catalytic activity in NTS ozonation does not depend on the amount but rather on the type of mineral present in each carbon. This is due to the fact that some components present in the carbon do not contribute to its catalytic activity (Table 2) in the ozonation process (Mg, Na, K, Si). The concentrations of these components vary between one activated carbon and another. Interestingly, the mineral matter with the greatest catalytic activity in the NTS ozonation process was that present in the Ceca (k hetero ) demi ((mol/l) -1 s -1 ) (a) (k hetero ) demi ((mol/l) -1 s -1 ) (b) Basic sites (µeq/g) Basic sites (µeq/ g m 2 ) Fig. 7. Relationship between the (k hetero ) demi value and the concentration of basic groups in the activated carbons.

8 M. Sánchez-Polo et al. / Carbon 43 (2005) GAC, Sorbo and Filtrasorb 400 carbons. Because of the chemical complexity of the mineral matter of activated carbon, much greater efforts must be applied to the identification of the specific metal compounds that participate in catalytic ozonation. 4. Conclusions In general, the presence of activated carbon in the ozonation process increases the NTS degradation rate. In accordance with the Weisz Prater criterion, it was determined that for particle sizes above lm the NTS degradation rate is controlled by intraparticular diffusion limitations. The kinetic equation that represents the heterogeneous reaction rate depends on the concentrations of contaminant and dissolved ozone. The results indicated that the reaction order with respect to both NTS and ozone is 1. Furthermore, the proposed general kinetic model adequately represents the experimental results, allowing quantification of the catalytic activity of the carbon in the NTS ozonation process. The k hetero constant values determined ranged from 94.2 (mol/ L) 1 s 1 for Witco to (mol/l) 1 s 1 for Norit carbon. The k hetero values were related to the chemical properties of the activated carbon, and its catalytic activity was observed to be directly related to the basicity and to the mineral matter content. On the other hand, studies with demineralized carbons revealed that the basicity of the activated carbon is mainly responsible for the catalytic activity in the NTS ozonation process. Moreover, these results show that not all components of the mineral matter contribute positively to the catalytic activity in ozonation. Because of the chemical complexity of the mineral matter, much greater efforts must be applied to elucidate the components involved and the mechanism by which they act in the ozonation process. Acknowledgments The authors are grateful for the financial support provided by MCT-DGI and FEDER (Project: CTQ C02-01/PPQ). M. Sánchez-Polo expresses his gratitude to the Andalusian Regional Government for providing a research fellowship. References [1] Perry RH, Green D. PerryÕs chemical engineerõs handbook 6th ed. New York: McGraw Hill Inc.; [2] Metcalf J, Eddie C. Waste water engineering: treatment, disposal and reuse, 3rd ed. New York: McGraw Hill Inc.; [3] Bermond A, Camel V. The use of ozone and associated oxidation processes in drinking water treatment. Water Res 1998;32(11): [4] McKay G, McAleavey G. Ozonation and carbon adsorption in a three-phase fluidised bed for colour removal from peat water. Chem Eng Res Des 1988;66(3): [5] Zaror CA. Enhanced oxidation of toxic effluents using simultaneous ozonation and activated carbon treatment. J Chem Tech Biotech 1997;70(1):21 8. [6] Rivera-Utrilla J, Sánchez-Polo M. Ozonation of 1,3,6-naphthalenetrisulphonic acid catalyzed by activated carbon in aqueous phase. Appl Catal B: Environ 2002;39(4): [7] Sánchez-Polo M, Rivera-Utrilla J. Effect of O 3 /reaction on the catalytic activity of activated carbon during degradation of 1,3,6- naphthalenetrisulphonic acid with ozone. Carbon 2003;41(2): [8] Sánchez-Polo M, Rivera-Utrilla J, Zaror CA. Advanced oxidation with ozone of 1,3,6-naphthalenetrisulphonic acid in aqueous solution. J Chem Tech Biotech 2002;77(2): [9] Rivera-Utrilla J, Sánchez-Polo M. The role of dispersive and electrostatic interactions in the aqueous phase adsorption of naphthalenesulphonic acids on ozone-treated activated carbon. Carbon 2002;40(14): [10] Bader H, Hoigné J. Determination of ozone in water by the Indigo method. Water Res 1981;15(4): [11] Fogler H, Scott L. Elements of chemical reaction engineering. 3rd ed. New Jersey: Prentice Hall; [12] Leyva-Ramos R, Geankoplis CJ. Diffusion in liquid filled pores of activated carbon. I. Pore volume diffusion. Can J Chem Eng 1994;72(2): [13] Reid RC, Prausnitz JM, Poling BE. Properties of gases and liquids. 4th ed. New York: McGraw- Hill; [14] Hoigné J, Bader H. Rate constants of the ozone with organic and inorganic compounds. I. Non dissociating organic compounds. Water Res 1983;17(2): [15] Rivera-Utrilla J, Sánchez-Polo M, Zaror CA. Degradation of naphthalenesulphonic acids by oxidation with ozone in aqueous phase. Phys Chem Chem Phys 2002;4(7): [16] Gracia R, Aragues JL, Ovelleiro JL. Mn(II)-catalysed ozonation of raw Ebro river water and its ozonation by-products. Water Res 1998;32(1): [17] Beltran FJ, Rivas FJ, Montero-de-Espinosa R. Catalytic ozonation of oxalic acid in an aqueous TiO 2 slurry reactor. Appl Catal B: Environ 2003;39(3): [18] Canton C, Esplugas S, Casado J. Mineralization of phenol in aqueous solution by ozonation using iron or copper salts and light. Appl Catal B: Environ 2003;43(2): [19] Moreno-Castilla C, Carrasco-Marin F, Maldonado-Hódar F, Rivera-Utrilla J. Effects of non-oxidant and oxidant acid treatments on the surface properties of an activated carbon with very low ash content. Carbon 1998;36(1 2):