Activated Carbon Promoted Ozonation of Polyphenol Mixtures in Water

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Ind. Eng. Chem. Res. 2007, 46, 8241-8247

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Activated Carbon Promoted Ozonation of Polyphenol Mixtures in Water: Comparison with Single Ozonation Ine´ s Gira´ ldez, Juan F. Garcı´a-Araya, and Fernando J. Beltra´ n* Departamento de Ingenierı´a Quı´mica y Quı´mica Fı´sica, UniVersidad de Extremadura, AVenida de ElVas S/N, 06071 Badajoz, Spain

A mixture of polyphenol compounds (gallic acid, tyrosol, and syringic acid) is treated with ozone in water in the presence of activated carbon (AC). Individual (O3) and combined (O3/AC) ozonations have been carried out following the concentrations of initial compounds, intermediates, ozone, and hydrogen peroxide and total organic carbon (TOC). AC ozonation processes significantly improve both polyphenol conversion and mineralization. Hydrogen peroxide formed during the process seems to play an important role in accelerating the oxidation rate. Different carboxylic acids are formed as intermediate products. Consumption of ozone per unit mass of carbon removed is reduced in the combined ozonation process. 1. Introduction In a previous paper1 activated carbon and ozone were simultaneously used to study the removal of gallic acid, a polyphenol compound usually present in different food and cherry liquor processing wastewaters.2 This work showed the beneficial effect of activated carbon to enhance the mineralization of gallic acid and byproducts. The results seemed also to confirm others reported early on the activated carbon ozonation of different compounds (i.e., surfactants, carboxylic acids, etc.3-5). Direct ozonation of gallic acid leads to the appearance of hydrogen peroxide in a first step and then activated carbon improves both the conversion of ozone and hydrogen peroxide likely into radicals that react with intermediates to reach, in many cases, full mineralization. Many wastewaters are constituted by compounds of different natures with phenols as one of the main groups. In particular, wastewater generated during some food and liquor processes (olive oil, table olives, wine distilleries, etc.2) contain polyphenols that constitute the main polluting problems of these industries. The fact that ozone reacts very fast with phenols and that activated carbon improves the oxidation process with the generation of hydroxyl radicals (through hydrogen peroxide formation) makes the combination of these two agents very suitable for removal of phenols and even mineralize the wastewater treated. However, so far, no attempt has been made to study this process for phenol wastewaters. In this work a polyphenol mixture is prepared in water from three main polyphenols, usually present in the wastewaters above indicated: gallic acid, syringic acid, and tyrosol (see structures in Figure 1). The synthetic wastewater is treated with ozone in the presence and absence of activated carbon. The polyphenols studied contain different nucleophilic positions in their structures which make them suitable to react quickly with ozone.6 In an ozone process the presence of activated carbon will usually enhance the mineralization rate (that is, removal of intermediates). The objective of this work has been to confirm, with a synthetic polyphenol wastewater, the beneficial effects of the ozone-activated carbon combination compared to single ozonation for further practical application. In a following paper a kinetic study will be presented. * To whom correspondence should be addressed. Tel.: +0034924289387. Fax: +0034924289385. E-mail: [email protected].

Figure 1. Molecular structures of polyphenols studied. Gallic acid: R1 ) OH, R2 ) COOH, and R3 ) OH. Syringic acid: R1 ) OCH3, R2 ) COOH, and R3 ) OCH3. Tyrosol: R1 ) H, R2 ) CH2CH2OH, and R3 ) H.

2. Experimental Section All compounds used in this work (polyphenols and intermediates as shown in Table 1) were obtained from Sigma-Aldrich (Spain) and used as received. Hydrogen peroxide was from Panreac (Spain). Ozone was generated in a laboratory ozone generator from pure oxygen. Commercial P100 Hydraffin activated carbon was supplied by Lurgi (Donan Carbon GmbH&Co, Germany). The AC was dried to remove moisture and kept in a water-free atmosphere before use. The main characteristics of the AC were as follows: 967 m2 g-1 surface area determined by BET method with a Quantachrome autosorb 1 automated gas adsorption system and pHpzc of 10.4, calculated from the method of Noh and Schwarz.7 Other characteristics of this activated carbon are presented in a previous work.1 Aqueous solutions of polyphenols (both separately and in mixtures) were prepared in buffered ultrapure (Millipore Q Millipore system) water. A mixture of NaH2PO4 and Na2HPO4 was used to fix the pH at 5 and ionic strength at 0.1 M. It should be noted that pH around 5 is the usual one in distillery or even olive oil wastewaters.9 Experiments were carried out in the 750 cm3 semibatch reactor depicted in detail in a previous work.1 The reactor was submerged in a thermostatic bath to keep the temperature constant at 25 ( 0.1 °C and was supplied with mechanical agitation operating at 200 rpm. The ozone-oxygen mixture was fed to the reactor through a porous plate situated at the reactor bottom. Five hundred cubic centimeter aqueous solution containing one polyphenol or mixtures of the three was charged. Also, in some experiments a given amount of activated carbon (5 g of 1-1.6 mm particle size, in most of the cases) was also charged. Samples were withdrawn from the reactor and analyzed for the parent compound and intermediate content, organic carbon and hydrogen peroxide concentration evolution, and ozone gas and dissolved ozone concentrations. To simulate polyphenol wastewater concentrations, each polyphenol had a

10.1021/ie0708881 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007

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Table 1. Intermediates Detected in the Ozonation of Polyphenols Studieda ozonation of

muconic acid

tartaric acid

gallic acid syringic acid tyrosol

X X

X X

a

malic acid

X

malonic acid

pyruvic acid

ketomalonic acid

oxalic acid

acetic acid

formic acid

X

X X

X X X

X X X

X X

X

All intermediates were detected in the ozonation of the polyphenol mixture regardless of the presence and absence of activated carbons.

10-3 M concentration in the starting aqueous solution both in the experiments with individual polyphenols or in their mixtures. Concentrations of polyphenol compounds and intermediates were determined from HPLC (Hewlett-Packard Series 1100) with a 25 cm long, 4.6 mm i.d. Hydro-RP 80A Phenomenex Synergi column. Mobile phase was varied depending on the chemical analyzed. Thus, for polyphenols in synthetic wastewater a 25/75 v/v acetonitrile-water (with 0.1% phosphoric acid) mixture was used and detection was made at 270 nm. In the ozonations of individual polyphenols, the mixture was 35/ 65 v/v acetonitrile/water with 0.5 mL min-1 flow rate and 268, 265, and 273 nm for detection of gallic and syringic acids and tyrosol, respectively. For intermediates, except muconic acid (determined as the polyphenol mixture), water (with 0.1% phosphoric acid) was used as mobile phase with 210 nm as detection wavelength. In this case, after 12 min of injection the mobile phase was slowly changed to 35/65 v/v acetonitrile/water to elute the polyphenols if they still remained in the sample. Hydrogen peroxide concentration was determined by applying the Eisemberg method10 based on the yellow color of pertitanic acid formed in the reaction between hydrogen peroxide and Ti4+. Dissolved ozone was measured by following the method proposed by Bader and Hoigne´11 based on the decoloration of the 5,5,7-indigotrisulphonate. Ozone concentration in the gas phase was monitored by means of an Anseros Ozomat ozone analyzer. The analysis was based on the absorbance at 254 nm. Total organic carbon was obtained by an I/O Analytic 1010 IR carbon analyzer after persulfate oxidation of the aqueous samples. 3. Results and Discussion 3.1. Ozonation of Individual Polyphenols. First, a series of ozone reactions of aqueous solutions of individual polyphenols with and without activated carbon was carried out. Figures 2-6 show some of the results obtained regarding the evolution with time of parent polyphenol compounds, intermediates, ozone, and hydrogen peroxide concentrations, and total organic carbon. Figures 2 and 3give the time profiles of the remaining concentration of polyphenols (separately treated with ozone) and total organic carbon, respectively, in the presence and absence of activated carbon. As can be seen, tyrosol was the most refractory phenol to ozonation. After 5 min of ozonation about 55% conversion of gallic and syringic acids are noticed in the ozone alone process while only approximately 30% removal of tyrosol is achieved at the same conditions. These percentages are slightly improved when activated carbon is present: in the case of gallic and syringic acids, about 65% conversion, in the case of tyrosol, a 52% removal was noticed. These results are likely due to the direct ozone reactions6 which are very fast for phenols, especially at the concentration applied. The similarity of results in gallic and syringic acid ozonations are likely due to their similar structures with two nucleophilic points in carbons placed at the ortho positions with respect to the carboxylic group (see Figure 1). For tyrosol, however, the presence of the ethylalcohol substituent makes the molecule

more refractory to ozone direct attack. As shown in a previous paper,1 during the first minutes of gallic acid ozonation there was no dissolved ozone which confirms the fast reactions in water. Similar results were obtained with the other two phenols, especially in the case of syringic acid (not shown). For the case of tyrosol, Figure 4 shows the evolution of the dissolved ozone concentration with time corresponding to both ozone alone and ozone-activated carbon reactions. In this case, there is some dissolved ozone at the beginning of reaction, which is the sight of a lower reactivity with the polyphenol compound. Then, with the exception of tyrosol, differences between both types of ozonations (with and without activated carbon) can be due to slight adsorption contribution. In a previous work,1 adsorption contribution to the removal of gallic acid in the presence of ozone was found negligible versus its fast reaction with ozone. Notice that in the absence of ozone, adsorption allows polyphenol removals of about 36% in 10 min. However, when any polyphenol is simultaneously treated with ozone and activated carbon, a different scenario is present. The absence or very low dissolved ozone presence undoubtedly indicates that fast gas-liquid reactions (the ozonepolyphenol reactions) develop. In this kind of kinetic regime, ozone is consumed in the proximity of the gas-liquid interface, that is, in the so-called liquid film. As a consequence, no ozone reaches the bulk water or the surface of activated carbon, if present. Then, this eliminates the presence of free radical reactions coming from ozone decomposition. Also, the polyphenol compound is mainly consumed with ozone and this diminishes the polyphenol concentration driving force to be adsorbed on the activated carbon. The result is that differences in the consumption of polyphenol with ozone in the presence and absence of activated carbon are very small. For example, in the case of the ozonation of gallic acid after 10 min of ozonation, about 97 and 95% gallic acid removal was observed. This means that only about 2% removal can be due to adsorption. Similar results present the ozonation of syringic acid. In the case of tyrosol, it is also possible that some free radical reactions already develop due to the lower reactivity of ozone with tyrosol and possible ozone decomposition in free radicals by reacting on the carbon surface or with the hydrogen peroxide formed (see Figure 4). Following back to Figure 2, it can also be observed that gallic and syringic acids are nearly completely removed in about 20 min while 12 and 5% tyrosol still remains in solution in the ozone alone and ozone-activated carbon runs. In the case of tyrosol, differences between both ozone processes can be seen more clearly from the results shown in Figure 4 regarding the evolution of hydrogen peroxide concentration with time. It is seen that, in the activated carbon free process, hydrogen peroxide concentration increases during approximately 2 h of reaction and then seems to slowly diminish. Similar results were observed for the case of syringic acid (not shown) but not in gallic acid ozonation. In this latter case1 hydrogen peroxide concentration decreases more quickly and no oxidant is present at 2 h reaction time. This difference, however, can likely be attributed to the buffer ionic strength applied that was unable to keep constant the pH of water. Thus, during syringic acid

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Figure 2. Time evolution of concentrations of gallic acid, syringic acid, and tyrosol during their individual ozonations in water in the presence and absence of activated carbon. Conditions: 25 °C; pH 5; ionic strength, 0.1 M; gas flow rate, 30 L h-1; agitation speed, 200 rpm; inlet ozone gas concentration, 40 mg L-1; initial polyphenol concentrations, 3 × 10-3 M; activated carbon concentration, 10 g L-1; particle size, 1.3 mm (average). (Numbers in figure: 1, O3-gallic acid; 2, AC/O3-gallic acid; 3, O3syringic acid; 4, AC/O3-syringic acid; 5, O3-tyrosol; 6, AC/O3-tyrosol.

Figure 3. Time evolution of total organic carbon during the individual ozonations of gallic acid, syringic acid, and tyrosol in water in the presence and absence of activated carbon. Conditions and numbers as in Figure 2.

Figure 4. Time evolution of concentrations of ozone and hydrogen peroxide formed during the ozonation of tyrosol in water in the presence and absence of activated carbon. Conditions as in Figure 2. (Black symbols correspond to AC/ozonation): ozone (O, b); hydrogen peroxide (0, 9).

and tyrosol ozonations pH decreased from 5 to 3.5 as a consequence of the high concentration of carboxylic acids formed. As is known, the lower the pH of water, the lower the reactivity of hydrogen peroxide with ozone.12 In the activated carbon run, however, hydrogen peroxide concentration reaches a maximum value, after about 30 min of reaction, and then clearly decreases with time up to total disappearance. These trends have also been observed in the case of gallic and syringic

acids, although with gallic acid the decrease of hydrogen peroxide concentration after reaching the maximum concentration is faster as shown in a previous work.1 Formation of hydrogen peroxide is due to two routes: direct ozone attack to carbon double bonds of phenols and likely unsaturated carboxylic acids (i.e., dipolar addition of ozone, Criegge mechanism6) and ozone decomposition on the carbon surface,13 although the former way is more plausible during the first minutes of ozonation. The improvement of hydrogen peroxide decomposition can be associated with the removal of tyrosol and intermediates (see Figures 3 and 6), that is, hydrogen peroxide reactions on the carbon surface and with ozone to yield free radicals that are consumed by polyphenol and intermediates. Regarding the evolution of TOC, Figure 3 presents the changes observed during the individual ozonation experiments in the presence and absence of activated carbon. Here, however, some differences can be deduced for reactivity. Notice that reactivity refers to both parent compound and intermediates. Thus, from Figure 3 it is observed that gallic acid ozonation reaches about 80% TOC removal (without activated carbon) and nearly complete mineralization (with activated carbon) after 3 h of treatment. The second most reactive process is the ozonation of tyrosol that after 3 h of treatment allows nearly 50% mineralization (without activated carbon) and, as in the case of gallic acid, nearly complete removal of TOC in the presence of activated carbon. Syringic acid ozonation seems to be the most refractory process with mineralization of 57 and 12% with and without activated carbon, respectively. Differences in TOC removal are likely due to the nature of intermediates formed and reactivity of the parent compound. Table 1 presents the intermediates detected during the ozonations of polyphenols treated. As can be seen, for gallic acid only two intermediates were detected: oxalic and ketomalonic acids.1 In this case, ozone and free radical reactions are very fast and only two intermediates were detected. In the case of the other two polyphenol ozonations six and nine intermediates were identified. In syringic acid ozonation, intermediates were as follows: muconic, tartaric, pyruvic, acetic, oxalic, and ketomalonic acids, while in tyrosol ozonation, in addition to these six intermediates, formation of malic, malonic, and formic acids was also observed. The high number of intermediates detected during the ozonations of syringic and tyrosol explain that these processes were more refractory than that of gallic acid. Finally, Figures 5 and 6 show the time evolution of intermediate concentrations during the ozonation of syringic acid and tyrosol with and without activated carbon (for the case of gallic acid, see previous work1). As observed from these figures, concentrations of intermediates are always lower during the activated carbon ozonation process and regardless of the nature of initial polyphenol, with only oxalic acid remaining in solution after 180 min reaction. The trends shown in the figures suggest a complex series-parallel reacting system with ozonated compounds yielding new byproducts. Again, contribution of adsorption alone is very low when ozone is present. In the absence of ozone, about 15% oxalic acid removal was noticed due to adsorption after 120 min when the starting oxalic acid concentration was the maximum reached in the ozone-activated carbon process. However, in the ozonation of gallic acid in the presence of activated carbon, the maximum concentration of oxalic acid is formed at about 40 min reaction and after 120 min oxalic acid has nearly completely disappeared. This means that, in the ozone-activated carbon process, oxalic acid at the maximum concentration observed only has 80 min to nearly disappear from water; that is, in only 80 min, contribution of adsorption should

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Figure 5. (a) Time evolution of concentrations of intermediates during the ozonation of syringic acid in water in the absence of activated carbon. Conditions as in Figure 2. Tartaric acid, O; pyruvic acid, 3; ketomalonic acid, 4; oxalic acid, 0; acetic acid, ]. (b) Time evolution of concentrations of intermediates during the ozonation of syringic acid in water in the presence of activated carbon. Conditions as in Figure 2. Tartaric acid, b; pyruvic acid, θ; ketomalonic acid, 2; oxalic acid, 9; acetic acid, [.

be much lower than 15%, an adsorption percentage removal reached, in the absence of ozone, and after 120 min. Similar results have been observed with other intermediates. Then, it is concluded that activated carbon adsorption contribution of intermediates is also low when ozone is applied. In Figure 7, a plot of the theoretical remaining TOC determined from HPLC analysis of intermediates and parent compound against the corresponding measured TOC, in dimensionless form, via IR analysis corresponding to different experiments is presented. It is seen that, with some exception, regardless of the nature of polyphenol and presence of activated carbon, points are situated around the diagonal line, thus suggesting there were no more intermediates in solution but those detected in this work. 3.2. Ozonation of Mixtures of Polyphenols. Second, a series of ozonations of polyphenol mixtures were also carried out in the presence and absence of activated carbon. Figures 8-12 show some of the results obtained. First, in Figure 8the evolution of initial polyphenol concentrations with time is shown. As is deduced, the order of reactivity of these polyphenols with ozone is similar to that observed in Figure 2 for the case of individual ozonations: gallic acid > syringic acid > tyrosol. Again, the presence of activated carbon reduces the content of polyphenols compared to the activated carbon free ozonation. After 20 min of reaction, both gallic and syringic acids are nearly completely removed (about 95% removal) and only tyrosol remains in water (70 and 83% removals in the absence and presence of activated carbon,

Figure 6. (a) Time evolution of concentrations of intermediates during the ozonation of tyrosol in water in the absence of activated carbon. Conditions as in Figure 2. Tartaric acid, O; ketomalonic acid, 4; oxalic acid, 0; acetic acid, ]; formic acid, 3; malic acid, +; malonic acid, /. (b) Time evolution of concentrations of intermediates during the ozonation of tyrosol in water in the presence of activated carbon. Conditions as in Figure 2. Tartaric acid, b; ketomalonic acid, 2; oxalic acid, 9; acetic acid, [; formic acid, θ; malic acid, ×; malonic acid, 0.

Figure 7. Theoretical TOC/TOC0 determined from initial polyphenol and intermediate HPLC concentrations against TOC/TOC0 from IR analyzer. Conditions as in Figure 2. (Black symbols correspond to AC/ozonation.) Gallic acid (4, 2); syringic acid (], [); tyrosol (0, 9).

respectively). If TOC is considered, as shown in Figure 9, the presence of activated carbon results in significant decreases of this parameter. Thus, after 180 min, 52 and 88% removals are achieved in the absence and presence of activated carbon, respectively. 3.3. Ozone Consumption. Ozone consumption is a fundamental parameter to establish the suitability of the ozonation process. In this work, ozone consumption has been calculated in both individual and mixture ozonations of polyphenols as two forms: as the mass ratio between ozone consumed and TOC

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Figure 8. Time evolution of concentrations of gallic acid, syringic acid, and tyrosol during the ozonation of an aqueous mixture of them in the presence and absence of activated carbon. Conditions as in Figure 2 with initial polyphenol concentration of 10-3 M for each polyphenol. Numbers 1, 3, and 5 in figure correspond to ozonation in the absence of activated carbon. 1 and 2 for gallic acid, 3 and 4 for syringic acid, and 5 and 6 for tyrosol.

Figure 10. Time evolution of TOC/TOC0 during the ozonation of the polyphenol mixture in the presence and absence of activated carbon at pH 2.5 and 5. Conditions as in Figure 2. (Black symbols correspond to AC/ ozonation.) pH 2.5 (0, 9); pH 5 (O, b).

Figure 9. Time evolution of TOC/TOC0 during the ozonation of an aqueous mixture of polyphenols in the presence and absence of activated carbon. Conditions as in Figure 2. Black bars correspond to ozonation in the presence of activated carbon.

removed and as the mass ratio between ozone fed and TOC removed. The first way allows us to establish the stoichiometry of the process while the second one gives a better idea of the consumption of ozone. Table 2 presents the results obtained. As seen from Table 2, in all cases but one and regardless of ozonation time and polyphenol nature, the presence of activated carbon allows great reductions in the consumption of ozone. The exception is the case of tyrosol that, after 180 min of reaction, ozone consumptions are 8.0 and 8.7 g/g TOC for the ozonations in the absence and presence of activated carbon, respectively. It should be highlighted that at long reaction times the figures obtained are not relevant since remaining TOC is very low, especially when activated carbon is present. For the same case, and shorter reaction times, the presence of activated carbon is beneficial in reducing the content of dissolved carbon (TOC) in wastewater. In fact, for the case of tyrosol at lower times the main reductions of ozone consumption are noticed (see for example after 30 min reaction with ozone consumptions of 14.6 and 3.6 g/g TOC for the ozonation in the absence and presence of activated carbon, respectively). If the ozone fed to TOC consumed ratio is considered, the results are more favorable to the application of activated carbon. In the case of the polyphenol mixture ozonation the beneficial effect of activated carbon is noticed at any reaction time. It should be reminded that two main mechanisms constitute the ozone processes studied. The first one is the removal of initial polyphenols which, in accordance with their high ozone reactivity and low influence of activated carbon presence, is likely due to fast direct ozone reactions (in fact, none or very

Figure 11. Time evolution of hydrogen peroxide concentration during the ozonation of the polyphenol mixture in the presence and absence of activated carbon at pH 2.5 and 5. Conditions and symbols as in Figure 10.

Figure 12. Time evolution of ozone concentration during the ozonation of the polyphenol mixture in the presence and absence of activated carbon at pH 2.5 and 5. Conditions and symbols as in Figure 10.

little dissolved ozone was noticed during this initial period of reaction, thus confirming fast ozone reactions). Ozone reactions break the aromatic rings and double bonds of first unsaturated carboxylic acids formed and yield hydrogen peroxide6 and saturated carboxylic acids with reductions of pH in cases of weak buffer ionic strength. Thus, oxidation of saturated car-

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Table 2. Ozone Consumption (and Ozone Fed) per TOC Removed (mg/mg) during the Water Ozonation of Polyphenols Studieda reaction time 30 min galic acid syringic acid tyrosol mixture a

reaction time 60 min

reaction time 120 min

reaction time 180 min

O3

O3/AC

O3

O3/AC

O3

O3/AC

O3

O3/AC

4.1 (12.2) 5.5 (11.7) 14.6 (32.5) 4.1 (5.3)

3.8 (8.2) 3.5 (6.8) 3.6 (6.3) 2.0 (2.9)

5.5 (19.5) 6.1 (19.1) 9.6 (31.3) 4.5 (8.3)

4.2 (11.7) 4.3 (11.3) 4.8 (10.7) 2.5 (4.7)

5.7 (28.3) 8.3 (36.4) 8.6 (40.9) 4.8 (13.7)

- (20.3) 5.6 (18.8) 6.4 (16.6) 3.4 (8.0)

6.0 (38.0) 8.9 (50.5) 8.1 (51.5) 4.6 (17.7)

- (30.0) 6.4 (25.1) 8.7 (24.3) 4.0 (10.2)

Figures in parentheses refer to the amount of ozone fed per consumed TOC.

boxylic acids mainly due to free radical reactions is the second mechanism of the ozone process, which is particularly important when activated carbon is present (see Figure 9). In the absence of activated carbon, the only way for free radical formation is the reaction between ozone and hydrogen peroxide12 but this reaction is favored with pH increase. Since pH diminishes because of carboxylic acids formation, free radical oxidation is rather inhibited without activated carbons. In the presence of activated carbon, however, free radical formation is also likely due to decomposition reactions of hydrogen peroxide and ozone on the carbon surface,1 which explains the higher mineralization achieved. Then, the results suggest the suitability of the process for real polyphenol wastewaters that will be the subject of future works. 3.4. pH Effect. As stated above, in ozonation processes pH is a variable of paramount importance since its value allows the appearance of free radical reactions.14 To check the pH effect in the case of the ozonation of a polyphenol mixture, some experiments were carried out at pH 2.5. The results are shown in Figures 10-12. In Figure 10 the time evolution of TOC in ozonation experiments carried out in the presence and absence of activated carbon at two pH values (2.5 and 5) is shown. As can be seen, in the absence of activated carbon (single ozonations) the decrease of pH leads to a decrease of TOC removal which is in accordance with what is expected: Increase of pH leads to an increase of ozone decomposition mainly due to its reaction with the ionic form of hydrogen peroxide that is formed in first direct ozone reactions on the aromatic ring (cyclopolar addition reactions):6

PPhOH + O3 f Intermediates + H2O2

(1)

H2O2 a HO2- + H+

(2)

O3 + HO2- f HO2• + O3-• f ... f HO•

(3)

Intermediates + HO• f ... f CO2 + H2O

(4)

Thus, the higher the pH, the higher the formation of free radicals through reaction (3) and, hence, the higher the removal rates of organics in water through reaction (4). Notice that pK of hydrogen peroxide dissociation equilibrium in water is 11.3.6 In the presence of activated carbon a different situation is presented. Now, the decrease of pH leads to increases of TOC removal as observed from Figure 10. In this system, hydrogen peroxide is not only formed from direct ozone reactions but also from ozone decomposition on the carbon surface. Then, formation of free radicals comes from both hydrogen peroxide decomposition and, likely, surface reaction with ozone.1 Figures 11 and 12 present the time concentration profiles of ozone and hydrogen peroxide corresponding to experiments of Figure 10.

In the absence of activated carbon, as is seen from Figures 11 and 12, the decrease of pH leads to an increase of both ozone and hydrogen peroxide concentrations. This is the consequence of the low ozone decomposition at acid pH and the lack of reaction between these two oxidants when hydrogen peroxide is in the molecular form.12 The result of this is the accumulation of both oxidants in water at pH 2.5. Notice that the concentration of ozone is negligible during approximately the first 50 min (especially at pH 5). During this initial period ozone is mainly consumed by fast or moderate reactions with polyphenols and first unsaturated intermediates. (This is the period of the first mechanism of ozonation.) Then, subsequent intermediates are less reactive toward ozone and accumulation of the oxidant in water starts to be noticed. In the presence of activated carbon, the concentration of ozone follows a similar trend with pH; that is, the decrease of pH leads to an increase (although slow) of ozone concentration. Now, during the first 150 min, and regardless of pH, the ozone concentration is rather low or negligible. Since polyphenols have already been removed at this time, the low ozone concentration is undoubtedly due to decomposition of ozone on the carbon surface. In this reaction, hydrogen peroxide is formed but the presence of activated carbon promotes its decomposition in free radicals. This would confirm the results reached during the ozone-activated carbon of gallic acid.1 If the concentration of hydrogen peroxide is followed with time, a first increase of concentration to reach a maximum value and a further decrease to become very low at long reaction times, especially at pH 2.5, are seen. Also, concentrations of hydrogen peroxide at pH 5 are always higher than those at pH 2.5. The time for which hydrogen peroxide concentration decreases approximately coincides with the reaction time when polyphenols have been nearly removed. Before this reaction time direct ozone reactions are likely the main responsible way of hydrogen peroxide formation. After the first 50 min, polyphenols are nearly removed and hydrogen peroxide decomposition starts to be more important than its formation (mainly through ozone decomposition on the carbon surface). The reactions of ozone and hydrogen peroxide seem to develop on the surface of the carbon; if not, a higher hydrogen peroxide accumulation in water should be noticed. These surface reactions lead to hydroxyl radicals that react with saturated intermediates (carboxylic acids) to increase mineralization. Notice that adsorption of intermediate acids, as starting products, on the activated carbon (not shown), in the absence of ozone, was very low (approximately a 15% removal was observed in 180 min) and independent of pH between 2.5 and 5. This also corroborates the importance of surface reactions of ozone and hydrogen peroxide as the source of free radicals. Then, in addition to reactions (1)-(4), the following reactions are proposed for mineralization or removal of saturated intermediates in the ozonation of polyphenols when activated carbon is present:

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O3 + AC a AC-O + O2 f ... f H2O2 or HO2- (5) H2O2 + AC h AC-H2O2 f ... f HO•

(6)

4. Conclusions The main conclusions reached in this work are the following: (a) The presence of activated carbon during the ozonation of water mixtures of polyphenols (specifically, gallic and syringic acids and tyrosol) mainly allow significant reductions of TOC. (b) Different carboxylic acids are detected as intermediates together with hydrogen peroxide. Evolution with time of the concentration of carboxylic acid intermediates and hydrogen peroxide is similar. They increase during the first 30-50 min while the parent compound is being removed, reach a maximum value, and decrease once the starting polyphenol has been nearly completely eliminated. (c) In the individual activated carbon ozonation of polyphenols, nearly complete mineralization is achieved after 180 min for gallic and syringic acids. Ozonation of the mixture allows, for the same reaction time, about 85% mineralization with similar initial concentration of polyphenols (10-3 M for each polyphenol). (d) Removal of parent polyphenols is mainly due to direct ozone reactions that yield hydrogen peroxide, while intermediates are eliminated via hydroxyl free radicals coming from ozone-hydrogen peroxide-activated carbon reactions with low contribution of adsorption. (e) Consumption of ozone (or amount of ozone fed) per gram of TOC consumed is, as a rule, lower when activated carbon is present. (f) The higher TOC removal at pH 2.5 compared to that pH 5 seems to confirm the previous statement. In fact, for pH 2.5 the results obtained could not be explained if hydroxyl radicals are produced from the direct reaction in solution between ozone and the ionic form of hydrogen peroxide (reaction (3)). However, at acid pH, hydrogen peroxide decomposition on the carbon surface, likely with the participation of ozone, is favored to yield free radicals. Given the potential benefits, this process presents treatment of polyphenol wastewater to increase the efficiency of ozonation; in a subsequent paper a kinetic study of these reactions will be presented. Also, attempts are also in progress to elucidate questions related to the mechanism of reactions such as where

hydroxyl free radicals react with intermediates (in solution or on the surface of activated carbon). Acknowledgment This work has been supported by the CICYT of Spain and the European Region Development Funds of the European Commission (Projects PPQ2003/00554 and PPQ2006/04745). Mrs. Gira´ldez thanks the Spanish Ministry of Science and Education for a FPU grant. Literature Cited (1) Beltra´n, F. J.; Garcı´a-Araya, J. F.; Gira´ldez, I. Gallic acid water ozonation using activated carbon. Appl. Catal., B 2006, 63, 249. (2) Lo´pez-Mateos, F.; Ovelleiro, J. L. Depuracio´n de aguas residuales de las destilerı´as de alcohol vı´nico. Ing. Quim. 1978, 109, 167. (3) Beltra´n, F. J.; Garcı´a-Araya, J. F.; Gira´ldez, I.; Masa, F. J. Kinetics of activated carbon promoted ozonation of succinic acid in water. Ind. Eng. Chem. Res. 2006, 45, 3015. (4) Valdes, H.; Zaror, C. A. Heterogeneous and homogeneous catalytic ozonation of benzothiazole promoted by activated carbon: Kinetic approach. Chemosphere 2006, 65, 1131. (5) Rivera-Utrilla, J.; Me´ndez-Dı´az, J.; Sa´nchez-Polo, M.; Ferro-Garcı´a, M. A. Removal of the surfactant sodium dodecylbenzenesulphonate from water by simultaneous use of ozone and powdered activated carbon: Comparison with systems based on O3 and O3/H2O2. Water Res. 2006, 40, 1717. (6) Beltra´n, F. J. Ozone reaction kinetics for water and wastewater systems; Lewis Publishers: Boca Raton, FL, 2003. (7) Noh, J. S.; Schwarz, J. A. Effect of HNO3 treatment on the surface acidity of activated carbons. Carbon 1990, 28, 675. (8) Alvarez, P. M.; Beltra´n, F. J.; Rodrı´guez, E. M. Integration of ozonation and an anaerobic sequencing batch reactor (AnSBR) for the treatment of cherry stillage. Biotechnol. Prog. 2005, 21, 1543. (9) Rivas, F. J.; Gimeno, O.; Portela, J. R.; Martı´nez de la Ossa, E.; Beltra´n, F. J. Supercritical water oxidation of olive oil mill wastewater. Ind. Eng. Chem. Res. 2001, 40, 3670. (10) Eisenberg, G. M. Colorimetric determination of hydrogen peroxide. Ind. Eng. Chem. Anal. Ed. 1943, 15, 327. (11) Bader, H.; Hoigne´, J. Detemination of ozone in water by the indigo method. Water Res. 1981, 15, 449. (12) Staehelin, S.; Hoigne´, J. Decomposition of ozone in water: Rate of initiation by hydroxyde ions and hydrogen peroxide. EnViron. Sci. Technol. 1982, 16, 666. (13) Alvarez, P. M.; Garcı´a-Araya, J. F.; Beltra´n, F. J.; Gira´ldez, I.; Jaramillo, J.; Go´mez-Serrano, V. The influence of various factors on aqueous ozone decomposition by granular activated carbons and development of a mechanistic approach. Carbon 2006, 44, 3102. (14) Staehelin, J.; Hoigne´, J. Decomposition of Ozone in Water the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions. EnViron. Sci. Technol. 1985, 19, 1206.

ReceiVed for reView June 28, 2007 ReVised manuscript receiVed September 7, 2007 Accepted September 7, 2007 IE0708881