An Attempt to Model the Kinetics of the Ozonation of Simazine in Water

Ind. Eng. Chem. Res. 2002, 41, 1723-1732

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An Attempt to Model the Kinetics of the Ozonation of Simazine in Water Fernando J. Beltra´ n,* Juan Fernando Garcıá-Araya, Vıćtor Navarrete, and Francisco J. Rivas Departamento de Ingenierıá Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain

A kinetic model for the ozonation of simazine in water in a laboratory bubble column is proposed. The model considers nonideal flow behavior, mass transfer, and chemical reactions; it assumes that ozone reactions develop in the slow kinetic regime of absorption; and it has been tested at different pH’s and in the presence of hydrogen peroxide. The model is based on a mechanism of reactions that considers both direct and free-radical means of ozone action on simazine and some of the intermediates identified or detected. A good fit between the experimental and predicted concentrations of simazine, intermediates, and ozone in the absence of hydrogen peroxide concentrations higher than 3.4 × 10-5 M is generally observed. For higher hydrogen peroxide concentrations, the model fails, especially, to give accurate values of the concentration of ozone in the gas leaving the column because the kinetic regime of ozone absorption changes to be moderate, a situation not accounted for by the ozone absorption rate equation applied. Comments about correcting this latter aspect are made. Introduction Because of the increasing concern for environmental issues, during the past several years, a great deal of research has dealt with treatment methods for the removal of micropollutants from water.1-3 Undoubtedly, herbicides constitute one of the pollutant families presenting water environmental problems. Herbicides have been detected in water at manufacturing sites and at points of application where they are employed, in many cases, in an extensive way for agricultural purposes. s-Triazine compounds are probably the most-used herbicides worldwide.4 In particular, simazine [2-chloro4,6-bis(ethylamino)-1,3,5-s-triazine] or CEET, is widely used for the removal of broadleaf and grassy weeds in corn and in olive tree areas that abound in Mediterranean countries such as Spain, Italy, and Greece. In Extremadura, an agricultural region in the Southwest of Spain, concentrations of simazine in water were very recently found to reach part per million levels, well above the 0.1 µg L-1 maximum contaminant level (MCL) established by the European Commission.5 Among treatment methods, chemical oxidation by ozone seems to be a promising approach to the removal of simazine from water, as has been reported in previous works.6,7 Ozone is a powerful oxidant that is able to react with the organic matter present in water through different mechanisms involving direct molecular reactions and hydroxyl radical oxidation.8 In the case of simazine, the rate constants of the reactions with ozone and hydroxyl radicals have already been reported.7 However, the following step, a kinetic model establishing the importance of ozonation in the removal of simazine from water, has not yet been investigated. Kinetic models of ozonation processes have been aplied to different pollutants such as polynuclear aro* Corresponding author: Fernando J. Beltrań, Departamento de Ingenierıá Quı´mica y Energe´tica, Universidad de Extremadura, 06071 Badajoz, Spain. Telephone number: 34924-289387. Fax number: 34-924-271304. E-mail address: [email protected].

matic hydrocarbons (PAHs)3 and volatile organochlorine compounds (VOCs).9 In most of these studies, however, reactions were carried out in assumed well-mixed tanks running semicontinuously by feeding ozone into a solution of water containing the pollutant. In practical applications, however, both the water and ozonated gas phases are fed continuously into reactors that usually present deviations from ideal flow (perfect mixing, plug flow). Because of the importance of simazine as an environmental aquatic pollutant and to gain some knowledge for a further scaled-up ozonation study, this work was undertaken. The main aims were to test the kinetic model for the ozonation of simazine in water in a laboratory bubble column. Experimental Section Reactions were carried out in a glass bubble column (with dimensions 30-cm height, 4-cm internal diameter) provided with a diffuser plate at its bottom for feeding the ozonated gas. A simazine aqueous solution buffered with phosphoric acid and sodium hydroxide was continuously pumped from a reservoir to the top of the column, while the ozonated gas circulated countercurrently from the bottom. Simazine was obtained from Sugelabor and used as received. Aqueous solutions of the herbicide were obtained from a saturated solution prepared with ultrapure water (Milli-Q system). For this stock solution, an excess amount of simazine was added; allowed to dissolve overnight; and finally, filtered through 45-µm Millipore membranes. Ozone was obtained from a laboratory ozonator from pure oxygen at a concentration between 5 and 15 mg L-1. Methylene blue (Merck) was used for experiments of nonideal flow with the water phase. The concentrations of ozone and methylene blue in water were determined by colorimetric methods, the former with the Indigo method,10 by measuring the absorbance at 600 nm. Ozone in the gas phase was

10.1021/ie010681m CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002

analyzed with a GM19 Anseros analyzer, which is based on measurements of the absorbance of the ozonated gas at 254 nm. The byproducts of ozonation [CEAT, 2-chloro4-ethylamino-6-amino-1,3,5-s-triazine, and CAAT, 2-chloro-4,6-bis(amino)-1,3,5-s-triazine] were detected in some cases, and their concentrations, like that of simazine, were determined by HPLC using a 15-cm-long, 0.4-cmi.d. C18 tracer column with a gradient flow of acetonitrile and water having the following characteristics: 100% water for the first 3 min, followed by a linear gradient flow to reach 35% acetonitrile at 25 min. A 1100 series Hewlett-Packard HPLC was used, with detection at 220 nm. Results and Discussion Although gas flow through the reactor presented some sort of dispersion, as indicated by tracer analysis (shown below), for comparative reasons, kinetic modeling of simazine ozonation was first studied by considering three possible cases with respect to the flow pattern of the gas phase: (a) well-mixed flow, (b) plug flow, and (c) axial-dispersion flow. On the other hand, the water phase was considered well-mixed, as experiments with nonideal flow confirmed (see below). Flow Pattern Determination. Experiments with nonideal flow, in triplicate, were first carried out for both the water and gas phases simultaneously circulating through the bubble column. For the water phase, methylene blue was used as the tracer with positive step input experiments.11 The results from these experiments corresponding to 20 L h-1 for the gas phase and 25 mL min-1 for the water phase were as follows: hydraulic residence time ) 12 min, mean residence time ) 15.8 min, variance of distribution ) 251 min2. From these values, the number of well-mixed agitated tanks that simulate the water flow pattern was found to be approximately 1. Hence, the water phase was considered to be well-mixed. For the gas phase, ozone was used as the tracer. Also, in this case, positive step input experiments were carried out. Because a small fraction of ozone was transferred into the water, the ozone concentrations in both the gas and the water were determined in these experiments. Also, the water was kept at pH 2 to diminish, as much as possible, the effects of ozone decomposition. It was found, however, that the concentration of dissolved ozone was less than 10% of that in the gas phase. This amount had no effect on the determination of the residence time distribution function. The average results of the gas-phase tracer experiments at the conditions above indicated were as follows: mean residence time ) 0.57 min, hydraulic residence time ) 1 min, variance of distribution ) 0.19 min2. According to these results, the gas-phase flow was found to behave as 1.8 well-mixed equally sized tanks in series. This means that the gas phase presented some dispersion while flowing through the column. The calculated dispersion number was found to be 0.106.11 However, because the small fraction of dissolved ozone during these experiments could lead to some uncertainty in the calculated dispersion number, three different flow patterns were considered for the gas phase in the kinetic model: (a) well-mixed flow, (b) plug flow, and (c) axial-dispersion flow with a value of 0.106 for the dispersion number. Kinetic Modeling. One of the factors that noticeably influence the performance of an ozonation kinetic model

is the role of intermediates. This is because intermediate compounds usually consume ozone and hydroxyl free radicals and can act as promoters or scavenging agents of these radicals, thus propagating or ending the chain mechanism.3,8 In the ozonation of simazine, depending on the experimental conditions, a series of intermediates was observed to form. On the whole, it was noted that the more intense the experimental conditions (increase of pH or presence of hydrogen peroxide), the higher the number of intermediates observed in the HPLC chromatograms. With the analytical techniques available for this work, however, only CEAT and CAAT were identified, and their concentrations quantified only in some cases. Other possible intermediates such as CDET (2chloro-4-acetamida-6-ethylamino-s-triazine), OAAT (4,6diamine-2-hydroxy-s-triazine), and OOOT (2,4,6-trihydroxy-s-triazine or cyanuric acid), reported in some other works where the hydroxyl radicals were used as the main oxidants of s-triazine herbicides in water,6,12-15 could not be identified because of the lack of available standards (in the cases of CDET and OAAT) or detection (in the case of OOOT). The literature includes different reports on the mechanism of advanced oxidation of s-triazine herbicides in water with atrazine as the most-studied compound.6,12-17 The formation of hydroxyl radicals, as the main oxidizing species, is the main common characteristic of these processes, although, depending on the oxidation system applied, other possible means of elimination can develop. Among them, direct ozonation (very negligible at pH > 5), direct photolysis, and oxidation with oxo-iron complexes have to be considered when using ozonation,15,16 UV radiation combined with ozone,6,17 semiconductor catalysts,13 or Fenton’s reagent.14 Two types of hydroxyl radical attack are considered: dealkylation and dechlorination. The former process consists of the abstraction of a hydrogen atom from the secondary carbon of the ethylamino side chain, followed by either reaction with oxygen to eventually yield a hydroperoxide compound that, after rearrangement, gives an amide or introduction of an oxygen atom from one hydroxyl radical to yield an unstable carbinol compound that gives an aldehyde and a N-dealkylated amine.18 The formation of amides and carbinolamine was reported by Ne´lieu et al.12 in the ozonation of atrazine under weak ozonation conditions. It seems that the two compounds are simultaneously formed in the first stages of ozonation and are subsequently oxidized to yield other intermediates and end products. According to their results, the amide is formed in a concentration higher than that of the carbinolamine, suggesting a preferential pathway of oxidation. The other means of hydroxyl attack is dechlorination at the position occupied by the chlorine atom to yield a hydroxyl group. Dechlorination can occur simultaneously with dealkylation, although the importance of one or the other reaction depends on the oxidation system type. Thus, dealkylation seems to be the main pathway in ozonation systems, whereas dechlorination is mainly observed when Fenton’s reagent or UV radiation are used. A possible sequence of intermediate formation in the ozonation of s-triazines, according to literature data,12 is dealkylation to yield some acetamide intermediate, then N-dealkylated amines, another acetamide formed from the second ethylamine group, and their corresponding N-dealkylated amine. Simultaneously, dechlorination can occur

Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002 1725

or carbinol compound that finally gives CAAT. Note that, for the experimental conditions applied in this work, CAAT was always an end product that accumulated in the water. This fact was also reported in the work of Nelieu et al.12 in the case of the ozonation of atrazine combined with hydrogen peroxide, although, in that case, OAAT was also reported as an end product. Another point considered for the preparation of the mechanism shown below is the promoting or inhibiting character of intermediates as far as the propagation or termination of the hydroxyl radical chain was concerned. Thus, in ozonation processes, it is well-known that some compounds can act as ozone decomposition promoters by reacting with hydroxyl radicals to yield superoxide ion radicals. These free radicals, in turn, react with ozone to form another hydroxyl radical and propagate the radical chain.8 Also, other compounds present an opposite behavior when reacting with hydroxyl radicals, in that they terminate the radical mechanism because they are unable to release the superoxide ion radical by reacting with the hydroxyl radical.8 In this work, both types of compounds are assumed to be formed, as indicated in the mechanism (see Figure 1 or the reaction mechanism in eqs 1-21). Thus, to account for the assumptions stated above, a fraction, R, of hydroxyl radicals was assumed to react with CEET and intermediates to yield superoxide ion radicals. The rest of the hydroxyl radicals were assumed to yield end products in their reactions with CEET and intermediates. Once the mechanism was established, the kinetic model was prepared from mole balance equations of species likely to be present in the gas and water phases. These species were ozone in the gas and water; CEET; and byproducts P1, CEAT (identified), P2, and CAAT (identified). Also, other compounds (reactions 20 and 21) were assumed to be formed as end products. One of them could be assigned to OAAT, which the literature also reports as end product at times less than 30 min. The proposed mechanism of reactions that follows the steps of Figure 1 is given below.

ozone direct reactions

Figure 1. Proposed sequence of simazine ozonation reactions.

at different stages to finally yield OAAT, usually accepted as the end product when oxidation times are less than 1 h.6,12 It should be noted, however, that cyanuric acid (2,4,6-trihydroxy-s-triazine) has also been reported as a final end product (the aromatic ring is never broken) during the photocatalytic ozonation of s-triazines but under very stringent conditions and after long reaction times (>1 h).13,15 In this work, the sequence of reactions given in Figure 1 has been considered to model the ozonation kinetic system: The first step involves the formation of a first intermediate, P1, which could be an amide or carbinol compound, that subsequently leads to CEAT. Then, CEAT is oxidized in the same way as CEET to yield a third intermediate, P2, which is likely another amide

O3 + CEET f P1 kD1 ) 8.7 M-1 s-1

(1)

O3 + P1 f CEAT kD2 ) ? M-1 s-1

(2)

O3 + CEAT f P2 kD3 ) 7.5 M-1 s-1

(3)

O3 + P2 f CAAT kD4 ) ? M-1 s-1

(4)

O3 + CAAT f P kD5 ) 0.2 M-1 s-1

(5)

ozone indirect reactions O3 + OH- f HO2- + O2 k1 ) 70 M-1 s-1

(6)

O3 + HO2- f HO2• + O3-• ki ) 2.8 × 106 M-1 s-1 (7)

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HO2• f O2-• + H+ k2 ) 5 × 1010 s-1

(8)

O2-• + H+ f HO2• k3 ) 8 × 105 M-1 s-1

(9)

O3 + O2-• f O3-• + O2 k4 ) 1.6 × 109 M-1 s-1 (10) O3-• + H+ f HO3• k5 ) 5 × 1010 M-1 s-1 (11) HO3• f HO• + O2 k6 ) 1.4 × 105 s-1

(12)

HO• + O3 f HO2• + O2 k7 ) 2 × 109 M-1 s-1 (13) HO• + H2O2 f HO2• + O2 k8 ) 2.7 × 107 M-1 s-1 (14) HO• + HO2- f HO2• + OHk9 ) 7.5 × 109 M-1 s-1 (15) RHO• + CEET f CEET• + ... f P1 + O2-• k10 ) 2.1 × 109 M-1 s-1 (16) RHO• + P1 f P1• + ... f CEAT + O2-• k11 ) ? (17) RHO• + CEAT f CEAT• + ... f P2 + O2-• k12 ) 2.1 × 108 M-1 s-1 (18) RHO• + P2 f P2• + ... f CAAT + O2-• k13 ) ? (19) RHO• + CAAT f P k14 ) 4.8 × 108 M-1 s-1 (1 - R)HO• + X f P′ kX ) ?

(20) (21)

where X represents CEET, P1, CEAT, P2, and CAAT and kX takes the values of the rate constants of their corresponding reactions with HO• (reactions 16-20, respectively). The rate constant values shown above were all taken from the literature.3,7,19 On the other hand, the rate constant for reactions 2 and 4 was taken as 7 × 109 M-1 s-1, and that for reactions 17 and 19 as 5 × 109 m-1 s-1, which are close to those of CEET, CEAT, and CAAT because of the assumed similarity of intermediate structures as reported in other works.12-14 Also, note that values of kX for this type of reaction are usually between 109 and 1010 M-1 s-1.20 Mole Balance Equations in the Water Phase. Because the water phase was always considered to be well-mixed, regardless of the kinetic model assumed, the mole balance equations in water were as follows. For the ith species dissolved in water, these equations follow the general form

βuLS(Cie - Ci) + βGi ) βSH

dCi dt

(22)

where uL is the water superficial velocity under emptycolumn conditions; S is the sectional area of the column; H is the height of the column occupied by the gas and water phases; β is the water-phase hold-up; Cie and Ci are the concentrations in water of the ith species at the column inlet and outlet, respectively; and Gi is the generation rate term, which varies depending on the

nature of ith species Thus, in the case of any species but ozone, this term is the chemical reaction rate, ri, times the reaction volume, whereas in the case of ozone, it is the sum of the ozone molar rate transferred from the gas phase plus the ozone molar decomposition rate, that is

GO3 ) (NO3 + rO3)SH

(23)

Note that eq 23 holds only when the kinetic regime of ozone absorption in water is slow. This situation develops when the Hatta number of the reactions of ozone with compounds present in water is lower than 0.3, as occurs in many aqueous micropollutant ozonation systems at pH < 12.21 Also note, however, that, in the presence of high concentrations of hydrogen peroxide, the kinetic regime could change, and these equations could become no longer applicable (see below). The chemical reaction rate equations (ri) of the species investigated were deduced by assuming the set of reactions shown above as elementary steps. On the other hand, the equation for the ozone molar transfer rate, NO3SH, varies depending on the gas flow pattern. Thus, NO3SH is defined as follows:

for the gas phase in plug flow or with axial dispersion NO3SH ) kLa

∫0H

[

]

CgRT - CO3 S dz He

(24)

for the well-mixed gas-phase NO3SH ) kLa

[

]

CgRT - CO3 SH He

(25)

where Cg and CO3 are the ozone concentrations in the gas and water phases, respectively, at any point in the column (which are constant values when phases are well-mixed); He and R are the Henry and ideal gas law constants, respectively; and T is the absolute temperature. Note that, in eqs 24 and 25, the ozone absorption rate is a function of kLa, the water-phase volumetric mass transfer coefficient, because ozone is a sparingly soluble gas in water and gas-phase resistance to mass transfer is negligible.22 The system of equations can be solved with the initial condition

t ) 0 Ci ) Ci0

(26)

with Ci0, the concentration at the start of ozonation, being Cie for simazine and 0 for any other species but hydrogen peroxide when present in the aqueous solution fed to the column. Mole Balance of Ozone in the Gas Phase. Three different equations were considered depending on the gas-phase flow pattern. The well-mixed gas-phase was represented by the CSTR design equation, similar to equation 22

uGS(Cge - Cg) - NO3SH

dCg β ) SH 1-β dt

(27)

where uG is the superficial gas velocity under emptycolumn conditions and Cge is the ozone concentration in the gas at the column inlet.


Similarly to the previous case, for plug-flow behavior, the corresponding plug-flow reactor design equation was applied

-uGS

∂Cg ∂Cg β - NO3S )S ∂z 1-β ∂t

(28)

Finally, when axial dispersion was considered, the following equation was used

-uGS

∂2Cg ∂Cg ∂Cg β + DS )S - NO3S 2 ∂z 1 β ∂t ∂z

(29)

where, for all cases, NO3 was obtained from eq 24 or 25 depending on the gas flow pattern considered. Equations 27-29 were solved with the following conditions:

t ) 0 Cg ) 0 (for any gas flow pattern) (30) for plug flow and axial-dispersion flow t > 0 z ) 0 Cg ) Cge

(31)

t > 0 z ) H Cg ) Cgs

(32)

where Cgs is an assumed value. In addition, values for kLa and He were also needed. The volumetric mass transfer coefficient was experimentally obtained from the absorption of ozone in an aqueous solution of indigo trisulfonate according to the method of Ridgway et al.23 A value of 0.05 s-1 was found for kLa. The Henry’s constant values were taken from literature.24 The system of equations 22-32 was then solved by using a specifically prepared Fortran program based on the Runge-Kutta and collocation methods.25 Figures 2-4 show, as examples, distribution curves of the experimental and calculated concentrations of ozone in the gas and dissolved in water and of simazine, respectively, at the column outlet. The results correspond to an experiment carried out at pH 7 with R ) 0.975, which leads to the best fit between the calculated and experimental concentrations of simazine when steady-state conditions are reached. According to this value, nearly all of the hydroxyl radicals (97.5%) are regenerated through reactions similar to steps 16-20. As shown in Figure 2, referred to the ozone concentration in the gas at the column outlet, the experimental and calculated concentrations at steady state are very close (maximum deviation is observed for perfect mixing conditions). The major difference among kinetic models is the time needed to reach the steady-state value. Thus, the axialdispersion model allows for the best fit between experimental and calculated concentrations, whereas the wellmixed and plug-flow models lead to calculated times shorter than the experimental time required to reach the steady-state concentration of ozone. The results obtained also confirm that only a small fraction of the ozone fed to the column is absorbed. Thus, for an inlet ozone concentration of 1.56 × 10-4 M, the experimental and calculated ozone outlet concentrations at steady state were 1.55 × 10-4 M (axial-dispersion conditions). As shown later, something different was observed when hydrogen peroxide was present at high concentration. For the water phase (see Figures 3 and 4), however, not much difference was found between the results obtained from one or another flow model, which is a

Figure 2. Calculated (from the plug-flow, perfect-mixing, and axial-dispersion models) and experimental concentrations of ozone gas at the column outlet as functions of time during the ozonation of simazine in water. Conditions: pH 7, 20 °C, CCEETo ) 3.5 × 10-5 M, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; curves and lines, calculated results.

Figure 3. Calculated (from the plug-flow, perfect-mixing, and axial-dispersion models) and experimental concentrations of dissolved ozone at the column outlet as functions of time during the ozonation of simazine in water. Conditions: pH 7, 20 °C, CCEETo ) 3.5 × 10-5 M, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; curves, calculated results for (‚‚‚) perfect mixing, (‚ - ‚ -) plug flow, (s) axial dispersion.

logical consequence of the fact that, in all cases, the water phase was considered to be well-mixed. Thus, the models accurately predict the steady-state concentrations of simazine and dissolved ozone. The main deviations were observed during the nonstationary period of ozonation. Finally, in Figure 5, the experimental and calculated concentrations of intermediates and/or end products with time using the axial-dispersion model are presented. It can be observed that the model predicts the trends of concentrations but generally overestimates the concentrations of CEAT and CAAT. However, it should be highlighted that the concentrations of these compounds were close to their detection limit (10-6 M), especially in the case of CAAT. Figure 5 also shows the evolution of the predicted concentrations for the assumed intermediates formed, P1 and P2. The calculated concentrations could be improved by making changes in the reaction mechanism, for example, by assuming parallel reactions from the first stages of ozonation. In this way, the predicted concentrations of CEAT and CAAT would be lower and closer to the experimental

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Figure 4. Calculated (from the plug-flow, perfect-mixing, and axial-dispersion models) and experimental concentrations of simazine at the column outlet as functions of time during its ozonation in water. Conditions: pH 7, 20 °C, CCEETo ) 3.5 × 10-5 M, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; curves, calculated results for (‚‚‚) perfect mixing, (‚ - ‚ -) plug flow, (s) axial dispersion.

Figure 5. Calculated (from the axial-dispersion model) and experimental concentrations of simazine and some intermediates at the column outlet as functions of time during the ozonation of simazine in water. Conditions: pH 7, 20 °C, CCEETo ) 3.5 × 10-5 M, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; numbered curves, calculated results for CEET (b, 1), P2 intermediate (2), CEAT (2, 3), P2 intermediate (4), CAAT (9, 5), unknown detected intermediate ([).

values. However, a better fit of the kinetic model would require improvements in the analytical techniques, such as quantifying concentrations of the more polar dechlorinated compounds. In Figure 5, the evolution of a third unknown intermediate detected but not quantified is also shown. The concentration of this compound was calculated by assuming a UV detection response similar to that of CEET. Following the trend observed, it can be concluded that this compound is formed after CEAT and before or in parallel with the formation of CAAT. At steady state, there was no difference between models, but the axial-dispersion model presented the best fit also during the nonstationary reaction period. Thus, for the rest of the simulation, the gas-phase flow was considered to follow the axial-dispersion model. Influence of pH. The pH of water is probably the most important parameter affecting ozonation processes. Thus, experiments at different pH values were carried out to test the kinetic model. Figures 6-8 show the changes with time of the calculated and experimental concentrations of simazine, dissolved ozone, and intermediates at three pH values. For simazine and dissolved ozone, the model predicts very well the

Figure 6. Calculated and experimental concentrations of simazine at the column outlet as functions of time during its ozonation in water at different pH. Conditions: axial-dispersion model, 20 °C, CCEETo ) 3.5 × 10-5 M, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; curves, calculated results for pH (b) 2.5, (2) 7, and (9) 9.

Figure 7. Calculated and experimental concentrations of dissolved ozone at the column outlet as functions of time during the ozonation of simazine in water at different pHs. Conditions: axialdispersion model, 20 °C, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; curves, calculated results for pH (b) 2.5, (2) 7, and (9) 9.

Figure 8. Calculated and experimental concentrations of CEAT and CAAT at the column outlet as functions of time during the ozonation of simazine in water at different pHs. Conditions: axialdispersion model, 20 °C, Cge ) 1.56 × 10-4 M, 20 L h-1. Symbols, experimental points; numbered curves, calculated results for CEAT (b, 1, pH 2.5), CAAT (O, 2, pH 2.5), CEAT ([, 3, pH 7), CAAT (], 4, pH 7), CEAT (2, 5, pH 9), and CAAT (4, 6, pH 9).

experimental simazine concentration provided that the appropriate R value was considered. The value of R was found to be 0.997 and 0.85 for pH 7 and 9, respectively. In the case of pH 2, this parameter did not affect the calculated results, which indicates that direct ozone attack was the main means of ozonation. Note that the R values were chosen to allow good agreement between experimental and calculated concentrations of simazine


Figure 9. Calculated and experimental concentrations of simazine at the column outlet as functions of time during its ozonation in water in the presence of hydrogen peroxide. Conditions: axial-dispersion model, pH 7, 20 °C, CCEETo ) 4.2 × 10-5 M, 20 L h-1. Symbols, experimental points; curves, calculated results for CH2O2T and Cge (M) ) (9) 3.44 × 10-5 and 1.4 × 10-4, (2) 4.21 × 10-4 and 1.35 × 10-4, and (b) 2.58 × 10-3 and 9.2 × 10-5.

once the steady state was reached (about 20 min). On the other hand, the concentrations of ozone in the gas leaving the column were found to be nearly coincident with their corresponding experimental values (not shown). In Figure 8, the predicted and experimental concentrations of CEAT and CAAT are plotted at different times and pH values. For pH 2.5, negligible concentrations were observed (both predicted and experimental), and good agreement was found. At pH 7, the predicted concentrations were somewhat higher than those experimentally measured, especially for the case of CAAT. At pH 9, on the other hand, the opposite result was observed. In any case, however, the trends of the concentrations are correctly predicted. Thus, it is seen that, at pH 2, both CEAT and CAAT are scarcely formed. At pH 7, CEAT reaches a plateau or maximum value at 25-30 min, whereas CAAT is accumulated in water, that is, the two compounds behave as intermediate and end product, respectively. Finally, at pH 9, it is clearly observed that CEAT behaves as an intermediate, with its concentration reaching a maximum value at about 5 min reaction time, whereas CAAT presents a plateau value at 15 min at a concentration of about 1.2 × 10-5 M. Influence of Hydrogen Peroxide Concentration. As shown in this work and others previously reported,6,12 hydrogen peroxide strongly influences the ozonation rate of s-triazine compounds in water. Consequently, the kinetic model was also tested under conditions when different concentrations of hydrogen peroxide were fed to the column (see Figures 9-12). Appropriate values of R for the cases shown were around 0.8. Compared to the ozonation results in the absence of hydrogen peroxide, the model predicts that a slighthly lower fraction of hydroxyl radicals is regenerated. Figure 9 shows the good agreement between the predicted and experimental concentrations of simazine. Note that both the kinetic model and the experimental results lead to an increase in the ozonation rate when the hydrogen peroxide concentration increases from 3.44 × 10-5 to 4.21 × 10-4 M and a slight decrease of ozonation rate when concentration of hydrogen peroxide is further increased to 2.58 × 10-3 M. These effects, also observed in the ozonation of other compounds such as tetrachloroethylene and trichloroethylene,9 are the logical consequence of the competitive character of hydrogen

Figure 10. Calculated and experimental concentrations of hydrogen peroxide at the column outlet as functions of time during the ozonation of simazine in water in the presence of hydrogen peroxide. Conditions: axial-dispersion model, pH 7, 20 °C, CCEETo ) 4.2 × 10-5 M, 20 L h-1. Symbols, experimental points; curves, calculated results for CH2O2T and Cge (M) ) (9) 3.44 × 10-5 and 1.4 × 10-4, (2) 4.21 × 10-4 and 1.35 × 10-4, and (b) 2.58 × 10-3 and 9.2 × 10-5.

Figure 11. Calculated and experimental concentrations of dissolved ozone at the column outlet as functions of time during the ozonation of simazine in water in the presence of hydrogen peroxide. Conditions: axial-dispersion model, pH 7, 20 °C, CCEETo ) 4.2 × 10-5 M, 20 L h-1. Symbols, experimental points; curves, calculated results for CH2O2T and Cge (M) ) (9) 3.44 × 10-5 and 1.4 × 10-4, (2) 4.21 × 10-4 and 1.35 × 10-4, and (b) 2.58 × 10-3 and 9.2 × 10-5.

peroxide in scavenging hydroxyl radicals.26 The predicted results for the remaining hydrogen peroxide and accumulated dissolved ozone concentrations were close to the experimental values (see Figures 10 and 11). Note, however, two aspects of Figure 11. First, for higher concentrations of hydrogen peroxide (>10-4 M), the model predicts slight increases in the ozone concentrations at low reaction times and the near absence of dissolved ozone at more advanced times when simazine has already disappeared (see also Figure 9), a situation similar to that observed in the experimental results (in these cases, no dissolved ozone was detected for t > 5 min). However, the model overestimates the concentrations of intermediates (not shown). In fact, the experimental concentrations of CEAT and CAAT were close to their detection limit (2 × 10-6 M), whereas the model predicted maximum concentrations of these compounds (hence both behaved as intermediates) at low reaction times (t < 7 min) for the highest concentrations of hydrogen peroxide. An analysis of the experimental samples indicated the presence of numerous intermediate compounds (concentrations were not measured because of a lack of standards or low peak areas) as a

1730

Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002 Table 1. Hatta Numbers of Ozone Reactions for the Ozonation of Simazine in the Presence of Hydrogen Peroxide at pH 7a

Figure 12. Calculated and experimental concentrations of ozone gas at the column outlet as functions of time during the ozonation of simazine in water in the presence of hydrogen peroxide. Conditions: axial-dispersion model, pH 7, 20 °C, CCEETo ) 4.2 × 10-5 M, 20 L h-1. Symbols, experimental points; curves, calculated results for CH2O2T and Cge (M) ) (2, 1) 3.44 × 10-5 and 1.4 × 10-4, (9, 2) 4.21 × 10-4 and 1.35 × 10-4, and (b, 3) 2.58 × 10-3 and 9.2 × 10-5.

difference from the results corresponding to hydrogen peroxide free oxidation. Thus, a more complex picture should be contemplated in the ozonation mechanism to account for more intermediates when hydrogen peroxide is present. This undoubtedly would result in lower predicted concentrations of CEAT and CAAT, hence bringing them closer to the experimental values. Another point of interest that would allow for a more rigorous explanation of discrepancies observed can be deduced from Figure 12, where the predicted and experimental concentrations of ozone in the gas at the column outlet corresponding to the mentioned experiments are shown as functions of time. It can be seen that, for applied concentrations of hydrogen peroxide higher than 3.44 × 10-5 M, the predicted remaining ozone gas concentrations are much higher than the corresponding experimental values. This is probably a result of the use of eq 24 for the ozone absorption rate term, which is only valid for the slow kinetic regime of ozone absorption. One of the characteristics of the slow kinetic regime is the presence of significant amounts of dissolved ozone (>10-5 M), which was only observed when a low concentration of hydrogen peroxide (3.44 × 10-5 M) was applied. Therefore, it was assumed that, for higher hydrogen peroxide concentrations in the feed water, the kinetic regime of ozone absorption was likely moderate or fast. For the slow kinetic regime to apply, the Hatta number for the ozone reactions should be lower than 0.3.27 The square of the Hatta number provides the ratio between the maximum chemical reaction rate in the liquid film and the maximum physical absorption rate27

Ha2 )

kDO3CB kL2

(33)

where k is the rate constant of the ozone reaction with compound or species B, CB is the concentration of species B, DO3 is the ozone diffusivity (1.7 × 10-9 m2 s-1 28), and kL is the individual liquid-side mass transfer coefficient (in this work, 5 × 10-5 m s-1, as calculated from a chemical method29). In the system studied, ozone reacts directly with s-triazine compounds, hydroxyl and hydroperoxide ions, and hydroxyl and superoxide ion radicals (see the reactions in the mechanism). However, the rate con-

Cge × 104 (M)

CH2O2T (M)

1.40 1.35 0.92 1.40 1.35 0.92 1.40 1.35 0.92

3.44 × 10-5 4.21 × 10-4 2.58 × 10-3 3.44 × 10-5 4.21 × 10-4 2.58 × 10-3 3.44 × 10-5 4.21 × 10-4 2.58 × 10-3

CB (M)

Hab

5.45 × 10-10 6.67 × 10-9 4.08 × 10-8 6 × 10-12 c 1.8 × 10-9 c 1.8 × 10-10 c 2.1 × 10-12 c 7.5 × 10-11 c 3 × 10-11 c

0.03 0.11 0.30 0.08 1.44 0.44 0.05 0.32 0.20

reaction species B 7

HO2-

10

O2-•

13

HO•

a C -5 M (average value), pH ) 7. b Calculated CEETo ) 4.2 × 10 from eq 33 with kL ) 5 × 10-5 m s-1 and DO3 ) 1.7 × 10-9 m2 s-1. c Calculated from the kinetic model at steady state.

stants of the ozone reactions with s-triazine and hydroxyl ion are very low, so that a possible change of kinetic regime would be due to reactions with hydroperoxide ion or free radicals. Thus, as a result of the very high rate constant values (>106 M-1 s-1) of reactions 7, 10, and 13, the kinetic regime can be moderate or even fast depending on the experimental conditions applied. All of these reactions can be considered as irreversible and second-order, so that their corresponding Hatta numbers can be calculated from eq 33 (see Table 1). Note that the concentrations of free radicals used to determine Ha for reactions 10 and 13 were calculated from the kinetic model under steady-state conditions. According to the values of Ha shown in Table 1, when the concentrations of hydrogen peroxide were higher than 3.44 × 10-5 M, the reactions of ozone with free radicals developed in the moderate to fast kinetic regime of absorption. Also, for the highest concentration of hydrogen peroxide applied, reaction 7 also developed in the moderate kinetic regime of absorption. Particularly important was the reaction between ozone and superoxide ion radical (reaction 10), which has a Ha value of 1.44. Hence, under these conditions, eq 24 is not suitable for the ozone absorption rate, which justifies the results shown in Figure 12. Instead, the correct equation should be

∫0H

NO3SH ) kLa

CgRT ES dz He

(34)

where E is the enhancement factor, that is, the ratio between the actual ozone absorption rate and the maximum physical absorption rate. For a single irreversible second-order gas-liquid reaction when Ha is between 0.3 and 3, E is somewhat higher than 1.27 However, for complex reaction systems with multiple series of parallel moderate reactions, such as the ozone/ hydrogen peroxide oxidation of simazine, depending on the ratio between the rate constants of the reactions involved, E could be even higher than 3.30 In this work, approximate experimental values of E were also calculated, under steady-state conditions, using

E)

NO3He kLaCgsRT

(35)

where Cgs corresponds to the ozone concentration at the gas outlet. Table 2 presents the results obtained. It is evident that the kinetic regime of ozone absorption was moderate and fast when the ozonation experiments were carried out in the presence of concentrations of hydrogen

Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002 1731 Table 2. Experimental Enhancement Factors for Ozone Absorption in Simazine Aqueous Solution in the Presence of Hydrogen Peroxidea Cgs × (M)

104

1.40 1.35 0.92

CH2O2T (M)

NO3, exp × 107 b (M s-1)

kLaCgRT/He × 107 c (M s-1)

Ed

3.44 × 10-5 4.21 × 10-4 2.58 × 10-3

0.49 2.21 3.22

5.44 2.03 1.35

0.09 1.09 2.38

CCEETo ) 4.2 × 10-5 M (average value), pH 7. b From experimental data at steady state. c From experimental ozone concentration at the column outlet at steady state. kLa ) 0.05 s-1. d From eq 35. a

peroxide in the feed simazine aqueous solution of 4.1 × 10-4 and 2.58 × 10-3 M, respectively. Conclusions Kinetic modeling of pollutant ozonation in water constitutes a suitable tool for predicting process efficiency and reactor design. For the case of simazine studied in this work, the kinetic model proposed was established by considering nonideal flow, mass transfer, and chemical reaction rates. Chemical reactions were also based on dealkylation as the main pathway of oxidation, and it was assumed that s-triazine compounds act as partial promoters of ozone decomposition. Also, after tracer experiments for nonideal flow, the kinetic model considered that the water phase was wellmixed, with some dispersion present in the gas phase. The latter possibility was compared to ideal-flow behavior. Although the dispersion effects were not very significant in the small bubble column used, nonetheless, the use of the axial-dispersion model is recommended to improve the results of this work in terms of the ozone concentration in the gas phase. On the whole, good agreement between the experimental and calculated concentrations of simazine, ozone, and intermediates was obtained at different pH values and in the presence of hydrogen peroxide for some of the compounds tested (simazine, dissolved ozone, hydrogen peroxide). At acidic pH, the kinetic model predicts the disappearance of simazine exclusively by direct ozonation, that is, without the participation of free radicals. In the presence of hydrogen peroxide (at concentrations of >10-4 M), although a good fit was found for the simazine, ozone, and hydrogen peroxide concentrations, the model was unable to fit the experimental intermediate and ozone gas concentrations. Probably, in practical situations, the concentrations of hydrogen peroxide are usually kept below 10-4 M. In any case, however, when hydrogen peroxide is present at high concentration, a more complex scheme of reactions should be considered, which requires improvements in the analytical techniques for the identification of intermediates and the availability of standards for the quantification of concentrations. For the ozone gas concentration, the experimental results confirmed that, at high hydrogen peroxide concentrations (in this work, above 10-4 M), the kinetic regime of ozone absorption changes to be moderate and even fast in some cases. Under these conditions, a different expression for the ozone absorption rate that accounts for the enhancement factor should be used. Studies are now in progress to identify a method for determining and theoretically calculating exact values of the enhancement factor to have a full picture of the ozonation kinetics for any regime of absorption.

Acknowledgment The authors thank the CICYT of Spain and Fondos Feder of EU for supporting this research through Project 1FD97/2222/C03/02. Literature Cited (1) Camel, V.; Bermond, A. The use of ozone and associated oxidation processes in drinking water treatment. Water Res. 1998, 32, 3208-3222. (2) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical processes for water treatment. Chem. Rev. 1993, 93, 671-698. (3) Beltrań, F. J.; Rivas, F. J.; Alvarez, P.; Alonso, M. A.; Acedo, B. A kinetic model for advanced oxidation processes of aromatic hydrocarbons in water: Application to phenanthrene and nitrobenzene. Ind. Eng. Chem. Res. 1999, 38, 4189-4199. (4) Plimmer, J. R. Herbicides. In Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; John Wiley & Sons: New York, 1978; Vol. 12, pp 297-351. (5) Sayre, J. M. International standards for drinking water. J. Am. Water Works Assoc. 1988, 80, 53-60. (6) Lai, M.; Jensen, J. N.; Weber, A. S. Oxidation of simazine: Ozone, ultraviolet and combined ozone/ultraviolet oxidation. Water Environ. Res. 1995, 67, 340-346. (7) Beltrań, F. J.; Garcıá-Araya, J. F.; Rivas, F. J.; Alvarez, P.; Rodrı´guez, E. Kinetics of simazine advanced oxidation in water. J. Environ. Sci. Health B 2000, B35, 439-445. (8) Hoigne´, J. Chemistry of aqueous ozone and transformation of pollutants by ozonation and advanced oxidation processes, In The Handbook of Environmental Chemistry; Hrubec, J., Ed.; Springer-Verlag: Berlin, 1998; Vol. 5, Part C. (9) Glaze, W. H.; Kang, J. H. Advanced oxidation processes. Description of a kinetic model for the oxidation of hazardous materials in aqueous media with ozone and hydrogen peroxide in a semibatch reactor. Ind. Eng. Chem. Res. 1989, 28, 1573-1580. (10) Bader, H.; Hoigne´, J. Determination of ozone in water by the indigo method. Water Res. 1981, 15, 449-456. (11) Fogler, H. S. Elements of Chemical Reaction Engineering, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 1999. (12) Ne´lieu, S.; Kerhoas, L.; Einhorn, J. Degradation of atrazine into ammeline by combined ozone/hydrogen peroxide treatment in water. Environ. Sci. Technol. 2000, 34, 430-437. (13) Pelizzetti, E.; Maurino, V.; Minero, C.; Carlin, V.; Pramauro, E.; Zerbinati, O.; Tosato, M. L. Photocatalytic degradation of atrazine and other s-triazine herbicides. Environ. Sci. Technol. 1990, 24, 1559-1565. (14) Arnold, S. C.; Hickey, W. J.; Harris, R. F. Degradation of atrazine by Fenton’s reagent: Condition, optimization and product identification. Environ. Sci. Technol. 1999, 29, 2083-2089. (15) Legube, B.; Guyon, S.; Dore´, M. Ozonation of aqueous solution of nitrogen heterocyclic compounds: Benzotriazoles, atrazine and amitrole. Ozone Sci. Eng. 1987, 9, 233-246. (16) Beltrań, F. J.; Garcıá-Araya, J. F.; Acedo, B. Advanced oxidation of atrazine in water. I. Ozonation. Water Res. 1994, 28, 2153-2164. (17) Beltrań, F. J.; Garcıá-Araya, J. F.; Acedo, B. Advanced oxidation of atrazine in water. II. Ozonation combined with ultraviolet radiation. Water Res. 1994, 28, 2165-2174. (18) Bailey, P. S. Ozonation in Organic Chemistry, Vol. II: Nonolefinic Compounds; Organic Chemistry Series, Vol. 39; Academic Press: New York, 1982. (19) Beltrań, F. J.; Garcıá-Araya, J. F.; Alvarez, P.; Rivas, F. J. Aqueous degradation of atrazine and some of its main byproducts with ozone/hydrogen peroxide. J. Chem. Technol. Biotechnol. 1998, 71, 345-355. (20) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of data constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (‚OH/‚O-) in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17, 513-886. (21) Beltrań, F. J. Theoretical aspects of the kinetics of competitive ozone reactions in water. Ozone Sci. Eng. 1995, 17, 163-181. (22) Beltrań, F. J.; Gonza´lez, M. Ozonation of aqueous solutions of resorcinol and phloroglucinol 3. Instantaneous kinetic regime. Ind. Eng. Chem. Res. 1991, 80, 2518-2522.

1732


(23) Ridgway, D.; Sharma, R. N.; Eanlay, T. R. Determination of Mass Transfer Coefficients in Agitated Gas-Liquid Reactors by Instantaneous Reactions. Chem. Eng. Sci. 1989, 44, 2935-2942. (24) Sotelo, J. L.; Beltrań, F. J.; Benı´tez, J.; Beltrań-Heredia, J. Henry’s law constant for the ozone-water system. Water Res. 1989, 23, 1239-1246. (25) Constantinides, A. Applied Numerical Methods with Personal Computers; McGraw-Hill: New York, 1987. (26) Christensen, H. S.; Sehensted, H.; Corfitzan, H. Reactions of hydroxyl radicals with hydrogen peroxide at ambient and elevated temperatures. J. Phys. Chem. 1982, 86, 55-68. (27) Charpentier, J. C. Mass Transfer Rates in Gas Liquid Absorbers and Reactors. In Advances in Chemical Engineering; Academic Press: New York, 1981; Vol. 11, pp 3-133.

(28) Johnson, P. N.; Davis, R. A. Diffusivity of ozone in water. J. Chem. Eng. Data 1996, 41, 1485-1487. (29) Danckwerts, P. S. V. Gas-Liquid Reactions; McGrawHill: New York, 1970. (30) Onda, K.; Sada, E.; Kobayashi, T.; Fujine, M. Gas absorption accompanied by complex chemical reactions. II. Consecutive chemical reactions. Chem. Eng. Sci. 1970, 25, 761-768.

Received for review August 15, 2001 Revised manuscript received December 10, 2001 Accepted December 11, 2001 IE010681M

An Attempt to Model the Kinetics of the Ozonation of Simazine in Water

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