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Ind. Eng. Chem. Res. 2006, 45, 166-174
Fluorene Oxidation by Coupling of Ozone, Radiation, and Semiconductors: A Mathematical Approach to the Kinetics F. Javier Rivas,* Fernando J. Beltra´ n, Olga Gimeno, and Marı´a Carbajo Departamento de Ingenierı´a Quı´mica y Energe´ tica, UniVersidad de Extremadura, AVenida de ElVas S/N, 06071 Badajoz, Spain
The kinetics of fluorene oxidation by systems that include ozone (O3), ozonation in the presence of titanium dioxide (O3/TiO2), photolytic ozonation (O3/UV-A), titanium dioxide photocatalysis (TiO2/UV-A), and titanium dioxide photocatalytic ozonation (UV-A/O3/TiO2) has been mathematically assessed by the proposal of a mechanism based on experimental results. The conventional free-radical mechanism involved in ozone processes was the starting point to model the different systems. Single fluorene ozonation simulation suggests the existence of a hydroxyl radical source other than ozone activation through hydroxyl anions and/or the ionic form of hydrogen peroxide. Combination of O3 with TiO2 or UV-A radiation involves a negligible effect in the first case and the improvement of the fluorene removal rate in the second case. In the latter process, ozone photolysis likely entails a higher formation of hydroxyl radicals. Because fluorene does not absorb UV-A light, photocatalytic oxidation of fluorene by the system TiO2/UV-A also indicates the development of surface radical reactions. Finally, the photocatalytic ozonation of the polycyclic aromatic hydrocarbon implies a synergistic effect of the single systems. 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a group of over 100 different chemicals that are mainly formed during incomplete combustion. Because of the widespread sources, PAHs are present almost everywhere.1 Data from animal studies indicate that several PAHs can induce many adverse effects, such as the well-documented carcinogenicity of several PAHs. Although PAHs are relatively insoluble and they are normally adsorbed to solid particulates, these compounds have also been detected in several types of waters.2,3 PAHs are very slowly biodegradable under aerobic conditions in the aqueous compartment. The biodegradation rates decrease drastically as the number of aromatic rings increases. Treatment technologies aimed at eliminating PAHs from water matrixes include adsorption,4 chemical oxidation,5-8 photolysis,9 etc. Modeling of the aforementioned processes has normally been conducted using general expressions of the type of first-order or pseudo-first-order kinetics when solids are not present or Langmuir-Hinselwood kinetics when a heterogeneous catalyst has been utilized.10 In a previous investigation, the efficiency of different oxidation processes (based on the combination of UV-A radiation, ozone, and titanium dioxide) on the fluorene removal and mineralization level achieved was shown.11 Fluorene was first used as a model PAH, because it slowly reacts with ozone so the kinetics of the different systems can be more efficiently monitored. It also shows an acceptable solubility (∼1 ppm), facilitating the analytical procedure. The work presented here is now focused on the mathematical simulation of the previously studied treatments. Because of the lack (in some cases) of available kinetic data, and the complexity of the systems studied, the mechanism used is a combination of well-known elemental reactions plus some pseudo-empirical expressions derived/ assumed from the experimental knowledge. Therefore, it must be emphasized that this is not a meticulous kinetic analysis (the * To whom correspondence should be addressed. Tel.: +0034924289385. Fax: +0034924289385. E-mail:
[email protected].
existence of some of the stages postulated has not been proven) but a mathematical approach to simulate the fluorene depletion profiles. 2. Experimental Section Fluorene (99.8%) and phosphoric acid (for pH adjustement) were obtained from Aldrich Chemical Company and used as received. Titanium dioxide (Degussa P25, 70% anatase and 30% rutile) was used in photocatalytic experiments. Ozone was generated from pure oxygen in a laboratory Sander ozonator. Experiments were performed in a 1-L-capacity tubular borosilicate glass photoreactor (450 mm long, 80 mm in diameter). In photolytic experiments, the aqueous solution was irradiated with a high-pressure mercury lamp (Heraeus, TQ 718, 700 W) immersed in a glass well (UV-B radiation not allowed) placed at the middle of the reactor. When TiO2 was added, the solid was maintained in suspension by magnetic stirring with a concentration of 1.5 g/L. Details of the experimental procedure can be found elsewhere.11 3. Results and Discussion 3.1. Preliminary Work. Calculation of the Mass Transfer Coefficient. The individual mass transfer coefficient (kla) in the liquid phase was determined by means of a double experiment of ozone absorption-decomposition in organic free water. Thus, in the semibatch system used in this work, because of the perfect mixing conditions applied, the differential equation that describes the ozone accumulation in the water bulk follows the expression
dCO3 dt
(
) kla(C*O3 - CO3) - kDCO3 ) kla CO3g out
RT - C O3 H kDCO3 (1)
)
where CO/ 3, CO3, and CO3g out represent the ozone concentrations in equilibrium with the exiting gas, that dissolved in the liquid
10.1021/ie050781i CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005
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ozonation of fluorene at pH 2 (at higher pHs, the fluorene depletion rate was too fast to obtain reliable kinetic data). The model adopted to simulate the ozonation system consisted of the classic radical chain mechanism first introduced by Hoigne´, Staehelin, and Bu¨hler (in Chelwoska et al.14). The main reactions constituting the mechanism are as follows. (Units are M-1 s-1 or s-1.)
O3 + OH- f HO2° + °O2O3 + °O2- f O2 + °O3H+ + °O3- a HO3°
-
HO° + °O2 f O2 + OH
[
( τt )]
Ci ) R 1 - exp -
(2)
where R and τ are the adjustable parameters of the mathematical expression (Ci is dissolved ozone or ozone at the gas outlet). Differentiation of eq 2 allowed for the calculation of dCi/dt. After the ozone accumulation rate dCO3/dt and a time expression for CO3g out were computed, the values of kla and kD were fitted/ optimized to model both the ozone accumulation and the ozone decomposition concentration curves adequately. Thus, using the ozone accumulation profile (modeled by eq 1), a value for kla and a value for kD were obtained. The value of kD was used thereafter to simulate the ozone decomposition curve. The optimization consisted of minimizing a square error function accounting for the differences between experimental and model results of both the ozone accumulation and decomposition profiles. Figure 1 illustrates the experimental and fitted profiles corresponding to ozone accumulation and decomposition (Figure 1A) in water and ozone leaving the reactor and accumulation rate (Figure 1B). From the optimization procedure, the following values were obtained: kla ) 9.5 × 10-3 s-1 and kD ) 4.9 × 10-5 s-1. The previous figures are in the range of typical values reported in the literature for these parameters.12,13 3.2. Fluorene Ozonation. 3.2.1. Classic Ozonolysis Mechanism. The first system kinetically studied was the single
(6)
-
(k ) 2.0 × 10 , k ) 1.0 × 10 ) (7) (8)
(k ) 1.0 × 1010) (9)
(k ) 5.0 × 109) -
HO° + HO3° f O2 + H2O2 HO3° + HO3° f 2O2 + H2O2 HO3° + °O2 f 2O2 + OH
2
(k ) 2.8 × 104)
HO° + HO° f H2O2
phase, and that in the gas leaving the reactor, respectively. In addition, R is the universal gas constant, T the temperature, H Henry’s constant, and kD the first-order ozone decomposition rate constant. The procedure to calculate kla was as described in the following discussion. In the absorption run, dissolved ozone that accumulated in the liquid and left the reactor was directly measured and fitted to a mathematical equation of the type
(4)
9
HO2° + HO4° f O2 + O3 (+ H2O)
-
(k ) 1.6 × 109)
(k ) 5.0 × 104)
HO4° f O2 + HO2°
Figure 1. Absorption-decomposition of ozone in ultrapure water. Experimental conditions: CO3g inlet ) 1 × 10-4 M; pH 2; temperature T ) 293 K; volume V ) 0.9 L; gas flow rate ) 50 L/h. In panel A, experimental (symbols) and fitted (dotted line) accumulated and decomposed ozone concentrations are shown. In panel B, experimental (symbols) and fitted (dotted line) ozone gas outlet concentration are shown. Symbols corresponding to the axis “dCO3/dt” were obtained from the left term in eq 1 after optimization of kla and kD.
(3)
(k ) 2.0 × 1010, k- ) 3.7 × 104) (5)
HO3° f O2 + HO° O3 + HO° a HO4°
(k ) 140/70)
-
HO4° + HO4° f 2O3 + H2O2 HO4° + HO° f O3 + H2O2
(10)
(k ) 1.0 × 10 ) (11) 10
(k ) 5.0 × 109) (12) (k ) 5.0 × 109)
(13)
(k ) 1.0 × 10 )
(14)
(k ) 5.0 × 109)
(15)
10
(k ) 5.0 × 109) (16)
HO4° + °O2- f O3 + O2 + OH-
(k ) 1.0 × 1010)
(17)
HO4° + HO3° f O3 + O2 + H2O2
(k ) 5.0 × 109)
(18)
(H2O) + °O3- a HO3° + OH(k ) 54.2, k- ) 1.0 × 1010) (19) HO2- + O3 f HO° + °O2- + O2
(k ) 2.8 × 106) (20)
HO2° a H+ + °O2-
(k ) 3.2 × 105, k- ) 2.0 × 1010) (21)
H2O2 a H+ + HO2-
(k ) 4.5 × 10-2, k- ) 2.0 × 1010) (22)
H+ + OH- a H2O
(k ) 1.0 × 1011, k- ) 1.0 × 10-3) (23)
The mechanism of reactions 3-23 was completed and numerically solved by considering the transfer of ozone from the gas to the aqueous phase and the reactions of fluorene with molecular ozone and hydroxyl radicals:
O3 (gas) a O3 O3 + F f P HO° + F f P
(kla)
(24)
(kO3-F)
(25)
(kHO°-F)
(26)
The values for rate constants in reactions 3-26 were taken from previous literature reports.5,7,15 Figure 2 illustrates the calculated dimensionless fluorene concentration profile (curve 1) versus time after solving the adopted mechanism (Model 1). As observed, Model 1 clearly underestimates the fluorene removal
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As a consequence of the previous analysis, it can be hypothesized that the radical source in the fluorene-ozone system comes from a different pathway than those reported in the classic chemistry of ozonolysis processes. 3.2.2. Modified Ozonolysis Mechanism. When oxygen is in excess, organic radical peroxides (ROO°) are easily formed in radical-mediated mechanisms. These organic radical peroxides arise from the combination of molecular oxygen and an organic radical. Further decomposition of ROOH (after a hydrogen abstraction step) might lead to release of HO°. Consequently, a new attempt to model the process was conducted by assuming the production of HO° by direct attack of HO° to fluorene (Model 2). Thus, a standard reaction would be of the type17 Figure 2. Ozonation of fluorene. Experimental conditions: CFo ) 7.8 × 10-6 M; CO3g inlet ) 1 × 10-4 M; pH 2; T ) 293 K; V ) 0.9 L; gas flow rate ) 50 L/h. Symbol legend: (b) dimensionless fluorene concentration, (2) ozone concentration at the reactor outlet, and (() dissolved ozone concentration; the lines represent model calculations. (Model 1, classic ozonation chemistry; Model 2, regeneration of HO° from HO° attack to organics; Model 3, generation of HO° from O3 and HO° attack to organics, and Model 4, similar to curve 3 but considering the competitive effects of intermediates.)
rate, even in the most favorable of the cases, i.e., by ignoring the competitive effect of intermediates (P) generated. It appears that, even at this low pH of 2, the role played by free hydroxyl radicals is higher than that expected from just considering the classic initiation stage (reaction 3). This could be experimentally verified by testing the influence of the addition of a well-known hydroxyl radical scavenger (tert-butyl alcohol) which partially inhibited the fluorene ozonation rate at acidic pH.5 The existence of parallel routes of radical generation at low pH has been suggested previously in aqueous ozonation processes.16 Apart from reaction 3, the other typical route of HO° formation in the presence of ozone is through the step described in reaction 20, that is, the reaction between the ozone molecule and the dissociated form of the hydrogen peroxide molecule. The presence of hydrogen peroxide in the system studied might be due to the decomposition of instable intermediates of the ozonolysis itself. Thus, Popov and Getoff17 suggested the release of one molecule of hydrogen peroxide after formation of the di-aldehyde from fluorene, according to
Similarly, the formation of 9-fluorenone (or 9-fluorenol) or dibenzofurane, which are two of the initial intermediates in fluorene ozonation,8 might involve the release of hydrogen peroxide:
Consequently, reaction 25 was modified and hydrogen peroxide was assumed to be steadily formed from fluorene ozonation. However, consideration of this stage did not influence the theoretical depletion profile of the parent compound, because the amount of H2O2 dissociated at pH 2 is too low (see reaction 22) to significantly initiate the chain mechanism through reaction 20.
Once more, curve 2 in Figure 1 indicates the inadequacy of the mechanism proposed. Just a slight improvement of fluorene conversion is observed after 20 min of reaction, which is insufficient, if compared to the experimental data. A further improvement of the model can be achieved by contemplating the enhancement of OH° production from ozone in the presence of some organic initiators. In this sense, Han et al.,18 using electron spin resonance (ESR) techniques, found a significant increase of OH° formation after ozone decomposition in the presence of some phenol derivatives and 5,5-dimethylpyrrolidine 1-oxyl (DMPO) as the radical trapping agent. The mechanism would be similar to reaction 30, i.e., through the formation of an organic peroxyradical. Curve 3 in Figure 2 was obtained by assuming that both the ozone and OH° attacks on fluorene led to the generation of one molecule of hydroxyl radical (Model 3). As observed from the proposed mechanism, now the model overestimates the reaction rate, showing fluorene conversion values of >95% in
