Degradation of the antimicrobial triclocarban (TCC) with ozone

Chemical Engineering and Processing 50 (2011) 637–643

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Degradation of the antimicrobial triclocarban (TCC) with ozone Chedly Tizaoui a,∗ , Naser Grima b , Nidal Hilal a a b

Centre for Water Advanced Technologies & Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom School of Engineering, Design and Technology, University of Bradford, Bradford, BD7 1DP, United Kingdom

a r t i c l e

i n f o

Article history: Received 26 October 2010 Received in revised form 29 March 2011 Accepted 31 March 2011 Available online 17 April 2011 Keywords: Ozone Triclocarban (TCC) Rate constant Oxidation kinetics Mass transfer with chemical reaction

a b s t r a c t Triclocarban (TCC) has been used as an antimicrobial compound for many decades due to its sanitising properties. Recent studies showed that TCC is toxic and causes disruption to the endocrine system. In this work, ozone oxidation of TCC was studied in 70% acetonitrile:30% water solutions. Ozone degraded TCC effectively and the reaction rates increased substantially with ozone gas concentration, pH and temperature. Second-order-reaction kinetics was suitable to describe the chemical reaction. The effect of temperature was described by the Arrhenius equation and the activation energy was found equal to 31.0 kJ/mol. The stoichiometry of the reaction was one and the value of the rate constant at pH 7 was 5 × 103 M−1 s−1 . © 2011 Elsevier B.V. All rights reserved.

1. Introduction During the past decade, there has been growing interest in understanding the fate and behavior of a selection of individual chemicals, such as endocrine disrupting chemicals and pharmaceutical and personal care products, in the environment and in wastewater treatment plants. It is common that these contaminants of concerns cannot be easily removed at wastewater treatment plants, which are not designed for such a purpose [1]. As a result, a range of new treatment options have been proposed and studied [2–5]. As a treatment option, ozonation has been found successful to remove a wide range of contaminants and its application is growing significantly. However, there is still much to be learnt about kinetics and mechanisms of ozone reactions with an ever growing number of potential chemicals of concerns in the aquatic environment. To this end, this study reports on results obtained for the ozonation of triclocarban, which is a micropollutant that has recently been identified as a potential endocrine disruptor and a cause for environmental concerns. Triclocarban or 3-(4-chlorophenyl)-1-(3,4-dichlorophenyl)urea (TCC), which is a poorly water-soluble substance, has antibacterial and antifungal properties. Hence it finds applications in disinfectants, detergents, cosmetics, soaps, etc. The industrial use of TCC dates back to the 1960s and has been growing ever since due to the popularity of the compound. In the United States alone, 84%

∗ Corresponding author. Tel.: +44 0 1792 606841; fax: +44 0 1792 295676. E-mail address: [email protected] (C. Tizaoui). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.03.007

of antimicrobial bar soaps contain TCC and its usage could reach an upper range as high as 750 metric tones/year [6,7]. Although TCC is classified as a High Production Volume (HPV) Chemical in the US and has been in use for half a century, it is only recently that concerns about its health and environmental effects have been reported [8–12]. Halden and Paull [10] have shown that TCC is toxic to humans and other animals since it increases methemoglobinemia. In another study carried out by Chen et al. [9], it was found that exposure to TCC exhibited androgenic activity by amplifying the bioactivity of endogenous hormones causing abnormal growth of the male sex accessory organs such as the prostate. Kinney et al. [11] assessed the transfer of organic anthropogenic waste indicators derived from land-applied biosolids or swine manure and have shown that triclosan, which is a similar antimicrobial to TCC, bioaccumulated in earthworms at high proportions up to a bioaccumulation factor of 27. Lawrence et al. [12] have recently shown that triclocarban and triclosan have significant effect on microbial composition, algal biomass, architecture and activity of the river biofilm communities they studied. TCC has also been found to affect algal growth [13] and potentially act as an endocrine disruptor [9,14]. Due to the large use of TCC for many decades now and the recent documented effects of the compound on humans and the environment, recent studies concerned with its fate in wastewater treatment plants and the environment have started to emerge [15–18]. It is clear from the research carried out so far that the removal of TCC with conventional wastewater treatment processes is difficult [18,19]. Therefore, new technologies, such as oxidation processes, must be studied for TCC removal. To this end, the number of

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studies concerned with such technologies is very limited and only the electro-Fenton degradation of TCC has been reported in the literature [20]. It is well established that ozone is effective in removing a wide range of organic and inorganic compounds and is applied successfully in treating both drinking and waste waters. Ozone may react in water through two mechanisms (i) direct reactions involving molecular ozone, and (ii) indirect reactions involving secondary • oxidants such as hydroxyl radicals ( OH) formed following ozone decomposition in water, a reaction initiated by for example OH− [21]. Knowledge of ozone reaction kinetic parameters including reaction rate constant k, the reaction stoichiometry, and the reaction order are essential to assess the effectiveness of ozone to remove TCC and to perform reactor design studies. The kinetic parameters are also important for understanding the effects of operating parameters and for developing models capable to optimize the treatment process. Accordingly, this work concerned the determination of the kinetic parameters of the ozone reaction with TCC, which are not currently available in the literature. Because TCC has limited water solubility (0.65 mg/L [22]), it was essential in this work to investigate its ozonation in acetonitrile/water mixtures at more concentrated solutions. Although acetonitrile reaction with molecular ozone is very slow (the second-order reaction rate constant is about 6 × 10−5 M−1 s−1 [23]), its reaction with hydroxyl radicals is nevertheless significant under the conditions used in this study. Indeed, knowing that the rate constant of ACN with hydroxyl radicals is about 2.2 × 107 M−1 s−1 [24] and assuming that the rate • constant of TCC with OH is in the order of 109 M−1 s−1 [5], the reac• tion rate of ACN with OH was calculated and was found to largely • exceed that of TCC with OH by over 1000 times. Hence under the conditions used in this study, ACN plays a scavenging role and only the reaction between TCC and molecular ozone is significant.

2. Material and methods High purity triclocarban (99%), acetonitrile (ACN) and t-butanol were purchased from Sigma–Aldrich Co., UK. Stock solutions of TCC at 100 mg/L were prepared by directly dissolving solid TCC in a mixture made of 70% ACN and 30% Milli-Q high purity water (>18 M cm). Reaction solutions were buffered using phosphoric acid and concentrated sodium hydroxide solutions. The pH values of the solutions were measured using an Inolab Terminal Level 3 digital pH meter (VWR, UK). In some experiments, t-butanol (0.2 M) was added into the TCC solution before bubbling ozone. All experiments were performed at a constant temperature of 20 ± 1 ◦ C if not otherwise stated. Experiments were carried out in a classical semi batch gas/liquid reactor filled with 500 mL of TCC solution of known initial concentration. Pure oxygen was used to feed the ozone generator at the working flow rate of 400 mL/min. An ozone generator Model Triogen LAB2B fitted with a manually adjustable ozone production control knob was used to generate ozone. Ozone gas concentration was measured using a BMT 963 ozone analyser that automatically converts ozone concentrations to NTP conditions (i.e. 0 ◦ C, 1 atm) and displays them in g/m3 NTP. Three different ozone gas concentrations (10, 20, 60 g/m3 NTP or 3.7, 7.5, 22.4 mg/min in terms of ozone production) were used in this study. Once the gas concentration reached steady-state, the stream of ozone in oxygen was directed to the reactor using a three-way valve. The gas was then diffused into the solution through a sintered glass diffuser and further dispersed within the volume of the reactor using a magnetic stirrer. The gas leaving the reactor was diverted to an ozone destructor and the whole system was placed inside a fume cupboard. TCC sampling was carried out manually using a syringe at different time intervals. The content of the syringe was

quickly transferred to initially prepared vials containing 2 mL of 0.005 M sodium thiosulphate (Na2 S2 O3 ·5H2 O) solution to quench any remaining aqueous ozone in the solution, which prevents further reaction before analysing for TCC concentration. The concentration of TCC was measured by HPLC (Waters 2695) with UV detection (Waters 2487 Dual ␭). The column was a reverse phase C18 Hypersil Gold column (150 × 4.6 mm, 5 ␮m) purchased from Thermo Scientific, UK, and the mobile phase was an isochratic solution of 70%ACN:30%H2 O at 1 mL/min. The injection volume was 20␮L and the retention time for TCC was found equal to 4.7 min. The maximum absorbance wavelength of TCC was determined in a 1-cm quartz cell using an Agilent HP8453 UV/vis spectrophotometer. 70%ACN:30%H2 O was used as blank solution and the obtained spectrum showed a maximum absorbance wavelength at 265 nm. Based upon this result, subsequent analysis of the TCC by HPLC/UV detector was carried out at a wavelength of 265 nm. Six standard solutions of TCC in 70%ACN:30%H2 O covering the range 0–100 mg/L were used for calibration and a very good linear relationship (R2 = 0.9990) between peak area and concentration was obtained. The limit of detection (LOD) was 20 ng and the limit of quantification (LOQ) was 66.7 ng as determined using a signal-tonoise ratios of 3:1 and 10:1 respectively.

3. Results and discussion 3.1. Ozone mass transfer parameters Determination of the ozone mass transfer parameters is essential for the analysis of ozone degradation of TCC in the gas/liquid reactor. This involved the determination of ozone saturation concentration (Css ), the volumetric mass transfer coefficient (kL a), and the specific interfacial area (a) in the solvent (i.e.70%ACN:30%H2 O). As discussed in the Introduction, due to the low solubility of TCC in water, a mixture of 70%acetonitrile and 30%water by volume was used as a solvent. At this ratio, it is assumed that the water is present at sufficient quantity to participate in the ozonation reaction and thereby mimic what would occur in ACN-free water. In their studies on ozone reaction kinetics, Hoigne and Bader [25] concluded that acetonitrile can be used as a co-solvent in ozone studies, but its scavenging effect of hydroxyl radicals should not be ignored. Since data on ozone saturation concentrations in 70%ACN:30%H2 O mixture is not available, an experimental study was carried out in this work for this purpose. Ozone gas concentration was fixed at a given value and the gas was bubbled in the liquid phase until saturation was reached (i.e. steady-state). The relationship between the equilibrium concentration (Ceq ) and the steady-state concentration (Css ) is Ceq = (1 + k/kL a)Css – where k is the ozone decay rate constant. Following addition of t-butanol (a strong radical scavenger), it is expected that the decay rate constant decreases and the volumetric mass transfer coefficient (kL a) increases [26], which leads to a decrease of the ratio k/kL a. In this study, measurement of kL a in the presence of t-butnaol was 1.8 times higher than in the absence of t-butanol. Hence the measured steady-state concentration in the presence of t-butanol approaches the equilibrium concentration. The values of the equilibrium concentrations were then determined from experiments where t-butanol was used. A UV spectrophotometer flow-through-cell was used in this work to measure ozone concentration in the liquid phase using a maximum absorbance wavelength max = 262 nm and an extinction coefficient ε = 3000 L/mol cm. Fig. 1 shows linear relationships between the molar ozone saturation liquid concentrations and the gas concentrations, hence Henry’s law is applicable. Ozone saturation in the 70%ACN:30%H2 O mixture was found higher than that in pure water obtained by experiments carried out in this study or calculated by Roth and Sullivan equation

C. Tizaoui et al. / Chemical Engineering and Processing 50 (2011) 637–643

1

70%ACN:30%H2O with t-butanol 70%ACN:30%H2O without t-butanol

6

0.8

DI water, this study Roth and Sullivan

C/C0 exp

104×CO3-liquid (mol/L)

8

639

4

2

0.6 0.4 0.2

0 0

20

40

60

0 0.0

CO3-gas (g/m 3) NTP

0.2

0.4

0.6

0.8

1.0

C/C0 model

◦

Fig. 1. Ozone saturation concentration in 70%ACN:30%H2 O (T = 20 C, pH 7).

Fig. 2. Comparison between results of the instantaneous chemical reaction model 3 and the experiments (C0 = 100 mg/L, CO3 = 10 g/m NTP, pH 2). in

[27]. Moreover, it was found, as expected, that the presence of t-butanol has slightly increased the saturation concentration of ozone in the 70%ACN:30%H2 O mixture, due to its effect on reducing ozone decomposition and increasing the volumetric mass transfer coefficient. Although, there is no data in the literature on ozone absorption in the mixture 70%ACN:30%H2 O to support the data obtained in this study, it is clear that acetonitrile has increased significantly the ozone saturation concentration as compared to pure water. The volumetric mass transfer coefficient was also determined using the classical instantaneous chemical reaction method by bubbling ozone in a 70%ACN:30%H2 O solution of indigo trisulfonate. The reaction between indigo and ozone is a second order reaction (i.e. 1st order with respect to ozone concentration and 1st order with respect to indigo concentration) with a rate constant, k2 = 9.4 × 107 M−1 s−1 and a stoichiometric ratio, b = 1 [28–30]. To ensure that the reaction between ozone and the indigo trisulfonate develops in the instantaneous kinetic regime, the concentrations of indigo trisulfonate and ozone gas were set so as to satisfy the criterion given in Eq. (1) for instantaneous regime [31].

After variable separation and integration, Eq. (5) is obtained which gives the change of the organic substance concentration as function of time.



CB = CB0 exp

3

Ha >5 Ei

(1)

where NO3 is the ozone absorption rate, CO3 ∗ is the equilibrium ozone concentration, Ha is the Hatta number (Eq. (2)), and Ei is the instantaneous reaction factor (Eq. (3)).

 Ha =

DO3 k2 CB kL

Ei = 1 +

CB DB bCO∗ DO3

(2)

   CB 

rB = −

1 dCB kL aDB Ha = NO3 = kL aCO∗ + CB for >5 3 Ei b dt bDO3

(4)

3

DB

 − exp

CB CB0





1 − exp

kL aDB − t DO3



(5)

2



=0

(6)

calc

  g 1/3   D 2/3 L O3 L

kL = 0.31

where k2 is the second-order reaction rate constant, CB is the concentration of the organic substance, B, (i.e. indigo trisulfonate in this case), kL is the liquid-side mass transfer coefficient, b is the reaction stoichiometric ratio of B relative to O3 , and DB is the diffusivity of the organic substance, B. Under the instantaneous reaction regime, the ozone reaction occurs at a plane on the liquid film and no ozone is transferred to the bulk liquid. With the assumption that all ozone transferred to the liquid phase reacts only with the organic substance, the rate of disappearance of compound B, rB , equals the rate of ozone absorption, NO3 , (Eq. (4)).

−

bCO∗ DO3

where the indices exp and calc mean experimental and calculated respectively. In order to check the validity of the instantaneous regime under the experimental conditions of this study, both Ha and Ei have to be calculated. The diffusivity of indigo trisulfonate was approximated to 8 × 10−10 m2 /s [33]. According to Eqs. (2) and (3), the only unknown parameter needed for calculating Ha and Ei is kL , which was determined by Eq. (7) [34]. A ratio of Ha /Ei equal to 25 was obtained, which clearly shows that the reaction develops indeed in the instantaneous reaction regime.

(3)

3



where CB0 is the initial concentration of the organic substance and t is time. Eq. (5) was used for the determination of the volumetric mass transfer coefficient, kL a, by fittings to the experimental data. The solver tool available in MS Excel was used for data fitting using kL a as parameter and the objective function as given by Eq. (6). Fig. 2 shows good agreement between the experimental results and those calculated by Eq. (5). A value of kL a equal to 0.0026 ± 8% s−1 was obtained in this study, which is well within the range of expected values for ozone mass transfer coefficients [32].

CB0 NO3 = kL aCO∗ Ei for

kL aDB − t DO3

L

L

(7)

where, ␮L is the solution viscosity, g is the gravity acceleration, L is the liquid density. In addition to the instantaneous reaction regime method, oxygen absorption method was also used to determine kL a so a check was made. Oxygen absorption in the mixture 70%ACN:30%H2 O was carried out in the semi-batch reactor. Eq. (8), which was derived from the double-film theory, was used to determine the volumetric mass transfer coefficient for oxygen, (kL a)ox . A plot of the left hand side of Eq. (8) as function of time should lead to a straight line with a slope equal to (kL a)ox . Indeed a linear plot was found and the coefficient of determination, R2 , was 0.9984. Ln

 C∗ − C oxO ox ∗ −C Cox ox

= (kL a)ox t

(8)

C. Tizaoui et al. / Chemical Engineering and Processing 50 (2011) 637–643

1

1

0.8

0.8

0.6

C/C0

C/C0 or CO3/CO3in

640

CO3/CO3in

0.4

0.6

10 g/m3 20 g/m3

0.4

60 g/m3

C/C0

0.2

0.2

0 0

20

40

60

0 0

5

10

15

20

time (min) Fig. 3. TCC degradation with ozone (C0 = 100 mg/L, CO3

3

in

= 60 g/m

DO3

(9)

Dox

where DO3 and Dox are the diffusivities of ozone and oxygen in the mixture respectively taken equal to the diffusivities in water. Although the diffusivities of ozone and oxygen in 70%ACN:30H2 O solution may be different from those in pure water, their ratios in the two liquids should be the same. This is true because at a constant temperature, the ratio of the diffusivities of two compounds in a liquid will not depend on the liquid properties (i.e. the ratio is proportional to only the inverse of the ratio of the compounds’ molar volumes elevated to power 0.6) [35]. Values of DO3 and Dox are in the ranges 1.75–1.82 × 10−9 m2 /s and 1.80–2.03 × 10−9 m2 /s respectively, thus a ratio of kL a to (kL a)ox equal to 0.93 was used in this study. Results obtained by the oxygen method agreed with those obtained by the instantaneous reaction regime with an error in the order of 20%.

Once the values of the volumetric mass transfer coefficient, kL a are determined by experiments and the values of the mass transfer coefficient, kL , are determined by the model presented in Eq. (7), the specific interfacial area was calculated using Eq. (10). A value of a equal to 23 ± 8% m2 /m3 was obtained. a=

kL a kL

140

discussed in the Introduction. Fig. 3 also shows that the ozone gas concentration at the outlet of the reactor evolves to a concentration lower than the inlet gas concentration also after about 10 min. This may occurred as a result of further reactions between ozone and the products formed from TCC oxidation. 3.2.1. Effect of ozone gas concentration on TCC degradation The effect of ozone gas concentration on TCC degradation was studied by bubbling ozone at different gas concentrations into the TCC solution. Fig. 4 shows that the degradation of TCC increases with increasing ozone concentration. Indeed a 90% reduction in TCC concentration was achieved at decreasing times of approximately 70, 40 and 10 min for increased inlet ozone gas concentrations of 10, 20 and 60 g/m3 NTP respectively. Increasing the inlet ozone gas concentration would increase the ozone saturation concentration, thus resulting in better ozone mass transfer to the liquid phase. As a result, the ozone liquid concentration also increases and leads to higher reaction kinetics, which are reflected in the sharper decrease of the TCC concentration shown in Fig. 4 for an ozone inlet gas concentration of 60 g/m3 NTP. 3.2.2. Effect of pH on TCC degradation In order to determine the optimum pH at which the maximum degradation of TCC may occur, experiments were carried out at different controlled pH values of 2, 7, and 9. The temperature was kept constant at 20 ± 1 ◦ C. The TCC concentration of the solution was 100 mg/L and an ozone concentration of 60 g/m3 NTP was used. Fig. 5 depicts the changes of the relative TCC concentration at different pH values. The figure shows that as pH decreased from 7 and 9 to 2, the degradation rate has significantly reduced. This can be explained by the fact that at pH 2, the reactivity of the amino-groups reduces significantly due to protonation [25,36]. In addition, protonation of the amino groups reduces activation for potential electrophilic attack of ozone on the ortho sites of the benzene rings, which leads

(10)

1.0 0.8

Different ozone gas concentrations (10, 20 and 60 g/m3 NTP) were used in this study to degrade a 100 mg/L TCC in 70%ACN:30%H2 O solution. The changes of TCC concentration divided by the initial TCC concentration (C/C0 ) and of ozone gas concentration at the outlet of the reactor divided by the inlet ozone gas concentration (CO3 /CO3 ) are shown in Fig. 3 as function of time.

0.6

Fig. 3 clearly shows that ozone was effective to achieve almost 100% degradation of TCC in about 10 min. It is more likely that the degradation of TCC occurred through molecular ozone (i.e. direct) reaction mechanism due to the scavenging effect of acetonitrile as

C/C0

3.2. TCC ozonation

in

120

NTP,

where (kL a)ox is the volumetric mass transfer coefficient of oxygen in the mixture 70%ACN:30%H2 O, Cox * is the equilibrium oxygen concentration determined from experiment, Cox0 is the initial oxygen concentration and Cox is the concentration of oxygen measured at a time t. The volumetric mass transfer coefficient of ozone in the mixture 70%ACN:30%H2 O, kL a, was then estimated according to the Danckwerts theory, Eq. (9).



100

Fig. 4. Concentration of TCC versus time at various reactor inlet ozone gas concentrations (C0 = 100 mg/L, pH 7).

22.4 mg/min O3 , pH 7).

kL a = (kL a)ox

80

Time (min)

pH = 2 pH = 7 pH = 9

0.4 0.2 0.0 0

5

10

15

20

25

30

time (min) Fig. 5. Effect of pH on TCC degradation (C0 = 100 mg/L, CO3

3

in

= 60 g/m NTP).

C. Tizaoui et al. / Chemical Engineering and Processing 50 (2011) 637–643

to reduction in ozone reactivity. By increasing pH, ozone decomposition increases leading to the formation of high levels of hydroxyl radicals [21,37–39]. However, since the scavenging effect of acetonitrile is significant, it is likely that under the conditions used in this study, no indirect (i.e. radical) reactions took place. Notwithstanding that in the absence of radical scavengers, it is likely that TCC degradation occurs through both molecular and radical reactions. Indeed, in their study on the degradation of triclocarban with an electro-Fenton system, Sires et al. [20] have shown that hydroxyl radicals were responsible for the oxidation of TCC. Fig. 5 also shows that as the pH increased from 7 to 9, a slight decrease in the degradation rate of TCC has occurred possibly due to competitive reactions of ozone decomposition at high pH.

k2

sented by the equation O3 + bTCC−→Products. This reaction was assumed as being a second order reaction, which is generally the case for ozone reactions with organic molecules [40]. The stoichiometric ratio b was determined in homogeneous system by measuring the change of TCC concentration after reaction with a given initial concentration of ozone (Eq. (11)) at pH 7. A value of b = 1 was obtained. This stoichiometric ratio is generally observed for ozone reactions with organic molecules. [TCC]0 − [TCC]f [O3 ]0

(11)

The rate constant of the reaction between ozone and TCC was determined using the fast pseudo-first-order method which is valid when 5Ha < Ei [31]. Once the rate constant was determined, the validity of the regime was checked by calculating the ratio Ei /Ha . Under the conditions of fast pseudo-first order reaction regime, the enhancement factor, E defined as the ratio between mass transfer with and without chemical reaction, can be assumed equal to Hatta number and the absorption rate of ozone can be described by Eq. (12).

3

for

5Ha < Ei

(12) ∗

where NO3 is the ozone absorption rate, CO3 is the equilibrium ozone concentration, Ha is the Hatta number (Eq. (2)), and Ei is the instantaneous reaction factor (Eq. (3)); in Eq. (3), k2 represents now the second-order reaction rate constant of ozone with TCC and the index B represents TCC. The diffusivity of triclocarban, DB , was estimated from Wilke–Chang equation [35] (Eq. (13)). A value of DB equal to 1.20 × 10−9 m2 /s was obtained, which falls within the range of diffusion coefficients for organic molecules. DB = 7.4 × 10−8

log 10(k2(M-1.s-1))

4 3 2 1 0 2

0

4

6

8

10

pH 3

in

= 60 g/m

NTP).

The chemical reaction between ozone and TCC can be repre-

NO3 = kL aCO∗ Ha

5

Fig. 6. Effect of pH on TCC degradation rate constant (C0 = 100 mg/L, CO3

4. Reaction models

b=

641

(s Ms )

0.5

T

(13)

s VB0.6

where ˚s is the association parameter for the solvent, Ms molecular mass of solvent, T is absolute temperature, s is viscosity of the solvent, VB is molar volume of the solute as liquid at its normal boiling point. With the assumption that all ozone transferred to the liquid phase reacts only with TCC and no ozone accumulates in the bulk of the reactor, the rate of disappearance reaction of TCC, rB , equals the rate of ozone absorption, NO3 , (Eq. (14)).

 1 dCB rB = − = NO3 = aCO∗ DO3 k2 CB for 3 b dt

5Ha < Ei

(14)

After variable separation and integration, the following relationship is obtained:



CB0

0.5

− CB

0.5

=

baCO∗ 3

k2 DO3 2

t

(15)

where CB0 is the initial concentration of TCC. A plot of the left hand side of Eq. (15) as function of time should lead to a straight line having a slope, s, given by Eq. (16). Rearrangement of Eq. (16) resulted in Eq. (17), which was used to calculate the rate constant, k2 . Once the value of k2 was obtained, the reaction regime was checked and it was found that the ratio Ei /Ha was at least 7 (i.e. Ei /Ha was always higher than 5), this proves the validity of the model.



s=

k2 DO3

baCO∗ 3

k2 =

1 DO3



2 2s baCO∗

(16)

2 (17) 3

The effects of pH and temperature on the reaction rate constant as determined by the method described earlier were studied and the results are discussed in the following sections. 4.1. Effect of pH on reaction rate constant The rate constant k2 was determined at different pH values and the results are shown in Fig. 6 in the form of log10 (k2 ) versus pH. The maximum rate constant value was approximately 5 × 103 M−1 s−1 observed at pH 7. At pH 2, the value of the rate constant was almost 16 times lower than that at pH 7, possibly due to protonation of the amino groups and deactivation of electrophilic attack of ozone. A slight drop in k2 was also observed at pH 9 indicating that competitive reactions with the main reaction may have taken place. These results indicate that TCC is oxidised rapidly with ozone, with a halflife time of approximately 1.7 s for an ozone concentration of 4 mg/L (i.e. 8.3 × 10−5 mol/L) at neutral pH. The values of the second-order rate constants for the reaction of triclocarban with ozone found in this work agree with the rate constant reported for the ozonation of the neutral form of triclosan, which has similar structure to TCC, (1.3 × 103 M−1 s−1 ) obtained by Suarez et al. [5]. Therefore, ozone can be used as an efficient treatment process for the removal of these substances from water. 4.2. Effect of temperature The effect of temperature on the reaction rate constant was studied at three temperatures (10, 20 and 30 ◦ C). The temperature in the ozonation reactor was controlled using a thermostatic water bath.

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C. Tizaoui et al. / Chemical Engineering and Processing 50 (2011) 637–643

7.5

Ln(k2)

7.2 6.9 6.6 6.3 6.0 3.2

3.3

3.4 103 ×1/T (K-1)

3.5

3.6

Fig. 7. Effect of temperature on the reaction rate constant (C0 = 100 mg/L, CO3

in

3

=

20 g/m NTP, pH 7).

It was found that the degradation rate constant of TCC increased with increasing temperature from 10 to 30 ◦ C. The classical Arrhenius law, which its logarithmic form is given by Eq. (18), was used to describe the variation of k2 with temperature. A plot of ln(k2 ) versus T−1 gave a straight line (Fig. 7), which proves the validity of the Arrhenius function. The values of the frequency factor (A = 2.98 × 108 L/mol s) and the activation energy (Ea = 31.0 kJ/mol) were calculated from the intercept and the slope respectively of the straight line. It was found that increasing the temperature by 10 ◦ C nearly doubled the rate constant, which is in agreement with the so-called van’t Hoff rule. The value of the activation energy (i.e. Ea = 31.0 kJ/mol) obtained for the oxidation of TCC with ozone lies well within the range of expected values of activation energies for ozone reactions with organics as reported in the literature [25,41,42]. Ln(k2 ) = Ln(A) −

Ea 1 × R T

(18)

where A is frequency factor, Ea is activation energy, R is the ideal gas law constant, and T is temperature in K. 5. Conclusions The results from the present laboratory experimental work suggests that triclocarban can be easily degraded with ozone. Second-order reaction was suitable to describe the kinetics of TCC degradation with ozone. The effect of ozone gas concentration, pH and temperature on the reaction rate constant was also studied. The results showed that the oxidation rates of TCC with ozone increased significantly by increasing pH, temperature and ozone gas concentrations. The degradation rate increased by about 16 times at pH 7 as compared to that at pH 2. The reaction stoichiometry was obtained equal to one mole of ozone per one mole of TCC. The effect of temperature on the oxidation of TCC with ozone was described by the Arrhenius equation and the activation energy was obtained equal to 31.0 kJ/mol. The results obtained in this study suggest that ozonation can be used as an effective treatment option to remove triclocarban from water. References [1] M. Carballa, F. Omil, J.M. Lema, M. Llompart, C. García-Jares, I. Rodríguez, M. Gómez, T. Ternes, Behavior of pharmaceuticals, cosmetics and hormones in a sewage treatment plant, Water Res. 38 (2004) 2918–2926. [2] M.C Dodd, H.P.E. Kohler, U. Von Gunten, Oxidation of antibacterial compounds by ozone and hydroxyl radical: elimination of biological activity during aqueous ozonation processes, Environ. Sci. Technol. 43 (2009) 2498–2504. [3] J.A. Giroto, A.C.S.C. Teixeira, C.A.O. Nascimento, R. Guardani, Photo-Fenton removal of water-soluble polymers, Chem. Eng. Process. 47 (2008) 2361–2369. [4] K. Ikehata, N.J. Naghashkar, M.G. El-Din, Degradation of aqueous pharmaceuticals by ozonation and advanced oxidation processes: a review, Ozone-Sci. Eng. 28 (2006) 353–414.

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