Scholarly article on topic 'Temperature and pH effects on the kinetics of 2-aminophenol auto-oxidation in aqueous solution'

Temperature and pH effects on the kinetics of 2-aminophenol auto-oxidation in aqueous solution Academic research paper on "Chemical sciences"

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Academic research paper on topic "Temperature and pH effects on the kinetics of 2-aminophenol auto-oxidation in aqueous solution"

Central European Science Journals

Central European Journal of Chemistry

www.cesj.com

CEJC 3 (2003) 233-241

Temperature and pH Effects on the Kinetics of 2-Aminophenol Auto-oxidation in Aqueous Solution

Abstract: The kinetics of the auto-oxidation of 2-aminophenol (OAP) to 2-amino-phenoxazin-3-one (APX) was followed in air-saturated aqueous solutions and the influence of temperature and pH on the auto-oxidation rate was studied. The kinetic analysis was based on a spectrophotometric method following the increase of the absorbance of APX. The process follows first order kinetics according to the rate law - d[OAP]/dt = k'[OAP]. The experimental data, within the pH range 4-9.85, were analyzed using both differential and incremental methods. The temperature variation of the overall rate constant was studied at pH = 9.85 within the range 25-50° C and the corresponding activation energy was evaluated. © Central European Science Journals. All rights reserved.

Keywords: auto-oxidation, aminophenol, phenoxazinone

1 Introduction

The oxidation of aqueous organic pollutants is an attractive removal method because of its high e±ciency and simplicity [1,2]. The oxidation products are usually low molecular weight oxygenated compounds that are easily biodegradable. Several new technologies based on advanced oxidation processes (AOP) were used in order to convert the pollutants into harmless chemicals [3,4]. Four major approaches to AOP are under development at present:

a) homogeneous UV photolysis of H2O2 or O3 used to generate 'OH and other radicals;

b) dark homogeneous oxidation involving the use of Fenton reaction [5], ozone at high pH [6], and ozone or peroxide [7]; c) heterogeneous UV photolysis on TiO2 [8] when solid

* E-mail: doan@gw-chimie.math.unibuc.ro t E-mail: mpuiu@gw-chimie.math.unibuc.ro

Dumitru Oancea*, Mihaela Puiuy

Department of Physical Chemistry, University of Bucharest, 4-12 bd. Elisabeta, sector 3, 70346, Bucharest, Romania

Received 7 March 2003; accepted 23 May 2003

particles of the semiconductor TiO2 absorb UV light and generate *OH and other radicals on the surface of the particles; d) radiolysis [9] using a source of high-energy radiation, when *OH, H*, hydrated electrons and other radicals are generated during the radiolysis of water.

Wet air oxidation - WAO [10] - is also a convenient and ecologically safe technology to treat aqueous waste streams containing organic and inorganic pollutants. WAO involves the liquid phase oxidation of organics at elevated temperatures (125-320 ° C) and pressures (0.5 - 20 MPa) using a gaseous source of dioxygen (usually air). Enhanced solubility of dioxygen in aqueous solutions at elevated pressures provides a stronger driving force for oxidation. The organic compounds are oxidized to CO2 and other innocuous end products.

Recent research work proved that APX is an important antineoplastic agent called questiomycin A [11]. This is one of the reaction products of OAP oxidation, a pollutant that is often found in wastewaters. OAP is a toxic compound, and is only partly biodegradable. There is therefore a justified need to develop effective methods for the degradation of OAP to less harmful compounds such as APX. The cost of OAP oxidation may be lowered if conventional oxidants are replaced by dioxygen from air, at ambient temperature and pressure.

It was observed that OAP undergoes an oxidation reaction leading to the formation of the corresponding o-quinone imine [12]. The final reaction product is not the o-quinone imine itself since it is highly reactive, but APX.

This reaction can also occur in the presence of different oxidizing agents such as manganese dioxide [13], potassium ferricyanide or dichromate [14]. Some metalloproteins like hemoglobin, tyrosinase [15] and catalase were found to be efficient catalysts. A kinetic model involving the so-called OAP "oxidative cascade" was also suggested and discussed [16,17,18]. According to this model, APX formation was preceded by the appearance of some active intermediates resulting in successive oxidation, conjugate addition, and tautomerisation reactions. The global equation for OAP oxidation is shown in Fig. 1 [19]:

Fig. 1 Overall process of 2-aminophenoxazinone formation

2 Experimental

Materials: OAP was obtained from Aldrich Chemical Co. and APX was prepared by the oxidation of 2-aminophenol with mercury oxide and recrystallization from ethanol, according to literature methods [20]. UV-VIS spectra were measured on a JascoW530

spectrophotometer equipped with kinetic software. The reaction mixture was also analyzed on a HPLC Shimadzu 10 AVP system using Shim Pack VP-ODS 150 x 4.6. Oxygen and Argon from Linde were 99.9% pure.

The oxidation was followed in an aqueous medium in a bubbling stirred tank reactor, using dioxygen from air as oxidizing agent and OAP as substrate in 0.9-5mM solutions. These concentrations were used due to the lower solubility in water of the substrate and the main reaction product. The air stream was passed through two thermostated saturation vessels containing water in order to avoid the evaporation of the solvent from the reactor. Under these conditions the solution was saturated with air. All experiments were performed at a constant total pressure of 100 kPa. The kinetic assays were run at 25, 30, 40, 50 and 58°C in a 0.2 M carbonate-bicarbonate buffer at pH 9.85, and at 30°C at pH 4.00, 6.00, 7.00, 7.45, 8.71, 9.37, 9.65, and 9.85. The other buffers used were as follows: high pH range (8.71-9.85) 0.2 M carbonate-bicarbonate; mid pH range (6.007.45) 0.01 M monosodium phosphate-disodium phosphate and low pH (4.00) 0.2 M acetic acid-sodium acetate. Since the reaction was very slow, the transformation was monitored for conversions ranging from 8% to 20%, depending on solution pH and temperature. To estimate the reaction order with respect to substrate, the initial rate method was used for substrate concentration ranging from 0.9 to 5 mM. In order to estimate the reaction order with respect to dioxygen, different air-oxygen mixtures (with molar fraction of oxygen equal to 0.21, 0.36, 0.68, 0.84 and 1) were used. The measurements were carried out at 25 ° C and a pH of 7.45. The stability of OAP in the absence of oxygen was verified by passing an argon stream through the reaction mixture, when no significant transformation of OAP was observed. APX formation was monitored at 434 nm (" = 23200 cm_1-M_1 for APX). The variation of molar absorption coe±cient with pH was verified by comparing the absorbance at 434 nm of 1:1 diluted 0.018 mM APX samples with distilled water and buffer solutions (pH 4.00, 5.00, 7.00, 8.10, 9.00, 9.85). Within the 4.00-7.00 pH range, the molar absorption coe±cient " was practically constant. At higher pH values (8.1-9.85) an increase of " was observed. The differences between measured absorbencies of samples diluted with distilled water and those diluted with buffer solutions were within 5% -10 % for this range.

3 Results

The chemical transformation of the substrate was studied by product (APX) analysis. For each experiment, 14 samples were removed from the reactor at different time intervals and the spectra of the reaction mixture were scanned between 200 and 800 nm. The formation of the product can be followed at 434 nm, where the substrate has no absorption (Fig. 2).

The HPLC analysis of the reaction mixture indicated the formation of APX as the main product and proved the appearance of a reaction product absorbing at A=340 nm, accumulated in an insignificant amount with respect to OAP and APX in the pH range 4.00-9.00. Until now we were not able to identify, isolate and characterize this

Fig. 2 UV/VIS spectra of the reaction mixture. Conditions: 0.01 M phosphate buffer, pH 6.00, 0.00445 M substrate at 30°C

compound. Its existence was previously reported in the literature [16]. At pH 9.85 and at 40 - 50?C, the absorbance of APX, after having reached a maximum value, undergoes a fast decrease and in a relatively short time the characteristic absorption band disappears, being replaced by a new one, absorbing at Xmax = 340 nm (Fig. 3).

Fig. 3 UV/VIS spectra of the reaction mixture. Conditions: 0.2 M carbonate-bicarbonate buffer, pH 9.85, 0.00445 M substrate at 40°C (1:10 diluted samples)

4 Discussion and Conclusions

A simple rate law for the oxidation reaction is given by:

1 d[OAP] d[APX]

= k ■ [OAP]m ■ [O2]r

2 dt dt

This equation can be simplified to Eq. (2) under degenerated order conditions (constant O2 concentration):

d[OAP ] d[APX ] = 2 •

= k' ■ [OAPf

dt dt with k = 2 • k • [O2]n (3)

The concentration of OAP was calculated from the mass balance using " = 23200 cm_1-M-1 for APX. From the linear dependencies of initial transformation rates (conversion of substrate 0.5-1%) on initial concentrations, the reaction order with respect to OAP was found to be one, with a correlation coefficient r = 0.9991(Fig. 4), for a significance level a = 0.05.

Fig. 4 Plot of reaction rate of OAP vs. its initial concentration (pH = 9.85, T = 25°C)

The experimental data, OAP concentration versus time at constant oxygen concentration, were used to fit different kinetic equations. The best fit was found for the exponential function:

c = c0 • exp(—k't) (3)

corresponding to a pseudo-first order process, which validates the first order reaction with respect to substrate concentration. The rate constant can be determined using the linear form of Eq.(3): lnc = ln c0 — k't which yields biased parameter estimates. A nonlinear regression was subsequently performed to improve the estimates [21, 22]. The kk values for different conditions, resulting from the integral kinetic analysis (dependent on " value), were practically the same with those calculated using both incremental [23] and differential [24] methods (independent of " value). In Table I the values of k' and the

corresponding statistical parameters r and Fstatistic are summarized (for a significance level a= 0.05). An increase of the auto-oxidation rate at higher pH values was observed.

pH 105 k'(s"1) r F statistic

7.0 0.11 § 0.00 0.9995 16499

8.7 1.70 § 0.16 0.9908 377

9.65 14.16 § 0.16 0.9983 3114

9.85 21.51 §1.68 0.9888 3188

Table 1 Variation of pseudo-first rate constant versus pH at 30°C According to similar models reported in the literature [25], the function:

k = A + B ■ [H + (4)

was fit to the experimental rate constant-[H+] data. Assuming that in aqueous solution dioxygen reacts with both OAP molecules and aminophenolate ions (similar to auto-oxidation of other phenols), an increase of the auto-oxidation rate at higher pH is expected. The fitting parameters were r = 0.9977, A = (1.97±2.30)-10"6s-1 and B = (3.04 § 0.05)-10-14mol-dm-3-s_1(for a significance level a = 0.05), as compared to A = 3.8T0_12s_1 and B = 5.8-10-11 mol-dm_3-s-1 reported for phenol auto-oxidation at 20 ° C [25], indicating a lower pH sensitivity of the auto-oxidation rate constant of OAP.

The reaction order with respect to dioxygen concentration was estimated from the plot of kk versus initial concentration of dissolved O2. The concentration of dissolved oxygen was calculated from the mole fraction solubility of oxygen from air in water [26], using the molar fraction of oxygen in air and air-oxygen mixtures (see Table II).

107 k'(s"1) X*O2 105CO2mol ■ dm"3

3.58§0.05 0.21 1.27

8.07§0.11 0.36 2.29

12.60§0.46 0.68 4.36

17.07§0.23 0.84 5.34

21.58§1.04 1.00 6.36

Table 2 Variation of pseudo-first order rate constant vs. initial concentration of O2 at pH 7.45 and 25 ° C

* Molar fraction of O 2 in input gas

The equation kk = 2k■ [O2]g was fit to the experimental data. The non-linear regression gave the reaction order n = 1.09§0.10 with r = 0.9949 and F = 400 (for a = 0.05), exceeding by far the critical value of F = 19.35. Similar values were obtained by using

the method of initial reaction rates, from the linear plot of reaction rate versus initial concentration of dioxygen.

The temperature variation of the reaction rate was analyzed, taking into account the solubility of dioxygen from air in water at different temperatures [26] and the first order dependence with respect to oxygen. The corrected rate constants were calculated using the pseudo-first order rate constants and the solubility of dissolved oxygen. The values of the rate constants are summarized in Table III (for a partial pressure of oxygen in air equal to 20.26 kPa). From these results, the overall activation energy of the oxidation process was estimated using the Arrhenius equation. A nonlinear regression analysis gave Ea = 56.82 § 3.17 kJ/mol and A = (1.15 § 1.43)-106 s"1 with a correlation coefficient r = 0.9974. No similar values of the activation energies were available in the literature for auto-oxidation of phenols.

T (K) 105 [O2] mol ■ dm"3 [O2 ]t [O2 ]2qs K 105ke Xp (s"1) 10 kcorr(s )

298.15 1.270 1.000 14.97§ 0.07 14.97 § 0.07

303.15 1.200 0.945 22.76 § 1.78 21.51 § 1.68

313.15 1.039 0.818 46.88§ 2.45 38.35 § 2.00

323.15 0.941 0.741 94.02 § 28.68 69.66 § 15.3

331.15 0.880 0.693 182.78 § 55.32 126.66§ 38.33

Table 3 Variation of pseudo-first rate constant with temperature at pH 9.85 The overall kinetic equation can be consequently written as:

- dd^ = 2k • [OAP] [O2] (5)

The possible mechanisms reported in the literature for the catalytic and enzymatic oxidation of OAP [17] take into account two pathways:

One pathway involves 2-electron oxidation of OAP to the highly reactive o—quinone imine, followed by several conjugate addition, 2-electron oxidation and tautomerisation reactions to form APX. In support of this pathway it has been previously demonstrated that chemically prepared quinone imine reacts with OAP to form APX [27] and that quinone imine is an intermediate in the oxidation of OAP by tyrosinase [15].

Another pathway considers 1 electron oxidation of OAP to a phenoxy radical. The OAP radicals undergo disproportionation to the o-quinone imine [19]. APX formation can then follow the first pathway. Equation (5) suggests that molecular oxygen and OAP are involved in the limiting reaction step and the phenoxy radical formation is a possible step for this auto-oxidation.

The oxidation of OAP to APX is a natural process, which occurs very slowly in air-saturated solutions, having an estimated half time of about 23 days under normal conditions (neutral pH, 25-30° C) and converts OAP from wastewaters into a less toxic product without consumption of any other oxidizing reagents.

Acknowledgements

The financial support of this study by CNCSIS grant no. 192/2001 is gratefully acknowledged.

We also thank Lucian Rotariu (Laboratory of Monitoring and Quality Control from Faculty of Chemistry, Bucharest University) for running HPLC analysis of OAP, APX and for helpful advice. The equipment was a donation from Japan International Cooperation Agency - JICA, Technical Cooperation.

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