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Kinetics and Mechanism of the Oxidation of Amaranth with Hypochlorite S. Nadupalli, N. Koorbanally, and S. B. Jonnalagadda* School of Chemistry, University of KwaZulu-Natal, Westville Campus, Chiltern Hills, P Bag X54001, Durban 4000, South Africa
bS Supporting Information ABSTRACT: The reaction mechanism of the oxidation of Amaranth dye (2-hydroxy-1-(4-sulfonato-1-naphthylazo) naphthalene-3,6-disulfonate) with hypochlorite under varied pH conditions was elucidated by a kinetic approach. Under excess concentration of oxidant, the reaction followed pseudofirst-order kinetics with respect to Amaranth, and the oxidation was found to occur through two competitive reactions, initiated by hypochlorite and hypochlorous acid. The reaction order with respect to both OCl ion and HOCl was unity. While the latter reaction was fast, the significance of the oxidation paths depended on the relative concentration of the two oxidizing species, which was dictated by the reaction pH. The role of the H+ ion in the reaction was established. For the hypochlorite ion and hypochlorous acid facilitated reactions, the second-order rate coefficients were 1.9 and 23.2 M1 s1, respectively. The energy parameters were Ea = 33.7 kJ mol1, ΔH‡ = 31.2 kJ mol1 and ΔS‡ = 190.6 J K1 mol1 for the OCl ion-driven oxidation, and Ea = 26.9 kJ mol1, ΔH‡ = 24.3 kJ mol1 and ΔS‡ = 222.8 J K1 mol1 for the reaction with HOCl-initiated oxidation. The major oxidation products for both the pathways were 3,4-dihydroxy naphthalene-2,7-disulfonic sodium salt (P1), dichloro-1,4-naphthoquione (P2) and naphtha(2,3)oxirene-2, 3-dione (P3). On the basis of the primary salt effect and other kinetic data, the rate law for the overall reaction and probable reaction mechanism was elucidated. The proposed mechanism was validated by simulations using Simkine-2.
’ INTRODUCTION With ever escalating population and anthropogenic activities, the demand for clean water is increasing incessant, and its availability is becoming increasingly scarce.1 The textile dyeing industry utilizes and pollutes huge quantities of water.2,3 The treatment of colored dye effluent streams has attracted the attention of environmentalists, technologists, and entrepreneurs because of its socio-economic and political dimensions.4,5 The nature of the pollution that accompanies the dyeing industry is primarily due to the nonbiodegradable nature of the dyes along with the strong presence of toxic metals and carcinogenic aromatics traceable in the effluents.6,7 More than 2000 azo dyes are known to exist,8 and over half of commercial dyestuffs are azo dyes, with some of the azo precursors and their degraded products (such as aromatic amines) being carcinogens.9,10 The azo dyes are broadly used in the textile, color solvent, ink, paint, varnish, paper, plastic, food, drug, and cosmetic industries.11 Amaranth (AM), used as dye in the textile industry, is a suspected carcinogen,12,13 and is an azo compound commonly referred to as Red No. 2, Food Red 9, Acid Red 27, and Azorubin S. It is a water-soluble dye, with a sharp absorption peak in the visible region (λmax = 620 nm, ε = 2.1 104 dm3 mol1 cm1),14 and it shows no shift due to pH change. The kinetics and reactivity of various azo dyes with different oxidants have been reported in the literature.15,16 Many organic substances can be mineralized with prolonged oxidation. During photocatalytic degradation using TiO2-mediated photo degradation of Amaranth dye, Karkmaz et al. suggested that the formation of lactic and formic acids at the beginning of irradiation-time is indicative of a fast and easy naphthalenic ring breaking.17 r 2011 American Chemical Society
Ghodake et al. reported the biological treatment of Amaranth and naphthalene sulfamide, hydroxyl naphthalene diazonium, and naphthalene diazonium salt as degradation products.18 Wu et al. investigated the rate of decolorization of Amaranth with UV/TiO2 and with O3 reported that lower pH favored the photo oxidation, while high pH favored the ozone-initiated oxidation.19 Sodium hypochlorite is a strong oxidizing agent, widely used in water treatment due to its cost effectiveness, less hazardous nature, and scope for easy transport. It is effective in the treatment of microorganisms20 and infectious deceases,21 as a household bleach,22 sewage treatment,23 oral treatment,24 and in endontonic irrigation. Understanding the reactivity and mechanism of hypochlorite under varied conditions is paramount for its usage for various purposes. The kinetics and reaction mechanism of the decolorization of Amaranth dye by hypochlorite in an aqueous solution was investigated in detail as a function of pH and other parameters that influence its reactivity. The kinetic investigations were supported by product characterization immediately after the decolorization and estimation of energy parameters. Plausible rate laws and reaction mechanisms for the reactions were elucidated.
’ EXPERIMENTAL SECTION Reagents. All chemicals were of Analar grade, and all the solutions were prepared in double distilled water. Amaranth Received: March 25, 2011 Revised: June 3, 2011 Published: June 07, 2011 7948
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The Journal of Physical Chemistry A (Aldrich), sodium hydroxide (BDH), 98% sulfuric acid (BDH), and anhydrous sodium sulfate (Univar) were used as supplied. The hypochlorite solutions were prepared by the procedure described by Nagypal et al.25 and standardized by an iodometric method. Kinetic Measurements. All the experimental runs were conducted with low concentration of dye and excess concentrations of the other reagents. Unless otherwise stated, all the experiments were carried out at (25 ( 0.1) °C. The rate of depletion of AM was monitored at 520 nm corresponding to the absorption maximum of the dye. At 520 nm, no interferences from the products or intermediates were observed. The reaction was monitored using the Hi-Tech SF-61 DX2 Double mixing micro volume stopped-flow equipment. The kinetic data acquired at single wavelength was analyzed using the KinetAsyst 3.10 software, which allows the matching of experimental results with different rate equations and to estimate the rate constants by choosing appropriate integrated rate equations.26 A HACH EC40 pH/ISE meter, with digital data output, was used to record the pH/potential changes.
’ RESULTS AND DISCUSSION Product Analysis and Stoichiometry. A mixture of Amaranth dye (500 mg) and a 100-fold molar excess of oxidant and with requisite amount of acid was continuously stirred using a magnetic stirrer at room temperature. Immediately after the decolorization of the dye, the reaction products from the aqueous reaction mixtures were extracted using diethyl ether as a solvent (3 200 mL). The solvent was dried over anhydrous calcium carbonate and evaporated to dryness. The crude extract was purified using silica gel column with hexane and ethylacetate (85:15) as eluent. The isolated products were characterized by gas chromatographymass spectrometry (GC-MS) and by NMR (1H and 13C), where possible. The major product was positively identified as 2-keto-hydroxynapthalene-3,6-disulfonic sodium salt (P1). Its 1H NMR spectra (Supporting Information, Figures S1 and S2) exhibited four aromatic protons as multiplets at δ7.67.8 and 7.98.0. Its identity was further confirmed by 13C NMR with the presence of the carbonyl carbon at δ166.0 and aromatic carbon resonances between δ100144. On the basis of the GC-MS data, two other major products had molecular masses m/z 227 and 174 and were in good agreement with molecular formulas C10H4Cl2O2 and C10H6O3, respectively. The fragmentation patterns of P2 (Supporting Information, Figure S3) and P3 (Supporting Information, Figure S4) had 99% match (the NIST library) with that of naptha(2-3)-oxirene-2-7-dione (P2) and dichloro-1,4napthaquinone (P3). Amaranth is a suspected carcinogen12,13 and is reported to have an LD50 of 1000 mg kg1 in rat and mouse models in MSDS data sheets,27 but no reported carcinogenicity to humans to date. Its oxidative degradation products identified in the current study, P1 and P2, are reported to be irritants on the skin and nontoxic to humans. However, P2 has a reported LD50 of 1300 mg kg1 in rat models, which shows that it is less toxic than Amaranth.27 The stoichiometry of the reaction was investigated using 1:1 and 1:5 molar ratios of Amaranth and oxidant and appropriate amounts of acid. The amounts of dye and hypochlorite reacted were estimated from the initial and residual amounts after the reaction. The stoichiometry was established to be about 1:3 ((10%) of AM to oxidant. Thus, taking into consideration the
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oxidation products, the stoichiometric equation for the overall reaction is proposed as AM + 3HOCl + 2H2 O f P1 + P2 + SO3 2 + Na+ + N2 + H+ + 3HCl where P1 = 2-keto-hydroxynaphthalene-3,6-disulfonic sodium salt, P2 = naphtha(2,3)oxirene-2,3-dione, and P3 = dichloro-1,4naphthoquinone.
Reaction Kinetics. Figure 1 shows the repetitive spectra illustrating the depletion of the dye as a function of time for a typical reaction with [AM] = 7.0 105 M, [OCl]t = 1.5 103 M, and pH = 9.0. All the experimental runs were conducted with low concentration of dye and excess concentration of the other reagents. To establish the reaction order with respect to the oxidant, a series of experiments were conducted with different initial concentrations of hypochlorite at a fixed initial pH 9.0 and ionic strength (0.128 M). Figure 2 shows the typical exponential depletion curves of Amaranth, with [AM]0 = 7.0 105 M at varied [OCl]0 conditions. The ln absorbance versus time plots were straight lines confirming the order with respect to dye is 1. The pseudo-first-order rate constant, k0 values obtained by analyzing their respective kinetic curves for different initial hypochlorite concentrations are summarized in Table 1. The plot of ln k0 versus ln[OCl] was a straight line with a slope equal to 1.10 (R2 = 0.99), suggesting that at pH 9.0, the order with respect to hypochlorite ion is 1. (a). Effect of pH on the Reaction Rate. The effect of added acid on the reaction rate was probed by the addition of varied amounts of sulfuric acid solution and maintaining the total ionic strength constant. The initial pH values were recorded. The kinetic data obtained for different experiments was analyzed using “KinetAsyst Fit Asystant”. The k0 values obtained at different pH values are illustrated in Figure 3. A perusal of Figure 3 shows that the increase in k0 was small at low [H+] and high with high [H+]. The plot of ln k0 versus ln [H+] gave a slope of 0.27 (R2 = 0.96). The observed partial reaction order with respect to acid clearly suggests that acid is not involved in the rate-limiting step. This behavior can be well explained in terms the role of acid in protonation of OCl to HOCl, and the resulting dynamic equilibrium between the two species, i.e., OCl + H+ h HOCl, with pKa = 7.4.28 With the presence of the two oxidizing species, the dye oxidation occurs simultaneously through its reaction with OCl and HOCl.29 The observed increase in k0 values with an increase in acid concentration indicates that the rate of oxidation by HOCl is much faster than with OCl. Of the chlorine-containing disinfecting agents, hypochlorous acid has the higher oxidation potential (Eo = 1.44 V). On the basis of that assumption, the kinetic curves were analyzed by using the equation {y = A exp (R1 x) + A exp(R2 x) + C} for the occurrence of two competitive first-order reactions. The simulated curve using the equation fitted quite well, with negligible residuals (Figure 4). For different [H+] conditions, the pseudo-first-order rate constants obtained are summarized in Table 2, where k10 represents the pseudo-first-order rate constant for the OCl-initiated oxidation, and k20 represents the corresponding value for the reaction by HOCl. Table 2 also includes the calculated equilibrium concentrations for H+, OCl, and HOCl for initial conditions at different pH values. 7949
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The Journal of Physical Chemistry A
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Figure 1. Typical depletion profile of Amaranth-repetitive spectra. Reaction conditions: [AM]0 (7.0 105 M) with [OCl]t (1.5 103 M) at pH = 9.0 and 520 nm. (4 scans/min).
Table 1. Reaction between Amaranth and Hypochlorite at Constant Ionic Strength [AM]0 (7.0 105 M), [OCl]t (0.85 104 5.1 103 M), pH = 9.0, and Ionic Strength (I = 0.128 M)
a
[OCl]t /103 M
k0 /s1a
0.50 1.70 2.55 3.40 5.10
0.041 0.093 0.145 0.175 0.322
Mean of four replicate experiments with relative standard deviation 0.97). At low pH conditions, [OCl]eq will be negligible, and the pseudo-first-order conditions do not prevail under those conditions. These results confirm that acid is not directly involved in the rate limiting reaction, but it influences the equilibrium between OCl and HOCl. An increase in [H+] at pH < 3 had a marginal effect on the rate (Table 2). Interestingly, under these conditions, all the hypochlorite exists in HOCl form, thus further increase in [H+] will have no effect on the reaction rate. (b). Kinetic Salt Effect at pH 9.0 and 4.0. The kinetic salt effect provides insight into the nature of the reacting species involved in the rate-limiting step. The reaction between hypochlorite and AM was investigated as a function of ionic strength (I), between 0.01 and 0.04 M with fixed initial concentrations of hypochlorite
Figure 3. Plot of ln k0 versus ln[H+] for the reaction of [AM]0 (7.0 105 M) with [OCl]t (1.45 103 M), [H+]eq (1.99 109 7.752 104 M).
and Amaranth. At pH 9.0, approximately 85% of oxidant exists in the form of hypochlorite and 15% as HOCl. Hence, the kinetic curves were analyzed for occurrence of two consecutive reactions, and the results obtained are summarized in Table 3. For the reaction between Amaranth and [OCl], the plot of log k0 versus I1/2 gave a 7950
dx.doi.org/10.1021/jp202812f |J. Phys. Chem. A 2011, 115, 7948–7954
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linear curve with a positive slope of 1.22 (R2 = 0.98), indicating that at pH 9.0 the rate-limiting step involves species of like charges, possibly AM and OCl ions. For the reaction of AM and HOCl, the slope was less than one (0.79 and R2 > 0.97). Under acidic conditions (pH 4.0), 99% of the oxidant will be in the form of the neutral polar species, HOCl. Theoretically, there should be no kinetic salt effect, but a linear relationship between the rate constant and ionic strength can be anticipated. Confirming the relationship, the plot of the
k0 versus I data gave a linear curve with a small gradient (slope = 0.13 and R2 = 0.95). Thus, under acidic pH, the rate-limiting step possibly involves AM and neutral species HOCl. (c). Activation Parameters. The activation parameters provide valuable information about the nature of the transition state and the reaction mechanism. A huge enthalpy of activation (ΔH‡) indicates that a large amount of stretching or breaking of chemical bonds is necessary for the formation of the transition state. The entropy of activation gives a measure of the inherent probability of the transition state, apart from energetic considerations.30,31 The energy parameters for the dye with HOCl and OCl reactions were studied by measuring the rate constants over the temperature range 1535 °C by using the Arrhenius and Eyring’s equations.32,33 Table 4 summarizes the calculated values of the energy parameters, namely, the energies of activation, enthalpy, and entropy for both the reactions. The HOCl reaction had a slightly lower energy of activation (26.87 ( 0.09 kJ mol1) compared to (33.65 ( 0.09 kJ mol1) for the OCl ion initiated reaction. The ΔH‡ value at 25 °C was found to be 31.2 kJ mol1 for the OCl ion initiated reaction and 24.3 kJ mol1 for the reaction with HOCl. The entropy of activation values for OCl and HOCl initiated oxidations were 190.6 J K1 mol1 and 222.8 J K1 mol1, respectively. The large negative entropies obtained suggest that the transition state requires the reacting molecules to orient into conformations and approach each other at a precise configuration,34 which confirms the formation of a compact activated complex. Rate Laws. The first-order dependence of the reaction rate on the reactants and the observed salt effect at pH 9.0 suggests that the rate limiting step involves one each of AM and OCl ions. Thus, the major pathways of the reaction involve HOCl or OCl possibly forming an activated complex, which undergoes Table 3. Effect of Ionic Strength on the Reaction Rate [OCl]t (1.45 103 M), [AM]0 (7.0 105 M), pH = 9.0
Figure 4. KinetAsyst double-exponential equation fit of two curves and residuals (lower sketch) for the two competitive first-order reactions for the reaction of [AM]0 (7.0 105 M) with [OCl]t (1.45 103 M), [H+]0 (9.96 109 M), and I (0.12 M). Function: 2 exp + C, y = A exp(R1 x) + A exp(R2 x) + C, rate constant 1 = 0.398 and rate constant 2 = 0.0472.
a
ionic strength/M
k10 /s1a
k20 /s1
0.0092 0.0167 0.0242 0.0317 0.0392
0.043 0.050 0.052 0.054 0.059
0.047 0.048 0.048 0.050 0.051
Mean of four replicate experiments with relative standard deviation
