Os(VIII) as an Efficient Homogeneous Catalyst for ... - ACS Publications

Mar 1, 2010 - Department of Post-Graduate Studies in Chemistry, Bangalore ... Bangalore-560 001, India, and Department of Apparel Technology and ...
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Ind. Eng. Chem. Res. 2010, 49, 3137–3145

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Os(VIII) as an Efficient Homogeneous Catalyst for the Oxidative Decolorization of Methylene Blue Dye with Alkaline Chloramine-T: Kinetic, Mechanistic, and Platinum Metal Ions Reactivity Studies K. N. Vinod,† Puttaswamy,*,† and K. N. Ninge Gowda‡ Department of Post-Graduate Studies in Chemistry, Bangalore UniVersity, Central College Campus, Bangalore-560 001, India, and Department of Apparel Technology and Management, Central College Campus, Bangalore UniVersity, Bangalore-560 001, India

One of the most important thiazine dyes is methylene blue (MB), which is extensively used in the dyeing industry. It also facilitates a number of biological applications. Methylene blue dye is not significantly hazardous, but it can lead to various harmful effects. Among the physicochemical processes developed to remove this dye from wastewater textile and dyestuff manufacturing industries, the oxidative decolorization method alone seems to ensure the advantage of a low cost, simple, and efficient process. So we investigated a detailed oxidative decolorization kinetic study of MB by sodium-N-chloro-p-toluenesulfonamide (chloramine-T or CAT) in alkaline medium catalyzed by Os(VIII) spectrophotometrically at 664 nm (λmax of the dye). The reaction rate showed a first-order dependence each on [CAT]o and [MB]o, a fractional-order dependence on [Os(VIII)], and an inverse-fractional-order dependence on [NaOH]. A decrease of the dielectric constant of the medium by the addition of methanol decreased the rate. The solvent isotope effect k′ (H2O)/k′ (D2O) was equal to 1.37. Activation parameters were computed. The kinetics of oxidation of MB by CAT was also studied with other platinum metal ions. The relative reactivity of these catalysts are in the order Os(VIII) > Ru(III) > Ir(III) g Rh(III) g Pt(IV) > Pd(II). This trend may be attributed to the different d-electronic configurations of the metal ions. It was found that the catalyzed reactions are about 3-fold to 10-fold faster than the uncatalyzed reactions. The mechanism proposed and the derived rate law are consistent with the observed kinetics. This simple and economic redox system can also be adopted for removing the MB dye present in industrial wastewater. 1. Introduction Methylene blue (MB), chemically known as 3,7-bis(dimethylamino)-phenazathionium chloride, is an important member of the thiazine class of dyes. It is extensively used as a colorant for paper, cotton, silk, and leather.1 The dye MB is also used as a stain, sensitizer, and sensor, and also facilitates a number of biological applications.2 Methylene blue though not regarded as actually toxic, can lead to various harmful effects.3 Therefore, removing MB dye from wastewater is essential. Oxidative decolorization is a simple and an economically feasible method for removal of this dye from wastewater.4 Several oxidative processes to minimize concentrations of this dye in wastewater have been reported in the literature, and the main oxidizing agent used in these processes is H2O2. This reagent requires stimulation by some activators. Banat et al5 investigated the oxidation of MB with H2O2 by UV radiation. Fenton’s reagent6 was also used in the oxidation of MB by H2O2. One major disadvantage of these methods is the formation of sludge, which results in a high disposal cost. Oxidation of MB by H2O2 in the presence of lignin was studied by Kling and Neto,7 but the disadvantage of this method is the cost of the enzyme. In the ozonation process,8 the major drawbacks are cost and the short half-time of the process. In the present research we have developed an oxidative decolorization process that is both simple and costeffective. The sodium salts of N-haloarylsulfonamides, generally known as organic N-haloamines, have attracted much attention in the * To whom correspondence should be addressed: Tel.: +91-8022961340. Fax: 91-80-22961335. E-mail: [email protected]. † Department of Post-Graduate Studies in Chemistry. ‡ Department of Apparel Technology and Management.

past few years because of their diverse behavior.9 These compounds have the ability to act as both bases and nucleophiles.9 As a result, these reagents react with a wide range of functional groups effecting an array of molecular transformations.9,10 Sodium-N-chloro-p-toluenesulfonamide (p-CH3-C6H4SO2NClNa.3H2O), well-known as chloramine-T (CAT), is a very important member of this class of compounds. It acts as an oxidizing agent in both acidic and alkaline media. It is commercially available, inexpensive, water tolerant, nontoxic, and easy to handle. Although the mechanistic aspects of many of its reactions have been well documented,9-12 very little information on the oxidation kinetics of organic dyes with this reagent is available in the literature.13,14 Platinum metal ion catalyzed reactions from their use in many important industrial processes such as hydrogenation, carbonylation, oxidation, and reduction reactions15 have generated an interest among researchers. The mechanism of catalysis is quite complicated because of the formation of different intermediate complexes, free radicals, and different oxidizing states.16,17 For these reasons, in recent years Os(VIII), Ru(III), Rh(III), Pd(II), Pt(IV), and Ir(III) have been widely employed as catalysts because these elements demonstrate strong catalytic influence in many reactions, and some of these systems have been found suitable for catalytic kinetic methods of analysis.18 The role of platinum metal ions in CAT oxidation of MB dye has not been examined so far. This gives us the incentive to investigate the oxidative decolorization of MB using CAT in the presence of platinum metal ions. We expect that these studies will highlight the interactions and relative reactivities of metal ions in these redox systems.

10.1021/ie900628r  2010 American Chemical Society Published on Web 03/01/2010

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Figure 1. GC-MS of 3,7-bis(dimethylamino)phenothiazin-5,5-dioxide, with molecular ion peak at 317 atomic mass units.

During the preliminary experiments, MB-CAT oxidation reactions were found to be sluggish under normal kinetic experimental conditions. These reactions become facile in the presence of a micro quantity (ca. 10-6 mol dm-3) of Os(VIII), Ru(III), Rh(III), Pt(IV), Ir(III) and Pd(II) catalysts. Os(VIII) catalyst showed 10 times greater catalytic activity than the other catalysts used in the present study. In view of these reasons, we have chosen to study the kinetics of MB-CAT redox system in alkaline medium in the presence of Os(VIII) catalyst. The main object of the present study was to establish the optimum conditions for Os(VIII) catalyzed oxidative decolorization of MB dye by CAT in alkaline medium. The study has also been extended to explore the mechanistic aspects of this oxidation and also to understand the role of platinum metal ions in the aforesaid redox system. An attempt has also been made to deduce the relevant rate law in consonance with the experimental results.

Spectrophotometer 166, Systronics, India) was used as a basic analytical approach and absorbance measurements were made at λmax ) 664 nm. The experimental procedure we followed was identical to that reported earlier.13 The kinetics of reaction was followed for more than 75% completion of the reaction. The absorbance readings of Do and Dt at the beginning of the reaction and at any time interval during the reaction, respectively, were recorded. The pseudo-first-order rate constants (k′, [)] s-1) calculated from the linear plots of log Do/Dt versus time were reproducible within (4-6%. The regression coefficient (R2) was calculated using a fx-100Z scientific calculator. 2.3. Stoichiometry of the Reaction. Varying ratios of CAT: MB in the presence of 5.0 × 10-3 mol dm-3 NaOH and 3.0 × 10-6 mol dm-3 Os(VIII) catalyst were kept at 298 K for 24 h. Determination of the residual oxidant showed that one mole CAT consumed one mole of the substrate. The stoichiometry obtained can be represented as

2. Experimental Section 2.1. Materials. Chloramine-T (E-Merck) was purified using the method of Morris et al.19 An aqueous solution of the compound was prepared, standardized periodically by the iodometric procedure, and preserved in brown bottles to prevent any photochemical deterioration.19 Methylene blue (Sd. Fine, India) was used without further purification, and aqueous solutions of the desired strength were prepared whenever required. Osmium tetroxide (Os(VIII)) (Sigma) solution was prepared in 20 mM NaOH. Ruthenium trichloride (Ru(III)) (Merck), rhodium trichloride (Rh(III)) (Merck), palladium chloride (Pd(II)) (Lancaster, England), chloroplatinic acid (Pt(IV)) (Merck), and sodium hexachloroiridate (Ir(III)) (Sigma) solutions were prepared in 20 mM HCl. Allowance was made for the amount of acid/alkali present in the catalyst solutions while preparing reaction mixtures for kinetic runs. Solvent isotope studies were made with D2O (99.4%) supplied by the Bhabha Atomic Research Centre, Mumbai, India. All other chemicals used were of analytical grade. Doubly distilled water was used throughout the studies. 2.2. Kinetic Measurements. The kinetic runs were performed under pseudo-first-order conditions by ensuring an excess of oxidant over substrate (∼10 times higher) at 298 K. In the present study, UV-visible spectrophotometry (Digital

2.4. Characterization of Products. The reaction mixture in the stoichiometric ratio was allowed to progress in the presence of NaOH and Os(VIII) catalyst for about 24 h at 298 K under stirred conditions. After completion of the reaction (monitored by thin layer chromatography), the reaction products were neutralized with dilute HCl and extracted with ethyl acetate. The organic products were identified as 3,7-bis(dimethylamino)phenothioazin-5,5-dioxide as the oxidation product of MB and p-toluenesulfonamide as the reduction product of CAT by TLC technique.20 Separation of these products was achieved20 using silica gel (60-100 mesh) column chromatography using hexane/ ethyl acetate (8:6, v/v) as mobile phase, with recrystallization by ethanol.

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Table 1. Effect of Varying CAT, MB, NaOH, and Os(VIII) Concentrations on the Reaction Rate at 298 K 104[CAT]o (mol dm-3)

105[MB]o (mol dm-3)

106[Os(VIII)] (mol dm-3)

103[NaOH] (mol dm-3)

2.0 4.0 8.0 10.0 12.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

8.0 8.0 8.0 8.0 8.0 4.0 6.0 8.0 10.0 12.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 0.5 1.0 3.0 5.0 7.0 3.0 3.0 3.0 3.0 3.0

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 2.0 3.0 5.0 7.0 9.0

a

104k′ (s-1)a 4.84 9.49 18.6 23.5 28.0 18.0 17.9 18.6 18.2 18.0 4.42 8.01 18.6 30.1 40.2 28.5 23.8 18.6 15.4 13.5

(2.42) (2.37) (2.32) (2.35) (2.33)

The values in the parentheses refer to second-order rate constants.

These products were confirmed by mass spectral analysis. The GC-MS data were obtained from a 17A Shimadzu gas chromatograph with a QP-5050 Shimadzu mass spectrophotometer. The mass spectrum showed a molecular ion peak at 317 atomic mass units (Figure 1), which clearly matches with the molecular mass of 3,7-bis(dimethylamino)-phenothioazin5,5-dioxide. p-Toluenesulfonamide was detected13 by paper chromatography. Benzyl alcohol saturated with H2O was used as the solvent with 0.5% vanillin in 1% HCl solution in EtOH as a spray reagent (retention factor, Rf ) 0.905). The mass spectrum showed a molecular ion peak at 171 atomic mass units confirming p-toluenesulfonamide. All the other peaks observed in GC-MS were consistent with the structure of the products. It was also noticed that there was no further reaction of this oxidation product under the present set of experimental conditions. 3. Results and Discussion The oxidation of MB by CAT was kinetically investigated at different concentrations of the reactants in NaOH medium catalyzed by Os(VIII) at 298 K. 3.1. Effect of Reactant Concentrations on the Rate of Reaction. With the oxidant in excess, at constant [CAT]o, [NaOH], [Os(VIII)], and temperature, plots of log (absorbance) versus time were linear (R2 > 0.9839), indicating a first-order dependence of rate on [MB]o. The values of pseudo-first-order rate constants (k′, [)] s-1) were found to be independent of [MB]o, confirming the first-order dependence on [MB]o (Table 1). The rate increased with the increase in [CAT]o (Table 1) and a log-log plot of rate versus [CAT]o gave a straight line (R2 ) 0.9999) with a slope of unity. This ensures that the order of the reaction with respect to CAT is 1. Further, a plot of k′ versus [CAT]o was linear and passed through the origin (R2 ) 0.9962), confirming the first-order dependence on [CAT]o and also showing the transient nature of the intermediate formed with the substrate. Furthermore, second-order rate constants k′′ ) k′/[CAT]o, were nearly the same establishing a first-order dependence of rate on the [CAT]o. The second-order rate constants are reported in Table 1 in parentheses. 3.2. Effect of NaOH and Os(VIII) Concentrations on the Rate of Reaction. Increase in [NaOH] decreased the reaction rate (Table 1) and a plot of log k′ versus log [NaOH]

Figure 2. Plots of log k′ versus 1/T for Os(VIII), Ru(III), Rh(III), Ir(III), Pt(IV), and Pd(II) catalysts.

was linear (R2 ) 0.9990) with a slope of -0.50, indicating an inverse-fractional-order dependence of the rate on [NaOH]. The rate was catalyzed with an increase in [Os(VIII)] (Table 1). A log-log plot of rate versus [Os(VIII)] was linear (R2 ) 0.9978) with a slope of 0.79, showing a fractional-order dependence of rate on [Os(VIII)]. 3.3. Effect of Dielectric Constant on the Rate of Reaction. The dielectric constant (D) of medium was varied by adding methanol (0-40% v/v) to the reaction mixture, with other experimental conditions being held constant. The reaction rates (104 k′ s-1) at 76.73, 72.37, 67.48, 62.71, and 58.06 of D were found to be 18.6, 15.1, 11.5, 8.71, and 6.13, respectively. A plot of log k′ versus 1/D yields a straight line (R2 ) 0.9956) with a negative slope. Blank experiments showed that there was no oxidation of methanol by CAT during the experimental period. Values of the dielectric constant of the methanol-water mixture reported in the literature21 were employed. 3.4. Effect of p-Toluenesulfonamide (PTS) on the Rate of Reaction. Addition of the reduction product of CAT, p-toulenesulfonamide, or TsNH2 (1.0 × 10-3 to 8.0 × 10-3 mol dm-3) to the reaction mixture had no significant effect on the rate, indicating that it is not involved in a pre-equilibrium to the rate determining step. 3.5. Effect of Temperature on the Rate of Reaction. All of the six metal ion catalyzed reactions were studied at five different temperatures (288, 293, 298, 303, and 308 K) with other experimental conditions being held constant. From the linear Arrhenius plots of log k′ versus 1/T (Figure 2; R2 > 0.9988), values of activation parameters such as energy of activation (Ea), enthalpy of activation (∆Hq), entropy of activation (∆Sq), Gibbs free energy of activation (∆Gq) and Arrhenius frequency factor (log A) for the overall reaction, for each temperature, were calculated. The average values of each parameter are reported in Table 2 along with errors for each catalyst. 3.6. Effect of Ionic Strength on the Rate of Reaction. The influence of ionic strength on the reaction rate was studied by adding 0.3 mol dm-3 NaClO4 solution. It was found that the ionic strength has negligible effect on the reaction rate, indicating the involvement of nonionic species in the rate

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Table 2. Effect of Varying Temperature on the Reaction Rate and Activation Parameters for the Oxidation of MB Dye by CAT in the Presence of Os(VIII), Ru(III), Rh(III), Pt(IV), Ir(III), and Pd(II) Catalystsa 104k′ s-1 Os(VIII) temperature (K) 288 293 298 303 308 activation parameters Ea (kJ mol-1) ∆Hq (kJ mol-1) ∆Gq (kJ mol-1) ∆Sq (JK-1 mol-1) log A a

Ru(III)

Rh(III)

Pt(IV)

Ir(III)

Pd(II)

10.3 14.3 18.6 23.9 33.4

6.01 8.15 11.6 16.1 23.6

3.83 6.17 8.52 13.6 19.1

3.85 6.19 8.49 13.7 19.2

3.79 6.20 8.59 13.5 19.0

2.04 3.81 6.23 8.54 14.2

42.6 40.1 ( 0.01 88.5 ( 0.27 -162 ( 0.07 4.75 ( 0.25

47.8 45.3 ( 0.01 89.4 ( 0.19 -148 ( 0.29 5.46 ( 0.13

51.8 49.3 ( 0.01 90.3 ( 0.19 -137 ( 0.11 6.12 ( 0.15

52.0 49.5 ( 0.01 90.3 ( 0.2 -136 ( 0.1 6.05 ( 0.13

51.6 49.1 ( 0.01 90.2 ( 0.21 -137 ( 0.07 5.99 ( 0.09

59.7 57.2 ( 0.01 91.5 ( 0.14 -114 ( 0.18 7.14 ( 0.11

[CAT]o ) 8.0 × 10-4 mol dm-3; [MB]o ) 8.0 × 10-5 mol dm-3; [Os(VIII)] ) 3.0 × 10-6 mol dm-3; [NaOH] ) 5.0 × 10-3 mol dm-3.

determining step. Hence, no attempt was made to keep the ionic strength of the medium constant during the kinetic runs. 3.7. Effect of Halide Ions on the Rate of Reaction. The rate remained constant with the addition of Cl- or Br- ions in the form of NaCl or NaBr (2.0 × 10-2 to 8.0 × 10-2 mol dm-3). This indicates that the halide ions play no role in the reaction. 3.8. Effect of Solvent Isotope on the Rate of Reaction. Solvent isotope studies were performed in D2O medium at 298 K. Values of k′ (D2O) and k′ (H2O) were found to be 1.35 × 10-3 and 1.86 × 10-3 s-1, respectively. The formal solvent isotope effect ratio k′ (H2O)/k′ (D2O) ) 1.37. 3.9. Polymerization Study. Addition of reaction mixture to aqueous acrylamide solution did not initiate polymerization showing the absence of free radical species. 3.10. Reactive Species of CAT. Chloramine-T acts as an oxidizing agent in both acidic and alkaline media. Generally, CAT undergoes a two-electron change in its reactions resulting in formation of the reduction products, p-toluenesulfonamide (PTS) and NaCl.22 The redox potential of the ChloramineT-PTS system is pH dependent,22,23 and it decreases with an increase in pH of the medium (1.139 V at pH 0.65, 0.778 V at pH 7.0, and 0.614 V at pH 9.7.). In aqueous solution, CAT behaves as a strong electrolyte,24 and, depending on the pH of the medium, CAT furnishes different types of reactive species.19,23-26 The possible oxidizing species in acidified CAT solutions are TsNHCl, TsNCl2, HOCl, and H2OCl+ (here Ts ) CH3C6H4SO2-). In alkaline solutions of CAT, dichloramine does not exist,26 and the possible species are the anions TsNCl- and OCl-, which would be transformed into more reactive oxidizing species TsNHCl and HOCl in the course of the reaction in alkaline medium. Under the present experimental conditions, the concentration of OCl- ion is small24 and hence does not make any significant contribution to the oxidation of MB. Further, Hardy and Johnston23 have reported the existence of the following equilibrium in alkaline solutions of CAT: K′ h TsNHCl + H2O y\z TsNH2 + HOCl

(2)

where Kh′ ) 4.21× 10-3 at 298 K. According to eq 2 the hydrolysis of TsNHCl is slight (Kh′ ) 4.21 × 10-3 at 298 K), and if HOCl is involved a first-order retardation of rate by the added p-toluenesulfonamide is expected. Since no such effect is noticed, HOCl is ruled out as an oxidizing species. A retarding influence of OH- ions on the reaction rate noticed in several haloaminometric reactions27-29 has been attributed to the formation of the conjugate acid

Scheme 1.

TsNHCl from TsNCl- in an alkali retarding step. An inversefractional-order dependence of rate on [OH-] observed in the oxidation of MB by CAT indicates that the hydrolysis of chloramine-T (TsNClNa) gives the free acid TsNHCl, which is the most likely oxidizing species. 3.11. Reactive Species of Os(VIII). Osmium tetroxide is known to be an efficient catalyst in the oxidation of several organic compounds by various oxidants in alkaline medium.30-32 It has been shown that osmium is stable in its +8 oxidation state.33,34 In alkaline solutions, the coordination of OH- and OsO4 takes place, and the formation of octahedral complexes [OsO4(OH)(H2O)]- and [OsO4(OH)2]2- has been reported by Griffith.35 OsO4 + OH- + H2O h [OsO4(OH)(H2O)]-

(3)

[OsO4(OH)(H2O)]- + OH- h [OsO4(OH)2]2- + H2O

(4) Both [OsO4(OH)(H2O)]- and [OsO4(OH)2]2- may not be able to form effective complexes with an oxidant. It is more realistic to postulate that OsO4, which possesses tetrahedral geometry, is the active catalyst species that can effectively form a complex with the oxidant species. In our earlier Os(VIII) catalyzed redox systems, OsO4 was considered as the active catalyst species.36,37 3.12. Formation of Complex between Oxidant and Catalyst. On the basis of the kinetic data, one can propose a reaction mechanism (Scheme 1) in which the oxidant interacts with the catalyst in the fast step to form the intermediate complex X. The formation of the intermediate complex X is demonstrated by spectroscopic studies. UV-visible spectra are recorded on a UV-3101PC, UV-vis-NIR scanning spectrophotometer (Shimadzu). Ultraviolet spectral measurements showed that CAT and Os(VIII) solutions have absorption bands at 220 and 320 nm, while a sharp band at 315 nm was noticed for the CAT-Os(VIII) mixture (Figure 3). A hypsochromic shift of 5

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Figure 3. UV-visible spectra of CAT, Os(VIII), and mixture of CAT + Os(VIII).

nm from 320 to 315 nm of the Os(VIII) suggests that the complexation occurs between CAT and Os(VIII). Further, for a general equilibrium K

M + n(CAT) y\z (M (CAT)n)

(5)

between two metal species, where M and (M (CAT)n) have different extinction coefficients, Ardon38 has derived eq 6: 1/∆A ) 1/[CAT]n{1/∆E[Mtotal]K} + 1/∆E[Mtotal]

(6)

where K is the formation constant of the complex, ∆E is the difference in extinction coefficient between two metal species, [Mtotal] is the total concentration of metal species, and ∆A is the absorbance difference between the solution in the absence of CAT and one that contains a certain concentration of CAT. Equation 6 is valid provided that [CAT] is many times greater than [Mtotal], that the amount of CAT tied up in the complex is negligible, or that it is subtracted from the initial concentration of CAT. According to eq 6, a plot of 1/∆A versus 1/[CAT] or 1/ [CAT]2 should be linear with an intercept in the case of the 1:1-type or the 1:2-type of complex formation between M and CAT. The ratio of intercept to slope of this linear plot gives the value of K. Os(VIII) in NaOH medium containing CAT showed an absorption peak at 315 nm (λmax of the complex) and the complex formation studies were made at this wavelength. In a set of experiments, the solutions were prepared by taking different concentrations of CAT (2.0, 4.0, 8.0, 12.0, and 16.0 × 10-4 mol dm-3) at constant concentrations of Os(VIII) (3.0 × 10-6 mol dm-3) and NaOH (5.0 × 10-3 mol dm-3). The absorbance of the solution in the absence of CAT was measured at the same wavelength. The difference of these absorbance values (in the presence and in the absence of CAT) provides the differential absorbance, ∆A. A plot of 1/∆A versus 1/[CAT]o was linear (Figure 4; R2 ) 0.9981) with an intercept indicating the formation of a 1:1 complex between Os(VIII) catalyst and CAT oxidant. From the slope and intercept of such a plot, the value of the formation constant, K, of the complex was found to be 4.1 × 102. 3.13. Reaction Scheme. An inverse-fractional-order dependence of the rate on [NaOH] clearly indicates the hydrolysis of CAT (TsNClNa) which results in the generation of the conjugate acid (TsNHCl). Under the present experimental conditions, it is quite likely that TsNHCl is the oxidizing species. In light of the above considerations and experimental facts, the Os(VIII)

Figure 4. Plot of 1/∆A versus 1/[CAT].

catalyzed MB-CAT oxidation in alkaline medium can be best explained by the mechanism shown in Scheme 1. A detailed mechanism involving the electron transfer during the oxidation of MB with CAT catalyzed by Os(VIII) in alkaline medium and the structures of the intermediates (X and X′) can be seen in Scheme 2. An initial equilibrium (step i) involves hydrolysis of TsNClNa, forming the conjugate acid TsNHCl in an OH- retarding step. In the next fast equilibrium step (step ii), this conjugate acid coordinates to the central metal of the catalyst activating CAT by stabilizing the charge on its nitrogen and polarizing the N-Cl bond to form a metal complex anion X. In a slow and rate determining step (step iii), the lone pair of electrons of sulfur of the substrate attacks the positive chlorine of the intermediate complex (X) to form another intermediate complex X′ with the elimination of p-tolunesulfonamide, and the regeneration of the catalyst species. This step is followed by several fast steps (step iv) leading to the formation of the product 3,7-bis(dimethylamino)phenazin-5,5-dioxide. 3.14. Kinetic Rate Law. Based in Scheme 1, the following rate law is obtained: rate )

K1K2k3[CAT]t[MB][Os(VIII)][H2O] [NaOH] + K1[H2O] + K1K2[Os(VIII)][H2O]

(7) Rate law 7 is in complete agreement with the experimental kinetic data, wherein a first-order dependence of rate on both [CAT]o and [MB]o, a fractional-order on [Os(VIII)], and an inverse-fractional-order in [NaOH] was noticed. The proposed scheme and the derived rate law are also supported by the experimental observations discussed below.

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Scheme 2. A Detailed Mode of Oxidation of Methylene Blue Dye by CAT in Alkaline Medium Catalyzed by Os(VIII)

3.15. Solvent Isotope Effect. When an atom is replaced by its isotope, there is no change in the potential energy surface for any reaction that it might undergo, but the rate of reaction changes because there is change in the average vibrational energy of the molecule and that of the activated complex. Many reactions in aqueous medium that are susceptible to acid-base catalysis have been studied in heavy water after equilibrium. Solvent isotope effect is the variation of the rate of the reaction after equilibrium, and it makes available valuable information on the type of bond breaking and bond making during a chemical reaction. Mild oxidation reactions of organic compounds involve the cleavage of the C-H bond. Thus, the deuterium isotope effect on such reactions gives information concerning the nature of the rate determining step.39 In the present investigations, as expected for a OH- retarded reaction, the rate of the reaction decreased in D2O medium and the solvent isotope effect is equal to 1.37. This study generally correlates with the fact that for a reaction involving a fast equilibrium OH- ion transfer,39,40 the rate increases in D2O since the OD-

ion is a stronger base (∼2-3 times greater) than OH-. The reverse holds true for reactions involving retardation by OHions as observed in the present case. Hence the proposed mechanism is supported by the rate decrease in D2O medium, indicating retardation due to OH- ion. The magnitude of the solvent isotope effect, however, is small (1.37) and is an indication of the inverse-fractional-order dependence of rate on OH-. 3.16. Dielectric Constant Effect. The kinetics of reaction between ions in solution are affected owing to electrostatic interactions. It is evident that electrostatic attraction or repulsion exerts a chemical effect on the kinetics of reaction. The influence of solvent on the rate and frequency factors of reactions in solutions can be explained on the basis of the theory of absolute reaction rate.41 The effect of varying solvent composition on the reaction rate has been described in several publications.41-44 From these observations, one can conclude whether ion-dipole or dipole-dipole is involved in the reaction sequence. Further, it is evident from the study that the rate constant increase or

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decrease on increasing the dielectric constant of the medium depends on whether the transition state bears a negative or positive charge. For the limiting case of zero angle of approach between two dipoles or an ion-dipole system, Amis43 has shown that a plot of log k′ versus 1/D yields a straight line, with a negative slope for a negative ion-dipole or dipole-dipole interaction; a positive slope results for a positive ion-dipole interaction. The negative dielectric effect in the present study supports the interaction between a negative ion and a dipole in the rate determining step (Scheme 2). It also signifies the charge dispersal in the transition state, which is less polar than the reactant state. 3.17. Ionic Strength Effect. Since ions exert considerable electrostatic forces on each other, the kinetics of reaction between ions deviates from such reactions which involve nonelectrolytes. The rate constants of ionic reactions depend upon the charges carried by the ions and also upon the ionic strength of the solution.45 Primary salt effect deals with the effect of ionic strength on the rate constant, whereas secondary salt effect on the rate constant refers to the actual change in the concentration of reacting ions by the addition of electrolytes. The primary salt effect on the reaction rates has been well described by the Bronsted-Bjerrum theory.45 According to this concept, the effect of ionic strength (µ) on the rate of a reaction involving two ions is given by the relationship log k ) log ko + 1.02ZAZBµ1/2

(8)

where A and B are the reacting ions, ZA and ZB are the charges on the respective species, k and ko are the rate constants in the presence and in the absence of the added electrolyte, respectively. According to eq 8, a plot of log k versus µ1/2 should be linear with a slope of 1.02ZAZB and intercept log ko. As the slope of the line depends on ZAZB, that is, charges of the reacting ions, three special cases may rise: (i) when ZA and ZB are of the same sign, then ZAZB will be positive and the rate constant will increase with the ionic strength, (ii) when ZA and ZB are of the opposite sign, then ZAZB will be negative and the rate constant will decrease with the ionic strength, and (iii) when one of the reactants is uncharged, then ZAZB will be zero and the rate constant will be independent of ionic strength. In the present investigations, variation of the ionic strength of the medium by adding NaClO4 (0.3 mol dm-3) solution does not alter the rate significantly and indicates that one of the reactant species is nonelectrolytic as shown in Scheme 2. Hence the observed ionic strength effect is consistent with the Bronsted-Bjerrum45 concept and the proposed scheme. 3.18. Catalytic Activity of Os(VIII). It has been pointed out by Moelwyn-Hughes46 that, in the presence of the catalyst, the uncatalyzed and catalyzed reactions proceed simultaneously, so that k1 ) ko + KC[catalyst]x

(9)

Here k1 and ko are the pseudo-first-order rate constants in the presence and absence of Os(VIII) catalyst, KC is the catalytic constant, and x is the order of the reaction with respect to Os(VIII), which is found to be 0.79 in the present study. The value of catalytic constant KC, at five different temperatures 288, 293, 298, 303, and 308 K, was calculated using the relationship KC ) (k1 - ko)/[Os(VIII)]x

(10)

The values of KC were found to vary for different temperatures, and activation parameters with respect to Os(VIII) catalyst

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Table 3. Effect of Varying Temperature on the Rate of Reaction and Activation Parameters for the Oxidation of MB Dye by CAT in Alkaline Medium in the Presence and Absence of Os(VIII) Catalyst. Also, Values of Catalytic Constants (Kc) for Os(VIII) at Different Temperatures and Activation Parameters Calculated using Kc Valuesa 104k′ s-1 Os(VIII) catalyzed temperature (K) 288 293 298 303 308 activation parameters Ea (kJ mol-1) ∆Hq (kJ mol-1) ∆Gq (kJ mol-1) ∆Sq (JK-1 mol-1) log A

uncatalyzed

10Kc s-1

10.3 14.3 18.6 23.9 33.4

0.72 1.30 1.80 2.80 4.56

2.52 3.41 4.40 5.53 7.56

42.6 40.1 ( 0.01 88.5 ( 0.27 -162 ( 0.07 4.75 ( 0.25

63.7 61.2 ( 0.01 94.2 ( 0.13 -110 ( 0.10 7.42 ( 0.13

38.2 35.7 ( 0.01 63.5 ( 0.15 -93.2 ( 0.04 8.34 ( 0.10

[CAT]o ) 8.0 × 10-4 mol dm-3; [MB]o ) 8.0 × 10-5 mol dm-3; [NaOH] ) 5.0 × 10-3 mol dm-3; [Os(VIII)] ) 3.0 × 10-6 mol dm-3. a

were deduced by a linear plot of log KC versus 1/T (R2 ) 0.9991). All these data are summarized in Table 3. Further, for the standard run at 298 K, a plot of k′ versus [Os(VIII)] (Table 1) was found to be linear (R2 ) 0.9906) with an intercept equal to ko, which was found to be 1.90 × 10-4 s-1. This value is in good agreement with the rate constant (k′ ) 1.80 × 10-4 s-1) determined experimentally for the uncatalyzed reaction at 298 K. This clearly signifies that both uncatalyzed and catalyzed reactions proceed concurrently. Further, the proposed mechanism is also supported by the moderate values of energy of activation and other thermodynamic parameters. The positive values of free energy of activation and enthalpy of activation indicate that the transition state is highly solvated and enthalpy controlled. The observed ∆Sq values are large and negative. It may be interpreted that a fraction of collisions become more stringent and form a rigid associative activated complex, and hence decomposition of the activated complex is quite a slow process. 3.19. Comparison of Catalyzed and Uncatalyzed Reactions. We thought, it would be worthwhile to compare the platinum metal ions catalyzed oxidation of MB by CAT with that of uncatalyzed reaction (without catalyst) under an identical set of experimental conditions. The observed rates of oxidation of MB in the presence of platinum metal ion catalysts revealed that the reactions are 3-fold to 10-fold faster than the uncatalyzed reactions (Tables 2 and 3). The difference in activation energies for the catalyzed and uncatalyzed reactions explains the catalytic effect on the reaction. This may be attributed to the formation of the intermediate complex (X) between Os(VIII) catalyst and the oxidant, which increases the oxidizing property of the oxidant. Further, Os(VIII) favorably modifies the reaction path by stabilizing the transition state, which in turn provides an alternative pathway having lower activation energy for the reaction. The same argument holds good for other metal ions also. 3.20. Relative Reactivities of Platinum Metal Ions. The orders of reactivities of the metal ions under investigation are Os(VIII) > Ru(III) > Ir(III) g Rh(III) g Pt(IV) > Pd(II), which are in conformity with the activation energies (Table 2). This may be attributed to the d-electronic configuration of the metal ions. Osmium having d0 electronic configuration has greater catalytic efficiency among the catalysts used in the present studies. Ru(III) having d5 electronic configuration shows second highest reactivity in the series. Pd(II) having d8 electronic

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Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010

configuration is expected to have the least catalytic efficiency. Rh(III), Pt(IV), and Ir(III) having d6 electronic configurations exhibit intermediate catalytic efficiency with virtually the same reaction rate. Hence, on the basis of d electron configuration of the metal ions, the reactivity decreases as the number of electrons increases in the d orbital as d0 (Os(VIII)) > d5 (Ru(III)) > d6 (Ir(III)) g d6 (Rh(III)) g d6 (Pt(IV)) > d8 ((Pd(II)). Such a behavior was noticed in our early work.47 It is likely that during the course of the reaction the metal ion momentarily undergoes reduction when the oxidant is attached to the metal ion, and after this the metal ion gets back to its original valence state as shown in Scheme 2. In conclusion, it can be said that Os(VIII) is the most competent of metal ions to catalyze MB-CAT redox system in alkaline medium. 3.21. Utility of the Current Work. In the present research program, an attempt has been made to develop optimum conditions for the Os(VIII) catalyzed oxidative decolorization of MB by CAT in alkaline medium. This method has the potential to successfully remove MB dye from wastewater. It offers a number of benefits such as short reaction times, cost effectiveness, the use of relatively nontoxic reagents, and the facility of this method to meet the industrial requirements. Hence, this method will be a valuable addition to the existing methods.

Appendix From Scheme 1, the total effective concentration of CAT, [CAT]t, is given by [CAT]t ) [TsNClNa] + [TsNHCl] + [X]

Solving for [TsNClNa] and [TsNHCl] from steps i and ii of Scheme 1, we get [TsNClNa] )

Acknowledgment K.N.V. is grateful to the Bangalore University, Bangalore, for the Research Fellowship under the Interdisciplinary Collaborative Research Project. The authors are also thankful to Prof. B. S. Sheshadri for helpful discussions.

[X][NaOH] K1K2[Os(VIII)][H2O]

(A2)

[X] K2[Os(VIII)]

(A3)

[TsNHCl] )

By substituting for [TsNClNa] and [TsNHCl] from eqs A2 and A3, respectively, into eq A1 and solving for [X], one obtains [X] )

K1K2[CAT]t[Os(VIII)][H2O] [NaOH] + K1[H2O] + K1K2[Os(VIII)][H2O]

(A4) From the slow/rate determining step (step iii) of Scheme 1, the rate of the reaction is rate ) k3[MB][X]. By substituting [X] from eq A4, the following rate law is obtained:

4. Conclusions On the basis of the experimental results, the following are the conclusive remarks: (1) In the present research, an attempt has been made to develop optimum conditions for Os(VIII) catalyzed oxidative decolorization of MB dye with CAT in alkaline medium. Among the methods developed to abolish MB dye in wastewater from the textile and dyestuff manufacturing industries, the oxidative decolorization technique has the most potential since it has significant advantages such as consisting of a simple process with a short reaction time, cost-effectiveness, the use of relatively nontoxic reagents, and the ability to be scaled up for industrial operations. (2) This research program was designed through an oxidation-kinetic study to reveal the mechanistic picture of the MB-CAT redox system and, in particular, the role of the platinum metal ions in the aforesaid reaction. (3) Os(VIII) catalyzed oxidation of MB by CAT in alkaline medium obeys the experimental rate law: -d[CAT]/dt ) k[CAT]o[MB]o[NaOH]-0.50[Os(VIII)]0.79. Activation parameters have been deduced, and the oxidation product of MB was identified. A suitable mechanism and a relevant rate law based on the kinetic results have been worked out. (4) It was found that Os(VIII), Ru(III), Ir(III), Rh(III), Pt(IV), and Pd(II) catalyzed MB-CAT reactions are 3-10-fold faster than the uncatalyzed reactions. This justifies the use of platinum metal ions for the facile oxidation of a MB-CAT redox system in alkaline medium. The relative ability of these catalysts is in the order Os(VIII) > Ru(III) > Ir(III) g Rh(III) g Pt(IV) > Pd(II), and this trend may be attributed to the different d electronic configuration of the catalysts.

(A1)

rate )

K1K2k3[CAT]t[MB][Os(VIII)][H2O] [NaOH] + K1[H2O] + K1K2[Os(VIII)][H2O]

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ReceiVed for reView April 20, 2009 ReVised manuscript receiVed February 1, 2010 Accepted February 14, 2010 IE900628R