Kinetic Study of Decolorization of Methylene Blue with Sodium Sulphite in Aqueous Media: Influence of Transition Metal Ions

The kinetics of decolorization of methylene blue (MB+) with sodium sulphite in aqueous media was investigated over the temperature range 20-40°C. The kinetic studies were carried out as a function of different variables like concentration, pH, and temperature on rate of decolorization. The rate of the reaction was found to be [H+] dependent and first order in both [MB+] and reductant. The empirical rate law conforms to the equation: 2 3 2 1 3 [ ] { [ ]} [ ][ ] [ ] obs d MB k k K H MB SO k MB dt + + + − + = − + = − Where K1 is the protonation constant, and k2 and k3 are overall pseudo-second order rate constants for the decolorization of MB+ with 2 3 SO − . The rate was found to increase linearly with temperature. The rate of decolorization decreased after addition of Co(II), Ni(II), Mn(II) and Zn(II), but increased after addition of Fe(II) and Cu(II). The activation parameters (Ea, ΔG #, ΔH# and ΔS#) of the decolorization reaction of MB+ with 2 3 SO − in absence and in presence of Fe(II) and Cu(II) were calculated.

These processes have found applications in numerous inventions like data recording holographic industries, optical data storage, food and pharmaceutical industries. Such reduction is also used in checking the purity of milk [51,52].
In the present study, an attempt is made to investigate the decolorization kinetics of methylene blue with sodium sulphite in aqueous media by UV-Visible spectrophotometry. The effect of various parameters such as initial pH, initial concentration of sulphite, MB + and transition met al ions concentration was studied.

Effect of initial concentration of MB +
The extent of MB + decolorization by Na 2 SO 3 with time at varying initial MB + concentrations (20-70 mg/dm 3 ) under identical conditions is shown in Figure 1. It is seen that the decolorization of MB + is relatively fast at the beginning and decreases as the reaction progresses. At low concentration (20 mg/dm 3 ), MB + is almost completely decolorized by Na 2 SO 3 within 60 s., which is also evident from the absorbance data recorded with reaction time (Figure 1a). The time profiles of MB decolorization show that the decolorization is substantially decreased with increase in initial MB + concentration. For example, the extent of decolorization after 60 s decreases from 85% to 29% with increase in concentration of MB + from 20 to 70 mg/dm 3 with Na 2 SO 3 .

Effect of initial concentration of
The effect of Na 2 SO 3 on the decolorization of MB + was investigated by varying initial concentration of Na 2 SO 3 from 0.4 to 1.5 mol dm -3 , the results are shown in Figure 2. It was observed that the decolourization efficiency increases from 69.6% to 80.9% as a consequence of increasing Na 2 SO 3 concentration from 0.4 to 1.5 mol/dm 3 within 60 sec. This is due to increasing reductive power of the sulphite. The maximum decolourization efficiency of about 99.4% was achieved with 1.5 mol/ dm 3 of Na 2 SO 3 within 5 min.

Kinetic study
The reductive decolorization of MB + by sulphite was a complicated process probably due to various reduced forms of MB + [22]. The process could not easily be depicted by simple reaction kinetics. To estimate the kinetics rate of decolorization, an equation form of power law is used: where k is the removal rate constant, [C] is the MB + concentration, m and n are the pseudo order of reaction with respect to MB + and 2 3 SO − respectively. The rate equation can be stated with observed rate constant (k obs ) as follows: For a zero-order reaction, the above equation after integration becomes: Also for a first-order reaction, the above equation after integration becomes: in which, C o is the initial MB + concentration and C t is the

Experimental Procedure
All chemical reagents used were of analytical grade. Methylene blue was obtained from M & B Laboratory Chemicals in the laboratory grade and used without further purification. Distilled water was used to prepare all solutions. A stock solution of Na 2 SO 3 and MB + were prepared by dissolving the required amount in distilled water. A Na 2 SO 4 solution was added to reaction mixture in order to maintain the ionic strength of the solution. The effect of pH was also studied by adjustment of pH of the reaction mixture prior to decolorization with NaOH or HCl and measured by a pH meter (JENWAY 3505 pH meter). The effect of temperature was studied and the temperature was controlled by a horizontal thermostated shaker (SM 101 by Surgafriend Medicals). The absorbance of the reaction mixture at different reaction times was recorded by measuring the absorption intensity at 661nm using Genesys 10 UV-VIS Scanning Spectrophotometer. The reaction mixtures were allowed to undergo complete reaction as evident from the constancy of repeated measurements of absorbance ) ( ∞ A at 661 nm. The concentration at any time t ) ( t C was obtained from the difference in the absorbance at time t ) ( t A and at infinity (1) The efficiency of MB + decolourization was calculated by using Where, C o and C t are MB + concentration at initial and any time concentration at reaction time t. For a second-order reaction, the integrated equation becomes: The absorbance-time data plot of the above reaction orders were carried out to determine the rate constant, k obs and best order for the reaction process. The zero-, first-and second-order kinetics rate constants for the decolorization of MB + with 2 3 SO − were obtained from the plots, and the results were shown in Table 2. It was observed that the correlation coefficients for the three models are different. The firstorder kinetics shows highest correlation (R 2 >0.9) than both zero and second-order kinetics (R 2 <0.9). It can therefore be concluded that the decolorization of MB + with 2 3 SO − fits the first-order reaction kinetics of the type: From the pseudo first-order kinetic plots (Figures 3a and 3b), the pseudo-first order rate constants with respect to MB + and 2 3 SO − were obtained ( Table 1).
The pseudo first-order rate constant decreased with increasing initial concentration of methylene blue (Figure 4a). The possible explanation is that at high dye concentrations, the reductive activity of sulphite might be reduced due to coverage of reducing ion species by the dye ions.
Similarly, the pseudo first-order rate constant increased with increasing concentration of This fact reveals also first-order dependence on the reductant. An overall rate equation can then be explained in the form: where k 2 is the overall second order rate constant.

Effect of pH
In view of the fact that pH of dye solution is a main parameter on the decolorization progress, the experiments were carried out to

12)
The above equation (12) suggests that the reaction occurs via aciddependent and acid-independent pathways. The high rate constant value observed at lower pH can be explained by the change in the MB + structure. The labile H atom makes the molecule of MB + dye vulnerable towards the attack of reductant. This has been explained in terms of protonation of 2 3 SO − in a fast step to give − 3 HSO which subsequently reacts with the substrate in a slow step to give the product [57].

Effect of transition metal ions
Different transition metal ions, Fe(II), Cu(II), Ni(II), Mn(II), Zn(II) and Co(II) were tested as a catalyst for the decolorization of MB + with 2 3 SO − . The results show that Fe(II) and Cu(II) accelerate the reaction rate with varying catalytic activity (Figure 6), while others have no catalytic effect on the rate of reaction (Table 2). It was observed that the rate of reaction between methylene blue and sodium sulphite was increased by the presence of trace amounts of Fe (II) and Cu(II) ions. The rate constant in the presence of Fe 2+ and Cu 2+ ions is greater than in the absence of these transition metal ions. This could be due to activation of sodium sulphite by these transition metal ions to produce sulphate anion radical 4 ( ) SO •− via sulfite radical anion [58][59][60]. The sulfite can be oxidized to sulphate by trace transition metal ions via free radicals. This metal-catalyzed autoxidation generates sulfur trioxide anion radical 3 ( ) SO •− in an initiating one-electron oxidation step through an oxygen-consuming chain reaction [61][62][63][64] in accordance with the following reaction scheme 2.

Effect of temperature
The effect of temperature on decolorization of methylene blue in the presence and absence of transition metal ions catalyst was investigated in the range of 293-313 K. The results in Figure 7 show that the decolorization efficiency of methelene blue gradually increased with increase in the temperature. The increase in temperature may lead to increasing reduction rate of methylene blue.
The activation parameters associated with the decolorization of methylene blue are calculated from the plot of 1/ obs In k versus T (Figure 7), which gives the value of activation energy ) ( a E , according to the Arrhenius equation: The values of can be calculated from the plot of the Erying equation [65] as follows: The rate constants and activation energies in the range of temperature studied (20-40°C) are listed in Table 3 while other thermodynamic parameters are given in Table 4. The positive values of # G ∆ for the reaction (Table 4)  The activation energy in presence of Fe(II) and Cu(II) is lower than in its absence ( Table 3). The decrease in the activation energy (E a ) in presence of these transition metal ions confirms their catalytic effects (metal activation).

Reaction mechanism
Based on the above experimental findings and observations, the reaction mechanism has been suggested utilizing the redox properties of Na 2 SO 3 . It has also been noted that redox reactions of many oxyanions are strongly acid dependent [66]. Under the present experimental conditions, it is reasonable to postulate that 2 3 SO − is protonated in a fast step to give − 3 HSO which then reacts with MB + in a slow step to give the products [56]. Also, the intercept obtained from the plot of k 2 versus [H + ] ( Figure 5) indicates that the unprotonated 2 3 SO − also reacts with MB + to form the products. Therefore, taking recourse to the experimental data, the following mechanistic steps have been postulated for the reaction: Putting equation (20)  where K 1 is the protonation constant, and k 2 and k 3 are the pseudosecond order rate constants for the protonation and deprotonation of MB + . Equation (22) shows acid dependence on the rate of decolorization of methylene blue with sodium sulphite and is similar to equation (12) with k 3 ='a'=1.32×10 -2 dm +3 mol -1 s -1 and k 2 K 1 ='b'=0.595 dm +6 mol -2 s -1 .

Conclusion
The color removal of the cationic dye methylene blue by the formation of a reduced form of MB + (colorless) is studied kinetically. The rate of color removal depends on the concentration of reactants, pH, and temperature. The decolorization of MB + is pseudo first order with respect to MB + and 2 3 SO − . The decolorization in creases with temperature, in acidic medium, after addition of Fe(II) and Cu(II), but decreases with the addition of Mn (II), Ni(II), Co(II) and Zn(II).