Oxidative Decolourisation of Rosaniline Hydrochloride (RAH)

Kinetic, thermodynamic studies for oxidation of rosaniline hydrochloride dye by persulphate in ambient temperatures

Z. M. Abou-Gamra*

Abstract

The kinetics of the oxidative decolourisation of rosaniline hydrochloride (RAH) by sodium persulphate was studied spectrophotometrically over pH range 3.5-9.5 at 30-45 ^oC. The reaction was second order with respect to dye and half order to persulphate. Increasing the pH of the medium increased the rate of decolourisation dramatically in alkaline medium. The Activation Parameters were found to be 62.11 kJ mol^-1, 90.33 kJ mol^-1 and -98.44J K^-1 mol^-1 with respect to activation energy, free energy and entropy respectively. Addition of sodium chloride and sodium sulphate had no effect on the rate of decolourisation.

Key wards: Kinetics, Mechanism, Degradation, Rosaniline, Persulphate.

1. Introduction

Textile dyeing process is significant source of environmental pollution. It produces large amounts of highly colored effluents, which generally toxic resistant to destruction by biological treatment methods. Many physical, chemical processes such as adsorption [1], electrochemical [2], photocatalytic [3] are used to remove the dyes from waste water. Chemical oxidative processes seem to provide an

opportunity for future use in industrial wastewater. Examples of such potentially effective chemical oxidants for oxidative processes include Fenton reagent [4-5], KBrO₃ [6-7] and KClO₃ [8].

*corresponding author e-mail: zanibabougamra@yahoo.com

The use of persulfate has recently attention as an alternative oxidant in the chemical oxidation of contaminants [9-12]. Persulphate (KPS) is one of the strongest oxidants known in aqueous solution and has a higher potential (E_o = 2.01 V) than H₂O₂ (E_o = 1.76 V) [13] Table 1. It offers some advantages over other oxidants as a solid chemical at ambient temperature with ease of storage and transport, high stability, high aqueous solubility and relatively low cost. It has great capability for degrading numerous organic contaminants through free radicals ( SO₄^-. and HO^.) generated in the persulphate system [12].

Basic dyes, such as, crystal violet, malachite green and roseaniline hydrochloride are used cotton tannin, mordant printing and dyeing in textile. Rosaniline is triphenylmethane dyes with amino group on each phenyl ring. Its structure is easily reducible where the chromophore group is destroyed and the compound loses its colour. Redox reaction of rosaniline hydrochloride by sulphite and nitrite ions are reported earlier [14-16].

This work focused on the kinetic study of oxidation of rosaniline hydrochloride with persulphate at ambient temperature spectrophotometrically. The effects of pH, dye, persulphate concentrations and temperature were studied. Also mechanism as well as rate law equation for the reaction is proposed.

2. Experimental

2.1. Reagents and materials

All chemicals were of pure grade and were used without further purification. Rosaniline hydrochloride BDH (molecular weight =337.8, _max = 540 nm). The chemical structure of (RAH) is given in (Fig.1). NaCl and Na₂SO₄ were purchased from Merck. All solutions were prepared using bidistilled water. Stock solutions of dye (1 mM), K₂S₂O₈ (100mM) were prepared. The pH is adjusted by HCl and NaOH solutions.

2.2. Kinetic experiments

All kinetic measurements were carried out using a Cecil 292 spectrophotometer equipped with a water-jacketed cell holder. The reactants (dye and K₂S₂O₈+NaOH) were thermostated for 15 min., then mixed thoroughly and quickly transferred to an absorption cell. The progress of the reaction was monitored at 540 nm. The pH of the reaction was adjusted using Griffin pH-meter fitted with a combined glass calomel electrode.

3. Results and Discussion

Kinetic study for oxidation of (RAH) by (KPS) was followed at _max= 540nm. Figure 2 shows the decreasing of absorbance with time. Figure 2 also shows that about 85% of rosaniline is removed in 60 minutes at temperature 25 ^oC.

3.1 Kinetic study

In the present study, zero-, first- and second-order reaction kinetics were used to study the decolourization kinetics of (RAH) by (KPS). The individual expression were represented below

C_t = C_o– k_ot

lnC_t = -k₁t + ln C_o

1/C_t = 1/C_o+ k₂t

where C_t is the concentration of (RAH) at reaction time t.

Regression analysis based on the zero-, first- and second-order reaction kinetics for the decolourization of (RAH) by (KPS) was conducted and the results were shown in (Fig. 3). Since plotting of A_t versus time did not give straight line zero-order kinetics is excluded. Comparing the regression coefficients (R²) obtained from (Fig. 3b) and (Fig. c), it can be seen that R² of the second-order reaction kinetics (Fig. 3c) was 0.9995, which was obviously much better than that based on the first-order (R² = 0.9394) reaction kinetics. The results indicated that the decolorization kinetics of (RAH) followed the second-order kinetics well.

Based on the above analysis, the second-order kinetic rate constants for the decolourization of (RAH) at different reaction conditions were shown in Table 2.

3.2. Effect of pH

A thermally activated persulphate oxidation system is known to involve SO4^–. and HO^. radicals depending on the pH of the medium. According to literature survey, SO4^–. is predominant oxidant radicals at pH 7, both SO4^–. and HO^. are present at neutral pH and HO^. is predominant radical at pH  9[9]. Keeping the concentration of (RAH), (KPS) and temperature constant and change the pH in range 3.5 to 9.5, the rate of reaction is increased by increasing the pH value, (Fig. 4). Increasing the pH in range 3.5 to 9.5 increased observed rate constant from 6.7x 10^-4 to 6.8 x 10^-3 mol dm³ s^-1, Table 2. This is probably attributed to the effect of hydroxyl ion on (RAH) which converting it to a carbinol base with no conjugation structure. All studies have done at pH = 9 since dyeing cotton performed in alkaline medium [17].

3.3. Effect of dye concentration

The effect of initial (RAH) concentration of aqueous solution of rosaniline on oxidation process by persulphate was investigated since pollutant concentration is important parameter in wastewater treatment. The observed rate constant decreases linearly with increasing the initial concentration of rosaniline, (Fig. 5). This is attributed to relatively lower of SO₄^-. and HO^. results from the increasing of rosaline concentration while concentration of persulphate and hydroxyl ions remains the same. The obtained results was in good agreement earlier reported [18-19].

3.4. Effect of persulphate concentration

Increasing the persulphate concentration in rang 4×10^-3 to 2.4x 10^-2 mol dm^-3 increasing the rate constant from 3.13×10^-3 to 9.92×10^-3 mol^-1dm³ s^-1 at pH= 9 and temperature 40^oC, Table 2. Plotting of log k_obs versus log [K₂S₂O₈] give straight line of slope equals 0.63 indicating the order of reaction with respect to persulphate is half, (Fig. 6). This is similar to results obtained by T. Mushinga and S. Jonnalagadda [20]. Also the fraction order ( n = 0.779) with respect to persulphate is obtained by M. Ahmadi et al[19].

3.5. Effect of temperature

The variation of the temperature in range of 303-318 K increases the rate of decolourization of rosaniline, (Fig. 7). The activation energy was calculated from Arrhenius plot and Eyring equation and was found to be 62.11 kJ mol^-1. The activation energy for decolourisation of RY84 was 45.84 kJ mol^-1[19] while for decolourisation of CV by persulphate was 28.9 kJ mol^-1[18]. Chen-Ju Liang and Shun-Chin Huang demonstrated that the activation energy for MB with persulphate was 87 and 90 kJ mol^-1 in acidic and alkaline medium respectively [9]. The other activation parameters were determined and are found to be 90.33 kJ mol^-1 and -98.44 JK^-1 for free energy and entropy respectively.

3.6. Effect of inorganic anions

The potent effect of persulphate as oxidizing agent in destroying the organic contaminants is high redox potential of sulphate free radical. The presence of other species in waste water such as chloride, sulphate and phosphate could reduce its oxidation efficiency. It is reported earlier [21] that chloride can react with sulphate free radicals according to the following mechanism:

Chloride concentrations had insignificant effect in studied range [0.008- 0.08 mol dm^-3]. Also the presence of sulphate did not reduce the decolourisation rate. Similar results are observed earlier [21-23].

4. Reaction Mechanism and Rate Law

The probable mechanism of reaction involves the break of conjugation of roseaniline hydrochloride. Since the fraction order of persulphate is observed, It likely that the initial reaction is thermal decomposition of persulphate,

Applying equilibrium approximation and assuming an equilibrium between the reactant and product of (eq. 1)

From (eqs.5& 6)

If the proposed mechanism and rate low are probable, so plotting of k_obs versus [S₂O₈^2-]^1/2 should yielded straight line passing by origin and the slope should be equals k x K^1/2. Using the data in Table 3 and the plot gave straight line passing by the origin with slope =0.065 (R2 = 0.9557) which support the proposed mechanism.

Conclusion

In this study, (RAH) was successfully degraded in aqueous solutions by the persulphate and it was found that the reaction of (RAH) degradation follows the second-order kinetic model with respect to (RAH) and half order to persulphate. The activation energy for (RAH) degradation with the persulphate was determined to be 62.11 kJ mol^-1.The presence of inorganic ions such as NaCl and Na₂SO₄ had no effects on the (RAH) degradation.

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