Kinetic and Thermodynamic Study of Oxidative Decolourisation of a Typical Food Dye (Tartrazine) in an Aqueous Environment

The study was carried out to describe the kinetics and thermodynamics of hydrogen peroxide oxidation of a typical food dye (Tartrazine). The effect of different operational factors were investigated spectrophotometricallyat wavelength460 nm under pseudo first order reaction.These included concentration of the oxidant and the dye, the pH, ionic strength and temperature of the reacting medium and the presence of transition metal ion as homogenous catalyst. A complete and smooth decolourisation was observed. The results showed that the rate of oxidation of dye increased with increasing in concentration of substrate and oxidant. Increasing in temperature, ionic strength and pH of the basic reaction medium also raised the reaction rate. The rate of oxidation also increased with increasing in the concentration of Fe (III) ion. Pseudo second order rate constant (k2) obtained was 1.95 x 10 Ms and 3.8 x10Ms in the absence and presence of Fe (III) ion respectively. The Arrhenius activation energy for the oxidation in the absence and presence of Fe (III) ion were 47.23 kJmol and 42kJmol respectively. Other thermodynamic parameters showed entropy of activation (ΔS), free energy of activation (ΔG) and Enthalpy of activation of the reaction (ΔH) in the presence of Fe (III) as -34.7 JKmol, 48.4 kJmol and 40.30 kJmol respectively. The results in the absence of Fe (III) ion were -24.6 JKmol, 51.2 kJmol and 44.0 kJmol respectively. The relative lower activation energy (Ea),fairly higher negative value of (ΔS) and higher (ΔG) , with higher rate constant in the presence of Fe(III) ion showed Fe(III) ion enhancement of rate of decolourisation. DOI:https://dx.doi.org/10.4314/jasem.v24i6.12 Copyright: Copyright © 2020 Okeola et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 27April 2020; Revised: 22May 2020; Accepted: 15June 2020

Nowadays foods and drinks contain various types of additives that are included for different desirable ends (Zeinab et al., 2017). Much studies have been focused on the use of additives and effect on man and his environment (Fonovich et al., 2012). Of course one the most widely used and dangerous additive is food dyes. Food dyes are often added to food stuff and drinks as expected or preferred by consumers in order to supply, intensify or restore the color to create desired colored appearance (Inetianbor et al., 2015). In United State of America, FDC (Food, Drug and Cosmetic) numbers are given to some colorant, for usein foods, drugs and cosmetics especially for synthetic food dyes that do not exist in nature. Also European number, (E no.) are used for all additives for both synthetic and natural approved for food application. (FDA, 2007;Zeinab et al., 2017). These dyes are environmentally persistent and have strong absorption band in the visible light regions. Their presence in bodies of water especially from effluents discharge generally reduces light transmission affecting aquatic biota (Meiden and Khalil, 2010); and Patel et al., 2016). Tartrazine (Tz) focused in this work is a common synthetic lemon yellow azo-dye which is used as food colouring agent. It is a commonly used colour all over the world, mainly for yellow, but can also be used with Brilliant Blue to produce various green shade. Products including Tz are confectionaries, soft drinks, flavor corn chips, cereal (corn flakes) cake mixes pastries, custard powder, ice cream potato chips, yogurt chewing gum, biscuit and many confectionary together with glycerine, lemon and honey product. Tz is referred in USA labeling as FD & C Yellow 5 and European tag as E102. Tartrazine is a water soluble food dye (Magda et al, 2016).Studies have been carried out on the controlling colouring agents and their colouration of aqueous environment. Biological approach (biodegradation) (Ghodake et al., 2011) and chemical methods (such chlorination, ozonation) (Shubha et al, 2014) are frequently used methods for the removal of dyes from aqueous solution. The oxidation process has been a method for the complete decolorization of dye, Harsa et al., in 2018 disclosed that oxidation process based on the generation of very reactive species such as radical ion (.OH) which then oxidizes a broad range of pollutants quickly and non-selectively. This research work describes the potential of Hydrogen peroxide in the oxidative decolourisation of tartrazine -a typical food additive. The resulting oxidized product could no longer absorb visible light. The paper deals with kinetics and thermodynamics of oxidation of tartrazine with Hydrogen peroxide in the presence of Fe (III) ion. Investigation was carried out on the effect of some working factors on the reaction rate during the oxidation process, such as concentration of substrate, oxidant and Fe (III) ion; ionic strength, pH and temperature of the reaction medium. The knowledge would serve in controlling the colouration of aqueous environment and the reduction of light transition effect on aquatic lives. Besides it could be of benefit to the workers in fishery and aqua culture, food dye industries and those who handle dye for purpose of staining (Falodun et al, 2015)

MATERIAL AND METHOD
Chemical reagents used in this study were hydrogen peroxide, Iron (III) chloride, tartrazine dye, sodium hydroxide, bicarbonate and sodium chloride salt, hydrochloric acid. They were MERCK and BDH products are analytical grade, they were used as received and where necessary subjected to further purification. Stock solutions of reagents were prepared in deionised water.
The substrates and the oxidant were also prepared in deionised water. The instrument used for this experiment includes spectrophotometer Beckman Coulter du 730, thermostated water bath and a pHmeter (CrisonMicroph 2000).the weighing balance (mettler p165) was used for all the weighing and thermometer (0-120 o c) was used to monitor the temperature.
Kinetic Measurement: Since change in concentration was monitored with spectrophotometer, the wavelength corresponding to the maximum absorbance (λ max) need to be ascertained from the absorption spectrum. The absorption spectrum of 0.01M aqueous solution of tartrazine was determined between the wavelength ranges of 410 nm-510 nm. Beer's law was verified between 2 × 10 -2 to 10 × 10 -2 moldm -3 of Tartarazine at (λ max) earlier ascertained and a plot of absorbance against concentration was made. Kinetic Measurement were made by preparing different set of reacting mixture. The kinetic runs were performed under pseudo-first order condition with the respective oxidants in excess over dye concentration Requisite quantity of each reaction mixture constituent was prepared. The reaction was initiated by rapid addition of the measured amount of oxidant solution to the rest reaction mixture and the entire contents were thoroughly mixed. The progress of reaction was monitor following the change in absorbance of Tz at λ max. The specific rates were evaluated for the reaction were obtained from plots of logarithm of absorbance (A) against time (t).The average initial rates kobs over three independent measurements were determined by linear regression from the slope of the concentration versus time plots. The rate constants were averages of at least three measurements. Wherek2 was the rate constant for the bimolecular reaction of Tz and H2O2, a and b were pseudo-order of the reaction with respect Tz andH2O2, the absorbance A at 640nm was proportional to the concentration of Tz. The slope of the ln A vs time defined the pseudofirst order rate constant kobs was determined. The second order rate (k2) was obtained from k2 = kobs/ (TZ).
The effects of the major system parameters on the kinetics oxidation of tartrazine dye such as the concentrations of the dye (tartrazine) and the oxidant (H2O2), ionic strength, pH and temperature of the solution were determine.
Experiments were also carried out in the presence and absence of Fe (III) ion to ascertain its catalytic influence of its presence in the reaction mixture. The quantities of testing parameter were varied while maintaining constant the amount of each of all other components in the system.
The kinetic procedure as describe above was then followed and pseudo-first order-order rate constant kobs were estimated from the slopes of the log absorbance versus time plots

Spectrum of Tartrazine:
The Absorption spectrum figure 1 shows the wavelength corresponding to the maximum absorbance (max) at 460nm. Verification of Beer's law between from a plot of absorbance against concentration, (Fig 2) gave a straight line graph that passed through the origin with ε = 3550 dm 3 mol -1 cm -3 . The second order rate constant k2 was obtained from the slope of the linear plot of pseudo-first order rate constant, kobs against respective dye concentration.
The result of the pseudo first order kobs as presented in table 1 shows that the rate of oxidation increased as the concentration of dye increasing both in the absence and presence of Fe (III) ion in reaction medium The value of pseudo second order rate constant (k2) obtained was 1.95x10 -3 M -1 s -1 and 3.8 x10 -3 M -1 s -1 in the absence and presence of Fe (III) ion respectively indicating the positive effect of Fe (III) ion. Such observation was made in a related work of Gamal et al 2017.    The oxidation rate which was observed to exhibit a steady increase with increasing in the ionic strength.
Such an influence of medium ionic strength on reaction kinetics have been observed  Effect of pH on dye oxidation: In the same vein rate constants were determined from varying the of pH of the reaction mixture from 8.70 and 11.20 while maintaining constant for other variables such as ionic strength, temperature, substrates (dye) and oxidant concentration. The rate constants observed kobs at respective pH are shown in Table 1 .The rate of oxidation have was observed to increase with increasing in pH (8.70 -11.20), the same trend have been observed in the work of Meiden and Khalil in 2010, suggesting that hydroxyl ions may be involved in the rate determining step of the oxidation reaction .

Effect of Fe (III) ion concentration on dye oxidation:
Also rate constants were determined from varying concentration of Fe (III) ion in the reaction mixture of oxidation of dye. The observed rate constants kobs was determined at different concentrations of Fe (III) ion As shown in Table 1  The other thermodynamic parameters evaluated include Enthalpy of activation of the reaction (ΔH # ), Gibbs free energy of activation (ΔG # ) and the entropy of activation (S # ) using following appropriate equations 6,7 and 8 (Mortimer et., al. 2002).   The value of the activation parameters are presented in table 3 for the decolorisation reaction in the absence and presence of Fe (III) ion respectively. The difference in the Arrhenius energy of activation (Ea) and a lower value with Fe (III) ion in reaction mixture showed an enhancement in the decolourisation reaction. The fairly higher negative value of entropy of activation (ΔS # ) and free energy of activation (ΔG # ) and lower Enthalpy of activation of the reaction (ΔH # ) also indicate this positive role of Fe (III) ion in the reaction medium.

Conclusion:
The kinetics of decolorisation of Tartrazine a typical food dye was studied in its oxidation reaction with hydrogen peroxide. The rate of reaction was found to increase with increasing in the concentration of oxidant and the dye. Increasing in temperature, ionic strength and pH of the reaction medium within the range in this study also increased the rate of oxidation reaction. The presence of Fe (III) ion also serves as positive catalyst. The reaction can therefore be applied appropriately to control the colouration of aqueous environment and can also be useful to those who handle dye for various purpose. Table 3 Arrhenius and thermodynamic activation parameters for the oxidation of dye in the absence and presence of Fe (III) ion by H2O2 at313K