Lebanese University, Faculty of Sciences (I), Chemistry Department, El Hadath, Lebanon

Abstract

Kinetic study of the indirect oxidation of methylene blue on Pt electrode in presence of several strong electrolytes is undertaken. Different operating conditions that affected the treatment process were studied in order to find the best conditions. The order with respect to methylene blue is zero order in presence of chloride, but it is second order in presence of bromide. The oxidation rate was affected by current density, halide concentration (KCl, KBr), nature of supporting electrolyte and initial pH. However, the initial dye concentration and temperature did not show a significant effect. The oxidation of methylene blue in presence of iodide, fluoride and sulfate is absent, but it is important in presence of chloride and bromide. The product of the indirect oxidation is the chloronated (bromonated) methylene violet bernthsen.

Keywords: Pt electrode, indirect oxidation, methylene blue, effect of supporting electrolyte.

Introduction

Methylene Blue (MB⁺) has many uses in a range of different fields such as biology and chemistry. Photosensitizer for singlet oxygen generation, antioxidant and antiseptic, stain for fixed and living tissues, antidote to cyanide and nitrate poisoning [1]. MB⁺ is used as optical oxygen sensors in the food industry [1, 2]. It is an organic dye usually used to dye cotton, wool, acrylic, and silk. When it is used recklessly, it can cause serious illness such as vomiting, hard breathing, mental disorder and sweating. Different methods are being developed to carry out the elimination of toxic compounds from wastewater [3, 4]. These include biological and physicochemical methods such as flocculation combined with flotation, adsorption, membrane filtration, coagulation, and ion exchange. The literature is rich in articles about the kinetic studies of the reduction and oxidation of MB⁺ chemically [5-8].

In recent years, electrochemical technologies have caused great interest because they offer effective means to solve environmental problems related to industrial processes and are more adaptable to a wide range of dye wastewaters [9-12]. Electrochemical treatment of textile wastewater with a high chloride concentration employing Ti/RuO₂, Ti/Pt and Ti/Pt/Ir electrodes was investigated by Polacro et al. [13]. Electrochemical methods for wastewater treatment mainly involve the direct and indirect electrochemical oxidation [14]. By indirect electrolysis, organic pollutants can be degraded by generating in situ strong oxidative intermediates that convert these pollutants into less hazardous products [15, 16]. The main oxidizing agents are active chlorine compounds, such as gaseous chlorine, hypochlorous acid, which are produced from original chlorides present in the wastewater [17]. The chemical and physical properties of chlorine in function of temperature, such as solubility and viscosity, are well studied [18]. Chlorine can electrochemically oxidize organic molecules quickly and irreversibly due to its intense oxidative activity. However, gaseous chlorine and hypochlorous acid can be easily interacted with organic molecules to form carcinogenic halogen compounds [19,20].

The efficiency of electrochemical oxidation is a function of electrode materials and supporting medium [14,17,19]. A brief review about the use of Boron Doped Diamond (BDD) in electrochemical oxidation provides total mineralization with high current efficiency of different organic molecules in real wastewaters [21]. The experimental results have indicated that the efficiency of electrochemical oxidation in chloride-mediated wastewaters is higher than that in chloride-free wastewaters for most of the electrodes [14]. The indirect oxidation of (MB⁺) using chloride resistant mixed oxide metal occurs by replacement of one of dimetylamino groups by oxygen accompanied by chlorination of the aromatic rings [19]. However, using other electrodes such as BDD, PbO₂/TiO₂ and TiRuO₂/Ti oxide leads to total mineralization and decolorization of MB⁺ solution [14,17,22].

Therefore, in this work, a full kinetic study of the indirect oxidation of methylene blue using platinum anode in the presence of different supporting electrolytes was undertaken to know the product of the reaction in such conditions and to find the best conditions of the MB⁺ oxidation.

Experimental

All chemical reagents used were of analytical grade. Methylene blue (MB⁺Cl -) is used as purchased from BDH (82 %, C₁₆H₁₈N₃SCl, MW: 319.85 g). Stock solution of MB⁺ was prepared by dissolving 100 mg in one liter of distilled water. The most electrolysis experiments are done at room temperature (293 K), in presence of 10 mg L^-1 MB⁺, 0.1 M KX (supporting electrolyte), where X is Cl^-, Br^-, with 10 mA, at pH 2. The experiments were carried out in a single combined electrolytic cell. The electrolysis is performed on 100 mL solution prepared by mixing 10 mL of 100 mg L^-1 of MB⁺, 10 mL of 0.1 M H₂SO₄, 10 mL of 1 M halide salt and 70 mL H₂O.

The cover of the cell is designed to hold multiple electrodes. The anode and the cathode are Pt wire from Taccusel. Each electrode measured 1.2 cm length and 0.15 cm diameter. The distance between the two electrodes was 5 cm. The electrolysis is done with a Chrono-Amperostat, type CEAMD-6, from Taccusel. The rate of the indirect oxidation of MB⁺ was followed by measuring the absorbance at 662 nm (max. wavelength of MB⁺). Absorption spectra were recorded on a double beam UV-Vis Specord 200 spectrometer, Analytic Jena AG (Germany). Measurements of pH were carried out using a Schott Gerate CG 819 pH-meter.

In order to determine the effect of the experimental parameters on the observed rate constant (Kobs) and the order with respect to MB⁺, the absorbance (A) of the solution was recorded after a predetermined time of electrolysis, during 10 minutes at 662 nm. The plot of A, Ln A and 1/A vs. time allows to determine the best order.

The indirect oxidation of MB⁺ was studied in function of several parameters such as initial pH (addition of diluted solutions of H₂SO₄ or NaOH), nature of electrolyte, concentration of MB⁺, temperature, current, and ionic force. Electrochemical oxidation efficiency of MB⁺ was determined in the presence of several strong electrolytes such as Na₂SO₄, NaF, KCl, KBr, KI. No variation of pH occurred during the experiments.

Results and discussions

Visible spectroscopy analysis

The oxidation at different concentrations of MB⁺ follows the same behavior and the slope of the line A vs. time remains approximately constant, so the order with respect to MB⁺ is zero, which means that the velocity of the reaction is independent of MB⁺ concentration.

Effect of strong electrolyte

The effect of the type of conductive electrolyte used in the electro degradation of toxic compounds affects strongly the efficiency of the process [22, 28]. The chloride salts give the best results since chloride plays in fact two roles: as strong electrolyte and as oxidant [29, 30]. It is interesting to study the effect of the nature of the strong electrolyte added on the oxidation rate. The electrolysis is done in presence of 0.1 M of the following supporting electrolytes: Na₂SO₄, NaF, KCl, KBr and KI (added separately). The results show that the decolorization in presence of Na₂SO₄ and NaF is absent. These results are expected since Na₂SO₄ and NaF are non-electro active in the conditions of the experiment, and confirm the absence of the direct oxidation of MB⁺ on Pt electrode.

As industrial wastewater contains already high amount of chloride, and the oxidation product in presence of KBr is similar to that in presence of KCl, we focused our study in the last part on the oxidation of MB⁺ in presence of KCl.

Effect of pH

This result confirms the decolorization of the organic compounds via the generated chlorine compounds [29, 30].

Effect of current intensity

This behavior can be explained by the decrease in chlorine solubility with the increase of salt concentration [18]. Similar result is also observed with the discoloration of MB⁺ in presence of sulfite [25].

Effect of temperature

It is clear that the variation of the rate of removal of color is negligible between 283 and 300 K; however, the reaction temperature above 303 K showed a decrease in the rate of color removal. The same result is observed with other dyes [20, 22]. The decrease in Cl₂ (g) solubility for T > 303 K is the major reason for the decolorization decrease [18]. Another reason may be the mass transport controlled reduction of hypochlorite according to the following cathodic loss reaction [32]:

Conclusions

The discoloration of MB⁺ by electrochemical oxidation was affected by the operating conditions such as pH, nature of strong electrolyte and many other parameters. The order with respect to MB⁺ follows zero order in presence of chloride, but it is second order in presence of bromide. The efficiency of the electrochemical oxidation of MB⁺ was proportional to chloride concentration and current density, but inversely proportional to the initial pH and ionic force. No significant effect of temperature and MB⁺ concentration on the rate constant has been detected under the experimental conditions. No direct oxidation of MB⁺ on Pt electrode in presence of Na₂SO₄ and NaF. The oxidation of MB⁺ by electro generated chlorine and bromine produced uncharged compound which probably corresponds to chlorinate or brominate methylene blue.

References