Electropolymerization of phenol on a vitreous carbon electrode in alkaline aqueous solution at different temperatures
Introduction
The formation of a passivating polymeric film on the electrode surface during anodic oxidation of phenols is of great importance since it interferes with the electrochemical process of oxidation involved in the treatment of phenolic wastes. Electropolymerization of phenols, occurring by direct electron transfer in the potential region of water stability, involves less than 2 electrons per molecule of monomer a value much lower than that required for a complete mineralization (28 e− per molecule in the case of phenol) [1]. Therefore, the removal of phenols based on anodic polymerization could be taken into consideration for waste water treatment as an alternative to the mineralization technique. Taking into account that electrochemical electropolymerization can be carried out at a lower potential than that of oxygen evolution, this technique appears as low demanding in energy.
It has been reported that electropolymerization of phenols occurs on different electrodes such as Au [2], glassy carbon [3], Pt [1], [4], [5], [6], [7], [8], PbO2 [9], [10], [11] and boron doped diamond (BDD) [12], [13], [14]. Polymer formation has been studied in several investigations [6], [7], [15], [16], [17], [18] and a global reaction mechanism is commonly accepted. Oxidation of phenol can be used as a model reaction; at pH value higher than pKa (9.89), during the first step of oxidation, the phenate anion gives rise to a phenoxy radical which can react with an another radical or with an unreacted phenate anion monomer, according to C–C or C–O–C couplings, to form dimeric products and then polymers. For example:C6H5O− → C6H5O + 1e−The film of polymer was considered by Bruno et al. [18] and Koile and Johnson [19] to be composed of a tightly adsorbed layer of products of oxidation and polymerization covered with polymeric and oligomeric layers; the tightly adsorbed layer was unaffected by oxygen evolution while the upper layers can be disrupted by gas evolution. Therefore, oxygen evolution is considered as beneficial to prevent complete deactivation of the electrode by a thick polymeric film; however, electron transfer remains hindered by a barrier at the electrode surface.
Electrochemical removal of phenols from aqueous solution based on anodic polymerization was attempted in batch type cell using carbon fiber [20], [21], [22] and granular activated carbon [23] as anodes having large surface areas; bulk electrolyses were carried out at a constant applied potential (around 50 mV more positive than the phenols oxidation peak potential) [20], [21], [22] and at very low constant anodic current density (0.017 mA cm−2) [23]. The electropolymerized phenols were immobilized on the anode surface. This method can be applied only for the removal of phenols present at very low concentration in water [20], [21], [22], [23]. Unfortunately, the anode used in this treatment of phenols must be regenerated or else disposed of by incineration or land filing. Otherwise, Zareie et al. [24] have shown that the removal of phenol from wastewater in the form of polymer suspended in the reactor can be achieved using carbon electrode and high anodic current density in the presence of NaCl (120 g dm−3); the authors suggested that the renewal of the oxidizing agent formed at the electrode by agitation caused most of the phenol oxidation to occur in the bulk solution without electrode fouling. However, this process uses a large amount of salt and would form toxic organohalogeno compounds.
In previous works [9], [10], [11], we have studied the electrochemical removal of phenol from aqueous solution based on anodic polymerization as a possible route that could improve the electrochemical wastewater treatment process. Results of bulk electrolysis conducted on Ta/β–PbO2 at high anodic current density (200 mA cm−2) and temperature (86 °C) have shown that 39% of the starting phenol can be removed as polymers dispersed in the reactor under the best operating conditions used [9], [10]. Using a high anodic current density (in the potential region of water decomposition) avoids anode fouling but simultaneously favors the oxidation of the polymeric film and oxygen evolution leading to current yield around 20% [9], [10]. It was also shown that the fraction of starting phenol converted into polymer increased substantially with temperature and that no electrode passivation (in that no noteworthy tension-cell increase was observed) occurred during electrolysis [10]. Gattrell and Kirk [3] have also observed during bulk electrolysis of phenol aqueous solution that passivation on Pt anode was reduced or prevented by increasing the temperature.
In the aim to find operating conditions that favor electropolymerization whereas avoiding anode fouling we have investigated phenol oxidation on a vitreous carbon electrode surface in alkaline aqueous solution at different temperatures [25–85 °C] using cyclic voltammetry and chronoamperometry. Our objective was to provide a better understanding of the effect of temperature and potential as the key factors in the film passivity.
Section snippets
Experimental
Cyclic voltammetry and chronoamperometry measurements were carried out in a conventional three-electrode cell (200 ml) using a computer controlled Eco Chemie Autolab Model 30 (Utrecht, The Netherlands). The working electrode was vitreous carbon disk with a geometric area of 0.0707 cm2. The counter and reference electrodes were respectively platinum spiral and Hg/Hg2Cl2/Cl− (sat.). Before each experiment, the working electrode was polished to a mirror with 1 μm alumina slurries on polishing sheet (3
Cyclic voltammetry
Fig. 1 displays cyclic voltammograms (first, second and sixth cycles) recorded at 25 °C on a vitreous carbon electrode in electrolytic aqueous solution containing potassium hydroxide at 1 M and phenol at 5 mM. The voltammogram obtained under the same experimental conditions but in the absence of phenol is also given for comparison in Fig. 1 (curve (1′)). Curve (1) shows the first positive direct scan; an anodic current peak corresponding to the oxidation of phenol is observed at 483 mV vs. SCE.
Conclusions
Electrochemical oxidation of phenol in alkaline aqueous solution has been studied on vitreous carbon electrode in the temperature range of 25–85 °C by cyclic voltammetry and chronoamperometry. The main results are as follows:
- 1.
One irreversible anodic peak is observed in the potential region of water stability corresponding to phenol oxidation by direct electron transfer. The potential shift of this peak towards the lower potential values and its height increase as the temperature increases are
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