New Electrode Based on Polymer Bacteria Modified Aluminum for Degradation of Phenol

The aims of this work were to examine the new electrode, based on polymer bacteria modified aluminum electrode for simultaneous production of electricity and degradation of phenol. This electrode is based aluminum modified by bacteria inserted in the polymer matrix, developed in situ on the surface. This electrode, designated subsequently by bacteria-polymer-aluminum, Showed stable response and was characterized with voltametric methods, as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The experimental results revealed that the prepared electrode could be a feasible for degradation of hazardous phenol pollutants. The sensor was successfully applied to the determination of phenol in a real sample with satisfactory results.


Introduction
Removal of phenolic hydrocarbons from wastewater prior to their discharge is a prerequisite in many chemicalmanufacturing industries since they can be fatal to the environment or human health even at ppb levels [1]. A number of studies have been carried out to decompose phenolic pollutants dissolved in wastewater during the last few decades. [2].Ultrasonic irradiation [3] in conjunction with other methods such as addition of TiO2, [4]. photochemistry, [5] and ozonation [6] was utilized for the decomposition of phenolic compounds, in which the cavitation process generates reactive free radicals responsible for chemical oxidation of organic pollutants. Biological treatments of wastewater such as microbial [7] and enzymatic [8] oxidations are probably the most environmentally compatible method, but it often requires pretreatment for high concentrations of organic pollutants and may result in the generation of hazardous byproducts. Direct or indirect electro-oxidation of organic compounds was also studied. While the electron transfer takes place between electrodes and decomposable species in direct electrochemical oxidation, indirect oxidation mainly uses electrochemically oxidized species as mediators for the destruction of organic compounds. For indirect electrooxidation, destruction of organic compounds by anodically generated chlorine and hypochlorite is well known. [9] Reactive and highvalent metal ions which are electrochemically generated from stable and low valent state were also utilized to degrade organic pollutants.
The aim of this work was to combine two methods of destruction of a toxic, aromatic, non-biodegradable product such as phenol. The electrochemical method has been used as a pre-reprocessing, which transforms organic products "non-biodegradable" to "biodegradable" products, which will be treated biologically thereafter. The bacterium used in this work has proved highly effective as a catalyst for the electrochemical degradation of phenol.

Reagents and apparatus
All chemicals were analytical grade and used without further purification. All solutions were prepared with distilled water. Voltammetric experiments were performed using a voltalab potentiostat (modelPGSTAT 100, Eco Chemie B.V., Utrecht, The Netherlands) driven by the general purpose electrochemical systems data processing software (voltalab master 4 software). The three electrode system consisted of a bacteria-polymer modified aluminum electrode as the working electrode a saturated calomel electrode (SCE) serving as reference electrode, and platinum as an auxiliary electrode. Prior to its modification the aluminum plate was polished with 0.05 μm alumina slurry for 2 min, rinsed with doubly-distilled water and sonicated in a water bath for 5 min.

Bacterial cultivation
The bacterial strain used in this study was Staphylococcus aureus ATCC 25923. The strain was cultured in Luria Burtani broth at 37°C for 24 h after culture; the cells were harvested by centrifugation for 15 min at 8400 xg and were washed twice with and resuspended in KNO3 solution with ionic strength 0.1 M. The physicochemical properties of this strain were measured by contact angle measurements. Provisions were made for oxygen removal by bubbling the solution with azotes gas for about 5 min then the solution was blanketed with azotes gas while the experiment was in progress. For reproducible results, a fresh solution was made for each experiment. The resuspended bacteria suspension was diluted with water to obtain needed suspension of different concentration before use.

Electrode preparation
The polymerized aluminum electrode (1 cm × 1 cm × 1 mm) has been immersed in a suspension containing the suspensions of the bacteria, after immobilization of the bacteria the polymer-aluminum matrix, we launch a scanning cyclic voltammetry, 10 cycles, in the potential range between -1.5 V and 1.5 V at 50 mV / s in a 1 M solution NaCl.

Electrodeposition of polymer Characterization by cyclic voltammetry
The polymerization of ε-caprolactone was carried out electrochemically, in a neutral electrolytic medium (0.1 M NaCl), the technique of successive scanning polymerization makes it possible to obtain stable films, the deposit can be controlled by following the decrease in intensity of current density. Figure 1represents a series of voltammograms obtained during the electropolymerization of ε-caprolactone on an aluminum plate electrode, recorded with a scanning speed equal to 80 mV / S, in the NaCl solution (0.1 mol / l ) containing 2 ml -caprolactam, in a range of potential between -2 V and 2 V. It can be seen that the decrease in the intensity of the currents during the sweeps accounts for the growth of the film deposited on the electrode surface with the number of cycles, the current densities of the voltammograms tend towards 0 indicating that the polymer developed has an insulating character [10]. Figure 2 represents the recorded voltammograms (CV) in an electrolytic medium (NaCl, 0.1 M) in the range of potential between -2 V and 2 V, respectively, by the electrode Al before (a) and after the polymerization (b) of the caprolactone monomer e at 100 mV.S -1. The voltammograms recorded for the two electrodes, in electrolytic medium, have different gaits, which suggests that the aluminum electrode is well modified by the polycaprolactone film.  Figure 3 shows the impedance diagrams (EIS) recorded, respectively for aluminum and aluminum electrodes modified by the polymer developed in situ on the surface of the aluminum electrode. The EIS recorded for the bare aluminum electrode has two time constants, the first at high frequency and corresponds to the transfer to the electron transfer at the metal / solution interface, the second at low frequency is a Warburg line who completes the diagram. The presence of a film on the surface of the electrode is illustrated by the appearance of two half loops entangled. The formation of the film on the surface of the electrode causes the reduction of the capacity of the double layer (Table 1) [11].   Figure 4 shows that the impedance diagrams recorded at the surface of the Al / polymer electrode in the absence and in the presence of phenol. In the cases the impedance diagrams are in the form of half-loops, the diameter of which corresponds to the electron transfer resistance, this resistance increases in the presence of the phenol, which suggests that the oxidation of the phenol is blocked by the formation of a polymer on the surface of the electrode.

Electrochemical study of the effect of phenol concentration
The concentration effect of phenol on the activity of the Al-polymer electrode towards the oxidation of phenol was studied by impedance spectroscopy. Figure 5 shows that the impedance spectroscopy curves have the shape of a semicircle for all concentrations of phenol in the high frequency region, which could be attributed to the electron transfer process. Increasing the concentration of phenol results in an increase in electron transfer resistance (Fig. 6).
- 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Figure 7 shows the recorded impedance diagrams, respectively for the Al / polymer and Al / Polymer / Bacteria electrodes, in aqueous NaCl (0.1 M) solution. In both cases, the diagrams have the form of a half-loop that appears at high frequency, and can be attributed to the process of electron transfer. The diameter of the half-loop corresponds to the transfer resistance of the electrons, the value of this resistance decreases in the case of presence of the bacteria at the electrode surface. This confirms the adhesion of microorganisms to the surface of the electrode that create a significant charge at the metal / solution interface (Table 2).

Electro oxidation of phenol on Al electrode / Polymer / Bacteria
In Figure 8, we illustrate the impedance diagrams recorded respectively at the surface of the Al / Polymer and Al / Polymer / Bacteria electrodes in the presence of phenol. We clearly see that the presence of a biofilm on the surface of the electrode causes a remarkable decrease in the electron transfer resistance deduced from the diameter of the after immobilization of bacteria before immobilization of bacteria half-loop appears at high frequency. This shows that the presence of bacteria significantly activates the oxidation of phenol. The electrochemical parameters deduced from the impedance diagrams are summarized in Table 3.

Influence of phenol concentration
The concentration effect of phenol on the Al-polymer-bacteria electrode was also studied by impedance spectroscopy. Figure 9 shows that the impedance spectroscopy curves are in the form of a semicircle for all concentrations of phenol in the high frequency region, which could be attributed to the electron transfer process. The increase in the concentration of phenol has a negative effect on the oxidation reaction, due to the formation of a polymer on the surface of the electrode Al / polymer / Bacteria, confirmed by the decrease in the capacity of the double layer ( Figure 10), which hinders the biodegradation of phenol. The increase in electron transfer resistance is linear ( Figure  11).  The AFM imaging taken respectively for the Al and Al / Polymer electrodes is given in Figure   12. The aluminum surface is relatively smooth; it has some defects, such as scratches that could be due to previous interventions. The surface of the Al / Polymer electrode has a different look, we see the disappearance of defects, the surface is homogeneous, and the organic film adheres to the entire surface of the electrode. The appearance of a very dense bacterial population in the case of an optimized surface, with a predominantly oval shape of particle size with a very important coverage rate of the surface occupied by the bacteria (fig13) [12]. These results show the studied electrodes Al, Al / Polymer and Al / Polymer / Bacteria, have very different surfaces, whose average roughness, shown in Table 5, and measured at places in the sample where the film was homogeneous without aggregates too large, varies significantly [9] (Table 4).

Practical application: in tap water
In order to evaluate the performance of bacteria-Polymer modified aluminum electrode by practical analytical applications, the determination of phenol was carried out in tap water without any pre-treatment. Figure 14: Impedance diagram obtained by the aluminum-polymer electrode (a) and Al polymer -bacteria electrode (b) in 100 ml tap water.

Electrochemical analysis of phenol
Electrochemical Impedance Spectroscopy (EIS) Figure 15 shows the recorded impedance diagrams, respectively for Al-polymer and bacteriapolymer-Al electrodes, in 100 ml medium of tap water contains 4 M of phenol, we can notice that the presence of bacteria to the surface of the electrode causes a decrease in the diameter of the semicircle, which corresponds to the decrease of the load transfer resistance. The     Electrochemical study of the effect of phenol concentration

Conclusion
The aim of this work was to group two methods of destruction of toxic organic products, of which phenol is the simplest molecule, the electrochemical method which is often blocked by the poisoning of the surface of the electrode by the formation of intermediates of the reaction and the biological method based on the biodegradation of phenol by bacteria. This biodegradation is limited by the nature of the toxic products that could expand the microorganisms hence the need to present a process of preliminary destruction of non-biodegradable toxic products. The Al / Polymer / Bacteria electrode has proved a great activity with respect to the oxidation of phenol. Electropolymerization took place in situ on the surface of the aluminum electrode by subjecting the Al electrode to a series of cyclic voltammograms. Deposition of the bacteria took place by contact of the Al / Polymer electrode with a solution containing the bacteria. AFM imaging has shown that the polymer adheres perfectly to the entire aluminum surface and the biofilm is composed of populations of bacteria dispersed over the entire surface.