CC BY-NC-ND
0Egyptian Journal of Petroleum (2013) 22, 211-216
Egyptian Petroleum Research Institute Egyptian Journal of Petroleum
www.elsevier.com/locate/egyjp www.sciencedirect.com
FULL LENGTH ARTICLE
Enhanced removal of Methylene Blue by electrocoagulation using iron electrodes
Mohamed S. Mahmoud a *, Joseph Y. Farah b, Taha E. Farrag
a Chemical Engineering Department, Faculty of Engineering, El-Minia University, El-Minia, Egypt b Department of Chemical Engineering and Pilot Plant, National Research Center, Cairo, Egypt
KEYWORDS
Wastewater; Electrocoagulation; Magnetic field; Methylene Blue
Abstract The removal of pollutants from effluents by electrocoagulation has become an attractive method in recent years. The study deals with the enhancement of removal of Methylene Blue dye by using an electromagnetic field during the electrocoagulation process. Effects of electrolyte concentration, dye concentration, intensity and the direction of the electromagnet on the decolorization efficiency have been investigated. The formed ferric hydroxide flocs trap colloidal particles and make solid-liquid separation easier during the next stage. The electrocoagulation stages must be optimized in order to design an economically feasible process. The results showed that the optimum electrolysis was 10-20 min at a current density of 8 mA/cm2, while the optimum concentration of the electrolyte (NaOH) was found to be 2 wt.% when the dye concentration was 50 mg/L. The utilization of an electromagnetic field enhanced the dye removal due to the induced motion of paramagnetic ions inside the solution. The power consumption required to remove the dye was reduced by 45% in the case of applying an electromagnetic field.
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1. Introduction
The effluents of many industries (textile, leather, pulp and paper, printing, photographs, cosmetics, pharmaceutical, food) contain dyes, which represent an important environmental problem [1]. As example, the textile industry utilizes about
* Corresponding author. Address: Chemical Engineering Department,
Faculty of Engineering, Minia University, Minia 61111, Egypt. Tel.:
+ 20 1227553150; fax: +20 862346674.
E-mail address: msmm122@yahoo.com (M.S. Mahmoud).
Peer review under responsibility of Egyptian Petroleum Research
Institute.
10,000 dyes and pigments [2]. While about 20-50% of reactive dyes used in textile fabrics can be released into waterways [3]. Lang [4] reports that dyes are normally found in dye house effluents at concentrations ranging from 10 to 50 mg/L. In fact, reactive dyes are hydrophilic, therefore, they have low affinity to adsorb to biomass and generally pass through conventional biological wastewater systems [5].
Several methods have been used for color removal from wastewaters [6]. This includes biological aerobic (e.g., activated sludge, SBR, bio-filter) or anaerobic treatments, enzymatic biodegradation (actinomycetes, fungi), chemical oxidation (e.g., H2O2/Fe2+ (Fenton), hypochlorite) or reduction (e.g., FeO), electrochemical oxidation (e.g., O3/UV, O3/H2O2, O3/UV/ H2O2, H2O2/UV), photo-degradation (e.g., TiO2/UV, photo-Fenton), adsorption (e.g., activated carbons, silica, biosorbents), membrane separation (e.g., microfiltration,
1110-0621 © 2012 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ejpe.2012.09.013
ultrafiltration, nanofiltration), chemical coagulation/floccula-tion (e.g., aluminum, iron or calcium salts), and electrolytic treatments, which include electro-oxidation, electro-floccula-tion, and electrocoagulation.
Electrocoagulation (EC) is a process consisting of creating metallic hydroxide flocs within the wastewater by electro-dissolution of soluble anodes, usually made of iron or aluminum. This method has been practiced for most of the 20th century with limited success. Recently, however, there has been renewed interest in the use of EC owing to the increase in environmental restrictions on effluent wastewater. In the past decade, this technology has been increasingly used in developed countries for the treatment of industrial wastewaters, by allowing the particles to react with: (i) an ion having an opposite charge; or (ii) a floc of metallic hydroxides generated within the effluent [7]. The EC process is highly dependent on the chemistry of the wastewater, especially its conductivity. In addition, other characteristics such as pH, particle size, and chemical constituent influence the process.
The application of an electromagnetic field (EMF) for conducting fluids is known as Magneto hydrodynamics (MHD) [8]. The idea of MHD is that magnetic fields can induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself. The effect of utilizing magnetic field (MF) in electrochemical reactions has been the subject of many investigations [9-15]. The main interest in most of these investigations is directed toward the regions where the rate-determining step of the reaction is diffusion (or at least, region of mixed kinetics). The application of the electromagnetic field was carried out to study the crystal growth under high magnetic field [10]. Other studies found that high magnetic field has an effect on the anodic behavior of a ferromagnetic electrode [11], and on electro-deposition of CoFe alloys [12]. Mass transfer was found to increase during stirring a suspension of magnetic particles by the use of an alternating magnetic field [13]. Bund et al. [14] studied the influence of an external magnetic field on the electrochemical behavior of several systems, they concluded that the increase of the limiting current density for copper depositions from a cell in the presence of a perpendicular magnetic field has been explained by the interplay of natural convection and the Lor-entz forces acting on the resulting flow profiles.
The study aims to investigate the effect of non-uniform EMF on the EC process to remove Methylene Blue from effluents. The iron core coil was assembled in three different positions to give variant directions of the electromagnetic field.
2. Theory
Electrocoagulation (EC) process involves the generation of coagulants in situ by dissolving iron ions from iron electrodes. The in situ generation of iron cations during the EC process takes place at the anode, whereas at the cathode, typically H2 production occurs. Various reactions take place in the EC process, where iron is used as the electrode [16]:
Fe + 2H+ ! Fe2+ + H2
2.1. Anodic reactions
Fe ! Fe2+ + 2e~
Also, in acidic pH, the electrode is attacked by H+ and enhances its dissolution by the following reaction:
Other reactions taking place in the vicinity of the anode are: Under alkaline conditions
Fe2+ + 2OH- ! Fe(OH)2 (3)
Under acidic conditions
Fe2+ + 0.2502 + 2.5H2O ! Fe(OH)3 + 2H+ (4)
Ferrous ions are oxidized to ferric ions by oxygen in the aqueous phase
Fe2+ + O.25O2 + O.5H2O ! Fe3+ + OH- (5)
The oxygen evolution reaction may also take place at the anode and is represented as:
2H2O ! O2 + 4H+ + 4e- (6)
2.2. Cathodic reactions
2H2O + 2e- ! H2 + 2OH-
The insoluble metal hydroxides of iron can remove pollutants by surface complexation or electrostatic attraction. The pre-hydrolysis of Fe3 + cations also leads to the formation of reactive clusters for wastewater treatment. The EC process is characterized by a fast and efficient rate of pollutant removal, compact size of the equipment, simplicity in operation, and low operating and equipment costs.
3. Materials and methods
The experimental setup is shown schematically in Fig. 1. The EC unit consists of a 2L electrochemical cell with iron electrodes. The dimensions of the electrodes are 0.04 x 0.08 m and inter electrodes distance was 0.02 m. The current density was maintained constant at 8 mA/cm2 by means of a precision DC power supply (ADAK-PS 808). The dyestuff used in the experiments was Methylene Blue (MB). The dyestuff was used as a commercial salt supplied by the Al-Nasr Company Egypt. The chemical structure of MB is shown in Fig. 2 where its properties are represented in Table 1. The dye was made in a stock solution of concentration 1000 mg/L and was subsequently diluted to the required concentration using distilled water. All samples were allowed to settle for 5 min and filtrated then analyzed using UV-Visible Spectrophotometer (8700 series Unican UV/V) that gave good linearity for the absorbance versus MB concentration at its maximum absorbance wavelength of 665 nm. Percentage of dye removal was calculated according to the following equation [9]:
Co - C Co
where Co and C are the initial and instant concentrations of the dye (mg/L), respectively. The iron-core coil was used to generate the magnetic field. The coil was placed at three different positions outside the EC unit namely: (1) below the unit, (2) perpendicular to the electrodes and in front of the EC unit, and (3) parallel to the electrodes and in front of the EC unit. This coil was connected to a DC power supply which generates a magnetic flux density up to 8 T. Consequently EMF was generated and its direction was determined by right hand rule. The total current and voltage were measured to calculate the
EC cell
Figure 1 Schematic diagram of the experimental setup.
Figure 2 The chemical structure of Methylene Blue (MB).
Table 1 Physical properties of Methylene Blue dye.
Property Value
Color Index (C.I.) 52030
Trade name Methylene Blue
Scientific name Basic Blue 9
Color Dark green to blue
crystals or powder
Maximum wavelength (kmax) (nm) 665
Molecular diffusivity (Dmol) (at 25 °C) 4.7 x 10~6 (cm2/s)
Solubility in water Soluble in water
Chemical formula C16H18N3OS-3H2O
Molecular weight 373.5 (g/gmol)
Molecular volume 390.2 (cm3/g mol)
power consumption. Since the air cooling of the coil was satisfactory, no additional cooling equipment was required. The whole apparatus was temperature controlled by a water bath and thermostat.
4. Results and discussion
4.1. Effect of electrolysis time
Reaction time influences the treatment efficiency of the electrolytic process. During electrolysis, the anodic electro-dissolu-
tion led to the release of the coagulating species. The dye removal efficiency depends directly on the concentration of metal ions produced on the electrodes. Increasing the time of electrolysis leads to an increase in both the concentration of metal ions and the accumulation of hydroxide flocs.
The effect of time of electrolysis was studied at constant current density of 8 mA/cm2 and initial pH 12. As shown in Fig. 3, an increase in the time of electrolysis from 10 to 60 min yields an increase in the dye removal efficiency from 68% to 92%. It also could be noticed that after 20min, only 10% further removal could be achieved which is not economical. Therefore we propose the efficient time of EC is 1020 min.
4.2. Effect of electrolyte concentration
Due to the chemical substances added at a high concentration from dyeing and finishing processes in the textile industry, the textile wastewaters have a broad variation in ionic strength. The greater ionic strength will generally cause an increase in current density at the same cell voltage, or the cell voltage decreases with increasing wastewater conductivity at constant current density [7]. Therefore, it is necessary to investigate the effect of wastewater conductivity on EC in terms of color removal. The conductivity of the wastewater is adjusted to the desired levels by adding an appropriate amount of NaOH. Kashefialasl et al. [7] reported that the type of electrolyte does not have any important effect against color removal efficiency, meanwhile, the applied voltage and conductivity of solutions provides the pronounced effect. NaCl is widely used as an electrolyte in industrial scale because it is cheap. However, in this study we used NaOH since it has higher conductivity than NaCl for the same concentration (e.g., at concentration of 0.1 wt.% and temperature of 250C, the conductivity of NaOH is 5820 is/cm compared with 1990 is/cm for NaCl) [17].
0 20 40 60 80 100
Time, min
Figure 3 Effect of electrolysis time on the removal efficiency of MB dye: current density = 8 mA/cm2; [NaOH] = 2 wt.%; initial pH =12; initial dye concentration = 50 mg/L, interelectrode distance = 0.02 m.
Figure 5 Effect of initial dye concentration on the removal efficiency of MB dye: [NaOH] = 2 wt.%; interelectrode distance = 0.02 m; current density = 8 mA/cm2.
Fig. 4 shows the effect of concentration of NaOH on the percentage color removal. It is noticeable that when the concentration of NaOH in solution increases, the solution conductivity increases. Also, there is an increase in removal efficiency up to 80% when electrolyte concentration was 2-4 wt.%.
4.3. Effect of initial dye concentration
The dye solution with different initial concentrations in the range of 25-100 mg/L was treated by EC in optimized current density and time of electrolysis values. According to the results in Fig. 5, percentage of dye removal decreases with increasing initial dye concentration, however this decrease was not distinguishable. Up to 20 min for all different concentrations, the adsorption capacity of flocs was not exhausted.
Adsorption on iron hydroxide is the main dye molecule removal pathway [6]. So, for a constant current intensity, there is obviously the same amount of electrogenerated iron cations and hence the same amount of coagulating species. It is more likely that with increasing the initial dye concentration, less adsorption sites are available to capture the extra organic dye molecules.
4.4. Effect of direction of electromagnetic field
In the present study, the effect of both intensity and direction of EMF was studied at electromagnetic flux density of 0-8 T. The results are depicted in Fig. 6. It is evident that after 10min, the percentage of color removal reached 98% with the application of EMF while it was 68% without it. The results also showed that placing the EMF below the EC unit gives fairly better dye removal efficiency than the other two positions. This finding is in harmony with that obtained by Krause et al. [18] who mentioned that the magnetic flux intensity parallel to the electrode surface [magnet perpendicular to the electrodes] is better than the perpendicular one [magnet parallel to electrodes].
As is generally accepted, magnetohydrodynamic (MHD) effects are induced in an applied magnetic field parallel to the surface [18]. The generated Lorentz force (~LN/m3) (Eq. (9)) leads to an agitation of the electrolyte:
Fl = J x ~ (9)
where J is the local flux of ions. As a result, the thickness of the diffusion layer in front of the electrode decreases and in consequence the limiting current density increases. The MHD effects are only important for diffusion-controlled reactions, which applies to the EC of dyes at low concentrated electrolytes.
Since Fe ions are paramagnetic, the effects of an applied magnetic field are mainly focused on both the induced motion of Fe ions due to Lorenz force and the effects over the process of mass transfer in solution. Several studies have reported the increasing mass transport rates under the influence of magnetic field. Such effects were due to the MHD flow produced by an applied magnetic field on metal/solution interfaces. Such a MHD flow in an electrochemical system is described by the force per unit volume acting on solution (Eq. (9)) [19].
Orientation of the magnetic field with respect to the ion flux will largely determine the influence of the magnetic field on the rate of mass transfer. Specially, (FL) and convective mass transport, are anticipated to be the largest when the external magnetic field is oriented perpendicular to the direction of the ion flux (i.e., B is parallel to the electrode surface).The placement of working electrodes and the way of the imposition of magnetic field during the experiments are previously shown
> 40--
t 20------
0 -----
0 1 2 3 4 5
Cnuh, wt%
Figure 4 Effect of electrolyte concentration on the removal efficiency of MB dye: initial dye concentration = 50 mg/L, interelectrode distance = 0.02 m, current density = 8 mA/cm2, time of electrolysis = 20 min.
>■ 20
- 1 H I
.....if
—♦— No magnetic field ■ Magnetic field below the electrodes
M agnetic field beside electrodes —6— M agnetic field perpendiular to elecrodes
JO 40 Time, min
Figure 6 Effect of application of EMF on the removal efficiency of MB dye: [NaOH] = 2wt.%; interelectrode distance = 0.02 m; current density = 8 mA/cm2, B = 8 T.
in Fig. 1. When the magnetic field is vertical and parallel to the working electrode surface, it will exert a Lorentz force and thus superimpose a MHD velocity ( ~mag) (m/s)on any charged particle moving in the interfacial diffusion layer. ( ~mag) is perpendicular to both the magnetic field direction and the electrode surface. The total velocity ( ~T) of a charged particle in the presence of a magnetic field can be expressed as the vector sum,
~T = ~mag + ~df
where ~df is the velocity of the charged particle due to the concentration gradient. There is ~T > ~df in the presence of magnetic field, i.e., the mass transfer rate is increased by the magnetic field.
The above results indicate that an increased mass transfer rate is caused by a convective flow at the electrode/solution interface produced by a magnetic field. Increasing the mass transfer rate could lead to thinning of the interfacial diffusion layer and change of the concentration gradients of reactive species.
4.5. Power consumption for EC
It is clear that a technically efficient process must also be economically feasible. The major operating cost of EC is associated with electrical energy consumption during the process. Although increasing current density and operating time enhances the efficiency of EC, it causes to raise the cell voltage, energy consumption and operating costs consequently.
In this investigation, we comparatively studied the application of different magnetic field intensity ranging from 0 to 8 T. The electrical energy required for the removal of MB dye was calculated in terms of kWh/kg dye removed using Eq. (11):
where U is the cell voltage (V), I is the current intensity (A), tEC is the efficient EC time (h), and m is the mass of dye removed during the efficient EC time (kg). The energy consumptions were presented in Fig. 7. The energy consumption was enhanced by 45% in case of application of 8 T (from 18 to 9.9 kWh/kg dye removed). Although we could not check these
Figure 7 Power consumption for removal of 1 kg dye Vs magnetic flux density: [NaOH] = 2wt.%; interelectrode distance = 0.02 m; current density = 8 mA/cm2, EC time = 10 min.
results from the viewpoint of repeatability, however these results showed that application of electromagnetic field in EC could be a promising tool for reducing the energy consumption and diversify the application of this method for wastewater treatment.
5. Conclusion
The enhancement of EC of dye solution (Methylene Blue) by means of application of electromagnetic field was studied and the following conclusions were obtained:
• EC was affected by the electrolyte concentration, initial dye concentration and time of electrolysis.
• The dye was effectively removed (80%) after 20 min.
• At 298 K, the optimum conditions for the removal of MB dye with an initial concentration of 50 mg/L are: 1020 min electrolysis time, current density of 8 mA/cm2, and interelectrode distance of 0.02 m.
• The application of eletcrmagnetic field enhanced the EC process due to both the induced motion of Fe ions and the influence of mass transport of the gross solution.
• The best position of electromagnetic field was perpendicular to the motion of ions in the electrolyte.
• The energy consumption for EC was enhanced by 45% in the case of application of EMF of 8 T compared to the energy consumption for EC without EMF; (from 18 to 9.9 kWh/kg dye removed).
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