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Latin American applied research

versión impresa ISSN 0327-0793

Lat. Am. appl. res. vol.42 no.2 Bahía Blanca abr. 2012

 

ARTICLES

Electrochemical technology for removing heavy metals present in synthetic produced water

K. R. Souza†, D. R. Silva§, W. Mata‡, C. A. Martínez-Huitle§ and A. L. M. L. Mata†

† Universidade Federal do Rio Grande do Norte, Centro de Tecnologia. Departamento de Engenharia Química - DEQ, Lagoa Nova CEP 59.072-970 - Natal, RN, Brasil. Tel/Fax.: +55 (84) 3215-3756. katiaregina@eq.ufrn.br, anadamata@eq.ufrn.br
‡ Universidade Federal do Rio Grande do Norte, Centro de Tecnologia. Departamento de Engenharia de Petróleo - DPET, Lagoa Nova - CEP 59.072-370 - Natal, RN, Brasil Brazil. Tel/Fax.: +55 (84) 3215 3905. wilson@ct.ufrn.br
§ Universidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra. Departamento de Química, Lagoa Nova CEP 59078-970 - Natal, RN, Brasil. Tel/Fax.: +55 (84) 3211-9224. carlosmh@quimica.ufrn.br, djalma@ccet.ufrn.br


Abstract - The performance of an electrocoagulation (EC) system with aluminium and iron electrodes for removing heavy metal ions (Cd2+, Cu2+, Cr3+, Sr2+ and Zn2+) present in synthetic produced water on laboratory scale was studied systematically. Experimental parameters such as applied current, flow effluent and sacrificial electrodes were investigated in order to understand their influence on the EC process. Increasing the current density accelerated the electrocoagulation process, but made it less efficient. Cd2+, Cu2+, Sr2+ and Zn2+ showed similar removal rates, under similar conditions, indicating a uniform electrochemical behavior. The study gave indications on the removal mechanisms of the investigated metals. Cd2+, Cu2+ and Zn2+ ions are hydrolyzed and co-precipitated as hydroxides. Cr4+, was proposed to be reduced first to Cr3+ at the cathode before precipitating as hydroxide. The process expenditure was estimated and reported showing the viability of this process as a green alternative, obtaining modest costs using Fe electrodes.

Keywords: Electrocoagulation; Wastewater Treatment; Heavy Metals; Removal Efficiency.


 

I. INTRODUCTION

The significance of oil and natural gas in modern civilization is well known. Nevertheless, like most production activities, oil and gas production processes generate large volumes of liquid waste. Oilfield wastewater or produced water contains various organic and inorganic components. Discharging produced water can pollute surface and underground water and soil (Rocha and Martinez-Huitle, 2011). Naturally occurring rocks, in subsurface formations are generally permeated by different underground fluids such as oil, gas, and saline water. Before trapping hydrocarbon compounds in rocks, they were saturated with saline water. Hydrocarbons with lower density migrated to trap locations and displaced some of the saline water from the formation. Finally, reservoir rocks absorbed saline water and hydrocarbons (oil and gas). There are three sources of saline water: (i) flow from above or below the hydrocarbon zone, (ii) flow from within the hydrocarbon zone and (iii) flow from injected fluids and additives resulting from production activities (Rocha and Martinez-Huitle, 2011).

The last category is called "connote water" or "formation water" and becomes "produced water" when saline water mixed with hydrocarbons comes to the surface. In oil and gas production activities, additional water is injected into the reservoir to sustain the pressure and achieve greater recovery levels. Both formation water and injected water are produced along with hydrocarbon mixture. At the surface, processes are used to separate hydrocarbons from the produced fluid or produced water. Then, produced water is considered to be one of the largest waste streams in the petroleum, oil and gas industry. Produced water from oil and gas industries often is permitted to be discharged to the environment. Water's toxicity and organic loading can generally characterize the impact of discharging produced water into the sea. Effects of produced water components on the environment are (Rocha and Martinez-Huitle, 2011): (i) increase in the salinity, (ii) dispersed and soluble oil contribution in marine ecosystems, (iii) inclusion of other compounds from treating chemicals, (iv) higher concentration of heavy metals than in seawater and (v) presence of radionuclides (Rocha and Martinez-Huitle, 2011). Most of the metals such as copper, nickel, chromium, silver and zinc are harmful when they are discharged without treatment. Due to their high to- xicity, petrochemical wastewaters are strictly regulated and have to be treated before being discharged. The most widely used method for the treatment of metal polluted wastewater is precipitation with NaOH and coagulation with FeSO4 or Al2(SO4)3 with subsequent time-consuming sedimentation (Kongsricharoern and Polprasert, 1995; Adhoum et al., 2004). Other methods include adsorption, ion exchange and reverse osmosis (Adhoum et al., 2004; and Meunier et al., 2004). Although precipitation is shown to be quite efficient in treating industrial effluents, the chemical coagulation may induce secondary pollution caused by added chemical substances (Adhoum et al., 2004). These disadvantages encouraged many studies on the use of electrocoagulation for the treatment of several industrial effluents (Adhoum et al., 2004).

The electrocoagulation process is based on the continuous in situ production of a coagulant in the contaminated water. It had been shown that electrocoagulation is able to eliminate a variety of pollutants from wastewaters, as for example metals and arsenic (Rocha and Martinez-Huitle, 2011; Kongsricharoern and Polprasert, 1995; Adhoum et al., 2004; Meunier et al., 2004; Kumar et al., 2004; Gao et al., 2005) clay minerals (Matteson et al., 1995; Holt et al., 2004), as well as oil (Xu and Zhu, 2004; Chen et al., 2000), chemical oxygen demand (Xu and Zhu, 2004; Murugananthan et al., 2004; Chen et al., 2000; Pouet and Grasmick, 1995), color (Jiang et al., 2002) and organic substances (Jiang et al., 2002; Vik et al., 1984). This technique does not require supplementary addition of chemicals, reduces the volume of produced sludge (Vik et al., 1984; Mollah et al. 2004; Holt et al. 2002; Meunier et al., 2006) and first economical studies indicate also a financial advantage compared to the conventional methods (Meunier et al., 2006).

The main objectives of the present work were to test the removal efficiency of heavy metals content in synthetic produced water by electrocoagulation process. Therefore, we studied (i) the potential and current density on the removal of Cd2+, Cu2+, Cr3+, Sr2+ and Zn2+ ions by electrocoagulation. We determined (ii) the removal rates of these metals and examined (iii) possible interactions between the metals during the coagulation process; and these results gave indications on the removal mechanisms by electrocoagulation. Finally, energy requirements and cost removal were estimated.

A. Theoretical approach of electrocoagulation (EC) process

Fundaments

A traditional physical-chemical treatment of phase separation for the decontamination of wastewaters before discharge to the environment is coagulation. It consists in the addition of coagulating agents such as Fe3+ or Al3+ ions, usually in the form of chlorides, for coagulate the suspended particles or precipitate and adsorb dissolved contaminants. The electrochemical technology can produce similar effects by means of the EC method. This technique uses a current to dissolve Fe (or steel) or Al sacrificial anodes immersed in the polluted water, giving rise to the corresponding metal ions that yield different Fe(II) (and/or Fe(III)) or Al(III) species with hydroxide ion depending on the medium pH (Martinez-Huitle and Brillas, 2009). These species act as coagulants or destabilization agents that bring about charge neutralization for pollutants separation from the wastewater. The coagulated particles can also be separated by electroflotation when they are attached to the bubbles of H2 gas evolved at the cathode and transported to the top of the solution where they can be separated. In general, the following main processes take place during an EC treatment:

(i) electrode reactions to produce metal ions from Fe or Al anodes and H2 gas at the cathode;

(ii) formation of coagulants in the wastewater;

(iii) removal of organic pollutants with coagulants by sedimentation or by electroflotation with evolved H2;

(iv) other electrochemical and chemical reactions involving reduction of organic impurities and metal ions at the cathode and coagulation of colloidal particles.

Many advantages for EC have been reported:

(i) More effective and rapid organic matter separation than in coagulation,

(ii) pH control is not necessary, except for extreme values,

(iii) the amount of chemicals required is small,

(iv) the amount of sludge produced is smaller when compared with coagulation. For example, the sludge formed in the EC method with Fe contains higher content of dry and hydrophobic solids than that produced in coagulation by the action of FeCl3 followed by the addition of NaOH or lime,

(v) sometimes the operating costs are much lower than in most conventional technologies (Espinoza-Quiñones et al., 2009).

However, this method presents as major disadvantages:

(i) Anode passivation and sludge deposition on the electrodes that can inhibit the electrolytic process in continuous operation mode,

(ii) high concentrations of iron and aluminium ions in the effluent that have to be removed.

Fe or steel anode

When an iron or steel anode is utilized in EC, Fe2+ is dissolved in the wastewater from Fe oxidation at the anode (standard potential E° = -0.44 V vs. SHE) as follows (Martinez-Huitle and Brillas, 2009):

Fe → Fe2+ + 2 e- (1)

whereas hydroxide ion and H2 gas are generated at the cathode from the reaction (E° = -0.83 V vs. SHE):

2 H2O + 2 e- → 2 OH- + H2(g) (2)

OH- production from reaction (2) causes an increase in pH during electrolysis. Then, insoluble Fe(OH)2 precipitates at pH > 5.5 and remains in equilibrium with Fe2+ up to pH 9.5 or with monomeric species such as Fe(OH)+, Fe(OH)2 and Fe(OH)3- at higher pH values. The formation of insoluble Fe(OH)2 can be written as:

Fe2+ + 2 OH- → Fe(OH)2(s) (3)

and the overall reaction for the electrolytic process from the sequence of reactions (1)-(3) is:

Fe + 2 H2O → Fe(OH)2(s) + H2(g) (4)

In the presence of O2, dissolved Fe2+ is oxidized to insoluble Fe(OH)3:

4 Fe2+ + 10 H2O + O2(g) → 4 Fe(OH)3(s) + 8 H+ (5)

and protons can be directly reduced to H2 gas at the cathode:

8 H+ + 8 e- → 4 H2(g) (6)

The corresponding overall reaction obtained by combining reactions (1), (5) and (6) is:

4 Fe + 10 H2O + O2(g) → 4 Fe(OH)3(s) + 4 H2(g) (7)

In acidic media of pH < 5, however, a greater quantity of Fe anode than that expected from Faraday law following reaction (1) is dissolved owing to the chemical attack of protons. Fe(OH)3 coagulates from pH > 1, i.e., it is present in much stronger acidic media than Fe(OH)2. This precipitate can then be in equilibrium with soluble monomeric species like Fe3+, Fe(OH)2+, Fe(OH)2+, Fe(OH)3 and Fe(OH)4- as a function of the pH range. Among them, hydroxo iron cations have a pronounced tendency to polymerize at pH 3.5-7.0 to give polymeric cations such as Fe2(OH)24+ and Fe2(OH)42+.

Once the insoluble flocs of Fe(OH)3 are produced, they can remove dissolved pollutants or metals by surface complexation or electrostatic attraction. The first mechanism considers that the pollutant can act as a ligand to bind a hydrous iron moiety of the floc yielding a surface complex:

pollutant-H + (HO)OFe(s) → pollutant-OFe(s) + H2O (8)

and the second one supposes that Fe(OH)3 flocs with surface complexes contain areas of apparent positive or negative charge that attract the opposite regions of the pollutant. Coagulation of these flocs forms particles that are separated from the wastewater by sedimentation or electroflotation.

Al anode

In the case of EC with Al, the anodic reaction leads to soluble Al3+ (E° = .1.66 V vs. SHE) (Martinez-Huitle and Brillas, 2009):

Al → Al3+ + 3e- (9)

and the cathodic reaction produces hydroxide ion and H2 gas:

3 H2O + 3 e- → 3 OH- + 3/2 H2(g) (10)

Al3+ is transformed into soluble monomeric species such as Al(OH)2+, Al(OH)2+, Al(OH)3 and Al(OH)4- depending on the pH range. Monomeric cations predominate in acid medium and Al(OH)4- in alkaline medium. The former can evolve to polymeric species such as Al2(OH)24+, Al6(OH)153+, Al7(OH)174+, Al8(OH)204+, Al13O4(OH)247+ and Al13(OH)345+. Soluble monomeric and polymeric cations are also converted into insoluble Al(OH)3 flocs by a complex precipitation kinetics. The overall reaction to give the latter product is:

Al + 3 H2O → Al(OH)3(s) + 3/2 H2(g) (11)

However, when an Al cathode is also utilized, it can be chemically attacked by OH- generated during H2 evolution at high pH values as follows:

Al + 3H2O + OH- → Al(OH) 4- + 3/2 H2(g) (12)

and treated wastewater contains higher amount of aluminium ions than those expected from reaction (9).

Similarly to EC with Fe, the removal of pollutants from wastewaters using Al can be explained by surface complexation, effect of the ionic strength rising and electrostatic attraction (Martinez-Huitle and Brillas, 2009). The surface complexes between pollutants and hydrous aluminium moieties are formed in the following manner:

Pollutant-H + (HO)OAl(s)→pollutant-OAl(s) + H2O (13)

In this case interaction attractions between pollutants (organic or inorganic) and monomeric and polymeric aluminium cations via precipitation and/or adsorption mechanisms become more important in the EC process. The large surface areas of freshly formed amorphous Al(OH)3 flocs can also adsorb soluble organic compounds and/or trap colloidal particles, which are thus separated from the aqueous solution.

II. METHODS

A. Model wastewater characteristics

All chemicals were of analytical grade and supplied by Aldrich, Brazil. Stock solutions was prepared from an standard solution multicomponent SpecSol® (Cd2+, Cu2+, Cr3+, Sr2+, Zn2+) by dissolving this solution in deionised water. The characteristics of the model wastewaters are: [Cd2+] = 5.272 ppm, [Cu2+] = 4.310 ppm, [Cr3+] = 3.798 ppm, [Sr2+] = 5.446 and [Zn2+] = 6.120. NaCl concentration was of 5,000 ppm in order to mimic the real concentration of salts dissolved in real produced water. Conductivity of the model wastewater was 20 mScm-1 and initial pH was 6.0.

B. Continuous experiments

Continuous experiments were carried out in a specific electrochemical reactor with a capacity of 2.5 L, as indicated in Fig. 1. Four electrodes were installed vertically with a spacer to ensure fixed distances of 4 cm in order to minimize the IR-drop (Mollah et al., 2004). The electrodes consisted of two aluminium plates (70 cm2) and two iron plates (128 cm2). To remove the oxide and/or passivation layer from the electrodes, the electrode surfaces were grinded with sandpaper before each experiment. The electrodes were operated in bipolar mode, so only the outer electrodes were connected to the power supply. The electrical contacts were established with crocodile clips. A defined current was applied by a DC power supply (DC Regulate Power Supply MPL 3305). During the experiments the direction of the current was reversed every 12 min to limit the formation of passivation layers (Barrera-Diaz et al., 2003). Every EC experiment was started with 2.5 L of metal solution; the desired current was applied and 5 mL samples were taken near the anode and the cathode from the bulk solution every 12 min. Different experimental conditions were tested, as reported in Table 1, in order to understand the behavior and determine the efficiency achieved at each experiment.


Fig. 1: Continuous recirculation flow electrochemical reactor: 1) power supply, 2) outlet, 3) inlet effluent, 4) sacrificial electrodes, 5) pump, 6) Continuous reservoir and 7) phase separator.

Table 1. Experimental conditions for each EC process, as a function of electrode material, current and flow rate.

C. Analytical measurements

The pH was monitored with a pH meter. Samples were filtered and acidized with HNO3 directly after the sampling. The total concentrations of Cd2+, Cu2+, Cr3+, Sr2+ and Zn2+ in solution were determined Inductively coupled plasma atomic emission spectroscopy (ICP-OES - Perkin Elmer). ICP-OES technique was optimized and standardized by USEPA (United States Environmental Protection Agency) e APHA (Standard Methods for the Examination of Water and Wastewater) methods.

Removal efficiency was estimated follows the equation:

(14)

where Ci and Cf are the average concentrations of heavy metals before electrolysis and after an electrolysis time t, respectively.

The EC reactor operational cost as R$ m-3 of the treated produced water effluent was calculated considering two parameters: the amount of energy consumption and the amount of electrode material used as preliminary economic study.

The electrical operational cost (EOC) of treated effluent and material cost (MC) were estimated in R$m-3, were included as part of the operational cost and calculated by applying equations 15 and 16 respectively, as follows (Espinoza-Quiñones et al., 2009).

(15)

where V is the applied voltage (V), i is the current density (Acm-2), Ae is the effective superficial area (cm2), t is the time of electrolysis (h), Veff is the sample volume of treated effluent (m3) and EEP is the electrical energy price (R$/kWh).

(16)

where i is the current density (Acm-2), Ae is the effective superficial area (cm2), t is the time of electrolysis (h), M is the relative molar mass of the concerned electrode (gmol-1), n is the number of electrons in oxidation/reduction reaction, F is the Faraday's constant (96,500 Cmol-1), EMP is the electrode material price (R$g-1) and Veff is the sample volume of treated effluent (m3).

The formation of MC takes into account the maximum possible mass of the iron theoretically dissolved by the anode per m3 of treated effluent (Espinoza-Quiñones et al., 2009).

III - RESULTS AND DISCUSSION

A. Removal efficiencies

As stated earlier, experiments have been carried out in a continuous recirculation flow electrochemical reactor in order to test the EC efficiency as alternative treatment for removing heavy metals from synthetic produced water. The removal efficiencies results are presented in Table 2 by varying operating conditions such as current, electrode material and flow rate. However, concentration decay data are included in the manuscript as Supplementary Electronic Material (SEM).

Table 2. Removal efficiencies of heavy metals from synthetic produced water as a function of experimental conditions.

From Table 2, it was observed that the removal of Cd2+, Cu2+, Cr3+ and Zn2+ was above 100% independent on current (experiment 2 and 4), but it was strongly dependent on flow rate and electrode material (Fe). At higher flow rates (4.3 ml/s), only Cr3+ was completely removed from synthetic effluent (experiment 1 and 6), while lower efficiencies were achieved for the other heavy metals (see Table 2, experiment 1 and 6). In contrast to the experiments using Fe electrodes, when Al electrodes were used removal efficiencies ranging from 50 to 70% were obtained for Cd2+, Cu2+, Cr3+ and Zn2+ metals (experiments 3, 5, 7 and 8). The Sr2+ decay concentration was independent on electrode material and current; however, the removal efficiency was improved when Fe electrodes was used at lower flow rates, achieving more than 70% of removal concentration.

Generally, an abundant evolution of H2 gas bubbles was observed at the cathode during all the EC experiments, whereas at the anodes only few O2 gas bubbles were evolved. The main reactions at the anode were aluminium dissolution and at the cathode hydrogen and hydroxyl ion formation. The minor oxygen formation at the anode competes with the aluminium dissolution and lowers the dissolved amount. This behavior gives explanation to the lower efficiencies obtained using Al electrodes (see, Table 2). These observations underline the following removal mechanisms for Zn2+, Cu2+ and Cd2+. Besides the direct reduction of the metal cations at the cathode surface, OH- ions produced at the cathode precipitate metal ions as hydroxides. This reaction buffers the pH as long as all metal ions are precipitated; consequently the pH increases. In fact, the pH increased within 5 min to values >9. It seemed that not all hydroxyl ions formed at the cathode were bound by Al3+ dissolved from the anode, which lead to a fast pH increase. This outcome is in agreement with the results reported by Jenke and Diebold (1984).

The removal mechanism for the chromate anion is different, depending on electrode material used. In the case of Fe electrodes, Cr3+ is oxidized to Cr4+ by Fe2+ an additional electrochemical reduction reaction at the cathode surface, as already indicated by other authors (Barrera-Diaz et al., 2003). In contrast, reduction of Cr4+ by Al3+ is not possible, the only mechanism with Al electrodes is the direct reduction at the cathode surface followed by precipitation as Cr(OH)3. This process would produce net 4 mol hydroxide ions per precipitated mol Cr, which explains the fast pH increase. Also, these assumptions explain the higher Cr3+ removal efficiencies obtained using Fe electrodes, and lower efficiencies achieved at Al electrodes (see, Table 2).

In these experiments the applied current (2.5 and 4.0 A) was varied to study the effect of different current densities on the metal removal. According to Faraday's law, increasing the current density leads to a higher Fe2+, Al3+ and OH- dosage by time. Thus the process can be accelerated. The question is if the higher current is completely converted into a higher coagulant dosage or if there are losses, which make the process less efficient, for example a higher oxygen formation at the anode. From results reported in Table 2, similar efficiencies (ranging from 50 to 60%) were obtained at Al electrodes varying applied current in all cases (experiments 3, 5, 7 and 8, Table 2). It confirms that probably part of current is loss by evolution of oxygen to anode. Perhaps, a different behavior could be observed applying lower current densities, increasing the removal efficiency, as already showed by other authors during EC with Zn, Cr and Cu (Adhoum et al., 2004; and Meunier, 2004). At the same time, metals compete for the hydroxide ions produced at the cathode and for sorption sites at the aluminium hydroxide surface (co-precipitation), decreasing the efficiency for each heavy metal.

On contrary, at Fe electrodes, the removal efficiency of heavy metals from synthetic produced water is independent on current density. Higher removal efficiencies were obtained at 2.5 or 4.0 A of applied current (above 100% for Cd2+, Cu2+, Cr3+ and Zn2+ and 70% for Sn2+); however, when flow rate was increased a decrease on the removal efficiency was observed (experiment 1 and 6). It can be due to a decrease in the interaction attractions between pollutants and monomeric and polymeric Fe cations via precipitation and/or adsorption mechanisms become important in the global EC process. The higher removal rates at lower current densities might be also explained with the lower reaction kinetics and concentration overpotential (Mollah et al., 2004). The concentration or mass transfer overpotential is caused by differences in electroactive species concentration near the electrode surface and the bulk solution due to the electrode reactions. Nevertheless, Cr3+ is only heavy metal to achieve higher removal efficiencies under all experimental conditions using Fe electrodes (100%, experiments 1, 2, 4 and 6); which confirms the different mechanism adopted by this ion respect to the other heavy metals.

It is important to comment that during EC with Fe electrodes, a change in a solution color was observed, increasing its brown color due to the ferrous oxides formation. Whereas, at Al electrodes, uncolored solution was obtained after EC treatment, but precipitate deposit was attained. Additionally, a modest increase in temperature was attained to all experiments, passing from 25°C to 29.5°C. This behavior is due to the resistance of interface electrode/solution during higher gas production (H2 or O2) that competes with EC process.

Tridimensional effect of experimental conditions for each heavy metal was projected, however, only a representative example is showed in Fig. 2 while additional graphs are reported in the SEM. In tridimensional surface analysis for Cd2+, the influence of each experimental condition on removal efficiency can be verified. Figure 2a shows that % of Cd2+ removal increases as a function of electrode used (aluminum or iron) by varying the applied current, but latter conditions did not influence this increase. In Fig. 2b, the increase in % removal occurs when the flow rate decrease. From Figures 2a and 2b, no influence of applied current on removal efficiency was observed. Figure 2c shows that using iron electrode and lower flow rate, higher removal efficiencies are favored.


Fig. 2: Tridimensional surface analysis for Cd2+ removal during EC process, as a function of experimental conditions: a) current, b) flow rate and c) electrode material.

For large application, it is also very important to estimate the treatment costs, as a function of volume of effluent treated. Table 3 presented the electrical operational cost (EOC) that is mainly composed of the electrical energy consumed and material cost (MC) to remove heavy metals from synthetic effluent under different conditions. As it can be observed, during the electrolyses of synthetic wastewaters, the energy consumption seems to be proportional to the applied current density. Nevertheless, energy requirements for removing heavy metals from synthetic solutions depend strongly on nature of electrode material. EC performed with Fe electrodes showed higher energy consumptions respect to Al electrodes experiments under similar conditions. Further it was also noticed that, higher removal efficiency was achieved using Fe electrodes than that Al electrodes, and it increases the energy requirements consequently a cost increase will be accomplished (see Table 3). Although EC is very effective, its energy consumption is relatively higher for practical application as the only treatment process due to the time employed; but it could be used as a pre-treatment of a refining technology divided in two step processes consisting in an electrochemical treatment followed by adsorption treatment or vice versa.

Table 3. Electrical operational cost (EOC) and material cost (MC) calculated from Eq. 15 and 16 respectively per volume of treated effluent during EC process for following experimental conditions (t = 0.2h, MFe = 55.84g/mol, MAl = 26.98g/mol, nFe = 2, nAl = 3, F = 96,500C/mol, Veff = 0.0025m3, EMPFe = 3.86E-03 R$/g, EMPAl = 0.017 R$/g ). Cost elimination per m3 (Brazilian currency - R$) and 1Ah = 3600C.

IV - CONCLUSIONS

The aim of the present work was to evaluate the applicability and the efficiency of the EC process. The results obtained confirm that it is possible to apply this method for the elimination of heavy metals from synthetic solutions of produced water. Removal levels and efficiency percentages were investigated as a function of applied current, electrode material and flow rate; obtaining % removal efficiencies from 43 to 100%, depending on experimental conditions. In the EC process a strong influence of nature of electrode material was observed, achieving higher removal efficiencies at Fe electrodes. By changing the flow rates, it was possible to carry out the EC obtaining good current and removal efficiency on Fe anode under lower flow rates. Color changes and turbidity, as a function of time during bulk electrolyses of synthetic wastewaters, was significantly affected by electrode material. The energy consumption makes inadequate EC for complete elimination of wastewaters polluted with heavy metals, but it can be a feasible as a pre-treatment process. Taking into consideration an electrical energy cost of about R$ 0.42 (Brazilian price, taxes excluded) per kWh (Agência Nacional de Energia Elétrica - ANEEL, Brazil), the process expenditure was estimated and reported in Table 3 in order to show the viability of this process as an alternative treatment, obtaining modest costs using Fe electrodes.

Finally, as iron and aluminum residues were generated after EC application, this waste also needs treatment and a appropriate destination. Therefore, a waste estimation production was obtained for each experiment, as reported in Table 4. As it can be observed, higher concentrations of Fe and Al were obtained when an increase on applied current and decrease on the flow rate were attained (Experiments 4 and 7, respectively; see Table 4). As it was commented, for EC to be suitable as an alternative technology for removing heavy metals, minor residual waste must be generated. For this reason, more experiments are in progress in order to improve the experimental conditions.

Table 4. Residue of iron and aluminum generated after coagulation.

ACKNOWLEDGMENTS
The authors gratefully acknowledges the NEPGN (Center for Research Studies on Petroleum and Natural Gas/Analytical Center), the LEAP (Laboratory of Advanced Petroleum Studies) for providing the infrastructure to conduct the experiments and analyses, the PPGEQ/UFRN (Postgraduate Program in Chemical Engineering) and the CNPq for financial support in the form of scholarships and grants.

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Received: December 9, 2010.
Accepted: April 19, 2011.
Recommended by Subject Editor Ricardo Gómez.

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