Electrochemically Dissolved Aluminum Coagulants - American

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Electrochemically Dissolved Aluminum Coagulants for the Removal of Natural Organic Matter from Synthetic and Real Industrial Wastewaters Khaled Mansouri,†Ahmed Hannachi,†Ahmed Abdel-Wahab,‡and Nasr Bensalah*,†,‡ †

Department of Chemical Engineering, National School of Engineering of Gabes, University of Gabes, 6072 Gabes, Tunisia Department of Chemical Engineering, Texas A&M University at Qatar, Education City, P.O. Box 23874, Doha, Qatar

‡

ABSTRACT: In this work, the effects of some experimental parameters (supporting electrolyte, current density, and initial pH) on the anodic dissolution of aluminum and on the electrocoagulation of tannic acid aqueous solutions as well as real industrial wastewaters containing tannic acid were investigated. Experimental results indicated that both chemical and electrochemical dissolution play an important role in the formation of hydroxo-aluminum species. The chemical dissolution of aluminum is strongly influenced by the solution pH. Corrosion studies have demonstrated that the presence of chloride ions in water accelerates dissolution of aluminum by pitting corrosion while phosphate ions inhibit the corrosion of aluminum by formation of a thick passive layer of aluminum hydroxide/phosphate on the aluminum surface. The results obtained can be used to better understand the mechanism of the electrocoagulation process due to the importance of the anode surface electrodissolution in the treatment process. Electrocoagulation using aluminum electrodes achieved high removal efficiency of chemical oxygen demand (≥80%) from aqueous solutions containing 0.51 g·L−1 tannic acid. The primary mechanism implicated in eliminating tannic acid from water by electrocoagulation using Al electrodes involves the adsorption of tannic acid molecules on the aluminum hydroxide surface. Also, results of the treatment of real wastewater obtained from the pulp and paper industry with an initial chemical oxygen demand (COD) concentration of 1450 mg·L−1 have shown that more than 60% of COD can be removed by electrocoagulation using Al electrodes under optimized experimental conditions. The specific energy required for the electrochemical process with Al electrodes was estimated to range from 1 to 2 kWh·m−3.

1. INTRODUCTION Electrocoagulation is a promising alternative to the conventional chemical coagulation.1−3 The electrochemical treatment method was successfully applied for turbidity,4,5 heavy metals,6,7 dyes,8,9 and phenols10,11 removals from synthetic and real wastewaters and for breaking oil/water emulsions12 at both laboratory scale and pilot plant scale. Conventional coagulation takes place by the addition of aluminum or iron salts to the water/wastewater, which causes destabilization and aggregation of smaller particles into larger particles,13 and subsequently, organic pollutants are entrapped and/or adsorbed at the surface of the insoluble hydroxo-metal species and can be easily removed by sedimentation or filtration.14 However, in electrocoagulation, coagulants are generated in situ by anodic dissolution of sacrificial electrodes, usually aluminum or iron electrodes. Anodic dissolution of the sacrificial anodes leads to the formation of hydrolysis products (hydroxo-metal species) that involve the destabilization of suspended, emulsified, or dissolved pollutants and/or the formation of insoluble particles that adsorb and enmesh the pollutants.15 Furthermore, the formation of hydrogen bubbles, from the reduction of water at cathode surface, promotes the flocculation process by the soft turbulence in the system and produces a soft mix.15−17 The electrogenerated gaseous bubbles help the destabilized particles to colloid and generate larger particles which facilitate separation of the flocculated pollutants by carrying the particles to the top of the solution, where they can be more easily removed by electroflotation. It was also reported in the literature that this electrochemical process had lower operating © 2012 American Chemical Society

costs compared to the conventional process due to the low electric current required.15−18 The passivity of the iron or aluminum electrodes was extremely problematic for the treatment of industrial effluents by electrocoagulation.15−18 The formation of a protective layer that adheres to the anode surface prevents the dissolution of metal and restricts the charge transfer between solution and electrodes. This leads to excessive consumption of electricity and reduces the treatment efficiency of wastewaters by electrocoagulation. Recently, several researchers have suggested overcoming this problem by addition of supporting electrolytes such as NaCl to destroy the passive layer and increase the conductivity of the medium or by application of a sinusoidal intensity to prevent the formation of a deposit on the electrode surface.19−23 The literature lacks attention to the effects of supporting electrolyte, pH, and current density parameters that significantly promote destroying the protective layer on the anode surface and influence the rates of the electrochemical reactions and mechanisms involved.24−26 Furthermore, the kinetics and the efficiency of electrocoagulation are reported to vary with pollutant and aqueous medium characteristics.25−27 The main goal of this work was to study the removal of tannins as natural organic matter (NOM) from synthetic and actual wastewaters by electrocoagulation using aluminum Received: Revised: Accepted: Published: 2428

September 25, 2011 December 19, 2011 January 11, 2012 January 11, 2012 dx.doi.org/10.1021/ie202188m | Ind. Eng.Chem. Res. 2012, 51, 2428−2437

Industrial & Engineering Chemistry Research

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equipped with a combined glass electrode (METROHM). Chemical oxygen demand (COD) was measured using a HACH DR2000 analyzer. The conductivity was measured with a conductivity meter (MeterLab, type CDM230) equipped with a conductivity cell of two platinum electrodes. Morphological observations of aluminum electrode surfaces after galvanostatic electrolyses were conducted with a metallurgical trinocular microscope (AuxiLab, model ZUZI 173/2). 2.3. Potentiodynamic Polarization. Potentiodynamic polarization experiments of aluminum were performed with a three-electrode cell (Radiometer C145/170) and a potentiostat/galvanostat (PGZ 301 20 V 1A) controlled with Voltalab software allowing data acquisition. A platinum plate and saturated calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively. The working electrode was a 2 cm2 aluminum plate inserted into a PTFE sample holder (Radiometer PEK 29). Before each experiment, the working electrode was mechanically ground using successively finer grades of abrasive paper, polished with 0.3 μm alumina, rinsed with a solution of 1.30 M HCl and then with deionized water, and then dried before being inserted into the electrochemical cell. 2.4. Electrocoagulation Experiments. Electrochemical coagulation experiments were carried out under galvanostatic conditions. Electrolyses were performed in a one compartment thermostatted electrolytic cell with two opposing aluminum plates serving as parallel−vertical electrodes with a constant anode/cathode gap of 2 cm. Aluminum plates were cut from a commercial grade aluminum sheet (99% purity) of 3 mm thickness. The effective area of each electrode used for electrolysis was 24 cm2. Aluminum electrodes were connected to a digital dc power supply with galvanostatic operational options (Monacor PS-430) providing current and voltage in the ranges 0−30 A and 0−20 V. The cell voltage was recorded using a potentiometer. During galvanostatic electrolyses, 200 mL of wastewater was treated under vigorous magnetic stirring and at 25 °C. Prior to each experiment, the electrodes were first mechanically polished under water with abrasive papers in order to ensure surface reproducibility, treated by rinsing with a solution of 1.30 M HCl in order to reject any effect due to the different prehistories of the electrodes, and rinsed with deionized water and dried prior to immersion in the electrolyte. Samples were taken at desired times and analyzed for COD, aluminum concentration, and pH. To study the influence of pH, the pH of the wastewaters was adjusted prior to electrocoagulation experiments, by additions of NaOH or HCl. Specific electrical charge (Q) and specific electrical energy consumption (SEEC) are calculated using the following equations:

electrodes. NOM is present in both natural water sources and industrial and domestic wastewaters.28−30 Many researchers28−34 reported that NOM can be considered as the main source of disinfection byproduct formation, such as trihalogenomethanes, haloacetic acids, and haloacetonitriles during drinking water disinfection processes.28,29 These halogenated compounds pose threats to human health and the environment due to their carcinogenic effects.28−34 Chemical coagulation was considered as one of the most effective physiochemical technologies used in the treatment of water and wastewaters contaminated with NOM.31−34 NOM can be removed from water by adsorption onto metal hydroxides and/or neutralization of charge by electrostatic attraction with cationic metal species, which results in the formation of reduced soluble complexes. However, little information was found in the literature concerning the optimization of operational conditions to promote the electrodissolution of sacrificial anodes.31−34 To improve understanding of the dissolution of aluminum during galvanostatic electrolyses, the effects of some experimental parameters (initial pH, current density, and supporting electrolyte nature) on aluminum electrodissolution were evaluated in this work. Tannic acid (TA) was chosen as a model molecule of tannins that can be present in natural water and industrial wastewaters, such as pulp and paper and tannery wastewaters. Tannins are natural organic molecules containing in their chemical structure a polyol carbohydrate core (usually D-glucose) esterified to phenolic acids such as gallic acid or ellagic acid, forming gallotannins and ellagitannins, respectively. Tannic acid is a gallotannin consisting of esters of gallic acid and glucose, containing galloyl groups esterified directly to the glucose molecule. Electrocoagulation was also applied for the treatment of pulp and paper industrial wastewaters (PPW) rich in NOM fragments.

2. MATERIALS AND METHODS 2.1. Chemicals. Tannic acid was obtained from Sigma− Aldrich (99%), and it has been used as received without further purification. All the other chemicals used were Sigma-Aldrich or Acros analytical grade reagents. Aqueous solutions were prepared with deionized water obtained from a Milli-Q system, with resistivity >18 MΩ·cm at 25 °C. Pulp and paper industrial wastewaters (PPW) were obtained by a pulp and paper industrial company located in Tunisia, collected in a closed container, and stored in the dark at 4 °C. The main characteristics of the PPW before treatment are presented in Table 1. Table 1. Physicochemical Characteristics of Paper and Pulp Industrial Wastewaters parameter

value

pH conductivity (mS·cm−2) COD (mg O2·L−1) TOC (mg C·L−1) total phenols (mg·L−1) chlorides (g·L−1)

11.2 2.42 1450 270 38 15

Q (A·h·L−1) =

It 3600V

SEEC (kW· h· m−3) = 1000UQ

(1) (2)

where I is the current intensity (in A), t is electrolysis time (in s), and V is the electrolyte volume (V = 0.2 L). 2.5. Chemical Coagulation. Chemical coagulation experiments followed the same procedure of electrocoagulation. However, in the case of chemical coagulation, aluminum was added by dosing a solution containing AlCl3 or Al2(SO4)3. Chemical coagulation experiments were carried out in a standard jar test experimental setup. In these experiments, a fixed amount of coagulant was added to the TA solution, and then the solution was vigorously stirred. The procedures of obtaining

2.2. Analytical Methods. Aluminum concentrations were determined by atomic absorption spectrometry (Zeeman spectrophotometer HITACHI Z-6100) after nitric acidification (4 N HNO3) and suitable dilution of samples taken at desired times. The pH was measured using an inoLab WTW pH-meter, 2429

dx.doi.org/10.1021/ie202188m | Ind. Eng.Chem. Res. 2012, 51, 2428−2437


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and analyzing the samples were the same as those used for electrocoagulation experiments.

The theoretical amount of dissolved aluminum is then given by the following equation (eq 7):

[Al (g · L−1)] =

3. RESULTS AND DISCUSSION 3.1. Dissolution of Aluminum during Electrocoagulation Experiments. According to the literature,35−38 the efficiency of the electrocoagulation process in treating wastewaters depends largely on the amount and speciation of aluminum dissolved during the electrochemical treatment. In order to study the dissolution of aluminum during galvanostatic electrolyses, the amount of aluminum dissolved (mexp) was measured and compared to the theoretical amount expected to be dissolved (mth), calculated using Faraday’s law. Figure 1

where [Al] is the amount of aluminum generated (in g·L ), I is the current intensity (in A), t is electrolysis time (in s), M is the molecular weight of aluminum (M = 26.98 g·mol−1), F is Faraday's constant (F = 96487 C·mol−1), V is the electrolyte volume (V = 0.2 L), and Q is the specific electrical charge (in A·h·L−1). Figure 1 shows that both the theoretical and experimental amounts of aluminum at a current density of 10 m·A·cm−2 increase linearly with specific electrical charge for Q ≤ 2.5 A·h·L−1. At the beginning of galvanostatic electrolysis, measured concentrations of aluminum correlate well with theoretical concentrations calculated from the aforementioned equation. However, for specific electrical charge greater than 0.5 A·h·L−1, experimental values exceeded theoretical values and the difference between them increased with increasing specific electrical charge. It should be noted that similar results were reported by other authors.35−38 The difference between measured aluminum concentrations compared to the theoretical amounts can be attributed to chemical dissolution of aluminum electrodes by corrosion in addition to anodic dissolution of aluminum. Several researchers have also attributed this behavior to the chemical dissolution of aluminum.15−18 However, to confirm this hypothesis, a set of experiments was conducted in which aluminum plates of known weights were immersed in slightly alkaline 0.1 M NaCl aqueous solutions (pH = 9). These conditions were selected to mimic pH conditions near the cathode where the chemical dissolution of aluminum is most likely to occur. Figure 2a shows changes of aluminum concentration with time, as determined by the gravimetric method during immersion of identical aluminum plates (24 cm2) in 0.1 M NaCl aqueous solutions at pH 9. As can be observed, aluminum concentration increased linearly with time until it reached a value of 0.014 g·L−1 at 480 min, after which it remained constant for the rest of the operating time. This behavior may be explained by the formation of a protective layer of aluminum hydroxide/oxide (Al(OH)3/Al2O3) on the aluminum electrode surface, which inhibits further chemical dissolution of aluminum.39−41 The formation of this protective layer in aerated solutions can be given by the following reactions:

presents the changes of the theoretical and measured concentrations of aluminum dissolved with a specific electrical charge (A·h·L−1) during galvanostatic electrolysis of a 0.1 M NaCl aerated aqueous solution using aluminum electrodes at a current density of 10 mA·cm−2. The theoretical aluminum concentration (in g·L−1) was calculated assuming that the available electrical energy was entirely used to oxidize Al to Al3+ according to the following reaction: Al ⇆ Al3+ + 3e−. According to Faraday’s Law, the electrical charge q is calculated using the following equation:

Al ⇆ Al3 + + 3e− 2H2O + 2e− ⇆ H2 + 2OH−

(3)

The overall chemical reaction is

By combining eqs 2 and 3, the aluminum concentration can be then given by

[Al (g ·L−1)] =

qMAl 3FV

2Al + 3H2O → 2Al(OH)3(solid) + H2 Also, in the presence of dissolved oxygen, aluminum can be directly oxidized into aluminum oxide, as shown by the following reaction:

(4)

During galvanostatic electrolyses, the current density is constant; thus, electrical charge (q) and specific electrical charge (Q) are defined respectively by the following equations:

q = It Q (A ·h ·L−1) =

4Al + 3O2(aq) → 2Al2O3(solid) These results confirmed that aluminum can be chemically dissolved, but the amount dissolved by chemical oxidation with dissolved oxygen at pH 9 is negligible compared to that dissolved by anodic oxidation. As such, the observed difference between experimental and theoretical concentrations under

(5)

It 3600V

(7) −1

Figure 1. Changes of theoretical and experimental concentrations of aluminum dissolved during galvanostatic electrolysis with a specific electrical charge. Operating conditions: initial pH = 7, j = 10 mA cm−2; supporting electrolytes: 0.1 M NaCl.

m q = 3F Al MAl

3600QMAl ItMAl = = 0.366Q 3FV 3F

(6) 2430



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surfaces, which makes it complex to quantify the amount of aluminum chemically dissolved. These results indicate that dissolved concentrations of aluminum measured during galvanostatic electrolyses using aluminum electrodes are attained by both electrochemical and chemical dissolution. The importance of chemical dissolution depends largely on the pH conditions near the electrodes’ surfaces. The superfaradic excess of dissolved aluminum is primarily related to the chemical dissolution of aluminum, which is more significant at pH > 11. Canizares et al.35−38 also demonstrated that the rate of chemical dissolution of aluminum in water is pH-dependent and is higher for pH values above 12. The preceding results showed that solution pH influences the rate of dissolution of aluminum during galvanostatic electrolysis of aqueous solutions using aluminum electrodes. This led us to investigate the influence of experimental parameters on the changes of pH during galvanostatic electrolysis between two aluminum plates. On the basis of the literature,23,31 it was reported that the initial pH, the current density, and the nature of the supporting electrolyte are the main parameters that can cause pH changes during galvanostatic electrolysis. Figure 3a presents the changes of pH vs time during galvanostatic electrolyses of 0.1 M NaCl aqueous solutions at different initial pH values using aluminum plates as electrodes at a current density of 10 mA·cm−2. This figure shows that pH changes over time depend on the initial pH value of the electrolyzed solution. These results demonstrate that, regardless of initial pH value, the pH evolution versus time follows two consecutive stages: a stage of rapid pH change over time followed by a stage in which pH remains almost constant. It is noteworthy that the curve of pH vs time is ascending for solutions of initial pH lower or equal to 9 and is descending in alkaline solutions (pH > 9). For acidic initial pH conditions, the increase in pH can be explained by the formation of OH− ions from the reduction of H+ or H2O on the surface of the cathode. In these circumstances, it appears that hydroxide ions formed at the cathode were not completely involved in the formation of hydroxo-aluminum species. Rather, the excess of OH− ions increased the pH of the medium. In contrast, for solutions with initial pH higher than 9, the decrease in pH could be attributed to the consumption of higher amounts of OH− ions to form anionic hydroxo-aluminum species such as Al(OH)4− and Al(OH)52−. It should be taken into account that, in highly alkaline conditions, the amount of Al3+ ions coming from chemical dissolution plays a significant role in the total amount of aluminum ions generated in situ toward the end of the electrolysis; the majority of the aqueous solution stabilizes at a pH value close to 9. As can be observed, the closer the initial pH is to 9, the more rapid the stabilization of the pH. The stabilization of the pH at a value around pH 9 for initial values in the range between 5 and 10 can be explained by a buffering effect of hydroxo-aluminum species that balances the quasi-static variation of the concentration of hydroxyl ions through the formation of monomeric and polymeric complexes of aluminum hydroxides.35−38 Several acid−base couples can be formed in the medium which buffer the pH to a value around pH 9. Figure 3b presents the changes of pH vs time during galvanostatic electrolyses at different current densities of 0.1 M NaCl aqueous solutions using aluminum plates as electrodes at initial pH 9. No significant change of pH with time was observed using different current densities, and the same behavior was obtained for all current densities applied. At the start of electrolysis, the pH drops by one unit and then increases with time

Figure 2. (a) Change of aluminum concentration with time during immersion of aluminum plates (24 cm2) in 0.1 M NaCl aqueous solutions at pH 9. (b) Changes of dissolved aluminum concentration with pH during 60 min immersion of aluminum plates (24 cm2) in 0.1 M NaCl aqueous solution.

these operating conditions cannot be sufficiently attributed to the chemical dissolution. Perhaps this behavior could be further explained by the fact that the operating conditions during the chemical dissolution of aluminum are different from the actual hydrodynamic conditions and the pH in the vicinity of the aluminum electrodes used during galvanostatic electrolysis. Particularly, the pH conditions at the cathode are strongly alkaline due to the reduction of water into H2 forming OH− ions. To better understand the behavior of chemical dissolution of aluminum as affected by solution pH in the electrocoagulation process, a set of experiments were conducted at different pH values. Figure 2b presents the concentration of dissolved aluminum at different initial pH values during a 60 min immersion of identical aluminum plates (24 cm2) in 0.1 M NaCl aerated aqueous solutions. This figure clearly shows that pH has a significant influence on the amount of chemically dissolved aluminum. The chemical dissolution of aluminum was negligible for pH < 10. However, for higher pH values, the concentration of aluminum dissolved in aqueous solution increased from approximately 0.03 g L−1 at pH 11 to 0.5 g L−1 at pH 12.5. These results demonstrate that the chemical dissolution of aluminum in an electrocoagulation process depends largely on pH in the vicinity of the electrodes. Nevertheless, it is very difficult to identify exactly the pH conditions at electrodes’ 2431



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increased continuously until it reached a value of 11.4, whereas, in the presence of NaCl and Na2SO4, pH decreased slightly at the beginning and then increased gradually until it stabilized at 9.3 and 10, respectively. This difference could be due to the nature and stability of aluminum complexes formed in each electrolyte. In particular, the presence of NaH2PO4 in the solution leads to the formation of stable complexes with Al3+ ions: Al(PO4), AlH(PO4)+, and AlH2(PO4)2+. As such, a major part of Al3+ ions formed during electrolysis react with phosphate ions. This leads to an excess of OH− available in the solution, which increases solution pH with time. It is notable that, in the presence of Na2SO4 and NaH2PO4, the pH of the solution stabilized at a value greater than 10, which facilitates the dissolution of Al(OH)3 into anionic hydroxo-aluminum species such as Al(OH)4− and Al(OH)52−, which makes flocculation and electroprecipitation difficult to occur. In the case of using NaCl, the pH remains in a range between 8.3 and 9.3 throughout the experiment. In this pH range, amorphous aluminum hydroxide Al(OH)3 is precipitated, which has a large surface area that is beneficial for coagulation.15−18 In order to better understand the influence of supporting electrolyte on the electrochemical corrosion of aluminum, potentiodynamic polarization experiments were performed in an attempt to explain the phenomena that occur on the anode surface. Results of open-circuit potential (OCP) experiments showed that OCP instantaneously increased from an initial value of −795 mV to reach an almost constant value with frequent fluctuations around −670 mV/SCE. The immediate increase of OCP can be explained by the growth of Al(OH)3 film on the surface of the Al electrode. The fluctuation of the OCP can be attributed to the changes of pH near the Al electrode surface. On the basis of these results, a corrosion potential value (Ecorr) of −670 ± 5 mV/SCE can be deduced for pure Al in aerated aqueous solutions. Figure 4a shows potentiodynamic polarization curves obtained from an aluminum plate immersed in aerated aqueous solutions of 0.1 M concentration of NaCl, Na2SO4, or NaH2PO4 under alkaline conditions (pH = 9) at a scan rate of 0.5 mV·s−1. The potentiodynamic polarization plot without supporting electrolyte was also presented in the same figure for the sake of comparison (in this case the current density is expressed in μA·cm−2). As can be seen, the anodic electrode behavior in the presence of supporting electrolyte was substantially different from that observed in aerated deionized water without supporting electrolyte. Also, the anodic polarization branches in the presence of supporting electrolyte did not exhibit the expected log/linear Tafel behavior over the whole applied range of potential. The curvature of the anodic branch may be attributed to the deposition of corrosion products on the aluminum surface to form a nonpassive surface film. Aluminum showed a classical passive region in which current remained almost unaffected by the change of applied potential. This can be seen as a current density plateau in Figure 4b. However, the current density increased abruptly after reaching a certain value of the electrode potential, which is the pitting potential (Epit). The pitting corrosion was preceded by uniform thinning of the hydroxide/oxide protective film that prevails over pitting corrosion prior to the pitting potential. After reaching Epit, the current density continued to increase very slightly with increasing potential. Pitting corrosion is responsible for the increase in current density that was observed right after the passivation plateau. The pitting potential of aluminum

Figure 3. Influence of (a) initial pH, (b) current density, and (c) supporting electrolyte on changes of pH vs time during galvanostatic electrolysis using aluminum electrodes.

until it reaches a value close to its initial value (pH = 9) and eventually stabilizes at this value. This negligible change of pH when varying current density during galvanostatic electrolysis can be explained by the fact that increasing the current density in the studied range caused only an increase of both the rate of aluminum anode dissolution and production of H2 at the cathode surface. Figure 3c presents the variation of pH vs time during galvanostatic electrolyses of aqueous solutions containing different supporting electrolytesNaCl, Na2SO4, or NaH2PO4using aluminum electrodes at a current density of 6 mA cm−2. Figure 3c clearly indicates that the nature of the supporting electrolyte has a significant effect on the variation of pH with time during galvanostatic electrolysis. In the presence of NaH2PO4, the pH 2432



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corrosion potential toward more negative values, indicating that corrosion of aluminum is catalyzed by the presence NaCl or Na2SO4. In contrast, the presence of NaH2PO4 increased the corrosion potential, which indicates that the presence of NaH2PO4 inhibits the corrosion of aluminum. Moreover, it can be seen that the corrosion rate of aluminum is highest in the presence of NaCl, as the corrosion current density is the largest. It appears that pitting corrosion also depends greatly on the nature of the supporting electrolyte, as the value of the pitting potential (Epit) varies between electrolytes. In the presence of NaCl, pitting corrosion occurs at a much lower potential than in the presence of Na2SO4 or NaH2PO4 electrolytes. This is most likely due to the size of the chloride ions, which are relatively smaller than sulfate and phosphate ions. These ions are characterized by their ability to penetrate through the Al(OH)3/Al2O3 film, forming soluble complexes.39−41 Also, the increase of chloride concentration decreases the corrosion potential further and minimizes the difference between pitting potential and corrosion potential. In the presence of sulfate ions, the pitting phenomenon is much lower than that observed in the presence of chloride ions. In the case of the sulfate solution, the passive layer of Al(OH)3/Al2O3 could be partially dissolved, and consequently, the rate of electrochemical corrosion of aluminum is lower, whereas, in the presence of phosphate ions, this phenomenon is practically absent due to the large size of the phosphate ions and the formation of a highly resistant and passive layer of aluminum hydroxide/phosphate (Al(OH)3/AlPO4) that inhibits corrosion of aluminum. 3.2. Effects of pH, Current Density, and Supporting Electrolyte on Tannic Acid Removal by Electrocoagulation Using Al Electrodes. Treatment of synthetic wastewater solutions containing 0.51 g·L−1 tannic acid (∼600 mg O2·L−1 of COD) by electrocoagulation using sacrificial aluminum anodes at room temperature was investigated. Effects of pH, current density, and supporting electrolyte on tannic acid removal efficiency were evaluated in order to identify the optimal operating conditions of the electrocoagulation process. The efficiency of the electrocoagulation process in removing tannic acid was evaluated in terms of chemical oxygen demand (COD). 3.2.1. Effect of Initial pH. Figure 5 illustrates the influence of initial pH on COD removal efficiency as a function of specific electrical charge during electrolysis of 0.1 M NaCl aqueous solutions containing 0.51 g·L−1 tannic acid (COD0 = 600 mg O2·L−1) using aluminum plate electrodes with a current density of 4 mA·cm−2. As can be observed, initial pH had a significant influence on the kinetics and overall percentage of COD removal. As pH increased from 2 to 7, the rate and percentage of COD removal were enhanced. In contrast, for pH > 7, the rate and efficiency of COD removal decreased with increasing pH. The most effective COD removal was observed at pH 7 (80%). For electrolysis done at initial pH 2, the evolution of medium pH progressed from pH 2 to 5.8. It can be argued that the increase of pH was due to the reduction of (excess) H+ ions at the cathode to dihydrogen (H2), which limits the formation of hydroxide and hence hydroxoaluminum species. Many researchers31−33 indicate that the removal of organic matter by electrocoagulation at this pH interval occurs due to coagulation by charge neutralization. Dissolved tannic acid reacts with Al3+ cations generated at the anode to form tannic acid−aluminum complexes. This reaction occurs due to the presence of negative charges around the tannic acid molecules that are a consequence of the deporotonation phenomenon of phenol groups, which in turn leads to the attraction between Al3+

Figure 4. (a) Potentiodynamic polarization curves of aluminum in aerated aqueous solutions containing different supporting electrolytes. (b) Different parts of the polarization curve: charge transfer, anodic passivation, and pitting corrosion. Operating conditions: anode, pure Al specimen (1 cm2); cathode, Pt wire; reference electrode, SCE; pH = 9; electrolyte concentration, 0.1 M; scan rate, 0.5 mV s−1; T = 25 °C.

is the potential of passivation breakdown for which there is an acceleration of the corrosion process by diffusion of anions through the pores of the Al(OH)3 layer. As can be observed, the nature of the electrolyte greatly affects corrosion current density and corrosion potential. The values of corrosion current density and corrosion potential in different electrolytes are shown in Table 2. The presence of NaCl or Na2SO4 shifted the Table 2. Results of Potentiodynamic Polarization of Aluminum Determined by the Tafel Extrapolation Method in the Presence of Different Supporting Electrolytesa electrolyte without supporting electrolyte 0.1 M NaCl 0.1 M Na2SO4 0.1 M NaH2PO4

corrosion potential, Ecorr (mV)

corrosion current density, jcorr (μA·cm−2)

−668

2.6

−1212.7 −1056.0 −518.5

21 8.7 4

a

Experimental conditions: pH = 9; electrolyte concentration, 0.1 M; scan rate, 0.5 mV s−1; T = 25 °C. 2433



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mechanism involved in electrocoagulation using aluminum electrodes is physical adsorption. It should be noted that recent studies have shown that the mechanism involved in electrocoagulation depends on the nature of target organic compounds. Canizares and Khemis42,43 demonstrated that COD removal in electrocoagulation of emulsions requires a critical amount of aluminum to neutralize repulsive forces between emulsion molecules. The study indicated that rapid rates and high percentages of COD removal occurred only after critical concentrations of coagulant were achieved. This suggests that the mechanism involved in the electrocoagulation of emulsions is achieved only by charge neutralization, where the electric charge is the main factor of repulsive forces between particles. Consequently, the agglomeration of emulsion molecules requires an optimal quantity of dissolved aluminum generated through galvanic electrolysis to reduce these repulsive forces. In contrast, Can and Bayramoglu44 indicated that the elimination of pollutants present in textile wastewater is mainly achieved through adsorption on the surface of insoluble aluminum hydroxide. 3.2.2. Effect of Current Density and Supporting Electrolyte. Figure 6a shows the influence of current density on COD

Figure 5. Influence of initial pH on the changes of (a) COD removal and (b) pH with specific electrical charge during electrocoagulation of synthetic solutions of tannic acid using Al electrodes (24 cm2). Operating conditions: COD0 = 600 mg O2·L−1; j = 10 mA·cm−2; supporting electrolyte 0.1 M NaCl.

cations and tannic acid molecules. This attraction leads to neutralization of tannic acid’s overall molecular negative charge, facilitating the bond between tannic acid and Al3+ and consequently leading to their precipitation. For electrolysis done at initial pH 7, the evolution of medium pH progressed from pH 7 to 7.5 in the first 30 min of experimentation. In this pH interval, Al3+ cations generated by anodic aluminum dissolution react with hydroxyl anions produced at the cathode to form amorphous aluminum hydroxide (Al(OH)3). Diagrams of prevalence of aluminum-hydroxyl complexes as a function of pH indicate that Al(OH)3 is the most prevalent species in this pH range.42 Accordingly, removal of tannic acid from aqueous solutions in this pH interval is carried out by adsorption of these organic pollutants on the surface of the amorphous hydroxidealuminum species through Van-Der-Waals interactions and hydrogen bonding. To confirm the nature of the adsorption involved in this treatment process, the solid obtained after the electrolysis was separated by filtration and then transferred into water having a temperature of 70 °C, and the color of the solution was observed. The solution color turned brown, indicating the reversibility of adsorption and demonstrating that the

Figure 6. Influence of (a) current density (supporting electrolyte 0.1 M NaCl) and (b) supporting electrolyte (current density 10 mA cm−2) on the changes of COD removal with specific electrical charge during electrocoagulation of a synthetic solution of tannic acid using Al electrodes (24 cm2). Operating conditions: COD0 = 600 mg O2·L−1; pH 7. 2434



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removal efficiency as a function of specific charge applied during galvanostatic electrolysis of synthetic solutions containing 0.51 g·L−1 tannic acid (COD0 = 600 mg O2·L−1) in the presence of 0.1 M NaCl supporting electrolyte at initial pH 7. COD removal efficiency increased with increasing current density. After consumption of 1.5 A·h·L−1 specific electrical charge, the COD removal was 71% for j = 3 mA cm−2, 79% for j = 4 mA·cm−2, and 86% for j = 10 mA·cm−2. The COD removal trend was similar for all three current densities. COD removal increased rapidly until consumption of approximately 0.4 A·h·L−1 specific electrical charge; then a slower increase in COD removal was observed until it reached certain efficiency, which remained constant after consumption of approximately 1.0 A·h·L−1 specific electrical charge. Additionally, it was found that current density affected the overall rate of COD removal efficiency. During the first hour of treatment under a current density of 10 mA·cm−2, 86% of COD was removed. In contrast, the COD removal efficiencies were 57% and 71%, respectively, for the same electrolysis time using applied current densities of 3 and 4 mA·cm−2. This demonstrates that both the efficiency and kinetics of tannic acid treatment by electrocoagulation using aluminum electrodes improved with an increase in current density applied. At lower current densities, the amount of aluminum cations generated is not sufficient to eliminate all tannic acid by the electrocoagulation process. However, at high current densities, the amount of aluminum generated through anodic dissolution is higher and consequently promotes effective COD removal. It should also be noted that increasing current density leads to an increase in the rate of hydrogen production and oxygen bubbles at the cathode. These bubbles promote the contact between electrogenerated coagulant and dissolved tannic acid, which facilitates the formation of flocks that can be eliminated by flotation. Figure 6b presents the influence of the supporting electrolyte (NaCl and Na2SO4) on COD removal efficiency as a function of specific electrical charge. The initial COD concentration and pH were the same as used in the effect of current density experiments. Results shown in Figure 8 indicate that using NaCl as supporting electrolyte resulted in more effective electrocoagulation than when using Na2SO4. After 45 min of treatment, the COD removal efficiencies were 84% and 70% for the chloride medium and the sulfate medium, respectively. The enhanced effectiveness of electrocoagulation using NaCl as supporting electrolyte could be due to the more effective destruction of protective layers formed on electrode surfaces in the presence of chloride ions (as shown in Figure 4). This in turn leads to the promotion of aluminum dissolution and more effective electrocoagulation.

Figure 7. Influence of initial pH on the changes of (a) COD removal and (b) pH with specific electrical charge during electrocoagulation of real effluent using Al electrodes (24 cm2). Operating conditions: COD0 = 1450 mg O2·L−1; j = 10 mA·cm−2.

10 mA cm−2. This figure indicates the same sharp increase of COD removal efficiency in acidic, alkaline, and neutral aqueous medium. However, initial pH had a significant influence on the kinetics and overall percentage of COD removal. As pH increased from 2 to 7, the rate and percentage of COD removal were enhanced. In contrast, the rate and percentage of COD removal decreased with an increase of initial pH from 7 to 11.2. These results can be explained by the behavior of aluminum hydroxide complexes at different pH values and the changes of pH during electrocoagulation experiments (see Figure 7b). In alkaline solutions, the solubility of Al(OH)3 is enhanced, which leads to the formation of soluble species: Al(OH)4−, Al(OH)52−.35−40 However, in acidic solutions, the formation of hydroxo-aluminum species is limited by the consumption of H+ cations to generate H2 at the cathode. In addition, Al(OH)3 does not precipitate at low pH.39−41 Consequently, the efficiency of electrocoagulation of organic matter removal is limited in both acidic and alkaline media. However, at initial pH 7, all aluminum cations generated by anodic dissolution were consumed to form aluminum hydroxide precipitate, which leads to more rapid removal of COD by adsorption on the surface of this species.

4. INFLUENCE OF EXPERIMENTAL PARAMETERS ON THE TREATMENT OF PPW BY ELECTROCOAGULATION The PPW studied in this work are characterized by their high conductivity values, high concentrations of chloride anions, and high concentrations of various complex organic structures. Due to these characteristics, electrocoagulation can be an adequate method to treat this wastewater. In this section, effects of experimental parameters (residence time, current density, initial pH, and supporting electrolyte) on COD removal efficiency were investigated. Figure 7 presents the changes of COD removal and pH with time at different initial pH values during the electrocoagulation treatment of PPW containing 1450 mg·L−1 O2 using aluminum plate electrodes under a current density of 2435



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water. Experimental results showed that initial pH value and supporting electrolyte concentration and type influence the dissolution of aluminum during galvanostatic electrolysis; however, current density does not seem to significantly affect the electrocoagulation process. Dissolved concentrations of aluminum measured during galvanostatic electrolyses using aluminum electrodes are attained by both electrochemical and chemical dissolution; but the importance of chemical dissolution depends largely on the pH conditions close to the surface of the electrode. The presence of chloride ions in water accelerates dissolution of aluminum by pitting corrosion while phosphate ions inhibit the corrosion of aluminum by formation of a thick passive layer of aluminum hydroxide/phosphate on the aluminum surface. Application of an electrocoagulation process using aluminum electrodes under optimized conditions can remove high amounts of COD from concentrated synthetic aqueous solutions of tannic acid and real industrial wastewaters. Removal of tannic acid from aqueous solutions occurs by adsorption of tannic acid molecules on the aluminum hydroxide surface. During electrocoagulation treatment, the pH conditions promote the formation of amorphous aluminum hydroxide, characterized by a large specific surface, a high density, and a good adsorption capacity. The specific electrical energy required for the treatment of PPW real effluents by an electrocoagulation process with Al electrodes varies from 1.0 kWh·m−3 with addition of 0.1 M NaCl to nearly 2.0 kWh·m−3 without addition of supporting electrolyte.

Figure 8a shows the evolution of COD removal during the electrocoagulation of PPW at pH 7 using a current density of

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AUTHOR INFORMATION

Corresponding Author

*Phone: +21675392600. Fax: +21675392421. E-mail: nasr. [email protected] and/or [email protected].

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ACKNOWLEDGMENTS The authors acknowledge Texas A&M University at Qatar and Qatar Foundation for providing partial financial support to accomplish this research work.

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REFERENCES

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Figure 8. Influence of addition of 0.1 M NaCl on changes of (a) COD removal with specific electric charge, and (b) specific electrical energy with COD removal during electrocoagulation of real effluent. Operating conditions: COD0 = 1450 mg O2·L−1; j = 10 mA·cm−2; initial pH = 7.

10 mA cm−2 with and without the addition of 0.1 M NaCl as supporting electrolyte. Sodium chloride was chosen as a supporting electrolyte for these runs since it was found to accelerate aluminum dissolution. As can be seen from Figure 8a, there is no significant effect of adding NaCl in the aqueous medium, which can be due to the presence of excessive amounts of chloride anions found naturally in the effluent. However, Figure 8b shows that the adition of NaCl leads to decreased specific electrical energy consumption (SEEC) for the same COD removal. This can be due to the fact that the addition of further NaCl decreases the cell voltage required to allow the same current density and accelerates dissolution of aluminum by pitting corrosion.

5. CONCLUSION Electrocoagulation using aluminum electrodes was evaluated for aluminum dissolution and removal of tannic acid from 2436



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Electrochemically Dissolved Aluminum Coagulants - American

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