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Continuous Electrocoagulation of Synthetic Colloid-Polluted Wastes P. Can ˜ izares, F. Martı´nez, M. Carmona, J. Lobato, and M. A. Rodrigo* Department of Chemical Engineering, Facultad de Ciencias Quı´micas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
The electrocoagulation of a synthetic wastewater has been studied in this work. The electrochemical process was carried out in a continuous single-flow electrochemical cell equipped with aluminum electrodes. Kaolin suspensions were used as a model of wastes polluted with colloids, as clays behave as hydrophobic colloids in water. The results obtained were useful to clarify the mechanisms that are involved in the electrocoagulation of this kind of waste and also to study the influence of the different operation conditions in the process. It has been found that the more important variables in the process were the aluminum concentration generated in the system and the pH. The concentration of aluminum generated in the electrochemical cell was always over the expected value (superfaradaic efficiencies) due to the important contribution of the chemical dissolution of the electrodes. This chemical dissolution of the electrodes depended strongly on the pH. A larger concentration of aluminum in the waste did not result in greater process efficiencies. For acidic pHs, a small concentration of aluminum achieved good coagulation efficiencies (80% removal of turbidity), while for alkaline pHs neither high nor low concentrations of aluminum yielded good coagulation results. For pHs close to neutral, a large concentration of aluminum was required to achieve good results. Two primary coagulation mechanisms can explain the experimental behavior of the system: at acid pH the neutralization of the superficial charge of the clays and at neutral pH (and also at high concentrations of aluminum) the enmeshment of the kaolin particles into a sweep floc. Introduction In recent years, several studies have focused on the study of electrocoagulation and its applications. Recent research has shown that electrocoagulation is a competitive technology for removing pollutants from supply water,1-4 urban wastewaters,5 and also in the treatment of actual and synthetic industrial effluents,6-24 as those generated in the agro-alimentary, metalworking, and textile industries. The results obtained allow us to classify this technique as one of the most promising methods for treating wastewater streams polluted with colloids or consisting of oil-in-water emulsions. Electrocoagulation involves the in situ generation of coagulants by electrolytic oxidation of an appropriate sacrificial anode (e.g., iron or aluminum) upon application of a direct current. The metal ions generated hydrolyze in the electrochemical cell to produce metal hydroxide ions and neutral M(OH)3. The low solubility of the neutral M(OH)3, mainly at pH values in the range of 6.0-7.0, promotes the generation of sweep flocs inside the treated waste and the removal of the pollutants by their enmeshment into these flocs.25-27 The charge of the metal hydroxide species promotes the neutralization of the charged pollutant particles, minimizing the electrical repulsion between them, and favoring their later flocculation.28,29 Through in situ generation of coagulants, electrocoagulation processes do not require any addition of chemicals. Other advantages of electrocoagulation include the promotion in the flocculation process caused by the electric field generated in the electrochemical cell,30,31 the promotion in the separation process caused by the improvements in the flotation of * To whom correspondence should be addressed. Tel.: +34 902204100. Fax: +34 926 29 53 18. E-mail:
[email protected].
the flocs with the hydrogen bubbles generated on the cathode, and the easy automation of the process, as the dosing of coagulant reagents depends directly on the cell potential (or current density) employed. Although electrocoagulation has been an available technique for more than a century, the design of an electrocoagulation cell is mainly based on empirical knowledge, with little consideration of the electrocoagulation mechanism.12 The goal of this paper is to evaluate coagulation as a continuous operation process, without promoting electroflocculation or electroflotation processes, and to study the influence of the different operating parameters in the electrocoagulation processes. To do this, kaolin suspensions are used as model wastewaters. Clays behave as hydrophobic colloids in water. These compounds consist of flat sheets of alternating layers of silicon oxides and aluminum oxides, held together by ionic attraction for cations sandwiched between the sheets. In water solutions, aluminum (+3) or silicon (+4) can be replaced with sodium (+1), potassium (+1), or ammonium (+1) ions resulting in an overall negative charge of the particle.31 This charge is responsible of the electric repulsion of kaolin particles and, thus, of the stability of the colloidal suspension. Experimental Section Experimental Devices. The electrocoagulation experiments have been carried out in a continuous benchscale plant, with a single-compartment electrochemical flow cell (Figure 1). Aluminum electrodes (HE 18) were used as the anode and cathode. Both electrodes were square in shape (100 mm sides) with a geometric area of 100 cm2 each and an electrode gap of 9 mm. The electrical current was applied using a dc power supply
10.1021/ie050527q CCC: $30.25 © 2005 American Chemical Society Published on Web 09/17/2005
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Results and Discussion
Figure 1. Layout of the bench-scale plant; detail of the electrochemical section.
(FA-376 PROMAX). The current flowing through the cell was measured with a 2000 digital multimeter (KEITHLEY). The electrolyte was stored in a 5000 mL glass tank stirred by an overhead stainless steel rod stirrer (HEIDOLPH RZR 2041) and thermostated by means of a water bath, which allowed the temperature to be maintained at the desired set point. The electrolyte was circulated through the electrolytic cell by means of a peristaltic pump. Experimental Procedure. Electrochemical coagulation experiments were carried out in a continuous operation bench-scale plant, under galvanostatic conditions. The electrolyte was pumped into the cell and collected in a tank. Samples were taken at the outlet of the cell, and after 20 min of settling, turbidity (using a 115 VELP SCIENTIFICA turbidimeter) and pH (using an inoLab WTW pH meter) were measured in the clarified liquid. The aluminum concentration was determined by dilution of samples 50:50 v/v with 4 N HNO3 and measurement using an inductively coupled plasma (LIBERTY SEQUENTIAL VARIAN) according to a standard method32 (Plasma Emission Spectroscopy). Preparative experiments were carried out to determine the amount of solution volume required to reach the steady state. In every experiment this final state was tested. The range of electrical charge passed was 0.0007-0.0138 A h dm-3. To determine the influence of the pH, the feeding solution pH was changed in the range of 1-12. To determine the influence of the kaolin concentration, kaolin suspension concentrations in the range of 250-2000 mg dm-3 were studied. To test the influence of the supporting electrolyte, sodium chloride concentrations in the range of 80-4900 mg dm-3 were tested. Previous to each experiment, the electrodes were treated with a solution of 1.30 M HCl in order to reject any effect due to the different prehistory of the electrodes. To determine the amount of aluminum electrogenerated by the applied current at the different operation conditions, several experiments were performed. In these assays, the electrolyte consists only of NaCl and NaOH or HCl added for any subsequent pH adjustment (without any kaolin). Aluminum and pH were then measured off-line.
Dynamic Response of the Continuous Electrocoagulation Process. Figure 2 shows the results of a typical electrocoagulation experiment. It can be observed that the aluminum concentration increases to reach a steady-state value and that turbidity decreases abruptly reaching its steady-state value even before that of the aluminum concentration. An increase in the pH of the outlet stream and a slight decrease of the cell potential can also be observed. This behavior is representative of the changes observed in the main parameters in most of the experiments, and the only remarkable modification that can be observed in other experiments is that there are some assays in which the turbidity presents a minimum before the steady state. This case will be discussed afterward as it gives important information about the coagulation mechanisms of kaolin with aluminum. In every case, the experimental time has shown to be sufficient to reach the steady-state conditions. The increase in the pH during the experiment can be interpreted in terms of the electrochemical and the chemical reactions that take place in the electrochemical cell. Cathodic water reduction (eq 1) and the chemical dissolution of aluminum sheets (eq 2)33,34 increase the pH value, while water oxidation (eq 3) decreases this parameter.
H2O + e- f 1/2H2 + OH-
(1)
2Al + 6H2O f 2Al3+ + 3H2 + 6 OH-
(2)
2H2O f O2 + 4H+ + 4e-
(3)
The chemical equilibria between the different aluminum species (eqs 4-8)34 also influence the value of this parameter.
Al(OH)4- + H+ a Al(OH)3
pKa ) 8.0
Al(OH)3 + H+ a Al(OH)2+ Al(OH)2+ + H+ a Al(OH)2+
pKa ) 5.7 pKa ) 4.3
Al(OH)2+ + H+ a Al3+ Al(OH)3(s) a Al3+ + 3OH-
pKa ) 5.0
(4) (5) (6) (7)
pKa ) 32.9 (8)
The amount of aluminum produced exceeds the value calculated if the process is considered to be only electrochemical (eq 9). This fact is observed in all the experiments carried out in this work.
Al f Al3+ + 3e-
(9)
Thus, for a pure direct electrochemical process, the maximum aluminum electrochemical dissolution rate (mmol Al3+ electrodissolved s-1) can be calculated using eq 10, where j is the current density (A m-2), A is the anodic surface (m2), F is the Faraday constant (96 500 C mol-1 e-), and n is the number of electrons involved in the anodic process (3 mol e-/mol Al3+ formed).
r)
jA nF
(10)
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Figure 2. (a) Turbidity, (b) concentration of aluminum, (c) pH, and (d) voltage profiles with time (dynamic response) generated in typical electrocoagulation experiments. Current density, 0.5 mA cm-2; temperature, 25 °C; flowrate, 19.8 dm3 h-1; initial pH, 4; kaolin concentration, 1000 mg dm-3; supporting media, 2450 mg dm-3 NaCl.
Figure 4. Scheme of the pH profile of the electrochemical cell. Table 1. Aluminum Generation as a Function of the Initial pH for the Range of pH Values Studieda aluminum generation rate (mg Al s-1 m-2 of electrode)
Figure 3. Aluminum chemical dissolution rates as a function of the pH.
According to this, for the case shown in Figure 2 the maximum expected aluminum concentration is 0.92 mg Al dm-3 (assuming no other anodic process), while the actual value that can be observed in the Figure 2 is 3.57 mg dm-3. This supposes a faradaic efficiency of 388%. These strange efficiencies (superfaradaic efficiencies) have been previously reported in the literature,33,34 and they have been interpreted in terms of the chemical dissolution of the aluminum electrodes. To confirm this point, and also to determine the chemical dissolution rate of the aluminum sheets, several experiments were carried out in batch operation using stirred beakers. These beakers were initially filled with different solutions of sodium chloride at different pHs, and a piece of aluminum was place inside each beaker. Figure 3 shows the chemical dissolution rates of aluminum sheets obtained as a function of the pH. It can be observed that there is the presence of a minimum close to neutral pH and that the dissolution rate is several orders of magnitude higher at alkaline pH. The direct estimation of the amount of aluminum chemically dissolved at the pH of the bulk solution in the cell, together with the aluminum electrogenerated according to eq 10, cannot justify the experimental values of the concentration of aluminum. To rationalize this value, it must be taken into account that the
pH of electrochemical electrochemical raw and chemical and chemical waste exptl electrodissolutionb dissolutionc dissolutiond 1 6 12
1.21 1.84 8.39
0.50 0.50 0.50
0.55 0.50 5.38
1.34 1.94 9.51
a Experimental conditions: electrical charge passed, 0.0029 A h dm-3; current density, 0.5 mA cm-2; supporting media, 2450 NaCl mg dm-3. b Considering the pure electrochemical process with a yield of 100%. c Obtained considering the electrodissolution and chemical dissolution with a constant pH in the whole cell.34 d Estimated taking into account the electrodissolution and chemical dissolution with the pH profile in the cell.34
electrochemical oxidation and reduction of water can modify importantly the pH on the anode and cathode surfaces with respect to the bulk pH. This is especially important on the cathode, where the pH can become strongly alkaline as schematized in Figure 4 (on the anode aluminum dissolution competes favorably with oxygen evolution, and the pH profile should be less marked). This can justify the important contribution of the chemical dissolution to the total dissolution rate. To confirm this role of the chemical dissolution, a model which considers both chemical and electrochemical processes has been recently proposed and applied to the description of the process.34 Table 1 shows the experimental versus the model values of the aluminum generation rate as a function of the pH for the range of pH studied in this work.
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Figure 6. Turbidity removal profile with time (dynamic response) generated in an electrocoagulation experiment, working at the following conditions: current density, 1.0 mA cm-2; temperature, 25 °C; flowrate, 19.8 dm3 h-1; initial pH, 4; kaolin concentration, 1000 mg dm-3; supporting media, 2450 mg dm-3 NaCl.
Figure 5. (a) Variation of steady-state aluminum concentration electrogenerated in the electrochemical processes and (b) removal of turbidity achieved in the steady state with the electrical charge passed. Temperature, 25 °C; initial pH, 4.0; flowrate, 19.8 dm3 h-1; kaolin concentration, 1000 mg dm-3; supporting media, 2450 mg dm-3 NaCl.
Influence of Operation Parameters. To study the influence of the different operation parameters on the removal of pollutants from a synthetic wastewater polluted with kaolin, a continuous electrocoagulation setup was used. The steady-state values of removal of turbidity and the aluminum generated have been taken for each experiment. Figure 5 shows the amount of aluminum generated in the electrochemical cell and the removal of turbidity (in percentage) as a function of the specific electrical charge passed (Q). To modify the specific electrical charge (I/q), the flow rate (q) was maintained constant and the applied intensity was modified in each experiment (I). Thus, the fluid dynamic conditions can be considered to be maintained almost constant in every case (except for the formation of bubbles due to oxygen or hydrogen evolution). It can be observed that, even at low electrical charge passed, high percentages of removal are attained. The removal of turbidity increases in an abrupt way initially, and after that it decreases slightly and reaches a constant value. Conversely, the aluminum concentration increases linearly with the specific electrical charge passed. The change observed in the turbidity removal versus charge graph seems to be related with the dynamic changes in the turbidity observed in several experiments (Figure 6), and all of them can be justified in terms of a predominant charge neutralization mechanism of coagulation. According to this, the dissolved aluminum is partially fixed on the kaolin surface compensating the negative charge of this surface (which is due to the initial exchange of aluminum and silicon atoms by monovalent ions (e.g., sodium, potassium) during the addition of kaolin to the water). An excess of aluminum ions fixed on this surface can change the net charge of the kaolin particles surfaces, and thus, it can decrease the coagulation yield. The prevalence of this mechanism is also supported by the low concentrations of aluminum required to treat the kaolin suspension, which are not high enough to create a sweep floc (another coagulation mechanism which consists of the precipitation of aluminum hydroxide and
Figure 7. (a) Concentration of aluminum, (b) pH, and (c) removal of turbidity profiles with time (dynamic response) generated in an electrocoagulation experiment working at the following conditions: current density, 2.5 mA cm-2; temperature, 25 °C; flowrate, 19.8 dm3 h-1; initial pH, 4; kaolin concentration, 1000 mg dm-3; supporting media, 2450 mg dm-3 NaCl. In part a, ([) indicates the total aluminum generated, (0) indicates the soluble aluminum.
the enmeshment of kaolin particles into this precipitate). In this sense, Figure 7 shows the results of an electrocoagulation experiment in which the high current density (and thus the high current charge passed) and the pH are adequate to generate aluminum hydroxide. It can be observed that despite the monotonic increase in the aluminum concentration, the turbidity removal presents two zones in which its value is high and one zone in which it decreases significantly. The first high-efficiency zone can correspond to a charge neutralization mechanism, while the second seems to be related with the formation of a sweep floc. In the lowefficiency zone both mechanisms coexist, but any of them is favored.
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Figure 10. Influence of the pH in the turbidity removal of kaolin without aluminum addition. Temperature, 25 °C; kaolin concentration, 1000 mg dm-3; supporting media, 2450 mg dm-3 NaCl; time of settlement, 20 min.
Figure 8. Influence of the steady-state pH in the removal of (a) turbidity and (b) variation of pH in the electrocoagulation experiments. Current density, 0.5 mA cm-2; temperature, 25 °C; flowrate, 19.8 dm3 h-1; kaolin concentration, 1000 mg dm-3; supporting media, 2450 mg dm-3 NaCl.
Figure 9. Influence of pH in the steady-state concentration of aluminum generated in the electrochemical processes. Temperature, 25 °C; current density, 0.5 mA cm-2; flowrate, 19.8 dm3 h-1; supporting media, 2450 mg dm-3 NaCl.
Figure 8 shows the influence of the waste pH on the turbidity removal percentage and on the steady-state pH, for a fixed specific current charge passed. It can be observed that acid pHs deal to high efficiencies, while neutral and especially alkaline pHs reduce drastically the efficiency of the process. A slight decrease in the pH at initial pH close to 4 and a slight increase when working at initial pH close to 9 can also be observed. All these changes of pH in the bulk solution are the consequence of the electrochemical and chemical reactions that take place in the cell (eqs 1-9). Figure 9 also shows the steady-state aluminum concentration measured in each case. As can be seen, this concentration increases with pH, this increase being especially marked for strongly alkaline pHs (due to the high influence of the chemical dissolution). It can be noticed that these higher concentrations of aluminum do not result in increases in the turbidity removal but in smaller removal efficiencies. This suggests an important influence of the pH in the obtained efficiencies. To clarify this point, Figure 10 shows the influence of pH in the turbidity removal of kaolin solutions
Figure 11. Removal of turbidity achieved in the electrocoagulation process as a function of the kaolin concentration. Current density, 0.5 mA cm-2; temperature, 25 °C; flowrate, 19.8 dm3 h-1; initial pH, 4; supporting media, 2450 mg dm-3 NaCl.
without addition of aluminum. It can be observed that strongly acid solutions of kaolin coagulate and reach similar turbidity removal efficiencies as those in which aluminum is added. This can only be explained in terms of the adsorption of protons in the kaolin surface and in the resulting surface charge neutralization. These observations confirm charge neutralization as the more important mechanism of coagulation for low concentration of electrodissolved aluminum. On the other hand, the high concentration of aluminum obtained at alkaline pH (Figure 8) has shown in other experiments to be enough to generate a sweep floc (Figure 6), but the small turbidity removal efficiency obtained in this case can be easily justified by the formation of soluble Al(OH)4- species in these conditions.35 Thus, the electrochemically assisted coagulation of kaolin suspensions depends strongly on both pH and aluminum concentration. The influence of the kaolin concentration in the removal of turbidity is shown in Figure 11. It can be observed that the removal percentage increases with the kaolin concentration and reaches a constant value for kaolin concentration higher than 1 g dm-3. This trend cannot be justified by the coagulation process but by the flocculation process. The flocculation is the association of coagulated particles by collision, and it is promoted with high concentrations of particles (due to the higher possibilities of collision). In Figure 12 is shown the influence of the electrolyte concentration on the turbidity removal and on the cell potential. It can be observed that the salinity does not have a marked influence on the coagulation in the range of concentration studied (only a small increase with increasing concentration is observed for low concentrations of NaCl). Thus, it can be assumed that the coagulation by the compression of the double layer (coagulation mechanism caused by an increase of the
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Acknowledgment This work was supported by the MCT (Ministerio de Ciencia y Tecnologı´a, Spain) and by the EU (European Union) through Project CTM2004-03817/TECNO. Literature Cited
Figure 12. Influence of the electrolyte concentration on (a) the turbidity removal and (b) the cell potential. Current density, 0.5 mA cm-2; temperature, 25 °C; flowrate, 19.8 dm3 h-1; initial pH, 4; kaolin concentration, 1000 mg dm-3.
ionic strength), does not play an important role in the overall process, at least in the sodium chloride concentration range used. On the other hand, the cell potential decreases strongly with increasing concentration of NaCl, due to the decrease in the ohmic losses. Conclusions The following conclusions can be drawn from the work described here: The electrocoagulation process can be applied to the treatment of wastes polluted with colloids. Removals of turbidity higher than 75% can be obtained with low currents. The more important variables in the process are the aluminum concentration generated in the system and the pH. The amount of aluminum generated in the process is always over the expected value (superfaradaic efficiency), and it is strongly influenced by the pH and the current density. Both chemical and electrochemical dissolution occur in the electrochemical cell. A larger concentration of aluminum in the waste does not result in greater process efficiencies, except for operation pHs close to neutrality. For acid pHs, small concentrations of aluminum achieve good coagulation efficiencies (80% removal of turbidity), while for alkaline pHs neither high nor low concentrations of aluminum yield good coagulation results. Two primary coagulation mechanisms can explain the experimental behavior of the system: at acid pHs the neutralization of the superficial charge of the clays and at neutral pHs (and also at high concentrations of aluminum) the enmeshment of the kaolin particles into a sweep floc. An increase of the kaolin concentration in the waste produces an increase of removal percentage in the electrocoagulation process, which reaches a constant value for kaolin concentrations higher than 1 g dm-3. This trend cannot be justified by the coagulation process but by the flocculation process. The salinity does not seem to have an influence in this particular case, so the compression of the diffuse ionic layer mechanism can be excluded as the primary mechanism in the coagulation process studied in this work.
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Received for review May 5, 2005 Revised manuscript received July 29, 2005 Accepted August 13, 2005 IE050527Q
