Hydrodynamic Effects on the Performance of Electro-coagulation

Mechanism of Dye Degradation during Electrochemical Treatment. ... Wastewater disinfection by electrocoagulation process and its interaction with abio...
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Ind. Eng. Chem. Res. 2005, 44, 7844-7853

Hydrodynamic Effects on the Performance of Electro-coagulation/ Electro-flotation for the Removal of Dyes from Textile Wastewater L. Szpyrkowicz* University of Venice, Department of Environmental Sciences, Dorsoduro 2137, 30123 Venice, Italy

The paper describes a study on the electro-coagulation/electro-flotation of textile wastewater bearing a mixture of three disperse dyes. The experiments were performed in a stirred reactor that was equipped with an aluminum sacrificial anode (100 cm2) and a stainless-steel cathode. Decolorization occurred in two well-distinguishable phases: a destabilization of colloids in the reactions with coagulating compounds formed in situ during anode dissolution (volumetric coagulation), and a subsequent gas flotation of particle agglomerates. Both phases proved to be dependent on the hydrodynamic conditions in the reactor and the applied current density. Model equations are derived from experimental data for the kinetics of the single phases of the process, for mechanical stirring and agitation by pumping. Introduction The efficient removal of pollutants from liquid effluents is often a problem for low- or medium-addedvalue industries, because it can significantly increase the overall plant and operational costs. The application of efficient and cost-effective methods for wastewater treatment is particularly important for textile factories in which the presence of dyes, which are generally characterized by low biodegradability, creates the need for pretreatment operations to be implemented ahead of a biological process of purification. Among the different processes for industrial wastewater treatment, coagulation constitutes one of the often-preferred options.1-7 This process may be wellsuited for the removal of disperse dyes, which are present in the wastewater in a colloidal form.8 Dosing the electrolytes causes the compression of a diffusive layer of these electrically charged particles, which reduces repulsive electrostatic forces and allows the particles to be brought together and settled or floated onto the surface of the liquid. The destabilization of the surface charge of the particles can be accomplished either by dosing the coagulants (chemical coagulation) or by generating them electrochemically in situ (electrocoagulation). In electro-coagulation, destabilization of colloids occurs both as a consequence of the electrical field existing between the electrodes (polarization coagulation) and due to the action of coagulating compounds produced by the oxidation of the anode. The first of these phenomena, an electrophoretic movement of the particles toward the electrode, is a slow process;9 hence, electro-coagulation is generally performed using sacrificial anodes. It has been observed that, with aluminum as the sacrificial electrode, the hydrolysis products of electrogenerated aluminum contain both monomer and polymeric compounds and their characteristics are between those of the products of hydrolysis of inorganic metal salts and pre-hydrolyzed compounds.10 These compounds are electrically charged, and they destabilize the colloidal system in which solid particles are initially * Tel.: +390412348667. Fax: +390412349591. E-mail: [email protected].

present, forming flocks of particles that can be easily separated by flotation. Flotation is enhanced by the formation of gas bubbles, which occurs during the electrolysis of the solvent (water). A constant flux of gas bubbles, whose dimensions can be as small as 20 µm,11 allows the elimination of very small solid particles, which would be otherwise difficult to separate by a gravitational sedimentation. The main processes that occur during electro-coagulation combined with electro-flotation are as follows: (1) Electrochemical reactions at electrode surfaces, resulting in generation of coagulants in the ionic form and of gas bubbles, with both processes being a function of the applied current; (2) the transport of coagulants into the bulk of the aqueous phase; (3) the neutralization of the zeta potential of colloids and the agglomeration of electrically neutral suspended particles into flocs (flocculation); and (4) the collection of flocks of solid particles by gas bubbles and their removal by flotation. The overall rate of the removal of solid particles by coagulation is a function of the hydrolysis rate of a coagulant, the destabilization rate of the colloids, and the transport rate of ions and particles in the system. The rate of flocculation is dependent on the nature of the coagulating species and an eventual presence of flocculating agents, generally chain-structured molecules, which act as bridges between coagulated colloids. Turbulence is an important factor in the processes of chemical coagulation and electro-coagulation, because it can increase the rate of transport of the coagulants, facilitating their contact with the colloids present in the bulk. Thus, enhancement of the mass transfer through the use of bubbles and additional stirring can potentially affect the time in which the coagulating agents are mixed within the reactor and interact with the electrically charged particles of colloids. On the other hand, stirring increasing the drag on the bubbles,12 thus reducing the bubble rise velocity. The bubbles are more easily entrained in the bulk of the solution, which results in an increase of the gas holdup and of the gasliquid interfacial area; hence, the collection efficiency of solid particles can also be positively impacted.

10.1021/ie0503702 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/03/2005

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7845

voltmeter. The cell potential was also followed. Its initial value was equal to 4.2, 6.8, and 10.0 V, respectively, for the experimental current densities of 100, 200, and 300 A/m2. The mixing time of the reactor was defined from the experiments performed with the addition of 1 M NaCl as a tracer and analysis of conductivity over time at six different locations inside the vessel. The variance of the measures was calculated according to20

Figure 1. Experimental setup for the study with mechanical agitation. Legend is as follows: 1, DC power source; 2, aluminum anode; 3, impeller; 4, stainless-steel cathode; 5, electrochemical cell; and 6, stirrer.

The effect that stirring exercises on chemical coagulation and flotation is well-known, and the rates of these processes are generally described as a function of the average shear rate in the vessel.13,14 Despite the fact that these relationships are well-established, the influence of hydrodynamics on the process of electro-coagulation and electro-flotation has been given less attention, even though numerous studies have been conducted on gas bubbles as turbulence promoters. The gas-liquid mass transfer in stirred reactors,15 the collection of solid particles in electro-flotation,16-19 and the effect of current intensity during electro-coagulation18,19 have been studied widely but separately. Consequently, a model that combines both the influence of the applied current and agitation on the electrocoagulation and electro-flotation rates has not yet been proposed. The study presented here was undertaken for the purpose of giving more insight into the performance of these two processes, which are occurring simultaneously, and to investigate the possibility of intensifying the overall rates of electro-coagulation/electro-flotation by enhancing the turbulence in the reactor by agitation. Two different ways of stirring are explored: mechanical mixing and pumping. Experimental Section The experiments were performed in a 0.7-L reactor that was equipped with an aluminum-alloy flat-sheet anode (1050 A, Alnor, Alluminio Nord, Italy; maximum quantity (%) of other elements: Si (0.25), Fe (0.40), Cu (0.05), Mn (0.05), Mg (0.05), Zn (0.07), and Ti (0.05)), and a stainless-steel cathode (both 100 cm2). Mixing in the reactor was performed by mechanical agitation using a Cole-Parmer model P-04554-10 mixer that was equipped with a glass, flat-blade homemade impeller of dimensions 20 mm × 5 mm, or using a peristaltic pump (Watson Marlow Model 313 F/D) that was equipped with two heads, operating under conditions of a full recycle. Because of changes in agitation, turbulence varied from laminar to transient conditions with the maximal Reynolds number of Re ) 9000. Stirring was enhanced also by the formation of gas bubbles (mainly of hydrogen) at the cathode. Figure 1 depicts the experimental setup used during electro-coagulation with mechanical agitation. The study was conducted under galvanostatic conditions at different current densities and stirring intensities. The anode potential was monitored during the experiments using a homemade saturated calomel electrode (SCE) as a reference electrode. It was connected to the working electrode by a high-impedance

σ2 )

1

i)n

∑(Ci - C∞)2

n - 1 i)0

(1)

and was used to define the mixing time under different conditions of turbulence. The mixing time was used as a reference parameter in the performance evaluation of the reactor during electro-coagulation. The electro-coagulation study was performed using a synthetic dyeing-bath wastewater, which was further diluted to obtain different initial concentrations of the dyes. It was prepared following the procedure for a green dyeing bath and contained a mixture of three disperse dyes: 0.181 g/L of Disperse Yellow 126 (Dispersol D-7G), 0.034 g/L of Disperse Red 74 (Foron S-BWFL) and 0.158 g/L of Disperse Blue 139 (Navy Blue Sumikaron S-2GL). Polyvinyl alcohol (PVA) (0.444 g/L) was added as a dispersing agent, together with 0.055 g/L of Nicca Sunsolt 7000 (anionic surfactant). The dyeing bath proved to have significant electrical resistance (the conductivity was equal to 134 µS/cm), resulting in the need to add a supporting electrolyte (0.1 M NaCl was used). The initial pH of the solution was adjusted to 9.0 by adding NaOH, in consideration of the results of a previous study on the coagulation of disperse dyes,21 which showed that alkaline pH favors electro-coagulation, probably due to enhancement of the generation of hydrolyzed aluminum compounds.22 The performance of the reactor was followed by analyzing the disappearance of the color of the wastewater. This was determined by measuring the absorbance at a fixed wavelength (420 nm), using a Hach DR 2000 single-beam spectrophotometer (extinction coefficient equal to 10.45 × 10-3 L mg-1 cm-1) after sample dilution (1:16). Samples whose pH differed from 9.0 were adjusted to this value by adding HCl or NaOH. Because the dyes were only partially soluble in water, this measurement gave the sum of both the turbidity and color. A calibration curve was prepared for various wastewater concentrations and was used to convert the absorbance measured at different time intervals into dye concentrations. This curve was compared to that obtained after the dissolution of the samples in ethanol (Figure 2). As can be seen, the results of the analysis of the water solution and the water-ethanol solution (1: 16 volumetric ratio) are well-correlated and there are no deviations from the linearity in the concentration rage used in the study. Although the extinction coefficient for the water-ethanol solution of the dyes was higher (14.7 × 10-3 L mg-1 cm-1) than that for the water-only solution, the use of water solutions was chosen for the determination of color, because of the simplicity of this procedure. A similar approach was also applied by other authors8 during studies on the removal of dyes by electro-coagulation. For the samples of raw and treated wastewater, UVVis spectroscopy was performed using a double-beam

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Power consumption and the average gradient of liquid velocity (G) were calculated using eq 2 from ref 20 and eq 3 from ref 23, respectively, for mechanical agitation and for pumping:

Figure 2. Calibration curve for determination of color: (9) EtOH solution, y ) 0.0294x, r2 ) 0.9992; and (2) H2O solution, y ) 0.0221x, r2 ) 0.9996.

P ) NpFN3D5

(2)

P ) FghQpumping

(3)

where P is the power dissipated during agitation (given in watts), Np the power number (assumed equal to 0.35),20 F the density of the solution (in units of kg/m3), N the rotation speed of the impeller (given in Hertz), D the impeller diameter (given in meters), g the gravitational constant (m/s2), h the pumping height (which is equal to the liquid level in a vessel, given in meters), and Qpumping the pumping rate (in units of m3/s). The average gradient of liquid velocity (shear rate), G (in units of s-1), was calculated from the relation12

G)

Figure 3. Mixing time under different conditions of turbulence: (a) mechanical stirring ((4) 3 Hz, (0) 5 Hz, and (b) 9 Hz) and (b) pumping ((0) 250 mL/min, (9) 500 mL/min, and (b) 1000 mL/min).

Perkin-Elmer 554 spectrophotometer. The spectrum of raw wastewater was the sum of the spectra of the single dyes. Other additives present in the dyeing bath influenced the spectrum to a far-lesser extent. UV-Vis spectra were also performed using wastewater samples containing only chlorides or sulfates at a time. The main difference regarding the spectra was a strong absorption peak at 210 nm observed in the presence of sulfates. The aluminum content in the samples was determined using a Perkin-Elmer Analyst 100 atomic absorption spectrophotometer. Results and Discussion Hydrodynamic Behavior of the Reactor. Figure 3 depicts the time trend of the variance of the tracer concentration for operating under different hydrodynamic conditions. The mixing time of the reactor was defined as the time needed for the variance to decay to 0.5% of its initial value. Table 1 shows the values of the characteristic mixing time obtained for conditions characterized by various stirring intensities. As can be seen, mechanical agitation was very effective in obtaining a quick homogenization of the reactor content, and, under the highest turbulence, the characteristic mixing time was as short as 0.3 min.

xVµP

(4)

where P is the dissipated power (in watts), V the volume (given in cubic meters), and µ the dynamic viscosity (in units of kg m-1 s-1). The values of G are depicted in Table 1. They oscillated between 19 s-1 and 169 s-1 and were comprised within the range characteristic for conventional chemical coagulation.24 Chemistry of the System. During the electrocoagulation experiments, anode potentials were monitored using an SCE in a Luggin capillary probe; values of 0.16, 0.17, and 0.31 V (SCE) were measured at current densities of 100, 200, and 300 A/m2. However, in the initial 10-20 s, the anode potentials were slightly higher, probably because of the presence of a thin, airformed film of oxide/hydroxide, which has very low solubility in noncomplexing solutions of pH ∼5-9, and low electronic conductivity, which is due to low defect concentrations.25 The solutions were not de-oxygenated prior to use; therefore, dissolved oxygen concentrations would have been ca. 0.25 mol/m3 under atmospheric pressure and ambient temperatures. Hence, oxygen reduction by reaction 5 could occur, both at the cathode, whose measured potential was (including any uncompensated IR drop) -1.35, -1.40, and -1.43 V (SCE) at 100, 200, and 300 A/m2, and, as EO2/H2O (SCE) ) 0.445 V, also at the anode:

O2 + 2H2O + 4e f 4OH-; jO2

(5)

Because reaction 5 generates hydroxyl ions, the local pH could have increased to >9.0, promoting the dissolution of Al(OH)3(s), which may be initially present on the anode surface, via reaction 6:

Al(OH)3(s) + OH - f [Al(OH)4]-

(6)

During the chemical dissolution of aluminum oxide/ hydroxide, other hydroxylated species (e.g., Al2O(OH)62-) could also have been formed. In chloride-rich media, corrosion of aluminum by pitting also could have been operative,25 considering that the apparent anode potential was higher than the

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7847 Table 1. Characteristic Mixing Time, Power Consumption, and Average Shear Rate for the Reactor Operating under Different Hydrodynamic Conditions Mechanical Mixing

Mixing by Pumping

rotation rate (Hz)

power consumption (× 103 W)

time of mixing (min)

average shear rate, G (s-1)

pumping rate (mL/min)

power consumption (× 103 W)

time of mixing (min)

average shear rate, G (s-1)

0 3 5 9

0 0.23 1.06 6.20

n.d. 4.3 1.7 0.3

0 19 41 99

0 250 500 1000

0 4.49 8.99 17.98

n.d. 10.8 7.0 2.8

0 84 115 169

critical pitting potential: -0.75 V (SCE) to -0.95 V (SCE), depending on the pH and Cl- ion concentration. It can be hypothesized that, under the conditions of the present study, three forms of pitting could have been operative: processes occurring on the passive film and at the boundary of the passive film, processes occurring within the passive film, and stable pit growth. Little is known about the first two processes, which can lead to breakdown of the passive film.25 A possible explanation is given by a point-defect model,25 which assumes that chloride ions are incorporated into the passive film by occupying anion vacancies; this results in a decrease of anion vacancies and accumulation of cation vacancies at the metal interface, causing breakdown of the film. Pitting corrosion of aluminum is initiated by electrostatic adsorption of chlorides on the oxide film,26,27 adsorption densities increasing with increasing electrode potential.25 The outermost surface of aluminum oxide in water is covered with a layer of hydroxyl groups and, when pH < pHpzc ) 9.5,27 the surface acquires a positive charge, which favors chloride adsorption, and promotes the following sequence of reactions, leading to pit formation:27 +

+

Al(oxide)OH + H f Al(oxide)OH2

(7)

Al(oxide)OH2+ + nCl- f Al(oxide)OH2+Cln-n

(8)

Transport of chloride ions in the film occurs via oxygen vacancies and, finally, local dissolution of aluminum ions at the metal/oxide interface occurs in three consecutive one-electron-transfer reactions.27 Application of electrode potentials more positive than the pitting potential in chloride-rich media leads to pit propagation and formation and growth of blisters below the passive film.27 pH values as low as 1 have been measured inside such pits,25 but calculated values can be even lower.27 Such an acidic pH in pits formed by chloride pitting corrosion creates conditions for the oxidative dissolution of aluminum,

AlfAl3+ + 3e

(jAl)

(9)

to be coupled to

2H+ + 2e f H2(g)

(jH2)

(10)

with the net current density being dependent on the local electrode potential. As a consequence, the pit propagates and, when the hydrogen pressure within the blister becomes sufficiently large, the blister ruptures and the corrosion pit is opened to the bulk electrolyte.

At a bare aluminum surface, reaction 9 is rapid and coupled to a series of hydrolysis reactions:25

Al3+ + H2O f H+ + Al(OH)2+

(11)

Al(OH)2+ + Cl- f Al(OH)Cl+ +

(12) +

Al(OH)Cl + H2O f Al(OH)2Cl + H

(13)

If localized dissolution occurs, large compositional and pH gradients develop at the anode, inside the hydroxide film, and inside the pits. The large local current densities (jpore) down the pores cause significant ohmic potential drops (∇φpore), depending on the local conductivity (σpore) of the solution in the pores, so that the effective potential (E - ∇φpore) at the base of the pore is much lower than the measured, apparent anode potential (E). It is probable that the effective electrode potential inside the pits was lower than the equilibrium hydrogen potential (EH2O/H2 (SCE) ) -0.777 V), so the following reaction could also occur:

2H2O + 2e f H2 + 2OH-

(14)

Hence, in the present study, the net anodic current could be the algebraic sum of aluminum oxidation current density, the oxygen reduction current density and hydrogen evolution, the last two being negative members in the sum A A A ) j AAl + j H +jO j appl 2 2

(15)

It follows that the actual current density (j AAl) was probably higher that the net applied current density A ) and, hence, apparently super-faradaic Al3+ cur(j appl rent yields17 would be expected, depending on the relative magnitudes of reactions 5, 10, and 14. An important aspect of the chemistry of the system is related to the hydrogen evolution process. For the cathode, reaction 14 was the primary reaction, whereas the rate of this reaction at the anode was dependent on the partial current density contributions in eq 15 during pitting corrosion.28 Generation of hydroxyl ions (reactions 5 and 14) and consumption of H+ (reactions 7 and 10) could be expected to produce, in a batch reactor, an increase of the pH with time. However, this was not observed in the present study. A rise to a pH value of 9.5 (from the initial pH, 9.0) was observed in the first 30 s, followed by a sharp decrease to pH 9.0, which stabilized at this value almost until the end of the process, when the pH was 9.2. Similar behavior of batch electro-coagulation under alkaline conditions was reported by other authors.29 Such a trend can be explained by the consumption of the alkalinity in reaction 6 and generation of H+

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Figure 4. Performance of electro-coagulation/flotation as a function of the charge; experiments were conducted under conditions of (9) 100 A/m2, (2) 150 A/m 2; (4) 200 A/m2, (O) 250 A/m2, and (b) 300 A/m2.

in reactions 11 and 13. Another explanation could be a possible formation of Al13 polymeric species, for which Al(OH)4- is known to be a precursor;30 these are reactions in which H+ is generated.31 Performance of Electro-coagulation/Flotation under Different Hydrodynamic Conditions. To verify whether the removal of the dyes could be influenced by the potential of the anode, assuming different values at various current densities, coagulation was performed under quasi-stagnant conditions, varying current density: 100, 150, 200, 250, and 300 A/m2. The results of single experiments, expressed as a function of the charge overlap, are shown in Figure 4, indicating that the mechanism of color removal was the same, despite the variation of electrochemical parameters. Figure 5 illustrates the time trend of the removal of color under different hydrodynamic conditions and current densities. Two different phases can be distinguished. In the initial phase, in which the effect of the anodic dissolution of aluminum prevails, the coagulating species of this metal are produced and further diffuse into the bulk solution, where they participate in the neutralization of the surface charge of the colloids. In this phase, which will be referred to as electro-coagulation, the effect of color elimination is not yet significant. In the subsequent second phase, neutral solid particles are brought together and, because of the van der Waals forces of attraction, form bigger flocks (process of flocculation), which are collected by gas bubbles and transported by them onto the liquid surface (process of flotation). The moment when the second phase initiates is dependent on the hydrodynamic conditions, as does also the rate of color elimination in this phase. A quasi-linear performance of the elimination of color in time during the first phase of coagulation is consistent with the Faraday low, because the quantity of aluminum that passes into the solution due to its anodic dissolution would be a linear function of time. The quantity of aluminum produced during the first phase of coagulation was determined experimentally, and the current efficiency (Φ) was calculated according to

Φ)

[Al]nFV ItMAl

(16)

where Φ is the anodic current efficiency, [Al] the concentration of aluminum in the solution and in the sludge at a time t (in units of g/L), n the number of exchanged electrons, F the Faraday constant (96 485 C/mol); V the electrolyzed volume (given in liters); t the

Figure 5. Performance of the reactor operating at different current densities and stirring conditions: (a) mechanical mixing, 100 A/m2 (([) 0 Hz, (b) 3 Hz, (2) 5 Hz, and (×) 9 Hz); (b) mechanical mixing, 200 A/m2 (([) 0 Hz, (b) 3 Hz, (2) 5 Hz; and (×) 9 Hz); (c) mechanical mixing, 300 A/m2 (([) 0 Hz, (b) 3 Hz, (2) 5 Hz, and (×) 9 Hz); and (d) pumping, 200 A/m2 ((b) 0 mL/ min, (9) 250 mL/min, ([) 500 mL/min, and (2) 1000 mL/min).

time of electrolysis (given in seconds), I the applied A Aa, where Aa is the current (given in amperes; I ) iapp surface area of the anode), and MAl the molecular weight of aluminum (in units of g/mol). Table 2 reports the total quantity of aluminum determined as the sum of aluminum present in the flocculated sludge and in the solution, sampled at different intervals of electro-coagulation performed at

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7849 Table 2. Quantity of Aluminum Present in the Solution and in the Sludge and the Current Efficiency of Its Anodic Generation time (min)

quantity of aluminum present (mg)

current efficiency, Φ

1 2 3 4 5 6

19.2 53.0 83.4 108.6 139.8 179.2

1.20 1.66 1.74 1.70 1.75 1.87

200 A/m2 under quasi-stagnant conditions. The Φ values calculated from eq 16 are also reported. As can be observed, the efficiency of the anodic process exceeded 100%; even a value as high as 187% was obtained after 6.0 min of the electrolysis. These super-faradaic yields can be explained considering that the effective current density at the anode was higher than the apparent value (see eq 15), because of oxygen reduction and hydrogen evolution, which are reactions that, under the conditions of the present study, probably occurred at the anode, simultaneous to the aluminum oxidation and its chemical dissolution, as discussed previously. An apparent efficiency of Φ >100% was also obtained by Mameri et al.32 during the electro-coagulation of fluoride-containing water. In Table 3, the time of coagulation (defined as the time elapsed from the beginning of the process until the moment that the flotation started to be operative), the time of flotation, and the total time needed to remove 99% of the color are reported for different experimental conditions. Comparing these data with the characteristic mixing time of the reactor, it can be observed that the process of decolorization is very rapid and at a high current density is completed in an even shorter time than the characteristic mixing time of the reactor. The data in Table 3 also prove that mechanical agitation is more efficient than pumping. The influence of the initial concentration of the dyes on the performance of the process was evaluated using the studies in solutions characterized by the initial absorbency varying over the range of 0.315-0.628. The results of the study are depicted in Figure 6, in terms of the time necessary to accomplish the first phase of coagulation in function of the initial absorbency of the solution. As can be observed, the necessary time is directly proportional to the concentration of the dyes, which confirms that this phase serves essentially to produce a coagulant.

Figure 6. Time necessary to accomplish electro-coagulation as a function of the initial absorbency of the solution (no mixing, 200 A/m2).

A Phenomenological Model of Electro-coagulation/Flotation. The decrease in the dyes concentration in the first phase of electro-coagulation was proportional to the time lapse and the decay followed:

Ct ) 1 - k1t C0

(17)

where C0 and Ct are the concentrations of the dyes at the beginning of the electrolysis and at time t (each reported in units of mg/L), respectively; k1 ) k′1/C0, and k′1 is the zero-order reaction rate constant (in units of mg L-1 s-1). The values of k1 were derived from the plots of Ct/C0 versus time. The performance of the second phase of electrocoagulation proved to be well-described by the pseudo first-order kinetics:

dC ) -k2C dt

(18)

where k2 is the first-order rate constant (in units of s-1). The values of the rate constant of the second phase of the process were determined from the slopes of the logarithms of the normalized dye concentration versus time, according to

ln

()

Ct ) -k2t C0

(19)

Table 4 reports the values of the rate constants k′1 and k2 calculated for the reactor performing under different experimental conditions. It can be observed that regarding the hydrodynamics of the system, it

Table 3. Time of Electro-coagulation and Electro-flotation under Different Hydrodynamic Conditions (a) Mechanical Mixing rotation rate (Hz) 0 3 5 9

Total Time to Remove 99% of the Dyes (min) 100 A/m2 200 A/m2 300 A/m2 14.0 9.5 6.4 5.1

7.1 5.2 3.7 2.6

5.5 4.7 2.8 2.0

Time of Electro-coagulation (min) 100 A/m2 200 A/m2 300 A/m2 6.8 5.6 4.1 3.3

3.5 2.4 2.3 1.4

2.3 2.2 1.8 1.1

Time of Electro-flotation (min) 100 A/m2 200 A/m2 300 A/m2 7.2 3.9 2.3 1.8

3.6 2.8 1.4 1.2

3.2 2.5 1.0 0.9

(b) Mixing by Pumping pumping rate (mL/min)

Total Time to Remove 99% of the Dyes (min)

Time of Electro-coagulation (min)

Time of Electro-flotation (min)

0 250 500 1000

7.2 5.6 4.6 3.4

3.4 3.2 2.5 1.9

3.7 2.4 2.1 1.5

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Table 4. Values of the Rate Constants for Electro-coagulation and Electro-flotation for Different Stirring Conditions Mechanical Mixing k′1

(×

10-4 mg

L-1

s-1)

k2 (× 10-2 s-1)

Mixing by Pumping (at 200 A/m2)

rotation rate (Hz)

100 A/m2

200 A/m2

300 A/m2

100 A/m2

200 A/m2

300 A/m2

pumping rate (mL/min)

k′1 (× 10-4 mg L-1 s-1)

k2 (× 10-2 s-1)

0 3 5 9

0.243 0.647 1.208 2.242

0.533 1.267 2.333 3.500

0.817 2.133 3.027 5.900

0.508 1.098 1.527 2.017

0.987 1.530 2.348 3.100

1.270 2.117 2.893 3.977

0 250 500 1000

0.533 2.183 2.500 3.100

0.987 1.756 1.874 1.919

influences in a significant way both the phases of the elimination of color. From results presented Table 2 it also appears that current density is another important parameter and its increase, in parallel with stirring, enhances the rates of both the phases of the process. In fact, the higher the current density, the higher the rate of aluminum production and the quantity of the evolved hydrogen also increases. This, in turn, improves the rates of flocculation and flotation by further augmenting the mass-transfer rate in the reactor due to the movement of gas bubbles and, probably, by increasing their total surface area, and so increasing the collection efficiency of solid particles. The following correlations were determined to well describe the variation of the value of the rate constant k1 with the applied current density iappl (A/m2) and the agitation intensity under the experimental conditions of the present study:

mechanical stirring: k1 ) (iappl0.9)(3 × 10-7N + 4 × 10-7) (20a)

During the process of electro-flotation, the particles of dyes attach to the gas bubbles and are floated onto the surface. The higher the total surface of the gas bubbles, the better the process performance. The surface of the gas bubbles is a function of the gas volume. Assuming that the gas bubbles have uniform dimensions and denoting the radius of a single bubble by the parameter r, the number of generated bubbles is given as

n)

Q H2 (4/3)πr3

(22)

In case the electro-flotation is not enhanced by additional stirring, Fukui and Yuu16 proposed the model in which the process rate is proportional to the flow rate of evolved hydrogen. The model equation, modified to include hydrogen evolution at both the electrodes, would be

( )

3QH2 dC′ )R C′ dt 4Asr

(23)

where C′ is the particle concentration, As the cross section area of the vessel, and R the collection efficiency

where N is the stirring rate (in Hertz), and

agitation by pumping: k1 ) (iappl)0.9(2 × 10-7(Qpumping)0.33 + 5 × 10-7) (20b) Figure 7 shows the plots of the values of the ratio k1/ iappl0.9 as a function of agitation intensity for all the experimental conditions of the present study. Simulated plots (solid lines) provide evidence of the adequacy of the proposed model given by eqs 20a and 20b. Regarding electro-flotation, in the analyses of the role of the gas bubbles, two phenomena, as already mentioned, must be distinguished: (i) the collection of the solid particles on the gas bubbles surface (thus, the total surface area of the gas-liquid interface would be important), and (ii) the enhancement of the masstransfer rates due to the rising of bubbles, which facilitates their contact with solid particles (the number of gas bubbles would be important). Considering that the gas is composed of hydrogen and is continuously evolved at the cathode and the anode, its flow rate (QH2) is given by

Q H2 )

(ΦH2I) V + Qa 2F M

(21)

where ΦH2 is the efficiency of hydrogen evolution on the cathode, I the total applied current (I ) iapplAc), Vm the molar volume of hydrogen at operating temperature, F the Faraday constant, and Qa the volumetric flow rate of hydrogen evolved at the anode.

Figure 7. Plot of the group k1iappl0.9 as a function of the intensity of agitation: (a) mechanical stirring (current density: ([) 100 A/m2; (9) 200 A/m2, (2) 300 A/m2) (b) and mixing by pumping (200 A/m2).

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7851

of a bubble. The last equation, after integration, gives

ln

() ( )

3QH2 C′0 )-R t ) -k′2 t C′t 4Asr

(24)

where C′0 and C′t are the initial particle concentration and the particle concentration after time t, respectively, and k′2 ) R(3QH2)/(4Asr). On the other hand it is recognized that, in a stirred vessel, the rate of flocculation is dependent on the average velocity gradient, according to12

ln

() ( )

C0 4βΨ )Gt ) -k′′2 t Ct π

(25)

where Ψ is the volume fraction of particles, β is the collision efficiency of the particles, and k′′2 ) 4βΨG/π. When both phenomena (i.e., electro-flotation and stirring enhanced flocculation) are operative simultaneously, a decrease of the concentration of solid particles with time can be supposed to be a function of the parameters that influence singly the above process: the applied current and the mean velocity gradient induced by stirring. Thus, the performance of the reactor during the present study is consistent with that previously discussed. A question arises whether these effects are simply additive or they are superimposed. Assuming the effects to be additive, the following equation, describing the decay of the dyes concentration, derived from a linear combination of eqs 24 and 25, should be valid:

ln

()

(

)

3QH2 4βΨG C0 ) -k2t ) f R t + Ct 4Asr π

(26)

Considering that the mean velocity gradient G is, according to eq 4, a function of the power dissipated during mixing, which, in turn, for mechanical agitation, is a function of the rotation rate of the impeller N (eq 2), and assuming hydrogen evolution being a function of the applied current, eq 26 can be rewritten:

ln

()

C0 ) -(a′Ic + b′N1.5)t Ct

(27)

where a′ and b′ are coefficients related to the collection efficiency of the gas bubbles R and the collision efficiency of the particles β. From eq 27, it follows that the rate of decolorization could be expected to increase at a factor of 1.5 times the intensity of agitation and with the applied current, in case the two phenomena are additive. The plots of k2 versus current density were not linear and this kinetic rate constant proved to be dependent on a factor of 0.847 times the current density (c ) 0.847). Application of the model eq 27 to the experimental data obtained with mechanical stirring showed that the rate of flotation was not simply a power function of N only. At this point, it is interesting to note that the effect of the gas bubbles on the increase of the mass-transfer rate due to their movement has been widely studied, and it is recognized that, in a vessel in which the gas is sparged, the volumetric gas-liquid mass-transfer coefficient kL,a was determined to be proportional not to the

impeller speed itself, but rather to a ratio between the impeller speed and the minimum impeller speed for a complete dispersion of the gas. Yawalkar et al.33 proposed the following correlation:

( )

kL,a ) f

N NCD

1.464

(28)

where N is the impeller speed (given in Hertz) and NCD is the minimum impeller speed for the complete dispersion of the sparged gas (also given in Hertz). The mixing process at any impeller speed below NCD would result in incomplete dispersion of the gas. In other words, at an impeller speed below this value, there is little or no gas in the central region of the tank and a part of it is wasted, which can result in a poor performance of flotation. The NCD value can be calculated from a correlation developed by Nienow et al.:34

NCD )

4(QH2)0.5T0.25 D2

(29)

where QH2 is the volumetric gas flow rate (in units of m3/s), T the tank diameter (given in meters), and D the impeller diameter (also given in meters). Under the conditions of the present study, with the diameter of the mixing paddle being D ) 3 × 10-2 m, assuming that the tank diameter T is equal to the distance between the electrodes (T ) 4.5 × 10-2 m), the minimum impeller speed for a complete dispersion of the gas evolved at the electrode would be NCD ) 0.37, 0.45, 0.52, 058, and 0.63 Hz, respectively, for 1.0, 1.5, 2.0, 2.5, and 3 A current intensities. This was calculated from eq 29, using the quantities of evolved H2 defined from eq 21 and assuming arbitrarily that the anodic evolution of hydrogen balanced the cathodic efficiency, which was