ARTICLE pubs.acs.org/IECR
Effect of the Ultrasonic Irradiation on the Cr(VI) Electroreduction Process in a Tubular Electrochemical Flow Reactor S. A. Martínez-Delgadillo* Depto. Ciencias Basicas, Universidad Autonoma Metropolitana-Azcapotzalco, Av. San Pablo 180, Azcapotzalco CP 07740, Mexico D.F., Mexico
V. X. Mendoza-Escamilla Depto. Electronica, Universidad Autonoma Metropolitana-Azcapotzalco, Av. San Pablo 180. Azcapotzalco, CP 07740, Mexico D.F., Mexico
H. R. Mollinedo-Ponce SEPI-ESIME-Zacatenco IPN, Av. IPN s/n Unidad Profesional Adolfo Lopez Mateos, Mexico D.F., Mexico
H. Puebla Depto. Energía, Universidad Autonoma Metropolitana-Azcapotzalco, Av. San Pablo 180, Azcapotzalco, CP 07740, Mexico D.F., Mexico
J. M. Mendez-Contreras Instituto Tecnologico de Orizaba 852, Tecnologico, Zapata 94320, Division de Estudios de Posgrado e Investigacion, Orizaba Veracruz, Mexico ABSTRACT: The electroplating, leather tanning, and textile industries are facilities that discharge wastewaters contaminated with high concentrations of Cr(VI), which is carcinogenic and has negative health effects. Low-cost technologies to remove this pollutant must be generated with high-efficiency reactors. Electrochemical Cr(VI) reduction is a low-cost alternative process, which has been studied and applied with success at the laboratory level. In this work, the hydrodynamic behavior of the fluid into the sonoelectrochemical tubular flow reactor with spiral-helicoid-wire low-cost electrode material (carbon steel) was evaluated. The fluid behavior was evaluated by computational fluid dynamics (CFD). In addition, the dispersion the reactor was evaluated experimentally and by CFD. The reactor presented large dispersion that was considered into the model to describe the sonoelectrochemical process to reduce the Cr(VI) in wastewaters. Moreover, an empirical model to describe the electrochemical reduction of the Cr(VI), with and without ultrasonic irradiation, was validated experimentally at different pHs.
’ INTRODUCTION The electroplating, leather tanning, and textile industries play an important role in Mexican industrial and economical activity. However, these industries commonly are small facilities that discharge wastewaters contaminated with high concentrations of Cr(VI), which is carcinogenic and has harmful health effects. In addition, these industries cannot afford for available technologies to remove chromium(VI) from wastewaters (i.e., the use of bisulfite, evaporation, ion exchange, and ferrous sulfate, among others), because of some drawbacks, such as high inversion and operation costs; in some technologies, i.e., ferrous sulfate1 a large sludge amount is generated, which must be treated, handled, and disposed, increasing the total costs of the processes. Low costs technologies must be generated with high efficiency reactors. Electrochemical Cr(VI) reduction is an alternative process, r 2010 American Chemical Society
which has been studied and applied with success to remove Cr(VI) from wastewater at the laboratory and pilot level.2-6 In this process, the Fe anode (anodic oxidation) releases Fe(II) into the solution when an electrical direct current is applied, then it reduces the Cr(VI) to Cr(III) and Fe(II) itself oxidizes to Fe(III), as shown in reaction 1: þ 3þ 6Fe2þ þ Cr2 O2þ 2Cr3þ þ 7H2 O ð1Þ 7 þ 14H f 6Fe
Special Issue: IMCCRE 2010 Received: January 17, 2010 Accepted: March 23, 2010 Revised: March 3, 2010 Published: March 31, 2010 2501
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Figure 2. Meshes near the sonoelectrochemical reactor inlet: (a) with gaskets and (b) without gaskets.
Figure 1. Schematic diagram showing a sonelectrochemical reactor.
In addition, there is a reduction reaction on the cathode that contributes with the Cr(VI) reduction, directly (as reaction 2) or indirectly (as reaction 3). In the last reaction, the Fe(II) is regenerated, and it again can reduce the Cr(VI) as in reaction 1.6 þ Cr2 O2f Cr2 O27 þ 6H þ 6e 4 þ 3H2 O
ð2Þ
Fe3þ þ 1e- f Fe2þ
ð3Þ
However, during the electrochemical process, the pH tended to increase, because of reaction 4, which occurs at the cathode:2 2H2 Oþ2e- f H2 þ 2OHð4Þ As known, pH has an important effect on the Cr(VI) reduction, because it affects the solubility of the iron and chromium ions.7-9 When the pH increases, the solubility of the iron and chromium species is reduced and insoluble oxides precipitate on the surfaces of the electrodes (electrode passivation) that reduces the mass and electrical charge transfers at the electrode surface/ solution; hence, the Fe(II) release rate into the solution and reduction rates are also lower (i.e., reduction of Cr(VI) to Cr(III) and Fe(III) to Fe(II) on the cathode). Then, passivation becomes an important drawback when low-cost electrode material, such as carbon steel, has been used. Ultrasonic irradiation produces acoustic cavitations that cause the formation, growth, and rapid recompression of vapor bubbles in the liquid that collapse and generate microscopic shock wave, which produces a microstirring effect. In sonochemical studies,10-14 it has been demonstrated that the effects depend on the frequency of the ultrasound used; at the higher frequencies (>350 kHz), pyrolytic pathways predominate, whereas mechanical effects predominate at lower frequencies (23 kHz). Although some studies using sonochemical methods to reduce the toxicity have been performed with other metals,15 there are no studies related to the electroreduction of Cr(VI) using ultrasonic irradiation. Moreover, the design of the reactor is a key component of the electrochemical processes. In fact, the behavior of the fluid into the reactor must be evaluated and introduced in the dynamic
modeling of electrochemical reactors, because the backmixing affects the reactor performance.16 In this work, the hydrodynamic behavior of the fluid into the reactor with two different arrangements, with and without gaskets, was evaluated and, based on the best hydrodynamic behavior in the reactor (lower dispersion), the performance of the electrochemical process to reduce the Cr(VI) from wastewaters using carbon steel in a novel tubular electrochemical continuous reactor, using ultrasonic irradiation ultrasound of low frequency (40 kHz), was evaluated. In addition, an empirical model to describe the electrochemical reduction of the Cr(VI), with and without ultrasonic irradiation, was validated experimentally at different influent pH values in an actual electrochemical reactor with high dispersion. The residence times (th) required to reduce the Cr(VI) from 1000 mg/L to 0.5 mg/L (the maximum concentration in the treated wastewaters permitted by Mexican environmental regulations17), at the different influent pHs, with and without ultrasonic radiation, were obtained at steady state with the validated model.
’ MATERIALS AND METHODS The industrial wastewater was sampled from the rinsing baths of a chromating industry; the amount used during the tests was 1000 mg of Cr(VI)/L. An electrochemical tubular reactor that was comprised of Plexiglas material with a tangential inlet, shown in Figure 1, was used during the experiments. The operation volume was 2.487 L, and its dimensions were a length of 1.035 m and an internal diameter (ID) of 0.054 m. A central polished carbon steel rod measuring 1.035 m served as the cathode, which was surrounded by a 5.356-m-long spiral helicoid wire of the same material, which served as the anode with an area of 0.101 m2. Both were isolated from each other. The reactor performance was evaluated by isolating the electrodes with and without rubber gaskets. When the rubber gaskets were used, they were located near the inlet, at the central part and near the exit, as seen in Figure 1. In Figure 2, the meshes used for the test with and without gaskets near the inlet are shown. A DC source was used to supply and control the current density at 108 A/m2. Six standard piezocomposite transducers with a resonance frequency of 40 kHz, connected to standard generator operating at 300 W, were bonded to the steel carbon plate that was attached to the anode to generate the ultrasound along it (see Figure 1). Each transducer has a maximum power capacity of 50 W. The wastewater was fed to the reactor by a peristaltic pump that was adjusted to drive the flow rate and regulate the hydraulic residence time (th) into the reactor. Before feeding the reactor with the wastewater with hexavalent chromium, the reactor was filled and fed by the peristaltic pump with tap water during a residence time to stabilize the reactor, and then the reactor was 2502
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fed with the wastewater with Cr(VI) adjusted to the required pH. At the same time, the DC source was turned on. When the effect of the ultrasonic irradiation was evaluated, the generator also was turned on. Samples were taken out at the reactor exit at different times, and, after the precipitation, the residual Cr(VI) was measured in triplicate into the liquid supernatant by the 1,5diphenylcarbazide method.18 The Cr(VI) reduction in the tubular reactor, with and without ultrasonic irradiation, was modeled under unsteady-state conditions based on eqs 5-10.19 Different influent pH values were tested during the experiments: 1.2, 1.5, 1.7, and 2.0. The model applied to describe the increase in pH due to the OH- production during the electrochemical process and the Cr(VI) removal in the tubular reactor at the nonsteady state is described. The constant rates were obtained to fit the experimental results adequately and the effect of pH on the process was evaluated. To obtain the residence times required to reduce the Cr(VI) from 1000 mg/L to 0.5 mg/L, simulations were performed until they reach steady state and the effluent (reactor outlet) Cr(VI) concentrations were e0.5 mg/L. The pH variation model is shown in eq 5, and the most largely used boundary conditions20 are shown in eqs 6 and 7. ! k1pH pH ∂pH D ∂2 pH ∂pH ¼ þ th th ð5Þ ∂t uL ∂z2 ∂z 1 þ k2pH pH pH -
D ∂pH - pHo ¼ 0 uL ∂z ∂pH ¼ 0 ∂z
ðat z ¼ 0Þ
ðat z ¼ 1Þ
ð6Þ ð7Þ
The Cr(VI) removal in the tubular reactor is described by eq 8, ∂Cr D ∂2 Cr ∂Cr k1 Cr ¼ - th th ð8Þ ∂t uL ∂z2 ∂z 1 þ k2 Cr and the boundary conditions20 are shown in eqs 9 and 10. Cr -
D ∂Cr - Cro ¼ 0 uL ∂z ∂Cr ¼ 0 ∂z
at z ¼ 0
at z ¼ 1
ð9Þ ð10Þ
where pHo is the influent pH, k1pH the constant rate of pH variation (k1pH = 0.009 min-1), k2pH the constant rate of pH variation (k2pH = 0.000332), Cr the Cr(VI) concentration in the reactor (given in units of mg/L), Cro the influent Cr(VI) concentration (also given in units of mg/L), k1 the constant rate of Cr(VI) removal (given in units of min-1) (k1 = A pH-1.1522), and k2 the constant rate of Cr (VI) removal (L/mg) (k2 = 0.0115(log pH) þB). When no ultrasonic irradiation was applied, the values of A and B are given as follows: At pH 1.2 and 1.5: A = 1.04 and B = 0.0005; For pH 1.7: A = 0.91 and B = 0.002566; and For pH 2.0: A = 0.7 and B = 0.002566. With ultrasonic (US) irradiation, the values of A and B are given as follows: At pH values of 1.2, 1.5, and 1.7: A = 1.19631 and B = -0.00198; and
At pH 2.0: A = 0.8031 and B = -0.000404. Other parameters are defined as follows: x = across position in the reactor (m) t = time (min) th = hydraulic residence time (min) θ = dimensionless time; θ = t/th z = x/L (dimensionless)| Nd = dispersion number; Nd = D/(uL) D = dispersion coefficient (m2/s) u = flow velocity (m/s) L = reactor length (m) E = exit age distribution function (dimensionless) The dispersion number (Nd) was evaluated by a tracer pulse in the inlet, and its concentration was evaluated at the exit (effluent) of the reactor.21 To perform the numerical simulation, commercial software FLUENT (version 6.2.16) has been used in all of the studies. The complete three-dimensional (3D) model of the reactor has been meshed using TGrid (tetrahedral cells), which involves an unstructured grid that was necessarily due to the complexity of the model. For the pressure-velocity coupling, a nonlinear procedure called semi-implicit pressurelinked equation (SIMPLE) algorithm was used; for pressure discretization, the standard scheme was used; and for the momentum discretization, the First-Order Upwind scheme was used. A maximum residual convergent goal of 10-5 was specified. The governing equations for the steady-state fluid flow simulation are given by the mass and momentum conservation equations, which are shown in eqs 12 and 13, respectively: r 3 vB ¼ 0
ð12Þ
F ð13Þ r 3 ðF vB B v Þ ¼ - rp þ r 3 ðτÞ þ F gB þ B where Bv is the velocity vector, F the density, hτ the stress tensor, FgB the gravitational body force, and B F the external force vector. For the pulse tracer simulations, a pressure-based solver, Gren-Gauss node-based gradient, and unsteady second-order implicit formulation was applied. The momentum and tracer equations were discretized with Second-Order Upwind and the pressure velocity coupling SIMPLE scheme. A time-stepping strategy was used.22-24 A tracer pulse of a time step of a 1s for one time step was applied at the reactor inlet until convergence (10-4); it then was suspended and the simulation with zero tracer concentration for a step of a 1s for one time step was obtained until convergence. The process then continued but with the step increasing to 2 s, 4 s, and 5 s, until convergence in each case. Then, a step of 5 s was maintained until the tracer reactor concentration at the outlet reached zero. The conservation equation takes the following general form of eq 14: ∂ ðFYi Þ þ r 3 ðF vBYi Þ ¼ - r 3 BJ i ∂t
ð14Þ
where Yi is the local mass fraction andBJ i is the diffusion flux of the species i. The diffusion flux can be written as eq 15: BJ i ¼ - FDi, m rYi
ð15Þ
where Di,m is the diffusion coefficient for species i in the mixture, F the density, and Bv the velocity vector. 2503
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Figure 3. Residence time distribution for the larger electrode without and with gaskets.
’ RESULTS The hydrodynamic behavior of the electrochemical reactor was evaluated. The exit edge (E) distribution curves obtained by CFD and the experimental data are shown in Figure 3. As seen, the model represents the experimental data adequately. The dispersion and the mean residence time in the reactor were obtained by applying a tracer pulse (during 1 s) at the reactor inlet, and then measuring its concentration at the reactor exit. The actual sonoelectrochemical reactor clearly does not have an ideal behavior. The dispersion number (Nd) obtained when the reactor was operated without gasket (LN) was 0.056 (as determined using computers), which means a large dispersion (Nd > 0.0121), and then the performance of the reactor will be affected. The three gaskets were placed into the reactor to separate the electrodes, as shown in Figure 1, and the dispersion again was evaluated. Figure 3 also shows the results of the residence time distribution curve for the reactor with gaskets (LG). In this case, the dispersion was reduced and an Nd value of 0.051 was obtained. As seen, more short-lived fluids are observed leaving the reactor when the reactor was operated without gaskets (LN) than in the reactor with gaskets (LG); more fractions of reactants then will be unconverted in the LN reactor. Moreover, the longer tail of the LN curve contributes to the larger dispersion without gaskets. Figure 4 shows the velocity axial vectors into the reactor with gaskets (LG) and without gaskets (LN). As shown, the LN reactor shows a higher zone of backwater near the inlet of the reactor than in the LG reactor. In the last case, the presence of the gasket avoids that several fluid streams return with negative velocity; hence the streams with negative axial velocity are reduced. Figure 5 shows a detail of these zones, where is seen that there is a larger zone with negative velocities in the reactor without gasket than LG. Figure 6 shows the comparison between axial velocities as a function of the position near the inlet for both configurations. The axial values from the inlet (position = 0) to a position 0.1 m into the reactor are superimposed. As seen, the reactor LN has a wider range of axial velocities and a larger zone with negative velocities than the LG reactor. Figure 7 shows the profiles along the reactor in the zone between the inlet and the outlet of the reactor (at positions 0.1-0.8 m into the reactor); the distribution of the axial velocity is more homogeneous in the LN reactor that in the LG reactor.
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As shown, in the reactor with gaskets (LG), the fluid moves with axial velocities between similar ranges as the LN reactor, except that there is a fast and very short change of the axial velocity, because of the central gasket (0.5 m); however, after this point, the fluid recovers their axial velocity values. In these zones, the fluids virtually do not have negative axial velocities in both cases. At the same time, Figure 4 shows that there is another zone with negative axial velocities near the outlet in the reactor without gaskets (LN), while in the reactor with gaskets (LG), this zone does not appear. Figure 8 shows the zones near the outlet reactor, where this effect and the velocity vectors for both cases are clearly shown. The LN reactor shows a zone with negative axial velocities. Figure 9 shows the axial velocity as a function of the position near the reactor outlet for both reactor configurations, where it is seen that, in the LN reactor, there is more variation in the axial velocities than in the LG reactor. The negative axial velocities reached in the LN reactor are located at a position between 0.925 m and 0.975 m, while in the case of the LG reactor, the zone of negative velocities is almost eliminated, which reduces the frontier effects. The percentage of negative axial velocity was evaluated in both cases: 3.80% for the reactor without gaskets (LN), while for the reactor with gaskets (LG) it was 2.92%. The axial velocity average without gaskets was lower (4676.27 10-7 m/s) than with gaskets (4698.76 10-7 m/s). Also, the average vorticity magnitude is lower with gaskets (0.1516543 s-1) than without gaskets (0.1590582 s-1). These effects caused greater dispersion in the reactor without gaskets (LN) was higher than in the reactor with gaskets (LG): the dispersion coefficients (D) evaluated were lower in the LG reactor than in the LN reactor (2.7453 10-5 m2/s vs 2.5165 10-5 m2/s). It is important to mention that, in both reactor configurations, the residence times obtained using RTD and those obtained with the average axial velocity evaluated by CFD are very similar, because the difference between them is e2.0%. This means that a good approximation of the residence time can be obtained simply with the evaluation of the average axial velocity by CFD without using the RTD curve. Based on the results, and because the reactor has a large dispersion, which has important influence on the sonoelectrochemical reactor performance, in the model (eqs 5-10), the reactor configuration with gaskets (LG), which has the smaller dispersion (Nd = 0.51), was used. Figure 10 shows the experimental data and the model obtained for the reactor with gaskets operated with and without ultrasonic radiation, at different influent pHs As shown, the Cr(VI) reduction process depends on the pH influent and the model predictions at the different pHs are in good agreement (a 95% confidence level) with experimental results, in both cases. The Cr(VI) reduction rate becomes slower as the pH increases. This effect is because the pH affects the solubility of the iron species that are generated during the process. At higher pH, the solubility of Fe3þ is reduced, then the Cr(VI) reduction rate is slower because the regeneration of ferrous ions (Fe3þ (aq) þ e w ) on the cathode also is diminished. When the pH in the Fe2þ (aq) bulk liquid increases at very high values (>7.0), the Fe2þ ions also become insoluble.8 Moreover, at higher pH, the ferric ions precipitate passivating the electrode and reducing the masstransfer rate between the electrodes and the bulk liquid. When ultrasonic irradiation was applied during the process, as shown in Figure 10b, the passivation effect is reduced. Figure 11 shows the comparison between the results obtained at the different influent pH values, for both cases. The Cr(VI) 2504
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Figure 4. Velocity vectors colored by axial velocity in the reactor with gaskets (LG) and without gaskets (LN).
Figure 5. Velocity vectors colored by axial velocity near the reactor inlet with gaskets (LG) and without gaskets (LN).
Figure 6. Axial velocity values near the reactor inlet with gaskets (LG) and without gaskets (LN).
reduction rate in the sonoelectrochemical reactor is higher than when the reactor was operated without ultrasonic radiation. This is explained because the ultrasonic radiation, at the frequencies (40 kHz) used in this work, produces “transient” cavitation bubbles that grow and collapse violently, producing the motion of solvent around the bubbles,10-14 near the electrodes. These effects make it possible that the insoluble oxide films of iron and chromium deposited on the electrodes surface25 can be removed. Moreover, it is possible that localized pH zones on
the electrode are reduced due to fluid motion that takes place at the electrode surface/solution, reducing the passivation and increasing the mass transfer26 between the electrodes and the solution, which also enhanced the reduction rate of Cr(VI) on the cathode,27 as shown in reaction 2. These effects caused the Cr(VI) reduction rate to increase, in comparison to the process without ultrasonic radiation. The percentage of reduction depends strongly on the influent pH. Based on the validated model, the treatment times to reduce the Cr(VI) concentration from 2505
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Figure 7. Axial velocity, as a function of the position reactor (from 0.1 m to 0.8 m).
Figure 8. Velocity vectors colored by axial velocity near the reactor outlet with gaskets (LG) and without gaskets (LN).
Figure 9. Axial velocity as a function of the position near the reactor outlet with gaskets (LG) and without gaskets (LN).
1000 mg/L to lower than the maximum of 0.5 mg/L (the limit permitted by the environmental regulations) were compared with those of the treatment without ultrasonic radiation. At the influent pHs tested (1.2, 1.5, and 1.7), the treatment time was
reduced; from 20 min to 13 min (a reduction of 35%), from 33 min to 22 min (a reduction of 33%), and from 65 min to 30 min (a reduction of 53.8%), without and with ultrasonic radiation, respectively. At an influent pH value of 2.0, it is not possible to 2506
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Figure 10. Model and experimental data of the variation of the Cr(VI) concentration, as a function of the time during the electrochemical process, (a) without and (b) with ultrasonic irradiation, at different influent pH values.
Figure 12. Variation of the pH during the sonoelectrochemical process at different initial pH values in to the reactor as a function of the dimensionless time (θ = t/th). Figure 11. Comparison between the electrochemical reactors without and with ultrasonic (US) irradiation, at different influent pHs.
reach a Cr(VI) concentration of 0.5 mg/L without ultrasonic irradiation. On the other hand, at the same influent pH, when ultrasound is applied, it is possible to reach that Cr(VI) concentration when the sonoelectrochemical reactor is operated with a residence time of 120 min, which is ∼9 times higher than when the process is operated with an influent pH (pHo) of 1.2 (13 min). In addition to operating the reactor at low influent pH values, it is important to initiate the process with the fluid within the reactor with low pH. The pH in the reactor when the process is initiated (the initial pH) plays an important role in the process. In Figure 12, the variation of the pH as a function of dimensionless time (θ) at different initial pH values for the four influent pH values is shown. As seen, because the initial pH is higher, then the pH reached at the first parts of the process also is higher, as the model predicts, and then the time to reach the steady state also is higher. The diagrams of predominance zones,7,8 for ferrous and ferric ions, show that both are insoluble at high pH; then, at higher initial
pHs, they can precipitate on the surface electrode, causing their passivation, which reduces the mass and electrical charges transfer, reducing the Cr(VI) rate reduction. Therefore, the initial pH affects the process performance as well as the influent pH. Based on the behavior of the pH, it is important to operate the reactor with ultrasonic radiation and low influent and initial pH (pH
