Copper electrodeposition in presence and absence of EDTA using reticulated vitreous carbon electrode

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1 Copper electrodeposition in presence and absence of EDTA using reticulated vitreous carbon electrode P. H. Britto a, L. A. M. Ruotolo a,b a Department of Chemical Engineering, Federal University of São Carlos, Rod. Washington Luiz km 235, São Carlos-SP, Brazil; Fax: ; Tel: ; britto_pedro@hotmail.com b Department of Chemical Engineering, Federal University of São Carlos, Rod. Washington Luiz km 235, São Carlos-SP, Brazil; Fax: ; Tel: ; pluis@ufscar.br Abstract Considering the expanding environmental awareness concerning industrial wastewater treatment, this work addresses the electrodeposition of copper ions in order to provide an efficient method to remove toxic metals from aqueous industrial effluents. It was studied the influence of the flow velocity and applied current on the copper electrodeposition in a reticulated vitreous carbon electrode (RVC) in presence and absence of EDTA as a complexing agent. The presence of EDTA leads to the decrease of the reaction kinetics and current efficiency compared to electrodeposition in its absence and the role of flow velocity on k m is very different considering the presence or absence of EDTA. While in the absence of EDTA the greater the flow velocity the greater k m, a surprising opposite effect was observed in the presence of EDTA, indicating that a phenomena other than mass transfer strongly affects the electrodeposition process. The study of the applied current revealed that the best current efficiency and energy consumption are obtained using high current density when EDTA is present in solution. Keywords effluent treatment; electrochemical reactor; electrodeposition 1. Introduction Industrial effluents containing toxic metals are very harmful to aquatic environment and human beings; moreover, they accumulate in the food chain. These effluents are, in most cases, treated by chemical precipitation, which only transfers the problem from the liquid to the solid phase, since a toxic sludge is generated, consequently requiring the correct disposal in very costly special landfills. Effluents containing metal ions are commonly found in the washing process of the metal finishing industry and, in many cases they can be complexed with an organic molecule. The metal concentrations in these processes are generally lower than 1. g L -1, which is considered very low for electrodeposition, hence making the kinetics of this process mass transfer controlled and, consequently, the use of flat electrodes unsuitable to be applied. In order to overcome this problem, the use of three-dimensional electrodes was proposed. The reticulated vitreous carbon (RVC) electrode has been considered as promising material, since it provides high porosity coupled with a high specific surface area, resulting in high mass transfer rates 1. At least four main advantages of the electrochemical technology over the conventional one can be pointed out: 1) metal can be removed in its pure solid form, i.e., avoiding the need for sludge transport and storage in landfills; 2) the metal can be sold or reused in the process; 3) electricity is considered a clean reagent and in many cases no by-products are generated, and 4) process automation is easy to be implemented since the operational variables (current and flow rate) can be easily controlled 2. This work addresses the copper electrodeposition since it is generated in many industrial processes, such as metal finishing, hydrometallurgy and in the printed circuit board industry. Page 1

2 The disposal of effluents containing copper, even in very low concentrations, is considered a drawback due to its toxicity; thus, the permitted copper concentration discharge in the sewage system, according to Brazilian regulatory laws, is 1.5 mg L -1. Accordingly, considering all the aspects aforementioned and the expanding environmental awareness concerning industrial wastewater treatment, including the use of our natural resources, this work addresses the electrodeposition of copper ions in order to provide an efficient method to remove toxic metals from aqueous industrial effluents. As in many processes the metal ions are present in solution in its complexed form, in this work it was studied the influence of the flow velocity and applied current on the copper electrodeposition on RVC electrodes in presence and absence of the complexing compound EDTA. Since the electrochemical process of aqueous effluents containing metal ions is mass-transfer controlled, the presence of EDTA can significantly modify the diffusion coefficient of the copper ions and, consequently, the mass transfer coefficient and the electrodeposition kinetics would be affected. 2. Experimental The experiments were carried out in the system schematically shown in Figure 1. Its main components are: 1) electrolyte reservoir; 2) centrifugal pump; 3) flow meter; 4) diaphragm valve; 5) voltmeter; 6) electrochemical reactor; 7) current source (Minipa 33D), and 8) thermostatic bath. Fig. 1 Schematic representation of the experimental system A more detailed view of the electrochemical reactor is shown in Figure 2. The acrylic pieces are assembled in a filter press configuration in order to facilitate its operation and simulated a commercial design reactor. Leakage was avoided using silicon rubber between the pieces. The porous cathode was a 45 ppi 4. cm x 7. cm x 1.27 cm RVC. Galvanostatic experiments were carried for different currents and flow rates. Electrolyte samples were collected throughout the process and the copper concentrations determined by atomic absorption spectrophotometry (Varian, SpectAA 2). The cell potential was also measured in order to calculate the energy consumption. Page 2

3 Fig. 2 Schematic side view of the electrochemical reactor. 1) current feeder (stainless steel); 2) RVC; 3) separator (polyethylene mesh covered by a polyamide fabric); and 4) counterelectrode (Ti/Ti.7 Ru.3 O 2 ). The arrows indicate electrolyte inlet and outlet Considering that current efficiency and energy consumption are the most important quantitative parameters to evaluate an electrochemical process 3, they were calculated using Equations (1) and (2), respectively. ICE is given in percentage and IEC in kwh kg -1. z F V dc ICE = M I dt I ΔU IEC = V dt ( dc ) (1) (2) In Equations (1) and (2) ICE and IEC are the instantaneous current efficiency and instantaneous energy consumption, respectively; z is the number of electrons of the electrochemical reaction, F the Faraday constant, V the electrolyte volume, M the molecular weight, I the current, and ΔU the cell potential. The global current efficiency (GCE) and global energy consumption (GEC) for the different experimental conditions were calculated using Equations (3) and (4), respectively. In these equations, t is the time necessary to reach a desired percentage of copper removal. GCE = t (ICE)dt t dt (3) GEC = t (IEC)dt t dt (4) Page 3

4 3. Results and discussion 3.1. Electrodeposition of Cu 2+ and Cu II /EDTA Figure 3 shows the results for copper electrodeposition in presence and absence of EDTA. The exponential pattern of these curves indicates that the reaction is mass transfer controlled, consequently, a limiting current and pseudo-first-order kinetics can be assumed in order to determine the mass transfer coefficient (k m ) 3. It can be observed that the presence of a complexing agent in the electrolyte makes the reaction kinetics slow, thus the time necessary to remove all copper from solution increased approximately 4 minutes. Considering the copper electrodeposition in absence of EDTA, the greater the flow velocity the faster the reaction kinetics, e.g., increasing the flow velocity from.55 to.11 m s -1, k m increases from 2.71 x 1-5 to 5,25 x 1-5 m s -1. Applying a flow velocity of.219 m s -1 (not shown), k m reaches the value of 7. x 1-5 m s -1 and for a further increase of flow velocity the k m enhancement is not expressive and would not justify the pumping cost. 1, removed mass (m/m ),8,6,4,2.55 m s m s m s m s -1, electrolysis time / s Fig. 3 Normalized mass of copper in solution against electrolysis time. Black Cu 2+ ; Red: Cu II /EDTA (1:1 molar). C = 163 mg L -1, Na 2 SO 4.5 M, ph 4, I = 1.5 A In presence of EDTA, the increase of flow velocity from.55 m s -1 to.11 m s -1 practically does not have influence on the electrodeposition kinetics (average k m = 2.31 x 1-5 m s -1 ). However, when the flow velocity is increased to.273 m s -1 (Figure 4) there is an unexpected drop of the electrodeposition rate (k m = 1.92 x 1-5 m s -1 ). Figure 4 shows an opposite effect of flow velocity on the reaction rate compared to that observed in the electrodeposition in absence of EDTA, i.e., the greater the flow velocity the lower the kinetics, indicating that other phenomena other than mass transfer controls the kinetics, such as adsorption or chemical reactions. Hence, the presence of additives and complexing agents like EDTA can substantially modify the copper electrodeposition in diluted solutions. Page 4

5 1. II/ II v =.55 m s -1 v =.19 m s -1 v =.164 m s -1 v =.219 m s -1 v =.273 m s time / min Fig. 4 Normalized Cu II concentration against time. C = 2 mg L -1, Na 2 SO 4.5 M, ph 4, I = 1.5 A. Copper:EDTA 1:1 (molar) Determination of the limiting current The limiting current (I L ) was calculated using Equation (5): I =.62 z F D ν c ω lim 1 (5) where z is the number of electrons in the electrochemical reaction, F the Faraday constant (C mol -1 ), M the molecular weight (g mol -1 ), A the electrode area (m 2 ), and C the mass concentration (g m -3 ) 3. The values of k m used in Equation 5 were those obtained using a flow velocity of.219 m s -1. Hence, the values of k m = 7. x 1-5 m s -1 and k m = 2.1 x 1-5 m s -1 were used to calculate the limiting currents: 4.3 A (C = 2 mg L -1 ) and 1.2 A (C = 184 mg L -1 ) for Cu 2+ and Cu II, respectively. Different ratios of the applied current and limiting current (α = I/I L ) were used in the galvanostatic electrodeposition of copper in presence and absence of EDTA Copper electrodeposition in absence of EDTA Figure 5 shows the kinetics of copper electrodeposition using values of α between.11 and.45. It can be observed in this figure the presence of two different patterns: exponential and linear, which correspond to mass transfer control and charge transfer control, respectively. The greater the α the faster the reaction kinetics; however, according to Figure 5, values greater than.45 probably would not increase significantly the reaction rate, leading to prohibitive values of current efficiency. Using the slopes of the concentration-time curves shown in Figure 5 and Equations (1) and (2), ICE and IEC against concentration were calculated and plotted in Figure 6 (a) and (b), respectively. It can be observed in Figure 6(a) that the current efficiency remains constant (CE ctc ), i.e., charge transfer controlled, until a concentration value in which the process starts to be controlled by mass transfer. This concentration was called transition concentration, C *. It can be noticed in Figure 6(a) that, on contrary that was observed by Ruotolo and Gubulin (22) 4 for copper electrodeposition at ph 2. using H 2 SO 4, 1% current efficiency does not occur at ph 4. and Na 2 SO 4 as support electrolyte. Page 5

6 2 2+ / mg L α =.11 α =.22 α =.34 α = time / min Fig. 5 Copper concentration against time. Na 2 SO 4.5 M, ph 4, v =.219 m s ICE / % C * Cu 2+ α =.11 α =.22 α =.34 α =.45 IEC / kwh kg α =.11 α =.22 α =.34 α =.45 (a) / mg L -1 (b) / mg L -1 Fig. 6 ICE (a) and IEC (b) against Cu 2+ concentration C = 2 mg L -1, Na 2 SO 4.5 M, ph 4, v =.219 m s -1 It would be expected that for currents lower than the limiting value, CE ctc would not be significantly affected by α. However, the results shown in Figure 6(a) suggest that when working with porous electrodes, the irregular potential and current distribution along the electric field plays an important role in the electrochemical process 5,6. Indeed, at the end of the electrodeposition experiment, it can be observed a more intense copper deposit on the RVC close to the counter-electrode. It was supposed that increasing α, despite the regions of the RVC close to the current feeder become more electrochemically active, this is not enough to overcome the bad effect of the very cathodic overpotentials emerging in the region close to the counter-electrode which favors the hydrogen evolution reaction (HER), thus being responsible for the CE ctc drop observed for the highest values of α. It is also important to observe that the C * depends on the applied α; hence the total time at which the process is carried out under CE ctc condition decreases and, consequently, GCE and GEC will be affect. Page 6

7 According to Figure 6(b), the greater α the greater the energy consumption, mainly due to the current efficiency loss observed for concentrations lower than C *. The small IEC increase verified at concentrations greater than C * is mainly due the cell potential increment in observed in this region. Using Equations (3) and (4) and the experimental data of ICE and IEC against time (not shown), GCE and GEC were calculated, respectively. In order to facilitate the comparison, the time used to integrate the ICE and IEC-time curves corresponded to that necessary to remove 95% of copper (t 95% ). Figure 7 shows GCE, GEC, and t 95% as a function of α. As expected, the greater α the lower GEC and t 95% ; thus considering only GCE, the choice of α would only depend on the desired operation time in order to perform a desired effluent treatment and minimize the capital cost. However, as can be seen in Figure 7(b), there is a value of α (.25) that optimizes the operational time and energy consumption, thus the best operational condition is obtained applying 1.7 A, with an energy consumption of 6. kwh kg -1, and 52 minutes would be necessary to remove 95% of copper. GCE / % (a) ,1,2,3,4,5 α GCE t 95% t 95% / min GEC / kwh kg -1 (b) 12 GEC t 95% 3,1,2,3,4,5 Fig. 7 GCE and t 95% against α (a); GEC and t 95% against α (b) for copper electrodeposition in absence of EDTA. C = 2 mg L -1, Na 2 SO 4.5 mol L -1, ph 4, v =.219 m s -1 α t 95% / min 3.4. Copper electrodeposition in presence of EDTA Following the same procedure previously described, experiments of copper electrodeposition in presence of EDTA were performed varying the value of α. Figure 8 shows that presence of EDTA changes completely the electrodeposition kinetics. For all values of applied α, the kinetics is slower than those observed in absence of EDTA, especially for α =.4. In this case, the kinetics is very slow and more than 3 hours is necessary to remove all copper from solution. It is also possible to verify two distinct kinetic patterns in the α =.4 curve, which can be better observed in the Figure 9(a) for the ICE. Initially, the kinetics is relatively fast, but after 1 hour there is a sharp drop in the reaction kinetics, revealing that other phenomena other than charge or mass transfer is occurring, such as a probably electrode passivation by adsorbed EDTA. The reaction kinetics can be greatly improved increasing the value of α, i.e., applying high values of current density. This enhancement of the reaction rate could have been obtained due to EDTA degradation in the anode at very high anodic potential (high anodic current densities), which would release Cu 2+ ions in solution, facilitating its electrodeposition 7. Indeed, chemical oxygen analysis (COD) carried out before and after the electrodeposition experiments showed a COD reduction of 8% and 31% for α =.6 and α =.8, respectively. Page 7

8 II / mg L α =.4 α =.6 α = time / h Fig. 8 Copper concentration against time. C = 184 mg L -1 ; Cu II /EDTA 1:1 (molar), Na 2 SO 4.5 mol L -1, ph 4, v =.219 m s -1 Figure 9 (a) and (b) shows the ICE and IEC as a function of copper concentration. The values of CE ctc (between 12% and 2%) for the electrodeposition in presence of EDTA were very low and much lower than those observed in absence of EDTA. These values of ICE suggests that the electrodeposition potential for complexed copper ions is dislocated to more negative values, thus the competition with RDE is more intense and the CE drops. It is also noteworthy that, on contrary of the observed in absence of EDTA, the greater the α the shorter is the region controlled by mass transfer, i.e., C * increases when the value of α decreases. Regarding the energy consumption, their values become prohibitive for values of α lower than.8. ICE / % α =.4 α =.6 α =.8 IEC / kwh kg α =.4 α =.6 α =.8 (a) (b) II / mg L -1 II / mg L -1 Fig. 9 ICE (a) and IEC (b) against Cu II concentration. C = 184 mg L -1, Na 2 SO 4.5 M, Cu II /EDTA 1:1 (molar), ph 4, v =.219 m s -1 The values of GCE and GEC, as well as the time necessary to remove 9% of Cu II (t 9% ), are show in Figure 1. The behavior of these curves are opposite to those shown in Figure 7, suggesting that values of α greater than.8 should be applied in order to maximize the GCE and minimize GEC. Page 8

9 GCE / % GCE t 9% t 9% / h GEC / kwh kg GEC t 9% t 9% / h (a) 4,4,5,6,7,8 α (b) 2,4,5,6,7,8 α Fig. 1 GCE and t 9% against α (a); GEC and t 9% against α (b) for copper electrodeposition in presence of EDTA. C = 184 mg L -1, Na 2 SO 4.5 M, Cu II /EDTA 1:1 (molar), ph 4, v =.219 m s Conclusions The presence of EDTA in the electrolyte decreases the electrodeposition rate and CE; consequently, the EC increases. The slow electrodeposition kinetics observed in presence of EDTA when the flow rate is increased suggests that reaction kinetics is not exclusively controlled by mass transfer. Finally, the results of GCE and GEC showed that EC can be optimized using α =.25 for Cu 2+ electrodeposition. Values of α greater than.8 should be applied in order to minimize the EC for Cu II electrodeposition. Acknowledgements The authors wish to thank the financial support provided by CNPq and FAPESP. References 1. D. Pletcher, I. Whyte, F. C. Walsh, J. P. Millington, J. Appl. Electrochem. 21 (1991) D. Pletcher, F. C. Walsh, Industrial Electrochemistry, 2nd ed., Chapman and Hall, London, F. Goodridge, K. Scott, Electrochemical Process Engineering, Plenum Press, New York, L. A. M. Ruotolo.; J. C. Gubulin, Braz. J. Chem. Eng. 19 (22) T. Doherty, J. G. Sunderland, E. P. L. Roberts, D. J. Pickett, Electrochim. Acta 41 (1996) M. R. V. Lanza, R. Bertazzoli, J. Appl. Electrochem. 3 (2) K. Katsuki, H. Nishida, S. Morooka, Y. Kato, J. Appl. Electrochem. 16 (1986) Page 9

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