Removal of Copper Ions from Electroplating Wastewater by Ion-exchange Membranes

Transcription

1 Removal of Copper Ions from Electroplating Wastewater by Ion-exchange Membranes SIMONA CAPRARESCU 1*, DANUT-IONEL VAIREANU 1, ANCA COJOCARU 1, IOANA MAIOR 1, ANDREI SARBU 2 1 Politehnica University of Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Applied Physical Chemistry and Electrochemistry, 132 Calea Grivitei, , Bucharest, Romania 2 INCDCP-ICECHIM, Bucharest, Department of Polymers, 202 Spl. Independentei,060021, Bucharest, Romania The electrodialysis technique for the treatment of a synthetic wastewater containing copper was studied using three-compartment electrodialysis cell using different type of ion-exchange membranes. In this paper conductivity, ph, gravimetric evaluation and percent extraction was determined for different values of the applied voltage. The best results were observed for A500, respective PPC100. Keywords: copper ions, electrodialysis, ion-exchange membranes, wastewater The release of industrial wastewater containing heavy metals to the environment is nowadays strictly controlled due to the toxic nature of these substances. In order to mitigate the environmental impact it is necessary to remove such heavy metals from wastewater before discharging the effluent industrial streams in the environment [1-14]. Electrodialysis started to be considered in various applications in the early 1900 s as an improved version of the dialysis process by the addition of 2 inert electrodes and the application of a direct current to increase the rate of dialysis process by speeding up the ions movement [2, 6, 7] This technique has been constantly improved over the years and since the 1940 s, and has developed into a newly independent technique that is considered as being different from classical dialysis in many ways, although it is still considered as a part of the membrane separation processes [6, 7]. Electrodialysis has been recently used more and more in sewage work processing to remove pollutants from water, especially where there are highly toxic low concentration contaminants, and it is preferred over other methods (e.g. reverse osmosis, freezing and flash distillation) on economic grounds. Heavy metal electrolytes have gained particularly attention for electrodialysis applications as they present a series of necessary particular properties (e.g. ionic conduction, small ionic volume, easy detection) in order to make this technique economic feasible [2, 3, 9]. Electrodialysis, as a separation technique, consists of series of ion-exchange/ion-selective membranes positioned between two electrodes where a d.c. potential gradient is imposed, as the driving force [6-9]. In an electrodialysis stack the cation-exchange and anion-exchange membranes are alternately arranged in a filter press-like system between two chemically and electrochemically inactive electrodes, positioned at the end of the stack, so that a diluted flow and concentrated flow are the results of the separation processes taking place. Another important membrane characteristic, besides the selectivity and ion-exchange capacity is also that of the membrane ionic conductivity, a high ionic conductivity being able to decrease the cell voltage drop and increase the energy efficiency [15]. The applied direct current field determines the transport of cations towards the cathode and anions towards the anode [2, 8, 11, 16]. During this transport process, anions are able to permeate through the anion selective membrane, but are blocked by the cation selective membrane. A similar process takes place with cations and the anion-exchange membrane [1]. The electrodialysis system is a very flexible one and may be operated in batch, semi-bath, or in a continuous mode [2]. In this paper one describes an electrodialysis cell stack consisting of three compartment cell, each compartment being separated from each other by ion-exchange membranes. The electrodialysis performance was evaluated in term of percent extraction [1, 7, 9], defined as: where C 0 is the initial ion concentration and C f is the ion concentration at the end of the experiment. Experimental part The electrodialysis was performed in a three compartments cell (fig. 1 and 2). The experiments were conducted using different types of ion-exchange membranes:anion-exchange membranes Purolite A400 and Purolite A500 and cation-exchange membranes Purolite C104, Purolite PPC100. The characteristics of these membranes are given in table 1 [10, 17]. Fig. 1. Schematic diagram of electrodialysis cell with three compartments (1) * scaprarescu@yahoo.com ; Tel.: (+40)

2 Fig. 2. Experimental electrodialysis cell Table 1 CHARACTERISTICS OF ION-EXCHANGE MEMBRANES USED IN ELECTRODIALYSIS The anion-exchange membrane was placed next to the anode and the cation-exchange membrane was placed next to the cathode. The leaks are prevented by the use of acid resistant O-rings placed on each side of the compartment body. The electrodes were made of lead 99.9%, in order to allow an easy construction and integration inside the cell. The metal preferred was lead chemically and electrochemically inactive with respect to the solution ions and electrode processes taking place (the anode does not release lead cations in solutions) [8]. The effective area of each membrane and electrode was of cm2. The distance between anode-cathode electrodes was 52 mm. Synthetic wastewater solutions, reassembling the wastewater from copper electroplating waste streams were prepared by dissolving copper sulphate (CuSO4. 5H2O) and sulphuric acid in deionised water to obtain concentrations between 1000 ppm and 4000 ppm of copper ions. The ratio between the cooper sulphate and sulphuric acid was 1:1. This ratio is the most defavourable case that one may encounter in real life applications. The heavy metal contaminated solution is pumped into the electrodialysis unit and the cell may be operated as batch/stagnant or continuously, at a controlled flow rate, depending on the chosen operating conditions. Under the influence of the applied electrical potential across the cell electrodes, the cations from solution migrate toward the cathode, and the anions toward the anode. For the above given membranes, when a cation encounters the cationexchange membrane, the cation is exchanged for H + ; similarly, anions migrate toward the anode and the process is taking place at an anion-exchange membrane, the sulphate anion being now exchanged for Cl - [1, 2, 7 and 19]. The effects of applied potential, and initial concentration on the percent extraction were studied in the above presented electrodialysis cell. One has also recorded the variations in ph and ionic conductivity in the cell compartments. The conductivity of the solutions was measured by a conductivity meter WA-100 ATC (Voltcraft, Germany) that was calibrated using KCl 0.02 n solution. The conductivity meter contains a temperature sensor that allows the automatic compensation with respect to the temperature variation [8]. The ph of the solutions was measured by a ph meter HI 8915 (HANNA Instruments, Germany). The ph meter contains also a temperature sensor that allows the automatic compensation with respect to the temperature variation. Results and discussions In these experiments the actual working volume which was subjected to the electrodialysis process at room temperature (25 C), for 3 h, under potentiostatic conditions (a constant anode-cathode applied voltage between 5 and 674

3 Fig. 3. Variation of cell current in time for different concentrated solutions and pairs of membranes at 5 V applied voltage Fig. 4. Variation of cell current in time for different concentrated solutions and types of membranes at 9 V applied voltage 9V) was of ml/compartment. The variation of current in time for the applied voltage (the polarization curve) was recorded for various initial concentrations of copper ions [8, 9, and 18] and various levels of applied voltage (fig. 3 and 4). Figures 3 and 4 show that the current initially increases with increasing time because the stack resistance gradually decreases due to ion transfer processes. The curves present a maximum value when the stack resistance was minimum (highest conductivity resulting from the addition of individual ionic species conductivity) and then decreases with increasing time (due to a decrease in the overall conductivity) reaching a steady state value after 120 min. As far as the initial concentration is concerned, the current increases with increasing concentration being explained by the increased ionic conductivity with increasing concentration; the increased current with increasing applied voltage is due to the ohmic behaviour of solution for increased applied voltage, the highest current being obtained for an applied voltage of 9 V. The wastewater conductivity of solutions from anodic and cathodic compartment was also measured. In table 2 and table 3 one may see that the values of conductivities for various solutions increase in the anodic compartment. This is due to the fact that following the oxygen evolution, the remaining excess protons will increase the overall proton concentrations increasing the value of conductivity in this compartment. The apparently abnormal decrease of conductivity in the cathodic compartment is a direct consequence of the electrodeposition reaction of copper on the cathode, decreasing the copper ions conductivity contribution, and consequently, the value of the respective conductivity. Similar results are also reported in [8, 9]. This is confirmed by the metallic copper layer deposited onto the cathode that can be visually noticed at the end of the experiment when the cell is disassembled and also by gravimetric estimation evaluation (table 4). The experimental results indicate that in the case of Purolite PPC100 one has obtained better results for the removal of copper ions comparing to that of Purolite C104 for all the applied electrical voltage values, as well as for both concentration levels studied (table 4), indicating that Purolite PPC100 has also an elevated copper loading than Purolite C104 - a membrane containing carboxylic functional groups; this is due to the fact that in Purolite PPC100 the sulphonic functional groups are able to provide a high degree of proton ionisation comparing to carboxylic groups contained in Purolite C104. Table 2 CONDUCTIVITY VALUES OF DIFFERENT SOLUTIONS AND MEMBRANES AT 5V APPLIED VOLTAGE 675

4 Table 3 CONDUCTIVITY VALUES OF THE DIFFERENT SOLUTIONS AND MEMBRANES AT 9V APPLIED VOLTAGE Table 4 GRAVIMETRIC EVALUATION FOR DIFFERENT SOLUTIONS ON DIFFERENT MEMBRANES AND VALUES OF POTENTIAL Table 5 ph VALUES AT VARIOUS CONCENTRATION SOLUTIONS AND MEMBRANES AT 5V APPLIED VOLTAGE Electrical voltage values, as well as for both concentration levels studied (table 4), indicating that Purolite PPC100 has also an elevated copper loading than Purolite C104 - a membrane containing carboxylic functional groups; this is due to the fact that in Purolite PPC100 the sulphonic functional groups are able to provide a high degree of proton ionisation comparing to carboxylic groups contained in Purolite C104. The lower capacity anion-exchange for sulphate anions of membrane Purolite A400 with respect to A500 is due to the fact that the first one is in fact a gel based and the diffusion process is slower in this case comparing to the macroporous membrane A500. The selectivity of A400 and A500 for sulphate anions and the rejection forces for copper cations are due to the presence of the quaternary ammonium functional groups that determines existence of the repulsive forces between A400 and A500 and copper cations [8]. Table 5 and table 6 present the ph values for the different concentration of solutions and type of membranes considered at different applied voltages measured after 3 h. The ph values decreased in the anodic compartment 676

5 Table 6 ph VALUES AT VARIOUS CONCENTRATION SOLUTIONS AND MEMBRANES AT 9V APPLIED VOLTAGE Fig. 5. Percent. extraction of copper ions after 3,5 h of treatment for different concentrated solutions at 5 V applied cell voltage due to the formation of H + as a result of oxygen evolution. The increase of ph in the cathodic compartment may be explained by a partial discharge besides copper ions of protons induced by an elevated overpotential caused by the increased applied voltage (5 and 9 V). The results are consistent to similar findings reported in [8, 12, 20]. The value of the percent extraction parameter was calculated using relation (1) in accordance with the established procedure [1, 7-9] in order to evaluate the efficiency of the electrodialysis process. Figures 5 and 6 show that the percent extraction increases with increasing initial concentration solutions and applied cell voltage. The best performance combination is obtained in the case of PPC100 and A500 pair. Conclusions The results presented here have shown that the Purolite PPC100 presents a high capacity for removal of copper ions from wastewater in comparison with Purolite C104 under the given conditions because of a limited ionization of sulphonic functional groups with respect to carboxylic ones and an increase water uptake. The value of the electrical current increases initially with increase concentration and applied voltage; the percent extraction of copper ions increases with increasing initial concentration of copper ions and increasing applied voltage reaching values above 80% for both membrane pair combinations, when a voltage above 5 V is applied. References 1. MARDER, L., SULZBACH, G. O., BRENARDES A. M., FERREIRA, J. Z., J. Braz. Chem. Soc., 14, no. 4, 2003, p VAIREANU, D. I., Electrochemistry and corrosion, AGIR Publishing House, Bucuresti, 2006 p. 102 Fig. 6. Percent. extraction of copper ions after 3,5 h of treatment for different concentrated solutions at 9 V applied cell voltage 3. CHOI, K. H., YOUNG, T., Korean J. Chem. Eng., 19, no. 1, 2002, p MIHALY COZMUTA, A., MIHALY COZMUTA, L., VARGA, C., Rev. Chim., 58, nr.12, 2007, p SIMONESCU, C. M., DELEANU, C., CAPATI, C., Rev. Chim. (Bucuresti), 58, nr. 11, 2007, p SANTAROSA, V. E., PERETTI, F., CALDART, V., ZAPPAS, J., ZENI, M., Desalination, 149, 2002, p MOHAMMADI, T., SADRZADEH, M., MOHEB, A., RAZMI, A., Ninth International Water Technology Conference, Egypt, IWTC9 2005, p CAPRARESCU, S., VAIREANU, D.-I., COJOCARU, A., MAIOR, I., PURCAR, V., SIRBU, A., U. P. B. Sci. Bull., in press 9. DALLA COSTA, R. F., KLEIN, C. W., BERNARDES, A. M., FERREIRA, J. Z., J. Braz. Chem. Soc., 13, no. 4, 2002, p JANCEVIÈIÛTË, R., GEFENIENË, A., J. Environ. Eng. Landsc. Manag, XIV, no. 4, ISSN , 2006, p GÜVENÇ, A., KARABACAKOÐLU, B., Desalination, 172, 2005, p KHAN, J., TRIPATHI, B. P., SAXENA, A., SHAHI, V. K., Electrochem. Acta, 52, 2007, p ABO-GHANDER, N. S., RAHMAN, S. U., ZAIDI, S. M. J., Portugaliae Electrochim. Acta, 24, 2006, p SATA, T., SATA, T., YANG, W., J. Membr. Sci., 206, 2002, p VAIREANU, D-I., MAIOR, I., GRIGORE, A., SAVOIU, D., Rev. Chim., 59, nr. 10, 2008, p VAIREANU, D. I., Metode electrochimice aplicate în protecþia mediului, Ed. Printech, Bucuresti, 2007, p KOTER, S., WARSZAWSKI, A., Polish J. of Environmental Studies, 9, no. 1, 2000, p NATARAJ, S. K., HOSAMANI, K. M., AMINABHAVI, T. M., Desalination, 217, 2007, p VAIREANU, D. I., Depoluarea electrochimicã a apelor reziduale, Ed. Printech, Bucuresti, 2000, p CHEN, X.-F., WU, Z.-C., J. of Zhejiang University Science, 6B(6), 2005, p. 543 Manuscript received: