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Electrochemical treatment of heavy metals (Cu2+, Cr6+, Ni2+) from industrial effluent and modeling of copper reduction
ARTICLE IN PRESS
Water Research 39 (2005) 610–616 www.elsevier.com/locate/watres
Electrochemical treatment of heavy metals (Cu2+, Cr6+, Ni2+) from industrial effluent and modeling of copper reduction M. Hunsoma,, K. Pruksathorna, S. Damronglerda, H. Vergnesb, P. Duverneuilb a Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Laboratoire de Genie Chimique-UMR 5503 Ecole Nationale Superieure des Ingenieurs en Arts Chimiques et Technologiques-INPT, France
b
Received 9 January 2004; received in revised form 4 October 2004; accepted 18 October 2004
Abstract An electrochemical technique was tested in a laboratory scale to treat heavy metals (Cu2+, Cr6+ and Ni2+) from plating industrial effluent. The experiments were performed in a membrane reactor having a capacity of 1 l. Stainlesssteel sheets with surface area of 0.011 m2 and titanium coated with ruthenium oxide were used as cathode and anode, respectively. The electrolyte was circulated at a constant flow rate (0.42 l/min) and the pH was kept constant at 1. Applied current densities were 10 and 90 A/m2. According to the experiment, it was found that a membrane reactor with plane electrode was capable for treating plating wastewater with low energy consumption (42.30 kWh/kg metal) and low operating cost (5.43 US$/m3). More than 99% of metal reduction was achieved and the final concentrations of copper, chromium and nickel in treated water were 0.10–0.13, 0.19–0.20 and 0.05–0.13 ppm, respectively. The brightener had no effect on copper reduction whereas hexavalent chromium had strong effect. The kinetic of copper reduction in the presence of hexavalent chromium was modeled as a two-step process with respect to copper concentration. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hexavalent chromium; Copper; Plating wastewater; Brightener
1. Introduction Plating wastewater contains various kinds of toxic substances such as acid cyanide, alkaline cleaning agent, degreasing solvents, oil and fat and heavy metals. Most of the heavy metals such as copper, nickel, chromium, zinc, etc. in wastewater are harmful when they are discharged directly to the environment. Hence, treatment of them before discharging becomes necessary.
Corresponding author. Tel.: +66 2 2187523 5; fax: +66 2 2555831. E-mail address: [email protected] (M. Hunsom).
Many previous works have attempted to find reasonable ways to remove heavy metals from wastewater. One of the most effective ways is electrochemical process. Copper ions were removed with high efficiency from a dilute industrial effluent in an electrochemical reactor with plate electrode (Solisio et al., 1999). A combination of electrochemical and biological processes was carried out to recover copper, nickel and chromium from sludge of publicly owned treatment works and industrial effluents (Pruksathorn et al., 1997, 1999). They demonstrated that around 86–91% of metals was recovered and the sludge could be further used in agriculture. A three-dimensional electrode such as carbon granule was used to recover zinc, copper and nickel on the
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.10.011
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
611
Nomenclature Eo F I j k0 k1 ni n Rt t
standard potential of metal (V) Faraday constant current (A) current density (A/m2) zero-order rate constant first-order rate constant initial mole of copper (mol) number of electron involved in the reaction (equivalent/mol) cupper reduction percentage at time t (%) electrolysis time (min)
electrochemical reactor known as ‘‘3PE reactor’’ (pulsed porous percolated electrodes) (Aguirre et al., 1994). It was found that the recovery percentage of all metals was approximately 93%, 99% and 82%, respectively; the treated effluents contained metal concentrations less than 0.5 g/l. Bertazzoli et al. (1997) used the same electrode to remove metals consisting of copper, lead and zinc from wastewater. They found that the best rate of metal removal was obtained at high cathode porosity and high electrolyte flow rate. The concentration of metal contained in the solution was reduced from 50 to 0.1 mg/l during the circulation time ranging from 20 to 40 min. The optimum pH of the selective electrodeposition of copper, lead, cadmium and zinc in membrane reactor was found to be at 1.5 with more than 99% purity (Doulakas et al., 2000). The deposited layer of copper, lead and cadmium showed a dendritric morphology, whereas compact morphology was observed in a zinc-deposited layer. In this work, experiments were carried out in an anionic membrane reactor to treat heavy metals (Cu2+, Cr6+, Ni2+) from plating industry. The effects of brightener and other metals such as nickel or chromium in wastewater on copper recovery were investigated. A kinetic model of copper reduction in the presence of chromium was also developed.
t* Wt Wi [ ]i [ ]t
transition time (min) weight of metal at time t (g) initial weight of metal (g) initial concentration of active species (mol/ m3) concentration of active species at time t (mol/m3)
Greek letters current efficient at time t (%)
Zt
membrane (IONAC type) having a total surface area of 7.74 103 m2. The volume of both anode and cathode compartments were equal to 0.5 l. The electrodes were located at the center of each compartment and the distance between them was fixed around 0.05 m. To achieve good mass transfer in the system, two magnetic pumps (Model NH-5PX type) were used to circulate the electrolyte in both compartments at a constant flow rate (0.42 l/min). Stainless-steel grid with surface area of 0.011 m2 and titanium grid coated with ruthenium oxide (Ti/RuO2) connected with the regulated DC power supply (ZS 3205-2X type) were used as cathode and anode, respectively. For each experiment, two optimum current density values were applied. The first was 10 A/m2 to remove copper in metallic state (Hunsom et al., 2001). After copper was completely removed, the current was consequently adjusted to 90 A/m2 to remove chromium and nickel in solution as metal hydroxide complex (Hunsom, 2001). Precisely, the amount of metallic ions in the solution was sampled and analyzed periodically by atomic absorption spectrophotometer (GBC AA Ver 1.1 type).
Table 1 Characteristics of industrial effluent
2. Materials and methods
Characteristics 2+
Wastewater containing copper, chromium and nickel ions coming from plating industrial plant in Thailand was used in this study and their characteristics are reported in Table 1. Fig. 1 shows the experimental setup used in this study, which was conducted at ambient temperature. The electrolytic cell was constructed with Plexiglas having a dimension of 0.10 0.10 0.11 m3 (1 l capacity). The cell was separated into two compartments, anodic and cathodic compartments, by anionic
Cu Cr6+ Ni2+ pH Conductivity CN Other metals Color Other anions (SO2 4 , HSO4 , 2 NO3 , CO3 , CN , BF4 )
Value 14.67 (mol/m3) 18.54 (mol/m3) 7.97 (mol/m3) 1 19–75 mS/cm No No Yellowish orange
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
V
A
2
− 3
Cu Ni Cr pH
80
4
8
8
7
7
5
metal reduction (%)
+
14
100
1
12 10
Anode
pH
60
8
Cathode
pH
612
40
6
j = 90 A /m2
j = 10 A /m2
4 20 2
9 6
9
0
0 0
6
Fig. 1. Schematic view of electrolysis cell with plane electrode: 1, DC power supply; 2, anode; 3, anionic membrane; 4, cathode; 5, reactor; 6, pump; 7, valve; 8, inlet stream; 9, outlet stream.
3. Results and discussion Fig. 2 shows the plot of metal reduction and the solution pH as a function of electrolysis time. As expected, the deposition of copper increased slowly during the first 6 h, then it increased dramatically. More than 99% of copper was reduced within 12 h whereas the percentage of chromium and nickel reduction had changed only slightly. After 12 h, the current density was changed to 90 A/m2. During this period, both chromium and nickel were reacted with hydroxide ions and precipitated rapidly. The precipitation of their ions started at 13.5 h corresponding to the increasing solution pH. More than 99.9% of chromium and nickel was reduced within 15 h. The remaining concentrations of copper, chromium and nickel in outlet solution were lower than 0.01–0.13, 0.19–0.20 and 0.05–0.13 ppm, respectively. In all cases, metal concentration was lower than the acceptable value limited by the Thai Government (Cuo1 ppm, Nio0.2 ppm, Cro0.5 ppm). The power consumption and total operating cost were approximately 42.30 kWh/kg metal and 5.43 US$/m3, respectively. Table 2 shows the comparison of operating costs of this work with other three previous works (Kongsricharoern and Polprasert, 1995; Chawalkitcharoen et al., 1993; Visvanathan et al., 1993). The results demonstrate that all treatment processes have almost the same operating cost. However, our operation should be easier than the others. In addition, our work succeeds to recover many metals (copper, chromium and nickel), whereas only one metal was removed by three other works.
2
4
8 10 time (hr)
12
14
16
Fig. 2. Metal reduction and pH evolution versus electrolysis time. [Cu]i ¼ 14.67 mol Cu/m3, [Ni]i ¼ 7.97 mol Ni/m3, [Cr]i ¼ 18.54 mol Cr/m3, pHi ¼ 1.
Table 2 Comparison of operating costs of this work and conventional methods Treatment process
Operating cost (US$/m3)
This worka Electrochemical precipitation processb Precipitation processc Ion exchange processd
5.43 5.22 5.71 5.29
a
Metal concentration ¼ 2364 mg/l (932 mg Cu/l, 964 mg Cr/l, and 468 mg Cr/l). b Metal concentration ¼ 3860 mg Cr/l. c Metal concentration ¼ 500 mg Cr/l. d Metal concentration ¼ 406.4 mg Cr/l.
the effective parameters for its reduction were also investigated. 3.1.1. Effect of brightener Effects of brighteners such as Elecopper 25A, Elecopper 25MU and Elecopper 25B (Okuno Chemical Industries Co., Ltd.) (Table 3) on copper reduction were investigated with synthetic copper solution. In each experiment, 0.15 vol% of each brightener was added at a current density of 10 A/m2 and a copper concentration of 15.74 mol Cu/m3 at pH ¼ 1. No significant difference in terms of the percentage of copper reduction were observed between the experiments with and without brightener (Fig. 3). More than 99% of copper was reduced within 6 h with more than 60% total current efficiency.
3.1. Effect of some parameters on copper reduction Because copper is a valuable metal and it can be removed as a metallic form as it is deposited on cathode,
3.1.2. Effect of Cr and Ni ions in solution The further experiment was conducted to observe the effect of other metals (Cr6+, Ni2+) on copper reduction
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616 Table 3 Composition of brighteners used in this study
100
Content
Weight (%)
Elecopper 25A
Di-methyl safranine T type dry Thioflavine De-ionized water
10.0 0.5 89.5
Polyethylene glycol 6000 8.0 Sodium 3-mercaptopropylsulfonate1.2 Copper sulfate 0.5 De-ionized water 90.3
Elecopper 25MUPolyethylene glycol 6000 Copper sulfate Thioflavine De-ionized water
80 copper reduction (%)
Type
Elecopper 25B
613
60
40 CuRec (NoCu brightener) % (pure) CuRec (Elecopper 25A)r % Cu (Elecoppe
20
4.0 1.0 0.5 94.5
CuRec (Elecopper 25MU) % Cu (Elecoppe r CuRec (Elecopper 25B)r % Cu (Elecoppe
0 0
at 10 A/m2 current density, pH ¼ 1. The experiments were done with three types of solution: copper solution (7.84 mol Cu/m3), mixture of copper and chromium solution (7.84 mol Cu/m3, 10.06 mol Cr/m3), and mixture of copper and nickel solution (7.84 mol Cu/m3, 8.26 mol Ni/m3). The solution having nickel ions gave results similar to those of the pure copper solution (Fig. 4). More than 99% of copper was removed within 4 h electrolysis time. In contrast, in a mixture of copper and chromium solution, the evolution of copper reduction percentage versus time was different from that of the pure copper solution, namely, the percentage of copper reduction was approximately 53% within 4 h and around 96% within 7 h electrolysis time. The reason was that chromium ions in effluent were found in hexavalent form (Cr6+) which reacts faster than copper ions due to its high standard potential (Eo41.1 V/NHE) compared with that of copper (Eo ¼ 0.3402 V/NHE). Unlike chromium, nickel had no effect on copper reduction because of its low standard potential (Eo ¼ 0.23 V/NHE). At pH between 1 and 6, chromium in solution was present in the forms of Cr2O2 7 and HCrO (Roberts et al., 2001). The possible 4 mechanism of hexavalent chromium reduction can be expressed by Eqs. (1) and (2). During the experiment, it can be actually observed that color of the solution is consequently changed from hexavalent chromium color (yellowish orange) to the trivalent chromium color (violet blue). þ 3þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ! 2Cr ðE o ¼ 1:33 V=NHEÞ;
(1)
þ 3þ þ 4H2 O HCrO 4 þ 7H þ 3e ! Cr o ðE ¼ 1:35 V=NHEÞ:
(2)
2
4 time (hr)
6
8
Fig. 3. Percentage of copper reduction versus time in the presence of brightener. [Cu]i ¼ 15.74 mol Cu/m3, 0.15 vol% brightener, pHi ¼ 1, j ¼ 10 A/m2.
Besides the above reactions, another kind of reaction known as side reaction can occur in cathode compartment. Generally, one of them is hydrogen evolution as shown by Eq. (3). This side reaction leads to loss of cathodic current efficiency. 2Hþ þ 2e ! H2 :
(3)
3.1.3. Effect of hexavalent chromium concentration Experiments were continuously performed at current density of 10 A/m2 and at constant electrolyte flow rate to investigate the effect of hexavalent chromium concentration on copper reduction. Different concentrations of hexavalent chromium were added in the copper solution at pH around 1. Fig. 5 displays the plot of copper reduction percentage in cathodic compartment versus electrolysis time in the presence of different hexavalent chromium concentrations. When no hexavalent chromium was added, more than 99% of copper was reduced within 6 h with current efficiency more than 90% at 80% reduction. However, in the presence of hexavalent chromium, the percentage of copper reduction was revealed to have an inverse relationship to the hexavalent chromium concentration. A higher concentration of hexavalent chromium led to lower percentage of copper reduction, lower current efficiency (25–95%) and longer electrolysis time (Fig. 6). Doubling the chromium concentration led to decrease in the current efficiency of the copper reduction to approximately 11%. Furthermore, electrolysis time was directly proportional to initial hexavalent chromium concentration. Increasing the initial hexavalent chromium concentration two-fold increased the electrolysis time about 20%.
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
80
80
60
current efficiency (%)
100
copper reduction (%)
100
Cu (pure) Cu (with Cr)
40
Cu (with Ni) Cr
600 500 400
60 300 40 80%
20
99%
0
0 2
4 time (hr)
100
90%
0
0
200
70%
Ni
20
6
3
100
6
9
0 12
chromium concentration (mol /m3)
8
Fig. 4. Percentage of copper reduction versus time in the presence of nickel and chromium. [Cu]i ¼ 7.84 mol Cu/m3, [Ni]i ¼ 8.26 mol Ni/m3, [Cr]i ¼ 10.06 mol Cr/m3, pHi ¼ 1, j ¼ 10 A/m2.
time (min)
614
Fig. 6. Evaluation of current density and time versus initial concentration of hexavalent chromium (data from Fig. 5).
At tpt*, the kinetic of copper reduction was the zeroorder reaction. The concentration profile of copper is expressed by ½Cu2þ t ¼ ½Cu2þ i k0 t:
(4)
*
At t4t , the kinetic of copper reduction was the firstorder reaction. The concentration profile of copper is expressed by
copper reduction (%)
80
½Cu2þ t ¼ ½Cu2þ i expfk1 ðt t Þg;
60
where k0 and k1 are the rate constants of the zero-order and the first-order reactions, respectively. In the absence of hexavalent chromium, we found that k0 ¼ 0.0608 mol/m3 min, k1 ¼ 0.0229 min1, and * t ¼ 122.4 min. However, in the presence of hexavalent chromium, the rate constant is a function of initial concentration of hexavalent chromium, the rate constant is
Cu (pure) 3
40
Cu (with 0.98 mol Cr/m ) Cu (with 2.02 mol Cr/m3) Cu (with 3.62 mol Cr/m3)
20
Cu (with 7.79 mol Cr/m3) Cu (with 12.17 mol Cr/m3)
0 0
2
4
6 time (hr)
8
10
(5)
12
Fig. 5. Percentage of copper reduction versus time in the presence hexavalent chromium. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
k0 ¼ 0:0608 0:0045½Cr6þ i þ 0:00015½Cr6þ 2i mol=m3 min;
ð6Þ
k1 ¼ 0:0229 0:0024½Cr6þ i þ 0:00023½Cr6þ 2i min1 ; (7)
3.2. Kinetic investigation
and t ¼ 24:52½Cr6þ i þ 122:4:
Regarding the kinetic of copper reduction in the presence of hexavalent chromium, we found that the kinetic of copper reduction occurs as a two-step process. On the one hand, when the solution contains high hexavalent chromium concentration, the kinetic was the zero-order with respect to copper concentration. On the other hand, at low hexavalent chromium concentration, the kinetic was the first-order with respect to copper concentration and the kinetic transferred from the zeroorder to the first-order at the transition time (t*).
(8)
The percentage of copper reduction (%R) and current efficiency (Zt) of this system can be calculated as a function of electrolysis time by Eqs. (9) and (10), respectively. Wi Wt %Rt ¼ 100; (9) Wi Zt ¼
nF ni ð%RÞt : It
(10)
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616 16
model (no Cr) model(0.98 (0.98mole/m3) mol/m3) model
14 copper concentration (mole/m3)
615
model (2.02 mole/m3) 12
model (3.62 mole/m3)
10
model (7.79 mole/m3) model (12.17 mole/m3)
8
experiment (no Cr) experiment (0.98 mg/l)
6
experiment (2.02 mole/m3)
4
experiment (3.62 mole/m3) experiment (7.79 mole/m3)
2
experiment (12.17 mole/m3)
0 0
120
240
360 480 time (min)
600
720
Fig. 7. Evaluation of copper concentration resulting from experiment and model. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
100
model (no Cr) model(0.98 (0.98mole/m3) mol/m3) model model (2.02 mole/m3)
copper reduction (%)
80
model (3.62 mole/m3) model (7.79 mole/m3)
60
model (12.17 mole/m3) experiment (no Cr) 40
experiment (0.98 mg/l) experiment (2.02 mole/m3) experiment (3.62 mole/m3)
20
experiment (7.79 mole/m3) experiment (12.17 mole/m3)
0 0
100
200
300 400 time (min)
500
600
700
Fig. 8. Evaluation of copper reduction resulting from experiment and model. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
The validity of both rate constants (k0, k1) is depending upon the hexavalent chromium concentration ranging from 2.02 to 12.17 mol Cr/m3. This understanding is valid to the behavior of metal elimination observed in this work because all experiments were performed in batch-type and under constant current condition, and so the concentration of cupric ions was continuously decreasing in the course of treatment. Therefore, the copper ions were eliminated on the cathode surface at the rate below the diffusion limiting current density in the initial stage of treatment, during which the reaction proceeds in the zero-order. However, the concentration of copper ions fell below the certain value where the limiting current was below the operating current, and thereafter the copper elimination showed the first order. Figs. 7, 8 and 9 show the plots of
the evaluation of the copper concentration, that of the copper reduction, and the current efficiency, respectively, for a comparison of the developed model and the experimental results. The best relationship among them was obtained with the standard deviation around 0.3–2.9. The novelty of this work is the development of the model equations to predict the concentration profile, reduction percentage and current efficiency of copper reduction system in the presence of hexavalent chromium.
4. Conclusion According to experimental results, we found that the classical membrane reactor with plane electrode was
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
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References
100 model (80 %) model (90 %)
current efficiency (%)
80
Experiment (80 %) Experiment (90 %)
60
40
20
0 0
3
6
9
12
chromium concentration (mole/m3)
Fig. 9. Evaluation of current density resulting from experiment and model. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
effective to treat wastewater from plating industry with energy consumption about 42.30 kWh/kg metal. The outlet copper, chromium and nickel concentration in treated water were 0.10–0.13, 0.19–0.20 and 0.05–0.13 ppm, respectively, which were lower than the value limited by law in Thailand. The presence of brightener, including Elecopper 25A, Elecopper 25MU and Elecopper 25B, does not have significant effect on copper reduction kinetics. On the other hand, the presence of hexavalent chromium in the solution would retard the copper reduction reaction because of its high standard potential. A higher concentration of hexavalent chromium led to a greater effect on copper reduction. The kinetic of copper reduction in the presence of hexavalent chromium was a two-step process. The kinetic was the zero-order with respect to copper concentration when the solution contained high hexavalent chromium concentration and it was the first-order with respect to copper concentration at low hexavalent chromium concentration. The equations for the copper concentration, the percentage of copper reduction, and the current efficiency, as calculated by the developed model, were in good agreement with the experimental results.
Acknowledgments The authors would like to thank The Royal Golden Jubilee Program of The Thailand Research Fund and Embassy of France in Thailand for the financial support to our project.
Aguirre, P., Gaballah, I., Fenouillet, B., Ivanaj, S., Lacoste, G., Solozabal, R., 1994. Recovery of zinc, copper, and nickel from industrial effluents generated by the electroplating units using the ‘‘3PE’’ technology. In: Misra M. (Ed.), Separation Processes: Heavy metals, Irons and Minerals. The minerals, Metal & Materials Society, pp. 257–268. Bertazzoli, R., Rosivania, C.W., Lanza, R.V., Di Iglia, R.A., Sousa, M.F.B., 1997. Electrolytic removal of metals using a flow-through cell with a reticulated vitreous carbon cathode. J. Braz. Chem. Soc. 8, 487–492. Chawalkitcharoen, P., Limpaseni, W., Boonyakitsombut, S., 1993. In-house vs. Central Waste Treatment Plant: A Case Study Seminar on Pollution Management Strategies for the Electroplating Industry, CDG-SEAPO, Bangkok, Thailand. Doulakas, L., Novy, K., Stucki, S., Comninellis, C., 2000. Recovery of Cu, Pb, Cd and Zn from synthetic mixture selective electrodeposition in chloride solution. Electrochem. Acta 46 (2–3), 349–356. Hunsom, M., 2001. Recovery of copper, chromium, and nickel from electroplating effluent by electrochemcial technique. A Dissertation of Doctor of Philosophy in Chemical Technology, Department of chemical technology, Faculty of Science, chulalongkorn university, Bangkok, Thailand, Chapter 5. Hunsom, M., Vergnes, H., Duverneuil, P., Pruksathorn, K., Damronglerd, S., 2001. Recovery of copper from synthetic solution by electrochemical technique. Proceedings of the Sixth World Congress of Chemical Engineering, Melbourne, Australia. Kongsricharoern, N., Polprasert, C., 1995. Electrochemical precipitation of chromium (Cr6+) from an electroplating wastewater. Water Sci. Technol. 31 (9), 109–117. Pruksathorn, K., Vergnes, H., Cantet, J., Duverneuil, P., 1997. Removal of metals in sludge of publicly owned treatment works and industrial effluents. Franco-Thai Symposium New Advances in Water and Wastewater Treatments, Chulalongkorn University, Bangkok, Thailand, pp. 281–295. Pruksathorn, K., Vergnes, H., Cantet, J., Duverneuil, P., 1999. Removal of metals in sludge of wastewater treatment. AIDIC Conf. Ser. 4, 147–152. Roberts, E.P.L., Hao Yu, E., Eager, S., 2001. Electrochemical treatment of effluents containing heavy metals using a carbon felts porous electrode. Proceedings of the Sixth World Congress of Chemical Engineering, Melbourne, Australia. Solisio, C., Panizza, M., Paganelli, P., Cerisola, G., 1999. Electrochemical remediation of copper (II) from an industrial effluent part I: monopolar plate electrodes. Resour. Conserv. Recycling 26 (2), 115–124. Visvanathan, C., Ha, N.T.L., 1993. Application of clean technology in small scale electroplating plant. A Case Study. Seminar on Pollution Management Strategies for the Electroplating Industry, CDG-SEAPO, Bangkok, Thailand.
Water Research 39 (2005) 610–616 www.elsevier.com/locate/watres
Electrochemical treatment of heavy metals (Cu2+, Cr6+, Ni2+) from industrial effluent and modeling of copper reduction M. Hunsoma,, K. Pruksathorna, S. Damronglerda, H. Vergnesb, P. Duverneuilb a Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Laboratoire de Genie Chimique-UMR 5503 Ecole Nationale Superieure des Ingenieurs en Arts Chimiques et Technologiques-INPT, France
b
Received 9 January 2004; received in revised form 4 October 2004; accepted 18 October 2004
Abstract An electrochemical technique was tested in a laboratory scale to treat heavy metals (Cu2+, Cr6+ and Ni2+) from plating industrial effluent. The experiments were performed in a membrane reactor having a capacity of 1 l. Stainlesssteel sheets with surface area of 0.011 m2 and titanium coated with ruthenium oxide were used as cathode and anode, respectively. The electrolyte was circulated at a constant flow rate (0.42 l/min) and the pH was kept constant at 1. Applied current densities were 10 and 90 A/m2. According to the experiment, it was found that a membrane reactor with plane electrode was capable for treating plating wastewater with low energy consumption (42.30 kWh/kg metal) and low operating cost (5.43 US$/m3). More than 99% of metal reduction was achieved and the final concentrations of copper, chromium and nickel in treated water were 0.10–0.13, 0.19–0.20 and 0.05–0.13 ppm, respectively. The brightener had no effect on copper reduction whereas hexavalent chromium had strong effect. The kinetic of copper reduction in the presence of hexavalent chromium was modeled as a two-step process with respect to copper concentration. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hexavalent chromium; Copper; Plating wastewater; Brightener
1. Introduction Plating wastewater contains various kinds of toxic substances such as acid cyanide, alkaline cleaning agent, degreasing solvents, oil and fat and heavy metals. Most of the heavy metals such as copper, nickel, chromium, zinc, etc. in wastewater are harmful when they are discharged directly to the environment. Hence, treatment of them before discharging becomes necessary.
Corresponding author. Tel.: +66 2 2187523 5; fax: +66 2 2555831. E-mail address: [email protected] (M. Hunsom).
Many previous works have attempted to find reasonable ways to remove heavy metals from wastewater. One of the most effective ways is electrochemical process. Copper ions were removed with high efficiency from a dilute industrial effluent in an electrochemical reactor with plate electrode (Solisio et al., 1999). A combination of electrochemical and biological processes was carried out to recover copper, nickel and chromium from sludge of publicly owned treatment works and industrial effluents (Pruksathorn et al., 1997, 1999). They demonstrated that around 86–91% of metals was recovered and the sludge could be further used in agriculture. A three-dimensional electrode such as carbon granule was used to recover zinc, copper and nickel on the
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.10.011
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
611
Nomenclature Eo F I j k0 k1 ni n Rt t
standard potential of metal (V) Faraday constant current (A) current density (A/m2) zero-order rate constant first-order rate constant initial mole of copper (mol) number of electron involved in the reaction (equivalent/mol) cupper reduction percentage at time t (%) electrolysis time (min)
electrochemical reactor known as ‘‘3PE reactor’’ (pulsed porous percolated electrodes) (Aguirre et al., 1994). It was found that the recovery percentage of all metals was approximately 93%, 99% and 82%, respectively; the treated effluents contained metal concentrations less than 0.5 g/l. Bertazzoli et al. (1997) used the same electrode to remove metals consisting of copper, lead and zinc from wastewater. They found that the best rate of metal removal was obtained at high cathode porosity and high electrolyte flow rate. The concentration of metal contained in the solution was reduced from 50 to 0.1 mg/l during the circulation time ranging from 20 to 40 min. The optimum pH of the selective electrodeposition of copper, lead, cadmium and zinc in membrane reactor was found to be at 1.5 with more than 99% purity (Doulakas et al., 2000). The deposited layer of copper, lead and cadmium showed a dendritric morphology, whereas compact morphology was observed in a zinc-deposited layer. In this work, experiments were carried out in an anionic membrane reactor to treat heavy metals (Cu2+, Cr6+, Ni2+) from plating industry. The effects of brightener and other metals such as nickel or chromium in wastewater on copper recovery were investigated. A kinetic model of copper reduction in the presence of chromium was also developed.
t* Wt Wi [ ]i [ ]t
transition time (min) weight of metal at time t (g) initial weight of metal (g) initial concentration of active species (mol/ m3) concentration of active species at time t (mol/m3)
Greek letters current efficient at time t (%)
Zt
membrane (IONAC type) having a total surface area of 7.74 103 m2. The volume of both anode and cathode compartments were equal to 0.5 l. The electrodes were located at the center of each compartment and the distance between them was fixed around 0.05 m. To achieve good mass transfer in the system, two magnetic pumps (Model NH-5PX type) were used to circulate the electrolyte in both compartments at a constant flow rate (0.42 l/min). Stainless-steel grid with surface area of 0.011 m2 and titanium grid coated with ruthenium oxide (Ti/RuO2) connected with the regulated DC power supply (ZS 3205-2X type) were used as cathode and anode, respectively. For each experiment, two optimum current density values were applied. The first was 10 A/m2 to remove copper in metallic state (Hunsom et al., 2001). After copper was completely removed, the current was consequently adjusted to 90 A/m2 to remove chromium and nickel in solution as metal hydroxide complex (Hunsom, 2001). Precisely, the amount of metallic ions in the solution was sampled and analyzed periodically by atomic absorption spectrophotometer (GBC AA Ver 1.1 type).
Table 1 Characteristics of industrial effluent
2. Materials and methods
Characteristics 2+
Wastewater containing copper, chromium and nickel ions coming from plating industrial plant in Thailand was used in this study and their characteristics are reported in Table 1. Fig. 1 shows the experimental setup used in this study, which was conducted at ambient temperature. The electrolytic cell was constructed with Plexiglas having a dimension of 0.10 0.10 0.11 m3 (1 l capacity). The cell was separated into two compartments, anodic and cathodic compartments, by anionic
Cu Cr6+ Ni2+ pH Conductivity CN Other metals Color Other anions (SO2 4 , HSO4 , 2 NO3 , CO3 , CN , BF4 )
Value 14.67 (mol/m3) 18.54 (mol/m3) 7.97 (mol/m3) 1 19–75 mS/cm No No Yellowish orange
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
V
A
2
− 3
Cu Ni Cr pH
80
4
8
8
7
7
5
metal reduction (%)
+
14
100
1
12 10
Anode
pH
60
8
Cathode
pH
612
40
6
j = 90 A /m2
j = 10 A /m2
4 20 2
9 6
9
0
0 0
6
Fig. 1. Schematic view of electrolysis cell with plane electrode: 1, DC power supply; 2, anode; 3, anionic membrane; 4, cathode; 5, reactor; 6, pump; 7, valve; 8, inlet stream; 9, outlet stream.
3. Results and discussion Fig. 2 shows the plot of metal reduction and the solution pH as a function of electrolysis time. As expected, the deposition of copper increased slowly during the first 6 h, then it increased dramatically. More than 99% of copper was reduced within 12 h whereas the percentage of chromium and nickel reduction had changed only slightly. After 12 h, the current density was changed to 90 A/m2. During this period, both chromium and nickel were reacted with hydroxide ions and precipitated rapidly. The precipitation of their ions started at 13.5 h corresponding to the increasing solution pH. More than 99.9% of chromium and nickel was reduced within 15 h. The remaining concentrations of copper, chromium and nickel in outlet solution were lower than 0.01–0.13, 0.19–0.20 and 0.05–0.13 ppm, respectively. In all cases, metal concentration was lower than the acceptable value limited by the Thai Government (Cuo1 ppm, Nio0.2 ppm, Cro0.5 ppm). The power consumption and total operating cost were approximately 42.30 kWh/kg metal and 5.43 US$/m3, respectively. Table 2 shows the comparison of operating costs of this work with other three previous works (Kongsricharoern and Polprasert, 1995; Chawalkitcharoen et al., 1993; Visvanathan et al., 1993). The results demonstrate that all treatment processes have almost the same operating cost. However, our operation should be easier than the others. In addition, our work succeeds to recover many metals (copper, chromium and nickel), whereas only one metal was removed by three other works.
2
4
8 10 time (hr)
12
14
16
Fig. 2. Metal reduction and pH evolution versus electrolysis time. [Cu]i ¼ 14.67 mol Cu/m3, [Ni]i ¼ 7.97 mol Ni/m3, [Cr]i ¼ 18.54 mol Cr/m3, pHi ¼ 1.
Table 2 Comparison of operating costs of this work and conventional methods Treatment process
Operating cost (US$/m3)
This worka Electrochemical precipitation processb Precipitation processc Ion exchange processd
5.43 5.22 5.71 5.29
a
Metal concentration ¼ 2364 mg/l (932 mg Cu/l, 964 mg Cr/l, and 468 mg Cr/l). b Metal concentration ¼ 3860 mg Cr/l. c Metal concentration ¼ 500 mg Cr/l. d Metal concentration ¼ 406.4 mg Cr/l.
the effective parameters for its reduction were also investigated. 3.1.1. Effect of brightener Effects of brighteners such as Elecopper 25A, Elecopper 25MU and Elecopper 25B (Okuno Chemical Industries Co., Ltd.) (Table 3) on copper reduction were investigated with synthetic copper solution. In each experiment, 0.15 vol% of each brightener was added at a current density of 10 A/m2 and a copper concentration of 15.74 mol Cu/m3 at pH ¼ 1. No significant difference in terms of the percentage of copper reduction were observed between the experiments with and without brightener (Fig. 3). More than 99% of copper was reduced within 6 h with more than 60% total current efficiency.
3.1. Effect of some parameters on copper reduction Because copper is a valuable metal and it can be removed as a metallic form as it is deposited on cathode,
3.1.2. Effect of Cr and Ni ions in solution The further experiment was conducted to observe the effect of other metals (Cr6+, Ni2+) on copper reduction
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616 Table 3 Composition of brighteners used in this study
100
Content
Weight (%)
Elecopper 25A
Di-methyl safranine T type dry Thioflavine De-ionized water
10.0 0.5 89.5
Polyethylene glycol 6000 8.0 Sodium 3-mercaptopropylsulfonate1.2 Copper sulfate 0.5 De-ionized water 90.3
Elecopper 25MUPolyethylene glycol 6000 Copper sulfate Thioflavine De-ionized water
80 copper reduction (%)
Type
Elecopper 25B
613
60
40 CuRec (NoCu brightener) % (pure) CuRec (Elecopper 25A)r % Cu (Elecoppe
20
4.0 1.0 0.5 94.5
CuRec (Elecopper 25MU) % Cu (Elecoppe r CuRec (Elecopper 25B)r % Cu (Elecoppe
0 0
at 10 A/m2 current density, pH ¼ 1. The experiments were done with three types of solution: copper solution (7.84 mol Cu/m3), mixture of copper and chromium solution (7.84 mol Cu/m3, 10.06 mol Cr/m3), and mixture of copper and nickel solution (7.84 mol Cu/m3, 8.26 mol Ni/m3). The solution having nickel ions gave results similar to those of the pure copper solution (Fig. 4). More than 99% of copper was removed within 4 h electrolysis time. In contrast, in a mixture of copper and chromium solution, the evolution of copper reduction percentage versus time was different from that of the pure copper solution, namely, the percentage of copper reduction was approximately 53% within 4 h and around 96% within 7 h electrolysis time. The reason was that chromium ions in effluent were found in hexavalent form (Cr6+) which reacts faster than copper ions due to its high standard potential (Eo41.1 V/NHE) compared with that of copper (Eo ¼ 0.3402 V/NHE). Unlike chromium, nickel had no effect on copper reduction because of its low standard potential (Eo ¼ 0.23 V/NHE). At pH between 1 and 6, chromium in solution was present in the forms of Cr2O2 7 and HCrO (Roberts et al., 2001). The possible 4 mechanism of hexavalent chromium reduction can be expressed by Eqs. (1) and (2). During the experiment, it can be actually observed that color of the solution is consequently changed from hexavalent chromium color (yellowish orange) to the trivalent chromium color (violet blue). þ 3þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ! 2Cr ðE o ¼ 1:33 V=NHEÞ;
(1)
þ 3þ þ 4H2 O HCrO 4 þ 7H þ 3e ! Cr o ðE ¼ 1:35 V=NHEÞ:
(2)
2
4 time (hr)
6
8
Fig. 3. Percentage of copper reduction versus time in the presence of brightener. [Cu]i ¼ 15.74 mol Cu/m3, 0.15 vol% brightener, pHi ¼ 1, j ¼ 10 A/m2.
Besides the above reactions, another kind of reaction known as side reaction can occur in cathode compartment. Generally, one of them is hydrogen evolution as shown by Eq. (3). This side reaction leads to loss of cathodic current efficiency. 2Hþ þ 2e ! H2 :
(3)
3.1.3. Effect of hexavalent chromium concentration Experiments were continuously performed at current density of 10 A/m2 and at constant electrolyte flow rate to investigate the effect of hexavalent chromium concentration on copper reduction. Different concentrations of hexavalent chromium were added in the copper solution at pH around 1. Fig. 5 displays the plot of copper reduction percentage in cathodic compartment versus electrolysis time in the presence of different hexavalent chromium concentrations. When no hexavalent chromium was added, more than 99% of copper was reduced within 6 h with current efficiency more than 90% at 80% reduction. However, in the presence of hexavalent chromium, the percentage of copper reduction was revealed to have an inverse relationship to the hexavalent chromium concentration. A higher concentration of hexavalent chromium led to lower percentage of copper reduction, lower current efficiency (25–95%) and longer electrolysis time (Fig. 6). Doubling the chromium concentration led to decrease in the current efficiency of the copper reduction to approximately 11%. Furthermore, electrolysis time was directly proportional to initial hexavalent chromium concentration. Increasing the initial hexavalent chromium concentration two-fold increased the electrolysis time about 20%.
ARTICLE IN PRESS M. Hunsom et al. / Water Research 39 (2005) 610–616
80
80
60
current efficiency (%)
100
copper reduction (%)
100
Cu (pure) Cu (with Cr)
40
Cu (with Ni) Cr
600 500 400
60 300 40 80%
20
99%
0
0 2
4 time (hr)
100
90%
0
0
200
70%
Ni
20
6
3
100
6
9
0 12
chromium concentration (mol /m3)
8
Fig. 4. Percentage of copper reduction versus time in the presence of nickel and chromium. [Cu]i ¼ 7.84 mol Cu/m3, [Ni]i ¼ 8.26 mol Ni/m3, [Cr]i ¼ 10.06 mol Cr/m3, pHi ¼ 1, j ¼ 10 A/m2.
time (min)
614
Fig. 6. Evaluation of current density and time versus initial concentration of hexavalent chromium (data from Fig. 5).
At tpt*, the kinetic of copper reduction was the zeroorder reaction. The concentration profile of copper is expressed by ½Cu2þ t ¼ ½Cu2þ i k0 t:
(4)
*
At t4t , the kinetic of copper reduction was the firstorder reaction. The concentration profile of copper is expressed by
copper reduction (%)
80
½Cu2þ t ¼ ½Cu2þ i expfk1 ðt t Þg;
60
where k0 and k1 are the rate constants of the zero-order and the first-order reactions, respectively. In the absence of hexavalent chromium, we found that k0 ¼ 0.0608 mol/m3 min, k1 ¼ 0.0229 min1, and * t ¼ 122.4 min. However, in the presence of hexavalent chromium, the rate constant is a function of initial concentration of hexavalent chromium, the rate constant is
Cu (pure) 3
40
Cu (with 0.98 mol Cr/m ) Cu (with 2.02 mol Cr/m3) Cu (with 3.62 mol Cr/m3)
20
Cu (with 7.79 mol Cr/m3) Cu (with 12.17 mol Cr/m3)
0 0
2
4
6 time (hr)
8
10
(5)
12
Fig. 5. Percentage of copper reduction versus time in the presence hexavalent chromium. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
k0 ¼ 0:0608 0:0045½Cr6þ i þ 0:00015½Cr6þ 2i mol=m3 min;
ð6Þ
k1 ¼ 0:0229 0:0024½Cr6þ i þ 0:00023½Cr6þ 2i min1 ; (7)
3.2. Kinetic investigation
and t ¼ 24:52½Cr6þ i þ 122:4:
Regarding the kinetic of copper reduction in the presence of hexavalent chromium, we found that the kinetic of copper reduction occurs as a two-step process. On the one hand, when the solution contains high hexavalent chromium concentration, the kinetic was the zero-order with respect to copper concentration. On the other hand, at low hexavalent chromium concentration, the kinetic was the first-order with respect to copper concentration and the kinetic transferred from the zeroorder to the first-order at the transition time (t*).
(8)
The percentage of copper reduction (%R) and current efficiency (Zt) of this system can be calculated as a function of electrolysis time by Eqs. (9) and (10), respectively. Wi Wt %Rt ¼ 100; (9) Wi Zt ¼
nF ni ð%RÞt : It
(10)
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model (no Cr) model(0.98 (0.98mole/m3) mol/m3) model
14 copper concentration (mole/m3)
615
model (2.02 mole/m3) 12
model (3.62 mole/m3)
10
model (7.79 mole/m3) model (12.17 mole/m3)
8
experiment (no Cr) experiment (0.98 mg/l)
6
experiment (2.02 mole/m3)
4
experiment (3.62 mole/m3) experiment (7.79 mole/m3)
2
experiment (12.17 mole/m3)
0 0
120
240
360 480 time (min)
600
720
Fig. 7. Evaluation of copper concentration resulting from experiment and model. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
100
model (no Cr) model(0.98 (0.98mole/m3) mol/m3) model model (2.02 mole/m3)
copper reduction (%)
80
model (3.62 mole/m3) model (7.79 mole/m3)
60
model (12.17 mole/m3) experiment (no Cr) 40
experiment (0.98 mg/l) experiment (2.02 mole/m3) experiment (3.62 mole/m3)
20
experiment (7.79 mole/m3) experiment (12.17 mole/m3)
0 0
100
200
300 400 time (min)
500
600
700
Fig. 8. Evaluation of copper reduction resulting from experiment and model. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
The validity of both rate constants (k0, k1) is depending upon the hexavalent chromium concentration ranging from 2.02 to 12.17 mol Cr/m3. This understanding is valid to the behavior of metal elimination observed in this work because all experiments were performed in batch-type and under constant current condition, and so the concentration of cupric ions was continuously decreasing in the course of treatment. Therefore, the copper ions were eliminated on the cathode surface at the rate below the diffusion limiting current density in the initial stage of treatment, during which the reaction proceeds in the zero-order. However, the concentration of copper ions fell below the certain value where the limiting current was below the operating current, and thereafter the copper elimination showed the first order. Figs. 7, 8 and 9 show the plots of
the evaluation of the copper concentration, that of the copper reduction, and the current efficiency, respectively, for a comparison of the developed model and the experimental results. The best relationship among them was obtained with the standard deviation around 0.3–2.9. The novelty of this work is the development of the model equations to predict the concentration profile, reduction percentage and current efficiency of copper reduction system in the presence of hexavalent chromium.
4. Conclusion According to experimental results, we found that the classical membrane reactor with plane electrode was
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References
100 model (80 %) model (90 %)
current efficiency (%)
80
Experiment (80 %) Experiment (90 %)
60
40
20
0 0
3
6
9
12
chromium concentration (mole/m3)
Fig. 9. Evaluation of current density resulting from experiment and model. [Cu]i ¼ 15.47 mol Cu/m3, pHi ¼ 1, j ¼ 10 A/m2.
effective to treat wastewater from plating industry with energy consumption about 42.30 kWh/kg metal. The outlet copper, chromium and nickel concentration in treated water were 0.10–0.13, 0.19–0.20 and 0.05–0.13 ppm, respectively, which were lower than the value limited by law in Thailand. The presence of brightener, including Elecopper 25A, Elecopper 25MU and Elecopper 25B, does not have significant effect on copper reduction kinetics. On the other hand, the presence of hexavalent chromium in the solution would retard the copper reduction reaction because of its high standard potential. A higher concentration of hexavalent chromium led to a greater effect on copper reduction. The kinetic of copper reduction in the presence of hexavalent chromium was a two-step process. The kinetic was the zero-order with respect to copper concentration when the solution contained high hexavalent chromium concentration and it was the first-order with respect to copper concentration at low hexavalent chromium concentration. The equations for the copper concentration, the percentage of copper reduction, and the current efficiency, as calculated by the developed model, were in good agreement with the experimental results.
Acknowledgments The authors would like to thank The Royal Golden Jubilee Program of The Thailand Research Fund and Embassy of France in Thailand for the financial support to our project.
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