ARTICLE pubs.acs.org/IECR
An Effective Electrochemical Cr(VI) Removal Contained in Electroplating Industry Wastewater and the Chemical Characterization of the Sludge Produced Sarai Velazquez-Pe~na,† Carlos Barrera-Díaz,*,† Ivonne Linares-Hernandez,‡ Bryan Bilyeu,§ and S. A. Martínez-Delgadillo|| †
)
Centro Conjunto de Investigacion en Química Sustentable UAEM UNAM, Carretera Toluca-Atlacomulco, km 14.5, Unidad El Rosedal, C.P. 50200, Toluca, Estado de Mexico, Mexico. ‡ Centro Interamericano de Recursos del Agua, UAEMex. Carretera Toluca-Atlacomulco, km 14.5, C.P. 50200, Toluca, Estado de Mexico, Mexico. § Department of Chemistry, Xavier University of Louisiana, Drexel Drive, Box 22, New Orleans, Louisiana 70125, United States Depto. Energía, Universidad Autonoma MetropolitanaAzcapotzalco, Av. San Pablo 180, Azcapotzalco, CP 07740, Mexico D.F., Mexico ABSTRACT: The goal of this work is to optimize the electrochemical reduction of hexavalent chromium in electroplating wastewater. The pH, electrolyte composition and concentration, and cathode metal were varied and the effect on reduction rate measured. Although all electrochemical systems studied reduced all Cr(VI) in the solutions, there were clear trends in the speed of the reactions. Because the reaction at pH 2 was faster than that at pH 4, acidity is favored. Higher electrolyte concentrations produced faster rates. Copper cathodes were faster than iron ones. Using the optimized conditions of pH 2 and a copper cathode, along with the high electrolyte concentration already present, actual electroplating wastewater was treated. The rate of Cr(VI) reduction was measured as a function of treatment time and was found to be even higher than that of the synthetic solutions. The 180 mg/L of Cr(VI) in the wastewater was completely reduced in about 15 min under optimal conditions. The sludge generated in the process was analyzed for morphology and elemental composition to provide insight into the mechanism of the reduction. The model electrochemical cell is as effective at reducing hexavalent chromium in actual electroplating wastewater as it is on synthetic solutions, so it can be effectively scaled up to industrial applications.
1. INTRODUCTION Hexavalent chromium is one of the most toxic compounds found in industrial effluents from the textile, oil refining, metal plating, leather tanning, and paint and pigment industries. Plating industry effluents contain Cr(VI) concentrations up to 200 mg L1.13 Although chromium can be present in different oxidation states in aquatic environments, the hexavalent and trivalent states are the most stable. Cr(VI) is highly mobile and a strong oxidant and potential carcinogen making it a thousand times more toxic than Cr(III), so reducing Cr(VI) to Cr(III) is environmentally beneficial.48 Because of its high toxicity and wide industrial use of chromium, different methods have been developed to remove it from wastewater such as adsorption, chemical reduction followed by precipitation, ion exchange, and reverse osmosis.915 However, new processes are being investigated to increase the Cr(VI) reduction efficiency and to lower the treatment cost. Electrochemical techniques are attractive because of their versatility and effectiveness. These techniques have been successfully applied to wastewater remediation,16,17 but the most important feature is their versatility. Electrochemical techniques can be applied to a wide variety of pollutants in a wide variety of environments, but the effectiveness depends on the pH, the electrolytes in solution, the applied current, and the electrodes.1820 Additionally, because the main reagents in the electrolytic redox r 2011 American Chemical Society
reactions are electrons, the process is very environmentally friendly and widely applicable. Electrochemical techniques have been successfully applied to Cr(VI) reduction in solution.2124 However, the redox reactions usually involve many simultaneous processes. When iron is the anode, electrochemical oxidation of the electrode generates Fe2+ (reaction 1). At the cathode, the polarization of the electrode involves simultaneously hydrogen evolution (reaction 2), reduction of water (reaction 3), iron reduction (reaction 4), and Cr(VI) reduction (reaction 5). Anode: FeðsÞ f FeðaqÞ 2þ þ 2e
ð1Þ
Cathode: 2HðaqÞ þ þ 2e f H2ðgÞ
ð2Þ
2H2 OðaqÞ þ 2e f H2ðgÞ þ 2OHðaqÞ
ð3Þ
Special Issue: AMIDIQ 2011 Received: May 4, 2011 Accepted: July 12, 2011 Revised: July 4, 2011 Published: July 12, 2011 5905
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Table 1. Characteristics of the Wastewater characteristics Cr(VI) (mg L1)
value 180
nitrates (mg L1)
0.6
sulfates (mg L1)
161
chlorides (mg L1)
185
pH
3.7
conductivity (ms cm1)
25.6
FeðaqÞ 3þ þ e f FeðaqÞ 2þ
ð4Þ
7HðaqÞ þ þ HCrO4ðaqÞ þ 3e f CrðaqÞ 3þ þ 4H2 O ð5Þ Among the cathode reactions, the reductions of iron(III) and hydrogen chromate in eqs 4 and 5 are the most favorable in terms of potential.25 Additionally, the iron(III) reduction produces iron(II), which is a reducing agent. Because of the favorable potential of these reactions, they and thus the overall Cr(VI) reduction depend on the current density.26 Solution reactions: 3FeðsÞ þ 14HðaqÞ þ þ 2HCrO4ðaqÞ T 2CrðaqÞ 3þ þ 3FeðaqÞ 2þ þ 8H2 OðlÞ
ð6Þ
Most of the Cr(III) and Fe(II) precipitate as follows: CrðaqÞ 3þ þ 3OHðaqÞ T CrðOHÞ3ðsÞ V
ð7Þ
FeðaqÞ 2þ þ 2OHðaqÞ T FeðOHÞ2ðsÞ V
ð8Þ
The goal of this research is to study the electrochemical reduction of Cr(VI) to Cr(III) from wastewater of the electroplating industry using two different configurations of electrodes: Fe/Fe and Fe/Cu. When we find the best conditions, we quantify the amount of sludge generated from the process. Then, the sludge was characterized by SEM, EDS, and XPS to determine which chemical species are present in the sludge and in which oxidation state.
2. MATERIALS AND METHODS 2.1. Synthetic Cr(VI) Solutions. Synthetic Cr(VI) solutions with a concentration of 100 mg/L were prepared with potassium dichromate adjusted to pH 2 and 4 with H2SO4 and NaOH with 0.01 M NaCl and 0.01 M Na2SO4 as supporting electrolytes. All reagents used were of analytical grade, and all synthetic solutions were prepared using deionized water. 2.2. Wastewater Samples Containing Cr(VI). Wastewater samples used in this study were collected from the effluent of a metal plating plant. The wastewater was characterized for pH, conductivity, SO42, NO3, Cl, and Cr(VI). Experimental data is presented in Table 1. 2.3. Experimental Device. Batch electrochemical experiments were carried out in a 250 mL reactor using vertically positioned electrodes immersed in the solution under continuous agitation with a magnetic stirrer. Electrodes were connected to a dc power supply (GW INSTEK model GPR-1820HD). All electrodes (iron or copper) were 20 mm 50 mm 3 mm and were positioned 30 mm apart in the reactor.
2.4. Cr(VI) Synthetic Samples. Synthetic and wastewater samples of approximately 100 mL batches were treated in the reactor under different conditions to optimize the parameters for Cr(VI) reduction. The pH, cathode metal, and supporting electrolyte were varied. After the electrodes were lowered into the solution and the agitation running, the reaction was started when the dc current was applied. Samples were removed and analyzed for Cr(VI) concentration at regular intervals. After each experiment, the electrodes were cleaned with dilute HNO3 and then rinsed with distillated water to remove oxide layers and deposits. The effect of the pH, supporting electrolyte, and cathode material on the Cr(VI) reduction was compiled. 2.5. Cr(VI) Concentration. Samples taken from the reactor during the treatment were analyzed for Cr(VI) concentration to determine the kinetics and extent of the electrochemical reduction. Cr(VI) concentration was measured using 15 diphenylcarbazid with a HACH DR 4000 spectrophotometer at 540 nm following the AWWA 3500-Cr D colorimetric method.27 2.6. Cyclic Voltammetry. The electrochemical behavior of cathodic Cr(VI) reduction was investigated by cyclic voltammetry. The tests were performed using a three-electrode glass cell connected to an Autolab potentiostat model PGSTAT302N. The system employs a platinum counter electrode, an Ag/AgCl calomel reference electrode, and an iron- or boron-doped diamond working electrode. The electrodes were abraded with fine emery paper, polished with alumina powder, and rinsed with distilled water. The working electrode was mounted into a PTFE holder that maintained a 0.2 cm2 surface area exposed to the solution. Measurements were performed on synthetic 100 mg L1 Cr(VI) solutions at pH 2. 2.7. Sludge Characterization. 2.7.1. Sludge Quantification. In order to quantify the total amount of sludge produced in the process, the treated solutions were adjusted to pH 8.5 with NaOH to precipitate all metals. The solutions were filtered, dried at about 100 °C, and then weighed and stored for additional analysis. 2.7.2. SEM and EDS Analysis. The sludge was examined with a JEOL 5900LV scanning electron microscope (SEM) at 20 keV in high-vacuum mode and an energy dispersive X-ray spectrometer (EDS). The SEM secondary electron images provide information on morphology of the sludge particles, while the EDS provides elemental analysis of localized areas and features. 2.7.3. XPS Analysis. X-ray photoelectron spectroscopy (XPS) provides information on elemental composition and the oxidation state of all compounds in the sludge generated in the treatment.28,29 XPS analyses of the sludge were carried out on a JEOL JSP-9200 spectrometer with a magnesium source to determine the atoms present in the sludge. The conditions for all analyses were 1 eV step, 100 ms of well time, and 50 eV of pass energy.
3. RESULTS AND METHODS 3.1. Cr(VI) Synthetic Samples. The concentration of Cr(VI) as a function of treatment time for different cathode metals and different supporting electrolytes at pH 4.0 and 2.0 are shown in Figures 1 and 2, respectively. Both cathodes (Fe and Cu) with either electrolyte (NaCl or Na2SO4) at both pH values (4.0 and 2.0) completely reduce all Cr(VI), but the kinetics are different. The reduction rates are faster at pH 2.0 than at pH 4.0. Also, the copper cathodes are significantly faster than iron ones, and the NaCl supporting electrolyte promotes a faster rate than Na2SO4. 5906
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Figure 1. Cr(VI) concentration as a function of treatment time at pH 4.0 with a current density of 0.1 A using different cathodes (Fe vs Cu) and electrolytes (NaCl vs Na2SO4). (() FeCu using 0.01 M NaCl, (0) FeCu using 0.01 M Na2SO4, (2) FeFe using 0.01 M NaCl, and (O) FeFe using 0.01 M Na2SO4.
Figure 2. Cr(VI) concentration as a function of treatment time at pH 2.0 with a current density of 0.1 A using different cathodes (Fe vs Cu) and electrolytes (NaCl vs Na2SO4). (() FeCu using 0.01 M NaCl, (0) FeCu using 0.01 M Na2SO4, (2) FeFe using 0.01 M NaCl, and (O) FeFe using 0.01 M Na2SO4.
At 15 min of treatment with the optimal conditions of pH 2.0, Cu cathode, and NaCl electrolyte, the Cr(VI) is completely reduced. When the pH is raised to 4.0, there is still about 5 mg L1 left at 15 min, so another 2 min is required for completion. The difference in cathode metal is even more dramatic, with iron having about 20 mg L1 more Cr(VI) than copper at any time during the treatment and an additional 5 min for complete reduction. NaCl appears to be a better supporting electrolyte than Na2SO4 but the difference was less dramatic, showing a small but noticeable difference. 3.2. Actual Wastewater Samples. Using the optimal pH and cathode from the synthetic solution study, an actual wastewater sample was treated in the reactor at two different current densities. There was no need to add a supporting electrolyte because the wastewater already contains significant levels of electrolytes and has a high conductivity, as shown in Table 1. The concentration of Cr(VI) as a function of treatment time for the wastewater at current densities of 0.1 and 0.2 A is shown in
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Figure 3. Cr(VI) concentration as a function of time using FeCu electrodes at pH 2.0 using a current density of (0) 0.2 A and (() 0.1 A.
Figure 4. Sludge particles (a) SEM image and (b) EDS spectra.
Figure 3. As expected for an electrochemical reaction, the higher current density completes the Cr(VI) reduction faster, but both values completely reduce the Cr(VI). Although the initial concentration of Cr(VI) in the wastewater is almost twice that of the synthetic solutions (180 vs 100 mg L1), the times to complete reduction are very similar. Thus, the process is effective on actual wastewater and is enhanced by the high conductivity. 3.3. SEM/EDS. After the Cr(VI) reduction process, the sludge of the FeCu process was examined using SEM. The surface morphology of the sludge particles (Figure 4a) is continuous and flaked. After the SEM images were taken, the sludge was also analyzed by EDS to determine the composition. The EDS spectrum presented in panel (b) of Figure 4 shows the semiquantitative elemental composition of the surface of the particles. As expected, the main components are iron, oxygen, and chromium, but there is also a large amount of copper. This copper on the sludge is likely due to the electrochemical reaction of the copper ions in solution with the iron surface. 5907
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Figure 5. XPS curve fitting spectra corresponding to (a) iron narrow scan of the sludge and (b) Cr 2p3/2 narrow scan of the chromium.
3.4. XPS. XPS analysis was performed in order to corroborate the presence of iron and chromium on the sludge and to identify their chemical state. The curve fitting results are as follows: FeO (25.5%), Fe2O3 (65.6%), and FeSO4 (8.8%). The chromium region was also analyzed. The XPS spectrum of the contact in the chromium binding energy range exhibits a peak that can be separated into two components (Figure 5). The two components are chromium(III) oxide (56.7%) and chromium(III) hydroxide (43.3%). It is interesting to note that there is no chromium(VI) signal in the spectra. Both signals correspond to chromium(III) with chromium(III) oxide in a slightly higher ratio than the hydroxide. The results show that the redox reaction between chromium and iron proceeds practically to completion. 3.5. Cyclic Voltammetry Test. In Figure 6, the cathodic direction, a reduction wave (R 1) is present in the case of the Cr(VI) solution. When the concentration of 100 mg Cr(VI)/L was tested, the Epc was displaced to 0.418 V and ipc to 0.0007 mA. Therefore, this cathodic peak could be associated to Cr(VI) reduction at the Cu electrode. A similar result has also been reported by Welch et al.,30 who studied the electrochemical detection of hexavalent chromium species in gold, glassy carbon (GC), and boron-doped diamond electrodes (BDD). 3.6. Mechanism. The Cr(VI) reduction process involves the reactions shown in Figure 7. At the anode, an electrochemical oxidation of the iron electrode generates Fe2+. At the cathode, the
polarization of the electrode involves the simultaneous hydrogen evolution and reduction of water. Additional important reactions at the cathode are iron reduction (reaction 9) and direct Cr(VI) electrochemical reduction (reaction 10).23 FeðaqÞ 3þ þ e f FeðaqÞ 2þ 7HðaqÞ þ þ HCrO4 þ 3e f CrðaqÞ 3þ þ 4H2 O
ð9Þ ð10Þ
These two cathodic reactions along with the generation of Fe2+ at the anode, are responsible for the Cr(VI) reduction. Therefore, the cathodic reactions are very influencial on the overall Cr(VI) reduction rate. While there are many possible explanations for the higher reduction rate for a Cu cathode over an Fe one, Goeringer et al.31 have reported copper directly reacting with hydrogen chromate, as shown in reaction 11. However when copper is used as a cathode, reaction 8 is not favorable because of the applied field. 2þ 2HCrO4 þ 3Cu0ðsÞ þ 14Hþ f 2Cr 3þ ðaqÞ þ 3CuðaqÞ þ 8H2 O
ð11Þ Direct Cr(VI) electrochemical reduction (reaction 10) at the cathode may be ascribed to the extent of polarization promoted by low electrical resistivity of copper, and bearing in mind their 5908
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Figure 6. Cyclic voltammogram obtained for a 100 mg/L Cr(VI) using copper electrodes.
performance, decreasing the time required for Cr(VI) reduction as compared with that of the iron electrode. There is a major effect on the Cr(VI) reduction of the aqueous solution pH and the type of electrolyte support in the media. Real wastewater can be treated using this methodology, making this electrochemical process a very attractive alternative to current environmental treatment technology.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: + (52)-(722)-2766611, + (52)-(722)-2766639. E-mail:
[email protected],
[email protected].
’ ACKNOWLEDGMENT The authors acknowledge the support given by the Universidad Autonoma del Estado de Mexico, specifically the Centro Conjunto de Investigacion en Química Sustentable and CIRA (Project PROMEP 436811). Support from CONACYT (Consejo Nacional de Ciencia y Tecnología) and supporting research by SNI (Sistema Nacional de Investigadores) are greatly appreciated. ’ REFERENCES
Figure 7. Schematic of electrochemical reactions in the FeCu reactor.
electrode potentials, the iron anode dissolution and electrochemical reduction of Cr(VI) are enhanced by higher copper cathode polarization. The evidence of a significant increase in the rate of removal suggests an overall process economy because the amount of sludge produced and electricity required diminished.
4. CONCLUSIONS There is a clear influence of the cathodic materials on the Cr(VI) reduction process; the copper cathode offers better
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