Environ. Sci. Technol. 2005, 39, 1875-1879
Electroreduction of Cr(VI) to Cr(III) on Reticulated Vitreous Carbon Electrodes in a Parallel-Plate Reactor with Recirculation FRANCISCO RODRIGUEZ-VALADEZ,† C A R L O S O R T I Z - EÄ X I G A , ‡ JORGE G. IBANEZ,§ ALEJANDRO ALATORRE-ORDAZ,† AND S I L V I A G U T I E R R E Z - G R A N A D O S * ,† Instituto de Investigaciones Cientı´ficas, Universidad de Guanajuato, Cerro de la Venada S/N, Pueblito de Rocha, 36080 Guanajuato, Gto., Mexico, and Centro Mexicano de Quimica en Microescala, Universidad Iberoamericana-Ciudad de Mexico, Prol. Reforma 880, 01210 Mexico, D. F. Mexico
The reduction of Cr(VI) to Cr(III) is achieved in a flow-by, parallel-plate reactor equipped with reticulated vitreous carbon (RVC) electrodes; this reduction can be accomplished by the application of relatively small potentials. Treatment of synthetic samples and field samples (from an electrodeposition plant) results in final Cr(VI) concentrations of 0.1 mg/L (i.e., the detection limit of the UV-vis characterization technique used here) in 25 and 43 min, respectively. Such concentrations comply with typical environmental legislation for wastewaters that regulate industrial effluents (at present time ) 0.5 mg/L for discharges). The results show the influence of the applied potential, pH, electrode porosity, volumetric flow, and solution concentration on the Cr(VI) reduction percentage and on the required electrolysis time. Values for the mass transfer coefficient and current efficiencies are also obtained. Although current efficiencies are not high, the fast kinetics observed make this proposed treatment an appealing alternative. The lower current efficiency obtained in the case of a field sample is attributed to electrochemical activation of impurities. The required times for the reduction of Cr(VI) are significantly lower than those reported elsewhere.
Introduction Chromium does not occur naturally in elemental form but only in compounds. It is mined as a primary ore product in the form of the mineral chromite, FeCr2O4. Major sources of Cr contamination include releases from electroplating processes (1) (where hexavalent chromium is the single most important pollutant) and the disposal of chromium-containing wastes (2). Cr(VI) is highly concentrated in the rinsing waters of electroplating processes and cannot be discharged in water bodies because of its high toxicity (3). International legislation limits Cr(VI) concentration in potable water supplies as well as groundwater to a few ppb * Corresponding author phone: (52) 473-73275555; fax: (52) 4737326252; e-mail:
[email protected]. † Universidad de Guanajuato. ‡ Deceased. § Universidad Iberoamericana-Ciudad de Mexico. 10.1021/es049091g CCC: $30.25 Published on Web 02/01/2005
2005 American Chemical Society
(4). For example, the maximum chromium level in drinking water in the U.S. is 0.1 ppm, and in discharges it is 3, as mentioned previously, E for the process becomes less positive with pH, and thus, ∆G becomes less negative (25) and (b) even though the pH in the bulk solution may be low, it is known that the local pH in the vicinity of some cathodes may be quite high, certainly high enough as to promote the production of insoluble Cr(III) hydroxide on the electrode surface, which may hamper electron transfer (26). Effect of Electrode Porosity. Experiments were conducted using 30, 45, 65, and 100 pores per inch (ppi) RVC plates at -0.8 V and pH ) 2. We found that the process requires 55, 45, 60, and >60 min to reach a virtually 100% Cr(VI) reduction when 30, 45, 65, and 100 ppi RVC is used, respectively. The effect of porosity can be better analyzed by calculating the mass transfer coefficient, km or better, the product kmAe (where Ae is the specific surface area, given by the ratio of the active area/unit volume of cathode). It has been reported that for a system like the one used in the present study, this product can be easily calculated on the basis of the equation of a simple batch reactor (19, 27, 28)
ln
[ ] (
)
Ve C(t) ) - kmAe t VR C(0)
(1)
where C(t) is the concentration of the electroactive species at a time t, C(0) is the initial concentration, Ve is the volume of the cathode, VR is the reactor volume, and t is the electrolysis time. By plotting ln[C(t)/C(0)] versus t for the different porosities, one obtains the value of kmAe. The values of kmAe and of km are shown in Table 2 as a function of porosity. It can be seen that both kmAe and km increase with porosity up to a maximum and then drop because of the increased difficulty for the solution to diffuse through smaller pores. The effective area decreases due to the sites occupied by evolved hydrogen and possibly also by chromium (III) VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1877
TABLE 3. Effect of Electrolyte Flow Rate on kmAea flow rate (mL/min)
time required for 90% reduction of Cr(VI) (min)
kmAe × 10-3 (s-1)
km × 10-6 (m s-1)
20 40 80 120
50 35 15 12
8.4 9.8 22.2 25.8
2.1 2.3 4.0 3.8
a
t ) 60 min, 65 ppi RVC.
FIGURE 5. Percent reduction of Cr(VI) with time in (b) synthetic sample under the original conditions (OC/SYS), (9) synthetic sample under the most favorable conditions (MFC/SYS), (4) field sample under the original conditions (OC/FS), and (]) field sample under the most favorable conditions (MFC/FS).
TABLE 4. Current Efficiencies and Electrolysis Times Required to Achieve Virtually Complete Reduction of Cr(VI) under the Original (OC) and the Most Favorable Conditions (MFC) Found in the Present Work, for Synthetic (SYS) and Field (FS) Samples FIGURE 4. Effect of the initial concentration of Cr(VI) on its reduction at -0.8 V (vs Ag/AgCl). t ) 60 min; RVC: 65 ppi; flow rate ) 80 mL/min, pH ) 1. deposits (which are visible to the naked eye in the opencircuit experiments). Effect of Electrolyte Flow Rate. The reduction of Cr(VI) at -0.8 V (at pH 2) was also studied at different electrolyte flow rates: 20, 40, 80, and 120 mL/min. We selected an intermediate RVC porosity (65 ppi) for this study. The times required for a 90% reduction of Cr(VI) and the values of kmAe and km are shown in Table 3. It is noteworthy that a very short electrolysis time (12 min) is required at 120 mL/min. Also, when the electrolyte flow increases from 20 to 80 mL/min, the value of kmAe (calculated from the initial slopes) also increases from 8.9 × 10-3 to 16.9 × 10-3 s-1. However, if the flow is increased even more (i.e., to 120 mL/min), the value of kmAe only changes by a small amount (to 16.0 × 10-3 s-1). For this reason, we selected 80 mL/min for further study. In addition, a flow rate increase translates into a decrease in residence time and an increase in turbulence near the electrode surface that reduces the possible formation and accumulation of Cr(III) insoluble products, thus avoiding electrode passivation and improving mass transfersas deduced from the kmAe and km values; therefore, Cr(VI) reduction is increased as well. Effect of the Initial Concentration of Cr(VI). To find a suitable Cr(VI) concentration range to be treated with our system, we tested the following initial concentrations, C(0): 50, 100, 200, 300, and 400 mg/L (ppm). Figure 4 shows the percent of Cr(VI) reduction (at t ) 60 min) as a function of C(0). One can note that with C(0) ) 400 mg/L, only 25% of Cr(VI) gets reduced, whereas when C(0) is 100 and 50 mg/L, we obtain reduction percentages of virtually 100%. This means that the system has a capacity limit for said reduction due to the reactor dimensions and the time selected (60 min). However, this limitation can in principle be solved by recirculating the solution until the desired final concentration is reached. 1878
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005
synthetic sample (SYS)
field sample (FS)
parameter
OC
MFC
OC
MFC
electrolysis time (min) % of Cr(VI) reduction current efficiency (%)
55 100 24
25 100 13
60 95 21
43 100 11
On the basis of these findings, we selected the following conditions as to get high reduction yields and low energy usage, which we will hereby call the most favorable conditions (MFC): pH 1, E ) -0.7 V, flow rate ) 80 mL/min, and 45 ppi RVC; with regard to concentration, we used 100 mg/L Cr(VI) for the synthetic samples (SYS) and 80 ppm for the field sample (FS). Figure 5 shows the results obtained with the OC (see Experimental Procedures) or under the MFC selected here. For the FS under the OC, we obtained a 90% reduction in 40 min, whereas the same reduction took only 25 min under the MFC. For the SYS under the OC, we eliminated 90% of the initial Cr(VI) in 35 min, whereas the same reduction took only 14 min under the MFC. Longer electrolysis times may reduce the final Cr(VI) concentration; analysis using a more sensitive method (i.e., the diphenylcarbazide standard method) must be used beyond the electrolysis times used here, to determine the maximum percent of Cr(VI) transformation. As a preliminary assessment of durability, we found that one electrode can be reused at least 20 times without a noticeable effect on the reduction of Cr(VI). According to these results, it is clear that by judiciously selecting the electrolysis parameters, one can obtain quite reasonable percentages of Cr(VI) reduction and reduce the required energy with short electrolysis times. Table 4 shows the current efficiencies and the energy consumption for each case. One can observe that a tradeoff has to be accepted since in order to obtain a 100% reduction for the SYS, the current efficiency drops from 24 (at OC) to 13% (at MFC) and from 21 to 11%, respectively, with the FS; such a phenomenon is likely due to a decrease in the residence time of the species in the reactor, as well as a higher electrode contact area concomitant with the higher porosity. In addition, parasitic
reactions due to the higher applied voltage very likely promote water decomposition and electrochemical activation of impurities in the field sample. The observed efficiencies are consistent with earlier reports (17, 18). We have shown that the reduction of Cr(VI) to Cr(III) can be accomplished by the application of relatively small potentials to RVC electrodes in a parallel-plate flow-by reactor. Synthetic samples and field samples were virtually 100% reduced in 25 and 43 min, respectively, thus complying with environmental norms that regulate industrial effluents. Even though current efficiencies are not high, they are compensated for by fast kinetics, which makes this proposed treatment an appealing alternative. The lower current efficiency obtained with the field sample is associated with the electrochemical activation of impurities. The required time for the reduction of Cr(VI) is significantly lower than that reported elsewhere.
Acknowledgments A.A.-O. acknowledges support from the National and State Science Foundations (CONACyT-CONCyTEG), Grant GTO2002-C01-6156.
Literature Cited (1) Pletcher, D.; Walsh, F. Industrial Electrochemistry, 2nd ed.; Chapman Hill: Cambridge, 1990. (2) Smith, L. A.; Means, J. L.; Chen, A.; Alleman, B.; Chapman, C. C.; Tixier, J. S.; Brauning, S. E.; Gavaskar, A. R.; Royer, M. D. Remedial Options for Metals-Contaminated Sites; Lewis Publishers: Boca Raton, FL, 1995. (3) Katz, S. A.; Salem, H. The Biological and Environmental Chemistry of Chromium; VCH Publishers: New York, 1994. (4) Standard Methods for the Examination of Water and Wastewater, 19th ed.; American Public Health Association/American Waterworks Association/Water Environment Federation: Washington, 1995. (5) http://www.epa.gov/safewater/contaminants/dw_contamfs/ chromium.html (01/2004); Groundwater and Drinking Water: Consumer fact sheet on chromium. (6) (a) NOM-002-ECOL-1996 (Mexican Ecological Official Norm); http://www.mexicanlaws.com/environmental_law.htm (02/ 2004); (b) Arreˆte´ du 2 fe´vrier 1998 relatif aux pre´le`vements et a` la consommation d’eau ainsi qu’aux e´missions de toute nature des installations classe´es pour la protection de l’environnement; Canadian Environmental Quality Guidelines; http://www. ccme.ca/assets/pdf/e1_062.pdf (02/2004). (7) http://www.epa.gov/ttn/atw/hlthef/chromium.html (02/2004); Chromium Compounds; Technology Transfer Network. (8) Patterson, R. R.; Fendorf, S.; Fendorf, M. J., Reduction of hexavalent chromium by amorphous iron sulfide, Environ. Sci. Technol. 1997, 31, 2039. (9) Walker, W. J.; Maslanka, C. K.; Pucik, L. E.; Schreier, C. G.; Brayley, R. In situ reduction of hexavalent chromium in groundwater and surface soil using acidified ferrous sulfate. J. Am. Chem. Soc. 1997, 2133, 64. (10) Singh, I. B.; Singh, D. R. Influence of dissolved oxygen on aqueous solutions Cr(VI) removal by ferrous ion. Environ. Technol. 2003, 23, 1347. (11) Leyva Ramos, R.; Juarez Martı´nez, A.; Guerrero Coronado R. Adsorption of chromium (VI) from aqueous solutions on
activated carbon. Water Sci. Technol. 1994, 30, 191. (12) Martı´nez, S. A.; Rodrı´guez, M. G.; Barrera, C. A kinetic model that describes removal of chromium VI from rinsing waters of the metal finishing industry by electrochemical processes. Water Sci. Technol. 2000, 42, 55. (13) Rodrı´guez, M. G.; Aguilar R.; Soto, G.; Martı´nez, S. A. Modeling an electrochemical process to remove Cr(VI) from rinse-water in a stirred reactor. J. Chem. Technol. Biotechnol. 2003, 78, 371. (14) Rodrı´guez, F. J.; Gutie´rrez, S.; Ibanez, J. G.; Bravo, J. L.; Batina, N. The efficiency of toxic chromate reduction by a conducting polymer (polypyrrole): Influence of electropolymerization conditions. Environ. Sci. Technol. 2000, 34, 2018. (15) Chaudhary, A.; Goswami, N. C.; Grimes, S. M. Electrolytic removal of hexavalent chromium from aqueous solutions. J. Chem. Technol. Biotechnol. 2003, 78, 877. (16) Golub, D.; Oren, Y. Removal of chromium from aqueous solutions by treatment with porous carbon electrodes: electrochemical principles. J. Appl. Electrochem. 1989, 19, 211. (17) Reussard, S.; Benezech, J. F.; Lacoste, G. Removal of hexavalent chromium converting to chromium hydroxide by treatment in an electrochemical reactor. Electrochem. Eng. Environ. 92nd Icheme Symp. Ser. 1992, 127, 97. (18) Abda, M.; Gavra, Z.; Oren, Y. Removal of chromium from aqueous solutions by treatment with fibrous carbon electrode: Column effects. J. Appl. Electrochem. 1991, 21, 734. (19) Sankaranarayanan, M.; Basha, C.; Vuayavalli, R. Electrochemical reduction of Cr6+ to Cr3+ using packed bed electrolysers. Bull. Electrochem. 1986, 2, 127. (20) Radwan, A.; El-Kiar, A.; Farag, H. A.; Sedahmed, G. H. The role of mass transfer in the electrolytic reduction of hexavalent chromium at gas evolving rotating cylinder electrodes. J. Appl. Electrochem. 1992, 22, 1161. (21) Ortiz, C.; Rodrı´guez, F.; Garcı´a, L. A.; Alatorre, A.; Gutierrez, S.; Direct electroreduction of Cr(VI) in a continuous system from industrial chromium-electroplating effluents. 15th Iberoam. Electrochem. Soc. Meeting 2002, 5320P. (22) Friedrich, J. M.; Ponce-de-Leon, C.; Reade, G. W.; Walsh, F. C. Reticulated vitreous carbon as an electrode material. J. Electroanal. Chem. 2004, 561, 203-217. (23) Guzman-Pantoja, J.; Ibanez, J. G.; Vasquez-Medrano, R. C.; Oropeza-Guzman, M. T. Direct electrochemical reduction of hexavalent chromium in a filter press reactor. Bull. Electrochem. 2004, 20(3), 107-114. (24) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed.; NACE International-CEBELCOR: Houston and Brussels, 1974. (25) Senthurchelvan, R.; Wang, Y.; Basak, S.; Rajeshwar, K. Reduction of hexavalent chromium in aqueous solutions by polypyrrole. J. Electrochem. Soc. 1996, 143, 44. (26) Roberts, E. P. L.; Yu, H. Chromium removal using a porous carbon felt cathode, J. Appl. Electrochem. 2002, 32, 1091. (27) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; Wiley: New York, 1999. (28) Pletcher, D.; Whyte, I.; Walsh, F. C.; Millington, J. P. Reticulated vitreous carbon cathodes for metal ion removal from process streams. Part I: Mass transport studies. J. Appl. Electrochem. 1991, 21, 659.
Received for review June 16, 2004. Revised manuscript received December 15, 2004. Accepted December 16, 2004. ES049091G
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1879
