Environ. Sci. Technol. 2007, 41, 1439-1443
Electrically Regenerated Ion Exchange for Removal and Recovery of Cr(VI) from Wastewater
ing processes, which is characterized by low Cr(VI) concentration and low pH value. The process of the ion exchange for removal and recovery of Cr(VI) is illustrated below (21):
YUNQING XING, XUEMING CHEN,* AND DAHUI WANG Environmental Engineering Department, Zhejiang University, Hangzhou 310027, China
R-HCrO4 + 2NaOH f R-OH + Na2CrO4 + H2O in regeneration (2)
Ion exchange is widely used for removal and recovery of Cr(VI) from wastewater. Generally, the exhausted ion exchanger is regenerated using chemicals. Although chemical regeneration is efficient, contaminants are introduced, leading to difficulty for the subsequent recovery of Cr(VI). To overcome such a problem, a new regeneration method, namely electrical regeneration, which is carried out on the principle of electrodialysis, is presented in this paper. Experimental results showed that the weak-base resin used could be effectively regenerated electrically. About 93% capacity of the resin was restored under a constant current of 0.25 A over a period of 24 h. The pure chromic acid was recovered in the anode chamber with a concentration of 5.03 g Cr(VI)/L. It was found that the weak-base resin regenerated electrically could remove Cr(VI) from wastewater as effectively as that regenerated chemically. The Cr(VI) concentration was reduced from initial 50 mg/L to lower than the detectable limit, 0.01 mg/L, after treatment.
Introduction Hexavalent chromium, Cr(VI), is widely presented in wastewaters from electroplating and metal finishing processes, pigment manufacture, tannery facilities, and chromium mining operations. Its concentration usually varies from tens to hundreds of mg/L (1). Cr(VI) species are very toxic. They can act as carcinogens, mutagens, and teratogens in biological systems (2, 3). Therefore, effective treatment of the wastewater containing Cr(VI) is very important. In addition, since chromium is expensive, simultaneous removal and recovery of Cr(VI) from wastewater are highly desired in industry. The principal conventional methods for treatment of the wastewater containing Cr(VI) include reduction followed by precipitation (4-6), adsorption (7-10), reverse osmosis (1113), electrodialysis (14-16), and ion exchange (17-23). The reduction followed by precipitation is usually used to treat the composite wastewater containing not only Cr(VI), but also some other heavy metal ions. This combined process is effective under good control, but sludge is produced, which may cause secondary pollution. Adsorption is also effective in removing Cr(VI), but its operation is complex. On the other hand, although reverse osmosis and electrodialysis are superior in recovering Cr(VI), it is difficult to reduce Cr(VI) in the effluent to an acceptable level. Relatively, ion exchange is an attractive approach in treating the wastewater containing Cr(VI), especially in treating the rinsing wastewater from electroplating and metal finish* Corresponding author phone:(86)57187951239; 57187952771; e-mail:
[email protected]. 10.1021/es061499l CCC: $37.00 Published on Web 01/12/2007
fax:(86)-
2007 American Chemical Society
R-OH + H2CrO4 f R-HCrO4 + H2O in treatment
Na2CrO4 + 2R-H f 2R-Na + H2CrO4 in recovery
(1)
(3)
where R-OH and R-H represent the OH-type anionic and H-type cationic exchange resin, respectively. The ion exchange has many advantages. First, it is very effective in removing Cr(VI) from wastewater. Gala´n (23) and co-workers reported that ion exchange could reduce Cr(VI) from the initial 377 mg/L to less than 0.1 mg/L. This can almost meet any strict environmental standards. Secondarily, the effluent is the pure water when cation exchange is employed together with anion exchange (20). This allows the rinsewater to be recycled completely. Furthermore, concentrated chromic acid solution can be recovered, and the secondary pollution can be eliminated. Despite these attractive advantages, a common problem of the currently available ion exchange system is the complexity in regenerating the resin and in recovering pure chromic acid. In general, the exhausted anionic exchanger is regenerated using alkali chemicals, such as NaOH and KOH. This unavoidably introduces unexpected contaminants, i.e., Na+ or K+. In order to recover pure chromic acid, an extra cation exchange column has to be fixed to remove the contaminants present in the previous elution. This cation exchange column, in turn, needs regeneration with acids after it is exhausted. Therefore, improvement of the regeneration method is necessary. In recent years, there are growing interests in electrodeionization (EDI), which incorporates electrodialysis with ion exchange. EDI is usually operated in a continuous mode, that is, electricity is supplied continuously, and ion removal relies mainly on electrical migration. The continuous EDI has been widely used for producing pure water in industry. This technique was also reported for the removal of Ni2+ and Co2+ from wastewater (24-26). In order to remove Cr(VI) and to recover concentrated chromic acid from wastewater, EDI was operated in batch in the present work. In this process, electricity was not supplied during wastewater treatment, and Cr(VI) removal relied totally on the ion-exchange resin. After the resin was exhausted, electricity was supplied to restore its exchange capacity. Obviously, such an EDI is essentially an electrically regenerated ion exchange process. The principle of electrical regeneration of the exhausted ionexchange resin is as below:
H2O f H+ + OH- at the CEM-resin interface
(4)
OH- + R-HCrO4 f R-OH +
HCrO4- in the center chamber (5)
2H+ + 2e f H2 v at the cathode
(6)
2H2O - 4e f 4H+ + O2 v at the anode
(7)
The H+ ions generated in reaction 4 transport toward the cathode chamber through a cation exchange membrane VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Characteristics of the Anion Exchange Resina type matrix porosity functional group total exchange capacity operating pH range a
weak-base anion exchanger polystyrene macro-porous -N(CH3)2‚H2O g1.4 mole /L in term of monovalent ions 0-8
Obtained from manufacturer.
FIGURE 1. Experimental setup. 1-anolyte reservoir; 2-anolyte feeder; 3-valve; 4-anode chamber; 5-AEM; 6-center chamber; 7-flowrate meter; 8-CEM; 9-cathode chamber; 10-thermostat; 11-pump; 12influent reservoir; 13-effluent reservoir. (CEM), whereas the OH- ions exchange with HCrO4- in the resin. Meanwhile, HCrO4- ions transport toward the anode chamber through an anion exchange membrane (AEM), and build up themselves there to form chromic acid with the H+ ions generated at the anode. This process can regenerate the saturated resin and recover the pure chromic acid simultaneously in a single step without need of chemicals. The major objectives of this study are to demonstrate the high efficiency of the electrical regeneration and the good performance of the regenerated resin for the subsequent treatment of the wastewater containing Cr(VI).
Experimental Section Experimental Setup. The experimental setup is schematically shown in Figure 1. The electrically regenerated ion exchange unit consisted of an anode chamber 4 (200 mm × 20 mm × 15 mm), a cathode chamber 9 (200 mm × 20 mm × 15 mm), and a center chamber 6 (200 mm × 20 mm × 9 mm). The macroporous-type weak-base anionic resin (D354, Zhengguang Co., Hangzhou, China) was packed in the center chamber as the ion exchanger. The electrode chambers and the center chamber were separated by an AEM 5 (Double Flower heterogeneous, SWTM Co., Shanghai, China) and a CEM 8 (Nafion117, DuPont Co.). Both membranes had an effective area of 40 cm2 (200 mm × 20 mm). The net spacing between AEM and CEM was 9 mm. Ti/IrO2-SnO2-Sb2O5 and titanium mesh, which had the same effective area as the membranes, were used as the anode and the cathode, respectively. The Ti/IrO2-SnO2-Sb2O5 anode was fabricated using IrCl4‚xH2O, SnCl4·5H2O, and SbCl3 as precursors. The detailed preparation procedures can be found elsewhere (27, 28). The electrodes were fixed 2 mm away from the membranes by inserting gaskets. A diaphragm-type metering pump 11 (Chem-Tech series 100, Pulsafeeder Co.) was employed to deliver the wastewater upward through the center chamber. A DC power supply (MPS-3002L-3, Matrix, Shenzhen, China) was used to maintain a constant current in regenerating the exhausted resin electrically. In order to increase conductivity, 0.05 M of H2SO4 solution was used as the catholyte during electrical regeneration, whereas the synthesized wastewater was employed as the anolyte and recycled artificially. After regeneration, the H2SO4 solution was discharged, and the synthesized wastewater was added to the cathode chamber. In order to obtain convincing results, all experiments were performed at least three times parallelly. Resin Treatment and Resistivity Examination. The characteristics of the macroporous-type weak-base anionic resin used are listed in Table 1. Before it was packed, the resin was soaked in 10% H2SO4 for 6 h , then washed with 1440
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FIGURE 2. Breakthrough curve at a flow rate of 1 L/h for the new ion-exchange resin regenerated fully with 5% NaOH solution. DI water, regenerated fully with 5% NaOH, and finally washed with DI water again. After treatment, 36 mL of the resin, which had be converted into the form of R-OH, was packed in the center chamber as an ion exchanger. The resistivity of the resin was obtained on the basis of the voltage-current relation, which was examined by packing the resin between two electrodes. Wastewater Preparation. Wastewater was synthesized by dissolving a proper amount of chromium trioxide (CrO3) in DI water. The concentration of Cr(VI) was 50 mg/L, and the pH value was about 3.0. Analysis. pH values were measured using a pH meter (230A+, Orion). Cr(VI) concentrations were analyzed with a spectrophotometer (DR/2500, Hach) according to the standard methods (29). The detectable limit was 0.01 mg/L.
Results and Discussion Performance of the New Resin for Cr(VI) Removal. For comparison purposes, the performance of the new resin regenerated fully using 5% NaOH solution was first investigated for Cr(VI) removal, and the result is shown in Figure 2. It was found that the Cr(VI) concentration in the effluent was below the detectable limit, i.e.,
