Plasma-Induced Reduction of Chromium(VI) in an Aqueous Solution

Oct 21, 2008 - Department of Chemistry, Zhejiang University,. Hangzhou 310027 P. R. China. Received June 23, 2008. Revised manuscript received...
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Environ. Sci. Technol. 2008, 42, 8492–8497

Plasma-Induced Reduction of Chromium(VI) in an Aqueous Solution LEI WANG AND XUANZHEN JIANG* Department of Chemistry, Zhejiang University, Hangzhou 310027 P. R. China

Received June 23, 2008. Revised manuscript received September 17, 2008. Accepted September 19, 2008.

An efficient reduction of hexavalent chromium [Cr(VI)] induced by gaseous glow discharge plasma (GDP) generated between a pointed platinum anode and the surface of an aqueous solution has been achieved for the first time. Experimental results show that Cr(VI) could be smoothly reduced to trivalent state [Cr(III)] by using GDP with 500 V as the optimum operating voltage. The rate of Cr(VI) reduction was enhanced by either decreasing the solution pH or adding radical scavengers to the solution. At initial pH 2.0, 100 mg/L of Cr(VI) was completely reduced within 10 min of GDP treatment in the presence of 100 mg/L phenol. A possible reaction mechanism was proposed based on the reduction kinetics. Energy efficiency of Cr(VI) reduction in GDP was compared with that in other competitive processes.

compatibility (8). Glow discharge plasma in aqueous solution (GDP) is a novel electrical process in which the plasma is sustained between a metal electrode and an electrolytic solution (9-11). Using a pointed anode in contact with the surface of an electrolytic solution, a sheath of vapor is formed due to Joule heating. If the applied voltage is sufficiently high, the vapor breaks down and GDP forms. During GDP, various chemically active species such as hydrogen atoms, hydroxyl radicals, and hydrogen peroxide are formed in the solution with yields much higher than those stipulated by Faraday’s law (9). These chemically active species, especially the hydroxyl radicals, can induce the decomposition of the organic pollutant dissolved in the solution. It has been found that phenols, aniline, nitrobenzene, and dyes, etc., can be oxidized to water and carbon dioxide by using GDP (12-16). As reported in reference 9, reducing species are also produced in GDP. The present study reports on an experiment that attempted to use Cr(VI) as a model pollutant to examine the reduction reactions in GDP. To the best of our knowledge, the use of GDP for reduction of aqueous Cr(VI) to Cr(III) has not been reported so far. The present study primarily focused on optimizing conditions for Cr(VI) reduction, and the possible reduction mechanisms are discussed.

1. Introduction Chromium compounds are widely used in electroplating, leather tanning, and other industrial processes. As a consequence, large quantities of aqueous chromium wastes are produced, which seriously pollute the environment without proper disposal. Chromium generally exists in water with two stable oxidation states: hexavalent [Cr(VI)] and trivalent [Cr(III)]. Cr(VI) is extremely toxic and has demonstrated to be carcinogenic (1), whereas Cr(III) is less toxic and can be readily precipitated out of solution in the form of Cr(OH)3. Current methods of treating Cr(VI) are by chemical reduction to Cr(III) in acidic condition followed by precipitation with alkali. The reducing agents are usually ferrous sulfate or sodium sulfite. However, these reduction methods have their respective shortcomings. When ferrous sulfate is used as the reducing agent, ferric hydroxide is produced as a solid waste, which requires complex subsequent disposal. With sodium sulfite as the reducing agent, sulfur dioxide will be formed in an acidic condition, which may cause air pollution as sulfur dioxide is toxic, odorous, and volatile. Furthermore, both the sodium sulfite and ferrous sulfate are not suitable for treating dilute Cr(VI) solution and often excessive chemicals are required (2). In this context, many alternative processes such as ionizing radiation (3, 4), photocatalysis (5, 6), and biological reduction (7) have been proposed. However, none of these processes have gained popularity over chemical reduction because of either process complexity or stringent reaction conditions. Recently, water treatment by electrical discharges has aroused considerable interest from environmental investigators because of its high removal efficiency and environmental * Corresponding author tel: 0086-571-87951611; fax: 0086-57187951611; email: [email protected]. 8492

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FIGURE 1. Schematic diagram of the GDP reactor for Cr(VI) reduction.

FIGURE 2. Effect of the applied voltage on Cr(VI) reduction (solution volume, 150 mL; initial conductivity, 5.0 mS/cm; pH0, 5.3; current, 100 mA). 10.1021/es8017286 CCC: $40.75

 2008 American Chemical Society

Published on Web 10/21/2008

FIGURE 3. Effect of initial pH on Cr(VI) reduction (solution volume, 150 mL; voltage, 500 V; current, 100 mA; initial conductivity, 5.0 mS/cm).

FIGURE 4. Variations of UV/vis spectra during GDP treatment (solution volume,150 mL; voltage, 500 V; current, 100 mA; pH0, 2.0; initial Cr(VI) concentration, 100 mg/L).

2. Materials and Methods The experimental apparatus consisted of a DC high-voltage power supply (variable voltage 0-600 V and current of 0-600

mA) and a reactor. The schematic diagram of the reactor is shown in Figure 1. The reactor was made of common glass, with inner diameter of 7.0 cm and length of 17.0 cm. The anode, from which the discharge emitted, was a pointed platinum wire (Φ ) 0.6 mm) sealed into a glass tube with about 10 mm exposed. The cathode was a stainless steel plate (surface area 2.0 cm2) placed in another glass tube and separated from the anodic compartment by a glass frit of medium porosity. The bulk solution in the jacketed reaction vessel was maintained at 298 ( 2 K by circulating water. The solution for treatment was prepared by dissolving a known amount of potassium dichromate in a sodium sulfate solution (conductivity 5.0 mS/cm) and a 150 mL portion was transferred to the reaction vessel for treatment. Sulfuric acid or potassium hydroxide was added to the solution to adjust the solution pH. The catholyte, sodium sulfate, is the same as the anolyte. DC voltage of 500 V was usually applied across the electrodes through a DC power supply to initiate the reaction. Depth of the anode immersed in the solution was carefully adjusted to average the current to 100 mA, with the current deviation less than ( 4%. The depth of the anode immersed for 100 mA was about 1.0 mm (550 V), 1.2 mm (500 V), 1.5 mm (450 V), 1.0 mm (400 V), 0.7 mm (350 V), and 0.5 mm (300 V), respectively. During the treatment, the solution was gently stirred with a magnetic stirrer. Spectra of the treated solution were monitored by a UV/vis spectrophotometer (JASCO, V-550), using the sodium sulfate solution with the same initial pH as the reference (17). Concentration of Cr(VI) remaining in the solution was determined colorimetrically at 540 nm using diphenylcarbazide as the color reagent (4). It was shown from the experiments that the amount of Cr(III) formed in the solution equaled that of Cr(VI) reduced by GDP. Ethanol or phenol was added to the solution as hydroxyl radical scavenger. HPLC (Agilent 1100) was used to analyze the byproduct formed from phenol or ethanol during the GDP treatment (14). The gases produced from each compartment were analyzed by gas chromatography (HP 6890). Waveforms of the voltage and current were measured by a digital oscilloscope (RIGOL, DS1102C).

3. Results and Discussion 3.1. Cr(VI) Reduction under Different Applied Voltages. In this experiment, the applied voltage was investigated from 300 to 550 V. Figure 2 shows the variations of Cr(VI) concentration under the different applied voltages. It can be seen from Figure 2 that the applied voltage imposed an apparent effect on the Cr(VI) reduction rate. For

FIGURE 5. Variations of pH (a) and conductivity (b) during GDP treatment (solution volume, 150 mL; voltage, 500 V; current, 100 mA; pH0, 2.0). VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Effect of initial concentration on Cr(VI) reduction (solution volume,150 mL; voltage, 500 V; current, 100 mA; treatment time, 5 min; conductivity, 5.0 mS/cm).

FIGURE 7. Effect of initial Cr(VI) concentration on H2 production (solution volume, 150 mL; voltage, 500 V; current, 100 mA; treatment time, 5 min; conductivity, 5.0 mS/cm). example, at 300 V, almost no Cr(VI) removal was observed after 15 min of treatment; while at 550 V, about 35% of Cr(VI) was reduced within 5 min of treatment. The phenomena can be explained from the fact that when the voltage is lower than 400 V, GDP is not fully grown and the amount of reactive species produced in the solution was trace and the Cr(VI) reduction rate was fairly low (9). This phenomenon indirectly demonstrated that the direct Cr(VI) reduction at the cathode (electrolysis) in GDP was negligible. There was an obvious difference in the Cr(VI) reduction rate between 450 and 500 V. It has been reported that H2O2 formation rate is not influenced by the applied voltage from 420 to 500 V if the current is the same (9). This may be due to the fact that Cr(VI) is an electron scavenger, which makes the second electron emission at the solution surface reduce and the voltage for the full GDP increases (only full GDP produces the largest amount of species) (11). It can also be observed from Figure 2 that the difference in Cr(VI) removals is not obvious between 500 and 550 V. If the applied voltage is over 550 V, the anode would melt. Therefore, 500 V is chosen as the optimum applied voltage in the present study. 3.2. Effect of Initial pH on Cr(VI) Reduction. pH is an important factor in water and wastewater treatment. Figure 3 shows the effects of initial pH on Cr(VI) reduction rate in GDP. It can be observed from Figure 3 that the Cr(VI) reduction rate was higher in acidic conditions than in neutral or basic 8494

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FIGURE 8. Effect of hydroxyl radical scavengers on Cr(VI) reduction (solution volume, 150 mL; voltage, 500 V; current, 100 mA; conductivity, 5.0 mS/cm).

FIGURE 9. Typical voltage and current waveforms of GDP (solution volume, 150 mL; voltage, 500 V; current, 100 mA; conductivity, 5.0 mS/cm; the upper trace is for voltage and the bottom trace is for current). conditions. For example, only 46% of Cr(VI) can be reduced with 10 min of GDP treatment when the initial pH is 8.0, while nearly 95% of Cr(VI) can be removed when the initial pH is 2.0 within the same treatment time. The above phenomena can be explained as follows. When GDP occurred, the liquid water molecules at the plasma/solution interface were decomposed to hydrogen atoms, hydroxyl radicals, and hydronium ions during the bombardment with the energetic H2O+ ions from the plasma. The primary reaction is (9) GDP

H2O++nH2O 98 H3O+ + n · OH + (n - 1) · H

(1)

followed by, OH + · H f H2O

(2)

·OH + · OH f H2O2

(3)

H + · H f H2 v

(4)

where n is the number of water molecules decomposed per H2O+ ion. The value of n is estimated to be as high as 12 (10). From reactions 1-4, it can be seen that the major products formed in GDP are hydrogen atoms, hydrogen peroxide, and hydroxyl radicals in the liquid phase. If Cr(VI) is present in

TABLE 1. Energy Efficiencies in GDP and Photo Processes for Cr(VI) Reduction C0/mg/L 50 100 100 100 36 42 60 50

method

GCr(VI)/mg/J

GDP, 50 W, pH 2.0 GDP, 50 W, pH 5.6 GDP, 50 W, pH 2.0 GDP, 50 W, pH 2.0, 100 mg/L phenol UV, 125 W, 0.5 g/L TiO2, pH 1.0 UV, 450 W, 1.0 g/L TiO2, EDTA, pH 2.0 UV, 100 W, 12 g/L ZnO, pH 6.0 electrolysis 1.0 A, 6.6 V, pH 5.0, Fe anode

10-4

5.0 × 2.5 × 10-4 6.1 × 10-4 7.9 × 10-4 1.0 × 10-5 3.9 × 10-6 7.9 × 10-6 1.2 × 10-4

reference this this this this 24 6 25 26

work work work work

the solution, it will be reduced to Cr(III) by hydrogen atoms via the following reaction steps (18) (eqs 5-7):

due to the gases released from the reactor; however, its influence on pH is negligible):

·H + Cr(VI) f Cr(V) + H+

(5)

+ 3+ Cr2O27 +6 · H + 8H f 2Cr +7H2O

(14)

Cr(V) + Cr(V) f Cr(IV) + Cr(VI)

(6)

+ 3+ HCrO4 +3 · H + 4H f Cr +4H2O

(15)

Cr(V) + Cr(IV) f Cr(III) + Cr(VI)

(7)

+ 3+ CrO24 +3 · H + 5H f Cr +4H2O

(16)

The stoichiometry of the reduction can be given by reaction 8: 3 · H + Cr(VI) f Cr(III) + 3H+

(8)

In general, dichromate (Cr2O72-), hydrogen chromate (HCrO4-), and chromate (CrO42-) are the major forms of Cr(VI) in the present experimental conditions. The mutual relations can be described by reactions 9 and 10 (eqs 9 and 10): + CrO24 + H a HCrO4 22HCrO4 a Cr2O7 + H2O

(9) (10)

Cr2O72- is predominant in acid media and CrO42- is predominant in neutral or basic media. As the rate constant of the reaction Cr2O72- and · H (2.0 × 1010 M-1 s-1, 298 K) is higher than that of CrO42- and · H (8.2 × 109 M-1 s-1, 298 K) (19), the Cr(VI) reduction rate increases as the solution pH decreases. On the other hand, as the oxidation potential of Cr(VI) increases with the decrease of solution pH, Cr(VI) can be slowly reduced to Cr(III) by the H2O2 produced in GDP in acidic medium (eqs 11-13) (20): + 3+ Cr2O27 +14H +6e f 2Cr +7H2O Φ(V) ) 1.33 0.138 pH (11) + 4H2O2+Cr2O27 +2H f 2CrO5+5H2O

(12)

2CrO5+6H+ f 2Cr3++2H2O + H2O2+3O2

(13)

At pH > 5.0, Cr(VI) can not be reduced by H2O2 (20). Therefore, an acidic medium is favorable for Cr(VI) reduction. To elucidate the Cr(VI) reduction in GDP, variations of UV/vis spectra, pH, and conductivity of the solution during GDP treatment are presented in Figures 4, 5a, and 5b, respectively. As shown in Figure 4, the peak height at about 350 nm, which is characteristic of the absorption band of Cr(VI) species (17), gradually decays within the treatment time. In the experiment, the color of the solution changed from orange to light yellow and finally to colorless. This can be also found from the decrease in absorption band at 440 nm. It can be seen from Figure 5a that the pH of solution containing Cr(VI) increases with treatment time. This can be explained by the fact that the Cr(VI) reduction consumes hydronium ions, as the reaction 8 can be represented by the following equations (23) (The solution volume decreases with treatment time

Figure 5b shows that the conductivity of solution containing Cr(VI) decreases with the treatment time. This is because the mobility of the hydronium ions is much higher than that of Cr3+. When the Cr(VI) reduction proceeds, the concentration of hydronium ions gradually decays and the overall conductivity decreases. The conductivity of the solution without Cr(VI) increases with treatment time, presumably due to the complex reactions taking place between the sulfate ions and GDP (9). 3.3. Effect of Initial Concentration on Cr(VI) Reduction. The effect of initial Cr(VI) concentration (50-1000 mg/L) on its reduction at initial pH 2.0 is shown in Figure 6. It can be seen from Figure 6 that higher initial concentration resulted in lower Cr(VI) removal efficiency, but higher amount of Cr(VI) removed. For example, when the initial Cr(VI) concentration was 50 mg/L, 97% of Cr(VI) was removed within 5 min of treatment and the amount of Cr(VI) removed is 7.28 mg. When the initial Cr(VI) concentration increased to 1000 mg/L, about 15.3% of Cr(VI) can be removed within the same treatment time, and the amount of Cr(VI) removed increased to 22.95 mg. This phenomenon can be explained by the fact that more Cr(VI) species are available for reduction by hydrogen atoms at the higher Cr(VI) concentration (9). The gaseous products in the anodic compartment were found to be H2 and O2, whereas the gas in the cathodic compartment was H2 during GDP treatment. The amounts of H2 produced in different initial concentrations of Cr(VI) solutions with 5 min of GDP treatment in each compartment are shown in Figure 7. It can be seen from Figure 7 that the amounts of H2 from the anodic compartment decreased with increasing initial Cr(VI) concentration. As more hydrogen atoms were consumed by Cr(VI) at the higher Cr(VI) concentrations, the amounts of H2 reduced. However, the amount of H2 yielded in the cathodic compartment was not influenced by the presence of Cr(VI) or its concentration, which further demonstrates that the contribution of electrolysis for Cr(VI) reduction at the cathode in GDP was minor. 3.4. Effect of Hydroxyl Scavengers on Cr(VI) Reduction. As can be seen from reaction 1, many hydroxyl radicals were produced in the solution. Normally the hydroxyl radical is an oxidizing species, which can reoxidize the lower states of chromium to higher states (21): ·OH + Cr(III) f Cr(IV) + OH- k17 ) 3.8 × 105M-1s-1 (298 K) (17) ·OH + Cr(V) f Cr(VI) + OH- k18 ) 1.5 × 109M-1s-1 (298 K) (18) VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The Cr(IV) produced in reaction 17 can be further transformed to Cr(VI) through reaction 7. In addition, the hydroxyl radicals can react with hydrogen atoms to produce water (eq 2), which will decrease the concentration of hydrogen atoms for Cr(VI) reduction. The most desirable way is to convert the hydroxyl radicals into organic radicals by reaction with organic solutes. Generally, the organic radicals possess reduction potentials in the range of -1 to -2 V and are thus capable of reducing Cr(VI) to lower oxidation states, which not only inhibits the reoxidation of Cr(III) and increases the concentration of hydrogen radicals, but also converts the oxidizing species to the reducing ones (22): ·OH + RH f R · + H2O

(19)

R · + Cr(VI) f Cr(V) + R+

(20)

Figure 8 shows the effects of phenol and ethanol on the Cr(VI) reduction rate in GDP, respectively. It can be observed from Figure 8 that the presence of phenol or ethanol enhances the Cr(VI) reduction rate. Phenol exhibits a better enhancing effect because it is a more efficient hydroxyl radical scavenger than ethanol (eqs 21 and 22) (19):

energy efficiency. The energy efficiencies of Cr(VI) reduction in GDP are higher than those in semiconductor photocatalysis and comparable to that in electrolytic reduction with Fe electrode. It should be noted that the experimental conditions in photocatalysis or electrolysis such as initial Cr(VI) concentration and pH are not the same as those in GDP. From the above discussions and experimental results, the Cr(VI) reduction by GDP in the acidic condition can be represented by the following equation: + 3+ + Cr2O27 + xH2O + 8H f 2Cr

(x + 7) O2 + (x + 4)H2 2 (25)

where x is a constant and its value is dependent upon the experimental conditions. It can be seen from reaction 25 that the byproducts in GDP are Cr3+, O2, and H2. H2 can be collected as a clean fuel. Cr3+ can be precipitated out of solution in the form of Cr(OH)3. In general, electric field in the order of MV/cm is needed to get plasma in water (27). However, 500 V is enough for operation in GDP, which helps to effectively improve the operation safety. The present study opens a possible way to the application of GDP for Cr(VI) removal in aqueous solution.

Acknowledgments This work was supported by the National Science Foundation of China (20476092). We express our appreciation to Professor Xiangning He for allowing us to use his digital oscilloscope and Professor Jixian Pang for the previous assistance.

•

·OH + CH3CH2OH f CH3CHOH + H2O k22 ) 1.8 × 109M-1s-1 (298 K) (22) Control experiments in Figure 8 show that, without GDP, there is no obvious Cr(VI) concentration decrease in phenol or ethanol solutions, which indicates that the direct Cr(VI) reduction by ethanol or phenol is negligible. HPLC analyses show that the byproducts from phenol and ethanol are present in only trace amounts (12). 3.5. Energy Efficiency. Energy efficiency is an important factor when compared with other competitive processes. In this work, the energy efficiency for Cr(VI) reduction (GCr(VI)) can be defined as (14):

GCr(VI) )

∫

1 CV 2 0 t1⁄2

0

(23)

U(t)I(t)dt

where C0 is the initial Cr(VI) concentration (mg/L), V is the solution volume (L), U(t) and I(t) are the applied voltage (V) and current (A) at given time t (s), respectively; t1/2 is the reaction time for half Cr(VI) removal. Figure 9 shows the typical voltage and current waveforms of GDP. It can be observed from Figure 9 that the pulsations of the voltage and current are very weak in GDP (the current was measured by a voltage drop across a 1.0 Ω resistor, thus 1 mV in CH2, Figure 9 corresponds to 1 mA through the reactor). Therefore, eq 23 can be represented by 1 CV 2 0 GCr(VI) ) UIt1⁄2

(24)

Table 1 shows the energy efficiencies of this work and those in photocatalysis and electrolysis (the values are calculated based on 50% conversion of Cr(VI)). It can be seen from Table 1 that the energy efficiency of Cr(VI) reduction in GDP increases with the increase of its initial concentration. Decreasing the solution pH and adding radical scavengers to the solution also help improve the 8496

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