Ind. Eng. Chem. Res. 2005, 44, 7071-7077
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Photocatalytic Oxidation of Ni-EDTA in a Well-Mixed Reactor Philippe Salama and Dimitrios Berk* Department of Chemical Engineering, McGill University, Montreal, Que´ bec, H3A-2B2, Canada
EDTA forms a strong complex with a variety of metals. These metal-EDTA complexes are very stable and often inert to conventional biological or chemical treatment methods. Photocatalytic oxidation has shown promising results in oxidizing metal-EDTA complexes. Thus, the objective of this research was to investigate the photocatalytic oxidation of the Ni-EDTA complex. For this investigation, experiments were carried out in a semi-batch reactor equipped with up to four UV lamps (with light intensities varying from 1.6 × 10-6 to 6.4 × 10-6 einstein/min). Preliminary studies demonstrated that the photocatalytic degradation of Ni-EDTA could not take place in the absence of a source of light energy, TiO2, and oxygen. From degradation experiments, it was found that the light energy and catalyst concentration limit the production of the electron/hole pair and thus the degradation of Ni-EDTA. The effect of the initial NiEDTA concentration was also investigated. The results display Langmuir-Hinshelwood type kinetic behavior. A similar trend was observed when the dissolved oxygen concentration was varied. The oxygen and Ni-EDTA concentrations were also found to limit the degradation of Ni-EDTA. In all experiments, the total organic carbon (TOC) measurements showed that minimal mineralization of the starting Ni-EDTA took place. Introduction EDTA (ethylenediamine tetraacetic acid) is widely used as a chelating agent in industrial processes involving metal treatment, photography, textile, pulp, paper, and microelectronics manufacturing, etc. EDTA has been known for its ability to form a strong soluble complex with a wide variety of metals in aqueous solution. A common example of the use of EDTA is in the microelectronics manufacturing industries where complexing agents are often used in metal plating solutions to facilitate homogeneous metal deposition. However, once the plating solution becomes depleted, it no longer has the ability for metal deposition. Since these solutions contain some amount of EDTA-metal complexes, proper waste treatment methods must be considered before sending them to a conventional metal removal process. Nickel is a common plated metal used in the microelectronics industry. When mixed with EDTA, the formed Ni-EDTA complexes are very stable and are often inert or slowly degraded by common biological methods such as activated sludge or aerated lagoons.1-4 Hence, solutions containing EDTA and metals such as nickel must be pretreated by a process that offers strong oxidizing capabilities. One of the most successful methods is photocatalysis using a semiconducting catalyst. Photocatalytic oxidation involves the use of a semiconducting material, such as TiO2, along with enough light energy to promote an electron from the valance band to the conduction band.5 The energy requirement for this reaction is equivalent to the band gap energy level of the material used. For TiO2, the band gap energy level has a value of 3.2 eV. The produced electron/hole pairs are used as reducing/oxidizing agents. In photocatalytic oxidation reactions, the positively charged holes oxidize the organic compounds adsorbed on the catalyst surface. In addition, hydroxyl radicals are * To whom correspondence should be addressed. Tel.: (514) 398-4271. Fax: (514) 398-6678. E-mail:
[email protected].
produced from the surrounding water molecules. An aqueous system will favor hydroxyl radical production because of the large amount of water molecules present as compared to organic molecules. The hydroxyl radicals serve as a strong oxidizing agent for the conversion of organic molecules to carbon dioxide. To prevent electron/ hole recombination from occurring, the electrons must also be consumed. Oxygen molecules are converted to superoxide molecules upon reacting with electrons. Therefore, oxygen is added, as an electron acceptor, to the reaction mixture as a way to minimize the recombination of electrons and holes. Studies of photocatalytic oxidation of various organic species are available in the literature.6-10 Recently, photocatalytic oxidation of EDTA and EDTA-metal complexes has been the subject of several reports in the literature. Babay et al. showed that the oxidation rate of uncomplexed EDTA was quite fast.11 Metal EDTA complexes of zinc, copper, cadmium, lead, and iron have shown degradability by photocatalytic oxidation.12-19 However, the literature offers very little information regarding the photocatalytic oxidation of Ni-EDTA. With photocatalytic reactions, the study of the light energy absorption distribution inside the reactor plays a major role in determining the rate. Even though the chemical species can be considered to be well-mixed, the same principle does not apply to the light energy or photons supplied by UV lamps. In general, with the UV lamps fixed inside the reactor, a decrease in the photon concentration away from the lamp is expected. This decrease is the result of photon absorption by TiO2 particles suspended around the UV lamps. In addition, light may be scattered, thus affecting the light distribution. Therefore, information on the light energy absorption distribution is required to discuss the effects of the light energy concentration on the rate of Ni-EDTA photocatalytic oxidation. The objective of the present study was to investigate the photocatalytic oxidation of the Ni-EDTA complex using TiO2 irradiated with UV light. Nickel was chosen
10.1021/ie050100j CCC: $30.25 © 2005 American Chemical Society Published on Web 08/06/2005
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Figure 1. Photocatalytic reactor setup.
as the metal ion for chelation due to its popular use in various types of industries, most particularly, the microelectronics industry. The investigated variables are the catalyst loading concentration (0.5-3.0 g/L), the amount of light energy available to the reacting medium (1.6 × 10-6 to 6.4 × 10-6 einstein/min), the dissolved oxygen concentration (0.055-0.62 mmol of O2/L), and the initial Ni-EDTA concentration (100-400 mg/L). From the resulting data, specific rates of Ni-EDTA degradation were calculated and compared. The total organic carbon (TOC) was analyzed to measure the amount of Ni-EDTA converted to CO2. In addition, a study of the amount of light energy absorbed by the catalyst was also carried out to determine the effects of the catalyst concentration on the light intensity distribution at various locations inside the reactor. Materials and Methods Experimental Setup. The photocatalytic experiments were performed using a batch reactor shown in Figure 1. The reactor was made of a Pyrex glass vessel (4 L) standing in a nonrefrigerated bath with cooling water circulation. The reactor could hold up to four quartz sleeves serving to protect the UV germicidal lamps from contacting the reacting mixture. Both the UV lamps and the quartz sleeves were purchased from the Atlantic Ultraviolet Corporation. The UV lamp measured 8 and 11/32 in. and supplied UV light at a wavelength of 254 nm. This wavelength supplies sufficient energy (3.2 eV) to generate the desired electron/ hole pairs. In addition, for the excitations of Degussa P-25 TiO2, Matthew and McEvoy have shown that a light source supplying a short wavelength (254 nm) is more efficient than a longer wavelength (350 nm).20 This particular study was carried out on the photocatalytic degradation of phenol and salicylic acid. Using the ferrioxalate actinometric method, the light intensity rate supplied by each lamp was found to be 1.6 × 10-6 einstein/min.21 This method involves the photochemical reduction of Fe(III) in an acidic oxalate solution to Fe(II). From the measured Fe(II) concentration and the known quantum efficiency of the reaction, a measurement of the light intensity rate can be obtained.
Mixing in the reactor was achieved with a stirrer attached to an electric motor. Oxygen was supplied via air from a compressor. The air was fed to the reactor from a sparger located below the mixing blade. A mixture of oxygen and nitrogen gas was used for experiments carried out at higher dissolved oxygen concentrations. Experimental and Analytical Procedures. All chemicals used were purchased from Fisher Scientific or Sigma-Aldrich. Prior to each experiment, the external surface of the glass Pyrex reactor was covered with aluminum foil to reflect the UV light back inside the reactor and thus minimize any light energy losses. Using reagent grade EDTA (disodium salt) and NiSO4‚ 6H2O, the desired amount of the 1:1 Ni-EDTA complex was prepared with 2.5 L of deionized water (18 MΩ). The desired quantity of TiO2 (Degussa P-25) catalyst was then added to the Ni-EDTA complex solution, and the air flow to the reactor was turned on. The temperature for each experiment was kept at 20 °C. The solution with the catalyst in suspension was then mixed for 15 min without any exposure to light. An initial sample was taken, and the reaction was initiated by turning on the power to the UV lamps. Samples were taken at predetermined time intervals and filtered using 0.2 µm size filter paper. The oxygen concentration was monitored using a DO meter (Cole Parmer DO100) and the pH using a pH meter (Fisher Scientific AR50). Once the experiment was complete, each sample was analyzed for the Ni-EDTA concentration and TOC. High-performance liquid chromatography (HPLC) was used to quantify the concentration of Ni-EDTA. The instrument was from Agilent Technologies (HP1100). The column used was the Zorbax SB-C18 column (also from Agilent Technologies) at a temperature of 15 °C. The solvent was composed of 20 mM ammonium phosphate buffer adjusted to pH 2.0, and the flow rate of the solvent was set to 1 mL/min. The detection wavelength was set to 210 nm. The TOC of each sample was measured using a TOC analyzer (Dohrmann DC-80). The light energy reduction due to absorption by the catalyst at radial distances of 1 and 2.5 cm and various axial positions in the photocatalytic reactor was measured using the previously described actinometric technique. The measured light intensity rate reduction does not represent the true amount of light energy absorbed by the catalyst as it includes light scattering effects inside the reactor. Thus, the fraction of the supplied light energy absorbed was obtained by comparing the measured light intensity rate of a TiO2 slurry (water/ TiO2) solution to a nonslurry solution (only containing water). These measurements were performed for catalyst concentrations of 0.5 and 2.0 g/L. Figure 2 displays the reactor setup and the axial positions at which the actinometric measurements were made. Results and Discussion Preliminary Experiments. Control experiments were carried out to study the Ni-EDTA degradation when the reactor was operated in the absence of oxygen, catalyst (TiO2), or light energy. Figure 3 shows that there is very little reduction in the Ni-EDTA concentration for all three experiments (less than 10%). The slightly higher decrease in the Ni-EDTA concentration observed for the two experiments carried out with TiO2 can be attributed to adsorption on the catalyst.
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Figure 4. Fraction of starting Ni-EDTA remaining using various TiO2 loadings with light intensity rate of 1.6 × 10-6 einstein/min. (Initial Ni-EDTA concentration: 0.52 mmol/L and TiO2 concentration: 0.5-3.0 g/L.)
Figure 2. Side view of the photocatalytic reactor with axial actinometric measurement positions.
Figure 5. Fraction of starting Ni-EDTA remaining using various TiO2 loadings with light intensity rate of 3.2 × 10-6 einstein/min. (Initial Ni-EDTA concentration: 0.52 mmol/L and TiO2 concentration: 0.5-3.0 g/L.)
Figure 3. Fraction of starting Ni-EDTA remaining without O2, TiO2, or light energy. (Initial Ni-EDTA concentration: 0.52 mmol/ L; TiO2 concentration: 2 g/L; and light intensity rate: 3.2 × 10-6 einstein/min.)
Photocatalytic Degradation Experiments. To investigate the effects of catalyst concentration, light intensity rate, dissolved oxygen concentration, and NiEDTA concentration, a series of experiments was carried out as described in the Materials and Methods. From the measured Ni-EDTA concentrations, the initial rate (mmol of Ni-EDTA/L min) was calculated from the slope of the Ni-EDTA concentration plotted against time taken at time zero. The initial rate was then divided by the catalyst concentration to get the initial specific rate (mmol of Ni-EDTA/g of TiO2 min). The calculated initial specific rate was chosen to quantify the rate of photocatalytic oxidation of Ni-EDTA. The initial specific rate was obtained at the early stages of the reaction when a small change in the Ni-EDTA concentration was measured and there was no change in the composition of the reacting mixture due to the accumulation of byproducts. Thus, the obtained rates were mainly changing due to variations in catalyst concentration, light intensity rate, or dissolved oxygen concentration. The mineralization of the starting Ni-EDTA solution was monitored from the measurements of TOC. In all experiments performed with air, the pH and oxygen concentration was found to remain constant at values of around 3.3 and 9.00 mg of O2/L, respectively.
Figure 6. Fraction of starting Ni-EDTA remaining using various TiO2 loadings with light intensity rate of 6.4 × 10-6 einstein/min. (Initial Ni-EDTA concentration: 0.52 mmol/ L and TiO2 concentration: 0.5-3.0 g/L).
Effect of Catalyst Loading and Light Intensity Rate. The catalyst (TiO2) and light energy play an important role in the photooxidation of organic compounds such as Ni-EDTA. Figures 4-6 display the fraction of the starting Ni-EDTA degraded over time for the experiments carried out using various light intensity rates. Figure 4 shows that for the lowest light intensity rate, there is a 10% decrease in the Ni-EDTA concentration for the lowest catalyst concentration (0.5 g/L). This value increases to 20% when the catalyst concentration is increased to 2.0 g/L. For the light intensity rate of 3.2 × 10-6 einstein/min (Figure 5), the percent degradation of Ni-EDTA is 10 and 40% for the
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Ind. Eng. Chem. Res., Vol. 44, No. 18, 2005 Table 1. Fraction of Total Light Intensity Rate Absorbed by the Catalyst at Various Axial Positions and 1 cm from the Lamp axial position (cm) (see Figure 2) fraction of the total light intensity rate absorbed by the catalyst with 2 g/L TiO2 fraction of the total light intensity rate absorbed by the catalyst with 0.5 g/L TiO2
7.7 0.95
12.2 0.93
16.7 0.90
0.66
0.65
0.70
Table 2. Fraction of Total Light Intensity Rate Absorbed by the Catalyst at Various Axial Positions and 2.5 cm from the Lamp Figure 7. Initial specific rate of Ni-EDTA degradation at various TiO2 loadings and light intensity rates. (Initial Ni-EDTA concentration: 0.52 mmol/L and TiO2 concentration: 0.5-3.0 g/L).
catalyst concentrations of 0.5 and 2.0 g/L, respectively. The results suggest that increasing the catalyst concentration gives a larger degradation of the Ni-EDTA for both light intensity rates. These results are expected since a larger amount of TiO2 particles allows for a higher electron/hole production rate. In addition, for the low catalyst concentration (0.5 g/L), no significant variation in the degraded Ni-EDTA was observed when comparing the light rates of 1.6 × 10-6 and 3.2 × 10-6 einstein/min. The overall degradation of Ni-EDTA does not increase because of an insufficient amount of electron/hole pair generated. In contrast, with the higher light intensity rate (Figure 6), increasing the catalyst concentration does not produce a significant change in the degradation of Ni-EDTA. When comparing the largest two light intensities (3.2 × 10-6 and 6.4 × 10-6 einstein/min) at a catalyst concentration of 2 g/L, it appears that there is no change in the extent of degradation in 25 min. To further investigate these results, the initial specific rate was calculated as described before. The resulting initial specific rates for the three light intensity rates were plotted in Figure 7 for the respective catalyst concentration. In contrast to the typical catalytic reactions, when the specific rate is independent of the catalyst concentration, a decrease in the specific rate is observed when the catalyst concentration is increased. The light intensity rate has the largest effect on the specific initial rate at the lowest TiO2 concentration. This implies that proportionally more catalyst is involved in the reaction because there is more light penetration in the reactor slurry. Therefore, on one hand, increasing the catalyst concentration increases the production rate of electron/hole pairs because the number of particles exposed to the light is increased. On the other hand, the presence of extra catalyst in the reactor produces a dark region, preventing light energy from activating the catalyst particles traveling in locations of the photocatalytic reactor far away from the lamp. In the dark, the holes quickly recombine with the electrons. This recombination of electron-hole pairs also explains that rates shown in Figure 7 are not greatly affected when the light intensity rate is increased from 3.2 × 10-6 and 6.4 × 10-6 einstein/min. Although a larger number of electron hole pairs is generated at the higher level, because of the intense mixing in the reactor, the catalyst is transferred to the dark region where the recombination occurs before the reactions leading to the oxidation of EDTA. Thus, the difference
axial position (cm) (see Figure 2) fraction of the total light intensity rate absorbed by the catalyst with 2 g/L TiO2 fraction of the total light intensity rate absorbed by the catalyst with 0.5 g/L TiO2
7.7 0.94
12.2 0.95
16.7 0.92
0.71
0.68
0.69
in the rates of EDTA degradation between these two high energy input levels is small. The presence of the dark region was confirmed by carrying out an actinometric study of the light energy absorbed by the catalyst close to the lamp. Tables 1 and 2 present the fraction of the total light intensity absorbed by the catalyst at three different axial positions and radial distances of 1 and 2.5 cm from the lamp. These values were obtained from experiments carried out with a catalyst (TiO2) concentration of 0.5 and 2.0 g/L as described in the Materials and Methods. The results show no significant variation in the absorbed light energy in the axial direction, indicating that the lamp is supplying light energy at a constant intensity and rate along its length. When comparing the two catalyst concentrations, an increase in the fraction of absorbed light energy is found when increasing the catalyst concentration. These results are expected considering that a higher catalyst concentration will offer a higher probability for photon absorption. Furthermore, for both catalyst concentrations, a relatively high fraction of the supplied light energy is absorbed at a very short distance from the lamp (1 cm). This indicates that a large portion of the reactor is not used in the photocatalytic degradation of Ni-EDTA. At a radial distance of 2.5 cm from the lamp, the highest catalyst concentration shows no significant decrease in the absorbed light energy when compared to the lower distance of 1 cm (as shown in Table 2). These results would indicate that since most of the photons have been absorbed by the catalyst within a close distance from the lamp (1 cm), not much are left to be absorbed within the remaining distance. The small unabsorbed light energy (>10%) would most likely be caused by scattering effects. When the catalyst concentration was decreased to 0.5 g/L, again no significant change in the light absorption was observed. However, when increasing the distance to 2.5 cm, the value of absorbed light (60%) did not increase further as would be expected from an absorbing medium. This expected increase in the absorbed light does not occur because of the high degree of scattering taking place within the 2.5 cm distance. The results show that light scattering effects are more significantly present at lower catalyst concentrations when more light can penetrate the reacting mixture.
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Figure 8. Fraction of starting Ni-EDTA remaining using various initial dissolved oxygen concentrations. (Initial Ni-EDTA concentration: 0.52 mmol/L; TiO2 concentration: 2.0 g/L; and light intensity rate: 6.4 × 10-6 einstein/min.)
Figure 10. Fraction of initial TOC reduction for experiments at various TiO2 loadings and light intensity rate of 6.4 × 10-6 einstein/min. (Initial Ni-EDTA concentration: 0.52 mmol/L and TiO2 concentration: 0.5-3.0 g/L.) Table 3. Fraction of Initial TOC for an Experiment Lasting 11 h time (h) [TOC]/[TOC]o
Figure 9. Initial specific rate of Ni-EDTA degradation at various initial dissolved oxygen concentrations. (Initial Ni-EDTA concentration: 0.52 mmol/L; TiO2 concentration: 2.0 g/L; and light intensity rate: 6.4 × 10-6 einstein/min.)
Dissolved Oxygen Concentration. As mentioned in the Introduction, oxygen was added as an electron acceptor to minimize the electron/hole pair recombination. Therefore, experiments at different oxygen concentrations were carried out by varying the supply of an oxygen/nitrogen gas mixture. The measured concentration of oxygen was found to remain constant for all experiments. Therefore, the oxygen utilization rate from electron scavenging was considerably slower than the oxygen supply rate. Figure 8 displays the fraction of initial Ni-EDTA concentration degraded over time at six different dissolved oxygen concentrations. The results show that increasing the oxygen concentration increases the amount of Ni-EDTA degraded over time. Once again, the initial specific rate of Ni-EDTA degradation was calculated. The obtained initial specific rate at each initial dissolved oxygen concentration was plotted in Figure 9. The specific rate plot indicates that the dissolved oxygen concentration seems to exhibit Langmuir-Hinshelwood type kinetics. Similar results were obtained for the photocatalytic degradation of phenol.8 At a high oxygen level (
