Environ. Sci. Technol. 1997, 31, 3475-3481
Oxidation of Metal-EDTA Complexes by TiO2 Photocatalysis THOMAS H. MADDEN, ABHAYA K. DATYE,* AND MELISSA FULTON Center for Micro-Engineered Materials and Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131-1341
TABLE 1. Photocatalytic Fates and EDTA Stability Constants for the Metals Used in This Study metal cation
photocatalytic fate
log EDTA Ks (17)
Cr(III) Cu(II) Ni(II) Pb(II) Zn(II) Cd(II)
no photodeposition (9) reductive photodeposition (9, 23) no photodeposition (9) oxidative photodeposition (7, 8) no photodeposition (9) no photodeposition (9)
23.40 18.80 18.62 18.04 16.50 16.46
MICHAEL R. PRAIRIE, SABIR A. MAJUMDAR, AND BERTHA M. STANGE Solar Thermal Technology Department, Sandia National Laboratories, MS 0703, Albuquerque, New Mexico 87185-0703
Ethylenediaminetetraacetic acid (EDTA), a common industrial agent for complexing metal ions in water, frequently inhibits conventional metals-removal technologies used in water treatment. This study investigated the use of TiO2 photocatalysis for the aqueous-phase oxidation of EDTA and several metal complexes of EDTA. Reactions were performed at 0.1 wt % loading of Degussa P-25 TiO2, a solute concentration of 0.8 mM and at a constant pH. The different metal-EDTA complexes exhibited widely different photocatalytic oxidation rates under equivalent conditions of pH ) 4 ( 0.1 in an aerobic system: Cu(II)-EDTA > Pb(II)EDTA >> EDTA > Ni(II)-EDTA ≈ Cd(II)-EDTA ≈ Zn(II)EDTA >>> Cr(III)-EDTA. Photoefficiency based on the Cu(II)-EDTA initial rate is nearly 60%. The rates of total organic carbon (TOC) removal and formaldehyde generation during photocatalytic EDTA oxidation indicate similarities to electrochemical oxidations of EDTA. Several means were explored to enhance the oxidation of Ni(II)-EDTA, whose behavior was taken to represent that of the slowly oxidizing complexes. Continuous addition of H2O2 solution during the photocatalytic treatment increased the oxidation rate for Ni(II)-EDTA as did the presence of homogeneous Cu2+. The presence of Cu2+ led to rapid ligand exchange transforming the Ni(II)-EDTA into Cu(II)-EDTA, which is easily oxidized.
Introduction Organic complexing agents are widely used in industrial applications involving dissolved metals. These chemicals are generally added to increase the metal ion solubility in aqueous solutions. Organic acids such as tartaric acid, oxalic acid, and ethylenediaminetetraacetic acid (EDTA) are used in electroless plating to facilitate homogeneous metals deposition (1). Processes that involve the removal of metal oxide scale from heat transfer surfaces, such as in boilers or nuclear reactors, also use these agents for their ability to dissolve metals (2). Recent publications in the environmental literature have also reported the use of organic complexing agents for removing toxic metals from contaminated soils. However, complexing metal ions with organics often results in increased inertness with respect to water treatment, and many common treatment technologies are unable to * Corresponding author telephone: (505)277-0477; fax: (505) 2771024; e-mail:
[email protected].
S0013-936X(97)00226-5 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Schematic of the reactions at the surface of illuminated TiO2. sufficiently remove these metals from wastewater streams. Precipitation technologies for metals removal, such as hydroxide and sulfide precipitation, are inhibited when the metals exist as metal-organic complexes (3). This is the case for EDTA, which forms stable 1:1 complexes with many metals over broad pH ranges. Specialized treatment processes can be employed for metal-EDTA complexes but are only applicable to certain complexes in specific situations (4). Therefore, finding a treatment technology able to treat many common metal-EDTA complexes is of interest. This study discusses the conditions for effective photocatalytic oxidation of EDTA and metal-EDTA complexes. For the metal-EDTA complexes, our study used several metals of environmental concern. The common fates of these metals as seen in previous photocatalytic studies are shown in Table 1. We have previously reported on some aspects of the photocatalytic treatment of metal-EDTA complexes (5). We showed that the byproducts of metal-EDTA oxidation by photocatalysis have a reduced capacity for binding these metals. Therefore, the metals can be more easily removed following an oxidative treatment to partially modify the EDTA ligand. Assessing the practical potential for photocatalysis to perform the EDTA oxidation is the continuing focus of our work. TiO2 photocatalysis, as illustrated in Figure 1, is unique among advanced oxidation processes in that simultaneous reduction and oxidation reactions can be performed on a particle of TiO2. The band gap of anatase TiO2 is 3.2 eV; therefore, excitation by UV radiation of wavelength 390 nm or less produces electron (e-) - hole (h+) pairs. The holes at the TiO2 valence band, having an oxidation potential of +2.6 V vs normal hydrogen electrode (NHE) at pH ) 7 (6), can migrate to the catalyst surface and either (a) oxidize an adsorbed species directly by direct hole oxidation, (b) oxidize water or hydroxide to produce hydroxyl radicals, which then proceed to oxidize other species, or (c) oxidize other species in solution, such as Pb2+ (7, 8). The electrons migrate to the surface to take part in reduction reactions. Having a reduction
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potential of -0.4 V vs NHE at pH ) 7 (6), the electrons can reduce a number of oxidants. These oxidants include oxygen, which can form superoxide or hydroperoxyl radicals, or certain aqueous metals that can be reduced to their metallic form onto the catalyst (9, 10). Several studies have shown that the rates of the electron reduction reaction and the hole oxidation reaction are intrinsically related (9, 11, 12). As in Figure 1, an oxidation reaction must occur simultaneously with a reduction reaction, or otherwise buildup of electrons or holes within the TiO2 particle will increase recombination rates, and lower photoefficiency.
Experimental Section Materials. All of the photocatalysis experiments were carried out in a typical recirculating photoreactor provided by the National Renewable Energy Laboratory (NREL) (5, 13). The reaction mixture was pumped from a large mixing vessel through the illuminated cylindrical annular reactor and back to the mixing vessel. The UV light source in the annulus center was a 13 in. long, 15 W, Phillips cylindrical blacklight. A relative emission spectrum recorded on a Hewlett-Packard 8452A diode-array UV/vis spectrophotometer showed that nearly all of the UV light is generated between 350 and 400 nm from a single band peaking at 368 nm. Other minor peaks were evident at 406 and 436 nm. Ferrioxalate actinometry measurements indicated a photon generation rate of 3.97 × 10-4 Einstein/min. Assuming monochromatic emission at 365 nm, the calculated power of 2.17 W indicates a lamp efficiency of 14.4%. This value is in good agreement with actinometry measurements performed on similar lamps (14). The 10 L/min flow rate from the 1/15 HP pump results in a calculated Reynold’s number of 4494 for flow in the annulus (15). The turbulent flow thus ensured efficient introduction of the solution and catalyst into the illuminated region. The inner annulus was kept free of TiO2 powder deposits by the use of an innovative interval scrubber to clean the surface every 10 min. Prior to the installation of the scrubber, we noticed that the different complexes resulted in different degrees of TiO2 deposition onto the surface. Although this TiO2 could be considered to be illuminated, the illumination and mass transfer characteristics of a thin film are different than that of a slurry, and different degrees of deposition could interfere with the ability to distinguish other kinetic phenomena. The inner tube was examined at the end of each run to ensure that only negligible film buildup had occurred, and 10-min scrubbing intervals were found to be adequate. Every reaction was performed with a 1300-mL solution charge to the reactor, which is the maximum volume of the reactor. All solutions of EDTA or of metal complexed 1:1 molar with EDTA were introduced initially at 0.8 mM. These concentrations correspond to around 50-150 mg/L of metal, depending on the metal. The following reagents were used: Na2EDTA, Ni(NO3)2‚6H2O, Pb(NO3)2, CuSO4‚5H2O or Cu(NO3)2‚2.5H2O, Cd(NO3)2‚4H2O, Cr(NO3)3‚9H2O, and Zn(NO3)2‚6H2O. All reagents were from J. T. Baker and were of reagent grade. The metal nitrate salts and dilute nitric acid (pH control) were used because the NO3- anion was shown to have very little effect on the photocatalytic degradation of salicylic acid, aniline, and ethanol at nitrate concentrations up to 0.1 M (16). Furthermore, there is no overlap between the absorbance bands of NO3- and the emission of our lamp for the NO3- concentrations used. This rules out significant contributions from NO3- related photochemistry. The reagents were introduced into the reactor as diluted stock solutions in deionized water. Degussa P-25 TiO2 was added to a concentration of 0.1 wt % in all reactions involving catalyst. CHN elemental analysis performed on virgin catalyst indicated a 0.069 wt % carbon average for three samples. A 45-min period of no-illumination prior to the reaction allowed for any complex adsorption onto the TiO2. This period was justified by adsorption studies that are discussed later.
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Experiments were generally conducted open to ambient air. Some experiments were conducted with an oxygen purge and are appropriately indicated when referenced. The mixing with the ambient gas took place in the mixing vessel where good entrainment of the gas in the liquid could be observed. The 10-mL samples were taken from the mixing vessel at specified times and were filtered through Gelman 0.2 µm Teflon syringe filters prior to analysis. The pH was maintained constant at (0.1 pH unit of the indicated value throughout the experiments in the mixing vessel by dropwise manual addition of 1.0 M HNO3 or 0.5 M NaOH solutions. Experiments were performed at pH ) 4 or greater as indicated. A pH of 4 was not low enough to disrupt any metal ion complexation by EDTA through protonation of the EDTA carboxylic acid sites. Hence all metal-EDTA experiments involved the fully complexed metal and not a mixture of complexed metal, free metal ion, and free EDTA. The relevant equilibrium equations were solved for this determination (15, 17). The extent of metal-EDTA complexation is determined by the stability of the metal-EDTA complex, reflected in the stability constant reported in Table 1 (17). Analytical Methods. Metal-EDTA complex concentrations were usually measured using a Dionex ISP 2000 ion chromatograph with an AS4A column and conductivity detector. Cr(III)-EDTA concentrations were measured on a Hewlett-Packard 8452A diode-array spectrophotometer at a peak absorbance of 544 nm and a molar extinction coefficient of 166.5 AU‚L/mol‚cm. When applicable, the concentration of the metal ion in solution was measured using a PerkinElmer Model 303 atomic absorption spectrophotometer. The aforementioned techniques were all calibrated using a series of standard metal-EDTA solutions prepared from the reagents and deionized water. The conversion of the EDTA ligand to inorganic carbon species was determined using a Shimadzu 5000 total organic carbon (TOC) analyzer. This instrument was calibrated with standard solutions of potassium hydrogen phthalate. Amounts of carbon on the catalyst were determined using a Perkin-Elmer Model 2400 CHN elemental analyzer. Formaldehyde was analyzed as the dinitrophenylhydrazine (DNPH) derivative using a Waters liquid chromatography unit with a UV/Vis monochromator detector. A Keystone Scientific Deltabond AK column was used for the separation. The derivitization procedure was provided by Waters technical support and was based on established references in this area (18, 19). Standard solutions of formaldehyde were derivitized in triplicate and measured using HPLC calibrated with solutions made from standard derivitized-formaldehyde solution purchased from Radian. The indicated recovery was 93%, so the quoted formaldehyde concentrations reflect this. Ferrioxalate actinometry was used to determine the output of the UV lamp used; the procedure was based upon classic papers on the subject (20, 21) with modifications (15).
Results and Discussion Effect of Complexed Metal on the EDTA Ligand Oxidation. Other studies have shown that dissolved metals can influence oxidation rates of organics in solution (9, 22-24). It was of interest to see if a metal complexed by EDTA would also affect its oxidation rate. Experiments were conducted on different metal-EDTA complexes and free EDTA under the same conditions of pH ) 4 and ambient air to determine their relative rates of photocatalytic oxidation and to characterize the metal ion influence. The experimental results are shown in Figure 2, where the compounds are listed in the legend in the order of decreasing disappearance rate. Two points for each series appear at t ) 0 for concentrations measured before and after the 45-min adsorption period. The Cu(II)-EDTA and Pb(II)-EDTA complexes are photocatalytically oxidized much faster than uncomplexed EDTA,
TABLE 2. Summary of Net % C on TiO2 Following the Given Reactions reaction (see Figure 2) Cu(II)-EDTA, pH ) 4, aerobic Pb(II)-EDTA, pH ) 4, aerobic EDTA, pH ) 4, aerobic Cd(II)-EDTA, pH ) 4, aerobic Ni(II)-EDTA, pH ) 4, aerobic
mg of C on TiO2 mg of C added to soln
% C on TiO2 after reaction
0.016
0.157
0.018
0.174
0.038
0.364
0.041
0.392
0.020
0.195
FIGURE 2. Comparison of disappearance rates for various metalEDTA complexes (0.8 mM each, pH ) 4, open to air).
FIGURE 4. Rates of EDTA and Ni(II)-EDTA disappearance, performed at the indicated pH values (0.8 mM, open to air).
FIGURE 3. Correlation of the average rates of TOC reduction to initial disappearance rates from Figure 2. whereas the Ni(II)-EDTA, Cd(II)-EDTA, and Zn(II)-EDTA complexes are oxidized much slower than uncomplexed EDTA. Interestingly, Cr(III)-EDTA seems unaffected by photocatalytic treatment. The TOC was also measured throughout these experiments as well. Figure 3 compares the average rates of TOC disappearance with the calculated initial rates of the compound disappearance. There is a positive correlation between the average rates of TOC removal and initial complex disappearance. At these conditions, no disappearance of any metal from the solution as measured by AA was observed with the exception of Pb, for which about 20% of the 0.8 mM Pb had disappeared from solution after 50 min. For some of the compounds in Figures 2 and 3, the net wt % C on the TiO2 following the experiments was determined. The catalyst samples were air-dried without washing and analyzed by CHN elemental analysis following the reaction. These data are given in Table 2. Blank Experiments. Some blank experiments were also performed to rule out significant contributions from homogeneous photochemistry. Experiments with the UV on and no photocatalyst were performed with Cu(II)-EDTA, Pb(II)EDTA, and Ni(II)-EDTA at otherwise equivalent conditions to those in Figures 2 and 3. No disappearance was seen for Cu(II)-EDTA or Ni(II)-EDTA over a 120-min period. Pb(II)EDTA showed ∼22% reduction after about 50 min, although no significant reduction was seen until after 20 min. As discussed later, in one experiment the photocatalytic oxida-
tion of Ni(II)-EDTA was augmented by a continuous addition of H2O2. Therefore, an equivalent experiment without photocatalyst was performed to rule out the possibility of significant UV-H2O2 chemistry. Only a 10% disappearance of Ni(II)-EDTA was seen over a 50-min period. No blank experiments were performed with uncomplexed EDTA or the other complexes. Although homogeneous photochemistry effects were present in some cases, their effect on the overall degradation rates are minor as compared to that of photocatalysis. Metal-EDTA Complex Adsorption Experiments. Adsorption experiments were performed to assess the time required for complete dark adsorption to occur. Cu(II)EDTA, Pb(II)-EDTA, Ni(II)-EDTA, and Cr(III)-EDTA at 0.8 mM were allowed to equilibrate with 0.1% wt TiO2 in a dark, agitated pot. In this case, AA was used to measure the M-EDTA concentration on filtered samples before and after TiO2 addition and thereby to determine the extent of adsorption. Fourty-five minutes was found to be the maximum time that any change could be detected. The extents of adsorption at pH ) 4 for Cu(II)-EDTA, Pb(II)-EDTA, Ni(II)-EDTA, and Cr(III)-EDTA (in order of decreasing oxidation) were approximately 0.044, 0.096,
