Environ. Sci. Technol. 2000, 34, 3789-3795
Photocatalytic Oxidation of Cu(II)-EDTA with Illuminated TiO2: Kinetics JAE-KYU YANG AND ALLEN P. DAVIS* Environmental Engineering Program, Department of Civil and Environmental Engineering, University of Maryland, College Park, Maryland 20742
Wastewater containing metal ions complexed with ethylenediaminetetraacetic acid (EDTA) is difficult to treat using conventional metal-removal techniques. In this work, photocatalytic oxidation kinetics of Cu(II)-EDTA using illuminated TiO2 has been evaluated with variation of Cu(II)-EDTA concentration from 2 × 10-5 to 1 × 10-3 M at pH 6 and with variation of pH from 3 to 8 at 10-4 M Cu(II)EDTA. Although the degree of adsorption varied from >50% to near zero, overall PCO rates varied only by a factor of 2, suggesting two parallel pathways: reaction on the surface and reaction with PCO-formed radicals in solution. Independent of pH, the initial degradation rate of Cu(II)-EDTA was well described by the expression, -dC/dt ) k1Caq/(1 + k2Caq) + kadsCads, where k1, k2, and kads are rate constants and Caq and Cads are aqueous and adsorbed concentrations of Cu(II)-EDTA, respectively. The success in describing the initial degradation rate of aqueous Cu(II)-EDTA by a hyperbolic rate expression suggests that [OH•]aq is a limiting reactant at higher Cu(II)EDTA concentrations. The initial rates of adsorbed Cu(II)EDTA were proportional to the adsorbed Cu(II)-EDTA concentration at different suspension pH values. Little pH dependence of the total degradation rate of Cu(II)EDTA was observed below pH 5. However, decreased destruction rates occurred above pH 5 where oxidation of Cu(II)-EDTA occurred primarily in the aqueous phase. During anaerobic photocatalysis, Cu2+ acted as an electron scavenger, albeit inefficient, instead of oxygen. Cu2+ that was decomplexed via PCO becomes adsorbed on the TiO2 at higher pH.
Introduction Several types of waste mixtures, including toxic/heavy metals and complexing agents such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), have been disposed at U.S. DOE waste burial sites and underground storage tanks (1). The fate of metals in groundwater and soils near waste sites can be greatly affected by the pH and the presence of complexing agents. Generally, the mobility of metal ions in subsurface systems is retarded due to significant adsorption. Chelating agents, however, can cause profound environmental problems by forming stable metal-organic complexes and increasing the mobility of metals to adjacent groundwater and soil systems (1-3). * Corresponding author phone: (301)405-1958; fax: (301)405-2585; e-mail:
[email protected]. 10.1021/es990874p CCC: $19.00 Published on Web 08/04/2000
2000 American Chemical Society
Due to the their stability, metal-EDTA complexes in water are difficult to treat using conventional metal-treatment technologies. As a promising technique, recently photocatalytic oxidation (PCO) with illuminated TiO2 has been evaluated for treating waters contaminated with metal-EDTA (4-7). Advantages of PCO for the treatment of metal-EDTA are (1) decomplexation of the metal, (2) partial mineralization of the EDTA to CO2 and other simple inorganic ions, and (3), if desired, removal of the resultant free metal ions via adsorption at neutral-high pH. Although a few studies have discussed the oxidation products and reaction pathways of EDTA or metal-EDTA (4-11), little information is available detailing the kinetics of metal-EDTA PCO. Madden et al. (4) studied the PCO of several types of metal-EDTA at 0.8 mM, pH 4 and found that the reactivity followed: Cu(II)-EDTA > Pb(II)-EDTA . EDTA > Ni(II)-EDTA ≈ Cd(II)-EDTA ≈ Zn(II)-EDTA . Cr(III)-EDTA. Kagaya et al. (5) reported that 3 mM Cu(II)EDTA decomposed completely after 20 min PCO at pH 5-6; however, residual total organic carbon (TOC) remained after 5 h PCO time. Davis and Green (6) and Vohra and Davis (7) investigated the PCO of Cd(II)-EDTA and Pb(II)-EDTA, respectively, with variation of pH and concentration. The total observed initial removal rates of both chelates were essentially independent of pH from 3 to 7. The present study expands upon the previous work through investigation of the PCO of Cu(II)-EDTA with variation of Cu(II)-EDTA concentration and suspension pH in the presence or absence of oxygen. The degree of Cu(II)EDTA adsorption varies significantly over the pH range of 3-8 (12), and the corresponding effect on PCO rates is studied. The focus is on developing a mathematical expression to describe the oxidation kinetics of Cu(II)-EDTA considering parallel reaction pathways for aqueous and adsorbed substrate. The feasibility of treating Cu(II)-EDTA using illuminated TiO2 is discussed in terms of the resulting rate expression and the fate of the metal and EDTA.
Experimental Section Experimental Procedure. All chemicals were analytical grade, and all solutions were prepared with deionized water (18 MΩ) from a Hydro-Service reverse osmosis/ion exchange apparatus (Model LPRO-20). The PCO experiments were conducted using procedures described in previous papers (6, 7). The UV source was a Spectronics lamp containing two integrally fitted UV tubes (15 W each, model XX-15A) with maximum emission at approximately 365 nm. Using modified ferrioxalate actinometry measurements (4), a photon generation rate of 1.3 × 10-4 einstein/min-L was determined at a flow rate of 75 mL/min. Assuming monochromatic emission at 365 nm, the calculated power of 0.50 W indicated a lamp efficiency of 1.7%. P-25 TiO2 (a 20/80 mixture of rutile and anatase) was obtained from Degussa Corp. Cu(II)-EDTA solution was prepared by dissolving equimolar amounts of Cu(ClO4)2‚6H2O (Aldrich Chemicals) and EDTA (disodium salt, J. T. Baker) in deionized water. NaClO4‚H2O (Fisher Scientific) was employed to provide a fixed ionic strength of 3 × 10-3 M. Before all experiments, TiO2 aqueous slurry without Cu(II)-EDTA was purged with oxygen for 1 h while being exposed to the UV light in order to remove any trace residual organic material in the system. Subsequently, the light was turned off, and previously prepared (concentrated) Cu(II)EDTA was added and mixed with the TiO2 suspension to VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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produce the desired concentrations of each (670 mL total volume). The slurry pH was adjusted and maintained using dilute HClO4 (Fisher Scientific) or NaOH (J. T. Baker). The TiO2 suspension was circulated at a flow rate of 75 mL/min. After 10 min, a suspension sample was taken as a control, and the photocatalytic oxidation was initiated by turning on the UV source. In experiments to measure CO2 production during PCO, O2 purging was stopped just before Cu(II)EDTA addition, and the system was tightly sealed using Parafilm. This sealing was confirmed by formation of positive pressure, visible through rise of acid/base solution in a pipet inserted into the reactor, and by total carbon balances (13). Although approximately 5.7 × 10-4 mol of oxygen are required for the stoichiometric mineralization of 670 mL 10-4 M EDTA to CO2 and ammonium, the expected initial dissolved oxygen is 8.4 × 10-4 mol after purging with pure oxygen. Also 80 mL of headspace can provide 3.3 × 10-3 mol of oxygen in the mixing reactor. Consequently, oxygen is not expected to be limited in the closed system. In other experiments, O2 purging continued throughout the entire run. PCO results from initial O2 purging only matched well with those found employing continuous O2 addition (13). To obtain adequate data points, two PCO runs were made for each experimental condition. Throughout each experiment, five or six samples (∼20 mL each), at different times for each PCO condition, were collected and immediately filtered through 0.2 µm membrane filters (Gelman Sciences) to separate the TiO2. All PCO experiments were completed at room temperature (22-25 °C). Analytical Methods. Cu(II)-EDTA concentrations were measured using a Dionex Model DX-100 ion chromatograph (IC) with a conductivity detector. An IonPac AS-5 anion column and an AG-5 guard were used employing an eluent of 1.3 mM Na2CO3/1.5 mM NaHCO3 (both J. T. Baker). The detection limit was 5 × 10-7 M Cu(II)-EDTA, and linear calibration was obtained through 10-4 M. A second linear calibration was used to measure Cu(II)-EDTA above 10-4 M. Concentrations of free cupric ion (Cu2+) were obtained from specific Cu2+ electrode (Orion) measurements. Total dissolved copper (Cu(II)) concentrations were measured using a Perkin-Elmer Model 5100ZL atomic absorption spectrophotometer (AAS). The optimum analytical concentration range using the flame method was from 1.6 to 63 µM. Dissolved organic carbon (DOC) was measured as total carbon using a Shimadzu total organic carbon analyzer (TOC, Model 5000). Experimental TOC recovery of 10-4 M EDTA was 12.3 ( 0.3 mg/L compared to the theoretical value of 12.0 mg/L. The TOC detection limit was 0.4 mg/L. DOC calibration was obtained with potassium biphthalate. Throughout selected PCO experiments, adsorbed Cu(II)EDTA was found by adding a few drops of 0.1 M NaOH to a portion of TiO2 slurry samples to desorb Cu(II)-EDTAads from the TiO2 at pH >8. After mixing for a short time, these samples were also filtered and dissolved Cu(II)-EDTA determined. Cu(II)-EDTAads was calculated as the difference between these measured values and the original concentrations. Greater than 96% desorption and recovery of 10-4 M Cu(II)-EDTA was observed at the initial condition. Aqueous carbon dioxide was analyzed in-situ employing a CO2 specific probe (Fisher Scientific) along with an Orion 501 digital analyzer. Calibration was obtained employing 10-4, 10-3, and 10-2 M HCO3- with adjustment to pH 4.0 and ionic strength 3 × 10-3 M. Reported CO2 measurements represent the CO2 remaining in the aqueous and headspace of the reactor during PCO time. System Species Analysis. Following analytical and data analysis procedures similar to Davis and Green (6) and Vohra 3790
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TABLE 1. Linear Kads Values for Copper Speciesa KadsCu(II)-EDTA pH t ) 0 min t ) 5 min t ) 10 min t ) 20 min t ) 60 min 3 4 5 6 7
1.88 1.07 0.34 0.06 0.05
2.45 NDb NDb NDb NDb
3.82 1.51 0.30 0.04 NDb
NDb 2.79 0.67 0.07 NDb
NDb NDb NDb 0.14 NDb
a Values for t > 0 correspond to determinations after PCO. not determined.
KadsCu2+
