Environ. Sci. Technol. 2000, 34, 3796-3801
Photocatalytic Oxidation of Cu(II)-EDTA with Illuminated TiO2: Mechanisms JAE-KYU YANG AND ALLEN P. DAVIS* Environmental Engineering Program, Department of Civil and Environmental Engineering, University of Maryland, College Park, Maryland 20742
Photocatalytic oxidation (PCO) using illuminated TiO2 was employed as a treatment procedure for Cu(II)-EDTA; heavy metal-chelates pose unique challenges for treatment and removal processes. The appearance and concentration of reaction products and intermediates were determined with variation of pH from 4 to 7 at 10-4 M Cu(II)-EDTA. From the mineralization of the EDTA (ethylenediaminetetraacetic acid), CO2 and NH4+ were formed as the major carbon and nitrogen species, respectively. Formaldehyde, formate, acetate, oxalate, and nitrate were minor products. Low concentrations of Cu(II)-IDA (iminodiacetic acid) were found at pH 7 only. The production ratio of CO2 to formaldehyde ranged from 4:1 to 10:1. A closed carbon mass balance was obtained with CO2, dissolved organic carbon, and adsorbed organic carbon during PCO. Lack of nitrogen recovery from total Kjeldahl nitrogen (TKN) and NO3measurements suggests the production of gaseous nitrogen compounds. It is proposed that PCO reactions are initiated at either a carboxyl or amine group, depending on pH and whether the Cu(II)-EDTA is adsorbed or in solution. At lower pH, the PCO reaction proceeded favorably via radical formation at a carboxyl group complexed to the TiO2 surface. Production of Cu(II)-IDA and oxalate only at pH 7 suggests some contribution from reaction at an amine group at neutral pH.
Introduction Photocatalytic oxidation (PCO) with illuminated TiO2 has recently received attention for treating water contaminated with metal-EDTA (ethylenediaminetetraacetic acid) complexes (1-5). A number of advantages are realized by this application. Although these works have identified some reaction products and reaction pathways have been discussed, little information has been presented on carbon and nitrogen balances during the PCO of metal-EDTA. Furlong et al. (6) reported the production of formic acid and formaldehyde from the PCO of EDTA using platinized TiO2. Equal production of CO2 and formaldehyde from the initial attack at an acetate group on the metal-EDTA complex has been reported (1, 4). Davis and Green (3) investigated the PCO of Cd(II)-EDTA with variation of pH and concentration and reported the formation of formaldehyde, formic acid, and acetic acid as major products. From the incomplete carbon balance and the lack of nitrate accumulation, they suggested the production of organic amines from the degradation of Cd(II)-EDTA. * Corresponding author phone: (301)405-1958; fax: (301)405-2585; e-mail:
[email protected]. 3796
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In our previous paper, in which the PCO kinetics of Cu(II)-EDTA with variation of pH and concentration were investigated, separate adsorbed and solution-phase pathways were proposed. The present study investigates degradation mechanisms of EDTA through the identification of reaction products and intermediates with a focus on closing carbon and nitrogen mass balances.
Experimental Section Experimental Procedure. The PCO experiments were conducted using procedures described in previous papers (35). The UV source was a Spectronics lamp containing two UV tubes (15 W each, model XX-15A) with maximum emission at approximately 365 nm. A photon generation rate of 1.3 × 10-4 einstein/min‚L was estimated using actinometric measurements (5). Cu(II)-EDTA solution was prepared by dissolving equimolar amounts of Cu(ClO4)2‚6H2O (Aldrich Chemicals) and ethylenediaminetetraacetic acid disodium salt (J. T. Baker) in deionized water. NaClO4‚H2O (Fisher Scientific) or NaCl (J. T. Baker) was employed to provide a fixed ionic strength of 3 × 10-3 M. The pH of the TiO2 slurry (670 mL total volume, 2 g/L P-25, Degussa Corp.) was adjusted using dilute HClO4 (Fisher Scientific), HCl (Fisher Scientific) or NaOH (J. T. Baker) solutions depending on the species used for background electrolyte. To obtain adequate data, two PCO runs were made for each experimental condition (except for CO2 production). Throughout each experiment, five or six samples (∼20 mL each) were collected (at different times for each experiment) and immediately filtered through 0.2 µm membrane filters (Gelman Sciences) to separate the TiO2. Analytical Methods. Cu(II)-EDTA, Cu(II)-IDA (iminodiacetic acid), nitrate, formate, acetate, and ammonium concentrations were measured using a Dionex Model DX100 ion chromatograph (IC) with a conductivity detector. Cu(II)-EDTA, Cu(II)-IDA, and nitrate were analyzed with an IonPac AS-5 anion column and an AG-5 guard employing an eluent of 1.3 mM Na2CO3/1.5 mM NaHCO3 (both J. T. Baker). For analyses of formate and acetate, an AS-4A analytical column and guard were used employing 5 mM borate (Na2B4O7‚10H2O, Fisher Scientific). Ammonium was measured using a CS-12A analytical column and guard with an eluent of 5.5 mN H2SO4 (Fisher Scientific). Standards for these analyses included the following: CH3COONa‚3H2O (J. T. Baker), HCOONa (J. T. Baker), NH4Cl (Fisher Scientific), and KNO3 (Fisher Scientific). Concentrations of Cu2+ (free cupric ion), Cu(II) (total dissolved copper), TOC, and CO2 were measured using ion selective electrode, atomic absorption spectrophotometry, TOC analyzer, and CO2 electrode, as described in the previous paper (5). As in Vohra and Davis (4), conversion of CO2 to HCO3-/CO32- was considered in the total CO2 production for PCO above pH 4. Also, CO2 in the reactor headspace was calculated by considering the headspace volume (from initial 80 mL to final 200 mL) and Henry’s constant (10-1.5) and included in total CO2 production. Formaldehyde was quantified by colorimetric analysis using Nash reagent (7). Three milliliters of the Nash reagent was mixed with 3 mL of sample and kept at 60 °C for 1 h. A Spectronics 21 UV-vis spectrophotometer was employed for absorbance measurements at 415 nm. HCHO (J. T. Baker) standards used were in the range of 1.1 × 10-5 M to 1.3 × 10-4 M. Adsorbed organic carbon was stripped off of the TiO2 by raising pH > 7 with NaOH and quantified using the TOC analyzer. The Kjeldahl methods (TKN) were used to determine the total concentration of organic nitrogen plus ammonium (8). 10.1021/es990875h CCC: $19.00
2000 American Chemical Society Published on Web 08/04/2000
FIGURE 1. Species measured in aqueous phase during PCO of 10-4 M Cu(II)-EDTA (2 g/L TiO2, 1-h O2, I ) 3 × 10-3 M NaClO4). All copper species presented as a percentage of total copper (10-4 M). DOC presented as a percentage of total initial carbon (10-3 M). a. Distribution of copper species, pH 4. b. Distribution of carbon-containing species, pH 4. c. pH 6. d. pH 7. Samples (100 mL) containing TiO2 or filtered solutions were used in the analysis. To confirm TKN recoveries, 10-4 M EDTA, 10-4 M Cu(II)-EDTA, and 2 × 10-4 M NH4Cl solutions with and without TiO2 were examined. TKN recoveries of all solutions were 2.7 ( 0.2 mg-N/L compared to the theoretical value of 2.8 mg-N/L. However, it was found that ClO4- used in samples for background electrolyte interfered with TKN recoveries. Therefore, in suspensions that required TKN analyses, the PCO solutions were made using CuCl2 and NaCl as Cu(II) and ionic strength, respectively. To evaluate gaseous products, O2 that was continuously introduced into the reactor was vented through an absorption solution. A colorimetric method was used to determine NO2(g) production by using an absorption solution made with N-(1-naphthyl)ethylenediamine dihydrochloride and sulfanilic acid in acetic acid (9). A Spectronics 21 UV-vis spectrophotometer was employed for absorbance measurements at 550 nm. Total nitrogen oxide, except N2O, was measured using an absorption solution containing 1 mL of H2O2 (3%) in 100 mL of H2SO4 (0.3%), which was used to convert these gaseous species to nitrate (9). Nitrate was analyzed using IC as described above. Symbols and calculated species were determined using mass balance expressions presented in the previous paper (5). In addition, total carbon and nitrogen mass balances were analyzed considering the following aqueous and adsorbed species
TOC ) DOC + AdOC + CO2
(1)
NT ) TKNaq + TKNads + NO2- + NO3- + ΣNu
(2)
TKN ) NH4+ + organic-N
(3)
where DOC and AdOC represent dissolved and adsorbed organic carbon, respectively. ΣNu represents unknown nitrogen-containing species produced from the photoca-
talysis of Cu(II)-EDTA. It was assumed that negligible adsorption of NO2- and NO3- onto the TiO2 occurred.
Results and Discussion PCO studies were conducted at pH 4, 6, and 7 with 1-h initial O2 purging in order to quantify CO2 production (Figure 1ad). Approximately 5.7 × 10-4 mol of oxygen are required for the stoichiometric mineralization of 670 mL of 10-4 M EDTA to CO2 and ammonia:
C10N2O8H16 + 8.5O2 f 10CO2 + 5H2O + 2NH3 (4) The initial dissolved oxygen was approximately 8.4 × 10-4 mol by considering saturated pure dissolved oxygen at 25 °C in 670 mL of sample volume. Also 80 mL of headspace could provide 3.3 × 10-3 mol of oxygen in the mixing reactor. This suggests no stoichiometric limitation of oxygen during closed system PCO. Determination of Reaction Intermediates and Products. Copper(II). Similar trends in copper species fates were observed between continuous O2 purging (5) and 1-h initial O2 purging, suggesting minor effects resulting from any possible O2 limitations (Figure 1a-d). The ΣCu-Yaq species (unidentified complexed copper, calculated using the copper mass balances described in the previous paper (5)) increased as PCO was initiated at all pH values with faster formation at lower pH (Figure 1a-d). The maximum concentration was nearly 60% of total copper at pH 4, which was followed by a decrease as the secondary organic compounds (Y) were reacted. The highest ΣCu-Yaq concentration was greater than the initial Cu(II)-EDTAaq concentration, indicating decomposition of Cu(II)-EDTAads and desorption of the products. On the other hand, production of ΣCu-Yaq at pH 6 and 7 wholly originated from the decomposition of Cu(II)-EDTAaq due to the negligible amount of Cu(II)-EDTAads present. This significant production of ΣCu-Yaq contrasts to that of Davis and Green (3) who reported negligible ΣCd-Yaq production VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Species measured in aqueous phase during PCO of 10-4 M EDTA (2 g/L TiO2, pH 4, 1-h O2, I ) 3 × 10-3 M NaClO4). Species presented as percentage of total initial carbon (10-3 M, all organic compounds) or total initial nitrogen (2 × 10-4 M, ammonium).
FIGURE 2. PCO Products of 10-4 M Cu(II)-EDTA (2 g/L TiO2, 1-h O2, I ) 3 × 10-3 M NaClO4). Species presented as a percentage of total initial carbon (10-3 M, all organic compounds), total initial nitrogen (2 × 10-4 M, ammonium), or total copper (10-4 M, Cu(II)-IDA). a. pH. 4. b. pH 6. c. pH 7. at the same concentration and pH values. The strong affinity of Cu2+ for organic complexing ligands, ΣYaq, as compared to Cd2+ may explain these results. Figure 2a-c shows identified products during PCO of 10-4 M Cu(II)-EDTA at pH 4, 6, and 7, respectively. Cu(II)-IDA was found only at pH 7 at low concentrations (Figure 2c, shown on a percentage copper basis). Cu(II)-IDA increased gradually during the PCO time and made up only 10% of the ΣCu-Yaq at 60 min, indicating the presence of other complexed copper species. This compound reacted slower than other ΣCu-Yaq species since 70% of the ΣCu-Yaq was Cu(II)-IDA at 180 min. Lockhart and Blakeley (10) reported the formation of ED3A (ethylenediaminetriacetic acid) during aerobic photodegradation of Fe(III)-EDTA. Other authors have reported Cu(II)-complexing intermediate products such as IDA, glycine (GLY), ethylenediaminediacetic acid (EDDA), nitrilotriacetic acid (NTA), and ethylenediamine (EN) in the ozonation or PCO of EDTA (6, 11). Although attempts were made to identify other ΣCu-Yaq species, they proved unsuccessful. At pH 4, after 150 min Cu2+ was approximately equal to Cu(II), indicating destruction of all ΣCu-Yaq (Figure 1a). The 3798
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steady-state Cu(II)aq concentration matched the equilibrium concentration obtained from 10-4 M Cu(II)/2 g/L TiO2 adsorption at pH 4 (12). At pH >4, all residual copper was adsorbed. Carbon. During PCO of 10-4 M Cu(II)-EDTA at pH 4, AdOC decreased sharply within 30 min, concomitant with destruction of Cu(II)-EDTA. Constant DOC was observed initially, and then it decreased slowly as reaction intermediates were gradually mineralized (Figure 1b). Calculated AdOC (eq 1) matched well with the experimental values. A notable increase in CO2 formation occurs concurrent with the significant drop in AdOC. Mineralization of DOC was not completed within 180 min; approximately 70% CO2 production (on a total carbon basis) was noted. CO2 production was slightly favored at pH 4 compared to that at pH 6 and 7 (approximately 60% CO2 production after 180 min, Figure 1c,d). Although some experimental variations were observed, a reasonable mass balance on carbon was maintained in all three studies. For comparison, Figure 3 shows the PCO of 10-4 M EDTA (without copper) at pH 4 with similar 1-h initial O2 purging. EDTA disappearance was slower than Cu(II)-EDTA. This same trend was also reported by Madden et al. (1). DOC increased as reaction products desorbed and then decreased after 60 min as oxidation progressed. Comparing the higher DOC profile with that of Cu(II)-EDTA suggests slower oxidation of ΣY from EDTA as compared to ΣCu-Y. AdOC (calculated) sharply decreased at initial PCO time and then remained relatively constant at 15-20% beyond 60 min, suggesting the formation of more sorbable intermediates than found with copper present. CO2 production was only 45%, much less than the 70% mineralization found with Cu(II)-EDTA at 180 min. Concentrations of carbon products were normalized to the total amount of carbon initially present in the system. Identified carbon products were formaldehyde, acetate, formate, and CO2 (Figure 2a-c). Oxalate was also found as a minor product (maximum concentration below 10-5 M,
