Environ. Sci. Technol. 2009, 43, 2937–2942
Advanced Oxidation Kinetics of Aqueous Trialkyl Phosphate Flame Retardants and Plasticizers MICHAEL J. WATTS Department of Civil and Environmental Engineering, Florida State University, Tallahassee, Florida 32310 KARL G. LINDEN* Department of Civil, Environmental, and Architectural Engineering, 428 UCB, University of ColoradosBoulder, Boulder, Colorado 80309
Received November 8, 2008. Revised manuscript received December 31, 2008. Accepted February 9, 2009.
Trialkyl phosphate esters are a class of anthropogenic organics commonly found in surface waters of Europe and North America, due to their frequent application as flame retardants, plasticizers, and solvents. Four trialkyl phosphate esters were evaluated to determine second-order rates of reaction with ultraviolet- and ozone-generated •OH in water. Tris(2-butoxyethyl) phosphate (TBEP) was fastest to react with •OH (kOH,TBEP ) 1.03 × 1010 M-1 s-1), followed sequentially by tributyl phosphate (TBP), tris(2-chloroethyl) phosphate (TCEP), and tris(2-chloroisopropyl) phosphate (TCPP) (kOH,TBP ) 6.40 × 109, kOH,TCEP ) 5.60 × 108, and kOH,TCPP ) 1.98 × 108 M-1 s-1). A two-stage process was used to test the validity of the determined kOH for TBEP and the fastest reacting halogenated alkyl phosphate, TCEP. First, •OH oxidation of TCEP and TBEP, in competition with nitrobenzene was measured in ozonated hydrogen peroxide solutions. Applying multiple regression analysis, it was determined that the UV/H2O2 and O3/ H2O2 data sets were statistically identical for each compound. The subsequent validated kOH were used to predict TCEP and TBEP photodegradation in neutral pH, model surface water after chemical oxidant addition and UV irradiation (up to 1000 mJ/cm2). The insignificant difference between the predicted TBEP and TCEP photodegradation and a best-fit of the firstorder exponential decay function to the observed TBEP and TCEP concentrations with increasing UV fluence was further evidence of the validity of the determined kOH. TBEP oxidation rates were similar in the surface waters tested. Substantial TCEP oxidation in the model surface water required a significant increase in initial H2O2.
Introduction Recent surveys of synthetic organic compounds in wastewater-receiving streams of Arkansas and Kansas found chlorinated and nonchlorinated trialkyl phosphates (TAPs) at concentrations ranging from 0.03 to 27 µg/L (1, 2). In addition, highway runoff can be a source of TAPs in the environment, as seven chlorinated and nonchlorinated alkyl phosphates were recovered from roadside snowbanks in Sweden (3). While the ecotoxic and human health effects of * Corresponding author phone: 303-492-4798; fax: 303-492-7317; e-mail:
[email protected]. 10.1021/es8031659 CCC: $40.75
Published on Web 03/13/2009
2009 American Chemical Society
aqueous TAPs are not clear, California has listed the chlorinated TAP tris(2-chloroethyl) phosphate (TCEP, CAS No. 115-96-8) among carcinogens and reproductive toxins since 1992 (4). Chlorinated TAPs have been shown to resist biodegradation, chlorination, and ozonation in treated waters (5, 6). With their resistance to conventional water treatment and relative abundance in the hydrosphere, it is apparent that alternative treatments for control of aqueous TAPs require investigation. Sampling before and after granular-activatedcarbon filtration of drinking water has shown significant removal of TAPs at full-scale treatment plants (7, 8). However, the application of other advanced water treatments, specifically geared toward the chemical destruction of TAPs in situ, warrants examination. Hydroxyl radical, with its relatively high redox potential (2.8 V), is often the oxidant of choice for engineered remediation of waters contaminated with trace anthropogenic organic compounds. The potential for •OH formation in UV-irradiated waters containing the oxidant H2O2 has led to the design and implementation of UV/H2O2 for oxidation of unwanted organics, at full-scale water treatment plants (9, 10). In a previous study of •OH oxidation of TCEP, the advanced oxidation process (AOP), O3/H2O2, more efficiently degraded the aqueous TAP when compared to O3 alone (11). The reported work assesses the potential of AOPs for remediation of surface waters containing TAPs. The advanced oxidation kinetics of TCEP were compared to a heavier molecular weight, chlorinated TAP, tris(2-chloroisopropyl) phosphate (TCPP, CAS No. 13674-84-5), and two nonhalogenated TAPs, tributyl phosphate (TBP, CAS No. 126-73-8) and tris(2-butoxyethyl) phosphate (TBEP, CAS No. 78-51-3). Their second-order rates of reaction with •OH (kOH) were assessed via a UV/H2O2 competition kinetics method (12). These rate constants were compared to assess the impact of alkyl chain structure on the •OH reaction rates. Figure 1 provides a visual comparison of the structural differences among the studied TAPs. A significant application of empirically determined kOH is for the modeling of contaminant oxidation in natural waters via engineered, •OH-generating processes. Additionally, using previously established UV/H2O2 kinetic models allows for validation of empirically determined kOH, by comparing the “goodness-of-fit” for kOH-based predictive models to observed kinetic data. Therefore, for the discussed work, the derived kOH was used to predict TAP oxidation in surface water for given solution conditions. The contaminated surface water was treated via sequential addition of a chemical oxidant, H2O2 or NaOCl, and a measured UV fluence (mJ/cm2). Previous work has highlighted the fast rate of •OH formation in acidic free chlorine (HOCl) solutions irradiated with monochromatic, 254 nm UV light (13). TCEP and TBEP serve as model surface water TAPs, and to ensure an accurate prediction of their oxidation rates, the derived kOH for these contaminants was validated using an alternative •OHgenerating advanced oxidation process, O3/H2O2.
Methods Chemicals. Nitrobenzene (NB) was supplied by Fluka (Buchs, Switzerland; 97%), while TCEP, TBEP, and TBP were acquired from ACROS (Geel, Belgium; 97%, 94%, and 99%, respectively). TCPP was purchased from Wako Chemicals (Osaka, Japan; purity unknown; see the discussion in the Analysis section below). The 30% H2O2 solution was procured from Fisher Chemical (Pittsburgh, PA). Hexachlorobenzene (HCB, >99%) was produced by Alfa Aesar (Ward Hill, MA). DichloVOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Chemical structures of TBP, TBEP, TCEP, and TCPP. romethane (99.9%), methanol (HPLC-grade), and chloroform (HPLC-grade with 50 ppm pentene preservative) were supplied by Fisher Scientific (Fair Lawn, NJ). All chemicals were used as purchased. Dilute solutions were prepared in laboratory-grade deionized water (HYDRO Ultrapure, Research Triangle Park, NC). UV/H2O2 Competition Kinetics. The bench-scale quasicollimated beam apparatus contained four low-pressure (LP) Hg UV lamps (General Electric #G15T8) emitting primarily at a wavelength of 253.7 nm. Calculating the UV fluence (mJ cm-2) applied to an aqueous solution required a measurement of the UV irradiance (mW cm-2) incident to the surface. A radiometer (IL 1700, SED 240/W, International Light, Peabody, MA), with a UV detector calibrated to 254 nm, was used to measure incident irradiance. Measures of incident irradiance varied between exposures due to changes in lamp condition with age and relative placement of the irradiation vessel. Therefore, the radiant exposure, or UV fluence, was averaged over the entire solution volume by multiplying an incident irradiance by factors accounting for the divergence of the collimated beam, reflection at the water surface, variation in the irradiance over the surface of the solution, and photon absorption with depth in the water column (14). In this way, multiple irradiations with variable exposure conditions can be compared. UV-irradiated, deionized-water solutions were spiked with NB and a TAP to 5 µM (of each) and H2O2 up to 50 mg/L and continuously mixed. Nitrobenzene is an ideal reference compound for UV competition kinetics studies due to the compound’s resistance to direct photolysis and relatively rapid rate of reaction with •OH (12, 15, 16). The total solution volume before exposure was 130 mL, with 30 mL samples taken from the bulk solution (the change in average irradiance caused by the reduction in solution volume and subsequent decrease in time required to reach the desired UV fluence were accounted for by recalculating the required time to reach subsequent UV fluences). Each 30 mL sample was placed into a 40 mL borosilicate vial, containing 1 mL of 2938
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dichloromethane, with a PTFE-lined cap. The borosilicate vials were mixed for 15 min on a rotary, end-over-end vial tumbler, and 250 µL of dichloromethane was removed for TAP and NB concentration analysis. O3/H2O2 Competition Kinetics. Ozone was supplied by an EFFIZON ozone generator (WEDECO) utilizing compressed medical-grade oxygen. Deionized water solutions of H2O2 and TCEP or TBEP were dosed with ozone via addition of a measured volume of an ozone-saturated solution (40 mg/L as O3). The molar ratio of H2O2 to O3 was consistently 1.2. To observe increased TAP and NB oxidation, initial concentrations of H2O2 (0.38-6.37 mg/L) were increased with proportional increases in O3 (0.45-7.5 mg/L). Ozone concentrations were measured using the indigo-colorimetric method described in Standard Methods for the Examination of Water and Wastewater (17). After a controlled period of oxidation, sodium thiosulfate (24.8 mg/mL) was added to quench the residual O3. A liquid-liquid extraction method, as previously described, was used to extract the analytes from the aqueous sample. Model Surface Water Irradiations. A model surface water was prepared by first adding extracts of Suwanee River humic acid (0.0256 g) and alginic acid (0.0532 g, International Humic Substances Society, http://www.ihss.gatech.edu) to 100 mL of 2.5 µM NaOH solution. The final “surface water” was prepared by mixing 0.09 g of NaHCO3, 600 µL of 1 g/L NO3-, 2 mL of 1 g/L HPO42-, and 20 mL of the buffered organic acid solution in 2 L of deionized water. Solution pH was adjusted using 0.16 N phosphoric acid (to pH 6.8). Dissolved organic carbon and inorganic anion concentrations are reported in Table 1. Significant •OH scavengers in this surface water (CO32-, HCO3-, and DOC) have well-understood •OHscavenging rates, such that the photooxidation of TAPs in this matrix could be predicted via established equations governing the rate of •OH formation under UV/H2O2 conditions. UV irradiations were conducted as previously described. TBEP or TCEP was added prior to irradiation (TAP0 ) 50
TABLE 1. Concentrations of DOC and Inorganic Anions in the Model Surface Water DOC, mg/L
NO3-, mg/L
HCO3-, mg/L
PO43-, mg/L
2.0
0.3
30
1.0
µg/L). Analyte extraction involved rapid vortexing of the entire irradiated solution volume (150 mL) with 1 mL of chloroform in a 250 mL centrifuge tube. A 250 µL aliquot of chloroform was taken for TAP concentration analysis. Analysis. The analytes were separated from the solvent (dichloromethane or chloroform) with gas chromatography followed by quadropole-MS detection (Shimadzu). After injection of 1-2 µL, the column temperature was held at 50 °C for 4.5 min; subsequent oven-temperature ramping increased the column temperature from 50 to 300 °C over 13 min. Single-ion monitoring was used to assess the detector response to the analytes. With regard to the purity of the TCPP standard, two peaks were detected when TCPP was the lone analyte. However, the second peak was consistently 5 times smaller (their relative retention times were also consistent); therefore, only the largest peak was used for response assessment. Andresen et al. also encountered multiple peaks for their technical grade TCPP (different supplier from the TCPP used in this study) and similarly reported the largest peak for quantification (18). The mass spectra for both peaks were similar, and it was assumed that the second-order •OH reaction rates are also similar for these potential isomers. H2O2 residuals were measured using the “I3-” method (19). Free chlorine residuals were analyzed according to the DPD colorimetric method (17).
Results and Discussion UV Photosynthesized •OH Oxidation in Laboratory-Grade Water. The molar absorption coefficients, ε (M-1 cm-1), were measured for all four compounds by diluting the neat standard in MeOH and measuring the solution absorbance (minus the background absorbance of MeOH) over the range of 200-400 nm (see Supporting Information for measured molar absorption coefficients) with a Cary UV-vis spectrophotometer (Varian, Inc., Palo Alto, CA). Ideally, the absorbance spectra would have been measured in water rather than MeOH, due to potential differences in solute-solvent interactions. However, the poor UV-C absorbance of the studied TAPs required dissolution of solute at concentrations well above reported water solubility. Direct photolysis of TBEP, TBP, TCEP, and TCPP was insignificant at the applied UV fluences, due to the almost negligible photon absorption (λ ) 254nm) by these alkyl-phosphates. In solutions where NB and the trialkyl phosphate are the only competitors for photosynthesized •OH, the linear correlation between the observed decay rate of [NB] and [TAP] represents the ratio of kOH(NB) to kOH(alkyl phosphate) (12, 15). Figure 2 illustrates the log(ln)-linear decrease in the TAPs relative to NB in UV/H2O2-treated deionized water solutions of TBP and NB, TBEP and NB, TCEP and NB, and TCPP and NB. The addition of a Cl atom on the alkyl chain (TCEP) significantly reduces H-atom availability relative to TBP and TBEP. Along with halogenation of the alkyl chain, the number of carbons and additional heteroatoms (O) on the chain also seems to affect the rate of •OH attack. The chain structure with the most carbons (TBEP) had the fastest rate of reaction. However, TCPP has one more carbon on each chain than TCEP, yet scavenged •OH at a slower rate, most likely due to the influence of an additional function group (-CH3) at the R-carbon of TCPP.
Ozone Synthesized •OH Oxidation in Laboratory Grade Water. An additional •OH-generating process, O3/H2O2, was used to measure the kOH and validate the values determined using UV/H2O2. TCEP and TBEP were selected for kOH validation as their respective rate constants are subsequently used to predict •OH oxidation in surface water (below). In ozonated aqueous solutions, the presence of H2O2 leads to accelerated decomposition of O3 to •OH. As the rates of O3 reactivity with NB [0.09 M-1 s-1 (20)], TCEP [0.3 min-1 in 9 mg/L O3 solution (11)], and TBEP (no observed oxidation for the applied ozone Ct) are negligible relative to the reaction rate of H2O2 with O3 [kO3,HO2- ) 2.8 ( 0.5 × 106 M-1 s-1 (21)], the assumptions underlining UV/H2O2 competition kinetics (no decay from UV or ozone alone) are also valid for O3/H2O2 competition kinetics. Figure 3 presents the correlations between [NB] and [TCEP] or [TBEP] in ozonated solutions of H2O2. The observed linear correlations between NB and TBEP or TCEP oxidation in ozonated H2O2 solutions were used to calculate kOH,O3 for TBEP and TCEP. Figure 4 presents the experimentally determined kOH,UV and kOH,O3 for both TCEP and TBEP. For both compounds, the UV/H2O2 competition kinetics method predicts a slightly higher rate of reaction with •OH, relative to O3/H2O2. To test the homogeneity of the competition-kinetics data from UV/H2O2 and O3/H2O2, a multivariate linear regression model
ln
( )
( )
( )
NB0 NB0 C0 ) β1 ln + β2z + β3 ln :z C NB NB
(1)
was applied to the combined UV/H2O2, O3/H2O2 data set. For this model, z is a dummy variable equal to 1 or 0 depending on whether the data point belongs to the UV/ H2O2 or O3/H2O2 data set. A single regression model can be fit to a combined UV/H2O2, O3/H2O2 data set when β2 ) β3 ) 0 (hypothesis of coincidence). Table 2 lists the parameter estimates and standard errors from the multivariate regression models for TCEP and TBEP oxidation. For both TCEP and TBEP, only β1 is significantly nonzero (R ) 0.05 for TCEP and R ) 0.01 for TBEP). Therefore, a single linear regression model can be used to determine kOH from the combined data sets of each contaminant. Table 3 summarizes the determined second-order •OH rate constants for TBEP and TCEP. TBEP and TCEP: Advanced Oxidation in a Model Surface Water. The measured •OH rate constants for TBEP and TECP were utilized to predict the degradation of the contaminants under two distinct UV-based AOPs: UV/ H2O2 and UV/HOCl. Theoretical first-order rate constants, k1 (s-1), for TCEP and TBEP degradation in a water matrix treated with an AOP can be predicted using established equations governing the simultaneous formation and scavenging of photosensitized •OH (22-24). The steadystate concentration of •OH ([OH]SS) is the ratio of the rate of •OH formation, for a given set of UV-irradiation and initial oxidant conditions (H2O2 or HOCl), to the sum of the products of •OH reactants and their reaction rates in the given water matrix (total •OH scavenging). The predicted k1 for TCEP and TBEP is the product of their derived second-order •OH reaction rates and [OH]SS. However, in UV-irradiated free chlorine solutions, the total •OH scavenging can be significantly affected by pH; as the ratio of OCl- to HOCl increases with pH, so does the fraction of [OH]SS that reacts with unphotolyzed free chlorine hypochlorite ion (25). Modifying the conventional equation VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. The linear correlations between the concentrations of tributyl phosphate (TBP), tris(chloroethyl) phosphate (TCEP), tris(butoxyethyl) phosphate (TBEP), tris(chloropropyl) phosphate (TCPP), and NB. •OH was the product of H2O2 photolysis under LP UV. Corresponding kOH listed in units of M-1 s-1.
TABLE 2. Parameter Estimates for Multiple Regression Models (eq 1) of TCEP and TBEP Oxidation, In Competition with NB for •OH TCEP
FIGURE 3. The linear correlations between the concentrations of TCEP/TBEP and NB. •OH was the product of O3/H2O2.
TBEP
coefficient
estimate
standard error
estimate
standard error
β1 β2 β3
0.13 0.03 0.04
0.01 0.03 0.03
2.61 0.32 -0.17
0.14 0.15 0.38
TABLE 3. Second-Order Reaction Rates (kOH) of TBEP and TCEP with •OH, Determined Using Combined UV/H2O2 and O3/H2O2 Competition Kinetics Datasets kOH, M-1 s-1 TBEP TCEP
FIGURE 4. A comparison of kOH,UV and kOH,O3 for TCEP and TBEP. Error bars present the upper and lower limits on the 95% confidence interval for the linear estimation of kOH. for [OH]SS to account for free chlorine speciation yields eq 2. [OH]SS ) ΦHOClfOHI0 fHOCl{1 - exp[-2.303b(εHOClCHOCl + ε1C1 + ...+εnCn)]} kOH,1C1 + ...+kOH,HOClCHOCl + kOH,OCl-COCl- +
(2)
...+kOH,nCn In eq 2, ΦHOClfOH is the quantum yield (λ ) 254nm) of HOCl to •OH [1.4 mol Es-1 (13)], I0 is the incident irradiance 2940
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1.03 ( 0.38 × 1010 5.60 ( 0.21 × 108
(Es L-1 s-1), fHOCl is the fraction of UV light absorbed by HOCl, b is the optical path length (cm), εHOCl is the molar absorption coefficient for HOCl [M-1 cm-1, λ ) 254nm (13)], C1-Cn are the concentrations of species 1 through n (M), and kOH,HOCl [8.46 × 104 M-1 s-1 (13)] and kOH,OCl- [8.0 × 109 M-1 s-1 (26)] are the second-order •OH rate constants for HOCl and OCl-. While OCl- photolysis products include •OH (26), consideration of OCl- in the rate of •OH formation term (numerator in eq 2) would not significantly impact [OH]SS [when pH
