Environ. Sci. Technol. 2000, 34, 2570-2575
Use of UV Spectroscopy To Characterize the Reaction between NOM and Free Chlorine CHI-WANG LI,† M A R K M . B E N J A M I N , * ,‡ A N D GREGORY V. KORSHIN‡ Department of Water Resources and Environmental Engineering, Tamkang University, Taipei, Taiwan, and Department of Civil and Environmental Engineering, Box 352700, University of Washington, Seattle, Washington 98195-2700
Simple and reliable relationships exist between the change in UV absorbance (∆A) of NOM when it is chlorinated and the formation of chlorinated byproducts. These relationships provide an approach for obtaining large amounts of data that can be used to interpret the kinetics, stoichiometry, and mechanism of the reactions. Analysis of these relationships suggests that the functional groups that are the major precursors for DBPs might be highly activated aromatic rings, as has been suggested previously, but that these groups have some fundamental differences from highly activated rings in pure compounds. The key evidence for this difference is that the UV absorbance of NOM decreases when dosed with even very low concentrations of chlorine, whereas the absorbance of pure compounds such as 3,5-DHBA and resorcinol increases. When hypochlorite species (HOCl and OCl-) are added to a solution containing NOM, between 1.6 and 4.1 Cl atoms become incorporated into NOM for each activated aromatic ring that is destroyed. The rate of Cl incorporation into organic molecules is very rapid initially and decreases steadily thereafter. Chlorine reduction on the other hand (or, equivalently, NOM oxidation) is negligible initially and then increases over time. The effect of these parallel processes is that the amount of Cl that becomes incorporated into organic molecules as a fraction of the amount of HOCl and OCl- consumed decreases from near 100% initially to near 20% over the course of the reaction. Considering that many of the carbon atoms in NOM probably do not participate in redox reactions with Cl, those that do must be oxidized quite substantially during the process, and it is likely that atoms other than C (particularly N) also provide some of the electrons to reduce Cl. Expanded use of ∆A to study DBP-forming reactions is likely to lead to more insights into key aspects of the reaction mechanisms.
Introduction Research into the formation of chlorinated disinfection byproducts (DBPs) by reactions between “free chlorine” * Corresponding author phone: (206)543-7645; fax: (206)685-9185; e-mail:
[email protected]. † Tamkang University. ‡ University of Washington. 2570
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000
(HOCl and OCl-, represented in the remainder of this paper simply as HOCl) and natural organic matter (NOM) is now nearing the end of its third decade. While a great deal has been learned in that time, the identities of the reactive sites on NOM molecules, the reaction pathways, and many of the end products are still only poorly understood. Several factors contribute to this state of affairs. First, NOM molecules comprise a varied and complex group of reactants, with a range of elemental composition, charge, and secondary and tertiary structure, all of which might affect their reactions with chlorine. Second, reactions of HOCl with NOM can lead to Cl substitution into the organic molecule, partial oxidation of the NOM (accompanied by reduction of HOCl to Cl-), or both. Third, many of the initial reactions between NOM and HOCl generate chlorinated intermediates that subsequently participate in other substitution, hydrolysis, or oxidation reactions, generating a very large range of reaction products. Finally, the very substantial analytical effort required to analyze for even the known, major products, let alone the many minor ones, serves as yet another impediment to deciphering the details of the reactions. The limited understanding of DBP formation is reflected in the mathematical models that have been proposed to predict the formation of individual DBPs such as trihalomethanes (THMs) or haloacetic acids (HAAs). While a few semimechanistic models have been proposed (1-4), the majority of models are of the form shown in eq 1 and include a dependence on the initial dissolved organic carbon (DOC) concentration, light absorbance at 254 nm (A254), time (t), temperature (T), chlorine dose, solution pH, and bromide concentration:
[THM] ) k1(DOC)a(A254)b(HOCl dose)c(t)d(T)e(pH - k2)f ((Br) + k3)g (1) Equations of this type lack any unifying theoretical framework and provide little insight into the reaction mechanism. When water containing NOM is chlorinated, the UV absorbance of the solution decreases at all wavelengths. In prior publications (5, 6), we have described relationships between the change in UV absorbance (∆A) induced by chlorination and the formation of DBPs that make it immensely easier to follow the progress of DBP-forming reactions. The essence of those relationships is summarized in Figures 1 and 2. In these figures, the change in absorbance at wavelength λ (∆Aλ) is defined as Aλ,final - Aλ,initial. Hence, since Aλ decreases upon chlorination, ∆Aλ is negative. (Note that this definition is opposite the one we used in previous publications. The current definition is more consistent with the usual convention for representing a change in any parameter.) Figure 1 shows that the decrease in Aλ upon chlorination follows a consistent pattern as a function of wavelength, with a peak (i.e., the maximum loss of absorbance) near 272 nm. This result has been obtained consistently for a very wide range of initial water quality and chlorination conditions. The upper line in Figure 2 shows that the amount of Cl incorporated into the NOM, quantified as the total organic halogen (TOX), is linearly related to ∆A272. This relationship indicates that for every µmol/L of Cl that is incorporated into an NOM molecule, an amount of absorbance equal to 3.01 ( 0.14 m-1 is destroyed at 272 nm. The relationship between TOX production and ∆Aλ is also approximately linear at other wavelengths throughout the UV region, but in our experience the amount of absorbance destroyed is always greatest at 272 ( 4 nm. 10.1021/es990899o CCC: $19.00
2000 American Chemical Society Published on Web 05/12/2000
FIGURE 1. Typical differential UV spectrum of a chlorinated water sample. FIGURE 3. Contributions of THMs, HAAs, and higher MW compounds to TOX as a function of reaction progress.
FIGURE 2. Formation of TOX and CHCl3 as a function of the change in the UV absorbance at 272 nm. The TOX relationship includes data from chlorination of 10 different waters with pH 5 to 11, DOC concentrations from 150 to 417 µmol/L (1.8 to 5.0 mg/L), HOCl:DOC dose ratios of 0.071 to 0.68 mol/mol, reaction times 0.5 min to 7 d, and Br- concentrations from zero to 500 µg/L. The CHCl3 curves are for chlorination of 417 µmol/L (5.0 mg/L) DOC with 280 µmol/L HOCl, with reaction times of 5 min to 6 d pH 7. The water for the CHCl3 tests was from Judy Reservoir (Mt Vernon, WA) and contained negligible Br-. Note that ∆A272 becomes increasingly negative from left to right. The lower line in Figure 2 indicates that the generation of chloroform (CHCl3) is also strongly related to ∆A272, but the relationship depends on pH and cannot be characterized by a straight line passing through the origin (6). A similar relationship applies to the formation of haloacetic acids (HAAs) (6). We have interpreted this result as indicating that the initial substitution of Cl into NOM forms relatively high molecular weight (MW) chlorinated compounds and that smaller compounds such as chloroform are not split from the parent molecule until a substantial amount of Cl has been incorporated. For the samples used to develop the THM relationship in Figure 2, the concentration of TOX reached ∼300 µg/L before any CHCl3 or HAA was formed. From that point forward, formation of higher-MW TOX was approximately balanced by the destruction of such compounds to form CHCl3 and HAAs, so the concentration of higher-MW TOX remained approximately constant (Figure 3). The results shown in Figure 2 indicate that ∆A272 can be used as an indicator of the overall progress of Cl incorporation reactions. The ease with which ∆A272 can be analyzed makes it a powerful tool for studying these reactions. In this paper, we use the TOX-∆A272 relationship to study the kinetics of DBP formation and HOCl consumption and to assess the relative importance of substitution and oxidation when HOCl reacts with NOM, and we use the spectroscopic results to speculate on the character of the DBP precursor sites.
Experimental Section All experiments were conducted using NOM from Judy Reservoir, the water supply source for Mt. Vernon, WA (DOC
) 3.0-4.5 mg/L). In some cases, the NOM was preconcentrated by sorption onto iron-oxide-coated sand, followed by elution with NaOH and neutralization by passage through an H+-saturated ion exchange column (6). The concentrate was then diluted with ultrapure water to ∼6 mg DOC/L prior to use in experiments. All other chemicals were reagent grade. The kinetics of TOX formation and HOCl consumption were studied in both batch and continuous flow experiments. In the batch experiments, HOCl was dosed into a well-mixed solution that contained NOM and that was buffered at pH 7.0 with phosphate (0.01 M PO4,tot). If the reaction time under investigation was less than 2 min, the reaction vessel was an open, well-mixed beaker. In experiments investigating longer reaction times, the solution was transferred to headspacefree amber vials immediately after the HOCl was added. Once the desired reaction time had passed, subsamples of the solution were dosed with either sodium sulfite or DPD to quench the residual oxidant. A272 was then determined in the sample quenched with sulfite, and the concentration of TOX was computed based on ∆A272, per Figure 2. A515 was measured in the sample quenched with DPD to determine the residual HOCl concentration (7). The latter correlation relies on an assumption that DPD does not react with Cl that has been incorporated into organic molecules. All experiments were conducted at the ambient temperature of 20 ( 2 °C. The continuous flow experiments followed essentially the same protocol, except that the sodium sulfite or DPD was injected into the reacting solution inline before it was pumped through a flow-through cell in a spectrophotometer. This setup allowed ∆A272 and the concentration of residual HOCl to be estimated at a frequency of approximately one analysis per 2 s, starting approximately 30 s after the HOCl was dosed into solution. UV absorbance was analyzed using a PerkinElmer Lambda 18 spectrophotometer. Absorbance at 515 nm (for residual HOCl) was determined using either that instrument or a Hach Model 4000 spectrophotometer. One experiment was conducted using an Applied Photophysics SX.18MV Stopped Flow Reaction Analyzer, which can thoroughly mix two solutions in a time frame of milliseconds and record the absorbance spectrum of the mixture at intervals of approximately 0.01 s thereafter. In this experiment, 2.5 × 10-5 mol/L 3,5-dihydroxybenzoic acid (3,5-DHBA) was dosed with 1.1 × 10-3 mol/L HOCl, and the absorbance of the mixture was followed for 50 s.
Results and Discussion Kinetics of TOX Formation and HOCl Consumption. Plots of TOX formation (based on measurements of ∆A272) and consumption of HOCl over time in batch experiments at four different HOCl doses are shown in Figure 4. The plots for both parameters indicate that the reaction proceeded to a significant extent before the first data point was collected. VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2571
FIGURE 4. Kinetics of (a) TOX formation and (b) HOCl consumption in systems containing Judy Reservoir NOM (4.0 mg/L as DOC) and various doses of HOCl. All systems at pH 7.0. The TOX values shown are based on measurement of ∆A272 and the correlation shown in Figure 2.
FIGURE 6. (a) Kinetics of HOCl consumption, TOX formation, and HOCl reduction and (b) differential and cumulative Cl incorporation efficiency as a function of time for batch chlorination of NOM. DOC ) 483 µmol/L (5.8 mg/L), HOCl dose ) 133 µmol/L. Data points are based on a running computation every 2 s, covering the preceding 8-s interval. The TOX values were calculated as described in the caption to Figure 4. collected in the latter system. The cumulative and differential Cl incorporation efficiency in this system are shown in Figure 6b. The differential value represents the incremental amount of Cl incorporated divided by the incremental amount of HOCl consumed over short periods of time, and the cumulative value represents, in essence, a running average of these values from time zero. Both curves have been smoothed by showing data points that have been averaged over four time steps (approximately 8 s).
FIGURE 5. Molar ratio of TOX formed to HOCl consumed during the experiments characterized in Figure 4. The TOX values were calculated as described in the caption to Figure 4. Thereafter, the reaction continued throughout the experimental period but at a steadily decreasing rate. Similar trends have been reported by Summers and co-workers (8, 9) for HOCl consumption and by Zou et al. (10) for TOX formation. The rate of TOX production declined much more rapidly than did the rate of HOCl consumption. The relationship between TOX formation and HOCl consumption in these experiments is shown in Figure 5. Also plotted in the figure is a line representing 100% Cl incorporation, i.e., the TOX that would be detected if all of the HOCl that was consumed became incorporated into NOM. The results indicate that Cl incorporation efficiency is near 100% initially (up to ∼8.5 µmol/L HOCl consumed in these experiments) and that it declines significantly thereafter. Qualitatively, results very similar to those in shown in Figure 4 were obtained when the experiment was repeated using a flow cell to analyze the absorbance semicontinuously (Figure 6a). However, immensely more data could be 2572
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000
As in the batch experiments, the Cl incorporation efficiency was close to 100% initially and declined rapidly thereafter. Presuming that loss of HOCl without formation of TOX corresponds to generation of Cl-, the curve for the “differential” incorporation efficiency indicates that within about 2 min, the stoichiometry of Cl usage shifted from almost 100% incorporation and 0% reduction to only 20% incorporation and 80% reduction The cumulative incorporation efficiency changes less dramatically, because the high incorporation efficiency at the beginning of the test contributes to all subsequent calculations. It is this cumulative (average) value that has been reported in the relatively few previous studies of Cl incorporation efficiency (e.g., ref 11). However, in those cases, only a single data point has been reported, typically after a substantial amount of reaction had occurred, i.e., conditions corresponding to the flat part of the curves in Figure 6b. The average reported Cl incorporation efficiency has usually been near 25%. The result obtained here at the end of the test period (18 min) was somewhat higher (36%), but the cumulative incorporation efficiency was still declining at that time. Linear correlations for predicting TOX or THM concentrations based on the amount of HOCl consumed by a sample have been proposed by a number of researchers (12, 13). The results in Figures 2 and 6 demonstrate that such relationships
are approximately valid during the later stages of the reaction but not at the very beginning. Furthermore, if the goal is to estimate TOX or THM concentrations based on an easily measured surrogate, it can be accomplished more easily and with greater accuracy using ∆A272 rather than HOCl consumption as the surrogate parameter. Using UV Absorbance and ∆A To Characterize the ClReactive Functional Groups in NOM. NOM molecules are widely represented as containing a few aromatic rings linked via short aliphatic chains. Most of the rings are thought to be activated by the presence on the ring of one or more hydroxyl groups. These activated aromatic rings are thought to be responsible for the majority of the UV absorbance of the NOM (14-16) and also to be the primary Cl-reactive sites in the molecules (10, 17, 18) so characterization of the number of such rings and their spectral properties before and after chlorination can provide substantial insight into the NOM-Cl reaction. It is not possible to evaluate the molar absorptivity of NOM directly, since NOM is a mixture of many different compounds with a range of molecular weights. Instead, the absorbance of NOM is commonly normalized to the concentration of DOC in a sample ((absorbance units (a.u.)/ m)/(mg C/L)). In water treatment literature, this ratio is referred to as the specific UV absorbance (SUVA). That is, at wavelength λ
SUVAλ )
Aλ DOC
(2)
Estimates based on 13C NMR spectroscopy (11, 16) suggest that approximately 10-30% of the carbon in NOM is present in aromatic rings, with 25% being a typical value. Attributing all of the UV absorbance of the NOM to such rings, the molar absorptivity at wavelength λ (λ) of the activated aromatic rings in an NOM sample can be estimated, as follows:
λ )
(
)
1 mg C 0.25 mg activated aromatic C Aλ 72 000 mg activated aromatic C mol activated aromatic ring DOC
(
λ ) 288 000*SUVAλ
)( ) (3)
where λ and SUVAλ are expressed in units of m-1/mol activated aromatic ring and m-1/(mg C/L), respectively. Historically, SUVA has been reported most frequently for λ ) 254 nm. The range of SUVA254 in natural freshwaters is from approximately 2 to 5 m-1/(mg C/L), with values between about 2.5 and 3.5 m-1/(mg C/L) being most common. SUVA254 for the water studied in the current work was 3.22 m-1/(mg C/L). Using eq 3 in conjunction with the range of SUVA254 values in natural waters, we estimate the molar absorptivity at 254 nm of activated aromatic rings in NOM to be in the range 5.8 × 105 0) with time at 250 < λ < 275 nm, whereas it always decreases when NOM is chlorinated. In the range, 275 < λ < 320 nm, the absorbance of the chlorinated DHBA also increases initially (∆A > 0 at t ) 1 s), but it decreases continuously thereafter, so that ∆A < 0 within 50 s of reaction over much of this range. At still higher wavelengths (320 < λ < 480 nm), the reverse trend is observed: ∆A is negative after 1 s but increases steadily for the next 50 s, eventually becoming positive. None of this complex behavior occurred with chlorination of NOM, in which case the absorbance decreased (∆A < 0) steadily over time over the entire wavelength range investigated. Increasing absorbance with increasing incorporation of chlorine, as observed for DHBA, is typical for other pure phenolic compounds as well (e.g., phenol, resorcinol) over at least some range of UV wavelengths. At large HOCl doses, of course, the ring is likely to break, so absorbance decreases. The results presented here suggest either that, unlike the case for pure phenolic compounds, Cl incorporation into NOM does not increase the absorptivity of the NOM or that the chromophores in NOM are much more susceptible to oxidation and ring cleavage than are the model compounds that have been used as NOM analogues, so that the decrease in absorptivity associated with NOM oxidation masks any increase due to Cl incorporation. Oxidation vs Substitution in NOM-HOCl Reactions. The reactions of NOM with Cl can be further elucidated by exploring the reaction stoichiometry, parts of which can be inferred from the data in Figure 2. Specifically, as noted above, Figure 2 indicates that absorbance at 272 nm decreases by 0.301 m-1 per µmol/L of Cl incorporated into NOM compounds. Also, the molar absorptivity at 272 nm (272) of the activated aromatic groups in NOM was estimated above as ranging from 4.9 × 105 to 1.2 × 106 m-1/(mol activated aromatic ring/L). Assuming that all of the decline in A272 can be attributed to destruction of activated aromatic rings, we can estimate the number of Cl atoms that are incorporated VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2573
TABLE 1. Table of Molar Absorptivities of Pure, Activated Aromatic Compounds
a
λmax in nm, in m-1/(mol/L).
into NOM molecules each time an activated aromatic ring is destroyed as
m-1 4.90 × 105-1.22 × 106 mol activated aromatic ring/L ) m-1 0.301 µmol Cl incorporated/L mol Cl incorporated 1.6-4.1 mol activated aromatic ring destroyed For the NOM used in the current research, the corresponding estimate is in the middle of this range, at approximately 2.4 mol of Cl incorporated per mol of activated aromatic rings destroyed. This result, derived using independent approaches for estimating the absorbance of activated aromatic groups (eq 3) and the amount of absorbance destroyed when Cl is incorporated (Figure 2), is satisfying in terms of providing an estimate that is chemically reasonable for the number of chlorine atoms that are incorporated into an aromatic structure when it breaks. The result might be interpreted as 2574
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000
FIGURE 7. Differential spectrum for chlorination of 2.5 × 10-5 mol/L 3,5-dihydroxybenzoic acid (3,5-DHBA) dosed with 1.1 × 10-3 mol/L HOCl. The thick curve is the spectrum collected after 1 s of reaction. indicating either that more than one Cl atom must be incorporated into the ring before it breaks or that the ring breaks when one Cl atom is incorporated, but that additional Cl atoms become incorporated immediately thereafter.
Additional insights into the reaction stoichiometry can be gained by combining the above information with the results in Figure 6. Since the slope of the TOX vs ∆A272 relationship is constant throughout the reaction period, the estimate that 2.4 Cl atoms are incorporated per activated ring destroyed is also applicable throughout that period. However, Figure 6 indicates that the Cl incorporation efficiency declines from near 100% to less than 20% as the reaction proceeds. Thus, the total number of HOCl molecules consumed by reaction with the NOM apparently increases from approximately 2.4 per activated aromatic ring destroyed at the beginning of the reaction to more than 12 per activated aromatic ring destroyed near the end. In other words, toward the end of the reaction, 9.6 HOCl molecules were consumed without being incorporated for every activated aromatic ring that was destroyed. Assuming that the Cl in these HOCl molecules was reduced to Cl-, approximately 19 electrons must have been transferred from the NOM to Cl atoms in this process. The average oxidation number of carbon in fulvic acid has been approximated to be +0.3, based on an elemental composition of CH1.08N0.04O0.74S0.02 (19), and it is likely that the oxidation number of carbon in the aromatic portions of the molecule is somewhat larger than this. Assuming an average oxidation state of +0.3 for carbon in the rings that are attacked and that one or two carbon atoms from the ring are released as part of a chlorinated DBP molecule without significant change in their redox state, release of 19 electrons from a single ring would require that, on average, each of the remaining carbon atoms in the ring (those not released as small chlorinated DBPs) release three to four electrons. Thus, the final average oxidation state of these carbon atoms would be at least +3.3, i.e., they would be very highly oxidized, and a substantial fraction of them would be mineralized to CO2. However, such highly oxidized compounds have not been reported to form in substantial amounts in chlorinated waters. Therefore, it seems likely that much of the Cl that gets reduced oxidizes atoms elsewhere in the NOM molecules. Furthermore, considering the fact that the ∆A272-TOX relationship is virtually identical regardless of whether Cl reduction is negligible (early in the Cl-NOM reaction) or very substantial (late in the reaction), it appears that much of the Cl reduction (and NOM oxidation) occurs independently of the Cl incorporation reaction. Reckhow et al. (11) have suggested that some of the electrons acquired by chlorine when the latter is reduced are contributed by organic nitrogen. For NOM with a typical N content of 2%, complete oxidation of the nitrogen from the -3 to the +5 oxidation state could account for approximately
one-third of the electrons consumed by Cl. While the participation of organic N in the reactions with Cl is unlikely to be that extreme, our results lend weight to Reckhow’s suggestion and indicate that N might play a significant role in the Cl consumption reaction.
Acknowledgments This work was supported by the American Water Works Association Research Foundation (Research Project 159), the USEPA (R826645), and HDR Engineering, Inc.
Literature Cited (1) Garcia-Villanova, R. J.; Garcia, C.; Gomez, J. A., Garcia, M. P.; Radanuy, R. Water Res. 1997, 31, 1299. (2) Garcia-Villanova, R. J.; Garcia, C.; Gomez, J. A., Garcia, M. P.; Radanuy, R. Water Res. 1997, 31, 1405. (3) Amy, G. L.; Chadik, P. A.; Chowdhury Z. K. J.sAm. Water Works Assoc. 1987, 79(7), 89. (4) Nokes, C. J.; Fenton, E.; Randall, C. J. Water Res. 1999, 33, 3557. (5) Korshin, G. V.; Li, C. W.; Benjamin, M. M. Water Res. 1997, 31, 946. (6) Li, C. W.; Korshin, G. V.; Benjamin, M. M. J.sAm. Water Works Assoc. 1998, 90(8), 88. (7) Standard Methods for the Examination of Water and Wastewater; 19th ed.; APHA, AWWA, and WPCF: Washington, DC, 1995. (8) Koechling, M. T., et al. Presented at the AWWA Annual Conference, Atlanta, GA, June 1997. (9) Bocelli, D., et al. Presented at the AWWA Annual Conference, Atlanta, GA, June 1997. (10) Huixian, Z.; Sheng, Y.; Xu, X.; Ouyong, X. Water Res. 1997, 31, 1536. (11) Reckhow, D. A.; Singer, P. C.; Malcolm, R. L. Environ. Sci. Technol. 1990, 24, 1655. (12) Singer, P. C.; Obolensky, A.; Greiner, A. J.sAm. Water Works Assoc. 1995, 87(10), 83. (13) Peters, C. J.; Young, R. J.; Perry, R. Environ. Sci. Technol. 1980, 14, 1391. (14) Novak, J.; Mills, G. L.; Bertsch, P. M. J. Envir. Qual. 1992, 21, 144. (15) Traina, S. J.; Novak, J.; Smeck, N. E. J. Envir. Qual. 1990, 19, 151. (16) Croue, J.-P.; Korshin, G. V.; Benjamin, M. M. Isolation, Fractionation and Characterization of Natural Organic Matter in Drinking Water; AWWA Research Foundation and AWWA: Denver, CO, 2000. (17) Boyce, S. D.; Hornig, J. F. Environ. Sci. Technol. 1983, 17, 202. (18) Hanna, J. V.; Johnson, W. D.; Quesada, R. A.; Wilson, M. A.; Xiao-Qiao, L. Environ. Sci. Technol. 1991, 25, 1160. (19) Krasner, S. W.; Croue, J.-P.; Buffle, J.; Perdue, E. M. J.sAm. Water Works Assoc. 1996, 88(6), 66.
Received for review August 4, 1999. Revised manuscript received March 8, 2000. Accepted March 29, 2000. ES990899O
VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2575
