Spectroscopic Studies of the Roles of Distinct Chromophores in NOM

Chapter 11

Spectroscopic Studies of the Roles of Distinct Chromophores in NOM Chlorination and DBP Formation Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: August 5, 2008 | doi: 10.1021/bk-2008-0995.ch011

Gregory V. Korshin and Hyun-Shik Chang Department of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195-2700

This chapter describes the results of in situ examination of chlorination in several waters (Lk. Washington, WA; Cincinnati, OH; Florence, NE; Sioux Fall, SD; Hillsborough River, FL) using the method of differential absorbance spectroscopy. The chlorination of NOM was observed to involve distinct groups of slowly and rapidly reacting chromophores. The fast chromophores in all water sources have identical spectroscopic characteristics while the properties of the slow chromophores exhibit some variability. The fast chromophores are consumed at the beginning of NOM chlorination, but their destruction is not associated with a significant release of total trihalomethanes (TTHM) or haloacetic acids (THAA). The engagement of the slow chromophores is closely associated with the generation of THMs and HAAs. Correlations between TTHM or THAA concentration and the differential absorbance associated with the slow chromophores were similar for all studied waters.

158

© 2008 American Chemical Society In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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159 Since the recognition in the 1970s of chloroform and other trihalomethanes (THMs) as ubiquitous disinfection by-products (DBP) formed in drinking water, numerous volatile and hydrophilic DBP species have been found (1,2). THMs and haloacetic acids (HAAs) are by far the largest DBP groups, typically contributing ca. 50% of the total organic halogen (TOX) in chlorinated water (1,3). Mechanistic models of DBP formation assume that the chlorination of NOM involves stepwise chlorine incorporation into predominantly aromatic reactive groups characteristic for NOM, followed by breakdown of these groups to release smaller DBPs (4-6). Such models can be employed to develop a kinetic description of NOM halogenation, but this approach is limited by uncertainties regarding the chemical and kinetic properties of the reactive N O M functionalities. Phenomenological models of DBP formation (e.g., those based on regressions relating DBP formation to DOC, chlorine dose, pH, etc.) can achieve a tolerable goodness-of-fit, but only by inclusion of a number of sitespecific empirical fitting parameters for each DBP species (5,7). These complexities have necessitated the use of surrogate parameters such as absorbance of light at 254 nm (Λ254) and the specific absorbance at that wavelength (SUVA ) to estimate NOM reactivity towards chlorination (8,9). We previously introduced an alternative approach, in which the intensity of the differential absorbance at 272 nm (ΔΛ272) is measured during the course of chlorination and correlated with the formation of specific DBPs and/or TOX concentrations generated in the solution (10-12). The differential spectra of chlorinated NOM from many natural waters exhibit common features (10-12), most notably a prominent band with a maximum located between 265 to 275 nm. The intensity of this band increases at higher reaction times and chlorine doses. Its shape also exhibits subtle changes as the reaction progresses (13), but the significance of these changes have not been exhaustively examined. In our recent study (14), we presented a view that changes of the shape of the differential spectra of NOM in a representative surface water (Lk. Washington, WA) are indicative of the engagement of spectroscopically and kinetically distinct functionalities which were termed "fast" and "slow" chromophores due to their appearance in the initial phase of NOM chlorination and later in the reaction, respectively. It was also shown that the appearance of selected individual DBPs (exemplified in reference (14) by chloral hydrate and dichloroacetic acid) was closely correlated with the engagement of the slow chromophores, but not with that of the fast chromophores. In this paper, we will examine whether these conclusions are applicable to a more diverse group of waters and individual DBP species and whether the technique of differential spectroscopy can be amended to determine yields of the most important groups for DBP species, total THMs and HAAs (TTHM and THAA) based on in situ measurements. 254

In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

160


Experimental Experiments reported in this study were carried out using water from Lake Washington (LW) located in Seattle, WA and water samples provided by Cincinnati, Florence and Sioux Falls water treatment plants (WTP). High-DOC water from Hillsborough River (Tampa, Fl) was also used in the study, after six fold dilution with deionized water. Main characteristics of the water samples are summarized in Table I. In all cases, chlorination was carried out at 20°C, pH 7.0 with 0.03 M phosphate buffer. All experimental procedures followed established practices that have been described in previous publications (e.g., 11). Chlorine was added as HCIO and is reported here as the equivalent amount of Cl . The chlorine dose is presented in terms of the weight ratio of C l added to the concentration of dissolved organic carbon (Cl :DOC). Reaction time was varied from 5 minutes to 7 days. Absorbance spectra were measured with a dual-beam Perkin-Elmer Lambda-18 spectrophotometer using 5-cm quartz cells. 2

2

2

Table I. Selected Characteristics of Waters Used in the Study Name (Location)

DOC, mg/L 3.0 2.3

170

5.0

0.024

230

8.5

0.023

85

27.5

0.049

}

Lake Washington (Seattle, Wash.) Cincinnati WTP (Cincinnati, OH) Missouri R./ Florence WTP (Metropolitan Utilities District, Omaha, Neb.) Big Sioux Aquifer (Sioux Falls Public Works, Sioux Fall, S.D.) Hillsborough R. (City of Tampa, Tampa, Fla.)*

SUVA , cm~'mg~'L 0.022 0.020

Alk, mg/L as CaCO 40 70

254

Hillsborough R. water was diluted 6 times prior to experiments

Results and Discussion The differential spectra of all waters examined in this study exhibited typical features observed in prior research (10-12). Specifically, the intensity of the differential absorbance at λ>250 nm increased with increasing reaction time and Cl :DOC ratio, and all differential spectra had a characteristic band with a 2


161 maximum around 272 nm (Figure 1). The shapes of the overall differential spectra underwent consistent changes in response to increases in reaction time and/or chlorine dose. When the differential spectra generated at varying reaction times were normalized by dividing all values by the maximum value of -ΔΑ in that spectrum, the characteristic band steadily broadened as the reaction progressed, as shown in Figure 1 for Missouri River water sampled at the Florence WTP. A similar trend in the width of the normalized spectrum characterized all the water sources investigated. We quantified this trend by tracking the wavelength at which the normalized differential absorbance was 0.5, as illustrated in Figure 1 for chlorinated Florence WTP water. This parameter is denoted henceforth as λ .5> ΗΊ,


0

Wavelength, nm Figure 1. Changes of normalized differential absorbance spectra of chlorinated Florence WTP water at varying reaction times. Cl DOC= 1.5, pH 7.0,20°C 2

For all examined waters, λ increased with reaction time and intensity of the differential absorbance,-A4 72- Correlations between À" .5 and -AA for four waters are shown in Figure 2. The curves are nearly linear and, although their slopes appear to be close, the intercepts exhibit some site-specificity that is prominent for Sioux Falls water. The reason that the curve for the Sioux Fall site is offset from the others is not clear, but Sioux Falls water is the only groundwater in the group, and it is possible that its NOM is therefore different from the NOM in the other waters. The monotonie increase of A .5 Figure 2 indicates a steady broadening of the characteristic band in the differential spectra of NOM at increased reaction 0 5

D

2

0

272

ND

0


162 330 325 + 320 315

• Cincinnati ο Florence • Hillsborough A Sioux Falls

310 305


300 295 290 0.00

•a? -f-

0.02

0.04

0.06

0.08

-Differential absorbance at 272, cm'

0.10 1

ND

Figure 2. Comparison of changes of X .5 values as a function of overall differential absorbance values measured at 272 nm. 0

times and/or chlorine doses. It has been shown in our studies that this phenomenon is a manifestation of the presence of two distinct components in the differential spectrum whose contributions can be quantified via numerical deconvolution (14). Figure 3 shows the normalized spectra of these components obtained for chlorinated Lk. Washington water. In reference (14), it was also established for Lk. Washington water (and confirmed in this study, as will be described in the detail in the sections that follow) that these components are associated with chromophoric groups that either rapidly react with chlorine or emerge later in the reaction, and for that reason these components are referred to as fast and slow chromophores, respectively. Because the differential spectra can be characterized using only two independent components, their contributions can be discerned using two wavelengths at which the relative absorbances of these components are sufficiently different. One possible such combination uses the wavelengths 272 and 350 nm. The former wavelength is close to the maxima of both components shown in Figure 3, while at the latter wavelength, the difference in the normalized absorbances of these components is pronounced. In our prior publication, we utilized the ratio of the differential absorbances at 272 and 350 nm to track the evolution of these components. A more mathematically transparent approach can be used to examine the behavior of the slow and fast chromophores. For instance, one can use the values of relative normalized absorbances of the fast and slow chromophores at 272 and 350 nm (denoted as et?, ε(%' and ^ ' r , ε* ™ for the fast and slow 1


163 chromophores, respectively) and measurements of the differential absorbances at 272 and 350 nm to calculate, using appropriate matrix algebra, the values of differential absorbances assigned to these functionalities and denoted as AA and AS ii , respectively. (Note that s/ and e values are not true molar absorbances but rather indicators of the relative shapes of the two dissimilar absorbance spectra.) Mathematically, this concept can be expressed via the following two equations that define the values of differential absorbance at 272 and 350 nm for any combination of initial chlorine dose or reaction time: ghast

ast

a

slow

ow

À

e%M +e%M Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: August 5, 2008 | doi: 10.1021/bk-2008-0995.ch011

fasl

slow

=M

(1)

212

egMjH+sSïAA+.-Mn

(2)

Numerical deconvolution of the differential spectra generated for chlorinated Lk. Washington water showed that the values of e*£, ε££, ε ^ and ^7 were 1.00, 0.15, 0.99 and 0.38, respectively. 5

as

A major hypothesis explored in this study was that the e[ \ ef matrices are similar for all studied waters. To test this hypothesis, it was assumed that the evolution of Mf and AA as a function of the overall -AA values for all waters will have a pattern similar that observed for Lk. Washington. That is, during the initial phase of chlorination, only the fast chromophores characterized by a sharp peak close to 272 nm and a low value of ε{^ would be present. ast

slow

272

1

However, for -AA values above a certain threshold value (e.g., -0.008 cm" for Lk. Washington), the contribution of the fast chromophores is expected to reach a plateau while that of the slow chromophores to increase rapidly. When ε[ \ ε * matrices with components practically identical to those for Lk. Washington were applied for Florence and Cincinnati WTP waters (only the value of ^ w a s adjusted to 0.35 and 0.37, for Florence and Cincinnati, 272

α$

1θλν

λ

respectively), a pattern of behavior closely matching that described above was observed (Figure 4). Specifically, the contribution of the fast chromophores was at a plateau for all reaction periods, except the shortest reaction times for which chlorination experiments and absorbance measurements could be carried in the conventional mode (5 and 10 minutes). At the same time, the contribution of the slow chromophores emerged and increased quasi-linearly only in a -ΔΛ range exceeding a threshold value of ca. -0.006 and 0.010 for Cincinnati and Florence WTP water, respectively (Figure 4). (The region of very small ~M values was not probed in these experiments because that would require using very small chlorine doses and extremely short reaction times). A virtually instantaneous emergence and ensuing stabilization of the contribution of the fast chromophores 272

272



164

2

0.00 -I 250

1

h

270

.

1

290

h~

1

1

310

330

350

1

h

370

390

Wavelength (nm) Figure 3. Normalized spectra of the fast and slow chromophores contributing to f the differential spectra of chlorinated NOM, as inferred by deconvolution of the differential spectra from the chlorination ofLk. Washington water. (Reproducedfrom reference 14. Copyright 2007 American Chemical Society.)

0.000

0.010

0.020

0.030

0.040

0.050

1

-ΔΑ 72> cm" 2

Figure 4. Development of contributions offast and slow chromophores as a function of overall differential absorbance measured at 272 nm. Data for chlorinated Florence and Cincinnati water, pH 7.0.


165 and a steady development of the slow chromophores can be additionally demonstrated when the values of -ΔΑ272, AA and AA i are represented as a function of reaction time, as exemplified in Figure 5 for Florence WTP water. Kinetic profiles represented in that figure clearly show that the former group emerges immediately following the introduction of chlorine and does not appreciably change throughout the entire reaction period. The slow chromophores also emerge quite rapidly but their contribution increases monotonically with time. fasl

s ow


0.050

S 0.000 -I 0

1 1000

1 1 2000 3000 Reaction time, minutes

1 4000

Figure 5. Time profile of the differential absorbance at 272 nm and contributions of the fast and slow chromophores in Florence WTP water chlorinated at pH 7.0, Cl /DOC ratio 1.5. 2

Data similar those shown in Figure 4 and Figure 5 were obtained for Sioux Falls WTP and Hillsborough River water. In all cases, practically identical s/ , s/ matrices were used, and only the value of ε^ζ was adjusted to 0.34 and ast

iow

0.46 for Sioux Falls and Hillsborough waters, respectively. It is unclear why the normalized relative absorbance of the slow chromophores at 350 nm exhibits some variability. It was noted, however, that the value of f'Jjj" was considerably higher in Hillsborough water that also had the highest S U V A 2 5 4 value (see Table I). Whether this is a manifestation of some sensitivity of the spectroscopic properties of the slow chromophores to the overall NOM aromaticity needs to be explored separately.



166 In sum, the quantitation of the behavior of the slow and fast chromophores in a variety of natural waters using measurements of differential absorbances at 272 and 350 nm and largely identical matrices of normalized absorbance coefficients indicated a similarity of the observed patterns. Given that, we proceeded to examine a related hypothesis, which was concerned with relationships between the emergence of the slow chromophores and generation of THMs and HAAs. This hypothesis assumes that that the behavior of the slow chromophores and release of individual DBPs, most importantly THMs and HAAs, should be closely correlated. Indeed, our measurements carried out for dichloroacetic acid and chloral hydrate formed in Lk. Washington water indicated the existence of such correlations, which were more linear than those for the latter DBP species and, on the other hand, overall -M 72 values (14). Prior research established that the correlation between the concentration of virtually any stable DBPs and the overall -ΔΑ 72 value is invariably very strong. At the same time, these correlations are non-linear and have a threshold -ΔΑ value below which little individual DBPs are generated (10-14). Major trends in the behavior of TTHM and THAA concentrations formed in several waters and plotted in the -ΔΛ domain (Figure 6 and figure 7; data for Hillsborough River water are not shown for simplicity) were identical to those described in prior research. To test whether the formation of TTHMs and THAAs is associated with that of the slow chromophores, the TTHM and THAA concentrations were also represented vs. their respective AA values (Figure 8 and Figure 9). Examination of these figures shows that there are three features that distinguish these plots from those shown in Figure 6 and Figure 7. Fist, in comparison with the behavior of TTHM and THAA as a function of overall -AA values, the TTHM or THAA vs. M relationship appear to be more linear. At the same time, the intercept of these correlations with the y-axis is non zero. This is likely to indicate that some level of TTHM and THAA formation takes place when the fast chromophores interact with chlorine. Second, the threshold -M values prominent in Figure 6 and Figure 7 below which little DBPs are formed, are absent in the AA i plots. Third, the slopes of THAA and THM vs. the -AA correlations are comparable and generally follow one imaginary trend line. This behavior is distinct from that shown in Figure 6 and Figure 7, where the data for different waters are positioned in different areas of the graph. The similarity of the relationships between the yield of major DBP classes and AA values is additionally shown in Figure 10. In that case, sums of THAA and THM concentrations for all the studied waters are compared. To demonstrate the similarity of these correlations, a best fitting line developed for the combination of all TTHM and THAA data obtained for all waters is shown in Figure 10. The experimental data for individual waters shown in Figure 10 do not deviate significantlyfromthat best-fit line. This can be interpreted to signify that 2

2

272

272

slow

272

slow

272

s ow

slow

simv


167

180 Δ Florence • Lk. Washington 140 + u Sioux Falls

'ft, Ο) 160

J •2

S

100

S

80

Ο Ο

60

S Χ

40

\Z

20

c Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: August 5, 2008 | doi: 10.1021/bk-2008-0995.ch011

120

0 0.000

0.010

-4-

-f-

0.020

0.030

0.040

0.050

0.060

0.070

1

ΔΑ72> cm" 2

Figure 6. Relationships between the formation of total trihalomethanes (TTHM) and the intensity of differential absorbance at 272 nm in Florence, Lk. Washington and Sioux Falls water chlorinated at pH 7.0.

Δ Florence • Lk. Washington » Sioux Falls ο Cincinnati

Ο Ο

J? Ο

0.000

1 0.010

·

Δ

6

Δ

0.020

-+-

-+0.030

0.040

0.050

0.060

0.070

1

-ΔΑ 72> cm" 2

Figure 7. Relationships between the formation of total haloacetic acids (THAA) and the intensity of differential absorbance at 272 nm in Cincinnati, Florence, Lk. Washington and Sioux Falls water chlorinated at pH 7.0.


168

180 Δ Florence • Lk. Washington ο Sioux Falls

Ο) 140

α Δ

Ο 120


±5 100

8

80

g

60

S

40

t

20

Β

β

0.000

«

β

Ο

*Δ·

1

1

0.005

0.010

· Δ

Û

1

H

0.015

0.020

0.025

1

0.030

h

0.035

0.040 -1

Differential absorbance of slow chromophores, cm

Figure 8. Comparison ofrelationships between the formation of total trihalomethanes and the differential absorbance associated with the slow chromophores in Florence, Lk. Washington and Sioux Falls water chlorinated at pH 7.0.

200 D) 180

Δ Florence • Lk. Washington Ο Sioux Falls Ο Cincinnati

160 140 120 Φ Ο C Ο ο

Spectroscopic Studies of the Roles of Distinct Chromophores in NOM

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