Chapter 8
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The Effect of Structural Characteristics of Humic Substances on Disinfection By-Product Formation in Chlorination 1
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Wells W. Wu , Paul A. Chadik , William M. Davis , Joseph J. Delfino , and David H. Powell 1
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Departments of Environmental Engineering Sciences and Chemistry, University of Florida, Gainesville, FL 32611 Environmental Laboratory, E S - P , Waterways Experiment Station, Vicksburg, MS 39180 2
The influence of structural characteristics of humic substances on disinfection by-product (DBP) formation was investigated for seven humic substances isolated from aquatic and terrestial sources. The structural characterizations included elemental analysis, total acidity titration, ultraviolet/visible (UV/VIS) spectroscopy, and C nuclear magnetic resonance (NMR) spectroscopy. The aqueous humic substances were chlorinated at p H 7.0 and 8.5, with and without the presence of the bromide ion to produce trihalomethanes and haloacetic acids. C/H and C/N ratios, correlated well with D B P formation for the humic substances investigated. Based on the results of computation with respect to total acidity, the contribution of carboxylic groups to the C/H ratio of the humic substances studied was not significant. The decrease in UV absorbance at 254 nm (∆UV254) and U V absorbance at 254 nm varied linearly with the formation of both trihalomethanes and haloacetic acids, with the ∆UV254 consistently showing better correlation. Both aromatic content and phenolic character determined by C-NMR correlated well with DBP formation. 13
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The relationship between aromatic content determined by C-NMR and other techniques were evaluated. Aromaticity data by C -NMR correlated well with those obtained by other methods that are independent of N M R spectroscopy. This paper presents techniques other than N M R that would be attractive alternatives for predicting D B P formation. 13
© 2000 American Chemical Society
In Natural Organic Matter and Disinfection By-Products; Barrett, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
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Introduction Chlorine, the most widely used disinfectant in the United States, reacts with humic substances to form disinfection by-products (DBPs) (/). D B P formation during chlorination has been correlated with several non-specific parameters characteristic of humic substances such as dissolved organic carbon and chlorine demand (2); however other structural characteristics, less frequently used in characterizing D B P formation, may provide additional insight regarding the type and amount of specific DBPs formed in a given chlorination condition. The nature and extent of interaction between humic substances and disinfectant species depend on numerous structural factors. For example, disinfectants such as chlorine are electrophiles and tend to react with electronrich sites such as aromatic structures in humic substances. Accordingly, methods to characterize these electron-rich sites in terms of aromaticity may be useful in predicting and understanding D B P formation (3). The objective of this research was to investigate the relationship between selected structural characteristics of seven humic substances and the DBPs formed under a variety of chlorination conditions. The structural characterizations used in this research included elemental analysis, total acidity titration, ultraviolet/visible (UV/VIS) spectroscopy, and C nuclear magnetic resonance (NMR) spectroscopy. The target DBPs were four trihalomethanes (THMs) and nine haloacetic acids (HAAs). , 3
Materials and Methods Humic Substances Seven humic substances were extracted and purified from water, soil, sediment, and commercial sources (4): four aquatic humic substances, Santa Fe River (SFR), St. Marys River (St MR), Newnans Lake (NL DOC) and a surficial groundwater source from Orange Heights (OH DOC); a soil humic substance from Orange Heights soil (OHS); a sediment humic substance from Newnans Lake bottom sediment (NLS); and a commercial source (Aldrich). The extraction technique involved acidification and concentration on a macroporous resin, and elution of the humic substance by backflushing the column with dilute base. The humic substances were then freeze dried until reconstituted with purified water. The objective of this research was to investigate how selected characteristics of humic substances can be related to D B P formation. In addition to the aquatic humic substances, soil, sediment, and commercial humic substances were included to increase diversity.
Reagents A l l D B P calibration standards, internal standards, and surrogate were purchased from either AccuStandard Inc., Aldrich Chemical Co., Supelco Inc., or U L T R A Scientific. A l l other chemicals employed in this study were reagent or certified grade (Fisher Scientific Co.). The reagent water used was Type 1 water (J) produced by
In Natural Organic Matter and Disinfection By-Products; Barrett, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
Ill Barnstead NANOpure ultrapure water system. Sodium hypochlorite solutions prepared from reagent grade NaOCl were used as a source offreechlorine.
Characterization of Humic Substances The seven humic substances were characterized by a variety of techniques: including elemental analysis, total acidity titration, U V / V I S spectroscopy, and C - N M R spectroscopy. The results of these characterizations are presented in Tables I and II.
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Table I. Results of characterization of the humic substances: elemental composition (percent), total acidity titration, and C / H , C / N and CJH^ ratios (4) Elements Source
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b
c
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Mois -ture
Ash
Aldrich
2.06
1.50
50.8
3.57
0.60
45.0
1.19
98.9
14.5
1.24
NLS
0.81
15.6
42.3
4.76
2.51
50.4
0.74
19.7
19.9
0.69
OHS
2.60
12.6
47.7
4.41
0.74
47.2
0.90
75.1
18.7
0.88
SFR
1.65
1.15
49.4
3.69
0.77
46.1
1.12
74.9
17.0
1.14
St M R
0.62
0.43
51.2
3.82
0.61
44.4
1.12
97.8
18.1
1.15
OHDOC
1.20
2.35
50.5
4.10
0.49
44.9
1.03
120.3
15.5
1.04
N L DOC
0.28
6.62
50.2
4.60
0.79
44.4
0.91
74.3
15.2
0.89
C
H
Ν
Ο
C/H
b
C/N
b
Total Acidity
C
rest
/
c
^rest
Corrected for moisture and ash content. Molar ratios meq H 7 g humic carbon.
Elemental analyses of C, H , and Ν were carried out on a Carlo Erba Model 1106 C H N analyzer. The oxygen content was determined by difference based on the C, H , and Ν content, after these values had been corrected for moisture and ash content. The moisture and ash content of each humic substance were measured by drying and ignition, respectively. The elemental composition of all samples of humic substances presented in Table 1 were analyzed in duplicate; the C/H, C/N, and C / H ratios will be discussed later. The Ba(OH) titration method (
