Environ. Sci. Technol. 19Q0,24, 1655-1664
Atmospheric Administration Contract 83-ABD-00012to J.M.T. and grants in aid of research to A.E.M. from the Andrew W . Mellon Foundation through the Coastal Research Laboratory at the Woods Hole Oceanographic Institution, the National
Wildlife FederationlAmerican Petroleum Institute Environmental Conservation Fellowship program, and Sigma X i . Contribution No. 6825 of the Woods Hole Oceanographic Institution.
Chlorination of Humic Materials: Byproduct Formation and Chemical Interpretations David A. Reckhow'
Department of Civil Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Phlllp C. Slnger
Department of Environmental Science and Engineering, University of North Carolina, Chapel Hill, North Carolina 275 14 Ronald L. Malcolm U S . Geological Survey, Denver, Colorado
Ten aquatic humic and fulvic acids were isolated and studied with respect to their reaction with chlorine. Yields of TOX, chloroform, trichloroacetic acid, dichloroacetic acid, dichloroacetonitrile, and l,l,l-trichloropropanone were measured at pH 7 and 12. Humic acids produced higher concentrations than their correspondingfulvic acids of all byproducts except l,l,l-trichloropropanone.Chlorine consumption and byproduct formation were related to fundamental chemical characteristics of the humic materials. A statistical model was proposed for activated aromatic content based on 13CNMR and base titration data. The values estimated from this model were found to be well correlated with chlorine consumption. Specific byproduct formation was related to UV absorbance, nitrogen content, or the activated aromatic content.
Introduction Since the early work of Rook (1, 2) and Stevens (3), aquatic humic materials have been strongly implicated as the principal organic precursors for trihalomethanes (THMs) in drinking water. The connection between humics and THMs is based in part on similarities in the rates of THM formation and THM yields for natural waters and extracted aquatic humic substances. In a similar fashion, humic materials have also been linked to the formation of total organic halide (TOX) in drinking waters (4-6). With recent advances in the identification of chlorination byproducts and their health effects, a greater appreciation for the non-THM organic halides exists. Several investigators have shown that a major fraction of the nonvolatile TOX produced by chlorination can be accounted for by trichloroacetic acid (TCAA) and dichloroacetic acid (DCAA) (6-8). The major volatile chlorination byproducts (aside from the THMs) include the haloketones (e.g., l,l,l-trichloroacetone or TCAC) (9) and the haloacetonitriles (e.g., dichloroacetonitrile or DCAN (10, 11). Members of these groups of halogenated compounds are currently under consideration by the U.S. EPA for possible regulation. Numerous other volatile (12) and nonvolatile (7) chlorination byproducts have been identified at trace levels. Some of these lesser byproducts, e.g., 3-chloro-4(dichloromethyl)- 5-hydroxy-2(5H)-furanone (MX), are major contributors to the overall mutagenicity of chlorinated waters (13). 0013-936X/90/0924-1655$02.50/0
The chemical basis for organic halide formation from the chlorination of humic materials is not well understood. Aquatic humic materials are thought to have a moderate aromatic character (- 25% of the total carbon) with large numbers of carboxyl groups, some phenolic groups, alcohol OH groups, methoxyl groups, ketones, and aldehydes. Degradative structural studies have suggested the presence of significant phenolic content, especially when the OH groups are protected by prior methylation (14). This may be important, because activated aromatic structures such as phenolics are known from model compound studies to be especially reactive with chlorine, producing large amounts of chlorinated byproducts (15-1 7). Rook (18) proposed that resorcinol structures in aquatic humic materials may be the major THM precursor in colored waters. Similarities in the higher molecular weight byproducts of resorcinol derivatives and humic acids provide circumstantial evidence in favor of this hypothesis (19). However, the presence of resorcinol structures in aquatic humic substances has not been firmly established. Also, many other structures considered to be plausible humic acid monomers can form significant amounts of chloroform, although their molar yields are generally less than with resorcinol. Considering other principal chlorination byproducts besides THMs, it is clear that a wide range of structures could be important. For example, while resorcinol produces relatively small yields of trichloroacetic and dichloroacetic acids, another likely fulvic acid monomer, syringaldehyde (20),does give significant yields of these halo acids (17). A number of researchers have tried to correlate humic acid or raw water characteristics to trihalomethane formation potential (THMFP) in an effort to find a useful surrogate parameter for THMFP or to better understand the chemical nature of THM formation from natural organic matter. Studies using raw and treated natural waters have shown excellent correlations between THMFP and UV absorbance (254 nm) (21,22). These correlations have been found to be stronger than the corresponding correlations between THMFP and TOC. Oliver and Thurman (23) conducted a study of the relationship between THMFP and a variety of fundamental characteristics &e., UV absorbance, molecular size, carboxylic and phenolic OH content) of 12 aquatic fulvic acids. They found that the strongest correlations were with color. Molecular size was also well correlated with THMFP, probably because
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, NO. 11, 1990
1655
of its strong degree of covariance with color. Phenolic content showed a rather poor correlation with THMFP. As a result, these authors concluded that THM precursors are more likely to be the highly conjugated chromophoric systems rather than the hydroxylated aromatic structures. This paper presents new information on the relationship between certain structural characteristics of natural organic material and THM formation. In addition, it relates these characteristics to broader questions of chlorine reactivity and major byproduct formation. The experimental objectives of this study were as follows: (1)to investigate the variability of different aquatic humic substances with respect to organic halide formation; (2) to relate organic halide formation in the humic materials to selected fundamental properties of the humics; and (3) to develop simple, yet chemically meaningful models for chlorinehumic reactions and byproduct formation. The chlorination byproducts chosen for study were the trihalomethanes (THMs), total organic halide (TOX), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA), 1,1,1-trichloroacetone (TCAC), and dichloroacetonitrile (DCAN). A total of 10 aquatic humic and fulvic acids were used in this study. All 10 humic materials were characterized with respect to elemental composition, 13CNMR spectroscopy, low-angle X-ray scattering, acid-base titration, and UV-vis absorbance.
Experimental Section Fulvic and Humic Acids. All aquatic humic and fulvic acids were isolated according to the hydrophobic resin procedure of Thurman and Malcolm (24). However, two of the humic samples (Ogeechee River humic acid and Black Lake fulvic acid) used in the first chlorination experiment (in which reaction time was varied) were prefiltered with cellulose cartridge filters rather than silver membrane filters. The Black Lake fulvic acid from this first experiment was prepared in the laboratories of the University of North Carolina. This particular humic material has been extensively characterized (6, 7, 17, 25-30). The Black Lake fulvic acid used in subsequent experiments and all nine remaining humic materials were isolated during the Spring season by Ronald L. Malcolm of the US. Geological Survey. These U.S. Geological Survey aquatic humic materials have also been the subject of intensive investigations (31-38). For the first set of experiments (varying reaction time), humic solutions of 10 mg/L dry weight were prepared (dried over CaCl,). Due to large variabilities in ash content, the organic carbon concentrations ranged from 1.9 to 5.5 mg/L. For the second set of experiments (effect of pH), solutions of equal carbon concentration were prepared based on the ash content and elemental analysis. This procedure incorporated a more rigorous drying step using p2°5*
Chlorination Conditions. Sample chlorinations were performed according to the following set of conditions unless otherwise noted: (1)pH 7.0, phosphate buffer (I = 0.028); (2) 20 mg/L chlorine dose; (3) Incubation at 20 "C in darkness; (4) at the end of the prescribed incubation period, quench with 120% of the requisite stoichiometric dose of sodium arsenite. Residual chlorine was measured in duplicate by the DPD-ferrous titrimetric procedure (39) immediately prior to quenching. Organic Halide Analyses. Volatile and nonvolatile specific organic halides were quantified in duplicate extractions by two separate solvent extraction-GC-ECD methods (6). Following sample cleanup, haloacids were extracted with diethyl ether at pH 0.8 and methylated with 1656
Environ. Sci. Technol., Vol. 24, No. 11, 1990
diazomethane. Volatile organohalides were extracted directly in pentane from NaC1-saturated aqueous samples at pH 7 and analyzed without further treatment. Total organic halides were determined in triplicate by the standard adsorption-pyrolysis-microcoulometryprocedure (39).
Base-hydrolyzable chloroform was measured by a procedure similar to that of Morris and Baum (40). Immediately after being quenched with arsenite, each sample was divided into two aliquots. One aliquot was analyzed immediately for CHCl,, and the other was spiked with sufficient NaOH to raise the pH to 12 and analyzed 3 h later. Base-hydrolyzable chloroform, or "base-CHCl;, was calculated as the difference between the two determinations. Physicochemical Analyses of Humic Materials. Carboxyl content was determined by rapidly titrating the proton-saturated material with NaOH to pH 8.0 (41). Phenolic content was then estimated at 2 times the alkali consumption observed during rapid titration from pH 8.0 to 10.0 (41). Comparison of this method with more sophisticated methods affording high resolution of oxygenated functional groups (42,43) has shown good agreement for the analysis of aquatic humic substances (44). The 13C NMR solid-state spectra were obtained during a cross-polarization (CP) experiment while spinning at 4 KHz at the magic angle (MAS). As a result of a variable contact time experiment, the TcHand T Icross-polarization parameters for the most quantitative spectra were obtained at a l-ms contact time and a repeat time of 1 s. The CPMAS 13C NMR spectra were recorded on a custommade spectrometer at the laboratory of Dr. Gary Maciel at the Regional NMR Facility at Colorado State University. The IH frequency was 90.1 MHz, the 13Cfrequency was 22.6 MHz, acquisition time of 1024 ms, sweep width of 531.11 ppm, and line broadening of 39.999 Hz. The number of scans varied from 3500 to 10000. Aromatic carbon content was estimated from the peak areas between 108 and 162 ppm. Aliphatic carbon was considered to comprise the remainder of the spectrum. Elemental analyses of humic and fulvic acids were performed by Huffman Laboratories, Golden, CO. All analyses are direct independent determinations including oxygen. Molecular size was determined by small-angle X-ray-scattering techniques (45). Ultraviolet absorbance (254 nm) and color (400 nm) were measured on a Cary Model 25 double-beam UV-vis spectrophotometer with 1-and 5-cm quartz cells, respectively.
Results For the first set of experiments,aquatic humic and fulvic acids were dissolved in a pH 7 buffer and chlorinated with sodium hypochlorite. At the end of the reaction period (1,3, or 7 days), samples were quenched and analyzed for chloroorganic byproducts. Figure 1 shows the results for chloroform,TOX, TCAA, DCAA, and TCAC with 7 of the 10 aquatic humic materials. To facilitate comparison of the humic substances, chloroorganic concentrations have been normalized to a common initial TOC concentration of 1mg/L. A linear relationship between THM formation and TOC in samples diluted with high-purity water is well established (4,5,8,18,46-48). The data in Figure 1 indicate that (1)there is a significant difference among the humic materials with respect to specific chloroorganic yield and (2) there are strong similarities among the humic materials with respect to the shapes of the formation curves. Because of these similarities, it was decided that only one reaction time need be studied in order to assess the relative
Table I. Elemental Composition of Ten Extracted Aquatic Humic Substancesa source Black Lake
F H F H F H F H F H
Coal Creek Ogeechee River Ohio River Missouri River a
F, fulvic acid: H. humic acid.
elemental analysis,b % 0 N S
ash, %
C
H
0.47 5.54 1.26 5.61 0.39 3.85 0.38 1.49 0.26 41.80
55.01 55.63 52.80 56.09 54.08 54.61 55.03 54.99 54.55 52.46
4.58 4.13 4.45 4.01 4.82 4.90 5.24 4.84 5.17 5.57
38.43 35.43 38.39 36.47 38.49 36.80 36.08 33.64 36.94 39.75
1.16 1.97 0.95 1.38 0.93 1.56 1.42 2.24 1.39 2.46
0.57 0.58 0.70 1.45 1.27 1.78 2.00 1.51 1.64 2.22
-
P
X
ND ND ND 0.05 0.20 0.36 0.34 0.06 0.64 1.45
0.11 0.25 0.06 0.07 0.05 0.08 0.18 0.13 0.10 0.02
ND, not determined.
Table 11. Characterization of Ten Extracted Aquatic Humic Substancesn 13C NMR 70 arom % aliph
source
F H F H F H F H F H
Black Lake Coal Creek Ogeechee River Ohio River Missouri River a
17 35 19 31 19 31 14 34 14 30
acidic groups, mequiv/g carboxyl phenolic
mol size, A
83 65 81 69 81 69 86 66 86 70
6.1 11.2 7.3 10.3 9.2 12.3 5.5 9.0 7.3 9.2
5.7 4.4 6.1 5.0 5.8 4.0 5.0 3.8 5.7 2.8
1.5 1.6 2.1 2.6 1.8 2.0 1.0 1.5 0.8 5.4
specific absorbance, nm 254
400
0.0412 0.0736 0.0428 0.0540 0.0370 0.0486 0.0286 0.0544 0.0300 0.0594
0.0039 0.0147 0.0047 0.0084 0.0038 0.0071 0.0026 0.0081 0.0029 0.0089
F, fulvic acid; H, humic acid.
Table 111. Chlorination of Ten Extracted Aquatic Humic Substances at pH 7 (Raw Data)a chlorine consumed, mg/mgof TOC
TOX
CHC1,
F H F H F H F H F H
1.48 2.28 1.64 2.02 1.52 2.12 1.24 2.14 1.10 2.14
208 288 232 268 216 262 161 232 136 230
48.6 68.2 50.8 59.6 49.8 59.6 32.6 47.2 30.8 49.8
50.2 98.6 66.6 92.8 60.6 90.6 30.6 70.6 23.6 62.6
17.8 30.0 25.0 27.4 22.0 32.0 16.6 27.2 11.8 24.4
F
1.40 2.14
191 256
42.6 56.8
46.2 83.0
18.6 28.2
source Black Lake Coal Creek Ogeechee River Ohio River Missouri River av
H a
specific yield, fig/mg of TOC TCAA DCAA TCAC
DCAN
base-CHC1,
1.5 1.8 1.4 1.0 1.6 0.8 1.5 1.3 1.7 1.2
0.64 1.54 0.74 1.06 0.60 0.90 0.66 1.62 0.58 2.24
10.0 13.6 13.0 12.0 12.0 12.8 9.0 12.8 8.0 11.2
1.5 1.2
0.64 1.48
10.4 12.4
F, fulvic acid; H, humic acid.
differences among the humic materials for long-term organohalide formation. Accordingly, the second set of chlorination experiments were conducted using a 3-day reaction period only. The second chlorination experiment was designed to examine differences in chloroorganic yields and the efect of high pH on these yields. The humic materials used for this work were characterized as to their aliphatic and aromatic content (13C NMR), molecular size (small-angle X-ray scattering), elemental analysis, carboxyl and phenolic content (potentiometric titration), and UV-vis spectral absorbance (see Tables I and 11). Results from the chlorination of these materials are shown in Tables I11 and IV. Note that in addition to the specific organohalides and TOX, chlorine consumption and base-hydrolyzed chloroform (base-CHC1,) data are also shown. TCAC, DCAN, and base-CHC1, are not shown in Table IV, be-
Table IV. Chlorination of Ten Extracted Aquatic Humic Substances at pH 12 (RawData)" chlorine
specific yield, pg/mg of TOC mg/mgofTOC TOX CHC13 TCAA DCAA consumed,
source Black Lake CoalCreek
F H F H
Ogeechee River F H Ohio River F Missouri River av
H F
H F
H
1.28 1.74 1.40 1.60 1.20 1.56 1.06 1.54 0.76 1.52
119 138 126 123
0.5 0.8 0.7 0.7 0.5
121
66.0 73.4 68.6 68.0 67.6 63.6 54.2 68.2 42.6 65.4
1.14 1.60
108 130
59.8 67.8
0.6 0.8
129
128 96 138 69
0.6
0.7 0.9 0.3 0.9
20.8 31.8 27.2 28.0 27.6 30.6 16.0 25.2 13.2
24.6 21.0 28.0
F. fulvic acid: H. humic acid. Environ. Sci. Technol., Vol. 24, No. 11, 1990
1657
100-
80 --
K N
60 --
.Coal C r HA OOgeschee R. HA ACoal Cr FA AOgcechee R FA .Ohio R FA OMlssourl R FA
40 --
$g
20 --
50 0
t--
3 --
10
2 --
1 --
0 0
50 100 150 200 Reaction Time (hours)
0
50
100 150 200
Reaction Time (hours)
0
50 100 150 200 Reaction Time (hours)
Formation of chlorination byproducts as a function of reaction time from six aquatic humic materials. All concentrations are in micrograms of byproduct per milligram of C. Byproducts are total trihalomethanes (TrHM), total organic halide (TOX), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA), and trichloroacetone (TCAC); reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 20 O C . Flgwe 1
I
cause TCAC and DCAN were always below the detection limit (generally C0.2 pg/L), and base-CHC1, is meaningless for alkaline chlorination. The average yields for all five fulvic acids and all five humic acids are shown at the bottom of both tables.
Discussion Characterization of Humic and Fulvic Acids. The elemental analysis of the 10 aquatic humics (Table I) shows a typical composition, about 52-56% carbon, 590 hydrogen, 35-4090 oxygen, 1-2% nitrogen, and 1-2'37 sulfur by weight (32,35,38,49,50).Phosphorus and halide contents were minor. The humics were slightly richer in nitrogen and ash, whereas the fulvics tend to be richer in oxygen. The 13C NMR data (Table 11) indicate 14-19% aromatic carbon for the fulvic fractions, with the majority of the carbon being aliphatic in nature. This is in agreement with the work of others (20, 32, 35, 38). The humic fractions show a much larger aromatic content (30-35%) with a corresponding smaller aliphatic component. Molecular size measurements range from 5.5 to 9.2 A for the fulvics and from 9.0 to 12.3 A for the humics. These values are typical for low-angle light scattering on aquatic humic materials (31,34). The titration data show a phenolic OH content of 0.8-2.1 mequiv/g for the fulvics and 1.5-5.4 mequiv/g for the humics. The value for the Missouri River humic acid is probably too high due to hydrolysis of the ash cations (51).The carboxyl acidity ranges from 5.0 to 6.1 mequiv/g for the fulvics and from 2.8 to 5.0 mequiv/g for the humics. These values are typical for aquatic humic substances (32, 38, 49), and they reflect the greater aliphatic nature of the fulvics and their associated oxygenated functional groups. Note that ultraviolet absorbance is greater for the humic acids, reflecting the higher aromatic content and greater molecular size. Chlorination at pH 7. Table I11 shows that, in all cases, the chloroform, TOX, TCAA, DCAA, and DCAN yields were greater for the humic acids than for their 1658
Environ. Sci. Technol., Vol. 24, No. 11, 1990
corresponding fulvic acids from the same source. The observation that humic acid fractions tend to have higher THM yields then their corresponding fulvic acids has been reported by others (23,48). The range of specific THM yields found with this research (31-68 pg of CHCl,/mg of TOC) falls within the range reported by Oliver and Thurman (23)for a set of aquatic fulvic acids (25-156 pg of CHC13/mg of TOC). Table V shows organohalide yields calculated from the data in Table 111, expressed as a percent of TOX. Total-CC13is defined as the sum of all trichloromethyl species: CHCl,, base-CHC13, and TCAA. The table also shows percent incorporation values, which are an estimate of the percent of the chlorine consumed that remained covalently bound to carbon. Percent incorporation was calculated as follows: TOXCoTr/35 500 % incorp = chlorine consumption/71 x 100
(1)
where TOXcorr= TOX (pg/L as C1) +
Because the TOX recovery for chloroform was low (85%), it was felt that a better estimate for the true percent chlorine incorporation could be made by using the measured TOX value corrected for incomplete chloroform recovery (i.e., TOXcorrin accordance with eq 2). The TOXcorrvalues were similarly used to calculate percent of TOX. Although there was a moderate degree of variability among the sources with respect to specific organic halide yields (Table 111), the percent of TOX values were much more uniform (Table V). For example, chloroform ranged from 17.5% to 20.4% of the TOXcorrfor all 10 humic materials chlorinated a t pH 7. Percent incorporation was
Table V. Chlorination of Ten Extracted Aquatic Humic Substances at pH 7 (Relative Yields)" % ' of TOX"*
% incorp
CHC13
TCAA
DCAA
TCAC
DCAN
base-CHC1,
total CCl,
sum
F H F H F H F H F H
28.9 26.0 28.9 27.3 29.1 25.5 26.5 22.2 25.2 22.1
20.2 20.4 19.0 19.3 20.0 19.6 17.5 17.6 19.6 18.8
15.2 21.6 18.2 21.9 17.8 21.8 12.0 19.3 11.0 17.2
4.6 5.6 5.8 5.5 5.4 6.5 5.5 6.3 4.6 5.6
0.46 0.40 0.39 0.24 0.47 0.20 0.60 0.36 0.33
0.19 0.33 0.20 0.25 0.17 0.21 0.26 0.44 0.27 0.61
4.2 4.1 4.9 3.9 4.8 4.2 4.8 4.8 5.1 4.2
40.0 46.5 42.4 45.3 43.0 45.9 35.0 42.0 36.4 40.6
44.8 52.4 48.3 51.0 48.6 52.6 40.7 48.1 41.3 46.8
F H
27.7 24.6
19.2 19.1
14.8 20.4
5.2 5.9
0.54 0.31
0.22 0.37
4.7 4.2
39.4 44.1
44.8 50.3
source Black Lake Coal Creek Ogeechee River Ohio River Missouri River av a
0.80
F, fulvic acid: H, humic acid. 5 "
-
0 Humic Acids
25 v
.c
20
A
15
0 Humic Acids
1
'
01 i?
A Fuivic Acids
0 0
10
20
30
40
0
1
2
3
4
5
% Aromatic Carbon
Figure 2. Relationship between chlorine consumption and aromatic content. Reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 OC.
also relatively uniform, ranging from 22.1% to 29.1%. Trichloroacetic acid, which showed the greatest variability, averaged 17.6% of the TOX- at pH 7, with values ranging from 11.0% to 21.9%. The sum of all byproducts measured (chloroform, TCAA, DCAA, TCAC, DCAN, and base-chloroform) averaged 44.8% and 50.3% of the TOXcorrfor the fulvic and humic fractions respectively. Relationship between Chlorine Consumption and Humic Properties. In an effort to understand the reactions between chlorine and aquatic humic materials on a more fundamental level, correlations between observed byproduct yields and physicochemical properties of the humic materials were examined. Although good correlations between THMFP and various properties of the humics were found, the correlations between these properties and chlorine consumption were generally stronger. The tendency of an organic molecule to react with aqueous chlorine is probably less sensitive to specific molecular structure than is its tendency to form any specific chlorine byproduct (e.g., THMs). As a result, it is more likely that meaningful generalizations regarding structure vs chlorine consumption can be made than for structure vs THM formation. Since model compound work has shown that, in general, only the activated (i.e., electron-rich) aromatics have high (i.e., >4 M HOCl/M compound) chlorine reactivities, it is likely that humic materials with greater numbers of activated aromatic centers will be more reactive with chlorine. Figure 2 shows the relationship between percent aromatic carbon and chlorine consumption for the 10 humics tested. The fulvic acids show a linear tendency, but the relationship seems to deviate from linearity for the humic acids.
Phenolic-OH (peq/mg-C)
Figure 3. Relationship between chlorine consumption and phenolic content. Reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 O C .
Another measure that may be related to the density of activated aromatic centers is the phenolic OH content. Figure 3 shows that the correlation between phenolic OH and chlorine consumption is good for the fulvic acids, but the humics do not follow the same trend [the anomalous high-ash-content Missouri River humic acid has been excluded from this figure]. The reason for this may be that the humic acids have a much higher aromatic carbon content than the fulvics, suggesting that the humic acid aromatic rings are, on the average, less populated with phenolic OHs (i.e., they have lower phenolic OH/aromatic C ratios). Although no direct measure of the activated aromatic content of these materials is available, one can estimate their concentration on a probabilistic basis. For this purpose it was assumed that only OH- and nitrogen-substituted aromatics are important and that additional substitutions on the aromatic nucleus, whether they be electron donating or electron withdrawing, are of lesser importance. Again, model compound studies seem to indicate that this is true (16). The final form of this model is given in eq 3 (derived in the Appendix). Activated [Act Ar-R1 = 6
[Ar-C]
aromatic concentrations have been calculated for each of the 10 humic materials, and they are presented in Table VI as millimolar activated aromatic rings per gram of carbon. It may be easier to comprehend the significance of these numbers by converting them to units of percent Environ. Sci. Technol., Vol. 24, No. 11, 1990
1650
35,
Table VI. Activated Aromatic Content of Ten Extracted Aquatic Humic Substanceso Act Ar-R mM/g of C 70Act.
source Black Lake Coal Creek Ogeechee River Ohio River Missouri River
F H F H F H F H F H
1.8 2.8 2.2 3.2 2.1 2.9
76 58 84 74
1.3
67 57 62
8
.-0 .t-
?
0 Q
L
0
0
c 8
80 67
2.7 1.2 4.2b
20
0.00
1OOb
35 ,--.
0
F
\
I
30
25
a
v
C
._ Y
20
0
15
t 0
10
a, C
5
0
Activated Aromatic Rings (pM/mg-C)
Flgure 4. Relationship between chlorine consumption and concentration of activated aromatic rings. Reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 O C .
activated aromatic carbon to total aromatic carbon (i.e., 70 Act.). 70 Act. = [Act Ar-R] (mM/g of C ) (&)6/(
yo
(4)
As expected, eq 4 predicts that, on average, more than three-quarters of the fulvic aromatic nuclei are activated (7770),whereas for the humics the figure is slightly below two-thirds (64%, excluding the Missouri River samples). Figure 4 shows the correlation between calculated activated aromatic content in micromolar per milligram of carbon vs chlorine consumption in micromolar per milligram of carbon. Omitting, once again, the anomalous Missouri River humic acid, a good linear correlation exists. The model represented in Figure 4 has the following form. C1, consumption = a’+ xlAct Ar-R] (5) Note that the slope in Figure 4 (x’in eq 5) is -7.9 pM/pM suggesting that, on the average, 7.9 molecules of chlorine react with each activated aromatic center. This is in excellent agreement with the stoichiometry observed between chlorine and OH- and NH,-activated aromatics from the literature (15-17). This is also quite close to the molar ratios observed for lignin monomers, vanillic acid and syringaldehyde (8.5 and 8.3, respectively) (17). However, due to uncertainties in interpreting 13CNMR and titration data and the speculative nature of eq 3, the arguments presented above cannot be considered conclusive in proving a direct relationship between activated aromatic centers and chlorine consumption. Nevertheless, they do strongly suggest that phenolic structures and aromatic 1660
Environ. Sci. Technol., Vol. 24, No. 11, 1990
0.05
0.10
0.15
Nitrogen/Chlorine Cons. (M/M)
F, fulvic acid; H, humic acid. *These estimates are high due to an anomalous value for phenolic group content.
I
DH 7
Figure 5. Relationship between nitrogen content and percent incorporation. Reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 OC.
amines could account for much of the chlorine demand based on our present knowledge of aquatic humic structure and chlorine chemistry. Relationship between TOX Formation and Humic Properties. The next step in understanding the chlorine-humic reaction is to develop a conceptual model to describe the relative amount of chlorine incorporation as compared to chlorine-induced oxidation (Le., chloride formation). While the differences in percent incorporation are small (see Table V), some useful information may be contained in these values. For example, it is clear that many chlorination byproducts are susceptible to nucleophilic dehalogenation (52, 53). A recent study with an aquatic fulvic acid indicated that as much as 50% of the TOX produced upon chlorination is susceptible to basecatalyzed dehalogenation (54). In particulqr, nitrogenous compounds are known to form highly labile N-chloroorganics upon chlorination. These N-chloroorganics may be readily dehalogenated upon the addition of excess reducing agent, yet they often are not measured as free residual chlorine by the DPD procedure (55). Thus, they may represent a significant chlorine demand without any apparent accompanying formation of TOX. In the experimental results reported here, N-chloroorganics may have been destroyed by the sodium arsenite reducing agent, or they may have been reduced by reaction with activated carbon during the TOX analysis. This being the case, one would expect to find an inverse relationship between fulvic or humic acid nitrogen content and percent chlorine incorporation. To investigate this, one could propose a correction to eq 5 so that the contribution of N-chloro compounds would be explicitly incorporated: C1, cons = a + z[N] + x[Act ArHR] (6) where z is the amount of chlorine consumed per mole of nitrogen. Furthermore, one could presume a simple stoichiometry for TOX formation based on the reactivity established in Figure 4. Equation 7 shows TOX as being the product of chlorine reactivity from eq 6 times a constant fractional incorporation, b. Note that the term for nitrogen is not included here, because we are presuming that direct reaction of chlorine with nitrogen does not form measurable TOX. TOXcor*= b(a + x[Act Ar-R]) (7) Combining eqs 1,6, and 7, one obtains a linear eq 8 that can be used to test the models represented by eqs 6 and 7. % incorp = 0.2b - 0.2bz
-
(Cl::ins)
(8)
n
s
kY
1 0 Humic Acids A Fulvic Acids
2*o
1.5--
/ oA
W
2
E
\
8
1.0-
A
A
0.5--
0.0 0.00
0.03
0.06
0.09
Specific UV Absorbance (L/c m*mg -C)
Figure 6. Relationship between TCAA/TTHM ratio and specific UV absorbance. Reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 OC.
Figure 5 shows a plot of the percent incorporation versus the ratio of nitrogen content to chlorine consumption in accordance with eq 8 for all 10 humic materials. It appears that the humic and fulvic acids define two separate lines with two different slopes. The fulvic acids give a slope of -65 which, when divided by the intercept, gives a stoichiometric ratio ( z ) of 1.9 mol of Cl/mol of N. Such a consumption ratio is not unreasonable as free and humic-bound amino acid nitrogen would be expected to consume 2-3.6 chlorine atoms while being converted to nitrile derivatives or undergoing deamination and breakpoint chlorination (56). Under certain conditions, proteins can undergo a stepwise degradation in the presence of residual halogen to give free amino acids (57). Oxidation of protein nitrogen is also possible through prior attack of chlorine at reactive carbon centers (e.g., tyrosine). If this occurs to any extent in these systems, one would also expect a consumption of 2-3.6 chlorine atoms per peptide nitrogen. Heterocyclic nitrogen may consume from 0 to 1.6 chlorine atoms (57,58),and simple amide nitrogen may consume 2 or more chlorine atoms (59). The humic acid stoichiometric ratio of 3.4 is less easily explained. Perhaps the presence of a nitrogen catalyzes the further oxidation of the molecule, or perhaps other processes are occurring. Finally, one should note that the intercepts in Figure 5 (i.e., 0.2b) represent a type of intrinsic percent incorporation; the percent incorporation one would expect to measure if formation of N-chloroorganics was insignificant. Assuming z = 2 in the revised chlorine consumption model (eq 6), the coefficients, x and a are naturally found to differ slightly from their counterparts in eq 5 (i.e., x’ and a?. The revised model gives a slightly better correlation (r2 = 0.96) with a slope (x) of 7.3 pM/pM and an intercept (a) of 0.30 mg/mg of C. If one picks 32.5% as the intercept of the fulvic line in Figure 6 (Le., 0.2b), eq 7 becomes TOXcorr(pg/mg of C) = 50 + 85[Act Ar-R]
(9)
Equation 9 gives excellent agreement with the fulvic acid data (r2= 0.96). However, since the humic acids have an apparent nitrogen dependence that is different from the fulvics (Figure 5), the humics give a different regression equation for chlorine consumption and TOX. Chemical Fate of Organic-Bound Chlorine. Factors influencing the distribution of TOX into its major species are complex. For example, the relative concentrations of the major byproducts will depend on the specific geometry of reacting groups in the precursor structures. Surpris-
ingly, the fraction of incorporated chlorine (i.e., TOX) that ends up as CHC&seems to be quite consistent at -20%. The reasons for this are not clear, as model compounds show a very wide range in chloroform/TOX ratio (17). Nevertheless, for aquatic humic and fulvic acids, one can estimate THMFP by multiplying eq 9 by 0.2. THMFP (pg/mg of C) = 10 + l7[Act Ar-R] (10) Trichloroacetic acid, on the other hand, shows a very pronounced difference in yield between humic and fulvic fractions, independent of the differences in TOX yield (Table V). Because dichloroacetic acid does not readily undergo chlorine substitution to give TCAA, it is likely that the immediate precursors to TCAA are trichloroacetyl derivatives. These structures should also be susceptible to base-catalyzed hydrolysis to produce chloroform. The relative rates of hydrolysis vs oxidative cleavage will determine whether a particular trichloroacetyl structure will give primarily chloroform or TCAA. It was hypothesized that TCAA formation is favored in the presence of an a-OH or a conjugated system (17). Such a structure would help to stabilize the carbonium ion formed upon the oxidative loss of trichloroacetyl. Interestingly, De Leer and co-workers (19) reported the isolation of a class of trichloroacetyl byproducts of humic acid. These compounds all had conjugated systems capable of donating electrons to the carbonyl group. The authors proposed that, for this reason, these compounds experienced reduced rates of hydrolysis. It is likely, therefore, that molecules having a high degree of conjugation will preferentially lead to TCAA formation over chloroform formation. One semiquantitative measure of the degree of conjugation is a compound’s UV absorbance. Figure 6 shows that there is a significant positive correlation between the TCAA/ TTHM ratio and specific UV absorbance (254 nm) for the 10 humic and fulvic acids. A least-squares linear regression on these data gave the following relationship. TC AAFP /TTHMFP (pg/ pg) = 0.6 + 14 (specific UV abs.) r2 = 0.55 (11) The high yield of TCAC for the fulvic fractions is noteworthy, because it is the only organic halide in the group studied here that actually showed higher yields for the fulvic fractions over the humic fractions. This may be the result of a higher methyl ketone content in the unreacted fulvic fractions. Studies using proton NMR with derivitized aquatic humic and fulvic acids have indicated that the fulvic fractions have a much higher methyl ketone content (35). It has been proposed that dichloroacetonitrile forms from activated amino acids and derivatives (10). As one might expect, DCAN correlates well with nitrogen content (see Figure 71, giving the following regression equation: DCANFP (pg/L) = -2.3 + 0.053(0rg-N) (pg/L) r2 = 0.87 (12) While this is interesting from a predictive standpoint, it fails to take into account variabilities among the humic substances with respect to chlorine reactivity. A more appropriate relationship, in light of the models presented to this point, would be between DCAN/TOX and organic N/organic C (see Figure 8). Indeed, this gives a slightly better correlation with a smaller x intercept. DCANFP/TOXFP (pM/pM) -0.00038 + 0.068(0rg-N/Org-C) (mg/mg) r2 = 0.89 (13) Such a relationship would be expected from a random Environ. Sci. Technol., Vol. 24, No. 11, 1990
1661
Table VII. Chlorination of Ten Extracted Aquatic Humic Substances at pH 12 (Relative Yields)a % of TOXcorr
7’0 incorp
CHC1,
TCAA
DCAA
total-CC1,
sum
F H F H F H F H F H
20.1 17.0 19.3 16.5 22.8 17.4 19.3 19.2 19.9 17.1
48.5 46.6 47.7 48.7 46.2 43.6 49.6 43.3 53.8 47.3
0.27 0.37 0.36 0.37 0.25 0.30 0.47 0.42 0.28 0.48
9.4 12.5 11.7 11.6 13.0 9.0 9.9 10.3 11.0
48.8 47.0 48.1 49.1 46.4 43.9 50.0 43.8 54.1 47.8
58.2 59.4 59.8 61.4 58.1 56.9 59.1 53.6 64.4 58.8
F H
20.3 17.4
49.2 45.9
0.32 0.39
10.4 11.8
49.5 46.3
59.9 58.0
source Black Lake Coal Creek Ogeechee River Ohio River Missouri River av a
12.4
F. fulvic acid; H, humic acid. 12.0
9.0
0 Humic Acids, pH 7 A Fulvic Acids, pH 7
0 Humic Acids A Fulvic Acids
A Fulvic Acids,.pH 12
-I
m
6.0 -U
-
3.0 --
-I-‘
t-
I;;iL 1 loo
A
72 Hours
0
0.0 1
0
Organic - N ( p g / L )
Figure 7. Relationship between dichloroacetonitrile formation potential and organic nitrogen: reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 O C .
-
0 Humic Acids A Fulvic Acids
5
a
\
5 a LL x
e
0.003 --
0.002--
a LL
=
40
0.001
--
0.000 I 0.00 0.01 0.02 0.03 0.04 0.05
Figure 8. Relationship between dichloroacetonitrilelTOX ratio and the ratio of organic nitrogen to organic carbon. Reaction conditions: 20 mg/L chlorine dose, pH 7 phosphate buffer, 72 h at 20 ‘C.
distribution of both nitrile precursors and TOX precursors. pH Effects. As expected from earlier studies (5,6),the aquatic humic materials showed a reduced reactivity a t high pH (compare Tables I11 and IV). Furthermore, a fairly uniform reduction in percent incorporation was seen for all 10 humic and fulvic acids (compare Tables V and VII). This may be interpreted as a shift in chlorination mechanisms (from substitution to oxidation) as the degree of protonation of the reacting species changes. Alternatively, the drop in percent incorporation may be due to accelerated base-catalyzed hydrolysis of C-Cl bonds (54). 1662
Environ. Sci. Technol., Vol. 24, No. 11, 1990
500
TOX
1000
1500
(FdL)
Figure 9. Total trichloromethyi content vs TOX as a function of pH. Reaction conditions: 20 mg/L chlorine dose, 72 h at 20 ‘C.
Tables V and VI1 indicate that the fraction of TOX bound in the THMs greatly increases with increasing pH. This has been observed by others ( 5 , 6 0 4 2 ) .Despite the greater formation of THMs a t pH 1 2 as compared to pH 7, the total CCl, content remained relatively constant with respect to TOX formation. Figure 9 shows the pH 7 and pH 12 total-CC1, data plotted as a function of TOX on the same set of axes. Recall that these experiments were performed in the absence of added bromide, so that the formation of brominated TOX species should be insignificant. It has been proposed that chloroform and trichloroacetic are derived from a common group of precursors, and that the relative abundance of these species is defined by pH and certain structural factors (17). On the basis of a haloform-type mechanism, increasing pH should accelerate TOX and R-CC1, formation by increasing the stability of carbanions. Conversely, high pH will also lead to a shift in the chemistry of chlorine from the potent halogenating agent, HOCl, to to the less reactive OC1-. Furthermore, alkaline conditions may lead to significant hydrolysis of carbon-halogen bonds, and therefore additional loss of TOX and CCl, groups. Figure 9 suggests that both TOX and total-CC1, decrease with increasing pH to the same relative extent. Note that the tendency for humic acids to have higher specific yields of chloroform than their corresponding fulvic acids is less pronounced at pH 12. Apparently, the higher pH of chlorination increases the fulvic acid yields of chloroform more than for the humic acids. This may explain why Oliver and Visser (63) found a riverine fulvic fraction to give higher specific chloroform yields at pH 11 than the corresponding humic fraction. These authors
found the reverse to be true for lake- and swamp-derived humic materials. The results of their work at pH 11 showed specific yields of 93-151 pg/mg of C for the fulvic fractions and 108-171 pg/mg of C for the humic fractions. The dichloroacetic acid showed no significant net change in going from pH 7 to pH 12. Earlier studies (17,301 have suggested that the formation of DCAA has a rather complex behavior with respect to pH. Thus, the constant yield observed here may just be a fortuitous result of the pH's chosen. In accordance with earlier studies (11,17)TCAC and DCAN were not detected a t pH 12. This is likely a result of their instability (base-catalyzed hydrolysis) a t alkaline pHs.
Thus, the probability of any one aromatic nucleus not having a phenolic OH group is
Summary and Conclusions Aquatic humic acids consume more chlorine, and produce more TOX, chloroform, trichloroacetic acid, dichloroacetic acid, and dichloroacetonitrile on a per carbon basis than aquatic fulvic acids extracted from the same source water. Aquatic fulvic acids produce more l , l , l trichloroacetone and show a greater ratio of TOX production to chlorine consumption than their corresponding humic acids. A set of semimechanistic models can be formulated for chlorine consumption and byproduct formation from aquatic humic substances. These incorporate an estimate of the activated aromatic content of humic materials based on data from alkaline titrations and 13CNMR. Subsequent calculations suggest, but do not prove, that most of the chlorine consumption is due to reaction with activated aromatic structures. The average chlorine consumption after 3 days at pH 7 (20 mg/L chlorine, 20 "C) was estimated to be 7-8 mol of active chlorine/mol of activated aromatic rings. Much of the variability in the ratio of TOX production to chlorine consumption can be accounted for by the nitrogen content of the humic substances. This may be interpreted as due to the formation and subsequent destruction of organic chloramines. Chloroform was found to represent a nearly uniform fraction (20%) of the TOX formed in all of the humic materials tested. Humic materials with a higher specific UV absorbance were found to form larger amounts of trichloroacetic acid as compared to THM formation. Dichloroacetonitrile formation was strongly correlated with organic nitrogen content. The TOX of solutions chlorinated at pH 12 was roughly half of that produced at pH 7. Much of this loss of TOX may be due to alkaline hydrolysis of halogenated byproducts. Chlorination at high pH did not change the fraction of TOX that is in the form of trichloromethyl species. It did, however, change the speciation of trichloromethyl compounds from trichloroacetic acid to chloroform.
P(N/C) = [Nl/[Cl
Acknowledgments We thank Theodore Walters, Carol Bowles, Myron Brooks, and George Aiken for their assistance in the isolation and characterization of the aquatic samples.
Appendix Calculation of Activated Aromatic Content. A model for activated aromatic content may be most easily formulated by first calculating the content of nonactivated aromatic nuclei and subtracting this from unity. Assuming complete independence, the probability ( P ) of any one aromatic carbon being bound to an OH group is simply the ratio of the molar concentrations of phenolic groups (Ar-OH) to aromatic carbon (Ar-C): P(Ar-OH/Ar-C) = [Ar-OH]/[Ar-C] (Al)
P(no Ar-OH/Ar-R) = [l - P(Ar-OH/Ar-C)I6 =
Assuming all nitrogen is present as NH2 groups (or structures that are equally ring-activating) and assuming random distribution of these groups, the probability that any given carbon is bound to an activating nitrogen group is simply equal to the ratio of the molar concentrations of nitrogen to carbon: (A3)
The probability that any given group of six carbon atoms (e.g., an aromatic ring) does not contain an activating nitrogen group is
Since the probability of an aromatic nucleus not containing an activating group is the intersection of eqs A2 and A4, the concentration of activated aromatic rings is equal to 1 minus the intersection times the total molar concentration of aromatic rings: [Act Ar-R1 = [Ar-OH] 6 [Ar-C]
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Received for review November 1, 1989. Revised manuscript received June 11,1990. Accepted June 26,1990. This research was supported by the US.EPA under Research Grant R-810235.
