Reactions of Chlorine with Selected Aromatic Models of Aquatic Humic Material Daniel L. Norwood, J. Donald Johnson, and Russell F. Christman" Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, Chapel Hill, N.C. 27514
J. Ronald Hass and Marie J. Bobenrieth National Institute for Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, N.C. 27709
A series of compounds designed to model the monomeric components of aquatic humic material was reacted with aqueous chlorine a t pH 7. Chloroform production and chlorine demand were measured for each compound over varied time periods. All compounds studied produced measurable amounts of chloroform with resorcinol derivatives showing the greatest yields. In addition, the chlorination reactions of resorcinol and 3-methoxy-4-hydroxycinnamicacid were studied in depth with GC/MS. T h e resorcinol reaction was found to proceed through several chlorinated intermediates, of which the most abundant was 3,5,5-trichlorocyclopent3-ene-1,2-dione, to chloroform and chlorinated acids. Chlorination of the cinnamic acid derivative roduced chlorinated substitution products and chloropheno , which broke down upon further reaction to chloroacetic acids.
L
I t has been shown that chlorination of natural waters containing humic substances produces chloroform (1-3), and, given the ubiquitous occurrence of this and other trihalomethanes in chlorinated U.S. drinking waters (2,4-ll ), it is attractive to assume that waterborne humic substances are a principal precursor. Unfortunately, little is known of the chemical structural properties of aquatic humic material and even less is known regarding their reactions with aqueous chlorine. Chemical degradation studies (22-14) of aquatic and soil humic material using extensive base hydrolysis or oxidation with KMn04 or alkaline CuSO4 have suggested that the core structure includes various phenols and phenolic acids (resorcinol, catechol, vanillic acid, syringic acid, 3,5-dihydroxybenzoic acid, protocatechuic acid, and p - hydroxybenzoic acid). The formation of chloroform as a stable chlorination product of resorcinol was first noted by Zincke ( 1 5 ) . Much later, Moye (16) proposed a mechanism for resorcinol chlorination in chloroform solvent which included a pentachlororesorcinol intermediate that was shown to be unstable in aqueous solution. More recently, Rook ( 2 ) postulated that the m- dihydroxy structure of resorcinol was the principal chloroform precursor in aquatic humic materials and proposed a reaction mechanism ( 3 ) based on Moye's work. A reaction in intermediate (3,5,5-trichlorocyclopent-3-ene-1,2-dione) the reaction of HOCl with resorcinol was identified by Christman et al. (121, who noted that the formation of this compound was inconsistent with Rook's proposed mechanism. Other workers have documented the production of chloroform from the chlorination of phenols, uracil (17),and from hydroxylated aromatic acids (18, 1 9 ) . T h e available literature suggests that a variety of compounds produce chloroform upon chlorination, although there has not been a systematic study of HOCl reactions with those degradation products known to derive from aquatic humic material. Furthermore, the mechanisms of these reactions and the nature of any nontrihalomethane-chlorinated end products have not been established. The objectives of the research reported here were to measure the chloroform yields and chlorine demands for structures representative of humic degradation products and to identify in selected cases other chlorinated end products. 0013-936X/80/0914-0187$01.00/0
Experimental Model Compound Selection. Compounds chosen for this study are representative of the three aromatic substitution patterns (Figure 1) known to exist in humic degradation products a t the initiation of this study. In addition, each substitution pattern was modeled with respect to alkyl substituent (propyl) and unsaturation where possible. The propyl side chain was selected because of the dominance of this unit in natural lignitic woody tissue which also contains the 3methoxy-4-hydroxy and 3,5-dimethoxy-4-hydroxy aromatic substitution patterns. Kinetic Studies. Each model compound was permitted to react with HOCl in a pH 7.0 buffer (KzHP04/KH*P04) over total time periods ranging from 20 to 250 min. The initial concentration of each model compound and the chlorine to carbon ratio (HOCl/C) employed are shown in Table I. Individual reactions were initiated by mixing standardized concentrations prepared from known dilutions of stock 5% NaOCl (at pH 7.0) and model compound/buffer solutions in chlorine demand free water. Reaction vessels were held a t ambient temperature (25 "C) except for resorcinol, which was held a t 5 "C in a constant temperature water bath. Reactions were terminated a t desired time increments by the addition of standardized 0.2 N sodium arsenite, and the solutions were stored in a headspace free condition prior to chloroform and chlorine demand analyses. Chloroform analysis was carried out using the method of Bellar and Lichtenberg (20)and employed a Tracor MT-220 gas chromatograph equipped with a Hall electrolytic conductivity detector. Chlorine demand was measured using a back titration of the excess sodium arsenite with standard iodine solution. Mechanistic Studies. Reaction procedures for the mechanistic studies were similar to those employed in the kinetic studies, except that an approximate 10-fold increase in initial concentration of model compound was used. At the end of each reaction time, free available chlorine was removed by addition of crystals of sodium arsenite instead of 0.2 N solution to minimize dilution. The chlorine to carbon ratios chosen were 0.5 and 2 for 3-methoxy-4-hydroxycinnamicacid and 2 for resorcinol. The procedure for the 0.5 chlorine to carbon reaction of resorcinol was described by Christman et al. (12). At the completion of each reaction, the aqueous solution was lowered to pH 1 with HC1 and extracted with small volumes of diethyl ether. The ether extract was then dried with sodium sulfate, and an aliquot was methylated using an ether solution of diazomethane generated from N-methyl-N-nitroso-N- nitroguanidine and sodium hydroxide. Methylated and nonmethylated samples were subjected to GC/MS analysis. Gas Chromatography/Mass Spectrometry. Low-resolution mass spectra were obtained by means of a Finnigan 3300 mass spectrometer interfaced to a Finnigan 9500 gas chromatograph. Electron impact spectra resulted from 70-eV ionization of the sample with 0.5-mA trap current. Chromatographic introduction of the sample was achieved through the use of 2 mm X 3 m glass columns containing either 3% OV-101 or 3% OV-17 on Chromosorb W-HP. Instrument
@ 1980 American Chemical Society
Volume
14, Number 2,
February 1980 187
VANILLIC ACID SUBSTITUTION PATTERNS:
Table 1. Results of Kinetics Studies C H = C H COOH
@OYH3 OH
OH
@OCH3 OH compound
S Y R I N G IC A C ID S U B ST ITU T ION PATT ERNS : CH2CH2COOH
OH
H3C0
H3C0G OH O C H 3
C H = C H COOH
H3CO@OCH3 OH
3,5-D HBA SUB STI TUT I 0 N PATTER N S :
OH
HO
Figure 1. Aquatic humic model compounds
I
H3COWOCH3
on
Figure 2. Relative chloroform yields from model compounds after 40-min chlorination
control and data acquisition were by means of a Systems Industries S/150 data system. Exact mass measurements and collisional activation (CA) mass spectra were obtained through the use of a VG-Micromass ZAB/2F mass spectrometer. The ion source was operated at 210 "C with a trap current of 200 pA and an electron energy of 70 eV. Sample introduction was by means of a heated direct probe inlet system. Helium was used as the collision gas for the CA spectra with the collision cell operated at approximately Torr pressure. This gave 50% alteration of the main beam intensity. Exact mass measurements were made a t a resolving power of 3500 using perfluorokerosene as the reference compound.
Results and Discussion The results of kinetic experiments are summarized in Table I and Figure 2, from which it is clear that the m-dihydroxy substitution pattern of resorcinol produces chloroform very rapidly. T h e chloroform yield is almost 93% for the carbon between the two hydroxyl groups. It may be seen that this carbon is responsible for chloroform production by noting the structure of the cyclic diketone (12) also formed, shown in Table 11. It is not surprising to note that substitution of hydrogen by the simplest alkyl group (CHJ to form orcinol (see Figure 2) does not significantly affect chloroform production. Replacement of this methyl group by carboxyl, however, produces a slight decrease in chloroform yield. In all of these cases a carbon is available between two hydroxyl groups with keto-enol resonance stabilization of the carbanion possible. The vanillic acid structure produces much less chloroform, although the chlorine demand remains relatively high. This 188
Environmental Science & Technology
1,3-dihydroxybenzene (resorcinol) 3,5-dihydroxybenzoic acid 3,5-dimethoxybenzoic acid 3,5-dihydroxytoluene (orcinol) 3,5-dimethoxy4-hydroxybenzoic acid (syringic acid) 3,5-dimethoxy4-hydroxycinnamic acid 3-(3,5-dimethoxy4-hydroxyphenyl)propionic acid 3-methoxy-4-hydroxybenzoic acid (vanillic acid) 3-methoxy-4hydroxycinnamic acid 3-methoxy-4hydroxyhydrocinnamic acid a
inil concn (buffer solution), mM
c12/C
CHC13 yield, mol of CHC13/ mol of compd a
CI conaumptlon, mol of CIZ/ mol of cbmpd a
6.10
1.93
0.877
6.60
4.36
1.95
0.450
7.06
0.534
2.00
0.009 00
3.00
0.570
1.82
0.852
6.26
3.35
1.76
0.005 02
5.18
0.553
1.85
0.022 8
6.09
0.552
1.93
0.033 0
6.06
0.573
1.73
0.037 5
5.38
0.553
1.71
0.109
7.63
0.553
1.53
0.009 79
4.94
Forty minutes
decrease can be explained by the loss of a doubly. activated carbon between two free hydroxyls. T h e number 5 carbon is still available for chloroform production, and because of the p-hydroxy configuration the molecule will probably undergo oxidative decarboxylation (29) with substitution of chlorine in place of carboxyl. Continued chlorination and final cleavage could then occur a t either of these chlorination sites. T h e syringic acid substitution pattern produces approximately 10-fold less chloroform than the vanillic structure. There is no longer an available position cy t o the O H for chlorination, and Larson has shown oxidative decarboxylation with chlorine substitution does not occur with syringic acid (19). T h e mdimethoxybenzoic acid structure in fact produces more chloroform than syringic acid, probably due to the fact that the carbon between the two methoxy groups is available for chloroform formation, although this production is markedly less than resorcinol due to the lack of keto-enol stabilization. The chlorine demand data are higher for the syringic structure, which may be due to the activating influence of the hydroxyl group. Substitution of the carboxyl group by unsaturated alkyl side chains results in a general increase in both chloroform production and chlorine demand. In the case of the 3-methoxy4-hydroxycinnamic acid structure, we have identified decarboxylation products with and without ring-substituted chlorine. This is consistent with Larson's observations (19) on the chlorination products of vanillic acid, as well as the observations of Hsu and Shimzu on the chlorination of the phenylpropanoids (21) TVhen the side chain is saturated on the vanillic structure, chloroform production is reduced as is chlorine demand. With the syringic acid structure an apparent anomaly exists in that higher chloroform production and chlorine demand data were obtained for the saturated alkyl side chain.
Table II. Reaction Products of Model Compounds and HOC1 at 25 O C at 2.0C12 /C
REACTANT
CHCI,
I ,3- DIHYDROXYBENZENE
F I F I
ZiOH
HO-C - C = C - C - O H I
/
H CI CCI jCOOH
Figure 3. Chloroform production and chlorine demand vs. time for orcinol
~
CH=CHCOOH 3-METHOXY-4-HYDROXYCINNAMIC ACID
CHZCHCOOH
@ow, OH
1"
/9/
PRODUCTS IDENTIFIED
CHC13
kOH c:; CH=CHCOOH
CHC12COOH
OH
CClaCOOH
k? CH :CHCI
OOCH3 OH
CH:CHCI
kCi3 OH
CHC13
0
10
20 TIME (min I
30
40
Figure 4. Chloroform production and chlorine demand vs. time for 3,5-dimethoxy-4-hydroxycinnamic acid
The actual rates of CHC13 production and chlorine consumption varied with each model compound studied, although the overall data exhibited two distinct patterns. The first pattern is typified by the orcinol data (Figure 3) and reflects a generally rapid and simultaneous exertion of both chlorine demand and CHC13 production. Compounds exhibiting this pattern include, in addition to orcinol, resorcinol, 3,5-dihydroxybenzoic acid, 3-methoxy-4-hydroxycinnamicacid, and 3,s-dimethoxybenzoic acid. These data suggest that CHCl3 is the primary reaction product for these model compounds. T h e second pattern is typified by the 3,5-dimethoxy-4-hydroxycinnamic acid data (Figure 4) and reflects an initial chlorine demand in excess of CHC13 production. Compounds exhibiting this pattern include 3-methoxy-4-hydroxyhydrocinnamic acid, syringic acid, vanillic acid, and /3-(3,5-dimethoxy-4-hydroxypheny1)propionicacid. These data suggest that CHC1:j is a minor reaction product for these model compounds. The nature of reaction products other than CHC13 was investigated for two of the model compounds: resorcinol (high CHC1:j and high C12 demand) and 3-methoxy-4-hydroxycinnamic acid (low CHCl:
