Factors Influencing the Formation of Haloforms in the Chlorination of Humic Materials Christopher J. Peters, Robert J. Young, and Roger Perry" Public Health and Water Resource Engineering Section, Imperial College of Science and Technology, London SW7 2AZ, United Kingdom
The reactions with chlorine, under controlled conditions of pH and temperature, of humic and fulvic fractions extracted from the River Thames are described. Chloroform production, monitored by a direct aqueous injection chromatographic technique, indicates that total chloroform is composed of two components: one produced by thermal decomposition of unidentified chlorinated intermediates (residual chloroform) and the other dissolved chloroform. pH is shown to be the dominant factor influencing both the production of total chloroform and the relative proportions of these components. Chloroform production takes place in two phases, the first of which is rapid and strongly pH dependent, whereas the second phase is slower and less dependent on pH. A linear relationship exists between chlorine consumption and total chloroform production. This reaction accounts for only a small proportion of the chlorine consumed. A superficial kinetic treatment is presented with possible reaction pathways. Introduction
The United States survey on trihalomethanes in potable waters ( I ) stimulated considerable research on this topic, and it is now generally accepted that chlorine reacts with humic materials to form chloroform, with brominated trihalomethanes being produced in addition when bromide is present in the raw water. Humic material used in this study was extracted from the River Thames. I t was composed of -4% humic acid, 13%hymatomelanic acid, and 83%fulvic acid as opposed to reported studies ( 2 , 3 )where the humic acid content ranged between 0.1 and 16%.The technique used to extract the humic material from Thames water did not separate the hymatomelanic and humic acid fractions (2). Quantitative comparison of the reactivity of humic and fulvic acids requires material extracted from the same source, as waters from different areas may well contain differing compositions of these acid fractions. The measurement of the trihalomethanes was carried out by direct aqueous injection (DAI), a method first reported by Nicholson and Meresz ( 4 ) .DAI measures not only the "dissolved" chloroform but also a "residual" chloroform level (see text) which results from the breakdown in the injection port of organochlorine intermediates. Other workers ( 5 ) in comparing a DAI method of analysis for chloroform in drinking water with head space (6) and gas stripping techniques ( 7 ) have commented on the presence of these intermediates. The purpose of this paper therefore is to relate the relative contributions of humic and fulvic acid fractions to haloform (principally chloroform) formation and to examine the relationship between residual haloforms and total haloform formation from these acid fractions under conditions generally encountered in water treatment.
Reaction Vessel and Sampling. A 1-L reaction vessel, illustrated in Figure l, was used without stirring. Sample withdrawal was organized by injecting nitrogen into the head space, thus displacing the requisite volume of sample for analysis. A high water-to-head space ratio was maintained which minimized evaporative losses of dissolved chloroform (as a percentage of total dissolved chloroform) from solution. In order to reduce photolytically induced reactions, we enclosed the reaction vessel in a darkened chamber. Water Chlorination and Chlorine Analysis. Sodium hypochlorite, low in bromine, was used as the chlorinating agent. Chlorine concentration was measured iodometrically by using 25-mL samples as opposed to the 500-mL samples described in the UK standard method ( 8 ) . Haloform Analysis. Chlorine was quenched before haloform analysis by addition of sodium thiosulfate. Haloform analysis was carried out with a Hewlett-Packard 5713A gas chromatograph incorporating an electron capture detector, with direct aqueous injection of 5-pL samples. Separation was achieved on a glass column, 4 mm X 1 m, packed with Chromosorb 101 (100-120 mesh) by using 95% argon/5% methane as the carrier gas a t a flow rate of 25 mL min-l. The detector temperature was 300 "C and that of the injection port 200 "C. Fine nickel powder (0.3-0.4 g) was introduced before the glass wool packing at the injection end of the analytical column. The column temperature was isothermal a t 140 "C for the analysis of chloroform and bromodichloromethane and 180 "C for dibromochloromethane and bromoform. Dissolved and Residual Chloroform. Total chloroform analysis was evaluated by injection of a 5-pL sample into the chromatograph under the above conditions. Residual chloroform analysis was carried out by a 5-pL injection of a previously purged water sample (5 mL) under the same conditions. Purging was carried out by using nitrogen a t a flow rate of 150 mL min-' for 10 min. Dissolved chloroform was calculated by difference as total chloroform = residual chloroform dissolved chloroform.
Experimental Section
Humic and Fulvic Acids. The humic and fulvic acids isolated from the River Thames by the method of Hall and Packham (2) were supplied by the Water Research Centre, UK. 0013-936X/80/0914-1391$01,00/0
@ 1980 American Chemical Society
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Results and Discussion The DAI analytical technique described proved satisfactory for the rapid and reproducible analysis of water samples for chloroform, dichlorobromomethane, dibromochloromethane, and bromoform, with the only sample pretreatment being the addition of a crystal of sodium thiosulfate. The detection limits were as follows: chloroform, 1pg L-'; bromodichloromethane, 0.5 pg L-l; dibromochloromethane, 1 pg L-I; and bromoform, 2 pg L-l. Brominated haloforms were not expected to be formed, as sodium hypochlorite, low in bromine, was used as a chlorinating agent. Occasionally, however, relatively high bromodichloromethane (up to 10 pg L-l) levels were detected, and, like chloroform, these were composed of distinct dissolved and residual components. The reduction of the injection port temperature to 140 "C had little effect on the residual chloroform and bromodichloromethane levels. Heating a sample in a digestor for 10 min a t 140 "C caused a slight reduction in total chloroform levels, possibly owing to evaporative losses. No residual chloroform was detected, indicating that all the Volume 14, Number 11, November 1980
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STAINLESS STEEL S A W I N G TUBE
VALVE
I
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Figure 1. Reaction vessel.
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9
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Z
l
L
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6
"
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Figure 3. Effect of pH on dissolved chloroform formation from humic acid at 10 'C.
IO'< 5'r
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i;, 1
TIME
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Figure 2. Effect of temperature on dissolved chloroform formation from humic acid at pH 8.
Table 1. Ratios of Chloroform Concentrations after 2-Day Reaction (CHC13)io d ( C H C M 5 oc
(CHCMi5 d(CHC13)io oc
PH
ratio
6 7 8 6 7 8
1.15 1.15 1.15 1.29 1.28 1.22
chloroform was removed by purging and that complete breakdown of the dissolved organochlorine intermediate had occurred in the digestor a t 140 "C (this temperature being below that normally used in the injection port of the gas chromatograph). Preliminary studies revealed that a significant loss of haloforms occurred as the reaction progressed. This loss was attributed mainly to evaporation into the head space and subsequent diffusion out of the reaction vessel. Other factors which may have contributed to such losses including adsorption onto the glass walls of the vessel, hydrolytic breakdown, and further reactions of the haloforms could not be experimentally evaluated although it was shown that overall losses of up to 6 pg L-' for bromodichloromethane and 8 pg L-l for chloroform per day occurred when using spiked samples in a glass-stoppered flask maintained a t 10 "C. Accordingly the reaction vessel and sampling procedure employed were designed so as to minimize evaporative losses. The presence of nickel powder in the injection port resulted 1392
Environmental Science & Technology
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Figure 4. Effect of pH on dissolved chloroform formation from fulvic acid at 10 'C.
in a "cleaner" chromatogram and more consistent breakdown of intermediates to produce residual chloroform. It is important to stress that the measurement of dissolved chloroform relied on the nonpurgeability of the organochlorine intermediates and that residual chloroform was formed by breakdown of these intermediates and was not the result of an accelerated reaction with remaining free chlorine as the latter had been quenched by sodium thiosulfate before analysis. Dissolved Chloroform. Figure 2 shows the relationship between dissolved chloroform formation and temperature, and the appropriate data are summarized in Table I. The activation energy for chloroform production based on these data was calculated to be between 20 and 30 kJ mol-l, a value which compared to the activation energy for the chlorination of polycyclic aromatic hydrocarbons (PAH) (9). The rate of dissolved chloroform production was pH dependent and increased with pH for both the humic and fulvic fractions (Figures 3 and 4). The trends in chloroform formation from humic and fulvic acids with variations in pH and temperature were similar. There is also a definite relationship between the amounts of chloroform produced on chlorinating humic and fulvic fractions under similar reaction conditions. The mean variation of the ratios of chloroform produced from humic acid to that produced from fulvic acid, under the range of reaction conditions used, were after 1 day 1.68 and after 2 days 1.64. Trussel (10)has reported similar findings. Oliver (11) found no difference in chloroform formation on chlorinating humic and fulvic acids, although it is not clear whether the acid fractions originated from the same source. Residual Chloroform. I t can be seen from Figures 5 and 6 that reductions occurred in residual chloroform concen-
4 8
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, 02
, 04
, 06
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04
,
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14
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16
, 18
, 21)
,
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22 24 TIME ( &ys )
Figure 5. Effect of pH on residual chloroform from humic acid (5 mg L-') at 10 O c .
tration utilizing both humic and fulvic materials when the pH was increased from 7 to 9. The kinetics of formation of residual chloroform from humic and fulvic acids were similar, both giving characteristic curves between p H 7 and 9. Residual chloroform formation is considered to involve two stages, an initial rapid phase within the first 5-7 h of the reaction, followed by a slow nearly linear secondary phase. This initial phase of the reaction yielded over 80% of the total residual chloroform intermediates produced over a 2-day period, while over the same time interval only 50% of the chloroform in solution had been formed. This two-stage reaction was compatible with the rate of fall of the chlorine concentration. Rook (12) noted a similar change in the reaction kinetics of chloroform formation (not residual chloroform) on chlorinating fulvic acid although he observed a phase change within the first 15 min of the reaction. A t p H 6 the rate of change of residual chloroform concentration from both acid fractions did not follow the characteristic curves obtained under neutral or alkaline conditions. This could have been the result of a change in mechanism and kinetics of the haloform reaction, or other reactions involving organochlorine intermediates, under acid conditions. At the higher p H levels there was a slow but constant increase in residual chloroform concentration after an initial rapid phase of production (Figures 5 and 6). After 2 days under similar reaction conditions, the ratio of the concentration of residual chloroform produced from humic acid to that produced from fulvic acid was approximately 2.5:l. For fulvic acid the levels of total chloroform, dissolved chloroform, and residual chloroform after 2 days of reaction a t 10 "C are given in Table 11. For increasing p H there was a nearly linear decrease in the percentage contribution of residual chloroform to the total chloroform level. A similar dependence on p H has also been reported (23, 14) for total organochlorine (TOC1) formation. At a higher fulvic acid concentration (25 mg L-I) the free chlorine was rapidly exhausted, and both residual and dissolved chloroform production were limited by the availability of chlorine. Residual chloroform, which is a breakdown product of chlorinated compounds heated in the injection port of the gas chromatograph, is considered as an indirect measure of the concentration of chlorinated intermediates of the haloform reaction. The high temperature of the injection port of the gas chromatograph was clearly conducive to a rapid rate of breakdown of these intermediates. I t is not known, however, whether all of the intermediates necessarily break down to produce dissolved chloroform a t ambient temperature or indeed whether complete breakdown occurs after prolonged storage under normal conditions. Studies on upland waters
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1D
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20 22 2 4 TIME ( d ~ y s )
Figure 6. Effect of pH on residual chloroform from fulvic acid (5 mg L-I) at 10 OC.
Table II. Total, Dissolved, and Residual Chloroform Concentrations (pg L-') from Fulvic Acid (5 mg L-') Reaction at 10 OC residual total
pn
cnc13
6 7 8 9
37 47 49 44.5
( % 01 total)
dissolved CHC13 ( % 01 total)
18 (49) 18 (38) 14.5 (29) 9 (20)
19 (51) 29 (62) 34.5 (71) 35.5 (80)
cnc13
rich in humic material do indicate that intermediates detected as residual chloroform decompose a t ambient temprrature to form dissolved chloroform. It is considered that the stability and resistance toward purging of the intermediates might be dependent on the nature and therefore source of the humic material. Total Chloroform. A relationship was found between total chloroform formation (residual and dissolved) and chlorine consumption for both humic and fulvic acids over a range of conditions. The relationship illustrated in Figure 7 can be represented by a linear equation (T.Chlor)t = 0.022[(C12)0- ( c l ~ ) ~ ]
(1)
where (T.Chlor)t = total chloroform concentration (mg L-I) a t time t , ( C ~ Z=)chlorine ~ concentration (mg L-l) a t time t , and (Cl& = initial chlorine concentration (mg L-1). The lower yield of dissolved and residual chloroform from fulvic acid compared to humic acid was compensated by a lower overall chlorine consumption. Total chloroform formation reaches a maximum between p H 7 and 8, consistent with the maximum rate of chlorine consumption. However, it is evident from data presented in Table I1 that this dependence on p H is much weaker than the marked effect of pH on the relative proportions of dissolved and residual components. Rate of Reaction. A first-order dependence of the rate of chlorine consumption on chlorine concentration during reaction with humic acid has recently been reported (10). I t is doubtful whether the derivation of a general rate equation for both chlorine consumption and haloform formation is possible owing to the complexity of the reactions and the variations in the structure of humic substances present in different waters. A rate equation may, however, be formulated for a specific source of water to predict the effect of changing the chlorination conditions on haloform formation and chlorine consumption. The data reported here were derived from experiments Volume 14, Number 11, November 1980
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150
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Total CHCL, pg 1.’
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/
/
\ Chlorination
Oxidation dependmt on HOC1 and OCI-
( Hfast O0 CHI -)
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OHslow
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Figure 7. Change in total chloroform formation with chlorine consumption.
designed to examine the variation of chloroform (residual and dissolved) production over a range of reaction conditions. The experimental design was not such as to facilitate rigorous kinetic interpretation. Selection of appropriate groups of data does, however, permit a simple representation of the kinetics involved. The overall chloroform reaction may be represented by eq 2 and 3, which involve the formation and subsequent breakdown of intermediates detected as residual chloroform (R.Chlor) FA
ki + nCl2 --+ OH-
R.Chlor
(2)
kz
R.Chlor +CHC13 OH-
(3)
where R.Chlor = residual chloroform and FA = fulvic acid. The rate of change of residual chloroform concentration and the rate of dissolved chloroform formation are given by eq 4 and 5 , respectively d(R.Chlor)ldt = hl(FA)x(Cl~)y(OH-)z - kz(R.Chlor) (4) d(CHCl3)ldt = k*(R.Chlor) (5) Equation 6 is the rate of total chloroform formation which is equal to the sum of the rates of formation of residual and dissolved chloroform d(T.Chlor) - d(R.Chlor) d(CHC13) dt dt dt where (T.Chlor) = total chloroform. Thus +
d(T.Chlor)ldt = kl(FA) (C12)y (OH-)
(6)
(7)
The overall chlorine consumption for fulvic acid after the 2-day reaction was not greater than 25% of the initial chlorine dose. Thus, it may be assumed that, under these conditions, chlorine remains in excess throughout the reaction. At constant pH a pseudo-first-order rate equation may be derived for the rate of total chloroform formation dependent on the fulvic acid concentration (eq 8) or, more correctly, dependent on the number of active sites available for the haloform reaction. d(T.Chlor)/dt = hl’(FA)X
(8)
For a first-order rate equation x = 1. For a given quantity of fulvic acid in excess chlorine, the chloroform will reach a limiting value. This maximum chloroform concentration is a measure of the active sites in the acid structure which give the haloform reaction. The effective fulvic acid concentration may therefore be represented in terms of this limiting chloroform concentration by eq 9 1394
Environmental Science & Technology
Figure 8. Reaction scheme for chloroform formation.
(FA),* 0: (T.Chlor),,,
- (T.Chlor)t
(9)
where (FA),* = effective fulvic acid concentration a t time t . Substituting eq 9 into eq 8 gives d(T.Chlor)ldt = k~”[(T.Chlor),,, - (T.Ch10r)~l (10) and integrating eq 10 gives (T.Chlor),,, - (T.Chlor),) = -kl”t (11) In (T.Chlor),,, The two basic assumptions for eq 11 are (a) that chlorine is in excess and (b) that all chloroform is produced via intermediates measured as residual chloroform. The calculation of the rate constant k l ” has not been attempted as the value (T.Chlor),,, has not been determined. An arbitrary but realistic value of 100 pg L-l may be assumed for (T.Chlor),,,, and straight-line plots were obtained for eq 11for pH 7,8, and 9 after 8 h of reaction. These plots also indicated that the initial rate of total chloroform formation is fast and does not follow the first-order rate equation. If the breakdown of intermediates is rapid (hz >> k l ) , no residual chloroform would be detected. However, the presence of residual chloroform is a clear indication that the hydrolysis of the organochlorine intermediates (eq 3) is slow and possibly dependent on factors such as steric hindrance and mesomeric and inductive effects. This is comparable to some simple acetyl-containing compounds that produce relatively stable trichloroacetyl derivatives (CC13CO-) under acid conditions (15) which undergo rapid hydrolytic attack under alkaline conditions to produce chloroform. A reaction scheme involving a minimum of two possible pathways to chloroform is presented in Figure 8. A fast initial step leads to the formation of an intermediate (CH&)A*, containing a trichloroacetyl group which is measured as residual chloroform (path 1).The rate-determining step in the formation of this intermediate is proton dissociation consistent with the classical haloform reaction although steric hindrance and other kinetic controlling factors might also involve the chlorinating species HOCl or H20Clf. The second path (path 2) involves a slow initial oxidation step, which is rate determining, to form an active carbon atom (HM)*.This subsequently undergoes the haloform reaction to produce either intermediate (CHC13)** or (cHC13)~*. This secondary phase to total chloroform formation is clearly independent of p H in that both HOCl and OC1- are involved in oxidation. These intermediates are considered to display differences in the ease with which they break down to yield chloroform, the rate being dependent upon pH.
(
I t is implicit from a general consideration of reaction mechanisms that the basic reaction system is complex and that it is further complicated by the presence of such species as bromide, ammonia, and amines. Summary
I t is apparent that more than one route to chloroform formation is available and that the relative rate and extent to which a route is followed depends upon several reaction parameters. Total chloroform formation is not significantly affected by p H compared to the strong pH dependence of both the dissolved and residual chloroform components. Several different moieties within the humic structure are capable of undergoing the haloform reaction. The higher molecular weight humic acid fraction, which contains a greater number of active sites than fulvic acid, gives rise to higher chloroform yields. Chlorinated intermediates detected by DAI as residual chloroform are a potential source of chloroform and must be taken into account when attempting to control trihalomethanes. The relationship between these laboratory studies and water treatment practice is currently being evaluated, and preliminary indications are that the amount of trihalomethanes detected in tap waters (including the intermediates of the haloform reaction) will depend upon the following: (i) the quality of the source water, (ii) the process conditions during chlorination, (iii) p H with respect to the balance between dissolved and residual chloroform, (iv) bromide concentration with respect to the balance between brominated and chlorinated trihalomethanes involving both residual and dissolved components, and (v) the conditions of water storage and distribution with respect to evaporative losses and breakdown
of halogenated organic intermediates of the haloform reaction to yield additional trihalomethanes. Acknowledgment
We are grateful to the Water Research Centre, UK, for funding this project and to Mr. M. Fielding for valuable discussion, and one of us, c. J. Peters, acknowledges the Science Research Council for a studentship. Literature Cited (1) Symons, J. M.; Bellar, T. A.; Carswell, J . K.; De Marco, J.; Kropp, K. C.; Robeck, G. G.; Seeger, D. R.; Slocum, C. J.; Smith, B. L.; Stevens, A. A. J . Am. Water Works Assoc. 1975,67, 634-47. (2) Hall, E. S.; Packham, R. F. J . Am. Water Works Assoc. 1965,57, 1149-66. ( 3 ) Black, A. P.; Christman, R. F. J . Am. Water Works Assoc. 1963, 55, 897-912. (4) Nicholson, A. A,; Meresz, 0. Bull. Enuiron. Contam. Toxicol. 1975,14, 453-6. (5) Pfaender, F. K.; Jones, R. B.; Stevens, A. A.; Moore, L.; Hass, J . R. Enuiron. Sei. Technol. 1978,12, 438-41. (6) Rook, J. J . Water Treat. Exam. 1974,23, 234-43. ( 7 ) Bellar, T . A,; Lichtenberg, J. J. J . Am. Water Works Assoc. 1974, 66, 739-44. (8) “Analysis of Raw, Potable and Waste Waters”; Department of the Environment, U.K., HMSO, 1972,91-5. (9) Harrison, R. H.; Perry, R.; Wellings, R. A. Enuiron. Sci. Technol. 1976,10, 1156-60. (10) Trussell, R. R.; Umphres, H. D. J . Am. Water Works Assoc. 1978,70, 604-12. (11) Oliver, B. G.; Lawrence, J . J. Am. Water Works Assoc. 1979,71, 161-3. (12) Rook, J. J. Enuiron. Sci. Technol. .1977,11, 478-82. (13) Oliver, B. G. Can. Res. 1978,11, 21-2. (14) Sander, V. R.: Kuhn. W.: Sontheimer. H . Z. Wasser Abwasser Forsch. 1977,10, 155-60. (15) Fuson, R. C.; Bull, B. A. Chem. Reu. 1934,15, 275-309.
Receiued for review June 29,1979. Accepted J u n e 25, 1980.
NOTES
Development of the FACTS Procedure for Combined Forms of Chlorine and Ozone in Aqueous Solutions John Liebermann, Jr., and Nina Matheny Roscher Department of Chemistry, The American University, Washington, D.C. 200 16
Eugene P. Meier’ and William J. Cooper’2 U.S. Army Medical Bioengineering Research and Development Laboratory, Fort Detrick, Frederick, Maryland 21701
Chlorine. Specificity is by far the most important characteristic of any analytical procedure for free available chlorine (FAC). However, interferences from impurities such as combined chlorine are common problems in most existing procedures ( I ) . An acceptable procedure must also be convenient, rapid, accurate, and precise over a wide range of temperatures, and adaptable to the wide variety of waters encountered in water and waste water treatment. Most of the existing procedures do not meet all of these criteria. Commercially available colorimetric test procedures have been evaluated (I, 2). Of the methods tested, the DPD (N,N-diethyl-p-phenylenediamine) method was the most
Present address: Methods Development and Analytical Support, U.S. EPA Environmental Monitoring and Support Laboratory, P.O. Box 15027, Las Vegas, NV 89114. 2Present address: Drinking Water Research Center, Florida International University, Tamiami Campus, Miami, FL 33199.
accurate and precise while the syringaldazine (liquid) method was the most specific for FAC. A modified syringaldazine (liquid) method called FACTS (free available chlorine test with syringaldazine) was designed for two ranges of FAC concentration ( 3 ) :the FACTS I procedure was designed for lower concentrations of FAC (to 5 mg/L) whereas the FACTS I1 procedure was developed for a higher concentration range to (10 mg/L). Early in its development, it was observed that the FACTS method could be modified to measure total available chlorine. This involved the preliminary reaction of both FAC and combined available chlorine with iodide to give iodine, which was then analyzed by the FACTS procedure. The present research was undertaken to exploit these observations by developing a “FACTS” procedure for determining combined and total chlorine. Ozone. Although ozone is widely used for water treatment in Europe, this is not the case in the United States ( 4 ) .Several factors have prevented its use. These include the high cost of
This article not subject to US. Copyright. Published 1980 American Chemical Society
Volume 14, Number 11, November 1980
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