Effect of Water Chlorination upon Levels of Some Polynuclear Aromatic Hydrocarbons in Water Roy M. Harrison’, Roger Perry*, and Roger A. Wellings Public Health Engineering, Imperial College, London SW7 2AZ, England
Laboratory studies of the effect of water chlorination processes upon the levels of eight polynuclear aromatic hydrocarbons (PAH) were undertaken. The effects of pH, temperature, initial levels of PAH and chlorinating agent, and of contact time were independently evaluated. A high efficiency for degradation of PAH was found, and the results were examined in the light of the measured removal of PAH a t the various stages of a water treatment works.
Some polynuclear aromatic hydrocarbons (PAH) are of proven carcinogenic activity; consequently, the World Health Organization has recommended a maximum permissible concentration for these compounds in drinking water ( 1 ) .In waters of acceptable purity, the concentration of six representative PAH compounds [fluoranthene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, benzo(ghi) perylene, and indeno( 1,2,3-cd)pyrene]should not, in general, collectively exceed 0.2 pg 1.-l ( I ) . The levels encountered in raw waters can be quite substantial. Whereas groundwaters usually contain very low concentrations of PAH, surface waters typically contain 0.005-0.5 pg 1.-’ of individual PAH, according to the degree of pollution. In exceptional instances, especially in the presence of urban run-off water or industrial discharges, still higher levels may be found (2). Consequently, a high efficiency of removal of PAH during water treatment is desirable. As part of a comprehensive study of the levels of PAH in water, aspects of the effects of water treatment processes upon levels of PAH have been investigated. The efficiency of chlorination processes in reduction of PAH levels in water has been systematically evaluated, and the results have been assessed in the light of PAH removal a t a water treatment works.
Experimental Laboratory Chlorination Experiments. Water was purified by distillation and passage through an activated charcoal column according to the method of Acheson et al. ( 3 ) . When 20 1. had been prepared, it was mixed and divided into four aliquots ( 5 1.). The temperature of the water was adjusted to a predetermined temperature within the range 5-20 “C ( f 0 . 5 “C) by heating or refrigeration. The p H was adjusted by addition of sulfuric acid (N/100), taking account of the anticipated change upon addition of chlorinating agent. PAH’s were then added to three aliquots as a solution in dichloromethane (0.1-0.4 ml), and the fourth aliquot was kept as a blank. The solutions and blank were then each mixed thoroughly with an Ultra-Turrax (Scientific Instrument Co., London) for 2 min a t 50% full revolutions. Aliquots (0-30 ml) of sodium hypochlorite solution (Hopkin and Williams Ltd., ca. 12% available chlorine, diluted 50:l with water) were then added with stirring to achieve the required level of “free chlorine”. At this point, the p H of the mixture was checked (f0.05) by pH meter, and an aliquot withdrawn for analysis of free chlorine by a standard iodometric titration procedure ( 4 I. The reaction mixtures were stood in the absence of light Present address, Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, England.
for the required contact time (0-25 min). The reaction was then terminated by addition of sodium bisulfite solution (5.25 g l.-l) in an equal volume to that of chlorinating agent, and the mixture stirred. Distilled dichloromethane (300 ml) was added, and the liquids were vigorously emulsified with the Ultra-Turrax a t 80%full revolutions for 5 min. After standing overnight in the absence of light, the dichloromethane layer was separated, concentrated, and analyzed by gas-liquid chromatography according to the method described by Acheson et al. for analysis of PAH in purified water (3). Experiments were performed at the various values of temperature, contact time, and reagent concentrations required. Extraction efficiencies using the Ultra-Turrax were determined according to the method of Acheson et al. ( 3 ) ,with particular regard to the effects of standing for the duration of the reaction and the presence of chlorinating and dechlorinating agents. Collection and Analysis of Samples from Water Treatment Works. The samples (5 1.) were collected in glass vessels and extracted and analyzed by gas-liquid chromatography according to the method described by Acheson et al. for determination of PAH in environmental water samples (3).
Discussion and Results Various disinfecting agents are used by the water industry and studies of the effects of chlorine (5-9), chlorine dioxide ( I O ) , and ozone (8,9,11,12)upon levels of PAH in water have been reported. Chlorination is used extensively in Europe; consequently, considerable attention has been paid to this reagent. An examination of the experimental results, however, reveals little consensus, it being especially evident that the results reported apply to only one set of conditions and may not be extrapolated to other conditions. Graf and Nothhafft ( 5 ) and Trakhtman and Manita (6) studied the removal of benzo(a)pyrene by chlorine, but in both instances, concentrations of B(a)P substantially higher than those normally encountered in raw waters were employed. In the work of Sforzolini et al. (7),extremely high initial levels of PAH were used with detergents added to aid solubilization, and in later studies (8, 9) relatively high PAH levels were again used. As well as the variable levels of reagents used in the above studies, in only one instance was any measurement of temperature or pH reported, and it appears that no account was taken for the variable efficiency of the extraction and analytical procedures. Since in earlier work experience had been accrued in the extraction and analysis of PAH, as well as considerable data upon the factors affecting extraction efficiency ( 3 ) ,it was decided to initiate a study of the effects of chlorination processes upon PAH, altering one variable factor in each experiment, with the other parameters fixed. Laboratory Chlorination Experiments. Chlorine gas, the most usual form of the chlorinating agent in water works, undergoes the following reaction upon solution in water Clp
+ H20
F==
HOCl
+ HC1
(1)
Equilibrium is rapidly established, and except a t pH less than 4, conversion to HC1 and HOCl is virtually complete. Hydrochloric acid is highly ionized in water, whereas hypochlorous Volume 10, Number 12, November 1976
1151
acid, the active chlorinating agent, is weakly acidic and ionizes only partially HOCl * H+
+ OC1-
(2)
The latter equilibrium is highly dependent upon pH; hence, any study lacking adequate control of this variable is unlikely to yield reproducible results. For the purposes of this study, sodium hypochlorite, often used for chlorination in water works, was chosen as the chlorinating agent because it was convenient for preparation of reliable and repeatable standard solutions. At pH values above 4, chlorination is effectively the same as with gaseous chlorine as chlorinating agent, because of the position of the equilibria affecting aqueous chlorine.
-
+ OC1OC1- + HzO * OH- + HOCl NaOCl
Na+
Table I . Extraction Efficiencies for PAH from
Purified Water Extraction effi- No. of ciency,% readings
Concn, pg I.-'
PAH
Fluoranthene Pyrene Benzo(a)anthracene Benzo(k)fluoranthene Benzo(a)pyrene Pery le ne Indeno(l,2,3-cd)pyrene Benzo(ghi)perylene
73 84
0.44
0.32 0.21 0.15 0.13 0.16
SD. %
7 5 10
88
64
9
70
10 13 12 11
0.07
69 80
0.12
80
(3) (4)
In the water works, sulfur dioxide is frequently used to limit chlorine concentrations before distribution of water to the consumer. In laboratory chlorination experiments, sodium bisulfite was added to terminate chlorination after a predetermined contact time HOCl
+ NaHS03
-
NaHS04
+ HCl
(5)
Concentrations of chlorinating agent were assayed by a standard iodometric procedure ( 4 ) ,and by convention concentrations are expressed as "free chlorine", Le., the equivalent in molecular chlorine. Sodium hypochlorite has the same oxidizing capacity as a solution of molecular chlorine on a molar basis; hence, 1 mg 1.-l sodium hypochlorite is equivalent to 1 X 71/74.5 = 0.95 mg 1.-l of "free chlorine". The concentrations of "free chlorine" used in this study were in the range 0-12 mg l.-I, reflecting the levels found in the water treatment works, and contact times were limited to a maximum of 25 min during which no significant changes in "free chlorine" concentrations were observed. Addition of sodium hypochlorite to water causes an increase in pH due to formation of hydroxyl ions in reaction ( 4 ) .Changes in pH for a given addition of chlorinating agent were reproducible, and reported values were those found after addition of hypochlorite. Eight PAH's were selected to be representative of those commonly encountered in the water cycle, while covering a wide range of molecular weights and being readily analyzed by gas-liquid chromatography. The concentrations selected (30-860 ng 1.-l of individual compounds) were representative of those found in polluted surface waters. The methods of extraction and analysis after chlorination were those proved in earlier work ( 3 ) ,and the water was purified by distillation and activated charcoal to the same high standard. The efficiency of extraction of the PAH from pure water a t the concentrations used in the chlorination studies was found to lie in the range 64-88% for the compounds studied, with coefficients of variation between 5 and 13%on repetitive analysis (Table I). The accuracy of the analytical procedure was tested by addition of PAH to a dichloromethane extract of purified water with the customary addition of octacosane as an internal standard. The quantities measured were correct to within f 5 % for the compounds studied, and it was concluded that no materials extracted from the purified water caused a significant interference with the analysis. Earlier work has shown that several factors influence the efficiency of extraction of PAH from water ( 3 ) .Two influences not investigated in the previous study were the presence of chlorinating and dechlorinating agents in the water, and the standing of the PAH solution prior to analysis. Figures 1-4 show the measured variation in extraction efficiency with concentration of chlorinating and dechlorinating agent (added simultaneously a t zero time) and with time of standing for the 1152
Environmental Science 8, Technology
5
IS
30
2s
20
M
35
LO
45
Time
*t.nd,"Q
Mn
Figure 1. Effects of standing time and chlorination reagents on extraction of fluoranthene and pyrene
5
10
IO
15
IS
St."d,"Q
10
35
40
45
7,m.
M,"
Figure 2. Effects of standing time and chlorination reagents on extraction of benzo(a)anthraceneand benzo(k)fluoranthene
80.
0
2 2
4 4
6 6 (E."*
5
lo
I5
8 8 (10 l l i Chior~".l~on P..g.nts F," Cnior,".',
$54
116
,e8
35
40
IS
10 mpi 1 0 ,
10 SI."d
25 "(i T
50
rn
Urn"
Figure 3. Effects of standing time and chlorination reagents on extraction of benzo(a)pyreneand perylene
initial concentrations of PAH given in Table I. It was impractical to investigate these effects for all concentrations of PAH encountered, and it was assumed that the effects were of the same relative size (as a percentage of the PAH concentration present) for all PAH levels. In the estimation of extraction efficiency for a given experiment, the two effects were assumed to be independent and additive in reducing extraction efficiencies from 100%.Hence, "corrected reductions" in PAH concentrations due to chlorination were calculable (Le., the difference between actual recovery of PAH and the recovery calculated for zero degradation allowing for standing time and reagents) and included some account of extraction efficiency. This "corrected reduction" is only an approximate figure, however, since it takes no account of the variation of extraction efficiency with the concentration of PAH in solution, which can be quite significant (3). Chlorination experiments were performed at pH 6.8, except in those experiments where the effect of pH variation was studied. This value lies within the range normally encountered in the water works. In all experiments, parallel blank runs were performed in which no PAH's were added to the water, in order to obtain a measure of background organic materials which could then be subtracted in the gas-liquid chromatographic analysis. The influence of the many variable factors upon the reduction in PAH concentrations by chlorination was then investigated. Contact Time. Using standardized conditions (pH = 6.8;
temperature = 20 "C; "free chlorine" = 2.2 mg l.-l) reductions in PAH levels were measured for different contact times up to 25 min, and the results are expressed graphically in Figures 5 and 6. Concentration of ChlorinatingAgent. A contact time of 5 min was used for these experiments as it gave results of a convenient order of magnitude. Again, a standard temperature (20 "C) and pH (6.8) were used, while "free chlorine" concentrations were varied between 0.44 mg typical of residual chlorine remaining in the water before distribution, and 13.2 mg l.-I, close to the maximum level used in the water works. Estimated reductions in PAH concentrations are shown in Figures 7 and 8. Temperature. By use of standard conditions, the temperature of the aqueous PAH solution was varied within the range 5-20 "C, and estimated reductions in PAH concentrations determined. Control experiments showed no significant effect of temperature upon extraction efficiency. The results (Figures 9 and 10) show increased degradation with increased temperature as might be anticipated from the Arrhenius equation for the rate constant of a chemical reaction. Acidity. The range of pH 4.5-7.6 was investigated. The higher values (6.5-7.6) reflected those encountered in a water treatment works, whereas the lower pH values were used to investigate more fully the effect of pH upon reaction rates and in particular whether greater degradations could be achieved at lower pH. Control experiments indicated no significant effect of pH upon the efficiency of extraction of PAH. Esti-
Sf."d,rn US" Ilrn
Figure 4. Effects of standing time and chlorination reagents on ex-
0
traction of benzo(ghi)peryleneand indeno(1,2,3-~d)pyrene
0
5
15
10
20
,
Contact Y," T i m
Figure 6. Effect of contact time upon corrected removal of PAH from
water
',I 30
1
FM Cnwln 1mpu.t-
2 Pmsl-'
2uc 68
ConI.st Yh Tm
Figure 5. Effect of contact time upon corrected removal of PAH from
water
IF,..
C",.,,".
mg, 3
Figure 7. Effect of "free chlorine" concentration upon corrected removal of PAH from water
Volume 10, Number 12, November 1976 1153
mated reductions in PAH levels increase considerably with decreasing pH (Figures 11 and 12), perhaps due to an increased hydrogen ion concentration affecting the hypochlorous acid dissociation equilihrium (Equation 2). A distinct bend appears in the curves a t approximately pH 6.5, possibly betokening a change in reaction mechanism. I n i t i a l PAH C o n c e n t r a t i o n s . These were varied within practicable limits, and chlorination was performed. Because of the anticipated variations in extraction efficiency with concentration, control analyses were performed in which
Y
11
v."c;:,"..m
