pated from laboratory studies. Further investigation is neeessary to establish the nature of the products of the chlorination reactions in view of present concern over possible health hazards due to halogenated organic compounds (16). Literature Cited (1) World Health Organization, “International Standards for Drinking Water”, 3rd ed., Geneva, Switzerland, 1971.
(2) Harrison, R. M., Perry, R., Wellings, R. A., Water Res., 9,331-46 (1975). (3) Acheson, M. A., Harrison, R. M., Perry, R., Wellings, R. A,, ibid., 10,207-12 (1976). (4) HMSO, “Analysis of Raw, Potable and Waste Waters”, PP 94-5, Londor, England, 1972. ( 5 ) Graf, W.2 Nothhafft, G., Arch. HYg. Bakteriol., 147, 135-46 il963). ( 6 ) Trakhtman, N. N., Manita, M. D., Hyg. Sanit., 31, 316-9 (1966).
(7) Scassellati Sforzolini, G., Savino, A., Merletti, L., Boll. Soc. Ital. Sper., 469 903-6 (lg70). (8) Scassellati Sforzolini, G., Savino, A., Monarca, S., Lollini, M. N., Ig, Mod,, 66, 309-35 (1973), (9) Scassellati Sforzolini, G., Savino, A., Monarca, S., ibid., pp 595-619. (10) Reichert, J. K., Arch. Hyg. Bahteriol , 152,37-44 (1968). (11) Il’nitskii, A. P., Khesina, A. Y., Cherkinskii, S. N., Shabad, L. M., H ~sanit,, ~ , 33,323-7 (1968). (12) Borneff, J., Gas Wasser, 110,29-34 (1969). (13) Borneff, J., Kunte, H., Arch. Hyg. Bahteriol., 148, 585-97 (1964). (14) Rei&&, J. K., Kunte, H., Engelhardt, K., Borneff, J., ibtd., 155, 18-40 (1971). , ibid., 146, 183-97 (1962). (15) Borneff, J,, ~ i s c h e rR., (16) Morris, J. C., “Environmental Health Effects Research Series”, Environmental protection A ~~ p ~ - 6~ 0 0 / 1 -~7 5 - 0 01975, ~2 , ~
Received for revieu, Februar) 24, 1976 Accepted J u n e 9, 1976.
Chemical Kinetics of Chlorination of Some Polynuclear Aromatic Hydrocarbons Under Conditions of Water Treatment Processes Roy M. Harrison’, Roger Perry*, and Roger A. Wellings Public Health Engineering, Imperial College, London SW7 2A2, England
w The chemical kinetics of chlorination of the polynuclear aromatic hydrocarbons, pyrene and benzo(ghi)perylene, in dilute aqueous solution were investigated. The active chlorinating agent was hypochlorous acid, and first order dependence of the reaction rate upon the concentration of this species and upon the PAH concentration was found. Above pH 6.5 there was no dependence of reaction rate upon the hydrogen ion concentration, apart from that which results from changes in the hypochlorous acid dissociation equilibrium. At lower values of pH the reaction rate was further increased apparently due to a change in mechanism which causes an additional dependence upon the concentration of hydrogen ions. Reaction rate constants and activation energies were estimated.
Polynuclear aromatic hydrocarbons (PAH) in aqueous solution are degraded by addition of chlorine ( 1 - 5 ) , chlorine dioxide ( 6 ) ,or sodium hypochlorite ( 7 ) .In the case of sodium hypochlorite, the rate of degradation has been shown to be dependent not only upon the initial concentrations of chlorinating agent and PAH, but also upon the temperature and pH of the solution. No reported investigation, however, has sought to identify the chemical agent with prime responsibility for reaction with the PAH, nor have any fundamental reaction rate data been established. Such data, if available, would allow prediction of the optimum conditions for removal of PAH during water treatment. In view of the known health hazard associated with PAH, it was the aim of this work to provide such necessary data. Experimental
Solutions of pyrene and benzo(ghi)perylene in purified water were chlorinated with sodium hypochlorite and analyzed in a manner identical to that described by Harrison e t
al. ( 7 ) .The temperature and pH of the solution, the initial concentrations of PAH and chlorinating agent, and the contact time were each varied independently. After chlorination the solutions were extracted with dichloromethane, and the quantity of unchanged starting material in the extract was determined by gas-liquid chromatography. This was related to the concentration remaining in aqueous solution after chlorination by means of the extraction efficiency data collected in earlier work (7,8).The calculation was performed as follows: Suppose a pg is the weight of a PAH added to the water which is extracted and analyzed after reaction. Allowance is made for the effect of standing time and the presence of chlorinating and dechlorinating agents. The two effects are assumed to be independent and additive. If the investigation of extraction efficiency ( 7 )showed that a given time of standing (Le,, the time from addition of PAH to commencement of extraction) caused a change in extraction efficiency of c% from that at zero time, and the presence of the appropriate concentration of chlorinating and dechlorinating agents caused a change of d % from that at zero reagent concentration, then the “corrected recovery”, b pg, is given by b = a X e + (e c d ) , where e is the extraction efficiency for zero standing time and no addition of reagents. The curves for variation of efficiency of extraction from purified water vs. PAH concentration given in the work of Acheson et al. (8)are then referred to. I t is assumed that b pg is the weight of compound which would have been extracted from a solution in purified water, and the curves allow this to be related to the weight present in the aqueous solution. This is then the actual weight remaining after reaction and allows calculation of the actual reduction in concentration resulting from the chlorination reaction. The results of the various experiments appear in Figures 1-4.
+ +
Discussion and Results Present address, Department of Environmental Sciences, University of Lancaster. Lancaster LA1 4Y4, England. 1156
Environmental Science & Technology
Data have been accumulated regarding the removal of eight PAH from water by chlorination with sodium hypochlorite
,
e
0
Figure 1. Variation of actual reduction in PAH concentration with contact time
L o
,,
11
s2
1,
I$ FI"
6 1
77
lla
ss
110
(11
I32
wI C**xln.'
Figure 2. Variation of actual reduction in PAH concentration with "free chlorine" concentration
5
20
PAH's were extracted into dichloromethane and analyzed by gas-liquid chromatography according to procedures which have been thoroughly investigated by Acheson et al. (8).The amount of PAH extracted was converted to a concentration in the aqueous solution by use of the extraction efficiency for that compound in that approximate concentration, after correction of the recovery for the effects upon the extraction efficiency of added chlorinating and dechlorinating reagents, and the time of standing of the solution. Control experiments indicated a negligible effect of temperature or pH upon the extraction. I t was then possible to perform experiments in which one of the many variables was altered independently, and to construct curves such as Figure 2 which shows the actual reduction in concentration of pyrene and benzo(ghi) perylene in a 5-min reaction time with different concentrations of chlorinating agent, assayed as "free chlorine" at fixed values of p H and temperature. The chlorinating agent used in these studies was sodium hypochlorite which is highly ionized in aqueous solution, with formation of hypochlorous acid favorable. NaOCl OC1-
3
5
Figure 4. Variation of actual reduction in PAH concentration with temperature
3 501
0 T.np"t"r.c
-+
Na+
+ OC1-
(1)
H+ * HOCl
30-1
l 0
4 0
4 s
$ 0
5 5
110
e 5
I O
7 5
8 0
d.
Figure 3. Variation of actual reduction in PAH concentration with pH
( 7 ) . The efficiency of solvent extraction of PAH from water has been shown to be highly dependent upon the concentration of PAH present (81, and unless full data regarding efficiencies of extraction are available, it is not possible to calculate the concentration of PAH in a given reaction mixture. In the case of pyrene and benzo(ghi)perylene, however, such data have been reported (8);hence, actual concentrations may be estimated and true rates of reaction calculated with some confidence. Pyrene and benzo(ghi)perylene were chlorinated in dilute aqueous solution by use of concentrations of PAH and the chlorinating reagent, sodium hypochlorite, reflective of those found in a water treatment works. After stopping the reaction after a set time by dechlorination with sodium bisulfite, the
Thus, the predominant chlorine-containing species in solution is HOC1, and it appears that this species is a far stronger chlorinating agent than OC1-, although chlorination by ClOH2+ and C1+ has been postulated (9).Consequently, in an examination of the kinetics of reaction, a rate equation of the form below was assumed as a starting point, although subsequent modification was not precluded. -d Rate = - (PAHJ= h { H + ) x ( H O C l } ~ ( P A H ) 2 (4) dt
The values of the reaction orders x, y , and z are unknown. For experiments performed a t constant pH (= 6.8), clearly {H+}is constant. Also since {HOCl)is far greater than {PAH) (by ca. lo4 times), its value can be considered unchanged through the course of a reaction, Thus, Rate = hobs(PAH}2 (5) where h& is the observed rate constant for a reaction during which {HOClJand pH do not change. (The term hobs is used at various points in this paper to describe different pseudo rate constants). In this instance hobs = h{H+)X{HOC1)J Volume 10, Number 12, November 1976
(6) 1157
Since previous studies of the chlorination of monocyclic aromatic compounds had indicated first order kinetics with respect to (HOC1)( 9 ) ,it was decided to test for a first order reaction by plotting In c,/ct against chlorination time from the data from reactions in which the effect of varying contact times was investigated (c, is the initial PAH concentration and ct the concentration a t time, t ) . From the data in Figure 1,the plots for pyrene and benzo(ghi)perylene appearing in Figure 5 were derived using the calculated "actual concentrations". Despite some scatter which is a result of the low PAH concentrations used and the uncertainties of the extraction and analyses, the plots show reasonable linearity, pass through the origin, and do not exhibit any apparent curvature which might indicate kinetics other than first order. Values of hobs were obtained by estimating the gradients of these plots by least-squares analysis. Hence z = 1 and the rate equation may be written as Rate = h{H+}x{HOCl)~{PAHJ
(7)
For reaction a t constant pH,
c o n t r t ,,me ~ 8 "
' wren. 0 8en2oIphilpryUn FR. cn,Or,m 22m1 T.mpr.l"r. 20 c DH
0
e5
10 I"
5
1
68
20
2
Figure 5. Plot of In co/ct against contact time for pyrene and benzo(ghi)perylene 50-
Rate = hl{HOCl}~{PAHJ
(8)
where
h 1 = k{H+IXand hobs = hl{HOC1)?
(9)
From the series of experiments in which the concentration of chlorinating agent, measured as its equivalent in "free" (Le., molecular) chlorine, was independently varied, calculated values of the observed rate constant, hobs,were plotted against the concentration of the chlorinating agent, HOCl. Good linear plots passing through the origin (Figure 6) were obtained indicating direct proportionality of hobs and {HOClJ,hence showing that y = 1and further confirming the first order dependence of rate upon {PAH}.Thus, Rate = h{H+)x(HOC1){PAH)
(10)
As mentioned previously, IHOC1)is dependent upon pH due to the equilibrium (2). In the series of experiments in which pH was varied independently, this variation will cause a change in {HOCl)as well as possibly affecting the reaction in other ways. Now, since hobs
= h{H+)X{HOCIJ
(11)
if there is no dependence upon (H+Jin the rate equation (Le., x = 0), then a plot of {HOCl)calculated from the dissociation equilibrium constant, K , vs. hobs should give a h e a r plot through the origin irrespective of changes in pH. Such plots appear in Figures 7 and 8. Above pH 6.5 an approximately linear plot which upon extrapolation passes through or close to the origin is found, but a t lower p H the plot curves steeply upward betokening a change in mechanism. Since hobs = h{H+)X{HOCIJ for all values of pH log hobs = log h
+ x log ( H + Jf log (HOCl)
(11) (12)
and loghDbt (HOC1)= log h
+ x log (H+I
Thus, using the same data, log10 kob,/{HOC1~was plotted against loglo (H+).The resultant plots (Figures 9 and 10) exhibit considerable scatter but appear approximately linear in the region pH 4.5-6.5, confirming a dependence upon (H'}. The intercept on the ordinate is equal to loglo h , and the gradient to x . For pyrene x = 0.24, and for benzo(ghi)perylene x = 0.16 approximately. Due to the considerable scatter, it would ap1158
Environmental Science & Technology
[ H O C I I ~ X l O5nmolI-'l
Plot of observed rate constant, kobs,against hypochlorous acid concentration Figure 6.
pear appropriate to approximate the value of x to 0.2 in both cases. Thus, to summarize, for values of pH greater than ca. 6.6, the only effect of acidity is to alter (HOClJvia its dissociation equilibrium; hence, Rate = kll{HOCIJ{PAH) (14) From Figure 5, by least-squares analysis for pyrene, hll = 34.4 1. mol-l s-l a t 20 "C and for benzo(ghi)perylene h I 1 = 23.2 1. mol-' s-l a t 20 "C. For pH less than 6.5, the acidity has a further effect, and in the region investigated, pH 4.5-6.4, Rate = k(HOC1}(PAH}{H+Jo~2
(15)
for pyrene h = 686 1.* mole2 s-l a t 20 "C, and for benzo(ghi) perylene h = 463 L2 mol-2 s-l a t 20 "C. This equation appears to imply the intervention of a chlorinating species such as ClOH2+ a t lower pH, and the latter rate equation is probably a combination of two terms, Le.,
+
Rate = h2]HOCl}(PAH) k~(HOClJ(PAHJ{H+)(16)
Investigation over a larger range of pH would elucidate the full kinetics, but is not relevant to the current study. The Arrhenius equation,
k = Ae-EJRT
Figure 7. Variation of observed rate constant, kobs, with changes in hypochlorous acid concentration caused by altering pH for pyrene
(17)
describes the variation of rate constant with temperature and includes two constants, A and E, which are relatively independent of temperature ( R is the gas constant). The energy of activation for the reaction, E,, may be determined by plotting In k vs. UT, when the gradient is equal to -E,IR. For the chlorination reactions in which temperature was independently varied, the gradient is unaltered for a plot of In kobs vs. l / T and this has been plotted. Both plots (Figure 11)reveal considerable scatter because of the small range of temperatures investigated. Calculation of approximate values of E, leads to values of
E, = 32 kJ mol-' for pyrene E, = 13 kJ mol-' for benzo(ghi)perylene although it must be emphasized that there is considerable uncertainty associated with both these values. These figures are consistent with the observed reaction, i.e., a reaction proceeding with moderate rapidity a t ambient and subambient temperatures and very high dilution.
Figure 8. Variation of observed rate constant, kobs. with changes in hypochlorous acid concentration caused by altering pH for benzo(ghi) perylene rJO
i'O
I
Conclusions From the data collected, by appropriate corrections using extraction efficiency data, the chemical kinetics of chlorination of two PAH have been elucidated, and some understanding of the reaction mechanism has been gained. I t appears that the active chlorinating agent is hypochlorous acid, and the reaction rate has a first order dependence upon the concentration of this species and upon the PAH concentration. The dependence upon acidity is more complex. At above pH 6.5 the only dependence of reaction rate upon the hydrogen ion concentration is due to changes in the hypochlorous acid dissociation equilibrium. At lower p H values an additional dependence is evident, probably as a result of the intervention of another chlorinating agent such as ClOH*+. Hence, in the water treatment works, efficient removal of PAH during chlorination is favored by high concentrations of chlorinating agent, low values of pH, and elevated temperatures. The results presented here should allow estimation of reaction rates under different physical conditions and with different reagent concentrations. Such knowledge is valuable
Figure 9. Plot of log,, kob,/{HOCI) against loglo (H+] for pyrene $l103)(K11
-66
3.5
3.4
3.6
PH
37
68
6 ,o(i
Iii*1
Figure 10. Plot of loglo kobs/(HOCI)against loglo (H+l for benzo(ghi) perylene
Figure 11. Plot of log,
kobs
against inverse temperature
Volume IO, Number 12, November 1976
1159
since removal of PAH from potable waters is desirable. However, it must be borne in mind that little is known of the reaction products from the chlorination of PAH, and the concentrations of these compounds in drinking water as well as their toxicological properties should be investigated. Literature
Cited
(1) Graf, W., Nothhafft, G., Arch. Hyg. Bakteriol., 147, 135-46
(1963). (2) Trakhtman, N. N., Manita, M. D., Hyg. Sanit., 31, 316-9 (1966). (3) Scassellati Sforzolini, G., Savino, A., Merletti, L., Boll. SOC.Ztal. Biol. Sper., 46,903-6 (1970).
(4) Scassellati Sforzolini, G., Savino, A., Monarca, S., Lollini, M. N., Zg Mod., 66,309-35 (1973). (5)-Scassellati Sforzolini, G., Savino A., Monarca S., ibid., pp 595619. (6) Reichert, J. K., Arch. Hyg. Bakteriol., 152,37-44 (1968). ( 7 ) Harrison, R. M., Perry, R., Wellings, R. A., Enuiron. Sci. Technol., 10,1151 (1976). (8) Acheson, M. A., Harrison, R. M., Perry, R., Wellings, R. A., Water Res., 10,207-12 (1976). (9) Hine, J . , “Physical Organic Chemistry”, 2nd ed., pp 358-63, McGraw-Hill, New York, N.Y., 1962.
Received for review February 24,1976. Accepted June 9, 1976.
Aerosol Formation Threshold for HCI-Water Vapor System Donald L. Fenton* and Madhav B. Ranade IIT Research Institute, 10 West 35th Street, Chicago, Ill. 60616
The conditions under which liquid droplet growth (aerosol formation) occurs in the presence of the hygroscopic vapor, hydrogen chloride (HCl),are experimentally investigated. The HC1 vapor has no impact on the nucleation process itself. The HC1-water equilibrium tables accurately predict the conditions where droplet growth occurs for HC1 concentrations ranging from 10 to 40 mg/m3. In the presence of HC1, a relative humidity of 81%promotes droplet growth. With the advent of NASA’s Space Shuttle Program, considerable quantities of hydrogen chloride (HC1) will be released in the earth’s atmosphere. The estimated mass of HC1 generated is approximately 18 000 kg/launch for the ammonium perchlorate propellant planned to be used. An environmental assessment is therefore necessary concerning this airborne HCl. Under the support of NASA, tropospheric washout of HC1 due to rainout was investigated by Knutson et al. ( 1 ) . Important in the study was the partitioning of HCl between the vapor and liquid phases under appropriate conditions of temperature and relative humidity. If the HC1 was mostly in the liquid phase (droplets), a considerable reduction in the HC1 washout occurred relative to gaseous HCl absorption by the rain. This prediction was based on the recent work of Kerker and Hampl ( 2 ) ,which gives an empirical expression for the capture of small airborne droplets by falling raindrops. Since HC1 is a hygroscopic vapor, mixing with a moist air stream can form an aerosol (liquid droplets) under the appropriate conditions. [Formation of an aerosol is taken to mean the significant growth of droplet size (to -0.3 pm) from the initial size of the nucleating particles.] Formation of the aerosol is normally due t o the nucleation of a supersaturated mixture of a condensable vapor in air. The nuclei may have any origin whatsoever-minute crystals of a foreign material or molecular clusters of the condensable vapor, which have an excess of surface energy sufficient to produce the aggregate as a liquid phase. In the literature thus far, experimental work relating to the aerosol formation threshold of the HC1-water vapor system is limited. Rhein ( 3 ) ,however, did perform a chemical equilibrium analysis where the conditions were determined under which HC1 and water vapor in the Space Shuttle’s exhaust cloud should form an aerosol. The temperatures and relative humidities a t which the aerosol exists were predicted for 1160
Environmental Science & Technology
various weight ratios of air-to-exhaust. Twomey ( 4 ) performed experiments to carefully measure the conditions under which HC1 aerosol droplets grow in size. Gillespie and Johnstone ( 5 )made measurements of particle-size distributions with various hygroscopic aerosols, including HCl. Relative humidity of the air, addition of foreign nuclei, concentration of the aerosol particles, and time were studied in reference to the particle-size distribution. The experimental system used is somewhat similar to the one constructed in this work. Aerosol formation was noted to result for relative humidities greater than 78% with the presence of both sodium chloride and ammonium chloride (soluble in HC1) nuclei material. Measurement of the aerosol’s size distribution, though difficult, indicated a mean diameter (mass basis) of 5.5 pm for a loading of 1 g/m3. Since none of the above studies was concerned with the conditions of actual HCl aerosol formation (significant droplet growth), an experimental program was initiated. The relative humidity, HCl mass concentration, and addition of foreign nuclei were varied in the experiment performed here. The criterion for aerosol formation was visual observation of the particles by means of light scattering. Formation of an HC1 aerosol has importance on precipitation scavenging because larger droplets are scavenged less efficiently by falling raindrops. Experimental Apparatus
A mixing tube was constructed to generate the HC1 aerosol. A glass tube with an inside diameter of 25.4 mm was used. A porous brass plate was located near the point where the humid air was introduced to reduce nonuniformities in the flow field a t the point of the HCl gas injection. The length of the mixing tube was approximately 30 cm and of sufficient length for liquid aerosol formation. A schematic diagram of the apparatus is shown in Figure 1.All the experiments were conducted a t 25 “C-ambient temperature. Gaseous HCl was generated by bubbling clean, dry air through 19% hydrochloric acid. The Zeisberg (6) HC1-HpO vapor-liquid equilibrium tables give the amounts of water vapor and HCl gas added to the air flow. To check the average HC1 concentration, a sample from the mixing tube was passed through a bubbler containing distilled water. Analysis of the water indicated agreement with the calculated HC1 concentration to within 5% over the full range of flow rates. The flow rate varied from 0.1 to 2.3 lpm through the HCl acid bubbler. The HCl bubbler was situated adjacent to the mixing tube to
