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Dissolution of non-aqueous phase liquids in groundwater
Journal of Contaminant Hydrology, 8 (1991) 23-42 Elsevier Science Publishers B.V., Amsterdam
23
Dissolution of non-aqueous phase liquids in groundwater D o n a l d M a c k a y a'*, W a n Ying Shiu a, Aila Maijanen a and Stan Feenstra b
aDepartment of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ont. M5S 1A4, Canada Waterloo Centre .for Groundwater Research, University of Waterloo, Waterloo, Ont. N2L 3G8, Canada (Received October 30, 1989; revised and accepted January 31, 1991)
ABSTRACT Mackay, D., Shiu, W.Y., Maijanen, A. and Feenstra, S., 1991. Dissolution of non-aqueous phase liquids in groundwater. J. Contam. Hydrol., 8: 23-42. A theoretical analysis is presented describing the equilibria and kinetics of the dissolution of organic components present in a non-aqueous phase liquid (NAPL) contacted by groundwater. Expressions are derived for the concentration of these components in water as a function of NAPL composition as it changes with time. Experimental laboratory "generator column" data are presented in support of the theory employing synthetic hydrocarbon and chlorinated hydrocarbon mixtures and crude oils. It is suggested that the "generator column" technique is a valuable experimental approach for investigating the dissolution characteristics of NAPLS.
INTRODUCTION
A situation of environmental concern arises when a water-immiscible or non-aqueous phase liquid (NAPL) enters the subsurface environment, contacts and contaminates groundwater. Such situations arise as a result of spills from underground tanks and pipelines, from surface spills which infiltrate through the vadose zone to the saturated region, and from "insecm'e" dumps or landfills. The occurrence and general behavior of such NAPLS has been reviewed by Van Dam (1967), Dracos (1987), Ptacek et al. (1987), Schwille (1981) and Feenstra (1990). In this study, we present a theoretical treatment and experimental results concerning the equilibria and some kinetic aspects of NAPL dissolution in groundwater. A procedure is suggested for testing the dissolution characteristics of a specific NAPL.
* Corresponding author.
24
D. MACKAY ET AL.
NOTATION
C f JR F FM Fv Ki Kvi k Kp me M n~ N QM Qv RF S T TM v V wi x y z ? p
aqueous concentration, mol/m 3 fugacity reference fugacity of pure liquid component fugacity ratio mass flow rate, mol/h volume flow rate, m 3/h ratio of activity coefficient ratio of activity coefficient on volume fraction basis rate constant h partition coefficient mass of component i, kg molecular mass, kg/mol number of moles Enl, total number of moles mass ratio, number of moles of water to number of moles of NAPE volume ratio, ratio of volume of water to initial volume of YAPL retardation factor aqueous solubility, g/m 3 system temperature, K melting temperature, K volume of individual component, m 3 volume of NAPL, m 3 mass fraction mole fraction in NAPL mole fraction in aqueous phase volume fraction of solid activity coefficient density, kg/m 3
Subscripts i ith component N in NAPL phase A in aqueous phase w in water phase
The aim of this work is to contribute towards answering the following questions: (1) What will be the concentrations of dissolved organic chemicals in water which has contacted a NAPL of defined composition? (2) How will these aqueous phase and Nape phase concentrations change with time as a result of depletion of the chemicals by dissolution from the NAPL? (3) How much water must contact the NAPE for a defined fraction (e.g., 50%) of the mass of the NAPL or a component in it, to dissolve, assuming intimate water-NAPL contacting?
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
25
(4) What is a convenient experimental proc.edure for contacting a sample of NAPE with water at defined water/NAPE volume ratio conditions? The answers to these questions may be valuable in determining remedial measures, including justification for excavation or other remedial treatments, as distinct from leaving the spill to disperse and degrade naturally, i.e., the "do nothing" approach. THEORETICAL
Theoretical aspects of the general issue of dissolution of NAPES have been discussed previously by Billington et al. (1988) and Shiu et al. (1988). The following is an expanded and modified treatment of these discussions which has the objective of setting out the fundamental physical chemical relationships surrounding dissolution of NAPES in the hope that they may be more fully exploited in groundwater science. If a volume of NAPE of Vm 3 is present in soil and consists of several pure components, each volume vi m 3, density pi kg/m 3 (i.e. p/lOOOg/mL or 1000p g/m3), mass v~pi kg, and molecular mass M i kg/mol (i.e., molar volume is M~/p~m3/mol), then the number of moles present of each component ni is v~p~/M~, and the total number of moles present N is Eni. The NAPE composition expressed on a volume fraction basis is vi/V or a mole fraction basis (ni/N) designated xi, or on a mass fraction basis v~pi/Y_,v~p~.It may also be expressed on a mass/volume basis as vip~/V, e.g. 10kg/m 3, equivalent to 10 g/litre. If this liquid is contacted with water and equilibrium or saturation is achieved, a specific dissolved concentration of each component in the water phase will be achieved. This may be expressed as a mole fraction y~ or concentration YiPA/MA mol/m3 o r yipgMi/MA kg/m 3, where PA is the density of the water solution (kg/m 3), and MA is the molecular mass (kg/mol) of the solution such that (MA/PA) is the molar volume of the solution (m3/mol) (usually approximately 18 × l 0 - 6 m3/mol). Subscript A specifies that the quantities are in the aqueous phase. Often, the solution is sufficiently dilute that the density and molecular mass of the aqueous solution are approximately those of pure water. It is assumed here that no emulsified NAPE is present in the water, i.e., only a true solution exists. The primary task is to relate the NAPE phase concentrations to the aqueous concentrations. Physical chemical conventions dictate that the linkage be made between x~ and y~. In this linkage it is convenient to use the equilibrium concept of fugacity, which is related to chemical potential, using the property that the fugacity of component i will approach equal values in each phase. The problem is then to link fugacity to the concentrations x and y. Applying the fugacity equation
D MACKAYETAL.
26
for liquid-liquid equilibrium (Prausnitz, 1969) to component i gives: =
xiT,Nf
=
yi
i
f.
(1)
w h e r e f is fugacity, 7N and YAare the Raoult's law activity coefficients in the NAPL and aqueous phases, respectively (such that 7 becomes unity when mole fraction becomes unity), and fR is the reference fugacity of the pure liquid component at the temperature of the system. These activity coefficients express the degree of non-ideality experienced by the chemical in the system: yi
=
XiTiy/YiA
=
(2)
SincefR cancels the task becomes that of defining the activity coefficients. K i is the ratio of activity coefficients and is a ratio of mole fractions (water to NAPL). Its value is normally much less than 1.0, e.g. 10 -3 to 10 -6. A considerable literature exists on methods of measuring, correlating, and predicting activity coefficients, reviewed for example, by Walas (1985), Prausnitz (1969), Van Ness and Abbott (1982), Fredenslund et al. (1977), Hala et al. (1967) and Reid et al. (1977). In the organic phase, the simplest approach is to assume that )'iN is unity, which is equivalent to assuming ideality or Raoult's law to apply in the organic phase. This is a reasonable assumption if the species in question are chemically similar, for example, all are alkanes, or all are chlorobenzenes. Chemical dissimilarity generally (but not always) causes ~iN to exceed unity. If desired, data can be sought for the system in question from the vapor-liquid or liquid-vapor equilibrium literature, or predictive approaches such as UNIQUACor UN1FAC,can be used (Walas, 1985). Often, the error incurred by assuming ideality is less than the uncertainty of other parameters considered later. The activity coefficient, 7iA, in the aqueous phase must be estimated, since it may adopt values ranging from approximately 20 to many million and even higher. It proves to be the critical and most variable determinant of NnPLwater partitioning. As has been discussed by Mackay and Shiu (1981), it may be estimated from the solubility of the organic chemical in water by applying the fugacity equation (1) to the pure chemical i, in which case xi and 7iA are unity, thus y~ the mole fraction solubility in water becomes 1/'/iA . It is a fair assumption that the activity coefficient remains constant in the dilute concentration range from zero to saturation, provided that the saturation or solubility limit is less than a mole fraction of 1%. The reason for this is that the behavior of the dissolved chemical is dominated by solute-water interactions, with less significant solute-solute interactions being possible because of the high dilution. For higher-solubility chemicals, such as alcohols, it is necessary to estimate activity coefficients at near "infinite dilution" conditions, using extrapolated
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
27
data or correlations of the type developed by Deal and Derr (1968) and reviewed by Reid et al. (1977). Using the solubility in pure water to estimate 7iA implies that there are no influences of co-solvents, surfactants or electrolytes. Organic cosolvents, e.g., alcohols or ketones, usually increase the solubility (decrease 7) by making the water matrix more "organic". Fu and Luthy (1986a,b) have discussed this effect and methods of quantifying it. Electrolytes usually "salt out" organic solutes (increase 7), as discussed by Aquan-Yuen et al. (1979). These effects are particularly important in landfill leachates which may contain substantial concentrations of numerous polar and ionic species. If desired, 7 can be estimated experimentally by measuring the solubility of the solute in the leachate. It is presently difficult to estimate the effect of co-solvent mixtures on 7iA, although progress is being made in this area as reported by Rao et al. (1985) and Nkedi-Kizza et al. (1985, 1987, 1989). In summary, for sparingly soluble organic substances which form ideal NAt'L phase solutions the relationship between y~ and x~ can be established by assuming: ];iN I/7iA
~
(3)
1
= S~MA/PAM,
(4)
then YilXi
=
S, MAIpAMi
=
(5)
where Si is the solubility of component i in water (kg/m3). Note that if x~ is unity, the organic liquid is pure, then yi becomes the mole fraction solubility of the chemical in water, i.e., S~MA/PAMg. For example, a NAPL consisting of 1% by volume benzene in n-octane has properties as given in Table 1. Table 1 clearly shows that solubilities and activity coefficients in water vary by many orders of magnitude as do partition coefficients. The dissolved NAPS is mainly benzene while the NAPL is mainly octane. The dissolved concentrations can be assumed to be xiSi where xi is the mole fraction in the NAPL and S~ the pure liquid solubility. A situation frequently arises in which the identities or concentrations of some of the organic components in the NAPL are in doubt, thus the mole fractions can not be ascertained. Examples are crude oils, fuel oils, solvents, or solutions of one chemical or a class of chemicals of concern (such as PCB) in a complex mixture (such as mineral oil). It may be necessary to estimate properties in such cases, but if necessary a simple expedient is to assume that all organic chemicals present have equal molar volumes, thus mole fraction and volume fraction are equal. Alternatively, if the mass concentration (e.g., 5g/kg) is known, the mass and mole fractions may be equated. These assumptions should not be introduced unless it is necessary to do so, but it
28
D. M A C K A Y E T AL.
TABLE 1 Properties of a mixture of benzene and n-octane at 25°C Benzene
n-Octane
Total
Molecular mass, Mi, kg/mol Density, pi, kg/m 3 Molar volume, cm 3/mol Aqueous solubility, S,, kg/m 3 Aqueous solubility, Ci, mol/m 3 Aqueous solubility, mole fraction y~ Activity coeff, in water 7~A
0.078 879 88.8 1.78 22.8 4.12 x 10 4 2424
0.1257 700 179.6 0.000682 0.0054 9.72 x 10 8 10.2 × l 0 6
_
Volume, m ~ Mass, kg Mass fraction, wi Number of moles Mole fraction xi (in NAPL phase)
0.01 8.79 0.012 112.5 0.02 4.12 × 10 4 8.24 × 10 -.6 0.46 35.9 8790 4.1 × 10 3
0.99 693 0.988 5513 0.98 9.72 × 10 8 9.52 × 10 8 0.0053 0.66 693000 9.5 × 1 0 - 7
1.0 702 1.0 5625 1.0 _ 8.33 × 10 6 0.465 36.5
Ki or yi/xi Yi (water phase) C,A mol/m 3 (water) S~A g/m 3 (water) SiN g/m 3 (in NAPL phase) Kvt (water/NAPL)
-
o f t e n t r a n s p i r e s t h a t the e r r o r i n t r o d u c e d ( o f p o s s i b l y a f a c t o r o f three in c o n c e n t r a t i o n ) is less t h a n o t h e r u n c e r t a i n t i e s in the c a l c u l a t i o n . Since liquid m i x t u r e s are o f t e n c h a r a c t e r i z e d b y v o l u m e f r a c t i o n , it is usually c o n v e n i e n t to r e p l a c e the m o l e f r a c t i o n b y v o l u m e f r a c t i o n w h e n c a l c u l a t i n g the c o n c e n t r a t i o n in a q u e o u s solution. I f this is d o n e in T a b l e 1 a n e r r o r o f a f a c t o r o f t w o is i n t r o d u c e d b e c a u s e the m o l a r v o l u m e s o f b e n z e n e a n d o c t a n e differ b y this factor. F o r p r a c t i c a l p u r p o s e s , it is c o n v e n i e n t to define a s e c o n d v o l u m e t r i c p a r t i t i o n coefficient Kv, w h i c h is the r a t i o o f " a m o u n t p e r unit v o l u m e " c o n c e n t r a t i o n s in units o f g / m 3, k g / m 3 or m o l / m 3 or e v e n v o l u m e fractions. Since Cm the c o n c e n t r a t i o n in the NAPL is Xi/VN, w h e r e VN is the m e a n m o l a r v o l u m e o f NAPL ( m 3 / m o l ) a n d CiA the w a t e r p h a s e c o n c e n t r a t i o n is yi/VA it follows that:
Kvi
=
CiA/CiN
=
(yi/xi)(VN/'UA)
:
Ki(MN/pN)/(MA/PA )
=
Kz(MN/pN)/18 x l0 -6
(6)
w h e r e M y a n d PN a r e the m e a n m o l e c u l a r m a s s a n d density, respectively. G e n e r a l l y , the m o l a r v o l u m e o f the NAPL will be a f a c t o r o f a p p r o x i m a t e l y 10 g r e a t e r t h a n t h a t o f w a t e r thus Kvi will be g r e a t e r b y this f a c t o r t h a n K~.
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
29
This is illustrated in Table 1. A problem with Kv~ is that it depends on the composition of the NAPL, specifically on its mean molecular mass or volume, thus as the NAPL composition changes Kv~ changes. On the other hand, Ki is a function only of the chemical, and not of the NAPL, provided that the activity coefficient in the NAPL phase is unity. A complication arises when the pure organic chemical is solid at the temperature of interest but is in liquid solution in the NAPL. In such cases, the correct saturation solubility S~ to use in equation 4 is that of the subcooled liquid chemical. This solubility is larger than that of the solid at temperatures below the melting point by the multiple ( l / F ) where F is the fugacity ratio which can be estimated from: F =
exp[(6.79(1 -
(7)
TM/T)]
where T is the system temperature (K) and TM is the solid's melting point (K) (Prausnitz, 1969; Yalkowsky, 1979). For example, naphthalene with a solid solubility of 33 g/m 3 and a melting point of 80.2°C, has a fugacity ratio F of 0.28 at 25°C, thus the subcooled liquid solubility is 118 g/m 3. This is the solubility which naphthalene would have if it was liquid at 25°C and is the "reference solubility" which applies in equation 4. The dissolved concentration S can never exceed the solid solubility, because it is impossible to dissolve naphthalene in the organic phase to the extent the xe exceeds the fugacity ratio or 0.28 in this case. Attempts to form more concentrated solutions result in precipitation of the solid chemical. DISSOLUTION KINETICS If a volume of multicomponent NAPE is contacted with clean water of flowrate F Vm3/h and equilibrium is reached, then the depletion of each component from the NAPE can be expressed as: dn~/dt
=
(8)
--F,C~
But n i is x i N and Ci is yiPAMA mol/m 3 and yi is K~x~ thus: d(xiN)/dt
=
-- F ~ K i X i P A / M A
--
FMKiX i
(9)
where Fv P A / M A is the flowrate of water in units of mol/h and is designated FM. In practical situations, equilibrium is rarely achieved, but this concept can still be used and applied to a real groundwater flow F R by applying an "efficiency of approach to equilibrium" E of say 10%. Fv is then E F R . Rigorous solution of this differential equation can only be done numerically because N varies as each component dissolves. If the solute of interest is one of the more soluble components and is present in low concentration, N
30
D. M A C K A Y ET AL.
can be assumed to be constant and the equation reduces to: dxi/dt
=
- ( K f M/N)x i
(10)
which can be readily solved from an initial condition xio to give: )(,
=
Xio
exp(--KiFMt/N)
(11)
The depletion rate constant is thus KiFM/N (h -I) and the characteristic time for x to be reduced to 37% (i.e., e x p ( - 1 ) ) of its initial value is N/KiFM. Alternatively, the quantity of water which must be saturated to achieve this extent of dissolution is FM t or N/Ki moles. This can be used to express the different depletion rates of NAPL components of different water solubilities. A sparingly soluble component of very low K,. (say 10 -6) will require 100 times more water to achieve a reduction to 37% than a more soluble component of K/equal to 10 -4. This characteristic time is useful because it gives an immediate indication of the time of depletion. For example, a 100-m 3 volume of sandy soil may contain 10 L/m 3, i.e. 1 m 3 of a petroleum NAPL with a mean molecular mass of 100 g/tool and a density of 700 kg/m 3. There is thus 700 kg or 7000 mol of product. If it contains 0.8% benzene by mass this corresponds to 5.6kg or 72 mol (molecular mass 78 g/mol) or approximately 1 mol %. The hydraulic conductivity and gradient could indicate a flow of 18 L/hour or 1000 mol/hour of water which will be in intimate contact with the NAPL. The characteristic time to dissolve 63% of the benzene will then be N/K~FM or 16 800 hours (700 days) since K~ is 1/2400. During this time, the concentration of benzene in the water will drop from an initial mole fraction of 4.2 x 10 -6 (concentration 18g/m 3) to 1.54 x 10 -6 (6.6g/m3). Clearly, the dominant quantity is K~ the activity coefficient ratio. Another convenient form of this equation is: x~ = Xio exp(--QMK~)
(12)
where QM is F MtiM and is the ratio of moles of water to the number of moles of NAPL. This suggests the use of a modified version which may be more convenient for practical calculations, i.e.; xi = Xio e x p ( - Q v K v i )
(13)
where Qv is the ratio of volume of water to initial volume of NAPL and gvi is the volumetric partition coefficient defined such that QMK~ equals Qv Kv~. K~ and Kv~are related through the ratio of molar volumes as discussed earlier. A more accurate estimation of the extent of dissolution may be obtained using Qv if the prevailing NAPL volume is used instead of the initial value. It may be useful to express these equations in terms of times and rate constants. Equations 11 and 12 show that the rate constant k is either
31
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
KeFM/N or KviFv/V where Fv is the volumetric flow rate of water and V is the YAPL volume. It should be noted that Kvi changes as the NAPL composition changes. The most serious error in using the simple first order expression occurs when the solute is intermediate in solubility within the group of components present in the mixture. This is best illustrated by an example in which an equimolar YAPL mixture of benzene, toluene, and p-xylene is dissolved in a flow of water under the conditions specified in Table 2. The results of a rigorous numerical simulation, which are given in Figure 1, show that the benzene concentration in the water decreases monotonically, but the toluene concentration rises to a m a x i m u m then falls. This is due to the initial decrease in toluene mole fraction as the benzene is depleted. We suggest a simple expedient for obtaining an approximate analytical solution in such cases. The concentrations are designated as being of the solute of interest x2, the more soluble components x, and the less soluble components x3, thus the sum of x,, x2, and x 3 is 1.0. At times when there is appreciable loss of x,, but relatively little loss of x2, the mole fraction of x 2 will have risen to approximately x2/(x2 + x3). This occurs when QvKvi or QMKi are approximately 0.1 or equivalently Qv is Kvi/lO or QM is Ki/IO. We thus assume that xe rises from Xio to X~o/(Xgo+ x3) during this period. There will be loss of only 5% of the solute i during this period. It is suggested that w o t e r / o i l volume ratio, q 200 I
600
400 I
600 '
800 I
1000 I
| 200 I
1400 I
1600 I
1800 i
2000 I
500 .J
400 E ~,~ 300 E u
c o
p-xylene
200
100
0
I 0
I 20000
I
I 40000
I
I 60000
time, hours
Fig. 1. Computed changing concentrations of hydrocarbons in water exposed to an equimolar mixture of benzene, toluene and p-xylene as a function of water to oil volume ratio or time.
32
D. MACKAY ET AL.
TABLE 2
Composition and aqueous solubility of equimolar mixture at 25°C used to obtain Figure 1
Molecular mass, M, kg/mol Solubility in water, S, kg/m 3 Density, kg/m 3 no. of moles, n mass, kg volume, m 3 mole fraction x0, in N A P L K,
mole fraction, yi0, in water Si0, initial concentration in water, kg/m 3 K,,
water flow Qv at 90% depletion =
2.3/Kv~
benzene
toluene
p-xylene
0.078 1.780 879 1000 78 0.0887 0.333 4.12 x 10 4 137 × 10 -6 0.593
0.092 0.515 867 1000 92 0.1061 0.333 1.0 × 10 4 33.3 × 10 -6 0.172
0.106 0.185 861 1000 106 0.1231 0.333 0.31 x 10 4 10.5 × 10 -6 0.062
2.43 x 10 .3 0.01 m3/h 946
5.93 × 1 0 - 4
1.85 x 10 4
3900
12600
total
3000 276 0.3179
0.827
x2 will change with time approximately as: x2 = =
n2/(n, + n2 + n3) n20 exp ( - k2 t)/[n~o exp ( - k 1t) + n20 exp ( - k 2t) + n30 exp ( - k 3t)] (14)
The rate constants kl, k2, and k3 are arbitrarily selected to give a larger rate constant to the more soluble material. If information is available on the solubility o f this material, an average number can be used. Otherwise, k~ can be set at possibly five times k2. The third term k3 could be set at zero. The equation causes Xz to rise from x20 to approximately x20/(x20 + x30) then to fall in a first-order fashion. It is suggested that these equations may be used to estimate the changing concentrations in the NAPI~ and thus the corresponding change in equilibrium water concentrations. The question arises of the extent of approach to equilibrium. Since contact times are long (hours or days), it is likely that water which is in intimate contact with the NAPe, becomes saturated, but much o f the water flow may "by pass" the NApL, is not saturated, but may later mix, or exchange diffusively or dispersively with the saturated water. Samples of water taken downstream may therefore be subsaturated. A convenient method of quantifying this dilution is to assume two water flows to be present, one which is saturated and one which is unaffected by the NAPE, but these are later mixed by dispersion and diffusion, resulting in a more dilute solution. It can be
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
33
shown that this approach is mathematically equivalent to introducing a transfer resistance between NAPL and the total water flow. This undersaturation or dilution effect should not influence the proportions of the components present in the groundwater, only the absolute concentration is affected. Subsequent sorptive retardation may, of course, change the relative concentrations of specific components. Having established this theoretical background, we test its validity by conducting small-scale laboratory experiments on dissolution of two NAPLS, a crude oil, and a chlorobenzene mixture. It transpires that the method used may be valuable as a protocol for experimental investigation of the dissolution characteristics of a NAPL sample.
EXPERIMENTAL
Four series of experiments were undertaken using a miniature system to simulate the contact between a stationary volume of NAPL and a flow of water. The technique used was the dynamic coupled column, or generated column technique described by May et al. (1978a, b). The procedures have also been described by Billington et al. (1988) and Shiu et al. (1988). The NAVLS used were: (1) a synthetic hydrocarbon mixture with the composition shown in Table 3; (2) two crude oils; (3) a diesel fuel oil, and (4) a synthetic chlorobenzene mixture with the composition shown in Table 4. In all cases a known volume of the NAVI~ was exposed to known flow of water and the water analysed periodically to determine the concentrations of dissolved organic chemicals. Only in the case of the synthetic mixtures was it possible to predict the changing water composition theoretically. The general aims were to confirm the validity of the technique and confirm that the aqueous compositions agree with the theoretical predictions, both in terms of absolute concentrations and the dependence of the changing concentrations on the volume of water contacted with the NAVL. Chemicals 99 mol % benzene, toluene, and certified grade naphthalene were obtained from Fisher Scientific Biphenyl and fluorescent-grade phenanthrene were obtained from Eastman Kodak Co. 99% pure chlorobenzene, 99% 1,2,4trichlorobenzene, 99% 1,2,3,5-tetrachlorobenzene, and 98% pentachlorobenzene were obtained from Aldrich Chemical Co. 98.5% p-xylene and 99.5% hexachlorobenzene were obtained from BDH Chemicals Canada Ltd. riVLC grade methanol was obtained from Caledon Laboratories, Ontario. Mill-Q ultrapure deionized water was used for all experiments. Chromosorb 750
3
* E s t i m a t e d from m o l a r v o l u m e o f N A P L .
a q u e o u s c o n c e n t r a t i o n , Kvi*
initial a q u e o u s conc., So, g / m J subcooled liquid solubility, SOL, g/m 3
3.8 × 10 -3
1.67 × 10 -5 4.10 x 10 4 72.44 72.44
Ki
mole fraction in water, y,
benzene
9.4 x 10 4
1.67 × 10 4
1.25 × 10 6 1.80 x I0 -4 12.83 12.83
106.2 4.22 0.0397 0.0693 185 1.74
p-xylene
5.74 × 10 5
4.09 × 10 8 6.24 × 10 -6 1.79 6.83
128.2 4.14 0.0323 0.0402 31.7 0.944
naphthalene
of the synthetic oil at 25°C
5.14 x 10 6 1.01 x 10 -4 26.27 26.27
92.13 2.69 0.0292 0.051 515 5.59
toluene
solubility of components
78.11 1.82 0.0233 0.0407 1780 22.8
and aqueous
Mol. wt., M, g / m o l % mass no. o f moles, n mole fraction in oil, x i a q u e o u s solubility, S, g / m 3 subcooled liquid solubility, CL, mol/m 3
Composition
TABLE
8.05 × 10 6
4.85 × 10 9 8.73 × 10 7 0.35 0.996
154.2 4.14 0.0268 0.0468 7.48 0.138
biphenyl
l.ll
× 10 6
4.35 × 10 -~° 1.20 x 10 7 0.048 0.267
178.2 4.14 0.0232 0.0405 1.18 0.037
phenanthrene
5.78 × I0 9
6.27 x 10-l0 0.0048 0.0048
198.4 78.94 0.398 0.695 0.00696 0.000037
tetradecane
119.6
0.5725 1.0
total
0.136 0.265 8.34 x 10 -8 1.77 x 10 6 1.20 x 10 5
18.84 18.84 1.87 x 10 -6 4.57 x 10 6 3.11 x 10 5
234.8 234.8 3.75 x 10 -5 7.47 x 10 -4 5.11 X 1 0 3
215.9 54.5 0.0675 0.0046 0.047 2.89 0.0262
1,2,3,5-
181.5 16.96 0.49 0.0401 0.409 46.1 0.068
1,2,4-
112.6 45.6 0.37 0.0492 0.502 470 4.17
* Estimated from average molecular weight a n d density.
Kvi*
Mol. wt mp, °C % mass no. o f mols, n mole fraction, xi0 aqueous solubility, S s, g/m 3 subcooled liquid aqueous solubility CL, mol/m 3 initial aqueous c o n c e n t r a t i o n So, g/m 3 subcooled liquid initial a q u e o u s concentration SOL, g/m 3 mole fraction, Y~0 K~
Chlorobenzene
C o m p o s i t i o n and aqueous solubility o f the c h l o r o b e n z e n e mixture at 25°C
TABLE 4
1.48 x 10 9 5.93 x 10 8 4.03 x 10 7
0.0206 0.10
250.3 86 0.034 0.00244 0.025 0.83 0.0162
penta-
5.69 × 10 12 3.16 x 10 -I° 2.15 × 10 9
0.00009 0.0087
284.8 230 0.034 0.00176 0.018 0.005 0.0017
hexa-
253.8 254.0
0.0981 1.0
Total
>
7 (7
O
_.u ,O
), ,0
,z
©
z
5
,q
36
D. MACKAY ET AL.
(60/80 mesh) and Chromosorb W (30/60 mesh) were purchased from John-Mansville. N o r m a n Wells crude oil and an Alberta crude oil were obtained from Esso Canada. The diesel fuel oil was obtained from BP Canada.
Methods (1) Purge-and-trap gas chromatograph analysis The volatile hydrocarbons in the water samples were determined by purgeand-trap GC. A small generator column was prepared by packing a 26-cm long, 6-mm OD glass tube with Chromasorb 750 coated with approximately 30% by weight of chemical mixture (NAPL). Only 30% of the Chromosorb was soaked with oil to prevent overflowing. The column was plugged with glass wool at both ends and with a 100 mesh stainless steel screen in the outlet. The column was enclosed in a water jacket which was temperature controlled to within +0.1°C by a Neslab Exacal Ex-100 temperature bath. Water was pumped through the column with a Beckman metering p u m p and was collected and stored in the refrigerator for later analysis by GC or LC. A Hewlett-Packard GC model 5840, equipped with a flame ionization detector, and a HP 7675A purge and trap sampler was used. The water sample was purged with N 2 and passed through a Tenax-GC trap. By thermodesorption the sorbed chemicals were swept directly onto the analytical column by the carrier gas. The analytical column was a 0.53-mm OD, 30-m long, J & W DB-1 mega bore fused silica capillary column. The GC oven was initially set at 50°C, held for 10 min, was then temperature programmed to 200°C at a rate of 5°C/min. The oven was held at 200°C for 20 min. The peak areas were integrated with a HP 5840A GC terminal.
(2) Generator column-HPLC analysis The generator column was a 0.64-cm OD, 30-cm long stainless steel tubing packed with prewashed chromasorb W (30/60 mesh) coated with 0.3mL chlorobenzene mixture. The column was thermostated in an Alltech water jacket, and the temperature was controlled to 25 ° _ 0.02°C by a Neslab Endocal RTE-S temperature bath. An Analabs Model B-100-S high-pressure metering p u m p was used to deliver water through the generator column. The water solution was then passed through an extractor column (0.64-cm OD, 6 c m long) stainless steel tubing packed with 30-50 #Bondapak/Corasil (Water Associates). By switching an eight-port Valco valve, the chemicals were extracted by the mobile phase (methanol:water 85:15 by volume in isocratic mode) and directly injected onto the analytical column. The instrument was a Waters' high-performance liquid chromatography (HPLC) system, consisting of a Model 6000 solvent delivery system, a Model M45
37
D I S S O L U T I O N O F N O N - A Q U E O U S P H A S E L I Q U I D S 1N G R O U N D W A T E R
solvent delivery system, a Model 720 system flow controller, a Model 440UV absorbance detector with 254- and 280-nm kits. The analytical column was a Waters' 3.9-mm OD, 300-mm long/~Bondapak Ct8 column. Integration of the peak areas was recorded by a Waters Model 730 data module. RESULTS A N D DISCUSSION
Figures 2 to 6 show the time courses of water concentration of specific chemicals as the NAPLS became depleted. Also shown on Figures 2 and 6 are the theoretically predicted and experimental concentrations of selected chemicals. Figure 2 shows that when the synthetic oil is contacted with water the benzene concentration in water falls rapidly to about 1 mg/L when Q is about 500. The less soluble toluene dissolves slightly more slowly and p-xylene more slowly still. Biphenyl, which is not shown, remained constant until a volume ratio about 12 000 then started to fall and phenanthrene, the least soluble, was unchanged even at Q of 24 000. A factor of ten depletion is expected theoretically when QvKvi is 2.3 (equation 13). The values of Ki and Kvi are given in Table 2, based on the solubilities and estimates of the oil mean molecular mass (92 g/mol) thus the Qv corresponding to this factor of ten depletion is predicted as 2.3/Kv. The values are approximately benzene 1000, toluene 4000 and xylene 13 000 which are in good agreement with the data shown in Figure 2. go
experimental data:
80
+ Benzene 6 Toluene
70
9 Ethylbenzene &x'ylenes (~
6O
C 0
5O
~
40
E
benzene
r~c o
30 A
20
0
~
, 1000
A
,
J., 2000
.~ 30~00
~
4000
water/oil volume ratio, q Fig. 2. Water concentrations in contact with the synthetic oil showing rapid depletion of the more soluble hydrocarbons.
38
D. M A C K A Y ET AL. 40
• Total
35
+ Benzene o Toluene
_..1
30
E
25
& Ethylbenzene & xylanea x Naphthalene
•
d 0
20 C Q) 0 c 0 0
15
+ ++
10 o
oO
5
o
o
o
A
o
+
&
0
~
&
i
0
&
I
i
200
i
i
400
i 800
i
600
X 1000
i
water/oil volume ratio, Q Fig. 3. Dissolution characteristics of Norman Wells crude oil.
A useful "rule of thumb" thus emerges from the theoretical treatment earlier, and is confirmed by experiment that for 90% loss a Qv of approximately 2.3/K v is needed. Figures 3 and 4 give similar data for two actual crude oils showing rapid depletion of benzene and toluene in the range of Q values to 400. As expected, 28 • Total 26 + Benzene 24
o Toluene 22
_J
& Ethylbenzene+xylenes
20 O~
E
x Naphthalene
la
E 16
0 k~
14-
-~
12-
0 C 0 0
10-
8
¢>
4~ o
o
6 4 £A
2 0
L~
A
+
÷
,- ~ -'~-i-'~0
"~'1 400
o &
I x
&
I 800
I ~
I0 12 0
1
I I 1600
!
water/oil volume ratio, Q Fig. 4. Dissolution characteristics of
Alberta
crude
oil.
I X
2000
I
|
2400
I
2800
39
D I S S O L U T I O N O F N O N - A Q U E O U S P H A S E L I Q U I D S IN G R O U N D W A T E R
• Total + Benzene o Toluene
3.5
& Xylenea x Naphthalene
3 ET=
E
2.5
0
P C Q) 0 c
2
•
1.5
0 0
I A/~
A
& &
& &
0.5
oo 2 o
& o
&
.+
o
~x~xx
x
i
x°
i~ 1000
0
water/oil
i
volume
xl 2000
3000
ratio, Q
Fig. 5. Dissolution characteristics of a diesel fuel oil.
the naphthalene concentration is relatively low and constant in this range. Figure 5 gives the results for a diesel fuel oil again showing rapid loss of the small quantities of benzene and toluene. Figure 6 presents data for the mixture of chlorobenzenes (CBs), these compounds being of particular concern as subsurface contaminants and 240
+
220
~. -p
200 ._J "~
+ chlorobenzene +
& 1,2,4-TCB
180
v 1,2,5.5-TeCB
160
E .
140
:~
120
c
100
c o u
80
g P
60
4o 20 0 0
10000
20000
30000
40000
woter/CB mixture volume ratio, Q Fig. 6. Dissolution characteristics of the synthetic chlorobenzene mixture.
40
D. M A C K A Y ET AL.
present in dumps such as the Love Canal site in Niagara Falls, NY. There is obvious rapid depletion of the more soluble mono-CB with 90% loss by Q of 7000. The corresponding value for tri-CB is about 25 000. The drop in tri-CB concentration is quite sudden, possibly as a result of onset of final depletion of the solute from the column. The other, less soluble CBs were not appreciably changed at the end of the experiment. The near logarithmic linear drop in mono-CB concentration is consistent with first-order kinetics. It is important to emphasize that the theory presented here assumes the YAPL and aqueous phases to be homogeneous. Even in the laboratory-scale generator column there is likely to be depletion of the more soluble components from the "upstream" end of the NAPL. Exact correspondence with theory is thus not expected. In an actual spill site a similar, but probably more pronounced, effect is likely to occur, with concentration changes developing in the YAPL phase. There may be a "chromatographic" effect involving differing net velocities of each component as they exchange between the water phase and the NAPL, as well as the mineral and organic matter present in the soil. To simulate these phenomena requires a more complex spatially resolved model of partitioning and transport. CONCLUSIONS The theoretical analysis and the experimental results are consistent, suggesting that the dissolution kinetics and equilibria of sparingly soluble chemicals in NAPES behave according to well established physical-chemical principles and are thus basically predictable. Returning to the questions posed in the Introduction, the concentration of chemicals present in NAPE and dissolved in water can be estimated using (1) a partition coefficient Ki or K~i obtained from solubility data and (2) information on the chemical concentration in the NAPE. The thermodynamics of systems have been reviewed, the principal uncertainties being the "co-solvent" effects of other soluble compounds in the leachate. Second, if an estimate is available for the volume of NAPE and the volumetric flowrate of water which contacts it, the quantity of water (or time) necessary to deplete the NAPE of a specified fraction of a chemical can be estimated using the "rule of thumb", namely: Fraction remaining =
exp ( -
Ovgvi)
where Qv is the ratio of water to NAPE volume and Kvi is the volumetric partition coefficient. Third, the time necessary to accomplish a defined extent of dissolution may be estimated if the water flowrate is known and intimate NAPE-Water contact
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
41
is assumed. A m e t h o d of treating chemicals intermediate in solubility has been suggested. An implication of this work is that investigations of leachate concentrations at various distances from a source can in principle be used to estimate the nature of the source, provided that information is also available on retardation factors. It is noteworthy that the more soluble species will have the smallest retardation factors. For example, the presence of a " h a l o " of benzene at relatively high concentrations some distance from the source may indicate a source which has become depleted of benzene. F r o m a knowledge of site specific flowrates, retardation factors and concentrations it should be possible to piece together a consistent picture of the likely general behaviour of a series o f contaminants present in and leaching from a particular source. It is hoped that this study will be of value to contaminant hydrogeologists by exposing some o f the physical-chemical principles which underlie the dissolution of NAPE components and helping to elucidate the behavior of, and remedial actions for, these complex systems. ACKNOWLEDGEMENT The authors are indebted to NSERC, PACE and the Association of American Railroads for financial support. REFERENCES Aquan-Yuen, M., Mackay, D. and Shiu, W.Y., 1979. Solubility of hexane, phenanthrene, chlorobenzene and p-dichlorobenzenein aqueous solutions. J. Chem. Eng. Data, 24: 30-34. Billington, J.W., Huang, G.L., Szeto, F., Shiu, W.Y. and Mackay D., 1988. Preparation of aqueous solutions of sparingly soluble organic substances, I. Single component systems. Environ. Toxicol. Chem., 7:117-124. Deal, C.H. and Derr, E.L., 1968. Group contributions in mixtures. Ind. Eng. Chem., 60: 29-38. Dracos, T.H., 1987. Immiscibletransport of hydrocarbons infiltrating in unconfined aquifers. In: J.H. Vandermeulen and S.R. Hrudey (Editors), Oil in Freshwater: Chemistry, Biology, Countermeasure Technology. Pergamon Press, Oxford-New York, pp. 161-175. Feenstra, S., 1990. Evaluation of multi-component DNAPL sources by monitoring of dissolved phase concentrations. Paper presented at the Conference on Subsurface Contamination by Immiscible Fluids. Int. Assoc. of Hydrologists, Calgary, Alberta, April 16-20, 1990. Fredenslund, A., Gmehling, J. and Rasmussen, P., 1977. Vapor Liquid Equilibria Using UNIFAC. Elsevier, Amsterdam. Fu, J.-K. and Luthy, R.G., 1986a. Aromatic compound solubility in solvent/water mixtures. J. Environ. Eng., 112: 328-345. Fu, J.-K. and Luthy, R.C., 1986b. Effect of organic solvent on sorption of aromatic solutes onto soils. J. Environ. Eng., 112: 346-366. Hala, E., Pick, J., Fried, V. and Vilim, O., 1967. Vapor-Liquid Equilibrium. Pergamon Press, Oxford. Mackay, D. and Shiu, W.Y., 1981. A critical review of Henry's law constants for chemicals of environmental interest. J. Phys. Chem. Ref. Data, 10:1175-1199.
42
D. MACKAY ET AL.
Mackay, D.M., Roberts, P.V. and Cherry, J.A., 1985. Transport of organic contaminants in groundwater. Environ. Sci. Technol., 19: 384-392. May, W.E., Wasik, S.P. and Freeman, D.H., 1978a. Determination of the aqueous solubility of polynuclear aromatic hydrocarbons by a coupled column liquid chromatographic technique. Anal. Chem., 50: 175-179. May, W.E., Wasik, S.P. and Freeman, D.H., 1978b. Determination of the solubility behavior of some polycyclic aromatic hydrocarbons in water. Anal. Chem., 50: 997-1000. Nkedi-Kizza, P., Rao, P.S.C. and Hornsby, A.G., 1985. Influence of organic cosolvents on sorption of hydrophobic organic chemicals by soils. Environ. Sci. Technol., 19: 975-979. Nkedi-Kizza, P., Rao, P.S.C. and Hornsby, G., 1987. Influence of organic cosolvent in leaching of hydrophobic organic chemicals through soils. Environ. Sci. Technol., 21:1107-1111. Nkedi-Kizza, P., Brusseau, M.L., Rao, P.S.C. and Hornsby, A.G., 1989. Nonequilibrium sorption during displacement of hydrophobic organic chemicals and calcium-45 through soil columns with aqueous and mixed solvents. Environ. Sci. Technol., 23: 814-820. Prausnitz, J.M., 1969. Molecular Thermodynamics of Fluid-Phase Equilibria. Prentice-Hall, Englewood Cliffs, NJ. Ptacek, C., Cherry, J.A., Gillham, R.W. 1987. Mobility of dissolved petroleum-derived hydrocarbons in sand aquifers. In: J.H. Vandermeulen and S.E. Hrudey (Editors), Oil in Freshwater: Chemistry, Biology, Countermeasure Technology. Pergamon Press, OxfordNew York, pp. 195-216. Rao, P.S.C., Hornsby, A.G., Kilcrease, D.P. and Nkedi-Kizza, P., 1985. Sorption and transport of hydrophobic organic chemicals in aqueous and mixed solvent systems: model development and preliminary evaluation. J. Environ. Qual., 14: 376-383. Reid, R.C., Prauznitz, J.M. and Sherwood, T.K., 1977. The Properties of Gases and Liquids, 3rd ed. McGraw-Hill, New York, NY. Schwille, F., 1981. Groundwater pollution in porous media by fluids immiscible with water. Sci. Total. Environ., 21: 172-185. Shiu, W.Y., Maijanen, A., Ng, A.L.Y. and Mackay, D., 1988. Preparation of aqueous solutions of sparingly soluble organic substances, II. Multicomponent systems - - hydrocarbon mixtures and petroleum products. Environ. Toxicol. Chem., 7: 125-137. Van Dam, J., 1967. The migration of hydrocarbons in water-bearing stratum. In: P. Hepple (Editors) Proc. Syrup. on Joint Problems of the Oil and Water Industries. Elsevier, Amsterdam, pp. 55-88. Van Ness, H.C. and Abbott, M.M., 1982. Classical Thermodynamics of Non-Electrolyte Solutions with Application to Phase Equilibria. McGraw-Hill, New York, NY. Walas, S.M., 1985. Phase Equilibrium in Chemical Engineering. Butterworth Publishers, Boston, MA. Yalkowsky, S.H., 1979. Estimation of entropies of fusion of organic compounds. Ind. Eng. Fundam., 18: 108-111.
23
Dissolution of non-aqueous phase liquids in groundwater D o n a l d M a c k a y a'*, W a n Ying Shiu a, Aila Maijanen a and Stan Feenstra b
aDepartment of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ont. M5S 1A4, Canada Waterloo Centre .for Groundwater Research, University of Waterloo, Waterloo, Ont. N2L 3G8, Canada (Received October 30, 1989; revised and accepted January 31, 1991)
ABSTRACT Mackay, D., Shiu, W.Y., Maijanen, A. and Feenstra, S., 1991. Dissolution of non-aqueous phase liquids in groundwater. J. Contam. Hydrol., 8: 23-42. A theoretical analysis is presented describing the equilibria and kinetics of the dissolution of organic components present in a non-aqueous phase liquid (NAPL) contacted by groundwater. Expressions are derived for the concentration of these components in water as a function of NAPL composition as it changes with time. Experimental laboratory "generator column" data are presented in support of the theory employing synthetic hydrocarbon and chlorinated hydrocarbon mixtures and crude oils. It is suggested that the "generator column" technique is a valuable experimental approach for investigating the dissolution characteristics of NAPLS.
INTRODUCTION
A situation of environmental concern arises when a water-immiscible or non-aqueous phase liquid (NAPL) enters the subsurface environment, contacts and contaminates groundwater. Such situations arise as a result of spills from underground tanks and pipelines, from surface spills which infiltrate through the vadose zone to the saturated region, and from "insecm'e" dumps or landfills. The occurrence and general behavior of such NAPLS has been reviewed by Van Dam (1967), Dracos (1987), Ptacek et al. (1987), Schwille (1981) and Feenstra (1990). In this study, we present a theoretical treatment and experimental results concerning the equilibria and some kinetic aspects of NAPL dissolution in groundwater. A procedure is suggested for testing the dissolution characteristics of a specific NAPL.
* Corresponding author.
24
D. MACKAY ET AL.
NOTATION
C f JR F FM Fv Ki Kvi k Kp me M n~ N QM Qv RF S T TM v V wi x y z ? p
aqueous concentration, mol/m 3 fugacity reference fugacity of pure liquid component fugacity ratio mass flow rate, mol/h volume flow rate, m 3/h ratio of activity coefficient ratio of activity coefficient on volume fraction basis rate constant h partition coefficient mass of component i, kg molecular mass, kg/mol number of moles Enl, total number of moles mass ratio, number of moles of water to number of moles of NAPE volume ratio, ratio of volume of water to initial volume of YAPL retardation factor aqueous solubility, g/m 3 system temperature, K melting temperature, K volume of individual component, m 3 volume of NAPL, m 3 mass fraction mole fraction in NAPL mole fraction in aqueous phase volume fraction of solid activity coefficient density, kg/m 3
Subscripts i ith component N in NAPL phase A in aqueous phase w in water phase
The aim of this work is to contribute towards answering the following questions: (1) What will be the concentrations of dissolved organic chemicals in water which has contacted a NAPL of defined composition? (2) How will these aqueous phase and Nape phase concentrations change with time as a result of depletion of the chemicals by dissolution from the NAPL? (3) How much water must contact the NAPE for a defined fraction (e.g., 50%) of the mass of the NAPL or a component in it, to dissolve, assuming intimate water-NAPL contacting?
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
25
(4) What is a convenient experimental proc.edure for contacting a sample of NAPE with water at defined water/NAPE volume ratio conditions? The answers to these questions may be valuable in determining remedial measures, including justification for excavation or other remedial treatments, as distinct from leaving the spill to disperse and degrade naturally, i.e., the "do nothing" approach. THEORETICAL
Theoretical aspects of the general issue of dissolution of NAPES have been discussed previously by Billington et al. (1988) and Shiu et al. (1988). The following is an expanded and modified treatment of these discussions which has the objective of setting out the fundamental physical chemical relationships surrounding dissolution of NAPES in the hope that they may be more fully exploited in groundwater science. If a volume of NAPE of Vm 3 is present in soil and consists of several pure components, each volume vi m 3, density pi kg/m 3 (i.e. p/lOOOg/mL or 1000p g/m3), mass v~pi kg, and molecular mass M i kg/mol (i.e., molar volume is M~/p~m3/mol), then the number of moles present of each component ni is v~p~/M~, and the total number of moles present N is Eni. The NAPE composition expressed on a volume fraction basis is vi/V or a mole fraction basis (ni/N) designated xi, or on a mass fraction basis v~pi/Y_,v~p~.It may also be expressed on a mass/volume basis as vip~/V, e.g. 10kg/m 3, equivalent to 10 g/litre. If this liquid is contacted with water and equilibrium or saturation is achieved, a specific dissolved concentration of each component in the water phase will be achieved. This may be expressed as a mole fraction y~ or concentration YiPA/MA mol/m3 o r yipgMi/MA kg/m 3, where PA is the density of the water solution (kg/m 3), and MA is the molecular mass (kg/mol) of the solution such that (MA/PA) is the molar volume of the solution (m3/mol) (usually approximately 18 × l 0 - 6 m3/mol). Subscript A specifies that the quantities are in the aqueous phase. Often, the solution is sufficiently dilute that the density and molecular mass of the aqueous solution are approximately those of pure water. It is assumed here that no emulsified NAPE is present in the water, i.e., only a true solution exists. The primary task is to relate the NAPE phase concentrations to the aqueous concentrations. Physical chemical conventions dictate that the linkage be made between x~ and y~. In this linkage it is convenient to use the equilibrium concept of fugacity, which is related to chemical potential, using the property that the fugacity of component i will approach equal values in each phase. The problem is then to link fugacity to the concentrations x and y. Applying the fugacity equation
D MACKAYETAL.
26
for liquid-liquid equilibrium (Prausnitz, 1969) to component i gives: =
xiT,Nf
=
yi
i
f.
(1)
w h e r e f is fugacity, 7N and YAare the Raoult's law activity coefficients in the NAPL and aqueous phases, respectively (such that 7 becomes unity when mole fraction becomes unity), and fR is the reference fugacity of the pure liquid component at the temperature of the system. These activity coefficients express the degree of non-ideality experienced by the chemical in the system: yi
=
XiTiy/YiA
=
(2)
SincefR cancels the task becomes that of defining the activity coefficients. K i is the ratio of activity coefficients and is a ratio of mole fractions (water to NAPL). Its value is normally much less than 1.0, e.g. 10 -3 to 10 -6. A considerable literature exists on methods of measuring, correlating, and predicting activity coefficients, reviewed for example, by Walas (1985), Prausnitz (1969), Van Ness and Abbott (1982), Fredenslund et al. (1977), Hala et al. (1967) and Reid et al. (1977). In the organic phase, the simplest approach is to assume that )'iN is unity, which is equivalent to assuming ideality or Raoult's law to apply in the organic phase. This is a reasonable assumption if the species in question are chemically similar, for example, all are alkanes, or all are chlorobenzenes. Chemical dissimilarity generally (but not always) causes ~iN to exceed unity. If desired, data can be sought for the system in question from the vapor-liquid or liquid-vapor equilibrium literature, or predictive approaches such as UNIQUACor UN1FAC,can be used (Walas, 1985). Often, the error incurred by assuming ideality is less than the uncertainty of other parameters considered later. The activity coefficient, 7iA, in the aqueous phase must be estimated, since it may adopt values ranging from approximately 20 to many million and even higher. It proves to be the critical and most variable determinant of NnPLwater partitioning. As has been discussed by Mackay and Shiu (1981), it may be estimated from the solubility of the organic chemical in water by applying the fugacity equation (1) to the pure chemical i, in which case xi and 7iA are unity, thus y~ the mole fraction solubility in water becomes 1/'/iA . It is a fair assumption that the activity coefficient remains constant in the dilute concentration range from zero to saturation, provided that the saturation or solubility limit is less than a mole fraction of 1%. The reason for this is that the behavior of the dissolved chemical is dominated by solute-water interactions, with less significant solute-solute interactions being possible because of the high dilution. For higher-solubility chemicals, such as alcohols, it is necessary to estimate activity coefficients at near "infinite dilution" conditions, using extrapolated
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
27
data or correlations of the type developed by Deal and Derr (1968) and reviewed by Reid et al. (1977). Using the solubility in pure water to estimate 7iA implies that there are no influences of co-solvents, surfactants or electrolytes. Organic cosolvents, e.g., alcohols or ketones, usually increase the solubility (decrease 7) by making the water matrix more "organic". Fu and Luthy (1986a,b) have discussed this effect and methods of quantifying it. Electrolytes usually "salt out" organic solutes (increase 7), as discussed by Aquan-Yuen et al. (1979). These effects are particularly important in landfill leachates which may contain substantial concentrations of numerous polar and ionic species. If desired, 7 can be estimated experimentally by measuring the solubility of the solute in the leachate. It is presently difficult to estimate the effect of co-solvent mixtures on 7iA, although progress is being made in this area as reported by Rao et al. (1985) and Nkedi-Kizza et al. (1985, 1987, 1989). In summary, for sparingly soluble organic substances which form ideal NAt'L phase solutions the relationship between y~ and x~ can be established by assuming: ];iN I/7iA
~
(3)
1
= S~MA/PAM,
(4)
then YilXi
=
S, MAIpAMi
=
(5)
where Si is the solubility of component i in water (kg/m3). Note that if x~ is unity, the organic liquid is pure, then yi becomes the mole fraction solubility of the chemical in water, i.e., S~MA/PAMg. For example, a NAPL consisting of 1% by volume benzene in n-octane has properties as given in Table 1. Table 1 clearly shows that solubilities and activity coefficients in water vary by many orders of magnitude as do partition coefficients. The dissolved NAPS is mainly benzene while the NAPL is mainly octane. The dissolved concentrations can be assumed to be xiSi where xi is the mole fraction in the NAPL and S~ the pure liquid solubility. A situation frequently arises in which the identities or concentrations of some of the organic components in the NAPL are in doubt, thus the mole fractions can not be ascertained. Examples are crude oils, fuel oils, solvents, or solutions of one chemical or a class of chemicals of concern (such as PCB) in a complex mixture (such as mineral oil). It may be necessary to estimate properties in such cases, but if necessary a simple expedient is to assume that all organic chemicals present have equal molar volumes, thus mole fraction and volume fraction are equal. Alternatively, if the mass concentration (e.g., 5g/kg) is known, the mass and mole fractions may be equated. These assumptions should not be introduced unless it is necessary to do so, but it
28
D. M A C K A Y E T AL.
TABLE 1 Properties of a mixture of benzene and n-octane at 25°C Benzene
n-Octane
Total
Molecular mass, Mi, kg/mol Density, pi, kg/m 3 Molar volume, cm 3/mol Aqueous solubility, S,, kg/m 3 Aqueous solubility, Ci, mol/m 3 Aqueous solubility, mole fraction y~ Activity coeff, in water 7~A
0.078 879 88.8 1.78 22.8 4.12 x 10 4 2424
0.1257 700 179.6 0.000682 0.0054 9.72 x 10 8 10.2 × l 0 6
_
Volume, m ~ Mass, kg Mass fraction, wi Number of moles Mole fraction xi (in NAPL phase)
0.01 8.79 0.012 112.5 0.02 4.12 × 10 4 8.24 × 10 -.6 0.46 35.9 8790 4.1 × 10 3
0.99 693 0.988 5513 0.98 9.72 × 10 8 9.52 × 10 8 0.0053 0.66 693000 9.5 × 1 0 - 7
1.0 702 1.0 5625 1.0 _ 8.33 × 10 6 0.465 36.5
Ki or yi/xi Yi (water phase) C,A mol/m 3 (water) S~A g/m 3 (water) SiN g/m 3 (in NAPL phase) Kvt (water/NAPL)
-
o f t e n t r a n s p i r e s t h a t the e r r o r i n t r o d u c e d ( o f p o s s i b l y a f a c t o r o f three in c o n c e n t r a t i o n ) is less t h a n o t h e r u n c e r t a i n t i e s in the c a l c u l a t i o n . Since liquid m i x t u r e s are o f t e n c h a r a c t e r i z e d b y v o l u m e f r a c t i o n , it is usually c o n v e n i e n t to r e p l a c e the m o l e f r a c t i o n b y v o l u m e f r a c t i o n w h e n c a l c u l a t i n g the c o n c e n t r a t i o n in a q u e o u s solution. I f this is d o n e in T a b l e 1 a n e r r o r o f a f a c t o r o f t w o is i n t r o d u c e d b e c a u s e the m o l a r v o l u m e s o f b e n z e n e a n d o c t a n e differ b y this factor. F o r p r a c t i c a l p u r p o s e s , it is c o n v e n i e n t to define a s e c o n d v o l u m e t r i c p a r t i t i o n coefficient Kv, w h i c h is the r a t i o o f " a m o u n t p e r unit v o l u m e " c o n c e n t r a t i o n s in units o f g / m 3, k g / m 3 or m o l / m 3 or e v e n v o l u m e fractions. Since Cm the c o n c e n t r a t i o n in the NAPL is Xi/VN, w h e r e VN is the m e a n m o l a r v o l u m e o f NAPL ( m 3 / m o l ) a n d CiA the w a t e r p h a s e c o n c e n t r a t i o n is yi/VA it follows that:
Kvi
=
CiA/CiN
=
(yi/xi)(VN/'UA)
:
Ki(MN/pN)/(MA/PA )
=
Kz(MN/pN)/18 x l0 -6
(6)
w h e r e M y a n d PN a r e the m e a n m o l e c u l a r m a s s a n d density, respectively. G e n e r a l l y , the m o l a r v o l u m e o f the NAPL will be a f a c t o r o f a p p r o x i m a t e l y 10 g r e a t e r t h a n t h a t o f w a t e r thus Kvi will be g r e a t e r b y this f a c t o r t h a n K~.
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
29
This is illustrated in Table 1. A problem with Kv~ is that it depends on the composition of the NAPL, specifically on its mean molecular mass or volume, thus as the NAPL composition changes Kv~ changes. On the other hand, Ki is a function only of the chemical, and not of the NAPL, provided that the activity coefficient in the NAPL phase is unity. A complication arises when the pure organic chemical is solid at the temperature of interest but is in liquid solution in the NAPL. In such cases, the correct saturation solubility S~ to use in equation 4 is that of the subcooled liquid chemical. This solubility is larger than that of the solid at temperatures below the melting point by the multiple ( l / F ) where F is the fugacity ratio which can be estimated from: F =
exp[(6.79(1 -
(7)
TM/T)]
where T is the system temperature (K) and TM is the solid's melting point (K) (Prausnitz, 1969; Yalkowsky, 1979). For example, naphthalene with a solid solubility of 33 g/m 3 and a melting point of 80.2°C, has a fugacity ratio F of 0.28 at 25°C, thus the subcooled liquid solubility is 118 g/m 3. This is the solubility which naphthalene would have if it was liquid at 25°C and is the "reference solubility" which applies in equation 4. The dissolved concentration S can never exceed the solid solubility, because it is impossible to dissolve naphthalene in the organic phase to the extent the xe exceeds the fugacity ratio or 0.28 in this case. Attempts to form more concentrated solutions result in precipitation of the solid chemical. DISSOLUTION KINETICS If a volume of multicomponent NAPE is contacted with clean water of flowrate F Vm3/h and equilibrium is reached, then the depletion of each component from the NAPE can be expressed as: dn~/dt
=
(8)
--F,C~
But n i is x i N and Ci is yiPAMA mol/m 3 and yi is K~x~ thus: d(xiN)/dt
=
-- F ~ K i X i P A / M A
--
FMKiX i
(9)
where Fv P A / M A is the flowrate of water in units of mol/h and is designated FM. In practical situations, equilibrium is rarely achieved, but this concept can still be used and applied to a real groundwater flow F R by applying an "efficiency of approach to equilibrium" E of say 10%. Fv is then E F R . Rigorous solution of this differential equation can only be done numerically because N varies as each component dissolves. If the solute of interest is one of the more soluble components and is present in low concentration, N
30
D. M A C K A Y ET AL.
can be assumed to be constant and the equation reduces to: dxi/dt
=
- ( K f M/N)x i
(10)
which can be readily solved from an initial condition xio to give: )(,
=
Xio
exp(--KiFMt/N)
(11)
The depletion rate constant is thus KiFM/N (h -I) and the characteristic time for x to be reduced to 37% (i.e., e x p ( - 1 ) ) of its initial value is N/KiFM. Alternatively, the quantity of water which must be saturated to achieve this extent of dissolution is FM t or N/Ki moles. This can be used to express the different depletion rates of NAPL components of different water solubilities. A sparingly soluble component of very low K,. (say 10 -6) will require 100 times more water to achieve a reduction to 37% than a more soluble component of K/equal to 10 -4. This characteristic time is useful because it gives an immediate indication of the time of depletion. For example, a 100-m 3 volume of sandy soil may contain 10 L/m 3, i.e. 1 m 3 of a petroleum NAPL with a mean molecular mass of 100 g/tool and a density of 700 kg/m 3. There is thus 700 kg or 7000 mol of product. If it contains 0.8% benzene by mass this corresponds to 5.6kg or 72 mol (molecular mass 78 g/mol) or approximately 1 mol %. The hydraulic conductivity and gradient could indicate a flow of 18 L/hour or 1000 mol/hour of water which will be in intimate contact with the NAPL. The characteristic time to dissolve 63% of the benzene will then be N/K~FM or 16 800 hours (700 days) since K~ is 1/2400. During this time, the concentration of benzene in the water will drop from an initial mole fraction of 4.2 x 10 -6 (concentration 18g/m 3) to 1.54 x 10 -6 (6.6g/m3). Clearly, the dominant quantity is K~ the activity coefficient ratio. Another convenient form of this equation is: x~ = Xio exp(--QMK~)
(12)
where QM is F MtiM and is the ratio of moles of water to the number of moles of NAPL. This suggests the use of a modified version which may be more convenient for practical calculations, i.e.; xi = Xio e x p ( - Q v K v i )
(13)
where Qv is the ratio of volume of water to initial volume of NAPL and gvi is the volumetric partition coefficient defined such that QMK~ equals Qv Kv~. K~ and Kv~are related through the ratio of molar volumes as discussed earlier. A more accurate estimation of the extent of dissolution may be obtained using Qv if the prevailing NAPL volume is used instead of the initial value. It may be useful to express these equations in terms of times and rate constants. Equations 11 and 12 show that the rate constant k is either
31
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
KeFM/N or KviFv/V where Fv is the volumetric flow rate of water and V is the YAPL volume. It should be noted that Kvi changes as the NAPL composition changes. The most serious error in using the simple first order expression occurs when the solute is intermediate in solubility within the group of components present in the mixture. This is best illustrated by an example in which an equimolar YAPL mixture of benzene, toluene, and p-xylene is dissolved in a flow of water under the conditions specified in Table 2. The results of a rigorous numerical simulation, which are given in Figure 1, show that the benzene concentration in the water decreases monotonically, but the toluene concentration rises to a m a x i m u m then falls. This is due to the initial decrease in toluene mole fraction as the benzene is depleted. We suggest a simple expedient for obtaining an approximate analytical solution in such cases. The concentrations are designated as being of the solute of interest x2, the more soluble components x, and the less soluble components x3, thus the sum of x,, x2, and x 3 is 1.0. At times when there is appreciable loss of x,, but relatively little loss of x2, the mole fraction of x 2 will have risen to approximately x2/(x2 + x3). This occurs when QvKvi or QMKi are approximately 0.1 or equivalently Qv is Kvi/lO or QM is Ki/IO. We thus assume that xe rises from Xio to X~o/(Xgo+ x3) during this period. There will be loss of only 5% of the solute i during this period. It is suggested that w o t e r / o i l volume ratio, q 200 I
600
400 I
600 '
800 I
1000 I
| 200 I
1400 I
1600 I
1800 i
2000 I
500 .J
400 E ~,~ 300 E u
c o
p-xylene
200
100
0
I 0
I 20000
I
I 40000
I
I 60000
time, hours
Fig. 1. Computed changing concentrations of hydrocarbons in water exposed to an equimolar mixture of benzene, toluene and p-xylene as a function of water to oil volume ratio or time.
32
D. MACKAY ET AL.
TABLE 2
Composition and aqueous solubility of equimolar mixture at 25°C used to obtain Figure 1
Molecular mass, M, kg/mol Solubility in water, S, kg/m 3 Density, kg/m 3 no. of moles, n mass, kg volume, m 3 mole fraction x0, in N A P L K,
mole fraction, yi0, in water Si0, initial concentration in water, kg/m 3 K,,
water flow Qv at 90% depletion =
2.3/Kv~
benzene
toluene
p-xylene
0.078 1.780 879 1000 78 0.0887 0.333 4.12 x 10 4 137 × 10 -6 0.593
0.092 0.515 867 1000 92 0.1061 0.333 1.0 × 10 4 33.3 × 10 -6 0.172
0.106 0.185 861 1000 106 0.1231 0.333 0.31 x 10 4 10.5 × 10 -6 0.062
2.43 x 10 .3 0.01 m3/h 946
5.93 × 1 0 - 4
1.85 x 10 4
3900
12600
total
3000 276 0.3179
0.827
x2 will change with time approximately as: x2 = =
n2/(n, + n2 + n3) n20 exp ( - k2 t)/[n~o exp ( - k 1t) + n20 exp ( - k 2t) + n30 exp ( - k 3t)] (14)
The rate constants kl, k2, and k3 are arbitrarily selected to give a larger rate constant to the more soluble material. If information is available on the solubility o f this material, an average number can be used. Otherwise, k~ can be set at possibly five times k2. The third term k3 could be set at zero. The equation causes Xz to rise from x20 to approximately x20/(x20 + x30) then to fall in a first-order fashion. It is suggested that these equations may be used to estimate the changing concentrations in the NAPI~ and thus the corresponding change in equilibrium water concentrations. The question arises of the extent of approach to equilibrium. Since contact times are long (hours or days), it is likely that water which is in intimate contact with the NAPe, becomes saturated, but much o f the water flow may "by pass" the NApL, is not saturated, but may later mix, or exchange diffusively or dispersively with the saturated water. Samples of water taken downstream may therefore be subsaturated. A convenient method of quantifying this dilution is to assume two water flows to be present, one which is saturated and one which is unaffected by the NAPE, but these are later mixed by dispersion and diffusion, resulting in a more dilute solution. It can be
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
33
shown that this approach is mathematically equivalent to introducing a transfer resistance between NAPL and the total water flow. This undersaturation or dilution effect should not influence the proportions of the components present in the groundwater, only the absolute concentration is affected. Subsequent sorptive retardation may, of course, change the relative concentrations of specific components. Having established this theoretical background, we test its validity by conducting small-scale laboratory experiments on dissolution of two NAPLS, a crude oil, and a chlorobenzene mixture. It transpires that the method used may be valuable as a protocol for experimental investigation of the dissolution characteristics of a NAPL sample.
EXPERIMENTAL
Four series of experiments were undertaken using a miniature system to simulate the contact between a stationary volume of NAPL and a flow of water. The technique used was the dynamic coupled column, or generated column technique described by May et al. (1978a, b). The procedures have also been described by Billington et al. (1988) and Shiu et al. (1988). The NAVLS used were: (1) a synthetic hydrocarbon mixture with the composition shown in Table 3; (2) two crude oils; (3) a diesel fuel oil, and (4) a synthetic chlorobenzene mixture with the composition shown in Table 4. In all cases a known volume of the NAVI~ was exposed to known flow of water and the water analysed periodically to determine the concentrations of dissolved organic chemicals. Only in the case of the synthetic mixtures was it possible to predict the changing water composition theoretically. The general aims were to confirm the validity of the technique and confirm that the aqueous compositions agree with the theoretical predictions, both in terms of absolute concentrations and the dependence of the changing concentrations on the volume of water contacted with the NAVL. Chemicals 99 mol % benzene, toluene, and certified grade naphthalene were obtained from Fisher Scientific Biphenyl and fluorescent-grade phenanthrene were obtained from Eastman Kodak Co. 99% pure chlorobenzene, 99% 1,2,4trichlorobenzene, 99% 1,2,3,5-tetrachlorobenzene, and 98% pentachlorobenzene were obtained from Aldrich Chemical Co. 98.5% p-xylene and 99.5% hexachlorobenzene were obtained from BDH Chemicals Canada Ltd. riVLC grade methanol was obtained from Caledon Laboratories, Ontario. Mill-Q ultrapure deionized water was used for all experiments. Chromosorb 750
3
* E s t i m a t e d from m o l a r v o l u m e o f N A P L .
a q u e o u s c o n c e n t r a t i o n , Kvi*
initial a q u e o u s conc., So, g / m J subcooled liquid solubility, SOL, g/m 3
3.8 × 10 -3
1.67 × 10 -5 4.10 x 10 4 72.44 72.44
Ki
mole fraction in water, y,
benzene
9.4 x 10 4
1.67 × 10 4
1.25 × 10 6 1.80 x I0 -4 12.83 12.83
106.2 4.22 0.0397 0.0693 185 1.74
p-xylene
5.74 × 10 5
4.09 × 10 8 6.24 × 10 -6 1.79 6.83
128.2 4.14 0.0323 0.0402 31.7 0.944
naphthalene
of the synthetic oil at 25°C
5.14 x 10 6 1.01 x 10 -4 26.27 26.27
92.13 2.69 0.0292 0.051 515 5.59
toluene
solubility of components
78.11 1.82 0.0233 0.0407 1780 22.8
and aqueous
Mol. wt., M, g / m o l % mass no. o f moles, n mole fraction in oil, x i a q u e o u s solubility, S, g / m 3 subcooled liquid solubility, CL, mol/m 3
Composition
TABLE
8.05 × 10 6
4.85 × 10 9 8.73 × 10 7 0.35 0.996
154.2 4.14 0.0268 0.0468 7.48 0.138
biphenyl
l.ll
× 10 6
4.35 × 10 -~° 1.20 x 10 7 0.048 0.267
178.2 4.14 0.0232 0.0405 1.18 0.037
phenanthrene
5.78 × I0 9
6.27 x 10-l0 0.0048 0.0048
198.4 78.94 0.398 0.695 0.00696 0.000037
tetradecane
119.6
0.5725 1.0
total
0.136 0.265 8.34 x 10 -8 1.77 x 10 6 1.20 x 10 5
18.84 18.84 1.87 x 10 -6 4.57 x 10 6 3.11 x 10 5
234.8 234.8 3.75 x 10 -5 7.47 x 10 -4 5.11 X 1 0 3
215.9 54.5 0.0675 0.0046 0.047 2.89 0.0262
1,2,3,5-
181.5 16.96 0.49 0.0401 0.409 46.1 0.068
1,2,4-
112.6 45.6 0.37 0.0492 0.502 470 4.17
* Estimated from average molecular weight a n d density.
Kvi*
Mol. wt mp, °C % mass no. o f mols, n mole fraction, xi0 aqueous solubility, S s, g/m 3 subcooled liquid aqueous solubility CL, mol/m 3 initial aqueous c o n c e n t r a t i o n So, g/m 3 subcooled liquid initial a q u e o u s concentration SOL, g/m 3 mole fraction, Y~0 K~
Chlorobenzene
C o m p o s i t i o n and aqueous solubility o f the c h l o r o b e n z e n e mixture at 25°C
TABLE 4
1.48 x 10 9 5.93 x 10 8 4.03 x 10 7
0.0206 0.10
250.3 86 0.034 0.00244 0.025 0.83 0.0162
penta-
5.69 × 10 12 3.16 x 10 -I° 2.15 × 10 9
0.00009 0.0087
284.8 230 0.034 0.00176 0.018 0.005 0.0017
hexa-
253.8 254.0
0.0981 1.0
Total
>
7 (7
O
_.u ,O
), ,0
,z
©
z
5
,q
36
D. MACKAY ET AL.
(60/80 mesh) and Chromosorb W (30/60 mesh) were purchased from John-Mansville. N o r m a n Wells crude oil and an Alberta crude oil were obtained from Esso Canada. The diesel fuel oil was obtained from BP Canada.
Methods (1) Purge-and-trap gas chromatograph analysis The volatile hydrocarbons in the water samples were determined by purgeand-trap GC. A small generator column was prepared by packing a 26-cm long, 6-mm OD glass tube with Chromasorb 750 coated with approximately 30% by weight of chemical mixture (NAPL). Only 30% of the Chromosorb was soaked with oil to prevent overflowing. The column was plugged with glass wool at both ends and with a 100 mesh stainless steel screen in the outlet. The column was enclosed in a water jacket which was temperature controlled to within +0.1°C by a Neslab Exacal Ex-100 temperature bath. Water was pumped through the column with a Beckman metering p u m p and was collected and stored in the refrigerator for later analysis by GC or LC. A Hewlett-Packard GC model 5840, equipped with a flame ionization detector, and a HP 7675A purge and trap sampler was used. The water sample was purged with N 2 and passed through a Tenax-GC trap. By thermodesorption the sorbed chemicals were swept directly onto the analytical column by the carrier gas. The analytical column was a 0.53-mm OD, 30-m long, J & W DB-1 mega bore fused silica capillary column. The GC oven was initially set at 50°C, held for 10 min, was then temperature programmed to 200°C at a rate of 5°C/min. The oven was held at 200°C for 20 min. The peak areas were integrated with a HP 5840A GC terminal.
(2) Generator column-HPLC analysis The generator column was a 0.64-cm OD, 30-cm long stainless steel tubing packed with prewashed chromasorb W (30/60 mesh) coated with 0.3mL chlorobenzene mixture. The column was thermostated in an Alltech water jacket, and the temperature was controlled to 25 ° _ 0.02°C by a Neslab Endocal RTE-S temperature bath. An Analabs Model B-100-S high-pressure metering p u m p was used to deliver water through the generator column. The water solution was then passed through an extractor column (0.64-cm OD, 6 c m long) stainless steel tubing packed with 30-50 #Bondapak/Corasil (Water Associates). By switching an eight-port Valco valve, the chemicals were extracted by the mobile phase (methanol:water 85:15 by volume in isocratic mode) and directly injected onto the analytical column. The instrument was a Waters' high-performance liquid chromatography (HPLC) system, consisting of a Model 6000 solvent delivery system, a Model M45
37
D I S S O L U T I O N O F N O N - A Q U E O U S P H A S E L I Q U I D S 1N G R O U N D W A T E R
solvent delivery system, a Model 720 system flow controller, a Model 440UV absorbance detector with 254- and 280-nm kits. The analytical column was a Waters' 3.9-mm OD, 300-mm long/~Bondapak Ct8 column. Integration of the peak areas was recorded by a Waters Model 730 data module. RESULTS A N D DISCUSSION
Figures 2 to 6 show the time courses of water concentration of specific chemicals as the NAPLS became depleted. Also shown on Figures 2 and 6 are the theoretically predicted and experimental concentrations of selected chemicals. Figure 2 shows that when the synthetic oil is contacted with water the benzene concentration in water falls rapidly to about 1 mg/L when Q is about 500. The less soluble toluene dissolves slightly more slowly and p-xylene more slowly still. Biphenyl, which is not shown, remained constant until a volume ratio about 12 000 then started to fall and phenanthrene, the least soluble, was unchanged even at Q of 24 000. A factor of ten depletion is expected theoretically when QvKvi is 2.3 (equation 13). The values of Ki and Kvi are given in Table 2, based on the solubilities and estimates of the oil mean molecular mass (92 g/mol) thus the Qv corresponding to this factor of ten depletion is predicted as 2.3/Kv. The values are approximately benzene 1000, toluene 4000 and xylene 13 000 which are in good agreement with the data shown in Figure 2. go
experimental data:
80
+ Benzene 6 Toluene
70
9 Ethylbenzene &x'ylenes (~
6O
C 0
5O
~
40
E
benzene
r~c o
30 A
20
0
~
, 1000
A
,
J., 2000
.~ 30~00
~
4000
water/oil volume ratio, q Fig. 2. Water concentrations in contact with the synthetic oil showing rapid depletion of the more soluble hydrocarbons.
38
D. M A C K A Y ET AL. 40
• Total
35
+ Benzene o Toluene
_..1
30
E
25
& Ethylbenzene & xylanea x Naphthalene
•
d 0
20 C Q) 0 c 0 0
15
+ ++
10 o
oO
5
o
o
o
A
o
+
&
0
~
&
i
0
&
I
i
200
i
i
400
i 800
i
600
X 1000
i
water/oil volume ratio, Q Fig. 3. Dissolution characteristics of Norman Wells crude oil.
A useful "rule of thumb" thus emerges from the theoretical treatment earlier, and is confirmed by experiment that for 90% loss a Qv of approximately 2.3/K v is needed. Figures 3 and 4 give similar data for two actual crude oils showing rapid depletion of benzene and toluene in the range of Q values to 400. As expected, 28 • Total 26 + Benzene 24
o Toluene 22
_J
& Ethylbenzene+xylenes
20 O~
E
x Naphthalene
la
E 16
0 k~
14-
-~
12-
0 C 0 0
10-
8
¢>
4~ o
o
6 4 £A
2 0
L~
A
+
÷
,- ~ -'~-i-'~0
"~'1 400
o &
I x
&
I 800
I ~
I0 12 0
1
I I 1600
!
water/oil volume ratio, Q Fig. 4. Dissolution characteristics of
Alberta
crude
oil.
I X
2000
I
|
2400
I
2800
39
D I S S O L U T I O N O F N O N - A Q U E O U S P H A S E L I Q U I D S IN G R O U N D W A T E R
• Total + Benzene o Toluene
3.5
& Xylenea x Naphthalene
3 ET=
E
2.5
0
P C Q) 0 c
2
•
1.5
0 0
I A/~
A
& &
& &
0.5
oo 2 o
& o
&
.+
o
~x~xx
x
i
x°
i~ 1000
0
water/oil
i
volume
xl 2000
3000
ratio, Q
Fig. 5. Dissolution characteristics of a diesel fuel oil.
the naphthalene concentration is relatively low and constant in this range. Figure 5 gives the results for a diesel fuel oil again showing rapid loss of the small quantities of benzene and toluene. Figure 6 presents data for the mixture of chlorobenzenes (CBs), these compounds being of particular concern as subsurface contaminants and 240
+
220
~. -p
200 ._J "~
+ chlorobenzene +
& 1,2,4-TCB
180
v 1,2,5.5-TeCB
160
E .
140
:~
120
c
100
c o u
80
g P
60
4o 20 0 0
10000
20000
30000
40000
woter/CB mixture volume ratio, Q Fig. 6. Dissolution characteristics of the synthetic chlorobenzene mixture.
40
D. M A C K A Y ET AL.
present in dumps such as the Love Canal site in Niagara Falls, NY. There is obvious rapid depletion of the more soluble mono-CB with 90% loss by Q of 7000. The corresponding value for tri-CB is about 25 000. The drop in tri-CB concentration is quite sudden, possibly as a result of onset of final depletion of the solute from the column. The other, less soluble CBs were not appreciably changed at the end of the experiment. The near logarithmic linear drop in mono-CB concentration is consistent with first-order kinetics. It is important to emphasize that the theory presented here assumes the YAPL and aqueous phases to be homogeneous. Even in the laboratory-scale generator column there is likely to be depletion of the more soluble components from the "upstream" end of the NAPL. Exact correspondence with theory is thus not expected. In an actual spill site a similar, but probably more pronounced, effect is likely to occur, with concentration changes developing in the YAPL phase. There may be a "chromatographic" effect involving differing net velocities of each component as they exchange between the water phase and the NAPL, as well as the mineral and organic matter present in the soil. To simulate these phenomena requires a more complex spatially resolved model of partitioning and transport. CONCLUSIONS The theoretical analysis and the experimental results are consistent, suggesting that the dissolution kinetics and equilibria of sparingly soluble chemicals in NAPES behave according to well established physical-chemical principles and are thus basically predictable. Returning to the questions posed in the Introduction, the concentration of chemicals present in NAPE and dissolved in water can be estimated using (1) a partition coefficient Ki or K~i obtained from solubility data and (2) information on the chemical concentration in the NAPE. The thermodynamics of systems have been reviewed, the principal uncertainties being the "co-solvent" effects of other soluble compounds in the leachate. Second, if an estimate is available for the volume of NAPE and the volumetric flowrate of water which contacts it, the quantity of water (or time) necessary to deplete the NAPE of a specified fraction of a chemical can be estimated using the "rule of thumb", namely: Fraction remaining =
exp ( -
Ovgvi)
where Qv is the ratio of water to NAPE volume and Kvi is the volumetric partition coefficient. Third, the time necessary to accomplish a defined extent of dissolution may be estimated if the water flowrate is known and intimate NAPE-Water contact
DISSOLUTION OF NON-AQUEOUS PHASE LIQUIDS IN GROUNDWATER
41
is assumed. A m e t h o d of treating chemicals intermediate in solubility has been suggested. An implication of this work is that investigations of leachate concentrations at various distances from a source can in principle be used to estimate the nature of the source, provided that information is also available on retardation factors. It is noteworthy that the more soluble species will have the smallest retardation factors. For example, the presence of a " h a l o " of benzene at relatively high concentrations some distance from the source may indicate a source which has become depleted of benzene. F r o m a knowledge of site specific flowrates, retardation factors and concentrations it should be possible to piece together a consistent picture of the likely general behaviour of a series o f contaminants present in and leaching from a particular source. It is hoped that this study will be of value to contaminant hydrogeologists by exposing some o f the physical-chemical principles which underlie the dissolution of NAPE components and helping to elucidate the behavior of, and remedial actions for, these complex systems. ACKNOWLEDGEMENT The authors are indebted to NSERC, PACE and the Association of American Railroads for financial support. REFERENCES Aquan-Yuen, M., Mackay, D. and Shiu, W.Y., 1979. Solubility of hexane, phenanthrene, chlorobenzene and p-dichlorobenzenein aqueous solutions. J. Chem. Eng. Data, 24: 30-34. Billington, J.W., Huang, G.L., Szeto, F., Shiu, W.Y. and Mackay D., 1988. Preparation of aqueous solutions of sparingly soluble organic substances, I. Single component systems. Environ. Toxicol. Chem., 7:117-124. Deal, C.H. and Derr, E.L., 1968. Group contributions in mixtures. Ind. Eng. Chem., 60: 29-38. Dracos, T.H., 1987. Immiscibletransport of hydrocarbons infiltrating in unconfined aquifers. In: J.H. Vandermeulen and S.R. Hrudey (Editors), Oil in Freshwater: Chemistry, Biology, Countermeasure Technology. Pergamon Press, Oxford-New York, pp. 161-175. Feenstra, S., 1990. Evaluation of multi-component DNAPL sources by monitoring of dissolved phase concentrations. Paper presented at the Conference on Subsurface Contamination by Immiscible Fluids. Int. Assoc. of Hydrologists, Calgary, Alberta, April 16-20, 1990. Fredenslund, A., Gmehling, J. and Rasmussen, P., 1977. Vapor Liquid Equilibria Using UNIFAC. Elsevier, Amsterdam. Fu, J.-K. and Luthy, R.G., 1986a. Aromatic compound solubility in solvent/water mixtures. J. Environ. Eng., 112: 328-345. Fu, J.-K. and Luthy, R.C., 1986b. Effect of organic solvent on sorption of aromatic solutes onto soils. J. Environ. Eng., 112: 346-366. Hala, E., Pick, J., Fried, V. and Vilim, O., 1967. Vapor-Liquid Equilibrium. Pergamon Press, Oxford. Mackay, D. and Shiu, W.Y., 1981. A critical review of Henry's law constants for chemicals of environmental interest. J. Phys. Chem. Ref. Data, 10:1175-1199.
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Mackay, D.M., Roberts, P.V. and Cherry, J.A., 1985. Transport of organic contaminants in groundwater. Environ. Sci. Technol., 19: 384-392. May, W.E., Wasik, S.P. and Freeman, D.H., 1978a. Determination of the aqueous solubility of polynuclear aromatic hydrocarbons by a coupled column liquid chromatographic technique. Anal. Chem., 50: 175-179. May, W.E., Wasik, S.P. and Freeman, D.H., 1978b. Determination of the solubility behavior of some polycyclic aromatic hydrocarbons in water. Anal. Chem., 50: 997-1000. Nkedi-Kizza, P., Rao, P.S.C. and Hornsby, A.G., 1985. Influence of organic cosolvents on sorption of hydrophobic organic chemicals by soils. Environ. Sci. Technol., 19: 975-979. Nkedi-Kizza, P., Rao, P.S.C. and Hornsby, G., 1987. Influence of organic cosolvent in leaching of hydrophobic organic chemicals through soils. Environ. Sci. Technol., 21:1107-1111. Nkedi-Kizza, P., Brusseau, M.L., Rao, P.S.C. and Hornsby, A.G., 1989. Nonequilibrium sorption during displacement of hydrophobic organic chemicals and calcium-45 through soil columns with aqueous and mixed solvents. Environ. Sci. Technol., 23: 814-820. Prausnitz, J.M., 1969. Molecular Thermodynamics of Fluid-Phase Equilibria. Prentice-Hall, Englewood Cliffs, NJ. Ptacek, C., Cherry, J.A., Gillham, R.W. 1987. Mobility of dissolved petroleum-derived hydrocarbons in sand aquifers. In: J.H. Vandermeulen and S.E. Hrudey (Editors), Oil in Freshwater: Chemistry, Biology, Countermeasure Technology. Pergamon Press, OxfordNew York, pp. 195-216. Rao, P.S.C., Hornsby, A.G., Kilcrease, D.P. and Nkedi-Kizza, P., 1985. Sorption and transport of hydrophobic organic chemicals in aqueous and mixed solvent systems: model development and preliminary evaluation. J. Environ. Qual., 14: 376-383. Reid, R.C., Prauznitz, J.M. and Sherwood, T.K., 1977. The Properties of Gases and Liquids, 3rd ed. McGraw-Hill, New York, NY. Schwille, F., 1981. Groundwater pollution in porous media by fluids immiscible with water. Sci. Total. Environ., 21: 172-185. Shiu, W.Y., Maijanen, A., Ng, A.L.Y. and Mackay, D., 1988. Preparation of aqueous solutions of sparingly soluble organic substances, II. Multicomponent systems - - hydrocarbon mixtures and petroleum products. Environ. Toxicol. Chem., 7: 125-137. Van Dam, J., 1967. The migration of hydrocarbons in water-bearing stratum. In: P. Hepple (Editors) Proc. Syrup. on Joint Problems of the Oil and Water Industries. Elsevier, Amsterdam, pp. 55-88. Van Ness, H.C. and Abbott, M.M., 1982. Classical Thermodynamics of Non-Electrolyte Solutions with Application to Phase Equilibria. McGraw-Hill, New York, NY. Walas, S.M., 1985. Phase Equilibrium in Chemical Engineering. Butterworth Publishers, Boston, MA. Yalkowsky, S.H., 1979. Estimation of entropies of fusion of organic compounds. Ind. Eng. Fundam., 18: 108-111.