(8)Umback, C. R.. Lembke, W. E.. 1965 Winter Meeting of the
A , = projected area of a spherical droplet normal to path, m2 CD = empirical drag coefficient, dimensionless D, = spray droplet diameter, m Dp = mass average droplet diameter, m erfo() = value of the error function with argument, x FD,,, = drag force in the x or z direction, N g = gravitational acceleration, m/s2 g, = dimensional constant, 1 (kg.m)/(Nd) rn(D,) = mass fraction of spray droplets having diameters less than or equal to D,, dimensionless Re,p = particle Reynolds number, dimensionless Ri = Richardson number, dimensionless T A = ambient air temperature, K uX,* = component droplet velocity in the x or z direction, m/s Uf = average wind velocity, m/s u s = terminal settling velocity of droplet, m/s n = downwind distance measured from leeward edge of first swath, m X , = swathwidth,m y = crosswind distance, m Y , = swath length, m z = vertical distance above grade level, m 2, = height at which active material is released, m r = adiabatic lapse rate, 0.0098 K/m v f = kinematic viscosity of air, m2/s x = pi, 3.141592.. . p f = air density, kg/m3 p, = spray droplet density, kg/m3 0, = standard deviation of spray droplet diameters, pm 4 = objective function
American Society of Agricultural Engineers, Chicago; 1965; A.S.A.E. Paper No. 65-702. (9) Garrett, A: J., 1968 Annual Meeting of the American Society of Agricultural Engineers, Utah; 1968; A.S.A.E. Paper No. 68-138. (10) Friedlander, S. K., Johnston, H. F., Ind. Eng. Chem. 1957,49, 1151-1156. (11) Threadgill, E. D., Smith, D. B., 1971 Winter Meeting of the American Societv of Amicultural Eneineers. Chicaeo: - , 1971: A.S.A.E. Paper N;. 71-66?. (12) Ware, G. W., Cahill. W. P.. Estesen. B. J.. J . Econ. Entomol. 1975,68, 329-330. (13) Glotfelty, D. E., Caro, J . H., ACS Symp. Ser. 1975,17, 42-62. (14) Lapple, C. E., Shepherd, C. B., Ind. Eng. Chem. 1940, 32, 605-61 7. (15) Hughes, R. R., Gilliland, E. R., Chem. Eng. Prog. 1952,48(10), 497-504. (16) Yen, C. Y., Yu, T. H., Chem. Eng. Prog. S y z p . Ser. 1966,62(62), 100-1 11. ( 1 7 ) Sinclair, D., “Handbook of Aerosols”; U.S.A.E.C.: Washington, D.C., 1950; p p 64-76. (18) Ranz, W.E., U.S. Public Health Service Research Grant S-19, 1956, Bulletin No. 66, Technical Report No. 1. 119) Davies. C. N.. “Surface Contamination”: Academic Press: New York, 1966; pp 393-445. (20) Davies. C. N., “Surface Contamination”: Peraamon Press: New York, 1967; pp 115-121. (21) Fuchs, N. A., “The Mechanics of Aerosols”; Pergamon Press: Yew York, 1964; pp 250-287. (22) Pasquill, F., “Atmospheric Diffusion”, 2nd ed.; Wiley: New York, 1974; pp 252-269,327. (23) Chamberlain, A. C., Int. J . Air Pollut. 1960, 3, 63-88. (24) Seinfeld, J. H., Roth, P. M., Reynolds, S. D., Chem. Eng. Comp u t . 1972,1, 17-31. (25) Bencala, K. E., Seinfeld, J. H., Atmos. Enuiron. 1976, 10, 941-950. (26) Apt, K. D., Atmos. Enuiron. 1976,10, 941-950. (27) Mugele, R. A., Evans, H. D., Ind. Eng. Chem., 1951,43, 13171324. (28) Himmelblau, D. M., “Process Analysis by Statistical Methods”; Wiley: New York, 1970; pp 10-42. (29) Abramowitz, M., Stegun, I. A,, Eds., “Handbook of Mathematical Functions”; Dover: New York, 1965; p 299. (30) Berry, F. A., Bollay, E., Beers, N. R., “Handbook of Meteorology”, McGraw-Hill: New York, 1973. (31) Byers, H. R., “General Meteorology”, 4th ed.; McGraw-Hill: New York, 1974. 132) Sutton. 0. G.. “Micrometeoroloev”: McGraw-Hill: New York. -” 1953; pp 56-104. (33) Yates. W. E.. Akesson. N. B.. Coutts. H. H.. 1964 Winter Meeting of the American Society of Agricultural Engineers, New Orlean; 1964; A.S.A.E. Paper No. 64-609-A.
-
I
Literature Cited (1) Akesson, N. B., Yates, W.E., “The Use of Aircraft in Agriculture”; F.A.O. Agricultural Development Paper No. 94; Food and Agriculture Organization of the United Nations: Rome, 1974. ( 2 ) Brandes, G. A,, Agric. Chem. 1967,22(1),43-47. (3) Grumbles, J. B.. Rangeman’s J . 1975,2(2),50-56. (4) LaMer, V. K., Hochberg, S., Chem. Rev. 1949,44, 341-352. (5) Johnstone, H. F., Winsche, W. E., Smith, L. W., Chem. Reu 1949, 44, 353-371. (6) Sexsmith, J. J., Hopwell, W. W., Anderson, D. T., Russel, G. C., Hurtig, H., Can. J. Plant Sci. 1957,37, 85-96. ( 7 ) Coutts, H.‘H., Yates, W. E., 1965 Annual Meeting of the American Society of Agricultural Engineers, University of Georgia; 1965; A.S.A.E. Paper No. 65-157.
Received for recieu: J u n e 8 , 1979. Accepted March 4,1980
Groundwater Leaching of Organic Pollutants from in Situ Retorted Oil Shale. A Mass Transfer Analysis Gary L. Amy’”, Anthony L. Hines2, Jerome F. Thomas, and Robert E. Selleck Department of Civil Engineering, University of California, Berkeley, Calif. 94720
Oil shale is a mineral material from which oil can be produced by pyrolysis of organic matter that occurs within the oil shale matrix. Most of the organic matter found in oil shale consists of Kerogen, a high molecular weight, three-dimensional polymer. The oil produced is in the form of condensible hydrocarbon vapors, which are subsequently condensed and cooled to produce a semiviscous liquid. Present address, Department of Civil Engineering, University of Arizona, Tucson, Ariz. 85721. Present address. Department of Chemical and Petroleum-Refining Engineering, Colorado School of Mines, Golden, Colo. 80303. 0013-936X/80/0914-0831$01.OO/O
Not all of the organic matter originally present in oil shale is converted to oil. During pyrolysis, various carbonaceous byproducts are formed. One of the most important byproducts of pyrolysis, from an environmental viewpoint, is retorted oil shale or spent shale (i.e., the residual solid material remaining after pyrolysis). After pyrolysis, some residual organic matter remains associated with the spent shale. A portion of this residual organic matter resides within the pores of the spent shale matrix, partly as a cokelike substance, partly as oil that was not extracted, partly as miscellaneous organic compounds produced as byproducts of pyrolysis. In addition, some residual organic
@ 1980 American Chemical Society
Volume 14, Number 7, July 1980
831
The potential degradation of groundwater quality by organic pollutants leached from in situ retorted oil shale was assessed by conducting a mass transfer analysis of laboratory data. Two unique samples of in situ retorted oil shale were investigated in a series of laboratory experiments. Results of these laboratory experiments were subjected to a mass transfer analysis to determine the rate-limiting step involved in the leaching of organic pollutants by groundwater. This mass transfer analysis suggested that, for one of the samples, internal diffusion through the porous infrastructure of re-
torted oil shale particles is the rate-controlling mass transfer step. For the other sample, no single rate-controlling step was conclusively identified, although it appeared that internal diffusion played a key role in the overall leaching phenomenon. Since internal diffusion is an extremely slow mass transfer process, it is anticipated that low, relatively constant levels of organic pollutants will appear in groundwater and persist for long periods of time. This implies that there may be a chronic, long-term groundwater pollution problem in areas of future in situ oil shale development.
material is associated with the exterior surface of the spent shale matrix. Overall, the resultant spent shale may contain up to 5% organic carbon residue by weight ( I ) . It has been demonstrated that some of this organic residue is soluble or sparingly soluble in water (2). The process of converting oil shale to oil is referred to as retorting, and the reaction chamber in which the conversion occurs is called a retort. Oil shale may be retorted above ground or in situ (i.e., in place below the ground surface). Owing to both economic and environmental considerations, recent emphasis has been placed on in situ retorting of oil shale. Other sources (2, 3 ) have rigorously discussed the preparation of in situ retorts and the alternative methods of providing heat for pyrolysis. To briefly summarize, heat is supplied to an in situ retort by either (a) an external heat source such as a hot inert gas or (b) an internal heat source whereby some of the residual organic material remaining on the spent shale is combusted. In geographical areas of rich deposits, oil shale often occurs as a continuous, relat,ively impermeable layer situated between two groundwater aquifers ( 3 , 4 ) .One of the major environmental hazards associated with in situ oil shale development is that it creates a potential for groundwater pollution. Several researchers have indicated that various inorganic and organic pollutants can be leached from spent shale as groundwater migrates into and possibly through an exhausted in situ retort (2,5-8). The objectives of this research were (a) to conduct a mass transfer analysis to define the mechanisms by which organic matter is leached from spent shale by groundwater and (b) to comment on the environmental significance of the leaching phenomenon.
a time period of 30 days was based on a series of preliminary batch experiments which assessed the kinetics of the leaching phenomenon. These experiments monitored TOC over time periods ranging from 0.5 to 30 days and demonstrated the establishment of “pseudoequilibrium” conditions at the end of 30 days (2). Spent Shale Samples. Two unique samples of spent shale were examined during the laboratory leaching experiments. One sample of spent shale, hereafter referred to as type 1,was produced during combustion retorting, while the other sample of spent shale, hereafter referred to as type 2, was produced during inert gas retorting. These spent shale samples were obtained from Lawrence Livermore Laboratory’s pilot-scale 125-kg simulated in situ retort (9).Each of the above spent shale samples was characterized according to various physical and chemical characteristics. The bulk densities of types 1 and 2 spent shale were 0.86 and 1.08 g/cm3, respectively, while each sample consisted of a particle size range of 0.14-0.64 cm. The rationale for selecting this specific particle size range is that spent shale samples were derived from a small pilot-scale retort. The spent shale produced by the retort ranged in size from “dust” (less than 200 mesh size) to approximately 1.0 cm. This shale was sieved to provide the aforementioned particle size range. It should be noted that larger particles and a broader particle size range will occur in an actual in situ retort. The total bulk porosity of the type 1 spent shale was 57%, while that of type 2 was 51%. Thus, the spent shale sample produced during inert gas retorting was characterized by a lower bulk porosity than spent shale produced during combustion retorting, This is likely due to the burning of the residual organic material inside the shale matrix during combustion retorting, thereby creating additional “internal” porosity (i.e., porosity due to the semiporous infrastructure of individual shale particles). Thus, one would expect the internal porosity to be greater for the combustion retorted sample than for the inert gas retorted sample. An extremely important characteristic of spent shale samples is their solid-phase organic carbon content, as expressed on a percent by weight basis. The solid-phase organic carbon content of the type 1 spent shale was 0.2% or 2 mg/g, while that of the type 2 spent shale was 1.8%or 18 mg/g. It is interesting to note that the type 1 spent shale contained substantially less organic carbon than the type 2 spent shale. Intuitively, one would expect the type 1 spent shale to contain less organic carbon since it was produced during combustion retorting in which residual organic material associated with the shale matrix was combusted to provide heat for pyrolysis.
Experimental Laboratory Leaching Experiments. Two types of laboratory experiments were required to generate data for a mass transfer analysis: continuous-flow column experiments and equilibrium batch experiments. In the continuous-flow experiments, 2.5 cm diameter columns of 15and 30 cm lengths were each filled and compacted with a particular type of spent shale. After compaction, peristaltic pumps were used to pump 20 O C distilled water through each packed column a t a flow rate of 0.6 mL/min for a 96-h time period. Time-averaged composite samples of leachate were collected in an enclosed container at designated time intervals. These composite samples were subsequently analyzed for total organic carbon (TOC), an indicator of the total organic matter present in the leachate. The equilibrium batch experiments involved placing a constant mass of spent shale into a series of flasks and subsequently adding a variable volume of distilled water to each flask. After initial preparation, each flask was sealed and allowed to sit quiescently at 20 O C for a period of 30 days. At the end of 30 days, the leachate was decanted and analyzed for total organic carbon (TOC). Data derived from the equilibrium batch experiments enables development of equilibrium isotherms. It should be noted that the rationale for selecting 832
Environmental Science & Technology
Results and Discussion The results of the continuous-flow column experiments are presented in Figure 1. As seen in Figure 1,a smooth “best fit” curve was drawn through each group of data points. (Initially, there were several attempts a t mathematical curve fitting, resulting in curves which, in the opinion of the authors, did not accurately portray data trends.) These results suggest that the rate at which organic material is leached was greatest
Type 1 Spent S h a l e 3 0 c m column length
0 1 5 c m column length
T y p e 1 Spen’ Shole
Type 2 Spent S h a l e 3 0 c m column IengTh
CI
A
1 5 c m column length 0
”tdI I
5 hours
A 7 5 hours
3 1 C boas
a
I123 t o d i s
€Z 2 5 nours
‘ 5 hours
3 TOC Irrg/ll
lot
2
C TOC img/l 1
3c
15
2 Column Length I c w l
Flgure 3. TOC concentration vs. column length: type 1 spent shale
Time t ihoursl
Figure 1. TOC concentration vs. time
z Calimn
LengtP I C I P J
Figure 4. TOG concentration vs. column length: type 2 spent shale
$
17.94
B(g%)
Table 1. Results of Equilibrium Batch Experiments type 1 spent shale
2 .oo
+
”I/
,g l! 1.95
0
TOC, mgJL
50g+30mL 50g+50mL
18 11 5.0
50 g 50 g
1.97
1.9L’
shale water mlxture
“
10
”
”
20
I
30
’
LO
’
J
50
C Irng/l)
Figure 2. Estimated equilibrium isotherms during initial periods of leaching. During subsequent time periods, the rate decreased and TOC concentrations approached a constant rate which is controlled by one or more mass transfer mechanisms. Data derived from the equilibrium batch experiments are presented in Table I. These data, in conjunction with initial solid-phase organic carbon measurements, were used to develop equilibrium isotherms. Equilibrium ‘isotherms, describing the desorption/leaching phenomenon for type l and type 2 spent shale, are presented in Figure 2. An equilibrium isotherm is defined as a relationship between the mass of solute per unit mass of sorbent and the equilibrium concentration of the solute remaining in solution at constant temperature (IO). In Figure 2, the parameter C is defined as the equilibrium TOC concentration in leachate, expressed in terms of milligramdliter, while the parameter q is defined as the equilibrium solid-phase organic carbon remaining on the spent shale per unit mass of spent shale (expressed in terms of mg of TOC/g of spent shale). It should be recognized that
+ 100 mL + 200 mL
TOC leached from shale, mg/100 g
3.9
1.1 1.1 1.o 1.6
type 2 spent shale TOC, mgJL
25 19 11 5.4
TOC leached from shale, mg/100 g
1.5 1.9 2.2
2.2
the isotherms presented in Figure 2 represent “calculated” isotherms derived from mathematical modeling of experimental data. Experimental data was modeled according to both the Freundlich and Langmuir models. For a detailed discussion of the development of the equilibrium isotherms presented in Figure 2 , see ref 2 . Plots of TOC concentration vs. column length, as shown in Figures 3 and 4, were developed by extracting relevant information from the plots of TOC concentration vs. time. These plots of TOC concentration vs. column length represent “concentration profiles”, describing the instantaneous concentration throughout the entire length of a 30-cm column at a designated time. Mass Transfer Analysis
Theory. The leaching phenomenon involves the mass transfer of organic compounds from a solid phase (spent shale) to a liquid phase (leachate). The sequence of steps involved in the overall mass transfer process includes (1) desorption of water-soluble organic compounds from an intraparticle surface, (2) diffusion of desorbed molecules through an intraparticle liquid surface film and into liquid contained within the pores and interstices of the shale matrix, (3) transport of desorbed molecules from internal pores to the exterior of the shale matrix, and (4)diffusion of molecules through a liquid Volume 14, Number 7, July 1980 833
surface film situated along the exterior of the shale matrix and into the bulk leachate (11). The overall rate of mass transfer during leaching is a function of one or more of the following mechanisms: (a) internal diffusion, (b) surface reaction of the desorption process at the liquid-solid interface, and (c) external mass transfer. Although two or more of these mechanisms may simultaneously limit the rate of mass transfer, only one of these mechanistic steps controls the overall rate in many cases (12). A rigorous mathematical model for leaching in a packed column requires (a) a mass balance on the component being transferred over a finite volume of the column, (b) a relationship for the equilibrium distribution of the component between liquid and solid phases (Le., an equilibrium isotherm), and (c) an expression for the rate of leaching. The first of the above steps, a mass balance over a finite volume, will yield a partial differential equation which can theoretically be integrated if both the equilibrium isotherm and the rate of leaching are mathematically evaluated. This differential equation, which has been derived elsewhere (2, 12-18), is presented as follows: pA
(z),
+
= F [E) dz t A
($)
where F = water flow rate (L/h); A = cross-sectional area of column (cm2);z = column length (cm); p = bulk density of spent shale in packed column (g/cm3); t = time (h); C = TOC concentration (mg/L); q = mass of solid-phase organic carbon per unit mass of spent shale (mg/g); d q l d t = rate of leaching of organic carbon (mg/(g.h)); dC/dz = rate of TOC concentration change with respect to column length (mg/(L.cm)); dC/dt = rate of TOC concentration change with respect to time (mg/(L-h)).The above expression can theoretically be integrated if the equilibrium isotherm and the rate of leaching can be mathematically evaluated. However, this integration is extremely difficult, involves various simplifying assumptions, and usually requires a computer-assisted solution. Owing to the complexity involved in integrating the above expression, this paper will attempt only to assess the rate of leaching by employing several kinetic rate models. Thus, in relation to this paper, the most important parameter in the differential mass balance equation is the term ( d q l d t ) , , the rate of leaching. As previously stated, one of the following mechanisms may be rate limiting: (a) external mass transfer; (b) surface reaction; or (c) internal mass transfer. A precise mathematical evaluation of ( d q / d t ) , would take into account all of the above mechanisms. However, in order to simplify the mathematics involved, it is often advantageous to assume that one particular mechanism is rate controlling over (a) the entire time period investigated or (b) certain ranges of time within the entire time period. Several rate models have been postulated in the literature to describe the rate of adsorption. These models can also be applied to the leaching phenomenon and are summarized below, classified according to the rate-controlling mechanism. External Mass Transfer. If external mass transfer is the rate-controlling mechanism, the rate of leaching can be described by (12): (dqldt), = KE(C
- C*)
(2)
where ( d q l b t ) , = rate of leaching (mg of TOC/g of shale-h), K E = external mass transfer coefficient (L/g-h); C = actual measured TOC concentration in bulk leachate (mg/L); C* = theoretical TOC concentration in equilibrium with actual solid-phase organic carbon on shale particles (mg/L), as estimated from equilibrium isotherm. In the external mass transfer model, the expression (C - C*) represents the driving force for mass transfer. 834
Environmental Science & Technology
Internal Mass Transfer. If internal mass transfer is the rate-controlling mechanism, the following rate model, known as the Glueckauf model, can be employed to describe the rate of leaching (12): (3)
where K G = Glueckauf rate constant (h-l), q* = theoretical solid-phase organic carbon on shale particles in equilibrium with actual measured TOC concentration in bulk leachate (mg of TOC/g of shale) as estimated from equilibrium isotherm, and q = actual solid-phase organic carbon on shale particles (mg of TOC/g of shale). In the Glueckauf model, the parameter (q* - q ) represents the driving force for mass transfer. Surface Reaction. The following expression, known as the Bohart and Adams model, can be used to describe the rate of leaching when the rate-limiting mechanism is surface reaction (13): (4) where KBA= Bohart and Adams rate constant (L/mg-h);q m = maximum potential solid-phase organic carbon on shale particles (mg of TOC/g of shale), as estimated from equilibrium isotherm. In the Bohart and Adams model, the parameter (9, - q ) represents the driving force for mass transfer. Application. The various rate models along with descriptions of the parameters required to test these models are summarized above. The procedure for evaluating the rate models consists of first estimating all parameters in each rate model except the rate constant and then solving for the rate constant. For each model, the rate constant is then estimated at several different values of time and the resulting estimates compared to see if they approach a constant value. The parameters which must be estimated to enable an assessment of the various rate models include ( d q l d t ) , , (dC/ B z ) ~ ,(dC/dt),, q, q,, q*, C, and C*. Estimates of these parameters are derived from analysis of (a) the TOC concentration vs. time curves presented in Figure 1,(b) the equilibrium isotherms presented in Figure 2, (c) the “concentration profiles” presented in Figures 3 and 4, and (d) the general differential equation presented as Equation 1. In this paper, the mass transfer analysis was based specifically on a column length of 15 cm (i.e., z = 15 cm). A similar analysis can be conducted for other column lengths, as demonstrated in ref 2 . Each of the parameters specified in the various rate models was estimated for eight different values of time ranging from 5 to 40 h. A lengthy compilation of the actual parameter estimates can be found in ref 2 , along with a detailed discussion of the methodology employed for estimating each parameter. Estimates of rate constants for each model are presented in Table 11;estimates are provided for time periods ranging from 5 to 40 h. An examination of Table I1 suggests that, for type 1 spent shale, the most likely rate-controlling mechanism is internal diffusion. The rate model which assumes that internal diffusion is rate limiting, the Glueckauf model, yielded fairly constant values for KG during a time period ranging up to and including 30 h. During this time period, values of K G ranged from 0.052 to 0.078 h-l. After 30 h, the value of K G dramatically increased, suggesting that either (a) internal mass transfer was no longer limiting or (b) two or more mechanisms simultaneously controlled the rate of leaching after 30 h. The values of K E presented in Table I1 indicate that a t no time did external mass transfer control the rate of leaching from type 1 spent shale. Furthermore, Table I1 reveals that values of KBAvaried significantly except for later time periods. During later time periods, the trend in data indicates that
Table II. Estimates of Rate Constants spent shale sample
5
7.5
10
the, h 20
15
25
30
40
av
Estimates of KG (Internal Oiffusion Rate Constant) for Column Length of 15 cm (Units: h-’) type 1 type 2
0.062 0.030
0.058
0.024
0.056 0.019
0.052 0.018
,
0.054 0.016
0.064 0.014
0.078 0.012
0.170 0.013
0.074 0.018
Estimates of KE(External Mass Transfer Rate Constant) for Column Length of 15 cm (Units: L/(g.h)) type 1 type 2
0.000 008 0 0.000 001 8
0,000015 0 0,000002 1
0.000027 0 0.000002 2
0.000090 0 0,000250 0 0.000003 7 0,000005 7
0.000470 0 0.000009 2
0.000820 0 0.000015 0
0.002 900 0 0.000580 0 0.000040 0 0.00001 1 0
Estimates of KBA(Surface Reaction Rate Constant) for Column Length of 15 cm (Units: L/(mg.h)) type 1 type 2
-0.080
-0.050
-0.061 -0.035
-0.048 -0.027
-0.036 -0.025
values of KBA were approaching a constant near the end of the time period investigated. This suggests that (a) surface reaction may have become the rate-controlling mechanism after 30 h or (b) both internal diffusion and surface reaction simultaneously may have controlled the rate of leaching at later time periods, However, the trends in data are not strong enough to substantiate either of the above claims. Overall, it is hypothesized that internal mass transfer is the rate-controlling mechanism for type 1 spent shale. This is likely due to the high internal porosity of the type 1 spent shale. As previouarly reported, its total bulk porosity was 57%, and its internal (Le., intraparticle) porosity was probably a significant portion of the total bulk porosity. As a consequence of its high internal porosity, leached material must diffuse through a “maze” of internal pores and interstices prior to appearing in the bulk leachate. In contrast to the results derived from type 1spent shale, the results presented in Table I1 for the type 2 spent shale reveal that none of the proposed rate models accurately describes the rate of leaching over the entire range of time considered. Although inconclusive, the data suggest that internal mass transfer may have become rate limiting during later time periods after 10 h of leaching. During the period of 10 to 40 h, values of KG ranged from 0.013 to 0.019 h-l. An examination of Table I1 indicates that values of KBA, the rate constant for the surface reaction model, were relatively constant during the latter time periods of leaching. After 10 h of leaching, values of K B Aranged from -0.019 to -0.027 L/(mg.hr). These data suggest that surface reaction may have served as a rate-limiting step during subsequent time periods. At first glance, the values of K E presented in Table I1 suggest that external mass transfer was not a rate-controlling mechanism. How(ever,close scrutiny of the data suggests that external mass transfer may have been initially rate controlling during the first 10 h over which values of K E remained fairly constant. If external mass transfer was indeed the rate-controlling step during the first 10 h, this is likely due to the high concentration of TOC in the bulk leachate during the initial leaching period, serving as an external resistance to mass transfer. Although results for type 2 spent shale are inconclusive, the following hypotheses are presented, although further experimentation is needed to verify these hypotheses. Overall, for time periods greater than 10 h, it is hypothesized that leaching from type 2 spent shale was controlled by (a) internal diffusion, (b) surface reaction, or (c) a combination of internal diffusion and surface reaction acting simultaneously. Conversely, it is hypothesized that external mass transfer controlled the leaching process during the first 10 h.
-0.036 -0.022
-0.027 -0.022
-0.024 -0.027
-0.020 -0.019
-0.041 -0.028
S u m m a r y and Conclusions The mass transfer analysis revealed that, for combustionretorted spent shale, internal diffusion is the most likely rate-limiting mass transfer mechanism. Although inconclusive, results for inert gas-retorted spent shale suggest that internal diffusion plays a key role in the leaching phenomenon. Internal diffusion is an extremely slow mass transfer process that may result in low, relatively constant levels of organic pollutants appearing in groundwater and persisting for long periods of time. The leaching phenomenon represents a potentially chronic pollution problem that may irreversibly impair water quality and preclude beneficial uses of groundwater in areas of in situ oil shale development. Acknowledgments Lawrence Livermore Laboratory supplied the samples of spent shale. Literature Cited (1) Schmidt-Collerus, Josef “The Disposal and Environmental Effects of Carbonaceous Solid Wastes from Commercial Oil Shale Operations”; Denver Research Institute, 1974. (2) Amy, Gary L. Ph.D. Thesis, University of California at Berkeley, 1978. (3) Sladek, T. A. Colo. Sch. Mines, Min. Ind. Bull. 1974,17, 1. (4) Weeks, J. B. US.Geol. Suru. Prof. Pap. 1974, No. 908. (5) Margheim, Gary Ph.D. Thesis, Colorado State University, 1975. (6) Ward, J. C.; Margheim, G. “Water Pollution Potential of Spent Oil Shale Residues”; Washington, D.C.: EPA, 1971. (7) Parker, Harry “Simulated Groundwater Leaching of in Situ Retorted or Burned Oil Shale”; Texas Tech University, Lubbock, Tex., 1976. (8) Jackson, L. P.; Poulson, R. E.; Spedding, T. J.; Phillips, T. E.; Jensen, H. B. Proc. Colo. Sch. Mines Oil Shale Symp. 1975, 105-134. (9) Sandholtz. Willis Lawrence Livermore Laboratorv. Dersonal communication, 1977. (10) Weber, Walter “Phvsicochemical Processes for Water 0ualitv .~ Control”; Wiley-Interscience: New York, 1972. (11) Sherwood, Thomas; Pigford, Robert L.; Wilke, Charles R. “Mass Transfer”; McGraw-Hill: New York, 1975. (12) Rimpel, Auguste; Camp, David T.; Kostecki, John A.; Canjar, Lawrence N. Chem. Eng. Prog. Symp. Ser. 1967, No. 74. (13) Kostecki, John Ph.D. Thesis, Carnegie Institute of Technology, Pittsburgh, Penn., 1964. (14) Camp, David Ph.D. Thesis, Carnegie Institute of Technology, Pittsburgh, Penn., 1963. (15) Klotz, Irving Chem. Rev. 1946,30, 241. (16) Antonson, C. R.; Dranoff, J. S. Chem. Eng. Prog. Symp. Ser. 1969, No. 96. (17) Rosen, J . B. Znd. Eng. Chem., 1954,46, 1590. (18) Rosen, J. B. J. Chem. Phys. 1952,20, 387.
