Environ. Sci. Technol. 1991, 25, 1578-1584
Efficiency of Capping Contaminated Bed Sediments in Situ. 1. Laboratory-Scale Experiments on Diffusion-Adsorption in the Capping Layer X. 0. Wang,+ L. J. Thibodeaux," K. T. Valsaraj, and D. D. Reible
Hazardous Waste Research Center and Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803-7303
A laboratory study was performed on the transport of the hydrophobic organic compound, 2,4,6-trichlorophenol (TCP), through a layer of clean sediment placed over contaminated sediment inoculated with TCP. A capped simulator cell (CSC) was designed for the study and four capping materials of 0.7-cm depth with different organic carbon contents and textures were used. A conceptual model of TCP fate combining molecular diffusion in interstitial porewater and partitioning onto fresh sediment was developed to assess the data. TCP breakthrough times and effective diffusivities for the various capping materials gave good comparisons with theoretical predictions. W
Introduction The existence of contaminated bed sediment in a natural aquatic system usually results in the overlying water column receiving some contamination that eventually leads to multimedia environmental exposures. Capping contaminated sediment with clean sediment to reduce the ecological impact has been utilized on an experimental basis by the New England Division, New York District, and the US. Army Corps of Engineers. Monitoring of capped disposal sites has shown that capping is technically feasible and stable under normal tidal and wave conditions (1,2). However, field studies have been inconclusive regarding the efficiency of capping to prevent contaminant uptake by water column and organisms (4-6). Before capping of contaminated sediment becomes a practical alternative technology for isolating pollutants from the environment, the physical, chemical, and biological impacts of capping must be better understood. The prime concern about the acceptability of capping as a chemical isolation method is its efficiency in retarding the movement of toxic chemicals to water and to pelagic and benthic biota. Several works have addressed this concern (3-6). Brannon et al. (5) developed an experimental methodology for testing the effectivenessof capping contaminated dredged material by using nonsorbing chemicals such as oxygen, ammonia, and orthophosphate phosphorus as tracer compounds. For hydrophobic pollutants, transient movement in pore water is highly retarded by sorption and therefore dependent on the partition constant between the aqueous and sediment phases. The rate of chemical movement via porewater may be attenuated several orders of magnitude relative to nonsorbed chemical species. For these types of chemicals, transport in the sorbed state (via sediment transport) may become quite significant (7). Theoretically, the high adsorption property of hydrophobic compounds onto sediment should play virtually no role in the steady-state transport through a cap material. However, this property does play a significant role in the approach to the steady-state flux and associated breakthrough time. This property of hydrophobic organic Present address: Environmental Engineering Research and Development Center, University of Petroleum, P.O. Box 902, Beijing 100083, China. 1578
Environ. Sci. Technol., Vol. 25, No. 9, 1991
compounds was not tested, although theoretical models have been proposed that may be used for predictive calculations of chemical breakthrough times and pollutant fluxes through the cap (8). In addition, no data have been published that provide a direct comparison of the leaching behavior of a sediment contaminated with a hydrophobic substance under capped and uncapped conditions. Figure 1is a simplified sketch that illustrates the original sediment (a) and the capped sediment (b). Figure l a shows the three zones in the bed that regulate the contaminant release process before capping. On the water side just above the sediment-water interface is the benthic boundary layer (9). Immediately below is the layer of bioturbation, and beyond is the molecular diffusion layer. Figure l b presents the same situation after capping except that the bioturbation zone is eliminated at the original sediment-water interface and moves upward into the top of the capping layer. In this study the transport of 2,4,6-trichlorophenol (TCP) through capping layers with different organic carbon contents and textures was investigated in a continuous-flow capping simulator cell (CSC). The studies were conducted in the absence of bioturbation and advection. Water flow during compaction of fine-grained sediments is very slow and in sands it is almost nonexistent (IO). Although in the field groundwater flow and local pressure variations on the sediment surface cannot be neglected because of advection and dispersion processes due to flow, they are not a part of this study. Under these circumstances, at depths below the zone of bioturbation, the only transport process of importance is molecular diffusion. The transport mechanisms and transport models that apply in the cap are identical with those that apply in the original contaminated bed sediment. Fick's first and second laws are appropriate for the steady-state and transient processes, respectively. The primary objectives of this study were (a) to develop a laboratory device capable of simulating some basic features of contaminated bed sediment capping, (b) to obtain experimental evidence of the efficiencies of a clean sediment cap in retarding hydrophobic organic transport, and (c) to test some theoretical principles of chemical transport in bed sediment as it applies to the capping process.
Experimental Section Capping Materials Selection. Four capping materials chosen for the study provided a range of organic carbon contents associated with compositional differences; no effort was made to collect samples that would characterize a particular source or geographical region. The University Lake sediment was obtained from the lake by the same name on the Louisiana State University campus in Baton Rouge. The Tao River sediment sample was obtained from the bed of the Yellow River in the Peoples Republic of China. The quartz sand was obtained from a local supplier of aggregates. The balsam sand is a proposed cap material from a quarry near New Bedford, MA. Characteristics of Sediments. The physicochemical characterization of the four sediments revealed a broad
0013-936X/91/0925-1578$02.50/0
0 1991 American Chemical Society
BENTHIC BOUNDARY LAYER
AIR WATER
\
/
ORIGINAL S/W INTER FACE
(a) Uncapped C o n t a m i n a t e d Sediment AIR c
WAT E R BENTHIC BOUNDARY LAYER
\
-NEW
S/W
(b) Capped W i t h Clean Sediment Flgure 1. I n situ containment of contaminated bed sediment.
Table I. Sediment Characteristics sediment
organic carbon, %
sand
composition, w t % silt
clay
porosity
bulk density, g.cm-*
Kp,”Lqkg-’
UL TR QS BS
1.73 0.26 50 pm), coarse silt (50-20 pm), medium silt (20-5 pm), fine silt (5-2 pm) and clay (> 1 for hydrophobic organics, Dt < DEE. The time t it takes a chemical species in transient transport to achieve a certain concentration C at a point x into an initially clean semiinfinite slab with a constant surface concentration Co is given by (8)
r
The semiinfinite geometry aspect of the bed is obviously violated if eq 4 is used for a finite cap thickness, L. Equations of this form are convenient however for developing correlations to estimate the time of chemical breakthrough in the cap, tB,and the time required to reach a steady-state chemical flux through the cap, tE8.
Results and Discussion Linearity of Isotherm and Effect of Particle Size. The adsorption data for TCP on all sediments showed a linear behavior over a broad range of aqueous-phase concentrations (Figure 4). That is (23-26) Kp = cs/cw (5) where C, is the concentration of TCP on the sediment relative to dry weight (mgkg-'1, C, is the equilibrium aqueous concentration of TCP (mgL-'), and Kp is the partition constant (L-kg-l). The sorption appeared reversible in all systems. Linear least-squares fits of the isotherm gave intercepts near zero in comparison to the scale of the ordinate.
2
0 0
'0
CONCENTRATION
I N WATER, mg/l
Figure 4. Sorption isotherms for TCP on various sediments. Balsam ( Boston ) 0 Quartz Sond
I /
;E-03
I E-02
IE-01 IE +00 P A R T I C L E D I A M E T E R , mm
IEtOI
Flgure 5. Size distribution for various sediments.
Organic carbon contents and textures associated with the sediment particle size distribution were the two sorbent properties investigated. For hydrophobic chemicals it has been shown that the value of K is a function of the organic carbon content, f,, of the se&ment (23). The partition constants can be normalized as
Kp = K d ,
(6)
where K , is the organic carbon based partition constant for the chemical. Correlations are available that relate K , to the octanol-water partition constant, KO,. For polynuclear aromatic compounds, including some chlorinated ones on sediments, we have (27) log K, = 0.921 log KO,- 0.23 (7) Using eqs 6 and 7, we found close agreement between the predicted and measured K p for the University Lake sediment (25.6 versus 27.5 Lskg-l) and for Tao River sediment (3.8 versus 4.6 Lakg-l), whereas predicted and measured values differed considerably for the Balsam sediment (6.1 versus 0.98 L-kg-l). The organic carbon content for balsam sediment is higher than that of Tao River sediment but shows a lower Kp for TCP. In the case of sediments with low organic carbon contents, the specific surface area and the nature of the mineral surfaces have a greater impact on the degree of sorption. Schwarzenbach and Westall (28) found that over 85% of sorption takes place on the size fraction 4 < 125 pm. From the results shown in Figure 5 it is clear that the size fraction 4 < 125 pm for Balsam sediment is less than 20% in contrast to more than 90% for Tao River sediment. The effect of particle concentration on TCP partition constant was not explored. Studies on naphthalene (32) showed that at a very high particle concentration up to bed sediment concentration (10000-1 000000 mgL-'), K , was Environ. Sci. Technol., Vol. 25, No. 9, 1991
1581
25 C A =~ l $ O r n g / l 0 Uncapped Contamlnoted Sedlmenl
I
.
" E
UL)
B
5
Y N
P
-
15-
LL
c'
0
*3
IO
-
T I M E , day
Figure 8. Flux versus time for contaminated sediments capped with BS and TR sediments.
i i 5 -
0'
1
4
k
e
Ib 1' 2
Ib
I 6 ;I 20 2 1 2: T I M E , day
26 2 6 36 3'2 34 3;
Flgure 8. Flux versus time from the exposed surface of uncapped contaminated sediment.
?
'I
C A ~ =2 5 0 m 9 / 1
Cap Depth = 0 . 7 c m Cho= 1 5 0 m g / l A Balsam Sediment 0 Taa River Sediment U L Sediment
0 Quartz Sand A U n i v e r r i t y Loke Sediment
T I M E , day
5
Figure 9. Flux versus time for contaminated sediments capped wtth QS and UL sediments.
TIME, day
Fbure 7. Flux versus time for different cap materials for the first few days.
constant. We used a relatively high value, 100 000 mgL-', for the evaluation of Kp in order to represent the in-bed conditions. Capping Efficiency Studies. The flux of TCP through the cap was the primary measurement taken during the simulation experiments. The flux F, in eq 1 is directly proportional to the concentration of TCP in the aqueous stream leaving the capping simulator cell. The data presented in terms of the flux F, (mgcm-2-s-') as a function of time is shown in Figures 6-9. The discussion that follows is divided into two sections. The first is concerned with the measured flux and what the experimenta revealed about the capping process. The second is concerned with testing the theoretical transport models for chemical breakthrough time, tg, time to steady-state flux, tw, and the steady-state flux equation. The effect of capping can be observed by comparing the flux curves with and without caps. Comparing Figures 6 and 7, it is obvious that capping drastically reduced flux of TCP to the water. The average flux of TCP for uncapped contaminated sediment was 18.4 mgcm-2.s-1 for the first day of leaching, whereas they were 3.3 and 0 mgcm-2.s-1 when capped with balsam sediment (f, = 0.35%) and clean University Lake sediment (f, = 1.73%), respectively. The shape of the curve in Figure 6 for the uncapped contaminated University Lake sediment should be con1582 Environ. Sci. Technoi., Voi. 25, No. 9, 1991
trasted with those of Figures 8 and 9 capped with clean balsam sediment (BS),Tao River sediment (TR), quartz sand (QS), and clean University Lake sediment (UL). The uncapped situation clearly displays the molecular diffusional transport of TCP as has been observed in similar cases (29). The initial flux is very large because the TCP concentration gradient is large at the sediment-water interface. Rapid loss reduces the mass of TCP in the upper layers and correspondingly the concentration gradient, reducing the rate thereafter. Theoretically it should fall proportional to t-1/2(see eq 3 of ref 8). In those cases with clean sediments a distinct maximum occurs in the elution curves, as is evident in Figures 8 and 9. Initially, due to the cap, the flux of TCP is zero. Quantities move from the contaminated underlayer into the clean cap layer with TCP sorption on the clean sites. Eventually, breakthrough occurs and the rate increases with time as the sorption capacity for TCP in the cap material is exhausted. At this time the flux reaches steady state and a maximum value. In time the flux falls again. It decreases because of the depletion of the source mass of TCP in the contaminated bed. The flux should fall proportional to t-'I2. The organic carbon content of the cap has the most significant effect on the time of chemical emergence. Figures 8 and 9 reveal that those caps with the lowest organic carbon content were the least effective in retarding the rate of breakthrough. Achievement of the maximum TCP flux occurred within 4 days for QS, BS, and TR sediments. A much larger time, approximately 4 weeks, was required for the UL sediment. The organic carbon content for the UL sediment was 1.73%, while it was less than 0.4% for the other three sediments (Table I). It appears to take twice as long for TCP to move through the TR sediment as it does for the BS sediment. The greater sorptive capacity of the TR sediment would suggest about 4 times slower. Greater sorption may be partially offset by the higher porosity of the TR sediment (0.5versus 0.38 for BS). Time for chemical emergence has been used in a very general sense to this point and a more precise definition is needed if a theoretical basis for capping is to be developed.
Table 11. TCP Breakthrough Time and Time to Steady State' sediment breakthrough time, t b calcdb measd time to steady state, t, calcdc measd
BS
time. h QS TR
UL
7.1 5
9.3 6
68.8 48
49 48
70 63
11.7 11
77 80
Table 111. Steady-State Diffusion Coefficients, D,, time, days
467 650
'The depth of the capping layer for QS was 0.6 cm. The rest were 0.7 cm. Calculated from eq 8. Calculated from eq 9.
The time for TCP breakthrough and the time required for achieving a steady-state flux can be determined from data such as shown in Figure 7. Here the flux of TCP through the cap is displayed during the first few days. Experimental breakthrough time, tg, is defined as the measured flux that is 5% of the steady-state flux or the maximum flux. Time to achieve steady-state flux, t , is defined as the measured flux that is 95% of the steadystate flux or the maximum flux. It should be noted that the maximum flux displayed on the flux versus time graphs is selected as the best representative of the steady-state flux. In fact a steady-state flux is difficult to maintain with very thin caps because they provide scant resistance to TCP transport and the concentration of TCP in the contaminated source begins to fall, yielding constantly decreasing fluxes as shown in Figures 8 and 9. On the other hand, thicker caps retard the emergence times and produce very low TCP concentrations at steady state, introducing analytical difficulties. A 0.6-0.7-cm cap appears to be the best compromise when working with TCP on the selected sediments. TCP breakthrough time, tg, and times to achieve steady-state flux through the cap, t,,, are presented in Table I1 for the four sediments. Both measured and calculated values are shown. The measured values follow the trend predicted by eq 4; both tBand t, being inversely proportional to D,.From eq 3, D,is inversely proportional to K and hence the tB and t,, values should directly correfate with Kp. Comparing the values in Tables I and I1 reveals this correspondence exactly. For the solution to Fick's second law for a finite layer, the ratio of the flux at any time to the steady-state flux is given by x(t-@) This solution assumes that the contaminated sediment concentration is essentially constant, the overlapping water concentration remains essentially zero, and the capping layer is initially free of contamination. The time to breakthrough and steady state can be determined from eq 8 by setting the flux equal to 5% and 95%, respectively. For the time to steady-state flux, only a single term in the infinite series is required and the time can be written as
The above equation shows that the time to steady state is proportional to the square of the depth of the cap and inversely proportional to the transient retarded diffusivity. This calculation was performed and the results appear in Table I1 along with the measured values. The agreement
4 5 6 7 8 9 10 11 12
13 14 15 av D, (theor) D, (theor)
QS
22.9 22.2 20.9 21.3 22.0 21.7 20.4 20.5 20.7 20.4 20.7 19.9 21.1 19.3 5.91
D,, X lo7 c m 2 d TR BS 27.4 27.0 26.0 23.5 22.9 22.3 23.8 24.1 24.1 19.8 19.4 20.0
23.4 28.8 6.63
22.8 23.3 21.4 19.2 19.1 19.6 19.8 19.2 18.6 15.8 15.9 15.7 19.2 20.0 10.5
UL' 23.5 23.3 23.5 22.8 23.9 26.2 27.4 25.8 23.7 25.1 23.7 22.6 24.4 25.0 1.09
"Time in this column is day 28-39.
between the calculated and measured results is generally very good. Breakthrough time estimates appear to be more scattered presumably due to transients that might occur early during the experiments. The time to steady-state flux is estimated quite well by eq 8 except for the UL sediment, for which both the capped and contaminated sediments are identical. In this experiment the time scale for depletion of the contaminated sediment is the same as that for transport into the cap, invalidating the assumption of a constant concentration in the sediment layer. The steady-state flux of TCP through various cap materials is similar and independent of Kpfollowing 3-4 weeks of testing (Figures 8 and 9). This is in agreement with theory, because the flux expression, eq 2, is K , independent. Once all sorption sites in the cap material are saturated, the same quantity of TCP entering the cap leaves the cap and a steady-state flux is achieved. This occurs in 48 h for BS, 63 h for QS, 80 h for TR, and 650 h for UL. These are the measured t,, values reported in Table 11. From the flux measurements it is possible to compute D,,, the observed values of the steady-state transport coefficient from eq 2
D,
= F,L/(C, - C )
(10)
The values appear in Table 111. Run times of day 4-15 were chosen as the steady-state flux period for QS, TR, and BS and day 28-39 were chosen for the UL sediment cap. These periods represent maximum and/or nearconstant flux values beyond the initial transient period and before the falling rate period. Also appearing in Table I11 are the theoretical values of D, computed as Dt4I3. D = 7.24 X lo4 c m 2 d was used for TCP in water at 25 "C. The correspondence between the average experimental values of D,, and the theoretical values is good. The effective transport coefficient is independent of Kp. This is to be expected since only the transient transport coefficient is dependent on Kp (see eq 3). Table I11 also contains this coefficient computed by using eq 3. The observed D, values do not follow the trend of the theoretical Dt values. The effect of bed thickness on chemical emergence times and steady-state flux was not investigated. It is a very important parameter of choice when capping is considered for a particular site. There is considerable evidence to support the L2 dependence in chemical emergence times (eq 9) and an L-' dependence in the steady-state flux (eq Environ. Sci. Technol., Vol. 25, No. 9, 1991
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Table IV. NHIN Flux at Steady State (3) cap thickness L, cm 0 2 4 6 8 10 12 14 18 22 26
flux, F,, mgm-2.day-1 310 180 140 120 80 50 30 20 25
10 0
F A mgcm-m-2.day-1 360 560 720 640 500 360 280 450 220
2). Typically D t / L 2 is a dimensionless scaling parameter for concentration in a plane sheet, infinite and semiinfinite (14, 15). It has been employed in correlating laboratory data on the transport of hydrophobic chemicals both into and out of bed sediment (8,29,33). Brannon et al. (3)and Sturgis and Gunnison ( 4 ) reported on a total of six steady-state flux experiments with varying cap thicknesses. All the data plotted as flux versus cap depth displayed the flux approximately proportional to the L-l functionality of eq 4. The most complete data set was for a sand cap with NH4-N, which is summarized in Table IV. Note from eq 2 that if D,(C, - C ) is constant then the product F,L is also constant. This functionality, a characteristic of steady-state flux, is evident and the other five experiments displayed this same general trend. No measurable flux resulted for the L = 26 cm bed and the lowest flux value of the series was reported for L = 22 cm. This is not surprising. Use of eq 9 with NH3-H20 diffusivity, D = 2.49 X lom6 cm2.s-l at 25 "C (30)yields t, = 84 days for L = 22 cm and t,, = 117 days for L = 26 cm. Assuming NH,-N is a nonadsorbing species and t = 0.37 for the sand, the calculation results in very long experiment run times. In the case of L = 26 cm, the investigators may not have waited long enough.
Conclusions A capping simulator cell was developed to investigate the transport of a hydrophobic compound through clean gap layers placed over a contaminated sediment. Four cap materials including two sediments and two sands were tested, and TCP flux measurements were obtained. The flux data was used to obtain chemical breakthrough time and the time to achieve steady-state flux. The times to achieve steady state and breakthrough through the cap were directly dependent on the equilibrium partition coefficient, K,. The larger the K,, the longer the time required for chemical emergence from the cap. The steady-state flux through the four cap materials was independent of Kp' Cap porosity and depth were the dominant parameters at steady state. A theoretical algorithm for the transport coefficient agreed very well with those extracted from the flux and concentration gradient data. Registry No. TCP, 88-06-2.
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1584 Environ. Sci. Technol., Vol. 25, No. 9, 1991
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Received for review June 4,1990. Revised manuscript received March 25, 1991. Accepted May 1, 1991.
