Environ. Sci. Technol. 1988, 2 0 , 483-490
search Triangle Park, NC, Dec 3-4, 1984; pp 45-76. Scheff, P. A.; Wadden, R. A. In “Proceedings of the APCA International Specialty Conference on Receptor Methods for Source Apportionment”;Williamsburg, VA, March, 1985; in press. Sexton, K.; Westberg, H. Atmos. Environ. 1983, 17, 467. Sexton, K.; Westberg, H. Environ. Sci. Technol. 1980,14, 329. Wetherold, R.; Provost, L. “Emission Factors and Frequency of Leak Occurrence for Fittings in Refinery Process Units.” 1980, EPA Report EPA-60012-79-044. Turner, D. B. “Workbook of Dispersion Estimates”. 1970, PHS Publication No. 999-AP-26 (NTIS PB-191482). Gifford, F. A. Atmos. Environ. 1982, 16, 883. Gunst, R. F.; Mason, R. L. “Regression Analysis and Its Applications: A Data-Oriented Approach; Marcel Dekker: New York, 1980. Henry, R. C. In “Receptor Models Applied to Contemporary Air Pollution Problems”; Air Pollution Control Association: Pittsburg, PA, 1982. Watson, J. G.; Cooper, J. A.; Huntzicker, J. J. Atmos. Environ. 1984, 18, 1347.
Registry No. Ethane, 74-84-0;ethylene, 74-85-1;acetylene, 74-86-2;propane, 74-98-6;propylene, 115-07-1;isobutane, 75-28-5; n-butane, 106-97-8; isopentane, 78-78-4; n-pentane, 109-66-0; 2-methylpentane, 107-83-5;3-methylpentane,96-14-0; n-hexane, 110-54-3; benzene, 71-43-2; toluene, 108-88-3; ethylbenzene, 100-41-4;p-xylene, 106-42-3;m-xylene, 108-3&3;o-xylene,95-47-6. Literature Cited Miller, M. S.; Friedlander, S. K.; Hidy, G. M. J. Colloid Interface Sei. 1972, 39, 165. Gordon, G. E. Environ. Sci. Technol. 1980, 14, 792. Kowalczyk, G. S.; Choquett, C. E.; Gordon, G. E. Atmos. Environ. 1978, 12, 1143. Scheff, P. A.; Wadden, R. A.; Allen, R. J. Environ. Sei. Technol. 1984, 18, 923. Mayrsohn, H.; Crabtree, J. H. Atmos. Environ. 1976, 10,
137. Feigley, C. E.; Jeffries, H. E. Atmos. Enuiron. 1979,13,1369. Nelson, P. F.; Quigley, S. M.; Smith, M. Y. Atmos. Environ. 1983, 17, 439. Wakamatsu, S.;Ogawa, Y.; Murano, K.; Goi, K.; Aburamoto, Y. Atmos. Environ. 1983, 17, 827. Uno, I.; Wakamatsu, S.; Suzuki, M.; Ogawa, Y. Atmos. Environ. 1984, 18, 751. Uno, I.; Wakamatsu, S.; Wadden, R. A.; Konno, S.; Koshio H. Atmos. Environ. 1985, 19, 1283. Wakamatsu, S.; Uno, I.; Wadden, R. A. In “Proceedings of the Eighth US.-Japan Conference on Photochemical Air Pollution”; U.S. Environmental Protection Agency: Re-
Received for review February 5, 1985. Revised manuscript received November 4,1985. Accepted November 21,1985. Partial support was provided by a Senior International Fellowship from the Fogarty Center, U S . Public Health Service, and by a Travel Fellowship from the World Health Organization to R.A. W.
Adsorption of Actinides by Marine Sediments: Effect of the SedimenVSeawater Ratio on the Measured Distribution Ratio Jennifer J. W. Higgo” and Lovat V. C. Rees
Department of Chemistry, Imperial College of Science and Technology, London SW7 2A2, U.K.
The effect of the solid/solution ratio on distribution ratios measured by using a standard batch technique has been investigated. Two different deep-sea sediments and the actinides americium, neptunium, and plutonium have been studied. It is shown that the apparent drop in distribution ratio with increasing sediment concentration can frequently be explained by the presence of a small amount of nonseparable or “low Rdnspecies and that by measuring the Rd at different solid/solution ratios it is possible to estimate the proportion of low Rd species present. These may be colloidal or stable complexes. Plutonium exhibited anomalous behavior with the high-carbonate sediment, and the distribution ratio dropped from >lo4 to lo2 mL/g as the solid/solution ratio increased from 2 to 20 g/L. At high sediment concentrations the plutonium in solution was mainly in the oxidized Pu(V + VI) form and it is postulated that Pu(V) or Pu(V1) carbonates were the main species present. Introduction
In experimental studies concerned with the fate of radionuclides in the environment major efforts have been devoted to the determination of the distribution coefficient, Kd, or more frequently the distribution ratio, Rd, between a solid adsorbent and natural waters. (Rd is numerically equal to Kd but is an empirical measured value. It does not imply equilibrium or reversibility, and no attempt is made to relate it to any specific sorption mechanism ( I ) . ) The values obtained are then used in two types 0013-936X/86/0920-0483$01.50/0
of model: (i) those designed to assess the suitability of various waste repositories in geological formations either on land or beneath the sea bed and (ii) models describing the movement of nuclides that have entered the environment from fallout, nuclear reprocessing facilities, or the occasional accident. This pragmatic approach is defended on the grounds that natural systems are so complicated that no other procedure is capable of providing the figures needed by the modelers. It has, however, met with limited success as the actual values obtained in the laboratory have been shown to be highly dependent upon the experimental procedure used (2). In theory, at trace loadings, the solid/solution ratio should have no effect on the distribution ratio. However, it has frequently been observed that the distribution ratio changed as the solid/solution ratio increased, and various explanations have been offered. Thus, Sanchez et al. (3) suggested that some surface sites serve as “nucleation sites” for precipitation of actinides and that an increase in the number of such sites would not necessarily increase the radionuclide removal from solution. Cremers ( 4 ) has suggested that the apparent variation in Kdwith changing solid/solution ratio can be explained in terms of changing composition of both phases. A practical explanation comes from Ames and McGarrah (5). They found not only that was it very difficult to prevent colloidal clay particles from remaining suspended in the solution phase but also that colloids consisting, at least partially, of hydrated ferric and manganese oxyhydroxides were generated during the shaking period.
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 5, 1986
483
These strongly sorbed the radionuclides being studied but remained in the solution phase after phase separation and lowered the apparent &. This theme is taken up by Sheppard et al., (6, 7) who studied the sorption of Cs, Sr, and Am on solids. They concluded that the distribution ratios of radionuclides are a “function of the soils studied and the particle size resolution technique used”. They found that for their set of soils three broad classes of particles determined the observed distribution ratios. These were (i) ionic ones, i.e., particles containing radionuclides with radii less than 1 nm, (ii) complexes of humic matter, possibly humic acid polymers with molecular weights between 8000 and 50 000 (2-3-nm radii), and (iii) larger soil particles with radii in the 10-50-nm range. O’Connor and Donolly (8)reviewed the literature covering studies involving a number of organic and inorganic solutes and sorbents and reported that “linear partition coefficients are inversely dependent upon the concentration of solids in the system”. Voice et al. (9, 10) studied their data and initiated a study to assess the effects of solids concentration on the partitioning of hydrophobic compounds. They concluded that the observed change in partitioning behavior could be attributed to a transfer of sorbing, or solute binding, material to the liquid phase during the course of a partitioning experiment. This material whether dissolved, macromolecular, or microparticulate was not removed from the solution phase during the separation procedure and was capable of stabilizing the compound of interest in solution. The importance of organic complexation has been emphasized by other workers. Thus, Nelson et al. (11) studied the effect of colloidal organic carbon (COC) and showed conclusively that plutonium associates with any COC that may be present in the water. The COC-Pu complex is poorly sorbed by the particulate matter and remains with the solution during phase separation resulting in a lowered Rd. Henrion et al. (12) found that organic material in Boom clay formed complexes with the actinides, and this lowered the measured Kd value. Clearly if distribution ratios determined by the batch method are to be of use in modeling radionuclide movement, it is essential that the situation be clarified. If necessary, models will have to deal separately with the movement of ions, polymer-size molecules, and microparticulates. We, therefore, decided to investigate the problem by undertaking, first, a mathematical analysis to determine how sensitive the measured distribution ratio would be to the presence of some nonseparable nuclide and, second, an experimental study in which we determined distribution ratios for Am, Pu, and Np over as wide a range of sediment-seawater concentrations as possible. The results of this work now follow.
Theoretical Section Experimentally it is difficult to distinguish between a “two-site’’ situation in which sorption occurs on sorption sites with different selectivities and a “two-species’’situation in which the ion in solution is present in two stable forms, e.g., one ionic and another liganded. In this analysis we have concentrated on a two-species model because we are working at very low nuclide concentrations (at which Henry’s law is obeyed) and high sediment concentrations. It is unlikely, therefore, that there will be competition for even the more highly selective sites. Furthermore, a two-species model is more appropriate for the situation in which the second species is simply particulate material that has not been removed during phase separation. An excellent theoretical study of the two-site model has been 484
Environ. Sci. Technol., Vol. 20, No. 5, 1986
made by Triolo and Lietzke (13). Calculations Showing the Effect That the Presence of Two Stable Species Has on the Measured Distribution Ratio. (i) Fixed Proportion of the Original Nuclide with a Low Distribution Ratio. In most batch-distribution experiments the nuclide in the solution phase is measured before and after sorption and the Rd calculated from the formula
where Co = initial concentration of the radionuclide in solution, C = concentration of radionuclide in solution after sorption, u = volume of liquid phase, and m = mass solid phase. Now let xC, = initial concentration of the species that is adsorbed with a high Rd (RH). Then (1- x)Co = initial concentration of the species that is adsorbed with a low Rd (RL). Let c = CH + CL where CH and CL are the final concentrations of the high and low Rd species, respectively. Then from (l),C = Co z / ( R d+ z ) where z = u / m so that CH = xCg/(RH + z ) and C L = (1- x)COz/(RL
+ 2):
and
(ii) Amount of Low R d Species Proportional to Sediment Concentration. In this case (1- x ) a m; Le., (1- x) l/z,(1- x) = k / z , and x = (z-h)/z. Substituting in eq 2 gives
c,-- c C
(Z
-
(RH+ ~ ) ( R+L2 ) k)(RL - RH) + RH
+z -1
(5)
The effect of changing the different variables in these equations is illustrated in Figures 1and 2. In each figure two sets of graphs are given, viz., log Rd vs. log (m/u)and log [(C,- C ) / C ]vs. log (m/u). The latter graphs are often more informative than the former since from the definition of Rd it can be deduced that if there is only one species present and if it is all adsorbed with the same effective Rd, a plot of log [(C, - C ) / C ] = log ( m / v )should give a straight line with gradient one and intercept log Rd [since by definition Rd = [(c,- C ) / C ) ( u / m )i.e., , log [(c,- c)/c] = log ( m / v )+ log Rd]. The presence of a second species changes both slope and intercept. Figure 1shows these graphs for the situation where there is a fixed proportion of low Rd nuclide, and Figure 2 shows what happens when the amount of low Rd nuclide is proportional to the sediment concentration. For the purpose of illustration the distribution ratio of the high Rd species (RH)is taken as lo6 and of the low Rd (RL)species as 0.1. It can be seen from Figure 1that if RH is as high as lo6, the observed distribution ratio is extremely sensitive to
Table I. Properties of Sediments CEC, SA, mequiv/ chemical composition, % m2/g 1OOg CaCOs Si A1 Fe Mn
mineralogy major
minor
10399 7K calcite 10400 8 K quartz, illite, Fe oxyhydroxides (510%) SH1578 smectite (30-40%), Fe oxyhydroxides @-lo%)
:
kaolinite, illite, smectite kaolinite, chlorite, smectite
85 190 396
17 43 124
65 3 4
9.3 3.4 1.8 0.10 24.6 8.8 5.0 0.32 12.0 2.0 14.6 4.3
a
1
\k
= 0.1
" 5b 4-
b
-6 0
-5
n
$1
0
-3-
8
_I
-6
-5
0
1
-4 2
-3 3
-2 4
-1
5
I 0 '6
1 log rnlv 17 'log mg/l
Figure 1. Effect of a fixed proportion of a second species that is not adsorbed by the sediment on the relationship between log ( m / v ) and (a) log R , and (b) log (C, - C)/C] for different percentages of low R , species (RH = 10Q., R L = 0.1).
the presence of even minute amounts of low Rd species. At low solid/solution ratios the observed Rd will tend to be RH but as the solid/solution ratio increases the observed Rd will drop sharply. Figure 2 shows that if the amount of low Rd species increases with increasing sediment concentration, then the distribution ratio drops more sharply with increasing solid/solution ratio than it does when there is simply a fixed amount present and the gradient of the graph of log [(C, - C)/C] vs. m / v actually becomes negative. Similar graphs can be drawn for other values and combinations of RH and RL, and it can be shown that the effect of a small amount of low Rd species decreases with decreasing RH. Thus, 0.05% of a species with Rd = 0.1 will have no detectable effect if RH is lo3 or less. It is worth noting a t this point that in deep sea sediments the porosity ranges from 65% to 85%: i.e., the solid concentration ranges from about 450 to 1400 g of sediment/L of seawater ( m / v between 0.45 and 1.4, i.e., log mlv between -0.35 and 0.15). Finally we should consider the effect that more than one species will have on the desorption distribution ratio. It has become common practice after measuring a distribu-
1
-4 2
-3 3
-2 4
-1
5
0 6
1 log m/v 7logrngA
Flgure 2. Effect of the presence of a second species that is not sorbed by the sediment and whose concentration Is proportional to the sediment concentration on the relationship between log ( r n l v )and (a) log R , and (b) log [(C, - C)/C] for different proportionality constants (RH = 10'; R , = 0.1).
tion ratio to replace the solution phase with fresh groundwater and then to determine the desorption Rd. Frequently desorption Rd values are an order of magnitude, or more, greater than sorption Rd values, and this is generally interpreted as demonstrating that the sorptiondesorption reaction is irreversible. Numerous theories to explain this effect have been suggested (2, 4 ) , and it is probable that one or more of the proposed mechanisms are involved in different experimental systems. It is also possible that the presence of some low Rd species (frequently the result of poor phase separation) has often magnified this effect. These would lower the observed sorption Rd values but would be removed after the sorption experiment and would not be present to lower the observed desorption Rd.
Experimental Section The effect of the solid/solution ratio on the sorption behavior of 242Am,238Pu,and 237Npon two sediments taken from possible waste-disposal sites in the mid-Atlantic was investigated. These sediments were (i) no. 10400 8K, a red clay, and (ii) no. 10399 7K, a high-carbonate (65% CaCO,) Environ. Sci. Technol., Vol. 20, No. 5, 1986
485
sediment. Details of their mineralogy, chemical composition, surface areas, and cation-exchange capacities are given in Table I. Distribution ratios were determined by using a standardized procedure based on the method of Relyea et al. (14). Briefly, a known weight of sediment was placed in a 50-mL Oakridge centrifuge tube together with 30 g of spiked seawater. The tubes were placed horizontally in a water bath at 4 OC and shaken gently at 120 strokes/min. After the required time interval the tubes were centrifuged at 7000 rpm (5280 g) for 90 min. The solution phase was then filtered rapidly through 0.22- or 0.10-pm filters. Aliquots were removed for analysis, the remainder was added to fresh sediment, and a "second RC determination was begun. The aim of this second Rd determination was to determine whether the nuclide remaining in solution after the first sorption behaved in the same way as that originally present in the spiked seawater. Fresh unspiked seawater was added to the solid phase, and desorption experiments were carried out. If solid/ solution ratios were high, it was necessary to remove the entrained solution by washing before beginning the longterm desorption. In general solid/solution ratios ranged from 1to 600 g/L. At these concentrations sorption to the container walls is negligible. Lower concentrations were not used because, in the absence of sediment, the actinides sorb strongly to the container walls. It is difficult to make blank corrections for this sorption because it is much lower in the presence of sediment, which provides competing surfaces, than when sediment is absent. From the time of collection the sediments were sealed and stored a t 4 OC. Samples were taken by mixing portions of sediment with slurry and then pipetting off representative portions. The spiked seawater was prepared by adding 100-200-pL portions of stock solution to 1 L of filtered (0.22 pm) seawater. The pH was readjusted to 8.2 with a few drops of 0.1 M sodium hydroxide and the solution allowed to stand for 6-8 weeks. Immediately before use it was filtered through 0.22-pm filters and analyzed for the nuclide of interest. In order to simulate conditions likely to occur in the far field after waste disposal and because the actinides have very low solubilities in environmental waters (17--19),initial concentrations were kept as low as possible (1 X M 241Am,2.3 X 10-loM 238Pu, and 3.4 X lo-* M "'Np). It was therefore necessary to analyze the solutions by 01 spectrometry and a simple technique involving solvent extraction into 0.2 M 2-thenoyltrifluoroacetone (TTA) in benzene or cyclohexane, and the preparation of evaporated sources from the organic layer was used. It was assumed that americium would be in the I11 oxidation state, and we had shown earlier (15) that all the neptunium in solution was in the V state both before and after sorption. Plutonium, on the other hand, can exist in several oxidation states, and the oxidation state distribution of the Pu in solution was determined a t each stage by using a combination of solvent extraction (TTA) and precipitation (LaF) techniques (16).
Results and Discussion Each nuclide and each sediment behaved differently. A large quantity of data was amassed during this investigation and has been reported in an internal DOE report (20). Only a selection of typical examples is given here. Errors quoted in the tables and error bars in the graphs are standard deviations on the basis of counting statistics. Where more than one determination was made, weighted means are given. 486
Environ. Sci. Technol., Vol. 20, No. 5, 1986
- --
..-- -x - -
___---
Q -
,,l_----
,,' I F -0'
r
*/'X
xI I
I
I
I0 2 2 ~ x 0 1 0 ~ a 010,u
-3 24days 27days 54days
l w .-_.._ 999
il
8 % R ~ = Z x l OO ~02%R~=O1
9997% RH = 1 X 106 003% RL = 0 1 -6 0
-5 1
-4 2
-3 log n/v -2 3 log rng/l 4
-1
5
1 0 6
Flgure 3. Effect of solid/solution ratio on the value of log [(Co- C)/C] for americium on the high-carbonate sediment 10399 7K.
Americium. The effect of increasing the solid/solution ratio on the americium distribution ratio is shown in Figure 3 where log [(C, - C)/C] has been plotted vs. log (rnlu). Superimposed on the experimental points are theoretical curves calculated by the method described under Theoretical Section. It can be seen that the theoretical and experimental curves are very similar in shape, and it seems highly likely that the reason for the observed drop in distribution ratio is that some low Rd species remained with the solution after phase separation. There is no indication that the proportion of this species increased with increasing sediment concentration, and it is probable that this nonseparable americium was attached to microparticulates. From the graphs it can be estimated that the distribution ratio of the high Rd americium was about lo6 for the carbonate sediment and that when filtered through 0.22pm filters approximately 0.05% of the original americium remained with the aqueous phase as compared with 0.02-0.03% when filtered through 0.1-pm filters. Similar curves were obtained with the red clay. The difference between the filters was not as marked for the red clay where it appears that the distribution ratio for the high Rd species was between 1 X lo6 and 2 X lo6 and the amount of nonseparable americium was about 0.02% and 0.03% when filtered through 0.22- and 0.10-pm filters, respectively. In Figure 4 sorption and desorption Rd values are plotted on the same axes. It can be seen that the desorption distribution ratios were higher than the sorption distribution ratios although a t low solid/solution ratios the desorption Rd values were not significantly different from RH, i.e., the value which would have been observed if no low Rd species had been present. I t seems probable that the americium sorption-desorption reaction is not as irreversible as has hitherto been supposed and that the large difference between sorption and desorption distribution ratios that has been observed by ourselves and others (15, 3,5,21) can be at least partially explained by the presence of some low Rd americium that was removed after the sorption stage. The presence of some low Rd species was confirmed when the solution remaining after the first sorption was contacted with fresh sediment. When this was done Rd values ranged from 40 to 400 mL/g; Le., they were several orders of magnitude lower than were the first Rd values. Neptunium. Because 237Npwas used instead of 236Np the concentration of neptunium was 4 orders of magnitude higher than in previous experiments (15). Nevertheless, distribution coefficients were unchanged (Le., about 500 for the red clay and about 4 X lo3 for the high-carbonate
X
6a
I
X
a 5‘0
0,
I
4-
I
sorption
X
I
U
a
x desorption
0
3
2
-3
I 9
o First sorption
I
.
I X
5[ -I
4
I
I
I
sorption
I
x desorption
21 -6 0
-5 1
-4 2
-3 3
I
-2
-1
4
5
I
L
0 log m/v 6 log mg/l
Flgure 4. Sorption and desorption R , values at different solid/solution ratlos for americium on the (a) high-carbonate sediment, 10399 7K, and (b) red clay, 10400 8K.
4-
-4
--_-__High carbonate
----?-- ----?-- -k--& r
---I--
X
3-
-3 U
Red clay
a
8 2-
I
-2
Sorption
x Desorption
-5 0
1
-4 2
-31og m/v -2 31og mg/l 4
-1 5
6
Figure 5. Sorption and desorption R , values at different solid/solution ratios for neptunium on the (a) high-carbonate sediment, 10399 7K, and(b) red clay, 10400 8K.
sediment), showing that Henry’s law is still being obeyed a t this much higher loading level. Henry’s law was also found to be obeyed with americium and plutonium (15, 16) at these very low concentrations. Figure 5 shows the graphs of log Rd vs. log ( m / u )for both sediments. (a) High-Carbonate Sediment. The final concentration in solution was low after phase separation and the analytical precision poor. I t appears, however, that the distribution ratio did not change with increasing solid/ solution ratio and was the same for both sorption and desorption. It should be pointed out, however, that with an Rd of only lo3, as compared with the lo6 found for americium, the system would be far less sensitive to the presence of small amounts of a low Rd nuclide and less than about 0.2% of a species with Rd = 0.1 would not have been detectable. The second distribution ratios obtained with the solution remaining after the first Rd determination were not significantly different from the first distribution ratios, so in none of the three sets of experiments is there Environ. Sci. Technol., Vol. 20, No. 5, 1986
487
Table 11. Plutonium Distribution Ratios on Carbonate Sediment 10399 7K: Sorptiona % in reduced 111 and IV
tube no.
shaking time, days
sediment concn, g/L
solution concn after sorption, Bq/L
distribution ratio, mL/g
370 376 376 371 377 377 372 378 378 373 379 379 374 380 380 375 381 381
24 29 128 24 29 128 24 29 128 24 29 128 24 29 128 24 29 128
205.5 171.6 206.0 89.14 80.81 96.97 41.57 40.02 48.02 15.64 19.77 23.73 7.71 7.67 9.20 1.85 1.81 2.17
1175 f 62 1257 f 89 969 f 75 1503 f 161 2041 f 298 1578 f 83 2022 f 142 2120 f 88 2441 f 124 1863 f 59 115 f 8 73 f 4 1103 f 73 1615 f 115 142 f 8 233 f 18 314 f 26 140 f 7
133 f 9 148 f 7 166 f 8 237 f 28 190 f 9 212 f 7 372 f 31 368 I 1 7 270 f 13 406 f 24 14596 f 795 19731 f 1030 3788 f 303 2562 f 120 26016 f 1282 76489 f 600 57920 f 2765 107 208 f 5218
"Before sorption: 238Puconcentration = 34133 f 1502 Ba/L, Le.. (2.337 f 0.104) X
form after sorption
pH after sorption
4.1 f 1.2 2.7 f 0.3
7.84
3.1 f 0.5 3.0 f 0.2
8.07
4.1 f 0.3
3.0 f 0.2
8.07
69 f 6 45 i 5
8.09
21 f 2 43 f 4
8.11
49 f 5 71 f 5
8.18
mol/L: DH 8.2: % in reduced form
N
25%.
Table 111. Plutonium Distribution Ratios: Solution Removed after First Sorption Contacted with Fresh Sediment Carbonate Sediment 10399 7K sediment concn, g/L first second sorption sorption 171.63 80.81 40.02 19.77 7.67 1.81 205.5 89.14 41.57 15.64 7.71 1.85
6.11 5.76 6.22 4.86 5.26 4.67
solution concn, Bq/L before starting after second sorption second sorption 797 f 62 1436 f 76 2161 f 110 69 f 6 138 f 8 140 f 7 1173 f 63 1502 f 164 2018 f 141 1860 f 70 1103 f 82 234 f 23
28 f 1 75 f 1 168 f 12 28 f 2 60 f 4 77 2 60 f 3 169 f 6 245 f 9 212 f 12 165 f 7 127 f 6
*
plained by the presence of some low Rd species, and we must therefore conclude that the sorption-desorption reaction is to a large extent irreversible. (b) High-Carbonate Sediment. Table I1 shows sorption distribution ratios together with the percent reduced plutonium in solution after phase separation (90-min centrifugation a t 5280g followed by filtration through 0.22-wm filters). It can be seen that for solid/solution ratios greater than about 0.04 (40 g/L) the distribution ratios are surprisingly low (100-200 mL/g), and there was no significant increase as the contact time increased from 24 to 128 days. Furthermore, at these high solid/solution ratios the plutonium in solution was virtually all in the oxidized (V VI) forms. At low solid/solution ratios, on the other hand, the plutonium in solution was mostly in the reduced (I11 + IV) forms, and the distribution ratios were greater than lo4. Similarly low distribution ratios have been observed in Lake Mono (22-24) whose waters are high in carbonate and bicarbonate. The obvious explanation for the low distribution ratios in the presence of large quantities of highcarbonate sediment is that carbonate complexes are being formed. These would be sorbed less strongly than are the Pu02+and Pu(OH)~+ ions which are probably present in seawater in the absence of sediment. No attempt was made to distinguish between the V and VI forms, but the fact that neptunium did not behave in the same way indicates that we were probably dealing with Pu(V1) com-
+
488
Environ. Sci. Technol., Vol. 20, No. 5, 1986
distribution ratio, mL/g first second 148 f 7 190 f 9 368 f 17 14596 f 795 2562 f 120 57920 f 2765 133 f 9 237 A 28 372 f 31 406 f 24 3788 f 304 76484 f 600
134 f 14 189 f 16 253 f 23 64 f 9 142 f 18 353 f 44 3053 f 190 1384 f 168 1104 f 95 1604 f 73 1076 f 92 180 f 32
70 in reduced (111 + IV) forms after after first sorption second sorption
~
4.1 f 1.2 3.1 f 0.5 4.1 f 0.3 69 f 6 21 f 2 49 f 5
2.7 f 0.3 3.0 f 0.2 3.0 f 0.3 45 f 5 43 f 4 71 f 5 43 f 2 23 f 2 22 f 2 37 f 3 53 f 4 101 f 1
plexes since neptunium had been shown to be in the V form in seawater and in our laboratory experiments (15). This is in agreement with the predictions of Skytte-Jensen (25) whose Eh-pH diagrams indicate that P U O ~ ( C O ~ ) ~ ~ and Pu02(C03)are the most probable carbonate species at pH 8 and relatively high Eh. Desorption distribution ratios were also determined. At high solid/solution ratios the sorption Rd values were not very different from the desorption Rd values (i.e., very low), and the desorbed plutonium in solution was also in the oxidized (V + VI) forms. The plutonium carbonate sorption-desorption reaction appears, therefore, to be reversible. At low solid/solution ratios behavior was similar to that with red clays, and desorption Rd values were significantly higher than sorption Rd values. The results of the experiments ih which the solutions remaining after the first distribution ratio determination were contacted with fresh sediment are given in Table 111. They confirmed that different species were present a t different solid/solution ratios and threw some further light on their behavior. The two extreme cases will be considered separately. (i) Low Solid/Solution Ratio in the First R d Experiment. Here the behavior was similar to that observed with the red clay; i.e., second Rd values were very much lower than first Rd values, indicating that most of the high Rd species had been removed in the first sorption. The P u remaining in solution after the first sorption was mostly
the low Rd species, and the effect of this was to lower the observed Rd by 2 orders of magnitude. (ii) High Solid/Solution Ratio i n t h e First R d Experiment. When the solid/solution ratio was high for both first and second sorptions, the distribution ratio was the same both times, and virtually all the Pu in solution was in the (V + VI) forms after both sorptions. This must mean either that only one plutonium species was present in solution or that equilibrium between different species was rapidly reestablished as one species was removed from solution. When the solid solution ratio in the second sorption was low the Rd rose to >lo3 and the percentage of oxidized Pu(V + VI) dropped. Clearly, in the absence of large quantities of sediment some of the plutonium carbonates have reverted to more readily sorbable species. The fact that the second Rd values are intermediate between lo2and lo4 mL/g indicates that both high and low Rd species were now present. It appears that the presence of high concentrations of sediment was necessary to keep the plutonium in the low Rd form. The carbonate and bicarbonate contents of the solution phases were obviously the same at the end of the first sorption experiment and the beginning of the second, so unless they decreased during the second sorption period the presence of the low Rd species cannot be entirely a function of carbonate and bicarbonate concentration. Possibly, complexes are formed at the surface of the sediment and resolubilized. Once in solution they could revert to other forms that could either be adsorbed or once again form low Rd complexes.
S u m m a r y and Conclusions It has been shown that when distribution ratios are determined by the batch method, it is possible to detect the presence of more than one species in solution and to make an estimate of the proportions in which they occur by measuring the distribution ratios over a wide range of solid/solution ratios. Very often the low Rd species are probably simply microparticulate material that has remained with the solution after phase separation, but complexes may be formed with constituents of either the liquid or the solid phase. If more than one stable species is present, then, a t low solid/solution ratios, the observed Rd will be close to that of the species with the highest Rd, and it will decrease as the solid/solution ratio increases. The behavior of plutonium when contacted with highcarbonate sediments is particularly noteworthy. At high solid/solution ratios the distribution ratio dropped to 100 mL/g possibly because Pu(V) or Pu(V1) carbonates were formed. Values as low as this must significantly alter predictions by modelers assessing waste disposal scenarios. From the waste disposal point of view it is clear that modelers cannot base their assessments on distribution ratios determined at an arbitrarily chosen solid/solution ratio. Effects such as that observed for plutonium in high-carbonate sediments are probably unusual but if ignored might lead to optimistic predictions. The possibility that the presence of microparticulates or organic complexes have lowered results should always be considered. In general the effect of such material will be to lower the observed Rd values of the main species, and predictions on the basis of such results will be conservative. However, the mobilities of the microparticulates and complexes, themselves, must be considered (assuming that they are not simply generated during shaking). They are likely to be neutral or negatively charged and could travel a considerable distance before being converted to a more readily sorbable form. The size of such particles could be of
critical importance particularly if diffusion is the main transport mechanism (26).
Acknowledgments The work described in this report has been carried out for the Department of the Environment as part of its waste management research program. The results will be used in the formulation of Government Policy but a t this stage do not necessarily represent that policy. Registry No. Am, 7440-35-9; Np, 7439-99-8; Pu, 7440-07-5; Pu5+, 22541-69-1; Pu6+, 22541-41-9.
Literature Cited (1) “Sorption: Modelling and Measurement for Nuclear Waste Disposal Studies”. summary of an NEA workshop, Paris, June 6-7, 1983. (2) Serne, R. J.; Relaya, J. F. “The Status of Radionuclide Sorption-Desorption Studies Performed by the WRIT Program”; Pacific Northwest Laboratory: Richland, WA, 1982: PNL-3997 UC-70. (3) Sanchez, A. L.; Schell, W. R.; Sibley, T. H. “Distribution Coefficients for Plutonium and Americium on Particulates in Aquatic Environments”. Proceedings of the International Symposium on Environmental Migration on Long-Lived Radionuclides, Knoxville, TN, July 27-31, 1981. (4) Cremers, A. “Ion Exchange Equilibria and Kd Values: Some Critical Comments”. presented at the NEA Workshop on Experimental Methodologies in Radionuclide Sorption Studies, Paris, June 6-7, 1983. (5) Ames, L. L.; McGarrah, J. E. “Basalt-Radionuclide Distribution Coefficient Determinations”. Pacific Northwest Laboratory, Richland, WA, 1980, PNL-3146 UC-70, FY1979 Annual Report. (6) Sheppard, J. C.; Campbell, M. J.; Kittrick, J. A. Environ. Sci. Technol. 1979, 13, 580-584. (7) Sheppard, J. C.; Campbell, M. J.; Kittrick, J. A. Enuiron. Sci. Technol. 1980,14, 1349-1353. (8) O’Connor, D. J.; Donnolly, J. P. J . Water Res. 1980, 14, 1517-1523. (9) Voice, T. C.; Rice, C. P.; Weber, W. J. Enuiron. Sci. Technol. 1983,17,513-517. (10) Voice, T. C.; & Weber, W. J. Enuiron. Sci. Technol. 1985, 19,789-796. (11) Nelson, D. M.; Larsen, R. P.; Penrose, W. R. “Chemical Speciation of Pu in Natural Waters”. h o c . Symp. Enuiron. Res. Actinide Elements, 1984, in press. (12) Henrion, P. N.; Monsecour, M.; Fonteyne, A. “Proceedings, Scientific Seminar on the Application of Distribution Coefficients to Radiological Assessment Models”; Louvain-La-Neuve, Belgium, Oct 7-11, 1985, in press. (13) Triolo, R.; Lietzke, M. H. J. Inorg. Nucl. Chem. 1980,42, 9 13-9 17, (14) Relayea, J. F.; Serne, R. J.; Rai, D. “Methods for Determining Radionuclide Retardation Factors”. Pacific Northwest Laboratories, 1980, PNL-3349, status report. (15) Higgo, J. J. W.; Rees, L. V. C.; Cronan, D. S. Radioact. Waste Manage. Nucl. Fuel Cycle 1983, 4, 73-102. (16) Higgo, J. J. W.; Rees, L. V. C.; Cronan, D. S. Radioact. Waste Manage. Nucl. Fuel Cycle 1985, 6 , 1-9. (17) Rai, D. R.; Strickert, R. G.; Moore, D. A.; Ryan, L. J. Radiochim. Acta 1983, 33, 201-206. (18) Strickert, R. G.; Rai, D.; Fulton, R. W. ACS Symp. Ser. 246, 1983, 135-145. (19) Rai, D. Radiochim. Acta 1984, 35, 97-106. (20) Higgo, J. J. W.; Rees, L. V. C.; Cronan, D. Department of the Environment, London, SWlY 3PY, 1984, DOE Report DOE IRW 1841084. (21) Barney, G.’S.; Brown, G. E. “The Kinetics and Reversibility of Radionuclide Sorption Reaction”. Third Contractor Meeting Proceedings, 1980, Task 4, RHO-ST-29. (22) Simpson, H. J.; Trier, R. M.; Olsen, C. R. Science (Washington, D.C.)1980,207, 1071-1073. (23) Anderson, R. F.; Bacon, M. P.; Brewer, P. G. Science (Washington, D.C.) 1982,216, 514-516. Environ. Sci. Technol., Vol. 20,No. 5, 1986
489
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(24) Cleveland, J. M.; Rees, T.F.; Nash, K. L. Science (Washington, D.C.) 1983, 222, 221-223. (25) Skytte-Jenson,B. "Migration Phenomena of Radionuclides into the Geosphere"; Harwood Academic Publishers: London, 1982. (26) Neretnieks, F. "Diffusivitiesof Some Dissolved Constituents
in Compacted Wet Bentonite Clay-MX 80 and the Impact on Radionuclide Migration in the Buffer"; Royal Institute of Technology: Stockholm, Sweden, 1982; 1982-10-29. Received for review June 6, 1985. Revised mansucript received November 4, 1985. Accepted December 30, 1985.
Tetrachlorodibenzodioxin: Rates of Volatilization and Photolysis in the Environment R. Thomas Podoll," Helen M. Jaber, and Theodore Mlll Chemistry Laboratory, SRI International, Menlo Park, California 94025
rn The vapor pressure of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) at 25 "C is (7.4f 0.4) X torr. This value
Table I. Vapor Pressure Measurement for TCDD at 25 O C
together with published values of aqueous solubility, octanollwater partition coefficient, and photolysis quantum yields provides a basis €or estimating the half-lives for movement and transformation of TCDD in water, air, and soil.
sample no.
sampling time, min
1 2
2832 2832 2832 2880 2880
~
Polychlorinated dioxins (PCDDs) now are widely distributed in the environment and cause great concern because of the extreme toxicity of some of the congeners, particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). PCDDs form in the manufacture of chlorinated intermediates and pesticides and in the incineration of chlorinated wastes (I). However, application of pesticides containing trace quantities of TCDD does not appear to be a significant source of these compounds (2). During the past 10 years environmental chemists have developed increasingly reliable methods for measuring rates and pathways for movement and transformation of organic chemicals in air, water, and soil (3-5). The objective of this paper is to use new quantitative information about the vapor pressure and photolysis of TCDD to estimate rate constants for its movement and transformation in the environment and, by analogy, the fate of TCDD congeners as well. The volatilization rate of dissolved TCDD from water can be estimated if Henry's constant is known (6). Henry's constant of a low-solubility organic compound is given with good accuracy as the ratio of the vapor pressure and aqueous solubility of the pure chemical (7). The solubility of TCDD has recently been measured at about 19.3 parts per trillion (ppt) (6 X 10-l1 M) (8). The vapor pressure of TCDD at 25 "C had not been reported prior to this investigation, and the other new reports of vapor pressure (9, 10) agree well with the value reported here. Several workers report that photolysis of TCDD in organic solvents is rapid with half-lives of 3-4 h and forms less chlorinated congeners that have significantly lower toxicity (11-13). Studies a t Saveso, Italy, following widespread distribution of TCDD in the surrounding soil and air showed that soil plots exposed to sunlight over a period of 10 days in Sept 1976 showed as much as a 10-fold decrease in TCDD content in the grass (14). Recently quantum yields for TCDD were measured for photolysis in water and in hexane (15) from which we can calculate rate constants for aqueous photolysis in sunlight and an upper limit for the atmospheric photolysis rate constant. However, no data are available for photolysis of TCDD in the atmosphere. 490
Environ. Sci. Technol., Vol. 20, No. 5, 1986
3 4
5
average flow rate, 10-~L/min
volume, L
7.6
21.5
5.9
16.8
3.2 4.6 3.0
13.4 8.66
9.10
weight collected? ~ 1 O - gl ~
vapor pressure,* ~10-'0torr
260 227 111 178 11
7.0 7.7 7.1 7.7 7.7 av 7.4 f 0.4
a Includes corrections for oxidizer efficiency, background, and quench. *Calculated from P = (w/322 g/mol) (62.36 L.torr/(mol K)) (298 K/ V).
Experimental Results and Discussion Volatility. We have now measured the vapor pressure of 14C-labeledTCDD a t 25 "C using the gas saturation technique (16) which involves passing inert gas though a sample slowly enough to saturate the carrier gas with sample vapor. A known volume of nitrogen gas was passed through an activated carbon sorbent trap where the TCDD was collected and then analyzed by combustion to 14C02, and then the TCDD concentrations were quantitated. The vapor pressure was calculated by assuming that the total pressure of a mixture of gases is equal to the sum of the pressures of the component gases. The partial pressure of the sample can be calculated from the ideal gas law, P = nRT/ V, where P is the sample vapor pressure, n is the number of moles collected on the sorbent, and V is the volume of the carrier gas passed through the sorbent. These experimental parameters and results are shown in Table I. The vapor pressure of TCDD was measured at several flow rates ranging from 3 to 7.6 mL/min with no change in vapor pressure, indicating that the carrier gas was indeed saturated. Analysis of the backup charcoal sections show a breakthrough of 0-1670 with no correlation of TCDD loading to flow rate. The observed average vapor torr (5.2% SE). Rordorf pressure was (7.4 f 0.4) X (9) and Schroy et al. (10) report vapor pressures at 30-200 "C which are in good agreement with our value at 25 "C. The volatility of TCDD depends upon the value of its Henry's constant (H) which we estimated from the ratio of measured values of vapor pressure (P) and aqueous solubility (5') (7). torr/6 X M= 12 torr M-l H = P / S = 7.4 X (1) The volatilization rate of TCDD from water can be estimated from the two-film model using estimated diffu-
00 13-936X/86/0920-0490$0 1.50/0
0 1986 American Chemical Society
