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Modelling the effect of salinity on radium desorption from sediments
Geochimica el Cosmochimica Acta. Vol. 59, No. 12, pp. 2469-2476, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA.All rights resected
Pergamon
0016-7037/95 $9.50+ .OO 0016-7037( 95)00141-7
Modelling the effect of salinity on radium desorption from sediments IAN T. WEBSTER,’ GARY J. HANCOCK,~and ANDREWS. MURRAY* ‘CSIRO Centre for Environmental Mechanics, GPO Box 821, Canberra, ACT 2601, Australia ‘CSIRO Division of Water Resources, GPO Box 1666, Canberra, ACT 2601, Australia (Received August 25, 1994; accepted in revised form March 20, 1995 )
Abstract-The
desorption of the four naturally occurring radium isotopes 223Ra,224Ra,226Ra,and 228Ra from estuarine sediments is investigated. These isotopes are created within sediments by the radioactive decay of insoluble thorium parents. Due to competition from other ions for the occupation of adsorption sites on the sediment grains, radium desorption is a function of salinity. A model is developed to describe radium desorption as it is affected by salinity, grain size, and sediment concentration. It is shown that to model the desorption of radium for a particular sediment requires the estimation of two independent parameters. One of these parameters is the concentration of radium available for exchange on the sediment; the other depends on major ion and radium exchange coefficients. Desorption experiments performed on sediments from the bed of the Bega River, Australia, are used to validate the model and to evaluate the parameters needed for model application. INTRODUCTION
the desorption behaviour of radium in sediments. Having a model that combines all of these effects means that the number of experiments needed to characterise desorption, when two (or three) of salinity, particle size, or liquid/solid ratio are varied, is much reduced. To describe desorption behaviour, the model requires the evaluation of two parameters, namely an exchange coefficient (a/b) and an exchangeable concentration (Co) which are site-specific since they will depend on the mineralogy and on the exposure of the sediments to ion-exchange processes. In the following, we demonstrate how desorption experiments on sediments from the bed of the Bega River validate the model and how they might be used to derive the two parameters required by the model to characterise desorption. Although the subject of our investigation is the desorption behaviours of z23Ra,224Ra,226Ra,and “*Ra, we expect that the model should be applicable to other trace cations as well.
The four naturally occurring isotopes of radium have great
potential as tracers in estuarine and ocean mixing processes (Bollinger and Moore, 1984; Moore et al., 1986; Moore and Todd, 1993). Because of their half-lives of 1600 years, 6 years, 11 days, and 4 days (for 226Ra,***Ra,“3Ra, and *“Ra respectively), mixing processes having a wide range of timescales can be studied. Radium isotopes are continuously created by the decay of their radioactive parents, 23’%, ?h, **‘Th, and “%, which are all members of the uranium and thorium decay series. Uranium and thorium are widely distributed in nature, and the four isotopes of radium are continuously forming in all continental and oceanic sediments. In a closed-system the concentrations of these isotopes will reach secular equilibrium with the appropriate parent nuclide when the rate of formation equals the rate of decay. Secular equilibrium rarely occurs in surficial material (Ivanovich and Harmon, 1982). In particular, as fluvial sediments encounter the estuarine salinity gradient, radium isotopes preferentially desorb (compared to their thorium parents) because of ion exchange competition with the major cations present in seawater (Li et al., 1977). After sedimentation in an estuary or on the continental shelf, this process continues, with the desorbed radium being continuously replaced by the decay of the sediment-bound thorium parents. The importance of biochemical reactions occurring within sediments to the chemical state of estuarine waters depends on the rate of transfer of solutes across the benthic boundary. We have used the distributions of naturally occurring radioactive tracers to estimate the exchange rate between sediment porewaters and the water column in the estuary of the Bega River in Australia (Webster et al., 1994). There have been a number of studies concerned with radium distributions in estuaries, but few attempt to utilise radium distribution data to quantify sediment-water cohunn exchange rates. In order to model the exchange process, we needed a way of describing the effects of salinity on desorption. This paper outlines a model which incorporates the effects of salinity, sediment particle size, and solid/liquid ratio on
DE8ORPTION MODEL Consider a sediment grain surrounded by saline water (Fig. 1). We shall suppose that there is continuous exchange of sodium ions between the grain surface and the water and that ion exchange equilibrium has been attained. Since sodium is the major cation in seawater, we shall assume it to be the major participant in ion exchange interactions. Sayles and Mangelsdorf ( 1977) demonstrated that the primary exchange reaction between fluvial clays and seawater is the replacement by sodium of their exchangeable cations. The flux of sodium ions adsorbing to the grain surface will be proportional to the number of sites available for cation adsorption on the surface. Also, we expect that the adsorption flux will increase as the concentration of sodium ions increases, that is as the salinity S increases. Thus, we can express the adsorption flux F,, as (1) where y., is a rate constant for the flux of sodium ions to the grain surface, NIMAxis the maximum number of sodium ions 2469
1. T. Webster, G. J. Hancock, and A. S. Murray
2470
r
Na+ L \
of the sediment grains in volume V, and N, to be the total number of exchangeable radium ions on this area. For a sediment comprised of uniform grains, the ratio NJA remains constant whether one or many sediment grains are being considered. Suppose a representative grain diameter within the sediment is d. We can specify that
Ra+ + * \
r
FIG. 1. Schematic of ion exchange between a sediment grain and surrounding saline water.
able to be adsorbed to a grain surface of area A, and N, is the actual number adsorbed to this surface. We have assumed that sodium so dominates the competition for vacant sites that the effect of minor cations such as radium on their availability is negligible. Consequently, the area1 concentration of vacant sites is equal to ( NsMAX- N,)A -I. The flux of sodium ions desorbing into the water Fd. will be assumed to be proportional to the area1 concentration of sodium on the grain surface. Thus, Fds
=
y
,
where I,LI is a factor which depends upon the grain shape, packing, aggregation, and the degree of fracture of the sediment. In our experiments, we added a volume of saline water VW to the sediment. The small amount of water originally present in the sediment was negligible and has been ignored in the following calculations. We mixed the sediment and water sufficiently thoroughly and for long enough that the exchange of sodium and radium between the water and the sediment achieved equilibrium. Because of the salinity of the added water, there was desorption of radium from the sediment into solution. If Co was the original concentration of exchangeable radium in the sediment before water was added then the conservation of radium mass implies
c,v, + c,v, = cov,.
(8)
We define R, the ratio of the total water volume to the sediment volume, as
(2) (9)
where ?,+ais another rate constant. At equilibrium, the adsorbing and desorbing fluxes of sodium to the grain surface must balance one another so that Fds = F...
(3)
Combination of Eqns. 1,2, and 3 yields an expression for the area1 concentration of vacant sites on the grain surface as
If V,,, were that volume of water which exactly filled the sediment’s pores, then R would equal the sediment porosity 4. Using Eqns. 4, 5, 6, 7, and 9, we can eliminate C, from Eqn. 8 to obtain
r
1-' co.
N.5MAX
-
h’s
A
(10)
YdsNsMAx
= A(?‘,
+ ?‘o& .
Equality between the fluxes of radium to and from the grain surface implies
With a and b defined as a = YorNsMAxm
&&“d
and
b=Y.., yds
(11)
Eqn. 10 can be simplified to The LHS is the desorbing flux of radium defined analogously to Eqn. 2 and the RHS is the adsorbing flux defined analogously to Eqn. 1. N, is the number of exchangeable radium ions on grain surface ama A and C, is the concentration of ions in the water expressed as mass per unit volume of water. Consider a volume of sediment V,. If C, is the concentration of exchangeable radium (presumably held on the surfaces of sediment grains) expressed as a mass per unit volume of sediment, then the mass of exchangeable radium per unit area of the grains’ surface will be
mN, -- G K A
A’
(6)
where m is the mass of one radium ion. In this expression and in Eqn. 7 which follows, we take A to be the total surface area
c,=
[R+&-J-‘co.
(12)
The parameters a and b are measures of the degrees of attraction of radium and sodium ions, respectively, to the sediment surface. MODEL APPLICATION The desorption experiments described below were undertaken to investigate the importance of various sediment and water characteristics on the desorption of radium (Hancock, 1993). Three types of experiments were performed. The first (A) investigated the effects of salinity on the desorption of four isotopes of radium from two sediment samples having different grain size ranges. A second series of experiments
Effect of salinity on Ra desorption (B) examined the influence of sediment to water ratios on radium desorption, and the third (C) investigated the effects of repeated sediment washings on desorbed radium concentrations. The experiments utilised Bega River bed sediments. These sediments had been collected upstream of the estuary and so had not been exposed to saline water. The sediments were wet sieved to separate the particle size fractions. An amount of the sieved sediment slurry corresponding to a known weight of dry sediment was weighed into a polypropylene bottle and an appropriate volume of saline water added. The bottle was shaken for 20 h, a period of time sufficient for radium to achieve ion-exchange equilibrium (Benes, 1990; Hancock, 1993). The sediment suspension was then centrifuged and the supematant filtered through a 0.45 pm membrane filter. Radium activities were measured on the filtrates and sediments by high resolution alpha-particle spectrometry following radiochemical separation as described in Hancock and Martin ( 199 1) . The cation exchange capacities of the sediments were determined by the method of Gillman (1977). For the analysis of radium within the sediment, samples were first dissolved using pyrosulfate fusion, and their radium, thorium, and uranium activities determined by alpha-particle spectrometry (Martin and Hancock, 1992). The activity of a particular radium isotope is to the mass present. In the following, we refer to centrations rather than mass concentrations. The concentration as activity concentration does not lidity of the theoretical analysis.
proportional activity condefinition of alter the va-
Experiment Series A In this series of experiments, sediment was mixed with saline water to a concentration of 12.5 g L-‘. Two grain size ranges were investigated: one having a particle size d < 63 pm and the other having 125 pm < d < 500 pm. The <63 pm size fraction contained the clay minerals kaolinite and illite together with quartz and felspar. The 125 to 500 pm fraction was predominantly quartz and felspar with some muscovite. The density of the solid phase for both sediment types was 2.5 g mL_’ . The smaller grained sediment had a measured porosity of 0.36; the larger grained sediment had a porosity of 0.40. Grain densities and sediment porosities are required for the calculation of the sample volumes. For each sediment, the dissolved concentrations of the four radium isotopes were measured at a number of discrete salinities. Assuming that the theory outlined in the previous section is valid, the variation in the dissolved radium concentration
(C,) with salinity should be. described by Bqn. 12. In our applications of Bqn. 12, we will assume that S is expressed as a percentage of a nominal full seawater concentration of 36 parts per thousand salt concentration by weight. From the physical characteristics of the sediments, we calculate R to be 128. In Bqn. 12, the parameters a and b and the concentration of exchangeable radium in the undiluted sediment, Co, are unknown; they were estimated from the experimental results using a least-squares fitting procedure. When the fitting procedure was applied to the data, it turned out that in all cases the optimal value of 6 was large enough that bSi + 1. In these circumstances, Eqn. 12 can be approximated by
2411
c, =
(13)
To fit Bqn. 13 to the measurements, only C,, and the ratio a/b need to be determined. Figure 2 shows the results of fitting Bqn. 13 to the measurements for the four radium isotopes Z23Ra224Ra,226Ra,and “*Ra for the sediments with d < 63 pm. The bars are plus and minus one standard error of the expected range of measurements based on the counting statistics. The experimental results show that radium desorption increases with salinity over the measured salinity range. For all the isotopes, the fitted curves are easily consistent with the uncertainties, and so the fitted curves provide good agreement with the measurements. Table 1 presents the values of a/b and Co obtained for the best fit between measurements and Bqn. 13. Since each measurement has an uncertainty associated with its counting statistics, uncertainty in the fitted a/b and C,, results. Table 1 includes the estimated 95% confidence limits for each fit. The optimal a/b ranges between 1.0 X lo4 for 228Raup to 1.6 X lo4 for 223Ra.The parameter b should be independent of the isotope number since it is dependent on the desorption properties of the sodium ions (Bqn. 11). Conversely, a will vary if the desorption properties of the isotopes differ from one another. The dashed lines in Fig. 2a, b, and d show the theoretical desorption curves assuming that a/b = 1.2 x lo4 for these isotopes. The value a/b = 1.2 X lo4 is the average of the fitted a/b for the three heavier isotopes. The 95% confidence interval is considerably narrower for these isotopes than it is for *z3Ra.Nevertheless, a/b = 1.2 X lo4 fits well within the 95% confidence limits for **‘Ra. The degree of agreement between the theoretical curves calculated with a/b = 1.2 X lo4 is not much less than it is for the curves calculated using the values fitted for each isotope. If there are differences between the desorption properties of the four isotopes, our measurements and analysis are not sensitive enough to differentiate them. This result is not surprising given the identical chemical behaviour expected of isotopes of the same element. Figure 3 shows the results of the desorption experiments performed for 223Ra, 224Ra, 226Ra, and *‘*Ra on the larger grained sediment 125 pm < d < 500 pm. For all the isotopes, the concentration of desorbed radium is an order of magnitude less for the larger grained sediment than for the smaller grained sediment. Also, the rate of increase of desorption with increasing salinity tapers off rather more for the larger grained sediment. With the exception of the results for 223Ra,the representations of the measurements by the theoretical desorption curves are good considering the uncertainties of the measmements. The measurements for 223Rado not show a smooth functional form for desorption. For this isotope the individual measurements are quite uncertain due to poor counting statistics. For 226Ra and 228Ra,the values of the optimal a/b are both 1.6 X lo3 (Table l), whereas the values for =Ra and *“Ra are about half as large. Nevertheless, the fit obtained using a/b = 1.6 X lo3 for 224Rastill lies mostly within the uncertainty bars for the data (Fig. 3b). The fit obtained using a/b = 1.6 X lo3 is not much worse than the optimal fit (Fig. 3a) for “‘Ra. The value a/b = 1.6 X lo3 is eight times smaller
2412
I. T. Webster, G. J. Hancock, and A. S. Murray
Ob’2iJAl tieIdYlb’ % Seawater FIG. 2. The effects of varying salinity on the measured and modelled dissolved radium concentrations for d < 63 pm (Experiment Series A). The measurements are dots; the solid line is the model’s best fit; the dashed line is the model result for a/b = 1.2 x 10’. The isotopes are (a) *=Ra; (b) *=Ra; (c) “%a; and (d) *%Ra.
than the average for the smaller grained sediment. The nominal mean grain diameters for the smaller and larger grained sediments differ by a factor of ten being 30 and 300 pm, respectively. The reduction in the experimentally determined value of o/b for the larger grained sediment is approximately consistent with the expected inverse dependence of a on grain diameter (Eqn. 11) . Table 1 . Values of ExperimentIsotope A-223
(I
/b
The values of Co for the smaller grained sediment are fifteen, eighteen, nine, and eight times larger than the corresponding values for the larger grained sediment for *23Ra. ‘%Ra, usRa, and ““Ra, respectively. These differences are due to the different radium content (Table 2), mineralogy, and the sediment grain surface to volume ratio of the two size fractions. Most of the exchangeable radium is likely to reside on
and Co obtained from model tit to measurements
d @nun) Best fit o /b
Best tit Co (mBqL-1) 3.2 x ld
95% Confidence Limits co (mBqL_‘) 2.1-7.2xld
Desorbable Proportion 0.45
<63
1.6 x 10’
95% Confidence Limits a/b 0.8 - 4.9 x 104
- 224
< 63
1.4 x 10’
1.1 - 1.9 x 10’
1.1 x lo-’
0.9 - 1.3 x Id
0.59
- 226
< 63
1.2 x 10’
1.0-1.5x10’
3.2 x 10’
2.9 - 3.7 x 10’
0.26
- 228
< 63
1.0x 10’
0.8 - 1.4 x 10’
5.4 x 10’
4.6 - 6.6 x 10’
0.30
- 223
125 - 500
0.9 x 10’
O.l-25x10)
2.2 x 102
l.7-2.9xld
0.18
- 224
125 - 500
0.9 x 10’
0.4 - 1.5 x ld
6.1 x Id
5.4 - 6.9 x ld
0.24
- 226
125 - 500
1.6 x lo3
1.2-2.1 x Id
3.5 x 103
3.2 - 3.7 x IO’
0.16
- 228
125 - 500
1.6 x ld
1.2-2.2x
7.0 x ld
6.4 - 7.8 x ld
0.30
B - 226
< 63
0.7 x IO’
0.4 - 1.1 x 10’
1.6 x 10’
1.1 - 2.2 x 10’
0.13
C - 226
< 63
1.2 x 10’
1.1 -1.4x
2.3 x 10’
2.2 - 2.5 x 10’
0.19
10’
10’
Effect of salinity on Ra desorption the surfaces of the sediment grains. The relative values of C, for the two size fractions compare well with the relative values of the cation exchange capacity (CEC) (Table 2). The measured cation exchange capacity was nine times larger for the smaller grained sediment than for the larger grained sediment.
2413
Table 2. Radium isolope conccnuarions and cation exchange capacities of the sediments used in the desorption expximents
U’Ra
“‘Ra
(&q/g,
Experiment B In Experiment B, we investigated the effects of changing the concentration of the sediment on the desorption characteristics of 226Ra.For this experiment, we used the smaller grained sediment d < 63 pm only and we kept the salinity constant at a value of 20% seawater. Changing the sediment concentration changes the value of R in the desorption equation (Eqn. 13), but a/b and Co should hold constant. The smallest concentration used (0.25 g L-’ ) corresponds to an R value of 6,400, whereas the largest concentration (50 g L-’ ) corresponds to an R value of 32. Figure 4 shows the results of fitting a theoretical desorption curve obtained from Eqn. 13 to the measurements for this experiment. This time the optimal value for a/b is 0.7 x lo4 which is somewhat lower than the value obtained for =Ra in Experiment Series A. However, the theoretical curve determined for the value of a/b derived from Experiment Series A ( 1.2 X 104) is also consistent with the experimental data. When we set a/b = 1.2 x lo4 for Experiment B, we obtain Co = 2.5 X IO4 m Bq L-’ as the best fit to the measurements.
(&q/g)
<63~
4.4 ~00.6
117 ti
125500pm
0.8 5~00.2
17.3 a.5
‘URa
CEC
tmBq/gj
wJq/g)
@woog)
76.2 il.8
114 is
27
15.5 tO.7
3
“Ra
14.6 AI.3
Weights arc dry weights. “‘Ra activity was calculakd assuming secular equilibrium
with ‘“U
and assuming the ratio of u’U to l”lJ to be 0.046. “‘Ra activity was obtained
by assuming seadar equilibrium with 9l1
This value is more consistent with Co = 3.2 X lo4 m Bq L-i determined from Experiment A for 226Ra. Experiment
C
Experiment C investigated the effects of repeated leaching of the sediment on the desorbed concentration of 226Rasuch as occurs due to tidal flushing of estuarine sediments. In this experiment, a quantity of the smaller grained sediment (d < 63 pm) was shaken with water of a salinity equivalent to 20% seawater to a concentration of 12.5 g L-l. The sediment suspension was subsequently centrifuged and the supematant
I
0
0
20
40
30
% !seawatef
30
im
0’
0
’ 20
’ 40
’ 80
’ 80
100
% seawater
FIG. 3. The effects of varying salinity on the measured and modelled dissolved radium concentrations for 125 pm < d <: 500 pm (Experiment Series A). Ihe measurements are dots; the solid line is the model’s best fit; the dashed line is the model result for a/b = 1.6 x 10’. The isotopes are (a) =Ra; (b) %Ra; (c) 2*6Ra;and (d) =I&.
I. T. Webster, C. J. Hancock, and A. S. Murray
2474
C”, = Q”-‘C!,,.
5oI
0’
’
0
’
’
’
10
20
30
’
I’
40
50
’
Sediment concentration
(gr’)
FIG.4. The effects of varying sediment concentration on the measured and modelled dissolved concentrations of “%a for d < 63 pm (Experiment B). The measurements are dots; the solid line is the model’s best fit; the dashed line is the model result for a/b = 1.2 x l@.
drained off and filtered. New saline water was added to the original sediment and the process repeated a number of times. The concentration of 226Rawas measured in the filtered water after the first, fourth, seventh, tenth, and eleventh draining. As the number of filling-draining cycles increased, the concentration of desorbed radium decreased as more and more of the exchangeable radium was leached from the sediment (Fig. 5). In order to investigate this process using our model of radium desorption, an extension of the analysis is required. At the beginning of a sediment filling-draining cycle, let the concentration of exchangeable radium in the volume of the sediment V, be Co as before. This radium has an activity of C,,V,. Once water has been added the concentration of radium in the water will be approximated by Eqn. 13. If the water is then drained, the loss of water will be VWso that the loss of radium will be C,,,V,. After one cycle of filling and draining the ratio of the exchangeable radium activity within the volume compared to the exchangeable activity at the beginning of the cycle will therefore be
Activity,, Activity,,,
=l-!!5. = CQK - C.NVXV COK
(16)
Note that the superscript n - 1 on Q in Eqn. 16 represents exponentiation. Figure 5 compares the theoretical concentration curve computed from Eqn. 16 to the measurements of the desorbed concentration of 226Ra.Note that Eqn. 16 only applies to an integral number of washings, but it has been presented in Fig. 5 as a continuous function for clarity. As in Experiments Series A and Experiment B the ratio a/b was varied to achieve an optimal fit to the data. The optimal value for a/b was determined to be 1.2 X lo4 which is the value used for the curve plotted. This value is the same as that determined in Experiment Series A for the desorption of *%Rafrom the same sediment size fraction. The last column in Table 1 presents the ratios of Co to the total activity concentrations of the sediment. The latter concentrations include contributions from the exchangeable fraction and from the nonexchangeable fraction bound in the mineral matrix within the interiors of the sediment grains. The ratios vary by a factor of five from 0.13 to 0.59. One would expect that the concentration ratio might depend on the surface area to volume ratio of the sediment grains. Table 2 shows that for Experiment A the smaller grained sediment does tend to have bigger concentration ratios than the larger grained sediment as one might expect. Also, since the likelihood of a desorption event significantly affecting exchangeable radium concentrations increases with the half-life of a particular isotope, we expect that concentration ratios should decrease with increasing half-life of the isotope. **‘jRa,which has a half-life of 1600 y, has smaller concentration ratios than 223Ra,224Ra,and ***Rawhich all have much shorter half-lives. COMPARISON WITH OTHER MEASUREMENTS The conventional exchange coefficient, K,, is defined as the ratio of the concentration of a chemical species in the solid phase expressed as mass per unit mass of sediment to con-
(14) 0
The desorbed concentration after one filling-draining cycle will be reduced in proportion to the reduction in exchangeable activity. Thus, the reduction factor Q for C, after one such cycle will be C”‘” Q=+=I-R
w
-I
[
.
R+;
1
(15)
Use has been made of Eqn. 13 to reduce the ratio CJC, in Eqn. 14. If CL is the desorbed concentration when the Fediment was first diluted, then after n filling-draining cycles, the desorbed concentration will be
0’
0
’ 2
’
’ 4
’ 6
’ 8
’ 10
s ”
12
Number of Washings FIG. 5. The effects of multiple washings on the measured and modelled dissolved concentrations of *%Rafor d < 63 pm (Experiment C). The measurements are dots; the solid line is the model’s best fit.
Effect of salinity on Ra desorption Table 3. Predicted and measured pore-water concentrations for Bega estuary sediment with mean grain size of
lsolOpe
COMPARISON
2475 WITH FIELD MEASUREMENTS
1mm. Measured Cw
CO
Predicted Cw
(mBqL_‘)
UnBqL-‘)
(mBqL_‘)
224
800 *ISO
26.2 i2.2
25.3 ti4.7
226
160 +30
5.5 HI.3
5.1 ko.9
228
400 GO
13.8 *1.3
12.6 zt2.5
centration in the dissolved phase expressed as mass per unit mass of water. Since the proportion of exchangeable radium to the total radium, which includes nonexchangeable radium held by the mineral lattice within the sediment grains, is not fixed we shall here define a KD in terms of the exchangeable radium only. With this definition, the formalism used to derive Eqn. 13 yields the following expression for KD K D
=&a ps bS’
(17)
where pw and p. are the densities of water and sediment. Li and Chan ( 1979) determined the distribution coefficient from <63 pm sediments collected from the Hudson estuary. Their value KD = 235 was pertinent to seawater. For our smaller grained sediment having the same nominal grain size, we calculate KD = 75 from Eqn. 17 using a/b = 1.2 X 104, S = 100, and pwIps = 0.63. We expect that the threefold difference between our value of KD and that of Li and Chan ( 1979) is due to differences in the grain-size distribution and in the mineralogy between the two sediment types. Elsinger and Moore ( 1984) determined the salinity dependence of the desorption of 226Ra from sediments collected from the Pee Dee River. We have found that the desorption function, Eqn. 13, fits well to the data presented in their Fig. 5. For the Pee Dee sediments, we estimate a/b = 2.0 x lo3 a value which lies between those we obtain for the two sediment grain sizes we have examined. The grain-size analysis on the Pee Dee sediment showed that 60% of it had a grain size < 62 pm. With the majority of this sediment being silt or clay, we might have expected that a/b would have been closer to the value we obtained for our smaller grained sediment. As with the results of Li and Chan (1979), the discrepancy is likely due to differences in mineralogy and in grain-size distributions within the particle size category. In Eqn. 17, KD is apparently independent of the solid to liquid ratio. Benes (1990) has noted a variation in KD with solid to liquid ratio in freshwater sediments presumably caused by grain-grain interactions. In our formalism such effects would reveal themselves as variations in $ (Eqn. 7) and therefore in a/b (Eqn. 11). If we assume that the effective value of A will decrease, a/b and Co decrease proportionally, then from Eqn. 13 these effects will tend to cancel one another. A variation in effective grain area by a factor of two through the range of solid to liquid ratios of Fig. 4 produced an insignificant variation in the modelled desorption.
The model was applied to a sediment and porewater sample collected from the Bega estuary at low tide. The salinity of the porewater was 6%0 ( 16% seawater). C,, was determined as the sum of the total desorbed radium from sequential washings. This determination was performed at a sufficiently high liquid to solid ratio that complete loss of radium occurred after two washings. The mean grain diameter of this sediment was 1 mm which is about three times that of 300 pm, the nominal grain diameter of the 125-500 pm sediment used in Experiment A. We assumed that a/b scaled inversely with particle grain size as in Eqn. 11. Thus, if a/b = 1.6 X 10’ for the 125-500 pm sediment, then we can estimate a/b = 5 X lo* for the 1 mm sediment obtained from the estuary. Table 3 compares predicted porewater concentrations calculated using Eqn. 13 with measured porewater concentrations for *“Ra, 226Ra,and ***Ra.A meaningful measure of C, and hence of C, was not obtained for 223Radue to its low concentration. For the remaining three isotopes, the predicted porewater concentrations agree with the measurements. This agreement is consistent with the scaling of a/b with sediment grain size and the application of the theory to sediment porosities which are encountered in the environment. The desorption model described here is a key component of our analysis of sediment flushing in the Bega estuary (Webster et al., 1994). In that analysis, radium desprbed to porewaters is transported into the water column by the tidal filling and draining of sediments. Using our desorption model and a model of radium transport, the radium distribution along the estuary was predicted. Figure 6 shows an example of the results. The measured and predicted concentrations of 223Ra are plotted as functions of water column salinity derived from Webster et al. ( 1994). The results of the analyses for 224Ra, 226Ra,and ‘**Ra showed similar good degrees of agreement between measurements and predictions confirming further the validity of the desorption model for applications to the real world.
1.2 -
0
20
40
60
80
100
% Seawater FIG. 6. Comparison of measured (0) and modelled (-) butions of ‘13Ra in the Bega estuary.
distri-
2416
1. T. Webster, G. J. Hancock, and A. S. Murray CONCLUSIONS
The main purpose of this work was to develop and demonstrate the validity of a model to describe the desorption of radium isotopes as a function of salinity. It has also been shown that within the sensitivity of the experiments there is no difference in the desorption behaviour of the four isotopes when the salinity alone is changed. Increasing the mean particle size of the sediment reduced both the exchangeable radium concentration (CO) and the ratio of the exchange equilibrium constants of the sodium and radium (a/b). The relative values of C,, for two sediments compared well with the relative cation exchange capacities. The change in a/b for the two sediments was consistent with an effective grain diameter of the larger grained sediment being about ten times that of the smaller grained sediment. This change is consistent with the known macroscopic particle size ranges. Only a portion of the total radium associated with the sediments was in ion-exchangeable form. The ratio of COto the total radium varied with isotope and with sediment grain size. The longest-lived isotope, Z26Ra,had the smallest ratios. The ratios were also smaller for the larger grained sediment. Both these results are consistent with expectations. The consistency of the independent estimates of C,, and of the ratio a/b for *%Ra from three experiments in which the salinity, the sediment to water ratio, and the number of sediment washings were separately varied clearly demonstrates the robustness of the model. This also shows that characteristic sediment parameters can be obtained from laboratory experiments, from which the dependence of water column concentrations on the important field variables, such as salinity and porewater exchange rates, can be predicted for all radium isotopes. Two caveats need to be considered in applications of the model to estuarine sediments. First, it is certain that radium desorption will depend upon the redox state of the sediments
which is known to influence the chemistry of grain surfaces through diagenic reactions. Coefficients obtained from oxic desorption experiments, such as ours, are unlikely to pertain in the reduced zone of the sediment column. Second, it is possible that desorption of radium from sediments is a two stage process. The first stage representing the release of surface-bound ions by ion exchange processes occurs fairly quickly, whereas the second stage may involve the slow release of ions diffusing from the interiors of the sediment grains. The rapid release occurs over timescales of minutes to hours; slow release occurs over timescales of days to months and perhaps even years. However, Nyffeler et al. (1984) found experimentally that for barium, an element with similar chemical properties to radium, partitioning was well described by an equilibrium adsorption-desorption reaction suggesting that for this element slow release was not significant. The
desorption process described in our analysis is assumed to be rapid release by ion exchange. Since the time scale for desorption from the sediments of the Bega estuary is the tidal period of 12 h, we expect that in this application fast-release dominated the desorption process. Acknowledgments-We very much appreciate the comments and suggestions on earlier versions of this manuscript by W. S. Moore, Y.-H. Li, and the three anonymous reviewers. Editorial handling: B. P. Boudreau REFERENCES Benes P. ( 1990) Radium in (continental) surface water. In The Environmental Behaviour of Radium, Vol. 1, 313-418. IAEA, Vienna. Bollinger M. S. and Moore W. S. (1984) Radium fluxes from a salt marsh. Nature 309,444-446. Elsinger R. J. and Moore W. S. ( 1984) “6Ra and “‘Ra in the mixing zones of the Pee Dee River-Winyah Bay, Yangtze River and Delaware Bay estuaries. Estuarine Coastal Shelf Sci. 18, 601-613. Gillman G. P. ( 1977) A proposed method for the measurement of exchange properties of highly weathered soils. Australian J. Soil Res. 17, 129-139. Hancock G. J. ( 1993) The effect of salinity on the concentrations of radium and thorium in sediments. M.Sc. thesis, Australian Natl. Univ. Hancock G. J. and Martin P. (1991) The determination of radium in environmental samples by alpha-particle spectrometry. Intl. .I. Appl. Radiat. Isot. 42,63-69. lvanovich M. and Harmon R. S. ( 1982) Uranium Series Disequilibrium: Applications to Environmental Problems. Clarendon. Li Y.-H. and L.-H. Chan (1979) Desorption of Ba and U6Ra from river-borne sediments in the Hudson estuary. Earth Planet. Sci. Lett. 43,343-350. Li Y.-H., Mathieu G. G., Biscaye P., and Simpson J. J. ( 1977) The flux of 2*6Rafrom estuarine and continental shelf sediments. Earth Planet. Sci. Len. 37.237-241. Martin P. and Hancock G. J. ( 1992) Routine Analysis of Naturally Occurring Radionuclides in Environmental Samples by AlphaParticle Spectrometty. Re. Rep. 7. Supervising Scientist for the Alligator Rivers Region, Australian Government Publishing Service, Canberra. Moore W. S. and Todd J. F. ( 1993) Radium isotopes in the Orinoco Estuary and Eastern Caribbean Sea. J. Geophys. Res. 98,22332244. Moore W. S., Sarmiento J. L., and Key R. M. (1986) Tracing the Amazon component of surface Atlantic water using *“Ra. salinity and silica. J. Geophys. Res. 91, 2574-2580. Nvffeler U. P.. Li Y.-H.. and Santschi P. H. ( 1984) A kinetic ao-preach to describe trace-element distribution between particles aud solution in natural aquatic systems. Geochim. Cosmochim. Acta 48, 1513-1522. Sayles F. L. and Mangelsdorf P. C. ( 1977) The equilibration of clay minerals with seawater: exchange reactions. Geochim. Cosmochim. Acta 41,95 l-960. Webster I. T., Hancock G. J.. and Murray A. S. (1994) On the use of radium isotopes to estimate sediment flushing rates in an estuary. Limnol. Oceanogr. 39(8), 1917-1927.
Pergamon
0016-7037/95 $9.50+ .OO 0016-7037( 95)00141-7
Modelling the effect of salinity on radium desorption from sediments IAN T. WEBSTER,’ GARY J. HANCOCK,~and ANDREWS. MURRAY* ‘CSIRO Centre for Environmental Mechanics, GPO Box 821, Canberra, ACT 2601, Australia ‘CSIRO Division of Water Resources, GPO Box 1666, Canberra, ACT 2601, Australia (Received August 25, 1994; accepted in revised form March 20, 1995 )
Abstract-The
desorption of the four naturally occurring radium isotopes 223Ra,224Ra,226Ra,and 228Ra from estuarine sediments is investigated. These isotopes are created within sediments by the radioactive decay of insoluble thorium parents. Due to competition from other ions for the occupation of adsorption sites on the sediment grains, radium desorption is a function of salinity. A model is developed to describe radium desorption as it is affected by salinity, grain size, and sediment concentration. It is shown that to model the desorption of radium for a particular sediment requires the estimation of two independent parameters. One of these parameters is the concentration of radium available for exchange on the sediment; the other depends on major ion and radium exchange coefficients. Desorption experiments performed on sediments from the bed of the Bega River, Australia, are used to validate the model and to evaluate the parameters needed for model application. INTRODUCTION
the desorption behaviour of radium in sediments. Having a model that combines all of these effects means that the number of experiments needed to characterise desorption, when two (or three) of salinity, particle size, or liquid/solid ratio are varied, is much reduced. To describe desorption behaviour, the model requires the evaluation of two parameters, namely an exchange coefficient (a/b) and an exchangeable concentration (Co) which are site-specific since they will depend on the mineralogy and on the exposure of the sediments to ion-exchange processes. In the following, we demonstrate how desorption experiments on sediments from the bed of the Bega River validate the model and how they might be used to derive the two parameters required by the model to characterise desorption. Although the subject of our investigation is the desorption behaviours of z23Ra,224Ra,226Ra,and “*Ra, we expect that the model should be applicable to other trace cations as well.
The four naturally occurring isotopes of radium have great
potential as tracers in estuarine and ocean mixing processes (Bollinger and Moore, 1984; Moore et al., 1986; Moore and Todd, 1993). Because of their half-lives of 1600 years, 6 years, 11 days, and 4 days (for 226Ra,***Ra,“3Ra, and *“Ra respectively), mixing processes having a wide range of timescales can be studied. Radium isotopes are continuously created by the decay of their radioactive parents, 23’%, ?h, **‘Th, and “%, which are all members of the uranium and thorium decay series. Uranium and thorium are widely distributed in nature, and the four isotopes of radium are continuously forming in all continental and oceanic sediments. In a closed-system the concentrations of these isotopes will reach secular equilibrium with the appropriate parent nuclide when the rate of formation equals the rate of decay. Secular equilibrium rarely occurs in surficial material (Ivanovich and Harmon, 1982). In particular, as fluvial sediments encounter the estuarine salinity gradient, radium isotopes preferentially desorb (compared to their thorium parents) because of ion exchange competition with the major cations present in seawater (Li et al., 1977). After sedimentation in an estuary or on the continental shelf, this process continues, with the desorbed radium being continuously replaced by the decay of the sediment-bound thorium parents. The importance of biochemical reactions occurring within sediments to the chemical state of estuarine waters depends on the rate of transfer of solutes across the benthic boundary. We have used the distributions of naturally occurring radioactive tracers to estimate the exchange rate between sediment porewaters and the water column in the estuary of the Bega River in Australia (Webster et al., 1994). There have been a number of studies concerned with radium distributions in estuaries, but few attempt to utilise radium distribution data to quantify sediment-water cohunn exchange rates. In order to model the exchange process, we needed a way of describing the effects of salinity on desorption. This paper outlines a model which incorporates the effects of salinity, sediment particle size, and solid/liquid ratio on
DE8ORPTION MODEL Consider a sediment grain surrounded by saline water (Fig. 1). We shall suppose that there is continuous exchange of sodium ions between the grain surface and the water and that ion exchange equilibrium has been attained. Since sodium is the major cation in seawater, we shall assume it to be the major participant in ion exchange interactions. Sayles and Mangelsdorf ( 1977) demonstrated that the primary exchange reaction between fluvial clays and seawater is the replacement by sodium of their exchangeable cations. The flux of sodium ions adsorbing to the grain surface will be proportional to the number of sites available for cation adsorption on the surface. Also, we expect that the adsorption flux will increase as the concentration of sodium ions increases, that is as the salinity S increases. Thus, we can express the adsorption flux F,, as (1) where y., is a rate constant for the flux of sodium ions to the grain surface, NIMAxis the maximum number of sodium ions 2469
1. T. Webster, G. J. Hancock, and A. S. Murray
2470
r
Na+ L \
of the sediment grains in volume V, and N, to be the total number of exchangeable radium ions on this area. For a sediment comprised of uniform grains, the ratio NJA remains constant whether one or many sediment grains are being considered. Suppose a representative grain diameter within the sediment is d. We can specify that
Ra+ + * \
r
FIG. 1. Schematic of ion exchange between a sediment grain and surrounding saline water.
able to be adsorbed to a grain surface of area A, and N, is the actual number adsorbed to this surface. We have assumed that sodium so dominates the competition for vacant sites that the effect of minor cations such as radium on their availability is negligible. Consequently, the area1 concentration of vacant sites is equal to ( NsMAX- N,)A -I. The flux of sodium ions desorbing into the water Fd. will be assumed to be proportional to the area1 concentration of sodium on the grain surface. Thus, Fds
=
y
,
where I,LI is a factor which depends upon the grain shape, packing, aggregation, and the degree of fracture of the sediment. In our experiments, we added a volume of saline water VW to the sediment. The small amount of water originally present in the sediment was negligible and has been ignored in the following calculations. We mixed the sediment and water sufficiently thoroughly and for long enough that the exchange of sodium and radium between the water and the sediment achieved equilibrium. Because of the salinity of the added water, there was desorption of radium from the sediment into solution. If Co was the original concentration of exchangeable radium in the sediment before water was added then the conservation of radium mass implies
c,v, + c,v, = cov,.
(8)
We define R, the ratio of the total water volume to the sediment volume, as
(2) (9)
where ?,+ais another rate constant. At equilibrium, the adsorbing and desorbing fluxes of sodium to the grain surface must balance one another so that Fds = F...
(3)
Combination of Eqns. 1,2, and 3 yields an expression for the area1 concentration of vacant sites on the grain surface as
If V,,, were that volume of water which exactly filled the sediment’s pores, then R would equal the sediment porosity 4. Using Eqns. 4, 5, 6, 7, and 9, we can eliminate C, from Eqn. 8 to obtain
r
1-' co.
N.5MAX
-
h’s
A
(10)
YdsNsMAx
= A(?‘,
+ ?‘o& .
Equality between the fluxes of radium to and from the grain surface implies
With a and b defined as a = YorNsMAxm
&&“d
and
b=Y.., yds
(11)
Eqn. 10 can be simplified to The LHS is the desorbing flux of radium defined analogously to Eqn. 2 and the RHS is the adsorbing flux defined analogously to Eqn. 1. N, is the number of exchangeable radium ions on grain surface ama A and C, is the concentration of ions in the water expressed as mass per unit volume of water. Consider a volume of sediment V,. If C, is the concentration of exchangeable radium (presumably held on the surfaces of sediment grains) expressed as a mass per unit volume of sediment, then the mass of exchangeable radium per unit area of the grains’ surface will be
mN, -- G K A
A’
(6)
where m is the mass of one radium ion. In this expression and in Eqn. 7 which follows, we take A to be the total surface area
c,=
[R+&-J-‘co.
(12)
The parameters a and b are measures of the degrees of attraction of radium and sodium ions, respectively, to the sediment surface. MODEL APPLICATION The desorption experiments described below were undertaken to investigate the importance of various sediment and water characteristics on the desorption of radium (Hancock, 1993). Three types of experiments were performed. The first (A) investigated the effects of salinity on the desorption of four isotopes of radium from two sediment samples having different grain size ranges. A second series of experiments
Effect of salinity on Ra desorption (B) examined the influence of sediment to water ratios on radium desorption, and the third (C) investigated the effects of repeated sediment washings on desorbed radium concentrations. The experiments utilised Bega River bed sediments. These sediments had been collected upstream of the estuary and so had not been exposed to saline water. The sediments were wet sieved to separate the particle size fractions. An amount of the sieved sediment slurry corresponding to a known weight of dry sediment was weighed into a polypropylene bottle and an appropriate volume of saline water added. The bottle was shaken for 20 h, a period of time sufficient for radium to achieve ion-exchange equilibrium (Benes, 1990; Hancock, 1993). The sediment suspension was then centrifuged and the supematant filtered through a 0.45 pm membrane filter. Radium activities were measured on the filtrates and sediments by high resolution alpha-particle spectrometry following radiochemical separation as described in Hancock and Martin ( 199 1) . The cation exchange capacities of the sediments were determined by the method of Gillman (1977). For the analysis of radium within the sediment, samples were first dissolved using pyrosulfate fusion, and their radium, thorium, and uranium activities determined by alpha-particle spectrometry (Martin and Hancock, 1992). The activity of a particular radium isotope is to the mass present. In the following, we refer to centrations rather than mass concentrations. The concentration as activity concentration does not lidity of the theoretical analysis.
proportional activity condefinition of alter the va-
Experiment Series A In this series of experiments, sediment was mixed with saline water to a concentration of 12.5 g L-‘. Two grain size ranges were investigated: one having a particle size d < 63 pm and the other having 125 pm < d < 500 pm. The <63 pm size fraction contained the clay minerals kaolinite and illite together with quartz and felspar. The 125 to 500 pm fraction was predominantly quartz and felspar with some muscovite. The density of the solid phase for both sediment types was 2.5 g mL_’ . The smaller grained sediment had a measured porosity of 0.36; the larger grained sediment had a porosity of 0.40. Grain densities and sediment porosities are required for the calculation of the sample volumes. For each sediment, the dissolved concentrations of the four radium isotopes were measured at a number of discrete salinities. Assuming that the theory outlined in the previous section is valid, the variation in the dissolved radium concentration
(C,) with salinity should be. described by Bqn. 12. In our applications of Bqn. 12, we will assume that S is expressed as a percentage of a nominal full seawater concentration of 36 parts per thousand salt concentration by weight. From the physical characteristics of the sediments, we calculate R to be 128. In Bqn. 12, the parameters a and b and the concentration of exchangeable radium in the undiluted sediment, Co, are unknown; they were estimated from the experimental results using a least-squares fitting procedure. When the fitting procedure was applied to the data, it turned out that in all cases the optimal value of 6 was large enough that bSi + 1. In these circumstances, Eqn. 12 can be approximated by
2411
c, =
(13)
To fit Bqn. 13 to the measurements, only C,, and the ratio a/b need to be determined. Figure 2 shows the results of fitting Bqn. 13 to the measurements for the four radium isotopes Z23Ra224Ra,226Ra,and “*Ra for the sediments with d < 63 pm. The bars are plus and minus one standard error of the expected range of measurements based on the counting statistics. The experimental results show that radium desorption increases with salinity over the measured salinity range. For all the isotopes, the fitted curves are easily consistent with the uncertainties, and so the fitted curves provide good agreement with the measurements. Table 1 presents the values of a/b and Co obtained for the best fit between measurements and Bqn. 13. Since each measurement has an uncertainty associated with its counting statistics, uncertainty in the fitted a/b and C,, results. Table 1 includes the estimated 95% confidence limits for each fit. The optimal a/b ranges between 1.0 X lo4 for 228Raup to 1.6 X lo4 for 223Ra.The parameter b should be independent of the isotope number since it is dependent on the desorption properties of the sodium ions (Bqn. 11). Conversely, a will vary if the desorption properties of the isotopes differ from one another. The dashed lines in Fig. 2a, b, and d show the theoretical desorption curves assuming that a/b = 1.2 x lo4 for these isotopes. The value a/b = 1.2 X lo4 is the average of the fitted a/b for the three heavier isotopes. The 95% confidence interval is considerably narrower for these isotopes than it is for *z3Ra.Nevertheless, a/b = 1.2 X lo4 fits well within the 95% confidence limits for **‘Ra. The degree of agreement between the theoretical curves calculated with a/b = 1.2 X lo4 is not much less than it is for the curves calculated using the values fitted for each isotope. If there are differences between the desorption properties of the four isotopes, our measurements and analysis are not sensitive enough to differentiate them. This result is not surprising given the identical chemical behaviour expected of isotopes of the same element. Figure 3 shows the results of the desorption experiments performed for 223Ra, 224Ra, 226Ra, and *‘*Ra on the larger grained sediment 125 pm < d < 500 pm. For all the isotopes, the concentration of desorbed radium is an order of magnitude less for the larger grained sediment than for the smaller grained sediment. Also, the rate of increase of desorption with increasing salinity tapers off rather more for the larger grained sediment. With the exception of the results for 223Ra,the representations of the measurements by the theoretical desorption curves are good considering the uncertainties of the measmements. The measurements for 223Rado not show a smooth functional form for desorption. For this isotope the individual measurements are quite uncertain due to poor counting statistics. For 226Ra and 228Ra,the values of the optimal a/b are both 1.6 X lo3 (Table l), whereas the values for =Ra and *“Ra are about half as large. Nevertheless, the fit obtained using a/b = 1.6 X lo3 for 224Rastill lies mostly within the uncertainty bars for the data (Fig. 3b). The fit obtained using a/b = 1.6 X lo3 is not much worse than the optimal fit (Fig. 3a) for “‘Ra. The value a/b = 1.6 X lo3 is eight times smaller
2412
I. T. Webster, G. J. Hancock, and A. S. Murray
Ob’2iJAl tieIdYlb’ % Seawater FIG. 2. The effects of varying salinity on the measured and modelled dissolved radium concentrations for d < 63 pm (Experiment Series A). The measurements are dots; the solid line is the model’s best fit; the dashed line is the model result for a/b = 1.2 x 10’. The isotopes are (a) *=Ra; (b) *=Ra; (c) “%a; and (d) *%Ra.
than the average for the smaller grained sediment. The nominal mean grain diameters for the smaller and larger grained sediments differ by a factor of ten being 30 and 300 pm, respectively. The reduction in the experimentally determined value of o/b for the larger grained sediment is approximately consistent with the expected inverse dependence of a on grain diameter (Eqn. 11) . Table 1 . Values of ExperimentIsotope A-223
(I
/b
The values of Co for the smaller grained sediment are fifteen, eighteen, nine, and eight times larger than the corresponding values for the larger grained sediment for *23Ra. ‘%Ra, usRa, and ““Ra, respectively. These differences are due to the different radium content (Table 2), mineralogy, and the sediment grain surface to volume ratio of the two size fractions. Most of the exchangeable radium is likely to reside on
and Co obtained from model tit to measurements
d @nun) Best fit o /b
Best tit Co (mBqL-1) 3.2 x ld
95% Confidence Limits co (mBqL_‘) 2.1-7.2xld
Desorbable Proportion 0.45
<63
1.6 x 10’
95% Confidence Limits a/b 0.8 - 4.9 x 104
- 224
< 63
1.4 x 10’
1.1 - 1.9 x 10’
1.1 x lo-’
0.9 - 1.3 x Id
0.59
- 226
< 63
1.2 x 10’
1.0-1.5x10’
3.2 x 10’
2.9 - 3.7 x 10’
0.26
- 228
< 63
1.0x 10’
0.8 - 1.4 x 10’
5.4 x 10’
4.6 - 6.6 x 10’
0.30
- 223
125 - 500
0.9 x 10’
O.l-25x10)
2.2 x 102
l.7-2.9xld
0.18
- 224
125 - 500
0.9 x 10’
0.4 - 1.5 x ld
6.1 x Id
5.4 - 6.9 x ld
0.24
- 226
125 - 500
1.6 x lo3
1.2-2.1 x Id
3.5 x 103
3.2 - 3.7 x IO’
0.16
- 228
125 - 500
1.6 x ld
1.2-2.2x
7.0 x ld
6.4 - 7.8 x ld
0.30
B - 226
< 63
0.7 x IO’
0.4 - 1.1 x 10’
1.6 x 10’
1.1 - 2.2 x 10’
0.13
C - 226
< 63
1.2 x 10’
1.1 -1.4x
2.3 x 10’
2.2 - 2.5 x 10’
0.19
10’
10’
Effect of salinity on Ra desorption the surfaces of the sediment grains. The relative values of C, for the two size fractions compare well with the relative values of the cation exchange capacity (CEC) (Table 2). The measured cation exchange capacity was nine times larger for the smaller grained sediment than for the larger grained sediment.
2413
Table 2. Radium isolope conccnuarions and cation exchange capacities of the sediments used in the desorption expximents
U’Ra
“‘Ra
(&q/g,
Experiment B In Experiment B, we investigated the effects of changing the concentration of the sediment on the desorption characteristics of 226Ra.For this experiment, we used the smaller grained sediment d < 63 pm only and we kept the salinity constant at a value of 20% seawater. Changing the sediment concentration changes the value of R in the desorption equation (Eqn. 13), but a/b and Co should hold constant. The smallest concentration used (0.25 g L-’ ) corresponds to an R value of 6,400, whereas the largest concentration (50 g L-’ ) corresponds to an R value of 32. Figure 4 shows the results of fitting a theoretical desorption curve obtained from Eqn. 13 to the measurements for this experiment. This time the optimal value for a/b is 0.7 x lo4 which is somewhat lower than the value obtained for =Ra in Experiment Series A. However, the theoretical curve determined for the value of a/b derived from Experiment Series A ( 1.2 X 104) is also consistent with the experimental data. When we set a/b = 1.2 x lo4 for Experiment B, we obtain Co = 2.5 X IO4 m Bq L-’ as the best fit to the measurements.
(&q/g)
<63~
4.4 ~00.6
117 ti
125500pm
0.8 5~00.2
17.3 a.5
‘URa
CEC
tmBq/gj
wJq/g)
@woog)
76.2 il.8
114 is
27
15.5 tO.7
3
“Ra
14.6 AI.3
Weights arc dry weights. “‘Ra activity was calculakd assuming secular equilibrium
with ‘“U
and assuming the ratio of u’U to l”lJ to be 0.046. “‘Ra activity was obtained
by assuming seadar equilibrium with 9l1
This value is more consistent with Co = 3.2 X lo4 m Bq L-i determined from Experiment A for 226Ra. Experiment
C
Experiment C investigated the effects of repeated leaching of the sediment on the desorbed concentration of 226Rasuch as occurs due to tidal flushing of estuarine sediments. In this experiment, a quantity of the smaller grained sediment (d < 63 pm) was shaken with water of a salinity equivalent to 20% seawater to a concentration of 12.5 g L-l. The sediment suspension was subsequently centrifuged and the supematant
I
0
0
20
40
30
% !seawatef
30
im
0’
0
’ 20
’ 40
’ 80
’ 80
100
% seawater
FIG. 3. The effects of varying salinity on the measured and modelled dissolved radium concentrations for 125 pm < d <: 500 pm (Experiment Series A). Ihe measurements are dots; the solid line is the model’s best fit; the dashed line is the model result for a/b = 1.6 x 10’. The isotopes are (a) =Ra; (b) %Ra; (c) 2*6Ra;and (d) =I&.
I. T. Webster, C. J. Hancock, and A. S. Murray
2474
C”, = Q”-‘C!,,.
5oI
0’
’
0
’
’
’
10
20
30
’
I’
40
50
’
Sediment concentration
(gr’)
FIG.4. The effects of varying sediment concentration on the measured and modelled dissolved concentrations of “%a for d < 63 pm (Experiment B). The measurements are dots; the solid line is the model’s best fit; the dashed line is the model result for a/b = 1.2 x l@.
drained off and filtered. New saline water was added to the original sediment and the process repeated a number of times. The concentration of 226Rawas measured in the filtered water after the first, fourth, seventh, tenth, and eleventh draining. As the number of filling-draining cycles increased, the concentration of desorbed radium decreased as more and more of the exchangeable radium was leached from the sediment (Fig. 5). In order to investigate this process using our model of radium desorption, an extension of the analysis is required. At the beginning of a sediment filling-draining cycle, let the concentration of exchangeable radium in the volume of the sediment V, be Co as before. This radium has an activity of C,,V,. Once water has been added the concentration of radium in the water will be approximated by Eqn. 13. If the water is then drained, the loss of water will be VWso that the loss of radium will be C,,,V,. After one cycle of filling and draining the ratio of the exchangeable radium activity within the volume compared to the exchangeable activity at the beginning of the cycle will therefore be
Activity,, Activity,,,
=l-!!5. = CQK - C.NVXV COK
(16)
Note that the superscript n - 1 on Q in Eqn. 16 represents exponentiation. Figure 5 compares the theoretical concentration curve computed from Eqn. 16 to the measurements of the desorbed concentration of 226Ra.Note that Eqn. 16 only applies to an integral number of washings, but it has been presented in Fig. 5 as a continuous function for clarity. As in Experiments Series A and Experiment B the ratio a/b was varied to achieve an optimal fit to the data. The optimal value for a/b was determined to be 1.2 X lo4 which is the value used for the curve plotted. This value is the same as that determined in Experiment Series A for the desorption of *%Rafrom the same sediment size fraction. The last column in Table 1 presents the ratios of Co to the total activity concentrations of the sediment. The latter concentrations include contributions from the exchangeable fraction and from the nonexchangeable fraction bound in the mineral matrix within the interiors of the sediment grains. The ratios vary by a factor of five from 0.13 to 0.59. One would expect that the concentration ratio might depend on the surface area to volume ratio of the sediment grains. Table 2 shows that for Experiment A the smaller grained sediment does tend to have bigger concentration ratios than the larger grained sediment as one might expect. Also, since the likelihood of a desorption event significantly affecting exchangeable radium concentrations increases with the half-life of a particular isotope, we expect that concentration ratios should decrease with increasing half-life of the isotope. **‘jRa,which has a half-life of 1600 y, has smaller concentration ratios than 223Ra,224Ra,and ***Rawhich all have much shorter half-lives. COMPARISON WITH OTHER MEASUREMENTS The conventional exchange coefficient, K,, is defined as the ratio of the concentration of a chemical species in the solid phase expressed as mass per unit mass of sediment to con-
(14) 0
The desorbed concentration after one filling-draining cycle will be reduced in proportion to the reduction in exchangeable activity. Thus, the reduction factor Q for C, after one such cycle will be C”‘” Q=+=I-R
w
-I
[
.
R+;
1
(15)
Use has been made of Eqn. 13 to reduce the ratio CJC, in Eqn. 14. If CL is the desorbed concentration when the Fediment was first diluted, then after n filling-draining cycles, the desorbed concentration will be
0’
0
’ 2
’
’ 4
’ 6
’ 8
’ 10
s ”
12
Number of Washings FIG. 5. The effects of multiple washings on the measured and modelled dissolved concentrations of *%Rafor d < 63 pm (Experiment C). The measurements are dots; the solid line is the model’s best fit.
Effect of salinity on Ra desorption Table 3. Predicted and measured pore-water concentrations for Bega estuary sediment with mean grain size of
lsolOpe
COMPARISON
2475 WITH FIELD MEASUREMENTS
1mm. Measured Cw
CO
Predicted Cw
(mBqL_‘)
UnBqL-‘)
(mBqL_‘)
224
800 *ISO
26.2 i2.2
25.3 ti4.7
226
160 +30
5.5 HI.3
5.1 ko.9
228
400 GO
13.8 *1.3
12.6 zt2.5
centration in the dissolved phase expressed as mass per unit mass of water. Since the proportion of exchangeable radium to the total radium, which includes nonexchangeable radium held by the mineral lattice within the sediment grains, is not fixed we shall here define a KD in terms of the exchangeable radium only. With this definition, the formalism used to derive Eqn. 13 yields the following expression for KD K D
=&a ps bS’
(17)
where pw and p. are the densities of water and sediment. Li and Chan ( 1979) determined the distribution coefficient from <63 pm sediments collected from the Hudson estuary. Their value KD = 235 was pertinent to seawater. For our smaller grained sediment having the same nominal grain size, we calculate KD = 75 from Eqn. 17 using a/b = 1.2 X 104, S = 100, and pwIps = 0.63. We expect that the threefold difference between our value of KD and that of Li and Chan ( 1979) is due to differences in the grain-size distribution and in the mineralogy between the two sediment types. Elsinger and Moore ( 1984) determined the salinity dependence of the desorption of 226Ra from sediments collected from the Pee Dee River. We have found that the desorption function, Eqn. 13, fits well to the data presented in their Fig. 5. For the Pee Dee sediments, we estimate a/b = 2.0 x lo3 a value which lies between those we obtain for the two sediment grain sizes we have examined. The grain-size analysis on the Pee Dee sediment showed that 60% of it had a grain size < 62 pm. With the majority of this sediment being silt or clay, we might have expected that a/b would have been closer to the value we obtained for our smaller grained sediment. As with the results of Li and Chan (1979), the discrepancy is likely due to differences in mineralogy and in grain-size distributions within the particle size category. In Eqn. 17, KD is apparently independent of the solid to liquid ratio. Benes (1990) has noted a variation in KD with solid to liquid ratio in freshwater sediments presumably caused by grain-grain interactions. In our formalism such effects would reveal themselves as variations in $ (Eqn. 7) and therefore in a/b (Eqn. 11). If we assume that the effective value of A will decrease, a/b and Co decrease proportionally, then from Eqn. 13 these effects will tend to cancel one another. A variation in effective grain area by a factor of two through the range of solid to liquid ratios of Fig. 4 produced an insignificant variation in the modelled desorption.
The model was applied to a sediment and porewater sample collected from the Bega estuary at low tide. The salinity of the porewater was 6%0 ( 16% seawater). C,, was determined as the sum of the total desorbed radium from sequential washings. This determination was performed at a sufficiently high liquid to solid ratio that complete loss of radium occurred after two washings. The mean grain diameter of this sediment was 1 mm which is about three times that of 300 pm, the nominal grain diameter of the 125-500 pm sediment used in Experiment A. We assumed that a/b scaled inversely with particle grain size as in Eqn. 11. Thus, if a/b = 1.6 X 10’ for the 125-500 pm sediment, then we can estimate a/b = 5 X lo* for the 1 mm sediment obtained from the estuary. Table 3 compares predicted porewater concentrations calculated using Eqn. 13 with measured porewater concentrations for *“Ra, 226Ra,and ***Ra.A meaningful measure of C, and hence of C, was not obtained for 223Radue to its low concentration. For the remaining three isotopes, the predicted porewater concentrations agree with the measurements. This agreement is consistent with the scaling of a/b with sediment grain size and the application of the theory to sediment porosities which are encountered in the environment. The desorption model described here is a key component of our analysis of sediment flushing in the Bega estuary (Webster et al., 1994). In that analysis, radium desprbed to porewaters is transported into the water column by the tidal filling and draining of sediments. Using our desorption model and a model of radium transport, the radium distribution along the estuary was predicted. Figure 6 shows an example of the results. The measured and predicted concentrations of 223Ra are plotted as functions of water column salinity derived from Webster et al. ( 1994). The results of the analyses for 224Ra, 226Ra,and ‘**Ra showed similar good degrees of agreement between measurements and predictions confirming further the validity of the desorption model for applications to the real world.
1.2 -
0
20
40
60
80
100
% Seawater FIG. 6. Comparison of measured (0) and modelled (-) butions of ‘13Ra in the Bega estuary.
distri-
2416
1. T. Webster, G. J. Hancock, and A. S. Murray CONCLUSIONS
The main purpose of this work was to develop and demonstrate the validity of a model to describe the desorption of radium isotopes as a function of salinity. It has also been shown that within the sensitivity of the experiments there is no difference in the desorption behaviour of the four isotopes when the salinity alone is changed. Increasing the mean particle size of the sediment reduced both the exchangeable radium concentration (CO) and the ratio of the exchange equilibrium constants of the sodium and radium (a/b). The relative values of C,, for two sediments compared well with the relative cation exchange capacities. The change in a/b for the two sediments was consistent with an effective grain diameter of the larger grained sediment being about ten times that of the smaller grained sediment. This change is consistent with the known macroscopic particle size ranges. Only a portion of the total radium associated with the sediments was in ion-exchangeable form. The ratio of COto the total radium varied with isotope and with sediment grain size. The longest-lived isotope, Z26Ra,had the smallest ratios. The ratios were also smaller for the larger grained sediment. Both these results are consistent with expectations. The consistency of the independent estimates of C,, and of the ratio a/b for *%Ra from three experiments in which the salinity, the sediment to water ratio, and the number of sediment washings were separately varied clearly demonstrates the robustness of the model. This also shows that characteristic sediment parameters can be obtained from laboratory experiments, from which the dependence of water column concentrations on the important field variables, such as salinity and porewater exchange rates, can be predicted for all radium isotopes. Two caveats need to be considered in applications of the model to estuarine sediments. First, it is certain that radium desorption will depend upon the redox state of the sediments
which is known to influence the chemistry of grain surfaces through diagenic reactions. Coefficients obtained from oxic desorption experiments, such as ours, are unlikely to pertain in the reduced zone of the sediment column. Second, it is possible that desorption of radium from sediments is a two stage process. The first stage representing the release of surface-bound ions by ion exchange processes occurs fairly quickly, whereas the second stage may involve the slow release of ions diffusing from the interiors of the sediment grains. The rapid release occurs over timescales of minutes to hours; slow release occurs over timescales of days to months and perhaps even years. However, Nyffeler et al. (1984) found experimentally that for barium, an element with similar chemical properties to radium, partitioning was well described by an equilibrium adsorption-desorption reaction suggesting that for this element slow release was not significant. The
desorption process described in our analysis is assumed to be rapid release by ion exchange. Since the time scale for desorption from the sediments of the Bega estuary is the tidal period of 12 h, we expect that in this application fast-release dominated the desorption process. Acknowledgments-We very much appreciate the comments and suggestions on earlier versions of this manuscript by W. S. Moore, Y.-H. Li, and the three anonymous reviewers. Editorial handling: B. P. Boudreau REFERENCES Benes P. ( 1990) Radium in (continental) surface water. In The Environmental Behaviour of Radium, Vol. 1, 313-418. IAEA, Vienna. Bollinger M. S. and Moore W. S. (1984) Radium fluxes from a salt marsh. Nature 309,444-446. Elsinger R. J. and Moore W. S. ( 1984) “6Ra and “‘Ra in the mixing zones of the Pee Dee River-Winyah Bay, Yangtze River and Delaware Bay estuaries. Estuarine Coastal Shelf Sci. 18, 601-613. Gillman G. P. ( 1977) A proposed method for the measurement of exchange properties of highly weathered soils. Australian J. Soil Res. 17, 129-139. Hancock G. J. ( 1993) The effect of salinity on the concentrations of radium and thorium in sediments. M.Sc. thesis, Australian Natl. Univ. Hancock G. J. and Martin P. (1991) The determination of radium in environmental samples by alpha-particle spectrometry. Intl. .I. Appl. Radiat. Isot. 42,63-69. lvanovich M. and Harmon R. S. ( 1982) Uranium Series Disequilibrium: Applications to Environmental Problems. Clarendon. Li Y.-H. and L.-H. Chan (1979) Desorption of Ba and U6Ra from river-borne sediments in the Hudson estuary. Earth Planet. Sci. Lett. 43,343-350. Li Y.-H., Mathieu G. G., Biscaye P., and Simpson J. J. ( 1977) The flux of 2*6Rafrom estuarine and continental shelf sediments. Earth Planet. Sci. Len. 37.237-241. Martin P. and Hancock G. J. ( 1992) Routine Analysis of Naturally Occurring Radionuclides in Environmental Samples by AlphaParticle Spectrometty. Re. Rep. 7. Supervising Scientist for the Alligator Rivers Region, Australian Government Publishing Service, Canberra. Moore W. S. and Todd J. F. ( 1993) Radium isotopes in the Orinoco Estuary and Eastern Caribbean Sea. J. Geophys. Res. 98,22332244. Moore W. S., Sarmiento J. L., and Key R. M. (1986) Tracing the Amazon component of surface Atlantic water using *“Ra. salinity and silica. J. Geophys. Res. 91, 2574-2580. Nvffeler U. P.. Li Y.-H.. and Santschi P. H. ( 1984) A kinetic ao-preach to describe trace-element distribution between particles aud solution in natural aquatic systems. Geochim. Cosmochim. Acta 48, 1513-1522. Sayles F. L. and Mangelsdorf P. C. ( 1977) The equilibration of clay minerals with seawater: exchange reactions. Geochim. Cosmochim. Acta 41,95 l-960. Webster I. T., Hancock G. J.. and Murray A. S. (1994) On the use of radium isotopes to estimate sediment flushing rates in an estuary. Limnol. Oceanogr. 39(8), 1917-1927.