Environ. Sci. Techno/. 1995,29, 1365-1372
Solid-Solution Distributions of Radionuclides in Acid Soils: Application of the WHAM Chemical Speciation Model EDWARD TIPPING,*,+COLIN WOOF,+ MICHAEL K E L L Y , * KEITH BRADSHAW,' AND JANE E. ROWE* Institute of Freshwater Ecology, Ambleside, Cumbria LA22 OLP, United Kingdom, and Institute of Environmental and Biological Sciences, Uniuersity of Lancaster, Luncaster LA1 4YQ, United Kingdom
The Windermere Humic Aqueous Model (WHAM) is tested with the data from laboratory experiments on three acid soils from the British Uplands. The model is reasonably successful at blind-predicting values of KO (solid-solution distribution coefficients) of Co, Sr, and Am on the basis of their interactions with soil humic matter. The competitive effects of aluminum are calculated to exert a strong influence on trace element binding. Organic matter interactions only explain the data for Cs in the case of the most organicrich soil; the KO values are substantially underestimated in the other cases, presumably reflecting the presence in the soils of minerals bearing sites with high specificity toward Cs+. Calculations for soils in situ show the importance of binding by dissolved organic matter in determining the KOvalues for strongly complexed radionuclides such as Th and Am.
The interactions of trace elements with soil solids exert a major control on their persistence, bioavailability, and transfer to surface waters and groundwaters. To obtain a proper description of the interactions, it is necessary to take into account other chemicalcomponents, most notably competing major ions like sodium, calcium, and aluminum, and inorganic and organic ligands that may form aqueousphase complexes with the trace elements. The more influence these other components have on a particular trace element, the less useful is a single-valued solid-solution distribution coefficient, KD[= (moles of element adsorbed per gram of solid)/ (moles of dissolved element per cm3 of solution)], The Windermere Humic Aqueous Model (WHAM) (1) has been developed to account for multiple chemical interactions in waters, sediments, and soils dominated by organic matter. The present work describes the testing of the model's ability to account for the interactions of trace amounts of Co, Sr, Cs, and Am with organic-rich acid soils. This can be regarded as an attempt to account for the observed variability in KD. The main effort is directed toward the explanation of new results with soils from the British Uplands. In addition, literature data published by Sanchez et al. (2) are considered. Windermere Humic Aqueous Model (WHAM). The model has previously been described in detail, and its approximations and shortcomings discussed (1). A summary of the main features is given here. WHAM is a combination of several submodels. These are Humic IonBinding Model V (3-6') and models of inorganic solution chemistry, precipitation of aluminum and iron oxyhydroxides, cation-exchange on a representative clay, and the adsorption-desorption reactions of fulvic acids (7). The interrelationships among these submodels are depicted in Figure 1. Features of the submodels, values of model parameters, and a list of inorganic species relevant to the present study are summarized in the Appendix. In Humic Ion-Binding Model V, humic compounds are represented by hypothetical size-homogeneous molecules, which carry proton-dissociating groups that can bind metal ions either singly or as bidentate pairs. The interactions are described by combinations of intrinsic equilibrium constants and electrostatic terms. The proton-binding groups of the humic substances are heterogeneous, having a range of intrinsic pKvalues. Competitive metal binding takes place at single proton-binding sites (monodentate) and at bidentate sites formed by pairs of proton-dissociating sites. The modification of binding strength at specific sites by electrostatic effects is due to the accumulation of an excess of counterions in a diffuse layer adjacent to the molecular surface, and this nonspecific process can be considered to contribute to the total binding. For concentrated systems like soils, where humic concentrations can be very high (tens of grams per liter of soil water), counterion accumulation may account for the greatest part of the binding of some species. In Model V, the diffuse layer is regarded as a zone of defined thickness around the humic molecules, and average concentrations of counter-
+
4
0013-936X/95/0929-1365$09.00/0
@ 1995 American Chemical Society
Institute of Freshwater Ecology. Institute of Environmental and Biological Sciences.
VOL. 29, NO. 5, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
1365
~
DISCRETE SITES
' ~
"4
DIFFUSE LAYER
SOIL/SEOIMENT SOLIOS
1
~
FIGURE 1. Functional relationships in WHAM. Sea the text and Appendix for more details.
ions within that zone are considered. Co-ions are completely excluded. Ionic distributions are calculated with Donnan expressions, requiring that the total counterion charge within the diffuse layer balances the charge due to the humic ionizable groups, as modified by their specific binding of protons and metal ions. The bulk solution is also charge-balanced (Figure 1). There is a degree of selectivity among nonspecifically bound counterions (1, 8).
A fixed charge mineral cation exchanger can be included in WHAM to allow some account to be taken of the presence of soil and sediment clays. The material is assumed to have a diffuse layer only, which is treated similarly to the humic diffuse layer. In the present work, the mineral exchanger was assumed to be present for initial data analysis, but the fits obtained were no better than those obtained with clay assumed absent. WHAM allows aluminum and iron(II1)oxyhydroxides to form iftheir solubility products are exceeded, but these reactions were not found to be relevant to the soils studied here. WHAM includes a submodel to calculate the distribution of fulvic acid (FA) between the aqueous phase and the soil solids (see Appendix). In this study, dissolved FA was estimated from directly measured values of dissolved organic carbon (DOC) when modeling the experimental results, assuming the FA to be 50%carbon. The FAsorption submodel was used when analyzing the data published by Sanchez et ul. (2) and for speculative calculations. The aqueous phase is considered to be comprised of fulvic acid, together with its diffuse layer,and a bulk solution in which the formation of inorganic complexes consisting of up to three inorganic master species occurs. Equilibrium constants and enthalpies for the reactions considered were obtained from several literature sources; the default inorganic chemistry data base for soils and sediments includes nearly 400 different species and reactions (1). 1366
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 5, 1995
ssetiea The soil samples used in this study were collected from the summit of Great Dun Fell in the Enghsh Pennines (U.K. Ordnance Survey Grid Reference NY 7103231. They are classified as an acidic brown earth (soil 92C), a peaty gley (soil92D),and a humic stagnogley (soil 92E). Two samples were collected at each site, from the OIA horizon (10-20 cm deep) and from the underlying mineral horizon. The horizons are distinguished by the letters 0 (organic) and M (mineral), respectively (e.g., soil samples 92CO and 92CM). The mineral horizons are stonyloams, comprising 20-30% clay (9). All the samples were acid; when suspended in 0.001 M NaCl, the organic horizon samples gave pH values of cu. 3.5 and the mineral horizons gave pH values of cu. 4.0. The samples were characterized by determining water content (drying at 100 "C) and carbon content (CarloErba elemental analyzer) or loss-on-ignition and by extracting metal cations with BaC12, as detailed in ref 8. Acidbase batch titrations of the soils were performed by suspending field-moist soil samples in 0.001 M NaCl to which known volumes of 0.1 M solutions of HC1, NaOH, or CaC4 had been added. For each suspension, four replicates each of totalvolume cu. 35 cm3were prepared in screw-cap centrifuge tubes. Two of the tubes were kept at the Windermere Laboratory of the Institute of Freshwater Ecology for the study of major components (pH, DOC, Al, base cations), while the other two were transported on the day of preparation to the Radiochemical Laboratory at Lancaster University. At Lancaster, the suspensions were spikedwith cocktails of 6oCo,*%r, 134Cs,and 241Am in 0.1 M HCl. The resulting radionuclide concentrations,in moles per liter of total water, were as follows: Co 1-3 x lo-", Sr 2 x 10-13, Cs 3-15 x 10-13,Am3-ll x 1O-l0, Thesuspensionsweremaintained at room temperature (ca. 20 "C) under constant agitation for 24 h, after which time they were centrifuged (12 000 rpm, 90 min) and the supernatants were transferred to preweighed glass vials. The vials were weighed, and cu. 0.5 cm3 of concentrated HC1 and 100 yL of carrier solution (containing 25 g L-l each of Co, Sr, and Cs and 5 mg L-l Nd, as chlorides in dilute HCl) were added to each. The solutions were then evaporated to dryness and taken for y spectrometry. Evaporated samples subsequently chosen for a-spectrometric determination of 241Amwere redissolved in a small quantity of concentrated HN03, and a known amount of standardized 243Am solution was added as a yield tracer for the subsequent purification steps. The samples were evaporated to dryness twice with concentrated "03 and redissolved in dilute HCl. Americium was separated by anion- and cation-exchange columns followed by electrodeposition onto planchettes. Samples were analyzed for y-emitting radionuclides by y-spectrometryusingan HPGe detector and a4Kmultichannel analyzer. a-Counting was done under vacuum with solid-state silicon surface barrier detectors. Amounts of the radionuclides adsorbed were calculated by the difference between total and supernatant amounts of radioactivity, and the values of KD(cm3g-l) were computed by taking the ratio of bound to free radionuclide divided by the concentration of soil solids. In some cases, it was found that supernatant concentrations of Am were too low for adequate determination by y-spectrometry, and so KD values could not be obtained. a-Spectrometry gave better detection limits for
Am and was used to estimate KDfor suspensions that had no added acid or base, i.e., those closest to the soils in situ. The suspensions kept at Windermere were spiked with the same amounts of radionuclide-free HC1, then agitated overnight at room temperature, and centrifuged. The supernatants were analyzed for pH, monomeric aluminum, and dissolved organic carbon. Good agreements in measurements of supernatant pH by the two laboratories were taken to justify combining the other analytical data to describe the major and minor element chemistry of the suspensions.
Results WHAM was applied to the experimental results with the
assumption that the solid-solution interactions in the soil suspensions are governed entirely by the soil humic substances. The model was used to fit the observed results for solution pH and aluminum concentration in the acidbase titrations by optimizing two quantities for each soil-the “active”contents of humic substances (CHS) and total monomeric aluminum (CAl)-while keeping all model constants (see Appendix) frxed. In this approach, the ionbinding properties of a soil sample are described in terms of average properties of humic acid and fulvic acid, derived from laboratory experiments with isolated materials. The model-estimated values of CHS and CAl have been found to be reasonably similar to estimates made on the basis of chemical extractions (81, although they are generally lower. This is presumed to reflect incomplete exposure of binding sites as a result of humic aggregation or adsorption to mineral surfaces. It should be noted that CHS is the weight content of active humic substances in the soil; for a given soil, therefore,it is a constant and not, for example, avariable measuring the soil content of ionized weak acid groups. The degree of ionization and extents of interaction with metal ions are computed by WHAM using the (frxed) constants given in the Appendix. The model calculates the distributions of chemical species, given total measured concentrations of base actions, strong acid anions, and dissolved organic carbon as inputs together with trial values of CHS and CAl; more details are given by Tipping et al. (8). In the present work, the humic substances in the organic horizons were assumed to be 84% humic acid (HA) and 16% fulvic acid (FA), while those in the mineral horizon were assumed to be 50%HA and 50% FA these proportions are averages from the results for a number of acid soils (8). Aqueous concentrations and activities of protons, aluminum species, and base cations (Na+,MgZf,Kf, Ca2+,NH4*) were computed for each point in a set of titration data (Table 21, and the value of the expression
was computed. (The second term is used instead of the perhaps more obvious residual in p[Al,,] to maintain consistency with previous work (7, 8) in which aqueousphase aluminum concentrations near to zero precluded the use of the logarithmic values.) The optimized pair of CHS and CAl was taken to be that for which the expression was minimal. Optimization was carried out with the BASIC code for the Nelder-Mead polytope method published by Nash and Walker-Smith (10).
TABLE 1
Analytical Data and Modeled Estimates of Soil Contents of Humic Substances and Monomeric Aluminuw soilcode
92CO
organic matter 0.26 C d a 0.7 GaMg 1.3 CBaAI 20 CB~K 0.2 C d a 1.5 C~a”4 0.1
9200
92E0
92CM
Measured Values 0.22 0.80 0.08 0.7 1.5 23 0.5 1.5 0.2
3.1 3.5 41 1.0 17.8
0.8
0.3 0.3 18 0.1 0.3 0.1
920M 0.12 0.4 0.2 21 0.1 0.2 0.0
92EM
0.08 0.4 0.4 20 0.1 1.5 0.1
Optimized Values CHS CAI
0.068 0.11 0.21 0.012 0.019 0.014 47 53 107 15 24 16
aThe BaC12-extractablecontentsofcations(CB,Na,CBaMg,CB,K, CB,Ca, C6.NH4, &.AI) and the modeled soil content of AI are in pmol (g of soil)-’. The soil organic matter content and the modeled contents of humic substances are in g (g of soil)-’.
The results are summarized in Table 1, while observed and calculated values of pH and p[Al,,] are shown in Table 2. The agreements between observed and calculatedvalues of pH and ~ [ A l q are ] considered acceptable, given that only two quantities are adjusted to achieve the best fits. For the organic horizons, the optimized soil contents of humic substances (CHS)represent between 26 and 50% of the soil organic matter, whereas for the mineral horizons the range is 15-18%. Given that organic matter in the mineral horizons of acid soils is generally considered to be more humified than that in the organic horizons ( 1 11,higher percentages for the mineral horizons might be expected, and therefore the relatively low values obtained may reflect the nonavailability of humic functional groups due to interactions with mineral surfaces (see above). With WHAM optimized on the basis of the interactions of protons and aluminum species with the humic matter, blind predictions of the distributions of the added radionuclides were made. This was done by including the total radionuclide concentrations in the data input to the model and by computing the equilibrium distributions of all components. The model output gives extents of specific binding (complexation) of the radioelements, based on values of the ModelVparameter, PKMM,and ofnonspecific cation-exchange binding, based on Donnan distributions and the net humic charge. Predictions are compared with observations in Table 2 and Figure 2. The predicted results refer to the actual radionuclide concentrations used in the experiments (all < 2 x mol L-l), but additional calculations showed that the same KDvalues were obtained if total concentrations as high as mol L-I were assumed. Thus, from the model’s perspective, it is evident that the radionuclides were present at trace levels. There are good agreements between observed and calculated values of KD for the interactions of Co and Sr with organic horizon samples. The model is able to predict the magnitudes of KD,the pH dependence, and the effects of the added competitor (Ca). There is a tendency for the KDvalues for the mineral horizons to be underpredicted by the model, but the trends are reproduced. In the cases of Co and Sr, the success or otherwise of the model derives VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1
1367
soil
[soil]
TB - TA
92CO
307
-2.9 -0.3 2.9 -13.3
92DO
351
-3.0 -0.3 3.0 -13.8
92EO
156
-2.4 -0.1 2.7 -11.6
92CM
515
cs
3.2
0.8
0.7
3.2
3.6
0.4
0.5
2.6
3.5
4.1
1.4
1.3
3.5
2.7 0.0 3.1
3.4
4.7
1.2
7.3
3.1
0.7
4.1
4.5
2.3
2.2
3.1
3.4
3.9
4.5
2.3
2.4
3.2
1.4
3.5
3.4
0.8
0.6
2.9
2.7
2.6
-0.6
6.6
3.2
3.2 4.7 21.9
7.0
8.4
0
9.3
3.3
3.6
0.0
0.0
3.1
3.4
0.7
0.7
2.9
3.4
0.5
0.7
2.5
0.2
4.2
1.2
1.2
3.5
2.9
9.1
3.4
3.1
4.1
1.4
1.5
3. I
0.8
0
34.9
3.9
4.6
2.1
3.0
3.2
3.2
3.5
4.4
2.3
2.4
3.1
1.4
7.3
11.7
3.3
3.4
0.6
0.5
3.0
2.6
3.0
3.4
2.4
-0.4
18.8
3.1
3.9
3.2
3.9
3.5
4.8
3.4
4.3 4.5
0.1 1.7 1.2 2.2 1.9 2.5 2.7 1.4 0.7 0.3 -0.8 1.1 0.6 1.9 1.3 0.3 - 0.3 0.3 -0.6 1.2 0.8 1.7 1.3 0.3 -0.1 0.5 -0.5 1.5 0.7 2.1 1.4 0.4 -0.4
0. I
0 0
20.0
0
30.1
3.9
3.8
4.4
9.2
23.6
3.2
3.7
3.2
3.8 3.0
3.2
3.0
-0.4
0
4.5
3.9
4.4
3.9
3.9
0
9.1
4.5
5.3
4.2
4.4
7.7
7.6
3.8
3.2
3.8
3.4
0
2.6
3.3
3.0
3.4
3.1
-4.7
-15.5 -5.1 -0.4 0.9 -18.7
2.6
0
3.3
0.7
510
0
Am
~[Alaql
3.2
-0.5
92EM
0
Sr
PH
0
-15.5 504
0
co
[DOC1
-4.6
0.8
92DM
Tca
0
3.0
3.8
4.2
3.9
3.9 5.0
0
7.2
4.2
4. I
4.3
7.7
3.1
3.7
3.2
3.8
3.4
0
3.9
3.1
3.2
3.2
3.2
3.8
4.7
3.9
4.2
10.6
4.2
5.0
4.2
4.5
2.8
3.7
3.5
3.6
3.4
0 0 9.2
4.3
1.5
1.8
1.3
3.2
0.7
2.1
3.9
1.7
2.1
3.5
1.3
2.4
2.9
2.0
2.8
3.5
1.1
1.7 1.5
0.8
3.1
0.2
0.3
3.0
3.1
- 1.8
1.5
1.2
3.6
0.7
2.4
1.9
0.2 3.7
I. 5
2.8
0.7
0.3
2.6
3.2
-0.6
1.9
- 1.3
0.3
3.1
- 1.7
1.8
1.2 0.8 1.7 1.4 0.2 -0.5 0.5 -0.6 1.5 0.8 2.1 1.6 0.2 -0.6
3.5
3.4
2.6
0.3
3.2
3.7
2.9
0.7
2.7
3.1
2.1
- 1.2
1.7
- 1.0
3.4
3.3
2.9
2.7
0.3
2.9
3.6
2.8
0.7
2.8
2.9
1.9
- 1.2
TB- Tn (mmol L-1 of total water) is the concentration of net added base (added [Nal added [CI]), T,, (mmol L-’) is the concentration of added Ca. [DOC] is in rng L-l, [AI,,] is in mol L-l, and KD is in cm3g-’. The figures in normal typescript are observations, italicized values of pH and p[Al,,l were obtained by fitting with WHAM; italicized values of loglo KO are predictions. a
from its ability to predict the (variable) cation-exchange capacity of the soils at different pH since very little of the uptake is calculated to be due to specific binding. The increase in loglo KD with pH reflects the progressive deprotonation of the solid-phase humic matter and the accompanyingincrease in net humic charge,Z, as illustrated in Figure 3. The underprediction of KD for the mineral soils possibly reflects errors in the calculation of Z, which depends primarily on the relatively small differencebetween the extents of humic deprotonation and aluminum binding. Inspection of the results in Table 2 shows that the worst cases of underprediction are at the lowest pHvalues, where Z is near to zero (Figure 3). Regarding the application of WHAM to environmental systems, the incorrect prediction of low values of KD is perhaps the least serious form of model failure. 1368 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5,1995
~
Neither the observed nor the calculated KD values for Am show a strong trend with pH (Table 2). This arises
primarily because at the highest pH values, fulvic acid desorbs from the soil solids (see [DOC] values in Table 21, taking complexed Am with it. Thus, the solution concentration ofhincreases, and this prevents KDfrom increasing with pH. Calculations with WHAM with the aqueous concentration of FA set to zero give loglo KDvalues of 4-5 at the highest pH values. Thus, WHAM is able to predict the KD for Am reasonably well, first because it deals appropriately with the binding by the solid phase and second because it accounts for Am complexation by dissolved fulvic acid. Note that Am differs from Co and Sr in that essentially all the binding (to the solid phase and to dissolved FA) is due to specific complexation as opposed to nonspecific cation exchange.
50
n
c
al
0
I
-
0
O
E
-50
a
organic horizons 0 92C 0 920 0 92E
CI
N -100
-150
0 1
0
-
2.5 3
2
5
4
3.0
_I
3.5 1
.'
4
bl
n c
mineral horizons
I0
-
0
E
a
Y
-
N
-5
- mineral 0
I u -
3
horizons
92E
~~
3.5
4.5
4.0
PH .
-2 0
,
I
1
,
I
2
,
1
3
,
1
4
FIGURE 3. Plots of calculated net soil charge (2) versus pH for the organic and mineral horizon soil samples. Note the different scales on the ordinate.
,
5
logloKD observed FIGURE 2. Observed and calculated values of loglo KOfor the Great Dun Fell soils. The 1:l lines are shown for guidance. The four points for Cs closest to the 1:l line in panel a refer to soil 92EO.
The observations considered so far, for both major and minor components, have been explained reasonably well by assuming that the sorption properties of the Great Dun Fell soils are due entirely to organic matter. However, this is not the case for the results with cesium. Only for soil 92E0, the most organic-rich (Table 1) soil, was the model found to predict KDto within an order of magnitude (Table 2, Figure 2). For the other soils, KDwas underestimated by several orders of magnitude. This is presumably due to the presence in the soils of clay minerals such as illite with sites of high selectivity for Cs (see ref 12). When such sites are present, the organic matter plays a minor role because it can only bind Cs by nonspecific ion exchange. A study by Sanchez et al. (2)provides further data with which to examine the ability of WHAM to describe radionclide-organic soil interactions. These authors suspended samples from two peat soils in artificial acid rain (13)at concentrations of 0.2 g L-l and added spikes of Co,
Sr, and Cs. The pH values of the suspensions were ad) :ted to pH 4.0, and KDvalues were determined by the centrifugation-depletion technique. One of the soils had a much greater carbon content than the other and low contents of mineral matter and aluminum. These features allow the composition of the experimentalsuspension to be estimated reasonably well, and therefore WHAM was applied to the results for this sample. Table 3 shows observed values of loglo KD and calculated values for two assumed values of CHS, both of which are within the range we have found for more than 20 different organic soils (8). The calculated values of KD are seen to be in reasonable agreement with the observations. This is true even for Cs, suggesting that there were very few Cs-specific sites on mineral surfaces in this soil. It is noteworthy that the KD values for this soil for Co and Sr are approximately an order of magnitude greater than found in our own experiments. This is mainly because the soil suspensions are low in aluminum; the effect of this metal can be seen from the plot in Figure 4, in which the calculated dependence on aluminum concentration of the KDfor Co is shown. The extent of binding of Al by the humic substances is characterized in Figure 4 by the bound aluminum ratio (BAR), a variable introduced by Walker et al. (14). BAR is the ratio of equivalents of aluminum bound per carboxyl group content. The values of BAR for the VOL. 29, NO. 5 , 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1
1369
TABLE 3
TABLE 4
Comparison of Results of Sanchez et ai. (2) with WHAM Predictions"
Calculations with WHAM for Soil 92E in SiftP
calcd loglo KO CHS = 0.1 g g-l CHS = 0.2 g g-l
range of obs loglo KO
metal
3.4 - 3.9 3.3 - 3.9 1.8 - 3.3
co Sr
cs
3.4 3.5 2.8
3.8 4.0 3.1
d
4
KO
pH
[FA.,] (mg L-l)
Co
Sr
UOZ
Th
Am
92E0
3.9
0
3.3 3.3 2.3 2.0 2.0 1.8
3.5 3.5 2.3 2.2 2.2 1.9
4.6 4.6 3.4 3.2 3.2 2.9
8.3 4.7 2.1 7.1 3.9 2.2
5.7 5.0
4.5
1 329 0 1 44.7
92EM
a The rangesofobservedvalues include resultsfrom both adsorption and desorption experiments for two depths of a peat soil. The experimental pH was 4.0, and the background electrolyte was artificial acid rain as formulated by Singh et a/. ( 73). The dissolved organic carbon concentrations in the experimental supernatants were not measured; values < I mg L-' are estimated with WHAM, which would be insufficient appreciably to influence KO values for the radionuclides in question.
2.6 4.3 3.9 2.4
a The assumed soil concentrations were 330 g L-' of soil water for theorganic horizon and 20009 L-'forthemineral horizon. Theassumed soil contents of HS and cationic species were as in Table 1; assumed aqueous concentrations of CI-, NO3-, and Sod2-were 200, 30, and 50 pmol L-', respectively. The highest concentration of aqueous-phase fulvicacid, [FA.,], in eachcasewascalculated usingvaluesofyestimated in previous work (8); y = 1.5 for soil 92E0, y = 1.9 for soil 92EM (see Appendix). For comparative purposes, calculations were also performed for fixed (lower) values of [FA,,].
3
mineral horizons were 0.9-1.0. It is apparent from these results that competition by aluminum exerts a strong influence on trace element binding.
2
Discussion
1
0 0
3
50
100
150
I
I
I
200
zn 8 ' 0
1.2
0.8 lmO
Om2
I
t
1
1
50
100
150
200
I
I
1
I
I
I
1I
50
100
150
200
t t/
0" 0
cl
FIGURE 4. Calculated effect of soil aluminum content on log,o KO for Co, pAl, and bound aluminum ratio (BAR). The results refer to a hypothetical titration with AI of the peat studied by Sanchez et a/. (2)with pH fixed at 4.0.
calculations in Figure 4 are 0-1.0; those for the Great Dun Fell organic horizons were 0.4-0.6,while those for the 1370
h i o
soil
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5,1995
The present study has shown that WHAM is able to explain to a considerable extent the experimental results for the interactions of several important radionuclides with acid organic soils. This represents a partial validation of the model, although more extensive data sets for more trace elements are desirable for further testing. In applying the model to major element chemistry in acid soils, some failures have been encountered (81, and there may be a need for modification of the model as more data become available. The results obtained in the present worksuggest that one aspect of the present version of WHAM that could be improved is the description of aluminum and proton binding under conditions where the net charge on the humic substances is near to zero since predicted nonspecific cation exchange is highly dependent upon the charge. This affects the binding of Co and Sr in particular (Table 2, Figure 2).
The main reason for developing and testing WHAM is to estimate the chemical behavior of soils in situ. For illustrativepurposes, calculations have been performed for one of the Great Dun Fell soils, assuming equilibrium under field conditions. This requires the high effective concentrations of the soil solids to be taken into account. The in situ concentrations are estimated to be ca. 330 g L-' for the organic horizon (ca. 75% water) and 2000 g L-l for the mineral horizon (ca. 25%water). The results are shown in Table 4. Speculative values for Th and UOz are included, based on PKMWvalues for humic and fulvic acids derived previously from published data (see Appendix and ref 6). An important feature of the calculations is the estimation of the concentration of aqueous-phase fulvic acid [Fkq1. The model includes a submodel to estimate [Fkq1, and this predicts a high value (329 mg L-l) for the organic soil horizon. To explore the influence of [Fhq]on KD,calculations were performed for [Fkq1 = 0 and 1 mg L-l as well as for the values estimated with WHAM. It is seen (Table 4) that in the organic soil the KDvalues for all radionuclides except Cs depend substantially on [F&,]. This is especially the case for Th and Am, the most strongly organicallybound elements. The same trends are seen for the mineral soil
horizon, but here the model-predicted value of [Fhq]is much lower (ca. 10 mg L-l). From these calculations, it is concluded that although organic-rich acid soils will bind certain radionuclides strongly, thereby tending to immobilize them, binding by organic compounds (FA) in the aqueous phase can be responsible for fairly low values of KD. The mobilities of some radionuclides in soils may therefore be critically dependent upon organic matter dynamics. This is in agreement with the observations of Nishita and Haug (15) that fulvic and humic acids appreciably increased the extractabilityof Pu(W that had been added to mineral soils but is not in agreement with those of Cleveland and Rees (161, who reported little solubilization of either Pu or Am from a contaminated mineral soil. Further experimental studies with different soils, including an investigation of possible aging effects, would help to clarify the issue. Another possibly significantpoint that arises from the results of Table 4 is that even low concentrations of F h q can substantially affect the value of KD so that when experimental studies are conducted to estimate possible solubility controls on radionuclides (e.g., 171, an account should be taken of possible organic complexation in the aqueous phase. WHAM is potentially useful in simulating radionuclide transport in acid soils. By combining the equilibrium soil model with a suitable model for hydrology, radionuclide dynamics could be explored. Work toward this objective is in progress.
Acknowledgments We thank R. Comans, J. Hilton, and M. Hornung for helpful discussions and an anonymous referee for constructive criticism. This work was funded by the United Kingdom Ministry of Agriculture, Fisheries and Food.
Appendix: Summary of WHAM Submodels and Parameters Humic Ion-Binding Model V. This submodel describes the specific and nonspecific binding of ions by humic acid (HA) and fulvic acid (FA). The main parameter set deals with proton binding and electrostatics: Model V parameter
mol of type A protonbindin sites per g Of H 8 (ne = 0.5fin) PKA central pKvalue for type A sites ~ K B central pKvalue for type B sites ApKa spread factor for type A pK’s ApK8 spread factor for type B pK’s P electrostatic parameter fpr proximity factor mol weight mol radius (nm) PKMHB in terms of PKMHA nA
HA
FA
3.29 x 10-3
4.73 10-3
4.02
3.26
8.55
9.6
1.78
3.34
3.43
5.52
-374 0.5 15000
-103 0.4 1500
1.72
0.8
3 PKMHA -3
3.96 ~ K M H A
Interactions with cations are described with p K ~ mand Ksei. The former is the negative logarithm of the equilibrium constant for cation-proton exchange; the latter is the selectivity coefficient for cation exchange (accumulation of counterions). Values of Ksel have only been estimated for alkali and alkaline earth metals; the default value of 1.0
is used for other species:
1 .o
NH4+ Na+ Mg2+ AP+, A I O H ~ + K+
3.3 1.3
2.2 0.4
Ca2+ Co2+, CoOH+
3.2 2.7 2.8
2.2 1.7 2.3
SrZ+ cs+ U02’+, U020H+ Th4+, ThOH3+ Am3+,AmOHZ+
1.3
0.25 0.75 0.5 1 .o
1 .o 1 .o
1.5
1.5 1 .o 1 .o 1 .o
0.9
--0.4 0.3
0.6 1.2
Mineral Cation Exchanger. The exchanger has a specific surface area of 100 m2g-’ and an exchange capacity of mol g-l. Precipitation and Dissolution of Al(0H)s. This can occur by the reaction Al(OH)3 3H+ = A13+ 3H20, for which a solubility product of lo9 and an enthalpy of -25 kcal mol-’ are assumed. Sorption Reactions of Pulvic Acid. The FA are considered to comprise 10 fractions, each of which adsorbs to the soil solids according to
+
+
FALad,= [FAi,aq] $czi-z) where FAi,adsis the amount of FA fraction i adsorbed in g (g of soil)-I, [FAi,aq]is the concentration of dissolved FA fraction i, fi is a constant, Zj is the modulus of the characteristic charge of FA fraction i defining the buildup of charge needed to overcome the fulvic hydrophobicity, and Z is the modulus of the actual charge on the fulvic fraction. The values of Zi are given by
nA Z j = -(i - 0.5) 10
where n~ is the fulvic content of type A (COOH) groups. The total soil contents of the different fulvic fractions (CFAi; g of FAi per g of soil) are given by
where CFA is the total FA content of the soil (allfractions). Limitations on Diffuse Layer Volume. It is assumed that in a soil there is shrinkage of the diffuse layers of HA, FA, and the mineral cation exchanger compared to the freely extending diffuse layers that would exist at infinite dilution. The model constrains the total volume of diffuse layers to be no more than 25%of the total volume of water. A factor is also included to make the diffuse layer volume shrink further at low net humic charge; this attempts to take into account the breakdown of the assumption that the humic charge is evenly distributed over all the molecular surface. It is arranged that as the magnitude of Z decreases the diffuse layer volume decreases as well. Inorganic Species. The species considered in the present work are listed below; the equilibrium constants VOL. 29, NO. 5 , 1 9 9 5 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
1371
and enthalpies for the formation reactions are given in ref 1. HN H4+ Na+ Mg2+ MgHC03' MgC03 MgSO4 ~ 1 3 +
AIOH2+ AI(OH)z+ AI(0H 14AIS04+ Ca2CaHC03+ CaC03 Cas04
co2+ CoOH+ Co(0H)z CoHC03+ COCO3 COS04 CoCI+ SrZ+ SrHC03' SrC03 SrS04
Th4+ ThOH3+ Th(OH)Z2+ Th(OH)3+ Th(OH)4 ThC032+ ThCl+ Am3+ AmOH2+ Am(OH)z+ A m (OH)3 Am (0H)rAmC03' Am(C0d-
OHCO& HC03HzC03 CINos-
sod2-
Literature Cited (1) Tipping, E. Comp. Geosci. 1994, 20, 973-1023. (2) Sanchez, A. L.; Schell, W. R.; Thomas, E. D. Health Phys. 1988, 54, 317-322. (3) Tipping, E.; Hurley, M. A. Geochim. Cosmochim.Acta 1992,56, 3627-3641.
1372 m ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5,1995
Tipping, E. Enuiron. Sci. Technol. 1993, 27, 520-529. Tipping, E. Colloids Surf: A 1993, 73, 117-131. Tipping, E. Rudiochim. Acta 1993, 62, 141-152. Tipping, E.; Woof, C. J. Soil Sci. 1991, 42, 437-448. ( 8 ) Tipping, E.; Berggren, D.; Mulder, J.; Woof, C. Eur. J. Soil Sci., in press. (9) Hornung, M. The Morphology, Mineralogy and Genesis of Some Soils on the Moor House National Nature Reserve. Ph.D. Thesis, University of Durham, 1968. (10) Nash, J. C.; Walker-Smith, M. Nonlinear Parameter Estimation. An Integrated System in BASIC; Dekker: New York, 1987. (11) Kononova, M. M. SoilOrgunicMatter; Pergamon: Oxford, 1961. (12) Sawhney, B. L. Soil Sci. Soc. Am. Proc. 1964, 31, 183-186. (13) Singh, B. R.; Abrahamsen, G.; Stuanes, A. Soil Sci. Soc. Am. J. 1980, 44, 75-80. (14) Walker, W. J.; Cronan, C. S.; Bloom, P. R. Soil Sci. SOC. Am. J. 1990, 54, 369-374. (15) Nishita, H.; Haug, R. M. Soil Sci. 1979, 128, 291-296. (16) Cleveland, J. M.; Rees, T. F.Environ. Sci. Technol. 1976,10,802(4) (5) (6) (7)
806.
(17) Rai, D.; Serne, R. J.; Moore, D. A. Soil Sci. Soc. Am. J. 1980, 44, 490-495.
Received for review October 14, 1994. Revised manuscript received January 19, 1995. Accepted February 1, 1995.@
ES940632J @
Abstract published in Advance ACS Abstracts, March 15, 1995.
