Effect of Particle Concentration and Fixation on Radiocesium Sorption

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Environ. Sci. Technol. 1996, 30, 2892-2898

Effect of Particle Concentration and Fixation on Radiocesium Sorption JAN WAUTERS* AND ADRIEN CREMERS Laboratory for Colloid Chemistry, Department of Interface Chemistry, Faculty of Agricultural and Applied Biological Sciences, K. U. Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium

A study was carried out on the effect of particle concentration on the solid-liquid distribution coefficient of radiocesium in suspended particles. The study was based on sorption-desorption batch equilibrium experiments in rigorously controlled ionic conditions. No concentration effect could be detected upon adsorption even though the range of particle concentrations covered 3 orders of magnitude. Trace cesium adsorption isotherms were linear. Particle concentration effects were noticeable in desorption tests and indicated that a fraction of the cesium is irreversibly fixed upon adsorption. This pool appeared to be a constant fraction of the initially adsorbed amount of radiocesium rather than of the cesiumselective sites. These findings were backed up by results from consecutive desorption experiments and quantified in an equilibrium model based on linear sorption isotherms and incorporating sorption irreversibility. Successful application of the model to sorption data on four more substrates widened its scope.

Introduction Some 10 years after Chernobyl, it is generally acknowledged that the understanding of the geochemical behavior of radiocesium in terrestrial and aquatic ecosystems is of key importance in the assessment of the long-term radiation risk. Such behavior is most often characterized by the solid/ liquid distribution coefficient (KD ) Bq g-1/Bq mL-1). Its value depends on a number of variables such as the ionic composition of both liquid and solid phases, the mineralogical composition of the substrate, and the intrinsic affinity for radiocesium of the solid phase. Together with the particle concentration (Cp ) amount of solid phase (g)/amount of liquid phase (mL)), KD determines the fraction of radiocesium associated with the particulate phase (F s):

F s ) KDCp/(1 + KDCp)

(1)

The fraction F s is strongly related to the nature of the ecosystem under study and is an important determinant for the residence time of the nuclide in the system (1). For * Corresponding author telephone: 32.16.321457; fax: 32.16.321998; e-mail address: [email protected].

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bottom sediments and soils, an increase in KD yields a proportional decrease of the fraction of radiocesium in the liquid phase (F l), but does not alter F s to any significant extent (F s . F l). The inverse scenario is encountered for suspended particles in aquatic systems: a KD increase entails a proportional increase in F s but hardly alters F l (F l . F s). Hence, countermeasures that increase KD should reduce the activity level in soil solutions and lead to a decreased plant uptake, whereas in aquatic systems, KD variations would hardly alter the activity levels that fauna and flora are exposed to but would shorten the period of time needed for the radionuclide to be discarded from the water column. Less evident is the controversial issue of the effect of Cp on KD often reported in the literature (2, 3). The so-called “particle concentration effect” is an unexpected and not readily explainable increase in KD with decreasing Cp. The importance of this phenomenon is far from academic: Depending on the relationsif anysbetween those two parameters, predictions of F s, and hence of residence times, can vary widely, as was clearly demonstrated by Honeyman and Santschi (1). In this paper, we are not referring to experimental artifacts related to changing ionic conditions. Ionic strength and the cationic composition of the liquid and solid phases (especially [K+] and [NH4+]) are important determinants for KD (4-8). They are influenced by the dilution of soluble salts and the heterovalence effect. To single out genuine Cp effects, all experiments reported below were carried out in solutions of rigorously controlled ionic composition. Some authors have explained Cp effects as an experimental artifact due to incomplete phase separation (9-12) or as indirect effects caused by changes of the cesium occupancy on the pool of cesium-selective sites (4). We will argue that Cp effects can be quantified as indirect effects due to partially irreversible sorption behavior of cesium. To that effect, we performed four experiments. First of all, we looked for Cp effects upon adsorption. Not having found any, we proceeded to investigating desorption Cp effects; results were elaborated upon in a consecutive desorption experiment and, finally, validated for additional substrates.

Materials and Methods Unless specified differently, experiments were performed using suspended particles from the Meuse River. The sample was collected at Hastie`re, Belgium, by the Institute for Hygiene and Epidemology (I.H.E.), air-dried, and passed over a 200-µm sieve. Its organic matter content was close to 30%. CEC was determined by the AgTU method (13) and equaled 295 µequiv g-1. The capacity of the pool of sites that is extremely selective for cesium (ln K cHAS(Cs/K) = 10.5), the “high affinity sites” or HAS, equaled 0.22 µequiv g-1. The high affinity sites are situated on the frayed edges of illitic clay particles and represent a substrate-specific fraction of the capacity of the frayed edge sites (FES) of about 10% (4, 14). Such values were obtained using different masking agents for the non-Cs-specific sites. The study on these HAS has been submitted. All KD measurements were performed in solutions of known ionic composition. Phase separation was achieved by high speed centrifugation (20 min at 27000g) or by the

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use of dialysis membranes (dialysis tubing, visking size 2 18/ in.; Medicell International Ltd.). Liquid-phase activities 32 were measured in a liquid scintillation analyzer (Packard, Tri-carb Model 1600CA). The 137Cs tracer was purchased from Amersham and had a specific activity of about 1010 dpm µmol-1. Adsorption Experiment. Various amounts of sample and solution were mixed to cover a relatively broad range of Cp values (2 × 10-5-1 × 10-2 g mL-1). For Cp exceeding 1 × 10-3 g mL-1, phase separation was performed by centrifugation; dialysis membranes were used at lower Cp. Three solutions were tested: 0.01 M KCl, synthetic seawater without NaCl, and synthetic seawater without bivalent cations. The composition of synthetic seawater (SW) was the following: 0.01 M KCl, 0.0102 M CaCl2, 0.0504 M MgCl2, and 0.470 M NaCl. The experiment was performed in duplicate. Single Desorption Experiment. Three subsamples of Meuse River sediment of approximately 10, 25, and 50 mg were introduced into three dialysis membranes. They were suspended in 10 mL of 1.0 × 10-3 M KCl (inside the membranes), and the membranes were transferred into three polyethylene vials containing 250 mL of 1.0 × 10-3 M KCl labeled with 137Cs. Systems were allowed to equilibrate for 24 h on an end-over-end shaker. The same procedure was repeated at 1.0 × 10-2 M KCl, thus producing six different Cp - [K+] combinations. The whole procedure was duplicated to yield a total of 12 observations. KD was measured by monitoring the liquid-phase activity in the outer solution before and after equilibration. As a result, KD values at three levels of Cp and two K+ concentrations were obtained in duplicate. Afterwards, the outer solution was discarded and replaced by 250 mL of either synthetic seawater (SW) or synthetic seawater without bivalent ions (SW - M2+). Thus, each Cp - [K+] combination applied during the adsorption was desorbed once in SW and once in (SW - M2+). Consecutive Desorption Experiment. Subsamples (0.0500 ( 0.0005 g) of Meuse River sediment were introduced into dialysis membranes with 10 mL of 1.0 × 10-3 M KCl. The membranes were brought into polyethylene vials containing 25 mL of 1.0 × 10-3 M KCl labeled with 137Cs. Systems were allowed to equilibrate for 24 h on an end-over-end shaker. A first desorption was performed by introducing the membranes in 25 mL of an unlabeled 1.0 × 10-2 M KCl solution and equilibrating them for 48 h. This step was repeated in 30 mL of 1.0 × 10-2 M KCl. From the third to the sixth desorption, 250 mL of a 1.0 × 10-2 M KCl solution was used; the equilibration time on the end-overend shaker was at least 72 h. The experiment was set up in 10-fold. Validation Experiment. Samples from two lake sediments from the Lake District, U.K. (Devoke and Esthwaite), a Po River sediment from Cornale, Italy, and a reference illite clay from Silver Hill, MO, were stored air-dry, ground, and sieved (200 µm). Values of the cation exchange capacity (AgTU method) are included in Table 3. Subsamples (0.250 ( 0.002 g) of each substrate were introduced into dialysis membranes together with 10 mL of solution (1.0 × 10-3 M in KCl and 2.0 × 10-3 M in CaCl2) and thoroughly equilibrated with a solution of the same ionic composition. The membranes were brought into polyethylene vials containing 25 mL of the same solution labeled with 137Cs. The systems were allowed to equilibrate for 72 h on an end-over-end shaker. A first desorption was performed on

FIGURE 1. Influence of the particle concentration (g mL-1) on the distribution coefficient (mL g-1) for the adsorption of radiocesium on suspended particles of the Meuse River in 0.01 M KCl (9), synthetic seawater without NaCl (SW - NaCl) (() and synthetic seawater without calcium and magnesium (SW - M2+) (2). Symbols refer to experimental data (averages for two measurements, differing by less than 10%).

all samples by introducing the membranes in 240 mL of (the same but) unlabeled solution and equilibrating them for 1 week on an end-over-end shaker. This step was repeated nine times. For two of the four substrates (the Esthwaite sediment and the illite sample), the volume of the desorption solution was increased to 1000 mL in the fourth desorption run and decreased to 500 mL in the subsequent runs. For each run, the attainment of desorption equilibrium was verified by monitoring the liquidphase activity three to five times during the 1-week desorption period. The experiment was set up in triplicate.

Results and Discussion Adsorption KD. The results of the adsorption experiment are shown in Figure 1. It is apparent that, within each of the three liquid-phase compositions and for a Cp range covering nearly 3 orders of magnitude, KD variations are quite small and that measurements were not influenced by the method used for phase separation (dialysis or centrifugation). On the other hand, the cationic environment did influence KD greatly, as expected (see Introduction). The statistical average KD values (in mL g-1) are

10-2 M KCl SW - NaCl SW - M2+

dialysis-membrane KD

centrifugation KD

overall KD

2195 ( 96 1666 ( 88 918 ( 55

1990 ( 49 1631 ( 133 831 ( 188

2093 ( 131 1649 ( 103 875 ( 133

It is worth noting that the cesium occupancy of the HAS HAS HAS (Z Cs ) changes with Cp. Although Z Cs varied relatively widely (between 0.1% and 3.5% for the set of data in Figure 1), KD remained unaffected. This would entail that trace cesium adsorption isotherms are linear and, moreover, that the explanation referred to in the Introduction that Cp effects HAS are indirect effects caused by changes in the Z Cs is unsubstantiated. Single Desorption KD. KD data are shown in Figure 2. In contrast with the abscence of a Cp effect during the

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s T F fix,ads ) F fix,des

(3)

The fraction of 137Cs that no longer participates in ion exchange reactions can further be expressed with respect to the capacity of the sites that rule radiocesium sorption, the HAS: HAS ) F fix

Qfix [HAS]

(4)

This fraction will turn out to be of interest; it can be rewritten as HAS ) F fix

FIGURE 2. Influence of the Cp on KD,ads (9) and KD,des (() for suspended particles of the Meuse in 0.001 M KCl, 0.01 M KCl, synthetic seawater (SW), or synthetic seawater without bivalent ions (SW - M2+). Symbols refer to averages of two measurements differing by less than 10% (adsorption) or to single measurements (desorption).

adsorption phase, an increase in KD with decreasing Cp was noted for the desorption step. Since no Cp effect was detected for the adsorption step of this experiment, none was to be expected a priori for the desorption step either. That one did occur nonetheless proves that sorption issat least in some respectsan irreversible process. This induces us to discard the hypothesis that incomplete phase separation accounts for the apparent Cp effect since the influence of colloids should be equally noticeable in both adsorption and desorption. A possible way of rationalizing the observed Cp effects is as follows. It is plausible that during the adsorption phase a certain fraction of the adsorbed cesium becomes fixed and thus no longer participates in ion exchange reactions on the time scale of weeks to months. Such fraction can be expressed with respect to all the cesium present in the systemson the solids and in the solutionsas: T F fix )

Qfix s

Q +Q

l

)

(KD - KD,rev)Cp 1 + KDCp

(2)

T where F fix is the fraction of cesium fixed, expressed with respect to all cesium present; Qfix is the amount of cesium fixed; Qs is the amount of cesium on the solid phase; Ql is the amount of cesium in the liquid phase; KD is the overall distribution coefficient for cesium; KD,rev is the distribution coefficient confined to the exchangeable fraction of cesium in the system; and Cp is the particle concentration. [Here and in what follows, F xy denotes the fraction of Cs with respect to the pool y with property x; Q xy denotes the y amount of Cs in pool y with property x; and K D,x denotes the KD for pool y with property x; y can be either T (total Cs present), s (Cs on solids), l (Cs in liquid phase), or HAS (capacity of the high affinity sites); x can be rev (reversible) or fix (fixed) and/or ads (adsorption phase) or des (desorption phase).] T s In our experimental setup, it holds that Qdes ) Qads since the liquid phase of the adsorption equilibrium is discarded and replaced by a radiocesium-free desorption solution, so that all of the 137Cs present in the desorption system T (Qdes ) originates from the adsorbed 137Cs at the end of the s ). As a consequence, it can be stated adsorption step (Qads that

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s s Qads F fix,ads

[HAS]

(5)

s s where Qads is measurable and F fix,ads is computable via eqs 3 and 2. To apply eq 2 to the experimental data in Figure 2, values for KD,rev have to be estimated first, since they cannot be measured directly. Estimates were made on the basis of RIP theory (14, 15). This theory on trace cesium sorption introduced a substrate-specific parameter, the radiocesium interception potential (RIP), that makes it possible to predict KD values when the cationic composition of the liquid phase is known (7). Two of the experimental conditions that are prescribed in the RIP determination protocol strongly favor reversible sorption, namely, the short adsorption time (24 h) and the fact that the high-affinity sites are homoionically saturated with potassium (see ref 16 on the effect of adsorption time and saturating cation on the cesium sorption kinetics on illite clay). Hence, values for KD estimated via RIP should approximate KD,rev very closely. For the K+ concentration in this experiment (1.0 × 10-2 M), RIP theory yields a KD for suspended particles of the Meuse River of

KD (10-2 M K+) ) 1900 mL g-1 This value, which is valid for a Cs-labeled homoionic 1.0 × 10-2 M K+ system, was then used to determine KD(SW) and KD(SW - M2+). Work by Pollaert (17) has shown that fixed ratios exist between KD values for these systems over a wide range of aquatic sediments:

KD(SW) KD(10-2 M K+)

) 0.50 ( 0.05

and

KD(SW - M2+) KD(SW)

) 1.18 ( 0.06

The KD values as estimated above can be inserted into eq T 2 to yield F fix . Table 1 shows the results of these calculations and the numerical values of all KD values for the various systems studied. It is apparent that a fairly constant value emerges T for the fixed fraction, F fix (0.094 ( 0.009), irrespective of Cp T values and ionic compositions. F fix is furthermore insensitive to variations of Cs occupancy in the HAS HAS (Z Cs covering a range of 0.02-0.35). In the light of mechanistic considerations on the nature of the 137Cs sorption/fixation process (4, 16), Cs should have been fixed in a certain constant portion of the HAS. In other words, a constant amount of Cs should have been fixed, irrespective of the total amount of Cs in the system,

TABLE 1

Results of Single Desorption Experimenta adsorption

desorption

fixation

Cp (g mL-1)

solution

HAS Z Cs

KD,ads

solution

HAS Z Cs

KD,des

T F fix

HAS F fix

0.00022 0.00021 0.00011 0.00010 0.00005 0.00004 0.00021 0.00021 0.00010 0.00010 0.00005 0.00003

0.01 M KCl 0.01 M KCl 0.01 M KCl 0.01 M KCl 0.01 M KCl 0.01 M KCl 0.001 M KCl 0.001 M KCl 0.001 M KCl 0.001 M KCl 0.001 M KCl 0.001 M KCl

0.019 0.028 0.032 0.024 0.027 0.030 0.082 0.064 0.145 0.195 0.232 0.356

1784 1892 1875 1882 1834 2139 13780 14508 13356 12648 14933 12950

SW SW - M2+ SW SW - M2+ SW SW - M2+ SW SW - M2+ SW SW - M2+ SW SW - M2+

0.005 0.008 0.007 0.005 0.005 0.006 0.020 0.018 0.024 0.034 0.031 0.045

1576 1805 1985 2277 2904 4225 1567 1863 1958 2136 3174 4375

0.10 0.10 0.09 0.10 0.09 0.11 0.10 0.11 0.08 0.08 0.09 0.09

0.002 0.003 0.003 0.002 0.002 0.003 0.008 0.007 0.012 0.016 0.021 0.031

a F T was calculated on the basis of eq 2 with K rev (SW) ) 950 mL g-1 and K rev (SW - M2+) ) 1120 mL g-1. K values are single measurements. D fix D D KD,ads (0.01 M KCl) ) 1900 ( 120. KD,ads (0.001 M KCl) ) 13700 ( 890.

TABLE 2

Results of Consecutive Desorption Experimenta ads des1 des2 des3 des4 des5 des6 a

eq time (h)

Cp (g mL-1)

KD (mL g-1)

HAS Z Cs

T F fix

HAS F fix

24 48 24 72 168 120 377

0.0100 0.00143 0.00125 0.000192 0.000192 0.000192 0.000192

22853 ( 2150 4458 ( 284 4247 ( 230 9068 ( 653 16482 ( 1499 44056 ( 4047 63738 ( 8582

0.00429 ( 0.00002 0.00374 ( 0.00004 0.00328 ( 0.00005 0.00218 ( 0.00008 0.00170 ( 0.00010 0.00153 ( 0.00010 0.00142 ( 0.00010

0.35 ( 0.03 0.44 ( 0.03 0.50 ( 0.05 0.67 ( 0.09 0.86 ( 0.11 0.90 ( 0.17

0.0074 ( 0.0007 0.0081 ( 0.0006 0.0082 ( 0.0008 0.0073 ( 0.0009 0.0073 ( 0.0009 0.0069 ( 0.0013

Equilibration times are given in the second column. All data are average values (n ) 10).

HAS i.e., at different Cp levels F fix would have been constant T HAS (and not F fix). This, however, was not the case as F fix varied between 0.2% and 3.1%. A constant fraction of the initially adsorbed amount of radiocesium no longer participates in ion exchange reactions on the time scale of days to weeks. A straightforward way to determine that this amount of radiocesium is really fixed upon adsorption is to measure whether the same amount of Qfix remains fixed during consecutive desorptions. Consecutive Desorptions. Table 2 shows data for six consecutive desorptions in 1.0 × 10-2 M KCl solutions for suspended particles of the Meuse River loaded with 137Cs in a 1.0 × 10-3 M KCl solution. The fixed fraction with respect to the total amount of cesium remaining in the T system (F fix ) increases with each new desorption together with KD (from 0.35 to 0.90). The fixed fraction with respect HAS to the capacity of the HAS (F fix ) remains fairly constant (0.0075 ( 0.0005), indicating (with eq 4) that Qfix indeed represents a fixed fraction, as expected. The effect of particle concentration on KD is most obvious if one compares the KD data for the second and third desorptions, where Cp changes substantially (from 1.25 × 10-3 to 1.9 × 10-4 g mL-1): the lower the particle concentration during desorption, the more drastically KD increases with each new desorption due to the important fractions of the exchangeable cesium present in the liquid phase discarded after each desorption, leaving an increasing fraction of irreversibly sorbed cesium in the system. To recapitulate the salient observations from these three experiments: (a) KD,des increases with decreasing Cp whereas KD,ads remains unaffected by Cp; and (b) in each of a series of consecutive desorptions KD,des increases, and this effect is more pronounced at lower particle concentrations.

Quantification of these phenomena showed that during the adsorption process a certain fraction of the radiocesium becomes fixed on the solid phase. In other words, it appears that, irrespective of the cesium occupancy of the HAS, a certain fraction of the adsorbed radiocesium does no longer participate in ion exchange reactions on the time scale of days to weeks. In an attempt to present some overall picture of the process, the next section introduces an equilibrium model for radiocesium sorption. Equilibrium Model. A comprehensive picture for irreversibility and Cp effects on Cs desorption behavior can be based on an adapted version of the equilibrium model outlined by Di Toro and Horzempa (18) for PCB adsorption-desorption isotherms. Basically, the adapted model can be summarized by two sets of equations describing an adsorption and a (consecutive) desorption isotherm: s s s qads ) qrev,ads + qfix

(6)

s ) KD,adsql qads

(7)

s ) (KD,rev,ads + KD,fix)ql qads

(8)

s s s qdes ) qrev,des + qfix

(8)

s qdes ) KD,desql

(9)

and

s s qdes ) qfix + KD,rev,desql

(10)

where qs is the concentration of Cs on the solid phase (nequiv kg-1), ql is the concentration of Cs in the liquid

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s FIGURE 3. F fix,ads in 0.01 M KCl (9) and 0.00 1 M KCl (() as a function HAS of Z Cs for suspended particles of the Meuse River; symbols refer to single measurements.

phase (nequiv L-1), and other modifiers are as in footnote above. The first set of adsorption equations (6-8) is built on two assumptions: (a) Cesium adsorption isotherms are linear, KD,ads being a constant for a given adsorption scenario. For trace quantities, that is, for cesium occupancies well below HAS saturation, as encountered in natural scenarios, this linearity was demonstrated: data in Table 1 show that KD,ads remains constant although the cesium occupancy of the HAS increases by a factor of 4; (b) The adsorption isotherm can be broken down into a reversible component characterized by KD,rev,ads and an irreversible component characterized by KD,fix. This assumption will turn out to be acceptable when confronted with experimental evidence. The second set of equations (9-11) demonstrates that consecutive desorption patterns are dependent on the preceding adsorption and that KD,des is not a constant, since the desorption isotherm does not pass through the origin (see eq 11). As already stated previously, values for KD,rev were approximated on the basis of RIP theory, but they should also be obtainable from a multiple desorption isotherm (KD,rev,des) by simple linear regression on the basis of eq 11; they will be equal for adsorption and subsequent (multiple) desorption if identical (cationic) conditions prevail in both scenarios. Since both KD,ads and KD,fix are constants for a given adsorption isotherm, the ratio of KD,fix to the overall KD,ads is a constant as well. It equals the fixed fraction of the cesium adsorbed at the end of the adsorption phase: s F fix,ads )

KD,fix KD,ads

(12)

It is possible to determine this ratio, valid only for the adsorption phase, using single desorption data since s T F fix,ads ) F fix,des

(13)

As is demonstrated in Table 2 (where single desorption T T T data are shown and therefore F fix,des ) F fix ), F fix,des is fairly constant across a broad range of (trace) cesium occupancies of the HAS. This backs up the assumption that KD,rev,ads and KD,fix are constants, that is, both the reversible and the irreversible adsorption isotherms of trace cesium are linear. This is illustrated in Figure 3 showing the ratio of KD,fix over KD,ads as a function of cesium occupancies on the HAS on the basis of data from the single desorption experiment (Table 1). Attention is drawn to the fact that these data originate from two different adsorption scenarios (different ionic composition of the liquid phase): the KD,ads values

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FIGURE 4. Graphical representation of the equilibrium model: cesium concentration in the solid phase (q s) as a function of cesium concentration in the liquid phase (q l); lines passing through the origin are hypothetical total (tot.ads.), reversible (rev.ads.), and “fixed” (fix.ads.) adsorption isotherms. Symbols represent averages of 10 measurements (percentage standard deviation less than 7%).

differ almost by an order of magnitude, which establishes that the above ratio is fairly insensitive to the absolute value of KD,ads. Figure 4 visualizes the data for the consecutive desorption experiment in accordance with the model. The desorption isotherm is linear and conforms to eq 11. The fraction of cesium fixed at the end of the adsorption phase s (F fix,ads ) can easily be determined graphically as the ratio s of the intercept of the desorption isotherm (qfix ) over s qads and equals KD,fix/KD,ads (eq 12). On the basis of eqs 7 and 8, KD,rev,ads can easily be calculated (KD,rev,ads ) KD,ads - KD,fix), which enables us to construct the hypothetical component adsorption isotherms for the reversible and the irreversible subprocesses, shown in Figure 4. Validation of the Model. The model was developed from and applied to experimental data obtained for only one type of substrate in a desorption scenario (a homoionic 1.0 × 10-2 M KCl solution) that differed from the adsorption conditions (1.0 × 10-3 M KCl). To validate the model, a consecutive desorption experiment was set up on four more substrates. Identical ionic conditions, including for calcium [the presence of bivalent cations may influence irreversibility (16, 19)] were maintained throughout. KD,des values increased gradually with the number of desorption runs for all four substrates (data not shown). The ratios of KD,des for the last desorption run over KD,ads are given in Table 3. They are inversely proportional to the KD,rev estimates on the basis of RIP theory. The table also s lists values for F fix,ads . It is interesting to note that s F fix,ads (a measure for the fixation potential) of some natural substrates exceeds that of pure illite, the type of mineral that is responsible for specific radiocesium sorption in almost all sediments and soils (20-23). Desorption isotherms are shown in Figure 5. The most striking characteristic is the high degree of linearity of the isotherms (only the Devoke sediment shows some tendency toward curvilinearity), confirming the hypothesis underlying the model outlined above. The slope of the isotherms equals KD,rev,des, and hence KD,rev,ads equals KD,rev,des since adsorption and desorption took place under the same ionic conditions. The KD,rev values obtained by linear regression are compared with KD,rev estimates based on RIP theory in Table 3. For three substrates, the agreement is very good; only in the case of the Po sediment, the radiocesium

TABLE 3

Results of Validation Experimenta parameter

CEC (µequiv g-1)

KD,des9/KD,ads (CV e 17%)

KD,rev estimate (mL g-1)

KD,rev slope (mL g-1)

s F fix,ads

Lake Devoke Po River Lake Esthwaite illite

260 ( 25 102 ( 9 397 ( 36 163 ( 14

5.4 3.7 2.7 1.5

1100 1252 3650 12600

1196 ( 48 2633 ( 353 4764 ( 369 9455 ( 329

0.16 ( 0.012 0.35 ( 0.027 0.36 ( 0.038 0.24 ( 0.016

a Cation exchange capacity, the ratio of the overall K at the end of the last (ninth) desorption (percentage standard deviation CV e 15%) over D s overall adsorption KD (CV e 8%). KD,rev as estimated via RIP theory; KD,rev obtained as the slope of the desorption isotherm; F fix,ads for four different substrates.

not the whole story however. Although no Cp effects were found upon adsorption in the present study, they did occur upon desorption. Yet, RIP theory does not predict any Cp effects at all. We argue, therefore, that Cp effects can be accounted for as a side effect resulting from partial irreversibility of the sorption process. If some (even though perhaps little) radiocesium becomes irreversibly fixed onto the soil particles after contamination, its concentration with respect to the solid phase will increase rapidly relative to the concentration of cesium in the liquid phase when the particles are suspended in large amounts of nearly cesiumfree river water; KD, of course, will rise accordingly.

Acknowledgments This research was funded by the European Community (Radiation Protection Programme) under Contract FI3PCT92-0029, FI3P-CT92-0010, COSU-CT92-0016, COSUCT93-0040, and COSU-CT94-0078. J.W. acknowledges a aspirant fellowship from the N.F.W.O. The assistance of three anonymous referees and of M. van Steenbergen in reworking the manuscript is gratefully acknowledged.

Literature Cited

FIGURE 5. Desorption isotherms for three sediments (Lake Devoke, Lake Esthwaite, and the Po River) and a reference illite sample in a solution of 0.001 M KCl and 0.002 M CaCl2; symbols refer to single measurements.

interception potential underestimates KD,rev by more than a factor of 2. This substantiates our conjecture that RIP theory may be useful to a priori approximate KD,rev. More importantly for our present purposes is, however, that the quantification of the model was successful and that the model proved to be applicable to sorption data on a range of substrates, so that we can outline the following overall view. Imagine an erosion-prone soil contaminated with radiocesium. Measure its KD and find a value between 500 and 5000 mL g-1. Let part of the soil work its way into the river nearby and determine KD again for the suspended particles; it will eventually rise to something like 100 000 mL g-1 (24). To explain this, people have resorted to introducing a “particle concentration effect”. Part of the reason for the KD increase has since been shown (RIP theory) to be the lower K+ concentration in the river (only 10-410-5 M as against about 10-3 M in the soil) (4, 8). This is

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(24) Belli, M.; Sansone, U. E. N. E. A. Italy, personal communication, 1990.

Received for review September 11, 1995. Revised manuscript received May 13, 1996. Accepted May 20, 1996.X ES950675F

X

Abstract published in Advance ACS Abstracts, July 15, 1996.