Sequential Extractions for the Study of Radiocesium and

To study radiostrontium (RSr) and radiocesium (RCs) aging in soils, three sequential extraction schemes were used on Mediterranean loamy and loam-sand...
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Environ. Sci. Technol. 1999, 33, 887-895

Sequential Extractions for the Study of Radiocesium and Radiostrontium Dynamics in Mineral and Organic Soils from Western Europe and Chernobyl Areas ANNA RIGOL, MARIA ROIG, MIQUEL VIDAL,* AND GEMMA RAURET Departament Quı´mica Analı´tica, Universitat de Barcelona, Martı´ i Franque`s 1-11, 08028 Barcelona, Spain

To study radiostrontium (RSr) and radiocesium (RCs) aging in soils, three sequential extraction schemes were used on Mediterranean loamy and loam-sandy soils, podsols and peaty podsols from the area near Chernobyl, and peats from Western Europe. Aging was quantified by changes in radionuclide distribution. Two factors were thought to affect radionuclide distribution: time elapsed since contamination and drying-wetting cycles. Changes in radionuclide distribution were of low significance in Mediterranean loamy and loam-sandy soils in the short term, even after drying-wetting cycles. In the short term, podsols and peaty podsols showed a decrease in the RSr exchangeable fraction in the laboratory samples (2025% decrease), whereas samples taken 6 years after contamination did not show any further decrease. For RCs in podsols and peaty-podsols, the application of dryingwetting cycles for 9 months led to observe a 2-3-fold decrease in the exchangeable fraction, whereas time alone did not lead to any change. No RCs aging was observed in peats with a low or almost negligible content of mineral matter, low base saturation and low interception potential for RCs, even after drying-wetting cycles. Finally, changes in the radionuclide exchangeable fraction over time in these soils corresponded to changes in transfer factors over a similar period.

themineral matter, especially the presence of illitic materials (4). The role of organic matter is more significant than the one of the mineral phase in organic soils with high organic matter content and low interception potential. In this case, the organic components may affect the interaction of RCs with the specific sites in the mineral phase (5). For radiostrontium (RSr), which interacts with soils through an ion exchange process, it is the organic matter that controls this process in organic soils (4). The reversibility of this interaction also depends on the radionuclide considered. In general, the reversibility of the RCs sorption process is greater in organic soils than in mineral soils (6), whereas the interaction of RSr with organic acids may decrease the fraction of this radionuclide directly involved in the equilibrium between the solid phase and soil solution (7). RCs aging in mineral soils has been partially attributed to solid-state migration into specific sites in the interlattice positions (8). In organic soils, aging should occur if RCs interaction is controlled by the mineral phase (9, 10). If, on the contrary, RCs interaction is controlled by the organic phase, no changes over time would be expected in RCs distribution. For RSr, less is known about its aging, and no clear conclusions can be drawn from previous studies (4). Aging studies are based on either adsorption experiments, in which an increase in the solid-liquid distribution coefficient is observed over time (11), or desorption studies, based on single and sequential extractions (12). The use of a given sequential extraction scheme shows that the most significant fractions are the exchangeable and the residual fractions, and intermediate fractions are clearly less significant and, often, basically operational. Despite this operational feature, extraction techniques may still throw some light on the aging processes in soils, as long as the desorption technique is applied to the same soil with the same experimental conditions over time (13). Here the dynamics of the interaction of RCs and RSr in soils were studied through sequential extractions from soils kept in laboratory conditions, a few days after soil contamination with soluble radionuclide to different times later. The influence of drying-wetting cycles on the dynamics of the radionuclide distributions was also evaluated. When possible, the validity of this approach was checked through comparisons with field sample distributions obtained some time after radioactive contamination. The ability to predict soil-to-plant transfer changes over time was also assessed.

Introduction

Experimental Section

The interaction between radionuclides and the solid phase in soils can be understood as a two-step process. First, a fast interaction occurs in which a given percentage of the total radionuclide content can be exchanged and a fixed fraction cannot. Second, this fixed fraction increases, and so the pool of radionuclides that might contribute to reestablishing soil equilibrium through eventual root uptake into the plant decreases (1). This process is called aging in the literature, and its quantification over a given time scale is helpful in explaining processes such as the change in the radionuclide soil-plant transfer over time (2, 3). Radiocesium (RCs)-soil interactions are mostly controlled by the mineral phase, quantified in terms of its RCs interception potential (RIP) (1). It depends on the quality of

Soil Characteristics. Mineral Soils. Two types of mineral soil were studied: soils from the Mediterranean area and podsols from areas near Chernobyl NPP.

* Corresponding author telephone: 34-93402 12 81; fax: 34-93402 12 33; e-mail: [email protected]. 10.1021/es980720u CCC: $18.00 Published on Web 02/09/1999

 1999 American Chemical Society

The Mediterranean soils used in this study were a loamy (Calcic Luvisol) and a loam-sandy (Fluvisol) soil, both with a low organic matter (OM) content, and illite being the main component in the clay fraction (14). The four podsols were Haplic Podsols, with a low OM content, low cation exchange capacity (CEC), and a clay fraction of less than 5% of the mineral fraction. The podsol 1 contained hot-particles due to fuel deposition (15). Organic Soils. The soils included in this study are highly organic and can be classified into two categories: peaty podsols (Terric Histosol), from areas close to the Chernobyl NPP (10), and peats from Ireland and the United Kingdom (4). Some soil characteristics are as follows: VOL. 33, NO. 6, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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peaty podsol 1. CEC 103 cmolc kg-1, OM 69%, and

-1

RIP 520 mmol kg -1

peaty podsol 2. CEC 114 cmolc kg , OM 84%, and

RIP 260 mmol kg-1

peat 1. CEC 106 cmolc kg-1, OM 97%, and

RIP 7 mmol kg-1

peat 2. CEC 84 cmolc kg-1, OM 88%, and

RIP 48 mmol kg-1

The podsols and peaty podsols were taken from plowed plots in 1992, six years after the Chernobyl fallout, and the peats were taken from undisturbed plots. All soils were air-dried and screened through a 2-mm sieve before analysis. Sequential Extraction Schemes. Three schemes were applied. The scheme applied to the Mediterranean loamy and loam-sandy soils was based on previous studies on heavymetals in sediments. It is designated as the common scheme in the present paper and adapted for radionuclides in soils (16). For the podsols and peaty podsols an acid scheme, which has been widely used in countries of the former Soviet Union, was used (15). Finally, an organic scheme focusing on the organic phase was also applied to the four organic soils (9). The extractant reagents and the experimental conditions are summarized in Table 1. Soil Samples. The Effect of Time on Radionuclide Distribution. Soil samples were contaminated with soluble 85Sr and 137Cs using radioactive solutions supplied by Damri (85SR-ELSB45 and 137CS-ELSB45). The two peats were contaminated only with RCs. After being contaminated, soil samples were kept at 10 °C in polyethylene bottles. The common scheme was applied to Mediterranean loamy and loam-sandy soils 4 days (4D), 40 days (40D), and 90 days (90D) after contamination with soluble radionuclide which allowed the short-term aging to be studied. The acid scheme was applied to podsols and peaty podsols 4 days (4D), 1 month (1M), 8 months (8M), and 17 months (17M) after contamination with soluble radionuclide. The distributions obtained were compared with the distributions obtained from 6-year samples (6Y) contaminated by Chernobyl fallout, thus allowing the long-term aging to be studied. The organic scheme was also applied to peaty podsols 4 days (4D) and 1 month (1M) after contamination and to the 6-year field samples. The organic scheme was applied to peats 4 days (4D), 1 month (1M), 8 months (8M), 20 months (20M), and 25 months (25M) after contamination. The Effect of Drying-Wetting Cycles on Radionuclide Distribution. The distributions obtained at 10 °C were compared with those obtained with samples submitted to drying-wetting cycles at different temperatures. The objective was to test the overall effect of such cycles, not the individual effect of each variable, trying to simulate the field conditions. The cycles consisted in subjecting the soils to several temperatures (-20 °C, room temperature, and 50 °C) at the same time as they were dried and rewetted at saturated paste. For podsols and peaty podsols, the cycles were applied to 8-month (after contamination) samples during four months obtaining the 12-month (12M*) sample, and during nine months to produce a 17-month (17M*) sample. A shorter experiment was conducted in Mediterranean loamy and loam-sandy soils. Four-day (4D) samples were exposed to a 40-day drying-wetting cycles, so producing the 40-day distributions (40D*). Finally, the 20-month samples of the peats underwent similar cycles for five and ten months, so extending the sample periods to 25 months (25M*) and 30 months (30M*), respectively. Gamma Spectrometry. The radionuclide activity concentration in samples contaminated with Chernobyl fallout 888

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was measured by high-resolution gamma spectrometry, using an intrinsic Ge detector (ORTEC GMX 15200-P) and a multichannel analyzer (ORTEC 5600) with 8192 channels. Samples contaminated with soluble radionuclides were analyzed in 20-mL capacity polyethylene vials by gamma spectrometry, using a solid scintillation detector (PACKARD MINAXI 5000 Series). Determination of 90Sr. The activity concentration of 90Sr in the extracts of the samples contaminated by Chernobyl fallout was determined by measuring the Cerenkov radiation of 90Y (17), with a Wallac 1220 Quantulus Ultra-Low-Level Analyzer.

Results and Discussion Radiostrontium Dynamics. Short-Term Aging in the Soils from the Mediterranean Area: Use of the Common Scheme. Figure 1 shows the RSr distributions obtained for the loamy and loam-sandy Mediterranean soils by applying the common scheme at several times after contamination (4D, 40D, and 90D) and after the application of drying-wetting cycles for 40 days (40D*). The lack of a distribution for the 90D samples is due to the short half-life for 85Sr, the levels of which were too close to detection limits when the experiments were carried out. The RSr distributions in the two soils were broken down into the sum of the desorption yields with water and NH4Cl (exchangeable fraction), the desorption yield with CH3COOH (radionuclide bound to carbonates and released in an acidic media), and the sum of the desorption yields with NH2OH‚HCl, H2O2, and the Residue (radionuclide bound to reducible phases, to oxidizable organic matter and the residual fraction). These last three were taken together due to their low yields. The largest proportion of RSr was found in the exchangeable fraction. A difference between the two soils was the small but significant amount of RSr in the CH3COOH fraction in the loamy soil, which may be due to the presence of carbonates in this soil and its slightly higher organic matter content. Protonation of the exchangeable sites in this phase could lead to a release of the bound RSr. With respect to the effect of time, the 4D and 40D RSr distributions were not statistically different, indicating that no aging occurred under these experimental conditions. For 90D samples, the only data available were the exchangeable fraction desorption yields. Despite its high associated errors, the mean values indicated a degree of aging. To check whether the poor aging was due to experimental conditions, the two soils were exposed to drying-wetting cycles for 40 days. The comparison between the 40D and 40D* distributions suggests that no significant RSr aging occurred as a consequence of the drying-wetting cycles applied. Long-Term Aging in Podsols and Peaty Podsols from the Chernobyl Area: Use of the Acid Scheme. Figure 2 shows the RSr distributions for the four podsols and the two peaty podsols after contamination with soluble 85Sr using the acid scheme (4D, 1M, and 8M). The distributions were divided into the sum of the desorption yields obtained with water and CH3COONH4 (exchangeable fraction), the extraction with HCl, and the sum of the extraction with HNO3 and the Residue (fixed fraction). As can be seen in Figure 2a-d, the 4D and 1M distributions were similar to each other for the four podsols, the exchangeable fraction being the most significant, and the fixed fractions being negligible. In the 8M distributions a change was noticed, with a significant RSr decrease in the exchangeable fraction that dropped to 65-75%, accompanied by an increase in the HCl and the fixed fractions. Interestingly, HCl fraction played an intermediate role, with higher increases than those in the fixed fraction. Therefore, an aging

TABLE 1. Experimental Conditions of the Sequential Extraction Schemes fraction

F1 F2 F3 F4 F5

reagent

ratio (mL g-1)

treatment

time (h)

temp (°C)

(b) (c)

Common Scheme (Loamy and Loam-Sandy Mediterranean Soils) (16) H2O 40 end-over-end shaking -1 1 mol L NH4Cl pH 7 40 end-over-end shaking 0.11 mol L-1 CH3COOH 40 end-over-end shaking 0.04 mol L-1 NH2OH‚HCl (25% in HAc) 10 digestion 30% H2O2 pH 2 (pH 2 with HNO3) 10 digestion digestion 30% H2O2 pH 2 (pH 2 with HNO3) 10 digestion 1 mol L-1 CH3COONH4 pH 2 (pH 2 with HNO3) 50 end-over-end shaking

16 16 16 6 1 1 1 16

room room room 80 room 85 85 room

(a) (b)

Acid Scheme (Podsols) (15) H2O 10 1 mol L-1 CH3COONH4 pH 7 10 6 mol L-1 HCl 10 8 mol L-1 HNO3 (plus drops of H2O2) 10 8 mol L-1 HNO3 (plus drops of H2O2) 10

end-over-end shaking end-over-end shaking digestion digestion digestion

24 24 1.5 1.5 1.5

room room 85 85 85

(a) (b)

Acid Scheme (Peaty Podsols) (15) H2O 20 -1 1 mol L CH3COONH4 pH 7 20 6 mol L-1 HCl 10 8 mol L-1 HNO3 (plus drops of H2O2) 20 8 mol L-1 HNO3 (plus drops of H2O2) 20

end-over-end shaking end-over-end shaking digestion digestion digestion

24 24 1.5 3 16

room room 85 85 85

16 16 16 2 4 16 16

room room room room 85 85 room

(a)

residue F1 F2 F3 F4 residue F1 F2 F3 F4 residue F1 F2 F3 F4

(a) (b) (c)

Organic Scheme (Peaty Podsols and Peats) (9) 1 mol L-1 CH3COONH4 pH 7 40 end-over-end shaking 0.1 mol L-1 Na4P2O7 40 end-over-end shaking 0.1 mol L-1 NaOH (N2) 40 end-over-end shaking 30% H2O2 pH 2 (pH 2 with HNO3) 10 digestion digestion 30% H2O2 pH 2 (pH 2 with HNO3) 10 digestion 1 mol L-1 CH3COONH4 pH 7 50 end-over-end shaking

residue

FIGURE 1. Radiostrontium distributions obtained with the common scheme for the loamy and the loam-sandy Mediterranean soils (mean value ( standard deviation, n ) 3; ND - not detected). All distributions correspond to laboratory-contaminated samples. process in the time scale studied (eight months), associated with a decrease of around 20% in the exchangeable fraction, was shown. The reported distribution of 90Sr in 6-year samples obtained using the same scheme can be compared with our data (18). Only the distributions obtained in podsols with condensed deposition (podsols 2-4) allowed the comparison with the distributions obtained after contamination with soluble radionuclide. In these podsols, the exchangeable fraction was around 60-70%, the HCl fraction around 2035%, and the HNO3 fraction between 10 and 15%. Comparing the 4D and 6Y distributions, the aging effect thus accounts for around a 20% decrease in the exchangeable fraction, which is clear evidence of an aging process for RSr, although no a further aging was noticed from the 8M distribution. So, these comparisons showed that aging is significant around one

year after contamination but is negligible from this time onward. Figure 2e,f show the RSr distributions obtained for the two peaty podsols. 85Sr was not detectable in any of the fractions of the 17M, 12M*, and 17M* samples due to its short half-life. RSr distributions were similar in the two peaty podsols. In the 4D samples the RSr exchangeable fraction was initially the largest, with desorption yields around 65-70%. These values are lower than those for podsols. The intermediate HCl fraction was also relatively large, with desorption yields around 30%. With no carbonates in these soils and under the experimental conditions used, the high values in this fraction can be explained both by the release of RSr because of protonation in the organic matter sites and by a potential remobilization of fixed RSr because of the acidic medium. The fixed fraction was the VOL. 33, NO. 6, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Radiostrontium distributions obtained with the acid scheme for podsols and peaty podsols (mean value ( standard deviation, n ) 3). All distributions correspond to laboratory-contaminated samples. smallest, with desorption yields lower than 5%, mostly from extraction with HNO3, since the level of RSr in the residue was negligible. As for podsols, the 4D and 1M distributions were similar to each other, indicating that no aging occurred during the first month after contamination. However, in the 8M distribution there was also a significant decrease in the exchangeable fraction of the two peaty podsols and an increase in the fixed fraction, which reached values around 20%. The percentage of RSr extracted with HCl remained constant. Therefore, it was clear that an aging process was taking place in the peaty podsols solely because of the time elapsed since the contamination event. This aging was quantified as a 25% decrease in the exchangeable RSr for both peaty podsols. The pattern observed in samples contaminated in the laboratory was compared with data on the RSr (90Sr) distribution in these soils 6 years after the Chernobyl fallout and in field conditions (18, 19). In this case, the RSr in the exchangeable fraction was 45-55% for peaty podsol 1 and 40-50% for peaty podsol 2, and in the fixed fraction it was 25% for peaty podsol 1 and 10-20% for peaty podsol 2. Thus, the RSr distribution in the two peaty podsols 6 years after the Chernobyl fallout showed the same degree of aging as that observed in the 8M samples (a decrease of 25% in the exchangeable fraction). This suggested that in these soils and for RSr, after a considerably short time period, aging reached a plateau, as previously observed for podsols. The 890

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aging process cannot be easily explained by the ion exchange mechanisms that control RSr interaction in soils, although specific interactions have been postulated in clay-organic matter complexes and isomorphic substitutions in Ca-bearing minerals (4). Long-Term Aging in Peaty Podsols from the Chernobyl Area: Use of the Organic Scheme. For this scheme fractions were distributed in four groups (Figure 3a,b): the fraction obtained with CH3COONH4 (exchangeable radionuclide), the sum of the fractions obtained with Na4P2O7 and NaOH (radionuclide associated with organic acids), the fraction extracted with H2O2 (radionuclide related to oxidizable material such as the most inert organic matter), and the final Residue (radionuclide irreversibly adsorbed). The RSr distribution in the 4D samples showed that the exchangeable fraction was the largest, as observed with the acid scheme. The desorption yields of the exchangeable fraction were quite similar in the two schemes, with slightly higher values for the organic scheme due to its higher ratio between the extractant solution and soil. The level of RSr in the residue was negligible in both schemes. However, the use of the organic scheme allowed for association of the intermediate fractions of the scheme with soil phases. As can be seen in Figure 3a,b, the desorption yield of the Na4P2O7 + NaOH fraction was around 20%, highlighting the association of RSr with organic acids. Moreover, the global desorption yield of the three organic fractions was similar to the desorption yield of the HCl fraction in the acid scheme,

FIGURE 3. Radiostrontium distributions obtained with the organic scheme for peaty podsols (mean value ( standard deviation, n ) 3; ND - not detected). The 6Y distribution corresponds to field samples and the other to laboratory-contaminated samples.

FIGURE 4. Radiocesium distributions obtained with the common scheme for the loamy and the loam-sandy Mediterranean soils (mean value ( standard deviation, n ) 3). All distributions correspond to laboratory-contaminated samples. showing that the latter also contained the RSr released because of the acidification of the organic matter sites. No aging was observed after 1 month, as found in the acid scheme. However, comparison with the RSr (90Sr) distribution in these soils 6 years after the Chernobyl fallout showed an aging of the same degree as that observed in the 8M and 6Y samples in the acid scheme. This confirmed again that RSr underwent significant aging in a considerably short time, after which the exchangeable fraction remained constant. Moreover, the yields of the organic fractions suggested that organic phases and especially organic acids controlled RSr retention in organic soils. This fact plus the similar yields found for Na4P2O7 + NaOH + H2O2 in the organic scheme and for HCl + HNO3 in the acid scheme confirmed that RSr aging occurred due to a specific interaction in the clay-organic matter complexes, with the breaking of these complexes, caused by the experimental conditions of the scheme, being responsible for its solubilization. Sequential Extraction as a Tool to Predict Changes in Radiostrontium Transfer Over Time. In a previous field study, a reduction in RSr transfer in a sandy soil was observed (2), suggesting that an aging process had occurred. However, in other field studies the decrease in RSr transfer observed in sandy and loam soils in a 5-year experiment at lysimeter level was explained by other factors than changes in the exchangeable fraction (7) or was the same as seasonal variations (20). Although the long-term scale of these field studies made difficult the comparison with our laboratory findings, these latter results are consistent with the low importance of the aging process observed in the loamy and loam-sandy soils. In podsols, some authors state that, when dealing with condensed deposition, changes in RSr transfer are negligible some years after the contamination (3), this being consistent with our findings. For organic soils, RSr transfer has been observed to be quite constant over time when condensed deposition was present (7, 20), but in all cases data were obtained more than one year after contamination. Therefore, although aging can

be deduced from our laboratory experiment, it cannot be easily related to a similar reduction in RSr transfer in the field due to the lack of short-term transfer data for these samples. However, the constant value for RSr transfer predicted from the constant exchangeable fraction from 8M samples onward is consistent with the field transfer data. Radiocesium Dynamics. Short-Term Aging in Soils from the Mediterranean Area: Use of the Common Scheme. Figure 4 shows the RCs distributions for the Mediterranean loamy and loam-sandy soils by applying the common scheme several times after contamination (4D, 40D, and 90D) and also shows the distributions obtained after 40 days of dryingwetting cycles (40D*). The results are shown as the sum of the extractions with water and NH4Cl (exchangeable fraction), whereas the rest of the fractions are plotted separately. The desorption yields associated with the exchangeable fraction in the 4D samples were around 40% (loamy soil) and 35% (loam-sandy soil). The other significant fractions were the H2O2 fraction, with desorption yields around 20%, and the residual fraction (desorption yields around 30%). The higher values obtained in the H2O2 fraction cannot be completely related to RCs associated with the organic matter phase, since a significant fraction of this radionuclide fixed by the mineral fraction can be also solubilized due to the operational character of this extraction (16). In the time scale of this study, the 4D, 40D, and 90D distributions for both soils were similar, showing that aging was a low significant process in these soils. However, a slight trend of aging can be observed in the two soils if 4D and 90D exchangeable fractions and residual-fixed fractions are compared. If not only the time elapsed since contamination is considered but also the effect of drying-wetting cycles, the comparison of the 40D and 40D* distributions showed that even after the drying-wetting treatment only a significant aging occurred in the loam-sandy soil. Therefore, for these soils in the short-term, neither time nor drying-wetting cycles had any key effect on the distributions. This lack of aging can be partially explained by the high initial rates of fixation that VOL. 33, NO. 6, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Radiocesium distributions obtained with the acid scheme for podsols and peaty podsols (mean value ( standard deviation, n ) 3; ND - not detected). The 6Y distribution corresponds to field samples and the other to laboratory-contaminated samples. drastically reduces the amount of RCs that can be fixed in the second, slower step of the RCs-soil interaction. Moreover, soils with collapsed interlayers have a lower RCs migration into inner specific sites (6), thus minimizing the existence of an aging process. Long-Term Aging in Podsols and Peaty Podsols from the Chernobyl Area: Use of the Acid Scheme. Figure 5 shows the RCs distributions obtained by applying the acid scheme to the podsols and peaty podsols after contamination with soluble radionuclide (4D, 1M, 8M, and 17M). This figure also includes the distribution in soil samples taken 6 years after the Chernobyl accident (6Y distribution) and after dryingwetting cycles (12M* and 17M* distributions). The distributions were divided as for RSr studies. The 4D distributions obtained for each of the four podsols had a similar pattern. The desorption yields of the exchangeable fraction were the highest. The desorption yields of the HCl fraction were also high, ranging from 35% (podsol 3) to 45% (podsol 2), and the fixed fraction was the lowest. No significant aging was observed in the soil samples contaminated with soluble RCs and kept in laboratory conditions, since distributions from 4D, 1M, 8M, and 17M samples were reasonably similar. On the other hand, the comparison between the 6Y and the former distributions indicated that aging had occurred, with a 3-4-fold decrease in the exchangeable fraction, and a similar increase in the HCl and in the fixed fractions. For podsol 1 the changes were even higher, due to the fact that the main source of radioactivity in this soil was fuel deposition and not related to a higher aging. 892

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Comparing the distributions obtained for samples without and with cycles, a similar behavior by each of the four podsols was observed. Considering the 8M samples as the starting point of the application of the drying-wetting cycles, it can be seen that there were significant changes in the distributions, with a 20-30% reduction in the exchangeable fraction from the 8M to 12M* samples, related to an increase in the HCl and fixed fractions. There was a further decrease in the exchangeable fraction and increase in the fixed RCs fraction in the 17M* distributions, due to the higher number of drying-wetting cycles. These results are consistent with published data, explicable by a change in the lattice layers in clays, especially changes in the structure of the frayed edge sites, which eventually enhance the entrapping of cesium ions (1). From these data it is easy to follow the dynamics of the RCs distributions by comparing the distributions obtained for the 4D, 12M*, 17M*, and 6Y samples. It can be concluded that for these soils time was not the most significant nor the only parameter involved in the aging process and that drying-wetting cycles increased the aging process, apart from the fact that they simulated better the field conditions. For the peaty podsols (Figure 5e,f) the use of this scheme on 4D samples showed a RCs distribution which varied slightly depending on the soil. The fixed fraction in peaty podsol 1 had the highest desorption yield, whereas the exchangeable and HCl fractions had similar yields (around 25%). For peaty podsol 2, with a higher OM content and a lower RCs interception potential, the exchangeable fraction had the highest desorption yield, while the desorption yields

FIGURE 6. Radiocesium distributions obtained with the organic scheme for peaty podsols and peats (mean value ( standard deviation, n ) 3; ND - not detected). The 6Y distribution corresponds to field samples and the other to laboratory-contaminated samples. of the fixed and HCl fractions were lower. Comparison of the distributions obtained in the laboratory samples (4D, 1M, 8M, and 17M) showed a similar pattern in both peaty podsols: a certain degree of aging was deduced from the decrease observed in the exchangeable fraction and the increase in the fixed fraction between the 1M and 17M distributions. To complete the dynamics study, the RCs distributions in the samples contaminated in the laboratory were compared with the distributions in the field samples taken 6 years after the Chernobyl fallout. These distributions (6Y) were totally different from the ones of the laboratory samples kept at constant temperature and moisture. The exchangeable fraction in the field samples was almost negligible, and the RCs was contained mainly in the fixed fraction. The aging process observed in these soils is consistent with published data on organic soils, in which a decrease over time in the exchangeable RCs was observed, although there were variations depending on the soil (4). In such soils the aging process also depended on the RCs interception potential of the soils and on the ionic status of the exchange complex. The higher the value of the interception potential and the saturation of the exchange complex, the more significant was the aging. Thus, the aging observed in the present study for the peaty podsols can be attributed to its high RCs interception potential because of the presence of illite in the mineral phase (5, 21) and its high base saturation (around 61% for peaty podsol 2 and 75% for peaty podsol 1) (15). The distributions obtained at constant temperature and moisture were compared with distributions after dryingwetting cycles. For the two peaty podsols, taking the 8M samples as a reference, there was a clear decrease in the exchangeable RCs at 12M* and an associated increase in the fixed fraction. This fact, also observed in mineral podsols, indicated that the mineral phase was responsible for RCs aging in these peaty podsols and that cycles enhanced the diffusion and subsequent adsorption of RCs through specific sites in the mineral matter (1). The effect of cycles was less significant on peaty podsol 2 than on peaty podsol 1, as a consequence of its lower RCs interception potential and base saturation, which was also related to a higher percentage of

OM. This tendency was also noticed from 12M* to 17M* samples, the latter having a much lower exchangeable fraction than the corresponding 17M (without cycles) samples. Therefore, in these soils time played a less significant role than the drying-wetting cycles, although the increase in the fixed fraction did not correlate with the number of cycles. In 12M* and 17M* distributions the two factors (time elapsed since contamination and drying-wetting cycles) affected the final distribution, and they could be considered as intermediate between the laboratory samples kept at constant temperature and moisture and field samples. Long-Term Aging in Peaty Podsols and Peats: Use of the Organic Scheme. The organic scheme was applied to the two peaty podsols and to the two peats. For the two peaty podsols fractions were plotted in four groups following the same criteria as for RSr (Figure 6a,b), whereas for the two peats the three organic phases (Na4P2O7, NaOH, and H2O2) were plotted together (Figure 6c,d). As previously shown by the acid scheme, no changes in RCs distribution between the 4D and 1M samples were observed for the peaty podsols with the organic scheme (Figure 6a,b). A comparison of these distributions with those obtained with the acid scheme shows that the exchangeable fractions were similar in the two schemes, as observed for RSr. RCs desorption yields of the intermediate fractions in the organic scheme were quite low, demonstrating a poor specific association of RCs with the organic phase, as shown in previous studies (5). The percentage of RCs remaining in the residue in the organic scheme was extremely high in comparison with the acid scheme, in which the residue was almost negligible in the 4D and 1M distributions. This could be explained by the fact that the organic scheme was less aggressive than the acid one, since in this latter the RCs that was associated with the mineral phase was partially released in the HCl and HNO3 extractions. The application of the organic scheme to the 6-year field samples again showed that RCs was mainly associated with the fixed fraction, which demonstrated the key role of the mineral phase in the aging process. These results were consistent with the results obtained with the acid scheme. VOL. 33, NO. 6, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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RCs distributions were also obtained after drying-wetting cycles for the peaty podsols. The tendency was similar to that observed with the acid scheme: a drastic decrease in the exchangeable fraction with a related increase in the fixed fraction. It can be concluded that both sequential extraction schemes give similar information about the dynamics of the RCs-soil interaction and that, although the time scale should be taken into account, the drying-wetting cycles that soils may undergo in natural conditions are a more influential factor in the RCs aging process. Therefore, despite the high organic matter content, in soils with a high interception potential the mineral phase controls RCs aging. The question arising is whether even in soils with a low or almost negligible mineral content, a low RCs interception potential and a low base saturation, there would be an aging process for RCs. To study this, the dynamics of the RCs distribution in peats 1 and 2 was obtained for a period of 30 months (Figure 6c,d). In this case, there was no final distribution to compare, but there were distributions obtained in drying-wetting conditions that enabled any aging occurring in these experimental conditions to be observed. The pattern of the initial distribution (4D samples) in the two peats was similar. For peat 1, the desorption yield of the exchangeable fraction was higher than 90%, whereas for peat 2 it was around 85%. In peat 2, the Residue was slightly higher, with values near to 10%. Despite the high OM content in these soils, there was poor specific association of the RCs with the organic phase, as shown by the low desorption yields associated with the Na4P2O7 + NaOH + H2O2 fraction. It is important to emphasize again that the differences in RCs distributions between organic soils cannot be explained solely by the OM content. This was very clear when comparing the 4D distributions for peaty podsol 2 and peat 2, both of which had a similar OM content, but quite different distributions. Thus, differences in distributions should be explained by differences in the RCs interception potential, which reflects the retention capacity of the mineral phase. If only time is considered, comparison among the distributions obtained up to 25 months after contamination indicated that no aging was taking place in the peats, since 25M distributions were similar to those at 4D, with the RCs remaining mainly exchangeable. If drying-wetting cycles are considered it can be seen that they had no effect in peat 1. This supports the hypothesis that aging is a process controlled by the mineral matter in soils and that, if the level of the specific sites is negligible, as shown by the extremely low value of the RCs interception potential in peat 1, the RCs adsorption is controlled by sites of the regular exchange complex (5). This is consistent with previous results in which RCs adsorption in peat 1 was not controlled by illite (21). The low base saturation (lower than 25%) in this peat also explained the absence of aging (4). A similar conclusion could be drawn for peat 2, although in this case a possible tendency for aging was observed in the 30M* distribution, that could be explained by the higher content of mineral matter and RCs interception potential in this soil than in peat 1. This finding also confirms previous results, which showed that in peat 2 adsorption was only partially controlled by the specific sites of the illitic material (21). Sequential Extraction as a Tool To Predict Changes in Radiocesium Transfer Over Time. For RCs in loamy and loamsandy soils, the distributions obtained showed that even after the drying-wetting treatment no significant aging occurred. Published data show that the variation in RCs transfer to some crops was negligible in a 3-year period in loamy soils (7) or in a 2-year experiment with sandy and loamy soils (20). Moreover, no variation in transfer to dry meadows was observed for heavy loam and clay soils (22). However, results in clay soils showed a 4-10-fold decrease in transfer factors in a 1-year experiment, although no data on the exchangeable 894

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fraction were included (2). Therefore, with the exception of these latter experiments, field results on transfer dynamics are consistent with the pattern found in the loamy and loamsandy soils. Comparison of our findings for podsols with transfer data in the literature shows that the decrease in the exchangeable fraction over the 6-year period fits neatly with changes in transfer over a similar period (3, 22). For peaty podsols a decrease in RCs transfer over time was observed after the Chernobyl fallout with a rapid decline over the initial 3-4 years followed by a slower decline thereafter (3, 22). Therefore, in podsols and peaty podsols, the time trend observed in the soil-to-plant transfer in the field is parallel to that of the exchangeable fraction observed in our experiment using drying-wetting cycles. Thus, this laboratory approach may be useful for predicting a decrease over time in the RCs soil-to-plant transfer factors, although it does not allow predictions of the real time period needed to reach the maximum aging, since it depends on the environmental conditions in the field. Finally, field experiments showed that, for soils with high organic matter content and low interception potential for RCs, soil-to-plant transfer remained high and constant over time (23), which validated the conclusions arising from the use of the organic scheme in peat soils.

Acknowledgments This work was funded by a DGICYT project. The authors would like to thank Dr. E. Valcke for characterizing the peats and J. M. Torres (UB) for his help in 90Sr determination. A. Rigol thanks MEC for a grant received.

Supporting Information Available Tables of data for radiostrontium and radiocesium distribution with the common, acid, and organic schemes. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) Sweeck, L. Ph.D. Thesis, Katholieke Universiteit Leuven (KUL), Belgium, 1996. (2) Haak, E.; Lo¨nsjo, H. Mitt. O ¨ sterr. Bodenkundl. Ges. 1996, 53, 129. (3) Krouglov, S. V.; Filipas, A. S.; Alexakhin, R. M.; Arkhipov, N. P. J. Environ. Radioact. 1997, 34, 267. (4) Valcke, E. Ph.D. Thesis, Katholieke Universiteit Leuven (KUL), Belgium, 1993. (5) Rigol, A.; Vidal, M.; Rauret, G.; Shand C. A.; Cheshire, M. V. Environ. Sci. Technol. 1998, 32, 663. (6) Hird, A. B.; Rimmer, D. L.; Livens, F. R. J. Environ. Radioact. 1995, 26, 103. (7) Nisbet, A. F.; Shaw, S. J. Environ. Radioact. 1994, 23, 1 and references therein. (8) Absalom, J. P.; Crout, N. M. J.; Young, S. D. Environ. Sci. Technol. 1996, 30, 2735. (9) Shand, C. A.; Cheshire, M. V.; Smith, S.; Vidal, M.; Rauret, G. J. Environ. Radioact. 1994, 23, 285. (10) Vidal, M.; Roig, M.; Rigol, A.; Llaurado´, M.; Rauret, G.; Wauters, J.; Elsen, A.; Cremers, A. Analyst 1995, 120, 1785. (11) Absalom, J. P.; Young, S. D.; Crout, N. M. J. European J. Soil Sci. 1995, 46, 461. (12) Kennedy, V. H.; Sanchez, A. L.; Oughton, D. H.; Rowland, A. P. Analyst 1997, 122, 89R. (13) Vidal, M.; Tent, J.; Llaurado´, M.; Rauret, G. J. Radioecol. 1993, 1, 49. (14) Vidal, M.; Rauret, G. Intern. J. Environ. Anal. Chem. 1993, 51, 85. (15) The transfer of radionuclides through the terrestrial environment to agricultural products, including the evaluation of agrochemical practices; Rauret, G., Firsakova, S., Eds.; European Comission: Luxembourg, 1996; EUR 16528 EN. (16) Roig, M.; Vidal, M.; Rauret, G. Analyst Submitted for publication. (17) Torres, J. M.; Garcı´a, J. F.; Llaurado´, M.; Rauret, G. Analyst 1996, 121, 1737.

(18) Askbrant, S.; Melin, J.; Sandalls, J.; Rauret, G.; Vallejo, R.; Hinton, T.; Cremers, A.; Vandecastele, C.; Lewyckyj, N.; Ivanov, Y. A.; Firsakova, S. K.; Arkhipov, N. P.; Alexakhin, R. M. J. Environ. Radioact. 1996, 31, 287. (19) Torres, J. M. Universitat de Barcelona, Spain, personal communication, 1995. (20) Ehlken, S.; Kirchner, G. J. Environ. Radioact. 1996, 33, 147. (21) Wauters, J.; Vidal, M.; Elsen, A.; Cremers. A. Appl. Geochem. 1996, 11, 595. (22) Sanzharova, N.; Belli, M.; Arkhipov, A.; Ivanova, T.; Fesenko, S.; Perepelyatnikov, G.; Tsvetnova, O. In The radiological conse-

quences of the Chernobyl accident; Karaoglou, A., Desmet, G., Kelly, G. N., Menzel, H. G., Eds.; European Comission, Brussels, 1996; EUR 16544EN; pp 507-510. (23) Beresford, N. A.; Howard, B. J.; Barnett, C. L.; Crout, N. M. J. J. Environ. Radioact. 1992, 16, 181.

Received for review July 15, 1998. Revised manuscript received December 14, 1998. Accepted December 15, 1998. ES980720U

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