Environ. Sci. Technol. 2003, 37, 2820-2828
Assessment of the Suitability of Soil Amendments To Reduce 137Cs and 90 Sr Root Uptake in Meadows MARTA CAMPS, ANNA RIGOL,* MIQUEL VIDAL, AND GEMMA RAURET Departament de Quı´mica Analı´tica, Universitat de Barcelona, Av. Diagonal 647, 3a Planta, 08028 Barcelona, Spain
In situ remediation strategies are an alternative approach in the management of radioactive contaminated areas, especially when based on modification of soil properties by the addition of amendments. Here, this strategy is applied to reduce 137Cs and 90Sr soil-plant transfer in meadows from areas of Russia, Belarus, and Ukraine affected by the Chernobyl fallout. Meadows were established on podzolic and peaty soils. Amendments covered a wide range of materials, such as loamy and sandy soils, polygorskite clay, phosphorite, turf, and sapropel. Field experiments showed the poor efficiency of most of the materials: only the polygorskite clay provoked a notable reduction (1.5-2fold) in 137Cs root uptake. Subsequent laboratory characterization showed the lack of significant changes in the radiocesium interception potential and soil solution composition in the amended soils, a fact that helped to explain the lack of effect on the reduction of transfer. Moreover, a laboratory methodology based on the quantification of the adsorption potential of the amendments and the reversibility of the adsorption process was applied. This methodology was first proposed for the correct selection of the suitable materials to be used to decrease radionuclide root uptake in future remediation actions and then validated with data of the previous field experiments.
Introduction After the Chernobyl Nuclear Power Plant (NPP) accident, vast territories of Ukraine, Belarus, and Russia were subjected to an intensive radioactive contamination, mainly by radiostrontium (RSr) and radiocesium (RCs) isotopes. The investigations carried out in post-Chernobyl activities showed that natural and seminatural meadows represented one of the main sources of radionuclide transfer into the food chain. This is so because meadows are often established on unfertile and/or organic soils, i.e., precisely the type used as pastures for grazing animals and as hay production for ruminant feeding (1). In these conditions, radionuclides become easily bioavailable and may represent an environmental threat to the health of local populations, which necessitates remediation actions aimed at decreasing radionuclide mobility in the ecosystem (2). The basic principles behind the design of remediation actions must consider the knowledge of the mechanisms controlling radionuclide-soil interaction, have an optimum cost-effectiveness balance (especially related to natural attenuation), and control potential detrimental or beneficial * Corresponding author phone: 34-934021281; fax: 34-934021233; e-mail:
[email protected]. 2820
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secondary effects, both radioecological and social. In situ remediation strategies through natural processes, such as phytoremediation, redefinition of land use, and modification of the properties of the soils, have resulted in lower cost and higher efficiency than many technology-based, ex-situ approaches, such as soil removal, which are not an alternative when facing large areas of contamination (3-5). In the past, a few remediation actions have been carried out in meadow ecosystems, although most efforts were dedicated to agricultural systems and animals (6, 7). The use of materials such as soil amendments was one of the most promising in situ strategies (8-10). Eventually, the amendment influences the radionuclide uptake in two ways: by sorbing and reducing the displacement of the radionuclide to the soil solution from the new solid phase created by the mixture of soil plus amendment and by modifying the composition of the soil solution. Amendments must increase the radionuclide adsorption potential of the soils, that is, the solid-liquid distribution coefficient (KD). Concretely for RSr, this is achieved by increasing the cation exchange capacity (CEC), since its adsorption is controlled by regular sites in the soil exchange complex (11). RCs adsorption is controlled by specific sites in the expanded 2:1 phyllosilicate clays in most soils (Frayed Edge Sites, FES), with the exception of soils with a high organic matter content (over 90%). In the past, the product of the FES content and the radiocesium trace selectivity coefficient to a monovalent cation M (i.e., K+ or NH4+) was shown to be constant from a concentration of the monovalent cation onward and equal to the product of the radiocesium KD obtained at a given concentration of the monovalent cation and the concentration of this cation, as shown by the following equation.
[FES]KC(Cs/M) ) KDmM This product was defined as the Radiocesium Interception Potential (RIP). This parameter was revealed to be very useful to compare the affinity of a set of soils to adsorb RCs because, unlike the common KD, the RIP values do not depend on the solution composition (12). Therefore, this parameter must be increased by the addition of the amendment to the target soils (13). Considering that only low doses of amendments can be used to have an economically suitable remediation strategy, CEC and RIP (or the derived KD values) of the used amendments should exceed by various orders of magnitude their values in the soils in order to lead to a significant effect at field level. To date, the selection of materials has been based less on quantifying soil-interaction parameters and more on local availability and low cost of the materials. Reasons for this may lie in a general lack of knowledge as to which methodology may be applied to select the materials and to predict their short and medium-term effectiveness. Moreover, only a few studies have been carried out at both laboratory and field level (8, 14). Laboratory backup must be based on low-cost experiments, capable of being applied on a routine basis. These experiments must provide information on the adsorption-desorption behavior of the radionuclides in soils and materials as well as on the dynamics of the interaction, especially when facing medium and long-term scenarios. The optimal dose of material to be used must also be calculated to justify a remediation action that is economically sustainable (15). Results reported in this work describe the use of soil amendments as a remediation strategy in contaminated meadows to decrease 137Cs and 90Sr transfer. First, field 10.1021/es026337d CCC: $25.00
2003 American Chemical Society Published on Web 05/15/2003
TABLE 1. List of the Field Sites and Experimentsa fertilizer dose (kg ha-1)
liming dose (t ha-1)
field site
soil description
control plot
Sawichi
haplic podsol
D + Pl + NPK + L
Belarussian Sites 90:100:150 15.6
Dublin
terric histosol
D + Pl + NPK + L
90:60:130
2
sapropel turf
40-80 40-80
14.3-28.6 14.3-28.6
mineral soil
100-200
167-333
sandy soil loamy soil
100-200 100-200
62.5-125 62.5-125
phosphorite polygorskite phosphorite polygorskite
0.06-0.18 10-30 0.06-0.18 10-30
0.03-0.08 4.5-13.6 0.04-0.11 6.2-18.8
Ukrainian Site
Mateyki
terric histosol
D + Pl
VIUA
haplic podsol
D + Pl + NPK
Russian Sites 80:60:80
Rudnuy
haplic podsol
D + Pl + NPK
80:60:80
a
amendment
basic-extra amendment doses (t ha-1) (g kg-1)
D: disking; Pl: ploughing; NPK: NH4NO3 + P2O5 + KCl fertilizers; L: liming.
experiments were performed in areas of Ukraine, Belarus, and Russia affected by the Chernobyl fallout. Second, a laboratory methodology based on previous studies was improved and subsequently applied to characterize field samples and materials; to explain data obtained in field experiments; and to predict, in quantitative terms, the suitability of candidate materials to be used in future remediation actions.
Experimental Section Field Plots and Experiments. The experimental work at field level was carried out in meadows of Russia, Belarus, and Ukraine. At the Belarussian and Ukrainian sites, 90Sr and 137Cs were the target radionuclides. At the Russian sites, 137Cs was the only radionuclide of concern. Plots were selected among those having a soluble radionuclide deposition from the Chernobyl fallout, thus avoiding those areas with the existence of hot particles with uranium oxide matrices. As previous studies showed that the use of tillage techniques (disking and ploughing) in natural meadows led to a significant decrease in radionuclide transfer (16, 17), these techniques were applied to all sites along with the addition of soil amendments. At some sites, a dose of fertilizer including potassium, nitrogen, and phosphorus minerals (NH4NO3 + P2O5 + KCl, NPK) and of lime was also added before the application of the amendments. Doses of fertilizers and lime were selected on the basis of the previous practices at the experimental sites, to obtain a reasonable biomass with the lowest cost. The sole application of these agricultural practices, without the addition of amendment, allowed the establishment of control plots in every site. Additionally, two doses of amendment were applied to study the effect that an increasing amount of the corresponding material could have on radionuclide transfer, thus defining the amended plots. Table 1 summarizes the field experiments. Amendments were selected taking into account their cost, local availability, practicability, and easy application. Mixtures of amendments were not tested. The addition of mineral materials was expected to be suitable for decreasing RCs transfer in the organic soils by increasing their RCs adsorption potential, whereas the addition of materials with a predictable high CEC (such as turf or sapropel) was chosen for decreasing RSr transfer in mineral soils. Uncommon local materials were phosphorite, which is a local rock phosphate from the Bryansk region commonly used as a mineral fertilizer; polygorskite clay, from the Kaluga region, which initially seemed to be a good choice to reduce RCs transfer; turf, which is a mixture of organic material and topsoil; and sapropel, which is a
material that derives from lake sediment, and it is composed of organic material with a naturally high mineral content (18). For every treatment, four randomly located plots were used as replicates (plot size about 3.5 × 5 m2). At harvest, four plant subsamples with a surface of 0.5 × 1 m2 were taken from each plot and mixed into one bulk sample providing four plant replicates per treatment. Soils were sampled from 0- to 10-cm depth. Four soil replicates per plot were taken and mixed into one bulk soil sample per plot, thus also leading to four soil replicates per treatment. Soil samples were air-dried, sieved throughout 1 mm, and homogenized before analysis. They were characterized in terms of general parameters such as organic matter content (OM), pH, CEC, exchangeable cations, bulk density, clay content, and texture (19, 20). 137Cs and 90Sr activity concentrations were determined in plant and soil samples originated from field experiments. In short, 137Cs was determined by high-resolution gamma spectrometry using a HPGe detector (ORTEC GMX series) coupled to a multichannel analyzer (IN-1200). Efficiency calibration was carried out with a 152Eu standard source. 90Sr was determined after an acid extraction of the samples with HCl + HNO3 and subsequent radiochemical separation leading to isolate its daughter product 90Y as yttrium hydroxide. This was then redissolved, reprecipitated as oxalate, ignited, and then counted as yttrium oxide with a low-level beta counter (Canberra 2404), with an efficiency of 48% (21). Laboratory Experiments. Field soil samples were analyzed after finishing field experiments to determine changes in specific soil properties that affect radionuclide root uptake. The examined properties were the Radiocesium Interception Potential (RIP) and soil solution composition. In addition to this, a complementary methodology based on adsorptiondesorption experiments was applied to predict the effectiveness and suitability of soil amendments to decrease radionuclide transfer. Determination of the Radiocesium Interception Potential (RIP). The RIP was determined in soils and amendments after preequilibrating the samples (1 g) with 50 mL of a solution containing 100 mmol L-1 in Ca2+ and 0.5 mmol L-1 in K+ (mK ) 0.5), for 24 h. After three preequilibrations, samples were equilibrated with the same solution but labeled with 134Cs, for 24 h. RCs distribution coefficients (KD(Cs)) were obtained in this scenario by measuring 134Cs level in the supernatant, before and after the equilibration, using a solid scintillation detector (PACKARD MINAXI 5000 Series). The calculated product KDmK quantified the RIP value (22). VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Description of the Soil Samples from Control Plots and Soil Amendments Tested in Field Experiments (Mean of Field Replicates)a soil amendment
137Cs
90Sr
(Bq kg-1) (Bq kg-1)
Sawichi sapropel turf Dublin mineral soil
5900
950
4000
1050
Mateyki sandy soil loamy soil
1100
VIUA Rudnuy phosphorite polygorskite
3600 3600
a
pH
OM (%)
bulk CEC Ca2+exch Mg2+exch K+exch NH4+exch (cmolc (cmolc (cmolc sand density (cmolc (cmolc (kg dm-3) kg-1) kg-1) kg-1) kg-1) kg-1) (%) Belarussian Sites 25 3.7 2.8 86 27 3.7 85 38 10 140 39 7.0 8.2 0.50 0.27
4.4 7.9 4.8 38 5.9 45 6.3 78 6.5 0.1
1.4 nd nd 0.3 nd
100
6.4 44 5.9 0.2 6.1 0.8
0.8 nd nd
Ukrainian Site 57 30 4.4 0.24 10 4.5
nd nd
6.7 3.5 5.6 13 6.8 2.4 7.7 4.3
1.1 0.8 nd nd
Russian Sites 9.7 3.4 15 6.2 8.6 7.3 12 8.4
clay (%)
texture
0.26 0.59 0.44 0.68 0.08
0.22 0.92 2.1 0.50 0.08
71.7 5.3 sandy-loam 47.4 26.8 sandy-clay loam 62.8 0.7 sandy-loam nd 1.3 82.9 0.6 loamy-sand
2.3 0.06 1.2
0.24 0.06 0.14
0.08 0.08 0.08
nd 11.9 90.2 1.6 sandy 55.9 12.9 sandy-loam
0.65 1.8 0.35 0.35
0.12 0.13 0.40 0.41
0.20 0.11 0.09 0.08
79.8 3.4 loamy-sand 90.2 3.1 sandy 26.9 20.1 loamy-silt 1.4 81.1 clay
RSD < 20%. nd: not determined.
Determination of Field Capacity and Major Elements in Soil Solution. Dried soil samples were saturated with water and centrifuged at 0.33 bar. The water content of the resulting soil sample was determined by the loss of weight at 105 °C, allowing the field capacity to be determined (23). Soil solution composition was determined following a method based on previous studies (24). After rewetting the air-dried soil samples up to field capacity, the wet soil was left for 24 h, placed in the upper part of a cylinder with a porous plate, and subsequently centrifuged. The solution, separated and recovered in the base of the cylinder, was then filtered through a 0.45 µm-porous membrane, acidified, and stored in polyethylene vials before analyses. Ca2+, Mg2+, and K+ were determined by ICP-AES (Thermo-Jarrell Ash 25). NH4+ was determined by UV-vis spectroscopy (Perkin-Elmer Lambda 19 UV/VIS) by an automated colorimetric method based on the formation of indophenol blue (25). Adsorption-Desorption Pattern of Soils and Amendments. The effect of the addition of an amendment at a given dose in a soil was studied by measuring RCs and RSr solid-liquid distribution coefficientssKD(Cs) and KD(Sr)sin a medium simulating the soil solution of the corresponding control soil. This was done in soil-amendment mixtures and in individual soil and amendment samples. In brief, mixtures of soil (5 g) and amendments (at doses similar to those used at field level) were preequilibrated with 200 mL of a solution containing K+, Ca2+, Mg2+, and NH4+ at concentrations representative for the respective soil solutions, by continuous end-overend shaking and regular replacement of fresh solution. After 3-4 equilibrations, the mixtures were again equilibrated with the same solution but labeled with 134Cs and 85Sr. After 1, 7, 14, and 21 days of end-over-end shaking, the 134Cs and 85Sr solution activities were measured using a solid scintillation detector to calculate KD(Cs) and KD(Sr) values. The KD for the soil or the amendment was determined by the same procedure but using 1.25 g of soil or amendment and 50 mL of the corresponding soil solution (15). The reversibility of the adsorption in the amendments was estimated by single extractions using 1 mol L-1 CH3COONH4. Briefly, the labeled amendment samples originating from the 24 h adsorption experiments were dried at room temperature, and 50 mL of the extracting solution was added to the amendment residues. The suspension was endover-end shaken for 16 h at room temperature, and the supernatant was separated after centrifugation. The 134Cs and 85Sr were determined in the supernatant. The extraction 2822
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process was repeated twice, and the sums of the extraction yields were referred to as the initial activity concentration of the amendment. Changes in the RCs reversibility pattern in the medium term in amendments were predicted by the application of drying-wetting cycles to samples coming from adsorption experiments. Drying-wetting cycles consisted of subjecting the labeled amendments to several temperatures (-20 °C, room temperature, and 50 °C) at the same time as they were dried and rewetted at field capacity (26). The procedure described above was applied to determine the extraction yield after the application of drying-wetting cycles.
Results and Discussion Main Characteristics of Soils and Amendments. Data from the controls of field soil samples are given in Table 2. Large differences were observed in the organic matter content of the soils considered (from 3.5% at the VIUA site to 78% at the Dublin site) and in the CEC, which mostly depended on the organic matter content. All soils were acidic with pH values between 4.4 and 6.7. Soil textures ranged from sandy to sandy-loam. Mateyki soil presented the highest clay content (around 11.9%), despite being the soil with the second highest organic matter content, whereas the Dublin soil had the lowest clay content (around 1.5%). Table 2 also shows the main characteristics of the selected amendments in each site. In general, these characteristics do not provide information on the suitability of a set of amendments to achieve a reduction in radionuclide soilplant transfer. However, materials with a high CEC could be predicted to be useful to decrease RSr root uptake, and those with a high clay content are suitable for RCs. From the data in Table 2, sapropel and turf have higher CEC values than that of the soil in which they were applied, thus being the best materials for RSr. For the rest of the materials, the polygorskite clay can be anticipated as the best candidate material for RCs, followed by sapropel and phosphorite. Effect of Soil Amendments: Field Experiments. Figures 1 and 2 summarize the changes in the soil-to-plant transfer factor (TF) (expressed as Bq kg-1 in plant/kBq m-2 in soil) obtained at the Belarussian, Ukrainian, and Russian sites for RCs and RSr, respectively. Transfer factors in the control (hatched area) and amendment plots (vertical bars) are compared (27). With respect to RCs (Figure 1), the addition of amendments did not lead to a decrease in transfer in most of the treatments. Only at the Russian experimental sites
FIGURE 1. Effect of soil amendment addition on 137Cs transfer factor (TF). TFs are mean values of four field replicates; error bars indicate 1 SD. Control treatments are represented by a hatched area that includes the mean ( 1 SD.
FIGURE 2. Effect of soil amendment addition on 90Sr transfer factor (TF). TFs are mean values of four field replicates; error bars indicate 1 SD. Control treatments are represented by a hatched area that includes the mean ( 1 SD. could a positive effect of both amendments (phosphorite and especially polygorskite) be determined. This was clearer when comparing the mean values: RCs transfer was reduced by a factor of around 1.5-2 at the VIUA site, using the highest dose of polygorskite, and at the Rudnuy site, using the two amendments. At the Belarussian sites, a negligible or even negative effect was observed when comparison was made with the control plot, as seen at the Dublin site. With respect to the Mateyki site, a negligible effect was observed by the two amendments tested. A similar trend was noticed for RSr at those sites where RSr was a target radionuclide (see Figure 2). A nonsignificant effect was observed for all the amendments at all sites, thus indicating the poor efficiency of the materials tested in reducing RSr root uptake. The use of the amendments led to a significant secondary effect, because the plant biomass increased in most of the amended plots, with the exception of the Mateyki site (see
Table A, Supporting Information). Although it has been reported that there exists a negative correlation between plant biomass production and radionuclide transfer (27), here this beneficial secondary effect did not provoke a significant decrease in transfer. The lack of effectiveness of soil amendments applied at field level in soil-amendment mixtures not previously tested at laboratory scale has been previously reported in the literature. While clay materials are expected to decrease RCs transfer, its use at field level has shown inconsistent results, with a maximum 2.5-fold reduction (6, 17, 18). The use of sapropel has been reported to provoke up to a 6-fold reduction but without any characterization of the soilamendment mixture and with application rates of more than 150 t ha-1 (18). Other materials, such as ammoniumferrichexacyano-ferrate (AFCF), have been used at the greenhouse level, with results depending on the soil type and time elapsed since RCs deposition. Doses of 10 g m-2 VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Changes in the RIP in Field Soil Samples and Amendmentsa Sawichi
VIUA
Rudnuy
treatment
RIP
treatment
RIP
treatment
RIP
control (C) sapropel C + sapropel C + 2 sapropel turf C + turf C + 2 turf
400 (20) 332 (8) 400 (70) 360 (90) 65 (1) 460 (140) 390 (100)
control (C) phosphorite C + phosphorite C + 3 phosphorite polygorskite C + polygorskite C + 3 polygorskite
24 (3) 810 (40) 29 (10) 20 (1) 7700 (810) 23 (1) 17 (4)
control (C) phosphorite C + phosphorite C + 3 phosphorite polygorskite C + polygorskite C + 3 polygorskite
130 (100) 810 (40) 210 (190) 250 (90) 7700 (810) 180 (80) 200 (70)
Dublin
a
Mateyki
treatment
RIP
treatment
RIP
treatment
RIP
control (C) mineral soil C + mineral soil C +2 mineral soil
161 (37) 15 (1) 119 (9) 121 (26)
control (C) sandy soil C + sandy soil C + 2 sandy soil
207 (39) 31 (5) 170 (50) 250 (30)
loamy soil C + loamy soil C + 2 loamy soil
455 (2) 260 (60) 320 (90)
µmol g-1; mean values, standard deviation (SD) in parentheses.
provoked a 25-fold decrease in sandy soils, although only a 64% reduction in loamy soils. For aged RCs, reduction decreased to 3.5 times in sandy soils, with the same amendment dose (28, 29). Therefore, our field results and others previously reported strengthen the need of a previous detailed characterization step and laboratory back-up for the selection of suitable amendments for a given soil type. Changes in Soil Properties after Field Treatments. The effect of the addition of amendments on soil properties was investigated to relate changes in various properties to variations in radionuclide soil-plant transfer. First, changes in the RIP in the soil-amendment mixtures were examined. In addition to this, alterations in the composition of the soil solution were also studied, especially of those elements that may compete for root uptake with RCs and RSr. Changes in the RIP Values of Field Soil Samples. Table 3 summarizes the RIP data of control, amendments, and amended plots. The high variability observed for the RIP values did not permit us to draw statistically significant conclusions. However, RIP values of the amended soils seemed to increase in a few scenarios. This was the case of the Mateyki site when adding the highest dose of the loamy soil and of the Rudnuy site when adding the polygorskite clay and phosphorite. The improvement in the RIP values was related to the much higher values of the RIP of the amendment related to the control soils. The low doses prevented a significant, positive effect in increasing the RIP and then reducing RCs transfer in most cases, although the increase in RIP values at the Rudnuy site when phosphorite and polygorskite were applied corresponded to the decrease in RCs transfer observed at field level. The lack of changes in RIP at the VIUA site makes it difficult to explain the observed decrease in RCs transfer at this site solely on the basis of a significant change in the adsorption potential. The Dublin site defined a different scenario because the RIP values seemed to decrease when adding the mineral material. This could be initially surprising, considering that a mineral material was added to an organic soil. However, the mineral soil had a sandy texture, with a lower clay content than the Dublin soil. Moreover, the sites of this amendment also showed a lower affinity for RCs, as can be seen by its lower RIP with respect to the Dublin control soil. Therefore, the decrease in RIP may help to explain the increase in RCs transfer observed at this site. Changes in the Soil Solution Composition. The effect of the amendment addition on the soil solution was estimated by quantifying the concentration of Ca2+ and Mg2+, competitive species for RSr root uptake, and K+ and NH4+, competitive species for RCs. Table 4 includes the composition 2824
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of the soil solution for all control and treated sites. It is generally accepted that an increase in Ca2+ and Mg2+ concentrations in the soil solution may have a positive effect in decreasing RSr soil-plant transfer in those scenarios having an initial low concentration of Ca2+ + Mg2+, although no consistent results are found in the literature (30). Ca2+ + Mg2+ concentrations remained unchanged in most scenarios, although at some sites (Dublin and Mateyki) a significant decrease was observed, this fact having a low influence in the eventual RSr transfer, because high concentrations of these elements were maintained in all scenarios (31). Although a high variability was observed for all scenarios, the results indicated that the addition of the amendments did not provoke a significant variation in K+ concentration in the amended soils. Considering the low K+ concentrations in some of the control plots (see VIUA, Rudnuy, and Mateyki sites), it would have been interesting to increase the fertilizer rate along with the use of the amendments because it is currently accepted that for K+ concentrations under a given threshold (around 0.5-1 mM), an increase in the K+ concentration in the soil solution would lead to a decrease in RCs root uptake (32). With respect to the NH4+, an increase in its concentration in the soil solution generally leads to an increase in RCs root uptake due to the different mechanisms affecting the related decrease in the KD(Cs) and dilution effect (13). The comparison between the NH4+ concentrations in control and amended soils showed a significant decrease in the NH4+ concentration only at the VIUA site when amended with polygorskite. This suggested a beneficial secondary effect when using this amendment, thus explaining the decrease in the RCs transfer at the VIUA site when using polygorskite, since both RIP and K+ concentration remained constant. In summary, the evaluation of changes in RIP and soil solution composition in the field samples made it possible to explain the general lack of a significant effect of the tested amendments in reducing radionuclide root uptake in most scenarios, thus with the advice to perform a previous characterization step prior to field experiments. Prediction of the Effectiveness of Soil Amendments from Laboratory Experiments. Data and conclusions from the former section show that a laboratory back-up for the selection of the materials is a recommended previous step before performing field experiments to apply this remediation strategy. Radionuclide transfer through root uptake depends on its level in the soil solution, corrected by a factor that includes plant physiological aspects, related to nutrient uptake and selectivity. This factor depends on both the plant and the radionuclide considered, and it is often assumed as inversely proportional to the concentration of
TABLE 4. Changes in Soil Solution Composition in Field Soil Samplesa Sawichi treatment control (C) C + sapropel C + 2 sapropel C + turf C + 2 turf
Ca2+ss + Mg2+ss 3 (2) 5.1 (0.4) 5 (1) 4.4 (0.8) 5 (1)
Dublin K+ss
NH4+ss
treatment
1.2 (0.3) 0.9 (0.2) 1.1 (0.3) 0.9 (0.4) 0.9 (0.5)
0.8 (0.3) 0.7 (-) 0.6 (0.2) 0.4 (0.2) 0.6 (0.2)
control (C) C + mineral soil C + 2 mineral soil
K+ss
NH4+ss
0.9 (0.2) 1.2 (0.1) 1.3 (0.2)
0.5 (0.2) 0.6 (0.1) 0.6 (0.2)
Ca2+ss + Mg2+ss 13 (2) 6.8 (0.5) 5.5 (0.5)
VIUA
Rudnuy
treatment
Ca2+ss + Mg2+ss
K+ss
NH4+ss
treatment
Ca2+ss + Mg2+ss
K+ss
NH4+ss
control (C) C + phosphorite C + 3 phosphorite C + polygorskite C + 3 polygorskite
8 (2) 4 (1) 6 (2) 5 (1) 4 (1)
1.0 (0.5) 0.6 (0.1) 0.5 (0.2) 0.7 (0.6) 0.5 (0.3)
2 (1) nd 1.5 (0.2) nd 0.4 (0.1)
control (C) C + phosphorite C + 3 phosphorite C + polygorskite C + 3 polygorskite
6 (3) 4 (3) 6 (4) 5 (3) 3 (1)
0.6 (0.1) 0.3 (0.1) 0.4 (0.1) 0.4 (0.2) 0.3 (0.1)
1.7 (0.3) 2.2 (-) 2.0 (-) 2.0 (0.4) 1.9 (0.4)
Mateyki
a
treatment
Ca2+ss + Mg2+ss
K+ss
NH4+ss
control (C) C + sandy soil C + 2 sandy soil C + loamy soil C + 2 loamy soil
4 (1) 2.6 (0.4) 2.8 (0.5) 2.9 (0.5) 2.7 (0.9)
0.06 (0.02) 0.03 (0.01) 0.05 (0.01) 0.04 (0.01) 0.05 (0.01)
0.18 (0.06) 0.18 (0.01) 0.17 (0.03) 0.2 (0.1) 0.15 (0.05)
mmol L-1; mean values, standard deviation (SD) in parentheses.
TABLE 5. Radiostrontium Distribution Coefficients (KD(Sr)) of Soils, Amendments, and Mixturesa soils amendments
individual KD(Sr) (mL/g) 1D 21 D
Sawichi turf
11 (1) 184 (9)
10.7 (0.4) 85 (2)
sapropel
115 (1)
113 (1)
Dublin mineral soil
34 (2) 0.9 (0.4)
29 (2) 0.22 (0.01)
VIUA phosphorite-V
5.1 (0.5) 8.9 (0.3)
4.4 (0.9) 10.9 (0.3)
polygorskite-V
16.0 (0.1)
15.1 (0.2)
Rudnuy phosphorite-R
9.0 (0.3) 9.5 (0.2)
9.1 (0.1) 12.1 (0.2)
polygorskite-R
20.2 (0.5)
19.3 (0.3)
Mateyki sandy soil
103 (1) 0.4 (0.1)
94 (1) 1.0 (0.1)
mixture KD(Sr) (mL/g) 1 D-calc 21 D-exp
21 D-calc
14.2 (0.3) 16.9 (0.1) 11.5 (0.2) 13.1 (0.3)
13 (1) 16 (1) 12 (1) 14 (1)
12.7 (0.1) 14.6 (0.3) 9.9 (0.2) 10.7 (0.2)
11.7 (0.4) 12.9 (0.4) 12.0 (0.4) 13.7 (0.4)
17 33
27.1 (0.8) 17.7 (0.5)
28 (2) 23 (1)
23 (3) 12.6 (0.2)
24 (2) 20 (1)
1.0 3.1 0.5 1.4
5.3 (0.1) 5.3 (0.2) 5.4 (0.4) 5.5 (0.4)
5.1 (0.1) 5.2 (0.5) 5.2 (0.5) 5.3 (0.5)
4.6 (0.2) 4.8 (0.1) 4.5 (0.1) 4.6 (0.1)
4.5 (0.9) 4.6 (0.9) 4.5 (0.9) 4.5 (0.9)
1.4 4.2 0.7 1.9
11.1 (0.4) 11.2 (0.5) 9.7 (0.2) 12.0 (0.8)
9.0 (0.3) 9.0 (0.3) 9.1 (0.3) 9.2 (0.3)
11.5 (0.7) 11.9 (0.3) 11 (2) 12 (1)
9.1 (0.1) 9.2 (0.1) 9.2 (0.1) 9.3 (0.1)
6.3 12.6
101 (1) 96 (3)
97 (1) 90 (1)
88 (2) 88 (3)
88.1 (0.9) 82.3 (0.9)
doses (%)
1 D-exp
1.4 3.0 1.3 2.9
a Mean values, SD in brackets. Abbreviations: 1D, 1 day (24 h) of equilibration time; 21 D, 21 days of equilibration time; exp, experimental value; calc, calculated value from the linear combination of individual values.
radionuclide competitive species in the soil solution (14). The level of a given radionuclide in the soil solution, in turn, depends on the radionuclide concentration and on the sorption-desorption pattern of the radionuclide in the solid phase (the solid-liquid distribution coefficient and the amount of radionuclide reversibly adsorbed) (13, 14). If we assume that no significant changes are induced in the soil solution due to the addition of soil amendments, as seen in the former section, the prediction of the potential effect of soil amendments on root uptake can be quantified in terms of changes in the adsorption-desorption pattern of a given radionuclide in the new soil-amendment mixture. Therefore, here we present a laboratory methodology based on the comparison of the radionuclide sorption properties of the amendments with respect to the soil as a tool to select the
suitable materials and their doses. No data are given for the loamy soil, because this material was not available for laboratory experiments. Radionuclide Distribution Coefficient. Amendment field doses (t ha-1) were converted into a percentage weight (g of amendment per 100 g of soil), taking into account the bulk density of the soil and assuming the addition of the amendment to a depth of 20 cm. The resulting doses for phosphorite were too low to perform laboratory experiments (lower than 0.1%), so their dose percentages were increased. KD data are summarized in Tables 5 and 6 for RSr and RCs, respectively. Focusing on results obtained after 1 day of equilibration (1D), only for the amendments used at the Sawichi site the KD(Sr) was much higher than the soil KD(Sr) (around 10-fold VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 6. Radiocesium Distribution Coefficients (KD(Cs)) of Soils, Amendments, and Mixturesa soils amendments
individual KD(Cs) (mL/g) 1D 21 D
Sawichi turf
45 (2) 36.0 (0.3)
60.0 (0.9) 288 (5)
sapropel
116 (2)
130 (3)
Dublin mineral soil
40 (2) 9.6 (0.5)
61 (5) 13 (1)
VIUA phosphorite-V
25 (2) 73 (3)
33 (3) 87 (3)
polygorskite-V
376 (3)
609 (7)
Rudnuy phosphorite-R
99 (5) 173 (6)
186 (12) 196 (1)
polygorskite-R
902 (36)
1420 (53)
Mateyki sandy soil
577 (11) 137 (8)
3473 (115) 279 (1)
mixture KD(Cs) (mL/g) 1 D-calc 21 D-exp
doses (%)
1 D-exp
21 D-calc
1.4 3.0 1.3 2.9
44.9 (0.1) 47 (1) 40 (1) 39.0 (0.9)
45 (2) 45 (2) 46 (2) 47 (2)
64 (1) 67.1 (0.6) 49 (3) 48.4 (0.8)
63.2 (0.9) 66.8 (0.9) 60.9 (0.9) 62.0 (0.9)
17 33
29 (1) 21 (2)
35 (2) 30 (1)
69 (4) 135 (3)
53 (4) 45 (3)
1.0 3.1 0.5 1.4
25.5 (0.2) 26.0 (0.5) 26.5 (0.6) 28.6 (0.1)
26 (2) 26 (2) 27 (2) 30 (2)
36.0 (0.1) 37.4 (0.5) 38.1 (0.6) 40.5 (0.4)
34 (3) 35 (3) 36 (3) 41 (3)
1.4 4.2 0.7 1.9
104 (6) 104 (4) 101 (2) 133 (10)
100 (5) 102 (5) 105 (5) 114 (5)
239 (25) 238 (14) 212 (11) 299 (20)
186 (12) 186 (12) 195 (12) 209 (12)
6.3 12.6
546 (8) 523 (24)
549 (10) 522 (10)
2035 (595) 3115 (422)
3272 (108) 3071 (100)
a Mean values, SD in brackets. 1D, 1 day (24 h) of equilibration time; 21 D, 21 days of equilibration time; exp, experimental value; calc, calculated value from the linear combination of individual values.
higher). This is related to the fact that CEC values of these amendments are higher than those of the control soil. For RCs, only in the case of polygorskite clay at the Russian sites the KD(Cs) was 1 order of magnitude higher than in the soil. The need to precharacterize the materials before they are used at field level was clearly demonstrated because the amendments often presented a much lower sorption capacity than that of the corresponding soil. This was the case of the mineral soil at the Dublin site and the sandy soil at the Mateyki site, both for RCs and RSr. The KD of the soil + amendment mixtures confirmed the lack of the suitability of the materials and doses tested. Promising materials, such as polygorskite at the Russian sites, were used at such low doses that their effect was diminished. Other materials, such as the mineral soil at the Dublin site, were used in higher doses, but their low KD led to a decrease in the KD of the mixture with respect to the initial soil. The KD corresponding to the mixtures studied was seen to follow a linear combination of the individual values, corrected by the dose, because an excellent correlation between the experimental and calculated KD values was observed in most cases. The Pearson correlation coefficient was equal to 0.998 for KD(Sr) and to 0.999 for KD(Cs). This fact makes this methodology even easier to apply, because it is only necessary to calculate the individual values of the materials to predict the KD values of mixtures with a reasonable agreement. Tables 5 and 6 also show the final KD(Sr) and KD(Cs) values, after 21 days of equilibration. The information on the changes in KD with the equilibration time is fully described in Figure A, in the Supporting Information. The time effect did not lead to any significant change in the overall conclusions drawn by examining the KD obtained after 24 h. The effect of equilibration time was almost negligible for RSr because only a significant change in the KD(Sr) was observed in the turf and mineral soil. This agreed with previous findings that indicated that the dynamics of its interaction in soils had a minor significance in most soils (26, 33). For RCs, the time effect led to an increase in KD(Cs) in many soils and materials, a fact that could be anticipated as beneficial with respect to a related decrease in the root uptake in the medium term. The increase in the KD(Cs) could be explained by the increase in the fixed fraction of RCs (34). This means that the initial 2826
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KD(Cs) can be considered as representing a scenario where most of the RCs was reversibly adsorbed, whereas the increase in the KD(Cs) indicates that the fraction of irreversibly adsorbed RCs also increases. Prediction of the KD(Sr) and KD(Cs) from Soil and Amendment Properties. The previous methodology permits us to estimate the adsorption potential of an amendment by quantifying the KD in the soil solution of the target soil. Based on the knowledge of the mechanisms underlying RSr and RCs interaction in soils, KD(Sr) and KD(Cs) could be predicted from soil parameters as an alternative, easier approach to estimate the adsorption potential of the materials. In soils with a saturated exchange complex, KD(Sr) can be predicted from the ratio of the CEC versus the sum of the concentrations of Ca2+ and Mg2+ in the soil solution. For soils with an unsaturated exchange complex, such as those of this study, the overall CEC should be substituted by the sum of exchangeable bases, that is, the sum of exchangeable Ca2+, Mg2+, Na+, K+, and NH4+, as seen in eq 1 (13, 14).
KD(Sr) )
Ca2+exch + Mg2+exch + K+exch + NH4+exch + Na+exch Ca2+ss + Mg2+ss
(1)
Since RCs adsorption is controlled by the specific FES, the KDFES(Cs) accounts for more than 80% of the total KD (13). The KDFES(Cs) is predicted by dividing the RIP by the sum of K+ and NH4+ concentrations in the soil solution, the latter amplified by the NH4+-to-K+ trace selectivity coefficient (KC(NH4/K)). If a total KD is calculated (KDTOT(Cs)), a second term must be added to account for the RCs adsorption in regular exchange sites (KDREC(Cs)) by dividing the sum of the exchangeable K+ and NH4+ by the sum of K+ and NH4+ concentrations in the soil solution (35). The equation derived may be written as follows:
KDTOT(Cs) ) KDFES(Cs) + KDREC(Cs) ) RIP K+ss + KC(NH4/K)‚NH4+ss
+
K+exch + NH4+exch K+ss + NH4+ss
(2)
FIGURE 3. Variation of RCs desorption yields in amendment samples exposed to 1, 2, and 3 drying-wetting (D + W) cycles. Desorption yields are mean values of two replicates; error bars indicate 1 SD. The calculation of the predicted KD(Sr) and KD(Cs) is straightforward from data obtained in this work, with the exception of the values of the KC(NH4/K). However, this parameter ranges from 4 to 8 for soils in which specific sites control RCs adsorption and down to 2 in those soils where the adsorption in regular sites becomes significant (13, 35, 36). The values of the predicted KD(Sr) and KD(Cs) may be found in Tables B and C of the Supporting Information. For RSr, the ratio of the KD(Sr) obtained in the laboratory experiments versus the predicted values had a mean value of 1.7 (SD ) 0.6). The two sets of values showed an excellent correlation, with a Pearson correlation coefficient of 0.993. These results agree with previous findings, in which the KD(Sr) showed an adequate correlation to a parameter calculated from the ratio of the CEC versus the electrical conductivity in the soil solution (37). For RCs, the ratio of experimental and calculated KD(Cs) had a mean value of 1.3 (SD ) 0.7, without the VIUA sample), with a Pearson correlation coefficient of 0.962. From these data, it is clearly concluded that the estimation of the adsorption potential of the materials can be done either on the basis of general properties (soil solution composition, CEC and RIP) or by obtaining the KD from laboratory experiments. Short- and Medium-Term Radionuclide Adsorption Reversibility in the Amendments. The reversibility of the radionuclide adsorption was better evaluated by desorption experiments. RSr extraction yields ranged from 85% (SD ) 3) in the sapropel to 100% (SD ) 11) in the mineral soil, with the exception of the phosphorite (52%; SD ) 9). For RCs the initial extraction yields ranged from 3.6% (SD ) 0.7) in the polygorskite to 97% (SD ) 1) in the sapropel, as seen in Figure 3. Therefore, initial extraction yields were generally high in all the amendments tested, showing that the initial adsorption was almost reversible, with the exception of the polygorskite clay, which showed an extremely low RCs extraction yield at the initial stage. Additionally, there may be a significant decrease in the reversibility of the RCs adsorption in the medium and long term, a fact that is responsible for a decrease in the RCs root uptake with time and that must be considered for a better assessment of the eventual amendment performance (26,
38). Figure 3 also shows the variation of RCs extraction yields in amendment samples exposed to drying-wetting cycles, which have shown in the past to be a good laboratory approach to simulate dynamics at field level (26). Results from the application of the cycles show that the extraction yields decreased proportionally to the number of cycles applied, indicating that the amount of RCs fixed by the amendment could increase with time. Therefore, dynamics were significant for all samples, with a decrease in the extraction yields of up to 1 order of magnitude (see the turf material). Again, the polygorskite clay was an exception to this pattern: its final extraction yield was only slightly lower than the initial one because this initial yield was already low. Prediction of the Suitability of the Amendments. The extent of the effect of the addition of amendments, in terms of changes in the adsorption-desorption pattern of the soil, can be rationalized based not only on the changes in the adsorption potential of a radionuclide in the soil-amendment mixture but also by correcting the fraction of radionuclide reversibly adsorbed at a given time. As stated before, changes in the radionuclide adsorption-desorption pattern in the soil will be the predominant effect affecting root uptake, considering that only small changes in the exchangeable complex and in the soil solution were quantified due to the low amendment doses used. The equation for a numerical estimation may be written as follows
effect ) APs‚(1 - wamend)/frev,s + APamend‚wamend/frev,amend (3) APs/frev,s in which APs refers to the radionuclide adsorption potential of the soil, APamend refers to the adsorption potential of the amendment, wamend is the dose in grams of amendment per gram of soil-amendment mixture, frev,s is the fraction of radionuclide reversibly adsorbed in the soil, and frev,amend is the fraction of radionuclide reversibly adsorbed in the amendment. As stated above, this equation is time dependent, especially for RCs, since its reversibly adsorbed fraction will significantly change with time. For the types of soil analyzed in the present work, the reversible fraction for VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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RCs may be around 10% 5 years after the contamination event and about 1% 10 years after (26). Here, we consider the actual values obtained in a previous study, which are 2.1% for Sawichi, 2.4% for Dublin, 2.6% for VIUA, 2.0% for Rudnuy, and 1.3% for Mateyki (39). With respect to the amendments, we can consider the final extraction yields given in Figure 3 to predict the medium-term effect. As the dynamics of the interaction is covered by the reversible fraction, we can take the distribution coefficients after 1 day of equilibration as good estimators of the adsorption potentials of soils and amendments. Equation 3 is useful to calculate the amendment dose to achieve a given effect or to predict an effect with an affordable dose. The results of the prediction exercise for RCs at mediumterm, with doses used at field level, indicate that the effect of these soil amendments is almost insignificant, with values around 1.0 in most cases. Even in a few scenarios, there were predicted negative effects (0.7-0.8 with the mineral soil at the Dublin site), whereas the use of polygorskite was the sole amendment that led to a clearly beneficial effect (1.2 when applied at the VIUA site). As highlighted before, the low KD(Cs) of most materials continues to be the factor responsible for the poor performance of these amendments. This prediction nicely agrees with the results obtained at field level, thus showing that besides considering significant secondary effects that may affect radionuclide root uptake (such as changes in the exchangeable complex and in the soil solution composition), this methodology is an excellent and necessary tool to improve the design of remediation strategies based on the in situ modification of soil properties.
Acknowledgments This research was funded by the European Community (Nuclear Fission Safety Program, contract No. IC15-CT960212) and by project AMB99-0430 (Spanish CICYT). M. Camps thanks the Spanish government for the concession of a training grant in research. Authors would like also thank Dr. S. Firsakova and Dr. N. Grebenshikova (Research Institute of Radiology, Belarus), Dr. N. Sanzharova and Dr. S. Fesenko (Russian Institute of Agricultural Radiology and Ecology, Russia), and Dr. Y. Ivanov and Dr. S. Levchuk (Ukrainian Institute of Agricultural Radiology, Ukraine) and their research groups for their responsibility for the field experiments.
Supporting Information Available Changes in KD(Cs) and KD(Sr) with the equilibration time (Figure A), plant biomass in control and amended plots (Table A), prediction of the KD(Sr) from general properties (Table B), and prediction of the KD(Cs) from general properties (Table C). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review November 17, 2002. Revised manuscript received March 20, 2003. Accepted March 31, 2003. ES026337D
