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DIVISION S-9—SOIL MINERALOGY

Specific Retention of Radiocesium in Volcanic Ash Soils Devoid of Micaceous Clay Minerals

^a CNRS–UMR 6532 HydrASA, Faculté des Sciences, Université de Poitiers, 40, avenue du Recteur Pineau, 86022 Poitiers cedex, France
^b Université Catholique de Louvain, Unité des Sciences du Sol, 2/10, Place Croix du Sud, 1348 Louvain-la-Neuve, Belgium

^* Corresponding author (emmanuel.joussein{at}hydrasa.univ-poitiers.fr).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
The environmental availability of trace radiocesium (¹³⁷Cs) was studied in soils devoid of weathered micas. The soils were developed from basaltic ash, within a sequence Udand Tropept Udalf Udult from Cameroon. Tropepts and Udalfs are halloysite-rich and they exhibit a large cation exchange capacity (CEC) and a strong exchange selectivity for K probably due to the presence of halloysite-smectite mixed-layered clays. The ¹³⁷Cs mobility was evaluated in the B horizons through (i) a physicochemical approach using the radiocesium interception potential concept (RIP) and a sequential sorption–desorption procedure, (ii) a biological test assessing the ¹³⁷Cs rhizospheric mobilization (¹³⁷Cs-RM). In a constant K⁺–Ca²⁺ background solution, one of the Tropepts and the Udalfs fixed about 76% of the initial ¹³⁷Cs loading. The second desorption phase in acidic conditions was more discriminating: the Udalfs fixed about 40%, while the other soils fixed 5 to 20% of the initial ¹³⁷Cs⁺ loading. The ¹³⁷Cs-RM was generally small (3–15%) in all samples and was negatively correlated with the RIP (439–1836 µmol g^–1). The specific retention of ¹³⁷Cs in these soils was thus largely similar to that obtained in soils that contain weathered micas. It demonstrates the presence of ¹³⁷Cs specific sites in halloysitic soils devoid of such minerals. These sites might be associated with halloysite-smectite mixed-layered clays. They were probably formed following wetting-drying cycles in soils heavily fertilized with K.

Abbreviations: ¹³⁷Cs-RM, ¹³⁷Cs rhizospheric mobilization • CEC, cation exchange capacity • FES, frayed-edge sites • K_G, Gapon selectivity coefficient • PAR, potassium adsorption ratio • RES, regular exchange sites • RIP, radiocesium interception potential • TF, ¹³⁷Cs rhizosphere-to-plant transfer factor • TGA, thermal gravimetric analysis

	INTRODUCTION

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RADIOCESIUM is a major airborne radiopollutant spread out after the Chernobyl accident (Avery, 1996). Its dissemination is of great environmental concern because ¹³⁷Cs is a long-lived radioisotope exhibiting a biogeochemical behavior similar to K, a major plant nutrient (Nimis, 1996). Radiocesium rapidly accumulated in the soil or sediment compartments of contaminated terrestrial, or marine and lacustrine ecosystems (Delvaux et al., 2001). In soils and sediments, the ¹³⁷Cs specific retention is controlled by micaceous clay minerals, as they have frayed-edge sites (FES) exhibiting very large selectivity for Cs⁺ (Sawhney, 1972; Evans et al., 1983; Cremers et al., 1988). In fact, these FES selectively sorb monovalent cations with low hydration energy such as Cs⁺, Rb⁺, K⁺, and NH⁺₄ (Brouwer et al., 1983; Wauters et al., 1996). They are located at wedge zones of exfoliating mica particles (Sawhney, 1972; Cremers et al., 1988).

The assessment of the RIP quantifies the specific action of these sites and measures the selective retention of trace Cs in soils and sediments (Sweeck et al., 1990; Cremers et al., 1990; Wauters et al., 1996). The RIP is defined as follows:

[1]

where K_C^FES is the trace Cs⁺ to K⁺ selectivity coefficient in the FES, [FES] is the amount of FES (FES capacity).

The RIP thus accounts for the mineralogical signature of weathered micas (Cremers et al., 1990) and is directly related to the amount of vermiculitic sites in soils (Delvaux et al., 2000).

Halloysitic soil clays devoid of micas can exhibit unusual CEC for 1:1 clays (Bailey, 1990) as well as a very strong ion exchange selectivity for NH⁺₄ (Okamura and Wada, 1984) and K⁺ (Fontaine et al., 1989; Delvaux et al., 1990a; Takahashi et al., 2001; Ndayiragije and Delvaux, 2003). So far, no unique mechanism explains these peculiar ion exchange properties. Strong K⁺ selectivity has been measured in halloysitic soil clays where interstratified smectite layers were identified (Delvaux et al., 1990b; Delvaux et al., 1992) or in soil without 2:1 clay minerals (Takahashi et al., 2001; Ndayiragije and Delvaux, 2003).

In this paper, we studied the mobility of trace Cs in volcanic ash soils exhibiting strong K⁺ selectivity. These soils are devoid of micaceous clay minerals, but they are rich in halloysite (Delvaux et al., 1990b). We used a physicochemical approach based on sorption-desorption properties (Maes et al., 1999) and RIP assessment (Sweeck et al., 1990), and a biological approach based on a rhizospheric assay depicting the ¹³⁷Cs rhizosphere-to-plant transfer (Kruyts et al., 2000).

	MATERIALS AND METHODS

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Soil Materials
The soils were previously studied by Delvaux et al. (1989). They derived from basaltic ash under humid tropical conditions in Cameroon (West Africa). This ash is rich in volcanic glass, contains Ca-plagioclases, pyroxenes, and olivine; and is very poor in native K-feldspars (Dumort, 1968; Sieffermann, 1973). The soils are associated with the mineralogical sequence allophane halloysite kaolinite, in which halloysite is interstratified with smectite (Delvaux et al., 1990a; Delvaux et al., 1990b; Delvaux et al., 1992).

We used soil samples (<2 mm) from the B horizons of seven soils representative of a soil sequence Udand-Tropept-Udalf-Udult. Some of the major properties of these horizons are presented in Table 1 for the fine earth fraction (<2 mm). Clay samples (<2 µm) were collected by sedimentation after H₂O₂ oxidation of organic matter. Free Fe oxides were removed with dithionite-citrate-bicarbonate (DCB). The main characteristics of the clay fraction (<2 µm) are summarized in Table 2.

With increasing weathering stage, allophane disappears, the C content and total reserve in bases (TRB) decrease, whereas clay content increases. Ammonium-extractable K is particularly large (0.6–2.7 cmol_c kg^–1 soil) and contributes to 3 to 25% of the effective CEC of the B horizons. This is caused by heavy K fertilization, as these soils are used for intensive banana (Musa sp.) cropping since the 1950s. Halloysite is the dominant clay mineral in the Tropepts (SN4, SN5) and, to a lesser extent, in the Tropudalfs (SN2, IR13), whereas kaolinite is the dominant 1:1 layer silicate in the Paleudult MK1. The halloysite-rich soils exhibit a high CEC and an especially high exchange selectivity for K. The values of the Gapon selectivity coefficient (K_G) were calculated from the experimental data as follows:

[2]

where E_K and E_Ca denote equivalent fractions of adsorbed K⁺ and Ca²⁺, and ( ) denote ion activities in the equilibrium solution.

As measured by the Gapon coefficient K_G,0.04 (value of the Gapon selectivity coefficient computed by Eq. [2] at a 0.04 K⁺ equivalent fraction in the equilibrium solution) and the estimated amount of K-specific sites, K affinity increases from the allophanic (LO1, NA13) to the halloysitic stage (SN4, SN5, SN2, IR13) and then decreases at the kaolinite stage (MK1). The comparison of K_G,0.04 values and estimated amounts of K-specific sites in the fine earth and clay fractions indicates that colloidal components are responsible for K⁺ selectivity. In these soils, the strong K⁺ selectivity is linked with the relative halloysite content, with respect to kaolinite (H/[H+K]), and probably to the presence of halloysite-smectite mixed-layered clays (H-Sm) (Delvaux et al., 1990b; 1992). The authors combined XRD, CEC, and TGA data for determining and inferring smectite content. The presence of 2:1 clays minerals in intermediate weathering stage of volcanic ash soils have also be noted in other studies (Shoji et al., 1981; Takahashi et al., 1993; Chorover et al., 1999).

Sorption-Desorption Experiments and Radiocesium Interception Potential Measurement
Trace ¹³⁷Cs sorption-desorption experiments were performed according to Maes et al. (1999). Briefly, soil samples (<2 mm) were previously equilibrated with a KCl-CaCl₂ solution (mK = 0.5 mM, mCa = 100 mM) with a potassium adsorption ratio (PAR) value of 0.05 mM^1/2 [PAR is defined as (K)/(Ca) where ( ) refers to ion concentration in mM]. After equilibration, the liquid phase was labeled with trace ¹³⁷Cs⁺ (Amersham, quasi carrier free). Sorption yields were defined by ¹³⁷Cs sorbed by soil materials after a 168-h equilibration time, expressed as a percentage of the initial ¹³⁷Cs loading. The solid-liquid distribution coefficient of ¹³⁷Cs (K^Cs_D) was computed after 168 h by -counting of the solution phase for 5 min using NaI scintillator -counter (Auto- 5000 series, Canberra, Australia; statistical error <5%). This measurement led to the determination of the RIP using Eq. [1] and as detailed in Wauters et al. (1996):

[3]

where K^Cs_D refer to the ¹³⁷Cs solid-liquid distribution coefficient measured in an electrolyte background, and mK, the fixed K molar concentration in the equilibrium solution (0.5 mM).

Desorption (adapted from Wauters et al., 1994) was performed sequentially, as described by Maes et al. (1999). A first desorption (D1) was performed with 5 g of K⁺–Ca²⁺–saturated Amberlite IR120, previously equilibrated with the KCl-CaCl₂ solution (PAR = 0.05 mM^1/2), using 1 g of soil in 100 mL of KCl-CaCl₂ solution. A second desorption (D2) was achieved in an acidic background, using 5 g of H⁺-saturated Amberlite IR120 (10^–3 N, pH 3), and 1 g of soil in 100 mL of KCl-CaCl₂ solution. The solution and the resin were replaced and counted for their activity. The ¹³⁷Cs net retention was defined as the percentage of the initial ¹³⁷Cs loading retained by the soil sample at the end of each desorption process (D1, D2). Desorptions D1 and D2 were monitored for 25 and 51 d, respectively. All experiments were performed in triplicate.

Rhizospheric Mobilization of Radiocaesium
The methodology used to determine the ¹³⁷Cs-RM was adapted from Niebes et al. (1993) and described by Kruyts et al. (2000). Seeds (1 g) of ryegrass (Lolium multiflorum cv. Lemtal) were germinated in a small ring container, on a water saturated sieve surface (20 µm mesh size), during a period of 4 d in darkness. Then, the seedlings were supplied with a K-free nutrient solution (3.5 mM Ca(NO₃)₂ · 4H₂O, 1 mM NaH₂PO₄, 1mM MgSO₄ · 7H₂O, 0.1 mM FeEDTA, 0.02 mM MnCl₂, 0.01 mM H₃BO₃, 0.2 µM ZnSO₄ · 7H₂O, 0.2 µM CuSO₄ · 5H₂O, and 0.2 µM Na₂MoO₄ · 2H₂O) and were placed in a controlled greenhouse for 3 d (light intensity: 8.4 W m^–2 of 350- to 750-nm wavelength 14 h a day; temperature: 20°C, and relative humidity of 95%). The root mat was then brought into contact with the ¹³⁷Cs-contaminated soil and with the K-free nutrient solution for 4 d (five replicates). One week before contact, each soil sample (1 g), previously equilibrated with a Ca²⁺ homoionic solution, was contaminated with trace ¹³⁷Cs⁺ (Amersham, quasi carrier free). Both shoots and roots were separated, weighed, and -counted using a NaI scintillator -counter (Auto- 5000 series, Canberra, Australia; statistical error <5%). The soils were also -counted, as well as the nutrient solution. The proportion of ¹³⁷Cs in plant (shoot and root) relative to the total ¹³⁷Cs contamination is defined as the ¹³⁷Cs-RM. The ¹³⁷Cs rhizosphere-to-plant transfer factor (TF), as measured on a weight basis, was computed as follow:

[4]

with a 5000 Bq g^–1 initial contamination of soil, and the plant dry material involving both roots and shoots (Kruyts et al., 2000).

	RESULTS

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RIP and ¹³⁷Cs Sorption-Desorption Yields
The RIP values, as measured according to Eq. [3], sorption yields and ¹³⁷Cs net retention are presented in Table 3. The results, performed in triplicate, showed a good reproducibility as indicated by standard deviations. The ¹³⁷Cs sorption-desorption step data are illustrated in Fig. 1 .

View larger version (25K): [in this window] [in a new window]   Fig. 1. The 137Cs+ sorption–desorption experiments. S: sorption; D1: first desorption by K+-Ca2+–saturated resin; D2: second desorption by H+-saturated resin. The different symbols refer to the different soils.

The values of ¹³⁷Cs sorption yield (Table 3) were large and similar for all soils. After 7 d of contact, the sorption yield ranged between 89 and 97% of the ¹³⁷Cs input. The maximum values were measured in one of the Tropepts (SN4: 97.3%) and the Udalfs (SN2: 97.1%; IR13: 97.3%).

The ¹³⁷Cs net retention values, as measured after each desorption phase discriminated the different horizons (Table 3 and Fig. 1). After the first desorption phase D1 with K⁺–Ca²⁺–saturated resin, three types of ¹³⁷Cs fate were observed (Fig. 1). The three samples with largest RIP (SN4, SN2, and IR13) exhibited ¹³⁷Cs net retention values of about 77%. The two samples with lowest RIP (NA13, MK1) had net retention values of 35 to 38%. The two horizons with intermediate RIP (LO1, SN5) showed intermediate ¹³⁷Cs net retention values (58%).

The second desorption phase D2, using H⁺–saturated resin, led to a strong decrease of the ¹³⁷Cs net retention (Fig. 1 and Table 3). The ¹³⁷Cs net retention value measured after D2 desorption was about 42% in the Udalfs (SN2, IR13) and ranged from 5 to 20% in the other soils (NA13, MK1, SN5, LO1, and SN4). The acidic desorption (D2) thus clearly discriminated the Udalfs (SN2 and IR13) from the others soils. The former retained much more ¹³⁷Cs than the latter.

Rhizospheric Mobilization of Radiocesium
The dry matter yield of ryegrass (shoot and root) was remarkably uniform whatever the soil type: 0.681 ± 0.010 g. For each sample, the reproducibility of the ¹³⁷Cs-RM values was good, due to strictly controlled experimental conditions (Delvaux et al., 2000; Kruyts et al., 2000). The ¹³⁷Cs-RM values varied between 3 and 14% (Table 3) of initial ¹³⁷Cs soil contamination despite a fairly uniform biomass. This means that the differences in ¹³⁷Cs-RM were linked with their distinctive Cs retention properties. The values of ¹³⁷Cs-RM were lowest (3–7%) in the Tropepts (SN4, SN5) and the Udalfs (SN2, IR13), and largest (10–14%) in the Udands (NA13, LO1) and the Tropudult (MK1).

Dry matter yields and the ¹³⁷Cs-RM were used to compute the TF on a weight basis according to Eq. [4]. The TF values ranged from 0.047 to 0.243 g g^–1 (Table 3).

	DISCUSSION

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Mobility of Trace Cesium in Soils
The high ¹³⁷Cs sorption yields (89–97%) denote large ¹³⁷Cs sorption capacities of the various exchangers. They are similar to those obtained for biotite and mica-derived vermiculite (Maes et al., 1999). After desorption, the largest ¹³⁷Cs net retention values, as measured in the Udalfs SN2 and IR13 (77% after D1, 42% after D2), are slightly lower than in mica-derived vermiculite (54% after D2 in Maes et al., 1999), but much larger than in unweathered biotite (1% after D2 in Maes et al., 1999). These results suggest the presence of ¹³⁷Cs specific sites in the studied soils, which do not contain micaceous clay minerals. Even if trace of micaceous clay minerals from aeolian deposits can be conceivable (Chorover et al., 1999), they cannot explain RIP values as large as 500 to 2000 µmol g^–1 (Kruyts et al., 2003). Moreover, we can assume that aeolian deposits are concentrated near the soil surface and decreased with depth. In our case, we studied the B horizons of soils and we can assumed that these horizons are very poor in micaceous clays, if any.

The presence of ¹³⁷Cs specific sites is supported by the typical range of RIP for soils containing such minerals (Wauters et al., 1996; Delvaux et al., 2001). Though our soils are devoid of native micas (Dumort, 1968; Sieffermann, 1973, Delvaux et al., 1990a), they possess exchange sites exhibiting the same behavior as the ones where the specific retention of trace Cs is due to FES originating from exfoliating mica particles. Applying the RIP concept (Sweeck et al., 1990) and using a mean value of K^FES_C = 10³ (Wauters et al., 1996) would lead to a hypothetical estimated FES capacity of about 1.8 µmol g^–1 soil, that is, around 1% of the cation exchange capacity of the Udalfs SN2 and IR13. Such estimated FES proportion is similar to the one estimated in soils containing weathered mica (Cremers et al., 1988; Wauters et al., 1996). Our results would thus suggest that the RIP concept (Sweeck et al., 1990) could also apply to fine clayey soils devoid of FES bearing micaceous clay minerals.

The small ¹³⁷Cs-RM values (3–15%) denote limited mobility of trace Cs in the vicinity of active plant roots. These values are in the low range of a large set of ¹³⁷Cs-RM values measured for a wide variety of European soils (1–69%, Delvaux et al., 2000). As plant growth was uniform in the various soil horizons, the differences in ¹³⁷Cs-RM values could be attributed to distinct ¹³⁷Cs retention properties. Indeed, the lowest RM values (3%) were measured in the soils with largest RIP, that is, the Tropept SN4 and the Udalfs SN2 and IR13 (Table 3). In fact, the ¹³⁷Cs-RM decreased with the ¹³⁷Cs net retention values, as measured after both D1 and D2 desorptions (Fig. 2) . In other words, Cs-specific sites in our volcanic ash soils control the mobility of ¹³⁷Cs in the vicinity of plant roots. This assessment is supported by Fig. 3 , plotting the TF from rhizosphere-to-plant against RIP. As illustrated in this figure, the ¹³⁷Cs TF covers two orders of magnitude (0.047–0.243 g g^–1) and is negatively correlated with RIP. Moreover, our experimental points strongly fit within the TF-RIP relationship obtained in a very large variety of 46 soil horizons developed in various parent materials containing detectable micaceous clay minerals (Delvaux et al., 2000). As most of these soils contain to some extent weathered micas, this further confirms that Cs-specific sites, with similar apparent reactivity as FES, control the rhizosphere-to-plant transfer of trace Cs in our volcanic ash soils devoid of weathered mica.

View larger version (12K): [in this window] [in a new window]   Fig. 2. Relationship between the net retention and the 137Cs+ rhizospheric mobilization (137Cs-RM) expressed in percentage of initial 137Cs soil contamination. : net retention after desorption by K+–Ca2+–saturated resin (D1);: net retention after desorption by H+–saturated resin (D2).

View larger version (11K): [in this window] [in a new window]   Fig. 3. Log-log plot of rhizospheric 137Cs+ transfer factor on a weight base (TF) against the radiocesium interception potential (RIP) in soil. : this study; : collection of soils with widely varying properties (adapted from Delvaux et al., 2000).

Most probably, the differences in electrolyte background and the non-similarity between FES sensu stricto and FES-like sites, as suspected here (see below), explain the large discrepancy between (i) exceptional K⁺ selectivity (Tables 1 and 2), and (ii) medium RIP values (Table 3), as compared with those measured in high Cs-fixing soils and sediments (see review in Delvaux et al., 2001).

In the soil sequence under study, the K⁺ selectivity was attributed to the dominance of hydrated halloysite and the presence of halloysite-smectite (H-Sm) mixed-layered clay (Delvaux et al., 1990a, 1990b). The RIP is larger in soils rich in hydrated halloysite and H-Sm (SN4, SN2, IR13), than in soils rich in kaolinite (MK1) or in allophane and organic matter (LO1, NA13); however, the SN5 sample does not behave in the same way. Though exhibiting the large K⁺ selectivity and largest content in H-Sm mixed layered clays, SN5 does not show a high RIP value (864 µmol g^–1), as compared with the ones measured in soils with detectable micaceous clays (10³–10⁴ µmol g^–1; Delvaux et al., 2001). Thus, smectite content was not significantly correlated with specific retention data.

Possible origin of FES-Like Specific Sites for Trace Cs
Though the average value of K^FES_C is about 10³ in FES originating from exfoliating micas (Wauters et al., 1996), the value of the Cs⁺ to K⁺ selectivity coefficient in regular exchange sites (RES) K^RES_C is about one in humic substances and five in clay minerals (in both planar sites and hydrated interlayers), where Ca²⁺ is selectively sorbed against K⁺ and Cs⁺ ions (Bruggenwert and Kamphorst, 1976). Consequently, the specific retention of trace Cs would be poor in smectite, which occurs in interstratification with halloysite in our soils (Table 2, Delvaux et al., 1990a). It could, however, largely increase through K⁺ fixation, generated by smectite layer-to-layer rearrangement of the stacking order induced by wetting-drying cycles (Mamy and Gaultier, 1978; Maes et al., 1985).

In this respect, it is interesting to note that the largest RIP and lowest ¹³⁷Cs-RM (Tables 3 and 4) are measured in soil horizons exhibiting the largest contents of total K in the Na⁺–exchanged clay fractions (SN4, SN2, IR13: Table 2), where total K could be considered as fixed K in 2:1 interlayers. Such K fixation could create wedge zones similar to the ones in weathered micas. Wetting-drying cycles applied on K-saturated smectites can indeed induce the formation of mica-like structures (Mamy and Gaultier, 1978), thereby creating specific retention sites for trace Cs similar to the FES, as shown by Maes et al. (1985). These authors demonstrated that (i) these sites can exhibit exceedingly high selectivity for trace Cs, and (ii) they can be formed after a limited number of wetting-drying cycles (5–10). The formation of mica-like structures in smectite increases K^FES_C by two orders of magnitude (Maes et al., 1985) and thereby involves large decrease of ¹³⁷Cs soil-to-plant transfer (Vandenhove et al., 2003).

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
We assessed the mobility of trace Cs in basaltic ash soils devoid of mica by measuring the specific retention of ¹³⁷Cs as well as the ¹³⁷Cs rhizosphere-to-plant transfer. The Cs specific retention was measured through sorption–desorption experiments and RIP assessment. Our results show that ¹³⁷Cs mobility in these soils is much like that in soils containing micas and their vermiculitized minerals. In particular, Cs specific sites, acting similarly to FESs located at exfoliating mica particles, control the mobility of ¹³⁷Cs in our volcanic ash soils, and its mobility in the vicinity of active plant roots.

The largest Cs retention was measured in soils rich in halloysite, which is interstratified with smectite layers, but with largest fixed K content in their clays. We believe that specific Cs⁺ sites may have formed in these soils because of the combination of the occurrence of smectite, wetting-drying cycles and intensive K fertilization over long periods. This combination is indeed known to generate mica-like structures in smectite units.

	ACKNOWLEDGMENTS

 
We express our deep gratitude to ladies Anne Iserentant and Claudine Givron for their major technical help. We also thank Prof. Adrien Herbillon for fruitful discussions and Mrs. Marie-Christine Depaue for editorial assistance. This research was supported by the Belgian Fonds National de la Recherche Scientifique FNRS (Contracts FRFC N°2.4595.98 and 2.4599.00F), the Fonds Spécial de Recherche of the Université Catholique de Louvain (FSR 1999), and by CNRS (DRI) << projet en collaboration bilatérale >> CNRS/CGRI-FNRS. N. Kruyts thanks the FNRS support to her position of scientific research worker (Collaborateur scientifique FNRS).

Received for publication December 31, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 

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