ISSN 1066-3622, Radiochemistry, 2014, Vol. 56, No. 2, pp. 222–225. © Pleiades Publishing, Inc., 2014. Original Russian Text © L.N. Maskalchuk, A.A. Baklai, A.V. Konoplev, T.G. Leontieva, 2014, published in Radiokhimiya, 2014, Vol. 56, No. 2, pp. 189–192.

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Migration of 90Sr in the Solid Phase of the Soil–Soil Solution–Plant Systems and Ways to Reduce It

L. N. Maskalchuk*а, A. A. Baklaiа, A. V. Konoplevb, and T. G. Leontievaа

а Joint Institute for Power and Nuclear Research–Sosny, National Academy of Sciences of Belarus, ul. A.K. Krasina 99, Minsk, 220109 Belarus;

* e-mail: leonmosk@tut.by b Taifun Research and Production Association, ul. Lenina 82, Obninsk, Kaluga oblast, 249038 Russia;

e-mail: konoplev@obninsk.com

Received May 30, 2013

Abstract—A model of 90Sr migration in soil–soil solution–plant system was developed, and an expression was obtained for estimating the strontium migration. In accordance with the model, the 90Sr migration in the soil–plant system can be reduced by liming of acidic soils only to a certain limit determined by the cation-exchange capacity of the soil. According to the estimations, for the 90Sr migration into plants on limed soil to be effi-ciently reduced by a factor of 2, taking into account the economically acceptable level of 1–4 wt % for sorbent introduction into soil, it is necessary that the ratio of the sorption potentials of the sorbent and soil should be no less than 25.

Keywords: strontium-90, migration model, soils, sorbents

Nuclear weapons tests and accidents at nuclear power engineering objects (Kyshtym, Chernobyl, etc.) led to environmental pollution with 90Sr. Because of high toxicity and capability to be readily involved in geochemical and biological migration processes, 90Sr is one of the most hazardous radionuclides [1]. During the time that passed after the Chernobyl accident, transformations of the species in radioactive fallout resulted in a 5–10-fold increase in the mobility and bioavailability of 90Sr in the ecosystems where no remediation was performed [2]. Published data indi-cate that the ion-exchange mechanism of the 90Sr sorp-tion by soil components prevails and that the strontium amount in soil solutions is relatively high compared to Cs, Pu, and Am radionuclides [1, 3]. As a result, 90Sr is redistributed in ecosystems more intensely, migrates along food chains, and gets into human body, produc-ing the internal irradiation dose along with 137Cs.

The migration (transfer) of 90Sr in the solid phase of the soil–soil solution–plant system is described using the concentration factor (CF) defined as follows:

DOI: 10.1134/S1066362214020167

activities of 90Sr in the plant and soil, respectively, Bq kg–1.

The 90Sr CF depends on a number of parameters characterizing the properties of soil, plant, and the macroanalog (Ca). As noted in [4], the CF values for the same plant species growing on different soils can differ by a factor of more than 40. Therefore, the use of averaged CF values leads to considerable differ-ences in evaluation of doses and risks for residents of a territory contaminated with radionuclides.

One of the ways to solve this problem can be the development of a model that takes into account the effect of physicochemical and biological processes on the 90Sr migration in the solid phase of the soil–soil solution–plant system. The use of such a model would allow correct estimation of CF from soil characteristics and evaluation of the efficiency of various procedures for remediation of 90Sr-contaminated soils.

The goal of this study was the development of a model of 90Sr migration in the solid phase of the soil–soil solution–plant system, based on physicochemical parameters of the soil, and analysis of ways to reduce the strontium migration within the framework of this model.

CF = [90Sr]p/[90Sr]s, (1)

where [90Sr]р and [90Sr]s are the equilibrium specific

MIGRATION OF 90Sr IN THE SOLID PHASE OF THE SOIL–SOIL SOLUTION–PLANT SYSTEMS 223

EXPERIMENTAL

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portional to its content in the exchange complex of roots;

(3) the 90Sr content in the exchange complex of roots is mainly determined by the Са2+ concentration in the soil solution;

(4) the 90Sr concentration in the soil solution is in-versely proportional to the exchange distribution coef-ficient of 90Sr between the solid and liquid phases of soils (Kd

ex);

(5) the exchange distribution coefficient of 90Sr is determined by the cation-exchange capacity (CEC) of the soil solid phase and by the cationic composition of the soil solution (mainly by the Са2+ content);

(6) the fixed form of 90Sr does not participate in the exchange with the soil solution.

According to this model, under equilibrium condi-tions the 90Sr concentration in the plant can be deter-mined as follows:

Modeling object. The object of mathematical mod-eling is a system consisting of a solid phase of the soil, a soil solution, and a plant. The conceptual scheme of the 90Sr migration in the solid phase of the soil–soil solution–plant system is shown below.

Model of 90Sr migration in the solid phase of the soil–soil solution–plant system. According to the modern views, 90Sr migrates from a soil solution into a plant, like other mineral elements, in the ionic form. The 90Sr2+ radionuclide is not an element of mineral nutrition of a plant, but it is taken up by a plant via transport systems for its macroanalog, Са2+. The 90Sr transfer from a solid phase of the soil into a plant is determined by the specific features of the strontium migration in the solid phase of the soil–soil solution–plant system. In this connection, the complex process of ion uptake by a plant from a soil can be subdivided into two steps [5]: distribution of ions between the solid and liquid (soil solution) phases of soil and up-take of simple ions by plant roots from the soil solu-tion. Correspondingly, there are two level of control of the 90Sr transfer in the solid phase of the soil–soil solu-tion–plant system: soil level (solid phase of the soil–soil solution), quantitatively characterized by the ex-change distribution coefficient (Kd

ex) [6], and biological (or physiological) level (soil solution–plant), evaluated by CF, which is largely determined by the concentrations of Sr2+, K+, Ca2+, and Mg2+ ions in the soil solution.

Processes and reactions that occur in the solid phase of the soil–soil solution system and are essen-tially physicochemical determine the 90Sr mobility and its potential availability to the uptake by plant roots. The mechanisms and processes of ion transport in up-take by plant roots are associated with the control on the biological level [5, 7].

Thus, the 90Sr transfer from a soil into a plant is determined by a set of various physicochemical and biological processes whose formalization is possible only within the framework of a mathematical model. The main assumptions forming the basis of the mathe-matical model are as follows:

(1) 90Sr in a soil solution is in dynamic equilibrium with two ion exchangers: solid phase of the soil and exchange complex of roots (root system);

(2) 90Sr is taken up by a plant from the soil solution, and the radionuclide concentration in the plant is pro-

Exchange complex of roots(plant roots)

Soil solution

Exchangeable form (CEC)

Soil solid phase Fixed form

Ion exchange

ion exchange

[90Sr2+]aq–[Са2+]aq

Kdex(90Sr)

Available 90Sr

Fixed 90Sr

Plant

Conceptual scheme of 90Sr migration in the solid phase of the soil–soil solution–plant system.

[90Sr]p = k[90Sr2+]w/[Ca2+]w, (2)

where k is the proportionality coefficient between the amount of 90Sr taken up by the plant and the 90Sr amount in the exchange complex of roots, charac-terizing the efficiency of the 90Sr uptake by a plant; [90Sr2+]w is the 90Sr concentration in the soil solution, Bq L–1; and [Са2+]w is the concentration of calcium cations in the soil solution, meq L–1.

The fixation and distribution of 90Sr in the solid phase of the soil are mainly determined by the relation-

The model was developed taking into account the experimental data [1, 2, 4, 5] on the 90Sr migration in the solid phase of the soil–soil solution–plant system and the assumption that 90Sr in the soil solution is in dy-namic equilibrium with two ion exchangers: solid phase of the soil and exchange complex of roots (plant roots).

The key parameters of the model are as follows: ratio αex/[Kc(

90Sr2+/Ca2+)], characterizing the capability of a soil to fix 90Sr; total concentration of 90Sr in the solid phase of the soil, [90Sr], Bq kg–1; and concentra-tion of exchangeable Ca2+ cations in the solid phase of the soil, [Са2+]ex, meq kg–1.

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RESULTS AND DISCUSSION

ships characteristic of its chemical macroanalog, stable calcium (Са2+), which allows determination of the ratio [90Sr2+]w[Ca2+]w from the ion-exchange equilibrium in accordance with the expression

where [90Sr2+]ex is the concentration of exchangeable 90Sr cations in the soil solid phase, Bq kg–1; [Са2+]ex, concentration of exchangeable calcium cations in the solid phase of the soil, meq kg–1; Kc(

90Sr2+/Ca2+), ex-change selectivity factor for the Sr/Ca cation pair in the solid phase of the soil.

After appropriate transformations using (2) and (3), we obtain

(3) [90Sr2+]ex/[90Sr2+]w = Kc(

90Sr2+/Ca2+)[Ca2+]ex/[Ca2+]w,

The concentration of exchangeable 90Sr in soil can be expressed via its total concentration as follows:

(4) [90Sr]p = k[90Sr2+]ex/{Kc(90Sr2+/Ca2+)[Ca2+]ex}.

Expression (6) allows prediction of the degree of plant contamination with 90Sr at varied levels of soil contamination with this radionuclide, which is impor-tant both at NPP accidents and for the control of the dynamics of the 90Sr accumulation and migration in the soil–plant system.

(5) [90Sr2+]ex = αex[90Sr],

where [90Sr] is the total concentration of 90Sr in the solid phase of the soil, Bq kg–1, and αex is the fraction of exchangeable 90Sr in the soil, %.

Taking into account (5), expression (4) can be pre-sented as follows:

(6) [90Sr]p = kαex[90Sr]/{Kc(

90Sr2+/Ca2+)[Ca2+]ex}.

The parameter CF determining the migration of 90Sr from a soil to a plant is considered as variable.

For acidic soils contaminated with 90Sr, the tradi-tional way of their remediation, i.e., of reduction of the migration in the solid phase of the soil–soil solution–plant system, is liming, provided that the soil has suffi-cient potassium content for the growth and develop-ment of plants.

For convenience of the analysis, let us transform expression (6) using the following relationship:

(7) [Са2+]ex = β × CEC,

where CEC is the cation-exchange capacity, meq kg–1, and β is the degree of filling of the cation-exchange capacity of soil with exchangeable calcium.

As a result, we obtain:

As shown in [8], the content of exchangeable Mg2+ in the majority of soils is less than 20% of the content of exchangeable Ca2+, and for Ca2+ the exchange selec-tivity on the exchange complex of plant roots is 2– 3 times higher than for Mg2+. Therefore, the migration (transfer) of 90Sr from a soil to plants after saturation of the cation-exchange capacity of the soil with Ca2+ due to liming (CEC ≈ [Са2+]ex, β ≈ 1) can be deter-mined from the following relationship:

(8) CF = kαex/[Kc(90Sr2+/Ca2+)β × CEC].

CF = kαex/[Kc(90Sr2+/Ca2+) × CEC]. (9)

Analysis of expression (9) shows that the possibil-ity of reducing the 90Sr migration in the solid phase of the soil–soil solution–plant system by liming acidic soils is not unlimited and is restricted by CEC of the soil, which is confirmed by the practice [9, 10].

Expression (9) suggests new possibilities for further reduction of the 90Sr migration from soils to plants on limed soils that already have the optimum acidity.

For example, in accordance with formula (9), CF does not depend on the density of soil contamination with 90Sr and is determined by the reciprocal value of Kc(

90Sr2+/Ca2+) × CEC. Therefore, further decrease in the 90Sr migration from soil into plants in limed soils is possible only by changing their sorption characteristics via introduction of sorbents with high values of Kc(

90Sr2+/Ca2+) × CEC. By analogy with 137Cs, let us term the quantity Kc(

90Sr2+/Ca2+) × CEC the sorption potential (SP) [11].

REFERENCES

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Taking into account the economically acceptable dosage of sorbents to be inserted into a limed soil, 1–4 wt %, the sorption potential of the sorbent for de-creasing CF of a plant with respect to 90Sr by a factor of 2 can be estimated from expression (9).

The change in the sorption potential of the soil with respect to 90Sr after introducing the sorbent can be cal-culated on the basis of the additivity rule:

where SP(sr), SP(s), and SP(s + sr) are the sorption potentials of the sorbent, soil, and soil with the intro-duced sorbent, meq kg–1, respectively; msr and ms are the sorbent and soil weights, kg.

For example, for 1 wt % dosage of the sorbent, we obtain from expressions (9) and (10) after simple trans-formations SP(sr)/SP(sl) ≈ 100 times.

Peat, organic sapropels, and zeolites can be consid-ered today as sorbents for further reducing the 90Sr mi-gration into plants from limed soddy podzolic and sandy loam soils.

Based on the suggested conceptual model of 90Sr migration from soils to plants, an expression was ob-tained for estimating the strontium migration. This ex-pression includes a combination of the key characteris-tics of soil (content of exchangeable Ca2+ and fraction of exchangeable 90Sr2+ in soil, exchange selectivity coefficient for the 90Sr2+–Ca2+ cation pair).

An advantage of the suggested model is small num-ber of parameters, which have clear physical sense and can be determined using the existing standard physico-chemical methods.

The model developed shows that the possibility of reducing the 90Sr migration in the soil–plant system by liming of acidic soils is limited by the cation-exchange capacity of the soil. It also follows from the model that further reduction of the 90Sr migration into plants in a limed soil is possible only by introducing into soil sor-

(10) SP(s + sr) = SP(sr)msr/ms + SP(s),

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veshchestv v pochve. Mekhanisticheskii podkhod (Bio-availability of Nutritional Substances in Soil. A Mecha-nistic Approach), Moscow, 1988.

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8. Sentenac, H. and Grignon, C., Plant Physiol., 1981, vol. 68, pp. 415–419.

9. Putyatin, Yu.V., Minimizatsiya postupleniya radionukli-dov 137Cs i 90Sr v rastenievodcheskuyu produktsiyu (Minimization of the Ingress of 137Cs and 90Sr Radionu-clides into Plant Growing Products), Minsk, 2008.

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bents with high sorption potential (SP), defined as the product of Ks(

90Sr2+/Ca2+) and CEC.

According to the estimate, to efficiently decrease the 90Sr migration into plants by a factor of 2 on a limed soil, taking into account the economically ac-ceptable sorbent dosage of 1–4 wt %, it is necessary that the ratio of the sorption potentials of the sorbent and soil should be no less than 25.