Distribution of radionuclides in soils modelling the ...Distribution of radionuclides in soils –...

Landesmessstelle für

Radioaktivität

Fachbereich

Physik/Elektrotechnik

Institut für Umweltphysik

Distribution of radionuclides in soils –

modelling the dependence on soil

parameters

Volker Hormann and Helmut W. Fischer

6th INSINUME Symposium, Brussels, 13.06.2012

Landesmessstelle für Radioaktivität

Volker Hormann 2 of 20

Contents

• model description

• model verification

• sensitivity study (variation of soil parameters)

• (redox sensitivity)

• discussion



Model description

Geochemical code: PHREEQC (Parkhurst and Appelo 1999)

Complexation models used with PHREEQC: Dzombak and Morel (1990)

Bradbury and Baeyens (2009, 2009)

Tipping (Model VI, 2002)

Model components: • Ni,U,Se: oxalate extractable hydrous ferric oxides (HFO) DM

• Cs,Ni,U: clay minerals (illite as representative material, including frayed edge sites) BB

• Ni,U: immobile organic matter T

• Ni,U: dissolved organic matter (DOM) T

• soil solution

• solid phases



Model Verification

Study of 18 different U-contaminated soils: Vandenhove et al. (2007)

Two Cs-contaminated soils: Nisbet (1995)

1,0E-09

1,0E-08

1,0E-07

1,0E-06

1,0E-05

1,0E-09 1,0E-08 1,0E-07 1,0E-06 1,0E-05

measured U concentration (mol/l)

ca

lcu

late

d U

co

nc

en

tra

tio

n (

mo

l/l)

0

20

40

60

80

100

120

sand loam

acti

vit

y o

f C

s-1

34 i

n B

q/l

initial, experimental values

final, experimental values

final, PHREEQC model

Figure 1. Comparison of measured and calculated U concentrations in soil solution

Figure 2. Activity of 134Cs in soil solution before and after treatment with 11.5 m potassium



Reference soils (RefeSols)

Refesol sand silt clay pH Corg CECeff Feox Alox type

02-A 2 83 15 6.6 1.3 133.2 3.54 0.69 Stagnic luvisol (loam)

04-A 85 11 4 5.1 2.9 85.7 0.63 1.51 Gleyic podsol (sand)

Table 1: characteristics of the reference soils, texture and Corg in %, CECeff (exchangeable Ca, Mg, H and Na) in mmolc/kg, oxalate-extractable oxides in g/kg, source: K.H. Weinfurtner, Fraunhofer Institute for Molecular Biology and Applied Ecology, Schmallenberg, Germany

used in the Reference Biosphere Project in collaboration with BfS, HelmholtzZentrum Munich and GRS background: long-term radioecological risk assessment of nuclear waste disposal



Assumptions for the Refesol model

The composition of the soil solution of the Refesols is not yet known

use of a „standard“ soil solution for modeling:

(concentrations are geometric means of ranges of frequent values,

Scheffer/Schachtschabel 2010):

Na K Mg Ca NH4 Fe Al Si Cl N P S DOM

6.3 9.5 11 80 0.9 - - 10 24.5 20 0.01 39 54

Table 2: Composition of the „standard“ soil solution (values in mg/l)

• Fe and Al determined by equilibrium with ferrihydrite and gibbsite

• no phosphate fertilization important for comparison with literature values

• DOC 27 mg/l, org. C 50% of org. matter

• concentration of contaminating nuclides: 1 Bq/kg DW (135Cs,63Ni, 238U and 79Se)



Calculated distribution coefficients – comparison with literature values

• calculation using „standard“ soil solution from Table 2

• equilibration of initial soil solution with surface assemblage equilibration with contaminated soil solution

Fig. 3: Logarithmic distribution coefficient compared to literature values (IAEA Tecdoc 1616)

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

Cs Ni U Se

log

Kd

RefeSol 2 (loam)

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

Cs Ni U Se

log

Kd

RefeSol 4 (sand)



Important soil parameters and processes

• contaminant concentration

• dilution/evaporation

• clay (mineral) content

• Fe-/Al-oxides (oxalate extractable)

• immobile organic matter

• dissolved organic matter

• pH

• (redox state)

conditions for modelling: saturated soil, no oxygen in solution



Concentration

0

20

40

60

80

100

120

0,1 1 10 100 1000 10000

ch

an

ge

in

%

concentration in Bq/kg

Concentration dependence of Kd

Cs

Ni

U

Se

g saturation effects



Dilution

1.0E+02

1.0E+03

1.0E+04

0 20 40 60 80 100

Kd

in l/k

g

% of original soil solution

Dilution (Mixing with rainwater)

Cs

Ni

U

Se

dilution but constant activity: g less competition by major ions



Evaporation

1.0E+01

1.0E+02

1.0E+03

1.0E+04

0 0.2 0.4 0.6 0.8 1

Kd

in l/k

g

water fraction

Evaporation

Cs

Ni

U

Se

evaporation at constant activity: g more competition by major ions

must not be confused with the case of unsaturated soil (relative concen-trations constant, same chemistry)



Clay content

Kd Ni: 150 l/kg (0 % clay) 200 l/kg (55 % clay)

complexation on clay highly significant for Cs, moderate for Ni, negligible for U and Se (no clay sorption model for Se as yet)

0.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

2.5E+04

0 10 20 30 40 50 60

Kd in

l/k

g

clay content in %

Clay content

Cs

note linear scale!



Content of Fe/Al oxides

1.0E+01

1.0E+02

1.0E+03

1.0E+04

0 2 4 6 8

Kd

in l/k

g

Fe in g/kg dry soil mass

Fe/Al hydroxides(oxalate extractable)

Ni

Cs

U

Se

red: values for Luvisol (Refesol 2)

strong influence of Fe/Al oxides for U and Se



Immobile organic matter

1.0E+01

1.0E+02

1.0E+03

1.0E+04

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Kd

in l

/kg

org C in %

Organic matter content

Cs

Ni

U

Se

red: values for Luvisol (Refesol 2) Soil density effects and blocking of mineral surface sites by org. matter not included

strong influence on Ni, moderate on U, no model for Se binding as yet



Dissolved organic matter

65 % of DOM is active average content of active DOM: 35.6 mg/l

moderate to strong influence on Ni and U, none on Cs, no model for Se binding as yet

1.0E+01

1.0E+02

1.0E+03

0 50 100 150 200

Kd in

l/k

g

active DOM in mg/l

Dissolved organic matter

Ni

U



CO2 pressure (organic activity)

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Kd

in l

/kg

- log CO2

CO2 - pressure

Cs

Ni

U

Se

red: values for Luvisol (Refesol 2)

formation of carbonate complexes that keep U in solution, competition effects by carbonate sorption

on Fe/Al oxides



Variation of pH

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

4 5 6 7 8

Kd

in l/

kg

pH

pH (equilibrium with Calcite) SI Gibbsite (pH >6) = 2

Cs

Ni

U

Se

pe

no precipitation reactions pe = 13.5….4.5

pH dependence of Kd(U) in batch experiments (Vandenhove et al. 2007)



Comparison of ranges

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

Cs Ni U Se

Kd

in l

/kg

Kd ranges

concentration

evaporation

dilution

clay

Feox

pCO2

pH/pe

DOM

org. C



Conclusions – sensitivity study

• in many cases the distribution of radionuclides strongly depends on soil parameters

• the variation of a single parameter may change the Kd by more than an order of magnitude

• the Kd variations can reasonably be modelled by PHREEQC

• Kd variability is important for predicting the influence of environmental conditions on radionuclide distributions in soils



Thank you!

This work was funded by the German Federal Agency for Radiation Protection (Bundesamt für Strahlenschutz)



Appelo, C.A.J. und Postma, D. 2005. Geochemistry, Groundwater and Pollution (2nd ed.). CRC Press, Boca Raton, Florida. Ashworth D.J., Moore J. und Shaw G. 2008. Effects of soil type, moisture content, redox potential and methyl bromide fumigation on Kd values of radio-selenium in soil. Journal of Environmental Radioactivity 99, pp.1136-1142. Bradbury, M.H. und Baeyens B. 2000. A generalised sorption model for the concentration dependent uptake of caesium by argillaceous rocks. Journal of Contaminant Hydrology 42, pp. 141-163. Bradbury, M.H. und Baeyens B. 2009. Sorption modelling on illite Part I: Titration measurements and the sorption of Ni, Co, Eu and Sn. Geochimica et Cosmochimica Acta 73, pp. 99-1003. Bradbury, M.H. und Baeyens B. 2009. Sorption modelling on illite. Part II: Actinide sorption and linear free energy relationships. Geochimica et Cosmochimica Acta 73, pp. 1004-1013. Dzombak, D.A. und Morel F.M.M. 1990. Surface Complexation Modeling: Hydrous Ferric Oxide. Wiley-Interscience, New York. IAEA 2010. Technical Reports Series No. 472. Handbook of parameter values for the prediction of radionuclide transfer in terrestrial and freshwater environments. – Vienna : International Atomic Energy Agency Nisbet A.F. 1995. Effectiveness of soil-based countermeasures six months and one year after contamination of five diverse soil types with caesium-134 and strontium-90. Contract Report NRPB-M546. Chilton: National Radiation Protection Board. Scheffer/Schachtschabel 2010. Lehrbuch der Bodenkunde (16. Auflage), Spektrum Akademischer Verlag Heidelberg. Tipping, E. 2002. Cation Binding by Humic Substances. Cambridge University Press, Cambridge, UK. Vandenhove H., Van Hees M., Wouters K. und Wannijn J. 2007. Can we predict uranium bioavailability based on soil parameters? Part 1: Effect of soil parameters on soil solution uraniumconcentration. Environmental Pollution 145, pp. 587-595.

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