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RADIUM DISTRIBUTION IN SOILS, ANALYSED WITH SEQUENTIAL EXTRACTION, AND ITS EFFECT ON RADON EMANATION

Cecilia Edsfeldt
Royal Institute of Technology, Division of Engineering Geology, SE-100 44 Stockholm, Sweden.
Phone: +46 8 790 68 07; Fax: +46 8 790 68 10; e-mail: ceciliae@ce.kth.se

The radium distribution of the soil is one important parameter governing radon emanation. The
present study’s main objective is to investigate radium distribution in different Swedish soils, using
chemical selective sequential extraction, and to compare the outcome with the radon emanation of the
sampled soil types. Investigated samples were unconsolidated sediments (till, sand and clay) from the
Stockholm area. In the developed selective sequential extraction procedure, ammonium chloride and
hydrochloric acid were used to extract an exchangeable and an oxide-bound fraction from grain-size
separated samples. The soil residues were digested after extractions. Radium-226, uranium, iron,
manganese, calcium and barium were analysed from the extractant solutions and the digested samples.
Radon emanation was measured on unleached samples. In pedogenic soil phases concentrations of all
elements, as well as Rn emanation, increased with decreasing grain size and increasing specific
surface area. Radium was very well correlated with Ca and Ba, implicating an association between
these elements in the sampled soils. A major portion of the Ra was bound on the surfaces of grains.
Radium in oxide bound phase was the primary source of emanating radon, although exchangeable
radium was also important. Radium in the oxide bound was very well correlated with iron, which
indicates that this Ra was adsorbed to or co-precipitated with iron oxides. However, since iron in soils
can be well correlated with organic matter, it can not be ruled out that this Ra was partly associated
with organic matter that was leached in the same extraction. Radium bound in the crystal lattice of
minerals did not seem to contribute to the radon emanation. The sequential extraction method for
characterising radium distribution is promising. With the sequential extraction method, it is possible to
identify in which soil phase radium resides, and to gain information about the complicated soil system
that controls the emanation of radon.
Keywords: 222 Rn, 226 Ra, radium, radium distribution, radon emanation, radon risk, radon, sequential
extraction, soil, soil geochemistry