Effect of moisture content on emanation at different grain size fractions – A pilot study on granitic esker sand sample

It is known that in soils and sediments moisture adsorbed on particle surfaces and in the pore system significantly affects the behaviour of recoiling radon (222Rn) atoms after decay of parent 226Ra, leading to increased 222Rn emanation. As a first step in an effort to characterize the 222Rn source term in mineralised sediments in the present study, complementing previous studies in the area, granitic esker sand samples were collected in order to test how moisture content affects 222Rn emanation at different grain size fractions. Emanation fractions measured for natural samples were compared with theoretical calculations. Six different grain size fractions were studied at 0%, 5% and 10% moisture contents relative to the mass of solids. In a further study necessary complementary information on the chemical and structural distribution of 226Ra was gained by selective leaching experiments. The results showed that 226Ra concentration increases from 50 Bq/kg at grain size 1–2 mm to 200 Bq/kg at grain size <0.063 mm. Respectively, the emanation factor increased from 0.12 to 0.30 at 5% moisture content. Both emanation factor and radium concentration increased significantly when grain size was below 0.125–0.250 mm. Above this fraction, the emanation fraction was approximately constant, 0.13 at 5% moisture content. In most of the grain size fractions, emanation reaches its maximum at 5% moisture content, being twice as high as in a dry sample. For the small particles (<0.063 mm) the 226Ra distribution is rather complex and depends on the mineral composition compared to larger particles wherein emanation from the internal pore system and the adjacent matrix is dominating over the contribution from external surface.

Research highlights
► The results showed that 226Ra concentration increased from 50 Bq/kg at grain size 1–2 mm to 200 Bq/kg at grain size <0.063 mm. Respectively, the emanation factor increased from 0.12 to 0.30 at 5% moisture content.
► Both emanation factor and Ra concentration increased significantly when grain size was below 0.1–0.2 mm. Above this fraction the emanation fraction was approximately constant, 0.13 at 5% moisture content.
► In most of the grain size fractions, emanation reaches their maxima at 5% moisture content and it is twice as high at 5% moisture content as in a dry sample. For larger particles, emanation from the internal pore system and the adjacent matrix is dominating over the contribution from external surface.
► For the small particles the contribution is rather complex and depends on the mineral composition.
► These results are not in contradiction with the theoretical estimations proposed by Morawska and Phillips (1993).