Sorption of inorganic 14C on to calcite, montmorillonite and soil

Although 14C occurs naturally, it is also a waste product of the nuclear industry, and can be important because of its long half-life, high mobility as an anion, and ready incorporation into biota. Some aqueous inorganic species are anionic with migration minimally retarded by most geological and soil materials. Substantial retardation is expected when calcite is present, but there are few data to quantify this effect. The present study measured partition coefficient values, Rd (concentration on solids divided by concentration in liquids), of 8–85 l kg−1 for a series of calcite materials and for a carbonated soil. In contrast, Rd was zero for montmorillonite. The series of calcite materials varied in particle size. In order to investigate the effects of particle size, dissolution and degassing of 14C and 12C were monitored as pH was slowly decreased. The change in pH with addition of acid was strongly affected by particle size, as expected, but there was no systematic effect of particle size on the relative dissolution rates of 14C vs 12C, or on Rd. Apparently, surface area was not a limiting factor in the interaction of 14C with these materials. The 14C in soil behaved most like the very fine calcite, indicating that the specific surface of the soil carbonate was similar to that of the very fine calcite.