The effect of different treatment processes on estimates of radionuclide distribution coefficients in freshwater sediments

Concentrations of several major ions in pore waters, extracted by centrifugation after manipulation in air, were significantly lower than those obtained after similar treatment under a nitrogen atmosphere. These differences are probably due to the oxidation of ferrous sulphide in air, which increases SO4 in solution. The corresponding reduction in Fe2+ reduces competition for sorption sites on the solids so that more major cations are sorbed. Even more determinands (major ions and nutrients) showed significantly higher concentrations in samples frozen under air compared to unfrozen samples manipulated in air. In frozen samples, cell rupture probably released additional CO2 into solution which resulted in higher Ca and alkalinity concentrations. The higher Ca concentrations increased competition for sorption sites with consequent increases in other cations in solution. Frozen and unfrozen samples manipulated under nitrogen showed no significant differences in major ion chemistry due to the large variability between samples. The latter probably results from a variable re-equilibration of CO2 with CO2-free nitrogen compared with air containing CO2. In the samples studied here, kdSr in frozen samples is likely to be a factor of 2 lower than in unfrozen samples. For Cs, kdCs could be up to an order of magnitude lower in frozen samples, due to release of ammonia. Hence separation of pore water by freeze-thawing is not a reliable method for in situ kd estimation.