On the variability of plant bio-concentration factors (BCF) of environmental radionuclides: a case study on the effects of surface film and free space on the interpretation of (TcO4-)-Tc-99m sorption in duckweed

The present paper addresses plant bioaccumulation factor (BCF) variability, and specifically focuses attention upon the handling of duckweed (Lemna gi bba) material, sampled from experimental media, especially considering accu mulation/kinetic studies with (TcO4-)-Tc-99m. In these short-term studies, relatively small BCF-values may be encountered, with related interferences in its assessment due to the presence of (TcO4-)-Tc-99m in the surface film medium (SF) and in the cellular water free spaces (FS). The sample handlin g methods used to remove the SF + FS component of the accumulated (TcO4-)-T c-99m, consisted of blotting, centrifugation and rinsing. The three methods were investigated using D-[1-C-14]mannitol, K-42(+), Br-82(-) and (TcO4-)- Tc-99m radioisotopes, which were measured by beta- and gamma-spectrometry, in both solution and solid samples. Centrifugation seems the most promising method to remove SF + FS (TcO4-)-Tc-99m. Results based on both mass analys is and radioactivity determinations in centrifugated fluids are independent of applied concentrations (10(-11) to 10(-3) mol m(-3) (TcO4-)-Tc-99m), an d are invariably compatible with the conceptual idea of the FS as a free-en trance phase for solutes. Blotting results in an overestimation of BCF valu es (up to factor 3 for the (TcO4-)-Tc-99m experiments performed), probably due to the incomplete removal of the SF + FS, and is suggested to yield irr egular results, leading to high variances in BCF values obtained. The appli cation of an efflux/rinsing period is indicated to result in an underestima tion of BCF values (up to factor 10 for the (TcO4-)-Tc-99m, experiments per formed), probably due to excess removal of (non-SF + FS) components of accu mulated solutes. Here we advocate centrifugation as a routine sample handli ng method to avoid SF + FS interferences in short-term (kinetic) (TcO4-)-Tc -99m uptake studies in duckweed. Moreover, the results suggest a more gener al applicability of centrifugation as a sample handling method to avoid SF + FS interferences in short-term element accumulation studies; centrifugati on approaches should, however, be adjusted to plant cell characteristics. ( C) 2000 EIsevier Science B.V. All rights reserved.