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 99mTcO 4− sorption in duckweed

Abstract

The present paper addresses plant bioaccumulation factor (BCF) variability, and specifically focuses attention upon the handling of duckweed ( Lemna gibba) material, sampled from experimental media, especially considering accumulation/kinetic studies with 99mTcO 4 −. In these short-term studies, relatively small BCF-values may be encountered, with related interferences in its assessment due to the presence of 99mTcO 4 − in the surface film medium (SF) and in the cellular water free spaces (FS). The sample handling methods used to remove the SF+FS component of the accumulated 99mTcO 4 − consisted of blotting, centrifugation and rinsing. The three methods were investigated using d-[1- 14C]mannitol, 42K +, 82Br − and 99mTcO 4 − radioisotopes, which were measured by - and γ-spectrometry, in both solution and solid samples. Centrifugation seems the most promising method to remove SF+FS 99mTcO 4 −. Results based on both mass analysis and radioactivity determinations in centrifugated fluids are independent of applied concentrations (10 −11 to 10 −3 mol m −3 99mTcO 4 −), and are invariably compatible with the conceptual idea of the FS as a free-entrance phase for solutes. Blotting results in an overestimation of BCF values (up to factor 3 for the 99mTcO 4 − experiments performed), probably due to the incomplete removal of the SF+FS, and is suggested to yield irregular results, leading to high variances in BCF values obtained. The application of an efflux/rinsing period is indicated to result in an underestimation of BCF values (up to factor 10 for the 99mTcO 4 − experiments performed), probably due to excess removal of (non-SF+FS) components of accumulated solutes. Here we advocate centrifugation as a routine sample handling method to avoid SF+FS interferences in short-term (kinetic) 99mTcO 4 − uptake studies in duckweed. Moreover, the results suggest a more general applicability of centrifugation as a sample handling method to avoid SF+FS interferences in short-term element accumulation studies; centrifugation approaches should, however, be adjusted to plant cell characteristics.