Abstract :
A quantitative and sensitive biosensing method has been developed for the determination of γ-ray emitting radionuclides in surface water. The method is based on the concept that at equilibrium the specific radioactivity in the biosensor is equal to the specific radioactivity in water. The method consists of the measurement of both the radionuclide and the element in the biosensor and the determination of the element level in water. This three-way analysis eliminates problems such as unpredictable biosensor behaviour, effects of water elemental composition or further abiotic parameters: what remains is the generally high enrichment (bioaccumulation factor BCF) of elements and radionuclides in the biosensor material.
Measurements were performed with floating water plants (Azolla filiculoides Lamk., Spirodela polyrhiza/Lemna sp.) and the fully submerged water plant Ceratophyllum demersum L., which were sampled from ditch water. Concentrations of elements and radionuclides were determined in both water and biosensor plants, using Neutron Activation Analysis (NAA), ICP-MS, and γ-ray spectrometry, respectively. For the latter, both 1 litre samples (Marinelli-geometry) and 1 cm3 samples (well-type detectors) were applied in measurements.
The analysis comprised the determination of BCF-values, and the measurement and calculation of activity concentrations and/or sensitivities for 7Be, 40K, 51Cr. 54Mn, 59Fe, 60Co, 65Zn, 75Se, 99Mo, 125Sb, 129I, 137Cs, 140La, 144Ce, 210–212,214Pb, 212,214Bi, 28Tl, 227,228,230,231,234Th and 228Ac in plants and in water. Based on comparisons between the results of the direct water analysis and the biosensing method, Ceratophyllum may be regarded as fully applicable as a biosensor plant; the results for Azolla and Spirodela should be interpreted with care when dealing with radionuclides for which substantial contributions from atmospheric deposition might be suspected (7Be, 210Pb).
The method is shown to be three to five orders of magnitude more sensitive than the direct analysis of water. The results show that the biosensing method, under the 24 h measurement conditions applied, permitted analyses generally below mBq/l concentrations, and showed sensitivities for radionuclides like 54Mn, 59Fe, 75Se, 99Mo, 140La, 144Ce, and 212,214Pb, 214Bi and 208Tl down to μBq/l levels.
