BIOAVAILABILITY OF TC-99 AS AFFECTED BY PLANT-SPECIES AND GROWTH, APPLICATION FORM, AND SOIL INCUBATION

Bioavailability of Tc-99 to plants is believed to decrease with time i n aerobic environments. This study was carried out to follow the fate of Tc-99 in the soil-plant systems according to the form of applied Tc -99 and the time of incubation in the soil. Two series of experiments were conducted: (i) (NH4TcO4-)-Tc-99 was applied to two inceptisols (A and B) at the levels of 0, 2.5, 25, and 250 kBq kg(-1) dry soil, and ryegrass (Lolium perenne L.) (A and B) and winter wheat (Triticum aest ivum L.) (A) were grown; and (ii) two forms of Tc-99 ((NH4TcO4)-Tc-99 and Tc-99 bio-incorporated in wheat leaves) were added to Soil A. Then ryegrass was grown on this soil incubated for 0, 1, 3, and 6 mo. Plan ts were harvested at maturity for wheat and monthly for ryegrass. Resu lts showed that ryegrass shoots accumulated 62 to 78% of Tc-99 supplie d as (TcO4-)-Tc-99. The fraction of soil Tc-99 taken up (y) fitted an exponential model of biomass production (x):y = a + e(cx), showing tha t bioavailability decreased with biomass production and not with time. In wheat, 92 to 95% was accumulated in the leaves and less than 1.1% in the grain. Technetium-99 bioincorporated in wheat leaves was highly extractable by water (73%), and as available to ryegrass as (TcO4-)-T c-99, suggesting that (TcO4-)-Tc-99 was the predominant form in leaves . Total uptake of Tc-99 by ryegrass was not affected by incubation, sh owing that mobility of Tc-99 was not changed by aerobic microbial acti vity.