A Study on the Behavior of Cadmium in the Soil Solution–Plant System by the Lysimeter Method Using the 109Cd Radioactive Tracer

In soils, cadmium (Cd) and its compounds, originating from industrial activities, differ both in mobility as well as in their ability to permeate the soil solution from naturally occurring cadmium compounds (native Cd). Therefore, the determination of the parameters of cadmium mobility in soils and its accumulation by plants in the soil-soil solution-plant system is very important from both scientific and practical viewpoints. 109Cd was used as a radioactive tracer to study the processes of the transition of Cd into the aqueous phase and its uptake by plants over the course of a vegetative lysimeter experiment. Using sequential extraction according to the Tessier-Förstner procedure and modified BCR schemes, certain patterns were determined in the distribution of Cd/109Cd among their forms in various compounds in the soil, along with the coefficients of the enrichment of native stable Cd with radioactive 109Cd. It was shown that the labile pool of stable Cd compounds (29%) was significantly smaller than that of radioactive 109Cd (69%). The key parameters characterizing the migration capacity of Cd in the soil-soil solution-plant system were determined. It was found that the distribution coefficient of native Cd between the soil and the quasi-equilibrium lysimeter solution exceeded the similar value for the 109Cd radionuclide by 2.2 times, and the concentration coefficients of Cd and 109Cd in the barley roots were 9 times higher than in its vegetative parts. During the experiment, the average removal of Cd (109Cd) from the soil by each barley plant was insignificant: 0.002 (0.004)%. Based on the results of 13C nuclear magnetic resonance (NMR) spectroscopy of a lyophilized sample of the high-molecular-weight dissolved organic matter (HMWDOM) of the soil solution, its components were determined. It transpired that the isolated lyophilized samples of HMWDOM with different molecular weights had an identical structural and functional composition. The selective sorption parameters of the HMWDOM and humic acid (HA) with respect to Cd2+ ions were determined by the isotope dilution method.