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

Abstract

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.

Keywords: Cd; NMR; barley; flow-through lysimeter; forms; high-molecular-weight dissolved organic matter; mass (volumetric) activity density; migration; parameters; specific activity.

Figure 1

¹³C MAS NMR spectra of lyophilized HMWDOM extracted from a soil solution (a) Mw ≥ 14 kDa, (b) Mw ≥ 7 kDa, (c) Mw ≥ 3.5 kDa.

Figure 2

Experimental setup for studying Cd/¹⁰⁹Cd migration parameters in the soil–lysimeter solution–plant system: (a) vegetation stand assembly; (b) lysimeter setup.

Figure 3

Successive steps of the preparation of a sample of dry HMWDOM-H from a soil solution: (a) Dialysis of the soil solution concentrate with MEMBRA-CEL dialysis bags with different pore sizes, (b) Control of the specific electrical conductivity (EC) of the sequential washings with easily soluble salts from the soil solution concentrate by dialysis; (c) Lyophilization of the HMWDOM dialysate of soil solution for subsequent analysis of its makeup by ¹³C MAS NMR; (d) Finished sample of lyophilized HMWDOM-H.

Figure 4

Determination of the cation exchange capacity (CEC) and selectivity coefficients K_s^Cd/Ca of a HMWDOM sample (3.5 kDa): (a) The general view of a dialysis tube with a “window” made of a MEMBRA-CEL membrane (3.5 kDa) and an external container; (b) The completed assembly (inside the dialysis tubes there is a suspension of 30 mg of HMWDOM (3.5 kDa) + 10 cm³ of the equilibrating solution).