Estimation of the Influence of Humic Acid on Radionuclide Binding to Solid Residuals in Deposits of Radioactive Waste

Abstract

In contaminated water reservoirs, the sorption and binding of radionuclides to solids (SR) determines their bioavailability and transport and thus human and ecosystem exposure. In this work, the influence of organic matter (OM) on binding of the radionuclides 90Sr, 137Cs, sum of 235U, 238U, and sum of 239Pu, 240Pu to solids are investigated, using experimental data derived from ecological monitoring of radioactive waste deposits in South Ural (Russia). OM in several surface water reservoirs mainly consists of humic substance (HS) which forms humates and fulvates with radionuclides and binds to solids via different mechanisms, such as coordinating bond or covalent bond. These processes are strongly dependent on the phase of HS, which can be colloidal or soluble high-molecular compounds. Based on the spatial distribution of radionuclides, SR and OM in waste deposits, we assumed a specific influence of humic acids (HA) on the binding of radionuclides to SR, and quantified it with invariant values of a modified partitioning coefficient. The mathematical form of this invariant value emphasizes a significant impact of the local mass of HA (mHA/V) and local surface area of SR (s = Ssorb/V) per volume V on the processes involved in binding radionuclides to SR. These processes may retard radionuclide migration into groundwater.