Geo&Bio (Dec 2019)
Influence of palygorskite structure on the adsorption of tritium from aqueous solutions
Abstract
The aim of the work is to determine the possibility of using the palygorskite adsorbent in removing tritium from protium-tritium water (HTO). The article is based on results of experiments using palygorskite from the Cherkassy deposit (Ukraine) and tritiated water ("HTO"). The performed studies evaluated the adsorption of tritium from an aqueous solution and the effect of hydrogen isotope fractionation in a stationary closed system "HTO-palygorskite". The mechanisms of delaying the super heavy hydrogen isotope in the structure of a natural clay mineral are explained. The ribbon-channel structure of palygorskite provides the possibility of accumulating tritium in the raw mineral up to 3.3 Bq × g-1 (or 9.19 × 10‑15 g / g). It is established that the accumulation of tritium is a multistage process. It is determined that the adsorption capacity of palygorskite is due to uncompensated charges on the surface of mineral particles, the presence of zeolite and coordinately bound water in the channels, where molecular HTO → H2O exchange occurs between the mineral and tritiumated water, and also by ionic OT → OH substitution in the octahedral layers of the mineral. During surface adsorption processes, due to their dynamic nature, the magnitude of adsorption, that is, the delay of molecules by the surface is a kinetic factor that determines the ratio of adsorbed and desorbed molecules. This creates conditions for the possible fractionation of molecules with different molecular masses (H2O and HTO), with the predominant retention on the adsorption surface of mineral particles of more inertial NTO molecules. As a result, up to 59 % of tritium absorbed by the mineral accumulates in the surface adsorbed form in the original palygorskite. The distribution of tritium is determined in four structural positions: in a surface-adsorbed, coordinately bound, zeolite, and strongly bound forms. The predominance of a strongly bound form (22 % of the total tritium content in the mineral) over channel water was revealed. The rationality of using palygorskite for the accumulation of tritium from aqueous solutions was determined.
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