Composition of Chemical Elements and Ion Exchange Complex of Acid- and Alkali-Modified Natural Zeolites From the Sokyrnytsky Deposit

Abstract

Natural zeolites are abundant and inexpensive resources. They are crystalline hydrated aluminosilicates with a framework structure that has pores and channels occupied by water, alkali, and alkaline earth cations. Having high cation exchange capacity, acting as a molecular sieve, natural zeolites have been widely utilized in recent decades as adsorbents in separation and purification processes. Modification of natural zeolite increases its adsorption capacity of environmental pollutants, in particular, radionuclides from low-level liquid radioactive waste. The article presents results obtained from a study of the chemical composition of the structural elements and ion exchange complexes of natural, acid-modified and alkali-modified zeolites from the Sokyrnytske deposit. The main rock-forming mineral of the Sokyrnytske zeolite is clinoptilolite. The zeolite was modified by a 5.5 M HCl solution for 2 hours at 100 oC using a backflow condenser. The ratio of solid to liquid phases was 1:2. For alkaline modification, 1.4 M NaOH solution was used. The exposure time was 2.75 hours. The conditions and phase ratio were similar to those in acidity modification. By composition of the ion exchange complex, natural zeolite belongs to potassium-calcium-sodium (K > Ca > Na) clinoptilolites. In the process of acidity and alkaline modification of the natural zeolite, redistribution of the exchangeable cations is observed and the content of structural cations in the clinoptilolite lattice changes. In alkali-modified zeolite, the content of exchangeable Na and Ca cations increases, and the content of K and structural Al cations decreases. In the acid-modified zeolite, the number of exchangeable Na, Mg, Ca, K cations decreases. At the same time, the content of Fe and Al decreases and the relative amount of Si in the lattice increases. The Si/Al ratio increases in the following succession: natural zeolite → alkali-modified zeolite → acid-modified zeolite. The specific surface area of the modified zeolites increases compared to the natural ones. The largest increase is observed for the acid-modified zeolite. The textural characteristics and mineral composition of the studied samples indicate that the natural, acid-modified and alkali-modified zeolites from the Sokyrnytske deposit may be used for removal of radionuclides from low-level liquid radioactive waste.