Acid and thermal treatment of natural heulandite

Acid treatment of natural zeolites is considered to be an effective method of “improving” their structure and properties, among which thermal stability occupies a special place, especially for catalytic applications of zeolites. The influence of hydrochloric acid solutions with concentrations up to 2 mol/L and calcination at temperatures up to 1100 °C on the structure and properties of heulandite-containing tuff from the Georgian Dzegvi-Tedzami deposit, selected for the creation of new bactericidal zeolite filter materials for purification and desinfection of water from various sources, was studied by the X-ray energy dispersion spectra, diffraction patterns and thermal analysis, as well as by adsorption of water, benzene and nitrogen methods. It has been found that an acidic environment leads to significant dealumination (Si/Al molar ratio increases from 3.6 to 9.5) and decationization (total charge of metal ions per Al atom decreases from 1 to 0.68) of the sample; solutions of hydrochloric acid do not lead to amorphization of the zeolite microporous crystal structure, but gradually dissolve it. As a result of acid treatment, there is also a sharp increase in the volume of micropores available for large molecules (from ≈ 7 to 80–90 mm3/g) and surface area (from ≈ 13 to 120–175 m2/g), as well as changes in the mesoporous system, leading to the prevalence of pores with a diameter of up to 4 nm. Heulandite heating leads to stepwise dehydration proceeding up to ≈ 800 °C, amorphization starting at ≈ 250 °C, and structural changes: the transition to methastable heulandite B phase at ≈ 340 °C is not fixed, but at ≈ 500 °C wairakite (Ca(Al2Si4O12)·2H2O) is formed, at temperatures above ≈ 1000 °C, amorphous aluminosilicate contains crystalline inclusions of cristobalite (polymorph of SiO2), α-quartz, albite (Na(AlSi3O8), hematite (Fe2O3) and magnetite (FeO·Fe2O3); heating-induced changes in micro- and mesopore systems are insignificant. It is also shown that heat treatment increases the acid resistance of heulandite, which is expressed in a decrease in the degree of dealumination after acid treatment of calcined samples. Thus, acid and heat treatment of heulandite make it possible to obtain materials with different sorption and ion-exchange properties.