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Wpływ modyfikacji zeolitów na ich właściwości fizykochemiczne

Świderska-Dąbrowska R., Schmidt R.

Rocznik Ochrona Środowiska

2012

Tom 14

460--470

Zeolites are crystalline, hydrated aluminosilicates of alkali and alkaline earth metals (Na, K, Mg, Ca, Sr, Ba), in which the pore size is 0.3-1.2 nm and depends on the structure of the zeolite. They consist of tetrahedrons of [AlO4]-5- and [SiO4]4-, which make ordered crystal lattice of rings, channels and chambers. Variable degree of oxidation of silicon (IV) and aluminum (III) causes their electronegative character. The size of the charge may determine the adsorption, ion exchange and catalytic capacities of zeolite. The structure made of ring tetrahedron units contains open tunnels, making zeolites also molecular sieves. Zeolites are mostly used for adsorption of ammonium ions from aqueous solutions, removal of radioactive elements and heavy metals, and recent studies suggest the possibility of application of zeolites for removal of chloroorganic compounds and oily pollutants. The most widely used zeolite in practice for water and wastewater treatment is clinoptilolite, which is characterized by large ion exchange and adsorption capacity. Authors of many works explain the adsorption process with ion exchange. But when interpreting the results the authors do not consider that the zeolites have also tunnels that make them also molecular sieves. As a result of ion adsorption on clinoptilolite in hydrogen or sodium form, pH of the solution decreases, resulting in possible formation of electropositive, electronegative complexes and dissociated ions which make up aquaions, increasing or decreasing the effectiveness of their removal. During usage of modified zeolites for the adsorption of ions from aqueous solutions, many accompanying processes take place, such as precipitation, coprecipitation and catalytic adsorption. The influence of the physicochemical properties of natural zeolite and zeolite in the hydrogen form on an aqueous solution in the process of washing out is presented in the paper. Presented results clear prove that the decisive factor affecting the quality of the aqueous solution after the washing out process is initial pH of the solution. Depending on method of modification, physicochemical properties of zeolites can vary considerably. Transformation of natural zeolite into the hydrogen form increases its acidity and physical and chemical resistance. Modification of zeolite with HCl may also increase the content of pure zeolite by dissolution of other, non-zeolite materials. Zeolite H has a greater content of of silicon and aluminum compounds in relation to total mass which is a result of introduction of protons into the zeolite. This is confirmed by lower content of alkali metals and alkaline earth metals (sodium, calcium and potassium) in ZH. Exchange of ions of sodium, potassium, calcium for protons is a result of modification of ZN with HCl. Examination of washing out process showed that there is a small migration of impurities (Fe and Ca) from ZH as compared to ZN. At the same time increase of iron content in the solution increases its turbidity. And also turbidity depends on the initial pH of the solution. It's highest value was observed when the initial pH was equal to 6. Significant increase of turbidity as a function of pH is mainly caused by abrasion of zeolite grains in the process of shaking and by formation of precipitable complexes of iron and calcium. The paper presents also the results of zeta potential measurements. potential of H zeolite in the investigated range of pH has a much higher absolute values compared to ZN and vary in the range from -20 mV to -30 mV. H zeolite has a higher electrostatic activity than natural zeolite. Potential creating ions such as H+ and OH- have decisive influence on the zeolite surface charge.