Photocatalytic activity of clinoptilolite rich zeolitic tuffs from different regions

Abstract

Due to their unique structural properties, low cost and environmental compatibility, natural zeolites have been recognized as effective supports for photocatalytic particles such as transition metal oxides (e.g. TiO2, NiO, ZnO, CuO, SnO2, Fe2O3) in the catalytic degradation of complex organic dyes present in water media [1, 2]. The lattice of zeolite prevents the aggregation of oxide species but also contributes to the electron-hole recombination. It has been noticed that pure clinoptilolite (CLI) exhibits a photocatalytic activity in the degradation of rhodamine B. This is explained by the presence of Ti and/or Fe species, which are usually impurities in zeolitic tuffs [3]. The present study focuses on CLI-rich natural zeolitic tuffs and their photocatalytic activity in the degradation of cationic organic dyes such as methylene blue (MB). CLI from different regions and deposists was tested: a) Serbia – Slanci (SCLI), b) Turkey – Gördes (TCLI), c) Iran – Semnam (ICLI), d) Romania – Baia Mare (RCLI) and e) Slovakia – Nižný Hrabovec (SKCLI). Samples were sieved to obtain the particles with the size in the range 63 and 125 μm mesh, washed with deionized water, dried at 105 °C overnight and tested wi thout any additional treatments. Photodegradation tests were performed using a batch reactor system equipped with a 50 cm3 Pyrex glass cell and by a circulating water jacket to keep the constant temperature at 25 °C. All tests were done at atmospheric pressure. Suspension contained water solution of MB (C0= 10 mg dm–3) and photocatalyst in the concentration of 0.2 g dm–3 at different pH (pH= 3, 6, or 9), was continuously stirred by magnetic stirrer for 30 min to achieved an adsorption/desorption equilibrium and then irradiated for 300 min. Osram Ultra Vitalux lamp (300 W) served as visible light source, positioned 10 cm above the photocatalytic reactor. The concentration of MB was measured colorimetrically at λ = 664 nm by using an UV/VIS spectrometer. The obtained results showed that the overall degradation of MB by the photocatalytic process can be attributed to the joint effect of initial adsorption and degradation under visible light irradiation. The adsorption amounts indicated that CLI has two roles: adsorption of MB during the dark phase and photocatalytical. The adsorption increased with pH. The photocatalytic degradation is also affected by pH showing the best activity for all studied zeolitic tuffs at pH= 6. The rate of MB degradation varied from about 36-47% while the total degradation was 70-91%. The photocatalytic activity was attributed to the presence of Fe species showing a linear increase with the Fe content in the tuffs. The MB photodegradation follows the Langmuir–Hinshelwood kinetic model. The recyclability tests showed that the degradation rate decreased from 91 to 69% during three reaction cycles, demonstrating a promising potential of CLI in the treatment of textile industry wastewaters.