Role of surface hydroxyl groups of acid-treated natural zeolite on the heterogeneous catalytic ozonation of methylene blue contaminated waters

The effect of surface properties of zeolites on the kinetics of heterogeneous catalytic ozonation of organic pollutants is still under discussion. In this work, the influence of hydroxyl groups (Z-OH2+, Z-OH, Z-O−) of acid-treated natural zeolite on the catalytic ozonation of methylene blue (MB) contaminated waters is analysed. A Chilean natural zeolite was acid-treated using HCl (2.44 M). Acid-treated zeolite was chemically and physically characterised by N2 adsorption at 77 K, X-ray fluorescence, acidimetric–alkalimetric titration, and by NH3 and CO2 temperature-programmed desorption methods. Fourier transform infrared spectroscopy of pyridine adsorption was used to elucidate the nature and strength of acidic sites resulting from the acid treatment of natural zeolite. Experimental results obtained here are kinetically modelled using a set of two homogeneous and three heterogeneous surface reactions. Moreover, the quantitative effects of single ozonation, adsorption and coupled treatment on MB removal rate, together with the effect of pH and the presence of radical scavengers are analysed. Brønsted acid sites in the form of proton-donating OH groups (Z-OH) of acid-treated zeolite are claimed here to play an important role on the catalytic ozonation of MB in water, acting as active sites for the adsorption of reacting species. The higher catalytic activity is observed at pH above the pH of point of zero charge (pHPZC) and could be related to the presence of surface hydroxyl groups in the deprotonated form.

► Acid treatment enhances catalytic ozonation activity of natural zeolite.
► Acid treatment of natural zeolite decreased the pHPZC.
► Zeolite acidic nature was elucidated by IR spectroscopy of pyridine adsorption.
► Brønsted acidity increases significantly on acid-treated zeolite.
► Brønsted acid sites are claimed to be the main active centres for catalytic ozonation.