BTX abatement using Chilean natural zeolite: the role of Brønsted acid sites.

In wastewater treatment facilities, air quality is not only affected by conventional unpleasant odour compounds; toxic volatile organic compounds (VOCs) are also found. In this study, the adsorptive capacity of Chilean natural zeolite toward VOC removal was evaluated. Moreover, the influence of zeolite chemical surface properties on VOC elimination was also investigated. Three modified zeolite samples were prepared from a natural Chilean zeolite (53% clinoptilolite, 40% mordenite and 7% quartz). Natural and modified zeolite samples were characterised by nitrogen adsorption at 77 K, elemental analyses and X-ray fluorescence (XRF). Chemical modifications of natural zeolite showed the important role of Brønsted acid sites on the abatement of VOCs. The presence of humidity has a negative effect on zeolite adsorption capacity. Natural zeolites could be an interesting option for benzene, toluene and xylene vapour emission abatement.

Oxidation of nitrobenzene by ozone in the presence of faujasite zeolite in a continuous flow gas-liquid-solid reactor.

This work investigates the oxidation of nitrobenzene (NB) by ozone in the presence of faujasite zeolite. Experiments were carried out in a gas-liquid-solid reactor were ozone transfer and NB oxidation took place at the same time. Three configurations of the reactor were compared: empty, filled with inert glass beads and filled with faujasite pellets. First, ozone transfer coefficient (k(L)a) and decomposition rate constant (k(C)) were determined for each configuration. In presence of solid, k(L)a was 2.0 to 2.6 times higher and k(C) was 5.0 to 6.4 times higher compared to the empty reactor. Then, the various configurations were evaluated in terms of NB removal and chemical oxygen demand (COD) decrease. The faujasite reactor showed higher removal of NB and decrease of COD compared to other configurations under the same conditions suggesting that the faujasite increases the oxidation rate of NB. Oxidation of NB in presence of faujasite also proved to be limited by the transfer of ozone from the gas to the liquid phase.

Enhanced bio-recalcitrant organics removal by combined adsorption and ozonation.

Removal of bio-recalcitrant and toxic compounds from wastewaters has been a major objective of industrial manufacturers for a few years. Due to the potential risk toward public health, regulations are becoming increasingly strict and classical treatments like biological treatments are not efficient. Other techniques such as incineration, oxidation or adsorption provide higher levels of removal but with a high energy and capital cost. A coupled process involving adsorption and oxidation is studied. Four adsorbents are tested and compared according to two objectives, their adsorption capacity and their capability to decompose ozone into powerful hydroxyl radicals. Two model compounds were chosen: 2,4-dichlorophenol and nitrobenzene. Experimental results allow comparing coupled process with results obtained during ozonation alone. Zeolite (Faujasite Y) gave disappointing results in term of both adsorption kinetics and ozone decomposition. On the contrary, activated carbons showed fast adsorptions and important capabilites to decompose ozone into radicals, almost in nitrobenzene experiments. S-23 activated carbon proved to be the most interesting adsorbent for better mechanical and chemical stabilities over time. Sequential adsorption/ozonation experiments were conducted, showing a strong loss of adsorption efficiency after the first operation, but the positive point is that the adsorption capacity remains almost constant during further cycles.

Transition metals-incorporated zeolites as environmental catalysts for indoor air ozone decomposition.

The present study aimed to prepare catalysts of Fe- and Cu-loaded zeolite via ion-exchange technique using dilute solutions of metal nitrate precursors followed by calcination at 600°C in the air for 4 h. Commercial zeolite ZSM-5 with specific surface area of 400 m2/g and diameter particle of 1.2-2 mm was used as a parent support. The prepared catalysts were characterized by Fourier transform infrared spectroscopy analysis. The IR absorbed bands of Cu-ZSM-5 and Fe-ZSM-5 revealed a shift in the frequency and a reduction in the intensity framework. This indicates that both catalysts have a significant change in the number of the zeolite structure bonds. The catalytic activity of the prepared materials compared to the parent zeolite was evaluated for the catalytic ozone decomposition. The ozone stream of the initial concentration (13 g/m3) with air flow rate (Q) of 0.18 m3/h was passed through a glass jacket column reactor filled with a fixed bed of 40 g zeolites. It was showed that the ozone removal efficiency by Cu-ZSM-5 and Fe-ZSM-5 was obviously higher than that found with the parent ZSM-5. In terms of O3 removal efficiency, zeolite samples could be ranked as follows: Fe-ZSM-5 > Cu-ZSM-5> parent ZSM-5. The results revealed about 90% O3 removal efficiency for Fe-ZSM-5 and 70% for Cu-ZSM-5 as compared to nearly 40% for the parent zeolite. Consequently, the incorporation of Fe and Cu metals onto the zeolite surface plays a key role for enhancing the gaseous ozone elimination.

Mass transfer in VOC adsorption on zeolite: experimental and theoretical breakthrough curves.

From experimental results of adsorption of volatile organic compounds (VOCs) on zeolite, we propose simulations of the breakthrough curves based on the Linear Driving Force model. Experiments were run on fixed beds of hydrophobic commercial zeolites. Pollutants chosen are from several chemical classes with different polarities. A good agreement between experimental and numerical results is found when an adjustable value of the internal mass-transfer coefficient is used. A constant value of effective diffusivity is found independent of the nature and the amount of VOCs adsorbed. A relation linking intrapellet mass-transfer coefficient and equilibrium constant is proposed, including the average effective diffusivity, to make predictions of breakthrough curves for any kind of volatile organic pollutant in gaseous effluents.