Héctor Valdés

Catalytic ozone aqueous decomposition promoted by natural zeolite and volcanic sand.

Chemical oxidation using ozone in the presence of appropriate catalysts offers an attractive option for removing poorly biodegradable pollutants. However, the effect of such catalysts on ozone aqueous decomposition is not well documented. This study compares experimental results on ozone aqueous decomposition promoted by zeolite and volcanic sand. These are two low costs natural materials identified as suitable catalysts for heterogeneous ozonation. The effects of pH (2-8) and free radical scavengers (acetic acid) on the dissolved ozone decay rate are evaluated at laboratory scale. Results show that the ozone decay rate increases in the presence of natural materials tested here. Under alkaline conditions, the effect of free radical scavengers on ozone self-decomposition is significantly reduced when such materials were present, particularly in the case of volcanic sand. Results suggest that ozone aqueous decomposition in the presence of zeolite and volcanic sand take place mainly on the natural material surface. The presence of metal oxides in zeolite and volcanic sand might catalyse ozone decomposition into more active radicals, thus enhancing the oxidation of organic pollutant.

Natural zeolite reactivity towards ozone: the role of compensating cations.

Among indoor pollutants, ozone is recognised to pose a threat to human health. Recently, low cost natural zeolites have been applied as alternative materials for ozone abatement. In this work, the effect of compensating cation content of natural zeolite on ozone removal is studied. A Chilean natural zeolite is used here as starting material. The amount of compensating cations in the zeolite framework was modified by ion exchange using an ammonium sulphate solution (0.1 mol L(-1)). Characterisation of natural and modified zeolites were performed by X-ray powder diffraction (XRD), nitrogen adsorption at 77K, elemental analysis, X-ray fluorescence (XRF), thermogravimetric analysis coupled with mass spectroscopy (TGA-MS), and temperature-programmed desorption of ammonia (NH(3)-TPD). Ozone adsorption and/or decomposition on natural and modified zeolites were studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Results show that the zeolite compensating cation content affects ozone interaction with zeolite active sites. Ammonium ion-exchange treatments followed by thermal out-gassing at 823 K, reduces ozone diffusion resistance inside the zeolite framework, increasing ozone abatement on zeolite surface active sites. Weak and strong Lewis acid sites of zeolite surface are identified here as the main active sites responsible of ozone removal.

Application of high silica zeolite ZSM-5 in a hybrid treatment process based on sequential adsorption and ozonation for VOCs elimination

In this study, a hydrophobic synthetic zeolite, namely ZSM-5 is chosen as an adsorbent/catalyst for toluene removal. Experimental results showed that toluene adsorption onto ZSM-5 was favourable, following a Langmuir adsorption isotherm model. ZSM-5 zeolite was regenerated using gaseous ozone at low temperature. Adsorbed toluene was oxidised, releasing mainly CO2 and H2O. Traces of oxidation by-products such as acetic acid and acetaldehyde were formed and remained adsorbed after the oxidativate regeneration with ozone. After four successive cycles of adsorption/ozonation, the adsorption efficiency was not affected (92%-99%). These results showed that volatile organic compound (VOC) removal by adsorption onto ZSM-5 zeolite followed by ozone regeneration could be used as a promising hybrid process for the control of VOC emissions in terms of efficiency.

Oxidative regeneration of toluene-saturated natural zeolite by gaseous ozone: The influence of zeolite chemical surface characteristics

In this study, the effect of zeolite chemical surface characteristics on the oxidative regeneration of toluene saturated-zeolite samples is investigated. A Chilean natural zeolite (53% clinoptilolite, 40% mordenite and 7% quartz) was chemically modified by acid treatment with hydrochloric acid and by ion-exchange with ammonium sulphate. Thermal pre-treatments at 623 and 823K were applied and six zeolite samples with different chemical surface characteristics were generated. Chemical modification of natural zeolite followed by thermal out-gassing allows distinguishing the role of acidic surface sites on the regeneration of exhausted zeolites. An increase in Brønsted acid sites on zeolite surface is observed as a result of ammonium-exchange treatment followed by thermal treatment at 623K, thus increasing the adsorption capacity toward toluene. High ozone consumption could be associated to a high content of Lewis acid sites, since these could decompose ozone into atomic active oxygen species. Then, surface oxidation reactions could take part among adsorbed toluene at Brønsted acid sites and surface atomic oxygen species, reducing the amount of adsorbed toluene after the regenerative oxidation with ozone. Experimental results show that the presence of adsorbed oxidation by-products has a negative impact on the recovery of zeolite adsorption capacity.

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.

Natural Zeolite Reactivity Towards Ozone: The Role of Acid Surface Sites

Abstract Ground-level ozone is an air pollutant which interferes with plant photosynthesis, stunts overall growth of some plant species, and harms animal respiratory systems. Activated carbon, synthetic zeolites, metal oxides and noble metals have been used as catalysts to destroy ozone. However, the high costs of these materials impair their application at full-scale. Low cost natural zeolites could be an interesting option for ozone elimination. In this work, the influence of Chilean natural zeolite chemical surface properties on gaseous ozone removal was evaluated. Chemical modifications of Chilean natural zeolite showed the role of acid surface sites and the Si/Al ratio on gaseous ozone abatement. Samples were characterised by nitrogen adsorption at 77 K, elemental analyses, X-ray fluorescence (XRF), temperature-programmed desorption (TPD), and hygroscopic analyses. The influence of air humidity on gaseous ozone removal using natural and modified zeolite samples was also evaluated. The results showed that lower Si/Al ratio corresponds to a higher density of Lewis acid sites. It was found that increasing the zeolite surface area does not improve gaseous ozone removal. The conversion of Brønsted acid sites into Lewis acid sites at 823 K led to an increase on ozone removal.

Methylene blue removal from contaminated waters using O3, natural zeolite, and O3/zeolite

This paper compares experimental results on methylene blue (MB) removal systems based on ozone oxidation, zeolite adsorption, and simultaneous adsorption-oxidation using ozone in the presence of natural zeolite. The effect of pH (2-8), and the presence of radical scavengers (sodium acetate) on process rates and removal efficiencies are assessed at laboratory scale. The experimental system consisted of a 1 L differential circular flow reactor and an ozone generator rated at 5 g O3/h. Results show that ozone oxidation combined with zeolite adsorption increases the overall MB oxidation rate with respect to ozonation process and zeolite adsorption. In presence of free radical scavenger, only a 25% of reduction on MB removal rate are observed in the simultaneous treatment, as compared with 70% when ozonation treatment is used, suggesting that MB oxidation reactions take mainly place on the zeolite surface.

Methylene blue removal from contaminated waters using heterogeneous catalytic ozonation promoted by natural zeolite: mechanism and kinetic approach

This paper presents experimental results of a methylene blue removal system based on heterogeneous catalytic ozonation in the presence of natural zeolite. The effect of pH (2–10) and the presence of radical scavengers (acetate ions) on homogeneous and heterogeneous ozonation systems are assessed at laboratory scale. Results show that heterogeneous catalytic ozonation using natural zeolite increases methylene blue removal rate with respect to the single ozonation process at all pH values. The inhibitory effect of free-radical scavengers is impaired by zeolite, suggesting that methylene blue oxidation reactions take place mainly on the zeolite surface. The increase in methylene blue removal rate could be related to ozone interaction with hydroxyl groups present on the zeolite surface ( , S‒OH, S‒O−), which plays a key role in the reaction mechanism.

Influence of Chemical Surface Characteristics of Natural Zeolite on Catalytic Ozone Abatement

In this article, the effect of zeolite chemical surface properties on gaseous ozone removal was investigated. Zeolite samples with different surface chemical composition were prepared from a Chilean natural zeolite. Such treatments included acid de-alumination, and thermal treatment at 400 °C and 550 °C prior to contact with ozone. Chemical modification of natural zeolite showed that the generation of new Lewis acid sites on zeolite surface increases ozone removal. It is concluded that Lewis acid sites present on zeolite surface play a key role enhancing the gaseous ozone elimination.

Effect of Zeolite Chemical Surface Properties on Catalytic Ozonation of Methylene Blue Contaminated Waters

Heterogeneous catalytic ozonation using natural zeolite has been recently reported. However, there is a lack in the information related to the influence of zeolite active surface sites in this combined system. This work presents experimental results on the effect of zeolite chemical surface properties on catalytic ozonation. Zeolite samples with different chemical surface compositions were prepared from natural zeolite. The effect of pH, and the presence of radical scavengers were assessed at laboratory scale. Results obtained here indicate that hydrous oxide sites present on zeolite surface (S–OH2 +, S–OH, S–O−) play a key role on the catalytic ozonation mechanism.