Research Outputs
Removal of benzothiazole from contaminated waters by ozonation: The role of direct and indirect ozone reactions
Benzothiazoles are emerging chemical pollutants mainly coming from leather, paper and rubber industries; due to their use as: herbicides, corrosion inhibitors, anti-freezers, and vulcanisation accelerators. This article presents experimental data on ozone treatment of benzothiazole contaminated waters. The effect of the initial concentration of benzothiazole, ozone dosage, temperature (10-30 °C), and pH (2-9), on ozonation removal rate were assessed at bench scale. Experimental results show that reaction between ozone and benzothiazole could be approximated to a second-order kinetic law. Kinetic parameters for direct and indirect ozone reactions are estimated and temperature dependence of rate parameters is evaluated. Moreover, an initial degradation pathway of benzothiazole ozonation is proposed.
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.
Gaseous ozone decomposition over high silica zeolitic frameworks
For several decades, it has been known that ozone emissions are harmful to humans, plants, and animals. Heterogeneous catalytic decomposition is an efficient process for removing ozone from air. This study examines the effect of the zeolite's framework and pore width on efficiency for decomposing gaseous ozone. Four highly hydrophobic zeolites are used: a large cavity zeolite (Faujasite/H‐FAU), a medium pore zeolite with parallel channel (Mordenite/H‐MOR), and two medium pore zeolites with interconnected channels (H‐ZSM‐5/H‐MFI and Na‐ZSM‐5/Na‐MFI). Experiments were conducted in fixed‐bed flow reactors loaded with zeolite at ambient conditions (20 °C and 101 kPa). Zeolite surfaces were analyzed during the experiments in order to understand the influence of physical and chemical surface properties on the ozone decomposition mechanism. A higher amount of ozone is eliminated using H‐MOR, compared with the zeolite samples H‐FAU, H‐MFI, and Na‐MFI. Pore width and micropore framework size distribution (channel and cages) appear to be key factors. A narrow channel or cage, slightly larger than the ozone molecule size, seems to promote ozone interactions with Lewis acid sites. Fourier transform infrared spectroscopy shows that Lewis acid sites (LAS), located on the walls of zeolite pores, decompose ozone. This leads to the formation of atomic oxygen species that could react with another ozone molecule to form dioxygen. Hence, LAS are regenerated, ready to decompose another ozone molecule once more.
Catalytic ozonation of toluene using Chilean natural zeolite: the key role of brønsted and Lewis acid sites
The influence of surface physical-chemical characteristics of Chilean natural zeolite on the catalytic ozonation of toluene is presented in this article. Surface characteristics of natural zeolite were modified by acid treatment with hydrochloric acid and ion-exchange with ammonium sulphate. Prior to catalytic ozonation assays, natural and chemically modified zeolite samples were thermally treated at 623 and 823 K in order to enhance Brønsted and Lewis acid sites formation, respectively. NaturalandmodifiedzeolitesampleswerecharacterisedbyN2 adsorptionat77K,elementalanalysis, X-ray fluorescence, and Fourier transform infrared (FTIR) spectroscopy, using pyridine as a probe molecule. The highest values of the reaction rate of toluene oxidation were observed when NH4Z1 and 2NH4Z1 zeolite samples were used. Those samples registered the highest density values of Lewis acid sites compared to other samples used here. Results indicate that the presence of strong Lewis acid sites at the 2NH4Z1 zeolite surface causes an increase in the reaction rate of toluene oxidation, confirming the role of Lewis acid sites during the catalytic ozonation of toluene at room temperature. Lewis acid sites decompose gaseous ozone into atomic oxygen, which reacts with the adsorbed toluene at Brønsted acid sites. On the other hand, no significant contribution of Brønsted acid sites on the reaction rate was registered when NH4Z1 and 2NH4Z1 zeolite samples were used.
Use of combination of coagulation and adsorption process for the landfill leachate treatment from Casablanca city
This article presents a combination of coagulation–flocculation and powder activated carbon (PAC) adsorption as a treatment process for landfill leachate. Leachates were collected from a municipal solid waste landfill in Mediouna site, Casablanca city. Ferric chloride (FeCl3) is used here as a coagulant to study the optimum conditions for the removal of chemical oxygen demand (COD), colour, total suspended solids (TSS) and turbidity in jar tests. This coagulant showed the highest removal efficiency in terms of COD (62.5%), turbidity (92.5%), colour (80%) and least sludge volume generation (30% v/v) for an optimum coagulant dose of 12 g Fe3+ L–1. Combining coagulation with adsorption process onto PAC enhances the removal of COD, turbidity and colour reduction by a 77%, a 99% and a 99.7%, respectively. These results show that coagulation-adsorption could be used as a promising hybrid process for the treatment of landfill leachates.
Oxygen vacancies in nano-sized TiO2 anatase nanoparticles
Anatase nanoparticles containing surface oxygen vacancies (VO) and Ti3+ are of great importance for applications in photocatalysis, batteries, catalysis, sensors among other uses. The properties of VO and their dependence on the size of nanoparticles are of great research interest and could allow obtaining advanced functional materials. In this work, a complete set of oxygen vacancies in an anatase nanoparticle of size 1.1 nm was investigated and compared to those of a twice larger nanoparticle, having the same shape and surface hydroxylation pattern. It turned out that the decrease in the size of the anatase nanoparticle strongly facilitated creation of surface oxygen vacancies and Ti3+. After their creation, oxygen vacancies undergo three transformation paths — (1) small repulsion of surrounding Ti cations with retention of the vacancy, (2) transfer of oxygen anion, leading to the movement of oxygen vacancy to a more stable position, and (3) collapse of oxygen vacancy accompanied by structure deformation towards Magneli-like phase.
Design of active sites in zeolite catalysts using modern semiempirical methods: the case of mordenite
Zeolites are widely used for numerous processes for production of a vast number of chemicals, fuels and commercial goods. Preparation of zeolite catalysts that have improved selectivity for the desired products, operate at lower temperature and possess increased stability is therefore of great interest. The key to such improved zeolite catalysts is in the design of active sites and facilitation of mass transfer via optimization of the porous structure. At the same time, undesirable sites that inhibit desirable properties of the active sites need to be removed or blocked. The strength and structure of either the Brønsted or Lewis acid sites, directly determines their catalytic activity and selectivity for each reaction. In the present study, the structure and acidity of active sites in zeolites are investigated for the example of mordenite using modern semiempirical methods pm7 and scc-dftb (dftb2). Models AlHSi95O192 and Al2H2Si94O192 are used for Brønsted acid sites and Al2Si94O191 for Lewis acid sites. In agreement with previous studies, the stability of T1, T2, T3 and T4 sites is similar. Many different configurations of pair-wise located Al atoms were studied. In the present work it was found that some of the pair-wise located Al atoms possess Brønsted acid sites with strength much higher than that for single Brønsted acid sites. However, since their stability is not the highest among other double sites, special preparation methods need to be developed for selectively obtaining these very active sites. The stability of different Lewis acid sites is also considered.
Zeolites as recyclable adsorbents/catalysts for biogas upgrading: Removal of octamethylcyclotetrasiloxane
Natural and synthetic zeolites with different properties (porous structure, SiO2/Al2O3 ratio, acidity and Fe-loading) were evaluated as adsorbents/catalysts for octamethylcyclotetrasiloxane (D4) removal in dynamic adsorption tests. BEA type zeolites, with high content of Lewis and Brønsted sites, promoted the catalytic D4 ring-opening leading on the formation of smaller a-x-silanediols, which are narrower molecules able to diffuse into the channel system. Wet oxidation processes were used for the regeneration of a spent BEA zeolite, including ozonation and Fenton-like treatment. Both treatments were optimized to recover almost completely the D4 uptake of the iron-exchanged Fe-BEA in the first use. Thus, its feasibility to be reused was evaluated in successive adsorption/oxidation cycles, recovering up to 80% in at least three subsequent steps. However, in further cycles the accumulation of D4 and/or by-products led to a successive decline in the catalytic activity of the zeolites, hampering not only the capacity to transform D4 into lineal silanediols, thus reducing the adsorption capacity, but also the catalytic activity towards promoting Fenton-like reactions during regeneration.
Adsorption of a cationic dye from aqueous solution using low-cost Moroccan diatomite: adsorption equilibrium, kinetic and thermodynamic studies
This article presents the adsorption of Methylene Blue (MB) onto diatomite, in order to develop a low-cost treatment technology as a process alternative for dye removal. Diatomite used in this work was taken from the Nador area in the northeast of Morocco. Diatomite is characterised by different physical–chemical methods (X-ray diffraction, nitrogen adsorption-desorption isotherm, scanning electron microscopy and Fourier transform infrared). Results showed that the adsorption of MB onto diatomite mineral is affected by various operating parameters like contact time, initial dye concentration, adsorbent dosage, pH and temperature. Adsorption equilibrium is reached after 1.5 h of contact time. Maximum MB removal is obtained at pH = 12. MB removal rate decreases as pH decreases. Adsorption equilibrium data are fitted to Langmuir, Freundlich, Redlich–Peterson and Toth models. Adsorption data are well described by Langmuir isotherm model indicating that a homogeneous adsorption occurs. A maximum adsorption capacity (or monolayer coverage) of 11 mg g–1 is obtained at 45°C. A value of the enthalpy of adsorption of 12.78 kJ mol–1 is found confirming the endothermic nature of adsorption process, while a Gibb’s free energy change (∆G°) falling in the range −30.8 to −35.34 kJ mol–1 confirms the spontaneity of the process. Adsorption kinetics are fitted to a pseudo-second-order kinetic model. Experimental results indicate that the Moroccan diatomite could be used as a potential adsorbent for the removal of cationic dye molecules, at lower cost.
Toluene, Methanol and Benzaldehyde removal from gas streams by adsorption onto natural clay and Faujasite-Y type Zeolite
A great number of pollution problems come as a result of the emission of Volatile Organic Compounds (VOCs) into the environment and their control becomes a serious challenge for the global chemical industry. Adsorption is a widely used technique for the removal of VOCs due to its high efficiency, low cost, and convenient operation. In this study, the feasibility to use a locally available clay, as adsorbent material to control VOCs emissions is evaluated. Natural clay is characterised by different physical-chemical methods and adsorptive interaction features between VOCs and natural clay are identified. Toluene (T), methanol (M) and benzaldehyde (B) are used here as representatives of three different kinds of VOCs. Adsorption isotherms onto natural clay and faujasite-Y type zeolite (Fau Y) are obtained at room temperature. According to Langmuir model data, maximum adsorption capacities (qm) of Fez natural clay and zeolite toward methanol (M), toluene (T) and benzaldehyde (B) at 300 K are 8, 0.89 and 3.1 mmol g–1, and 15, 1.91 and 13.9 mmol g–1 respectively. In addition, the effect of temperature on the adsorption of toluene onto natural clay is evaluated in the range from 300 to 323K. An increase on temperature reduces the adsorption capacity of natural clay toward toluene, indicating that an exothermic physical adsorption process takes place. The enthalpy of adsorption of toluene onto Fez natural clay was found to be –54 kJ mol–1. A preliminary cost analysis shows that natural clay could be used as an alternative low cost adsorbent in the control of VOCs from contaminated gas streams with a cost of US$ 0.02 kg–1 compared to Fau Y zeolite with US$ 10 kg–1.