Water pollution has jeopardized human health, and a safe supply of drinking water has been recognized as a worldwide issue. The increase in the accumulation of heavy metals in water from different sources has led to the search for efficient and environmentally friendly treatment methods and materials for their removal. Natural zeolites are promising materials for removing heavy metals from different sources contaminating the water. It is important to know the structure, chemistry, and performance of the removal of heavy metals from water, of the natural zeolites to design water treatment processes. This review focuses on critical analyses of the application of distinct natural zeolites for the adsorption of heavy metals from water, specifically, arsenic (As(III), As(V)), cadmium (Cd(II)), chromium (Cr(III), Cr(VI)), lead (Pb(II)), mercury(Hg(II)) and nickel (Ni(II)). The reported results of heavy-metal removal by natural zeolites are summarized, and the chemical modification of natural zeolites by acid/base/salt reagent, surfactants, and metallic reagents has been analyzed, compared, and described. Furthermore, the adsorption/desorption capacity, systems, operating parameters, isotherms, and kinetics for natural zeolites were described and compared. According to the analysis, clinoptilolite is the most applied natural zeolite to remove heavy metals. It is effective in removing As, Cd, Cr, Pb, Hg, and Ni. Additionally, an interesting fact is a variation between the natural zeolites from different geological origins regarding the sorption properties and capacities for heavy metals suggesting that natural zeolites from different regions of the world are unique.

In the present study, facile and cost-effective modification methods, including alkali treatment and acid treatment, were evaluated to improve the adsorption capacity of NaY zeolite for diclofenac sodium (DFS) elimination from the aqueous solution. The performance of unmodified NaY, sodium hydroxide treated NaY (SHT-Y), hydrochloric acid treated NaY (HAT-Y), and sulfuric acid treated NaY (SAT-Y) was compared, followed by optimization of sulfuric acid concentration since sulfuric acid was recognized as the preferred modifying agent. The modified NaY with a concentration of 0.1 M of sulfuric acid exhibited the highest adsorption capacity. Analyses of FE-SEM, EDS, FTIR, BET, and XRD were used to determine the characteristics of NaY and treated zeolites. All treatments changed the Si/Al ratio of NaY. Silanol and bridging hydroxyl functional groups were detected in all zeolitic samples. Acidic modifications enhanced the specific surface area of NaY more than the alkali treatment. The influence of the major factors on the DFS adsorption, including adsorbent dosage, contact time, pH of the solution, and initial concentration of DFS was investigated and optimized in detail. The maximum adsorption capacity was obtained at pH 11 at the equilibrium time of 180 min. The hydrogen bond formation and cation-π interaction were recognized as DFS adsorption mechanisms. From the results of kinetic studies, the Elovich model has better agreement with experimental data, suggesting that the adsorbent surface is energetically heterogeneous and chemisorption leads the DFS uptake onto the adsorbent, indicative of hydrogen bond formation. In accordance with the equilibrium modeling results, the Sips isotherm better expresses the DFS adsorption mechanism onto the surface of the modified zeolite. According to the Langmuir isotherm model, the maximum adsorption capacity obtained for 0.2 g/L of treated zeolite was 117.77 mg/g. Eventually, sulfuric acid treatment was found as an effective and facile method for enhancing zeolite performance in DFS removal from aqueous solution.

Mechanism of MFI film growth from seed crystals and evolution of defects during the film growth were investigated. The hydrothermal growth of colloidal silicalite-1 crystals of 50 nm on silicon substrate was used to reveal the MFI film formation mechanism at different time intervals using high resolution-transmission electron microscopy (HR-TEM) and extreme high resolution-scanning electron microscopy (XHR-SEM). It was found that the MFI seeds started to grow in the early stages of hydrothermal treatment from 50 nm to 75 nm in size and connect to the crystals in the vicinity, showing the onset of film formation. The film growth mechanism was led by the sub-colloidal particles in the synthesis solution arriving at the crystal surfaces contributing to the film growth. A continuous film with a thickness of 100 nm was formed after 12 h of hydrothermal treatment containing pinhole defects. Pinhole defects disappeared after 24 h of hydrothermal treatment with a film thickness of 200 nm and grain boundaries thickness of 2 nm were formed. Furthermore, mesoporous defects were found in the grains of the film, which appeared due to the film growth by sub-colloidal particles of synthesis solution. The growth rate of the MFI film was calculated to 0.007 (μm/h) and compared with the crystal growth inside the synthesis solution.

Se investigó el mecanismo de crecimiento de películas de MFI a partir de cristales semilla y la evolución de los defectos durante el crecimiento de dichas películas. La síntesis hidrotérmica a partir de cristales coloidales de silicalita-1 de 50 nm sobre sustrato de silicio se utilizó para revelar el mecanismo de formación de la película de MFI en diferentes intervalos de tiempo, utilizando microscopía electrónica de transmisión de alta resolución (HR-TEM) y microscopía electrónica de barrido de extrema alta resolución (XHR-SEM). Se observó que las semillas de MFI comenzaron a crecer en las primeras etapas del tratamiento hidrotérmico de 50 a 75 nm de tamaño y se conectaron a los cristales en las cercanías, evidenciando el inicio de la formación de la película. El mecanismo de crecimiento fue dirigido por las partículas subcoloidales en la solución de síntesis que llegaron a las superficies de los cristales, contribuyendo al crecimiento de la película. Se formó una película continua con un espesor de 100 nm después de 12 h de tratamiento hidrotérmico que contenía orificios. Estos desaparecieron después de 24 h de este mismo tratamiento, habiendo alcanzado un espesor de película de 200 nm, y se formaron límites de grano de 2 nm. Además, se encontraron defectos mesoporosos que aparecieron debido al crecimiento de la película a partir de partículas subcoloidales presentes en la solución de síntesis. La tasa de crecimiento de la película de MFI se calculó en 0,007 (μm/h) y se comparó con el crecimiento de cristales dentro de la solución de síntesis.

ZSM-5 is one of the most well-known zeolites. It has been synthesized for many different applications, including as catalyst for catalytic reactions and as a membrane in separation processes.  The main goal of this thesis is to investigate and characterize the defects in crystal and film growth of ZSM-5 for modification purposes.

Primarily, ZSM-5 crystals with carefully controlled thickness were synthesized in fluoride and hydroxide media. These synthesized catalysts were then characterized and the two synthesis routes were compared with each other in terms of stability. Then, in the synthesized catalysts, the role of defects during reaction was investigated. Furthermore, the growth of ZSM-5 nanocrystals and subsequent film formation was investigated to better understand how particles formed in synthesis solution (a hydrolyzed mixture of H₂O-TEOS-TPAOH), followed by how these crystals interact to form a zeolite film. Finally, pre- and post-production modification of ZSM-5 and PHI membranes was investigated. Since, the roughness surface of the support could play a crucial role in having an even film, it was done as a pre-treatment technique for membrane synthesis. Additionally, a technique was developed in an attempt to plug the defects of grain boundaries to increase the membrane performance.

Two main types of morphological features resulting in surface roughness were observed on alumina discs used as supports for zeolite membranes. These features can be described as hills and pits and it was shown that defects as cracks formed in the zeolite film at these locations of the support. It was demonstrated that the roughness of the support can be reduced significantly by a polishing strategy developed in this paper. Finally, zeolite MFI membranes grown on the polished support shows remarkably improved quality as compared to films grown on non-polished supports.

This work presents the synthesis of nearly defect-free ZSM-5 nanosized crystals, prepared in fluoride medium by seeding with silicalite-1. This material was carefully characterized and its catalytic performances in the methanol to hydrocarbons (MTH) reaction were assessed. Such fluoride-based material was compared to a reference ZSM-5, produced through a conventional alkaline synthesis but from the same seeding. Despite both the materials show closely identical morphology and they have a comparable acid site population, the catalyst prepared using the fluoride route showed significantly longer lifetime in MTH compared to the catalyst prepared using conventional synthesis at high pH. The slower deactivation for the samples prepared using the fluoride route was ascribed, thanks to a thorough in situ IR spectroscopy study, to its lower density of internal defects. According to the UV-Raman characterization of coke on the spent catalyst, the fluoride-based ZSM-5 catalyst produces less molecular coke species, most probably because of the absence of enlarged cavities/channels as due to the presence of internal defects. On the basis of these observations, the deactivation mechanism in the ZSM-5 synthesized by fluoride medium could be mostly related to the deposition of an external layer of bulk coke, whereas in the alkali-synthesized catalyst an additional effect from molecular coke accumulating within the porous network accelerates the deactivation process.

ZSM-5 zeolite crystals with carefully controlled thicknesses in the range 20–110 nm, i.e. in the colloidal domain, were synthesized in fluoride and hydroxide media. The crystals were treated in steam at high temperature to evaluate the stability and evaluated by SEM, XRD, NMR and NH3-TPD. The results showed that the framework of crystals synthesized in fluoride media was more stable than the framework of crystals synthesized in hydroxide media. This should be an effect of lower concentration of structural defects and silanol groups in the former zeolites as reported by other groups. However, independently of the synthesis conditions, all crystals dealuminated rapidly when treated with steam at the conditions investigated in the present work.

Commercial application of zeolite membranes has been hindered by the challenge of preparing defect-free membranes. Herein, we report a facile method able to selectively plug grain boundary defects in high-flux MFI zeolite membranes by coking of iso-propanol at 350 °C. After modification, the permeance via defects was reduced by 70%, whereas that via zeolite pores was reduced by only 10%.