Removal of Organic Pollutants by Modified Zeolite/Clay Based Adsorbents/Photocatalysts

Abstract

The thesis entitled “Removal of Organic Pollutants by Modified Zeolite/Clay Based Adsorbents/Photocatalysts” is divided into eight chapters. Chapter 1: This chapter portrays the introduction of noxious organic pollutants and their removal techniques. The impact of organic pollutants on living beings was elaborated. The effect of toxic organic pollutants on human beings and the environment was discussed along with their sources, health effects, and their removal methods. The noxious organic pollutants such as methylene blue (MB), fipronil (FIP), rhodamine B dye (RhB), and metronidazole (MET) were briefly introduced along with their structures and harmful effects. The different types of removal techniques for treating wastewater were compared to know the best technique. The adsorption and photodegradation processes for EDCs, dyes, or other pollutants were summarized.

Chapter 2: It deals with the literature survey of the different noxious contaminants (EDCs, pharmaceuticals, or dyes) which were removed by zeolite-based composites with adsorption or photocatalysis techniques. The % removal of various zeolites has been compared with other noxious contaminants. The pros and cons of various materials such as carbonaceous materials, metal-organic frameworks, zeolites, carbon nanotubes, adsorbents, activated carbon materials were briefly discussed. The structural properties of zeolite frameworks along with their significance in adsorption and photocatalytic degradation were explored. The properties of photoactive materials were also discussed. In comparison to other support materials zeolites were found to be an appropriate substance for the hybrid adsorption and photocatalysis process, because it has a huge surface area for adsorption, tunable surface properties, porosity, ion-exchange capability, and the abundance of acidic-basic sites which can be able to decrease the chances of electron/hole pairs recombination. Chapter 3: In this chapter, the chemicals used for the synthesis and modification of zeolite framework were discussed, along with the characterization methods of prepared composites. The various characterization like N2 adsorption-desorption analysis, X-ray diffraction analysis (XRD), Scanning electron microscopy (SEM), High-Resolution Transmission electron microscopy (HRTEM), Energy-dispersive X-ray spectroscopy analysis (EDX), X-ray photoelectron spectroscopy (XPS) were performed to evaluate the structural properties of the material in detail. The procedure for adsorption or photocatalytic degradation processes and formulas adopted to calculate the adsorption capacities, rate constants, and other parameters were elaborated. The experimental data of adsorption were further verified with adsorption isotherms. The adsorption mechanism describing adsorbate and adsorbent interactions was evaluated by fitting the data in the Langmuir, Freundlich, Harkins-Jura, Dubbin-Radushkevich (D-R), and Halsey adsorption isotherm models. Different kinetic models such as Pseudo-first order, Pseudo-second order, Elovich model, and intra-particle diffusion were also investigated. Based on R2 (Pearson correlation coefficient) values their accuracies were estimated. A thermodynamic study was carried out to know the enthalpy, entropy, and heat of the reaction. Chapter 4: It covers the adsorptive removal of FIP, an endocrine disrupter insecticide from its aqueous solution by cerium-modified ZSM-5 zeolite adsorbent. A series of cerium modified H-ZSM-5 zeolite adsorbents such as Ce5ZSM-5, Ce10ZSM-5, Ce15ZSM-5, Ce20ZSM-5, Ce25ZSM-5, and Ce30ZSM-5 were prepared from the cerium salt of concentration (w/v%) 5%, 10%, 15%, 20%, 25%, and 30% respectively by simple refluxing treatment. The adsorptive performance of all of these prepared adsorbents was compared and Ce25ZSM-5 was found to be the best adsorbent for FIP removal and characterized with XRD, N2-adsorption, HRTEM, FESEM, EDS, and XPS techniques to study the porosity, crystallinity, and surface properties in detail. The batch adsorption experiments were performed to know the maximum adsorption capacity and % removal and found to be 598.80 mg/g and ~93% at optimum conditions (t: 120 min, pH 3, temperature: 25 °C, dose: 3.5 g/L, speed: 250 rpm, pollutant conc.: 600 mg/L). There was monolayer adsorption as the data fitted well in the Langmuir adsorption isotherm model and in kinetics, the pseudo-second-order model was followed with a high rate constant. The thermodynamic studies were carried out and ΔG°, ΔH°, and ΔS° parameters were calculated which confirms that the adsorption process is feasible, spontaneous, and exothermic. Chapter 5: It deals with the metal-free catalyst g-C3N4 which is a visible light active material with a bandgap value of 2.7 eV. It can act as an efficient photocatalyst but its less surface area may hinder the photocatalysis process, therefore zeolite material with a huge surface area has been used to build an effective photocatalyst. The synthesized g-C3N4/H-ZSM-5 nanocomposite has sufficient surface area (~172 m2/g) for the adsorption of pollutants with effective photodegradation activity. The composite was synthesized by mixing followed by facile calcination at 550 °C and properties of synthesized photocatalyst were explored with, BET, HRTEM, FESEM, EDS, and XPS characterization techniques. The photoactivity of as-prepared g-C3N4/H-ZSM-5 nanocomposite was tested against the model MB dye and FIP insecticide. From the scavengers study •OH and O2•- were found to be the responsible species for MB and FIP degradation. The maximum %degradation calculated were ~93% and ~89% for MB and FIP photocatalytic degradation. From these results, it has been discovered that this visible light-active photocatalyst is an effective composite for the degradation of harmful pollutants, and it's also easy to prepare. Chapter 6: The adsorptive performance of cerium-modified different categories of zeolites was compared for the removal of FIP from its aqueous solution. The Ce-BETA, Ce-Mordenite, and Ce-13X adsorbents were prepared by a simple ion exchange method. The as-prepared different zeolites were characterized by XRD, BET, SEM, and EDS analysis for crystallinity, surface properties, and elemental compositions. The average crystallite size ‘D’ of cerium oxide is 6.67 nm for Ce-beta zeolite and 6.42 nm for Ce-Mordenite and 4.48 nm for Ce-13X zeolite, calculated with the Debye-Scherrer formula. The XPS data suggests that the cerium is present in two oxidation states i.e. Ce(IV) and Ce(III) with % amount of 71.10% and 28.29%, respectively, indicating that the Ce(IV) is more prominently. The d-spacing of cerium oxide was calculated from HRTEM analysis and their corresponding planes were matched with XRD following the JCPDS No. 34-0394. The various parameters of adsorption were investigated for FIP removal like pollutant concentration, adsorbent dose, pH, contact time, stirring speed, and temperature. The equilibrium data were fitted in various adsorption isotherms to know the type of adsorption. The best-fitted kinetic model signifies a higher adsorption rate. Thermodynamic parameters of adsorption revealed the spontaneous and exothermic adsorption processes. The structural comparison was also made between the 13X, Mordenite, and Beta zeolite frameworks. Chapter 7: It deals with MET, a recalcitrant antibiotic from the nitro-imidazole family which contributes majorly to water pollution. The one-pot synthesis of Cdots@zeolite (CDZ) nano-composite by a hydrothermal treatment was carried out to photo-degrade MET molecules. The different ratios of CDZ composites (1:1, 1:3, 1:5, 5:1, and 1:7) were prepared by mentioned method and characterized by XRD, BET, EDS, and XPS for their crystallinity, surface area, elemental compositions, and surface chemical states respectively which confirms crystalline nature, incorporation of Cdots into zeolite frameworks with sufficient surface area. The morphology was analyzed by SEM analysis, d-spacing along with planes were characterized by HR-TEM and SAED analysis. The PZC (point of zero charges) value for the CDZ composite was determined to be at pH, 3.4. The maximum photocatalytic degradation of ~79% was achieved at an optimum dose of 0.2 g/L and pH 4 for MET and that of RhB was ~90% at a catalyst dose of 0.4 g/L. The comparison of sunlight, Visible, and UV sources was performed with rate constants ‘k’ 0.0081, 0.0041, and 0.0101 min-1respectively. The scavengers experiment was carried and found that electrons were the responsible active species for degradation of MET pollutant. The plausible photocatalytic degradation mechanism was explored and the intermediates or degradation pathway was drawn with the help of the m/z value obtained from GC-MS analysis. Chapter 8: This chapter presents the conclusions and the future perspective of the present work. In the end, the references cited in the thesis have been listed. The modification of zeolite with cerium oxide was done with refluxing by ion exchange method. The increased cerium content in zeolite increases the active sites in zeolite which favours adsorption process. The Ce25ZSM-5 adsorbent was found to have maximum removal because after that the saturation of active sites of zeolite occurs and the % removal remains almost constant. The cerium modified beta zeolite was found to possess maximum % removal of ~94% due to its huge surface area, porosity, and high Si/Al ratio. The efficient photocatalysts g-C3N4/H-ZSM-5 and Cdots@zeolite nano-composites were also prepared to degrade MB/FIP and MET/RhB pollutants effectively. The GC-MS analysis for MET was also carried out to determine the intermediates during the photocatalysis process.