Aluminosilicate Waste derived Novel Zeolite A based Nanocomposite for Treating Wastewater

Abstract

Rapid industrialization without sufficient emphasis on ecologically sound practices contributes significantly to solid refuse and water pollution. The current waste management system mainly focuses on end-of-pipe solutions and short-term effects, which may be reactive but fail to achieve sustainability. The Cradle-to-Cradle approach promotes a transition from the linear model of "take-make-dispose" to a circular model that encourages resource conservation and regeneration. Thus priority is to move towards a circular economy approach in waste management by following the steps (i) reclamation of solid industrial waste (such as fly ash and red mud) into useful zeolite material by recovering resources within wastes, (ii) reduction of organic and inorganic contaminant by green technologies without involving in any secondary pollution, (iii) treated water can be recycled into the environment protecting its safeguards. This research contributes to a more sustainable future by reducing waste, conserving resources, and minimizing the negative environmental impacts of waste pollutants. The entire thesis work is divided into four major chapters. The first part of the thesis is the sustainable synthesis procedure of zeolite 4A utilizing resources from industrial solid wastes without adding external chemical precursors and its structural and morphology control optimization study using a central composite design. High-modulus silicate and aluminate are extracted from fly ash and red mud, abundantly found in the Eastern part of India, Odisha, by low-temperature alkali fusion followed by ultrasonication. The colloidal aluminosilicate sol to a stable crystal of zeolite 4A by the hydrothermal method was investigated to observe morphology changes and tentative crystal growth mechanism. A highly pure zeolite was obtained under the optimized FA/RM extract=1.02, crystallization temperature=90°C, crystallization time=10.25hrs. Three different types of surface morphology of zeolite 4A are observed sharp edge cube when FA/RM extract is higher than 1.02 (Run 19), truncated edge cube when FA/RM extract is equal to 1.02 (Run 2), and rounded edge cube when FA/RM extract is less than 1.02 (Run 18). From the desirability function evaluation, we can conclude that the desired goal of maximum crystallinity has been achieved by optimizing three independent variables at a 95% confidence level. The next objective of the thesis is to synthesize a novel core-shell nanocomposite for multiple refractory organic in wastewater using waste-derived zeolite A as core material. We developed a novel core-shell zeolite A@oxygen-deficient ZnO (ZA@ZnO1-X) nanocomposite for the degradation of PAHs mixture containing fluorene, phenanthrene, and anthracene. The hierarchical structure helps to reduce the recombination of photogenerated electron and holes pair, thereby availing more active species for photodegradation. The enhanced photodegradation efficacy is due to the synergistic effect of oxygen-defect sites in ZA@ZnO1-X and photoactive species. Core-shell ZA@ZnO1-X shows improved catalytic properties with a band gap value of 2.65eV lower than the sole ZnO nanosheet (3.03eV). The porous shell of the ZnO1-X structure provides an enhanced adsorption site and fully utilizes photon energy (visible light) with no aggregation because of ZA support. The maximum photocatalytic degradation is achieved by optimizing reaction parameters of 96%, 95.1%, and 93% of fluorene (0.02436min-1), phenanthrene (0.02421min-1), and anthracene (0.02102min-1), respectively in 2 hours at neutral pH and catalyst load 1g/l. The primary active species responsible for PAHs degradation is h+, followed by HO•, O2•- radicals analyzed by quenching experiment. The degradation intermediates and degraded products analysis by GC-MS gives a plausible general reaction pathway of PAHs and reveals primary intermediate is a phthalate derivative. The regeneration of active catalytic sites in the ZA@ZnO1-X photocatalyst shows its stability and reusability over five consecutive cycles. The subsequent section of the thesis involves toxic heavy metal Chromium removal of both state Cr(VI) and Cr(III) by ZA@ZnO1-X nanocomposite. ZA@ZnO1-X has removed 98.3% of total Cr, achieving a discharge limit of 0.05ppm of Cr(VI) rather than transforming it to less harmful Cr(III) after the photoreduction process. The effect on photocatalytic reduction efficiency (PRE) is systematically studied on varying operational variables like pH, citric acid and initial Cr(VI) concentration, and catalyst load. ZA@ZnO1-X removed Cr(VI) (2.5 ppm) and photo-reduced Cr(III) simultaneously by 98.5% PRE with catalyst load 0.4g/100ml, CA=5mM, pH=5.06 under halogen light irradiation(300W, 240V) for 50mins. This experimental study evaluates the prominent role of oxygen vacancy of ZA@ZnO1-X for photocatalytic reduction of Cr(VI) and enhanced adsorption of Cr(III). The kinetic study reveals adsorption of both Cr(VI) and Cr(III) follows pseudo-second-order kinetics, whereas photoreduction of Cr(VI) follows pseudo-first-order kinetics. A possible mechanism of total Cr removal is sketched, supporting enhanced adsorption due to unsaturated Zn atoms and unpaired e- at the oxygen defect site, followed by photoreduction by photogenerated e- and CO2-• radical. The ZA@ZnO1-X performance is remarkable (reduced by 2.5%) after five consecutive runs without deformation. The easy regeneration process makes it suitable for toxic total Cr removal, avoiding any secondary pollution. In the final objective, hybrid technology has been studied using a fabricated membrane photoreactor in a continuous mode of operation on a bench scale. The work is based on the synergistic effect of photocatalyst, microbubble, and membrane separation for enhanced degradation of Bismarck Brown R, which is otherwise very low in the standalone process. The fabricated bench-scale membrane setup is of submerged type, a modified tubular membrane with the novel ZA@ZnO1-X nanocomposite, and detailed analytical characteristics are investigated. The thin layer coating (average thickness 24.6μm) of oxygen-deficient ZnO1-X photocatalyst gives a high photocatalytic efficiency under visible light without hampering the rejection rate. Here, the hybrid process treated Bismarck Brown R contaminated wastewater in the prototype setup with simulated and natural wastewater collected from a local dying factory. The operational parameters such as Bismarck Brown R concentration, solution pH, temperature, and flux were varied and analyzed optimized reaction conditions favorable for actual wastewater treatment. The hybrid ZA@ZnO1-X nanocomposite-based tubular membrane improved degradation to 95.4% decolorization and 94% COD removal rate in 90 mins at pH 8.15, solution flux 120 ml/min, and temperature 30°C. The scavenging experiment resolved responsible reactive species for organic dye degradation: holes and HO• radical. The tentative mechanistic degradation pathway involved homolytic cleavage of the azo bond followed by phenyl radical generation to a small intermediate of hydroxyquinol. Thus the work represents an efficient alternative to conventional membrane technology with lesser membrane fouling tendency and enhanced catalytic efficiency in assistance with microbubbles simultaneously.