Green Synthesis of Metal Oxides Based Nanomaterials for Enviornmental Applications

Abstract

The nanoscale dimensions based structures showed excellence across diverse domains over the bulk materials which constitutes merely one morphological manifestation. Initially, metal oxides like nanosize TiO2 and ZnO have been used vastly due to considerable band gap exceeding 3.0 eV that showed their activeness in UV-light. Use of UV light, encounter challenges associated with photo-corrosion. Furthermore, widely used nanosize TiO2 and ZnO have caused bio-enrichment in the environment. These limitations have restricted their applicability across various industries, compelling demand for advanced new sunlight active and alternative metal oxide based, achieved through purposeful modification or doping strategies. The modification of metal oxides have been done by formation of hetrojunction with other metal oxides/sulfides and incorporation in moieties. To enhance stability and properties of nanocomposites, metal oxides have been incorporated into different matrix such as polymers, reduced grapheme oxide and many more. It results in reduction of the probability of oxidation, aggregation and enhanced stability due to the expansion of magnitude of electrostatic interaction The main target behind using natural polymers with nanomaterials is to reduce the toxicity or leaching of nanomaterials in the environment. The major use of metal oxides based nanomaterials in the removal of toxic and hazardous pollutants from water via photocatalytic degradation. Water scarcity continues to be a significant global concern due to high demand of pure water over the available supply in a particular region. Rapid urbanization often outpaces the development of infrastructure to supply clean water to growing cities. The complete processing of industries such as textile, food processing, farming and aquaculture is based on hazardous chemicals. Wastewater discharged from industries usually contains pesticides, dyes and plastic additives as recalcitrant pollutants which have caused major environmental issues. Usage of those chemicals beyond a limit causes their accumulation in the environment. India, being an agriculture based country, uses large amount of pesticides for improving the quantity and quality of the crops. Tones of pesticides used in various sectors annually without apprehending their consequences to fulfill the hassles of the world population. Only 0.1% of the applied pesticides were absorbed by the target while the rest were dispersed off in the environment leading to destruction in its quality. Owing to their hazardous and toxic nature, pesticides and their intermediates stand on the ninth position in the list of twelve most dangerous and persistent chemicals. As per global production and demand, Europe is the highest consumer, followed by Asia. Among pesticides, endosulfan, atrazine, DDE and lindane were consumed extensively and most regularly found in ground-water. Moreover their bioaccumulation and toxicity have beenfrequently reported. Consequently, the regulated amount of selected pesticides ranges from 0.001 mL-1 to 8 mg L-1 in water. Besides pesticides, dyes are also more hazardous and carcinogenic because of more stability towards heat, light, and oxidizing agents and their resistance to degradation than the others. Presently, more than 100,000 dyes are available commercially with 100 lakh tonnes of production per annum. Azo dyes has extensive use (75%) in the textile industries and their uncontrolled degradation produced carcinogenic and toxic intermediates. Several scientific studies reported that 10–12% of dyes like are utilized annually in textile industries of which a major portion (~20%) is lost during synthesis and processing operations and ends up in wastewater. As per International Agency for Research on Cancer, ingestion, inhalation and skin contact of some azo dyes causes acute and chronic poisoning. The toxicity of methylene blue, rhodamine B, congo red, eriochrome black T and auramine O reported in living beings. Therefore, their removal is also necessary from the environment. Nowadays, pollution by plastic additives is demanding tremendous attention due to use of lot plastic products during COVID-19 severe toxicity and perservance in water. TBBPA is a brominated flame retardants with largest production in the world. BPA is a degradation intermediate of TBBPA which is basically used as a monomer in the polycarbonate plastics materials. The annual production of TBBPA was found to be nearly 170,000 tons in the US, Japan and Israel, which is nearly more than half of the production of all BFRs. Similarly, HBCD commonly found in thermoplastic and styrene-based polymers and proven to be hepatotoxicant, developmental neurotoxin, and endocrine disruptor in human beings. Various traditional waste water techniques includes volatilization, hydrolysis, adsorption, oxidation, photocatalysis and microbial degradation are used (Soni et al., 2022). Cost effectiveness, energy, limit the use of these procedures and time consumption are some disadvantages associated with these method. Therefore, photocatalysis, involving semiconductor nanomaterials, has advantages of reactive species without using costly or toxic oxidizing chemicals. TiO2 (3.2 eV), ZnO (3.4 eV) and other metal oxides have shown photocatalytic performance for pollutant removal. Despite this, they have limited exploration at the industrial level owing to their high cost, bioaccumulation, large band gap, and need of a UV source for excitation. Therefore, development of new photocatalytic system with enhanced activities under natural sunlight is being emphasized. In the present work, doped nanomaterials such as Bi2O3@TiO2, TiO2-CdS, CeO2@ZnO, CdMgFe2O3@TiO2, Al-ZnFe2O4@rGO, GG-CdMgFe2O4@TiO2 and GG-CaO@SiO2 were synthesized using plant extract (Murraya koenigii and Sapindus mukorossi). Phytochemicals present in plant extract plays an important role as reducing and capping agents for synthesis of nanomaterials. The selection of pollutants for present work is based on their global utilization, bioaccumulation, occurrence and frequent toxicity. For the sake of clarity and convenience, the work done in the thesis has been divided into the following five chapters I. Introduction and literature review II. Experimental Methodology III. Bimetallic-based nanocomposites for photocatalytic degradation of organic pollutants IV. Metal ferrites-based nanocomposites for removal of organic pollutants V. Guar gum based nanocomposites for removal of organic pollutants Chapter I deals with the introduction of functionalization in metal oxide based nanomaterials employed for the treatment of wastewater. The importance of metal oxide based materials with different metallic frameworks and polymers as well as their importance in various fields have been described. The significance of plant extract in the synthesis of nanomaterials has been highlighted. Based on literature review and the advantageous properties of metal oxides/ metal sufides, metal ferrites and polymer to form heterojunction, interfacial charge transfer, better incorporation and high adsorption tendency after doping emphasized us for the synthesis of various nanocomposites (Bi2O3@TiO2, TiO2-CdS, CeO2@ZnO, CdMgFe2O3@TiO2, Al-ZnFe2O4@rGO, GGCdMgFe2O4@ TiO2 and GG-CaO@SiO2). The selection of pollutants was cleared through their health hazards and negative effects on the environment. Conventional techniques used for the removal of contaminants were explained with their limitations. This chapter further explains importance of green synthesis of metal oxide based nanomaterials and compliment green chemistry principles in nanomaterials synthesis. Chapter II consists of the mechanism of nanomaterials synthesis and the different synthesis strategies involved. The reaction conditions involved in synthesizing multiple nanomaterials using green methods have been discussed. Murraya koenigii and Sapindus mukorossi-based synthesis of transition metal oxides, metal sulfides, metal ferrites, polymer coating, and their doped nanomaterials have been explained in this chapter. These nanomaterials were then employed to remove hazardous contaminants, such as dyes, pesticides and plastic additives. The methodology used for degrading these pollutants has been discussed. Optimization of reaction conditions for maximum removal of pollutants are imperative. The details of the instruments used for characterization of those nanomaterials and analysis of pollutants treated for photodegration have been provided. Powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FT-IR), diffuse reflectance spectroscopy (DRS) measurement, zeta potential measurement, BET (Brunauer– Emmett–Teller) surface area analysis, photoluminescence (PL) analysis and gas chromatographymass spectrometry (GC-MS)/ Liquid chromatographymass spectrometry (LC-MS) analysis were adopted. Chapter III deals with explaining synthesized metal oxide nanomaterials properties through the results obtained from different characterization techniques. The different metal oxide heterojunction (Bi2O3@TiO2, TiO2-CdS and CeO2@ZnO) have been employed for the removal of dyes (methylene blue, congo red, rhodamine B, eriochrome black T), plastic additive (BPA) and pesticide (endosulfan). Optimization of reaction conditions for maximum removal of pollutants are imperative. Hence, the degradation of pollutant was found best at neutral pH, lowest pollutant concentration and under sunlight irradiation. Under conditions of meticulous optimization, the nanocomposite exhibited superior outcomes in comparison to its constituent nanoparticles, owing to its maximal surface area, enhanced semiconducting attributes and heightened stability. The first-order kinetics followed for the degradation of all pollutants. Various adsorption models were tried for pollutants removal and found best fitted for Langmuir isotherm. Qualitative studies on identifying metabolites formed from pollutant degradation revealed the formation of minor and non-toxic by-products. The stability of nanocomposites was explained through their reusability, CeO2@ZnO showed upto six cycle, Bi2O3@TiO2, and TiO2-CdS showed upto eight cycle. Chapter IV deals with the removal of plastic additives (TBBPA, BPA) by employing green synthesised crystalline CdMgFe2O4@TiO2 nanocomposite and semi-crystalline Al- ZnFe2O4@rGO used for the removal of AO and AT. The plant extract of Murraya koenigii employed for the synthesis of both CdMgFe2O4@TiO2 and Al-ZnFe2O4@rGO nanocomposite. The shifting in peaks was observed in PXRD pattern because of distortion in lattice while FT-IR, XPS, FE-SEM, EDS analysis supported the effective formation of Al-ZnFe2O4@rGO and CdMgFe2O4@TiO2 nanocomposite. The higher negative zeta potential value (- 36 eV), larger surface area (163 m2g−1), lower band gap (2.1 eV) makes Al-ZnFe2O4@rGO nanocomposite showed good photocatalyst as compared to CdMgFe2O4@TiO2 nanocomposite (surface area: 90 m2g-1, lower band gap: 1.9 eV and zeta potential: -20.0 mV). The photoluminescence results indicates high photoactivity of doped nanocomposite supported by more charge separation as compared to parent nanoparticle followed by the production of massive hydroxyl radicals for the extensive degradation of pollutants under direct sunlight. The scavenger analysis confirmed the degradation of pollutants due to involvement of hydroxyl radical over other active charge species. Photocatalytic degradation pathways of organic pollutant have been confirmed by GC-MS/LC-MS analysis via oxidation and hydrolysis mechanisms. The reusability of the nanocatalysts showed better recyclability of Al-ZnFe2O4@rGO (up to nine cycle) as compared to CdMgFe2O4@TiO2 (up to eighth cycle). Chapter V deals with the application of Guar gum (GG) based polymeric nanocomposite GG-CdMgFe2O4@TiO2 and GG-CaO@SiO2 for the treatment of simulated water containing toxic pesticides such as endosulfan, DDE, lindane and plastic additive HBCD. Spectroscopic and microscopic studies confirmed the wrapping of nanocomposite in GG matrix. The TGA analysis showed the better thermal stability of GG-CdMgFe2O4@TiO2 nanocomposite as compared to GG-CaO@SiO2 nanocomposite due to higher thermal stability of ferrites as compared to metal oxides. The maximum removal of all pollutants were achieved at minimum concentration of pollutant and neutral pH. The findings were based on the better surface activity of nanocatalysts and the amount of active species available under these conditions. Under optimized conditions, nanocomposite showed better results than its parent nanoparticles due to its better semiconducting properties and stability. Under solar light exposure, degradation of organic pollutants followed by the 1st- order kinetics and the sorption behavior followed the Langmuir adsorption model. Qualitative studies on identifying metabolites formed from pollutants revealed the formation of minor and non-toxic by-products. The reusability analysis of polymeric nanocompsoite concluded that GG-CaO@SiO2 showed more reusability cycles for the removal of pollutants (upto 10th cycle) as compared to GG-CdMgFe2O4@TiO2 (upto 9th cycle). The thesis completes with a brief overview of the outcomes of the current studies. The present study will help to provide a suitable methodology for generating different kinds of metal oxide based based nanomaterials. These nanomaterials are characterized with various analytical techniques, resulting in good catalytic efficiency towards harmful wastewater pollutants into safer metabolites. Systematic investigation on the breakdown of these pollutants will provide a better knowledge of the function of many environmental conditions in selecting this period. The results may be helpful for further research in nanomaterials and wastewater treatment techniques.