Background: Azo dyes are recognized as non-decomposable and recalcitrant compounds and can be depleted into more dangerous secondary products in anaerobic environments. In current scenario different water treatment strategies including adsorption, photo-catalysis and advanced oxidation processes based practices are facing different limitations. Method: A literature survey was accomplished by searching the scientific data from different search engines including Scopus, PubMed, Science Direct, Springer, Taylor & Francis,Google Scholar, Blackwell-Synergy, Wiley-Interscience and Research-Gate etc. This article has been compiled after intensively reviewing about 231 research papers, reviews and book chapters in the fields of industrial effluents, hazardous materials and water treatment strategies with their advantages and limitations. Results: Molecular oxygen and other active species such as O2•−, HO2•, H2O2 and •OH play a significant role in degradation of dyes in AOPs and photo-catalyst utilizes sunlight energy and accelerates some chemical reactions depending upon the activation energies. Different reaction parameters;calcination temperature, pH, initial dye concentration, and catalyst dosage have significant impact on photo-catalytic degradation performance.Characterization of degradation processes of dye-stuffs could be carried out by state of the art analytical techniques i.e. UV-Visible spectroscopy, powdered XRD, FTIR (ATR), EDX-SEM, BET and differential pulse voltammetry. GC-MS and LC-MS investigation of photo-degradation by-products and intermediates could provide identification and possible degradation pathway for target dye molecules. This review covers research related to photocatalytic degradation of azo dyes by TiO2 and ZnO, widely used photo catalysts and various combination of zeolites. Conclusion: It can be concluded that the combination of nano-sorbents (Fly Ash Zeolites) and photo-catalysts not only enhances the degradation but also removes toxic dye molecules and theirby-products, effectively. The review explains the suitability of synergic applications of catalysts (TiO2, ZnO) and catalytic bed (zeolites) for different industrial effluents and waste water treatment as a significant pace towards green technology.

Massive production of carcinogenic fly ash waste poses severe threats to water bodies due to its disposal into drains and landfills. Fly ash can be a source of raw materials for the synthesis of adsorbents. Rag fly ash as a new class of raw materials could be a cheap source of Al and Si for the synthesis of Na-zeolites. In this work, NaOH activation, via a prefusion- and postfusion-based hydrothermal strategy, was practiced for the modification of rag fly ash into Na-zeolite. Morphology, surface porosity, chemical composition, functionality, mineral phases, and crystallinity, in conjunction with ion exchangeability of the tailored materials, were evaluated by SEM, ICP-OES, XRF, FTIR, XRD, and cation exchange capacity (CEC) techniques. Rag fly ash and the synthesized Na-zeolites were applied for the removal of Pb (II) from synthetic wastewater by varying the reaction conditions, such as initial metal ion concentration, mass of adsorbent, sorption time, and pH of the reaction medium. It was observed that Na-zeolite materials (1 g/100 mL) effectively removed up to 90–98% of Pb (II) ions from 100 mg/L synthetic solution within 30 min at pH ≈ 8. Freundlich adsorption isotherm favors the multilayer heterogeneous adsorption mechanism for the removal of Pb (II). It is reasonable to conclude that recycling of textile rag fly ash waste into value-added Na-zeolites for the treatment of industrial wastewater could be an emergent move toward achieving sustainable and green remediation.

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