Fly ash-based zeolites: from waste to value – A comprehensive overview of synthesis, properties, and applications

The innovative utilization of fly ash for synthesizing zeolites presents notable environmental and economic benefits, positioning it as a pivotal area of research. This review distinguishes itself by offering a critical and up-to-date synthesis of the latest advancements in fly ash-based zeolite production, surpassing existing literature in both depth and breadth. It meticulously explores not only the fundamental principles and conventional synthesis methods—such as hydrothermal and sol-gel techniques—but also delves into emerging approaches like microwave-assisted, ultrasound-enhanced, fusion and melting, and ionothermal synthesis. A unique contribution of this paper is its comprehensive analysis of the interplay between synthesis parameters (composition, temperature, reaction time, and pH) and their nuanced effects on the crystal structure, morphology, and functional properties of the resulting zeolites. Furthermore, the review introduces novel classifications of zeolites derived from varied fly ash sources, highlighting their tailored applications in cutting-edge fields such as advanced soil remediation, precision nutrient retention systems, next-generation wastewater treatment technologies, and efficient removal of emerging gaseous pollutants. By identifying and discussing the latest trends and gaps in the current research landscape, this paper not only synthesizes existing knowledge but also proposes future directions, including the development of hybrid zeolite materials and the integration of machine learning techniques for optimized synthesis. This comprehensive and forward-looking perspective significantly advances the understanding of fly ash-derived zeolites and sets the stage for future innovations in sustainable material science.