Speciation-Driven Cascade Technologies for Valorizing Hazardous Incineration Ash into a Stable Resource: A Critical Review

Fly ash from municipal solid waste incineration is a hazardous waste whose safe disposal and resource recovery are obstructed by a fundamental trilemma: no single treatment simultaneously immobilizes heavy metals permanently, destroys persistent organic pollutants, and valorizes the residual matrix. Here we resolve this trilemma through a speciation-driven cascade strategy, a design principle in which chemical, thermal, and physical unit operations are precisely matched to the distinct chemical fractions and volatilities of each metal. We systematically review the speciation and occurrence of heavy metals in fly ash and propose a classification of multi-coupled technologies based on how they manipulate the acid-soluble, reducible, and residual fractions. We distill the four recurring interaction mechanisms (i.e., phase-transfer promotion, kinetics alteration, secondary-pollutant co-control, and resource–recovery balancing) that enable synergistic performance, demonstrate that the cascade framework explains why certain couplings succeed while others fail, and derive general rules for process integration. Bottlenecks in multi-variable regulation, secondary pollution, long-term stability evaluation, and engineering scale-up are dissected. We then chart a roadmap toward intelligent, low-carbon, and zero-waste valorization via selective recovery and all-component utilization. This review reframes fly ash treatment as a tunable sequence of speciation-modifying steps and provides a structured framework for hazardous-waste-to-resource conversion across diverse industrial residues.