Redox-synergistic mechanisms and engineering implementation of microscale zero-valent iron-based processes for...

Accelerated global industrialization intensifies industrial wastewater pollution, positioning zero-valent iron (ZVI)-based technologies as effective solutions due to their environmental friendliness and multi-pathway pollutant elimination capability. Microscale ZVI (mZVI) offers dual advantages in cost-effectiveness and application performance compared to large-size ZVI (e.g., scrap iron, iron shaving, sponge iron, and iron-carbon composite) and nanoscale ZVI (nZVI). With a cost range approaching that of economical large-size ZVI and significantly lower than nZVI, mZVI simultaneously overcomes the limitations of large-size ZVI (slow reaction kinetics in fixed-bed reactors) and nZVI (severe aggregation and rapid self-passivation). Therefore, mZVI-based wastewater treatment processes demonstrate great potential for efficient treatment of refractory organic wastewaters. This review focuses on the structural-directed degradation mechanisms and engineering applications of organic contaminants by mZVI-based processes. The multifunctional mechanisms of mZVI-based processes are systematically elucidated, including direct reduction, oxidant activation for generating reactive oxygen species, and adsorption-coagulation. Moreover, the synergistic relationship between contaminant molecular structural characteristics and the dominant removal pathways is established. Synergistic removal mechanisms are revealed for co-existing composite pollution systems in actual wastewaters, including heavy metals-organic contaminants and NO₃^--organic contaminants. Based on the rotating fluid reactor, four types of mZVI-based processes are further summarized to provide targeted solutions for different refractory organic wastewaters. Future research necessitates addressing critical bottlenecks in fundamental mechanisms and engineering applications by strategies including advanced electron efficiency optimization, reactor and process innovations, and process customization for different refractory organic wastewaters to promote the progress of industrial wastewater treatment technology.