Microwave Pyrolysis of Spent Activated Carbon-Laden Sludge: A Critical Review of Regeneration, Cross-Phase Pollutant Transfer, and Environmental Reactivity

Spent activated carbon-laden sludge (SACLS) is a composite residue containing a potentially recoverable porous carbon matrix together with organic contaminants, mineral phases, and heavy metals, thereby creating simultaneous resource-recovery and environmental-risk challenges. This review critically evaluates microwave pyrolysis as a strategy for activated-carbon regeneration and SACLS valorization, with particular emphasis on particulate formation, cross-phase pollutant transfer, heavy-metal redistribution, and environmentally persistent free radicals (EPFRs). Peer-reviewed evidence from spent activated-carbon regeneration, sewage-sludge microwave pyrolysis, particulate formation, metal speciation, and EPFR chemistry was critically synthesized into a unified process-level framework. The analysis indicates that selective microwave absorption by carbonaceous domains can accelerate adsorbate desorption, cracking, and pore redevelopment; however, heterogeneous energy deposition may also generate interfacial hot spots that promote tar conversion, mineral transformation, particle nucleation, metal volatilization and condensation, carbon loss, and radical stabilization. Fine particles should therefore be regarded as mobile carriers of metals, aromatic radical precursors, and redox-active mineral phases rather than merely as secondary dust. Existing studies commonly evaluate only the solid char, whereas particles, condensates, reactor deposits, and time-dependent EPFR evolution are rarely included in complete mass balances and integrated risk assessments. Safe SACLS valorization consequently requires a regeneration-risk-constrained operating window that jointly considers adsorption recovery, carbon preservation, energy demand, particle-size-resolved metal fate, organic contaminant redistribution, and EPFR/reactive oxygen species (ROS) reactivity. Standardized feedstock characterization, spatially resolved thermal diagnostics, and phase- and time-resolved analyses are essential for scalable application.