Why radical oxidation prevails: Dual advantages in sludge valorization and ARG suppression over nonradical pathways

While the efficacy of advanced oxidation processes (AOPs) in sludge valorization is well-established, their associated ecological risks—particularly concerning antibiotic resistance genes (ARGs)—remain a critical blind spot. This knowledge gap is especially significant given the fundamental mechanistic dichotomy between radical and nonradical pathways. Herein, we systematically evaluate two representative systems—sludge-derived biochar-activated peracetic acid (SBC/PAA, nonradical/¹O₂) and nanoscale zero-valent iron-activated peracetic acid (nZVI/PAA, radical/·OH and ·CH₃)—focusing on their dual impacts on short-chain fatty acid (SCFA) production and ARG fate. A stark contrast was observed: the mild ¹O₂ from SBC/PAA resulted in only a modest increase in SCFA yield but inadvertently increased ARG dissemination potential by enriching mobile genetic elements (MGEs), amplifying host bacteria, and activating stress-induced horizontal gene transfer pathways (oxidative stress, signal transduction and secretory system). In contrast, the highly reactive radicals from nZVI/PAA achieved a dual victory, maximizing SCFA production through efficient cell disruption while concurrently suppressing intracellular and extracellular ARGs via profound genomic damage and functional pathway disruption, with intracellular ARGs (iARGs) and extracellular ARGs (eARGs) further reduced by 46.07% and 45.21% during fermentation. Thus, the distinct chemical effects exerted by the oxidants propagated through the microbial ecosystem, shaping divergent ARG risk trajectories by differentially governing MGEs, hosts, and functional pathways. This insight provides a critical mechanistic foundation for overcoming the trade-off between resource recovery and risk mitigation, shifting the paradigm from the indiscriminate application of AOPs to the strategic selection of oxidation mechanisms for a sustainable and safe sludge treatment future.