Ligand-modulated Fe(III)/Fe(II) cycling enables dual sulfide control via accelerating abiotic oxidation and ROS-mediated microbial suppression.

Sulfide (S(-II)) accumulation in engineered systems poses persistent risks, yet its in situ oxidation by O2 is severely hindered by intrinsically sluggish kinetics and is continuously counteracted by microbial sulfur reduction. Here, we demonstrate the application of Fe(III)-ligand (L) complexes as a novel dual-functional strategy for S(-II) control, which involves (i) accelerating abiotic oxidation and (ii) suppressing microbial S(-II) regeneration via boosting reactive oxygen species (ROS) production. By identifying a "thermodynamic-kinetic framework", we decipher how ligands modulate Fe(III)/Fe(II) redox cycling to mediate S(-II) oxidation, wherein efficient complexes (e.g., Fe(III)-EDTA) accelerate S(-II) oxidation by up to 76-fold (kobs, S(-II) = 6.3 h-1). Crucially, these efficient Fe(III)-L complexes break the efficiency bottleneck of ROS production, elevating •OH/H2O2 conversion efficiency from ∼2% to nearly 45%. The generated ROS in sediments during oxidation could suppress subsequent microbial sulfur-reducing activity by inducing oxidative damage. 16S rRNA sequencing and qPCR analyses further revealed that ROS exposure could decrease the abundance of sulfur metabolism microbes and associated functional genes. Our findings reveal that ligand-modulated Fe(III)/Fe(II) redox cycling drives abiotic-biotic sulfide dynamics, providing an effective strategy for S(-II) control in engineered environments (e.g., sewers, landfills, and polluted rivers), with broader implications for Fe-driven sulfur biogeochemical cycling.