Fe(III)-sulfonate complexes as autocatalytic engines: enabling long-lasting peroxydisulfate activation for refractory sulfonic azo compounds elimination.

Conventional iron-based peroxydisulfate (PDS) advanced oxidation processes often suffer from sluggish Fe(III)/Fe(II) cycling and iron sludge deposition. This study proposed a novel self-sustaining PDS activation strategy via pollutant triggered autocatalysis to overcome these limitations. Reactive Black 5 (RB5), a representative sulfonic azo contaminant, was employed as a model compound to elucidate the underlying autocatalytic mechanism. RB5 served dual functions as both an electron donor and a ligand, facilitating the reduction of Fe(III) and forming Fe(III)-complexes including Fe(III)-sulfonate and a hexacyclic naphthol-azo-Fe(III) complex. Then the generated Fe(II) and Fe(III)-sulfonate complex served as critical sites for PDS activation to generate reactive oxygen species (ROS) such as SO4•-, O2•-, and [FeIVO]2+. Meanwhile, the d-π conjugation in the naphthol-Fe(III)-azo hexacyclic complex directed O2•- to selectively cleave the -N=N- bond. The RB5 triggered PDS activation resulted in 90 % RB5 itself being removed, exhibiting autocatalysis performance. Notably, intermediates such as quinone from RB5 degradation as a "secondary engine" accelerated Fe(III)/Fe(II) cycling, sustaining ROS regeneration and long-lasting oxidative capacity. Further investigation demonstrated that the Fe(III)/PDS system can selectively autocatalyze sulfonic azo pollutants, particularly those containing a higher number of sulfonate groups. This study presents a promising pre-treatment strategy for the disposal of refractory dyeing wastewater.