Biochar-Chitosan Composite Microspheres for Non-radical Persulfate Activation: Efficient Azo Dye Degradation and Continuous Flow Application

Conventional approaches to azo dye wastewater treatment face challenges including poor recyclability of powdered biochar (BC), low mechanical strength of chitosan (CS), and limited efficiency of radical-dominated pathways. To address these issues, we developed novel biochar–chitosan composite microspheres (CSBC) via a green sol–gel method. The CSBC catalyst achieved 92.02% methyl orange (MO) removal within 60 min, with a pseudo-first-order kinetic constant of 3.720 h −1 , which is 1.71- and 2.46-fold higher than those of BC and CS alone, respectively. Mechanistic studies, including quenching experiments and electron paramagnetic resonance (EPR) analysis, revealed a dominant non-radical pathway primarily mediated by singlet oxygen ( 1 O 2 ), with superoxide radicals (O 2 ·− ) serving as critical intermediates, while contributions from ·OH and SO 4 ·− were minor. In a continuous-flow fixed-bed system (3 mL min −1 , 0.5 cm bed height), the catalyst maintained 86.78% MO removal efficiency. After five consecutive cycles, it retained 77.80% of its initial activity. Thus, CSBC microspheres represent a promising metal-free, non-radical persulfate activation system for efficient dye wastewater remediation.