The iron-based biochar activating chlorite (ClO₂⁻) driven by mechanochemical ultrasonic: piecewise kinetics, biomimetic catalytic-mechanism, and novel advanced redox process

Chlorite (ClO₂⁻ or COI) is used to establish the advanced reduction and oxidation process (AROP). The iron/biochar-based particles (iron-based hydrothermal carbon with hinge-like structure, Fe_bHCs, 20 mg/L) can be utilized to activate COI (2 mmol/L) to present selective oxidation in removing triphenylmethane derivatives (15 min, 90%). The protonation (H⁺ at ∼10² µmol/L level) played a huge role (k-2^nd = 0.136c-H⁺ - 0.014 (R²-adj = 0.986), and r_app = − 0.0876/c-H⁺ + 1.017 (R²-adj = 0.996)) to boost the generation of the active species (e.g., high-valent iron oxidizing species (HVI=O) and chlorine dioxide (ClO₂)). The protonation-coupled electron transfer promoted Fe-substances in Fe_b/HCs activating COI (the calculated k_obs ranging from 0.066−0.285 min⁻¹). The form of ClO₂ mainly attributed to proton-coupled electron transfer (1e/1H⁺). The HVI=O was generated from the electron transfer within the coordination complex. Moreover, carbon particles in Fe_bHCs serve as the bridge for electron transfer. The above roles contribute to the fracture and formation of coordination-induced bonds between L_x-Fe^II/III and ClO₂⁻ at phase interface to form AROP. The ultrasonic (US) cavitation enhanced the mass transfer of active species in bulk solution, and the HVI=O and ClO₂ attack unsaturated central carbon atoms of triphenylmethane derivatives to initiate selective removal. Furthermore, the scale-up experiment with continuous flow (k values of approximately 0.2 min⁻¹, COD removal efficiency of approximately 80%) and the reactor with COMSOL simulation have also proved the applicability of the system. The study offers a novel AROP and new insights into correspondingly heterogeneous interface activation mechanisms.