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

Highlights

	● A novel process of sonocatalysis/iron-carbon particles activating ClO2− was provided.
	● Activation mechanism of ClO2− attributed to the protonation coupled electron transfer.
	● Dominant active species involved high-valent iron oxidizing species and ClO2.
	● Ultrasonic played the roles of “carrying” and “driving” forces.

Abstract

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.

Graphical abstract

Keywords

Chlorite / Iron-based biochar / Ultrasonic / Chlorine dioxide / High-valent iron oxidizing species

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Acknowledgements

This work was supported by the Natural Science foundation of the Jiangsu Higher Education Institutions (No. 24KJB610019), the Scientific Research Foundation of Yancheng Teachers University (No. 204060047), and the Horizontal Topic Research “Developing the environmental remediation technology by acoustic catalysis” (No. 203060199).

Conflict of Interests

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-025-1944-4 and is accessible for authorized users.

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