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• A MoS 2 /rGO heterojunction is introduced to a graphite felt electrode. • The electrode utilizes both dissolved O 2 and gaseous O 2 in 2e− ORR. • The MoS 2 /rGO heterojunction is a nanoscale p-n junction. • Gaseous O 2 adsorbing onto sulfur vacancies is reduced by electrons from rGO. • OH generated by autocatalysis of HAFA promotes As(III) and Cr(III)oxidization. Accumulation of high-arsenic fly ash (HAFA) poses a serious environmental threat due to the toxicity of As and release of other heavy metals especially Cr. In this work, a novel graphite felt (GF) cathode modified with the nanoscale MoS 2 /reduced graphene oxide (rGO) heterojunction is prepared by blending with PTFE emulsion for efficient synergistic oxidative dissolution of As(III) and Cr(III) in HAFA. By taking advantage of the p-n junction characteristics of the heterojunction and appropriate hydrophobicity of the PTFE coating, the modified GF efficiently utilizes both dissolved O 2 and gaseous O 2 in the 2e− oxygen reduction reaction (ORR). Our theoretical assessment indicates that gaseous O 2 adsorbs stably on sulfur vacancies and is reduced by electrons transmitted from rGO. Experimentally, the modified GF shows superior ORR catalytic activity as exemplified by a high peak current density of 8.41 mA cm−2 and onset potential of 0.53 V vs. RHE. OH generated by the Cr and Fe-triggered autocatalysis mechanism promotes oxidization of As(III) and Cr(III) in detoxification of HAFA resulting in 96.1% As removal as well as 70.74% Cr removal in 135 min. The modified GF with excellent stability and durability has immense industrial prospect in detoxification of HAFA and treatment of other types of As-containing hazardous wastes. [ABSTRACT FROM AUTHOR]