Graphite felt incorporated with MoS₂/rGO for electrochemical detoxification of high-arsenic fly ash

1. Introduction – Accumulation of high-arsenic fly ash (HAFA) poses a serious environmental threat due to the toxicity. As(III) oxidation is a necessary step for HAFA detoxification and in situ H₂O₂ electrogeneration via the two-electron-pathway (2e^-) oxygen reduction reaction (ORR) is considered a promising technique for As(III) oxidation [1, 2]. MoS₂ has been reported to have attractive 2e^- ORR catalytic activity [3]. However, there are still some problems to be solved: (i) its conductivity needs to be improved, (ii) gaseous O₂ is difficult to be effectively utilized, and (iii) the assembled electrode is not flexible and durable enough to treat solid waste.
2. Experimental - the experimental flow is schematically illustrated in Image. 1. 
3. Results and Discussion - A novel graphite felt (GF) modified with the MoS₂/reduced graphene oxide (rGO) heterojunction is developed and demonstrated as an efficient electrode for electrochemical detoxification of HAFA, as shown in Image 2(a). The modified GF has abundant microchannels (Image 2(b)), desirable surface properties, and fast electron transfer capability thereby enabling effective utilization of both dissolved O₂ and gaseous O₂ in 2e^- ORR. The p-n junction consists of the p-type rGO substrate and uniformly distributed n-type MoS₂ nanoflowers, seeing Images 2(c) and 2(d). Theoretical calculation indicates that gaseous O₂ adsorb stably on sulfur vacancies and is reduced by electrons transferred from rGO. The modified GF exhibits superior ORR catalytic activity such as a high H₂O₂ yield of 47.53 mg·L^-1·h¹·cm^-1 (Image 2(e)) and onset potential of -0.12 V vs. SCE. The ·OH generated by the autocatalytic mechanism promotes oxidative dissolution of As(III) during detoxification of HAFA and the As removal percentage is 96.10% after electrolysis for 135 min, displayed in Images 2(f) and 2(g). The modified GF with excellent stability and durability has large industrial potential in detoxification of HAFA and other As-bearing hazardous wastes. 
4. Conclusions - This study makes significant contributions. Firstly, the gaseous O₂ reaction path in ORR is confirmed by the combination of experiment and calculation. Secondly, 2e ORR is innovatively applied to detoxification of high-arsenic solid waste.