Nitrogen-sulfur co-doped industrial graphene as an efficient peroxymonosulfate activator: Singlet oxygen-dominated catalytic degradation of organic contaminants

Abstract This paper presents for the first time the doping of nitrogen (N) and sulfur (S) into industrial reduced graphene oxide (i-rGO) to synthesize the catalyst, named as i-rGO-NS, to activate peroxymonosulfate (PMS). Co-doping of N and S into the catalyst was investigated by many surface techniques. The i-rGO-NS catalyst had the higher content of graphitic N (˜34%) than only N-doped rGO (i.e., i-rGO-N). The i-rGO-NS showed high activation of PMS for catalyzing oxidation of target contaminant, methyl paraben (MP), an endocrine disruptor in water under various conditions (reaction temperature, catalyst loading and PMS dosage). In comparison with the conventional GO and its N-doped or N/S-co-doped composites and classical metal catalysts (e.g., Co3O4 and Fe3O4), i-rGO-NS has superior effectiveness to activate PMS to degrade contaminants even under the conditions of less dosage of the catalyst (i-rGO-NS) and oxidant (PMS). Results suggest that the N doping, and especially additional S doping were of pivotal in enhancing catalytic performance. Transformation pathways of MP in the i-rGO-NS/PMS system were tentatively proposed that agree with the identified intermediates and frontier electron density calculations. Quenching tests and electron paramagnetic resonance (EPR) studies showed that the singlet oxygen (1O2) was the main reactive oxygen species, revealing that MP degradation follows predominantly the nonradical oxidation pathway. The i-rGO-NS/PMS system not only exhibited considerable removal efficiency of MP in real waters, but also showed the rapid degradation of other pollutants (e.g., UV filter benzophenone-4 (BP-4) and phenol) in water. The newly developed nonradical i-rGO-NS/PMS process is highly effective in decontamination of water.