Activation of persulfate by graphene/biochar composites for phenol degradation: Performance and nonradical dominated reaction mechanism

Recently, metal-free carbon catalysts have attracted extensive attention in sulfate radical (SO4 center dot-)-based advanced oxidation processes (SR-AOPs). In this paper, graphene/biochar composites (GBCs) were prepared by blending two-dimensional graphene and bulk biomass followed by pyrolysis, and were employed to activate persulfate (PS) to degrade phenol. The effects of different factors (e.g., graphene/biomass mass ratio, GBC dosing, phenol concentration, pH values, reaction temperature, and background substrates in water) on the removal of phenol and the mechanism in the GBC/PS system were investigated. The results showed that 100% removal of phenol can be achieved in the GBC0.6 (i.e., the mass ratio of 0.6%) /PS system within 30 min (k(obs) = 0.1634 min(-1)). GBC0.6/PS system also demonstrated various advantages of adapting to a broad pH range (3 similar to 9), the lower activation energy of 12.13 kJ/mol, strong resistance to inorganic ions and natural organic matters, and higher tolerance to the background water matrices. The adsorption of phenol in the GBC0.6/PS system was related to the pi-pi* EDA interaction of phenol with GBC0.6, while the degradation mainly relied on the nonradical pathway (singlet oxygen (O-1(2)) and electron transfer) and secondarily on the radical pathway, the role of aqueous SO4 center dot- and hydroxyl radical (center dot OH) was negligible. Meanwhile, spectroscopic characterizations confirmed that the porous structure, defects, multilayer graphite structure, and carboxyl group (-COOH) of GBC0.6 were also involved in the removal of phenol. This study provides a new idea for the removal of micropollutants from the water environment by a novel green and environment-friendly metal-free carbon catalyst via persulfate activation.