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•−)-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 (kobs = 0.1634 min−1). GBC0.6/PS system also demonstrated various advantages of adapting to a broad pH range (3 ∼ 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 π-π * EDA interaction of phenol with GBC0.6, while the degradation mainly relied on the nonradical pathway (singlet oxygen (1O2) and electron transfer) and secondarily on the radical pathway, the role of aqueous SO4•− and hydroxyl radical (•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.