Molten salt synthesis of porous graphene-like carbons as peroxydisulfate catalyst for the efficient removal of rhodamine B dye.

Carbon materials have been receiving considerable attention as effective green catalysts for peroxydisulfate (PDS) activation to degrade organic pollutants. Herein, the porous graphene-like carbons (PGCs) were synthesized by pyrolyzing a nitrogen-rich biomass (peanut shell, PS) in the eutectic mixture of FeCl3 and ZnCl2. The results suggested that involvement of molten salts attributed the biochar the amazing properties such as high specific surface area (SBET = 2529.4 m2 g-1), abundant structural defects, high nitrogen content (6.5%), and oxygen-containing functional groups on its surface. Especially when pyrolyzed at activation temperature of 800 °C, mass ratio of 1:3:15 (PS:ZnCl2:FeCl3), and activation time of 2 h, the optimized PGCs-op exhibited outstanding performance in the catalytic degradation of rhodamine B (RhB). Almost all of RhB (99.02%) was removed in 40 min and basically not influenced by initial pH in the range of 3.00 to 9.98. Although the RhB degradation was influenced by anions (Cl-, HCO3-, HPO42-), the inhibition would be significantly alleviated within 120 min unless these substances were high in concentration. Furthermore, the quenching tests revealed that the reactive species were involved in RhB degradation in the sequence of 1O2 > O2∙-  > SO4∙-  > ∙OH, among which singlet oxygen played a crucial role. Combined with characterization analysis, a possible mechanism of RhB degradation in PGCs-op/PDS system was proposed. Overall, this study provided a promising metal-free catalyst for the removal of organic pollutants while achieving reutilization of the waste biomass.