Efficient Adsorption of Azo Dye Acid Brilliant Red on Graphite Carbon Nitride in Aqueous Solution

In this study, two types of graphite carbon nitrides were prepared by directly calcinating urea (U-g-C₃N₄) and melamine (M-g-C₃N₄) in a muffle furnace. Their adsorption performances on acid brilliant red (ABR) from aqueous solution were examined and compared. Results showed that, at the optimum calcination temperature of 580 °C, both the adsorption capacity of U-g-C₃N₄ and that of M-g-C₃N₄ increased strongly with decreasing solution pH. U-g-C₃N₄ exhibits higher adsorption capacity than M-g-C₃N₄ at an initial pH > 2.0. However, at an initial pH of 1.0, M-g-C₃N₄ displayed a much higher adsorption capacity than U-g-C₃N₄, where the maximum adsorption capacity of M-g-C₃N₄ can reach 25 635.64 mg g^-1, being the highest reported to date. Adsorptions of both adsorbents followed pseudo-second-order kinetic models and the Langmuir adsorption isothermal models. The adsorption is spontaneous and exothermic and occurs mainly through electrostatic attraction between the protonated g-C₃N₄ and the negatively charged ABR. In addition, the used U-g-C₃N₄ can be easily regenerated with ethanol and the renewed U-g-C₃N₄ possesses comparable adsorption capability of its original form, showing its superior recyclability and broad industrial application prospects.