Synergistic adsorption-photocatalysis processes of graphitic carbon nitrate (g-C3N4) for contaminant removal: Kinetics, models, and mechanisms

The synergy between adsorption and photocatalysis has been well recognized in contaminant photodegradation; however, the governing mechanism is still not clear. The main objective of this work is to understand the kinetic processes of synergic adsorption-photocatalysis of graphitic carbon nitrate (g-C3N4), a visible light responsive photocatalyst with a planar graphitic-like structure, in organic contaminant removal. An anionic dye Reactive Red 120 (RR120), which has six sulphonate groups and a complex aromatic molecular structure, was selected as a model contaminant compound. A range of experiments were conducted to determine the kinetics of adsorption, photodegradation, and the integrated process. Various kinetic models were used to simulate and interpret the experimental data and thus to unveil the governing mechanisms. The integrated adsorption and photodegradation of the dye by g-C3N4 was mainly controlled by: 1) adsorption of dye onto g-C3N4 surface, 2) photodegradation of dye in bulk solution, and 3) photodegradation of adsorbed dye on g-C3N4 surface. Both experimental and modeling results showed that the adsorption kinetic rate (3.37 min(-1)) was faster than the photodegradation kinetic rates. In addition, the surface photodegradation kinetic rate of adsorbed dye (0.149 min(-1)) was faster than that in solution (0.005 min(-1)). Adsorption process thus can promote the photodegradation of contaminants by g-C3N4. On the other hand, photodegradation of dye-laden g-C3N4 regenerated its adsorption capacity for multiple times, suggesting photocatalysis process can also promote the adsorption of contaminates on g-C3N4.