Photocatalytic degradation of dyes using phosphorus-containing activated carbons

Highlights: • P-containing activated carbon is microporous with surface pyrophosphate and carboxyl groups. • The successive introduction of phosphorus, Cu and Co cations results in narrowing the bandgap. • Joint introduction of phosphorus and oxygen improves the photocatalytic activity in visible light. • Cation-containing carbons are the most active in photodegradation of dyes under visible light. • Activated carbon granules can be used at least 3 cycles in photocatalysis. Phosphorus-containing carbon was obtained by modification of coconut shell-based activated carbon with phosphoric acid at 800 °C. It has been shown that the introduction of phosphorus alters the surface chemistry of activated carbon. Phosphorus in the structure of carbon exists in the form of pyrophosphates, which gives the carbon surface an acidic character and the ability to adsorb metal cations. Moreover, the introduction of phosphorus leads to a narrowing of the bandgap from 4.1 eV to 3.2 eV and an even greater narrowing to 2.8–2.9 eV after adsorption of copper and cobalt. This change pre-determines the photocatalytic activity of phosphorus-containing activated carbon in rhodamine B and safranin T photodegradation under visible light irradiation. Phosphorus-containing carbons with adsorbed Cu²⁺ and Co²⁺ cations are more active in comparison with the same phosphorus-free carbons even though their specific surface area is 37% lower, and the photocatalytic testing was conducted in the form of granules with the size of 0.6 mm. The latter makes it easy to separate the photocatalyst from the liquid phase and reuse three times without loss of activity. The main reason for the increase in photocatalytic activity is narrowing the bandgap and increasing the adsorption of dyes at carbon surface.