Concentrated sulfuric acid-assisted synthesis of tubular g-C3N4 for photoactivating peroxymonosulfate and the synergistic degradation mechanism of rhodamine B.

Graphitic carbon nitride (g-C3N4) has attracted considerable attention in photocatalytic peroxymonosulfate (PMS) activation for organic pollutant degradation owing to its metal-free nature, structural stability, and visible-light response. However, conventional bulk g-C3N4 still suffers from a low specific surface area, limited active sites, and severe recombination of photogenerated charge carriers, which restrict its photocatalytic activity and PMS activation efficiency. In this work, a series of tubular g-C3N4 photocatalysts (TCN) were prepared using melamine as the precursor via a concentrated sulfuric acid-assisted hydrothermal treatment followed by thermal polymerization. The results showed that an appropriate amount of concentrated sulfuric acid could regulate the precursor condensation process, promote the formation of a tubular porous structure, and improve the local chemical environment and photoelectrochemical properties of the obtained materials. Among the prepared samples, TCN-3 exhibited the best photocatalytic PMS activation performance, achieving nearly complete degradation of RhB within 15 min under simulated sunlight irradiation, with an apparent reaction rate constant of 0.42477 min-1, approximately 15.3 times higher than that of pristine MCN. Structural and photoelectrochemical characterizations revealed that TCN-3 possessed a larger specific surface area, a narrower band gap, suppressed recombination of photogenerated charge carriers, and enhanced interfacial charge transfer ability. Reactive species trapping experiments and ESR analysis demonstrated that 1O2 and ˙O2 - were the dominant reactive species responsible for RhB degradation in the TCN-3/light/PMS system, while h+, ˙SO4 -, and ˙OH also participated in the reaction process. In addition, the system maintained good degradation performance over a wide pH range (3-9), exhibited satisfactory tolerance toward most coexisting anions, and showed good cycling stability. This work provides a useful strategy for constructing efficient metal-free g-C3N4-based photocatalysts and developing photocatalytic PMS activation systems for advanced oxidation processes.