Process optimization and economic calculation of photocatalytic degradation of methylene blue over sulfur-doped g-C3N4 Scientific paper

Abstract

Sulfur doping has been considered an effective strategy to enhance the photocatalytic activity of graphitic carbon nitride (g-C₃N₄). However, there are no reports on the process optimization and economic calculation to determine the optimal photocatalytic parameters and compare the operating costs, respect­ively. In this work, sulfur-doped g-C₃N₄ (S/g-C₃N₄) was prepared in situ by a one-pot pyrolysis approach using thiourea as the precursor and sulfur source. S/g-C₃N₄ was characterized by various instrumental techniques. Response sur­face methodology (RSM) was employed to determine the optimal photocatalytic conditions for the degradation of Methylene blue (MB) over S/g-C₃N₄. A central composite design (CCD) with four factors at five levels was established, com­prising 30 experimental runs. The optimal conditions were determined as an ini­tial MB concentration of 27.6 mg L^-1, a catalyst dosage of 4.1 g L^-1, a solution pH of 8.7 and a reaction time of 127 min. Under these conditions, the predicted degradation efficiency could reach 99.94 %, while the experimental value was 99.5 %. Cost accounting indicated that the optimization substantially reduced direct costs, greatly improved equipment utilization, and enhanced operational flexibility. This work will lay key foundation for cost control and efficiency improvement in the scale-up application of g-C₃N₄-based photocatalysts.