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

The sulfur doping has been considered as 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 in situ prepared by 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 27.6 mg L-1 initial MB concentration, 4.1 g L-1 catalyst dosage, solution pH 8.7, and 125 min reaction time. 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 the key foundation for the cost control and efficiency improvement of the scale-up application of g-C3N4-based photocatalyst.