A unique Si-doped carbon nanocatalyst for peroxymonosulfate (PMS) activation: insights into the singlet oxygen generation mechanism and the abnormal salt effect

Abstract

The heteroatom-doped, carbon-activated peroxymonosulfate (PMS) system proceeding via the non-radical oxidation pathway involving singlet oxygen (¹O₂) represents a promising advanced oxidation process (AOP) due to the resistance of ¹O₂ to background anions in solution. However, the performance level of pollutant removal is not yet satisfied, and the mechanism of ¹O₂ generation remains elusive. Herein, we report the development for the first time of a unique Si-engaged heteroatom (i.e., N)-doped carbon (Si/N–C) nanomaterial for PMS activation, which exhibits outstanding performance in the removal of various kinds of pollutants and an appreciably wide working pH range of 2.5 to 11. Taking rhodamine B (RhB) as an example, the Si/N–C@PMS system achieved a much higher reaction rate (0.64 min⁻¹) than the Si–C (0.047 min⁻¹), N–C (0.017 min⁻¹), SiO₂ (0.021 min⁻¹), and Co₃O₄ (0.026 min⁻¹; a standard catalyst) systems. In addition, the results of electron spin resonance (ESR) and scavenging experiments solidly support the conclusion that the mechanism of ¹O₂ generation in this system is via the recombination of superoxide radicals O₂˙⁻, a pathway seldom reported for metal-free, catalyst-activated PMS systems. More importantly, we observed apparent inhibition effects of the anions (e.g., HCO₃⁻, HPO₄²⁻, and PO₄³⁻) on the degradation performance, inconsistent with many previous reports suggesting that the ¹O₂-mediated oxidative system possesses a high selectivity toward pollutant degradation in the presence of these anions. Further studies with the anions, including Cl⁻, HCO₃⁻, and H_xPO₄^(3−x) (x = 0, 1, 2), suggest that the reaction between O₂˙⁻ and these anions is the main reason for the abnormal salt effect. This work will deepen the understanding of ¹O₂ formation via the intermediate of O₂˙⁻ and provide a new insight into its salt resistance capacity.