The activation mechanism of peroxymonosulfate and peroxydisulfate by modified hydrochar: Based on the multiple active sites formed by N and Fe.

Modified hydrochar (NHC@Fe), with multiple functional groups and transition metal oxide-containing surface, was successfully synthesized by one-step hydrothermal method. The differences in its catalytic activity for peroxymonosulfate (PMS) and peroxydisulfate (PDS) activation were studied in detail. Experimental and DFT studies showed that abundant active sites, namely, transition metals and functional groups on NHC@Fe provided multiple effective pathways for the activation of persulfate (PS). The NHC@Fe/PMS and NHC@Fe/PDS systems could degrade about 80% of tetracycline hydrochloride (TC) in 120 min and were found to be better than those modified by iron or nitrogen alone. This emphasized the advantage of N–Fe co-modification in persulfate activation. Although the Fe²⁺/Fe³⁺ cycle accelerated the activation, the activation of PMS mainly relied on Fe³⁺, whereas that of PDS mainly relied on Fe²⁺. Moreover, Fe–N, pyrrolic N, pyridine N, C–O, and O–C O groups also played a key role in the activation process, but the dominant action sites were not the same. Multiple free radicals, such as SO 4 ^•—, ^•OH, O 2 ^•—, and ¹O 2 were generated in PMS and PDS activation systems. ¹O 2 induced non-free radical pathway was mainly involved in the degradation of TC in both activation systems, but the generation pathway of ¹O 2 was more direct and rapid in the PDS system. This study provides detailed DFT models of the active sites activated by PMS and PDS and discusses the activation pathways of PMS and PDS along with the similarities and differences in ROS reaction mechanisms. [Display omitted] • The induction of multiple active sites in hydrochar by N and Fe has been elucidated. • Enhanced activation of PMS and PDS on multiple active sites was explored. • Adsorption and electron transfer to PMS and PDS of each site were contrasted. • Different mechanisms were found for generating ¹O 2 in PMS and PDS activation. [ABSTRACT FROM AUTHOR]