Enhanced degradation of DDT using a novel iron-assisted hydrochar catalyst combined with peroxymonosulfate: Experiment and mechanism analysis

Comprehensive Study on the Adsorption and Degradation of Dichlorodiphenyltrichloroethane on Bifunctional Adsorption-Photocatalysis Material TiO2/MCM-41 Using Quantum Chemical Methods

The adsorption and degradation capacities of dichlorodiphenyltrichloroethane (DDT) on a photocatalyst composed of TiO2 supported on the mesoporous material MCM-41 (TiO2/MCM-41) were investigated using density functional theory and real-time density functional theory methods. The van der Waals interactions within the PBE functional were adjusted by using the Grimme approach. The adsorption of DDT was evaluated through analyses involving adsorption energy, Hirshfeld atomic charges, Wiberg bond orders, molecular electrostatic potential, noncovalent interaction analysis, and bond path analysis. The findings reveal that DDT undergoes physical adsorption on pristine MCM-41 or MCM-41 modified with Al or Fe due to the very small bond order (only about 0.15-0.18) as well as the change in total charge of DDT after adsorption is close to 0. However, it chemically adsorbs onto the TiO2/MCM-41 composite through the formation of Ti···Cl coordination bonds because the maximum bond order is very large, about 1.0 (it can be considered as a single bond). The adsorption process is significantly influenced by van der Waals interactions (accounting for approximately 30-40% of the interaction energy), hydrogen bonding, and halogen bonding. MCM-41 is demonstrated to concurrently function as a support for the TiO2 photocatalyst, creating a synergistic effect that enhances the photocatalytic activity of TiO2. Based on the computational results, a novel photocatalytic mechanism for the degradation of DDT on the TiO2/MCM-41 catalyst system was proposed.

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-CO groups also played a key role in the activation process, but the dominant action sites were not the same. Multiple free radicals, such as SO4•-, •OH, O2•-, and 1O2 were generated in PMS and PDS activation systems. 1O2 induced non-free radical pathway was mainly involved in the degradation of TC in both activation systems, but the generation pathway of 1O2 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.