Nanostructured NiCo₂S₄ immobilized on nickel foam as recyclable catalyst for activating peroxymonosulfate toward doxycycline degradation: Performance and mechanism

The existing powder catalysts used in sulfate radical-based advanced oxidation processes (SR-AOPs) commonly suffer from severe limitation due to the difficulty of recycling and inadequate exposure of the active site. In this work, spinel sulfide (NiCo₂S₄) was uniformly grown on 3D nickel foam (NF) through a one-step hydrothermal route for the activation of peroxymonosulfate (PMS) toward highly efficient degradation of doxycycline (DOX). Due to the synergistic effect of Ni and Co in the NiCo₂S₄, the NiCo₂S₄/NF (0.6 g/L) combined with PMS (3.0 mmol L^-1) can promote a nearly complete degradation (98.2 %) toward 0.3 mmol/L DOX within 6 min. Density functional theory (DFT) calculations illustrated that the NiCo₂S₄/NF can adsorb PMS through a unique Ni-S-Co structure and “conducting bridge” mode of NF, accelerating the electron transfer from NiCo₂S₄/NF to PMS and the subsequent O-O bond cleavage. Especially, the Co/Ni dual-site enhanced the adsorption of PMS and promotes electron transfer to form surface-activated PMS complex (catalyst-PMS*), leading to the generation of surface-bound reactive oxygen species (ROS) on the NiCo₂S₄ surface. Quenching experiments and ESR tests demonstrated that radicals such as SO₄^•-, ¹O₂, ^•OH and surface-bound ROS play a vital role in DOX degradation. Moreover, the potential mechanism and pathway of DOX degradation are reasonably deduced by investigating and analyzing the intermediates with liquid chromatography-mass spectromsetry (LC-MS). Consequently, this study provides a deep insight on the systhsis of spinel sulfide and its intrinsic mechanism of PMS activation toward organic contaminant degradation.