Strongly coupled Bi₂S₃/CoTiO₃ heterostructure with alginate encapsulated hydrogel for boosting peroxymonosulfate activation: Reaction sites and activation mechanism

Heterostructure-based catalysts have garnered significant attention for peroxymonosulfate (PMS) activation to degrade refractory organic pollutants. However, their broader application is greatly hindered by metal ion leakage and the poor recoverability of powder materials. In this study, a Bi₂S₃/CoTiO₃ heterostructure was synthesized using a simple hydrothermal strategy and encapsulated within an alginate biopolymer through ionotropic gelation to form alginate/Bi₂S₃/CoTiO₃ (Alg/BC) hydrogels. The alginate serves as a potential backbone of the crystal structure via hydrogen bond formation, provides a porous scaffold, and acts as a stabilizing carrier. The resultant Alg/BC system demonstrated exceptional and steady PMS activation for sulfamethoxazole (SMX) degradation. Under optimal conditions, 97.2% (k = 0.0552 min⁻¹) of SMX was degraded within 60 min, achieving over 90% removal efficiency after six sequential cycles. The dynamic change of Co²⁺/Co³⁺ redox cycle triggered PMS activation for generating reactive oxidative species (ROS), thereby accelerating SMX degradation. Radical trapping tests and electron paramagnetic resonance analysis elucidated the ROS production mechanism and its contribution to SMX removal. Additionally, the reaction sites and degradation routes of SMX were deduced based on the density functional theory. This study offers new insights into the fabrication of heterostructure-based 3D hydrogels for PMS activation in sustainable wastewater treatment.