Design of highly stable and scalable three-dimensional alginate-encapsulated cobalt(III) oxide-graphitic carbon nitride-bismuth(III) oxide beads for peroxymonosulfate photoactivation toward antibiotic oxidation in a real water matrix.

Peroxymonosulfate (PMS)-based advanced oxidation processes are often limited by unstable activation and reliance on continuous mechanical stirring to sustain high photocatalytic performance during pollutant degradation. Herein, a ternary Co3O4-graphitic carbon nitride (GCN)-Bi2O3 dual S-scheme (CoCNBi) was successfully immobilized within calcium alginate (Alg) hydrogel beads (CoCNBi@Alg) via a 3D cross-linking strategy. The biopolymer structure aids dispersion/recovery; despite low BET surface area (3.9 m2/g), enhanced electroactive area and charge-transfer supported efficient degradation. CoCNBi@Alg beads showed excellent photo-electrochemical performance, achieving substantial mineralization of Sulfamethoxazole (SMX) (up to 76.3% total organic carbon (TOC) removal) under simulated sunlight without external stirring, outperforming bare GCN, Bi2O3, Co3O4, and non-immobilized CoCNBi powder. Additionally, CoCNBi@Alg demonstrated a strong performance under real-water matrices (>98% SMX degradation) and maintained ∼91% efficiency after 10 cycles, with minimal metal leaching (Co: 0.05 mg/L; Bi: 0.03 mg/L). Mechanistically, the CoCNBi dual S-scheme heterojunction, coupled with Co/Bi redox cycling, facilitates effective e-/h+ separation and transfer, thereby enhancing PMS activation. In this framework, the Alg matrix acts as a 3D stabilizing scaffold, ensuring better interfacial contact and streamlining mass transfer, thereby triggering synergistic non-radical (1O2) and radical (SO4•-, •OH, O2•-) oxidative species. Liquid chromatography-mass spectrometry (LC-MS) data reveal a complex degradation process with multiple routes, including the cleavage of the sulfonamide bond, hydroxylation, and ring opening, while the reduced toxicity of intermediates was confirmed by ECOSAR predictions. This study demonstrates that the encapsulation of dual S-scheme structures within Alg provides a dependable pathway for complete mineralization.