Photocatalytic degradation of rhodamine-B by visible light assisted peroxymonosulfate activation using the Z-scheme MIL-100(Fe)/Bi2S3 composite: a combined experimental and theoretical approach

Abstract

The photocatalytic efficiency of the binary MIL-100(Fe)/Bi₂S₃ (MIL-BS) composite was utilized towards visible light assisted peroxymonosulfate (PMS) activation and degradation of Rhodamine-B (RhB) dye. The binary catalyst, with 10 wt% Bi₂S₃ (MIL-BS(10)), showed improved activity relative to that of only MIL-100(Fe), Bi₂S₃ and sole PMS. Under ambient conditions and neutral pH, 0.025 g l⁻¹ MIL-BS(10) and 0.15 g l⁻¹ PMS generated abundant SO₄˙⁻, O₂˙⁻ and OH˙ that mineralize 81.4% RhB in the medium, with complete degradation. Surface-bound Fe²⁺/Fe³⁺ and Bi³⁺/Bi⁴⁺ are involved in local redox interconversions to facilitate charge transport and generation of reactive radicals. A first-principles based modeling approach for photocatalytic oxidation, including pore diffusion-surface reaction effects, was employed to simulate the temporal variation of bulk-averaged RhB concentration in the medium. Also, the degradation kinetics, under the influence of various operational parameters, was successfully predicted using the developed model. The determined mass transfer and kinetic parameters, i.e., mass transfer coefficient, effective diffusivity and kinetic rate constant, are useful for prediction of dye concentration and efficiency of the process. An in-depth RhB degradation pathway was designed based on the intermediate analysis and the critical degradation steps were identified. The catalyst showed good reusability for six consecutive cycles. Overall, the MIL-BS catalyst could be employed as a visible light responsive PMS activator to efficiently mineralize typical persistent organic contaminants, i.e., RhB, in aqueous medium.