Photocatalytic decomposition of methylene blue by persulfate-assisted Ag/Mn₃O₄ and Ag/Mn₃O₄/graphene composites and the inhibition effect of inorganic ions

Persulfate- (PS-) assisted photocatalytic degradation of Methylene Blue (MB) in the hydrothermal synthesis of Ag/Mn₃O₄ and Ag/Mn₃O₄/graphene composite catalysts under visible-light irradiation was highlighted in this study. The influence of graphene on the photocatalytic performance was evaluated by comparing the activity on both composites. Based on the results, the photocatalytic performance was enhanced by graphene. Specifically, the optimal photocatalytic activity was observed for Ag-to-Mn₃O₄ molar ratio of 1 : 0.5 in Ag/Mn₃O₄ composites, as well as in Ag/Mn₃O₄/graphene composites with 5 wt.% graphene concentration. Moreover, the results indicated a significant improvement in the photocatalytic performance of both composites with the addition of persulfate (PS) under visible-light irradiation. Accordingly, for PS concentrations of up to 12 mM, the initial MB degradation rate constant increased from 0.0129 to 0.0337 min⁻¹ and from 0.0414 to 0.1627 min⁻¹ in 2 h for Ag/Mn₃O₄ and Ag/Mn₃O₄/graphene composites, respectively. Furthermore, the influence of inorganic ions, including Cl⁻, SO₄²⁻, NO₃⁻, H₂PO₄⁻, CO₃⁻, K⁺, Ca²⁺, and Mg²⁺, which are usually present in industrial wastewaters, was likewise investigated. The inhibition effect for MB degradation by these ions was in the order H₂PO₄⁻ > CO₃²⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻ for the anions and Ca²⁺ > Mg²⁺ > K⁺ for the cations.