Reduction of hexavalent chromium and degradation of tetracycline using a novel indium-doped Mn₂O₃ nanorod photocatalyst

This study investigates the photocatalytic reduction of hexavalent chromium (Cr(VI)) and degradation of tetracycline (TC) via visible-light-active In-doped Mn₂O₃ photocatalysis. Mn₂O₃ photocatalysts loaded with different In doses are prepared using a simple hydrothermal method, and the results indicate the formation of Mn₂O₃ nanorod-like structures with good crystallinity. The most significant photocatalytic parameters, namely the catalyst and substrate concentrations, pH, and co-existing ions for the Cr(VI) reduction and TC degradation reactions are systematically examined. Result demonstrates that the Cr(VI) reduction and TC mineralization efficiencies of 52% and 40%, respectively are achieved at the optimum pH of 7, undoped Mn₂O₃ (10 mg/L), and Cr(VI) or TC concentration of 50 mg/L. However, these efficiencies are remarkably increased to 95% and 93%, respectively, when 10 mg/L of 5% In-doped Mn₂O₃ is used as the photocatalyst under the same reaction conditions. Moreover, the co-existing HCO₃⁻ anions and Ca²⁺ and Mg²⁺ divalent cations considerably deteriorate the performance of the In-doped photocatalysts compared with the SO₄²⁻ and Cl⁻ anions and Na⁺ and K⁺ monovalent cations. Liquid chromatography–mass spectrometry analysis reveals that the photodegradation of TC is mainly driven by the elimination of the −CH₃ group followed by the subsequent cleavage of the primary –NHCH₃ group.