Decolorization of RhB dye by manganese oxides: effect of crystal type and solution pH

Organic dye pollution in water has become a major source of environmental pollution. Mn(III/IV) oxides have attracted a great deal of attention to remove organic dye pollutants due to their unique structures and physicochemical properties. Numerous studies have reported the removal of dye by various Mn(III/IV) oxides through catalytic degradation and adsorption. The crystalline structures of manganese oxides and solution pH may exert substantial impact on the removal of dyes. However, few studies have focused on the oxidative degradation of RhB dye using Mn(III/IV) oxides with different crystal structures during a spontaneous reaction. In the present study, three manganese oxides with different crystal type (α-MnO₂, β-MnO₂, and δ-MnO₂) were prepared by refluxing process to decolorize RhB dye in various pH solutions.

The results showed that the decolorization efficiencies of RhB for the three manganese oxides all increase with decrease solution pH. α-MnO₂ exhibited highest activity and could efficiently degrade RhB at pH 2–6. The degradation of RhB by β-MnO₂ and δ-MnO₂ could be observed at pH 2–3, and only little adsorption RhB on manganese oxides could be found at pH 4–6. The UPLC/MS analysis suggests that the decolorization of RhB by manganese oxides consists of three main stages: (1) cleavage of the ethyl groups from RhB molecular to form Rh; (2) further destruction of –COOH and –CNH₂ from Rh to form the small molecular substances; (3) mineralization of the small molecular substances into CO₂, H₂O, NO₃ ^? and NH₄ ⁺.

Overall, these results indicate that α-MnO₂ may be envisaged as efficient oxidants for the treatment of organic dye-containing wastewater under acid conditions.