Supplementary material from "Degradation of methylene blue by natural manganese oxides: kinetics and transformation products"

In this study, natural manganese oxides (MnO_x), a potential environmental material with high redox potential, were used as a promising low-cost oxidant to degrade a widely used dyestuff methylene blue (MB) in aqueous solution. Though the surface area of MnO_x was only 7.17 m²g⁻¹, it performed well in the degradation of MB with a removal percentage of 85.6% in pH 4. It was found that MB was chemically degraded in low-pH reaction system and the degradation efficiency was correlated negatively with the pH values (4–8) and initial concentration of MB (10–50 mg l⁻¹), but positively with the dosage of MnO_x (1–5 g l⁻¹). The degradation of MB fitted well with the second-order kinetics. Mathematic models were also built for the correlation of the kinetic constants with the pH value, the initial concentration of MB and the dosage of MnO_x, respectively. Furthermore, several transformation products of MB were identified with HPLC-MS, which was linked with the bond energy theory to reveal that the degradation was initiated with demethylation.