Abstract
In this study, natural manganese oxides (MnOx), an environmental material with high redox potential, were used as a promising low-cost oxidant to degrade the widely used dyestuff methylene blue (MB) in aqueous solution. Although the surface area of MnOx was only 7.17 m2 g−1, it performed well in the degradation of MB with a removal percentage of 85.6% at pH 4. It was found that MB was chemically degraded in a low-pH reaction system and the degradation efficiency correlated negatively with the pH value (4–8) and initial concentration of MB (10–50 mg l−1), but positively with the dosage of MnOx (1–5 g l−1). The degradation of MB fitted well with the second-order kinetics. Mathematical models were also built for the correlation of the kinetic constants with the pH value, the initial concentration of MB and the dosage of MnOx. 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.
Highlights
Nowadays, a great deal of wastewater from industrial production is discharged into the natural environment, leading to a certain degree of contamination [1]
The crystalline structure of manganese oxides (MnOx) was characterized by X-ray diffraction (XRD)
According to the XRD patterns shown in figure 1a, the diffraction peaks (2u) of 12.748, 37.638, 49.908, 56.188, 60.248, 65.528 and 73.078 correspond, respectively, to the (1 1 0), (3 1 0), (3 0 1), (4 1 1), (6 0 0), (5 2 1) and (4 5 1) crystal planes of a-MnO2 (JCPDS card no. 72-1982) [39]
Summary
A great deal of wastewater from industrial production is discharged into the natural environment, leading to a certain degree of contamination [1]. It has been estimated that 10–15% of dyestuff in the dyeing process is drained into natural water via sewage plants [3]. There is an urgent need to find a method with high efficiency, economical feasibility and ease of operation to degrade dyestuff wastewater. It has been reported that manganese dioxide possesses high oxidation–reduction potential (1.29 V, 258C) and that synthetic MnO2 has been applied to degrade phenol and aromatic amines [26,27]. The high cost and complex synthetic steps of synthesized MnO2 have limited its application in wastewater treatment. Studies have demonstrated the good degradation efficiency of organic contaminants by natural manganese oxides (MnOx), such as diclofenac [34], melanin [35] and paracetamolin [36] and emerging organic contaminants [37]. A degradation pathway is proposed with the transformation products identified with high-performance liquid chromatography–mass spectrometry (HPLC-MS)
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