Mesoporous MnO₂ polymorphs as sorbent materials for removal of cationic dyes from water

ABSTRACT

Scientists have become interested in treating dye-contaminated water due to the health issues and ecological imbalances caused by the presence of these injurious compounds in water. Herein, two MnO₂ polymorphs i.e. ε- and γ-MnO₂ were prepared by the comproportionation reaction between KMnO₄ and MnCl₂. In comparison to γ-MnO₂, ε-MnO₂ has greater surface area, pore width, and pore volume. According to the thermal study, ε-MnO₂ contains more surplus mass in the form of negatively charged surface hydroxyl groups than γ- phase. The adsorption capability of both polymorphs was examined for the removal of both cationic (methylene blue, malachite green and crystal violet) and anionic dyes (eosin yellow). The efficient adsorption for the cationic dyes using ε-MnO₂ was occurred in the neutral medium, where it had a highly negative surface as determined by zeta potential measurements. It only took few minutes to complete the removal. The electrostatic interactions of the cationic dye molecules with the OH- groups account for this result. While, complete dye removal (40 min) was occurred in the acidic medium using γ-MnO₂. On the other hand, in the neutral medium the adsorption of the anionic dye molecules on the surface of both MnO₂ phases was hampered by the repulsion forces. As a result, despite extending the contact time to 180 min and the adsorbent dose to 50 mg, no anionic dye removal was seen. On the contrary, in the acidic medium very fast removal occurs for Eosin Y as the surface of both MnO₂ phases acquired a positive charge. The influence of the ionic strength on MB elimination was investigated. The results showed that as the content of NaCl was increased, the removal effectiveness decreased from 100% to 35%. The kinetics of cationic dye adsorption on the ε-MnO₂ surface was studied, and it was discovered that the mechanism follows a pseudo-second order model.

KEYWORDS:

• Manganese oxides polymorphs
• porous materials
• dye remediation
• adsorption kinetics

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