The adsorption activity and mechanism of common tourmalines for typical anionic and cationic dyes

The adsorption activity and mechanism of four common kinds of tourmalines (magnesium-based tourmaline: µm-Mg TM and nm-Mg-TM; iron-based tourmaline: Low-Fe TM and High-Fe TM) for typical cationic dyes (methylene blue: MB and Rhodamine B: RhB) as well as anionic dye (methyl orange: MO) in aqueous solution were investigated. It was found that all of the tourmalines are negatively charged in aqueous solution due to their dissolution of Fe or Mg ions. Accordingly, four types of tourmalines displayed obvious electrostatic adsorption for cationic dyes like MB and RhB, but very slight adsorption activity for MO (anionic dye). Since the cationic group of RhB is too large to be absorbed into the ring structure of tourmalines, weaker RhB absorption activity was observed on the tourmalines, relative to for the absorption of MB. Through thermal treating at 350 °C, the absorbed MB on/in high-Fe TM and nm-Mg TM can be well desorbed.