Characterization of anion–cationic surfactants modified montmorillonite and its application for the removal of methyl orange

► The anion–cationic surfactant modified montmorillonite (MMT) were prepared. ► Anionic surfactant can be intercalated into the interlayer spaces of MMT. ► Organic carbon content of MMT can be improved by anion–cationic modification. ► The anion–cationic surfactant modified MMT has the largest sorption capacity. ► The adsorption of methyl orange is spontaneous and exothermic process. Anion–cationic, anionic and cationic surfactants modified montmorillonite (MMT) are prepared by incorporating cationic surfactants cetyl trimethyl ammonium bromide (CTMAB), anionic surfactants sodium stearate (SSTA) and their mixture to montmorillonite. The modified samples are characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and Thermogravimetry–differential thermal analysis (TG–DTA). XRD result shows that both the anionic and cationic surfactants can be intercalated into the MMT, making the interlayer spaces of MMT expand. TG–DTA studies indicate the organic carbon contents of anion–cationic surfactants modified MMT are higher than the montmorillonite modified by anionic or cationic surfactants solely. The adsorption experiments indicate that CTMAB/10SSTA–MMT had the largest adsorption capacity compared with CTMAB–MMT and 10SSTA–MMT due to the formation of a highly effective partition medium by anion–cationic surfactant micelle. The adsorption kinetic of methyl orange onto the modified MMT samples can be best described by the pseudo-second-order model, and the adsorption isotherm is in good agreement with the Langmuir equation. The activation energy, change of free energy, enthalpy and entropy of anion–cationic modified MMT are also evaluated for the adsorption of methyl orange. Thermodynamic results indicate that the adsorption is a spontaneous and exothermic process.