Coal-fly-ash magnetic sphere based magnetic adsorbent for multiple-dye adsorption

Abstract

Owing to the synergetic characteristics of effective adsorption and magnetic separation, adsorption using magnetic adsorbents was regarded as one of the most promising methods for synthetic dye removal. Herein, a green, cheap, and efficient magnetic adsorbent was synthesized by a simple chemical oxidation in situ polymerization method. Processed coal-fly-ash magnetic spheres (CMS) were employed as the magnetic core material. A polyaniline (PANI) layer was covered on the CMS@SiO2 surface to form a core–shell structural magnetic adsorbent, CMS@SiO2@PANI. Careful investigations showed that the PANI layer was composed of nanorods of dozen nanometers in width and several microns in length, which forms an interconnected network and covers the overall surface of the core. The saturation magnetization of CMS@SiO2@PANI is as high as 30.99 emu·g−1, enabling efficient magnetic separation. It showed high selective adsorption for various synthetic dyes. The highest adsorption capacity for cationic dyes Methyl Orange and Congo Red was tested as 216.72 and 413.49 mg g−1 at pH 3, respectively, while that for anionic dyes Methylene Blue and Rhodamine B were 108.65 and 41.49 mg g−1 at pH 11, respectively. Systematic adsorption studies suggest that the pH value of the water has a significant influence on the surface properties of PANI, as well as the molecular structure and functional groups of the dyes. This could affect the interaction between PANI and dye molecules significantly. Thus, the multiple-dye adsorption performance of CMS@SiO2@PANI could be adjusted by adjusting the pH value. The adsorption kinetics and isothermal simulation results show that the dye adsorption is dominated by the monolayer chemical adsorption, and electrostatic attraction plays a vital role in the adsorption process. The multiple-dye adsorption of CMS@SiO2@PANI could be explained as a two-step process.