Synthesis of magnetic hollow carbon nanospheres with superior microporosity for efficient adsorption of hexavalent chromium ions

Microporous hollow carbon nanospheres were prepared through the polymerization of 2,4-dihydroxybenzoic acid and formaldehyde in the presence of ammonia and tactfully using chelating zinc species as dynamic porogens during the carbonization step to create extra micropores. The Cr(VI) maximum adsorption capacity of microporous hollow carbon spheres consequently increase from 139.8 mg g(-1) of pristine hollow carbon spheres to 199.2 mg g(-1). Owing to the presence of the carboxyl groups in the polymer matrix, Zn2+ ions can be easily introduced into the hollow polymer spheres through complexation process. During carbonization, high temperature treatment results in the reduction of Zn2+ to metallic Zn and subsequent evaporation of Zn, consequently forming nanospaces and nanopaths in the carbon shell. As little as 8.6 wt.% Zn2+ in the polymer matrix can increase the micropore volume by 133% and the specific surface area by 86%. The microporous hollow carbon spheres can be made magnetic by anchoring them to 14.0 wt.% gamma-Fe2O3 nanoparticles, thus producing a highly efficient Cr(VI) adsorbent. The maximum adsorption capacity measured was 233.1 mg g(-1), which is significantly higher than other reported carbon-based adsorbents. After adsorption, the magnetic microporous hollow carbon spheres can be flexibly separated using an external magnet.