[19], polymer-based composites [20][21][22][23], a graphene oxide (GO)-based composite adsorbent [24], and activated carbon [25][26][27][28] have been explored and used in the removal of Cr(VI). Among these adsorbents, carbonaceous porous materials have been successfully used in the removal of Cr(VI) because of their acid and alkali corrosion resistance, excellent thermal and chemical stability and high adsorption capacity for heavy metals [29].In contrast to bulk materials, the high specific surface area of nanoscale materials provides more surface active sites, and good dispersability in solution to help facilitate mass transfer [1,30]. In addition, micropores inherently possess a strong adsorption potential, and can strongly trap and adsorb guest species from the external environment [31]. One can thus envisage that nanosized carbon materials with abundant micropores directly open to the environment could provide fast kinetics and a high adsorption capacity. Furthermore, to easily retrieve these nanosized adsorbents from solution, a functionalized nanocarbon with a magnetic response would be ideal. Although several magnetic carbon adsorbents [12,[31][32][33][34][35][36] have been reported for the removal of chromium, there is still a need to improve such adsorbents to have a high adsorption capacity.It is worth mentioning that recent studies have shown that zinc species is good dynamic molecular porogens to create extra micropores [37][38][39]. These results show that Zn ions turn into ZnO during pyrolysis process. Further temperature increase can lead to the reduction of ZnO nanoparticles in the presence of carbon materials (carbothermal reduction), accompanied by the evaporation of Zn, CO 2 , and CO. The evaporation of the Zn species would create additional nanochannels that contribute to the adMicroporous 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, Zn 2+ ions can be easily introduced into the hollow polymer spheres through complexation process. During carbonization, high temperature treatment results in the reduction of Zn 2+ to metallic Zn and subsequent evaporation of Zn, consequently forming nanospaces and nanopaths in the carbon shell. As little as 8.6 wt.% Zn 2+ 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.% γ-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 adsorben...
