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

ARTICLES SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 19 August 2015 | doi: 10.1007/s40843-015-0076-8 Sci China Mater 2015, 58: 611–620 Synthesis of magnetic hollow carbon nanospheres with superior microporosity for efficient adsorption of hexavalent chromium ions Lu-Hua Zhang, Qiang Sun, Chao Yang and An-Hui Lu* 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.% 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 adsorbents. After adsorption, the magnetic microporous hollow carbon spheres can be flexibly separated using an external magnet. INTRODUTION Hexavalent chromium is of high global environmental concern due to its high solubility in water, toxicity, non-biodegradation and its tendency to accumulate in living organisms. Various approaches have been developed for the removal of Cr(VI), including ion exchange, chemical precipitation, solvent extraction, membrane filtration, electrochemical treatment and adsorption. Among the various removal processes, fundamental studies have shown that adsorption is the most promising method for industrial applications due to its easy and safe mode of operation, low cost and wide availability of adsorptive materials [1]. A variety of materials including raw and modified ligno- cellulosic materials [2–4], TiO2 microspheres [5], iron oxide [6–8], Fe@Fe2O3 [9], γ-Fe2O3 [10–12], Fe3O4 [13–17], mixed maghemite-magnetite nanoparticles [18], Al2O3 [19], polymer-based composites [20–23], a graphene oxide (GO)-based composite adsorbent [24], and activated carbon [25–28] 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 [12,31–36] 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 [37–39]. 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, CO2, and CO. The evaporation of the Zn species would create additional nanochannels that contribute to the ad- State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China * Corresponding author (email: ) 611 August 2015 | Vol.58 No.8 © Science China Press and Springer-Verlag Berlin Heidelberg 2015 ARTICLES SCIENCE CHINA Materials ditional micropore. Those methods not only removes zinc species without the necessity of acid washing but also results in accessible nanoscale porosity. Compared to conventional chemical activation, the method mentioned above can significantly reduce the dosage of zinc salt. Under these considerations, we report a simple and efficient strategy for the synthesis of magnetic hollow carbon spheres with superior microporosity by using zinc species as dynamic molecular porogens, which show a superior maximum adsorption capacity of 233.1 mg g−1 for Cr(VI) from water and fast kinetics compared to other previously reported carbon-based adsorbents. EXPERIMENTAL SECTION Materials 2,4-Dihydroxybenzoic acid (DA) was obtained from Aldrich. Oleic acid, formaldehyde, ammonia solution (25%), zinc chloride, Fe(NO3)3⋅9H2O, 1,5-diphenylcarbazide, phosphoric acid (85%) and potassium dichromate were obtained from the Sinopharm Chemical Reagent Co. All chemicals were used as received without any further purification. Synthesis of hollow polymer and carbon nanospheres Hollow polymer nanospheres (HPSs) were synthesized following the previously reported method [38]. An emulsion solution was prepared by mixing oleic acid/ammonia (forming ammonium oleate) and DA/formaldehyde solution, and then transferred into an autoclave and hydrothermally aged for 4 h at 120°C. Hollow carbon nanospheres (HCSs) were obtained by heating the HPSs to 910°C with a heating rate of 5°C min−1 and holding them at that temperature for 3 h under an argon atmosphere. Synthesis of microporous hollow carbon nanospheres HPSs were impregnated in aqueous ZnCl2 for 12 h to load the Zn2+ to obtain HPSs-Zn. The resultant HPSs-Zn were retrieved by centrifugation, washed with deionized water and ethanol to remove the ZnCl2 located outside the spheres, with final drying at 50°C for 12 h. The obtained polymeric spheres HPSs-Zn were heated at 5°C min−1 to 910°C, where they were pyrolyzed for 3 h under an argon atmosphere. Finally, microporous hollow carbon nanospheres were synthesized, and these are called HCSsZn-910. For comparison, another sample, HCSs-Zn-750, was produced by pyrolysis at 750°C, also for 3 h. Synthesis of magnetic microporous hollow carbon spheres The microporous hollow carbon spheres (HCSs-Zn-910) were immersed in aqueous Fe(NO3)3⋅9H2O (0.24 M) for 4 h at 30° (...truncated)