Size-tunable graphitized carbon spheres for water defluoridation

Abstract

We fabricated non-catalytic carbon spheres with controllable dimensions (diameter 0.3–1.5 µm) by chemical vapour deposition at 1273 K using C2H2 (carbon source) and N2 (dilutant) gas precursors. By varying the flow rates of precursor gases, spherical or pseudo-spherical morphologies with chain-like hierarchical structures are discerned. The X-ray diffraction data show hexagonally graphitized networks. The mechanism of carbon sphere formation is not yet fully resolved largely due to the ambiguity associated with the nucleation step, viz. pentagon or heptagon route. According to molecular dynamics calculations, the formation of carbon pentagons is energetically more favorable than heptagons. Pentagon structures seem to form by closing Fjord regions of the graphitized network as evidenced in IR bands at 635 cm−1 and 796 cm−1. The X-ray photon spectroscopic (XPS) measurements show adventitious oxygen on the carbon sphere’s surface sites with -COOH and -C-CO groups. The -COOH and -C-CO groups are polar; hence, the carbon spheres’ surface sites readily hydroxylate (where ≡GOHδ represents surface sites with a fractional charge). The carbon sphere synthesized under C2H2N2 = 56 gas flow ratio (viz., CS3) shows the highest fluoride removal capacity, viz., 0.095 ± 0.01 mmol.m2-. The comparable zeta potential values observed between the carbon spheres and fluoride laden carbon spheres point to an anion exchange of surface hydroxyl ions for F- sorption. In agreement with the XPS data, the broad IR band at 3440 cm−1 due to H-bonded surface hydroxyl stretching vibrations has resolved into three discrete bands, e.g., 3640, 3752, and 3738 cm−1, which evidenced fluoride adsorption. Compared to activated carbon, the retention of fluoride on carbon spheres is over 60 times higher, which shows its potential in water treatment in removing fluoride.