Modified carbon spheres as universal materials for adsorption of cationic harmful substances (paraquat and dyes) in water

In this study, carbon spheres (CS) with ideal particle sizes (similar to 700 nm) and regular morphology were prepared by extending the Stober method followed by oxidative modification of nitric acid to obtain carbon spheres (MCS) with abundant carboxyl and nitryl functional groups. Characterisation by SEM, BET, XRD, Raman spectroscopy, FTIR, XPS, and elemental analysis confirmed that these materials possessed regular morphology, large specific surface area (636 m(2)/g for CS and 339 m(2)/g for MCS), a good degree of graphitisation and many carboxyl groups. Because of abundant functional groups, MCS showed excellent adsorption properties for cationic targets (paraquat (PQ), chrysoidine G (CG) and malachite green (MG)), where the maximum adsorption capacities for PQ, CG and MG were 134.63, 299.44 and 326.93 mg/g, respectively. Compared with the linearity of Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models and the pseudo-first-order, pseudo-second-order, Elovich and intra-particle diffusion kinetics models, the adsorption models for PQ, CG and MG were best represented by the Temkin, Langmuir, and Freundlich isotherms, respectively, and the adsorption kinetics all fitted well with the Elovich model. Moreover, the good regeneration demonstrated in this study (the adsorption capacity remained at more than 80% after five cycles) makes MCS an outstanding candidate for water purification.