Hollow carbon fibers and flakes derived from Calotropis procera as adsorbents for dye removal from aqueous solutions

Plant fibers represent a rich and inexpensive source of easily renewable carbon fibers with various potential applications. In this study, nitrogen-doped carbon flakes (NCFs) and nitrogen-doped hollow carbon fibers (NHCFs) were prepared from Calotropis procera fibers (CPFs) by controlling the conditions of the carbonization process. The pre-oxidation of CPFs plays a vital role in maintaining their morphology after carbonization at high temperature. Scanning electron microscopy (SEM) images confirmed that the carbonized CPFs at 400 οC without pre-oxidation produced NCFs-4, but after carbonization at 500 and 600 οC via the pre-oxidation process, the hollow-fibers retained their morphology and produced NHCFs-5 and NHCFs-6, respectively. Structural characterization of the NCFs and NHCFs was conducted by Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDX), Raman spectroscopy and X-ray diffraction (XRD) and BET analysis. All samples showed a mixture micro/mesoporous structure with a BET surface area of 474.66, 469.29 and 987.80 m2/g for NCFs-4, NHCFs-5 and NHCFs-6, respectively. The prepared NCFs and NHCFs were applied as effective adsorbents for the removal of cationic crystal violet dye (CV) from polluted water. The obtained results showed that the NHCFs-6 sample exhibited a maximum adsorption capacity of 160.18 mg/g. The prepared samples follow the order NHCFs-6 > NCFs-4 > NHCFs-5. The nonlinear Redlich-Peterson model successfully simulated the adsorption isotherm of CV dye removal. The kinetics study demonstrated that the pseudo-first-order (PFO) model is the best model for simulating adsorption kinetics on the NHCFs-6 surface, while the pseudo-second-order model was the best model on the surface of NCFs-4 and NHCFs-5.