Hollow carbon nanofibers (HCNFs) were found to have formed during microwave pyrolysis of palm kernel shell (PKS) at 500 and 600 degrees C. The formation of HCNFs was,only observed during microwave pyrolysis and not the fixed-bed pyrolysis, indicating that microwave played a key role in formation of HCNFs. X-ray diffraction (XRD) analysis of microwave bio-chars showed typical carbon diffractions at 2 theta = 26.3 degrees and 43.2 degrees, indicating good graphitic structure of HCNFs, especially for bio-chars prepared at 600 degrees C. High resolution transmission electron microscope (HRTEM) results revealed multiwall nature of the HCNFs with carbon layer spacing of 0.34 nm. Two major tubular and bamboo-shape structures were observed for HCNFs. With increasing microwave pyrolysis temperature from 500 to 600 degrees C, the yield of HCNFs increased from 5.85 wt.% to 9.88 wt.%. Raman spectroscopy analysis showed that with increasing pyrolysis temperature to 600 degrees C, the /D//G ratio decreased from 0.95 to 0.86, indicating higher order of the carbon layers of the HCNFs. Energy dispersive X-ray spectroscopy (EDS) showed the presence of Fe, K, and Ca in HCNFs structure which may have played a catalytic role during their formation and growth. Mechanism of formation and growth of HCNFs under microwave irradiation were proposed and discussed. The HCNFs-coated bio-char has great potential for removal of heavy metals from waste water. (C) 2017 Elsevier Ltd. All rights reserved.