Chemical oxidation is still the major approach to the covalent functionalization of carbon nanotubes (CNTs). Theoretically, the defects on CNTs are more reactive than skeletal hexagons and should be preferentially oxidized, but conventional oxidation methods, e.g., HNO3/H2SO4 treatment, have poor reaction selectivity and inevitably consume the C=C bonds in the hexagonal lattices, leading to structural damage, pi-electrons loss and weight decrease. In this work, we realized the nondestructive covalent functionalization of CNTs by selective oxidation of the defects. In our method, potassium ferrate (K2FeO4)-O-VI was employed as an oxidant for CNTs in H2SO4 medium. The CNT samples, before and after K2FeO4/H2SO4 treatment, were characterized with colloid dispersibility, IR, Raman spectroscopy, FESEM and XPS. The results indicated that (i) CNTs could be effectively oxidized by Fe (VI) under mild condition (60 degrees C, 3 h), and hydrophilic CNTs with abundant surface COOH groups were produced; and (ii) Fe (VI) oxidation of CNTs followed a defect-specific oxidation process, that is, only the sp3-hybridized carbon atoms on CNT surface were oxidized while the C=C bonds remained unaffected. This selective/nondestructive oxidation afforded oxidized CNTs in yields of above 100 wt%. This paper shows that K2FeO4/H2SO4 is an effective, nondestructive and green oxidation system for oxidative functionalization of CNTs and probably other carbon materials as well. (C) 2015 Elsevier B.V. All rights reserved.