Preparation of highly-conductive pyrogenic carbon-supported zero-valent iron for enhanced Cr(Ⅵ) reduction

[Display omitted] •ZVI catalyzed PC carbonization with more graphitic C and greater surface area;•PC/ZVI is electrochemical reactive with greater electron transfer rate & quantity;•Electron transfer of PC/ZVI was over 220 times greater than PC;•Electron shuttling is attributed to conductance of PC/ZVI due to graphitic C;•Reduction accounted for over 80 % of Cr(Ⅵ) detoxification. In this work, electron transfer (ET) moiety of PC was ascertained in chromate (Cr(Ⅵ)) reduction by zero-valent iron supported by pyrogenic carbon (PC) (ZVI/PC) prepared by pyrolysis of hematite (α-Fe2O3)-treated pinewood. X-ray diffraction analysis suggested successive phase transformation of α-Fe2O3→magnetite (Fe3O4)→wustite (FeO)→ZVI (Feo). Raman spectra and Brunauer–Emmett–Teller analysis revealed that ZVI/PC is characterized with more ordered graphitic carbon and greater surface area than pristine PC. Maximal Cr(Ⅵ) removal capacity (pH = 3) as predicted by Langmuir isotherm model were 5.78, 36.12 and 8.39 g kg−1 for PC, ZVI/PC and ZVI, respectively. ZVI/PC maintained significantly greater Cr(Ⅵ) removal capacity than ZVI and PC at pH 3–9, but Cr(Ⅵ) removal dropped rapidly to 6.78 g kg−1 at pH 4 and above. X-ray photoelectron spectroscopy and successive desorption of Cr-laden ZVI/PC and ZVI showed trivalent Cr was the dominant species, suggesting reduction was an important mechanism for Cr(Ⅵ) detoxification. Electrochemical analysis demonstrated that ZVI/PC exhibited greater Tafel corrosion rate and ET quantity, with lower electrical resistance. Besides, Cr(Ⅵ) reduction showed reversal trend with electrical resistance of ZVI/PC. To conclude, ET capacity was closely associated with electrical conductivity of ZVI/PC due to intensified conductive graphitic carbon structure of PC at higher pyrogenic temperatures.