Molecular-level investigation on removal mechanisms of aqueous hexavalent chromium by pine needle biochar

Pine needle biochar produced via oxygen-limited pyrolysis at 800 °C for 2 h was used for removing aqueous hexavalent chromium [Cr(VI)] from water. The optimality of the pseudo-second order kinetic model indicated the chemisorption dominated the removal process. Strong acid environment (pH = 2.0) was more conductive to Cr(VI) removal. The functional groups (C–O, O–C–O, C = O) on biochar contributed to Cr(VI) elimination from aqueous solution. The governing removal mechanisms involved electrostatic attraction, reduction, and complexation. Especially, confocal micro-X-ray fluorescence (μ-XRF) mapping indicated that Cr ions and functional groups were primarily distributed on the surface of biochar. This study firstly provided a deep interpretation of Cr speciation and spatial distribution on pine needle biochar, which provided important information on removal mechanisms of Cr(VI) by pine needle biochar.