N-doped biochar from chitosan gel-like solution: Effect of hydrothermal temperature and superior aqueous Cr (VI) removal performance

N-doped biochar was prepared by gel-like chitosan solution via hydrothermal carbonization followed by low-temperature oxidation, where the effect of hydrothermal temperature on the structure of biochar was studied. The removal of Cr (VI) on biochar was studied as a function of pH and contact time via batches of adsorption experiments. Results indicated that the as-made biochar included numbers of N, O-based groups with limited surface areas of ~50 m2/g and main mesopores, and Cr (VI) removal efficiency was largely affected by hydrothermal temperature. Adsorption kinetics and isotherms of Cr (VI) were well-described by pseudo-second order and Langmuir model, respectively, and an initial pH of two was most favorable for Cr(VI) removal. Significantly, OCS-160 sample had an unexpected maximum Cr (VI) capacity up to 190.48 mg/g at 25 °C with good recyclability, showing great potential as inexpensive and highly efficient adsorbents for Cr (VI) removal from wastewater. N-doped biochar was prepared from chitosan-based gel-like solution via hydrothermal carbonization and low-temperature oxidation, showing superior Cr (VI) removal from water with the maximum capacity of highly up to 190.48 mg/g at 25 °C, following the Langmuir model. [Display omitted] •N-doped biochar was prepared by hydrothermal carbonization and oxidation.•N-doped biochar showed unexpected high Cr (VI) capacity of 190.48 mg/g at 25 °C.•Removal of Cr (VI) was mainly chemical mechanism followed by Langmuir model.•Biochar can be efficiently regenerated by NaOH with good cycling performance.