One-step hydrothermal carbonization of amine modified black liquor and lignin for efficient Cr(VI) adsorption

Amine modified black liquor (BL) and purified lignin (PL) derived biochar were successfully synthesized by facile one-step hydrothermal carbonization (HTC) with hexamethylenediamine (HMDA). A variety of analytical techniques were used to characterize the physico-chemical properties of biochar. The effects of experimental parameters (initial concentration of pollutant, contact time, solution pH and coexisting ions) on the adsorption performance of biochar were systematically investigated. The adsorption capacity of Cr(VI) on biochar prepared by BL and PL was significantly different under the optimal hydrothermal temperature. The maximum adsorption amount of BL-180 (hydrothermal temperature 180 °C) was 741.74 mg/g, while that of PL-150 (hydrothermal temperature 150 °C) was only 165.16 mg/g, as calculated by Langmuir model at 45 °C. Therefore, the preparation of biochar by direct HTC of BL showed more significant advantages and potential than PL in removing Cr(VI) from wastewater. Further, amine modification enhanced the adsorption performance of biochar, especially for PL biochar. The maximum adsorption amount of N-BL-180 was increased to 859.43 mg/g, whereas that of N-PL-150 was significantly increased to 583.77 mg/g. Moreover, the modified biochar exhibited excellent reusability. The removal mechanism of Cr(VI) was attributed to electrostatic interaction, complexation as well as redox reaction. This work provides a successful attempt of amine-modified BL biochar under mild condition, which provided a novel idea for the treatment of Cr(VI)-containing wastewater.