O, N-doped porous biochar by air oxidation for enhancing heavy metal removal: The role of O, N functional groups

Oxygen- and nitrogen-doped porous oxidized biochar (O,N-doped OBC) was fabricated in this study. Biochar (BC) can be enriched in surface functional groups (O and N) and the porosity can be improved by a simple, convenient and green procedure. BC was oxidized at 200 °C in an air atmosphere with quality control via oxidation time changes. As the oxidation time increased, the O and N contents and porosity of the materials improved. After 1.5 h of oxidation, the O and N contents of O,N-doped OBC-1.5 were 54.4% and 3.9%, higher than those of BC, which were 33.4% and 1.8%, respectively. The specific surface area and pore volume of O,N-doped OBC-1.5 were 88.5 m2 g?1 and 0.07 cm3 g?1, respectively, which were greater than those of BC. The improved surface functionality and porosity resulted in an increased heavy metal removal efficiency. As a result, the maximum adsorption capacity of Cu(II) by O,N-doped OBC was 23.32 mg L?1, which was twofold higher than that of pristine BC. Additionally, for a multiple ion solution, O,N-doped OBC-1.5 showed a greater adsorption behavior toward Cu(II) than Zn(II) and Ni(II). In a batch experiment, the concentration of Cu(II) decreased 92.3% after 90 min. In a filtration experiment, the O,N-doped OBC-based filter achieved a Cu(II) removal capacity of 12.90 mg g?1 and breakthrough time after 250 min. Importantly, the chemical mechanism was mainly governed by monolayer adsorption of Cu(II) onto a homogeneous surface of O,N-doped OBC-1.5. Surface complexation and electrostatic attraction were considered to be the chemical mechanisms governing the adsorption process. ? 2022 Elsevier Ltd