Trace nitrogen-doped hierarchical porous biochar nanospheres: Waste corn roots derived superior adsorbents for high concentration single and mixed organic dyes removal

Seeking high performance adsorbents for highly efficient treatment of wastewater containing organic dyes has become increasingly imperative worldwide. Herein, with a specific surface area (SSA) of 2,745.4 m²·g⁻¹, trace N-doped porous biochar nanospheres (NPBs) are derived for the first time from affluent waste corn roots, via a hydrothermal conversion followed by a mild calcined activation by K₂CO₃ (KC) in the presence of low virulent melamine. Melamine acts as N source and synergistic activator for significant promotion in SSA, pore volume, and surface defects. The obtained NPBs (CHC-0.5N-4KC-900) are confirmed as superior adsorbents for removal of organic dyes rhodamine B (RhB, q_m = 1,630.7 mg·g⁻¹) and Congo red (CR, q_m = 1,766.2 mg·g⁻¹) as well as their mixtures, within not only a low (< 50 mg·L⁻¹) but also a high (> 50, esp. 250–1500 mg·L⁻¹) concentration range. The values for q_m are far beyond commercially activated carbon (AC) as well as most reported biomass derived carbons, undoubtedly revealing the NPBs as great promising candidate adsorbents for disposal of real industrial wastewater. In addition, the adsorption of RhB is fitted by Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models. The kinetic analysis indicates that the adsorption before equilibrium conforms to the pseudo-second-order model, and the hydrogen bonding, electrostatic attraction, and esp. π–π interaction have contributed to the superior adsorption performance of the NPBs.