Tailored design of graphitic biochar for high-efficiency and chemical-free microwave-assisted removal of refractory organic contaminants

Energy-saving, chemical-free, and high-efficiency microwave (MW)-assisted water treatment can be greatly facilitated via tailored design of an economical, sustainable, and ‘green’ carbonaceous catalyst. In this study, various biochars (BC) were pyrolyzed from two lignocellulosic waste biomasses, oak (O) and apple tree (A), at a high temperature (900 °C) and under different gases (N₂ and CO₂). The holistic characterization by advanced spectroscopic techniques demonstrated that CO₂ pyrolysis of feedstock with more lignin (i.e., oak), produced biochar with increased aromaticity and degree of carbonization. CO₂ modification created a hierarchical porous structure, improved surface hydrophilicity, polarity, and acidity, and provided higher densities of near-surface functionalities of the biochar. Without MW irradiation, ABC-900C (1 g L⁻¹) provided the highest adsorption (52.6%, 1 min) of 2,4-dichlorophenoxy acetic acid (2,4-D) ascribed to large specific surface area, high micropore content, appropriate pore size, and abundant active groups. OBC-900C (1 g L⁻¹) enabled significantly increased 2,4-D removal (81.6%, 1 min) under MW irradiation (90 °C) in contrast with an oil bath (55.7%, 90 °C, 1 min) and room temperature (33.9%, 1 min) conditions, due to its highest graphitization degree and medium-developed microporous structure. The MW-induced thermal effect formed “hot spots” on the biochar surface as evidenced by elevated temperature of the bulk solution and lowered energy consumption of the MW reactor in the presence of OBC-900C, compared to those of the other biochars. The scavenging tests suggested that the generation of highly oxidative hydroxyl (^•OH), anionic superoxide (O₂^•−), and singlet oxygen (¹O₂) radicals contributed to the removal of 2,4-D. This study has demonstrated that biochar with customized structure and high organic adsorption capacity can act as an effective MW absorber suitable for rapid and improved removal of toxic organics.