<p>Designing simple and efficient biochar adsorbents for removing chemical oxygen demand (COD) is of great significance for environmental protection and human health. In this study, phenol was used as a model pollutant. K₂CO₃-modified soybean straw biochar (CSBC) with efficient adsorption properties was simply synthesized for the removal of COD from wastewater. K₂CO₃ generated CO gas with carbon and formed bubbles in the biochar. When the bubbles ruptured, a substantial number of pores emerged on the cover of biochar, providing a mass of adsorption sites. Furthermore, the modification of K₂CO₃ caused more oxygen-containing functional groups to appear in the biochar, which greatly improved the adsorption performance. The physicochemical structure of CSBC was studied by a battery of characterization methods. The adsorption experiments showed that CSBC had a good removal effect on COD, and the removal rate could reach 91.07% after 25&#xa0;min of adsorption. Density functional theory (DFT) analysis revealed that the potential interaction forces between CSBC and phenol were hydrogen bonding, π-π interaction, and electrostatic interaction. In addition, CSBC still has good adsorption effect after 5 times of recycling. It may provide new ideas for the development of economical, high-efficiency and reusable adsorbents to remove COD from wastewater.</p> Graphical Abstract <p></p>