Porous biochar derived from walnut shell as an efficient adsorbent for tetracycline removal

In this study, a high-performance porous adsorbent was prepared from biochar through a simple one-step alkali-activated pyrolysis treatment of walnut shells, and it was effective in removing tetracycline (TC). The specific surface area (SSA) of potassium hydroxide-pretreated walnut shell-derived biochar pyrolyzed at 900 °C (KWS900) increased remarkably compared to that of the pristine walnut shell and reached 1713.87 ± 37.05 m2·g−1. The maximum adsorption capacity of KWS900 toward TC was 607.00 ± 31.87 mg·g−1. The pseudo-second-order kinetic and Langmuir isotherm models were well suited to describe the TC adsorption process onto KWS900. The KWS900 exhibited high stability and reusability for TC adsorption in the presence of co-existing anions or cations over a wide pH range of 1.0–11.0. Further investigations demonstrated that the proposed adsorption mechanism involved pore filling, hydrogen bonding, π–π stacking, and electrostatic interaction. These findings provide a valuable reference for developing biochar-based adsorbents for pollutant removal.