Turning nitrogen vacancy and chemisorption sites by co-doping Na and N into cow dung biochar ultrathin nanosheets for removal of oxytetracycline

Biochar has recently attracted considerable interest in wastewater treatment owing to its remarkable adsorption capabilities, low cost, and abundant raw materials. However, traditional biochar suffers from issues that include a relatively small specific surface area and few adsorption sites, limiting the potential applications in practice. Herein, ultrathin biochar nanosheets with varying degrees of nitrogen defects were synthesized via a one-step molten-salt process to remove oxytetracycline, a typical antibiotic in livestock wastewater. Substituting partial carbon atoms in the carbon skeleton with nitrogen atoms generated nitrogen-containing functional groups (e.g., pyridinic-N, pyrrolic-N, graphitic-N), which significantly enhanced the surface polarity of biochar. Meanwhile, sodium cations (Na⁺) form ion pairs with negatively charged nitrogen sites, reducing the surface charge shielding effect and increasing the density of effective adsorption sites in biochar. This synergy significantly boosted the biochar’s adsorption capacity for oxytetracycline to 82.7 mg/g, which was 1.76 times that of the untreated biochar at 46.9 mg/g. Furthermore, its packed column consecutively ran for 311 h with a dynamic sorption capacity of 196.1 mg/g, demonstrating significant potential for scaling up wastewater treatment processes. Primary mechanisms underlying this efficient adsorption process encompass H-bonding, ππ interaction, electrostatic attraction, and pore-filling. This study provided a novel strategy for designing a high-adsorption capacity, environment-friendly adsorbent and offered a theoretical foundation for treating oxytetracycline in livestock wastewater using biochar derived from cattle dung.