One-pot hydrothermal synthesis of magnetic N-doped sludge biochar for efficient removal of tetracycline from various environmental waters

Efficient removal of zwitterionic tetracycline (TC) from water is a critical environmental challenge which is not fully addressed by conventional treatment technologies. A magnetic N-doped sludge biochar (MNSBC) was the first time synthesized by a simple one-pot hydrothermal method. The maximum adsorption capacity of MNSBC for TC was 197.3 mg/g at 298 K. Solution pH, ionic species/strength and humic acid concentration were the critical factors affecting TC adsorption by MNSBC. Elovich and Freundlich models better describing the experimental data illustrated that TC adsorption onto MNSBC was a multi-layer physicochemical adsorption process. Lewis acid-base, π-π conjugation, electrostatic interactions and pore filling were the main adsorption mechanisms. MNSBC also exhibited excellent adsorption performance for TC in various environmental waters, which achieved removal rates of up to 91.6%, 89.0%, 82.0% and 80.8% in mineral, tap, lake and river waters, respectively. The magnetic susceptibility of MNSBC allowed it to be easily collected after adsorption. Regeneration using NaOH could maintain its sustainable adsorption performance. Furthermore, MNSBC showed a very low release levels of iron and total nitrogen at all pH ranges (from 3 to 11), which suggested its suitability for water treatment applications. This study developed a simple technology for synthesis of effective TC adsorbent for different environmental waters and identified a circular economy pathway to reuse of water industry wastes.