Adsorption of methylene blue and tetracycline on nitrogen-doped cotton stalk-based biochar and mechanism research

To address the pressing issue of tetracycline (TC) and methylene blue (MB) contamination in water, most prior research on cotton stalk-based biochar has concentrated on optimizing process parameters (e.g., carbonization temperature and activator ratio), with less systematic investigation into regulating the material’s surface chemistry via heteroatom doping. In this study, nitrogen-doped biochars (NKBC-10, NKBC-5, and NKBC-1) with different melamine/KOH/cotton straw mass ratios (1:10:10, 1:5:5, and 1:1:1) were prepared by co-pyrolysis using cotton straw as the raw material and melamine as the nitrogen source, and the synergistic mechanism of N doping on the structural properties and adsorption performance of the materials was systematically elucidated. The results indicated that NKBC-5 possessed an excellent microporous-mesoporous structure (average pore size of 2.006 nm) and abundant surface nitrogen-containing functional groups, achieving maximum adsorption capacities of 1328.636 mg/g for TC and 917.170 mg/g for MB at 308.15 K. The adsorption process aligned with the quasi-secondary kinetics and Langmuir monolayer adsorption models. The mechanism involved a synergistic mix of Lewis acid-base interactions, π-π stacking, hydrogen bonding, and electrostatic interactions. The regeneration experiments revealed that the adsorption capacity of NKBC-5 for TC and MB remained 91.26% and 93.54% of the initial time after four cycles of recycling, indicating good stability. This study employs an N-doping strategy to synergistically optimize the pore structure and surface chemistry of cotton stalk-based biochar, offering a novel material design concept and technical foundation for the efficient and sustainable remediation of water contaminated with dyes and antibiotics.