Facile synthesization of NaOH activated hierarchical porous biochar from cucumber straw for the effective removal of doxycycline in aqueous solution

The effective utilization of biomass waste for the fabrication of biochar adsorbents has aroused significant interest. In this work, a novel hierarchical porous biochar (HPBC) was successfully synthesized by dry mixing combined with one-step pyrolysis method using cucumber straw as a raw material and NaOH as an activator. The prepared HPBC was then applied to remove doxycycline (DOX) from aqueous solutions. HPBC comprised hierarchical porous structures with excellent specific surface area (1409.75 m 2  g −1 ), high pore volume (0.6549 cm 3  g −1 ), and small average pore diameter (1.8582 nm). HPBC was found to contain multiple functional groups involving hydroxyl, carbonyl, amine, and aromatic structure. The adsorption kinetics and isotherm of DOX on HPBC were well described by the Avrami fractional order model and Sips model, suggesting that the adsorption process involved multiple kinetics as well as monolayer and multilayer adsorptions. HPBC exhibited an excellent adsorption capacity for DOX with the maximum value of 552.30 mg g −1 at 25 °C (Sips model). The possible adsorption mechanisms of DOX on HPBC included pore filling, π-π interactions, hydrogen bonding, and electrostatic interactions. This study provided a new approach for resource utilization of straw waste and effective removal of antibiotics from water.