Steam activation of biochars facilitates kinetics and pH-resilience of sulfamethazine sorption

Purpose: Sulfamethazine (SMT) is increasingly detected in environmental matrices due to its versatile use as antibiotics. We aimed to investigate the benefits and roles of steam activation of biochars with respect to SMT sorption kinetics and equilibrium sorption. Materials and methods: Biochars were produced from burcucumber plant and tea waste using a pyrolyzer at a temperature of 700 °C for 2 h. The biochar samples were treated with 5 mL min⁻¹ of steam for an additional 45 min for post-synthesis steam activation. The SMT sorption on the unmodified and steam activated biochars were compared. Results and discussion: The time taken to reach equilibrium was significantly less for steam activated biochars (∼4 h) than non-activated biochars (>24 h). Up to 98 % of SMT could be removed from aqueous solutions by steam activated biochars. The sorption kinetic behaviors were well described by the pseudo-second model and SMT sorption rates of steam activated biochars (k₂ ∼ 1.11–1.57 mg g⁻¹ min⁻¹) were significantly higher than that of the unmodified biochars (k₂ ∼ 0.04–0.11 mg g⁻¹ min⁻¹) because of increased availability of accessible porous structure with averagely larger pore diameters. Moreover, the equilibrium sorption on the unmodified biochars was significantly influenced by increasing solution pH (∼30–50 % reduction) because of speciation change of SMT, whereas steam activated biochars manifested much stronger sorption resilience against pH variation (∼2–4 % reduction only) because the enhanced porosity offset the effect of unfavorable electrostatic repulsion. Conclusions: The observed features of steam activated biochars would render their applications more versatile and reliable in field throughout changeable environmental conditions.