Highly effective removal of sulfamethoxazole by Na2CO3-modified biochar derived from sorghum straw and sewage sludge

Abstract

A biochar composite (SSBC) was prepared with sorghum straw and sewage sludge as raw materials by modification with Na₂CO₃ for the removal of sulfamethoxazole (SMX) from water. The morphology and chemical composition of SSBC were evaluated using scanning electron micrography (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) surface area. The specific surface area and total pore volume were 519 m² g⁻¹ and 0.210 cm³ g⁻¹, respectively, calculated from the N₂ adsorption–desorption isotherm. The effects of pH, ionic strength, contact time and initial concentration on the adsorption of SMX on the SSBC were investigated. Combined with the species distribution of SMX at different pH levels, the pH-dependent adsorption was well explained by hydrophobic and electron–donor–acceptor (EDA) interactions. At pH 3–6, neutral SMX molecule (SMX⁰) adsorption was mainly through hydrophobic action. When pH > 7.0, the adsorption of anionic SMX species mainly depended on the negative charge-assisted hydrogen bond. The experimental data were analyzed using two-parameter models (Langmuir, Freundlich, and Temkin) and three-parameter models (Redlich–Peterson, Sips, and Toth). The Langmuir model showed the best consistency with experimental data, and the maximum adsorption capacity of SMX on SSBC at pH 6.0 and 20 °C was calculated to be 139.8 mg g⁻¹. Adsorption kinetics were investigated by using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models. The possible adsorption mechanisms of SMX on the SSBC mainly included pore filling, π–π EDA interactions, electrostatic interaction, hydrophobic interaction, and charge-assisted hydrogen bonds. Moreover, the high-performance SSBC with highly efficient regeneration is well suited for removing SMX from water and presents an alternative material for treating water polluted by antibiotics.