Sorption of antibiotic sulfamethoxazole varies with biochars produced at different temperatures

Sorption of sulfonamides on biochars is poorly understood, thus sulfamethoxazole (SMX) sorption on biochars produced at 300–600 °C was determined as a function of pH and SMX concentration, as well as the inorganic fractions in the biochars. Neutral SMX molecules (SMX0) were dominant for sorption at pH 1.0–6.0. Above pH 7.0, although biochars surfaces were negatively-charged, anionic SMX species sorption increased with pH and is regulated via charge-assisted H-bonds. SMX0 sorption at pH 5.0 was nonlinear and adsorption-dominant for all the biochars via hydrophobic interaction, π–π electron donor–acceptor interaction and pore-filling. The removal of inorganic fraction reduced SMX sorption by low-temperature biochars (e.g., 300 °C), but enhanced the sorption by high-temperature biochars (e.g., 600 °C) due to the temperature-dependent inorganic fractions in the biochars. These observations are useful for producing designer biochars as engineered sorbents to reduce the bioavailability of antibiotics and/or predict the fate of sulfonamides in biochar-amended soils. •Sulfamethoxazole (SMX) sorption on biochars at pH 5.0 was adsorption-dominant.•Removal of inorganic fractions in low-temperature biochars reduced SMX sorption.•Removal of inorganic fractions in high-temperature biochars enhanced SMX sorption.•Anionic SMX was adsorbed on negatively charged biochar via charge-assisted H-bond. Solution pH and biochar property control the sorption amount and mechanisms of antibiotic sulfamethoxazole.