Contribution of different sulfamethoxazole species to their overall adsorption on functionalized carbon nanotubes

Antibiotics pose environmental risks, but their adsorption mechanisms are still unclear. Identifying the contributions of different mechanisms is vital in predicting antibiotic environmental behavior and consequently understanding their environmental risks. This study used functionalized carbon nanotubes (CNTs), namely hydroxylized (MH), carboxylized (MC), and graphitized (MG) multiwalled CNTs, as adsorbents and sulfamethoxazole (SMX) as an adsorbate to study the adsorption mechanisms of ionizable organic contaminants on solid particles. At pH around 3.7, SMX always showed the highest adsorption on different CNTs and the adsorption followed the order of MH > MG > MC. Combining the results on SMX specie analysis, the pH-dependent adsorption is well-explained by hydrophobic and electron-donor-acceptor interactions. The adsorption of neutral SMX is always dominant by contributing generally over 80% to the overall adsorption. A significant contribution of cationic SMX at pH < 3.5 suggested significant contribution of hydrogen bonds to SMX adsorption. The strength of bisphenol A (BPA) inhibiting SMX adsorption was dependent on the concentration ratio of BPA to neutral SMX, instead of to the overall SMX concentration. This result emphasized the importance of identifying the dominant mechanisms or species for the adsorption of antibiotics.