Adsorption of sulfamethoxazole and 17β-estradiol by carbon nanotubes/CoFe2O4 composites

• SMX and E2 adsorption by two types of CNTs/CoFe2O4 composites were investigated.
• The rate-limiting steps of SMX/E2 adsorbed by two types of composites are different.
• Hydrophobic interactions and π–π interactions are the main adsorption mechanisms.
• CNTs/CoFe2O4 had high magnetization, good reusability, and high stability.

Two types of multi-walled carbon nanotubes (CNTs)/CoFe2O4 composites were prepared using carboxylic CNTs (CNTs-C) and amino CNTs (CNTs-N) via a hydrothermal method. The adsorption of sulfamethoxazole (SMX) and 17β-estradiol (E2) by the two types of CNTs/CoFe2O4 composites were investigated. Kinetic data show that film diffusion is the main rate-limiting step of SMX and E2 adsorption by the CNTs-N/CoFe2O4 composites, whereas both liquid film diffusion and intra-particle diffusion are the rate-limiting steps of SMX and E2 adsorption by the CNTs-C/CoFe2O4 composites. The adsorption isotherms were well described by the Freundlich model. The adsorption potentials (ε) of SMX on the CNTs/CoFe2O4 composites were higher than those of E2, indicating the stronger interaction of SMX with the CNTs/CoFe2O4 composites than E2. On the contrary, the site energy distributions (Φ(ε)) of E2 were higher than SMX, suggesting that the CNTs/CoFe2O4 composites adsorbed E2 more effectively than SMX. The SMX adsorption decreased with increasing pH from 3.1 to 10.4, whereas little change was found for E2 adsorption. This difference may be attributed to their different pKa values. FTIR analysis suggested that, in addition to hydrophobic interactions, π–π electron donor–acceptor interactions may also act simultaneously. The CNTs/CoFe2O4 composites could be easily separated using a simple magnetic process and thermally regenerated at 300 °C. Therefore, the CNTs/CoFe2O4 composites could be used as a potential adsorbent for removing organic pollutants from water.

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