N-propyl functionalized spherical mesoporous silica as a rapid and efficient adsorbent for steroid estrogen removal: Adsorption behaviour and effects of water chemistry

To achieve an enhanced and selective adsorption of steroid estrogens, the n-propyl functionalization was applied to the mesoporous silica material (MCM-41) according to the physico-chemical property analysis of steroid estrogens. Adsorption behaviour and water chemistry effects were evaluated with the most concerned steroid estrogens: estrone (E1), 17β-estradiol (E2) and 17α-ethinyl estradiol (EE2) based on the materials characterization. The results showed the uptakes of E1, E2, and EE2 onto the modified MCM-41 were enhanced and accelerated by the n-propyl functionalization, which was positively correlated with the hydrophobicity of the synthesized materials. Kinetic data fitted the pseudo-second-order model well. Based on the Langmuir model, the maximum adsorption capacities of the n-propyl modified MCM-41 were up to 119.87, 88.38, and 86.91 mg g^-1 for EE2, E1, and E2, respectively. Importantly, both acid and neutral solutions were beneficial to estrogen removal, but ionic strength and humic acid did not affect the estrogen adsorption. The above results suggested that the n-propyl functionalized MCM-41 would be a promising adsorbent for the rapid and efficient removal of estrogens with the selectivity from natural organic matter like humic acid. Mechanism analysis showed the key role of hydrophobic interaction, and it also confirmed the contribution of the carbonylic lone pair electrons of E1, which helped the formation of stronger hydrogen bonds with silicon hydroxyls and enhanced the dipole-dipole interaction between E1 and the synthesized materials.