Removal of bisphenol a and 17β-estradiol by single-walled carbon nanotubes in aqueous solution: Adsorption and molecular modeling

Single-walled carbon nanotubes, both before (SWNTs) and after treatment (t-SWNTs) with acidified ammonium persulfate, were successfully used to adsorb bisphenol A (BPA) and 17β-estradiol (E2) from aqueous systems. The surface characteristics of the SWNTs and t-SWNTs were analyzed by measuring their surface charge and by imaging their morphological properties through transmission electron microscopy. The extent of defects on the SWNT scaffold generated through acid etching was analyzed by Raman spectroscopy. A total of 19.4, 15.4, and 14.3 mg/g of BPA was adsorbed on SWNTs, while a total of 8.0, 6.4, and 5.1 mg/g was adsorbed on t-SWNTs with a 72-h contact time at 280, 295, and 315 K, respectively. A significantly high fraction of E2 (27.2 mg/g) was absorbed by both SWNTs and t-SWNTs, as compared to BPA. The adsorption kinetics was analyzed using a pseudo-second-order model. Sorption experiments showed that t-SWNTs adsorbed less than half as much BPA as SWNTs, but their E2 adsorption was similar. The sorption mechanism was investigated by performing molecular-level calculations. Adsorption energies calculated using density functional theory show preferential sorption of E2 to SWNTs and graphene (-26.2 kcal/mol on SWNT and -34.1 kcal/mol on graphene) compared to BPA (-17.1 kcal/mol on SWNT and -22.5 kcal/mol on graphene), which were consistent with the experimental findings. Thus, ab initio calculations can mechanistically explain the adsorption differences of BPA and E2 on SWNTs.