Roles of hydrogen bond and ion bridge in adsorption of two bisphenols onto montmorillonite: an experimental and DFT study

Reactions occurring at silicate minerals play a significant role in controlling phenolic endocrine disrupting chemicals' bioavailability and mobility. However, the underlying molecular mechanisms for BPA (bisphenol A) and BPS (bisphenol S) onto montmorillonite (MMT) remain poorly understood. To decipher the molecular-level interfacial configuration and dominant reaction mechanism, batch experiments, X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), X-ray photoelectron spectroscopy (XPS) measurements, and density functional theory (DFT) calculations were performed. Our batch adsorption experiments showed that MMT exhibits a higher adsorption capacity for BPS than for BPA. The XRD analysis also suggested that BPA and BPS could enter the interlayer of MMT. FTIR and XPS results revealed that The Ca atoms and the oxygen atoms in the SiO4 tetrahedra were involved in BPA and BPS adsorption process via non-covalent interactions. The electrostatic potential calculations suggested that the sulfonyl O site and the hydroxyl H site on the BPS molecule are more favorable for interacting with the MMT surface than the hydroxyl O and H sites of the BPA molecule. DFT calculations further confirmed that stronger binding of BPS than BPA onto the (001) surface and the interlayer of MMT. BPA tends to form Ca ion bridges through hydroxyl groups onto MMT. In contrast, BPS prefers to form Ca ion bridges onto MMT through its sulfonyl group and form hydrogen bonds via hydroxyl groups. This study provides new insights into the molecular-scale interactions that control BPA and BPS adsorption onto an important and environmentally-relevant clay mineral, and which affect the fate and behavior of BPA and BPS in soil systems.