Adsorption of bisphenolic xenoestrogens on graphene: A peculiar adsorbate concentration dependence on the conformation of graphene

• Efficient removal of ultralow-level endocrines from water is achieved with graphene adsorption.
• Unusual changes were observed in the adsorption isotherms of bisphenols in the tested wide range.
• The adsorption decreased the surface grooved regions but increased the flat regions of graphene.
• An adsorption mechanisms of bisphenols on graphene was proposed.

Graphene was used to remove low-level environmental endocrines in micro-polluted water with the aid of an investigation about the adsorption of bisphenols at low levels in a very wide range from 0.0005 to 70 mg L−1. It was found that over the wide concentration range, the adsorption isotherms could be divided into two regions: they followed the Freundlich model in the lower concentration region (correlation coefficient R2 > 0.99), but obeyed the Langmuir model in the higher concentration region (R2 > 0.99). This was attributed to the adsorption-induced changes in the morphology of graphene, which created different adsorption sites on the surface of graphene. The obtained values of thermodynamic parameters ΔG0, ΔH0 and ΔS0 indicated that the adsorption was a spontaneous, exothermic and entropy-decreasing process. The absolute values of these parameters tended to be decreased with increasing bisphenol concentration, suggesting stronger affinity of the binding sites on graphene for the adsorption of the bisphenols at lower concentrations. As confirmed by some characterization and monitoring experiments, the increased bisphenol adsorption decreased the surface grooved regions formed by wrinkles and increased the flat regions. The highly and lowly-wrinkled (or flat) surface states of graphene accounted for the observed two-stage adsorption isotherms.

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