Rigidity and Flexibility: Unraveling the Role of Fulvic Acid in Uranyl Sorption on Graphene Oxide Using Molecular Dynamics Simulations.

Using molecular dynamics simulations, this work targets a molecular understanding on the rigidity and flexibility of fulvic acid (FA) in uranyl sorption on graphene oxide (GO). The simulations demonstrated that both rigid Wang's FA (WFA) and flexible Suwannee River FA (SRFA) can provide multiple sites to cooperate with GO for uranyl sorption and act as "bridges" to connect uranyl and GO to form GO-FA-U (type B) ternary surface complexes. The presence of flexible SRFA was more beneficial to uranyl sorption on GO. The interactions of WFA and SRFA with uranyl were primarily driven by electrostatics, and the electrostatic interaction of SRFA-uranyl was significantly stronger owing to the formation of more complexes. The flexible SRFA could markedly enhance the bonding strength of uranyl with GO by folding itself to provide more sites to coordinate with uranyl. The rigid WFAs tended to be adsorbed on the GO surface in parallel due to π-π interactions, whereas the flexible SRFAs took more slant configurations resulting from intermolecular hydrogen bonds. This work provides new insights into the sorption dynamics, structure, and mechanism and addresses the effect of molecular rigidity and flexibility, with great significance for FA-based remediation strategies of uranium-contaminated sites.