Evaluation of graphene oxide-coated sand adsorption on ammonium, lead and FA with molecular dynamic simulation and spectral induced polarization

Graphene oxide (GO) exhibited notable efficacy in adsorbing diverse contaminants. However, considering the economic constraints and the challenges in applying GO in natural environments, the prospect of coating it onto a three-dimensional framework presents a promising avenue for expanding its applications. In this study, abundant silica sands, which possess minimal adsorption capabilities, were employed as the framework for surface coating with GO. Batch and column tests, along with spectral induced polarization (SIP) monitoring, were conducted to investigate the adsorption process and the interaction mechanisms of GO–sand composite (GOS) with typical contaminants found in landfill-contaminated groundwater, i.e., Pb²⁺, fulvic acid (FA), and NH₄⁺. The geochemical findings indicated that GOS exhibited the highest effectiveness in adsorbing and retarding Pb²⁺ (7.49 mg/g; Rd = 2.2), and the least effectiveness on NH₄⁺ (0.37 mg/g; Rd = 1.4) under flow-through conditions. The SIP signals successfully revealed variations in the adsorption mechanisms among Pb²⁺, FA, and NH₄⁺. Specifically, the imaginary conductivity responses indicated that the interaction between NH₄⁺ and GO was weaker compared to Pb²⁺, possibly because water films could attract NH₄⁺ close to the media surface through a less tightly bound adsorption mechanism. Molecular dynamic simulations demonstrated that Pb²⁺ had a propensity to be adsorbed near oxygen atoms, with a priority order of carboxyl ≈ epoxy > hydroxyl > carbonyl. This adsorption process occurred rapidly and extensively due to the formation of Pb–O bonds. FA exhibited a tendency to maintain a parallel structure with the GO sheet due to π–π interactions and polar interactions, while the functional groups present in both GO and FA contributed to their mutual attraction, as evidenced by the discovery of coordination C–O bonds.