Recent studies reported that ion adsorption equilibrium was achieved in 20–30 min for GO membranes26–28. In contrast, the efficient adsorption equilibrium of M–GO was achieved within 1 min, which was attributed to the large specific surface area and high dispersibility of the M–GO sheets in the solution. Notably, Co2+, Mn2+, and Sr2+ ions efficiently adsorbed on M–GO can be effectively desorbed by the addition of Al3+ ions at a concentration of less than 0.4 mM (10 mg/L Al3+ in mixture solutions). It is important to note that graphene–based materials14,29 and biosorbents16,17 exhibit strong ion adsorption, and high multivalent metal ions, such as Co2+, Cu2+, Cd2+, Cr2+, and Pb2+ interact particularly strongly with graphene sheets10 or biosorbents16,17. The conventional methods for the desorption of these ions require high volumes of highly concentrated (0.1–0.2 M) acids and bases such as HCl and NaOH16,17, and cannot be used to treat M–GO because of the simultaneous dissolution of the Fe3O4 nanoparticles present. Thus, our results demonstrate the rapid and thorough desorption of Co2+, Mn2+, and Sr2+ ions on M–GO through the addition of Al3+ ions. Remarkably, the eluted concentration of Al3+ was reduced by a factor of at least 250 compared with the conventional desorption method.
In addition, we analyzed the effects of Al3+ concentration on ion desorption of Co2+, Mn2+, and Sr2+ on M–GO. As shown in Fig. 3, there was significant desorption of 40~60% for Co2+, Mn2+, and Sr2+ ions (10 mg/L in mixtures), even with the addition of a very limited amount of Al3+ (~2 mg/L in mixtures). The desorption rate increased with increasing concentration of Al3+ ions and reached up to ~95% when the addition of Al3+ was ~8 mg/L. The results further confirmed the efficient ion desorption on M–GO by our method of Al3+ ion treatment.
We further performed quantum chemistry calculations to illustrate the underlying physical mechanism occurring on the surface of graphene, using Co2+ and Al3+ as examples. The hydrocarbon C54H18 was used as a model for graphene, and the complexes were named [email protected] (X = Co2+–(H2O)6 and Al3+–(H2O)6). We thereafter calculated the adsorption energies (ΔEX), which are defined as follows:
where EX@G denotes the total energy of the hydrated cation adsorbed on graphene, whereas EG and EX denote the energies of the isolated graphene and hydrated cation, respectively. As shown in Fig. 4, both hydrated ions can be adsorbed stably on the surface of graphene, and the corresponding adsorption energies are –66.36 kcal/mol for Co2+–(H2O)6 and –120.35 kcal/mol for Al3+–(H2O)6. The interaction of graphene with Al3+ is much stronger than that with Co2+. Here, the adsorption energy is mainly due to the interaction between the hydrated cation and the aromatic rings in graphene, namely the hydrated cation–p interaction12,18,30. The existence of these interactions was confirmed by ultraviolet absorption spectroscopy (Fig. S4).
Then, we can estimate the adsorption probability of Al3+ on graphene (PAl), relative to that of Co2+ (PCo), as follows:
where kB denotes Boltzmann's constant and T the temperature. At 300 K, the calculated PAl/PCo is . This result demonstrates that the adsorption probability of Co2+ is completely negligible compared to that of Al3+, indicating that the Co2+ ions adsorbed on graphene can be rapidly desorbed by Al3+ ions, which is consistent with our experimental observations. Considering that universal monovalent and divalent ions should have smaller adsorption energies than that of Al3+ 18, we suggest that the method proposed in the present study could be used to enrich a wider range of ions.
In summary, we have experimentally achieved effective ion (Co2+, Mn2+, and Sr2+) adsorption and desorption of M–GO by adding trace amounts of Al3+. Unlike conventional desorption methods which use large amounts of HCl or NaOH solutions with high concentration, our desorption method involving the addition of trace amounts of Al3+ is facile, convenient, and has a low consumption of reagent. Importantly, we demonstrated the effective enrichment of radioactive 60Co from the solution by the controllable ion adsorption and desorption on M–GO. Density functional theory calculations indicate that these facile adsorption and desorption processes originate from the hydrated cation–π interaction between the ions and π–conjugated system in the graphitic surface, which promotes ion–surface adsorption, and the huge difference in adsorption probability between Al3+ ions and other ions. We also noted that monovalent and divalent ions should have smaller adsorption energies than Al3+ has, indicating that this method could be used for the adsorption and desorption of a wider range of ions. Thus, these findings represent a facile step for the high–efficiency desorption, extraction, and concentration of ions with potential applications, including nuclear energy, medicine, agriculture, and nuclear wastewater treatment.