Adsorption separation technology is one of the most effective and economical separation methods for high-efficiency extraction, concentration, and purification. Strong ion-surface adsorption between cations and graphene-based materials usually leads to certain difficulty in the desorption of ions adsorbed on sorbents due to strong cation–π interactions. Conventional desorption methods require a high consumption of HCl or NaOH solutions with concentrations as high, and exhibit ineffective or slow desorption for high multivalent metal ions. Moreover, these methods cannot be used to treat some functional graphene–based materials with superior performance, such as magnetite–graphene oxide (M–GO). Therefore, ion desorption in graphene-based materials remains a challenge.

    In this work, we proposed a facile, convenient, low reagent consumption desorption method, which achieved the rapid and efficient desorption of ions on magnetite-graphene oxide (M-GO) by adding a small amount of Al³⁺(at a volume ratio of 1:500).

    First, M-GO with high magnetic properties was prepared, and enrichment experiments of radioactive ions using controllable ion desorption on M-GO were performed using radioactive ⁶⁰Co as an example. Experimental results showed that we achieved effective enrichment of radioactive ⁶⁰Co and reduced the volume of concentrated ⁶⁰Co solution by approximately 10 times compared to the initial solution. (Fig. 1)

Fig. 1 | Enrichment of radioactive ⁶⁰Co from the solution by the controllable ion adsorption and desorption on M-GO. aSchematic of ⁶⁰Co enrichment. b TEM image and c high-resolution TEM image of M-GO. d Magnetization curve at room temperature (298 K) for the M-GO. Inset is a photograph of separation of M-GO with magnets in solution. e Radioactivity of ⁶⁰Co at each step of the enrichment experiments. Error bars indicate the standard deviation from three different samples.

       Then we further performed corresponding kinetics experiments on the desorption for typical bivalent ions of Co²⁺,Mn²⁺, and Sr²⁺. Results showed that Co²⁺, Mn²⁺, and Sr²⁺ adsorbed by M-GO can be completely desorbed within ~1 min when trace Al³⁺ solution was added, and the desorption rate reached 99.9 ± 0.1%, 97.0 ± 2.1%, and 98.3 ± 2.6% for Co²⁺, Mn²⁺, and Sr²⁺ solutions, respectively, yielding a desorption performance superior to that of the conventional desorption methods reported to date. (Fig. 2) Remarkably, we found that Al³⁺ ions adsorbed on M-GO can be effectively desorbed by adding a small amount of NH₃·H₂O, thus achieving the easy recycling M-GO without compromising its adsorption efficiency and magnetic performance. In addition, density functional theory calculations show that the interaction of graphene with Al³⁺ is stronger than with divalent ions, which promotes ion-surface adsorption and accounts for the huge difference in adsorption probability between Al³⁺ ions and other ions.

Fig. 2 | Ion adsorption and desorption of M-GO. a Adsorption kinetics of 10 mg/L Co²⁺, Mn²⁺, and Sr²⁺ by M-GO, as well as adsorption kinetics of 10 mg/L Al³⁺ added to the salt solutions (Co²⁺, Mn²⁺, and Sr²⁺) at 298 K, respectively. q_t denotes the adsorption capacity of M-GO with time. Light purple and light orange are highlighted to distinguish between adsorption and desorption. b Desorption rate of Co²⁺, Mn²⁺, and Sr²⁺ from M-GO by the subsequent addition of Al³⁺. c Adsorption kinetic parameters of Al³⁺during the ion desorption of Co²⁺, Mn²⁺, and Sr²⁺ by a pseudo-second-order rate model. Error bars indicate the standard deviation from three different samples.

    The method proposed represents a facile step for the high-efficiency desorption, extraction, and concentration of ions, and could be used to enrich a wider range of ions in the fields of energy, biology, environmental technology, and materials science.

    For more details on this work, please read our paper in Nature Communications at https://www.nature.com/articles/s41467-022-35077-9.