Unexpectedly efficient ion desorption of graphene–based materials

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Unexpectedly efficient ion desorption of graphene–based materials

https://doi.org/10.21203/rs.3.rs-577721/v1

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Ion desorption is extensive and extremely challenging for adsorbents with superior performance, for which conventional desorption methods involving high acid or base concentrations and large consumption of reagents are widely used. Here we experimentally demonstrated the unexpectedly rapid and efficient desorption of ions on magnetite–graphene oxide (M–GO) by adding trace amounts of Al³⁺. The corresponding concentration of Al³⁺ used was reduced by a factor of at least 250 compared with the conventional desorption method. The desorption rate reached up to ~ 97.0% for the typical radioactive and bivalent ions of Co²⁺, Mn²⁺, and Sr²⁺ within ~ 1 min. Importantly, we achieved the effective enrichment of radioactive ⁶⁰Co, with the volume of the concentrated ⁶⁰Co solution reduced by approximately 10 times compared with the initial solution. Density functional theory calculations revealed that the interaction of graphene with Al³⁺ was much stronger than that with divalent ions, yielding the adsorption probability of Al³⁺ superior to Co²⁺, Mn²⁺, and Sr²⁺ ions, suggesting that the proposed method could be used to enrich a wider range of ions in the fields of energy, biology, environment, and materials science.

Nuclear Engineering

Nanoscience

Adsorbents

Consumption of Reagents

Magnetite-graphene Oxide

Effective Enrichment

Density Functional Theory

Divalent Ions

Adsorption separation technology is one of the most effective and economical separation methods for high–efficiency extraction¹, concentration², and purification^3–5. Ion–surface adsorption between cations and graphene–based materials^6–10 results in strong adsorption due to the one–atom–layer thickness and perfect aromatic ring structure of graphene¹¹. Notably, this strong adsorption of ions can precisely fix the interlayer spacing of graphene membranes for water desalination^12,13, forming NaCl crystallization on the graphene surface in salt solutions with concentrations far below the saturated concentration¹⁴. The strong adsorption can potentially be exploited in graphene–based technology for applications such as high–efficiency extraction, concentration, and purification.

For the desorption of ions adsorbed on sorbents, which is one of the most important steps in adsorption separation technology, the conventional methods involve the addition of acids and bases including HCl and NaOH. These methods require the high consumption of HCl or NaOH solutions with concentrations as high as 0.1–0.2 M and a long desorption time of approximately 1–2 h^14–17. High multivalent metal ions such as Co²⁺, Cu²⁺, Cd²⁺, Cr²⁺, and Pb²⁺, in particular, interact strongly with the graphene sheet¹⁸ or biosorbents^16,17 and exhibit ineffective or slow desorption¹⁵ using these conventional methods. Moreover, these methods cannot be used to treat some functional graphene–based materials with superior performance, such as magnetite–graphene oxide (M–GO) with Fe₃O₄ nanoparticles, because of the simultaneous dissolution of functional groups^19–22. Therefore, it is difficult to achieve ion desorption with facile, convenient, and low consumption of reagents in graphene–based materials owing to strong ion–surface adsorption. These challenges hinder the potential ion adsorption applications of graphene–based membranes.

In this study, we observed the unexpectedly rapid and efficient desorption of ions adsorbed on M–GO by adding trace amounts of Al³⁺. The desorption rate reaches up to ~ 97.0% for typical radioactive and bivalent ions of Co²⁺, Mn²⁺, and Sr²⁺ within ~ 1 min, yielding a desorption performance superior to that of the conventional desorption methods reported so far. Interestingly, 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 revealed that monovalent and divalent ions should have lower adsorption energies than Al³⁺ has¹⁸, suggesting that the proposed method could be used to enrich a wider range of ions.

M–GO was prepared through chemical co–precipitation of magnetic iron oxide nanoparticles grown on graphene oxide (GO) sheets with Fe³⁺ and Fe²⁺ under alkaline conditions^23,24 (see Supplementary Information section PS1). Fe₃O₄ nanoparticles with an average particle size of 11.6 nm were well distributed in the GO sheets (Fig. 1b). High–resolution transmission electron microscopy (HRTEM) images (Fig. 1c) and X–ray diffraction (XRD) patterns (Fig. S1a) show that the Fe₃O₄ nanoparticles have a face–centered cubic structure with a lattice spacing of 0.257 nm. The Fe₃O₄ nanoparticles grown on the GO sheets were further demonstrated using Raman and X–ray photoelectron spectroscopy (XPS) spectra, as shown in Fig. S1b and Fig. S2, respectively. The magnetization performance of M–GO was measured at room temperature (298 K). As shown in Fig. 1c, the prepared M–GO had high magnetic properties with a saturation magnetization of 47.8 emu/g. The inset of Fig. 1c shows that M–GO can be rapidly attracted and separated within ~ 10 min using an external magnet, demonstrating its easy separation from aqueous solutions through magnetic solid/liquid separation.

Enrichment experiments of radioactive ions by the controllable ion desorption on M-GO were performed, using radioactive ⁶⁰Co as an example. As illustrated in Fig. 1a, step 1, the prepared M–GO (60 mg) was added to a 300 mL solution of 15 Bq/L ⁶⁰Co and 1.0 mg/L Co²⁺. Step 2: The mixtures were stirred at 298 K for 5 min and then separated through magnetic separation. Next, the M–GO, which was adsorbed with ⁶⁰Co and Co²⁺ ions (⁶⁰Co@M–GO), was removed and the ⁶⁰Co@M–GO was dispersed with deionized water to a final volume of 30 mL. In step 3, a negligible volume of Al³⁺ solution was subsequently added such that the concentration of Al³⁺ in the mixtures was 20 mg/L, stirred at 298 K for 5 min, and then separated through magnetic separation and filtration. The radioactivity of ⁶⁰Co in the filtrates was determined using a high–purity germanium γ spectrometer. The results are shown in Fig. 1e; the radioactivity of ⁶⁰Co was only 1.8 ± 0.4 Bq/L for the supernatants after magnetic separation in step 2, showing an efficient ⁶⁰Co removal of the M–GO. The final 30 mL solution after desorption exhibited high radioactivity up to 124.7 ± 4.1 Bq/L, with the volume of the solution reduced by a factor of about 10 compared with the initial ⁶⁰Co solution. This demonstrates the effective enrichment of radioactive elements through controllable ion adsorption and desorption.

Kinetics experiments on the desorption for typical radioactive and bivalent ions of Co²⁺, Mn²⁺, and Sr²⁺ adsorbed by M–GO using Al³⁺ solutions, were further performed. The prepared M–GO (200 mg) was added to 200 mL solutions of 10 mg/L Co²⁺, Mn²⁺, and Sr²⁺, and were stirred at 298 K for 125 min. Then, a negligible volume of highly concentrated Al³⁺ solutions was subsequently added such that the concentration of Al³⁺ in the mixtures was 10 mg/L, and stirred at 298 K for another 125 min. At designated time intervals ranging from 0 to 250 min, 5 mL of the solution was taken each time for filtration separation and measurement of the residual ion concentration. The adsorption capacities (q_t) were calculated (see Supplementary Information section PS2); the results of the kinetic experiments are shown in Fig. 2a. Rapid ion adsorption of Co²⁺, Mn²⁺, and Sr²⁺ adsorbed by M–GO occurred within 1 min after the addition of ions. The adsorption capacities (q_t) and the equilibrium adsorption capacities (q_e) of M–GO (Fig. S3) for Co²⁺, Mn²⁺, Sr²⁺, and Al³⁺ solutions are consistent with those of previous reports^23–25. Interestingly, for the subsequent addition of 10 mg/L Al³⁺ ions at 125 min, thorough desorption of Co²⁺, Mn²⁺, and Sr²⁺ ions originally adsorbed on M–GO occurred, along with the corresponding rapid adsorption of Al³⁺ ions. The desorption rate by the addition of Al³⁺ ions reached 99.9 ± 0.1%, 97.0 ± 2.1%, and 98.3 ± 2.6% for Co²⁺, Mn²⁺, and Sr²⁺ solutions, respectively (Fig. 2b).

Recent studies reported that ion adsorption equilibrium was achieved in 20–30 min for GO membranes^26–28. 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 materials^14,29 and biosorbents^16,17 exhibit strong ion adsorption, and high multivalent metal ions, such as Co²⁺, Cu²⁺, Cd²⁺, Cr²⁺, and Pb²⁺ interact particularly strongly with graphene sheets¹⁰ or biosorbents^16,17. 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 NaOH^16,17, 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 C₅₄H₁₈was used as a model for graphene, and the complexes were named X@G (X = Co²⁺–(H₂O)₆ and Al³⁺–(H₂O)₆). We thereafter calculated the adsorption energies (ΔE_X), which are defined as follows:

where E_X_@G denotes the total energy of the hydrated cation adsorbed on graphene, whereas E_G and E_X 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 interaction^12,18,30. The existence of these interactions was confirmed by ultraviolet absorption spectroscopy (Fig. S4).

Then, we can estimate the adsorption probability of Al³⁺ on graphene (P_Al), relative to that of Co²⁺ (P_Co), as follows:

where k_B denotes Boltzmann's constant and T the temperature. At 300 K, the calculated P_Al/P_Co 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 Al^{3+ 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.

Experimental operation. GO was prepared from natural graphite powder using a modified Hummers method³¹. M–GO was prepared through chemical co–precipitation of magnetic iron oxide nanoparticles by coating GO with Fe³⁺ and Fe²⁺ under alkaline conditions^23,24 (see Supplementary Information section PS1). The M–GO was characterized by TEM, XRD, Raman spectroscopy, XPS and VSM (corresponding parameters see Supplementary Information section PS1). In the adsorption and desorption of radioactive ⁶⁰Co for enrichment, the radioactive ⁶⁰Co solution was prepared from a ⁶⁰Co standard solution (National Institute of Metrology). The activity concentration of ⁶⁰Co in the solution was determined using a high–purity germanium γ spectrometer (GEM–100). The enrichment experiment was performed according to the steps shown in Fig. 1a. The activity concentrations of ⁶⁰Co in the adsorbed and desorbed solutions (steps 2 and 3, respectively) were detected after magnetic separation and filtration, respectively. The enrichment multiple (n) was calculated from the activity concentrations of the initial and desorbed solutions (see Supplementary Information section PS2). In addition, the details of experimental operation of adsorption and desorption study of Co²⁺, Mn²⁺, and Sr²⁺can be found in Supplementary Information section PS2.

Theoretical calculation. The hybrid DFT B3LYP^32–34 with Grimme’s dispersion correction³⁵ and 6–31G(d) basis set³⁶ was employed for geometric optimization, frequency, and energy calculations. All minima have no virtual frequency, and the energies are corrected using zero–point vibrational energies. All electronic calculations were performed using the Gaussian 09 program package³⁷. The structures were visualized using the CYL view³⁸.

Data availability

The authors declare that all the data supporting the findings of this study are available within the article (and its Supplementary Information file), or available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing financial interests.

Author Information

Xinming Xia, Feng Zhou and Jing Xu contributed equally to this work.

Author Contributions

H.F. and L.C. conceived the ideas. L.C., X.X. and F.Z. designed the experiments, simulations and co-wrote the manuscript. X.X., F.Z. and J.L. performed the experiments and prepared the data graphs. F.Z. performed the radioactive ⁶⁰Co enrichment experiments. J.X, Z.W. and Y.Y performed the simulations. All authors discussed the results and commented on the manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12074341, U1832150, 11875236, 11975206, 12075211, 11905186), the Fundamental Research Funds for the Provincial Universities of Zhejiang (2020TD001), and the Scientific Research and Developed Fund of Zhejiang A&F University (No. 2017FR032).

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Unexpectedly efficient ion desorption of graphene–based materials

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