Unexpectedly efficient ion desorption of graphene-based materials

Ion desorption is extremely challenging for adsorbents with superior performance, and widely used conventional desorption methods involve high acid or base concentrations and large consumption of reagents. Here, we experimentally demonstrate the rapid and efficient desorption of ions on magnetite-graphene oxide (M-GO) by adding low amounts of Al3+. The corresponding concentration of Al3+ used is reduced by at least a factor 250 compared to conventional desorption method. The desorption rate reaches ~97.0% for the typical radioactive and bivalent ions Co2+, Mn2+, and Sr2+ within ~1 min. We achieve effective enrichment of radioactive 60Co and reduce the volume of concentrated 60Co solution by approximately 10 times compared to the initial solution. The M-GO can be recycled and reused easily without compromising its adsorption efficiency and magnetic performance, based on the unique hydration anionic species of Al3+ under alkaline conditions. Density functional theory calculations show that the interaction of graphene with Al3+ is stronger than with divalent ions, and that the adsorption probability of Al3+ is superior than that of Co2+, Mn2+, and Sr2+ ions. This suggests that the proposed method could be used to enrich a wider range of ions in the fields of energy, biology, environmental technology, and materials science.

In this manuscript, the authors studied the desorption of Co (II), Mn (II) and Sr (II) from M-GO by the addition of Al 3+ . The kinetic desorption and DFT calculation about the interaction of Al 3+ and other metal ions with M-GO were also carried out to understand the desorption of Co (II), Mn (II) and Sr (II). The kinetic desorption of bivalent metal ions from GO or M-GO by adding trivalent metal ions is a general information. The DFT calculation only gave the adsorption energies simply. Based on the contents, I think the manuscript does not merit the publication requirement in Nature Communication.
Reply: We sincerely thank the referee for his/her very helpful suggestions and very constructive comments.
As the referee said, it is general information that the interaction of graphene with Al 3+ is much stronger than that with divalent ions. However, the highlights of our work are mainly reflected in two aspects. First, one of the chief advantages is the efficient desorption of ions on M-GO; that is, the desorption rate of typical bivalent ions of Co 2+ , Mn 2+ , and Sr 2+ can reach 97.0% by adding very low amounts of Al 3+ (with a volume ratio of 1:500, 10 mg/L). The desorption performance is superior to that of the conventional desorption methods reported to date, which usually require the addition of acids and bases, including HCl and NaOH, and consume large amounts of HCl or NaOH solutions with concentrations as high as 0.1~0.2 M. The second chief advantage is the ultrafast ion desorption, within ~1 min, which is superior to that of conventional desorption methods with a long desorption time of approximately 1~2 h [Water Res. 39, 2273-2280(2005; Chem. Eng. J. 151, 113-121 (2009)]. Such rapid and efficient desorption or adsorption of ions on M-GO is unexpected and attributed to their superior dispersibility and the high specific surface area of monolayer GO in water [Carbon 52, 171-180, (2013); Chem. Eng. J. 226, 336-347 (2013)] as well as the strong cation-π interactions of the cations with the aromatic rings in the GO surfaces [Nature 550, 380-383 (2017)].
In response to the referee's suggestions, the revised manuscript reports on new DFT calculations of the binding of the cations on M-GO, including optimized geometries, energy adsorption distances, partial charges, and electron density differences. The new DFT calculation results show that all hydrated ions can be stably adsorbed on the surface of the M-GO model with considerable absorption energies, and Al 3+ has the strongest interaction with graphene compared with other cations studied here. In addition, ab initio molecular dynamic (AIMD) simulations were also performed to illustrate the underlying physical mechanism taking place in the kinetic sorption of Al 3+ on M-GO. The AIMD results of adsorption time and mean square displacements (MSD) show a fast adsorption speed of Al 3+ to graphene.
Further and importantly, we have achieved the reuse of M-GO by Al 3+ ion desorption in our revised work. From the cycle sustainability of Al 3+ @M-GO, the higher trivalent metal ions of Al 3+ interact strongly with the M-GO sheet over other bivalent metal ions, which introduced a more difficult desorption. Surprisingly, we found that the Al 3+ ions adsorbed on M-GO can be effectively desorbed by the addition of a small amount of NH3·H2O, such as 75 μL NH3·H2O (25~28%) added to the ~30 mL of Al 3+ @M-GO mixture solution (pH ~10). Yet, when the same amount of NH3·H2O was added to Co 2+ @M-GO, the desorption of Co 2+ ions cannot be achieved.
Importantly, the recycled M-GO could be reused easily multiple times without compromising its adsorption efficiency and magnetic performance, using Co 2+ as an example as shown below in Fig. R1.

Reply:
We thank the referee for his/her comments.
Our revised manuscript has been edited by a professional English editing service.
There are two types of chromium ions, Cr 2+ and Cr 3+ . We checked the reference we cited in the main text [Sci. Rep. 3, 3436 (2013)], in which adsorption energy between Cr 2+ and graphene was studied.
2. The authors mentioned "unexpectedly rapid and efficient desorption", it is necessary to give the desorption kinetic parameters to understand the rapid desorption. (1) where k (g/mg min −1 ) is equilibrium rate constant of the pseudo-second-order rate for Al 3+ , qt (mg/g) is the amount of Al 3+ adsorbed on the M-GO at time t (min), and qe (mg/g) is the adsorption capacity at equilibrium.

Changes made:
(1) We added a description and figure for the adsorption kinetic parameters of Al 3+ during the ion desorption of Co 2+ , Mn 2+ , and Sr 2+ by a pseudo-second-order rate model in the revised manuscript (pages 7-8 and Fig. 2c     (2) We further performed AIMD simulations on the kinetic process of Al 3+ adsorption on the graphene surface. Figure R5a shows the initial system containing a graphene sheet, 83 water molecules, 3 Cl − anions, and 1 Al 3+ cation. Initially, the Al 3+ cation is placed 0.80 nm from the graphene sheet. To rapidly minimize the approximate energy of the system, we performed a 200-step geometry optimization with the Al 3+ cation fixed. Then, based on the energy minimization result, we performed six AIMD simulations. Both the geometry optimization and AIMD simulations were performed using the CP2K 9.1 package, through the PBE method and DZVP-GTH basis. During the AIMD simulations, we placed a position constraint on the 3 Cl − anions, so that the Al 3+ would not be attracted by the Cl − anions and approach them. The temperature was set at 300 K by Nosé-Hoover thermostat.

Changes made:
We added the most stable structures, adsorption energies, cation adsorption distances, cation partial charges, and electron density differences of all systems, with the corresponding discussions and theoretical calculation method, to the revised manuscript ( Fig. 5 and pages 10

4.
It is very strange that the sorption of Sr 2+ is the highest, but the desorption percentage is also the highest.

Reply:
We thank the referee for his/her comments. To respond to the referee's comments, we checked and further repeated the experiments (Fig. R6), obtaining results that are consistent with the results in our main text (Fig. 3). In the previous version, the adsorption capacity is evaluated in mg/g (Fig. R7a). It should be noted that, compared with the adsorption capacity in mg/g, the adsorption capacity in mmol/g takes into account the number of ions adsorbed by M-GO with a certain specific surface area, and this can provide a more accurate adsorption difference between ions. As can be seen from Fig. R7b, the adsorption performances of M-GO for the three cations are very similar. This also can be confirmed by our DFT computations, showing that the corresponding adsorption energies of X@G are very close (Fig. R4).
For Sr 2+ , the relatively low adsorption capacity in mmol/g and adsorption energy correspond to a higher desorption rate. If it requires addition of a large amount of HCl or NaOH, a desorption problem that the authors claimed in the introduction section will occur again.
(2) In addition, desorption selectivity upon addition of Al 3+ ions should be discussed. Namely, analyze desorption amount of metal ions using M-GO which adsorbed two (or more) different metal ions.

Reply:
We sincerely thank the referee for these constructive suggestions and very positive comments regarding the novelty of our manuscript.
(1) The referee raised an important concern on the cycle sustainability of the M-GO after desorption with Al 3+ ions. Clearly, it is a greater challenge to achieve Al 3+ ion desorption, as the higher trivalent metal ions of Al 3+ interact more strongly with the M-GO sheet than bivalent metal ions.
Fortunately, we found that the Al 3+ ions adsorbed on M-GO can be effectively desorbed by the addition of a small amount of NH3·H2O. In detail, 75 μL NH3·H2O (25~28%) was added to the 30 mL mixture solutions to adjust the pH to 10, and then the mixture underwent magnetic separation and filtration. The separated M-GO was desorbed again with 30 mL DI water (containing 75 μL NH3·H2O). The concentrations of Al 3+ in the filtrates were determined. The desorption rates of Al 3+ on M-GO reached 78.5 ± 4.0% and 99.9 ± 0.1% for the two desorption steps, indicating that recycled M-GO could be achieved. However, with the same amount of NH3·H2O added to Co 2+ @M-GO, the desorption of Co 2+ ions cannot be achieved. Importantly, the recycled M-GO could be reused easily multiple times without compromising its adsorption efficiency and magnetic performance (Fig. R8). Furthermore, the corresponding concentration of NH3·H2O used here was two to three orders of magnitude smaller than that for the conventional acid-base desorption method.
We noted that alkaline conditions generally increase the ion adsorption of GO [J.  The total desorption rate for all mixed ions was 98.6 ± 1.3%, which corresponds to desorption rates of 98.6 ± 1.6%, 99.9 ± 0.1%, and 97.3 ± 4.7% for Co 2+ , Mn 2+ , and Sr 2+ , respectively. Therefore, the rapid adsorption, especially the efficient mixed ions desorption on M-GO by adding very low amounts of Al 3+ , is still consistent with those of the single-salt solutions.

Changes made:
(1) We added a description and figure for the Al 3+ ion desorption and the reusability of M-GO for Co 2+ adsorption in the revised manuscript (pages 2, 3, 10, and 13 and Fig. 4) (Fig. 4).
Furthermore, the corresponding concentration of NH3·H2O used here was two to three orders of magnitude smaller than the conventional acid-base desorption method." "We noted that the alkaline conditions generally increase the ion adsorption of  Fig. 4) Other points: Phrases such as "adding trace amounts of Al 3+ " (page 3, line 4), "a negligible volume of Al 3+ " (page 5, line 7) are overstatements. Analytically, these amounts are not negligible.

Reply:
We thank the referee for his/her comments. In response to the referee's comments, we revised "trace amounts" by "very low amounts" and added a detailed description for "very low amounts" and "a negligible volume" in our revision. In our experiments, the volume ratio of the Al 3+ solution to the mixed solution was 1:500. For example, 60 μL of Al 3+ was added to 30 mL mixed solution (10 mg/L Al 3+ in the prepared mixed salt solution) or 400 μL of Al 3+ was added to 200 mL mixed solution (10 mg/L Al 3+ in the prepared mixed salt solution).

Changes made:
We revised "trace amounts" by "very low amounts" and added a detailed description of "very low amounts" and "a negligible volume" in the revised manuscript (page 3, 5, and 6) and revised Supplementary Information (Section 1): (2) References cited in the introduction section are a bit biased. The authors cite only uranium examples for general high-efficiency extraction and concentration.
Furthermore, references 6-10 are papers discussing cation-π interactions or noncovalent interactions, but not showing graphene-based materials.

Reply:
We thank the referee for his/her comments. To respond to the referee's suggestion, we have cited the relevant papers in the introduction of our revised manuscript.

Changes made:
We cited relevant papers in the introduction of our revised manuscript (Page 2).
"Adsorption separation technology is one of the most effective and economical separation methods for high-efficiency extraction 1-4 , concentration 5-7 , and purification 8-11 . Ion-surface adsorption between cations and graphene-based materials 12-18 results in strong adsorption due to the one-atom-layer thickness and perfect aromatic ring structure of graphene 19 ." Referee #3 (Remarks to the Author): I recommend rejecting this paper based on poor English grammar usage. The manuscript in its current form is substandard. I am not commenting on the scientific merits of the research; rather, I am unable to understand the paper sufficiently to judge its scientific merits. The authors are encouraged to submit this manuscript for professional English technical editing by a fluent speaker.

Reply:
We thank the referee for his/her comments. Our revised manuscript has been edited by a professional English editing service (San Francisco Edit).
We sincerely thank the referees for their very helpful suggestions and very constructive comments, which helped us to improve our manuscript. In accordance with the suggestions of the referees, we added new calculations involving the binding of the cations on M-GO and the kinetic sorption of Al 3+ . We found that the interaction between Al 3+ and graphene is the strongest, and Al 3+ can be adsorbed very rapidly as seen from the dynamic simulations. Notably, we experimentally demonstrated the rapid and efficient desorption of typical bivalent ions on M-GO by adding very low amounts of Al 3+ and further were able to conveniently reuse M-GO based on the unique hydrolysis of Al 3+ . Our new experimental results confirmed the efficient ion desorption of graphene-based materials with a method that is facile, convenient, and reusable and consumes low amounts of reagents. This advancement will greatly improve the adsorption applications of Al 3+ ion desorption in energy, biology, environment, and materials science fields.
We think that all ambiguities in the manuscript have been clarified. We hope that

Changes made:
(1) We added the high-resolution TEM images and magnetization curves of M-GO after each cycle in the revised Supplementary Information Section 7 ( Supplementary   Fig. 6).
(2) We added a description in the revised manuscript (page 10): "Importantly, the recycled M-GO can be reused easily multiple times without compromising its adsorption efficiency and magnetic performance ( Fig. 4 and Supplementary Fig. 6)."    Table 1). We note that the k of Al 3+ in the desorption process of Cu 2+ is higher than those in other divalent ions (Co 2+ , Mn 2+ , Sr 2+ , and Cd 2+ ), which we attribute to the less amount of Cu 2+ desorption, allowing a shorter time to reach desorption equilibrium, thus resulting in a faster adsorption kinetic parameter. In all, these results indicate that the rapid desorption of the Cu 2+ and Cd 2+ ions also can be achieved by our method, indicating a wide application range of the method in this work.

Changes made:
(1) We added a description for the ion adsorption and desorption of M-GO for Cu 2+ and Cd 2+ in the revised manuscript (page 7): "Furthermore, similar rapid desorption of the Cu 2+ and Cd 2+ ions also can be achieved by our method (see Supplementary Information, Section 4), showing a wide range of applications of the method in this work." (2) We added a corresponding detailed description and figure in the revised Supplementary (Section 4).

"The ion adsorption and desorption kinetics experiments of M-GO for Cu 2+ and
Cd 2+ were further carried out, and the experimental procedures were consistent with those of Co 2+ , Mn 2+ , and Sr 2+ , refer to Section 1. 10 mg/L Cu 2+ and Mn 2+ solutions were prepared with CuCl2·2H2O and CdCl2·2.5H2O, respectively.
As shown in Supplementary Fig. 4a ions also can be achieved by our method, indicating a wide application range of the method in this work." (3) The k values of Al 3+ during the desorption of Cu 2+ and Cd 2+ ions were listed in the Supplementary Table 1. 3. In addition, make sure that figure numbers quoted in the manuscript are correct: for example, in lines 90 and 92 on page 4 ' Fig. 1c' should be Fig. 1d.
Reply: Many thanks. The errors have been corrected in the revised manuscript and we checked the manuscript clearly again. Now, we think that all ambiguities in the manuscript have been clarified. We hope that the referee will be satisfied with our response and will now recommend its publication in Nature Communications.