Highly selective uptake of Ba²⁺ and Sr²⁺ ions by graphene oxide from mixtures of IIA elements

Abstract

We show here that graphene oxide selectively gathers heavy IIA group elements in the order of Ba²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺ with enrichment factors >100.

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Strontium 90 (⁹⁰Sr) is a radioactive isotope which the human body incorporates into its bone structure because it is not distinguishable from Ca²⁺. Such incorporated ⁹⁰Sr cannot be removed from the organism and, having a half-life of 28.8 years, causes cancer. Given the large release of ⁹⁰Sr into the environment during the last two major nuclear power plant accidents, i.e., Chernobyl (⁹⁰Sr released into the atmosphere) and Fukushima (⁹⁰Sr released into seawater), there is an urgent need for efficient remediation. Barium is also present in fission products in the form of ¹³¹Ba and ¹³³Ba isotopes. Ba²⁺ is a highly toxic element (BaSO₄ is non-toxic only because of its extremely low dissociation constant)¹ and it can affect the human nervous system by blocking potassium ion channels in cell membranes.² Removal of barium from the environment, and from industrial waste, is very important as it has broad uses in paint, fireworks, and special ceramics.³ However, separation of Ca²⁺, Sr²⁺, and Ba²⁺ pose a great challenge to ion-exchange materials.⁴ Recently, an interesting report was made about green algae that was capable of separating Sr²⁺ from Ca²⁺ by coprecipitation with barite.⁵ However, this method possesses major limitations because a particular cell culture is required.⁵ High purity Ba and Sr compounds are important for the production of ferroelectric materials and other mixed oxide compounds. Similarly, preferential sorption of Pb(II) ions further showed the complex preference among first series transition metal ions.⁶ This interesting work shows (as we confirm in this report) that preferential sorption selectivity increases down the group; note that the novelty of our paper is that it is focused on alkaline earth elements.

Graphene oxide (GO) has a very large surface area (up to 2630 m² g⁻¹),⁷ which is significantly larger than that of single-walled carbon nanotubes (1315 m² g⁻¹)⁸ and graphite (10–20 m² g⁻¹).⁹ GO can be readily prepared from inexpensive graphite and contains a large number of oxygen-containing groups (C : O ratio ∼2). Furthermore, GO can be easily dispersed in water but it is easier to separate it from suspension via simple filtration, because the size of the GO flakes is in the order of micrometers. Finally, GO can be used as a highly permeable membrane for water filtration.¹⁰

Here, we show that GO exhibits an extremely different uptake of alkaline earth metal cations, with uptake dramatically increasing with the increasing atomic mass of the elements in the IIA group, increasing in the order Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. We also show that GO is capable of exhibiting highly preferential removal of trace amounts of Ba²⁺ from Sr(NO₃)₂, which are present as impurities in ACS purity class chemicals. Such highly preferential removal of Sr²⁺ and Ba²⁺ from other IIA group elements using inexpensive GO can be applied to environmental remediation.

We have prepared GO by oxidizing graphite using a mixture of sulphuric acid, nitric acid and potassium chlorate (Hofmann's method¹¹) to yield graphite oxide, which was consequently ultrasonicated to produce GO. 100 mL of a 1 mol L⁻¹ solution of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, produced from their respective nitrates, was then mixed with 100 mL of a suspension containing 0.1 g of GO, and stirred for 24 hours. The solution was filtered and the remaining material was dried. This material was labelled GO–IIA. Full characterization of the GO prepared by this method was published by our group previously. Briefly, the elemental composition of GO was 50.81 at.% of C, 20.89 at.% of H, 0.31 at.% of S and 27.99 at.% of O, based on combustible analysis.¹² Consequently, it was heated to 1000 °C to create a stable material, which was not susceptible to swelling by hydration from atmospheric humidity.¹³ The materials are indicated in the following text as G–IIA (general) or G–Me²⁺, where Me is substituted for the particular IIA element. The resulting G–IIA materials were analyzed for their IIA ion content using X-ray fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and SEM/EDX. In the following text, we will first show individual IIA ion removal from solution, followed by the high selectivity of GO toward heavier IIA elements.

Individual ion uptake

GO–IIA and G–IIA materials were analyzed by X-ray fluorescence as it is precise and the general method for determining elemental composition. GO contained 0.110% (wt) Mg, 0.261% Ca, 0.541% Sr, and 2.192% Ba. The concentration of IIA ions in aqueous solution was 33 mmol L⁻¹. The increase in sorption capacity corresponded to the trend in ionic radius increase from 86 pm for Mg²⁺, 114 pm for Ca²⁺, 132 pm for Sr²⁺, and 149 pm for Ba²⁺.¹⁴ However, other factors such as the tendency to form coordination compounds and the strength of the bonds with oxygen functionalities, play roles in the sorption capacity of both graphite oxide and graphene oxide. Heated samples of G–IIA contained 0.240% (wt) Mg, 0.277% Ca, 0.881% Sr, and 6.283% Ba. A clear trend in the uptake of the heavier elements by GO, especially of Ba²⁺, is observed. The uptake of IIA elements is likely due to their chelation by the oxygen-containing groups in GO. These metals are also strongly bonded and remain in graphene even after high-temperature treatment.

The elemental composition of the G–IIA samples was investigated using XPS. According to the wide scan spectra shown in Fig. 1, among the four sample graphenes, only Ba can be found in the wide scan spectrum (Fig. 1D) as 0.087 at.%. A high-resolution scan for four of the IIA group metals was then performed. These are shown in Fig. 1E–H. From the results, it is obvious that Mg and Ca cannot be detected by XPS whereas Sr and Ba can be detected; the amount of absorbed Ba is significantly higher than that of Sr. These results confirm the trend suggested by the XRF data. One should note that XPS is highly surface sensitive. The XRF results provide better overall information about the elemental composition of the samples. SEM/EDX analysis of the G–IIA samples is presented and discussed in the ESI (see Fig. S1†).

Fig. 1 Left: wide scan spectra of (A) G–Mg, (B) G–Ca, (C) G–Sr, and (D) G–Ba. Right: high-resolution XPS spectra of the absorbed group 2 metals (same scale); (E) Mg, (F) Ca, (G) Sr, and (H) Ba.

It is very important to assess the ability of GO to selectively gather individual elements from their mixtures. The total concentration of all IIA group ions was 10 g L⁻¹ for all binary mixtures. We have exposed GO to various mixtures containing barium and strontium nitrates. GO selectively binds (takes up) heavier elements. The selectivity towards the uptake of Ba²⁺ was tested for a solution of nitrates with Ba²⁺/Sr²⁺ ratios of 0.5, 0.1, and 4 × 10⁻³. The original mixture ratio of Ba²⁺/Sr²⁺ in solution was 0.5. After the uptake of Ba²⁺ and Sr²⁺ by GO, their ratio increased to 34.1 in solid GO. Similarly, a solution containing a mixture of Ba²⁺ and Sr²⁺ in a ratio of 0.1 resulted in preferential uptake of Ba²⁺, with a Ba²⁺/Sr²⁺ ratio of 16 in solid GO and an enrichment factor of 160. Finally, we demonstrated the highly preferential uptake of Ba²⁺ over Sr²⁺ in the case of a trace concentration of Ba²⁺, which is typically present in ACS p.a. (for analysis) purity grade strontium nitrate. These types of chemicals typically have about 0.1–0.3 wt% of Ba²⁺ (the Ba²⁺/Sr²⁺ ratio in ACS p.a. pure Sr(NO₃)₂ in our case was 4 × 10⁻³). Even for such a low concentration, we observed a strong increase in the Ba²⁺/Sr²⁺ ratio after the uptake by solid GO (Fig. 2), which had an enrichment factor of 139. Similar behavior was observed for mixtures containing other lighter elements from the IIA group and also the IA group. We performed tests with a ratio of 0.1 in the liquid phase for the Sr²⁺/Ca²⁺, Sr²⁺/Mg²⁺ and Ca²⁺/Mg²⁺ mixtures. For the Sr²⁺/Ca²⁺ mixtures, we observed a ratio of 0.64 in solid GO. For the Sr²⁺/Mg²⁺ mixture, the ratio was 0.45 and for the Ca²⁺/Mg²⁺ mixture, the ratio was 0.28. In all cases, we obtained an enrichment of the heavier ion concentration in GO, compared to the ion ratio in solution. Finally, we tested a complex mixture containing Na⁺, K⁺, Mg²⁺, Ca²⁺, and Sr²⁺ with equal concentrations of 2 g L⁻¹. In this case, we also observed enrichment of the heavier elements in GO compared to their concentrations in solution. The element ratio in GO was 0/0.28/0.39/0.60/1 for Na⁺, K⁺, Mg²⁺, Ca²⁺, and Sr²⁺, respectively. Ions with a 1+ charge only have minimal affinity towards GO, and we observed that only trace amounts had been taken up from the solutions. The concentration of Na was below 10 ppm and the concentration of K was about 30 ppm. The higher uptake of potassium is related to the large ionic radius of K⁺ (152 pm). The ability to form coordination compounds is also very important, and a lower concentration of 1+ ions compared to 2+ ions was observed. This effect is related to the low tendency of alkali metals towards coordination with oxygen functional groups.

Fig. 2 Sorption ratio for Ba²⁺/Sr²⁺ mixtures in solution and the sorption ratio when taken up by graphene oxide. Note that the y-axis is logarithmic.

In summary, we observed an increase in graphene oxide affinity towards alkaline earth ions with an increase in atomic number. The greatest difference was observed in the case of Ba²⁺ ion uptake. We have shown that even trace amounts of barium can be selectively removed from alkaline earth elements and alkali metals. A high enrichment ratio was also observed in the case of strontium toward lighter alkali earth elements (calcium and magnesium), as well as alkali metals. The sorption capacity for monovalent ions was found to be negligible. Barium belongs to a group of highly toxic elements and graphene oxide can be used for the removal of such elements from the environment. Other applications are in the field of alkaline earth compound purification. Graphene oxide can be applied to the preparation of high purity strontium compounds where barium is a very common impurity. The enhanced sorption capacity toward strontium in comparison with lighter elements can also be applied to the separation of strontium. This is very important for ⁹⁰Sr radioactive isotopes. From the increase in sorption activity for heavy elements, an extremely high affinity toward Ra²⁺ ions may also be likely, and possible applications for separating radium from mixtures are apparent. This characteristic can lead to broad applications for the separation of radium from uranium ore.

Experimental section

Materials

Graphite (2–15 μm, 99.99995% purity) and strontium nitrate (99.95%) were obtained from Alfa Aesar, Germany. Deionised water with a conductivity of 18.2 MΩ cm was used in the preparation of solutions. Sulfuric acid (98%), nitric acid (68%), potassium chlorate (>99%), hydrochloric acid (37%), silver nitrate (>99.8%), barium nitrate (>99%), magnesium nitrate hexahydrate (>99%), calcium nitrate tetrahydrate (>99%), strontium nitrate (>99%), sodium nitrate (>99.5%) and potassium nitrate (>99%) were obtained from Lach-Ner, Czech Republic. Nitrogen (99.9999%) was obtained from SIAD, Czech Republic.

Apparatus

Scanning electron microscopy (SEM) images were obtained using a JEOL 7600F field emission scanning electron microscope (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) samples were prepared by compacting a uniform layer of the materials on a carbon tape. These samples were measured using a monochromatic Mg X-ray radiation source (SPECS, Germany) and a Phoibos 100 spectrometer in order to obtain survey and high resolution C1s, O1s and metals spectra. An Axios sequential WD-XRF spectrometer (PANalytical, the Netherlands) was used to perform the XRF analysis. It was equipped with a Rh anode end-window X-ray tube fitted with a 50 μm Be window. All 11 scans, covering 79 measured elements, were collected using software SuperQ in vacuum. The obtained data were evaluated by standardless software Omnian. The analyzed powders were pressed into H₃BO₃ pellets without any binding agent and covered with 4 μm supporting polypropylene (PP) film. The measurement time was 20 min.

Synthesis

Graphite oxide prepared by the Hofmann method¹¹ was named HO-GO. 87.5 mL of concentrated sulphuric acid and 27 mL of nitric acid were added to a reaction flask containing a magnetic stir bar. The mixture was subsequently cooled at 0 °C and 5 g of graphite was added. The mixture was vigorously stirred to avoid agglomeration and to obtain a homogeneous dispersion. While keeping the reaction flask at 0 °C, 55 g of potassium chlorate was slowly added to the mixture in order to avoid a sudden increase of temperature and the formation of explosive chlorine dioxide gas. Upon the complete dissolution of potassium chlorate, the reaction flask was then loosely capped to allow the escape of the gas evolved and the mixture was continuously stirred vigorously for 96 h at room temperature. On completion of the reaction, the mixture was then poured into 3 L of deionized water and decanted. Graphite oxide was then redispersed in HCl (5%) solutions to remove sulphate ions, and repeatedly centrifuged and redispersed in deionized water until a negative reaction on chloride and sulphate ions (with AgNO₃ and Ba(NO₃)₂ respectively) was achieved. A graphite oxide slurry was then dried in a vacuum oven at 50 °C for 48 h before use.

For the measurement of sorption capacity, 0.2 g of graphene oxide (GO) was dispersed in 100 mL water by ultrasonication (400 W, 60 minutes). 50 mL of 0.1 M solution of alkaline earth nitrate (Mg, Ca, Sr and Ba) was added to the suspension of formed GO and stirred for 24 hours. The coagulation of the GO suspension starts immediately after addition of the nitrate solution. The reaction mixture was suction filtered and repeatedly washed using deionized water. The obtained GO composite was dried for 48 hours in a vacuum oven at 50 °C before characterization. For the testing of selective sorption, 0.2 g GO and 1 g of the total amount of ions in various ratios was used.

The thermal exfoliation of the GO composite was carried out at 1000 °C for 12 minutes, by placing the GO inside a porous quartz glass capsule, which was connected to a magnetic manipulator in a vacuum tight quartz reactor under a controlled atmosphere. This system provided a temperature gradient of over 1000 °C min⁻¹. The sample was then flushed repeatedly with pure nitrogen and subsequently inserted into a preheated reactor under a nitrogen (99.9999% purity) atmosphere (pressure: 100 kPa), to give the noble metal doped graphene hybrid material. The flow of the nitrogen was 1000 mL min⁻¹, resulting in the removal of the by-products of the reactions.

Acknowledgements

This research was supported by a Specific University Research grant, MSMT no. 20/2014, Czech Republic. M.P. acknowledges the Tier 2 grant (MOE2013-T2-1-056; ARC 35/13) from the Ministry of Education, Singapore.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02640c

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