Keywords

1 Introduction

Uranium content is about 3 ppb in the East crust [1] and ocean water [2]. This element is unevenly distributed in rocks. It is concentrated mainly in minerals, which contain a high amount of silicon, such as uraninite, carnotite, and torbernite. Some water sources contain heightened amount of uranium due to both its leaching from rocks and anthropogenic effect (working of nuclear power plants, mines, and mineral processing plants) [1]. Uranium also occurs in environment due to testing nuclear weapon, combustion of coal, and so on.

Since uranium is not only radioactive but also toxic, the maximal allowable concentration (MAC) of its soluble compounds is 15 ppb; smaller MAC values are also suggested [3]. Uranium and decay products attack all types of living cells causing organ pathologies, especially kidneys are poisoned. When uranium is accumulated in human organism, a risk of radiation sickness is enhanced. Thus, it is necessary to avoid the appearance of uranium in water sources, especially in drinking water. Among different methods, sorption technique is wide spread to remove small amounts of uranium from aqueous media.

Among a huge variety of inorganic and organic adsorbents of natural and synthetic origin, carbon materials are widely used for uranium removal, since they can be produced from cheap and available feedstock [4,5,6]. Moreover, activated carbon, particularly biochar, possesses considerable adsorption capacity, and resistance against acids and alkali. These materials can be easy utilized by means of combustion.

Recently, advanced carbon nanomaterials are in a focus of attention. Nanotubes, CNTs (single walled, SWCNTs, and multiwalled, MWCNTs) [7], and oxidized graphene (GO) [8] as well as graphene-like carbon nanomaterials occupy special position (Fig. 1). This is due to their hardness, ductility, light weight, and fast ion transport. Highly developed surface is attributed to them, this enhances adsorption. Both carbon nanotubes and GO can be easy modified. Moreover, GO initially contains considerable amount of −COOH groups, which are able to exchange cation. The disadvantage of carbon nanomaterials is their fine dispersion. Thus, it is difficult to separate solid and liquid, especially in the case of purification of a large volume of water.

Fig. 1
figure 1

Schematic images of SWCNT (a) MWCNT (b) and GO (c)

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In order to overcome this disadvantage, carbon nanomaterials are included into composites, the base of which forms large granules. Earlier we applied this approach to partially unzipped MWCNTs and GO-they were inserted to hydrated zirconium dioxide (HZD) [9,10,11] and zirconium hydrophosphate (ZHP) [9, 10]. It should be mentioned that HZD and ZHP were also used as modifiers of ion exchange resins [12,13,14,15]: selectivity of composites towards U(VI) compounds is considered from the point of view of a change of porous structure of the polymer support [12, 15]. Moreover, formation of surface complexes of U(VI) is also taken into account similarly to d-metal ions like Ni2+ [16]. Hydrated oxides of multivalent metals are also applied to preparation of inorganic composites for U(VI) sorption [17,18,19] and also to modifying polymer and ceramic membranes of wide functional purpose [20,21,22].

Adsorption of uranium compounds strongly depends on their speciation in solutions, acidic-basic properties of polymer or inorganic support of carbon nanomaterials, and chemical composition of the surface of carbon nanomaterials. In this review, the effect of these factors on sorption is considered briefly.

2 U(VI) Speciation

The content of one or other form of U(VI) is affected by the solution composition. As an example, Fig. 2 illustrates the simplified diagrams for nitrate and sulfate solutions [17], which were calculated based on [23]. More complex diagrams are given in [24]. Regarding nitrate solutions, they contain mainly cations in acidic media. When pH > 3, UO22+ ions are transformed into UO2(OH)+ species. Colloidal UO2(OH)2 particles dominate in alkaline region. In the case of sulfate solutions, cationic, neutral, and anionic forms co-exist in acidic media; insoluble UO2(OH)2 compound is formed in alkaline liquid.

Fig. 2
figure 2

U(VI) speciation in nitrate (a) and sulfate (b) solutions. Reproduced from [14] according to the permission of Springer Publisher

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Speciation of U(VI) compounds strongly affects sorption behavior of carbon nanomaterials and their composites. Besides U(VI) speciation, surface of carbon nanomaterials plays a key role for adsorption. This information is summarized in a review [25].

3 Carbon Nanotubes

Nanotubes are related to an allotrope modification of carbon. They are empty cylinder, a diameter of which is within the interval of several angstroms to several tens nanometers. Their length is from micron to several centimeters. The cylinder walls are rolled graphene planes. CNTs are intertwined forming large bundles due to strong Van der Waals interactions (Fig. 3a, [26]).

Fig. 3
figure 3

SEM images of pristine MWCNTs (a) and MWCNTs (b) modified with amidoxime. Reproduced from [26] according to the permission of Elsevier Publisher

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The adsorption properties of non-modified CNTs are affected by their kinds (for instance, a number of walls), purity. pores, surface area, etc. CNTs contain different adsorption centers: inner and outside surface, grooves, interstitial channels, and contacts of nanotubes in bundles. As shown in the example of gas adsorption, the outer surface is occupied before the filling internal surface [27]. The information about the removal of toxic metal ions from water with participation of CNTs is summarized in [28, 29], the data about MWCNTs in nuclear waste management are given in [30].

Pristine MWCNTs contain phenolic, lactonic, and carboxyl groups, and their total amount is 0.8 mmol g−1 [31]. The presence of oxygen-containing functional groups on the surface of purchased CNTs is reported also in [32]. However, adsorption behavior of pristine nanotubes is different. The equilibrium time of U(VI) adsorption is reached during ≈2 min [32] and 1 h [31] under similar conditions. This is caused by different content of functional groups, with which U(VI) forms surface complexes [33]. As a result, kinetic curves are approximated by the model of chemical reaction of pseudo-secondorder.

In order to enhance adsorption ability of CNTs, they are additionally oxidized with nitric acid [33,34,35] or plasma [36].

Different modifiers can be attached to the outer CNTs surface to provide adsorption ability towards toxic metal ions, particularly U(VI) compounds. For instance, MWCNTs modified with amidoxime were obtained by irradiation of the nanotubes followed by acrylonitrile grafting and its conversion into the adsorptive active component [26]. In general, modifying causes merging CNTs (see Fig. 3b). The models of Langmuir and chemical reaction of pseudo-second order were applied to U(VI) adsorption. The maximal capacity, which is realized at pH 3–5 (optimal conditions), is 68 mg g−1. The equilibrium is reached during 60 min. It means, the adsorption rate is comparable with that for ion exchange resins and their composites [12,13,14], but slower comparing with ion exchange fibers [37]. In the last case, the equilibrium time is about several minutes.

SWCNTs surface can be treated with nitric acid, −COOH groups are formed under these conditions [33]. Further, the nanotubes were modified with poly(amic-acid) by means of polymerization in-situ. The resulting material contained amide and carboxyl groups. Before the modifying with polymer, adsorption capacity toward U(VI) was 65 mg g−1, the capacity was higher in three times after modifying. It remained practically the same after five cycles of adsorption–desorption. Adsorption of U(VI) in a presence of alkaline metal ions, hardness ions and also Mn2+, Sr2+, Ni2+, Zn2+, Sm2+ was investigated. The degree of U(VI) removal was 80% in the case of the modified SWCNHs. At the same time, no selectivity has been found for SWCNTs containing only −COOH groups. It was suggested the complex formation between UO22+ on the one hand, amide and carboxyl groups on the other hand. The mechanism of U(VI) sorption on oxidized MWCNTs combines cation exchange and outer-sphere surface complexation in acidic media, precipitation occurs in neutral solutions [35]. Formation of complexes with amide groups is slower comparing with −COOH groups [36]. The reaction rate is comparable with that for aminogroups [38]. As opposed to [33], the materials [36, 38] show faster adsorption rate, when the equilibrium time is 1 h and higher. The equilibrium time is about 5 min. The potential of zero charge (PZC) has been estimated as 4.4 (pristine MWCNTs) and 5 (oxidized MWCNTs).

Chitosan molecules were used for MWCNTs modifying by means of low temperature plasma method, which allows us a possibility to graft them to the surface [39]. As found, sorption of U(VI) on obtained materials is affected by the solution pH, but it is not influenced by ionic strength. Adsorption is accompanied by the formation of inner-sphere surface complexes: this mechanism dominates over ion exchange or outer-sphere complexation.

Functionalization of MWCNTs with fragments containing carbonyl, amino, and amidogroups was considered in [40]. Depending on the solution composition, the modified material shows maximal capacity at pH 7, or plateau in neutral and weakly alkaline media, where colloidal particles are formed. The equilibrium time is very short (5–10 min); this is comparable with the data of [37] for ion exchange fibers. The kinetic data are approximated by the model of chemical reaction of pseudo-second order: it means high rate of the complex formation.

MWCNTs functionalized with N,N-dihexyl amide are considered in [41]. The capacity of 32 mg g−1 toward U(VI) has been found. The adsorbent possesses higher selectivity toward Th4+ comparing with U(VI). The structure of adsorbed UO22+ species has been proposed (Fig. 4).

Fig. 4
figure 4

Structure of UO22+ species adsorbed on MWCNTs modified with N,N-dihexyl amide [41]. Reproduced according to the permission of RSC Publisher

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Adsorption characteristics of MWCNTs are improved after impregnation with [2-(5-Bromo-2-pyridylazo)-5-(diethylamino)phenol] [42] or functionalization with dihydroimidazole [43]. Electronic and band structure of SWCNT with U atom at the outer surface were calculated by means of the density functional theory [44].

Since CNTs are finely dispersed, they cannot be related to perspective adsorbents. However, they can be used as an active component for composite preparation. This will be considered further.

4 Composites with CNTs

The composites including CNTs and magnetic nanoparticles are in a focus of attention, since magnets provide separation of finely dispersed carbon nanomaterials from liquid. The polyphosphazene-based composite containing CNTs and Fe3O4 nanoparticles (Fig. 5) was obtained and applied to U(VI) removal from aqueous solutions [45]. In this case, adsorption capacity is much higher comparing with that for pure CNTs. The maximal value, which was estimated from the Langmuir isotherm, reaches 606 mg g−1. The capacity of 20 mg g−1 has been found for CNTs. As opposed to CNTs, pH 5 corresponds to maximal adsorption.

Fig. 5
figure 5

Interaction of uranyl-ions with polyphosphazene, to which CNTs containing magnetic nanoparticles are incorporated [45]. Reproduced according to the permission of ACS Publisher

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When the initial concentration of U(VI) is 100 mg dm−3, no sufficient effect of NaCl on adsorption has been found (the concentration of this electrolyte was 0.1–0.5 mol dm−3). If the solution contains ions of d- and f-metals. U(VI) compounds are sorbed preferably. Adsorption on the composite is expectedly faster comparing with CNTs. The adsorbent can be regenerated with HNO3 solutions. In order to reach the desorption degree of 90% and higher, the solution concentration has to be 0.3–0.5 mol dm−3. From cycle to cycle of adsorption–desorption, the capacity of the composite decreased: its loss was ≈12%. Moreover, magnetic nanoparticles are dissolved during the treatment with acid. Their positive effect is providing convenient separation of solid and liquid.

The adsorbent of similar composition was obtained in [46]. However, aminogroups were inserted into polyphosphazene. The pH of PZC is 3.5, the maximal adsorption capacity is at pH 4–5. The capacity obtained from the Langmuir isotherm is 250 mg g−1. This is lower comparing with the value of [45], In the case of [45, 46], the time of equilibrium state is 80 min, U(VI) complexation with phosphate [45] and aminogroups [46] is suggested.

Magnetic composite based on MWCNTs were synthesized [47]. Preliminarily the carbon nanomaterial was oxidized with H2SO4 and modified with polyethylenimine. Magnetic nanoparticles (up to 20 nm) are seen in TEM image as dark spots (Fig. 6).

Fig. 6
figure 6

TEM images of pristine MWCNTs (a) and composite based on MWCNTs modified with polyethylenimine containing magnetic nanoparticles (b) [47]. Reproduced according to the permission of Elsevier Publisher

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As opposed to [45], the maximal adsorption capacity (450 mg g−1) is achieved at pH 6 [47]. At pH 2–3, the capacity is up to 50 mg g−1, further fast growth is observed. After pH 6, the capacity decreases down to 350 mg g−1. The pH of PZC is 6.2, and the equilibrium state is reached during 15 min. Increasing the temperature from 20 to 40 ℃ depresses adsorption evidently due to enhancement of U(VI) hydrolysis and transformation of ions into insoluble colloidal particles. The composite is more easily regenerated than that described in [45]: the concentration of nitric acid is only 0.01 mol dm−3.

The rod-like dual-shell composite consisting of polypyrrole, cobalt ferrite and MWCNTs has been reported in [48]. The material was synthesized with a hydrothermal method. In fact, MWCNTs are encapsulated into the polymer shell, there are magnetic nanoparticles on its surface. As found, the highest adsorption capacity is realized at pH 7 (≈150 mg g−1). It is suggested that U(VI) ions are attracted by π-electrons densities of the graphene structure. Other way is the UO22+ → H+ exchange, since MWCNTs are weak acceptors of protons. Electrostatic attraction of U(VI) and CoFe2O4 is also suggested. At last, surface complexation of U(VI) with nitrogen of polypyrrole is considered.

The composites of CNTs with polyvinylalcohol [49] and Cu2O—CuO [50] are reported. They are used both for U(VI) removal from water [49] and its analytical determination [50].

5 Unzipped CNTs

Among advance carbon nanomaterials, unzipped CNTs occupy special position. Graphene nanoribbons are graphene sheets with a width of tens nanometers [51] (Fig. 7). Their length corresponds to that for nanotubes. The nanoribbons can be related to the quasi one-dimensional CNTs, they derive functional properties from both CNTs and graphene. Chemical and electrochemical synthesis methods provide oxygen-containing functional groups on the surface of nanoribbons [52]. Three-dimensional aerogel consisting of graphene nanoribbons for uranium (VI) removal from water is reported in [53].

Fig. 7
figure 7

TEM image (a) and scheme of unzipped nanotubes, which contain functional groups (b) [51]. These groups provide U(VI) adsorption. Reproduced according to the permission of Elsevier Publisher

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The nanoribbons were used for U(VI) adsorption [51]. As opposed to MWCNTs, which demonstrate considerable adsorption capacity only at the pH interval of 6–10 (maximal capacity is 90 mg g−1), nanoribbons show wider pH diapason, where their adsorption ability is sufficient. For different samples, the capacity reaches 5–40 (pH 2), 30–95 (pH 3), and 45–130 (pH 4) mg g−1, when the initial concentration is 60 mg dm−3. This value depends on the amount of functional groups. Further, no sufficient change of the capacity is observed. Based on isotherms, the maximal capacity is 430 mg g−1.

The HZD-and ZHP-based composites containing partially unzipped MWCNTs were obtained in [9, 10]. Increase of their content in the composites results in a decrease of granule size. The optimal amount of the modifier is 2%; rather large granules (0.3–0.35 mm) are formed under these conditions. The carbon additions expand the pH interval, where adsorption capacity is sufficient. The unzipped MWCNTs increase sorption capacity of HZD at pH 3–4. Regarding ZHP, the addition is effective at pH 5–7. As found, the isotherms are modeled by Dubinin-Radushkevich equation indicating micropores, a size of which is comparable with uranyl ions. Different amount of carbon additions was inserted into inorganic matrixes. The ZHP-based composite removes U(VI) from aqueous media completely, when water contains hardness ions. In this case, the model of chemical reaction of the first order can be applied. When one-component solution is investigated, the model of pseudo-second order is the most suitable. The regeneration degree reaches 92%, when HNO3 solution is used.

6 Oxidized Graphene

The possibility to use GO for U(VI) removal from water is intensively investigated. GO is attractive due to its thermal and radiation stability, a large content of functional groups, which are able to ion exchange. Highly developed porosity and large specific surface area are also attributed to GO, it should be noted that the value of specific surface area, which is determined with a method of adsorption–desorption of nitrogen, is from 560 [54] to 900 [55] m2g−1, even lower values were obtained for partially unzipped MWCNTs (230 m2g−1). This is much lower than the theoretical magnitude for the isolated graphene flakes (≈2600 m2 g−1 [56]). This discrepancy is due to overlapping, curling, and agglomeration of the flakes. As a result, a part of the surface is unavailable for N2 molecules. The method of standard contact porosimetry gives 2000–2400 m2 g−1, when water is used as a working liquid [57]. Disjoining pressure provides the liquid penetration between graphene flakes. For comparison, the values of 550 and 325 m2 g−1 were obtained for SWCNTs and MWCNTs, respectively. The developed surface as well as functional groups provide excellent adsorption performance of GO.

Three methods of GO preparation from commercial exfoliated graphite were performed: chemical oxidation (Hummers method), electrolysis, and ball milling [58] (the techniques of preparation of GO-like materials from the feedstock of biological origin are given in [59]). Analysis of XRD patterns allows us to conclude that chemical treatment provides the most complete exfoliation and isolation of GO flakes. The peak at 10.2° corresponds to the interlayer distance of 0.87 nm. In the case of GO obtained by ball milling or electrolysis, the reflex at 22.5° indicates the distance of 0.4 nm. The compactness of structure decreases within the order: pristine carbon material > sample obtained by ball milling > sample obtained electrochemically > sample obtained by chemical oxidation (Fig. 8). Disordering structure is confirmed by Raman spectroscopy. XPS spectroscopy shows carbonyl, carboxyl and C − O groups for the chemically oxidized sample. Only C − O groups have nee found for other GO samples.

Fig. 8
figure 8

SEM images of the pristine carbon material (evidently exfoliated graphite) (a) and samples obtained by ball milling (b), electrochemical (c) and chemical (d) oxidation [58]. Reproduced according to the permission of Springer Publisher

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For three samples, the equilibrium state for U(VI) adsorption is reached after 120 min. However, the highest adsorption capacity (about 250 mg g−1) has been found for the sample obtained by chemical oxidation. Other samples show lower values (≈70–80 mg g−1). Slightly smaller value (≈50 mg g−1) has been found for reduced GO (rGO) [60], similar magnitude has been reported for sulfonated GO [61]. However, the value of 300 mg g−1 was suggested for GO obtained with Hummers method, the capacity for rGO is about 50 mg g−1. Much lower capacity is attributed to GO doped with Fe and Ni (25 mg g−1) [62]. All the data [58,59,60,61,62,63] were calculated from the Langmuir isotherms.

As found, adsorption curves for GO samples obtained with different methods obey the model of pseudo-second order [58]. The constant rate of the chemically obtained sample is lower in ten times comparing with other materials. It means, high oxidation degree, which is achieved by chemical oxidation, provides considerable U(VI) adsorption. However, adsorption is slower comparing with other samples due to the interaction of ions with the surface oxygen-containing groups. Unfortunately, no data about specific surface area are given in [58].

In order to improve adsorption properties of GO, it is modified with functional groups similarly to CNTs. For instance, aminogroups were grafted to the GO surface [64]. The adsorption capacity of GO and GO-NH2 was found to be 97 and 215 mg g−1, respectively, under ambient temperature. Functionalized adsorbent shows higher capacity than the pristine sample at pH 4.5–6.5. No sufficient improvement of adsorption is observed outside this pH interval. The rate constant of adsorption is lower in ≈10 times for the modified GO. Functionalized adsorbent can be regenerated with a 0.5 M HCl solution. The loss of capacity is 10% after 3 cycles of adsorption–desorption. No effect of ionic strength (NaCl) on adsorption has been found.

More complex modifier, such as diethylenetriaminepentaacetic phenylenediamine, has been proposed in order to increase the GO capacity [65]. In this case, adsorption capacity of GO reaches about 500 mg g−1. At the same time, the constant of adsorption rate is two orders less.

It is noted that π-conjugation system makes difficult the techniques for GO modifying [66]. A novel strategy has been proposed through post-decoration with amidoxime functionalized diaminomaleonitrile. This approach allows one to activate the inert sites in GO flakes. Adsorption capacity toward U(VI) reaches 935 mg g−1, no sufficient change of this value is observed at pH 6–10. Unexpected result has been obtained: the time of the adsorption equilibrium is one third of that for GO. Preferable adsorption of U(VI) is realized, when the solution contains alkaline, alkaline earth, and d-metal ions.

Amidoxime-functionalized β-cyclodextrin/GO aerogel was synthesized in [67] using hydrothermal procedure (Fig. 9). The resulting material possesses higher porosity caused by the cross-linkage of GO flakes. At the same time, β-cyclodextrin increases the interlayer spacing avoiding overlapping by π − π stacking interactions. This also leads to loosening GO structure.

Fig. 9
figure 9

Change of GO morphology during GO modifying and scheme of the resulting aerogel (a), mechanism of U(VI) adsorption [67]. Here, DMAO−amidoxime-functionalized diaminomaleonitrile, CMCD−carboxymethyl-β-cyclodextrin. Reproduced according to the permission of ACS Publisher

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U(VI) adsorption reaches equilibrium after 60 min, the maximal adsorption capacity evaluated from Langmuir isotherms is 650 mg g−1. The aerogel shows excellent adsorption performance in a presence of competitive cations, anions, and organic substances (oil). The capacity reaches 19.7 mg g−1 after 21 days of the aerogel exposition in 50 dm3 of natural seawater. Cyclodextrine-modified GO was also used for the removal of both U(VI) and humic acids from water [68].

Aside from surface modifying, other approach was used-GO aerogel was impregnated with Tri-n-butyl phosphate (TBP)/n-dodecan [69]. Higher adsorption capacity (316 mg g−1) has been reported comparing with [58,59,60,61,62,63]. The disadvantage of the impregnation technique is a leakage of solvent.

Besides U(VI), adsorption of compounds containing uranium of other valency, for instance, UC2, is considered [70].

It should be stressed that despite excellent adsorption performance, practical application of GO (except probably aerogel) is limited due to its fine dispersion. In order to overcome this difficultness, GO-containing composites are synthesized. This will be considered further.

7 Composites Containing GO

Last ten years, GO is in a focus of attention as a material for composite preparation. This addition provides high specific surface area of the support and, as a result, considerable adsorption capacity. The support forms large granules. It means, the composite can be used as a filler of ion exchange columns. It should be stressed that carbon additions lead to a decrease of granule size, when inorganic matrix is used [9, 71]. It means large content of GO is undesirable from the technological point of view.

As shown in the example of HZD, GO makes its structure less compact at nano- and microlevel, since GO flakes cover HZD particles [72] preventing their cross-linkage, deteriorating mechanical properties of granules and decreasing their size. GO possesses superhydrophilic properties similarly to ion exchange polymers, i.e., its porosity in water is higher than that in octane (ideally wetting liquid) [57]. Similar property is attributed to the HZD-GO composite [72]. Since GO contains only cation exchange groups, it enhances adsorption of cations on composites and depresses anion exchange.

In order to facilitate separation of solid from liquid, magnetic GO-containing composites were synthesized [73,74,75,76]. For instance, tea wastes (TW) were used as a support of rGO [76]. Following reaction occurred during the composite preparation:

$${\text{GO}} - {\text{TW }} + {\text{ Fe}}^{{{2} + }} \to {\text{Fe}}^{{{3} + }} + {\text{ rGO}} - {\text{TW}}$$
(1)
$${\text{Fe}}^{{{3} + }} + {\text{H}}_{{2}} {\text{O}} \to {\text{FeOOH}}$$
(2)
$${\text{rGO}} - {\text{TW}} + {\text{Fe}}^{{{\text{n}} + }} \left( {{\text{n}} = 2{, }3} \right) + {\text{FeOOH}} + {\text{NH}}_{4} {\text{OH}} \to {\text{rGO/Fe}}_{{3}} {\text{O}}_{{4}} - {\text{TW}}$$
(3)

The maximal value of adsorption capacity is realized at pH 5, the equilibrium time is about 60 min. Adsorption capacity, which was estimated from the Langmuir isotherms, is ≈90 (TW), 80 (GO), and 110 (GO-TW) mg g−1. In other words, synergetic effect is observed. Reduction of GO causes a decrease of the capacity down to 100 mg g−1. Despite lower capacity comparing with GO (see above) the composite is attractive, since it requires not so much GO, the support is low cost and cheap. At last, the adsorbent can be easy removed from liquid. Similar capacity magnitude has been obtained for the composite based on MnO2 containing Fe3O4 and rGO (≈90 mg g−1 under ambient temperature) [73]. In this case, the equilibrium is achieved after 6 h. As opposite to the materials containing rGO, the capacity of GO-containing adsorbent based on ferberite (FeWO4) is much higher (455 mg g−1) [74].

Synthetic polymers are also used as a support. For instance, the composite based on amidoximated polyacrylonitrile containing GO was obtained [77]. As shown in Fig. 10, GO flakes are transformed into dendrites after modifying. The maximal capacity of the composite towards U(VI) is about 200 mg g−1.

Fig. 10
figure 10

SEM images of GO and the composite including Change of GO morphology during GO modifying and scheme of the resulting aerogel (a), amidoximated polyacrylonitrile [77]. Reproduced according to the permission of Elsevier Publisher

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Besides synthetic inorganic compounds and organic polymers, such natural materials as clay mineral [78], biopolymers [79,80,81] and even microorganisms [82] are considered as a support of graphene. The composites of graphene with these materials are effective adsorbents of U(VI) compounds.

8 Conclusions

This chapter is devoted to advanced carbon nanomaterials, such as graphene, nanotubes, and nanoribbons produced from them, for U(VI) removal from aqueous media. It is noted that the adsorption capacity of these materials is up to 100 mg dm−3. Special approaches, such as oxidation and grafting organic fragments allow us to increase the capacity in several times. Carbon adsorbents possess high selectivity toward U(VI) compounds; they are not sensitive to ionic strength of a solution. This is due to U(VI) complexation with surface functional groups. However, adsorption is strongly affected by the solution pH: the highest removal degree is reached at pH > 4–5 due to the features of U(VI) speciation. In all cases, the rate-determining stage of adsorption is chemical reaction of pseudo-second order. It is stressed that the carbon nanomaterials are finely dispersive, this makes impossible their usage as a filler of ion exchange columns. In order to overcome this disadvantage, they are inserted into supports, such as inorganic ion-exchangers, synthetic or biopolymers and even microorganisms. It is possible to obtain large granules by this manner. The carbon additions change morphology of a support providing high specific surface area, which is necessary for providing high adsorption capacity.

Further development of sorbents containing advanced carbon nanomaterials, which are selective toward U(VI) compounds, is a choice of suitable support. The support should possess a complex of needed functional properties: high adsorption capacity, selectivity, chemical stability as well as ability to form large durable granules. The synergetic effect is expected, when carbon nanomaterials would be inserted into such support.