Hongjuan
Liu
ab,
Tianyu
Fu
a,
Ziying
Cao
a and
Yuanbing
Mao
*b
aSchool of Nuclear Science and Technology, University of South China, Hengyang, 421001, China
bDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA. E-mail: ymao17@iit.edu; Tel: +1 312 567 3815
First published on 5th March 2024
Uranium is a nuclear contaminant possessing radioactivity and chemical toxicity. It can be discharged into the environment from multiple sources such as uranium ore mining and hydrometallurgy, used nuclear fuel disposal and nuclear accidents. Uranium could be a growing threat to human survival and biodiversity if it is released into the environment without treatment. As one of the treatment technologies, uranium adsorption, therefore, has become an important research area. The traditional eco-friendly hydroxyapatite (HAP) and emerging MXenes have been proved to serve as potentially ideal adsorbents for uranium while there is no review about their uranium adsorption. In this paper, the recent research status of HAP and MXenes as uranium adsorbents is overviewed. The uranium adsorption capacity, adsorption influencing factors, and interaction mechanisms of these two types of materials are discussed. In addition, MXenes are a new class of two-dimensional materials, and their synthesis methods are constantly updated. Thus, the latest progress of the preparation methods of MXenes is reviewed in detail. Furthermore, we have pointed out some challenges in their use for uranium adsorption and suggested possible future research directions.
Environmental significanceUranium is one of the nuclear pollutants with radioactivity and chemical toxicity, and it threatens human survival and biodiversity after its release into the environment without treatment. The traditional eco-friendly hydroxyapatite (HAP) and emerging MXenes have been proved to be potentially ideal adsorbents for uranium. Nevertheless, there are few reviews about the two materials for uranium adsorption. In order to fill this gap, we reviewed the research progress of HAP- and MXene-based materials in uranium adsorption. We believe that this review will attract much attention of relevant researchers and give them enlightenment. |
HAP is an eco-friendly adsorbent for U(VI) uptake with its strong adsorption ability, excellent biocompatibility and biodegradability, low cost, easy attainability, outstanding stability, and low solubility. Therefore, uranium adsorption by HAP-based materials has been broadly investigated. Phosphate in HAP could control the transformation and migration of U(VI) and generate uranyl phosphate precipitates, which has been confirmed by many related studies.4–6 The precipitation of U–phosphate phases of chernikovite, meta-autunite or autunite is the mechanism of the immobilization of uranyl for HAP due to the dissolution–precipitation reaction. Furthermore, reactions of HAP with U(VI) have been found to include three main adsorption mechanisms: ion exchange, surface complex formation, and/or solution–precipitation.7–9 It was also found that the occurrence and the degree of the dissolution–precipitation reaction depend largely on the simultaneous reaction and the water environment, such as U(VI) concentration, pH, and alkalinity.10 In addition, the lattice of HAP is so flexible to allow the substitution of other cations or anions for Ca2+, PO43−, or OH− in the HAP, which was found to influence the structure and properties such as the uranium adsorption capacity and acidic stability of HAP.11–14 Initial HAP is easily decomposed in an acidic environment and easily aggregated in aqueous solution, and its uptake ability and selectivity lessen in the actual wastewater with numerous co-existing ions. To advance the adsorption properties of HAP, various means have been applied to modify HAP. Grafting functional groups such as amine or tributyl phosphate into HAP generally enhances the adsorption ability and selectivity for uranium. Magnetic iron oxide combined with HAP can be beneficial to its reusability. However, it is still challenging to achieve high selectivity for adsorption of uranium due to the strong interference caused by various coexisting ions in actual ambient water. Therefore, improving the adsorption capacity and selectivity of HAP in complex and harsh wastewater environments by further research is still needed.
As newly emerging two-dimensional materials, MXenes have recently attracted extensive attention and shown potential application in multiple fields such as supercapacitors,15 gas sensors,16 catalysis,17 lithium-ion batteries,18 hydrogen storage,19 electromagnetic shielding,20,21 environmental remediation,22,23 and water desalination.24 MXenes, written as a chemical formula of Mn+1XnTx (M represents transition metals, X represents C or N, T represents the surface terminations such as –O, –OH, and/or –F, and n = 1–3), are commonly produced by selective etching of group A elements (Al, Ga, In, Si, Ge, Sn, Pb, P, As, S or Cd) from their three-dimensional MAX phase precursors (Mn+1AXn).25 The synthesis procedures of most MXenes generally include the utilization of toxic and dangerous concentrated HF, which has hindered the development and further applications of MXenes. Thus, the development of milder and less hazardous etchants for exfoliation and delamination of MAX precursors is necessary and an important research direction in the future.26,27 So far, less dangerous fluoride salt-based etchants and fluorine-free etchants have been developed for exfoliation and delamination in the synthesis process of MXenes. In addition, MXenes have a heterogeneous surface composition containing fluoro, hydroxyl, and oxo terminations, and the properties of MXenes are highly sensitive to the surface composition and structure causing suboptimal performance. Therefore, Jawaid et al. developed an efficient etching method that utilizes halogens (Br2, I2, ICl, IBr) in anhydrous media to synthesize MXenes with homogeneous surfaces from Ti3AlC2 at room temperature in 2021.28 This solvent-based halogen etching enables exciting opportunities for widespread applications. However, there are still challenges in developing non-hazardous and optimization synthesis techniques and expanding a broader range of MAX-phase precursors. Additionally, synthetic methods greatly influence the surface functional groups and properties of MXenes, which subsequently affect their uranium pollution treatment. Therefore, termination groups and properties of MXenes should be considered when MXenes are prepared.
MXenes have several merits of high oxidation–reduction activity, large specific surface area, great surface hydrophilicity, excellent radiation resistance and thermal stability, which make them desirable adsorbents for U(VI).29 For instance, Wang et al. predicted through density functional simulation that V2CTx has efficient U(VI) adsorption capacity (174 mg g−1), excellent selectivity, and rapid adsorption kinetics.22 Zhang et al. embedded amidoxime chelating groups onto MXene, which enhanced the U(VI) adsorption capacity from 294.0 mg g−1 to 626.0 mg g−1.30 Deng et al. prepared a Ti3C2/SrTiO3 heterostructure with excellent photocatalytic and adsorption properties for U(VI).31 In recent years, some MXene-based materials for uranium adsorption have been mostly predicted by theoretical simulations. More MXene-based materials should be experimentally synthesized and investigated as adsorbents of uranium in the future. In addition, MXenes tend to oxidize in water and degrade under certain conditions. Evaluating the stability of MXene adsorbents and obtaining high stability and excellent uranium adsorption properties of MXenes are worthy of further study.
We should mention that reviews covering the syntheses, structure, functionalization, and application of HAP in heterogeneous catalysis, biomedicine and pollution control have been recently published.32–34 For MXenes, their application in lithium-ion batteries, hydrogen storage, supercapacitors, and environmental governance has been reported.25,29,35–38 Although the traditional eco-friendly HAP and newly emerging MXenes have been investigated as admirable adsorbents for uranium (Fig. 1), there are few reviews about these two types of adsorptive materials for uranium adsorption. Thus, the research status of HAP-based and MXene-based adsorbents in U(VI) uptake is reviewed here. We believe that the focused review for summarizing the application of HAP-based and MXene-based materials in uranium uptake is of significance for the development of uranium decontamination in the future. It will be helpful for researchers to deeply understand the interaction mechanisms between the two materials and uranium and the better application of the two materials as excellent adsorbents in uranium pollution treatment.
This review will not go into the synthesis methods of HAP as these have been discussed in previous reviews.33,34 Nevertheless, as emerging two-dimensional materials, the synthesis methods for MXenes are constantly and rapidly updated. Thus, the latest progress of the preparation methods of MXenes is introduced in this review. Moreover, the uranium adsorption capacities, adsorption influencing factors, and uranium uptake mechanisms of these two types of materials are discussed (Fig. 2). In addition, some challenges in their use for uranium adsorption and possible future research directions are pointed out. We hope that this review will present an imperative reference for further study and actual application in the treatment of uranium-containing wastewater with HAP-based and MXene-based materials.
Fig. 3 Projections of (a) the unit cell of HAP according to the (001) plane, (b) the arrangement of [Ca(1)O6] octahedra, (c) the sequence of octahedral [Ca(1)O6] and tetrahedral [PO4], and (d) the sequence of octahedral [Ca(1)O6] and [Ca(2)O6] and tetrahedral [PO4] in the HAP structure.34 |
HAP is an environmentally friendly adsorbent with a large specific surface area and high stability in both reducing and oxidizing environments. It has a strong affinity for actinides and heavy metals.41–43 HAP has been investigated by researchers for its uranium pollution treatment.7,10,44 The microstructure (morphology, pore size and specific surface area, defects, vacancies, etc.) of HAP materials was found to be closely related to the adsorption performance of uranium. Different morphologies such as fibrous and plate-like, agglomerate-like, rod-like, needle-like, and spherical or hollow spherical HAP materials were prepared under different synthesis conditions.45,46 The morphology of HAP plays a great role in its U(VI) adsorption performance. Hierarchical hollow HAP microspheres with high specific surface area were found to display a rapid adsorption response of only 5 min and an excellent U(VI) adsorption capacity of 2659 mg g−1.46 Zheng et al. synthesized HAP samples with different morphologies and structures.47 The morphology of HAP prepared at 120 °C (HAP-120) was irregular and rough nanosheets. HAP-120 was mainly composed of CaHPO4, Ca8H2(PO4)6·5H2O and Ca10(PO4)6(OH)2. The ratio of CaHPO4 and Ca8H2(PO4)6·5H2O for HAP prepared at 150 °C (HAP-150) decreased considerably, while the Ca10(PO4)6(OH)2 phase increased tremendously, and the morphology of HAP-150 was nanoribbons. When the hydrothermal temperature reached 180 °C, the HAP sample completely translated into Ca10(PO4)6(OH)2 and the morphology of HAP-180 was nanoblocks. HAP-180 showed efficient U(VI) removal of 99.3% within 30 s and the maximum adsorption capacity of 2024 mg g−1, which is higher than those of HAP-120 and HAP-150. The morphology and structure of HAPs are tunable, and subsequently affect their adsorption of uranium. Additionally, the crystal lattice of HAPs is very flexible and can tolerate defects and vacancies caused by the replacement of cations and anions. The replacement including ion exchange of Ca2+ by Ag+, Zn2+, Sr2+, and La3+ cations, the replacement of PO43− by CO32−, SiO44−, and VO43− anionic groups, and the occupancy of the OH− site by anions such as F−, Cl−, and O2− are interesting and important methods to influence the chemical and physical properties of HAPs.11,12,34 Skwarek et al. found that the replacement of Ca2+ (Ca-HAP) and Ag+ (Ag-HAP) impacted the structure and properties of nano-HAPs and greatly enhanced their U(VI) uptake ability.13 Similarly, occupancies of PO43− groups by carbonate ions (C-HAP) and phosphate ions (P-HAP) were found to significantly influence the crystal structure and properties of HAPs and their U(VI) adsorption performance.14 Besides, the replacement of OH− by F− in HAP can boost the stability of HAP in acidic aqueous solution.10
Pristine HAP has drawbacks of simple decomposition in acidic water, the decrease of adsorptive uptake capacity and selectivity in harsh environments with plenty of coexisting ions, easy aggregation, and difficulty in recycling.10,11,48,49 Hence, it needs further research to elevate its acid-resistant stability and adsorption performance in complex wastewater environments. Various modifications have been made to HAP for the sake of increasing these properties of HAP. For example, grafting functional groups (i.e. amine or tributyl phosphate) onto HAP can usually enhance its U(VI) uptake ability and selectivity.48,50 The combination of magnetic iron oxide with HAP can simplify its reusability.49,51
In recent years, U(VI) adsorptive uptake by HAP-based materials has been focused on and investigated extensively. Consequently, they have great application prospects in uranium contamination treatment.7,10,44 In Table 1, we summarized their uranium adsorption capacities, kinetics, isotherms, and thermodynamic analysis results. As shown in Table 1, P-HAP exhibits extremely high U(VI) adsorption capacity (7750 mg g−1) on account of its great affinity for U(VI) and the formation of poorly soluble uranium phosphates.14 In general, the U(VI) uptake process of HAP-based adsorbents was spontaneous and endothermic, and their U(VI) adsorption conformed to the pseudo-second order equation and Langmuir isotherm equation as seen from Table 1.
HAP-based adsorbents | pH | T (K) | Q max (mg g−1) | Equilibrium time (min) | Isotherm model | Thermodynamics | Kinetics | Ref. |
---|---|---|---|---|---|---|---|---|
Q max (mg g−1) represents the maximum U(VI) adsorption capacity obtained by fitting the adsorption data from the Langmuir model, and n.a. represents not available. | ||||||||
HAP–ZVI | 4.0 | 298 | 155.8 | 150 | Langmuir | Endothermic/spontaneous | Pseudo-second-order, intra-particle diffusion | 52 |
Isotherm | ||||||||
Mg/Fe-LDHs@nHAP | 6.0 | 298 | 845.16 | 30 | Langmuir | n.a. | Pseudo-second order | 53 |
Isotherm | ||||||||
Ca-HAP | 6.0 | 293 | 1927.8 | 120 | Langmuir | Endothermic/spontaneous | Pseudo-second order | 13 |
Isotherm | ||||||||
Ag-HAP | 6.0 | 293 | 1713.6 | 120 | Langmuir | Endothermic/spontaneous | Pseudo-second order | 13 |
Isotherm | ||||||||
P-HAP | 6.0 | 293 | 7750 | 120 | Langmuir–Freundlich | n.a. | Pseudo-second order | 14 |
C-HAP | 6.0 | 293 | 1770 | 300 | Langmuir–Freundlich | n.a. | Pseudo-second order | 14 |
HAP | 6.0 | 293 | 800 | 300 | Langmuir–Freundlich | n.a. | Pseudo-second order | 14 |
Calcined magnetic layered double hydroxide/HAP (CMLH) | 6.0 | 298 | 207.9 | 120 | Langmuir | Endothermic/spontaneous | Pseudo-second order | 14 |
Isotherm | ||||||||
Porous HAP | 3.0 | 298.15 | 111.4 | 20 | Langmuir | n.a. | Pseudo-second order | 54 |
Isotherm | ||||||||
Magnetically modified HAP (MNHA) | 5.0 | 298 | 312 | 120 | Langmuir | Endothermic | Pseudo-second order | 49 |
Isotherm | ||||||||
HAP | 3.0 | RT | 2659 | 5 | Langmuir | n.a. | Pseudo-second order | 46 |
Isotherm | ||||||||
TBP-HAP | 8.0 | 298.15 | 38 | 1440 | Langmuir | n.a. | n.a. | 48 |
Isotherm | ||||||||
PAAm-HAP | 4.5–3.0 | 298 | 678.3 | 70 | Langmuir | Endothermic/spontaneous | Pseudo-second order | 55 |
Isotherm | ||||||||
HAP-180 | 3 | 298 | 2024 | 2 | Langmuir | n.a. | Pseudo-first-order and pseudo-second order | 56 |
Isotherm | ||||||||
BR/HAP | 5.5 | 298 | 428.25 | 30 | Langmuir | Endothermic | Pseudo-second order | 57 |
Isotherm | ||||||||
HAP/white clay | 5 | 295 | 570.8 | 180 | Langmuir–Freundlich | Endothermic/spontaneous | n.a. | 58 |
HAP-AC-alginate hydrogel beads | 6 | 298 | 18.66 | 420 | Langmuir | n.a. | Pseudo-second order | 59 |
Isotherm | ||||||||
COF-HAP | 3 | 298 | 392.2 | 240 | Langmuir | Exothermic/spontaneous | Pseudo-second order and Elovich model | 60 |
Isotherm |
For uranium adsorption by HAP-based adsorbents, the most favorable pH is 2–6 as shown in Table 1. The change in the favorable pH range depends on the chemistry properties of different adsorbents. This change could be due to the variation of uranyl species in the solution with varying pH values, which is also related to U(VI) concentration. The most favorable pH is also related to the PZC of different HAP adsorbents. At pH > PZC, the adsorbent is negatively charged and the interaction between the uranium cation and HAP adsorbent is more likely to occur. Conversely, at pH below PZC, the affinity to anions from HAP-based adsorbents is greater due to their positive surface charge.61,62
The Langmuir, Freundlich, Brunauer–Emmett–Teller, Dubinin–Radushkevich and Redlich–Peterson isothermal adsorption models were extensively applied to obtain significant information of the adsorption behavior. Among these models, the Freundlich and Langmuir ones are the most used. The Langmuir model is an empirical one assuming that the adsorbent surface and the distribution of the adsorption site are uniform.63 The Langmuir adsorption process is monolayer and homogeneous adsorption with constant adsorption activation energy and enthalpy. All adsorption sites have equal affinity to adsorbates without adsorbate transmigration on surface planes of adsorbents. Even at adjacent sites, there are no steric hindrance and lateral interaction between adsorbed molecules.64 The adsorption sites on adsorbents are in adsorption equilibrium. On the other hand, the Freundlich isotherm model is applied to multilayer adsorption, and the adsorption heat and affinities of this model do not need to be evenly distributed on the heterogeneous surface.65
The linear (eqn (1)) and nonlinear forms (eqn (2)) of the Langmuir isotherms are expressed as:
(1) |
(2) |
(3) |
(4) |
Generally, uranium adsorption by HAP-based materials is mostly fitted well by the Langmuir model as seen from Table 1, indicating that the adsorption is mostly a monolayer process. For example, You et al.60 found that the adsorption data fitting results had better description by the Langmuir model than by the Freundlich one (Fig. 4a and b), demonstrating that uranium adsorption by COF-HAP was mainly a monolayer adsorption process.
Fig. 4 (a) Langmuir sorption isotherms and (b) Freundlich sorption isotherms for U(VI) adsorption onto COF-HAP.60 (c) The isotherms of U(VI) adsorption on Ca-HAP and Ag-HAP adsorbents with pH = 6 at T = 293, 313, and 333 K, where q is the amount of U(VI) per unit weight of the adsorbent (mmol g−1) and ceq is the equilibrium concentration of U(VI) (mmol dm−3).13 (d) Effect of uranium concentration on the adsorption of uranium by CMLH with an adsorption dosage of 0.05 g, C0 = 10–500 mg L−1, reaction time = 2 h, T = 25–45 °C, and pH = 6.51 |
ΔG0 = −RTlnK | (5) |
(6) |
In common, the U(VI) adsorption process of HAP-based adsorbents is endothermic and spontaneous as seen from Table 1. For example, Skwarek et al. found that the adsorbed amount of U(VI) slightly enhanced at temperatures of 293–333 K,13 which revealed that high temperature improved the U(VI) uptake of the HAP adsorbents (Fig. 4c). The positive ΔH0 for U(VI) adsorption on Ag-HAP and Ca-HAP suggested that the process of U(VI) adsorption was endothermic. When ΔG0 was negative, it demonstrated that the adsorption was spontaneous. Similarly, Li et al. reported that the adsorption capacity of U(VI) onto CMLH improved from 207.9 mg g−1 to 261.1 mg g−1 at temperatures of 298–318 K (Fig. 4d).51 The negative ΔG0, the positive ΔH0, and the positive ΔS0 demonstrated that the adsorption process was endothermic and spontaneous, and favorable at high temperature. U(VI) adsorption onto HAP–ZVI and PAAm-HAP was also found to be an endothermic and spontaneous process.52,55
More interestingly, the adsorption of U(VI) by CMLH consisted of different phases when the initial U(VI) concentration was 50 mg L−1 and 200 mg L−1, respectively. The adsorption capacity enhanced with the further adsorption and reached adsorption equilibrium within 60 min when the initial U(VI) concentration was 50 mg L−1. The U(VI) adsorption kinetics was divided into a transient adsorption stage or an outer surface adsorption stage, and a stable adsorption stage where intra-particle diffusion controlled the adsorption rate until equilibrium. Differently, the U(VI) adsorption process included an initial faster phase, a decreasing phase, and a slower phase at the U(VI) concentration of 200 mg L−1.51
In recent years, HAP materials with hollow microspheres have also been prepared for high specific surface area and plenty of active adsorption sites, and therefore, they can reach fast uranium removal.68 For example, U(VI) adsorption by HAP hollow microspheres can reach apparent equilibrium after 5 min reaction and a removal efficiency of U(VI) close to 100% by extending the contact time.69,70 On the basis of this evidence, HAP hollow microspheres are powerful materials for fast and highly efficient adsorption of U(VI) from radioactive wastewater while the adsorption equilibrium of most HAP-based materials for uranium commonly takes tens of minutes to hours as shown in Table 1.46
The kinetic study is helpful to describe reaction pathways and adsorption equilibrium times. Various common kinetic models have been used such as pseudo-first order, pseudo-second order, and Weber & Morris adsorption kinetic models (i.e., intraparticle diffusion model), and the Elovich model. The pseudo-first order kinetic model assumes that adsorption is governed by diffusion steps, meaning that the dominant process is physical adsorption.71 On the other hand, the pseudo-second order kinetic model is based on the assumption that chemical adsorption is the dominant process in which interactions such as electron transfer and sharing between the adsorbent and adsorbate determine the adsorption rate.72 If the plot of qtversus t0.5 is a straight line and passes through the origin of the axis, the intraparticle diffusion is responsible for adsorption and becomes the rate-controlling step. If multi-linear plots of qtversus t0.5 are presented, two or more stages such as external diffusion and intraparticle diffusion control the adsorption process.62
The linear forms of the pseudo-first order model and pseudo-second order model are presented in eqn (7) and (8):
ln(qe − qt) = lnqe − k1t | (7) |
(8) |
qt = kintt0.5 + C | (9) |
qt = ln(α·β)/β + (1/β)lnt | (10) |
The reported results of kinetics in Table 1 showed that uranium adsorption of HAP-based materials generally follows the pseudo-second order model, indicating that chemical adsorption is the dominant process. For example, when the experimental data of U(VI) adsorption by magnetically modified HAP nanoparticles (MNHA) were analyzed by the kinetic models, the process of U(VI) adsorption fitted the pseudo-second-order model (R2 = 0.98) better than the Elovich model (R2 = 0.90).49 These results displayed that the U(VI) adsorption process was chemisorption, including the reaction of uranium with phosphate groups on the MNHA. Moreover, when the intraparticle diffusion model was used to fit the experimental data, it was found that the U(VI) adsorption process of MNHA includes the reaction stage between uranium and the functional groups on MNHA, and the diffusion stage in the internal pores of MNHA particles.
The effect of ionic strength on U(VI) removal efficiency by the MNHA adsorbent was monitored with various aliquots of U(VI) solutions containing NaCl solution at different concentrations from 0.1–0.7 mol L−1.49 The results demonstrated that the existence of NaCl had a slight influence on the U(VI) adsorption. This finding uncovered that the adsorption process taking place via complex formation mechanisms is independent of ionic strength whereas that via ion-exchange mechanisms is dependent on ionic strength.49,74 Kim et al. carried out adsorption experiments using HAP in simulated seawater (0.7 M NaCl solution). And the experiments demonstrated that TBP-coated HAP prepared at pH = 7 was an efficient adsorbent to recover U from seawater.48 Also, the study on the effect of ionic intensity on U(VI) removal by a polyacrylamide–hydroxyapatite composite in the existence of CaCl2 (0.01–0.50 M) demonstrated that the existence of Ca2+ had no effect on U(VI) adsorption. The adsorption amount of U(VI) in a solution of 1000 mg L−1 was completely adsorbed in a time interval not different from the adsorption time obtained in the absence of Ca2+.55
The adsorption selectivity of HAP for uranium can be improved after being compounded with other composites or modified by other functional groups. However, extracting uranium from complex real wastewater and seawater, which contain large amounts of coexisting ions, salts, and low concentrations of uranium, remains a paramount challenge. It is still necessary to develop more efficient HAP-based materials with excellent selectivity for uranium under severe conditions.
HAP-based adsorbents | Adsorption mechanisms | Analysis techniques | Ref. |
---|---|---|---|
HAP–ZVI | Dissolution–precipitation, reduction, ion exchange and surface complexation | XRD, FTIR and XPS analyses | 52 |
Mg/Fe-LDHs@nHAP | Ion exchange, surface complexation and dissolution–precipitation | XRD, XPS, EDX and FT-IR analyses | 53 |
Ag-HAP | Dissolution–precipitation, sorption, and ion exchange | Batch experiments | 53 |
Ca-HAP | Dissolution–precipitation, sorption and ion exchange | Batch experiments | 53 |
CMLH | Ion exchange and surface complexation | Batch experiments, XRD analysis | 51 |
Porous HAP | Dissolution–precipitation, surface complexation | XRD analysis | 54 |
MNHA | Chemical reaction and intraparticle diffusion | Batch experiments | 49 |
HAP | Surface complexation and incorporation | XRD, XPS, and FT-IR analysis | 46 |
PAAm-HAP | Complexation reaction | Batch experiments | 55 |
HAP-180 | Surface complexation and dissolution–precipitation | SEM, FT-IR, XRD, and XPS analyses | 56 |
mHAP | Surface complexation and dissolution–precipitation | SEM-BSE, XRD analyses | 41 |
HAP-coated quartz | Dissolution–precipitation, surface complexation | SEM-EDS, XRD and XPS analyses | 43 |
BR/HAP | Ion exchange and surface complexation | FT-IR and XPS analyses | 57 |
Fig. 5 The possible mechanisms of U(VI) extraction on LDHs@nHAP.53 |
Ca10(PO4)6(OH)2 → 10Ca2+ + 6PO43− + 2OH− | (11) |
2H+ + 2UO22+ + 2PO43− + nH2O → H2(UO2)2(PO4)2·nH2O (chernikovite) | (12) |
Ca2+ + 2UO22+ + 2PO43− + nH2O → Ca(UO2)2(PO4)2·nH2O (autunite) | (13) |
Fig. 6 XRD patterns of (a) LDHs@nHAP before and after U(VI) adsorption,53 (b) BC-HAP1, BC-HAP2, BC-HAP3, and HAP after U(VI) adsorption,7 (c) HAP hollow microspheres before and after U(VI) adsorption,46 and (d) Bio-HAP600 and Bio-HAP600 after adsorption of U(VI) for (e) 2.5 min and (f) 15 min.44 |
In addition, the occurrence of the dissolution–precipitation reaction is significantly related to the concentration of U(VI) interacting with HAP. U(VI)-phosphate solid phases were not observed in HAP with 4700 ppm adsorbed U(VI). In contrast, U(VI) species adsorbed over 7000 ppm formed a crystalline chernikovite solid phase on HAP, and autunite formation occurred at a total U:P molar ratio ≥0.2.75 The precipitation of H2(UO2)2(PO4)2·10H2O or Ca(UO2)2(PO4)2·10–12H2O was a dominant mechanism at high uranium concentrations of the solution. Nevertheless, at low uranium concentrations, other reaction mechanisms such as adsorption or ion exchange played an important role in the uranium adsorption process by HAP.13
pH and alkalinity are also important factors to the occurrence of the dissolution–precipitation reaction. For example, groundwater with high carbonate alkalinity could inhibit U–P formation and facilitates the formation of aqueous uranyl carbonate complexes. On the other hand, groundwater with low carbonate alkalinity and pH could drive the precipitation of chernikovite.41 Similar results were found by Krestou et al. in a HAP–U(VI) system in which the precipitates were very stable under acid and neutral conditions but not under alkaline conditions.80
Ca2+ + UO22+ → UO22+ + Ca2+ | (14) |
OH + UO22+ → O–UO2+ + H+ | (15) |
O3P–OH+ + UO22+ → O3P–O–UO22+ + H+ | (16) |
Fig. 7 (a) XPS survey spectra of HAP hollow microspheres before and after U(VI) adsorption. (b) XPS Ca 2p spectra of HAP hollow microspheres before and after U(VI) adsorption. XPS O 1s spectra of HAP hollow microspheres (c) before and (d) after U(VI) adsorption.46 |
Like the other two adsorption mechanisms discussed above, the occurrence of surface complexation between U(VI) and HAP-based materials strongly depends on the solution pH and the U(VI) concentration. At different pH values, the surface groups are protonated and deprotonated, generating negative or positive groups which have a forceful affinity for U(VI) species with opposite charge. The association and stability of U(VI) with the groups are closely related to the properties of the adsorbent and U(VI) which are controlled by pH.43,87 For the effect of U(VI) concentration on the occurrence of surface complexation, Fuller et al. found that the formation of chernikovite and autunite precipitates can hardly occur at low U(VI) concentration in the adsorption process while U(VI) adsorption by functional groups as surface complexes was the main removal mechanism at low U(VI) concentration.76 Similar studies reported that adsorption of uranium on the surface of HAP by surface complexation was the only removal process in the whole experiment because the low level concentration of U(VI) could not cause the formation of uranium phosphate phases.51
In summary, uranium uptake by HAP adsorbents commonly obeys the pseudo-second order kinetic equation and Langmuir isotherm equation. The adsorption process of HAP-based materials for uranium was spontaneous and endothermic as shown in Table 1. HAP-based materials have been confirmed to have great potential in treating uranium-containing wastewater. Nonetheless, more research is required to enhance their acid stability, recyclability, extraction ability and selectivity in the harsh environment. In general, the combination of functional groups such as amine or tributyl phosphate with HAP can improve the uranium adsorption capacity and selectivity. HAP is combined with magnetic iron oxide to achieve reusability. Interestingly, the substitution of other cations or anions for Ca2+, PO43−, or OH− in the HAP lattice can influence the structure and properties such as the adsorption capacity and acidic stability of HAP. Moreover, the three major reaction mechanisms of dissolution–precipitation, surface complexation, and ion exchange were previously found by researchers for U(VI) adsorption of HAP-based materials. The mechanism of U(VI) removal of HAP adsorbents requires deeper consideration in the case of the coexistence of various anions and cations, different pH values, and uranium concentrations, which will lead HAP-based materials much closer to practical uranium-containing wastewater remediation.
Fig. 8 (a) Schematics of M2AX, M3AX2, and M4AX3 crystal structures.35 (b) Schematic of the exfoliation process for Ti3AlC2: (b1) Ti3AlC2 structure, (b2) Al atoms replaced by OH after the reaction with HF, and (b3) breakage of hydrogen bonds and separation of nanosheets after sonication in methanol.88 |
A variety of synthesis methods were applied for obtaining MXenes. Top-down and bottom-up syntheses are the two primary strategies for synthesizing MXenes. The top-down approach involves exfoliating bulk crystals into fewer or single-layer MXene sheets. On the other hand, the bottom-up one involves obtaining MXenes from the growth of atoms or molecules. Mostly, MXenes are produced by wet etching of MAX phases followed by exfoliation as a top-down synthesis strategy (Fig. 8). As M–A bonds in MAX phases are metallic, MXenes are difficult to prepare mechanically, so the higher electrochemical activity of M–A bonds than M–X bonds becomes the basic conditions for obtaining MXenes from MAX phases.35
Typically, MXenes are prepared by etching a layer from MAX phases at room temperature using aqueous HF by the following reactions (eqn (17)–(19)):29
Mn+1AXn + 3HF(aq) = Mn+1Xn(s) + AF3(s) + 3/2H2(g) | (17) |
Mn+1Xn(s) + 2H2O(l) = Mn+1Xn(OH)2(s) + H2(g) | (18) |
Mn+1Xn(s) + 2HF(aq) = Mn+1XnF2(s) + H2(g) | (19) |
Fig. 9 (a) Schematic describing the synthesis process of MXenes from MAX phases.36 (b) Schematic diagram of exfoliation of Ti3C2 MXene proceeding via LiF + HCl.89 (c) Etching and exfoliation of Ti3C2 MXene.90 |
The principal shortcoming of using HF as an etchant in the synthesis of MXenes is the involvement of corrosivity, toxicity and dangerousness. Thus, milder and less dangerous fluoride salt-based etchants (e.g., NH4HF2, NH4F, LiF, NaF, KF, CaF2, or tetrabutylammonium hydroxide + HCl or H2SO4) and fluorine-free etchants (e.g., NH4OH, TMAOH, NaOH, KOH, ZnCl2, CuCl2, Br2, and Cl2) have been developed for exfoliation and delamination in the synthesis process of MXenes.28,29,35 As reported by Bian et al. and Xiao et al., the Al layer in MAX phases (e.g., Ti3AlC2) was slowly and selectively etched by less toxic LiF + HCl, and MXene (Ti3C2) was prepared after delamination (Fig. 9b and c).89,90 LiF + HCl as an etchant is milder than HF and the prepared MXene product commonly shows better mechanical stability, a larger size, lower defects, fewer –F termination groups and additional –Cl termination groups.89,91,92 Similar research studies used a less hazardous and milder etchant such as ammonium bifluoride (NH4HF2) to etch epitaxial Ti3AlC2, resulting in collateral intercalation of ammonium ions of –NH3/–NH4+ into the interlayers belonging to Ti3C2Tx with a simultaneous increase in lattice parameters.27 Besides, other fluoride salts including CaF, KF, NaF, CsF and tetrabutyl ammonium fluoride can also be used with either HCl or H2SO4 to prepare aqueous etchants for the preparation of multi-layered transition metallic carbides with various compositions and properties. It has also been reported that the properties of MXenes are subjected to variation by certain modifications of the surface chemistry and diversified use of pre-intercalated ions.29,37 Considering environmental friendliness, the fluorine-containing etching method for synthesis of MXenes is not a good option for their environmental applications such as uranium pollution treatment. Furthermore, the –F termination groups are undesirable when MXenes are used as adsorbents in most cases.36
Hence, safer and more environmentally friendly nonfluoride synthesis methods were developed for MXenes. NH4Cl is an ideal candidate as a fluoride-free etchant. Yang et al. developed an effective method to prepare MXene using electrochemical etching.93 The electrolyte is composed of 1 M NH4Cl and 0.2 M tetramethylammonium hydroxide (TMA·OH). The selective removal of aluminum followed by intercalation of NH4OH results in the formation of single or bilayer MXene sheets (Ti3C2Tx, Tx = O and OH) with high yields (>90%) and large average dimensions (Fig. 10a). The consequence is comparable with or better than that obtained from etching means using HF or LiF/HCl.
Fig. 10 (a) Schematic of the etching and delamination process from Ti3AlC2 to form single or bilayer MXene sheets of Ti3C2Tx.93 (b) Schematic illustration of the etching and exfoliation process for Ti3AlC2.94 (c) Schematic of the reaction between Ti3AlC2 and NaOH water solution under different conditions.95 |
Alkalis such as KOH and NaOH with high concentration and temperature are another class of fluoride-free etchants and can effectively extract the Al layers, especially from Ti3AlC2, because of the strong binding ability of OH– to the Al element. Li et al. prepared Ti3C2(OH)2 through etching the Ti3AlC2 precursor using etchant KOH in a small amount of water.94 The OH– group replaced the Al layer in Ti3AlC2 and H2 is formed as a by-product during the etching process. Single layer Ti3C2(OH)2 sheets could be efficiently obtained via a simple washing process (Fig. 10b). Li et al. reported an NaOH-etching hydrothermal strategy to synthesize MXene Ti3C2Tx.95 The inspiration for this means came from Bayer technology which had a wide range of applications in bauxite refining. The etching process is fluorine-free and yields multilayer Ti3C2Tx with 92 wt% in purity via 27.5 M NaOH at 270 °C (Fig. 10c). However, such high alkali concentration and high temperature conditions could also be dangerous.
Methods using Lewis acidic molten salts were also generalized to synthesize fluorine-free MXenes by etching MAX precursors. Li et al. initially demonstrated a universal means for synthesizing a variety of Cl-terminated MXenes derived from the reaction between ZnCl2 and MAX precursors. In this means, the Zn atom occupied the A layer in the MAX phases of Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC, and then novel MAX phases of Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. In the presence of excess ZnCl2, MXenes containing Ti3C2Cl2 were acquired through the stripping of Ti3ZnC2, as fused ZnCl2 is a strong Lewis acid (Fig. 11). Ti2NCl2 and V2CCl2 were not acquired since the M–A bonding strengths of the Zn-MAX phases were stronger than those of Ti3ZnC2 and Ti2ZnC. The etching of Lewis acid for molten salt provides a feasible path to fabricate MXenes without HF.96
Fig. 11 Zn in molten ZnCl2 occupied the A layer in the MAX precursor of Ti3AlC2 to obtain Ti3ZnC2. Cl-terminated MXenes (Ti3C2Cl2) were obtained by exfoliation of Ti3ZnC2 with excess ZnCl2.96 |
Besides, a layer like the Si, Ga layer in the MAX phase Ti3SiC2 was difficult to etch by fluorine solutions. Thus, Li et al. proposed a universal means for etching MAX precursors to obtain MXenes through redox coupling between the A element and the Lewis acid molten salt cation. Taking Ti3SiC2 as an example, the Cu/Cu2+ redox potential is −0.43 eV while the Si/Si4+ redox potential is −1.38 eV in the copper chloride molten salt at 700 °C. The Lewis acid Cu2+ in the molten salt can easily oxidize Si atoms to Si4+, leading to the formation of the volatile SiCl4 phase and reduction of Cu2+ to the Cu elemental substance. The residual Cu particles in the product can be removed from the Ti3C2Cl2 MXene surface by ammonium persulfate (APS) solution, and finally Ti3C2Tx (Tx = Cl, O) MXene can be obtained (Fig. 12).97
Fig. 12 Schematic of Ti3C2Tx MXene preparation. (a) The MAX phase of Ti3SiC2 is immersed in CuCl2 Lewis molten salt at 750 °C. (b and c) The reaction between Ti3SiC2 and CuCl2 results in the formation of Ti3C2Tx MXene. (d) MS-Ti3C2Tx MXene is obtained after further washing in ammonium persulfate solution.97 |
Based on the same principle, Li et al. obtained a series of MXenes (Ti2CTx, Ti3C2Tx, Ti3CNTx, Nb2CTx, Ta2CTx, Ti2CTx, Ti3C2Tx) from the corresponding MAX phases (Ti2AlC, Ti3AlC2, Ti3AlCN, Nb2AlC, Ta2AlC, Ti2ZnC and Ti3ZnC2) by using different chloride molten salts (CdCl2, FeCl2, CoCl2, CuCl2, AgCl, NiCl2) as shown in Fig. 13.97
Fig. 13 Generalization of the Lewis acid etching route to a large family of MAX phases. (a) Gibbs free energy mapping (700 °C) guiding the selection of Lewis acid Cl salts according to the electrochemical redox potentials of A-site elements in MAX phases (x axis) and molten salt cations (y axis) in Cl melts. Stars mark the corresponding MXenes demonstrated in the study. All symbols (stars and spots) were calculated using the same approach; the star symbols were experimentally verified in this paper while spot symbols remained as theoretical prediction. (b–g) SEM images reveal the typical accordion morphology of MXenes from different MAX phases etched by various Lewis acid Cl salts, such as (b) Ti2AlC by CuCl2, (c) Ti3ZnC2 by CuCl2, (d) Ti3AlCN by CuCl2, (e) Ti3AlC2 by NiCl2, (f) Ti3ZnC2 by FeCl2, and (g) Ta2AlC by AgCl. Scale bars, 2 μm.97 |
MXenes with different terminations possess different properties, which could affect uranium adsorption. Thus, it is important to regulate termination groups on the surface of MXenes. Kamysbayev et al. used a variation of molten Lewis acid (CdCl2 and CdBr2) to prepare single-layered MXenes with uniform Cl− and Br− termination groups, respectively.98 The Cl and Br termination groups of MXenes can be replaced with other terminations, such as –O, –Se, –S, and –NH2, and bare MXenes without terminations were also synthesized (Fig. 14).97
Fig. 14 Surface reactions of MXenes in molten inorganic salts. (a) Schematics for etching of MAX phases in Lewis acidic molten salts. (b) Atomic resolution high-angle annular dark-field (HAADF) image of Ti3C2Br2 MXene sheets synthesized by etching the Ti3AlC2 MAX phase in CdBr2 molten salt. The electron beam is parallel to the zone axis. (c) Energy dispersive X-ray (EDX) elemental analysis (line scan) of Ti3C2Br2 MXene sheets. HAADF images of (d) Ti3C2Te and (e) Ti3C2S MXenes obtained by substituting Br for Te and S surface groups, respectively. (f) HAADF image of Ti3C2□2 MXene (□ stands for vacancy) obtained by reactive elimination of Br surface groups. All scale bars are 1 nm.98 |
Nonetheless, it is still a big challenge to etch specific MAX phases such as Mo2Ga2C into Mo2C MXenes with high quality and a large scale. Inspired from the strong binding ability of OH–/Cl– and “A” elements, fluoride-free Mo2C MXenes with a high efficiency of about 98% were obtained from etching by sole hydrochloric acid solution with appropriate external power (Fig. 15a).99 MXenes such as Ti3C2Tx are hydrolytically unstable. Moreover, the MXene surface is compositionally heterogeneous terminations resulting in suboptimal performance. Molten salt etching has been developed to avoid aqueous etchant-containing approaches. However, the molten salt method requires high temperatures (up to 550 °C) limiting its utilization. Safe, energy saving and efficient synthesis of MXenes with homogeneous surfaces is needed. Thus, Jawaid et al. presented a room-temperature etching method that utilizes halogens (Br2, I2, ICl, IBr) in anhydrous media to remove the A-layer from Ti3AlC2 to synthesize Ti3C2 MXenes with a homogeneous Cl, Br, or I surface. Hazardous by-products were readily quenched by the addition of stabilizers avoiding any undesired exposure and improving the safety in this synthesis process (Fig. 15b).28
Fig. 15 (a) Fluoride-free Mo2CTx prepared by the sole HCl-assisted hydrothermal etching strategy. (a) Schematic illustration of the preparation procedure for fluoride-free Mo2CTx.99 (b) Halogen etching of MAX phases: (b1) generalized process for the formation of delaminated, halogen-terminated MXenes. (b2) Addition of Br2 to Ti3AlC2 in anhydrous cyclohexane produces a deep red solution. (b3) As Br2 reacts with the Al interlayer, the supernatant turns pale-yellow, reflecting the depletion of Br2 and the production of AlBr3 species. AlBr3 is rendered inert by addition of stabilizers (tetrabutylammonium bromide, TBAX). (b4) The MXene crude is purified via repeated redispersion in a nonpolar solvent (i.e., CHCl3). (b5) The purified size-selected MXene is obtained via centrifugation and dispersed in THF.28 |
In summary, the synthesis methods of MXenes have evolved from hazardous and toxic fluorine-containing etching to safer and more environmentally friendly fluorine-free etching methods. However, great challenges remain in developing non-hazardous and optimal etching methods and establishing a broader range of MAX-phase precursors. In addition, the surface functional groups and properties of MXenes are highly dependent on synthetic methods, which subsequently influence their environmental applications such as uranium pollution treatment. In general, the –F termination is undesirable when MXenes are used as adsorbents of metal ions in most cases while a hydrophilic surface (e.g. –O and –OH groups) is more beneficial for the adsorption of metal ionic species.36 Thus, the relationship between etching methods and termination groups and properties of MXenes should be cautiously considered when preparing them. In addition, MXenes with a specific morphology such as nanotubes and nanocages are rarely reported, so it is recommended to synthesize them and study their uranium properties.
MXene-based adsorbents | pH | T (K) | Adsorption capacity (mg g−1) | Equilibrium time | Isotherm model | Thermodynamics | Kinetics | Ref. |
---|---|---|---|---|---|---|---|---|
V2CTx | 4.5 | RT | 174 | 270 | Freundlich isotherm | n.a. | Pseudo-second order | 22 |
V2C(OH)2 | n.a. | n.a. | 536 | n.a. | n.a. | n.a. | 22 | |
Ti3C2Tx–DMSO-hydrated | 5.0 | RT | 214 | 360 | Freundlich isotherm | n.a. | n.a. | 108 |
Ti3C2(OH)2 | n.a. | n.a. | 595.3 | n.a. | n.a. | n.a. | n.a. | 109 |
Ti3C2Tx | 3 | 298 | 470 | n.a. | n.a. | n.a | n.a. | 110 |
Ti2CO2 | n.a. | n.a. | 1890 | n.a. | n.a. | n.a. | n.a. | 101 |
V2CO2 | n.a. | n.a. | 1090 | n.a. | n.a. | n.a. | n.a. | 101 |
Cr2CO2 | n.a. | n.a. | 1070 | n.a. | n.a. | n.a. | n.a. | 101 |
Amidoxime functionalized Ti3C2Tx | 5 | 298 | 626 | 5 | Langmuir model | n.a. | n.a. | 30 |
MXene/graphene oxide nanocomposites | 6 | 298 | 1003.5 | 40 | Langmuir model | Endothermic and spontaneous | Pseudo-second order | 111 |
MXene-based adsorbents | Adsorption mechanisms | Analysis techniques | Ref. |
---|---|---|---|
V2CTx | Ion-exchange | DFT computations, EXAFS analysis | 22 |
V2C(OH)2 | Bidentate inner-sphere, complexation | DFT computations | 22 |
Ti3C2Tx–DMSO-hydrated | Imprisonment of U(VI) through rational control of interlayer space (d-spacing) | SEM, TGA and XRD analyses | 108 |
Ti3C2(OH)2 | Bidentate coordination, hydrogen bond | DFT computations | 109 |
Ti3C2Tx | Chemical adsorption, reduction, bidentate binding, and ion exchange | XANES, EXAFS, XRD and XPS analyses | 110 |
Ti2CO2 | Physical and chemical adsorption | DFT computations | 101 |
V2CO2 | |||
Cr2CO2 | |||
Amidoxime functionalized Ti3C2Tx | Bidentate chelation | EXAFS analysis | 30 |
Ti3C2/SrTiO3 | Photocatalytic reduction | DFT computations and XPS analysis | 31 |
MXene/graphene oxide nanocomposites | Reduction-induced immobilization, complexation reaction, electrostatic interaction and intraparticle diffusion | SEM, XRD, FTIR, XPS analysis | 111 |
Among the reported MXenes, titanium-based Ti3C2Tx and Ti2CTx are more commonly used in uranium adsorption. Wang et al. developed a hydrated intercalation strategy to enlarge the interlayer space of multilayered Ti3C2Tx under hydrated conditions.108 The U(VI) uptake of hydrated Ti3C2Tx is significantly higher than that of dry Ti3C2Tx, which is principally on account of the larger interlayer space and stronger flexibility of hydrated Ti3C2Tx. More UO22+ can be easily contained and confined into the Ti3C2Tx channel by controlling the interlayer space rationally. The authors also proposed a fast post-adsorption calcination approach for the encapsulation of uranium inside Ti3C2Tx (Fig. 16a). Zhang et al. predicted Ti3C2(OH)2 to be an effective adsorbent for uranium in aqueous solution by using DFT simulations.109 The calculations revealed that the bidentate coordination of UO22+ to the Ti3C2(OH)2 surface was more beneficial than other configurations. And UO22+ was more likely to bind with adsorption sites of the deprotonated O than the protonated one. The chemisorption and hydrogen bond formation were the primary interaction mechanisms during the U(VI) uptake onto Ti3C2(OH)2. Zhang et al. reported that the amidoxime functional group grafted onto Ti3C2Tx could efficiently coordinate with uranyl ions in a stable chelate structure with great selectivity.30 Given the excellent conductivity of MXenes, the authors demonstrated that the amidoxime functionalized Ti3C2Tx exhibited electrochemical adsorption for uranium by the application of an electric field. Deng et al. developed a novel Ti3C2/SrTiO3 heterostructure as a photocatalyst to efficiently adsorb U(IV) through photocatalytic reduction.31 They also found that the multilayered Ti3C2 could promote charge transport and inhibit the recombination of electrons in the conduction band (Fig. 16b). The deep study on the mechanism of Ti3C2/SrTiO3 for photocatalytic reduction and removal of U(VI) would bring new insights into photocatalysis and optoelectronic applications.
Fig. 16 (a) Rational control of the interlayer space of multilayered Ti3C2Tx enables the MXene to exhibit an excellent U(VI) sorption capacity and an exceptional radionuclide encapsulation performance.108 (b) The photocatalytic mechanism and charge transfer processes of the Ti3C2/SrTiO3 hybrid system under simulated sunlight irradiation. The photogenerated electrons and holes are marked by red (−) and blue (+) spheres, respectively.31 |
Titanium-based MXenes easily transform into TiO2 which can bring about photocatalysis and reduction immobilization of uranium. MXene (Ti3C2Tx)/graphene oxide nanocomposites (MGN) for removing U(VI) were prepared by the combination of the ice-templating method and freeze-drying technology as shown in Fig. 17a.111 From the possible adsorption mechanism for U(VI) on MGN as shown in Fig. 17b, U(VI) adsorption by MGN was mainly ascribed to the oxygen-containing functional groups on MGN while the photocatalytic reduction immobilization of uranium was attributed to the TiO2 nanoparticles which were derived from the oxidation of the MXene in MGN. Meanwhile, the electrostatic interaction between MGN and U(VI) could further improve the U(VI) loaded on MGN. Wang et al. demonstrated that U(VI) interacted with MXene Ti2CTx through simultaneous adsorption and reduction in a wide pH range.110 At low pH, the reduced U(IV) species was mononuclear and bidentately bound to the Ti2CTx; however, the UO2+x phase nanoparticles adsorbed onto some Ti2CTx transformed into amorphous TiO2 at near-neutral pH. Their study highlighted the reduction-induced immobilization of U(VI) by Ti2CTx MXene, which included a pH-dependent reduction mechanism that could promote the use of titanium-based materials for the removal of other oxidized contaminants (Fig. 18).
Fig. 17 (a) Schematic illustration for the synthetic process of MXene/graphene oxide nanocomposites (MGN). (b) Possible adsorption mechanism for U(VI) on MGN.111 |
Fig. 18 Schematic of the U(VI) reduction and sequestration by Ti2CTx MXene.110 |
Vanadium-based MXenes were also investigated for U(VI) uptake.22 The adsorption mechanism of U(VI) onto V2CTx was described by heterogeneous adsorption models because of the existence of heterogeneous binding sites such as –OH, –O, and –F on MXenes. The interaction between V2CTx and U(VI) was deeply elaborated from the molecular aspect through DFT calculations and EXAFS. The analysis results demonstrated that UO22+ was adsorbed by V2CTx through forming bidentate inner-sphere compounds at –OH sites bonded to V atoms. The deprotonation of –OH after attaching with U(VI) revealed an ion-exchange reaction during the adsorption process. Zhang et al. studied the adsorption behavior of uranyl species toward V2C(OH)2 nanosheets using DFT calculation and molecular dynamics (MD) simulations (Fig. 19).23 They revealed that UO22+ could stably and effectively bond to V2C(OH)2. The powerful adsorption is accomplished by forming two U–O bonds with the V2C(OH)2 nanosheets and hydrogen bonds between the axial O atoms from UO22+ and H atoms from the nanosheets. While the F termination could remarkably cut down the U(VI) uptake ability of V2C nanosheets, U–F bonds were analyzed to be overall weaker than the U–O ones, manifesting that MXenes with the F termination are less favorable for U(VI) uptake.
Fig. 19 (a and b) Two models for investigating the effects of terminal groups on the adsorption of [UO2(H2O)3] on V2C MXene and (c and d) the corresponding ELF analysis.23 |
In conclusion, MXenes have emerged as a new class of two-dimensional layered transition metal carbides and/or nitrides that have attracted considerable attention due to their unique properties such as high specific surface area, high hydrophilicity, abundant functional groups, large ion-exchange capacity, tunable interlayer spacing, and excellent resistance to acids. Thus they have therefore been explored as ideal adsorbents for uranium. Titanium-based MXenes and vanadium-based MXenes are the main types of MXenes for uranium adsorption, which was investigated by theoretical simulation or experimental methods. The mechanisms of adsorption of uranium by MXenes have been proposed to be ion-exchange, complexation, coordination, d-layer manipulation, physisorption, chemisorption, reduction, and hydrogen bonding. Also, ion-exchange, complexation, and coordination are the main mechanisms of U(VI) adsorption by MXenes.
Nevertheless, there are still some key points that need to be paid special attention to in future studies about HAP-based and MXene-based materials for uranium adsorption.
(1) Since the U(VI) concentration under realistic conditions is much lower than other coexisting substances, improving the selectivity of HAP-based and MXene-based materials for uranium adsorption under harsh conditions still needs further investigations.
(2) RE-doped HAP materials have been found to detect uranium in water.112,113 Thus, the development of HAP-based materials with bifunctional features of adsorption along with detection is a meaningful research direction.
(3) The mechanism of uranium removal and immobilization is complicated due to the influence of different environmental conditions (such as pH, uranium concentration and multiple co-existing ions) and synchronous reactions in the system, and has not been fully revealed. Considering the complexity of actual uranium-containing wastewater, it is urgent to strengthen the research on interactions between uranium and HAP-based and MXene-based materials under the influence of multiple factors, so as to further explore and expand the removal mechanism of uranium.
(4) The synthesis procedures for most MXenes include the utilization of toxic HF. Therefore, development of synthetic strategies without using HF is essential for their broader applications as adsorbents for uranium removal and remediation.
(5) MXene-based adsorbents for U(VI) uptake were majorly predicted by theoretical simulations, but relatively few experimental research studies were accomplished. More MXene-based materials should be experimentally synthesized and investigated as uranium adsorbents.
(6) MXenes oxidize easily in aqueous solution and degrade in certain applied environments. For instance, Ti3C2Tx MXene was discovered to oxidize to titania in aqueous solution for a long period of time. The stability of MXene adsorbents under different conditions is crucial and needs to be further evaluated. Obtaining high stability and excellent adsorption capacities of MXenes is important for their practical applications.
(7) The adsorption of uranium can be affected by the interlayer spacing of MXenes which is worth being regulated by the introduction of intercalants and the cross-linking of molecules to be suitable for U(VI) encapsulation to increase the adsorption capacity.
(8) MXenes with specific morphologies such as nanotubes and nanocages are rarely reported, which are recommended to be synthesized and investigated for their uranium performances.
(9) Further research about the toxicity of MXenes should be carried out for their safe application in the treatment of uranium pollution since Ti3C2Tx MXene was found to have an influence on the viability of cells.
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