Eco-friendly hydroxyapatite and emerging MXenes for uranium adsorptive uptake

Abstract

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.

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Environmental significance

Uranium 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.

1. Introduction

Due to the extensive use of fossil fuels and the shortage of traditional energy, the world is facing serious environmental problems, speeding up the development of new efficient and clean energy. Nuclear energy is one of the important new energy sources to solve the energy shortage and alleviate environmental problems.^1–3 Uranium is a crucial strategic resource extensively used for developing nuclear energy. Nevertheless, on account of the enhancement of nuclear energy activities (uranium mining, spent nuclear fuel treatment, etc.), a lot of uranium-containing wastewater was generated. The adsorption method has the merits of low cost, convenient operation, great efficiency, strong versatility, etc. It is a promising method to lessen or eliminate the harm of uranium to the environment and human health. Development of highly effective adsorbents for treatment of uranium wastewater is a quite essential endeavor that scientists are undertaking these days. Among investigated adsorbents for uranium, hydroxyapatite (HAP) and MXenes regarded as efficient and promising adsorbents for uranium removal have stood out as discussed below.

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 M_n+1X_nT_x (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 (M_n+1AX_n).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 V₂CT_x 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.

Fig. 1 The number of publication and citations on uranium adsorption by (a and b) MXene-based and (c and d) HAP-based materials through the Web of Science. The key words are “hydroxyapatite” and “uranium adsorption” for HAP-based materials, and “MXene” and “uranium adsorption” for MXene-based materials.

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. 2 Schematic diagram summarizing the main content of this review.

2. Hydroxyapatite-based materials

2.1. Properties and uranium adsorption capacities of hydroxyapatite-based materials

HAP is the major inorganic constituent of human and animal bones. It is an environment-friendly material with biocompatibility and non-toxicity.¹¹ HAP has a hexagonal columnar structure with lattice parameters of a = b = 9.418 Å, c = 6.881 Å, α = β = 90°, and γ = 120°, and the space group of P6₃/m.³⁹ Its unit cell consists of ten Ca²⁺, six PO₄³⁻, and two OH⁻. In consequence, the common chemical formula of HAP can be written as Ca₁₀(PO₄)₆(OH)₂. Its crystal network consists of a tight combination of tetrahedral PO₄ groups, in which the P⁵⁺ ions are located in the center of the tetrahedron, and their tops are taken by four oxygen atoms. Every PO₄ tetrahedron is shared by a column and divided into two kinds of unconnected channels (Fig. 3).³⁴ Two different Ca²⁺ sites, namely Ca1 and Ca2, are contained in the unit cell. Ca1 (4 per cell) is coordinated with the nine oxygen atoms of the PO₄ tetrahedron and located in the center of the Ca–O octahedron consisting of six oxygen atoms, and Ca2 (6 per unit cell) is in 7 coordination with six atoms of oxygen belonging to the PO₄ tetrahedron and one from an OH anion (Fig. 3a and b). OH⁻ groups are along the c axis to balance the positive charge of the HAP lattice, and OH⁻ appears in columns perpendicular to the unit cell face. Different projections in the HAP structure are shown in Fig. 3a–d.³⁴ The stoichiometric HAP has a Ca/P molar ratio of 1.67 while many highly non-stoichiometric calcium phosphate compounds exist. When the Ca/P molar ratio is lower or higher than 1.67, the HAP is expressed as calcium deficient HAP (D-HAP) or calcium rich HAP (R-HAP), respectively. The chemical formula of D-HAP is described as Ca_10−x(HPO₄)_x(PO₄)_6−x(OH)_2−x (x = 0–1) and that of R-HAP is described as Ca_10−x(PO₄)_6−x(CO₃)_x(OH)_2−x or Ca_10−yNa_y[(PO₄)_6−y(CO₃)_y][(OH)_2−2x(CO₃)_x] if Na exists.^11,40

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.

Table 1 Adsorption capacities, isotherm models, thermodynamics, kinetics, and principal parameters of HAP-based adsorbents for U(VI) uptake

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

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2.2. Effect of different factors, adsorption isotherms, kinetics, and thermodynamic studies of HAP-based materials

2.2.1. Influence of the pH value and adsorption isotherm analysis of HAP-based materials. The pH value plays a great role in U(VI) removal, since it impacts the activity sites in HAP-based adsorbents and the existing form of UO₂²⁺. For instance, the U(VI) removal rate of calcined magnetic layered double hydroxide/hydroxyapatite (CMLH) was highly dependent on the pH of the solution.⁵¹ The removal rate increased from 89% to 98.5% at pH 2.0–6.0 and gradually dropped to 97% at pH 12. U(VI) removal was low at pH < 6.0 since the CMLH adsorbent and uranyl ions have positive charges, many H⁺ ions competed with uranyl ions for adsorption sites on the adsorbent, which is not conducive to adsorption. As the pH increased, the number of H⁺ of the adsorbent surface decreased, allowing UO₂²⁺ to take advantage of the binding site. The U(VI) removal rate decreased when the pH value was greater than 6, which is probably because hydroxide and carbonate compounds formed. As another example, layered double hydroxide@nHAP (LDHs@nHAP) kept high U(VI) adsorption efficiency in a pH range of 4 < pH < 8 and reached the maximum efficiency at pH = 6.⁵³ The U(VI) adsorption efficiency was low at pH < 4.0, which might be due to the damage and dissolution of some hydroxides and phosphate groups of the LDHs@nHAP under pH < 4.0 conditions. At the same time, too many H⁺ could cause protonation of adsorbents, leading to lower removal efficiency. The surface zeta potentials of LDHs@nHAP-U were negative in the range of pH 3–11. When pH > 8, U(VI) transformed into negatively charged species such as UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻. Electrostatic repulsive force occurred since negative charges of both adsorbents and adsorbates were charged, bringing about lower U(VI) removal efficiency at pH > 8. Similarly, the U(VI) adsorption capacity of magnetically modified HAP (MNHA) gradually improved with the increase of pH, it reached the maximum at pH 5 and then decreased.⁴⁹ Also, Wu et al. found that the ability of HAP to adsorb U(VI) was much lower at pH 2.0, then enhanced fleetly and reached a maximum at pH 3.0.⁴⁶ The adsorption capacity decreased obviously with the further increase of the pH value. Under the conditions of pH < 3.0, UO₂²⁺ dominated in the solution. The higher concentration of H⁺ caused competition with UO₂²⁺ for the binding sites of the adsorbents. The higher concentration of H⁺ competed with UO₂²⁺ for the binding sites of HAP. The zero-charge point (PZC) of HAP is 3.03. The electrostatic repulsion might hinder the interaction between UO₂²⁺ and HAP when the pH value was less than 3.0. With the increase of the pH value, the surface charge decreased, resulting in a decrease in the electrostatic repulsion between UO₂²⁺ and HAP. When the pH value was greater than 3.0, UO₂²⁺ started to hydrolyze, forming hydroxy uranyl such as [UO₂(OH)]⁺, [(UO₂)₂(OH)₂]²⁺, [(UO₂)₃(OH)₄]²⁺, [(UO₂)₃(OH)₅]⁺ and [(UO₂)₄(OH)₇]⁺, which were usually unfavorable for the adsorption due to the existence of –OH groups on HAP.

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:

The linear (eqn (3)) and nonlinear forms (eqn (4)) of the Freundlich isotherms are expressed as:where c_e (mg L⁻¹) is the adsorption equilibrium concentration of adsorbates, q_max (mg g⁻¹) is the maximum number of adsorbates per unit weight of adsorbents, and q_e (mg mg⁻¹) is the amount of adsorbates per unit weight of adsorbents at equilibrium. k_l (L mg⁻¹) is the equilibrium constant of adsorption strength. The larger the value of k_l is, the stronger the adsorption capacity is. Furthermore, k_f (L mg⁻¹) represents the adsorption intensity and capacity of adsorbents. The value of n represents the difficulty level of adsorption. When 1/n = 0–1, adsorption is easy to occur. When 1/n > 1, adsorption is difficult to occur.

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 c_eq 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

2.2.2. Influence of temperature and thermodynamic analysis of HAP-based materials. Temperature can affect the adsorption process between the adsorbent and adsorbate. Variations in temperature can impact chemical and physical adsorption behaviors and the adsorption capacity. The properties and feasibility of the adsorption process can be evaluated by calculating thermodynamic parameters such as Gibbs free energy (ΔG₀), enthalpy change (ΔH₀), and entropy change (ΔS₀). A negative value of ΔG implies that an adsorption process is favorable and spontaneous. A negative value of ΔH and a positive value of ΔH signifies that an adsorption process is exothermic and endothermic, respectively. Since ΔS represents the measure of chaos in the system, a positive ΔS value indicates that there is an increase in the randomness/disturbance of the system. These thermodynamic parameters can be acquired using the two equations below (eqn (5) and (6)):

ΔG⁰ = −RT ln K (5)

where K, K₁, and K₂ (L mol⁻¹) are the adsorption equilibrium constants at absolute temperatures T, T₁, and T₂ (K), respectively, and R is the ideal gas constant (8.314 J mol⁻¹ K⁻¹), and K_d is the distribution coefficient (mL g⁻¹).

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

2.2.3. Influence of adsorption time and kinetic analysis of HAP-based materials. Studying the influence of the adsorption time on the adsorption process is significant since it is helpful to evaluate the adsorption properties. Kinetic studies can fully understand the rate of the adsorption process and help to explain the mechanism of adsorption.⁶⁶ For example, El-Maghrabi et al. observed a rapid increase of U(VI) removal by magnetically modified hydroxyapatite nanoparticles (MNHA) within the first 50 min and a stable increase till equilibrium after 120 min.⁴⁹ This phenomenon might be on account of plentiful adsorption sites on MNHA in the initial phase of adsorption. As the adsorption proceeded further, the removal rate enhanced gradually until equilibrium on account of the gradual reduction of active sites. The kinetic model is used to analyze experimental data, and it is found that the U(VI) adsorption process conforms to the pseudo-second-order model. The adsorption mechanism was also explained by applying the intra-particle diffusion model. The results revealed that the adsorption of U(VI) on the MNHA composite experienced the chemical reaction of UO₂²⁺ with the surface functional group of MNHA, and then the intra-particle diffusion process of uranium into MNHA.⁶⁷

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 q_tversus t^0.5 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 q_tversus t^0.5 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(q_e − q_t) = ln q_e − k₁t (7)

The form of the intraparticle diffusion model is presented in eqn (9):

q_t = k_intt^0.5 + C (9)

The form of the Elovich model is presented in eqn (10):

q_t = ln(α·β)/β + (1/β)ln t (10)

where q_t (mg g⁻¹) is the amount of adsorbates per unit weight of adsorbents at time t (min), q_e (mg g⁻¹) represents the amount of adsorption at equilibrium, k₁ (1 min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the rate constants of the first order and second order models, respectively, k_int is the rate constant of intraparticle diffusion, C is a constant dependent on the thickness of the interface and boundary value, α (mgg⁻¹ min⁻¹) is the initial rate of adsorption, and β is the extent of surface coverage.

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.

2.2.4. Effect of co-existing ions and ionic strength of HAP-based materials. The effect of co-existing ions and ionic strength on U(VI) adsorption by HAP adsorbents has been investigated to mainly inspect their U(VI) adsorption selectivity. The results of adsorption experiments indicated that U(VI) adsorption by Mg/Fe-LDHs@nHAP composites was less affected by other co-existing ions, revealing the high selectivity to U(VI) by Mg/Fe-LDHs@nHAP composites.⁵³ Su et al. investigated the effect of co-existing ions on U(VI) removal by porous HAP and found that the U(VI) removal efficiency was kept high even in the existence of 0.1 M Na⁺, K⁺, Ca²⁺, Mg²⁺, CO₃²⁻, Cl⁻, NO₃⁻, and SO₄²⁻ ions.⁵⁴ These results indicate that these co-existing ions did not significantly influence the U(VI) removal capacity of porous HAP. Similarly, El-Maghrabi et al. studied the effect of cations such as Na⁺, K⁺, Ca²⁺, and Mg²⁺ on U(VI) removal by a MNHA adsorbent.⁴⁹ The U(VI) adsorption percentage under such competitive conditions was 93% in comparison with 100% under non-competitive conditions. The adsorption efficiency of Na⁺, K⁺, Ca²⁺, and Mg²⁺ cations was only 22, 0.05, 13, and 33%, respectively. These results demonstrate that the adsorption process is carried out through the formation of complexes rather than through ion exchange reactions, and is effective for the removal of U(VI) even in the presence of coexisting ions.⁷³

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.

2.3. Adsorption mechanisms of HAP-based materials

The possible interaction mechanisms of U(VI) between HAP adsorbents are summarized in Table 2. In different adsorption systems, U(VI) adsorption onto HAP adsorbents consists of various reactions such as dissolution–precipitation, surface complexation, ion exchange, adsorption, and redox. Most studies have indicated that there are three major reaction mechanisms of U(VI) adsorption by HAP-based materials: (1) dissolution–precipitation, (2) surface complexation, and (3) ion exchange.^{7,10,41,43,44} For example, the adsorption of U(VI) on LDHs@nHAP included these three possible mechanisms as shown in Fig. 5.⁵³ These reactions may happen simultaneously and can vary in the extent depending on the environmental factors in aqueous solution and the concentration of U(VI). For instance, below a threshold U(VI) concentration, UO₂²⁺ forms inner-sphere complexes on the surface of HAP while chernikovite (H₂(UO₂)₂(PO₄)₂·nH₂O) and autunite Ca(UO₂)₂(PO₄)₂·nH₂O have been observed to form above the threshold due to the dissolution–precipitation reaction.^75,76 We introduced the three major reaction mechanisms, i.e., dissolution–precipitation, surface complexation, and ion exchange, in detail below.

Table 2 Adsorption mechanisms of HAP-based materials for uranium

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

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Fig. 5 The possible mechanisms of U(VI) extraction on LDHs@nHAP.⁵³

2.3.1. Dissolution–precipitation. Dissolved phosphate ions from HAP can drive the transformation of HAP-based materials and the reaction with U(VI).^4,5 The precipitated U–P containing minerals have been confirmed by plenty of researchers.^6,41,77 As dissolution of HAP happens, it provides phosphate ions which can precipitate UO₂²⁺, thus creating new U(VI)-phosphates such as chernikovite or autunite. These reaction processes can be expressed by the following equations (eqn (11)–(13)):^78,79

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)

The new phase H₂(UO₂)₂(PO₄)₂·8H₂O was precipitated and formed from Mg/Fe-LDH@nHAP due to the dissolution of HAP which supplies phosphate ions having good affinity with UO₂²⁺. As shown in Fig. 6a, some new peaks emerged in the XRD pattern of LDHs@nHAP after U(VI) adsorption, indicating that a new phase was formed as the adsorbed U(VI) species. The emerging new XRD peaks after uranium adsorption can be assigned to the (001), (101), (110), and (204) planes of chernikovite (H₂(UO₂)₂(PO₄)₂·8H₂O) (PDF No. 09-0296). These results showed that phosphate ions were involved in the formation and precipitation of chernikovite.⁵³ Chernikovite and autunite phases were detected after U(VI) adsorption onto BC-HAP1, BC-HAP2, and BC-HAP3 as seen from the XRD patterns in Fig. 6b.⁷ Ca(UO₂)₂(PO₄)₂·3H₂O formed by the interaction between UO₂²⁺ and HAP hollow microspheres was found from the XRD analysis in Fig. 6c.⁴⁶ Similar reaction products were found by Han et al. after uranium adsorption by Bio-HAP.⁴⁴Fig. 6d–f display the XRD patterns of Bio-HAP600 with characteristic peaks of HAP at 2θ = 31.765°, 32.194°, 32.896°, 39.790°, 46.693°, and 49.489°, which gradually dispersed after uranium adsorption for 2.5 min and 15 min. The new peaks appearing at 10.523°, 16.527°, 18.009°, 24.738°, 25.576°, 27.724°, and 35.880° can be attributed to the formation of autunite (Ca(UO₂)₂(PO₄)₂(H₂O)₆) (PDF No. 72-2117) and correspond to its crystal planes of (001), (101), (110), (102), (200), (201), and (212), respectively. The results demonstrated that autunite was formed and HAP disappeared after Bio-HAP600 reacted with U(VI). These results indicated that the formation of the chernikovite or autunite solid phase is one unique reaction mechanism of U(VI) adsorption onto HAP-based materials.

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

2.3.2. Ion exchange. HAP has a strong cation exchange capacity. Consequently, ion exchange is another common adsorption mechanism between U(VI) and HAP-based materials, in which UO₂²⁺ substitutes Ca²⁺ present in the HAP lattice through a reaction process expressed by eqn (14) below:⁸¹

≡Ca²⁺ + UO₂²⁺ → ≡UO₂²⁺ + Ca²⁺ (14)

The ion exchange of Ca²⁺ with UO₂²⁺ played an important role in the adsorption of U(VI) on Ag-HAP and Ca-HAP.¹³ Zeng et al. also found that one of the adsorption mechanisms between U(VI) and HAP–ZVI was ion exchange.⁵² Similarly, the higher than expected effluent Ca concentrations observed in the experiment by Lammers et al. were likely ascribed to ion exchange of UO₂²⁺ for Ca²⁺ among incongruent dissolution of a metastable form of hydroxyapatite (mHAP) to produce chernikovite.⁴¹ Interestingly, the pH and bicarbonate concentrations of solutions can influence ion exchange of uranium onto HAP. Neuman et al. found rapid adsorption of U(VI) by bone powder containing HAP through an ion exchange mechanism, which reached a maximum at pH 6.55 and low bicarbonate concentrations while the amount of U(VI) adsorption decreased with increasing either pH or bicarbonate concentrations.^82–85

2.3.3. Surface complexation. Another adsorption mechanism is the surface complexation reaction of U(VI) with reactive functional groups on HAP-based materials. Two main types of surface functional groups (≡OH and ≡O₃P–OH⁺) were identified to be located on HAP. UO₂²⁺ can be coordinated with the functional groups via the reactions shown in eqn (15) and (16):^13,43,81

≡OH + UO₂²⁺ → ≡O–UO₂⁺ + H⁺ (15)

≡O₃P–OH⁺ + UO₂²⁺ → ≡O₃P–O–UO₂²⁺ + H⁺ (16)

Based on the literature, surface complexation reactions between uranium and HAP-based materials have been reported widely. For example, Wu et al. found that chemical changes took place on HAP hollow microsphere samples before and after U(VI) uptake by using XPS analyses (Fig. 7).⁴⁶ As seen from Fig. 7a, C 1s (285.78 eV), O 1s (531.30 eV), Ca 2p (346.37 eV), and P 2p (132.74 eV) peaks were detected from the samples. A new double peak representing uranium was observed for the samples after U(VI) uptake. The binding energy value of Ca 2p shifted from 347.1 eV to 347.3 eV after U(VI) uptake, which revealed the interactions between U(VI) and Ca (Fig. 7b). As seen from Fig. 7c and d, the O 1s peak was divided into peaks as anion oxide (530.8 eV, O²⁻), hydroxyl bonded to calcium (531.2 eV, Ca–OH), and adsorbed H₂O (532.5 eV). The area ratio of Ca–OH peaks dropped from 64.4% to 60.8% after U(VI) uptake, which demonstrated that Ca–OH shared electrons for formation of U–O bonds.⁸⁶ Zheng et al. also observed that the area ratio of –OH decreased from 44.22% to 39.02% which showed that the –OH group played a significant role in the surface complexation of uranium.⁵⁶ Moreover, the surface complexation by –OH with the outer- and inner-layers of LDHs@nHAP with UO₂²⁺ ions was confirmed by FT-IR and XPS.⁵³

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.

3. MXene-based materials

3.1. Structure and synthesis of MXenes

MXenes are a new class of two-dimensional (2D) layer-structured transition metal carbides and/or nitrides. The general chemical formula of MXenes can be written as M_n+1X_nT_x (n = 1–3), where M represents transition metals (e.g. Sc, Ti, V, Cr, Zr, Mo, Nb, Ta, and Hf), X represents carbon or nitrogen, and T_x denotes surface terminal groups (–O, –OH, and/or –F).^29,88 The precursors of MXenes are MAX phases [M_n+1AX_n (n = 1–3)] which have a layered hexagonal structure space group P6₃/mmc and an anisotropic and laminated structure (Fig. 8a). A mostly refers to Ti, In, Al, S, Si, Cd, P, Ga, As, Ge, Sn, and Pb elements. The M–A bond is metallic and thus is weaker and more reactive than the M–X bond with a mixed covalent/metallic/ionic character, making it possible to obtain MXenes by removing the A layers from MAX.²⁹ The structure of the widely known MXene Ti₃C₂TX produced from Ti₃AlC₂via HF etching followed by sonication is shown in Fig. 8b. Al atoms were extracted from Ti₃AlC₂ after the reaction with HF and new 2D Ti₃C₂ exfoliated layers with OH and/or F surface groups formed after sonication in methanol (Fig. 8b₁–b₃). The first MXene (Ti₃C₂T_x) was produced by Naguib et al. in 2011.⁸⁸ So far, about more than 30 MXenes have been experimentally synthesized including Ti₃C₂T_x, Ti₂CT_x, Nb₄C₃T_x, Ti₃CNT_x, Ta₄C₃T_x, Nb₂CT_x, V₂CT_x, Nb₄C₃T_x, etc., of which Ti₂CT_x and Ti₃C₂T_x are the most widely used ones for environmental protection applications owing to their element abundance and non-toxic properties.

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

M_n+1AX_n + 3HF(aq) = M_n+1X_n(s) + AF₃(s) + 3/2H₂(g) (17)

M_n+1X_n(s) + 2H₂O(l) = M_n+1X_n(OH)₂(s) + H₂(g) (18)

M_n+1X_n(s) + 2HF(aq) = M_n+1X_nF₂(s) + H₂(g) (19)

As seen from eqn (17), the Al layer in MAX phases is removed by the reaction with HF and M_n+1X_n is generated. And then newly formed M_n+1X_n reacts with HF and H₂O to form surface terminated –OH and –F groups on M_n+1X_n through reactions shown in eqn (18) and (19). Finally, a few or single layer MXenes are obtained after ultrasonic treatment (Fig. 9a).

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 Ti₃C₂T_x 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 (Ti₃C₂T_x, T_x = 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 Ti₃C₂T_x.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 Ti₃C₂T_x.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 Ti₃C₂T_x 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 Ti₃C₂T_x (T_x = Cl, O) MXene can be obtained (Fig. 12).⁹⁷

Fig. 12 Schematic of Ti₃C₂T_x 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 Ti₃C₂T_x MXene. (d) MS-Ti₃C₂T_x MXene is obtained after further washing in ammonium persulfate solution.⁹⁷

Based on the same principle, Li et al. obtained a series of MXenes (Ti₂CT_x, Ti₃C₂T_x, Ti₃CNT_x, Nb₂CT_x, Ta₂CT_x, Ti₂CT_x, Ti₃C₂T_x) 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 Ti₃C₂T_x 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 Mo₂CT_x prepared by the sole HCl-assisted hydrothermal etching strategy. (a) Schematic illustration of the preparation procedure for fluoride-free Mo₂CT_x.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.

3.2. Adsorption capacities of MXene-based materials

MXenes have garnered considerable attention recently 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.²⁹ At the same time, MXenes exhibit excellent irradiation tolerance and great thermal stability.¹⁰⁰ Thus, they have been explored as ideal adsorbents for a plethora of radionuclides such as uranium (²³⁸U),^30,101 thorium (²³²Th),¹⁰² cesium (¹³⁷Cs),^103,104 strontium (⁹⁰Sr),^105,106 and europium (^152,154Eu).¹⁰⁷ At present, MXenes have been explored for uranium adsorption relatively more than other radionuclides. Studies have shown that MXenes exhibit excellent uranium adsorption capabilities as listed in Table 3. For instance, the theoretical capacity of uranium onto Ti₂CO₂ could reach 1890 mg g⁻¹. This implies that MXenes can be employed as superior potential candidates for uranium removal and enrichment. However, the uranium adsorption capacity of most MXene-based materials was predicted by theoretical simulations. The adsorption kinetics and thermodynamics of MXene-based materials for uranium have not been widely investigated by batch adsorption experiments. This must be due to that most of the MXene synthesis processes involve the use of toxic HF, making the experiment not well carried out. Thus, the development of a fluorine-free synthetic strategy of MXenes is imperative, which can facilitate the experimental synthesis of more MXene-based materials and their use as uranium adsorbents.

Table 3 Adsorption capacities and main parameters of uranium adsorption by MXene-based materials

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

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3.3. Influence factors of MXene-based materials

3.3.1. Effect of the solution pH value of MXene-based materials. Wang et al. demonstrated that the U(VI) adsorption processes by multilayered V₂CT_x are significantly pH dependent.²² Specifically, V₂CT_x multilayers were tested by measuring the zeta potential to be negatively charged within the pH range from 2.5 to 7.0. As the pH value increased, the zeta potential of V₂CT_x became more negative, causing a stronger electrostatic interaction between V₂CT_x and U(VI). Thus, higher U(VI) removal capacity was obtained with increasing pH, i.e., its removal capacity increased rapidly with the pH increase from 3.0 to 5.0 and reached the maximum capacity of 174 mg g⁻¹ at a pH value of 5.0. Similarly, the same research team found that U(VI) adsorption onto Ti₂CT_x was obviously influenced by the solution pH value.¹¹⁰ Specifically, the uranium adsorption increased dramatically at first at a low pH range of 1.5 to 3.0. As the pH_PZC of Ti₂CT_x was approximately 2.75, the variation of U(VI) uptake behavior with pH could be well explained by the conditions of Ti₂CT_x surface charge. For pH < 2.5, the electrostatic repulsion force between UO₂²⁺ and Ti₂CT_x with positive charge gave rise to a low U(VI) adsorption efficiency. For pH values of 3.0 to near 7, Ti₂CT_x with negative charge could well uptake cationic U(VI), which brought about U(VI) removal efficiency >90%. At pH > 8.0, a small drop of U(VI) uptake could be attributed to generation of anionic UO₂²⁺ species (UO₂(OH)₃⁻, (UO₂)₃(OH)₇⁻, etc.), which were not conducive to adsorption onto Ti₂CT_x with negative charge. Hence, the U(VI) uptake of MXene was greatly affected by pH mainly because pH can bring about the variation of surface charge of MXene and U(VI) species.

3.3.2. Effect of contact time of MXene-based materials. Adsorption time is an important factor affecting the adsorption capacity of MXenes for uranium. Zhang et al. showed that more than 95% of UO₂²⁺ was removed by amidoxime-functionalized Ti₃C₂T_x (F-TC) after 5 min with the kinetic data of U(VI) adsorption onto the F-TC.³⁰ The rapid kinetics implied the excellent hydrophilicity and abundant binding active sites of F-TC. Wang et al. demonstrated that U(VI) adsorption for six Ti₃C₂T_x MXenes with different enlarged c lattice parameters (Δc-LPs) reached equilibrium in approximately 360 min and Ti₃C₂T_x–DMSO-hydrated exhibited the highest adsorption capacity of U(VI) among the six MXenes, indicating a significantly elevated adsorption capability due to intercalation and hydration activation.¹⁰⁸ Wang et al. revealed that the U(VI) adsorption of V₂CT_x was mainly controlled by a chemical sorption process.²² The adsorption kinetics of V₂CT_x were divided into a rapid adsorption phase within the first 20 minutes, followed by a slower phase that achieved equilibrium after approximately 4.5 h. They also found that the pseudo-second order model fitted the adsorption kinetic data of V₂CT_x better. Similarly, the same group reported that the adsorption kinetic experimental data of Ti₃C₂T_x for U(VI) were fitted much better by the pseudo-second order model, indicating that the U(VI) uptake was a chemical adsorption process.¹¹⁰

3.3.3. Effect of co-existing ions and ionic strength of MXene-based materials. There are some studies on the impact of co-existing ions on U(VI) uptake by MXenes. For instance, to test the ion selectivity of V₂CT_x, Wang et al. performed U(VI) removal tests from solutions containing a series of competing metal cations such as Co²⁺, Ni²⁺, Zn²⁺, Sr²⁺, La³⁺, Nd³⁺, Sm³⁺, Gd³⁺, and Yb³⁺ at two different pH values of 4.5 and 5.0.²² They found that there was no obvious reduction of the U(VI) adsorption capacity onto V₂CT_x by these tested competing cations, revealing that V₂CT_x selectively adsorbed uranium over the other tested metal cations. Similarly, Wang et al. conducted co-existing ion and ionic strength experiments. Ti₃C₂T_x–DMSO-hydrated exhibited an excellent selectivity for U(VI) uptake in the presence of cations (Na⁺, Mg²⁺, Ca²⁺, etc.) with high concentrations, which could be ascribed to the groups of [Ti–O]⁻H⁺ on Ti₃C₂T_x having strong U(VI) affinity.¹⁰⁸ In addition, Zhang et al. carried out experiments of U(VI) removal by F-TC under high salinity of 0.6 M NaCl and demonstrated that the adsorption behavior was independent of ionic strength, suggesting that UO₂²⁺ was complexed by F-TC on the inner-sphere surface.³⁰ For coexisting anions, Wang et al. found a drastic reduction of U(VI) adsorption efficiency (only 7%) caused by Na₂CO₃ (0.01 M) due to generation of UO₂(CO₃)₃⁴⁻ at pH 10.94 as U(VI) was equilibrated with Na₂CO₃ before contact with Ti₂CT_x. In contrast, U(VI) uptake by M–Ti₂CT_x was independent of the anions (Cl⁻, NO₃⁻, ClO₄⁻, etc.).¹¹⁰

3.4. Adsorption mechanisms of MXene-based materials

For uranium removal by MXenes, various interaction mechanisms have been proposed as listed in Table 4, including ion-exchange,²² complexation or coordination,²²d-layer manipulation,¹⁰⁸ physisorption, chemisorption, reduction,¹¹⁰ and hydrogen bonding.^13,23 Among these mechanisms, ion-exchange, complexation, and coordination are the main mechanisms for uranium removal by MXenes. Etched surface groups with –O, –OH, and –F terminations play important roles in uranium capture. Additionally, the layer d-spacing of MXenes adjusted by appropriate intercalants is beneficial to encapsulate uranium in the adsorption process. Sometimes, more than one or two interaction mechanisms occur simultaneously depending on reaction conditions and the type of MXene. In the following section, we will discuss the adsorption mechanisms of recent research work about uranium removal by MXenes in detail.

Table 4 Adsorption mechanisms of MXene-based materials for uranium

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

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Among the reported MXenes, titanium-based Ti₃C₂T_x and Ti₂CT_x are more commonly used in uranium adsorption. Wang et al. developed a hydrated intercalation strategy to enlarge the interlayer space of multilayered Ti₃C₂T_x under hydrated conditions.¹⁰⁸ The U(VI) uptake of hydrated Ti₃C₂T_x is significantly higher than that of dry Ti₃C₂T_x, which is principally on account of the larger interlayer space and stronger flexibility of hydrated Ti₃C₂T_x. More UO₂²⁺ can be easily contained and confined into the Ti₃C₂T_x channel by controlling the interlayer space rationally. The authors also proposed a fast post-adsorption calcination approach for the encapsulation of uranium inside Ti₃C₂T_x (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 Ti₃C₂T_x 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 Ti₃C₂T_x 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 Ti₃C₂T_x 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 (Ti₃C₂T_x)/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 Ti₂CT_x 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 Ti₂CT_x; however, the UO_2+x phase nanoparticles adsorbed onto some Ti₂CT_x transformed into amorphous TiO₂ at near-neutral pH. Their study highlighted the reduction-induced immobilization of U(VI) by Ti₂CT_x 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 Ti₂CT_x MXene.¹¹⁰

Vanadium-based MXenes were also investigated for U(VI) uptake.²² The adsorption mechanism of U(VI) onto V₂CT_x 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 V₂CT_x and U(VI) was deeply elaborated from the molecular aspect through DFT calculations and EXAFS. The analysis results demonstrated that UO₂²⁺ was adsorbed by V₂CT_x 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.

4. Conclusions and prospects

In summary, HAP is a conventional promising adsorbent for the treatment of uranium pollution due to its excellent adsorption capacities, acid–base adjustability, and good thermal stability. Moreover, HAP is low cost, biodegradable, environmentally friendly, and easily attainable. The main reaction mechanisms of U(VI) adsorption by HAP-based materials include ion exchange, dissolution–precipitation, and surface complexation. MXenes have emerged as a new class of 2D layer-structured transition metal carbides and/or nitrides, which have garnered considerable attention due to their unique properties and thus they are explored as ideal adsorbents for uranium. 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. Thus, the relationship between synthesis methods and termination groups and properties of MXenes should be cautiously considered when preparing them. Titanium-based and vanadium-based MXenes are the main types of MXenes investigated for uranium adsorption by theoretical simulation prediction or experimental methods. Interaction mechanisms for uranium removal 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 major mechanisms for U(VI) uptake 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, Ti₃C₂T_x 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 Ti₃C₂T_x MXene was found to have an influence on the viability of cells.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the China Scholarship Council (CSC201908430311), the Natural Science Foundation of Hunan Province (2021JJ30563), and the IIT startup funds.

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