Remobilization of ZnO–TA nanoadsorbent for U(VI) and Th(IV) extraction: adsorption optimization through the Box–Behnken design model

Abstract

The rapid expansion of nuclear technology has led to increasing volumes of radioactive wastewater, threatening the environment and necessitating the removal of contaminants in order to protect the surroundings. This report presents the sequestration of U(VI) and Th(IV) ions through an adsorption technique using a highly efficient ZnO–TA nanoadsorbent, driven by ionic interactions with the surface-active functional groups of the adsorbent. This nanoadsorbent was successfully synthesized using a hydrothermal process and structurally analyzed through various analytical techniques. Batch experiments were performed to study different parameters such as the influence of pH, amount of sorbent, time, initial concentration of sorbate, and interference of other ions. Adsorption study parameters were optimized via the response surface methodology and systematic batch studies. The highest removal efficiencies of 99.98% (U(VI)) and 98.80% (Th(IV)) ions were obtained at pH =4, using 10 mL of 50 mg L⁻¹ adsorbate concentration with 3 mg and 4 mg of adsorbent within 15 min, respectively. Moreover, the maximum adsorption capacities under optimum conditions were evaluated to be 909.09 mg g⁻¹ and 380.62 mg g⁻¹ for U(VI) and Th(IV) ions, respectively. These results were derived using the Langmuir isotherm model adhering to pseudo-second-order kinetics. The dominant adsorption mechanism of U(VI) and Th(IV) onto the ZnO–TA nanoadsorbent is explained through the interplay of electrostatic interactions and hydrogen bonding between the functional group active binding sites. Therefore, the results stated that the synthesized adsorbent, with its excellent recyclability, is effective in uranium and thorium uptake.

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1. Introduction

The release of radioactive or nuclear waste from nuclear reactors and fuel-processing plants is the primary contributor radioactive pollution in the current time.^1,2 Uranium is the main source of fuel for generating electricity in nuclear power plants, whereas thorium can be utilized as an alternative fuel by undergoing a conversion process to produce ²³³U.^3–5 The most common oxidation states of uranium and thorium are U(IV) and U(VI) and Th(IV), respectively.^6,7 The high chemical toxicity and radioactivity of uranium and thorium make them hazardous heavy metals.⁸ Activities associated with uranium and thorium extraction, processing, and disposal can potentially lead to underground water contamination.⁹ Their presence in water resources poses significant environmental concerns. Uranium and thorium species enter aquatic systems through various routes such as industrial waste discharge from mining operations and nuclear power generation processes, air pathways via precipitation or dust, and soil leaching.^10,11 According to WHO provisional guidelines, the maximum allowable concentrations for uranium and thorium in drinking water are 0.03 mg L⁻¹ and 0.0225 mg L⁻¹, respectively.^12,13 Due to the prolonged half-lives of the most abundant isotopes of uranium and thorium viz. 4.468 × 10⁹ years and 14.1 × 10⁹ years, respectively, they continue to harm the environment for a long time, as evidenced by the Hiroshima–Nagasaki incidents of 1945.^14–17

Apart from its uses in nuclear power production, uranium has been widely used in various applications such as additives in colored glass, medical and industrial applications, defense purposes, etc.^18,19 Thorium, on the other hand, is used for the production of ceramics, camera lenses, welding rods, radiotherapy, etc.²⁰ Such numerous applications of uranium and thorium have resulted in an increase in their usage over time. Inhalation of uranium and thorium exerts adverse effects on the liver, kidneys, bones, and reproductive tissues.^21,22 Upon exposure, normal functioning of the vital organs is disrupted. Given these significant issues, it is important to develop advanced and precise techniques capable of detecting and selectively removing ultra-trace amounts of these toxic elements. Over the years, several methods have been used to eliminate U(VI) and Th(IV) ions such as solvent extraction,²³ ion exchange,²⁴ lime softening,²⁵ coagulation,²⁶ and adsorption.²⁷ Among these decontamination techniques, adsorption is considered to be advantageous due to its affordability, high efficiency, high selectivity, availability of various sustainable adsorbents,²⁸ absence of harmful byproducts^29,30 and ease of handling.^31–33

A diverse range of adsorbents has been developed to address this issue such as activated carbons,³⁴ zeolites,³⁵ silica,³⁶ clays,³⁷ radioactive residues,³⁸ and hydrogels.³⁹ Besides these materials, metal oxide-based nanocomposites have emerged as a promising solution owing to their high surface area, chemical affinity towards contaminants, and abundance. Tannic acid is a non-toxic, naturally occurring polyphenol known for its antioxidant properties.^40,41 Due to their numerous functional groups, tannic acid molecules are highly effective at adsorbing metal ions.⁴² Kaptanoglu et al. reported GO–ZnO and rGO–ZnO composites for the adsorption of uranium ions at pH 5 and 5.5 and the obtained adsorption capacities were 476.19 mg g⁻¹, 256.41 mg g⁻¹, respectively.⁴³ De-Bin Ji et al. synthesised a Nano-ZnO enhanced SA/CMC-ZnONPS-AO1 hydrogel, which was explored for the removal of uranium ions, achieving an adsorption capacity of 641.7 mg g⁻¹.⁴⁴ Kaynar et al. reported Co-doped ZnO nano-material for the adsorption of thorium ions with an adsorption capacity of 121.29 mg g⁻¹.⁴⁵

Thus, this report successfully outlines the synthesis of ZnO–TA nanoadsorbent through a hydrothermal process for extracting U(VI) and Th(IV) ions in aqueous media. The presence of functional groups was confirmed by FTIR analysis and FESEM and EDS analysis of the synthesised adsorbent revealed spherical shaped particles containing zinc, carbon, and oxygen elements. The presence of micropores on the surface of the ZnO–TA nanoadsorbent was confirmed by BET analysis. Systematic batch studies and the Box–Behnken design model were used to optimize adsorption parameters such as the mass of adsorbent, pH, contact time, and the initial concentration of adsorbate. The adsorption process was well-defined using adsorption kinetic and isotherm models, in which the adsorption process followed the pseudo-second-order model and Langmuir model with maximum adsorption capacities of U(VI) (909.09 mg g⁻¹) and Th(IV) ions (380.62 mg g⁻¹). The ZnO–TA nanoadsorbent was also demonstrated in regeneration studies to be reusable up to seven adsorption–desorption cycles. The materials utilized in the formation of the adsorbent are eco-friendly, non-toxic, sustainable, and have a high surface area. Moreover, this adsorbent excels at removing the most toxic radionuclides with a high removal efficiency. Therefore, this research explores the nanoadsorbent's capability for effectively adsorbing radioactive ions.

2. Experimental section

2.1 Materials

Zinc acetate (Zn(CH₃CO₂)₂·2H₂O), tannic acid, and arsenazo-III dye were purchased from Sigma Aldrich (99.9% purity). Nitric acid (HNO₃), sodium acetate (CH₃COONa), chloroacetic acid (ClCH₂COOH), and sodium hydroxide pellets (NaOH) were procured from Fischer Scientific (99.5% purity), while thorium nitrate tetrahydrate (Th(NO₃)₄·4H₂O) and uranium nitrate hexahydrate (UO₃(NO₃)₂·6H₂O) were purchased from M&B and BDH (the British Drug houses) (99.8% purity), respectively. All the chemicals were used as received with no further purification. All the necessary instrument details are listed in Text S1 (ESI†).

2.2 Synthesis procedure

In the synthesis of the ZnO–TA nanoadsorbent, a thorough procedure was followed to ensure the precise formation of the nanoadsorbent. The synthesis commenced with the preparation of a 3 g zinc acetate solution in a solvent mixture comprising ethanol (20 mL) and distilled water (20 mL). Simultaneously, 1 g tannic acid solution was prepared using the same solvent mixture. The two solutions were then combined by the dropwise addition of tannic acid solution to the zinc acetate solution, which was stirred for 30 minutes, initiating the formation of zinc oxide (ZnO) nanoparticles. To further enhance the functionality of ZnO nanoparticles, the entire mixture of zinc oxide nanoparticles was subjected to a controlled hydrothermal process at 130 °C for 16 h, leading to the formation of the ZnO–TA nanoadsorbent. This thermal treatment facilitated the stabilization of the nanoadsorbent structure. After that, the obtained ZnO–TA nanoadsorbent was washed with distilled water and dried in the oven at around 80 °C for 15 h. A schematic of the synthetic steps of the ZnO–TA nanoadsorbent is shown in Scheme 1.

Scheme 1 A scheme depicting the formation process of the ZnO–TA nanoadsorbent.

2.3 Adsorption behaviour of U(VI) and Th(IV) ions

Systematic batch adsorption studies were carried out to optimize several factors including pH, mass, time of contact, initial concentration of adsorbate, and interference of different ions. A standard stock solution of uranium and thorium (1000 ppm each) was made using distilled water diluted to multiple running solutions according to the specific requirements. Experimental studies were carried out under optimum conditions by mixing 3 mg and 4 mg of nanoadsorbent in 10 mL of 50 ppm of U(VI) and Th(IV) at pH = 4, respectively. The conical flask containing solutions was then agitated on a shaker for a particular duration and the resulting mixtures were filtered using a nylon syringe filter (0.22 μm). To evaluate the concentrations of U(VI) and Th(IV), the UV-visible spectroscopic method was used at 649 nm and 668 nm wavelengths of U(VI) and Th(IV), respectively, and the absorbance was verified by the ICP-OES technique for both ions. The adsorption capacity (q_e) and removal efficiency (%) for both ions can be evaluated using the following expression:^46,47here, C₀ and C_e denote the initial concentration and equilibrium concentrations of the ions, respectively, v (L) is the volume of the solution, and m (g) is the mass of the adsorbent.

2.4 The Box–Behnken design model (BBDM)

The response surface methodology (RSM) approach is proposed to optimize the removal process efficiently. It is the most flexible, efficient, and easy statistical technique and offers advantages over the traditional methods.^48,49 RSM consists of a collection of valuable statistical methods for modeling the problems with the BBDM being used to model the response surface for RSM.^50,51 The main objective is to optimize a response surface influenced by different parameters including pH, time, and mass of adsorbent. A series of 17 experimental runs were conducted and ‘Design Expert 13’ was used to plot the data where significance was provided using analysis of variance (ANOVA) analysis. A quadratic model is used to simulate the functional relationship between the input factors and output response, as described by the given equation:⁵²where β₀ represents a constant and β_i, β_ii, and β_ij denote the coefficients for linear, quadratic, and three-way interaction terms, respectively.

2.5 Reusability studies

The regenerative capabilities of an adsorbent majorly affect its value. To investigate this, our research focused on conducting experiments on the recyclability and regeneration of the adsorbent. For this, an adsorption experiment was done in which 10 mL of 50 mg L⁻¹ of U(VI) and Th(IV) ion solutions were agitated with 3 mg and 4 mg of ZnO–TA nanoadsorbent at room temperature at a pH of 4 for 15 min, respectively. The agitation was carried out until the adsorption equilibrium was attained. Following this, 0.1 M HNO₃ solution was utilized to desorb the U(VI) and Th(IV) ions from the adsorbent. Lastly, the ZnO–TA nanoadsorbent was cleansed with distilled water and dried in an oven at 60 °C for 18 h and used for another cycle.

3. Results and discussion

3.1 Characterization

The BET method of analysis is used to analyze the surface area, pore size, and pore volume of the ZnO–TA nanoadsorbent. The nitrogen adsorption–desorption graph of the nanoadsorbent is shown in Fig. 1(a), which also exhibits an H3 hysteresis loop representing the groove pores. The nanoadsorbent exhibits a high surface area of 55.55 m² g⁻¹ which facilitates the interaction of adsorbate ions with the adsorbent surface, enhancing the adsorption capacity. The pore volume of the nanoadsorbent is 0.495 cc g⁻¹, and the average pore size of 1.82 nm reveals the microporous nature of the nanoadsorbent, facilitating the binding of U(VI) and Th(IV) ions onto functional active sites. The presence of functional groups in the nanoadsorbent was determined by the FTIR investigations, and the spectra of the ZnO–TA nanoadsorbent are shown in Fig. 1(b), which reveal the typical peaks of the functional groups present in the nanoadsorbent. The broad peak observed at 3303 cm⁻¹ represents the –OH stretching mode of tannic acid present in the nanoadsorbent. The characteristic peaks at 1555 cm⁻¹, 1415 cm⁻¹, 1014 cm⁻¹, and 751 cm⁻¹ contribute to the –C=C, –C–O, –C–H, and –C–O–C– stretching modes, respectively.⁵³ The peak at 552 cm⁻¹ is assigned to the Zn–O stretching vibrations in the ZnO–TA nanoadsorbent. All of these functional groups of the nanoadsorbent enhance interactions with U(VI) and Th(IV) ions and therefore increase the removal efficiency.

Fig. 1 (a) The nitrogen adsorption–desorption graph and (b) FT-IR spectra of the ZnO–TA nanoadsorbent.

The surface morphology of the nanoadsorbent was analyzed through micrographs obtained via field emission scanning electron microscopy (FESEM), and the elemental composition was determined through EDS analysis. The FESEM images of the synthesised ZnO–TA nanoadsorbent show spherical shaped particles with a smooth surface due to the presence of the polymer, which is shown in Fig. 2(a and b). The presence of zinc, oxygen, and carbon was also confirmed by EDS analysis (Fig. 2(c)), and the EDS mapping results revealed the uniform distribution of different elements in the synthesized ZnO–TA nanoadsorbent (Fig. 2(d–f)).

Fig. 2 (a) and (b) FESEM images, (c) EDS spectra, and (d)–(f) EDS mapping of the prepared ZnO–TA nanoadsorbent.

High-resolution transmission electron microscopy (HR-TEM) was used to define the particle shape and size of the synthesized nanoadsorbent. The micrograph of the HR-TEM analysis of ZnO–TA nanoadsorbent reveals the spherical shape of the ZnO nanoparticles on which tannic acid is functionalized on its surface (Fig. 3(a)). The SAED pattern is irregular, which shows the polycrystalline nature of the synthesized nanoadsorbent (Fig. 3(b)). The interplanar d-spacing between particles is 1.4 nm, which is presented in Fig. 3(c).

Fig. 3 (a) The HR-TEM image, (b) SAED pattern, and (c) interplanar distance of the ZnO–TA nanoadsorbent.

3.2 Adsorption of U(VI) and Th(IV) ions

To obtain the ideal adsorption conditions for the highest removal of U(VI) and Th(IV) ions using the ZnO–TA nanoadsorbent, various batch studies were carried out by changing several parameters such as solution pH, contact time, initial concentration of adsorbate, and dosages of nanoadsorbent.

The initial pH of the solution has a significant impact on achieving a high sorption capacity during the process as it reflects the diversity of ions present and surface charge distribution resulting from protonation or deprotonation. Therefore, a batch study was conducted by varying the pH of the U(VI) and Th(IV) solutions (10 mL of 50 ppm) in the range of 2–10 without changing any of the other variables. The removal efficiencies of U(VI) and Th(IV) ions increased between pH 2 to 4, then declined after pH 4 (Fig. S1, ESI†). For lower pH values (<4), the dominant species were UO₂²⁺, UO₂OH⁺, UO₂(OH)₂²⁺, Th⁴⁺, and Th(OH)₂²⁺, which show binding on the surface of the synthesized nanoadsorbent and at pH 4, the maximum removal efficiencies were 99.98% and 98.80% for U(VI) and Th(IV) ions, respectively. At higher pH (>4) values, the presence of active sites is compromised due to the formation of carbonate and hydroxy complexes of the U(VI) and Th(IV) ions, which inhibits the sorption process due to electrostatic repulsion.^54,55 Therefore, the maximum removal efficiency was observed at pH = 4. So, this was used as the optimum pH for further adsorption studies.

The mass of the nanoadsorbent is a key parameter as it influences the adsorbent's effectiveness in removing a particular concentration of radioactive ions. In this experiment, the dosages of nanoadsorbent were varied from 1 to 7 mg to determine the effect on the removal of U(VI) and Th(IV) ions in a solution of 10 mL of 50 mg L⁻¹ at pH = 4 under agitation for 3 h. As shown in Fig. 4(a and b), the removal efficiency % increased at the initial stage with the addition of more adsorbent due to the growing accessibility of adsorption sites, whereas the plot for adsorption capacity showed the opposite trend due to the increasing mass quantity.^56,57 Hence, 3 mg and 4 mg dosages of nanoadsorbent were used for the U(VI) and Th(IV) ions adsorption studies, respectively.

Fig. 4 A graphical depiction of the effect of mass on (a) U(VI) and (b) Th(IV) adsorption using ZnO–TA nanoadsorbent.

To study the impact of contact time, the kinetics of the interactions between an adsorbent and an adsorbate are investigated to estimate the actual duration of the adsorption process. Fig. 5(a and b) shows the increasing adsorption of U(VI) and Th(IV) ions within the initial time and thereafter, adsorption equilibrium was reached after 15 min. Initially, the accessible surface sites facilitate adsorption. However, achieving equilibrium becomes challenging as residual unoccupied sites are less readily filled, possibly due to repulsive forces between U(VI) ions and Th(IV) ions, which increases the removal efficiencies of both ions.⁵⁸

Fig. 5 Graphical depiction of the influence of contact time on (a) U(VI) and (b) Th(IV) ions adsorption, and the effect of the initial concentration of adsorbate on (c) U(VI) and (d) Th(IV) ions adsorption by the ZnO–TA nanoabsorbent.

It is stated that the ability to adsorb U(VI) and Th(IV) ions relies upon the initial concentration of the following ions in an aqueous solution. Thus, experimental studies were conducted to study the impact of initial concentration in various initial U(VI) and Th(IV) concentrations in the range of 10–400 mg L⁻¹, and other parameters, including pH, the mass of adsorbent, and contact time, were kept constant. Fig. 5(c and d) indicates the reduced percentage of U(VI) and Th(IV) ion adsorption with increasing concentrations of adsorbate.⁵⁹ For further adsorption studies, 50 mg L⁻¹ was chosen as the optimum initial concentration of adsorbate.

3.3 Adsorption kinetics

Adsorption kinetics can be studied to facilitate the exploration of the adsorption mechanism by interpreting the sorbate-sorbent interactions. The adsorption kinetics of U(VI) and Th(IV) ions onto the ZnO–TA nanoadsorbent were assessed using different models, such as the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The assessment of parameters for these models is based on the correlation coefficient and the agreement between the observed and calculated values of q_e, which are enabled through the fitting of the adsorption experimental data. The mathematical expressions for the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models are described in the ESI† (Text S2 and eqn (S1)–(S3)).

Fig. 6 and Fig. S2 (ESI†) depict the linear correlation of the kinetic models for adsorption of U(VI) and Th(IV) ions on the ZnO–TA nanoadsorbent with variables for both models detailed in Table 1. The higher correlation coefficient with R² = 0.9949 is observed for the pseudo-second-order, as compared to the pseudo-first-order model, indicating that the second model offers a superior fit to the adsorption data.^60,61 The experimental adsorption capacity is closely related to the calculated adsorption capacity obtained from the pseudo-second-order model. The intra-particle diffusion model can be used for the transportation mechanism and to influence the adsorption control steps. However, this fitting curve does not pass through the origin, indicating that it is not the only influential factor for justifying the adsorption process. Thus, the adsorption process adheres to a pseudo-second-order kinetic model describing the adsorbate–adsorbent chemical interactions.

Fig. 6 Kinetics graphs for the adsorption of (a) and (b) U(VI) ions and (c) and (d) Th(IV) ions onto the ZnO–TA nanoadsorbent.

Table 1 Parameters of kinetic models for U(VI) and Th(IV) ions adsorption

Kinetic model	Parameter	U(VI)	Th(IV)
Pseudo-first-order	K 1 (min^−1)	0.46	0.064
	q e (cal) (mg g^−1)	69.26	43.81
	R ^2	0.6840	0.9293
Pseudo-second-order	K 2 (mg g^−1 min^−1)	0.0033	0.003
	q e (cal) (mg g^−1)	87.26	103.09
	R ^2	0.9949	0.9825
Intra-particle diffusion model	K id (mg g^−1 min^−1/2)	3.85	3.19
	C (mg g^−1)	44.90	104.8
	R ^2	0.8138	0.8472
q e (exp) (mg g^−1)		83.3	135.63

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3.4 Adsorption isotherm

Adsorption isotherms are essential for understanding the mechanism behind the adsorption process and the adsorbent's affinity towards radioactive ions. This analysis typically involves using various isotherm equations, with a significant focus on established models like Langmuir and Freundlich isotherm models. The Langmuir model considers a monolayer process, which indicates homogeneity in the distribution of sorption sites across the adsorbent surface.⁶² The Freundlich model describes multilayer adsorption processes and the flexibility of this isotherm is used to describe sorption on heterogeneous surfaces, making it valuable for systems where multilayer adsorption is expected.⁶³ The linear equations of these models are described in the ESI† (Text S3 and eqn (S4), (S5)).⁶⁴

Fig. 7(a–d) illustrates the linear correlation observed in the U(VI) and Th(IV) ions adsorption isotherm models on the ZnO–TA nanoadsorbent. The variables of all adsoprtion isotherm models are recorded in Table 2. The linear plots of the adsorption isotherm provide data showing that the Langmuir model yields the highest correlation coefficient (R² = 0.9849 and 0.9815) for both ions. This indicates that, among the models considered, the Langmuir model best describes the adsorption mechanism. The maximum adsorption capacities obtained from this model are 909.09 and 380.62 mg g⁻¹ for U(VI) and Th(IV) ions, respectively. To demonstrate the superiority of the prepared ZnO–TA nanoadsorbent in terms of adsorption capacity for both U(VI) and Th(IV) ions compared to different adsorbents described in the literature, Table 3 presents a comparative analysis.

Fig. 7 Linear plots of the adsorption isotherm models for (a) and (b) U(VI) and (c) and (d) Th(IV) ions on the ZnO–TA nanoadsorbent.

Table 2 Isotherm model parameters for the adsorption of U(VI) and Th(IV) ions

Isotherm model	Parameters	U(vi)	Th(iv)
Langmuir model	Q m (mg g^−1)	909.09	380.62
	b (L mg^−1)	0.99	0.03
	R ^2	0.9849	0.9815
Freundlich model	K f (mg g^−1)	84.03	31.50
	1/n	0.26	0.46
	R ^2	0.8062	0.9657

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Table 3 A comparative analysis of adsorption capacities and parameters documented in different studies for the removal of U(VI) and Th(IV) ions using various adsorbents

S. no.	Adsorbent	Adsorbate	pH	Time	Adsorption capacity (mg g^−1)	Ref.
1.	rGO–ZnO	U(VI)	5.5	30	256.41	65
2.	GO–ZnO	Th(IV)	3	—	243.90	66
3.	Polyaniline nanocomposites	Th(IV)	3.2	300	68.49	67
4.	TiO2/Fe3O4/GO nanocomposite	Th(IV)	2.5	60	29.97	68
5.	Fe3O4@PAA	Th(IV)	3	60	122.5	69
6.	ZnO particles	Th(IV)	3.5	30	150	70
7.	Al doped ZnO particles	Th(IV)	—	—	192.3	71
8.	ZnO–TA nanoadsorbent	U(VI)	4	15	909.09	This work
		Th(IV)	4	15	380.62

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3.5 Optimization using the Box–Behnken design model

The RSM approach is explored to evaluate the optimal adsorption parameters by optimizing the process using the BBDM. Here, three independent variables, pH, contact time, and mass of the adsorbate, were optimized to attain the highest removal efficiency of radioactive ions. The data for the 17 sets of experiments has been recorded in Table 4.

Table 4 Experimental data of the Box–Behnken design model to optimize the removal efficiency of U(VI) and Th(IV) ions

Run	A: pH	B: mass (mg)	C: time (min)	Removal efficiency uranium (%)	Removal efficiency thorium (%)
1	4	6	10	99.2	97.4
2	6	4	20	45.23	95.2
3	4	4	15	99.98	98.8
4	2	6	15	25.67	85.43
5	4	4	15	99.98	98.8
6	4	2	10	87.98	76.78
7	4	4	15	99.98	98.8
8	4	4	15	99.98	98.8
9	2	4	20	17.65	84.34
10	6	6	15	45.64	95.32
11	2	2	15	13.25	74.32
12	4	6	20	99.9	98.9
13	2	4	10	14.4	81.76
14	4	4	15	99.98	98.8
15	4	2	20	99.12	85.43
16	6	2	15	36.68	78.89
17	6	4	10	40.12	82.34

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The obtained results are outlined in Tables S1 and S2 (ESI†). ANOVA analysis confirming the model's validity for U(VI) and Th(IV) ions adsorption onto the ZnO–TA nanoadsorbent was based on F-values and p-values. In the case of U(VI) ions, the F-value of 671.02 and p-value of 0.0500 indicate the significance of this model. In the case of uranium adsorption, A, B, C, BC, A², and C² are significant model terms. In the case of thorium adsorption, A, B, C, AC, BC, A², B², and C² are significant model terms. The predicted (0.9815) and actual (0.9974) values of R² are somewhat close, with a minor difference (Fig. S3, ESI†). Adeq. Precision assesses the signal-to-noise ratio, favouring ratios above 4. The value of Adeq. Precision is 60.742, which suggests that this model is well-suited to exploring design space. While for Th(IV) ions, the F-value of 101.95 and p-value of 0.0500 describe the significance of the model, and the predicted (0.8789) and actual (0.9827) values of R² are somewhat close with a difference of >0.2 (Fig. S4, ESI†). The value of Adeq. Precision is 27.185, indicating a sufficient signal, which indicates that this model offers a framework for exploring the design space. Removal efficiencies of U(VI) and Th(IV) ions are as follows:

Removal efficiency of uranium = −254.88000 + 143.08125 pH + 9.33625 mass + 3.99700 time − 0.216250 pH × mass + 0.046500 pH × time − 0.261000 mass × time − 17.1085 pH² − 0.308750 mass² − 0.087800 time²

Removal efficiency of thorium = −14.29000 + 15.46125 pH + 16.79562 mass + 4.37825 time + 0.332500 pH × mass + 0.257000 pH × time −0.178750 mass × time −2.37844 pH² − 1.44906 mass² − 0.135050 time²

The RSM plots revealed that the maximum removal efficiencies of U(VI) and Th(IV) ions are 99.99% and 98.74%, respectively, achieved under the optimal conditions of pH = 4.2, mass = 3.1 mg, and time = 17.4 min for uranium, and pH = 4.31, mass = 4.12 mg, and time = 16.3 min for thorium, which closely aligns with the experimental values, highlighting the model's accuracy (Fig. 8). The contour plots depicting the removal efficiency of both ions under optimal conditions are given in Fig. S5 and S6 (ESI†). Hence, this model proves valuable for predicting the % efficiency of radioactive ions.

Fig. 8 RSM 3D graphs illustrating the impact of pH, time, and mass of adsorbent on the removal efficiency of (a)–(c) U(VI) and (d)–(f) Th(IV) ions using the ZnO–TA nanoadsorbent.

3.6 Adsorption mechanism

Adsorption of radioactive ions onto the ZnO–TA nanoadsorbent is comprehensively elucidated by FT-IR spectroscopy, and FESEM-EDS elemental mapping. The FTIR spectra effectively describe the interaction, various bonding modes, and the probable binding of U(VI) and Th(IV) ions onto the nanoadsorbent. In the FTIR spectra, peaks associated with the –OH group, –C=O, and –C–O, stretching mode showed reduced peak intensity after the adsorption process and slightly shifted. This is because the functional active sites are now occupied by the U(VI) and Th(IV) ions (Fig. 9(a)). Furthermore, the agglomerated surface of the ZnO–TA nanoadsorbent becomes apparent in the FESEM image following the adsorption of the radioactive ions onto the nanoadsorbent. It implies that the ZnO–TA nanoadsorbent adsorbs a large number of U(VI) and Th(IV) ions during adsorption (Fig. 9(b)). Fig. 9(c), illustrating the EDS mapping after adsorption, confirms the uniform distribution and successful adsorption of U(VI) and Th(IV) ions. This demonstrates the presence of zinc, carbon, oxygen, uranium, and thorium. According to the results, uranium and thorium are adsorbed onto the ZnO–TA nanoadsorbent primarily through electrostatic interactions and the active functional binding sites.

Fig. 9 (a) FT-IR analysis of the nanoadsorbent before and after adsorption and (b) the FESEM image and (c) EDS mapping of the ZnO–TA nanoadsorbent after the adsorption of U(VI) and Th(IV) ions.

3.7 Interference of ions

The aim is to examine the adsorption capability of ZnO–TA of nanoadsorbent for adsorbing U(VI) and Th(IV) ions, considering the presence of interfering ions like Cl⁻, F⁻, Ca²⁺, Mg²⁺, CO₃²⁻, and NO₃⁻ under optimum conditions with optimal adsorption parameters including pH, time, mass of adsorbent, and initial concentration of adsorbate, which were obtained from the adsorption studies. As shown in Fig. 10(a), no substantial decline was observed in the removal efficiency of the radioactive ions when various ionic salts were present. Therefore, the nanoadsorbent exhibited effective adsorption performance despite the existence of other ions, which suggests that the adsorbent is a highly effective material for the adsorption of U(VI) and Th(IV) ions.

Fig. 10 Graphical depiction of (a) the interference study and (b) the reusability study of ZnO–TA nanoadsorbent.

3.8 Reusability study

The ability to reuse and recover is crucial for reducing operational costs, thereby enhancing the attractiveness of the adsorption process for wastewater treatment. The ZnO–TA nanoadsorbent was investigated for its ability to be regenerated across seven consecutive adsorption and desorption cycles, as illustrated in Fig. 10(b). This experiment involves initially adsorbing U(VI) and Th(IV) ions followed by desorption. The nanoadsorbent was desorbed in 0.1 M HNO₃ and agitated on a shaker at room temperature for 3 h. Further, the nanoadsorbent was rinsed with distilled water, oven-dried and prepared for subsequent cycles of use. The adsorbent can effectively be used for up to seven cycles, making it a suitable material for adsorbing radioactive ions.

4. Conclusions

To conclude, this research article has demonstrated the successful synthesis of a nanoadsorbent i.e. ZnO–TA for the selective extraction of radioactive ions. Various analytical techniques were employed to characterize the nanoadsorbent. The ideal adsorption parameters were obtained from batch studies, 10 mL of 50 mg L⁻¹ U(VI) and Th(VI) solutions was adsorbed within 15 min at pH = 4 using 3 mg and 4 mg of nanoadsorbent, respectively. Two adsorption isotherm models were evaluated, and the Langmuir adsorption model proved to be the best fit, providing the maximum adsorption capacity of 909.09 mg g⁻¹ and 380.62 mg g⁻¹ for U(VI) and Th(IV) ions, respectively. The ZnO–TA nanoadsorbent demonstrates selectivity and can be reused for as many as seven cycles, underscoring its sustainability. The optimization process was conducted using the BBDM in which the predicted output factor matched with the actual value obtained in the adsorption experiment. These factors collectively signify the nanoadsorbent's efficacy in the sorption of radioactive ions.

Data availability

Data will be available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Manish Sharma is grateful for the financial assistance of MNIT Jaipur and Defence Laboratory, Defence Research and Development Organization (DRDO), Jodhpur, 342011, India, for ICP-OES. The authors would also thank the Materials Research Centre (MRC), MNIT Jaipur, for materials characterization.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04199b

‡ Both authors contributed equally.

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