Retracted Article: A carboxymethyl cellulose modified magnetic bentonite composite for efficient enrichment of radionuclides

Abstract

The radio-frequency (RF) plasma-induced grafting technique was employed to fabricate a carboxymethyl cellulose (CMC) grafted magnetic bentonite composite (CMC-g-MB) in Ar conditions. The grafted CMC could improve the composite dispersion ability as well as the chemical stability in strong acidic conditions. The CMC-g-MB composite exhibited fast adsorption kinetics and good adsorption capacities in the enrichment of Cs(I), Sr(II) and Co(II) from aqueous solutions, showing broad applicability for radionuclide removal. The enrichment efficiencies of radionuclides on the CMC-g-MB composite decreased in the order of Cs(I) > Sr(II) > Co(II). On the basis of the thermodynamic parameters, radionuclide adsorption on the CMC-g-MB composite was thermodynamically favorable and endothermic. Considering the non-toxicity and biodegradation of CMC, the CMC-g-MB composite presented promising potential in radioactive pollution management.

---

1. Introduction

With the rapid development of nuclear power plants, nuclear waste has aroused considerable attention owing to its long half-life, high toxic and carcinogenicity.^1,2 The presence of fission products can pose serious damage to the functions of biological systems. The discharge of untreated radioactive wastewater into the sea by Fukushima Daiichi Nuclear Power Plant in Japan (2011) brought sewage disposal to the frontier of environmental science and technology once again. As the main products of spent fuels, Cs(I) and Sr(II) with relatively high fission yields are distributed in almost all of the contaminated radioactive wastewater. Co(II) with a half life of 5.27a and a high gamma decay energy is produced in nuclear facilities as an activation product. Given their great threat to environmental safety and public health, the elimination of Cs(I), Sr(II) and Co(II) from contaminated liquid waste is in urgent need. Many approaches such as adsorption,³ electrolysis,⁴ biodegradation,⁵ photocatalytic oxidation,⁶ and membrane⁷ have been studied to eliminate radionuclides. Among them, adsorption technique is commonly used in the enrichment of radionuclides owing to its convenient operation, economic feasibility and broad adaptability.⁸

Bentonite is always used as a traditional inorganic exchanger for cations because of its relatively high exchange capacity, low penetrability and high swelling property. It is consisted of two silica tetrahedral sheets fused to one central alumina octahedral sheet, which is defined as: E_x(H₂O)₄{(Al_2−x,Mg_x)₂[(Si,Al)₄O₁₀](OH)₂}, where E is the exchangeable cations (i.e., Na⁺, Ca²⁺ and K⁺) between the layers.⁹ Herein, bentonite can be treated as an effective candidate for the management of radioactive wastewater.^10–12 However, the inconvenient separation of bentonite from solutions greatly limits its further applications. Thus the synthesis of magnetic bentonite (MB) by introducing Fe₃O₄ particles can solve this problem via an external magnetic field. It is worth noting that Fe₃O₄ nanoparticles are found to be susceptible to air oxidation, unstable at low pH, and easy to form an aggregation. There needs to coat a protective layer to ensure the chemical stability of MB.

CMC, a derivative of cellulose, presents low-cost, renewable, hydrophilic, biodegradable and non-toxic properties. The reaction of hydroxyl and carbonyl oxygen with metal ions makes it possible to be served as an excellent adsorbent to remove metal ions.¹³ CMC can be employed as the protective layer to stabilize MB and functionalize it further. By unifying the complementary characteristics of bentonite and CMC, CMC-g-MB composite should exhibit a good performance to enrich radionuclides.

For all we know, conventional chemical approaches were applied to modify adsorbents by complicated procedures with multiple steps. Meanwhile, large amounts of used chemicals would cause environmental pollution. With this aim in mind, we applied RF plasma-induced grafting technique to synthesize the composite. Dry plasma process is solvent-free, low-temperature and environmentally friendly, and irradiated free radicals formed on the backbone of bentonite in pre-irradiation process can act as active sites to initiate the indirect grafting of cellulose.^14,15 Moreover, the plasma grafting process can introduce functional groups onto the surfaces of various substances without converting their main nature. Numerous active sites introduced by low-temperature RF plasma technique would induce bonds with polymers easily and then initiate their graft to bulk materials.^16–18

In this paper, we utilized an effective method to functionalize MB with CMC via low-temperature plasma-induced technique in Ar condition. The as-synthesized composite was analyzed using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS). Then CMC-g-MB composite was served as an adsorbent to enrich Cs(I), Sr(II) and Co(II) from aqueous solutions. The effects of environmental factors including contact time, ionic strength, pH and temperature were investigated in details.

2. Experimental

2.1 Materials

Bentonite was collected from Gaomiaozi County (Inner Mongolia, The People's Republic of China). Bentonite powder was rinsed by 5% HCl solution for 12 h, and then washed by Mill-Q (Millipore, Billerica, MA) water till chloride was undetected in supernatant with 0.01 M AgNO₃ solution. The obtained materials were put in vacuum at 60 °C overnight for further usage.

2.2 Preparation of CMC-g-MB composite

The deposition of Fe₃O₄ nanoparticles on bentonite was performed by the reaction between FeCl₃·6H₂O and ethylene glycol.^19,20 Briefly, 1.0 g FeCl₃·6H₂O was dissolved into 50 mL ethylene glycol, and 500 mg bentonite was dispersed with vigorous stir and ultrasonication for 2 h. Afterwards, 4.5 g sodium acetate and 1.25 g sodium dodecyl sulfate were added with vigorous stir within 1 h. The mixture was kept in a Teflon-lined stainless steel autoclave at 200 °C for 15 h. Then the sample was diluted with Milli-Q water and ethanol respectively and dried overnight.

The CMC-g-MB composite was synthesized by an inductively coupled RF plasma device in Ar condition with two successive steps. Firstly, 1.0 g MB was pretreated under continuous stirring by Ar plasma for 15 min at a pressure of 5.0 Pa and RF power of 120 W. Secondly, 100 mL CMC solution (2.0 g L⁻¹, 80 °C) was injected into the plasma-induced grafting reactor immediately and stirred under 80 °C vigorously overnight. The obtained samples were collected by a permanent magnet and diluted with 0.01 mol L⁻¹ H₃PO₄ solution and Mill-Q water thoroughly. Then the sample was dried in a vacuum oven at room temperature.

2.3 Characterization

CMC-g-MB composite was characterized by SEM, STEM, FT-IR, TGA, XPS and XRD techniques. The SEM measurements were conducted on a JEOL JSM-6330F at the beam energy of 15.0 kV. FT-IR spectra were performed on a Nicolet Magana-IR 750 spectrometer over a range from 400 to 4000 cm⁻¹. STEM measurements were performed using a JEOL JEM-2100F operated at 200 kV. TGA was recorded on a Shimadzu TGA-50 thermogravimetric analyzer from 20 to 700 °C at the heating rate of 10 °C min⁻¹ with an air flow rate of 50 mL min⁻¹. XPS measurements were conducted in an ESCALab220i-XL surface microanalysis system equipped with two ultrahigh vacuum chambers. XRD patterns were carried out by using a diffractometer with Cu Kα source (λ = 1.54178 Å). The zeta potential was measured using Zetasizer Nano ZS Analyzer. The zero point charge (pH_zpc) of CMC-g-MB composite was obtained by interpolating the zeta potential value to zero.

2.4 Batch adsorption experiments

All the adsorption experiments were carried out by batch technique in 10 mL polyethylene tubes. 3.0 mg adsorbent was added to 6 mL aqueous solution containing CsCl/SrCl₂/CoCl₂, background electrolyte, and trace quantities of ¹³⁷Cs/⁹⁰Sr/⁶⁰Co radiotracer. All the adsorption experiments in this paper were carried out by employing single adsorbate. Negligible amounts of HCl or NaOH solutions were used to adjust the suspension pH values. Afterwards these centrifuge tubes were oscillated for 48 h on an oscillator to ensure the adsorption equilibrium. The solid/liquid phases were separated by a permanent magnet. The counts of ¹³⁷Cs/⁹⁰Sr/⁶⁰Co radiotracer were determined by liquid scintillation counting (Packard 3100 TR/AB Liquid Scintillation analyzer, Perkin-Elmer) with a scintillation cocktail (ULTIMA GOLD ABTM, Packard) after the supernatant was filtered through a 0.45 μm membrane filter. The adsorbed quantity of Cs/Sr/Co was obtained by calculating the distinct values of the initial activity of ¹³⁷Cs/⁹⁰Sr/⁶⁰Co radiotracer (A_tot) in Cs/Sr/Co stock solution and that in supernatant (A_L) (i.e., R% = 100% × (1 − A_L/A_tot)). All these experimental data were the averages of at least triplicate determinations.

3. Results and discussion

3.1 Characterization of CMC-g-MB composite

The surface morphologies of bentonite and CMC-g-MB composite were analyzed using SEM technique. The image of pristine bentonite (Fig. 1A) clearly exhibits the rough surfaces with many crumples. As for the morphology of CMC-g-MB composite (Fig. 1B), one can see that the crumpled structure and less compact surfaces are exhibited, and the layers of bentonite are partially exfoliated (as the arrow shown in Fig. 1B) and rearranged. The filmy and loose nature of CMC covering on the multiple layers is also observed, demonstrating that CMC was introduced on the surfaces of bentonite successfully. STEM image (Fig. 1C) shows that Fe₃O₄ nanoparticles with diameters around 50 nm to 100 nm attach on the bentonite layers. By magnification of bentonite structure (Fig. 1D), the intercalation in bentonite layers of CMC-g-MB composite is clearly exhibited, and the interlayer spacing is measured to be ∼1.45 nm which is obviously larger than the d-spacing of bentonite (∼1.25 nm).¹¹ Fig. 1E shows that the magnetic Fe₃O₄ nanostructures are wrapped by amorphous CMC layers to protect the core from acid/base corrosion.

Fig. 1 SEM images of bentonite (A) and CMC-g-MB composite (B); STEM images of CMC-g-MB composite (C–E); typical XRD patterns (F and G).

As a key factor of adsorbents, good dispersion would provide large numbers of binding positions to enrich radionuclides from aqueous solutions. The dispersion properties of bentonite, MB, and CMC-g-MB composite (Fig. S1†) show that the CMC-g-MB composite is still well dispersed even after settling for 24 h. The poor dispersion of MB can be attributed to the deposition of magnetic Fe₃O₄ particles on bentonite, while the graft of soluble CMC enhances the dispersion of the composite greatly due to its abundant oxygen functional groups including hydroxyl and carboxyl groups. The excellent dispersion property results in that the CMC-g-MB composite cannot be separated directly from the aqueous solution within a relatively short time only by gravity, indicating that the magnetic separation is needed. Moreover, the grafted soluble CMC can improve the chemical stability of CMC-g-MB composite in extremely acidic solutions, which is qualitatively evaluated at pH = 1.0 and 2.0. After magnetic separation, the color of MB supernatants is yellowish at pH = 2.0 and becomes deeper as the solution acidity increases from pH 2.0 to 1.0, owing to the dissolved Fe₃O₄ nanoparticles by hydrochloric acid. However, the CMC-g-MB supernatants are transparent, indicating that Fe₃O₄ nanoparticles in CMC-g-MB composite are more stable than those in MB. Thus, the grafted CMC can be applied to improve the composite dispersion ability and chemical stability in acid conditions as well.

The typical XRD patterns of bentonite and CMC-g-MB composite are shown in Fig. 1F. For pristine bentonite, the peaks of montmorillonite, quartz, calcite and feldspar are observed. Note that the (001) diffraction peak at 2θ = 7.08° is observed in pristine bentonite. For Fe₃O₄, the characteristic peaks at 30.32°, 35.76°, 43.42°, 57.46° and 62.96° are ascribed to (220), (311), (400), (511) and (440) reflections of a cubic magnetite phase (JCPD no. 75-0033), respectively. Similar diffraction peaks are found in XRD pattern of CMC-g-MB, indicating the successful disposition of Fe₃O₄ on bentonite. We record XRD at a slow scan rate and the obtained patterns are exhibited in Fig. 1G. It can be seen that the (001) diffraction peaks of MB and MB treated by RF plasma both locate at the same position (2θ = 7.08°) as that of bentonite exhibited in Fig. 1F. However, the (001) diffraction peak shifts to 5.95° after the graft of CMC. According to the Bragg's Law (2d sin θ = nλ), the interlayer spacing d₀₀₁ increases from 1.25 to 1.48 nm, suggesting that the interaction of CMC molecules in the interlayer region of bentonite occurred in the grafting process.

TGA technique was applied to evaluate the thermal characteristic of CMC-g-MB composite as shown in Fig. S2.† The initial weight loss below ∼150 °C is corresponded to the moisture loss (∼6% weight loss). The next obvious weight loss from ∼150 to ∼480 °C is related to the decomposition of oxygen-containing functional groups and the complete combustion of the CMC backbones (∼16% weight loss). Moreover, the continuous weight loss from ∼480 to 700 °C is due to following reasons: (1) as the phase transition from Fe₃O₄ to FeO shown in the phase diagram of Fe–O system,²¹ FeO is the thermodynamically stable phase above 570 °C; (2) the self-dehydroxylation of hydroxyl groups on the bentonite surfaces. Taken the weight loss of moisture into consideration, the weight content of CMC in the composite is calculated to be ∼17.0%. By employing the similar calculation method, we study the effect of RF powers (i.e., 30, 60, 90, 120 and 180 W) and duration time (i.e., 5 and 15 min) on the grafting content of CMC in CMC-g-MB composite. The corresponding results are shown in Fig. S3A.† When the RF power increases from 30 W to 180 W, the CMC grafting content in CMC-g-MB composite increases from ∼2.0% to ∼12.0% for the treatment of 5 min, and keeps at a relatively high level around ∼12.5% to ∼17.0% for the treatment of 15 min. As we know, more Ar meta-stable radicals would be generated to activate the bentonite and induce the CMC graft further with the increasing RF power. Considering the cost of the treatment and the stability of generating plasma, we apply 120 W of RF power and 15 min of processing time to guarantee the maximum grafting content of CMC in this paper.

XPS technique can provide important information to investigate the valence and electronic structure of elements, and surface compositions in materials. The XPS spectra of CMC-g-MB composite are shown in Fig. 2. Fig. 2A shows that the presence of Si, Fe, C and O on the surface of CMC-g-MB composite. The peaks of Si 2s and Si 2p are attributed to the silica sheets consisted in bentonite.²² Three peaks deconvoluted from C 1s spectrum (Fig. 2B) are located at 284.6 eV (C–C), 286.4 eV (C–O) and 288.6 eV (O–C=O). The existed C–O and O–C=O peaks can be attributed to the abundant hydroxyl and carboxyl functional groups of CMC. The appearance of Fe 2p peak (Fig. 2C) with multiple splitting (i.e., Fe 2p_3/2 at 710.9 eV and Fe 2p_1/2 at 724.8 eV) confirms the formation of Fe₃O₄. Fe 2p_3/2 peak is deconvoluted into two components, i.e., Fe²⁺ at 709.7 eV and Fe³⁺ at 711.9 eV. The intensity ratio of atomic Fe³⁺ and Fe²⁺ (I_Fe³⁺/I_Fe²⁺ = 0.64 : 0.36) is close to the Fe³⁺/Fe²⁺ ratio of pure Fe₃O₄ (0.67 : 0.33 or 2 : 1).²³ Thus, Fe₃O₄ was successfully deposited onto the bentonite surfaces in our experiment. It is quite interesting to note that as the treated time of RF plasma (120 W) increases from 0 min to 15 min (Fig. S3B†), the intensity of Si–Si bond increases and that of Si–O bond decreases gradually. The reduction of Si–O bond corresponding to the silica tetrahedral sheets in the layered structure of bentonite indicates that the deformation of silica tetrahedral sheets occurs gradually with the increasing treatment time of RF plasma. The activated O radical remained in the silica tetrahedral sheets would play an important role in the grafting of CMC.

Fig. 2 XPS spectra of CMC-g-MB for survey scan (A), C 1s spectrum (B) and Fe 2p spectrum (C).

To analyze the functional groups of CMC-g-MB composite, FT-IR results are shown in Fig. 3A. The characteristic bands at 3622 and 3436 cm⁻¹ can be attributed to O–H stretching vibration of silanol (Si–OH) groups and H₂O molecules adsorbed on solid surfaces. The band at 3620 cm⁻¹ is not obvious because of the increased content of CMC in the composite. The band at 1630 cm⁻¹ corresponds to an overtone of H₂O bending vibration. The bands at 1034 and 914 cm⁻¹ are assigned to Si–O bending vibration and Al–O(OH)–Al stretching vibration, respectively.²⁴ The appearance of bands at 1418 cm⁻¹ (O–C=O) and 1325 cm⁻¹ (–OH) can be attributed to carboxyl and hydroxyl functional groups of grafted CMC, respectively.²⁵ The bands at 519 and 461 cm⁻¹ are due to Al–O–Si and Si–O–Si bending vibrations, respectively. Compared with the spectrum of bentonite, an additional band located at 587 cm⁻¹ is corresponded to Fe–O bond in the deposited Fe₃O₄, which is consistent with the XPS results.

Fig. 3 FT-IR spectra (A), zeta potentials of CMC-g-MB composite (B).

To comprehend the surface property further, the zeta potential of CMC-g-MB composite was investigated. The surfaces of CMC-g-MB composite are considered to be amphoteric, indicating that the adsorbent surfaces are positively charged at relatively low pH due to the protonation reaction (SOH + H⁺ ↔ SOH₂⁺, where S represents the CMC-g-MB surface, and OH represents the oxygen-containing functional groups on the CMC-g-MB surface), and negatively charged at relatively high pH due to the deprotonation reaction (SOH ↔ SO⁻ + H⁺). The latter is favorable to bind positively charged radionuclide ions. Fig. 3B shows that the electrostatic point of CMC-g-MB composite is about 5.6. The surfaces of CMC-g-MB composite are positively charged at pH < 5.6, and negatively charged at pH > 5.6.

3.2 Evaluation of adsorption kinetics

Fig. 4A shows the adsorption kinetics of Cs(I), Sr(II) and Co(II) on CMC-g-MB composite. The percentages for these three radionuclides increase rapidly in the first 6 h and achieve the equilibriums at 6 h. The kinetic results were modeled by the pseudo-first-order and pseudo-second-order rate equations (Fig. 4B). The Q_e values calculated by the pseudo-second-order model are also closer to the experimental ones (Q_e,exp) as shown in Table S1,† suggesting that the rate-controlling mechanism for Cs(I), Sr(II) and Co(II) adsorption on CMC-g-MB composite is chemisorption rather than mass transport.²⁶

Fig. 4 Effect of contact time on the radionuclide adsorption (A) and the comparison of the two kinetic models of Cs(I), Sr(II) and Co(II) on CMC-g-MB composite (B). m/V = 0.5 g L⁻¹, I = 0.01 M NaCl, pH = 6.0 ± 0.1, T = 298 K, C_initial = 20 mg L⁻¹ for all cations.

3.3 Effect of pH and ionic strength

pH-dependence is an important characteristic for the adsorption of radionuclides. The change of pH in solutions can promote/suppress the adsorption by affecting the radionuclide distribution in solutions and the surface charges of the adsorbents through dissociating functional groups.²⁷ The effect of pH on Cs(I), Sr(II) and Co(II) adsorption by CMC-g-MB composite is shown in Fig. 5.

Fig. 5 The proposed mechanism of radionuclide adsorption on CMC-g-MB composite.

The adsorption percentage of Cs(I) ions increases gradually with increasing pH (Fig. S4A†). Cs(I) ions don't hydrolyze or form complexes readily in the broad pH range. Hence, the pH-dependent adsorption can be attributed to the competition between H⁺ and Cs(I). At low pH, the concentration of H⁺ is high which makes the adsorption capacity relatively low, and most hydroxyl groups in CMC-g-MB composite are also protonated to induce the electrostatic repulsion between Cs(I) ions and positively charged surfaces of the composite. As pH increases, the competition between H⁺ and Cs(I), and the protonation of hydroxyl groups will be both weak, leading to the rising amount of exchangeable sites. It should be noted that the adsorption percentage of Cs(I) ions at low pH values is much higher than that of Sr(II) and Co(II) (Fig. S4B and S4C†). For example, the adsorption percentage of Cs(I) ions is ∼74% while only ∼45% of Sr(II) ions and ∼15% of Co(II) ions are adsorbed (pH = 3.0, m/V = 0.5 g L⁻¹, T = 298 K, C_initial = 20 mg L⁻¹ for all cations). This result can be attributed to the smallest hydrated ionic radii (0.329 nm) of monovalent Cs(I) ions compared with those of Sr(II) (0.412 nm) and Co(II) (0.423 nm).²⁸ Cs(I) has a thinnest hydration shell which makes a closest approach to move into and out of the bentonite channels freely.

The hydrolysis constants of Sr(II) (log β₁ = −13.29 and log β₂ = −28.51) indicate that Sr(II) presents as Sr²⁺ at pH < 11.0 and no Sr(OH)₂ precipitation forms at pH < 13.²⁹ As for Sr(II) adsorption, the dissociation of hydroxyl groups is restricted at low pH conditions, and only a small amount of Sr(II) is adsorbed. The surfaces become negatively charged gradually as pH increases, and then the adsorption percentage is enhanced due to electrostatic attraction effect (Fig. S4B†).

The adsorption curves of Co(II) ions are shown in Fig. S4C.† The species distribution of Co(II) ions calculated from the hydrolysis constants (log K₁ = −9.6, log K₂ = −9.2, and log K₃ = −12.7) indicates that Co(II) ions are present as Co²⁺, Co(OH)⁺, Co(OH)₂ and Co(OH)₃⁻ at different pH.³⁰ The adsorption percentage of Co(II) is relatively low at pH < 5 due to the highly concentrated hydronium ions which would compete with Co(II). The electrostatic repulsion between the positively charged adsorbent surfaces and Co(II) would also suppress the adsorption percentage. Moreover, the Na⁺ ions added before the injection of Co(II) have equilibrated with CMC-g-MB composite and the adsorption of Co(II) ions can be treated as the exchange of Co²⁺ with Na⁺. At pH > 8.48, the dominant species of Co(II) are Co(OH)⁺ and Co(OH)₂, which tend to enrich on negatively charged adsorbents. Most Co(II) ions have been adsorbed on CMC-g-MB composite at pH 8.0. Therefore, Co(II) enrichment is assigned to adsorption process rather than Co(OH)₂(s) precipitation.

The effect of ionic strength on radionuclide enrichment on CMC-g-MB composite was evaluated by changing NaCl concentrations. As shown in Fig. S4,† the adsorption of radionuclides on CMC-g-MB composite is strongly correlated with ionic strength. Na⁺ concentrations affect the double electrode layer thickness and the interface potential, and will influence on the binding species further. Inner-sphere surface complexation is always not sensitive to ionic strength variation compared to cation exchange and outer-sphere surface complexation.³¹ The adsorption percentage of radionuclides reduces as the ionic strength increases from 0.001 to 0.1 M. Therefore, outer-sphere surface complexation with hydroxyl groups and cation exchange both play important roles in Cs(I), Sr(II) and Co(II) adsorption on CMC-g-MB composite, which is schematically proposed in Fig. 5.

3.4 Adsorption isotherms and thermodynamics

The adsorption isotherms can evaluate the radionuclide distribution relationships in liquid/solid phases. Fig. 6 shows the temperature-dependent adsorption isotherms of radionuclides on CMC-g-MB composite at three temperatures (i.e., 298, 318 and 338 K). The adsorption isotherms of radionuclides on MB at 298 K are also shown for comparison. To comprehend the adsorption mechanism and optimize the design of adsorption process for radionuclide enrichment, the analytical data were handled with Langmuir and Freundlich models.

Fig. 6 Temperature-dependent adsorption isotherms of Cs(I), Sr(II) and Co(II) on MB and CMC-g-MB composite. The solid lines are Langmuir model simulation, and the dash lines are Freundlich model simulation. m/V = 0.5 g L⁻¹, I = 0.01 M NaCl, pH = 6.0 ± 0.1.

The correlation coefficients (Table S2†) indicate that Langmuir model fits the experimental data much better than Freundlich model, suggesting that the radionuclides were removed by a monolayer adsorption process. According to the parameters simulated by Langmuir model, the Q_s,max of Cs(I), Sr(II) and Co(II) ions on CMC-g-MB composite are 80.5, 63.0 and 41.1 mg g⁻¹ (pH = 6.0 ± 0.1 and T = 298 K) respectively, higher than those of Cs(I) (48.5 mg g⁻¹), Sr(II) (39.5 mg g⁻¹) and Co(II) (26.3 mg g⁻¹) on MB, suggesting that the grafted CMC can improve the adsorption performance of MB for radionuclides. The adsorption capacities of these radionuclides are relatively high compared with many others adsorbents under similar experimental conditions as shown in Table 1.

Table 1 Comparison of Cs(I), Sr(II) and Co(II) adsorption capacity of CMC-g-MB composite with other adsorbents

	Adsorbents	Experimental conditions	Qs,max (mg g^−1)	Ref.
Cs(I)	Taiwan laterite	pH = 7.0, T = 298 K	41.1	32
	CNTs	pH = 7.0, T = 293 K	32.2	24
	Prussian blue	pH = 8.0, T = 298 K	131.6	33
	Walnut shell	pH = 7.0, T = 298 K	4.94	34
	Clinoptilolite	pH = 5.0, T = 293 K	49.3	27
	CMC-g-MB	pH = 6.0, T = 298 K	80.5	This study
Sr(II)	Montmorillonite	pH = 5.0, T = 298 K	13.3	35
	Activated carbon	pH = 4.0, T = 293 K	44.4	36
	Graphene oxide	pH = 4.0, T = 293 K	16.3	37
	Oxidized MWCNTs	pH = 7.0, T = 298 K	6.62	38
	Amorphous MnO2–ZrO2	pH = 4.0, T = 293 K	16.9	39
	CMC-g-MB	pH = 6.0, T = 298 K	63.0	This study
Co(II)	Sepiolite	pH = 7.8, T = 293 K	11.0	40
	Al-pillared bentonite	pH = 6.0, T = 303 K	38.6	41
	Zeolite	pH = 6.0, T = 298 K	14.4	42
	Activated carbon	pH = 6.0, T = 303 K	1.20	43
	CMC-g-MB	pH = 6.0, T = 298 K	41.1	This study

---

The relatively thermodynamic parameters are listed in Table 2. The negative values of ΔG⁰ indicate that these adsorption processes are in spontaneity. The decreasing values of ΔG⁰ means the radionuclide adsorption are more favorable with the increasing temperature. The positive ΔH⁰ values of 24.8 kJ mol⁻¹ for Cs(I), 14.4 kJ mol⁻¹ for Sr(II) and 23.0 kJ mol⁻¹ for Co(II) show clearly that the adsorption of Cs(I), Sr(II) and Co(II) are all endothermic processes. Due to the complete dissolution of radionuclides in water, the hydration sheaths of the radionuclides are supposed to be destroyed when the cations are adsorbed on CMC-g-MB composite. The energy required for this dehydration process exceeds the energy released during the attachment of cations to the solid surfaces, leading to the positive value of enthalpy.⁴⁴ Moreover, the positive ΔS⁰ values of 105 J mol⁻¹ K⁻¹ for Cs(I), 67.0 J mol⁻¹ K⁻¹ for Sr(II) and 93.3 J mol⁻¹ K⁻¹ for Co(II) demonstrate that the structures of the CMC-g-MB composite complex have been changed and the degrees of freedom increase at the solid–liquid interface during the adsorption processes.

Table 2 Thermodynamic parameters for Cs(I), Sr(II) and Co(II) adsorption on CMC-g-MB composite

Adsorbate	T (K)	ΔG^0 (kJ mol^−1)	ΔS^0 (J mol^−1 K^−1)	ΔH^0 (kJ mol^−1)
Cs(I)	298	−7.12	105	24.8
	318	−8.60
	338	−11.3
Sr(II)	298	−5.94	67.0	14.4
	318	−6.93
	338	−8.62
Co(II)	298	−4.85	93.3	23.0
	318	−6.68
	338	−8.58

---

3.5 Reusability study

The recycling availability of CMC-g-MB composite for the adsorption of Cs(I), Sr(II) and Co(II) were investigated through adsorption–desorption cycles. In brief, the desorption experiments were carried out by rinsing the radionuclides-loaded CMC-g-MB composite adsorbed radionuclides with 1.0 M of HCl for 12 h under continuous stirring.⁴⁵ The separated samples were rinsed with Milli-Q water thoroughly, vacuum dried at room temperature overnight and reused. As shown in Fig. 7, the adsorption of radionuclides on CMC-g-MB composite decreases gradually with the increasing recycle times. The decline occurs after the recycle for five times (i.e., ∼10.2% for Cs(I), ∼9.5% for Sr(II), ∼8.5% for Co(II)), indicating that CMC-g-MB composite are stable and can be reused.

Fig. 7 Recycling of CMC-g-MB composite in the removal of Cs(I), Sr(II) and Co(II). m/V = 0.5 g L⁻¹, I = 0.01 M NaCl, pH = 6.0 ± 0.1, T = 298 K, C_initial = 45 mg L⁻¹ for all cations.

3.6 Characterization of CMC-g-MB composite after adsorption

As shown in Fig. 1F, the (001) peak of CMC-g-MB composite locates at 2θ = 5.95° and its corresponding interlayer distance is estimated to be 1.48 nm. Then we analyzed the XRD patterns of the samples before and after radionuclide desorption as exhibited in Fig. 8A. One can see that there is no shift for the (001) peaks of bentonite, indicating that the structures of CMC-g-MB composite almost remain intact and are not damaged during these adsorption/desorption processes. Similar results are reported by Shahwan et al.⁴⁶

Fig. 8 XRD patterns (A) and XPS wide scan spectra (B) before and after the desorption of radionuclides; XPS high-resolution spectra of Cs 3d (C), Sr 3d (D) and Co 2p (E) after the enrichment of radionuclides.

The XPS spectra of CMC-g-MB composite before and after adsorption of radionuclides were recorded as shown in Fig. 8B. The peaks of Cs 3d, Sr 3d and Co 2p are clearly presented in the wide scan spectra of radionuclide-anchored CMC-g-MB composite. The high-resolution spectra of Cs 3d, Sr 3d and Co 2p are shown in Fig. 8C, D and E, respectively. The doublet characteristic peak of Cs 3d appears at 724.3 eV and 738.1 eV, assigned to 3d_5/2 and 3d_3/2, respectively. The Sr 3d_5/2 (133.8 eV) and 3d_3/2 (135.5 eV) peaks are also clearly exhibited in the spectrum of CMC-g-MB-Sr(II). The intensities of Co 2p_3/2 and 2p_1/2 peaks shown in the CMC-g-MB-Co(II) are relatively weaker as compared with those of Cs 3d and Sr 3d peaks, which is consistent with the adsorption capacities of Cs(I), Sr(II) and Co(II) shown in Fig. 6. The typical Na 1s peaks at 1071 eV and Na KLL peak at 500 eV disappear after the enrichment of Cs(I), Sr(II) and Co(II), indicating that the sodium ions are almost completely exchanged with these radionuclides.⁴⁷ As the smallest cation, H⁺ is commonly applied to regenerate adsorbents, resulting in the highest priority in the ion exchange. After the desorption of anchored radionuclides using HCl solutions, the peaks of Cs 3d, Sr 3d and Co 2p disappear in the XPS spectrum, demonstrating that the anchored radionuclides are successful desorbed and the electronic structures of CMC-g-MB composite surfaces remain stable.

4. Conclusions

In summary, a facile, green and low-cost low-temperature RF plasma-induced approach was proposed to synthesize CMC-g-MB composite with a good dispersion property. The results of SEM, STEM, XRD, TGA, FTIR, and XPS demonstrated that CMC was successfully grafted onto MB. The as-synthesized CMC-g-MB composite exhibited fast adsorption kinetics and good adsorption amounts toward the preconcentration of Cs(I), Sr(II) and Co(II) from aqueous solutions, and the adsorption processes were strongly dependent on pH and ionic strength of the suspensions. The adsorption kinetics could be described well by the pseudo-second-order model. The thermodynamic studies indicated that the interactions of radionuclides with CMC-g-MB composite were endothermic and spontaneous. The cation exchange and the coordination of radionuclides with oxygen-containing functional groups can be summarized as the mechanisms for radionuclide adsorption by CMC-g-MB composite. After the adsorption–desorption experiment, the CMC-g-MB composite remain stable and can be reused. The design and synthesis of CMC-g-MB composite will also broaden the applications of low-temperature RF plasma technique to combine excellent properties of different materials, especially in environmental field of wastewater treatment.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21307135, 21225730, 91326202), Chinese National Fusion Project for ITER (No. 2013GB110004), Anhui Provincial Natural Science Foundation (1508085MB29 and 1608085QB44), the Science Foundation of Institute of Plasma Physics, Chinese Academy of Sciences (DSJJ–13–YY01) are acknowledged.

References

1. T. B. Scott, G. C. Allen, P. J. Heard and M. G. Randell, Geochim. Cosmochim. Acta, 2005, 69, 5639–5646.
2. L. M. Camacho, S. G. Deng and R. R. Parra, J. Hazard. Mater., 2010, 175, 393–398.
3. A. D. Ebner, J. A. Ritter and J. D. Navratil, Ind. Eng. Chem. Res., 2001, 40, 1615–1623.
4. X. Y. Li, Y. H. Cui, Y. J. Feng, Z. M. Xie and J. D. Gu, Water Res., 2005, 39, 1972–1981.
5. M. H. El-Naas, S. A. Al-Muhtaseb and S. Makhlouf, J. Hazard. Mater., 2009, 16, 720–725.
6. M. C. Long, W. M. Cai, J. Cai, B. X. Zhou, X. Y. Chai and Y. H. Wu, J. Phys. Chem. B, 2006, 110, 20211–20216.
7. A. Bódalo, J. L. Gómez, M. Gómez, G. León, A. M. Hidalgo and M. A. Ruíz, Desalination, 2008, 223, 323–329.
8. R. Hu, X. K. Wang, S. Y. Dai, D. D. Shao, T. Hayat and A. Alsaedi, Chem. Eng. J., 2005, 260, 469–477.
9. J. Ma, J. Zou, L. Li, C. Yao, T. Zhang and D. Li, Appl. Catal., B, 2013, 134–135, 1–6.
10. T. Kozaki, H. Sato, S. Sato and H. Ohashi, Eng. Geol., 1999, 54, 223–230.
11. S. B. Yang, N. Okada and M. Nagatsu, J. Hazard. Mater., 2016, 301, 8–16.
12. X. M. Ren, S. W. Wang, S. T. Yang and J. X. Li, J. Radioanal. Nucl. Chem., 2010, 283, 253–259.
13. T. S. Anirudhan, A. R. Tharun and S. R. Rejeena, Ind. Eng. Chem. Res., 2011, 50, 1866–1874.
14. D. D. Shao, J. Hu, C. L. Chen, G. D. Sheng, X. M. Ren and X. K. Wang, J. Phys. Chem. C, 2010, 114, 21524–21530.
15. C. L. Chen, A. Ogino, X. K. Wang and M. Nagatsu, Appl. Phys. Lett., 2010, 96, 131504.
16. R. Hu, D. D. Shao and X. K. Wang, Polym. Chem., 2014, 5, 6207–6215.
17. R. Hu, S. Y. Dai, D. D. Shao, A. Alsaedi, B. Ahmad and X. K. Wang, J. Mol. Liq., 2015, 203, 80–89.
18. X. Ren, D. Shao, G. Zhao, G. Sheng, J. Hu, S. Yang and X. Wang, Plasma Processes Polym., 2011, 8, 589–598.
19. B. P. Jia, L. Gao and J. Sun, Carbon, 2007, 45, 1476–1481.
20. H. Deng, X. L. Li, Q. Peng, X. Wang, J. P. Chen and Y. D. Li, Angew. Chem., Int. Ed., 2005, 44, 2782–2785.
21. H. S. C. O'Neill, Am. Mineral., 1988, 73, 470–486.
22. F. Dietsche, Y. Thomann, R. Thomann and R. Mülhaupt, J. Appl. Polym. Sci., 2000, 75, 396–405.
23. T. Yamashita and P. Hayes, Appl. Surf. Sci., 2008, 254, 2441–2449.
24. S. B. Yang, C. Han, X. K. Wang and M. Nagatsu, J. Hazard. Mater., 2014, 274, 46–52.
25. S. Mishra, G. U. Rani and G. Sen, Carbohydr. Polym., 2012, 87, 2255–2262.
26. W. C. Song, X. X. Wang, Q. Wang, D. D. Shao and X. K. Wang, Phys. Chem. Chem. Phys., 2015, 17, 398–406.
27. S. B. Yang, D. D. Shao and C. L. Chen, Environ. Sci. Technol., 2011, 45, 3621–3627.
28. I. Smiciklas, S. Dimovic and I. Plecas, Appl. Clay Sci., 2007, 35, 139–144.
29. T. Cole, G. Bidoglio, M. Soupioni, M. O'Gorman and N. Gibson, Geochim. Cosmochim. Acta, 2000, 64, 385–396.
30. H. Yuzer, M. Kara, E. Sabah and M. S. Celik, J. Hazard. Mater., 2008, 151, 33–37.
31. C. L. Chen and X. K. Wang, Appl. Radiat. Isot., 2007, 65, 155–163.
32. T. H. Wang, M. H. Li, W. C. Yeh, Y. Y. Wei and S. P. Teng, J. Hazard. Mater., 2008, 160, 638–642.
33. A. K. Vipin, B. Hu and B. Fugetsu, J. Hazard. Mater., 2013, 258–259, 93–101.
34. D. H. Ding, Y. X. Zhao, S. J. Yang, W. S. Shi, Z. Y. Zhang, Z. F. Lei and Y. N. Yang, Water Res., 2013, 47, 2563–2571.
35. B. Ma, S. Oh, W. S. Shin and S. J. Choi, Desalination, 2011, 276, 336–346.
36. S. Chegrouche, A. Mellah and M. Barkat, Desalination, 2009, 235, 306–318.
37. A. Y. Romanchuk, A. S. Slesarev and S. N. Kalmykov, Phys. Chem. Chem. Phys., 2013, 15, 2321–2327.
38. R. Yavari, Y. D. Huang and A. Mostofizadeh, J. Radioanal. Nucl. Chem., 2010, 285, 703–710.
39. S. J. Ahmadi, N. Akbari, Z. Shiri-Yekta, M. H. Mashhadizadeh and A. Pourmatin, J. Radioanal. Nucl. Chem., 2014, 299, 1701–1707.
40. M. Dogan, Y. Turhan, M. Alkan, H. Namli, P. Turan and O. Demirbas, Desalination, 2008, 230, 248–268.
41. D. M. Marlohar, B. F. Noeline and T. S. Anirudhan, Appl. Clay Sci., 2006, 31, 194–206.
42. E. Erdem, N. Karapinar and R. Donat, J. Colloid Interface Sci., 2004, 280, 309–314.
43. A. H. Subaymon, B. A. Abid and J. A. Al-Najar, Chem. Eng. J., 2009, 155, 647–653.
44. G. D. Sheng, J. X. Li, D. D. Shao, J. Hu, C. L. Chen, Y. X. Chen and X. K. Wang, J. Hazard. Mater., 2010, 178, 333–340.
45. T. Wen, Z. Zhao, C. Shen, J. Li, X. Tan, A. Zeb, X. Wang and A. Xu, Sci. Rep., 2016, 6, 20920.
46. T. Shahwan and H. N. Erten, Radiochim. Acta, 2005, 93, 225–232.
47. F. Liu, Y. Jin, H. Liao, L. Cai, M. Tong and Y. Hou, J. Mater. Chem. A, 2013, 1, 805–813.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10990j

---