Introduction

In recent years, nuclear power generation, as an economical, efficient and clean energy source, is rapidly developing around the world [1]. Thorium is three to four times more abundant than uranium in nature [2], which can be used as a source of secondary nuclear fuel. A large number of wastewater with chemical and radiotoxicity is produced with the use of nuclear energy [3, 4]. Exposure to these radionuclides will bring about radiation hazards to the surrounding environment and human health. Therefore, from the perspective of the development and utilization of thorium resources, environmental protection and human health, it is urgent to study the high-efficiency separation and enrichment of thorium in aqueous solution.

In the past several decades, many adsorption media have been studied to separate, enrich and recover thorium. At present, the adsorption media used to separate and enrich thorium mainly include carbon materials, silicon materials, polymers, magnetic materials and biomass [5,6,7,8,9,10]. Nanometer-wide ribbons of graphene, namely graphene nanoribbons (GNRs), are a new favorite in the field of carbon nanomaterials in recent years. Its unique structure and a series of excellent properties have stimulated the research enthusiasm of researchers. GNRs are easier to adjust and have higher application value than graphene, and have excellent properties of both carbon nanotubes (CNTs) and graphene [11]. The oxidative chemical unzipping of multi-walled carbon nanotubes (MWCNTs) developed by Tour’s team, providing a unique way for mass production of GNRs, these nanoribbons are called GONRs [12]. Besides, nanomaterials have potential toxicity in the ecological environment due to their difficulty in solid-liquid phase separation after practical application. Therefore, the practical application of nano-dispersed GONRs in wastewater purification may be limited.

Chitosan is a non-toxic, bioactivity, bio-affinity and biodegradable polysaccharide [13]. Chitosan can form metal complexes with a variety of metal ions because of its molecular chain structure containing a large number of unpaired electron active groups (–NH2, –OH). There are attractive prospects in applications such as wastewater treatment and metal catalysts [14,15,16]. In this work, an eco-friendly GONRs-CTS composite was synthesized, which reduced the potential toxicity of nanomaterials in the environment. Then, the adsorption efficiency of thorium in aqueous solution by GONRs-CTS was investigated under the influence of various experimental parameters.

Experimental

Materials

All chemicals were of AR grade, which include H2SO4 (98%), KMnO4, Chitosan, H2O2 (30%), epichlorohydrin and ethanol. Their chemicals were purchased from Xilong Scientific Co., Ltd. (China). MWCNTs (The purity of 95%) were purchased from Chengdu organic chemicals Co., Ltd. (China). The Th(IV) stock solutions (1000 mg L−1) were prepared from Th(NO3)4·4H2O (AR, Aladdin) after dissolution and dilution with deionized water. The stock solution was diluted to prepare working solutions as required.

Instrumentation

Th(IV) as Th(IV)-arsenazo(III) complex at 660 nm were determined by UV–spectrophotometer (721-type spectrophotometer). The structure and morphologies of GONRs and GONRs-CTS were identified using SEM (Model: FEI Nova NanoSEM450) at the accelerating voltage of 5.0 kV. Fourier transform infrared (FTIR) spectra were recorded on a IR-843 spectrophotometer using KBr disc. The X-ray diffraction pattern of the materials were recorded by X-ray powder diffraction (D8 advance) diffractometer and samples were scanned from 5° to 40° (2θ).

Synthesis of GONRs

Synthetic procedure of GONRs was the same to our previous paper [17].

Synthesis of GONRs-CTS composites

Chitosan (1 g) was dissolved entirely in glacial acetic acid (1%, 100 mL) and stirred magnetically at 45 °C for 2 h. Then the pH was adjusted to 5 with of NaOH (0.1 mol L−1). GONRs (1 g) was added to 100 mL deionized water for ultrasonic treatment for 30 min and the suspension was formed by magnetic stirring at 60 °C for 30 min. The chitosan solution and GONRs solution were mixed at a ratio of 5∶1 and stirred at 60 °C for 4 h. The above mixed solution was uniformly coated in a culture dish and dried in an oven at 60 °C for 5 h to form film. The film was immersed in a 5% ethanol solution of epichlorohydrin and dried at 60 °C for 20 h. The film was washed several times with deionized water and dried overnight. The solid product was denoted as GONRs-CTS.

Adsorption experiments

The adsorption behavior of GONRs-CTS for Th(IV) was investigated by batch mode experiments. The effects of different parameters such as pH, contact time, initial Th(IV) concentration, temperature and adsorbent dosage were examined. The adsorption mechanism was studied using kinetics, adsorption isotherms and thermodynamic models. GONRs-CTS (0.005–0.045 g) was weighed and transferred into a 50 mL glass vials containing 20 mL Th(IV) solution (25 mg L−1). The pH of the thorium solutions was adjusted to be in the range of 1–7 by the addition of HNO3 or NaOH solutions. Then, the mixture was shaken for different periods at room temperature to achieve an adsorption equilibrium. The GONRs-CTS adsorption capacity (qe, mg g−1) and removal efficiency (p, %) for Th(IV) were calculated by Eqs. (1) and (2), respectively.

$$q_{\text{e}} = \frac{{C_{0} - C_{\text{e}} }}{m} \times V$$
(1)
$$p = \frac{{C_{0} - C_{\text{e}} }}{{C_{0} }} \times 100\%$$
(2)

where C0 and Ce (mg L−1) represent the initial and equilibrium solution concentrations of Th(IV), respectively. m and V designate the weight (g) of the GONRs-CTS and the solution volume (L), respectively.

Results and discussion

Characterization of GONRs-CTS

The morphologies of GONRs and GONRs-CTS were observed by SEM, as shown in Fig. 1. The GONRs are nanometer-wide ribbons of graphene (Fig. 1a) [18]. The surface of GONRs-CTS is rough and have many folds, which may be due to the effect of CTS on overlapping the GONRs (Fig. 1b, c).

Fig. 1
figure 1

SEM images of GONRs (a), GONRs-CTS (b, c)

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The main groups on the surface of GONRs, CTS and GONRs-CTS composites were determined by FT-IR analysis and the results are presented in Fig. 2. The FT-IR spectrum of GONRs displays the characteristic peaks at 3460 cm−1 (–OH vibrations), 2930 and 2860 cm−1 (C–H vibrations), 1610 cm−1 (C=O vibrations), 1053 cm−1 (C–O vibrations) [19]. As previously reported [20], the FT-IR spectra of CTS showed the characteristic peaks at 3435 cm−1 (–OH and –NH vibrations), 2920 and 2863 cm−1 (–CH and –CH2 vibrations), 1605 cm−1 (–NH and –NH2), 1420 and 1383 cm−1 (–CO), 1078 cm−1 (C–O vibrations). Finally, the FT-IR spectra of GONRs-CTS showed the characteristic peaks at 3440 cm−1 (–OH vibrations), 2943 and 2871 cm−1 (C–H), 1625 cm−1 (–NH vibrations), 1112 cm−1 (C–O) [21].

Fig. 2
figure 2

FTIR spectra of GONRs, CTS and GONRs-CTS

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Figure 3 shows the XRD patterns of GONRs and GONRs-CTS. The specific 2θ peak of GONRs at 10.2° corresponds to (001). The (002) diffraction peak of graphite appears at 2θ = 25.4°, which was attributed to the MWCNTs not being wholly unzipped in the oxidizing environment [22]. The diffraction peaks of carbon nanomaterials are consistent with the standard card JCPDS #49-1721. In the case of GONRs-CTS, the intense (001) peak decreased, while broad diffraction peaks appear between 2θ = 21.7°–26.1°, indicating that the chitosan is well compounded on the surface of the GONRs. It may be related to the amorphous state and crystal structure of chitosan [23].

Fig. 3
figure 3

XRD patterns of GONRs and GONRs-CTS

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Effect of pH on thorium adsorption onto GONRs-CTS

The pH of solution is a critical parameter affecting the sorption of thorium on adsorbent because it influences the species of thorium ions and the active sites of the sorbent in the solution. The adsorption of GONRs-CTS was investigated in the pH range of 1.0–7.0 (Fig. 4). As it could be seen clearly from Fig. 4, the adsorption capacity of Th(IV) increased sharply with an increase of pH from 1.0 to 5.0. When the pH increased beyond 5.0, the adsorption capacity remained constant. The adsorption capacity of Th(IV) is lower at pH value below 1.0–3.0 because the adsorbent surface was positively charged thus resulting in the competition between H+ and Th4+ ions for the same adsorption site [24]. As the pH of the solution increases, the electrostatic repulsion of the GONRs-CTS surface disappears. However, thorium ions begin to hydrolyze into hydroxide complex [Th(OH)3+, Th(OH) 2+2 , Th(OH) +3 and precipitation] at high pH value [25]. The initial pH was optimized to 3.0 in order to obtain high adsorption capacity and avoid precipitation of Th(IV).

Fig. 4
figure 4

Effect of pH on the adsorption of thorium onto GONRs-CTS, m = 15 mg, C0 = 50 mg L−1, V = 20 mL, t = 12 h, and T = 298.15 K

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Effect of the adsorbent dosage on adsorption

The effect of GONRs-CTS dosage (varied from 0.005 to 0.045 g) on Th(IV) adsorption was shown in Fig. 5. The Th(IV) adsorption capacity decreases as the amount of GONRs-CTS dosage was further increased. However, as the amount of adsorbent increases, the active site on the surface of the GONRs-CTS increases, and the adsorbent tends to be saturated. Therefore, the removal efficiency of Th(IV) from aqueous solution on GONRs-CTS increases. Hence, in order to maintain a balance between removal efficiency and adsorption capacity, 15 mg was optimized as the adsorbent dosage for all adsorption experiments.

Fig. 5
figure 5

Effect of adsorbent dosage on the thorium adsorption, pH = 3.0, C0 = 50 mg L−1, V = 20 mL, t = 12 h, and T = 298.15 K

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Effect of contact time and kinetic studies

The effect of the contact time on the adsorption of Th(IV) onto GONRs-CTS is shown in Fig. 6. Experiments were performed to study the effect of the contact time on Th(IV) sorption on GONRs-CTS at different times varying from 10 to 1080 min. Figure 6 shows that as the contact time increases, the adsorption capacity of Th(IV) on GONRs-CTS increases rapidly and then tends to equilibrium. The adsorption was initially fast and then reached equilibrium within 12 h and the removal efficiency at equilibrium was 82.4%. Therefore, 12 h was selected from the above results as the optimum contact time.

Fig. 6
figure 6

Effect of contact time on the thorium adsorption, pH = 3.0, C0 = 50 mg L−1, V = 20 mL, m = 15 mg, and T = 298.15 K

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Adsorption kinetics is an important aspect of determining the efficiency of the adsorption process. The pseudo-first-order and pseudo-second-order were employed to describe the kinetic characteristics of the sorption of Th(IV) on GONRS-CTS. The kinetic models can be expressed as the following Eqs. (3) and (4).

$$\ln \left( {q_{\text{e}} - q_{\text{t}} } \right) = \ln q_{\text{e}} - k_{1} t$$
(3)
$$\frac{t}{{q_{\text{t}} }} = \frac{1}{{k_{ 2} q_{\text{e}}^{ 2} }} + \frac{t}{{q_{\text{e}} }}$$
(4)

where qe and qt (mg g−1) are the amounts of Th(IV) ions adsorbed of GONRs-CTS at equilibrium and at any time t, respectively. k1 (min−1) and k2 (g mg−1 min−1) are the adsorption rate the constants of pseudo-first-order and pseudo-second-order, respectively.

Kinetic parameters are calculated and shown in Table 1 and Fig. 6. The theoretical value of the adsorption capacity was calculated from the slope and intercept of the fitted curve. The highest correlation coefficient value of pseudo-second-order model (R2 = 0.9988) and the closest qe,cal (55.30 mg g−1) to qe,exp (54.94 mg g−1) show the second-order nature of the present sorption process.

Table 1 Kinetic parameters for the adsorption
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Adsorption isotherm

The adsorption properties of the adsorbent are described by adsorption isotherms which is plots of adsorption capacity (qe) and final equilibrium solute concentration (Ce). Therefore, experimental data were evaluated by both Langmuir isotherm and Freundlich isotherm models (Fig. 7). The Langmuir adsorption isotherm assumes that the adsorbed molecules form an adsorption layer of one molecule in thickness and all sites are equal, resulting in equal energy and adsorption enthalpy. The Freundlich adsorption isotherm is obtained by assuming a non-uniform surface with a non-uniform adsorption heat distribution on the surface. The linear equations of the Langmuir and Freundlich isotherm models can be respectively expressed as

$$\frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{1}{{q_{\text{m}} K_{\text{L}} }} + \frac{{C_{\text{e}} }}{{q_{\text{m}} }}$$
(5)
$$\log q_{\text{e}} = \log K_{\text{F}} + \frac{1}{n}\log C_{\text{e}}$$
(6)

where Ce (mg L−1) is the equilibrium concentration, qe (mg g−1) is the amounts of Th(IV) adsorbed per gram of sorbent at equilibrium, qm (mg g−1) and KL (L mg−1) are the Langmuir constants associated with adsorption capacity and adsorption energy, respectively. KF (mg1−1/n L1/n g−1) and n are the Freundlich characteristic constants representing adsorption capacity and adsorption intensity respectively.

Fig. 7
figure 7

Langmuir (a) and Freundlich (b) isotherm plot of thorium adsorption for GONRs-CTS

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The parameters for the isotherm models are listed in Table 2. As can be seen, the values of R2 show that the Langmuir (R2 = 0.9930) model can describe the experimental data better than the Freundlich (R2 = 0.9804). This indicates that the adsorption process of Th(IV) in GONRs-CTS is monolayer adsorption.

Table 2 The isothermal adsorption model parameters
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Another basic feature of the Langmuir model can be described by the separation factor (RL). The separation factor can be expressed as the following Eqs. (7).

$$R_{\text{L}} = \frac{1}{{\left( {1 + K_{\text{L}} C_{0} } \right)}}$$
(7)

The separation factor calculated according to KL which gave an indication for the possibility of the adsorption process to proceed as follows: RL > 1 is an unfavorable process; RL = 1 is a linear process; RL < 1 shows favorable adsorption [26]. The values of RL lie between 0.5414 and 0.0557, indicating that Th(IV) from aqueous solution adsorbent on GONRS-CTS is favorable.

Adsorption thermodynamics

The adsorption thermodynamics is very important to understand the adsorption mechanism of Th(IV) on the adsorbent. The values of ΔH° and ΔS° listed in Table 3 were calculated from the slope and intercept of the plots of ln Kd versus T−1 (Fig. 8). The thermodynamic parameters such as the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) can be calculated by the following Eqs:

$$K_{\text{d}} = \frac{{C_{ 0} - C_{\text{e}} }}{{C_{\text{e}} }} \times \frac{V}{m}$$
(8)
$$\ln K_{\text{d}} = \frac{{\Delta S^{o} }}{R} - \frac{{\Delta H^{o} }}{RT}$$
(9)
$$\Delta G^{o} = \Delta H^{o} - T\Delta S^{o}$$
(10)

where Kd (mL g−1) is the distribution coefficient, T (K) and R (8.314 J mol−1 K−1) are the absolute temperature in Kelvin and the gas constant.

Table 3 Thermodynamic parameters of thorium adsorption on MGONRs
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Fig. 8
figure 8

The adsorption thermodynamics of Th(IV) on GONRs-CTS, pH = 3.0, C0 = 50 mg L−1, V = 20 mL, t = 12 h, and m = 15 mg

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As shown in Table 3, the negative values of ΔG° increased with increasing temperature and the absolute value increases, indicating that the adsorption process was spontaneous and irreversible. The positive value of ΔH° (23.06 kJ mol−1) confirmed that the adsorption process was an endothermic reaction, and the adsorption process was more favorable at higher temperatures. At the same time, the positive values of ΔS° (150.60 J mol−1 K−1) suggested an increase in the degree of freedom of the solid-solution interface during the adsorption process.

Effect of coexisting anions

Figure 9 shows the effect of coexisting anions on the adsorption of Th(IV) from aqueous solutions onto GONRs-CTS. The addition of Cl− had no effect on the adsorption of Th(IV) on GONRs-CTS. CO32− and HCO3− has a coordination exchange on the surface of GONRs-CTS, which increases the negative charge and adsorption sites on the surface and promotes the adsorption of Th(IV) on GONRs-CTS. Under acidic conditions, SO42− and HSO4− forms a complex with Th4+, which leads to an increase in Th4+ in solution and a decrease in Th4+ on the surface of GONRs-CTS.

Fig. 9
figure 9

Effect of coexisting anions(0.01 M) on adsorption of Th(IV)

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Conclusion

The batch adsorption results show that GONRs-CTS composite material has excellent adsorption performance of thorium from aqueous solution. Thorium adsorption on the GONRs-CTS was a pH dependent, spontaneous, endothermic and a pseudo-second-order process. The maximum adsorption capacity calculated by Langmuir isotherm equation was 140.6 mg g−1. This eco-friendly GONRs-CTS sorbent is expected to be applied to thorium from radioactive waste water.