Introduction

Thorium is an important component of the fuel in nuclear breeder reactors and thorium fuel cycle can be used in most of the reactors already operated [1]. The presence of thorium in the environment not only originates from the nuclear industry, but also from other human activities such as lignite burning in power plants, ore processing and the use of fertilizers. The presence of thorium ions in fresh water supplies has been of health concern due to their detrimental effects associated with radioactivity and/or toxicity to biological systems [2].

Several methodologies including chemical precipitation, liquid–liquid extraction, ion exchange, extraction chromatography and adsorption [3–8] have been developed to remove thorium from aqueous solutions. However, most of these methods suffer from technical, economic and health problems related to low selectivity, long time of extraction and large quantity of hazardous materials used. Adsorption is by far the easiest way for removing radioactive ions from aqueous solutions, due to its high selectivity, the ease of handling and the environmental safety [8].

In recent years, the use of sorbents composed of polysaccharides such as chitosan and its derivatives has received particular attention, since chitosan is a low-cost and effective sorbent compared to activated carbons and other sorbents [9]. Chitosan contains a large number of functional groups (e.g. primary amino and hydroxyl groups) that are capable of coordinating with different metal ions. However, chitosan resins present some disadvantages that limit their use in practical wastewater treatment applications, such as their weak mechanical properties, poor chemical resistance, low surface area and difficult separation from the liquid phase. Magnetic sorbents on the other hand, especially in particulate form, have a relatively high surface area and are easy to separate and recover by using a magnetic field. Their advantages prevail over the difficulties normally associated with other powdered sorbents [9, 10].

Various types of magnetic particles have recently been synthesized and used as sorbents for actinide removal, as reviewed by Rojo et al. [10]. He et al. [11] investigated the separation and recognition of Th(IV) by magnetic Th(IV)-ion imprinted polymers (the maximum adsorption capacity q m = 42.5 mg Th(IV)/g). Hritcu et al. [12] reported the magnetic chitosan composite particles for the adsorption of thorium and uranyl ions (q m = 666.67 mg UO2 2+/g and 312.50 mg Th4+/g). And Wang et al. [13] studied the adsorption of uranyl ions from aqueous solutions onto ethylenediamine-modified magnetic chitosan (q m = 82.8 mg UO2 2+/g).

Grafting is a simple technique in which functional groups are introduced into the back bone of chitosan for the sorption of actinide ions. Some of the attempts were to introduce groups like amino and azole groups which have chelating capacity for anchoring actinide ions [13, 14]. Cross-linked chitosan resins modified with catechol, iminodiacetic acid, iminodimetylphosphonic acid, phenylarsonic acid, or serine were also reported for the collection and concentration of uranium(VI) [15]. The sorption of actinide ions can be enhanced by grafting functional groups such as amine groups onto chitosan-based matrix [14].

In this work, the magnetic chitosan resins was prepared by adding the basic precipitant of NaOH solution into a W/O emulsion system and subsequently modified with triethylene-tetramine. The obtained magnetic sorbents possess some excellent characteristics such as high adsorption capacity and fast kinetic toward Th(IV) due to the high concentration of active sites and the small size of the reins. The effect of different parameters on the adsorption of Th(IV) was investigated. The adsorption kinetics, isotherms, regeneration and reusability of the adsorbent for the adsorption of Th(IV) from aqueous media were also discussed.

Materials and methods

Chemicals and reagents

Chitosan (CTS) with 40 mesh, 90 % degree of deacetylation and molecular weight of 1.3 × 105 was purchased from Yuhuan Ocean Biology Company (Zhejiang, China). Triethylene-tetramine and epichlorohydrin were Aldrich products and were used as received. Th(NO3)4·4H2O (analytical reagents) were supplied by Jingan Uranium Company (Fuzhou, China). Stock radioactive simulated solutions (500 mg L−1) were prepared by dissolving the corresponding amount of Th(NO3)4·4H2O in distilled water, after adding some drops of HNO3. All other reagents were of analytical grade and used without further purification.

Preparation of magnetic chitosan sorbents (MCS)

The magnetic chitosan sorbents (MCS) were prepared by adding the basic precipitant of NaOH solution into a W/O emulsion system, which was composed of chitosan and ferrous salt as water phase, and cyclohexane, n-hexanol (11:6, v/v) and an emulsifier (Triton X-100) as oil phase. The W/O ratio of the W/O emulsion was 4: 17 (v/v). The detail preparation procedure was described in previous work [16]. The size distribution of MCS could be controlled by changing the dosage of Triton X-100. The magnetization of MCS demands the presence of an extremely low concentration of dissolved oxygen which oxidize Fe(OH)2 to Fe3O4 without forming essentially nonmagnetic Fe(OH)3. After reaction, the magnetic resins were recovered from the reaction mixture using a permanent magnet, washed thoroughly with distilled water.

Preparation of triethylene-tetramine grafted magnetic chitosan sorbents (TETA-MCS)

The MCS (5.0 g) were suspended in 70 mL isopropyl alcohol to which 5 mL epichlorohydrin dissolved in 100 mL acetone/water mixture (1:1 v/v) was added. The contents were stirred for 24 h at 60 °C. The solid obtained was separated and washed several times with ethanol followed by water and labeled as ECH -MCS.

The ECH -MCS obtained in the previous step was suspended in 100 mL ethanol/water mixture (1:1 v/v), then triethylene-tetramine (5 mL) was added. The reaction mixture was stirred at 60 °C for 12 h, then the product (TETA -MCS) was washed with ethanol followed by water, and dried in vacuum. The synthesis of the TETA-MCS was shown in Scheme 1.

Scheme 1
scheme 1

Synthesis of TETA-MCS

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Characterizations of the magnetic chitosan sorbents

The morphology of TETA-MCS was observed by a Leica Cambridge S360 scanning electron microscope (SEM). FTIR spectra was measured on a Nicolet, Magna-550 spectrometer. Thermal gravimetric analysis of the TETA-MCS was conducted on Shimadzu TGA-50H with the heating rate of 10 K/min in the nitrogen flow. The concentration of the amine active sites in the obtained resins was estimated using the volumetric method [17]. The zeta potential measurements were performed using a zeta potential analyzer 2000.

Adsorption experiments

Batch experiments were carried out by shaking 50 mg of sorbents with 100 mL Th(IV) solution of known concentration for 1 h and 150 rpm. The initial pH between 2.0 and 6.0 of the solutions was adjusted using 0.1 M HNO3 or NaOH solutions. After equilibrium, the aqueous phase was separated from the solid phase by centrifugation at 12,000 rpm. The final Th(IV) concentration in the supernatant was analyzed spectrophotometrically at wavelength of 665 nm by using Th(IV)-arsenazo(III) complex method [18]. The adsorbed amount of Th(IV) ions was calculated from the difference between initial and final concentrations. Effect of pH on the adsorption of Th(IV) was studied in the pH range 2.0–6.0 and at 25 °C using initial concentrations of 20 and 50 mg/L. Kinetic experiments were performed at the optimum pH with different Th(IV) concentrations ranging from 20 to 100 mg/L at 25 °C. At appropriate time intervals, solutions were centrifuged, and the supernatant was analyzed to obtain Th(IV) concentrations. For the isotherm experiments, the initial solution pH was kept at 4.0, with varying Th(IV) concentration ranging from 10 to 100 mg/L at 25, 35, and 45 °C, respectively.

The selective adsorption behavior of Th(IV) from multi-component mixtures with La(III), Sr(II), Fe(III), Zn(II), Ni(II), Co(II), and Cu(II) was obtained by shaking 50 mg resins with 100 mL multi-component solution for 1 h and 150 rpm. The initial concentration of each cation in the mixture was 500 μg/L. The cation concentration after adsorption was analyzed using Inductively Coupled Plasma (ICP, ARL-340, ICP-AES Fison instruments).

The selectivity coefficient is defined as:

$$ \beta_{{{\text{Th}}^{4 + } /{\text{M}}^{n + } }} = \frac{{D_{{{\text{Th}}^{4 + } }} }}{{D_{{{\text{M}}^{n + } }} }} $$
(1)

where \( D_{{{\text{Th}}^{4 + } }} \) and \( D_{{{\text{M}}^{n + } }} \) are the distribution ratios of Th(IV) and other coexistent cations, respectively. The distribution ratio (D) was calculated by using the following expression:

$$ D = \frac{{C_{0} - C_{\text{e}} }}{{C_{\text{e}} }} \times \frac{V}{W} $$
(2)

where C 0 and C e are the concentrations of Th(IV) in the initial solution and equilibrium solution (mg/L), respectively, V is the volume of the aqueous solution (L), and W is the mass of the resins (g).

All the adsorption experiments were carried out in duplicate and the average values were used.

Desorption

After performing adsorption experiments with 100 mg/L Th(IV) solution, the resulting Th(IV)-loaded TETA-MCS was separated and washed. The spent sorbent was gently washed to remove the un-adsorbed Th(IV) ions. The exhausted sorbent was reintroduced to the desorption medium and agitated for 1.5 h. The sorbent was removed by centrifugation and the final concentration of Th(IV) in the desorption medium was estimated.

Results and discussion

Sorbent characterization

The TETA-MCS was found to be insoluble in organic solvents as well as in mineral and organic acids. The SEM micrograph of the TETA-MCS is shown in Fig. 1a. It can be observed that the TETA-MCS particles are uniformly dispersed and have a diameter range of 0.3-0.5 μm. The zeta potential measurements for TETA-MCS and MCS are shown in Fig. 1b. The point of zero charge of MCS and TETA-MCS occurs at pH 4.3 and 4.7, respectively, below which most of the amine sites become protonated and the surface charge of the resin is reversed. The higher zeta potential of TETA-MCS compared to MCS is due to the chemical binding of the basic functional amine groups to the surface of TETA-MCS.

Fig. 1
figure 1

a SEM image of the TETA-MCS, b Zeta potential of MCS and TETA-MCS, and c XRD pattern of TETA-MCS

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The XRD pattern (Fig. 1c) for the TETA-MCS exhibited eight characteristic peaks for Fe3O4 marked by their indices [(111), (220), (311), (400), (422), (511), (440), and (622)]. These peaks are consistent with the database in JCPDS file (PDF No. 65-3107) and confirm the presence of pure Fe3O4 particles with a spinel structure in TETA-MCS. The thermogravimetric analysis indicated that the principle chains of chitosan began to degrade at about 240 °C, and the final decomposition temperature was around 700 °C. The average mass content of Fe3O4 in TETA-MCS by TGA was 38.5 %. The magnetization measurement performed with VSM indicated that the saturation magnetization of the EMCN was 26.4 emu/g. As mentioned in a previous report, this magnetic susceptibility value is sufficient for this resin to be used in wastewater treatment [19].

The IR spectra of MCS, TETA-MCS, and Th(IV)-loaded TETA-MCS are shown in Fig. 2. The peak at 580 cm−1 is contributed to the Fe–O bond of Fe3O4. The adsorption band around 3,420 cm−1 reveals the stretching vibration of N–H group bonded with O–H group in chitosan, and at around 1,650 cm−1 confirms the N–H scissoring from the primary amine [20]. The bands at 1,450, 1,380 and 1,075 cm−1 are related to amide II, C–O group and O–H group, respectively. The increasing intensity at 1,650 and 1,450 cm−1 in the spectrum of TETA-MCS (Fig. 2b) compared to MCS (Fig. 2a) indicates that TETA-MCS has more amine groups than MCS. The concentration of amine groups on the resins are determined as 5.84 mmol/g for TETA-MCS and 2.53 mmol/g for MCS, respectively. The higher amine group concentration of TETA-MCS compared to MCS indicates that the amine moieties are covalently bonded to the magnetic resins. After the adsorption of Th(IV) (Fig. 2c), the band at 3,420 cm−1 becomes narrower, and the peaks at 1,650, 1,450 and 1,076 cm−1 are shifted to 1,642, 1,436, and 1,065 cm−1, respectively, especially the intensity of these peaks decreases significantly compared to the TETA-MCS before adsorption. These results indicate that both amine and hydroxyl groups are involved in the adsorption process. The obvious change in the peak position and intensity in the 800–400 cm−1 region in the IR spectra of Th(IV)-loaded TETA-MCS was observed due to Th–O and O–Th-O vibrations [21].

Fig. 2
figure 2

FTIR of MCS (a), TETA-MCS (b), and Th(IV)-loaded TETA-MCS (c)

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Effect of pH and adsorption mechanism

As shown in Fig. 3, the adsorption of Th(IV) increases up to pH 2.0–4.0 and then remains constant. The maximum adsorption occurred at pH 4.0 for both 20 and 50 mg/L initial Th(IV) concentration. The lower uptake in an acidic medium may be attributed to the partial protonation of the active groups and the competition of H3O+ in the solution for the adsorption sites of TETA-MCS. Also the hydrolysis of the metal ions is suppressed by higher acid concentrations. With increasing the pH values, the surface positive charge on the sorbent decreases which lowers the coulombic repulsion for the adsorbing actinide ions. It is well known that besides three monomers [Th(OH)]3+, [Th(OH)2]2+, and [Th(OH)3]+, the hydrolysis of Th4+ involves extensive formation of polynuclear complexes, such as Th2(OH) 6+2 , Th3(OH) 7+5 and Th4(OH) 8+8 [22]. Humelnicu et al. [23] reported that low soluble mononuclear as well as polynuclear hydrolysis products is formed at pH 4.5. The maximum adsorption in the pH range 4.0 may be due to the formation of Th(IV) complexes with amine groups and hydroxyl groups of TETA-MCS. Liu et al. [24] calculated the distribution of thorium species as a function of pH values in aqueous solution, and their results were: Th4+ >70 % and Th(OH)3+ <30 % at pH <2.5; Th4+ <35 % and Th(OH)3+ <45 % at pH <3.0; Th(OH)3+ <17 % and Th(OH) 2+2 <54 % at pH <4.0. At higher pH values, Th(OH)4 precipitation could be formed. Hence we concluded that at the optimum pH 4.0, the dominant species in the adsorption process are Th (OH) 2+2 and Th(OH)3+ [25]. The pHpzc of TETA-MCS was found to be 4.7 above which the surface charge is negative. Hence the positively charged Th(IV) ions are adsorbed by electrostatic interaction at pH >4.7.

Fig. 3
figure 3

Effect of initial pH on the adsorption of Th(IV) ions onto TETA-MCS

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Scheme 2 illustrates the possible mechanism for Th(IV) adsorption onto the TETA-MCS. There are many amino groups from grafting by triethylene-tetramine and hydroxyl groups from reacting with epichlorohydrin for the preparation of the TETA-MCS, thus the lone pair on nitrogen and oxygen atoms could be supplied to the empty atomic orbital of Th(IV), which makes O, N–Th(IV) complex formation in the active sites of the sorbent. This mechanism could partly be confirmed by the FTIR analysis of TETA-MCS and Th(IV)-loaded TETA-MCS (as discussed in “Sorbent characterization” Section), which has demonstrated that amine and hydroxyl groups are involved in the adsorption of Th(IV) onto TETA-MCS .

Scheme 2
scheme 2

The mechanism of Th(IV) adsorption onto the TETA-MCS (Mn+=Th(OH) 2+2 and Th(OH)3+)

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In general, metal ions bind to chitosan resins via the available functional groups (hydroxyl, carboxyl, thiol and amine groups) on the resins. These functional groups can react with the different metallic species through chelation and/or ion-exchange [9]. Based on FTIR spectral analysis, Chen et al. [26] reported that the mechanism for the adsorption of Sr(II) onto magnetic chitosan beads was likely via the amine groups. Similarly, Chen and Wang [27] used FTIR to show that copper ions bind to the –NH2 and –OH groups in chitosan. Atia A. [14] proposed that uranyl (II) formed complex structure with the –NH2 and –OH groups of chitosan/amine and chitosan/azole resins. Other literatures [14, 28] also indicated that the actinide ions could chelate with the hydroxyls of the resins to form complex structure. Accordingly, it is reasonable to infer that except from the amine groups, the hydroxyl groups of TETA-MCS should also be involved in the adsorption of Th(VI).

Effect of contact time and initial Th(IV) concentration

The effect of contact time on the extent of adsorption of Th(IV) at different concentrations are shown in Fig. 4. The adsorption was rapid at first and then reached equilibrium within 60 min for all the three Th(IV) concentrations (20, 50, and 100 mg/L). The short equilibrium time indicates high efficiency and economic feasibility of the sorbent for industrial applications. The adsorption capacity (mg/g) increased from 38.8 to 125.4 mg/g as the initial Th(IV) concentration increased from 20 to 100 mg/L. This might be due to the fact that the adsorption sites with higher energy will be occupied by Th(IV) as increasing the initial Th(IV) concentration.

Fig. 4
figure 4

Effect of adsorption time on the uptake of Th(IV) ions at different Th(IV) concentrations

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In order to investigate the controlling mechanism of adsorption process, the pseudo-first-order and pseudo-second-order kinetic models were used to evaluate the experimental data obtained from batch Th(IV) removal experiments. Pseudo-first-order kinetic model and pseudo-second order kinetic model were shown in Eqs. (3) and (4) [29]:

$$ {\text{Pseudo-first-order}}:{ \ln } (q_{\text{e}} - q_{t} ) = { \ln } q_{\text{e}} - k_{ 1} t $$
(3)
$$ {\text{pseudo-second order}}:\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{\text{e}}^{2} }} + \frac{1}{{q_{\text{e}} }}t $$
(4)

where q e and q t are the amounts of Th(IV) adsorbed (mg/g) at equilibrium and at time t (min), respectively. k 1 (min−1) and k 2 (g/mg/min) are the rate constants of pseudo-first-order and pseudo-second-order rate constants, respectively.

Kinetic parameters were calculated and were shown in Table 1. Results clearly indicate that the rate of adsorption of Th(IV) onto TETA-MCS depends on the initial concentration of Th(IV). The applicability of these models was quantified from the coefficient of determination, R 2 values. The values of R 2 show that the pseudo-second-order fits the experimental data better than the pseudo first-order model. Moreover, the theoretical q e values calculated from the pseudo-second-order model are very close to the experimental data. These results confirm the validity of the pseudo-second-order model to the adsorption system, suggesting the main adsorption mechanism of chemical adsorption [13, 19]. The TETA-MCS is characterized by its high content of amine groups, which are the main active sites for the adsorption of Th(IV). The q e values increase with the concentration of Th(IV) ions, whereas the values of k 2 decrease. Hence at lower Th(IV) concentrations, the probability to get bonded on the active sites of the sorbent increases [10].

Table 1 Kinetic parameters of the adsorption of Th(IV) onto TETA-MCS at different concentrations
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Adsorption isotherms

The adsorption isotherm reveals the specific relation between the concentration of the adsorbate and the adsorption capacity of a sorbent at constant temperature [30]. The effect of temperature on the adsorption equilibrium of Th(IV) onto TETA-MCS is depicted in Fig. 5. It can be seen that the extent of adsorption increased with increase in temperature. The adsorption sites are homogeneous and have different adsorption energy. As increasing temperature, the adsorption sites with higher energy can interact with Th(IV), resulting the increase of adsorption capacity.

Fig. 5
figure 5

Adsorption isotherms of Th(IV) ions adsorption onto TETA-MCS at different temperatures

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All the isotherm curves of Th(IV) adsorption onto TETA-MCS belong to the L class (Langmuir type), subgroup 2 [31]. The equilibrium isotherm data were analyzed with the Langmuir, Freundlich, and Temkin models.

The Langmuir isotherm model assumes that a monomolecular layer is formed when adsorption takes place without any interaction between the adsorbed molecules. The Langmuir model can be represented as:

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

where C e is the equilibrium concentration (mg/L), q e the amount of metal ion sorbed (mg/g), q m is the maximum monolayer adsorption capacity (mg/g), K L is a constant related to the affinity of the binding sites (L/mg).

The Freundlich isotherm model is an empirical equation assuming that the adsorption process takes place on heterogeneous surfaces and adsorption capacity is related to the concentration of metal ion at equilibrium. This isotherm model is defined by the equation below:

$$ \ln q_{\text{e}} = { \ln }k_{\text{F}} + 1/n\,{ \ln }C_{\text{e}} $$
(6)

where K F and n are empirical constants those indicate the relative sorption capacity and sorption intensity, respectively.

The derivation of the Temkin isotherm is based on the assumption that the decline of the heat of sorption as a function of temperature is linear rather than logarithmic, as implied in the Freundlich equation. It can be described as follows:

$$ q_{\text{e}} = \frac{RT}{b}(\ln A) + \frac{RT}{b}(\ln C_{\text{e}} ) $$
(7)

where b is the Temkin constant related to heat of sorption (J/mol). A is the Temkin isotherm constant, R is the gas constant [8.314 (J/(mol/K)] and T is the absolute temperature (K).

Table 2 summarized the parameters of Langmuir, Freundlich, and Temkin isotherm models at different temperatures. The R 2 >0.99 in case of Langmuir model indicates favorable nature as compared to other two isotherm models. The q e values obtained from Langmuir model are also in good agreement with the experimental values. Formation of a monolayer coverage on adsorption indicates that the process is chemisorption. The degree of suitability of sorbent toward metal ions is estimated from the values of separation factor constant (R L ), which gives indication for the possibility of the adsorption process to proceed. The essential characteristics of the Langmuir equation can be expressed in terms of dimension factor R L , which is defined by the following equation [19]:

$$ R_{L} = \frac{1}{{1 + K_{L} C_{0} }} $$
(8)
Table 2 Isotherm model parameters for the adsorption of Th(IV) by TETA-MCS and MCS
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where C 0 is the initial Th(IV) concentration (mg/L). The value of R L indicates the nature of adsorption as favorable (0 < R L  < 1), unfavorable (R L  > 1), linear (R L L = 1), and irreversible (R L  = 0). The R L values (0.011 < R L  < 0.182) obtained in this work indicate that the adsorption of Th(IV) onto TETA-MCS is more favorable.

The maximum adsorption capacity (q m) for the adsorption of Th(IV) on TETA-MCS in this work was compared with that of other sorbents. The reported q m values for the adsorption of Th(IV) on magnetic imprinted polymer with CPMA on silica gel [6], Th(IV)-ion imprinted polymers [11], poly(MAGA-EDMA) [32], Th(IV)-IIP with MAA on silica gel [33], Th(IV)-imprinted chitosan phthalate [34], chitosan/poly(acrylamide) hydrogel [35] were found to be 36, 42, 40, 33, 61,118 mg/g, respectively. Compared to these sorbents reported in the literatures, the TETA-MCS exhibited higher adsorption capacity (133.3 mg/g) for Th(IV) ions. However, the q m of chitosan/clinoptilolite composite [12] was reported to be 328.3 mg/g for Th4+, which was higher than the q m values obtained in this work. The relatively higher adsorption capacity of TETA-MCS toward Th(IV) ions may be attributed to the formation of extended polymeric film of the resin over the magnetite particles resulting in a large number of exposed active sites and the incorporation of the amine groups facilitating the adsorption of Th(IV) ions.

Thermodynamic parameters

The increase in adsorption with a rise in temperature can be explained on the basis of thermodynamic parameters such as enthalpy (ΔH°, kJ/mol), entropy [ΔS°, J/(mol/K)],and Gibbs free energy (ΔG°, kJ/mol), which could be calculated using the following relationships [36].

$$ \ln K_{L} = \frac{{ - \varDelta H^{\circ} }}{RT} + \frac{{\varDelta S^{\circ} }}{R} $$
(9)
$$ \varDelta G^{\circ} = \varDelta H^{\circ} - T\varDelta S^{\circ} $$
(10)

where T is the absolute temperature (K) and K L is the Langmuir equilibrium constant (L/mol). ΔH° and ΔS° were obtained from the slope and intercept of the linear Van’t Hoff plots of ln K L versus 1/T: ln K L  = −3094/T + 21.902 (R 2 = 0.975). The positive ΔH° (= 25.7 kJ/mol) indicated the endothermic nature of the adsorption interaction whereas the positive ΔS° [= 182 J/(mol/K)] showed the increased randomness at the solid–solution interface during the adsorption process. The ΔG° values are −28.5, −30.3, −32.2 kJ/mol at 298, 308, and 318 K, respectively. These results indicated the spontaneous nature of adsorption and the more negative values at higher temperature showed that it was more favored.

Adsorption selectivity

The distribution ratios and selectivity coefficients with respect to other cations (that are likely to coexist with Th(IV) in natural sources) using MCS and TETA-MCS are shown in Table 3. It can be seen that the distribution ratio (D) of TETA-MCS for Th(IV) was much greater than that of MCS. Furthermore, the relative selectivity coefficient of TETA-MCS for the cations was far greater than 1. Based on the results shown in Table 3, it is evident that TETA-MCS can selectively adsorb Th(IV) ions from other cations present in aqueous solutions.

Table 3 Selective adsorption of Th(IV) from multi-component mixtures by MCR and TETA-MCS
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Test with simulated nuclear industry wastewater

Simulated nuclear industry wastewater was used to test the effectiveness of TETA-MCS for the removal of Th(IV). The compositions of the simulated wastewater are: Th(IV) (15 mg/L), NaCl (41.5 g/L), Na2SO4 (6.9 g/L), KCl (1.1 g/L), NaHCO3 (0.34 g/L), KBr (0.16 g/L), H3BO3 (0.16 g/L), NaF (5 mg/L), MgCl .2 6H2O (18.79 g/L), CaCl .2 2H2O (2.63 g/L), SrCl .2 6H2O (0.04 g/L). Almost complete removal was possible with 0.65 g of the sorbent from 1L of the wastewater while for the treatment of the sample containing only 15 mg/L Th(IV) ions an sorbent dosage of 0.50 g is required. The increase in the sorbent dosage for the treatment of simulated wastewater may be related to antagonistic response such as competition of the co-existed ions with the binding sites.

Desorption and regeneration of the sorbents

The spent TETA-MCS was regenerated by shaking Th(IV)-loaded TETA-MCS with different types of desorbing agents such as 0.3 M NaOH, 0.3 M NaNO3, and 0.3 M HNO3, and 0.2 M HNO3–0.1 M EDTA. The desorption percentage for corresponding desorbing agents was 64.3, 68.4, 81.2, and 97.4 %, respectively. Among these, 0.2 M HNO3–0.1 M EDTA was proved to be the most suitable desorbing agent. When the Th4+-loaded TETA-MCS was added into 0.2 M HNO3–0.1 M EDTA solution, EDTA could form stable complex with Th(IV), and H+ from HNO3 could result in the protonation of amino groups, which makes part of sites occupied by metal ions may be replaced by H+, thus Th(IV) ions are desorbed and returned to the solution. The TETA-MCS was assessed for deterioration by subjecting to repeated adsorption–desorption experiments with 0.2 M HNO3-0.1 M EDTA. The adsorption capacity of the TETA-MCS decreased from 96.4 % in the first cycle to 92.5 % in the fifth cycle. Apparently the performance of the sorbent was not appreciably deteriorated after repeated use and regeneration for five cycles, which indicated that the TETA-MCS could be potentially used for the adsorption of Th(IV) from aqueous solution.

Conclusions

The efficiency of triethylene-tetramine modified magnetic chitosan sorbents (TETA-MCS) for the adsorption of thorium ions from aqueous solution was investigated in batch adsorption system. The optimum pH for the adsorption of Th(IV) was Ph 4.0 and the adsorption equilibrium was achieved within 60 min. Kinetic study showed that the pseudo-second order model was appropriate to describe the adsorption process, indicating the chemical adsorption. The FTIR analysis indicates that amine and hydroxyl groups are the active sites for the formation of O, N–Th(IV) complex. Among different models used for describing equilibrium isotherm data, Langmuir model is in good agreement with the experimental data with high R 2. The adsorption of Th(IV) dependence on temperature was investigated and the thermodynamic parameters ΔG°, ΔH°, and ΔS° were calculated. The results showed a feasible, spontaneous and endothermic adsorption process. The suitability of the sorbent for the adsorption of thorium was tested by using simulated wastewater. The adsorption–desorption cycle results demonstrated that the regeneration and subsequent use of the TETA-MCS would enhance the economics of practical applications for the removal of Th(IV) from water and wastewater.