Study on Th(IV) Adsorption Properties on Natural and Surface-Modified Red Soil

Abstract

In order to study the effects of inorganic and organic modification on the adsorption properties of red soil (RS), ferric chloride (FC) and hexadecyl trimethyl ammonium bromide (HTAB) surface-modified red soil have been prepared and characterized by SEM, FT-IR and X-ray diffraction. Adsorption properties of Th(IV) on natural red soil (NRS), FC surface-modified red soil (FC-RS), and HTAB surface-modified red soil (HTAB-RS) have been investigated by batch technique. The inorganic and organic modification have been found to greatly improve the adsorption kinetics and thermodynamic properties and increase the adsorption capacity as compared with NRS. The adsorption of Th(IV) on the three adsorbents have been strongly dependent on pH and ionic strength, and intra-particle diffusion was the rate-controlling step. The non-linear pseudo-second-order kinetic model could fit the kinetics much better compared with the linear forms, and the linear and non-linear Langmuir expressions could fit the thermodynamics. The results obtained denote on successful application of inorganic and organic modification to treatment of water samples spiked to Th(IV) ions.

INTRODUCTION

Thorium is an important actinide in view of nuclear energy production brings some environmental problems, which has aroused great interest in terms of the radioactive waste disposal [1, 2]. Therefore, the adsorption or recovery of radionuclides in soil and water is essential for minimizing their destructive effects to human health and the environment [3, 4]. Studies of Th(IV) adsorption on different adsorbents have been investigated for finding a material with high efficiency and high adsorption capacity to solve the pollution problem [5–7]. Meanwhile, the adsorption of Th(IV) on clay minerals was found to be one of the most important efficient ways in reducing the mobility of Th(IV) in the natural environment [8, 9]. Red soil is a widely distributed soil and the clay fraction mineralogy of red soil is dominated by kaolinite, gibbsite, hematite, and amorphous minerals, which carry both positive and negative charge on their surfaces. Meanwhile, the studies on the adsorption of radionuclides on red soil have been reported a lot [10, 11]. As an adsorbent, the adsorption capacity of red soil is very limited, compared with other adsorption materials, because of its weak capacity of ion exchange. In order to improve the adsorption properties and capacity of natural material, surface modification methods are proposed and applied. Houhoune et al. prepared a bentonite sample modified with cationic alkylammonium surfactant, hexadecyltrimethylammonium bromide (HDTMA-Br), and found the organic modification changes the bentonite material behavior towards cationic and anionic pollutant species, furthermore, the presence of HDTMA in the interlayer space of the bentonite greatly increased the retention capacity of uranium [12]. Clark et al. studied the effects of aging on the pH-dependence and reversibility of uranium and thorium binding by a modified bauxite refinery residue, and the result show that modified bauxite refinery residues, particularly seawater-neutralised residues may be utilised in the removal of U(VI) from wastewaters [13]. Comparison of adsorption behavior of Th(IV) and U(VI) on quinizarin (1,4-dihydroxyanthraquinone, QNZ) modified Amberlite XAD-16, and the adsorption behavior of Th(IV) and U(VI) on/in the modified resin was compared with that of the resin prepared via the conventional method, the result suggested that adsorption capacity and adsorption rate of the modified resin are significantly greater than the conventional one [14].

In this study, surface-modified red soil samples with ferric chloride (FC) (FC-RS) and hexadecyl trimethyl ammonium bromide (HTAB) (HATB-RS) were prepared and characterized, adsorption properties of Th(IV) on modified red soil samples were investigated, compared with natural red soil (NRS), and the adsorption kinetics and thermodynamics were studied to discuss the adsorption mechanism.

EXPERIMENTAL

NRS. The NRS sample was taken from the suburb of Nanchang city, Jiangxi province, East China, and the particle size is less than 0.01 mm by grinding.

FC-RS. Weight a certain amount of NRS sample, added into NaOH solution, then added a certain volume of FeCl₃ solution slowly. After 24 h, centrifuged and rinsed the sample to no chloride ion, then dried for 24 h and got FC-RS sample.

HTAB-RS. Weight a certain amount of NRS sample, added into solution of HTAB and stirred for 5 h, removed the supernatant fluid after the suspension layered, and rinsed the sample to remove bromide ion, dried the sample for 24 h.

During the experiments, we used a Scanning electron microscope (SEM), an X-ray diffractometer (XRD), an Infrared spectrometer, a Potential coulometric titrator, a Nitrogen adsorption-desorption instrument, a Visible Spectrophotometer (721E), an Arsenazo III solution: 1.0 g/L; Th(IV) standard solution: 500 mg/L solution. In the experiments, materials and reagents are all analytical grade, and all the water is distilled water.

Adsorption procedure of Th(IV) on NRS, FC-RS, and HTAB-RS and the data processing methods are the same as the previous study [15]. Adsorption percentage (S %) and the distribution coefficient (K_d) were calculated by the following formula:

$${\text{Sorption}}\,\,{{\% }} = \frac{{{{q}_{{\text{e}}}}}}{{{{c}_{0}}}}\frac{m}{V}\,\, \times \,\,100,$$

((1))

$${{K}_{{\text{d}}}} = \frac{{({{c}_{0}} - {{c}_{{\text{e}}}})}}{{{{c}_{{\text{e}}}}}}\,\, \times \,\,\frac{V}{m},$$

((2))

where c₀ (mg/L) is the initial concentration of Th(IV) in liquid phase, c_e (mg/L) is Th(IV) concentration in liquid phase after the adsorption equilibrium, V (L) is the volume of the suspension and m (g) is the mass of red soil [15].

RESULTS AND DISCUSSION

Characterization of NRS, FC-RS and HTAB-RS

The SEM images of NRS, FC-RS and HTAB-RS are shown in Fig. 1. It can be seen from Fig. 1 that NRS presents a layered and porous structure, and the diameter of the pores are about 0.2–2.0 μm, which leads to the extra-large surface area. Meanwhile, FC-RS shows an irregular, developed hole structure and complex surface, and the BET surface area can be increased, and the same is true in total pore volume values, compared with NRS, which suggests that this structure offer the possibility of higher adsorption capacity of FC-RS. Moreover, the surface structure of HTAB-RS is similar to NRS.

Fig. 1.

SEM micrographs of NRS (a), FC-RS (b) and HTAB-RS (c).

In order to further explore the surface properties of the three red soil samples, the specific surface area (BET) of NRS, FC-RS and HTAB-RS were determined by N₂ adsorption-desorption instrument, and the values of BET, total pore volume and mean pore diameter are listed in Table 1. Compared with other two samples, BET value of FC-RS is the biggest among the three samples, this indicates FC surface-modification increase the specific surface area greatly and could promote the adsorption reaction. Furthermore, the mean pore diameter value of HTAB-RS is three times bigger than NRS, suggests the organic modification causes the change of surface structure of NRS by the insertion of function groups.

Table 1. BET and Pore volume values of NRS, FC-RS and HTAB-RS

Isoelectric point (IEP) is one of the most important properties of absorbent surface, and it can affect the protonation-deprotonation process of the surface. Figure 2 shows the zeta potential titration curves of NRS, FC-RS and HTAB-RS, which are measured by a particle charge titrator (Particle Metrix Stabino), and determined from the Zeta Potentials versus pH curve at the pH value when zeta potentials equal to zero. In Fig. 2, the values of IEP are about 5.3 for NRS, 3.8 for FC-RS and 1.9 for HTAB-RS, respectively. The IEP value of NRS (5.3) is closed to that of kaolinite [16]. This result of surface zeta potential suggests that the surface-modification of FC and HTAB lead to the decrease of the IEP and make it possible to get surface deprotonation at lower pH, which changes the surface properties of NRS significantly. In addition, HTAB-RS can make electronegativity in the solutions and get deprotonation easier due to the organic functional groups of HTAB.

Fig. 2.

Isoelectric points (IEP) of NRS, FC-RS and HTAB-RS samples. T = 298 K.

FTIR spectrum reflects the functional groups in different samples. Figure 3 shows FTIR spectra of the three red soil samples. In Fig. 3a, the peaks of 692, 756, 998, 1645, 3619, and 3696 cm^–1, etc. are the characteristic absorption peaks of NRS, and belong to Al–OH, Si–O–Al, Si–O–Si, O–H, etc., which represent kaolinite’s characteristic infrared spectrum. Compared with NRS, the FC-RS FT-IR spectrum has little change, but the spectra of HTAB-RS has two new peaks: 2850 and 2925 cm^–1, they represent –CH₂, this shows that the surface-modification introduces new organic functional groups. This result further confirms the conclusion of surface potential titration.

Fig. 3.

FT-IR spectra of NRS (a), FC-RS (b) and HTAB-RS (c).

XRD analysis was carried out to study the effect of surface modification on sample structures. Figure 4 shows the XRD images of the NRS, FC-RS, and HTAB-RS, respectively. The result indicates that surface-modification does not change the mineral components of the samples, that is, kaolinite and quartz are the main components of the samples. In addition, there are a certain number of gibbsite, montmorillonite and ilmenite in the soil samples. This result shows that the operation of FC and HTAB surface-modification has no significant influence on the basic structure and mineral composition of NRS.

Fig. 4.

X-ray powder diffraction patterns of (a) NRS, (b) FC-RS, and (c) HTAB-RS.

Effect of pH and the Protonation–Deprotonation Process

Effect of pH on the adsorption of Th(IV) on the three soil samples are shown in Fig. 5. The results indicate that the adsorption appears in a low pH in the three sample systems, and the adsorption percentage grow fast from 10 to 95% in the range of pH 2.0–6.0. With the growth of pH, the adsorption percentage of Th(IV) remains relatively stable at pH > 6.0. This suggests that the adsorption of Th(IV) is closely related to pH value. Furthermore, in the whole adsorption pH range, and no other background ion that cause interference (I = 0), FC-RS adsorption system shows the higher adsorption percentage than NRS system, and HTAB-RS shows the same result as FC-RS at pH < 6.0, while gives the lower adsorption percentage than NRS at pH > 6.0. Meanwhile, different ionic strength has a big impact on the adsorption, e.g., in FC-RS system, it can be seen that divalent ion Ca²⁺ has the greatest effect, and the effect of K⁺ is larger than that of Li⁺ and Na⁺, which indicates that the stronger the cationic polarity, the greater the impact on ion exchange is.

Fig. 5.

Adsorption of Th(IV) on NRS, FC-RS and HTAB-RS samples as a function of pH. T = 293 K, m/V = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

The main relative distribution of Th(IV) speciation in aqueous solution (Fig. S1, see Supporting Information) which is calculated from the hydrolysis constants with PHREEQC software. The speciation is Th⁴⁺ at pH < 4.0, ThOH³⁺, Th(OH)$_{2}^{{2 + }}$ and Th(OH) at pH 4.0–6.0, Th(OH)₄ at pH > 3.0, respectively. Combined the previous characterization, these results reflect the protonation–deprotonation process of the surface ($\equiv {\text{XOH}}$) and ion exchange or inner sphere complexation of the adsorption at pH < 6.0:

Below pH_PZC,

$$ \equiv {\text{X}}{{{\text{O}}}^{ - }} + {{{\text{H}}}^{ + }} \leftrightarrow \,\, \equiv {\text{XOH}}{\text{,}}$$

((3))

$$ \equiv {\text{XOH}} + {\text{T}}{{{\text{h}}}^{{4 + }}} \leftrightarrow {{( \equiv {\text{XOTh}})}^{{3 + }}} + {{{\text{H}}}^{ + }},$$

((4))

$$ \equiv {\text{XOH}} + {{({\text{ThOH}})}^{{3 + }}} \leftrightarrow {{( \equiv {\text{XOThOH}})}^{{2 + }}} + {{{\text{H}}}^{ + }},$$

((5))

$$ \equiv {\text{XOH}} + ({\text{ThOH}})_{2}^{{2 + }} \leftrightarrow {{( \equiv {\text{XO}}{{({\text{ThOH}})}_{2}})}^{ + }} + {{{\text{H}}}^{ + }},$$

((6))

$$ \equiv {\text{XOH}} + ({\text{ThOH}})_{3}^{ + } \leftrightarrow \,\, \equiv {\text{XO}}{{({\text{ThOH}})}_{3}} + {{{\text{H}}}^{ + }}.$$

((7))

Higher than pH_PZC,

$$ \equiv {\text{XOH}} + {\text{O}}{{{\text{H}}}^{ - }} \leftrightarrow \,\, \equiv {\text{X}}{{{\text{O}}}^{ - }} + {{{\text{H}}}_{2}}{\text{O}}{\text{,}}$$

((8))

$$ \equiv {\text{XOH}} + {\text{T}}{{{\text{h}}}^{{4 + }}} + {\text{O}}{{{\text{H}}}^{ - }} \leftrightarrow {{( \equiv {\text{XOTh}})}^{{3 + }}} + {{{\text{H}}}_{2}}{\text{O}}{\text{,}}$$

((9))

$$\begin{gathered} \equiv {\text{XOH}} + {{({\text{ThOH}})}^{{3 + }}} + {\text{O}}{{{\text{H}}}^{ - }} \\ \leftrightarrow \,\,{{( \equiv {\text{XOThOH}})}^{{2 + }}} + {{{\text{H}}}_{2}}{\text{O}}{\text{,}} \\ \end{gathered} $$

((10))

$$\begin{gathered} \equiv {\text{XOH}} + ({\text{ThOH}})_{2}^{{2 + }} + {\text{O}}{{{\text{H}}}^{ - }} \hfill \\ \leftrightarrow {{( \equiv {\text{XO}}{{({\text{ThOH}})}_{2}})}^{ + }} + {{{\text{H}}}_{2}}{\text{O}}{\text{,}} \hfill \\ \end{gathered} $$

((11))

$$\begin{gathered} \equiv {\text{XOH}} + ({\text{ThOH}})_{3}^{ + } + {\text{O}}{{{\text{H}}}^{ - }} \\ \leftrightarrow \,\, \equiv {\text{XO}}{{({\text{ThOH}})}_{3}} + {{{\text{H}}}_{2}}{\text{O}}{\text{.}} \\ \end{gathered} $$

((12))

At pH > 6, the main speciation of Thorium is Th(OH)₄, which precipitates on the surface, precipitation and surface complexes enhance the adsorption and the adsorption maintains the maximum adsorption percentage. It is obviously that the speciation of Th(IV) in aqueous solution is the internal factor affecting the adsorption pH boundary [17, 18].

Effect of Humic Acid (HA)

Figure 6 shows the effect of HA on the adsorption of Th(IV) on NRS, FC-RS and HTAB-RS, respectively. It can be seen from Fig. 6, the adsorption reaction is enhanced in the presence of HA at pH < 5.0 (In HTAB-RS system, it is pH < 4.0), and reduces the adsorption at pH > 5.0 (In 20 mg/L HA-HTAB-RS system, it is pH > 4.0, and 10 mg/L HA system is close to a system in which humic acid is absent in the whole adsorption pH range). In the range of pH < 5.0, the presence of HA enhanced the adsorption capacity of red soil, because the surface of the red soil adsorbs HA and the charge density is reduced on the surface, which makes the adsorption of Th(IV) have a more favorable electrostatic environment and also strengthen the formation of ternary complex of Th-HA-NRS (FC-RS/HTAB-RS) [19]. Furthermore, the negative effect of HA at pH > 5.0 is attributed to the strong soluble complexes of HA–Th(IV) in solution, respectively. At about pH 8.0, all of the three systems show a peak valley (Except 10 mg/L HA-HTAB-RS system), as the surface charge of red soil samples are negative and the adsorption of negatively charged HA on the negatively charged samples surface decreases with increasing pH due to electrostatic repulsion. At pH > 8.0, the three red soil HA systems remained the adsorption percentages or increased to a certain extent again, as the physical adsorption plays a main role.

Fig. 6.

Effect of HA on the adsorption of Th(IV) on (a) NRS, (b) FC-RS, and (c) HTAB-RS. T = 293 K, m/V = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

The solid surface contains a number of variable charge sites which could be either protonated and deprotonated. The adsorption mechanism of Th(IV) on red soil samples in the presence of HA could be postulated by following reactions:

$$\begin{gathered} \equiv {\text{XOH}} + {\text{HA}}-{\text{COOH}} + {\text{T}}{{{\text{h}}}^{{4 + }}} \\ \leftrightarrow \,\, \equiv {\text{XO}}-{\text{OC}}-{\text{HA}}-{\text{T}}{{{\text{h}}}^{{4 + }}} + {{{\text{H}}}_{2}}{\text{O}}{\text{,}} \\ \end{gathered} $$

((13))

$$\begin{gathered} \equiv {\text{XO}}-{\text{OC}}-{\text{HA}}-{\text{T}}{{{\text{h}}}^{{4 + }}} + {{{\text{H}}}_{2}}{\text{O}} \\ \leftrightarrow \,\, \equiv {\text{XOH}} + {\text{HA}}-{\text{COOH}} + {\text{T}}{{{\text{h}}}^{{4 + }}}, \\ \end{gathered} $$

((14))

$$\begin{gathered} \equiv {\text{XO}}-{\text{T}}{{{\text{h}}}^{{4 + }}} + {\text{HA}}-{\text{COOH}} \\ \leftrightarrow \,\, \equiv {\text{XOH}} + {\text{HA}}-{\text{COO}}-{\text{T}}{{{\text{h}}}^{{4 + }}}. \\ \end{gathered} $$

((15))

The analysis mentioned above shows that, due to of the modification, HTAB-RS contains a lot of organic functional groups, so the lower concentration of HA (10 mg/L) has less effect on the adsorption.

Adsorption Kinetics

Adsorption of Th(IV) on NRS, FC-RS and HTAB-RS at different temperatures are showed in Fig. 7a, and the linear fitting curves of pseudo-first-order and pseudo-second-order kinetic equations at 323 K are showed in Fig. 7b. It can be seen from Fig. 7a that the adsorption of Th(IV) took about 12 h to reach the adsorption equilibrium in NRS and HTAB-RS systems, 20 h in FC-RS system. After the equilibrium time (12 or 20 h), the percentage of adsorption remains substantially unchanged. Temperature can promote the adsorption because the adsorption percentage is higher at high temperature, in general. In NRS system, the percentage at 308 K is the lowest, it shows that temperature could reduce the adsorption in the presence of organic matter which decomposes at high temperature. This phenomenon also appears in the HTAB-RS system.

Fig. 7.

Adsorption kinetics of Th(IV) on NRS, FC-RS and HTAB-RS. pH 4.0, m/V  = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

The adsorption equilibrium percentage of Th(IV) on FC-RS is overall higher than that of Th(IV) on NRS and HTAB-RS under the experimental conditions; meanwhile, from Fig. 7a, HTAB-RS system is the fastest to achieve adsorption equilibrium. The results show FC and HTAB in the soil are the major factors which enhance the adsorption capacities and rates of NRS.

Figure 7b shows the pseudo-first-order and pseudo-second-order kinetic equations linear fitting curves on RS, FC-RS and HTAB-RS. Table 2 lists the parameters related to the two kinetic equations, the chi-square (x²) and linear correlation coefficient (R²).

Table 2.   Linear pseudo-first-order and pseudo-second-order kinetic parameters for adsorption of Th(IV) on NRS, FC-RS and HTAB-RS

Pseudo-first-order equation [20]:

$$\ln ({{q}_{{\text{e}}}} - {{q}_{{\text{t}}}}) = \ln {{q}_{{\text{e}}}} - {{K}_{1}}t.$$

((16))

Pseudo-second-order equation [21]:

$$\frac{t}{{{{q}_{{\text{t}}}}}} = \frac{1}{{{{K}_{2}}q_{{\text{e}}}^{2}}} + \frac{1}{{{{q}_{{\text{e}}}}}}t,$$

((17))

where K₁ (h^–1) is the adsorption rate constant of pseudo-first-order, q_t (mg/g of dry weight) is the adsorption quantity of Th(IV) at time t (h), and q_e (mg/g) is the calculated equilibrium adsorption capacity, K₂ (g mg^–1 h^–1) is the pseudo-second-order rate constant of adsorption

$${{x}^{2}} = \sum {\frac{{{{{(q_{{\text{e}}}^{'} - {{q}_{{\text{e}}}})}}^{2}}}}{{{{q}_{{\text{e}}}}}}} ,$$

((18))

where $q_{{\text{e}}}^{'}$ (mg/g) and q_e (mg/g) are the measured adsorption capacity at time t and the calculated adsorption capacity at equilibrium, respectively [22].

In Fig. 7b, the linear plot feature of ln(q_e – q_t) vs. t (the small window in the figure) and the linear plot feature of t/q_t vs. t on NRS, FC-RS and HTAB-RS are achieved, the results show that the three linear curves of pseudo-first-order kinetic equation could not fit the experimental data well because of the relatively small linear correlation coefficients (0.7889, 0.5474, and 0.0861) and relatively big x² (0.2582, 6.698, and 3.321).

According to Table 2, the adsorption rate constant K₂ of Th(IV) on HTAB-RS is much bigger, and the adsorption capacity at equilibrium of FC-RS is also much higher, compared with NRS system. The results indicate that the operation of surface-modification is successful and surface-modification can greatly increase the adsorption capacity (FC-RS) and adsorption rate (HTAB-RS).

Weber–Morris intraparticle diffusion equation was used to imitate the experimental data to better understand the control factors that influence the Th(IV) adsorption kinetics. Weber-Morris intraparticle diffusion model [23]

$${{q}_{{\text{t}}}} = {{k}_{{\text{p}}}}{{t}^{{0.5}}} + C,$$

((19))

where k_p (kg/(mg h^0.5)) is the constant of intraparticle diffusion rate and C is the intercept.

The fitting curves of Weber-Morris intraparticle diffusion model of Th(IV) adsorption on NRS, FC-RS and HTAB-RS at 323 K are shown in Fig. 8. From figure, it can be seen that intraparticle diffusion of Th(IV) adsorption appears in two parts: an initial rapid adsorption part and slow or balance absorption part. The first part belongs to the diffusion of adsorbate to the adsorbent samples surface through the solution or diffusion of solute molecules, this part is mainly physical adsorption. The second part suggests this portion is mainly chemical adsorption process. The location of the three curves indicates that the adsorption capacity of FC-RS is higher than NRS and HTAB-RS according to the q_t values. The result shows that the diffusion rate is most influenced by the surface-modification.

Fig. 8.

Fitting curves of Weber–Morris intraparticle diffusion model for Th(IV) adsorption on NRS, FC-RS and HTAB-RS. pH 4.0, m/V = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

Adsorption Thermodynamics (Effect of Temperature)

Langmuir and Freundlich isotherm model are used to describe the adsorption process, and the linear adsorption isotherms of Th(IV) on NRS, FC-RS and HTAB-RS at 293, 308 and 323 K are drawn in Fig. 9, and the related correlation coefficients are listed in Table 3, respectively.

Fig. 9.

Linear adsorption isotherms for Th(IV) adsorption on NRS, FC-RS and HTAB-RS at different temperatures. (a) Langmuir model, (b) Freundlich model. pH 4.0, m/V = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

Table 3. Langmuir and Freundlich correlation coefficients and lnK values for Th(IV) adsorption on NRS, FC-RS and HTAB-RS at different temperatures

Langmuir isotherm expression [24]:

$$\frac{{{{c}_{{\text{e}}}}}}{{{{q}_{{\text{e}}}}}} = \frac{1}{{b{{q}_{{\max }}}}} + \frac{{{{c}_{{\text{e}}}}}}{{{{q}_{{\max }}}}}.$$

((20))

Freundlich isotherm expression [25]:

$${\text{log}}{\kern 1pt} {{q}_{{\text{e}}}} = {\text{log}}{\kern 1pt} a + {{\;}}\frac{1}{n}{\text{log}}{\kern 1pt} {{c}_{{\text{e}}}},$$

((21))

where q_e (mol/g) is the equilibrium adsorption capacity, c_e (mol/L) is the concentration of Th(IV) in liquid phase, q_max (mol/g, mg/g) is the maximum adsorption capacity, b is a constant of Langmuir model that relates to the heat of adsorption, a(mol^1 – n g^–1 Lⁿ) and n are constants of Freundlich model, respectively.

Figure 9a shows Langmuir isotherms of Th(IV) adsorption on NRS, FC-RS and HTAB-RS, and the result indicates that the experimental data can not fit Langmuir model very well, due to the relatively large R² (Table 3), compared with that of Freundlich isotherm. Meanwhile, Fig. 9b suggests that Freundlich model can fit the experimental data well, combined with the R² values in Table 3.

The thermodynamic parameters (∆G⁰, ∆S⁰, and ∆H⁰) for Th(IV) adsorption on NRS, FC-RS and HTAB-RS can be calculated from the equations [26]:

$$\Delta {{G}^{0}} = - RT\ln K,$$

((22))

$${{\left( {\frac{{\partial {{G}^{0}}}}{{\partial T}}} \right)}_{p}} = - \Delta {{S}^{0}}$$

((23))

$$\Delta {{H}^{0}} = \Delta {{G}^{0}} + T\Delta {{S}^{0}},$$

((24))

where K is the adsorption equilibrium constant, R (8.3145 J mol⁻¹ K⁻¹) is the ideal gas constant, T (K) is the Kelvin temperature. Values of lnK are obtained by plotting ln K_d vs. c_e. and extrapolating c_e to zero (Fig. 10) and listed in Table 3, too. The thermodynamic parameters are listed in Table 4.

Fig. 10.

Linear plots of lnK_d vs. c_e. pH 4.0, m/V = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

Table 4. Thermodynamic parameters of Th(IV) adsorption on NRS, FC-RS and HTAB-RS

The results show that the adsorption is a spontaneous process under the experimental conditions because the Gibbs free energy changes (ΔG⁰) of the reaction are all negative. The overall absolute values of ΔG⁰ for NRS, FC-RS and HTAB-RS increase with the increasing of temperature, respectively, which indicates that temperatures can promote the adsorption reaction for the value of ΔG⁰ appears an absolute maximum at 323 K in all the tree sample systems. The values of standard enthalpy (ΔH⁰) of the three temperatures are all positive, which indicates that the adsorption process is endothermic process. Values of entropy (ΔS⁰) are all positive, this reflects that the presence of red soil has the positive effect of attracting Th(IV) in solution, and also further illustrate that the adsorption reaction is a spontaneous process. Among the three systems, the values of ΔH⁰ and ΔS⁰ of HTAB–RS system are all the biggest under the same experiment conditions, this shows that the influence of organic modification on adsorption is the largest.

Furthermore, combined with the results of adsorption kinetics and thermodynamics, it can be concluded that the adsorption process can be divided into at least two stages, and it can be simulated by Dubinin–Radushkevich (D–R) isotherm model [6, 27]:

$$\ln {{q}_{{\text{e}}}} = \ln {{q}_{{\max }}} - \beta {{\varepsilon }^{2}},$$

((25))

where β is a constant related to the mean free energy of adsorption (mol² J^–2), q_max is the theoretical saturation capacity, and ε is the Polanyi potential,

$$\varepsilon = RT\ln \left( {1 + \left( {\frac{1}{{{{c}_{{\text{e}}}}}}} \right)} \right).$$

((26))

The constant β gives an idea about the mean free energy E (kJ mol^–1) of adsorption and can be calculated using the relationship:

$$E = \frac{1}{{{{{(2\beta )}}^{{1/2}}}}}.$$

((27))

The value of E gives the information about adsorption mechanism as chemical ion-exchange or physical adsorption. With the magnitude of 8 kJ mol^–1 < E < 16 kJ mol^–1, the adsorption process follows chemical ion-exchange, while for the values of E < 8 kJ mol^–1, the adsorption is a physical process.

Figure 11 shows that D–R model fits the HTAB-RS system experimental data well, and E are calculated and listed in the following according to Eqs. (26) and (27):

$$\begin{gathered} {\text{293 K}}{\text{,}}\,\,E = {\text{8}}{\text{.362 kJ mo}}{{{\text{l}}}^{{ - 1}}}, \\ {\text{308 K}}{\text{,}}\,\,E = {\text{9}}{\text{.586 kJ mo}}{{{\text{l}}}^{{ - 1}}}, \\ {\text{323 K}}{\text{,}}\,\,E = {\text{13}}{\text{.33 kJ mo}}{{{\text{l}}}^{{ - 1}}}. \\ \end{gathered} $$

Fig. 11.

D–R adsorption isotherms for Th(IV) adsorption on HTAB-RS at different temperatures. pH 4.0, m/V = 0.02 g/L, c_{Th(IV)initial} = 500 mg/L.

From the results, E is between 8 and 16 kJ mol^–1, this indicates that the adsorption mechanism is chemical ion-exchange at different temperatures. The values of E increase with the increasing of temperature, this suggests that the adsorption of Th(IV) on HTAB-RS is close to physical adsorption under low temperature (8.362 kJ mol^–1), and high temperature promotes the reaction of chemical adsorption (13.33 kJ mol^–1).

CONCLUSIONS

This work prepared FC and HTAB surface-modification red soil, discussed kinetics and thermodynamics of Th(IV) adsorption on NRS, FC-RS and HTAB-RS. According to the results of Th(IV) adsorption at different experimental conditions, the following conclusions could be drawn:

(1) Characterization results show that the surface-modification changed the composition and surface structure of red soil. FC surface-modification improves the surface area and total pore volume values of NRS, and increases the adsorption capacity and the rate of adsorption. HTAB surface-modification has introduced organic functional groups into red soil, which greatly improves the performance of adsorption, but the adsorption performance at high temperature is decreased.

(2) Ion exchange, outer surface complexation and inner surface complexation are the main mechanism of the adsorption.

(3) The surface-modification affects the adsorption kinetics and thermodynamics greatly, and the kinetics can be described with linear pseudo-second-order kinetic equation, and the adsorption thermodynamics of Th(IV) on red soil could be described well by Freundlich model. The calculation results of the thermodynamic functions suggest the adsorption is a spontaneous and endothermic process.

(4) The adsorption of Th(IV) experiences two parts, and the intraparticle diffusion is one of the controlling step. The adsorption of Th(IV) on red soil is close to physical adsorption under low temperature and close to chemical adsorption at high temperature.

Supporting information includes thorium speciation in deionized water (Fig. S1).