Effect of Environmental Conditions on Strontium Adsorption by Red Soil Colloids in Southern China

Abstract

The fate of radionuclides in the environment is attracting increased attention. The effect of various environmental effects on the adsorption behavior of the strontium ion (Sr²⁺) by red soil colloids in Southern China was studied by a series of batch experiments, and the adsorption mechanism was briefly investigated as well. With the increase in the solid–liquid ratio and the concentration of Sr²⁺, the adsorption efficiency increased gradually. The effect of pH and ionic strength on adsorption was strong, while temperature had little effect. The adsorption data fitted to the Langmuir model indicates that the process is monolayered and homogeneous. The thermodynamic parameters also show that the adsorption of Sr²⁺ on red soil colloids is a spontaneous and exothermic process. The aim of this work is to gain insight into the role of red soil colloids on the fate of radionuclides in the field.

1. Introduction

Increased attention has been paid to the environmental behavior of radionuclides, especially since the Fukushima nuclear accident in 2011. In 2005, the activity value of ⁹⁰Sr in Fukushima Prefecture was 3.6 Bq·kg⁻¹, while the activity value increased to hundreds or more Bq·kg⁻¹ near Fukushima Daiichi nuclear power plant after the nuclear accident [1]. As a typical artificial radionuclide, ⁹⁰Sr with a half-life of 28.5 years, is produced by ²³⁵U and ²³⁹Pu nuclear fission, and is commonly released into the environment due to nuclear weapon tests, nuclear power plant accidents, and nuclear fuel reprocessing industries [2].

As one of the most toxic radionuclides, ⁹⁰Sr ion has a chemical behavior similar with calcium [3], which could potentially cause great external radiation doses and internal radiation to humans and other living things [4,5,6]. ⁹⁰Sr eventually enters the food chain by various ways from soil, under some conditions, and could further harm the health of plants, animals, and humans [7,8]. Therefore, it is crucial to study the behavior of Sr²⁺ in the soil since the stable Sr²⁺ presented a similar chemical migration with radionuclide ⁹⁰Sr²⁺.

Soil colloids, as the most active component in the soil with a size of 1 nm–1000 nm, perform a significant role in the migration of pollutants in the soil [9]. According to the composition, soil colloids are categorized into inorganic colloids, organic colloids, and organic-inorganic composite colloids. The behavior of Sr²⁺ adsorbed by inorganic colloids of Na-rectorite was studied using the static batch method. It was found that the adsorption process was affected by the pH value and ionic strength [10]. It was also reported that bentonite colloids had a significant retardation of the migration of Sr²⁺ with very slow flow rates of 1 mL·h⁻¹ [11]. For the organic colloids, most studies focused on the effects of the reprehensive organic matter of humic acids (HA), and it was found that HA had different effects on the migration of radionuclides under different conditions [12,13,14]. The effect of HA on the adsorption of ¹³⁷Cs, ¹³³Ba, and ¹⁵⁴Eu was studied [15]. It was found that adding HA has no obvious effect on the adsorption of Cs⁺ at pH 3–10. For Ba²⁺, HA promoted the adsorption when the pH value was lower than 6.4. However, HA inhibited the adsorption when the pH was higher than 6.4. Furthermore, HA had a significant impact on the adsorption of Eu³⁺. Therefore, it can tell that HA had different effects on different nuclides. In fact, the organic-inorganic composite colloid is the most ubiquitous colloid in nature. However, more attention has been paid on the artificially synthetic composite colloids in the lab [16]. There are relatively few reports on red colloids as organic-inorganic colloids extracted from arable soils. The current studies on the adsorption of Sr²⁺ were mainly focused on the artificial inorganic colloids, the synthetic composite colloids or natural whole soil without considering the effect of size [17]. The behavior of U⁶⁺ on the red soil colloid was studied by a static method, the effects of various factors on adsorption processes have been discussed, and the significant role of organic matters in the migration of U⁶⁺ in soil has been confirmed [18]. Red soil is the most extensively distributed soil in Southern China [19]. However, there is no study about the adsorption of strontium in red soil colloids in China [20]. To further assess the effect of strontium on the ecosystem, it is necessary to study real soil colloids on the fate of Sr²⁺ in the field.

In the present study, the adsorption behavior of Sr²⁺ on soil colloids extracted from red soil is investigated to check the effect of environmental effects on Sr²⁺ adsorption by a series of batch experiments. The sorption mechanism between colloids and Sr²⁺ is explored, which is of some significance for the future study of the migration behavior of radionuclides in soil.

2. Materials and Methods

2.1. Materials

Materials: Strontium nitrate, nitric acid, sodium hydroxide, and hydrogen peroxide (Analytical grade, Beijing chemical plant). Deionized water (>18.2 MΩ cm⁻¹) was obtained from the Milli-Q system, and anhydrous ethanol were obtained from commercial sources.

2.2. Sampling and Colloid Extraction

The arable surface red soil was collected in Hunan Province (28°19′ N, 112°98′ E), which is a typical red soil area in southern China. The study area is a subtropical monsoon climate with abundant rainfall. Soil colloids were prepared by the method of sedimentation siphoning performed in accordance with Stokes law [21]. In our study, we selected the stable Sr element to simulate the behavior of radioactive Sr. There were two Sr compounds that could be used in our study, which were Sr(NO₃)₂ and SrCO₃. However, the solubility of Sr(NO₃)₂ is better that SrCO₃. Therefore, Sr(NO₃)₂ was selected for the adsorption study. Deionized water was used in the experiments.

2.3. Inorganic Colloid

The effect of organic matter on the adsorption of Sr²⁺ from soil colloids was studied by using H₂O₂ to remove the organic matter in the soil colloids [21,22]. A certain amount of H₂O₂ was added into the soil colloids, and the soil was put in a heated water bath to evaporate until no bubbles were present. Finally, the supernatant was discarded by means of centrifugation. The matter at the bottom of centrifuge tube was the inorganic soil colloids.

2.4. Batch Adsorption Experiments

All experiments were conducted using a batch method on a thermostated oscillator. A centrifuge tube with 4 mg of soil colloids and 100 mg·L⁻¹ of Sr (NO₃)₂ was placed in the water bath thermostated oscillator with a contact time of 12 h at room temperature (25 °C). Then, the solution was filtered through a 0.1 μm filter.

The adsorption efficiency and the adsorption capacity (Q_e, mg g⁻¹) was calculated by Equations (1) and (2). The data were measured by ICP-MS (Inductively coupled plasma mass spectrometry, Thermo-X7, Thermo Scientific, Waltham, MA, USA).

$$Adsorptionefficiency=\frac{C_0-C_t}{C_0}\times 100 \%$$

(1)

$$Q_e=\frac{C_0-C_t}{m}\times V$$

(2)

where C₀ is the concentration of Sr²⁺ solution before adsorption (mg·L⁻¹), and C_t is the concentration of Sr²⁺ at a certain time after adsorption (mg·L⁻¹). m and V are the weight (g) of soil colloid, and the volume (L) of the solution used in the adsorption experiment [23].

The effects of these factors on the adsorption were studied by changing the amount of soil colloids and varying the strontium nitrate concentration, ionic strength, temperature, pH values, and organic carbon.

2.5. Characterization

The particle size was measured by a Nano particle size analyzer (Zetasizer Nano ZS90, Malvern, England). The contents of the major elements in the soil colloids were analyzed by X-ray Fluorescence (XRF, Eagle III XXL μ-Probe, EDAX Inc., Pleasanton, CA, USA). The content of the soil’s total C, N, and S was measured using an organic elemental analyzer (Vario EL Cube, Elementar, Germany).

3. Results and Discussion

3.1. The Properties of Soil Colloid

In the colloidal extraction process, we took the last extract and diluted it 10–20 times to characterize the particle size.

Table 1. Basic properties of the red soil colloids (%).

Soil	TN	TS	TC	SiO2	Al2O3	Fe2O3	CaO
%	0.30	0.03	2.87	53.15	30.92	11.95	0.49

Remarks: TN represents the total nitrogen in soil colloids; TS represents the total sulfur in soil colloids; TC represents the total carbon in soil colloids.

shows the analytical results of the basic properties of the soil colloids.

3.2. Colloid Dosage

The initial concentration of Sr²⁺ was 100 mg·L⁻¹, and the amount of soil colloids were 2, 4, 6, 8, and 10 mg, respectively; these quantities were used to calculate the adsorption efficiency. As is shown in Figure 1, with the increase in the solid-to-liquid ratio, the adsorption capacity gradually increased. In the first five minutes of the reaction, the adsorption efficiency increased rapidly, the maximum adsorption efficiency increased from 0% to 13%, which may be attributed to more available adsorption sites at the beginning of the reaction. After half an hour, the adsorption reaction basically was kept in an equilibrium.

When the colloid dosage was increased from 2 mg to 10 mg, the adsorption efficiency increased from 2% to 13% with the adsorption capacity of 8.42, 7.51, and 6.41 mg·g⁻¹, respectively. An increase in the amount of colloids provided more adsorption on the active sites or participation of the number of adsorption functional groups, and enhanced the colloidal surface adsorption capacity. The adsorption rate was faster when the concentration changed from 2 mg to 4 mg, resulting in the steep rise of adsorption efficiency. Additionally, more soil colloids would result in the adsorption capacity when the same Sr concentration was used. Since the concentration of Sr was stable, the adsorption capacity decreased when the adsorption efficiency increased, which is consistent with a similar previous study of U(VI) [24].

3.3. Effect of pH

As shown in Figure 2, the adsorption efficiency was approximately 2% at pH = 5, and it increased to 7% with an increase in the pH to 7. At a pH of 3–10, Sr was present in the form of Sr²⁺; therefore, the adsorption behavior was dependent on the negative charge on the soil colloid surface [25]. With the increase of the pH value, the dissociation of negative minerals on the clay minerals, organic matter, and oxide surface in the soil colloid was promoted, thereby increasing the number of negative charges on the colloidal surface [26]. Therefore, the subsequent experimental pH = 6 value was selected.

3.4. Effect of Ionic Strength

The effect of ionic strength on the sorption process is shown in Figure 3. It was clearly found that the ionic strength has a significant inhibiting effect on the adsorption. As the ionic strength increased from 0.001 mol·L⁻¹ to 0.1 mol·L⁻¹, the adsorption efficiency decreased from 10% to 3%. One reason for this may be that the ionic strength affected the electrostatic interaction of the colloidal surface. A high ionic strength would weaken the repulsive force, causing agglomeration, which decreases the number of available adsorption sites. On the other hand, the presence of electrolytes on the adsorption, mainly due to the cationic ions and Sr²⁺ produce competitive adsorption sites for the adsorption on the colloidal surface [27,28]. The results obtained were similar to the sorption of Sr²⁺ on a magnetic adsorbent and on montmorillonite [29,30]. It was also reported that the adsorptions of the Sr²⁺ and U⁶⁺ on the Na-rectorite decreased with an increase in ionic strength [27].

3.5. Effect of Initial Sr²⁺ Concentration

As shown in Figure 4, the adsorption efficiency increased from 2% to 17% with the increase in the initial concentration of Sr²⁺. When the interaction between the colloids and the strontium equilibrated, the adsorption efficiency slowed down, and as the initial concentration increased, the residual unreacted strontium ion concentration increased, and the adsorption efficiency decreased.

To explore the adsorption mechanism of Sr²⁺ by colloids, the adsorption data was fitted with Langmuir and Freundlich adsorption isotherm models [31,32]. The soil colloid adsorption of different concentrations of strontium nitrate was the data used for the isothermal fitting [33]. The modeled results are provided in

Table 2. Parameters for the Langmuir and Freundlich isotherm models at different initial concentrations.

Langmuir			Freundlich
Qm (mg·g−1)	b (L·mol−1)	R2	KF (mol1−n·Lng−1)	n	R2
11.27	29.40	0.9346	1.32	3.30	0.9062

. The correlation coefficient (R²) calculated by Langmuir model was higher, which suggests that the Langmuir isotherm model is more suitable to describe the adsorption process and the sorption sites on the colloids are homogenously distributed. The major sorption mechanism is monolayer sorption. The modeling adsorption capacity Q_max was 11.27 mg·g⁻¹, which was similar with the real soil colloids of 11.50 mg·g⁻¹.

3.6. Effect of Organic Matter

As shown in Figure 5A, the adsorption efficiency was significantly decreased by 40% after removal of soil organic matter. The adsorption efficiency increased with the increase in the concentration of HA. As shown in Figure 5B, the concentration of HA from 0 to 100 mg·L⁻¹, the adsorption efficiency increased by approximately 36%. These may be attributed to the polar groups in the soil organic matter (such as hydroxyl, carboxyl, etc.), which enhanced the adsorption of Sr²⁺ by providing a large number of negative charges as important complexing agents [34,35].

This study confirms the important role that organic matter performs in the adsorption of Sr²⁺ by real soil colloids, which were proposed by previous studies on synthetized colloids [10,36].

3.7. Sorption Kinetic

To examine the controlling mechanism of the adsorption process, the pseudo-first order kinetic and pseudo-second order kinetic models were employed to examine the experimental data at an initial concentration of 100 mg·L⁻¹ at pH 6, 25 °C, and the soil-liquid rate of 0.80 g·L⁻¹ according to Equations (2) and (3) [37,38]:

$$Q_t=Q_e\cdot \left(1-\exp \left(-K_{1}t\right)\right)$$

(3)

$$\frac{t}{Q_t}=\frac{1}{K_2{Q}_e^2}+\frac{1}{Q_e}t$$

(4)

where Q_t (mg·g⁻¹) and Q_e (mg·g⁻¹) represents the amount of Sr²⁺ adsorbed at a moment and at equilibrium time, t (min) is the shaking time, and K₁ (min⁻¹) and K₂ (g·min⁻¹·mg⁻¹) are pseudo-first order kinetic and pseudo-second order kinetic adsorption parameters, respectively.

The kinetic results and parameters are shown in Figure 6 and

Table 3. Values of kinetics parameters for the adsorption of Sr on soil colloid.

First-Order Kinetic			Second-Order Kinetic
Qm (mg·g−1)	K1 (min−1)	R2	Qm (mg·g−1)	K2 (g·min1·mg−1)	R2
5.24	0.58	0.9579	5.63	0.40	0.9918

. It was found that a better fit was obtained for the pseudo-second order model (R² = 0.9918) compared with the pseudo-first order model (R² = 0.9579), which indicates that Sr²⁺ was chemically adsorbed by the colloids. The value of Q_m was 5.63 mg·g⁻¹, which was much higher than the red soil colloids analyzed in this study (2.90 mg·g⁻¹).

3.8. Adsorption Thermodynamics

As shown in Figure 7, the adsorption quantity gradually increased with the increasing of temperature. The data were fit to the thermodynamic model. Thermodynamic parameters are listed in

Table 4. Values of thermodynamic parameters for the adsorption of Sr on soil colloid, T = 293/298/303/318/328 K, soil-liquid rate = 0.8 g·L⁻¹, pH = 6 ± 0.1, strontium nitrate concentration = 100 mg·L⁻¹.

Temperature (K)	ΔG (KJ·mol−1)	ΔS0 (KJ·(mol·K)−1)	ΔH0 (KJ·mol−1)
293	−8.96	0.1130	24
298	−9.53
303	−10.09
318	−11.78
328	−12.90

, include the Gibbs energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were calculated from the following equations [39,40]:

$$ln{K}_D=-\frac{\mathsf{\Delta }{H}^0}{RT}+\frac{\mathsf{\Delta }{S}^0}{R}$$

(5)

$$\mathsf{\Delta }G=\mathsf{\Delta }H-T\mathsf{\Delta }{S}^0$$

(6)

where K_D is solid–liquid partition coefficient; Q_e is adsorption equilibrium when adsorption; C_e is concentration of adsorption equilibrium; R (8.314 J (mol·K)⁻¹) is the ideal gas constant; and T is the Kelvin temperature.

The positive ΔH indicates that the adsorption of Sr²⁺ by soil colloids is an endothermic process in the environment. With the increase in temperature, the adsorption efficiency increased, which is consisted with the above reported results. In addition, when ΔH was less than 30 KJ·mol⁻¹, it meant that most of the adsorption reactions under this condition are physical adsorption. The positive ΔS and negative ΔG suggests that the adsorption process is a spontaneous and entropy increased reaction at the solid/solution colloid interface during the adsorption. It was found in the adsorption behavior of Sr on bentonite that the negative ΔH and ΔG indicated that the adsorption progress was exothermic and spontaneous [41].

4. Conclusions

The adsorption of a typical radionuclide of Sr²⁺ by composite colloids extracted from red soil in China was investigated for the first time. It was found that the soil organic matter played an important role in the process and the pH. The adsorption process in nature was in accordance with second order kinetics and the Langmuir adsorption isotherm, and spontaneously proceeded. This work provides new data for assessing the fate of radionuclides in the soil.