Environ Eng Res > Volume 23(3); 2018 > Article
Lee, Na, and Park: Adsorption characteristics of strontium onto K2Ti4O9 and PP-g-AA nonwoven fabric

### Abstract

This study investigated the possibility of using potassium titanate oxide (K2Ti4O9) and acrylic acid-grafted polypropylene fabric (PP-g-AA) as adsorbents capable of removing strontium from aqueous solutions. K2Ti4O9 showed the highest rate of strontium removal in the weak alkaline range, while the PP-g-AA increased strontium removal in the neutral range. Moreover, the adsorption capacity of the K2Ti4O9 was not affected by the coexistence of K and Na ions, while the adsorption capacity decreased when Ca and Mg ions were present at the same concentration as that of strontium. When coexisted at the same concentration as strontium, Na, K, Ca, and Mg ions strongly reduced the adsorption capacity of the PP-g-AA. The results also indicated that the adsorption of strontium on K2Ti4O9 was consistent with both the Langmuir and Freundlich adsorption isotherms. In contrast, the adsorption of strontium on the PP-g-AA was more consistent with the Langmuir isotherm model. Moreover, the adsorption equilibrium time of K2Ti4O9 was generally 12 h, while that of the PP-g-AA was 5 h, indicating that the adsorption rates were consistent with the pseudo-second order kinetics model. K2Ti4O9 and the PP-g-AA could be regenerated by simple washing with 0.5 N HCl.

### 1. Introduction

The purpose of this study was to evaluate the possibility of using K2Ti4O9 and acrylic acid-grafted polypropylene (PP-g-AA) nonwoven fabric as adsorbents for the removal, separation, and recovery of strontium. The PP-g-AA nonwoven fabric employed herein was prepared by photo-irradiation polymerization of acrylic acid onto polypropylene fiber. The sorption capacity and sorption selectivity for strontium of K2Ti4O9 and the PP-g-AA nonwoven fabric were determined and compared. In addition, their adsorption characteristics were evaluated using adsorption equilibrium experiments and the influence of pH on the removal of strontium ions using these adsorbents was investigated.

### 2.1. Materials

#### 2.1.1. Materials for the preparation of the PP-g-AA nonwoven fabric

Non-woven polypropylene (PP) fabric (180 g/m2, Samsung Non-Woven Fabric Co., Korea) was used as the substrate polymer for grafting. In the preparation, acrylic acid (AA) served as a monomer, benzophenone (BP) as a photosensitizer, while sulfuric acid (H2SO4) and iron sulfate (FeSO4.7H2O) were used as a graft accelerator and a homo-polymer inhibitor. The solvent used for removing the homo-polymer was methanol.

#### 2.1.2. Materials for the preparation of K2Ti4O9

K2CO3 (99.9% purity, Sigma-Aldrich, US) and TiO2 (98.5%, purity, TaiHai, Japan) were used as the main materials for the synthesis of the adsorbent.

### 2.2. Methodology

#### 2.2.1. Preparation of the PP-g-AA nonwoven fabric

A 400 W high-pressure mercury lamp (H400, Mia Electric, Korea) was used as a light source for photografting. The distance between the sample and light source was set to 10 cm. The graft polymerization was carried out by adding 30 mL of a methanol/water (30/70 vol.%) solution. A mixed solution containing AA at a final concentration of 3.6–14.4 wt.%, 0.2 wt.% benzophenone, 0.4 N H2SO4, and 5.0 × 10−3 M FeSO4 (~400 mg) was immersed in a Pyrex tube. Nitrogen gas was flushed through the mixture for 5 min to remove any dissolved oxygen. The solution was then irradiated uniformly at 60°C for 60 to 90 min. The PP nonwoven fabric grafted with AA was subjected to Soxhlet extraction through washing with hot water and methanol for 24 h to remove the homo-polymer and unreacted monomer adhered to the surface of the sample. This process was followed by drying at 60°C for 24 h. After drying, the graft yield was calculated from the weight gain. The aforementioned experimental conditions resulted in a PP-g-AA nonwoven fabric with a graft of 100 wt.%, which was then used as an adsorbent.

#### 2.2.2. Preparation of K2Ti4O9

K2O and TiO2 were mixed in a molar ratio of 1:4 and isopropyl alcohol was added to the mixture. The powder mixture was then placed in a platinum crucible and heated to a temperature of 900 to 1,150°C. The sample was taken out from the furnace at a temperature of 1,150°C and then quenched in air.

In the adsorption experiment, 1 L of the experimental solution with known ionic species and concentration was placed in a 1 L high-density polyethylene (HDPE) sample bottle. Then, 1 g of adsorbent was added to the sample, after which the bottle was sealed. After a batch-wise shaking at 120 rpm and 25°C using a thermostatic stirrer, the pH of the test solution was adjusted to 2.5–11.5 using 0.1 N HCl and NaOH solutions. After shaking it for the required amount of time, the solution was immediately filtered using a 0.45-um membrane filter and the ion concentration remaining in the filtrate was measured. The adsorption of ions, q (mg/g), was calculated using the following equation:
##### (1)
$q=(C0-Ce) VW$
where, C0 and Ce are the initial and post-adsorption strontium concentrations (mg/L) of the filtrate, respectively, V is the volume of the test solution (mL), and W is the dose (g) of adsorbent. After the desorption experiments, the filtered adsorbent was washed several times with distilled water and dried at 90 ± 5°C. Next, 0.1 g of dried strontium adsorbed-adsorbent was added to 100 mL of 0.5 N HCl solution. The resulting mixture was shaken at 25°C and 120 rpm for 1 h, and subsequently filtered. The concentration of strontium ions in the filtrate was measured using mass spectrometry. The desorption efficiency (%) was then calculated in relation to the adsorption amount.

#### 2.2.4. Properties and chemical analysis

The zeta potential of each sample was measured using a flat board cell of an electrophoresis apparatus (Otsuka ELS-8000, Japan). This measurement was conducted in order to investigate the change of the surface potential according to the pH of the adsorbent. The particle size of K2Ti4O9 was analyzed using a laser particle size analyzer (Mastersizer S, Malvern Instruments Ltd., UK). The surface of the PP-g-AA nonwoven fabric and K2Ti4O9 was observed by a scanning electron microscope (SUPRA 55VP, Carl Zeiss, Germany). The PP-g-AA nonwoven fabric was subjected to Fourier-transform infrared spectroscopy (FTIR; Nicolet 6700, Thermo Scientific, USA) analysis in order to confirm the structural changes before and after the adsorption. The mineral composition of K2Ti4O9 was observed using X-ray diffraction (XRD; Panalytical xpert pro mpd, Netherlands).

### 3.1. Physical Properties of the Adsorbent

#### 3.1.1. K2Ti4O9

The mineral composition of K2Ti4O9 used in the adsorption experiments was observed by XRD (Fig. 1). The resulting solution from the heating of the mixture of K2O and TiO2 to 1,150°C and subsequent quenching in air contained a low amount of K2Ti6O13. However, it was confirmed that K2Ti4O9 was the main synthetic product and the material synthesized with K2Ti4O9 was observed using a scanning electron microscope (Fig. 2) [11]. The morphology of the K2Ti4O9 compound was prismatic and showed an undivided crystal, which is consistent with previous reports on the K2Ti4O9 crystal structure. The average particle size of K2Ti4O9 was found to be 2,373 nm, and the Brunauer-Emmett- Teller (BET) surface area was 2.5282 m2/g.

#### 3.1.2. The PP-g-AA nonwoven fabric

Fig. 3 shows the FTIR spectrum of the PP-g-AA nonwoven fabric, in which the peak ~1,740 cm−1 was attributed to the C=O double bond stretching of the AA grafting. Moreover, a peak arising from the −OH group was also detected at around 3,200–3,600 cm−1. Fig. 4 shows a comparison of the scanning electron microscope (SEM) photographs of raw PP nonwoven fabric and the PP-g-AA nonwoven fabric with a graft ratio of 100 wt.%. The original PP nonwoven fabric had a thin and smooth surface, whereas the PP grafted with AA showed increased thickness and became rough and inhomogeneous simultaneously [16].

### 3.2. Effect of pH

In order to investigate the effect of pH on the strontium adsorption capacity of PP-g-AA nonwoven fabric and K2Ti4O9, adsorption experiments were carried out by adjusting the pH of the test solution using NaOH and HCl. Measurements of the final pH of the test solution indicated that the final pH tended to be lower than that the initial value. Correspondingly, it was difficult to properly determine the effect of pH. Therefore, this study investigated the effect of pH on the adsorption capacity, using a buffer solution suitable for each pH range in an attempt to maintain a constant pH, by minimizing the changes in pH that arise from release of hydrogen ions. The following buffers were employed for this aim: primary phosphate (pH = 2), secondary phosphate (pH = 7), tertiary phosphate (pH = 12), secondary carbonate (pH = 10), and acetate (pH = 4). In order to minimize the effect of cation-forming possible salts, bases of weak acids (in the form of Na) were used as cations. A 0.01 M buffer solution for each pH range was prepared such that the acid to base ratio was 1:1. Also, a solution with a strontium concentration of 50 mg/L was prepared using the buffer solution. The adsorption experiment was conducted by mixing 100 mg of the PP-g-AA fabric with a graft ratio of 100 wt.% and 100 mg of K2Ti4O9, and the results are shown in Fig. 5.
The results confirmed that the precipitation of insolubilized strontium increased with the rise in pH. More specifically, a blank test system that did not contain any adsorbent was prepared with the same solution as the one used in the adsorption experiments, and the same procedure was employed. Then, the solutions were filtered through a 0.45-μm filter paper and their concentrations were measured. After the adsorption was completed, the solution was subjected to the same procedure as the solution of the blank test system in order to determine the final residual concentration. The results showed that strontium tends to precipitate strongly in the alkaline region; hence, it is difficult to precisely investigate the effect of pH unless the experiment is conducted at a concentration that can exist in a dissolved phase in all pH regions. Therefore, the results shown in Fig. 5 were not compared at a constant concentration. In addition, since the buffer system differed depending on the pH, it was difficult to completely exclude the effect. However, it is generally shown that strontium has excellent adsorption capacity in the neutral region. These results suggested that the adsorption competitiveness of the ion can be changed according to the pH of the solution. The findings also confirmed the possibility of using PP-g-AA nonwoven fabric and K2Ti4O9 as adsorbents for the separation and purification of strontium.

##### (2)
$Ceqe=1bqm+Ceqm$
where, q is the equilibrium concentration (mg/L), qe is the equilibrium adsorption (mg/g), qm is the maximum monolayer adsorption (mg/g), and b is the Langmuir constant (L/mg) associated with the adsorption energy.
The Freundlich adsorption isotherm is a multilayer adsorption model, expressed in the following linear form (Eq. (3)):
##### (3)
$ln qe=ln KF+1nlnCe$

In order to investigate the adsorption kinetics of the PP-g-AA nonwoven fabric and K2Ti4O9 compound, 100 mL of a solution containing 100 mg/L of strontium was immersed and stirred at 120 rpm at room temperature for 24 h. As shown in Fig. 8, the adsorbed amount of strontium increased significantly for up to 3 h for the PP-g-AA nonwoven fabric, and the amount levelled off after 3 h. The corresponding value with the K2Ti4O9 compound continuously increased up to 10 h and then levelled off. Furthermore, the strontium adsorption reached equilibrium after about 5 h for the PP-g-AA nonwoven fabric and at least 12 h for the K2Ti4O9 compound. The adsorption was analyzed using Eq. (4) and Eq. (5), which represent the first adsorption kinetics equation and second adsorption kinetics equation, respectively [20, 21].
##### (4)
$ln (qe-qt)lnqe-k1t$
##### (5)
$tqt=1k2qe2+tqe$
where, qe and qt are the equilibrium adsorption amount and the adsorption amount (mg/g) at time t, respectively, and k1 and k2 are the first adsorption rate constant (min−1) and the second adsorption rate constant (g/mg·min), respectively. As shown in Fig. 9, the strontium adsorption on the K2Ti4O9 compound and PP-g-AA nonwoven fabric could not be explained by the first adsorption rate equation. According to this equation, a straight line can be seen for the K2Ti4O9 compound after 60 min, while that for the PP-g-AA nonwoven fabric appears 360 min earlier. The theoretical qe values of the K2Ti4O9 compound and PP-g-AA nonwoven fabric obtained from the straight lines are shown in Table 2. The kinetic data for both the K2Ti4O9 compound and PP-g-AA nonwoven fabric have well-converged into linear line at the second adsorption kinetics equation, resulting in a high linear regression constant (r2 > 0.99) (Fig. 9(b)). The theoretical qe values of K2Ti4O9 and the PP-g-AA nonwoven fabric obtained from the linear equation were comparable to the experimental qe value. This indicated that the strontium adsorption by the K2Ti4O9 compound and PP-g-AA nonwoven fabric can be explained better by the second adsorption rate equation.

### 3.6. Regeneration Efficiency and Durability

The strontium adsorption and regeneration experiments were repeated in order to investigate the regeneration efficiency of the PP-g-AA nonwoven fabric and K2Ti4O9 compound. The regeneration experiment was carried out via an acid washing method, in which the adsorbent was immersed and stirred in 100 mL of a 0.5 N HCl solution for 1 h. The desorbed adsorbent was regenerated by repeated washing with distilled water until its pH became neutral and subsequent drying at low temperature. The adsorption-regeneration process was repeated five times, and the amount of adsorbed strontium was measured (Fig. 10).

### 4. Conclusions

In this study, the adsorption characteristics of K2Ti4O9 and PP-g-AA nonwoven fabric, as adsorbents for strontium removal, were investigated through batch experiments. The results are summarized as follows:
The K2Ti4O9 compound showed the highest strontium removal in the weak alkaline range of pH 7–8, while the PP-g-AA nonwoven fabric exhibited increased strontium removal in the neutral range of pH 5–8.
The strontium adsorption capacity of K2Ti4O9 was not influenced by the coexistence of the K and Na ions. However, when Ca and Mg ions were present at the same concentration of strontium, the adsorption capacity decreased to 40% and 70%, respectively. In the case of the PP-g-AA nonwoven fabric, the capacity was affected by the coexisting cations. For the same concentration of strontium, the adsorption capacity of Na ions was lowered to 35%, while the corresponding values for K ions, Ca ions, and Mg ions were 40%, 60%, and 70%, respectively.
The strontium adsorption on K2Ti4O9 satisfied both the Langmuir and Freundlich adsorption isotherms. In contrast, the strontium adsorption on the PP-g-AA nonwoven fabric was more consistent with the Langmuir isotherm model. The result indicated that the adsorption mechanisms of strontium on K2Ti4O9 and the PP-g-AA nonwoven fabric were mostly dominated by ionic shifts.
The adsorption equilibrium time was generally 12 h for the K2Ti4O9 compound and 5 h for the PP-g-AA nonwoven fabric. The adsorption rates were consistent with the pseudo-second order kinetics model.
A simple washing process can help regenerate the PP-g-AA nonwoven fabric more than five times without deteriorating the adsorptivity potential and its physical properties. In the case of K2Ti4O9, the adsorption efficiency decreased by about 15% after one round of regeneration. However, the decrease in adsorption capacity was considerably smaller after two rounds of the regeneration process. In conclusion, both K2Ti4O9 and the PP-g-AA nonwoven fabric could be regenerated by simple washing with 0.5 N HCl.

### Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2015R1C1A2A01052334).

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##### Fig. 1
XRD patterns of compounds of K2O and TiO2.
##### Fig. 2
SEM images of K2Ti4O9.
##### Fig. 3
FT-IR spectra of PP and PP-g-AA fabric.
##### Fig. 4
SEM images of PP (a) and PP-g-AA fabric (b).
##### Fig. 5
Comparison of adsorption capacity according to change in pH.
##### Fig. 6
Effect of competing metal ions on the strontium adsorption capacity of the adsorbents a) PP-g-AA, b) K2Ti4O9.
##### Fig. 8
Kinetic plots of strontium onto PP-g-AA fabric and K2Ti4O9.
##### Fig. 9
(a) Pseudo first-order and (b) pseudo second-order kinetics plots for strontium adsorption by adsorbents.
##### Fig. 10
Regeneration efficiency of adsorbents via washing with 0.5 N HCl.
##### Fig. 11
SEM photographs of K2Ti4O9 and PP-g-AA fabric. (a) K2Ti4O9 adsorbed on strontium, (b) K2Ti4O9 after regeneration with 0.5 HCl, (c) PP-g-AA fabric adsorbed on strontium, (d) PP-g-AA fabric after regeneration with 0.5 HCl.
##### Table 1
Langmuir and Fruendlich Isotherm Parameters
Sample Langmuir Isotherm Freundlich Isotherm

qm (mg/g) b r2 KF n r2
K2Ti4O9 105.26 4.24 0.99 1.55 1.66 0.98
PP-g-AA 60.97 5.80 0.98 1.67 0.54 0.83
##### Table 2
First- and Second-order Kinetics Parameters
Sample qe (mg/g) Pseudo-first-order kinetics Pseudo-second-order kinetics

k1 (min−1) qe (mg/g) r2 k2 (g/mg·min) qe (mg/g) r2
K2Ti4O9 69.6 0.33 21.67 0.9914 0.02 70.9 0.9998
PP-g-AA 45.26 0.33 4.55 0.9203 0.58 46.5 0.9999
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