1. Introduction
Recently, there has been renewed interest in thorium’s possible application in the nuclear fuel cycle [
1,
2] as it has several advantages compared with uranium. Thorium is four times more abundant in nature than uranium and, therefore, potentially meets the substantial and expanding demand for energy resources across the world. Moreover. thorium is a naturally occurring radioactive heavy element, widely distributed in nature as an easily exploitable resource in many countries. It found in a variety of minerals, such as monazite, xenotime, zircon, and ilmenite [
3,
4,
5]. Some human activities, such as nuclear fuel plants, ore mining, tin processing, rare-earth extraction process, production of phosphate fertilizer, phosphate rock processing, industrial boilers, coal-fired utilities, and laboratories dealing with radioactive substances, contributed to an increase in the concentration of thorium in our environment as a result of waste generated from such activities [
6,
7,
8,
9]. Thus, the removal of thorium from radioactive waste is regarded as a crucial issue in the treatment of such waste because it is extremely dangerous and can harm the environment and human health due to its radiotoxicity and chemical toxicity, as well as its long half-life [
10]. In addition to this, the removal of thorium from radioactive waste by using an appropriate treatment will minimize the amount of waste that has to be disposed of, leading to a reduction in disposal costs and an improvement in the disposal site efficiency [
11,
12,
13,
14,
15].
Treatment techniques used to remove radionuclides from radioactive wastes include adsorption, chemical precipitation, ion exchange, evaporation, reverse osmosis, micro-filtration, ultra-filtration, electrosorption, and others [
10,
16]. However, among these treatment techniques, adsorption is the most common technique because it is convenient, efficient, simple, inexpensive, has a large capacity, and produces no sludge. It has been extensively used to treat radioactive wastes [
10,
17]. Many types of materials have been utilized for thorium adsorption, such as activated carbon [
18], gibbsite [
19], illite [
20], perlite [
21], bentonite [
22], and zeolite [
23].
In the 1950s, zeolite was first considered for the treatment of radioactive wastes [
24]. Zeolites have been employed in low level radioactive waste treatment techniques. However, zeolites have recently been used in the treatment of high and medium level wastes [
25]. These materials have a high radiological stability to beta, alpha, and gamma irradiations, have a high ion exchange capacity, have an excellent chemical and thermal stability, with almost no reactivity to chemicals, are selective, affordable, and abundant. Moreover, the uniform presence of channels and pores in zeolite is an important advantage. That is why this mineral is known for its high absorption abilities, owing to its high surface-to-volume ratio. These unique properties make zeolites presently the most widely utilized adsorbents for the treatment of radioactive wastes [
25,
26,
27]. Zeolites are crystalline aluminosilicates with three-dimensional crystal frameworks built of tetrahedral silica (SiO
4) and alumina (AlO
4) which are connected to each other via sharing oxygen atoms. There are many types of natural zeolites known around the world, such as mordenite, clinoptilolite, chabazite, phillipsite, analcime, laumontite, and stilbite [
28].
Among the 40 identified types of natural zeolites, clinoptilolite is considered the most important and abundant natural zeolite. It is found in large deposits all around the world and is widely utilized on a global scale in different sorption treatment studies of industrial wastewater [
29,
30]. Clinoptilolite is a high silica member of the heulandite group natural zeolites. The typical chemical formula for clinoptilolite is given by Na
6[(AlO
2)
6(SiO
2)
30]·24 H
2O [
31]. However, this chemical composition of clinoptilolite is generally variable in both the extra framework cation number and its framework [
32]. The types and physical and chemical properties of natural clinoptilolites rely on the environment and the place of the deposits [
33]. The low cost, availability, and favorable ion exchange capacity of clinoptilolite which make it particularly suitable to be used as an adsorbent material for environmental protection. The cost of the clinoptilolite would be negligible in comparison with the cost of activated carbon, as the cost of clinoptilolite is approximately 0.06–0.08 US
$ per kg [
32,
34,
35,
36].
There are many researchers and scientists looking for ways to enhance the adsorption of thorium through modifying the surface of natural zeolite. From the point of view of adsorption performance and cation exchange capacity, modified zeolites have higher performance and capacity than the natural zeolites [
36,
37,
38]. Several studies on zeolite modification have been conducted in order to further improve its adsorption efficiency for thorium removal [
39,
40,
41,
42]. However, to the best of our knowledge, there are no reports on the removal of thorium from an aqueous solution using natural zeolite modified with sulphate and phosphate. The adsorption of phosphate and sulphate anions would increase the negative charge on the natural zeolite surface and, as result of that, the removal efficiency of zeolite towards thorium could be increased.
In the present study, the natural zeolite (clinoptilolite) has been modified with sulphate and phosphate in order to enhance the removal of thorium ions from aqueous solutions. Here, X-ray Diffraction (XRD), N2 adsorption–desorption (BET), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM), and EDX were used to characterize the natural zeolite (NZ), phosphate-modified zeolite (PZ), and sulfate-modified zeolite (SZ). The equilibrium adsorption data have been analyzed using common isotherm models, namely Langmuir, Freundlich, and Dubinin–Radushkevitch (D–R), and the equilibriums parameters have been calculated. The findings acquired in this investigation may be useful for future research and demonstrate the practical uses of the adsorbent in the remediation of thorium industrial residue.
2. Materials and Methods
2.1. Materials
The natural zeolite (clinoptilolite) used in this research was supplied by Heiltropfen (Heiltropfen Lab. LPP, 27 Old Gloucester Street, WC1N 3AX, London, UK). According to the manufacturer, the product is 100% natural volcanic mineral from Slovakia, EU, with more than 90% clinoptilolite content. Potassium dihydrogen phosphate (KH2PO4, ≥99.0%, Mw = 136.09 g/mol) and sodium sulfate (Na2SO4, anhydrous, ≥99.0%, Mw = 142.04 g/mol) that were used for surface modification of the natural zeolite were purchased from Sigma-Aldrich. The stock standard solution of 1000 mg/L of thorium nitrate [Th(NO3)4] was purchased from AccuStandard, New Haven-CT, USA. Different concentrations of thorium solution were prepared from the stock solution by appropriate dilution. Barium chloride dihydrate (BaCl2, ≥99.0%, Mw = 244.26 g/mol) and iron (III) chloride (FeCl3, anhydrous, ≥99.0%, Mw = 162.20 g/mol), which were used to test for the ions of sulphate and phosphate, were purchased from Sigma-Aldrich. Dilute solutions of sodium hydroxide (NaOH) and nitric acid (HNO3) were used to adjust the pH to the required value. All materials and chemicals were used as obtained from the suppliers without additional purification and modification, unless stated.
2.2. Modification and Characterization of the Adsorbent Materials
The modification of the natural zeolite (clinoptilolite) was carried out by mixing 100 g of the natural zeolite (clinoptilolite) with 1000 mL of 200 mg/L of sodium sulfate (Na
2SO
4) and potassium dihydrogen phosphate (KH
2PO
4) in a 2000 mL beaker. The zeolite suspensions were stirred on a magnetic stirrer (a Cimarec 1 model, Thermolyne Barnstead, Dubuque, IA, USA) for 24 h at room temperature (25 °C), after which they were been filtered off by using Whatman filter papers (Whatman’s No. 1, GE Healthcare Life Science, Hatfield, UK). The solid phase was washed several times with 1000 mL portion of deionized water in order to remove excess sulphate and phosphate ions. Tests for sulphate and phosphate in the filtered solution were confirmed negative. The samples of PZ and SZ were subsequently dried in an oven at 105 °C and were packed into glass containers and stored in the desiccator at room temperature for further use. The modification procedure of natural zeolite using sulphate and phosphate is illustrated in
Figure 1.
The natural and modified adsorbents were characterized using X-ray diffraction (XRD, Bruker AXS Germany, a D8 Advance model, fitted with a scintillation counter) to confirm the identity, purity, and crystallinity of the adsorbent materials. The X-ray diffraction (XRD) data of the natural and modified zeolites were collected using a Bruker AXS Advance D8 diffractometer with a monochromatic Cu Kα radiation source (wavelength (λ) = 1.5406 Å) consisting of Soller slits (0.02 rad), a fixed divergence slit (0.3°), and operated at a tube current of 40 mA and a tube voltage of 40 kV with a scanning speed of 2°/min, at room temperature. The samples were scanned between 2θ = 5° and 50° with a scintillation counter detector.
The specific surface area (SBET) of the adsorbent materials has been determined by using a micromeritics accelerated surface area and porosimetry analyzer system (ASAP 2020, Micromeritics, Atlanta, GA, USA), while the nitrogen adsorption/desorption isotherms were measured at 77 K. The natural and modified adsorbents were first degassed under vacuum conditions at 300 °C to avoid damaging the structure of zeolite for 10 h before conducting the BET analysis. The nitrogen adsorption/desorption isotherm measurements were recorded at relative pressures (P/Po−1) between 0.01 and 0.99. The specific surface area (SBET) of the adsorbent materials was determined using the BET method. The pore size distributions were determined from the desorption branch using the Barrett–Joyner–Halenda (BJH) method.
Fourier transform infrared spectroscopy of the natural and modified adsorbents has been studied using an FTIR analysis instrument (Perkin Elmer, spectrum 400 FT-IR NIR spectrometer) to identify the functional groups present in the adsorbent materials. This instrument is equipped with an ATR single reflection diamond crystal. The Fourier transform infrared spectra (FTIR) of the natural and modified zeolite samples were collected in the range of 650–4000 cm−1.
The surface morphology of the adsorbent materials has been observed by using field emission scanning electron microscopy (FESEM, Merlin ZEISS GEMINI 2, Oberkochen, Germany) under the following analytical conditions: EHT = 3.00 kV, Signal A = SE2, WD = 8.9, 9.2, and 9.3 mm at different magnifications (1000×, 10,000×, 20,000×, and 30,000×), respectively. The field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (EDX) was used to ensure the modification procedure of the natural zeolite and to determine the dispersion of desirable species in the adsorbent materials. Concentrations and positions of different elements have been determined using X-ray elemental mapping (dot mapping) analysis.
2.3. Adsorption Experiments
The adsorption experiments were carried out using the batch method. In these batch adsorption experiments, accurately weighed masses (~0.03 g) of natural zeolite (clinoptilolite), PZ, and SZ were placed in 2 mL polypropylene vials, and an aliquot (2 mL) of each different concentration (50, 100, 150, 200, 300, 400, and 600 mg /L) of thorium solution was added. The polypropylene vials were directly placed vertically on a shaker (high-speed microplate shaker, Illumina, a 945190 model, San Diego, CA, USA) at 775 rpm for one day to reach equilibrium at room temperature (25 °C), at a constant pH (3), being controlled by using a diluted solution of sodium hydroxide NaOH (1M) or nitric acid HNO3 (1M). A pH meter (a EUTECH ph700 model, Thermo Scientific, Waltham, WA, USA) was used to monitor the pH value. The aqueous phase was separated by using centrifugation (with a model TGL-16 C centrifuge, Shanghai Longyue Instrument Equipment Co., Ltd., Shanghai, China) at 13,400 rpm for ten minutes. One milliliter of the supernatant was taken from each polypropylene vial to be analyzed using inductive coupled plasma–mass spectrometry (ICP–MS, Perkin Elmer Sciex Elan 9000, USA) in order to determine the equilibrium concentration of thorium remaining in the solutions after the adsorption processes. The schematic illustration of the batch adsorption experiments is presented in
Figure 2.
All experiments were conducted in triplicate, and only the average results were reported. The difference between the initial and equilibrium concentration of thorium has been used to calculate the amount of the thorium adsorbed on the surface of the adsorbent (qe) by using the following Equation (1):
where Co is the initial concentration of the adsorbate molecules in the solution (mg/L), Ce is the equilibrium concentration of adsorbate molecules remaining in the solution (mg/L), V is the volume of solution (L), m is the mass of absorbent material (g), and qe is the adsorption capacity of adsorbent (mg/g).
The removal efficiency of the adsorbent material can be obtained using the following Equation (2) [
10]: