Investigation of the Segregation of Radiocesium from Contaminated Aqueous Waste Using AMP-PAN Extraction Chromatography

Abstract

Removing the hazardous and unstable radioactive isotopes has been considered an arduous task, though they are in minimal concentrations. Cesium-137 (¹³⁷Cs⁺) is a primary fission product produced by nuclear processes. Even at low concentrations, such radioactive material is a menacing source of contaminants for the environment. The current study aims to separate ¹³⁷Cs⁺ from a real contaminated aqueous solution via an ion exchange mechanism using ammonium molybdophosphate–polyacrylonitrile (AMP-PAN) resin loaded in an extraction chromatographic column that possesses considerable selectivity toward cesium ion (Cs⁺) due to the specific ion exchange between ¹³⁷Cs⁺ and NH⁴⁺. Additionally, the proposed interaction mechanism between ¹³⁷Cs⁺ with APM-PAN resin has been illustrated in this study. The results disclosed that the optimum efficient removal of ¹³⁷Cs⁺ (91.188%) was obtained by the AMP-PAN resin using 2 g·L⁻¹, while the distribution adsorption coefficient (129.359 mL·g⁻¹) was at pH 6. The isothermal adsorption process was testified through the Langmuir and Freundlich models. The estimated maximum adsorption capacity reached 140.81 ± 21.3 mg·g⁻¹ for the Freundlich isotherm adsorption model. Finally, AMP-PAN resin could eliminate ¹³⁷Cs⁺ from water effectively through adsorption.

1. Introduction

Nuclear technology is regarded as one of the most reliable and consistent sources of electrical energy for countries worldwide through electro-nuclear power plants [1]. Additionally, it is vital in various industries, including manufacturing, agriculture, medicine, energy, and fuel. The more benefits supplied by electro-nuclear stations, the more challenges emerge; for instance, the vast amounts of high-level liquid radioactive wastes (HLWs) generated from the nuclear power plants (NPPs) during the reprocessing of spent nuclear fuel at radiochemical enterprises, which necessitates treatment of this radioactive wastewater to eliminate the hazard of isotopes dissolved in wastewater [2].

Radioactive cesium (¹³⁷Cs,¹³⁴Cs), along with radioactive strontium (⁹⁰Sr) and cobalt (⁶⁰Co), is one of the nuclear fission products dissolved in water in NPPs. Because of its long half-life and high fission yield potential, ¹³⁷Cs (half-life: 30.17 y) represent one of the essential hazardous radionuclides involved in radiological damage to humans and the environment. As one of the upwards 30 cesium isotopes, ¹³⁷Cs is a fission product of uranium (²³⁵U) [3,4,5]. The decay of ¹³⁷Cs to the short-lived barium ^137mBa (half-life: 2.61 m) emits beta rays with utmost energies of 512 (94.0%) and 1176 kilo electron volts (keVs) (6.0%), respectively. ^137mBa emits 661 keV gamma rays and transforms into stable ¹³⁷Ba.

There are two major environmental sources of ¹³⁷Cs. The first is from the overall stockpile of 545–765 PBq ¹³⁷Cs from atmospheric nuclear weapon tests from the 1950s to the 1980s [6]. The second source is from nuclear accidents; for example, the 1986 Chernobyl nuclear power plant accident in the former Soviet Union and the 2011 Fukushima Daiichi nuclear power plant accident in Japan [7]. Extensive research has revealed that in addition to the physical radiological properties of cesium and its chemical similarity to potassium (K⁺), cesium is vulnerable to being transferred into the human body via the food chain and replacement for K⁺ during mobility in the biological membranes [8]. Due to the gamma radiation from its daughter ^137mBa, ¹³⁷Cs have serious adverse effects on the human body. Since ¹³⁷Cs are primarily racked up in bone and muscle tissue and they can cause cancer in soft tissue cells. Consequently, eliminating radiocesium is critical, especially after the Chornobyl incident and the Fukushima Daiichi nuclear plant mishaps, in which massive volumes of ¹³⁷Cs were released into the environment [9,10]. Therefore, it is urgent to treat this radioactive isotope for the hazard mentioned above and dispose of wastewater streams generated in many activities in NPPs before releasing it within safe release standards. Numerous researchers have sought various techniques to circumvent and reduce the hazards of heavy metal and radionuclides in recent decades, including evaporation, solvent extraction, chemical precipitation, electrochemical process, membranes, adsorption, and extraction chromatography [11,12,13].

Traditional methods for Cs⁺ separation include chemical precipitation and solvent extraction. Although they hold the advantages of simple processes and wide applicability on a large scale, they generate secondary waste, including solid waste residue and organic extractants, which have an unavoidable harmful impact on the environment [14,15,16]. Šebesta and Štefula [16] found that incorporating ammonium molybdophosphate in a polyacrylonitrile matrix has minimal impact on its uptake kinetics, which remain fast through the AMP-PAN extraction chromatography column. The resin remains chemically stable under extreme conditions with a constant adsorption coefficient, sorption kinetics, and capacity.

Extraction chromatography utilizes the high selectivity of solvent extraction with column chromatography’s simple and multiple features, necessitating simple operation, easier material handling, and potentially lower costs. More notably, extraction chromatography can considerably minimize the volume of highly radioactive solutions and the quantity of solid waste, thereby significantly lowering the harmful effects of radioactive waste on human health and the environment [17]. The most common resin in extraction chromatography is ammonium molybdophosphate (AMP), which has a high adsorption capacity and radioactive stability, particularly with radiocesium. AMP (NH₄)₃P(Mo₃O₁₀)₄·3H₂O) is a hetero-poly salt cation exchanger with NH4⁺ (3.31 Å) associated in the gap of the (P(MO₁₂O₄₀)⁻³ spherical anion structure. AMP has been identified as a perfect Cs⁺ (3.29 Å) adsorbent due to its high adsorption capacity and selectivity. Furthermore, to enhance the flow pattern of packed columns, AMP is embedded in the PAN, leading to rapid kinetics and high levels of radiation stability, with no change in absorption observed for doses up to 1000 kGy [18,19]. Kamenk et al. [19] reported that the composite absorbers of AMP-PAN and KNiFC-PAN were effective in determining cesium concentrations in large seawater samples, making AMP-PAN resin a potential candidate for treating radioactive waste solutions due to its high selectivity for cesium under harsh chemical conditions. Brewer et al. [20] tested AMP-PAN resins for removing ¹³⁷Cs from acidic high-active liquid radioactive waste. After elution with 5-M NH₄NO₃, the Cs recoveries were 87%. The resins were found to be highly resistant to high salt concentrations and could analyze ¹³⁴Cs and ¹³⁷Cs in seawater. In another study, Pike et al. [21] used AMP-PAAN to concentrate and purify Cs from 20-liter seawater samples, with experiments showing a removal rate of 93.5% +/− 5.0% (n = 55).

In the present study, an actual sample was chosen to confirm the presence of ¹³⁷Cs-contaminated wastewater in the tanks of the Tuwaitha nuclear site’s radiochemistry building. The samples were measured and evaluated before and after treatment with AMP-PAN-loaded extraction chromatography columns employing high-purity germanium gamma spectroscopy (HPGe) equipment at the Central Laboratories Directorate (CLD). Furthermore, the extraction chromatographic column method was utilized to examine the effect of the primary feed concentration and pH on the absorption coefficient (K_d) and removal efficiency (R%).

2. Materials and Methods

2.1. Materials and Equipment

The AMP-PAN extraction chromatography column was supplied by European Union (EU) scholarship from the Triskem Company (Bruz, France). It contained 2 mL columns (HC-C50-M) with particle sizes of 100–600 micrometers (µm), a CS capacity of 64 mg/g dry resin, and a density of 0.27 g/mL. A column rack-vacuum box system, part: AR-12-BOX (12-hole box) and vacuum gauge (AR-12-BOX) included a valve for controlling the vacuum.

The inner liner part was AR-12-Liner for the 12-hole box. In experimental tests, the vacuum pump (volt/ frequency: 220 V/50 Hz, discharge volume: 28 L/min) was employed to obtain vacuum pressure. Nitric acid (HNO₃ 63%, M.Wt. = 63.012 g/mol, density = 1.48 g/cm³) and sodium hydroxide (NaOH 99%, M.Wt. = 40 g/mol) were purchased from Riedesel-de Haen AG, Germany. Ammonium chloride (NH4Cl, 99%, M.Wt. = 53.49 g/mol) was supplied by a Loba Chemie company in India and has been used to regenerate the AMP- PAN resin after each run. A pH meter (HQ411d pH/mV) was employed to verify the acidity or alkalinity of the main feed solution. A high-purity germanium gamma spectrometer (HPGe 60%) was supplied from a Canberra company, Toledo, OH, USA, and was utilized to identify the activity of ¹³⁷Cs.

2.2. Sample Pretreatment

Initially, the actual wastewater sample was retrieved from the AL-Tuwaitha nuclear site’s radiochemical tanks and coded (HLLW). Firstly, the HLLW sample was filtered through a 0.45-micron filter before use. Then, the initial activity of ¹³⁷Cs at HLLW was measured by high-purity germanium (HPGe) was 568,200 ± 1353 Bq/L; also, the pH was measured and found to be about 7 at room temperature for the feed solution. Five diluted samples were analyzed under the same conditions, as shown in

Table 1. The initial activity of the ¹³⁷Cs of the diluted samples.

Sample Code	The Initial Activity of 137Cs+ (Bq/L)	The Initial Activity of 40K+ (Bq/L)	pH	Technique
STD-1	46,432 ± 386	3340 ± 265	6.9	HPGe gamma spectroscopy
STD-2	23,040 ± 273	2850 ± 148	7.1
STD-3	12,352 ± 199	1870 ± 121	6.9
STD-4	6785 ± 147	967 ± 87	7.0
STD-5	4001 ± 113	856 ± 65	7.0

, to study the influence of the initial concentration of ¹³⁷Cs on the AMP-PAN extraction chromatography column’s distribution adsorption coefficient (K_d) and removal efficiency (R%).

2.3. Experimental Work

The buffer solution was eluted from the column (HC-C50-M), and 3 M HNO₃ 10 mL was progressively added to each cylindrical column, which ordinarily contains 2 g of AMP-PAN resin, as shown in Figure 1. Then, 25 mL of a ¹³⁷Cs-containing feed solution was put on five chromatographic extraction columns arranged in a vacuum box. A total of 5 mL of the first portion of each sample was discarded, and 20 mL of a permeate-contaminated feed solution was collected in a graduated cylinder tube. The flow rate of each extraction chromatographic column was set to 2 mL/min [22,23], and the contact duration of each sample with cesium absorption was 10 min at room temperature. This method was used to investigate the influence of the initial concentration of cesium on K_d and R%.

The same approach was used to examine the influence of pH on the adsorption coefficient and removal efficiency with the contaminated feed solution created in the pH ranges (4, 6, 7, 9, and 11). After each run, the clean-up procedure in Section 2.4 was used [20].

2.4. Clean-Up Procedure

The clean-up procedure is critical when using adsorption materials loaded into extraction columns to separate radiocesium from contaminated aqueous solutions. During the separation of cesium from the polluted solution, a series of steps were taken to clean up the column. First, the column was washed with distilled water (DW). Then, 5 mL (0.1 M) of HNO₃ was passed through the columns followed by 10 mL of DW for acidity removal.

An alkali solution was also used for the cleaning, where 5 mL of 0.1 M NaOH was used for each column, followed by 10 mL of DW for rinsing. Finally, 10 mL (5 M) of NH4Cl was poured into the columns to reactivate the solid resin and compensate for the absence of ammonium ions during the separation process [20].

2.5. Adsorption Isotherm and Kinetic Model

Ion exchange equilibrium under isothermal equilibrium can be used to characterize the extraction chromatography process. Numerous parameters, such as feed concentration, pH, contact time, resin weight, and sorbent concentration, can influence this equilibrium under experimental conditions at a given temperature. Many theoretical models have been presented to describe equilibrium in solid-fluid systems. Relevant empirical equations to experimental findings were ordinary in early attempts to describe ion exchange equilibrium. These equations are often adapted to the mass action law, Langmuir, or Freundlich adsorption isotherms. Despite being derived from various models and assumptions, most of these equations will produce a satisfactory match to experimental isotherm data if the constants are chosen correctly. The Langmuir and Freundlich isotherm equations were applied to fit the equilibrium data. The Langmuir and Freundlich equations are expressed by Equations (1) and (2) [24]:

$$\frac{1}{qe}=\left(\frac{1}{Kl q_{max}}\right)\times \frac{1}{Ce}+\frac{1}{q_{max}}$$

(1)

$$\log qe=\frac{1}{n}\log Ce+\log K$$

(2)

where K_l, and K_f are the equilibrium constants for the reaction (mg·L⁻¹), q_max is the maximum adsorption capacity (mg·g⁻¹), n is the Freundlich isothermal model constant, and Ce is the equilibrium Cs in the fluid phase. By monitoring the ¹³⁷Cs concentration solution and pH before and after treatment, the distribution coefficient (K_d) of the adsorption selectivity for ¹³⁷Cs in different environments was estimated using Equation (3) [25].

$$K_d=\frac{Co-Ce}{Ce}\times (\frac{v}{m})$$

(3)

where K_d is the distribution adsorption coefficient (mL·g⁻¹), Co and Ce are the initial and final specific activity of ¹³⁷Cs (Bq/L), v is the volume of contaminated feed solution (mL), and m is mass of solid phase AMP-PAN resin (g).

The removal efficiency R (%) for ¹³⁷Cs through AMP-PAN resin was calculated utilizing Equation (4) [12].

$$R\left(\%\right)=\left(1-\frac{Ce}{Co}\right)\times 100$$

(4)

where R (%) is the removal efficacy of ¹³⁷Cs by the extraction chromatography column and Co and Ce are the initial and final specific activity of ¹³⁷Cs (Bq/L).

2.6. Measurement of the Sample

A gamma spectrometry system (high-purity germanium (HPGe), ORTEC company, Bremen, Germany) with 65% relative efficiency with Gamma Vision-32 software version 6 was used in this study to analyze gamma ray spectra, which has a resolution (>1.9) keV based on measurements of (1.332) an MeV gamma ray photo peak of 60 Co. This system consists of a coaxial high-purity germanium detector with an operation-positive voltage of 1500 V. A standard multi-gamma radioactive source was used for energy and efficiency calibration type (CCPS) and has certification No. 9031-OL-505/13, which contains 13 radioisotopes with different gamma rays energies from 59.5 keV, which belong to 241 Am to 1836.08 keV (88 Y) to cover all radionuclides of NORM samples. Gamma Vision software performs reports that include information such as dead time, isotope and its activity of each energy, minimum detection activity (MDA) that depends on the standard deviation of count number where blank sample measure is a background level, and compound relative uncertainty for each radionuclide that depend on many factors such as the efficiency calibration, peak area determination, calibration of equipment, and correction factors.

3. Results and Discussion

3.1. Effect of the Initial Activity of ¹³⁷Cs on R (%) and K_d

Figure 2 demonstrates the influence of the initial activity of ¹³⁷Cs on the removal efficiencies R (%) and distribution coefficient (K_d) using extraction chromatography columns in 25 mL of the different starting activity feed solutions (as explained in Section 2.2 and represented in Table 1). Figure 2 reveals that the removal efficiencies (R (%)) were 90.01%, 89.87%, and 91.188% for STD-5, STD-4, and STD-3, respectively. However, the AMP-PAN beads showed remarkable selectivity for ¹³⁷Cs⁺ adsorption under the same circumstances where the removal efficiencies reached more than 90% at STD-3. Figure 2 shows that the value of K_d varies for five samples due to the ¹³⁷Cs⁺ being in the same group with ⁴⁰K⁺ in the periodic table, and thus the physicochemical properties converge to each other, such as ionic radius and hydrogen energy, so ⁴⁰K⁺ will compete with ¹³⁷Cs⁺ and works to displace the ion exchange between the ¹³⁷Cs⁺ and AMP-PAN resin, causing the observed dissimilarity in the value K_d value for samples ranging from STD-1 to STD-5 [26].

The results obtained in this study on the influence of the initial concentration for ¹³⁷Cs⁺ are consistent with the findings of other researchers [26,27,28]. Liu et al. [26] investigated the mechanism of ¹³⁷Cs⁺ adsorption with K⁺, NH₄⁺, and Rb⁺ ions on sediments from the United States Hanford site. This mechanism explains how high-concentration K, NH₄, and Rb ions (electrolytes) may induce the collapse of “frayed-edge sites” (FES) where high-affinity Cs⁺ exchange is thought to occur. Because their strong ion exchange selectivity produces high mass-action pressure, these specific electrolytes were utilized to induce Cs⁺ desorption. Electrolyte-induced edge collapse, on the other hand, may diminish the apparent “exchangeable fraction” of the sorbed Cs ion. Another study by Ding et al. [27] to remove cesium from an aqueous solution using nickel hexacyanoferrate (NiHCF) via an adsorption method revealed a correlation between the Cs⁺ adsorbed and the K⁺ released during the Cs⁺ adsorption process. The rise in adsorbate dose and initial Cs⁺ concentration leads to an increase in Cs⁺ adsorbed and K⁺ released, exhibiting affinity between them; hence, another conclusion can be made based on Ding’s investigations that K⁺ displaces the Cs⁺ ion with the AMP-PAN resin via adsorption processes causing the variance in the value of K_d. Furthermore, Bouzidi et al. [28] evaluated the sorption behavior of cesium in the ground close to the Es-Salam nuclear facility in Ain Oussera. Sorption followed pseudo-second-order kinetics with an excellent regression coefficient (R² = 0.999). The activation energies were 11.26 and 15.21 kJ mol⁻¹, respectively, corresponding to an ion exchange sorption process. Additionally, it clarifies that the existence of rival cations in groundwater, including potassium K⁺ and calcium Ca²⁺ ions, can drastically diminish Cs⁺ ion adsorption. From the aforementioned, STD-3 was selected to investigate another parameter in this work.

3.2. Effect of pH

The aqueous solution’s pH is a critical parameter influencing adsorption, particularly for monovalent cations, because of competitive adsorption between H⁺ ions and radioactive ions, such as ⁴⁰K⁺ and ¹³⁷Cs⁺. As illustrated in Figure 3, the preliminary pH values have a low ¹³⁷Cs⁺ adsorption ability because the active sites of the AMP/PAN resin adsorbent are mostly neutralized, making them less available for cations (¹³⁷Cs⁺). The existing sites for cations and the adsorption ability of ¹³⁷Cs⁺ by AMP/PAN resin adsorbent increase as pH increases, reaching a maximum at pH 6. Furthermore, it is evident that as pH increases, the adsorption efficiency decreases; this phenomenon can be attributed to the formation of stable ¹³⁷Cs⁺ complexes. As a result, pH 6 is considered the best pH for scientific tests. The removal efficiency R (%) and distribution adsorption (K_d) decreased slightly because the NH₄⁺ was released during adsorption and was then hydrolyzed to produce H⁺. Meanwhile, when the concentration of H⁺ in the solution was high, the hydrolysis reaction of NH₄⁺ was inhibited (low pH). The low distribution coefficient values in an acidic medium are attributed to the replacement of adsorbed ¹³⁷Cs⁺ ions by H⁺ ions, which reduces ¹³⁷Cs⁺ ion adsorption capacity. This finding matches with other investigation studies [29,30,31]. Nilchi and colleagues [30] synthesized AMP-PAN composite adsorption material that has been tested and is being explored for use in removing radiocesium from solutions. Cesium adsorption on a composite adsorbent was investigated using multiple adsorption factors, one of which was pH. The findings demonstrate that pH has an ascending influence on the cesium distribution coefficient, which reaches 7. The produced composite adsorbent dissolves in the aqueous solution in alkali media, reducing adsorption efficiency. Also, the result observed at the acidic test shows that low distribution coefficient values in an acidic medium are related to the displacement of adsorbed Cs⁺ ions by H⁺ ions, which reduces Cs⁺ ion adsorption capability. In another work, Chen et al. [31] employed AMP developed for identifying ¹³⁷Cs⁺ in seawater. The optimal pH range for recovery was shown to be between 4.3 and 5.6, with average recovery rates of ¹³⁷Cs⁺ exceeding 97%. A decreased recovery of ¹³⁷Cs⁺ was also found at pH values of less than 4.3, suggesting that the strong acidity of the solution caused the dissolution of APM particles and the creation of yellow-colored particles during the adsorption and sedimentation process. Based on the preceding, it can be speculated that the present findings converge with previous research, indicating that the effect of high acidity and basicity on the structure of APM-PAN resin and that competition between H⁺ and OH⁻ ions with cesium ions leads to the displacement of ¹³⁷Cs⁺ ions, resulting in low adsorption coefficient values and removal efficiency.

3.3. Isothermal Adsorption

The adsorption capacity of AMP-PAN composition resin loaded in extraction chromatographic columns was calculated using adsorption isotherm models, specifically Langmuir and Freundlich (Equations (1) and (2)). The experiment data for AMP-PAN resin fit the Freundlich model better than Langmuir for the STD-3 sample, as shown in Figure 4a,b. The plot of Ce/Qe against Ce for the Langmuir model fits the liner Equation (1), (Ce/Qe) = 0.000327 + 0.564 × Ce, with R² = 0.767. The Freundlich isotherm model appears to be dominant for AMP-PAN resin, as shown in Figure 4 by the plot of log (Ce/Qe) against Ce for the experiment data, where the liner Equation (2) is log (Qe) = −2.0606 + 1.0257 × log (Ce) with R² = 0.998, which is better than the Langmuir isotherm model. The Freundlich isotherm model fits the ¹³⁷Cs⁺ adsorption on the AMP-PAN resin in extraction chromatography column 3 thoroughly. The results show that the adsorption capacity of ¹³⁷Cs⁺ ions by AMP-PAN resin is appropriate when using the Freundlich isotherm model’s extraction chromatography technique. In general, if the value of n was equal to unity, adsorption was linear; if the value was less than unity, adsorption was chemical; and if the value was greater than unity, adsorption was a favorable physical process [32]. Freundlich exponent n values in the 1–10 range indicated suitable adsorption [32,33,34] through kinetic and isothermal adsorption studies represented by Langmuir and Freundlich’s isothermal adsorption model. The results revealed that the Freundlich (R² = 0.998) model outperformed the other model. Furthermore,

Table 2. Comparison of adsorption capacities of 137Cs+ adsorption on various adsorbents.

Adsorbents	Adsorption Conditions	q Max (mg·g−1)	Adsorption Isothermal	References
Bentonite	pH > 5, t = 24 h, mads = 5 g·L−1, Co = 10 mg·L−1	40	Langmuir	[35]
NaSM zeolite	pH 7, t = 6 h, mads = 10 g·L−1,·Co = 2.28 × 104 Bq·L−1 *	159.4–284	Langmuir	[27]
Copper hexacyanoferrate polyacrylonitrile	pH 9, t = 4.7 h, mads = 10 g·L−1, Co = 10 mg·L−1	7.31–11.46	Freundlich	[30]
PB-Fe3O4	pH 5.5, t = 1 h, mads = 1 g·L−1,·Co = 0.01–0.5 mg·L−1	16.2	Freundlich	[36]
PB/Fe3O4/GO	pH 4–10, t = 2 h, mads = 2.5 g·L−1,·Co = 25–150 mg·L−1	43.52	Pseudo-second-order	[37]
AMP-PAN	pH 2.5–7, t = 24 h, mads = 0.1–0.5 g·L−1, Co = 1–200 mg·L−1	138.9	Langmuir	[38]
PAN-KNiCF	pH 1.7–10.52, t = 24 h, mads = 0.124 g·L−1, Co = 20–250 mg·L−1	157.279	Langmuir	[39]
AMP-PAN	pH 6, t = 10 min, mads = 2 g·L−1, Co = 12,352 Bq·L−1 *	140.81	Freundlich	This work

Not mentioned in the literature: * 1 M Bq·L⁻¹ (137Cs) is equivalent to 9.044 × 10^–4 mg·L⁻¹.

shows that the result of this study was compared with other adsorption materials to select ¹³⁷Cs⁺ from the contaminated solution. According to the Freundlich isothermal adsorption in Figure 4b, the adsorption occurred at relatively low pressures, indicating that the amount of the adsorbed material was small, as suggested by the linear relationship in Figure 4b. This behavior explains the ion exchange between cesium and ammonium on the surface of AMP-PAN, which is shown in the description as a close simulation of the mechanism of exchange on the surface of the adsorbent at a small weight quantity. Thus, it is suitable for the large process of applications in terms of high adsorption efficiency at a low cost and low pressure.

3.4. Regeneration

In the present study, the AMP-PAN composite material loaded in the extraction chromatography column technique was reused for five adsorption–desorption cycles, as shown in Figure 5. As the cycle number increased, the removal efficiency significantly decreased. After five cycles, the removal efficiency of 137Cs⁺ for the AMP-PAN composite material in the STD-3 sample ranged from 86.31% to 66.15% at a 15 min regeneration time. Furthermore, the reactivation time was examined for 30 min. The influence on removal efficiency altered significantly with the first test. As a result, the AMP-PAN resin has a high regeneration capacity; this finding matches other investigation studies by Ding et al. [38].

There was no significant decline in removal efficiency among the ten cycles studied. The AMP-PAN composite membrane could eliminate around 90% Cs⁺ from the aqueous solution within ~70 s under low pressure. Moreover, to test the AMP-PAN membrane’s reusability, 10 successive filtration cycles (100 mL/cycle) were conducted, and the removal efficiencies were measured. The results showed no significant decrease in the removal efficiency among the 10 cycles, indicating the high capacity of the membrane. Moreover, the filtration duration remained relatively stable among the 10 cycles under low pressure. The match between the recycling process and the results obtained from the previous investigation was attained by applying low pressure. During the adsorption process utilizing a vacuum box and extraction chromatography column in this present work, the applied pressure reached 0.00167 MPa (0.5 inHg), while the applied pressure on the composite membrane reached 0.03 MPa. These circumstances led to obtaining the most prolonged period for the ionic exchange between NH⁴⁺ ions (APM-PAN resin) and Cs⁺ ions in the solution during all adsorption processes. Therefore, a significant decline in the removal efficiency value was not observed during the five cycles of resin regeneration.

3.5. Ion Exchange Mechanism

If an ion exchange reaction primarily caused adsorption, the number of released cations (in gram equivalent) would be close to the adsorbed target [27]. The relationship between the ¹³⁷Cs⁺ adsorbed and the NH₄⁺ released during the ¹³⁷Cs⁺ adsorption process using an AMP-PAN extraction chromatography column is shown in Figure 6. The results showed that NH₄⁺ was released much more than ¹³⁷Cs⁺ was adsorbed and was most liable due to AMP-PAN resin dissolution. Furthermore, a close relationship between ¹³⁷Cs⁺ adsorbed and NH₄⁺ released was observed at all contact times except 10 min, indicating that the dissolution rate of AMP at 10 min was the highest, with R (%) reaching 91.188% and K_d being 129.36 mL·g⁻¹ for the STD-3 sample.

4. Conclusions

The specific removal of ¹³⁷Cs⁺ from aqueous solutions was investigated using the AMP-PAN resin extraction chromatography technique. The European Union supplied the AMP-PAN resin as a promising material to examine the possibility of radiocesium segregation from a contaminated aqueous solution at the AL-Tuwaitha nuclear site for the first time.

The present study showed that the AMP-PAN beads (with particle sizes ranging from 100 to 600 μm) had a high predicted maximum adsorption capacity (as high as 91.118% removal efficiency and distribution adsorption of 129.36 mL·L⁻¹). Additionally, using a fast method, the AMP-PAN resin could effectively remove ¹³⁷Cs⁺ quickly and perform well in reusability.

The effect of pH on removal efficiency (R% = 91.73) and distribution adsorption coefficient (K_d = 138.768 mg·L⁻¹) was evident, with practical experiments yielding the best results at pH 6. In contrast, there is a dramatic reduction in R (%) and K_d when the acidity and alkalinity are high.

This study shed light on using chromatographic column separation technology in the form of a pilot project for contaminated wastewater treated from chromatographic separation columns as a preliminary step, followed by a reverse osmosis system as a final stage, to significantly reduce the risks of cesium in nuclear research facilities and ensure that access to clean wastewater can be launched without restriction. Furthermore, experts and researchers in this field can be encouraged to test composite materials made (highly efficient) from green and eco-friendly materials in place of commonly used manufactured materials, like AMP-PAN, at high costs, lowering the project’s overall cost.