Removal of Radio and Stable Isotopes of Cobalt and Cesium from Contaminated Aqueous Solutions by Isatin-Derived Ligand

Abstract

This study investigated the effectiveness of a ligand known as (2-Mercapyo-phenylimino)-1,3-dihydro-indol-2-one-based ligand, in removing stable/radioactive cesium and cobalt from contaminated wastewater. Several parameters, such as contact duration, temperature, adsorbent quantity, pH of the medium, and concentration of adsorbate, have been investigated as primary active parameters impacting the adsorption process. Regarding the stable isotopes, the concentrations of Co²⁺ and Cs⁺ were measured before and after the treatment processes using the Optical Emissions Spectroscopy with Inductively Coupled Plasma (ICP-OES) technique. Additionally, kinetic and equilibrium isotherm models were applied to understand the equilibrium data. Both Cs⁺ and Co²⁺ were ideally eliminated after 120 and 60 min, respectively. The optimal pH for Cs⁺ was 6.3, while that for Co²⁺ was 5. The results indicate that the adsorption process is endothermic for Co²⁺ and exothermic for Cs⁺. Three thermodynamic parameters (∆G°, ∆H°, and ∆S°) were calculated. The reported R² values for the Freundlich and Langmuir models showed that the adsorption process for Cs⁺ and Co²⁺ always followed these isotherms, regardless of the temperature used. For Cs⁺, the maximum single-layer capacity (q_max) was 15.10 mg g⁻¹, while for Co²⁺, it was 62.11 mg g⁻¹. When the aqueous medium was spiked with both radioisotopes individually, the elimination of ⁶⁰Co and ¹³⁴Cs achieved maximum values of 99 and 86%, respectively, within 120 min. It can be concluded that the ligand effectively removed cobalt and cesium from wastewater, with higher adsorption for cobalt.

1. Introduction

The widespread presence of heavy metal ions in water sources is a major environmental concern that endangers all living things. This is because of their natural tendency to accumulate in biological tissues and their resistance to degradation [1,2]. One of the main causes of water contamination is the presence of heavy metals in IMEW, or industrial and municipal effluent water. The primary concern associated with IMEW is its contamination and deterioration in quality, particularly in areas with limited water resources [3]. Adsorption has been known to humans for a long time. Purification and separation are two of the most common methods for reducing water contamination by heavy metals. When a material with pores draws a multi-component fluid (gas or liquid) to its surface through physical or chemical interactions, the process is called adsorption. A high micropore volume in a solid porous material increases its adsorptive capacity. Adsorbate molecules must cross the microspore volume of the porous solid medium, which has small pores. It is imperative that the adsorbent possesses excellent adsorption capacity and kinetics, as adsorption efficiency is dependent on these properties. Materials that can adsorb substances with very small pores, moderate porosity, large microspore volume, and extensive pore networking allow molecules to access the interior, aiding adsorption [4].

Therefore, developing highly effective techniques to treat water contaminated with heavy metals, particularly industrial effluents, has become a critical priority for safeguarding aquatic ecosystems. Heavy metal species in water can be classified into two main categories based on their coordination structure: the unstable state and the stable state. When weak ligands like H₂O and Cl⁻ are present, the coordination structures of heavy metal ions are weakened, which is why they are so susceptible to environmental influences. Thus, these volatile heavy metals can be easily removed using conventional methods such as ion exchange, adsorption, and alkaline precipitation [5]. Radioactive and toxic waste containing metals is accumulating at an alarming rate due to the proliferation of nuclear power and conventional industries. The increasing discharge of industrial effluents, proliferation of human habitation, persistence of urbanization, use of technology, and other human-caused phenomena frequently lead to the contamination of aquatic streams. Because of the adverse effects on human health caused by the accumulation of dangerous components in the food chain, the elimination of these pollutants has been examined and assessed using more effective and advanced methods [6].

The adsorption process using porous materials is widely regarded as a superior method for water purification due to its cost-effectiveness, operational simplicity, design simplicity, straightforward design, environmentally friendly characteristics, and high efficiency [7,8]. Hence, it is necessary to remove them from the environment in order to reduce their negative impacts and maintain their concentration levels that are safe for living organisms. To achieve this objective, numerous metal treatment techniques have been thoroughly studied, including electrodeposition, ion exchange, chemical precipitation, coagulation, adsorption, and membrane filtering. For wastewaters with low concentrations of metals, sorption is frequently considered the most cost-effective alternative treatment option [9]. Hence, the efficient elimination of this hazardous metal from wastewater or polluted water is of utmost significance and constitutes a critical concern from environmental, health, and economic perspectives. Consequently, it is essential to discover methods to eliminate it [10].

Adsorption technology has been identified as the best option after a thorough evaluation due to its high efficiency, low operating costs, and ease of use [11]. Additionally, adsorbent materials that successfully remove metal ions from polluted water have been produced using chelating agents [12].

Radionuclide removal from water streams is crucial for removing metal ions from wastewater for chemical analysis or lowering metal concentrations to values below toxicity thresholds. Chemists around the world are interested in selectively removing Cs⁺ and Co²⁺ from solutions, particularly at low concentrations, due to their significance in treating radioactive liquid effluents [13], then immobilize the collected radiocontaminants in cementitious materials [14,15,16,17].

Cesium and cobalt have been effectively removed through adsorption, which can be achieved using either natural or synthetic materials. As traditional adsorbents, ion exchangers have limitations when it comes to dealing with liquid radioactive waste since they compete with other monovalent cations like potassium and sodium. Cesium and other divalent cations that compete for cobalt adsorption sites may have their efforts thwarted by this competition. The development of new, environmentally friendly materials that are highly effective at removing stable or radioactive pollutants from wastewater is a pressing need due to the increasing demand for cation exchange sites for metal adsorption [18]. The adsorption of Cs⁺ ions in inner-sphere coordination near the smectite surface is favored because of the low hydration energy of Cs⁺ ions, and their large ionic radius [19].

It is imperative to identify a procedure that can adequately mitigate this type of pollution. Numerous techniques, such as adsorption, have been developed to remove these harmful ions from solutions containing water, chemical reduction, microfiltration, ion exchange, and chemical precipitation. Evidence suggests that adsorption, which employs environmentally friendly materials and lowers costs, is an effective method for ion removal [20].

Among the various adsorbents studied, isatin-derived ligands have demonstrated a high affinity for metal ions due to their unique chelating functional groups and stable coordination structures. The presence of carbonyl and amine groups allows for strong binding interactions with cobalt and cesium ions, enhancing their selectivity and adsorption efficiency. Isatin and its metal complexes are extremely successful in biological and pharmacological applications and are highly sought after in the medical industry. Isatin metal complexes are effective chelating agents because they are simple to synthesize, versatile, and exhibit the distinctive action of the -C=N (azomethine) moiety. They may also form chelates with a range of metal ions and change the ligation features by adjusting basicity and density. In the current study, unlike conventional adsorbents such as activated carbon and zeolites, isatin-derived ligands offer enhanced selectivity for Co²⁺ and Cs⁺ due to their specific coordination chemistry. Their high adsorption capacity, chemical stability, and ease of regeneration make them promising candidates for industrial wastewater treatment. This study provides new insights into the adsorption mechanism and highlights the potential of isatin-based materials for addressing heavy metal contamination in challenging industrial effluents.

2. Experimental Approach

2.1. Materials

2.1.1. Adsorptive Materials

All metal salts used were obtained from Sigma-Aldrich (St. Louis, MO, USA), and the Schiff base ligand was previously prepared and crystallized using spectroscopic techniques. The novel 3-(2-mercaptophenylimino)-1,3-dihydro-indol-2-one-based ligand was investigated by thermogravimetric analysis (TGA) using a Rheometric Scientific STA 1500 instrument from Rheometric Scientific, Inc., Piscataway, New Jersey, United States to determine the water content and thermal stability. After the adsorption procedure, the remaining concentrations of Co²⁺ and Cs⁺ in the solution were determined using ICP-OES, inductively coupled plasma optical emission spectroscopy (Prodigy High Dispersion ICP, Leeman in the United States).

2.1.2. Preparation of Adsorbent Ligand (HL)

A 20 mL solution of isatin (2.942 g, 0.02 mol) in ethanol was heated with 3-aminothiophenol (3.207 g, 0.02 mol) to produce the Schiff base ligand HL. The mixture was then subjected to a flux bath for four hours to complete the reaction, as shown in Figure 1. An 87% yield of the orange ligand was achieved by collecting the precipitate after chilling it via filtration, processing it through a series of alcohol and ether washes to remove impurities, followed by drying in a desiccator over anhydrous CaCl₂. The experimental procedure for synthesizing the adsorbent ligand (HL) is described in detail in a recent study by the authors [21].

2.1.3. Adsorbate Elements

High-purity analytical-grade cesium and cobalt salts were obtained from Sigma-Aldrich. The concentrations of the stable elements were diluted to a range of 5–100 mg/L to obtain suitable values for subsequent tests. Furthermore, to adjust the solutions to various pH values, the acidity or alkalinity was modified by adding NaOH and 0.1 N HCl in a stepwise manner. The radioactive isotopes ¹³⁴Cs and ⁶⁰Co were produced by the Egyptian Atomic Energy Authority in Egypt.

2.2. Experimentation

2.2.1. Analyses of Adsorption

All experiments were performed in a bathwater solution with a constant shaking speed to examine the impact of temperature, pH value, adsorbent weight, concentration of Cs⁺ or Co²⁺, and contact time as shown in Figure 2.

Under equilibrium conditions, in proportion to the number of adsorbed ions that have been eliminated, the quantity of ligand (HL) was determined using Equation (1) [22]. The Langmuir isotherms under equilibrium conditions were expressed using the Hanes-Woolf equation, as shown in Equation (2) [23].

$$qₑ=(C₀-Cₑ)\times {\displaystyle \frac{V}{m}}$$

(1)

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{K_L\times q_{max}}}+{\displaystyle \frac{C_e}{q_{max}}}$$

(2)

An alternative absorption model, the Freundlich adsorption model, was employed (Equation (3)). According to Equation (3), this model is valuable in demonstrating that the adsorption energy value on a uniform surface is not influenced by the extent of the surface coverage.

$$qₑ=K_f\times C_e^{1/n}$$

(3)

C₀ is the initial ion concentration, C_e is the equilibrium concentration, m is the adsorbent mass, V is the solution volume, and q_e is the quantity of ions adsorbed on the ligand (HL).

The Langmuir constant (K_L) that relates to the adsorption energy value, and the maximum number of ions adsorbed on the surface of the ligand (q_max) are both utilized. We calculated q_max and K_L by analyzing the slope and crossing intercept of the plot of C_e/q_e versus C_e, respectively.

Where K_f is the Freudenberg model constant (mg/g), n is the adsorption intensity, C_e is the remaining concentration (mg/L at equilibrium), and q_e is the amount of ions adsorbed onto the ligand (HL) at equilibrium [24].

Contact Time

This experiment aimed to determine the influence of time on the adsorption of Cs⁺ or Co²⁺ ions (50 mg/L) onto the ligand and its metal complexes (0.1 g). A constant temperature of 25 °C, pH value of 7, and shaking speed of 100 rpm were utilized to conduct studies for a range of two-hour durations (15, 30, 45, 60, 90, and 120 min). The concentration of non-adsorbed ions in 1 mL of the transparent solution was measured at each contact period using an atomic absorption spectrophotometer.

pH Effect

After 60 min, 0.1 g of the complexes (ligand and its metal complexes) were extracted to evaluate the elimination efficiency of both ions at a concentration of 50 mg/L across a range of pH values from 2 to 8. The elimination efficiency of both ions (50 mg/L concentration) at different pH levels (2 to 8) was assessed by extracting 0.1 g of complexes (ligand and its metal complexes) after 60 min.

Adsorbent Dose

Six bottles containing 100 mL of Cs⁺ or Co²⁺ were mixed with seven different weights of dry ligand (HL) at a temperature of 25 °C: 0.025, 0.05, 0.07, 0.1, 0.15, 0.2, and 0.3 g. We used a 100 rpm shaker to agitate the bottles, with a pH value of 7 for Cs⁺ and 6 for Co²⁺, for a duration of 60 min. Next, a volume of 1 mL was taken from each bottle and examined to ascertain the residual concentration of each component remaining following the completion of the process.

2.2.2. Optical Emissions Spectroscopy with Inductively Coupled Plasma (ICP-OES)

Using the Prodigy High Dispersion ICP apparatus from Leman in the US, the initial concentrations of Co and Cs, along with the concentration of the remaining metal in the solution, were calculated using inductively coupled plasma optical emission spectroscopy (ICP-OES). The apparatus used nebulization to transform liquid into an aerosol. Following this, the aerosol sample was transported to the plasma and subjected to several processes, such as desolvation, vaporization, atomization, excitation, and ionization. Using a gadget, the spectral radiation that energized atoms and ions produced was collected and organized by wavelength [25].

The control of analytical reagent blank levels, instrument sensitivity, memory effects, interferences in the spectrum, cleanliness of the digesting vessel, and other factors were largely considered while determining the limit of detection (LOD). Determining the lowest quantities that are reliably detectable and quantifiable is within reach.

To determine the Limits of Detection (LODs) for each element, we multiplied the multi-run reagent blank solutions (3% v/v HNO₃, including internal standard pulses) by dividing the standard deviation of the ion counts by three times the concentration [26]. Here are some aspects of ICP-OES: Three different views (radial, axial, and dual) are at your disposal; ranging from 165 to 1100 nanometers in wavelength; precision less than 0.008 nm; Superb elemental ratio readings with a relative standard deviation of only 0.01–0.02 percent.

2.2.3. Adsorption Isotherms and Data Analysis

For contact lengths of 45 and 90 min, respectively, the adsorption isotherms for Cs and Co ions were estimated using different temperatures, constant pH, and shaking speed (pH = 7, 100 rpm). Various beginning concentrations (Co) of Cs or Co ions were added to 100 mL of 100 mL HL at temperatures of 25, 35, 45, and 55 degrees Celsius, respectively, until equilibrium was reached. The initial concentrations were 10, 25, 50, 100, and 150 mg/L. Using atomic absorption equipment, a 1 mL portion of the remaining clear solution was examined to identify the ions that were not adsorbed under different conditions.

Kinetic studies in pseudo-first-order, pseudo-second-order, and intra-particular diffusion were conducted using batch data from different contact durations and initial metal ion concentrations.

Ion adsorption potentials were also studied by employing several isothermal absorption models, such as the “Langmuir” and “Freundlich” methods.

2.2.4. Adsorption of Radioisotopes (⁶⁰Co or ¹³⁴Cs)

Various starting activity concentrations of ¹³⁴Cs (1400 to 4350 Bq) and ⁶⁰Co (3800 to 11,000 Bq) were used in 50 mL solutions to monitor the absorption processes at a constant room temperature. The experiment relied on a range of adsorbent concentrations (0.05, 0.07, 0.1, and 0.15 g) employed as ligands (HL) in conjunction with ⁶⁰Co and ¹³⁴Cs. The conditions included a 120 min contact duration, uniform shaking at 100 rpm, and a constant pH value of 7. A multichannel analyzer with a NaI detector, manufactured in Europe by PCAP, was used to regularly measure the radioactivity level of the residual solution in order to evaluate the elimination of radionuclides.

2.2.5. Statistical Analysis

Data are presented as means ± standard error of the mean (SEM) and regression models using the statistical tool SPSS (IBM SPSS Statistics for Windows, Ver. 19.0 Armonk, NY, USA: IBM Corp.). When comparing several groups, we utilized multivariate ANOVA and the Least Significant Difference (LSD) test. When the alpha probability was less than 0.05, we concluded that there were statistically significant differences.

3. Results and Discussions

3.1. Characterization of the Synthesized Materials

Information about the infrared spectra of the ligand HLwas obtained from previously published works. To determine which coordination sites may be involved in chelation, the complex infrared spectra were compared to those of the free ligand’s (see Figure 3). The spectra of the ligands exhibited several significant peaks that served as useful indicators for achieving this objective. Anticipated chelation is likely to induce changes in the location and/or magnitude of these peaks.

The infrared spectra of the ligands show a distinct peak at 1625 cm⁻¹, attributed to the v (C=N) frequency of the azomethine group. In all the metal complexes, this band was shifted to a range of 1687–1675 cm⁻¹, with a shift of 50–62 cm⁻¹. This shift suggests that the azomethine nitrogen coordinates with the metal atom [27]. Another notable distinction is the shift in the characteristic vibrations associated with the (C=O) unit of the isatin moiety from 1705 to 1755–1778 cm⁻¹. It appears that the oxygen atoms in the ligand HL, were used to coordinate the metal ions, as this change was very noticeable [28].

In the complexes, the band at 2831 cm^−1, which corresponds to the (S-H) bond, was weaker than that in the ligand spectra, indicating that the thiol sulfur was involved in its deprotonated form. The displacement of the symmetric bands to 742–715 cm⁻¹ and the asymmetric bands to 820–806 cm⁻¹ provides further evidence that the thiol sulfur atom is involved in the formation of the complexes [29].

Infrared spectra of the ligand HL exhibited a prominent and distinct band at 3309 cm⁻¹, attributed to the NH moiety. This band remained unchanged in the complexes, suggesting that NH did not participate in coordination. A band between 3502 and 3448 cm⁻¹ was observed, confirming the presence of coordinated water molecules in the complexes.

As shown in Figure 4, there are two distinct phases in the breakdown of the ligand (HL) in the TGA data, which occur between 260 and 561 °C. In the first stage, (2C₄H₄, CN, SO) molecules are lost at temperatures between 260 and 289 °C, with a mass loss of around 70.78% (calculated 70.38%). The second one involves the mass loss of (C₂H₂,1/2N₂) molecules between 535 and 561 °C, which is estimated to be 15.67% (calculated to be 15.72%). remains after calculating the total weight reduction, which is 13.54% (or 14.01%).

3.2. Contact Time

At the maximum adsorption value, the equilibrium point was largely determined by the contact period. When evaluating different adsorption concentrations, the length of time required to achieve the highest removal percentage is known as the optimal contact time [30]. Under controlled conditions of constant pH (7) and temperature (25 °C), the adsorption capabilities of both ions (Cs⁺ and Co²⁺) on the complexes were investigated. Figure 5 shows that initially, the rate of adsorption increases with increasing contact length. The ions that have been adsorbed form aggregates, which allow them to slowly but surely reach higher energy sites within the adsorbent structure. After this point, the prices will remain the same. By reducing the diffusion of aggregated ions in the adsorbents, the pore filling process diminishes the impact of contact time [31]. However, after 120 min and 60 min, respectively, the maximal sorption capacity was reached, leading to an enhanced elimination efficiency. For the sorption of Cs+, the optimal period was 120 min of contact, while for the sorption of Co²⁺, it was 60 min. Sorption rates are directly proportional to the biosorbent mass, since an increase in the biosorbent mass results in an increase in the number of sorption sites.

Both pseudo-first-order and pseudo-second-order kinetic models, as illustrated in Equations (4) and (5), were built using the computed adsorption/time data.

$$\log \left(q_e-q_t\right)=\log \left(q_e\right)-{\displaystyle \frac{k_1}{2.303}}t$$

(4)

$${\displaystyle \frac{t}{q_e}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(5)

If the pseudo-first-order rate constant of adsorption is k_l (min⁻¹), then the number of ions adsorbed at equilibrium is qe (mg/g) and the number adsorbed at time t is qt (mg/g). K₂ (g mg⁻¹ min⁻¹) is the empirical value for the pseudo-second-order adsorption rate constant.

The appropriate model was assessed by computing the straight-line correction coefficient (R²). Figure 6a shows that compared to the pseudo-first-order model, the pseudo-second-order model had an identical regression coefficient (R²). As a result, pseudo-second-order events involving the adsorption of Cs⁺ ions were identical to pseudo-first-order ones. Additionally, it appears that the adsorption of Co²⁺ ions processes are identical to the second-order model than the first-order model, as indicated by the greater value of the regression coefficient (R²) for the pseudo-second-order model in Figure 6b. [32].

(a)

Cs⁺ adsorption; the pseudo-first-order kinetic model, showing a linear relationship with a correlation coefficient (R²) of 0.9691, indicating good fitting to the model but suggesting that the adsorption rate is relatively slow. The pseudo-second-order kinetic model, which exhibits an excellent fit with R² = 0.9899, suggests that Cs⁺ adsorption follows a chemisorption mechanism dominated by valence forces or electron sharing.

(b)

Co²⁺ adsorption; the pseudo-first-order model with an R² value of 0.9884, indicating reasonable fitting but showing a negative slope, which might reflect a desorption tendency or low interaction strength. The pseudo-second-order model, showing a perfect linear fit (R² = 1), suggests that Co²⁺ adsorption is highly consistent with a chemisorption mechanism, indicating strong binding between the ligand and Co²⁺ ions. These results confirm that the adsorption of both Cs⁺ and Co²⁺ is better described by the pseudo-second-order kinetic model, implying that chemical interactions are the primary driving force for metal ion uptake.

3.3. pH Effect

A critical parameter for adsorption processes is pH, as H⁺ ions impact the sorption of Cs⁺ and Co²⁺ ions in the complexes. [33]. Figure 7 shows that between 2 and 8, the adsorbent surface characteristics and the relative distribution of ions in the solution are significantly affected by pH. The adsorption of the complexes was greatly influenced by the various solution pH values; it was found to be practically constant for the Co solution and to rise sharply from 3 to 5 pH. After reaching a maximum at pH 7, the Cs solution elimination capability rapidly declined at pH levels higher than 7.0. Lower pH levels hinder the adsorption of Cs⁺ and Co²⁺ ions due to competition for the limited number of adsorption sites, which explains the observed behavior. Some heavy elements are more soluble in water under low pH conditions, which improves their dispersion. Heavy metal migration across the liquid and solid phases is highly dependent on the optimal pH [34]. Increasing the pH of a solution is another factor that could improve metal ion adsorption since it makes it easier to precipitate metal ions as hydroxides [35]. Experiments at different pH levels revealed that the sorption process was most effective at pH 6.3 for Cs⁺ and pH 5 for Co²⁺.

3.4. Dosage Weight of Dried Ligand (HL)

Economic considerations necessitate the use of complex chemicals and the execution of several tests, thereby increasing the overall cost. Consequently, it is necessary to incorporate economic cost with efficiency in order to effectively eliminate pollutants. A comprehensive grasp of how the dosage of an adsorbent impact’s adsorption is crucial for achieving optimal adsorbent utilization. Figure 8 illustrates the batch experiments conducted to determine the most effective adsorbent dosage by varying the quantities of ligand (HL). Co and Cs exhibited a positive correlation between the adsorbent dosage and adsorption capacity (q_e), which remained consistently high. Undoubtedly, the number of adsorption sites and surface area can be increased by increasing the dosage of the adsorbent. Therefore, the increase in active sites is proportional to the improvement in clearance with increasing dosage [36].

The decrease in removal at a high dosage could be attributed to the aggregation of adsorbent particles, and the subsequent continuous removal serves as an indication of adsorption equilibrium [37].

3.5. Comparison of the Study Findings with Other Similar Published Work

The adsorption process and its many adsorbent types have been the subject of substantial scholarly investigation regarding their environmental and financial impacts. In order to determine their adsorption capability, the adsorbents listed in

Table 1. Comparison of the adsorption capabilities of ligand (HL) and other previously studied adsorbents.

Material	Adsorption Capacity (qe) of Co, (mg/g)	Literature
Kaolinite	0.92	[38]
Soil	1.50	[39]
Marine bacterium	4.38	[40]
Nedalco sludge	11.71	[40]
Eerbeek sludge	12.34	[41]
Coir pith	12.82	[42]
Brown seaweed	20.63	[43]
Myriophyllum spicatum	43.40	[44]
Mixed waste	37.45	[18]
Complexes	62.11	Current study
Material	Adsorption Capacity (qe) of Cs, (mg/g)	Literature
Ceiling tiles	0.21	[45]
Coal and chitosan	3.00	[46]
Bure mudrock	13.30	[47]
Modified akadama clay	16.10	[48]
Kaolinite clay	17.10	[49]
Coir pith	32.00	[50]
Bentonites	92.3	[51]
Myriophyllum spicatum	58.00	[44]
Mixed waste	48.30	[18]
Complexes	15.10	Current study

were compared to the ligand (HL) that was used in the study for each of the two ions.

The adsorption capacities vary across different studies according to various factors, such as the material’s surface area, porosity, functional groups, pH, temperature, and metal ion competition. Analyzing these parameters would provide a deeper understanding of the adsorption mechanisms and material performance.

3.6. Adsorption Isotherm at Different Temperatures

The results show that at 55 °C and 35 °C, respectively, the adsorption behavior reached its maximum uptake values for Cs⁺ and Co²⁺, as shown in Figure 9. A larger radius (Cs) ion has a competitive advantage over a smaller one (Co) because the cation’s interaction energies with water molecules across layers and with the charges on the biomass layer’s surface are different [52]. There is some evidence from the Cs experiment that the cation diffusion coefficient changes with temperature.

$${\displaystyle \frac{1}{q_e}}={\displaystyle \frac{1}{k_L{q}_{max}{C}_e}}+{\displaystyle \frac{1}{q_{max}}}$$

(6)

If we choose q_e to be the uptake at equilibrium concentration (in mg/g), and C_e to be the concentration at which C_e is present (in mg/L), and q_max to be the maximum number of ions needed to form a monolayer (in mg/g), then the equilibrium data were analyzed using the linearized Langmuir adsorption isotherm, as shown in Figure 10.

Table 2. Value of Langmuir parameters for adsorption of both ions separately on ligand (HL).

Temp.	Cs+				Co2+
	qe	qmax	kL	R2	qe	qmax	kL	R2
25 °C	59	9.36	0.019	0.987	40.066	30.1120	0.037	0.908
35 °C	57.5	7.48	0.017	0.963	134	62.118	0.046	0.937
45 °C	56.5	7.62	0.023	0.982	142.8	43.859	0.160	0.868
55 °C	61	15.10	0.012	0.979	142.224	24.096	0.159	0.868

displays the Langmuir constants (K_L) and monolayer sorption capacity (q_max) that were computed using the slope and intercept of the curve that passed through 1/q_e and 1/C_e. Langmuir plots yielded quantum maximum (q_max) values that disagree with the experimental data [53].

An empirical equation used to define heterogeneous schemes is the Freundlich isotherm. The “Freundlich” equation is represented as

$$ln{q}_e=ln{K}_f+{\displaystyle \frac{1}{n}}ln{C}_e$$

(7)

Equation (7) provides the linear formulation of the Freundlich equation, with K_f and n being the Freundlich constants. The degree of favorable adsorption progression is represented by n, and the adsorption capacity is K_f (mg/g (L/mg) 1/n). A positive value for the exponent, 1/n, indicates a favorable adsorption environment for the adsorbate. If n is greater than 1, the adsorption conditions are favorable.

Table 3. Value of Freundlich parameters for adsorption of both ions separately on ligand (HL).

Temp.	Cs+			Co2+
	n	Kf	R2	n	Kf	R2
25 °C	0.702	0.005	0.995	0.985	0.695	0.9125
35 °C	0.692	0.007	0.984	0.726	7.450	0.96
45 °C	0.64	0.003	0.977	0.671	60.140	0.95
55 °C	0.711	0.004	0.974	0.517	17.926	0.948

shows the calculated K_f and n values from the curves in Figure 11, using their intercept and slope, respectively. The R² values were the same for both the Freundlich and Langmuir isotherms, suggesting that the adsorption processes for Co²⁺ and Cs⁺ ions followed a similar behavior at all temperatures [54].

Processing kf values at varying temperatures in accordance with the van’t Hoff Equation (8) allowed us to obtain the thermodynamic characteristics of the adsorption reaction:

$$\ln \left({\displaystyle \frac{K_2}{K_1}}\right)=-{\displaystyle \frac{∆H^0}{R}}({\displaystyle \frac{1}{T_2}}-{\displaystyle \frac{1}{T_1}})$$

(8)

where R is the gas constant, and the changes in enthalpy and entropy are denoted as ∆H⁰ (KJ.mol⁻¹) and ∆S⁰ (KJ.mol.k⁻¹), respectively. Figure 12 shows that when graphing k_f versus 1/T, a linear relationship with a slope of −∆H⁰/R and an intercept of ∆S⁰/R is formed.

Table 4. Thermodynamic parameters of adsorption of both ions on ligand (HL).

Element	∆S°	∆H°	T (K)	T∆S°	∆G°
Co2+			298	43.020	−8.771
	0.144	51.814	308	44.46	7.884
Endothermic			318	45.90	4.845
			328	47.351	5.014
Cs+	−60.829381	−8.467809	298	−18.1272	9.819
Exothermic			308	−18.7354	10.433
			318	−19.3437	9.9732
			328	−19.952	12.0610

shows the computed values of ∆H⁰ and ∆S⁰ for the samples.

The endothermic adsorption of Co²⁺ ions is indicated by positive values of ∆H⁰, whereas the exothermic adsorption of Cs⁺ is indicated by negative values. Equation (9) was used to estimate the Gibbs free energy of adsorption, as shown in Table 4.

As shown in Table 4, there was minimal variation in T∆S⁰ at all temperatures for Co²⁺ ions, and T∆S⁰ < ∆H⁰. These results suggest that enthalpy, rather than entropy, is the primary factor influencing adsorption. The positive ∆G⁰ values discovered provide support for the prior theory’s postulation that the adsorption process is not spontaneous [55]. The fact that T∆S⁰ > ∆H⁰ suggests that the adsorption process for Cs⁺ is defined by an entropic alteration instead of an enthalpic one. The positive value of ∆G⁰ indicates that the Cs⁺ ion adsorption process was not spontaneous.

$$∆G^o=∆H^o-T∆S^o$$

(9)

3.7. Adsorption of Radioisotopes (⁶⁰Co and ¹³⁴Cs) on Ligand (HL)

Figure 13 shows the adsorption of radioactive cesium and cobalt. At equilibrium, the efficacy of removal is constant, but it increased with time. After 120 min, the elimination rates of ⁶⁰Co using various weights of ligand-based (0.05, 0.07, 0.1, and 0.15 g) were 94%, 95%, 97%, and 99%, respectively. The elimination rate of ¹³⁴Cs after 120 min with varying weights of ligand-based (0.05, 0.07, 0.1, and 0.15 g) ranged from 77% to 86%. Since radioactive and stable elements are chemically indistinguishable, the adsorption pattern observed in this study is consistent with that of a previous study on radioisotopes [56]. Because cesium and cobalt have intrinsically different radii, the biosorbent surface experiences slower diffusive mobility of ¹³⁴Cs from the solution to its surface in aqueous conditions compared to ⁶⁰Co [57].

It could be stated from the current study and previous studies that the higher adsorption capacity for Co²⁺ over Cs⁺ could be due to many reasons such as a small ionic radius of Co²⁺ that enhance move through pores more easily than larger Cs⁺, high charge density of Co²⁺ that strongly impose its electrostatic interactions and binding affinity, achieve more stable bonds with adsorption sites due to stronger inner-sphere complexes of Co²⁺ or the high mobility of Co²⁺ that accelerate the diffusion and adsorption kinetics [18,44].

4. Conclusions

This work presents a comprehensive study of the elimination of Co²⁺ and Cs⁺ ions using the ligand (HL) as an effective adsorbent. The adsorption capacities reached 96% and 99% for ligand quantities of 0.2 g and 0.3 g, respectively. The degree of adsorption onto the ligand (HL) was slightly higher for Co²⁺ than for Cs⁺ in wastewater, highlighting the ligand selectivity toward divalent ions. pH tests revealed that the most significant sorption occured at a pH of 4 for Co²⁺ and 6.3 for Cs⁺, demonstrating that the adsorption mechanism was pH-dependent. The optimal contact durations for maximum sorption efficiency were determined to be 60 min for Co²⁺ and 120 min for Cs⁺. Furthermore, the increase in the biosorbent mass directly enhanced the sorption capacity due to the increased availability of active sites. The maximum monolayer adsorption capacities (q_max) for Co²⁺ and Cs⁺ were 62.11 mg/g and 15.10 mg/g, respectively. The adsorption data were well described by both the Freundlich and Langmuir isotherm models, with correlation coefficients of 0.96 and 0.93, respectively. These findings confirm that the ligand (HL) exhibits a strong binding capacity and high efficiency for the removal of Co²⁺ and Cs⁺ ions, making it a promising candidate for practical wastewater treatment. Future work should focus on enhancing the ligand adsorption capacity and selectivity by modifying the functional groups, improving the structural stability, and developing composite materials. Investigating reusability, real-world wastewater performance, and economic feasibility will help optimize ligands for practical applications.