Efficient adsorption performance of an alginate-based dental material for uranium(vi) removal

2.1 Reagents

U O 2 ( N O 3 ) 2 · 6 H 2 O (Merck KGaA, Darmstadt, Germany) and Kromalgin (Vannini Dental Industry, Florence, Italy) were used in batch experiments. To ensure appropriate pH levels in the diluted test solutions before the adsorption experiments, 0.50 mol L⁻¹ HNO₃ (Merck KGaA, Darmstadt, Germany) solution and solid Na₂CO₃ (Merck KGaA, Darmstadt, Germany) were used. 1,3-Diphenyl-1,3-propanedione, pyridine, 2-propanol, and tri-n-octyl phosphine oxide (all from Merck KGaA, Darmstadt, Germany) were used in the spectrophotometric determination of U(VI).

2.2 Instrumentation

A thermostated shaker bath (ST402, Nuve, Ankara, Turkey) was used to perform batch adsorption tests. The pH of each solution was measured using a microprocessor pH meter (Inolab pH 537, WTW, Weilheim, Germany). The samples were centrifuged using a digital centrifuge (Universal 16A, Hettich, Tuttlingen, Germany). The samples were dried in a laboratory oven (M420, Electro-Mag, İstanbul, Turkey). The concentration of U(VI) ions in the solution was determined using an ultraviolet–visible (UV–vis) spectrophotometer (UV–vis 1601, Shimadzu, Kyoto, Japan). FT-IR spectra were recorded using an FT-IR spectrometer (Spectrum BX, PerkinElmer, Waltham, MA, USA). The surface properties and morphology of the adsorbent were examined using a scanning electron microscope (Thermo Scientific Phenom, Waltham, MA, USA).

2.3 Preparation of the adsorbent

The adsorbent material used in the study was Kromalgin, which was obtained commercially. The composition of the material is sodium alginate (10–12%), calcium sulfate (∼10%), magnesium oxide (3–5%), diatomaceous earth (60–70%), dipotassium hexafluorotitanate (0.5–2.5%), zinc oxide (0.5–2.5%), and minor components such as retardants, stabilizers, pigments, and flavoring agents. To purify the adsorbent from undesirable materials, Kromalgin was washed with distilled water and alcohol and then stirred thoroughly until a uniform density was achieved. Subsequently, it was dried in an oven at 105 ± 5°C for 6 h. After grinding, the resulting adsorbent was crushed using a mortar and then passed through a 150 μm sieve. The stages of the experimental procedure are illustrated in Figure 1.

Figure 1

Flowsheet of the preparation of the adsorbent and experimental procedure.

2.4 Batch sorption experiments

Batch procedures were used to conduct experimental studies. A polyethylene (PE) bottle containing 0.05 g of adsorbent was stirred with 10 mL of a known concentration of U(VI) solution in a thermostatically controlled shaking water bath at constant temperature. After shaking for a specific duration, the adsorbent was separated, and the amount of adsorbed U(VI) ions was analyzed spectrophotometrically using tri-n-octylphosphine oxide–dibenzoylmethane (TOPO–DBM) method at 405 nm [38].

2.5 Data analysis

The following equations were used to calculate the adsorption percentage ( % ) and distribution coefficients (K _d) of uranium ions adsorbed on the adsorbent [28,39].

The symbols used in the above equations represent the following: C _i (mg L⁻¹) is the concentration of uranium ions before adsorption, C _e (mg L⁻¹) is the concentration of uranium ions remaining in the solution after adsorption, V (mL) is the volume of the liquid phase, and m (g) is the weight of the adsorbent.

Langmuir, Freundlich, and D–R isotherm models were used in the adsorption studies. Constant values in these models were calculated using the equations given below.

Langmuir isotherm equation [40,41]:

The symbols in the equation represent the following: C _e is the equilibrium concentration of metal ions in the solution, q _e is the amount of metal ions adsorbed onto the adsorbent Q _max is the monolayer adsorption capacity, and b _L is the adsorption energy.

Freundlich isotherm equation [42]:

In the Freundlich equation, q _e is the amount of adsorbate adsorbed per mass of adsorbent at equilibrium. K _F and n _F are Freundlich constants, which express the adsorption capacity and intensity, respectively.

D–R isotherm equation [43]:

The mean energy of adsorption E (kJ mol⁻¹) was calculated using the following equation [43]:

The symbols used in the above equations are as follows: C _ads is the concentration of the adsorbed metal ion, X _m is the maximum adsorption capacity, β is the activity coefficient related to adsorption energy, ε is the Polanyi potential, R is the universal gas constant, (8.314 J mol⁻¹  K⁻¹), and T is the absolute temperature.

The equations used to calculate the thermodynamic parameters are given by equations (8) (van’t Hoff equation) and (9) (Gibbs free energy equation) [44].

In the above equations, K _d is the distribution coefficient, ∆H ⁰ is the enthalpy, ∆S ⁰ is the entropy, K is the absolute temperature, R is the gas constant, and ∆G ⁰ is the Gibbs free energy.

3.1 Characterization of the adsorbent

Characterization of the adsorbent has been performed using FTIR spectroscopy and SEM analysis. FT-IR spectra were recorded in the range of 4,000–400 cm⁻¹ at a resolution of 2 cm⁻¹. The FTIR spectra presented in Figure 2 reveal structural variations between commercial Kromalgin and its U(VI)-loaded water/alcohol-treated forms. A prominent peak around 1,068 cm⁻¹, observed in all spectra, is attributed to C–O [31,45,46] or C═O stretching vibrations within the carboxylate groups of sodium alginate [46] originating from the structure of Kromalgin. The common peaks at 620 and 791 cm⁻¹ are assigned to Si–O–Si bending and symmetric stretching vibrations, respectively, with similar vibrations reported in the range of 600–825 cm⁻¹ [47,48], attributable to the silicate structures originating from the diatomite content in Kromalgin. The peaks around 1,412 and 1,616 cm⁻¹ correspond to the symmetric and asymmetric stretching vibrations of the carboxylate (–COO⁻) groups from sodium alginate [32,45,46]. The O–H stretching vibration appears within the range of 3,400–3,700 cm⁻¹, corresponding to hydroxyl groups [31,32,37,45,46,47], and the peaks at 2,903 and 2,978 cm⁻¹ suggest interactions involving alkyl groups [37,46,47,49,50].

Figure 2

IR spectra of Kromalgin before and after U(VI) adsorption.

Following the adsorption of uranium(vi), a significant change in the intensity of several peaks in the spectrum was observed, indicating enhanced interactions of the functional groups and structural changes in the material. The peaks at 3,677 cm⁻¹ suggest a possible interaction between hydroxyl groups and U(VI) ions. The increased intensity observed at 2,903 and 2,978 cm⁻¹ is notable, and a similar enhancement is seen at 1,412 and 1,616 cm⁻¹, collectively suggesting changes in the surface structure of alginate upon U(VI) binding. These results suggest a possible role of hydroxyl and carboxylate groups in the adsorption process, while alkyl groups may influence the interaction indirectly by modifying the surface properties of the adsorbent. In summary, the FTIR spectra presented in Figure 2 suggest the adsorption of U(VI) ions by the alginate-based Kromalgin. The observed changes in the characteristic functional group peaks imply coordination interactions between UO 2 2 + and alginate functional groups during the adsorption process.

SEM analysis was performed to investigate the surface morphology of Kromalgin before and after adsorption. Figure 3(a) and (b) presents SEM images of Kromalgin before adsorption at 300 and 150 µm scales, respectively, and Figure 3(c, e, and f) presents SEM images of Kromalgin samples treated with water and Figure 3(d) of Kromalgin samples treated with alcohol after U(VI) adsorption. The images reveal morphological differences that arise from both the treatment method and the comparison between the states before and after adsorption, highlighting the pore structures of the water-treated sample in greater detail.

Figure 3

SEM images of Kromalgin samples (a) and (b) Kromalgin before adsorption (300 and 150 µm); (c) water-treated Kromalgin after adsorption (150 µm); (d) alcohol-treated Kromalgin after adsorption (150 µm); (e) water-treated Kromalgin after adsorption (200 µm); (f) water-treated Kromalgin after adsorption, showing pore structures (200 µm).

As shown in Figure 3(a) and (b), before the adsorption of U(VI) ions, Kromalgin exhibited a less compact structure with numerous surface pores, which diminished after adsorption. The rough and non-uniform structure of the adsorbent plays a role in providing more binding sites and increasing the contact area, which reduce the mass transfer resistance and facilitate the diffusion of metal ions during the adsorption process [47]. After U(VI) ion adsorption, as shown in Figure 3(c)–(e), the surface of Kromalgin becomes smoother, and the pores are relatively reduced, which can be attributed to the adsorption of U(VI) ions. In the SEM images after adsorption, more wrinkles and small particles are observed on the surface. These particles can be attributed to the adsorption of U(VI) ions, suggesting their attachment to the surface of Kromalgin during the adsorption process. Particle size measurements were conducted on a representative SEM image (Figure 3f), revealing particle diameters ranging from 6.81 to 35.07 μm, suggesting a heterogeneous structure with a broad particle size distribution.

3.2 Effect of pH

Because pH affects the charge on the active site of the adsorbent surface, it is the key variable in the metal adsorption process in solution. Throughout the adsorption of U(VI), the surface charge of the adsorbent may undergo changes depending on the pH. This variability in pH can influence the ability of UO 2 2 + to hydrolyze and form polymeric species, subsequently affecting its adsorption properties [51]. At low pH, the high concentration of H⁺ ions increases the positive charge of the adsorbent’s active sites, which leads to a decrease in the number of metal ions that can bind to these sites. As the pH increases, the active sites become neutralized, resulting in an increased amount of adsorbed metal.

Batch adsorption experiments were performed at various pH levels to investigate the impact of pH on the adsorption of U(VI) ions onto Kromalgin. Three parallel series of experiments were conducted on the adsorbent: (1) commercially available form; (2) alcohol-treated adsorbent; (3) water-treated adsorbent. Washing with water aimed to remove water-soluble impurities and residual salts that could block active adsorption sites, while washing with alcohol targeted the removal of organic contaminants and enhanced surface cleanliness. The untreated sample served as a control to observe the baseline adsorption capacity. This approach allowed a comparative assessment of how different pretreatment methods influence the availability of binding sites and the overall efficiency of uranium adsorption. Experiments were carried out at pH 2.5, 3, 4, 5, 6, 7, and 8 with an initial concentration of 150 mg L⁻¹ U(VI), at a temperature of 30°C for 60 min of adsorption time (Figure 4).

Figure 4

Effect of pH on adsorption of U(VI) on Kromalgin (C _i: 150 mg L⁻¹, m: 0.05 g, V: 10 mL, t: 1.0 h, and T: 30°C).

According to the experimental study results, commercial, alcohol-treated, and water-treated adsorbents showed maximum adsorption at pH 3 under the specified conditions with the adsorption percentage of 75.06, 86.15, and 89.70%, respectively. The small difference in adsorption efficiencies is due to the fact that the Kromalgin adsorbent material, free of water-soluble impurities, exhibits better adsorption performance compared to when alcohol-soluble impurities are present. Therefore, the subsequent experiments were conducted using the water-treated adsorbent.

When the distribution profiles of uranium species at different pH values reported by Zhang et al. are examined; it can be observed that pH 3 is a value where the hydrolysis of uranium(vi) ions is minimal, and thus UO₂ ⁺² ions are the dominant species in the solution [52]. Therefore, experiments conducted at pH 3 provide ideal conditions for investigating the adsorption efficiency of uranium(vi) ions.

3.3 Effect of time

The effect of contact time on U(VI) adsorption was investigated by experiments conducted at varying contact times (5–1,440 min) with an initial concentration of 150 mg L⁻¹ U(VI) solution at pH 3 (Figure 5). Figure 5 shows that as the contact time increases, the adsorption percentage and equilibrium constant of the ion increase for the Kromalgin adsorbent. During a 30 min agitation period, the adsorption percentage and distribution coefficient of U(VI) ions were found to be 92.35% and 2415.73 mL g⁻¹, respectively. Subsequent periods showed only a slight increase in the adsorption percentage reaching 94.56% adsorption and the equilibrium constant reaching a 3473.25 mL g⁻¹ distribution coefficient after 300 min. In the agitation periods of 600–1,440 min, there was little change in the adsorption percentage and equilibrium constant. Considering the economy of the applied process, a 1-hour mixing time is deemed efficient.

Figure 5

Effect of contact time on the adsorption of U(VI) on Kromalgin (C _i: 150 mg L⁻¹, m: 0.05 g, V: 10 mL, pH: 3, and T: 30°C).

3.4 Effect of temperature

To examine the effect of various temperatures on U(VI) adsorption on Kromalgin, 0.05 g of adsorbent was shaken with a U(VI) solution in a thermostat water bath at temperatures of 298.15, 303.15, 308.15, and 313.15 K for 24 h. The results are shown in Figure 6.

Figure 6

Effect of temperature on the adsorption of U(VI) on Kromalgin (C _i: 150 mg L⁻¹, m: 0.05 g, V: 10 mL, pH: 3, and t: 1.0 h).

Thermodynamic parameters of adsorption were calculated using van’t Hoff graphs (Figure 7) drawn on the basis of the Nernst distribution law and the van’t Hoff equation. The results are given in Table 1. The rise in temperature increased the adsorption capacity, indicating that the adsorption process is endothermic. This was also supported by the positive standard enthalpy (52.71 kJ mol⁻¹). The positive value of enthalpy change (ΔH ⁰) between 298.15 and 313.15 K reflects the difference in bond energy between coordinated water and the adsorbent [53]. The sorption-free energy value indicates that the chemisorption process is the dominant mechanism [54] in the U(VI) adsorption process on water-treated Kromalgin. The entropy change (ΔS ⁰) value was found to be 0.234 kJ mol⁻¹ K⁻¹ for the adsorption of U(VI) ions. A positive entropy value indicates that random adhesion at the solid/solution interface increases during the adsorption process [55]. The fact that the free energy change (ΔG) reaches smaller values with increasing temperature explains that the adsorption process is spontaneous and favorable at high temperatures. In addition, negative values of ΔG ⁰ indicate that the adsorption nature is thermodynamically feasible.

Figure 7

Van’t Hoff plot for adsorption of U(VI) on Kromalgin in aqueous solution.

3.5 Effect of concentration

To examine the effect of initial concentration on UO 2 2 + adsorption on Kromalgin, studies were conducted on 0.05 g adsorbent with 150, 200, 300, 400, and 500 mg L⁻¹ uranium solutions separately. Other parameters (solution pH: 3.0; temperature: 30°C; contact time: 1 h) were kept constant. The amount of U(VI) adsorbed by the adsorbent at these concentrations and the calculated K _d values are shown in Figure 8. At lower metal ion concentrations, the removal efficiency increases because there is a high ratio of available adsorption sites on the adsorbents to the initial quantity of U(VI) ions. However, as the concentration of U(VI) ions increases, there is a shortage of adsorption sites, leading to a decrease in metal uptake [45]. According to the figure, the highest adsorption efficiency for the Kromalgin adsorbent at 150 mg L⁻¹ uranium concentration was calculated as 91.68%, and the distribution coefficient of uranium was 2203.88 mL g⁻¹. At lower concentrations, uranium molecules occupy all the accessible active sites on the surface of the adsorbent. As the amount of the adsorbent increases, the active sites become saturated, leaving no vacant sites on the adsorbent surface.

Figure 8

Effect of concentration on the adsorption of U(VI) on Kromalgin (m: 0.05 g, V: 10 mL, pH: 3, t: 1.0 h, and T: 30°C).

3.6 Sorption isotherms

The adsorption isotherm serves as a method for characterizing the equilibrium state between the concentration of solutes adsorbed on a solid surface and the corresponding amount of adsorption under constant temperature conditions. Adsorption continues until the concentration of solute remaining in the solution reaches a dynamic equilibrium with the concentration of solute adsorbed on the surface. In this state of equilibrium, there is a distinct distribution between the solid and liquid phases of the solute. This distribution rate is a measure of the equilibrium state in the adsorption process. The interaction between the adsorbent and the adsorbate can be described by various adsorption isotherms, including the Freundlich, Langmuir, and D–R isotherms. These three adsorption isotherms are commonly used for solid–liquid phase adsorption. Specifically, the Langmuir and Freundlich models are commonly used to correlate experimental equilibrium adsorption data of heavy metals onto alginate-based adsorbents [26]. The Langmuir adsorption isotherm model is commonly used to describe adsorption on a homogeneous surface, assuming monolayer coverage without interactions between the adsorbed molecules [40]. The Freundlich isotherm is employed to characterize heterogeneous adsorption, which assumes a surface that is not energetically uniform [42]. The D–R isotherm serves as an empirical model that aids in understanding the adsorption process, distinguishing between chemical and physical adsorption [43].

Graphs of Langmuir, Freundlich, and D–R isotherms were created with the equilibrium data obtained for U(VI) ions in the concentration range of 150 to 500 mg L⁻¹ at 30°C (Figure 9). The linearity of the curves represented by R ² was found to be 0.9968, 0.9825, and 0.9836 for Langmuir, Freundlich, and D–R isotherms, respectively. This indicates a good correlation between the experimental data and the mathematical model equations for the three isotherm models. Isotherm data derived from the adsorption of U(VI) ions onto Kromalgin are presented in Table 2.

Figure 9

Sorption isotherms of adsorption of UO 2 2 + on Kromalgin: (a) Langmuir, (b) Freundlich, and (c) D–R.

According to the Langmuir isotherm, the adsorption capacity for U(VI) ions was determined to be 142.86. The perfect fit of the isotherm graphs to the Langmuir isotherm indicates that the adsorption is homogeneous, single-layered, and that chemical adsorption is dominant. This result is compatible with that obtained from the adsorption enthalpy. The D–R isotherm was employed to estimate the mean free energy of adsorption (E). If the E value is between 1.00 and 16.00 kJ mol⁻¹, adsorption is physical, and when it is above 16.00 kJ mol⁻¹, it is chemical adsorption [54]. The mean energy of adsorption was calculated as 17.81 kJ mol⁻¹. The adsorption energy E is greater than 16.00 kJ mol⁻¹ for U(VI) ions, showing that the adsorption of U(VI) ions on Kromalgin proceeds through the chemical interaction mechanism.

Various adsorbents, including activated carbon [56], metal–organic frameworks (MOFs) [57], clays [58], and bio-based materials [59] have been extensively studied for U(VI) removal. Among these, activated carbon is widely used due to its high surface area and well-developed porosity, with adsorption capacities ranging from 2.3 to 639.77 mg g⁻¹ under optimal conditions, as reported by Mittal et al. [60]. However, alginate-based materials offer advantages such as cost-effectiveness, biodegradability, and ease of modification [61]. The alginate-based adsorbent materials can be used as unmodified or functionalized with various compounds. Studies have shown that the enhancement in characteristics and functional properties, including mechanical strength and adsorption capacity, is evident compared when with its unmodified form when alginate is functionalized with organic compounds [26]. Characteristics of alginate can also be improved by compounding with organic substances, oxides, or nanoparticles [62,63,64]. Many studies have been conducted on U(VI) adsorption using alginate-based adsorbents. In Table 3, some examples from U(VI) adsorption studies using alginate-based adsorbents in the literature are presented in comparison with the current study. Although a direct comparison is not possible, the Kromalgin adsorbent used in this study provides significant results, rivaling the alginate-based adsorbent materials in the literature.

3.7 Desorption studies

Recovering uranium from the loaded adsorbent is crucial for cost-effectiveness and plays a key role in the overall efficiency of the uranium removal process from aqueous media [65]. Desorption experiments were carried out by treating 0.05 g of U(VI)-loaded adsorbents (150 mg L⁻¹) with 10 mL of HNO₃ and NaOH solutions at concentrations of 0.5 and 1.0 mol L⁻¹. After a contact time of 1 h, the aqueous phase was sampled for analysis. Experiments with NaOH at concentrations of 0.5 and 1.0 mol L⁻¹ resulted in relatively low desorption efficiencies of 28 and 41%, respectively. In contrast, HNO₃ achieved significantly higher efficiencies of 72.35 and 81.41% at concentrations of 0.50 and 1.0 mg L⁻¹, respectively. These findings demonstrate that HNO₃ is an effective desorbing agent for uranium, achieving significantly higher recovery rates compared to NaOH based on a single desorption cycle.