Introduction

Uranium is a metal that is released from nuclear power plants, scientific research laboratories, various industrial production factories, and natural sources and poses a danger to humans and the environment if it is found in the environment and wastewater, as well as its chemical and radioactive properties. If uranium in the form of hexavalent uranium (UO22+) in an aqueous environment is taken into human metabolism, it can form chelates with various biochemical molecules and accumulate in organs, leading to organ failure and death. In this respect, the removal of uranium, which has a daily intake dose of 0.6 µg kg−1 of body weight per day, particularly from wastewater, is of vital importance [1]. On the other hand, the use of natural materials as markers for the detection of possible uranium contamination is possible by identifying materials that are found in nature and have an interest in uranium. In particular, subsoil plant structures, which can be found in soil and aqueous environments, can be used as markers for the determination of spontaneous uranium enrichment and environmental uranium concentration.

In contrast, the recovery of uranium, a precious metal with a concentration of 3.3 ppb in seawater is economically important [2]. Although there are many physicochemical methods for uranium enrichment or recovery from aqueous media, most of them are not sustainable because of the need for advanced technology, cost, and low efficiency [3]. Among these methods, the recovery and removal of metals from aqueous solutions by the adsorption method has come to the forefront because of its low cost. The adsorption method is widely used for the removal of many pollutants because parameters such as adsorbent design, selectivity, and adsorption rate can be controlled. Among adsorbents, natural materials [4, 5], cellulose [6], chitosan [7, 8], carbon biomass [9], synthetic polymers [10], MOF structures [11], and composite materials [12, 13] can also be used.

Crocus is a plant species belonging to the Iridaceae family. The corm tunic structure (Fig. 1), which is a component of the underground root structure of this plant, can grow under many different conditions and act as a root sheath. The CT structure of this plant, which has different species, is fibrous woody and has styloid crystals, which vary depending on the species. The fibrous structure has a very large surface area and crystals on it, and the soft and layered root structure, especially toxic oxalate ions, are suggested to be retained and deposited on the surface of the tunic in the form of calcium oxalate [14]. No study has been conducted in the literature regarding the affinity of this structure, which forms the contact surface of the plant with soil and water, to metals. Although its structural characteristics are given in many articles, it has the same structure as saffron, which belongs to the same family and is widely produced. It is seen that it will be possible to use it as a natural economic adsorbent if it is possible to use the CT structure, which is highly produced and discarded as waste, as an adsorbent [15].

Fig. 1
figure 1

The photo of corm tunic structure of Crocus (Iridaceae) used in this study

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In this study, the adsorption properties of the corm tunic structure (CTC) of crocus were investigated for uranium. The structural properties of CTC were described by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) analyses, and the adsorption properties were determined using synthetic solutions containing uranyl ions. The effects of ambient conditions such as concentration, temperature, time, pH, and adsorbent dose on adsorption were evaluated within the scope of this research. The results showed that CTC can be used effectively for uranium enrichment because of its nature in both subaqueous and aqueous environments.

Materials end methods

Reagents

Crocus (Iridaceae) was collected from its natural habitat in Sivas, Turkey. 2-pyridylazo resorcinol (PAR) was purchased from Sigma (St. Loius, MO, USA). UO2(NO3)2·6H2O and the remaining of the chemicals were obtained from Merck (Germany). Ultra-pure water was used during all experiments. All experiments were performed in duplicates and within ± 5% experimental error limit.

Instrumentation

In the characterization studies, FTIR analyses were carried out in the range of 400–4000 cm−1 and Bruker Tensor II. SEM analyses were performed using the TESCA Mira 3. Uranium equilibrium concentrations were measured using a T60 UV spectrophotometer (PG Instruments), and pH measurements were made using Selecta pH2005.

Determination of uranium

Uranium concentrations were determined spectrophotometrically using the PAR method [16]. In this method, uranyl ions form a selective complex with (4-(2-pyridylazo) resorcinol) at pH 8.5. For this purpose, 3.5 × 10−3 M of PAR was prepared in Tris/HCl buffer (pH 8.5). Then, 50 µL of sample solution was added to 3 mL of PAR solution and the absorbance of the formed complex was followed at 530 nm. After a calibration graph was plotted, the concentration of uranyl ions was determined in the supernatants.

Preparation of adsorbent

Crocus is a subspecies of the Iridaceae family and is known to grow in a wide variety of habitats. Like other species of the Iridaceae family, it has tuberous roots and a corm tunic, which is a sheath surrounding the root structure. A photograph of the adsorbent used in this study is shown in Fig. 1. The samples collected from the wild were washed several times with distilled water, and the parts that did not belong to the corm tunic structure were removed. The sample was dried and after grinding and sieving, the particle size was approximately 0.5 mm in the adsorption process.

Adsorption studies

The batch method was used for adsorption studies with a 100 mg sample and 10 mL solution. The adsorbent was exposed to uranium solutions with concentrations ranging from 8.5 × 10–4 to 4.63 × 10–3 molL−1 and allowed to equilibrate at a constant temperature of 25 °C for 24 h. Uranium concentration in equilibrium solutions was determined using the method described in “Determination of uranium” section. The concentration of uranium was set to a constant value of 1.85 × 10–3 molL−1 for kinetic and other studies.

Results and discussions

Characterization

The FTIR spectra of CTC before and after uranyl adsorption are shown in Fig. 2. When the pre-adsorption FTIR spectrum was analyzed, it was concluded that the peaks at 2919 cm−1 and 2853 cm−1 were due to -C-H stretching vibrations, while the peak at 1603 cm−1 was due to ester and acid groups, and the peak at 1318 cm−1 was due to aromatic C = C and was caused by lignin and cellulose content in the structure. The decrease in the intensity of the peak at 777 cm−1 after adsorption and the presence of the newly formed peak at 914 cm−1 appear to be due to the introduction of uranium into the structure [15].

Fig. 2
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FTIR spectrum of CTC

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SEM images were obtained to analyze the surface morphology of the materials before and after adsorption and to determine the surface composition, and EDX diagrams were obtained to understand the elemental distribution, as shown in the figure. In the SEM image of CTC before adsorption, the fiber structure was clear (Fig. 3a). The surface gives the impression of a highly porous and hard surface. After adsorption, the surface appeared soft (Fig. 3b). The increase in brightness after adsorption can be considered a result of the electron interaction of uranium attached to the surface. In the EDX diagram before adsorption (Fig. 3c), Ca peaks appeared quite strong, whereas after adsorption (Fig. 3d), an increase in U peaks was observed with a decrease in the presence of Ca peaks. This result is evidence that adsorption occurs as a result of adsorption by the displacement of Ca ions on the surface.

Fig. 3
figure 3

SEM images of CTC (a) before adsorption 500 magnification (a') 20 k magnification (b) after adsorption 500 magnification (b') 20 k magnification (c) EDX diagram before adsorption (d) after adsorption

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The elemental map image of CTC after the adsorption of uranyl ions shows that uranium is present on the surface of CTC and is homogeneously distributed on the surface (Fig. 4).

Fig. 4
figure 4

SEM image and elemental mapping of after uranyl adsorption of CTC

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PZC point of adsorbent and pH effect on adsorption

Because adsorption takes place at the solid–liquid phase interface, the charge of the surface and the adsorbed species are important factors in adsorption. In particular, in the adsorption studies of ions, the charge distribution on the surface significantly affects adsorption. In this respect, it is important to understand the dependence of the surface charge on the ion concentration in the environment. One method used to determine the surface charge is to determine the surface charge of the solid phase by measuring the equilibrium pH of the material interacting with solutions at different pH values. With this method, the distribution of the surface charge and the point zero charge (PZC) value can be determined [17]. The PZC value of the material above this value indicates that there is a relative excess of negative charge, and below this value, there is a positively charged surface. To determine the PZC value of the material, solutions at different pH values were interacted, the equilibrium pH was measured after 24 h, and the PZC point was determined, and the results are shown in Fig. 6a. To maintain the ionic strength of the medium constant, 0.1 M KNO3 solution was used. The results show that the surface charge of the adsorbent remained constant between pH 6–10. The surface charge is positive at pH < 6 and negative at pH ≥ 10.

To measure the effect of solution pH, which is one of the factors affecting adsorption, research was carried out at different pHs at constant uranyl ion concentration and the results are given in Fig. 6b. Adsorption increases with increasing pH. This is due to the positive charge of the surface at low pH and the presence of H+ ions reduce adsorption. Both the competition between H+ ions and uranyl ions and the effect of repulsive forces due to the positive charge of the solid surface result in a decrease in adsorption. The increase in adsorption with increasing pH is explained by the partial shift of the surface from positive to negative charge. As the PZC point is approached, the adsorption is maximized. Another important parameter is the species of uranyl ions changing with pH. While UO22+ is the dominant species at low pHs, polycationic species are formed with increasing pH and this causes an increase in adsorption [18]. Since polyanionic forms of uranium formed at high pHs cause precipitation, no study was carried out at high pHs because adsorption and precipitation cannot be distinguished at these pHs (Fig. 5).

Fig. 5
figure 5

pH-dependent species change simulation of uranium using Minteq 3.0 (Curanyl: 1000 ppm)

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Effect of uranyl concentrations on adsorption and isotherms

To investigate the adsorption properties of CTC, the effect of the concentration of adsorbed species, which is one of the factors affecting adsorption, was investigated within the scope of this study. For this purpose, a fixed amount of CTC interacted with uranium solutions at different concentrations (1.85 × 10–4–4.63 × 10–3 molL−1) at constant temperature and constant time and the concentration of uranyl ions in equilibrium was determined UV-spectrophotometrically by the method described in the “Determination of uranium section”. The results are shown in Fig. 6, and the adsorption parameters are listed in the table.

Fig. 6
figure 6

Experimental results of adsorption (a) PZC point of CTC (b) pH effect to adsorption (c) adsorption isotherms and fitting to theoretical models (d) effect of time to adsorption and fit of results to theoretical models (e) adsorbent dosage effect to adsorption (f) temperature effect to adsorption

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As shown in Fig. 6d, The adsorption isotherm corresponded to the L-type adsorption isotherm in the Giles classification. This type of adsorption behavior shows that the adsorption shows high amounts as a result of the high number of active centers at low concentrations, while at high concentrations it reaches equilibrium with the decrease in the number of adsorption centers. Langmuir, Freundlich, and DR models, which are widely used theoretical adsorption models, were used to evaluate the adsorption isotherms [19]. The nonlinear forms of the models are presented in Table 1. One of the parameters found from the Langmuir model, XL, is the maximum adsorption capacity, and the adsorption capacity of CTC for uranyl ions was found to be quite high. The Langmuir model predicts that the surface is homogeneous, and that adsorption occurs through the active centres [20]. Considering the fiber structure of CTC, it can be said that adsorption may occur as a result of displacement with calcium oxalate ions in the CTC structure as well as interaction with cellulose and lignin in the structure [21]. From this perspective, it can be predicted that CTC, which is in contact with the aqueous environment in its natural environment, can be used as a marker for measuring the environmental concentration of uranium. It was also concluded that CTC can be used as an effective adsorbent considering its annual production and waste amount.

Table 1 Adsorption equations and parameters
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The parameters obtained from the other commonly used adsorption equations are listed in Table 1. The value of β obtained from the Freundlich equation, which is a useful parameter for adsorption, is considered an indicator of surface heterogeneity. Usually, it cannot exceed 1. This indicates that the heterogeneity increases as it approaches zero. Xf value is interpreted as α parameter related to adsorption interest [22]. This was related to the slope of the hyperbolic curve at low concentrations. As the value increases, it can be interpreted as a measure of the increase in interest between the adsorbent and the adsorbate. The EDr value obtained from the DR model is a parameter related to the adsorption energy and is used to determine whether adsorption is physical or chemical. The EDR value for the study was found to be 21.88 kJmol−1 and this value indicates that the interaction of uranyl ions with CTC is chemical that is, complex, formation or ion exchange [23].

Influence of time on adsorption and kinetic studies

The change in adsorption over time is important for predicting the adsorption process. Adsorption kinetics are difficult to explain using simple models such as homogeneous reactions. Many events occur independently during the adsorption process, and the kinetics of each event determines the kinetics of the entire process. Initially, the process begins with the transfer of adsorbed species to the surface and ends with surface film formation, diffusion to the pores, and binding of active binding sites [24]. When it is difficult to explain the kinetics of each event with a single model, the adsorption kinetics can be explained using different kinetic models. To reveal the time variation of uranyl ions to CTC and to determine the adsorption parameters, adsorption studies were carried out at a constant uranyl concentration (1.85 × 10–3 molL−1) and constant temperature, and equilibrium concentrations at different times, and the amount of adsorbed was determined spectrophotometrically. The results and their agreement with the theoretical models are shown in Fig. 6c, and the adsorption kinetic parameters obtained from these models are listed in Table 1. The nonlinear forms and parameters of the common theoretical models, pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich model (EM), and intraparticle diffusion (IP) models are listed in Table 1 [25].

The results of the adsorption study reveal a prompt initiation of adsorption, followed by a gradual deceleration until reaching equilibrium. Examination of the fit curve coefficients derived from theoretical models establishes the PFO model as an apt explanation for the adsorption phenomenon. An essential criterion in model selection lies in the model’s capacity to accurately predict the adsorption equilibrium. From this perspective, the PFO model emerges as the more adept choice. Despite the comparatively lower R2 value in the fit curve of the PSO model in comparison to the PFO model, it remains sufficiently robust to furnish valuable insights into adsorption kinetics. Although not attaining the precision of equilibrium adsorption prediction demonstrated by the PFO model, the PSO model proves instrumental in ascertaining pivotal parameters such as the adsorption initial rate (H) and adsorption half-life (t1/2).

Despite the lower R2 value observed in the curve fitting of the intraparticle diffusion (IP) model in comparison to other models, the positive value of the “c” parameter signifies the influence of boundary layer effects on adsorption [26]. The outcomes of the experimental data fitting to the Elovich model are presented in Table 1. In this context, α represents the adsorption rate, and β denotes the desorption constant [27]. Specifically, α was determined to be 0.029 molKg−1 min−1 for the adsorption of uranyl ions on CTC, indicating a commendable initial rate in contrast to numerous adsorbents employed in uranyl adsorption.

Effect of adsorbent amount to adsorption

To investigate the effect of adsorbent dosage on adsorption, different adsorbent amounts interacted with uranyl ions at constant concentration, and the results are given in Fig. 6e. % adsorption value increases with increasing adsorbent amount. This can be explained by the increase in both surface area and active centers with increasing adsorbent amount.

Influence of temperature on adsorption

To determine the thermodynamic parameters of adsorption, investigations were conducted at various temperatures while maintaining a constant uranyl ion concentration. The results are shown in Fig. 6f. The equilibrium concentrations were measured, and the adsorption thermodynamic parameters were calculated using the equations and thermodynamic parameters outlined in Table 1. The enthalpy of adsorption was positive, indicating that the transition from the solution to the solid phase favors energy consumption. Furthermore, the positive entropy of adsorption suggests an increase in disorder during the process. While a decrease in entropy is typically anticipated in solution adsorption during the transition from an aqueous solution to a solid phase, additional processes associated with adsorption contribute to the overall entropy of the process. Concomitant with adsorption, phenomena such as dehydration, ion exchange, dissociation, and coalescence are posited to augment disorder in the entire process [28]. The negative Gibbs free enthalpy value observed at 298 K aligns with our expectations, indicating that the adsorption process occurs spontaneously.

Conclusion

The results of the study showed that CTC can be used as an adsorbent for the removal/recovery of uranyl ions. The morphological and structural properties of CTC before and after adsorption were elucidated by SEM images and FTIR analyses. The optimum conditions of adsorption were analyzed in detail and it was observed that maximum adsorption was achieved at the natural pH of uranium. The maximum adsorption capacity was found to be 0.286 molKg−1 from the Langmuir model and this shows a high adsorption capacity when compared with similar materials in Table 2. Adsorption kinetics were found to be by the PFO model. From the adsorption thermodynamic parameters, it was concluded that adsorption is an energy-consuming, entropy-increasing, and spontaneous process. It was concluded that the CTC material, which is the root sheath of the Crocus plant of the Iridaceae family, can be used both as an adsorbent and as a marker for measuring environmental uranium activity due to the ability of the plant to grow in many different conditions and the direct interaction of CTC with the aqueous environment. Most of the components of the commercially produced saffron (Crocus sativus L) plant, which belongs to the Crocus subspecies of the Iridacea family and has the same structure as the biomaterial used in the study, except the flower part, are wasted without being utilized [29]. This study, in which the corm tunic structure of this family was used as an adsorbent, has shown that it is possible to use this structure, which is agricultural waste and has no use as an adsorbent.

Table 2 Comparison table of similar materials as plant wastes
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