Decontamination of actual radioactive wastewater containing 137Cs using bentonite as a natural adsorbent: equilibrium, kinetics, and thermodynamic studies

Batch adsorption treatment using Iraqi bentonite as a natural adsorbent was adopted in this study to decontaminate actual 137Cs radioactive wastewater from the Al-Tuwaitha Nuclear Research Center, located south of Baghdad. The bentonite characterization was applied before and after treatment, using chemical compositions analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer–Emmett–Teller (BET) surface area analysis and Fourier-transform infrared spectroscopy (FT-IR). The batch adsorption mode was applied with the initial radioactivity concentration (1440.5 Bq/L), solid/liquid ratio (1 g/L), pH (6–8), contact time (1.5 h), and temperature (298°K). The adsorption experiments showed a decontamination removal efficiency of about 95.66% of 137Cs. A Freundlich adsorption isotherm model was approved for the adsorption of 137Cs, with a coefficient of determination R2 = 0.998. A pseudo-second-order model fitted well with the adsorption of 137Cs, with R2 = 0.983. The positive value of ΔH° in the thermodynamic results indicated that the adsorption process was endothermic physisorption (ΔH° = 15.01 kJ mol−1), spontaneous and favorable (ΔG° = −7.66 kJ mol−1 K−1), with a very low degree of disorder (ΔS° = 0.076 kJ mol−1 K−1).


Scientific Reports
| (2022) 12:13837 | https://doi.org/10.1038/s41598-022-18202-y www.nature.com/scientificreports/ adsorption of 137 Cs such that for availability, inexpensive cost, and simple application, raw bentonite can be employed without any physical or chemical pretreatment 18 . In this study, there are two main features that distinguish it from other similar ones. The first of these is that the Cs-137 radioactive liquid used in the research is different due to the properties it contains. The second is that the natural bentonite used is specific to a site. Iraqi raw bentonite was investigated as an adsorbent for the uptake of 137 Cs from contaminated liquid at the Al-Tuwaitha site. Bentonite was characterized using chemical composition analyses, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), particle size distribution, surface area, and Fourier-transform infrared (FT-IR) spectroscopy. Investigations studied the removal efficiency% of the effects of adsorption parameters, such as the adsorbent/ liquid ratio, pH, sorbate concentration, contact time, and temperature. Additionally, evaluations were conducted of the influence of the adsorption capacity on the kinetic mechanisms, isotherm models, and thermodynamics.

Materials and methods
Sorbate preparation. The source of the radioactive waste of Cs-137 is back to the previous activities of the Al-Tuwaitha nuclear site which was attacked during the Gulf war in 1990 and 2003. This radioactive wastewater contains in addition to the 137 Cs traces of different types of short half-life radionuclides. All samples were measured and analyzed using gamma spectrometry analysis system with a closed end, coaxial, p-type model GEM65P4-95/ORTEC as shown in Fig. 1, which consists of a 65% relative efficiency of High purity germanium detector (HPGe), with the advanced gamma vision software. The energy and efficiency of the measuring system were calibrated using a multi-energy measuring source (MGS5.1045), using special containers for sec 3600. The dominant radionuclide of ( 137 Cs) was measured at the energy of 661 keV. The specific activity of 137 Cs for the stock liquid was 4.5 MBq/L, which was calculated using the below equation 19 : where A: specific activity in units (Bq/L). B.G: Back ground. V: the volume of the model. eff%: The percentage of efficiency. Iγ % : Intensity of gamma rays. t: the measurement time.
The contaminated liquid was diluted to a safe limit of about 7201.8 Bq/L. Analytical solutions of HCl and NaOH were used to regulate the pH of the 137 Cs solutions for batch experiments using a gamma spectroscopy system HPGe detector. The total samples numbers are 25, and the experiment was repeated two times for each sample.
Sorbent preparation and characterization. A bentonite clay sample was supplied from a deposit in Wadi Bashira of the Western Desert (Iraq). The clay sample was crushed by a jaw crusher (Retsch BB 1, Germany), then milled in a rotating cylinder ball mill and sieved in a 75-micron sieve opening. Bentonite clay was used without any physical or chemical pretreatment. Chemical wet analyses were carried out on bentonite to identify the composition of the contents by the Iraqi Geological Survey Laboratories. Mineralogical analyses were inspected using the Ital structure model MPD 3000. A MIRA3 TESCAN high-resolution analytical scanning electron microscope (SEM) with EDX was used to investigate the bentonite's morphology. The particle size distribution analyses were accomplished by Brookhaven Instruments. The specific surface area (SSA) and the cation exchange capacity (CEC) for bentonite were obtained from technical reports of the Iraqi Geological Survey 20 . FT-IR analysis was conducted by ultraviolet and visible absorption spectrophotometer by a Bomem MB-Series FT-IR spectrometer according to ASTM E 1252-98.

Specific Activity
Bq   2, 4, 6, 8, and 10). Bentonite was added with various solid/liquid (S/L) ratios (0.5-2.5 g/L) in glass containers. The mixtures were combined using a shaking water bath at 200 rpm and at different bath temperatures (25-50 °C). At the end of the mixing times (which varied from 0.5 to 3 h), the samples were filtered using a 0.45-micron Whatman™ filter, and the radioactivity concentrations (specific activity Bq/kg) of 137 Cs in the filtrates were assayed by an HPGe detector. The concentrations of 137 Cs (µg/L) in the filtrates were estimated using Eqs. (1)-(4) 21 .
where λ is the radioisotope decay constant (s −1 ), Aav is the Avogadro's number (6.02 × 10 23 nuclei/mol), w is the weight (g), and M is the atomic weight (g/mol).
where v is the volume of sample (L). The removal efficiency (R%) and adsorption capacity (q e ) in (mg/g), were determined using Eqs. (5) and (6), respectively 22,23 : where C o and C e are the initial and equilibrium concentrations of radioactive cesium (Bq/L), respectively; V is the volume of the solution (L); and M is the weight of the clay mineral materials (g).
Adsorption isotherm. Various types of adsorption isotherms models are mentioned in the literature, and the most remarkable ones have proceeded as linearized isotherm forms of either the Langmuir (Eq. 7), Freundlich (Eq. 8), Temkin (Eq. 9), or Hill (Eq. 10) models 24 . The adsorption isotherm rule at equilibrium presents the relationship between the adsorbent concentration and ion concentration in solution as follows: where C e is the activity concentration of cesium alone in solute (mg/L), q e is the adsorbed material on the adsorbent surface (mg/g), q max is the maximum monolayer capacity (mg/g), and b is the Langmuir constant (L/g).
where K f is the Freundlich constant (mg/g) (which approximately indicates the adsorption capacity), and 1/n is the intensity of adsorption (which evaluates the adsorption process).
where b is the isotherm Temkin constant (which is related to the adsorption temperature), A is the binding constant of the equilibrium (L/g), R is the ideal gas constant (8.314 j/mol. K), and T is the absolute temperature (°K).
where n h Hill cooperativity coefficient of the binding interaction, q sh and K D Hill isotherm constant. Furthermore, in this model if n h =1 that means the binding is hyperbolic or non-cooperative, if n h > 1, binding has positive cooperativity, while negative cooperativity occurs when n h < 1.
Adsorption kinetics. The mechanism of 137 Cs adsorption on bentonite surfaces was evaluated by using the data resulting from the effect of contact time. The evaluation is based on the application of three kinetic adsorption linearized models: pseudo-first-order (Lagergren model), pseudo-second-order (Ho model), and intraparticle diffusion (Weber-Morris model), which are represented by Eqs. (11), (12), and (13), respectively 25 . where q t and q e are the adsorption capacity (mg/g) at time t (min) and at equilibrium, respectively; K 1 and K 2 are pseudo-first order (min −1 ) and pseudo-second order (g/mg min) adsorption rate constants, respectively; K P is the constant of the intraparticle diffusion rate (mg/g min 0.5 ); and C is the constant of the intraparticle diffusion.
Adsorption thermodynamics. Thermodynamic studies were conducted by evaluating the physicochemical properties and thermochemical nature of Cs adsorption onto bentonite. K c values were estimated by Eq. (14) (15) and (16) to generate Eq. (17), which is called van't Hoff model 26 .
where Kc is the equilibrium constant, R is the ideal gas constant (8.314 J/mol. K), T is the absolute temperature (°K), ΔG° is the adsorption Gibbs free energy change (kJ/mol), ΔH° is the adsorption free enthalpy change (kJ/ mol), and ΔS° is the adsorption free entropy change (kJ/mol K).

Results and discussion
Sorbent characterizations. The results of the chemical and mineralogical analyses (shown in Table 1 and Fig. 2) indicated that the applied bentonite type was low-grade bentonite ore. There was a high content of impurities of major minerals (e.g., silica, calcite, and gypsum) as displayed in Fig. 2, while the clay minerals, (Ca, Mg and Na montmorillonites) were present as minor minerals. The bentonite had not been previously purified or modified, which is clearly shown in these results. Scanning electron microscopy (SEM) images for bentonite are shown in Fig. 3. The SEM showed the morphology of the bentonite structure before adsorption, as shown in Fig. 3a, with a fine and close plate-like shape structure. In contrast, after adsorption (Fig. 3b), the layered structure was disordered, and the particles were almost flat.
Energy-dispersive x-ray analysis (EDX) was achieved at point (a) on the bentonite surface after adsorption, as shown in Fig. 3b. The EDX spectrum detects the presence of 137 Cs by Lα and Lβ in the range of 4-5 keV (see Fig. 4). The quantitative analysis at the same point indicates the main composition percentage of bentonite clay and the loaded 137 Cs.
The particle size of bentonite was investigated using a Brookhaven NanoBrook 90Plus particle size analyzer, which uses dynamic light scattering (DLS) principles. The mean particle size of the bentonite is presented in Fig. 5. It was found that the 30.9 nm particle had the highest intensity value, with the specific surface area of bentonite listed in Table 2.
Montmorillonite (smectite) has a 2:1-layer structure consisting of an octahedral alumina sheet sandwiched between two opposing tetrahedral silica sheets. FT-IR spectra of bentonite before and after the cesium adsorption process were measured to investigate the structural changes. However, the spectrum of bentonite before the adsorption in Fig. 6b showed the following bands: a stretching vibration Al-OH/OH, bending vibration H-OH, stretching vibration Si-O-Si, and a stretching vibration Al-OH-Al at 3421, 1636, 1026/776, and 912 cm −1 , respectively. Figure 6a shows the spectrum after the cesium adsorption, with a slight shift towards a higher frequency on the Al-OH-Al band than in the raw bentonite (from 3421 to 3427 cm −1 ) 27 .

Batch adsorption parameters.
Effect of the solid/liquid ratio on adsorption. The effect of the solid/ liquid (S/L) ratio on the adsorption of 137 Cs is shown in Fig. 7. The bentonite dosage was 0.5, 1, 1.5, 2, and  26 studied the adsorption rate of Cs + by changing the liquid/solid ratio from 1000:1 mL/g to 50:1 mL/g, which decreased the adsorption rate from 63.5 to 98.0%. This can be explained by the fact that the odds of Cs + colliding on the surface of the montmorillonite increased with additional adsorbent active sites. The increase in adsorbent amounts meant an increase in the surface area and the number of active sites of adsorbent surface exposed for ion exchange, as found by Abou-Lilah et al. (2020) 27 .
Effect of pH on adsorption. The investigation of the initial pH effect on the 137 Cs adsorption removal by bentonite is depicted in Fig. 8. The uptake of 137 Cs from wastewater occurred at higher pH levels (6-7), while it decreased as the acidity increased from 6 to 4 and the alkalinity increased from 8 to 10. This result can be explained due to the negative electrostatic charge of the natural bentonite clay sheet surfaces, which attracts the positive charge of the 137 Cs to its surface to encourage ion exchange. When the acidity increased, the H + ions increased, competing with the 137 Cs ions on the surface of the clay. On the other hand, the high alkalinity creates      , which indicated that the adsorption percentage approaching 97% was reached at pH levels between 6 and 8. Therefore, for maximum Cs-adsorption, the disposal of high-level radioactive wastewater should be carried out at a pH of 7 28 .

Effect of contact time and adsorption kinetics.
The influence of contact time on the 137 Cs uptake percent from wastewater using bentonite is plotted in Fig. 9a. The adsorption quickly increased in the first 30 min and reached equilibrium at 1.5 h, with a removal % of 95.66%. The effect of contact time on the 137 Cs adsorption using raw bentonites has been studied 29,30 . The faster 137 Cs uptake percent was associated with the initial presence of many empty active sites for adsorption on the clay surfaces. Then, with increasing contact time, the uptake percent gradually increased because most active sites were already occupied and remained in that condition until equilibrium was reached, when all active site became vacant 31,32 . Kinetic models were employed to simulate the experimental data in the contact time study in relation to the adsorption capacity (q t ). The linear forms of the pseudo-first-order model (Eq. 11), pseudo-second-order model (Eq. 12), and intraparticle diffusion model (Eq. 13) are plotted in Fig. 9b-d, respectively. For further discussion, the kinetic characteristics were estimated from the slopes and intercepts of these linear models and tabulated in Table 2. The pseudo-first and secondorder models have a good fit to the 137 Cs adsorption mechanism, with high regression coefficients (R 2 ) of 0.983 and 0.978, respectively. The theoretical amount of 137 Cs adsorbed at equilibrium q e suggested by the pseudofirst-order model was about 2.36 × 10 −6 mg/g, which was closer to the experimental q e of about 2.15 × 10 −6 mg/g than that of the pseudo-second-order model, which was approximately 2.54 × 10 −6 mg/g, as shown in Table 3.
Although there is only a small difference in the q e between the first-and second-order models, the regression coefficient R 2 best fits the second-order models. The intraparticle diffusion model does not fit with the experimental data because the regression coefficient R 2 had a low value (0.757), indicating poor diffusion and that adsorption mainly occurred on the surface. The kinetics of 137 Cs of adsorption fitted with the pseudo-second-order model, meaning that the controlling mechanism step was ion-exchange by electrostatic interactions, which played a key role. Most of the previous studies on ion adsorption on clays were represented by the pseudo-second-order kinetic model [32][33][34] . Film www.nature.com/scientificreports/ diffusion can be considered the rate-controlling step if the adsorption process is marked by inefficient mixing, small solute size, and low concentration; otherwise, intraparticle diffusion controls the process.

Effect of 137 Cs initial concentration and adsorption isotherm.
This study investigated the effects of the initial concentrations of adsorbate on the amount of Cs + ions sorbed onto the surface of the clay, finding that increasing the initial adsorbate concentrations caused an increase in the equilibrium concentration. Thus, the amount of 137 Cs isotope sorbed onto the surface of the clay increased, as illustrated in Fig. 10. The adsorption isotherm illustrates the adsorption situation at equilibrium and reveals the influence of the adsorbate ion concentration in the bulk solution on the total adsorbed amount. Figure 11a-d clarify the application of the linear form of either the Langmuir (Eq. 7), Freundlich (Eq. 8), or Temkin (Eq. 9), Hill (Eq. 10) adsorption isotherms of 137 Cs. To verify the experimental data, the correlation coefficient parameters of these isotherms was estimated from the slopes and intercepts of linearization, as shown in Table 4. From the obtained correlation coefficient (R 2 ) values for these models, it was evident that the isotherm model's applicability for cesium removal using raw bentonite follows this order: Freundlich > Langmuir > Temkin > Hill, as shown in Table 4. The R 2 value was greater than 0.99 for both the Langmuir and Freundlich isotherm models, making them more suitable to elucidate the adsorption isotherm of the adsorbed ions than the Temkin model, with R 2 = 0.963. While Hill model with n h = 4.5 > 1 indicate that positive cooperative adsorption, this mean influencing of other binding sites on the same adsorbent, but the data did not fit with Hills model at R 2 = 0.736 35 . Depending on the comparison between the correlation coefficient parameters of the isotherm models, for the Langmuir model, the maximum monolayer adsorption capacity q max was 5.18E-6 mg/g and R L (the dimensionless equilibrium parameter value) was close to 0.058. An R L value in the range from 0-1 indicates that the adsorption process is favorable. However,    These favorable characteristics support the validity of using the Langmuir adsorption isotherm model. The adsorption by raw bentonite is monolayered sorption predominantly on distinct localized adsorption sites, with no transmigration of the adsorbate in the plane of the surfaces and assumes uniform energies of monolayer sorption onto the adsorbent surface, which was also concluded for the adsorption by raw bentonite by Abou-Lailah et al. (2020) 27  Effect of temperature and adsorption thermodynamics. The effect of temperature on 137 Cs adsorption is shown in Fig. 12a, which indicates that 137 Cs removal increased slightly with the increasing temperature of the adsorption process from 298-323°K. Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) provide an evaluation of the adsorption process spontaneity, the adsorption process nature, and the applicability of the adsorbent, respectively. The equilibrium constant (Kc) was determined from Eq. (14), while ΔH° and ΔS° were evaluated from the slope and intercept of a graph of Ln K c versus 1/T (van't Hoff), as illustrated in Fig. 12b. The thermodynamic parameters for 137 Cs adsorption are listed in Table 5.
The negative value of ΔG° confirms the spontaneous nature of the adsorption process. The values of ΔG° (−7.66 kJ/mol) indicated that the process of physisorption had occurred, as values of ΔG° for the physisorption process are generally between 20 and 0 kJ/mol. The positive value of ΔH° (15.01 kJ/mol) confirmed the endothermic nature of the process. The positive value of ΔS° (0.076 kJ/mol/°K) signified that randomness among the solid-liquid interfaces in the bulk solution had increased during adsorption equilibrium 38,39 . The numerical value of ΔH° supplies the type of sorption features, such that if the ΔH° value is from 2-21 kJ/mol, the adsorption process is physisorption, but if it ranges from 80-400 kJ/mol, it is chemisorption 40 . Based on the range of the ΔH° value, the 137 Cs adsorption occurred using an ion-exchange mechanism 41 .

Comparison study
The optimum equilibrated conditions resulting from the batch adsorption experiments were a solid:liquid ratio of 1 g/L, pH (6-8), initial cesium activity concentration (7201.8 Bq/L), contact time (1.5 h), and temperature (298°K). The adsorption efficiency achieved about 95.66% at the optimum condition. The results of this study were compared with some previous studies using the same approach. The batch adsorption conditions are listed in Table 6. The results of this study are similar to the adsorption efficiency of bentonite found in other studies 42 . Also, the Langmuir isotherm fit with an adsorption isotherm of 137 Cs mentioned by 43 . It should be noted that the raw bentonite in this study was untreated or modified.

Conclusions
Iraqi raw bentonite without any pretreatments successfully decontaminated 137 Cs from the Al-Tuwaitha radioactive wastewater by batch adsorption. The adsorption experiments indicated that about 95.66% of 137 Cs was decontaminated under the following conditions: initial activity concentration (1440.5 Bq/L), solid:liquid ratio (1 g/L), pH (6)(7)(8), contact time (1.5 h), and temperature (298°K). The adsorption in this 137 Cs isotherm study proved the validity of using the Langmuir adsorption isotherm model. The kinetics of the adsorption of 137 Cs fitted with a pseudo-second-order model, meaning that the controlling mechanism step was ion-exchange by electrostatic interactions on the bentonite clay surface. The positive value of ΔH° in the thermodynamic study confirmed the endothermic nature of the process. The ΔH° range value indicated that 137 Cs adsorption