2.1 Materials

The basaltic volcanics used in the present study were taken from the Manisa-Kula region of Turkey. Kula volcanics are classified as basic according to the rate of SiO₂ production, and basaltic according to their output temperature [23]. Kula basaltic volcanics were selected because they are abundant in nature, cheap, and an accessible sorbent.

A cationing surface modifying agent (N,N-Diethylethanolammonium chloride DEEA-Cl (C₆ H₁₃ NO^.HCl, WA 151.64 g/mol)), which is a quaterner ammonium salt, was used. A standard solution of 1000 mg/L U (VI) was prepared by dissolving a suitable amount of UO₂ (CH₃ COO)₂ 2H₂ O (Merck, Germany) in ultrapure water. The initial pH solutions were changed by HNO₃ or NaOH. U (VI), and the concentrations were determined by Arsenazo III [24]. A PG (UK) instrument T80 UV-Vis Spectrophotometer was used. All reagents and solvents used were of an analytical grade.

2.2 Preparation of organo-volcanics

The organo-volcanics were prepared by treating the volcanics with an aqueous solution of DEEA) (Fig 1). A total of 3 g of volcanics with 30 mL DEEA solution of different concentrations were added in a flask, which was gently shaken for 24 h at 20 ^oC. Then, the DEEA-modified volcanics were centrifuged at 4000 rpm, washed five times with ultrapure water, and dried at 60 ^oC [8]. Thus, the volcanics were modified with the addition of DEEA amounts of 5, 10, 15, and 20 mM.

Fig. 1Full-screen View

N,N-Diethylethanolammonium chloride

2.3 Characterization of organo-volcanics

The surface structure of the made organo-volcanic was examined under a FEI QUANTA 400F SEM scanning electron microscope at 1.2 nm resolution. The dried powders (approximately 0.01 g) were placed on a sticky carbon tape on standard Al mounts, then sputter coated with a thin conductive layer of gold. The chemical composition of the basaltic original volcanic samples was analyzed by X-Rays Fluorescence Spectroscopy (XRF), using a Spectro IQ II instrument. The interactions between DEEA and the volcanics were examined by FTIR spectroscopy. The FTIR spectra was obtained by using KBr pellets with a Perkin Elmer FT-IR System/Spectrum BX. XRD patterns of the unmodified volcanite and the DEEA-uranium modified volcanite were obtained by using X-ray diffraction analyses (PANALYTICAL Emperian diffractometer), and the samples were scanned from 2 to 88 2-Theta in step sizes of 0.0130 and scanned at a step time of 148.92 s.

The surface areas of the volcanics were found by using the BET equation to the physical adsorption data for the nitrogen at 77 K. The unmodified and DEEA-volcanic samples were degassed in a vacuum at 573 K and 323 K for 8 hours respectively. The (BET) analysis using the Autosorb 6 by Quantachrome Corporation was performed. The SEM-EDS images and BET were performed at the Central Laboratory of the Middle East Technical University in TURKEY.

2.4 Adsorption experiments

The uranyl acetate solution was added to the organo-volcanics. The sorption studies were carried out using the batch method. The mixture was shaken in a shaker (GFL 1083 model). The uranium (VI) percentage and adsorption uptake in the equilibrium, q_e(mg/g) were found by using the following equations:

where, C_o, is the initial concentration of U (VI) in the solution, (mg/L); C_e is the equilibrium concentration of the uranium in the solution, (mg/L); V is the solution volume (L), and m is the dry adsorbent mass (g). The distribution coefficient, K_d (mL/g), was calculated using Eq. (3):

3.1 Characteristic analysis

The chemical composition of the original basaltic volcanic sample was found to be composed of SiO₂, Al₂ O₃, Fe₂ O₃, MnO, MgO, CaO, Na₂ O, K₂ O, TiO₂, P₂ O_5, and other trace elements at the percentages of 48.4%; 17.66%; 8.648%; 0.085%; 8.538%; 6.234%; 3.989%; 2.881%; 1.85%, 0.625%, and 1.09 %, respectively. The morphologies, as well as the structural ordering of the natural and modified volcanic, are presented in Fig. 2a and Fig. 2b, respectively.

Fig. 2Full-screen View

SEM-EDS pictures of the Kula volcanics (a) and DEEA modified of Kula volcanics (b)

There are many crystals in the cavities of unmodified volcanics (Fig. 2a). It was observed that the morphology of the surface was changed by the DEEA surfactant. The volcanic surface was covered with the DEEA organic. Carbon traces on the surface were seen, according to the findings of EDS (Fig. 2b). The specific surface areas of the unmodified and DEEA modified volcanics were found as 2.265 and 3.689 m²/gby the multipoint BET method, respectively.

The FTIR spectras of DEEA organo-volcanic and U (VI)-DEEA organo-volcanic are shown in Fig.3. For the DEEA organo-volcanic, the broad band around 3385 cm^-1 in the DEEA organo-volcanic was attributed to the O-H stretching vibrations. Generally, the C-H stretching bands of the alkylammonium cations are found in the 3020-2800 cm^-1area [25]. For the modified volcanic, two weak bands were noted at 2945 and 2845 cm^-1. The HOH deformation peak at 1654 cm^-1 was present in the FTIR spectrum of the DEEA organo-volcanic. Generally, the adsorbed material was water contributing to the bending region in the 1600-1700 cm^-1 area [24].The strong band around 1031 cm^-1 was ascribed to the Si-O stretching vibration. The stretching and bending vibrations of the SiO₄^2- tetrahedral were the peaks at 537 and 471 cm^-1. The absorption band at 695 cm^-1 was attached to the coupled Al-O and Si-O out of plane vibrations [26].

Fig. 3Full-screen View

FTIR spectra of organo-volcanic, and U (VI) adsorbed to the organo-volcanic.

In the case of the U (VI)-DEEA organo-volcanic, peaks similar to that of the organo-volcanic structure were detected in the region of 3200-3600 cm^-1where it is indicated by Si-O, N-H and O-H stretching vibrations. It was observed that the H-O-H deformation peak shifted from 1654 cm^-1 to 1641 cm^-1 with adsorption. The changes of the absorption band related to the H-O-H deformation of water molecules adsorbed on the volcanic were shown by this band. The characteristic absorption peak of Si-O of the organo-volcanic was observed at 1031 cm^-1, and the Si-O of the uranium adsorption was expanded to the same range, which showed that the uranium is adsorbed on the volcanic surface. In additon, the peak at 695 cm^-1was shifted to the 654 cm^-1region.

According to the XRD patterns of natural volcanics, such minerals as quartz (SiO₂ ), Coesite (SiO₂ ), potassium sodium alumosilicate (K_.667 Na_..333 ) (AlSiO₄ ), magnesium iron alumosilicate (Mg_0.34 Fe_1.66 Al₄ ), Sekaninaite (Si₅ O₁₈ ), calsium magnesium catena-silicate diopside high (Ca Mg Si₂ O₆ ), and ferropargasite (NaCa₂ Fe₄ Al(Si₆ Al₂ )O₂₂ (OH)₂ were determined in the samples. The XRD patterns of the unmodified volcanite and the DEEA-uranium modified volcanite are shown in Figure 4. The unmodified volcanite sample mostly consisted of the SiO₂ amorphous phases (Fig.4). The X-ray pattern of the unmodified volcanite was different to the pattern of the treated one. A new few peaks of the organic cation are seen on the X-ray pattern of the DEEA-uranium modified material. They could originate from the DEEA cations and uranium deposited on the volcanite surface. Also, the uranium peak appeared on the pattern 34, 39 2-Theta^o. The uranium peaks are similar to those in the literature [27,28].

Fig. 4Full-screen View

X-ray diffraction patterns of the unmodified, the DEEA- modified and DEEA- U(VI) modified of volcanite: (a) Unmodified volcanite; (b) DEEA modified volcanite; (c) DEEA-U modified volcanite

The surface area was calculated by using the multipoint BET method. The BET surface areas of unmodified volcanics and DEEA modified-volcanics were found as 2.265 m²/g and 3.689 m²/g, respectively.

The zeta potentials of the unmodified and modified volcanics were measured in the pH range of 3–8. The surfaces of both the volcanics were found to be negatively charged. We believe that there was an electrostatic attraction on the volcanic surface between the negative charge of the SiO and AlO groups and the positive charge of the organic chain. However, the volcanic surface was also negatively charged because the DEEA concentration was low. In the literature, it has been emphasized that cationic surfactants covered all the clay surfaces and displacing expanded by entering of the organic molecules between the sheet layers of the clay minerals [29].

3.2 Batch adsorption experiment

3.2.2 Effect of contact time

The time dependence of the U (VI) adsorption experiments is given in Fig. 5 under the conditions of 1 g adsorbent, a volume of 30 mL, 20 mg/L U (VI), a pH of 3.5, and a temperature of 25 °C. The sorption of the U (VI) was increased from 61.15% to 76.30 % after 4 hours. Equilibrium was reached within 4 hours.

Fig. 5Full-screen View

Effect of contact time on removal of U(VI).

3.2.3 Effect of initial pH values

The effect of the initial pH on the U (VI) sorption was examined by adjusting it to different values ranging from 2.0 to 6.0 (30 mL solution and 20 mg/L U (VI)). The solution and volcanics were mixed at 25 °C for 4 hours. The U (VI) sorption strongly depended on the pH solution, and increased with the increasing pH (Fig. 6.). When organic cation concentration reached 5 mM micelle formation, it is more efficient than the interaction of the hydrocarbon chains. After this critical micelle point, the absorption capacity decreases [24].

Fig. 6Full-screen View

Effect of initial pH

At a low pH (below pH 4), when the dissociation of the Si-OH bonds are suppressed, the adsorption of the U (VI) is low. The UO₂²⁺ in the acidic solutions is the only complex-forming uranium species ((UO₂ )₂ CO₃ (OH)₃^-, UO₂ OH⁺, UO₂ (OH)₂, (UO₂ )₂ (OH)₂²⁺, (UO₂ )₃ (OH)₅⁺ and UO₂ CO₃, etc) [14]. (UO₂ )₂ (OH)₂²⁺) and (UO₂.OH)⁺ hydrolysis species were found between pH 3.0 and 4.0. Also, the (UO₂ )₃ (OH)₅⁺ forms at pH 4.0, and becomes dominant at the pH values exceeding 4.5 [30]. In this study, the optimum pH was found to be 5.0.

3.2.4 Effect of initial U (VI) concentration

The effect of the initial concentration of U (VI) was examined by contacting a mass of DEEA organo-volcanics (1 g) at 25 °C and an initial pH=5.0 using a range of U (VI) concentrations (20, 40, 60, 80, and 100 mg/L) (Fig. 7). The removal of the U (VI) was increased by increasing the initial U (VI) concentration between 20 mg/L and 40 mg/L. Moreover, the adsorption capacity of the DEEA-volcanics for U (VI) was decreased by increasing the initial uranium concentration (after 40 mg/L). The optimum concentrationwas found to be40mg/L.

Fig. 7Full-screen View

Effect of initial U(VI) concentration on the removal of U(VI)

3.2.5 Adsorption isotherms

Adsorption isotherms show the equilibrium relationships between the adsorbed ion concentrations and ion concentrations in a solution. In our work, the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm adsorption models, the most commonly used models, were also employed to describe the sorption behaviors. The Langmuir isotherm model assumes that the sorption occurs on a homogeneous surface by the monolayer sorption. The Langmuir adsorption isotherm model is represented as Eq. (4),

$\frac{C_{\text{e}}}{q_{\text{e}}}=\frac{1}{K_{\text{L}}{q}_{\text{m}}}+\frac{C_{\text{e}}}{q_{\text{m}}},$ (4)

where, q_m(mg/g) is the maximum amount of adsorption of U (VI) per unit mass of the adsorbent, and K_L (L/mg) is the Langmuir affinity constant that represents the affinity between the adsorbent and the adsorbate.

Freundlich isotherm, the linear equation is expressed by Eq. (5),

$\log q_e=\log K_{\text{F}}+\frac{1}{n}\log C_{\text{e}},$ (5)

where K_F [(mg/g)(L/mg)^1/n] is the constant related to the adsorption capacity of the adsorbent, and 1/n is the constant related to the adsorption intensity of the adsorbent. The Freundlich isotherms were based on the adsorption on the heterogeneous solid surfaces. The Dubinin–Radushkevich isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface. The model has often successfully fit high solute activities and the intermediate range of concentrations data well.

$\ln q_{\text{e}}=\ln (q_{\text{s}})-{(K_{\text{ad}}{\epsilon }^2)}_{,}$ (6)

where q_e is the amount of adsorbate in the adsorbent at equilibrium(mg/g); q_s is the theoretical isotherm saturation capacity (mg/g); K_ad is the Dubinin–Radushkevich isotherm and is constant (mol² /kJ² ), and ε is the Dubinin–Radushkevich isotherm and is constant. The approach is usually applied to distinguish the physical and chemical adsorption of metal ions with its mean free energy. The E per molecule of the adsorbate (for removing a molecule from its location in the sorption space to the infinity) can be computed by the relationship (7):

$E={\left[\frac{1}{\sqrt{2B_{\text{DR}}}}\right]}_{,}$ (7)

where B_DR is denoted as the isotherm constant. Meanwhile, the parameter, ε, can be calculated by Eq. (8):

$\epsilon =RT\ln {\left[1+\frac{1}{C_{\text{e}}}\right]}_{,}$ (8)

where R, T, and C_e represent the gas constant (8.314 J/mol K), the absolute temperature (K), and adsorbate equilibrium concentration (mg/L), respectively.

We also tested the Langmuir, Freundlich, and D-R adsorption isotherms, and the results indicated that the D-R adsorption isotherm (R²> 0.96) fitted the data better than the Langmuir and Freundlich adsorption isotherms. All the results are listed in Table 1.

Table 1Full-screen View

Equilibrium isotherm parameters of sorption of U(VI) by DEEA organo-volcanics.

Langmuir			Freundlich			D-R
qm(mg/g)	KL(L/mg)	R2	KF(mg/g)	1/n	R2	qs(mg/g)	Kad(mol2/kJ2)	E (KJ/mol)	R2
400	0.0144	0.3131	2.2	0.9061	0.9564	74.24	0,0040	11,18	0.9646

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From this research work, the maximum monolayer coverage capacity (qm) from the Langmuir Isotherm model was determined to be 400 mg/g, K_L (Langmuir isotherm constant) is 0.014 L/mg, and the R² value is 0.31.

The Freundlich isotherm constants k_f and n were determined from the intercept and slope of a plot of log qe versus logC_e (Fig. 8).

Fig. 8Full-screen View

The linearized isotherms for adsorption of U(VI) by DEEA organo-volcanics

From the data in Table 1, it can be seen that the value of 1/n is 0.9061, the sorption of U(VI) on to DEEA organo-volcanics is favourable, and the R² value is 0.9564. Thus, the adsorption process is multilayered.

In addition, from the linear plot of the D-R model, q_s was determined to be 74.24 mg/g, the mean free energy, E was 11.18 KJ/mol indicating a chemical adsorption process, and the R²was 0.9646 higher than that of the Freundlich.

Essentially, the DEEA organo-volcanic shows very good sorption performance for U(VI) in comparison with other adsorbents reported in the literature in Table 2.

Table 2Full-screen View

The comparison of the Freundlich adsorption capacities (K_F; mg/g) of different natural adsorbents towards U (VI) in the literature.

Adsorbent	KF(mg/g)	pH	References
Wood powder	0,461	8	[31]
Wheat straw	0,346	7	[31]
Hematite	1,63	7	[2]
Talc	2,19	5	[32]
Natural clinoptilolite zeolite	0,087	6	[33]
Goethite-coated sand	0,064	9	[34]
Polyacrylamidoxime resin coated quartz sand	1,0	6	[35]
DEEA organo-volcanics	2,2	5	This Study

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Uranium adsorption onto DEEA organo-volcanics is shown schematically in Fig. 9.

Fig. 9Full-screen View

(Color online) Schematic illustration of the U(VI) adsorption on DEEA organo-volcanics.

3.2.6. Adsorption kinetics

The kinetics of adsorption is an important aspect in defining the efficiency of the adsorption process. The kinetic data in this study was modeled using pseudo-first-order [36] and pseudo-second-order [2] according to:

$\text{Pseudo-first-order }\ln q_{\text{t}}=\ln q_{\text{e}}-k_{1}t,$ (9) $\text{Pseudo-second-order }{q}_{\text{t}}=\frac{1}{k_2{q}_{\text{e}}^{\text{2}}}+\frac{1}{q_{\text{e}}t},$ (10)

where qe and qt are the adsorption capacity (mg/g) at equilibrium and time 't’, respectively, k₁ and k₂ are the pseudo-first-order and pseudo-second-order constants of the adsorption, respectively. The relationships of ln(qt-qe) to t and t/qt to t were plotted according to the experimental values (Fig. 10).

Fig. 10Full-screen View

Pseudo-second-order kinetic plots for the adsorption of U(VI) ions from aqueous solutions onto DEEA organo-volcanics.

The adsorption kinetic data was ideally described by the pseudo-second-order equation (R²=0.9999). qe and k₂ were obtained and represented in Table 3.

Table 3Full-screen View

The calculated parameters of the pseudo second order kinetic models

Temperature (K)	qe (mg/g)	k2 (g/mg min)	R2
298	0.458	0.183	0.999

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