Introduction

Jordanian governments tend to build nuclear reactors for peaceful purposes such as research purposes and to generate electricity. Uranium is considered the main source for the operation of nuclear plants, as a nuclear fuel in nuclear water, to generate energy. However, it is possible to produce amounts of the nuclear power reactor effluents containing uranium ion [1, 2]. When conditions are available and sufficient to oxidize uranium element turns into enriched uranyl ion (UO22+) [3,4,5]. Generally, uranium exists in solution as soluble UO22+ ion in the form of carbonate complexes (UO2)2CO3(OH)−3 , UO2CO◦3 , UO2(CO3)2−2 , UO2(CO3)4−3 and possibly (UO2)3(CO3)6−6 [6] and hydrolysis ion such as [UO2(OH2)5]2+ [5, 7, 8]. However, uranium and uranium ion can cause a threat for both the human body and environment because it has chemical toxicity and radioactivity. The presence of a high concentration of UO22+ ion in drinking water can accumulate in some human body organs such as kidney, liver, lung, and bone, causing serious risks like cancer disease and renal frailer (kidney damage) [9,10,11,12,13,14]. The EPA has set the level of safe contamination of uranium ion to be lower than 30 μg L−1. Thus, the concentration of uranium ion in water (e.g. UO22+ ion) must be reduced to the recommended limit as well as selectively capturing within a safe framework structure. This research work can help us to remove the toxic uranium ions from nuclear water effluents as well as to re-concentrates it for another use.

Various techniques are used to remove UO22+ ion from water, including solvent extraction [15, 16], coagulation-filtration [17], lime softening [18], electrolytic reduction [19], ion exchange [20,21,22], chemical precipitation [23], reverse osmosis [24], altra-filtration [25], membrane and electrodyalysis [26, 27], chromatographic extraction [28], flotation [29] and adsorption. Different adsorbents are widely utilized for uranium (VI) ion from water [30]. The importance of utilizing different type of adsorbents is due to the efficiently, flexibility, safely, economical visibility, simplicity and lower environmental health impact. There are pieces of examples such as mesoporous silica [31], nanoporous silica [32], synthetic resin [33], covalent organic framework [34], chitosan and cross-linked chitosan [35, 36], polypyrole [37], composite adsorbents [38],silicon dioxide nanopowder [39], inorganic oxides nanosheets or nanofibers [40, 41], biomasses [42,43,44], hydroxyapatite [45, 46], activated carbon [47,48,49], carbon nanotubes [50], graphene oxide and its amine-functionalized composite [51], zero-valent iron [52, 53], polyamidoxime-functionalized colloidal particles [54], amino-functionalized urea–formaldehyde framework mesoporous silica [55], titania nanoparticles covalently functionalized with simple organic ligands [56], porous magnetic N-doped Fe/Fe3C at carbon matrix and its highly efficient uranium(VI) remediation [57], and iron oxides [58,59,60,61], hematite [62] oxine functionalized magnetic Fe3O4 particles [63] and activated silica gel [64]. Uranium extraction by sulfonated mesoporous silica derived from blast furnace slag [65] montmorillonite, [66] hydroxide/graphene hybrid material [67] Manganese Oxide coated zeolite [68] amidoxime functionality within a mesoporous imprinted polymer material [69], Tendurek volcanic tuff [70].

These methods do not succeed in terms of selectivity and high cost, as well as are not easy to handle. From another direction, the natural adsorbents and their biomasses are economically effective and easy to handle. Nevertheless, in terms of selectivity, it is not. To achieve higher selectivity in capturing uranium ions from water, the silica gel functionalized organic or inorganic entities could be the best choice. Recently, the removal of uranium(VI) from water has been reviewed [71]. In terms of selectivity, the removal of uranium(VI) ion was investigated using amidoxime silica [72]. Other recent pieces of works on the market deal with utilizing silica gel functionalized organic or inorganic entities for capturing uranium ion from water. For example, silicate nanotubes [64, 73, 74], ethylene-di-amine-tri-acetate [75], amine-modified silica gel [76], murexide [77], silica gel or silica-gel-bound macrocycles [78, 79] and organic or inorganic polymeric ion exchangers [80,81,82,83].

Recently, Huang et al. have used tris(2-aminoethyl)amine ligand to modify the surface of silica gel. Wherein, it shows a high affinity for selective sorption Cr(III), Cd(II), and Pb(II) ions from water [84]. Besides, our group has found a highly efficient of such SG-TAEA-NH2 material for capturing of a single ion phase of the ferric ion [85]. As the continuity of this recent work, we are still studying the efficiency of this adsorbent to capture a single phase of metal and heavy metal ions from water. It is important to indicate that we have not found any study related to the utilization of silica gel particles functionalized tris(2-aminoethyl)amine moiety (SG-TAEA-NH2) for the sorption of aqueous UO22+ ion. Herein, we are still studying the efficiency of this adsorbent to capture a single phase of U(VI) ion from water. The novelty of this study lies in understanding the mechanism of adsorption and the bonding of uranium ion into the surface of SG-TAEA-NH2. This can provide us a complete knowledge and view regarding the selectivity capturing uranium ion from water, which can help us to understand the sorption mechanism in case of the binary ions phase of the next study. In this contribution, we use SG-TAEA-NH2 solid particles for capturing and sorption the aqueous UO22+ ion from water. The distribution of uranium(VI) ion between the aqueous phase and the SG-TAEA-NH2 solid phase can be investigated. The adsorption data of the various experiments can be analyzed by adsorption kinetics and isotherm models.

Materials and methods

Preparation of silica gel functionalized tris(2-aminoethyl)amine moiety (SG-TAEA-NH2)

The silica gel functionalized tris(2-aminoethyl)amine moiety (SG-TAEA-NH2) was prepared according to the reported literature [84]. A summary of the preparation method has been recently published [85]. The surface proposed structure of the tris(2-aminoethyl) amine-functionalized silica gel (SG-TAEA-NH2) is demonstrated in Scheme 1.

Scheme 1
scheme 1

The proposed structure of the tris(2-aminoethyl) amine-functionalized silica gel (SG-TAEA-NH2) [85]

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Reagents

The analytical grade uranyl acetate (UO2(CH3COO)2·2H2O) was provided from Polysciences GmbH. Other reagents, the preparation of the stock solution (100 mg L−1) of UO22+ ion, and the methodology of the experiments have been reported [85]. Arsenazo-III (3,6-bis[(2-arsonophenyl)azo]-4,5-dihydroxy-2,7-naphthalenedisulfonic acid) (Fluka Chemie, Buchs, Switzerland). Perchloric acid (70%) purchased from Sigma-Aldrich.

Apparatus and instruments

All absorbance measurements of the electronic spectra were made on A Shimadzu UV/Vis-1800 spectrophotometer (Shimadzu, Tokyo, Japan) with 1-cm quartz cells was used for measurement of the absorbance. The concentration of U was investigated by a spectrophotometric method using arsenazo(III) as a chromogenic reagent. Arsenazo III (Sigma-Aldrich): (0.07% w/v) solution was prepared in 3 M HClO4.

Uranium standard solution (1000 mg L−1 U (w/v)): This solution can be prepared by dissolving uranyl nitrate hexahydrate (Riedel-de-Haen, Switzerland) in 3 M HClO4. In a 10 ml standard volumetric flask, 2 ml of the sample containing U(VI) and 3 ml of the 0.07% (w/v) arsenazo III solutions were mixed. The contents of the flask were made up to the mark with 6 M HClO4 solution to ensure a final concentration of 3 M for HClO4. After shaking the sample, the absorption can be measured against the reagent blank at 651 nm [86].

Equilibrium studies

The adsorbed amount of the UO22+ ion onto the SG-TAEA-NH2 at equilibrium and in a specific t is \(q_{\text{e}} \,{\text{and}}\, q_{\text{t}}\) (mg g−1), which are calculated by Eqs. 1 and 2, respectively.

$$q_{\text{e}} = \frac{{(C_{\text{i}} - C_{\text{e}} )}}{S}$$
(1)
$$q_{t} = \frac{{(C_{i} - C_{t} )}}{S}$$
(2)

where Ci, Ce, and Ct are the initial, the equilibrium, and the final concentration at a specific time of UO22+ ion in the aqueous solution (mg L−1), respectively.

The dosage (S) of SG-TAEA-NH2 can be calculated by Eq. 3:

$$S = \frac{m}{v}$$
(3)

where ν and m are the initial volume of UO22+ ion solution and the mass of SG-TAEA-NH2 adsorbent, respectively.

The percentage removal of uranium ion can be calculated by Eq. 4:

$$\%\, {\text{Removal}}\,{\text{of}}\,{\text{U}}\left( {\text{VI}} \right)\,{\text{ion}} = \frac{{C_{\text{i}} - C_{\text{e}} }}{{C_{\text{i}} }} \times 100\% .$$
(4)

The distribution ratio (Kd) can be clarified in Eq. 5:

$$K_{\text{d}} = \frac{{{\text{Amount}}\,{\text{of}}\,{\text{U}}\left( {\text{VI}} \right)\, {\text{ion}}\,{\text{in}}\,{\text{SG - TAEA - NH}}_{2} }}{{{\text{Amount}}\,{\text{of}}\, {\text{U}}\left( {\text{VI}} \right){\text{ion}}\,{\text{in }}\,{\text{solution}}}} \times \frac{1}{S}.$$
(5)

wherein, the relationship between the adsorption percentages and Kd (L g−1) is presented in Eq. 6:

$$\% \,{\text{of}}\,{\text{Adsorption}} = \frac{{1000K_{\text{d}} }}{{K_{\text{d}} + \frac{1}{S}}}.$$
(6)

Batch sorption experiment

The sorption performance of SG-TAEA-NH2 material toward the sorption of the uranium (VI) ion was tested by using the batch system at specific T = 35 °C (± 1 °C) with changing in Ci (1, 5, 10, and 20 mg L−1) or at specific Ci = 10 with changing T (25, 35, 45, and 55 °C). The closed sorption system containing 2 g L−1 of SG-TAEA-NH2 was shaken vigorously (rpm of the thermostatic mechanical shaker = 80) of up to 180 min. Afterward, the supernatant solution must be filtered by using filter paper (Whatman No. 41). The filtrate solutions can be analyzed by recording the average of at least triplicate measurements as mention in our reported experiment [85].

Result and discussion

FT-IR spectra

The silica gel functionalized tris(2-aminoethyl)amine moiety (SG-TAEA-NH2) are mainly composed of silica gel and organic entities, which contain a variety of functional groups. The main functional groups in the FTIR spectrum are Si–O–Si and CH2, primary and secondary NH2, which have been recently discussed [85]. After heavy uranium (VI) ion sorption, some changes can be observed in the FTIR spectrum of SG-TAEA-NH2 material. Therefore, the FTIR spectra can be also used as evidence of the chemisorption and complex formation in the surface by controlling the shift in the stretching frequency of the primary and secondary NH2. In particular, Fig. 8 shows the bending vibration of primary amine (δ primaryN–H ) at 1594.4 cm−1, which is shifted by ∆δ = 69 cm−1 in contrast to the SG-TAEA-NH2 (δprimaryN–H  = 1525 cm−1) before adsorption. The stretching (νprimaryN–H ) is 3321 cm−1, which is cannot be observed due to the weakness of peak transmittance. These changes and shifts in peak positions suggest the presence of the chemical interaction and a possible complexation reaction between the uranium(VI) ion and NH2 moiety in the SG-TAEA-NH2 surface [87]. These results are consistent with the mechanisms proposed for the removal of iron ion [85] (Fig. 1).

Fig. 1
figure 1

ATR-IR spectrum for the SG-TAEA-NH2-U(VI) after 20 min of sorption Ci = 10 mg L−1 at 25 °C

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Effect of pH

Figure 2 shows that the sorption of uranium (VI) ion by the SG-TAEA-NH2 matrix increase dramatically with increasing pH to the maximum 99% at pH = 7–8, as no further increase above this pH value. This result matches the reported one regarding the adsorption of uranium on the amidoxime resins [88], modified carbon nanotubes [89], and polyacrylhydroxamic acid sorbent [90].

Fig. 2
figure 2

The effect of pH on the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25  C, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1, t = 5 min

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The pH has a significant role in the sorption and binding of U(VI) ion with amino-entity in the inorganic silica gel of the SG-TAEA-NH2 surface. Furthermore, the pH effects in both the stability of the U(VI) ion (complex species) and the SG-TAEA-NH2 surface morphology (protonation of the amino-entity). Therefore, to understand the effect of pH on the sorption behavior of U(VI) ion onto the SG-TAEA-NH2 surface, we have studied the sorption using the basic, neutral, and acidic medium.

In an acidic medium (pH < 7), there is a high concentration of H+ cation, which competes UO22+ ion (pH < 5) for binding in the amino-entity active sites. However, the functional amino-entity is protonated to be –NH3+. This can block and shield the attraction of di-cationic UO22+ ion toward the positively charged surface. However, the competition of proton (H+) cation for the amino-entity active sites decreases by increasing the pH (1 > pH > 7). Wherein, this can enhance the interaction and sorption of U(VI) ion onto the SG-TAEA-NH2 surface forming SG-TAEA-NH2…U complex.

In a basic medium (pH > 7), uranium (VI) ion gradually hydrolyzed to form a mono-cationic complex of UO2(OH)+. The mono-cationic complex has less affinity of interaction with amino-entity on the surface than the di-cationic uranyl ion (UO22+: pH < 5). Therefore, the sorption affinity of UO2(OH)+ cation with the SG-TAEA-NH2 matrix at basic is less than acidic and neutral medium. This agrees with the reported work regrading the sorption of uranium (VI) ion from nuclear industrial effluent by using nanoporous silica adsorbent [91]. At 7 ≤ pH ≤ 11, Uranium (VI) can be found in the form of UO2(OH)2·H2O neutral molecule [92], and wood powder and wheat straw [93].

The acidic medium can affect directly the SG-TAEA-NH2 matrix forming a shield of positive charges against the sorption of di-cationic UO22+ ion. From another direction, the basic medium can decrease this positive shield on the SG-TAEA-NH2 matrix, but forming a mono-cationic UO2(OH)+The acidic medium can afect directly complex. These two opposite factors guiding us to conclude that the capturing of uranium (VI) is not favorable in both acidic and basic medium to achieve the maximum removal. Therefore, we have chosen pH = 7–8 as an optimum pH for capturing of uranium (VI) onto the SG-TAEA-NH2 matrix.

Effect of initial concentration

The batch sorption technique is used for all experiments at 35 (± 1 °C). Different initial concentrations of UO22+ ion (Ci = 1, 5, 10, and 20 mg L−1) are used of up to 3.0 h. All other sorption parameters are kept constant (e.g. pHi = 7, T = 25 °C and 2 g L−1 dosage of SG-TAEA-NH2 materials). Figure 3 shows the effect of initial concentration, wherein the percentage of capturing uranium (VI) ion from solution decreases with increasing the initial concentration of uranium (VI) ion in solution. For example, the maximum percentage of capturing uranium (VI) ion achieve 98% at Ci = 1 mg L−1, while it is 93% at Ci = 20 mg L−1. This is due to the availability the sufficient amino-active sites into the SG-TAEA-NH2 matrix. For a given SG-TAEA-NH2 particle dose, the total number of amino-active sites available is fixed; wherein, the fixed amount adsorb almost the equal amount of uranium (VI) ion. This results in a decrease in the removal of uranium (VI) ion inconsequent to an increase in the initial uranium (VI) ion concentration. Furthermore, due to the increasing of competing of U(VI) cation onto the active sites of the SG-TAEA-NH2 matrix. This agrees with what reported previously regarding the utilization of wood powder and wheat straw [93].

Fig. 3
figure 3

The effect of initial concentration (Ci) on the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25 °C, dosage = 2 g L−1, 80 rpm, and Ci = 1, 5, 10 and 20 mg L−1, pH = 7, t = 5 min

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Effect of temperature

The effect of temperature is studied over a variant T = 25, 35, 45, and 55 °C. Wherein, 50 mL of 10 mg L−1 of UO22+ ion and 2 g L−1 of TAEA-SiO2 solid materials were shaken vigorously and controlled through a period of up to 3 h and pHi = 7. The percentage of capturing uranium (VI) ion increase with the increase of temperature as shown in Fig. 4. The maximum percentage of capturing uranium (VI) ion is 99% at T = 55 °C, while it is 93% at T = 25 °C. This is in line with what has been recently published regarding the sorption of uranium(VI) ion from aqueous solutions by using amidoxime-silica [72]. This indicates the endothermic sorption reaction and capturing process. We notice that no highly significant effect of temperature has appeared. Therefore, from the economic view, we use T = 25 °C for all experiment batches.

Fig. 4
figure 4

The effect of Temperatures on the capture of uranium (VI) ion from water by using Silica Gel particles functionalized tris(2-aminoethyl)amine moiety, T = 25, 35, 45, and 55 °C, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1, t = 5 min, pHi = 7.0)

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Thermodynamic isotherm

The sorption isotherms is a function of the uranium (VI) ion amount adsorbed into the SG-TAEA-NH2 surface. To investigate the distribution of uranium(VI) ion between the aqueous phase and the SG-TAEA-NH2 solid phase, the adsorption data were analyzed by using the Langmuir [94] and the Freundlich [95] adsorption isotherms. The isotherm experiments were conducted by using 1, 5, 10, and 20 mg L−1 of UO22+ ion solutions. All other sorption parameters are kept constant for each experiment (e.g. pHi = 7, T = 25 °C and 2 g L−1 dosage of SG-TAEA-NH2 material.

The Langmuir isotherm model of the uniform monolayer adsorption onto a surface can be linearized as given by Eq. 7:

$$\frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{1}{{q_{ \hbox{max} } b}} + \frac{1}{{q_{ \hbox{max} } }}C_{\text{e}}$$
(7)

where \(q_{\hbox{max} }\) is the adsorption capacity and the maximum capturing of uranium (VI) ion per unit dosage of SG-TAEA-NH2 material (mg g−1); and the energy of adsorption b is Langmuir constant (L g−1), which is exponentially proportional to the heat of adsorption and the affinity of binding sites. The linear plot of (Ce/qe) versus the equilibrium concentration (Ce) gives a straight line. The slope of the plot gives \(\frac{1}{{q_{{\max} } }}\), while the intercept of the plot gives \(\frac{1}{{ q_{{\max} } b}}\).

The Freundlich model represents the heterogeneous sorption of U(VI) ion on the surface of SG-TAEA-NH2 material followed by a condensation effect resulting from strong U(VI)-U(VI) ions interaction. The linear form of the Freundlich model is also given by Eq. 8:

$$\ln q_{\text{e}} = \ln K_{\text{F}} + \left( {\frac{1}{n}} \right)\ln C_{\text{e}}$$
(8)

Where qe represents the amount adsorbed U(VI) ion per amount of SG-TAEA-NH2 material at the equilibrium (mg g−1), Ce represents the equilibrium concentration (mg L−1), \(K_{\text{f}}\) (mg g−1) and n is the sorption capacity of the SG-TAEA-NH2 material and the intensity of sorption, respectively. The plot of the \(\ln q_{\text{e}}\) versus \(\ln C_{\text{e}}\) gives the slope (= \(\frac{1}{n}\)) and intercept (= ln \(K_{\text{f}}\)). The higher \(\frac{1}{n}\) the value indicates the more favorable of the chemisorption. The Freundlich adsorption correlation coefficients (R2), \(K_{\text{f}}\) and n are presented in Table 1.

Table 1 Freundlich constants of \(K_{\text{f}}\) and n
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Figures 5 and 6 exhibit Freundlich and Langmuir plots, respectively, for the sorption of U(VI) cation into the SG-TAEA-NH2 surface. The Langmuir sorption isotherm gives \(R^{2}\) < 0.800, which measure a bad goodness-of-fit for the experimental results. While the results reveal that the Freundlich sorption isotherm is the best model (R2 > 0.99). From the chemical point of view regarding the behavior of the complex reaction system on the surface, it should behave like a monolayer behavior following the Langmuir sorption isotherm model. Nevertheless, what we found is the opposite. The suggestion reason is that the SG-TAEA-NH2 surface does not have complete coverage by TAEA-NH2 entities. With this visualization, there is a small part of (≡ Si‒O–H) on the surface of silica gel still active, wherein most of the surface is covered with TAEA-NH2 entities. Through the sorption process, part of the U(VI) ion is adsorbed by ≡ Si‒O–H entities and other bulk ions captured by TAEA-NH2 entities. For this, we find that adsorption is heterogeneous and follows Freundlich sorption isotherm. Nearly similar results have been reported [85, 96]. Wherein, the heterogeneous layer of uranium (VI) ion is formed on the surface of powdery aerobic activated sludge including carboxyl (‒COOH), Hydroxyl (‒OH), Amino (‒NH2) achieving Freundlich isotherm [96]. Besides, our results are not in good qualitatively agreement with those found from adsorption of the iron ion with the SG-TAEA-NH2 material [85]. The reason may be due to the interaction affinity of iron and uranium with SG-TAEA-NH2 material and ≡ Si‒O–H entities in the acidic and neutral medium, respectively.

Fig. 5
figure 5

Frunedlich isotherm on the sorption of uranium (VI) ion by using Silica Gel particles functionalized tris(2-aminoethyl)amine moiety T = 25, 35, 45, and 55 °C, dosage = 2 g L−1, 80 rpm, and Ci = 1, 5, 10, and 20 mg L−1, t = 10 min, pH = 7.0

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Fig. 6
figure 6

Langmuir isotherm on the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25, 35, 45, and 55 °C, dosage = 2 g L−1, 80 rpm, and Ci = 1, 5, 10, and 20 mg L−1, t = 10 min, pH = 7.0

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The thermal parameters ΔH and ΔS can be calculated by using Vant Hoff linear Eq. 9:

$$\ln k_{\text{c}} = \frac{{\Delta S^{\text{o}} }}{R} - \frac{{\Delta H^{\text{o}} }}{RT}$$
(9)

The linear plot of the Vant-Hoff (\(\ln k_{\text{c}}\) vs \(\frac{1}{T}\)) gives the slope = \(\frac{{ H^{\text{o}} }}{R}\) and the intercept = \(\frac{{S^{\text{o}} }}{R}\) (see Fig. 7 and Table 2).

Fig. 7
figure 7

The plot of ln Kc versus I/T regarding the capturing of uranium (VI) ion by using Silica Gel particles functionalized tris(2-aminoethyl)amine moiety, t = 2 min, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1, pH = 7.0

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Table 2 Thermodynamic parameters of capturing U(VI) ion by SG-TAEA-NH2 material
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The positive value of enthalpy change (ΔH = + 48.71422 kJ mol−1) suggests the possibility of a strong binding between the UO22+ ion and the active site on the surface of the SG-TAEA-NH2 as an endothermic process. That means, the positive value of ∆H° further confirms the endothermic nature of the sorption. The positive value of entropy change (ΔS = + 0.1846623 kJ mol−1 K−1) reflects a good affinity of uranium (VI) ion towards the SG-TAEA-NH2 surface and increases the randomness at the solid–liquid interface during the sorption. At high temperature, the water molecules surrounded uranium (VI) ion decrease; this leads to increase the water molecules freedom. From other directions, less positive ΔS is due to the decreasing number of free molecules to add in on particle molecule of SG-TAEA-NH2-U6+, they were two fragments and be one fragment.

The spontaneity of sorption can be measured by the Gibbs free energy (ΔG). Based on obtaining isotherm results, the best choice to calculate ΔG is Eq. 10:

$$G = \Delta H - T\Delta S$$
(10)

The calculated ΔG are found to be a negative in sign (ΔG < 0) based on the changing in temperature values (25, 35, 45, and 55 °C, see Table 2). This indicates the spontaneous interaction of uranium ion into the SG-TAEA-NH2 surface as shown in Fig. 8. Furthermore, ΔG° values are found more negative and higher than − 40 kJ/mol, which indicates a chemisorption process.

Fig. 8
figure 8

Plot of The value of standard Gibbs free energy change (\(G)\) versus temperatures (T = 25, 35, 45, and 55 °C) for capturing of uranium (VI) ion from water by using the SG-TAEA-NH2

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The DKR isotherm is reported to be more general than the Langmuir and Freundlich isotherms. It helps to determine the apparent energy of adsorption. The characteristic porosity of adsorbent toward the adsorbate and does not assume a homogenous surface or constant sorption potential [97]. The Dubinin–Kaganer–Radushkevich (DKR) model has the linear form as in Eq. 11:

$$\ln q_{\text{e}} = \ln q_{\hbox{max} } - \beta \varepsilon^{2}$$
(11)

wherein \(q_{\text{e}}\) is DKR monolayer capacity (mg g−1), β is adsorption energy constant, \(q_{\hbox{max} }\) is the amount of U(VI) ion adsorbed per unit weight of SG-TAEA-NH2 material (mg g−1), and ε is the Polanyi potential. The Polanyi potential can be calculated by using Eq. 12:

$$\varepsilon = RT\ln \left( {1 + \frac{1}{{C_{\text{e}} }}} \right)$$
(12)

wherein \(C_{\text{e}}\) is the equilibrium concentration of U(VI) ion in aqueous solution (mg L−1), R is the gas constant, T is the temperature (K).

The sorption energy E can be calculated by Eq. 13:

$$E = - \frac{1}{{\sqrt { - 2\beta } }}$$
(13)

wherein The Dubinin–Kaganer–Radushkevich sorption energy E can confirm the adsorption mechanism as follow [98]:

E (kJ mol−1)

Indication

− 1 to − 8

Physisorption

− 8 to − 16

Ion-exchange

− 20 to − 40

Chemisorption

The slope of the plot of lnqe versus ε2 gives β (mol2 J−2) and the intercept yields the sorption capacity, \(q_{\hbox{max} }\) as shown in Fig. 9. The values of β and E, as a function of temperature, are listed in Table 3 with their corresponding value of the correlation coefficient, R2. The E value obtained shows that the sorption follows the chemisorption mechanism.

Fig. 9
figure 9

Dubinin–Kaganer–Radushkevich isotherm on the capture of uranium (VI) ion from water by using Silica Gel particles functionalized tris(2-aminoethyl)amine moiety, T = 25, 35, 45, and 55 °C, dosage = 2 g L−1, 80 rpm, and Ci = 1, 5, 10, and 20 mg L−1, t = 10 min, pH = 7.0

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Table 3 Dubinin–Kaganer–Radushkevich sorption parameters
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Adsorption kinetic

Effect of contact time

To study the kinetic models of such a sorption system, it is worth describing the sorption rate of aqueous U(VI) ion as a function of time-based (1) changing the initial concentration or (2) changing the Temperature.

Figure 10a–c show the effect of contact time onto the sorption process that controlled by changing initial concentration, temperatures, and pHi. In all cases, we find that the removal rate of uranium (VI) ion is high in the first 5 min. This is due to the availability of the uncovered surface area of the SG-TAEA-NH2 surface. During the next 5 min, the sorption equilibrium starts yielded a maximum removal of 99% (approx.). This finding is found better than what has been recently published regarding the removal of the U(IV) ion by using an NH2-functionalized ordered silica [99] and amidoxime silica [72].

Fig. 10
figure 10figure 10

a The effect of contact time controlled by changing initial concentration on the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety (T = 25 °C, dosage = 2 g L−1, 80 rpm, and Ci = 20, 10, 5, and 1 mg L−1, pH = 7). b The effect of contact time controlled by changing temperatures on the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety (T = 25, 35, 45, and 55 °C, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1, pH = 7). c The effect of contact time controlled by changing pH on the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety (T = 25 °C, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1)

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Sorption kinetic model

The rate constant and rate-determining step of the reaction can be measured by using the sorption kinetic models. In this respect, two commonly kinetic models of the sorption are used. Wherein, the pseudo-first-order kinetic model and its integral can be expressed by Eq. 14 [100, 101]:

$$\ln (q_{\text{e}} - q_{\text{t}} ) = \ln q_{\text{e}} - k_{1} t$$
(14)

where qe and qt (mg g−1) are the amounts of adsorbed U(VI) ion at equilibrium and at the time (t), respectively, k1 (min−1) is the pseudo-first-order rate constant, and t (min) is contact time. The linear plot of \(\ln (q_{\text{e}} - q_{\text{t}} )\) versus t provides the slop of the \(k_{1}\) and the intercept of \(\ln q_{\text{e}}\).

Besides, the pseudo-second-order kinetic model and its integral form are expressed by Eq. 15 [102, 103]:

$$\frac{t}{{q_{\text{t}} }} = \frac{1}{{k_{2} q_{\text{e}}^{2} }} + \frac{t}{{q_{\text{e}} }}$$
(15)

where k2 is the rate constant of the pseudo-second-order kinetic model (g mg−1 min−1). The value of k2 and \(q_{\text{e}}\) can be determined from the intercept and slop that can be generated by a plot of \(\frac{t}{{q_{\text{t}} }}\) versus t.

Firstly, the kinetics sorption describes the removal rate of aqueous U(VI) ion based on the changing of the initial concentration (Ci = 1, 5, 10, 20 mg L−1) at constant other parameters (pHi = 7, 80 rpm, dosage = 2 g L−1 of SG-TAEA-NH2 surface and T = 25 °C). Figures 11 and 12 show the pseudo-second-order kinetic and pseudo-first-order kinetic models, respectively. The pseudo-first-order kinetic model has the coefficient of determination value (R2), which is less than 0.020. While, the pseudo-second-order kinetic model has a correlation coefficient R2 = 1.0, which approve the results.

Fig. 11
figure 11

Pseudo-second order model related to the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25 °C, dosage = 2 g L−1, 80 rpm, and Ci = 20, 10, 5 mg L−1, pH = 7.0

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Fig. 12
figure 12

Pseudo-first order model related to the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25 °C, dosage = 2 g L−1, 80 rpm, and Ci = 20, 10, 5 mg L−1, pH = 7.0

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Secondly, the kinetic sorption describes the removal rate of aqueous U(VI) ion as a function of changing the temperatures (T = 25, 35, 45 and 55 °C) at constant other parameters (pHi = 7, 80 rpm, dosage = 2 g L−1 of SG-TAEA-NH2 surface and Ci = 10 mg L−1). The pseudo-second-order kinetic model has a linear plot with R2 > 0.995 as shown in Fig. 13. While the pseudo-first-order kinetic model has the coefficient of determination value R2 < 0.026. Therefore, the kinetic adsorption data were satisfactorily fitted to the pseudo-second-order model.

Fig. 13
figure 13

Pseudo-second order model related to the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25, 35, 45 and 55 °C, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1, pH = 7.0

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Based on kinetic adsorption experiments as a function of changing the temperatures or initial concentration vs. contact time, the kinetic sorption data indicate that the pseudo-second-order kinetic model can perfectly describe the sorption reaction of uranium ion onto SG-TAEA-NH2 surface. This means that the U(VI) ion can be chemi-adsorbed by the sorption sites onto the SG-TAEA-NH2 surface as shown in Schemes 2 and 3. We use tow experiment functions to be further proof that adsorption exhibits chemisorption behavior. The good matching between the experimental and the calculated (qexp and qcal) support the finding results of kinetic models as listed in Tables 4 and 5. This result was found matching with the reported one by utilizing amidoxime silica [72] and adsorbing iron by SG-TAEA-NH2 material [85].

Scheme 2
scheme 2

Reaction progress of sorption uranium (VI) ion onto the SG-TAEA-NH2 surface

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Scheme 3
scheme 3

Schematic representation of chemisorptions of U(VI) ion by the sorption sites of the SG-TAEA-NH2 surface

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Table 4 The parameters of the pseudo-Second order kinetic model
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Table 5 The parameters of the pseudo-second order kinetic model
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Figure 14 shows the plot of the Arrhenius equation (\(\ln k_{2} \,{\text{versus}}\, \frac{1}{T}\) as in Eq. 16), which gives the slop (= \(- \frac{{E_{\text{a}} }}{R}\)) and intercept (\(\ln A\)). Wherein, the Ea and A is the activation energy and collision frequency, respectively.

$$\ln k_{2} = \ln A - \frac{{E_{\text{a}} }}{RT}$$
(16)

The calculated activation energy (Ea = − 192.144854 kJ mol−1) has a negative sign. In this case, it is possible. There are attractive forces between the amino groups in the surface and uranium ion, there are no barrier, or the barrier is submerged. This means that barrier-less or there are complexes on the potential energy surfaces between the U(VI) ion and the surface of the adsorbent.

Fig. 14
figure 14

The Arrhenius plot related to the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety

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The Weber-Moris intraparticle diffusion model regarding the sorption of the U(VI) cation into the SG-TAEA-NH2 surface can be discussed through the plot of the qtvs.\(\sqrt t\) as mention in Eq. 17 [104]:

$$q_{\text{t}} = k_{\text{int}} \sqrt t + C$$
(17)

wherein kint is the intraparticle diffusion rate constant (mg g−1\(\min^{{\frac{1}{2}}}\)). The straight line of the plot confirms intraparticle diffusion sorption. A plot presents multi-linearity, which indicates that three steps occur. These are the external surface adsorption (0–2 min), intraparticle diffusion (3 up to 20 min), and final equilibrium stage (t > 20 min). Herein, we could say that the intraparticle diffusion could be the rate-controlled (kint = 0.0479 mg g−1\(\min^{{\frac{1}{2}}}\) and R2 = 0.8031, see Fig. 15). This finding matches our recent study related to using SG-TAEA-NH2 material for the sorption of iron ion from water [85].

Fig. 15
figure 15

Weber-Moris intra-particular diffusion model related to the capture of uranium (VI) ion from water by using Silica Gel functionalized tris(2-aminoethyl)amine moiety, T = 25 °C, dosage = 2 g L−1, 80 rpm, and Ci = 10 mg L−1, pH = 7.0

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Conclusion

Capturing of uranium (VI) ion from water by the silica gel functionalized tris(2-aminoethyl)amine moiety (SG-TAEA-NH2) is effective in the pH range of 7–9. The capturing percentage increases by increasing the temperature and decreasing the initial concentration. The maximum capturing is 99% based on the sorption parameters of T = 25 °C, dosage = 2 g L−1, 80 rpm, and low initial concentration, t = 5 min. The sorption equilibrium can be reached within 5–10 min in maximum. The obtained experimental data have excellent fits within the Freundlich isotherm (R2 > 0.999) proving that the surface of SG-TAEA-NH2 is un-uniform. The kinetic sorption data fits very well with the pseudo-second-order model indicating chemisorption behavior. Depending on the logic of coordination chemistry, sorption isotherm, FTIR, and kinetic model, the metal complex spheres can be formed spontaneous, favorable on the surface through the chemical interaction between the primary amine active sites (–NH2) and U(VI) ion as heterogonous-layers. The new finding is the utilization of SG-TAEA-NH2 as a good potential material for the removal of uranium ion from water.