Introduction

With the development of nuclear energy, uranium has been comprehensively applied as nuclear fuel in nuclear plants [1, 2]. Due to its radioactivity and strong biological toxicity, uranium is known as a dangerous metal element in natural environment, which can finally reach the top of the food chain and be assimilated by humans, subsequently causing severe and irreversible kidney or liver injury and even death due to gradual accumulation in human [3,4,5]. Thus, the contamination of natural water sources by uranium is a well-known environmental problem and has been a public health concern for many years [6,7,8].

World Health Organization guidelines regulated that the maximum concentration of uranium in drinking water should be below 0.03 mg L−1. The permissible emission level of uranium for nuclear plants ranges from 0.1 to 0.5 mg L−1 [9, 10]. Therefore, it is important to remove uranium from water samples. However, the separation of uranium ions in the presence of relatively high concentration of various ions is a challenging work. In addition, traditional method, such as evaporation method, is an energy-intensive and inefficient process. Consequently, the development of a new material to adsorb uranium effectively from the aqueous solution is imperative.

In the past decades, a number of techniques have been used for the separation of dangerous metal ions, including precipitation [11], liquid–liquid extraction [12] and solid-phase extraction (SPE) [13,14,15,16]. Among these techniques, solid phase extraction has received much attention in recent years [4]. Compared with traditional technologies, SPE process has a number of advantages over other processes due to flexibility, simplicity, inexpensive, low consumption of reagents and less pollution to the environment [17,18,19,20,21,22,23,24]. Thus, various adsorbents, such as Amberlite XAD resin [25], chelating adsorbents [26, 27], glycerol–silica gel [28], modified chitosan resin and other adsorbents [29,30,31,32,33,34] have been developed and used to adsorb metal ions from aqueous solutions. However, the synthesis of new extractant is expensive and not easy due to the requirement of uncommonly available chemicals and complex process. Therefore, it is desired to find out more efficient, cheap and commonly available extractants for the adsorption of U(VI). The use of impregnated resins is particularly convenient because it is easy to prepare. Active ingredients of drugs are commonly available in the market and have received little attention for their application as metal extractants.

As an extractant, nalidixic acid (HNA) (1-ethyl-1, 4-dihydro-7-methyl-4-oxo-1, 8-naphthyridine-3-corboxylic acid) has been employed for selective adsorption of uranium(VI) from various aqueous solutions [35]. Nalidixic acid is a weak acid and its pKa value is about 5.9–6.3 [36]. It has been commonly utilized as an antibacterial drug to remedy several bacterial diseases [37,38,39]. The existence of carbonyl group at position 4 and carboxylic group at position 3 in this compound make it feasible to form complexes with some metals [40, 41]. Previously, several studies about the potential efficiency of HNA for the extraction of diverse metal ions have been reported [35, 42]. However, the application of HNA as an extractant impregnated into solid silica-based macroporous material has not been studied.

In this work, HNA/SiO2-P was prepared by impregnating HNA into a macroreticular styrene–divinylbenzene (SDB) copolymer which was immobilized in porous silica support (SiO2-P) with a diameter of 60 µm [43]. The new synthesized solid adsorbent was superior to original extractant (HNA) in some aspects, such as better mechanical strength, acid and radiation resistance [44]. The prepared adsorbent was characterized and used for extracting U(VI) from aqueous solutions by batch experiments. In this study, the adsorption properties, chemical stability of the adsorbent and effects of interference ions on uranium adsorption were investigated. The adsorption kinetics, isotherm and thermodynamic of U(VI) onto the adsorbent were also studied in detail.

Experimental

Materials

All chemicals and reagents used for experiments were procured from suppliers and were of analytical grade. Nalidixic acid employed in experiments was commercial reagent from J&K Chemical suppliers. The chemical structure of nalidixic acid is shown in Fig. 1. U(VI) solutions were prepared by dissolving uranyl nitrate hexahydrate [UO2(NO3)2·6H2O] with deionized water. Stock solutions of diverse metal elements were prepared by the high purity salts of the cations. The required pH values of the solutions were adjusted by adding appropriate quantity of nitric acid and sodium hydroxide solutions, which were checked by laboratory pH meter. All of HNA/SiO2-P adsorbents employed in this work were synthesized in our laboratory.

Fig. 1
figure 1

Chemical structure of nalidixic acid (a) and SEM image of HNA/SiO2-P (b)

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Preparation of HNA/SiO2-P adsorbent

Silica based polymer was synthesized by a method reported by Wei et al. [43]. Preparation of the adsorbent was conducted as the following procedure: Firstly, the SiO2-P particles were washed with methanol in a conical flask at room temperature for 1 h. Such operation was repeated for 3 times, and the residue was dried in vacuum oven at 40 °C for 24 h. Subsequently, 10.0 g of HNA were placed in flask and dissolved by dichloromethane. Afterwards, 20.0 g of dried SiO2-P were added into the solution and the mixture was rotated for 1 h at room temperature. Then the flask was immersed in water bath and rotated at around 40 °C. Meanwhile, the pressure was reduced by a rotary evaporator to impregnate HNA molecules into the pores of SiO2-P. After drying the remainder in vacuum oven for 12 h at 40 °C, HNA/SiO2-P adsorbent was obtained and sealed in cool and dry place. The experimental flow chart of synthesizing adsorbent is plotted in Fig. 2. The microstructure of the synthesized HNA/SiO2-P adsorbent was characterized by scanning electron microscope (SEM, Sirion 200, FEI COMPANY) and the SEM image is illustrated in Fig. 1b, in which the spherical particle with a mean diameter of 60 μm was confirmed.

Fig. 2
figure 2

Experimental flow chart of synthesizing adsorbent

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Batch experiments

In batch experiments, weighed adsorbent particles were contacted with measured volume of various solutions in a glass vial, which subsequently shaken mechanically in a gas bath thermostatic oscillator for a given time. The adsorbent was separated from solution by filter and corresponding concentrations of metal ions before and after absorption were analyzed by inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu ICPS-7510). Adsorption capacity (Q, mg g−1), adsorption efficiency (R, %) and distribution coefficient (K d, mL g−1) were calculated as follows:

$$ Q = \frac{{\left( {C_{0} - C_{\text{e}} } \right)V}}{m} $$
(1)
$$ R = \frac{{C_{0} - C_{\text{e}} }}{{C_{0} }} \times 100\% $$
(2)
$$ K_{\text{d}} = \frac{{C_{0} - C_{\text{e}} }}{{C_{\text{e}} }} \times \frac{V}{m} $$
(3)

where C 0 and C e denote the concentrations of metal ions before and after absorption in the liquid phase in mg L−1, respectively. V and m are the volume of liquid phase in mL and weight of adsorbent in g.

Results and discussion

Thermal analysis of HNA/SiO2-P

The thermal stability of HNA/SiO2-P was evaluated by thermal gravimetry analyzer (TG–DTA, Shimadzu DTG-60) at the operating temperature range of 30–600 °C, with a heating rate of 2 °C min−1 under oxygen atmosphere. The results are shown in Fig. 3. As seen, the curves of SiO2-P appeared a weight loss at around 280 °C. It was assumed to be a burning of SDB copolymer. The finally weight loss of SiO2-P was estimated to be 21.9% which indicates that corresponding weight percentage of SDB had been polymerized inside the SiO2 substrate. According to thermal analysis of HNA, the curves of HNA/SiO2-P showed a large weight loss at around 220–230 °C which could be explained as thermal decomposition of HNA. The estimated overall weight loss of HNA/SiO2-P was 48.5%. Thus, the ratio of copolymer and HNA in adsorbent can be calculated by the ratio of SiO2 in HNA/SiO2-P. Consequently, the component (wt%) of HNA/SiO2-P adsorbent was determined as 34.1% HNA, 51.5% SiO2 and 14.4% organic copolymer, respectively.

Fig. 3
figure 3

TG–DTA curves of SiO2-P support (a), HNA extractant (b) and HNA/SiO2-P adsorbent (c) in an atmosphere of O2 (20 cm3 min−1) at a heating rate of 2 °C min−1

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Spectroscopic studies

The FT-IR spectra of SiO2-P, HNA and HNA/SiO2-P were investigated in the spectral wavenumber range 3200–1330 cm−1. The obtained IR spectra are illustrated in Fig. 4. It was found that the IR spectrum of HNA/SiO2-P showed the peak of framework of nalidixic ring gave rise to a band at about 1450 cm−1. In addition, the stretching vibration band of C=O due to carboxyl group as a peak appeared at near 1715 cm−1. The absorption peak of ring C=O stretching vibration was found to be at about 1620 cm−1. The aromatic C–H stretching vibrations of nitrogen heterocyclic aromatic compounds band appeared at 3100–3010 cm−1. Therefore, the analytical results demonstrated that nalidixic acid had been impregnated into porous silica-based polymer after preparation.

Fig. 4
figure 4

FT-IR spectra of SiO2-P (a), HNA/SiO2-P (b) and HNA (c)

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Chemical stability of the HNA/SiO2-P adsorbent

In order to study the chemical stability of SiO2-P and HNA/SiO2-P, weighted amount of adsorbents were contacted with aqueous solutions with different pH value in glass vials and shaken at room temperature. After determined contact time (96 h), the aqueous phase was separated from adsorbents and the concentration of total organic carbon (TOC) in aqueous phase before and after contact was measured by a TOC analyzer (TOC-V, Shimadzu, Japan).

The influence of pH on leakage percentage of extracting agent is shown in Fig. 5. It was found that SiO2-P is quite stable in experimental conditions, however the leakage percentage of HNA/SiO2-P increased with the increasing pH value at room temperature. The maximum leakage percentage in aqueous phase was about 83% at pH 13 after 96 h contact time. It is assumed that the leakage is on account of the solubility of the extracting agent in aqueous solution due to protonation [45]. According to the Eq. (4), these results can be attributed to the change of concentration of H3O+ which leaded to equilibrium shift of the equation. Thus, HNA/SiO2-P is considered as a relatively stable adsorbent at lower pH value. However, the adsorbent gained a weaker chemical stability within pH values higher than 9.

(4)
Fig. 5
figure 5

Effect of pH on leakage percentage in aqueous solution (phase ratio: 0.1 g/35 cm3; contact time: 96 h; temperature: 25 °C)

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Effect of pH

Considering that the pH of sample solution is one of the important variables for the adsorption of metal ions by adsorbent, the effect of this factor on the recovery was investigated. The pH value of the solution was studied in range of 1–12 and the results are presented in Fig. 6. The pH value of maximum and quantitative absorption was found to be 3.5–10.0. When pH value was lower than 3.5, the recovery of uranium increased with the pH value. However, an opposite tendency was observed at the pH ranges of 10–12. The obtained results can be explained by the appearance of various species of U(VI) at the different pH values in the solution. The lower adsorption at lower pH (0–3.5) could be due to the high concentration of H+ which caused a competition against to UO2 2+. The maximum absorption at pH value higher than 3.5 is assumed attributed to the formation of other ions, such as [UO2(OH)]+, [(UO2)2(OH)2]2+ and [(UO2)3(OH)5]+ [46]. With the increase of pH value, it was easy for UO2 2+ to form a stable precipitation with OH− at pH value higher than 10.0, which caused a reduction of concentration of UO2 2+.

Fig. 6
figure 6

Effect of pH on the recovery of uranium adsorbed by HNA/SiO2-P (initial concentration: 20 mg L−1; phase ratio: 0.01 g/5 cm3; contact time: 180 min; temperature: 25 °C)

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Effects of interfering ions

The influence of co-existing ions is a very important factor in the adsorption of uranium in natural samples. Hence, in order to assess the selectivity of the synthesized adsorbent, the property of the selective adsorption of U(VI) in the presence of several commonly existing cations and anions was examined by measuring the recovery of uranium under optimized conditions. The main interfering ions investigated in this work were: Na+, K+, Ca2+, Mg2+, Mn2+, Cr3+, Co2+, Ni2+, Cu2+, SO4 2−, Cl− and NO3 −. A certain amount of U(VI) were mixed with different amounts of foreign ions in 5 ml aqueous solution and contacted with adsorbent. Concentrations of foreign ions [C 0 (mg L−1)] are listed in Table 1. The obtained results of experiments are plotted in Fig. 7. As can be seen, under the conditions specified in the procedure, all co-existing ions had no obvious effect on the adsorption process in aqueous solutions containing 20 and 40 mg L−1 uranium.

Table 1 Concentrations of foreign ions [C 0 (mg L−1)]
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Fig. 7
figure 7

Influence of foreign ions on the adsorption of uranium (phase ratio: 0.01 g/5 cm3; pH 6; contact time: 180 min; temperature: 25 °C)

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Kinetic study

Adsorption kinetic experiments were performed by shaking 20, 40 and 80 mg L−1 of uranium solutions in the shaker at 25 °C for different time. The results of experiments are shown in Fig. 8. It was evident that the percentage of adsorbed uranium onto adsorbent increased with contact time at the beginning. However, the adsorption rate decreased with the concentration of uranium. The uptake of uranium reached equilibrium state after about 60 min of contact. It seems that the adsorption process occurred in two steps. The first step was rapid ion adsorption within 40 min of contact. The subsequent step was a relatively longer time before the uptake equilibrium reached. In order to explain the adsorption kinetics of U(VI), the pseudo-second order equation was applied to analyze the experimental data. The expression of equation was shown as follow:

$$ \frac{t}{{q_{t} }} = \frac{1}{{K_{2} Q_{\text{eq}}^{2} }} + \frac{t}{{Q_{\text{eq}} }} $$
(5)

where q t and Q eq (mg g−1) denote the amounts of ions absorbed onto adsorbent at time t (min) and at equilibrium state, respectively. K 2 is the rate constant of pseudo-second order equation.

Fig. 8
figure 8

Effect of contact time on uranium recovery by HNA/SiO2-P (initial concentration: 20, 40, 80 mg L−1; pH 6; phase ratio: 0.01 g/5 cm3; temperature: 25 °C)

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The plots of t/Q t versus t are illustrated in Fig. 9. The values of parameters were calculated from the intercept and slope of the plots and are summarized in Table 2.

Fig. 9
figure 9

Pseudo-second-order kinetic fitting for adsorption of uranium onto HNA/SiO2-P (initial concentration: 20, 40, 80 mg L−1; pH 6; phase ratio: 0.01 g/5 cm3 at 25 °C)

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Table 2 Kinetic parameters of uranium adsorption onto HNA/SiO2-P at 25 °C
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The values of correlation coefficient are very high in the case of pseudo-second-order kinetic model. The values of the rate constant (K 2) at different initial concentration are 0.0378, 0.00750, 0.00193 g mg−1 min−1, respectively. These results are compliance with the experimental results. Thus, this model is suitable to indicate the kinetic behavior of the adsorption. Since the chemisorption processes are in accord with the pseudo-second-order kinetic model, it can be inferred that the adsorption of uranium onto the adsorbent can be well explained by the pseudo-second-order equation, indicating that rate-controlling step might be chemisorption [47].

Adsorption isotherm

In order to evaluate the adsorption capacity of uranium onto HNA/SiO2-P, the batch experiments were conducted at optimum condition. A certain amount of resin (0.01 g) was contact with the 5 mL of solution containing different initial concentration of uranium respectively. The maximum capacity of HNA/SiO2-P for U(VI) ion was determined and the result is shown in Fig. 10. The maximum adsorption capacity of adsorbent was found to be 35.4 mg g−1.

Fig. 10
figure 10

Capacity of uranium adsorbed onto HNA/SiO2-P (pH 6; phase ratio: 0.01 g/5 cm3; temperature: 25 °C; contact time: 180 min)

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The adsorption isotherm implies the relationship between the amount of adsorbate ions onto adsorbent and metal ions concentration at equilibrium state. The Langmuir equation is based on the assumption that the reaction of adsorption is a monolayer adsorption with constant energy, and without movement of adsorbate in the plane of surface [18]. The Langmuir equation is given as follow: [48]

$$ \frac{{C_{\text{e}} }}{{Q_{\text{e}} }} = \frac{{C_{\text{e}} }}{{Q_{\hbox{max} } }} + \frac{1}{{K_{\text{L}} Q_{\hbox{max} } }} $$
(6)

where C e (mg L−1) and Q e (mg g−1) denote the equilibrium concentration of U(VI) in aqueous and solid phases, respectively. K L (L mg−1) is the Langmuir constant, and Q max (mg g−1) is the maximum amount of U(VI) adsorbed on adsorbent. The data of experiments were plotted in a figure, and the linear plot of C e/Q t versus C e is shown in Fig. 11. The values of K L and Q max are also calculated from the plot and listed in Table 3. The correlation coefficient (R 2) for the Langmuir plot was found to be 0.999, which indicates that the Langmuir isotherm was suitable for the adsorption. As we all know, uranium can also be adsorbed by macroporous silica, therefore, it is necessary to make a study on the isotherm of uranium adsorption onto SiO2-P. The results are plotted in Fig. 12, and the values of parameters are listed in Table 3.

Fig. 11
figure 11

Adsorption isotherm of uranium on HNA/SiO2-P at 25 °C with Langmuir model (pH 6; phase ratio: 0.01 g/5 cm3; contact time: 180 min)

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Table 3 Isotherm parameters of uranium adsorption onto HNA/SiO2-P and SiO2-P at 25 °C
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Fig. 12
figure 12

Adsorption isotherm of uranium on SiO2-P at 25 °C with Langmuir model (pH 6; phase ratio: 0.01 g/5 cm3; contact time: 180 min)

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Moreover, an essential parameter of Langmuir isotherm, R L, also expresses whether Langmuir treatment favorable or not. It can be called equilibrium parameter. The equation can be expressed as follow: [49]

$$ R_{\text{L}} = \frac{1}{{1 + K_{\text{L}} C_{0} }} $$
(7)

where C 0 represents initial concentration of U(VI); K L is Langmuir constant. The values of R L are shown in Table 4. Consequently, the values of R L appeared between 0 and 1, which meant that the adsorption of U(VI) onto HNA/SiO2-P was favorable.

Table 4 Langmuir equilibrium parameter (R L)
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Adsorption thermodynamic

In order to understand the influence of temperature on the adsorption process, uptake of uranium onto HNA/SiO2-P was measured at temperatures of 298, 308, 318 and 328 K. The experimental data are depicted in the plot of lnK d versus 1/T which are shown in Fig. 13. The thermodynamic parameters of the adsorption reaction such as standard enthalpy change (ΔH°), standard entropy change (ΔS°) and standard Gibbs free energy (ΔG°) were calculated by the Van’t Hoff equation and the Gibbs–Helmholtz equation [50]:

$$ \ln \left( {K_{\text{d}} } \right) = \frac{{\Delta S^{ \circ } }}{R} - \frac{{\Delta H^{ \circ } }}{RT} $$
(8)
$$ \Delta G^{ \circ } =\Delta H^{ \circ } - T\Delta S^{ \circ } $$
(9)

where T (K) is the absolute temperature; R [8.314 (J mol−1 K−1)] denotes the gas constant and K d is the distribution coefficient on equilibrium. The values of the calculated thermodynamic parameters are listed in Table 5.

Fig. 13
figure 13

Van’t Hoff plots for the adsorption of uranium on HNA/SiO2-P in aqueous solution (phase ratio: 0.01 g/5 cm3; pH 6; contact time: 180 min)

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Table 5 Thermodynamic parameters for uranium adsorption on HNA/SiO2-P
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The positive ΔH° indicates that the adsorption reaction is an endothermic reaction. Meanwhile, the negative value of ΔG° means that the adsorption process was spontaneous under experimental conditions.

Conclusions

A novel HNA/SiO2-P adsorbent was synthesized by impregnating HNA into a macroporous silica material (SiO2-P). The characterization of adsorbent illustrated that HNA had been impregnated into SiO2-P composite. Adsorption behavior of uranium from aqueous solution onto HNA/SiO2-P adsorbent was systematically investigated.

It was found that the good performance of U(VI) adsorption could be attained in a broad pH range. Moreover, the adsorption kinetics data were in accordance with pseudo-second-order equation, which indicates that rate-controlling step of adsorption was chemisorption. Furthermore, the adsorption of U(VI) onto adsorbent was well described by the Langmuir isotherm. In addition, adsorption thermodynamic study suggested that the adsorption reaction was an endothermic reaction and higher temperature was in favor of the adsorption process. Consequently, the obtained HNA/SiO2-P showed a great potential to be an effective adsorbent for U(VI) adsorption from aqueous solutions containing a high concentration of commonly existing cations and anions. These results have demonstrated that the macroporous silica material impregnated by the active ingredients of some common drugs can also be effective for the adsorption of metal ions.