Preparation and characterization of a core–shell KNO₃@alginate-Ca particle with uranium-removal and slow-release of KNO₃

Abstract

A novel core–shell KNO₃@alginate-Ca particle with uranium-removal and slow-release of KNO₃ was prepared by a simple method of coaxial electrospinning. The feasibility of the core–shell KNO₃@alginate-Ca particle used for uranium removal and slow release of potassium was investigated. The main factors affecting the removal of uranium, including pH, initial concentration, temperature and contact time were investigated. The core–shell KNO₃@alginate-Ca particle displayed a high adsorption capacity of 0.825 mmol g⁻¹ at a pH of 5, initial concentration of 0.963 mmol L⁻¹ and a temperature of 318.15 K, and the maximum uranium absorption frequency was 91.143% at pH 5, 318.15 K and an initial concentration of 0.074 mmol L⁻¹. The adsorption data were fitted well with the non-linear Langmuir isotherms and non-linear pseudo-first-order kinetics. The presence of other cations like K(I) ions, Na(I) ions, Zn(II) ions and Mg(II) ions had almost no effect on the uranium adsorption. The uranium adsorption process was feasible and spontaneous. The adsorption of uranium on the core–shell KNO₃@alginate-Ca particle was mainly attributed to exchange between the calcium and uranium. The study of nutrient slow release revealed that the particle showed excellent slow-release properties. Thus, the core–shell KNO₃@alginate-Ca particle was a promising adsorbent for uranium removal and a slow-release material for potassium release.

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1. Introduction

Uranium (U), which has a number of adverse effects and unforeseeable consequences for ecosystems and human health, is a radioactive and chemotoxic heavy metal.¹ Major uranium sources come from the widespread use of nuclear energy, ore processing, military activities and coal combustion.² Thus, the removal of uranium from waste waters and soil is of great importance.

A variety of techniques including physical, chemical and biological methods have been developed or applied for the removal of uranium.³ Among them, phytoremediation, as a modern technique, is widely used for the remediation of media contaminated with uranium, depending on several factors including its simplicity in operation, reliability, preventing destroying of soil structure and function and lower cost.^4–6 Plants' growth is one of the main factors that influence the efficiency of phytoremediation and fertilizers is one of the vital input materials for the growth of plants. KNO₃ is the most widely used fertilizers around the world as plant nutrients,⁷ because of its nutrient-rich which contains not only nitrogen but also potassium, therefore KNO₃ is selected as a nutrient for release study in this thesis. It is very important to use the fertilizers effectively. However, the normal fertilizers without coating materials cannot effectively reach to the plant, it is washed off by rain and irrigation water,⁸ which causes serious environmental pollution. Especially, about 40–70% of nitrogen and about 50–70% of potassium of the applied normal fertilizers is lost to the environment.⁹

Considering such drawbacks, KNO₃ with coating materials that release nutrients at controlled rates to maintain plants' maximum growth and fertilizers' minimum losses have gained much attention.^10,11 Currently, the degradability of coating materials of slow-release fertilizers is an important focus of the research in the field of slow-release fertilizers, because the degradable materials in fertilizer possess environmentally friendly properties.^12,13 Sodium alginate is a biodegradable material that is generally regarded as a safe substance due to its low cost, simplicity of handling, nontoxicity and can easily bind divalent cations such as Ca(II) ions to form stable gel particle in the water solution.^14,15 In spite of the advantages of alginate applied as fertilizer carriers, the weak mechanical strength and easily released nutrient hinder its application.

To solve the problem, one of the alternatives is to add humic acid (HA) in gel-forming process of alginate polymers to fabricate a composite to control the release of nutrient. The introduction of humic acid not only enhances nutritive, but also reduces loss of fertilizer and protects the environment.¹⁶ For further improving the performance of slow-release fertilizer, glutaraldehyde is used as the crosslinking agent to make the molecular chains relatively stable.¹⁷ Moreover, alginate possesses numerous carboxyl and hydroxyl, has adequate affinity for uranium, so that the material can be used as an adsorbent to efficiently remove uranium from wastewater.

On the basis of the above background and our previous studies on biomass materials, in this study (i) a novel core–shell KNO₃@alginate-Ca particle with uranium-removal and slow-release of KNO₃ was prepared by a facile method of coaxial electrospinning, whose core was the mixture of HA, KNO₃ and alginate, and the outer shell was crosslinked calcium alginate. Core–shell KNO₃@alginate-Ca particle could adsorb uranium and have slow-release property of KNO₃ at the same time. (ii) The effects of various parameters such as pH, contact time, temperature and different initial uranium molar concentrations on uranium adsorption were studied. The adsorption isotherm and adsorption kinetics were investigated to understand the adsorption process in detail. The slow release property of KNO₃ in the particle also has been examined. (iii) The adsorption mechanism of uranium and release mechanism of KNO₃ were elucidated by means of SEM, FT-IR, EDX and XPS.

2. Material and methods

2.1 Materials

Sodium alginate (NaAlg), calcium chloride anhydrous (CaCl₂), potassium nitrate (KNO₃), humic acid (HA), dichloromethane (DCM), magnesium hexahydrate (Mg(NO₃)₂·6H₂O), sodium nitrate (NaNO₃), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and glutaraldehyde (C₅H₈O₂) used in the experiment were of analytical grade, all of which were purchased from Kelong Co., Ltd without further purification. Uranium nitrate hexahydrate (UO₂(NO₃)₂·6H₂O) is obtained from HuBei Chushengwei Chemistry Co., Ltd. Stock solution of uranium with the concentration of 3.704 mmol L⁻¹ is prepared by dissolving 1.8588 g UO₂(NO₃)₂·6H₂O in 1000 mL deionized water, and the required concentrations are obtained by diluting stock solution before they are used. pH was adjusted to the required value using 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH.

2.2 Synthesis of core–shell KNO₃@alginate-Ca particle

Core–shell KNO₃@alginate-Ca particle were fabricated according to the process schematically shown in Fig. 1 using a coaxial electrospinning machine (DT-200, Dalian ding tong technology development Co. Ltd) equipped with two syringe pumps connected to a concentric nozzle. The shell fluid was sodium alginate aqueous solution at a concentration of 2 wt%. The core fluid was prepared by mixing 5 mL of dichloromethane, 10 g of KNO₃, 1.2 g of HA, 1.5 g of sodium alginate with 100 mL of distilled water. The core and shell fluids were separately loaded into two 20 mL plastic syringes, and then pumped drop wise into a gelling bath of 5% (w/v) CaCl₂ aqueous solution for 1 h through the concentric nozzle, the shell syringe and core syringe at a rate of 0.020 mm s⁻¹, 0.167 mm s⁻¹, respectively. After that, 3 mL of C₅H₈O₂ was added into the gelling bath and the crosslinking process of the particle was carried out for another 24 h. The fabricated particle was washed thrice with deionized water to remove any residual ions, and dried at 313.15 K for 10 h.

Fig. 1 Schematic diagram of the set-up of coaxial electrospinning.

2.3 Batch adsorption experiments

The batch experiments were carried out in order to evaluate the adsorption processes of uranium. All the adsorption experiments were performed with 0.05 g of core–shell KNO₃@alginate-Ca particle suspended in 50 mL of uranium solution in a conical flask at selected pH, and shaked in a thermostated shaker at a speed of 140 rpm. The effect of initial pH of the solution on uranium adsorption was studied with the initial uranium molar concentration of 0.074 mmol L⁻¹ and the pH between 2 and 7 adjusted by 0.1 mol L⁻¹ NaOH and 0.1 mol L⁻¹ HCl. The same initial concentration was used for the investigations of kinetics of the U(VI) adsorption, and the equilibrium concentrations were measured after a predetermined contact time. For the isotherm experiments, the experiments were conducted at varying uranium concentration ranging from 0.074 to 0.963 mmol L⁻¹ in the different temperatures (288.15 K, 298.15 K, 308.15 K, 318.15 K). In the experiment assessing the effect of co-existing cations, the uranium molar concentration was fixed at 0.074 mmol L⁻¹ and the concentrations of the co-existing cations K(I) ions, Na(I) ions, Zn(II) ions and Mg(II) ions ranged from 0 to 3.0 mmol L⁻¹. Uranium adsorption was conducted in double component systems U/K, U/Na, U/Pb, U/Zn and U/Mg to determine the influence of different cations. The initial and residual uranium concentrations in solution were analyzed by UV-Vis spectro-photometer (UV-3900, Hitachi Corporation). All experiments were replicated thrice and the average values were used in calculations. The adsorption capacity (q_e) was calculated using the following formula:¹⁸

Absorption frequency based on the percentage of uranium removal was calculated by following formula:¹⁸

where q_e (mmol g⁻¹) is the adsorption capacity of adsorbent, C₀ (mmol L⁻¹) and C_e (mmol L⁻¹) are the uranium molar concentration of the initial solution and equilibrium solution, respectively, m (g) is the mass of the adsorbent and V (L) is the volume of uranium solution.

2.4 Slow-release behavior of KNO₃ in core–shell KNO₃@alginate-Ca particle

To study the slow-release behavior of KNO₃ in core–shell KNO₃@alginate-Ca particle, the following experiment was carried out: 0.1 g samples were added into conical bottles containing 50 mL of distilled water. Then, the bottles were kept at 298.15 K in an incubator for the duration of the experiment. At certain time intervals (1 or 2 h), 5 mL of solution were sampled for potassium determination and an additional 5 mL of water was carefully injected into the bottles to maintain a constant amount of solvent. The potassium concentrations in the solution were measured by means of atomic adsorption spectrometer analyzed (AAS). The release experiments were carried out in triplicate, and the average value was taken as the result. The cumulative release was obtained by the following formula:¹³where C_K⁺ (mmol L⁻¹) is the accumulative release potassium concentration of the particle, V_E (L) is the sampling volume, V₀ (L) is the initial volume of release media, C_i and C_n are the potassium concentration (mmol L⁻¹) at the sampling times i and n, respectively.

2.5 Characterization of core–shell KNO₃@alginate-Ca particle

The mechanical properties were determined by a stable micro systems texture analyser (TA-XT plus, Stable Micro System Ltd). The test was conducted in a compression schemes, pre-test speed was 10 mm s⁻¹, test speed was 10 mm s⁻¹, post-test speed was 5 mm s⁻¹, target stress was 50 g mm⁻¹ and the trigger force was 0.005 kg. All measurements were performed on 12 beads and the average values were used in calculations. The morphology and the structure of the core–shell KNO₃@alginate-Ca particle was obtained by scanning electron microscopy (SEM, Ultra 55, Zesis Corporation), and then the energy dispersive spectrometer (EDX) was employed to understand the element composition and distribution of the particle before and after adsorption, before and after release potassium. Core–shell KNO₃@alginate-Ca before and after adsorption was analysed using a Fourier transform-infrared spectrometer (FT-IR, Spectrum One Autoima, USA) in the wavenumber range of 400–4000 cm⁻¹. The possible adsorption and release mechanism of the particle were studied by X-ray Photoelectron Spectroscopy (XPS, Thermo Scientific Escalab 250, Thermo Fisher Corporation).

3. Results and discussion

3.1 Uranium adsorption studies

3.1.1 Effect of pH on adsorption. The effect of pH on uranium adsorption by core–shell KNO₃@alginate-Ca particle was analyzed over the initial pH range from 2 to 7. As shown in Fig. 2, the adsorption capacity and absorption frequency of core–shell KNO₃@alginate-Ca particle for uranium, constant at the range of pH 3 to pH 6, adsorption capacity was equal to 0.065 mmol g⁻¹ and absorption frequency reaches up to 96.50%. While the pH value decreased from 3 to 2, a significant decline in uranium adsorption capacity and absorption frequency appeared, which is probably due to that more hydrogen ions will compete with UO₂²⁺ to bind with active sites.¹⁹ While the pH value increased from 6 to 7, there was a significant decline in the adsorption capacity and absorption frequency of the adsorbent for uranium, which may be ascribed to the presence of the hydrolysis products such as UO₂(OH)⁺, (UO₂)₂(OH)₂²⁺ and (UO₂)₃(OH)⁵⁺.²⁰

Fig. 2 Effect of solution pH on uranium adsorption by core–shell KNO₃@alginate-Ca (initial uranium molar concentration: 0.074 mmol L⁻¹, contact time: 15 h, temperature: 298.15 K, adsorbent dose: 0.05 g).

3.1.2 Effect of contact time on adsorption and adsorption kinetics. Fig. 3 shows the effect of contact time on uranium adsorption by core–shell KNO₃@alginate-Ca particle. It could be seen that the adsorption capacity and absorption frequency of the particle for uranium increased with an increase of contact time. Then, it rose slowly and reached equilibrium within 15 h. The initial rapid increased of adsorption capacity and absorption frequency can be attributed to that the surface of the adsorbent had many reaction sites, which were available for the adsorption of U(VI) ions.²¹ Along with the uranium ions occupation of the active binding sites on the adsorbent, lead to a slower adsorption rate.²²

Fig. 3 Effect of contact time on uranium adsorption by core–shell KNO₃@alginate-Ca adsorbent (initial uranium molar concentration: 0.074 mmol L⁻¹, pH: 5, temperature: 298.15 K, adsorbent dose: 0.05 g).

To analyze the adsorption kinetics of U(VI) ions on the core–shell KNO₃@alginate-Ca particle, the pseudo-first-order and pseudo-second-order kinetics were investigated, respectively.

The linear and non-linear forms of pseudo-first-order kinetic are expressed as following:²³

ln(q_e − q_t) = ln q_e − K₁t (4)

q_t = q_e(1 − e^−K₁t) (5)

The linear and non-linear forms of pseudo-second-order kinetic are expressed as following:²⁴

where t (h) is the contact time, q_e and q_t (mmol g⁻¹) are the uranium adsorption capacity at equilibrium and contact time t, respectively. K₁ (h⁻¹) and K₂ (g mmol⁻¹ h⁻¹) are the pseudo-first-order and pseudo-second-order model rate constant, respectively, and calculated from the intercept and slope of the plots.

The results are presented in Fig. 4 and Table 1. It could be seen that all the four models has a high values of the correlation coefficient (R²), but the R² value of non-linear pseudo-first-order rate model was relatively higher than those of other models. Furthermore, adsorption capacity (0.077 mmol g⁻¹) was calculated by non-linear pseudo-first-order rate model was closed to the result of experiment (0.077 mmol g⁻¹). These results demonstrated that uranium adsorption on the core–shell KNO₃@alginate-Ca particle was described by the non-linear pseudo-first-order model, which indicated that the rate-determining step in adsorption process might be physisorption. The similar phenomenon was reported by other authors.²⁵

Fig. 4 (a) and (b) Linear of pseudo-first-order kinetic and pseudo-second-order kinetic, (c) and (d) non-linear of pseudo-first-order kinetic and pseudo-second-order kinetic.

Table 1 Model parameters of pseudo-first-order and pseudo-second-order kinetics

Type	Pseudo-first-order kinetic			Pseudo-second-order kinetic
	qe (mmol g^−1)	K1 (h^−1)	R^2	qe (mmol g^−1)	K2 (g mmol^−1 h^−1)	R^2
Linear model	0.075 ± 0.001	0.215 ± 0.003	0.994	0.090 ± 0.013	3.290 ± 0.344	0.994
Non-linear model	0.077 ± 0.004	0.223 ± 0.007	0.998	0.088 ± 0.001	3.572 ± 0.317	0.990

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3.1.3 Effect of initial uranium concentration and temperature on adsorption. The effects of initial uranium concentration and temperature, varying from 288.15 K to 318.15 K, at an vary initial uranium concentration of 0.074 mmol L⁻¹, 0.185 mmol L⁻¹, 0.296 mmol L⁻¹, 0.407 mmol L⁻¹, 0.519 mmol L⁻¹, 0.741 mmol L⁻¹, 0.963 mmol L⁻¹, on uranium adsorption were conducted. As shown in Fig. 5, the uranium adsorption capacity increased with increase of initial uranium concentration and rose slowly at the higher concentration. The uranium adsorption capacity reached to the maximum of 0.825 mmol g⁻¹ at initial concentration of 0.963 mmol L⁻¹ and 318.15 K. The initial relatively high increase of adsorption capacity of the particle for uranium may be due to that a large number of adsorption sites on the particle were available for adsorption. Later on, the adsorption tends to saturate at higher concentration because of the less active sites.²⁶ However, the absorption frequency decreased with increase of initial uranium concentration, which was perhaps due to that the adsorption active sites on the adsorbent were saturated at higher concentration. The maximum uranium absorption frequency was 91.143% at pH 5, 318.15 K and initial concentration 0.074 mmol L⁻¹. Temperature also influenced the uranium adsorption, and the adsorption capacity and absorption frequency increased with the increase of temperature, at the same initial uranium concentration. This may be due to that the adsorption process of uranium was endothermic reaction in nature.²⁷ Considering the factors of adsorption capacity and energy consumption, room temperature (298.15 K) is selected as the optimum adsorption temperature for the adsorbent.

Fig. 5 Effect of temperature and initial uranium concentration on the uranium adsorption by core–shell KNO₃@alginate-Ca particle (pH: 5, contact time: 15 h, adsorbent dose: 0.05 g, initial uranium molar concentration: 0.074–0.963 mmol L⁻¹, temperature: 288.15–318.15 K).

3.1.4 Thermodynamic study. In order to further evaluate the effect of temperature on the adsorption and investigate the possible mechanism involved in the adsorption process, the thermodynamic parameters were calculated from the following equations:²⁸

ΔG⁰ = −RT ln K₀ (9)

where R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is the adsorption temperature (K), K₀ equals to q_m × K_L of the Langmuir isotherm, and ΔH⁰ (kJ mol⁻¹) is the enthalpy change, ΔS⁰ (J mol⁻¹ K⁻¹) is the entropy change and ΔG⁰ (kJ mol⁻¹) is the Gibbs free energy change. The values of ΔH⁰ and ΔS⁰ can be obtained from the slope and intercept of a plot of ln K₀ vs. 1/T.

The results are presented in Fig. 6 and Table 2, the positive values of ΔH⁰ at all temperatures confirmed that the adsorption process was endothermic. The values of ΔG⁰ were negative at all temperatures, confirmed the spontaneous of the adsorption process. Furthermore, the positive value of ΔS⁰ implied an increased randomness at the solid–solution interface during the adsorption process of uranium by core–shell KNO₃@alginate-Ca particle.²⁹

Fig. 6 The plot of ln K₀ versus 1/T for uranium adsorption by core–shell KNO₃@alginate-Ca particle.

Table 2 Thermodynamics parameters for the adsorption of uranium by core–shell KNO₃@alginate-Ca particle

T (K)	ΔH^0 (kJ mol^−1)	ΔS^0 (J mol^−1 K^−1)	ΔG^0 (kJ mol^−1)
288.15	15.648	71.453	−4.911
298.15	15.648	71.453	−5.837
308.15	15.648	71.453	−6.087
318.15	15.648	71.453	−7.217

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3.1.5 Adsorption isotherm studies. To explore the maximum adsorption capacity of adsorbent and understand the adsorption mechanism, the Langmuir and Freundlich models were investigated.

The Langmuir model assumes that adsorption occurs on a homogenous surface and all adsorption sites are uniform and energetically equivalent.³⁰ The linear and non-linear forms of the Langmuir equations are given as follows:³¹

where q_e is the adsorption capacity of core–shell KNO₃@alginate-Ca for uranium at equilibrium concentration (mmol g⁻¹), C_e is the equilibrium concentration of uranium (mmol L⁻¹), q_m is the maximum adsorption capacity (mmol g⁻¹), K_L is the Langmuir constant (L mg⁻¹).

The adsorption possibility can be evaluated by calculation of the separation factor constant (R_L) according to the following equation:³²

The R_L parameter can imply different types of isotherm: R_L > 1.0, unfavorable adsorption; R_L = 1, linear adsorption; 0 < R_L < 1, favorable adsorption; R_L = 0, irreversible adsorption.

The Freundlich model is based on the assumption that there is adsorption onto the heterogeneous surface.³³

The linear and non-linear forms of the Freundlich equations are given by follows:³³

q_e = K_FC_e^1/n (non-linear) (14)

where K_F is the Freundlich constant, n is the Freundlich constant indicating adsorption intensity.

The equilibrium data fitted by the Langmuir and Freundlich models are presented in Fig. 7, Tables 3 and 4. It was shown that the correlation coefficient for non-linear Langmuir model was higher than those of other models, the adsorption capacity calculated by non-linear Langmuir model was closed to the result of experiment. Therefore, the adsorption processes were approximated more favorably by non-linear Langmuir isotherm model, confirmed the monolayer coverage of U(VI) onto core–shell KNO₃@alginate-Ca. Furthermore, it can be observed that all the values of R_L listed in Table 4 are between 0 and 1, indicated the adsorption of uranium(VI) by core–shell KNO₃@alginate-Ca was a favorable process.³⁴

Fig. 7 (a) and (b) Linear and non-linear Langmuir adsorption isotherm model, (c) and (d) linear and non-linear Freundlich adsorption isotherm model.

Table 3 Linear and non-linear of Langmuir and Freundlich model for the adsorption of uranium by core–shell KNO₃@alginate-Ca particle

T (K)			288.15	298.15	308.15	318.15
Langmuir	Linear model	qm (mmol g^−1)	0.395 ± 0.220	0.398 ± 0.106	0.575 ± 0.295	0.554 ± 0.425
		KL (L mmol^−1)	19.656 ± 0.014	26.441 ± 0.006	18.703 ± 0.006	27.642 ± 0.008
		R^2	0.928	0.972	0.994	0.903
	Non-linear model	qm (mmol g^−1)	0.558 ± 0.082	0.711 ± 0.068	0.816 ± 0.100	0.876 ± 0.187
		KL (L mmol^−1)	9.707 ± 3.021	6.552 ± 1.519	7.917 ± 1.405	11.083 ± 2.771
		R^2	0.984	0.974	0.984	0.932
Freundlich	Linear model	KF/(mmol g^−1) (L mmol^−1)^1/n	0.847 ± 0.088	0.858 ± 0.105	2.565 ± 0.208	2.226 ± 0.375
		n	1.850 ± 0.030	2.115 ± 0.037	1.364 ± 0.057	1.581 ± 0.093
		R^2	0.981	0.963	0.971	0.881
		qm (mmol g^−1)	0.619	0.604	1.057	0.977
	Non-linear model	KF/(mmol g^−1) (L mmol^−1)^1/n	0.743 ± 0.039	0.896 ± 0.021	1.787 ± 0.157	1.920 ± 0.289
		n	1.983 ± 0.087	2.035 ± 0.070	1.541 ± 0.089	1.658 ± 0.117
		R^2	0.986	0.973	0.961	1.056
		qm (mmol g^−1)	0.555	0.811	1.138	0.876
		qm(exp) (mmol g^−1)	0.537	0.621	0.798	0.825

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Table 4 Equilibrium parameters, R_L

Temperature (K)	Uranium concentration (mmol L^−1)
	0.074	0.185	0.296	0.407	0.519	0.741	0.963
288.15	0.582	0.357	0.258	0.202	0.168	0.122	0.097
298.15	0.673	0.452	0.340	0.273	0.227	0.171	0.137
308.15	0.630	0.405	0.299	0.237	0.196	0.146	0.116
318.15	0.549	0.327	0.233	0.181	0.148	0.109	0.087

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3.1.6 Effect of co-existing cation. The adsorption behavior of uranium ions in the presence of K(I) ions, Na(I) ions, Zn(II) ions and Mg(II) ions has been examined. The concentration of cations was ranged from 0 to 3.0 mmol L⁻¹ and the pH of the solution was kept at 5 during these studies. As shown in Fig. 8, the co-existing cation exerted almost no effect on the adsorption of uranium in the concentration range of 0.5–3.0 mmol L⁻¹, suggested that the core–shell KNO₃@alginate-Ca particle has a good selectivity to uranium ions in aqueous solutions with several other ions.

Fig. 8 Effect of co-existing cations on uranium adsorption by the core–shell KNO₃@alginate-Ca particle.

3.2 Slow-release behavior of KNO₃ in core–shell KNO₃@alginate-Ca particle

Neutral environment is conducive to plant growth. Therefore, pH value of 6, 7, and 8 were chosen to evaluate the effect of different pH on the release behavior of potassium from core–shell KNO₃@alginate-Ca particle. In order to demonstrate the speed of release potassium from core–shell KNO₃@alginate-Ca particle, the KNO₃@alginate-Ca particle (non-HA and uncrosslinked particle) was selected as a control. The result is depicted in Fig. 9. It was observed that the pH of 6, 7 and 8 had little influence on the core–shell KNO₃@alginate-Ca for release of potassium. As shown in Fig. 9, the curve can be roughly divided into three phases. In the first phase, the rate of potassium released from the particle was low in the first 7 hours. This may be due to that the coating material, alginate-Ca, will be slowly swollen in the solution. In the second phase, the rate of potassium released from the particle was fast between 7 hour and 27 hour. In this phase, the free water in the coating material (the outer shell of the particle) migrated to the core, the potassium in the core was dissolved and diffused into the outer shell and then released into the solution. Meanwhile, the use of alginate and HA in the particle made the potassium release slowly because of the physical barrier of the hydrogel matrix. At last, the release of potassium from the particle attained equilibrium rate after 27 h. However, the release behavior of potassium from KNO₃@alginate-Ca particle (non-HA and uncrosslinked particle) was rapid and reached equilibrium after 6 h, as shown in Fig. 9. The results clearly demonstrated that the core–shell KNO₃@alginate-Ca particle had excellent slow-release ability compared with the KNO₃@alginate-Ca particle.

Fig. 9 Potassium slow-release behavior of KNO₃@alginate-Ca particle in solution (pH: 7) and potassium slow-release behavior of core–shell KNO₃@alginate-Ca particle in solution (pH: 6, 7, 8).

3.3 Material characterization

3.3.1 Mechanical properties. Compressive strength of the particles was measured in order to study whether addition of HA and crosslinking by glutaraldehyde contribute to improvements in mechanical strength, and the results are shown in Table 5, it can be observed that the compressive strength of the core–shell KNO₃@alginate-Ca particle is higher than that of KNO₃@alginate-Ca particle (non-HA and uncrosslinked particle), which means that the core–shell KNO₃@alginate-Ca particle has relatively high mechanical properties in the adsorption and release process.

Table 5 Mechanical properties of the particles

Samples	KNO3@alginate-Ca particle (non-HA and uncrosslinked particle)	Core–shell KNO3@alginate-Ca particle
Force (kg)	0.810	1.921

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3.3.2 Surface morphology analysis. The structure and morphology of core–shell KNO₃@alginate-Ca particle were investigated by SEM. The SEM image of the overall shape of core–shell KNO₃@alginate-Ca particle before and after adsorption of uranium, and release of potassium ions are shown in Fig. 10a1, b1 and c1, the average size of the particles was 853 μm after drying. As shown in Fig. 10, there are obvious rough and uneven bumps on the surface of core–shell KNO₃@alginate-Ca (Fig. 10a2), which are favorable for adsorption of uranium ions from aqueous solutions.³⁵ From Fig. 10a3, it can be found that there are obvious boundaries between core and shell in the particle, which confirms that core–shell KNO₃@alginate-Ca particle are successfully acquired and the core of KNO₃ is effectively encapsulated by alginate shell. The surface and cross section of the core–shell KNO₃@alginate-Ca after adsorption are exhibited in Fig. 10b2 and b3, it can be clearly seen that the surface and shape change are not obvious after adsorption. Fig. 10c2 and c3 showed there were no structure collapse but numerous shallow cracks existed on the surface and cross section of the core–shell KNO₃@alginate-Ca after release of KNO₃ into the solution. The cracks appeared on the surface may be resulted from the release of potassium ions.

Fig. 10 SEM images of (a1), (b1), (c1) core–shell KNO₃@alginate-Ca particle before and after adsorption, after release, (a2) the surface (a3) cross section of core–shell KNO₃@alginate-Ca particle, (b2) the surface (b3) cross section of core–shell KNO₃@alginate-Ca particle after uranium adsorption, (c2) the surface (c3) cross section of core–shell KNO₃@alginate-Ca particle after release potassium.

3.3.3 FT-IR. FT-IR spectrum of core–shell KNO₃@alginate-Ca before and after adsorption are shown in Fig. 11. For core–shell KNO₃@alginate-Ca, the band at 3399 cm⁻¹ is assigned to –OH stretching vibrations and –NH₂ stretching vibrations, which indicated the HA existed in the particle.³⁶ The peak at 2938 cm⁻¹ was attributed to C–H antisymmetric stretching vibration.³⁷ The peaks at 1629 cm⁻¹ and 1426 cm⁻¹ were corresponded to antisymmetrical stretching of –COO⁻ groups and symmetrical stretching of –COO– groups, and the peaks at 1083 cm⁻¹ and 1032 cm⁻¹ were attributed to –CO stretching vibrations, which indicated alginate existed in the particle.³⁸ The band at 1384 cm⁻¹ was assigned to NO₃⁻ of KNO₃. Overall, the FT-IR spectra of core–shell KNO₃@alginate-Ca before and after adsorption were remarkably similar. Yet, faint changes in the positions of individual bands such as 3399 cm⁻¹ (2), 2938 cm⁻¹ (10), 1629 cm⁻¹ (13), 1083 cm⁻¹ (6), 1033 cm⁻¹ (10) were observed, suggested that hydroxyl and carboxyl groups participated in the adsorption process.²⁵ In addition, the special peak at 916 cm⁻¹ belongs to the stretching vibrations of uranyl ions,³⁹ appears in FT-IR spectrum of core–shell KNO₃@alginate-Ca after adsorption, which confirms the uranyl ions has been adsorbed by core–shell KNO₃@alginate-Ca particle.

Fig. 11 FT-IR of core–shell KNO₃@alginate-Ca particle before and after adsorption.

3.3.4 Energy dispersive X-ray measurement. The EDX spectra of core–shell KNO₃@alginate-Ca before and after adsorption of uranium, and release of potassium ions are shown in Fig. 12. From Fig. 12a1 and b1, there is the presence of C, O, Ca, Cl in the shell of the particle, whereas in the core of the particle there is the appearance of C, O, Ca, Cl, K, Si, Al. The results confirmed that K and HA existed in the core of the particle. After adsorption of uranium, a new peak of U(VI) appeared in EDX spectrum of the shell of the particle, confirmed that U(VI) was adsorbed onto the shell of the particle (Fig. 12a2 and b2). Comparing Fig. 12b1 and b2 with Fig. 12b3, the peak of potassium disappeared in the EDX spectrum of the core of the particle after release of potassium ions. The disappearance of potassium in the core of the particle indicated that the potassium ions were released from the particle.

Fig. 12 EDX spectra of (a1) the shell (b1) core of core–shell KNO₃@alginate-Ca particle, (a2) the shell (b2) core of core–shell KNO₃@alginate-Ca particle after uranium adsorption, (a3) the shell (b3) core of core–shell KNO₃@alginate-Ca particle after release potassium.

Furthermore, the percentage of atomic content obtained by EDX analysis was given in Table 6. It was found that the atomic content of Ca in shell of core–shell KNO₃@alginate-Ca reduced from 4.55% to 2.78%, besides, a new component of U(VI) with content of 1.89% appeared in the shell of the particle after adsorption. These revealed that a possible adsorption mechanism is due to the exchange between the calcium of alginate of the particle in the shell and the U(VI) in solution.⁴⁰ The atomic content of K in the core of the particle reduced from 8.34% to 2.37% after adsorption, which was attributed to the release of K in the core of the particle during the process of adsorption of uranium. Meanwhile, the K release in the process of adsorption of uranium resulted in an increase from 0 to 0.15% of potassium in the shell of the particle, which was assigned to the conglutination between potassium and the shell of the particle. Moreover, the further release of K in the core of the particle resulted in the content of K decreased from 8.34% to 0 and an increase in the relative percentage of other elements, such as the content of Ca increased from 2.78% to 3.90%, 4.55% to 5.02% in the core and shell of the particle, respectively.

Table 6 Atomic content on the particle obtained using EDX

Sample	Atomic content (%)
	Si	Al	Ca	K	U
Core of the particle	1.23	0.98	2.78	8.34	0
Core of the particle after adsorption	1.36	1.07	2.67	2.37	0
Core of the particle after release	1.29	1.06	3.90	0	0
Shell of the particle	0	0	4.55	0	0
Shell of the particle after adsorption	0	0	2.78	0.15	1.89
Shell of the particle after release	0	0	5.02	0	0

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3.3.5 XPS. The results of XPS analysis of the core–shell KNO₃@alginate-Ca particle before and after adsorption of U(VI) and release of potassium are given in Fig. 13. Fig. 13a presented that the main elements in the shell of the core–shell KNO₃@alginate-Ca particle were O, Ca, C, Cl. As for core of the particle, Fig. 13b showed the existence of K, Al, Si, besides the basic element in the shell of the particle, indicated that HA and K were successfully encapsulated by coaxial electrospinning. Upon U(VI) adsorption, the Ca 2p peak in the shell of the particle weakened, while U 4f peak emerged, indicated the adsorption was mainly an ion-exchange process between Ca²⁺ in the shell of the particle and U(VI) cations (Fig. 13a). The XPS spectrum of U(VI) is given in Fig. 13c. The U 4f spectrum in Fig. 13c contained two main peaks at 381.16 eV and 392.65 eV, which were assigned to the U 4f_7/2 and 4f_5/2 spin states, respectively.^41,42 The results confirmed the uranium adsorbed on the particle was only in the form of hexavalent uranium(VI).⁴³ Compared XPS spectra of the core of the particle before and after release (as shown in Fig. 13b), the K 2p spectrum in the core of the particle disappeared, demonstrated the potassium had been released.

Fig. 13 XPS spectra of (a) the shell of core–shell KNO₃@alginate-Ca particle (before adsorption, after adsorption, after release), (b) the core of core–shell KNO₃@alginate-Ca particle (before adsorption, after adsorption, after release), (c) U 4f in the shell of the particle.

3.4 Mechanism analysis

Combining the adsorption models with the analysis of SEM, EDX, FT-IR, XPS in this study, the mechanism of core–shell KNO₃@alginate-Ca particle formation and uranium adsorption by the particle can be inferred (shown in Fig. 14). (i) It could be concluded that ion-exchange reactions between Na⁺ in the alginate and Ca²⁺ ions in solution lead to formation of the insoluble gel particle of KNO₃@alginate-Ca. The further linking of the particle by glutaraldehyde, whose linking group was the hydroxyl group in alginate-Ca, resulted in the formation of more space grid structure, and improved the gel intense of the particle.⁴⁴ (ii) The uranium ions in the solution may be removed by the core–shell KNO₃@alginate-Ca particle through the ion exchange of the uranium ions in the solution with the calcium ions in the shell of the particle.

Fig. 14 Mechanism of the formation of particle and uranium adsorption on core–shell KNO₃@alginate-Ca particle.

Combining the slow-release behavior of core–shell KNO₃@alginate-Ca particle with the analysis of SEM, EDX, XPS in this study. It can be concluded that physical barrier of the alginate and HA in the particle make the potassium release slowly, which is shown in Fig. 15.

Fig. 15 Slow release of potassium mechanism of core–shell KNO₃@alginate-Ca particle.

4. Conclusions

The core–shell KNO₃@alginate-Ca particle with slow-release of KNO₃ and uranium-removal was prepared. Its core was KNO₃ in the mixture of alginate and humic acid, the shell (coating) was crosslinked calcium alginate. Core–shell KNO₃@alginate-Ca particle is highly efficient in adsorption of uranium. The optimum adsorption condition was as following: contact time was 15 h and the pH was 3–6. The experimental data were fitted well by the non-linear Langmuir isotherm and non-linear pseudo-first-order model, the maximum uranium adsorption capacity was 0.825 mmol g⁻¹ at pH 5, 318.15 K, initial concentration of 0.963 mmol L⁻¹. The uranium adsorption was almost not affected by the co-existing cations of K(I) ions, Na(I) ions, Zn(II) ions and Mg(II) ions at the concentrations range from 0 to 3.0 mmol L⁻¹. The process of uranium adsorption was spontaneous, endothermic and of increased disorder. Moreover, the core–shell KNO₃@alginate-Ca particle exhibits a preferable slow-release property of KNO₃. The adsorption of uranium onto the particle may be related to the ion exchange between calcium of the particle and uranium in the solution, and the slow release of potassium due to the physical barrier of the shell of the particle.

Acknowledgements

This work was supported by National Key Scientific Projects for Decommissioning of Nuclear Facilities and Radioactive Waste Management (14zg6101) and Professional Scientific Research Innovation Team Building Fund Projects of Key Research Platform of Southwest University of Science and Technology (No. 14tdsc02). The authors want to thank the technology support of Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology.

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