Removal of Uranium (VI) from Acidic Aqueous Solution by Fluorapatite

doi:10.15541/jim20210255

Abstract

Abstract:

Removal of uranium (VI) from uranium (VI) containing waste water is urgently needed with the global nuclear energy exploitation. The present work completed synthesis of fluorapatite and removal of uranium (VI). Fluorapatite was synthesized using calcium fluoride, calcium pyrophosphate and calcium hydroxide as the raw materials. The fluorapatite before and after adsorption of uranium (VI) was characterized. The experimental results show that, at condition of 308 K, pH=3, solid-liquid ratio of 0.12 g/L, equilibrium time of 120 min and initial uranyl ion concentration of 100 mg/L, the adsorption capacity of uranium (VI) by fluorapatite reaches 655.17 mg/g. Adsorption process of uranium (VI) by fluorapatite is in accordance with the pseudo-second-order kinetics and Langmuir isotherm adsorption model. Adsorption of uranium (VI) by fluorapatite is a spontaneous and endothermic reaction. The removal of uranium (VI) by fluorapatite is ascribed to surface mineralization. After uranium (VI) is absorbed, a new phase of meta-autunite [Ca(UO₂)₂(PO₄)₂·6H₂O] is generated on the surface of fluorapatite. The meta-autunite can maintain high stability in the pH≥3 aqueous solution. Above results indicate that fluorapatite can be used as a promising mineralizer for purification and solidification of uranium (VI) containing waste water.

Key words: fluorapatite, uranium (VI), removal, acidic aqueous solution, surface mineralization mechanism

CLC Number:

X591

MA Lei, HUANG Yi, DENG Hao, YIN Hang, TIAN Qiang, YAN Minghao. Removal of Uranium (VI) from Acidic Aqueous Solution by Fluorapatite[J]. Journal of Inorganic Materials, 2022, 37(4): 395-403.

Figures/Tables 10

Fig. 1 XRD pattern(a), FT-IR spectrum(b), particle size distribution (c) and SEM image (d) of synthetic fluorapatite powder

Fig. 2 (a) Distribution of uranium (VI) species in the solution with different pH (C0=100 mg/L, T= 308 K); (b) Adsorption capacity and removal rate of uranium (VI) by fluorapatite in the solution with different pH; (c) Zeta potential of fluorapatite in the solution with different pH; (d) Effect of solid-liquid ratio on adsorption of uranium (VI) by fluorapatite (pH3); (e) Change of the adsorption of uranium (VI) by fluorapatite with adsorption time (pH3, solid-liquid ratio at 0.12 g/L); (f) Effect of initial uranium (VI) concentration on adsorption of uranium (VI) by fluorapatite (pH3, solid-liquid ratio at 0.12 g/L, adsorption time=120 min)

Fig. 3 Solubility of fluorapatite (a) and fluorapatite adsorbed with uranium (VI) (b) in the solutions with different pH

Fig. 4 (a) Adsorption capacity and removal rate of uranium (VI) by fluorapatite in the temperature range of 303-323 K; (b) Linear fitting of thermodynamic model for adsorption process; (c) Nonlinear fitting of pesudo-first-order kinetic model; (d) Nonlinear fitting of pesudo-second-order kinetic model; (e) Nonlinear fitting of Langmuir isotherm adsorption model; (f) Nonlinear fitting of Freundlich isotherm adsorption model

Table 1 Fitting parameters of thermodynamic model for adsorption

Materials	∆H/(kJ∙mol^-1)	∆S/(J∙mol^-1•K^-1)	∆G/(kJ•mol^-1)
Materials	∆H/(kJ∙mol^-1)	∆S/(J∙mol^-1•K^-1)	303 K	308 K	313 K	318 K	323 K
Fluorapatite	8.41	88.65	-8.47	-8.91	-9.31	-9.8	-10.24

Table 2 Fitting parameters of pesudo-first-order and pesudo-second-order kinetic models

Materials	Pseudo-first-order		Pseudo-second-order
Materials	K₁/min^-1	R²	K₂/ (g∙mg^-1∙min^-1)	R²
Fluorapatite	0.016	0.986	1.85×10^-5	0.999

Table 3 Fitting parameters of Langmuir and Freundlich isothermal adsorption models

Materials	Langmuir			Freundlich
Materials	q_max/(mg∙g^-1)	R²		n	R²
Fluorapatite	1300.35		0.998	2.18	0.969

Fig. 5 SEM images of fluorapatite before (a) and after (b) adsorption of uranium (VI), and element mappings (c-g) of the surface of fluorapatite after adsorption of uranium (VI)

Fig. 6 (a) XPS spectra of fluorapatite before and after adsorption of uranium (VI) and (b) U4f on the fluorapatite after adsorption of uranium (VI) (b), and XPS spectra of Ca2p(c), O1s(d), P2p(e) and F1s(f) on the fluorapatite before and after adsorption of uranium (VI)

Fig. 7 (a) XRD pattern and (b) FT-IR spectra of fluorapatite before and after adsorption of uranium (VI)

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