Ternary Layered Double Hydroxide Supported Sulfide NZVI: Efficient U(VI) Elimination and Mechanism

doi:10.15541/jim20190365

Abstract

Abstract:

Nanoscale zero-valent iron (NZVI) has been widely applied to eliminate radionuclide U(VI). However, poor stability and low efficiency restrict the employment of pure NZVI. In this study, surface passivation and dispersion technology were employed together. Ca-Mg-Al layered double hydroxide supported sulfide NZVI (CMAL-SNZVI) was synthesized and applied for U(VI) elimination. Macroscopic and microscopic investigations demonstrate the outstanding physicochemical properties, high reactivity and excellent performance for U(VI) removal. The reaction process can be achieved equilibrium within 2 h and the maximum elimination capacity reaches 175.7 mg·g ^-1. The removal mechanism of U(VI) on CMAL-SNZVI is the synergistic effect between adsorption and reduction, through which U(VI) can be adsorbed by CMAL base and the SNZVI surface via inner-sphere surface complexation, U(VI) can be reduced into less toxic and insoluble U(IV) by Fe ⁰ inner core. Overall, the synthetization of CMAL-SNZVI can lead a new direction of NZVI modification. In the meantime, the outstanding performance of U(VI) removal indicate the potential of CMAL-SNZVI as excellent material for environment remediation.

Key words: SNZVI, layered double hydroxide, U(VI), adsorption, reduction

CLC Number:

X591

PANG Hongwei, TANG Hao, WANG Jiaqi, WANG Xiangxue, YU Shujun. Ternary Layered Double Hydroxide Supported Sulfide NZVI: Efficient U(VI) Elimination and Mechanism[J]. Journal of Inorganic Materials, 2020, 35(3): 381-389.

Figures/Tables 9

Fig. 1 SEM images of NZVI (A), SNZVI (B), CMAL (C) and CMAL-SNZVI (E); TEM images of CMAL (D) and CMAL- SNZVI (F)

Fig. 2 Element mapping of Ca(A), Mg(B), Al(C), Fe(D), S(E), and O(F), and EDS pattern (G) of CMAL-SNZVI

Fig.3 XRD patterns (A) of CMAL-SNZVI, CMAL and SNZVI, N2 adsorption-desorption isotherm (B)and magnetization curve (C) of CMAL-SNZVI

Table 1 Kinetic modeling of U(VI) adsorption on CMAL and CMAL-SNZVI

Material	Pseudo-first-order model			Pseudo-second-order model
Material	k₁/min^-1	Q_e/(mg·g^-1)	R²	k₂/(g·mg^-1·min^-1)	Q_e/(mg·g^-1)	R²
CMAL	0.049	62.62	0.95	0.0015	65.36	0.99
CMAL-SNZVI	0.121	87.60	0.94	0.0036	90.09	0.99

Fig. 4 Adsorption kinetics (A), pseudo-first-order (B) and pseudo-second-order kinetic plots (C), adsorption isotherms (D) of U(VI) adsorption on CMAL-SNZVI and CMAL

Table 2 Parameters calculated from the Langmuir and Freundlich models for U(VI) adsorption on CMAL-SNZVI at 25 ℃

Adsorbents	Langmuir model			Freundlich model
Adsorbents	Q_max/(mg·g^-1)	K_L/(L·mg^-1)	R²	K_F/(mg^1-ⁿ·Lⁿ·g^-1)	n	R²
CMAL	129.8	0.36	0.975	51.5	3.89	0.877
CMAL	175.7	0.95	0.954	89.5	4.60	0.933
-SNZVI	175.7	0.95	0.954	89.5	4.60	0.933

Table 3 Comparison of CMAL-SNZVI with other typical adsorbents for U(VI) decontamination

Adsorbents	pH	Equilibrium time/h	Removal capacity /(mg·g^-1)	Ref.
nZVI/C composite	4.0	Not given	103.1	[25]
g-C₃N₄@Ni-Mg-Al-LDH	5.0	6	99.7	[26]
Ca-Mg-Al-LDH	5.0	24	132.5	[27]
GO@LDH	4.5	10	159.7	[28]
Magnetic biochar	3.0	12	54.4	[29]
CMAL	5.0	2	129.8	This study
CMAL-SNZVI	5.0	2	175.7	This study

Fig. 5 Effect of pH (A) and ionic strength (C) on U(VI) adsorption of CMAL-SNZVI and CMAL, Zeta potential values of CMAL-SZVI as a function of pH (B), species distribution of U(VI) as a function of pH by Visual MINTEQ (D)

Fig. 6 XPS survey spectra of CMAL-SNZVI (before and after U(VI) adsorption) (A), High -resolution XPS spectra of U4f (B), Fe2p (C), S2p (D), Ca2p (E), Mg1s (F) and Al2p (G), and removal mechanisms of U(VI) on CMAL-SNZVI (H)

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