Factors affecting uranium adsorption on aquifer sandstone

doi:10.13745/j.esf.sf.2021.2.19

Abstract

Abstract:

During and after in situ uranium (U) mining, the diffusion of U-containing leachate can potentially have an impact on the groundwater quality. The range and magnitude of such impact determines the natural attenuation of U(VI) in groundwater in sandstone aquifers. This study investigates the influence of solution chemistry characteristics and pyrite content in sandstone on the U(VI) adsorption/transformation by aquifer sandstone taken from a uranium mining area in northern China. Adsorption equilibrium in sandstone particles can be reached after 12 h. The linear adsorption isotherm fits will with the observed data. The adsorption efficiency generally decreases with increasing grain size but stabilizes under grain size larger than 0.20-0.25 mm. The solution pH is the main factor affecting adsorption. By controlling the U(IV) complexes in solution as well as rock particle surface charge, adsorption is promoted at pH < 6 and inhibited at pH > 6. $HCO 3 -$ shows the most negative effect on U(VI) adsorption, followed by Ca²⁺, Mg²⁺, and $SO 4 2 -$ . $HCO 3 -$ inhibits U(VI) adsorption mainly by forming negatively charged $UO 2 2 +$ complexes. At near neutral pH, the increase of pyrite content significantly promotes U(VI) removal by adsorption and reduction, as the adsorbed Fe²⁺ reduces U(VI) to UO₂₊_x by electron transfer on the surface of the adsorbent. Adsorption and reduction are mutually reinforcing processes. Under weakly alkaline condition, increasing pyrite content does not significantly affect U(VI) removal, as hydrolytic precipitation and low adsorption inhibit the reduction of U(VI) by pyrite.

Key words: uranium, sandstone, pyrite, adsorption, reduction

CLC Number:

P574.2
X143

HUI Shujun, YANG Bing, GUO Huaming, LIAN Guoxi, SUN Juan. Factors affecting uranium adsorption on aquifer sandstone[J]. Earth Science Frontiers, 2021, 28(5): 68-78.

Figures/Tables 14

Fig.1 XRD pattern for sandstone samples

Fig.2 Variations of U(VI) concentration and pH with time

Fig.3 Isothermal adsorption curves for sandstone samples with different particle sizes

Table 1 Fitting coefficients and adsorption partition coefficients of isothermal adsorption curves for sandstone samples with different particle sizes

粒径/mm	R²	K_d/(L·g^-1)
<0.10	0.995 1	0.030
0.10<0.15	0.994 3	0.038
0.15<0.20	0.997 9	0.032
0.20<0.25	0.993 6	0.029
0.250.50	0.997 8	0.029

Fig.4 Effects of pH on U(VI) adsorption on absorbent samples

Table 2 List of factors affecting U(VI) adsorption at different pH

因素	不同pH值条件下的影响因素情况
因素	pH<4	pH=46	pH>6
固体表面带电荷情况	有正电荷也有负电荷	主要带负电荷	几乎都带负电荷
U的络合形态	$UO 2 2 +$	$UO 2 2 +$ 、UO₂OH⁺、 (UO₂)₃(OH $) 5 +$ 、 (UO₂)₄(OH $) 7 +$ 等	UO₂CO₃、UO₂(OH $) 3 -$ 、 (UO₂)₃(OH $) 7 -$ 、UO₂(CO₃ $) 2 2 -$ 、 UO₂(CO₃ $) 3 4 -$ 等
库仑力	引力为主	引力	斥力增强
H⁺竞争作用	较强	减弱	几乎没有

Table 2 List of factors affecting U(VI) adsorption at different pH

因素	不同pH值条件下的影响因素情况
因素	pH<4	pH=46	pH>6
固体表面带电荷情况	有正电荷也有负电荷	主要带负电荷	几乎都带负电荷
U的络合形态	$UO 2 2 +$	$UO 2 2 +$ 、UO₂OH⁺、 (UO₂)₃(OH $) 5 +$ 、 (UO₂)₄(OH $) 7 +$ 等	UO₂CO₃、UO₂(OH $) 3 -$ 、 (UO₂)₃(OH $) 7 -$ 、UO₂(CO₃ $) 2 2 -$ 、 UO₂(CO₃ $) 3 4 -$ 等
库仑力	引力为主	引力	斥力增强
H⁺竞争作用	较强	减弱	几乎没有

Fig.5 Variations of morphological distribution of U(VI) with pH

Fig.6 Effects of ion concentration on U(VI) adsorption for different ions

Fig.7 Variations of U (a) and Fe2+ (b) concentrations with time

Fig.8 XPS spectra of uranyl minerals in solid samples containing 10% FeS2 reacting with U(VI) for 10 days

Fig.9 Changes of U concentration with time in solutions containing different amounts of FeS2

Table 3 Percentages of U (VI), U (IV), Fe(II), and Fe(III) on solid sample surface containing 10% FeS2 reacting with U(VI) for 10 d under different experimental conditions

不同试验条件	U(VI)含量占比/%	U(IV)含量占比/%	Fe(II)含量占比/%	Fe(III)含量占比/%
原始黄铁矿			98.9	1.1
近中性pH值	68	32	95.8	4.2
弱碱性pH值	100	0	98.5	1.5

Fig.10 Iron 2p XPS spectra for solid samples containing 10% FeS2 reacting with U(VI) for 10 d under different experimental conditions

Fig.11 Sulphur 2p XPS spectra for solid samples containing 10% FeS2 reacting with U(VI) for 10 d under different experimental conditions

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