不同因素对砂岩含水层介质吸附铀的影响

doi:10.13745/j.esf.sf.2021.2.19

摘要/Abstract

摘要：

铀矿开采过程中及井场退役后,含铀浸出液的扩散会对地下水造成一定影响,该影响范围和程度决定了铀的天然衰减特征。本文以北方某地浸铀矿区砂岩含水介质为研究对象,研究了溶液化学特征和黄铁矿含量对砂岩吸附和转化铀的影响。结果表明,砂岩颗粒对U(VI)的吸附基本在12 h可达平衡。线性等温吸附模型可以很好地描述吸附特征;砂岩颗粒对U(VI)的吸附率总体随着粒径增大而减小,当岩石粒径增大到0.200.25 mm时,吸附率趋于稳定。溶液pH值是影响吸附的主要因素,通过控制溶液中U(VI)的络合形态和岩石颗粒表面带电荷情况,在库仑力的作用下促进或者抑制吸附,在pH=6时,达到最佳吸附状态。共存离子对U(VI)吸附的抑制程度是: $HCO 3 -$ >Ca²⁺>Mg²⁺> $SO 4 2 -$ 。 $HCO 3 -$ 主要通过与 $UO 2 2 +$ 络合形成带负电荷的络合阴离子抑制U(VI)吸附。近中性pH值试验条件下,黄铁矿含量的增加对U的去除起到显著的促进作用,这种促进作用体现在吸附和还原作用,被吸附的Fe²⁺在吸附剂表面通过电子转移还原吸附态的U(VI),还原产物是UO₂₊_x,吸附和还原是个相互促进的过程;在弱碱性pH值试验条件下,黄铁矿对U(VI)去除的影响不明显,水解沉淀作用和较低的吸附率抑制了还原反应的发生。

关键词: 铀, 砂岩, 黄铁矿, 吸附, 还原

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

中图分类号:

P574.2
X143

惠淑君, 杨冰, 郭华明, 连国玺, 孙娟. 不同因素对砂岩含水层介质吸附铀的影响[J]. 地学前缘, 2021, 28(5): 68-78.

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.

图/表 14

图1 砂岩样品XRD测试图谱

Fig.1 XRD pattern for sandstone samples

图2 溶液中U(VI)浓度和pH值随时间的变化

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

图3 不同粒径砂岩样品的等温吸附曲线

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

表1 不同粒径砂岩样品等温吸附曲线拟合系数和吸附分配系数

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

图4 pH值对样品吸附U(VI)的影响试验 a—初始pH值试验组;b—恒定pH值试验组。

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

表2 不同pH值条件下影响U(VI)吸附的因素

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

表2 不同pH值条件下影响U(VI)吸附的因素

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

图5 不同pH值条件下U(VI)的形态分布模拟条件:[U(VI)]=1 mg/L,[NaNO3]=10 mmol/L,与大气CO2平衡,wateq4f.dat数据库。

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

图6 不同离子的浓度对U(VI)吸附的影响

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

图7 溶液中U (a)和Fe2+浓度(b)随时间的变化

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

图8 10% FeS2含量组反应10 d的固体样品U(4f)XPS谱图

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

图9 不同FeS2 含量溶液中U浓度随时间的变化

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

表3 不同试验条件下10% FeS2组反应10 d的固体表面U(VI)、U(IV)及Fe(II)、Fe(III)含量占比

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

图10 不同试验条件下10% FeS2组反应10 d的固体Fe(2p)XPS光谱 a—原始黄铁矿;b—近中性pH值;c—弱碱性pH值。

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

图11 不同试验条件下10% FeS2组反应10 d的固体S(2p)XPS光谱 a—原始黄铁矿;b—近中性pH值;c—弱碱性pH值。

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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