砂岩含水介质中铀的吸附和迁移行为研究

doi:10.13745/j.esf.sf.2022.1.30

摘要/Abstract

摘要：

地浸铀矿山退役后,含水层中残留的含铀浸出液随着地下水的运动向下游迁移扩散,存在对周边地下水污染的风险。本文设计了若干组批实验和柱实验,研究铀在北方某地浸铀矿山砂岩含水介质中的吸附和迁移行为。实验结果表明,砂岩对铀的吸附在12 h以内达到平衡,铀初始浓度越高,砂岩的铀吸附容量越大;砂岩对铀的吸附为吸热反应,温度升高有利于吸附反应的进行。溶液pH值和共存 $HCO 3 -$ 浓度会对铀的吸附作用产生强烈的影响:pH值在7左右时,铀的吸附量最高; $HCO 3 -$ 浓度越高,铀的吸附量越低。这些影响是通过改变溶液中铀的络合形态和砂岩矿物表面的电荷性质实现的。柱实验表明,pH值、铀浓度、流速和 $HCO 3 -$ 浓度是影响铀在饱和砂岩含水介质中迁移的重要因素。pH值≤7时,pH值越高,砂岩柱越不易被铀穿透;而铀浓度、流速、 $HCO 3 -$ 浓度越高,砂岩柱越易被铀穿透。两点非平衡模型可以很好地拟合不同条件下铀在砂岩柱中的迁移过程。批实验获得的分配系数是柱实验的1.16.6倍。通过对比实验条件、含水层特性和地下水化学特征,确定分配系数为48.1 mL/g时,较适合描述研究区内砂岩含水层中的铀迁移。上述认识为地浸铀矿山地下水铀的反应运移过程和天然自净化机理提供了理论依据。

关键词: 地下水, 铀, 吸附, 柱实验, 两点模型, 分配系数

Abstract:

After the decommissioning of a in-situ leach uranium mining site, residual uranium leaching fluid in the aquifer migrates and diffuses downstream posing pollution risks to the surrounding groundwater. In this study, a series of batch and column experiments were designed to investigate the adsorption and migration behavior of uranium in aquifer sandstone in an in-situ leach uranium mining area in northern China. In batch experiments, uranium adsorption on sandstone reached equilibrium within 12 h, and in the process a positive correlation between the initial uranium concentration and sandstone’s uranium adsorption capacity was observed. It was found that the uranium adsorption process was endothermic and the increase in temperature was beneficial for uranium adsorption. The pH of the eluent and $HCO 3 -$ concentration have a strong influence on uranium adsorption. Maximum uranium adsorption was reached at around neutral pH, and higher $HCO 3 -$ concentration led to lower level of adsorption. These effects are due to the alteration of uranium complexation in the solution and the surface charge properties of sandstone. In column experiments, pH, uranium concentration, flow rate, and $HCO 3 -$ concentration were the most important factors affecting uranium migration. Weaker acidity (pH≤7) was found to be associated with fewer late uranium breakthrough in the sandstone column; whereas higher uranium concentration, flow rate, and $HCO 3 -$ concentration might cause early uranium breakthrough. The two-point non-equilibrium model could well describe the uranium migration process in the sandstone column under variable conditions. The partition coefficients obtained from batch experiments were 1.1-6.6 fold higher than those obtained from column experiments. According to the aquifer characteristics and hydrogeochemical characteristics, we suggest that the distribution coefficient of 48.1 mL/g would be suitable for describing uranium migration in sandstone aquifers in the study area. Overall, this study provided a theoretical basis for the reactive transport process and for the natural remediation of polluted groundwater in the in-situ leaching uranium mining areas.

Key words: groundwater, uranium, Adsorption, column experiments, two-site chemical non-equilibrium model, partition coefficient

中图分类号:

X523

崔迪, 杨冰, 郭华明, 连国玺, 孙娟. 砂岩含水介质中铀的吸附和迁移行为研究[J]. 地学前缘, 2022, 29(3): 217-226.

CUI Di, YANG Bing, GUO Huaming, LIAN Guoxi, SUN Juan. Adsorption and transport of uranium in porous sandstone media[J]. Earth Science Frontiers, 2022, 29(3): 217-226.

图/表 11

表1 柱实验条件

Table 1 Experimental conditions in column experiments

柱编号	pH值	铀浓度/ (mg·L^-1)	流速/ (mL·min^-1)	$HCO 3 -$ 浓度/ (mmol·L^-1)
1	4	2	0.1	0
2	7	2	0.1	0
3	8.5	2	0.1	0
4	7	1	0.1	0
5	7	5	0.1	0
6	7	2	0.5	0
7	7	2	0.3	0
8	7	2	0.1	0.5
9	7	2	0.1	1.0

表1 柱实验条件

Table 1 Experimental conditions in column experiments

柱编号	pH值	铀浓度/ (mg·L^-1)	流速/ (mL·min^-1)	$HCO 3 -$ 浓度/ (mmol·L^-1)
1	4	2	0.1	0
2	7	2	0.1	0
3	8.5	2	0.1	0
4	7	1	0.1	0
5	7	5	0.1	0
6	7	2	0.5	0
7	7	2	0.3	0
8	7	2	0.1	0.5
9	7	2	0.1	1.0

表2 水力学特征参数

Table 2 Characteristic hydraulic parameters

柱号	饱和含水率θ	流速 v/(cm·min^-1)	弥散系数 D/(cm²·min^-1)	弥散度 λ/cm	R²
1	0.333	0.309	0.011	0.034	0.998
2	0.411	0.308	0.012	0.037	0.998
3	0.457	0.270	0.009	0.034	0.993
4	0.342	0.330	0.028	0.085	0.982
5	0.346	0.342	0.018	0.053	0.995
6	0.420	0.959	0.087	0.091	0.999
7	0.389	0.585	0.011	0.019	0.993
8	0.405	0.297	0.021	0.072	0.985
9	0.447	0.287	0.018	0.064	0.999

图1 不同初始浓度下溶液中铀浓度随时间变化曲线

Fig.1 Temporal variation of uranium concentration in solution under different initial uranium concentrations

表3 准一级和准二级动力学模型拟合参数表

Table 3 Parameters of psudo first order and psudo second order kinetic models for uranium adsorption on sandstone

U初始浓度/ (mg·L^-1)	实验q'_e/ (10² mg·g^-1)	准一级动力学	准二级动力学
U初始浓度/ (mg·L^-1)	实验q'_e/ (10² mg·g^-1)	R²	q_e/(10² mg·g^-1)	k₂/(g·mg^-1·h^-1)	R²
1	2.763	0.237	2.794	69.973	0.997
2	5.268	0.443	5.357	24.928	0.993
5	22.555	0.338	22.367	12.562	0.999

表4 Langmuir和Freundlich等温吸附模型拟合参数

Table 4 Langmuir and Freundlich constants for uranium adsorption on sandstone at different temperatures

温度/℃	Langmuir等温吸附方程			Freundlich等温吸附方程
温度/℃	K_L	q_max	R²	K_F	1/n	R²
10	4.309	0.088	0.990	0.076	0.582	0.979
25	7.276	0.070	0.980	0.076	0.553	0.994
35	10.263	0.085	0.992	0.088	0.500	0.978
45	30.325	0.105	0.994	0.149	0.453	0.969

表5 热力学模型拟合参数

Table 5 Thermodynamic parameters for uranium adsorption on sandstone

T/℃	K_L	ΔG⁰/ (kJ·mol^-1)	ΔH⁰/ (kJ·mol^-1)	ΔS⁰/ (J·mol^-1·K^-1)
10	4.31	-3.44	7.10	37.23
25	7.28	-4.92		40.32
35	10.26	-5.96		42.40
45	30.33	-9.02		50.69

图2 pH值对砂岩吸附铀的影响

Fig.2 Effects of pH on uranium adsorption on the sandstone

图3 HCO 3 -浓度对砂岩铀吸附率及溶液pH的影响

Fig.3 Effects of HCO 3 - concentration on the uranium adsorption capacity of sandstone and pH

图4 不同条件下的铀穿透曲线

Fig.4 Breakthrough curves of uranium adsorption in sandstone columns under different conditions a—CU=2 mg/L, Q=0.1 mL/min, C H C O 3 -=0; b—pH=7, Q=0.1 mL/min, C H C O 3 -=0 mmol/L; c—pH=7, CU=2 mg/L, C H C O 3 -=0; and d—pH=7, CU=2 mg/L, Q=0.1 mL/min。

表6 模型拟合参数及拟合度系数

Table 6 The fitted parameters of breakthrough curves of uranium adsorption in sandstone column

条件	k'_d/(mL·g^-1)	ω/(10³ min^-1)	R²
pH=4	9.82	1.720	0.995
pH=7	117	0.182	0.997
C_U=1 mg/L	159	0.122	0.936
C_U=2 mg/L	117	0.182	0.997
C_U=5 mg/L	23.9	0.828	0.989
流速0.1 mL/min	117	0.182	0.974
流速0.3 mL/min	49.5	0.604	0.977
流速0.5 mL/min	39.5	1.228	0.997
$C H C O 3 -$ =0	117	0.182	0.997
$C H C O 3 -$ =0.5 mmol/L	58.9	0.483	0.996
$C H C O 3 -$ =1.0 mmol/L	48.1	4.759	0.964

表6 模型拟合参数及拟合度系数

Table 6 The fitted parameters of breakthrough curves of uranium adsorption in sandstone column

条件	k'_d/(mL·g^-1)	ω/(10³ min^-1)	R²
pH=4	9.82	1.720	0.995
pH=7	117	0.182	0.997
C_U=1 mg/L	159	0.122	0.936
C_U=2 mg/L	117	0.182	0.997
C_U=5 mg/L	23.9	0.828	0.989
流速0.1 mL/min	117	0.182	0.974
流速0.3 mL/min	49.5	0.604	0.977
流速0.5 mL/min	39.5	1.228	0.997
$C H C O 3 -$ =0	117	0.182	0.997
$C H C O 3 -$ =0.5 mmol/L	58.9	0.483	0.996
$C H C O 3 -$ =1.0 mmol/L	48.1	4.759	0.964

表7 柱实验和批实验分配系数对比

Table 7 Partition coefficients from column and batch experiments

条件	柱实验分配系数 k'_d/(mL·g^-1)	批实验分配系数 k_d/(mL·g^-1)
pH=4	9.81	65.1
pH=7	117	637
C_U=1 mg/L	159	189
C_U=2 mg/L	117	133
C_U=5 mg/L	23.9	55.5
$C H C O 3 -$ =0	117	133
$C H C O 3 -$ =0.5 mmol/L	58.9	83.1
$C H C O 3 -$ =1.0 mmol/L	48.1	69.0

表7 柱实验和批实验分配系数对比

Table 7 Partition coefficients from column and batch experiments

条件	柱实验分配系数 k'_d/(mL·g^-1)	批实验分配系数 k_d/(mL·g^-1)
pH=4	9.81	65.1
pH=7	117	637
C_U=1 mg/L	159	189
C_U=2 mg/L	117	133
C_U=5 mg/L	23.9	55.5
$C H C O 3 -$ =0	117	133
$C H C O 3 -$ =0.5 mmol/L	58.9	83.1
$C H C O 3 -$ =1.0 mmol/L	48.1	69.0

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