利用X射线K吸收边成像法测量碘离子在岩石中的孔隙扩散系数

The Pore Diffusion Coefficient of Iodide Ion in Rock Samples Using X-ray K-edge Imaging

污染物质在岩层孔隙水中的扩散过程决定了污染物质在地下水与岩层之间迁移的速率,并进而影响其在环境中的迁移-转化过程。利用碘离子作为示踪剂,通过X射线成像法可以获得碘离子在岩石中随扩散距离和时间变化的扩散曲线,进而通过人工拟合获得孔隙扩散系数(Dp)以及孔隙度(φ)等重要参数,这是研究污染元素扩散行为的基础。但是在X射线成像过程中,岩石本底对X射线的吸收也会对示踪剂碘离子的成像造成干扰。本文使用X射线能谱CT利用X射线K吸收边成像法,在碘的K吸收边(33keV)两侧的能量区域(27~32keV和34~39keV)对样品进行成像,有效地减少了岩石背景干扰,获得碘离子的扩散曲线,并通过软件利用近似公式对扩散曲线进行拟合获得了灰岩中碘离子的孔隙扩散系数。在实验过程中,本文还通过改进扩散装置,对低浓度端溶液采用连续接收的方法,避免了管路中溶液死体积的影响。通过实验获得碘离子的岩石孔隙扩散系数Dp=(1.12±0.22)×10^(−11)m^(2)/s,并计算得到岩石孔隙度φ=0.02。此实验结果与文献中灰岩孔隙度范围(0.005~0.042)相符合。通过本次工作,验证了应用改进的扩散装置和X射线K吸收边成像方法测量碘离子在岩石中的孔隙扩散系数的可行性。

BACKGROUND:The diffusion process is important for environmental studies and pollution monitoring.When contaminants are released into groundwater in the natural environment,they can diffuse into pore water in the rock matrix driven by the concentration gradients.When the contamination source is removed after remediation,the contaminated porewater can be a long-term contamination source to the groundwater in fractures via back diffusion.The diffusion coefficients are the key parameters for the studies of the diffusion-reaction process of contaminants in the environment,especially in natural porous rocks.The diffusion method,which measures the steady-state diffusion flux of non-sorbing tracers,such as I−and HTO,has been widely used for the determination of diffusion coefficients.X-ray radiography techniques,which are non-destructive and can monitor temporal changes of tracers in the porous rock medium,have been developed to obtain the pore diffusion coefficients in rock samples by fitting a solution of Fick's second law of diffusion to profiles ofΔμ(the change in the X-ray attenuation coefficient).The value ofΔμis a function of the mass of a tracer along the X-ray path.However,the X-ray attenuation by the rock matrix interferes with the determination of tracer signals,due to the low concentration of the tracer in the porewaters.The method of X-ray K-Edge imaging using a photon counting detector(PCD),which takes images from the two energy bins on both sides of the theoretical K-edge position of the tracer element and obtains the image subtraction,can be used for the determination of diffusion coefficients to reduce the interferences from the rock background.This work focuses on the diffusion process of iodide ion in limestone samples.X-ray K-edge imaging with energy-resolved computed tomography(CT)has been employed to image the iodide ion tracer in limestone samples.The energy range is on both sides of the iodine K-edge(33keV)for X-ray K-edge imaging.