Obtaining pore scale knowledge about retention mechanisms of nanoparticles (NP) is inherently difficult and can in turn restrict accurate forward prediction. Herein, an X-ray microcomputed tomography (μ-CT, 0.5 μm pixel size) approach is described which is capable of resolving sulfidated nanoscale zerovalent iron (S-nZVI) retention at the pore scale, by using difference images of pre- and post-injection scans to account for local background variations around grains on a per pixel basis. The type of information that can be obtained from this approach including its limitations is discussed based on a first set of S-nZVI transport experiments performed in columns filled with irregular fine sand, where μ-CT images were obtained before and after S-nZVI injection at three positions along the column. A total of 5 column experiments were performed testing the effect of three different superficial injection velocities (5.8 × 10⁻⁴ m s⁻¹, 2.9 × 10⁻⁴ m s⁻¹ and 1.5 × 10⁻⁴ m s⁻¹) and three different S-nZVI concentrations (5.0 g L⁻¹, 10 g L⁻¹ and 20 g L⁻¹) on S-nZVI retention behaviour, while the total injected S-nZVI mass was kept constant across experiments. The results clearly show that S-nZVI retention is determined by physical straining. Column depth and S-nZVI aggregate size dependency during straining appears to play a role, yet depth extension seems limited. Ripening deposition likely also occurred and increased with a decrease in injection velocity and/or increase in S-nZVI concentration. The local pore geometry and the flow regime strongly impacted S-nZVI attachment and retention behaviour, which suggests that pore space descriptors and velocity should be included in future predictive models. Together these results provide new perspectives for further studies of nZVI-based particle retention and transport in porous media.

中文翻译：

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X 射线 μ-CT 可视化多孔介质中硫化 nZVI (S-nZVI) 的保留——孔隙空间几何形状的相关性

获得有关纳米粒子 (NP) 保留机制的孔隙尺度知识本质上是困难的，并且反过来会限制准确的前向预测。在此，描述了一种 X 射线微计算机断层扫描（μ-CT，0.5 μm 像素大小）方法，该方法能够通过使用前后的差异图像来解决硫化纳米级零价铁 (S-nZVI) 在孔隙尺度上的滞留问题。 - 注入扫描以在每个像素的基础上考虑晶粒周围的局部背景变化。基于在填充有不规则细砂的柱子中进行的第一组 S-nZVI 传输实验，讨论了可以从这种方法获得的信息类型，包括其局限性，其中 μ-CT 图像是在 S-nZVI 注射之前和之后获得的在沿柱的三个位置。^-4 ms ^-1、2.9 × 10 ^-4 ms ^-1和 1.5 × 10 ^-4 ms ^-1）和三种不同的 S-nZVI 浓度（5.0 g L ^-1、10 g L ^-1和 20 g L ^-1) 对 S-nZVI 保留行为的影响，而总注入的 S-nZVI 质量在整个实验中保持不变。结果清楚地表明，S-nZVI 保留是由物理应变决定的。应变期间的列深度和 S-nZVI 聚集体大小依赖性似乎起作用，但深度扩展似乎有限。随着注射速度的降低和/或 S-nZVI 浓度的增加，成熟沉积也可能发生并增加。局部孔隙几何形状和流动状态强烈影响 S-nZVI 附着和保留行为，这表明孔隙空间描述符和速度应该包括在未来的预测模型中。这些结果共同为进一步研究基于 nZVI 的颗粒在多孔介质中的保留和传输提供了新的视角。