Retention of sulfidated nZVI (S-nZVI) in porous media visualized by X-ray μ-CT - the relevance of pore space geometry

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.