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Chlorinated organic compounds (COCs) are widely used industrial chemicals that pose significant environmental risks due to their toxicity, volatility, instability, and limited solubility in groundwater, often leading to persistent secondary contamination [1, 2]. Recent studies have highlighted the potential of Pickering emulsion injection as an innovative strategy for soil and groundwater remediation [3]. Pore-scale experiments have shown that chlorinated solvents can be efficiently displaced by tailored emulsions, followed by removal of residual contaminant blobs through compositional ripening—a process in which contaminants diffuse across thin liquid films into surrounding emulsion droplets under no-flow conditions. In parallel, zero-valent iron (ZVI), particularly nano zero-valent iron (nZVI), has long been recognized for its strong reactivity toward COCs [4]. However, practical application of nZVI is hindered by rapid oxidation, aggregation, and sedimentation, which significantly reduce its reactivity, mobility, and effective surface area [5]. To address these limitations, this study explores the use of Pickering emulsions to encapsulate nZVI, thereby protecting it from corrosion while enhancing its transport through porous media. The objectives are to investigate emulsion transport behavior, fluid phase distribution at the pore scale, and the mechanisms of trichloroethylene (TCE) removal using well-controlled microfluidic experiments.
The experimental setup consisted of three main components: the fluid injection, the optical, and the microfluidic control systems. A schematic diagram of the setup is shown in Figure 1a. The microchip used was water-wet (Figure 1b), with a pore-width distribution ranging from 4 to 440 µm (Figure 1c) and a constant depth of 20 µm. Its porosity and absolute permeability were 0.52 and 2.5 Darcy, respectively. Pickering emulsions were formulated using either rapeseed oil or castor oil as the dispersed phase, with and without nZVI (5 g/L). Silica nanoparticles (2.5 wt%) or sodium caseinate (NaCas, 13.5 wt%) were employed as stabilizers. Dyed TCE was injected into the chip and brought to residual saturation after each experimental step. Fluid distributions were monitored using optical microscopy, and images were segmented and analyzed using ImageJ.
Results show that all emulsion droplet diameters were significantly smaller than the pore widths, minimizing droplet breakup during transport (Figure 1c). Rheological measurements indicate that silica-stabilized emulsions exhibit strong yield stress and shear-thinning behavior, whereas NaCas-stabilized emulsions behave as Newtonian fluids. (Figure 1d). Following comparable initial pollutant distributions after water imbibition (Figures 2a1, 2b1 and 2c1), rapeseed oil–based emulsions demonstrated more effective physical displacement (Figures 2a2, 2b2 and 2c2) and compositional ripening of TCE than castor oil–based emulsions (Figures 2a3, 2b3 and 2c3). This difference is attributed to the higher water affinity of castor oil droplets, which preferentially invaded water-saturated pores rather than contaminant-filled regions. During the final water flooding stage (Figures 2b4 and 2c4), emulsions stabilized with NaCas were largely recoverable, whereas silica-stabilized emulsions exhibited aggregation and high viscosity, limiting their recovery.
Overall, this study demonstrates the strong potential of Pickering emulsions for enhanced TCE remediation. Systematic comparison of four emulsion formulations provides new insights into emulsion design and transport behavior, offering practical guidance for future environmental applications.
| Country | France |
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