Direct visualization of colloid transport over natural heterogeneous and artificial smooth rock surfaces.

Colloid transport in fractured rock formations is an important process impacting the fate of pollutants in the subsurface. Despite intensive and outstanding research on their transport phenomena, the impact of small-scale surface heterogeneity on colloid behavior at the fracture scale remains difficult to assess. In particular, there is relatively little direct experimental evidence on the impact of natural fracture surface heterogeneity on colloid transport. To investigate this, we developed an experimental setup allowing the direct visualization of fluorescent colloid transport in a flow cell containing a natural chalk rock sample while simultaneously monitoring effluent colloid concentrations. We used samples containing both a natural fracture surface and an artificially made smooth surface from the same chalk core. We characterized the roughness and chemical composition of both surface types and numerically calculated each surface's velocity field. From the experiments, we obtained direct images of colloid transport over the surfaces, from which we calculated their dispersion coefficients and quantified the residual deposition of colloids on the rock surface. We also measured the colloid breakthrough curves by collecting eluent samples from the flow cell outlet. The natural fracture surface exhibited larger physical and chemical heterogeneity than the smooth, artificially generated surface. The aperture variability across the natural surface led to preferential flow and colloid transport which was qualitatively apparent in the fluorescent images. The colloid transport patterns matched the calculated velocity fields well, directly linking the surface topography and aperture variation to colloid transport. Compared to the artificially made surface, the natural surface also showed higher dispersion coefficients, which corresponded to the colloids' earlier breakthrough from the flow cell. While we found differences between the elemental composition of the natural and artificially smooth surfaces, we could not observe their impact on the colloids' surface attachment and retention. The main novelty in this work is the coupling of direct colloid transport imaging, breakthrough curve measurements, and colloid surface deposition analyses, in a flow cell containing a natural carbonate rock sample. Our experimental setup can be used to further investigate the link between surface heterogeneity, both chemical and physical, and colloid transport and deposition in natural rock fractures.

Drying in a microfluidic chip: experiments and simulations.

We present an experimental micro-model of drying porous media, based on microfluidic cells made of arrays of pillars on a regular grid, and complement these experiments with a matching two-dimensional pore-network model of drying. Disorder, or small-scale heterogeneity, was introduced into the cells by randomly varying the radii of the pillars. The microfluidic chips were filled with a volatile oil and then dried horizontally, such that gravitational effects were excluded. The experimental and simulated drying rates and patterns were then compared in detail, for various levels of disorder. The geometrical features were reproduced well, although the model under-predicted the formation of trapped clusters of drying fluid. Reproducing drying rates proved to be more challenging, but improved if the additional trapped clusters were added to the model. The methods reported can be adapted to a wide range of multi-phase flow problems, and allow for the rapid development of high-precision micro-models containing tens of thousands of individual elements.

Combined effects of biosolids application and irrigation with reclaimed wastewater on transport of pharmaceutical compounds in arable soils.

Pharmaceutical compounds (PCs) are introduced into the agricultural environment through irrigation with treated effluents and application of biosolids. Transport processes can determine the fate of PCs and the associated risks related to their exposure in the environment. The aim of this work was to evaluate the combined effects of biosolids application and irrigation with treated effluents on the mobility of PCs in soil and to elucidate the main mechanisms affecting their transport. Column-leaching experiments revealed that application of biosolids generally increased the retardation of PCs, whereas treated effluents increased the mobility of weakly acidic PCs in the biosolids-amended soils. Experiments conducted at environmentally relevant PC concentrations (≈ 1 µg/L) highlight the importance of irreversible sorption as a possible mechanism for low leachability. Data generated from this study suggest that (i) transport behavior of PCs can be affected by common biosolids application to arable land; (ii) treated effluents increase the mobility of weakly acidic PCs mainly by increasing of the soil solution pH and not due to complexation of the PCs with dissolved organic matter; and (iii) it is highly important to evaluate transport behavior at environmentally relevant concentrations and not to base modeling on data obtained from experiments conducted in high concentrations.