Impact of gravity, collector surface roughness and fracture orientation on colloid retention kinetics in an artificial fracture

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.

Micro- and nanoplastics retention in porous media exhibits different dependence on grain surface roughness and clay coating with particle size

The presence and/or coating of natural colloids (e.g., clays and metal oxides or hydroxides) on collector surfaces has frequently been demonstrated to enhance the retention of engineered colloids that are negatively charged due to favorable electrostatic interactions. However, this work demonstrates that the presence of natural clay coating can lead to reduced or nonmonotonic retention of micro- and nanoplastics (MNPs). Column experiments were carried out to systematically investigate the transport of MNPs with different sizes in relatively smooth and rough sands that had various clay coating fractions. These coating fractions on the collector were found to significantly influence MNP retention in a complex manner that changed with the colloid size and the roughness properties of the sand. This observation was attributed to the impact of clay coatings on the roughness and morphology properties of collector surfaces that were dominant over surface charge. Scanning electron microscopy and interaction energy calculations on surfaces with pillars or valleys indicate that mechanisms that contributed to MNP retention changed with the colloid size. In particular, retention of nanosized plastics was mainly controlled by interactions on convex/concave locations that changed with the solution chemistry, whereas microsized plastics were also strongly influenced by the applied hydrodynamic torque and straining processes. Additionally, the significant sensitivity of MNP retention under a low-level ionic strength also reflects the importance of roughness and charge heterogeneities. These observations are important for investigating the mechanisms of colloid transport in natural systems that ubiquitously exhibit clay coating on their surfaces.

Cotransport of micro- and nano-plastics with chlortetracycline hydrochloride in saturated porous media: Effects of physicochemical heterogeneities and ionic strength

Global production and use of plastics have resulted in the wide dissemination of micro- and nano-plastics (MNPs) to the natural environment. Potentially acting as a vector, the role of MNPs on the fate and transport of environmental pollutants (e.g., antibiotics such as chlortetracycline hydrochloride; CTC) has garnered global concern recently. Herein, the cotransport of MNPs and CTC in columns packed with uncoated sand or soil colloid-coated sand (SCCS) under different degrees of physicochemical heterogeneity and ionic strength was systematically explored. Our results show that MNPs and CTC inhibit the transport of each other when they coexist. The adsorption of CTC onto sand grains, soil colloids, and MNPs, as well as the aggregation of MNPs in the presence of CTC could be the major contributors to the enhanced retention of CTC and MNPs. In SCCS with different degrees of soil colloid coating, the adsorption of CTC on soil colloids is critical to influence the transport of CTC, and the nonlinear retention of MNPs to soil colloids is mainly attributed to the alteration of collector surface roughness by soil colloids. High ionic strength slightly facilitates CTC transport due to the competition for adsorption sites and the formation of CTC macromolecules, but significantly inhibits MNPs transport by suppressing the electrostatic double layers based on colloid stability theory. Consequently, the cotransport of MNPs and CTC is governed by the coupled interplay of collector surface roughness and chemical heterogeneity, due to the soil colloid coatings and the adsorbed CTC on the surfaces associated with solution chemistries such as ionic strength. Increased cotransport of MNPs and CTC occurred under a higher concentration of MNPs due to a larger number of adsorption sites for CTC. Our findings advance the current understanding of the complex cotransport of MNPs and antibiotics in the environment. This information is valuable for understanding contaminant fate and formulating strategies for environmental remediation due to the contamination of MNPs and co-occurring contaminants.

Favorable and unfavorable attachment of colloids in a discrete sandstone fracture

The transport of cationic amine-modified latex (AML) and anionic carboxylate-modified latex (CML) microspheres through a discrete sandstone fracture with mineralogical heterogeneity and roughness was studied. Two microsphere sizes (200 nm and 1000 nm), two ionic strengths (5 mM and 10 mM), and two specific discharges (0.35 mm.s-1 and 0.70 mm.s-1) were tested to observe the impact on transport under favorable and unfavorable conditions. The difference in retention between AML (net favorable) and CML (net unfavorable) microsphere attachment was 25% for the 200 nm microspheres and 13% for the 1000 nm microspheres. Less than 50% of the AML microspheres were retained in the fracture, postulated to be due to the effects of mineralogical heterogeneity and fracture surface roughness. The effect of an increase in ionic strength in increasing retention was significant for unfavorable attachment, but was not significant for favorable attachment conditions. The effect of specific discharge was minor for all but the 200 nm CML microspheres at 10 mM ionic strength. When flushing the fracture first with cationic microspheres, then with anionic microspheres, the recovery of anionic microspheres resembled favorable attachment presumably due to interaction with cationic microspheres that remained attached to the sandstone surface. Colloid breakthrough curves could be fit well with a two site attachment model, with reversible and irreversible sites.

Analytical description of colloid behavior in single fractures under irreversible deposition

OBJECTIVES

Irreversible colloid deposition in groundwater-saturated fractures is typically modeled using a lumped deposition coefficient (κ) that reflects the system physiochemical conditions. A mathematical relationship between this coefficient and the physicochemical conditions controlling deposition has not yet been defined in the literature; thus, κ is typically fitted using experimental observations. This research develops, for the first time, an analytical relationship between κ and the fraction of colloids retained in single fractures (F_r). This relationship could be subsequently integrated with available models relating F_r to the system's physicochemical properties to develop an explicit mathematical relationship between κ and these properties.

METHOD

The F_r-κ analytical relationship was developed through conceptualizing irreversible deposition as first-order decay, as both lead to permanent mass loss, and coupling this with the advection-dispersion equation. The model estimates of colloid deposition were compared to observations from laboratory-scale colloid tracer experiments. A variance-based global sensitivity analysis was applied to identify the parameters controlling deposition.

FINDINGS

The analytical relationship efficiently replicated the experimental observations, and the global sensitivity analysis revealed that colloid deposition variability is controlled by fracture length, aperture size, and deposition coefficient; this supports the accepted understanding that colloid deposition is controlled by the system's physicochemical properties.

Induction of Au-methotrexate conjugates by sugar molecules: production, assembly mechanism, and bioassay studies

Au-methotrexate (Au-MTX) conjugates induced by sugar molecules were produced by a simple, one-pot, hydrothermal growth method. Herein, the Au(III)-MTX complexes were used as the precursors to form Au-MTX conjugates. Addition of different types of sugar molecules with abundant hydroxyl groups resulted in the formation of Au-MTX conjugates featuring distinct characteristics that could be explained by the diverse capping mechanisms of sugar molecules. That is, the instant-capping mechanism of glucose favored the generation of peanut-like Au-MTX conjugates with high colloidal stability while the post-capping mechanism of dextran and sucrose resulted in the production of Au-MTX conjugates featuring excellent near-infrared (NIR) optical properties with a long-wavelength plasmon resonance near 630-760 nm. Moreover, in vitro bioassays showed that cancer cell viabilities upon incubation with free MTX, Au-MTX conjugates doped with glucose, dextran and sucrose for 48 h were 74.6%, 55.0%, 62.0%, and 63.1%, respectively. Glucose-doped Au-MTX conjugates exhibited a higher anticancer activity than those doped with dextran and sucrose, therefore potentially presenting a promising treatment platform for anticancer therapy. Based on the present study, this work may provide the first example of using biocompatible sugars as regulating agents to effectively guide the shape and assembly behavior of Au-MTX conjugates. Potentially, the synergistic strategy of drug molecules and sugar molecules may offer the possibility to create more gold-based nanocarriers with new shapes and beneficial features for advanced anticancer therapy.

Transport of silver nanoparticles in single fractured sandstone

Silver nanoparticles (Ag-NP) are used in various consumer products and are one of the most prevalent metallic nanoparticle in commodities and are released into the environment. Transport behavior of Ag-NP in groundwater is one important aspect for the assessment of environmental impact and protection of drinking water resources in particular. Ag-NP transport processes in saturated single-fractured sandstones using triaxial flow cell experiments with different kind of sandstones is investigated. Ag-NP concentration and size are analyzed using flow field-flow fractionation and coupled SEM-EDX analysis. Results indicate that Ag-NP are more mobile and show generally lower attachment on rock surface compared to experiments in undisturbed sandstone matrix and partially fractured sandstones. Ag-NP transport is controlled by the characteristics of matrix porosity, time depending blocking of attachment sites and solute chemistry. Where Ag-NP attachment occur, it is heterogeneously distributed on the fracture surface.