Transport of Non-Newtonian Suspensions of Highly Concentrated Micro- And Nanoscale Iron Particles in Porous Media: A Modeling Approach

The use of zerovalent iron micro- and nanoparticles (MZVI and NZVI) for groundwater remediation is hindered by colloidal instability, causing aggregation (for NZVI) and sedimentation (for MZVI) of the particles. Transportability of MZVI and NZVI in porous media was previously shown to be significantly increased if viscous shear-thinning fluids (xanthan gum solutions) are used as carrier fluids. In this work, a novel modeling approach is proposed and applied for the simulation of 1D flow and transport of highly concentrated (20 g/L) non- Newtonian suspensions of MZVI and NZVI, amended with xanthan gum (3 g/L). The coupled model is able to simulate the flow of a shear thinning fluid including the variable apparent viscosity arising from changes in xanthan and suspended iron particle concentrations. The transport of iron particles is modeled using a dual-site approach accounting for straining and physicochemical deposition/release phenomena. A general formulation for reversible deposition is herein proposed, that includes all commonly applied dynamics (linear attachment, blocking, ripening). Clogging of the porous medium due to deposition of iron particles is modeled by tying porosity and permeability to deposited iron particles. The numerical model proved to adequately fit the transport tests conducted using both MZVI and NZVI and can develop into a powerful tool for the design and the implementation of full scale zerovalent iron application

Transport of ferrihydrite nanoparticles in saturated porous media: role of ionic strength and flow rate

The use of nanoscale ferrihydrite particles, which are known to effectively enhance microbial degradation of a wide range of contaminants, represents a promising technology for in situ remediation of contaminated aquifers. Thanks to their small size, ferrihydrite nanoparticles can be dispersed in water and directly injected into the subsurface to create reactive zones where contaminant biodegradation is promoted. Field applications would require a detailed knowledge of ferrihydrite transport mechanisms in the subsurface, but such studies are lacking in the literature. The present study is intended to fill this gap, focusing in particular on the influence of flow rate and ionic strength on particle mobility. Column tests were performed under constant or transient ionic strength, including injection of ferrihydrite colloidal dispersions, followed by flushing with particle-free electrolyte solutions. Particle mobility was greatly affected by the salt concentration, and particle retention was almost irreversible under typical salt content in groundwater. Experimental results indicate that, for usual ionic strength in European aquifers (2 to 5 mM), under natural flow condition ferrihydrite nanoparticles are likely to be transported for 5 to 30 m. For higher ionic strength, corresponding to contaminated aquifers, (e.g., 10 mM) the travel distance decreases to few meters. A simple relationship is proposed for the estimation of travel distance with changing flow rate and ionic strength. For future applications to aquifer remediation, ionic strength and injection rate can be used as tuning parameters to control ferrihydrite mobility in the subsurface and therefore the radius of influence during field injection

Shear thinning fluids to optimize the injection of engineered microparticles for groundwater remediation

Nanoscale and microscale zerovalent iron particles (NZVI and MZVI) are promising materials for the remediation of contaminated aquifers. These particles are dispersed in water-based slurries and injected in the subsoil to generate a reactive zone. However, the successful injection of MZVI and NZVI may be significantly hindered by the reduced colloidal stability, and therefore mobility in the porous medium, due to the fast sedimentation (MZVI) and aggregation (NZVI) of the particles when dispersed in water. To overcome this issue the use of stabilizing agents wax proposed: shear thinning solutions of green biopolymers have been recently studied as kinetic stabilizers and viscous carrier fluids for the delivery of MZVI and NZVI. Shear thinning fluids exhibit high viscosity in static conditions, improving the colloidal stability, and lower viscosity at high flow rates enabling the injection at limited pressures. In this work the use of guar gum is presented. Polymeric solutions (1.5 to 7 g/l) were prepared following different procedures, and their efficacy in stabilizing highly concentrated dispersions of MZVI (20 g/l, average size 1.2 um) was evaluated. Ideally, the optimal guar gum suspension should (i) keep the MZVI suspended for a time sufficient for its injection; (ii) be easily degradable, to avoid possible negative effects on MZVI reactivity; (iii) do not clog the porous medium due to residual undissolved guar gum. With these targets in mind, a detailed rheological characterization of the guar gum, both in the bulk and when injected in a porous medium, was carried out. A modified Cross model, linking guar gum concentration and bulk shear viscosity, was derived based on bulk rheological measurements. Column filtration tests were then performed, and a modified Darcy law was derived to predict pressure gradients arising during guar gum injection. The kinetics of guar gum degradation was studied to investigate the correct enzymes dosage required to achieve a complete breakdown of the suspensions, and a modified Stokes law for the prediction of the sedimentation rate of the MZVI was proposed and validated. All derived empirical relationships (namely, rheological model, modified Stokes law and Darcy law) were finally included in MNMs (www.polito.it/groundwater/software/MNMs.php), a software for particle transport simulation in 1D (column) and radial domain. MNMs can be used as a tool for a preliminary design of the field injection of MZVI/NZVI - guar gum mixtures, providing an estimate of particle transport and pressure build up associated to the injection at the pilot scale. Funded by: EU FP7 Aquarehab Grant Agreement No. 22656

Simulating nanoparticle transport in 3D geometries with MNM3D

The application of NP transport to real cases, such as the design of a field-scale injection or the prediction of the long term fate of nanoparticles (NPs) in the environment, requires the support of mathematical tools to effectively assess the expected NP mobility at the field scale. In general, micro- and nanoparticle transport in porous media is controlled by particle-particle and particle-porous media interactions, which are in turn affected by flow velocity and pore water chemistry. During the injection, a strong perturbation of the flow field is induced around the well, and the NP transport is mainly controlled by the consequent sharp variation of pore-water velocity. Conversely, when the injection is stopped, the particles are transported solely due to the natural flow, and the influence of groundwater geochemistry (ionic strength, IS, in particular) on the particle behaviour becomes predominant. Pore-water velocity and IS are therefore important parameters influencing particle transport in groundwater, and have to be taken into account by the numerical codes used to simulate NP transport. Several analytical and numerical tools have been developed in recent years to model the transport of colloidal particles in simplified geometry and boundary conditions. For instance, the numerical tool MNMs was developed by the authors of this work to simulate colloidal transport in 1D Cartesian and radial coordinates. Only few simulation tools are instead available for 3D colloid transport, and none of them implements direct correlations accounting for variations of groundwater IS and flow velocity. In this work a new modelling tool, MNM3D (Micro and Nanoparticle transport Model in 3D geometries), is proposed for the simulation of injection and transport of nanoparticle suspensions in generic complex scenarios. MNM3D implements a new formulation to account for the simultaneous dependency of the attachment and detachment kinetic coefficients on groundwater IS and velocity. The software was developed in the framework of the FP7 European research project NanoRem and can be used to predict the NP mobility at different stages of a nanoremediation application, both in the planning and design stages (i.e. support the design of the injection plan), and later to predict the long-term particle mobility after injection (i.e. support the monitoring, final fate of the injected particles). In this work MNM3D an integrated experimental-modelling procedure is used to assess and predict the nanoparticle transport in porous media at different spatial and time scales: laboratory tests are performed and interpreted using MNMs to characterize the nanoparticle mobility and derive the constitutive equations describing the suspension behavior in groundwater. MNM3D is then used to predict the NP transport at the field scale. The procedure is here applied to two practical cases: a 3D pilot scale injection of CARBO-IRON® in a large scale flume carried out at the VEGAS facilities in the framework of the NanoRem project; the long term fate of an hypothetical release of nanoparticles into the environment from a landfill is simulate

Zerovalent iron micro and nanoparticles for groundwater remediation: from laboratory to field scale

The poster presents an overview of laboratory tests, field applications and modelling approaches for the development of an innovative groundwater remediation technique based on the injection of zerovalent iron micro and nanoparticles dispersed in shear thinning fluids

Water Chemistry Affects the Efficacy of Concentrated Suspensions of Iron Oxide Nanoparticles Used for Aquifer Reclamation

Nanosized colloids of iron oxide enhance the subsurface microbial degradation of a wide range of organic contaminants and they are used as a technology to clean-up contaminated aquifers. These particles are synthesized with a coating of humic acids to increase their stability in aqueous suspension, a crucial property to ensure the subsurface mobility of concentrated slurries of this material. This study investigates particle properties, stability, and sedimentation in water as a function of chemistry and ionic composition. Goethite particles display high stability in different electrolyte solutions of NaCl and MgCl2, consistent with a negative zeta potential of strong magnitude that implies an effective electrostatic stabilization. While goethite particles follow the predicted DLVO behavior in NaCl and show a high critical coagulation concentration, their aggregation is fast in the presence of calcium, even at very low ionic strengths (< 1 mM). This result is rationalized with the occurrence of bridging phenomena related to the interaction of calcium with adsorbed chains of humic acid, inducing fast flocculation and sedimentation of the suspensions. The dose of calcium, i.e., the concentration of calcium ions with respect to that of particles in the dispersion, is found to be the parameter governing these stabilization mechanisms. This result implies that more concentrated slurries may be more stable than dispersions of low particle concentration under certain conditions. Stability results correlate well with the extent of slurry transport within a column of saturated sand. These results suggest the possibility to design an effective remediation strategy for each specific site geochemical condition

Pore-scale simulation of fuid flow and solute dispersion in three-dimensional porous media

In the present work fluid flow and solute transport through porous media are described by solving the governing equations at the pore-scale with finite-volume discretization. Instead of solving the simplified Stokes equation (very often employed in this context) the full Navier-Stokes equation is used here. The realistic three-dimensional porous medium is created in this work by packing together, with standard ballistic physics, irregular and polydisperse objects. Emphasis is placed on numerical issues related to mesh generation and spatial discretization, which play an important role in determining the final accuracy of the finite-volume scheme, and are often overlooked. The simulations performed are then analyzed in terms of velocity distributions and dispersion rates in a wider range of operating conditions, when compared with other works carried out by solving the Stokes equation. Results show that dispersion within the analyzed porous medium is adequately described by classical power laws obtained by analytic homogenization. Eventually the validity of Fickian diffusion to treat dispersion in porous media is also assessed

Monitoring the injection of microscale zerovalent iron particles for groundwater remediation by means of complex electrical conductivity imaging

The injection of microscale zerovalent iron (mZVI) particles for groundwater remediation has received much interest in recent years. However, to date, monitoring of mZVI particle injection is based on chemical analysis of groundwater and soil samples and thus might be limited in its spatiotemporal resolution. To overcome this deficiency, in this study, we investigate the application of complex electrical conductivity imaging, a geophysical method, to monitor the high-pressure injection of mZVI in a field-scale application. The resulting electrical images revealed an increase in the induced electrical polarization (∼20%), upon delivery of ZVI into the targeted area, due to the accumulation of metallic surfaces at which the polarization takes place. Furthermore, larger changes (>50%) occurred in shallow sediments, a few meters away from the injection, suggesting the migration of particles through preferential flowpaths. Correlation of the electrical response and geochemical data, in particular the analysis of recovered cores from drilling after the injection, confirmed the migration of particles (and stabilizing solution) to shallow areas through fractures formed during the injection. Hence, our results demonstrate the suitability of the complex conductivity imaging method to monitor the transport of mZVI during subsurface amendment in quasi real-time

Metodi per la delineazione automatica di aree di cattura per pozzi idropotabili e sistemi Pump and Treat

L'articolo propone un confronto critico fra due approcci per la determinazione delle aree di cattura di pozzi in pompaggio. Il primo metodo proposto è il metodo APA, di tipo deterministico, basato sul particle tracking, e pertanto sui soli fenomeni advettivi. Rispetto ai comuni codici di tracciamento di particelle, APA fornisce un algoritmo di ottimizzazione del calcolo dei percorsi e un algoritmo di post-processing per la perimetrazione automatizzata delle aree. Il secondo approccio considerato è di tipo probabilistico, basato su un modello inverso che consente di defi nire le aree di salvaguardia in termini di mappe di probabilità di cattura, con costi computazionali ridotti rispetto ai metodi di tipo stocastico, e includendo, oltre all'advezione, anche i meccanismi di dispersione idrodinamica

Modelling the transport of iron-based colloids in saturated porous media

The aim of this work is to develop a numerical model able to describe the transport of highly concentrated suspensions of iron particles, in particular during their injection in the subsurface, and in the early stages of migration through the porous matrix. The model is intended to be used as a tool in the development of an efficient injection technology for field-scale applications of nano- and microscale iron suspensions. Due to the complexity of phenomena governing the transport or iron colloidal suspensions, the first part of this study (Chapters 1 to 4) focuses on assessing the influence of hydrochemical parameters on the mobility of simple, well-known colloidal systems, namely latex microspheres. Latex colloids were chosen as they are often used in the literature as model particles when studying the transport of natural nano- and microparticles in porous media. Colloid transport is governed by advection-dispersion mechanisms, filtration, particle-particle and particle-soil interactions. The latter result in dynamic deposition of the particles on (and release from) the grains of the porous medium. Hydrochemical parameters are known to greatly influence colloid deposition/release phenomena, but little or no systematic studies are found in the literature that explicitly include these effects in a numerical model. Consequently, in the first part of this work the influence of transients in ionic strength on particle deposition and re-entrainment is systematically investigated. Semi-empirical relationships are proposed that tie the deposition and release kinetics to the salt concentration, and embedded in a one-dimensional transport model. The work is structured as follows: • Chapter 1. Phenomena controlling colloid transport in saturated porous media are described, and model equations are stated to simulate colloid deposition and release during transients of ionic strength. The effects of changes in the solution ionic strength are explicitly included into the set of model equations. • Chapter 2. The transient model is solved numerically with the implementation of a finite-differences code, called MNM1D (Micro- and Nanoparticle transport Model in porous media). The partial differential equations for the salt and the colloid concentrations are solved simultaneously, and coupled with empirical functions describing the explicit dependence of deposition and release coefficients on the ionic strength. MNM1D was validated against well-established analytical and finite-elements transport codes that solve colloid transport under stationary hydrochemical conditions, proving to be adequately stable and robust. • Chapter 3. A complete characterization of colloidal suspensions and porous medium are presented, and the protocol adopted for column tests of colloidal mobility under different hydrochemical conditions is described: each experiment included a deposition step at constant ionic strength, followed by release steps induced by abrupt changes in porewater chemistry. • Chapter 4. Experimental results are discussed, and fitting of the breakthrough curves with MNM1D are presented. First, the model is applied to the initial part of the experiments (i.e. colloid deposition at constant ionic strength), and the semi-empirical relationships for deposition and release kinetic parameters are derived from the fitting results. Then, MNM1D is applied for the fitting of the whole experiment. In the second part of the work (Chapters 5 to 7), the model is extended to iron micro- and nanoparticles. In this case, particle deposition and rheological properties of the highly concentrated slurry of iron colloids play a major role (concentrations are up to 20 g/l, and particles are dispersed in non-Newtonian viscous fluids for improving stability). Consequently, the hydrodynamic parameters and fluid properties are no longer independent on the concentration of deposed and suspended colloids, and flow and transport become coupled problems. • Chapter 5. Model equations for the description of coupled flow and transport phenomena of non-Newtonian suspensions of micro- and nanosized iron particles are presented. They are developed from the commonly used equations of solute transport in porous media, and modified to account for changes in matrix properties due to particle deposition (i.e. clogging) and for the non-Newtonian nature of the carrier fluid. • Chapter 6. First, the properties of iron micro- and nanoparticles, porous medium and dispersant fluid used in laboratory transport tests are described. Then, the protocol adopted in column experiments is detailed. Column tests were lead under different hydrochemical conditions. A first deposition step with injection of iron particles dispersed in a xanthan slurry was followed by a release step obtained by flushing the column with water. • Chapter 7. The model equations are implemented in a finite differences code, that represents the extension of MNM1D, and are applied for the fitting of the experimental data, proving adequate to simulate particle transport. Before this study, no model was available in the literature for the simulation of iron transport under the conditions described in the experimental section: although solutions exist for the simulation of clogging in deep bed filtration, or changes in pore space geometry due to mineral precipitation, none of these models can simulate the non-Newtonian nature of the carrier fluid, nor the influence of the concentration of suspended colloids on the fluid properties. The numerical model developed in this work, although implemented for the simulation of 1D laboratory column tests, could be extended to more complex geometries, thus becoming a useful tool for the design of the injection and early stages of migration of iron slurries in field application