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

Modelling the transport of iron micro and nanoparticles in saturated porous media

In the framework of groundwater remediation, the injection of nanoscale and microscale zerovalent iron particles (NZVI and MZVI, respectively) for the generation of reactive zones proved effective, and represents a promising remediation technology for the treatment of contamination sources and dissolved plumes. To improve colloidal stability and mobility in the subsurface, the use of biopolymers is usually required. Polymers are dosed in low concentrations to modify surface properties and increase particle-particle repulsion (mainly for NZVI) or in high concentration to form shear-thinning fluids preventing particle sedimentation and improve delivery (mainly for MZVI). Shear thinning fluids exhibit high viscosity at low flow rates (which improves colloidal stability in static conditions) and lower viscosity at high flow rates, corresponding to the injection in the subsurface, when low viscosity (and consequently low pressures) is required. In this work a modelling approach is described to simulate the transport in porous media of nanoscale iron slurries, implemented in MNMs (www.polito.it/groundwater/software). Colloid transport mechanisms are controlled by particle-particle and particle-collector interactions, typically modelled with kinetic terms of deposition onto the porous medium and corresponding release. Ionic strength, flow rate and fluid viscosity all play a major role in determine the interactions between particles and porous medium, and therefore deposition and release mechanisms and kinetics. The key aspects included in MNMs are the influence of salt concentration on attachment and detachment kinetics (both under constant and transients in I.S), clogging phenomena (i.e. reduction of porosity and permeability due to particles deposition), and the rheological properties of the carrier fluid. Colloid transport is modelled with a dual-site (physico-chemical interactions plus straining) advection-dispersion-deposition equation. A general formulation for attachment/detachment dynamics is adopted. The influence of colloid transport on porosity, permeability, and fluid viscosity is explicitly embedded into the model through correlations from the literature, or derived on purpose. The software also implements a tool for the simulation of particle transport in radial geometry, for the estimate of the radius of influence of the slurry injection

Bimetallic Fe/Cu nanoparticles for groundwater remediation: optimized injection strategies via transport modelling in porous media

A field-scale remediation of a contaminated aquifer by means of nanoscale zero valent iron (NZVI) requires an accurate assessment of the mobility of such particles in saturated porous media. Thanks to their reduced size, such particles can be effectively injected in the form of concentrated colloidal dispersions into the subsurface to target contaminated zones and sources. However, NZVI slurries faced critical problems for applications in porous media due to colloidal instability. NZVI aggregation is caused by strong particle-particle attraction, and results in short travel distances and pore plugging, especially when NZVI is injected at high concentrations. More stable suspensions of NZVI can be obtained by adding polymeric surface modifiers or anionic surface chargers or directly modifying the particle surface during synthesis by addition of noble metals. Bimetallic NZVI showed much higher degradation rates towards all contaminants traditionally treated by millimetric iron. Several studies investigated the effects of several factors (eg. particle stabilization methods, groundwater ionic strength, particle size and composition, etc.) on the transport and retention of NZVI in well-controlled lab-scale columns. Conversely, few studies have been devoted to understand the role of the injection strategy (flow rate, NZVI concentration, injection duration and alternation with flushing) on NZVI mobility. In this study, a quantitative analysis is presented on how the management of the injection of NZVI water-based slurries can optimize the mobility of the particles. In particular, the impact of injected NZVI concentration, flow rate, and number, duration, and alternation of injection and flushing periods is considered. NZVI transport simulations in 1D domains were performed using E-MNM1D for bimetallic nano-Fe/Cu particles, whose transport was previously assessed by the authors in laboratory column tests. Several injection scenarios were considered, including single-step injections (injection followed by flushing), and multi-steps injections (repetition of injection+flushing steps) with constant and variable particle concentration. The performance of each scenario was quantified in terms of travel distance, changes in porous medium porosity, permeability, and overpressure during injection. The results of this study indicate that, when injecting under conditions typical of a full-scale aquifer remediation, nanoparticle mobility and distribution are optimized and clogging is minimized by using high flow rates, low concentrations, and frequent injection steps without intermediate flushing

Integrating NZVI and carbon substrates in a non-pumping reactive wells array for the remediation of a nitrate contaminated aquifer

The work explores the efficacy of a biochemical remediation of a nitrate-contaminated aquifer by a combination of nanoscale zero-valent iron (NZVI) and bacteria supported by carbon substrates. Nitrate removal was first assessed in batch tests, and then in a laboratory bench-scale aquifer model (60cm length×40cm width×50cm height), in which a background flow was maintained. Water and natural sandy material of a stratified aquifer were used in the tests to enhance the reliability of the results. An array of non-pumping-reactive wells (NPRWs) filled with NZVI (d50=50nm, and SSA=22.5m(2)/g) mixed with carbon substrates (beech sawdust and maize cobs) was installed in the bench-scale aquifer model to intercept the flow and remove nitrate (NO3(-) conc.=105mg/l). The NPRW array was preferred to a continuous permeable reactive barrier (PRB) since wells can be drilled at greater depths compared to PRBs. The optimal well diameter, spacing among the NPRWs and number of wells in the bench-scale model were designed based on flow simulations using the semi-analytical particle tracking (advection) model, PMPATH. An optimal configuration of four wells, 35mm diameter, and capture width of 1.8 times the well diameter was obtained for a hydraulic conductivity contrast between reactive materials in the wells and aquifer media (KPM/Kaq=16.5). To avoid excessive proximity between wells, the system was designed so that the capture of the contaminated water was not complete, and several sequential arrays of wells were preferred. To simulate the performance of the array, the water that passed through the bench-scale NPRW system was re-circulated to the aquifer inlet, and a nitrate degradation below the limit target concentration (10mg/l) was obtained after 13days (corresponding to 13 arrays of wells in the field). The results of this study demonstrated that using the NZVI-mixed-carbon substrates in the NPRW system has a great potential for in-situ nitrate reduction in contaminated groundwater. This NPRW system can be considered a promising and viable technology in deep aquifers

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

Modelling nanoparticle transport in porous media across the scales: from pore network models to simulation of filed injection

Transport and deposition of colloidal particles in saturated porous media are of great importance in many fields of science and engineering. A thorough understanding of particle filtration processes is essential for predicting the transport and fate of colloidal particles in the subsurface environment. Particles migrating through a porous medium can remain in suspension and be transported due to advection and dispersion phenomena, or be retained due to filtration and deposition onto the porous matrix. In particular, in the framework of the FP7 project Nanorem (G.A. Nr. 309517), the application of nanoparticles for groundwater remediation is the key research question. Colloid transport is a peculiar multi-scale problem, and pore-scale processes have an important impact on the transport at the larger scale. In this study, colloid transport modelling was carried out at different scales, from the pore scale (applying pore-network models) up to the full field scale. Assessing the mechanisms that control the mobility of reactive nanoparticles is of pivotal importance in the design, implementation, and performance evaluation of field applications. While numerical models for the simulation of dissolved contaminants transport are widely available, field scale models of nanoparticles with proven predictive ability are yet to be developed. This is mainly because the fundamental controlling mechanisms for the transport of nanoparticles in the subsurface at the field scale are not well understood. Using pore network modelling we simulate fluid flow and transport of colloids within a network of interconnected pores (Raoof et al., 2013). Colloidal processes such as deposition and aggregation are implemented at the scale of individual pores. Averaging over the network domain composed of several pores, we derive macro-scale parameters to be used within field scale models (Raoof et al., 2010). Transport of concentrated nanoparticle suspensions in porous media is affected by the rheological properties of the dispersing fluid (shear thinning) and by particle deposition and filtration in the porous matrix, which result in porous medium clogging (i.e. reduction of porosity and permeability). Moreover, the kinetics of particle retention is strongly influenced by the ionic strength of the pore water. Up to date, modelling of colloid transport in the presence of such complex interaction phenomena has been mainly faced in one-dimensional Cartesian coordinates for the simulation of laboratory column tests (Tosco et al., 2009; Tosco and Sethi, 2010), or at larger scales in simplified radial domains (Tosco et al., 2014), as implemented in MNMs (www.polito.it/groundwater/software/MNMs.php). In this work, a modelling tool for the simulation of colloid injection and transport under transients in ionic strength in more complex scenarios is developed and validated. To this aim colloid transport equations were implemented in the well-known transport model RT3D (Clement et al., 1998). The tool can be used for multi-dimensional simulations, and the approach is validated through comparison of results from MNMs and RT3D for a one-dimensional domain

Field-scale modeling of nanoparticle transport in aquifer systems

Despite their great potential for technological applications, engineered nanoparticles (NPs) can represent a significant, and still largely unknown, environmental hazard. NPs-containing products are already widely available on the market, or expected in few years, likely leading to the dissemination of large amounts of NPs in the environment. Moreover, injection into the subsurface of suspensions of iron-based micro and nanoparticles have proved promising for groundwater remediation. In both cases, a full understanding of the mechanisms governing the transport of NPs in the subsurface and quantitative modeling tools are necessary, for both the design of NP-based remediation technologies, and long-term fate prediction. NP transport in porous media is controlled by particle-particle and particle-collector interactions, typically modelled with kinetic terms of deposition onto the porous medium and corresponding release. Ionic strength and flow rate play a major role in both. In this work, two modeling tools, MNMs and MNM3D, are proposed to simulate NP transport in porous media, respectively at laboratory and field scale, under space- and time-variable ionic strength and flow velocity. Applications of the tools are also presented. MNMs is a Matlab-based software (www.polito.it/groundwater/software/MNMs.php) implementing numerical solutions for lab-scale (1D columns) and pilot-scale NP transport (injection through a single well, radial geometry). MNM3D is a modified version of the well-known transport model RT3D, and solves particle transport in 3-dimensional geometries. MNM3D can be used for multi-dimensional simulations and employed in many practical field-scale applications, eg. the preliminary design of in situ aquifer remediation via NP injection, and the estimate of long-term fate of NPs released in landfill leachate. The work is co-funded by the FP7 EU project NANOREM (GA 309517)

On the failure of upscaling the single-collector efficiency to the transport of colloids in an array of collectors

Defining the removal efficiency of a filter is a key aspect for colloid transport in porous media. Several efforts were devoted to derive accurate correlations for the single-collector removal efficiency, but its upscaling to the entire porous medium is still a challenging topic. A common approach involves the assumption of deposition being independent of the history of transport, that is, the collector efficiency is uniform along the porous medium. However, this approach was shown inadequate under unfavorable deposition conditions. In this work, the authors demonstrate that it is not adequate even in the simplest case of favorable deposition. Computational Fluid Dynamics (CFD) simulations were run in a vertical array of 50 identical spherical collectors. Particle transport was numerically solved by analyzing a broad range of parameters. The results evidenced that when particle deposition is not controlled by Brownian diffusion, nonexponential concentration profiles are retrieved, in contrast with the assumption of uniform efficiency. If sedimentation and interception dominate, the efficiency of the first sphere is significantly higher compared to the others, and then declines along the array down to an asymptotic value. Finally, a correlation for the upscaled removal efficiency of the entire array was derived

MNM3D: a modelling tool for simulation of nanoparticle injection and transport in 3D geometries

The design of a field-scale injection of engineered nanoparticle (NP) suspensions for the remediation of a polluted site requires the development of quantitative predictive models for the system design and implementation. 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. Particle transport is modelled by a modified advection-dispersion equation, which accounts for particle deposition and release phenomena. During the injection, a strong perturbation of the flow field is induced around the well, and the particle transport is mainly controlled by the consequent sharp variation of pore-water velocity, and by the hydro-chemical properties of the injected fluid. 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 support nanoremediation design. 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 Nanoparticles 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. To the authors' knowledge, MNM3D is the first 3D numerical code embedding a specific routine for the continuous calculation, cell by cell and at the time-step level (for transient flow simulations), of the attachment and detachment kinetics. 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 stage (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 is used to model a 2D pilot scale injection of CARBO-IRON® in a small scale flume carried out at the VEGAS facilities in the framework of the NanoRem project. Moreover, the long term fate of an hypothetical release of nanoparticles into the environment from a landfill is simulated. The work is co-funded by the FP7 EU project NANOREM (GA 309517)

Transport models for risk assessment of natural and engineered nanoparticles in groundwater

Nanosized particles of several materials, such as TiO2, graphene, zero-valent iron, iron oxides, carbon nano-tubes, etc., are commonly used in industrial processes and for the manufacturing of commercial products. They can be released into the environment and, in particular, into groundwater throughout their entire life cycle, thus representing a potential risk for human health. Once released, nanoparticles can exhibit inherent toxicity, or play a role in enhancing the mobility of many contaminants, acting as a mobile solid phase which accelerates the transport of strongly sorbing contaminants (colloid-facilitated contaminant transport). It is therefore extremely important to develop approaches and tools suitable to predict the long-term fate of these emerging contaminants and the associated potential risk. The quantification of the toxic and carcinogenic risk towards potential receptors requires the application of transport models for the evaluation of the nanoparticle concentration at the exposition point, Cpoe. Nanoparticle transport in porous media is usually described by a modified advection-dispersion equation that takes into account the mass exchanges between liquid and solid phase due to physical and physico-chemical interactions. According to the degree of detail of the risk assessment procedure, Cpoe can be calculated via analytical formulations, Tier 2, or numerical tools, Tier 3. In this work, NP-specific transport models are simplified and adapted to the analytical solutions commonly adopted in the conventional Tier 2 RBCA approach. The analytical solutions commonly used for the calculation of dissolved contaminant Cpoe, are here extended to account for particle-porous-medium interaction. Moreover, the numerical tool MNM3D is proposed for the simulation of nanoparticle transport applied to Tier 3 risk assessment. MNM3D is a modified version of the well-known RT3D code, which implements numerical solutions to the NP transport equations porous media, accounting for the dependency of the attachment and detachment kinetic coefficients on transients in pore water ionic strength and velocity. The analytical solutions and the numerical code MNM3D are here both applied to a synthetic case of release of silver nanoparticles and the results are compared. Finally the same release in a more complex hydrogeological scenario is simulated using the MNM3D code to highlight the effects of hydrochemical heterogeneities on the long term fate of nanoparticles in the environment