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

Sensitivity Analysis on the Performance of a Ground Source Heat Pump Equipped with a Double U-pipe Borehole Heat Exchanger

AbstractGround Source Heat Pumps (GSHP) are economically and environmentally advantageous for the heating and cooling of buildings, provided that the long-term sustainability of the thermal exploitation of the soil is ensured. In particular, the performance of a closed-loop Borehole Heat Exchanger (BHE) strongly depends on the geometrical and physical properties of its components and on the thermo-hydrogeological properties of the surrounding soil. In this work, we present the results of a series of simulations of a double U-pipe Borehole Heat Exchanger, carried out with the finite-element flow and heat transport modelling software FEFLOW to assess the relative influence of these parameters on the operation of a GSHP. The analysis confirms that the length of the borehole is the main design parameter, but the thermal conductivity of the grout, the pipe spacing, the heat carrier fluid and its flow rate also have an important effect on the energy efficiency of the system. The thermal conductivity of the soil is another fundamental variable in the design of a GSHP, and hence it is better to rely on site-specific data, rather than adopting values from the literature. Although most design methods neglect it, the presence of a subsurface flow results in an enhancement of the performance of the system. Thermal dispersion also enhances the efficiency of the system but, since it has not yet been adequately studied, relying on it is not advised for the design of BHE fields

Modelling thermal recycling occurring in groundwater heat pumps (GWHPs)

The performance of a Ground Water Heat Pump (GWHP) is often impaired by the thermal recycling between the injection and the extraction well(s), and hence this phenomenon should be evaluated in the design of open loop geothermal plants. The numerical flow and heat transport simulation of a GWHP requires an expensive characterization of the aquifer to obtain reliable input data, which is usually not affordable for small installations. To provide a simple, fast and inexpensive tool for preliminary and sensitivity analyses, an open-source numerical code was developed, which solves the hydraulic and thermal transport problem of a well doublet in the presence of a subsurface flow. The code, called TRS (Thermal Recycling Simulator), is based on a finite-difference approximation of the potential flow theory. The method was validated through the comparison with flow and heat transport simulations with FEFLOW. Subsequently, TRS was run with different values of the aquifer and plant parameters. The correlation observed between some characteristic non-dimensional quantities permitted an empirical correlation to be developed, that describes the time evolution of the extracted water temperature. An example is given for the use of the numerical code and the formula in the dimensioning of an open loop geothermal plant

Territorial Analysis for the Implementation of Geothermal Heat Pumps in the Province of Cuneo (NW Italy)

The efficiency of Geothermal Heat Pumps (GHPs) strongly depends on the site-specific parameters of the ground, which should therefore be mapped for the rational planning of shallow geothermal installations. In this paper, a case study is presented for the potentiality assessment of low enthalpy geothermal energy in the Province of Cuneo, a district of 6900 km2 in Piedmont, NW Italy. The available information on the geology, stratigraphy, hydrogeology, climate etc. were processed and mapped, and conclusions were drawn on the geothermal suitability and productivity of different areas of the territory surveyed

Microscopic zero valent silver for dye removal in wastewater

In the present study the removal efficiency towards dyes of microscopic metal silver, produced with a sodium borohydride reduction, was tested. In absence of stabilizers the synthesis proved to be fast, with completion in less than 5 minutes and yield of 99.7 %. The product has micrometric dimensions, while when citrate is added in the reactor particle size is reduced to 10-100 nm and stability results highly increased. The removal efficiency was tested on two dyes, Methylene Blue (MB) and Bromophenol Blue (BPB) for the micrometric particles only. Both dyes are removed in less than 1 hour, with 94 % efficiency on MB and 90 % of BPB. The promising performances shown by our material, which are very good removal efficiency and the evidence of removing the dye by degradation rather than its adsorption, suggest the possibility to overcome the health risk posed by nanometric silver particles (AgNPs), obtaining a material which presents both catalytic and antibacterial properties plus an easier removal from the treated effluent

G.POT: A quantitative method for the assessment and mapping of the shallow geothermal potential

GSHPs (Ground source heat pumps) exchange heat with the ground to provide sustainable heating or cooling. Their technological feasibility and economic viability depend on the site-specific thermal properties of the ground and on the usage profile of the plant. These parameters influence the shallow geothermal potential, which is defined as the thermal power that can be efficiently exchanged by a BHE (Borehole Heat Exchanger) of a certain depth. We present a general method (G.POT) for the determination of shallow geothermal potentials. This method was derived using a comprehensive set of analytical heat transfer simulations, performed by varying (i) the thermal properties of the ground, which comprise its thermal conductivity and capacity, (ii) the thermal properties of the borehole, and (iii) the operational and design parameters of the plant, namely, the BHE length, the threshold temperature of the heat carrier fluid, the duration of the heating/cooling season and the simulated lifetime. Therefore, the G.POT method is a simple and flexible tool that can be implemented in a wide range of different scenarios for large-scale mapping of geothermal potentials. We also assess G.POT by discussing its application to map the geothermal yield in the Province of Cuneo (Piemonte, NW Italy)

A Normalized and Extended Correlation Equation for Predicting Single-Collector Efficiency in Physicochemical Filtration in Saturated Porous Media

The colloidal transport and deposition are phenomena involved in different engineering problems. In the environmental engineering field the use of micro- and nano-scale zerovalent iron (M-NZVI) is one of the most promising technologies for groundwater remediation. Colloid deposition is normally studied from a micro scale point of view and the results are then implemented in macro scale models that are used to design field-scale applications. The single collector efficiency concept predicts particles deposition onto a single grain of a complex porous medium in terms of probability that an approaching particle would be retained on the solid grain. Different approaches and models are available in literature to predict it, but most of them fail in some particular conditions (e.g. low fluid velocity and/or very small or very big particle dimension) because they predict efficiency values exceeding unity. By analysing particle fluxes and deposition mechanisms and performing a mass balance on the entire domain, the traditional definition of efficiency was reformulated and a novel total flux normalized correlation equation is proposed for predicting single-collector efficiency under a broad range of parameters. The new equation has been formulated starting from a combination of Eulerian and Lagrangian numerical COMSOL Multiphysics® simulations, performed under Smoluchowski-Levich conditions in a geometry which consists of a sphere enveloped by a cylindrical control volume (Figure 1). The normalization of the deposited flux is performed accounting for all of the particles entering into the control volume through all transport mechanisms (not just the upstream convective flux as conventionally done) to provide efficiency values lower than one under any possible combination of transport mechanisms. The particle fluxes onto the collector and through the control volume have been described mathematically as a summation of terms. In order to guarantee the independence of each term, the correlation equation is derived through a rigorous hierarchical parameter estimation process, accounting for single and mutual interacting transport mechanisms. The new correlation equation provides efficiency values lower than one over a wide range of parameters (Figure 2) and it is valid both for point and finite-size particles. Moreover the correlation equation is extended to include porosity dependence and reduced forms are also proposed by elimination of the less relevant terms without losing the main features of the full equation

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

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