Acid/base front propagation in saturated porous media: 2D laboratory experiments and modeling

Abstract

We perform laboratory scale reactive transport experiments involving acid-basic reactions between nitric acid and sodium hydroxide. A two-dimensional experimental setup is designed to provide continuous on-line measurements of physico-chemical parameters such as pH, redox potential (Eh) and electrical conductivity (EC) inside the system under saturated flow through conditions. The electrodes provide reliable values of pH and EC, while sharp fronts associated with redox potential dynamics could not be captured. Care should be taken to properly incorporate within a numerical model the mixing processes occurring inside the electrodes. The available observations are modeled through a numerical code based on the advection-dispersion equation. In this framework, EC is considered as a variable behaving as a conservative tracer and pH and Eh require solving the advection dispersion equation only once. The agreement between the computed and measured pH and EC is good even without recurring to parameters calibration on the basis of the experiments. Our findings suggest that the classical advection-dispersion equation can be used to interpret these kinds of experiments if mixing inside the electrodes is adequately considered.

Mixing and Reactive Fronts in the Subsurface

“…Multidimensional laboratory flow-through experiments are instrumental for the investigation of mixing and mixing-controlled reactions in porous media. Different reactive systems have been investigated to study the interactions between mixing and reaction dynamics in porous media, including abiotic reactions between dissolved reactants (e.g., Gramling et al 2004;Katz et al 2010;Rolle et al 2009), transport of pH fronts and electrostatic interactions (e.g., Loyaux-Lawniczak et al 2012;Muniruzzaman et al 2014;Muniruzzaman and Rolle 2015), precipitation/dissolution reactions (e.g., Tartakovsky et al 2008;Poonoosami et al 2015;Haberer et al 2015;Battistel et al 2019), and microbially-mediated reactions (e.g., Thullner al. 2002;Bauer et al 2009a;Song et al 2014).…”

On the importance of diffusion and compound-specific mixing for groundwater transport: An investigation from pore to field scale

“…Intermediate-scale flow-through chambers are increasingly used to study key processes affecting solute transport such as the impact of heterogeneity on conservative transport (e.g., Barth et al, 2001;Levy and Berkowitz, 2003;McNeil et al, 2006;Silliman et al, 1998), mixing in the longitudinal (e.g., Gramling et al, 2002) and transverse direction (e.g., Haberer et al, 2011Haberer et al, , 2012Rolle et al, 2009); abiotic reactions (e.g., Katz et al, 2011;Loyaux-Lawniczak et al, 2012) and biodegradation (e.g., Bauer et al, 2009aBauer et al, , 2009bHuang et al, 2003;Rolle et al, 2010;Thullner et al, 2002). In this work we refer to the study presented by Bauer et al (2009b) and, in particular, to the data collected during an initial phase of conservative transport and during a selected phase of aerobic bioreactive transport.…”

Measurements and models of reactive transport in geological media

“…In a different set of experiments, Loyaux‐Lawniczak et al [] employed a horizontal flow cell with external dimensions of 167 cm (length) × 68 cm (width) × 4 cm (thickness), packed with glass beads (diameters of 0.5–0.75 mm). Two reactant solutions with different pH and Eh (redox potential) values—containing nitric acid (HNO 3 ) and sodium hydroxide (NaOH) solutions colored in red and blue, respectively, by inert food coloring agents—were introduced into the cell along the inlet face, in separate, parallel streams.…”

“…Reactive transport experiments laboratory flow cells, with reactant solutions flowing in parallel, are presented in Tartakovsky et al [2008] and Katz et al [2011] (recall that similar inlet conditions were employed in a bimolecular micromodel system [Willingham et al, 2008] and a pH-controlled system [Loyaux-Lawniczak et al, 2012], discussed in sections 4.1 and 4.2, respectively). In both experiments, solutions of Na 2 CO 3 and CaCl 2 were injected in parallel into two halves of a flow cell, yielding formation of a narrow calcium carbonate (CaCO 3 ) precipitate layer in the mixing zone between the two injected solutions.…”

“…With minimal calibration, agreement between the computed and measured pH and EC compared favorably, although early and late time tailing behavior of the measured BTCs, in particular, was not well matched. Loyaux‐Lawniczak et al [] concluded that such controlled experiments can provide effective tools for model verification.…”