Transport and retention of microparticles in packed sand columns at low and intermediate ionic strengths: experiments and mathematical modeling

Functional relationships correlating particle filtration coefficients and porewater ionic strength are herein proposed and validated, based on deposition experiments of micrometer-sized particles onto siliceous sand. Experiments were conducted using one-dimensional laboratory columns and stable monodisperse aqueous suspensions of negatively charged latex particles with a mean size of 1.90 lm. The role of ionic strength was systematically investigated and six different monovalent salt concentrations (1, 3, 10, 30, 100, 300 mM) were employed by addition of sodium chloride to the aqueous solution. A mathematical advection-dispersiondeposition transport model was adopted assuming that attachment and detachment of particles in the porous medium are concurrent mechanisms of particle filtration, and including a Langmuir-type blocking function to account for availability in deposition sites. The system of equations modeling colloid transport was solved numerically. Attachment rate and detachment rate coefficients were thereby determined for each employed ionic strength, as well as a blocking coefficient in the form of a maximum particle concentration in the solid phase. Therefore, functional relationships expressing the dependence of these coefficients on ionic strength were proposed, based on literature findings and present experimental observations. The existence of a critical salt deposition concentration (and release concentration) separating a favorable attachment (and detachment) regime from an unfavorable condition is assumed. In respect to the blocking coefficient, a power-law dependence on ionic strength is hypothesized. The proposed functional relationships proved adequate to reproduce the coefficient trends extrapolated from data fitting by the transport model. They may represent a powerful tool to describe and predict microparticle mobility in saturated porous media if embedded a priori in the related mathematical transport model

Mechanistic, mechanistic-based empirical, and continuum-based concepts and models for the transport of polyelectrolyte-modified nanoscale zerovalent iron (NZVI) in saturated porous media

Controlled emplacement of polyelectrolyte-modified NZVI at a high particle concentration (1–10 g/L) is needed for effective in situ subsurface remediation. For this reason, a modeling tool capable of predicting polyelectrolyte-modified NZVI transport is imperative. However, the deep bed filtration theory is invalid for this purpose because several phenomena governing the transport of polyelectrolyte-modified NZVI in saturated porous media, including detachment, particle agglomeration, straining, and porous media ripening, violate the fundamental assumption of such a classical theory. Thus, this chapter critically reviews the literature of each phenomenon with various kinds of nanoparticles with a special focus on polyelectrolyte-modified NZVI. Then, each phenomenon is elaborated using three kinds of mathematical models, including mechanistic (such as extended DLVO theory), mechanistic-based empirical (correlations to predict NZVI agglomeration and deposition), and continuum-based (Eulerian continuum-based models). These proposed modeling tools can be applied at various scales from column experiments (1-D) to field-scaled operations (3-D) for designing NZVI injection and emplacement in the subsurface