RESUMO
Transportation process of nano scale zero valent iron (nZVI) in clay-rich soils is complicated and crucial for in-situ remediation of contaminated sites. A coupled computational fluid dynamic and discrete element method (CFD-DEM) was used to investigate the interplays of repulsive and attractive forces and the injection velocity of this process. The screened Coulomb's law was used to represent the electrostatic interaction, and surface energy density was introduced to represent the effects of the van der Waals interaction. A phase diagram was constructed to describe the interplay between injection velocity and repulsive force (in terms of charge of colloids). Under the boundary and initial conditions in this study, clogging formed at low repulsive force (colloidal charge = -1 ×10-15 C), where increment of injection velocity (from 0.002 m/s to 0.02 m/s) cannot prevent clogging, as in the case of bare nZVI transportation with limited mobility; On the other hand, excessive repulsive force (charge = -4 ×10-14 C) is detrimental to nZVI-clay transportation due to repulsion from the concentrated colloids in pore throats, a phenomenon as in the overuse of stabilizers and was defined as the "membrane repulsion effect" in this study. At moderate charge (-1 ×10-14 C), injection velocity increment induced clogging due to aggregates formed at the windward of cylinder and accumulated at the pore throats.
RESUMO
The pick-up, migration, deposition, and clogging behaviors of fine particles are ubiquitous in many engineering applications, including contaminant remediation. Deposition and clogging are detrimental to the efficiency of environmental remediation, and their mechanisms are yet to be elucidated. Two-dimensional microfluidic models were developed to simulate the pore structure of porous media with unified particle sizes in this study. Kaolin and bentonite suspensions were introduced to microfluidic chips to observe their particle deposition and clogging behaviors. Interactions between interparticle forces and particle velocity profiles were investigated via computational fluid dynamics and discrete element method simulations. The results showed that (1) only the velocity vector toward the micropillars and drag forces in the reverse direction were prone to deposition; (2) due to the negligible weight of particles, the Stokes number implied that inertia was not the controlling factor causing deposition; and (3) the salinity of the carrying fluid increased the bentonite deposition because of the shrinkage of the diffused electrical double layer and an increase in aggregation force, whereas it had little effect on kaolin deposition.
RESUMO
Owing to its high surface area and high surface charge density, clay, either contaminated with heavy metal ions or modified with organic additives as barrier materials, is difficult to assess and monitor. Complex dielectric permittivity (κ*) showed promising potential in tackling the above issues. In this study, the complex dielectric permittivity (κ*) of clays modified with a surfactant, four polymers, and four metal ions was measured at frequencies from 0.2-20 GHz. With the addition of polymer and metal ions, increasing frequency caused a slight decrement in real permittivity but a significant decrement in effective permittivity. A modified linearity polynomial equation, which considered the particle conductivity, was developed to fit the relationship between effective conductivity (σeff) and porosity ranging from 0.7 to 1.0. A three-dimensional Cole-Cole plot (κ'-κeffâ³-w) shows Cole-Cole circle expansion at higher water content. The resonance strength of modified clays was observed to increase with water content, which suggests that the number of water molecules in the diffuse layer of polymer or metal ions saturated clay increased. However, sorbed polymer and metal ions have an insignificant influence on the resonance time τs and stretching exponent 1-α. κ* can provide nondestructive characterization of metal or polymer modified clays.
