Transport and removal of a passive tracer in porous media employing surface washing

TL;DR

This study experimentally analyzes how surface washing removes a passive tracer from porous media, revealing a three-stage mass-transport process and providing insights for optimizing washing protocols.

Contribution

It identifies the three-stage removal process and explores how washing parameters affect tracer removal, offering practical guidance for industrial and environmental applications.

Findings

Three-stage mass-transport process identified

Removal rate influenced by washing film and permeability

Accelerated removal occurs when tracer reaches boundary

Abstract

This experimental study investigates the dynamics of surface washing to remove a passive tracer from a porous plate by a gravity-driven liquid film across its surface. A disodium fluorescein tracer is introduced at the surface of a water-saturated porous plate and allowed to diffuse into the plate for a number of hours before a film of water solution flows over its surface to extract and transport the tracer away. The removal rate of the tracer is monitored quantitatively by using fluorescence measurements to determine the concentration in the effluent from the washing process. These measurements are supplemented by dye-attenuation imaging, which provides mainly qualitative insights about the tracer's concentration distribution on the porous plate surface. Our findings reveal a three-stage mass-transport process consisting of an initial period of rapid removal of the tracer found within the surface roughness, followed by a period of slower removal, which appears to be limited by vertical diffusion, and a third stage of accelerated advection-dominated removal when the tracer-rich region that was transported downstream during the second stage reaches the downstream boundary of the porous plate. A parametric study explores the influence of the characteristics of the washing film, the permeability of the porous plate, the amount and initial spatial extent of the tracer on the porous plate and the tracer's diffusive penetration depth on the mass removal rates. Our insights offer practical guidance for optimising surface washing protocols for porous systems in industrial and environmental applications.