Visualizations and Optimization of Iron–Sand Mixtures for Permeable Reactive Barriers

Abstract

AbstractDiluting granular iron with sand is a common practice performed to minimize clogging and to reduce the cost of permeable reactive barrier (PRB) installations. This study used a pore‐scale image analysis technique and a reanalysis of previously published data to test the hypothesis of Bi et al. (2009) that the mixing of 15% by weight sand with a commercial, platy‐grained iron medium opens the pore space between grains, exposes more reactive grain surface to flowing water, and leads to a more reactive medium (i.e., promoting faster transformation rates per unit volume of medium) than 100% by weight granular iron. Four mixing ratios (100, 85, 75 and 50% iron by weight) were compared on the basis of two morphological parameters measured in section: (1) total grain area, which correlates with the total amount of iron present, and (2) grain perimeter, which is governed by both the mobile solution‐available surface and the total amount of iron present. As expected, grain areas exposed in section were highest for 100% iron packings and decreased with increasing sand content. The estimated iron grain effective perimeters (i.e., accessible to mobile water) for 85% iron‐by weight mixtures were similar to 100% iron and decreased in 75 and 50% iron mixtures. The section confirmed that the presence of 15% sand by weight opened up the pore structure, likely improving the mobile‐water to iron contact. The analysis of kinetic column experimental data indicated that the same trend was present in the sorption capacity term in the Kinetic Iron Model (KIM) equation, providing corroborating evidence that the iron surface availability was higher in the 85% iron medium than the 100% iron medium on a per gram of iron basis.