PERMEABILITY VERSUS SURFACE REACTIVITY IN DETERMINING THE LONGEVITY OF GRANULAR IRON REACTIVE BARRIERS

Granular iron permeable reactive barriers (PRBs) are an increasingly important remediation technology for groundwater pollutants. However, there remains concern over the life span of these barriers. The longevity of iron barriers is expected to be determined by either loss of permeability or by loss of reactivity of iron. As the iron reacts with contaminants, precipitates will form and over time this can cause the hydraulic conductivity to decrease and the PRB to clog. In addition to decreasing hydraulic conductivity, the precipitates can decrease the surface reactivity of the iron by covering reactive sites with less reactive solid substances. The chief objectives of this study are to determine the relative and combined importance of changes to surface reactivity and hydraulic conductivity in a granular iron medium by conducting experiments under constant head and gravity driven flow conditions.

A Plexiglas block with multiple flow-through channels connected by an internal manifold was constructed to permit the simultaneous flow of solution through a granular iron medium and non-reactive media. In addition, a viewing channel, used to observe the pore spaces of the iron grains, was machined into the block and covered with a thin mylar film affixed to the block with silicon adhesive. Initially, 8mM aerobic perchlorate solution was passed through the apparatus. As precipitates formed in the iron filled channel and the trench due to corrosion, hydraulic conductivity was lost and the relative flow through the non-reactive channels increased. The onset of this flow diversion provides information about the sensitivity of flow to precipitate formation in PRBs.

In addition to monitoring flow for the effects of precipitate formation, the disappearance of a probe compound, 4-chloronitrobenzene, was monitored by comparing influent and effluent concentrations. These experiments were underway at the time of writing, and are expected to establish whether permeability or reactivity are first to cause the granular iron channel to fail. Later experiments will address the same issue in geochemically different waters, including carbonate rich solution, sulfate solution, chloride solution and nitrate solution.