Mineral precipitation and porosity losses in granular iron columns

As permeable reactive barriers containing zero-valent iron are becoming more widely used to remediate contaminated groundwaters, there remains much uncertainty in predicting their long-term performance. This study focuses on two factors affecting performance and lifetime of the granular iron media: plugging at the treatment zone entrance and precipitation in the bulk iron media. Plugging at the system entrance is due principally to mineral precipitation promoted by dissolved oxygen in the influent groundwater and is an issue in aerobic aquifers or in above-ground canister tests. Designs to minimize plugging in field applications where the groundwater is oxygenated include the use of larger iron particles and admixing sand of comparable size with the iron particles. Beyond the entrance zone, the groundwater in anaerobic and mineral precipitation leads to porosity losses in the bulk iron media, potentially reducing flow through the treatment zone. The nature of the mineral precipitation and the factors that affect extent of mineral precipitation have been examined by a variety of tools, including tracer tests, aqueous inorganic profiles, and surface analytical techniques. At short treatment times, porosity losses as measured by tracer tests are due mainly to Fe(OH) 2 precipitates and possible entrapment of a film of hydrogen gas on the iron surfaces. Over longer treatment times, precipitation of Fe(OH) 2 and FeCO 3 in low carbonate waters and of Fe(OH) 2, FeCO 3 and CaCO 3 in higher carbonate waters begin to dominate porosity losses. The control of pH within the iron media by addition of ferrous sulfide was shown not to reduce significantly calcium and carbonate precipitates, indicating that mineral precipitation is controlled by more than simple carbonate equilibrium considerations.