First results of operating and monitoring an innovative design of a permeable reactive barrier for the remediation of chromate contaminated groundwater

Assessment of Long-Term Performance and Chromate Reduction Mechanisms in a Field Scale Permeable Reactive Barrier

“…This alteration mainly took place near the inflow of the cylinder and at the depth of level 2 (transition zone). These visual characteristics of the reactive material were in line with the groundwater monitoring at these locations over the time span of four years (12) Investigation of Corrosion Rinds. The zerovalent iron shavings collected from the cylinders were always entirely covered by corrosion coatings, with the metallic iron core never exposed directly to the surface.…”

“…In contrast, precipitate layers rich in poorly crystalline Fe III -hydroxides, as primarily observed in the oxidized zone, are expected to interfere much less with the ZVI corrosion (32). Still, we observed geochemical changes within the cylinders: over four years, the oxygen concentration measured at the inflow of the barrier constantly increased and finally Cr VI was detected within the first few cm of the cylinders (12). This observation indicates that the reduction rates which can be sustained by the corrosion of the ZVI shavings declined over time.…”

“…The microanalytical findings (i.e., Cr-containing precipitates on the gravel surfaces and solids with a Cr/Fe ratio of 1/3 in the corrosion rinds) strongly suggest that, as intended in the design process, homogeneous Cr VI reduction is an important pathway within the zone impacted by ZVI corrosion. Homogeneous reactions may also take place in the mixing zone between the Fe II plume formed in the back parts of the cylinders (see below) and flowpaths initially bypassing the reactive media (12).…”

Characterizing the reactivity of metallic iron in Fe0/EDTA/H2O systems with column experiments

“…Despite a broad consensus on reductive transformations [1] whereas metals, metalloids and radionuclides may be removed via reductive precipitation, surface adsorption or complexation, or co-precipitation with the Fe oxyhydroxides that are generated in the system [2][3][4]. The validity of this concept is progressively questioned [6][7][8][9][10][11][12].…”

Progress in Understanding the Mechanism of CrVI Removal in Fe0-Based Filtration Systems

“…The PRB had an innovative design, consisting of two different components: (1) a single row of cylinders for lower expected Cr VI concentrations; and (2) an offset double row of cylinders for higher expected Cr VI -concentrations. The reactive filling inside the cylinders (d = 1.3 m) was installed from 12 to 23 m below ground surface, and consisted in a mixture of Fe 0 shavings (5-20 mm) and gravel (2-5 mm) in the ratio of 1:3 (by weight); this ratio was selected to ensure an initial permeability of the reactive material approximately three times larger than the surrounding subsoil, and to prevent the rapid clogging of the barrier due to precipitation of secondary phases in pore spaces [59,60]. The double row of cylinders successfully treated the Cr VI contamination at normal groundwater flow velocities (residual Cr VI concentrations < 0.01 mg/L); however, during events of exceptionally high groundwater levels (which result in a substantial mobilization of Cr VI ) the remediation effectiveness was only 96%.…”