Impacts of Fe(0) grain sizes and grain size distributions in permeable reactive barriers

Permeable reactive barriers consist of heterogeneous Fe(0) shavings that are provided as fixed bed to remove a number of different pollutants from contaminated aquifers. Chemical properties, such as reactivity, and physical parameters, such as porosity or flow velocity, influence the contaminant removal and, in the long term, the susceptibility to clogging and passivation. The grain size and the grain size distribution are important characteristics that can be expected to influence chemical and physical processes. In the present work a typical granular cast iron material has been separated into grain size fractions that have been characterized with regard to physical and, in column experiments, chemical properties. Determined bulk densities and porosities revealed that increased pore volumes and thus increased residence times can be achieved by fractionation. The pore structures of the original grain size composition and of two grain size fractions (0.4–0.5 mm and 0.71–0.8 mm) have been visualized. Column tests with 100 g reactive Fe(0) of different grain size fractions (0–0.4 mm, 0.4–0.71 mm, 0.71–1.0 mm, 1.0–2.0 mm and original grain size distribution) were conducted over a period of 160 d to compare the reactivity with regard to reduction of trichloroethylene, hydrogen gas generation and precipitation of carbonate and calcium. Fractionation of the heterogeneous grain size distribution can be expected to improve the long term efficiency of a permeable reactive barrier. Finally a simple but promising modification of Fe(0) permeable reactive barriers is discussed.

► Large grain size range of typical granular cast iron for Fe0 fixed bed filters.
► Significantly varying porosities of grain size fractions.
► Visualized pore geometry varies with no significant anisotropy.
► Highest reactivity for column test with finest grain size fraction.
► Grain size fractionation for clogging reduction in the influent section.