Corrosion product and precipitate distribution in two-component Fe(0) permeable reactive barriers

• Secondary minerals preferentially accumulate on corroding surfaces.
• Pore space of porous filter materials only partly filled with precipitates.
• Complete conversion of ferric minerals to magnetite.

In situ Fe(0) permeable reactive barriers are suitable for the passive treatment of various hazardous substances in groundwater. Unavoidable secondary minerals on cast iron granules (Fe(0)) in permeable reactive barriers cause passivation and reduction of pore volume. Precipitate agglomeration on mixing components might reduce the passivating layers on Fe(0). Mixtures of Fe(0) with different conventional, porous or reactive granular materials (sand, gravel, pumice, anthracite, wood, granular ferric hydroxide, calcite) were analyzed with different methods after 200 days exposure to contaminated groundwater in column studies. Gravimetric analyses indicate that the weight increase of the Fe(0) granules was greater than that of all mixing materials except calcite indicating preferential formation of secondary minerals on Fe(0) granules. A greater weight increase compared to Fe(0) was only observed for calcite, which was transformed during the experiment into large and compact aragonite crystals on the granule surfaces. Microscopic analyses of cross-sections showed a corrosion build-up on Fe(0) but only minor precipitate layers on mixing materials. Akaganeite, a non-magnetic ferric hydroxide as major component of the granular ferric hydroxide, was transformed into magnetite during ground water treatment as revealed by X-ray diffraction. Hence iron hydroxides are intermediate phases that further transform to the final product magnetite.