Degradation of TCE, Cr(VI), sulfate, and nitrate mixtures by granular iron in flow-through columns under different microbial conditions

Flow-through aquifer columns packed with a middle layer of granular iron (Fe⁰) were used to study the applicability and limitations of bio-enhanced Fe⁰ barriers for the treatment of contaminant mixtures in groundwater. Concentration profiles along the columns showed extensive degradation of hexavalent chromium Cr(VI), nitrate, sulfate, and trichloroethene (TCE), mainly in the Fe⁰ layer. One column was bioaugmented with Shewanella algae BRY, an iron-reducing bacterium that could enhance Fe⁰ reactivity by reductive dissolution of passivating iron oxides. This strain did not enhance Cr(VI), which was rapidly reduced by iron, leaving little room for improvement by microbial participation. Nevertheless, BRY-enhanced nitrate removal (from 15% to 80%), partly because this strain has a wide range of electron acceptors, including nitrate. Sulfate was removed (55%) only in a column that was bioaugmented with a mixed culture containing sulfate-reducing bacteria. Apparently, these bacteria used H₂ (produced by Fe⁰ corrosion) as electron donor to respire sulfate. Most of the TCE was degraded in the zone containing Fe⁰ (50-70%), and bioaugmentation with BRY slightly increased the removal efficiency to about 80%. Microbial colonization of the Fe⁰ surface was confirmed by scanning electron microscopy.