Degradation of chlorinated organics from water by membrane-based nanosized metallic systems and by hydroxyl radical reaction

Groundwater contamination with chlorinated organics is quite widespread in various locations. We have successfully evaluated highly effective methods for the destruction of toxic, chlorinated organics through comprehensive mechanistic probing of both oxidative (free-radical reaction pathways) and reductive (zero-valent nanoscale metals) dechlorination systems. For oxidative pathway Fe(II), a chelate (citric acid or gluconic acid), and hydrogen peroxide are needed for free radical production. Highly effective dechlorination was obtained with TCE (trichloroethylene), TCP (trichlorophenol), and selected PCBs. Both H2O2 and gluconic acid were generated by reacting glucose in membrane pore nanodomain by immobilizing Glucose oxidase enzyme in layer-by-layer assembled poly-electrolytes. Work involving reductive dechlorination involved the use of bimetallic (Fe/Ni, and Fe/Pd) nanoparticle systems, both membrane-supported and direct aqueous-phase symthesis. The significant findings are: (1) direct synthesis of bimetallic nanoparticles with controlled diameters < 40 nm using membrane-based supports derived from polyligand functionalization and ion exchange, and phase inversion synthesis, (2) demonstrated complete (with product and intermediates analysis) dechlorination of trichloroethylene (TCE) and selected PCBs by nanosized metals. High catalytic activity of Pd was confirmed by the low activation energy (experimentally evaluated) compared with other catalytic systems. Our lab group has quantified the hydrogen generation from the iron corrosion reaction. For both bimetallic systems, hydrogen generation by iron oxidation depends strongly on the surface coverage of the second metal. Based on these findings, it is likely that the primary step of the reaction mechanism associated with bimetallic dechlorination involves the generation of reactive hydrogen (H) by the primary metal (Fe). Active hydrogen then reacts with the chlorinated organic on the surface of the second-metal, which is typically a hydrogenation-promoting catalyst such as Pd or Ni. This research is funded by the NIEHS-SBRP program and by KRCEE-DOE.