Superfund Research Program
Chloro-Organic Degradation by Polymer Membrane Immobilized Iron-Based Particle Systems
Project Leader: Dibakar Bhattacharyya
Grant Number: P42ES007380
Funding Period: 2000-2019
This project has successfully demonstrated 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. Dr. Bhattacharyya and his lab have made highly significant advances in two areas of free-radical -based oxidative destructions: (1) immobilization of iron-chelate for biphenyl destruction using functionalization of either silica particles or polymeric membranes, (2) a new approach to make iron chelate (gluconic acid) and hydrogen peroxide enzymatically for potential, on-site Superfund site applications. Such on-site oxidant generation system would eliminate the need for hazardous material transport and storage. For example, the researchers' laboratory results showed over 80% dechlorination of dichlorobiphenyl and formation of various non-toxic organic acids.
Work involving reductive dechlorination involved the use of bimetallic nanoparticle systems, both membrane-supported and direct aqueous-phase symthesis. Three significant developments are: (1) direct synthesis of bimetallic nanoparticles with controlled diameters < 40 nm using membrane-based supports derived from polyligand functionalization and ion exchange, phase inversion synthesis, or electrodeposition under applied-voltage, (2) established methods to characterize bimetallic nanoparticles in the presence of a polymer support using electron microscopy to quantify the role of the second metal for bimetallic reduction mechanisms of toxic organics, (3) demonstrated complete (with product and intermediates analysis) dechlorination of trichloroethylene (TCE) and selected PCBs by nanosized metals and developed reaction model involving bimetallic (Fe/Pd, Fe/Ni) nanoparticles. High catalytic activity of Pd was confirmed by the low activation energy (experimentally evaluated) compared with other catalytic systems. Bhattacharyya's 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.
For PCB's the best strategy for Superfund sites would involve a reductive step (with Fe/Pd nanoparticles) to form biphenyl followed by oxidative treatment (iron + chelate + hydrogen peroxide) to breakdown the conjugated ring to non-toxic organic acids. In this way the formation of any intermediate chloro-organic acid could be avoided.