ZERO VALENT IRON
Chlorinated organic compounds, such as trichloroethene (TCE; C2HCl3) are in an oxidized state because of the presence of chlorine. Zero valent iron (Fe0), a strong reducing agent, reacts with the chlorinated organic compounds through electron transfers:
3Fe0 → 3Fe2+ + 6e–
C2HCl3 + 3H+ + 6e– → C2H4 +3Cl–
net: 3Fe0 + C2HCl3 + 3H+ → C2H4 + 3Fe2+ +3Cl–
When measurable, chloride mass balances close to 100% are typically obtained in column experiments with granular iron and contaminated groundwater. In the case of TCE and its breakdown products, dechlorination is complete with ethene and ethane as the final carbon-containing compounds. Ethene/ethane mass balance of 80% and higher have been reported from closed-system tests with chlorinated ethenes and ethanes.
The figure to the right shows two competing pathways for dechlorination of chlorinated ethenes in iron systems; β-elimination and hydrogenolysis. The β-elimination pathway dominates the reaction and produces chloroacetylene intermediates, which are unstable and rapidly reduced to ethene. Typically about 90% of the TCE is degraded through the β-elimination pathway (as opposed to biological degradation where hydrogenolysis is the dominant pathway).
The use of ZVI for groundwater remediation is well established with applications at over 150 sites around the world since the early 1990’s.