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Biological Benefits of Insitu Zero-Valent Iron Technology

Treatment of chlorinated volatile organic compounds and metals in the subsurface by Zero-Valent Iron (ZVI) has been proven and widely accepted as an effective insitu remediation technology. Recent studies have found that there is potentially a biological component to the overall degradation of the compounds.

Focusing on the conventional permeable reactive barriers (PRBs) with ZVI, experiments conducted at the University of Iowa[1] has shown that a variety of anaerobic bacteria in the presence of the elemental iron can increase the rate and extent of transformation of certain common contaminants.

The oxidation of ZVI releases electrons that in turn reduce the susceptible pollutants (such as TCE, PCE, DCE, VC, Pb, Cr and PCB’s). Some of the electrons also reduce water-derived protons to generate hydrogen gas, which is an excellent energy source for a wide variety of anaerobic bacteria. Consumption of the hydrogen gas by the microbes enhances the oxidation of iron and the production of more hydrogen as well as promoting the growth of the bacteria that can potentially degrade the contaminants. The removal of the hydrogen gas layer from the surface of the iron and the formation of hydrogen bubbles also serves to maintain the reactivity of the iron and enhances the permeability of the treatment zone.

The research team further investigated the application of bio-augmentation in conjunction with the ZVI technology and found that these two technologies can have a symbiotic effect on each other.

The findings of the research can play an important role in regulatory agencies’ perception and acceptance of ARS’s ZVI technology (Feroxsm) as an overall site closure strategy. In dealing with the permitting of ZVI injection, the New Jersey Department of Environmental Protection (NJDEP) has proposed an accelerated and streamlined process by classifying the ZVI as an enhancement of biodegradation for permitting purposes.

Recent data from both the laboratory and the field coupled with the regulatory perception of the technology seem to favor the use of ZVI in conjunction with a natural attenuation approach as an effective closure strategy at sites with large plume areas.

On several full-scale ZVI systems, ARS has incorporated biological parameters into the monitoring programs to further examine these phenomena. Parameters such as nitrate/nitrite, sulfate/sulfide,total/dissolved metals DO, pH and ORP are valuable indicators of the geochemical conditions of the subsurface that strongly influence the microbial activities in the presence of ZVI.

Reference
1. Parkin, G.F., Alvarez, P.J., Scherer M.M. and J.L. Schnoor. 2000. Role of Microbes in Remediation with Feo Reactive Barriers. Ground Water Currents USEPA 542-N-00-002 Issue No. 35. pp. 2-3.

 

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