Longevity of EHC ® Treatment

EHC ® is an integrated combination of controlled-release plant-derived carbon plus micro-scale zero valent iron (ZVI) particles used for stimulating in situ chemical reduction of otherwise persistent organic compounds in groundwater. EHC-M ® uses the same combination plus a source of sulphate (if needed) to promote the precipitation of dissolved metals. In evaluating the longevity of EHC, several consumption mechanisms of the reactive components need to be considered:-ZVI oxidation and carbon fermentation due to consumption (reduction) of terminal electron acceptors (TEAs), such as dissolved oxygen (DO), nitrate and sulfate;-Anaerobic corrosion of ZVI;-Organic contaminant reduction; and-Trace metal reduction and precipitation. These processes are not independent of one another and are also controlled by site conditions such as groundwater flow velocity, inorganic aqueous concentrations, VOC concentrations and temperature. Although it is somewhat difficult to separate the longevity of the iron from that of the carbon given that they are present together, theoretical calculations supported by results from long-term laboratory studies and mature field sites can be used to estimate the effective lifetime of EHC. To evaluate potential rates of ZVI consumption in EHC, the following theoretical calculations due to reactions with VOCs and inorganic electron acceptors are presented for a hypothetical site, but similar calculations can be performed using site-specific data. The hypothetical site used in the calculations contains 2 mg/L DO, 5 mg/L of trichloroethene (TCE), 5 mg/L nitrate and 20 mg/L sulphate. The groundwater flow velocity at the site is assumed to be 0.10 m/day and aquifer porosity 0.25. An assumed EHC-amended zone is 6 m wide was amended with 0.5% EHC (0.2%ZVI) per soil mass. The ZVI consumption calculations are based on a unit cell with a face surface area of 1 m 2 of the EHC-amended zone or a volume of 6 m 3 of EHC-amended aquifer, containing about 22 kg of ZVI. A worse case scenario was assumed; that is the carbon electron donor in EHC was not included in the electron acceptor consumption calculations. Water corrosion (oxidation) of granular iron is an important ZVI consumption process, resulting in ferrous iron, hydrogen gas and hydroxide ion production and an increase in pH and decline in Eh: Fe 0 + 2H 2 O → Fe 2+ + H 2(aq) + 2OH-(1)