DNAPL source control by reductive dechlorination with iron-based degradative solidification/stabilization



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Iron-based degradative solidification/stabilization (Fe(II)-DS/S) is a treatment method that could be economically applied to smaller DNAPL-contaminated sites and to those sites with impermeable soils. Reductive dechlorination is achieved by compounds that are formed by reaction of ferrous iron with components of Portland cement or with defined chemicals (FeCl3 + Ca(OH)2). These dechlorinating agents can effectively degrade chlorinated hydrocarbons (PCE, TCE, and 1,1,1-TCA) that are dissolved in aqueous solution. This research investigated the application of Fe(II)-DS/S to remove chlorinated hydrocarbons that are present as DNAPLs in source zones and to compared the reactivity of ferrous iron in different mixtures, including the conventional mixture with cement (Fe(II)+C) and an iron-solid mixture (ISM) that was synthesized without the addition of cement. The modified first-order model, which the rate was proportional to the concentration of target in the aqueous phase and it was also nearly constant when DNAPL was present, was developed to describe dechlorination kinetics. The modified second-order model assumed that the rate was proportional to the product of the concentration of target in the aqueous phase and the concentration of reductive capacity of the solid reductant. The modified first-order model was used to describe degradation of target compounds with ISM, and the modified second-order model was used to describe removals for TCE and 1,1,1-TCA with Fe(II)+C. Results of experiments on PCE dechlorination with ISM indicated that the increase of Fe(II) in ISM increased rate constants and decreased the solubility of targets. The half-life was increased with increasing total PCE concentration. The product analysis implied that degradation of PCE with ISM was via a combination of the hydrogenolysis and ?-elimination pathways. A comparison of the types of targets and reductants indicated that Fe(II)+C had better reactivity for chlorinated ethenes (PCE and TCE) than ISM. However, ISM could dechlorinate a chlorinated ethane (1,1,1-TCA) as rapidly as Fe(II)+10%C. The ratio of [RC]o/[Fe(II)]o implied that Fe(II) in Fe(II)+C was more involved in reducing chlorinated ethenes than was Fe(II) in ISM. Dechlorination of a DNAPL mixture followed the same order of reactivity as with individual DNAPLs with both reductants.