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

摘要

Iron-based degradative solidification/stabilization (Fe(II)-DS/S) is a treatmentmethod that could be economically applied to smaller DNAPL-contaminated sites and tothose sites with impermeable soils. Reductive dechlorination is achieved by compoundsthat are formed by reaction of ferrous iron with components of Portland cement or withdefined chemicals (FeCl3 + Ca(OH)2). These dechlorinating agents can effectivelydegrade chlorinated hydrocarbons (PCE, TCE, and 1,1,1-TCA) that are dissolved inaqueous solution. This research investigated the application of Fe(II)-DS/S to removechlorinated hydrocarbons that are present as DNAPLs in source zones and to comparedthe reactivity of ferrous iron in different mixtures, including the conventional mixturewith cement (Fe(II)+C) and an iron-solid mixture (ISM) that was synthesized withoutthe addition of cement.The modified first-order model, which the rate was proportional to theconcentration of target in the aqueous phase and it was also nearly constant whenDNAPL was present, was developed to describe dechlorination kinetics. The modifiedsecond-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 capacityof the solid reductant. The modified first-order model was used to describe degradationof target compounds with ISM, and the modified second-order model was used todescribe removals for TCE and 1,1,1-TCA with Fe(II)+C. Results of experiments onPCE dechlorination with ISM indicated that the increase of Fe(II) in ISM increased rateconstants and decreased the solubility of targets. The half-life was increased withincreasing total PCE concentration. The product analysis implied that degradation ofPCE 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 betterreactivity for chlorinated ethenes (PCE and TCE) than ISM. However, ISM coulddechlorinate 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 reducingchlorinated ethenes than was Fe(II) in ISM. Dechlorination of a DNAPL mixturefollowed the same order of reactivity as with individual DNAPLs with both reductants.