ABSTRACT

DNAPL Source Control by Reductive Dechlorination with Iron-based Degradative Solidification/Stabilization

Chlorinated hydrocarbons are one of the main sources of sub-surface contamination in the U.S. and the most serious type of contamination occurs when they are present as dense non-aqueous phase liquids (DNAPL). The presence of contaminant DNAPL results in extended times for remediation, because the DNAPL continuously dissolves and thereby contaminates large volumes of groundwater. Therefore, effective remediation of a contaminated aquifer usually requires removal of DNAPL in order to remove the source of contamination. Several technologies can be applied to treating DNAPL in a source zone, but they tend to be capital intensive and poorly applicable to soils that are not highly permeable. An attractive alternative that could be more economically applied to smaller sites and to those sites with impermeable soils is abiotic reductive dechlorination. Iron-based degradative solidification/stabilization (fe-ds/s) is a treatment process developed with funding initiated by the GCHSRC that combines reductive dechlorination with immobilization. Immobilization is achieved by reactions of Portland cement, which is the primary reagent used by conventional solidification/stabilization. Dechlorination is achieved by a compound formed by reaction of ferrous iron with components of Portland cement. Current research indicates that it will be possible to produce this dechlorinating agent directly using specific chemicals so that Portland cement will not be required. Research has shown that fe-ds/s can effectively dechlorinate compounds such as PCE when they are present in aqueous solution. However, the process has not been investigated as a technology for treating such compounds when present as a DNAPL in source zones. Therefore, the overall goal of the proposed research is to demonstrate the ability of modified fe-ds/s to remove chlorinated solvents present as DNAPL in source zones and to determine operational variables that will optimize the process. This goal will be achieved by a two-part experimental plan. The first task will determine optimal conditions for producing active reductants for DNAPL dechlorination. The effects on reductive activity of Fe(II)/Fe(III), OH/Fe(III), Cl/Fe(III), Al(III)/Fe(III), SO₄/Fe(III) and reaction time will be investigated in batch reactors. Activity of the dechlorinating agent will be quantified in batch reactors as the amount of PCE removed after a specific reaction time. More extensive kinetic experiments will be conducted to measure rate constants for dechlorinating agents with highest activity. The second research task will determine the effectiveness of fe-ds/s in remediating chlorinated organics present as DNAPLs. Solid-phase experiments will be conducted with contaminated soils. Experimental variables to be investigated are: soil type (loamy sand, loam, silty clay), target chlorinated organic type (PCE, TCE, TCA), target organic concentration (3,000, 10,000 mg/kg), reductant type (Fe(II), preformed reductant), and reductant dose (3, 10, 30, 100 times stoichiometric). Portland cement doses will be chosen to achieve optimal porewater pH. Samples will be taken over time and target compounds and products will be analyzed by electron-capture gas chromatography after solvent extraction. Kinetic coefficients will be determined by non-linear regression using an appropriate rate model (first-order, modified Langmuir-Hinshelwood). Partitioning experiments will be conducted so that kinetic coefficients can be reported as being independent of the sorption capacity of the soil. Products of dechlorination reactions will be measured to document effective destruction. The fe-ds/s process should have costs that are similar to that of conventional s/s, which range from $60 to $290/ton. They would be on the order of $10/ton higher due to costs of the reductant.