In situ chemical reduction of aquifer sediments: enhancement of reactive iron phases and TCE dechlorination.
In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.
Szecsody
Jim E
JE
Pacific Northwest National Laboratory, Richland, Washington 99352, USA. jim.szecsody@pnl.gov
Fruchter
Jonathan S
JS
Williams
Mark D
MD
Vermeul
Vince R
VR
Sklarew
Debbie
D
D004227:Dithionite / Q000737:chemistry*; D005290:Ferric Compounds / Q000032:analysis / Q000737:chemistry*; D005296:Ferrous Compounds / Q000032:analysis / Q000737:chemistry*; D005618:Fresh Water; D019015:Geologic Sediments / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D013696:Temperature; D013997:Time Factors; D014241:Trichloroethylene / Q000737:chemistry*
D005290:Ferric Compounds; D005296:Ferrous Compounds; D004227:Dithionite; D014241:Trichloroethylene