In situ redox manipulation by dithionite injection: Intermediate-scale laboratory experiments

The goal of in situ redox manipulation (ISRM) is to create a permeable trea tment zone capable of removing redox-sensitive contaminants from ground mat er. The objective of this study was to evaluate the effectiveness of one pr omising ISRM technology: chemical reduction of aquifer sediments by sodium dithionite (Na2S2O4) injection. The technology was evaluated in intermediat e-scale laboratory experiments designed to investigate the kinetics of Fe(I II)-reduction and dithionite-disproportionation reactions in a radial flow field over similar transport distances (similar to 7 m) and time scales (si milar to 72 hours) as those used Zn a field trial for remediation of chroma te contaminated ground water at the Department of Energy Hanford site in Wa shington state. Four hundred liters (similar to 1 pore volume) of 0.1 M Na2 S2O4 in a 0.4 M K2CO3/0.04 M KHCO3 buffer were injected at a rate scaled to field values. Dithionite breakthrough curves at sampling ports were approx imately described by the advection-dispersion equation with a two-part reac tion model containing first-order rate coefficients for dithionite reaction with sediment Fe(III) (k(1) = 0.13 hr(-1)) and dithionite disproportionati on (k(2) = 0.05 hr(-1)). Analyses on sediment cores collected from the phys ical model indicated that substantial Fe(III) was reduced to Fe(III) and th at the dithionite-treated sediment was capable of removing 2 mg/L chromate from similar to 100 column pore volumes of synthetic ground water. These re sults indicate that the ISRM technology is a potentially feasible method fo r removing chromate from Hanford ground water.