In Situ Redox Manipulation by Dithionite Injection: Intermediate‐Scale Laboratory Experiments

Abstract

Abstract The goal of in situ redox manipulation (ISRM) is to create a permeable treatment zone capable of removing redox‐sensitive contaminants from ground water. The objective of this study was to evaluate the effectiveness of one promising ISRM technology: chemical reduction of aquifer sediments by sodium dithionite (Na 2 S 2 O 4 ) injection. The technology was evaluated in intermediate‐scale laboratory experiments designed to investigate the kinetics of Fe(III)‐reduction and dithionite‐disproportionation reactions in a radial flow field over similar transport distances (∼ 7 m) and time scales (∼ 72 hours) as those used in a field trial for remediation of chromate contaminated ground water at the Department of Energy Hanford site in Washington state. Four hundred liters (∼ 1 pore volume) of 0.1 M Na 2 S 2 O 4 in a 0.4 M K 2 CO 3 /0.04 M KHCO 3 buffer were injected at a rate scaled to field values. Dithionite breakthrough curves at sampling ports were approximately described by the advection‐dispersion equation with a two‐part reaction model containing first‐order rate coefficients for dithionite reaction with sediment Fe(III) (k 1 = 0.13 hr −1 ) and dithionite disproportionation (k 2 = 0.05 hr −1 ). Analyses on sediment cores collected from the physical model indicated that substantial Fe(III) was reduced to Fe(II) and that the dithionite‐treated sediment was capable of removing 2 mg/L chromate from ∼ 100 column pore volumes of synthetic ground water. These results indicate that the ISRM technology is a potentially feasible method for removing chromate from Hanford ground water.