Long-term stability of dithionite in alkaline anaerobic aqueous solution

Abstract Closed-system experiments were conducted to investigate the decomposition of sodium dithionite in aqueous solutions under varying pH and starting concentrations to simulate the deployment of dithionite as an in-situ redox barrier. Co-determination of dithionite and its degradation products was conducted using UV–Vis spectrometry, iodometric titration, and ion chromatography. In unbuffered solutions, dithionite reacted rapidly, whereas in near-neutral solutions (pH ∼7), it persisted for ∼ 50 days and in alkaline solution (pH ∼9.5) for >100 days. These are the longest lifetimes reported to date, which we attribute to not only excluding oxygen but also preventing outgassing of H 2 S. Thoroughly constraining the reaction products has led to the following hypothesized reaction: 4 S 2 O 4 2−  + H 2 O → HS −  + SO 3 2− +2 SO 4 2−  + S 4 O 6 2−  + H + which represents relatively rapid degradation at near-neutral pH values. At the more alkaline pH, and over longer time scales, the reaction is best represented by: 3 S 2 O 4 2− + 3 H 2 O → 2HS -  + SO 3 2− +3 SO 4 2− + 4 H + the following kinetic rate law was developed for the pH range studied: dC i dt = S i 10 − 4.81 { H + } 0.24 { S 2 O 4 2 - } , where dC i dt is the rate of change of the i t h chemical component in the simplified equation (mole L −1 s −1 ) and S i is the stoichiometric coefficient of the ith chemical. The kinetic rate law was used to calculate a pseudo first order half-life of 10.7 days for near-neutral pH and 33.6 days for alkaline pH. This work implies that if hydrogen sulfide is contained within the system, such as in the case of a confined aquifer below the water table, dithionite decomposes more slowly in alkaline aqueous solution than previously thought, and thus it may be more cost-effectively distributed in aquifers than has been previously assumed.