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 H2S. Thoroughly constraining the reaction products has led to the following hypothesized reaction:4 S2O42− + H2O → HS− + SO32−+2 SO42− + S4O62− + 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 S2O42− + 3 H2O → 2HS- + SO32−+3 SO42−+ 4 H+the following kinetic rate law was developed for the pH range studied:dCidt=Si10−4.81{H+}0.24{S2O42-},where dCidt is the rate of change of the ith chemical component in the simplified equation (mole L−1 s−1) and Si 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.