Acidity generation accompanying iron and sulfur transformations during drought simulation of freshwater re-flooded acid sulfate soils

Remediation of acid sulfate soil (ASS) wetlands by re-flooding with freshwater generates alkalinity and leads to the reformation of reduced inorganic sulfur (RIS) and Fe(II) species in surface soil layers. These contemporary RIS/Fe(II) species are vulnerable to drought-induced oxidation. However, the rates and magnitude of acid-generating reactions and competing acid-neutralisation reactions during such oxidation events are unknown. In this study, ten surface soil samples (0–0.2 m depth) were collected from two freshwater re-flooded ASS wetlands and subjected to oxidative incubation for up to 130 days. The objective was to examine the rate and magnitude of acidity generation and compare this to changes in RIS/Fe speciation. During the incubation, soil pH decreased rapidly by ~ 2–3 units, while titratable actual acidity increased, largely as a result of H⁺generation from oxidation of RIS species. RIS species (primarily small pyrite framboids and dispersed sub-micron sized, euhedral pyrite crystals), decreased over time in all soil samples while the reactive pool of Fe(III) minerals (e.g. schwertmannite) increased. Importantly, the highest rates of acidity generation occurred within the first 20 days, suggesting that surface soil layers in these remediated wetlands are prone to rapid acidification during future droughts. Variations in the magnitude of soil acidification largely reflect differences in both initial RIS content and acid neutralisation capacity (ANC). Spatial variations were evident, with sites located at lower elevations generally containing higher initial RIS and generating more acidity in a shorter time period. However, the magnitude of acidity generation was consistently less than that predicted by theoretical calculations. These results provide new information that is directly relevant to the future management and mitigation of risks to water quality associated with freshwater re-flooding of ASS wetlands.