Deoxygenation and acidification dynamics in acid sulfate soil landscapes: flood-induced oxidation of sulfidic drain materials

Document Type

Conference publication

Publication details

Fyfe, DM, Bush, RT, Burton, ED & Sullivan, LA 2006, 'Deoxygenation and acidification dynamics in acid sulfate soil landscapes: flood-induced oxidation of sulfidic drain materials', Abstracts and programme: Soil science solving problems : ASSSI-ASPAC-ACMS National Soils Conference, Adelaide, SA, 3-7 December, Australian Society of Soil Science Inc., the Australasian Soil and Plant Analysis Council, in association with the Australian Clay Minerals Society.


Drain sediments in acid sulfate soil landscapes often contain large amounts (1–6% dry mass) of highly reactive iron monosulfide minerals. The mobilisation and interaction of these materials with floodwaters is thought to contribute to the intense deoxygenation and acidification that is a common phenomenon in coastal lowland rivers. We examined changes to sediment and water chemistry during laboratory experiments designed to simulate flood-induced sediment re-suspension events. Our results demonstrate that iron monosulfide minerals oxidised within minutes, rapidly consuming available oxygen. Where oxygen was continuously bubbled through the suspension, the half-life of the iron monosulfide was < 15 minutes, with it completely reacting within 4 hours. The complete oxidation of iron monosulfide to sulfate is an acid liberating process. Curiously, pH remained near-neutral during iron monosulfide oxidation with extreme acidification (pH < 3.5) developing only after 4 days. The deoxygenation and acidification processes appear to occur in two distinct steps. The first step involves the rapid oxidation of iron monosulfides and formation of elemental sulfur. This is followed by acidity released during the oxidation of elemental sulfur to sulfate and the formation of characteristic ferric iron precipitates. These results clarify important linkages between water quality and the management of low lying coastal floodplain landscapes.