Diane M Fyfe

Occurrence and abundance of monosulfidic black ooze in coastal acid sulfate soil landscapes

Organic oozes enriched in iron monosulfides are called monosulfidic black ooze (MBO). The occurrence and abundance of MBO were quantified in natural tributaries and man-made drainage canals on a typical coastal river floodplain. Thick deposits of MBO occurred in drains and the protected upper reaches of tributaries in acid sulfate soil (ASS) areas. Acid-tolerant lilies grew prolifically in these areas, contributing large amounts of decaying organic debris. The MBO contained up to 6.3% acid-volatile sulfur (equivalent to 18% iron monosulfide), an order of magnitude greater than reported previously for natural sediments. It also contained up to 6% pyritic sulfur (equivalent to approximately 11% pyrite). Sulfate and iron from surrounding ASS combined with abundant organic matter and protected flow in ASS drains provide excellent conditions for sulfate reduction, leading to iron sulfide precipitation and the accumulation of organic detritus. When mixed with water, the iron monosulfide in MBO can react within minutes to completely consume dissolved oxygen. There are thousands of drains in ASS areas that may have thick deposits of MBO. The MBO has the potential to cause rapid and severe effects on water quality. The challenge is to develop management practices that reduce the formation and accumulation of MBO in acid sulfate landscapes.

The process of sulfide oxidation in some acid sulfate soil materials

The process of sulfide oxidation in acid sulfate soils (ASS) is complex, involving the formation of numerous oxidation products. In this study the sulfide oxidation process was examined in 2 ASS materials over a period of 36 days using laboratory incubation experiments. Both ASS materials experienced substantial sulfide oxidation and acidification during incubation. The oxidation of pyrite was the primary cause of acidification in these ASS materials. Although a decrease in magnetic susceptibility (χ) over the initial 4 days of incubation suggested the rapid oxidation of ferromagnetic iron monosulfide greigite (Fe3S4), the total acid volatile sulfur (SAV) fraction increased in concentration by an order of magnitude over the initial 8 days of incubation. Oxygen (O2) concentration profiles indicated the presence of anoxic conditions in the centre of the incubating materials even after 16 days of exposure to the atmosphere enabling SAV formation to occur. The oxidation of the SAV fraction did not result in substantial acidification. A large proportion of the water-soluble iron released by sulfide oxidation was precipitated as iron oxides and hydroxides. Sulfate (SO42–) was the dominant sulfur species produced from sulfide oxidation in both ASS materials, although water-soluble SO42– was a poor indicator of the extent of sulfide oxidation. The sulfoxyanion intermediates, thiosulfate (S2O32–) and tetrathionate (S4O62–), were detected only in the early stages of incubation, with minimal amounts being detected after the initial 4 days. The relative abundance of these 2 intermediate sulfur species appeared to be dependent on the soil pH, with S4O62– dominating S2O32– in the more acidic ASS material (i.e. pH <6) as has been observed in previous studies. The diminishing presence of sulfoxyanion intermediates as oxidation progressed was indicative that ferric ion (Fe3+) and bacterial catalysis were driving the oxidation processes. While these sulfoxyanion intermediates only constituted a small percentage of the reduced inorganic sulfur (RIS) fraction, they accounted for up to 9.3% of the total soluble sulfur fraction. Elemental sulfur (S0) was not an important sulfide oxidation product in the ASS materials examined in this study.

Redistribution of monosulfidic black oozes by floodwaters in a coastal acid sulfate soil floodplain

The observations presented in this paper illustrate that significant amounts of monosulfidic black oozes (MBO) were eroded from flood mitigation drainage canals and redistributed across a coastal floodplain during a flood event associated with extreme deoxygenation and a massive fish kill. MBO are organic materials enriched in iron monosulfides and thick layers can accumulate in drains affected by acid sulfate soils. Laboratory studies have demonstrated that MBO can react rapidly when brought into suspension to completely consume dissolved oxygen. The abundance of MBO in flood mitigation drains and their extreme reactivity implicated MBO in the acute deoxygenation of the Richmond River, north-eastern New South Wales, Australia, following a major flood in February 2001. The field observations of MBO redistribution provide valuable evidence to help explain how these materials may interact and contribute to the deoxygenation of floodwaters.