Oxidation pathways of monosulfidic black ooze

Monosulfidic Black Ooze (MBO) describes an organic material, enriched with iron monosulfides, commonly found in Acid Sulfate Soil (ASS) landscapes. Dominating the surficial sediment layer, this highly mobile material is easily incorporated into the water column when disturbed, resulting in severe deoxygenation and acidification. MBO has recently been implicated in massive fish kills associated with flood events in north eastern NSW, Australia. The process of acidification and the formation of intermediate sulfur species have been examined in ASS; however, there is little information on the chemical behaviour of MBO originating from drainage channels in ASS landscapes. Intermediate sulfur species are environmentally significant because their presence promotes the conversion of highly reactive monosulfides to more stable disulfides (e.g. pyrite). If present in substantial quantities, sulfur intermediates may be an important indicator of MBO oxidation, particularly when the oxidation is associated with extreme deoxygenation but not with acidification. This research was designed to examine the oxidation pathways of MBO and to characterise and quantify the reduced inorganic sulfur (RIS) species formed during a simulated re-suspension and land application of MBO. The MBO had an extremely high acid volatile sulfur (AVS) concentration of 703 μmol S ^g-1 and a pyrite concentration of 528 μmol S ^g-1. The oxidation of monosulfides and pyrite over the incubation period contributed to a steady increase in sulfate (SO₄^2-); the major water-soluble sulfoxyanion detected (600 mg L^-1). Thiosulfate (S₂O₃^2-) and sulfite (SO₃^2-) were below detection levels during the entire incubation period however, tetrathionate (S₄O₆^2-) was observed throughout the incubation period. A large proportion of the water-soluble iron released by sulfide oxidation was precipitated as ferric (oxy)hydroxides. A detailed understanding of the oxidation pathways of MBO will ensure that the most appropriate and effective drain management techniques to minimise the export of acidity and deoxygenating drain oozes will be developed.