Chemical changes during oxidation of iron monosulfide-rich sediments

Abstract

Iron monosulfide-rich sediments are commonly found in drains in acid sulfate soil (ASS) areas in eastern Australia. Sulfides represent an important sink for contaminants (including acidity and metals) and play an important role in improving water quality. However, these sediments are prone to oxidation during drain cleaning or drought, releasing acidity and metals into the drainage system, and are a potential environmental hazard in ASS landscapes. Chemical changes during oxidation of iron monosulfide-rich sediments have been studied in an incubation experiment. The effect of lime and mill mud/ash (an alkaline sugar mill byproduct) as neutralising agents has been examined. Oxidation of untreated sediments resulted in decreased pH, the production of acidity, and release of SO4 and metals, including iron, aluminium, and manganese. The large concentration and highly reactive nature of iron monosulfides suggest that the chemical changes observed are mostly a result of iron monosulfide oxidation, although the contribution of pyrite oxidation cannot be totally excluded. Lime appears to be an effective neutralising agent by maintaining pH and low acidity concentrations throughout the incubation experiment. The mill mud/ash initially prevented oxidation but its effectiveness was limited. These findings are important in terms of drain management in ASS environments.

Zinc speciation in mining and smelter contaminated overbank sediments by EXAFS spectroscopy

“…When exposed to oxygen, oxidation and dissolution of sulphides occurs, as shown for example by Isaure et al (2005), Panfili et al (2005) and Schuwirth et al (2007). This chemical reaction results in acidification of the pore water (Smith, 2004), analogous to the processes of acid mine drainage and acid sulphate soils. Here, the decrease in pH is relatively small (at most one unit), because of the large pH buffering capacity of carbonates.…”

Treatment of Acid Sulfate Soil Drainage By Direct Application of Alkaline Reagents

“…In this anaerobic environment, bacterial sulfate reduction generates hydrogen sulfide that reacts with dissolved iron precipitating FeS (Appelo and Postma, 1999). The formation of FeS and other sulfides can be effective as a temporary sink for acidity and dissolved metals; however, with drying or aeration of the sediment, the sulfides can oxidise and release the acidity and dissolved metals (Smith, 2004).…”

Hydrochemistry of episodic drainage waters discharged from an acid sulfate soil affected catchment

“…The iron monosulfides, once mobilized, can oxidize and potentially deoxygenate the water column and release other oxidation products. Unlike natural drainage systems, however, hydraulic velocity in the channels under investigation here is small and relatively constant due to pumping which, in turn, maintains a uniform channel profile with limited scouring or deposition (Smith, 2004). Nevertheless, there is a possibility that, during the sporadic operation of the catchment pump, the sudden back flow of water may disturb the iron monosulfide-rich sediment.…”

“…It has been suggested by Smith (2004) that iron monosulfide-rich sediments occur widely in drain bases and remain unoxidised provided a constant water level is maintained in the drains. Oxidation of these relatively reactive minerals presumably occurs when the overlying waters are maintained oxic.…”

“…The iron monosulfides, when oxidized (Eq. (3)), can lower pH Eq. (2)) and could potentially release iron and aluminium into waterways (Smith, 2004).…”