Role of temperature on the development of hypoxia in blackwater from grass.

In eastern Australia the development of hypoxic blackwater/floodwater and its detrimental consequences are more common in summer than winter. This study examined the effect of temperature on the development of hypoxic conditions which was determined as biochemical oxygen demand (BOD) in floodwater when pasture grass (a source containing labile organic carbon) was inundated. Labile dissolved organic carbon (DOC) in blackwater is one of the main factors that contribute to the development of hypoxic conditions. Temperature plays a key role on the microbial mineralization of labile DOC and hence the development of hypoxic conditions. Prolonged flooding at different seasons (having different temperatures) was simulated in the laboratory by incubating fresh pasture grass cuttings with river water and soil at three different temperatures (20 °C, 27.5 °C and 35 °C) for 20 days. Although this study shows that elevated ambient temperatures can result in more rapid development of hypoxic conditions during the first week of flood peak, it is evident that blackwater formed at relatively moderate ambient temperatures (e.g. 20 °C) has a similar potential to deoxygenate the receiving water bodies, especially after one week's duration of flood peak.

Chromium(VI) formation via heating of Cr(III)-Fe(III)-(oxy)hydroxides: A pathway for fire-induced soil pollution.

Mixed Cr(III)-Fe(III) (oxy)hydroxides are important Cr-bearing phases in natural, unpolluted soil. Fires frequently affect large areas of land around the world, causing the temporary development of elevated soil temperatures. This study examines the hypothesis that heating Cr(III)-Fe(III) (oxy)hydroxides at temperatures which occur in surface soils during fires can drive rapid oxidation of Cr(III) to hazardous Cr(VI). To test this, poorly-ordered Cr(III)x-Fe(III)1-x (oxy)hydroxides, with x spanning 0.1 to 0.9, were heated at up to 800 °C for 2 h. Heating at 400-800 °C produced a highly crystalline hematite-eskolaite solid-solution (FeIII2-nCrIIInO3, where n ranges from 0 to 2). Chromium K-edge X-ray absorption spectroscopy showed that during heating up to ∼40% of the initial Cr(III) was oxidized to Cr(VI), with the greatest extent of Cr(VI) formation occurring at 200-400 °C. At these temperatures, a substantial proportion (17%-70%) of the newly-formed Cr(VI) was exchangeable (i.e. extracted by a pH 7.2, 10 mM PO43- solution). This suggests that much of the Cr(VI) formed by heating of Cr(III)x-Fe(III)1-x (oxy)hydroxides at 200-400 °C is likely to be relatively mobile in fire-impacted soils. The results of this study provide new insights into a potentially-important pathway for the in-situ formation of Cr(VI) in soil.

Effect of schwertmannite and jarosite on the formation of hypoxic blackwater during inundation of grass material.

This study focused on understanding the effect of schwertmannite and jarosite, commonly found in floodplains containing acid sulfate soil materials, on the characteristics of the hypoxic blackwaters that can form when floodplain vegetation experiences prolonged inundation. The formation of these 'blackwaters' was simulated in the laboratory by inundating flood-intolerant pasture grass leaf material in both the presence of schwertmannite/jarosite (schwertmannite and jarosite treatments) minerals and their absence (control treatment) at 27.5 °C for 32 days. The presence of either schwertmannite or jarosite was able to decrease the concentrations of DOC, nutrients (e.g. NH3 and PO43-) and the biological oxygen demand (BOD) in the incubating water compared to the control treatment. Being fresh and labile, the pasture grass material liberated DOC immediately following inundation with a concomitant decrease in dissolved O2 thereby resulting in anoxic and reducing conditions in the incubating water. With the onset of anoxic and reducing conditions, the biogeochemical cycling of DOC in schwertmannite and jarosite treatments might have proceeded via microbially mediated iron(III) and sulfate reduction and electron shuttling processes. Under anoxic, slightly acidic conditions, microbially mediated iron(III) reduction and subsequent dissolution of schwertmannite and jarosite were triggered by liberating Fe2+, SO42- and alkalinity to the incubating water. The resultant increase in pH led to SO42- reduction in schwertmannite, and the Fe2+ catalysed transformation of both schwertmannite and jarosite to goethite. Schwertmannite almost completely transformed to goethite within two weeks of incubation. Iron(III) in goethite (formed from schwertmannie transformation) was also reduced and likely proceeded via direct microbial reduction or via electron shuttling using the humic acids in the incubating water derived from pasture grass. These findings are highly useful in managing the coastal low lying acid sulfate soils landscapes which are subject to frequent flooding during wet seasons.