Conversion of naturally occurring schwertmannite to hematite by controlled heating: implications for soluble acidity and titratable actual acidity

Naturally occurring schwertmannite (Fe₈(OH)_5.5(SO4)_1.25), a result of acid sulfate soils (ASS) oxidation and severe acidification, has been identified as a labile source of iron, sulfate and acidity. Schwertmannite forms surface accumulations on coastal flood plains of eastern Australia. These flood plains are subject to wild fire events and fire-stick farming practices. Four samples of naturally occurring schwertmannite, from New South Wales coastal catchments, were converted to hematite by controlled heating. Complete conversion of schwertmannite to hematite (α-Fe2O3) occurred after exposure to temperatures of ≥500oC. The unheated schwertmannite and corresponding hematite samples were analysed for soluble acidity and Titratable Actual Acidity (TAA). In addition to acidity, the samples were measured for pH, EC and dissolved and total metals and cations. The schwertmannite and hematite samples were characterised by Scanning Electron Microscope (SEM) & Energy Dispersive X-ray micro-analysis (EDX), powder X-Ray Diffraction (XRD), LECO, inductively coupled plasma optical emission spectroscopy (ICP-OES) and Fourier Transform InfRared (FTIR). The hematite presented as a pseudomorph after schwertmannite. The schwertmannite contain appreciable amounts of soluble acidity and titratable actual acidity. The hematite contained no acidity measurable as soluble acidity and or titratable actual acidity. Theoretical acid loads have been calculated and the environmental implications of fire in areas containing schwertmannite accumulations are discussed. This study provides the first evidence of schwertmannite transformation to hematite as a result of wild fire events and therefore presents an original and significant insight into the iron cycle in ASS landscapes.