The influence of naturally occurring schwertmannite on sulfur biomineralization in coastal acid sulfate soils landscapes

In acid sulfate soil affected landscapes the iron oxide mineral schwertmannite (Fe8 (OH)5.5(SO4)1.25) is often abundant, forming coatings on ground surfaces and soil peds. Recent research has demonstrated that schwertmannite is a labile source of iron, sulfate and acidity, and may be the major mineral phase controlling water quality in these landscapes. We report experimental evidence that schwertmannite may also have an important role in the contemporary sulfur cycle, particularly sulfur reduction and accumulation of Monosulfidic-Black-Ooze (MBO). In-vitro anaerobic incubations were undertaken over 203 days to examine the influence of naturally occurring schwertmannite precipitates on the reduction and precipitation of sulfur. Four treatments, incorporating combinations of drain water, vegetation and naturally occurring schwertmannite were used. Sampling and analysis occurred on day 0, 13, 100 and 203. Measurements included pH, Eh, EC, Iron; ferrous, dissolved and total, sulfur; hydrogen sulfide, Acid Volatile Sulfur (AVS), Chromium Reducible Sulfur (CRS), and elemental sulfur, dissolved and total metals and cations. Schwertmannite was characterized 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). Schwertmannite effectively moderated pH and Eh. Both pH and Eh are known to control the kinetics of sulfur mineralization. Schwertmannite appears to enhance the formation of Reducible Inorganic Sulfur (RIS) (AVS, pyrite and elemental sulfur). This was evident in both the quantity and kinetics of sulfide accumulation in this experiment. The data suggests that schwertmannite moderates sulfur mineralization and promotes an increased formation of MBO in Actual Acid Sulfate Soils (AASS) landscapes. The presence of schwertmannite and its effect on the sulfur cycle have implications for land management and water quality.