Sulfur biomineralisation across a spatio-temporal hydrogeochemical gradient in an acid sulfate soil wetland

The formation of iron sulfides in wetland soils is an acidconsuming process, which can sequester a range of trace metals and metalloids. In acid-sulfate soil wetlands, iron sulfide formation can therefore aid in neutralisation of acidity and immobilization of contaminants. For this reason, iron sulfide formation as a result of reductive S biomineralisation processes is an attractive site remediation objective. In this study, we quantified the in situ rates and products of dissimilatory SO4 2- reduction across a landscape-scale spatio-temporal hydrogeochemical gradient in an acid-sulfate soil wetland. The study site is an 800 ha tidal wetland that was extensively drained in the 1970’s. Drainage triggered in situ pyrite (FeS2) oxidation and acidification of surface-waters and shallow groundwaters. In 2001-2002, a remediation program was initiated which involved progressively re-flooding the site via controlled tidal inundation. We examined spatio-temporal dynamics in S biogeochemistry at the current fringe of tidal inundation about 5 years after commencement of remediation activities. In situ SO4 2- reduction was confined to near-surface soil layers (to ~60 cm below ground surface) and occurred at rates up to ~300 nmol cm-3 day-1. Elemental S was the main short-term product of SO4 2- reduction, as a result of (1) reaction between S (-II) and abundant jarosite-derived Fe (III), and (2) shortterm redox oscillations near the soil surface. Sulfur K-edge XANES spectroscopy showed that S (0)(s) was abundant in near-surface soils, which corroborated selective extraction data showing S (0)(s) up to ~40 μmol g-1. The iron sulfide thiospinel, greigite (Fe3S4), was also an important biomineralisation product, as evident from XRD, XANES spectroscopy and analytical electron microscopy. The results are discussed in terms of thermodynamic and kinetic constraints on the spatio-temporal behaviour of S biomineralisation products, especially S (0)(s) and greigite. Dynamic tide-induced redox oscillations in near-surface soil have a central role in the formation and fate of these observed S biomineralisation products.