Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils

摘要

We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), Fe-III-rich (27-138 mu M) surface water with low molar Cl:SO4 ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30% of aqueous surface water Fe being present as the (FeSO4+)-S-III species). Subsurface sediments contained abundant pore-water HCO3 (6-20 mM) and were reducing (Eh < -100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence. pore-water Fell concentrations were high (> 2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO4-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water Fell, precipitation-dissolution of disordered mackinawite maintained very low (i.e. < 0.1 mu M) S-II concentrations. Such low concentrations of S-II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (< 300 mu mol g(-1) as Fe) compared to disordered mackinawite (up to 590 mu mol g(-1) as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways. (c) 2006 Elsevier Inc. All rights reserved.