Phosphate as a control on schwertmannite stability, iron mineralogy and anaerobic biogeochemistry in acid-sulfate systems

Schwertmannite is a common iron (Fe) mineral in acid sulfate soils and acid mine drainage systems, where it plays an important role in controlling acidity dynamics, iron and sulfur biogeochemistry, and trace metal mobility. Phosphate (PO₄^3-) is commonly found in schwertmannite-bearing systems (i.e. from runoff or internal phosphorus cycling) and readily sorbs with schwertmannite.

The overarching objectives of this thesis were 1) to examine the effects of PO₄^3- on schwertmannite stability and its transformation to secondary iron minerals under different environmental scenarios, and 2) to determine the fate of PO₄^3- as schwertmannite undergoes mineralogical and biogeochemical transformations.

To address these objectives, schwertmannite was dosed with varying PO₄^3- loadings and exposed to (1) a natural assemblage of microorganisms to examine schwertmannite stability under acidic Fe(III)-reducing conditions, (2) strongly reducing conditions to assess schwertmannite stability and mineralogical transformations in the presence of microbial Fe(III)- and sulfate (SO₄^2-)- reduction, and (3) abiotic Fe(II)-rich conditions which drive the Fe(II)-accelerated transformation of schwertmannite. In these experiments, aqueous- and solid-phase properties were monitored using standard analytical techniques and selective extractions, with Fe solid-phase speciation evaluated using x-ray diffraction, Mössbauer spectroscopy and synchrotron x-ray absorption spectroscopy.

Phosphate stabilized schwertmannite under acidic, Fe(III)-reducing conditions, with the most microbial Fe(III)-reduction occurring at intermediate PO₄^3- loadings. This discovery reflects a balance between the rapid transformation of schwertmannite to less bioavailable goethite in the absence of PO₄^3-, and PO₄^3- stabilization of schwertmannite against microbial Fe(III)-reduction at higher PO₄^3- loadings.

Following extensive Fe- and SO₄^2- -reduction (i.e. the second study), secondary mineral precipitation was dictated by PO₄^3- loading. The presence of PO₄^3- facilitated formation of sulfate green rust. The formation of mackinawite, a product of microbial SO₄^2- -reduction, decreased at higher loadings of PO₄^3- due to PO₄^3- stabilizing schwertmannite against transformation to goethite, which thereby causes microbial Fe(III)-reduction to outcompete microbial SO₄^2- -reduction. Vivianite became increasing important as PO₄^3- loading increased.

In the abiotic, Fe(II) rich schwertmannite system, sorption of PO₄^3- triggered rapid partial transformation of schwertmannite to a micro-crystalline Fe(III) oxyhydroxide. Following Fe(II) addition, PO₄^3- loading determined the extent of more crystalline mineral formation (i.e. lepidocrocite and goethite), with PO₄^3- inhibiting schwertmannite transformation. This experiment shows, for the first time, that PO₄^3- has contrasting effects on schwertmannite stability, whereby it both induces and inhibits schwertmannite transformation depending on environmental conditions. Under all three experimental settings, PO₄^3- remained bound to the solid-phase, reflecting its strong affinity for schwertmannite.