Phosphorous (P) is a key agricultural nutrient whose mobility in sediments and water sources governs eutrophication risk and long-term nutrient burial. In anoxic environments, dissolved phosphate can be immobilized as vivianite (Fe3(PO4)2·8H2O), coupling P retention to Fe(II) availability and redox cycling. There is growing recognition of vivianite occurrence in sediments, as well as in wastewater treatment plants (WWTPs). However, predicting where it forms and how long it persists remains challenging because crystal growth, surface oxidation, and dissolution kinetics may be strongly modified by common sediment constituents, namely dissolved silica and organic matter (OM).
In our study, we aim to investigate how silica and OM regulate vivianite precipitation, oxidation, and dissolution pathways. We will pursue two complementary trajectories: Vivianite dynamics under controlled anoxic conditions and in complex natural matrices such as sewage sludge. For this purpose, synthetic vivianite will be precipitated anaerobically across a silicate gradient (and subsequently with OM additions). Products will be characterized by XRD, XPS, and TEM to quantify crystallinity, particle size/morphology, and the extent/composition of oxidized surface layers. Complementary dissolution experiments will test how silica and surface passivation influence vivianite solubility and the extent of P release. In parallel, reducing sludge/sediment matrices will be collected across Israel and analyzed to evaluate how natural geochemical complexity controls vivianite occurrence and reactivity relative to synthetic analogues.
Preliminary results indicate that surface oxidation inhibit vivianite dissolution, while increasing silicate did not measurably disrupt bulk crystallinity under the conditions tested (Si/Fe<0.1). Ongoing analyses will resolve silica/OM partitioning between bulk and surface phases and link these controls to P retention capacity. This study will improve mechanistic understanding of Fe–P mineral transformations and help predict sedimentary P burial versus remobilization under changing redox regimes.