Multi-Step Crystallization Pathway of Vivianite

Vivianite is a poorly soluble iron (II) phosphate phase found in modern and ancient anoxic soils and sediments. It plays an important role in the biogeochemical cycling of Fe (II) and P and is also a promising P recovery source from waste water. Despite this, the mechanisms and kinetics of vivianite formation are still unknown. Here we present results from a study where we followed the pathway of nucleation and growth of vivianite in solution, starting from aqueous iron (II) and phosphate ions. We aimed to shed light on whether vivianite – like many other sparingly soluble phases – forms via a classical or a non-classical nucleation and growth pathway. To address this, we employed a series of cross-correlated in-situ and ex-situ approaches to followed the time resolved formation reactions using UV-vis spectrophotometry, scanning and transmission electron microscopy, powder X-ray diffraction, infrared spectroscopy, simultaneous thermal gravimetric-differential thermal analyses and synchrotron-based X-ray absorption experiments. These complementary approaches helped us unravel the various stages of nucleation and crystallization of vivianite at high and low supersaturation. Our data indicates that vivianite nucleates via distinct stages in solution and not via a single step, thus following a non-classical nucleation and crystallization pathway. We also demonstrated how one can stabilize and characterize the structure, chemical composition of the intermediate phases and discuss the factors influencing their stability and transformation to the more thermodynamically stable crystalline vivianite.