Nucleation and Crystallization of Ferrous Phosphate Hydrate via an Amorphous Intermediate

Abstract

The fundamental processes of nucleation and crystallization are widely observed in systems relevant to material synthesis and biomineralization; yet most often, their mechanism remains unclear. In this study, we unravel the discrete stages of nucleation and crystallization of Fe3(PO4)2·8H2O (vivianite). We experimentally monitored the formation and transformation from ions to solid products by employing correlated, time-resolved in situ and ex situ approaches. We show that vivianite crystallization occurs in distinct stages via a transient amorphous precursor phase. The metastable amorphous ferrous phosphate (AFEP) intermediate could be isolated and stabilized. We resolved the differences in bonding environments, structure, and symmetric changes of the Fe site during the transformation of AFEP to crystalline vivianite through synchrotron X-ray absorption spectroscopy at the Fe K-edge. This intermediate AFEP phase has a lower water content and less distorted local symmetry, compared to the crystalline end product vivianite. Our combined results indicate that a nonclassical, hydration-induced nucleation and transformation driven by the incorporation and rearrangement of water molecules and ions (Fe2+ and PO4 3–) within the AFEP is the dominating mechanism of vivianite formation at moderately high to low vivianite supersaturations (saturation index ≤ 10.19). We offer fundamental insights into the aqueous, amorphous-to-crystalline transformations in the Fe2+–PO4 system and highlight the different attributes of the AFEP, compared to its crystalline counterpart.

Direct Nanoscale Imaging Reveals the Mechanism by Which Organic Acids Dissolve Vivianite through Proton and Ligand Effects

“…Previous studies have explored the formation mechanism of the inorganic Fe–P precipitate, its environmental influencing factors (such as pH, temperature, and the presence of organic matter), and its distribution and role in the natural environment. ,− These studies reveal the potential effects of inorganic Fe–P precipitates on the P cycle, ecosystem productivity, and water eutrophication. However, inorganic Fe–P precipitates are also potential P sinks in nature .…”

“…In a relatively anoxic aquatic environment, Fe is often in a reduced state (Fe 2+ ). At a K sp value of 1.0 × 10 –36 , − P binds with Fe 2+ to form Fe­(II)-P precipitates such as vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O). However, regardless of whatever Fe–P binding inorganic form is in the environment, it is important to probe the biogeochemical cycle of Fe and P and their respective availabilities.…”

Nonclassical Crystallization Pathway of Transition Metal Phosphate Compounds

“…Although the synthesis and precipitation of Mg or Ca phosphate-bearing phases are relatively well-characterized, 17−19 investigations of the crystallization of transition metal phosphate materials are scarce and focus on mineralogical aspects. 20 Most of the available studies concentrate on the incorporation or adsorption of transition metals and metalloids 10,21−24 rather than their complete substitution in phosphate-based compounds. 25−27 However, TMPs exhibit major differences in their crystallization pathways, growth kinetics, thermodynamic stabilities, or transformations among each other and differ from their related maingroup analogues.…”

“…For example, transition metal phosphates, such as Zr, Sn, Ni, or Co phosphates, garnered attention as low-cost, chemically stable, and tunable proton conductors for fuel cells. − However, to effectively harvest metal and phosphate in the form of TMPs and convert them to functional materials, one must elucidate their nucleation and crystallization pathways from aqueous solutions. Although the synthesis and precipitation of Mg or Ca phosphate-bearing phases are relatively well-characterized, − investigations of the crystallization of transition metal phosphate materials are scarce and focus on mineralogical aspects . Most of the available studies concentrate on the incorporation or adsorption of transition metals and metalloids ,− rather than their complete substitution in phosphate-based compounds. − However, TMPs exhibit major differences in their crystallization pathways, growth kinetics, thermodynamic stabilities, or transformations among each other and differ from their related main-group analogues. , …”

Non-classical crystallization pathway of transition metal phosphate compounds

“…However, to effectively harvest metal and phosphate in a form of TMPs and convert them to functional materials, one needs to elucidate their nucleation and crystallization pathways from aqueous solutions. Although, the synthesis and precipitation of Mg or Ca phosphate-bearing phases are relatively well characterized [17][18][19] , investigations of the crystallization of transition metal phosphate materials are relatively scarce and focus on mineralogical aspects 20 . Most of the available studies concentrate on the incorporation or adsorption of transition metals and metalloids 10, 21-24 rather than their complete substitution in phosphate-based compounds [25][26][27] .…”