Abstract
Adsorption and subsequent precipitation of dissolved phosphates on iron oxides, such as hematite and goethite, is of considerable importance in predicting the bioavailability of phosphates. We used in situ atomic force microscopy (AFM) to image the kinetic processes of phosphate-bearing solutions interacting with hematite or goethite surfaces. The nucleation of nanoparticles (1.0–4.0 nm in height) of iron phosphate (Fe(III)-P) phases, possibly an amorphous phase at the initial stages, was observed during the dissolution of both hematite and goethite at the earliest crystallization stages. This was followed by a subsequent aggregation stage where larger particles and layered precipitates are formed under different pH values, ionic strengths, and organic additives. Kinetic analysis of the surface nucleation of Fe-P phases in 50 mM NH4H2PO4 at pH 4.5 showed the nucleation rate was greater on goethite than hematite. Enhanced goethite and hematite dissolution in the presence of 10 mM AlCl3 resulted in a rapid increase in Fe-P nucleation rates. A low concentration of citrate promoted the nucleation, whereas nucleation was inhibited at higher concentrations of citrate. By modeling using PHREEQC, calculated saturation indices (SI) showed that the three Fe(III)-P phases of cacoxenite, tinticite, and strengite may be supersaturated in the reacted solutions. Cacoxenite is predicted to be more thermodynamically favorable in all the phosphate solutions if equilibrium is reached with respect to hematite or goethite, although possibly only amorphous precipitates were observed at the earliest stages. These direct observations at the nanoscale may improve our understanding of phosphate immobilization in iron oxide-rich acid soils.
Highlights
Phosphorus (P) is a limiting nutrient for plant growth [1,2] due to both its low P bioavailability in soils and the increasingly limited mineable resources of P-containing rocks [3]
The dissolution of hematite or goethite in the presence of phosphate solutions provided a source of Fe3+ ions, which resulted in a supersaturation of the interfacial fluid with respect to a Fe-P phase and its nucleation on the dissolving hematite or goethite surfaces
In situ atomic force microscopy (AFM) imaging demonstrated that the observed nanoparticles/nanoclusters were present with a height of 1.0–4.0 nm and 2.0–3.0 nm on hematite (Figure 1A–C,G) and goethite (Figure 1D,F,H), respectively, at the earliest nucleation stages, i.e., within minutes of the phosphate solution contacting the mineral surfaces
Summary
Phosphorus (P) is a limiting nutrient for plant growth [1,2] due to both its low P bioavailability in soils and the increasingly limited mineable resources of P-containing rocks [3]. Minerals 2018, 8, 207 surface reaction kinetics of solutions containing phosphate with iron oxides remain limited. The objective of this study was to observe the kinetics of hematite and goethite dissolution and the coupled precipitation of new phases in the presence of aqueous solutions of phosphate. To achieve these goals, we used in situ atomic force microscopy (AFM) to image the real-time kinetics of hematite and goethite dissolution and the subsequent formation of new Fe-P phases. We chose hematite (Fe2 O3 ) and goethite (α-FeOOH) because they have been recognized as two of the most abundant of the naturally occurring iron oxides in soils [15,16]. We directly compared the surface nucleation rates of Fe-P phases on hematite and goethite under the same solution conditions and calculated solution speciation and thermodynamics of precipitating phases using PHREEQC modeling
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