Despite years of study, quantitative models for the nucleation and growth of metal oxyhydroxide nanoparticles from aqueous solution have remained elusive. The problem is complicated by surface adsorption, which causes the stoichiometry of the nucleus to differ from that of the bulk precipitate and causes the surface tension of the precipitate-water interface to depend upon solution chemistry. Here we present a variation of classical nucleation theory that can accommodate surface adsorption, and apply it to understand the nucleation of β-FeOOH (akaganeite) nanoparticles from aqueous FeCl3 solutions. We use small-angle X-ray scattering (SAXS) to quantify nucleation rates over a range of concentrations (5-200 mM FeCl3) and temperatures (47-80 °C), then apply our model to estimate the critical nucleus size and surface tension at each condition. The surface tension varies from 0.07 J/m2 in 200 mM solutions to 0.16 J/m2 in 5 mM solutions. This behavior indicates that the nuclei contain an excess of Cl- and H+ relative to the ideal FeOOH stoichiometry, and the coadsorption of Cl- and H+ is critical for reducing surface tension into a range where classical nucleation pathways can operate. Furthermore, we find that the surface tension can be roughly estimated from aqueous solubility data alone, which may help to understand systems where surface tension data is unavailable.

Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO3 and then 0.1 M NaOH), creating new Ca-P via formation of Ca phosphates between NaOH-desorbed phosphate and exchangeable Ca2+ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca2+ regardless of the presence or absence of CaCO3, whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of Ca-P simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure.

Iron oxides and oxyhydroxides form via Fe3+ hydrolysis and polymerization in many aqueous environments, but the pathway from Fe3+ monomers to oligomers and then to solid phase nuclei is unknown. In this work, using combined X-ray, UV-vis, and Mössbauer spectroscopic approaches, we were able to identify and quantify the long-time sought ferric speciation over time during ferric oxyhydroxide formation in partially-neutralized ferric nitrate solutions ([Fe3+]=0.2M, 1.8