Interfacial precipitation of phosphate on hematite and goethite

© 2018 by the author. Licensee MDPI, Basel, Switzerland. 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 NH4H2PO4at pH 4.5 showed the nucleation rate was greater on goethite than hematite. Enhanced goethite and hematite dissolution in the presence of 10 mM AlCl3resulted 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

The role of magnesium in the crystallization of calcite and aragonite in a porous medium

Morphological development of calcite crystals is related to supersaturation conditions during growth. Crystallization of calcium carbonate (calcite and aragonite l as well as Mg-calcite was studied under controlled supersaturation conditions by the counter diffusion of Ca2+ and co,2 - ions through a porous transport medium (a column of silica gel). U~der our experimental conditions, where ion transport is constrained to be diffusion controlled, nucleation and growth take place under conditions of high supersaturation, the actual threshold value of the supersaturation depending on the supersaturation gradient. In the pure CaC03 system, calcite grows at lower supersaturation than aragonite. The calcite develops relatively simple rhombohedra while the aragonite grows as spherulites. Presence of Mg2+ in the interstitial fluid inhibits nucleation, increasing the threshold supersaturation at which crystallization begins. The resulting Mg-calcite crystals show a range of morphologies depending on the Mg content and the upersaturation at the point of crystallization. At high values of supersaturation, up to 15 mol % MgC03 is incorporated into the calcite and the crystals form spheres. At lower supersaturations, Mg content decreases and morphologies change progressively through a well-defined and reproducible sequence from spheres to dumbbell-like forms to wheat-sheaf-like bundles and eventually single crystals with steep rhombohedral faces. The crystals are compositionally zoned, showing both sector and oscillatory zoning. The compositional evolution is related to the supersaturation and interface roughness during crystal growth

The composition of solid solutions crystallising from aqueous solutions: the influence of supersaturation and growth mechanisms

In this paper we present a new approach to the problem of the crystallisation in solid solution–aqueous solution (SS–AS) systems, in which two main controlling factors have been considered: (i) the supersaturation state of the multicomponent solution in contact with the growing crystal and (ii) the growth mechanisms, operating at a molecular scale on the various faces of the crystal. Supersaturation has been evaluated as a function of the solid solution (the β function) and the transitional supersaturation between spiral growth and two dimensional nucleation mechanisms has been considered as a linear function of the solid composition (the β* line). By superimposing β functions and β* line on a supersaturation–solid composition diagram, we can define compositional regions growing according to different growth mechanisms. In order to test our model, a number of in situ Atomic Force Microscope (AFM) experiments have been conducted in the Ba2+–Sr 2+–SO24–H2O system, using barite (001) as the substrate. The general growth behaviour observed is consistent with the predictions given for a number of initial aqueous solution compositions. Microprobe analysis shows that the new (001) layers grown under conditions where the maximum supersaturation corresponded to intermediate compositions of the (Ba,Sr)SO4 solid are very Sr-rich. A qualitative explanation for such a compositional shift is given on the basis of nucleation rate calculations in the Ba2+–Sr 2+–SO424–H2O system. Finally we discuss the effect of the substrate on the formation and distribution of two-dimensional (Ba,Sr)SO4 nuclei on a barite (001) surface

Microtopography of the barite (001) face during growth: AFM observations and PBC theory

Under moderate supersaturation conditions, crystal growth on the barite (0 0 1) surface takes place by the development of two-dimensional nucleation simultaneously with the advancement of molecular-height cleavage steps on the surface. The most frequent growth steps have a height of a half-unit cell, as has been predicted by periodic bond chain (PBC) theory, and they are parallel to the S1 2 0T PBC directions. Along opposite directions the velocity of S1 2 0T growth steps is strongly anisotropic. Moreover, the directions of fast growth alternate for successive elementary growth layers. The anisotropy of the growth rates can be explained by taking into account the crystallographic features and orientation of the complete PBC within each (0 0 2) elementary growth slice. On the other hand, the alternation of the fast growth direction for S1 2 0T steps in successive d002 growth layers is related to the existence of a 21 screw axis perpendicular to the (0 0 1) surface. Two-dimensional nucleation on the barite (0 0 1) surface is characterized by the development of islands with a circular sector shape and half-unit cell in height. The two-dimensional islands nucleated on the initial surface show the same orientation. As growth proceeds, islands coalesce and a homogeneous layer with a thickness of 3.5 A is formed. Nucleation on this new surface produces islands oriented in the opposite sense to those in the previous layer. Goniometric measurements and X-ray di¤raction experiments conÞrm that the straight edges of the islands are parallel to the [1 2 0] and [1 2 0] crystallographically equivalent directions. The third side of each island is curved, rough and tangent to [010]. Both the morphology and development of two-dimensional nuclei on the barite (001) face clearly indicate that the growth process is structurally controlled. The asymmetry of [120], [120] and [010] PBCs and their crystallographic features can be considered as responsible for the geometry and spread of the circular sector islands formed on each elementary (002) growth layer

Microscopic and spectroscopic investigation of the calcite surface interacted with Hg(II) in aqueous solutions

The interaction of the {101¯4} cleavage surface of calcite with Hg(CH3COO)2 aqueous solutions with concentration of 5 mM Hg(II) (pH ≈3.5), was investigated using microscopic and spectroscopic techniques. In situ atomic force microscopy experiments showed that surface microtopography changes significantly as a result of the interaction, and that the initial rhombic etch pits induced by H2O dissolution are rapidly transformed to deeper etch pits exhibiting an unusual triangular shape. The growth of these etch pits is strongly anisotropic, moving faster along the [22¯1] direction than along the [010] direction (with step-retreat velocities of ~12 nm s –1 and ~4 nm s–1, respectively). The modified etch pits are due to Hg(II) sorption in the surface, rather than due to the effect of the acetate anion. The sorption (adsorption and probably absorption also) of Hg(II), in the first minutes of the interaction, is shown by X-ray photoelectron spectroscopy. After ~2 h, the triangular etch pits are interconnected to form larger hexagonal etch pits, while Hg(II)-bearing phases (confirmed later by SEM-EDS) grow onto the surface through a heterogeneous nucleation process. The crystal growth of orthorhombic (montroydite-type) hydrated Hg(II) oxide (HgO·nH2O) on the surface of calcite was confirmed by XRD patterns and FT-IR spectra from samples exposed for longer times to Hg(CH3COO)2 solution

Mineral Surface Rearrangement at High Temperatures: Implications for Extraterrestrial Mineral Grain Reactivity

Mineral surfaces play a critical role in the solar nebula as a catalytic surface for chemical reactions and potentially acted as a source of water during Earth's accretion by the adsorption of water molecules to the surface of interplanetary dust particles. However, nothing is known about how mineral surfaces respond to short-lived thermal fluctuations that are below the melting temperature of the mineral. Here we show that mineral surfaces react and rearrange within minutes to changes in their local environment despite being far below their melting temperature. Polished surfaces of the rock and planetary dust-forming silicate mineral olivine ((Mg,Fe)2SiO4) show significant surface reorganization textures upon rapid heating resulting in surface features up to 40 nm in height observed after annealing at 1200 °C. Thus, high-temperature fluctuations should provide new and highly reactive sites for chemical reactions on nebula mineral particles. Our results also may help to explain discrepancies between short and long diffusion profiles in experiments where diffusion length scales are of the order of 100 nm or less.This work was funded by a Deutsche Forschungsgemeinsschaft grant awarded to A. Putnis (PU153/16-1) and a Humboldt fellowship funding a short stay for H.St.C.O. at the University of Mü nster, Germany. All analytical and experimental procedures were carried out at the Institut fü r Mineralogie, University of Mü nster, Germany. H.E.K. acknowledges funding through the European Marie Curie Actions International Outgoing Fellowship (TMuPiFe 2012-328731). O.P. acknowledges The Netherlands Organisation for Scientific Research (NWO) Veni Grant (No. 863.13.006). C.V.P. and A.P. acknowledge funding through the EU Marie Curie International Training Networks, CO2REACT, FlowTrans and MINS

Metastable phenomena on calcite {1014} surfaces growing from Sr2+–Ca2+–CO3 2- aqueous solutions

In situ atomic force microscopy (AFM) experiments, scanning electron microscopy (SEM) imaging and composition analysis, and X-ray diffraction have provided information about the growth, dissolution and transformation processes promoted by Sr2 + –Ca2 + –CO3 2 aqueous solutions in contact with calcite {101¯4} surfaces. Experiments have shown a wide variety of surface phenomena, such as the influence of the Sr-bearing newly-formed surface on the subsequent growth (template effect), the growth and subsequent dissolution of surfaces and the nucleation of secondary three-dimensional nuclei on calcite surfaces. These phenomena reveal the metastability of the crystallisation system and are a consequence of the interplay between thermodynamics (the relative stability of the two calcite and aragonite structure solid solutions that can be formed), supersaturation of the aqueous solution with respect to the two possible solid solutions, and the crystallographic control of the surfaces on cation incorporation

Nucleation of solid solutions crystallizing from aqueous solutions

The study of nucleation and growth mechanisms of salts from aqueous solutions, as a function of supersaturation, is described using both macroscopic and microscopic experiments. In situ observations in a fluid cell in an atomic force microscope (AFM) reveal phenomena not accounted for in standard crystal-growth theories, specifically on the role of the crystal structure of the substrate in controlling spiral growth and two-dimensional nucleation. As a model example, the crystallization of two isostructural salts, BaSO4 and SrSO4, is described. The growth of solid-solution crystals is considerably more complex. The supersaturation of a given aqueous solution relative to a solid solution is different with respect to each solid composition, and it leads to the possibility that different compositions can simultaneously grow by different mechanisms on the same crystal face. Oscillatory compositional zoning is another consequence of the interplay between the thermodynamics and the kinetics of nucleation. The factors which control nucleation and growth of the solid solution (Ba,Sr)SO4 from an aqueous solution are described. The predictions made from the theory are compared with direct observations of crystal growth in an AFM

Nanoscale phenomena during the growth of solid solutions on calcite {101¯4} surfaces

This work deals with the growth behaviour of calcite {101¯4} surfaces in contact with multicomponent aqueous solutions containing divalent cations (Ba2+, Sr2+, Mn2+, Cd2+, or Mg2+). The result is the formation of solid solutions, with calcite or aragonite as one of the end-members. In situ atomic force microscopy has revealed a wide variety of surface phenomena occurring during the formation of these solid solutions. Among them are: (1) the thickening of growth steps and the subsequent dissolution of surfaces followed by the nucleation of secondary three-dimensional nuclei on calcite surfaces, (2) the transition between growth mechanisms, (3) the formation of an epitaxial layer that armours the substrate from further dissolution and (4) the inhibitory effect of the newly formed surface on the subsequent growth (template effect). The two last phenomena can considerably limit coprecipitation as an effective mechanism for divalent metal uptake. All the phenomena described are a consequence of the interplay between thermodynamics, supersaturation of the aqueous solution with respect to the possible solid solutions and the crystallographic control of the surfaces on the cation incorporation, and indicates that there are many differences between the crystal growth of solid solutions and phases with fixed composition