Dirk Bosbach

The dissolution of apatite in the presence of aqueous metal cations at pH 2–7

Apatite dissolution was studied at 25°C in a series of batch experiments carried out within the pH range of 2–7 with or without the presence of aqueous Pb2+ or Cd2+. The synthetic, microcrystalline hydroxylapatite used in the majority of the experiments was found to have a significantly higher solubility than natural fluorapatite, but a lower dissolution rate. The dissolution rates of both phases increased with decreasing pH. When Pb2+ was present in solution in contact with synthetic hydroxylapatite its concentration decreased over a time interval ranging from several days to several weeks, to a steady state minimum. The rate of Pb2+ loss from solution was sensitive to acidity, and progressed faster at lower pH, but maximum loss was independent of pH. Calcium release to solution matched aqueous lead loss on a mole for mole basis. By the end of each experiment mass calculations suggest that all apatite had been consumed regardless of reaction rate and pH. The solid residue was newly crystallised Pb–hydroxylapatite. This reaction was also observed in situ using Atomic Force Microscopy (AFM) and was found to take place epitaxially onto apatite surfaces. The concentration of aqueous Cd2+ in solution was also reduced in the presence of hydroxylapatite. Cadmium losses were, however, substantially lower. Unlike Pb2+, the maximum amount of Cd2+ lost from solution was a function of pH, and was higher as solution composition approached neutral pH. Cadmium was present in the solid residue at the end of these experiments, probably as a Ca–Cd phosphate solid solution. This work suggests that the interaction between apatite and metals in solution is controlled by apatite dissolution and results in the precipitation of new metal phosphates. The new phosphates nucleate heterogeneously onto the hydroxylapatite surfaces, which acts as a catalyst for the reaction.

Chlorite dissolution in the acid pH-range: A combined microscopic and macroscopic approach

Abstract The dissolution of chlorite with intermediate Fe-content was studied macroscopically via mixed flow experiments as well as microscopically via atomic force microscopy (AFM). BET surface area normalized steady state dissolution rates at 25 °C for pH 2 to 5 vary between 10−12 and 10−13 mol/m2.s. The order of the dissolution reaction with respect to protons was calculated to be about 0.29. For pH 2 to 4, chlorite was found to dissolve non-stoichiometrically, with a preferred release of the octahedrally coordinated cations. The additional release of octahedrally coordinated cations may be due to the transformation of chlorite to interstratified chlorite/vermiculite from the grain edges inward. In-situ atomic force microscopy performed on the basal surfaces of a chlorite sample, which has been preconditioned at pH 2 for several months, indicated a defect controlled dissolution mechanism. Molecular steps with height differences which correspond to the different subunits of chlorite, e.g. TOT sheet and brucite like layer, originated at surface defects such or compositional inhomogenities or cracks, which may be due to the deformation history of the chlorite sample. In contrast to other sheet silicates, at pH 2 nanoscale etch pits occur on the chlorite basal surfaces within flat terraces terminated by a TOT-sheet as well as within the brucite like layer. The chlorite basal surface dissolves layer by layer, because most of the surface defects are only expressed through single TOT or brucite-like layers. The defect controlled dissolution mechanism favours dissolution of molecular steps on the basal surfaces compared to dissolution of the grain edges. At pH 2 the dissolution of the chlorite basal surface is dominated by the retreat of 14 A steps, representing one chlorite unit cell. The macroscopic and microscopic chlorite dissolution rates can be linked via the reactive surface area as identified by AFM. The reactive surface area with respect to dissolution consists of only 0.2% of the BET-surface area. A dissolution rate of 2.5 × 10−9 mol/m2s was calculated from macroscopic and microscopic dissolution experiments at pH 2, when normalized to the reactive surface area.

Molecular-scale mechanisms of crystal growth in barite

Models of crystal growth have been defined by comparing macroscopic growth kinetics with theoretical predictions for various growth mechanisms,. The classic Burton–Cabrera–Frank (BCF) theory predicts that spiral growth at screw dislocations will dominate near equilibrium. Although this has often been observed,, such growth is sometimes inhibited,, which has been assumed to be due to the presence of impurities. At higher supersaturations, growth is commonly modelled by two-dimensional nucleation on the pre-existing surface according to the ‘birth and spread’ model. In general, the morphology of a growing crystal is determined by the rate of growth of different crystallographic faces, and periodic-bond-chain (PBC) theory, relates this morphology to the existence of chains of strongly bonded ions in the structure. Here we report tests of such models for the growth of barite crystals, using a combination of in situ observations of growth mechanisms at molecular resolution with the atomic force microscope, and computer simulations of the surface attachment of growth units. We observe strongly anisotropic growth of two-dimensional nuclei with morphologies controlled by the underlying crystal structure, as well as structure-induced self-inhibition of spiral growth. Our results reveal the limitations of both the BCF and PBC theories in providing a general description of crystal growth.

Structure and reactivity of the calcite-water interface

The zetapotential of calcite in contact with aqueous solutions of varying composition is determined for pre-equilibrated suspensions by means of electrophoretic measurements and for non-equilibrium solutions by means of streaming potential measurements. Carbonate and calcium are identified as charge determining ions. Studies of the equilibrium solutions show a shift of isoelectric point with changing CO(2) partial pressure. Changes in pH have only a weak effect in non-equilibrium solutions. The surface structure of (104)-faces of single crystal calcite in contact to solutions corresponding to those of the zetapotential investigations is determined from surface diffraction measurements. The results reveal no direct indication of calcium or carbonate inner-sphere surface species. The surface ions are found to relax only slightly from their bulk positions; the most significant relaxation is a ∼4° tilt of the surface carbonate ions towards the surface. Two ordered layers of water molecules are identified, the first at 2.35±0.05Å above surface calcium ions and the second layer at 3.24±0.06Å above the surface associated with surface carbonate ions. A Basic-Stern surface complexation model is developed to model observed zetapotentials, while only considering outer-sphere complexes of ions other than protons and hydroxide. The Basic-Stern SCM successfully reproduces the zetapotential data and gives reasonable values for the inner Helmholtz capacitance, which are in line with the Stern layer thickness estimated from surface diffraction results.

In situ investigation of growth and dissolution on the (010) surface of gypsum by Scanning Force Microscopy

Abstract The kinetics of crystal growth and dissolution on the (010) surface of gypsum were investigated in situ by Scanning Force Microscopy (SFM). It could be shown that growth and dissolution on the (010) surface in aqueous solution is a layer-by-layer process. Monolayer steps parallel to [001], [100], and [101], move with a distinctive anisotropy in velocity. During dissolution experiments, etch pits develop at surface regions, where local structural defects may occur. Therefore, etch pits are getting deeper at the same lateral position. As a consequence of the anisotropy in velocity of step movement, the etch pits are elongated in [001]. Isolated holes remain stable over a long period during growth experiments. Such holes can occur at the same position in several overgrowing monolayers. This kind of memory effect might also be explained by local structural defects. Local surface topography has a strong influence on the velocity of step movement. At surface regions with a high step density, the velocity of monolayer step movement is reduced compared to isolated monolayer steps. Isolated [100] monolayer steps move with a velocity of up to 30.0 nm s−1, whereas [010] steps move with up to 9.5 nm s−1, and [001] steps 2.5 nm s−1, in an undersaturated aqueous solution (9.8 mmol L−1). Imaging monolayer steps with lateral molecular resolution reveals the molecular arrangement at mono-layer steps. Individual kink sites can be observed. The kink site formation energy along [001] monolayer steps is 4.1 ± 0.7 KJ mol−1 in saturated aqueous solution. Observed kink site density agrees with predicted values from Monte Carlo simulations.

MINERAL PRECIPITATION AND DISSOLUTION IN AQUEOUS SOLUTION: IN-SITU MICROSCOPIC OBSERVATIONS ON BARITE (001) WITH ATOMIC FORCE MICROSCOPY

Abstract Crystal growth and dissolution mechanisms on the barite (001) surface have been observed in-situ in aqueous solution as a function of the saturation state at molecular scales using Atomic Force Microscopy (AFM). On freshly cleaved barite (001) surfaces, cleavage steps with a step height of 7 A occur, representing the height of a unit cell in the c direction. Growth and dissolution occurs via the advance and retreat of steps with a step height of half a unit cell (3.5 A). In supersaturated BaSO4 solution, monolayer step growth increases linearly with the BaSO4 concentration. Step velocities of one BaSO4 half-unit cell layer are faster in [100] relative to [−100], whereas in the underlying BaSO4 half-unit cell layer, step velocities are faster in [−100] than in [100]. A 21 screw axis parallel to [001] causes the directional growth in opposite directions in different half-unit cell layers. At low to moderate supersaturations, up to about 80 μM BaSO4, spiral growth is the dominant growth mechanism, whereas at higher BaSO4 concentrations, surface nucleation dominates the growth process. In the presence of a crystal growth inhibitor (NTMP, nitrilotri(methylenephosphonic) acid), the nucleation rate as well as step growth is retarded depending on the inhibitor concentration. The morphology of monolayer step edges becomes irregularly curved and jagged, thus suggesting that NTMP molecules attach preferentially to step edges. In pure water, shallow etch pits form with a morphology defined by monolayer steps parallel to [010] and [110]. All etch pits within one BaSO4 layer point in the same direction, whereas etch pits in the underlying BaSO4 layer point in the opposite direction, as a result of the 21 screw axis. Chelating agents, such as EDTA, ethylenediaminetetraacetic acid, dissolve barite effectively by the formation of Ba-EDTA surface complexes and their desorption resulting in an increased etch pit formation rate. Different types of etch pits could be distinguished: (i) shallow etch pits are defined by steps parallel to [010] and [110], (ii) whereas deep etch pits are defined by steps parallel to [100] and [010]. Step velocities of retreating monolayer steps parallel to [110] retreat faster than steps parallel to [010] resulting in an elongated etch pit morphology. With increasing EDTA concentration from 0.1 mM to 100 mM, the velocity of steps parallel to [110] is reduced by one order of magnitude. Etch pit morphology as well as monolayer step kinetics suggest that EDTA molecules attach to the barite (001) surface and detach a BaEDTA2− complex preferentially from steps parallel to [110].

Natural attenuation of TCE, As, Hg linked to the heterogeneous oxidation of Fe(II): an AFM study

Abstract Hydrous ferric oxide (HFO) colloids formed, in strictly anoxic conditions upon oxidation of Fe 2+ ions adsorbed on mineral surface, were investigated under in situ conditions by contact mode atomic force microscopy (AFM). Freshly cleaved and acid-etched large single crystals of near endmember phlogopite were pre-equilibrated with dissolved Fe(II) and then reacted with Hg(II), As(V) and trichlorethene (TCE)-bearing solutions at 25 °C and 1 atm. HFO structures are found to be of nanometer scale. The As(V)–Fe(II) and Hg(II)–Fe(II) reaction products are round (25 nm) microcrystallites located predominantly on the layer edges and are indicative of an accelerated Fe(II) oxidation rate upon formation of Fe(II) inner sphere surface complexes with the phyllosilicate edge surface sites. On the other hand, TCE–Fe(II)–phlogopite reaction products are needle-shaped (45 nm long) particles located on the basal plane along the Periodic Bond Chains (PCBs) directions. Experiments with additions of sodium chloride confirm the importance of the Fe(II) adsorption step in the control of the overall heterogeneous Fe(II)–TCE electron transfer reaction. Kinetic measurements at the nanomolar level of Hg° formed upon reduction of Hg(II) by Fe(II) in presence of phlogopite particles provide further convincing evidence for reduction of Hg(II) aq coupled to the oxidation of Fe(II) adsorbed at the phlogopite–fluid interface, and indicate that sorption of Fe(II) to mineral surfaces enhances the reduction rate of Hg(II) species. The Hg(II) reduction reaction follows a first-order kinetic law. Under our experimental conditions, which were representative of many natural systems, 80% of the mercury is transferred to the atmosphere as Hg° in less than 2 h. The reduction of a heavy metal (Hg), a toxic oxyanion (arsenate ion) and a chlorinated solvent (TCE) thus appear to be driven by the high reactivity of adsorbed Fe(II). This is of environmental relevance since these three priority pollutants are that way reductively transformed to a volatile, an immobilizable and a biodegradable species, respectively. Such kinetic data and reaction pathways are important in the evaluation of natural evaluation scenarios, in the optimization of Fe(II)/mineral mixtures as reductants in technical systems, and in general, in predicting the fate and transport of pollutants in natural systems.

Barite scale formation and dissolution at high ionic strength studied with atomic force microscopy

Abstract In-situ atomic force microscopy (AFM) experiments are used to demonstrate the influence of the ionic strength of the solution on growth and dissolution of barite with some comparative experiments on celestite. Growth and dissolution rates, as determined from monolayer step edge velocities, increase with increasing background electrolyte (NaCl) concentration. The electrolyte effect is interpreted as a consequence of decreased interfacial tension between barite and supersaturated aqueous solution at high ionic strength. The changes in the reaction rates do not only depend on the ionic strength but also on the crystallographic orientation of the monolayer step edges. We found that in solutions with high ionic strength, the relative stability of [010] steps is increased in comparison to the 〈120〉 direction, evident as growth islands and etch pits which are elongated in the [010] direction under growth and dissolution conditions, respectively. This indicates a specific interaction between the background electrolyte and certain sites on the mineral surface. The increased relative stability of steps parallel to [010] relative to those parallel to 〈120〉 can be explained by the formation of stabilized NaSO4NaSO4 or ClBaClBaCl chains along the step which are less polar than terraces bounded by either SO42− or Ba2+. The most likely explanation for the increased growth velocity is that Na+ ions in solution can attach to preexisting growth islands to start a new growth row, which is the rate limiting step for growth in solutions with a low salinity. For both minerals under investigation, barite and celestite, we found that surface features such as two-dimensional nuclei, growth spirals and etch pits which have formed in pure BaSO4/SrSO4 solution can be distinguished from surface features which have formed in solutions of high salinity.

Monazite as a suitable actinide waste form

Abstract The conditioning of radioactive waste from nuclear power plants and in some countries even of weapons plutonium is an important issue for science and society. Therefore the research on appropriate matrices for the immobilization of fission products and actinides is of great interest. Beyond the widely used borosilicate glasses, ceramics are promising materials for the conditioning of actinides like U, Np, Pu, Am, and Cm. Monazite-type ceramics with general composition LnPO4 (Ln = La to Gd) and solid solutions of monazite with cheralite or huttonite represent important materials in this field. Monazite appears to be a promising candidate material, especially because of its outstanding properties regarding radiation resistance and chemical durability. This article summarizes the most recent results concerning the characterization of monazite and respective solid solutions and the study of their chemical, thermal, physical and structural properties. The aim is to demonstrate the suitability of monazite as a secure and reliable waste form for actinides.

Probing disorder in isometric pyrochlore and related complex oxides

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.