J.P Quirk

Crystal nucleation in Fe(III) solutions and hydroxide gels

Abstract Fe(III) hydroxide gels were produced from a series of 0·5 molal Fe(III) nitrate solutions hydrolyzed for varying times and at pH values up to 1·5. The types and sizes of crystals produced by aging the gels at 60°C in the presence of excess NaOH were examined qualitatively by electron microscopy, selected area diffraction and X-ray diffractometry. Acicular goethite crystals were nucleated in the gel by extremely small goethite crystals which had formed during slow hydrolysis in the parent iron solution. Goethite crystals occurred as cruciform twins showing an apparent (021) twin plane when epitaxial growth commenced on nuclei which were extremely small crystals of hematite formed during very rapid hydrolysis at precipitation. According to conditions of pH the hematite nuclei grew to hematite, to hematite with epitaxial growths of goethite, or to twinned goethite crystals apparently free of hematite.

The point of zero charge of hydroxyapatite and fluorapatite in aqueous solutions

Abstract The points of zero charge (pzc) of synthetic hydroxyapatite and fluorapatite were measured by a titration technique. The titrations were reversible if the pH value did not go below 7.5 for hydroxyapatite and 5.5 for fluorapatite. The pzc value obtained for hydroxyapatite in KCl, which was shown to be indifferent, was 8.5 and unaffected by the surface area of the sample and the use of KNO 3 , KC1O 4 and (CH 3 ) 4 NCl in place of KCl. Aging of the suspension from 16 to 504 hr did not affect the pzc although there was some increase in charge on either side of the pzc at a given pH value with increasing equilibration time. There was a shift in pzc to 7.6 when NaCl was used as supporting electrolyte indicating substitution of Na + for Ca 2+ on the surface. The presence of 5 × 10 −4 M F − moved the pzc to 6.8, a value close to that obtained for fluorapatite.

Crystal morphology and the dissolution of goethite

Abstract An electron microscope has been used to examine the characteristics of goethite crystals and the nature of acid attack on these crystals in aqueous solutions. The (100) face of synthetic acicular goethite crystals makes up 50–60 per cent of the total surface area of a sample. The smaller the crystal, the greater is the contribution of the (010) and (001) faces to the surface area. Very large acicular goethite crystals have a type of substructure. Dissolution of synthetic goethite crystals is anisotropic, two of the faces, (010) and (001) dissolving more rapidly than (100), as a result of an unequal distribution of the reactive sites over the different crystal faces. Dissolution rates are related to the size and shape of the crystals. Star-shaped twinned crystals and the large acicular crystals are rapidly attacked along “grain” boundaries. Pronounced acid attack also occurs at the twin planes. Haematite-containing twins dissolve in two stages, an initial rapid removal of the haematite being followed by much slower dissolution of the goethite.

The specific adsorption of inorganic Hg(II) species and Co(III) complex ions on goethite

The specific adsorption of Hg(II), as Hg2+ and HgCl20, on goethite has been measured as a function of pH. The results show that the mercuric ion is not a typical heavy metal cation since its adsorption is considerably less than for other cations at pH values near their respective p*K values. Complexing the mercuric ion with chloride to give HgCl20 was found to reduce adsorption to negligible amounts. It is concluded that hydroxo ligands are essential for Hg(II) adsorption and it is probable that two hydroxo bridges are formed between the oxide surface and the adsorbed Hg(II) atom. Reduction of the positive charged surface. The considerable covalency of the Hg-OH bonds is believed to cause the relatively low affinity of the mercuric ion for the oxide by polarizing the surface-OH-Hg bonds and therefore minimizing the strength of the surface-OH bonds. The complex ions [Co(NH3)6]3+, [Co(NH3)5H2O]3+, and [Co(en)2(H2O)2]3+ were not specifically adsorbed even when the latter two were hydrolyzed to hydroxo species. Their inability to adsorb is again attributed to the covalency of the Co(III)-OH bonds of the hydroxo ions which prevents the formation of stable hydroxo bridges between the oxide surface and the Co(III) species.

Characterization of hydrolyzed ferric ion solutions a comparison of the effects of various anions on the solutions

Abstract The spherical ferric-hydroxy polycations were isolated from a ferric nitrate solution. By adding nitrate or chloride ion to the isolated species, the subsequent aging processes could be modified. This provided evidence that the spherical polycations had a structure independent of the anion present during their formation, but subsequent aging processes were modified by the anion. The results of this study of the polycations was compared with earlier studies. Some disagreements were found and reasons advanced for them. Also the earlier results of this study involving ferric nitrate, perchlorate and chloride solutions have been compared and contrasted.

Kinetics and mechanisms of the acid dissolution of goethite (α-FeOOH)

Abstract The initial rate of dissolution of several crystalline forms of goethite (α-FeOOH) has been measured as a function of solution conditions. These include hydrogen ion concentration, anion concentration (chloride or perchlorate) and temperature. For dissolution in HClO 4 the rate of reaction is first order with respect to hydrogen ion concentration and is independent of perchloric ion concentration. The rate determining step is thus a protonation of the surface. The activation energy in the presence of perchlorate ion is 23 kcal/mole and the rate of dissolution can be described by the simple collision theory. For dissolution in HCl the kinetics are more complex, the rate again being first order with respect to hydrogen ion concentration but also a function of chloride ion concentration. The results indicate that an adsorption complex is formed before dissolution occurs.

Kinetics of isotopic exchange of phosphate at the α-FeOOH-aqueous solution interface

Abstract The kinetics of heterogeneous isotopic exchange of phosphate between 32P-labelled dilute orthophosphate solutions and Fe(III) phosphate complexes on the surfaces of goethite (α-FeOOH) crystals in dilute suspension are described by the Elovich Equation. Concentration-independent rate constants are obtained at 25° for the pH range 2·6–12. The reference states for the rate constants are defined with respect to an approximation for the distribution of activation energies for exchange. Rates of exchange are first order with respect to the concentration of Fe(III) phosphate surface complexes, show little or no dependence on the concentration of phosphate in solution, and are subject to acid-base catalysis involving fractional powers of H+ and OH− concentrations with a minimum rate at pH 9·0. The inert character of Fe(III) phosphate complexes is attributed to the formation of a briding ligand structure in binuclear complexes on the predominantly-exposed (100) crystal surfaces. The rate-determining step is postulated to be SN1 dissociation or SN2 bimolecular solvolysis of phosphate from Fe(III) co-ordination, in which the mechanism of catalysis is related to a hydrogen bonding stabilisation of the transition state.

A titrimetric and electrophoretic investigation of the pzc and the iep of pigment rutile

Electrophoresis and titration experiments were used to study the effect of impurities (Zn2+ and SO42−) arising as a result of the preparation process, on the pzc and iep of commercial rutile pigment produced by hydrolysis of the sulfate. The presence of these impurities caused the pzc and the iep of the unwashed pigment to be noncoincident; anionic impurities made the pzc move up and the iep move down the pH scale, while cationic impurities had the opposite effect. Successive acid/base/water washings gradually removed the impurities from the surface layers of the oxide and the pzc and the iep of the sulfate base rutile then coincided at pH 5.5 The effect of impurities on the pzc and the iep of commercial chloride base rutile and on hydrous rutile was considered briefly for the purpose of comparison. Hydrous rutile that contained no detectable impurity had an iep/pzc of 6. Heating at high temperatures has been shown to lower the pzc/iep of rutile considerably, but it was found here that although the sulfate base rutile had been heated at 800–1000°C during preparation, the pzc/iep of the clean sample was only 0.5 pH units lower than that of the hydrous rutile.

Incongruent dissolution and surface complexes of hydroxyapatite

Abstract Calcium/phosphorus ratios in solution were determined when samples of synthetic hydroxyapatite (HA) were subject to acid (HCl) dissolution (pH 4.5) in 0.1 M KCl at various solid-to-solution ratios. At a ratio of 500g in 1 liter, the Ca/P approached 2.0, but when the HA solid was washed in electrolyte and resuspended at approximately the same solid: solution ratio, dissolution at pH 4.5 was congruent (Ca/P = 1.67). Incongruent dissolution was associated with the initial contact between solid and solution. At ratios of less than 8g HA/liter the initial dissolution appeared congruent because the absolute amount of excess calcium or deficit of phosphorus was less than the experimental accuracy of estimation of the elements. At the highest solid: solution ratio used, all the calcium and phosphorus involved in the production of Ca/P greater than 1.67 could be attributed to variations in the composition of the surface layer of unit cells. However, the underlying units are of normal composition. The changes in this surface layer are thought to occur at the end of the preparation of the HA, and may be associated with the acetone washing before drying of the crystals.

The limited crystalline swelling of smectites in CaCl2, MgCl2, and LaCl3 solutions

Abstract The limited crystalline swelling, or transition of the basal spacings from ∼15.5 to ∼19.0 A, has been studied for some well-defined Mg-, Ca- or La-saturated montmorillonites equilibrated with corresponding salt solutions. The swelling occurs over a range of salt concentrations depending upon the magnitude and structural origin of the clays surface charge. The higher its charge, the lower the salt concentration at which the clay swells. Ca-saturated smectites with high surface charges, developed principally from Al3+ for Si4+ substitutions in tetrahedral sites, have restricted swelling in water. The swelling phenomenon involves a balance between cation hydration forces and interlayer electrostatic forces. The transition from 15.5 to 19.0 A spacings can be considered an osmotic process; this is discussed.