Showing papers on "Dissolution published in 1993"

Wet chemical etching of silicate glasses in hydrofluoric acid based solutions

Abstract: The etching of silicate glasses in aqueous hydrofluoric acid solutions is applied in many technological fields. In this review most of the aspects of the wet chemical etching process of silicate glasses are discussed. The mechanism of the dissolution reaction is governed by the adsorption of the two reactive species: HF and HF 2 - and the catalytic action of H+ ions, resulting in the breakage of the siloxane bonds in the silicate network. The etch rate is determined by the composition of the etchant as well as by the glass, although the mechanism of dissolution is not influenced. In the second part of this review, diverse applications of etching glass objects in technology are described. Etching of SiO2 and doped SiO2 thin films, studied extensively for integrated circuit technology, is discussed separately.

Erosion kinetics of hydrolytically degradable polymers.

Abstract: Degradable polymers are beginning to play an increasing role as materials for environmental and medical applications. Understanding factors that control erosion, such as bond cleavage and the dissolution and diffusion of degradation products, will be critical to the future development of these materials. Erosion kinetics, photomicroscopy, and infrared spectroscopy were used to understand the erosion mechanism of two families of degradable polymers, polyanhydrides and polyesters. Polyanhydrides exhibit behavior more characteristic of surface erosion, whereas the polyesters exhibit bulk erosion patterns. Control of erosion times from a few days to several years can be achieved by a judicious choice of monomer units and bond selection.

Mass transfer from nonaqueous phase organic liquids in water-saturated porous media

Abstract: Results of dissolution experiments with trapped nonaqueous phase liquids (NAPLs) are modeled by a mass transfer analysis. The model represents the NAPL as isolated spheres that shrink with dissolution and uses a mass transfer coefficient correlation reported in the literature for dissolving spherical solids. The model accounts for the reduced permeability of a region of residual NAPL relative to the permeability of the surrounding clean media that causes the flowing water to partially bypass the residual NAPL. The dissolution experiments with toluene alone and a benzene-toluene mixture were conducted in a water-saturated column of homogeneous glass beads over a range of Darcy velocities from 0.5 to 10 m d(-1). The model could represent the observed effluent concentrations as the NAPL underwent complete dissolution. The changing pressure drop across the column was predicted following an initial period of NAPL reconfiguration. The fitted NAPL sphere diameters of 0.15 to 0.40 cm are consistent with the size of NAPL ganglia observed by others and are the smallest at the largest flow velocity.

Formation of thermodynamically unstable solid solutions in the Cu-Co system by mechanical alloying.

Abstract: Cu-Co has been chosen as a model system to study phase formation during mechanical alloying in systems with components exhibiting a positive heat of mixing. Phase formation is monitored by scanning electron microscopy, x-ray diffraction, and calorimetric measurements. Although the occurrence of a single fcc Bragg pattern cannot definitely prove the alloy formation on an atomic scale, the quantitative agreement of the heat release upon decomposition of the alloy with the heat of mixing, calculated from the available thermodynamic data of the system, is regarded as clear evidence for the formation of an fcc solid solution. As a mechanism of alloy formation the dissolution of Cu and Co particles during milling is discussed. It is proposed that the formation of a homogeneous solid solution is energetically favored if the particle size is reduced below a critical value, which amounts to about 1\char21{}2 nm, depending on the composition of the alloy and the temperature. The driving force is provided by the chemical contribution of the Cu/Co interface enthalpy and by the high configurational entropy of the randomly distributed atoms in the solid solution.

Leaching and reconstruction at the surfaces of dissolving chain-silicate minerals

Abstract: THE pathways by which silicate minerals transform to solutes, clays and amorphous solids are relevant to a wide range of natural, industrial and even medical concerns. For example, weathered layers on silicate may have a high sorptive capacity, affecting nutrient and contamination retention in soils; less obviously, such layers on inhaled silicate grains might affect their interaction with lung tissue. Here we report the observation, in dissolution experiments on a range of chain-silicate minerals, of the formation of a near-surface amorphous region enriched in silicon and hydrogen, and depleted in other metals. Raman spectroscopy and ion-beam elemental analysis show that portions of the polymeric silicate anion in this region spontaneously reconstruct to form a network that contains four-member silicate rings and areas of incipient crystallization. If hydrolysable metals interact with the silicate anion during this reconstruction, clays and amorphous products may form directly. This process complements traditional dissolution-precipitation pathways of mineral diagenesis1, as the silicon does not have to be present in solution before being incorporated into a growing secondary phase.

Dynamics of small molecules in dense polymers subject to thermal motion

Abstract: A new transition‐state theory model has been proposed that is based on the concept that the dynamics of small molecules dissolved in dense polymers is coupled to the elastic thermal motion of dense polymers but can be treated separately from their structural relaxation. The model has been used to study the dynamics of light gases dissolved in atomistic microstructures of poly(isobutylene) and bisphenol‐A‐polycarbonate. Short‐time scale MD runs have been used to characterize the elastic thermal motion of the host matrix. This information on mobility is then used for a stochastic simulation of solute dynamics up to approximately 10 μs. Three time regimes have been observed: a short‐time, high‐mobility domain, a time domain of anomalous diffusion, and a diffusive regime at long times. From the long‐time data diffusion coefficients for He, H2, Ar, O2, and N2 have been estimated. Comparison with experimental data has resulted in satisfactory agreement indicating that the mechanisms of the motion of small gases...

Biogeochemical processes during the infiltration of river water into an alluvial aquifer

Abstract: Summary and Conclusions The chemical changes in the infiltration flow path from the Lot River to an adjacent aquifer were followed using chloride as an indicator of physical mixing of the river water with groundwater. The chemical reactivity close to the river is very significant with respect to the aquifer water composition (Figure 7). Two types of processes occur. A first series leads to irreversible reactions: weathering produces enriched Ca, Mg, bicarbonate, and Si concentrations, reductive reactions lead to denitrifi- cation, and adsorption reactions remove zinc from solution (11,22). A second series comprises processes which lead to the formation of the so-called reduced zone, and most of them are reversible. As long as microbial activity, which is very intense in the river sediments, consumes more oxygen than what is brought by the water coming from the river, the environment becomes reduced. This leads to dissolution of manganese oxides. Further along, when the microbial activity decreases, and when additional oxygen can be supplied by the permeable unsaturated zone, the water environment becomes oxidized again. Manga- nese is removed from solution by a combination of adsorption and surface oxidation reactions.

Coal tar dissolution in water-miscible solvents : experimental evaluation

Abstract: Coal tar, a dense nonaqueous phase liquid (NAPL), is a common subsurface contaminant at sites of former manufactured gas plants. A proposed remediation technology is water-miscible solvent extraction, which requires understanding of the effect of water-miscible solvents on the solubility of coal tar. This study investigated this effect and the extent to which multicomponent coal tar could be represented as a pseudocomponent in thermodynamic modeling. The coal tar used in this study showed a predominance of polycyclic aromatic hydrocarbons nvith no single compound accounting for more than 4% (nvt). The bulk solubility of coal tar in water was estimated to be 16 mg/L using composition data and Raoult's law assumption for aqueous solubility

Anomalous solubility behaviour of C60

Abstract: SOLUBILITIES of solids in liquids exhibit a temperature dependence that is correlated with the energetics of dissolution. For organic solutes in organic solvents the common experience is that solubility increases on warming (that is, dissolution is normally endothermic), forming the basis for purification by crystallization by slow cooling of hot solutions1. Interactions between inorganic compounds and water are often more energetic, and may lead to complicated temperature-dependent solubilities, often associated with the formation of hydrated solid phases2. We have investigated the temperature-dependent solubility of C60 in hexane, toluene and CS2. We observe a solubility maximum near room temperature (around 280 K) for all three solvents. Although the solubility of C60 in these three solvents differs by several orders of magnitude, the temperature dependence of the relative solubilities is much the same in each case. We conclude that dissolution is endothermic below room temperature and exothermic above. We interpret this change as being due to a phase change in solid C60 presumably the phase change observed previously in the absence of a solvent3–5, modified by solvent wetting. A solubility maximum (or minimum) for organic compounds in non-electrolytes is highly unusual, and may be unprecedented. The effect may have consequences for solvent-extraction techniques of fullerene purification.

Curvature effects in alloy dissolution

Abstract: An elemental metal can dissolve at kink sites at low over potentials producing no new interfacial area. When an ideal solid solution alloy undergoes selective dissolution, this situation is not possible owing to atomic-scale disorder. Dissolution of the less noble constituent can proceed only by injection of regions of negative curvature into the solid surface, which increases the interfacial area. The authors present a thermodynamic analysis which accounts for these capillary effects in alloy dissolution. The phenomenon of the critical potential for macroscopic selective dissolution is analyzed in terms of a kinetic roughening transition. This transition results from a competition between curvature-dependent dissolution and surface diffusion. An expression for the critical potential as a function of alloy composition is developed. The dealloying threshold corresponds to a critical composition on the line of critical potentials defining the roughening transition.

Adsorption of Metal Ions on Chitosan and Crosslinked Copper(II)-Complexed Chitosan

Abstract: A fundamental investigation on the adsorption behavior of metal ions on chitosan from aqueous ammonium nitrate and hydrochloric acid solutions was conducted by batch-wise examining the effects by various factors: the pH, concentrations of ammonium nitrate and hydrochloric acid, and initial metal concentration on the amount of metals adsorbed. In order to prevent the dissolution loss of chitosan into an acidic aqueous solution, crosslinked copper(II)-complexed chitosan was prepared and its adsorption behavior was compared with that of the original chitosan. The amount of adsorption, not only of metal ions, but also of the hydrogen ion, was decreased by crosslinking in the adsorption from an ammonium nitrate solution. The decrease, however, was least for copper(II), compared with the other divalent metal ions examined: oxovanadium(II), zinc(II), nickel(II), and cobalt(II). It was considered that this result may be attributable to the “template” effect by the copper(II) ion, which was employed during crossli...

Dissolution and Recrystallization in Modern Shelf Carbonates: Evidence from Pore Water and Solid Phase Chemistry: Discussion

Abstract: We present an overview of geochemical data from pore waters and solid phases that clarify earliest diagenetic processes affecting modern, shallow marine carbonate sediments. Acids produced by organic matter decomposition react rapidly with metastable carbonate minerals in pore waters to produce extensive syndepositional dissolution and recrystallization. Stoichiometric relations among pore water solutes suggest that dissolution is related to oxidation of H$\_{2}$S which can accumulate in these low-Fe sediments. Sulphide oxidation likely occurs by enhanced diffusion of O$\_{2}$ mediated by sulphide-oxidizing bacteria which colonize oxic/anoxic interfaces invaginating these intensely bioturbated sediments. Buffering of pore water stable isotopic compositions towards values of bulk sediment and rapid $^{45}$Ca exchange rates during sediment incubations demonstrate that carbonate recrystallization is a significant process. Comparison of average biogenic carbonate production rates with estimated rates of dissolution and recrystallization suggests that over half the gross production is dissolved and/or recrystallized. Thus isotopic and elemental composition of carbonate minerals can experience significant alteration during earliest burial driven by chemical exchange among carbonate minerals and decomposing organic matter. Temporal shifts in palaeo-ocean carbon isotope composition inferred from bulk-rocks may be seriously compromised by facies-dependent differences in dissolution and recrystallization rates.

Photoreduction of iron(III) in marine mineral aerosol solutions

Abstract: Although there have been a number of studies of the solubility of Fe in marine mineral aerosols, there have been few studies of the oxidation states of the soluble iron fraction and of the factors that affect the solubility of iron in aerosol solutions. In this paper we present measurements of the concentrations of total Fe (including particulate), total soluble Fe and total soluble Fe(II) in marine aerosol particles. Only 1% of the total Fe and 7.5% of the soluble Fe was in the Fe(II) form. Photolysis experiments were performed with solutions extracted from aerosol filter samples and with solutions of Fe(III) in acidic sodium chloride. In both systems, Fe(II) concentrations increased rapidly when the solutions were exposed to sunlight and they attained steady state within an hour. However, in all cases Fe(II) is only a minor component even when conditions are favorable for photolysis. Fe(II) formation is hindered at the low pH that is believed to be characteristic of marine aerosol solutions. Solutions with added oxalate yielded greatly increased concentrations of Fe(II); this is probably related to the fact that Fe(III) oxalate complexes have strong ligand to metal charge transfer bands in the tropospheric solar UV-visible region. However, the presence of oxalate also leads to the formation of H2O2; when the radiation level decreased, the Fe(II) was partially or totally oxidized back to Fe(III) due to reactions with H2O2. The photoreduction of Fe(III) to Fe(II) did not appear to significantly increase the dissolution of Fe(III) from the dust mineral matrix.

Reversible photoreductive deposition and oxidative dissolution of copper ions in titanium dioxide aqueous suspensions

Abstract: Particulate titanium dioxide was used to remove and concentrate Cu(II) ions in aqueous solutions through a cyclic process of photodeposition, separation, and oxidation. Illuminated, nitrogen-purged solutions containing copper sulfate, excess sodium formate (pH 3.6), and titanium dioxide formed a purple Cu-TiO[sub 2] species. Cu(II) concentrations in the supernatant were driven from 51 to [le] 0.018 [mu]g/mL. Upon purging with oxygen, this purple Cu-TiO[sub 2] system reverted back to white along with a corresponding increase in the Cu(II) supernatant concentration. The photodeposition step and the air oxidation step were utilized to demonstrate a volume reduction process. Eighty-six percent of the Cu(II) in a synthetic waste stream was concentrated to an organic-free solution having 7% of the initial volume. The remaining waste solution contained only 1% of the initial Cu(II), and in a subsequent step, the remaining formate ion was destroyed using conventional TiO[sub 2] photocatalytic oxidation. The overall process demonstrated the ability to separate copper ions from organics using only light and air. The reversible photoreduction deposition of Cu(II) from solution was observed in the pH range 1.84-6.60. The reversible photoreduction deposition of copper (II) was dependent on the organic used to scavenge holes and independent of the copper salt used.more » 33 refs., 4 figs., 4 tabs.« less