Surface evolution of aluminosilicate glass fibers during dissolution: Influence of pH, solid-to-solution ratio and organic treatment.

Materials made of synthetic vitreous mineral fibers, such as stone wool, are widely used in construction, in functional composites and as thermal and acoustic insulation. Chemical stability is an important parameter in assessing long term durability of the products. Stability is determined by fiber resistivity to dissolution, where the controlling parameters are solid surface area to solution volume ratio (S/V), pH and composition of the fibers and organic compounds used as binders. We investigated stone wool dissolution under flow through conditions, far from equilibrium, at pH range of 2 to 13, as well as under batch conditions, close to equilibrium, for up to 28 days, where S/V ranged from 100 to 10000 m-1. The dissolution rate of stone wool shows minimum at pH 8.5 and increases significantly at pH < 4.5 and pH > 12. In close to equilibrium conditions, S/V defines the steady state concentration for the leached components. Decreased dissolution rate could result from evolution of a surface leached layer or the formation of secondary surface phases or both. We suggested three dissolution rate controlling mechanisms, which depend on pH. That is, dissolution is controlled by: a SiO2 rich surface layer at pH < 4.5; by adsorption of an Al and Al-Si mixed surface layer at 5 < pH < 11 and by divalent cation adsorption and formation of secondary phases (silicates, hydroxides) at pH âˆ¼ 13. The organic compounds, used to treat the stone wool fibers during manufacture, had no influence on their dissolution properties.

Surface Reactivity and Dissolution Properties of Alumina-Silica Glasses and Fibers.

The ability of bulk glass and fibers to react in aqueous solution, with organic polymers and coupling agents, depends on the surface charge, reactivity, and adsorption properties of the glass surface, i.e. the character and density of surface -OH groups, whereas glass and fiber chemical stability and biosolubility depend on the resistance to dissolution. If glass dissolution products are accumulated in a media, they can change the surface properties by specific adsorption. We determined the -OH surface concentration, reactivity, adsorption, and dissolution properties of aluminosilicate glasses containing various modifiers and compared the results with the behavior of complex mineral wool fibers. Using proton consumption and element release from batch surface titration experiments, over the range 5 < pH < 10, surface -OH adsorption properties were modeled with the FITEQL program. During titration, network modifiers in the glass subsurface are preferentially replaced by protons, resulting in cation accumulation in the solution and formation of a leached layer enriched with Si on the solid. The behavior of Al was different. At 5 < pH < 9, only very small amounts of Al were found in the leachates, which can be explained by almost complete Al adsorption as stable surface complexes, i.e. >XOAl(OH)2 (where X = Si or Al and > represents the surface). At pH > 9, divalent cations adsorbed specifically, as >XOMe+ complexes (Me = Ca or Mg). This deeper understanding of the surface behavior of glasses and fibers is important for the design of composite materials, for applications in biology and medicine and in materials production in general, as well as for understanding natural processes, such as global uptake estimates of CO2 during rock weathering.

Nickel adsorption on chalk and calcite.

Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption studies. The goal was to understand the difference in sorption behavior between synthetic and biogenic calcite. Experiments at atmospheric partial pressure of CO2, in solutions equilibrated with calcite and chalk and pH ranging from 7.7 to 8.8, explored the influence of initial concentration and the amount and type of sorbent on Ni uptake. Adsorption increases with increased surface area and pH. A surface complexation model describes the data well. Stability constants for the Ni surface complex are log KNi=-1.12 on calcite and log KNi=-0.43 and -0.50 on the two chalk samples. The study confirms that synthetic calcite and chalk both take up nickel, but Ni binds more strongly on the biogenic calcite than on inorganically precipitated, synthetic powder, because of the presence of trace amounts of polysaccharides and clay nanoparticles on the chalk surface.

Eu3+ uptake by calcite: preliminary results from coprecipitation experiments and observations with surface-sensitive techniques.

A lack of information in databases for contamination risk assessment about the transport behaviour of the trivalent f-orbital elements in groundwater systems where calcite is at equilibrium motivated this study of Eu(3+) uptake. The free drift technique was used to examine the effects of Eu(3+) concentration, presence of Na(+) or K(+) and temperature, as well as calcite nucleation and precipitation kinetics, on the partitioning of calcite. Changes in surface composition and morphology resulting from exposure of single crystals of Iceland spar to Eu(3+)-bearing solutions were observed with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). First results confirm that calcite has high affinity for Eu(3+). Rates of nucleation and precipitation strongly affect the extent of uptake but the presence of Na(+) and K(+) has no effect, suggesting formation of solid solution as CaCO(3)-EuOHCO(3). Surface-sensitive techniques prove that Eu(3+) is adsorbed to calcite even when the surface is dissolving and adsorption is not accompanied by precipitation of a separate Eu(3+)-solid phase. Adsorbed Eu modifies calcite's dissolution behaviour, roughening terraces and rounding step edges, and producing surface morphology where some surface sites appear blocked. Results imply that Eu(3+) concentrations in natural calcites are limited by Eu(3+) availability rather than by a lack of ability to fit into calcite's atomic structure. This behaviour can probably be expected for other trivalent rare Earth elements (REE), actinides and fission products whose behaviour is similar to that of Eu(3+). These elements are likely to be incorporated within the calcite bulk in systems where it is precipitating and the demonstrated strong partitioning ensures some uptake even where calcite is at or under saturation.