BIOLOGICAL EXTRACTION OF METALS: THE RATE-CONTROLLING STEP

Although an enormous amount of work has been done in the area of biohydrometalJurgy, the influence of the inert solid matrix on metal extraction, and consequently the rate-controlling mechanism have not been fully understood. Actually, most studies have mainly focused on the bioextraction of metals from high-grade mineral concentrates. Such an approach is important in elucidating the diffusion and reaction phenomena that are taking place on the surface of the mineral. In reality, however, the mineral is embedded within an inert solid matrix which will have significant effects on diffusion and chemical reaction rate processes. In this work, a detailed experimental kinetic study was conducted to incorporate this factor in an attempt to quantify the rate-controlling mechanism involved. The biological dissolution of pyrite in the presence of acidophilic Thiobacillus ferrooxidans was investigated in a batch experiments on three distinct mine waste rocks. The experiments were conducted on two particle sizes and at three operating temperatures. Based on particle size and activation energies results, it was found that intraparticle mass transfer resistances were solely the rate-controlling mechanism for this process This new finding is different from that reported by other investigators who underestimated the importance of intraparticle mass transfer resistances. Also, because similar processes are ubiquitous in nature, this work should have broad applications.     

Assessment of degree of fusion of rigid PVC from inverse gas chromatographic measurements

Inverse gas chromatography of alkanes has been used to assess the degree of fusion of rigid polyvinyl chloride (PVC) extrudates. The relative degree of fusion is provided by ratios of the retention volumes of the alkane probe molecules in the bulk of the PVC. This method is nonintrusive and can be used to measure degree of fusion quantitatively if a completely fused reference material can be provided. The present analysis involves evaluations of the contributions to the overall retention volume of the alkane probes, resulting from adsorption and from bulk diffusion. The diffusivity of the alkanes was clearly dependent on the processing history of the PVC extrudate. A direct variation was observed between the size of the diffusing molecules and their residence time as adsorbates on the polymer surface. © 1995 John Wiley & Sons, Inc.     

Gel permeation chromatographic analyses of resole phenolic resins

Tetrahydrofuran solutions of resole polymers were analyzed by gel permeation chromatography (GPC) on crosslinked polystyrene gel packings. The best separation was obtained with low solvent flow rates in low porosity columns. Irregular elution volumes were observed, but the effects of this erratic behavior can be eliminated by referencing retention times to that of a marker compound such as benzene or phenol. A calibration and data analysis method are presented which utilizes hydrodynamic volumes. Phenolic polymers vary in shape and ability to form hydrogen bonds with solvent; hence their molecular weights cannot be estimated from GPC data. Separation of the constituent species of resole samples is shown to be incomplete, because of aggregation between the various phenol derivatives. Particular peaks in the GPC chromatogram could generally not be assigned to individual species. Despite these limitations, GPC is a useful tool for charcterizing phenolics, and several applications are reviewed here.     

Formation and magnetic properties of Fe-B-Si metallic glasses

The equilibrium solidification behaviour of Fe-rich Fe-B-Si alloys has been characterized in terms of primary and secondary crystallization, and correlated to metallic glass formation and properties. A eutectic trough extends from the binary eutectic on the Fe-B edge of the ternary composition diagram toward the binary eutectic on the Fe-Si edge. Metallic glass forming compositions closely follow the path of the eutectic trough, extending from the Fe-B edge to compositions containing approximately 20 at% Si. The relative ease of glass formation and the thermal stability of the glassy state are maximized at compositions along the eutectic trough. The magnetic coercive forces of both as-cast and annealed metallic glasses are minimized along the eutectic trough.     

The scaling of iron in oxygen and sulfur-containing gases

The following possible conditions for the simultaneous formation of oxide and sulfide on iron in oxygen and sulfur-containing gases are considered: (7) simultaneous thermodynamic stability of oxide and sulfide; (2) temporal changes in gas composition; (3) intermediary cracking of the oxide film; (4) solubility and diffusion of sulfur in the oxide (lattice diffusion); (5) kinetic conditions for simultaneous oxide and sulfide formation at the scale-gas interface.     

Enzymic browning in green olives and its prevention

The browning of green olives, which results from mechanical injury, was followed by measuring the reflectance from the fruit surface at 545 nm. The process is enzymic, apparently catalysed by catechol oxidase. It could not be prevented by inhibitors of the enzyme or by reducing agents, due to the impermeability of the whole fruit to these compounds. Dipping the fruit in 0.4 % NaOH prevented the formation of brown spots following mechanical injury.     

The World's Great Lakes

Most of the large lakes lie in glacial scour basins in the northern hemisphere. These lakes are closely similar in physicochemical characteristics and in their biota. Most of the other large lakes are tectonic in origin and they differ greatly among themselves and from other lakes since they occur in a diversity of terrestrial environments under a broad range of climatic conditions. Large lakes have a great diversity of habitats resulting in great species diversity and endemism in ancient lakes. The pronounced horizontal gradients in physicochemical conditions in large lakes contribute to the diversity of habitats. Conditions found in Lakes Michigan and Skadar are examples.