Showing papers by "Michael R. Hoffmann published in 1981"

Characterization of soluble and colloidal phase metal complexes in river water by ultrafiltration. A mass-balance approach.

Abstract: Ultrafiltration, anodic stripping voltammetry, and atomic absorption spectrophotometry have been used to determine the distribution and degree of association of major trace metals with discrete size fractions of dissolved organic matter which passes initially through a 0.4-pm membrane filter. A novel mass-balance approach has been developed for fractionation of operationally defined, dissolved organic matter and organometallic complexes into well-defined molecularsize ranges. The highest concentrations of trace metals were found in intermediate molecular weight fractions ( 10^3-10^4) with no detectable free or labile metal as determined by anodic stripping voltammetry.

Kinetics of the removal of iron pyrite from coal by microbial catalysis.

Abstract: Different strains of Thiobacillus ferrooxidans and Thiobacillus thiooxidans were used to catalyze the oxidative dissolution of iron pyrite, FeS2, in nine different coal samples. Kinetic variables and parametric factors that were determined to have a pronounced effect on the rate and extent of oxidative dissolution at a fixed Po2 were: the bacterial strain, the nitrogen/phosphorus molar ratio, the partial pressure of CO2, the coal source, and the total reactive surface area of FeS2. The overall rate of leaching, which exhibited a first-order dependence on the total surface area of FeS2, was analyzed mathematically in terms of the sum of a biochemical rate, {nu}1, and a chemical rate, {nu}2. Results of this study show that bacterial desulfurization (90 to 98%) of coal samples which are relatively high in pyritic sulfur can be achieved within a time-frame of 8 to 12 days when pulp densities are ≤20% and particle sizes are ≤74 µm. The most effective strains of T. ferrooxidans were those that were isolated from natural systems, and T. ferrooxidans ATCC 19859 was the most effective pure strain. The most effective nutrient media contained relatively low phosphate concentrations, with an optimal N/P molar ratio of 90:1. These results suggest that minimal nutrient additions may be required for a commercial desulfurization process.

Thermodynamic, kinetic, and extrathermodynamic considerations in the development of equilibrium models for aquatic systems.

Abstract: Thermodynamic and kinetic criteria are examined for the general application of equilibrium models to dynamic natural waters. The hypolimnion of a dimictic lake is used as a specific example. The majority of chemical reactions given consideration in equilibrium models are shown to be kinetically rapid in comparison to the chemical residence times in hypolimnetic waters. Extrathermodynamic relationships are explored as a method for extrapolating kinetic and thermodynamic constants from a limited set of experimental data.

Development of a computer-generated equilibrium model for the variation of iron and manganese in the hypolimnion of Lake Mendota.

Abstract: A computer-generated chemical equilibrium model has been developed to predict the unusual seasonal variation of iron and manganese in the hypolimnetic water above the sediments in Lake Mendota. Models for the detailed chemical speciation of Mn and Fe have been developed as functions of pH, redox level, adsorption, and organic complexation. Results indicate that pH is the primary variable controlling the preferential release of Mn due to simultaneous pH dependent dissolution of solid MnCO_3 and desorption of Mn(II) from oxide surfaces. Progressive appearance of Fe is predicted in terms of a pH-dependent dissolution of iron oxides as influenced by organic complexation. Using the maximum observed concentrations of major chemical components as input data, we have reproduced experimentally determined variations of Mn and Fe as a function of pH over the range 8.7-6.9.