Removal of metal ions from dilute aqueous solutions by ion flotation

Ion flotation is a separation technique that allows economical and efficient removal of toxic metal ions from dilute aqueous solutions. Metal ion removal kinetics and selectivity are two key issues in better understanding the ion flotation process. These are systematically studied in this dissertation from a thermodynamic perspective. It was found that the adsorption of surfactants at the vapor/aqueous interface reached equilibrium very rapidly. Typically, it took about 0.3 to 8 seconds to establish equilibrium adsorption of sodium dodecylsulfate (SDS), sodium tetradecylsulfate ( STS) and sodium hexadecylsulfate (SHS), with longer times required for longer hydrocarbon chain surfactants. The equilibrium adsorption densities predicted from surface tension data were incorporated into a mass transfer kinetic equation, which was able to quantitatively predict the experimentally observed metal ion removal kinetics in alkylsulfate- Cu2+ systems.; It was shown that surface tension data could also be used to predict the selectivity order for different metal ions, but the selectivity coefficients calculated this way deviated considerably from the experimental values. A different thermodynamic selectivity model was developed, based on the Grahame adsorption equation taking into account the electrical, chelating, hydrophobic and dehydration terms of the overall Gibbs free energy for adsorption. Additional studies were then done to explore the effect of each of these terms on the adsorption behavior of metal ions at a solution/vapor interface. The effect of the Gibbs free energy for dehydration during adsorption on the selectivity between metal ions was studied using three different approaches. The Gibbs free energy for dehydration was calculated theoretically by defining a fractional dehydration parameter using a geometrical analysis. The selectivity coefficients estimated this way were consistent with the experimental values. The hydrophobic interaction between metal-triethylenetetraamine (Trien) complexes at the solution/vapor interface was examined. Finally, the chelating interaction between collectors and colligends was studied by using a chelating surfactant, 3 dodecyldiethylenetriamine (Ddien). The results were qualitatively explained by speciation diagrams for analogous diethylenetriamine ( Dien)metal ion systems, invoking the fact that metal-(Ddien )₂ complexes are much more surface active than metal- Ddien species. Surface tension measurements for the Ddien -metal ion systems at various pH values provided further evidence that different surface-active species could be produced at different pH ranges. (Abstract shortened by UMI.)...