A Rotating Disk Study of the Mechanisms of Calcite Dissolution in the Presence of Environmentally Benign Polyaspartic Acid

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Title: A Rotating Disk Study of the Mechanisms of Calcite Dissolution in the Presence of Environmentally Benign Polyaspartic Acid
Author: Burns, Kathie Lee
Advisors: Dr. John van Zanten, Committee Member
Dr. Saad Khan, Committee Member
Dr. Christine S. Grant, Committee Chair
Abstract: A rotating disk technique was used to investigate the mechanisms of calcite (CaCO3) dissolution using environmentally benign polyaspartic acid (PASP) under controlled hydrodynamic conditions. Additional techniques including scanning electron microscopy and dynamic light scattering explored the specific role of PASP in the dissolution process. Using this approach, rates of dissolution were evaluated as a function of pH, rotating speed, polymer concentration and molecular weight. In this research, it was determined that PASP is an effective dissolving agent for calcite mineral over a range of pHs (3.5-10.0), rotating speeds (150-1500 rpm), PASP concentrations (0.001-0.1M) and PASP molecular weights (3,000 and 10,000 Mw). An enhancement factor, ηenh, was developed to quantify the effect of PASP on dissolution behavior. It is defined as the rate of dissolution in PASP over the rate in water. Maximum enhancement was observed at pHs in the range of 4-5 for high concentrations and low molecular weights of PASP. Results demonstrate that dissolution is governed primarily by interfacial phenomena, including adsorption and surface reaction, at high pHs (>7), while limited chiefly by mass transport at low pHs (<7). Dissolution at high pHs proceeds via a surface complexation mechanism involving the chelation of calcium by PASP. At the high pHs, dissolution is inhibited by small amounts of PASP (0.001-0.01M) and enhanced by large quantities (0.1M) of PASP. In contrast, at low pHs, dissolution occurs predominantly by acid attack, or the reaction of hydrogen ion with calcite. At low pHs, PASP enhances dissolution over the entire concentration range (0.001-0.1M). For the two molecular weights studied, the lower molecular weight (3,000) is the most efficient dissolving agent at low pH, while both molecular weights dissolve calcite at comparable rates at high pH. Finally, a model was developed based on fundamental calcite and sequestration chemistry to predict the dissolution kinetics of calcite in the presence of PASP at pHs above 7. The model agrees closely with experimental dissolution rates at pH 10 and shows that the water reaction with calcite dominates dissolution at low PASP concentrations while the PASP ligand reaction with calcite is the primary interfacial reaction at high PASP concentrations.
Date: 2002-09-04
Degree: MS
Discipline: Chemical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/126

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