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Effects of surface chemistry on kinetics of coagulation of submicron iron oxide particles (α-Fe_2O_3) in water

Liang, Liyuan (1988) Effects of surface chemistry on kinetics of coagulation of submicron iron oxide particles (α-Fe_2O_3) in water. California Institute of Technology , Pasadena, CA. (Unpublished)

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Particles in the colloidal size range, i.e. smaller than 10^(-6) meter, are of interest in environmental science and many other fields of science and engineering. Since aqueous oxide particles have high specific surface areas they adsorb ions and molecules from water, and may remain stable in the aqueous phase with respect to coagulation. Submicron particles collide as a result of their thermal energy, and the effective collision rate is slowed by electric repulsion forces. A key to understanding particle stability and coagulation is the role of simple chemical changes in the water altering the electrostatic repulsion forces between particles. Experiments using hematite particles (α-Fe_2O_3, 70nm in diameter) reveal important features of coagulation dynamics. Three experimental techniques are employed: (1) Light scattering measurements to yield quantitative information on the rate of the initial coagulation process; (2) electrokinetic measurements to provide information about the sign and magnitude of the electrical charge on the aqueous oxide particles; (3) acid-base titration and equilibrium adsorption to obtain the intrinsic equilibrium constants for surface species. The acid-base titration data indicate that the pH_(zpc) of the synthesized hematite colloid is 8.5. This is also supported by the electrophoretic mobility measurements. In the presence of non-specific adsorbing ions (such as Na^+ and Ca^(2+), etc.), the coagulation of a hematite colloid is achieved mainly by compression of the diffuse layer and Schulze-Hardy Rule is exhibited for non-specific electrolytes. Specifically adsorbed counter ions (such as phosphate) are able to reduce the surface charge of aqueous oxide particles, and the critical coagulation concentrations are dependent on the value of the pH, and are much less than those predicted by DLVO theory. In inorganic media, we found that the order of the effectiveness in causing hematite particles to coagulate is: phosphate>sulfate>chloride at pH<pH_(zpc) and magnesium>calcium>sodium~potassium at pH>pH_(zpc) The adsorption study reveals that phthalate ions specifically adsorb on hematite particles. The process is most likely due to carboxylic group bonding to the surface. Hematite coagulation rates in the presence of poly-aspartic acid (PAA) demonstrate that the polyelectrolyte is very effective in causing the colloid to coagulate. When the PAA concentration is increased beyond the critical coagulation concentration, the particles are stabilized; this is attributed to the reversal of surface potential as a result of the adsorption of PAA. Similar features are observed in the initial coagulation rates when naturally occurring organics (fulvic and humic acid from Suwannee River) are used. The adsorption of lauric acid on hematite was investigated and the results interpreted in terms of the energy contributed by the specific chemical, electrostatic and hydrophobic interactions. The initial coagulation rates of hematite particles and the electrophoretic mobilities with respect to fatty acid concentration both show systematic variations as a function of the numbers of carbons in the acid. Hydrophobic interaction may account for these observations since the specific chemical energy appears to be the same for all the fatty acids studied, and the electrostatic contribution is also similar at the same extent of adsorption.

Item Type:Report or Paper (Technical Report)
Additional Information:I would like to express my appreciation for the help and fellowship I have received from my friends and colleagues during the course of my research. I am particularly grateful to Professor J. J. Morgan for his guidance, help and tolerance, and for introducing me to the world of surface chemistry. I would also like to thank Professor Norman Brooks for his encouragement, and the other members of my examination committee: Professors M. Hoffmann, R. Flagan, and G. Rossman. Professor J. Westall of Oregon State University and Professor F. M. M. Morel of MIT provided helpful insights into the application of surface chemical models. Professors W. Stumm of EAWAG, P. Gschwend of MIT, and C. O'Melia of Johns Hopkins University also gave useful suggestions. I would like to thank the following people. Simon Davies for helping me to start my laboratory work; Sue Larson for introducing me to the use of Mie theory; Bill Munger for assisting me in using the IC and TOC instruments; Scott Northrop and Ranajit Sahu for making the BET measurements. Elton Daly, Joe Fontana and Rich Eastvedt played an invaluable role in making effective instruments according to the vaguest specifications. Rueen-Fang Wang helped me get familiar with the idiosyncrasies of the Keck computers, and made numerous helpful suggestions throughout my research. Her kindness and friendship provided vital support throughout the many vicissitudes of my work. The following people have aided my research, either by giving specific help, or by generally improving the quality of life in Keck Laboratory, and are much appreciated: Elaine Granger, Joan Matthews, Rayma Harison, Gunilla Hastrup, Bob Koh, Sandy Brooks, Chi Kin Ting, Imad Hannoun, Pratim Biswas, Dennis Lyn, Kit Yin Ng, David James, Terri Olson and Michael Scott. I am grateful for the financial support provided by a fellowship from Jessie Smith Noyes, Inc., and for funding from the Mellon Foundation. I would like to thank my parents whose spiritual support was always apparent in spite of the great distance separating us across the Pacific Ocean. Finally, I would like to dedicate this thesis to my husband, David, who has supported me with love, patience, and good humor throughout my work. This report was submitted to the California Institute of Technology in May 1988 as a thesis in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering Science.
Group:W. M. Keck Laboratory of Hydraulics and Water Resources
Record Number:CaltechKHR:AC-5-88
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ID Code:26000
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Deposited On:09 Dec 2009
Last Modified:03 Oct 2019 03:10

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