| Particles in the colloidal size range 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 ({dollar}alpha{dollar}-Fe{dollar}sb2{dollar}O{dollar}sb3{dollar}, 70nm in diameter) reveal important features of coagulation dynamics. The acid-base titration data indicate that the pH{dollar}sb{lcub}rm zpc{rcub}{dollar} of the synthesized hematite colloid is 8.5, which is also supported by the electrophoretic mobility measurements. In the presence of non-specifically adsorbed ions (such as Na{dollar}sp+{dollar} and Ca{dollar}sp{lcub}2+{rcub}{dollar}), the coagulation of a hematite colloid is achieved mainly by compression of the diffuse layer. 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.; The adsorption study reveals that phthalate ions specifically adsorb on hematite particles through carboxylic group bonding to the surface. Hematite coagulation rates in the presence of polyaspartic 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 fulvic and humic acid 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. |
