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Lyotropic ion effects in guar gum adsorption on various minerals Ma, Xiaodong

Abstract

Adsorption of guar gum was studied on a number of oxide and clay minerals. The tests were performed as a function of salt concentration and pH from solutions of lithium, sodium, potassium, and cesium chlorides. The four salts allowed an assessment to be made of the significance of lyotropic ion phenomena in controlling the adsorption of the polysaccharide. The results showed that the adsorption of the polymer was independent of pH and acidity/basicity of the minerals. Therefore, no evidence of acid-base type of chemical interactions was observed and hydrogen bonding was suggested as the adsorption mechanism. It was also observed that the adsorption of guar gum on quartz, kaolinite and illite proceeded differently from lithium and sodium chloride solutions in comparison to potassium and cesium chlorides. In contrast, no significant effect of salt type and concentration was observed in guar gum adsorption on titania, hematite, and alumina. It was postulated that the presence or absence of an extensive hydration layer at the mineral-solution interface was the dominant factor and that the interfacial water created a barrier against guar gum adsorption. Therefore, the role of ions of a given lyotropic series is to destabilize the interfacial water structure and promote guar gum adsorption. On the other hand, the absence of an extensive hydration layer on titania, alumina, and hematite allowed guar gum to freely interact with the surfaces and thus no lyotropic ion effect was observed. Therefore, lyotropic ion effects are very strong only in the case of strongly hydrated quartz and, apparently, clay minerals. It was also shown that guar gum undergoes extensive aggregation in concentrated solutions of kosmotropic salts (LiCl and NaCl) while chaotropic potassium and cesium chlorides turned out to be very powerful solvents for the polymer. As a result, it can be postulated that individual guar gum molecules adsorb on minerals from concentrated KCl solutions, while entire guar gum aggregates adsorb from concentrated NaCl solutions. Such findings are of particular interest to potash flotation which is carried out in saturated salt solutions. It was stressed that any analysis of the adsorption results using traditional approaches and models must be performed with extreme caution since guar gum solutions are inherently heterogeneous due to the presence of undissolved, colloidal polymer aggregates.

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