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In this paper, the precipitation-redissolution behavior of A1C13 and the dependence of it on pH in sodium dodecyl benzenesulfonate and sodium dodecanesulfonate solutions are discussed. As in the case of bivalent metal ion-sulfonate systems, the redissolution is found to be caused by micelles. In contrast to the bivalent metal ion systems, precipitation in AlCl3-sulfonate solutions is influenced markedly by pH. The results indicate that the competition among sulfonate monomers, micelles, and hydroxyl ions for Al species controls the system behavior. The thermodynamic solubility product for the precipitation of aluminum trisulfonates and the dependence of onset of redissolution on A1C13 concentration have been determined.

A series of fixed and fluidised bed ion exchange column runs were conducted to identify the ability of natural clay minerals, sepiolite and clinoptilolite, to remove ammonia from a contaminated drinking water reservoir. Ion exchange column tests using both fixed and fluidised bed were initially carried out with synthetic water composed of NH4Cl. Breakthrough curves as a function of flow rate, particle size, and initial ammonia concentration reveal that sepiolite does not have as high ion exchange capacity as clinoptilolite but maintains a steady adsorption up to higher bed volumes. The adsorption capacity was modified by using regeneration cycles at both acidic and alkaline pH. Furthermore, fluidised bed runs with clinoptilolite utilising water and air as fluidiser resulted in inferior results compared to those of fixed bed runs. This was respectively ascribed to the presence of ammonia in the circulating water and competition of exchangeable ions released in water and the ability of air to adsorb nitrogen. Tests conducted with natural raw water contaminated with sewage indicated that clinoptilolite adsorbs ammonia the same as the synthetic water. Regenerated clinoptilolite is capable of removing ammonia from both synthetic and actual raw water at a much higher rate than the untreated clinoptilolite.

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The presence of maxima and sometimes minima in the adsorption isotherms of surfactant on minerals has been attributed to various mechanisms involving micelle exclusion, impurities, surfactant composition, mineral morphology, etc. This study on the precipitation of sulfonates with various inorganic ions (Na, K, NH 4 , Ca, Mg, and AI) and the subsequent redissolution when the surfactant concentration is increased shows a precipitation maximum to occur in the same surfactant concentration range where an adsorption maximum is obtained. Simultaneous abstraction, precipitation, and adsorption experiments have been conducted using chromatographically purified dodecylbenzenesulfonate and Nakaolinite, and the results show the "abstraction vs. surfactant concentration" curves to exhibit a much more pronounced maximum than the real adsorption (abstraction minus precipitation) maximum. We suggest that the total abstraction of surfactant from the solution on contact with mineral phase, usually called "adsorption," is a summation of both the "real" adsorption and precipitation and that the precipitationiredissolution phenomenon is one of the major reasons for the adsorption maxima. Introduction of oil into the system reduces the precipitation because of partitioning of the sulfonate between the oil and aqueous phases. Precipitation and adsorption data are used to derive possible mechanisms for the precipitationiredissolution phenomena.

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