Study of an exchange reaction between an anion and hydroxyl groups in the soils or clay minerals by isotope tracer method Public Deposited

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  • The mechanism of phosphate adsorption by clay minerals was reported by Stout. He showed that the loss of water of kaolinite in the presence of KC1, K₂SO₄ or KH₂PO₄ was due to an exchange reaction between anion and hydroxyl groups from clay minerals. Also, Kelly and Midgley reported that the increase in pH of a colloidal system caused by phosphate fixation is taken as evidence for the exchange of phosphate ions with hydroxyl ions. Thomas found that the amount of sulfate which is held by an acid red soil increased with time and later Chao, Harward and Fang reported the soil constituents and properties on the adsorption of sulfate ions. A mechanism for sulfate adsorption by soil was proposed by Chang and Thomas. They reported that the adsorption of sulfate was due to the exchange reaction with hydroxyl ions on the soil. An attempt was made to tritiate the soils and clay minerals. It is believed that if the hydrogen of the hydroxyl ions in the soil or clay minerals can be labeled with tritium, then the exchange reaction between hydroxyl ions of soils or clay-minerals and anion may be measured directly from the tritium activity in the solution phase. The radioactivity in aqueous solutions was measured with a Packard Tri-Carb Liquid Scintillation Counter. Tritium counts were obtained at high voltage tap 7 and the amount of tritium released frorn one gram of tritiated soil after shaking with salt solutions of varying concentration is reported. In order to determine the nature of tritium in soil after an exchange reaction, aliquots of tritiated Aiken soil were heated separately at various temperatures (110-800°C) for one hour. The tritium activity which remained in the soil sample could be released by shaking the soil either with water or with salt solutions. This proved definitely the labeling of tritium in the OH group of soil-clay minerals. By the equilibration procedure and using double isotope technique the amount of sulfate or chloride adsorbed by the soil and tritium released was directly determined in the single sample. Results from radio salt showed that the tritium release was unchanged, but a positive adsorption for sulfate for both tritiated Aiken and Willamette soils was observed. The magnitude of sulfate adsorption increased with the increase of sulfate concentration. With a more concentrated solution, it did not induce an additional OH³ release of tritiated soils. Hence, the mechanism of exchange of sulfate adsorption and hydroxyl group in soil clay mineral as theorized by Chang and Thomas cannot be applied here. When the tritium activity is high in the form of hygroscopic or combined water, then the tritium activity in the hydroxyl ion released by an exchange with the adsorbed anion may not be large enough to be detected. In the chromatographic procedure, all hygroscopic and combined tritiated water was completely removed before the addition of 0.2 N salt solution. For this purpose water was percolated through the column and two mls increments of the effluent solution were colIected, The tritium activity in each increment was determined. In general, 30 to 35 mls of water were required for the column to reach a steady state. At this time, the salt solution was added and tritium release was measured. Results from chromatographic method using 0.2 N KH₂PO₄, K₂SO₄ or KC1 solutions did not support the concept of exchange of hydroxyl ions from clay minerals and anion.
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