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Abstract

Phosphate adsorption by noncrystalline aluminosilicate gels and their mixtures with kaolin and bentonite was studied. The infrared and ion-exchange data indicated that the noncrystalline gels were chemically bonded to the crystalline minerals. The noncrystalline aluminosilicates showed a high phosphate adsorption capacity which increased with increasing Al in the gels. Phosphate adsorption was initially fast due to adsorption on Al-OH polymer surfaces and then slowed due to the breakdown of Al-OH polymers and/or displacement of structural silicate by phosphate groups. Similar studies of the claygel mixtures showed that noncrystalline material played a dominant role in phosphate adsorption by increasing the adsorption capacity by 8.5 and 3.8 times for kaolin and bentonite, respectively. Similar adsorption capacities of both clay-gel mixtures indicated that the phosphate reactive sites on the crystalline minerals appeared to be blocked by the noncrystalline material. In these mixtures phosphate adsorption appeared to be mainly on the noncrystalline aluminosilicate gels.

Fluoride ion was sorbed by the noncrystalline gels in a manner similar to that for the phosphate ion. The amount of hydroxyls released during the fluoride reaction varied linearly with the amount of phosphate adsorbed by the systems. This relationship may be useful in characterizing soils for their content of noncrystalline material and phosphate adsorption capacity.

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