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Reactions of Aluminosilicates, Aluminum Hydrous Oxides, and Aluminum Oxide with o-Phosphate: The Formation of X-ray Amorphous Analogs of Variscite and Montebrasite¹

ABSTRACT

The formation of X-ray amorphous analogs of variscite and Namontebrasite was demonstrated to occur when Al₂O₃ · nH₂O, synthetic allophanes, and allophanic soils were reacted with sodium o-phosphates, Na_3-AH_APO₄, of varying acidity A. The formation of the two amorphous Al-phosphates, Al(OH)₂H₂PO₄ and AlOHNaPO₄, respectively, was favored both by an increase in lability of Al (i.e., by large values of the hydration number n in Al₂O₃·nH₂O or the presence of Si-O-Al bonds in the aluminosilicates) and by an increase in the acidity A of the added o-phosphate solution.

Evidence for the formation of amorphous Al-phosphates by secondary precipitation rather than the formation of a surface o-phosphate phase by adsorption was indicated by the slowness of the reaction between o-phosphate and Al-containing material, by the immediate and significant increase of Si in solution when aluminosilicates were reacted, and by the large amounts of P reacted (e.g., 82% of the P added as equimolar H₃PO₄ reacted with synthetic allophane). The presence of amorphous Al(OH)₂H₂PO₄ in the products was demonstrated by showing that a solid phase, Al-P compound was formed in all of the reactions studied that had the same pH stability limites (2.5 pH 9) and solubility product pK_so (28.1 ± 0.1) as were observed in a previous investigation for amorphous Al(OH)₂H₂PO₄ formed directly from the reaction of Al-hydroxy-chlorides and o-phosphate. The presence of amorphous AlOHNaPO₄ in the products was inferred from an analysis of their formation curves (P in a solid phase vs. pH) and from measurements of the amount of Na bound with Al and P, corrected for adsorption of Na, as a function of pH. The formation of the amorphous, basic, Na-containing AlOHNaPO₄ begins at pH 6 and ends with the appearance of Al(OH)₄^- at pH 10.

¹ Contribution from the Institut für Ökologie-Bodenkunde-der Technischen Universität, Berlin, Engler-Allee 19–21, 1 Berlin 33, West Germany, and the Dep. of Soil and Environ. Sciences, Univ. of California, Riverside, CA 92521.

² Soil Scientist and Associate Professor of Soil Science, respectively. Present address of the senior author: Institut für Anorganische Chemie der Freien Universität Berlin, Fabeckstrasse 34/36, 1 Berlin 33, West Germany.

Received for publication December 19, 1975. Accepted for publication May 11, 1977.

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