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

Abstract

AbstractThe 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, Na3‐AHAPO4, 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 pKso (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.