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

Abstract

Abstract The formation of X‐ray amorphous analogs of variscite and Namontebrasite was demonstrated to occur when Al 2 O 3 · n H 2 O, synthetic allophanes, and allophanic soils were reacted with sodium o ‐phosphates, Na 3‐ A H A PO 4 , of varying acidity A . The formation of the two amorphous Al‐phosphates, Al(OH) 2 H 2 PO 4 and AlOHNaPO 4 , respectively, was favored both by an increase in lability of Al (i.e., by large values of the hydration number n in Al 2 O 3 · n H 2 O 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 H 3 PO 4 reacted with synthetic allophane). The presence of amorphous Al(OH) 2 H 2 PO 4 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) 2 H 2 PO 4 formed directly from the reaction of Al‐hydroxy‐chlorides and o ‐phosphate. The presence of amorphous AlOHNaPO 4 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 AlOHNaPO 4 begins at pH ≃ 6 and ends with the appearance of Al(OH) 4 ‐ at pH ≃ 10.