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

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, 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.

Interaction between phosphate and acid-activated neutralized red mud during adsorption process

“…Therefore, we could conclude that the Al(OH) 3 reacted with phosphate by the precipitation mechanism. The result was consistent with that from Veith and Sposito [42], who proposed that the precipitate formed during the phosphate adsorption onto Al(OH) 3 was an amorphous analog of variscite. Lookman et al [43] found the formation of an amorphous aluminum phosphate phase on the Al(OH) 3 surface through 27 Al and 31 P solid-state MAS NMR.…”

Effects of soil organic matter on pH-dependent phosphate sorption by soils

“…This AI-P gel is also observed when P is applied to clay fractions of pumice (Nanzyo 1988) and allophanic Andisols (Nanzyo 1987). Veith and Sposito (1977b) suggested the formation of X-ray amorphous analogs of variscite (Al(OH)2H2P04) and montebrasite (AIOHNaP0 4 ) when AIP3·nHp, synthetic allophanes, and allophanic soils reacted with P. Although the formation of surface complexes (adsorption) and precipitates should be differentiated, both reactions (sorption) are likely to be based on the same kind of ligand exchange reaction of aquo or hydroxo ions on Al and / or Fe with P. These P sorption characteristics might be basically identical among all forms of Al and Fe in soils. However, co-existing SOM in soils may affect the P sorption characteristics, because SOM could be sorbed on metal (hydr)oxides by the same mechanism as P (Parfitt et al 1977;Stevenson 1994).…”

Sorption of divalent metal ions on CrPO4

“…Veith and Sposito [21] have also suggested a similar dissociation of adsorbed water (H 2 O) on AlPO 4 and FePO 4 .…”