Retention of phosphate by hydroxyaluminosilicate- and hydroxyaluminum-montmorillonite complexes

Hydroxyaluminosilicate (HAS) ions were prepared from solutions containing hydroxyaluminum (HyA) ions and orthosilicic acids at an Al concentration of ≈4 mM; Si/Al molar ratios of 0, 0.25, 0.5, and 1.0; and NaOH/Al molar ratios of 1.0, 2.0, and 2.5. Phosphate retention behavior of montmorillonite (Mt) carrying nonexchangeably adsorbed HyA, HAS ions, or both (HyA/HAS-Mt complexes) was studied at equilibrating solution pH of ≈5.0. The HyA/HAS-Mt complexes showed larger phosphate retention capacities than untreated Mt. Retention of phosphate by these complexes at 0.5 to 50 mM initial P concentrations decreased with increasing NaOH/Al and Si/Al ratios of the hydroxy materials adsorbed on Mt. Phosphate, probably retained initially through surface complexation and ligand exchange reactions principally with OH or OH₂ groups, coordinated with Al atoms at the broken edges of HyA or HAS polymers adsorbed on the external planar surfaces and in the interlayer spaces of Mt. This was followed by expulsion of the phosphated hydroxy materials from the Mt interlayers to the ambient solution, and finally, precipitation as amorphous Al-phosphate solid phases. Results of this study thus merit attention in P-fertility management as well as indicate a probable P-induced weathering process in acidic nonallophanic Andisols and in Red-Yellow soils (Hapludalfs and Hapludults) with a predominance of hydroxy-interlayered minerals.