RETENTION OF PHOSPHATE BY HYDROXYALUMINOSILICATE-MONTMORILLONITE AND HYDROXYALUMINUM-MONTMORILLONITE COMPLEXES

Hydroxyaluminosilicate (HAS) ions were prepared from solutions contain ing hydroxyaluminum (HyA) ions and orthosilicic acids at an Al concent ration of approximate to 4 mill; Si/Al molar ratios of 0, 0.25, 0.5, a nd 1.0; and NaOH/Al molar ratios of 1.0, 2.0, and 2.5. Phosphate reten tion behavior of montmorillonite (Mt) carrying nonexchangeably adsorbe d HyA, HAS ions, or both (HyA/HAS-Mt complexes) was studied at equilib rating solution pH of approximate to 5.0, The HyA/HAS-Mt complexes sho wed larger phosphate retention capacities than untreated Mt. Retention of phosphate by these complexes at 0.5 to 50 mM initial P concentrati ons decreased with increasing NaOH/Al and Si/Al ratios of the hydroxy materials adsorbed on hit. Phosphate, probably retained initially thro ugh surface complexation and ligand exchange reactions principally wit h OH or OH2 groups, coordinated with Al atoms at the broken edges of H yA or HAS polymers adsorbed on the external planar surfaces and in the interlayer spaces of Mt. This was followed by expulsion of the phosph ated hydroxy materials from the Mt interlayers to the ambient solution , and finally, precipitation as amorphous Al-phosphate solid phases. R esults of this study thus merit attention in P-fertility management as well as indicate a probable P-induced weathering process in acidic no nallophanic Andisols and in Red-Yellow soils (Hapludalfs and Hapludult s) with a predominance of hydroxy-interlayered minerals.