Phosphate Adsorption Behavior of Hydroxyaluminum- and Hydroxyaluminosilicate-Vermiculite Complexes.

Abstract

Phosphate adsorption behavior of vermiculite (Vt) complexed with hydroxyaluminum (HyA) and hydroxyaluminosilicate (HAS) ions were studied at equilibrating solution pH ∼ 5.0. The HAS ions were prepared from solutions containing HyA ions and orthosilicic acids at an Al concentration of ∼4 mM; Si/Al atomic ratios of 0, 0.25, 0.50, and 1.0; NaOH/Al molar ratios of 1.0, 2.0, and 2.5. At initial P concentration of 0.25 to 50 mM, the adsorption of phosphate by these complexes decreased with increase in the NaOH/Al and Si/Al ratios of hydroxy materials fixed on Vt. When adsorption data were evaluated using the Scatchard plots of Langmuir equation, multiple linear relationships were observed which suggested the presence of multiple population of adsorption sites that have a differing affinity for phosphate ions. Both adsorption maxima and binding energy for the adsorption sites in the higher (region I) and lower (region II) affinity regions of different complexes were in decreasing trends with increasing NaOH/Al and Si/Al ratios of the fixed ions. The total phosphate adsorption maxima of different complexes ranged from 0.40 to 0.96 mmole g -1 in comparison to only 0.17 mmole g -1 as found for untreated Vt. Ligand exchange reactions in which phosphate ions displaced OH or OH 2 groups from coordination shells of structural Al on the edges of Vt and HyA or HAS polymers fixed on external planar surfaces and/or interlamellar spaces of Vt may be involved in the phosphate adsorption processes in region I. In region II, phosphate adsorption may be occurred through ligand exchange predominantly with OH groups linking Al atoms inside the polymeric structure of the fixed HyA or HAS ions. In the HAS-Vt complexes, upon phosphate adsorption, the resultant HAS-phosphate complexes were substantially to almost entirely expelled from the interlamellar spaces of Vt. While, only a partial expulsion of the interlayer HyA-phosphate complexes was noticed following phosphate adsorption by the HyA-Vt complexes. Very high phosphate adsorption capacities of the complexes as evaluated in the present study merit attention in P-fertility management in acidic and nonallophanic Andisols and related soils.