Tuesday, November 14, 2006

Phosphate Bonding to Fe(III) and Al(III) in Soil Organic Matter.

Dean Hesterberg and Kimberly Hutchison. NC State Univ, Dept of Soil Science, Box 7619, Raleigh, NC 27695-7619

Solids containing iron and aluminum are important for phosphate sorption in soils.  Both Fe(III) and Al(III) can be associated with natural organic matter (NOM) as metal-NOM complexes. Our objective was to determine phosphate sorption mechanisms of metal-NOM complexes in single and binary metal systems. Acid-washed peat was reacted with aqueous Fe(III), Al(III), or Fe(III)/Al(III) mixtures under acidic conditions at varying concentrations and pH. Moist peat was analyzed using transmission-mode Fe K-EXAFS spectroscopy to characterize bound Fe(III). Phoshorus K-XANES analysis was used to characterize phosphate bonding to the metal-peat complexes. Phosphate sorption was also determined on samples of metal-NOM at various P/metal molar ratios. Iron K-EXAFS data showed clustering of Fe(III) atoms into hydroxy-Fe polynuclear species or precipitation of a poorly-ordered ferrihydrite-like phase at Fe(III) concentrations greater than 900 mmol/kg. The Fe(III)-NOM complexes sorbed phosphate up to a P/Fe molar ratio of 0.6.  Phosphorus K-XANES data of peat containing a mixture of bound Fe(III) and Al(III) showed binding of phosphate to both metals. These results are important for predicting phosphate binding capacity of organic-matter rich soils with varying concentrations of Fe(III) and Al(III), and in understanding reductive dissolution of Fe(III) and associated phosphate in water-saturated, organic soils.