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.