Wednesday, November 7, 2007
288-7
Study of Phosphate-iron-organic acid Ternary Systems.
Fiona Kizewski, Chemistry, North Carolina State University, 2011 Quaker Landing, Apt#303, Raleigh, NC 27603
Binding of phosphate into ternary complexes with organically-bound Fe(III) and Al(III) serving as cation bridges is a potentially important phosphate retention mechanism in soil organic matter. However, the molecular details of this retention mechanism and its impact on phosphate dissolution under changing pH and redox conditions are not understood. Using simple carboxylic and phenolic acids as analogues for these functional groups in soil organic matter, our objective was to synthesize crystalline ternary compounds that can be used to better characterize the coordination and bonding stability of phosphate in organic matter. Aqueous solutions containing 6 mmol PO43-, 2 mmol Fe(III) and a concentration series of organic acids having different binding affinities for Fe(III); including oxalate, citrate, tartrate, succinate, malonate, benzoate and phenol; were reacted under hydrothermal conditions at pH 5.5 and 170вк C. Selected solid-phase precipitates were characterized by x-ray powder diffraction and x-ray absorption spectroscopy at the Fe and P K-edges, and stoichiometry of solids was estimated from aqueous solution composition following reaction. In the absence of organic acid, x-ray diffraction confirmed the presence of Fe(H2PO4)3 and one or more unknown compound(s) in the solid-phase products. Similar diffraction patterns were obtained for all solid products regardless of the type and amount of organic acid added. However, new diffraction peaks appeared for systems containing greater than 2 mmol of tartrate, oxalate or citrate, indicating the formation of additional crystalline compound(s). Our results will help to define how the relative affinities of organic ligands and phosphate for metal cations affect the binding affinity of phosphate in ternary complexes.