Binding Mechanism of Fe(III) in Organic Matter as Affected by Mode of Addition.

Phosphorus dissolution from soils depends in part on the amounts and forms of Fe and Al in oxide minerals and organic matter.  Retention of phosphate by Fe and Al in soil organic matter is likely affected by the molecular bonding environment of these metal cations with organic functional groups.  The reduction and oxidation of soil Fe may alter Fe (and Al) bonding to organic matter.   The objective of this research was to determine how the mode of addition of Fe(III) affected molecular binding of Fe to organic matter and binding of phosphate.  Two modes of Fe(III) addition to Pahokee peat were made at pH 6.8 for a range of Fe concentrations: direct addition of Fe(III) (method 1), and addition of Fe(II) followed by Fe(II) oxidation (method 2). Phosphate binding capacities were evaluated through a series of adsorption isotherms.  As expected, maximum retention of P in the solid phase by each mode of Fe addition was correlated to the amount of Fe initially bound in the solid phase.  Up to 13 % more of the total added Fe remained bound following addition of Fe by method 1 than by method 2, however 1 to 30 % more of the added phosphate added was retained in the solid phase by method 2.  Dissolved organic carbon levels significantly increased as Fe concentrations decreased (r² = 0.97).  EXAFS analysis indicated minor differences in the Fe coordination environment between the two modes of Fe addition.  Notwithstanding differences in soil organic matter formed under varying redox conditions, our results suggest that Fe(III) incorporated into soil organic matter during periodic reduction-oxidation cycles could have a different molecular configuration and phosphate retention properties than organic-matter bound Fe(III) in a well-drained soil.