Iron oxides are considered one of the more important sorbents for oxyanions such as phosphate and arsenate in soils. However, published research indicates that oxyanions also bind as ternary complexes to Fe(III) ions associated with natural organic matter (NOM). In some cases, maximum sorption capacities of Fe-laden NOM approaches a 1:1 molar ratio of bound phosphate to NOM-bound Fe(III). Formation of such ternary complexes could be an important mechanism of oxyanion sorption, particularly in organic soils and organic-rich mineral soil horizons. We hypothesize that the oxyanion sorption capacity of Fe(III)-laden NOM would depend in part on whether the Fe(III) is bound as individual ions dispersed throughout the organic matrix, or as hydroxy-Fe(III) clusters or a precipitate associated with the organic matrix. Our research objective was to determine how Fe(III) concentration and pH affect clustering or precipitation of Fe(III) associated with NOM.

Aqueous Fe(III) was reacted under acidic conditions (pH <2.5) with Pahokee peat from the International Humic Substances Society. The peat had been hydrated and acid washed to diminish residual Fe(III) and Al(III). Concentrations of added Fe(III) ranged from 0 to 10800 mmol Fe/kg peat (Fe/C molar ratios of 0 to ~0.25) in a 0.1 M KCl background electrolyte. The pH of the samples was slowly increased to pH 5.5, 6.0, or 7.0 with 0.1 M KOH solution. After equilibrating for 42 h at the target pH and at 25 °C, the solid-phase peat was separated by centrifugation. The moist peat samples were analyzed using transmission-mode Fe K-EXAFS spectroscopy at Beamline X-18B at the National Synchrotron Light Source, Brookhaven National Laboratory in Upton, New York. EXAFS data showed two trends that were evident in the radial structure functions (RSFs): (1) With increasing added Fe(III) concentration, the amplitude of an EXAFS peak near 1.6 Å (not corrected for phase shift) corresponding to first-shell Fe-O coordination decreased toward the amplitude found for ferrihydrite (and hematite). (2) Two overlapping peaks developed between 2.5 and 3.5 Å (not corrected for phase shift) in RSFs for samples containing 900 mmol Fe(III)/kg; the amplitude of these peaks increased with increasing Fe(III) concentration until they essentially matched corresponding peaks for ferrihydrite at >1800 mmol Fe(III)/kg peat. EXAFS data for control samples of aqueous Fe(III) brought to pH 5.5 or 7.0 in the absence of NOM exhibited similar peaks between 2.5 and 3.5 Å. These peaks result from the presence of Fe atoms in the higher coordination environment of x-ray absorber Fe atoms, and indicate hydroxy-Fe clustering or precipitation of a poorly-ordered ferrihydrite-like phase in the Fe/NOM mixtures. The Fe(III) concentration at which the transition to a cluster (or poorly-ordered precipitate) occurred was largely independent of pH between 5.5 and 7.0. In essence, the EXAFS results indicated a transition between separated Fe(III) ions bound to NOM and either hydroxy-Fe(III) clusters or a poorly-ordered solid forming in the presence of the NOM. One would anticipate that the oxyanion sorption capacity of Fe-laden NOM would vary, depending on the form of Fe(III).