Characterization of Fe–humic complexes in an Fe-enriched biosolid by-product of water treatment

The fertilizing potential of Fe-enriched biosolids has been attributed to Fe associations with humic substances contained therein. In this study, alkaline and near-neutral aqueous extractions of humic substances from an Fe-enriched biosolid were followed by gel chromatographic fractionation and characterization (CHNS elemental analysis; UV/visible and FTIR spectroscopy; FAAS analysis). The alkaline bulk humic extract had a strong fulvic character and Fe was predominantly associated with the higher molecular weight (∼50 000 Da) molecules, possibly including organic-coated Fe oxides from which Fe may be released more slowly. Under both near-neutral and alkaline conditions, associations with lower molecular weight humic molecules were also observed, indicative of the presence of Fe in more readily available forms. Thus the biosolid appears to have good short- and long-term fertilizing potential, particularly for alkaline, Fe-deficient soils.