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Characterization of Iron and Zinc in Albuquerque Sewage Sludge¹

ABSTRACT

Chemical forms of Fe and Zn present in anaerobically digested sewage sludge from Albuquerque, NM were identified and quantified. Water-soluble Fe and Zn were speciated based on charge and stability of metal complexes, and on their degree of association with soluble organics. Chemical forms of Fe and Zn present in the solid phase of the sludge were characterized with a sequential extraction procedure. Soluble Fe was predominantly present as negatively charged slowly labile organic complexes of molecular weights > 1000 daltons. Zinc was associated with negative and neutral species that were very or moderately labile, according to the techniques used. The contribution of soluble organics to the chemistry of Fe and Zn was evaluated using gel chromatography. Iron was associated with a larger portion of sludge organics and with larger molecular weight fractions than Zn complexes. The percentage of total sludge Fe and Zn found in readily soluble forms was very small (<1%). The amounts may nevertheless represent significant pools of metal forms available for plants. Zinc was found in larger concentrations than Fe in the potentially more labile, organic, and carbonate fractions of the sludge. The Albuquerque sludge is an excellent source of Fe and Zn to plants because it contains readily soluble forms of these metals that may be maintained in soil solution through organic complexation.

Key Words: micronutrient availability • metal complexes • lability • gel chromatography

¹ Journal article 1221, Agric. Exp. Stn., New Mexico State Univ., Las Cruces, NM 88003. This study was supported by funds from the Dep. of Energy under contract no. DE-AC04-83AL21776.

² Research Specialist and Professor, respectively, Dep. of Crop and Soil Sci., New Mexico State Univ., Las Cruces, NM 88003.

Received for publication December 16, 1985.