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Chemistry of Water-Soluble, Metal-Complexing Ligands Extracted from an Anaerobically-Digested Sewage Sludge¹

ABSTRACT

Organic ligands extracted with deionized water from a representative municipal sewage sludge were studied by a variety of chemical techniques. The use of deionized water as an extractant of soluble organic matter from sewage sludge offered two advantages over NaOH: (i) the water soluble extract (WSE) represents an environmentally significant fraction, and (ii) counterion and inorganic ligand levels are comparable to those in FA extracts that have undergone tedious, extensive purification steps. Approximately 8% of the total sludge C was extracted with H₂O. A charge balance calculation, including the major inorganic components of the WSE, yielded a value of 5 mol_c kg^–1, for the cation exchange capacity (CEC) of the extracted organic matter. The molar organic C/N ratio for the WSE was found to be 6, indicating that the mixture is rich in polypeptide, amino sugar, and/or nitrogeneous base-containing compounds. The presence of amino groups and carbohydrates was confirmed both by infrared (IR) identification and by direct chemical analysis. Amino acids accounted for 25% of the WSE organic carbon (C_TS), with polysaccharides, hexosamines, and a steam-volatile component accounting for 12, 9, 8.5% of the C_TS, respectively. In total, 55% of the C_TS in the WSE was identified as organic compounds that can form soluble complexes with transition metals. Gel filtration experiments suggested that approximately one-half of the organic compounds in the WSE have relative molecular masses of < 1,500 daltons. The low molecular mass fraction of the WSE is composed primarily of aliphatic organic acids.

Key Words: humic substances • fulvic acid • trace metals • soil organic matter

¹ Contribution from the Dep. of Soil and Environmental Sciences, University of California, Riverside, CA 92521.

² Former Graduate Student (now Assistant Professor, Dep. of Soil Science, Oregon State Univ., Corvallis, OR 97331) and Professor of Soil Science, University of California, Riverside, respectively.

Received for publication February 10, 1982.

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