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Proton and Metal Complexation by Water-soluble Ligands Extracted from Anaerobically Digested Sewage Sludge¹

ABSTRACT

A water-soluble extract (WSE) of an anaerobically-digested sewage sludge containing 19.5 mol m^–3 (19.5 mM) total soluble carbon (C_TS) was acidified to pH 3 and titrated with NaOH to pH 11 in a 50 mol m^–3 (50 mM) NaClO₄ background at 25°C. Conditional protonation constants were determined from the formation function computed from the titration data. The common logarithms of the average protonation constants for functional groups titrated between pH 3 and 11 were 4.2, 7.2, and 9.6. Trace metal-WSE complexes were chromatograpbed on a molecular size exclusion gel. Copper formed the more stable complexes with the ligands in the WSE when compared to Ni or Cd. Cadmium was present predominately as the free metal ion or in inorganic complexes. Adsorption of Cu by montmorillonite was significantly reduced in the presence of the WSE. Since most of the organic carbon present in the WSE remained in solution during the adsorption experiments, the primary factor controlling Cu solubility was concluded to be the formation of soluble organo-Cu complexes. A "mixture model" was developed to simulate qualitatively the proton binding of the inorganic and organic components of the WSE. This model included organic and inorganic ligands whose relative concentrations were previously analytically determined for the WSE. The "mixture model" ligands were employed to model Ca and Cd speciation for gel filtration and adsorption experiments. The "mixture model" qualitatively explained the observed speciation in both experiments.

Key Words: Potentiometric titrations • gel filtration • GEOCHEM • humic substances

¹ Contribution from the Dep. of Soil Science, Oregon State Univ., Corvallis, OR 97331, and the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Assistant Professor of Soil Science and Professor of Soil Science, respectively.

Received for publication December 3, 1984.