Characterization of copper(II) binding by molecular weight fractions of a soil fulvic acid

Abstract

Cu(II) binding characteristics of bulk, <30,000, <10,000, <5,000, <1,000, and <500 dalton molecular weight (MW) fractions of soil fulvic acid were determined from Cu(II)/dissolved organic matter interactions. Manual complexometric and potentiometric titrations measured Cu(II) MW fraction binding capacities and acidities. Ionic strength, temperature, and pH were varied. Cu averaged 7.3 x 10⁻⁴ micromoles per liter and carbon ranged between 0.5 and 3.5 ppm. Total Cu(II) binding capacity differed among all but bulk and <30,000 fractions due to site concentration differences; binding was insignificant for the <500 fraction. A three-site discrete ligand model determined conditional stability constants 10⁷˙⁰ 10¹¹˙⁰ and 10¹³˙⁸ and site concentrations 1.8 x 10⁻⁶ 8.5 x 10⁻⁶ and 7.6 x 10⁻⁵ M L⁻¹ for the bulk and 30,000 fractions (pH 6.2, 0.01 M, 22°C). Similar stability constants were determined for lower MW fractions. Cu(II) complexation capacity was pH-dependent; Ionic strength effects were significant at pH 7.0, not at 6.2. Temperature effects were insignificant.