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Calcium Ion Complexation by Fulvic Acid Extracted from Sewage Sludge-soil Mixtures¹

ABSTRACT

The complexes formed between Ca²⁺ and fulvic acid extracted from sewage sludge were investigated by potentiometric titration of K/H-fulvate with Ca(NO₃)₂ at pH 7 and 25°C, in an ionic medium of 0.1N KNO₃. Formation functions, giving the number of moles of Ca bound per mole of K/H-fulvate, were calculated from the titration data and plotted against the concentration of Ca²⁺, with the concentration of K/H-fulvate being a fixed parameter for each plot. The molarity of K/H-fulvate, which was held rigorously constant during each titration, was determined with the help of separate experiments in which the number-average molecular weight of the K/H-fulvate sample employed was measured by vapor pressure osmometry. The formation functions were interpreted as evidence of: (i) a relatively strong ion pair that is formed between Ca²⁺ and the most acidic functional groups in fulvic acid, and (ii) a weaker ion pair that is formed between Ca²⁺ and the more weakly acidic functional groups. The properties of the latter complex depend both on pH and on the concentration of fulvic acid, since these variables in part determine the number of ionized, weakly acidic functional groups that can react with Ca²⁺. The Ca²⁺ ion was shown to have a modest ability to displace protons from undissociated acidic functional groups in sludge-derived fulvic acid.

¹ Contribution from the Dept. of Soil and Environ. Sci., Univ. of Calif., Riverside 92521.

² Professor of Soil Science, Research Associate IV, and Research Associate I, respectively.

Received for publication December 19, 1977. Accepted for publication April 3, 1978.