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Proton Binding and Cadmium Complexation Constants for a Soil Humic Acid Using a Quasi-particle Model

Scarborough Campus, Univ. of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada

S. Sjöberg

Dep. of Inorganic Chemistry, Univ. of Umeå, S-901 87 Umeå, Sweden

L. J. Evans

Dep. of Land Resource Science, Univ. of Guelph, ON, N1G 2W1, Canada

*Corresponding author (bolton{at}wave.scar.utoronto.ca).

ABSTRACT

In order to better understand the fate of Cd in soils it is important to ascertain the influence of soil humic substances on the retention of Cd in soils. In this study the acid-base and Cd complexation properties of a humic acid fraction extracted from a soil were investigated using potentiometric titrations. A number of electrostatic and nonelectrostatic equilibrium models were considered in order to obtain the simplest model that would best describe the titration data. All proton binding and Cd complexation constants were determined using the optimization procedure in the computer program FITEQL. The model that best fit the experimental data was a nonelectrostatic model, which included two diprotic acids, H₂ A and H₂B. The dissociation constants for H₂A were calculated to be log ß_-1,0,1,0 = –4.00 ± 0.02 and log ß_-2,0,1,0 = –9.32 ± 0.03 and for H₂B were calculated to be log ß_-1,0,0,1 = –7.43 ± 0.24 and log ß_-2,0,0,1 = –16.66 ± 0.31. The model indicated the presence of four Cd complexes, CdHA⁺, CdA⁰, CdH_-1A^– and CdHB⁺, with complexation constants calculated to be log ß_-1,1,1,0 = –1.29 ± 0.09, log ß_-2,1,1,0 = –5.92 ± 0.07, log ß_-3,1,1,0 = –14.39 ± 0.09, and log ß_-1,1,0,1 = –3.72 ± 0.18.

Received for publication February 28, 1995.