Stability of Ca2+-, Cd2+-, Cu2+-[illite-humic] complexes and pH influence

Decomposition of fresh plant residues in soil is expected to produce humic fractions varying in molecular size. It was hypothesized that metal adsorpt ion by soil, to some degree, will depend on humic acid content and molecula r size. The latter is expected to vary in number and type of functional gro ups. In this study, illite-humic complexes were used to evaluate Ca2+, Cd2, and Cu2+ adsorption and how this adsorption was affected by humic acids, differing in molecular size, under various pH values. Potentiometric titrat ion using ion-selective electrodes with a stop-and-go procedure was employe d to evaluate metal-[illite-humic] complex formation. The results showed th at illite-humic complexes exhibited at least two types of metal-ion adsorpt ion sites (low and high affinity) and molecular size of humic fractions had a large potential influence on total metal adsorption but a relatively sma ller influence on metal-complex stability. Relative strength of metal-ion-[ illite-humic] complexes followed the order of Cu2+> Cd2+> Ca2+ and were aff ected by pH, especially for low metal-ion affinity sites. Magnitude of meta l-[illite-humic] stability constants, depending on molecular size of humic fraction and pH, varied on a log-scale from 3.52 to 4.21 for Ca2+, 4.38 to 5.18 for Cd2+ and from 5.23 to 5.83 for Cu2+. There was an approximate 5-fo ld difference in these stability constants between the three different size s of humic fractions. The larger the humic fraction, the lower the metal-[i llite-humic] stability constant.