COMPLEXATION OF CADMIUM(II) WITH HUMIC ACIDS - EFFECTS OF PH AND HUMIC-ACID ORIGIN

A comparative study on cadmium (II) complexation with three well chara cterized humic acids (SHA: soil humic acid from the Okchun Metamorphic Belt; AqHA: aquatic humic acid from Gorleben underground aquifer, Ger many; CoHA: commercially available humic acid from the Aldrich Co.) wa s carried out in 0.1 M NaClO4 at different solution pH (5.0, 5.5, and 6.0) using the ultrafiltration technique. The maximum binding ability (MBA) of the humic acids for cadmium (II) was observed to vary with th eir origins and solution pH. The results suggest that 1 : 1 complex pr edominates within the experimental range, and the conditional stabilit y constants were calculated based on the assumption of cooperative bin ding, yielding log K values that were quite similar (CoHA: 4.17 +/- 0. 08; AqHA: 4.14 +/- 0.07; SHA: 4.06 +/- 0.12 1 mol-1 at pH 6.0) irrespe ctive of humic acid origins or pH. By contrast a nonlinear Schatchard plot was obtained, using the cadmium (II) ion selective electrode spec iation analysis method, which indicated that humic acid may have two o r more classes of binding sites, with log K1 and log K2 of 4.73 +/- 0. 08 and 3.31 +/- 0.14 l mol-1 respectively.