Evaluation of the copper(II) complexing ability of ultrafiltered humic acid by the solvent extraction method.

A commercial humic acid dissolved in water was fractionated to nine samples by means of ultrafiltration (UF); the nominal molecular weight used for UF membranes was 1 k-200 kDa. Concerning the nine samples, copper(II) complexing capacities (CuCC) and conditional stability constants (beta) of the formed copper(II) complexes were measured by a solvent extraction method. A total organic carbon (TOC) and the UV-VIS absorption ratio (E350 nm/E450 nm) were also measured. From a comparison of these data, it was found that a) humic acids in each fraction formed two kinds of copper(II) complexes with different stability; b) the beta values obtained from each fraction were almost the same; c) large CuCC values were observed in the molecular weight range from 10 kDa to 20 kDa and below 1 kDa; d) molecules with molecular weight higher than 50 kDa scarcely had any copper(II) complexing ability; e) the values of CuCC/TOC of each fraction were in the range from 1.7 to 3.4 x 10(-7) mol mg(-1).