Copper(II) binding abilities of molecular weight fractionated humic acids and their mixtures

It was found that the intermolecular interaction between molecular weight fractionated humic acids (HAs) affected the copper(II) binding abilities. The mean values of copper(II) binding constants (μav) and the total binding capacities (Nt) of the molecular weight fractionated HAs were in the range of 6.0–6.6 and 4.7–5.4 (meq g−1 of C), respectively. The μav and Nt values of the unfractionated HA, which could be regarded as a mixture of the fractions, were smaller than those of the fractionated ones (5.6 and 3.8). Moreover, when the fractionated HAs were mixed in the various mixing ratio, the μav and Nt values of all mixtures were not identical to arithmetic mean values of the fractions, but were smaller than those values of the fractions (5.5–5.7 and 2.2–3.6). These results showed the decrease of the copper(II) binding abilities by mixing the fractions. In order to explain these results, the UV-visible absorption, fluorescence, Fourier transform infrared spectra and gel permeation chromatogram of the fractionated HAs and their mixtures were investigated. From the obtained results, we concluded that the decrease of binding abilities in the mixtures was due to the shielding effect of the copper(II)-binding sites by intermolecular interaction between the molecular weight fractions.