Effects of selected chemical and physicochemical properties of humic acids from peat soils on their interaction mechanisms with copper ions at various pHs

The main aims of this study were to investigate the interactions of copper ions (Cu(II)) at pH 5 and 7 with humic acids (HAs) isolated from four peat soils at different humification stages, as well as to determine the physicochemical properties of HAs that may have an influence on the above interactions. For this purpose, a number of parameters were determined for the qualitative and quantitative analysis of peat soils, HAs, and HA-Cu(II) interactions. Processes in the HA-Cu(II) systems were studied using fluorescence spectroscopy (for chemical complexation) and atomic absorption spectrometry with carbon measurements (for coagulation processes). The influence of HA properties on their interactions with Cu(II) ions was evaluated using matrices of correlation coefficients between some HA properties and parameters describing the HA-Cu(II) interactions. Results showed that the complexation capacity was higher at pH 7 than at pH 5. The coagulation mechanism at pH 7 appeared to be based on the precipitation of Cu(II)-humates following the neutralization of HA functional groups by Cu(II) ions, while coagulation at pH 5 appeared to be controlled by the ionic strength. The complexation capacity increased significantly with increasing content of carboxylic and phenolic groups, O/H atomic ratio, degree of internal oxidation and surface negative charge of HAs. The binding of Cu(II) ions was stronger for HAs characterized by a higher aromaticity and humification degree (expressed respectively as absorbance at 280 nm and E4/E6 ratio). The stability constants were higher at pH 7 than at pH 5 and in most cases they did not correlate significantly with HA properties.