Effects of calcium on the chromophores of dissolved organic matter and their interactions with copper.

The binding of Ca(2+) and Cu(2+) by dissolved organic matter (DOM) exemplified by Suwannee River Humic Acid (SRHA) was examined in this study. Metal-DOM interactions were quantified using linear and log-transformed DOM absorbance spectra acquired at varying pHs and total Ca(2+) and Cu(2+) concentrations. The binding of Cu(2+) by DOM was examined in the presence of three different Ca(2+) concentrations. Interactions of Ca(2+) with DOM chromophores were accompanied by the emergence of features in the differential spectra that were dissimilar from those characteristic for the binding of Cu(2+). The amount of Ca(2+) and Cu(2+) bound by DOM was ascertained using changes of the slopes of log-transformed absorbance spectra in the range of wavelength 350-400 nm (denoted as S350-400) and comparing the data with model predictions. The observed effects indicated the occurrence of the replacement of a characteristic number of protons by the bound Ca(2+) and Cu(2+) ions.