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

The binding of Ca2+ and Cu2+ 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 Ca2+ and Cu2+ concentrations. The binding of Cu2+ by DOM was examined in the presence of three different Ca2+ concentrations. Interactions of Ca2+ with DOM chromophores were accompanied by the emergence of features in the differential spectra that were dissimilar from those characteristic for the binding of Cu2+. The amount of Ca2+ and Cu2+ 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 Ca2+ and Cu2+ ions. (C) 2015 Published by Elsevier Ltd.