Examination of effects of Cu(II) and Cr(III) on Al(III) binding by dissolved organic matter using absorbance spectroscopy

Effects of Cu(II) and Cr(III) ions on the binding of Al(III) onto Dissolved Organic Matter (DOM) exemplified by Suwannee River Humic Acid (SRHA) at pH 6.0 were quantified in this study using linear and log-transformed SRHA absorbance spectra acquired at varying Al(3+) concentrations and Cu(2+) or Cr(3+) levels. The competition between Al(3+) and Cu(2+)/Cr(3+) for the binding sites in DOM was ascertained by examining the intensity and shapes of the metal-specific differential spectra of DOM. The results indicated that the binding of Al(3+) onto SRHA is little influenced in the cases of in presence of 1.0 and 10.0 μM background Cr(3+) and in presence of 1.0 μM background Cu(2+), but it is significantly depressed in presence of 10.0 μM Cu(2+). Changes of the spectral slope of the log-transformed absorbance spectra in the 350-400 nm wavelength range (S350-400) were unambiguously correlated with the total amount of DOM-bound metals. The concentrations of Me-DOM complexes were determined using the NICA-Donnan Model. The results demonstrate that differential absorbance measurements provide quantitatively interpretable information concerning the nature and mechanisms of metal-DOM interactions and effects of metal cations competition on these processes.