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

• Differential spectra of DOM caused by metal binding exhibit metal-specific features.
• Little competition exist between Cr3+ and Al3+ interaction with DOM binding sites.
• Cu2+ exerts a strong influence on and depresses Al3+ binding by DOM.
• Deconvolution of the differential spectra provides unambiguous information about the Al3+/Cu2+ competition.

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 Al3+ concentrations and Cu2+ or Cr3+ levels. The competition between Al3+ and Cu2+/Cr3+ 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 Al3+ onto SRHA is little influenced in the cases of in presence of 1.0 and 10.0 μM background Cr3+ and in presence of 1.0 μM background Cu2+, but it is significantly depressed in presence of 10.0 μM Cu2+. 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.

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