Effects of pH on the differential absorbance spectra, d-d transition bands and structural properties of copper complexes with humic substances and model compounds.

Interactions between metal cations, notably Cu(II), and humic substances (HS) affect their mobility, bioavailability, and toxicity. This necessitates a molecular-level determination of the nature of HS functional groups binding Cu(II) (Cu-HS) and effects of pH on them. This study investigates the pH effects on the spectroscopic and structural properties of the complexes of Cu(II) with HS and representative model compounds using differential absorbance spectroscopy (DAS), examination of the properties of the d-d transition band characteristic for Cu(II) ions, and quantum chemical (QC) calculations. DAS of Cu-HS show distinct bands at 240, 275, 310 and 400 nm, while absorbance features located from 600 to 800 nm correspond to the d-d transitions in Cu(II). Similar features appear in copper complexes with the model compounds of salicylic acid (Cu-Sal) and poly(4-styrenesulfonic acid-co-maleic acid) (Cu-PSM). Increasing pH resulted in consistent changes of the DAS and the d-d band of Cu(II) which exhibited a hypsochromic shift and increased intensity. Deconvolution of the d-d bands into discrete Gaussian bands was indicative of transitions between dominant species at increasing pH. Cu-Sal and Cu-PSM structures that were modeled successfully by QC calculations. These results demonstrate the sensitivity of DAS spectra and d-d band to the modes of Cu(II) binding by HS and open a possibility of further elucidation of the functional groups engaged in the binding of heavy metals by HS.