Fluorescence behaviour of Zn and Ni complexes of humic acids from different sources

Conventional fluorescence spectroscopy in the excitation, emission and synchronous scan modes and three-dimensional fluorescence spectroscopy in the form of an excitation–emission matrix (EEM) of fluorescence intensity as a function of excitation and emission wavelengths have been applied to the study of three humic acids (HAs) extracted from soil (SHA), peat (PHA) and compost (CHA) and their interaction products with Zn(II) and Ni(II) ions. Fluorescence spectra of HAs appear to be related to the nature and origin of the sample. A strong reduction of intensity of all peaks is observed in the spectra of HAs–metal complexes as compared to those of untreated HAs. Ni(II) exhibits greater quenching ability than Zn(II). Fluorescence quenching measured for complexes of HAs at increasing Ni(II) concentrations was linearly correlated with metal ion concentration. The different capacity to interact with metal ions showed by various HAs is attributed to their different molecular complexity.