Spectroscopic in situ examination of interactions of rare earth ions with humic substances

This study utilized the methods of fluorescence quenching and differential absorbance to probe in situ the extent and the nature of the interactions between rare earth ions (REIs) and humic substances. Experiments were conducted with the standard Suwannee river humic acid (SRHA) in the presence of varying amount of lanthanum, europium and terbium. The data of differential absorbance showed that the mechanism of SRHA-metal complexation was largely the same for all the examined REIs. In all cases several discrete bands whose properties were discerned via numerical decomposition of the differential spectra absorbance were observed. Their nature was examined based on the comparison of the experimental data and those of NICA-Donnan modeling carried out for Eu3þ. The observed effects suggested that the changes of SRHA absorbance induced by REIs binding are likely to be caused by a bathochromic shift of the absorbance bands associated with such chromophores. The intensity of the Gaussian band with a maximum at 387 nm was observed to be proportional to the total concentration of SRHA-bound REIs. The data obtained in this study demonstrate the existence of complex yet quantifiable changes of the spectroscopic properties of humic species in the presence of REIs and their utility to quantify modes of interactions in such systems.