Molecular and Quantitative Analysis of Metal Ion Binding to Humic Acids from Sewage Sludge and Sludge-Amended Soils by Fluorescence Spectroscopy

摘要

Humic acids (HAs) isolated from sewage sludge (SS) and control and SS-amended soils were characterized by fluorescence spectroscopy.The main feature of fluorescence spectra was a broad band with the maximum centered at excitation/emission wavelengths that were much shorter for SS-HA (340/438 nm) than for any soil HA (440/510 nm).Titration with Cu(ll),Zn(ll),Cd(ll),and Pb(ll) ions decreased fluorescence intensities of HAs.Titration data were fitted to a single-site fluorescence quenching model,and metal ion complexing capacities of each HA sample and stability constants of metal ion-HA complexes were calculated.The binding capacities of HAs and strengths of metal ion-HA complexes followed the order Pb(ll) > Cu(ll) > Cd(ll) > Zn(ll).With respect to the control soil HA,SS-HA,which showed the smallest contents of acidic functional groups and the lowest humification degree,was characterized by much smaller binding capacities and stability constants.The binding capacities and affinities of SS-amended soil HA were intermediate between those of native soil HA and SS-HA but closer to the former,thus suggesting a partial incorporation of HA fractions of SS into native soil HAs.These effects are expected to have a great impact on the behavior of metals in SS-amended soils.