Fluorescence properties of the <5kDa molecular size fractions of a soil humic acid

International audienceSoil humic acids are complex mixtures displaying several still unexplained spectral properties. In this work, we aimed to investigate the fluorescent properties of soil humic acid macromolecules of molecular sizes less than 5 kDa. To recover the fraction < 5 kDa, we employed a protocol based on repeated ultrafiltration of a soil humic acid HA with a 5 kDa membrane. The obtained retentate did not show any fluorescence. In contrast, the ultrafiltrate, which accounted for 20% by mass of the starting HA, contained a significant amount of the fluorescent components. This ultrafiltrate was in turn ultrafiltered through 3 kDa and 1 kDa membranes, and three subfractions were obtained. Their emission spectra, measured at an absorbance of 0.1 at 300 nm, were different in shape and intensity. In particular, the contribution of the long-wavelength emissions decreased and the intensity of emission at the maximum increased as the molecular weight decreased. These results show that a large portion of the fluorescent moieties of the soil HA are contained in the fraction < 5 kDa and that the repartitioning of fluorophores depends on their molecular size. This finding represents an interesting starting point for further studies