LASER-INDUCED FLUORESCENCE OF MARINE SEDIMENTARY INTERSTITIAL DISSOLVED ORGANIC-MATTER

Flourescence spectroscopy is investigated on dissolved organic matter (DOM) of marine sedimentary interstitial waters (IW). These were collected near urban outfalls and a river month in the Mediterranean, having a high interstitial dissolved organic carbon content (DOC). A self-made experimental laser-crystal fluorimeter and two classical spectrofluorimeters are employed (Perkin Elmer LS-2 and LS-50). For an excitation wavelength at 366 nm, a definite emission ''bump'' is found for LS-2 and LS-50, which is caused by specific fluorophores. Two kinds of curves are reported with the experimental laser, the ''laser induced fluorescence'' produced at a fixed time (LIF) and the ''time resolved emission spectra'' (TRES), which are three dimensional space graphs. Photochemical decay of fluorescent organic matter (FOM) results in an almost exponential decrease of the complete LIF spectrum. A correlation exists between two peaks of the spectra, the broad band around 475 nm and the narrow peak at 720 nm, depending on DOC. The emission spectra (EFS, LIF, TRES) are systematically compared with three different factors, being filtration, pH and added aluminium ion. (1) filtration affects the fluorescence of organic matter by minimizing the scattering interference. (2) pH affects the ''fine structure'' of the broad band around 475 nm. As concerning the increase at 530 nm at stronger pH, one can consider that there is a competition between H+ ion and other cationic species: the anionic form favors the formation of chelate species. This is due to stereostructural changes of FOM, involving neutral, anionic or chelate species. (3) Aluminium enhances fluorecence intensities near 530 nm, which is attributed to a colloidal organo-aluminium complex formation. Additions of Al3+ in excess, at constant salinity, result in essential modifications of the equilibrium between spheroidal and unfolded geometries of FOM. Since a decreasing intensity of this ''bump'' at 530 nm is observed after re-filtration, this confirms the possible colloidal state of the Al-FOM complex. Emission fluorescent spectra (EFS) obtained by LS-2 due to Al3+ addition, at natural pH and natural salinity, are treated with a linear regression method; an apparent stability constant is calculated at 530 nm, K = 3.58.