Solid-phase-fluorescence intensity of clay-fulvic acid complexes: differences based on amount of fulvic acid adsorbed...

Solid-phase-fluorescence (SPF) excitation-emission matrix (EEM) spectroscopy is a non-destructive soil analysis method for understanding the interaction between organic matter and minerals in soil and it does not require alkali extraction. However, the relationship between the amount of organic matter and fluorescence intensity is not well understood. Herein, SPF-EEM spectra of complexes that simulate soils with different amounts of fulvic acid (FA) adsorbed on montmorillonite, kaolinite, and goethite are compared. The apparent EEMs of the tested complexes differ across these clay types, probably because adsorbed FA is influenced by the different adsorption sites and extinction properties of the clays themselves during complex formation. Results obtained using the Kubelka-Munk (KM) function suggest that the absorbance of the samples increased as the amount of FA adsorbed on the complexes increased. However, these increases did not shift the apparent fluorescence maxima of the complexes. Rather, in some other cases, the fluorescence intensity decreased, possibly because of the increasing inner-filter effect. To correct this, the use of theoretical and empirical equations based on KM functions is examined. However, since a reasonable relationship in which a higher concentration of adsorbed FA yields higher fluorescence intensity could not be identified, further study is necessary.