Concentration and pH effects on the fluorescence spectra of humic acid-like soil fungal polymers

Abstract

Concentration- and pH-dependence effects have been studied on the fluorescence spectra measured in the emission, excitation and synchronous-scan excitation modes of five humic acid-like polymers synthesized by soil fungi in culture media. Fluorescence intensity of humic polymers synthesized by S. atra and A. sydowi fungi exhibit a nearly linear, positive relationship with concentration, while the polymers from the fungi H. toruloidea , A. glaucus and E. echinulatum show a “threshold” concentration value in the range 50–70 mg/L for optimal fluorescence intensity. Generally, the pH affects only limitedly, and especially in the emission mode, some fluorescence peaks of A. glaucus and E. echinulatum polymers, while a differentiated maximum for the fluorescence intensity at pH values around neutrality (pH = 6–8) characterizes the remaining polymers. These effects are principally ascribed to different ionization degree, inter- and intra-molecular bonding, and configuration and structural rearrangement achieved, as a function of pH, by the groups responsible for the fluorescence in the humic macromolecules.