pH-Dependent adsorption of fractionated peat humic substances on different silver colloids studied by surface-enhanced Raman spectroscopy

The adsorption of several fractions of humic and fulvic acids on silver colloids was studied by means of surface-enhanced Raman spectroscopy. The short-range mechanism of this technique was applied in the determination of the chemical groups which are directly attached to the metallic surface. Adsorption of humic substances was found to be very sensitive to pH, to the electric charge of both the humic substances and the metallic surface, and to the size of the adsorbed macromolecules. From the obtained results, a model for the adsortion mechanism of these macromolecules is proposed on the basis of a conformational change and a variation of their electric charge on modifying the pH. FTIR spectra of some of the extracted fractions and SERS spectra of 0-, m-, and p- hydroxybenzoic acids, used as molecular models for humic substances, were also obtained in order to better attribute the SERS spectra of these humic substances.