Protonation of NOM from the Rio Negro Basin: Comparison of Results of Potentiometric and Differential Absorbance Titrations

The deprotonation of NOM originating from the Rio Negro river basin can be quantified based on measurements of the effects of pH on its absorbance spectra, which contain multiple bands with maxima located at ca. 240, 280, 315 and >350 nm. The emergence of these features is associated with the engagement of phenolic, carboxylic and other chromophores. Correlations between the accumulation of charge in NOM molecules measured by potentiometric titrations and evolution of their differential absorbance spectra show that for wavelengths <300 nm changes of the absorbance occur predominantly in the range of charges < – 2 meq/g, while for wavelenghts > 300 nm the absorbance of NOM is sensitive to the deprotonation in the entire range of electrostatic charges, with notable differences between fulvic acids and colloidal fractions of NOM.