Effects of charging on the chromophores of dissolved organic matter from the Rio Negro basin

• Dissolved organic matter (DOM) from the Rio Negro area was isolated and characterized.
• Colloidal and hydrophobic fractions were major DOM constituents.
• Deprotonation of these fractions was tracked using differential spectroscopic methods.
• Result of this approach is consistent with those of NEXAFS and potentiometric methods.
• Changes of spectra were correlated with the charges of phenolic and carboxylic groups.

This study demonstrates that the deprotonation of dissolved organic matter (DOM) originating from a small creek characteristic for DOM-rich waters located in the Rio Negro basin can be quantified based on measurements of pH effects on its absorbance spectra. The method was ascertained by the data of Near-Edge X-Ray Absorbance Spectroscopy (NEXAFS), potentiometric titration to quantify the structural and compositional differences between the colloidal and hydrophobic fractions that contribute 91% of black-water creek DOM. Changes in the absorbance spectra of the DOM fractions caused by deprotonation quantified via numeric deconvolution which indicated the presence of six well-resolved Gaussian bands in the differential spectra. The emergence of these bands was determined to be associated with the engagement of carboxylic and phenolic functionalities and changes of inter-chromophore interactions in DOM molecules. Interpretation of the data based on the NICA-Donnan approach showed that behavior of DOM chromophores was consistent with results of potentiometric titrations. Similar trends were observed for changes of the spectral slope of the DOM absorbance spectra in the range of wavelengths 325–375 nm (DSlope325–375). The behavior of DSlope325–375 values was modeled based on the NICA-Donnan approach and correlated with potentiometrically-estimated charges attributed to the carboxylic and phenolic groups. The correlations between DSlope325–375 and charges of low- and high-affinity protonation-active groups in DOM were monotonic but not linear and had important differences between the colloidal and hydrophobic fractions.

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