Functional Group Chemistry of Humic Substances

With the help of NMR, vibrational spectroscopy, and pyrolysis, significant information on different humic substance (HS) functional groups isolated from various natural materials has been obtained. However, several of these spectroscopic methods are not useful for in-situ characterization of aqueous and mineral sorbed humics without sample modification. For example, the presence of Fe and Mn oxides, and silicates in soil samples interfere with the organic molecule spectral features and thus limit the characterization of HS at dilute concentrations. Significant overlap of the vibrational spectral features of humic functional groups prevents their identification and characterization of coordination environment.

The objective of our study is to explore the in-situ functional group chemistry of aqueous and mineral sorbed HS (protonation, mineral, metal and contaminant complexation) using element-specific X-ray spectroscopy methods and other conventional spectroscopic and spectro-microscopy methods. We are evaluating the molecular structure of HS on mineral surfaces and their influence on the sorption of trace elements and inorganic and organic contaminants at mineral-water interfaces. In the last year we have conducted a detailed examination of the C, N, P, and S-functional groups in HS, and different structural models related to the HS functional groups. These studies are conducted at the Stanford Synchrotron Radiation Laboratory (SSRL) in Stanford, CA, and the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory in Berkeley, CA. Some of the spectroscopic investigations on the S K-absorption edge are shown here. Interested researchers can contact us for more details.

S-Functional Groups

Using X-ray absorption spectroscopy techniques (specifically, near-edge x-ray absorption fine structure spectroscopy, or NEXAFS), several researchers have examined the S-functional groups in various HS, and reported that HS contain reduced S (mono- and di-sulfides, sulfoxide), sulfonate, and ester sulfates as the dominant S forms. Our experimental results are in good agreement with these previous investigations for the most part, though our results differed in the observed concentrations of sulfonate and some reduced S forms. A brief discussion of these results is presented here.

The S-NEXAFS spectra of organo-S compounds primarily depend on the local symmetry of S groups and the electronegativity of the atoms to which they are connected. This is noticeable in the case of S compounds sensitive to pH, and metal complexation. For this reason, sulfonate and sulfate, and some reduced S forms could not be identified unequivocally in the natural samples. For instance, S in the sulfate ion (SO42-) exhibits 1s → t2* transitions at ~2480 eV. The protonated form of sulfate (HSO4-) showed an intense shoulder at ~2478 eV on the lower energy side of the white line. This is caused by the 1s → a1* and e* electronic transitions associated with the symmetry changes in the SO42-tetrahedron. Although the aqueous solutions containing HSO4- exhibit similar features, the intensity of this low-energy feature is not as pronounced as in solid NaHSO4, which may have been caused by the faster exchange rates of protons with water in aqueous solutions. Attachment of a C atom to one of the oxygen atoms of the SO42- group (as in -C-OSO3, e.g. lauryl sulfate) also showed the same spectral features as those of solid bisulfate.

When compared to the energy positions in sulfonate and organo-SO42-, the white line in sulfonate is at the same location as the low energy feature in organo-SO42- (such low energy feature also exists in the case of organo-PO43-, but this is at a higher-energy than that of phosphonate, and is not as strong as that of organo-SO42-). Although organo-SO42- and sulfonate can be separated by treating the samples with Ba-salts (e.g. barium trifluoroacetate) this restricts the in-situ examination of natural samples. Hence the spectral features at 2478 eV in the S-NEXAFS spectra of HS may not be completely attributable to the sulfonate alone.

When compared to the isolated humic substances, the S-NEXAFS spectra of natural organic molecules in soils showed a much higher concentration of oxidized-S forms. These results indicate that the isolation procedures preferentially concentrate the reduced-S forms. Hence studies with the isolated humic materials perhaps do not represent the true S-chemistry of HS in natural systems.