Elemental compositions, FT-IR spectra and thermal behavior of sedimentary fulvic and humic acids from aquatic and terrestrial environments

Studies on elemental analysis, Fourier transform infrared spectroscopy (FT-IR) and thermal decomposition (thermogravimetry, TG; and derivative thermogravimetry, DTG) of sedimentary fulvic (FA) and humic acids (HA) from marine, estuarine, lacustrine and terrestrial environments are presented. In general, H/C and N/C atomic ratios, as well as the infrared spectra, reflected the extent of the influence of algal and/or terrestrial organic matter sources on the samples, the aquatic (both, marine and freshwater) humic substances (HS) being richer in nitrogen and more saturated, than terrestrial materials. Comparison of properties of FA and HA from the same parent sediments showed that the latter are relatively richer in nitrogen and unsubstituted aliphatic chains and poorer in carboxylic groups. FT-IR spectra showed that the nitrogen present is mostly as forming part of amide groups. Concerning thermal degradation, two main steps were observed for all samples: the first, relative to the loss of moisture, being located between 40degreesC and 100degreesC and the second between 270degreesC and 440degreesC. FT-IR spectra of the samples which had been heated to 90degreesC, 400degreesC and 900degreesC showed that, upon heating, the carboxyl content decreases (especially for FA), the aliphaticity decreases and the aromaticity increases (especially for HA), indicating that the 270degreesC-470degreesC degradation step might be related to decarboxylation and unsaturation losses. In spite of this, both kinds of HS, were shown to be highly thermo-resistant materials retaining most of their typical original infrared spectral features, even after being heated to 400degreesC.