Spectroscopic investigations of lignin, humic substances and peat

Abstract

The close structural and genetic relationships between lignin, humic substances and further products of the coalification process, often supported by strong arguments and well-based considerations, is generally accepted. Such relationships can be particularly well demonstrated by absorption of light by these substances in the ultraviolet and visible regions (250–700 mμ), in the near infrared (0.7–2.5 μ), and finally in the infrared region of the NaCl-prism (2–14 μ). In fact, the similarity among these substances also became apparent in the general gradation of these spectra; such gradation may take on particular significance if interpretation of the spectra is to be used for genetic, chemical, or even physical studies. In any case, if it is recognized that the spectra have different sources in different regions, then the spectra of individual regions must be very different. In some cases, special atomic groups are associated with characteristic bands, and in many instances additional use of the near infrared region (harmonics) can be useful. Ultraviolet spectra are usually completely uncharacteristic, with absorption intensity dropping in going from shorter to longer wavelengths. Nevertheless, the general impression of the ultraviolet spectrum, along with other prominent maxima and minima from other spectra, and additional analytical data permit a certain delineation in the lignification-humification-coahfication sequence. Therefore, it is obvious that it is necessary to study the materials in question over the entire accessible region of light absorption. As a further point, it should be noted that a spectrum is frequently difficult to interpret and become useful only when a reference material can be used. The intermediate position of humic substances is not surprising inasmuch as they certainly arise from lignin, judging from the decomposition products of lignin, and, moreover, its natural conversion in the coalification process leads first to peat and the usual related substances. The particularly uncharacteristic ultraviolet spectrum of humic substances should be noted. If reactive acidic complexes (humic acid precursors) can be separated from the true, essentially unreactive humic acids, then it should be possible to determine their differences experimentally by absorption techniques using suitable preparations. If this study could then be generalized, it might illustrate that the further advanced a material is in the course of this natural process, the more uncharacteristic its ultraviolet spectrum becomes. A number of maxima in the ultraviolet spectrum were taken as being characteristic for lignin, and in this way the substance could be differentiated from humic acid; the humic acid precursors gave the same general impression. It should be noted that for many lignins there is a displacement of certain maxima dependent upon pH, an effect that is not generally noted for humic acid precursors. The spectrum for lignin also shows a general decrease in the intensity of light absorption with increasing wavelength. As already stated, infrared spectra are particular informative. For the materials in question, characteristic groups can be identified in spite of a more or less marked background absorption. Thus, the fewer the number of bands in the infrared spectrum or the further along the material is in the coalification process, the more intense the background absorption becomes. Attempts are being made to define most low molecular weight constituents that do not at the outset enter into the lignification or humification process and to compare their absorptions with those for lignin or humic substances. These materials may play a special key role in the alteration of lignin or humic substances and there is also the possibility that something may be learned about the nature of their bonding with lignin or humic substances. It is felt that if these questions are answerable by the application of spectroscopic techniques, that later a solution to the overall picture may be attainable.