Humic substances spectra

Matrix Effects. pH. Numerous factors such as sample pH, ionic strength, humic substances, and relaxation agents can modify the NMR spectrum. For example, monoester phosphate chemical shifts are pH-de-pendent (44-46), and we showed (44) for several monoester phosphates that as the sample pH is increased, their chemical shifts also increase (Figure 3). This behavior is caused by the ability of monoester phosphates to undergo protonation-deprotonation. Because the monoester phosphate chemical shift is pH-dependent, the curves resulting from plotting pH versus chemical shift are analogous to a titration curve. Thus, monoester phosphate pKa values can be measured from these pH-chemical shift curves (44-46). 

The transient absorption spectrum in Fig. 7 shows the formation of a radical anion of oxazine 725 following the excitation of humic acid adsorbed on Ti02 particles. A similar approach has also been employed recently to reduce Cr(VI) ions using humic acid/ZnO system [250]. The use of humic acid in such a semiconductor mediated reduction process has significant environmental implications. The presence of naturally occurring metal oxide semiconductors in soil systems along with humic substances presents a possible pathway for natural reductive processes to clean up the environment. 

The adsorption-desorption reaction in Eq. 4.3 has been applied to soils in an average sense in a spirit very similar to that of the complexation reactions for humic substances, discussed in Section 2.3.11 Although no assumption of uniformity is made, the use of Eq. 4.3 to describe adsorption or desorption processes in chemically heterogeneous porous media such as soils does entail the hypothesis that effective or average equilibrium (or rate) constants provide a useful representation of a system that in reality exhibits a broad spectrum of surface reactivity. This hypothesis will be an adequate approximation so long as this spectrum is unimodal and not too broad. If the spectrum of reactivity is instead multimodal, discrete sets of average equilibrium or rate constants—each connected with its own version of Eq. 4.3—must be invoked and if the spectrum is very broad, the sets of these parameters will blend into a continuum (cf. the affinity spectrum in Eq. 2.38). 

Figure 10.11 shows the fluorescence emission spectrum of pyrene in the absence and presence of two humic substance concentrations. The fluorescence intensity of pyrene decreases with increasing humic substance concentration. This decrease results from the binding of pyrene on the humic substance. 

All soils would be expected to contain a broad spectrum of humic substances, as depicted in Figure 7. However, distribution patterns will vary from soil to soil and with depth in the soil profile. The humus of forest soils... 

Figure 1 gives a typical infrared spectrum of lake humic acid. The interpretation of infrared spectra of humic substances is discussed in depth by -MacCarthy and Rice in Chapter 21. The similarity of infrared spectra of humic acids from different lakes suggests a similarity of the aspects of chemical structure that are related to their infrared absorptions. However, infrared spectroscopy is not sensitive enough to uncover minor structural differences among humic acids. In fact, humic acids were separated by organic solvents (chloroform, methylethylketone, methanol, dimethylformamide) into various fractions (Ishiwatari, 1969b, 1973). Infrared spectra of two of these fractions, the chloroform-soluble fraction and the methylethylketone-... 

Allow us to obtain information about the range of molecular properties encountered within the spectrum of humic substances. 

The applicability of spectroscopic methods (other than NMR) for determining functionality in humic substances is reviewed. Spectroscopic methods, like all other investigational techniques, are severely limited when applied to humic substances. This is because humic substances are comprised of complicated, ill-defined mixtures of polyelectrolytic molecules, and their spectra represent the summation of the responses of many different species. In some cases only a small fraction of the total number of molecules contributes to the measured spectrum, further complicating the interpretation of spectra. The applicability and limitations of infrared spectroscopy, Raman spectroscopy, UV-visible spectroscopy, spectrofiuorimetry, and electron spin resonance spectroscopy to the study of humic substances are considered in this chapter. Infrared spectroscopy, while still very limited when applied to humic substances, is by far the most useful of the methods listed above for determining functionality in these materials. Very little information on the functionality of humic substances has been obtained by any of the other spectroscopic methods. 

In the spectrum of the Cu -humic acid complex (Fig. 12B) the intensity of the 1710 cm band has decreased considerably compared to that of the humic acid. This provides direct evidence for the participation of carboxyl groups in complexation by humic substances in the aqueous environment. 

A typical ESR spectrum for humic acid is presented in Figure 16 (Steelink and Tollin, 1967). This is similar to a spectrum reported for a soil fulvic acid by Schnitzer and Skinner (1969). The ESR spectrum of humic acid or fulvic acid consists of a single line identified by its position and width. In general, the ESR spectra reported for humic substances are devoid of hyperfine... 

The ESR spectrum of a simple free radical, 2,5-dimethylquinone, is shown in Figure 17. This spectrum has a total of 21 lines (seven triplets), and since each line is characterized by two parameters (position and width) there are a total of 42 bits of data from which to deduce information about this one relatively small molecule. In contrast, the ESR spectra of humic substances are characterized, in general, by only a single line, and one is then left with only two pieces of data from which to deduce information, functional or structural, concerning the nature of the complicated, multicomponent mixtures in humic substances. This, again, puts into perspective, the limitations imposed on various investigational methods when applied to humic substances. 

The reported spin concentrations correspond to one free radical per 1100 molecules (number average molecular weight of 951) for an untreated fulvic acid in the work of Schnitzer and Skinner (1969) and one free radical per 250 molecules of humic acid (1.4 x 10 spins/g, with a molecular weight of 20,000) in the work of Steelink (1964). Consequently, the ESR spectrum is providing data on only a small fraction of the total molecules in the humic mixture, as pointed out by Riffaldi and Schnitzer (1972). Hayes et al. (1975) found that the free radical contents of humic and fulvic acids vary with the solvent used in the extraction, suggesting that the free radical in a humic substance may be an artifact of the extractive procedure. With the severity of these limitations in mind, any generalization concerning the functionality or structural nature of humic substances based on ESR data must be conservative. 

In conclusion, the ESR spectra of humic substances contain relatively little data from which to deduce any detailed information concerning the nature of these materials. The ESR spectrum results from only a small fraction of the total number of molecules that comprise the humic or fulvic acid, further complicating any attempts to interpret the spectra on a microscopic basis. 

It is equally difficult to arrive at concrete conclusions concerning the functionality of humic substances based on their ESR spectra. When compared to the ESR spectra of discrete free radical molecules, the ESR spectra are seen to be exceedingly crude. In addition, the ESR spectrum results from only a very small fraction of the molecules present in the system, further complicating any interpretation. The ESR spectra of humic substances have been interpreted in terms of the presence of semiquinone moieties. While such interpretations are reasonable, and consistent with what is known about these substances, no definitive proof of the nature of the free radical entities in humic substances has yet been provided. 

The basic problem in the interpretation of NMR spectra of humic substances is that for quantitation, as we have pointed out above, the integrated area under a given band in a NMR spectrum is not only a function of the number of carbon atoms resonating at that frequency, but is also a... 

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