Humic substances elemental composition

Fig. 5. Elemental composition of humic substances representing various solid phase organic matter (SP0M> number of samples = 52)...

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

Elemental Analysis. The elemental analyses are presented in Table IV. The atomic ratios H/C for all drinking water samples (nos. 1-10) were between 1.28 and 1.39. These values were comparable to humic acid derived from lake sediments. However, H/C ratios were much lower when compared to the chlorinated model humic substances (e.g., 1.04-1.08 for CFH-1 and CFH-2). Bromine was present in almost negligible quantities, whereas Cl varied between 0.3 and 2.4, and S varied between 0.9 and 2.7 in the drinking water organic matter. All fractions from drinking water showed similar elemental composition. However, they differed from the elemental composition of the CFH samples in all respects, especially in chlorine content. 

Jince the time of Berzelius, chemists have proposed structures for the amorphous, black substance known as humic acid. In the past 150 years, much experimental work has appeared on the nature of humic acid, most of it based on classical chemical and microbiological studies. Very little information about the molecular structure of humic add has resulted from these studies however. Some of the problems plaguing investigators in this field have been (a) variation in the source of humic acid, (b) variation in the definition of humic fractions of soil and coal, (c) lack of crystallinity of the samples, (d) uncertainty of molecular weight measurements, (e) variation in extraction techniques, and (f) variation in elemental composition. The little unambiguous information that exists today is based on extensive degradation of the humic acid polymer and represents only a small fraction of the total molecule. 

Hunt, A. P., J. D. Parry, and J. Hamilton-Taylor. 2000. Further evidence of elemental composition as an indicator of the bioavailability of humic substances to bacteria. Limnology and Oceanography 45 237—241. 

Rice, J. A., and MacCarthy, P. (1991). Statistical evaluation of the elemental composition of humic substances, Org. Geochem. 17, 635-648. 

Steurmer, D. H., Peters, K. E., and Kaplan, I. R. (1978). Source indicators of humic substances and protokerogen. Stable isotope ratios, elemental compositions and electron spin resonance spectra. Geochim. Cosmochim. Acta. 42, 989-997. 

Proton and C-NMR data compare well with each other and suggest that surface ocean HMWDOM has a H C ratio of approximately 1.8—1.9 (Aluwihare, 1999 Benner et al, 1992) and an 0 C ratio between 1 and 1.1. These H C and 0 C ratios are very close to those of a pure carbohydrate with a general hexose structure (e.g., C6H12O6). In comparison, humic substances isolated from seawater have an H C ratio between 1.2 (direct elemental analyses) and 1.4 (based on NMR estimates) and are therefore, relatively C-rich (Hedges et al, 1992). The H C and 0 C composition of phytoplankton as estimated by NMR spectroscopy is approximately 1.7 and 0.3, respectively (Hedges et al, 2002). In comparison to phytoplankton... 

Humic substances are ubiquitous in the environment, occurring in all soils, waters, and sediments of the ecosphere. Humic substances arise from the decomposition of plant and animal tissues yet are more stable than their precursors. Their size, molecular weight, elemental composition, structure, and the number and position of functional groups vary, depending on the origin and age of the material. Humic and fulvic substances have been studied extensively for more than 200 years however, much remains unknown regarding their structure and properties. 

TABLE 10. Elemental Composition of Humic Substances from Various Organic Soils with Standard Errors of the Means... 

Elemental composition, functional group analyses, spectral properties, and characterization of acid hydrolyzates have shown that peatland humic acids are similar to those in mineral soils. NMR spectroscopy has revealed that peat fulvic acids are largely carbohydrate in nature while the residue of alkaline extraction is not all humin. It is suspected but not proven that humic substances in peatlands are more soluble, due to lack of clays and mineral elements, than those in mineral soils. 

NATURE OF HUMIC SUBSTANCES IN GROUNDWATER Elemental Composition... 

Elemental composition is one of the most essential characteristics of humic substances. Average elemental composition of humic substances was calculated and results are presented in Table 3. As shown, data for fulvic acid and humin are rare compared to those for humic acid. Interestingly, the average... 

GEOCHEMISTRY OF HUMIC SUBSTANCES IN LAKE SEDIMENTS 153 TABLE 3. Average Values of Elemental Composition of Humic Substances from Lake Sediments" ... 

The ratios of humic to fulvic acids in estuarine and coastal sediments range from 0.4 to 3.4, the higher values being associated with areas or sediments having a terrestrial influence (Palacas et al., 1968 Brown et al., 1972 Hue and Durand, 1973 Pelet and Debyser, 1977 MacFarlane, 1978). These values are also consistent with those from other marine and terrestrial environments (Ishiwatari, 1966 Kononova, 1975 Stuermer et al., 1978 Cronin and Morris, 1982). Other parameters measured on coastal humic substances, such as elemental composition, spectral properties, organic components, stable isotope ratios, or C ages (Pelet and Debyser, 1977 Stuermer et al., 1978 Benoit et al., 1979 Nissenbaum, 1979) are consistent with terrestrial or marine humic compounds, or a mixture of these two endmembers. 

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