Humic acids structure

Fig. 9. Schematic of humic acid structure. From Schulten and Schnitzer [48], with permission... 

Figure 9.10 (a) Schematic soil humic acid structure proposed by Schulten and Schnitzer (1997). Note that the symbols stand for a linkages in the macromolecules to more of the same types of structure. (b) Schematic seawater humic substances structure proposed by Zafiriou et al. (1984). (c) Schematic black carbon structure proposed by Sergides et al. (1987). 

Figure 1.4. The Haworth concept of humic acid structure.

Miikki, V., Senesi, N., and Hanninen, K. (1997). Characterization of humic material formed by composting of domestic and industrial biowastes, Part 2 Spectroscopic evaluation of humic acid structures. Chemosphere 34,1639-1651. 

In this study we have investigated selective oxidation as a potential organic sulfur characterization approach. In particular we have used the peroxyacetic acid oxidation procedure. Although this selective oxidant has received some attention in the study of lignin (24) and humic acid structures (25), its application to the study of coal has been limited to only a few instances (26.27J with very little information about organic sulfur species being reported. 

Figure 1.22. Fuchs model for humic acid structure. From Swain (1963). Reprinted by permission of Pergamon Press.

Nevertheless, the major drawback to the investigation of coal structure has bear the incomplete solubihty of the material, which has in many cases dictated that structural determinations be carried out on extracted material. Even then, the answer may not be complete. For example, coal structure is often considered to analogous to humic acid structure humic acids are considered to be soluble molecular entities that are produced during the formation of coal. However, humic acid structure is not well known and has been, in the past, represented as involving large condensed nuclear systems (Figure 10.24). 

FIGURE 10.19 Humic acid structure proposed by (With kind permission from Springer Science+Business Media, Naturwissenschaften, Schulten, H.-R., and M. Schnitzer, 80,1993,... 

MATHUR S.P. and PAUL E.A. 1966. A microbial approach to the problem of soil humic acid structure. Nature, London, 212,... 

Humic substances in sediments and soils have commonly been, defined as heteropolycondensates of decomposing plant and animal detritus 46. For lack of a better structural definition, these macromolecular substances have been divided into three categories fulvic acids and humic acid and humin. Fulvic acids and humic acids are soluble in dilute alkaline solutions, whereas humin is insoluble. 

Various destructive and non-destructive methods of analysis have been tested and H-l and C-13 NMR have, among other techniques provided valuable structural information on soluble humic acids and fulvic acids 48, Humin, on the other hand has withstood detailed non-destructive analysis. 

Rates of hydrolysis may be influenced by the presence of dissolved organic carbon, or organic components of soil and sediment. The magnitude of the effect is determined by the structure of the compound and by the kinetics of its association with these components. For example, whereas the neutral hydrolysis of chlorpyrifos was unaffected by sorption to sediments, the rate of alkaline hydrolysis was considerably slower (Macalady and Wolf 1985) humic acid also reduced the rate of alkaline hydrolysis of 1-octyl 2,4-dichlo-rophenoxyacetate (Perdue and Wolfe 1982). Conversely, sediment sorption had no effect on the neutral hydrolysis of 4-chlorostilbene oxide, although the rate below pH 5 where acid hydrolysis dominates was reduced (Metwally and Wolfe 1990). 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

This organism is able to oxidize acetate to CO2 under anaerobic conditions in the presence of Fe(III). A study of the intermediate role of humic and fulvic acids used ESR to detect and quantify free radicals produced from oxidized humic acids by cells of G. metallireducens in the presence of acetate. There was a substantial increase in the radical concentration after incubation with the cells, and it was plausibly suggested that these were semiquinones produced from quinone entities in the humic and fulvic structures (Scott et al. 1998). 

M. Schnitzer, A chemical structure for humic acid. Chemical, C NMR, colloid chemical, and electron microscopic evidence. Humic Sukstances in the Global Environment and Implications on Human Health (N. Senesi and T. M. Miano, eds.), Elsevier, Amsterdam, 1994, p. 57. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

Malcolm RE, Vaughan D (1979) Effects of humic acid fraction on invertase activities in plant tissues. Soil Biol Biochem 11 65-72 Malcolm RL (1989) Application of solid-state 13C NMR spectroscopy to geochemical studies of humic substances. In Hayes MHB, MacCarthy P, Malcolm RL, Swift RS (eds) Humic substances II. In search of structure. Wiley, Chichester, UK, pp 339-372... 

The surfaces of sorbent materials, e.g., oxide particles in soil, are often less complex than the exterior of protein molecules. However, if such particles are (partly) covered with organic materials, e.g., humic acids and/or fulvic acids, their surface chemistry may be very complex as well. Also, surfaces of biological structures, such as those of plant roots, may be heterogeneous. 

Table 1. Structural carbon distribution (%) of the humic acids extracted from soil horizons, adopted from Xing (2001). The distribution was calculated from solid state 13C Cross-Polarization Magic-Angle-Spinning (CP/MAS) NMR spectra. Chemical shift assignment for carbon functional groups alkyl 0-50 ppm O-alkyl 50-117 ppm aromatic 107-165 ppm.

Simpson M, Chefetz B, Hatcher P. (2003) Phenanthrene sorption to structurally modified humic acid. J Environ Qual 32 1750-1758... 

Cook, R. L. and Langford, C. H. (1998). Structural characterization of a fulvic acid and a humic acid using solid-state ramp-CP-MAS 13C nuclear magnetic resonance, Environ. Sci. Technol., 32, 719-725. 

Many researchers have attempted to unravel the mystery of the structure of humus. One approach has been to isolate fractions by extracting humus using various extraction procedures. These procedures result in the isolation of three or more fractions humic acid, fulvic acid, and humin. Humic material is isolated from soil by treating it with alkali. The insoluble material remaining after this treatment is called humin. The alkali solution is acidified to a pH of 1.0 and the precipitate is called humic acid, while the soluble... 

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