Fulvic acids structure

Fulvic Acid Structures from End-Member Environments... 

Murray, K. and Linder, P.W. (1983) Fulvic acids structure and metal binding. I. A random molecular model./. Soil Sci., 34, 511-523. 

One must be cautious in interpreting data on carbohydrates, hydroxyl acids, and uronic acids in humic substances. As Thurman and Malcolm 11983) have pointed out, the amount of these materials in a fulvic acid is a function of the way that it was isolated. They have shown that nonassociated carbohydrates, uronic acids, and hydroxyl acids may be separated from fulvic acid by adsorption chromatography on XAD resins. For example, a prairie soil fulvic acid contained 20% carbohydrate before XAD adsorption chromatography and only 5% after chromatography. In the fulvic acid isolated by Sposito et al. (1978) adsorption chromatography was not used in the purification process and therefore some of the carbohydrate that they report may not be an integral part of the fulvic acid structure. 

Figure 1.24. Schnitzer and Kahn s (1972) model for fulvic acid structure. Reprinted by permission of Marcel Dekker, Inc.

Figure 3.21 Assumed models of humic and fulvic acid structures (Buffle J. et al., 1977)...

Alvarez-Puebla, R. A., C. Valenzuela-Calahorro, and J. J. Garrido. 2006. Theoretical study on fulvic acid structure, conformation and aggregation a molecular modelling approach. Science of the Total Environment 358, no. 1-3 243-254. 

Aharoni, C., and D. L. Sparks. 1991. Kinetics of soil chemical reactions—A theoretical treatment. In Rates of Soil Chemical Processes, eds. D. L. Sparks and D. L. Suarez, 19-59. SSSA Special Publications 27. Madison, Wl Soil Science Society of America, https // dl.sciencesocieties.oig/publications/books/pdfs/sssaspecialpubl/ratesofsoilchem/19. Alvarez-Puebla, R. A., C. Valenzuela-Calahorro, and J. J. Garrido. 2006. Theoretical study on fulvic acid structure, conformation and aggregation. A molecular modelling approach. The Science of the Toted Environment 358, no. 1-3 243-254. 

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. 

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). 

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. 

DePaolis F, Kukkonen J (1997) Binding of organic pollutants to humic and fulvic acids Influence of pH and the structure of humic material. Chemosphere 34 1693-1704... 

The designation of certain classes of organic materials as humic and fulvic acids unfortunately implies a certainty and regularity of structure which... 

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. 

Leenheer, J. A., Wershaw, R. L. and Reddy, M. M. (1995). Strong-acid, carboxyl groups in fulvic acid from the Suwannee River, Georgia. 1. Minor structures,... 

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... 

In spite of this variation in molecular weights and solubilities humic acid and fulvic acid have a very similar chemical composition. These acids consist of aromatic moieties such as phenols, benzenepolycarboxylic acids, hydroxybenzenepolycarbo-xylic acids, 1,2-dihydroxybenzene carboxylic acids, together with more complex condensed structures and polycylic compounds. It is conjectured that these various units are joined together by aliphatic chains (45, 54) the distribution of functional groups is presented in Table 5. 

Recall from Chapter 23.2.4 that humic substances are isolated from seawater by adsorption on a hydrophobic resin followed by elution using solvents of varying pH. The desorbed compounds are fractionated into two classes, humic acids fulvic acids based on their solubility behavior. A model structure for a humic acid is illustrated in Figure 23.10a in which fragments of biomolecules, such as sugars, oligosaccharides. 

Model chemical structure of a soil, (a) Humic acid and (b) part of a fulvic acid. Source-. From (a) Kleinhempel, D. (1970). Albrecht Thaer Archiv 14, 3-14 and (b) Rashid, M. A. (1985). Geochemistry of Marine Humic Compounds, Springer-Verlag, p. 75. 

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