Humic acid in soil

Cooke, J. D., Hamilton-Taylor, J., and Tipping, E. (2007). On the acid-base properties of humic acid in soil. Environ. Sci. Technol. 41, 465 70. 

The work of Swaby and Ladd (1962) dealing with the nature and origin of humic acids in soils is especially noteworthy because of some new ideas that they proposed. In their work with soil humic acid preparations they detected small amounts of phenolic substances and numerous carboxyl groups but no proteins. Over 20 amino acids, some ammonia and traces of indoles were detected after acid hydrolysis. They concluded, as have many other workers, that lignin constitutes a negligible proportion of humic acid even in peats. 

Soils containing high levels of organic matter may be characterized by Py-GC techniques. The presence of humic acids in soils infers that there should be potential for soil differentiation by THM techniques. 

The formation of humic acid in soil may involve oxidation of phenolic vegetable substances such as tannins while adsorbed on silica. A solution of pyrogallol passed rapidly over silica gel produces brown oxidation products. The silica surface plays a role since it is inactivated by being heated to high temperature or by absorption of Al, Ca, or Mg ions (167). The mechanism of the reaction was examined by Zeichmann (168). 

Davies G., Ghabbour E.A., Cherkasskiy A. Tight metal binding by solid phase peat and soil humic acids. In Humic Substances and Chemical Contaminants, C.E. Clapp, M.H.B. Hayes, N. Senesi, P.R. Bloom, and P.M. Jardine, eds. Madison, WI Soil Science Society of America, Inc., 2001. 

Organic matter is also the essential component of natural soils and its association with microorganisms may influence the behavior and fate of toxic metals. A variety of batch complexation experiments were performed by Borrok et al. (2007) in single, binary and ternary systems for the three components natural organic matter (NOM), bacterium (B. subtilis) and metals (Pb, Cu, Cd, and Ni) to determine the significance of ternary complexation. They found that the formation of bacteria-metal-NOM complex is a rapid, fully-reversible chemical process. The stability of bacteria-metal-NOM complexes increases with the decrease of pH. All NOM fractions form ternary complexes to similar extents at circumneutral pH, but humic acid becomes the dominant NOM fraction in ternary complexes at low pH. The abundance of humic acid in ternary form is greatest with Ni or Cd systems and less with Pb and Cu systems. Their results suggest that... 

Spectrofluorimetric methods are applicable to the determination of aliphatic hydrocarbons, and humic and fulvic acids in soil, aliphatic hydrocarbons polyaromatic hydrocarbons, optical whiteners, and selenium in non-saline sediments, aliphatic aromatic and polyaromatic hydrocarbons and humic and fulvic acids in saline sediments. The only application found in luminescence spectroscopy is the determination of polychlorobiphenyl in soil. Generally speaking, concentrations down to the picogram (pg L 1), level can be determined by this technique with recovery efficiencies near f00%. 

NMR has been applied to the determination of organomercury compounds in non-saline sediments and humic and fulvic acids in soil and saline sediments. 

Humic and fulvic acids contain various types of phenolic and carboxylic functional (hydrophilic) groups as well as aromatic and aliphatic moieties which import hydro-phobic properties to these substances. Fig. 4.12 gives a schematic idea on the composition of these substances. We refer to the book of Thurman (1985) and Aiken et al. (1985) for a description of the various properties of humic and fulvic acids in soils and waters and the book by Buffle (1988) for the coordinating properties of humus and humic acids. 

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. 

Perdue, E. M. 1985. Acidic functional groups of humic substances. In Humic Substances in Soil, Sediment and Water Geochemistry, Isolation, and Characterization, (G. R. Aiken,... 

Haider, K., and Martin, J. P. (1967). Synthesis and transformation of phenolic compounds by Epicoccum nigrum in relation to humic acid formation. Soil Sci. Soc. Am. Proc. 31, 766-772. 

Hausler, M. J., and Hayes, M. H. B. (1996). Uses of the XAD-8 resin and acidified dimethyl-sulfoxide in studies of humic acids. In Humic Substances and Organic Matter in Soil and Water Environments Characterization, Transformations and Interactions, Clapp, C. E., Hayes, M. H. B., Senesi, N., and Griffith, S. M., eds., IHSS, University of Minnesota, St. Paul, 25-32. 

Chefetz, B., Tarchitzky, J., Desmukh, A. P., Hatcher, P. G., and Chen, Y. (2002). Structural characterization of soil organic matter and humic acids in particle-size fractions of an agricultural soil. Soil Sci. Soc. Am. J. 66,129-141. 

Figure 15.3. Overlaid HSQC spectra of biopolymers on IHSS peat. (A) Biopolymers lignin (gray), amylopectin (red), albumin (blue) and cuticle (green) overlaid on each other. (B) All biopolymers are illustrated in black. (C) IHSS humic acid extract from peat. (D) Biopolymers (black) overlaid on IHSS peat (green). The highlighted areas in 2D are those not well represented by biopolymers in the HA, namely complex carbohydrates and p- ydroxybcnzoatcs from lignin [see Kelleher and Simpson (2006) for more details]. See color insert. Reprinted from Kelleher, B. R, and Simpson, A. J. (2006). Humic substances in soils Are they really chemically distinct Environ. Sci. Technol. 40,4605-4611, with the permission of the American Chemical Society.

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