Humic Substances HS

Humic acids (HA) and fulvic acids (FA) are the main components of humic substances (HS), which are the most chemically and biochemically active and widely spread fractions of nonliving natural organic matter in all terrestrial and aquatic environments. They comprise a chemically and physically heterogeneous group of substances with colloidal, polydis-persed, polyelectrolyte characteristics and mixed aliphatic and aromatic nature (Senesi and Loffredo 1999). 

Note Humic substances (HS) are operationally defined as DOC that is retained on XAD-2 or XAD-8 resins. UDOM is operationally defined as DOC that is retained by a membrane with a 1 nm pore size and 1000 Da molecular weight cutoff. 13C-NMR spectroscopy was used for analysis of carbon functional groups, which are presented as a percentage of the total organic carbon in the sample. THNS, total hydrolyzable neutral sugars THAA, total hydrolyzable amino acids. 

Achard (1786) is regarded as the first to isolate and to make a fractionation of humic substances (HS). He treated peat with potassium hydroxide, added acid to the dark solution, and obtained an amorphous dark precipitate. More precipitate was obtained from the darker (more humified) layers than from the less rotted residues in the upper layers. Later, Vauquelin (1797,1798) isolated in alkali solution humic-type substances from elm wood infected by fungi, and Thomson (1807) proposed the term ulmin for the isolates. 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

The presence of humic substances (HS) in the environment has long been recognized (Kononova, 1966 Schnitzer and Khan, 1972 Orlov, 1985 Frimmel and Christman, 1988). The term humic substances refers to a category of naturally occurring materials found in, or extracted from, soils, sediments, and natural waters and constitute one of the most abundant forms of organic matter (OM) on the surface of the earth (Woodwell and Houghton, 1977 Woodwell et al., 1978). 

Figure 9.3. Model for the action of humic substances (HS) on plasma membrane-bound targets of a root hair cell. Besides the well-known effects on plasma membrane H+-ATPase (P) and carriers (C) of mineral nutrients, the envisaged alteration of membrane lipid environment and the possible interaction with an hypothetical membrane receptor (R) for humic molecules which allows transduction of the signal for induction and expression of genes involved in nutrient uptake and root hair development are also presented.

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

Humic substances (HS) were determined in water using catalytic cathodic stripping voltammetry (CSV).113 This method was based on the adsorptive properties of iron-HS complexes on the mercury drop electrode at natural pH. HS were also determined in water using a method based on the binding of a dye, Toluidine Blue (TB), to HS molecules to produce a dye-HS complex, which caused a decrease in absorbance at 630 nm.114 The method was rapid, sensitive, and practicable. 

Humic substances (HS) are polymeric oxidation products that result from the decomposition of plant and animal residues. As a consequence of their colloidal state in natural waters, they play an important role in the transport of organic pollutants. Thus hydrophobic organic pollutants such as polycyclic aromatic hydrocarbons, DDT, and PCBs are known to bind well to humic substances, thereby enhancing the former s water solubility. One important characteristic of... 

Characterization of complex organic matter like humic substances (HS) is a formidable task (7). A variety of destructive and non-destructive methods have been applied. Among the non-destructive methods, spectroscopic methods, such as NMR and FT-IR have proven to be very useful in providing information about the structure of these materials as a whole. Of these, NMR has proven to be the best method for bulk characterization, especially solid-state I R, where the relative contribution of specific carbon types can be made (2). Different structural parameters (aromaticity,... 

An important source of error in the quantification of PAHs is the ubiquitous presence of humic substances (HS) in natural waters (Kile et al., 1994). These substances have been shown to form associations with PAHs and as such to play an important role in PAH transport and fate in the environment (McCarthy and Jimenez, 1985 Kile et al., 1994 Kopinke et al., 2001). These interactions are the basis of competitive complex-ation that reduces the uptake of PAHs by solid phase extraction (Stum et al., 1998 Li and Kee Lee, 2001). This effect is illustrated in Figure 4.3.4 from Algarra et al. (2005), where the impact of humic substances is shown in a Stem-Volmer plot of the pyrene signal obtained by using the present method against the concentration of Suwanne River Humic acid. In addition to the humic substances many other organic substances and particulate matter may interact with PAHs and thus could significantly... 

Classical studies of membrane filtration have tended to consider humic substances (HS) and multivalent ions. Initial experiments (designed to concentrate NOM as described in detail in Appendix 1) showed a drastic flux decline of the microfiltration process. Analysis of the fouling layer (discussed in Chapter 2) suggested the inclusion of inorganic colloids in the study. 

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