Humic materials functional groups

Figure 7.2 C DPMAS NMR spectra of the three humic fractions isolated from a peat soil using the procedure outlined in Figure 7.1. Carbon-type distributions generally used to describe samples are aliphatic (0-50 ppm), carbohydrate (50-110 ppm), aromatic (110-190 ppm) and carboxyl (190-220 ppm). Elemental data [6] are average elemental compositions for each humic fraction. Functional group analyses are for these specific samples [7]. There is considerable variation in these chemical characteristics among samples, and between environments, for each fraction. For descriptions of compositional variations in humic materials from different environments the reader is referred to the references [1-5].

Humic materials absorbed in the UV-vis spectral region because of an aromatic structure containing phenolic and carboxylic functional groups. 

Concurrent studies in these laboratories (15) indicate that humic acids heated to temperatures around 300°C. (or higher) will, even in the presence of oxygen, tend to lose their acidic functional groups and revert to an essentially coal-like material. In view of this and observations made in the course of the investigations reviewed here, the overall reactions occurring during dry oxidation of coal can be put in the form ... 

If it is also recalled that alkali soluble material (humic acid) builds up much more slowly than acidity (and always markedly dependent on T and [O]), and that the distinctly acidic parent coal is effectively insoluble in alkali, it becomes evident that acidity and alkali solubility are not necessarily covariant, and that accepted definitions of humic acid are, chemically speaking entirely arbitrary. Under the conditions of this study the oxidation appears to involve two simultaneous but seemingly unrelated reactions which result in the development of acidity and in molecular (skeletal) breakdown, respectively, and this suggests that alkali solubility is mainly a consequence of degradation which is only coincidentally connected with the formation of acidic functional groups. Figure 20 illustrates this concept qualitatively and leads to the inference that the wide spread in molecular weights of humic acids reported... 

The XAD-8 resin separation of hydrophobic and hydrophilic components of WSOM was also employed by Sannigrahi et al. (2006). The 13C-NMR results indicated that WSOM in urban atmospheric particles is mostly aliphatic in nature (-95% C mass) with major contributions from alkyl and oxygenated alkyls (-80%), carboxylic acid (-10%), and aromatic functional groups (-4%). The authors also found that urban aerosol WSOC are only qualitatively similar to aqueous humic material in terms of functional group distribution. 

Reductive dissolution of transition metal oxide/hydroxide minerals can be enhanced by both organic and inorganic reductants (Stone, 1986). There are numerous examples of natural and xenobiotic organic compounds that are efficient reducers of oxides and hydroxides. Organic reductants associated with carboxyl, carbonyl, phenolic, and alcoholic functional groups of soil humic materials are one example. However, large... 

Aluminum occurs widely in nature in silicates such as micas and feldspars, complexed with sodium and fluorine as cryolite, and in bauxite rock, which is composed of hydrous aluminum oxides, aluminum hydroxides, and impurities such as free silica (Cotton and Wilkinson 1988). Because of its reactivity, aluminum is not found as a free metal in nature (Bodek et al. 1988). Aluminum exhibits only one oxidation state (+3) in its compounds and its behavior in the environment is strongly influenced by its coordination chemistry. Aluminum partitions between solid and liquid phases by reacting and complexing with water molecules and anions such as chloride, fluoride, sulfate, nitrate, phosphate, and negatively charged functional groups on humic materials and clay. 

In recent years the understanding of colloid surfaces and soils constituents has increased tremendously. Surface coordinating functional groups on particulate inorganic and humic materials are viewed as complexant ligands (Stumm, 1992). 

The study of Lovgren et al. (1987) provides an example of the application of a discrete functional group approach to model the complexation of aluminium with humic substances found in bog-water. The acid-base titration behaviour of the humic material found in Swedish bog-water was modelled as a diprotic acid with the following reactions and acid dissociation constants ... 

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