Humic acids 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... 

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,... 

TABLE 4.3. Acidic Functional Group Contents of Humic Acids (HAs) and Fnlvic Acids (FAs) Isolated from Cattle Manure (CM), Sewage Sludge (SS), Municipal Solid Waste Compost (MSWC), Liquid Swine Manure (LSM), Soils Amended with 25tha 1yr 1 of CM for 4 Years (SO + CM25), 25,50, and lOOtha yr1 of SS for 4 Years (SO + SS25,... 

Ritchie, J. D., and Perdue, E. M. (2008). Analytical constraints on acidic functional groups in humic substances. Org. Geochem. 39, 783-799. 

Electrostatic discrete functional group models. The development of charge on the surface of the humic macromolecule decreases the tendency to dissociate protons from the acid functional groups. To overcome this problem an electrostatic correction factor is introduced into the acid dissociation and complexation constants. This is similar to the approach adopted for the SCMs for inorganic surfaces. 

Oiganic colloids (mainly humic substances) also possess large specific surfaces and are generally negatively charged due to the acidic functional groups that tend to dissociate or polarize. Examples of such groups include carboxyls (R-COOH), quinones (-Ar=0), phenols (-Ar-OH), and enols (R-CH=CH-OH). (See Section 5.4.2 for a more complete discussion of humic substances.)... 

The technique of titration calorimetry has been successfully used to determine the nature and abundances of a variety of acidic functional groups in proteins ( ). Several investigators have made rather limited efforts to use titration calorimetry to study humic substances, usually as a method to determine the cation exchange capacity or titratable acidity of humic substances (23, 24). Choppin and Kullberg ( ) have recently used titration calorimetry to determine the enthalpies of neutralization of acidic functional groups in humic substances and have combined that data with pH titration data to obtain AG, AH, and AS values... 

Because of the considerable potential of titration calorimetry as an analytical technique for characterization of the acidic functional groups of humic substances, our studies have been extended to river water humic substances. In this paper, results are presented for the thermochemical characterization of the acidic functional groups of river water humic substances from two quite different river systems 1) the Satilla River in southeastern Georgia, and 2) the Williamson River in southern Oregon. 

Concentrations of Acidic Functional Groups in River Water Humic Substances. The results of the total acidity, carboxyl... 

Table I. Concentrations of acidic functional groups in river water humic substances and soil humic acid.

Perdue E. M., Reuter J. H., and Ghosal M. (1980) The operational nature of acidic functional group analyses and its impact on mathematical descriptions of acid-base equilibria in humic substances. Geochim. Cosmochim. Acta 44, 1841-1851. 

The product of interaction with the herbicides of 3 humic acids of synthetic, coal and peat origin, indicated as synthetic-2, coal-2 and peat-2 in Table 2, will specifically examined in the first part of this paper. The 3 original HA samples do not differ sensibly in their elemental and acidic functional group composition, but they show quite different ash content (Table 2). 

At lower pH values, the adsorption capacity rises for 2 reasons. The acid functional groups become less ionized and hence the neutral molecules adsorb better. Also the solubility, especially of the humic acid fraction, becomes low in an acid environment. In a buffered solution, not only the kind but also the amount of buffer will influence the adsorption. Snoeyink et al [6] demonstrated that at pH 7, the higher phosphate buffer concentrations show higher adsorption capacities for fulvic acid. 

Humic and fulvic acids form both soluble and insoluble complexes with polyvalent cations, depending on degree of saturation. Because of their lower molecular weights and higher contents of acidic functional groups, metal complexes of fulvic acids are more soluble than those of humic acids. Attempts have been made to subdivide humic acids on the basis of molecular weight by fractional precipitation with ammonium sulfate at pH 7 (Theng et al., 1968). 

Relative to soil humic substances, humic substances from Lake Celyn, Wales, and fulvic acids from lakes near Mt. St. Helens contain larger amounts of reactive acidic functional groups (especially carboxyl groups). The reason for this is not known. In Lake Celyn, 24% of the humic acid carbon is carboxyl and 40% is aromatic, suggesting that the Lake Celyn humic acids are largely of terrestrial origin (M. A. Wilson et al., 1981a). 

---