Humic acids carboxyl groups

About half of the dissolved organic carbon may appear in humic or fulvic acids. These are high-molecular weight organic compounds of a composition which is somewhat uncertain. They contain aromatic hydroxyl and carboxyl groups which have the ability to bind to metal ions. Rivers and estuaries typically contain 10 mg/liter of acid with an exchange capacity of 5-10 mmol/g, mainly due to carboxylic... 

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. 

Comparison of an alkalimetric titration curve of an equimolar (1 O 4 M) solution of acetic acid (pK = 4.8) and phenol (pK = 10) with a humic acid that contains 10 4 M carboxylic groups. 

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. 

The concentrations of HS fractions, i. e., fulvic acids, humic acids, and humin expressed as mol Cl kg of SPH s in terms of various functional groups such as carboxyl, phenolic OH, alcoholic OH, and carbonyl are shown in Fig. 10. 

The total acidity is highly variable but in general it is lower than that of soil humic matter acidity in marine humic compounds is largely a function of carboxyl groups the phenolic hydroxyl content is generally low the restricted aeration of bottom sediments favors greater accumulations of carbonyl groups... 

Fiumic acid binds Fe " " predominantly or exclusively through carboxylate groups, though there may be a very small amount of ligation by phenolate. Some qualitative observations on stabilities and complex formation and dissociation reactivities are available for humic acid and for fulvic... 

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