Humic acids acidic-exchange capacity

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... 

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... 

Both clay and organic matter contribute to the cation-exchange capacity (CEC) of the soil. The contribution from humic and fulvic acids is due largely to the ionization of COOH groups, although some contribution from phenolic OH and NH groups is expected. 

Our results show that the adsorption on suspended matter is increased in the presence of suspended humic acid (Figure 10). This is supported by the finding of other authors (53,54) showing that organic flocculant coatings greatly affect the cation exchange capacities of sediment and suspended matter. 

Fulvic acids are organic substances which remain in the solution after acid precipitation of humic and hymatomelanic acids from the extract of humic acids. The chemical structure and physico-chemical properties of these substances are similar to those in humic acids. The aromatic character of the molecule is less distinct, and side chains predominate. They also differ from humic acids in their lower molecular weight, yellow to light-brown colour, better solubility in water and easier hydrolysis in acid media. They are easily eluted from soils and peat. They are more acid than humic acids (pH of solution with c = 0.01 mol 1 is about 2.6). They manifest complex-forming properties and exchange capacity. 

Calculation based on humic cation-exchange capacity data obtained by Stuermer and Harvey (1974, 1977) fulvic acid (FA) concentration in Sargasso Sea = 0.134 mg 1" (49.98% C) equivalent weight = 473 g the FA carbon represents 5.1% of the DOC it is assumed that recovery of FA is 100%, and that FA is the only metal-binding ligand. Lower recoveries and/or the presence of other non-humic ligands would tend to increase values shown. 

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