Carboxyl groups, humic substances

Humic substances are anionic polyelectrolytes of low to moderate molecular weight (500 - 20 000) their charge is due primarily to partially deprotonated carboxylic and phenolic groups (see Fig. 4.15). 

The basic structure of humic substances involves a backbone composed of alkyl or aromatic units crosslinked mainly by oxygen and nitrogen groups. Major functional groups attached to the backbone are carboxylic acids, phenolic hydroxyls, alcoholic hydroxyls, ketones, and quinones. The molecular structure is variable as it is dependent on the collection of DOM available in seawater to undergo the various polymerization, condensation, and oxidation reactions and reaction conditions involved in humification, as well as the ambient physicochemical reaction conditions, such as temperature and light availability. 

H NMR data from these seven sites are presented by spectral peak-height ratios in Table III. The sites were listed in order of increasing aromatic plus olefinic carbon percentages. Fulvic acids from all the lake samples are much lower in aromatic plus oleflnic carbon content than those from river samples. These results confirm the hypothesis that autothonous inputs result in dissolved humic substances that have a low aromatic plus oleflnic carbon content. The lake samples also are lower in the ratios of peak 2 (carboxylated chains and aliphatic ketones), peak 3 (carbohydrates), and peak 4 (phenolic tannins and lignins) to peak 1 (branched methyl groups and alicyclic ali-phatics) than are the river samples. 

In this chapter the sorption of two organic substances, EDTA and valine amino acid, are discussed. They were selected for two reasons as seen in Figure 2.8, EDTA can be sorbed on the edge sites of montmorillonite. The other reason is the presence of characteristic functional groups in both molecules. The carboxylic and amine groups are very important in organic syntheses and also in humic substances. 

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