8 - Cation binding sites in humic substances

Summary

This chapter summarises information about the chemical groupings responsible for the binding of protons and metal cations by humic substances. The bonding atoms themselves are considered, together with structural arrangements at the molecular level (conformation). Such information complements the quantitative binding data reviewed in Chapter 7, and can help in the formulation of mechanistic models of cation binding by humic matter (Chapters 9–11).

Proton-dissociating groups

The proton dissociation properties of humic substances (Section 7.1) point very strongly to the presence of carboxylic acid groups in their structures. Direct evidence for the chemical nature of groups dissociating in the acid-to-neutral pH range was provided by Cabaniss (1991), who used Fourier-transform infra-red (FTIR) spectroscopy to show that the groups were nearly all carboxylic acids (Fig. 8.1).

Leenheer et al. (1995a, b) carried out detailed studies to investigate the chemical natures of carboxylic acid groups in Suwannee River fulvic acid. They used chemical modifications specific to individual groupings, titration data, model compound studies, NMR, and theoretical arguments, to estimate the contributions of different groups. They pointed out that a significant number of the groups are quite strongly acidic. For the fulvic acid sample in question, they estimated that one-third of the carboxyl groups had pK values of 3.0 or less. The main aim of their work was to account for such acidity. Table 8.1 summarises their findings. It was deduced that the strong carboxyl acidity is associated with polycarboxylic α-ether and α-ester structures.