Humic substances complexes with metals

Humic substances can form complexes with metals, including cationic micronutrients (36), thanks to the presence of electron-donor functional groups in these molecules. It therefore appears evident that due to these properties, humic substances can contribute to the regulation of the chemical balances of metals, thus influencing their solubility (5). With regard to plant availability, the molecular dimension and solubility of humic substances are very important. 

On the other hand, at a pH value of 5.0 there were no large differences in the strength of bonds between humic acids and metals such as Ca, Mg, Mn, Co, Ni, and Zn, whereas bonds with Pb, Cu, and Fe were stronger than with other metals (Schnitzer and Kahn, 1972) this behavior indicates that at different pH values, metal humic substance complexes of different stability are formed in the soil. This aspect is of particular relevance in an environment such as the rhizosphere, where dynamic pH gradients are present mainly due to the availability of nutrients and to their selective uptake by roots. With regard to plant availability, great importance lies in the molecular dimension and solubility of humic substances (Briimmer and Herms,... 

Figure 3.19 Schematic presentation of metal-humic substances complexation (adapted and modified from Stevenson, 1982, with permission). 1, Electrostatic interaction 2, inner-sphere complexation 3, weak water bridging.

Fulvic acids are the primary type of humic substances that bind metal ions since they contain many oxygen-containing functional groups. Many of the low-molecular-mass ligands, however, can form more stable complexes with polyvalent cations than do fulvic acids. 

Due to the various health risks of different element species, there are a multitude of applications for natural water samples in this field (e.g., Cr and Sb speciation or Br and I determination).19 The investigation of heavy metal complexes with humic substances by isotope dilution SEC-ICP-MS has been described, for example, by McSheehy and Mester.20 Copper, zinc and molybdenum were found to form complexes with similar size fractions of humic substances in seepage water samples from soils. Sturgeon s group proposed the use of solid phase microextraction (SPME)... 

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