Metal association, fulvic acid

ESR examination of nonchemically isolated fulvic acids showed that Mn2+ was the primary paramagnetic species observable (60, 61). Most likely, the soluble-colloidal fraction we identified in the speciation studies consisted primarily of such complexes. Because the ESR spectral characteristics of Mn in fulvic acid complexes are quite similar to Mn(H20)62+, Alberts et al. (62) suggested that the metal-fulvate interaction was weak. Stronger interaction would be expected to lead to changes in peak shape. This view leaves unexplained the ability of the complexes to survive the isolation procedure s long ultrafiltration steps, because weak interactions are usually associated with reversible complexation. 

Transport of metals in the environment is critically dependent on speciation. Often metals are associated with colloidal materials, fulvic acid, or humic acid. Extensive research is ongoing to study the uptake capacity of these materials as well as minerals, the distribution of elements in natural water samples, and species-dependent means of remediation. Speciation of yttrium and lanthanides in natural water has been reported [415] using size exclusion chromatography and ICP-MS. Multistage filtration has also been used with ICP-MS measurement of the separated fractions [416]. Kinetic studies of metal uptake rate as a function of EDTA, NTA, and fulvic acid concentrations in the water have been described [417]. 

The presence of at least two fluorophores, and possibly a third, associated with metal ion binding in fulvic acid strongly suggests the need for multiple binding site models. Existing linear and nonlinear models will be reviewed for both fluorescence quenching and enhancement. A new modified 1 1 Stem - Volmer model will be introduced as well as two site and multiple site models. Application of the models to Cu binding by fulvic acid and certain well defined model systems are discussed. 

Cu, have shown toxic effects on a diverse assortment of aquatic biota (6 8). The very same metal ions, on the other hand, portray a reduction or complete eradication of toxic effects when complexed with natural organic matter. Fate and transport of metal ions in the environment are also governed by associations with fulvic acid material. Therefore, determination of stability constants between FA ligand sites and potentially hazardous metal ions should be considered fundamentally important. 

Steinberg, C. and Baltes, G. F. (1984). Influence of metal compounds of fulvic acid/molybde-num blue reactive phosphate associations. Arch. Hydrobiol. 100, 61-71. 

The empirically defined terms "humic acids" and "fulvic acids" have been used to denote important macromolecular constituents of aquatic systems. Even though their structures are incompletely resolved (Section 3.2.3 and 4.5.3), their association with low-molecular-mass organic compounds — and with metal cations — is important in determining the bioavailability and hence biodegradability and toxicity of organic compounds. Their role in the generation of OH radicals under anaerobic conditions is noted in Section 4.1.1, and as intermediates in electron-transfer reactions under anaerobic conditions in Section 5.5.5. 

The mesh structure formed by humic substances is capable of trapping smaller chemical species. For example, minor amounts of acyclic alkanes are found in most samples of humic and fulvic acids, and some of the fatty acids associated with humics may be similarly trapped components rather than bonded to the macromolecular backbone. Humic substances also usually contain a variety of metals, which are incorporated into the macromolecular structure. Metal ions can be surrounded by and bonded to suitable chelating groups, chiefly carboxylic acids, on humic molecules that stabilize the ions and allow them to be transported with the organic material. This important property of humic substances is examined again later (Section 7.6.5) in relation to the environmental fate of heavy metals. 

Of the other forms of nonresidual associations, some metal-organic compounds, for example, fulvic and humic acids, have been shown to be particularly effective in the transfer of (toxic) metals from inorganic matter into organisms 51). According to Table II, where data of humate extractions with O.IN NaOH are compared, these effects should be more relevant for iron, copper, zinc, and lead in the sample from the Rhine River than in the sediment material from Lake Constance. The other nonresidual metal associations (easily reducible, carbonates, moderately reducible forms) partly indicate higher percentages in Lake Constance sediments (Mn and Pb), whereas others (chromium, copper, and zinc) are enriched in the Rhine sample. 

Humic substances undoubtedly have the ability to combine with (or bind) considerable quantities of metal ions. For example, many metals can be enriched into peat (which is a type of soil organic matter of a humic nature) from soil waters so that the concentration of the metal in the peat is up to 10,000 times that of its concentration in the water. It has also been observed that in many natural waters, Fe(III) is associated with the color causing organics. Schnitzer has estimated that a solution containing 100 mg of fulvic acid/liter can maintain in solution 8.4 mg Fe(III) and 4.0 mg Al(lII)/liter. The concentration of Fe(III) is more than two orders of magnitude greater than that expected in solution in a water at pH 7 in equilibrium with ferric hydroxide. 

Soluble fulvic acid complexes of metals may be important in natural waters. They probably keep some of the biologically important transition-metal ions in solution and are particularly involved in iron solubilization and transport. Fulvic acid-type compounds are associated with color in water. These yellow materials, called Gelbstoffe, frequently are encountered along with soluble iron. 

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