Fulvic acid ligands

Gamble, D. S., Underdown, A. W. and Langford, C. H. (1982). Copper(II) titration of fulvic acid ligand sites with theoretical, potentiometric and spectro-photometric analysis, Anal. Chem., 52, 1901-1908. 

The way in which conditional stability constants are used to calculate the distribution of chemical species can be illustrated by consideration of the forms of dissolved Cu(II) in a dilute, acidic soil solution. Suppose that the pH of a soil solution is 6.0 and that the total concentration of Cu is 0.1 mmol m 3. The concentrations of the complex-forming ligands sulfate and fulvic acid have the values 50 and 10 mmol m 3, respectively. The important complexes between these ligands and Cu are CuS04 and CuL where L refers to fulvic acid ligands (see Section 2.3). These illustrative complexes are not the only ones formed among Cu, S04, or L, nor are the three ligands the only ones that form Cu complexes in soil solution.29 Under the conditions assumed, the equation of mole balance for Cu is (cf. Eqs. 2.11 and 2.30)... 

Intensity changes in the natural fluorescence of fulvic acid (FA) caused by the binding of metal ions have been well documented. Various quantitative models have been developed relating the measured fluorescence signal to the amount of metal ion bound to fulvic acid. Stem-Volmer, linear, and nonlinear models developed for 1 1 binding between metal ions and fulvic acid ligand sites have been used to calculate concentrations of FA binding sites (CJ, and conditional stability constants (K). However, the ability of these models to describe metal complexation by the polydispersed fulvic acid system is somewhat limited. 

Fulvic acid plays a major role in the transport and deposition of Fe, AI, and other metals in soils. The acid is produced by organic decay in the top of the soil s A horizon. Fulvic acid ligands can form soluble complexes with Fe + and AP+ and other metals, which facilitates metal movement downward through the soil. As a rule of thumb, if the molar ratio of metals/fulvic acid is less than 1/1, the metals are water soluble and mobile (Schnitzer 1971). If that ratio exceeds 1/1, the metals become insoluble and immobile. Thus, as fulvic acids are destroyed by aerobic decay or other processes during downward percolation, the metals precipitate, typically in the soil s B horizon. Precipitation of Fe and Al (and also Mn) oxyhydroxides, in turn, leads to coprecipitation and concentration of trace metals such as Cu, Cd, Zn, Co, Ni, and Pb in the soil (cf. Suarez and Langmuir 1975). 

The actinide cations can form strong complexes with humic and fulvic acid ligands. This may reflect the presence of phenolic, amine, and alcoholic OH-groups, and also carbonate groups in... 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

Using the aforementioned methodology, the electrode reaction of Mo(VI) has been studied in the presence of phenanthrohne and an excess of fulvic acids [105]. Both ligands exhibit a synergetic effect toward adsorption of the mixed complex of... 

Humic and fulvic acids are the main natural ligands acting in the subsurface aqueous solution. An example of a metal species that may occur in natural waters as a result of potential inorganic and organic ligands is presented in Fig. 6.3. It is... 

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

Also present in many natural waters are humic/fulvic acid, citric acid, and the like. These organics also can complex actinides. In Figure 15.18, we show the relative stability constants for the first complexation reaction of various ligands with actinides of different oxidation states. Clearly, the carbonate and humate ions along with hydrolysis dominate the chemistry. The tetravalent actinide ions will tend toward hydrolysis reactions or carbonate complexation rather than humate/fulvate formation. 

Many studies have been carried out concerning the stability constants of humic and fulvic acid complexes.188 190,191 Stability constants vary considerably with pH and ionic strength213 and this, together with the variable nature of the ligands involved, accounts for the range of values reported for individual metal ions in the literature. However, the stabilities of divalent metal complexes generally follow the well-known Irving-Williams order Mg < Ca < Mn < Co < Zn = Ni < Cu < Hg. 

---