Fulvic acid complexes with

Tiseanu, C.-D., Kumke, M. U., Frimmel, F. H., Klenze, R., and Kim, J.-I. (1998).Time-resolved fluorescence spectroscopy of fulvic acid and fulvic acid complexed with Eu3+—A comparative study. /. Photochem. Photobiol. A Chemistry 117,175-184. 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

Elkins, K. M., and D. J. Nelson. 2001. Fluorescence and FT-IR spectroscopic studies of Suwannee river fulvic acid complexation with aluminum, terbium and calcium. Journal of Inorganic Biochemistry 87, no. 1-2 81-96. doi 10.1016/S0162-0134(01)00318-X. 

Humic acids and fulvic acids interact with a wide variety of cations. In addition to interacting with iron and aluminium, the species with which they are complexed in soils (57), they also form stable complexes with zirconium, thorium, the lanthanides and the uranyl ion. In the case of uranium it has been suggested that humic acids could be of considerable importance in the geological formation of secondary deposits of uranium (58). 

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

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. 

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. 

The fluorescence properties of two fulvic acids, one derived from the soil and the other from river water, were studied. The maximum emission intensity occurred at 445-450 nm upon excitation at 350 nm, and the intensity varied with pH, reaching a maximum at pH 5.0 and decreasing rapidly as the pH dropped below 4. Neither oxygen nor electrolyte concentration affected the fluorescence of the fulvic acid derived from the soil. Complexes of fulvic acid with copper, lead, cobalt, nickel and manganese were examined and it was found that bound copper II ions quench fulvic acid fluorescence. Ion-selective electrode potentiometry was used to demonstrate the close relationship between fluorescence quenching and fulvic acid complexation of cupric ions. It is suggested that fluorescence and ion-selective electrode analysis may not be measuring the same complexation phenomenon in the cases of nickel and cobalt complexes with fulvic acid. 

In the course of time it has been unambiguously demonstrated that humic- and fulvic acids interact with metal cations by forming rather stable, and often soluble complexes(1 2). The increasing awareness of a possible pollution of the environment, e.g. in connection with the disposal of nuclear waste, emphasizes the need for additional knowledge about the interaction between relevant metal ions, e.g. radionuclides commonly present in nuclear waste, and humic substances. The possible presence of soluble and rather stable complexes may play an important role in determining the migration behavior of the metal ions under shallow land burial conditions. The influence of humic- and fulvic acids on the migration behavior of metal ions has been discussed previously (2-6),... 

Senesi et al. (1977), using the methods of electron spin resonance and Mossbauer spectroscopy in conjunction with chemical methods, established that at least two and possibly three forms of binding of Fe occur in humic materials. Ferric iron is firmly bound and protected in tetrahedral or octahedral coordination this form of binding of iron is resistant to chemical complexing and reduction. Fe adsorbed on the outer surfaces of humic materials is less firmly bound. The iron-fulvic acid complexes studied contain from 5.5 to 50.1% Fe, but a large part of the iron is bound to the surficial octahedral position. 

Indeed, the metal humic acid complexation constants obtained for trace metals correlate reasonably well with the hydroxide and carbonate stability constants of the metals (Turner et al., 1981). Humic and fulvic acid complexation is therefore most likely to be significant for those cations that are appre-... 

Mixed ligand complexes may also exert a significant influence on metal speciation and concentration in natural waters. Fulvate-phosphate complexes of Cu", Pb", Cd" and Zn" are more stable than the metal-fulvic acid complexes. Incorporation of these species into models in order to estimate free metal ion concentrations gave estimates which were in excellent agreement with concentrations determined experimentally using ion-specific electrodes. However, mixed ligand complex formation with humic and fulvic acids has not received widespread attention, despite these results and their potential importance. 

Humic and FULVIC acids, along with other organic colloidal materials, are fascinating substances that can have profound environmental consequences. Their abilities to complex radionuclides and toxic metals have been recognized for some time by researchers interested in the migration and mobilization of nuclear and industrial waste at contaminated sites. The micellar properties of humic and fulvic acids also give them the ability to play important roles in the solubilization and transport of hydrophobic pollutants. 

The free and complexed Cd (II) are separated by two 25 cm HPLC columns of Sephadex G-10 (a cross-linked dextran gel of 40-120 p bead diameter). The mobile phase was distilled deionized water. Sephadex G-10 xerogel has an exclusion limit 700, that is, it can be used to fractionate species of molecular weight less than 700. The larger Cd-fulvic acid complex is unretained and elutes before hydrated Cd (II). As with the phosphorus esters above, SEC is a viable method not only for separating these complexes for analysis but also for purification. 

A comparison of the formation constants for metal-fulvic acid complexes shown in Table 5-13 with those values for the complexes of the same metals with NTA (Table 5-10) would seem to indicate that the metal-fulvic acid complexes are much weaker than the metal-NTA complexes. Indeed some of the values of the constants for metal-fulvic acid complexes approach the values for the very weak inorganic ion pairs (e.g., MgCOs , log = 2.9 CaS04 logff = 3.4). Morgan has pointed out that the values for the metal-fulvic acid complexes were measured at much lower pH values than the K values for other complexing agents such as NTA and EDTA. The stability constants for NTA complexes reported in Table 5-10 are for the totally deprotonated species,... 

The relatively recalcitrant dissolved poly-phenolic compounds resulting from breakdown of higher plants (fulvic and humic acids) complex with many bacterial and algal enzymes, but particularly phosphatases (Wetzel, 1992). The formation of such complexes inactivates phosphomonoesterase (Boavida and Wetzel, 1998), which is inhibited both competitively and non-competitively (Wetzel, 1992). Phosphorus-limited cells therefore need to expend more energy on phosphatase synthesis, and enhanced phosphomonoesterase activity has been reported for waters with increased humic materials (Stewart and Wetzel, 1992b). Wetzel (1992) pointed out these results with frequent observations (e.g. Jones, 1990) that the primary production in humic-rich waters is consistently lower than in clear waters with comparable loadings and light availability. 

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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