Fulvic acid chemical properties

TABLE III Differences in Chemical Properties of Suwannee River Fulvic Acid Sorbed onto Iron Oxide (Sorbable) and Suwannee River Fulvic Acid Remaining in Solution (Nonsorbable)... 

Molecular fluorescence spectroscopy is a commonly employed analytical method that is sensitive to certain chemical properties of FA (9-13). Fulvic acid s molecular fluorescence is principally due to conjugated unsaturated segments and aromatic moieties present in the macromolecule (14). Several types of fluorescence spectra can be measured, including an excitation emission matrix or total luminescence spectrum, constant offset synchronous fluorescence, excitation spectra, and emission spectra, furnishing the researcher with useful data. The ability to resolve and select multiple fluorescent species makes these approaches extremely useful for studying FA relative to its chemical reactivity. 

Soil DOM comprises a wide variety of organic substances. Specific microbial or plant exudates are released as a consequence of metabolic activity of soil biota. Transformations of biogenic compounds, through partial degradation and polymerization, result in the formation of a structurally ill-defined mixture of humic substances, a term used to include both humic and fulvic acids (20, 21). Nonuniform effects of DOM components on metal mobilization may be anticipated because of the heterogeneity of their chemical structures and properties. 

General Chemical Properties of Humic and Fulvic Acids... 

Elemental composition is an important chemical property which can be used to establish the nature and source of humic substances. When the percent composition data are displayed as the atom ratios H C, 01C, and NIC, some general characteristics become visible. Soil, coal, marine, and aquatic humates may be distinguished, one from the other. Structural trends may be identified in specific environments, such as lake sediments and soil profiles. Nonhumate contaminants can be detected. Atom ratios may also aid the investigator in proposing hypothetical structures for humic and fulvic acids and serve as a guide in the synthesis of artificial humates. 

The fate of trace metal radionuclides in the aquatic environment and their participation in the biogeochemical cycle depend strongly on the chemical and physico--chemical form in which radionuclides are introduced in natural waters. The abundance of natural humic substances and their ability to form metal complexes and to adsorb on suspended matter and sediment makes these substances especially important in transport, availability and accumulation of trace metal radionuclides in natural water environments. In that sense complexatlon of di- and tri-valent metal radionuclides with humic and fulvic acids of different origin was studied. The sorption properties of natural suspended matter and undissolved humic acid for the sorption of some radionuclides was also studied. 

Although this scheme of separation appears to be somewhat crude and arbitrary, it does achieve a degree of segregation of polymeric materials as gauged by their more important chemical properties. Table 2.5 summarizes some of these properties, showing that the fulvic acid-humic acid-humin sequence represents a continuum of... 

Chemical characterization of DOM has been studied extensively, providing a wealth of information regarding its chemical properties [17,20,25,60,64-89]. CDOM is usually characterized as aquatic humic substances, such as humic and fulvic acids, owing to the presence of multiple double bonds in aromatic, aldehyde, and ketone groups. Isolation of humic substances involves their separation by adsorption on macroporous resins (e.g., XAD-8 or XAD-4) and elution at various pH [90]. Humic acids are soluble above a pH of 2, while fulvic acids are soluble at any pH. Solid phase extraction (SPE) onto Cig resin is also employed to isolate CDOM [44,73]. Amador and coworkers have shown that... 

Leppard, G. G., J. Buffle, and R. Baudat (1986), A Description of the Aggregation Properties of Aquatic Pedogenic Fulvic Acid Combining Physico-chemical and Microscopical Observations, Water Res. 20, 185-196. 

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. 

Fulvic acids are organic substances which remain in the solution after acid precipitation of humic and hymatomelanic acids from the extract of humic acids. The chemical structure and physico-chemical properties of these substances are similar to those in humic acids. The aromatic character of the molecule is less distinct, and side chains predominate. They also differ from humic acids in their lower molecular weight, yellow to light-brown colour, better solubility in water and easier hydrolysis in acid media. They are easily eluted from soils and peat. They are more acid than humic acids (pH of solution with c = 0.01 mol 1 is about 2.6). They manifest complex-forming properties and exchange capacity. 

Humic substances have been arbitrarily divided into three diverse groups of compounds on the basis of their solubility in dilute acid and dilute base, Fulvic acids are soluble in both dilute acid (pH 1) and dilute base. Humic acids are soluble in dilute base but are precipitated by dilute acid (pH 1). Humin is insoluble in both dilute acid and dilute base. Some authorities divide humic substances into only two groups the humic acids and the fulvic acids, stating that the humins have the same characteristics as humic acids but that their solubility in base is hindered because they are associated with clay minerals in natural waters.The gross chemical and physical properties of humic acids and fulvic acids are presented in Table 5-12. It is the fulvic acid fraction that appears to... 

Humic and fulvic acids are highly active soil organic polyelectrolytes which interact chemically and physically with pesticides and with other organic pollutants that are in natural water systems. In practically all instances the chemical and physical mechanisms of these interactions have not been elucidated. The elucidation of these interactions will require the use of well defined, homogeneous soil polyelectrolyte fractions. A better understanding of the chemical structure of humic and fulvic acids will also be necessary. With a more detailed understanding of interactions of pesticides with all of the components of natural water systems, it should then be possible to tailor their properties so that a minimum of undesirable alteration of environment will result from their use. 

These measurements may also aid In the characterization of the physical and chemical properties of the humic and fulvic acid fractions of dissolved organic matter. The present study describes an experimental setup for determination of luminescence lifetimes and presents values obtained for some natural organic materials. 

---