Humic fractions

Any factor that affects the size or shape of a molecule, the hindered movement of a fluorophore within a molecule, or the energy transfer within the molecule will affect the measured depolarization of its fluorescence emission. Therefore, the conformation of humic fractions in solution can be studied as a function of pH, ionic strength, temperature, and other factors by depolarization measurements. The principle of the method is that excitation of fluorescent samples with polarized light stimulates... 

Equations 9 and 10 were used to obtain the molar volumes of the humic fractions. 

Measurements of tan A as a function of temperature for the six humic fractions are plotted in Figures 2a-f. These measurements were made at pH 6.5. These plots show a maximum between 6 °C emd 25 °C. Equation 15 offers another method for the calcuation of the molar volume based on the emisotropy of the molecule and the temperature and viscosity at the experimental maximum of the tan A plots. The consequences of obtaining the volume in this manner are discussed by Mantulin and Weber U8J. 

Humic Fraction Vapparent (cm3/mole) VPerrin (cm ) (cm3/mole) VPerrinA apparent... 

Figures 3a-f show the emission and excitation spectra for all six humic fractions. The excitation and emission maxima are listed in Table III along with the maxima of the phase-resolved emission spectra. In each case the emission spectrum was scanned with the excitation maximum wavelength held constant, and the excitation spectrum was scanned with the emission maximum wavelength held constant. Several interesting features are noted. The two humic samples ( Figures 3a,b) each have two excitation maxima and it appears that a double peak has been merged into the emission scan as evidenced by the shoulder on the high energy side of the emission peak. Similarly it seems evident that the exaggerated shoulders in the emission spectra of all the fractions point to the inclusion of two emission peaks in each spectrum. This evidence suggests the presence of two chromophores in each humic fraction.

Table III. Humic Fractions Fluorescence Excitation and Emission Maxima and...

Humic Fraction Excitation Maxima (in nm) Emission Maxima (in nm) Emission Maxima Phase Resolved (in nm)... 

Humic materials fractionated on the basis of hydrophobicity and proton affinity continue to exhibit two fluorophores as discussed in the section "Exciation-Emission Spectra. Strong evidence to establish the existence of at least two chromophores is seen in the phase-resolved spectra. These spectra are shown in Figures 4 a-f. They consist of the phase-resolved emission spectrum of each of the two fluorophores plotted separately and the normal emission spectrum of the humic fraction. If the nulling out of one fluorophore is exact then the sum of the two separate phase resolved spectra should be additive to equal the normal spectrum. In these figures the normal emission spectrum was measured separately from the two phase resolved emision spectra. The phase resolved spectra were then superimposed onto the scan of the normal emission spectrum. 

Figure 4. Phase-resolved plots of the six humic fractions superimposed on the normal emission scan for each fraction. The emission spectrum of the first fluorophore was suppressed and a scan was made of the second fluorophore then the second fluorophore was suppressed and an emission scan was made of the first fluorophore. Fractions hydrophobic humic weak (a) and strong (b) acids.

Other than a nutritional role linked to mineralization processes, humic compounds have been hypothesized to directly affect plant nutrition, since it has been suggested that roots may take up low-molecular-weight humic molecules (21). Interestingly, plants have been ob.served to express carriers for amino acids (22) and small peptides (23) at the root level. Certain components of the humic fraction have been found inside root cells and were, moreover, translocated to the shoots (24,25). Recent experiments performed on rice cells in suspension culture seem to suggest that they may use carbon skeletons from humic molecules to synthesize proteins and DNA (26). 

G. Concheri, S. Nardi, A. Piccolo, G. Dell Agnola, and N. Rascio, Effects of humic fractions on moiphological changes related to inverta.se and peroxidase activities in wheat seedling roots. Humic Substances in the Global Environment and Implications on Human Health (N. Senesi, and T. M. Miano, eds.), Elsevier. Amsterdam, 1994, p. 257. 

A typical fluorescence EEM results for leachate samples from R-landfill demonstrate five distinctive and intense fluorescence peaks in Figure 2, such as at Ex/Em=230-250/400-440 nm (labeled as A ), which was relative to UV humic fraction identified in location to the diagnostic fluorescence centre observed previously at Ex/Em=220-230/340-370 nm (labeled as D ), a poorly understood fluorescent centre widely attributed to a component of the UV fulvic-like (Coble 1996) at Ex/Em=320-350/400-440 nm (labeled as C ), which can be attributed to aromatic and aliphatic groups in the DOM fraction and commonly labeled as fulvic-like (Coble 1996) at Ex/Em=350-400/420-460 nm (labeled as E ), which is attributed to humic-like and a final fluorescence centre at Ex/Em= 275-280/350-360 nm (labeled as B ), which is attributed to the protein tryptophan, and widely observed in polluted river waters (Baker 2001 2002) and clean estuaries (Mayer et al. 1999). 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

The ratio of fuivic to humic acid varies between soils and between horizons of the same soil. Humic fractions are involved with solubilization of the sesquioxides (especially gibbsite, AI(OH)3 goethite, FeOOH haematite, Fe Oj and ferrihydrite, Fe Oj.nH O). It is therefore desirable to determine the Al and Fe associated with these fractions. The scheme of separations is shown in Fig. 5.1. 

Jince the time of Berzelius, chemists have proposed structures for the amorphous, black substance known as humic acid. In the past 150 years, much experimental work has appeared on the nature of humic acid, most of it based on classical chemical and microbiological studies. Very little information about the molecular structure of humic add has resulted from these studies however. Some of the problems plaguing investigators in this field have been (a) variation in the source of humic acid, (b) variation in the definition of humic fractions of soil and coal, (c) lack of crystallinity of the samples, (d) uncertainty of molecular weight measurements, (e) variation in extraction techniques, and (f) variation in elemental composition. The little unambiguous information that exists today is based on extensive degradation of the humic acid polymer and represents only a small fraction of the total molecule. 

It is suggested here that the greater insolubility of the humic fraction of coal may be the result, in part, of polymerization by complexing with metals, particularly aluminum and silicon. These elements are suggested because of their presumed greater availability. The minor elements also play a part, however, only in relation to their availability compared with major elements such as aluminum and silicon. 

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