Studies of Semiquinone Free Radicals by ESR in the Whole Soil , HA , FA and Humin Substances

In this work it was studied soil organic matter from a gley soil, using ESR (electron spin resonance spectroscopy). The studied soil samples were collected in Rio de Janeiro, Brazil. Humic and fulvic acid, humin macromolecules, and whole soil samples were analyzed. The results showed that the amount and line width of semiquinone free radical from whole soil samples had good correlation with humin fraction and no correlation with humic and fulvic acids contents.


Introduction
Soil organic matter (SOM) is a mixture of simple and complex organic compounds including macromolecular structures usually classified as humic acid (HA), fulvic acid (FA) and humin (HU).They are basically classified in relation to their solubility in alkali and acid. 1 The use of conventional chemical methods and advanced physico-chemical techniques including capillary electrophoresis, fourier transform infrared (FTIR), fluorescence, electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopies have allowed the elucidation of several aspects of the structures, functionalities and reactivity of HA. [2][3][4][5][6] ESR can detect free radical compounds, determine its concentration and, sometimes, their origin.Humic substances contain very stable semiquinone free radicals that are supposed to be related with polymerizationdepolymerization reactions and to interact with pesticides and toxic poluents. 7,8The semiquinone free radicals (spins), which are believed to be stabilized by condensed aromatic structures, [9][10][11] have been associated with humification degree of soil humic substances. 5,6,12am et al., 13 using ESR technique to investigate litter samples, showed a small increasing in spin concentrations (semiquinone free radical) and a small decreasing in line width to the depth of the sample.This fact suggested a more advanced humification of the litter in relation to the depth of the sample is found in soil profiles.
Martin-Neto et al. 5 showed that the spin concentration of the HA samples exhibits a highly significant positive correlation with mean annual rainfall in samples from the Argentine Pampa.
Generaly, studies of humic substances [(HA), (FA) and (HU)] concentrate in one of its fractions, usually HA.The FA(s) fraction, many times appear in small amounts in many soils, and are related to structural aspects. 14In litter studies, it was found that is difficult to dissociate humin fraction from the mineral part of the soil and, consequently, very few information from this fraction can be available.
This work studied and compared the fractions of Humic Substances (HA, FA and HU), and Whole Soil (WS) samples through ESR spectroscopy.

Samples
The analysed Gley soil, seven samples from horizon A, 0-20 cm layer, were collected in Rio de Janeiro -Brazil, in a region named "Região dos Lagos".The percentage of carbon of these samples is show in Table 1.

Extraction of humic substances
The extraction of humic substances from the soil samples was made followng the methodology suggested by International Humic Substances Society (IHSS), 15 using NaOH (0.1mol L -1 ) as extraction agent.The extracted HA was dialyzed against water and silver nitrate was used to test for removal of excess chloride ions. 16The resulting sample was stored as a homogenized freeze-dried powder.The ash content of HA was about 6.0 %.The fraction FA was passed in the resin XAD 8 in pH approximately 2 and washed with NaOH (0.1 mol L -1 ).The obtained fractions were HA, FA and HU, all were dried in pH around 4.0.The fractions HU and FA were not purified so have high ash content.The elemental analysis (C) of this sample was done using Carlo Erba equipment.

ESR spectroscopy
ESR spectra were recorded for freeze-dried WS, AH, AF, and HU samples using a Bruker -EMX EPR spectrometer with a rectangular cavity operating at X-band (9.4 GHz).Experimental conditions for the ESR experiments were: magnetic field centred at 0.34 T, 0.2 mW microwave power, and 0.02 mT amplitude modulation.The absolute concentration for semiquinone free radical was obtained using ruby as a secondary standard, calibrated with strong pitch reference of known free radical content obtained by Bruker. 6

Results and Discussion
The spectra of semiquinone free radical sign to all investigated fractions are shown in Figure 1.Higher amount of soil organic carbon and the reduced quantity of paramagnetic ions permitted us to obtain very good spectra to whole soil samples, as well as to all humic fractions. 17he signs were similar to all the samples and, the obtained g value, approximately 2.004 (see Figure 1), shows that the paramagnetic species (whole soil and HA, FA, HU fractions) have the same origin. 18In other words, they were originated from semiquinone free radical. 11n Table 2, values of line width and the amount of free radical per mass are shown.It can be seen that the largest value of the line width was observed to FA and the smallest value was observed to HU.This result can be explained by knowing that the semiquinone free radical in FA is less protected, permiting it to interact with its neighbors what decreases its relaxing time, and increases its line width.This conclusion was also confirmed using potency saturation measurements (the intensity of the sign is proportional P 1/2 until the limit of the saturation of the sign 19 ), as show in Figure 2. Potency saturation is smaller for HU, indicating a larger time of relaxation and a smaller value associated to the line width. 20he line width value, ∆H, for HU was smaller in all the samples when compared with the other humic fractions, and also close to value related to the whole soil (Table 2).It suggests that the humin is the largest fraction for gleysoil, and produces the largest contribution for the sign of free radical in the whole soil sample.It can be observed in Table 2 that the fraction AH is the fraction associated to the largest amount of free radicals.It is because the fractions HU and AF were not purified and therefore, possess as the smallest amount of C. Table 3 shows the amount of free radical per gram of C in the whole soil and in the HU, AH and AF fractions.Table 3, shows the normalized amount of C of each fraction.It can be seen that AF possesses 10 16 -10 17 spin/gC while AH and HU possesses about 10 17 -10 18 spin/gC, (it was found that HU exhibit the largest amount of all the investigated fractions).It can be explained because the fraction HU has a larger molecular mass and high condensation degree.It means that HU has a larger amount of free radicals. 1,11In Figure 3, the large value of r (r=0.91)shows a good correlation between semiquinone free radical and humin quantities.The amount of semiquinone free radical of the whole soil samples had direct correlation to humin fraction but no correlation to humic and fulvic acids (Figure 3).

Conclusion
ESR measurements from whole soils, and macromolecule humic acids, fulvic acids and humin

Figure 1 .
Figure 1.EPR spectra of semiquinone free radicals in HA, FA and HU fractions and, Whole Soil (WS).H indicating line width and I the intensity of the signal.

Tabela 3 . 13 Figure 3 .
Figure 3. Graph showing the correlation among amount of semiquinone free radical among Whole Soil (WS) and Humin (HU).The numbers identify the samples.

Table 2 .
Line Width (∆H) in Gauss and the amount of free radical (FR) in 10 17 (Spin (S) / g) to the whole soil (WS), HA, FA, and HU fractions

Table 1 .
Percentage of organic carbon in the soils samples