IRspec HA Peat

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COMPARATIVE IR SPECTRAL CHARACTERISTICS

OF HUMIC ACIDS FROM PEATS OF DIFFERENT ORIGIN

IN THE TOMSK AREA

M. V. Gostishcheva,1 M. V. Belousov,1 M. S. Yusubov,1

R. R. Ismatova,2 and S. E. Dmitruk 1

Translated from Khimiko-Farmatsevticheskii Zhurnal, Vol. 43, No. 7, pp. 44 – 47, July, 2009.

Original article submitted February 11, 2008.

Results obtained from comparative studies of the structures of humic acids from peats of different origins inthe Tomsk area by infrared spectroscopy are presented. These data showed that the IR spectra of humic acids

extracted from different types of peat had high levels of similarity, demonstrating similarities in their chemicalstructures. Humic acid from lowland woody-grassy peat had very significant differences from other types of

peat and consisted of compounds with high proportions of aliphatic fragments and, perhaps, lower benzenoidlevels, as well as higher contents of active acid groups (carboxyl, hydroxyl).

Key words: Humic acids, peat, IR spectra.

Humic acids (HA) from peat are high molecular weightnitrogen-containing compounds with cyclic structures andconsist of mixtures of dark-colored organic, high molecular weight, mainly aromatic, methoxy-containing, hydroxyoxo-carboxylic acids sharing a common type of structure but hav-ing various differences related to their origins and means of extraction [1]. The literature contains detailed descriptions of the molecular structures of humic acids from European peatsand their chemical properties in general terms, though thereare virtually no such data for Siberian peats, which makes thestudies reported here relevant.

The great diversity of structural units and functionalgroups and the multiple ways in which they are combined,along the variation in the composition and molecular weightof humic acids currently make direct identification of their structures impossible. Infrared spectroscopy (IR spectros-copy) provides abundant information on the set of the most

important groups of atoms and types of bonds, as well as onthe actual positions of some groups. The method is non-de-structive, which allows the correspondence of humic acid

preparations with native substances present in peat to be as-sessed [2 – 4].

The aim of the present work was to perform a compara-tive study of the structure of humic acids from peats of dif-ferent origins in the Tomsk area using IR spectroscopy.

Humic acid structures were evaluated by collecting rep-

resentative peats of the highland, intermediate, and lowlandtypes, with different botanical compositions, levels of degra-dation, and ash contents from oligotrophic and eutrophictypes of marshes in the Tomsk area; their characteristics are

presented in Table 1.Humic acids were extracted with 0.1 N NaOH as de-

scribed in [5]. The infrared absorption spectra of humic acidswere recorded using a Nicolet 5700 IR Fourier spectrometer (Thermo Electron Corp., USA) in KBr tablets at a ratio of 1:100, over the range 500 – 4000 cm – 1.

RESULTS AND DISCUSSION

Analysis of IR spectra (Fig. 1) from the humic acids col-lected showed that they had absorption bands typical of humic acids from peat [3, 4, 6 – 16]. The wide, intense ab-sorption band with a peak at 3500 – 3300 cm – 1 was associ-

ated with the valent vibrations of hydroxyl groups (nOH) of different types, mainly those bound to hydrogen bonds. Thiswas supported by the fact that the wave number decreased to3400 – 3300 cm – 1 as a result of a reduction in the vibrationfrequency due to formation of hydrogen bonds (peak inten-

4180091-150X/09/4307-0418 © 2009 Springer Science+Business Media, Inc.

Pharmaceutical Chemistry Journal Vol. 43, No. 7, 2009

1 Siberian State Medical University, Tomsk, Russia.2 Kazan’ State Medical University, Tatarstan, Russia.



sity at 3650 – 3585 cm – 1 is characteristic of unbound, freeOH groups). Intermediate absorption was seen at thelong-wave tail of the main band, at about 3250 – 3200 cm – 1,forming a step (flexure), due to N–H vibrations (n NH) inamides and amines bound via hydrogen bonds.

Intermediate absorption bands were clearly visible at2926 – 2920 cm – 1 and 2852 cm – 1, due to valent vibrationsof side chain –CH3 and –CH2 groups in humic acid mole-

cules. These bands were mainly due not to terminal methylgroups, but rather to methylene groups, as indicated by thealmost perfect coincidence of wave numbers with the stan-

dard values (2926 and 2853 ± 10 cm – 1) [2 – 4]. The pre-dominance of methylene groups is also indicated by the na-ture of the bands at 1480 – 1380 cm – 1, which indicated thatthe methylene groups cannot be in any kind of long paraffinchains. Such chains of –(CH2)

n – groups, with n ³ 4, give

strong absorption bands at 720 cm – 1, due to non-valent vibrations of –CH2 groups. In the samples studied here, veryweak absorption was seen in this range only for humic acidsfrom intermediate sedge peat; there was none in the other

preparations. The low intensities of bands corresponding to

–CH2 and –CH3 groups are consistent with indications of theminor role of alkanes in humic acid structures and the factthat the aliphatic (hydrolyzable) components consist almostentirely of amino acids and carbohydrates [8 – 10, 17, 18].

All humic acids showed weak absorption at2600 – 2500 cm – 1, typical of carboxylic acids. This wide

band is regarded as very characteristic for valent vibrationsof hydroxyl groups (nOH) involved in forming strong hydro-

gen bonds in dimeric forms of carboxylic acids. A clear high-intensity peak was seen in all spectra at1725 – 1650 cm – 1. This band is typical of valent vibrationsof carbonyl groups (nC=O) in ketones, aldehydes, carboxylicacids, and their functional derivatives [3, 4, 8, 9, 12,14 – 16].

The IR spectra of all humic acid samples showed astrong band at 1650 – 1600 cm – 1, due to planar vibrations of conjugated carbon-carbon (aromatic, nC=C) and carbon-oxy-gen (carbonyl groups linked by hydrogen bonds, carboxylateions, nC=O) bonds in the aromatic skeleton and quinones. Ar-omatic nC=C bonds are taken to be responsible for this band,

whose peak is at 1610 cm – 1. In the humic acids studied here,

Comparative IR Spectral Characteristics of Humic Acids 419

140

130

120

110

100

90

80

70

60

50

40

4000 3000 2000 1500 1000 500

cm –1

3403

2923

2852

17171610

1634

1513

1453

1420

1371

1352

1225

1166

11241034

833

720698

772

698

1

2

3

4

5

T r a n s m i s s i o n

Fig. 1. IR spectra of humic acids: 1) lowland woody-grassy; 2) lowland grassy; 3) lowland grassy-mossy; 4) intermediate sedgy; 5) highland

pine-cottongrass peat.

TABLE 1. General Characteristics of Peats from the Tomsk Area.

Type of peatDepth of sample

collection, cmLevel of degradation,

% by weightAsh content,% by weight

HA content,% by weight

Highland pine-cottongrass 10 … 30 35 6.3 38.4

Intermediate sedgy 150 … 200 45 4.8 40.4

Lowland grassy-mossy 200 … 250 40 4.3 39.2

Lowland grassy 230 … 250 45 10.4 31.0

Lowland woody-grassy 10 … 30 25 8.1 43.6



the peak was at 1630 cm – 1, which may be due to superimpo-sition of the amide group band (n NH2

amide II). The absorp-tion band close to 1513 cm – 1 (nC–C) demonstrates the pres-ence of non-condensed aromatic compounds in humic acids,

bound to nitrogen and oxygen atoms. The band peaks of asymmetrical and symmetrical deformational vibrations at1480 – 1380 cm – 1, as noted above, are associated with C–H

bonds in –CH2 and –CH3 groups.Vibrations at 1264 – 1225 cm – 1 are due mainly to valent

(nC–O) and deformational vibrations (dO–H) of bonds. Ab-sorption in the shortwave part of the spectrum at1150 – 1000 cm – 1 is due to valent vibrations of alcohol andcarbohydrate hydroxyl groups (nOH). In the range around1050 cm – 1 (nC–O), radiation is absorbed by primary alco-hols; at 1100 cm – 1 (nC–O) by secondary alcohols, and at1150 cm – 1 (nC–O) by tertiary alcohols. Absorption in thisarea can also be due to valent vibrations (nC–O–C) of theglycoside bonds of carbohydrates and the C–O–C bonds in

cyclic and aliphatic ethers. Weak absorption bands were seenat wave numbers of 1000 to 600 cm – 1, possibly due toextraplanar deformation vibrations (dC–H) in aromatic ringswith two or more unsubstituted hydrogen atoms.

Absorption band peaks in the spectra of humic acidsfrom peats were seen for hydroxyl and carbonyl (carboxyl)groups and for aliphatic and aromatic fragments.

Quantitative assessment [11] of the content of functionalgroups based on IR spectroscopy using the ratios of the opti-cal densities of the absorption bands of oxygen-containinggroups to the optical densities corresponding to aromaticmultiply conjugated systems at 1610 cm – 1 and aliphatic sub-

stituents at 2920 cm – 1

is presented in Table 2.The main oxygen-containing functional groups in humicacids from peats are hydroxyl (nOH 3400 – 3300 cm – 1) andcarboxyl (nC–O 1150 – 1000 cm – 1) groups and their func-tional derivatives (nC=O 1725 – 1700 cm – 1 and nC–O

1260 – 1225 cm – 1), along with ether groups (nC–O–C1050 – 1035 cm – 1) [3, 4, 9]. The ratio of the optical densi-ties of the absorption bands of functional oxygen-containinggroups and alkyl substituents to aromatic fragments demon-

strated a predominance of aromatic structures over alkyl, carbonyl, hydroxyl, and ether structures. The exclusion washumic acids from lowland woody-grassy peat, the moleculesof which had a ratio of aromatic and aliphatic components of unity, as well as the highest content of carboxyl groups:

AC=O1720/ AC=C1610 = 1.05 and AC=O1225/ AC=C1610 = 0.88. Thespectral coefficient of the of the ratio A

C-O1720

/ AC-C1610

inhumic acid molecules from lowland woody-grassy peat wasgreater than unity (1.05), possibly because of a predomi-nance of aliphatic fragments with carboxyl groups and their functional derivatives in the humic acids, as well as a greater degree of substitution of aromatic ring hydrogen atoms bycarboxyl groups both overall and in comparison with thehumic acids of other peats. Slightly lower carboxyl groupcontents in humic acid molecules were noted for the twoother lowland types of peat; the lowest was seen in the humicacids of the highland pine-cottongrass type of peat. Thus,comparing the carboxyl group contents in the humic acidsfrom the different types of peat from which they were ex-

tracted showed decreases going from peats of the lowlandtype to those of the intermediate and highland types.

There were no particular differences in the contents of al-cohol, carbohydrate, and ether fragments (nC–O and nC–O–C)

between the humic acids of different types of peat; all hadvirtually identical values of 0.74 – 0.76; humic acids of theintermediate type of peat had the lowest value (0.64). Therelative total content of hydroxyl groups AOH3400/ AC=C1610 inhumic acids from different peats changed very significantly(from 0.97 to 1.15). The highest values were characteristic of humic acids of the highland (1.15) and intermediate (1.07)types of peat. Lowland woody-grassy peats were also charac-

terized by high hydroxyl group contents (1.05), wherebythey differed from the other two types of lowlandgrassy-mossy and grassy types of peat, which had the lowestvalues (0.97 – 0.99).

The data presented in Table 2 show that the humic acidmacromolecules of all types of peat showed predominance of hydroxyl groups over alkyl fragments; the AO-H3400/ ACalk2920ratio was greater than unity for all samples, while the twolowland peat types – grassy and woody-grassy – showed a

420 M. V. Gostishcheva et al.

TABLE 2. Ratios of Optical Densities of Absorption Bands at Defined Wavelength Measured by IR Spectroscopy.

Type of peat Lowland woody-grassy Lowland grass Lowland grassy-mossy Intermediate sedgyHighland

pine-cottongrass

AO-H3400/ AC=C1610 1.05 0.97 0.99 1.07 1.15

AC=O1720/ AC=C1610 1.05 0.97 0.93 0.91 0.87

AC-Î1225/ AC=C1610 0.88 0.87 0.84 0.79 0.77

AC-O and C-O-C1035/ AC=C1610 0.74 0.74 0.76 0.64 0.74

ACalk 2920/ AC=C1610 1.00 0.90 0.94 0.97 0.94

AO-Í3400/ ACalk 2920 1.05 1.11 1.07 1.11 1.24

AC=O1720/ ACalk 2920 1.05 1.11 0.93 0.93 0.93

AC-O1225/ ACalk 2920 0.88 0.99 0.89 0.82 0.83

AC-O1035/ ACalk 2920 0.74 0.84 0.83 0.66 0.79



predominance of carboxyl groups over alkyl substituents( AC=O1720/ ACalk2920). This may be evidence that the number of oxygen-containing groups in all types was greater than thenumber of aliphatic C–H bonds for the humic acids of differ-ent peats.

The IR spectra obtained here show that humic acids ex-

tracted from different types of peat have molecules of identi-cal structures, containing aromatic carbohydrate nuclei with peripheral aliphatic chains bound to them. The peripheralaliphatic structures mostly contain hydrolyzable (peptide andcarbohydrate) compounds; the contribution of non-hydroly-zable compounds with –CH3 and –CH2 groups in thealiphatic compounds is negligibly small. Thus, the IR spectraof the humic acids from peats of different origins studiedhere had high levels of similarity (the main characteristic ab-sorption peaks for humic acids were seen in all samples), in-dicating similarities in their chemical structures; these resultsare consistent with data from other authors on humic acids

from peats [3, 4, 6 – 16].The studies also established that humic acids show sig-

nificant differences from each other depending on the type of peat from which they were extracted. Thus, humic acid mol-ecules from highland and intermediate types have differentcontents of carboxyl groups as compared with lowland peattypes, i.e., woody-grassy, grassy, and grassy-mossy types,which contain the largest amounts of carboxyl groups (Ta-

ble 2). Conversely, humic acid molecules from highland andintermediate types, as compared with humic acids from low-land peats, contained more hydroxyl groups; the exceptionwas one type of lowland peat, the woody-grassy type, inwhich the hydroxyl group content was also high. Overall, thehumic acids from lowland woody-grassy types of peatshowed very significant differences from other types of peatand were compounds with high proportions of aliphatic frag-ments and, possibly, lower benzenoid levels, along with highcontents of acidic groups (carboxyl, hydroxyl).

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Comparative IR Spectral Characteristics of Humic Acids 421

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