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Extraction of soil organic matter by supercritical fluids
Org Geochem Vol. 8,No 1,pp. 111-113,1985 Printed in Great Britain. All rightsreserved
0146-6380/85 $03.00+00 00 Copyright © 1985 Pergamon Press Ltd
Extraction of soil organic matter by supercritical fluids MICHAEL SPITELLER Institut fuer Bodenkunde und Waldernaehrung der Universitaet Goettingen, Buesgenwcg 2, 3400 Goettingen, F.R.G.
In the last few years a new extraction procedure at supercritical conditions has been developed for the isolation of organic matter from coal, cellulose from wood, etc. We applied this technique to the extraction of soil samples using supercritical CO2 and organic solvents at supercritical conditions. A mobile phase used at supercritical or slightly above supercritical conditions offers three main advantages: - - l o w viscosity - - h i g h diffusion coefficients - - d e n s i t y usually characteristic of liquids Supercritical fluids have a unique combination of physical properties that are neither exclusively gas-like nor liquid-like but which represent more of an intermediate state. The experimental conditions used for the extraction of soil samples with different organic solvents are listed in Table 1. The apparatus for the supercritical fluid extraction (SFE) can be easily built from commercially available H P L C and G C components (K611 et al., 1978). Table 1. Experimental conditions for the extraction of soil samples with supercritical gases Fraction 1
n-Pentane
2
abs. Ethanol
3
Ethanol/H20 40 6O Ethanol/H20 40:60
4-5
Pressure/ temperature Extractiontime
Solvent
100 bar 250°C 100 bar 250°C 100 bar 250°C 100-200bar 250°C
12(1mm 12(1min 311min 120 min
Pentane usually dissolves only fats and waxes, whereas supercritical polar alcohol is able to dissolve phenols, soluble lignin components and hemicellulose. Cellulose and lignin as well as other high molecular weight components will be dissolved effectively by using an ethanol/water mixture. A f t e r a 12 hour treatment of a spruce organic layer sample by this method, more than 80 wt% can be dissolved. The n u m b e r average molecular weight of the extract varies between 600 and 800. Table 2 shows the yields of organic matter and elemental composition of extracts from a podzol obtained by using the S F E method in comparison to the corresponding data for humic and fulvic acids obtained using NaOH/HC1.
Table 2. Total yieldsand elemental contents of Humic Acids (HA) and FulvicAcids (FA) compared to supercritlcal fired extracts (SFE) HA Sample Podzol O1 Podzol OhAh Podzol B,h
FA
HA + FA yield (wt%)
%C
%N
%C
18 3 15.6 4.7
56 9 55.8 55.3
3.4 25 2.6
48.2 47 2 49.1
*Solvent. acetone/H20 40:60 Pressure rises from 100 to 200 bar within 3 hr Temperature" 250°C Time of experiment' 6 hr. lll
SFE %N
SFE* yield(wt%)
%C
%N
2.1 11 0.9
53.8 40 7 11.1
53.3 49.4 54 3
2.6 1.4 2.2
112
MICHAELSPITELLER
All fractions were subjected to GC/MS-analysis and the detailed results are published elsewhere (Spiteller et al., 1982). Another application of the SFE-method is in the extraction of soils which are usually extracted very poorly by sodium hydroxide (Griffith et al., 1975). Some chemical characteristics of the soils investigated
are listed in Table 3. Table 3 Origin and characteristics of 3 volcanic soils and an oxisol Order substrate
Sample No.
lncept~sol andesit~c ash lnceptisol andesmc ash Molhsol andesmc ash Oxisol glacial depos.
1 2 3 4
Geographical origin
Depth of horizon
pH H_,O
%C
%N
Domimca
0-15 cm
5.6
12.6
0.9
Domimca
0-20 cm
4.5
10 7
0.9
Columbia
0-20 cm
50
9.2
0.6
Columbia
0-10 cm
3.6
2.8
0.1
Compared with humic and fulvic acids, 3-5 times more organic carbon and nitrogen can be extracted (Table 4). Carbon and nitrogen content of SFE's is slightly above that of the humic and fulvic acids (Table 5). This may be due to a solvolytic loss of methanol or water. Table 4. Yields of humlc acids (HA), fulvic acids (FA) and supercritlcal fluid extracts (SFE) as percentages of total carbon and nitrogen Sample No.
% C % C SFE % C SFE % N SFE % N SFE H A + F A M e O H / H 2 0 Aceton/H20 M e O H / H 2 0 Acetone/H.~O
1 2 3 4
9 7 10 1 9.4 7.2
50.3 36.5 55.1 62 7
55.9 43.6 43 7 73.2
69.9 63.5 77.7 72.4
72 8 66.8 66 8 69.2
Table 5. Carbon and nitrogen contents of HAs, FAs and SFEs
Sample No
1 2 3 4
HA
SFE MeOH/HEO
FA
SFE Acetone/H20
%C
%N
%C
%N
%C
%N
%C
%N
54.9 54.4 53 1 52.2
4.4 5.5 3.9 3.8
50.6 42 8 44.2 43.9
3.3 2.0 29 2.1
55.3 54.1 47 3 49.6
7.3 69 3.8 5.5
57.3 58.1 49 4 53.3
5.9 63 4.6 3.6
After permanganate oxidation of SFE's many aromatic compounds can be recovered, which are the main constituents of these samples. Because of the drastic extraction conditions for supercritical organic solvents we also used supercritical CO2 as a mild solvent. The extract--a white powder----consists mainly of fatty material which represents only 1-2% of the total carbon (Table 6). Table 6. Carbon and nitrogen contents of samples obtained by supercritical fired extraction with CO2 Sample
Solvent
%C
%N
Podzol B,h Podzol B,h lnceptlsol An
CO2 COJMeOH CO2/MeOH
65.9 65.3 59.3
0.4 0.3 0.5
Pressure m the extractor: 260 bar. Temperature" 75°C. Pressure in the condenser: 60 bar. Temperature: 40°C. Time of experiment: 4 hr.
Extraction of soil organic matter by supercritical fluids
113
The addition of organic modifiers does not increase the yield of organic carbon but some of the fatty esters are hydrolyzed. Supercritical fluid extraction provides a powerful tool for the extraction of natural products such as humic material from soils. Despite the thermal and solvolytic degradation of compounds this method, together with spectroscopic techniques, provides new incentives for structural studies of humic substances. REFERENCES
Griffith S. M. and Schnitzer M. (1975) Can. J. Soil Sci. 55, 251-267. Koll P. and Metzger J, (1978) Angew. Chem. 90, 802-803. Spiteller M. and Ashauer A. (1982) Z. Pflanzenernaehr. Bodenkd. 145, 567-575.
0146-6380/85 $03.00+00 00 Copyright © 1985 Pergamon Press Ltd
Extraction of soil organic matter by supercritical fluids MICHAEL SPITELLER Institut fuer Bodenkunde und Waldernaehrung der Universitaet Goettingen, Buesgenwcg 2, 3400 Goettingen, F.R.G.
In the last few years a new extraction procedure at supercritical conditions has been developed for the isolation of organic matter from coal, cellulose from wood, etc. We applied this technique to the extraction of soil samples using supercritical CO2 and organic solvents at supercritical conditions. A mobile phase used at supercritical or slightly above supercritical conditions offers three main advantages: - - l o w viscosity - - h i g h diffusion coefficients - - d e n s i t y usually characteristic of liquids Supercritical fluids have a unique combination of physical properties that are neither exclusively gas-like nor liquid-like but which represent more of an intermediate state. The experimental conditions used for the extraction of soil samples with different organic solvents are listed in Table 1. The apparatus for the supercritical fluid extraction (SFE) can be easily built from commercially available H P L C and G C components (K611 et al., 1978). Table 1. Experimental conditions for the extraction of soil samples with supercritical gases Fraction 1
n-Pentane
2
abs. Ethanol
3
Ethanol/H20 40 6O Ethanol/H20 40:60
4-5
Pressure/ temperature Extractiontime
Solvent
100 bar 250°C 100 bar 250°C 100 bar 250°C 100-200bar 250°C
12(1mm 12(1min 311min 120 min
Pentane usually dissolves only fats and waxes, whereas supercritical polar alcohol is able to dissolve phenols, soluble lignin components and hemicellulose. Cellulose and lignin as well as other high molecular weight components will be dissolved effectively by using an ethanol/water mixture. A f t e r a 12 hour treatment of a spruce organic layer sample by this method, more than 80 wt% can be dissolved. The n u m b e r average molecular weight of the extract varies between 600 and 800. Table 2 shows the yields of organic matter and elemental composition of extracts from a podzol obtained by using the S F E method in comparison to the corresponding data for humic and fulvic acids obtained using NaOH/HC1.
Table 2. Total yieldsand elemental contents of Humic Acids (HA) and FulvicAcids (FA) compared to supercritlcal fired extracts (SFE) HA Sample Podzol O1 Podzol OhAh Podzol B,h
FA
HA + FA yield (wt%)
%C
%N
%C
18 3 15.6 4.7
56 9 55.8 55.3
3.4 25 2.6
48.2 47 2 49.1
*Solvent. acetone/H20 40:60 Pressure rises from 100 to 200 bar within 3 hr Temperature" 250°C Time of experiment' 6 hr. lll
SFE %N
SFE* yield(wt%)
%C
%N
2.1 11 0.9
53.8 40 7 11.1
53.3 49.4 54 3
2.6 1.4 2.2
112
MICHAELSPITELLER
All fractions were subjected to GC/MS-analysis and the detailed results are published elsewhere (Spiteller et al., 1982). Another application of the SFE-method is in the extraction of soils which are usually extracted very poorly by sodium hydroxide (Griffith et al., 1975). Some chemical characteristics of the soils investigated
are listed in Table 3. Table 3 Origin and characteristics of 3 volcanic soils and an oxisol Order substrate
Sample No.
lncept~sol andesit~c ash lnceptisol andesmc ash Molhsol andesmc ash Oxisol glacial depos.
1 2 3 4
Geographical origin
Depth of horizon
pH H_,O
%C
%N
Domimca
0-15 cm
5.6
12.6
0.9
Domimca
0-20 cm
4.5
10 7
0.9
Columbia
0-20 cm
50
9.2
0.6
Columbia
0-10 cm
3.6
2.8
0.1
Compared with humic and fulvic acids, 3-5 times more organic carbon and nitrogen can be extracted (Table 4). Carbon and nitrogen content of SFE's is slightly above that of the humic and fulvic acids (Table 5). This may be due to a solvolytic loss of methanol or water. Table 4. Yields of humlc acids (HA), fulvic acids (FA) and supercritlcal fluid extracts (SFE) as percentages of total carbon and nitrogen Sample No.
% C % C SFE % C SFE % N SFE % N SFE H A + F A M e O H / H 2 0 Aceton/H20 M e O H / H 2 0 Acetone/H.~O
1 2 3 4
9 7 10 1 9.4 7.2
50.3 36.5 55.1 62 7
55.9 43.6 43 7 73.2
69.9 63.5 77.7 72.4
72 8 66.8 66 8 69.2
Table 5. Carbon and nitrogen contents of HAs, FAs and SFEs
Sample No
1 2 3 4
HA
SFE MeOH/HEO
FA
SFE Acetone/H20
%C
%N
%C
%N
%C
%N
%C
%N
54.9 54.4 53 1 52.2
4.4 5.5 3.9 3.8
50.6 42 8 44.2 43.9
3.3 2.0 29 2.1
55.3 54.1 47 3 49.6
7.3 69 3.8 5.5
57.3 58.1 49 4 53.3
5.9 63 4.6 3.6
After permanganate oxidation of SFE's many aromatic compounds can be recovered, which are the main constituents of these samples. Because of the drastic extraction conditions for supercritical organic solvents we also used supercritical CO2 as a mild solvent. The extract--a white powder----consists mainly of fatty material which represents only 1-2% of the total carbon (Table 6). Table 6. Carbon and nitrogen contents of samples obtained by supercritical fired extraction with CO2 Sample
Solvent
%C
%N
Podzol B,h Podzol B,h lnceptlsol An
CO2 COJMeOH CO2/MeOH
65.9 65.3 59.3
0.4 0.3 0.5
Pressure m the extractor: 260 bar. Temperature" 75°C. Pressure in the condenser: 60 bar. Temperature: 40°C. Time of experiment: 4 hr.
Extraction of soil organic matter by supercritical fluids
113
The addition of organic modifiers does not increase the yield of organic carbon but some of the fatty esters are hydrolyzed. Supercritical fluid extraction provides a powerful tool for the extraction of natural products such as humic material from soils. Despite the thermal and solvolytic degradation of compounds this method, together with spectroscopic techniques, provides new incentives for structural studies of humic substances. REFERENCES
Griffith S. M. and Schnitzer M. (1975) Can. J. Soil Sci. 55, 251-267. Koll P. and Metzger J, (1978) Angew. Chem. 90, 802-803. Spiteller M. and Ashauer A. (1982) Z. Pflanzenernaehr. Bodenkd. 145, 567-575.