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Soil Organic Matter Studies – necessity to understand better the fate
of pollutants and the way to improve quality of risk assessment
Dagmar Gajdošová1, Eladia M. Peña-Méndez2 , Ma. De Lourdes Pacheco H.3 and J. Havel1
1Department
of Analytical Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37-Brno, Czech Republic
2 Department of Analytical chemistry, Nutrition and Food Science, Faculty of Chemistry, University of La Laguna, Spain
3 Universidad Autónoma del Estado de Hidalgo, Centro de Investigaciones Químicas, Hidalgo, Mexico
RESULTS AND DISCUSSIONS
Soil organic matter (SOM) represents the organic part of soil. It includes high molecular weight organic
material (polysaccharides, proteins, etc.), small molecules (amino acids, sugars, etc.) and humic
substances (humic acids, fulvic acids and humin). Humic substances (HS), natural compounds widely
distributed in nature, play important role in the environment. Several humic acids (HA) extracted from
soil of different sources and countries and all continents, including Antarctica were analyzed and
characterized by MALDI-TOF mass spectrometry and Capillary Zone Electrophoresis (CZE).
We have found, that extraction process can be eliminated and by direct laser ablation of soil mass
spectra of low molecular weight HA can be obtained. Spectra measured were compared with those of
extracted humic acids.
Additionally, also HA-xenobiotics (pesticides, heavy metals, uranium etc.) interactions were studied.
The study was based on capillary electrophoresis. The complexation of HA- xenobiotics is strong. The
interactions were found to be of different kind depending on the structure of the xenobiotic molecule
and on its charge (neutral, positive or negative). Metal ions are strongly bound, but e.g. in case of
uranium several complexes are formed and it was proved that at high excess of HA-uranium (VI) is
mobilized. Even such ions like chloride, nitrate are partly bound to HA.
Soil composition
• Mineral- inorganic (stones, gravel, sand, dust, clay)
• Organic matter: 1. High molecular-weight organic material (polysaccharides, proteins)
2. Simpler substances such as sugars, amino acids, and other small molecules
3. Humic substances: fulvic acids – the fraction that is soluble in water under all pH conditions
humic acids - the fraction that is not soluble in water under acidic conditions (pH < 2)
(lignin, polyphenols, proteins and amino acids, cellulose and other
polysaccharides)
humin - the fraction that is not soluble in water at any pH value and in alkali
• Water
• Gas
• Living organisms (gnathostoma, insects, bacteria, protozoa, fungi, algae).
MALDI-TOF MS
A: Study of SOM and humic acids, especially (capillary electrophoresis, mass spectrometry)
%Int.
Rapid characterization of soil low molecular weight organic matter
EXPERIMENT
813.41
100
MALDI-TOF MS
Matrix Assisted Laser Desorption Ionization-Time Of Flight Mass
Spectrometry is a powerful tool for
the determination of molecular
weight
of biomolecules.
90
815.37
80
%Int.
338 mV 53 mV
70
310.01
360.00
337.99
104.95
128.85
B
40
180.91
20
A
104.89
158.83180.83
122.88
140.86
100
120
412.87
164.87
103.85
140
160
180
200
278.92
250.99
303.98325.84
212.16
256.04 284.04304.97
212.08232.85
289.00
220
240
260
280
300
320
375.88
424.74
368.04 396.03
360.91
408.69
338.92
369.07
391.88 425.05 452.10
340
360
380
400
420
440
460
Mass/Charge
%Int.
Intensity (rel.)
337.93359.88
60
476.91
B
816.40
811.43
810.34
10
814.45
50
0
810
811
90
691.2
793.3
70
10
0
880.7
749.4
658.51
100
837.1
60
50
80
763.7 807.5
689.0
30
642.80
60
B
20
674.49
757.0
522.97550.96
550
660.74
618.77
590.82
647.74
600
650
704.73 734.65
700
792.65
750
800
836.60
850
880.58
750
817
818
819
Axima-CFR, MS MALDI TOF
(Kratos Analytical, Shimadzu
Group Company, Manchester,
United Kingdom)
812
813
814
815
816
817
818
819
Mass/Charge
814.6 858.3
1013.1
1027.5
1100.9
901.9
0
700
20
953.4
810.4
712.9
816
909.5
766.0
684.6
10
40
811
968.8
40
814
815
Mass/Charge
817.46
810.42
810
925.0
851.9
813
Mass spectrum of Soil HA IHSS standard
20
734.4
812
816.46
812.43
30
100
80
500
20
60
817.42
30
70
500
A
812.38
815.46
705.4
41 mV 4.9 mV
A
80
40
480
%Int.
658.74
0
814.35
50
40
80
0
811.36
60
90
309.90
100
MODEL
813.44
100
B
800
850
900
Mass/Charge
950
1000
1050
1100
1[c].C22
Mass spectra modeling
of a group of the peaks around m/z 813.4 M+H+
924.51
900
950
(at least 5 compounds with a similar structure are present
1000
Mass/Charge
A
Possible combination of empirical formulas:
A humic acid extracted from pine forest (La Esperanza, Tenerife, Spain)
B soil from pine forest (La Esperanza, Tenerife, Spain)
m/z 810. 42 = C45H62O13+ , 811.43 = C45H63O13+ , 812.43 = C45H64O13+ , 813.44 = C45H65O13+ , 815.46 = C45H67O13+
•Mass spectra of low molecular weight organic matter can be obtained rapidly by direct soil ablation (extraction procedure is eliminated).
•Mass spectra modelling shows complexity of mass spectra, some empirical formulas are proposed (HA are low molecular weight and contain at least
several hundreds of compounds).
Fig. A
0.03
Neutral
Marker
Capillary zone electrophoresis (CZE) B. Study of humic acids interactions with:
0.02
Absorbance (AU)
:xenobiotics (insecticides – p,p´-DDT, p,p´-DDE, Paraquat,
Beckman (Model PACE) System 5500
Diquat; toxic inorganic compounds, nitrate,)
(Palo Alto, CA, USA) equipped with
diode array detection (DAD) system,
:heavy metal ions interaction (Hg (II), Cu (II), Cd, uranyl, etc).
UO22+-HA
complex
0.02
HA
0.01
automatic injector, fluid cooled
0.01
column cartridge and System Gold
[UO22+]
Data station. Fused silica capillary
0.00
tube of 37 cm (30.5 cm to the
1.2
1.3
*
1.4
1.5
1.6
1.7
1.8
1.9
2
Migration time (min)
detector) y 75 um I.D. was used.
Example of toxic metal interaction with humic acids:
Fig.
0.15
B
[UO2(CO3)3-X(HA)X]z-
[UO2(CO3)3-X(HA)X]z complex
complex
HATot (mM)
Peat HA IHSS standard
5.0
0.06
Chemapex HA standard
(AU)
0.05
Brazilian HA
Antarctic HA
Absorbance (AU)
0.10
A
“hump” of HA
6
Migration time (min )
Electropherograms of humic acids extracted from soil of various origin
Conditions: background electrolyte BTE (pH 8.5)
4.5
3.5
0.04
3.0
2.5
0.02
1.5
0.0
0.00
4
UO22+Tot (mM)
Absorbance (AU)
Soil HA IHSS standard
POSITIVE UO22+- HA SPECIES [UO2 2+]tot= 0.2 mM / [HA]tot=1.6 mM, pH = 4
Fig.
3.0
1.0
0.6
0.4
0.2
0.0
0.020
0.016
0.012
0.008
“hump” of HA
0.004
0.000
0.00
2
4
4
Migration time (min)
UO2(CO3)34-
5
6
7
8
Migration time (min)
A: Migration of UO22+-(CO3)-HA species increasing [HA]tot [UO22+ ]tot = 5 mM, carbonate buffer 20 mM, pH = 10.5
B: Migration of UO22+- (CO3) -HA species at various [UO22+]tot [HA]tot=5 mM, carbonate buffer 20 mM, pH = 10.5
•The complexation of HA-uranium (VI) leads to several complexes, uranyl migrates as cationic species at low HA concentration (Fig. A) . Electrophoretic mobilites are
decreasing with increasing HA concentration. At [HA] higher than 3 mM negatively charged uranium species is formed. However, migration of UO2 2+ depends also on
carbonate content in system (Fig. B, C).
•Study of HA interaction with organic and inorganic xenobiotics shows that it is of various kind (complexation, adsorption, intercalation, supramolecular complexation,
etc.) and it is quite different for each xenobiotics.
CONCLUSION: These results should be taken into account for evaluation of risks and proposal of adequate soil (sediments) remediation.
REFERENCES:
[1] D. Gajdošová, K. Novotná, P.Prošek and J. Havel: ”Separation and characterization of humic acids from Antarctica by capillary electrophoresis and MALDI TOF mass spectrometry. Inclusion complexes of humic acids with
cyclodextrins”, J Chromatogr A, (2003), 1014.
[2] D. Gajdošová, L. Pokorná, K. Láska , P. Prošek, J. Havel: “Are there humic acids in Antarctica?” in Humic Substances Structures, Models and Functions, Cambridge: RSC, (2001). 11 s.
[3] M.L. Pacheco, E. M. Peña-Méndez, J. Havel: “ Supramolecular interactions of humic acids with organic and inorganic xenobiotics studied by capillary electrophoresis“ , Chemosphere 51 (2003) 95-108
[4] E. M. Peña-Méndez, D. Gajdošová, K. Novotná, P. Prošek and J. Havel : “Mass Spectrometry of Humic Substances of different origin including those from Antarctica. A comparison study“ – (Talanta 11/2004 )
C