5 Chemistry of Humic Substances in Relation to Coalification WOLFGANG FLAIG Downloaded via TUFTS UNIV on July 10, 2018 at 19:07:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Institut für Biochemie des Bodens, Braunschweig,
Germany
Plant constituents decompose at different rates. Since phenolic substances are more resistant to microbial attack, they are regarded as material essential in forming
humic substances.
During
humification the methoxyl content decreases and nitrogen increases. Lignin decomposition products which are formed undergo different oxidation reaction leading to phenols or quinones and polymerization products. A scheme for the decomposition
of
lignin
and
humification
according
to
experimental results with labelled lignin decomposition products is discussed. Under the conditions of coalification the formation of polymerization products derived from quinonoid substances may occur which leads finally to a higher degree of aromatization.
Q u r i n g humification various plant constituents decompose at different rates. Carbohydrates a n d proteins serve as an energy source for microorganisms or as materials for forming their body substances while lignin a n d other phenolic compounds decompose more slowly than the two other main constituents. Therefore, lignin a n d other phenolic compounds may be regarded as materials essential for forming the dark colored substances i n humus. W e have investigated the humification of straw, as an example, under constant conditions of humidity a n d temperature i n a climatic chamber (4,18) and separated different fractions according to a modified method of W a k s m a n s proximate analysis. T h e amount of nitrogen present i n these processes is the factor limiting the rate of decomposition. Therefore w e added nitrogen i n form of ammonium nitrate i n a quantity of 1 % of straw d r y weight to a nutrient solution i n one experiment while the nutrient solution h a d no nitrogen i n a parallel experiment. Table I shows the calculated data. U n d e r the condition chosen cellulose decomposes m u c h faster than lignin. A d d i n g nitrogen accelerates the decomposition of holocellulose as w e l l as 58
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
5.
FLAIG
Humic Substances and Coalificaiion
Table I.
Days
59
Décomposition of Straw, Holocellulose, and Lignin with and without Added Nitrogen Nitrogen Added
0
Org. Subst.
120 180 240
b
Lignin H S(V 0 0.4 7.2 9.0 24.0 13.3 37.7 24.6 48.1 2
0 10.4 49.3 27.7 66.4 36.3 76.4 48.7 80.9
0 ί ο 1 0 1 0 1
70
Decrease of Fractions in % Holocellulose 0 20.2 67.5 36.7 84.5 44.7 92.4 57.4 94.7
• I N = 1% N as NH4NO8 per dry weight of straw * Mean value of three analyses
lignin. T h e lignin fractions have been isolated according to a modified method of Klason w i t h sulfuric acid ( 2 3 ) . T h e values of the elementary analysis are shown i n Table II. Since carbon values decreased and oxygen values increased, oxidative reac tions must have occurred during decomposition. Nitrogen values more than triple on adding ammonium nitrate. T h e more* the straw is rotted, the less nitrogen can be hydrolyzed w i t h hydrochloric a c i d . A part of the nitrogen i n the lignin fractions is fixed as α-amino nitrogen. T h e quantity decreases from 6 0 % of total nitrogen content i n fresh straw to about 2 0 % i n rotted straw. A remarkable decrease could be observed for the methoxyl content. This fact can be explained b y an enzymatic demethylation process. T h e decomposition of lignin is caused b y oxidation a n d demethylation; furthermore, fixation of nitrogen occurs. Nitrogen can be derived from pep tides, amino acids, or ammonia. A l l these compounds can be formed b y the decomposition of plant proteins or during the autolysis of the microorganisms. F o r further consideration some additional remarks must be made. In coalification not only might lignin alteration itself play an important role, but Table II. Days
nitro gen
0 70 120 180 240
0 V 0 1 0 1 0 1
Values of Elementary Analysis of the Lignin Fractions from Experiments with and without Added Nitrogen* c %
Η %
o%
N%
S %
OCHz %
ash %
62.73 62.29 61.42 62.13 60.93 62.20 60.94 62.14 59.61
5.64 5.48 5.25 5.42 5.41 5.41 5.38 5.27 5.13
30.55 31.20 30.20 31.41 31.20 31.30 31.15 31.03 32.77
0.53 0.54 1.44 0.56 1.68 0.56. 1.74 0.56 1.88
0.55 0.49 0.69 0.48 0.78 0.53 0.79 0.97 0.61
17.08 15.53 12.78 14.99 11.33 14.37 10.95 13.46 9.93
6.15 7.65 9.00 8.62 10.02 9.31 11.24 9.36 11.69
* Calculated for ash-free substances » I N = 1% N as NH4NOS per dry weight of straw
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
COAL GEOCHEMISTRY
60
also possible further reaction of the various primary lignin decomposition products that might occur subsequently during humification. O n e part of these further decomposition products can be used as an energy source by the hgnin-decomposing microorganisms (6). Another part reacts alone or w i t h other compounds to form dark colored humic substances (7, 8). D u r i n g the rotting of plant materials, different phenolic compounds can be identified (14, 16, 18, 28) as shown i n Figure 1. These are aromatic com pounds w i t h side chains of one or three carbon atoms. T h e y may be derived from the polymeric lignin by cleavage or oxidation reactions whereby the side chain is shortened. R m - CHO
OH
p-hydroMyb.mald.hyd. l|
R * -COOH p-hydro*yb.nxoic
acid
R m-CHmCH-COOH
3
0CH
acid
S
£
0
Q
,
n
2
n
C
0
H
Figure
acid
9
,
C
" «
OCH3
-°~\J)--C»
H
-CHmCH-COOH
2
HC
HCOH
R*
Rm-CH0H-CH0H-CH 0H guoiocyl-gtyc.rot R.-CH2-C0-C00H guaiacyl-pyruvic odd
H
H C0H
vanillin vanillic acid
Rm-CHmCH-CHO conif.rylald.hyd. f.rulic
I 0 C H 3
3
Rm -CHO »- -COOH
L U "V" *
p-hydronycinnamic H C0
J^
= C»-CH 0H
guaiacylglyc.ro! β-conif.ryl.th.r
I.
dmhydro-
2
Structure
-
J*. ] *,C0^V OH
I II divonillin V S c * , OH
of identified lignin decomposition
products
Guaiacylglycerol-/3-coniferyl ether is a dimeric decomposition product of coniferous lignin; dehydrodivanillin may be also formed by dimerization of vanillin after formation from lignin. A m o n g the lignin decomposition products are mono-, d i - and triphenol derivatives w h i c h are derived from the different types of lignin of needle trees, deciduous trees, or graminées. In our above mentioned experiment the straw lignin h a d a ratio of vanillic to syringic components of 1.7 : 1. This ratio has been determined by oxidizing the lignin w i t h m-nitrobenzenesulfonic acid (24). T h e lignin has been isolated according to the method of Brauns. T h e question arises: w o u l d this ratio be constant during the humification process? Kratzl and Claus (17) analyzed the original and rotted straw and determined vanilloylacetyl and syringoylacetyl quantitatively after ethanolysis. They found that the ratio 1.6 : 1 (to be compared w i t h the value 1.7 : 1 that we found w i t h the other method) changed to 2.7 : 1 i n the rotted straw after 240 days. This means that during humification the lignin decomposition products of syringyl type are transformed more rapidly than those of guaiacyl type. However, i n the lignin fraction of the rotted straw, the ratio was 0.5 : 1 at the end of the decomposition experiment. Therefore, the syringyl components are split off more slowly from the polymeric lignin than the guaiacyl
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
5.
FLAIG
61
Humic Substances and Coalification
components. Both results agree w i t h other experiments. Thus, syringic acid is more sensitive to oxygen treatment i n alkaline solution than vanillic acid. Furthermore the C2 atoms of the side chain of three carbon atoms are preferred for connecting the monomers of the phenylpropenyl type ( I , 2 0 ) , especially syringyl type. W e were able to confirm this result also with labelled compounds. In Figure 2 the further reactions of the lignin decomposition products, w h i c h involve biological oxidation i n the soil or i n cultures or microorganisms, are summarized. A l l of the secondary reaction products could be identified. T h e essential reactions ( 5 ) during the further transformations of the de composition products of lignin are: (e) hydroxylation ( V - I V ) (a) cleavage of the side chain (f) formation of quinones ( I I I (I or I I - I I I ) XIII, I V - X I , V - X , VII-XII) (b) decarboxylation ( I I I - X I I I , (g) dimerization ( V I I - V I I I or I X , VII-IX) XIII-XIV) (c) demethylation ( I I I - I V , (h) polymerization ( I I - ( I I ) * a n d VI-VII) others such as Χ , X I , X I I ) (d) demethoxylation ( I I I - V ) T h e dimerization a n d polymerization of the resulting quinones w i l l be discussed more extensively later i n relation to the formation of compounds i n w h i c h the aromatic structure increases successively during the reactions. .OCH
3
OH
OH
YI
V
CH
CH
2
II II
1
CO
CH
1
1
COOH
1
COOH
η
OH
1. 11. lit and V identified in toil* and composts tho other compounds in cultures of m icroorgonis ms
CO
H
0
^ ^ \
Figure 2. I II III IV V VI VII
0
0
H
Oxidation of lignin decomposition
Guaiacyl-pyruvic acid Ferulic acid Vanillic acid (vanillin) Protocatechuic acid ( -aldehyde ) p-Hydroxybenzoic acid (-aldehyde) Syringic acid Gallic acid
products
VIII Ellagicacid IX Purpurogallin X p-Benzoquinone XI Hydroxy-p-benzoquinone XII 3-Hydroxy-o-benzoquinone XIII Methoxy-p-benzoquinone X I V 3,3'-Dimethoxy-diphenyl-diquinone-2,5,2' ,5'
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
t
62
COAL OfOCHIMISTRY
Polymerizing lignin decomposition products under oxidizing conditions depends on substitution on the phenyl ring, whether there is one O H group alone or whether it is associated w i t h one or two methoxyl groups. Further more i t is important to know whether the side chain is composed of aldehyde, carboxyl, v i n y l alcohol, acrylic acid, or glycerol groups. F o r three carbon atoms i n the side chain the polymer linking can occur through the side chains, through the rings, or through a side chain and a ring i n form of a C - C linkage or a C - O - C linkage. I n the case of phenyl derivatives w i t h one carbon atom i n the side chain the only possibility that exists is linkage between the rings ( I I ) (Figure 3 ) .
COOH
HO-f~\^—'
C = c — COOH 3 2 1 βΛΦηοΙοΜίό*—»
% activity in polymer*
at '
'
40
w
* 100
tOO
WO
0
0
-
SO
PlêurotuM %
J
.
—tr+ctut
activity Me,
"c
2
