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3 Oxidative degradation of HAs, FAs, and humins

3 Oxidative degradation of HAs, FAs, and humins

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158



Morris Schnitzer and Carlos M. Monreal



conditions with peracetic and nitric acids (Schnitzer, 1978). Other oxidants

used included alkaline nitrobenzene, sodium hypochlorite, and H2O2

solutions. Degradations with Na2S and phenol have also been done (Hayes

and O’Callagham, 1989).

Major components produced by the oxidation of methylated and

unmethylated HS from widely differing pedological and geographical

origins under alkaline as well as under acidic conditions were aliphatic

carboxylic, phenolic, and benzenecarboxylic acids (Figs. 4À6) (Schnitzer,

1978). The most prominent oxidation products were aliphatic mono,

di-, tri-, and tetra-carboxylic acids, phenolic acids with between 1 and 3

OH groups and between 1 and 5 CO2H groups, and the tri-, tetra-, and

penta-benzenecarboxylic acids. From the oxidation products identified, it

appeared that in the initial HS the aromatic rings were linked by paraffinic

CH3(CH2)14CO2H

CH3(CH2)16CO2H

CO2H

(CH2)n, n = 0 – 8



O



R5



R2



CO2H



CH2

CH

CH2



CO2H

CO2H



R3



R4



CO2H



Figure 4 Major aliphatic oxidation products of HS. (Source: From Schnitzer,

1978, Fig. 7, p. 28. With permission of the publisher.)



CO2H



CO2H



CO2H

CO2H



CO2H



CO2H



CO2H



HO2C



CO2H



CO2H

CO2H

CO2H



HO2C



CO2H



CO2H



CO2H



HO2C

CO2H



CO2H



HO2C



CO2H



CO2H



HO2C



CO2H

CO2H



Figure 5 Major benzenecarboxylic acid oxidative degradation products of HS.

(Source: From Schnitzer, 1978, Fig. 8, p. 29. With permission of the publisher.)



159



Quo Vadis Soil Organic Matter Research? A Biological Link to the Chemistry



chains (Fig. 7). On oxidation, the aliphatic C’s closest to the aromatic rings

were converted to C’s of the CO2H groups which remained bonded to the

rings, while the C’s in aliphatic chains were oxidized to aliphatic acids. The

purpose of methylation prior to oxidation was to protect OH groups against

attacks by electrophilic oxidants. This made it possible to isolate and

identify phenolic acid in addition to benzenecarboxylic and aliphatic acids.

The oxidation products from soil HS separated from soils from all the earth’s

surface were remarkably similar.

Two conclusions can be drawn from the oxidative degradation experiments: (1) isolated aromatic rings are important structural units in all HS

CO2H



CO2H



OH

OH



OH



HO



OH



CO2H



CO2H



CO2H



CO2H

OH



OH

CO2H



HO



CO2H



OH



OH

CO2H



HO2C



CO2H



CO2H



HO2C



CO2H



CO2H



CO2H



CO2H



Figure 6 Major phenolic oxidation products of HS. (Source: From Schnitzer,

1978, Fig. 9, p. 29. With permission of the publisher.)



OH



OH



HO



OH

OH



COOH



COOH



HOOC



COOH



HO



COOH



OH

OH HOOC

OH



OH



COOH

OH



Figure 7 Chemical structure for HS based on oxidative degradation products.

(Source: From Schnitzer, 2000, Fig. 12, p. 23. With permission of the publisher.)



160



Morris Schnitzer and Carlos M. Monreal



and in SOM and (2) aliphatic chains are linking aromatic rings to form

an alkylaromatic network.



7.4. Reductive degradation

Compared to oxidative degradation of HS and SOM, reductive degradation

has been less successful in obtaining structural information. The methods

most widely used for this purpose were Na-amalgam reduction and Zn-dust

distillation (Stevenson, 1994). Reduction of HS with Na-amalgam produced

phenols and phenolic acids. On the other hand, Zn-dust distillation and

Zn-dust fusion (Hansen and Schnitzer, 1969) yielded small amounts of

methyl-substituted naphthalene, anthracene, phenanthrene, pyrene, and perylene. The methyl groups on these polycyclic rings were probably remains of

longer alkyl chains which linked the polycyclics in HS and in SOM; or from

microbial methylation during polyketide (PK) synthesis (see Section 11).



8. Spectrometric and Spectroscopic

Characteristics of HS

8.1.



13



C NMR spectrometry of HS



One of the advantages of 13C NMR is that it indicates the presence in HS a

wide variety of C types whose determination by other methods would either

be laborious and time consuming or not at all possible. In this sense, the use

of 13C NMR offers unique possibilities. Of considerable interest is a comparison of solid-state or CP-MAS (cross-polarization magic angle spinning)

13

C NMR spectra of HA (extracted with 0.5 M NaOH solution from the

Ah horizon of a Haploboroll) and FA (extracted with 0.5 M NaOH solution

from the Bh horizon of a Spodosol). Both extracts were freeze-dried prior

to NMR analysis. The CP-MAS 13C NMR spectrum of HA (Fig. 8A)

shows several distinct signals in the aliphatic (0À105 ppm), aromatic

(106À150 ppm), phenolic (151À170 ppm), and carboxyl (171À185 ppm)

C regions. The signals at 17, 21, 25, 27, and 31 ppm are due to alkyl C. The

resonance at 17 ppm is characteristic of terminal CH3 groups and that at

31 ppm of (CH2)n in straight paraffinic chains. The peak at 40 ppm could

include contributions from both alkyl and amino acid-C. The broad signal

at 53 ppm and the sharper one at 59 ppm may arise from C in OCH3

groups, but amino acid-C may also contribute to signals in this region

(Breitmaier and Koelter, 1978). Carbohydrates in HA would be expected to

produce signals in the 60À65, 70À80, and 90À104 regions, although other

types of aliphatic C bonded to O would also do so. The aromatic region



Quo Vadis Soil Organic Matter Research? A Biological Link to the Chemistry



200



100



161



0



(A)



(B)



ppm



200



100



0



Figure 8 13C NMR spectra of: (A) a Mollisol HA and (B) a Haplaquod Bh FA.

(Source: From Schnitzer and Schulten, 1998, Fig. 8-4, p. 158. With permission of

the publisher.)



exhibits a relatively sharp maximum near 130 ppm, due to alkylaromatics.

The signal at 155 ppm shows the presence of O- and N-substituted aromatic

C (phenolic OH and/or NH2 bonded to aromatic C). The broad signal near

180 ppm is due to C in CO2H groups, although amides and esters could

also contribute to this resonance.

The CP-MAS 13C NMR spectrum of the FA (Fig. 8B) consists of

a number of aliphatic resonances in the 20À50 ppm region, followed by

signals from C in OCH3 groups and amino acids between 50 and 60 ppm

and from carbohydrates between 61 and 105 ppm. The broad signal between

130 and 133 ppm indicates the presence of C in alkylaromatics. The strong

signal between 170 and 180 ppm shows the presence of C in CO2H groups.

In general, fewer sharper signals are observed in the spectrum of the FA than

in that of the HA, possibly because of the occurrence of more H-bonding in

the FA.

The 13C NMR spectra in Fig. 8 show that the HA is slightly more

aromatic than the FA, but the FA is significantly richer in CO2H groups,

which appears to be the main difference between the two substances and

accounts for the water solubility of the FA in contrast to the HA which is

insoluble in water. Other differences are that the HA is richer in paraffinic C but poorer in carbohydrate C than the FA. It is noteworthy that

the main structural features such as aromaticity and aliphaticity are similar.

Little is known about the chemical structure of humin, which is

that portion of SOM which is left behind after extraction of the soil

with dilute alkali. Preston et al. (1989) deashed a humin fraction separated

from the surface horizon of the Bainsville soil and allowed it to stand



162



Morris Schnitzer and Carlos M. Monreal



with an aqueous solution of HClaHF (1.16 M HCl 12.88 M HF) at

room temperature for a prolonged period of time, changing the solvent

at fixed intervals. With progressive deashing, the humin became more

soluble in 0.5 M NaOH solution. After extensive deashing, the CP-MAS

13

C NMR of the residue and its elemental composition were identical to

those of the HA extracted with 0.5 M NaOH solution from the same

soil. This indicates the humin is HA bound strongly to minerals and metal

oxides and hydroxide in the soil, so that instead of three fractions, SOM

contains only two fractions, that is, HA and FA. As has been mentioned

before, since in most soils FA represents ,10% of the alkaline extract,

and because FA is essentially a partial oxidation product of HA, it is clear

that HA is the major component of SOM.



8.2. Effect of hot acid hydrolysis on the

spectrum of HA



13



C NMR



Figure 9A shows the solution-state 13C NMR spectrum of a HA extracted

with 0.5 M NaOH solution from the Ah horizon of a Chernozem from

Central Alberta, Canada (Schnitzer and Preston, 1983). To facilitate the

analysis of the 13C NMR data, the spectrum was divided into the

following regions: 0À40 ppm (C in straight chain, branched and cyclic

alkanes, and alkanoic acids); 41À60 ppm (C in branched aliphatics,

amino acids, and OCH3 groups); 61À105 ppm (C in carbohydrates and

in aliphatics containing C bonded to OH, ether oxygens, or occurring in

five- or six-membered rings containing O; 106À150 ppm (aromatic C);

151À170 ppm (phenolic C); and 171À190 ppm (C in CO2H groups).

The 13C NMR spectrum of the HA after hydrolysis with hot 6.0 M HCl

for 24 h is shown in Fig. 9B. This spectrum is simpler than that in Fig. 9A,

with distinct maxima remaining only at 18.7, 25.1, 31.3, 40.0, and 58.7 ppm

in the aliphatic region and at 131 and 180 ppm in the aromatic region. The

13

C NMR of the same HA, which had been hydrolyzed first by standing in

contact with 12.0 M H2SO4 for 16 h at room temperature, and then refluxed

with 0.5 M H2SO4 for 5 h, is shown in Fig. 9C. Note that curves 9B and

9C are very similar. In the aliphatic region of curve 9C, small signals can be

seen at 16.5, 25.2, 31.4, 40.1, and 58.4 ppm. The persistence of the

distinct signal at 58.7 in (B) and at 58.4 ppm in (C) as well as at 58.5 ppm in

(A) suggests that this signal is due to C in OCH3 groups. Resonances at

56.2, 63.5, and 73.4 ppm in Fig. 9A are no longer present in the spectra of

the hydrolyzed HA (Fig. 9B and 9C). Chemical analysis for amino-N and

carbohydrates indicated that the two acids had removed most of the proteinaceous compounds and carbohydrates from the acid-treated HAs. On the

basis of these observations, and from data published in the literature, we

could now assign the following bands in Fig. 9A: 56.2 ppm (C in amino



Quo Vadis Soil Organic Matter Research? A Biological Link to the Chemistry



200



100



163



0



131.8

31.3

16.5



121.6



63.5

73.4



40.2



25.0



58.5



180.4



(A)



130.6

31.3

25.1



40.0

58.7



179.3



(B)



131.2

31.4

40.1



25.2

16.5



171.0



58.4



178.3



(C)



75.2



ppm



200



100



0



13



Figure 9

C NMR spectra of (A) Chernozem HA; (B) the same HA hydrolyzed

with 6 M HCl; (C) the initial HA hydrolyzed first with 12 M H2SO4 and then with

0.5 M H2SO4. (Source: From Schnitzer and Preston, 1983, Fig. 1, p. 204. With

permission of the publisher.)



acids), 63.5 and 73.4 ppm (C in carbohydrates). One striking observation

was that the relative intensity of the aromatic regions in spectra B and C was

greater than that in spectra A, although the position of the maximum

remained near 130 ppm. For the initial HA, the HCl-hydrolyzed HA, and

the H2SO4-hydrolyzed HA (spectra A, B, and C), aromaticities were 41%,

62%, and 53%, respectively. Aromaticities were computed by expressing areas

due to aromatic C (105À165 ppm) as percentage of the total area

(0À165 ppm) but omitting contributions from carboxyl carbons

(166À185 ppm).

Acid hydrolysis also reduces the intensity of the carboxyl region

(170À180 ppm). Area integration of the carboxyl regions (170À180 ppm)

in spectra A, B, and C showed CO2H contents of 4.5, 3.0, and 3.1 meq g21,

respectively. Thus, aside from lowering the CO2H by acid decarboxylation



164



Morris Schnitzer and Carlos M. Monreal



and by removing amino acids and carbohydrates, the hot acids also increase

the aromaticity of the HA by removing aliphatic compounds such as amino

acids and carbohydrates.

To establish whether hot acid hydrolysis would affect other HAs and

also FA in the same way as the Chenozemic HA, Schnitzer and Preston

(1983) passed a HA extracted with 0.5 M NaOH solution from the Ah

horizon of a Humic Gleysol and a FA extracted from the Bh horizon of

Spodosol through the same procedure. The results obtained were identical

to those described herein.

Thus, hot acid hydrolysis removes proteinaceous materials and carbohydrates from HAs and FAs but leaves aliphatic and aromatic compounds

intact. These findings may have important structural implications: (1) they

suggest that proteinaceous compounds and carbohydrates are not structural

compounds of HA, FA, or SOM but are adsorbed on their surfaces and in

their voids. If they were structural components, they would have resisted

dissolution by the hot acids; (2) the strong signal at 130 ppm in the 13C

NMR spectra is due to C in aromatic rings not substituted by electrondonating O and N but by C, as in alkylaromatics, which indicates that the

latter are very significant building blocks of HA, FA, and SOM.



8.3. Curie-point pyrolysis-gas chromatography-mass

spectrometry of HAs

Curie-point pyrolysis-gas chromatography-mass spectrometry (CpÀGCÀMS)

is a valuable method for structural studies on HS (Schulten and Schnitzer,

1992) because the transfer of thermal energy from the wire to the sample is

fast with temperature rises on the order of milliseconds. The resulting

thermal shock produces small, stable organic molecules as pyrolysis products whose identification is based on two independent data sets: (1) gas

chromatographic retention times and (2) computer-assisted library searches

of standard mass spectra libraries.

Schulten and Schnitzer (1992) did CpÀGCÀMS analyses of two HAs

which had previously been examined by Py-FIMS. As shown in Table 3,

the major compounds produced from the two HAs were benzene and

alkylbenzenes, ranging from C6H6 to C6H5 Á C22H45 (docosylbenzene).

Other compounds produced were naphthalene and alkylated naphthalenes

and phenanthrene and alkylated phenanthrenes. The alkylaromatics identified in Table 3 consisted of aromatic rings which were covalently linked

to aliphatic groups or chains, and these building blocks were released

during pyrolysis from an alkylaromatic structural network that was made

up of the constituents listed in Table 3. A chemical structure for a HA

skeleton, without functional groups, based on alkylbenzenes is shown in

Fig. 10. This structure contains voids of varying dimensions which can

trap and bind organic and inorganic components.



165



Quo Vadis Soil Organic Matter Research? A Biological Link to the Chemistry



Table 3 Building blocks of Bainsville and Armadale HAs identified by Curie-point

pyrolysis GC-MS

Intensity

Armadale



Bainsville



Compounds



11111

11111

1111

1

11

1

1

1

11

1

11

1

11

1

1

1

11

1

1

1

1

1

1

1

1

1

1

1

111

1

11

111

1

11

11

11

11

1

1



111

11111

1111

1

11



Benzene

Toluene

Ethylbenzene, xylenes

Ethylmethylbenzene

Propylbenzene

Butylbenzene

Methylpropylbenzene

Tetramethylbenzene

Pentylbenzene

Hexylbenzene

Octylbenzene

Methyloctylbenzene

Decylbenzene

Methylnonylbenzene

Undecylbenzene

Methyldecylbenzene

Dodecylbenzene

Methylundecylbenzene

Tridecylbenzene

Tetradecylbenzene

Pentadecylbenzene

Hexadecylbenzene

Heptadecylbenzene

Octadecylbenzene

Nonadecylbenzene

Eicosylbenzene

Hemicosylbenzene

Decosylbenzene

Styrene

Methylstyrene

Indene

Indane

Fluorene

Naphthalene

Methylnaphthalenes

Dimethylnapthalenes

Trimethylnaphthalenes

Tetramethylnaphthalenes

Pentamethylnaphthalene



1

1

1

1

1

1



111

1

1

11

1

1

1

1

1



(Continued)



166

Table 3



Morris Schnitzer and Carlos M. Monreal



(Continued )



Intensity

Armadale



1

1

1

1

1



Bainsville



Compounds



Phenanthrene

Methylphenanthrene

Dimethylphenanthrene

Trimethylphenanthrene

Tetramethylphenanthrene



Intensity of peak height: 1 1 1 1 1 , 80À100%; 1 1 1 1 , 60À80%; 1 1 1 , 40À60%; 1 1 ,

20À40%; 1, observed.

Source: From Schulten et al. (1991), Table 1, p. 312. With permission of the publisher.



(CH3)0-4



(CH3)0-5



(CH3)0-5



Figure 10 Chemical structure of HA based on alkylaromatic building blocks

without functional groups. (Source: From Schulten et al., 1991, Fig. 1, p. 311.

With permission of the publisher.)



An inspection of the data in Table 3 shows that Armadale HA

produced a greater variety of alkylaromatics than the Bainsville HA.

This may be related to differences in the origins of the two HAs. The

Armadale HA was extracted from the Bh horizon of a Spodosol, about

25 cm below the soil surface while the Bainsville HA was extracted

from the surface horizon of the Bainsville soil, a Haplaquoll. One of the

striking features of a Spodosol Bh horizon is its low microbial activity,

which may have led to a better preservation of the alkylaromatics listed

in Table 3.



Quo Vadis Soil Organic Matter Research? A Biological Link to the Chemistry



167



8.4. X-ray analysis of FA

The X-ray diffraction pattern of a nonoriented flat powder specimen of a FA

extracted with 0.5 M NaOH from the Bh horizon of a Spodosol exhibited a

diffuse band at about 4.1 A˚, accompanied by a few minor humps (Kodama

and Schnitzer, 1967). The experimental intensity curve was corrected for

polarization and absorption. Figure 11, curve A, shows the corrected intensity

curve as a function of (sin θ)/λ over the range 0.02À0.5. Curve A was normalized to the total independent scattering curve B for FA in such a way that

curves A and B approached each other. Curve B was obtained by adding the

independent coherent scattering curve C to the incoherent scattering curve D.

Calculations for the independent coherent scattering curve C and the incoherent scattering curve D were based on C28H23O19 (the molecular formula of

FA) and included contributions from all atoms.

8.4.1. Radial distribution analysis

The radial distribution analysis of FA is based on the generalized Fourier

method for polyatomic substances (Warren et al., 1936). For more details

on how this method was used see Kodama and Schnitzer (1967). Figure 12

4

×103

3



I, (e.u.)



(A)



2

(B)

(C)

1



(D)

0



0.2



0.4



0.6



(sin θ)/λ



Figure 11 X-ray intensity curve for FA in electron units per

C20H12(CO2H)6(OH)5(CO)2 (the elemental and functional group composition of

FA). (A) The corrected intensity curve; (B) the total independent scattering

curve; (C) the independent coherent scattering curve; and (D) the incoherent

scattering curve. (Source: From Kodama and Schnitzer, 1967, Fig. 1, p. 90. With

permission of the publisher.)



168



Morris Schnitzer and Carlos M. Monreal



8

×104



Σ km 4πr 2 ρm (r)



6



4



2

16 700

0



6000

2



4



6



r (Å)



Figure 12 Radial distribution curve for FA. (Source: From Kodama and Schnitzer,

1967, Fig. 2, p. 91. With permission of the publisher.)



shows the existence of two peaks with maxima at 1.6 and 2.9 A˚ and two

˚.

shoulders at 4.2 and 5.2 A

Since FA resembles low-rank coals in a number of properties, it seemed

worthwhile to compare our results with those for carbon black as reported

by Warren et al. (1936). His curve showed four distinct maxima at distances

of separation at 1.5, 2.7, 4.05, and 5.15 A˚, indicating that carbon black was

not amorphous but consisted of graphite-like layers. These four distances

are in good agreement with the respective peak positions for FA. The

broadness of the peaks and the similarity of their positions as compared to

carbon black suggest that FA has a considerable random structure in which

carbon atoms are arranged in a manner similar to that in carbon black.

More dedicated analysis of each peak is complicated because in addition

to carbon atoms, oxygen atoms also form part of the structure of FA.

Contributions of hydrogen atoms can be neglected because they constitute

less than 10% of the total number of electrons.

Interatomic distances of bond pairs according to the Interatomic

Tables (Hypercube Inc., 1962) are 1.54À1.40 A˚ and 1.47À1.36 A˚

for a single bond or a partial double bond for carbonÀcarbon and

carbonÀoxygen, respectively. Assuming loose packing, the peak at 1.6 A˚

may include contributions from both atom pairs. The area under the peaks

corresponds, therefore, to the sum of the electrons from carbonÀcarbon and

carbonÀoxygen bond pairs. One numberÀaverage molecular weight of FA

contains 28 carbon atoms, 13 of which are either in functional groups or

linked to the functional groups and may be diatomically bonded to



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