Journal of Nuclear and Radiochemical Sciences, Vol. 17, pp. 23-29, 2017

ArticlesMössbauer Characterization of Iron in Ancient Buried Trees Excavated from the Foothillsof Mt. Chokai
Shigeru Yamauchia,*, Yasuji Kurimotoa, and Yoichi SakaibabInstitute of Wood Technology, Akita Prefectural University, Noshiro, Akita 016-0876, Japan
Department of Chemistry, Daido University, Nagoya, Aichi 457-8530, Japan

Received August 18, 2017; Accepted December 26, 2017; Published online January 25, 2018
The oxidation number and spin state of Fe in sample of ancient trees (umoregi trees) excavated from the foothillsof Mt. Chokai in Japan were determined from the 57Fe Mössbauer spectra. Moreover, basic structures of the Fecompounds were investigated using Mössbauer spectroscopy and complementary analytical techniques. The
umoregi wood samples were prepared from the trunks of four tree species: Japanese cedar and Japanese zelkova
which were brown and dark green, respectively; and Japanese oak and Japanese chestnut which were a pure black
tone. Treatment with oxalic acid removed these characteristic colors from the samples. Mössbauer spectra wererecorded at 293 K and 78 K, and their absorption line-shapes were analyzed using a curve-fitting method. At 293
K and 78 K, the respective isomer shifts of Fe were in the range of 0.21(3)-0.43(1) mm/s and 0.44(3)-0.53(1) mm/s,
and the respective quadrupole splitting values were in the range of 0.62(6)-1.06(2) mm/s and 0.84(5)-1.07(1) mm/s.
Although the resulting Mössbauer parameters confirmed that Fe in the umoregi wood was trivalent and its spin
momentum number was 5/2 (i.e. high-spin state), no changes in spectral shape due to paramagnetic relaxation
were observed even at 78 K. From this comprehensive investigation, it was determined that Fe3+ in Japanese oak
and Japanese chestnut umoregi wood forms multinuclear complexes and has some differences in its first coordination sphere from Fe3+ in Japanese cedar and Japanese zelkova umoregi wood. Furthermore, it was suggested
that the molecular structure of the Fe3+ complexes significantly affects the umoregi wood color tone.

1. Introduction
The Japanese term umoregi refers to ancient trees buried orsubmerged for a long period of time. In particular, this term
implies that the trees, which have not been entirely denatured
due to their isolation from sunlight and oxygen, were buried orsubmerged by natural phenomena (e.g., landslide, debris avalanche, ground subsidences) more than several hundred years
ago. Umoregi generally refers to the trunk of the ancient trees.
In English, bog wood, fossil wood (sub-fossil wood), and lignite have been used as translations for umoregi, however, they
are not fully equivalent. This Japanese term does not belong
to any scientific vocabulary and includes some ambiguity. In
this study, we define umoregi wood as the trunk of an ancientburied tree that contains neither carbonized nor petrified components, although ancient trees are usually called umoregi
even if they have undergone partial carbonization.
Most umoregi wood is brown or black in color, and sometimes deep green. Sufficiently sturdy umoregi wood can beused for construction and furniture, and they are traded as premium woody materials, because their characteristic colors orhues enhance their commercial value. The colors are due tochemical changes in the wood components that proceed slowly
over a long period of time.
Umoregi trees have been found throughout Japan, and thedegree of denaturation in them appears to be dependent on thetree species and the burial conditions, including the burial
time and the soil type. In the last decade, Narita and co-workers have used chemical approaches to examine the essential
oils contained in umoregi wood excavated in Japan.1-5
However, these studies did not discuss the wood coloring*Corresponding author. E-mail: sigeru@iwt.akita-pu.ac.jp, Fax:
+81-185-52-6924

mechanisms.
From studies of iron-gall ink6-10 and dark spots on timber
due to Fe contamination,11-13 it has been recognized that thecharacteristic color of umoregi wood arises from reactions
between Fe and polyphenols such as tannin. Only two studies14, 15 on Fe in umoregi wood have been reported so far, andthey made no mention of the coloring mechanism, although
they investigated the chemical states of Fe in the wood using
Mössbauer spectroscopy.
Our aim in this study is to determine the oxidation and spinstate of Fe in umoregi wood and to obtain fundamental information about the molecular structure of Fe-containing chemical species. Here, we use 57Fe Mössbauer spectroscopy andcomplementary analytical techniques to examine samples ofumoregi wood obtained from trees excavated from the foothills of Mt. Chokai.
2. Experimental
2.1. Umoregi and raw wood samples. The umoregi trees
used in this study were excavated in 2015 from Nikaho City in
Akita Prefecture, Japan, located to the north-west of Mt.
Chokai. The excavation site was within a construction area of
the Nihonkai-Tohoku Expressway Nikaho interchange. It isconceivable that large-scale landslides occurred there and natural forests on the north-west side of Mt. Chokai to the present
coast of Nikaho city were covered with soil and stone ca. 2500
years ago.16, 17
The umoregi trees were Japanese oak (OA; Quercus section
Prinus, e.g. Quercus serrata or Quercus crispula), Japanesechestnut (CH; Castanea crenata), Japanese cedar (CE;
Cryptomeria japonica), and Japanese zelkova (ZE; Zelkova
serrata).
Hereafter, we refer to the tree trunks obtained by removing© 2018 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 01/25/2018

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J. Nucl. Radiochem. Sci., Vol. 17, 2018

Yamauchisoil and bark from the excavated umoregi trees as “umoregi
wood”. After drying at ambient temperature for several
months, umoregi wood was cut into ca. 9 × 9 × 1 cm3 blocks.
The trunk samples of OA (Quercus serrata), CH, CE, and ZE,
which are modern species, were prepared to compare withumoregi wood samples. We refer to the trunks of the modern
trees as “raw wood”.

sealed rotary vacuum pump during measurements. X-ray irrad iat ion was done at room temperat u re a nd t he tot almeasurement time was 2000 s. The X-ray tube voltages were
15 kV (1000 s) and 50 kV (1000 s). Relative mass percentagesof elements to total elements detected were estimated using astandardless fundamental parameter method18, 19 from characteristic X-ray intensities.

2.2. Treatment with oxalic acid. Oxalic acid is widelyused as a timber bleaching agent for Fe contamination. It is
generally accepted that the bleaching is due to formation of Fe
oxalates instead of Fe tannates, because Fe oxalates exhibit ayellow or pale-yellow hue, and hence we treated the umoregi
wood with oxalic acid solution. The umoregi wood blocks
were cut into smaller blocks and dried in air at 105 °C for 24hours in an electric oven. The small blocks were immersed inaqueous oxalic acid solution (3 w/w%), and the glass beakerholding the solution and blocks was subsequently placed in a
vacuum desiccator. The internal pressure was sufficiently
reduced using a water-jet pump, and the desiccator was sealedfor 3 hours. The blocks were taken out from the solution anddried in air for more than 3 days. Special-grade oxalic acidwas obtained from Wako Pure Chemical Industries, Ltd.

2.5. Infrared photoacoustic (IR-PA) spectroscopy. The
IR-PA spectra were obtained at 22±1 °C using a spectrometer(JIR7000W, JEOL, Ltd.) equipped with an IR-PA spectroscopy
module (IR-PAS 1000, JEOL, Ltd.). The spectra wereobtained from 300 accumulations with a spectral resolution of
ca. 4 cm-1. A thin cell (diameter, 5 mm) was filled with thewood powder, and photoacoustic signals were collected upon
purging a cell bench with He gas. The moving-mirror velocity
was fixed at 0.16 cm/s. Carbon black was used as a reference.

2.3. Preparation of sample powders for spectroscopic
analyses. The small blocks of umoregi wood and raw wood
were crushed to powder using a ceramic ball mill. Wood powder that could be passed through an 18.5-mesh-sieve was used
as a sample for the spectroscopic measurements. Soil was collected at the excavation site of a ZE umoregi tree. The soil
was ground in a ceramic mortar after air-drying, and the soil
powder was analyzed by X-ray fluorescence spectrometry.
2.4. X-ray fluorescence spectrometry. Elements withatomic numbers greater than 11 in umoregi wood and soil
samples were determined by X-ray fluorescence spectrometry
using a compact spectrometer (MESA-500, Horiba, Ltd.).
Umoregi wood (0.36 - 0.79 g) or soil (2.3 g) powder was placedin a cylindrical polytetrafluoroethylene cell with a diameter of
20 mm. The sample chamber was evacuated using an oil-

2.6. Mössbauer spectroscopy. 57Fe Mössbauer measurements were carried out in a conventional transmission mode
on a Mössbauer spectrometer (Model-222, Topologic System
Co.) with a 57Co(Rh) source (925 MBq). Measurement temperatures were 293 K and 78 K and they were regulated using
an Oxford cryostat DN-1726 with an ITC-601 temperature
controller. Curve fitting of the Mössbauer spectra was perfor med by a nonlinear least-squares method using the
MossWinn 4.0Pre. program, assuming that all spectra werecomposed of a doublet of Lorentzian-shaped peaks. The isomer shift (IS) and Doppler velocity scale were calibrated withrespect to the sextet of α-Fe at room temperature.
The umoregi wood sample thicknesses of CE and the otherthree were ca. 43 mg/cm2 and ca. 30 mg/cm2, respectively, andthese allowed us to discuss Mössbauer absorption intensities
semi-quantitatively.
3. Results and Discussion
3.1. Umoregi wood colors and bleaching with oxalic acidtreatment. Figure 1 shows photos of the umoregi and raw
wood samples, and the umoregi wood samples treated with

a

bb

c

d

a’
a’

b’
b’

c’

d’

a’’
a’’

b’’
b’’

c’’

d’’

Figure 1. Umoregi and raw wood samples, and umoregi wood samples treated with oxalic acid. The top row is umoregi wood (a, CE; b, ZE; c,
OA; d, CH), the middle row is raw wood (a’, CE; b’, ZE; c’, OA; d’, CH), and the bottom row is umoregi wood treated with oxalic acid (a’’, CE;b’’, ZE; c’’, OA; d’’, CH).

J. Nucl. Radiochem. Sci., Vol. 17, 2018

Mössbauer Characterization of Iron in Ancient Buried Trees Excavated

25

TABLE 1: Relative mass percentage of elements in umoregi wood and soil samples
SampleCE* umoregi
ZE* umoregi
OA* umoregi
CH* umoregi
Soil***

Mg
2.2**

Al
2.0(1)
1.4(2)
4.8(1)
0.7**
11.6(1)Si
12.8(1)22.4(2)11.7(1)
4.4**
61.5(1)P
0.2(1)
0.5(1)-

Element wt%
SCl
0.3(2)
1.4(2)
13.5(1)
1.2**-

K
1.1(1)
1.0(1)
0.8**
1.1**
4.5**

Ca
39.9(2)34.0(2)14.4(1)21.5(1)
4.2**

Mn
1.9(1)
0.1**
1.0**
0.3**
0.2**

Fe
41.2(2)38.6(2)53.7(1)72.1(1)
13.3**

Ashcontent %
0.62
1.38
1.31
2.73
-

*CE, Japanese cedar; ZE, Japanese zelkova; OA, Japanese oak; CH, Japanese chestnut.
**Standard deviation is less than 0.1.
***Gray soil was present at the excavation site of the ZE umoregi tree.

oxalic acid. In the top row, CE and ZE umoregi wood
appeared as brown (khaki) and dark green (olive green),
respectively. Umoregi wood of OA and CH, which belong to
the family of Fagaceae, ex hibited a pu re black tone.
Therefore, the colors of umoregi wood may vary with woodspecies. By comparing with the raw wood shown in the middle row, we saw that the wood color changed drastically withthe burial time, and the color of umoregi wood was unrelated
to that of the raw wood.
The umoregi wood samples shown in Figure 1 were prepared about 15 months after tree excavation, and their
Mössbauer and IR-PA spectra were recorded at roughly thesame time. The color tones of CE and ZE umoregi wood hadchanged to brown and dark green, respectively, immediately
after the excavation. However, we judged that their color tones
slightly and gradually became deeper with time. As for OA
and CH umoregi wood, no color tone changes with time could
be observed by the naked eye.
As mentioned above, although no studies are available on
the coloring mechanism of umoregi wood based on immediate
evidence, it is commonly recognized that the color features ofumoregi wood are due mainly to Fe compounds interactingwith polyphenols such as tannin. This has been deduced fromstudies of the historical ink (iron-gall ink)6-10 or stains on timber caused by Fe contamination.11-13 The stains showed darkcolors similar to those of umoregi wood. Moreover, it has
been reported that the dark stains occur on timber owing toCu, Cr, and Mn as well as Fe.20-22
The bottom row shown in Figure 1 presents umoregi woodpieces treated with oxalic acid which were cut from the umoregi wood blocks shown in the top row. In all the umoregi
wood pieces, their characteristic colors vanished dramaticallythrough this treatment. The CE and ZE umoregi wood turned
colors close to those of their raw wood, and the pure black tone
faded away from the OA and CH umoregi wood. From thedisappearance of dark color, we anticipated that the coloringmechanisms in umoregi wood were similar to those in timbercaused by Fe contamination.
3.2. Elements analysis of umoregi wood and soil samples.
Table 1 summarizes the relative mass percentage of elementsfrom Mg to Pb in umoregi wood and soil samples collectedfrom the excavation site.
Tsuchiya et al.23 reported the content of minor elements inthe wood and bark of various raw tree species including CE,
ZE, and OA using inductively coupled plasma emission spectrometry. Wood and bark are organic matter, and the major
elements constituting them are C, O, and H. According totheir study, K and Ca were major elements among metals,
exceeding 1000 mg/kg in more than half of the wood samples.
Mg, Si, and P were classified into a secondary group among
the minor elements and their contents ranged from dozens toseveral hundred mg/kg. The Fe content in the wood samples

was under 10 mg/kg and less than Al and Sr.
The umoregi wood had not been degraded markedly as
mentioned later, and hence C, O, and H should also be major
elements. In contrast, Fe was a major element in all the umoregi wood samples among the elements detected using X-ray
fluorescence spectrometry, and, in particular, the relative masspercentage of Fe in CH umoregi wood was more than 70%.
The elemental analysis indicated that Fe content in the soil wasmuch higher than that in the raw wood samples. Thus, it canbe assumed that Fe was transferred from the soil to the umoregi wood during the long burial period. Other transition metals, which are capable of causing color changes in umoregi
wood, were negligible except for Mn. Although the contentsof Mn were larger than those of Cu, Ni, or Pb, particularly in
CE and OA umoregi wood, the relative mass percentages ofMn were an order of magnitude lower than those of Fe. Most
Ca contained in the umoregi wood was expected to have been
already present in the trees when they were buried.
We noted that the relative mass percentages of Al in theumoregi wood were much less than those of Fe despite thevalue of Al being close to that of Fe in the soil sample. Thismight be explained as follows. In general, Al3+ shows many
similarities in chemical behavior to Fe3+ and all of the Al contained in the soil must be trivalent (Al3+). However, our previous Mössbauer measurement revealed that the Fe2+/Fe3+ ratio
in the soil was ca. 3/7, 24 and therefore Fe2+ might transfer toumoregi wood more easily than Fe3+.
Major elements of raw wood are C, O, and H, as mentioned
above, and the sum of their contents is about 99 %. These elements cannot be detected by X-ray fluorescence spectrometry.
Thus, the relative mass percentages shown in Table 1 should
be much larger than the absolute mass percentages. We
adopted a simple calculation method to estimate roughly theabsolute mass percentage of Fe in the umoregi wood samples.
In particular, assuming that only elements with atomic numbers smaller than 11 were completely removed from ash byburning, we calculated the tentative absolute mass percentagesof Fe based on Eq. (1):
TPFe = RPFe × A × 1/100(1)where TP Fe, RP Fe, and A are the tentative absolute mass percentage, relative mass percentage, and ash content in a percent
unit, respectively. The ash contents of CE, ZE, OA, and CH
umoregi wood samples were 0.62, 1.38, 1.31, and 2.73%,
respectively, as reported in our previous work.24 The TPFe values obtained from Eq. (1) of CE, ZE, OA, and CH umoregi
wood samples were approximately 0.3, 0.5, 0.7, and 2%,
respectively.
Eq. (1) is an approximate calculation because metal elements should be contained as oxides in ash. We expect, however, that TPFe obtained from Eq. (1) is a useful tool to estimate
the chemical states of Fe in the umoregi wood.

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J. Nucl. Radiochem. Sci., Vol. 17, 2018

Yamauchi

3.3. Infrared photoacoustic (IR-PA) spectroscopy. Figure
2 depicts the IR-PA spectra over the 2000 - 750 cm-1 range ofthe umoregi and raw wood samples. It is difficult to obtain afine IR spectrum from black or dark-colored samples such asdegraded wood and charcoal using ordinary techniques
because of non-resonant absorption. Photoacoustic spectroscopy is a suitable technique to measure the IR spectra of such
samples. For example, by means of the IR-PA spectroscopic
technique, we have studied the weathering of tropical woods,
revealing that this method is a useful tool to follow chemicalchanges in functional groups, especially carbonyl, carboxyl,and phenyl groups.25
As seen in Figure 2, the IR-PA spectrum of the umoregi
wood samples was analogous to that of the same species ofraw wood except for the regions of 1800 - 1620 cm-1 and 1300 1200 cm-1. We noted that there were no significant spectral
differences between raw and umoregi wood samples in thevibrational bands due to phenyl groups of lignin (1592 cm-1
and 1503 cm-1) 26, 27 and saturated cyclic groups of cellulose or
hemicellulose (1450 - 1320 cm-1 and 1180 - 1000 cm-1, respectively).26 Therefore, we could assume that the degradation ofumoregi wood had not proceeded drastically during the2500-year-burial of these trees.
Remarkable spectral differences, however, appeared in two
regions. A broad band around 1740 cm-1 was assigned mainly
to C=O stretching vibrations in carboxyl 26 and unconjugatedcarbonyl 28 groups, and the band intensity of umoregi wood
was much less than that of raw wood. The band of stretchingvibrations attributed to conjugated C=O groups28 was detected
at ca. 1650 cm-1, and its intensity in umoregi wood seemed tobe slightly less than in raw wood. Furthermore, it seemed thatthe band intensities in the region of 1300 - 1200 cm-1 for the

1592

1270
1230

1650 1503
1740

a
b

Relative intensity

c
d
e
f
g
h
200

1600

1200

Wavenumber /cm

800

1

Figure 2. IR- PA spectra of raw and umoregi wood samples: a, CE(raw wood); b, CE (umoregi wood); c, ZE (raw wood); d, ZE (umoregi wood); e, OA (raw wood); f, OA (umoregi wood); g, CH (rawwood); h, CH (umoregi wood).

umoregi wood samples except for CE were smaller than thoseof the raw wood samples. Several vibrational modes havebands in this region and one of them will be a stretching vibration of C-O in carboxyl groups.26 Thus, the IR-PA spectra of
the umoregi wood samples reinforced the suggestion that somechemical changes occurred in carboxyl and/or carbonyl groups
during the long burial period.
There are two possible chemical changes. One is decomposition of the functional groups, and the other is the occurrenceof chemical interactions between the functional groups andtheir neighboring species. If O atoms included in the functional groups form hydrogen, ionic or coordinate bonds, thebands at 1740 and 1650 cm-1 would be shifted to considerably
lower wavenumbers.29 It was, however, difficult to determine
from the IR-PA spectra whether such shifts occurred.
3.4. Mössbauer spectroscopy. Figures 3 and 4 show the
Mössbauer spectra of umoregi wood samples recorded at 293
K and 78 K, respectively. The absorption intensities are substantially weak for almost all umoregi wood samples becauseof the low Fe contents (0.3 to 2 %) as described in Section 3.2.
Absorption lines in all the Mössbauer spectra consisted of a setof doublet lines, and no magnetic hyperfine splitting wasobserved even at 78 K. Table 2 summarizes the Mössbauer
parameters (IS; quadrupole splitting, QS; and line width, LW)
calculated by curve fitting. The LW values were slightly but
clearly larger (~0.26 mm/s in the present work) compared withordinar y Fe compounds. This en hancement of LW is
explained later. In addition, from Figures 3 and 4, theMössbauer absorption intensity was considered to be in thefollowing order: CE < ZE < OA < CH, and this was in agreement with the order of TPFe (see Section 3.2).
The IS values of 0.21 to 0.43 mm/s at 293 K indicated that
Fe in all of the umoregi wood samples was high-spin Fe3+ or
low-spin Fe2+.30-32 The assignment to high-spin Fe2+ should be
excluded, since the IS and QS values at 293 K (Table 2) ranged
from 0.2 mm/s to 0.5 mm/s and from 0.6 mm/s to 1.1 mm/s,
respectively, and they were much smaller than those of mosthigh-spin Fe2+ compounds.33, 34 It seemed that all the IS and QS
values at 293 K of Fe in the umoregi wood samples were
within the shared ranges between high-spin Fe3+ and low-spin
Fe2+.33, 34 However, the assignment to low-spin Fe2+ should also
be excluded, because the umoregi wood and soil samples
would contain little or no amounts of ligands inducing a large
ligand field splitting such as CN-, NO2-, or NH3. Therefore, theFe in the umoregi wood samples was most likely to be highspin Fe3+. However, the QS values were somewhat larger thanvalues typically seen for high-spin Fe3+ compounds.
The four types of umoregi wood were classified into twogroups based on their IS and QS, as shown in Figure 5. Group
1 consisted of CE and ZE, and Group 2 consisted of OA and
CH. IS and QS of Group 1 were significantly smaller thanthose of Group 2. Moreover, the two groups differed fromeach other in color; Group 1 samples were brown or darkgreen, while Group 2 samples were pure black as seen in
Figure 1. Hence, the first coordination sphere of Fe in Group 1
would be expected to be somewhat different from that inGroup 2, even though Fe in both groups exists as high-spin
Fe3+ species.
Group 1 showed a small but clear temperature-dependence
for QS, although the QS values of Group 2 were independentof temperature. This suggested that Fe3+ species in Group 1
may undergo some structural change from 293 K to 78 K,
because QS of high-spin Fe3+ shows little or no temperaturedepndence. 31 Moreover, the differences in IS of Group 2
between 293 K and 78 K were ca. 0.1 mm/s as shown in Table
2, which was consistent with the second-order Doppler shift.30
However, the increases in IS of Group 1 at 78 K were too large
to be accounted for as the second-order Doppler shift alone,J. Nucl. Radiochem. Sci., Vol. 17, 2018

Mössbauer Characterization of Iron in Ancient Buried Trees Excavated

a

a

b

×4

c

× 1.5

×5

Relative intensity / a. u.

Relative intensity / a. u.

×3

d

0



Doppler velocity / mm s



× 1.5



CH*

IS**
mm s-10.26(3)0.44(3)0.21(3)0.44(3)0.42(1)0.51(1)0.43(1)0.53(1)QS***
mm s-10.64(5)0.84(5)0.62(6)0.88(4)0.99(2)0.98(1)1.06(2)
1.07(1)

LW****
mm s-10.66(8)0.68(7)0.82(15)0.70(7)0.60(3)0.70(2)0.70(3)0.67(1)*CE, Japanese cedar; ZE, Japanese zelkova; OA, Japaneseoak; CH, Japanese Chestnut.
**Isomer shift, ***quadrupole splitting, ****line width.

1.1

CH

1.0

OA

0.9

ZE

0.8

CE

0.7
ZE

CH
OA

CE

0.6
0.2

and it appeared that there was some structural change taking
place between 293 K and 78 K.
It is well recognized, at room temperature, that IS is in therange of 0.3 – 0.6 mm/s if coordinating atoms to Fe3+ form anoctahedron (six-coordinated structure), and in the range of 0.2– 0.4 mm/s if a tetrahedron (four-coordinated structure) is
formed.32 The IS of Fe3+ in Group 2 umoregi wood samples
was in the former range, whereas the IS of Fe3+ in Group 1
umoregi wood samples was slightly less than 0.3 mm/s. It is,however, known that most Werner type Fe3+ complexes havean octahedral coordination geometry, except for halogenocomplexes.
On the other hand, for high-spin Fe3+ species, it has beenreported that QS ranges at room temperature corresponding to
the six-coordinated and four-coordinated structure ranges areapproximately 0 – 1.1 mm/s and 0 – 0.75 mm/s, respectively.34
As listed in Table 2, our QS values supported the finding thatthe coordination number of Fe3+ in Group 2 umoregi wood
samples was six; however, they did not suggest a coordination



1

1.2

1

Temp.
K
293
78
293
78
293
78
293
78

0

Figure 4. Mössbauer spectra at 78 K of umoregi wood samples: a,
CE; b, ZE; c, OA; and d, CH. Numerals in the spectra represent a relative magnification of intensity when the spectrum of CH is used asa reference. The percent absorption intensity is about 1.6 % at the
maximum for CH.

Quadrupole splitting / mm s

Umoregi wood
sample

Doppler velocity / mm s

TABLE 2: Mössbauer parameters of umoregi wood samples

OA*

d

1

Figure 3. Mössbauer spectra at 293 K of umoregi wood samples: a,
CE; b, ZE; c, OA; and d, CH. Numerals in the spectra represent a relative magnification of intensity when the spectrum of CH is used asa reference. The percent absorption intensity is about 0.8 % at the
maximum for CH.

ZE*

c

×1



CE*

b

×3

×1



27

0.3

0.4

0.5

0.6

0.7

1

Isomer shift / mm s

Figure 5. Isomer shift and quadrupole splitting at 293 K (♦) and 78
K (■) of Fe in umoregi wood samples. The red band shows the IS
range for high-spin Fe3+ at room temperature32, 34, and the blue bandshows that at liquid nitrogen temperature estimated from the secondorder Doppler shift30 on the basis of the former range.

number of Fe3+ in Group 1 umoregi wood samples.
Accordingly, the Fe3+ in Group 2 umoregi wood is certainly
located at the center of an octahedron structure. In contrast, it
is difficult to determine from Mössbauer parameters whether
the first coordination sphere of Fe3+ in Group 1 umoregi wood
is a tetrahedral or octahedral structure.
Several studies have addressed the reaction between Fe andpolyphenols, and discussed the structure of polyphenol-Fe
complexes. These studies are divided into two categories.
The first is research into the effects of polyphenols in wood on

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J. Nucl. Radiochem. Sci., Vol. 17, 2018
6-10

Fe corrosion. Researchers have supposed that two kinds ofmononuclear Fe(III) complexes bonded to tannins as a bidentate ligand are formed by the reaction with tannins extracted
from wood, on the basis of Mössbauer spectroscopic analysis.
The second category is a series of studies of historical ink(iron-gall ink).35-37 From both categories, it should be notedthat a basic structure for a model compound for gallic acidFe(III) complex was determined using an X-ray diffraction
method.6, 37 The gallic acid-Fe(III) complex has polynuclear
structures, and a molecule of gallic acid forms three bonds
between its three hydroxyl groups and two Fe atoms, and achelate bond between its carboxyl group and another Fe atom.
Fe atoms are located at the center of the octahedral structures.
However, it will be very difficult to determine the structure of
Fe compounds contained in the umoregi wood samples by
X-ray diffraction methods, because Fe concentrations are low
and the crystals are microscopical if Fe compounds are crystallized.
Our Mössbauer data suggested that Fe species in Group 2umoregi wood had multinuclear structures for which a unit
contained two or more Fe atoms, because no changes in spectral shape due to paramagnetic relaxation were observed evenat 78 K despite the presence of high-spin Fe3+. However, it wasdifficult from our Mössbauer spectral data to determine
whether or not their molecular structures were analogous tothat of the gallic acid-Fe(III) complex.
Paramagnetic relaxation phenomena in Mössbauer spectroscopy have been interpreted as the f luctuation of magnetic
fields produced by electronic spins in paramagnetic compounds, and the f luctuation rate decreases as temperature
decreases. This phenomenon is observable in high-spin Fe3+
and Eu 2+ whose orbital momentum vanishes (L≈0) as changes
in Mössbauer spectra. Well-resolved magnetic hyperfine
splitting clearly appears in Mössbauer spectra when a fully
f i xed mag net ic f ield ex ist s at a Mössbauer nuclide.
Meanwhile, a magnetic field has no influence on Mössbauer
absorption line-shape if its fluctuations are sufficiently fast.
In both cases, Mössbauer spectra can be expressed as a combination of Lorentzian curves. However, at intermediate spin
fluctuation rates comparable to the Larmor precession rate of
the Mössbauer nucleus, it is known that the Mössbauer absorption line-shape becomes complicated through the relaxation
effect and cannot be precisely approximated with Lorentzianor Gaussian curves.30
We previously reported the Mössbauer spectra of high-spin
Fe3+ 38-43 and Eu 2+ 44, 45 compounds and their frozen solutions,and indicated that the electron-spin fluctuation rate decreases
as the Fe-Fe or Eu-Eu distance increases due to less spin-spin
dipole interaction. Specifically, we experimentally examined
the Mössbauer line-shapes of tris(β-diketonato)Fe(III)38, 39 andhexakis(alkylurea)Fe(III)42 complexes, which have a six-coordinate octahedral structure in terms of the paramagnetic relaxation effect. The Mössbauer spectra of all these complexes
showed an apparent single absorption line, but the line hadwings on both sides and its line-shape was distinctly nonLorentzian. The bulkier the ligand of the Fe complex, thelarger the wings were. In other words, the absorption lineshape broadened with increasing average Fe-Fe distance in thecomplexes. The absorption line at liquid nitrogen temperature(78K) exhibited more obvious broadening compared with thatat room temperature in each complex. Moreover, splittings
due to magnetic hyperfine structures were observed in the
Mö s s b a u e r s p e c t r a wh e n Fe ( I I I ) (a c a c) 3 (a c a c, C H 3
COCHCOCH3-) complexes were homogeneously dispersed infrozen solutions.38, 40
In contrast, it was confirmed that the Mössbauer absorption
li ne -shape of [(μ- CH 3 O)Fe(III)(dpm) 2 ] 2 (dpm, (CH 3) 3
COCHCOCH(CH3)3-), which is a dinuclear Fe complex, showsno significant changes when dispersed in a frozen solution

Yamauchi3+

despite the fact that the Fe in the complex is in the high-spinstate and located at the center of an octahedral structure.46
This was interpreted as an indication that the spin-spin dipole
interaction was hardly weakened by dispersion in frozen solution, because the Fe-Fe distance in the dinuclear Fe complex
unit was constant.
Accordingly, the Mössbauer absorption line of Fe in Group
2 umoregi wood samples should exhibit broad wings or somedifferences from Lorentzian curves if the Fe3+ species weremononuclear complexes. The reasons are as follows.
(1) Fe3+ species in Group 2 umoregi wood certainly havesix-coordinate octahedral structures.
(2) Polyphenols, which will be bulkier than β-diketone andalkylurea molecules, likely form chemical bonds with Fe3+ asligands.
In addition, the wings at 78 K should be broader and larger
than those at 293 K. Nevertheless, neither wings nor differences from Lorentzian curves were observed in the Mössbauer
absorption of Fe in the umoregi wood samples and the absorption lines shown in Figures 3 and 4 were well explained by asuperposition of two identical Lorentzian curves.
The wood components with pyrogallol groups that can form
cross-linked structures by an O-Fe-O-Fe-O bond include several polyphenols containing tannins. As a result of the
Mössbauer spectroscopic measurements, we concluded that
Fe 3+ formed multinuclear complexes with polyphenols inGroup 2 umoregi wood. Furthermore, the large LW values
allowed us to predict co-existence of two or more kinds of Fe
complexes with similar structures, and this prediction wasapplicable to Group 1 umoregi wood.
On the other hand, the absence of a paramagnetic relaxation
effect on the Mössbauer absorption line-shape could not beeasily accepted as evidence that Fe3+ species in Group 1 umoregi wood were multinuclear complexes, because the coordination atoms to Fe3+ may form tetrahedral structures.
Our Mössbauer spectroscopic data indicated that Fe3+ species in Group 1 umoregi wood differ significantly from thosein Group 2 umoregi wood in terms of their molecular structure, thereby reinforcing the suggestion that the molecular
structure of Fe3+ species is an important factor determining thecolor tone of the umoregi wood. Further study is necessary tounderstand completely the molecular structures of Fe3+ species
in different types of umoregi wood.
4. Conclusion
The results of the Mössbauer spectroscopic analysis
described here confirmed that Fe in the umoregi wood samples had a 3d 5 electron configuration and its spin quantum
number (S) was 5/2 (high-spin state), and they indicated thatthe high-spin Fe3+ species in OA and CH umoregi wood samples exhibited octahedral symmetry and could be attributed tomultinuclear compounds. Moreover, the temperature-dependence of IS and QS suggested that some structural changes
might occur in Fe3+ species in CE and ZE umoregi wood samples between 293 K and 78 K.
Comprehensive investigation of all the experimental results
indicated that the Fe3+ complexes included polyphenols with
pyrogallol groups as ligands, and suggested that their molecular structure was a governing factor for the umoregi woodcolor tone.
Acknowledgement
We are grateful to the Nikaho City Board of Education forthe donation of the umoregi wood samples.

Mössbauer Characterization of Iron in Ancient Buried Trees Excavated

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