Industrial Hemp (Cannabis sativa L.) Growing on Heavy Metal Contaminated Soil: Fibre Quality and Phytoremediation Potential

Abstract

Hemp (Cannabis sativa L.) was used to examine its capability as a renewable resource to decontaminate heavy metal polluted soils. The influence of heavy metals on the fibre quality was of special interest. Determination of heavy metal content was carried out by means of atomic absorption spectroscopy (AAS). Four different parts of the plant were examined: seeds, leaves, fibres and hurds. In each case, the concentration relation was Ni>Pb>Cd. However, the heavy metal accumulation in the different parts of the plant was extremely different. All parts of hemp plants contain heavy metals and this is why their use as a commercially utilisable plant material is limited. We found that the highest concentrations of all examined metals were accumulated in the leaves. In this field trial, hemp showed a phytoremediation potential of 126 g Cd (ha vegetation period)−1. We tested the fibre quality by measuring the pure fibre content of the stems and the fibre properties after mechanical separation. In addition, the fibre fineness was examined using airflow systems and image analysis. The strength was measured by testing single fibre bundles with a free clamping distance of 3.2 mm using a universal testing device. Finally, we compared the results from the stems and fibres from trials on heavy metal polluted ground with hemp stems and fibres from non-polluted ground. Since there was no comparable unpolluted area near the polluted one, reference values were taken from an area quite far away and subsequently with a different soil composition and also exposure to different meteorological conditions. Thus, the observed differences are only partially caused by the heavy metal contamination.

Full text

Industrial Crops and Products 16 (2002) 33–42
Industrial hemp (Cannabis sati6aL.) growing on heavy
metal contaminated soil: fibre quality and phytoremediation
potential
P. Linger
a,
*, J. Mu¨ssig
b,
*, H. Fischer
b
, J. Kobert
a
a
Department
9
—Chemistry,Physiological Chemistry of Plants,Uni6ersity of Wuppertal,Gauss Str.
20
,
42097
Wuppertal,Germany
b
Faserinstitut Bremen e.V., FIBRE,PO Box
10 58 07
,
28058
Bremen,Germany
Accepted 19 December 2001
Abstract
Hemp (Cannabis sati6aL.) was used to examine its capability as a renewable resource to decontaminate heavy
metal polluted soils. The influence of heavy metals on the fibre quality was of special interest. Determination of heavy
metal content was carried out by means of atomic absorption spectroscopy (AAS). Four different parts of the plant
were examined: seeds, leaves, fibres and hurds. In each case, the concentration relation was Ni\Pb\Cd. However,
the heavy metal accumulation in the different parts of the plant was extremely different. All parts of hemp plants
contain heavy metals and this is why their use as a commercially utilisable plant material is limited. We found that
the highest concentrations of all examined metals were accumulated in the leaves. In this field trial, hemp showed a
phytoremediation potential of 126 g Cd (ha vegetation period)
−1
. We tested the fibre quality by measuring the pure
fibre content of the stems and the fibre properties after mechanical separation. In addition, the fibre fineness was
examined using airflow systems and image analysis. The strength was measured by testing single fibre bundles with
a free clamping distance of 3.2 mm using a universal testing device. Finally, we compared the results from the stems
and fibres from trials on heavy metal polluted ground with hemp stems and fibres from non-polluted ground. Since
there was no comparable unpolluted area near the polluted one, reference values were taken from an area quite far
away and subsequently with a different soil composition and also exposure to different meteorological conditions.
Thus, the observed differences are only partially caused by the heavy metal contamination. © 2002 Elsevier Science
B.V. All rights reserved.
Keywords
:
Cannabis sati6aL.; Fibre quality; Fibre bundle; strength; fineness; Optical fibre fineness analyzer; Airflow; Phytoremedi-
ation; Phytoextraction
www.elsevier.com/locate/indcrop
1. Introduction
In the future, we will have to deal with the
problem that increasing numbers of agriculturally
used areas will be contaminated by anthropo-
* Corresponding authors. Tel.: +49-202-439-2820; fax: +
49-202-439-3142.
E-mail addresses
:
linger@uni-wuppertal.de (P. Linger),
muessig@faserinstitut.de (J. Mu¨ssig).
0926-6690/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0926-6690(02)00005-5

P.Linger et al.
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Industrial Crops and Products
16 (2002) 33–42
34
genic-derived pollutants. Consequently, these ar-
eas will no longer be able to be used for food
production. One of the most important classes of
pollutants are heavy metals which stem from cad-
mium-containing phosphate fertilisers, the smelt-
ing industry or sewage sludge distribution
(Adriano, 1986, Saxena et al., 1999).
However, several approaches exist for cleaning
up contaminated soils (Rulkens et al., 1998; Chen
et al., 2000). One of the most interesting is phy-
toremediation, or strictly speaking phytoextrac-
tion (Cunningham et al., 1995; Cunningham and
Ow, 1996; Ernst, 1996; Chaney et al., 1997;
Raskin et al., 1997; Salt et al., 1998; Chen et al.,
2000; Meagher, 2000). In this case, plants grow on
polluted soil, extract the toxic substances and
accumulate them in the upper parts of the plant.
They are then harvested, and consequently the
soil is cleaned up. On the other hand, the greatest
shortcoming of this method is the long period of
time needed for decontamination. The plants used
in phytoremediation are generally annual herbs
which don’t have any economic value, but do
have a very high extraction potential, namely
hyperaccumulators (Robinson et al., 1997; Salt et
al., 1998; Wenzel et al., 1999).
The aim of our research is to combine phytore-
mediation with a crop of commercial interest,
with the view of achieving low price decontamina-
tion of soil by the production of a commercially
usable resource. The plant we chose for our ap-
proach was Cannabis sati6aL., hemp. Hemp has
been used by man for over 5000 years and is
known to have many uses. The fibres can be used
for clothes, insulating material, or as a composite
material (Nature, 1996). The seeds serve as an
excellent source of oil due to its composition of
unsaturated fatty acids (Theimer et al., 1997). The
oil has a range of uses including production of
colours, lacquer and in the cosmetic industry
(Karus et al., 1995; Rausch, 1995; Wirtshafter,
1995; Hupperts et al., 1997). The seeds also
provide a source of protein for man and animals
(Patel et al., 1994). Moreover, the compounds
resulting from hemp’s secondary metabolism are
of major interest to the pharmaceutical industry
(Karus, 1995; Robinson, 1996; Grotenhermen,
1998).
Here, we explore the following issues: (a) is
hemp a suitable plant for phytoextraction; (b) is
the commercially utilisable fibre material contami-
nated with heavy metals; and (c) if so, how does
this contamination influence fibre quality.
To find out the answers to these issues, we
carried out a field trial on a heavy metal contam-
inated field.
2. Materials and methods
2
.
1
.Plant material/experimental arrangement
The hemp cultivation experiments were or-
ganised in 1999 at the trial station in Hagen
(Nordrhein-Westfalen, Germany) using the hemp
variety C.sati6aUSO 31. The plot of the test was
3×4 m wide, the seed was sown on 12th June.
The seed rate was 250 seeds/m
2
(with no addi-
tional fertilisation), and the number of plants
shortly before harvesting was nearly 100 plants/
m
2
. The mature plants were harvested about 15
weeks later on 24th September, with an average
stem height of 183 cm. The stems were picked by
hand, dried and stored at 20 °C and 50% relative
humidity. The stems and the separated hemp fibre
bundles are named in accordance with the identifi-
cation code USO Hagen ’99. The soil of the trial
site had been polluted by sewage sludge distribu-
tion and contained 102 ppm cadmium (Cd), 419
ppm nickel (Ni) and 454 ppm lead (Pb) (Hygiene-
Institut des Ruhrgebietes, 1997).
We compared the fibre content and the fibre
properties from the trials on heavy metal polluted
ground with hemp stems from non-polluted
ground. Since there was no comparable unpol-
luted area near the polluted one, reference values
were taken from an area with different soil com-
position and meteorological conditions. Thus, the
observed differences are only partially caused by
the heavy metal contamination. We took samples
from the test location of the chamber of agricul-
ture Weser-Ems in the Wehnen district, in the
community of Bad Zwischenahn, near Oldenburg
in the northern part of Germany.
The experimental plants were subjected to 4
randomised repetitions. The plots of the test were

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35
3x6 m wide, and were sown also with hemp
variety USO 31 during the last week of April,
1999. The seed density was 200 seeds/m
2
with a
rate of nitrogen of 100 kg/ha, the germination
rate was on average 182 plants/m
2
(28th May).
The experimental plot was harvested on 27th Au-
gust, 1999 and hemp exhibited an average stem
height of 260 cm. However, the number of plants
reached the level of maturity required for harvest-
ing was much reduced. In 1999, the values of
plant density were only examined for the hemp
variety Fedrina 74. For this hemp variety, the
seed rate was also 200 seeds/m
2
and the number
of plants counted shortly before harvesting was 99
plants/m
2
. The stems of USO 31 were dried and
stored at 20 °C and 50% relative humidity
(Martens and Mu¨ssig, 2000). The stems and the
separated hemp fibre bundles are named in accor-
dance to the identification code USO Wehnen ’99.
The stems from both the ‘contaminated’trial in
Hagen and the ‘control’test trial in Wehnen were
unretted.
2
.
2
.Mechanical separation of fibre bundles
After crop collection the stems were dried and
stored. The fibre bundles were separated mechani-
cally with a BAHMER-FLAKSY laboratory
device. This equipment was developed by Bahmer
Maschinenbau GmbH (Germany) and consists of
four pairs of profiled, rotating rolls. The speed
control was adjusted to position 10, resp. 10
m/min transport speed. After six passages through
the machine, the hurdless fibre-bundles were
refined with a coarse separator. A self-developed
laboratory coarse separator was used. This ma-
chine consists of a serrated cylinder (Ø261 mm)
and is fed by a rotating roll (Ø32 mm). The
distance from the feeding roll to the rotating
serrated cylinder is 20 mm. The fibre transport
after coarse separation was organised by air. The
results of separation are comparable with indus-
trial separation techniques (Mu¨ssig, 2001).
2
.
3
.Chemical separation of fibre
The fibre content (pure fibre content) was ex-
amined by the method from Bredemann (1942)
Fig. 1. Various forms of hemp fibres open to testing (Mu¨ssig,
2001).
after chemical separation in 2% NaOH in an
autoclave at 2 bar pressure. From the test plots, a
random sample was taken and from this four
stems were cut into top, middle, and bottom
parts. The pure fibre content was measured for
the different sections.
2
.
4
.Tensile testing
The mechanically separated hemp fibres were
conditioned for 24 h at 20 °C and 65% relative
humidity. The strength of single hemp fibre bun-
dles was tested. Fig. 1 shows the differences be-
tween the various forms of hemp fibres which are
open to testing.
The single hemp fibre bundles were tested in an
Instron universal testing device 4502 with an In-
stron 2518–806/1 kN power cell. Samples were
fastened to a Pressley clamp with Plexiglas jaws.
The free clamping length was 3.2 mm and the test
speed was 2 mm/min. The device is shown in Fig.
2. The length of each tested single bundle was
Fig. 2. A single hemp fibre bundle fastened to the Pressley
clamp.

P.Linger et al.
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36
constant because of the thickness of the Pressley
clamps and the distance of the gauge length (3.2
mm). The length l
B
of each bundle was 15 mm.
The mass of the single bundles Mwas measured
with an accuracy of 0.01 mg. The mass related
fineness gF (in tex) of each bundle could be
calculated with the following sizes equation:
gF=
M
mg
N
B
l
B
mm
·10
−3
·10
6
=1000
(1)
With the calculated mass related fineness gF (in
tex) and the experimentally measured value for
the breaking force of the single bundle F(in N),
the strength R
H
(in cN/tex) could be calculated
according to Eq. (2).
R
H
u
F
N
gF
tex
·1
10 (2)
To obtain an acceptable level of accuracy for fibre
bundle strength, 100 valid results are necessary.
Experiments with bundle breaking inside or near
the clamping area or slipping effects were counted
as non-valid results.
2
.
5
.Fibre fineness testing
The fineness of fibre bundles was examined by
an optical fibre fineness analyzer (OFDA) and
airflow methods. The principle of the different
measurement methods is given in Fig. 3. The
OFDA was developed for measuring the diameter
of wool fibre (Baxter et al., 1992). This apparatus
efficiently measures width distribution of bast
fibre bundles and results have been found to
correlate well with those of other methods. Be-
cause of the large number of measurements taken,
our results could be well reproduced. Compared
with the OFDA, the airflow method does not
provide information about the distribution of
fineness for individual bundles. However, this
method is very rapid as well as being highly
reproducible (Drieling et al., 1999).
Prior to the experiment the hemp fibres were
conditioned for 24 h at 20 °C and 65% relative
Fig. 3. Airflow and OFDA methods to measure the fineness of
hemp fibre bundles (Martens and Mu¨ssig, 2000).
humidity. The pressure of the Airflow testing
device was adjusted to a water column of 120 mm
and calibrated after a recently developed method
to get comparable airflow values from different
labs and researchers for hemp testing (Mu¨ssig,
2001). For each sample, we used three specimens
each weighing 2.5 g. The measurement was taken
three times.
The FMT-Shirley device works in the same way
as the airflow, i.e. with indirect examination of the
fibre surface by the flow of air. The coarse hemp
fibre bundles were measured in the FMT at com-
pression stage P
L
. For each sample we used three
specimens each weighting 4 g. The measurement
was taken three times. In contrast to the airflow,
coarser fibres gave low and fine fibres high Shirley
values.
To examine the bundle width distribution with
the OFDA, the bundles were cut into 3 mm long
snippets. The snippets were prepared on a slide
using tweezers. We prepared 5 slides for each
hemp sample for both USO Hagen ’99 and USO
Wehnen ’99. Each slide was scanned twice.
2
.
6
.Determination of hea6y metal content
Determination of heavy metal content was car-
ried out by means of atomic absorption spec-
troscopy (AAS). Four different parts of the plant
were prepared as samples: (1) seeds; (2) leaves; (3)
Í
Á
Ä

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37
fibres; and (4) hurds. These samples were dried at
105 °C for 3 h prior to the analysis. For each
sample, 1.5–2 g were weighed in a 24-ml crucible,
and heated to 450 °C at a rate of 2 °C/min, and
kept at this temperature for 10 h.
After cooling, the residues were weighed again
to obtain the total residue amount, dissolved in 10
ml concentrated nitric acid (HNO
3
ROT-
IPURAN
®
, Roth no. 4989.1), and then diluted
with distilled water to a total volume of 50 ml.
This solution was used as the sample for AAS
determination of cadmium (Cd), lead (Pb) and
nickel (Ni) content in an AAS 5FL (Carl Zeiss
Technology). Heavy metal content of the original
sample was calculated back from the concentra-
tion of the measured solution.
3. Results and discussion
3
.
1
.Fibre properties
One problem we faced for this investigation was
that we could only use hemp plants from different
habitats. Many different factors are known to
influence hemp fibre quality and quantity such as
meteorological conditions, soil properties, fertili-
sation, sowing period, plant density and harvest
period. Therefore, it is difficult to compare the
results from plants of different origins. Neverthe-
less, the data could provide insight if soil contam-
inated with heavy metals has a significant impact
on fibre properties.
3
.
1
.
1
.Mechanical separation of fibre
Mechanical extractable fibre content after
decortication and mechanical separation was 26%
for USO Hagen ’99 and 36% for USO Wehnen
’99.
3
.
1
.
2
.Chemical separation of fibre/pure fibre
content
As described in Section 2.3, we examined the
fibre content (pure fibre content) using Brede-
mann‘s method. The results are shown in Fig. 4.
We compared the fibre content from the trials on
heavy metal polluted ground (USO Hagen ’99)
with hemp stems from non-polluted ground (USO
Wehnen ’99).
Fig. 4. Pure fibre content of hemp stems/USO Wehnen ’99 vs.
USO Hagen ’99.
As seen in Fig. 4, the value of the pure fibre
content in the stems of ‘contaminated’USO Ha-
gen ’99 samples is reduced in comparison to the
values of the ‘control’USO Wehnen ’99 samples.
This holds true for all parts of the stems.
Comparable results were achieved by mechani-
cal decortication and separation.
3
.
1
.
3
.Fibre fineness
We examined the fineness of the mechanically
separated hemp fibre bundles using two measure-
ment methods. The Airflow measurements are
shown in Fig. 5.
The Airflow values gave highly similar results
for both ‘contaminated’and ‘control’hemp sam-
ples, with a tendency for the USO Wehnen ’99
sample to be a little bit coarser. The same result
was observed with the FMT-Shirley values (Fig.
Fig. 5. Airflow values for the fineness of mechanically sepa-
rated hemp fibre bundles/USO Wehnen ’99 vs. USO Hagen
’99.

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38
Fig. 6. FMT-Shirley value for hemp fibre bundles/USO Weh-
nen ’99 vs. USO Hagen ’99.
Fig. 8. OFDA fibre bundle width of hemp sample USO Hagen
’99.
6). It should be pointed out that in general
coarser fibres show reduced Shirley values.
The results from the airflow and the FMT
measurements do not provide information about
the distribution of fineness of the bundles. How-
ever, these methods are very rapid and highly
reproducible. To find out about the distribution
of fibre bundle width, we used the OFDA system.
This apparatus efficiently measures width distri-
bution of bast fibre bundles. The distribution of
the coarse separated fibre bundles for the sample
USO Hagen ’99 is documented in Fig. 7.
The results of the OFDA measurement show a
wide distribution of fibre bundle width. For a
mechanically coarse separated unretted hemp
sample this is typical (Dreyer and Mu¨ssig, 2000).
The data of the USO Hagen ’99 samples are
summarised in the box-and-whisker chart in Fig.
8.
The nearly identical results from the different
fineness testing methods document the low differ-
ences between the fineness of the fibre bundles of
the two hemp samples USO Hagen ’99 and USO
Wehnen ’99.
3
.
1
.
4
.Strength of single fibre bundles
As described in Section 2.4, we tested single
fibre bundles and not collectives. The tensile prop-
erties of the coarse separated hemp fibre bundles
are documented in the box-and-whisker chart in
Fig. 9.
A wide range of values were obtained, and was
more distinct for the single bundles USO Wehnen
’99. The wide range of strength distribution of
hemp fibres has also been documented in the
work of Sankari (2000) and Keller et al. (2001).
Although the values of the strength of the fibre
bundles of USO Hagen ’99 are smaller than those
of USO Wehnen ’99, the values are still on an
acceptably high level and the distribution is more
homogeneous.
To summarize the results, heavy metal polluted
soil had no significant influence on fibre proper-
ties. All results seemed to be in natural occurring
range.
Fig. 7. Distribution of fibre bundle width of hemp USO Hagen
’99.

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39
Fig. 9. Strength of single hemp fibre bundles/USO Wehnen ’99
vs. USO Hagen ’99.
of the soil with these in the plants, we find that
the concentration of Ni in the soil is only four
times higher than that of Cd. Therefore, there
must be different mechanisms for uptake and
accumulation for nickel, lead, and cadmium in
general and also for the different plant parts
themselves. The highest concentrations of all ex-
amined metals were found in the leaves, which
indicates the transport of heavy metals via the
xylem sap. In the seeds, the concentrations of Cd
and Ni are relative high. However, the concentra-
tions found in hurds and fibres are comparably
small.
For Ni, the concentration relation was leaves\
seeds\hurds \fibres, whereas it was leaves \
fibres\hurds \seeds for Pb. For Cd, the relation
was leaves\seeds \fibres =hurds, but the Cd
concentrations found here are comparably small.
Though, the possibility can not be ruled out that
concentration levels could be different in soils
with a different heavy metal composition.
Another interesting aspect is the total metal
uptake of the plant, which depends on (a) the
concentrations found in its single parts, and (b)
on the relative mass constitution of the plant.
That is to say, the contribution of the relatively
high metal concentrations found in the seeds to
the total mass of metal in the plant is negligible,
since the seeds contribute only 8–10% of the
plant’s total mass (LBP, 1996). The leaf content
accounts for 13–15%, fibre content 25 –27%,
whereas the dominating part is the hurd content
with about 60%.
3
.
2
.Determination of hea6y metal content
For all samples Cd, Pb and Ni content were
examined by AAS. The complete results are dis-
played in Table 1. Heavy metals could be detected
in all parts of the plants which are of commercial
interest. In each case, the concentration relation
was Ni\Pb \Cd. It is obvious from the data,
that hemp shows a strong specificity for the accu-
mulation of various heavy metals. From this
study, the concentrations of Cd were eight to 26
times lower than the Ni concentrations. However,
when comparing the heavy metal concentrations
Table 1
Heavy metal contents of the hemp samples
WeightSample Residue Ni contentCd content Pb content
(g) (%)
ppm Average ppm Average ppm Average
1.732 12.962.6 0.78 2.60Hurds
2.8 9.99 11.51.973Hurds 2.6 0.76 0.8 2.94
7.421.537Fibres 3.5 0.85 3.97
6.96.323.93.780.8Fibres 0.793.31.575
63.8323.3Leaves 3.941.485 23.20
2.058 23.5 2.96 3.5Leaves 21.65 22.4 63.46 63.6
33.241.980.757 1.19Seeds 6.0
0.829 6.4 1.03 1.1 1.69 1.8Seeds 24.79 29.0

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These mass contribution may be slightly differ-
ent depending on variety, season, and crop region,
but it depicts clearly that the plant’s total metal
accumulation is dominated in the hurds on one
hand, due to their large contribution of over 50%
to the total mass, and in leaves on the other hand
due to their high individual metal uptake.
To calculate the phytoextraction potential, we
determined the absolute amount of Cd in one
plant, i.e. 126 mg Cd (plant)
−1
. By extrapolation
the total extraction of Cd by hemp could be
calculated as 126 g Cd (ha vegetation period)
−1
.
This implies about 126 g Cd (ha)
−1
in 3–4
months. Thlaspi caerulescens, one of the best stud-
ied hyperaccumulators for cadmium, is able to
accumulate 3000 ppm (Brooks et al., 1998) and to
extract up to 2 kg Cd (ha year)
−1
under optimal
growth conditions (Saxena et al., 1999). The phy-
toextraction potential of T.caerulescens is ap-
proximately 16-fold higher when compared with
USO 31, but hemp grows well under natural
conditions, and does not require the expensive use
of fertilisers nor the time- and money-consuming
control of optimal growth conditions. Another
point is, that T.caerulescens provides nothing,
which could be used as a raw material for com-
mercial purposes.
The question that has be answered is: Could
hemp, grown on contaminated soil, be used as a
supplier of food or raw material for commercial
applications? The WHO (1972) and FAO/WHO
(1995) pointed out that 70 mg Cd uptake per day
are not harmful to man. The WHO set a limit of
0.1 mg heavy metal (kg food)
−1
. These levels
would therefore disqualify hemp seeds or leaves to
be used in food production. However, hemp oil
could be utilized in lacquer or industrial oil pro-
duction. The use of hemp fibres for clothes pro-
duction would not be possible as the heavy metal
concentrations exceed the O
8
ko-Tex-Initiative
(2000) (Hohenstein), which are 0.1 ppm for Cd,
0.2–1.0 ppm Pb and 1.0 –4.0 ppm Ni.
The main use of contaminated fibres and hurds
might be in combine material, where the fibre are
embedded in polymers and could not be set free.
Another possible use of the plant material is for
energy production in thermal power stations, as
here the plant material falls below the restrictions
(Winkler, 1997). In addition, it might be possible
to recycle the metal from the ash, this process is
called phytomining (Chaney et al., 1995;
Robinson et al., 1997; Brooks et al., 1998; Ander-
son et al., 1999; Nedelkoska and Doran, 2000).
From our work, it seems unrealistic that hemp is
an economically viable option in regard to
phytomining.
4. Conclusion
The fact that C.sati6aaccumulates heavy
metals in all plant parts limits its use as a raw
material in clothes as well as in the food chain.
However, the high quality of the fibres and hurds,
which were not affected by the heavy metal con-
tamination, allows them to be used in special
products like combine material. Our experiments
provided no evidence of fibre damage due to
heavy metal contamination. Fibre bundle fineness
and strength from the contaminated as well as the
non-contaminated hemp are identical within the
limits of experimental error. The fibre content was
significantly lower in hemp grown on unpolluted
soil, however, it is not possible to decide if this is
only an effect of the contamination.
Hemp seems to be best suited for soils with a
low content of heavy metals due to its relative low
phytoextraction potential. Hemp’s commercial as-
pects together with its ability of extracting heavy
metals from the soil makes it an ideal candidate as
a profit yielding crop when used for phytoremedi-
ation purposes.
Acknowledgements
The authors thank Reent Martens and Jakob
Gatena from the Chamber of Agriculture Weser-
Ems in Oldenburg, Germany for the possibility of
using the samples USO 31 from the cultivation
trials in 1999 and for the detailed report of the
experiments. We are much indebted to Ute
Melville from the Institute of Applied and Physi-
cal Chemistry of the University of Bremen for her
support in the AAS measurements.

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