1. Environmental Geochemistry and Health (1998), 20, 19±27
Correlation of the methylating capacity of river
and marine sediments to their organic sediment
index
S.A. Hadjispyrou1
, A. Anagnostopoulos1
, K. Nicholson2*
, M.K. Nimfopoulos3
and
K.M. Michailidis4
1
Institute of Inorganic Chemistry, Department of Chemical Engineering, Polytechnic School, Aristotle
University, 540-06 Thessaloniki, Macedonia, Greece
2
Environment Division, School of Applied Sciences, The Robert Gordon University, Aberdeen AB1
1HG, Scotland, UK
3
Geochemistry Division, Institute of Geology and Mineral Exploration, 1 Fragon str., 546-26
Thessaloniki, Macedonia, Greece
4
Department of Geology, Division of Mineralogy, Petrology and Economic Geology, Aristotle
University, 540-06 Thessaloniki, Macedonia, Greece
Methylation experiments of the metals Sn, Pb and Hg were carried out using representative terrestrial and marine
sediment samples from the Axios river and Thermaikos Gulf in northern Greece. GC-FID, GC-TCD and GC-MS
were used. The experiments were carried out on sterilised and bioactive samples by adding pure metals and metal
salts (chloride, nitrate, oxalate, acetic, penicillaminic, methioninic and cysteinic). Except for sterilised HgCl2,
methylated derivatives of Sn, Pb and Hg were produced only from bioactive sediments, and therefore higher yields
were measured when nutrients were added to the sediments. Volatile products (CH4, CO2, H2S) of biological
activity range between 35 and 250 mg lÀI
. The correlation of methylation yield with organic sediment index (OSI),
determined as the wt% product of [organic carbon] Â [organic nitrogen], is positive for all the metals and metal salts
added in the sediments. Methylation yields for Hg are found to be four orders of magnitude higher than those of Pb
and Sn. In low OSI (terrestrial) sediments, the rate of Hg-methylation is higher than those of Pb and Sn. In high OSI
(marine) sediments, where methylation of most of the contained Hg has taken place, methylation of Pb is slightly
faster than Sn.
Keywords: methylating capacity, river-marine sediments, organic sediment index
Introduction
It is generally recognised that methyl derivatives of
heavy metals (especially Hg, Sn and Pb) are more
toxic than their inorganic counterparts (Wood, 1974).
In the field of environmental methylation of various
metals and metalloids, many natural methylating
factors have been studied. Besides the well-known
methyl-cobalamin (Pratt, 1972; Ashby and Craig,
1991), SAM (Wood, 1974) and N5
-THF (Wood et
al., 1977), a large number of other factors, such as
CH3J (Jarvie and Whitmore, 1981), (CH3)3S+
JÀ
,
betaine, acetylcholine, choline (Anagnostopoulos
and Hadjispyrou, 1993), among others have been
studied.
In all these cases, the methylating factors are either
natural organic products or metabolites from micro-
organisms and as such, they are related to the organic
fraction of an ecosystem, in this case, a sediment.
Neufeld and Hermann (1975) introduced the term
organic sediment index (OSI) to describe the organic
fraction of a sediment. According to them, the
methylation capacity of a sediment is related directly
to its OSI value, as far as methylation of Hg2+
is
concerned.
The OSI index is determined as the product of the
percentage of organic carbon (OC wt%) and the
percentage of organic nitrogen (ON wt%) in a sedi-
ment (Neufeld and Hermann, 1975):
OSI ˆ ‰OCwt%Š‰ONwt%Š
Metals and metalloids have different methylation
capacities, depending mainly on their oxidation/re-
duction potential in various oxidation states (Wood
et al., 1977; Craig, 1989). This means that some
metals are methylated easily, and others to a lesser
extent (and under strictly determined conditions) or
not at all (Krishnamurthy, 1992). The most widely
studied metal for which methylation has been estab-
0269-4042 # 1998 Chapman & Hall
*
To whom correspondence should be addressed.

2. lished, is mercury (Hg) (Moreton, 1983; Zhou et al.,
1990; Yu et al., 1992; Weber, 1993). The OSI index
concept has been tested so far only in relation to Hg
methylation (Neufeld and Hermann, 1975).
In this paper, the influence of the OSI value on the
methylating capacity of sediment samples from the
Axios river (low OSI) and Thermaikos Gulf (high
OSI) was investigated. The methylating capacity of
the sediments was tested for the metals Hg, Sn and
Pb in their M0
and M(II) oxidation states.
Study Area
The marine part of this research consists of three
geographical water zones (Figure 1). (1) the Bay of
Thessaloniki, which is situated in the direct vicinity
of Thessaloniki city; (2) the Gulf of Thessaloniki,
which is further from the city, communicating
through the Thermaikos Gulf with the north Aegean
Sea; and (3) the Gulf of Thermaikos which is located
in direct proximity to the north Aegean Sea. The
Gulf and the Bay of Thessaloniki consist of two
communicating basins which are linked to the Ther-
maikos Gulf and so through to the open sea by
narrow and shallow canals, which do not permit
coastal streams to carry waste into the open sea.
The Thermaikos Gulf has an overall perimeter of
approximately 70 km and a mean depth of between
20 m and 40 m. The gulf occupies a total area of 553
km2
. About 200 000 m3
dÀ1
of untreated sewage
water from the city of Thessaloniki, with a popula-
tion of about 1 500 000 inhabitants, are directly
released into the Bay of Thessaloniki. Totally or
partly treated industrial effluents (in the order of
80 000 m3
dÀ1
) are discharged on the north-west
coast of the Thermaikos Gulf, where the industrial
zone is located (Figure 1).
The springs and the major route of the Axios river
are located in former Yugoslavia. Only the lower part
of the river enters northern Greece, crossing the
industrial area of Thessaloniki and being discharged
into the Thermaikos Gulf. The actual amount of
effluents discharged into the river is not known,
due to lack of information from the industries located
either in the former Yugoslavia or in northern
Greece.
Anoxic Neogene marine and terrestrial sediment
samples were obtained at locations on the Thermai-
kos Gulf and the Axios river, shown in Figure 1. Site
A (Gulf of Thessaloniki, industrial zone) is contami-
nated by sewage and industrial effluents. Site B
(Thessaloniki harbour) is also heavily polluted by
ship discharges and continuous leaching of antifoul-
ing substances from ship hulls. Sites C (Gulf of
Figure 1 GEOMET satellite image of the area showing sampling sites and river routes. Agricultural areas surrounding the
river Axios can be distinguished.
20 S.A. Hadjispyrou et al.

3. Thessaloniki, near the airport) and D (airport) may be
typical of locations with less-contaminated sedi-
ments. Site E is located near the estuary of Axios
but represents, together with D, deep marine sedi-
ment sampling. Sites F and G are located in the
estuary of Axios. Sites H, J and K are located in the
polluted zone of the Axios river, where the river
receives domestic effluents (H) and run-off water
from agricultural and industrial areas (J and K).
Materials and Methods
Sample collection
A total of 100 samples were collected from the
sampling sites, following in general the procedures
described in Franson et al. (1989).
Each site was covered by a sediment-sampling net-
work involving an area of approximately 20 m2
. One
representative (optimal) sample of maximum biolo-
gical activity was selected from each study site for
the experiments. Gas-liquid chromatography was
used to determine biological activity of sediments.
Each of the 10 selected experimental samples was
split into 20 sub-samples in order to execute the
incubation experiments with various metal salts
(Table 1). For the marine sediments, sampling was
done using a 0.1 m2
Van Veen grab. Immediately
after collection, the samples were stored at À208C by
freezing in solid CO2 in situ and transferring to a
deep freezer in the laboratory.
Experimental design and analysis
Methodology. The way in which these experiments
have been designed is shown in Table 1. Before the
onset of the experimental run, methyl derivatives of
the metals Hg, Sn and Pb were confirmed by GC
analysis to be below detection limits (Table 2) for all
sediment samples.
Other workers (Rapsomanikis, 1983; Moreton, 1983)
have evaluated the optimum conditions for incuba-
tion experiments of this type. These were found to be
20 g of sediment, 30 ml total volume, 300 mg of each
nutrient (glucose or Oxoid code CM1), a minimum
incubation time of 21 d at room temperature condi-
tions and in the dark. Under these conditions, the
concentrations of CH4, H2S and CO2 of the samples
and the blanks are the same at the end of each
experimental run. This is an indication that the
samples were not affected by the presence of high
amounts (1 mmole) of the metals with which each
sediment sample was spiked.
All the methylation experiments were carried out
under anaerobic conditions. Incubation Hypovials
of 50 cm3
capacity with crimp-on, Teflon-lined
silicon rubber septa were used, 20 g of wet sediment
were added to each Hypovial and the volume made
up to 30 cm3
with distilled deionised water. Nutrient
broth (Oxoid code CM1) and glucose (300 mg) were
also added to encourage bacterial growth. The metal
salts were added in the form of solutions of the
appropriate dilution by injection of 1 cm3
via the
septum.
Control experiments with vials containing (a) sedi-
ment only and (b) sediment plus nutrient and glucose
were also prepared under identical conditions to the
incubated samples. One sample for each metal (Hg,
Sn, Pb) was sterilised by keeping it at 1218C for 40
min (Rapsomanikis, 1983). The metal salts were
added after sterilisation in a sterilised-glove box,
using a sterilised syringe and dilution water.
The OSI values of the samples were calculated by
determining the percentage of organic carbon (OC
wt%) and the percentage of organic nitrogen (ON
wt%) of each according to established procedures
(Franson et al., 1989).
Table 1 Incubation experiments of environmental sediments
with metal salts
Sample Amount of metal/salt added (mg)
1 Blank (20 g of sediment/or plus nutrient broth*
)
2 Ay 119 Sn0
3 Ax 190 SnCl2
4 A 237 Sn(OAc)2
z
5 A 350 SnCl4Á5H2O
6 A 207 Sn(COO)2
7 A 415 Sn[Meth]2
z
8 A 266 Sn-Penz
9 A 238 Sn-Cystz
10 A 207 Pb0
11 A 379 Pb(OAc)2Á3H2O
12 Ax 331 Pb(NO3)2
13 A 503 Pb[Meth]2
14 A 354 Pb-Pen
15 A 326 Pb-Cyst
16 A 319 Hg(OAc)2
17 Ax 271 HgCl2
18 A 496 Hg[Meth]2
19 A 347 Hg-Pen
20 A 319 Hg-Cyst
*
See text.
yA, 20 g sediment + nutrient broth (see text).
z(OAc) = acetic radical, ÀOCOCH3;
(Meth) = methionine, ÀCH3S(CH2)2 CH
j
NH2
-COO-;
(Pen) = penicillamine,
HS-C(CH3)2- CH
j
NH2
-COO-;
(Cyst) = cysteine, HS-CH2- CH
j
NH2
-COO-
xSamples 3, 12 and 17 were sterilised.
Correlation of the methylating capacity of river and marine sediments to their organic sediment index 21

4. Analyses of the head-space gases for (CH3)4M pro-
ducts were obtained periodically and at the end of
each experimental run by gas chromatography-flame
ionisation detection (GC-FID) and gas chromatogra-
phy-mass spectrometry (GC-MS) with 1 ml injec-
tions. Standards of methyl derivatives of the metals
were employed in the same matrix as the experi-
mental runs, so that matrix and pressure variations
were taken into account. The whole process gener-
ally followed the published methodology (Donard et
al., 1986; Rapsomanikis et al., 1986; Craig and
Moreton, 1986). Detection limits and %RSD are also
given (Table 2).
Chemicals employed. Simple salts of the metals in
divalent form, such as chlorides, nitrates, acetics and
oxalates are water soluble and therefore were
employed for the experiments. These salts, and
metals in elemental form (e.g. Sn0
, Pb0
), undergo
`oxidative addition' reactions through which metals
are bonded to methyls (Anagnostopoulos and
Hadjispyrou, 1993) and therefore were used in this
study.
Sulphur-containing amino acids such as cysteine
(Cyst), penicillamine (Pen) and methionine (Meth)
are ubiquitous in nature and therefore were employed
for the methylation experiments as ligands for the
metal complexes. The use of these amino acids
positively influences the methylation yield via the
formation of a metal±sulphur bond. This bond stabi-
lises the initially formed methyl metal against hydro-
lysis. The fully methylated product results from the
subsequent redistribution reactions (Craig and Mor-
eton, 1984).
Instrumentation. A Pye 105 gas chromatograph with
flame ionisation detector, fitted with a glass column
1800 mm in length by 0.4 mm internal diameter, was
used to analyse (CH3)4Sn and (CH3)4Pb. The column
was packed with 10% SP2100 on Chromosorb W80/
100 mesh; oven, detector and injector temperatures
were 60±70, 60±70 and 1008C, respectively. The
carrier gas was oxygen-free nitrogen and the flow rate
was 30 cm3
minÀ1
. Under these conditions the
retention times of (CH3)4Sn and (CH3)4Pb were 2.4
and 5.1 min respectively (Hadjispyrou, 1984). For
CH3Hg+
analysis, a Pye 104 gas chromatograph with
electronic detector (GC-ECD), fitted with a glass
column 1500 mm in length by 0.4 mm internal
diameter, and packed with 2.5% Carbowax 20M on
Chromosorb G-HP (AW-DMCS) 80/100 mesh, was
used. Oven, detector and injector temperatures were
175, 265 and 1108C. The carrier gas was oxygen-free
nitrogen, the flow rate was 120 cm3
minÀ1
and
retention time was 1.2 min.
A magnetic deflection Micromass 16-F instrument
was used for GC mass spectroscopy (GC-MS) and
thermal conductivity detection (GC-TCD), coupled
to a Pye 105 GC system fitted with a 1% SP2100 (on
Chromosorb 106 mesh) 50 m capillary column
(WCOT) with a He carrier-gas flow rate of 2 cm3
minÀ1
. Oven, detector and injector temperatures
were 60±1208C, 2508C and 1008C respectively.
The retention times of (CH3)4Sn and (CH3)4Pb were
3.55 and 3.96 min respectively. Gas (CH4, CO2 and
H2S) was analysed with a glass column 1800 mm in
length by 0.4 mm internal diameter on Chromosorb
106 mesh and a He carrier flow rate of 30 cm3
minÀ1
. Oven, detector and injector temperatures
were 458C, 1208C and 458C respectively. Retention
times were 2.06 (CH4), 3.86 (CO2) and 16.20 (H2S)
min.
Results and Discussion
OSI values
The OSI values of all sediment samples show an
increasing pattern in the order F to K in terrestrial
and A to E in marine samples (Table 3). However,
the sediments from sites D and E, as well as from J
and K, display an abrupt increase of OSI values, in
relation to the other marine and terrestrial samples,
respectively.
One reason for the abrupt increase in OSI at sites J
and K may be the large quantities of industrial
Table 2 Relative standard deviation (RSD) and detection
limits for methyl metal analysis
Compound Method %RSD*
Detection lim-
ity ("g LÀ1
)
(CH3)4Sn GC-FID 3.0 0.15
(CH3)4Pb GC-FID 8.5 0.13
CH3Hg+
GC-ECDz 7.0 0.40
*
Sn, Reproducibility for 15 ng as elemental tin (n=5); Pb, precision
evaluation of 200 pg of Pb as (CH3)4Pb (n=5); Hg, Five replicate
analyses of a sample gave a variation coefficient of 7%.
‡
Based on the background 3' value and a sample size of 100 ml
(n=10).
zGas chromatography±electron capture detection.
Table 3 OSI values of sediment samples from the Axios
river and Thermaikos Gulf.
Sampling site OC wt% ON wt% OSI
F Estuaries 1.2 0.03 0.04
G Estuaries 2.5 0.07 0.18
H Industrial area 3.1 0.08 0.25
J Industrial area 4.3 0.11 0.47
K Industrial area 4.7 0.12 0.56
A Industrial area 5.8 0.15 0.87
B Harbour 6.7 0.17 1.14
C Bay 8.1 0.20 1.62
D Airport 9.3 0.23 2.14
E Near estuaries 10.5 0.26 2.73
22 S.A. Hadjispyrou et al.

5. effluents discharged into the river Axios from the
nearby industrial zone of Thessaloniki (Figure 1).
Furthermore, in the same area, intense agricultural
activity and the associated application of agrochem-
icals ± for instance pesticides, herbicides and chemi-
cal soil improvers ± produces leachates of different
kinds which are introduced into the river water and
may also contribute to the OSI values of sediments in
sites J and K. As the river Axios approaches the
Thermaikos Gulf, the land becomes increasingly
agricultural.
Figure 2 Mean and maximum methylation yields for Pb, Sn and Hg versus increasing organic sediment index. Sampling
sites indicated for Hg are the same as for Pb and Sn. (a) Pb; (b) Sn; (c) Hg.
Correlation of the methylating capacity of river and marine sediments to their organic sediment index 23

6. Figure 3 Methyl derivatives of the studied metals from elemental (M0
) and salt M(II) form.
24 S.A. Hadjispyrou et al.

7. Sediments at D and E represent deep marine sam-
pling, where anoxic conditions prevail. According to
Neufeld and Hermann (1975) and Rapsomanikis
(1983), anoxic conditions such as those prevailing
in the deep sediments of sites D and E accelerate the
concentration of organic material, and therefore in-
crease the OSI value. Site E is also charged with
significant concentrations of waterborne materials,
which are steadily conveyed by the river Axios.
Methylation yields
The methylation yields of metals in the incubation
experiments of all the sample sites display a very
similar overall distribution. For illustration purposes,
representative incubation-experiment data from site
A of the Thermaikos Gulf are given (Table 4).
Furthermore, the variation of mean and maximum
methylation yield values for the three metals (Hg, Pb
and Sn), in relation to OSI, are given for all the
sampling sites (Figure 2). The yields increase from
site A through to site E for the Gulf samples, whereas
for the samples from the Axios river the yields
increase from site F to K for all the metals with
increasing OSI. It is very important that the methyla-
tion yields of Hg are four orders of magnitude higher
than those of Pb and Sn. A more detailed pictorial
presentation of methyl derivatives of the studied
metals and metal salts is given in Figure 3 as
absolute values. Pb and Sn methylation yields dis-
play a nearly smooth positive correlation with in-
creasing OSI values (Figure 3a, b). On the other
hand, Hg-methylation yield patterns (Figure 3c) are
different from those of Pb and Sn. Methyl Hg yields
originated from all the Hg-bearing compounds fol-
low increasing trends from F to K and A to E.
However, there is an abrupt increase of methyl metal
yields for marine samples compared to terrestrial
samples. Furthermore, there is a clearly discontinu-
ous trend for HgCl2 and Hg-cysteine yields between
the terrestrial and marine samples. Given that methyl
groups are absent from the structure of these two
substances, the difference in the HgCl2 and Hg-
cysteine behaviour may be attributed to structural
differences. Carbon (14
C) isotopic analyses of the
metal sources and the final methyl metal products
could assist in this interpretation.
Plotting on a binary diagram the absolute values of
the maximum methylation yields for the pairs Hg±Pb
and Hg±Sn (Figure 4) reveals the relative rates of
methylation of the three metals (Hg, Pb, Sn). Differ-
ent rates of methylation are shown for the terrestrial
(low OSI) and marine samples (high OSI). Given that
the methylation capacity of Hg is much higher than
that of the other metals (Wood, 1974), and taking
into account that methyl Hg yields were four orders
of magnitude higher than those of Pb and Sn, the
abrupt enhancement of methylation rates of Pb (Pb
methylation slightly faster) and Sn in marine sedi-
ments reveals that methylation of the major part of
Hg has taken place.
Methylated derivatives were produced only from
bioactive sediments, except in the case of HgCl2
methylation (Table 4; Figure 3c). In this case, a very
low methyl Hg product was detected in the sterilised
samples. This is in accordance with the fact that,
although Hg methylation follows the biotic pathway,
the abiotic one cannot be completely ruled out
(Hadjispyrou, 1984). The biological activity of the
non-sterilised (bioactive) sediments was confirmed at
Table 4 Methylation yields of sediments from the site A of Thermaikos Gulf.
Compound (1 mmole) (CH3)nMmÀn
(ng)*
Yield (Â10À4
) wt%
Sn0
50 (CH3)4Sn 0.28
SnCl2 17 (CH3)4Sn 0.09
Sn(OAc)2 188 (CH3)4Sn 1.05
Sn(COO)2 152 (CH3)4Sn 0.85
Sn-Pen 78 (CH3)4Sn 0.44
Sn-Cyst 86 (CH3)4Sn 0.48
SnCl2 (S) ± (CH3)4Sn ±
Pb0
400 (CH3)4Pb 1.50
Pb(NO3)2 62 (CH3)4Pb 0.23
Pb-Pen 115 (CH3)4Pb 0.43
Pb-Cyst 132 (CH3)4Pb 0.49
Pb(NO3)2 (S) ± (CH3)4Pb ±
Hg(OAc)2 388Á104
CH3Hg+
1.80Á104
HgCl2 110Á104
CH3Hg+
0.51Á104
Hg-Pen 301Á104
CH3Hg+
1.40Á104
Hg-Cyst 320Á104
CH3Hg+
1.49Á104
Hg[Meth]2 314Á104
CH3Hg+
1.46Á104
HgCl2 (S) 15Á103
CH3Hg+
0.01Á104
*
M = Sn, Pb, Hg: Pb and Sn, m = 4; Hg, m = 2; Pb and Sn, n = 4; Hg, n = 1.
(S), Sterilised samples.
Correlation of the methylating capacity of river and marine sediments to their organic sediment index 25

8. the end of each run. Volatile products of biological
activity (CH4, CO2, H2S) analysed by GC-TCD were
present in concentrations ranging from 35 to 250 mg
lÀ1
. In the head-space of the sterilised samples no
biogas was detected. This indicates an absence of any
associated biological activity.
In cases where no nutrients were added to samples,
lower values of methyl derivatives were obtained
although the same process was followed. This was an
indication that the biological activity of the non-
sterilised sediments was not depressed during the
experiments, i.e. the samples were not poisoned from
the added metals. On the contrary, it is very impor-
tant that nutrients assist in higher methylation yields.
This combined with the widely recognised toxicity of
Hg reveals serious implications in case of Hg pollu-
tion of sediments high in OSI from which drinking
water may be supplied to the communities.
The generality which is revealed in methylation of
the metals Hg, Sn and Pb and the related influence of
the OSI value on the sediment methylation capacity
is a challenge to investigate this behaviour with other
metals or metalloids, for instance As, Cd, Cr or Se,
on sediments taken from the same area or from areas
of the rivers Aliakmonas or Loudias which are also
discharged in Thermaikos Gulf (Figure 1). The above
elements have already been studied in other cases
(Craig, 1989; Krishnamurthy, 1992).
In conclusion, the OSI value of a sediment, which
represents its humic substances content (phenols,
phenolic acids, etc.), plays a determining, positive,
role in environmental methylation. OSI determina-
tion is very important in the effort to gauge the
methylating capacity of sediments, i.e. their potential
toxicity in case of heavy metal pollution (e.g. Hg, Pb
and Sn). Environmental methylation is complex in
nature and has a strong biological component rather
than an `abiotic' character (Hadjispyrou, 1984; Ana-
gnostopoulos and Hadjispyrou, 1994).
Conclusions
The following conclusions are drawn from this study:
(1) the OSI values of the sediments exhibit a positive
correlation with methyl metal production; (2) both
ecosystems (Axios river±Thermaikos Gulf) display a
similar pattern as far as the methylation is concerned;
(3) OSI determination is very important in the effort
to gauge the methylating capacity of sediments, i.e.
their potential toxicity in case of heavy-metal pollu-
tion (e.g. Hg, Pb and Sn); (4) the methylation yields
of Hg are four orders of magnitude higher than those
of Pb and Sn in both ecosystems; (5) Hg methylation
mainly precedes the methylation of Pb and Sn. Thus,
in low OSI sediments, the rate of Hg methylation is
higher than that of Pb and Sn, whereas in high OSI
sediments, the methylation of Pb is slightly faster
than Sn; (6) methylation of Hg, Pb and Sn predomi-
nantly follows the biotic pathway with the exception
of Hg, for which a minor abiotic process has been
detected.
Figure 4 Plot of Pb and Sn maximum yield values versus the corresponding values of Hg in an increasing OSI order (left to
right; sites F to E).
26 S.A. Hadjispyrou et al.

9. Acknowledgements
The authors wish to thank Professor P.J. Craig for
making available the facilities of his laboratory and
for valuable discussions. We would also like to thank
Dr S. Rapsomanikis for all his invaluable theoretical
and practical assistance during the stay of S.H. in
Leicester, two anonymous referees for valuable sug-
gestions which greatly improved the manuscript and
Dr K.G. Katirtzoglou (IGME) for the drafts.
References
Anagnostopoulos, A. and Hadjispyrou, S. 1993. Methy-
lation of Sn0
, Pb0
and Sn(II), Pb(II) inorganic salts by
carbonium ion donors. Toxicological and Environ-
mental Chemistry, 39, 207±15.
Anagnostopoulos, A. and Hadjispyrou, S. 1994. The
influence of Na2S on the methylation of trimethyltin
and trimethyllead in environmental sediments. Tox-
icological and Environmental Chemistry, 41, 175±86.
Ashby, J.R. and Craig, P.J. 1991. The methylation of
tin(II) chloride and tin(II) amino acid complexes by
methylcobalamin. Science of the Total Environment,
100, 337±46.
Craig, P.J. 1989. Biological and environmental methy-
lation of metals. In: Hartley, F.R. (ed.) The Chemistry
of the Metal±Carbon Bond, Vol. 5, pp. 437±63. J.
Wiley, Chichester, UK.
Craig, P.J. and Moreton, P.A. 1984. The role of sulphide
in the formation of dimethylmercury in the environ-
ment. Marine Pollution Bulletin, 15, 406±8.
Craig, P.J. and Moreton, P.A. 1986. Total mercury,
methyl mercury and sulphide levels in British es-
tuarine sediments. Water Research, 20, 1111±18.
Donard, O.F.X., Rapsomanikis, S. and Weber, J.H. 1986.
Speciation of inorganic tin and alkyltin compounds by
atomic absorption spectrometry using electrothermal
quartz furnace after hydride generation. Analytical
Chemistry, 54, 772±7.
Franson, M.A.H., Clesceri, L.S., Greenberg, A.E. and
Trussell, R.R. 1989. Standard Methods for the Ex-
amination of Water and Wastewater. American Public
Health Association (published jointly with Water
Works Association & Pollution Control Federation),
Washington DC.
Hadjispyrou, S. 1984. A study on the environmental
methylation of Sn, Pb and Hg in aquatic ecosystems
of northern Greece. PhD Thesis, Aristotle University
of Thessaloniki. (In Greek with English summary).
Jarvie, A.W.P. and Whitmore, A.P. 1981. Methylation of
elemental lead and lead(II) salts in aqueous solution.
Environmental Technology Letters, 2, 197-202.
Krishnamurthy, S. 1992. Biomethylation and environ-
mental transport of metals. Journal of Chemical
Education, 69, 347±50.
Moreton, P.A. 1983. A study on the environmental
methylation of mercury. PhD Thesis, Leicester Poly-
technic.
Neufeld, R.D. and Hermann, R.E. 1975. Heavy metal
removal by acclimated activated sludge. Journal of
Water Pollution Control Federation, 47, 310±29.
Pratt, J.M. 1972. The Inorganic Chemistry of Vitamin
`B12'. Academic Press, NY.
Rapsomanikis, S. 1983. A Feasibility study for the
Environmental Methylation of Sn and Pb. PhD thesis,
Leicester Polytechnic.
Rapsomanikis, S., Donard, O.F.X. and Weber, J.H. 1986.
Speciation of lead and methyllead ions in water by
chromatography/atomic absorption spectrometry after
ethylation with sodium tetraethylborate. Analytical
Chemistry, 58, 35±8.
Weber, J.H. 1993. Review of possible paths for abiotic
methylation of Hg(II) in the aquatic environment.
Chemosphere, 26, 2063±77.
Wood, J.M. 1974. Biological cycles for toxic elements in
the environment. Science, 183, 1049±52.
Wood, J.M., Ridley, W.P. and Dizikes, L.J. 1977.
Biomethylation of toxic elements in the environment.
Science, 197, 329±32.
Yu, C., He, Y., Zhao, Q. and Wang, S. 1992. Study on
methylation of Hg in fish (English abstract). Zhong-
guo Huanjing Kexue, 12, 236±40.
Zhou, Y., Peng, A. and Jia, J. 1990. Effects of Se(IV) on
the biological methylation of Hg (English abstract).
Zhongguo Huanjing Kexue, 9, 45±8.
[Manuscript No 422: Submitted June 26, 1995 and accepted
after revision June 24, 1996.]
Correlation of the methylating capacity of river and marine sediments to their organic sediment index 27