INICAL REPORT DATA
(Please read Wegenioas on the reverse before

completing)

T. REPORT NO. 2.
EPA-600/3-75-014

RECIPIENT'S ACCESSION NO.

4, TITLE AND SUBTITLE

METHYLATION OF MERCURY IN A TERRESTRIAL ENVIRONMENT

|. REPORT DATE
October 1975

|6. PERFORMING ORGANIZATION CODE

7, AUTHOR(S)
Robert D. Rogers

Is. PERFORMING ORGANIZATION REPORT NO.

9, PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development

U.S. Environmental Protection Agency

Las Vegas, Nevada 89114

10. PROGRAM ELEMENT NO,

1AA006 21BKN
TT, CONTRACT/GRANT NO. —

12, SPONSORING AGENCY NAME AND ADDRESS.

Same as above

13, TYPE OF REPORT AND PERIOD COVERED
Annual FY~-75

74. SPONSORING AGENCY CODE
EPA-ORD, Office of Monitor~-

ing and Technical Support

15, SUPPLEMENTARY NOTES

16. ABSTRACT

soil systems.

and time.
content, temperature, and mercury concentration.

methylmercury content with increasing time.

Methylation of applied divalent mercury ion was found to occur in terrestrial

The production of methylmercury was affected by soil texture, soil

moisture content, soil temperature, concentration of the ionic mercury amendment,
Methylation was directly proportional to percent clay content, moisture

After an initial buildup of
methylmercury in the soil, there appeared to be a mechanism that decreased the

17. KEY WORDS AND DOCUMENT ANALYSIS.

la. DESCRIPTORS

[b. IDENTIFIERS/OPEN ENDED TERMS

lc. COSATI Field/Group

Mercury (metal)

*Mercury organic compounds
*Soils

Methylmercury

07B
o7c
O6F

18. DISTRIBUTION STATEMENT

19. SECURITY CLASS (This Report)

UNCLASSIFIED 20
RELEASE TO PUBLIC 20. SECURITY CLASS (This page) 2a. PRICE
UNCLASSIFIED

21. NO. OF PAGES:

EPA Form 2220-1 (9-73)


sterile, non-sterile soils study of the preceding section, gives credence to
the hypothesis that biologically initiated loss of methylmercury can occur and
is, in fact, enhanced when conditions for biological proliferation are provided.

Table 4. METHYLMERCURY OCCURRENCE IN SOILS WITH VARIOUS
MOISTURE CONTENTS

Moisture-Holding Concentration

Soil Capacity (ng CHaHgt/50 g soil)

a 1 Week 3 Weeks
Sand 25 88 sample lost

50 98 sample lost

75 105 sample lost
Loam 25 223 130

50 188 62

75 212 54
Clay 25 277 144

50 256 108

75 195 51

The loam soil with standing water was analyzed after 3 weeks. Under
these conditions, the system contained from 4 to 10 times more methylmercury
(537 ng CH3Hgt per 50 g of soil) than the soil under the other moisture regimes.
Because this information seemed contradictory (the trend had been to increase
the loss of methylmercury with increased moisture content), studies using
mercury-203 tracer were carried out in order to determine what effects soil
moisture had on the loss of methylmercury from the loam soil. Low quantities
of mercury-203 labeled methylmercury were mixed into the soil. Fifty grams of
the soil was placed into wide-mouth jars. This soil was then adjusted to 102,
50%, and 100% moisture content in order to give even greater differences in
moisture content than used previously. In addition, one set of the saturated
soil contained 2 to 3 centimeters standing water. The mouth of each jar was
covered with a charcoal filter. Jars containing samples were then incubated
in the dark at 24°C. At the end of each week, the filters were replaced and
the used filter analyzed by means of a gamma radiation detector for the pres-
ence of mercury-203. By the end of the second week, the mercury-203 content
in the filters increased as the moisture content of the soils increased, while
the soil covered with standing water showed relatively little loss.

This information indicated that decreasing aerobiosis increased the
loss of methylmercury. The presence of standing water moderates this loss.
It is possible that more methylmercury was found in the loam soil with stand-
ing water because regardless of the form of the volatile mercury, it is not
easily lost through the water. Of course, it is also possible that the

8


CONTENTS

List of Tables iv
Introduction 1
Conclusions 1
Recommendations for Future Research 2
Materials and Methods 2
Results ; 6

Sterile and Non-Sterile Soil 6

Moisture Content 7

Incubation Temperature 9
Discussion 10
References 12

iii


REFERENCES

Alberts, J. J., J. E. Schindler, R. W. Miller and D. E. Nutter, Jr. "Elemental
Mercury Evolution Mediated by Humic Acid," Setence, 184, pp 895-897 (1974).

Beckert, W. F., A. A. Moghissi, F. H. F. Au, E. W. Bretthauer and J. C. McFarlane.
"Methylmercury: Evidence for Its Formation in a Terrestrial Environment,"
Nature, 249, pp 674-675 (1974).

Bertilssen, L., and H. Y. Neujahr. "Methylation of Mercury Compounds by Methyl-
cobalamin," Biochemistry, 10, pp 2805-2808 (1971).

Bisogni, J. J., Jr., and A. W. Lawrence. "Kinetics of Microbially Mediated
Methylation of Mercury in Aerobic and Anaerobic Aquatic Environments," USDA
Publication No. PB-222025 (1973).

Braman, R. S., and D. L. Johnson. "Ambient Forms of Mercury in Air," Proceedings
of the Second Annual NSF RANN Trace Contaminants in the Environment, pp 75-78
(August 29-31, 1974).

D'itri, F. M. The Environmental Mercury Problem, CRC Press, Cleveland, Ohio,
pp 63-67 (1972).

Fagerstrom, T., and A. Jernelov. "Formation of Methylmercury from Pure Mercuric
Sulphide in Aerobic Organic Sediment," Water Research, 5, pp 121-122 (1971).

Frissel, M. J., P. Poelstra, P. Reiniger and H. A. Das. "Contamination of the
Soil with Mercury," Radioecology Applied to the Protection of Man and His
Environment, Commission of the European Communities International Symposium,
pp 7-10 (1971).

Fujiki, M. "Studies on the Course that the Causative Agent of Minamata Disease
Was Formed, Especially on the Accumulation of the Mercury Compound in the Fish
and Shellfish of Minamata Bay," J. Kwnamoto Med. Soc., 39, p 494 (1963).

Imura, N., E. Sukegawa, S. Pan, K. Nagae, J. Kim, T. Kwan and T. Ukita.
"Chemical Methylation of Inorganic Mercury with Methylcobalamin, A Vitamin By2
Analog," Seience, 172, pp 1248-1249 (1971).

Jensen, S., and A. Jernelov. "Biological Methylation of Mercury in Aquatic
Organisms," Nature, 223, pp 753-754 (1969).

Jernelov, A. "Factors in the Transformation of Mercury to Methylmercury,"

Environmental Mercury Contamination, edited by R. Hartung and B. D. Dinman,
p 167 (1972).

12


sprayed with 2 ml of the mercury solution using an atomizer. The amended soil
was mixed with a spatula and poured into a 250-ml flask. The amount of water
necessary to adjust the moisture content of the soil to the desired level was
then added dropwise. The flask and soil were weighed and capped with a loose
fitting aluminum foil cap. On alternate days during the period of incubation,
the flask and its contents were reweighed and brought back to the initial
weight by the addition of distilled water. All soils were incubated in the
dark at 24 degrees Celsius (°C).

A modified Westoo (1966) method was used to extract methylmercury (CH3Hgt)
from the soils. It was found that 50 ml of 6W HCl per 50 g of soil resulted in
the best extraction of standard methylmercury chloride (CH3HgCl1) solution. The
soils were highly calcareous and unless care was exercised effervescence, as a
result of the acid addition, forced much material from the flask. Following
the addition of acid, the flasks containing the soil solutions were shaken for
1 hour on a reciprocating shaker. The resulting mixture was then filtered
under vacuum through Whatman No. 1 paper. The extracted soil was then rinsed
with three additional 5-ml acid washes. Next, nanograde quality benzene was
used to extract CH3Hgt+ from the soil leachate. The soil leachate was extracted
twice with two separate 50-ml quantities of the benzene using 250-ml separatory
funnels.

A 1% cysteine solution was used to extract CH3Hgt from the combined
benzene washes. It was found that the two separate extractions with 6-ml
quantities of the cysteine solution were necessary. The cysteine solution was
made in the following manner: dissolve 1.000 g of cysteine hydrochloride
(HSCH2CH(NH2)COOH+HC1-H20) , 0.775 g of sodium acetate (CH3COONa*3H20), and
12.500 g anhydrous sodium sulfate (Na2S0,), in that order, in 75 ml distilled
water. Additional distilled water was added to make up the volume to 100 ml.
This solution was adjusted to pH 8.3 with 5% sodium hydroxide just before use.

The two cysteine extracts were combined in a 60-ml separatory funnel,
acidified with 10 ml of 6W HCl (the resulting mixture must have a pH of 1 or
less), and extracted with 10 ml of benzene. This final benzene extract was
analyzed by gas chromatography.

In order to evaluate the effectiveness of the extraction procedure, stan-
dard solutions consisting of 1 wg Hg as CH3HgCl in distilled water were added
to the three soils. The soils were extracted using the above method and the
quantity of the extracted CH3Hgt was compared to the amount initially added.
From an average of nine replications for each soil, it was determined that with
the sand soil there was a 57.1% recovery of applied CH3Hgt, loam soil 52.2%,
and clay soil 38.9%. These findings were in general agreement with those re-
ported by Krenkel (1974) who indicated that a soil's affinity for CH3Hg* in-
creases with soil clay content. By knowing the percent recovery of CHsHgt
from these soils it was possible to calculate the amount of methylmercury
contained in the soil by applying the appropriate correction factor for that
soil.

The gas chromatograph used for these studies was a Hewlett-Packard Model
5713A with a nickel-63 linear electron capture detector. The attenuation of
the chromatograph was adjusted so that 1 ul of 0.1 ppm Hg as CH3HgCl in benzene
caused a 3/4-scale deflection on the recorder. Figure 1 shows a typical

4


INTRODUCTION

Since the time Fujiki (1963) first proposed the possibility that the
Minamata Disease was caused by natural methylmercury formation, the majority
of reports on this subject have dwelled on the occurrence of methylmercury in
anaerobic aquatic systems (Jensen and Jernelov, 1969; Wood et al., 1968;
Lander, 1970; Jernelov, 1972). However, there has also been some work on aer-
obic aquatic systems. Fagerstrom and Jernelov (1971) indicated that under
aerobic aquatic conditions mercuric sulfide can be shown as the initial sub-
strate for methylmercury synthesis. Methylmercury has also been found in aer-
obic microbial cultures (Parks et al., 1973). Bisogni and Lawrence (1973)
found that methylation rates for aerobic aqueous systems were higher than
those for corresponding anaerobic systems.

While many hypotheses on mercury cycling have become prevalent as a result
of these findings, the methylation of mercury in terrestrial environments has
only recently been found. Beckert et al., (1974) found methylmercury in desert
soils which had been amended with mercuric nitrate containing mercury-203. The
presence of methylmercury was discovered using thin-layer chromatography, but
the amount was not quantified. Methylmercury has also been found in the atmos-
phere above a soil amended with mercuric chloride (Braman and Johnson, 1974).

Coal and other fossil fuels earmarked for use in the nation's energy pro-
gram are known to contain elevated levels of the element mercury. As a result,
concern has been expressed over the lack of understanding of mercury cycling
in terrestrial environments, but it should also be noted that there are many
additional natural and man-made terrestrial mercury exposure pathways which
are not fully understood. This study was undertaken to confirm the findings
of occurrence of methylmercury in terrestrial soil systems, and to study the
kinetics involved in its production.

CONCLUSIONS
The following conclusions were drawn from the current study.
1. Methylation of mercury does occur in a terrestrial environment.

2. There is a mechanism available for the decrease in methylmercury
concentration with time.

3. It is possible that the methylation process could, in part, be abiotic.

4. Non-sterile systems have a net loss of methylmercury such that there
is less methylmercury in non-sterile soils than in the sterile soils.


LIST OF TABLES

Number
1 Physical and Chemical Properties of Soils.........

2 Methylmercury Occurrence in Sterile and Non-Sterile
Soil Systems After 1 Week of Incubation. ......+..

3 Methylmercury Occurrence in Sterile and Glucose-Amended
Non-Sterile Soil Systems After 1 Week of Incubation. . .

4 Methylmercury Occurrence in Soils with Various
Moisture Contents... . 2.2 eee ee ee ee ee ee

5 Methylmercury Occurrence in Soils Incubated
at Various Temperatures . . 2. 1. 6 6 1 ee ee ee ee ee

6 Methylmercury Occurrence in Soils Incubated for 1 Week
with Amendments of Varying Mercuric Nitrate Concentration

iv

10


Krenkel, P. A. "Mercury: Environmental Considerations, Part II," Critical
Reviews in Environmental Control, 4, pp 251-339 (1974).

Lander, L. "Biochemical Model for the Biological Methylation of Mercury Sug-
gested from Methylation Studies in vivo with Neurospera crassa," Nature, 230,
pp 452-453 (1970).

Magos, L., A. A. Tuffery and T. W. Clarkson. "Volatilization of Mercury by
Bacteria," Brit. J. Industr. Med., 21, pp 294-298 (1964).

McArthur, F., and L. E. Sommers. “Mercury Transformation in Lake Sediments,"
Agronomy Abstracts Annual Meetings, Chicago, Illinois (November 10-15, 1974).

McKeague, J. A., and B. Kloosterman. "Mercury in Horizons of Some Soil Profiles
in Canada," Can. J. Soil Set., 54, pp 503-507 (1974).

Parks, G. A., F. W. Dickson, J. 0. Leckie, P. L. McCarty, P. Berendson and
K. L. Pering. "Part of Trace Elements in Water: Origin, Fate, and Control,"
Progress Report, Stanford University, March 1972 to February 1973, pp 247 (1973).

Spangler, W. J., J. L. Spigarelli, J. M. Rose and H. M. Miller. "Methylmercury:
Bacterial Degradation in Lake Sediments," Science, 180, pp 192-193 (1973).

Talmi, V. "The Rapid Sub-Picogram Determination of Volatile Organo-Mercury
Compounds by Gas Chromatography with a Microwave Emission Spectrometric
Detector System," Analytica Chimica Acta, 74, pp 107-117 (1975).

Tonomura, K., K. Madeda and F. Futai. "Studies on the Action of Mercury
Resistant Microorganism on Mercurials, (II) The Vaporization of Mercurials
Stimulated by Mercury-Resistant Bacterium," J. Ferment. Technol., 46, pp 685-692
(1968). :

Tonomura, K., K. Furukawa and M. Yamada. "Microbial Conversion of Mercury
Compounds," Environmental Toxicology of Pesticides, edited.by.¥. Matsumura,
G. M. Bores and T. Misate (1972).

Van Faassen, H. G. "Effects of Mercury Compounds on Soil Microbes," Plant and
Soil, 38, pp 485-487 (1973).

Westoo, G. "Determination of Methylmercury Compounds in Foodstuffs, (I) Methyl-
mercury Compounds in Fish, Identification and Determination," Acta Chemica
Seandivica, 20, pp 2131-2137 (1966).

Wood, J. M., F. S. Kennedy and C. G. Rosen. "Synthesis of Methylmercury
Compounds by Extracts of a Methanogenic Bacterium," Nature, 220, pp 173-174
(1968),

13
GPO 967-357


northeast of Las Vegas and is primarily an agricultural locality. The soils
were collected in December 1974 and had supported crops during the previous
growing season. Depth of the soil collections was limited to the upper 10
centimeters (cm) of the Ap horizon*. The moist soil was processed through
a 2-millimeter sieve and stored at room temperature in plastic bags. The
physical and chemical properties of the soils are found in Table 1.

Table 1. PHYSICAL AND CHEMICAL PROPERTIES OF SOIL

Soil % % % meq per
(Texture Series Sand Cla Organic 100 g pH
Classification) Y carbon CEC*
Sand Bluepoint - a member 79.8 3.5 53 4.3 9.0

(Loamy sand) of the mixed, thermic
family of Typic

Torripsamment
Loam Calico - a member 53.9 10.8 1.30 12.7 8.6
(Fine sandy of the coarse-loamy
loam) over clayey, mixed

(calcareous), ther-
mic family of Aquic

Xerofluvents
Clay Overton - a member 14.7 50.0 3.44 29.0 7.8
(Silty clay of the fine mont-
loam) morillonitic,

calcareous, thermic
family of Mollic
Haplaquepts

* CEC = cation exchange capacity

In all cases, mercuric nitrate (Hg (NOs) 2) was used for the ionic mercury
(Hg2*) soil amendment and each treatment was carried out in triplicate. Be-
cause of the restricted amount of moisture which could be added to the soils
under some soil moisture regimes, the mercury amendments needed to be in a
highly concentrated solution. So that all studies would remain uniform with
respect to the volume of mercury amendment, all soils were amended with the
same volume of concentrated mercury solution. The volume of mercury solution
added to the soils was 2 milliliters (ml). ess specified otherwise, the
concentration of the mercury solution was parts per million (ppm) mer-
cury (Hg) as Hg(NO3)2, for a total addition of 25,000 micrograms (yg) Hg per
50 grams (g) of soil. In order to solubilize the Hg(NO3)2 in water, it was
found necessary to add 2 or 3 ml of concentrated hydrochloric acid (HC1) per
100 ml Hg solution.

The amendment and incubation process was carried out in the following
manner: 50 g of soil was spread thinly on a sheet of acetate and then

* "A" horizon, plow layer, ¢.e., agriculturally disturbed topsoil

3


EPA-600/3-75-014 :
October 1975 Ecological Research Series



chromatograph for CH3Hg* under the specified conditions.

Because of the hazard involved in using high concentrations of mercury,

special masks were worn. These masks were 3M mercury vapor respirator masks,
number 8707.

RESULTS

STERILE AND NON-STERILE SOIL

Biological mechanisms have been hypothesized as the causative agents for
the synthesis of methylmercury. In order to verify this premise in a terres-
trial environment, soils were sterilized by autoclaving and then amended with
25,000 micrograms of mercury as mercuric nitrate. Non-sterile soils amended
with 25,000 micrograms of mercury as mercuric nitrate were used as controls.
Both sets of soils were incubated in the dark at 24°C for 1 week.

Analyses of these soils produced striking results (Table 2). In every
case, the autoclaved soils produced substantially more methylmercury than did
the non-autoclaved soils. To substantiate these findings, the experiment was
repeated. This time, in an effort to produce more effective sterilization,
the soils were autoclaved at 4-hour intervals every other day for a period of
5 days. In addition, to enhance microbial growth, those soils used as non
sterile controls were amended with a 20% glucose solution at 4 milliliters per
50 grams of soil. The mercury amendment and conditions of incubation were the
same as those used in the initial experiment. Analytical results obtained
from these soils (Table 3) were similar to those reported in Table 2; however,
some differences were noted between the two sets of experiments. Those soils
which received the extended autoclaving, except for sand, appeared to have in-
creased concentrations of methylmercury over those found initially, while the
glucose-amended soils when compared to the other non-sterile soils, except for
sand, showed a decrease in methylmercury.

Table 2. METHYLMERCURY OCCURRENCE Table 3. METHYLMERCURY OCCURRENCE

IN STERILE AND NON-STERILE
SOIL SYSTEMS AFTER
1 WEEK OF INCUBATION

Concentration
Soil (ng CH3Hgt/50 g soil)
Sterile Non-Sterile
Sand 105 74
Loam 223 169
Clay 318 215

IN STERILE AND GLUCOSE-AMENDED
NON-STERILE SOIL SYSTEMS AFTER
1 WEEK OF INCUBATION

Concentration
Soil (ng CH3Hgt/50 g soil)
Sterile Non-Sterile
Sand 108 81
Loam 307 127

Clay 420 190



BENZENE

METHYLMERCURY

COLUMN: glass, 1.8mx4mmID

PACKING: 5% HIEFF (ethylene
glycol adipate) on 80-100 mesh
AW chromosorb W

INJECTION TEMPERATURE: 200°C

OVEN TEMPERATURE: 170°C
DETECTOR: °°Ni

GAS: 5% methane in argon
GAS FLOW: 60 cc/min

INJECTED VOLUME: 1 ul of a
0.1 ppm solution of Hg as
CH3HgC1 in benzene

Figure 1. Typical gas chromatograph tracing
for methylmercury


5. Standing water on the surface of the soil reduces the loss of
methylmercury.

6. The rate of conversion of mercury into methylmercury is dependent upon
ionic mercury concentration, soil texture, temperature, and soil moisture.

These conclusions open exciting possibilities for future research.

RECOMMENDATIONS FOR FUTURE RESEARCH

1. Determine the total mercury budget. It will be possible to do this
with the use of semi-closed systems. Soil amended with mercuric ion would be
maintained in a flask. At some predetermined rate, the flask atmosphere would
be flushed through a series of selective absorption tubes which will separate
the mercury in the air into mercuric ion (Hg2+) compounds, methylmercury (CH3Hg+)
compounds, metallic mercury, and dimethylmercury (Braman and Johnson, 1974).
It will also be possible, by conventional methods, to determine the total mer-
cury and synthesized methylmercury remaining in the soil environment. This
work will help in understanding not only the fate of applied mercuric ion, but
also the fate of synthesized methylmercury. The important fact is that this
system of analysis has been developed and is ready for use.

2. Conduct field work at terrestrial sites known to contain high back-
ground levels of mercury. Many areas fall into this category. For example, a
belt of mercury deposits runs through southern California, Nevada, Idaho, and
into Canada. Soils sampled from one part of the belt in Nevada were found to
contain a mercury concentration of 4,240 parts per billion. McKeague and
Kloosterman (1974) report finding soils in Canada containing mercury up to
14,000 parts per billion. Further work in such areas will be of great interest.

3. Use of a microwave emission spectrometric detector system (Talmi, 1975),
which will increase the detection limits for methylmercury by at least a factor
of 10, will make it possible to study the kinetics of methylmercury formation
using reduced quantities of initial mercury substrate.

4. Work should be expanded in the areas of mercury fixation in soils and
the effect of such variables as pH, temperature, moisture content, clay content,
and various soil elements.

5. There should be an attempt to isolate the postulated biotic system
responsible for methylation and the subsequent loss of methylmercury.

6. Because the possibility for abiotic methylation has been shown, further
investigation of this possibility is recommended.

MATERIALS AND METHODS

Soils used for this investigation were obtained from an area around
Logandale in the Moapa Valley of Nevada. The Valley is approximately 60 miles

2


DISCLAIMER

This report has been reviewed by the Environmental Monitoring and Support
Laboratory-Las Vegas, U.S. Environmental Protection Agency, and approved for
publication. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.

ii


mechanism for volatilization does not occur under standing water. In either
event, systems under standing water appear to be producing more methylmercury
because of a reduced mercury loss.

INCUBATION TEMPERATURE
The soils were incubated at various temperatures to ascertain the effect

of this variable upon the methylation process. Temperatures selected were 4°C,
24°C, and 36°C. Results of this work are shown in Table 5.

Table 5. METHYLMERCURY OCCURRENCE IN SOILS INCUBATED AT
VARIOUS TEMPERATURES

Incubation Concentration
Soil Temperature (ng CHsHgt/50 g soil)
°c 1 Week 3 Weeks
Sand 4 42 60
24 67 sample lost
36 123 109
Loam 4 65 196
24 169 62
36 300 46
Clay 4 128 174
24 179 107
36 195 36

Incubation of the soils for 1 week produced the expected results; z.e. ,
the production of methylmercury was directly proportional to the temperature.
After 3 weeks, however, except for sand, the concentration of methylmercury
was inversely proportional to the temperature. These data indicated that both
the formation and loss of methylmercury were temperature dependent. The soils
maintained at 4°C increased in methylmercury content over the first week.
Evidently, the mechanism for methylation is more active at a lower temperature
than the mechanism for mercury loss.

MERCURY CONCENTRATION

In order to understand the effect that mercury concentration has on the
kinetics of methylmercury production, the soils were amended with three increas-
ing concentrations of divalent mercury ion. These concentrations were 5,000,
12,500, and 25,000 micrograms of mercury as Hg(NO3)2 per 50 grams of soil. The
soils were incubated in the dark for 1 week. The results from this experiment

9


Temperature also had an effect upon the rate of methylation. Soils incu-
bated at lower temperatures contained less methylmercury at the end of 1 week
than soils incubated at elevated temperatures (Table 5). These findings were
in agreement with McArthur and Sommers (1974) who found that methylation rates
in two calcareous lake sediments were doubled by increasing the temperature
from 4°C to 25°C. At the end of 3 weeks, the soils at lower temperatures
showed a net increase in methylmercury, while soil maintained at higher
temperatures had a net decrease.

An increase in moisture content and the amount of available carbon also
increased the net loss of methylmercury with time (Tables 2, 3, and 4). An
increase in available carbon has also been shown to increase methylmercury loss
in calcareous lake sediments (McArthur and Sommers, 1974). Such losses under
alkaline conditions could partially explain the findings of D'Itri (1972). He
found that neutral and alkaline environments favor the formation of dimethyl-
mercury, which is more volatile than monomethylmercury. These factors all
indicate that the loss of methylmercury from soil was mediated by biological
systems. It is important to further define this pathway for methylmercury loss
from soil since the presence of such a pathway reduces the probability of
methylmercury buildup in the terrestrial environment.

Soil texture was found to be related to the occurrence of methylmercury in
both sterile and non-sterile soils. The clay soil contained the most methyl-
mercury, followed by the loam soil, and finally the sand. The cause for this
phenomenon has not yet been investigated. It is possible that methylmercury
production depends upon available surface area, since the same increase in
methylmercury content with increase in clay content was also noted for the
autoclaved soil. Also, the biological synthesis of methylmercury could be ex-
pected to be the greatest under conditions favorable for microbial growth.
Bacterial counts of the three soils used in this study showed that microbial
numbers increased with clay content. In addition to this, Van Faassen (1973)
found that in soils treated with mercuric chloride, microbial processes were
inhibited more strongly in sand than in clay soil. It was reasonable, then, to
expect the clay soil would have an elevated methylmercury content. The dis-
crepancies between the sand and the other soils, with respect to the loss of
methylmercury over time and increasing temperature (Tables 2, 3, and 4), are
further evidence of the biological amelioration of clay content and the
detrimental effect that mercury has on microbial populations associated with
the sand.

il


EPA-600/3-75-014
October 1975

METHYLATION OF MERCURY IN A TERRESTRIAL ENVIRONMENT

by

R. D. Rogers
Monitoring Systems Research and Development Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114

ROAP No. 21BKN
Program Element No. 14A006

U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
LAS VEGAS, NEVADA 89114


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These results appear to indicate the abiotic conversion of mercuric ion into
methylmercury. In addition, it was possible that there was also a biological medi-
ation occurring in the non-autoclaved soils leading to a reduction in methylmercury.
This was indicated because those non-sterile soils amended with glucose, except
sand, contained only about 40% the amount of methylmercury found in the ‘autoclaved
soil. However, the non-sterile soils without glucose amendment contained 70% as
much methylmercury (Tables 2 and 3). Because soile amended with glucose showed an
increase inthe metabolic activities of many microbial species, it was possible
that this resulting activity was responsible for the methylmercury loss.

It is tempting to speculate on the presence of a mercury cycle involving
the methylation of mercury coupled with demethylation into other unknown forms
of mercury. It was possible that biotic methylmercury was being produced as an
intermediate whose gross occurrence was not observed because of a subsequent
biotic mediated loss. Because there is much evidence for biotic demethylation
(Magos et al., 1964; Tonomura et al., 1968; Frissel et al., 1971: Tonomura et
al., 1972; Spangler et al., 1973; Alberts et al., 1974) this cycle would appear
to be a biotic possibility. This possibility was further enhanced by the obser-
vation that the CH,Hgt in these soils decreased with time and increased with
temperature. These results will be discussed in later sections of this report.
The possibility of a methylation-demethylation cycle occurring in soil, similar
to that reported for aqueous systems, would greatly expand the understanding of
mercury transformation.

MOISTURE CONTENT

A prevalent hypothesis has been that anaerobic conditions are necessary
for the maximum formation of methylmercury. Other evidence is available indi-
cating that methylmercury formation occurs at a higher rate in aerobic aqueous
systems than in anaerobic ones (Bisogni and Lawrence, 1973). Similar work with
soils is not available at this time.

Soils under varying oxygen tensions were examined for the production of
methylmercury. Oxygen content was adjusted by varying the amount of moisture
content in the soil. The higher the moisture content the lower the oxygen
tension. Sets of the three soils were developed which contained 25%, 50%, and
75% of the soil's moisture-holding capacity. Under this regime, the higher
the percentage of moisture in the soil, the lower the percentage of air spaces
in the soil. Therefore, at 100% moisture holding, the soil should contain no
air spaces, and it would be considered to be under a favorable anaerobic con-
dition. In addition to the three different moisture conditions described, one
set of the loam soil was maintained with 1 to 2 centimeters of standing water
over a period of 3 weeks. All soils were amended at a rate of 25,000 micro-
grams of mercury as mercuric nitrate.

Results of this work (Table 4) indicate that during the first week of in~
cubation there was little difference in the amount of methylmercury produced
in any soil regardless of the soil moisture content. At the end of the third
week, apparent differences were seen between treatments. The decrease in
methylmercury coincides with improving conditions for anaerobic microbial growth.
As a rule of thumb, optimum moisture-holding capacity for aerobic microbial
growth is about 60%. This information, coupled with the results from the

7


Table 6. METHYLMERCURY OCCURRENCE IN indicated that the amount of conver-

SOILS INCUBATED FOR 1 WEEK WITH sion of divalent mercury ion to
AMENDMENTS OF VARYING MERCURIC methylmercury was dependent upon the
NITRATE CONCENTRATION amount of applied mercury ion (Ta~
" ble 6). These same findings were
Soil Hg(NO3)2 CHsHg reported by Parks et aZ. (1973) and
Added Detected Jensen and Jernelov (1969). While

(ug Hg/50 gsoil) (ng/50 g soil) the loam soil showed a direct rela-
tionship between methylmercury pro-

Sand 5,000 28 duced and the amount of mercuric

12,500 56 nitrate used, such a relationship for

25,000 98 the other two soils was not as pro-

nounced. These data indicated that

Loam 1 eee ze the methylation process is also

25000 188 controlled by a rate-limiting step

? dependent upon substrate concentration.

Clay 5,000 41

12,500 67

25,000 256

DISCUSSION

The salient findings from this study are the confirmation of methylmercury
synthesis from applied divalent mercury ion in terrestrial systems. In addition,
there is a strong indication that a mechanism exists which prevents the accumu-
lation of quantities of methylmercury. It is not known whether the loss of
methylmercury is due to demethylation or volatilization, but this loss from
soil systems was influenced by time, temperature, soil moisture, available
carbon in the soil, and soil texture.

The site of methylmercury synthesis in soil has not been determined.
From work with apparently sterile soils, there was evidence that the process
could have been abiotic (Tables 2 and 3), but this evidence is circumstantial.
The absolute sterilization of soil systems by a gas procedure has yet to be
used, but absolute sterilization would not rule out the possibility that extra-
cellular enzyme systems and organic substrates could account for the occurrence
of methylmercury (Wood et al., 1968; Imura et al., 1971; Bertilsson and Neujahr,
1971). Whatever the process of methylation in this study, it was apparently
dependent upon the concentration of mercuric nitrate applied to the soil
(Table 6.)

Because only about 1 x 10 ° of the mercuric ion applied was detected as
methylmercury, there appeared to be an unfavorable equilibrium for methylmer-
cury production. However, it is possible that the mercury applied to the soil
was fixed in such a way that most of it was not available for methylation. In
support of this, it was found that after 1 week an acid loam soil amended with
25,000 micrograms of mercury as mercuric nitrate per 50 grams of soil contained
three times more methylmercury than the alkaline loam soil treated in the same
manner (612 ng Hg per 50 g of soil versus 188 ng Hg per 50 g of soil). The
higher acidity could have increased the availability of mercury ion.

10