Sci. Rep. Kanazawa Univ.
Vol.52, p.1-26, 2008

Clays and bacteria carry important role to releaseHg from small scale Au mine near Lake Victoria, Tanzania
Kazue TAZAKI1*, Ryuji ASADA1, Hiroaki WATANABE1, Koichi SHIRAKI1,
Takamasa IWAI1, Rie WAKIMOTO1, Majura A.M. SONGO2and Sospeter M. MUHONGO2
1 Graduate School of Natural Science and Technology, Kanazawa University,
Kakuma, Kanazawa, Ishikawa 920-1192, Japan
2 Department of Geology, University of Dar Es Salaam,
P.O. Box 35052, Dar Es Salaam, United Republic of Tanzania* Corresponding author
E-mail : kazuet@kenroku.kanazawa-u.ac.jp
Phone : +81-76-264-6512
Fax : +81-76-264-6545
Abstract : Pollution of Hg has attracted much attention in Au mines at North America,
Africa, and South America, because Minamata-diseases are still increasing in the 21st
century. Mercury fragments influence on not only human but also air and soil environments including natural microbial systems. Studies on bacterial mineralization have received much attention as well. However, only few studies have been conducted on the
Au mine in relation to the long-term bioremediation study. In this study, mineralogicalproperties of clay minerals, goethite, quartz, and calcite associated with heavy metals,
such as Fe, Au, Ag, Hg, Ba, Sb, and Pb from the small-scale Au mine, Geita, near Lake
Victoria, Tanzania, have been studied using electron micro techniques. The lateritic
reddish mud samples were collected from 7 locations of Hg panning ponds whereas air
dusts were collected from 35 locations from Dar Es Salaam to Geita for about 1000 km
away. Both the wet mud and air dust samples are mainly composed of clay minerals(smectite, halloysite, kaolinite), calcite, and goethite, associated with ppb ordered Hgon the surface or the structures. Observation of FE-TEM electron microscopy and elemental distribution maps revealed that the clay particles, calcite grains, and bacterialcolonies contain relatively high Hg and Fe on the surface and/or the structures, indicating Hg pollution release from the panning ponds in the Au mine after heating treatment
of Au-Hg amalgam. Volatilization of Hg carry to the air dusts associated with dried fineclays at all over the land in Tanzania. The highest Hg content in the dust was about
7785 ppb in Muguli, Southern parts of Lake Victoria. The soils near Hg panning pond
contains relatively high Hg as 1576 ppb in Muhama and 1183 ppb in Rwamagaza. The
results suggest that biodegradation of heavy metals by microorganisms associated withclays is one of the primary ways by which heavy metals are eliminated from contaminated sites. The finding of heavy metals-degrading bacteria and clay minerals in the Au

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Kazue TAZAKI et al.

mine may have a significant effect on the weathering processes during the long-termbioremediation in Tanzania.
Keywords : Au mine, Hg pollution, panning ponds, air dusts, soils, clay minerals, heavy metalsdegrading bacteria, bioremediation______________________________________________________________________

Introduction
Concern regarding mercury (Hg) as an environmental contaminant has recently cometo the forefront of public awareness. Because of this, many involved in Hg research are being increasingly called upon to help understand the sources and sinks for Hg in the environment, Hg biogeochemical cycle, and the implications for human and ecological health.
“Mercury : Sources, Measurements, Cycles and Effects”, was published the book by Parsons and Percival (2005) on this timely topic. Mercury is known to have detrimental effects
on human health, so it is surprising to read that it may not be worthwhile to regulate mercury release from U.S. power plants. Hg pollution in Au mine has attracted much attention
in Africa, North America, and South America because Minamata-diseases are still increasing in the 21st century. Mercury is considered a global pollutant because it is present asgaseous phases, rendering the atmosphere an important pathway by which it can cross national and international boundaries as well as oceans. Mercury contamination associatedwith small-scale gold mining and processing represents a major environmental and humanhealth concern in Eastern and Southern Africa. Approximately 200,000 - 300,000 persons
are involved in small-scale gold mining activities in Tanzania and >200,000 persons in
Zimbabwe. Mercury is used mainly for the processing of primary gold quartz veins and supergene gold mineralizations. Gravimetric material flow analyses show that 70-80% of the
Hg is lost to atmosphere during processing, 20-30% are lost to tailings, soils, stream sediments and water. For every 1g Au produced, 1.2-1.5g Hg is lost to the environment (vanStraaten 2000). Urinary mercury levels in gold mine workers frequently exposed to Hg vapor in amalgamation and burning of amalgam were significantly higher (mean : 241 ng/ml) than those in the general mine population not occupational exposed to Hg (mean : 2.6
ng/ml) (Ikingura and Akagi 1996).
Similarly, although chelating therapy may remove methyl- and ethyl mercury, it cannot reverse central nervous system damage, implying that prenatal mercury exposure leads
to lifelong lost benefits, irrespective of money spent on removing the causal agent from thebody after the damage has been done (Zeller and Boot 2005). In Africa, mercury pollutionof aquatic ecosystems can potentially be devastating, because there is both high dependence on untreated water and relatively few regulatory controls for monitoring and mitigating aquatic contaminants (Campbell et al. 2003). Monitoring of environmental and human
exposure to mercury in the Nungwe Bay area of the Lake Victoria goldfields, Tanzania,
has revealed low mercury concentrations in fish (range : 1.8 - 16.9 ppb, mean : 7.0 ppb)

Hg-release from Au mine by clays and bacteria

3and human hair (range : 156 - 5433 ppb, mean : 947 ppb) that represent background levels. Gold mining has not produced a significant increase in environmental methylmercury
that is available for bioaccumulation in aquatic food chains (Ikingura and Akagi 1996,
2003 ; Machiwa 2004).
Mercury fragments influence on not only human but also air and soil environments including natural microbial systems. Studies on bacterial mineralization have received muchattention as well. The biodegradation of heavy metals by microorganisms is one of the primary way by which Hg is eliminated from contaminated sites (Compeau 1985 ; Hughes
and Poole 1989, 1991). In general, heavy metal contamination, no matter of their sizes, influence natural microbial system around environments. Many approaches to heavy metal
contamination clean up attempted to accelerate natural biodegradation processes in promoting the activity of heavy metal-degrading microorganisms. This is an important perspective to present given the evolving nature of Hg biogeochemistry. The characteristics
facilitate the movement of Hg between air, soil, water, and biota, making the biogeochemical cycle extremely complex. While the limitations and uncertainties associated with thedevelopment of the Hg biogeochemical cycle create a field of research. Clays and microorganisms are ubiquitous on the Earth’s surface and, more often than not, are closely associated with one another. Indeed, microorganisms in soil tend to gather at or near clay surfaces as these are enriched in ions, water, and organic matter relative to the bulk soil. However, only few studies have been conducted both on the panning ponds and air dusts in relation to the long-term bioremediation study.
In this work, we repot the results of a mineralogical and biological investigation ofsamples from ppb-ordered Hg contaminated panning ponds, soils, and air dusts in Tanzania.
In addition, the Au and Hg associated with nm-order mineral grains and bacteria were
evaluated in comparison with the indigenous microorganisms using Field Emission microscopy equipped with STEM of Energy selection elemental mapping-TEM. We showed thatthe Hg-clay nm-crystals were made up of an Au-rich panning pond with bacteria. To ourknowledge, there are no previous reports on the natural bioremediation of Au and Hg
bioremediation in Au mine in Tanzania.
Geological setting
Precambrian rocks underlie most of central and western Tanzania. Archean granite
and greenstone rock assemblages from the central nucleus of the country, the Tanzania
Craton, distributed at southern part of Lake Victoria. The Tanzania Craton surrounded byProterozoic belts : the Palaeoproterozoic Usagaran-Ubendian belt, and the Mesoproterozoic Kibaran (Karagwe-Akolean). The name Lake Victoria Gold Fields (LVGF) refers collectively to a number of gold fields located to the south and east of Lake Victoria in northern Tanzania. Included in the LVGF are the South West Mwanza (Sukumaland) gold field
to the south of Lake Victoria, the Musoma gold field to the east of the lake and the IrambaSekenke gold field to the south east of the lake. These gold fields occur in Archean (>2400

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ma) terrane of weakly metamorphosed mafic and felsic volcanic rocks and graywackes.
The terrain belongs to the Nyanzian greenstone belt of Tanzania Craton.
Gold occur in four types of deposits namely : 1, auriferous quartz reefs and stockworks 2, disseminated gold associated with sulphides in banded iron formation (BIF) andsheared metavolcanic rocks (sulphide impregnated deposits) 3, alluvial gold in laterites,and 4, placer gold in river sediments. The first three are by far the most important in termsof grade tenor and spatial distribution. All the important auriferous reefs are associatedwith major shear zones within the greenstone belt (van Straaten 2000).
Lake Victoria Gold Fields is one of the major producers in the country. The gold production is at both small and large scales. Mercury pollution is dominant in small-scale gold
mines and it needs through environmental studies in order to understand the extent of the
problem and the control of the use of mercury. The research problem has been clearly identified. Briefly the problem deals with Hg toxicity to human beings. It highlights bioaccumulation by fish, which affects human beings of which the pathways and speciation of Hg
in the ecosystem to affects human beings of which the pathways and speciation of Hg inthe ecosystem to affect the people is not yet adequately determined.
Materials and methods
Field sites and sample collection
The 30 locations studied on the heavy metals-contaminated air dusts and soils in Tanzania (Fig. 1, Table 1). Heavy metal contaminated soils were collected from 6 panningponds at Geita City, Southern Lake Victoria, on June 2005 (Fig. 2). The field work carryon dry season, therefore the environmental conditions in Tanzania were very dusty
throughout the country (Fig. 3). Panning pond water and river water quality near smallscale Au mine at Geita City was performed at 6 locations (Fig. 4, Table 2). Measurementsof characteristics of the waters (pH, Eh, EC, Water Temperature and Dissolved Oxygen) in
the field are summarized as shown in Table 2.
Elemental analysis
Soils and the dusts were analyzed by X-ray fluorescence spectrometer (XRF), immediately after returning from the field. Briefly, water samples from panning pond were acidified with HNO3. The samples were air-dried up at room temperature and ground to fine
powder for ED-XRF analysis. The powder samples were pressed to make pellet andmounted on the Mylar film. Analysis was carried out by an energy dispersive X-ray fluorescence spectrometer (JEOL JSX-3201), using Rh Kα, which operated at an accelerating
voltage of 30 kV under a vacuum condition.
Hg quantitative analysis
The determination of mercury by official method and its improved methods was usedfor Hg quantitative analysis in this study (Korenaga et al. 1987 ; Yamada and Korenaga

Hg-release from Au mine by clays and bacteria

51992). Procedure of mercury quantitative analysis as follows ; Soil samples were analyzed
for total mercury by means of the Hydride Generation Atomic Absorption Spectrometry(HG-AAS). On addition of 1 ml of distilled water, 2 ml of 1 : 1 mixture of HNO3 andHClO4 , and 1ml of H2 SO4 to 0.5 g of soil samples, which were preheated at 200 ! for
two hours. The solid materials were removed from the extracts by the centrifugal separation. The measurements were carried out for 1 ml of the extracts using the HG-AAS apparatus, HITACHI HG-310.
X-ray powder diffraction analysis
The purpose of this study was to elucidate the interaction between minerals and indigenous microorganisms in the Hg panning ponds collected from the Au mines studied by
X-ray powder diffraction analysis (XRD ; Rigaku RINT1200 X-ray diffract meter). Powder samples were prepared by spreading 2 mg of the suspended clays over a 2.5 cm2 area ofFig. 1. Map of the research areas near Lake Victoria, Tanzania. The field work carry on June 2005.

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Table 1. Hg concentrate in soils and dusts of 30 locations in Tanzania.

a glass slide. Relative change in the basal spacing of smectite was measured by EG treatment, and observed using Cu-Kα radiation, generated at 40 kV and 30 mA, using a scan
speed 2°
/min. The powder samples on Mylar films were prepared for analysis by energy
dispersive X-ray fluorescence spectroscopy (ED-XRF ; JSX-3201), using Rh-Kα radiation
generated at 30 kV under vacuum. The wt% of each element in a sample was determinedon the elements heavier than elemental Na.

Hg-release from Au mine by clays and bacteria

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Fig. 2. Sampling points of dusts and soils at small-scale Au mines near Lake Victoria, Tanzania, on June
2005. The sampling point No. 6 is mainly studied on mud at panning pond.

Electron microscopic techniques
For dust and soil samples were directly set up on the sample stub with carbon double
sticky tape, and viewed on a JEOL JSM-5200LV scanning electron microscope. Bacterial
cells were fixed with 2.5% (v/v) glutaraldehyde for 24 h at 4 !, mounted on copper specimen grids, allowed to dehydrate at room temperature, and then viewed using a JEOL JEM2000EX at accelerating voltage of 120 kV and energy-filtering TEM (JEM-2010FEF) at

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Kazue TAZAKI et al.

A

B

Fig. 3. Field views of the reddish brown lateritic soils (A) and dusty leaves (B) collected from Muguli(sampling No. 1) on June 24, 2005.

Table 2. Characteristics of river water, panning pond water, and separation water measured in the field ofsmall-scale Au mine at 6 locations in Tanzania shown in Fig. 2

Hg-release from Au mine by clays and bacteria
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Kazue TAZAKI et al.

A

C

B

D

Fig. 4. Panning pond at small-scale Au mines in Muhama (sampling point No. 6). In the pan we can see
one small grain of Hg-Au amalgam (B).

the accelerating voltage of 200 kV. Energy dispersive spectroscopy (EDS) was also performed during TEM analysis to evaluate changes in elemental compositions of the soils
and dust samples in phosphate buffer at room temperature for 1 h. Dehydrated samples in a
graded ethanol series (50, 75, 95, and 100%), mounted on carbon-coated copper stubs, andviewed on a JEOL JSM-5200LV scanning electron microscope.
Farther more, ultra-thin sectioned samples after resinous fixation were cut with anultra-micro tome, and observed it with nm-scale observations on Formvar and carboncoated copper grids. The elemental distribution analyses of the samples performed with a
Field Emission Electron Microscope equipped with STEM detectors showing elemental
mapping of transmission electron microscope images (JEOL JEM-2010FEF). The JEM-

Hg-release from Au mine by clays and bacteria

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Fig. 5. X-ray powder diffraction patterns of the reddish brown mud sample collected from panning pondat sampling point No. 6.

2010FEF, a state-of-the art analytical electron microscope, is equipped with a 200 kV fieldemission gun (FEG), an in-column energy filter that allows elemental analysis and chemical analysis of specimens, a scanning TEM image generator (EM-Z90471) and an energy
dispersive X-ray spectrometer (EX-340251GT). A combination of high-resolution TEM,
scanning TEM, and electron energy-loss spectroscopy is a powerful tool in the study ofelemental associations in nm-scale particles. Identification of precipitates associated withbacterial cells was accomplished by selected area electron diffraction (SAED). The TEM isan essential tool in all field of basic nm-science including biotechnology and material science, and finds specific elemental distribution.
Results
Sampling points and Hg quantitative analysis of dusts and soils
Map of the 30 sampling points of soils and dusts across in Tanzania between Dar Es
Salaam and Lake Victoria are shown in Fig. 1 and Table 1. The specific research areas ofsmall-scale Au mines near Lake Victoria closely are shown in Fig. 2 indicating bar graphof Hg contents in soils and dusts. The Hg contents at the 30 research areas and 6 Au mines
list up in Table 1 and 2. Characteristics of river water, panning water, and separation watermeasured in the field of small-scale mine at 6 locations, Tanzania (Fig. 2, Table 2), showing variety pH of 4.3 - 10.2, relatively high oxidation and high EC (mS/cm) conditions.
The water temperature at the panning, separation, and river areas indicated about 20 - 30

12Kazue TAZAKI et al.

Fig. 6. X-ray fluorescence analyses of the reddish brown mud sample collected from panning pond at
sampling point No. 6. Water (A) and sediments (B) indicate presence of high S content in thewater whereas Al, Si, and Fe elements are due to clay minerals.

!. The fieldwork carry on dry season on June 24-30, 2005, to collect dust and soil samples at small-scale Au mines near Southern Lake Victoria (Fig. 2). The lateritic reddish
mud samples were collected from 6 locations of Hg panning ponds whereas air dusts werecollected from 30 locations from Dar Es Salaam to Geita for about 1000 km away. The
sampling point No. 6 in Fig. 2 is the typical panning pond to collect sediments and water,
showing extremely high Hg contents of 1576 ppb. The highest Hg content in the dust was
about 7785 ppb in Muguli. Both soil (30ppb) and dust (50 ppb) samples show high Hg
content in Arusha, Southern parts of Lake Victoria (No. 25). The soils near Hg panning
pond contains extremely high Hg of 1183 ppb in Rwamagaza. Both mercury emissions anddeposition are the highest site of panning pond is in Geita City (No. 6 ; Muhama, 1576

Hg-release from Au mine by clays and bacteria

A

B

Fig. 7. Transmission electron micrographs of ultra
thin sectioned mud sample collected fromthe panning pond at sampling point No. 6,
showing living bacteria which are tolerantto Hg and Au. The bacterial structure is
still preserved, such as cell wall, cell film,
and EPS. Clay particles are adhesive on thesurface of the bacteria.

C13ppb) (Fig. 2, Table 2). In the pan we can see one small grain of Hg-Au amalgam of less 1
cm in diameter (Fig. 4B).
The analytical data suggested that the concentration of Hg between soil and dust at thesame area is correlative conditions. Note that mercury emissions and deposition are high at
the urban areas, such as Dar Es Salaam (12 ppb for soil and 25 ppb for dust), Shingida (21ppb for soil), Shinyanga (31 ppb for soil), Mwanza (18 ppb for soil), Arusha (30 ppb forsoil and 50 ppb for dust) and Mkata (17 ppb for soil) are high, suggesting that the Hg release from Au mine to whole lands in Tanzania by wind. Field views of the reddish brownlaterite soils (A) and dusty leaves (B) collected at Shingida on July 24, 2005, showing 21
ppb Hg in the soil (Fig. 3).
X-ray powder diffraction and X-ray fluorescence analyses of mud from panning pond
X-ray powder diffraction patterns of the reddish brown mud sample collected frompanning pond at sampling point No. 6 (Fig. 5) identified as smectite (14.24Å), kaolinite(7.19Å), quartz (3.29Å), and goethite (4.14Å). The 14.24 Å d-spacing shifted to 16.66Å after EG treatment indicated presence of smectite. The 3.29, 4.21, and 2.03 Å identified
as quartz whereas the 4.14 and 2.62 Å can be identified as goethite as well. The goethite
poorly crystallized, showing weak intensities. The reddish brown sediments are character-14

Kazue TAZAKI et al.

N

0-loss

5μm

P

Ca

Al

Si

Fe

Au

Fig. 8. Transmission electron micrographs of ultra thin sectioned mud sample collected from the panning
pond at sampling point No. 6, showing zero-loss image and distribution maps of N, P, Ca, Al, Si,
Fe, and Au elements. Note that Au are associated with Fe and clay minerals (Al-Si).

Hg-release from Au mine by clays and bacteria

15

B

A

200nm

C

100nm

D

100nm

100nm

F

E

50nm

20nm

G

200nm

Fig. 9. Transmission electron micrographs of thin sectioned living bacteria (A - D), quartz (E), goethite(F), and kaolinite (G) with electron diffraction pattern (inset). High dense materials present on/ inthe bacteria suggesting bio-mineral products (C and D).

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Kazue TAZAKI et al.

A

B

C

D

E

Fig. 10. Scanning transmission electron micrographs of zero loss images of clay particle (A and B) andcalcite grain (C, D, and E), showing Hg distribution maps. Note that clay minerals with 10-12 Å
d-spacing and calcite accumulate Hg on the surface or inside structure. The Si content map (E)shows independence from Hg distribution.

Hg-release from Au mine by clays and bacteria

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istic kaolinite-rich lateritic soils.
X-ray fluorescence analyses of the water (A) was collected from panning pond at
sampling point No. 6 (Fig. 6) composed of high S content associated with traces of Na, Mg,
Al, Si, K, Ca, and Fe. While the sediment (B) composed of mainly Al, Si, and Fe which thechemical components agreed with clay minerals in lateritic soil using XRD analyses (Fig.
5).
Optical electron microscopy of mud from panning pond
Optical micrographs and TEM ultra thin section micrographs of the reddish brown
mud sample collected from panning pond at sampling point No. 6, show inhabitant of living bacteria with cell wall and EPS (external-cellular polymeric substance) are wrapped in
the clay particles (Fig. 7). The size of bacteria ranges from 400 nm to 1 μm in diameter.
The clay particles are oriented with lattice images of 10-12 Å indicating smectite after dehydration of 14 Å. Both optical light and epifluorescence micrographs showing browncolored clay minerals associated with blue light of coccus type bacteria.
FE-TEM transmission electron microscopy
Transmission electron micrographs of ultra thin sectioned mud sample collected fromthe panning pond at sampling point No. 6, showing clay minerals associated with Au nmparticles (Fig. 7, 8). The zero loss image and energy-dispersive X-ray elemental maps
showed distribution of N, P, Ca, Al, Si, Fe, and Au elements. The Au and Fe elements are
co-related in nm-sized particles made up of an Au-rich mud in the panning pond, and it
suggests that the Au electronically influences the catalytic properties of Fe (Fig. 8). STEMEDX maps of Al and Si composition are located at the clay nm- sized particles around bacteria. The p map shows co-related with clay composition of Al and Si maps. The N and Ca
distributions are not so clear relation ship between other elements. The living bacteria withcell structure (A, B) and bio-mineralized bacteria with high dense materials on/in the cell(C, D) can be seen with quartz grain (E), goethite grain (F), and kaolinite particles (G) thatthe minerals identified by characteristic electron diffraction patterns (Fig. 9). Zero loss image and Hg maps of clay particles indicated with precipitation of Hg on the particle, showing 10-12 Å lattice images (Fig. 10, A, B), where as calcite grain accumulated with Hg on
the surface (Fig. 10, C, D). The trace of Si adhesion is on the calcite surface (Fig. 10, E).
Scanning transmission electron micrographs and TEM zero loss images of soils (Fig.
11) and dust (Fig. 12) using energy dispersive X-ray analyses can be identified each grain,
as indicating elements and identified minerals. Scanning electron micrograph (A), transmission electron micrograph (B, C, D) of soil sample was collected from panning pond in
Muhama (Hg 1576 ppb), identify each grains, such as tube-like halloysite, hexagonal calcite, high dense of iron grain (goethite) and kaolinite/or smectite (Al, Si, and Fe)(Fig. 11).
A few granules are found composing pure Au (Fig. 11A, center). Abundant coccus typed
bacteria are associated with clays and organic matters (Fig. 11 B). Moreover, heavy metals

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Kazue TAZAKI et al.

A

C
Fe

Fe

Al, Si, Fe

Al, SiAl, Si,
Fe
Ca

0.5μm

Au

D
Al, Si, Fe

B
Na, Mg, Si,
S, Cl, K

Fe

5μm
Na, Mg, Al, Si,
S, K, FeAl, Si, Fe

Al, SiCa
Al, SiHalloysite

Al, Si, S

Ca

Al, Si0.5μm

organic material

0.5μm

Fig. 11. Scanning transmission electron micrographs and TEM zero loss images of soils in panning pond
(location No. 6), using energy dispersive X-ray analyses indicating elements and identified minerals. Scanning electron micrograph (A), transmission electron micrograph (B, C, and D) of soil
sample was collected from panning pond in Muhama (Hg 1576 ppb). The tube-like halloysite(D), and hexagonal calcite (Ca ; in C and D), high dense of iron grain (D ; goethite), kaolinite/or smectite grains (Al, Si, and Fe), and a few granules of pure Au (A, center). Abundant coccus
typed bacteria are associated with clays and organic matters (B).

Hg-release from Au mine by clays and bacteria

A

19

C
Si, ZrAl, Si
Ag, S
Ag, ClAg, ClAl, SiD

10μm

Al, Si

10μm
Ba

B

E

5μm
Al, SiPb
Si
Al, SiP, La, Ce

Fe

5μm

5μm

Fig. 12. Scanning transmission electron micrographs, TEM zero loss images, and energy dispersive Xray analysis of dust sample (location No. 1) was collected from Muguli (Hg 7785 ppb), showing
abundant heavy metals, such as Ag (A), Pb and Fe (B), Zr (C), Ba (D), and Ce (E), which are always aggregated with clay particles.

of Ag, Pb, Zr, Ba La, and Ce were found with clays (Al, Si) and quartz (Si) associated withorganic materials in the panning pond.
Scanning transmission electron micrographs, TEM zero loss images, and energy dispersive X-ray analyses of dust sample (Hg 7785 ppb), was collected from Muguli, can be
identified each grains as indicating elements and minerals in Fig. 12. Abundant heavy metals, such as Ag (A), Pb and Fe (B), Zr (C), Ba (D), and Ce (E), which are always associatedwith clay particles nm - μm in diameter, suggesting easily release to long distance in Tan-

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Kazue TAZAKI et al.

Fig. 13. Schematic diagram of release Hg from small-scale Au mine in Geita near Lake Victoria, in Tanzania, showing relationships between Au and Hg in panning pond, dusts, and soils. The Hg carries on the surface of clay minerals, calcite, and bacteria.

zania by dry wind (Fig. 12).
Schematic diagram of release Hg from mud panning pond at small-scale Au mine in
Geita near Lake Victoria, in Tanzania, showing relationships between Au-Hg amalgam,
volatilization of Hg by heating at out door, and produced Hg rich dusts and soils at thesmall-scale Au mine (Fig. 13). In the mud panning pond living bacteria which are tolerantto Hg and Au carry important role to adhere clay particles to the cell wall by EPS, as
shown in TEM micrographs. The results suggest that reason why Hg pollution spread tosoils and dusts at whole area in Tanzania.
Discussions
Environmental impacts and human health risks associated with mercury pollution
have prompted numerous studies, worldwide, that are geared to provide better understanding of mercury cycling and bioaccumulation in temperate and tropical ecosystem. In tropical regions, small-scale gold extraction by mercury amalgamation is a major source of mercury pollution. The data on atmospheric mercury pollution are still scarce, in spite of thefact that more than 55 % of mercury used in gold extraction is released to the atmosphere
during burning and re-burning of amalgams. In the absence of direct air mercury sampling
and measurement equipment, the use of locally available plant species in the assessmentand evaluation of air pollution patterns in the gold mining area was described (Ikingura andAkagi 2002). Lichens were selected for the assessment of mercury dispersion in air frompoint sources related to gold mining activities in Tanzania. Theoretically, mercury should
preferentially rain down in areas near to power plants. In this study attempts to determine
the fallout by wind flow have proved and clays carry important role to release Hg to largeHg-release from Au mine by clays and bacteria

21

urban city. A partial explanation for this dichotomy is the process by which elemental mercury becomes methyl mercury by microorganisms. For mercury to get methylated and enter the food web, it must be processed by bacteria, thrive on sulfate, a sulfur compound.
This means that dissolved organic matter and sulfur enhance methylation, as do panningwaters such as those in the small scale Au mine.
Tailings are local “hot spots” with high concentrations of As, Pb, Cu, and Hg. Lateraland vertical disperson of Hg lost to soils and stream sediments is very limited (laterally <260 m, vertically < 20 cm). Dispersion of mercury from tailings is low because Hg is transported largely in the elemental, metallic form. Fe-oxide rich laterites and swamps appear tobe natural barriers for the dispersion of metals in soils and streams. Ground and surface
water quality data indicate very low dispersion rates during the dry season (van Straaten
2000). High-resolution TEM-STEM-EELS study demonstrated the presence of nm-scalegraphitic carbon agglomerates host even finer (< 3 nm) particles, some of which appear to
be Fe-spinels, with typical soot-like onion-ring structures on the coarser fly ash particles ina Kentucky fly ash. The elemental analysis indicated an association of Hg, possibly in sulfide form, with carbon. Arsenic, Se, Pb, Co, and traces of Ti and Ba are associated withthese Fe-rich particles (Hower 2005).
Accumulation of heavy metals requires organic matter burial in sediments, which islargely controlled by the sheltering or preservational effects of clay minerals in the sediments. The bio-formation of magnetite and siderite plays an important role in the cyclingof iron and carbon as well as in carbon sequestration in natural environments. Biogeniciron-rich minerals and microbial Fe (III) may also serve as physical indicators of previous
biological activities (Kim et al. 2005). “Biomineralization” is the process by which livingorganisms convert ions in solution into solid minerals (Swannel and Head 1994). The process is the result of cellular activities that allow for certain physicochemical changes to occur, as shown in this study. Hong and Fie (2005) describe the characteristics of the minerals associated with gold in the Shewushan supergene gold deposit, China, that is mainlycomposed of quartz, kaolinite, halloysite, minor illite and goethite. In this study, STM
maps of mud in panning pond show that the introduction of Au to Fe improves selectivity,
and we believe that the surface of the bimetallic nm-particles will still contain some Au
with clay particles (Fig. 8), indicating that the Au electronically influences the catalytic
properties of Pd, by reported Enache et al. (2006).
Ore samples from five copper sulfide deposits associated with Jurassic ophiolites in
the Northern Apennine belt of Italy were found to contain Au, Ag and U (Garuti and Zaccarini 2005). They consist of fine-grained sphalerite (< 20 μm) spotted with interstitialpatches of quartz and calcite. Locally, sphalerite consists of closely packed spheroide (upto 100 μm in diameter) coated with a thin shell of silica, possibly representing replacementof radiolarian shells or bacterial colonies, suggesting post-depositional biogenically driven
reworking and clastic accumulation of sulfide debris in topographic lows (epigenesis). The
close association of pyrite and siderite places the conditions for the formation of hydromar-

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Kazue TAZAKI et al.

chite at Eh of -0.2 to -0.4 units and pH between neutral (pH=7) and moderately basic (8<pH< 9), compatible with the submarine environment.
The thematic issue on metals in the environment and mine wastes originated from the
MAC-sponsored Metal in the Environment symposium and the Environmental Studies ofMine Wastes session held at the GAC-MAC-SEG Annual Meeting in 2003. In this special
issue, Sidenko and Sherriff (2005) describe the mineralogical changes that occur in mine
wastes following their uncontrolled disposal in the paper entitled “Geochemical and mineralogical zoning of high-sulfide mine-waste at the Berikul mine-site, Kemerovo region,
Russia”. The composition and nature of the iron oxides in mine wastes from the giant gold
mine in Yellowknife, Northwest Territories in Canada, determined that maghemite andhematite contain both As5+ and As3+ species (Walker et al. 2005). The persistence of As3+ inroaster-derived maghemite in shallow oxidized shoreline tailings for over 50 years suggeststhat the arsenic is relatively stable under these conditions, even though As3+ is a reduced
form of arsenic, and maghemite is normally considered a metastable phase.
Siderophres are low molecular weight organic ligands produced by microorganisms
and some plants under Fe-limited conditions, and commonly present in soil systems. Siderophores have a high specificity for Fe (III), relative to Fe(II) ; siderophore Fe (III) 1 : 1complex stability constants range from 1023 to 1052. Siderophores can also form stable complexes with other metals, such as Al(III), Ga (III), In (III), Cd (II) and Pb (III). This ability
to form stable metal-ligand complexes suggests that siderophores may influence metal mobility in soils by affecting rates of mineral weathering (Hepinstall et. al. 2005). According
Fig.7-12 in this study, potential co-adsorption of siderophore-metal complexes to themineral-surface might enhance metal adsorption, such as Au, Fe, Hg, As and S. In panning
pond sulfur bacterial surface might enhance metal adsorption of Au, Hg, and Fe. Furthermore, cohesion materials on the bacterial surface effects on Au, Fe, and Hg adsorption to
clay minerals (smectite, mica clay minerals, and kaolinite). Kim et al (2005) reported theeffect of microbial Fe (III) reduction on smectite flocculation.
However, very little is known about the biodegradation of Au, Ag, Hg, and As etc.,
because of their complex structures, which are difficult to analyze nm-order scale. There isrecent evidence of As degradation in hot springs (Tazaki et al. 2003, 2004 ; Ohnuki et al.
2004). The mechanism of uptake of arsenic (As) in bacterial microhabitat, a biomat, fromdischarged arsenic mine water at pH 4.7 in field (As mine in Gunma, Japan) and laboratory
experiments. Especially iron bacteria of gallionella sp. was the predominant microorganism present in the biomat, accelerate the oxidation of Fe (II). The uptake of heavy metals
concurred with the oxidation of Fe (II) to Fe (III).
Some heavy ions, particularly low-molecular-weight fractions, can also be biodegraded at low concentrations. Korenaga et al. (1987) reported that elimination of interference by iodide in determination of mercury by cold-vapor AAS. Further research is stillneeded to understand the biodegradation of Au, Ag, and Hg compounds. In order to evaluate the potential influence of Fe-cycle bacteria on Fe cycling during post-deposition clay-

Hg-release from Au mine by clays and bacteria

23

weathering alteration, Fe (III)-reducing and/or Fe(II)-oxidizing microorganisms were examined in open-pit, subsurface mine samples from kaolin lenses and smectite formations
collected from sites in central Georgia (Shelobolina et al. 2005). All clay samples showed
greater numbers of Fe (II)-oxidizing bacteria (22-22 000 cells/g) than Fe (III)-reducing
bacteria (3-410 cells/g) as determined by MPN analysis. Anaerobic bacteria involved inboth oxidation and reduction of Fe exist in these subsurface clay formations. Crystallization carried out near room temperature or by the slow cooling or slow evaporation of a diluted solution (McCrone 1954). Kim et al. (2006) reported the investigation of changes inflocculation properties of Fe-rich smectite suspensions, including settling velocity, aggregate and floc architecture associated with microbial Fe (III)-reduction in the smectite structure. The increased net negative charge caused by microbial Fe (III) reduction significantly
promoted clay flocculation by increasing the electrochemical attraction in the smectite suspensions.
In this study, the high concentration Hg in soils, dusts, and panning ponds in small
scale mine in Tanzania. The nm-ordered studies suggest that clay minerals and minerals(smectite, kaolinite, quartz, and goethite) reacted with microorganisms. Several anaerobic
species may be used to bioremediate Hg and Au organic pollutants either individually or in
combination. It is also possible, however, that the indigenous microorganisms inhabiting
the panning ponds which are being exposed to heavy metals may also become resistant toor capable of transforming and detoxifying heavy. The presence of miscellaneous pollutants may present extreme challenges to the maintenance of a phylogenetically and functionally diverse microbial community (Shi et al. 2002). Here, we show biological and claygeo-chemical evidence for an increase in heavy metal deposition in small-scale Au mine in
Tanzania. Today most clay minerals originate in biologically active soils (Tazaki 2005).
Multiple enzymes produce fatty acids in cells, and carry out these bio-chemical syntheses.
Hanczyc et al. (2003) reported that montmorillonite catalyzes the polymerization of RNA
from activated ribosome mucleotide and that montmorillonite accelerates the spontaneousconversion of fatty acid micelles into vesicles.
Conclusion
This is an important perspective to present given the evolving nature of Hg biogeochemistry. The characteristics facilitate the movement of Hg between air, soil, water, andbiota, making the biogeochemical cycle extremely complex. While the limitations and uncertainties associated with the development of the Hg biogeochemical cycle create a fieldof research. Mercury fragments influence on not only human but also air and soil environments including natural microbial systems. Studies on bacterial mineralization have been
conducted on the small scale Au mines in Tanzania. Mineralogical properties of clay minerals, goethite, quartz, and calcite associated with heavy metals, such as Fe, Au, Ag, Hg,
Ba, Sb, and Pb from the small scale Au mine, Geita, near Lake Victoria, Tanzania, havebeen observed using electron micro techniques. The laterite soils of reddish brown mud

24

Kazue TAZAKI et al.

samples were collected from 6 locations of Hg panning ponds whereas air dried dusts werecollected from 30 locations from Dar Es Salaam to Geita for about 1000 km away. Boththe wet mud and air dust samples are mainly composed of clay minerals (smectite, halloysite, kaolinite), calcite, and goethite, associated with ppb ordered Hg on the surface or
the structures. Observation of FE-TEM electron microscopy and elemental distribution
maps revealed that the clay particles, calcite grains, and bacterial colonies contain relatively high Hg and Fe on the surface and/or the structures, indicating Hg pollution releasefrom the panning ponds in the Au mine after heating treatment of Au-Hg amalgam. Volatilization of Hg carry to the air dusts associated with dried fine clays at all over the land in
Tanzania. The highest Hg content in the dust was about 7785 ppb in Muguli, Southernparts of Lake Victoria. The soils near Hg panning pond contains the highest Hg as 1576
ppb in Muhama. The biodegradation of heavy metals by microorganisms is one of the primary ways by which heavy metals are eliminated from contaminated sites, such as in panning pond bacterial surface might enhance metal adsorption of Au, Hg, and Fe. Cohesionsof the bacterial cells might be effect on Au, Fe, and Hg adsorption to clay minerals (smectite, mica clay minerals, and kaolinite).
The finding of heavy metals-degrading bacteria had stuck to the clay minerals in the
Au mine, where clay may be less of a problem, and may have a significant effect on theprocesses during the long-term bioremediation. Bacteria use diverse small molecules for
extra- and intracellular signaling. Identifying significant sources and sinks of Hg is important for the recognition of global biogeochemical cycles. The primary concern for humanhealth is methyl Hg in aquatic systems, exposure through lateritic dust of clays to local, regional, and global ecosystems.
Acknowledgements
We would like to thank to Dr. Yoshimasa Michigami at Environment Preservation
Center, Kanazawa University, for Hg analysis, and Dr. Kurihara at High Technology Center of Kanazawa Medical University, for Technical assistant. This work was supported by agrant from the Japanese Ministry of Education, Culture, Science and Technology.
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