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Clay minerals as a scavengers of heavy metals in the soils and sediments and, formation of
metal sulphides – experimental study
E. HELIOS RYBICKA
University of Mining and Metallurgy, Al. Mickiewicza 30-059 Cracow, Poland
Abstract
The saturated with the metal ions (Cu, Pb, Cd, Zn, Tl, Ag, Hg) clay minerals were treated
by aqueous H2S solution under controlled Eh and pH conditions. The heavy metals ions
were released and precipitated at 80oC as sulphides, in most cases appeared to be
crystalline. They often form coatings or crystals overgrown on clay minerals or form tight
intergroths with clay particles. After 5 days the well crystallised sulphides are formed;
covellite - CuS, galena - PbS and sphalerite ZnS were identified by X-ray method. The
microscope observations revealed some other Cu-Fe-minerals like chalcopyrite - CuFeS2,
idaite, bornite. The native copper and pyrite were also observed. Cadmium formed
hawleyite - CdS, whereas the crystals of greenockite were rarely found in the reaction
products. Mercury react very quickly with S2- ions and in the heavy fraction of reaction
product the metacinnabar - HgS has been determined. The numbers of thallium sulphides
were obtained: TlS, Tl2S - carlinite, Tl2S5, Tl4S3 and TlFeS2 - raquinite, Cu3Tl2S5, Cu2TlS3,
AgTlS, Ag4Tl2S3 and AgTlS2.
Key words: clay minerals, syntheses, heavy metals, sulphides, ore microscopy, electron
microprobe.
1. Introduction
Clay minerals due to their active surface play an important role in the accumulation,
sorption/desorption, as well as in exchange processes of many trace metal ions [5], [1].
Fletcher and Sposito [3] described metal-cation sorption on montmorillonite, using a
binary-site model with complexation reactions. In different polluted environmental
compartments clay minerals act as scavengers of metals [8], [10], [6]. Under special
environmental conditions (pH, Eh, ionic strength) they may also act as metal sources
providing new formed phases e.g., sulphides with metal ions [2], [4], [7], [15].
The researchers have been trying to find explanation for the heavy metal amounts
and forms in the sediments especially in the clay fraction and moreover, for the formation
conditions of low-temperature sulphides in the Kupferschiefer ores. It has already been
indicated that the adsorption processes are of great importance for the solution of the first
problem. The sulphides characteristic of Kupferschiefer ores show partly very low
temperature stability.
Solubility and mobility of the sediment-bound metals can be increased by many
factors: i.e., lowering of pH and changing of redox conditions. Under anoxic conditions at
the presence of sulphur ions some metals can precipitate as sulphides. In the anoxic
polluted sediments the high portions of cadmium, copper, lead and zinc have been found
in a sulphidic fraction [9], [7].
Basing on these facts the attempts have been made to synthesise the heavy metal
sulphides in an aqueous medium through the solid phase solution interactions by means of
thermostat. As "sources " of the metal ions some clay minerals were used.
2. Material and Methods
As initial material for the experiments the following clay minerals were used illite - the < 2
µm grain size fraction from the illitic clay (San Juan, USA) and smectite - the < 2 µm
grain size fraction from the bentonite (Upper Silesia, Poland).
The clay samples (illite or smectite) were fully saturated during 24 h with the
aqueous solutions of (CH3COO)2Cd, (CH3COO)2Zn, (CH3COO)2Pb, (CH3COO)2Cu,
CH3COOAg, (CH3COO)2Hg, FeSO4 and TlNO3. Afterwards, the saturated clay samples
were treated with an aqueous H2S solution at 80 oC in order to receive the reaction
products in a crystalline form within the given time. The pH and Eh of the solution in the
column (Tab. 1) were measured several times daily to control a system. To avoid
oxidation process the columns were closed. During the reaction of H2S with the clay
minerals a part of iron present in the clay samples were also released, and involved in the
reaction of sulphides formation. The new formed sulphides were separated from clay
substrate using heavy liquids and/or proper sieve sizes. The concentrates obtained this
way have been investigated by X-ray diffraction, ore microscopy and electron microprobe
(ARL-SEMQ-microprobe, the following spectral lines were used: SK, BiM, FeK, TlL,
AgL, CuK).
3. Experimental Results
During the experiments carried out at room temperature after two days the formation of
colour rims of sulphide precipitate on smectite band were observed: yellow of CdS, white
of ZnS and black of CuS. These sulphide phases obtained at room temperature are X-ray
amorphous, whereas the sulphides formed at 80°C, on the other hand, in most cases
proved to be crystalline.
Table 1. Experimental conditions and synthesised metal sulphides.
Starting substances
solution - clay mineral
Reaction
time (days)
pH Eh Synthesised
sulphides
(CH3COO)2Zn smectite
(CH3COO)2Cd smectite
(CH3COO)2Cu smectite
(CH3COO)2Pb smectite
(CH3COO)2Hg illite
CH3COOAg illite
TlNO3 smectite
TlNO3 + FeSO4 smectite
TlNO3 + CuSO4 smectite
TlNO3 + CH3COOAg
smectite
5
5
5
5
9
9
22
26
20
26
7.4 -
8.8
7.5 -
5.8
4.0 -
4.6
3.5 -
3.9
4.2 -
2.5
4.6 - 4.9
9.7 - 10.3
4.9 -
10.3
4.8 -
10.0
5.3 - 9.8
-344.2 to -542.8
-380.5 to -632.4
-521.6 to -614.5
-482.0 to -582.3
-581,3 to -724.8
-467.5 to -524.1
-589.3 to -687.2
-624.5 to -753.6
-524,7 to -678.5
-564.2 to -628.6
sphalerite ZnS
greenockite and
havleyite CdS
covellite CuS,
native copper,
chalcopyrite, idaite,
pyrite, bornite
galena PbS
metacinnabar and
cinnabar HgS
acanthite Ag2S,
native silver
TlS, Tl2S5, Tl4S3,
Tl2S carlinite
raquinite TlFeS2,
TlS
(Tl,Cu)S,
TlCu2S3, Tl3CuS4
AgTlS2, Ag4Tl2S3,
(Ag,Tl)2S
3.1 SIMPLE SULPHIDES
Copper, lead, cadmium, zinc and mercury belong to the group of metals which react easily
with S2- ions (Tab. 1). After few days the sulphides were formed; white rims of ZnS and,
yellow of CdS macroscopically were visible. Covellite - CuS, galena - PbS are identified
by X-ray method and, ore microscopy. Copper reacts also with iron extracted from clay
particles to form the Cu-Fe sulphides like chalcopyrite - CuFeS2, idaite, bornite. Often
pyrite as the framboids revealed by microscopic observations. The native copper was also
observed. Cadmium formed hawleyite - CdS, whereas the crystals of greenockite were
rarely found in the reaction products. Mercury react very quickly with S2- ions and in the
heavy fraction of reaction product the metacinnabar - HgS has been determined. The
interaction between monovalent silver and H2S solution leads to the formation of acanthite
- Ag2S and native silver (Tab. 1), often as intergrowths. The typical alternating precipitates
of acanthite - illite - acanthite was observed [4]. The chemical composition of Ag2S
determined by electron microprobe lies between Ag2.3 S and Ag 2.0 S.
3. 2 THALLIUM - SIMPLE AND POLYSULPHIDES
Similar experiments have been carried out with thallium, a very toxic metal showing a
high affinity to sulphur. Smectite samples were fully saturated during 24 h with the
aqueous solution of TlNO3 and (1:1) with solutions of FeSO4, CuSO4 and CH3COOAg. As
a result of the experiments the numbers of thallium sulphides (Tab. 1) were synthesised:
TlS, Tl4S3, Tl2S - carlinite and Tl2S5 (Tl2.08S5 - Tl1.96S5) - in the system of a smectite
saturated with the aqueous solution of TlNO3;
TlS, Tl2S, TlFeS2 - raquinite and Fe2S - pyrite and marcasite - in the system of a
smectite saturated with the aqueous solution of TlNO3 and FeSO4 (Tab. 2);
Tl3CuS4, Cu2TlS3 and CuS - covellite in the system of a smectite saturated with the
aqueous solution of TlNO3 and CuSO4 (Tab. 3);
AgTlS2, Ag4Tl2S3 and Ag2S - acantite in the system of a smectite saturated with the
aqueous solution of TlNO3 and CH3COOAg (Tab. 4).
The reaction of Tl with other heavy metal ions leads to the formation of mentioned
above Tl-sulphides and also simple sulphides of Cu, Fe, Ag as the separate phases as well
as coatings on other minerals. Because of very low content of the new formed phases,
only some of them have been detected by X-ray method: TlS, TlFeS2 - raquinite and Fe2S
- pyrite, AgTlS2, native silver and Ag2S - acantite, CuS - covellite and FeS2 - pyrite.
Table 2. Microprobe chemical composition of raguinite - TlFeS2 (upper figures in wt.%,
lower figures - atomic proportions).
Sample S KBi MFe KCu KTl LTotal
SIX/4, K-3/2 16.73
1.9480
0.95 12.86
0.8596
0.02 54.75
1.0000
85.29
Tl1.0000Fe0.8596S1.9480
SIX/4, K-3/2b 17.33
1.9537
0.02 12.82
0.8297
0.03 56.54
1.0000
86.72
Tl1.0000Fe0.8297S1.9537
Table 3. Microprobe chemical composition of mineral phases of the Cu-Tl-S system
(upper figures in wt%, lower figures - atomic proportions).
Sample S KBi MFe KCu KTl LTotal
SX/1, K-1/1b 22.94
1.0000
0.02 0.05 29.29
0.6442
47.72
0.3264
100.00
Tl0.9791Cu1.9329 S3.0000
SX/1, K-1/1c 18.67
1.0000
0.02 0.06 24.07
0.6506
55.92
0.4699
98.72
Tl1.4097Cu1.9518 S3.0000
SX/1, K-1/4b 15.11
0.4712
1.0000
0.52 0.16 7.69
0.1210
0.2568
76.10
0.3723
0.7900
99.58
Tl3.1604Cu1.0272S4.0000
SX/1, K-2/5 18.64
1.0000
1.09
0.0089
0.05 14.02
0.3795
66.66
0.5609
100.46
Tl1.6827Cu1.1385S3.0000
SX/1, K-2/5b 17.15
0.5348
1.0000
0.02 0.06 11.37
0.1789
0.3345
68.64
0.3358
0.6279
97.22
Tl1.8836Cu1.0035S3.0000
SX/1, K-2/5c 18.31
1.0000
0.02 0.01 13.20
0.3637
67.23
0.5760
98.74
Tl1.7280Cu1.0911S3.0000
Table 4. Microprobe composition of the Ag-Tl-S system (upper figures: wt.%, lower
figures atomic proportions).
Sample Tl LS KAg LTotal
SIX/5, K-7/1b 39.34
0.1925
9.98
0.3109
47.88
0.4439
97.20
Ag1.4278Tl0.6192S1.0000
SIX/5, K-7/1c 41.76
0.2043
10.70
0.3337
46.11
0.4275
98.57
Ag1.2810Tl0.6122S1.0000
SIX/5, K-7/1d 52.17
0.2553
11.51
0.3589
33.65
0.3119
97.33
Ag0.8689Tl0.7112S1.0000
SIX/5, K-7/1e 41.76
0.2043
10.70
0.3336
46.11
0.4275
98.57
Ag1.2816Tl0.6124S1.0000
SIX/5, K-7/1f 52.16
0.2553
11.51
0.3589
33.65
0.3119
97.32
Ag0.8689Tl0.7112S1.0000
SIX/5, K-6/2 0.71
0.0035
12.68
0.3954
82.39
0.7638
95.78
Ag1.9315Tl0.0089S1.0000
SIX/5, K-6/3 15.57
0.0762
6.23
0.1943
75.50
0.6999
97.30
Ag3.6022Tl0.3922S1.0000
SIX/5, K-5/4b 88.82
0.4346
10.62
0.3312
0.86
0.0079
100.30
Tl3.9363Ag0.0716S3.0000
SIX/5, K-5/4d 86.45
0.4230
11.12
0.3468
1.16
0.0108
98.73
Tl1.2197Ag0.0311S1.0000
SIX/5, K-5/5 89.57
0.4383
9.83
0.3066
0.02 99.40
Tl1.4297S1.0000
SIX/5, K-5/5b 90.25
0.4416
10.21
0.3184
0.92
0.0085
101.38
Tl1.3868Ag0.0267S1.0000
SIX/5, K-4/7b 88.97
0.4353
10.21
0.3184
0.55
0.0051
99.73
Tl1.3670Ag0.0160S1.0000
SIX/5, K-1/10 45.84
0.2243
10.23
0.3190
45.11
0.4182
101.18
Ag1.3108Tl0.7030S1.0000
4. Final Remarks
Clay minerals due to ion exchange capability act as "acceptors" of metal ions, in both by
the formation of some ore deposits e.g., Kupferschiefer, and in a polluted environment.
Under specific conditions (e.g., pH, Eh) clay minerals act as sources of metals. The
released by H2S metals, because of the S2- ions present in a system, can form sulphides.
In the case of the Pb and Cu after few days well crystallised galena and covellite
were observed; occasionally the native copper under microscope has been detected. In the
case of silver the tendency to form native Ag increases, and is formed often as tight
intergroth with acanthite and/or argenthite.
Cadmium and Hg, form monosulphides. In the present experiments usually the
crystalline phases have been formed. Mercury precipitates mainly as high temperature
phase as metacinnabar, while the low temperature cinnabar only as traces was found at the
microscopic observations. In the case of Cd, the main phase is the high temperature
modification hawlayite, not the low temperature greenockite.
The results of the experiments with Tl showed that the numbers of Tl sulphides
were synthesised: TlS, Tl2S - carlinite, Tl2S5, Tl4S3 and TlFeS2 - raquinite, Tl3CuS4,
Cu2TlS3, AgTlS2 and Ag4Tl2S3. Some electron microprobe results doesn’t lit to 100% of
the sulphides chemical composition because of the analytical problems connected with
low-temperature minerals (see Tab. 2).
The system Tl-Fe-S was subject of a systematic study under dry conditions at 180-
300oC [14]. The experimentally established assemblage TlFeS2- TlFe2S3-FeS2 agrees with
the paragenesis observed in the natural environments. Our results showed that raquinite
TlFeS2 are also stable at lower temperature i.e. at 85 oC. The system Ag2S-Tl2S has been
investigated in the dry synthesis by several authors [11], [12], [13]. From all phases
obtained at the temperature from 358 to 200 oC, because of the contradictions between
different authors only AgTlS and Ag3TlS2 were considered to be well confirmed [13].
In our low hydrothermal experiments we were able to confirm by X-ray and
microprobe data the AgTlS2 phase. Besides this phase, among others reported in the
literature we have also obtained the new phase Ag4Tl2S3 and confirmed its composition by
microprobe analysis (Tab. 4). The obtained sulphides form often single crystals or
coatings overgrown on other minerals, or form tight intergrowths with clay matrix. Often
repeatedly typical alternating banded precipitates: sulphide - clay mineral - sulphide, or
different sulphides alternatively were observed.
The obtained results confirmed that precipitation of the insoluble sulphides is the
probable mechanism of removal of the metals from a leachate be produced during the
weathering processes of the polluted waste materials. In this case sulphides should be
beneficial for the heavy metals precipitating.
Acknowledgements. This research was supported by DFG Project and Humboldt
Foundation, and carried out in the Laboratory of Experimental Mineralogy at the
University Heidelberg (Germany).
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