www.elsevier.com/locate/scitotenv

Science of the Total Environm

Formation of HgS—mixing HgO or elemental Hg with S,

FeS or FeS2

Margareta Svensson a,b,*, Bert Allard a, Anders Duker a

a Man–Technology–Environment Research Centre, Orebro University, SE-701 82 Orebro, Swedenb SAKAB AB, SE-692 85 Kumla, Sweden

Received 15 October 2004; received in revised form 23 June 2005; accepted 12 September 2005

Available online 27 October 2005

Abstract

The aim of this study is to assess the feasibility for generation of the sparingly soluble solid HgS from HgO or elemental Hg by

mixing with a suitable sulphur source under various conditions (dry, wet at different pH, and room temperature). The formation of

mercury sulphide was confirmed in 14 of the 36 combinations of Hg and S sources. Mercury sulphide was generally formed under

alkaline conditions. Almost 100% HgS was obtained in anaerobic systems at high pH in the presence of elemental sulphur after

about two years. Thus, it is feasible to create an environment at room temperature that, with time, leads to the generation of HgS

from elemental Hg or HgO. This is relevant for the design of a repository for permanent geologic disposal of mercury. Choosing

wrong components and conditions can however lead to a reduction of Hg(II) to elemental mercury.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Cinnabar formation; Hg stabilization; HgS; Mercury; Sulphur; Pyrite; Troilite

1. Introduction

A directive from the Swedish Government is to

terminate essentially all use of mercury in society. All

existing mercury within industry and society shall be

collected and stored in a safe and controlled way (SOU,

2001). A geologic repository for permanent storage of

mercury and waste containing more than 1% of mercu-

ry will be designed and built in Sweden. The preferred

chemical state for mercury in a geologic repository

would be the sulphide HgS, which is a sparingly solu-

ble and stable crystalline solid and the dominating

mercury mineral (cinnabar) in nature. Mercury in

0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.scitotenv.2005.09.040

* Corresponding author. Man–Technology–Environment Research

Centre, Orebro University, SE-701 82 Orebro, Sweden. Tel.: +46 19

305217; fax: +46 19 577027.

E-mail address: Margareta.Svensson@sakab.se (M. Svensson).

waste would be present as soluble compounds, as

well as the oxide and elemental and possibly also as

the sulphide. Soluble mercury compounds should be

extracted and converted to an insoluble form prior to

the final storage according to The Swedish Environ-

mental Protection Agency. Thus, the stabilization of

mercury by conversion to sulphide is an important

issue to ensure its long-term immobilization in a per-

manent storage. Previous studies have evaluated the

formation of mercury sulphide in laboratory by stirring

elemental mercury and sulphur in various proportions at

200 (Oji, 1998) and at 40 8C (Fuhrmann et al., 2002).

An immediate production of meta-cinnabar and cinna-

bar was observed. It is desirable that the various chem-

ical forms of mercury in the waste can be transformed

into sulphide at room temperature. Pyrite, pyrrhotite

and elemental sulphur would react with mercury

oxide to form mercury sulphide (Swedish EPA,

ent 368 (2006) 418–423

Table 2

Experimental matrix of samples after 37 months

Chemical formulas=products identified by XRD, n.n.p.=no new crys-

talline phase indicated by XRD, X%=approximate percentagew /w of

HgS formed, sem=analysis performed by SEM and EDX, and red.=re-

duction of HgO to elemental Hg was observed.

M. Svensson et al. / Science of the Total Environment 368 (2006) 418–423 419

1997). In the case of elemental mercury, reaction with

elemental sulphur seems to be feasible. However, inter-

action between Hg (II) and partly oxidized pyrite may

also be due to the formation of surface complexes

between mercury and either the pyritic functional

groups or the Fe(III) oxyhydroxide sites (Ehrhardt et

al., 2000). These experiments were made under aerobic

conditions, which is not ideal since the Eh/pH stability

area of HgS corresponds to an anaerobic environment

(Andersson, 1979; Swedish EPA, 1997). The reactions

between pyrite grains sampled from pulps containing

silver or mercury ions under anaerobic conditions were

studied by voltammetry (Perdicakis et al., 1999). Pyrite

pulp or massive pyrite electrodes were conditioned

during two weeks with Hg2+ in a HClO4 medium. It

was assumed that the more labile species of the sorbed

mercury involved Hg–O bonds and the more stable

ones involved Hg–S bonds. No reduction of mercuric

ions to elemental mercury by pyrite was observed.

Long-term investigations (several years) of reactions

in mercury–sulphur systems have not been reported.

The objective of this study was to assess suitable

conditions at room temperature for the long-term gen-

eration of solid HgS from HgO or elemental Hg by

mixing with a suitable sulphur source.

2. Materials and methods

2.1. Materials and experimental procedure

Elemental mercury (Merck) and yellow mercury(II)

oxide (Aldrich) were mixed with elemental sulphur

(Merck) or iron sulphide (natural pyrite crystals FeS2(Kvarntorp Sweden), troilite FeS (Merck)) under vari-

ous conditions (dry and wet; neutral/acidic and alkaline

pH; aerobic and anaerobic, see Table 1). Milli-Q (Milli-

pore) water was used to wet samples and Ca(OH)2(Merck) was added to generate high pH conditions

(and to simulate the pH-buffering effect of cement or

concrete). The ingredients of each sample corresponded

to a S /Hg mole ratio of 1.5. Pyrite and troilite were

crushed into grains less than 1 mm and subsequently

ground in a mill. The constituents (gram quantities)

were transferred to 5-ml glass vials, sealed, mixed

and stirred. The anaerobic vials were cautiously filled

Table 1

Components and conditions of the samples

Sulphur source Mercury source

Sulphur, sublimated, S0 (+99%) Powdered yellow HgO (+99%

Synthetic troilite, fused sticks, FeS Elemental mercury (99.95%)

Natural pyrite crystals, FeS2

with nitrogen gas before they were sealed by melting,

whereas the vials for the aerobic tests were only loosely

sealed with a screw cap. The parameters in Table 1 gave

36 combinations. Samples of each combination were

thoroughly mixed in a shaker and stored in the dark at

room temperature (19–21 8C). Three identical samples

of each combination were prepared which gave the

opportunity to analyse at different times from start for

a period of up to 37 months. Thus, any formation of

HgS under well-defined conditions, starting with well-

defined solid sulphur and mercury phases, could be

demonstrated. The aerobic test tubes were stored in

separate plastic boxes with high or low humidity,

depending on whether the test tubes were wet or dry

inside. The samples were thoroughly mixed in a shaker

once a year.

2.2. Analytical procedures

The various phases present in the samples were

characterized by X-ray powder diffractometry (Philips

XRD, PW1729 with evaluation program Philips

Condition Atmosphere

) Dry Aerobic

Wet and neutral/acidic Anaerobic

Wet and alkaline, Ca(OH)2 (pa)

M. Svensson et al. / Science of the Total Environment 368 (2006) 418–423420

APD1700 (Vax)). The analyses were supplemented by

ocular microscopy (after each year) and by scanning

electron microscopy, SEM (Philips XL30 ESAM-FEG

equipped with an EDX detector). The samples were

analysed after 12 and 37 months except for samples E3

and E6 that were analysed after 24 and 37 months.

Measurement of pH was made after 37 months (on

wet samples) by a flat membrane combined pH glass

Fig. 1. X-ray powder diffractogram for selected samples in Table 2: A)

(metacinnabar) yield about 50%w /w, formation of the crystalline iron hydro

electrode, which was calibrated for measurement be-

tween pH 4 to 7 or 7 to 10.

3. Results

Initial and post-reaction composition and conditions

of the samples are given in Table 2. No visible reac-

tion had occurred after one year for sample E3 and E6,

F6–HgS (cinnabar, metacinnabar) yield about 100%. B) D1–HgS

xide goethite (a-FeOOH).

M. Svensson et al. / Science of the Total Environment 368 (2006) 418–423 421

so analysis was delayed until two years from start for

these samples. In seven cases (A1, C1, C3, C4, C6,

E2, and E4) the mercury had coalesced and mercury

beads were observed at the time of analysis. Quanti-

tative analysis was made whenever mercury sulphide

was indicated by XRD. By comparing the XRD in-

tensity from a synthetic sample with known HgS

concentration, the weight fraction of HgS was estimat-

ed. Formation of mercury sulphide, with various

yields, was confirmed in 14 of the samples. Both

red cinnabar and black meta-cinnabar were detected;

the sum of the two forms after 37 months is given in

Table 2. Almost 100% and 80%w/w red cinnabar were

formed in the anaerobic systems at high pH in the

presence of elemental sulphur (F6 and E6, respective-

ly). Mercury sulphide was formed predominantly

under alkaline conditions. Analyses performed after

12 and 37 months (24 and 37 months for samples

E3 and E6) gave similar result for the sum of red

cinnabar and black meta-cinnabar. Possibly there was

a slight difference in the distribution between the two

forms, with a higher fraction of cinnabar after 37

months.

Fig. 1A shows a diffractogram that demonstrates

the formation of cinnabar and meta-cinnabar (sample

F6, Table 2). Gypsum (CaSO4d 2H2O) was formed in

some of the samples, predominantly under aerobic/

alkaline conditions (see Table 2). Goethite (aFeOOH)

was detected in the wet sample of HgO and FeS at

ambient pH (D4 and D1, Fig. 1B and Table 2). Traces

of HgS were observed in the alkaline, anaerobic wet

sample containing elemental Hg and pyrite (F5).

Measured pH of the samples is given in Table 3. The

pH decrease in the aerobic/alkaline samples is probably

governed by equilibrium of the Ca–CO2–H2O system.

Table 3

pH of wet samples after 37 months

The acidification in the aerobic FeS2 system is expected

and related to the formation of H2SO4 through oxida-

tion of the sulphide.

The SEM pictures in Fig. 2A and B show elemental

sulphur grains in various mercury matrices. Initial com-

ponents in Fig. 2A were Hg(0) mixed with S(0). At the

time of analysis a small amount of HgS had been

formed as a surface coating on the sulphur grain. Fig.

2C shows a pyrite-covered mercury bead. The picture

in Fig. 2D confirms the XRD analyses in Table 2 (D5).

Pyrite grains and mercury oxide remain intact without

apparent reaction after 37 months.

From the SEM pictures it is obvious that mercury

had partially been reduced to its elemental form in some

samples initially containing HgO (Fig. 2E and F).

Reduction of HgO to Hg(0) was observed in about 6

samples (D2, D4, F1, F2, F4, and F5).

4. Discussion

Minor amounts of HgS were formed in the dry

sulphur–elemental mercury samples, but not indicated

in the other dry samples. In wet, neutral/acidic samples

containing HgO and (FeS), both HgS and a-FeOOH

were formed, but no reaction occurred in samples con-

taining S(0). Neither dry nor wet and neutral/acidic

samples containing pyrite led to formation of new

crystalline phases.

For samples containing FeS or FeS2, an oxidation of

Fe(II) to Fe(III) occurs in aerobic systems, leading to

the formation of amorphous brustQ (samples C1, D1, E1

and F1), as well as goethite (D1 and D4). No formation

of Hg(II) species by oxidation of Hg(0) was indicated

from SEM pictures or X-ray powder diffractograms

(C1, C2, C4 and C5). This means that in systems

containing elemental Hg and Fe(II) an oxidizing agent

other than oxygen is needed for the formation of HgS.

In aerobic, wet, neutral/alkaline systems an oxidation of

iron but not of elemental mercury was observed (C1).

The experimental results indicate that both iron phases

predominantly act as reducing agents for mercury

oxide, in aerobic as well as anaerobic systems (D2,

D4, F1, F2, F4, and F5).

Alkaline conditions clearly influence the forma-

tion of HgS (Table 2). In several of the alkaline

samples, HgS was formed in various yields—from

traces to almost 100%. Highest yields were observed

for the samples containing S(0), where both elemen-

tal Hg and HgO were transformed into HgS. Sul-

phur may disproportionate into S(� II) and S(VI)

under anaerobic conditions, which lead to more

efficient sulphide generation whereas aerobic condi-

Fig. 2. SEM-pictures for selected samples in Table 2: A) Sulphur grain covered with mercury sulphide (A3). B) Sulphur grain embedded in mercury

oxide (B3). C) Pyrite-covered mercury bead (C5). D) Pyrite grains embedded in mercury oxide (D5). E) Sample initially containing HgO and pyrite

that has been reduced to elemental mercury at alkaline pH under anaerobic conditions (F5). F) Sample initially containing HgO and pyrite that has

been reduced to elemental mercury at acidic pH under aerobic conditions (D2). Samples analysed after 37 months.

M. Svensson et al. / Science of the Total Environment 368 (2006) 418–423422

tions lead to increased oxidation and content of

sulphate, e.g., through;

4SðsÞ þ 3Hg0 þ 3=2O2 þ H2OX 3HgS þ SO2�4 þ 2HþDG0 ¼ �645kJ=mol

ð1Þ

4SðsÞ þ 3HgO þ H2OX 3HgS þ SO2�4 þ 2HþDG0 ¼ �471kJ=mol ð2Þ

Reactions (1) and (2) are both thermodynamically

favourable (Svensson et al., in preparation). The stoi-

chiometric ratio between Hg and S is important for

optimal formation of HgS. In the experiments it was

1.5 but 1.3 would be ideal (see reactions (1) and (2)).

The reactions indicate that oxidation of sulphur leads to

a pH decrease and an alkaline environment favours the

formation of HgS since deficiency of hydrogen ions

forces the reactions (1) and (2) to the right. Excess of

sulphur in the system would lead to a risk for formation

of soluble mercury disulphide (HgS22�) (Conner, 1990;

M. Svensson et al. / Science of the Total Environment 368 (2006) 418–423 423

Swedish EPA, 1997). This reaction is favoured by

alkaline conditions (see reactions (3) and (4)) similar

to reactions (1) and (2) (see above). The selected ratio

between Hg and S should be low (close to 1.3) in order

to reduce the formation of HgS22� (Svensson et al., in

preparation).

3SðsÞ þ HgO þ 1=2O2 þ 2H2OX HgS2�2 þ SO2�4

þ4HþDG0 ¼ �236kJ=mol ð3Þ

3SðsÞ þ Hg0 þ O2 þ 2H2OX HgS2�2 þ SO2�4

þ4HþDG0 ¼ �292kJ=mol ð4Þ

5. Conclusions

It is feasible to create an environment at room tem-

perature that, with time, leads to the generation of HgS

from Hg(0) as well as from HgO. Elemental sulphur in

an alkaline anaerobic environment (corresponding to the

long-term conditions in a closed repository containing,

e.g., cement/concrete) gave a yield of close to 100%

mercury sulphide already within two years. Minor

amounts of HgS were observed also under aerobic con-

ditions (corresponding to the initial open state of a

repository with presence of air). Thus, design of a

repository for permanent geologic disposal of mercury

is feasible and a cementitious wet anaerobic environ-

ment appears to be favourable. Choosing wrong com-

ponents and conditions can, however, lead to a reduction

of Hg(II) to elemental mercury.

Acknowledgements

The Knowledge Foundation, the SAKAB Kumla

Environmental Foundation and SAKAB AB supported

this work, which are gratefully acknowledged.

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