Slag dusts from Kabwe (Zambia): Contaminant mineralogy and oral bioaccessibility

Abstract

The former Pb-Zn mining town of Kabwe in central Zambia is ranked amongst the worst polluted areas both in Africa and in the world. The fine dust particles from the ISF and Waelz slags deposited in Kabwe represent a health risk for the local population. Here, we combined a detailed multi-method mineralogical investigation with oral bioaccessibility testing in simulated gastric fluid (SGF; 0.4 M glycine, pH 1.5, L/S ratio of 100, 1 hour, 37 °C) to evaluate the risk related to the incidental dust ingestion. The slag dust fractions contain up to 2610 mg/kg V, 6.3 wt.% Pb and 19 wt.% Zn. The metals are mainly bound in a slag glass and secondary phases, which formed during the slag weathering or were windblown from nearby tailing stockpiles (carbonates, Fe and Mn oxides, phosphates, vanadates). The bioaccessible fractions (BAFs) are rather high for all the main contaminants, with the BAF values generally higher for the ISF slags than for the Waelz slags: Pb (24-96%), V (21-100%) and Zn (54-81%). The results clearly indicate the potential risks related to the incidental slag dust ingestion. Even when a conservative value of the dust daily intake (100 mg/day) is considered, the daily contaminant intake significantly exceeds the tolerable daily intake limits, especially for Pb>>V>Zn. At higher ingestion rates, other minor contaminants (As, Cd) also become a health risk, especially for children. The slag heaps in Kabwe should be fenced to prevent local people entering and should be covered to limit the dust dispersion.

Full text

Slag dusts from Kabwe (Zambia): Contaminant mineralogy and oral
bioaccessibility
Vojt
ech Ettler
a
,
*
, David 
St
ep
anek
a
, Martin Mihaljevi
c
a
, Petr Drahota
a
, Radim Jedlicka
b
,
Bohdan K
ríbek
c
, Ale
s Van
ek
d
, Vít Pení
zek
d
, Ondra Sracek
e
, Imasiku Nyambe
f
a
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 00, Prague 2, Czech Republic
b
Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Albertov 6, 128 00, Prague 2, Czech Republic
c
Czech Geological Survey, Geologick
a 6, 152 00, Prague 5, Czech Republic
d
Department of Soil Science and Soil Protection, Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences Prague, Kamýck
a 129,
165 00, Prague 6, Czech Republic
e
Department of Geology, Faculty of Science, Palacký University in Olomouc, 17. Listopadu 12, 771 46, Olomouc, Czech Republic
f
Department of Geology, University of Zambia, School of Mines, P. O. Box 32379, Lusaka, Zambia
highlights graphical abstract
Slag dusts from Kabwe (Zambia)
contain high levels of contaminants.
Metals bound in slag glass, carbon-
ates, oxides and phosphates/
vanadates.
Bioaccessible fractions up to 96% (Pb),
100% (V) and 81% (Zn).
High health risk even under conser-
vative dust intake scenario (100 mg/
day).
article info
Article history:
Received 6 May 2020
Received in revised form
29 June 2020
Accepted 4 July 2020
Available online 11 July 2020
Handling Editor: Lena Q. Ma
Keywords:
Metal(loid)s
PbeZn slag Dusts
Mineralogy
Bioaccessibility
Kabwe
abstract
The former PbeZn mining town of Kabwe in central Zambia is ranked amongst the worst polluted areas
both in Africa and in the world. The fine dust particles from the ISF and Waelz slags deposited in Kabwe
represent a health risk for the local population. Here, we combined a detailed multi-method mineral-
ogical investigation with oral bioacce ssibility testing in simulated gastric fluid (SGF; 0.4 M glycine, pH 1.5,
L/S ratio of 100, 1 h, 37

C) to evaluate the risk related to the incidental dust ingestion. The slag dust
fractions contain up to 2610 mg/kg V, 6.3 wt% Pb and 19 wt% Zn. The metals are mainly bound in a slag
glass and secondary phases, which formed during the slag weathering or were windblown from nearby
tailing stockpiles (carbonates, Fe and Mn oxides, phosphates, vanadates). The bioaccessible fractions
(BAFs) are rather high for all the main contaminants, with the BAF values generally higher for the ISF
slags than for the Waelz slags: Pb (24e96%), V (21e100%) and Zn (54e81%). The results clearly indicate
the potential risks related to the incidental slag dust ingestion. Even when a conservative value of the
dust daily intake (100 mg/day) is considered, the daily contaminant intake significantly exceeds the
tolerable daily intake limits, especially for Pb [V>Zn. At higher ingestion rates, other minor con-
taminants (As, Cd) also become a health risk, especially for children. The slag heaps in Kabwe should be
fenced to prevent local people entering and should be covered to limit the dust dispersion.
©2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
*Corresponding author.
E-mail address: ettler@natur.cuni.cz (V. Ettler).
Contents lists available at ScienceDirect
Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere
https://doi.org/10.1016/j.chemosphere.2020.127642
0045-6535/©2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/
).
Chemosphere 260 (2020) 127642

1. Introduction
The mining and processing of metal ores generates large
amounts of dust particles especially in semi-arid or arid areas
(Ghorbel et al., 2010;Plumlee and Morman, 2011;Ettler et al.,
2019). These dust particles cause not only soil and crop contami-
nation (Ettler, 2016 and references therein), but can have a direct
effect on the staff working in the ore mining/processing industry
and/or populations living in the adjacent areas through the inci-
dental dust ingestion and inhalation (Plumlee and Morman, 2011;
Ettler et al., 2012,2014;Boisa et al., 2013;Cheyns et al., 2014;
Entwistle et al., 2019).
Kabwe, in central Zambia, has been repeatedly ranked amongst
the 10 worst polluted places on Earth (Blacksmith Institute and
Green Cross Switzerland, 2013) and, due to this reason, it has
attracted much media attention over the years (Branan, 2008;
Carrington, 2017;Filippelli et al., 2020). Lead (Pb) and zinc (Zn)
mining and processing in Kabwe lasted for almost ninety years
(1906e1994) and left a huge legacy of contaminated land. Soil
pollution originating primarily from the smelter dust was first re-
ported by Tembo et al. (2006) and indicated that the most
contaminated areas were located downwind of the mine and
smelter area. More recent studies have revealed that the concen-
trations of Pb and other contaminants in the Kabwe soils were very
high. Nakayama et al. (2011) reported that the Pb and Zn levels
attained 51 g/kg (median: 282 mg/kg) and 92 g/kg (median:
607 mg/kg), but the concentrations of other contaminants (As, Cd,
Cu) were also elevated. Bose-O’Reilly et al. (2018) compiled soil Pb
concentration data from previous screening projects and their own
measurements in the Kabwe townships adjacent to the former
mine area (n ¼339), found that the Pb concentration range was
139e62,142 mg/kg (geometric mean: 1470 mg/kg) and concluded
that in 25% of the samples, the Pb concentrations were >400 mg/kg,
being higher than the recommended tolerable soil Pb level. Our
team has recently published a detailed geochemical survey in
Kabwe and the soil data (topsoils: n ¼116; subsurface soils: n ¼40)
were used for the determination of the spatial distribution of the
major contaminants in the area (K
ríbek et al., 2019). In agreement
with previous studies, it was found that the highest contamination
was found near the former mine area and downwind (the Kasanda
and Makululu townships) with concentrations up to 41 g Pb/kg and
68 g Zn/kg (K
ríbek et al., 2019). In line with these soil-screening
studies, numerous researchers, especially Zambian-Japanese
teams, have reported the intoxication of local inhabitants based
on the high blood Pb levels (BLL; up to 428
m
g/dL highly exceeding
the maximum recommended value of 5
m
g/dL) (Yabe et al., 2015,
2020;Bose-O’Reilly et al., 2018) and in the urine and feces (Yabe
et al., 2018). In addition, the high levels of contaminants were
also found in domestic animals (cattle, goats, chicken) (Ikenaka
et al., 2012;Yabe et al., 2011,2013;Nakata et al., 2016) and small
mammals (wild rats) (Nakayama et al., 2011).
Lead exposure in Kabwe has been attributed to highly contam-
inated soils (affected mainly by the historical smelter emissions)
and dust particles dispersed from the historical mine area. The
bioaccessible testing indicated that Pb is the most important
contaminant, which can be leached out from the incidentally
ingested contaminated soil from Kabwe (Plumlee and Morman,
2011;K
ríbek et al., 2019). To decrease the Pb bioaccessibility/
bioavailability from the Kabwe soils, several measures such as the
chemical stabilization of the soil using phosphate (leading to the
formation of insoluble Pb-phosphate minerals) have been proposed
(K
ríbek et al., 2019). Since 2005, non-governmental organizations
have initiated the replacement of the contaminated soils in the
most polluted Kabwe townships with clean soil coming from
elsewhere (Carrington, 2017).
Apart from the soil contamination, the major source of
contaminated dust particles - the former mine area - is still there.
To mitigate the continuous formation of dust susceptible to wind
erosion and transport, the phytostabilization of the slag heaps us-
ing contaminant-tolerant species was suggested almost 20 years
ago (Leteinturier et al., 2001); however, the dump surfaces still
remain largely uncovered today (Fig. 1). Despite very promising
remediation techniques based on solidification and biocementation
processes, which have been recently tested on the Kabwe wastes
(Mwandira et al., 2019a,b), the area is partly unfenced and is still
accessible to locals. Adults dig for Pb on one of the old slag heaps
(called “Black Mountain”), children play there (Fig. 1) and become
directly exposed to the slag dusts with potentially high levels of Pb
and other metal (loid)s (Carrington, 2017).
The potential risk for human health related to this Kabwe slag
dust has not been quantified yet. Our previous investigation from
mining areas in northern Namibia underlined the key role of the
solid speciation of metal(loid)s on their extractability and bio-
accessibility from dust materials originating from the mining and
smelting activities (Ettler et al., 2019). Using a similar approach, the
aim of this study is to fill this gap and, for the first time, evaluate the
risk related to the incidental ingestion of fine slag dust particles
from Kabwe using a combination of (i) a detailed mineralogical
investigation and partitioning of the inorganic contaminants in the
slag dusts and (ii) oral bioaccessibility testing in simulated gastric
fluids.
2. Materials and methods
2.1. Site description
Kabwe is the capital of Central Province, located ca.110 km north
of Lusaka, the capital of Zambia. The population is approximately
220 000 (2010 census), the climate is mild and generally warm and
corresponds to Cwa according to K€
oppen-Geiger classification, with
average temperature of 20.6

C and 945 mm of annual precipitation
with a strong seasonal distribution (www.climate-data.org).
The PbeZn deposit in the Kabwe area (formerly known as
Broken Hill) was discovered in 1902 and was mined from 1906 until
1994, when most of the massive sulfide ores reserves were
exhausted. The deposit is formed in the carbonate rocks with pri-
mary mineralization composed of massive sulfides and the sec-
ondary mineralization composed of quartz (SiO
2
), willemite
(Zn
2
SiO
4
), cerussite (PbCO
3
), smithsonite (ZnCO
3
), goethite
(FeOOH), hematite (Fe
2
O
3
) and metal-bearing phosphates, vana-
dates and arsenates (Kamona et al., 1999;Kamona and Friedrich,
2007;Mondillo et al., 2018).
The early-years smelting technologies in Kabwe were based on
blast furnace smelting for the Pb recovery from the carbonate ores
followed later by the recovery of Zn by electrolysis from the pre-
dominantly Zn silicate ores (Barlin, 1972). Following the exploita-
tion of the sulfide orebody after 1938, the Imperial Smelting
Furnace (ISF) was installed in 1962 to process the primary sulfide
PbeZn ores (Barlin, 1972) and the Waelz process was installed in
1977 to process the non-sulfide ores. Both technologies (see tech-
nological details in Sinclair, 2005) were in operation until the mine
closure in 1994. Large amounts of slag were left on the dumps in the
vicinity of the former mine known today as the Sable Zinc Kabwe
Recovery site. The ISF slag tonnage has been estimated to be ca.
1482 kt grading 8.07 wt% Zn and 1.22 wt% Pb (BMR Group Plc,
2020). The amount of the Waelz slag is slightly lower with ca.
1105 kt grading 3.64 wt% Zn and 1.52 wt% Pb (BMR Group Plc,
2020). Both types of slags were granulated (water-quenched) dur-
ing the tapping and are mostly composed of fragments smaller than
5mm(Fig. 1b and c); however, larger rounded pellets of the Waelz
V. Ettler et al. / Chemosphere 260 (2020) 1276422

slags are also occasionally encountered (Fig. 1c).
2.2. Sample collection and processing
The representative grab samples of the slags, each weighing
2 kg, were collected on the slag dumps in the former mine area (ISF
slags: K1 and K3; Waelz slags: K5 and K7) (Fig. 1a). The samples
were air-dried and homogenized before further processing.
Whereas for the hand-to-mouth incidental soil ingestion, a
<250
m
m fraction has been traditionally used for the bio-
accessibility testing (e.g., US EPA, 2007;Cave et al., 2011), other
systematic investigations demonstrated that the size fractions
adhering to the hands are much smaller (e.g., Yamamoto et al.,
2006;Siciliano et al., 2009). Siciliano et al. (2009) suggested that
a 45-
m
m sieve should be used as the cut-off mesh size for the
sample preparation before the oral bioaccessibility testing in the
simulated gastric fluid (SGF). For this reason, we used a polyamide
sieve with the closest nominal mesh of 48
m
m (UHELON 120T, Silk &
Progress, Czech Republic) for the preparation of a <48
m
m dust
fraction. Moreover, a <10
m
m fraction (PM
10
) of the dusts was ob-
tained by dry sieving through a SEFAR NITEX 03e10/2 polyamide
sieve (SEFAR AG, Switzerland; nominal mesh of 10
m
m) to only
obtain particles that could potentially enter the airways. The
<48
m
m fraction accounted for 2.0 ±0.3% (ISF slag) and 8.5 ±2.5%
(Waelz slag) of the total sample weight. The <10
m
m fraction cor-
responded to 0.55 ±0.03% (ISF slag) and 1.1 ±0.2% (Waelz slag) of
the sample.
The aliquot parts of original slags and <48
m
m and <10
m
m slag
dust fractions were ground to an analytical fineness in an agate
mortar (Retsch PM 400 planetary mill) and subsequently used for
Fig. 1. (a) An aerial photo of the slag disposal sites and former mine area in Kabwe, Zambia (source: 2-D Google Earth); (b) The ISF slag deposit with detail of the granulated slag
material; (c) The Waelz slag deposit with details of the granulated slag material with pellets of variable size resulting from the smelting in a rotary kiln; (d) Children playing on the
slag dumps, note the dust adhering to their hands and legs.
V. Ettler et al. / Chemosphere 260 (2020) 127642 3

the determination of the bulk chemical and mineralogical
compositions.
2.3. Chemical and mineralogical investigations
The bulk chemical compositions of the slag samples were
determined after digestion in mineral acids (HClO
4
, HF, HNO
3
) ac-
cording to Ettler et al. (2009). The concentrations of As, Cd, Co, Cr,
Cu, Ni, Pb, Sb, V and Zn in the digests were determined using either
inductively coupled plasma optical emission spectrometry (ICP-
OES, Agilent 5110) or inductively coupled plasma mass spectrom-
etry (ICP-MS, ThermoScientific, iCAP™).
The bulk mineralogy was determined by X-ray diffraction
analysis (XRD; PANalytical X’Pert Pro diffractometer, CuK
a
radia-
tion, 40 kV, 30 mA, 2 theta range of 2e80

, step of 0.02

, counting
time of 150 s per step). The diffraction patterns were analyzed using
the X’Pert HighScore Plus 3.0 software coupled with the Crystal-
lography Open Database (COD) (Gra
zulis et al., 2012).
The slag dust samples (only fraction <48
m
m) were prepared as
polished sections and examined using and electron probe micro-
analyzer (EPMA; JEOL JXA-8530F) equipped with a field emission
gun electron source (FEG) for the scanning electron microscopic
(SEM) imaging and the energy dispersion spectroscopic (EDS) an-
alyses (spectrometer JEOL JED-2300F). We used the same instru-
ment for the quantitative chemical analyses of the individual
minerals. A total of ~230 EDS and ~160 EPMA spot analyses have
been performed. The analytical conditions, standards and detection
limits are given in Table S1.
2.4. Bioaccessibility testing and exposure calculations
We performed the oral bioaccessibility tests in a simulated
gastric fluid (SGF), because the incidental ingestion of the dust
particles corresponds to a major exposure route for the local pop-
ulation (Fig. 1d). This is valid for the dust fraction adhering to hands
(<48
m
m), as well as for the <10
m
m fraction, presumably entering
the lung compartments, because it has been demonstrated that
larger inhaled particles (~4e10
m
m) are cleared from the airways by
the mucociliary escalator, swallowed and transported into the
gastrointestinal tract and only particles <4
m
m can access the deep
lung (e.g., Brown et al., 2013;Boisa et al., 2014;Kastury et al., 2017).
Another reason for performing the oral bioaccessibility tests only is
the fact that the dissolution of the dust materials in the simulated
lung fluid (SLF), which exhibits a pH close to neutrality, isgenerally
significantly lower than that for the highly acidic SGF (see e.g.,
Plumlee and Morman, 2011;Ettler et al., 2014). As a result, the oral
bioaccessibility tests can be used to predict the “worse-case
scenario”.
The extractions were performed according to the US EPA’s
(2007) protocol known in literature as SBRC-G (gastric phase
extraction using the Solubility Bioaccessibility Research Con-
sortium assay). We selected this test because of its simplicity,
straightforwardness and suitability for Pb (being the major
contaminant in our case) and its previous successful validation
using in vivo experiments (US EPA, 2007;Deshommes et al., 2012).
The slag dust sample was extracted in a 0.4 M glycine solution
adjusted to pH 1.5 ±0.05 by HCl (reagent grade, Merck) at an L/S
ratio of 100. The mixture was gently agitated for 1 h at 37

C in a GFL
3032 incubator. The pH value was regularly checked to verify any
potential drifts larger than 0.5-pH units, which was not the case for
our samples. The pH and Eh of the final solution were recorded
using a WTW Multi 3620 IDS multimeter equipped with a SenTix®
940 pH electrode and a SenTix®ORP-T 900-P redox electrode. The
final pH was in the range of 1.63e1.82 and the final Eh in the range
of 660e873 mV. The obtained extracts were filtered through a 0.45-
m
m membrane filter, diluted and analyzed for the major cations and
contaminants such as, Cd, Co, Cr, Cu, Ni, Pb, Sb, V and Zn by ICP-OES
and/or ICP-MS. Subsequently, the bioaccessible concentrations of
the metal (loid)s were expressed in mg/kg and converted to the
bioaccessible fraction (BAF; percent amount of the total contents).
The daily contaminant intakes have been calculated as follows:
DI ¼ðBC=1000Þ
*
DIR
where DI is the daily intake of a given contaminant (in
m
g/day), BC is
the bioaccessible concentration obtained by the extraction tests in
SGF (in
m
g/g or mg/kg), DIR is the dust ingestion rate (in mg/day)
and 1000 is the unit conversion factor.
Given the fact that the Kabwe former mine area is unfenced and
accessible to the local inhabitants, including people who re-mine
the old dumps, the obtained daily intakes were compared with
tolerable daily intake (TDI) limits calculated for children (weighing
10 kg) and adults (weighing 70 kg), who are all considered potential
targets (Fig. 1;Blacksmith Institute and Green Cross Switzerland,
2013;K
ríbek et al., 2019). For the basic calculations, we used an
ingestion rate of 100 mg/day, a rather conservative value generally
used for children in this kind of exposure assessment (Ghorbel
et al., 2010;Bierkens et al., 2011). However, a very recent investi-
gation of dust ingestion rates in the mining areas of the Democratic
Republic of Congo by Smolders et al. (2019) indicated that the dust
ingestion rates in sub-Saharan African countries could be much
higher. As a result, in addition to the basic exposure calculations, we
used exposure scenarios with dust ingestion rates of 280 mg/day
(geometric mean for all the data set) and 1700 mg/day (average
value for children, age 4e15 years) (Smolders et al., 2019). For the
sake of consistency, the TDI limits were taken from Baars et al.
(2001) and Tiesjema and Baars (2009) (
m
g/kg
bw
/day): As (1), Cd
(0.5), Co (1.4), Cr (5), Cu (140), Ni (50), Pb (3.6), Sb (6), V (2) and Zn
(500) (bw ¼body weight). This has been undertaken despite the
fact that the European Food Safety Authority (EFSA) has recently
lowered the TDI for Cd to 0.36
m
g/kg
bw
(EFSA, 2009) and suggested
that the provisional tolerable weekly intake (TWI) limits for Pb (15
m
g/kg
bw
) and As (15
m
g/kg
bw
) are no longer appropriate due to the
toxicity of these elements (EFSA, 2010;EFSA, 2014).
2.5. Data processing and QC/QA
The Prism 6 software (GraphPad, USA) was used for plotting the
data and for the statistical data treatment. The speciation-solubility
modeling was carried out using the PHREEQC-3 geochemical code,
version 3.3 with a Notepadþþ 6.6.9 interface and a llnl. dat ther-
modynamic database (Parkhurst and Appelo, 2013).
The extractions were performed in duplicate and with the cor-
responding procedural blanks. Certified reference materials (CRMs)
were used to ensure the accuracy of the digestion procedure and
the subsequent analysis of the digests and leachates (CCU-1e, CZN-
4, NIST 1640). The quality control/quality assurance (QC/QA) results
are reported in Table S2; they indicate good agreement between the
measured and certified values (the recoveries were between 91 and
102%).
3. Results
3.1. Metal(loid) bioaccessibility from the slag dusts
The concentrations of the metal (loid)s in the original slags and
slag dust fractions <48
m
m and <10
m
m are reported in Table 1 (key
contaminants: Cd, Pb, V, Zn) and Table S3 (all data sets). With some
exceptions, probably related to their mineralogical form, the metal
(loid) concentrations in the fine slag dust fractions are generally
V. Ettler et al. / Chemosphere 260 (2020) 1276424

higher than in the original slag samples (Tables 1 and S3). The
average concentrations of the key contaminants (Cd, Pb, V, Zn) in
the fine dust fractions are 1.8e2.8 higher for the ISF slags than for
the Waelz slags (Table 1). Moreover, the metal(loid) concentrations
are significantly higher in the <10
m
m slag dust fractions than in the
<48
m
m slag dust fractions for both slag types (Wilcoxon matched-
pairs signed rank test, P <0.05).
The bioaccessible concentrations of the metal (loid)s obtained
by the extractions in the SGF are reported in Table 2 (main con-
taminants: Cd, Pb, V, Zn) and Table S5 (all data sets). The main
contaminants’bioaccessible concentrations were also higher in the
ISF slags than in the Waelz slags and varied as follows:
10.4e72.6 mg Cd/kg, 9762e60,439 mg Pb/kg, 424e2195 mg V/kg
and 14,042-154,8622 mg Zn/kg. The BAFs accounted for 42e82%
(Cd), 24e96% (Pb), 21e100% (V) and 54e81% (Zn) and were also
systematically higher for the ISF slag dusts (Tables 2 and S5). The
bioaccessible concentrations and BAF values were statistically
higher for the <10
m
m fractions than the <48
m
m fractions (Wil-
coxon matched-pairs signed rank test, P <0.05).
3.2. Mineralogical transformations during the bioaccessibility tests
The solid speciation of the metal (loid) contaminants in the slag
dusts is rather complex and significant changes in the mineralog-
ical composition occurred during the extractions in the SGF in all
the samples (Fig. S1). The slag glass fragments are ubiquitous in all
the studied slag samples (Fig. 2) and are highly enriched in Zn and
Pb (up to 48.6 wt% ZnO and 11.7 wt% PbO), but also contain some Cu
(up to 1.00 wt% CuO) and V (up to 0.51 wt% V
2
O
3
)(Table S4). The
elevated background in the XRD patterns (Fig. S1), mainly corre-
sponding to the slag glass, is not as pronounced in the dust residue,
which means that the metal-bearing slag glass has been substan-
tially dissolved during the extraction tests. The primary glass
fragments are partly or completely weathered particles to Zn-
bearing clay-like minerals (e.g., sauconite, Na
0$3
Zn
3
(Si,A-
l)
4
O
10
(OH)
2
$4H
2
O) or hemimorphite-like phases
(Zn
4
Si
2
O
7
(OH)
2
$2H
2
O) (Fig. 2;Table S4). Primary Zn-bearing sili-
cates (olivine, melilite, willemite) and oxides (spinels, wüstite,
zincite) are rather rare (Fig. 2b; Table S4), despite the fact that they
have often been reported in PbeZn slags from other sites (Ettler
et al., 2001,2019,2020;Puziewicz et al., 2007;Piatak and Seal,
2010;Vanaecker et al., 2014;Yin et al., 2016). The slag dusts also
contain large amounts geogenic particles of angular shapes (quartz,
micas, carbonates), probably windblown from the geological en-
vironments, soils or nearby disposal sites for the ore processing
wastes (tailings, slimes or leach plant residues) (Figs. 2 and S1).
Carbonates (calcite, dolomite) are metal-rich, with up to 18.7 wt%
PbO and 8.31 wt% ZnO (Table S4). All the studied slag samples
contain Fe(III) (hydr)oxides (goethite and/or lepidocrocite, FeOOH;
hematite, Fe
2
O
3
) as well as ferrihydrite-like phases (Figs. S1 and 2).
Especially, goethite and ferrihydrite are metal (loid)-rich (up to
14.9 wt% PbO, 4.46 wt% ZnO, 0.49 wt% CuO, 0.27 wt% V
2
O
5
and
0.75 wt% As
2
O
5
)(Table S4). Given the fact that metal-bearing
goethite and hematite were both reported from the Kabwe
oxidized ores (Kamona et al., 1999;Kamona and Friedrich, 2007;
Mondillo et al., 2018), it is impossible to determine whether these
phases are windblown from other materials deposited in the
former mine area or correspond to secondary weathering products
that formed on the PbeZn slags (e.g., Ettler et al., 2020). This am-
biguity is also valid for the complex vanadates and phosphates,
such as descloizite [(Pb,Zn)
2
(OH)VO
4
], pyromorphite [(Pb,Ca)
5
(-
PO
4
)
3
Cl] - vanadinite [Pb
5
(VO
4
)
3
Cl] series and corkite [PbFe
3
(PO
4
)
(SO
4
) (OH)
6
], which are also important metal (loid)-bearing hosting
phases in the slag dusts and might be of both origins (Fig. 2;
Table S4). The secondary Mn oxides of the birnessite group (ac-
cording to XRD) only occur in the ISF slags (Figs. 2b and S1) and
contain high levels of contaminants (up to 10.4 wt% PbO); in
contrast, the rarely encountered pyrolusite-like (MnO
2
) phases are
probably of geogenic origin and are metal-free (Table S4). Inter-
estingly, very few primary sulfides have been detected and except
for litharge (PbO) (Fig. 2b; Table S4), the other secondary carbon-
ates such as cerussite (PbCO
3
) or smithsonite (ZnCO
3
), frequent in
the original ores, have not been found in the slag dusts. No specific
Cd-bearing phases were found.
The carbonates, hemimorphite and gypsum (the latter found
only in the K5 sample) disappear from the XRD patterns during the
bioaccessibility extraction testing (Fig. S1). In contrast, quartz (and
cristobalite in the K7 sample) and phyllosilicates are relatively
stable during the extraction as well as the ubiquitous goethite and
hematite (and lepidocrocite in the K7 sample). This is also reflected
by the PHREEQC-3 calculations indicating that the saturation
indices for goethite and hematite are positive and these phases are,
thus, not susceptible to dissolution (Table S6). The manganese ox-
ides found in the ISF slag dusts generally dissolve during the
extraction in the SGF (Fig. S1). The presence of pyromorphite, not
found by XRD, but reported by SEM/EPMA in the original slag
samples (Figs. S1 and 2), appear in the XRD patterns of the slag dust
residues after the leaching tests. This fact indicates its high stability
even under highly acidic conditions (Fig. S1). Corkite is also very
stable during the extraction in the SGF. Interestingly, hydrozincite
[Zn
5
(CO
3
)
2
(OH)
6
] is found in the majority of the slag dust residues
after extraction in the SGF with major diagnostic peaks at d-values
of 6.83, 3.68 and 3.18 Å (2 theta values 13.0, 24.1 and 28.0

,
respectively) (Fig. S1). The reference hydrozincite pattern (96-900-
7482) has majord-values of 6.78, 3.67 and 3.16 Å, but Zachara et al.
(1989) reported that a secondary hydrozincite-like phase can have
Table 1
Concentrations (mg/kg) of Cd, Pb, V and Zn in the original slag samples and the slag dust fractions <48
m
m and <10
m
m. Other potentially toxic elements found in lower
concentrations are given in Table S3.
Sample/Element Cd Pb V Zn
K1 (ISF) bulk slag 9.1 8600 3000 81,100
<48
m
m 65.0 42,300 2240 108,700
<10
m
m 90.5 57,600 2190 154,300
K3 (ISF) bulk slag 7.2 7800 2350 79,000
<48
m
m 64.5 51,200 2410 129,300
<10
m
m 89.0 62,700 2420 190,300
K5 (Waelz) bulk slag 13.7 17,100 2690 43,000
<48
m
m 24.8 47,500 2410 82,500
<10
m
m 29.6 49,100 2610 102,600
K7 (Waelz) bulk slag 7.1 6700 2330 29,300
<48
m
m 24.5 18,400 848 25,900
<10
m
m 30.6 23,100 817 28,200
V. Ettler et al. / Chemosphere 260 (2020) 127642 5

d-values of 6.86, 3.67 and 3.18 Å, more similar to those observed in
our XRD patterns (Fig. S1). However, the precipitation of this phase
is only favorable under the circumneutral to alkaline conditions and
at a high CO
2
activity (Zachara et al., 1989;Preis and Gamsj€
ager,
2001); thus, it can hardly form as a secondary weathering prod-
uct in our slag dust residues. This is also confirmed by the
PHREEQC-3 thermodynamic calculations performed on our SGF
extracts, where all the potentially solubility-controlling phases
(except goethite and hematite) exhibit negative saturation indices
and tend to be dissolved (Table S6). Thus, we hypothesize that
hydrozincite has formed as a weathering product in the original
dust samples and, due to its low amount, has not been detected by
XRD and was overlooked during the SEM observations and subse-
quent EPMA analysis. Even through hydrozincite should be a
readily soluble phase under acidic conditions as shown by the
experimental dissolutions of pure compounds by Molina et al.
(2013), the persistence of this phase in our slag dust residues
seems to be (i) either determined by the kinetic constraints or (ii)
by its embedding in another phase.
3.3. Exposure estimates and human health implications
The potential exposure estimates are strictly dependent on the
amount of dust potentially ingested via the hand-to-mouth sce-
nario or via clearing the airways. The calculated daily intakes of Pb,
Zn and V are plotted in Fig. 3 and indicate that, at both exposure
scenarios of 100 and 280 mg/day, especially Pb is a key contami-
nant, which exceeds 27e470 the TDI values for both targets
(children and adults). The Zn intake exceeds the TDI values for
children mainly in the case of the ISF slag dusts and particularly for
Table 2
Bioaccessible concentrations (in mg/kg) and bioaccessible fractions (BAFs; in % of the total concentration) of Cd, Pb, V and Zn as obtained by the extraction in the SGF (mean;
n¼2). Values of less important potentially toxic elements are given in Table S5.
Sample/Element Cd Pb V Zn
mg/kg BAF (%) mg/kg BAF (%) mg/kg BAF (%) mg/kg BAF (%)
K1 (ISF) <48
m
m 45.8 71 31,314 74 2070 92 72,207 66
<10
m
m 71.9 79 52,769 92 2195 100 116,012 75
K3 (ISF) <48
m
m 51.2 79 47,294 92 1967 82 102,112 79
<10
m
m 72.6 82 60,439 96 1920 79 154,862 81
K5 (Waelz) <48
m
m 10.4 42 11,469 24 513 21 53,657 65
<10
m
m 16.1 54 14,464 29 559 21 72,837 71
K7 (Waelz) <48
m
m 15.1 62 9762 53 424 50 14,042 54
<10
m
m 21.2 69 13,549 59 523 64 16,467 58
Fig. 2. Scanning electron micrographs (SEM) in back-scattered electrons (BSE) of the Kabwe slag dusts (<48
m
m fraction). (a) The ISF slag (sample K1); (b) The ISF slag (sample K3);
(c) The Waelz slag (sample K5); (d) The Waelz slag (sample K7). Abbreviations: carbon ecarbonaceous particle (smelting furnace fuel residue), C eclay mineral [variable
composition], Cb ecarbonate [variable composition], Cor ecorkite [PbFe
3þ3
(PO
4
) (SO
4
) (OH)
6
], Cv ecovellite [CuS], Dol edolomite [CaMg(CO
3
)
2
], Fh eferrihydrite [simplified
composition: Fe(OH)
3
], Frk efranklinite [ZnFe
2
O
4
], Ghn egahnite [ZnAl
2
O
4
], Gth egoethite [FeOOH], Har ehardystonite [Ca
2
ZnSi
2
O
7
], Hem ehematite (Fe
2
O
3
), Hm ehemi-
morphite [Zn
4
Si
2
O
7
(OH)
2
$2H
2
O], Lep elepidocrocite [FeOOH], Lit elitharge [PbO], Ms emuscovite [KAl
2
(Si
3
Al)O
10
(OH,F)
2
], Po epyrrhotite [Fe
1-x
S], Pyr epyromorphite
[(Pb,Ca)
5
(PO
4
)
3
Cl], Qz equartz (SiO
2
), Sac esauconite [ideal formula: Na
0$3
Zn
3
(Si,Al)
4
O
10
(OH)
2
$4H
2
O], slag eglassy slag fragment, Van evanadinite [Pb
5
(VO
4
)
3
Cl], Wi ewillemite
[Zn
2
SiO
4
], Wue - wüstite [FeO].
V. Ettler et al. / Chemosphere 260 (2020) 1276426

the model calculation with a higher dust ingestion rate (by up to
8.7 )(Fig. 3;Table S5). The V intake exceeds the TDI value by up to
31 for children and by up to 4.4 for adults (Fig. 3;Table S5). The
intakes of metallic contaminants are significantly higher for the ISF
slag dust compared to the Waelz slag dust; the inter-comparisons
between the individual dust fractions indicate a slightly higher
intake for the <10
m
m slag fraction (Table S5). The intake of the
other contaminants (As, Co, Cr, Cu, Sb) is lower than the TDI values
except for Cd, whose calculated intake is relatively low for adults,
but exceeds the TDI values for children in the case of the ISF slag
dusts by up to 4.1 (Table S5).
The model calculation was also performed for the dust ingestion
rate of 1700 mg/day, which corresponds to the mean value of the
dust ingestion for children (4e15 years old) living in a mining area
of the DR Congo (Smolders et al., 2019) and represents an extreme,
but not impossible, exposure scenario. Such calculations indicate
that the metallic contaminant intake would exceed the TDI values
by up to 2850 (Pb), 187 (V), 53 (Zn) and 25 (Cd) and also
other minor contaminants, such as or Co, become important in a
cocktail of potentially harmful elements leached under gastric
conditions (Table S5).
4. Discussion
In abandoned mining areas, dust windblown from the uncov-
ered disposal sites of mine tailings and slags represent a key source
of soil pollution, but can also directly contribute to exposure of local
inhabitants and cause adverse health effects to humans (Ettler
et al., 2019;Entwistle et al., 2019). It has been demonstrated that
smelter-derived dusts and smelter-impacted soils generally exhibit
higher bioaccessible concentrations of metal (loid) contaminants
than those originating from the mining areas (Ettler et al., 2012,
2019;Boisa et al., 2013). The particle size distribution is an
important factor affecting the concentration levels and subsequent
leaching of the contaminants. In agreement with our data (Tables 1,
2, S3 and S5), other studies have reported similarly higher bulk
concentrations and bioaccessibilities for fine-grained slag dusts,
which are more reactive when exposed to leaching solutions
(Morrison and Gulson, 2007;Morrison et al., 2016). However, the
mineralogy of the contaminants in the studied material also in-
fluences their release. The PHREEQC-3 calculations indicate that,
from a thermodynamic point of view, the key metal (loid)s-hosting
phases tend to dissolve (Table S6) as also pointed out by Ghorbel
et al. (2010) for the SGF extracts from the mining wastes. Howev-
er, given the fact that the bioaccessibility tests are performed in the
time frame of 1 h, the dissolution kinetics also matters. In their
pioneering critical review, Ruby et al. (1999) showed that Pb car-
bonates, Pb oxides and Pb-bearing Mn oxides are highly soluble
under gastric conditions, whereas Pb-bearing Fe oxides and Pb
sulfates have much lower solubility, and galena (PbS) and pyro-
morphite are virtually insoluble. This was later confirmed by other
in vitro bioaccessibility studies performed on pure compounds
(Bosso and Enzweiler, 2008) and by in vivo investigations, which
measured the Pb uptake by swine fed with soils spiked by various
Pb-bearing phases (Casteel et al., 2006). These observations are in
line with our results showing that the primary Pb-bearing Mn ox-
ides have been dissolved during the extraction in the SGF to a
greater extent than Fe oxides and phosphates (pyromorphite and
corkite) (Fig. S1). Argyraki (2014) also found that pyromorphite and
corkite, minerals with very low solubility, are commonly found in
soils polluted by Pb mining and smelting and will contribute to the
bioaccessible Pb a little. Despite the fact that pyromorphite in our
slag samples probably originates from the local ore materials (and/
or is windblown from nearby tailing sites), it can also form
weathering rims on Pb-rich slag fragments as observed by Argyraki
(2014).
Experiments with pure Zn-bearing phases have been carried out
by Molina et al. (2013) and showed that hydrozincite, hemi-
morphite and zincite were practically fully dissolved under gastric
conditions, smithsonite to 72% and sphalerite only to 1.7% of the
total. In accordance with these results, our mineralogical in-
vestigations also show that hemimorphite has been completely
dissolved (Fig. S1). Despite the relatively high solubility of hydro-
zincite reported by Molina et al. (2013) for extraction in the SGF,
this phase was reported in most slag dust residues (Fig. S1).
Hydrozincite cannot form under highly acidic conditions due to
thermodynamic reasons (Table S6;Zachara et al., 1989;Preis and
Gamsj€
ager, 2001;Vanaecker et al., 2014); thus, we hypothesize
that it probably occurred in the original slag as a trace secondary
weathering product and did not fully dissolve during the bio-
accessibility test.
In smelting slags, soluble metal-rich phases (e.g., weathered
sulfides) are often embedded/encapsulated in less reactive silicate
Fig. 3. The calculated daily intake of Pb, Zn and V in
m
g/day, assuming a slag dust
intake of 100 mg/day and 280 mg/day and comparisons with the tolerable daily intake
(TDI) limits for a 10-kg child and a 70-kg adult, respectively. The TDI values were taken
from Baars et al. (2001) (
m
g/kg
bw
/day): Pb (3.6), Zn (500), V (2). The values obtained for
the fractions <48
m
m and <10
m
m are plotted.
V. Ettler et al. / Chemosphere 260 (2020) 127642 7

phases as demonstrated by Morrison et al. (2016), who used
automated SEM and classified the slag phases according to their
accessibility to be “liberated”,“accessible”and “locked”. In contrast,
Kabwe slags contain small and “liberated”fragments of presumably
reactive glass highly enriched in metals (with up to 48.6 wt% ZnO,
11.7 wt% PbO and 0.51 wt% V
2
O
3
). It has been previously demon-
strated that glass is more susceptible to weathering than crystalline
phases (Ettler et al., 2001). We, thus, hypothesize that the Zn-rich
slag glassy fragments are substantially dissolved during the bio-
accessibility testing of the Kabwe slags and are responsible for the
substantial release of Zn (and also Pb and V) into the solution
(Fig. S1).
The mean BAF values for the key contaminants (Cd, Pb, Zn) re-
ported for the Kabwe slag dusts are very high, with systematically
higher values for the ISF slags than for the Waelz slags (Cd: 78 and
57%, Pb: 89 and 41%, Zn: 75 and 62%) (Tables 2 and S5). These values
are much higher than those observed by Boisa et al. (2013) for the
Zn slags from Kosovo. The mean BAFs for V are also very high, 88%
for the ISF slag and 39% for the Waelz slag (Tables 2 and S5). So far,
no vanadium bioaccessibility data have been reported for slag
dusts. However, recent pH-dependent leaching data from the V-
bearing Waelz slags from Berg Aukas (Namibia) showed that the V
release under acidic conditions are quite low (<1% at pH 3) (Ettler
et al., 2020). The BAF values for the V obtained by the oral bio-
accessibility testing (gastric phase) of the titanomagnetite ore
concentrate (V concentration: 4219 mg/kg), mining-polluted soils
and dusts were also quite low, accounting only for <9, 5.5 and 3.8%,
respectively (Yu and Yang, 2019). In soils from an industrial area in
southern France with much lower V concentrations (up to 242 mg/
kg), the BAF values were up to 38% (Reis et al., 2014). To summarize,
the overall high BAFs for the metals released from the Kabwe slag
dusts are likely caused by fine-grained characteristics of the sam-
ples and deportment of the metals (Pb, Zn) in phases, which are
highly soluble under simulated gastric conditions (e.g., carbonates,
Mn oxides, hemimorphite, glass).
Even under a conservative exposure scenario (dust ingestion
rates of 100 mg/day) the metal (loid) contaminant intake exceeds
the TDI values for children and adults; however, the exposure to
dust in this former mining area will probably be much higher as
recently demonstrated by Smolders et al. (2019). Our data clearly
indicate that slag dusts in Kabwe represent an important risk for
the local population due to high bioaccessibility of Pb and other
contaminants and could contribute to the health problems re-
ported by Yabe et al. (2020) and references therein. Following our
recommendation of remediation measures mainly related to the
soil contamination in the Kabwe area (K
ríbek et al., 2019), we
emphasize that the former mine area with slag heaps should be
first carefully fenced off to prevent local people (including children)
from entering. The surface stabilization/solidification of the area
based on successful technical solutions (Mwandira et al., 2019a,b)
to prevent dust formation is also highly needed. However, in 2019,
Jubilee Metals Group Plc acquired the area known as the Sable Zinc
Kabwe Recovery site in view of the future recovery of metals from
the surface waste materials (more than 356 kt of Zn, 351 kt of Pb
and 1.26% equivalent V
2
O
5
are expected) (Jubilee Metals Group,
2020). This recent change in ownership of the former mining
area in Kabwe is a unique chance to reprocess old wastes in an
environment-friendly way. The residues obtained after the metal
recovery in hydrometallurgical circuits should then be placed in a
safe and sealed disposal site. In addition, special care should be
taken to limit the dust generation and dispersion during the me-
chanical disturbances of the existing waste material disposal sites
and transportation of these materials for the reprocessing and
metal recovery.
5. Conclusions
Old slags deposited in the area of an abandoned PbeZn mine in
Kabwe, Zambia, represent an important source of metal(loid)-
bearing dust particles. We used a combination of a detailed
mineralogical investigation and oral bioaccessibility tests under
simulated gastric conditions to assess the exposure risks to the
metallic contaminants for the local inhabitants, who are in direct
contact with this hazardous dust material. The high contents of
metals (Pb, V, Zn) in the fine slag dust fractions (<48
m
m and
<10
m
m) and their partitioning into some readily soluble phases
(metal-rich Mn oxides, slag glass, hemimorphite, carbonates) are
responsible for their very high bioaccessibilities. Even when using a
conservative dust exposure scenario (100 mg/day), the daily
contaminant intake significantly exceeds the TDI limits, especially
for Pb, V and Zn (in this order of importance). Urgent remediation
action is needed for the slag heaps (and nearby tailing stockpile) in
the former Kabwe mine area. First, the entire site should be care-
fully fenced off to prevent the local people from entering. A recent
change in ownership of the Kabwe mine area provides a unique
opportunity to reprocess old wastes in an environment-friendly
way and to remediate the site in view of minimizing the human
health impact and further dispersion of the contaminated dust in
the nearby townships and soil systems.
Credit authorship contribution statement
Conceptualization: Vojt
ech Ettler. Data curation: Vojt
ech Ettler,
David 
St
ep
anek, Martin Mihaljevi
c, Petr Drahota, Radim Jedlicka.
Formal analysis: Vojt
ech Ettler, David 
St
ep
anek, Martin Mihaljevi
c,
Petr Drahota, Radim Jedlicka. Funding acquisition: Vojt
ech Ettler,
Martin Mihaljevi
c. Investigation: Vojt
ech Ettler, David 
St
ep
anek,
Martin Mihaljevi
c, Petr Drahota, Radim Jedlicka. Methodology:
Vojt
ech Ettler. Project administration: Vojt
ech Ettler, Martin
Mihaljevi
c. Resources: Vojt
ech Ettler, Martin Mihaljevi
c, Bohdan
K
ríbek, Ale
s Van
ek, Vít Pení
zek, Ondra Sracek, Imasiku Nyambe.
Supervision: Vojt
ech Ettler. Validation: Vojt
ech Ettler. Visualiza-
tion: Vojt
ech Ettler. Roles/Writing - original draft: Vojt
ech Ettler.
Writing - review &editing: Vojt
ech Ettler, David 
St
ep
anek, Martin
Mihaljevi
c, Petr Drahota, Radim Jedlicka, Bohdan K
ríbek, Ale
s
Van
ek, Vít Pení
zek, Ondra Sracek, Imasiku Nyambe
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the Czech Science Foundation
(GA
CR) project no. 19-18513S, institutional funding from the Center
for Geosphere Dynamics (UNCE/SCI/006) and the Operational
Programme Prague eCompetitiveness (Project CZ.2.16/3.1.00/
21516). The kind support from Dr. Geoff Casson of Enviro Processing
Limited was highly appreciated. We thank numerous colleagues for
their support in the laboratories (V
era Von
askov
a, Lenka Jílkov
a,
Marie Fayadov
a) and Mr. Alan Harvey Cook for his review of the
English in the manuscript. The valuable comments of three anon-
ymous reviewers helped significantly to improve the original
version of the manuscript.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
V. Ettler et al. / Chemosphere 260 (2020) 1276428

https://doi.org/10.1016/j.chemosphere.2020.127642.
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