Insights into solid phase characteristics and release of heavy metals and arsenic from industrial sludge via combined chemical, mineralogical, and microanalysis

Environ Sci Pollut Res (2015) 22:2205–2218
DOI 10.1007/s11356-014-3438-y

RESEARCH ARTICLE

Insights into solid phase characteristics and release of heavy
metals and arsenic from industrial sludge via combined chemical,
mineralogical, and microanalysis
Tran Thi Thu Dung & Asefeh Golreihan &
Elvira Vassilieva & Nguyen Ky Phung &
Valérie Cappuyns & Rudy Swennen

Received: 16 May 2014 / Accepted: 10 August 2014 / Published online: 31 August 2014
# Springer-Verlag Berlin Heidelberg 2014

Abstract This study investigates the solid phase characteristics and release of heavy metals (i.e., Cd, Co, Cu, Cr, Mo, Ni,
Pb, and Zn) and arsenic (As) from sludge samples derived
from industrial wastewater treatment plants. The emphasis is
determining the influence of acidification on element mobilization based on a multidisciplinary approach that combines
cascade and pHstat leaching tests with solid phase characterization through X-ray diffraction (XRD), field emission gun
electron probe micro analysis (FEG-EPMA), and thermodynamic modeling (Visual MinteQ 3.0). Solid phase characterization and thermodynamic modeling results allow prediction
of Ni and Zn leachabilities. FEG-EPMA is useful for direct
solid phase characterization because it provides information
on additional phases including specific element associations
that cannot be detected by XRD analysis. Cascade and pHstat
leaching test results indicate that disposal of improperly treated sludges at landfills may lead to extreme environmental
risks due to high leachable concentrations of Zn, Ni, Cu, Cr,
Responsible editor: Philippe Garrigues
T. T. T. Dung : A. Golreihan : E. Vassilieva : V. Cappuyns :
R. Swennen
Department of Earth and Environmental Sciences, KU Leuven,

Celestijnenlaan 200E, 3001 Leuven, Belgium
V. Cappuyns
Faculty of Business and Economics, KU Leuven, Warmoesberg 26,
1000 Brussels, Belgium
T. T. T. Dung (*)
University of Science, Faculty of Environment, Vietnam National
University Ho Chi Minh City, 227 Nguyen Van Cu St., W4, D5, Ho
Chi Minh City, Vietnam
e-mail:
N. K. Phung
Department of Science and Technology, 244 Dien Bien Phu St., W7,
D3, Ho Chi Minh City, Vietnam

and Pb. However, high leachabilities under acid conditions of
Ni and Zn as observed from pHstat leaching test results may
provide a potential opportunity for acid extraction recovery of
Ni and Zn from such sludges.
Keywords Heavy metals and arsenic . Industrial sludge .
Cascade leaching test . pHstat leaching test . XRD .
FEG-EPMA

Introduction
All over the world, urbanization and industrialization have
increased the production of sludge (Kazi et al. 2005), especially in developing countries such as Vietnam. Sludge generated from wastewater treatment plants is usually treated as
solid waste and classified as hazardous or non-hazardous
based on its constituents and leachability (VMONRE 2013;
US EPA 2011).
In Vietnam, management options for sludge include landfill disposal, stabilization/solidification, and incineration
(VMOST 2004). The option of choice depends on the origin
of the sludge and leachable concentrations of contaminants

(VMONRE 2013). However, these management options apply only to sludge generated at factories registered at the
National Environmental Protection Agency, the source of
65 % by mass of total sludge production (LBCD & Experco
International 2010). The remaining sludge is disposed of at
illegal dump sites. Depending on its origin, sludge composition can vary from highly organic (domestic sludge) to
mineral-enriched (industrial sludge) (Perez-Cid et al. 2002).
Industrial sludge that contains high quantities of heavy metals
(HMs) and As can contaminate soil and groundwater, even
when located in landfills (Salado et al. 2008). In recent years,
due to the increasing volume of industrially derived sludge


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Environ Sci Pollut Res (2015) 22:2205–2218

and associated environmental concerns, HMs and arsenic in
sludge have become a popular research topic (Perez-Cid et al.
2002; Kazi et al. 2005; Hsieh et al. 2008; Jamali et al. 2009a;
Jamali et al. 2009b; Wu et al. 2009; Gao et al. 2012; Ozdemir
and Piskin 2012; van der Sloot and Kosson 2012; Huang et al.
2013; Milinovic et al. 2014). Studies mostly deal with sludge
bulk composition and leaching properties (Orescanin et al.
2009; van der Sloot and Kosson 2012; Milinovic et al.
2014) or possible treatment options—recovery of valuable
metals via vitrification (Huang et al. 2013; Chou et al.
2012), acid leaching (Li et al. 2010), solidification to immobilize HMs and As in Portland cement, or bio-stabilization via
use of iron reducing microorganisms (Papassiopi et al. 2009).
Understanding chemical and mineralogical properties of industrial sludge and the factors that may influence release of
HMs and As provides essential information for defining management options. The present study investigates leaching of

HMs (Cd, Co, Cu, Cr, Mo, Ni, Pb, and Zn) and As from
industrial sludge using the cascade and pHstat leaching tests.
The focus is determining the effect of acidification on element
mobilization. Given that the leaching behavior of solid materials is largely dependent on the mineralogical characteristics
of the solid phase (Ettler et al. 2003), the second objective
involves identification of possible mineralogical phases in
sludge via X-ray diffraction (XRD) and field emission gun
electron probe micro analysis (FEG-EPMA). Mineralogical
composition can be related to leaching of particular elements.
Furthermore, the environmental risk of HMs and As is strongly dependent on chemical speciation. We present predictions
of leachate HM and As species distributions based on Visual
MinteQ 3.0 thermodynamic models. In this study, the term
“heavy metals” denotes the elements Cd, Cr, Co, Cu, Ni, Pb,
and Zn. Arsenic (As), which is actually a metalloid, is mentioned separately.

Material and methods
Sampling and sample pretreatment
Three industrial sludge samples with different chemical composition originating from three wastewater treatment plants
were collected in Binh Duong and Dong Nai provinces
(southern Vietnam) in February 2013. These provinces host

chemical, garment, shoe and leather, metal plating, iron and
steel processing, and export industries (Dore et al. 2008). Two
of the industrial sludge samples were collected from centralized wastewater treatment plants in the industrial parks of
Dong Nai (sample SI1) and Binh Duong (sample SI2) that
accept the effluent from a variety of nearby industrial factories. The third industrial sludge (sample SE) originated from
an electroplating wastewater treatment plant in Binh Duong.
Electroplating sludge is regarded as a hazardous waste in
Vietnam (VMONRE 2013). The dried bulk sample SI2 collected for use in experiments weighed ∼1 kg, while the wet
samples SI1 and SE weighed ∼3 kg. After collection, the

samples were placed in sealed plastic bags and transported
to KU Leuven, Belgium, for further analysis. The moisture
contents of samples SI1 and SE were, respectively, determined
from the weight difference between the wet and the dry
samples (105 °C). The moisture content was 90 % for SI1
and 79 % for SE.
Each sample was divided into two portions. One portion
was then dried in an oven at 60 °C until constant weight. The
other portion was air-dried. The oven-dried samples were used
for bulk chemical and mineralogical sample characterization,
while the air-dried samples were used for leaching tests. Once
dry, the samples were ground and homogenized in a porcelain
mortar and sieved through a 2-mm sieve.
Chemical characterization
In this study, the term “total concentrations” applies to geochemical data collected using a three-acid digestion method,
where samples were digested by HNO3conc, HClO4conc, and
HFconc in a Teflon beaker on a hot plate. All glassware was
first rinsed with HNO3 0.5 mol/L and all reagents were of
analytical grade. Total concentrations of the elements Al, Ca,
Fe, K, Mg, P, S, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, and Zn
were then determined via ICP-OES (Varian 720ES). In order
to evaluate the quality of the analytical method, a certified
reference material (NIST2782, industrial sludge) and sample
duplicates were analyzed. The comparison between the measured concentrations with the certified data in the certified
reference material is given in Table 1. The pH of each sample
was measured (pH Hamilton single-pore electrode, calibrated
at pH 4 and 7) in a suspension solution of 5.0 g of sludge in
25.0 mL water, following shaking for 2 h. The content of

Table 1 Comparison between the measured concentrations with the certified data in the certified reference material (NIST2782, industrial sludge)

(average±standard deviation of two replicates)

Measured value
Certified value

As
(mg/kg)

Cd
(mg/kg)

Co
(mg/kg)

Cr
(mg/kg)

Cu
(mg/kg)

Mo
(mg/kg)

Ni
(mg/kg)

Pb
(mg/kg)

Zn

(mg/kg)

123±2
166

6±0
4.17

57±1
66.3

102±12
109

2,778±15
2,594

11±4
10.07

136±22
154.1

509±69
574

1,158±136
1,254

Environ Sci Pollut Res (2015) 22:2205–2218

organic matter was determined by the Walkley and Black
manual titration method (Nelson and Sommers 1982).
Solid phase characterization
We performed XRD analysis on the original samples and on
residues from the pHstat test in order to identify any changes in
the mineralogical composition of the solid phase during the
leaching test (Fig. 1). A Philips PW1830 diffractometer with
Bragg/Brentano θ–2θ setup, CuK radiation, 45 kVand 30 mA,
and graphite monochromator was used. Data from FEGEPMA analysis were used to complement the XRD characterization. For FEG-EPMA analysis, sludge samples were
embedded in a resin and prepared as polished thin sections
coated with a ∼14-nm-thick carbon layer. The polished thin
sections were examined with a Jeol JXA8530F machine, with
energy dispersive spectrometer (EDS) mode (in spot analysis)
or wavelength dispersive spectrometer mode (WDS) (in mapping mode).
Leaching tests
Cascade leaching test
We used a cascade leaching test (CLT, NEN 7349 1995) to
assess the extent of leaching as a function of liquid/solid (L/S)
ratio. This is a serial batch test in which material is successively extracted five times, resulting in L/S ratios of 20, 40, 60,
80, and 100 (L/kg). This test was replaced by compliance test
NEN-EN 12457-3:2002 in 1999 consisting of two successive
extractions at liquid-to-solid ratio 2 and 8 L/kg dry matter.

2207

However, because our interest is leaching characteristics rather than compliance with environmental standards, we chose to
use CLT in the present study. Extractions were carried out in
duplicate in acid-rinsed 50 mL polyethylene centrifuge tubes

with screw caps. Thirty milliliters of Milli-Q water, acidified
to pH 4 with HNO3 (ultrapure), was added to 1.5 g of dry
sample, shaken (for 24 h), centrifuged at 3,000 rpm for
10 min, and filtered (0.45 μm, Chromafil® PET-45/25,
Macherey). No pH adjustment was performed during the test.
pHstat leaching test
We performed a pHstat leaching test (CEN/TS 14429, 2004) at
a pH of 4 to assess the influence of acidic conditions on the
release of HMs and As from sludge. The test was based on an
automatic multititration system (Titro-Wico Multititrator,
Wittenfield and Cornelius, Bochum, Germany). Eighty grams
of dried sample was put in an erlenmeyer flask together with
800 mL of Milli-Q water (L/S ratio=10 L/kg). We continually
monitored the pH and adjusted it by automatic addition of
HNO3 solution. Preliminary experiments showed that when a
HNO3 solution with a concentration of 1 mol/L was used, a
very large volume of HNO3 1 mol/L solution was needed to
maintain the predefined pH (pH 4) for sample SE. Therefore,
we adapted the concentration of the HNO3 solution to the
different samples (SI1 and SI2 samples, 1 mol/L; SE sample,
2.5 mol/L). At regular time intervals (0, 1, 3, 6, 12, 24, 48, 72,
96 h), a sample of the suspension (10 mL) was taken over a
filter (0.45 μm, Chromafil® PET-45/25, Macherey-Nagel
GmbH & Co. KG, Germany) by means of a syringe attached
to a flexible tube.

Fig. 1 XRD patterns of original sample and sample after the pHstat test (pH=4, sample SI2)


2208

Analysis of leachates
Part of each leachate sample (CLT and pHstat test) was acidified with a drop of concentrated HNO3, for measurement of
Al, Ca, Fe, K, Mg, P, S, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb,
and Zn by ICP-OES whereas the other part was not acidified,
and stored in cool and dark conditions until anion (F−, Cl−,
SO42−, S2O32−, and PO43−) analysis by ion chromatography
(IC-Dionex ICS-2000). We used the colorimetric
diphenylcarbazide (DCB) method (USEPA 1995) to determine the Cr(VI) content in the leachates. Cr measured by
ICP-OES constitutes total Cr. Cr(III) was obtained by subtraction of Cr(VI) from total Cr.

Environ Sci Pollut Res (2015) 22:2205–2218

composition of industrial sludge may vary widely, depending
on industry type. As mentioned, sample SI1 and SI2 originate
from centralized industrial wastewater treatment plants of
industrials parks that actually treat mixed wastewaters from
factories within the industrials parks, making comparison to
published data difficult.
Regarding the electroplating sludge sample (SE), concentrations of major elements (Al, Ca, and Fe) and Zn are higher
and Mg, Mn, As, Cd, and Cu are lower compared to those
reported by other workers. Zn concentration is 75 times higher
than the value reported by Wu et al. (2012). Concentrations of
Cr, Ni, and Pb lie within the same range.
Solid phase characterization

Aqueous speciation based on modeling by Visual MinteQ,
version 3.0
We performed calculations based on Visual MinteQ version
3.0 to determine speciation of HMs in the leachate. Input data

include concentrations of elements (Al, K, Ca, Mg, Fe, Co, Cr,
Mn, Mo, Ni, Pb, Zn, S, P) and anions (F−, Cl−, SO42−, S2O32−,
and PO43−) measured in the leachate during the pHstat leaching
test (96 h). Concentrations of NO3− were derived from acid
dosage in the pHstat test. Measured Eh and pH values were
used in the calculation. The specified redox couples were
Fe2+/Fe3+, Co2+/Co3+, Cu+/Cu2+, Cr(OH)2+/CrO42−, HS−/
SO42−, and Mn2+/Mn3+.

Results
General sludge characteristics
Bulk chemical composition
The three sludge samples have a nearly neutral to slightly
alkaline pH (Table 2). Elevated concentrations of Al, Ca, Fe,
P, S, and organic matter (percentage level) were found in all
samples. These elements might derive from the use of chemical precipitants and coagulants (e.g., lime, ferrous sulfate,
ferrous phosphate, and poly aluminum chloride (PAC)) in
the chemical remediation step in wastewater treatment (Hsieh
et al. 2008). Sample SI2 has a notably high concentration of
Pb (8,130 mg/kg), likely associated with the battery and steel
processing factory in the industrial park. The electroplating
sludge (sample SE) has the highest concentrations of As and
HMs (Co, Cu, Mo, and Zn) and extremely high concentrations
of Cr and Ni (13,208 and 55,732 mg/kg, respectively)
(Table 2). Chemical data related to samples SI1, SI2, and SE
and a comparison with published data from electroplating
sludges (Sophia and Swaminathan 2005; Le 2007; Wu et al.
2012; Huang et al. 2013) appear in Table 2. The elemental

Understanding the correlation between solid phase properties

and leaching behavior of materials requires a detailed knowledge of the minerals present together with their chemical
composition (Bayuseno and Schmahl 2010). XRD analysis
reveals the presence of silicates, including quartz (SiO2 in all
three samples) and narcrite (Al2Si2O5(OH)4 in sample SI1),
and carbonates such as calcite (CaCO3 in SE). The occurrence
of quartz and gypsum in sample SE is in accordance with
results reported by Ozdemir and Piskin (2012), who also
reported the presence of quartz and gypsum as common
phases in metal plating sludge. Calcite is also a common byproduct of the neutralization step by lime in wastewater treatment systems due to its precipitation from high calcium content and carbonate fraction in industrial sludge (Zinck 2005).
Some phosphate and sulfate minerals are also present:
phosphosiderite (FePO4·2H2O in sample SI1), aluminum
phosphate (Al16P16O64 in SI2), gypsum (CaSO4·2H2O in all
three samples), and lanarkite (Pb2SO4O in SI2).
Mineral phases detected by XRD analysis are consistent
with high contents of Al, Ca, Fe, P, and S in all samples in
ICP-OES data. Apart from Pb, no HMs as discrete mineral
phases was detected by XRD. We applied FEG-EPMA as a
check on the consistency of XRD results and to identify the
phases that could not be detected by XRD. Table 3 provides
the chemical composition from selected EDS spot analysis by
FEG-EPMA.
In sample SI1, 62 spots were analyzed by FEG-EPMA to
examine elemental compositions of the phases. Results reveal
the presence of quartz (spots 1 and 2, Table 3 and Fig. 2),
phosphorus (spots 3, 4, 5, 6, 7, 12, and 13), and sulfur species.
Although some HMs (Cr, Ni, and Zn) were detected with
FEG-EPMA, no Cd or Co was detected. As was only detected
in one spot in a matrix of glass (Si-Ca-Mg-Na) (spot 8, Fig. 2).
A different distribution of Cr, Ni, and Zn was observed
(Table 3). Cr-rich spots were observed in Fe-rich spots (spots

9, 10, and 11) or in a matrix with Zn, Ni, P and small amount
of Ca, Al, and Fe (spot 12). Ni was found in Zn-rich spots
containing P, Al, Fe (spots 12 and 13), and Si (spot 14). Most


Cd
(mg/kg)
1.4±0.5
3.0±0.5
1.2±0.2
NA
NA
NA
30

As
(mg/kg)
11±2
10±2
35±2
NA
NA
NA
97.6

SI1
SI2
SE
Sophia and Swaminathan (2005)
Le (2007)

Wu et al. (2012)
Huang et al. (2013)

SI1
SI2
SE
Le (2007)
Wu et al. (2012)
Sophia and Swaminathan (2005)
Huang et al. (2013)

NA not available, DL below detection limit

Ca
(%)
8.01±0.04
7.06±0.04
9.82±0.16
NA
NA
NA
3.19

Al
(%)
4.36±0.05
7.93±0.06
3.36±0.06
NA
NA

NA
0.16
Co
(mg/kg)
260±6
11±1
230±1
NA
NA
NA
NA

Fe
(%)
3.63±0.09
2.91±0.03
5.36±0.04
NA
NA
NA
1.5
Cr
(mg/kg)
5,039±51
501±6
13,208±148
4,656–7,274
28,828–133
85,000±1,400
220

P
(%)
1.97±0.09
2.72±0.04
6.17±0.02
NA
NA
NA
NA
Cu
(mg/kg)
10,851±29
542±146
9,790±95
NA
NA
NA
19,000

S
(%)
7.98±0.02
2.19±0.02
1.30±0.01
NA
NA
NA
NA
Mo

(mg/kg)
DL
5±1
8±1
NA
NA
NA
NA

K
(mg/kg)
1,223±14
820±3
193±22
NA
NA
NA
220
Ni
(mg/kg)
14,118±159
174±6
55,732±388
50,229
308–4,976
590±76
267,000

Mg
(mg/kg)

2,702±83
1,727±6
5,153±113
5,600±360
NA
NA
2,6800
Pb
(mg/kg)
122±1
8,130±100
268±1
NA
424
100±14
5,700

Mn
(mg/kg)
427±20
168±3
157±2
NA
NA
NA
250

Zn
(mg/kg)
72,347±2,223

1,124±9
77,352±1,061
NA
1,036
20,100±720
17,300

OC
(%)
7.8±0.2
7.2±0.7
3.0±0.5
NA
NA
NA
NA

IR
(%)
8.5±0.9
9.2±0.7
14.0±1.3
NA
NA
NA
NA

pH
(H2O)
7.7±0.3

7.16±0.3
7.82±0.3
NA
NA
NA
NA

Table 2 Chemical characteristics of samples SI1, SI2, and SE (mean±standard deviation of 2 replicates) and data from electroplating sludge reported by Sophia and Swaminathan (2005), Le (2007), Wu
et al. (2012), and Huang et al. (2013)

Environ Sci Pollut Res (2015) 22:2205–2218
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Environ Sci Pollut Res (2015) 22:2205–2218

Table 3 Chemical composition (wt%) of selected spots by FEG-EPMA, the spot numbers refer to the numbers given in Fig. 2
Sample Spot P
SI1

SI2

SE

SE

Fe

O

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

55.56
55.57
16.45
43.17
14.63 1.72 33.15
16.21 10.95 43.59
16.48 10.69 43.1
16.96 9.57 43.21
44.72
85.6 1.13

87.84
85.92 1.06
8.43 5.82 37.4
8.41 5.14 40.16
3.99 4.93 38.71
58.41
43.85 38.47
12.84 12.14 43.76

18
19
20
21
22
23
24
25
26
27
28
29
30

13.4

13.21 44.37
24.07
24.06
2.13 1.33 48.77
11.47 6.74 26.64

51.76
50.04
3.7
26.55
10.51 37.44
39.79
25.72 22.09
10.05 34.22
14.59 35.9

Pb

N

Na

Mg

Al

Si

S

Ca

Ti

Mn Cr

Ni

Zn

As

Zr

Sn

44.44
44.43
10.27
0.69

5.18

3.73 2.66

38.2

6.95
7.95
5.5

21.64
10.62
12.09
6
12.02

61.84
59.15
1.13
0.66
7.53 12.25

19.6
41.59

7.37
3.47
4.15
4.66
5.49

1.07
1.14

3.42
12.29
7.22

Zn-rich spots occurred within P-Fe-rich spots (spots 12, 13
and 14) or in a complex matrix of Na, Al, Si, and Ca with
small amounts of S and Cl (data not shown). WDS mapping
was also used to deduce the micro-scale elemental distributions in a selected area. The map (Fig. 2) shows a variety of
phases, namely phases with high Zn-Ni-P-Fe and phases with
high Al-Si content. The coexistence of Zn, Ni, P, and Fe, as
observed in the map, suggests that Fe and PO43−-rich phases

are important host phases for Zn and Ni. The coexistence of
Al-Si suggests the presence of nacrite (Al2Si2O5) which was
also identified by XRD.
In sample SI2, 42 spots were analyzed in combination with
WDS mapping of selected areas. Representative EDS point
analyses are given in Fig. 2 and Table 3. In general, SI2
consists of a large variety of elements that are mostly P, Si,
Al, and S, most likely coinciding with quartz, aluminum
phosphate and Si, Al-rich phases (spots 15, 16, 17, and 18).
Pb was the only HM that was detected with FEG-EPMA, most

1.47

30.11
38.75
29.24
29.73
30.26

7.01

4.08

11.47
23.04 8.21

1.34

4.54

3.68
13.28
12.16
13.01
4.46 11.77 21.09
8.01 30.33
5.83 21.44

6.22

1.06 0.7
32.64
9.91 7.96
45.23
49.96
0.95 1.15
2.3
36.52
5.14

0.97
1.94
0.88
5.81

55.12 5.61
17.77

26.16
11.4

9.09

11.59 19.15
8.4
14.35

47.33
35.17

likely because of its elevated concentration (8,130 mg/kg Pb
in SI2) and the heterogeneous distribution of other HMs in this
sludge sample. In spot analyses, traces of Pb were observed in
Ti-rich spots with trace amount of P, Fe, Al, and Si (spot 21).
Pb-rich spots were also found in Al-Si-rich spots (spots 19 and
20) and in Al-, P-, S-rich spots containing small amounts of
Fe, Si, and Ca (spot 22). Mapping of selected areas revealed
the presence of quartz and lead sulfide by the coexistence of
Si-O and Pb-S in the selected area.
With regard to sample SE, 61 spot analyses were performed. Major phases in SE comprise calcite (spots 23 and
24) and metal-rich spots with Fe, Ni, Sn, and Zn (spots 29 and
30). Results reveal an elevated content of some HMs, including Ni, Zn, Cr, and Sn. Cd, Co, and Mo were not detected. Znrich spots were identified in 30 of the 61 analyzed spot which
is in accordance with the fact that Zn is the most abundant HM
in sample SE (see also Table 2). Ni- and Zn-rich spots with
varying contents of Al, Fe and Sn and, in some cases, traces of


Environ Sci Pollut Res (2015) 22:2205–2218

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Fig. 2 Graph of selected spots for EDS analysis and area for WDS mapping analyzed by FEG-EPMA. From left to right: a SI1; b WDS mapping of
selected area revealed the coexistence of Ni, P, and Fe in SI1; c SI2; d SE. The spot numbers refer to the numbers given in Table 3

Cr (spots 25, 26, and 28), S (spots 25 and 26), and As and Pb
(data not shown) were found. Arsenic was also observed in a
Ca-Mg-O matrix (spots 23 and 27).
Leaching tests
Although many different elements were measured, the following discussion mainly focuses on As, Cd, Co, Cr, Cu, Ni, Mo,
Pb, and Zn because of their potential toxicity. Major elements
(Al, Ca, K, Mg, P, and S) and anions (SO42− and PO43−) are
mentioned because of their relevance for interpretation of
release mechanisms of elements of interest.
Cascade leaching tests
Figure 3 shows pH and cumulative leached concentrations of
HMs and SO42− from the CLT. Concentrations of As in the
leachates are below detection limit in all three samples. In the
CLT, the final pH of the leachates varied from step to step
(Fig. 3). According to Cappuyns and Swennen (2008a), the
extent of pH change mainly depends on the acid neutralization
capacity (ANC) of the samples. From this result, we deduce
that sample SE has the highest ANC because its pH change
was smallest during the CLT. Below, “leachability” refers to

the cumulative leachability (sum of five extractions steps),
expressed in percent of an element leached relative to its total
content in a sample.
In sample SI1, the leachability is in the following order: S
(43 %)>Ca (32 %)>K (15 %)>Mg (9 %)>Co≈Mn (3 %)>Ni
(2 %)>Zn (1 %)>Cu≈P (0.1–0.2 %). The leachability of Al,

Fe, and Cr is negligible (<0.02 %). Even though a relatively
low cumulative leachability of Ni and Zn was observed (as %
of their total content in the sludge sample), special attention
should be paid to these elements because of the high absolute
concentrations (Ni, 322 mg/kg; Zn, 861 mg/kg) cumulatively
released during the CLT. Speciation analysis of Cr indicates
that the leachates of sample SI2 did not contain Cr(VI). The
concentrations of Pb in the leachates were below detection limit.
In sample SI2, we observed a considerable release for
almost all analyzed elements. Among HMs, Mo showed the
highest leachability (60 %). The lowest leachability was measured for Cu and Cr (2 %). In descending order, leachability
for the other HMs is as follows: Ni (14 %)>Co (9 %)>Zn
(4 %). Speciation analysis of Cr indicates that the leachates of
sample SI2 do not contain Cr(VI).
In general, sample SE displays a lower leachability of
elements compared to SI1 and SI2. Similar to SI2, Mo showed


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Environ Sci Pollut Res (2015) 22:2205–2218

Fig. 3 Cumulative amount leached of HMs including Cr(VI) (sample SE), and SO42−, and pH change during the cascade leaching test

the highest release among the HMs (23 % of the total content
was released). The leachability of Zn, Ni, and Cu is negligible
(<0.1 %) while leachabilities of Cr and Co are in the range 1–
2 %. The concentration of Pb in the leachates of sample SE is
below detection limit. The release of Cr appears to correspond
mainly to Cr(VI) because Cr(VI) is the main species measured

in the leachate. Therefore, Cr should be taken into consideration because of the high absolute cumulative concentration
leached (122 mg/kg) and the occurrence of Cr(VI) in the
leachate. The analysis of Cr speciation shows that cumulative
concentration leached Cr from sample SE was dominantly
Cr(VI) (108 mg/kg).

(5,627 mmol/kg), followed by samples SI1 (910 mmol/kg) and
SI2 (410 mmol/kg) (Fig. 4). The presence of calcite in sample
SE most likely contributes to the high ANC of this sample.
This result agrees with the CLT result that sample SE has the
highest ANC. In all three sludge samples, despite the high total
concentration, Al and Fe exhibit very low leachability (<1 %)
compared to other major elements, such as Ca, K, Mg, Mn, and
S (>6 %). This suggests that no significant dissolution of Al-Fecontaining minerals occurred during the pHstat leaching test.
Sample SI1 shows the lowest leachability of major elements
among the three samples. With regard to SO42− and PO43−, the
samples show the following order for released amount: SI2>
SE>SI1 for SO42− and SI1>SI2>SE for PO43−.

pHstat leaching test
Heavy metals and As
Figures 4 and 5 display release patterns of major elements,
ANC, anions, HMs, and As from the pHstat leaching test. In
the following section, leachability refers to leachable concentration after 96 h of pHstat leaching, expressed in percent
relative to total concentration.
Acid neutralization capacity, major elements, and anions
We calculated ANC based on the quantity of acid added to
maintain a particular pH value. During the pHstat leaching test,
the highest ANC (at 96 h) was observed in sample SE

During pHstat leaching at pH 4, Cd, Co, Ni, and Zn leachability is high (9–31 % of the total content). This is consistent with
Kazi et al. (2005), who report identifying Cd and Ni mainly in
the acid-soluble fraction of industrial sludge samples, suggesting release at low pH.
Cu exhibits a medium leachability (1–5 %), while Cr and
Pb show a low leachability (<1 %). Cr(VI) occurs as an anion,
which may account for its relatively stronger bonding to
positively charged components (e.g., surfaces of
Fe(hydr)oxides, organic matter). Speciation analysis on Cr


Environ Sci Pollut Res (2015) 22:2205–2218

2213

Fig. 4 Release of major elements, ANC and anions in pHstat leaching test at pH 4. Some components were multiplied by a factor of 10, 100, or 1,000 for
a better visualization

shows that most dissolved Cr in the leachates in SE is Cr(VI)
(up to 78–98 % relative to total dissolved Cr). We did not
detect Cr(VI) in the leachates of sample SI1 and SI2.
After 6 h during pHstat leaching, As was detected only in
small amounts (0.2–0.5 mg/kg) in sample SE. No As was
detected in samples SI1 and SI2. Mo concentrations were also
below detection limit for the three samples.
From an environmental point of view, if such types
of sludge are disposed of landfills without proper treatment, considerable release of Cu, Ni, and Zn may be
expected. Acidification of sludge implies extreme environmental risk, especially for sludges similar to samples
SI1 and SI2 that are characterized by low ANCpH4.
Sample SE is characterized by a much higher ANCpH4
(Fig. 4) and high concentrations of HMs (Zn, Ni, Cu,

and Cr) released at pH 4 (Fig. 5).

With regard to Co, Cu, Ni, and Zn, free metal (Me2+) ions
form the dominant species types in the leachates of all three
samples. Pb-sulfate is the main species in sample SI2, whereas
Pb-nitrate complex was observed as dominant species in
sample SE.
Cr is dominantly present as trivalent in all three samples.
Free Cr3+ (50–53 %) and Cr(III)-hydroxide (31–38 %) are the
main species in sample SI1 and SE, while Cr(III)-sulfate is the
dominant soluble species (45 %) in sample SI2.
Although Cr(VI) was measured in the pHstat leachate of
sample SE (2.8 mg/l) and accounted for up to 98 % of total Cr
measured by chemical analysis, thermodynamic modeling
predicts a negligible amount of Cr(VI) (<0.001 %).
Thermodynamic modeling results suggest that Cd mainly
occurs as free Cd2+ (48–72 %) in samples SI1 and SE, whereas Cd-thiosulfate is the most abundant species in sample SI2.

Aqueous speciation based on modeling by Visual MinteQ
version 3.0
Discussion
Thermodynamic modeling results reveal a great variety of
chemical species depending on the composition, pH, and
redox potential of leachates. Most elements form free metal
ions and complexes with hydroxides, sulfates, chlorides, nitrates, and oxyanions (Fig. 6).

Solid phase characterization
We performed XRD phase analysis on residual sludge following pHstat leaching (at pH=4) to assess leaching-related

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Environ Sci Pollut Res (2015) 22:2205–2218

Fig. 5 Release of HMs in pHstat leaching test at pH 4. The concentration of some elements was multiplied by a factor of 10, 100, or 1,000 for a better
visualization

changes to major solid phases. Changes include the disappearance of calcite (CaCO3) from sample SE and the appearance
of some peaks in gypsum (CaSO4) in all samples (Fig. 1),
consistent with the high Ca and SO42− content of leachates.
Izquierdo and Xavier (2012) noted that Ca-bearing species
such as gypsum dissolve easily from solid waste material in
the presence of water and acidic conditions.
XRD analysis prior to pHstat leaching revealed the presence
of lanarkite (Pb2SO4O) in sample SI2. We assumed that all Pb
released during pHstat leaching derived from lanarkite dissolution and then used Pb concentrations to calculate the mass of
lanarkite in the samples. Based on this assumption, we performed a Visual MinteQ calculation of lanarkite solubility at
pH 4 and found that lanarkite dissolves completely at pH 4.
Nevertheless, XRD analysis detected lanarkite at pH 4, showing that dissolution was partial.
XRD analysis alone is insufficient for understanding the
mineral characteristics of the sludge samples because XRD
generally detects only abundant (>2 % of dry sample weight
with Rietveld corrections) minerals with well-defined structures (Venditti et al. 2000). Also, XRD does not detect HMs
and As adsorbed on host minerals such as oxides, carbonates,
sulfates, and phosphates or as inclusions in amorphous phases
(Krupka et al. 2004; Ganne et al. 2006). EPMA does not share
these deficiencies. As mentioned in the previous section

(“Solid phase characterization”), XRD analysis led to the
identification of phosphosiderite; we used EPMA to identify

the coexistence of Ni and Zn with Fe and P (Fig. 2).
Phosphosiderite (FePO4·2H2O) seems to be a potential host
phase for Zn and Ni (sample SI1). Similar leaching patterns of
Ni and Zn in leaching tests (Figs. 3 and 5) support the
hypothesis that they come from the same host phase. Their
rapid release on exposure of the samples to acid
leaching (Fig. 5) suggests that Ni and Zn were not
incorporated within phosphosiderite but were adsorbed
on its surface (sample SI1).
We observed lead sulfide via FEG-EPMA, but not by
XRD, most likely because its amount is below the
detection limit (sample SI2). A large number of Znrich spots were observed by FEG-EPMA, although no
mineral related to Zn was identified by XRD, suggesting that sample SE contained a large quantity of poorly
crystalline Zn phases. This conclusion is supported by
the fact that a high leachability of Zn was observed in a
pHstat test (28 %) performed on this sample. Zinck
(2005) also concluded that Zn is commonly associated
with amorphous or poorly crystallized phases and easily
leached from industrial sludge. Our results also show
that FEG-EPMA can be applied to complement XRD
analysis by identification of phases that are not


Environ Sci Pollut Res (2015) 22:2205–2218

2215

Fig. 6 Aqueous speciation as predicted by Visual MinteQ for Cu, Ni, Pb, and Zn in pHstat leachates (at 96 h)

detectable by XRD analysis. Notice that in general,

FEG-EPMA results are in good agreement with mineral
phases identified by XRD.
Retention and time-dependent leaching behaviors
Retention of HMs
Understanding HM release is important for predicting the
environmental risks associated with these elements over time.
According to Cappuyns and Swennen (2008b), in the CLT, the
cumulative release of HMs can be fitted to the following
equation:
C x ¼ C 20 k à lnx þ a

ð1Þ

where x is the L/S ratio (L/kg), Cx is the concentration of an
element released at L/S=x (mg/kg), C20 is the concentration of
an element released at L/S=20 (mg/kg), k* is the retention
factor (dimensionless), and a is an constant. A higher retention
factor corresponds to a stronger binding of elements to solid
phases.
We estimated the retention factor of each element by fitting
experiment data from the CLT to Eq. 1. However, As and Cd
data in all three samples and Pb in samples SI1 and SE are

excluded from this analysis because their releases were below
detection limits. Regression coefficients (R) mostly range
from 0.85 to 1.00, indicating that Eq. 1 adequately describes
element release.
Co, Cr, and Ni in sample SI1 display the highest retention
factors of the three samples. This suggests that they are
released more slowly from sample SI1. In sample SI2, the

Zn retention factor is notably higher than in samples SI1 and
SE. In general, bonding of HMs (Co, Ni, and Zn) to solid
phases in sample SE is weaker than in samples SI1 and SI2.
Time-dependent leaching behaviors of HMs and As
In general, release patterns as a function of time during pHstat
leaching depend on the elements and pH. Kirby and Rimstidt
(1994) defined four types of element release patterns based on
release concentration versus time curves. The classification
scheme of Dijkstra et al. (2006) is based on five types. We
have observed three types for HMs and As, defined below.
Figures 4 and 5 display typical example curves.
1. Elements (i.e., of Co, Ni, and Zn) initially undergo rapid
release. Release then slows. Steady state is not attained.
Sometimes, the concentrations increased linearly after a
certain time (e.g., Cu in sample SI1). This pattern


2216

generally points to desorption/dissolution upon acidification as proposed by Lee et al. (2005) and Cappuyns and
Swennen (2008b).
2. Elements release happens initially, but there is some decrease in dissolved concentrations over the time (e.g., in
Cu, Pb (sample SI2), and Cr). This pattern is related to readsorption onto solid phases, often observed where
oxyanions form elements (Cr) (Vandecasteele and
Cornelis 2010) or during (co)precipitation. Based on speciation analysis of Cr, most Cr (sample SE) is present as
Cr(VI) in leachate, usually in the form of HCrO4− under
acidic conditions. This suggests that oxyanions of Cr may
be re-adsorbed on positively charged surfaces. Metalsulfate complexes (in case of Cu and Pb (sample SI2) as
predicted by modeling) might exhibit a similar readsorption behavior onto solid phases.
3. Element release is so low as to be undetectable at the

beginning of the pHstat experiment, but is detected after a
certain time (e.g., Cd and Pb (sample SE)). This pattern is
usually associated with slow dissolution of stable phases
where elements are initially locked in silicate matrix or
glass phases (Ganne et al. 2006). Quartz and glass phases
are present in all three samples, but their (partial) dissolution could not be detected based on XRD analysis of the
samples before and after pHstat leaching.

Environ Sci Pollut Res (2015) 22:2205–2218

these elements might be an efficient treatment option for associated wastes, as suggested by Lee
et al. (2005). However, high release of some HMs
at lower pH may pose a challenge should untreated
sludge be exposed to acid rain during landfill
disposal.
Group 2 Element leachability is lower in the pHstat leaching
test than in the CLT (Cr (SE, SI2), Cu (SI2), Mo
(sample SI2 and SE), and Pb (SI2)). This suggests a
lower sensitivity to acidification; however,
leaching at the natural pH of the sample is important. In a study on characterization leaching tests
and modeling based on electroplating sludge, van
der Sloot and Kosson (2012) concluded that Mo
leaching is controlled by solubility at a material’s
own pH. Elements with a tendency to form
oxyanions in industrial wastes, such as Cr (as
CrO42−) and Mo (as MoO42−), usually are more
mobile at higher pH (Kim and Kazonich 2004;
Hassett et al. 2005; Izquierdo and Xavier 2012).
In sample SI2, a higher S leachability in the CLT
(65 %) compared to the pHstat leaching test (36 %)

coincides with relatively higher dissolved concentrations of Cu and Pb in the CLT test. Sulfate
minerals in sample SI2, such as gypsum (as identified by XRD), therefore seem to be the host minerals for Cu and Pb.

Influence of acidification on release of HMs and As
We compared results from the CLT and pHstat leaching tests at
pH 4 in order to access the influence of acidification on HMs
and As release. Leaching of metals usually increases as pH
decreases (van der Sloot and Dijkstra 2004). The pH was fixed
at 4 during pHstat leaching, whereas pH was neutral to weakly
alkaline/acid during CLT. We distinguish two groups of elements based on the leachability of HMs and As from CLT
(cumulative leachability) and pHstat leaching tests (leachability at 96 h):
Group 1 Element leachability is higher in the pHstat leaching
test than in the CLT (Pb (sample SE), Cd, Co, Cu
(samples SI1 and SE), Ni, Zn, and Cr (sample
SI1)). This suggests that these elements are particularly sensitive to acidification. In general, HMs are
retained in industrial sludge as hydroxide precipitates and by adsorption onto solid phases (Zinck
2005; Wu et al. 2009). In the presence of acid (high
concentration of protons), HMs are exchanged by
protons and hydroxides also dissolve, causing HMs
release into solution. The high leachability of Ni
(14–18 %) and Zn (23–28 %) combined with high
total concentrations of these elements in the sludge
means that acid extraction to remove and/or recover

Although Cr(VI) was measured in the pHstat leachate (SE)
and accounts for up to 78–98 % of total Cr, Visual MinteQ
calculations predict negligible quantities of Cr(VI)
(<0.001 %). Calculation may therefore lead to underestimation of the environmental risk of Cr(VI). Measurement of
Cr(VI) is the most reliable method for collecting information
on Cr speciation. Stabilization following metal recovery offers

a potentially suitable treatment option with regard to Cr-rich
sludge (sample SE) remediation, due to the high HM content
of the sludge and low Cr leachability. Stabilization can be
performed by chemical or biological methods, as suggested by
Sophia and Swaminathan (2005) and Papassiopi et al. (2009).

Conclusions
We used a combination of leaching tests (CLT and pHstat
leaching), thermodynamic modeling (Visual MinteQ 3.0),
and solid phase characterization (XRD and FEG-EPMA) to
investigate the release behavior of HMs and As from industrial sludges of southern Vietnam. The sludges are chemically
distinct, making it possible to distinguish groups of elements
with different time-dependent leaching behaviors and potential leachability under acidic conditions. In general, Cd, Co Ni,


Environ Sci Pollut Res (2015) 22:2205–2218

and Zn in the sludges are more sensitive to acidification,
whereas Mo is less susceptible to acidification. For Mo,
leaching at the natural pH of the sample is more important
than leaching under acidic conditions because it is present
mainly as oxyanions. CLT and pHstat leaching tests both
indicate that if industrial sludges are disposed at landfills
without proper treatment, extreme environmental risk may
result due to high leachable concentrations of HMs such as
Zn, Ni, Cu, Cr (sample SE), and Pb (sample SI2). Therefore,
industrial sludges should be treated prior to landfill disposal.
Sample SE contains highly toxic and mobile Cr(VI) in its
CLT- and pHstat-derived leachates. This should be treated
carefully if the associated sludge is recycled. XRD analysis

detected no discrete mineral phases containing HMs or As.
However, FEG-EPMA provides information on phases containing HMs that were not detected by XRD analysis (e.g.,
lead sulfide in SI2 and a poorly crystalline Zn phase in SE)
and therefore supports in direct solid phase characterization.
The results of solid phase characterization can be used to
account for the leachability of some HMs (e.g., Ni and Zn in
sample SI1). Information from this study may be helpful in
designing a suitable treatment for related sludges. From an
environmental and economic perspective, recovery of Ni and
Zn (sample SI1 and SE) with acid extraction may be a promising option, due to their high leachability under moderately
acid conditions (pH 4).
Acknowledgments We acknowledge members of Geology Division,
Department of Earth and Environmental Sciences, KU Leuven, for their
support. In particular, we would like to thank Prof. Sarah Fowler for her
correction of English language, Herman Nijs for his help with thin section
preparation, and Rieko Adriaens for discussion regarding XRD. We also
thank personnel at wastewater treatment plants for providing samples.
This research was supported by the Belgian Technical Cooperation
(BTC).
Conflict of interest The authors have declared no conflict of interest.

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