j.jhazmat.2019.121136

Journal of Hazardous Materials 383 (2020) 121136
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Journal of Hazardous Materials
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Leaching behavior of metals from iron tailings under varying pH and lowmolecular-weight organic acids
T
Huanhuan Genga, Fei Wanga, , Changchun Yana, Zhijun Tianb, Huilun Chena, Beihai Zhoua,
Rongfang Yuana, Jun Yaoc
⁎
a
School of Energy & Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, 100083, Beijing, China
Beijing Geo-engineering Design and Research Institute, 6 East Yuanlin Road, Miyun District, 101500, Beijing, China
c
School of Water Resource and Environmental Engineering, Sino-Hungarian Joint Laboratory of Environmental Science and Health, China University of Geosciences
(Beijing), 29 Xueyuan Road, Haidian District, 100083, Beijing, China
b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Editor: Daniel C.W. Tsang
The migration of metals (e.g., Fe, Cd, Co, Cr, Cu, Mn, Ni, and Zn) in both of iron tailings under different pH
leachates was studied by laboratory static leaching experiments. The results indicated that Fe showed the highest
leaching concentration at an initial pH of 2, reaching 16.19 and 51.72 mg L−1 in the Qian'anling (Q0) and
Majuanzi (M0) iron tailings, respectively. Metal ions manifested a strong pH dependence. In addition, the
leaching behavior of Cd, Cr, Fe, and Cu for the two tailings was also evaluated under leaching by three lowmolecular-weight organic acids (LMWOAs). The results indicated the leaching of Cd and Fe followed the order of
citric acid > malic acid > oxalic acid and that the leaching order for Cr and Cu was citric acid > oxalic
acid > malic acid. The concentration of Fe was low in 5 mM oxalic acid leaching for 20 days because of the
hydrolysis precipitation of iron ions and the complexation with organic ligand. The crystal lattice on the tailings
was significantly damaged after leaching. The CO32- peak appeared in M0 with different treatments, and the
proportion of COO- fitting peak areas increased markedly after leaching with LMWOAs.
Keywords:
Iron tailing
Metals
pH
Low-molecular-weight organic acids
Leaching
⁎
Corresponding author.
E-mail address: wangfei@ustb.edu.cn (F. Wang).
https://doi.org/10.1016/j.jhazmat.2019.121136
Received 17 June 2019; Received in revised form 1 September 2019; Accepted 1 September 2019
Available online 05 September 2019
0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
1. Introduction
role in the activation and migration of metals in nature. However, few
of the influence of LMOWAs on metals leaching and the morphology
changes of iron tailings after leaching have been studied.
The aims of this work were to (i) understand the leaching behavior
of Cd, Co, Cr, Cu, Fe, Mn, Ni, and Zn from two kinds of iron tailings at
different initial pH and LMOWAs (citric acid, oxalic acid, and malic
acid), and (ii) explore the microstructure characterization of tailings
before and after pH static and LMWOA leaching using X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron
microscopy (HRTEM). It is useful for providing theoretical support for
leaching risk assessment of actively used and closed tailings impoundments and also providing guidance for protection measures for water
sources.
After the development of agriculture, mining is the most important
industry developed by human society. However, mine working, beneficiation, and other activities inevitably produce tailings. Because iron
tailings not only occupy a large amount of land resources but also often
contain many associated elements. In this case, the tailings diffuse into
surrounding areas through surface runoff and dust, which further results in the release of metal ions through weathering, rainwater, and
microorganisms and seriously pollutes the local soil, water, and atmospheric environments (Kang et al., 2019; Park et al., 2019; Hayes et al.,
2012). Thus, the metal pollution can threaten human production, life,
and physical health (Gil-Loaiza et al., 2018; Tepanosyan et al., 2018).
Tailings ponds are mainly distributed in the upper reaches of the Miyun
Reservoir in the Chaohe River and Baihe River basins. The Miyun Reservoir is the largest reservoir and the only source of drinking water in
Beijing. Hence, the status and degree of metals from tailings leaching is
associated with the water security of Beijing residents (Chen et al.,
2016; Ding et al., 2016).
Iron tailings usually contain As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and
Zn, some of which are highly toxic and carcinogenic. Leaching is the
process of extracting soluble substances from solids using leaching
agents (i.e., water, acid, alkali and salt solutions), and it is a crucial
hydrometallurgical method for metals extraction from various solids
(Ye et al., 2017; Zhang et al., 2019a; Wang et al., 2018a; Wiśniowska
and Włodarczyk-Makuła, 2018a). Mineral acids (Yang et al., 2009) and
low-molecular-weight organic acids (LMWOAs) are always chosen as
leaching agents for solid contaminants. However, the pH of acidic mine
wastewater, or rainwater, redox conditions, cation exchange capacity,
mineral composition, and the concentration of organic ligands can affect the release of metal elements in the actual environment (Gleyzes
et al., 2002; Komonweeraket et al., 2015; Dermont et al., 2008; Wang
et al., 2019; Naidu et al., 2019). Previous reports (Cappuyns and
Swennen, 2008; Houben et al., 2013) have demonstrated that pH is a
paramount parameter for controlling the migration of metal elements
from the solid phase to the liquid phase. For instance, Komonweeraket
et al. (2015) studied the release behavior of metal elements in a soil–fly
ash mixture at a pH values of 1.5 to 13 and found that the concentrations of Ca, Cd, Mg, and Sr monotonically decreased with increasing pH
values. However, Beiyuan et al. (2017) found that the increment of pH
increased the mobility in soil and plant availability of As, while the
effect on Pb was opposite. Some studies reduced the pH of the extract
by increasing the concentration of inorganic acids, such as hydrochloric
acid, sulfuric acid, and phosphoric acid, and then dispersed metal
compounds and dissolved metal mineral components to release metal
ions from tailings (Yang et al., 2009; Zhang et al., 2019b; Kim et al.,
2017).
LMWOAs, including oxalic acid, citric acid, malic acid, acetic acid,
succinic acid, and so on, are mainly produced by secretion by plant
roots, metabolism of microorganisms, and decomposition of organic
matter (Strobel, 2001). Among them, citric acid, oxalic acid and malic
acid are commonly used in many studies because of their high and
widespread content in the environment. Studies have shown that the
total concentration of organic acids in soil is 10−2–5 mM (Sposito,
1989). LMWOAs can dissolve out metal elements in soils, sludge, fly
ash, e-waste, and tailings (Schwab et al., 2008; Zeng et al., 2015; Wang
et al., 2018b; Burckhard et al., 1995; Wiśniowska and WłodarczykMakuła, 2018b). Hernández et al. (2007) used tartaric acid and oxalic
acid to leach about 70% of the Ni, 80% of the Co, and 30% of the Fe
from nickel tailings. Onireti et al. (2017) found that the mobilization of
As, Pb, and Fe was affected by organic acids in soil. Additionally, plant
growth-promoting bacteria exhibiting metal and pH tolerance have
been obtained from the rhizosphere of quailbush plants grown in
Klondyke tailings (Grandlic et al., 2008). Liu et al. (2019) also determined the metabolic activity of bacterial communities in non-ferrous
metal tailings in Guangxi. Therefore, LMOWAs should play a significant
2. Materials and methods
2.1. Study area and tailings sampling
The study sites are located in the Huairou region (40°41′–41°4′ N,
116°17′–116°63′ E), city of Beijing, China (Fig. S1). The region has a
warm temperate, and the average annual precipitation is 600–700 mm.
We selected the Qian'anling and Majuanzi iron tailings ponds because
these two tailings ponds have not been artificially repaired, have relatively little manmade interference, and because the latter has higher
levels of metals in the tailings than the former.
An "S"-shaped multi-point sampling method was used to collect
surface (0–20 cm) tailings. After stripping impurities, the same sampling point was sampled 5 times at different positions and mixed uniformly to form one sample weighing about 1.0–1.5 kg. The samples
were placed in polyethylene bags and transported to the laboratory.
Before analysis, the tailings were dried to a constant weight at room
temperature, ground until all particles passed through a 2 mm nylon
sieve, and subsequently homogenized.
2.2. Chemical and mineralogical analysis
Mine tailing pH was measured with a calibrated pH meter (Mettler
Toledo 8603, USA) in a 0.01 M CaCl2 solution (mine tailing:
water = 1:2.5) after shaking for 30 min (Williams et al., 2011). Organic
matter (OM) content was determined by the Walkley and Black wet
oxidation method (Walkley and Black, 1934). Water soluble salt (WS)
was determined by the gravimetric method. Alkaline nitrogen (AN) was
determined by the alkali-hydrolysis diffusion method. Available phosphorus (AP) was analyzed with inductively coupled plasma optical
emission spectrometry (ICP-OES, Thermo Fisher iCAP 7000, USA).
Available potassium (AK) was extracted with 1.0 mol/L NH4OAc and
measured with an atomic absorption spectrophotometer (Purkinje
General TAS-1901, China). Available sulfur (AS) was determined by the
phosphate−HOAc extraction-BaSO4 turbidimetric method. Tailing
particle size was analyzed by laser particle size analyzer (Mastersizer
3000, England). The tailings were digested in a microwave digestion
system (CEM MARS6, USA). The concentrations of metals (As, Cd, Co,
Cr, Cu, Fe, Mn, Ni, Pb, and Zn) were extracted by aqua regia digestion
and analyzed by ICP-OES. All glassware was acid washed before usage.
X-ray fluorescence spectroscopy (XRF, PANalytical Axios, Holland)
was used to analyze the percentages of total oxides and elements in
tailings qualitatively and quantitatively. X-ray diffraction (XRD, Bruker
D8 Advance, Germany) was used to analyze the main compositions of
the tailings, and based on the ICDD PDF-2 database. XPS (Ulvac-Phi
PHI-5000 Versaprobe, Japan) and HRTEM mapping (JEOL 2100 F,
Japan) was used to analyze the changes of chemical bonds and microstructure in tailings before and after leaching respectively.
2.3. pH-static leaching experiment
The method of the static leaching experiment for the iron tailings
2
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
was developed from the method of the leaching experiment for antimony ores described by Hu et al. (2016). The pH range of the leachate
of 2 to 9.5 (different experiments for initial pH of 2, 4, 6, 7.5, and 9.5)
was selected to study the leachability of metals in the tailings under
acidic, neutral, and weakly alkaline conditions. The initial pH values of
the lixivium were adjusted with 0.1 M HCl and NaOH. The detailed
experimental procedures were presented in Supplementary Material.
To understand the primary crystalline mineral constituents further,
XRD was performed (Fig. 1). The matrix compositions of the two tailings samples were similar, consisting mainly of quartz. Diffraction lines
of As, Cd, Co, Cr, Ni, and Pb were not detected due to low concentration
or presence in an amorphous phase. The main sulfides were manganese
sulfide and sphalerite in M0, which were easily soluble in dilute acid.
Meanwhile, it is likely to cause acid mine drainage (Naidu et al., 2019).
2.4. Low-molecular-weight organic acids leaching experiment
3.2. pH leaching characteristics of metals in the tailings
Citric acid, malic acid, and oxalic acid, as model LMWOAs, were
used in the experiment at concentrations of 1, 2, and 5 mM. The initial
pH and total organic carbon (TOC, Shimadzu TOC-VCPN, Japan) of the
LMWOA solutions was determined. The detailed information referred to
the Supplementary Material.
All chemical agents were of analytically pure, and purchased from
Sinopharm Chemical Reagent Beijing Co., Ltd.
Fig. S2 shows the pH change curves of the leachate at different initial pH values. It is clearly apparent acidic (pH 4 and 6) and basic (pH
8.5 and 9.5) interaction systems tended to become neutral and stable
(pH ˜7.5) after 1 day, except for pH 2 (2 days for Q0 and 4 days for M0).
This occurred because of hydration of minerals in the tailings to form
hydroxides (Hu et al., 2016) and dissolution of minerals by consuming
H+. It is interesting that the pH for M0 was maintained at ˜4 after 4
days. This may be due to a small amount of sulfide in M0 (Fig. S2b),
which can be oxidized by other means and is also affected by iron and
protons (Sánchez-Andrea et al., 2014). (Figs. 2 and 3)
The change curves of metals concentrations in the leachate of Q0
and M0 are shown in Figs. S3 and 2. Since the detection values of As
and Pb were very low (< 0.0001 mg L−1), and their leaching changes
were not analyzed here. It can be seen that metals concentrations are
closely related to the initial pH and leaching time. Cd, Co, Cr, Cu, Fe,
Mn, Ni, and Zn have the highest concentrations at the initial pH 2, and
Fe had the highest leaching concentration from Q0 and M0, 16.19 and
51.72 mg L−1, respectively. But these are very low at initial pH values
of 4, 6, 7.5, 8.5, and 9.5. The reason is mainly hydrolysis and coprecipitation of metal ions. This indicates that H+ was dominant in the
dissolution process of the metal ions (Komonweeraket et al., 2015).
Meanwhile, the lattices of Q0 and M0 changed significantly after
leaching (Figs. S4 and 3). Before leaching of Q0 (a-1, a-2) and M0 (d-1,
d-2), their lattice fringes were relatively intact and closely arranged.
Part of the lattice fringes were destroyed at the initial pH of 7.5 (b-1, b2, e-1, e-2), and obviously destroyed at the initial pH of 2 (c-1, c-2, f-1,
f-2). It is speculated that the lattice fringe damage may be more serious
at the edge. It may be that the edges were easily eroded by acidic liquids, causing metal ions to be released (Qian et al., 2019; Revesz et al.,
2015; Liu et al., 2006).
The leaching amounts of metals tended to decrease with increment
of initial pH and leaching time, although some metal elements were
also detected at alkaline conditions. Similar results were reported by
Cappuyns and Swennen (2008). It is likely that iron phase precipitation
occurs when the pH of a solution changes from acidic to neutral. The Fe
coating grows on the surface of the tailings and prevents the leachate
3. Results and discussion
3.1. Characteristics of the tailings
The physicochemical properties of the Qian'anling (Q0) and
Majuanzi (M0) iron tailings are shown in Table S1. Both tailings samples were weakly alkaline. OM content was slightly higher in Q0 (5.38 g
kg−1) than in M0 (4.32 g kg−1). OM is one of the vital factors influencing the bioavailability of metals in soil (Egli et al., 2010; Yoo et al.,
2018). Studies have shown that the molecular structure and functional
groups of different components in OM have adsorption or reduction
effects on metals (Zhang et al., 2019c; Liang et al., 2019), and it can
form complexes, thereby affecting the migration and transformation of
metals (Guo et al., 2017). Both of the tailings were mainly composed of
sand, which accounted for approximately 97.74% and 98.56% of the
total weight, respectively. The clay content was very low (< 1%). This
suggests that the tailings have high porosity and low water retention,
and may aggravate the leaching of metals by rain.
The contents of the metals in the tailings are summarized in Table
S2. It is obvious that Fe content was most highly enriched in Q0 and
M0. The As content was noticeably higher in M0 (0.748 mg kg−1) than
in Q0 (0.052 mg kg−1), and this difference may be associated with the
original mineral composition of the tailings. However, on the whole,
there was no significant difference in metals contents between the two
tailings.
The XRF results showed that SiO2 accounts for 65.49% of Q0 and
63.55% of M0, respectively (Table S3). Oxide compositions and contents of the two tailings were almost the same.
(a)
a Quartz
b Hematite
c Anorthite
d Dolomite
e Grossularite
f Calcite
g Ramsbeckite
a
(b)
a Quartz
b Hematite
c Dolomite
d Natrolite
e Arsenic manganese
f Calcite
g Ramsbeckite
h Manganese sulfide
i Spinel
j Sphalerite
a
a
a
a
a
a a
cd f
g
20
40
ae
2θ
a
60
b
a
ae a
a
a
J d ce f
g
80
20
a
40
a
h
2θ
Fig. 1. X-ray diffraction analysis of minerals in Q0 and M0.
3
a
a
J b bi
60
a
80
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
0.06
0.05
0.04
0.03
0.02
0.01
0.004
Co concentration (mg L -1 )
Cd concentration (mg L -1 )
0.005
0.003
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
0.002
0.001
0.0004
0.004
0.0002
0.002
0.0000
0.000
2
4
6
8
10
12
0.014
14
16
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
Cr concentratin (mg L -1)
0.012
0.010
0.008
0
4
6
8 10
Time (day)
12
14
16
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
0.05
0.04
0.03
0.02
0.006
0.004
0.01
0.002
0
2
4
6
8
10
Time (day)
12
14
0.00
16
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
40
30
0
2
4
6
8
10
Time (day)
12
10
3
2
1
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
0.1
0.2
0.1
0.0
2
4
6
8
10
Time (day)
12
14
16
4
20
0
14
5
Mn concentration (mg L -1)
50
Fe concentration (mg L -1)
2
Cu concentration (mg L-1)
0
0.0
16
0
2
4
6
8
10
Time (day)
12
14
16
0.06
0.018
0.05
0.015
0.04
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
0.012
0.009
0.006
0.03
0.02
0.01
0.003
0.0005
0.0000
0
2
4
6
8
10
Time (day)
12
14
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
Zn concentration (mg L-1 )
Ni concentration (mg L-1)
pH 2
pH 4
pH 6
pH 7.5
pH 8.5
pH 9.5
16
0.00
0
2
4
6
8
10
Time (day)
12
14
16
Fig. 2. The leached concentration changes of metals from Majuanzi iron tailings at different pH.
from eroding the tailings, thereby impeding the release of other metals
(Hu et al., 2016). It is worth noting that under the initial pH 2, the
concentration of metals leached from Q0 decreased monotonously and
that leached from M0 stabilized gradually. This was mainly caused by
the change of pH during the leaching process (Fig. S2). The metals for
Q0 and M0 reached a maximum after leaching for 1 day and 2–4 days,
respectively. This indicates that the tailings were exposed to acidic
water for 1–4 days and that the risk of metal dissolution was the
4
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
Fig. 3. HRTEM images of M0 before and after leaching: original tailings HRTEM images (d-1, d-2), HRTEM images of tailings after leaching under initial pH 7.5 (e-1,
e-2) and initial pH 2 (f-1, f-2) conditions.
highest. In this case, a longer hydraulic retention time is beneficial to
the reuse of flotation wastewater. This has great significance for the
management of existing tailings pond and the reuse of wastewater.
XPS spectra of the original tailings and the tailings after 16 days of
leaching in the initial pH 2 and 7.5 leachate are shown in Fig. S5. The
main characteristic peaks were C, O, and Si, and the characteristic
peaks of Fe, Ca, and Mg were weakened and the peak area was reduced
compared with the original tailing spectra. The ratio of Fe 2p content to
total element content in Q0 original tailings and the tailings of the initial pH 7.5 and pH 2 leaching experiments were 4.05%, 2.96%, and
2.88%, respectively at 16 days. The content of Fe in Q0 was strikingly
reduced after leaching. This result reveals that metal ions are dissolved
out of tailings after soaking, and are released preferably in an acidic
environment. Hence the surrounding soil and water are at high risk of
being contaminated by metals (Cánovas et al., 2019).
The XPS peak fitting parameters assembled from previous research
sources (Zhang et al., 2019b; Chi et al., 2019; Moreira et al., 2017) were
used to fit the O 1s, Fe 2p, and C 1s spectra shown in Figs. S6, S7, and 4.
The relative parameters are listed in Tables S4 and S5. In the highresolution spectra of the Fe 2p region, the Fe 2p3/2 spectrum of the
tailings was primarily fitted using Fe2O3 and Fe3O4 (II) peak. Moreover,
the satellite characteristics under different treatments were significantly different. The C1 s energy spectrum of Q0 with different
treatments can be divided into three peaks (Fig. 4a, b, c), which were CC, C–O and COO-, and their binding energy were 284.6 ± 0.1,
286.12 ± 0.1, and 288.58 ± 0.1 eV, respectively. The intensity of the
characteristic peak change after the tailings were immersed in the different pH solutions was non-significant. Fig. 4 (d–f) shows that the C 1s
energy spectrum of M0 with the different treatments can be divided
into CeC, CO, COO, and CO32ee−, and their binding energy were
284.6 ± 0.1, 286 ± 0.1, 288.6 ± 0.1, and 289.51 ± 0.2 eV, respectively. The peak of CO32− changed for the leached tailings compared with the original tailings, and the peak area of CO32− was almost
0 in the initial pH 2. The excess acid neutralizes CO32−, thus decreasing
its peak area.
3.3. Low-molecular-weight organic acid leaching characteristics of metals in
the tailings
The content of Fe was the highest content, up to 57,900 mg kg−1 for
for M0 (Table S2). Cu, also with a content, was a
common companion element in the iron ore. As, Pb, Cd, and Cr are
more toxic in nature, but the contents of As and Pb in the leachate were
low and not detected. Therefore, Fe, Cu, Cd, and Cr were determined to
explore their release from the tailings in three LMWOAs. Among these,
oxalic acid has the strongest acidity, followed by citric acid and then
malic acid (Table S6).
The variation trends of TOC and pH of Q0 and M0 were similar
under the same LMWOA leaching (Fig. 5). TOC ranked as citric
acid > malic acid > oxalic acid at a given concentration of LMWOA.
TOC of malic acid and citric acid extracts decreased slowly, whereas the
TOC of 5 mM oxalic acid extract decreased significantly, being reduced
by 78.26% and 74.43%, respectively, which was closely associated with
the consumption of the LMWOAs.
The leachate pH for the two tailings varied from strongly acidic to
weakly acidic or even neutral after 20 days. The pH increased rapidly
for the 1 mM and 2 mM LMWOA leaching solutions after 10 days of
leaching, and became weakly alkaline. The solution pH for Q0 and M0
after leaching in 5 mM citric acid for 20 days were 6.86 and 5.80.
Although the initial pH of the citric acid leachate was higher than that
of the other two acids, the solution pH was lower after leaching for 20
days. As malic acid and oxalic acid contain two carboxyl groups and
citric acid contains three carboxyl groups, citric acid can provide more
H+ at the same concentration than before two. Simultaneously, as the
two tailings contained a certain amount of alkaline substances (i.e.,
calcite and dolomite) and had strong acid buffering capacity, the final
pH of the leachate increased with the depletion of H+ (Wang et al.,
2018b).
It can also be seen that the concentration of the same metal in the
same LMWOA leaching solution of Q0 and M0 had a similar trend with
leaching time (Figs. S8 and 6). The concentration of free metal ions in
Q0 and 64,775 mg kg−1
5
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
284.6
C-C
Original
Fit
Background
C-C
C-O
COO-
Intensity (a. u.)
(a)
286.12
C-O
282
284
286
288
Binding energy (eV)
284.6
C-C
Intensity (a.u.)
(d)
284
282
290
288.6
COO-
286
288
Binding energy (eV)
288.58
COO-
284
286
288
Binding energy (eV)
284.6
C-C
(e)
Original
Fit
Background
C-C
C-O
COOCO23
286
C-O
282
286.12
C-O
288.58
COO-
284.6
C-C
(c)
pH 7.5
Fit
Background
C-C
C-O
COO-
284.60
C-C
(b)
286.12
C-O
290
282
284.6
C-C
288.6
COO289.51
CO23
CO23
290
282
284
286
288
Binding energy (eV)
290
282
284
290
pH 2
Fit
Background
C-C
C-O
COOCO23
286
C-O
288.6
COO- 289.51
289.51
CO23
288.58
COO-
284
286
288
Binding energy (eV)
(f)
pH 7.5
Fit
Background
C-C
C-O
COOCO23
286
C-O
pH 2
Fit
Background
C-C
C-O
COO-
286
288
Binding energy (eV)
290
Fig. 4. XPS survey spectra of C 1s on the original and leached at pH 7.5 and pH 2 for 16 days, Q0 (a–c) and M0 (d–f).
the 5 mM LMWOA leachates was significantly higher than that in the
1 mM and 2 mM leachates, suggesting that low concentrations of the
LMWOAs were insufficient to leach large amounts of metals. Therefore,
increasing the concentration of the LMWOAs in the leachate generally
contributed to the dissolution of metals in the tailings.
(a)
50
0
0
TOC (mg L -1)
100
-100
Malic
-200
-300
0
5
10
Time (day)
15
1mM TOC
2mM TOC
5mM TOC
1mM pH
2mM pH
5mM pH
20
TOC (mg L -1)
(d)
0
5
10
Time (day)
15
20
-200
Malic
-300
0
5
10
Time (day)
15
1mM TOC
2mM TOC
5mM TOC
1mM pH
2mM pH
5mM pH
20
8
-200
Citric
-300
-400
300
0
5
10
Time (day)
15
1mM TOC
2mM TOC
5mM TOC
1mM pH
2mM pH
5mM pH
Oxalic
10
8
0
5
10
Time (day)
15
20
-100
-200
Citric
-300
-400
0
5
10
Time (day)
Fig. 5. Characteristics of pH and TOC in LMWOAs leachate with time, Q0 (a–c), M0 (d–f).
6
2
12
0
-100
4
14
(f)
100
1mM TOC
2mM TOC
5mM TOC
1mM pH
2mM pH
5mM pH
6
20
200
-50
-150
10
-100
400
(e)
0
-100
-400
Oxalic
-100
1mM TOC
2mM TOC
5mM TOC
1mM pH
2mM pH
5mM pH
50
0
12
0
-50
-150
(c)
100
100
100
300
200
150
300
200
(b)
100
14
pH
200
400
150
pH
300
Except for Fe, the concentrations of metals in the 1 mM and 2 mM
LMWOAs were low or even nearly nonexistent. When leaching was
done with 5 mM malic acid and citric acid, the concentrations of Cd, Fe,
Cr, and Cu generally increased. Meanwhile, as the pH of the leachate
increased, the dissolution rate of metals in tailings slowed, indicating
15
1mM TOC
2mM TOC
5mM TOC
1mM pH
2mM pH
5mM pH
20
6
4
2
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
that Cd, Fe, Cr, and Cu are more likely to leach out under acidic conditions. Hence, the form of metals was changed, affecting the migration
and bioavailability of metals in the environment. The concentrations of
the four metals reached a maximum on the 5th day of leaching in 5 mM
oxalic acid, and the concentrations of Cd and Fe were almost 0 after 20
days. The order of release of Cd and Fe by the LMWOAs was citric
acid > malic acid > oxalic acid, and that for Cr and Cu was citric
acid > oxalic acid > malic acid. Additionally, The HRTEM images of
the Q0 and M0 leached in 5 mM LMWOAs (Figs. S9 and 7) show that
lattice fringes of both tailings were relatively intact and closely arranged before leaching. When soaked with the different LMWOAs for
20 days, the lattice fringes on the tailings were damaged to varying
degrees. The most serious damage was caused by citric acid. Therefore,
it was proven that citric acid leaches metals from tailings more easily
than the other two acids. Mappings of the iron tailings with different
treatments are shown in Fig. S10.
The concentration of Cd was much higher at the high concentration
of the LMWOAs (Figs. S8a and 6a), which might be attributable to
greater dissolution of metal-bearing minerals (carbonates and sulfates)
with increasing concentration of H+ (Tiruta-Barna et al., 2006;
Taghipour and Jalali, 2016). Overall, Cd of M0 was 0.0128 mg L−1 after
20 days of leaching in citric acid and was significantly higher than that
in malic acid or oxalic acid. Remarkably, the concentration of Cd leached by oxalic acid increased first and then decreased rapidly with
time. Because a high concentration of H+ competed with Cd2+ to
prevent Cd2+ accessible sites on tailings at lower pH values. With pH
increase, hydroxyl groups and carboxyl groups tended to be deprotonated, so positive metal ions could be adsorbed (Ash et al., 2016),
forming an insoluble complex reducing the toxic effect of Cd.
Fe (Fig. S8b and 6b) exhibited a leaching pattern similar to that of
Cd. The concentration of Fe in malic acid did not decrease dramatically
with increasing pH, supposedly because malic acid formed a stable
complex with the dissolved iron. In the 5 mM oxalic acid solution of Q0
and M0, Fe increased rapidly to 63.48 and 95.38 mg L−1 in 5 days, and
the ratio of leaching amount to total Fe content were 10.96% and
14.72% respectively. However, the concentration of Fe decreased
monotonously after 5 days. Especially after 20 days, the concentration
decreased to a very low level, 1.69 and 3.03 mg L−1, respectively. This
should be attributable to two main reasons for the reduction of Fe
concentration. Firstly, Fe3+ was prone to hydrolysis precipitation.
Secondly, oxalic acid was found to have good complexing properties
and strong reducibility; thus, the dissolved Fe3+ could be reduced to
Fe2+. In this case, the Fe2+ would combine with an organic ligand to
form iron (II) oxalate dihydrate, which is a water-insoluble precipitate
(Du et al., 2011; Jiang et al., 2019). The chemical reactions are shown
below (Taxiarchou et al., 1997; Ubaldini et al., 1996):
Fe2 O3 + 6H2 C2 O4 = 2Fe(C2 O4 )33 + 6H+ + 3H2 O
Jalali (2016). After leaching for 20 days, malic acid, oxalic acid, and
citric acid leached 0.0154, 0.0525, and 0.108 mg L−1 of Cr from Q0 and
0.0174, 0.0461, and 0.0677 mg L−1 from M0. The lost percentage of Cr
in the citric acid leaching solution of the two tailings were 6.11% and
4.82% respectively. The significant difference of Cr concentration can
be explained by the following: (i) the content of Cr in Q0 was greater
than that in M0 (Table S2); (ii) the structure of the LMWOAs (Table S6)
was relevant, such that different functional groups have different ability
to release metals; and (iii) there was interaction among metal ions
caused by the dissolution of multiple minerals (Wrobel et al., 2015;
Kwak et al., 2018). As reported previously, Cr was mainly present as a
Cr6+ compound at pH < 5.5, and HCrO4- was predominant (Hyks
et al., 2009). In addition, Wrobel et al. (2015) and Mu et al. (2018)
confirmed the catalytic action of Mn2+ on oxalic acid reduction of
Cr6+. That is, Cr6+ can be reduced to Cr3+ in the presence of Mn2+,
which can result in a final Cr3+ tri-oxalato complex that exists stably.
The equations are as follows (Puzon et al., 2005; Gupta and Babu,
2009):
2HCrO4 + 3HC2 O4 + 11H+
Cr 3 + + 3HC2 O4
Cr (III )
3+
2+
2Cr 3+ + 6CO2 + 8H2 O
(4)
(HC2 O4 )3
2+
(3)
2+
In addition, Fe , Fe , Cu , and Mn can also promote catalytic
reduction of Cr by citric acid (Marinho et al., 2016; Li et al., 2015;
Sarkar et al., 2013; Hug et al., 1997). Simultaneously, the ligand provided by citric acid can combine with the Cr3+ to produce a stable
chelation body. Consequently, LMWOAs have a remarkable impact on
the toxicity and bioavailability of Cr and reduce its impact on the
surrounding environment (Dai et al., 2010). Nevertheless, further research is needed to identify the mechanism of metal ion interaction and
the proportions of Cr in various speciations during the leaching process.
(Figs. 6 and 7)
Fig. S8d and 6d show that citric acid facilitated a greater capacity
for Cu mobilization (Pérez-Esteban et al., 2013). At the end of leaching,
the Cu concentration released by citric acid from Q0 and M0 was 0.156
and 0.143 mg L−1, and the ratio of leaching amount to total Cu content
were 20.75% and 26.18%, respectively. Whereas those released by
oxalic acid were 0.103 and 0.106 mg L−1 and those released by malic
acid were 0.092 and 0.078 mg L−1, respectively. A previous study reported that LMWOAs with more carboxyl groups have a stronger extraction capacity (Onireti et al., 2017; Suanon et al., 2016) because
LMWOAs with two or three −COOH can form stable chelates having a
5- or 6-membered ring structure (Qin et al., 2004); thus, citric acid can
dissolve more metal ions from tailings. However, the dissolution
amount of Cu from Q0 decreased after 5 days, possibly because the
newly formed metal-ligand complexes were re-adsorbed onto the surface of tailings minerals (Guo et al., 2018).
XPS was employed to elucidate the elemental composition and
element valence state of the iron tailings before and after leaching by
the LMWOAs. XPS full spectrum scans of Q0 and M0 and the tailings
after leaching with the three LMWOAs for 5 days and 20 days are shown
in Fig. S11. The main elements of the different treated tailings surfaces
were Si, C, O, K, Ca, Mg, and Fe, but the relative content of each element and the chemical composition varied. The peak area of most
elements was generally increased compared with the original tailings;
the LMWOAs destroyed the surface of the tailings and exposed various
minerals to the leachate. However, the generated metal hydroxide
precipitate and the weakly water-soluble chelate may cover the surface
of the tailings. For example, the Mg and Ca peaks of 5 mM oxalic acid
were significantly larger than original peak areas.
Figs. S12 and S13 exhibit the O 1s and Fe 2p peaks of the Q0 and M0
tailings in different conditions, and the proportions of the different
fitting peak areas are listed in Tables S7 and S8. As shown in Fig. 8, the
C 1s spectra of Q0 (Fig. 8 (a–c) with the different treatments can be
deconvoluted into three peaks, 284.60 ± 0.1, 286.12 ± 0.1, and
(1)
2Fe(C2 O4 )33 + 6H+ + 4H2 O= 2FeC2 O4 2H2 O+ 3H2 C2 O4 + 2CO2
(2)
Sobianowska-Turek et al. (2016) indicated that Fe and Cd were almost co-precipitated entirely when leaching waste batteries by oxalic
acid. Mazurek (2013) and Taxiarchou et al. (1997) also reported that
the content of iron compounds decreased systematically with increasing
concentration of OH− in the solution.
The increment of Fe concentration in citric acid was mainly due to
the dissolution of Fe-bearing minerals in low pH conditions and citric
acid formed a soluble complex with Fe. A previous study showed that
the dissolution of amorphous Fe oxides by citric acid results in the release of various toxic metals from soil (Schwab et al., 2008), and this
could be one of the reasons for the leaching of other metal ions in the
citric acid leachate.
The result of Cr from citric acid leachate had a better leaching effect
(Figs. S8c and 6c) was similar to Chen et al. (2018) and Taghipour and
7
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
(a) 0.015
0.015
Cd concentration (mg L-1 )
1mM malic
2mM malic
5mM malic
0.012
0.012
0.009
0.009
0.006
0.006
0.006
0.003
0.003
0.003
5
10
Time (day)
15
0
20
10
Time (day)
15
20
0
150
120
120
120
90
90
90
60
60
60
30
30
30
Fe concentration (mg L -1 )
150
0
0
5
10
Time (day)
15
20
0.08
5
10
Time (day)
15
20
C r co ncen tration (m g L -1 )
0
0.04
0.04
0.04
0.02
0.02
0.02
0.00
0.00
0.00
15
20
0
5
10
Time (day)
15
20
0
0.15
0.15
0.12
0.12
0.12
0.09
0.09
0.09
0.06
0.06
0.06
0.03
0.03
0.03
0.00
0.00
C u co n c en tra tio n (m g L -1 )
0.15
0
5
10
Time (day)
15
20
15
20
5
10
Time (day)
15
20
15
20
15
20
0.06
0.06
10
Time (day)
10
Time (day)
0.08
0.06
5
5
0
0
0.08
0
(d)
5
150
0
(c)
0.000
0.000
0
1mM citric
2mM citric
5mM citric
0.012
0.009
0.000
(b)
0.015
1mM oxalic
2mM oxalic
5mM oxalic
5
10
Time (day)
0.00
0
5
10
Time (day)
15
20
0
5
10
Time (day)
Fig. 6. The leached concentration changes of metals from Majuanzi iron tailings at different concentrations of LMWOAs with time.
288.58 ± 0.1 eV assigned to CeC, CO, and COO,ee respectively. The
C1 s spectra of M0 (Fig. 8 (d–f) with the different treatments can be
deconvoluted into four peaks, 284.60 ± 0.1, 286.12 ± 0.1,
288.58 ± 0.1, and 289.50 ± 0.2 eV assigned to C-C, C–O, COO-, and
CO32−, respectively. Apparently, the peak area ratio of CO32− decreased compared with that of the original tailings, and the proportion
of the COO- fitting peak areas increased markedly, indicating that COOfrom the organic acid complexes with the tailings (Tables S5 and S7).
To summarize, in comparison with the result of the pH-static
leaching, the LMWOAs more easily leached metals from the tailings.
They greatly enhance the migration and mobility of hazardous metal
elements in the environment.
4. Conclusion
The Qian'anling and Majuanzi iron tailings have similar mineral
composition. For the pH-static leaching study, the pH of the leachate of
the two tailings tended to be neutral or weakly alkaline after leaching,
except for M0 with an initial pH of 2; therefore, the two iron tailings
have a certain acid buffering capacity. pH is an important factor controlling the dissolution of the metals, and the leaching solution with
lower pH is beneficial to the release of metal ions. Compared with the
original tailings M0, the crystal lattice on the tailings was destroyed,
which indicated the dissolution of metallic minerals. LMWOAs can effectively activate metals and enhance their migration and fluidity.
8
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
Fig. 7. HRTEM images of M0 before and after leaching: original tailings HRTEM images (e-1, e-2), HRTEM images of M0 leached in 5 mM malic acid (f-1, f-2), 5 mM
oxalic acid (g-1, g-2), and 5 mM citric acid (h-1, h-2) for 20 days.
Higher concentration of the LMWOAs resulted in better digestion of the
metals. Under the same concentration of LMWOA, citric acid leaches
metals more easily from tailings. COO- from the LMWOAs complexes
with the tailings after leaching.
Compared with the pH-static leaching, the LMWOAs greatly promoted the dissolution of minerals and the release of metals. Overall,
(a)
284.60
C-C
286.12
C-O
282
(d)
5mM Malic
Fit
Background
C-C
C-O
COO-
288.58
COO-
286.01
C-O
284.60
C-C
5mM Oxalic
Fit
Background
C-C
C-O
COO-
286.12
C-O
284
286
288
Binding energy (eV)
284.59
C-C
(b)
this study has important guiding significance for the management of
existing tailings ponds and the reuse of wastewater. In addition, it
provides a direction for further study of the effects of plant and microbial secretions on metals migration and bioavailability in the real
environment.
290
5mM Malic
Fit
Background
C-C
C-O
COOCO23
282
(e)
286
C-O
288.53
COO289.49
CO23
282
284
286
288
Binding energy (eV)
290
282
290
5mM Oxalic
Fit
Background
C-C
C-O
COOCO 23
288.59
COO-
284
286
288
Binding energy (eV)
284.60
C-C
286.12
C-O
288.58
COO-
284
286
288
Binding energy (eV)
284.59
C-C
(c)
282
288.58
COO-
284
286
288
Binding energy (eV)
(f)
284.6
C-C
286
C-O
289.65
CO 23
290
5mM Cirtic
Fit
Background
C-C
C-O
COO-
282
5mM Citric
Fit
Background
C-C
C-O
COOCO23
288.6
COO- 289.57
CO23
284
286
288
Binding energy (eV)
Fig. 8. XPS survey spectra of tailings leached by LMWOAs for 20 days, Q0 (a–c) and M0 (d–f) of C 1s regions.
9
290
290
Journal of Hazardous Materials 383 (2020) 121136
H. Geng, et al.
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121136.
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