minerals

Article
Research on the Adsorption Behavior of Heavy Metal
Ions by Porous Material Prepared with
Silicate Tailings
Dongxiao Ouyang 1,† , Yuting Zhuo 2,† , Liang Hu 1 , Qiang Zeng 1 , Yuehua Hu 1 and Zhiguo He 1, *
1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China;
Ouyangdongxiao@csu.edu.cn (D.O.); huliang2018@csu.edu.cn (L.H.); 165601027@csu.edu.cn (Q.Z.);
hyh@csu.edu.cn (Y.H.)
2 School of Life Science, Central South University, Changsha 410012, China; znyutingzhuo@csu.edu.cn
* Correspondence: zghe@csu.edu.cn
† The first two authors contributed equally to this paper.




Received: 14 March 2019; Accepted: 8 May 2019; Published: 11 May 2019


Abstract: Tailings generated from mineral processing have attracted worldwide concerns due
to creating serious environmental pollution. In this work, porous adsorbents were prepared as
a porous block by using silicate tailings, which can adsorb heavy metal ions from the solution
and are easy to separate. The synthesized silicate porous material (SPM) was characterized by
X-ray diffraction (XRD), Brunner–Emmet–Teller (BET), and scanning electron microscope (SEM).
The material presented a surface area of 3.40 m2 ·g−1 , a porosity of 54%, and the compressive strength
of 0.6 MPa. The maximum adsorption capacities of Pb2+ , Cd2+ , and Cu2+ by SPM were 44.83 mg·g−1 ,
35.36 mg·g−1 , and 32.26 mg·g−1 , respectively. The experimental data were fitted well by the Freundlich
and Langmuir adsorption models. The kinetics of the adsorption process were fitted well by the
pseudo-first order kinetic equation. These results show that the porous materials prepared with
silicate tailings could act as an effective and low-cost adsorbent for the removal of heavy metal ions
from wastewater. This study may provide a new thought on the high-value utilization of tailing for
alleviating environmental pressure.

Keywords: heavy metals removal; solid waste; silicate porous material; water dispose; recycling

1. Introduction
According to the report of the United Nations Development Program, only one-third of the
world’s population can get clean water [1], and heavy metal ions are one of the main pollutants
that pollute water. Sources of heavy metal pollution are extensive, including paint, electroplating,
metal finishing, fertilizer, electrical, pigment industries, and wood manufacturing, etc., which impact
on the environment and human health [2]. Various wastewater treatment techniques have been
used for the removal of heavy metal ions [3], such as chemical precipitation [4,5], ion exchange [6],
adsorption [2,7], membrane filtration [8], and electrochemical treatment [9]. Among these methods,
adsorption treatment is considered as a low-cost, high-efficient, eco-friendly, and easily operated
method [10,11]. It is of great significance for exploring the proper adsorbents, since the severity of
heavy metal pollution and high processing costs are increasing.
Solid waste is another serious environmental issue. Global solid waste production was about 11
billion tons in 2011, and about 1.74 tons solid wastes for per capita were generated in one year [12].
Since rapid urbanization and industrialization need a lot of mineral resources, and these processes
will produce vast solid wastes (e.g., tailings), serious environmental problems are then raised. The
untreated solid wastes have not only occupied a large area of land resource, but also pollute water and

Minerals 2019, 9, 291; doi:10.3390/min9050291 www.mdpi.com/journal/minerals

Minerals 2019, 9, 291 2 of 16

air if not disposed properly. Landfill was the common method used for disposing waste solid in the
past, but this method was inefficient and easily caused secondary pollution [13]. Thus, the secondary
utilization of solid waste is a means of effectively relieving environmental pressure and realizing
recycling. The use of silicate tailings as an adsorbent is a desired secondary utilization method for
solid waste.
There are many studies that investigated the possibility of solid waste for heavy metal adsorption,
such as black liquor lignin, slag, flue dust, fly ash, blast furnace sludge, and red mud [14–16]. Similarly,
silicate tailings, as an eco-friendly material, were feasible for the removal of various harmful cations
due to their wide specific surface area and high adsorption capacity. In recent years, it has been widely
studied that natural silicates and modified silicates could be used as potential adsorbents to remove
various heavy metal ions from aqueous solvents [17–22]. However, the silicate adsorbents that have
been reported were powdered and granulated; these forms of adsorbents were difficult to separate
from the solution, and secondary pollution arose easily [23–25]. To find a simple and inexpensive
method for separating adsorbents from solution, preparing the adsorbent into a stable structure such
as a block with pores is a convenient way to realize this goal [26–28].
Sintering is a simple processing method that has been extensively applied and can observably
improve the adsorption capacity of silicate minerals [28]. Sintering minerals at high temperatures can
remove the combined water, hydration water, and the water participated in skeleton construction,
thus reducing the adsorption resistance of water film to pollutants and increasing its adsorption
performance [29]. At the same time, most of the silicate minerals have large specific surface areas, and
the surfaces are covered with Al–O– and Si–O–, which could be easily charged and combined with
heavy metal ions; these characteristics can also be kept after sintering. Therefore, it is a feasible and
beneficial method to sinter silicate tailings into block porous materials to remove heavy metal ions
from wastewater.
This work aimed to sinter a bulk porous adsorbent with silicate tailings, which have favorable
adsorption capacity and can be easily separated. Meanwhile, the adsorptive behaviors of heavy metals
by the synthesized silicate porous material (SPM) were further investigated to evaluate their possible
application in the heavy metals containing wastewater.

2. Materials and Methods

2.1. Reagents
The reagents used in this work were all analytical grade. The raw materials for the preparation of
SPM were silicate tailings, bauxite, bentonite, Fe2 (SO4 )3 , MgSO4 , and NaHCO3 . The silicate tailing
samples of flotation were obtained from Guangdong, China, and the bauxite was obtained from
Jiangxi, China. Chemical analysis of the raw materials mentioned above is summarized in Tables 1
and 2. Bentonite with more than 85 wt % of montmorillonite content was obtained from Henan, China.
The nitrate solution was dissolved in deionized water to a desired concentration in a stoichiometric
amount to prepare a heavy metal ions solution. The pH values of the solution were adjusted by using
0.1 mol·L−1 of NaOH or 0.1 mol·L−1 of HNO3 solution, and solution ionic strength was adjusted by
using NaNO3 solution.

Table 1. Chemical elemental contents in silicate tailing.

Element Oxide SiO2 Al2 O3 Fe2 O3 CaO TiO2 K2 O MgO

Chemical composition (%) 73.123 14.095 2.273 1.728 0.229 5.042 0.317

Table 2. Chemical elemental contents in bauxite.

Element Oxide SiO2 Al2 O3 Fe2 O3 CaO TiO2 K2 O MgO

Chemical composition (%) 28.609 53.231 3.255 2.883 3.734 3.715 1.338
Minerals 2019, 9, 291 3 of 16

2.2. SPMs Preparation

The SPM was prepared by a sintering method. Silicate tailings and bauxite were milled and
sieved through a 200-mesh screen. The raw materials of 64.35 wt % silicate tailings, 27.15 wt %
bauxite, 1.35 wt % Fe2 (SO4 )3 , 0.92 wt % MgSO4 , 1.7 wt % NaHCO3 , and 4.53 wt % bentonite were
finely mixed, and water (w:v = 10:3.5) was added after mixing the raw materials. A certain amount of
the mixture was uniaxially pressed into the mold (length 70 mm, width 50 mm, and height 30 mm)
to obtain the green body, which was then placed into the drying oven for 4 h at 80 ◦ C. In the drying
process, the green body was expanded with the releasing gas, and the interconnected pores were then
formed. Subsequently, the desiccated green body was placed into a muffle furnace with a ramp rate of
10 ◦ C·min−1 from the room temperature to 1140 ◦ C and kept for 20 min. After the sintering period, the
enamel on the material surface was removed by polishing (Figure S1). All the heat treatments of the
green body were conducted with normal pressure and temperature.

2.3. Characterization
Mineralogical analyses of the raw materials and SPM were performed by X-ray diffraction (XRD,
SeimensO8DISCOVER, Seimens, Berlin, Germany); the prepared samples were exposed to an X-ray
with the 2θ angle varying between 10–80◦ with Cu Kα radiation. The applied voltage and current were
40 kV and 30 mA, respectively. The major chemical compositions of SPM were analyzed by an X-ray
fluorescence (XRF) spectrometer (AxiosmAX Panalytical. B.V, Milford, CT, USA). The microstructure,
textural, and morphological characteristics of the SPM sample were observed by the JSM-6460LV
scanning electron microscope (Japan electronics corporation, Tokyo, Japan). The specific surface area
of the SPM was determined by an automatic specific surface analyzer (Quantachrome Instruments
Monosorb, Quantachrome, Düsseldorf, Germany). Fourier transform infrared (FTIR) spectra were
recorded on a Nicolet 6700 spectrometer (Nicolet, Waltham, MA, USA). The compressive strengths of
the samples, which were cut from the cubes with a normal size of about 20 mm × 20 mm × 20 mm,
were measured using an Inst LTD Hydraulic servo dynamic test system 8802 (Instron, London, UK).
The porosity of the SPM was measured by a pycnometer and calculated by the formula (Equation (1)):

ρg
!
n = 1− × 100% (1)
ρ

where ρg is the bulk density, ρ is the true density, and n is the porosity.

2.4. Determination Point of Zero Charge

To determine the point of zero charge [30], a series of 50-mL centrifugal tubes loaded 45 mL of
0.1 mol·L−1 of NaNO3 solution were transferred. The solution pH values were adjusted from 2.0 to
10.0 by adding either 0.1 mol·L−1 of HNO3 or 0.1 mol·L−1 of NaOH solution. The solution total volume
in each flask was adjusted to 50 mL exactly by adding the NaNO3 solution. Then, the pH0 of the
solutions were subsequently recorded. SPM (1 g) was immediately added to each centrifugal tube and
securely capped. Then, the suspension was manually agitated, and the pH values of the supernatant
liquid were recorded. The differences between the initial and final pH values (pH0 −pHf ) were plotted
through the pH0 .

2.5. Adsorption Experiments

Batch mode experiments were conducted on the adsorption of heavy metals by SPM from solutions
containing single heavy metal ions at an initial concentration of 125 mg·L−1 and an ionic strength of
0.01 mol·L−1 of NaNO3 at a temperature of 30 ◦ C. SPM adsorbents that were cut into small cubes
(0.5 g) were added into 50-mL capped polyethylene bottles that contained 40 mL of single metal ion
solutions; the competitive experiment used 40 mL of multiple metal ion solutions. The bottles were
sealed and agitated at 150 revolutions per minute (rpm) in an orbital shaker. The energy-dispersive
Minerals 2019, 9, 291 4 of 16

spectrum (EDS) was used to confirm that heavy metal ions have been adsorbed on the SPM. After the
adsorption process, the mixed solution was centrifuged at 5000 rpm for 10 min, and the supernatant
was reserved for Pb2+ , Cd2+ , and Cu2+ analyses by inductively coupled plasma mass spectrometry
(ICP-MS, PerkinElmer Elan 6000, PerkinElmer, Waltham, MA, USA). The experimental conditions are
shown in Table 3.

Table 3. Batch mode experimental conditions.

Experimental Conditions Experiment Parameter

Contacting time of 0.5, 1.5, 3.5, 7.5, 11.5, 15.5, 19.5, 24, 26 and 28 h;
Contacting time
initial concentration of 125 mg·L−1 ; pH = 4
Initial heavy metal ions concentrations of 20, 40, 60, 80, 100, 150, 200,
Initial heavy metal ions concentrations 400, 600, 800, 1000, 1200 and 1400 mg·L−1 ; contacting time 24 h;
pH = 4
Initial pH values of 2, 3, 4, 5, 6 and 7; initial concentration of
Initial pH values
125 mg·L−1 ; contacting time 24 h
Contain Pb2+ , Cd2+ and Cu2+ of 20, 40, 60, 80 and 100 mg·L−1 ,
Competitive adsorption
respectively; contacting time 24 h

All the experiments were performed in triplicate, and the procured data were analyzed by Origin
8.0 (OriginLab, Northampton, MA, USA). The adsorption amounts of heavy metal ions at equilibrium
state were calculated using Equation (2), and the removal efficiency was calculated using Equation (3).

(Co − Ce)V
qe = , (2)
M
C0 − Ce
Removal efficiency = × 100%, (3)
C0
where qe is the adsorption capacity (mg·g−1 ), C0 is the initial concentration of a heavy metal (mg·L−1 );
Ce is the equilibrium concentration of a heavy metal (mg·L−1 ); V is the volume of solution (L), and M
is the mass of adsorbent (g). The average value was obtained, and the error ranges were controlled
within ±5%. The amount of each ion adsorbed on SPM was calculated and expressed as a percentage
by using the difference between the amount of ion in the initial solution and the final solution.

2.6. Desorption Experiments

The desorption experiments were carried out with 0.1 mol·L−1 of HNO3 solution. First, 0.5 g
of SPM loaded with single heavy metals after adsorption was dried in the oven. After drying, the
SPM was added into 40 mL of HNO3 solution and shaken in a shaker at 30 ◦ C for 24 h. Next, the
mixture was centrifuged at 4000 rpm for 10 min, and the SPM precipitant was collected. The amount
of desorbed heavy metal ions was determined by the amount of heavy metal ions in the solution.

2.7. Adsorption Isotherm of SPM

Langmuir and Freundlich adsorption isotherm models were used to depict the equilibrium
between adsorbed Pb2+ , Cd2+ , and Cu2+ on SPM (qe ) and ions concentration in solution (Ce ) at constant
temperature (30 ◦ C). The ratio of adsorbent to solution volume was 12.5 g·L−1 , and the adsorption
amount kept rising during heavy metal ions concentration ranging from 20 mg·L−1 to 1000 mg·L−1 .

2.8. Kinetic Study of SPM

The pseudo-first order and pseudo-second order kinetic models were used to describe the
adsorption process at a heavy metal ions concentration of 125 mg·L−1 and temperature of 30 ◦ C.
The ratio of adsorbent to solution volume was 12.5 g·L−1 . The removal rate of heavy metals in the
solution was increased with time from 0 to 28 h.
 Based on the BET analysis, the SPM has a specific surface area of 3.40 m ·g , and a porosity of
54%. The compressive strength of SPM is 0.6 MPa. The above characteristics of SPM basically accorded
with the design requirements. SEM was used to observe the morphology of SPM, which was shown
in Figure 2. Some favorable features of SPM were found; the material surface had geometrical
properties with irregular shape and large agglomerates and the rugged surface, which would provide
Minerals 2019, 9, 291 5 of 16
more sites for adsorbing heavy metal ions.
The FTIR spectra of SPM, silicate tailings, bauxite, and bentonite were shown in Figure 3. The
peaks
2.9. of these
Column substances were mainly distributed in three regions: 3399–3695 cm−1, 1608–1873 cm−1,
Experiments
and 542–792 cm−1. The characteristic absorption peaks at 3399 cm−1 and 3695 cm−1 were assigned to
The columns were constructed from a polymethyl methacrylate cylinder (inner diameter 25 mm,
the Si–OH stretching vibration [30]. The peaks at 1629 cm−1 and 1873 cm−1 were attributed to C–O
length 100 mm) which were packed with 16 g of SPM to yield a bed height of 90 mm. Single heavy
lattice vibrations [34]. The adsorption bond at 542 cm−1 −1 represented the Si–O stretching vibration, and
metal ions solution with a concentration−1of 100 mg·L was pumped upward through the column at a
the adsorption bonds near 534–542 cm were –H deformation vibration [35]. The Si–O symmetric
filtration velocity of 5 mL·min−1 and controlled by a peristaltic pump.
stretching vibration occurred at 792 cm−1 [33,36]. The broad bonds near 1034–1047 cm−1 were assigned
to Results
3. stretching vibrations
and of the Si–O tetrahedral [36]. The peaks located at about 3695 cm−1 and 1822 cm−1
Discussion
were approximately the Mg3O–H and Fe–O stretching vibrations in the spectra of SPM. Mg3O–H and
3.1.
Fe–OCharacterization
are presented in of the
SPM,SPM
because MgSO4 and Fe2(SO4)3 were added during the preparation [35,37].
In the spectra of raw materials, included silicates, tailings, bentonite, and bauxite, peaks of C–O stretching
The zero-charge point was determined by the solid addition method, and the zero-charge point
vibration located at 1608–1629 cm−1, 1871–1873 cm−1, 1989 cm−1 and 2123 cm−1 were observed [37],
(pH0 ) of SPM was 6.7 (Figure 1a). The XRD patterns of SPM, bauxite, and tailings are shown in
which might be due to the existed impurities or a small amount of adsorbing carbon dioxide. The
Figure 1b. The crystalline phases of raw materials were mainly composed of quartz, orthoclase, and
adsorption bonds of C–O at 1989 cm−1 and 2128 cm−1 disappeared in SPM, which could be due to the
diaspore. After the sintering process, the main crystalline phases of SPM were quartz, corundum, and
bicarbonate decomposition at high temperatures. Therefore, it could be concluded that the hydroxyl
hematite, which have been previously shown to be capable of adsorbing heavy metal ions [31–33].
groups on the surface of SPM played an important role in the removal of heavy metals [38].

Figure Zero-chargepoint
1. Zero-charge
Figure 1. point of
of the
the silicate
silicate porous
porous material
material (SPM)
(SPM) (Figure
(Figure 1a)
1a) and
and X-ray
X-ray diffraction
diffraction (XRD)
(XRD)
patterns
patterns of
of SPM,
SPM, bauxite,
bauxite, and
and tailings
tailings (Figure
(Figure 1b,
1b, red
red for
for SPM,
SPM, blue for bauxite,
blue for bauxite, and
and black
black for
for tailings).
tailings).

Based on the BET analysis, the SPM has a specific surface area of 3.40 m2 ·g−1 , and a porosity of
54%. The compressive strength of SPM is 0.6 MPa. The above characteristics of SPM basically accorded
with the design requirements. SEM was used to observe the morphology of SPM, which was shown in
Figure 2. Some favorable features of SPM were found; the material surface had geometrical properties
with irregular shape and large agglomerates and the rugged surface, which would provide more sites
for adsorbing heavy metal ions.
The FTIR spectra of SPM, silicate tailings, bauxite, and bentonite were shown in Figure 3. The
peaks of these substances were mainly distributed in three regions: 3399–3695 cm−1 , 1608–1873 cm−1 ,
and 542–792 cm−1 . The characteristic absorption peaks at 3399 cm−1 and 3695 cm−1 were assigned to
the Si–OH stretching vibration [30]. The peaks at 1629 cm−1 and 1873 cm−1 were attributed to C–O
lattice vibrations [34]. The adsorption bond at 542 cm−1 represented the Si–O stretching vibration, and
the adsorption bonds near 534–542 cm−1 were –H deformation vibration [35]. The Si–O symmetric
stretching vibration occurred at 792 cm−1 [33,36]. The broad bonds near 1034–1047 cm−1 were assigned
to stretching vibrations of the Si–O tetrahedral [36]. The peaks located at about 3695 cm−1 and
1822 cm−1 were approximately the Mg3 O–H and Fe–O stretching vibrations in the spectra of SPM.
Mg3 O–H and Fe–O are presented in SPM, because MgSO4 and Fe2 (SO4 )3 were added during the
preparation [35,37]. In the spectra of raw materials, included silicates, tailings, bentonite, and bauxite,
peaks of C–O stretching vibration located at 1608–1629 cm−1 , 1871–1873 cm−1 , 1989 cm−1 and 2123 cm−1
were observed [37], which might be due to the existed impurities or a small amount of adsorbing
carbon dioxide. The adsorption bonds of C–O at 1989 cm−1 and 2128 cm−1 disappeared in SPM, which
Minerals 2019, 9, 291 6 of 16

could be due to the bicarbonate decomposition at high temperatures. Therefore, it could be concluded
that the hydroxyl groups on the surface of SPM played an important role in the removal of heavy
metals
Minerals [38].
2019, 9, x FOR PEER REVIEW 6 of 16
Minerals 2019, 9, x FOR PEER REVIEW 6 of 16

Figure2.
Figure 2. Micromorphology
Micromorphology of
of SPM
SPM 1000×
1000× (a) and 5000×
5000× (b).
(b).
Figure 2. Micromorphology of SPM 1000× (a) and 5000× (b).

Figure 3. Fourier
Figure 3. Fourier transform
transform infrared
infrared (FTIR)
(FTIR) spectra of SPM,
spectra of silicate tailings,
SPM, silicate tailings, bauxite,
bauxite, and
and bentonite.
bentonite.
Figure 3. Fourier transform infrared (FTIR) spectra of SPM, silicate tailings, bauxite, and bentonite.
3.2. Adsorption Properties
3.2. Adsorption Properties
3.2.1. Effect of
3.2.1. Effect of Contacting
Contacting Time
Time
3.2.1. Effect of Contacting Time
The
The uptake
uptake capacities
capacities of
of single
single heavy
heavy metal
metal ions
ions were
were investigated
investigated as as aa function
function ofof time
time toto
The
determine uptake
an capacities
optimum of
contact single
time heavy
for the metal ions
adsorption were
of investigated
heavy metal as
ions
determine an optimum contact time for the adsorption of heavy metal ions on SPM. The effects a
on function
SPM. of
The time to
effects
of
determine an optimum contact time for the adsorption of heavy metal ions on SPM.
contact time on adsorption amount of Pb2+, Cd2+, and Cu2+ by SPM are shown in Figure 4. The equilibrium The effects of
contact
time wastime on adsorption
found to be 20 h. amount of Pb2+, Cd
At equilibrium
2+, and Cu2+ by SPM are
conditions, 88.4% of Pb2+,shown
51.2% inofFigure 4. The
Cd2+, and equilibrium
41.3% of Cu2+
time was found to be 20 h. At equilibrium conditions, 88.4% of Pb2+, 51.2% of Cd2+, and 41.3% of Cu2+
were removed from the solution. When interacting with heavy metal ions, the adsorption site on SPM
were removed
gradually from the
decreased, butsolution. When interacting
the percentage of adsorption with heavy metal
depended ions,
on the the adsorption
amount of heavysite on ions
metal SPM
gradually decreased, but the percentage of adsorption depended on the amount of
transported from the bulk liquid phase to the actual adsorption site, so the percentage of adsorption heavy metal ions
Minerals 2019, 9, 291 7 of 16

of contact time on adsorption amount of Pb2+ , Cd2+ , and Cu2+ by SPM are shown in Figure 4.
The equilibrium time was found to be 20 h. At equilibrium conditions, 88.4% of Pb2+ , 51.2% of
Cd2+ , and 41.3% of Cu2+ were removed from the solution. When interacting with heavy metal ions,
the adsorption site on SPM gradually decreased, but the percentage of adsorption depended on the
amount of heavy
Minerals 2019, 9, x FORmetal
PEERions transported from the bulk liquid phase to the actual adsorption site, so
REVIEW 7 ofthe
16
percentage of adsorption increased with time until saturation [39]. SPM could remove these heavy
and the
metal ionsremoval abilities
at different of different
extents, metal ions
and the removal were in
abilities the ordermetal
of different of Pbions were in the order of Pb2+
2+ > Cd2+ > Cu2+, which could

>
beCddue2+ to> Cu
the2+difference
, which could in the
beion-exchange capacityinofthe
due to the difference theion-exchange
heavy metal capacity
ions adsorbed
of the on SPMmetal
heavy [40].
EDS adsorbed
ions results confirmed
on SPM [40]. that EDS
Pb ,results
2+ Cd , and
2+ Cu were
2+
confirmed 2+
that adsorbed
Pb , Cd on 2+ the Cu
, and 2+
SPM were
surface. As shown
adsorbed on thein
Figure
SPM S2, theAscharacteristic
surface. shown in Figure peaks S2,ofthe
Pb, Cd, and Cupeaks
characteristic were of observed
Pb, Cd, and in the
CuEDSwerespectra,
observed andin the
corresponding
EDS spectra, and peak
theintensities
corresponding on the energy
peak spectraon
intensities were
the increased compared
energy spectra were to blanks. compared
increased The removal to
percentage
blanks. Theremains
removalstagnant
percentage at 0.5remains
to 4 h, which might
stagnant be contributed
at 0.5 to 4 h, which bymight
the tiny
beand rich micropores
contributed by the
filledand
tiny in rich
SPM. After the filled
micropores addition of SPM,
in SPM. Afterthe
the heavy
additionmetal ionsthe
of SPM, wereheavyrapidly
metaladsorbed
ions wereonto the
rapidly
external surface
adsorbed onto the ofexternal
the SPM, since the
surface of theexternal surface
SPM, since thewas in direct
external contact
surface was with the solution.
in direct contact withOwingthe
to the narrow
solution. Owing diameter of micropores,
to the narrow diameter theofheavy metal ions
micropores, neededmetal
the heavy moreionstimeneeded
to permeatemore into
timetheto
pores, so it took a long time for SPM to get the adsorption saturation. Therefore,
permeate into the pores, so it took a long time for SPM to get the adsorption saturation. Therefore, the first time to reach
equilibrium
the first timecouldto reach indicate the adsorption
equilibrium saturation
could indicate the of the outer saturation
adsorption surface, and ofthe
thesecond equilibrium
outer surface, and
wassecond
the probably the adsorption
equilibrium saturation
was probably theof the inner saturation
adsorption surface of the pores.
of the innerThe amount
surface of adsorption
of the pores. The
when reaching
amount the second
of adsorption whenequilibrium
reaching the is second
significantly higher is
equilibrium than that of thehigher
significantly first time.
than It could
that be
of the
inferred
first time.that the amount
It could of heavy
be inferred metal
that the ions adsorbed
amount of heavy on theions
metal internal surface
adsorbed onofthe SPM was more
internal surfacethan
of
that adsorbed on the external surface.
SPM was more than that adsorbed on the external surface.

Effect of contact 2+ Cd 2+2+ 2+ at an initial

Figure 4.Effect
Figure 4. of contact timetime
on theon the adsorbed
adsorbed amountamount of2+Pb
of Pb2+, Cd , and,Cu at,an
and Cuconcentration
initial of
concentration of 125 speed −1
mg·L of, 150
agitation speed of 150 rpm, −1 and
125 mg⸱L−1, agitation rpm, each adsorbent doseeach
of 12.5adsorbent
g⸱L−1, anddose of 12.5 g·L
temperature of 30, °C.
temperature of 30 ◦ C.
3.2.2. Effect of Initial Concentration of Heavy Metal Ions
3.2.2. Effect of Initial Concentration of Heavy Metal Ions
The influence of initial concentrations of heavy metal ions on the adsorption of Pb2+, Cd2+, and
The influence of initial concentrations of heavy metal ions on the adsorption of Pb2+ , Cd2+ , and
Cu by SPM was investigated, and the results are shown in Figure 5. The adsorption capacity sharply
2+
Cu2+ by SPM was investigated, and the results are shown in Figure 5. The adsorption capacity sharply
increased with the increase of the initial concentration of heavy metals ions ranging from 20 mg·L −1
increased with the increase of the initial concentration of heavy metals ions ranging from 20 mg·L−1 to
to 1000 mg·L . The maximum adsorption capacities of Pb
−1 2+ , Cd , and Cu by SPM were 44.83 mg·g ,
2+ 2+ −1
1000 mg·L−1−1. The maximum adsorption capacities of Pb2+ , Cd2+ , and Cu2+ by SPM were 44.83 mg·g−1 ,
35.36 mg·g−1, and 32.26 mg·g−1 , respectively. The adsorption capacity of heavy metal ions increased,
35.36 mg·g , and 32.26 mg·g−1 , respectively. The adsorption capacity of heavy metal ions increased,
and the removal efficiency decreased with the increasing initial concentrations of heavy metal ions
and the removal efficiency decreased with the increasing initial concentrations of heavy metal ions [39].
[39]. This indicated that the increase of initial concentrations of heavy metal ions contributed to
This indicated that the increase of initial concentrations of heavy metal ions contributed to enhance the
enhance the driving force at the solid–liquid interface to increase the adsorption capacity until the
driving force at the solid–liquid interface to increase the adsorption capacity until the adsorption sites
adsorption sites were saturated [41,42]. In order to describe the adsorption behaviors well (i.e., the
were saturated [41,42]. In order to describe the adsorption behaviors well (i.e., the extent of adsorption
extent of adsorption and relevant mechanism), the experimental data were then fitted with the
Freundlich and Langmuir isotherm models in Section 3.5.
Minerals 2019, 9, 291 8 of 16

and relevant mechanism), the experimental data were then fitted with the Freundlich and Langmuir
isotherm models
Minerals 2019, in PEER
9, x FOR Section 3.5.
REVIEW 8 of 16

2+ 2+ and 2+ on the adsorbents

Figure 5.5. Adsorption
Figure Adsorption isotherms
isotherms for
for PbPb2+, Cd 2+,, and Cu2+ on the adsorbents at heavy metal metal ions
ions
concentrations from 20 to 1400 mg·L −1 , an adsorbent dose of 12.5 g·L −1 , an agitation speed of 150 rpm,
concentrations from 20 to 1400 mg⸱L , an adsorbent dose of 12.5 g⸱L
−1 −1

aa temperature ◦ C, and a contact time of 24 h.

temperatureof
of30
30 °C, and a contact time of 24 h.

3.2.3. Effect of pH Value on the Adsorption of Heavy Metal Ions

3.2.3. Effect of pH Value on the Adsorption of Heavy Metal Ions
During the adsorption process, the pH value is one of the important impact factors that can
During the adsorption process, the pH value is one of the important impact factors that can
influence the heavy metal ions binding to the surface of the adsorbent, since the pH value of the solution
influence the heavy metal ions binding to the surface of the adsorbent, since the pH value of the
would affect the binding sites on the surface of the adsorbent and the chemical nature of the adsorbent,
solution would affect the binding sites on the surface of the adsorbent and the chemical nature of the
and the hydrogen ions could strongly compete with heavy metal ions at low pH values [43,44]. In this
adsorbent, and the hydrogen ions could strongly compete with heavy metal ions at low pH values
work, the effect of pH value on the adsorption of heavy metal ions was investigated, and the results
[43,44]. In this work, the effect of pH value on the adsorption of heavy metal ions was investigated,
are shown in Figure 6. The adsorption curve results indicated that the effect of pH value on Cd2+
and the results are shown in Figure 6. The adsorption curve2+results indicated that the effect of pH
adsorption was not distinct, but the effect was obvious for Pb and Cu2+ , and the removal efficiency
value on Cd2+ adsorption was not distinct, but the effect was obvious for Pb2+ and Cu2+, and the
increased as the pH value increased from 2 to 6. The removal efficiencies of Pb2+ and Cd2+ reached their2+
removal efficiency increased as the pH value increased from 2 to 6. The removal efficiencies of Pb
maximum of 82% and 71% at pH 6, respectively, while the removal efficiencies dropped down to 57%
and Cd2+ reached their maximum of 82% and 71% at pH 6, respectively, while the removal efficiencies
and 63% when the pH value was higher than 6. The removal efficiency of Cu2+ reached a maximum of2+
dropped down to 57% and 63% when the pH value was higher than 6. The removal efficiency of Cu
98% at pH 7. The surface of SPM contained many active sites, and might become positively charged at
reached a maximum of 98% at pH 7. The surface of SPM contained many active sites, and might
a low pH value, thus increasing the competition between the heavy metal ions and H+ for available
become positively charged at a low pH value, thus increasing the competition between the heavy
adsorption sites. However, this competition decreased while these surface active sites became more
metal ions and H+ for available adsorption sites. However, this competition decreased while these
negatively charged with the increasing pH value, which enhanced the adsorption of the positively
surface active sites became more negatively charged with the increasing pH value, which enhanced
charged metal ions through electrostatic force [45,46]. To rule out the effect of SPM on the pH of the
the adsorption of the positively charged metal ions through electrostatic force [45,46]. To rule out the
solution, the results are shown in Table S1 and Figure S3. The adsorption results of SPM in this study
effect of SPM on the pH of the solution, the results are shown in Table S1 and Figure S3. The
are consistent with most of the previously reported silicate minerals. The Cu2+ removal efficiency of
adsorption results of SPM in this study are consistent with most of the previously reported silicate
98% from the solution at pH 7 was attributed to Cu2+ being favored to precipitate as Cu(OH) when
minerals. The Cu2+ removal efficiency of 98% from the solution at pH 7 was attributed to Cu22+ being
the pH value was above 6 [39]. This phenomenon further proved that electrostatic interaction played
favored to precipitate as Cu(OH)2 when the pH value was above 6 [39]. This phenomenon further
an important role in the adsorption of Pb2+ , Cd2+ , and Cu2+ . Thus, the optimal pH value for the
proved that electrostatic interaction played an important role in the adsorption of Pb2+, Cd2+, and Cu2+.
removal of Pb2+ , Cd2+ , and Cu2+ was determined to be 6 in this study.
Thus, the optimal pH value for the removal of Pb2+, Cd2+, and Cu2+ was determined to be 6 in this study.
Minerals 2019, 9, 291 9 of 16
Minerals 2019, 9, x FOR PEER REVIEW 9 of 16

Effect of pH value
Figure 6. Effect
Figure value on
on the
theadsorption
adsorptionof
ofPbPb2+ Cd2+
2+, Cd 2+,, and Cu2+2+onto
and Cu ontothe
theadsorbents
adsorbentsat at an
an ion
ion
−1
concentration of 125 mg·L , ,an anadsorbent
adsorbent dose
dose of
of 12.5
12.5 g·L −1
g·L , ,ananagitation
agitationspeed
speedofof150
150rpm,
rpm, aa solution
solution
concentration −1 −1

temperature of
of 30 ◦ C, and a contact time of 24 h.
30 °C,
temperature and a contact time of 24 h.

3.2.4. Competitive
3.2.4. Competitive Adsorption
Adsorption amongamong Three Three Heavy
Heavy Metal
Metal Ions
Ions
It is
It is of
of great
great significance
significance to to investigate
investigate the the interaction
interaction between
between heavy
heavy metal
metal ions
ions and
and SPM,
SPM, since
since
various heavy metal ions usually coexist in the wastewater. The effects
various heavy metal ions usually coexist in the wastewater. The effects of interaction among heavy of interaction among heavy
metal ions
ions on on the 2+ Cd2+ , and Cu2+ were tested, and the results are shown in Figure 7.
metal the adsorption
adsorption of of Pb
Pb2+,,Cd 2+, and Cu2+ were tested, and the results are shown in Figure 7.
−1 −1 the removal
When the initial concentration of metal ions
When the initial concentration of metal ions increased
increased fromfrom 20 20mg·L
mg·L−1 to to 100
100 mg·L
mg·L−1,, the removal
efficiency of heavy metal ions decreased significantly. The result indicated
efficiency of heavy metal ions decreased significantly. The result indicated that the adsorption that the adsorption amount
presented
amount a significant
presented differencedifference
a significant between single betweenmetalsingle
and multi-metal ions adsorbed
metal and multi-metal ionson the SPM. The
adsorbed on
adsorption capacity of Pb 2+ was larger than those of Cd2+ and Cu2+ . For example, the 42.8% of Pb2+
the SPM. The adsorption capacity of Pb2+ was larger than those of Cd2+ and Cu2+. For example, the
was adsorbed on the SPM when the multi-metal −1 , but the removal
42.8% of Pb2+ was adsorbed on the SPM when theions concentration
multi-metal was 100 mg·Lwas
ions concentration 100 mg·L−1, but
efficiencies of Cd 2+ and Cu 2+ were just 13.8% and 4.9% at the same concentration, respectively. In
the removal efficiencies of Cd and Cu were just 13.8% and 4.9% at the same concentration,
2+ 2+
single metal experiments, the removal efficiencies of Pb 2+ , Cd2+ , and Cu2+ were higher than those
respectively. In single metal experiments, the removal efficiencies of Pb2+, Cd2+, and Cu2+ were higher
of multi-heavy
than metal ions metal
those of multi-heavy experiments at the sameatconcentration.
ions experiments Since different
the same concentration. Sinceheavy metal
different ions
heavy
competed for adsorption sites, the removal efficiency in a single-metal system
metal ions competed for adsorption sites, the removal efficiency in a single-metal system was higher was higher than that of a
multi-metal ions system. The higher concentration of mixed heavy metal
than that of a multi-metal ions system. The higher concentration of mixed heavy metal ions resulted ions resulted in the stronger
competition
in the stronger among heavy metal
competition among ions. Pb2+metal
heavy couldions.
replace
Pb2+Cd
2+
couldand Cu2+ Cd
replace to be adsorbed
2+ and Cu2+ toon bethe surface
adsorbed
of the SPM; therefore, Pb 2+ was more competitive than Cd 2+ and Cu 2+ [47]. These different properties
on the surface of the SPM; therefore, Pb2+ was more competitive than Cd2+ and Cu2+ [47]. These
of heavy properties
different metal ions,ofsuch heavy as metal
ionic potential,
ions, suchelectronegativity,
as ionic potential,softness capacity for
electronegativity, hydroxylation,
softness capacity
and position in the Irving–Williams series [48] also made the difference
for hydroxylation, and position in the Irving–Williams series [48] also made the difference in removal in removal efficiency among
heavy metal ions adsorption. The preference of the sorbent for the Pb 2+ may be because the metal ions
efficiency among heavy metal ions adsorption. The preference of the sorbent for the Pb2+ may be
have the the
because largest atomic
metal weight
ions haveand theparamagnetic,
largest atomic andweight
the mostand electronegative
paramagnetic, ion and
has thethehighest
most
standard reduction potential as compared to Cu 2+ and Cd2+ [49]. Another reason may be related to
electronegative ion has the highest standard reduction potential as compared to Cu2+ and Cd2+ [49].
the hydration
Another reason energy
may be andrelated
hydrated ionic
to the radius ofenergy
hydration heavy andmetals, and theionic
hydrated lower hydration
radius of heavyenergy was
metals,
easier
and theforlower
the adsorption.
hydration The hydration
energy was easierenergy forranked as follows: The
the adsorption.
2+ < Cd < Cu
Pb hydration 2+ 2+
energy [36].
ranked as
follows: Pb2+ < Cd2+< Cu2+ [36].
Minerals 2019, 9, 291 10 of 16
Minerals 2019, 9, x FOR PEER REVIEW 10 of 16

Figure 7. Competitive
Figure adsorption of Pb2+
Competitive adsorption Cd2+2+
2+, , Cd , ,and
andCuCu2+2+ononthe
theadsorbents
adsorbentsatataaheavy
heavy metal
metal ions
ions
concentration of 100 mg·L −1 , each adsorbent dose of 12.5 g·L −1 , agitation speed of 150 rpm, temperature
concentration −1, each adsorbent dose of 12.5 g·L , agitation speed of 150 rpm, temperature
−1

of 30 ◦ C, and contact time of 24 h.

30 °C,
of and contact time of 24 h.

3.3. Desorption of Heavy Metal Ions

3.3. Desorption of Heavy Metal Ions
The result of single heavy metal ions desorption from SPM is shown in Figure 8. During the
The result of single2+heavy metal ions desorption from SPM is shown in Figure 8. During the
desorption process, Cd and Cu2+ presented greater desorption efficiency than Pb2+ . Heavy metal
desorption process, Cd2+ and Cu2+ presented greater desorption efficiency than Pb2+. Heavy metal ions
ions were released from the surfaces of SPM over the first nine hours of the desorption process, and
were released from the surfaces of SPM over the first nine hours of the desorption process, and 15.8%
15.8% of Cd2+ , 16.7% of Cu2+ , and 35.8% of Pb2+ were desorbed from SPM, respectively. The loosely
of Cd2+, 16.7% of Cu2+, and 35.8% of Pb2+ were desorbed from SPM, respectively. The loosely bound
bound metal from the surface of the adsorbent could not be washed away before the desorption
metal from the surface of the adsorbent could not be washed away before the desorption experiment,
experiment, so a very strong desorption occurred at the beginning of the desorption experiment. The
so a very strong desorption occurred at2+ the beginning of the desorption experiment. The desorption
desorption isotherm indicated that Cd and Cu2+ could be strongly bonded on the surface of the
isotherm indicated that Cd2+ and Cu2+ could be strongly bonded on the surface of the SPM, which
SPM, which should be due to the existence of sites for specifically adsorbing Cd2+ and Cu2+ on the
should be due to the existence of sites for specifically adsorbing Cd and Cu on the SPM surface.
2+ 2+
SPM surface. This result indicated that more Cd2+ and Cu2+ could be adsorbed on specific sites than
This result indicated that more Cd2+ and Cu2+ could be adsorbed on specific sites than nonspecific
nonspecific sites, while a higher proportion of Pb2+ was adsorbed on a non-specific site [40]. This
sites, while a higher proportion of Pb2+ was adsorbed on a non-specific site [40]. This also indicated that
also indicated that more non-specific sites were on the SPM surface, leading to the better adsorption
more non-specific sites were on the SPM surface, leading2+ to the better adsorption capacity for Pb2+ by
capacity for Pb2+ by SPM, but the binding stability for Pb was lower as compared with those of Cd2+
SPM, but 2+the binding stability for Pb2+ was lower as compared with those of Cd2+ and Cu2+ [48].
and Cu2019,[48].
Minerals 9, x FOR PEER REVIEW 11 of 16
The raw material for the preparation of SPM was silicate tailings, and the main components were
the silicate with great adsorption capacity to heavy metals ions. Table 4 shows the adsorption results
of reports on various silicate solid wastes. Compared with silicate solid wastes, SPM was sintered
into block, which retained the excellent adsorption capacity of silicate solid wastes and was easy to
separate from solution, avoiding secondary pollution. Thus, the preparation of SPM provides a way
for reusing solid waste such as silicate tailings.

Table 4. Adsorption of metals on other silicate solid wastes.

Adsorbent Ions Adsorption Capacity References

Natural clay Cu2+ 10.8 mg·g−1 [49]
Smectite-rich clay Pb 2+ 25.44 mg·g−1 [31]
Bentonite Cu2+ 42.41 mg·g−1 [36]
Coal fly ash Cd2+ 18.98 mg·g−1 [50]
Fly ash Pb2+ 18.8 mg·g−1 [51]
Fly ash zeolite Pb2+ 70.6 mg·g−1 [52]
Blast furnace slag Pb2+ 40.0 mg·g−1 [53]
Blast-furnace sludgeofofPb 2+
Cd2+,,2+Cu
2+ and Cu2+2+
16.07 mg·g −1 SPM after[54]
Figure8.8.The
Figure Thedesorption
desorption behaviors
behaviors Pb Cu2+,,and Cu from
from successivedesorption.
SPM after successive desorption.
Coal fly ash Cu2+ 20.92 mg·g−1 [55]
The raw material for the
Treated preparation
sewage sludge of CdSPM 2+ was 16.7
silicate
mg·g tailings,
−1 and the
[56]main components were
3.4. Adsorption Isotherm of SPM
the silicate with great adsorption capacity to Pbheavy metals
2+ ions. Table 4 shows the adsorption results
44.83 mg·g −1

The adsorption equilibriumsSPM are usually

Cd2+ described 35.36 by
mg·g isotherm
−1 equations
The study whose parameters
represent the surface properties and affinity Cu of the32.26
2+ adsorbent.
mg·g−1 The adsorption isotherms can be
generated on the basis of theoretical models, the most common of which are the Langmuir model and
the Freundlich model [45]. The Langmuir (Equation (4)) and Freundlich (Equation (5)) adsorption
isotherm models can be written as follows:
Minerals 2019, 9, 291 11 of 16

of reports on various silicate solid wastes. Compared with silicate solid wastes, SPM was sintered
into block, which retained the excellent adsorption capacity of silicate solid wastes and was easy to
separate from solution, avoiding secondary pollution. Thus, the preparation of SPM provides a way
for reusing solid waste such as silicate tailings.

Table 4. Adsorption of metals on other silicate solid wastes.

Adsorbent Ions Adsorption Capacity References

Natural clay Cu2+ 10.8 mg·g−1 [49]
Smectite-rich clay Pb2+ 25.44 mg·g−1 [31]
Bentonite Cu2+ 42.41 mg·g−1 [36]
Coal fly ash Cd2+ 18.98 mg·g−1 [50]
Fly ash Pb2+ 18.8 mg·g−1 [51]
Fly ash zeolite Pb2+ 70.6 mg·g−1 [52]
Blast furnace slag Pb2+ 40.0 mg·g−1 [53]
Blast-furnace sludge Cd2+ 16.07 mg·g−1 [54]
Coal fly ash Cu2+ 20.92 mg·g−1 [55]
Treated sewage sludge Cd2+ 16.7 mg·g−1 [56]
Pb2+ 44.83 mg·g−1
SPM Cd2+ 35.36 mg·g−1 The study
Cu2+ 32.26 mg·g−1

3.4. Adsorption Isotherm of SPM

The adsorption equilibriums are usually described by isotherm equations whose parameters
represent the surface properties and affinity of the adsorbent. The adsorption isotherms can be
generated on the basis of theoretical models, the most common of which are the Langmuir model and
the Freundlich model [45]. The Langmuir (Equation (4)) and Freundlich (Equation (5)) adsorption
isotherm models can be written as follows:
Ce 1 Ce
= + (4)
qe qm kl qm

1
ln qe = lnk f +
lnCe (5)
n
The Langmuir and Freundlich models can describe the adsorption mechanism on the surface well,
and they can account for these experimental results in a wider range concentration [57,58]. According
to the Langmuir adsorption model and Freundlich adsorption model, the values of qe followed the
sequence: Pb2+ > Cd2+ > Cu2+ , which matched to the batch experiment. In fact, the data obtained
from the experiment of adsorption fitted the two models well (Table 5), indicating that the adsorption
sites were uneven and non-specific. The adsorption coefficient is greatly consistent with the conditions
that support favorable adsorption. This result suggested that more than one type of active site took
part in the heavy metal ions adsorption, such as some specific adsorption sites that were consistent
with the speculation of Section 3.3 [42].

Table 5. Langmuir model and Freundlich model parameters.

Langmuir Model Freundlich Model

Ions
qmax kL b* R2 kF n R2
Pb2+ 44.832 0.068 17.732 0.982 2.473 0.992 0.984
Cd2+ 35.368 0.071 15.453 0.984 3.229 0.946 0.983
Cu2+ 32.561 0.038 13.367 0.981 4.183 0.853 0.981
* b is equilibrium constant for Langmuir model.
Minerals 2019, 9, 291 12 of 16

3.5. Kinetic Study of SPM

The pseudo-first order kinetic model and pseudo-second order kinetic model have been diffusely
used to describe the removal of pollutants from solution in different fields [28]. Here, the tested
pseudo-first order (Equation (6)) and pseudo-second order (Equation (7)) kinetics models can be written
as follows:
ln(qe − qt ) = lnqe − k1 t (6)
t 1 t
= + (7)
qt k2 qe 2 qe

where qe is the mass of heavy metal ions adsorbed at equilibrium (mg·g−1 ), qt is the mass of metal
adsorbed at time t (h), and k1 and k2 are the pseudo-first order model and the pseudo-second order
model rate constants of adsorption. The kinetic adsorption data were satisfactorily fitted to the
pseudo-first order model and pseudo-second order model (Table 6). The R2 values of the pseudo-first
order model and pseudo-second order model for Pb2+ , Cd2+ , and Cu2+ were ~0.915–0.984 and
~0.908–0.982, respectively, and the equilibrium adsorption capacities predicted from the models were
approximately equal to the corresponding experimental values. The diffusion process of heavy metal
ions will affect the interaction between heavy metal ions and the SPM surface; thus, the adsorption rate
will be affected. If the R2 values were considered, the pseudo-first order model appeared to fit the
data better than the pseudo-second order model. The better-fitted results suggested that the diffusion
process might be the prior rate-limiting step in the adsorption.

Table 6. Pseudo-first order kinetic model and pseudo-second order kinetic model parameters.

Pseudo-First Order Kinetic Model Pseudo-Second Order Kinetic Model

Ions
qe k1 R2 qe k2 R2
Pb2+ 44.832 0.068 0.982 4.439 0.051 0.908
Cd2+ 35.368 0.071 0.984 2.57 0.128 0.943
Cu2+ 32.561 0.038 0.981 2.093 0.278 0.982

3.6. Column Experiments

Column experiments were conducted for determining the single ion adsorption characteristics of
Pb2+ , Cd2+ , and Cu2+ in the dynamic adsorption process, respectively (Figure 9), and the adsorption
parameters are listed in Table 7. The Thomas model satisfactorily described the adsorption behavior of
the different heavy metal ions from SPM (R2 = 0.913~0.934). SPM had better adsorption ability for
Pb2+ as compared to Cd2+ and Cu2+ in the column experiments, and the time to reach the plateau
of Ct/C0 value was significantly longer for Pb2+ than other metal ions. The results can be obtained
from Figure 8 that the breakthrough speed was the slowest and the adsorption curve was the least
steep for Pb2+ . The breakthrough point of Cu2+ appeared at about 30 min, followed by Cd2+ at 60 min
and Pb2+ at 110 min. The adsorption capacity order of the column from the breakthrough curves was
Pb2+ > Cd2+ > Cu2+ , which suggested that Pb2+ was the most easily to bond to the adsorption sites of
SPM in the column experiment. This is consistent with the results of the batch experiment, because Pb2+
presented stronger adsorption properties due to its greater ion size, while Cu2+ and Cd2+ were lower
affinity to bind to the adsorption groups [59]. The maximal capacity of the column experiment was
significantly lower than that of the batch experiment, because the heavy metal ions flowing through
the columns probably had less contact time, and the adsorption equilibrium was not reached, unlike in
the batch equilibrium experiment [60]. Therefore, SPM is an appropriate adsorbent to adsorb heavy
metal ions from the contaminated aqueous media in column adsorption application.
Minerals 2019, 9, 291 13 of 16
Minerals 2019, 9, x FOR PEER REVIEW 13 of 16

Figure 9. Adsorption curves of column experiments.

Table 7. Thomas model parameters.

4. Conclusions
−1 2
kth materials for heavyRmetal adsorption
Preparing Ions
the silicate tailingsqintoe (mg·g )
foamed silicate porous
2+
from solution isPban effective way to 14.421
alleviate environmental pressures from tailings,
0.072 0.926 and can attain
2+
sustainability. Porous materials were successfully fabricated by silicate tailings 0.934
Cd 13.025 0.069 in this work, and it
2+ 11.653
could be used Cu as an adsorbent to remove Pb2+, Cd2, and0.061 Cu2+ from aqueous 0.913 solution with high
adsorption capacities. The adsorption performance of SPM was greatly affected by the pH value and
4.
theConclusions
other co-existence of heavy metal ions in the solution. Referring to the heavy metal ions on the
SPM,Preparing
the selective
the adsorption orderinto
silicate tailings of these
foamed metals wasporous
silicate Pb2+ > Cd 2+ > Cu2+. In addition, experimental
materials for heavy metal adsorption
data were suitable for the Langmuir model and the Freundlich model,
from solution is an effective way to alleviate environmental pressures from tailings, indicating that the
andadsorption
can attain
sites were uneven and less non-specific. The kinetic model fitting results
sustainability. Porous materials were successfully fabricated by silicate tailings in this indicated that the adsorption
work, and
process
it could was limited
be used as anbyadsorbent
heavy metal ions diffusion
to remove and
Pb2+ , Cd chemisorption
2 , and Cu2+ from processes. The experimental
aqueous solution with high
results indicate that the SPM prepared in this study is a potential adsorbent
adsorption capacities. The adsorption performance of SPM was greatly affected by the pH value for the removal of heavy
and
metal ions from solution.
the other co-existence of heavy metal ions in the solution. Referring to the heavy metal ions on the
SPM, the selective adsorption order of these metals was Pb2+ > Cd2+ > Cu2+ . In addition, experimental
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Photographs
data were suitable for the Langmuir model and the Freundlich model, indicating that the adsorption
of SPM, Figure S2: EDS spectra of SPM after adsorb heavy metal ions (a-Pb, b-Cd, c-Cu), Figure S3: Effect of pH
sites were uneven and less non-specific. The kinetic model fitting results indicated that the adsorption
value on the ion concentration, Table S1: The effect of SPM on the initial concentration of the solution.
process was limited by heavy metal ions diffusion and chemisorption processes. The experimental
Author indicate
results Contributions: Z.H.
that the SPMandprepared
Y.H. received thestudy
in this project;
is aZ.H. designed
potential the experiment;
adsorbent for the D.O. performed
removal the
of heavy
experiment; D.O., Y.Z.
metal ions from solution.and Q.Z. analyzed the data; D.O., Y.Z. and L.H. wrote and revised the manuscript,
respectively.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/5/291/s1,
Funding:
Figure S1:This work was supported
Photographs by the S2:
of SPM, Figure National
EDS Natural
spectra Science
of SPMFoundation of heavy
after adsorb China (31500091
metal ionsand 51774339),
(a-Pb, b-Cd,
c-Cu, d-blank), Center
Co-Innovation Figure for
S3: Clean
Effect and
of pH value on
efficient the ion concentration,
Utilization Table
of Strategic Metal S1: TheResources.
Mineral effect of SPM on the initial
concentration of the solution.
Conflicts of Interest: The authors declare no conflict of interest.
Author Contributions: Z.H. and Y.H. received the project; Z.H. designed the experiment; D.O. performed
the experiment; D.O., Y.Z. and Q.Z. analyzed the data; D.O., Y.Z. and L.H. wrote and revised the
Referencesrespectively.
manuscript,
Funding:
1. This work
Alqadami, A.A.;was supported
Naushad, by the National
M.; Abdalla, Natural T.;
M.A.; Ahamad, Science Foundation
Abdullah of China
ALOthman, (31500091
Z.; Alshehri, andGhfar,
S.M.; 51774339),
A.A.
Co-Innovation Center for Clean and efficient Utilization of Strategic Metal Mineral Resources.
Efficient removal of toxic metal ions from wastewater using a recyclable nanocomposite: A study of
Conflicts of Interest:
adsorption The authors
parameters declare no mechanism.
and interaction conflict of interest.
J. Clean. Prod. 2017, 156, 426–436.
2. Manzoor, Q.; Nadeem, R.; Iqbal, M.; Saeed, R.; Ansari, T.M. Organic acids pretreatment effect on Rosa
bourbonia phyto-biomass for removal of Pb(II) and Cu(II) from aqueous media. Bioresour. Technol. 2013,
132, 446–452.
3. Fu, F.; Wang, Q. Removal of heavy metal ions from wastewaters: A review. J. Environ. Manag. 2011, 92, 407–418.
4. Wang, X.; Jing, S.; Hou, Z.; Liu, Y.; Qiu, X.; Liu, Y.; Tan, Y. Permeable, robust and magnetic hydrogel beads:
Water droplet templating synthesis and utilization for heavy metal ions removal. J. Mater. Sci. 2018, 53,
15009–15024.
Minerals 2019, 9, 291 14 of 16

References
1. Alqadami, A.A.; Naushad, M.; Abdalla, M.A.; Ahamad, T.; Abdullah ALOthman, Z.; Alshehri, S.M.;
Ghfar, A.A. Efficient removal of toxic metal ions from wastewater using a recyclable nanocomposite: A study
of adsorption parameters and interaction mechanism. J. Clean. Prod. 2017, 156, 426–436. [CrossRef]
2. Manzoor, Q.; Nadeem, R.; Iqbal, M.; Saeed, R.; Ansari, T.M. Organic acids pretreatment effect on Rosa
bourbonia phyto-biomass for removal of Pb(II) and Cu(II) from aqueous media. Bioresour. Technol. 2013, 132,
446–452. [CrossRef]
3. Fu, F.; Wang, Q. Removal of heavy metal ions from wastewaters: A review. J. Environ. Manag. 2011, 92,
407–418. [CrossRef] [PubMed]
4. Wang, X.; Jing, S.; Hou, Z.; Liu, Y.; Qiu, X.; Liu, Y.; Tan, Y. Permeable, robust and magnetic hydrogel beads:
Water droplet templating synthesis and utilization for heavy metal ions removal. J. Mater. Sci. 2018, 53,
15009–15024. [CrossRef]
5. Ku, Y.; Jung, I.L. Photocatalytic reduction of Cr(VI) in aqueous solutions by UV irradiation with the presence
of titanium dioxide. Water Res. 2001, 35, 135–142. [CrossRef]
6. Kang, S.-Y.; Lee, J.-U.; Moon, S.-H.; Kim, K.-W. Competitive adsorption characteristics of Co2+ , Ni2+ , and
Cr3+ by IRN-77 cation exchange resin in synthesized wastewater. Chemosphere 2004, 56, 141–147. [CrossRef]
[PubMed]
7. Castro, L.; Blázquez, M.L.; González, F.; Muñoz, J.A.; Ballester, A. Hydrometallurgy Heavy metal adsorption
using biogenic iron compounds. Hydrometallurgy 2018, 179, 44–51. [CrossRef]
8. Landaburu-Aguirre, J.; García, V.; Pongrácz, E.; Keiski, R.L. The removal of zinc from synthetic wastewaters
by micellar-enhanced ultrafiltration: Statistical design of experiments. Desalination 2009, 240, 262–269.
[CrossRef]
9. Zhou, L.; Xiong, W.; Liu, S. Preparation of a gold electrode modified with Au–TiO2 nanoparticles as an
electrochemical sensor for the detection of mercury(II) ions. J. Mater. Sci. 2015, 50, 769–776. [CrossRef]
10. Gong, R.; Sun, Y.; Chen, J.; Liu, H.; Yang, C. Effect of chemical modification on dye adsorption capacity of
peanut hull. Dyes Pigment. 2005, 67, 175–181. [CrossRef]
11. Yen, C.H.; Lien, H.L.; Chung, J.S.; Yeh, H. Der Adsorption of precious metals in water by dendrimer modified
magnetic nanoparticles. J. Hazard. Mater. 2017, 322, 215–222. [CrossRef]
12. Song, Q.; Li, J.; Zeng, X. Minimizing the increasing solid waste through zero waste strategy. J. Clean. Prod.
2015, 104, 199–210. [CrossRef]
13. Petcherdchoo, A. Repairs by fly ash concrete to extend service life of chloride-exposed concrete structures
considering environmental impacts. Constr. Build. Mater. 2015, 98, 799–809. [CrossRef]
14. Zhang, L.; Zeng, Y.; Cheng, Z. Removal of heavy metal ions using chitosan and modified chitosan: A review.
J. Mol. Liq. 2016, 214, 175–191. [CrossRef]
15. Ahmaruzzaman, M. Industrial wastes as low-cost potential adsorbents for the treatment of wastewater laden
with heavy metals. Adv. Colloid Interface Sci. 2011, 166, 36–59. [CrossRef]
16. Lee, B.; Kim, Y.; Lee, H.; Yi, J. Synthesis of functionalized porous silicas via templating method as heavy
metal ion adsorbents: The introduction of surface hydrophilicity onto the surface of adsorbents. Microporous
Mesoporous Mater. 2001, 50, 77–90. [CrossRef]
17. You, W.; Hong, M.; Zhang, H.; Wu, Q.; Zhuang, Z.; Yu, Y. Functionalized calcium silicate nanofibers with
hierarchical structure derived from oyster shells and its application in heavy metal ions removal. Phys. Chem.
Chem. Phys. 2016, 18, 15564–15573. [CrossRef]
18. Huang, R.; Wu, M.; Zhang, T.; Li, D.; Tang, P.; Feng, Y. Template-free Synthesis of Large-Pore-Size Porous
Magnesium Silicate Hierarchical Nanostructures for High-Efficiency Removal of Heavy Metal Ions. ACS
Sustain. Chem. Eng. 2017, 5, 2774–2780. [CrossRef]
19. Huang, R.; He, L.; Zhang, T.; Li, D.; Tang, P.; Feng, Y. Novel Carbon Paper @ Magnesium Silicate Composite
Porous Films: Design, Fabrication, and Adsorption Behavior for Heavy Metal Ions in Aqueous Solution.
ACS Appl. Mater. Interfaces 2018, 10, 22776–22785. [CrossRef] [PubMed]
20. Petrella, A. Porous Alumosilicate Aggregate as Lead Ion Sorbent in Wastewater Treatments. Separations 2017,
4, 25. [CrossRef]
21. Kim, J.; Kwak, S. Efficient and selective removal of heavy metals using microporous layered silicate AMH-3
as sorbent. Chem. Eng. J. 2016, 313, 975–982. [CrossRef]
Minerals 2019, 9, 291 15 of 16

22. Chen, J.; He, F.; Zhang, H.; Zhang, X.; Zhang, G.; Yuan, G. Novel core-shell structured Mn-Fe/MnO2 magnetic
nanoparticles for enhanced Pb(II) removal from aqueous solution. Ind. Eng. Chem. Res. 2014, 53, 18481–18488.
[CrossRef]
23. Ou, Q.; Zhou, L.; Zhao, S.; Geng, H.; Hao, J.; Xu, Y.; Chen, H.; Chen, X. Self-templated synthesis of bifunctional
Fe3 O4 @MgSiO3 magnetic sub-microspheres for toxic metal ions removal. Chem. Eng. J. 2012, 180, 121–127.
[CrossRef]
24. Argane, R.; Benzaazoua, M.; Hakkou, R.; Bouamrane, A. Reuse of base-metal tailings as aggregates for
rendering mortars: Assessment of immobilization performances and environmental behavior. Constr. Build.
Mater. 2015, 96, 296–306. [CrossRef]
25. Zhao, Z.; Zhang, X.; Zhou, H.; Liu, G.; Kong, M.; Wang, G. Microwave-assisted synthesis of magnetic
Fe3 O4 -mesoporous magnesium silicate core-shell composites for the removal of heavy metal ions. Microporous
Mesoporous Mater. 2017, 242, 50–58. [CrossRef]
26. Gorakhki, M.H.; Bareither, C.A. Sustainable Reuse of Mine Tailings and Waste Rock as Water-Balance Covers.
Minerals 2017, 7, 128. [CrossRef]
27. Aprianti, E. A huge number of artificial waste material can be supplementary cementitious material (SCM)
for concrete production—A review part II. J. Clean. Prod. 2017, 142, 4178–4194. [CrossRef]
28. Qiu, H.; Lv, L.; Pan, B.; Zhang, Q.; Zhang, W.; Zhang, Q. Critical review in adsorption kinetic models.
J. Zhejiang Univ. A 2009, 10, 716–724. [CrossRef]
29. Sverjensky, D.A.; Sahai, N. Theoretical prediction of single-site surface-protonation equilibrium constants for
oxides and silicates in water. Geochim. Cosmochim. Acta 1996, 60, 3773–3797. [CrossRef]
30. Mohan, S.; Gandhimathi, R. Removal of heavy metal ions from municipal solid waste leachate using coal fly
ash as an adsorbent. J. Hazard. Mater. 2009, 169, 351–359. [CrossRef]
31. Chaari, I.; Fakhfakh, E.; Chakroun, S.; Bouzid, J.; Boujelben, N.; Feki, M.; Rocha, F.; Jamoussi, F. Lead removal
from aqueous solutions by a Tunisian smectitic clay. J. Hazard. Mater. 2008, 156, 545–551. [CrossRef]
32. Egodawatte, S.; Greenstein, K.E.; Vance, I.; Rivera, E.; Myung, N.V.; Parkin, G.F.; Cwiertny, D.M.; Larsen, S.C.
Electrospun hematite nanofiber/mesoporous silica core/shell nanomaterials as an efficient adsorbent for
heavy metals. RSC Adv. 2016, 6, 90516–90525. [CrossRef]
33. Zhang, Q.; Lin, B.; Hong, J.; Chang, C.T. Removal of ammonium and heavy metals by cost-effective zeolite
synthesized from waste quartz sand and calcium fluoride sludge. Water Sci. Technol. 2017, 75, 587–597.
[CrossRef] [PubMed]
34. Rethwisch, D.G.; Dumesic, J.A. Effect of Metal–Oxygen Bond Strength on Properties of Oxides. 1. Infrared
Spectroscopy of Adsorbed CO and CO2 . Langmuir 1986, 2, 73–79. [CrossRef]
35. Al-Oweini, R.; El-Rassy, H. Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared
using several Si(OR)4 and R00 Si(OR0 )3 precursors. J. Mol. Struct. 2009, 919, 140–145. [CrossRef]
36. Eren, E.; Afsin, B. An investigation of Cu(II) adsorption by raw and acid-activated bentonite: A combined
potentiometric, thermodynamic, XRD, IR, DTA study. J. Hazard. Mater. 2008, 151, 682–691. [CrossRef]
37. Mcdowell, R.S.; Horrocks, W.D.; Yates, J.T. Infrared spectrum of Co(CO)3 NO. J. Chem. Phys. 1961, 34, 530–534.
[CrossRef]
38. Bradl, H.B. Adsorption of heavy metal ions on soils and soils constituents. J. Colloid Interface Sci. 2004, 277,
1–18. [CrossRef] [PubMed]
39. Duan, P.; Yan, C.; Zhou, W.; Ren, D. Development of fly ash and iron ore tailing based porous geopolymer
for removal of Cu(II) from wastewater. Ceram. Int. 2016, 42, 13507–13518. [CrossRef]
40. Jiang, M.Q.; Jin, X. ying; Lu, X.Q.; Chen, Z. liang Adsorption of Pb(II), Cd(II), Ni(II) and Cu(II) onto natural
kaolinite clay. Desalination 2010, 252, 33–39. [CrossRef]
41. Wang, W.; Tian, G.; Zhang, Z.; Wang, A. A simple hydrothermal approach to modify palygorskite for
high-efficient adsorption of Methylene blue and Cu(II) ions. Chem. Eng. J. 2015, 265, 228–238. [CrossRef]
42. Adebowale, K.O.; Unuabonah, I.E.; Olu-Owolabi, B.I. The effect of some operating variables on the adsorption
of lead and cadmium ions on kaolinite clay. J. Hazard. Mater. 2006, 134, 130–139. [CrossRef]
43. Jin, X.; Yu, C.; Li, Y.; Qi, Y.; Yang, L.; Zhao, G.; Hu, H. Preparation of novel nano-adsorbent based on
organic-inorganic hybrid and their adsorption for heavy metals and organic pollutants presented in water
environment. J. Hazard. Mater. 2011, 186, 1672–1680. [CrossRef]
44. Vindevoghel, P.; Guyot, A. Suspended Emulsion Copolymerization of Acrylonitrile and Methyl Acrylate.
Polym. React. Eng. 1995, 3, 23–42. [CrossRef]
Minerals 2019, 9, 291 16 of 16

45. Unuabonah, E.I.; Adebowale, K.O.; Olu-Owolabi, B.I.; Yang, L.Z.; Kong, L.X. Adsorption of Pb (II) and Cd (II)
from aqueous solutions onto sodium tetraborate-modified Kaolinite clay: Equilibrium and thermodynamic
studies. Hydrometallurgy 2008, 93, 1–9. [CrossRef]
46. Uddin, M.K. A review on the adsorption of heavy metals by clay minerals, with special focus on the past
decade. Chem. Eng. J. 2017, 308, 438–462. [CrossRef]
47. Guo, S.; Dan, Z.; Duan, N.; Chen, G.; Gao, W.; Zhao, W. Zn (II), Pb (II), and Cd (II) adsorption from aqueous
solution by magnetic silica gel: Preparation, characterization, and adsorption. Environ. Sci. Pollut. Res. 2018,
25, 30938–30948. [CrossRef]
48. Adebowale, K.O.; Unuabonah, I.E.; Olu-Owolabi, B.I. Adsorption of some heavy metal ions on sulfate- and
phosphate-modified kaolin. Appl. Clay Sci. 2005, 29, 145–148. [CrossRef]
49. Zacaroni, L.M.; Magriotis, Z.M.; das Graças Cardoso, M.; Santiago, W.D.; Mendonça, J.G.; Vieira, S.S.;
Nelson, D.L. Natural clay and commercial activated charcoal: Properties and application for the removal of
copper from cachaça. Food Control 2015, 47, 536–544. [CrossRef]
50. Rao, M.; Parwate, A.V.; Bhole, A.G. Removal of Cr6+ and Ni2+ from aqueous solution using bagasse and fly
ash. Waste Manag. 2002, 22, 821–830. [CrossRef]
51. Diamadopoulos, E. As (V) Removal from aqueous solutions by. Water Res. 1993, 27, 1773–1777. [CrossRef]
52. Gan, Q. A case study of microwave processing of metal hydroxide sediment sludge from printed circuit
board manufacturing wash water. Waste Manag. 2000, 20, 695–701. [CrossRef]
53. Lopez, E.A.; Perez, C. The adsorption of copper (11) ions from aqueous solution on blast furnace sludge.
J. Mater. Sci. Lett. 1996, 15, 1310–1312. [CrossRef]
54. Zhai, Y.; Wei, X.; Zeng, G.; Zhang, D.; Chu, K. Study of adsorbent derived from sewage sludge for the
removal of Cd2+ , Ni2+ in aqueous solutions. Sep. Purif. Technol. 2004, 38, 191–196. [CrossRef]
55. Srivastava, S.; Gupta, V.K.; Mohan, D. Removal of lead and chromium by activated slag-A blast-Furnace
waste. J. Environ. Eng. 1997, 123, 461–468. [CrossRef]
56. Papandreou, A.; Stournaras, C.J.; Panias, D. Copper and cadmium adsorption on pellets made from fired
coal fly ash. J. Hazard. Mater. 2007, 148, 538–547. [CrossRef]
57. Treybal, R.E. Tryball, Mass Transfers Operations, 3rd ed.; McGraw: New York, NY, USA, 1980.
58. Juang, R.S.; Wu, F.C.; Tseng, R.L. The Ability of Activated Clay for the Adsorption of Dyes from Aqueous
Solutions. Environ. Technol. 1997, 18, 525–531. [CrossRef]
59. Shahbazi, A.; Younesi, H.; Badiei, A. Functionalized SBA-15 mesoporous silica by melamine-based dendrimer
amines for adsorptive characteristics of Pb(II), Cu(II) and Cd(II) heavy metal ions in batch and fixed bed
column. Chem. Eng. J. 2011, 168, 505–518. [CrossRef]
60. Nguyen, T.C.; Loganathan, P.; Nguyen, T.V.; Vigneswaran, S.; Kandasamy, J.; Naidu, R. Simultaneous
adsorption of Cd, Cr, Cu, Pb, and Zn by an iron-coated Australian zeolite in batch and fixed-bed column
studies. Chem. Eng. J. 2015, 270, 393–404. [CrossRef]

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