Selective removal of mercury from aqueous solutions using thiolated cross-linked polyethylenimine
Dalia M. Saad
Ewa M. Cukrowska
D. M. Saad E. M. Cukrowska (&) H. Tutu Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand
A successful approach to develop an insoluble form of polyethylenimine with a thiol-based functional group for selective removal of Hg(II) from aqueous solutions is reported. The selectivity of the modified polymer for Hg(II) as well as its ability to be regenerated for re-use has been studied. The synthesised polymer exhibited high selectivity for Hg(II) with high removal efficiency of up to 97 %, even in the presence of competing ions. The Freundlich isotherm was found to best fit and describe the experimental data. The pseudo-second-order equation explains the adsorption kinetics most effectively implying chemisorption. The thermodynamic study of the adsorption process revealed high activation energies [41 kJ mol-1, further confirming chemisorption as the mechanism of interaction between mercury ions and the polymer surface. The polymer exhibited good potential for re-use after many cycles of regeneration, giving good removal efficiency up to the fifth cycle.
Industrial development has left pronounced effects on the
environment generally and on water resources specifically.
Many industries generate waste products that contain high
concentrations of heavy metals which are discharged
directly or indirectly into water systems (Hang et al. 2009;
Velea et al. 2008; Zhuang et al. 2009). In South Africa, the
mining industry accounts for the largest portion of heavy
metal contamination through acid mine drainage (AMD).
The removal of these toxic metal ions is an important
challenge taking into account that the current methods of
remediating contaminated water such as ion-exchange
resin, electrolytic or liquid extraction, electrodialysis,
chemical precipitation, membrane filtration, and
biosorption are sometimes inadequate due to the large quantities of
these wastes, resulting in metal-bearing sludges which are
difficult to dispose of. Furthermore, most of these
traditional methods are expensive. Therefore, there is an urgent
need for new feasible and cost effective methods (Ahmed
et al. 2008; Ghoul et al. 2003).
There is also a need for such methods to be manipulated
for effectiveness in removing certain targeted pollutants.
This way, these techniques can be used in tandem with
other non-selective methods, thus obtaining efficiency and
Among the separation and remediation techniques,
polymeric adsorbents are some of the most efficient and
widely applied in separation processes (Kwon et al. 2000;
Shentu et al. 2007; Wang et al. 2001). They can be of
varying configurations, but mainly consist of a polymer
backbone and a ligand pendant on the backbone. Metal
ions are usually bound to the polymer ligand by a
coordinate bond (Kaliyappan and Kannan 2000). The advantage
of this chemistry is that a selective removal can be
achieved by choosing a suitable polymer ligand to target a
It was demonstrated in our previous articles that phosphate
and sulphate functional groups substituted in the backbone of
polyethylenimine act as strong adsorption sites for removing
specific metals in solution (Saad et al. 2012a, b). This study
was aimed at developing a polymeric adsorbent based on a
thiol functional group for selective removal of Hg from
aqueous systems. Hg exposure can affect kidney functions,
the central nervous system, and mental system. As such, its
removal from environmental systems has become a research
priority (Manohar et al. 2002; Rio and Delebarre 2003; Wahi
et al. 2009; Cai and Jia 2010).
Cross-linked polyethylenimine was functionalized
with ethylene sulphide (C2H4S) for targeted Hg(II)
This study gives a background to a wider study intended
to introduce polymers of this type for use in household
filter systems. It has emerged that, due to the scarcity of
water, certain households around Johannesburg use
minepolluted groundwater as a source of drinking water. This is
common in small plot holdings and farms (Dr. Carl
Albrecht, Cancer South Africa, personal communication). Such
water has been found to contain Hg and other toxic
elements (Lusilao-Makiese et al. 2012). Another configuration
would be to pack the polymer into small columns that can
be placed in household containers (e.g. water jars) and left
to contact with polluted water, allowing mass transfer of
the Hg to the adsorbent in the column.
To assess the stability of the thiolated CPEI (TCPEI) for
the above-mentioned purposes, the effects of some
parameters on adsorption of Hg(II) were studied, namely
pH, adsorbent amount, contact time, and the presence of
competing ions. Regeneration of the polymer was also
done so as to assess its re-usability.
Materials and methods
All the reagents and solutions were prepared using
reagentgrade chemicals from Sigma-Aldrich (South Africa)
without further purification.
Cross-linked polyethylenimine (synthesis reported by
Saad et al. 2011) and ethylene sulphide were used for the
synthesis of TCPEI. The Hg(II) solutions were prepared
from Hg(NO3)2. Competing metal ion solutions were
prepared from their nitrate salts, namely Zn(NO3)2, Pb(NO3)2,
Co(NO3)2, Cu(NO3)2, Fe(NO3)3 H2O, and Ni(NO3)2.
Adjustments of pH for the adsorption experiments were
conducted using 1 mol L-1 solutions of HNO3 and NaOH.
Deionised water (Millipore) was used for the preparation of
Cross-linked polyethylenimine (CPEI) of 5 g was
dissolved in 100 mL of water, and the pH of the solution was
adjusted to 7. The solution was purged with nitrogen for
20 min followed by the addition of 1.73 mL of ethylene
sulphide. The reaction mixture was then kept overnight
under reflux at 90 C. The collected product was rinsed
with abundant deionised water and dried in an oven. The
thiolation reaction scheme is shown in Fig. 1.
Fourier Transform Infra Red spectroscopy was used to
characterise the thiolated derivative of cross-linked
polyethylenimine (TCPEI) to confirm the introduction of thiol
group. The content of sulphur in the synthesised polymer
was determined using LECO-932 CHNS analyser from
LECO Corporation (Michigan, USA).
Batch adsorption studies
Batch adsorption experiments were conducted using a
1,000-mg L-1 stock standard solution of Hg(NO3)2 from
which 40 mg L-1 working standard solutions were
obtained by serial dilution. This concentration was
arbitrarily chosen as it represents a worst-case scenario of
pollution by most toxic elements (e.g. mercury, uranium,
arsenic and vanadium) in mining-impacted waters in the
Witwatersrand Basin goldfields (Tutu et al. 2009).
Adsorption experiments were performed in 50 mL flasks
at room temperature. Different amounts of synthetic TCPEI
were weighed out into each flask to assess the optimum
amount of the adsorbent; 40 mL of 40 mg L-1 solution of
Hg(NO3)2 standard was then added to each flask and stirred
using magnetic stirrer. Adsorption at different pH values
was assessed. At equilibrium, the solutions were filtered
and the equilibrium concentrations were determined using
an inductively coupled plasma optical emission
spectroscopy (ICP-OES) (Genesis Spectro, Germany). The same
procedure was followed using the multi-component
solution to assess the effect of competing ions. The amount of
ions adsorbed per unit mass of adsorbent was calculated on
the basis of the mass balance equation:
where qe (mg metal g-1 polymer) is the adsorption
capacity, Ci (mg L-1) is the initial concentration of Hg(II)
in the solution, Cf (mg L-1) the concentration of Hg(II) in
the filtrate, V (mL) the volume of initial solution and
P (g) is the amount of polymer used.
Effect of adsorbent amount
The amount of adsorbent was optimised to obtain the
optimal removal. Adsorption experiments were conducted
using different amounts of TCPEI, namely 0.03, 0.05, 0.1,
0.2, 0.5, and 1.0 g, and synthetic standard mercury
solutions of 40 mg L-1 at room temperature and fixed time.
Effect of contact time
Adsorption experiments were conducted at room
temperature (27 C) to obtain the optimal time required for
adsorption. Adsorption was studied at various time
intervals (10120 min) and fixed concentration (40 mg L-1).
The concentration of Hg(II) was determined at the end of
each time. The obtained equilibrium capacities (qe) were
then plotted against the equilibrium time for kinetic
Desorption of Hg(II) from TCPEI was studied by treating
the previously loaded polymer with an excess of extracting
reagent. HNO3 at different concentrations, namely 2, 3, 5,
and 7 mol L-1, was used as an extractant. During
regeneration, the mixtures were stirred for 1 h, filtered and the
polymer washed with deionised water and dried prior to
reuse, while the filtrates were analyzed by ICP-OES.
All experiments were repeated three times (n = 3), and
the limit of detection (LOD) was calculated as 3 9
standard deviation of the blank and the method quantitation
limit (MQL) was calculated as 10 9 standard deviation of
Modelling of analytical results
The results from adsorption studies were modelled
using kinetic (pseudo-first-order and pseudo-second-order
Fig. 2 Effect of adsorbent amount (a), contact time (b), and pH (c) on
the adsorption process. Initial concentrations = 40 mg L-1, contact
time = 30 min, at pH 3 and room temperature (27 C). Initial
concentrations = 40 mg L-1, adsorbent amount = 0.2 g, at pH 3 and
room temperature (27 C) . Initial concentrations = 40 mg L-1, contact
time = 30 min, adsorbent amount = 0.2 g, at room temperature (27 C)
Table 1 Removal efficiencies of elements in a multi-component solution
Initial concentration of the ions = 40 mg L-1
Cf Final concentration after adsorption, RSD relative standard deviation (n = 3), LOD limit of detection in mg L-1, MQL method quantitation
limit in mg L-1
equations), isotherms (Langmuir and Freundlich), and
Speciation modelling of the metal ions in solution was
conducted using MEDUSA software (KTH Royal Institute
of Technology, Sweden).
The kinetic models that were used to fit the experimental
data are as follows.
The pseudo first-order model was defined by the
qt log qe k1=2:303t
The plot of log (qe - qt) vs. t gives a straight line.
The pseudo-second-order model was defined by the
1=qt 1=k2qe2 1=qet
The plot of t/qt vs. t gives a straight line.
The parameters in the above equations are defined as
follows: qe (mg g-1) is the adsorption capacity at
equilibrium, qt (mg g-1) is the adsorption capacity at time t, and
k1 and k2 (1/min) are the rate constants for the
pseudo-firstorder and pseudo-second-order models, respectively. These
are obtained from the slope of the plot of log (qe - qt) vs. t.
Adsorption isotherms describe the nature of the adsorbent
adsorbate interaction as well as the specific relation
between the concentration of adsorbate and its degree of
accumulation onto the adsorbent surface (Gupta et al. 2003;
Li et al. 2008). To understand the adsorption mechanism of
Hg(II) onto the TCPEI surface, two adsorption isotherm
models, Langmuir and Freundlich were used to fit the
experimental data (Cozmuta et al. 2012; Freundlich 1926).
The experimental data for isotherm modelling were
obtained by conducting the adsorption experiments using
1 g of TCPEI and 40 mL solutions of different Hg(II)
concentrations under continuous stirring. At equilibrium,
the solutions were filtered and the non-adsorbed Hg(II) was
The Langmuir model is given by the following equation:
where qe (mg g-1) is the amount adsorbed per unit weight
of adsorbent at equilibrium, Ce (mg L-1) is the equilibrium
concentration of the adsorbate, and qm (mg g-1) is the
maximum adsorption capacity, and b (L mg-1) is the
constant related to the free energy of adsorption. The
values of maximum capacity (qm) and Langmuir constant
(b) were calculated from the intercept and the slope of the
The Freundlich model given by the following equation:
where KF (mg1-(1/n) L1/ng-1) is a constant correlated to the
relative adsorption capacity of the adsorbent and n is a
constant indicative of the intensity of the adsorption.
The thermodynamic study was done by conducting the
adsorption experiments at two different temperatures (15
and 27 C). The concentrations obtained after adsorption
were then used to calculate the activation energy (Ea)
according to the Arrhenius equation:
where Ea is the activation energy, R is the gas constant; T1
and T2 are the two different temperatures; k1 and k2 are rate
constants for the two temperatures.
The constants k1 and k2 were calculated using the
pseudo-second-order equation at each temperature.
The magnitude of the activation energy gives an idea
about the type of adsorption, namely physisorption (usually
Fig. 3 Comparison between pH
dependency of Hg(II) and
competing ions removal. Initial
concentrations = 40 mg L-1,
adsorbent amount = 0.2 g,
contact time = 30 min at room
temperature (27 C)
no more than 4.2 kJ mol-1) or chemisorption (Klekamp
and Urnbac 1993; O zcan et al. 2006).
Results and discussion
Characterisation of thiolated cross-linked
Sulphur content could be used as an indicator of the
thiolation of the polymer since polyethylenimine only contains
C, N, and H. The results from CHNS analysis confirmed
the introduction of the thiol group with a sulphur content of
9.1 % being recorded.
IR characterisation confirmed the presence of the thiol
group as the sharp peaks observed at 671.47 and
721.80 cm-1 which correspond to the stretching vibration
of CS (Coates 2000).
Effect of adsorbent amount
The results for the dependence of adsorption of Hg(II) on
the amount of the polymer are shown in Fig. 2a.
The results showed that adsorption increased sharply
from 81 % with an adsorbent amount of 0.03 and 0.05 g,
93 % with 0.1 g of adsorbent, and increased and remained
constant at 97 % for 0.21 g. Thus, 0.2 g was considered
as the optimum adsorbent amount required for optimal
Effect of contact time
The experimental runs measuring the effect of contact time
on the adsorption of Hg(II) were conducted at various time
intervals between 10 and 120 min and at room
temperature, pH 7, elemental concentration 40 mg L-1, and an
Fig. 4 Pseudo-second-order plot for the adsorption process. Initial
concentrations = 40 mg L-1, adsorbent amount = 0.2 g, at pH 3 and
room temperature (27 C)
adsorbent mass of 0.2 g, to obtain the optimal time
required for adsorption. The concentration of Hg(II) was
determined at the end of each time. The obtained
equilibrium capacities (qe) were then plotted against the
equilibrium time for kinetic modelling. Figure 2b shows the effect
of contact time on the adsorption of Hg(II).
Adsorption increased rapidly with an increase in contact
time from 10 to 30 min. A further increase in time had no
effect on the adsorption. A maximum contact time of
30 min was therefore considered as the optimum time for
Effect of pH
Synthetic standard solutions of 40 mg L-1 with a fixed
quantity of adsorbent, fixed time, and fixed temperature
were used. The pH of the synthetic solutions was adjusted
to five different pH values, namely 1, 3, 5, 7, and 9, using
1 mol L-1 sodium hydroxide and 1 mol L-1 hydrochloric
acid. Figure 2c shows adsorption at different pH values.
The adsorption percentages showed high removal
efficiency independent of pH. This independence of the
adsorption of Hg(II) on the pH could hinge on the high
affinity of the Hg(II) to thiol group on the polymer surface,
as such outperforming the hydrogen ions.
Effect of competing ions
To investigate the selectivity of TCPEI towards Hg(II),
adsorption experiments were performed in the presence of
competing ions using multi-component standard solutions
containing Co(II), Cu(II), Pb(II), Zn(II), Ni(II), Fe(III), and
Hg(II) at pH 7.3 and adsorbent amount of 0.2 g. Table 1
shows the final metal concentrations obtained after
adsorption from an initial concentration of 40 mg L-1 for
all metals. The adsorption capacity, efficiency (%) and the
relative standard deviation (RSD) are based on three
measurements (n = 3).
TCPEI showed good removal efficiency for all metals
except Fe. However, the removal of Hg(II) was still the
highest. The effect of competing ions was further studied at
low pH of 3 to assess the effect of pH on the removal of
Hg(II) in the presence of other elements. Figure 3 shows
the removal efficiency of elements in a multi-component
solution in acidic and basic conditions.
The results show that the removal of Hg(II) by TCPEI
was independent of pH, whereas that of other ions was
highly dependent. From speciation modelling using
MEDUSA, the following species were found to be
dominant at pH 3: Hg2?, Co2?, Cu2?, Pb2?, Zn2?, Ni2?, Fe3?
and FeOH2?. At pH 8, the following species were observed
to be dominant: HgOH?, Hg(OH)2; CoOH?, Co(OH)2,
Cu(OH)2, Pb(OH)2, Zn(OH)2, Ni(OH)2 and Fe(OH)3. It can
be observed that, at pH 3, the metals exist as divalent ions
in solution. Thus, any adsorption is influenced by the
affinity of the metal to the adsorbent surface. This binding
is based on the hardsoft Lewis acidbase theory where the
sulphur on the thiolated polymer acts as a Lewis base and
binds the metal (Lewis acid) by a coordinate bond (Pearson
1968). Since Hg(II), Co(II) and Cu(II) are soft acids, they
tend to preferentially bind to sulphur (a soft base)
compared to other metals. However, the affinity of thiol to
Hg(II) is superior compared to that for Co(II) and Cu(II), a
property that has earned thiol the name mercaptan or
mercury capturing (Dujardin et al. 2000). Ni(II), Zn(II) and
Pb(II) showed good adsorption as well since they are
considered moderate acids (or borderlines acids) and as
such they can bind to either soft or hard bases (Pearson
1968). Fe(III), on the other hand, is a hard acid, hence its
low removal efficiency. At pH 8, it can be observed that the
metals, except partly for Hg and Co, are completely
hydrolysed and form precipitates. Their adsorption at this pH
is largely due to precipitation, while that for Hg and Co
could be due to the combination of both precipitation and
interaction of HgOH? and CoOH? with sulphur.
Kinetic modelling of adsorption process
The high correlation obtained by plotting the linearised
form of pseudo-second-order model (R2 = 0.995)
compared to that for the pseudo-first-order model (R2 = 0.758)
demonstrated that the former gives a better fit, implying
that the adsorption occurs via a chemisorption process
(Antures et al. 2003). A plot of the linearised form of
pseudo-second-order model (t/qt vs. t) is given in Fig. 4.
The results from isotherm modelling suggest that the
Freundlich model fits the data better as shown by the
correlation coefficient of 0.963, whereas that for the Langmuir
correlation coefficient is 0.536. This result demonstrates
adsorption on a heterogeneous surface. It also assumes that
the adsorption capacity of the adsorbent increases with
increasing concentration of the adsorbate.
The calculated activation energy value (Ea) for the
adsorption of Hg(II) onto TCPEI surface was 55.97 kJ mol-1. This
elevated value implies that Hg(II) is adsorbed onto the
TCPEI via a chemisorption process which is consistent with
the results obtained from kinetic modelling. k1 and k2 values
are 0.027 and 0.021, respectively.
Desorption results for TCPEI by different nitric acid
concentrations are shown in Fig. 5. The results show the
concentrations of Hg(II) desorbed by 2, 3, 5, and
7 mol L-1 of HNO3.
The desorbed amount of Hg(II) increased with increasing
HNO3 concentration from 3.3 mg L-1 of 2 mol L-1 HNO3
to 31 mg L-1 of 5 mol L-1 HNO3, with no further
Fig. 6 Adsorption capacity after several subsequent desorptions.
Initial concentrations = 40 mg L-1, adsorbent amount = 0.2 g,
contact time = 30 min at pH = 3 and room temperature (27 C)
desorption beyond that. Subsequently, regeneration of
TCPEI was carried out using 5 mol L-1 acid concentration.
The regenerated TCPEI showed 88 % removal
efficiency and 7.022 mg g-1 removal capacity after one cycle
of desorption. This is a lower efficiency compared to that
for a fresh polymer (dropped by 9 %), which was still
Serial desorptions were conducted to assess the amount
of intractable Hg(II) that would remain bound to the
polymer. For example, after seven desorption cycles
(Fig. 6), the amount of Hg(II) adsorbed onto TCPEI
dropped by 5.48 mg g-1 (from 7.78 mg g-1 for the fresh
polymer to 2.00 mg g-1 after the seventh desorption). The
decrease of the removal efficiency could be attributed to
the fact that, after each desorption process, there will be an
amount of Hg(II) that would remain bound to the polymer
surface and hence reduce the available sites for adsorption.
The removal efficiency decreased in the following
percentages: 97 [ 88 [ 78 [ 63 [ 57 [ 50 [ 26 [ 25. This
suggests that TCPEI could be efficiently re-usable up to
five cycles, beyond which the removal efficiency drops
below 50 %.
Generally, the results obtained from this study showed
that the performance of TCPEI outperforms other reported
adsorbents in terms of selectivity and pH dependency.
Sreedhar and Anirudhan (1999) studied the removal of Hg
using polyacrylamide grafted onto sawdust which had a
superior removal capacity, but this was more dependent on
the pH (less removal efficiency was observed at pH below
5.5). This limits the use of the adsorbent for Hg recovery
from low pH solutions, e.g. AMD-impacted waters in
which pH can be lower than 3. Moreover, the removal
procedure was time consuming, taking 5 h to reach the
optimum removal. Another study by Kesenci et al. (2002)
demonstrated very fast adsorption using
poly(ethyleneglycol dimenthacrylate-co-acrylamide) beads, where the
largest amount of Hg was attached to the adsorbent within
the first 10 min with saturation gradually reached within
30 min. Beside the advantage of the fast kinetics, it also
showed some demerits such as the high dependency on the
pH as well as the poor selectivity towards Hg in the
presence of Pb.
Rio and Delebarre (2003) also reported the removal of Hg
using silico-aluminous fly ashes and sulfo-calcic fly ashes.
They obtained removal efficiencies of 54 and 81 %,
respectively. According to their explanation, the difference
on the removal efficiency is based on the chemical
composition of two fly ashes, in which the one with high
removal efficiency contains more sulphur than the other one
with poor removal. However, in both cases, the removal
process was time consuming, taking 72 h to reach the
equilibrium, while in our case, this was achieved in 30 min.
Liu and Guo (2006) reported the removal of Hg using
polyacrylamide-grafted attapulgite (PAM-ATP) with
removal capacity of 1.12 mg g-1, which is quite similar to
the results of this study. On other hand, some differences in
terms of efficiency, selectivity, influencing of different
conditions such as pH, and time were demonstrated.
In another study by the authors (Saad et al. 2012b), good
efficiency of sulphonated cross-linked polyethylenimine
was reported with up to 87 % removal of Hg(II), but Se
was found to highly compete with Hg(II). In this sense,
TCPEI represents a good alternative for Hg(II) removal
considering its high selectivity and independence of pH,
thus making it an efficient adsorbent for the intended
application of this study.
A new, efficient polyethylenimine derivative has been
developed by thiolation and successfully employed for the
removal of mercury from aqueous solutions under optimal
conditions, namely 30 min contact time, 40 mg L-1 initial
concentration, and 0.2 g adsorbent amount.
The modified polymer showed high efficiency and
selectivity towards Hg(II) independent of pH and
competing ions, thus outperforming most of the reported
adsorbents for Hg capture.
The Freundlich isotherm was found to be the best fit
describing the experimental data, suggesting that
adsorption occurred on a heterogeneous surface. The
pseudosecond-order model was found to explain the adsorption
kinetics most effectively. This model and the results of the
thermodynamic study showed that Hg(II) adsorption
occurred via chemisorption.
The possibility of re-using the developed polymer is also
a very significant factor especially with respect to cost
effectiveness of the removal process.
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