articulo de investigación ambiental

1. Ecological Engineering 60 (2013) 160–166
Contents lists available at ScienceDirect
Ecological Engineering
journal homepage: www.elsevier.com/locate/ecoleng
Adsorption, concentration, and recovery of aqueous heavy metal ions
with the root powder of Eichhornia crassipes
Xiaosen Lia
, Songlin Liua
, Zhongyuan Nab
, Diannan Lua
, Zheng Liua,∗
a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
b
Institute of Ecological Agriculture, Yunnan 650000, China
a r t i c l e i n f o
Article history:
Received 5 January 2013
Received in revised form 27 June 2013
Accepted 6 July 2013
Keywords:
Eichhornia crassipes
Heavy metal uptake
Adsorption/desorption mechanism
a b s t r a c t
We investigated the adsorption of aqueous Cu2+
and Cr3+
, as model heavy metal ions, by the root powder
of Eichhornia crassipes, followed by combustion to establish an economical route suitable for large-scale
recovery of heavy metal ions from wastewater. In the optimal pH range of 5.0–6.0, the adsorption reached
equilibrium after 30 min and could be described by Langmuir isotherms with maximum adsorption capac-
ities of 32.51 mg/g for Cu2+
and 33.98 mg/g for Cr3+
. A pseudo-second-order kinetic model was applied
to describe the adsorption kinetics. We concluded from Fourier transform infrared spectroscopy that
functional groups containing OH and COOH contributed to the adsorption. During adsorption, Ca2+
,
Mg2+
, and K+
were discharged from the root powder. Electrostatic interactions also played an important
role in the absorption process. X-ray photoelectron spectroscopy analysis suggested that this long-root
adsorbent appeared to chelate Cr3+
more strongly than Cu2+
, both of which appeared on the surface and
in the interior of the adsorbent; both were eluted best with H2SO4. Combusting the saturated adsorbent
generated a product with high concentrations of metal ions: 23% w/w Cu2+
and 30% w/w Cr3+
, values
equal or higher than the regular contents of mine ore (20–25% w/w). Thus, this process created a product
that was favorable for subsequent processing.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Recovering heavy metal ions from industrial or urban effluents,
particularly those toxic to human beings, animals, and plants, is
an important issue for both environmental safety and resource
sustainability. A number of methods to remove heavy metal ions,
such as precipitation, coagulation, solvent extraction, electroly-
sis, membrane separation, ion exchange, and adsorption (Bai and
Abraham, 2003; Bailey et al., 1999; Ucun et al., 2002) have been
developed and tested. These methods are not efficient for treat-
ing dilute solutions, and recovering valuable heavy metals is an
important but unfulfilled objective. Increasing efforts have been
made in recent years to find biodegradable adsorbents, such as
plants (Dhir and Srivastava, 2011; Zuo et al., 2012), fungi (Bingol
et al., 2004), lignin (Sciban et al., 2011), alginate (Singh et al., 2012),
algae (Areco et al., 2012; Aravindhan et al., 2004; Bulgariu and
Bulgariu, 2012; Cabatingan et al., 2001; Lee and Chang, 2011), and
other biomaterials (Sharma and Bhattacharyya, 2005; Schneegurt
et al., 2001) that can recover metal ions. These biodegradable adsor-
bents are advantageous over chemically synthesized adsorbents
∗ Corresponding author. Tel.: +86 10 6277 9876.
E-mail address: liuzheng@mail.tsinghua.edu.cn (Z. Liu).
because a saturated biodegradable adsorbent can be combusted
to remove the majority of the adsorbent, concentrating the heavy
metal species into a feedstock for subsequent processing. Because
of this advantage, the adsorption/desorption mechanism and the
microstructure of these adsorbents should be investigated more
thoroughly.
Long-root Eichhornia crassipes, termed because of its morpho-
logical characteristics, is being tested for the on-site treatment
of Dianchi Lake in China because of its extraordinary uptake of
aqueous nitrogen and phosphorus and its ability to adsorb other
hazardous compounds such as arsenic contaminants (Lin et al.,
2012). It is also expected that recovery and concentration of the
adsorbed metal ions from the saturated plant adsorbent can be
done conveniently. This method could provide an ecologically
friendly way to deal with eutrophic wastewater while recover-
ing valuable heavy metals. Thus, this study aims to examine the
adsorption behavior toward heavy metal ions, the mechanism of
adsorption, and the adsorbent microstructure of long-root E. cras-
sipes and subsequent recovery of the metal ions by combustion.
In this work, we prepared the root powder of long-root E. cras-
sipes as the adsorbent, and we chose Cu2+and Cr3+ as representative
heavy metal ions. We began the experimental study by examining
the adsorption of Cu2+ and Cr3+ as a function of pH and salt concen-
tration, followed by a study of adsorption kinetics and isotherms.
0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ecoleng.2013.07.039

2. X. Li et al. / Ecological Engineering 60 (2013) 160–166 161
Fourier transform infrared spectroscopy (FTIR) and X-ray photo-
electron spectroscopy (XPS) were used to explore the interactions
between the metal ions and the adsorbent. The adsorption of Cu2+
and Cr3+was accompanied by the discharge of Ca2+, Mg2+, and
K+ from the adsorbent, indicating an ion-exchange mechanism. A
scanning electron microscopy (SEM) with a back-scattered elec-
tron detector (BSE) was used to determine the adsorption sites. We
combusted the root powder of long-root E. crassipes saturated with
adsorbed metal ions to confirm the effectiveness of the process in
removing biomass while concentrating metal ions.
2. Materials and methods
2.1. Materials
The chemical reagents used in this work were of analytically
pure grade. The heavy metal solutions were prepared by dissolving
appropriate amounts of CuCl2·2H2O and CrCl3·6H2O in deionized
water. NaOH and HCl solutions were used to adjust the pH of
the heavy metal solutions. Standard metal solutions with con-
centrations of 1000 mg/L purchased from the National Institute of
Metrology (Beijing, China) were used to calibrate our flame atomic
absorption spectroscope. All glassware was soaked in 10% HCl and
washed with deionized water at least three times before use. The
particle size of the adsorbent prepared from the root powder of
long-root E. crassipes was 300–600 ␮m in diameter. The elemental
contents of C, H, O, and N were 47.23%, 4.72%, 44.4%, and 1.98%,
respectively.
2.2. Adsorption experiment
One gram of the adsorbent was added to a 100 mL aliquot
containing the Cu2+ or Cr3+solution at an appropriate pH and
concentration in a 300 mL flask. The initial concentration of the
metal was 100 mg/L. The flask was then placed in a BOD incuba-
tor shaker (Sky-111B, SuKun, China) at 30 ◦C with a rotation speed
of 175 rpm. The supernatants were sampled at given time inter-
vals. After filtering through a 0.45 ␮m membrane, we determined
the concentrations of Cu2+ and Cr3+ using flame atomic absorption
spectroscopy (FAAS; Z-5000, Hitachi, Japan). The wavelengths of
heavy metal were 324.8 nm for Cu2+ and 357.9 nm for Cr3+ with
operational currents of 5.0 mA and 7.5 mA, respectively. We inter-
preted the extent of adsorption according to the initial and final
concentrations of the solution. The removal ratio (R) was then cal-
culated using Eq. (1).
R =
Ci − Ceq
Ci
× 100% (1)
Where Ci and Ceq are the initial and equilibrium concentrations of
the solution, respectively. We performed adsorptions of Cu2+ and
Cr3+ ions at different pH values from 1.0 to 7.0. We studied the
effect of salt on the adsorption behavior at a pH of 5.0 with a NaCl
concentration of 0–300 mmol/L. Each experiment was carried out
in triplicate with deviation less than 5%.
2.3. Desorption experiment
The desorption of metal ions was carried out using 100 mL of
elution buffer containing 0.2 M HCl, 0.2 M HNO3, 0.2 M H2SO4,
0.2 M thiourea, 0.2 M HCl, and 0.2 M EDTA. During each run, the
above solution was mixed with the adsorbent saturated with metal
ions in a 300 mL flask. The flask was then placed in the BOD incu-
bator shaker at 30 ◦C with a rotation speed of 175 rpm for 2 h. The
solutions were filtered through a 0.45 ␮m membrane and the con-
centrations of metal ions were measured by FAAS as mentioned
Fig. 1. FTIR spectra of the root powders before and after adsorption.
above, based on which the recovery yield was obtained. Each exper-
iment was triplicate with deviation less than 5%.
2.4. Assays
We used FTIR (Nicoletis10, Thermo Scientific, USA), XPS (PHI-
5300, Perkin-Elmer, USA), and SEM-BSE (JSM-6460LV Japan) to
identify the major functional groups of the adsorbent and the sites
responsible for the adsorption. We first added the root-powder
adsorbents to the 300 mg/L metal solutions at a pH of 5.0. After
the samples reached absorption equilibrium, they were analyzed
by FTIR, XPS, and SEM-BSE.
The concentrations of Ca2+, Mg2+, and K+ in solution were mea-
sured by FAAS before and after adsorption; one adsorbent washed
with deionized water served as the control. For these experiments,
1 g of root powder was added into 100 mL of the metal solution
(500 mg/L) at a pH of 5.0. The pH value of the solution was deter-
mined using a pH meter (SevenEasy, Mettler, Switzerland). The net
release of cations (mM/g of absorbent) was confirmed using a solu-
tion without metal ions as the control. After combustion, we used
X-ray diffraction (XRD; D8-Advance, Bruker, Germany) and X-ray
fluorescence (XRF; XRF-1800, Shimadzu, Japan) to determine the
compositions of the residual materials.
3. Results and discussion
3.1. Characterization of the adsorption of Cu2+ and Cr3+
FTIR and XPS, which have been extensively used to character-
ize adsorption of metal compounds (Altenor et al., 2009; Hartono
et al., 2009; Wang et al., 2013), were used in this study to study
the adsorbent. As shown in Fig. 1 and Table 1, after adsorption
the OH peak shifted from 3412 cm−1 to around 3350 cm−1. This
shift could be caused by the formation of a complex between metal
ions and the OH groups of the adsorbent. The peak at 1321 cm−1
Table 1
List of peak shifts.
Peak Associated
functional
group
Shift
Before adsorption After adsorption
3412 3350 OH 62 ± 8
1321 disappeared COOH –

3. 162 X. Li et al. / Ecological Engineering 60 (2013) 160–166
Fig. 2. Binding energies of the 1s electrons of O on the powder.
disappeared after adsorption, indicating that the carboxyl group
also contributed to the formation of a complex with Cu2+ and Cr3+.
Fig. 2 shows how the binding energies of the 1s electrons of O
on the surface of the powder change after adsorption. The elec-
tron binding energy of O 1s decreased by an average of 0.370 eV
after adsorption of Cu2+ and decreased by 0.643 eV after adsorp-
tion of Cr3+. These results indicate that stronger chelation occurs
between Cr3+ and the adsorbent than between Cu2+ and the adsor-
bent, which leads to a different desorption efficiency for the two
ions (see Section 3.7).
The Ca2+, Mg2+, and K+ contents of the root powder were
determined by XRF. The variation of these ions before and after
adsorption was determined by FAAS. These results are shown in
Table 2, in which the net discharge of these ions after washing with
deionized water was measured as the control. As Table 2 shows,
Ca2+, Mg2+, and K+ were discharged from the root powder while
Cu2+ and Cr3+ were adsorbed. This behavior suggests that Cu2+ and
Cr3+ displaced Ca2+, Mg2+, and K+ from the adsorbent.
3.2. Distribution of adsorption sites
SEM-BSE was used to monitor the heavy metal ions being
adsorbed. The results are shown in Fig. 3(a)–(c).
Figs. 3(a)–(c) show the existence of adsorbed metal ions in the
white areas. Adsorbed Cu2+ and Cr3+ appeared both inside and out-
side the porous adsorbent. This behavior suggests that the Cu2+ and
Cr3+ ions were not exclusively adsorbed on either the inner or outer
surface of the plant cell wall, which was also observed by Conrad
(2008).
3.3. Effect of pH on adsorption
We carried out these experiments at a pH of 1.0–7.0 in order to
identify the optimal pH. The initial concentration of Cu2+ or Cr3+
was 100 mg/L. As shown in Fig. 4, increasing the pH significantly
increased the adsorption capacity for both Cu2+ and Cr3+. Similar
Table 2
Discharge of Ca2+
, Mg2+
, and K+
during adsorption.
Loading of Cu2+
and
Cr3+
(mM/g)
Net release (mM/g)
Mg2+
Ca2+
K+
Cu2+
0.411 0.333 0.025 0.042
Cr3+
0.507 0.327 0.051 0.106
Fig. 3. SEM (backscattering) images of adsorbents without adsorbed heavy metal
ions and those saturated with Cu2+
and Cr3+
.

4. X. Li et al. / Ecological Engineering 60 (2013) 160–166 163
Table 3
Regression parameters for adsorption of Cu2+
and Cr3+
.
Kinetic models Qe ± s (mg/g) R2
± s
Cu2+
Cr3+
Cu2+
Cr3+
Pseudo-second-order model 13.15 ± 0.26 13.00 ± 0.09 0.9944 0.9994
Pseudo-first-order model 13.00 ± 0.16 13.08 ± 0.26 0.9836 0.8589
adsorption behaviors have also been identified in other biomass
adsorbents such as algae, apple waste, and pine bark (Chen and
Yiacoumi, 1997; Crist et al., 1992, 1994; Kratochvil et al., 1995; Lee
and Yang, 1997; Seco et al., 1997; Yu and Kaewsarn, 1999). This pH
dependence of adsorption capacity may be caused by the increased
availability of OH and COOH, which chelate with the metal ions.
We carried out the adsorption at a pH of 5.0; thus, hydrolysis of
the metal ions is not significant and has little effect on our results
(Villaescusa et al., 2004).
3.4. Characterization of the adsorption kinetics at the optimal pH
Fig. 5 shows the adsorption of Cu2+ and Cr3+ over time at
the optimal pH of 5. After 20 min, an adsorption degree of 80%
was achieved. Other agricultural by-products used to absorb Cu2+
and Cd2+ have been found to have similar adsorption kinetics
Fig. 4. Adsorption of Cu2+
and Cr3+
at different pH values.
Fig. 5. Adsorption over time of Cu2+
and Cr3+
. Initial metal concentration is 100 mg/L.
(Kratochvil et al., 1995; Lee and Yang, 1997). This rapid adsorption
of metals allows for greater economy and efficiency.
We applied pseudo-first-order and pseudo-second-order
kinetic models (Chen and Wang, 2007; Ho et al., 1996; Unnithan
and Anirudhan, 2001) to simulate the above-mentioned adsorp-
tion process. These two kinetic models can be described as follows:
Qt = Qe 1 −
1
exp (V0t)
(2)
t
Qt
=
1
V0Q2
e
+
t
Qe
(3)
where Qe is the equilibrium adsorption capacity in solution, Qt is
the adsorption amount at time t, and V0 is the initial adsorption
rate.
The regression parameters of these two kinetic models are listed
in Table 3. The R2 values for the pseudo-first-order kinetic model
are 0.9836 and 0.8588 for Cu2+ and Cr3+, respectively; the R2 values
for the pseudo-second-order kinetic model are 0.9944 and 0.9994,
respectively. The better fit by the pseudo-second-order kinetic
model may be underpinned by the presence of two mechanisms in
the adsorption process: chemical sorption (rapid) and subsequent
slow ion exchange (slow) (Villaescusa et al., 2004), supported by
the observed release of Ca2+, Mg2+, and K+.
3.5. Adsorption isotherms at the optimal pH
We performed regression analysis using Langmuir and Freund-
lich isotherms on the data of Cu2+ and Cr3+adsorption at pH 5.0,
described by the following equations,
Qe =
QmaxbCe
1 + bCe
(4)
ln Qe =
1
n
ln Ce + ln KF (5)
where Qe and Qmax are the equilibrium and saturated adsorp-
tion capacities of the adsorbent, respectively. Ce is the equilibrium
concentration of the metals in the solution and b is the adsorp-
tion constant. KF and n are the empirical constant for Freundlich
isotherm.
As shown in Fig. 6 and Table 4, the Langmuir model offers a bet-
ter description of the adsorption behavior than does the Freundlich
model. Additionally, the Qmax of Cr3+ is greater than that of Cu2+.
The better agreement of the Langmuir model indicates that “mono-
layer coverage” occurs on the surface of the root powder by the two
metals. As Table 4 shows, the Qmax values of both Cu2+ and Cr3+
are above 30 mg/g of absorbent, much higher than those of other
reported biodegradable adsorbents such as pine bark (Al-Asheh and
Duvnjak, 1998) and grape stalks (Villaescusa et al., 2004).
3.6. Effect of salt concentration on adsorption
In practice, industrial wastewater and urban effluents contain
various salts that may affect adsorption. To examine how salt con-
centration affects adsorption of the heavy metal ions explored in

5. 164 X. Li et al. / Ecological Engineering 60 (2013) 160–166
Table 4
Comparison of Langmuir and Freundlich models for adsorption.
Metal ions R2
Qmax (mg/g)
Langmuir model Freundlich model Langmuir model Freundlich model
Cu2+
0.9788 0.9431 32.51 34.82
Cr3+
0.9819 0.9442 33.98 36.14
Table 5
Metal ions present after combustion determined by XRF.
Bare adsorbent Cu2+
saturated adsorbent Cr3+
saturated adsorbent
Element Content (%) Element Content (%) Element Content (%)
K 15.90 K 2.66 K 2.50
Si 14.46 Si 26.63 Si 25.55
Fe 13.98 Fe 16.13 Fe 22.34
Mg 11.05 Al 12.07 Al 14.50
Cu 0.04 Cu 30.70 Cu 0.06
Cr 0.11 Cr 0.05 Cr 23.41
Elements with contents below 10% are not listed.
this paper, we added sodium chloride to the solution. The con-
centration of NaCl was 0–300 mM. The initial concentration of the
metal was 100 mg/L and the pH was 5.0.
As Fig. 7 shows, increasing the NaCl concentration decreases
the adsorption capacity for the heavy metal ions, particularly Cr3+.
This behavior suggests that electrostatic interactions between the
metal ions and the adsorbent dominate the absorption process
and that copper and chromium are adsorbed through different
Fig. 6. Adsorption isotherm.
mechanisms (Flogeac et al., 2004, 2003). This same effect was also
observed when cork waste was used as an adsorbent of metals
ions (Villaescusa et al., 2000). It is known that the presence of salt
formed by strong acid and alkali increases the ionic strength of the
solution. This increase in ionic strength depresses the adsorption
based on electrostatic interactions, and thus reduces the adsorp-
tion capacities of Cu2+ and Cr3+, as shown in Fig. 7. The disparate
Fig. 7. Adsorption of Cu2+
and Cr3+
metal ions as a function of NaCl concentration.
Fig. 8. Desorption of Cu2+
and Cr3+
.

6. X. Li et al. / Ecological Engineering 60 (2013) 160–166 165
Fig. 9. XRD analysis of adsorbents after combustion.
change of the adsorption capacities of Cu2+ and Cr3+ may be rooted
in their difference in outer layer electron distributions, leading
to different affinities toward the COO− group (Villaescusa et al.,
2004).
3.7. Desorption of metal ions from the adsorbent
We performed batch desorption using elution buffer with 0.2 M
HCl, 0.2 M H2SO4, 0.2 M HNO3, 0.2 M thiourea, 0.2 M HCl, and 0.2 M
EDTA. These results are shown in Fig. 8. The elution of Cu2+ was
more complete than that of Cr3+. XPS shows a stronger interac-
tion of Cr3+ with the adsorbent compared to that of Cu2+ with the
adsorbent. Additionally, because of the Jahn-Teller effect, the coor-
dination compound formed by Cu2+ and the root powder is weaker
than that formed by Cr3+ and the root powder, leading to easy elu-
tion of Cu2+ (Parkman et al., 1999). When we compared the elution
solutions, we found that using H2SO4 gives the best recovery of
both Cu2+ (>90%) and Cr3+ (>50%).
3.8. Combustion of adsorbents saturated with metal ions
During this experiment, we added 1 g of the adsorbent to 100 mL
aliquots of Cu2+ or Cr3+ solutions, each with a pH of 5.0 and a
300 mg/L initial concentration of the metal (Cu2+ or Cr3+). After
adsorption, the metal contents of the loaded root powder are
30.3 mg/g for Cu2+ and 28.8 mg/g for Cr3+. We then combusted the
adsorbent as the first step of recovering the metal ions. We used
a muffle furnace at 700 ◦C for 2 h, which decomposed the long-
root powder adsorbent. We then used XRD and XRF to determine
the composition of the residual material. The results are shown in
Fig. 9(a)–(c).
Fig. 9 shows that K present in the bare adsorbent (Fig. 9(a)) dis-
appears in the adsorbents saturated with Cu2+ or Cr3+, as shown in
Fig. 9(b) and (c), respectively. We attribute this disappearance to
replacement by Cu2+ and Cr3+, as shown in Table 1.
We used XRF to determine the contents of ions in the adsorbents
after combustion, which are shown in Table 5.
Combusting the adsorbents induced weight loss: 90.2% for the
Cu2+-saturated adsorbent and 87.1% for the Cr3+-saturated adsor-
bent. The final contents of Cu2+ and Cr3+ are 30.70% w/w and 23.41%
w/w, respectively. These values are equal or greater than the reg-
ular content of mine ore, around 20–25% w/w (Chrysochoou and
Dermatas, 2006; Gu et al., 2005). Combustion’s ease of operation,
effective removal of biomass, and high concentration of metal ions
in the resultant concentrate make it a suitable choice for treating
long-root E. Crassipes saturated with metal ions. Similar adsorp-
tion and concentration behavior has been identified in other kinds
of biomass adsorbents such as Pteris vittata and E. acicularis (Kalve
et al., 2011; Sakakibara et al., 2011).
4. Conclusion
We validated the adsorption of Cu2+ and Cr3+as representa-
tive heavy metal ions in wastewater by the powder of long-root
E. crassipes. We attributed this adsorption to the formation of com-
plexes with the hydroxyl and carboxyl groups, supported by FTIR
analysis. XPS analysis revealed that the adsorbent more strongly
chelated to Cr3+than to Cu2+. SEM-BSE showed adsorption sites
distributed across both the inside and outside of the structure.
Adsorption was accompanied by release of Ca2+, Mg2+, and K+,
indicating an ion-exchange displacement mechanism. The adsorp-
tion could be described by the Langmuir isotherm. Optimizing the
pH to 5–6 gave maximum adsorption capacities of 32.5 mg/g and
34.0 mg/g for Cu2+ and Cr3+, respectively. The adsorbent reached
equilibrium within 30 min and could be described by a pseudo-
second-order kinetic model. Combusting the saturated adsorbent
at 700 ◦C removed most of the biomass and generated a prod-
uct with higher concentrations of the metal ions; thus, it was
favorable for subsequent refining. Our results provide an effective
way to treat wastewater contaminated by metal ions.

7. 166 X. Li et al. / Ecological Engineering 60 (2013) 160–166
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