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Ecological Engineering 60 (2013) 160– 166

Contents lists available at ScienceDirect

Ecological Engineering

j ourna l ho me pa g e: www.elsev ier .com/ locate /eco leng

dsorption, concentration, and recovery of aqueous heavy metal ionsith the root powder of Eichhornia crassipes

iaosen Lia, Songlin Liua, Zhongyuan Nab, Diannan Lua, Zheng Liua,∗

Department of Chemical Engineering, Tsinghua University, Beijing 100084, ChinaInstitute of Ecological Agriculture, Yunnan 650000, China

r t i c l e i n f o

rticle history:eceived 5 January 2013eceived in revised form 27 June 2013ccepted 6 July 2013vailable online 14 August 2013

eywords:ichhornia crassipeseavy metal uptake

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 powderof Eichhornia crassipes, followed by combustion to establish an economical route suitable for large-scalerecovery of heavy metal ions from wastewater. In the optimal pH range of 5.0–6.0, the adsorption reachedequilibrium 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 appliedto describe the adsorption kinetics. We concluded from Fourier transform infrared spectroscopy thatfunctional 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

dsorption/desorption mechanism role in the absorption process. X-ray photoelectron spectroscopy analysis suggested that this long-rootadsorbent appeared to chelate Cr3+ more strongly than Cu2+, both of which appeared on the surface andin the interior of the adsorbent; both were eluted best with H2SO4. Combusting the saturated adsorbentgenerated a product with high concentrations of metal ions: 23% w/w Cu2+ and 30% w/w Cr3+, valuesequal or higher than the regular contents of mine ore (20–25% w/w). Thus, this process created a productthat was favorable for subsequent processing.

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. Introduction

Recovering heavy metal ions from industrial or urban effluents,articularly those toxic to human beings, animals, and plants, isn important issue for both environmental safety and resourceustainability. A number of methods to remove heavy metal ions,uch as precipitation, coagulation, solvent extraction, electroly-is, membrane separation, ion exchange, and adsorption (Bai andbraham, 2003; Bailey et al., 1999; Ucun et al., 2002) have beeneveloped and tested. These methods are not efficient for treat-

ng dilute solutions, and recovering valuable heavy metals is anmportant but unfulfilled objective. Increasing efforts have been

ade in recent years to find biodegradable adsorbents, such aslants (Dhir and Srivastava, 2011; Zuo et al., 2012), fungi (Bingolt al., 2004), lignin (Sciban et al., 2011), alginate (Singh et al., 2012),lgae (Areco et al., 2012; Aravindhan et al., 2004; Bulgariu andulgariu, 2012; Cabatingan et al., 2001; Lee and Chang, 2011), and

ther biomaterials (Sharma and Bhattacharyya, 2005; Schneegurtt al., 2001) that can recover metal ions. These biodegradable adsor-ents are advantageous over chemically synthesized adsorbents

∗ Corresponding author. Tel.: +86 10 6277 9876.E-mail address: liuzheng@mail.tsinghua.edu.cn (Z. Liu).

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925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ecoleng.2013.07.039

© 2013 Elsevier B.V. All rights reserved.

ecause a saturated biodegradable adsorbent can be combustedo remove the majority of the adsorbent, concentrating the heavy

etal species into a feedstock for subsequent processing. Becausef this advantage, the adsorption/desorption mechanism and theicrostructure of these adsorbents should be investigated more

horoughly.Long-root Eichhornia crassipes, termed because of its morpho-

ogical characteristics, is being tested for the on-site treatmentf Dianchi Lake in China because of its extraordinary uptake ofqueous nitrogen and phosphorus and its ability to adsorb otherazardous compounds such as arsenic contaminants (Lin et al.,012). It is also expected that recovery and concentration of thedsorbed metal ions from the saturated plant adsorbent can beone conveniently. This method could provide an ecologicallyriendly way to deal with eutrophic wastewater while recover-ng valuable heavy metals. Thus, this study aims to examine thedsorption behavior toward heavy metal ions, the mechanism ofdsorption, and the adsorbent microstructure of long-root E. cras-ipes and subsequent recovery of the metal ions by combustion.

In this work, we prepared the root powder of long-root E. cras-

ipes as the adsorbent, and we chose Cu2+and Cr3+ as representativeeavy metal ions. We began the experimental study by examininghe adsorption of Cu2+ and Cr3+ as a function of pH and salt concen-ration, followed by a study of adsorption kinetics and isotherms.

gineering 60 (2013) 160– 166 161

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ietthe OH peak shifted from 3412 cm−1 to around 3350 cm−1. Thisshift could be caused by the formation of a complex between metalions and the OH groups of the adsorbent. The peak at 1321 cm−1

Table 1List of peak shifts.

Peak Associatedfunctional

Shift

X. Li et al. / Ecological En

ourier transform infrared spectroscopy (FTIR) and X-ray photo-lectron spectroscopy (XPS) were used to explore the interactionsetween the metal ions and the adsorbent. The adsorption of Cu2+

nd Cr3+was accompanied by the discharge of Ca2+, Mg2+, and+ from the adsorbent, indicating an ion-exchange mechanism. Acanning electron microscopy (SEM) with a back-scattered elec-ron detector (BSE) was used to determine the adsorption sites. Weombusted the root powder of long-root E. crassipes saturated withdsorbed metal ions to confirm the effectiveness of the process inemoving biomass while concentrating metal ions.

. Materials and methods

.1. Materials

The chemical reagents used in this work were of analyticallyure grade. The heavy metal solutions were prepared by dissolvingppropriate amounts of CuCl2·2H2O and CrCl3·6H2O in deionizedater. NaOH and HCl solutions were used to adjust the pH of

he heavy metal solutions. Standard metal solutions with con-entrations of 1000 mg/L purchased from the National Institute ofetrology (Beijing, China) were used to calibrate our flame atomic

bsorption spectroscope. All glassware was soaked in 10% HCl andashed with deionized water at least three times before use. Thearticle size of the adsorbent prepared from the root powder of

ong-root E. crassipes was 300–600 �m in diameter. The elementalontents of C, H, O, and N were 47.23%, 4.72%, 44.4%, and 1.98%,espectively.

.2. Adsorption experiment

One gram of the adsorbent was added to a 100 mL aliquotontaining the Cu2+ or Cr3+solution at an appropriate pH andoncentration in a 300 mL flask. The initial concentration of theetal was 100 mg/L. The flask was then placed in a BOD incuba-

or shaker (Sky-111B, SuKun, China) at 30 ◦C with a rotation speedf 175 rpm. The supernatants were sampled at given time inter-als. After filtering through a 0.45 �m membrane, we determinedhe concentrations of Cu2+ and Cr3+ using flame atomic absorptionpectroscopy (FAAS; Z-5000, Hitachi, Japan). The wavelengths ofeavy metal were 324.8 nm for Cu2+ and 357.9 nm for Cr3+ withperational currents of 5.0 mA and 7.5 mA, respectively. We inter-reted the extent of adsorption according to the initial and finaloncentrations of the solution. The removal ratio (R) was then cal-ulated using Eq. (1).

= Ci − Ceq

Ci× 100% (1)

here Ci and Ceq are the initial and equilibrium concentrations ofhe solution, respectively. We performed adsorptions of Cu2+ andr3+ ions at different pH values from 1.0 to 7.0. We studied theffect of salt on the adsorption behavior at a pH of 5.0 with a NaCloncentration of 0–300 mmol/L. Each experiment was carried outn triplicate with deviation less than 5%.

.3. Desorption experiment

The desorption of metal ions was carried out using 100 mL oflution buffer containing 0.2 M HCl, 0.2 M HNO3, 0.2 M H2SO4,.2 M thiourea, 0.2 M HCl, and 0.2 M EDTA. During each run, thebove solution was mixed with the adsorbent saturated with metal

ons in a 300 mL flask. The flask was then placed in the BOD incu-ator shaker at 30 ◦C with a rotation speed of 175 rpm for 2 h. Theolutions were filtered through a 0.45 �m membrane and the con-entrations of metal ions were measured by FAAS as mentioned

Fig. 1. FTIR spectra of the root powders before and after adsorption.

bove, based on which the recovery yield was obtained. Each exper-ment was triplicate with deviation less than 5%.

.4. Assays

We used FTIR (Nicoletis10, Thermo Scientific, USA), XPS (PHI-300, Perkin-Elmer, USA), and SEM-BSE (JSM-6460LV Japan) to

dentify the major functional groups of the adsorbent and the sitesesponsible for the adsorption. We first added the root-powderdsorbents to the 300 mg/L metal solutions at a pH of 5.0. Afterhe samples reached absorption equilibrium, they were analyzedy FTIR, XPS, and SEM-BSE.

The concentrations of Ca2+, Mg2+, and K+ in solution were mea-ured by FAAS before and after adsorption; one adsorbent washedith deionized water served as the control. For these experiments,

g of root powder was added into 100 mL of the metal solution500 mg/L) at a pH of 5.0. The pH value of the solution was deter-

ined using a pH meter (SevenEasy, Mettler, Switzerland). The netelease of cations (mM/g of absorbent) was confirmed using a solu-ion without metal ions as the control. After combustion, we used-ray diffraction (XRD; D8-Advance, Bruker, Germany) and X-rayuorescence (XRF; XRF-1800, Shimadzu, Japan) to determine theompositions of the residual materials.

. Results and discussion

.1. Characterization of the adsorption of Cu2+ and Cr3+

FTIR and XPS, which have been extensively used to character-ze adsorption of metal compounds (Altenor et al., 2009; Hartonot al., 2009; Wang et al., 2013), were used in this study to studyhe adsorbent. As shown in Fig. 1 and Table 1, after adsorption

group

Before adsorption After adsorption3412 3350 OH 62 ± 81321 disappeared COOH –

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

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Fig. 2. Binding energies of the 1s electrons of O on the powder.

isappeared after adsorption, indicating that the carboxyl grouplso contributed to the formation of a complex with Cu2+ and Cr3+.

Fig. 2 shows how the binding energies of the 1s electrons of On the surface of the powder change after adsorption. The elec-ron binding energy of O 1s decreased by an average of 0.370 eVfter adsorption of Cu2+ and decreased by 0.643 eV after adsorp-ion of Cr3+. These results indicate that stronger chelation occursetween Cr3+ and the adsorbent than between Cu2+ and the adsor-ent, which leads to a different desorption efficiency for the two

ons (see Section 3.7).The Ca2+, Mg2+, and K+ contents of the root powder were

etermined by XRF. The variation of these ions before and afterdsorption was determined by FAAS. These results are shown inable 2, in which the net discharge of these ions after washing witheionized water was measured as the control. As Table 2 shows,a2+, Mg2+, and K+ were discharged from the root powder whileu2+ and Cr3+ were adsorbed. This behavior suggests that Cu2+ andr3+ displaced Ca2+, Mg2+, and K+ from the adsorbent.

.2. Distribution of adsorption sites

SEM-BSE was used to monitor the heavy metal ions beingdsorbed. The results are shown in Fig. 3(a)–(c).

Figs. 3(a)–(c) show the existence of adsorbed metal ions in thehite areas. Adsorbed Cu2+ and Cr3+ appeared both inside and out-

ide the porous adsorbent. This behavior suggests that the Cu2+ andr3+ ions were not exclusively adsorbed on either the inner or outerurface of the plant cell wall, which was also observed by Conrad2008).

.3. Effect of pH on adsorption

We carried out these experiments at a pH of 1.0–7.0 in order todentify the optimal pH. The initial concentration of Cu2+ or Cr3+

as 100 mg/L. As shown in Fig. 4, increasing the pH significantlyncreased the adsorption capacity for both Cu2+ and Cr3+. Similar

able 2ischarge of Ca2+, Mg2+, and K+ during adsorption.

Loading of Cu2+ andCr3+ (mM/g)

Net release (mM/g)

Mg2+ Ca2+ K+

Cu2+ 0.411 0.333 0.025 0.042Cr3+ 0.507 0.327 0.051 0.106

Fig. 3. SEM (backscattering) images of adsorbents without adsorbed heavy metalions and those saturated with Cu2+ and Cr3+.

X. Li et al. / Ecological Engineering 60 (2013) 160– 166 163

Table 3Regression parameters for adsorption of Cu2+ and Cr3+.

Kinetic models Qe ± s (mg/g) R2 ± s

Cu2+ Cr3+ Cu2+ Cr3+

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Pseudo-second-order model 13.15 ± 0.26

Pseudo-first-order model 13.00 ± 0.16

dsorption behaviors have also been identified in other biomassdsorbents such as algae, apple waste, and pine bark (Chen andiacoumi, 1997; Crist et al., 1992, 1994; Kratochvil et al., 1995; Leend Yang, 1997; Seco et al., 1997; Yu and Kaewsarn, 1999). This pHependence of adsorption capacity may be caused by the increasedvailability of OH and COOH, which chelate with the metal ions.e carried out the adsorption at a pH of 5.0; thus, hydrolysis of

he metal ions is not significant and has little effect on our resultsVillaescusa et al., 2004).

.4. Characterization of the adsorption kinetics at the optimal pH

Fig. 5 shows the adsorption of Cu2+ and Cr3+ over time at

he optimal pH of 5. After 20 min, an adsorption degree of 80%as achieved. Other agricultural by-products used to absorb Cu2+

nd Cd2+ have been found to have similar adsorption kinetics

Fig. 4. Adsorption of Cu2+ and Cr3+ at different pH values.

ig. 5. Adsorption over time of Cu2+ and Cr3+. Initial metal concentration is 100 mg/L.

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13.00 ± 0.09 0.9944 0.999413.08 ± 0.26 0.9836 0.8589

Kratochvil et al., 1995; Lee and Yang, 1997). This rapid adsorptionf metals allows for greater economy and efficiency.

We applied pseudo-first-order and pseudo-second-orderinetic models (Chen and Wang, 2007; Ho et al., 1996; Unnithannd Anirudhan, 2001) to simulate the above-mentioned adsorp-ion process. These two kinetic models can be described as follows:

t = Qe

[1 − 1

exp (V0t)

](2)

t

Qt= 1

V0Q 2e

+ t

Qe(3)

here Qe is the equilibrium adsorption capacity in solution, Qt ishe adsorption amount at time t, and V0 is the initial adsorptionate.

The regression parameters of these two kinetic models are listedn Table 3. The R2 values for the pseudo-first-order kinetic modelre 0.9836 and 0.8588 for Cu2+ and Cr3+, respectively; the R2 valuesor the pseudo-second-order kinetic model are 0.9944 and 0.9994,espectively. The better fit by the pseudo-second-order kineticodel may be underpinned by the presence of two mechanisms in

he adsorption process: chemical sorption (rapid) and subsequentlow ion exchange (slow) (Villaescusa et al., 2004), supported byhe observed release of Ca2+, Mg2+, and K+.

.5. Adsorption isotherms at the optimal pH

We performed regression analysis using Langmuir and Freund-ich isotherms on the data of Cu2+ and Cr3+adsorption at pH 5.0,escribed by the following equations,

e = QmaxbCe

1 + bCe(4)

n Qe =(

1n

)ln Ce + ln KF (5)

here Qe and Qmax are the equilibrium and saturated adsorp-ion capacities of the adsorbent, respectively. Ce is the equilibriumoncentration of the metals in the solution and b is the adsorp-ion constant. KF and n are the empirical constant for Freundlichsotherm.

As shown in Fig. 6 and Table 4, the Langmuir model offers a bet-er description of the adsorption behavior than does the Freundlich

odel. Additionally, the Qmax of Cr3+ is greater than that of Cu2+.he better agreement of the Langmuir model indicates that “mono-ayer coverage” occurs on the surface of the root powder by the two

etals. As Table 4 shows, the Qmax values of both Cu2+ and Cr3+

re above 30 mg/g of absorbent, much higher than those of othereported biodegradable adsorbents such as pine bark (Al-Asheh anduvnjak, 1998) and grape stalks (Villaescusa et al., 2004).

.6. Effect of salt concentration on adsorption

In practice, industrial wastewater and urban effluents containarious salts that may affect adsorption. To examine how salt con-entration affects adsorption of the heavy metal ions explored in

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

Table 4Comparison 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 34.82Cr3+ 0.9819 0.9442 33.98 36.14

Table 5Metal 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 2.50Si 14.46 Si 26.63 Si 25.55Fe 13.98 Fe 16.13 Fe 22.34Mg 11.05 Al 12.07 Al 14.50

E

tcm

tTma

moiformed by strong acid and alkali increases the ionic strength of the

Cu 0.04 Cu

Cr 0.11 Cr

lements with contents below 10% are not listed.

his paper, we added sodium chloride to the solution. The con-entration of NaCl was 0–300 mM. The initial concentration of theetal was 100 mg/L and the pH was 5.0.As Fig. 7 shows, increasing the NaCl concentration decreases

3+

he adsorption capacity for the heavy metal ions, particularly Cr .his behavior suggests that electrostatic interactions between theetal ions and the adsorbent dominate the absorption process

nd that copper and chromium are adsorbed through different

Fig. 6. Adsorption isotherm.

sbt

F

30.70 Cu 0.060.05 Cr 23.41

echanisms (Flogeac et al., 2004, 2003). This same effect was alsobserved when cork waste was used as an adsorbent of metalsons (Villaescusa et al., 2000). It is known that the presence of salt

olution. This increase in ionic strength depresses the adsorptionased on electrostatic interactions, and thus reduces the adsorp-ion capacities of Cu2+ and Cr3+, as shown in Fig. 7. The disparate

ig. 7. Adsorption of Cu2+ and Cr3+metal ions as a function of NaCl concentration.

Fig. 8. Desorption of Cu2+ and Cr3+.

X. Li et al. / Ecological Engineer

cit2

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3

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aFr

a

CbwuDeiltoe

4

tEpacdAitp3esat 700 ◦C removed most of the biomass and generated a prod-

Fig. 9. XRD analysis of adsorbents after combustion.

hange of the adsorption capacities of Cu2+ and Cr3+ may be rooted

n their difference in outer layer electron distributions, leadingo different affinities toward the COO− group (Villaescusa et al.,004).

ufw

ing 60 (2013) 160– 166 165

.7. Desorption of metal ions from the adsorbent

We performed batch desorption using elution buffer with 0.2 MCl, 0.2 M H2SO4, 0.2 M HNO3, 0.2 M thiourea, 0.2 M HCl, and 0.2 MDTA. These results are shown in Fig. 8. The elution of Cu2+ wasore complete than that of Cr3+. XPS shows a stronger interac-

ion of Cr3+ with the adsorbent compared to that of Cu2+ with thedsorbent. Additionally, because of the Jahn-Teller effect, the coor-ination compound formed by Cu2+ and the root powder is weakerhan that formed by Cr3+ and the root powder, leading to easy elu-ion of Cu2+ (Parkman et al., 1999). When we compared the elutionolutions, we found that using H2SO4 gives the best recovery ofoth Cu2+ (>90%) and Cr3+ (>50%).

.8. Combustion of adsorbents saturated with metal ions

During this experiment, we added 1 g of the adsorbent to 100 mLliquots of Cu2+ or Cr3+ solutions, each with a pH of 5.0 and a00 mg/L initial concentration of the metal (Cu2+ or Cr3+). Afterdsorption, the metal contents of the loaded root powder are0.3 mg/g for Cu2+ and 28.8 mg/g for Cr3+. We then combusted thedsorbent as the first step of recovering the metal ions. We used

muffle furnace at 700 ◦C for 2 h, which decomposed the long-oot powder adsorbent. We then used XRD and XRF to determinehe composition of the residual material. The results are shown inig. 9(a)–(c).

Fig. 9 shows that K present in the bare adsorbent (Fig. 9(a)) dis-ppears in the adsorbents saturated with Cu2+ or Cr3+, as shown inig. 9(b) and (c), respectively. We attribute this disappearance toeplacement by Cu2+ and Cr3+, as shown in Table 1.

We used XRF to determine the contents of ions in the adsorbentsfter combustion, which are shown in Table 5.

Combusting the adsorbents induced weight loss: 90.2% for theu2+-saturated adsorbent and 87.1% for the Cr3+-saturated adsor-ent. The final contents of Cu2+ and Cr3+ are 30.70% w/w and 23.41%/w, respectively. These values are equal or greater than the reg-lar content of mine ore, around 20–25% w/w (Chrysochoou andermatas, 2006; Gu et al., 2005). Combustion’s ease of operation,ffective removal of biomass, and high concentration of metal ionsn the resultant concentrate make it a suitable choice for treatingong-root E. Crassipes saturated with metal ions. Similar adsorp-ion and concentration behavior has been identified in other kindsf biomass adsorbents such as Pteris vittata and E. acicularis (Kalvet al., 2011; Sakakibara et al., 2011).

. Conclusion

We validated the adsorption of Cu2+ and Cr3+as representa-ive heavy metal ions in wastewater by the powder of long-root. crassipes. We attributed this adsorption to the formation of com-lexes with the hydroxyl and carboxyl groups, supported by FTIRnalysis. XPS analysis revealed that the adsorbent more stronglyhelated to Cr3+than to Cu2+. SEM-BSE showed adsorption sitesistributed across both the inside and outside of the structure.dsorption was accompanied by release of Ca2+, Mg2+, and K+,

ndicating an ion-exchange displacement mechanism. The adsorp-ion could be described by the Langmuir isotherm. Optimizing theH to 5–6 gave maximum adsorption capacities of 32.5 mg/g and4.0 mg/g for Cu2+ and Cr3+, respectively. The adsorbent reachedquilibrium within 30 min and could be described by a pseudo-econd-order kinetic model. Combusting the saturated adsorbent

ct with higher concentrations of the metal ions; thus, it wasavorable for subsequent refining. Our results provide an effectiveay to treat wastewater contaminated by metal ions.

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