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Chemical Engineering Journal 183 (2012) 365– 371

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

ong-root Eichhornia crassipes as a biodegradable adsorbent for aqueous As(III)nd As(V)

en Lina, Guoxing Wanga, Zhongyuan Nab, Diannan Lua,∗∗, Zheng Liua,∗

Dept. of Chemical Engineering, Tsinghua University, Beijing 100084, ChinaYunnan Research Institute of Ecological Agriculture, Yunnan 610203, China

r t i c l e i n f o

rticle history:eceived 25 October 2011eceived in revised form1 December 2011ccepted 3 January 2012

eywords:

a b s t r a c t

A biodegradable adsorbent prepared from long-root Eichhornia crassipes has been tested for aqueousadsorption of As(III) and As(V). The surface properties and morphology of the root powder have beencharacterized by means of SEM, zeta-sizer, potentiometric titrimeter, FTIR, and XPS. Chemical composi-tion analysis confirmed that the hydroxyl, amino, and carboxyl groups on the surface of the root powderfacilitated the adsorption of As(III) and As(V) by either electron sharing or electron transfer. The optimalpH values for the adsorption of As(III) and As(V) were determined as 7.5 and 3.0, respectively. The effects

rsenic adsorptionichhornia crassipesseudo-second-order dynamic modelangmuir model

of co-ions and counter-ions on the adsorption of arsenite and arsenate have also been examined. It wasshown that the adsorption efficiencies of As(III) and As(V) remained unchanged in the presence of Cl−,SO4

2−, and NO3−. The presence of PO4

3−, however, which has a similar stereochemical structure to thatof the arsenic species, reduced the adsorption capacity substantially. The adsorption of As(III) or As(V)could be well described by the pseudo-second-order dynamic model at different initial concentrationsand the Langmuir model at different temperatures.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

As a metalloid, arsenic [1] is harmful to humans and animals dueo its toxicity, such as teratogenicity, carcinogenicity, and muta-enicity. Arsenite and arsenate [2] are the major forms of As inater. Since arsenite is more toxic and difficult to handle, oxida-

ion of arsenite to arsenate is often the first step in waste treatmentsn practice [3]. The treatments of arsenic-contaminated water thatave been applied to date can be grouped into six categories [4],amely chemical precipitation, ion-exchange adsorption, mem-rane separation, electrolysis, biosorption, and adsorption. Amonghese methods, adsorption has been extensively investigated [5],ince it is more economically efficient. Active metals and theirxides or hydroxides, metal alloys [6], metal-loaded polymers [7],are-earth materials [8], pure substances with numerous func-ional groups [5], minerals [9], and biodegradable waste [10] haveeen studied as adsorbents for arsenic in water. The adsorption

echanisms of metal oxides or hydroxides [11] and some organic

ubstances (e.g., chitosan [12], chitin [13], macrocyclic materials14]) have been delineated. Naturally occurring and biodegrad-

∗ Corresponding author. Tel.: +86 10 6277 9876.∗∗ Corresponding author. Tel.: +86 10 6278 3153.

E-mail addresses: ludiannan@mail.tsinghua.edu.cn (D. Lu),iuzheng@mail.tsinghua.edu.cn (Z. Liu).

385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.01.013

able adsorbents, such as spent grain [15], orange juice residue [16],shelled Moringa oleifera Lamarck seed powder [17], and so on [10],have been extensively studied in recent years because of their highabundances, low cost [18], and convenience of subsequent pro-cessing. While noteworthy efforts have been made onto improvingthe adsorption capacity [19,20] and understanding the underlyingmechanisms [21,22], the research into biodegradable adsorbent isfar from sufficient.

In the present work, a new kind of biodegradable adsorbent,root powder of long-root Eichhornia crassipes, has been tested forthe adsorption of arsenic. Long-root Eichhornia crassipes is distin-guished by its long root, which accounts for more than 80% of itsbiomass. Excellent availability and biodegradability make this rootpowder an ideal adsorbent for arsenic compounds. The objectiveof the present study has been to provide a detailed appraisal ofthe adsorption of arsenic compounds, with particular focus on theadsorption mechanism. The present work started with a character-ization of the structure of the root powder of long-root Eichhorniacrassipes. FTIR and XPS were then applied to probe the interac-tions between the adsorbent and As(III) and As(V), respectively.The effects of pH, temperature, as well as the presence of otherions were examined. The adsorption data were also subjected

to different adsorption models. Based on the results obtained,the adsorption mechanism was established, which is essential forthe subsequent application of long-root Eichhornia crassipes as abiodegradable adsorbent.

3 ering Journal 183 (2012) 365– 371

2

2

gAiditN

2p

2mmwHVcB

2

oflitvct

2

AdPri1

b

3

3

dtmsifcoict

Fig. 1. SEM images of the root powder of long-root Eichhornia crassipes.

Table 1Elemental analysis results of the root powder.

Element C H O N

of carboxyl [24] and amino [25] groups on the surface of the adsor-bent. Taking these results together with those of Fourier-transforminfrared spectroscopy (FTIR) of the adsorbent as shown, it was

Table 2

66 S. Lin et al. / Chemical Engine

. Materials and methods

.1. Materials

The chemicals used in this study were all of analytically purerade. All sample solutions were prepared with deionized water.n As(III) stock solution was prepared by dissolving arsenic triox-

de (As2O3) in 1 M hydrochloric acid. The As(V) source was sodiumihydrogen arsenate (NaH2AsO4·12H2O). All glassware was soaked

n 15% HNO3 before use. In all experiments, the initial pH of solu-ion was adjusted to certain value by using 0.05 M HCl and 0.05 MaOH.

.2. Characterization of the adsorbent prepared from the rootowder of long-root Eichhornia crassipes

The surface morphology was determined by SEM (FEIQuanta00, FEI, USA). The surface area and the pore diameter were deter-ined by means of the BET method and mercury porosimetryethod, respectively. The contents of the elements C, N, and Oere determined by using an organic element analyzer (UVAS,ACH, USA). The basic biopolymers contents were determined byansoest’s method [23] and potentiometric titration. The surface-harge properties were measured using a zeta-sizer (Delsa Nano C,eckman Coulter, USA).

.3. Adsorption

One gram of adsorbent was added to 100 mL aliquots of As(III)r As(V) solutions at a given pH and concentration in a 250 mLask. The flask was then placed in a temperature-controlled BOD

ncubator shaker (Sky-111B, SuKun, China) at 175 rpm at a givenemperature. The supernatants were sampled at given time inter-als. After filtrated through the 0.45 �m filter membrane, theoncentrations of arsenic were determined by using ICP-AES. Theemperature, unless stated otherwise, was set at 30 ◦C.

.4. Assays

The concentrations of arsenic in solutions were analyzed by ICP-ES (IRIS, ThermoElemental, USA). The pH value of solutions wasetermined by using a pH meter (SevenEasy, Mettler, Switzerland).otentiometric titrations were conducted by using a potentiomet-ic titrator (ZDJ-4A, REX, China). XPS spectra are obtained by XPSnstrument (PHI-5300, Perkin-Elmer, USA) with the pass energy of147.5 eV and the Al KR as the excitation source.

All experiments were performed in duplicate and the variationsetween parallel experiments were less than 5%.

. Results and discussion

.1. Physical and chemical properties of root powder

Scanning electron microscope (SEM) images of the root pow-er of long-root Eichhornia crassipes are given in Fig. 1, which showhe particle size to be 300−600 �m. The mean pore size was deter-

ined as 10.36 �m by the mercury intrusion method. BET resultshowed the specific surface area of the powder to be 1.8 m2/g,ndicating an absence of micropores. This highlights the major dif-erence of the root powder adsorbent in comparison with activatedarbon [13] and spent grains [15], which have specific surface areas

f 96.37 and 78.5 m2/g, respectively. The low specific surface areandicates that the macropores on the root powder surface, whichan be seen from the SEM images, may play a major contributiono the adsorption of arsenic compounds.

Content (%) 47.23 4.72 44.40 1.98

Table 1 shows the elemental composition of the root, in whichC, H, O, and N account for over 98% of the molar mass. Table 2 showsthe ingredients of the root powder, with cellulose, hemicellu-lose, and lignin constituting 18.04, 15.53, and 23.63%, respectively.This suggests that hydroxyl groups predominate in the root pow-der adsorbent. Potentiometric titration showed pKa values of5.18(±0.1) and 8.33(±0.1), corresponding to the inflection pointson the titration curve of the root powder, suggesting the presence

The ingredients of the root power.

Composition Cellulose Hemicellulose Lignin

Content (%) 18.04 15.53 23.63

S. Lin et al. / Chemical Engineering Journal 183 (2012) 365– 371 367

cgcb

znf1

3

troa

aoF

F(

Fig. 2. Zeta potential of the root powder suspensions against pH at 30 ◦C.

onfirmed that hydroxyl, amino, and carboxyl were the majorroups on the surface, with hydroxyl being the most abundant. Inontrast, amino groups are present at only a low level, as indicatedy the low content of N, i.e. 1.98%.

The isoelectric point was identified from measurements of theeta potential at different pH values. As shown in Fig. 2, the mag-itude of the zeta potential decreased in response to pH increasing

rom 1 to 13, from which the isoelectric point was determined as.9.

.2. Characterization of the adsorption by FTIR and XPS

Fig. 3 shows FTIR spectra of the root powder adsorbent (a) and ofhe root powder adsorbent with adsorbed As(III) (b) and As(V) (c),espectively. The presence of –OH, –NH2, and –COOH on the surfacef the root powder adsorbent was evidenced by absorbance bandst 3570, 3292, and 1645 cm−1 in Fig. 3(a).

The participation of the hydroxyl and amino groups in the

dsorption of arsenic was indicated by comparison of the FTIR dataf the respective samples. The absorbance at 3570 and 3292 cm−1 inig. 3(a) indicate the presence of –OH and –NH2, while the overlap

ig. 3. FTIR spectra of the root powder (a), the root powder with adsorbed As(III)b), and the root powder with adsorbed As(V) (c).

Fig. 4. The binding energies of O1s (a) and N1s (b) on the surfaces of the root pow-der (1), the root powder after adsorption of As(III) (2), and the root powder afteradsorption of As(V) (3).

at 3365 cm−1 after adsorption of As(III) in Fig. 3(b) suggests thebinding of H3AsO3 with these groups. In the case of As(V), similarabsorption bands appear at 3570 and 3292 cm−1, and the over-lap at 3381 cm−1 after adsorption is more significant, indicatingan intensified absorption upon binding with –OH and –NH2. TheFT-IR spectra of As(III) and As(V) are shown in Refs. [26,27] withcharacteristic adsorption at 797 cm−1 and 829 cm−1, respectively.Due to the low concentration of arsenic, their adsorption signals arecovered up by absorbent. Nevertheless, the overlapping of charac-teristic adsorption signal at 3570 and 3293 cm−1 can demonstratethe adsorption of arsenic on the root powder. On the other hand,the disappearance of the absorbance at 1319 cm−1 correspondingto the –OH of the carboxylic group in Fig. 3(c) suggests that –COOHcontributes to the adsorption of arsenate.

Fig. 4 shows the changes in the binding energies of the 1s elec-trons of O and N on the powder surface after the adsorptions. Theelectron binding energy of O1s increased by 0.382 eV on averageafter the adsorption of As(III), but decreased by 0.499 eV upon bind-ing of As(V), as shown in Fig. 4(a). Similarly as shown in Fig. 4(b),

the N1s binding energy increased by 0.733 eV on average after theadsorption of As(III) and decreased by 0.563 eV upon binding ofAs(V). When –OH and –NH2 combine with H3AsO3, the electrondensities in the 1s shells of O and N decrease, while the binding

3 ering Journal 183 (2012) 365– 371

ewwai

3

dabAoia1bipaaTpv

68 S. Lin et al. / Chemical Engine

nergies increase. On the contrary, the binding energies decreasehen –OH (including that in the carboxyl group) and –NH2 bindith H2AsO4

−, which leads to increases in electron densities. Thedsorption mechanism interpreted on the basis of these XPS datas accordance with that based on FTIR analysis.

.3. The effect of pH on the adsorption

Experiments on the adsorption of As(III) and As(V) were con-ucted as a function of pH in the range 1−13, in which thedsorption time was set at 12 h to ensure saturation of the adsor-ent. The initial arsenic concentrations were 10 mg/L throughout.s shown in Fig. 5, the initial pH values favorable for the adsorptionf arsenite and arsenate were 7–9 and 2–4, respectively. Accord-ng to the pKa values [28] of H3AsO3 (arsenite) (pK1 9.2, pK2 12.3,nd pK3 13.4) and H2AsO4

− (arsenate) (pK1 2.2, pK2 7.1, and pK31.5), as well as the magnitude of the zeta potential of the adsor-ent as a function of pH, as shown in Fig. 2, it was concluded that

n an acidic environment the functional groups –OH and –NH2 arerotonated by H+, which is not favorable for the adsorption. Thedsorption capacity for As(III) would also be reduced in an extreme

lkaline environment since the major existing form is not H3AsO3.hus there is optimal pH value, i.e., pH = 7.5, for the highest removalercentage of As(III) from solution. In the case of H2AsO4

−, the pKalues of arsenate are lower than those of arsenite, resulting in the

Fig. 6. Effects of competing anions on the ads

Fig. 5. Effect of pH on the adsorption of As(III) and As(V).

acidic environment will favor the adsorption of arsenate on the

surface of root powder. The root powder has negative charge on itssurface once pH >1.9 as shown in Fig. 2, the pK1 value of arsenateis 2.2, thus when pH > pK1, both the root powder and the arsenate

orption of As(III) and As(V) by the root.

S. Lin et al. / Chemical Engineering Journal 183 (2012) 365– 371 369

F ncenta g curv

hwfip

3

NAadv2

atctbabr

ig. 7. Kinetics of adsorption of As(III) and As(V) onto the root powder at initial cond As(V) at different contact time, respectively; (c) and (d) for the adsorption fittin

ave negative charge, giving rise to an electrostatic repulsive forceith the negatively charged H2AsO4

−. Thus, the optimal pH valuesor the adsorption As(V) were 3.0, as shown in Fig. 5. In the follow-ng studies, the adsorption of As(III) and As(V) on root powder iserformed at pH 7.5 and pH 3.0, respectively.

.4. The effect of ions on the adsorption

In practice, the presence of anionic species such as Cl−, SO42−,

O3−, and PO4

3− in wastewater may also affect the adsorption ofs(III) and As(V) [4]. Experiments on the effect of competing anionss substrate on arsenic removal were conducted at an adsorbentose of 1 g and with a contact time of 12 h. Concentrations werearied in the range 0−100 mmol/L, with an initial concentration of0 mg/L and initial pH values of 7.5 for As(III) and 3.0 for As(V).

As shown in Fig. 6, the adsorption percentages of arsenite andrsenate in the presence of Cl−, SO4

2−, and NO3− at different ini-

ial concentrations remained almost unchanged compared with theontrols, even at high concentrations of these anions. This suggestshat the adsorption of arsenite and arsenate occurs by chemical

inding or complex formation that is distinct from electrostaticdsorption or ion exchange, which would be impacted stronglyy competing ions. However, the presence of PO4

3− significantlyeduced the adsorption for both As(III) and As(V). The adsorption

rations of 1, 5, 10, and 20 mg/L: (a) and (b) for the adsorption efficiencies of As(III)es of As(III) and As(V) using the pseudo-second order dynamic model, respectively.

percentage of As(III) decreased from 43 to 15% at an initial NaH2PO4concentration of 20 mg/L, while that of As(V) was reduced from 52to 11%. According to its pKa, H3PO4 dissociates into HPO4

2− andH2PO4

− at pH 7.5 and 3, respectively [20]. Competition betweenHPO4

2− or H2PO4− and H3AsO3 or H3AsO4 for the adsorption sites

may account for the reduction in the adsorption. The results in fol-lowing section further confirmed that the adsorption of arseniteand arsenate by the root powder adsorbent occurs by chemicalbinding based on electron sharing or electron transfer between theadsorbent and the adsorbate. Thus, the adsorption is less affected byspecies with a structure distinct from those of arsenite or arsenate.

3.5. Kinetics of the adsorption process

The dynamic experiments were conducted with initial pH val-ues of 7.5 for As(III) and 3.0 for As(V), whose initial concentrationswere 1 mg/g, 5 mg/g, 10 mg/g, and 20 mg/g. The time courses of theadsorption of arsenite and arsenate by the root powder at differ-ent initial concentrations are shown in Fig. 7. It can be seen thatthe first 100 min corresponds to a rapid adsorption stage for both

arsenite and arsenate according to Fig. 7a and b. The adsorption ratethen decreases and the adsorbent is saturated in 3 h. The removalefficiencies of arsenite were 78, 61, 54, and 38% at initial concen-trations of 1, 5, 10, and 20 mg/L, respectively. The corresponding

370 S. Lin et al. / Chemical Engineering Journal 183 (2012) 365– 371

Table 3Pseudo-second-order model parameters for the adsorption of As(III) and As(V).

Initial concentration (mg/L) qe ± � (�g/g) �e ± � (�g/g min) R2

As(III) As(V) As(III) As(V) As(III) As(V)

1 77.74 ± 0.55 52.72 ± 0.35 10.13 ± 0.098 17.53 ± 0.028 0.98817 0.99486

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ac

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wta

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that of the traditional biodegradable adsorbents. For example, thearsenic adsorbance of bio-char [32], sea nodule [33], alginate bead[34], human hair [35], and so on are less than 0.1 mg/g. Thereforethe root powder obtained from Eichhornia crassipes is an effective

5 326.55 ± 0.53 318.84 ± 0.29

10 599.29 ± 1.87 432.42 ± 1.53

20 800.58 ± 1.21 912.32 ± 1.65

emoval efficiencies of arsenate were 89, 70, 63, and 51% at initialoncentrations of 1, 5, 10, and 20 mg/L, respectively. The adsorp-ion occurs mainly during the fast adsorption period and then theesidual sites on the surface of the root powder are competed for byrsenite or arsenate in aqueous form after the time, but a furtherncrease is hindered by the electrostatic repulsive force. It couldlso be concluded from comparison of the above adsorption resultshat the root powder adsorbent had a higher adsorption capacityor arsenate compared to arsenite.

A pseudo-second-order kinetic model was applied to fit thedsorption of arsenite and arsenate, showed in Fig. 7c and d, whichan be expressed as follows [29,30]:

t = t

((1/v0) + (t/qe))(1)

here qe is the adsorption capacity at equilibrium, qt is the quan-ity of arsenic adsorbed on the root, and �0 represents the initialdsorption rate.

As shown by the data in Table 3, this kinetic model agreed veryell with the experimental data at all initial concentrations. Theseudo-second-order kinetic model assumes the predominancef chemisorption in determining the adsorption rate, where thedsorption capacity of the sorbent is determined by the adsorptionites at which electron sharing or electron transfer between thedsorbent and adsorbate is possible. Here, the agreement betweenhe adsorption model and the experimental data indicates that thedsorption of arsenic and arsenate can be assigned to a chargeransfer complex pattern on the surface of the adsorbent, whichas consistent with the previous results of the adsorption of arsenicrovided by Grossl et al. [31], Guo et al. [11], and so on.

Based on the above-described results, the functional groups forhe adsorption of As(III) are hydroxyl and amino, while those forhe adsorption of As(V) are hydroxyl, amino, and carboxyl. Thedsorption occurs by chemical binding through electron sharing orlectron transfer and can be described by a pseudo-second-orderinetic model.

.6. Adsorption at different temperatures

Adsorption isotherms were conducted at different tempera-ures (20 ◦C, 30 ◦C, 40 ◦C, 50 ◦C, and 60 ◦C) with different initialrsenic concentration. The initial pH is 7.5 for As(III) while 3.0or As(V). After equilibrium for 6 h, the concentration of arsenicn supernatant was determined. Fig. 8(a) and (b) gives the absorp-ion isotherms of arsenite and arsenate at different temperatures,espectively. It is shown that both the adsorption rate and theapacity increase with temperature. The Langmuir isothermaldsorption model, which represents the monolayer adsorption ashown in Eq. (2) [26], was applied to fit the experiments.

e = QmaxbCe

1 + bCe(2)

here Qe is the equilibrium adsorbance of the root powder, Qmax

s the saturation adsorption capacity of the root powder at givenemperature, Ce is the equilibrium concentration of arsenic in theolution, and b is the adsorption constant. It is shown in Fig. 8

15.55 ± 0.088 92.77 ± 0.29 0.99215 0.9936521.92 ± 0.19 134.79 ± 0.85 0.98716 0.9935069.53 ± 0.66 204.59 ± 0.77 0.98666 0.99459

that Langmuir model can describe the adsorption behavior well(R2 > 0.96) for both of As(III) and As(V) at different temperatures.The adsorption constants evaluated from the isotherms at certainexperimental conditions are listed in Table 4. It is shown that theincrease of temperature favors the adsorption of both As(III) andAs(V) in the temperature range of 20–60 ◦C. The Qmax for As(III)is in the range of 1.14–1.92 mg/g, while the Qmax for As(V) in therange of 1.86–2.51 mg/g. Thus the removal capacity of arsenate isbetter than that of arsenite in the temperature range of 20–60 ◦C,which is consistent to previous studies [5]. The adsorption capacityof root powder for both arsenite and arsenate is much superior than

Fig. 8. Isotherm curves for the adsorption of As(III) (a) and As(V) (b) onto the rootpower at temperatures 20 ◦C, 30 ◦C, 40 ◦C, 50 ◦C, and 60 ◦C.

S. Lin et al. / Chemical Engineering Journal 183 (2012) 365– 371 371

Table 4Langmuir model parameters for the adsorption of As(III) and As(V).

Temperature (◦C) Arsenite Arsenate

Qmax (mg/g) b R2 Qmax (mg/g) b R2

20 1.1377 0.1203 0.98081 1.8623 0.1105 0.9841230 1.1677 0.1675 0.97553 1.7770 0.1417 0.98363

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40 1.2977 0.1792

50 1.7249 0.1258

60 1.9197 0.12325

orbent for the removal of arsenic, indicating the potential for thendustrial application in the future.

. Conclusions

As indicated by FTIR and XPS analyses, chemical binding withOH, −NH2, and −COOH, through electron sharing or electronransfer, is responsible for the adsorption of As(III) and As(V) on theurface of root powder of long-root Eichhornia crassipes. Examina-ion of the effects of pH and ions has further confirmed the proposed

echanism. The pseudo-second-order dynamic model has beenhown to be appropriate for the adsorption of both As(III) ands(V) at any initial concentration. The adsorption of As(III) or As(V)ould be fitted thermodynamics by the Langmuir model at differ-nt temperatures. The use of this root powder adsorbent as a kindf biodegradable adsorbent for arsenic might facilitate downstreamrocessing, e.g. the production of As2O3 for use as a biopreservative.or practical application, efforts should be extended to the formu-ation and enhancement of the adsorption capacity of this kind ofdsorbent.

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