me

ity,

ng

Received 8 February 2010

Received in revised form

14 September 2010

from wastewaters prior to discharge into aquatic environ-

including phytoextraction, reverse osmosis, electrodialysis,

ion exchange, membrane filtration and adsorption, have

also been developed to remove metals from industrial waste-

waters. While these methods are useful in removing Cr(VI)

valent iron (ZVI) include higher reactive surface area, faster

agglomeration of iron particles is often unavoidable due to the

extremely high-pressure drops occurring in conventional

systems, which along with its lack of durability and mechan-

ical strength limits theapplicationofnZVI (Cumbal et al., 2003).

* Corresponding author. Tel./fax: 86 591 83465689.

Avai lab le at www.sc iencedi rect .com

.e ls

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 8 8 6e8 9 2E-mail address: zlchen@fjnu.edu.cn (Z.-l. Chen).ments (Ju-Nam and Lead, 2008). Conventional remediation

techniques typically involve reduction of Cr(VI) to Cr(III) which

precipitates as chromium hydroxide or chromium iron

hydroxide at high pH, followed by disposal of the resulting

dewatered sludge (Ngomsik et al., 2005). Other treatments,

and more complete reactions, and better injectability into

aquifers (Li et al., 2006). However, there are still some technical

challenges associated with practical applications, such as the

aggregation of nZVI particles and limitations imposed by high

reactivity and low stability (Liu et al., 2007). Furthermore, the1. Introduction

Chromium (VI) is an industrial contaminant in both soil and

groundwaterand isalsoawell-knownhumancarcinogen (Katz

and Salem, 1994). Due to its toxicity, Cr(VI) must be removed

fromaqueoussolution, theyhavesomelimitationsand it is still

necessary to develop new and effective remediation tech-

niques (Mohan and Pittman, 2006). In recent years, nanoscale

zero-valent iron (nZVI) has been used to remove various

groundwater contaminants. Theadvantages ofnZVIover zero-Accepted 18 September 2010

Available online 1 October 2010

Keywords:

Bentonite

Nanoscale zero-valent iron

Cr(VI)

Wastewater0043-1354/$ e see front matter 2010 Elsevdoi:10.1016/j.watres.2010.09.025phase reduction. The orthogonal method was used to evaluate the factors impacting Cr(VI)

removal and this showed that the initial concentration of Cr(VI), pH, temperature, and

B-nZVI loading were all importance factors. Characterization with scanning electron

microscopy (SEM) validated the hypothesis that the presence of bentonite led to a decrease

in aggregation of ironnanoparticles and a corresponding increase in the specific surface area

(SSA) of the iron particles. B-nZVI with a 50% bentonite mass fraction had a SSA of 39.94m2/

g, while the SSA of nZVI and bentonite was 54.04 and 6.03 m2/g, respectively. X-ray

diffraction (XRD) confirmed the existence of Fe0 before the reaction and the presence of Fe

(II), Fe(III) and Cr(III) after the reaction. Batch experiments revealed that the removal of Cr

(VI) using B-nZVI was consistent with pseudo first-order reaction kinetics. Finally, B-nZVI

was used to remediate electroplating wastewater with removal efficiencies for Cr, Pb and Cu

> 90%. Reuse of B-nZVI after washing with ethylenediaminetetraacetic acid (EDTA) solution

was possible but the capacity of B-nZVI for Cr(VI) removal decreased by approximately 70%.

2010 Elsevier Ltd. All rights reserved.Article history: Bentonite-supported nanoscale zero-valent iron (B-nZVI) was synthesized using liquid-a r t i c l e i n f o a b s t r a c tRemoval of Chromium (VI) frobentonite-supported nanoscal

Li-na Shi a, Xin Zhang b, Zu-liang Chen a,*aSchool of Chemistry and Material Sciences, Fujian Normal Universb School of Medicine, Shanxi University of Chinese Medicine, Xianya

journa l homepage : wwwier Ltd. All rights reservewastewater usingzero-valent iron

Fuzhou 350007, Fujian Province, China

712000, Shanxi Province, China

ev ier . com/ loca te /wat resd.

heavy metal ions using B-nZVI and evaluation of reuse.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 8 8 6e8 9 2 8872. Materials and methods

2.1. Materials and chemicals

Bentonite was provided by Fenghong Co. Ltd, Anji, Zhejiang,

China, primarily as Na-Mt montmorillonite (>90%), the

chemical composition was 62.5% SiO2, 18.5% Al2O3, 1.75%

Fe2O3, 4.25% MgO, 0.95% CaO, and 2.75% Na2O. The cation

exchange capacity (CEC) was 75.4 meq/100g. After drying

overnight at 80 C, the raw bentonite was ground and sievedthrough a 200 mesh screen prior to use in experiments.

All the reagents were analytical grade (Shanghai Nanxiang

Reagent Co., Ltd., China) and distilled water was used in all

preparations. A stock solution containing potassium dichro-

mate (K2Cr2O7) was prepared by dissolving K2Cr2O7 with

deionized water and a series of solutions used during the

experiment were prepared by diluting the stock to the desired

concentrations.

2.2. Synthesis of nZVI and supported nZVI

The nZVI and B-nZVI were prepared using conventional

liquid-phase methods via the reduction of ferric iron by

borohydride without or with bentonite as a support material

(Celebi et al., 2007). Bentonite (2.00 g) was initially placed into

a three-necked open flask, and a ferric solution produced by

dissolving ferric chloride hexa-hydrate (9.66 g) in an ethanol-Recently, technologies have been developed using porous

materials as mechanical supports to enhance the dis-

persibility of nZVI particles (Uzum et al., 2009). Bentonite is

a traditional low-cost efficient adsorbent, which has high

potential for heavy metal removal from wastewaters due to

its abundance, chemical and mechanical stability, high

adsorption capability and unique structural properties

(Bhattacharyya and Gupta, 2008). Removal of metal ions using

bentonite is based on ion exchange and adsorption mecha-

nisms due to the materials relatively high cation exchange

capacity (CEC) and specific surface area (Bhattacharyya and

Gupta, 2008). In this paper, bentonite was used as a porous-

based support material for synthesized nZVI. More recently,

nZVI supported by zeolite (Li et al., 2007) and stabilized by

chitosan (Geng et al., 2009) has been reported to increase the

durability and mechanical strength of nZVI. However, only

a few studies have reported using natural clays as support

materials for nZVI (Uzum et al., 2009).

In this paper the removal of Cr(VI) from an aqueous solu-

tionwas investigated using B-nZVI and the objectiveswere: (1)

synthesis of bentonite-supported nanoscale zero-valent

(B-nZVI) by reduction of Fe3 ions with NaBH4, and charac-terization of the producedmaterial with SEM, XRD and BET-N2technology; (2) evaluation of the factors impacting on Cr(VI)

removal using an orthogonal method; nZVI and bentonite

were used for Cr(VI) removal individually as a control, and the

kinetics of Cr(VI) reduction by B-nZVIwere also evaluated; and

(3) remediation of electroplating wastewater including somewater solution (50 mL, 4:1 v/v) was added and stirred for

10 min. Subsequently, a freshly prepared NaBH4 solution(3.54 g of NaBH4 in 100 mL) was added drop-wise into the

mixture with constant stirring for 20 min after addition. The

whole process described above was performed under a N2atmosphere with vigorous stirring to avoid the oxidization of

B-nZVI. The formed suspension was filtered and the black

nanoscale precipitate was washed three times with pure

ethanol and dried overnight at 75 C under vacuum (Celebiet al., 2007; Uzum et al., 2009). The theoretical mass fraction

of bentonite in synthesized B-nZVI was 50%, and nZVI was

prepared under identical conditions but with bentonite

omitted. The nZVI and supported nZVI sampleswere stored in

brown, sealed bottles under dry conditions and were not

acidified prior to use.

2.3. Characterizations and measurements

Scanning electron microscopy (SEM) was performed using

a Philips-FEI XL30 ESEM-TMP (Philips Electronics Co., Eind-

hoven, The Netherlands). Images of various materials were

obtained at an operating voltage of 30 kV. The SSA of nZVI,

B-nZVI, and bentonite was measured via the BET adsorption

method (Uzum et al., 2009) using Micromeritics ASAP 2020

Accelerated Surface Area and Porosimetry Analyzer (Micro-

meritics Instrument Corp.,USA). The specific surface areas of

nZVI, B-nZVI and bentonite were 54.04, 39.94 and 6.03 m2/g,

respectively. X-ray diffraction (XRD) patterns of B-nZVI before

and after contacting Cr(VI) were performed using a Philips-

XPert Pro MPD (Netherlands) with a high-power Cu- Ka

radioactive source (l 0.154 nm) at 40 kV/40 mA.The concentration of total Cr in solution and the concen-

trations of different heavy metal ions in the wastewater were

determined using a flame atomic absorbance spectrometer

(VARIAN AA 240FS, USA), and the Cr(VI) concentration was

determined using the 1,5-diphenylcarbazide method (Geng

et al., 2009) on a 722N visible spectrophotometer (Shanghai

Precision & Scientific Instrument Co., Ltd, China).

2.4. Batch experiments

The orthogonal method was used to test the effects of

various factors on the reaction, and to optimize the condi-

tions for Cr(VI) removal using B-nZVI. The experimental

design was developed with the aid of the Orthogonal Design

Assistant, where the initial concentration of Cr(VI), B-nZVI

loading, temperature and pH were chosen as variables. Cr(VI)

solutions (25 mL) with a known mass of B-nZVI were sealed

in 50 mL centrifuge tubes and mixed for 4h before being

centrifuged prior to analysis of the aqueous phase for

residual Cr(VI).

In order to investigate the role that bentonite and Fe0

played in the B-nZVI system, nZVI, B-nZVI and bentonite were

all used in batch experiments examining Cr(VI) removal from

aqueous solutions at an initial concentration of 50 mg/L at

35 C and 250 r/min. As themass ratio of Fe0:bentonite was 1:1in the B-nZVI system, the dosages of nZVI and bentonite were

both set at 1.5 g/L, which was half the B-nZVI dosage. The

mixtures were filtered through 0.45 mm mixed cellulose

ester (MCE) membranes prior to determining the residualconcentrations of Cr(VI) after contacting for 3 h. In order to

investigate the effects of the different factors mentioned in

the orthogonal experiments further, the mixtures of Cr(VI)

solution and B-nZVI were mixed in 50 mL centrifugal tubes in

a rotary shaker for determined periods of time, the conditions

of which were initially set at 25 mL of 50 mg/L Cr(VI) solution,

3 g/L of B-nZVI, 35 C and 250 r/min. At selected timed inter-vals, the suspension was filtered through 0.45 mm MCE

membranes, and the concentration of Cr(VI) in the filtrate was

determined.

To explore the feasibility of removing heavy metal ions

from wastewater, B-nZVI was used to remediate electro-

on a rotary shaker at 35 C and 250 r/min for 4 h. Then the

mixtures were centrifuged at 3000 r/min for 10 min and the

(II) fromanaqueoussolution (Uzumetal., 2009).As indicated in

Fig. 1c, the sizes of iron nanoparticles increase prominently

after reacting with Cr(VI). This phenomenon could be attrib-

uted to the co-precipitation of Cr(III) and Fe(III) on the surface

of the nanoparticles (Ponder et al., 2000; Manning et al., 2007),

which occurs due to a redox reaction between Cr(VI) and Fe0

(Ponder et al., 2000; Manning et al., 2007).

The XRD patterns of synthesized materials were compared

with the XRD patterns obtained from standard materials, to

identify the apparent peaks attributable to different iron and

chromium compounds. The XRD patterns of B-nZVI before

reaction (Fig. 2a) with Cr(VI) showed an apparent peak of Fe0

(2q z 44.90), which weakened significantly after the reaction

Table 1eOrthogonal experimental design and the resultsobtained from the full 24 factorial experiment matrix.

T(C)

Cr(VI)ina(mg/L)

pHina B-nZVIina(mg/L)

Cr(VI)res(mg/L)

Removalefficiency (%)

1 25 20 2.0 2 0.11a 99.5

2 25 50 5.0 3 29 1 42.03 25 70 8.0 4 45.0 0.6 35.74 25 100 10 5 73 2 26.65 30 20 5.0 4 0a 100

6 30 50 2.0 5 0.06a 99.9

gonal test.

Variance Analysis

Ranges SSE DOF F-value F critical values

34.1 2518 3 0.63 3.49

54.4 7039 3 1.77 3.49

49.1 5912 3 1.49 3.49

13.7 422 3 0.11 3.49

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 8 8 6e8 9 2888upper aliquot collected to determine pH and the concentra-

tion of each heavy metal ion.

The potential to reuse B-nZVI for removing Cr(VI) from

aqueous solution was also evaluated. B-nZVI (0.1 g) was added

to 50 mg/L Cr(VI) solution (25mL) and themixture was shaken

on a rotary shaker (35 C and 250 r/min). After 3 h, centrifu-gations at 3000 r/min were performed for 10 min to obtain

solid-liquid separation. The supernatant was decanted care-

fully and used to determine the concentration of Cr(VI)

remaining in solution while the used B-nZVI was mixed with

different concentrations of EDTA. The residual B-nZVI-EDTA

solution was washed with distilled water three times and

shaken for another 3 h under identical conditions. The B-nZVI

treatedwith 50mg/L and 10mg/L of EDTAwas used to remove

Cr(VI) for 3 times in succession to test the efficacy of reuse.

In order to ascertain the accuracy, reliability and repro-

ducibility of the data, orthogonal experimentswere conducted

in quadruplicate (n 4) and other batch experiments werecarried out in triplicate (n 3) to minimize any experimentalerrors. The average values of the parallel measurements were

used in all analysis and together with the standard deviations

of these means were listed in Tables 1 and 3.

3. Results and discussion

3.1. Characterization

The SEM images of nZVI and B-nZVI showed the morphology

and nanoparticle distribution of nZVI in the absence or

Table 2 e Range analysis and variance analysis of the ortho

Factors Range Analysis

k1 k2 k3 k4

Temperature (C) 50.9 61.6 71.2 85.0Cr(VI)ina (mg/L) 99.7 71.0 52.7 45.3

pHina 99.7 56.4 61.8 50.7

B-nZVIina (mg/L) 59.1 66.7 72.8 70.1plating wastewater collected from an electroplating factorys

sewage outfall (Fuzhou, China). The wastewater was centri-

fuged at 3000 r/min for 10 min to remove any insoluble

impurities, prior to determining the initial pH and concen-

trations of total Cu, Cr, Pb and Zn. A batch of 50 mL bottles

containing wastewater (10mL) and B-nZVI (0.10 g) weremixedina - initial; SSE - the Square Sum of Errors; DOF - Degree of Freedom.presence of bentonite (Fig. 1). The synthesized nZVI without

bentonite as a support material showed that nZVI particles

were roughly globular and aggregated into a chain-like

conformation (Fig. 1a). The diameters of the nanoscale zero-

valent iron particles were in the range of 20e90 nm when

bentonite was introduced as a support material. Compared

with Fig. 1a, the aggregation of nZVI particles seemed to

decrease and their dispersity increase in Fig. 1b, where the

mass fraction of bentonite was 50%. A similar conclusion has

been drawn using kaolin as a support material to synthesize

kaolin supportednZVI,whichwasused to removeCu(II) andCo

7 30 70 10.0 2 55.3 0.9 21.08 30 100 8.0 3 75 2 25.39 35 20 8.0 5 0.04a 99.8

10 35 50 10.0 4 31.1 0.7 55.611 35 70 2.0 3 0.04a 99.9

12 35 100 5.0 2 71 2 29.413 40 20 10.0 3 0.10a 99.5

14 40 50 8.0 2 6.75a 86.5

15 40 70 5.0 5 32.0 0.9 54.316 40 100 2.0 4 0.24a 99.8

ina - initial; res - residual; a means the standard deviations are too

low to be listed.

and the F-value, the more significant the factor was and

optimal conditions for chromium removal were 40 C, 20mg/Lof initial Cr(VI) concentration, 4 g/L B-nZVI loading and pH 2.0.

3.3. Conditions affecting Cr(VI) removal

After contacting for 3 h under identical conditions, the

removal efficiencies of Cr(VI) were 5.5, 60.0 and 100.0%

respectively when bentonite, nZVI and B-nZVI were added

individually. Bentonite generally has poor adsorption of Cr(VI)

due to its negatively charged surface and the predominant

existence of Cr(VI) as anions (Bhattacharyya and Gupta, 2008).

Table 3 e Remediation of actual electroplatingwastewater by B-nZVI.

Wastewater TotalCr

Pb2 Cu2 Zn2 pH

c0 (mg/L) 73 2 13.1 0.6 33 1 284 4 1.9c (mg/L) after reaction a a 2.4 0.1 115 3 4.5Removal amount

(mg/g B-nZVI)

7.3 1.3 3.0 16.8 e

Removal percentage (%) 100 100 92.7 59.4 e

a means the concentration of the heavy metal was under the limit

of detection.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 8 8 6e8 9 2 889the greater the influence of the factor on Cr(VI) removal

Comparing the ranges and F-values in Table 2, factors influ-

encing Cr(VI) removal were (in order of decreasing influ-

encing): initial Cr(VI) concentration > pH > temperature > B-

nZVI loading. The K values from K1 to K4 represented each

level of each factor, from the lowest to the highest. The higher

the K value, the higher the removal efficiency, and the better

the level of the factor. Take temperature for example, the

highest K was K4, which represented the level 40 C, and thismade 40 C the optimum temperature. The change in K values(Fig. 2b) (Uzum et al., 2009). The XRD patterns of B-nZVI after

reaction (Fig. 2b) indicated the presence of g-Fe2O3 (2q 35.68),Fe3O4 (2q 35.45) and Cr2FeO4 (2q 35.50), which were notdetected in the sample before reaction (Chen et al., 2008). The

appearance of Fe(II), Fe(III) and Cr(III) in B-nZVI after reaction

demonstrated the occurrence of redox reactions between Fe0

and Cr(VI) where nZVI particles were acting as reductants,

which was consistent with previous literature (Ponder et al.,

2000).

3.2. Orthogonal test

The designed complex conditions and the final removal effi-

ciencies of Cr(VI) from aqueous solution by B-nZVI were listed

in Table 1. Results were processed using the Orthogonal

Design Assistant software (El Hajjouji et al., 2008) in Range

Analysis and Variance Analysis (Table 2). The higher the rangeindicated that chromium removal increasedwith temperature

and decreased as initial Cr(VI) concentration and pH rose. The

Fig. 1 e SEM images of laboratory synthesized iron particles wit

reaction with Cr(VI) solution; c. B-nZVI after reaction with Cr(VIThe activity of nanoscale zero-valent iron particles was

enhanced significantly when bentonite was introduced as

a support material, which confirmed the role bentonite played

as a dispersant and stabilizer in B-nZVI (Ponder et al., 2000).

Kinetics studies of Cr(VI) reduction using B-nZVI suggested

that the reactivity of nZVI particles supported on bentonite

were enhanced significantly due to an increase in SSA and

a decrease in aggregation. Reduction kinetics of Cr(VI) by

B-nZVI were described by a pseudo first-order reaction (Ponder

et al., 2000):

v dcdt

kSAasrmc (1)

Where c was the concentration (mg/L) of contaminant, kSAwas the specific reaction rate constant associated with the

SSA of the materials (L/h m2), as was the specific surface area

(m2/g), and rm was the mass concentration (g/L). For kSA, as,

and rm are constant for a specific reaction, the product of the

three can be expressed with one parameter kobs, which is

called the observed rate constant of a pseudo first-order

reaction (h1). Therefore Eq. (1) can be integrated into:

lncc0 kobst (2)

The kobs values under different conditions are equal to the

slope of the line achieved by plotting lnc=co versus timeunder various conditions.

In this study, the plots of lnc=co versus time producedlinear plots with correlation coefficients (R2) higher than 0.9

(Fig. 3). This indicated that the rate of Cr(VI) reduction by

B-nZVI fitted well the pseudo first-order model under various

conditions. Additionally, the reduction of Cr(VI) by B-nZVI

represented a solid-liquid inter-phase reaction, which agreed

with the pseudo first-order kinetics model.h and without a support material. a. NZVI; b. B-nZVI before

) solution. The scale bar in the figure is 500 nm.

3.3.1. Effect of initial Cr(VI) concentrationThe effect of initial Cr(VI) concentration on removal efficiency

was investigated in the range 20e100 mg/L. The plot fitted the

pseudo first-order model well (Fig. 3a), where the observed

rate constant decreased significantly as the initial Cr(VI)

concentration increased, which agreed with the orthogonal

test results. The equilibrium time became longer and the final

removal efficiency of Cr(VI) decreased as the initial Cr(VI)

concentration increased, so that while the percentage of Cr(VI)

removed within 20 min at a Cr(VI) concentration of 20 mg/L

was nearly 100%, it was only 30.4% within 60 min at a Cr(VI)

concentration of 100 mg/L. Generally, the slower rate and

lower efficiency of Cr(VI) removal from aqueous solution were

found at higher concentrations of Cr(VI). Based on the SEM

analysis and previous research, Cr(VI) reduction by nZVI could

be defined as a surface-mediated process (Ponder et al., 2000;

Rivero-Huguet and Marshall, 2009). The more the Cr(VI) ion

approached the surface of nZVI dispersed on the bentonite,

the faster Fe0 was oxidized into Fe(III) and the faster the co-

precipitation of Cr(III) and Fe(III) oxides/hydroxides occurred.

This reduced the reactivity of nZVI and subsequently resulted

Fig. 2 e X-ray diffractogram of B-nZVI. a. before the

reaction with Cr(VI) solution; b. after the reaction with Cr

(VI) solution.

Fig. 3 e Effects of various factors on Cr(VI) removal by fitting to

b. B-nZVI loading; c. pH value; d. temperature.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 8 8 6e8 9 2890the pseudo first-order model. a. initial Cr(VI) concentration;

ambient temperature. Fig. 3d highlights the relationship

between lnc=co and time, where the linearity suggested thatthe reduction of Cr(VI) at different temperatures in the pres-

ence of B-nZVI fitted pseudo first-order dynamics (Ponder

et al., 2000; Manning et al., 2007). The kobs was 0.020, 0.023,

0.026 and 0.030/min at four temperatures (25, 30, 35 and 40 C),showing that an increase in the reaction temperature resulted

in an improved reaction rate. The apparent activation energy

(Ea) of Cr(VI) reduction by B-nZVI was 24.9 kJ/mol, demon-

strating that it is a chemically controlled adsorption process

having an Ea value higher than 21 kJ/mol (Geng et al., 2009).

3.4. B-nZVI used to remove Cr(VI) from electroplatingwastewater and B-nZVI reuse

The data obtained from batch experiments where B-nZVI was

used to remove Cr(VI) and other metals from electroplating

wastewater are presented in Table 3, which indicated that B-

nZVI had the capacity to remove various heavy metals and

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 8 8 6e8 9 2 891in a decrease in kobs (Geng et al., 2009; Yuan et al., 2009). On the

other hand, the highest removal amount was obtained at an

initial Cr(VI) concentration of 70 mg/L, which could be

ascribed to the limited capacity of B-nZVI for Cr(VI) removal

determined by SSA.

3.3.2. Effect of B-nZVI loadingThe initial loadings of B-nZVI in Cr(VI) solutionwere 1, 2, 3 and

4 g/L. The Cr(VI) removal percentage increased as the B-nZVI

concentration increased (Fig. 3b). The removal percentage of

Cr(VI) was 54.6% using B-nZVI at 1 g/L for 120 min, but was

nearly 100% when the B-nZVI loading was over 3 g/L. Mean-

while, kobs increased as the B-nZVI loading increased. These

phenomena can be attributed to the increase in the available

active sites resulting from the elevation in B-nZVI loading,

where the reduction of Cr(VI) occurred (Geng et al., 2009; Yuan

et al., 2009). However, the concentration of Cr(VI) decreased

dramatically in the initial 10 min, then slightly declined in the

later reduction. A few researchers (Ponder et al., 2000;

Manning et al., 2007) reported a sorption phase during the

reaction which could also be supported by our SEM images,

suggesting that the overall mechanismwasmore complicated

than a simple chemical reaction.

3.3.3. Effect of the pH valueThe two dominant forms of Cr(VI) in aqueous solution were

HCrO4, between pH 1.0 to 6.0 and CrO4

2 above pH 6.0 (Mohanand Pittman, 2006). The dependence of the reaction rate

constant on pH was investigated by adjusting the solution pH

to 2.0, 4.0, 6.0 and 8.0 with either 0.1 M HCl or NaOH (Yuan

et al., 2009). Except for pH 2.0, the reduction of Cr(VI) can be

described using the pseudo first-order model well (Fig. 3c). A

remarkable increase in the removal rate occurred at pH 2.0,

where equilibrium was achieved within 1 min and the

residual Cr(VI) was below the detection limit, which made

kinetic fitting infeasible. The Cr(VI) removal percentage

decreased significantly with increases in the initial pH, so that

only 27.2% Cr(VI) was reduced at pH 8.0 in 20 min while nearly

100% Cr(VI) was removed in 1 min at pH 2.0. In addition, kobswas respectively 0.0275, 0.0163, and 0.0083/min, when the

initial pH value was 4.0, 6.0 and 8.0, indicating that the

reduction rate increased as pH decreased, which had also

been reported in other studies (Geng et al., 2009; Yuan et al.,

2009). These results demonstrated that a lower pH favored

Cr(VI) reduction, since at lower pH corrosion of nZVI was

accelerated and the precipitation of Cr(III) and Fe(III) hydrox-

ides on the surface of iron was consequently not as favorable,

which led to an increase in the reaction rate (Lee et al., 2003).

Furthermore, the increase in H concentration left the surfaceof bentonite less negatively charged, which reduced the

electrostatic repulsion between bentonite and Cr(VI) anions.

This consequently promoted the electron transfer between

zero-valent iron and Cr(VI) (Yuan et al., 2009).

3.3.4. Effect of temperatureTo assess the effect of different temperatures, batch experi-

ments were conducted at 25, 30, 35 and 40 C. The resultsshowed that 82.4% Cr(VI) was removed at 40 C while only

73.4% Cr(VI) was reduced at 25 C in 60 min. Thus zero-valentiron could have a positive effect on Cr(VI) reduction even atwas a potential promising candidate for applications to in situ

environmental remediation. After reacting 10 mL of the

wastewater with 0.1 g of B-nZVI for 4 h, the residual concen-

tration of each metal ion showed that 100% total Cr, 100% Pb

(II), 92.7% Cu(II), and 59.4% Zn(II) were removed, following

treatment with B-nZVI. Total Cr, Pb(II) and Cu(II) received

higher removal percentages due to their higher standard

reduction potentials compared to Fe(II) (fqFeII=Fe0 0:44V). In

contrast, a lower removal percentage of Zn(II) was obtained

because the standard reduction potential of Zn(II)

(fqZnII=Zn0 0:762V) was more negative than Fe(II) (Ladd,

2004).

The amount of Cr(VI) removed when using B-nZVI treated

with different concentrations of EDTA after being used four

times was calculated and it was shown that B-nZVIs ability to

remove Cr(VI) was dramatically reduced after being used only

once with an initial Cr(VI) concentration of 50 mg/L (Fig. 4).

The rapid deterioration of B-nZVI was ascribed to the inability

of the redox reaction between Cr(VI) and Fe0 to proceed

Fig. 4 e The variation of Cr(VI) removal amount by B-nZVI

after reusing four different times. The solutions of EDTAused for treatment of B-nZVI were 50 mg/L and 10 mg/L

respectively as marked in the figure.

further since this was a chemical controlled and irreversible

process. As confirmed by XRD analysis (Fig. 2), reaction

products were deposited on the surface of B-nZVI in the form

of oxide-hydroxide co-precipitation of Fe(II), Fe(III) and Cr(III),

which consequently decreased the activity of Fe0 (Chen et al.,

reduction of aggregation and increased SSA. Batch experi-

remediation material to remove Cr(VI) and other metals from

metals on natural and modified kaolinite andmontmorillonite: a review. Adv. Colloid Interface Sci. 140,

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study of the adsorption behavior of aqueous Ba2 ions onwastewater. However, further research must be carried out to

slow and control the degree of nZVI oxidation in the atmo-

sphere and as a consequence a more effective regeneration

method may emerge from such studies.

Acknowledgements

This work is supported by the Fujian Minjiang Fellowship

from Fujian Normal University (gs1). The authors also

gratefully acknowledge the significant contributions of Dr

Gary Owens in editing and improving the many revisions of

this manuscript, his suggestions and corrections have

undoubtedly significantly improved the quality of the final

manuscript.

r e f e r e n c e s

Bhattacharyya, K.G., Gupta, S.S., 2008. Adsorption of a few heavyments indicated that the removal rate increased as the

temperature and B-nZVI loading (4 g/L) increased, and fell asthe initial Cr(VI) concentration and pH increased, which

agreed with the result obtained from the orthogonal test.

Under the various operational conditions considered, reduc-

tion of Cr(VI) using B-nZVI was in accordance with a pseudo

first-order model. B-nZVI was effective in removing Cr(VI) and

other heavy metals, including Pb(II), Cu(II) and Zn(II) from

electroplatingwastewater. Since bentonite is a stable and low-

cost clay mineral, B-nZVI could be an efficient and promising2008). This phenomenon confirmed that the active ingredient

of B-nZVI was Fe0 which acted as a reductant, while bentonite

only played a role as a dispersant and stabilizer.

4. Conclusions

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Removal of Chromium (VI) from wastewater using bentonite-supported nanoscale zero-valent ironIntroductionMaterials and methodsMaterials and chemicalsSynthesis of nZVI and supported nZVICharacterizations and measurementsBatch experiments

Results and discussionCharacterizationOrthogonal testConditions affecting Cr(VI) removalEffect of initial Cr(VI) concentrationEffect of B-nZVI loadingEffect of the pH valueEffect of temperature

B-nZVI used to remove Cr(VI) from electroplating wastewater and B-nZVI reuse

ConclusionsAcknowledgementsReferences