Loai Bo Asen

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O R I G I N A L P A P E R

Removal of arsenic from aqueous solution by two types of nanoTiO2 crystals

L. Ma S. X. Tu

Received: 19 June 2010 / Accepted: 20 October 2010

Springer-Verlag 2010

Abstract Titanium dioxide (TiO2) is a promising sorbent

for As removal. There are two main and physico-chemi-cally distinct polymorphs of TiO2 in nature, namely anatase

and rutile. Since the difference of arsenic removal by the

two polymorphs of TiO2 is now well known, study on the

arsenic removal efficiency and the underlying mechanism

is of great significance in developing new remediation

strategies for As-polluted waters. Here batch experiments

were carried out in combination with instrumental analysis

of X-ray diffraction (XRD), Fourier-transform infrared

spectroscopy (FT-IR), and X-ray photoelectron spectros-

copy (XPS) to investigate the effects, influential factors and

mechanisms of As removal from aqueous solution by two

types of nano TiO2 crystals. The adsorption behavior of

anatase and rutile for As(V) and As(III) are well described

by Freundlich equations. Anatase had higher As removal

efficiency and adsorption capacity than rutile. Solution pH

had no influence on the As adsorption of anatase TiO2,

whereas the As removal by rutile TiO2 was increased by

718% with pH from 4 to 10. Presence of accompanying

anions such as phosphate, silicate, nitrate and sulfate,

decreased the As(V) and As(III) removal by both crys-

tals, with phosphate being the most effective. However,

removal of As by rutile TiO2 was greatly enhanced in the

presence of divalent cations i.e. Ca2? and Mg2?. Shading

of light decreased the removal of As(V) and As(III) of

anatase by 15.5% and 17.5%, respectively, while a slightincrease of As removal was observed in the case of Rutile

TiO2. FT-IR characterization of As(V) or As(III)-treated

nano TiO2 crystals indicated that both Ti-O and As-O

groups participated in As adsorption. Both FT-IR and XPS

analysis demonstrated that As(III) was photooxidated into

As(V) when adsorbed by anatase under the light condition.

Thus, the effect of crystal types and light condition on As

removal should be taken into consideration when nano

TiO2 is applied for As removal from water.

Keywords Anatase Rutile Sorption Photocatalytic

activity Titanium dioxide

Introduction

Recently, arsenic (As) contamination of groundwater has

been reported in more than ten provinces and municipali-

ties of China. Chronic health effects associated with arsenic

ingestion are endemic in Shanxi (Wang et al. 2004),

Xinjiang (Wang 1984), Inner Mongolia (Smedley et al.

2003), and in Taiwan (Lin et al. 2004) where As concen-

trations in the groundwater used for drinking exceed the

new China national maximum level of 10 lg L-1. Arsenic

is also ingested with agricultural products from farms that

use As-contaminated groundwater for irrigation or raising

farm animals. Similar situations exist in India, Bengal,

Australia, South America, and Japan where millions of

people suffer deleterious health effects from drinking

As-contaminated water (Alaerts and Khouri 2001; Mandal

and Suzuki 2002; Ohno et al. 2007). The search for safe

and effective ways to remove As from drinking water

requires an ongoing international effort.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10311-010-0303-1 ) contains supplementarymaterial, which is available to authorized users.

L. Ma S. X. Tu (&)Department of Environmental Sciences and Engineering,

Huazhong Agricultural University, 430070 Wuhan, China

e-mail: [email protected]

123

Environ Chem Lett

DOI 10.1007/s10311-010-0303-1
http://dx.doi.org/10.1007/s10311-010-0303-1http://dx.doi.org/10.1007/s10311-010-0303-1


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Arsenic can be removed from water by physical,

chemical, or biological separation processes. Physical

methods include enhanced coagulation (Song et al. 2006),

adsorption (Tuutijarvi et al. 2009), reverse-osmosis

(Walker et al. 2008), membrane separation (Ferella et al.

2007), and electro-adsorption (Basha et al. 2008). Chemi-

cal methods include anion exchange (Anirudhan and Un-

nithan 2007), pre-oxidation (Balarama Krishna et al. 2001),and oxidative adsorption (Bang et al. 2005a; Kanel et al.

2005). Biological methods include either bioremediation

(Katsoyiannis and Zouboulis 2004) or phytoremediation

(Ma et al. 2001; Tu et al. 2004).

Many adsorbents have been used to remove As from

water including iron oxide (Pierce and Moore 1982), iron

rock (Zhang et al. 2004), ferrous manganese ore (Chak-

ravarty et al. 2002), zero-valent iron (Zeng 2003), activated

carbon (Lorenzen et al. 1995), activated alumina (Hao et al.

2009), etc. More recently, nano-sized particles of materials

such as titanium dioxide (TiO2), zero-valent iron, cupric

oxide and maghemite have emerged as promising newadsorbents for As removal from waters (Pena et al. 2005;

Kanel et al. 2005; Martinson and Reddy 2009; Tuutijarvi

et al. 2009).

It is well known that TiO2 is a good adsorbent of As, with

anatase and rutile being the two main and physico-chemi-

cally distinct polymorphs of TiO2 in nature. Anatase has

much greater photocatalytic activity than rutile while it is

thermodynamically less stable (Fujishima and Honda 1972).

Studies have shown differences between nano-sized TiO2polymorphs in the photocatalytic degradation of organic

pollutants (Watanabe et al. 2003) and those in the photo-

chemical reaction of inorganic compounds (Kim et al.

2003). We hypothesized that differences would also exist in

the efficacy of nano-particles of TiO2 polymorphs to adsorb

As from water. Also, the factors such as accompanying ions,

pH, temperature, and light intensity would influence As

removal. Thus, the objectives of this study were (1) to

compare the As removal efficacy and As adsorption capacity

by nano anatase and rutile; (2) to understand the effect of

accompanying ions, pH, and temperature as well as light

condition on the As removal; and (3) to illustrate the

mechanisms of As removal by the two nano crystals of TiO2.

Materials and methods

Materials

Nano-sized rutile (VK-T25H) and anatase (VK-TA18H)

were obtained from Aldrich (Lot: 718467), U.S.A. The

pHzpc values for anatase and rutile powders were 6.42 and

2.72, respectively (see supporting information, Table S1

and Fig. S1), indicating that the titanyl group, Ti-OH, was

more acidic on rutile than on anatase (Pena et al. 2005).

The particle sizes of anatase and rutile crystals were 15 and

25 nm, respectively (Table S1; Fig. S2). The BET-deter-

mined surface area for anatase and rutile were 68.0 and

50.0 m2 g-1, respectively (Table S1). The X-ray diffrac-

tion patterns of anatase and rutile in the Joint Committee

for Powder Diffraction Studies database (JCPDF card

00-021-1272 and 00-004-0551, respectively) show differ-ent diagnostic peaks (Fig. S3).

Standard stock solution of As(V) containing 1,000 mg

As L-1 (CB/T601-2002) was purchased from Sinopharm

Chemical Reagent Co., Ltd, and standard stock solution of

As(III) was prepared in 18 MX purified water using ana-

lytical reagent As2O3. Test solutions containing different

concentrations of As (V) and As (III) were made by

diluting these stock solutions.

Batch experiments

The efficacy of As removal by anatase and rutile wasstudied using increasing concentrations of As(V) or As(III)

in aqueous solution. Studies on the efficacy of As removal,

light intensity, and pH were done using a sorbent concen-

tration of 8 g L-1 for both anatase and rutile. Except for

the batch adsorption isotherms and the efficacy of As

removal, a solution of 200 lg L-1 As(V) or As(III) was

used in all cases due to\200 lg L-1 of As concentration

in most of natural As-contaminated groundwater.

Each batch treatment was replicated three times in all

studies. All batch adsorption studies were done at pH 7.0

except for the experiment to determine pH effects on As. In

all cases, the pH of the initial As solution was adjusted

using 0.1 mol L-1 HCl or 0.1 mol L-1 NaOH. The 50-ml

sorbent/solution suspensions used for all studies were

prepared in 250-ml Erlenmeyer flasks, shaken for 1 h at

180 oscillations per minute at 25C, and centrifuged at

18,300 g acceleration for 5 min. The samples of the

supernatant were withdrawn for As analysis.

The concentrations of As(V) and As(III) in the super-

natant were determined using Atomic Fluorescence Spec-

trometer (AFS) using a Beijing Jitian, Model 8220 AF

spectrometer. Mixed reagent of 10% thiourea and 10%

ascorbic acid was used for pre-reduction of arsenate, and

hydrochloric acid (5%) was used for hydride generation.

The standard reference solution (1,000 mg L-1) from

Sinopharm (CB/T601-2002) was analyzed as part of the

quality assurance and quality control protocol. Reagent

blanks and internal standards were used where appropriate

to ensure accuracy and precision in the AFS analysis for

As.

The experiment for comparing the efficacy of As

removal by anatase and rutile and As adsorption isotherms

were obtained by adding 50 mL of 0.1, 0.2, 0.5, 1, 2, 4, 6,

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10, 20, 30, 40 mg L-1 of As(V) solution, and 0.1, 0.2, 0.5,

1, 2, 4, 6, 10, 15, 20, 30 mg L-1 of As(III) solution to

sorbent at 0.400 g of anatase or rutile nano-particles.

Effects of As removal was determined by dark, simulated

natural sunlight (using 10 W fluorescent lamp), and UV

(using 10 W SW-CJ-1D UV lamp).

To determine pH effects on As adsorption, 50 ml of

200 lg As L-1 solutions was adjusted to pH 4, 5, 6, 7, 8, 9,and 10 and then mixed with anatase or rutile nano-parti-

cles. To study the effect of different types and concentra-

tions of accompanying ions on As adsorption, anatase or

rutile nano-particles were added to 50 mL of 40 mmol L-1

of NaCl solution containing 200 lg L-1 of As. The fol-

lowing anions or cations were added based on Pena

et al.(2005) and Bang et al. (2005b): Ca2? (as CaCl2),

Mg2? (as MgCl2), SiO32- (as Na2SiO3), NO3

- (as

NaNO3), SO42-(as Na2SO4), H2PO4

- (as Na3PO4) at 0, 5,

10, 50, 100, 1,000 lmol L-1, respectively.

X-ray diffraction (XRD) analysis was made to charac-

terize the different nano-crystalline of TiO2 by using D8Advance X-ray power diffractometer (Bruker, Germany)

equipped with Cu Ka radiation at a scanning speed of 2/

min from 20 to 60, voltage of 40 kV, and applied

potential current 40 mv. The surface area of the TiO2particles was analyzed by Brunauer-Emmett-Teller (BET)

method, N2 gas adsorption at 77 K using a Quantachrome

Autosorb-1 automatic surface and pore size distribution

analyzer (Malvern Instruments, England). The pHzpc

analysis of the TiO2 was recorded using a 90 plus Particle

Size Analyzer Brookhaven Zeta-Plus system (the U.S.

Brookhaven Instruments Corporation). The FT-IR spectra

of pure TiO2 and As-adsorbed TiO2 were recorded in KBr

media using a AVATAR 330 FT-IR Thermo Nicolet

(German Blue Man Company). The As-adsorbed crystals

were acquired by reaction with As 100 mg L-1 solution

with pH of 7.0 at 25C for 1 h. Ten milligrams of the dried

samples were dispersed in 200 mg of spectroscopic grade

KBr to record the spectra. Forty scans were collected on

each sample at a resolution of 4 cm-1.The oxidation state

of As on the surface of TiO2 nano-particles was determined

by MULTILAB 2000 X-ray photoelectron spectroscopy

(XPS) (Thermo Fisher thermoelectric). The As-adsorbed

crystals were acquired after reaction with As 100 mg L-1

solution with pH of 7.0 at 25C for 1 h. The XPS pattern

showed a double-anode Al of 300 w and passage energy of

25 eV.

All data were the average of three replications. Sigma-

Plot 10.0 and Origin7.0 drawings were used to edit and

draw the figures. The tables were made by MS office 2003.

Freundlich models were computed using least square

regression. Regression analysis was carried out using the

SAS 8.1 software.

Results and discussion

As removal and adsorption capacity of two TiO2crystals

Anatase removed more As than rutile when adding 0.400 g

of the minerals in 50 ml solution spiked with 030 mg L-1

of As(V) or As(III) for 1 h (Fig. 1). The octahedra of rutileare slightly ortho-rhombic, while those of anatase are dis-

torted ortho-rhombic-type (Fujishima et al. 2000). In

addition, each octahedron is connected to 10 surrounding

octahedrons for anatase and 8 for rutile. These crystal

structural differences would influence the activity of

chemical reactions and might also contribute to greater

ability of anatase to remove As.

The As adsorption isotherm curves for anatase and rutile

varied with the TiO2 crystals and As species and were well fit

by the Freundlich equation (Qe = KfCe1/n) (R2 = 0.950.99)

(Thirunavukkarasu et al. 2003) (Fig. 1; Table 1).

Values of n and Kf in Freundlich models reflect thereaction speed and adsorption capacity (Thirunavukkarasu

As concentration in equilibrium solution (mg L-1

)

Asadsorptioncapacity(mgg

-1)

0

5

10

15

20

25

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 16

Anatase

RutileAs(V) As(III)

Fig. 1 Arsenic adsorption isotherms of nano-sized particles of

anatase and rutile. The reaction conditions included adding 0.400 g

of the crystals in 50 ml of As(V) or As(III) solutions at pH 7.0, 25 C

and simulated sunlight (10 W fluorescent lamp) for 1 h. Each value is

the mean of 3 measurements

Table 1 Parameters (n and Kf) and coefficients of determination (R2)

of Freundlich model (Qe = KfCe1/n) for As adsorption isotherms oftwo types of nano TiO2 crystals (n = 11)

Crystals As species n Kf R2

Anatase As(V) 2.972 15.417 0.9606**

As(III) 1.100 2.236 0.9532**

Rutile As(V) 1.139 0.682 0.9928**

As(III) 1.010 0.369 0.9537**

** Significant at P B 0.01, respectively

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As(III) removal by anatase was decreased by 15.2, 7.75,

5.08, and 3.97%, respectively. Generally similar effects of

the four anions on As removal were obtained in the case of

rutile (Fig. 3).

The obvious inhibition of As removal with H2PO4-

addition was mainly due to the similar chemical property of

As(V) and PO43-

, resulting in competitive adsorption(Je0ze0quel and Chu 2006). Gupta and Ghosh (2009) found

no significant effects on As(III) removal by a nano-sized

synthetic bimetal iron(III)-titanium(IV) oxide when phos-

phate was added but reduction from 83.0 to 23.4% in

As(V) removal was observed with a PO43- to As mole ratio

of 1.6. Removal of As (V) by CuO was decreased by more

than 10% when phosphate concentration was greater than

0.2 mmol L-1, but no decrease of As(III) removal by CuO

was found in the same conditions (Martinson and Reddy

2009). However, adding high concentrations of PO43-

dramatically decreased As (III) and As(V) adsorption by

ferric hydroxide (Meng et al. 2002; Bang et al. 2002).

Addition of Ca2? and Mg2? at 1,000 lmol L-1

increased As(V) removal by 9.2, 5.13% and As(III)

removal by 9.35 and 7.21% for rutile, respectively (Fig. 3).Je0ze0quel and Chu (2005) also reported that the addition of

Ca2? and Mg2? promoted As(V) adsorption on nano-sized

TiO2 due to formation of CaH(AsO4) and CaH2(AsO4)?

through a co-precipitation mechanism. As (V) adsorbed on

top of a TiO2-calcium layer, leading to an enhancement of

As (V) adsorption (Chusuei et al. 1999). In contrast,

additional As (V) in the form of HAsO42- was adsorbed

electrostatically with Ca2? acting as a bridge to initially

adsorbed As (V) which is a bidentate surface complex

(Ronson and McQuillan 2002). It is reasonable to assume

that these hypotheses regarding the cooperative effect of

calcium on As (V) adsorption could be extrapolated tomagnesium. Zhang et al. (2008) demonstrated that Ca2?,

Mg2?, and other divalent cations increased As removal by

manganese ore, quartz sand, and red mud.

Effect of light on As adsorption by two TiO2and its mechanism

Under the conditions of simulated sunlight or ultraviolet

light, anatase completely removed As (V) and As (III),

while the dark condition reduced the removal by 15.5 and

17.5%, respectively (Fig. 4). This suggested that UV andsunlight would improve As removal by anatase. For rutile,

however, the dark condition slightly increased As (V) and

As (III) removal by 1.5 and 5.7%, respectively (Fig. 4).

Fujishima and Honda (1972) suggested that anatase lat-

tices, containing more defects and dislocations than rutile,

cause more oxygen vacancies to capture electrons. Higher

photocatalytic activity might be the important reason for

Ultraviolet Natural light Dark

Asremovedfroms

olution

(%)

0

20

40

60

80

100

Ultraviolet Natural light Dark

Rutile

Anatase

As(V) As(III)Fig. 4 Effect of light

conditions on As(V) and As (III)

removal after 1-h equilibration

of 0.400 g nano-sized particlesof TiO2 polymorphs anatase and

rutile with 50 ml of a

200 lg L-1 As(V) or As (III)

solution at pH 7.0 and 25C.

Each value is the mean of 3

measurements

Asremovedfroms

olution(%)

75

80

85

90

95

100

H2PO4

-

SiO3

2-

NO3

-

SO4

2-

Ca2+

Mg2+ As(V), Anatase As(III), Anatase

As(V) RutileAs(III), Rutile

Concentration of accompanying ions (mol L-1

)

0 200 400 600 800 10000 200 400 600 800 100055

60

65

70

75

80

As(V), Rutile

Fig. 3 The effect of species and concentrations of accompanying ionsin solution on As removal by 0.400 g nano-sized particles of TiO2polymorphs anatase and rutile from 50 ml of 200 lg L

-1 As(V) or

As(III) solutions at pH 7.0 and 25C under simulated sunlight (10 W

fluorescent lamp) for 1 h. Each value is the mean of 3 measurements

Environ Chem Lett

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anatase to adsorb greater amount of As than rutile under

light.

FT-IR spectra of pure crystal of anatase showed the

characteristic peaks at the bands of 829.06 and

1,632.33 cm-1 which attributed to Ti-O bond extending

and cOH or dOH bond bending, respectively (curve a in

Fig. S5A and B). Under light (curve b in Fig. S5A) and

dark (the curve c in Fig. S5A) conditions, As(V)-adsorbedanatase crystal showed bands at 1,462.43 and 1,470.60 cm-1

by H2AsO4- ion stretching, the bands at 1,000.68 and

1,000.68 cm-1 by As-O-Ti stretching, the bands at

1,131.44 and 1,127.36 cm-1 by the stretching of As-O

bond in AsO43- group, and the bands at 1,262.63 and

1,282.63 cm-1 by As-O and H2AsO4- group, respectively.

For the spectra by As(III)-adsorbed anatase crystal, the

bands at 1,094.67 and 1,281.21 cm-1 were due to As-O/

AsO33- stretching and As-O/HAsO4

2- stretching, respec-

tively (Fig.S5B).

The characteristic peaks for the FT-IR spectra of pure

crystal of rutile were 778.94 and 1,635.62 cm-1 (curve a inFig. S5C and D). Under light (curve b in Fig. S5C) and

dark (curve c in Fig. S5C) conditions, As(V)-adsorbed

rutile showed the bands at 1,121.11 and 1,125.20 cm-1,

probably because of As-O and AsO43- stretching, the

bands showed at 1,000.44 and 1,000.48 cm-1 by As-O-Ti

stretching. In As(III)-adsorbed rutile crystals, FT-IR spec-

tra analysis indicated that the stretching of As-O and

AsO33- produced bands at 1,017.03 and 1,025.20 cm-1

(Fig. S5D).

The FT-IR analysis clearly indicated the formation of As-

O-AsO43- and As-O-H2AsO4

- groups in As(V)-adsorbed

anatase crystals, and the appearance of As-O-AsO43- in

As(V)-adsorbed rutile crystals under light or dark condi-

tions. In the case of As(III)-adsorbed anatase crystals,

however, the only As-O-AsO33- was found under dark

condition, while simultaneous formation of As-O-AsO33-

and As-O-HAsO42- occurred under light conditions, but As-

O-AsO33- was found in As(III)-adsorbed rutile under light

or dark conditions. The above suggested that light conditions

had no effect on the FT-IR spectra of As-adsorbed rutile

crystals, further indicating a possible photooxidation of

As(III) into As(V) on the anatase crystals (Fig. S5).

Further high-resolution analysis on the XPS spectra

(Fig. S6) at As3d peak of anatase TiO2 nano-particles

indicated that the binding energies (BE) of 45.1 eV for

As3d by As(V)-adsorbed anatase crystals under light and

dark conditions corresponded to the characteristic peak

position of AsO43- or H2AsO4

-. The BEs of 44.4 and

45.1 eV for As3d by As(III)-adsorbed anatase crystals

under light condition were in close agreement with those of

AsO33-, HAsO4

2-, so was the BE of 44.1 eV under dark

condition with that of AsO33- (Wagner et al. 1979).

Therefore, we assumed a transformation of As(III) into

As(V) under light condition, which was consistent with the

results by Roberts et al. (1975) and Bang et al. (2005c) who

reported possible oxidization of As(III) into As(V) in the

solution under light conditions (Fig. S6A).

There was no photooxidation in the case of rutile crys-

tals. The BEs for As3p by As(V)-adsorbed rutile crystals

under light and dark conditions were 144.3 and 144.0 eV,

respectively, which corresponded to the characteristic BEof TiO2-As2O5 system, while the BEs for As(III) under

light and dark conditions were 143.7 and 143.6 eV,

respectively, which are typical BEs of As2O3 based on the

reports by Borgmann et al. (1993) (Fig. S6B).

Conclusions

Both anatase and rutile adsorbent were very effective for

As removal from water, of which anatase had a higher

adsorption capacity across a wide range of As concentra-

tions, water pH, and water temperature. Increasing solutionpH increased As removal by rutile TiO2. The optimum

temperature for As removal by two TiO2 crystals was

around 25C. The As adsorption of accompanying anions,

such as H2PO4- decreased, but cations, like Ca2? and

Mg2?, increased. Sunlight and UV facilitated the removal

of As (V) and As (III) by anatase TiO2, but slightly reduced

the As removal by rutile TiO2. FT-IR analysis of

As-adsorbed TiO2 indicated that the adsorption As(V) and

As(III) on the two crystals of TiO2 through As-O-Ti and

As-O bonds. Both XPS and FT-IR analysis suggested that

As(III) be directly adsorbed by anatase TiO2 in dark con-

dition, but both As(V) and As(III) be adsorbed due to

photooxidation of As(III) under light condition.

Acknowledgments This research was supported by the National

High Tech Developmental Project (863 project) of China

(2007AA06Z332), and the Professional Developmental Project of

Chinese Ministry of Agriculture (200803034). The authors

acknowledge Zhang L and Qian X for their assistance in chemical and

instrumental analysis. We thank Professor Cao Xinde from Shanghai

Jiaotong University and Professor Naraine Persaud from Virginia

Tech. for valued comments and proofreading for this manuscript.

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