A zirconium based nanoparticle for significantly enhanced adsorption of arsenate: Synthesis, characterization and performance

Journal of Colloid and Interface Science 354 (2011) 785–792

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

A zirconium based nanoparticle for significantly enhanced adsorption of arsenate: Synthesis, characterization and performance Yue Ma, Yu-Ming Zheng, J. Paul Chen ⇑ Division of Environmental Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

a r t i c l e

i n f o

Article history: Received 16 July 2010 Accepted 19 October 2010 Available online 23 October 2010 Keywords: Adsorption Arsenate Nanoparticle Zirconium

a b s t r a c t In this study, a zirconium nanoparticle sorbent for significantly enhanced adsorption of arsenate (As(V)) was successfully synthesized. The characterization of the zirconium nanoparticle sorbent and its adsorption behavior for arsenate were investigated. The HRTEM micrographs showed that the sorbent was nanoscale with particle sizes ranging from 60 to 90 nm. The thermal gravimetric and elemental analyses indicated that the sorbent had a molecular formula of Zr2(OH)6SO43H2O. The X-ray diffraction study revealed that the sorbent was amorphous. The potentiometric titration study demonstrated the surface charge density of the sorbent decreased with an increase in solution pH, and the pH of zero point charge of the sorbent was around 2.85. The kinetics study showed that most of the uptake took place in the first 6 h, and the adsorption equilibrium was obtained within 12 h. The optimal pH for As(V) adsorption was between 2.5 and 3.5. The Langmuir equation well described the adsorption isotherm; the maximum adsorption capacity of 256.4 mg As/g was found at the optimal pH, better than most of sorbents available in the market. The presence of fluoride or nitrate did not obviously affect the adsorption of As(V) onto the sorbent; however, the existence of humic acid, phosphate or silicate in aqueous solution significantly reduced the uptake of As(V). The humic acid did not cause the reduction of the As(V). The FTIR and XPS spectroscopic analyses revealed that surface hydroxyl and sulfur-containing groups played important roles in the adsorption. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Arsenic is a notorious poisonous metalloid. It has been well documented that arsenic can be accumulated in the human body and cause irreversible damage to a number of biological systems even at its concentration of ppb level. Based on the research from International Agency for Research on Cancer, arsenic is classified as human carcinogens [1]. A series of researches have shown that arsenic could induce such diseases as skin, lung and liver cancers. Elevated arsenic level in drinking water has recently been regarded as the major cause of arsenic toxicity in the world [2]. United States Environmental Protection Agency and World Health Organization have stringently regulated the maximum allowable contamination level of arsenic in drinking water to be no more than 10 lg/L [3]. In particular, under directive 2000/60/CE, the European Community has already defined much more restrictive limits of 1.6 and 1.4 lg/L for superficial water to be achieved in 2008 and 2015, respectively [4]. Several technologies have been used for the purification of water contaminated with arsenic, including coagulation–precipita-

⇑ Corresponding author. Fax: +65 6774 4202. E-mail addresses: [email protected], [email protected] (J.P. Chen). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.10.041

tion, ion exchange, membrane filtration and adsorption. Among them, adsorption is still the most extensively used method because of its ease in operation as well as the availability of wide range of adsorbents [4–13]. A number of sorbents such as activated carbon [4], iron-containing activated carbon [6], titania [7], iron oxides [8], zero valent iron [9,10], zirconium [3,11] and lanthanum compounds [13] can remove the arsenic from aqueous solutions. In recent years, there is a growing interest in the application of nanoparticles as sorbents for pollutant removal [8,14–16]. Compared with traditional millimeter- or micron-sized materials (mm to lm), nanoscale particles have quite different physicochemical properties. The surface properties, electronic structure, coordination and other properties could be changed when the material becomes nano-scaled. Most of atoms on the surface of nanoparticles are unsaturated and can easily bind with other atoms. Due to the huge specific area and the absence of internal diffusion resistance, nanoscale sorbents may have superior performance for removing contaminants (e.g., high adsorption capacity and fast adsorption kinetics). The nanoparticles may be prepared through various techniques such as co-precipitation and hydrolysis [3,5,8,17]. The aims of this study were devoted to develop and characterize a nanoscale zirconium-based sorbent for arsenic removal from aqueous solution. High resolution transmission electron microscopy

786

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

(HRTEM), thermal gravimetric analysis (TGA) and potentiometric titration were used to investigate the properties of the sorbent, such as the surface morphology, microstructure, and surface charge density of the sorbent. A series of batch adsorption experiments, including isotherm and kinetics studies, pH and coexistence anions effects, was performed to better understand the adsorption behavior of the sorbent. Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were employed to investigate the interaction of the arsenic and sorbent in the adsorption. 2. Materials and methods 2.1. Materials All the chemicals used are of analytical grade. Di-sodium hydrogen arsenate heptahydrate (Na2HAsO47H2O, 98.5%) was supplied by Fluka (Switzerland). Zirconium (IV) oxychloride (ZrOCl28H2O, 99.5%) and the other chemicals used in this study were purchased from Sigma–Aldrich (Singapore). In the fabrication of the nanoscale zirconium-based sorbent, a mixture of ZrOCl28H2O and an acid was first prepared. Numerous white fine particles were then formed and suspended in the solution. The new particles were harvested from the suspension by centrifugation. After that, the collected particles were washed with deionized (DI) water. Finally, the purified particles were dried with a freeze dryer and used as a sorbent for subsequent experiments. 2.2. Characterization of sorbent The morphology of the sorbent was observed by using a high resolution transmission electron microscopy (JEOL JEM3010, Japan). The crystal structure of the sorbent was analyzed using an X-ray diffractometer (Philips, X’Pert PRO) with Cu Ka (k = 0.1542 nm) radiation. The thermal gravimetric study with 16.6 mg sample in an alumina pan was carried out by a Thermogravimetric–Differential Thermal Analyzer (TG–DTA) (TA Instruments, 2960 SDT V3.0F) under a nitrogen atmosphere. The temperature was ramped from 20 to 980 °C with a heating rate of 10 °C/min. The elemental analysis of the sorbent (carbon, hydrogen, nitrogen and sulfur) was determined using a CHNS elemental analyzer (Elementar Vario Micro Cube, Elementar, Germany). The dried and ground sorbent was combusted at 1000 °C. The combustion products (namely gases of CO2, H2O, NO2, and SO2) were detected by a thermal conductivity detector for quantitative analysis. The surface charge density (r) of the sorbent was determined by a potentiometric titration method. The following equation was used.

ro ¼

ðcA  cB þ ½OH   ½Hþ ÞF m

ð1Þ

where cA and cB (M) were the concentrations of acid and base needed to reach a point on the titration curve, [H+] and [OH] (M) were the concentrations of H+ and OH, F was the Faraday constant (96,490 C/mol), m (g/L) was the concentration of the sorbent. The acid–base titration was conducted. 0.1 g of sorbent was added into 100 mL CO2-free ultrapure water. A recorded volume of 0.1 M nitric acid or 0.1 M sodium hydroxide was added into the mixture. The pH of each addition of acid or base (in a time interval of 15 min) was recorded. The pH value of the suspension was measured using an ORION 525A pH meter. During the titration, the suspension was gently shaken on a rotary shaker (Daiki, DK-OS010) and high purity nitrogen gas was purged into the suspension throughout the whole process to eliminate the effect of

CO2 from the atmosphere. The experiments were carried out at 20 °C. The acid and base titration experiments were conducted separately. 2.3. Adsorption experiments A stock arsenate solution with concentration of 100 mg/L in As(V) was prepared by dissolving Na2HAsO47H2O in the DI water. The stock solution was diluted with the DI water to prepare As(V) solution with desired concentration for the subsequent batch adsorption kinetics and equilibrium experiments. In the kinetic experiments, 0.2 g sorbent was added into 2000 mL arsenate solution with an initial concentration of 20 mg/ L, of which the initial pH was adjusted as 6.8. The mixed solution was stirred at a constant rate. The samples were taken at different time intervals, and analyzed for arsenic concentration by an inductively coupled plasma emission spectrometer (ICP-OES; Perkin Elmer Optima 3000 DV). The equilibrium time for the adsorption equilibrium experiments was determined by the kinetics experiment. In the pH effect experiments, 100 mL of 20 mg/L arsenate solutions with different pH were prepared in glass bottles. HNO3 or NaOH was used to adjust the pH of arsenate solution. Ten milligram of the sorbent was added into the arsenate solution, and then the mixtures were shaken at room temperature for 24 h. At the end of the experiment, the samples were taken and filtered with a 0.45 lm filter. The filtrate was then analyzed for residual As(V) concentration and Zr concentration by the ICP-OES. In the adsorption isotherm experiment, 100 mL arsenate solutions with different concentration were prepared in glass bottles. The pH of arsenate solution was adjusted to be 3.2 using HNO3. The sorbent with a mass of 10 mg was added into each of the arsenate solution; the mixtures were shaken at room temperature for 24 h. Other procedures were the same as the pH effect experiments. In the humic acid and coexisting anions effect experiments, 100 mL arsenate solutions with different concentration of humic 2 3 acid or coexisting anions (F, NO 3 , SiO3 , and PO4 ) were prepared in glass bottles. The solution pH was adjusted to be 6.9 ± 0.1. 10 mg of the sorbent was added into each of the arsenate solution; the mixtures were shaken at room temperature for 24 h. Other procedures were the same as the pH effect experiments. To determine whether the arsenate was reduced to arsenite during the adsorption in the existence of HA, sample was collected after adsorption and analyzed by voltammetry (VA; 797 VA Computrace, Metrohm, Switzerland) using the method of standard addition with acetate buffer (pH = 4.5) as the background electrolyte. 2.4. Spectroscopic analysis The Fourier transform infrared spectra were measured by a FTS135 spectrometer (Bio-Rad). The dry particle sorbent was mixed with the spectrometry grade KBr with a mass ratio of [sorbent]/ [KBr] = 1:100, and then ground in an agate mortar. The resulting powder mixture was pressed to form a pellet for the FTIR spectra capture. The background obtained from scan of pure KBr was automatically subtracted from the sample spectra. The spectra were collected within the wave number range of 400 and 4000 cm1. The surfaces of sorbent before and after As(V) adsorption were analyzed using X-ray photoelectron spectroscopy (Kratos XPS system – Axis His –165 Ultra, Shimadzu, Japan), with a monochromatized Al Ka X-ray source (1486.6 eV). For wide scan XPS spectra, an energy range from 0 to 1100 eV was used with pass energy of 80 eV and step size of 1 eV. The high resolution scans were conducted according to the peak being examined with pass energy of 40 eV and step size of 0.05 eV. To compensate for charging effect, C 1s

787

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

signal of an adventitious carbon was used as reference at a binding energy (BE) of 284.8 eV. The XPS results were collected in binding energy form and fit using a non-linear least-square curve fitting program (XPSPEAK41 Software). For the element of oxygen, the spectra were deconvolved with the subtraction of a linear background and a Gaussian (80%)–Lorentzian (20%) mixed function. The peak’s full-width-at-half-maximum (FWHM) was fixed during the fitting.

Surface charge density (C/g)

800

3. Results and discussion 3.1. Characterization of sorbent

400 200 0 -200 -400 -600 2

4

6

8

10

pH Fig. 2. Surface charge density as a function of pH (m = 1 g/L).

100

0.5 TGA DTA

Weight (%)

The HRTEM was used to observe the surface morphology of the sorbent, and the obtained photos were shown in Fig. 1. The shape of the sorbent is irregular, which indicates it is not crystalline. The size of the sorbent ranges from 60 to 90 nm when it is well dispersed in solution, which indicates the sorbent is nanoscale. However, the sorbent particles may agglomerate together in the water solution when they are used for arsenate removal. The used sorbent particles can be removal using a filter with pore size of 0.45 lm. Fig. 2 shows the surface charge density of the sorbent as a function of pH. It was found that the surface charge density decreases when the solution pH is increased, and the pH of zero point charge (pHzpc) is around 2.85. The value of pHzpc implies that the surface of the sorbent is positively charged as solution pH is less than 2.85, while the surface of the sorbent of the sorbent is negatively charged as solution pH is higher than 2.85. The result of thermal gravimetric analysis is given in Fig. 3. The whole weight loss process can be decomposed into three steps: (1) in temperature range of 20–100 °C, the weight loss may be attributable to the loss of physically adsorbed water; (2) the weight loss within the temperature range of 100–300 °C is ascribed mainly to the loss of ‘‘lattice’’ water; (3) the biggest weight loss at higher temperature might be due to the loss of OH and SO2 4 groups. In the acidic ZrOCl2 solution, Zr is present mainly as a tetramer [18]. When the temperature is above 300 °C, OH would be converted to H2O, which contributes to the third weight loss step. Agarwal et al. [19] reported that SO2 could be removed at the 4 temperature above 500 °C. The sorbent was further investigated by elemental analysis to obtain its elemental composition. The result reveals that there is no carbon or nitrogen content in it. Hydrogen and sulfur elements are found with a composition of 2.34% and 8.82% (w/w), respectively. Base on the thermal gravimetric analysis and the elemental

600

90

0.4

80

0.3

70

0.2

60

0.1

50 0

200

400

600

800

0.0 1000

o

C

Fig. 3. TGA–DTA curves of the sorbent.

analysis, the molecular formula of the sorbent can be deduced as Zr2(OH)6SO43H2O.

3.2. Adsorption studies 3.2.1. Kinetics study The adsorption kinetics of As(V) on the sorbent is of great importance for designing appropriate adsorption technologies. It

Fig. 1. HRTEM micrograph of the zirconium nanoparticle: (a) 4000 and (b) 30,000.

788

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

can be found from Fig. 4 that most of uptake of As(V) rapidly takes place in the first 6 h, followed by a relative slow process. The uptake of As(V) reaches equilibrium within 12 h. The result is much better than many available adsorbents for anionic pollutants removal [4,8]. To better understand the adsorption kinetics of As(V) on the sorbent, an intraparticle diffusion model was employed to simulate the adsorption process. This model works with an assumption of ‘‘two-step mass transport mechanism’’: arsenic anion first transfers through the external liquid film from the bulk solution and subsequently diffuses inside the sorbent before finally being adsorbed by functional groups. The mathematical equations and corresponding initial and boundary conditions are expressed as follows [20,21]:

DS

  @ @q @q r2 ¼ r2 ; @r @r @t

q ¼ 0 0 6 r 6 ap ; @q ¼ 0; @r DS qp

0 6 r 6 ap ;

tP0

ð2Þ

t<0

ð3Þ

r¼0

ð4Þ

@q ¼ kf ðC  C  Þ @r

ð5Þ

where C and q are the concentration of the As(V) in bulk and in solid phases, respectively; C is the aqueous phase concentration at the particle surface, in equilibrium with the corresponding concentration in the solid phase q; DS is the surface diffusivity within the particle; qp is the particle density; r is radius distance measured from the center of particle; ap is the particle radius; kf is the external mass transfer coefficient, and t is the time. As shown in Fig. 4, the model well describes the experimental data. The good simulation result indicates the sorption process is mainly controlled by the surface diffusion, while the external mass transfer is less critical. The kf and DS values are determined as 2.1  104 m/s and 3  1010 m2/s, respectively.

3.2.3. Adsorption isotherm study Study on As(V) adsorption isotherm was conducted at pH 3.2 ± 0.1, the optimal pH for As(V) adsorption on the sorbent. Both Langmuir and Freundlich models were used to describe the relationship between the amount of As(V) adsorbed and its equilibrium concentration in solution. Langmuir model is applicable to homogeneous sorption, which the adsorption of each adsorbate molecule onto the surface has equal adsorption activation energy. Langmuir model can be expressed by the following equation:

qe ¼

qmax bC e 1 þ bC e

ð6Þ

where qmax and b are maximum adsorption capacity and adsorption reaction constants, respectively. Its linearized equation is shown as below:

Ce 1 1 ¼ Ce þ bqmax qe qmax

qe ¼ K F C 1=n e

ð8Þ

180

240

150

200

120

160

90

ð7Þ

The most important multisided adsorption isotherm for heterogeneous surfaces is the Freundlich adsorption isotherm, characterized by the heterogeneity factor 1/n. The Freundlich model is described by:

qe (mg/g)

q(mg/g)

3.2.2. pH effect study To evaluate the effect of pH on the As(V) adsorption, batch adsorption experiments were carried out at different initial pH values ranging from 1 to 9. The experimental result shown in Fig. 5 demonstrates the adsorption of As(V) on the sorbent is strongly pH dependent. The maximum As(V) uptake is observed in the pH range between 2.5 and 3.5. At extremely acid situation, the adsorption of As(V) increases with an increase in pH, and reaches a max-

imum uptake at pH around 3.0; while the uptake of As(V) decreases with a further increasing pH. As shown in the surface charge density study, at low pH values (<3), the sorbent surfaces are protonated and positively charged, which enhances the adsorption of the negatively charged species. On the other hand, according to the As(V) species distribution at different pH value, at extremely low pH range, more H3AsO4  converts into H2 AsO 4 as the pH is increased, and H2 AsO4 species becomes predominant. Hence, the uptake of As(V) increases with an increase in pH at extremely acid condition. However, the surfaces of sorbent become negatively charged, leading to a reduction in adsorption of As(V). Thus, the pH-dependent behavior of As(V) adsorption onto the sorbent indicates that electrostatic attraction might play an important role in the adsorption at low pH. In addition, the Zr concentration was measured. It was shown that the concentration was below the detection limit of ICP-OES. This further indicates that the aggregation of the nanoparticles causes stability of the sorbent and would not cause harmful effects to the treated water.

120

60

80

30

40 0

0 0

2

4

6

8

10

12

Time (hr) Fig. 4. Adsorption kinetic of As(V) onto the sorbent (sorbent dose = 0.1 g/L, initial concentration = 20 mg/L, pH = 6.8, T = 20 °C).

1

2

3

4

5

6

7

8

9

Initial pH Fig. 5. Effect of initial pH on As(V) adsorption (sorbent dose = 0.1 g/L, initial concentration = 20 mg/L, T = 20 ± 1 °C).

789

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

and its linearized expression is shown as below:

ln qe ¼ ln K F þ

1 ln C e n

ð9Þ

As shown in Fig. 6, the experimental data of the adsorption of As(V) onto the sorbent were fit with both Linearized Langmuir and Freundlich isotherms. The results are summarized in Table 1.

(a)

300

v2 ¼

240

qe (mg/g)

According to the correlation coefficient (r2), Langmuir model is found to be better fit the isotherm data. However, the transformation of non-linear isotherm equations to linear forms implicitly alters their error structure and may also violate the error variance and normality assumptions of standard least squares [22,23]. Therefore, it is necessary to analyze the data set using the nonlinear chi-square test (v2) in order to validate an appropriate isotherm for the sorption system. v2 is determined using the following equation:

ð10Þ

qe;m

where qe is the experimental equilibrium uptake and qe,m is the equilibrium uptake calculated using the model. The results of the non-linear chi-square test analysis are also presented in Table 1. The obtained results from v2 test are in agreement with that from r2. Therefore, the Langmuir model is used to fit the adsorption of As(V) on the sorbent. Based on the Langmuir model, the maximum adsorption capacity of As(V) onto the sorbent is calculated as high

180 2

120

X ðqe  qe;m Þ2

Langmuir equation (r =0.977) 2 Freundlich equation (r =0.899)

60

0 0

16

32

48

64

80

(a) 180

Ce (mg/L)

150

(b) 0.35 0.30

qe (mg/g)

120

Ce/qe (g/L)

0.25 0.20

90 60

0.15 0.10

30 y=0.0061+0.0039x 2 r =0.999

0.05

0

0.00

0 0

10

20

30

40

50

60

70

80

Ce (mg/L)

(c)

2

4

6

8

10

HA concentration (mg/L)

(b)

3.0

400

Sample As (V) As (III) As (V) and As(III) mixture

Current (nA)

log qe

2.5

2.0

300

As(III) peak

As (V) peak

200

1.5 100

y=1.905+0.338x 2 r =0.879

1.0 -1.0

-0.5

0.0

0.5

1.0

1.5

0

2.0

-0.1

log Ce

0.0

0.1

0.2

0.3

Voltage (V)

Fig. 6. Adsorption isotherm of As(V) onto the sorbent: (a) Experimental data and fitting by Langmuir and Freundlich isotherms; (b) linearized Langmuir isotherm; and (c) linearized Freundlich isotherm. The curves in Fig. 6a are based on the parameters obtained by the non-linear approach.

Fig. 7. Effect of humic acid on adsorption of As(V): (a) the adsorption capacity; (b) arsenic valency. Experimental conditions: sorbent dose = 0.1 g/L, initial concentration = 20 mg/L, pH = 6.9 ± 0.1, T = 20 °C.

Table 1 Langmuir and Freundlich isotherm constants for adsorption of As(V) onto the sorbent. Langmuir isotherm

Linear Non-linear

Freudlich isotherm

qmax (mg/g)

b (L/mg)

r2

v2

KF

n

r2

v2

256.4 243.7

1.564 0.907

0.999 0.977

N/A 183.6

80.35 106.8

2.958 4.484

0.879 0.899

N/A 789.1

790

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

as 256.4 mg/g, which is much higher than that of most materials reported in the literatures [24]. 3.2.4. Effect of natural organic matter Natural organic matter (NOM) is ubiquitous in shallow aquifers. Since NOM may have a high tendency to be adsorbed onto the surface of various materials, it might modify the properties of mineral surfaces and block the adsorption sites [10,25,26]. Hence, the presence of NOM may significantly influence the adsorption behavior of adsorbate on adsorbent in a natural aquatic environment. Moreover, the existence of organic matters may change the valency of arsenate during the adsorption. There may be a possibility that the organic matters reduce arsenate to arsenite. To get a better understanding on application of the sorbent for As(V) removal from natural aquatic environment, it is necessary to study the potential interference of NOM, represented by humic acid (HA), on the adsorption of As(V) onto the sorbent. Fig. 7a shows that adsorption capacity of the adsorbent for arsenate decreases by 27% and 74% in the presence of 2 and 10 mg/L HA, respectively. It demonstrates that the existence of HA significantly inhibits the adsorption of As(V) onto the sorbent. Similar observations have been reported on the adsorption behaviors of other sorbents, such as nanoscale zerovalent iron [10] and hematite [27]. In natural waters, arsenic mainly exists as arsenate or arsenite. The toxicity and mobility of arsenic are affected by the oxidation state. Arsenite is reportedly more toxic and mobile than arsenate in the aqueous environment. To investigate the effect of NOM on the reduction of arsenate into arsenite during the adsorption, the 200

3.2.5. Effect of coexisting anions Generally, arsenic contaminated water contains other anions, which may compete with arsenic for the active adsorption sites. Hence, it is important to investigate the potential influence of coexisting anions on arsenic adsorption onto the sorbent. Different 2 3 concentrations of anions, including F, NO 3 , SiO3 and PO4 , were prepared by dissolving certain amount of their sodium salts in 20 mg/L arsenate solution. It should be noted that the concentrations of coexisting anions prepared are referred to their actual concentration level in natural environment and calculated by the weight of target atoms. As shown in Fig. 8, there is no significant influence on the As(V) adsorption capacity caused by the presence of fluoride or nitrate. However, the uptake of As(V) is significantly hindered by the presence of phosphate or silicate. The similar adverse effects of phosphate or silicate on the adsorption of As(V) on metal/metal oxide were reported by other researchers [28–30]. Phosphate can be strongly adsorbed on metal oxide surfaces to form an inner sphere complex [29]. The decrease in the adsorption capacity of sorbent

(b)

200

160

160

120

120

q e (mg/g)

q e (mg/g)

(a)

voltammetry analysis was conducted. Fig. 7b shows the voltammogram of different arsenic species in the water solution. The singlespecies arsenate and arsenite have characteristic peaks with voltage of 0.07 and 0.11 V, respectively. Both peaks can easily be found in the As(V) and As(III) mixture. The solution sample after the adsorption has a peak with voltage of 0.07 V; this clearly indicates that the valency of the arsenate doesn’t change during its adsorption in the existence of NOM. It can be concluded that the NOMs don’t cause reduction in the arsenic.

80

40

80

40

0

0 0

2

4

6

8

10

0

-

10

15

20

25

SiO3 concentration (mg-Si/l)

200

(d)

200

160

160

120

120

q e (mg/g)

q e (mg/g)

(c)

5 2-

F concentration (mg/l)

80

80

40

40

0

0 0.0

0.5 3-

1.0

1.5

2.0

PO 4 concentration (mg-P/l)

2.5

0

5

10

15

20

25

-

NO3 concentration (mg-N/l)

Fig. 8. Effect of coexisting anions on the adsorption of arsenic onto the sorbent: (a) fluoride; (b) silicate; (c) phosphate; and (d) Nitrate (sorbent dose = 0.1 g/L, initial concentration = 20 mg/L, pH = 6.9 ± 0.1, T = 20 °C).

791

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

(a)

(a) Fresh Sorbent

O-S

1128

Intensity

620 452

Metal Oxide

845 647 450

3232

1631

3369

(b)

1214 1122

1637

Transmittance (a.u)

O-H

4000 3500 3000 2500 2000 1500 1000

(b) As-loaded Sorbent

500

Wavenumber (cm-1)

O-As O-H

Fig. 9. FTIR spectra of the sorbent: (a) virgin sorbent and (b) As(V) loaded sorbent.

Metal Oxide O-S

for As(V) in the presence of phosphate may be due to that the phosphate competes for the adsorption sites on the surface of sorbent.

534 533 532 531 530 529 528 527 526

Binding Energy (eV)

3.3. Spectroscopic analysis The transmission FTIR spectrum of the virgin sorbent was shown in Curve (a) of Fig. 9. The broad, strong absorption band in wave number of 3400–3600 cm1 (centered at 3269 cm1) corresponds to the O–H stretching vibration, which indicates the presence of hydroxyl group on the surface of the sorbent. The sharp peak at 1637 cm1 could be due to the existence of water of hydration, which is consistent with the band at 3400–3600 cm1. Peaks at 1214, 1122, and 620 cm1 demonstrate the sorbent contains SO2 4 [19,31]. Compared with the FTIR spectrum of virgin sorbent, an obvious new band at wave number of 845 cm1 appears in the spectrum of the sorbent after adsorption of As(V) (Curve (b) of Fig. 9). The new band can be assigned to the stretching vibration of As–O bands in H2AsO4 group [32,33], which indicates the uptake of As(V) on the sorbent. Curve (b) shows that the peak at 3369 cm1 shifts to O 1s

(a) Fresh Sorbent

Zr 3p3

Zr 3d

C 1s

S 2p

Zr 4p

Intensity

Zr 3p1

(b) As-loaded Sorbent O 1s

C 1s

Fig. 11. XPS O 1s spectra of the sorbent: (a) virgin sorbent; (b) As(V) loaded sorbent.

3232 cm1, indicating that –OH on the surface of sorbent may be involved in the As(V) adsorption. It must be double checked since this change also could be caused by the change of water of hydration, with peak diminution at 1631 cm1. XPS is a useful tool for the determination of existence and chemical state (i.e. valence) of element [3,5,8,34–36]. As shown in Fig. 10, the wide scan XPS spectrum of the fresh sorbent indicates that Zr, S, and O are present in the sorbent. While the wide scan of XPS spectrum of sorbent after adsorption demonstrates that three of As characteristic peaks, i.e. As 3d, As 3p and As LMM, are existing in the spectrum, which confirms the adsorption of As(V) onto the sorbent. Furthermore, the peak assigned to S 2p disappears in the spectrum of sorbent after the adsorption of As(V), which implies that sulfur-containing groups might be involved in the adsorption. The high resolution scan of O 1s spectrum of the fresh sorbent can be decomposed into four component peaks as shown in Fig. 11a. The peaks with binding energy of 528.4, 529.6, 530.4 eV can be assigned to the O in the forms of metal oxide (Metal-O), O–H, and O–S [34–36]. Compared with the spectrum of virgin sorbent, a new component peak with binding energy of 528.9 eV is identified in the spectrum of sorbent after adsorption of As(V), which can be assigned to As–O as illustrated in Fig. 11b [5]. It is found that the intensity of oxygen peaks in form of O–H and O–S decreases significantly after the adsorption, which is corresponding to the disappearance of S 2p in the wide scan spectra (Fig. 10b). These indicate that hydroxyl group and sulfur-containing group play important roles in the uptake of As(V).

AsLMM

4. Summary

Zr 3d As 3d

0

100

As 3p

200

In this study, a nanostructure zirconium-based sorbent was successfully developed for effective adsorption of As(V) from aqueous solution. The following conclusions could be drawn. 300

400

500

600

Binding Energy (eV) Fig. 10. XPS wide scan spectra of the sorbent: (a) virgin sorbent; (b) As(V) loaded sorbent.

(1) According to thermal gravimetric and elemental analyses, the molecular formula of the zirconium nanoparticle was identify as Zr2(OH)6SO43H2O.

792

Y. Ma et al. / Journal of Colloid and Interface Science 354 (2011) 785–792

(2) HRTEM observation showed the sorbent was nanoscale with a particle size ranging from 60 to 90 nm. XRD analysis indicated that the sorbent was amorphous. The surface charge density of the sorbent decreased with an increase in solution pH, and the pHzpc of the sorbent was found to be around 2.85. (3) The kinetics study showed that the adsorption equilibrium could be obtained with 12 h, which was better than most of sorbents available in the market for removing anionic pollutants from aqueous solution. (4) It was found that the optimal pH for As(V) adsorption was from 2.5 to 3.5. The Langmuir model could be well used to describe the adsorption of As(V) onto the sorbent. According to Langmuir equation, the maximum As(V) adsorption capacity of the sorbent was found to be as high as 243 mg/ g under optimal pH. (5) The presence of fluoride or nitrate did not significantly influence the adsorption of As(V) on the sorbent, however, the coexistence of humic acid, phosphate or silicate severely hindered the adsorption of As(V) onto the sorbent. (6) FTIR and XPS spectroscopic analyses demonstrated that the surface hydroxyl and sulfur-containing functional groups played import roles in the uptake of As(V).

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

Acknowledgment [29]

This work was supported by the Agency for Science, Technology and Research, Singapore (Grant No. 0 921 010 059, and R-288-000066-305). References [1] J. Siemiatycki, L. Richardson, K. Straif, B. Latreille, R. Lakhani, S. Campbell, M.S. Rousseau, P. Boffetta, Environ. Health Perspect. 112 (2004) 1447.

[30] [31] [32] [33] [34] [35] [36]

D.N.G. Mazumder, Indian J. Med. Res. 128 (2008) 436. Y.M. Zheng, S.F. Lim, J.P. Chen, J. Colloid Interface Sci. 338 (2009) 22. F. Di Natale, A. Erto, A. Lancia, D. Musmarra, Water Res. 42 (2008) 2007. S.F. Lim, Y.M. Zheng, J.P. Chen, Langmuir 25 (2009) 4973. Z. Gu, J. Fang, B. Deng, Environ. Sci. Technol. 39 (2005) 3833. M.E. Pena, G.P. Korfiatis, M. Patel, L. Lippincott, X.G. Meng, Water Res. 39 (2005) 2327. S.J. Zhang, X.Y. Li, J.P. Chen, Carbon 48 (2010) 60. S.R. Kanel, B. Manning, L. Charlet, H.C. Choi, Environ. Sci. Technol. 39 (2005) 1291. A.B.M. Giasuddin, S.R. Kanel, H. Choi, Environ. Sci. Technol. 41 (2007) 2022. T.M. Suzuki, J.O. Bomani, H. Matsunaga, T. Yokoyama, React. Funct. Polym. 43 (2000) 165. J.P. Chen, L. Yang, Langmuir 22 (2006) 8906. S.A. Wasay, J. Haron, A. Uchiumi, S. Tokunaga, Water Res. 30 (1996) 1143. G. Horányi, E. Kálmán, J. Colloid Interface Sci. 269 (2004) 315. C.F. Chang, P.H. Lin, W. Höll, Colloids Surf. A 280 (2006) 194. S.S. Banerjee, D.H. Chen, J. Hazard. Mater. 147 (2007) 792. T. Ohtani, Y. Nawa, A. Watanabe, Colloids Surf. 66 (1992) 97. A. Clearfield, Rev. Pure Appl. Chem. 14 (1964) 91. M. Agarwal, M.R. Guire, A.H. Heuer, J. Am. Ceram. Soc. 80 (1997) 2967. C. Tien, Adsorption Calculations and Modeling, Butterworth-Heinemann, Boston, 1994. S.F. Lim, Y.M. Zheng, J.P. Chen, Chem. Eng. J. 152 (2009) 509. D.A. Ratkowsky, Handbook of Nonlinear Regression Models, Marcel Dekker Inc., New York, 1990. Y.S. Ho, Carbon 42 (2004) 2115. D. Mohan, C.U. Pittman, Hazard. Mater. 142 (2007) 1. A.W.P. Vermeer, W.H. van Riemsdijk, L.K. Koopal, Langmuir 14 (1998) 2810. T. Saito, L.K. Koopal, W.H. van Riemsdijk, S. Nagasaki, S. Tanaka, Langmuir 20 (2004) 689–700. I. Ko, A.P. Davis, J.Y. Kim, K.W. Kim, J. Hazard. Mater. 141 (2007) 53. L.C. Roberts, S.J. Hug, T. Ruettimann, M. Billah, A.W. Khan, M.T. Rahman, Environ. Sci. Technol. 38 (2004) 307. K. Tyruvola, N.P. Nikolaidis, N. Veranis, N. Kallithrakas-Kontos, P.E. Koulouridakis, Water Res. 40 (2006) 2375. M.C. Ciardelli, H. Xu, N. Sahai, Water Res. 42 (2008) 615. N. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986. A. Voegelin, S. Hug, Environ. Sci. Technol. 37 (2003) 972. A. Gupta, V.S. Chauhan, N. Sankararamakrishnan, Water Res. 43 (2009) 3862. J. Riga, J.J. Verbist, P. Josseaux, A.K. Mesmaeker, Surf. Interface Anal. 7 (1985) 163. L.V. Duong, B.J. Wood, J.T. Kloprogge, Mater. Lett. 59 (2005) 1932. L. Yang, J.P. Chen, Bioresource Technol. 99 (2008) 297.