Cd2+sorption characteristics of iron coated silica

Desalination 277 (2011) 221–226

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Cd2+sorption characteristics of iron coated silica M. Waseem a,⁎, S. Mustafa a,1, A. Naeem a,1, G.J.M. Koper b, K.H. Shah a,1 a b

National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar-25120, Pakistan Delft University of Technology, Department of Chemical Engineering, (SAS), The Netherlands

a r t i c l e

i n f o

Article history: Received 29 November 2010 Received in revised form 9 April 2011 Accepted 12 April 2011 Available online 10 May 2011 Keywords: Cadmium Characterization Complexation Iron coated silica Sorption

a b s t r a c t The present work deals with the Cd2+ sorption studies on iron coated silica synthesized by sol-gel method. The coated media has been characterized before and after the adsorption of cadmium by surface area measurement, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), point of zero charge (PZC) and X-ray diffraction (XRD). X-ray diffraction patterns confirm the amorphous nature of the coated oxide. The uptake of Cd2+ ions is found to increase with increasing concentration, pH and temperature of solution. The Langmuir model is used to interpret sorption of cadmium ions on the solid surface. The values of both ΔH and ΔS are found to be positive, which explains the sorption process to be endothermic and spontaneous in nature. The silica dissolved from the solid surface plays a key role in the in the formation of ligand like surface ternary complexes. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In natural aquatic systems many reactions like dissolution, adsorption, precipitation and colloidal adhesion result in the formation of multi-component solids, which have bulk and surface properties that differ from those of the parent phases [1]. Various studies have shown the importance of surface coated materials in controlling the metal distribution in soils. Many researchers demonstrated the influence of surface coatings on the sorptive behavior towards heavy metal ions [2–5]. Edwards and Benjamin coated iron oxide onto the surface of ordinary filtered sand and found the product an efficient adsorbent for the removal of Cu, Cd, Pb, Ni and Zn ions [6]. Meng and Letterman tested oxide mixtures of Fe(OH)3/SiO2 and Al(OH)3/SiO2 for the adsorption of Cd2+ and Ca2+ ions [7]. They further observed that Cd2+ion sorption on Fe(OH)3/SiO2 mixed oxide is indistinguishable from the adsorption on a similar amount of pure Fe(OH)3, when the concentration of iron is higher than 0.3 mM. Further, it was found that the metal ion sorption on aluminum coated silica was greater than aluminum oxide or silica itself [8]. Xu and Axe [9] synthesized iron oxide coated silica by the adsorption and precipitation methods. They adsorbed Ni2+ ion on coated silica and found that coated goethite exhibits a higher adsorption capacity than the discrete goethite system. Uygur and Rimmer suggested that iron oxide present in the form of coating on calcite has high sorption capacity for Zn ions [10]. Moreover, the adsorption ability can be increased by decreasing the thickness of the coating. ⁎ Corresponding author. Tel.: + 92 91 9216766; fax: + 92 91 9216671. E-mail address: [email protected] (M. Waseem). 1 Tel.: + 92 91 9216766; fax: + 92 91 9216671. 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.04.034

In nature, oxides and hydroxides are mostly occurring in the form of coatings on other particles and exist in various amorphous and crystalline forms. We are interested in this particular coated oxide, because it constitutes the main part of the soil. So the study under consideration is more close to the natural system. The work presented here thus deals with the synthesis of iron coated silica for the effective removal of Cd2+ ions from aqueous solutions. Cadmium is selected because of its environmental importance. It enters into the natural systems by waste waters, discharged from industries such as plating and producing batteries, paints, pigments, etc. Some of its health effects on human life include diarrhea, nausea, muscle cramps and damage of bone marrow. However, it is considered as carcinogenic and found responsible for the formation of kidney stones [11]. To trace the mechanism, the coated oxide has been characterized before and after the adsorption of Cd2+ ions. 2. Materials and methods All the analytical grade reagents were used without any further purification. Fe(NO)3 (99.9%) and HNO3 (65%) from MERCK, Na2SiO3 (99.9%) from Scharlau, and NaOH (extra pure) from BDH chemical were employed for the synthesis of coated oxide. 2.1. Synthesis of iron coated silica Sol-gel method was employed for the synthesis of iron coated silica [12]. The respective HNO3 and Na2SiO3 were mixed drop wise in the ratio of 2:1 molar ratio leading to the formation of white gel. After 2 h of aging time, Fe(OH)3 was precipitated in the gel by the drop wise addition of NaOH and Fe(NO)3 in 3:1 molar ratio. The stirring speed

M. Waseem et al. / Desalination 277 (2011) 221–226

2.2. Adsorption, desorption and dissolution studies Batch adsorption studies were performed by taking 40 ml of solution having cadmium ion concentration ranging from 10.2 to 79.3 mg/L in 100 ml conical flasks. Afterward, 0.1 g of the powder sample was added to each flask and the pH values of the suspensions were adjusted to the initial values of 5 and 7. The sorption experiments were performed in the temperature range 288 (±1)– 318 (±1) K. The reaction vessels were equilibrated in a temperature controlled shaker bath model DAIHAN WSB-30 at a shaking speed of 120 rpm. The preliminary sorption experiments show that 2 h are sufficient for the system to attain the equilibrium. After equilibration, the suspensions were filtered and the concentration of Cd2+ ions was measured with atomic absorption spectrophotometer model AAS 800. Finally the metal ions adsorbed qe (mmol g− 1) were calculated from the difference between the initial and the equilibrium concentrations. The residue on the filter paper before and after the adsorption of cadmium ions was subjected for physicochemical analysis. Desorption studies were also performed by taking 40 ml of 5 M HNO3 as an extractant in a 100 ml conical flask. The suspension was shaken at 120 rpm for 30 min. The Cd2+ ions released and the silica dissolved from the surface of the solid were determined. 2.3. Characterization of the coated oxide The surface area of the solid was determined by nitrogen adsorption method at 77 K using a surface area analyzer model Quantachrome NOVA 1200e. Prior to analysis the sample was degassed at 373 K for 1 h. The specific surface area of the solid before and after the adsorption of cadmium ions on the surface was determined by Brunauer Emmett and Teller (BET) and Barrett, Joyner and Halenda (BJH) methods. SEM images were taken on SEM model JSM 5910 (JEOL Japan) equipped with an energy dispersive X-ray (EDX) model INCA 200 (UK) at 20 keV. FTIR spectra before and after Cd2+ adsorption were recorded using SHIMADZU 8201PC FTIR spectrophotometer. The samples were mixed with KBr powder in the ratio 1:100, dried and then subjected for IR observations. The XRD patterns of the powder samples were recorded over a range of 2θ from 10°to 80° with a step angle of 0.30° and a step time of 0.5 s using X-ray diffractometer model JEOL, JDX-3532 with Mn filtered Cu-Kα radiation. The PZC of coated oxide was determined by the method of Kinniburgh et al. [13], using a pH meter model BOECO BT-600 (Germany) with temperature probe and a pH electrode of research grade. The concentration of the background electrolyte (NaNO3) used was 0.01 M. 3. Results and discussions 3.1. Cadmium adsorption, dissolution and desorption data In the preliminary studies, sorption of Cd2+ ions has been performed on silica, Fe(OH)3 and the iron coated oxide (Fig. 1). It is observed that the coated oxide has high affinity towards the Cd2+ ions. The higher Cd2+ uptake on the coated oxide in comparison to its counterparts is due to the presence of two types of functional groups SiO− and FeO− on the surface. Further, the enhancement in the Cd2+ sorption may be due to the lowering of surface charge on account of silicate sorption on Fe(OH)3. The cadmium adsorption studies on the coated oxide at the pH values 5 and 7 are shown in Fig. 2. Sorption of

Silica Fe(OH)3

0.12

Iron coated silica

0.10 qe (mmolg-1)

during mixing of the ingredients was kept constant and the suspension thus obtained was aged for one day at pH 6.8 ± 0.1. After 24 h, the supernatant was decanted and the solid was washed several times with doubly distilled water. The final product thus obtained was dried, ground and stored in a polythene bottle after passing through 450 mesh sieve.

0.08 0.06 0.04 0.02 0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Ce (mmolL-1) Fig. 1. Cd2+ sorption on SiO2, Fe(OH)3 and iron coated silica at pH 5 and at 288 K.

Cd2+ ions onto the iron coated silica at pH 7 is found to be higher than at pH 5. This may be because of the point of zero charge (PZC) of the adsorbent. The PZC of the coated oxide is found to be 4.6, which lies between the PZC of silica (2.1) and iron hydroxide (6.5), as recently reported by us [14]. The surface being more negatively charged at pH 7 results in a higher uptake of Cd2+ ions. The comparison of the sorption data further reveals that the uptake of Cd2+ ions increases with increasing temperature of the solution [15]. It shows that the effect of temperature is almost as important as the pH of the electrolyte solution. The decrease in the equilibrium pH values (Fig. 3) with concentration of metal ions shows a direct relationship between exchange of protons with the Cd2+ ions on the surface. When the initial pH was selected to pH 5, an increase in the equilibrium pH values was observed (Fig. 3A) showing the competition between H+ and Cd2+ ions. However, when the initial pH was selected 7, a decrease in the equilibrium pH values was observed (Fig. 3B) which shows that ion exchange may take place along with the formation of ligand like ternary complexes on the surface of coated oxide [16]. The silica released from the surface of coated oxide at both the pH values was also determined. The silica released from surface at pH 5 was found in the range of 6.9 mg/L to 11.4 mg/L, which changes to 8.9 mg/L to 12.9 mg/L at pH 7, however, the release of iron from the surface was found almost negligible. It is interesting to note that the adsorption of cadmium on the coated oxide is directly related to the

0.26

pH 7

0.24

288 K 298 K 308 K 318 K

0.22 0.20 qe (mmolg-1)

222

0.18 0.16 0.14 0.12

pH 5

0.10

288 K 298 K 308 K 318 K

0.08 0.06 0.04 0.02 0.0

0.1

0.2

0.3

0.4

0.5

Ce (mmolL-1) Fig. 2. Cd2+ sorption on coated oxide at pH 5 and 7.

M. Waseem et al. / Desalination 277 (2011) 221–226

223

A 6.4 288 K 298 K 308 K 318 K

6.2 Equilibrium pH (pHeq)

6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 0

B

10

20

30 40 50 60 Initial concentration (Co)

70

4.8 288 K 298 K 308 K 318 K

4.6 Equilibrium pH (pHeq)

80

4.4 4.2 4.0 3.8 3.6 3.4 3.2 0

10

20

30 40 50 60 Initial concentration (Co)

70

80

Fig. 3. Plot of equilibrium pH vs. initial concentration of Cd2+ ions at (A) pH 5 and (B) pH 7. Fig. 4. Langmuir model for iron coated silica at (A) pH 5 and (B) pH 7.

extent of the silica released from the surface of the adsorbent [8]. From the desorption studies, about 93% Cd2+ ions are recovered. To evaluate the binding energy constants and sorption maxima, the Langmuir model was applied (Fig. 4) to the data in its linear form: Ce = qe = 1 = Xm Kb + Ce = Xm

ð1Þ

where Ce is the equilibrium ion concentration in the solution (mmol L− 1), qe the amount of metal ion sorbed on the surface of the sorbent (mmol g− 1). Xm is the maximum amount of metal ions sorbed with a monolayer coverage on the surface and Kb is the Langmuir adsorption constant (L g− 1). The values of maximum sorption capacity (Xm) and binding energy constant (Kb) are compiled in the Table 1. The Xm and K b values were observed to increase with both the pH and temperature of the system. The data (Table 1) revealed that the temperature effect is more profound on the Kb as compared to the Xm values. In current study, the Cd2+ ions sorption at both the pH values were found higher than those reported for silica [17] and hydrous iron oxide [18]. Further, the sorption capacity of this coated oxide (0.25 mmol/g) towards cadmium is higher than the value (0.1 mmol/g) recently reported by us [19]. Agrawal and Sahu observed similar trends for Kb values while studying cadmium adsorption on manganese nodules residue [20].

3.2. Characterization of coated oxide 3.2.1. Surface area and pore size distribution The BET surface area of the coated oxide is found to be 320 m2/g which after Cd2+ ion adsorption decreases from 284 m2/g to 186 m2/g at the pH values 5 and 7 respectively. Similarly, the decrease in the BJH surface area from 363 m2/g to 322 m2/g (pH 5) and 176 m2/g (pH 7) is observed after the uptake of Cd2+ by the solid. Nitrogen adsorption– desorption (Fig. 5) gives isotherms of type IV according to IUPAC classification with hysteresis (H1) and are the characteristic of the mesoporous materials [21]. A steep adsorption of N2 on the surface of coated oxide at the very beginning (P/Po b 0.04) (Fig. 6) suggests that the solid has micropores. However, the hysteresis loop for coated oxide before and after the Cd2+ sorption beginning from P/Po = 0.4, Table 1 Langmuir parameters for Cd2+ sorption on iron coated silica. Temperature (K)

pH 5 Xm (mmol.g− 1)

Kb (L.g− 1)

Xm (mmol.g− 1)

pH 7 Kb (L.g− 1)

288 298 308 318

0.137 0.140 0.145 0.155

11.44 15.26 20.86 24.06

0.213 0.217 0.222 0.225

13.71 21.40 35.66 59.16

224

M. Waseem et al. / Desalination 277 (2011) 221–226

450 After Cd

2+

Adsorption Desorption

adsorption

Volume N2 adsorbed (cm3/g)

375 2+

Before Cd

300

adsorption

Adsorption Desorption

225

150

75

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure P/Po Fig. 5. N2 adsorption isotherms of coated silica before and after Cd2+ sorption at pH 7.

illustrates that the micropores no longer exist and they evolved into mesopores. Further, in the solid, the position of the loop at relatively high pressure indicates the presence of large pores as such pores fills only at relatively high N2 pressure. However, after the sorption of Cd2+ the loop appeared at relatively low pressure, indicating that mesopores are occupied with the metal cations. The decrease in surface area after the sorption of Cd2+ results in the decrease in mesopore volume which can be seen from the plot of BJH desorption cumulative volume vs. pore size of the adsorbent (Fig. 6). The BJH average pore volume of the coated oxide is found to be 0.68 cc/g, which changes to 0.67 cc/g at pH 5 and 0.23 cc/g at pH 7. Furthermore, it can be seen from Fig. 6 that after the cadmium ion adsorption on the solid, the mesopore diameter decreases from the initial 6.2 nm to 6.0 nm (pH 5) and 2.4 nm (pH 7). The decrease in pore volume and the pore diameter is due to blocking of the pores after the sorption of Cd2+ ions [22]. 3.2.2. XRD, SEM and EDX analyses X-ray diffraction pattern of the coated oxide before and after sorption studies (not shown here) shows no peak, indicating the amorphous nature of coated oxide. From the SEM micrographs (Fig. 7), it is hard to detect any morphological change before and after the sorption of cadmium ions on the surface of solid, however, the EDX spectra (Fig. 8) provide the Fig. 7. SEM images (A) before and (B) after Cd2+ sorption at pH 5 and (C) pH 7.

0.30 Before Cd2+ sorption After Cd2+ sorption at pH 5

dv(log d) (cc/g-nm)

0.25

After Cd2+ sorption at pH 7

0.20

0.15

0.10

0.05

1

2

3

4 5 Pore size (nm)

6

7

8

Fig. 6. PSDBJH derived from the desorption wing of N2 adsorption–desorption isotherm.

spectroscopic evidence for the presence of Cd2+ ions on the surface of the solid. The percent Cd2+ ions on the surface after the adsorption at both the values 5 and 7 are found to be 1.44 (±0.08) and 4.76 (±0.12) respectively. 3.2.3. FTIR spectroscopy The IR spectra of Fe coated silica before and after sorption of cadmium ions are shown in Fig. 9. The spectrum before the Cd2+ sorption consists of the bands at 1630, 1380, 930–1200, 800, 700 and 465 cm− 1. The band at 1630 cm− 1 is assignable to OH bending vibration [23]. The band at 1380 cm− 1 which is an indication of the 3 presence of NO− anions, however, reduced in the spectra of 3 adsorbed cadmium ions. It is evident from the IR spectra that the peak intensity at 1380 cm− 1decreases in intensity when Cd2+ions are adsorbed at pH 5. However, it almost disappears after the sorption of Cd2+ ion at pH 7. This may be due to the replacement of interlayer nitrates with the silicate anions dissolved from the surface of coated oxide. Similar data on silicate sorption on oxide surface are reported elsewhere [24,25]. The band at 465 cm− 1 assigned to the bending

M. Waseem et al. / Desalination 277 (2011) 221–226

225

Fig. 9. FTIR spectra of coated silica (a) before and (b) after Cd2+ sorption at pH 5 and (c) pH 7.

Kababji et al. for Si–O–As and Si–O–Co respectively, while studying the sorption of these metal ions on silicates [29,30]. The peak appeared at 938 cm− 1 at pH 5, which was also assigned by Ponthieu et al. to dimeric silicate shifts to 916 cm− 1 at pH 7 due to the increased interaction of Cd2+ ions with dimeric silicate on the surface [31]. Thus these observations also point towards the formation of ligand like monomeric and dimeric complexes on the surface of coated oxide. 3.3. Thermodynamics of adsorption Enthalpy and entropy changes for the adsorption process were calculated by using the following relationship: ln Kb =

Fig. 8. EDX analysis (A) before and (B) after Cd2+ sorption at pH 5 and (B) pH 7.

mode of Si–O–Si has also been observed by other investigators [26,27]. The broad spectral region centered at 930–1200 cm− 1 which is assigned to the different Si–O–Si vibrations correspondingly splits into three peaks at 1128, 1025 and 938 cm− 1 and to 1130, 980 and 916 cm− 1 after the sorption of Cd2+ ions at pH 5 and 7. The appearance of these new peaks shows that Si–O–Si and Si–OH groups are complexed with the Cd2+ and form Si–O–Cd type complexes on the surface of mixed oxide. The new bands that appeared at 1128 and 1130 cm− 1 are assigned to the asymmetric stretching vibration of Si– O–Si and are due to the formation of silica dimers on the surface [16]. The appearance of bands at 1025 and 980 cm− 1 can also be assigned to the Si–O–Cd band after the breaking of Si–OH bands. Handke and Mozgawa had also assigned a band at 1044 cm− 1 to SiO− group while studying different silicates [28]. Similar band appearance at 1020– 1036 cm− 1 and 1023 cm− 1was reported by Abo-El-Enein et al. and

ΔS ΔH − R RT

ð2Þ

The values of thermodynamic parameters (Table 2) are calculated from the plot of ln Kb vs. 1/T (Fig. 10). The values of ΔH are found to be positive suggesting the process to be endothermic in nature while the positive ΔS suggests increased randomness at the solid/solution interface accompanied with the dehydration of Cd2+ ions [32–34]. Similar positive values of ΔH and ΔS are reported elsewhere [35,36]. Free energy changes were evaluated by using the Gibb's Helmholtz relation: ΔG = ΔH−TΔS

ð3Þ

The Gibb's free energy indicates the degree of spontaneity of the adsorption, where negative values reflect energetically favorable sorption processes. Similar ΔG values were reported elsewhere [37].

Table 2 Heat of adsorption (ΔH), entropy (ΔS) and free energy (ΔG) changes of Cd2+ sorption on iron coated silica. pH

ΔH (kJ mol− 1)

ΔS (J mol− 1 K− 1)

− ΔG (kJ mol− 1) 288 K

298 K

308 K

318 K

5 7

19.4 37.2

87 150

05.8 06.2

06.7 07.7

07.6 09.2

08.5 10.7

226

M. Waseem et al. / Desalination 277 (2011) 221–226

Fig. 10. Plot of Ln Kb vs. 1/T for iron coated silica at pH values 5 and 7.

4. Conclusions In the present study, we have synthesized iron coated silica by solgel method and characterized for a set of spectroscopic techniques before and after the adsorption of Cd2+ ions. The PZC value of coated oxide lies between its parent material silica and iron hydroxide. Cadmium sorption at pH 7 is found to be greater than that of pH 5. The decrease in the surface areas, pore radii and pore volumes may be associated to the presence of Cd2+ ions within the mesopores of the solid. Both Xm and Kb values increase with temperature showing that the sorption process is energetically more favored at higher temperatures. Higher Xm values reveal the higher selectivity of coated media for the Cd2+ ions. The values of both ΔH and ΔS are found to be positive, showing the process to be of endothermic nature. The negative values of ΔG confirm the spontaneity of the adsorption. IR studies point towards the formation of ligand like ternary surface complexes. Acknowledgement We greatly acknowledge Higher Education Commission (HEC) of Pakistan for supporting this work under the scholarship, International research support initiative program (IRSIP). References [1] P.R. Anderson, M.M. Benjamin, Surface and bulk characteristics of binary oxide suspensions, Environmental Science and Technology 24 (1990) 692–698. [2] L. Weng, E.J.M. Temminghoff, S. Lofts, E. Tipping, W.H. Van Riemsdijk, Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil, Environmental Science and Technology 36 (2002) 4804–4810. [3] L. Weng, E.J.M. Temminghoff, W.H. Van Riemsdijk, Contribution of individual sorbents to the control of heavy metal activity in sandy soil, Environmental Science and Technology 35 (2001) 4436–4443. [4] M.P. Papini, A. Bianchi, M. Majone, M. Beccari, Equilibrium modeling of lead adsorption onto a “Red Soil” as a function of the liquid-phase composition, Indian Engineering Chemistry Research 41 (2002) 1946–1954. [5] D. Dong, Y.M. Nelson, L.W. Lion, M.L. Shuler, W.C. Ghiorse, Adsorption of Pb and Cd on metal oxides and organic material in natural surface coatings as determined by selective extractions: new evidence for the importance of Mn and Fe oxides, Water Research 34 (2000) 427–436. [6] M. Edwards, M.M. Benjamin, Adsorptive filtration using coated sand: a new approach for treatment of metal-bearing wastes, Journal of Water Pollution Control Federation 61 (1989) 1523–1533.

[7] X. Meng, R.D. Letterman, Effect of component oxide interaction on the adsorption properties of mixed oxides, Environmental Science and Technology 27 (1993) 970–975. [8] X. Meng, R.D. Letterman, Modeling ion adsorption on aluminum hydroxidemodified silica, Environmental Science and Technology 27 (1993) 1924–1929. [9] Y. Xu, L. Axe, Synthesis and characterization of iron oxide-coated silica and its effect on metal adsorption, Journal of Colloid and Interface Science 282 (2005) 11–19. [10] V. Uygur, D.L. Rimmer, Reaction of zinc with iron oxide coated calcite surfaces at alkaline pH, European Journal of Soil Science 51 (2000) 511–516. [11] A. Penkova, J.M.M. Blanes, S.A. Cruz, M.A. Centeno, K. Hadjiivanov, J.A. Odriozola, Gold nanoparticles on silica monospheres modified by amino groups, Microporous and Mesoporous Materials 117 (3) (2009) 530–534. [12] C. Xu, R.I. Tang, Y.X. Hua, P.X. Zhang, Mesoporous silica materials synthesized via sol-gel methods modified with ionic liquid and surfactant molecules, Chinese Journal of Chemical Physics 21 (6) (2008) 596–600. [13] D.J. Kinniburgh, J.K. Syres, M.L. Jacksow, Soil Science Society of America Proceedings 39 (1975) 161. [14] M. Waseem, S. Mustafa, A. Naeem, G.J.M. Koper, S. ud-Din, Physiochemical properties of mixed oxides of iron and silicon, Journal of Non-Crystalline Solids 356 (2010) 2704–2708. [15] B.B. Johnson, Effect of pH, temperature, and concentration on the adsorption of cadmium on goethite, Environmental Science and Technology 24 (1990) 112–118. [16] A.S. Heintz, M.J. Fink, B.S. Mitchell, Silicon nanoparticles with chemically tailored surfaces, Applied Organometallic Chemistry 24 (2010) 236–240. [17] A. Heidari, H. Younesi, Z. Mehraban, Removal of Ni(II), Cd(II), and Pb(II) from a ternary aqueous solution by amino functionalized mesoporous and nano mesoporous silica, Chemical Engineering Journal 153 (1–3) (2009) 70–79. [18] R.R. Gadde, H.A. Laitinen, Studies of heavy metal adsorption by hydrous iron and manganese oxides, Analytical Chemistry 46 (13) (1974) 2022–2026. [19] S. Mustafa, M. Waseem, A. Naeem, K.H. Shah, T. Ahmad, S.Y. Hussain, Selective sorption of cadmium by mixed oxides of iron and silicon, Chemical Engineering Journal 157 (2010) 18–24. [20] A. Agrawal, K.K. Sahu, Kinetic and isotherm studies of cadmium adsorption on manganese nodule reside, Journal of Hazardous Material B 137 (2006) 915–924. [21] M. Schneider, M. Maciejewski, S. Tschudin, A. Wokaun, A. Baike, Vanadia-titania aerogels: I. Preparation, morphological properties, and activity for the selective catalytic reduction of NO by NH3, Journal of Catalysis 149 (1994) 326–343. [22] H. Yang, Q. Lu, F. Gao, Q. Shi, Y. Yan, F. Zhang, S. Xie, B. Tu, D. Zhao, Advanced Functional Materials 15 (2005) 1377–1384. [23] Z.H. Cheng, A. Yasukawa, K. Kandori, T. Ishikawa, FTIR study on incorporation of CO2 into calcium hydroxyapatite, Journal of Chemical Society Faraday Transactions 94 (1998) 1501–1505. [24] S.V. Prasanna, R.A.P. Rao, P.V. Kamath, Layered double hydroxides as potential chromate scavengers, Journal of Colloid and Interface Science 304 (2006) 292–299. [25] C.D. Christina, W.C. Hsiao, E. Marc, Modeling silica sorption to iron hydroxide, Environmental Science and Technology 36 (2002) 582–587. [26] R.M. Almeida, T.A. Guiton, C.G. Pantano, Characterization of silica gels by infra red by reflection spectroscopy, Journal of Non-Crystalline Solids 121 (1990) 193–197. [27] C.M. Parler, J.A. Ritter, M.D. Amiridis, Infra red spectroscopic study of sol-gel derived mixed-metal oxides, Journal of Non-Crystalline Solids 279 (2001) 119–125. [28] M. Handke, W. Mozgawa, Model quasi molecule silica as an approach in the IR spectra description glassy and crystalline framework silicates, Journal of Molecular Structure 348 (1995) 341–344. [29] S.A. Abo-El-Enein, M.A. Eissa, A.A. Diafullah, M.A. Rizk, F.M. Mohamed, Removal of some heavy metal ions from waste water by copolymer of iron and aluminum impregnated with active silica derived from rice husk ash, Journal of Hazardous Material 172 (2009) 574–579. [30] A.H. Kababji, B. Joseph, J.T. Wolan, Silica supported cobalt catalysts for Fischer– Tropsch synthesis: effect of calcinations temperature and support surface area on cobalt silicate formation, Catalysis Letter 130 (2009) 72–78. [31] M. Ponthieu, F. Juillot, T. Hiemstra, W.H. Van Riemsdijk, M.F. Benedetti, Metal ion binding to iron oxides, Geochimica et Cosmochimica Acta 70 (2006) 2679–2698. [32] Z. Elouear, J. Bouzid, N. Boujelben, M. Feki, F. Jamoussi, A. Montiel, Heavy metal removal from aqueous solutions by activated phosphate rock, Journal of Hazardous Material 156 (2008) 412–420. [33] M.M. Abu-Mesalam, Sorption kinetics of copper, zinc, cadmium and nickel ions on synthesized silico-antimonate ion exchanger, Colloids and Surface A: Physiochemical and Engineering Aspects 225 (2003) 85–94. [34] R.S. Juang, J.Y. Chung, Equilibrium sorption of heavy metals and phosphate from single- and binary-sorbate solutions on goethite, Journal of Colloid and Interface Science 275 (2004) 53–60. [35] P.N. Pathak, G.R. Choppin, Nickel(II) sorption on hydrous silica: a kinetic and thermodynamic study, Journal of Radioanalytical and Nuclear Chemistry 268 (3) (2006) 467–473. [36] S. Mustafa, M. Waseem, A. Naeem, K.H. Shah, T. Ahmad, Cd2+ ions removal by silica, iron hydroxide and their equimolar mixed oxide from aqueous solution, Desalination 255 (2010) 148–153. [37] T.K. Naiya, A.K. Bhattachayara, S.K. Das, Adsorption of Cd(II) and Pb(II) from aqueous solutions on activated alumina, Journal of Colloid and Interface Science 333 (2009) 14–26.