TiO2

Applied Surface Science 316 (2014) 649–656

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Adsorption characteristics of hexavalent chromium on HCB/TiO2 Li Zhang, Yonggang Zhang ∗ State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China

a r t i c l e

i n f o

Article history: Received 19 May 2014 Received in revised form 5 August 2014 Accepted 8 August 2014 Available online 17 August 2014 Keywords: HCB/TiO2 Sol–gel Adsorption Chromium(VI)

a b s t r a c t Sol–gel method was adopted to prepare HCB/TiO2 and its adsorption ability of hexavalent chromium, Cr(VI), and removal from aqueous solution were investigated. The samples were characterized by Power X-ray diffraction (XRD) and a transmission electron microscope (TEM) which showed that the TiO2 was deposited on the surface of HCB. FTIR was used to identify the changes of the surface functional groups before and after adsorption. Potentiometric titration method was used to characterize the zero charge (pHpzc ) characteristics of the surface of HCB/TiO2 which showed more acidic functional groups containing. Batch experiments showed that initial pH, absorbent dosage, contact time and initial concentration of Cr(VI) were important parameters for the Cr(VI) adsorption studies. The Freundlich isotherm model better reflected the experimental data better. Cr(VI) adsorption process followed the pseudo-second order kinetic model, which illustrated chemical adsorption. The thermodynamic parameters, such as Gibbs free energy (G), changes in enthalpy change (H) and changes in entropy change (S) were also evaluated. Negative value of free energy occurred at temperature range of 25–45 ◦ C, so Cr(VI) adsorption by HCB/TiO2 is spontaneous. Desorption results showed that the adsorption capacity could maintain 80% after five cycles. The maximum adsorption capacity for Cr(VI) was at 27.33 mg g−1 in an acidic medium, of which the value is worth comparable with other low-cost adsorbents. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The existence of hexavalent chromium, Cr(VI), in water is dangerous because of its toxicity to human beings and the environment [1,2]. According to the standard of the U.S.EPA, the Cr(VI) ion should be reduced to 0.05 mg L−1 in water [3,4]. Accordingly, wastewater containing chromium has to be treated to meet standards before discharged into the environment. Cr(VI) removal methods include ion exchange [5,6], adsorption [7,8], biological processes [9,10], and chemical reduction [11,12]. Among these techniques, adsorption has been confirmed to be an effective and reliable method [13]. In this regard, metal oxides (TiO2 ) and carbon composite materials may offer some possibility for wastewater treatment because of the large surface area and the high capacity for removing heavy metal ions, making the treatment more economical and effective [14,15]. K. Parida [19] has investigated titania loading on zirconium phosphate (Zr–P) and titanium phosphate (TiP) and it is well applied to photocatalytic reduction of Cr(VI) in aqueous solution. Meanwhile, compared with WO3 and ZnO which has been reported in literature,

∗ Corresponding author. Tel.: +13502182420; fax: +8395 5451. E-mail addresses: [email protected], [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.apsusc.2014.08.045 0169-4332/© 2014 Elsevier B.V. All rights reserved.

TiO2 is the most suitable semiconducting oxides for removal Cr(VI) because of its low cost of production, high activity, photocatalytic activity, conservative nature, and low toxicity [17,18]. Recently, the literatures have reported that titanium dioxide (TiO2 ) load on carbon microspheres possessed the capacity of removing hexavalent chromium ion [1,15]. Due to the confidentiality and complexity of the preparation technology of carbon microspheres and the high costs of carbon materials, in comparison, carbon black (CB) which is easily mass produced was adopted. Meanwhile, in order to improve dispersibility and hydrophilicity of CB, surface oxidation treatment was considered. The advantage of oxidation treatment is to obtain a relatively large number of oxygen-containing functional groups on the surfaces [20,21]. In addition, surface-modified carbon black can be used as high-efficiency absorbent for heavy metal ion removal [16]. In this study, the HCB/TiO2 was prepared by sol–gel method. As is evident in literatures [1,36], the sol–gel method is the most widely applied method which not only introduces an inorganic phase onto HCB but also has intimate mixing or chemical interactions between the HCB and TiO2 . The effects of the solution pH, contact time, adsorption kinetics, Cr(VI) adsorption isotherm were investigated.

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2. Materials and methods 2.1. Materials All the chemicals used were of analytical grade. The stock solution of Cr(VI) (1000 mg L−1 ) was prepared with dissolving 2.829 g of K2 Cr2 O7 in deionized water. Required test solutions were obtained by diluting the stock solution with deionized water. 2.2. Methods 2.2.1. Preparation of HCB/TiO2 The commercial carbon black (CB) was purchased from Qiushi Carbon Black Factory, Tianjin, China. The carbon black (10–70 nm) was oxidized by refluxing CB (5 g) in 200 ml H2 O2 (30%) (HCB) in a conical flask at 80 ◦ C for 150 min. The HCB was filtered and cleaned repeatedly by using deionized water so that the pH of the filtrate was almost neutral without residual H2 O2 . Then HCB was dried in a vacuum oven at 100 ◦ C for 10 h. Sol–gel method was used to prepare TiO2 and HCB/TiO2. In the first step, butyl titanate (25 ml) was added to the anhydrous ethanol (50 ml) for 10 min sealed stirring and solution A was formed; in the second step, distilled water (2.5 ml) and HCl (37%, 1 ml) were added to the anhydrous ethanol (25 ml) for 5 min sealed stirring, HCB (2 g) was then added and solution B was formed; in the third step, solution B was slowly added to solution A with stirring 10 min to form the desired sol HCB/TiO2 , which was then allowed to age 36 h to form gel and was dried at 60 ◦ C for 3 h. Then the sample was heated at 5 ◦ C min−1 for elevating temperature till 450 ◦ C in Muffle furnace under N2 and calcined at 450 ◦ C for 30 min. The TiO2 was prepared with the same method described above. 2.2.2. Batch experiments Batch equilibrium Cr(VI) adsorption experiments were operated by mixing a known dose of HCB/TiO2 or HCB or TiO2 (0.5–5 g) with 100 mL solutions of certain Cr(VI) concentration in a 250 mL Erlenmeyer flask, which was shaken at 150 rpm in a water bath shaker (SHA-B) at 25 ◦ C for 2.5 h to reach equilibrium. Then the mixture was filtered and the residual Cr(VI) concentrations were analyzed using the 1,5-diphenylcarbazide method [21] by the double beam UV–visible spectrophotometer (UV-1901, Beijing) at a wavelength of 540 nm. In pH studies, the initial solution pH was adjusted to 2–11 with 0.1 M HCl or 0.1 M NaOH solutions, and the pH values were measured by a pH meter (Model PHS-25). The uptake amount Qe (mg g−1 ) and removal efficiency of Cr(VI) at equilibrium can be calculated by Eqs. (1) and (2): Qe =

(Ci − Ce )V W

%removal =

100(Ci − Ce ) Ci

(1) (2)

where Ci and Ce (mg L−1 ) are the initial and equilibrium concentrations of Cr(VI), respectively. V (L) is the solution volume (100 ml) and W (g) represents the amount of HCB/TiO2 (0.2 g). Adsorption kinetic experiments were done to investigate the effect of contact time and evaluate the kinetic properties. The HCB/TiO2 (2 g L−1 ) was added to the initial concentrations of 10.0, 20.0 or 30.0 mg L−1 solution. The mixtures were operated in a water bath shaker (SHA-B) at 150 rpm and 25 ◦ C and the time intervals were 0–480 min. The adsorption amount at time t (min), qt (mg g−1 ) was obtained by Eq. (3): qt =

(Ci − Ct )V W

where Ct (mg L−1 ) are the concentrations of Cr(VI) at time t.

(3)

2.2.3. Analytical methods The point of zero charge (pHpzc ) is usually measured by potentiometric titration [22,23]. For the determination of pHpzc , the sorbent (0.1 g) was suspended in 50 ml 0.1 M NaCl solution, which was used as an inert/background electrolyte in 100 ml stopper conical flask. This set of experiments was performed at a pH interval of 2.0 and on each occasion the solution pH was adjusted to the desired value with 0.01 M HCl or 0.01 M NaOH. The suspension was allowed to equilibrate for 24 h at 150 rpm in a shaker bath at 25 ◦ C. After the equilibration time, the mixture was filtered and the final pH value of the filtrate was measured. The pHpzc of HCB/TiO2 (4.3) was obtained to be less than HCB (6.2). It can be concluded that the surface of HCB/TiO2 generates more acidic surface groups [16]. Powder X-ray diffraction (XRD) analysis of the samples were obtained with a DMAX-2500 (Japan) X-ray diffractometer with Cu K␣ radiation ( = 1.5406 nm). The samples for transmission electron microscopy (TEM) were prepared by dispersing the final powders in ethanol and the dispersion was dropped on carbon copper grids. Then, the obtained powders deposited on a copper grid were observed by a transmission electron microscope (TEM, H7650). The samples (HCB–HCB/TiO2 ) were measured by Fourier transform infrared spectrometer (FTIR-650). The sample (1 mg) was mixed with spectroscopically pure bromide (KBr) (100 mg). 2.2.4. Desorption experiment To investigate the reusability of HCB/TiO2 , HCB/TiO2 (0.2 g) was contacted with 100 mL of 10 mg L−1 Cr(VI) for 150 min. The chromium-loaded adsorbent was mixed with 50 ml of aqueous NaOH (0.04 M) solution and agitated for 2.5 h. Then it was washed gently with deionized water to remove any chromium and residual NaOH solution. Several such samples were prepared. The desorbed chromium was estimated as before. The above procedure was repeated for five times to test the reusability of the HCB/TiO2 . 3. Results and discussion 3.1. Characterization of materials The XRD patterns of HCB, TiO2 , HCB/TiO2 which were obtained by the sol–gel method are shown in Fig. 1. The diffraction peak at 2 = 25.42◦ is assigned to the (0 0 2) plane of the HCB. The identified diffraction peaks of TiO2 are in good agreement with the bodycentered tetragonal TiO2 (JCPDS standard card no. 89-4921). The six new peaks of HCB/TiO2 at 25.42◦ , 37.98◦ , 47.94◦ , 53.72◦ , 54.90◦ , and 62.68◦ were observed and the characteristic peak of the HCB remained unchanged. The average crystal sizes of the samples are calculated with the Scherrer equation [3]. The crystallite sizes of HCB, TiO2 respectively are 49 nm and 16.8 nm. The TEM images of TiO2 , HCB and HCB/TiO2 at the same magnifications are shown in Fig. 2. The TEM images of TiO2 and HCB show spherical particles (Fig. 2(a and b)). The TEM image of HCB/TiO2 shows the TiO2 highly dispersed on the surface of the HCB (Fig. 2(c)). The average size of TiO2 and HCB nanoparticles is approximately 16 nm and 50 nm. The results are consistent with XRD. FTIR spectroscopy was applied to investigate the changes of surface functional groups of adsorbents HCB/TiO2 . The FTIR spectroscopy of HCB–HCB/TiO2 and Cr(VI)-HCB/TiO2 are shown in Fig. 3. The FTIR spectroscopy of HCB–HCB/TiO2 shows that the O H adsorption peak is ascribed to 342 cm−1 . The peaks at 292 cm−1 and 2853.7 cm−1 ascribed to C H stretching vibration. The adsorption of 1630 cm−1 adsorption band may be attributed to the stretching vibration of C O [36]. The adsorption band at 1115 cm−1 were assigned to C O C bending vibration. The bands at 1048 cm−1 corresponding to C O stretching vibration.

L. Zhang, Y. Zhang / Applied Surface Science 316 (2014) 649–656

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Fig. 1. X-ray diffraction pattern (XRD) pattern of the as-prepared HCB (a) and TiO2 (b) and HCB/TiO2 (c).

The FTIR spectroscopy of Cr(VI)-HCB/TiO2 shows that the FTIR adsorption spectra have some substantial changes such as the appearance of new absorption peak, the weakening or even disappearance of absorption peak after Cr(VI) adsorption, thus adsorption process cannot be simply regarded as pure physical adsorption. The O H adsorption peak is shifted from 3421.8 to 3418.3 cm−1 and significantly weakened after Cr(VI) is adsorbed. The peak at 2922 cm−1 , 2853.7 cm−1 ascribed to C H vibration significantly weakened and even disappeared after Cr(VI) is adsorbed. The adsorption of 1630 cm−1 adsorption band will disappear, new peaks appeared at 1627 cm−1 and 1573 cm−1 , which was attributed to the stretching vibration of Cr O [36]. The adsorption band at 1385 cm−1 ascribed to COOH vibration significantly weakened. The adsorption band at 1115 cm−1 disappeared after the Cr(VI) adsorption and meanwhile two peaks at 1189 and 1100 cm−1 appeared, which were assigned to C O C bending vibration. The bands at 1048 cm−1 corresponding to C O stretching significantly weakened. The substantial changes in FTIR spectra indicate that the functional groups participated in the adsorption process of Cr(VI). At the same time, the changes may prove that part of the sorbed Cr(VI) in the process was reduced to Cr(III) on the surface of HCB/TiO2 . 3.2. Effects of the adsorption experimental 3.2.1. Effect of the initial solution pH The pH value is considered to be the most important control parameter in the adsorption experiments, on account of its influence on the metal ions forms and the adsorbent surface properties [25,29]. The effect of pH may be attributed to surface charge of

HCB/TiO2 adsorbed Cr(VI) species, and the presence of acid and base used to adjust the pH of the solution. The influence of pH value on removal rate is shown in Fig. 4. At pH 2, Cr(VI) removal efficiency was 97.4%. As the pH value increased from 3 to 11, removal efficiency decreased significantly from 97.4 to 7.0%. Similar adsorption phenomena were observed by other investigators [27,28]. The chromate has different forms in the solution with different pH values and chromium concentration [29]: H2 CrO4 ↔ H+ + HCrO4 − −

+

HCrO4 ↔ H + CrO4

2−

2HCrO4 − ↔ Cr2 O7 2− + H2 O

(R1) (R2) (R3)

At pH 2–6, there are different forms of chromium ions such as CrO4 2− , Cr2 O7 2− , HCrO4 − , however, HCrO4 − is the predominant form in solution [26,30]. Meanwhile, HCrO4 − is the more favorable adsorption because of the low adsorption free energy [27]. As the pH increases this form shifts to CrO4 2− and Cr2 O7 2− [30]. The low pH increases H+ ion concentration on HCB/TiO2 surface as a result of increasing the surface charges of the functional groups [31]. In the acidic range, where the Cr(VI) is predominately in the form of HCrO4 − , the removal is found to decrease exponentially with increased pH, since HCrO4 − ions have a greater affinity toward the H+ ion present on the HCB/TiO2 surface. Adsorption of Cr(VI) on HCB/TiO2 was not significant at pH values greater than 6 due to dual completion of the anions (CrO4 2− , Cr2 O7 2− , and OH− ) to be adsorbed on the surface of the adsorbent, of which OH− predominates. The other important influence factor for Cr(VI) adsorption removal is point of zero charge (pHpzc ) of HCB/TiO2 . When the pH

Fig. 2. TEM images of HCB (a), TiO2 (b), HCB/TiO2 (c).

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Fig. 5. Effect of adsorbent dose on the adsorption of Cr(VI) by HCB and HCB/TiO2 and TiO2 (conditions: Cr(VI) solution concentration = 10 mg L−1 ; adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm; pH 2). The dotted lines and solid lines correspond to Qe and Cr(VI) removal efficiency, respectively.

Fig. 3. The FTIR spectra of the HCB–HCB/TiO2 and the Cr(VI)-loaded HCB/TiO2 .

value was lower than pHpzc , the HCB/TiO2 surface was protonated and positively charged, and as a result Cr(VI) was removed by electrostatic attraction. When the pH value of the solution was higher than pHpzc , the positive charge of the surface of the adsorbent weakened, then the electrostatic attraction was reduced, thus removal efficiency (7.0%) was decreased (Fig. 4). It is also hypothesized that maybe Cr(VI) can be reduced to Cr(III) because of electron donor functional groups of HCB/TiO2 and high redox potential value (>+1.3 V at standard condition) of Cr(VI) [15,28] as well.

Fig. 4. Effect of initial solution pH on Cr(VI) ions adsorption by HCB/TiO2 (conditions: Cr(VI) solution concentration = 10 mg L−1 ; adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm).

3.2.2. Effect of adsorbent dosage The dosage of adsorbent for removing effect is shown in Fig. 5. When the dosage of the HCB/TiO2 increased from 0.5 to 2 g L−1 , Cr(VI) removal efficiency increased from 63.8% to 97.4%. However, over 2 g L−1 , the removal efficiency would not increase significantly, so 2 g L−1 was identified as the best adsorption. When compared to HCB and TiO2 , HCB/TiO2 shows much higher removal efficiency, and the adsorption capacity of Cr(VI) is three times that of the former at sorbent dose of 2 g L−1 . The high removal efficiency can be explained that HCB/TiO2 possesses more specific surface area and functional groups which may be due to the high surface area and the bigger number of hydroxyl groups present on the TiO2 sample at pH 2 [19]. The functional groups include acidic functional groups such as hydroxyl and carbanyl groups on the surface, then it provides more adsorption active sites for Cr(VI) [13]. 3.2.3. Effect of contact time and initial Cr(VI) concentration The relationship between the contact time and the removal of Cr(VI) is described in Fig. 6 when the Cr(VI) concentrations are 10.0, 20.0 and 30.0 mg L−1 . The rate of adsorption was found to be slow and finally approached equilibrium at 120 min. The maximum adsorption was found to be 2.31, 6.32, 9.93 mg g−1 , respectively. The removal efficiency of Cr(VI) increased significantly within 120 min, which may provide sufficient adsorbent as a contact point, and there is a big difference between the concentration of the aqueous phase and a solid phase [25].

Fig. 6. Effect of contact time for Cr(VI) adsorption onto HCB/TiO2 at three different concentrations (conditions: adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm; pH 2).

L. Zhang, Y. Zhang / Applied Surface Science 316 (2014) 649–656

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Fig. 7. Adsorption kinetic plots for adsorption of chromium on HCB/TiO2 . Pseudo-first-order kinetics (a); pseudo-second-order kinetics (b); intra-particle diffusion kinetics(c) (Conditions: adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm; pH 2).

3.3. Adsorption kinetics

due to physical adsorption or ion exchange at absorbent surface. Thus there may be more than one mechanism participating in the actual adsorption process. A similar conclusion has been reported in the adsorption of Cr(VI) onto activated carbons derived from wheat-residue derived black carbon [10].

The pseudo-first-order and pseudo-second-order were often applied as adsorption kinetics models [13,25] (Eqs. (4) and (5)): ln(qe − qt ) = ln qe − k1 t

(4)

1 1 t = + 2t qt k2 qe 2 qe

(5)

3.4. Intraparticle diffusion model Adsorption process may be not only a diffusion (external mass transfer), but it also may be controlled by the intraparticle diffusion [31]. The intraparticle diffusion model is used to confirm the diffusion mechanism. The equations can be defined by Eq. (6):

where qe and qt (mg g−1 ) are the amounts of Cr(VI) adsorbed at equilibrium and at time t (min), respectively, and k1 (min−1 ) is the rate constant of the pseudo-first-order model and k2 (g (mg min)−1 ) is the rate constant of the pseudo-second-order model. The parameters of qe and k1 and k2 can be calculated from the intercept and slope of a plot of ln(qe − qt ) versus t and (t/q) versus t, respectively. Adsorption kinetics data were discussed contact time and initial Cr(VI) concentration (10, 20, 30 mg L−1 ) by HCB/TiO2 (2 g L−1 ). The results of two kinetic models for the adsorption of Cr(VI) at different initial concentrations are presented in Fig. 7(a) and (b). All kinetic parameters are listed in Table 1. The correlation coefficients (R2 ) were compared to find that the pseudo-secondorder model fits the experimental data (R2 > 0.996) quite well and in agreement with qe and Qexp . The pseudo-second-order model is considered to be an ideal model for the adsorption of Cr(VI) by HCB/TiO2 . The pseudo-second-order equation may be used to explain chemisorption processes [10]. However, rapid growth stage in the first 120 min of adsorption rapid growth stage may be mainly

qt = kdif t 0.5 + C

(6)

where qt (mg g−1 ) is the amount of Cr(VI) adsorbed at time t (min), kdif (mg (g min0.5 )−1 ) is the intraparticle diffusion rate constant and C (mg g−1 ) is the intercept, which represents the thickness of the boundary layer. The greater the C value means that the greater the influence of boundary layer. The plot of qt versus t0.5 is a straight line that passes through the origin which illustrates the intra-particle diffusion process is the rate-limiting step [13]. Table 1 shows the intra-particle diffusion rate constant (kdif ) increased from 0.46 to 1.28 (mg (g min0.5 )−1 ) with the initial chromium concentration from 10 to 30 mg L−1 is due to the availability of free sites on the external surface and easily accessible site [16,31]. The larger kdif values suggest a better

Table 1 Comparison of rate constants calculated based on pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. Parameter −1

Ci (mg g

−1

10 mg g 20 mg g−1 30 mg g−1

)

First-order kinetics −1

Qexp (mg g 4.87 9.63 14.39

)

Second-order kinetics −1

K1 (min)

qe (mg g

0.03393 0.02783 0.03035

4.995 8.876 13.391

)

2

−1

Intraparticle diffusion kinetics −1

R

K2 (g(mg min))

qe (mg g

0.99407 0.99602 0.99726

0.00485 0.00232 0.00179

6.281 12.293 17.981

)

R

kdif (mg(g min−0.5 )−1 )

C (mg g−1 )

R2

0.99723 0.99837 0.99797

0.4591 0.8719 1.2763

0.31035 0.66815 1.2237

0.92305 0.93401 0.94022

2

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L. Zhang, Y. Zhang / Applied Surface Science 316 (2014) 649–656

Table 2 Isotherm parameters for the Cr(VI) removal by HCB/TiO2 . Langmuir equation

Freundlich equation

Qm (mg g−1 )

B (L g−1 )

R2

RL

Kf ((mg g−1 ) (mg L−1 )1/n )−1

n

R2

31.84

0.744

0.9745

0.093

78.17

2.04

0.9921

adsorption mechanism, which is related to an improved bonding between Cr(VI) ions and the adsorbent particles [31]. The relationship between HCB/TiO2 and Cr(VI) is non-linear at the entire time range, which suggests that maybe there is more than one process on the adsorption. The C values decrease with the increase of initial Cr(VI) concentration, the thickness of boundary layer is decreased and thus increase the likelihood of external mass transfer energy [24,35]. 3.5. Cr(VI) adsorption isotherm The adsorption isotherms were often used the Langmuir [19] and Freundlich [25] equations, respectively. The equations can be defined by Eqs. (7) and (8): qe =

qm bCe 1 + bCe

qe = kf Ce

1 n

(7) (8)

where qm (mg g−1 ) is maximum adsorption capacity and b (L g−1 ) is the Langmuir constant. The parameters of kf ((mg g−1 ) (mg L−1 )1/n)−1 and n are the constants of Freundlich equation, respectively. The most essential factor of Langmuir isotherm is the calculation of the equilibrium parameter (RL ) which can be represented as: RL =

1 1 + BC0

(9)

where B is the Langmuir constant and C0 is the initial Cr(VI) concentration. The RL values (0 < RL < 1) presented in Table 2 also indicate a favorable adsorption process [25]. The adsorption isotherms and isotherm parameters are shown in Fig. 8 and Table 2. Langmuir isotherm describes that the monolayer adsorption or adsorption occurs in a fixed number of sites, and all the adsorption sites are identical [13]. The Freundlich adsorption isotherm describes surface heterogeneity of the adsorbent and adsorption capacity is associated with the concentration of Cr(VI) in the solution at equilibrium [27]. The correlation coefficient (R2 = 0.992) (Fig. 8) of the Freundlich isotherm is superior to

Fig. 9. Effect of temperature on the adsorption capacity of Cr(VI) and Thermodynamic plot for the adsorption of Cr(VI) (Conditions: adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm; pH 2).

the Langmuir isotherm, indicating that the experimental data are more consistent with the Freundlich isotherm model. The better fitting of the Freundlich isotherm indicates that the surface of the HCB/TiO2 is likely to be heterogeneous. 3.6. Thermal analysis The effect of temperature and thermodynamic plot for the adsorption of Cr(VI) is shown in Fig. 9. The results show that the qe increased slightly with temperature rising from 20 to 45 ◦ C and indicate the removal process was endothermic. As the temperature increases, the mobility of Cr(VI) is also enhanced, which may acquire more energy to exchange Cr(VI) ions on adsorbent surface. Thermodynamic parameters including Gibbs free energy of adsorption (G), changes in enthalpy of adsorption (H) and changes in entropy of adsorption (S), are calculated from Eqs. (10) and (11) [34,35]: S −G + RT R

(10)

G + H − T S

(11)

ln k =

where Kd is the distribution coefficient for the adsorption, T is the absolute temperature, and R (8.314 J mol−1 K−1 ) is gas constant. A plot of ln Kd versus 1/T according to Eq is shown in Fig. 9. The values of H and S calculated from the slope and intercept of linear plot of ln Kd versus 1/T (R2 = 0.992) and the results are listed in Table 3.

Table 3 Thermodynamic parameters for the adsorption of Cr(VI) onto HCB/TiO2 .

Fig. 8. Freundlich isotherm of the Cr(VI) removal by HCB/TiO2 (Conditions: adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm; pH 2).

T (◦ C)

Kd

G (kJ mol−1 )

H (kJ mol−1 )

S (kJ mol−1 K−1 )

20 25 30 35 40 45

0.14 0.17 0.23 0.28 0.34 0.37

−5.74 −5.86 −6.01 −6.14 −6.27 −6.41

2.03

0.027

L. Zhang, Y. Zhang / Applied Surface Science 316 (2014) 649–656 Table 4 Comparison of adsorption capacity of Cr(VI) with other adsorbents.

655

4. Conclusions

Adsorbent

% adsorption

Qm (mg g−1 )

pH

C (mg L−1 )

Reference

C/TiO2 HDTMA BC RAC HCB/TiO2

94.8 90 86 89.2 97.4

18.1 17.6 21.34 44.05 27.33

2–6 3 1 2 2

40 50 100 200 30

[1] [3] [30] [11]

In this work, HCB/TiO2 was successfully prepared by sol–gel method and applied to remove Cr(VI) in the water. The HCB/TiO2 showed an effective Cr(VI) removal performance. The efficiency of adsorption removal of Cr(VI) has a close relationship with the pH, temperature, and the quality of absorbent. The maximum adsorption capacity for Cr(VI) was 27.33 mg g−1 at pH 2. The pseudo-second-order kinetic and Freundlich isotherm models were found to be more in line with the experimental data. The adsorption of Cr(VI) ions on the HCB/TiO2 , decreased from 25 to 45 ◦ C. The negative values of G indicated that the Cr(VI) adsorption process is spontaneous. The positive value of S revealed the increase in randomness at the solid solution interface by the adsorption of Cr(VI) onto the HCB/TiO2 . The positive values of H indicated an endothermic nature of the adsorption process. Desorption results show that the adsorption capacity can maintain to 80% after 5 times usage.

References

Fig. 10. Adsorption–desorption cycles (conditions: adsorbent dose = 2 mg in 100 mL solution; T = 25 ◦ C; string speed = 150 rpm; pH 2).

The positive values of H indicate an endothermic nature of the adsorption process. The negative values of G reveals that the degree of spontaneity of the adsorption process. The positive value of S suggests the increase in randomness at the solid solution interface by the adsorption of Cr(VI) onto the HCB/TiO2 .

3.7. Comparison of HCB/TiO2 with other adsorbents The adsorption capacity of Cr(VI) onto HCB/TiO2 was compared with other adsorbents reported in literature and is shown in Table 4. According to the data, it can be seen that a pH range of 1–3 was found to be an optimum in nearly all cases. This is in line with the fact that the Cr(VI) gets reduced to a large extent at an acidic pH[32,33]. HCB/TiO2 can be considered to be a viable adsorbent for removal of Cr(VI) from solutions.

3.8. Desorption performance of the HCB/TiO2 The reusability of the HCB/TiO2 is important from economic point of view as it may reduce the overall cost of the adsorbent. The solid was used for five successive adsorption-regeneration cycles. Because the lower pH is in favor of the Cr(VI) adsorption, the desorption of Cr(VI) ions from the adsorbent can be achieved by increasing the system pH values. Therefore, for the reusability study, NaOH should be used in experiments [31]. The recycling of HCB/TiO2 in the removal of Cr(VI) is shown in Fig. 10. The efficiency of adsorption removal of Cr(VI) still remained 80.0% after five cycles of reuse, which indicates that HCB/TiO2 has a good reusability for Cr(VI) adsorption. The efficiency of adsorption removal of Cr(VI) of the HCB/TiO2 decreased by 8.66% in the third adsorption desorption cycle, reflecting the high adsorption and stability of HCB/TiO2 . The desorption for the adsorbent treated with 10 mg L−1 of Cr(VI) at pH 2 and at a temperature of 323 K was less than 6%.

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