TiO2-Based Sorbent of Lead Ions - CORE

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ScienceDirect Procedia Materials Science 12 (2016) 147 – 152

6th New Methods of Damage and Failure Analysis of Structural Parts [MDFA]

TiO2-based sorbent of lead ions Jana Seidlerováa, Ivo Šafaříkb, Lucia Rozumováa, Mirka Šafaříkováb, Oldřich Motykaa* b

a VŠB –Technical University of Ostrava, 17. listopadu 15/2172, Ostrava-Poruba 708 33, Czech Republic Institute of Nanobiology and Structural Biology of GCRC AS CR, Na Sádkách 7, České Budějovice 370 05, Czech Republic

Abstract The non-magnetic TiO2 powder and magnetically modified TiO2 powder were employed for the sorption experiments. FeSO4.7H2O was used for synthesis of magnetically responsive TiO2. The non-magnetic and prepared magnetic materials were characterized by scanning electron microscopy and X-ray diffraction methods. The particle size and specific surface area were determined. A detailed study of the adsorption process performed using batch adsorption experiments was carried out with various concentrations of lead ions and contact time. A flame atomic absorption spectrometer was used for determination of Pb2+ ions concentration in solution. Adsorption process has been modeled by the Langmuir and Freundlich isotherms using linear and non-linear regression. The results showed that the adsorption of Pb2+ ions on the magnetic and non-magnetic TiO2 particles occurred in a monolayer. Presence of magnetic iron oxides particles on the surface of sorbents increased the adsorption rate, and increased the maximum amount of adsorbed Pb2+ ions per mass in comparison with adsorption on the non-magnetic TiO2 particles. The magnetically modified TiO2 particles allow magnetic separation of the sorbents with already adsorbed ions of pollutants. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava,Faculty of Metallurgy and Materials Engineering Keywords: TiO2, sorption, lead ions

1.Introduction Heavy metals are non-biodegradable pollutants with a great environmental impact. For the removal of such pollutants dispersed in wastewater, sorption is applied as one of the most important and effective techniques. Special kind of sorbents of metals ions are nanoparticles of metal oxides. Nanoparticles of titanium dioxide (occurring in three modifications: anatase, brookite and rutile) are considered not only as a suitable and effective photoactive

* Corresponding author. Tel.: +420-597-321-559; E-mail address: [email protected]

2211-8128 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the VŠB - Technical University of Ostrava,Faculty of Metallurgy and Materials Engineering doi:10.1016/j.mspro.2016.03.026

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Jana Seidlerová et al. / Procedia Materials Science 12 (2016) 147 – 152

material but also as an adsorbent for removal of toxic ions from aqueous solutions due to their high stability, low cost and safety toward both humans and the environment (Augugliaro, 1999; Klaasen and Watkins, 2010). The anatase can be used for water cleaning and in various technology applications such as photovoltaic solar cells, sensors and photocatalytic processes (Fujishima et al., 1999). Some authors have studied the sorption properties of titanium dioxide particles recently. The batch techniques were also used for the adsorption/desorption of Pb2+, Cu2+ and Zn2+ ions from commercially prepared TiO2 nanoparticles (anatase). The isothermal, kinetic and thermodynamic properties were investigated in the study of Hu et al. (2012). Titanium dioxide nanoparticles were employed for the sorption of Te4+ ions from aqueous solution. A detailed study of the process was performed by variation of the sorption time, pH, and temperature. The sorption was found to be rapid – equilibrium was reached within 8 min. The sorption data could be well interpreted by the Langmuir model with the maximum adsorption capacity of 32.75 mg.g−1 of Te4+ ions on nano-TiO2 (Zhang et al., 2010). Hydrous TiO2 materials and their application for the sorption of inorganic ions were described by Tsydenov et al. (Tsydenov et al., 2014). Hydrous TiO2 adsorbent samples were prepared and adsorption of ten different anions and cations from water was studied. Sorption of selenium oxyanions on rutile was studied by batch or column experiments and spectroscopic methods (Svecova et al., 2011). Sorption of other ions on TiO2 in aqueous solutions was described by Konstantinou et al. (2008), Choi et al. (2006) and Tan et al. (2007). Their studies focused not only on the common ions (Cu2+ or Pb2+ ions) but on uncommon ones as well, e.g. europium, thorium or uranium ions. To improve the separation of solid sorbents from liquids, magnetic sorbents were prepared. Magnetic nanoparticles, or magnetic particles in general, are exceptional adsorbent materials due to their unique magnetic properties and good adsorption capacity. For example, the magnetic hydroxyapatite nanoparticles are used for the removal of cadmium and zinc ions from aqueous solutions (Yuan, 2012). The magnetic nano-adsorbent was prepared from an agricultural waste (orange peel powder) by co-precipitating with Fe3O4 nanoparticles and it was used successfully for the removal of cadmium ions from aqueous solutions (Gupta and Nayak, 2012). Presented study is focused on the preparation of the magnetically modified TiO2 particles and verification of their sorption properties. Batch adsorption techniques were used to study sorption of Pb2+ ions from aqueous solution on both non-magnetic and magnetic TiO2 particles. 2.Experimental 2.1.Adsorbents and adsorbates TiO2 particles were prepared by titanyl sulphate (TiOSO4) thermal hydrolysis (Mamulová Kutláková, 2010). The first step was heating the solution up to 100 °C. The second step included 90 minutes thermal hydrolysis of solution at 100 °C performed by addition of the appropriate volume of warm water. After the thermal hydrolysis, the solution was cooled down. The prepared TiO2 was washed several times with distilled water. The obtained sample was dried at 105 °C (Mamulová Kutláková et al., 2010). The magnetically modified particles of TiO2 were prepared according to the described procedure. 0.36 g FeSO4.7H2O was dissolved in 100 ml of water in a 600–800 ml beaker. One gram of TiO2 was added and a solution of sodium hydroxide (1 mol.L-1) was dropped slowly under mixing until the pH value reached the value ca 12; during this process a precipitate of iron hydroxides was formed. Then the suspension was diluted up to 200 ml with water and inserted into a standard kitchen microwave oven (700 W, 2450 MHz). The suspension was treated for 10 min at the maximum power. The formed magnetically responsive composite was captured using an appropriate magnetic separator or NdFeB magnet. Then the magnetic TiO2 formed was washed with water and air dried at ca 60 oC (Safarik, 2013). Model solution of Pb2+ ions was prepared from Pb(NO3)2, analytical grade. 2.2.Methods of characterization The X-ray powder diffraction (XRPD) patterns were recorded under CoKα irradiation (λ = 1.789 nm) using Bruker D8 Advance diffractometer (Bruker AXS, Germany) equipped with fast position sensitive detector VÅNTEC 1. Measurements were carried out in reflection mode, powder samples were pressed in a rotational holder. Phase


composition was evaluated using database PDF 2 Release 2004 (International Centre for Diffraction Data). Shape of the non-magnetic and magnetic TiO2 particles was studied by scanning electron microscope (XL 30, with X-Ray Micro-Analyser EDAX, PHILIPS, Holland). The specific surface area was measured with the Sorptomatic 1990 using nitrogen and calculated by the Advance Data Processing software according to the BET isotherm. Particle size was measured by Laser Scattering Particle Size Distribution Analyzer (Horiba LA-950). The content of Fe in the magnetically modified TiO2 was determined by AES - ICP SPECTRO CIROS VISION after acid decomposition with acid mixture (HNO3, concentrated, analytical grade and HCl, concentrated, analytical grade). The content of FeO was determined using titration with 0.1 M solution of K2Cr2O7 after previous sample decomposition in concentrated HCl and concentrated HF (both analytical grade) in carbon dioxide atmosphere, according to the Czech standard CSN 722041, Part 1. 2.3.Sorption experiments Both the non-magnetic and magnetically modified powdered TiO2 were employed for the sorption experiments carried out with the 0.05 g of each sorbent suspended in 50 ml solution of different Pb2+ ions concentrations (1 2000 mg.L-1) at the laboratory temperature. The suspensions were stirred for one hour. The length of stirring was determined according to preliminary kinetic experiments. Then the adsorbents were separated by filtration through a 0.45 µm membrane filter (Whatman EO47). The concentration of Pb2+ ions in filtrate was determined by atomic absorption spectroscopy with flame atomization (UNICAM 969) after stabilization of solution by HNO3, analytical grade, in all experiments 2.4.Data processing The amount of adsorbed metal per gram of adsorbent (q expressed in mg.g-1) was calculated from the experimental data of Pb2+ ions concentration at equilibrium as follows: c −c (1) q= o m ads where co and c are the metal concentrations in liquid phase before and after adsorption experiments (expressed in mg.L-1), respectively, and mads is mass of adsorbent in the solution (expressed in g). The experimental data were fitted by adsorption isotherms or kinetic models to determine the isotherms coefficients. The isotherm/kinetic coefficients were calculated by regression statistics using ADSTAT (Meloun and Militký). The coefficient of determination (R2) was used to describe the best corresponding model calculated by linear regression. Nonlinear regression based on a trial and error procedure was also applied. The coefficient of determination (D2), Akaike information criterion (AIC) and mean square of error (MEP) were used to describe the best corresponding adsorption isotherm. 3.Results and discussion 3.1.Characterization of sorbents The amount of iron in magnetic TiO2 was determined to be 6.47 ± 0.25 wt. %. Powder diffraction proved that iron in magnetic material occurs in the form of Fe3O4. The magnetic TiO2 particles were different in shape. Particles of iron oxides were present on the TiO2 surface as particles smaller than 1 μm. The maximum size of both prepared types of TiO2 particles was different; while the size of non-magnetic particles ranged from 0.051 μm to 7.70 μm, the size of the magnetic TiO2 particles was from 0.115 μm to 6.72 μm. The specific surface area of the non-magnetic TiO2 particles (84.9 m2.g-1) is smaller than that of the magnetic TiO2 (173 m2.g-1), so probably the process of magnetization altered the size of particles.

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3.2.Adsorption isotherms The experimental data were processed by two-parameter theoretical models. The most commonly used isotherm is the Langmuir isotherm (Langmuir, 1918):

q = qm

K Lc

(2)

1 + K L .c

where q (expressed in mg.g-1) is the amount of the adsorbed metal ion per unit mass of sorbent and c (expressed in mg.L-1) is the unadsorbed lead ions concentration in final solution at equilibrium; qm is the maximum amount of the metal ion per unit mass of adsorbent to form a complete monolayer on the surface bound at high lead ions concentration and KL is a constant related to the affinity of the binding sites (expressed in L.mg-1). The Freundlich isotherm model (Freundlich, 1906), as an empirical equation based on sorption on a heterogeneous surface, is often used to describe the adsorption of ions/molecules on surfaces:

q = K Fc1/β

(3)

where q is the amount of metal ion adsorbed per unit mass of adsorbent, KF and β are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively, and c is the concentration of unadsorbed metal ion in final solution at equilibrium. The Langmuir and Freundlich constants can be calculated from experimental data by non-linear regression or by converting Eq. (2) and (3) into linear form. The values of Langmuir constants can be calculated from the plot between c/q versus c and the Freundlich constant from the plot between lnq versus lnc, respectively. Isotherm parameters and coefficient of determination calculated by linear and non-linear regression are presented in Table 1. The calculated amount of adsorbed metal per gram of adsorbent and the Langmuir isotherm lines (calculated by linear and no-linear regression) are presented in Fig. 1a. The maximum amount of Pb2+ ions per unit mass of the non-magnetic adsorbent, calculated both by linear regression (121 mg.g-1) and non-linear regression (127 mg.g-1), is very similar. The coefficient of determination of the Freundlich isotherm in the linearized form was relatively high as well (R2=0.969). Table 1. Isotherm parameters of lead ions sorption on the non-magnetic and magnetic TiO2 particles calculated by linear and non-linear regression. Sorbent non-magnetic TiO2 magnetic TiO2

non-magnetic TiO2

magnetic TiO2

Linear regression qm (mg.g ) KL (L.g-1) R2 qm (mg.g-1) KL (L.g-1) R2

Langmuir isotherm 121 0.010 0.996 161 0.038 0.991

qm (mg.g-1) KL (L.g-1) D2 AIC MEP qm (mg.g-1) KL (L.g-1) D2 AIC MEP

Langmuir isotherm 127 0.0081 0.981 38.7 53.2 162 0.021 0.577 67.6 1196

-1

Freundlich isotherm 6.10 KF (mg1-1/ .L1/ .g-1) 2.34 β R2 0.969 54.2 KF (mg1-1/ .L1/ .g-1) 6.38 β R2 0.693 Non-linear regression Freundlich isotherm 14.0 KF (mg1-1/ .L1/ .g-1) 3.34 β D2 0.904 AIC 54.7 MEP 266 72.1 KF (mg1-1/ .L1/ .g-1) 9.13 β D2 0.879 AIC 56.3 MEP 674 β

β

β

β

β

β

β

β


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Fig. 1. Experimental adsorption data () and isotherms of Pb2+ ions; (a) the Langmuir isotherms of sorption on the non-magnetic TiO2 calculated by linear regression (dashed line) and non-linear regression (continuous line); (b) the Langmuir adsorption isotherm calculated by linear regression (dashed line) and the Freundlich isotherm calculated by non-linear regression (continuous line) of Pb2+ ions on the magnetic TiO2

When the magnetically modified TiO2 particles were used as an adsorbent of Pb2+ ions, there were found differences between the results of linear and non-linear regressions adsorption isotherm models. Only linearized form of the Langmuir isotherm corresponded with adsorption data. However, the better fitting non-linear model was found to be the Freundlich isotherm (D2=0.879) whilst the Langmuir isotherm had the coefficient of determination significantly lower (D2=0.577). The Fig. 1b shows the experimental data and the Langmuir adsorption isotherm (calculated by linear regression) and the Freundlich isotherm (calculated by non-linear regression). On the other hand, the calculated maximum amount of Pb2+ ions adsorbed on the magnetic TiO2 by linear (161 mg.g-1) and nonlinear regression (162 mg.g-1) is similar and still higher than the maximum amount of Pb2+ ions adsorbed on the nonmagnetic TiO2. The conclusion is the same if KF calculated by linear and non-linear regression for the non-magnetic and magnetic TiO2 are compared. These results indicate that presence of iron oxides on sorbent surface affected adsorption of Pb2+ ions. Surface of magnetically modified TiO2 particles is heterogeneous and so the Freundlich isotherm is theoretically better model for describing adsorption of Pb2+ ions. This conclusion was confirmed by results of non-linear regression. Calculated isotherms parameters indicate that the adsorption of Pb2+ ions on the non-magnetic and magnetic TiO2 particles occurs in monolayer. 4.Conclusion The magnetically modified TiO2 powder was prepared from TiO2 using a microwave-assisted procedure. Both the non-magnetic and magnetically modified powder of TiO2 were employed for the study of sorption of Pb2+ ions from aqueous solution by batch technique. Adsorption process has been modelled by the Langmuir and Freundlich isotherms. The linear and non-linear regressions were used to calculate isotherms parameters. The Langmuir isotherm model fits well the adsorption on the non-magnetic TiO2. The maximum amount of Pb2+ ions per unit mass of the non-magnetic adsorbent, calculated by both linear regression (121 mg.g-1) and non-linear regression (127 mg.g-1), is very similar. Only the linearized form of the Langmuir isotherm corresponded with the adsorption on the magnetic sorbent. However, the better fitting non-linear model was found to be the Freundlich isotherm whilst the Langmuir isotherm had the coefficient of determination significantly lower. The results of non-linear regression correspond with theoretical assumption better because the Freundlich isotherms model describes adsorption on heterogeneous surface. On the other hand, the calculated maximum amount of Pb2+ ions adsorbed on the magnetic TiO2 by linear (161 mg.g-1) and non-linear regression (162 mg.g-1) is similar and still higher than maximum amount of Pb2+ ions adsorbed on the non-magnetic TiO2. The amount of absorbed Pb2+ ions per unit surface area of sorbent showed that that adsorption of Pb2+ ions occurred as monolayer. The magnetically modified TiO2 particles are suitable material for removing of lead from aqueous solution. In addition to the biodegradable and photocatalytic properties of TiO2 particles, the magnetically

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