In this study, nano-TiO 2 particles were synthesized by sol-gel method. The synthesized nanoparticles were characterized by Fourier Transform Infrared (FT-IR), X-ray diffraction (XRD), Transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET). The results showed that the average size of TiO 2 nanoparticles and their specific surface area were 21.1 nanometer and 55.35 m 2/gr, respectively. The effects of several variables such as adsorbent weight, pH and contact time on lead ions adsorption were studied in batch experiments and finally the optimum conditions for lead ions adsorption by synthesized nano-TiO 2 were obtained. The results showed that the synthesized nano TiO 2 had a good capacity to adsorb lead ion. The kinetic data were described by pseudo-first and second-order models. Freundlich and Langmuir isotherm models were used for the analysis of equilibrium data, and results showed that the Langmuir model was suitable for describing the equilibrium data of lead ion adsorption by nano TiO 2. Using the Langmuir isotherm, the maximum sorption capacity of Pb 2+ was estimated to be 7.41 (mg/g) at 25 °C.
The increase of pollutant concentrations in water resources is one of the most serious environmental problems worldwide. Heavy metals are one of the major chemical pollutants that cause acute toxicity [
Among the different adsorbent materials used for heavy metals removal, nano-sized materials are being widely used due to their properties [
The conducted studies have showed that TiO2 nanoparticles and composites containing TiO2 have a good capability to adsorb heavy metals from aqueous solutions. For example, adsorption of lead ion by bauxite containing 3.12% TiO2 was examined by Wang et al. (2008) [
In this study, the sol-gel method was applied to synthesize TiO2 nanoparticles. In this method, the nanoparticles were uniform both in size and shape and the synthesis procedure was simple to use. The aim of this study was to use synthesized nano TiO2 for the adsorption of lead from aqueous solution under batch conditions. The structure of the synthesized nanoparticles was characterized using TEM, XRD, BET and FT-IR. Moreover, the effects of pH, contact time and adsorbent weight on adsorption process were investigated along with studying the lead ions desorption. All the experiments were carried out in the laboratory of the Faculty of Environment and Energy, Science and Research Branch of Tehran Islamic Azad University.
All the selected reagents were of analytical grade and purchased from Merck. The stock solutions for preparation of lead solution were prepared by dissolving Pb(CH3OO)2∙3H2O in deionized water.TiCl4 and NH4 were used to synthesize TiO2 nanoparticles by the sol-gel method. For adjusting pH, 1 M HNO3, NaOH and NH3 solutions as well as a Metrohm pH meter model 744 were used. Sartorius Electrical Balans Model BP 221S, Laboren oven, mixer HT Infors AG model CH-4103-BOT Tmingen, Centrifuge model MSE ministral1000 were used to conduct the experiments, and analysis of heavy metals was carried out using Inductivity Coupled Plasma (ICP) model Optima 2000 DV.
TiCl4 (30 mL) was added to deionized water (1L) under vigorous stirring (1000 rpm). pH of the solution was adjusted by adding NH3 (drop wise) to reach 7.8, and the mixing was continued until gel was formed. The gel was left for 7 days until colloidal sediment of TiOH2 was formed. Then, TiOH2 sediment was separated by filtration. The reaction was performed as presented by Equation (1).
The separated sediment was placed in oven for 24 hours at 70˚C. Then, calcination was performed at 400˚C for 4 hours.
Adsorption experiments were performed by adding 0.15 g of adsorbent to 50 mL of solution with the initial Pb2+ ions concentration of 25 mg/L in a flask. The effect of pH on sorption ions was studied in the range of 3 - 6.5, at the temperature of 25˚C and contact time of 4 h. The effect of contact time was investigated by varying the time from 10 to 240 min, at a temperature of 25˚C, with the obtained pH values. The effect of adsorbent weight on sorption metal ions was studied in the range of 1 to 4 g/L (0.05, 0.10, 0.15 and 0.20 g of adsorbent in 50 mL of metal ions solution) at the contact time of 4 h, temperature of 25˚C and the obtained pH values.
The concentration of lead ions before and after equilibrium sorption was determined using ICP. The uptake percentages of the lead ions were calculated according to Equation (2) [
where, C0 and Ce are initial and equilibrium concentrations of ions (mg/L), respectively.
Characterization of crystalline size of the adsorbent was determined by XRD (model STV_MP STOE Company, Germany).For this purpose, Cu radiation (λCu = 1.5405 A) was used and the sample was scanned in a 2θ range of 8 - 108.5˚ at a scanning rate of 0.015˚/S. The crystalline size was determined from the characteristic peak at 2θ = 25.326˚ (corresponding to the 440 plane) using Scherrer formula crystalline size, nm = Kλ/W cosθ, where K is shape factor = 0.9, λ is wavelength of the X-ray used (1.5405), and W is (Wb-Ws, width of peak at half-height at 2θ = 25.326) the difference of broadened profile width of the experimental sample and the standard width of reference TiO2 sample(reference code 01-073-1764, pdf2-2003) [
The TEM images (taken by PHILIPS, EM 208) of synthesized nano TiO2 are illustrated in
Specific surface area was determined through nitrogen adsorption isotherms method. Using the BET (model
Quantachrome NOVA 2200e) method, the surface area of the sample was calculated to be 55.35 m2/g. Also, the pore size distribution was attained by Barrett-Joyner-Halenda (BJH method revealed the mesoporosity) [
Optimization of the initial pH value of the adsorption is an important parameter that allows for obtaining a high adsorption capacity. The effect of pH on lead ions adsorption is shown in
The effect of contact time on ions adsorption by nano-structured TiO2 was studied. Lead ions adsorption from aqueous solution, which had been adjusted to the nano-structured TiO2 (0.15 g in 50 mL) at optimum pH, was studied at different shaking times in the range of 10 - 240 min (
The effect of adsorbent amount on adsorption rate was examined by a series of experiments performed using different amounts of nano TiO2 (0.05, 0.10, 0.15 and 0.20 g) (
The results showed the high adsorption efficiency of nano TiO2 for lead ions. This could be ascribed to the uniformity of size and shapes of nanoparticles.
The aqueous solutions (50 mL) containing the lead ions (25 mg/L), kept under the optimum experimental condition, were stirred with 0.15 g of the adsorbent for 240 min at 25˚C. Then, desorption studies were carried out using 50 mL of 1 M HNO3 during 1-hour mixing time. It was found that the adsorbed ions could be quantitatively stripped by contacting nitric acid (
where, qe and qt are amounts of the lead adsorbed onto the sorbent (mg/g) at equilibrium and at time t respectively, and k1 is the rate constant of the first-order adsorption (min−1). The straight line plots of
where, K2 is the rate constant of adsorption (gr∙mg−1∙min−1), qe is the amount adsorbed at equilibrium, and qt is the amount adsorbed at any time. The equilibrium adsorption amount (qe) and the pseudo-second-order rate parameters (K2) can be calculated from the slope and intercept of t/qt plotted versus t. The values of constants and calculated correlation coefficients for pseudo-second-order are presented in
Metal Ion | Adsorption | Desorption |
---|---|---|
Pb2+ | 82.5% | 63% |
Pseudo-first-order | Pseudo-second-order | ||||
---|---|---|---|---|---|
qe (mg/g) | K1 (min−1) | R2 | qe (mg/g) | K2 (g/mg∙min) | R2 |
3.27 | 0.01 | 0.66 | 5.78 | 12.41 | 0.98 |
In adsorption of lead ions, correlation coefficient of the pseudo-second-order equation was larger than that of the pseudo-first-order equation, indicating that lead ion adsorption onto the TiO2 nanoparticles followed the pseudo-second-order kinetic model. It was observed that the predicted qe value for the pseudo-second-order model well agreed with the experimental value. Therefore, the pseudo-second-order kinetic model was found to be more suitable for predicting the kinetic sorption process of lead ion onto the TiO2 nanoparticles. The kinetic sorption fitted plots are illustrated in
Adsorption equilibrium is usually described by an isotherm equation whose parameters express the surface properties and affinity of the sorbent at a fixed temperature and pH. An adsorption isotherm describes the relationship between the amount of adsorbate on the adsorbent and the concentration of dissolved adsorbate in the liquid at equilibrium. Having this in mind, the adsorption isotherms for the removal of lead ions from aqueous solution by nano TiO2 were determined.
Langmuir sorption isotherm models the monolayer coverage of the sorption surfaces and assumes that sorption occurs on a structurally homogeneous adsorbent and all the sorption sites are energetically identical [
The linearized form of the Langmuir equation is given by Equation (5) [
where, qmax is the maximum sorption capacity (mg/g), and b is a constant related to binding energy of the sorption system (l/mg). The graphic presentations of (Ce/qe) versus Ce give those straight lines that the numerical values of constants qmax and b have evaluated form the slope and intercept of plots (
Freundlich equation is derived to model the multilayer sorption and for the sorption on heterogeneous surfaces. The logarithmic form of Freundlich equation can be described by Equation (6) [
where, Kf is a constant indicative of the relative sorption capacity of nano TiO2 (mg/g), and 1/n is a constant indicative of the intensity of sorption process. The numerical values of the constants 1/n and Kf are computed from the slope and the intercepts of log qe versus logCe curve. The correlation coefficient and other parameters
Iosotherm Equation | Langmuir | Freundlich | ||||
---|---|---|---|---|---|---|
Parameters | qmax (mg/g) | b (L/mg) | R2 | Kf (mg/g) | n | R2 |
Quantity | 7.41 | 0.35 | 0.97 | 2.73 | 3.39 | 0.82 |
obtained for the adsorbent are given in
Comparison of R2 values presented in
The results of Qmax and Kf values in Langmuir and Freundlich isotherms show the capability of sorption on an adsorbent. Comparing the quantities presented in
The Langmuir isotherm fitted well to the experimental data, probably because of the homogeneous distribution of active sites on nano-structure of TiO2 adsorbent. Based on the Langmuir model assumptions, adsorption energies are uniform and independent of surface coverage, and complete coverage of surface by amonolayer of adsorbate indicates the maximum adsorption.
The results indicated that nano-structured TiO2 synthesized by sol-gel method could be an effective adsorbent for the adsorption of Pb2+ ions from aqueous solutions under optimized conditions of pH 6, adsorbent weight of 3 g/L, contact time of 4 h and at room temperature (25˚C). All kinetic results suggested that sorption of Pb2+ by nano-structured TiO2 followed the second-order kinetics model relying on an assumption that sorption might be a rate-limiting step involving valence forces through sharing or exchange of electrons between adsorbent and sorbent. The adsorption isotherms for Pb2+ fitted well to the Langmuir adsorption isotherm equations. The maximum capacity of adsorbent was 7.41 mg∙gr−1 for Pb2+. Comparison of the results from this study and those from similar studies shows that lead ions removal by synthesized nano TiO2 is favorable and the synthesized adsorbent would be reusable with high efficiency after desorption process. The nano-structure of TiO2 exhibited a good capability to be used in water and wastewater treatment for the removal of lead ions.
Afshin Shokati Poursani,Abdolreza Nilchi,Amirhessam Hassani,Seyed Mahmood Shariat,Jafar Nouri, (2016) The Synthesis of Nano TiO2 and Its Use for Removal of Lead Ions from Aqueous Solution. Journal of Water Resource and Protection,08,438-448. doi: 10.4236/jwarp.2016.84037