Removal of Pb(II) by nano-titanium oxide investigated by batch, XPS and model techniques

Abstract

Nano-titanium oxide (nano-TiO₂) was synthesized and characterized by using XRD, TEM, XPS and FTIR. The effect of contact time, solid content, pH, foreign anions, ionic strength, humic acid (HA) and temperature on the removal of Pb(II) by nano-TiO₂ were investigated using a batch technique. The batch results showed that the adsorption of Pb(II) on nano-TiO₂ significantly decreased with increasing ionic strength at low pH conditions, whereas the ionic strength-independent adsorption was observed at high pH, indicating that the ion exchange and inner-sphere surface complexation dominated the adsorption of Pb(II) at low and high pH, respectively. The further evidences were provided by the effect of foreign anions and HA. The maximum adsorption capacity of Pb(II) on nano-TiO₂ calculated from a Langmuir model at pH 5.0 and 293 K was 30.8 mg g⁻¹, which was significantly higher than commercial P25 TiO₂ (15.0 mg g⁻¹). The thermodynamic parameters showed that the adsorption of Pb(II) on nano-TiO₂ was an endothermic and spontaneous process. The hydroxyl groups of nano-TiO₂ were responsible for the highly effective adsorption of Pb(II) by XPS analyses. Based on surface complexation modeling, a diffuse layer model gave excellent fitting for the pH-edge adsorption data. The results indicated that the nano-TiO₂ could be used as a suitable adsorbent for the removal of heavy metals from large volumes of aqueous solutions in environmental cleanup.

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1. Introduction

The rapid development of industries leads to massive discharges of various hazardous metal ions into water environments, which is serious threats to ecological systems, especially to human beings and other animals. Among these hazardous metal ions, Pb(II) contamination is one of the most serious environmental issues due to its bioaccumulation. The excessive discharge of Pb(II) from car exhausts, the combustion of fossil fuels, battery manufacturing, mining activity, and wastewater from iron and steel manufacturing have increased dramatically in the past several hundred years. Once people having intake of a wealth of Pb(II) could be resulted of the harmful effects such as anemia, headache and diarrhea.¹ Therefore, the elimination of Pb(II) from aqueous solutions is crucial for environmental pollution cleanup.

The adsorption technique is recently considered to be one of the most extensive and promising methods to remove heavy metal ions from aqueous solution, due to the low cost, simple design, easy operation, and high feasibility.^2–4 A great deal of effort has been made to develop high effective adsorbents for the removal of heavy metal ions from water in recent years.^5–9 TiO₂ is widely studied for its potential application in chemical sensors,¹⁰ photo-electrochemical cells,¹¹ photocatalytic oxidation.^12,13 Recently, TiO₂ has been investigated as adsorbent due to its high chemical stability in acidic and alkaline solutions and reasonably fast rate of adsorption and desorption.¹⁴ Belessi et al. claimed that the maximum adsorption capacity of TiO₂ was 86.96 mg g⁻¹ for Reactive Red 195.¹⁵ Janus et al. found that the adsorption capacity of carbon-modified TiO₂ for Direct Green 99 was enhanced to 96.77 mg g⁻¹, which was more than that of the unmodified TiO₂.¹⁶ It was also reported that TiO₂ possess high adsorption capacity of heavy metal ions.^17–20 Vu et al. investigated that the adsorption capacity of Cu(II) on nano-TiO₂ (anatase) prepared via electrospinning was about 12.8 mg g⁻¹.²⁰ However, few studies on the removal mechanism of Pb(II) on TiO₂ by XPS and surface complexation modeling techniques was available.

The objectives of this study are (1) to synthesize nano-TiO₂ and characterize their nanostructures by X-ray diffraction (XRD), transmission electron microscope (TEM), surface area and fourier transform infrared spectrometry (FTIR); (2) to investigate the adsorption behavior of Pb(II) on nano-TiO₂ as a function of pH, contact time, foreign ions, HA, temperature; (3) to determine the interaction mechanism between Pb(II) and nano-TiO₂ by X-ray photoelectron spectra (XPS) and surface complexation modeling. The highlight of this study is to demonstrate the interaction mechanism of Pb(II) and nano-TiO₂ by spectroscopic and modeling techniques.

2. Experimental section

2.1 Materials

All reagents were of analytical reagent grades and used without further purification. The Pb(II) solution was prepared by dissolving Pb(NO₃)₂ in Milli-Q water and then diluted to 60 mg L⁻¹. HA was extracted from the soil of Hua-Jia county (Gansu province, China), and were characterized in our previous studies in detail. The main components of HA were C (∼60 wt%), O (31 wt%), N (4.21 wt%) and S (0.52 wt%), revealing a variety of oxygen-, nitrogen- and sulfur-containing functional groups. Titanium isopropoxide (97%) and P25 were purchased from Sigma-Aldrich Chemical Co. Ltd and Sinopharm Chemical Reagent Co., Ltd, respectively.

2.2 Preparation of nano-TiO₂

The nano-TiO₂ was synthesized by hydrothermal methods. Briefly, 125 mL of titanium isopropoxide was dropwise added into 750 mL of 0.1 M nitric acid solution under vigorous stirring. Then slurry was heated to 80 °C for 8 h to achieve peptization. Milli-Q water was added into the filtrate to adjust the final solids concentration to ∼5 wt%. The nanoparticles were achieved under hydrothermal conditions in a titanium autoclave, which was heated for 12 h with the temperature of 200 °C. The nano-TiO₂ particles were obtained after heated to 450 °C for 30 min. The nano-TiO₂ particles was purified to remove the soluble impurity by adding 1 mol L⁻¹ HNO₃ and shaking for 24 h at room temperature and then was rinsed with Milli-Q water up to pH = 6.0. The nano-TiO₂ powder was dried in oven at 110 °C for 24 h.

2.3 Characterization

The mineralogy of nano-TiO₂ was characterized by XRD patterns using M18XHF diffractometer with Cu Kα radiation (λ = 0.15406 nm) and scanning rate 2° min⁻¹. The FT-IR spectra of the samples were recorded by the Nicolet 8700 FT-IR spectrometer at room temperature. The morphology and nanostructures of as-prepared nano-TiO₂ were characterized by TEM (JEOL-2010, Tokyo, Japan). The surface area was determined at −195.8 °C by N₂ adsorption–desorption isotherms using a Micromeritics ASAP 2020M + C accelerated surface area analyzer. The X-ray photoelectron spectra (XPS) were characterized by the thermo ESCALAB 250 electron spectrometer with multidetection analyzer using Al Kα X-ray source (1486.6 eV) at 10 kV and 5 mA under 10⁻⁸ Pa residual pressure. Surface charging effects were corrected with C 1s peak at 284.4 eV as a reference. The recorded lines (O 1s and Ti 2p) were fitted by using XPSPEAK41 program after subtraction of the background (Shirley baseline correction). The electrophoretic zeta potential of nano-TiO₂ was measured by ZetaSizer Nano-ZS90 with a laser Doppler electrophoresis technique.

2.4 Batch adsorption experiments

The adsorption of Pb(II) on nano-TiO₂ was carried out under ambient conditions by the batch technique. The mixtures of nano-TiO₂ suspension and NaNO₃ electrolyte solution were pre-equilibrated in the polyethylene tubes for 24 h, and then the desired concentration of Pb(II) solution was added. The pH values were adjusted by adding negligible amount of 1–0.01 M HNO₃ or NaOH solution. After the adsorption equilibrium (24 h), the solid phases were separated from the solutions by centrifugation at 9000 rpm for 20 min. According to the blank tests, the adsorption of Pb(II) on the polyethylene tube walls was negligible. The concentration of Pb(II) was analyzed by atomic absorption spectrophotometer (AAS-6300C, Shimadzu, Japan). The adsorption percentage (R%) and distribution coefficients (K_d) of Pb(II) on nano-TiO₂ could be defined as eqn (1) and (2), respectively:

R% = (C₀ − C_e)/C₀ × 100% (1)

K_d = (C₀ − C_e)V/C_em (2)

where C₀ (mg L⁻¹) and C_e (mg L⁻¹) were the initial concentration and equilibration concentration, respectively. m (g) was the mass of nano-TiO₂ and V (mL) was the volume of the suspension, respectively. All the experimental data were averaged by the triple determination data. The relative errors were controlled within ±5%.

3. Results and discussions

3.1 Characterization

Fig. 1A shows the XRD patterns of the nano-TiO₂. These peaks are well indexed to the structure of anatase (JCPDS card no. 89-4921).²¹ As shown in FTIR spectrum (Fig. 1B), the peak at 1642 cm⁻¹ and the wide peak at 3357 cm⁻¹ could be attributed to the stretching and bending of the hydroxyl group adsorbed on the nano-TiO₂ surface.²² The adsorption peaks between 550 and 653 cm⁻¹ refer to Ti–O stretching vibration.²³ The result from FTIR spectrum indicates the plentiful hydroxyl groups on nano-TiO₂ surfaces, which is easy to combine with heavy metals in aqueous solution. Fig. 1C and D show the TEM of nano-TiO₂ and P25. By TEM observation, the uniform particle size of nano-TiO₂ is observed (ca. 14 nm), while P25 presents the wide size distribution from 15 to 35 nm. As shown in Fig. 1E, N₂ adsorption–desorption curves display classical type IV with H2 hysteresis, indicating the presence of mesopores on nano-TiO₂ surface.²⁴ Based on Barrett–Joyner–Halenda (BJH) method, the peak pore size and the total pore volumes are calculated to be 8.02 nm (inset in Fig. 1E) and 0.4536 cm³ g⁻¹ for the nano-TiO₂, 1.21 nm (inset in Fig. 1F) and 0.4483 cm³ g⁻¹ for P25, respectively. The BET surface areas of nano-TiO₂ and P25 are 129.10 and 56.75 m² g⁻¹, respectively. The results indicate that the uniform particle size of nano-TiO₂ is obtained by using hydrothermal method.

Fig. 1 The characterization of the nano-TiO₂ and P25, (A) XRD pattern of nano-TiO₂; (B) FTIR spectra of nano-TiO₂; (C and D) TEM image of nano-TiO₂ and P25, respectively; (E and F) N₂ adsorption and desorption isotherms of nano-TiO₂ and P25; (G) Ti 2p XPS spectra of nano-TiO₂; (H) O1s XPS spectra of the nano-TiO₂.

The high resolution Ti 2p and O1s XPS spectra of nano-TiO₂ are shown in Fig. 1E and F, respectively. The two characteristic peaks at 458 eV and 464 eV are assigned to Ti 2p_3/2 and Ti 2p_1/2, respectively.²⁵ As shown in Fig. 1F, the peaks at 530.3, 531.2, 532.1 and 533.3 eV are assigned to the lattice oxygen, bridging hydroxyl, terminal hydroxyl and the surface adsorption of water, respectively.²⁶ The XPS results indicate that the surface of nano-TiO₂ presents massive hydroxyl groups, which is responsible for the Pb(II) adsorption.

3.2 Adsorption kinetics

Fig. 2A shows the adsorption of Pb(II) on nano-TiO₂ and P25 under varying time interval. As illustrated in Fig. 2A, the adsorption of Pb(II) on nano-TiO₂ and P25 increase sharply within 6 h, then high level adsorption are observed. Approximately 24.1% and 56.3% of Pb(II) are removed by P25 and nano-TiO₂, respectively, indicating that the as-prepared nano-TiO₂ displays the high adsorption performances compared to P25. It is observed that 24 h is enough to reach the adsorption equilibrium. The pseudo-first order and pseudo-second order kinetic models are adopted to fit the adsorption kinetics of Pb(II) on nano-TiO₂ and P25. The pseudo-first and pseudo-second order equation are written as eqn (3)²⁷ and eqn (4),²⁸ respectively:

log(q_e − q_t) = log q_e − k_ft/2.303 (3)

t/q_t = 1/k_sq_e² + t/q_e (4)

where q_e and q_t (mg g⁻¹) are the amount of adsorption of Pb(II) on nano-TiO₂ and P25 at equilibrium and t, respectively. k_f and k_s are the pseudo-first and pseudo-second rate constant, respectively.

Fig. 2 (A) Adsorption kinetics of Pb(II) on the nano-TiO₂, solid line: pseudo-second order kinetic model; (B) the effect of sorbent content on Pb(II) adsorption on the nano-TiO₂, C₀ = 10 mg L⁻¹, pH = 5.0, I = 0.01 mol L⁻¹ NaNO₃, T = 293 K.

The calculated parameters of two kinetic models are listed in Table 1. As shown in Table 1, it can be seen that the adsorption kinetics of Pb(II) on nano-TiO₂ and P25 by using pseudo-second order kinetic model (R² = 0.999, 0.999) are better than the pseudo-first order kinetic model (R² = 0.668, 0.946). These results indicate that the rate-controlling step for Pb(II) on nano-TiO₂ was chemisorptions instead of mass transport effect.^29,30

Table 1 Kinetic Parameters of Pb(II) adsorption on P25 and nano-TiO₂

Parameters	P25	nano-TiO2
Pseudo-first-order model
kf (h^−1)	0.283	0.255
qe (mg g^−1)	3.05	3.11
R^2	0.946	0.668
Pseudo-second-order model
ks (g (mg h)^−1)	0.242	0.159
qe (mg g^−1)	8.26	19.13
R^2	0.999	0.999

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3.3 Effect of sorbent content

The adsorption of Pb(II) on nano-TiO₂ as a function of sorbent content at pH 5.0 is showed in Fig. 2B. The adsorption of Pb(II) on nano-TiO₂ increases quickly from 25.4% to 65.5% with increasing sorbent content from 0.05 g L⁻¹ to 1.0 g L⁻¹, whereas the little decrease in K_d values is observed with increasing sorbent content. The slight decrease of K_d values is due to the aggregation of nano-TiO₂ particles.³¹ The collision and overcrowding of nano-TiO₂ particles at higher sorbent content increase the aggregation of particles, which may decrease the specific surface area and prolong Pb(II) diffusion path on nano-TiO₂. Although the aggregation of nano-TiO₂ particles happen, the more reactive sites can be available to combine Pb(II) when the amount of sorbent content increases, thereby, the total adsorption percentage of Pb(II) increases with increasing the sorbent dosage under the fixed initial concentration of Pb(II) (C_o = 10 mg L⁻¹). The similar phenomena is also described by Jin et al. that the adsorption of Co(II) on graphene oxide increased with improving solid content.⁸

3.4 Effect of pH and ionic strength

The pH value in the aqueous solutions plays an important role on the adsorption of Pb(II) on nano-TiO₂. Fig. 3A shows the pH dependence of Pb(II) adsorption on the nano-TiO₂. The adsorption of Pb(II) on nano-TiO₂ increases gradually at pH range 2.0–6.0, then reaches an adsorption equilibrium at pH 6.0–10.0, whereas the significantly decreased adsorption of Pb(II) on nano-TiO₂ is observed at pH above 10.0. According to the measurement of zeta potentials, the negatively charged nano-TiO₂ surface is observed at pH > 5.3. Fig. 3B shows the distribution of Pb(II) species in aqueous solutions at I = 0.01 mol L⁻¹ NaNO₃ by using hydrolysis constants (log k₁ = 6.48, log k₂ = 11.16, log k₃ = 14.16).³² The main species of Pb(II) is Pb²⁺ at pH < 6.0, Pb(OH)⁺ at pH 7.0–9.0, Pb(OH)⁰₂ at pH 9.0–11.0 and Pb(OH)₃⁻ at pH > 11.0, respectively. Therefore, the adsorption of Pb(II) on nano-TiO₂ at pH 2.0–5.5 could be resulted from the cation exchange reactions, which is further demonstrated by the fitted results of surface complexation modeling. The high adsorption of Pb(II) on nano-TiO₂ increases gradually at pH 5.0–6.0 can be attributed to the electrostatic attraction between negatively charged nano-TiO₂ and positive charged Pb²⁺ species. According to the calculation of solubility product of Pb(OH)₂(s) (K_sp = 1.2 × 10⁻¹⁵),³³ the precipitate (Pb(OH)₂(s)) is observed at pH = 8.7 and C_Pb²⁺ = 4.83 × 10⁻⁵ mol L⁻¹. However, 90% of Pb(II) ions have been adsorbed on the surfaces of nano-TiO₂ at pH 6.0, thereby, the high-level adsorption of Pb(II) on nano-TiO₂ at high pH values is not mainly caused by the precipitation of Pb(II). The strong surface complexation is responsible to the adsorption of Pb(II) on nano-TiO₂. Therefore, the adsorption of Pb(II) on nano-TiO₂ at pH 6.0–10.0 is ascribed to the formation of Pb(II) precipitate (Pb(OH)₂⁰) and inner-sphere surface complexation. The decreased adsorption of Pb(II) on nano-TiO₂ at pH > 10.0 is due to the electrostatic repulsion between negatively charged nano-TiO₂ surface and negatively charged Pb(II) species such as Pb(OH)₃⁻ species.

Fig. 3 The adsorption of Pb(II) on nano-TiO₂, (A) pH effect, C_o = 10 mg L⁻¹, m/V = 0.30 g L⁻¹ and T = 293 K. (B) distribution of Pb(II) species in aqueous solutions as a function of pH; (C) HA effect; (D) foreign anions effect.

The effect of ionic strength on the adsorption of Pb(II) on nano-TiO₂ at I = 0.001, 0.01, 0.1 M NaNO₃ solutions is also showed in Fig. 3A. One can see that the adsorption of Pb(II) on the nano-TiO₂ significantly decreases with increasing the concentration of ionic strength at low pH, whereas the adsorption of Pb(II) is independent of ionic strength at high pH. The change of the adsorption edges in different ionic strength solution implies the different adsorption mechanisms. Inner-sphere surface complexation is not sensitive to ionic strength variation as compared to outer-sphere surface complexation and cation exchange.^21,34 Therefore, it can be induced that the Pb(II) adsorption on the nano-TiO₂ may be due to cation exchange and/or outer-sphere surface complexation at low pH and inner-sphere surface complexation at high pH. From the FTIR and XPS analysis, there are many functional groups on nano-TiO₂ surface, which can form complexes with Pb(II) ions at the solid surfaces. The formation of surface complexes results in the adsorption of Pb(II) on nano-TiO₂ surfaces, which is also proved by the following surface complexation modeling.

3.5 Effect of HA and foreign anions

As one of the representatives for natural organic matters, HA has been widely used to simulate the effect of natural organic matters on many heavy metals adsorption.^35,36 Adsorption of Pb(II) on the nano-TiO₂ in the presence of HA as a function pH are shown in Fig. 3C. It is observed that the presence of HA increases the adsorption of Pb(II) to the nano-TiO₂ at pH < 6.0, whereas the suppressed adsorption is observed at pH > 7.0. At low pH values, the negative charged HA can be mostly adsorbed by the nano-TiO₂, so the strong complexation ability of surface adsorbed HA with Pb(II) should result in the adsorption of Pb(II) on nano-TiO₂ surface increasing at pH < 7.0. HA has a macromolecular structure, only a small fraction of the “adsorbed” group is free to interact with metal ions. The complexation between Pb(II) and HA is more stronger than that of Pb(II) and the nano-TiO₂. The free energy of the formation of HA–Pb(II) complex is smaller than that of nano-TiO₂–Pb(II). However, the adsorbed HA on the surface of the nano-TiO₂ can be dissolved with increasing pH, which is easy to combine with Pb(II) ion in aqueous solution. Therefore, the suppressed adsorption of Pb(II) on nano-TiO₂ at high pH conditions is attributed to the combination of HA with Pb(II) in aqueous solution, which inhibits the adsorption of Pb(II) on the surface of the nano-TiO₂.

In order to evaluate the influence of foreign anions, the adsorption of Pb(II) on nano-TiO₂ in 0.01 mol L⁻¹ of NaClO₄, NaNO₃, NaCl is showed in Fig. 3D. As can be seen from Fig. 3D, the presence of NO₃⁻ and ClO₄⁻ show similar influence on Pb(II) adsorption on the nano-TiO₂. The presence of Cl⁻ enhances Pb(II) adsorption at pH < 5.0, while affect slightly Pb(II) adsorption at pH > 5.5. When pH is under 5.0, the Cl⁻ may enhance the adsorption of Pb(II) on the nano-TiO₂ through the formation of ternary nano-TiO₂–Pb–Cl⁻ complexes. Relative to the free Pb²⁺ ion, the affinity of Pb–Cl⁻ complexes for the nano-TiO₂ surface is enhanced, which reduces the electrostatic barrier between Pb(II) and the positive nano-TiO₂ surfaces (pH < pHpzc ∼ 5).³⁷ Furthermore, the adsorption of Pb(II) on nano-TiO₂ in the presence of Cl⁻ is shifted to the lower pH values compared to that of NO₃⁻ and ClO₄⁻, which enhances the electronegativity of the nano-TiO₂ surface and thereby promotes the adsorption of positively charged Pb(II) ions due to electrostatic attraction.

3.6 Adsorption isotherms

The adsorption isotherms of Pb(II) on the nano-TiO₂ and P25 at 293, 308 and 323 K are shown in Fig. 4. The adsorption of Pb(II) on the nano-TiO₂ rises with increasing temperature. The experimental data are matched with the Langmuir and Freundlich models. The Langmuir model assumes identical sites on solid surface, no interaction among the adsorbate molecules and monolayer adsorption.³⁸ The Freundlich model is an empirical relationship describing the adsorption on the heterogeneous solid surface, which assumes that the surface sites of solid have a spectrum of different binding energies.³⁹ The Langmuir and Freundlich model are described by the eqn (5) and (6), respectively:

Q_e = Q_max × K_L × C_e/(1 + K_LC_e) (5)

Q_e = K_FC_eⁿ (6)

where Q_e (mg g⁻¹) and C_e (mg L⁻¹) are the amount of Pb(II) adsorption on the nano-TiO₂ and concentration of Pb(II) at solution after equilibrium, respectively. K_L (L mg⁻¹) and K_F ((mg g⁻¹) (mg L⁻¹)^−1/n) are the Langmuir constant and Freundlich constant, respectively. Q_max (mg g⁻¹) is the maximum adsorption capacity, and n is the degree of adsorption capacity with equilibrium concentration in aqueous solutions.

Fig. 4 Adsorption isotherms of Pb(II) on the nano-TiO₂ (A) and P25 (B) under the different temperature, pH = 5.0, I = 0.01 mol L⁻¹ NaNO₃, m/V = 0.30 g L⁻¹. The solid lines are Langmuir model simulation, and the dashed lines are Freundlich model simulation.

The relevant parameters calculated from the two models are listed in Table 2. The fitting results show that the adsorption behaviors of Pb(II) on the nano-TiO₂ are simulated by Langmuir model very well (R² > 0.98), indicating that monolayer adsorption is dominated between Pb(II) and the nano-TiO₂ and P25. The Q_max of the nano-TiO₂ and P25 calculated from Langmuir model at pH 5.0 and 293 K are 30.8 and 15.0 mg g⁻¹, respectively.

Table 2 Parameters for Langmuir and Freundlich models of Pb(II) adsorption on the nano-TiO₂ and P25

Samples	Langmuir model				Freundlich model
	T (K)	Qmax (mg g^−1)	KL (L mg^−1)	R^2	KF (mg^1−n L^n g^−1)	n	R^2
Nano-TiO2	293	30.8	0.294	0.995	8.17	0.458	0.965
	308	35.4	0.524	0.985	12.3	0.412	0.950
	323	41.2	0.767	0.991	16.7	0.359	0.933
P25	293	15.0	0.229	0.982	3.59	0.450	0.927
	308	19.1	0.290	0.980	5.22	0.426	0.909
	323	22.2	0.589	0.984	8.63	0.349	0.920

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The thermodynamic parameters (standard enthalpy change-ΔH°, standard entropy change-ΔS°, and Gibbs free energy changes-ΔG°) for Pb(II) adsorption on nano-TiO₂ can be calculated from the adsorption isotherms at three temperatures. The values of ΔG°, ΔH° and ΔS° can be calculated by the following eqn (7) and (8), respectively:

ΔG° = RT ln K_d = ΔH° − TΔS° (7)

ln K° = ΔS°/R − ΔH°/RT (8)

where R (8.314 J mol⁻¹ K⁻¹) and T (K) are the universal gas constant and temperature in Kelvin, respectively. The adsorption equilibrium constant, K°, can be calculated by plotting ln K_d versus C_e and extrapolating C_e to zero (Fig. 5A). The values of ΔH° and ΔS° can be obtained by the slope and intercept of the plot of ln K° versus 1/T (Fig. 5B). The values of ΔG°, ΔH° and ΔS° are tabulated in Table 3. As listed in Table 3, the positive values of ΔH° (19.33 for the nano-TiO₂ and 25.32 kJ mol⁻¹ for P25) are observed, which may be concluded that the adsorption of Pb(II) on the nano-TiO₂ is an endothermic process. It is well known that the dehydration from hydrous Pb(II) is an endothermic process, whereas the combination of dehydrated Pb(II) with the functional group of the nano-TiO₂ is an exothermic process. It is assumed that the endothermic energy of dehydration may surpass the exothermic energy of the dehydrated Pb(II) ions combining with the surface of the nano-TiO₂. The negative ΔG° (for nano-TiO₂ (−21.96 kJ mol⁻¹ at 293 K) and P25 (−18.78 kJ mol⁻¹ at 293 K)) is observed, showing that the Pb(II) adsorption process is a spontaneous process. The value of ΔG° becomes more negative with the increase of temperature, indicating that an increase in the temperature contributes to the adsorption Pb(II) on the nano-TiO₂ and P25. The values of ΔS° for nano-TiO₂ and P25 are 140.92 and 150.51 J (mol⁻¹ K⁻¹), which reflect that the randomness degree increases at the solid–liquid interface during the adsorption process. The thermodynamic results reveal that the adsorption of Pb(II) on the nano-TiO₂ and P25 are a spontaneous and endothermic process.

Fig. 5 Plots of ln K_d vs. C_e (A and C) and ln K° & 1/T (B and D) for the nano-TiO₂ and P25, pH = 5.0, C_o = 10 mg L⁻¹, I = 0.01 mol L⁻¹ NaNO₃, m/V = 0.30 g L⁻¹.

Table 3 Thermodynamic parameters of Pb(II) adsorption on nano-TiO₂ and P25

Samples	Temperature	ΔG° kJ mol^−1	ΔH° kJ mol^−1	ΔS° J (mol^−1 K^−1)
Nano-TiO2	293 K	−21.96
	308 K	−24.07	19.33	140.92
	323 K	−26.19
P25	293 K	−18.78
	308 K	−21.04	25.32	150.51
	323 K	−23.29

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3.7 Surface complexation modeling

The adsorption of Pb(II) on nano-TiO₂ is fitted by diffuse layer modeling with an aid of Visual MINTEQL software.⁴⁰ The protonation and deprotonation constants (log K⁺ and log K⁻) can be given by eqn (9) and (10), respectively:

SOH + H⁺ = SOH₂⁺, log K⁺ = log([SOH²⁺]/([SOH][H⁺])) (9)

SOH = SO⁻ + H⁺, log K⁻ = log([SO⁻][H⁺]/[SOH]) (10)

The values of log K⁺ and log K⁻ can be obtained by fitting the potentiometric titration of nano-TiO₂. The surface complexation reactions between Pb²⁺ and nano-TiO₂ are presumed by eqn (11)–(13):

2XNa + Pb²⁺ = X₂Pb + 2Na⁺ (11)

SOH + Pb²⁺ = SOPb⁺ + H⁺ (12)

2SOH + Pb²⁺ + 2H₂O = (SO)₂Pb(OH)₂²⁻ + 4H⁺ (13)

where XNa and SOH refer to the ion exchange and surface complexation sites, respectively. The log K values are optimized by trial and error and summarized in Table 4. The fitted results of surface complexation modeling of Pb(II) adsorption on nano-TiO₂ at I = 0.001 and 0.1 mol L⁻¹ NaNO₃ are showed in Fig. 6. As shown in Fig. 6A and B, the adsorption of Pb(II) on nano-TiO₂ can be satisfactorily fitted by diffuse layer model with an ion exchange and two surface complexation reactions. It is observed that the main adsorbed species are X₂Pb and SOPb⁺ species at pH < 4.0 and pH 4.0–6.0, respectively, whereas the (SO)₂Pb(OH)₂²⁻ species predominates the adsorption of Pb(II) on nano-TiO₂ at pH > 7.0. The fitted results from surface complexation modeling indicate that the adsorption of Pb(II) on nano-TiO₂ is ion exchange at pH < 4.0, whereas inner-sphere complexation dominates the Pb(II) adsorption on nano-TiO₂ at pH > 5.0. In addition, the transformation of mononuclear and monodentate inner-sphere complexes (SOPb⁺ species) into binuclear and bidentate inner-sphere complexes ((SO)₂Pb(OH)₂²⁻ species) is observed with increasing pH in aqueous solutions.

Table 4 The optimized parameters of surface complexation modeling of Pb(II) adsorption on nano-TiO₂

Reactions	log K
SOH + H^+ = SOH2^+	3.34
SOH = SO^− + H^+	−6.64
2XNa + Pb^2+ = X2Pb + 2Na^+	2.45
SOH + Pb^2+ = SOPb^+ + H^+	2.18
2SOH + Pb^2+ + 2H2O = (SO)2Pb(OH)2^2− + 4H^+	−11.24

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Fig. 6 The surface complexation modeling of Pb(II) on nano-TiO₂ at I = 0.001 (A) and 0.1 mol L⁻¹ (B) NaNO₃, C_o = 10 mg L⁻¹, m/V = 0.30 g L⁻¹ and T = 293 K.

4. Conclusions

In this study, the nano-TiO₂ was synthesized by hydrothermal method and characterized by batch techniques. The effect of environmental factors (contact time, pH, ionic strength, HA and temperature) on Pb(II) adsorption onto nano-TiO₂ was investigated. The results indicate that the adsorption of Pb(II) on nano-TiO₂ is well fitted by the pseudo-second-order model. The adsorption of Pb(II) on nano-TiO₂ significantly decreases with increasing the concentration of ionic strength at pH < 6.0, whereas the no effect of ionic strength is observed at pH > 6.0. The increased and decreased adsorption of Pb(II) on nano-TiO₂ in the presence of HA is observed at pH < 6.0 and pH > 7.0, respectively. The thermodynamic parameters indicate that the adsorption of Pb(II) on nano-TiO₂ is an endothermic and spontaneous process. Based on the surface complexation modeling, the adsorption mechanism between Pb(II) with nano-TiO₂ is mainly ion exchange at pH < 4.0, whereas inner-sphere surface complexation dominate the adsorption of Pb(II) on nano-TiO₂ at pH > 4.0. The findings of this study indicated that nano-TiO₂ is a suitable adsorbent for the removal of Pb(II) from large volumes of aqueous solutions.

Acknowledgements

Financial support from the National Natural Science Foundation of China (41273134, 21477133, 21377132 and 21173228) and Anhui Provincial Natural Science Foundation (1408085MB28), the Scientific Research Grant of Hefei Science Center of CAS (2015SRGHSC006 and 2015SRG-HSC009).

References

1. D. Ozdes, A. Gundogdu, B. Kemer, C. Duran, H. B. Senturk and M. Soylak, J. Hazard. Mater., 2009, 166, 1480–1487.
2. S. B. Yang, C. L. Chen, Y. Chen, J. X. Li, D. Q. Wang, X. K. Wang and W. P. Hu, ChemPlusChem, 2015, 80, 480–484.
3. Y. B. Sun, J. X. Li and X. K. Wang, Geochim. Cosmochim. Acta, 2014, 140, 621–643.
4. N. J. Wang, L. L. Zhou, J. Guo, Q. Q. Ye, J. M. Lin and J. Y. Yuan, Appl. Surf. Sci., 2014, 305, 267–273.
5. H. Yanagisawa, Y. Matsumoto and M. Machida, Appl. Surf. Sci., 2010, 256, 1619–1623.
6. Y. B. Sun, S. T. Yang, G. D. Sheng, Z. Q. Guo and X. K. Wang, J. Environ. Radioact., 2012, 105, 40–47.
7. J. S. Hu, L. S. Zhong, W. G. Song and L. J. Wan, Adv. Mater., 2008, 20, 2977–2982.
8. Z. X. Jin, J. Sheng and Y. B. Sun, J. Radioanal. Nucl. Chem., 2014, 299, 1979–1986.
9. C. C. Ding, W. C. Cheng, Y. B. Sun and X. K. Wang, Geochim. Cosmochim. Acta, 2015, 165, 86–107.
10. J. S. Chen, J. D. Zhang, Y. Z. Xian, X. Y. Ying, M. C. Liu and L. T. Jin, Water Res., 2005, 39, 1340–1346.
11. D. Wei and G. Amaratunga, Int. J. Electrochem. Sci., 2007, 2, 897–912.
12. B. M. Reddy, K. N. Rao, G. K. Reddy and P. Bharali, J. Mol. Catal. A: Chem., 2006, 253, 44–51.
13. W. S. Lee, B. Z. Wan, C. N. Kuo, W. C. Lee and S. Cheng, Catal. Commun., 2007, 8, 1604–1608.
14. W. Li, G. Pan, M. Y. Zhang, D. Y. Zhao, Y. H. Yang, H. Chen and G. Z. He, J. Colloid Interface Sci., 2008, 319, 385–391.
15. V. Belessi, G. Romanosa, N. Boukosa, D. Lambropouloud and C. Trapalis, J. Hazard. Mater., 2009, 170, 836–844.
16. M. Janus, E. Kusiak, J. Choina, J. Ziebro and A. W. Morawski, Desalination, 2009, 249, 359–363.
17. C. Z. Huang, Z. C. Jiang and B. Hu, Talanta, 2007, 73, 274–281.
18. X. W. Zhao, Q. Jia, N. Z. Song, W. H. Zhou and Y. S. Li, J. Chem. Eng. Data, 2010, 55, 4428–4433.
19. S. Bang, M. Patel, L. Lippincott and X. G. Meng, Chemosphere, 2005, 60, 389–397.
20. D. Vu, Z. Y. Li, H. N. Zhang, W. Wang, Z. J. Wang, X. R. Xu, B. Dong and C. Wang, J. Colloid Interface Sci., 2012, 367, 429–435.
21. X. L. Tan, Q. H. Fan, X. K. Wang and G. Bernd, Environ. Sci. Technol., 2015, 43, 3115–3121.
22. Y. Gao, H. Wang, J. Wu, R. H. Zhao, Y. F. Lu and B. F. Xin, Appl. Surf. Sci., 2014, 294, 36–41.
23. Y. F. Chen, C. Y. Lee, M. Y. Yeng and H. T. Chiu, J. Cryst. Growth, 2003, 247, 363–370.
24. P. D. Yang, D. Y. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater., 1999, 11, 2813–2826.
25. M. P. Casaletto, L. Lisi, G. Mattogno, P. Patrono and G. Ruoppolo, Appl. Catal., A, 2004, 267, 157–164.
26. Y. B. Sun, C. C. Ding, W. C. Cheng and X. K. Wang, J. Hazard. Mater., 2014, 280, 399–408.
27. S. Lagergren, K. Sven. Vetenskapsakad. Handl., 1898, 24, 1–39.
28. Y. S. Ho and G. McKay, Process Saf. Environ. Prot., 1998, 76, 183–191.
29. Y. B. Sun, S. T. Yang, G. D. Sheng, Q. Wang, Z. Q. Guo and X. K. Wang, Radiochim. Acta, 2012, 100, 779–784.
30. Y. B. Sun, Q. Wang, C. L. Chen, X. L. Tan and X. K. Wang, Environ. Sci. Technol., 2012, 46, 6020–6027.
31. Y. B. Sun, S. B. Yang, Q. Wang, A. Alsaedi and X. K. Wang, Radiochim. Acta, 2014, 102, 797–804.
32. S. W. Wang, J. Hu, J. X. Li and Y. H. Dong, J. Hazard. Mater., 2009, 167, 44–51.
33. G. D. Sheng, S. W. Wang, J. Hu, Y. Lu, J. X. Li, Y. H. Dong and X. K. Wang, Colloids Surf., A, 2009, 339, 159–166.
34. Y. B. Sun, S. B. Yang, Y. Chen, C. C. Ding, W. C. Cheng and X. K. Wang, Environ. Sci. Technol., 2015, 49, 4255–4262.
35. Y. B. Sun, C. L. Chen, D. D. Shao, J. X. Li, X. L. Tan, G. X. Zhao, S. B. Yang and X. K. Wang, RSC Adv., 2012, 2, 10359–10364.
36. Y. B. Sun, S. B. Yang, G. X. Zhao, Q. Wang and X. K. Wang, Chem.–Asian J., 2013, 8, 2755–2761.
37. J. D. Ostergren, G. E. Brown Jr, G. A. Parks and P. Persson, J. Colloid Interface Sci., 2000, 225, 483–493.
38. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361–1403.
39. H. Freundlich, Z. Phys. Chem., Stoechiom. Verwandtschaftsl., 1906, 57, 385–470.
40. H. P. van Leeuwen, Dynamic aspects of in situ speciation processes and techniques, John Wiley and Sons, Chichester, 2000.

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