Sorption of heavy metal ions from aqueous solution

Journal of Industrial and Engineering Chemistry 20 (2014) 1656–1664

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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Sorption of heavy metal ions from aqueous solution by a novel cast PVA/TiO2 nanohybrid adsorbent functionalized with amine groups Saeed Abbasizadeh a, Ali Reza Keshtkar b,*, Mohammad Ali Mousavian a a b

Department of Chemical Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran Nuclear Fuel Cycle School, Nuclear Science and Technology Research Institute, Tehran, Iran

A R T I C L E I N F O

Article history: Received 10 June 2013 Accepted 11 August 2013 Available online 19 August 2013 Keywords: PVA/TiO2 nano hybrid adsorbent Amin groups Casting Cadmium(II) Nickel(II) Uranium(VI)

A B S T R A C T

Sorption of Cd(II), Ni(II) and U(VI) ions onto a novel cast PVA/TiO2/APTES nanohybrid adsorbent with variations in adsorbent dose, pH, contact time, initial metal concentration and temperature has been investigated. The adsorbent were characterized by SEM and FTIR analysis. BET surface area, pore diameter and pore volume of adsorbent were 35.98 m2 g1, 3.08 nm and 0.059 cm3 g1, respectively. The kinetic and equilibrium data were accurately described by the double-exponential and Freundlich models for all metals. The maximum sorption capacities were 49.0, 13.1 and 36.1 mg g1 for Cd(II), Ni(II) and U(VI) ions with pH of 5.5, 5 and 4.5, respectively. Thermodynamic studies showed that the sorption process was favored at higher temperature. The adsorbent can be easily regenerated after 5 cycles of sorption–desorption. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction A wide range of industries such as mining, dye manufacturing and metal processing, releases heavy metal ions and dyes into the environment in the amounts potentially hazardous to human health [1–5]. Even at low concentrations, some heavy metals like cadmium (Cd(II)), nickel (Ni(II)) and uranium (U(VI)) because of their persistent and accumulative nature are toxic [6,7]. Chronic exposure to elevated levels of these heavy metals is known to cause the bone degeneration, the liver and the lung and blood damage [6]. Therefore, the development of clean-up technologies for removing heavy metals and dyes from industrial wastewaters is very important [8–12]. Toxic metals and dyes can be removed from wastewaters by a number of separation technologies, such as chemical precipitation [13], membrane process [14,15], solvent extraction [16], ion exchange [17], floatation [18], coagulation [19] and sorption process [20–30]. Among these methods, sorption is currently considered to be very suitable for wastewater treatment because of its high efficiency, simple operation and low cost [31]. A number of effective adsorbents have been prepared and reported in recent years [32,33]; among them, oxide of titanium (TiO2) is often applied because of its high surface area and its affinity to several

* Corresponding author. Tel.: +98 021 82064399; fax: +98 021 88221127. E-mail address: [email protected] (A.R. Keshtkar).

heavy metals. However, these nano-oxides are usually in the colloidal forms and have the drawback in obtaining the spherical beads of suitable size for practical application and are not suitable for water treatment. In recent years, this situation has led to a growing interest in the syntheses and the application of novel adsorbents by loading an oxide on another solid such as polymer and resin [20,34]. Sorption capacity of adsorbents for removal of heavy metal ions increases after modification with –NH2, –SH, – HSO3, tetrazine and phosphoric functional groups, which can react with the metal ions [20,35,36]. In this investigation, at first, TiO2 nanoparticles surface was modified with –NH2 functional group of aminopropyltriethoxyl silane (APTES) and polyvinyl alcohol (PVA) was chosen as a polymeric matrix because of its good film forming properties. Then, the novel mesoporous PVA/TiO2/APTES nanohybrid adsorbent was synthesized via casting method. Specific surface area of PVA/TiO2/APTES nanohybrid adsorbent was measured by the Brunauer–Emmett–Teller (BET). The functional groups and surface structure of the prepared nanohybrid adsorbent were analyzed using Fourier transform infrared (FTIR) and Scanning electron microscopy (SEM). The goal of the present paper was the sorption of Cd(II), Ni(II) and U(VI) on PVA/TiO2/APTES nanohybrid adsorbent. The influences of both TiO2 and APTES contents, pH, contact time, initial concentration, adsorbent dose and temperature on the sorption process were investigated. Also, to optimize the separation process, the appropriate kinetic models (pseudo-first-order, pseudo-second-order and double-exponential), isotherm models

1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.08.013

S. Abbasizadeh et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1656–1664

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(Freundlich, Langmuir and Dubbinin–Radushkevich) and sorption thermodynamics were established. Finally, the reusability of the PVA/TiO2/APTES nanohybrid adsorbent was also determined.

flasks were kept for 4 days and the final pH of the solution was measured using pH meter (827-pH lab, Switzerland). Graphs were then plotted for pHfinal vs. pHinitial.

2. Experimental

2.5. Batch sorption studies

2.1. Materials

All adsorption experiments were performed batch-wise. The effects of pH on sorption of Cd(II), Ni(II) and U(VI) ions were studied by varying the pH of heavy metal solution from 1 to 6. Sorption kinetics was studied by adding 0.1 g of the adsorbent to 100 mL of metal solution with an initial metal concentration of 30 mg L1 in a flask. Measurements of metal concentrations were made at the intervals of 0–6 h at temperature of 25 8C. The sorption isotherms were investigated in various concentrations of heavy metal ion solutions from 30 to 500 mg L1 and at different temperatures (25–45 8C). The residual concentration of Cd(II), Ni(II) and U(VI) ions was determined by an inductivity coupled plasma atomic emission spectrophotometer (ICP-AES, Thermo Jarrel Ash, Model Trace Scan). Analytical wavelengths were set at 231, 228 and 367 nm for Cd(II), Ni(II) and U(VI) ions, respectively. The sorption capacity for heavy metal ions was determined as follows:

PVA polymer (99% hydrolyzed, average MW = 72,000), APTES, HCl, HNO3 and acetone were provided by Sigma–Aldrich. Nanosized TiO2 powder was purchased from Aldrich Co. with average particle size of 10–20 nm. The solutions of Cd(II), Ni(II) and U(VI) ions were prepared by dissolving weighed amounts of cadmium, nickel and uranyl nitrates (Aldrich) in deionized water, respectively. Deionized water was utilized throughout this experiment. 2.2. Surface modification of TiO2 nanoparticles with amine groups The unmodified TiO2 nanoparticles were dried at 400 8C for 6 h before surface modification. Then 0.1 g of TiO2 nanoparticles was added into 10 mL acetone and different amounts of amine groups (5–20 wt%) were dissolved in 10 mL distillated water. After that, the mixture of nanoparticles and APTES was sonicated for 2 h. The reaction mixture was centrifuged and subsequently washed with distillated water two times to remove the unattached coupling agent molecules. Finally, the modified TiO2 nanoparticles were dried for 4 h.

qe ¼

C0 Ce m

(1)

where C0 and Ce (mg L1) are the initial and final metal ion concentrations, respectively; V is the volume of solution (L) and m is the weight of the adsorbent (g). All the experiments were repeated three times and the average values were reported.

2.3. Preparation of PVA/TiO2 and PVA/TiO2/APTES nanohybrids 3. Results PVA/TiO2 nanohybrid adsorbents were fabricated by casting method. At first, aqueous 10 wt% PVA solutions were provided by dissolving 1 g of PVA in 10 ml deionized water, after that, different amounts of unmodified TiO2 nanoparticles (5–30 wt%) were dispersed in PVA solution and then sonicated for 2 h. The resulting mixtures were cast in glass petri dishes. Bubbles were removed by shaking for 30 min. The cast petri dishes were kept at room temperature until dried. For preparation of PVA/TiO2/APTES nanohybrid adsorbents, first, different amounts of modified TiO2 with amine groups (5–20 wt%) were dispersed in PVA solution and then sonicated for 2 h. The mixture of modified nanoparticles and PVA solution was cast in glass petri dishes. Finally, the solvent of nanohybrid adsorbents was removed and the white solids were peeled out and dried in vacuum at 50 8C for 3 h. 2.4. Nanohybrid adsorbent characterization studies Fourier transform infrared spectroscopy (FTIR) spectra of the nanohybrid adsorbents were obtained by German spectrometer (Vector22-Bruker Company, Germany) in the range of 400– 4000 cm1 and a scanning electron microscopy (SEM, JEOL JSM6380) was used to observe the surface morphology of nanohybrid adsorbents. Specific surface area was measured by the Brunauer– Emmett–Teller (BET) method. Total pore volume and pore size were estimated using Brrett–Joyner–Halenda (BJH) method. The point of zero charge (PZC) was investigated for finding the surface charge of PVA/TiO2/APTES nanohybrid adsorbent. At this pH, the nanohybrid absorbent does not induce the release of either H+ or OH ion in solution, therefore, the surface does not undergo or acquire any charge through acid–base dissociation. The pHpzc of nanohybrid adsorbent was determined through the following procedure: 0.01 M NaCl was prepared and its pH was adjusted in the range of 1–6 by adding 0.1 M HCl and/or 0.1 M NaOH solution. Then, 50 mL of 0.1 M NaCl was transferred in a series of 100 mL flasks, and 0.1 g of the adsorbent was added to each solution. These

3.1. Adsorbent characterization The effects of unmodified TiO2 and modified TiO2 weight percentages in nanohybrid adsorbents were studied by scanning electron micrographs (SEM). Typical surface SEM photographs of PVA/TiO2 10 wt%, PVA/TiO2 20 wt% and PVA/TiO2 30 wt% nanohybrid adsorbents were illustrated in Fig. 1a–c. As can be seen, the TiO2 nanoparticles in the nanohybrid adsorbents with 10 and 20 wt% TiO2 content are more uniformly dispersed in smaller size in the PVA matrix than the nanohybrid adsorbent with 30 wt% TiO2. Fig. 1c showed that PVA/TiO2 30 wt% nanohybrid adsorbent aggregated with the nanoparticles. Fig. 1d shows the morphology of PVA/TiO2/APTES nanohybrid adsorbent. This will also indicate that the modified TiO2 with amine groups has excellent adhesion and strong interfacial bonding to the PVA beads [37]. Infrared techniques have been used for identification of PVA/TiO2/APTES nanohybrid adsorbent. For PVA/TiO2/APTES nanohybrid adsorbent spectrum, there is a broad band at around 3100–3600 cm1, which is assigned to O–H stretching for the strong hydrogen bonded hydroxyl groups. The sorption band at 1713 cm1 is due to the stretching vibration of the carbonyl (C5 5O) from the acetate group present after the production of PVA from hydrolysis of polyvinyl acetate. The bands of 1093, 1420 and 2941 cm1 are assigned to the C–O stretching, C–H bending and C–H stretching of PVA, respectively. A new broad band around 550–800 cm1 is assigned to the Ti–O–Ti band and a weak –NH peak is observed in the range of 1540–1580 cm1, which proves that amine groups have been successfully added to the PVA/TiO2/APTES nanohybrid adsorbent. The pHpzc value of PVA/TiO2/APTES nanohybrid adsorbent obtained was 4.1 (Figure is not presented). BET surface area of the nanohybrid adsorbent was 35.98 m2 g1. Based on BJH method, the average pore diameter and pore volume of PVA/TiO2/ APTES nanohybrid adsorbent were 3.08 nm and 0.059 cm3 g1, respectively.

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Fig. 1. SEM images of the prepared nano hybrids PVA/TiO2 (10%) (a) PVA/TiO2 (20%) (b) PVA/TiO2 (30%) (c) PVA/TiO2/APTES.

3.2. Effect of TiO2 content in PVA/TiO2 nano hybrid adsorbents To explain the effect of TiO2 weight percentage on sorption capacity of Cd(II), Ni(II) and U(VI) ions onto the fabricated nanohybrid adsorbent, PVA/TiO2 nanohybrid adsorbents were sensitized in different amounts of TiO2 nanoparticles (0–30 wt% with respect to the PVA weight). The sorption capacity of Cd(II), Ni(II) and U(VI) ions onto PVA/TiO2 nanohybrid adsorbent is shown in Fig. 2. As can be seen, the sorption capacity of Cd(II), Ni(II) and U(VI) ions increases with the increase of TiO2 amounts up to 20 wt% because specific surface area of the nanohybrid adsorbent is increased. Further increase in TiO2 amount leads to a decrease in adsorption capacity of nanohybrid adsorbents for Cd(II), Ni(II) and U(VI) metal ions because of the aggregation of TiO2 nanoparticles which decreases the available sites of the nanohybrid adsorbent. Furthermore, the agglomeration of nanoparticles causes the Cd(II), Ni(II) and U(VI) ions to diffuse with difficulty in the pores on the internal surface of the nanohybrid adsorbent. However, TiO2 (20 wt%) was the optimum amount which was added to PVA solution.

3.3. Effect of APTES content in PVA/TiO2/APTES nano hybrid adsorbents The effect of APTES weight percentage on sorption capacity of Cd(II), Ni(II) and U(VI) ions onto PVA/TiO2 nanohybrid adsorbent was investigated in different amounts of APTES (5–20 wt% with respect to the PVA weight). The results are shown in Fig. 3. As can be seen, the sorption capacity of Cd(II), Ni(II) and U(VI) ions increases with the increase of APTES amounts up to 10 wt% because of more regular pore structure and more uniform surface. Further increase in APTES amounts causes a decrease in sorption capacity of Cd(II), Ni(II) and U(VI) ions because the surface area, the pore diameter and the active sites of the mesoporous silica decrease, gradually. However, PVA/TiO2 20%/APTES 10% was selected for the next stage of the adsorption study. 3.4. Effect of pH Solution pH is an important parameter for the sorption experiments. Sorption capacity values of Cd(II), Ni(II) and U(VI)


Fig. 2. Effect of the TiO2 weight percentage in PVA/TiO2 nano hybrid adsorbent for sorption of U(VI) and Th (IV) ions.

ions onto the PVA/TiO2/APTES nanohybrid adsorbent as a function of pH are shown in Fig. 4. As can be seen, the sorption capacities were found to be low at lower pH values and increased with an increase in pH, and then decreased as the pH continued increasing. At a lower pH, because H3O+ vied with Cd(II), Ni(II) and U(VI) ions, the sorbent surface took up more H3O+ reducing heavy metal ions bound to the sorbent surface. H3O+ ions hindered the access of metal ions from the surface functional groups such as –NH2. For pH values between 2 and 4.5, the amount of Cd(II), Ni(II) and U(VI) ions sorption onto the PVA/TiO2/APTES nanohybrid adsorbent increased with the increasing pH because the competition between H3O+ ion and heavy metal ions decreased. There were the distinct maximums in the sorption capacity at pH 5.5,5 and 4.5 for Cd(II), Ni(II) and U(VI) ions, respectively. At higher pH of optimum, the formation of anionic hydroxide complexes decreased the concentration of the free heavy metal ions, thereby the sorption capacity of metal ions was decreased. The observed trend can be explained by the effect of the surface charge of nanohybrid adsorbent and pHpzc. The point of zero charge (PZC) of the PVA/TiO2/APTES nanohybrid adsorbent was 4.1. At pH < pHpzc, the dominant species having high positive charge density made the Cd(II), Ni(II) and U(VI) ions sorption unfavorable due to electrostatic repulsion. Also, stiff competition between H3O+ and heavy metal ions for the active sites would decrease Cd(II), Ni(II) and U(VI) sorption. But at pH > pHpzc, the nanohybrid adsorbent surface was negatively charged, the increasing electrostatic attraction between positive sorbate species and nanohybrid

Fig. 3. Effect of APTES weight percentage in PVA/TiO2/APTES nano hybrid adsorbent for sorption of Cd(II), Ni(II) and U(VI) ions.

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Fig. 4. Effect of pH on Cd(II), Ni(II) and U(VI) sorption onto the PVA/TiO2/APTES nano hybrid adsorbent.

adsorbent would lead to an increase sorption of heavy metal ions [38]. 3.5. Effect of adsorbent dose Adsorbent dosage has a major effect on the sorption process and defines the potential of the adsorbent through the number of binding sites available to remove heavy metal ions at a specified initial concentration. The effect of the adsorbent dosage on Cd(II), Ni(II) and U(VI) ions removal is shown in Fig. 5. In equilibrium, heavy metal sorption capacity increases with an increase in biomass dosage from 0.1 to 1 g L1 because there are sufficient available sorption sites on the PVA/TiO2/APTES nanohybrid adsorbent. But a decreased trend is observed in the range of the adsorbent dose >1 g L1. This decrease can be due to increasing of metal ion mass transfer limitations from bulk of liquid to the solid surface. Therefore, 1 g L1 of the nanohybrid adsorbent dosage is selected for further experiments as the optimum dosage. 3.6. Effect of contact time The effect of contacts time on the sorption of Cd(II), Ni(II) and U(VI) ions onto the PVA/TiO2/APTES nanohybrid adsorbent were indicated in Fig. 6. As can be seen, the sorption capacity shows a rapid increase during the first period then a slow increase followup until equilibrium state is reached. According to the experimental results, the possible sorption processes of the Cd(II), Ni(II) and

Fig. 5. Effect of adsorbent dose on Cd(II), Ni(II) and U(VI) sorption onto the PVA/ TiO2/APTES nano hybrid adsorbent.

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Fig. 6. Effect of contact time on Cd(II), Ni(II) and U(VI) sorption onto the PVA/TiO2/ APTES nano hybrid adsorbent.

U(VI) ions can be divided into two distinct steps: a very rapid initial sorption over 120 min, followed by a long period of much slower sorption. In general, about 90% of the total metal ions sorption was achieved within 120 min because all active sites on the adsorbents surface were vacant and the solution concentration was high. After 5 h, a lot of active sites of nanohybrid adsorbent became saturated and few active sites on the adsorbent were available. Therefore, a contact time of 5 h was selected for all batch sorption experiments. 3.7. Effect of initial concentration and temperature The change in sorption behavior of PVA/TiO2/APTES nanohybrid adsorbent with different metal ions concentrations from 30 to 500 mg L1 is shown in Fig. 7. As can be seen, the sorption capacity of nanohybrid adsorbent increases from 12.9 to 46.0, 4.6 to 12.0 and 8.8 to 32.0 mg g1 at 25 8C with increasing initial concentration of Cd(II), Ni(II) and U(VI) ions, respectively. This uptrend may be due to an increase in the initial ion concentration providing a larger driving force to overcome the whole mass transfer resistance between the solid and liquid phases, thus resulting in higher metal ion adsorption. The result may lead to more collisions between Cd(II), Ni(II) and U(VI) ions and active sites on the PVA/TiO2/APTES nanohybrid adsorbent [39]. In order to investigate the effects of temperature on the sorption capacity of Cd(II), Ni(II) and U(VI) ions onto PVA/TiO2/ APTES nanohybrid adsorbent, the experiments were performed at different temperatures (25–45 8C) the results of which are shown in Fig. 7. As can be seen, the sorption capacity increases with increasing temperature in all metal ions. This may be due to the increased mobility of heavy metal ions and to their tendency to adsorb from the aqueous solution onto the surface of the nanohybrid adsorbent as well as due to an intense activity of binding sites as the temperature increases. 3.8. Sorption kinetics Investigation of sorption kinetics provides valuable information about the mechanism of sorption. Sorption kinetics, demonstrating the solute uptake rate, is one of the most important aspects of the operation defining the efficiency of the process. The kinetic data were analyzed using two reaction-based kinetic models and a diffusion-based model by non-linear regression using the MATLAB software. The first reaction-based model is given by the pseudofirst-order rate equation. This non-linear kinetic model is described by the following rate equation [40]: Pseudo-first-order : qt ¼ qe ð1 expðk1 tÞÞ

(2)

Fig. 7. Effect of initial concentration and temperature on U(VI) (a) Cd(II), (b) and (c) Ni(II) sorption onto the PVA/TiO2/APTES nano hybrid adsorbent.

where qt and qe (mg g1) are the sorption capacities of metal ions onto PVA/TiO2/APTES nanohybrid adsorbent at time t and equilibrium, respectively; k1 (min1) is the pseudo-first-order rate constant. This model indicates that the rate of occupation of sorption sites is proportional to the number of unoccupied sites. The nonlinear second reaction-based model is described by the pseudo-second-order rate equation using the following equation [40]: Pseudo-second-order : qt ¼

k2 q2e t 1 þ k2 qe t

(3)

where k2 (g mg1 min1) is the pseudo-second-order rate constant. This model considers that the rate of occupation of adsorption sites is proportional to the square of number of unoccupied sites. The nonlinear diffusion-based model is described by the double exponential kinetic model using the following equation [41]: Double exponential : qt ¼ qe

D1 expðkD1 tÞ mads

D2 expðkD2 tÞ mads

(4)


where mads (g L1) is the adsorbent concentration, kD1 and kD2 (min1) are the double exponential rate constants, and D1 and D2 (g L1) are the equation constants which correspond to the rapid phase and slow phase, respectively. All model parameters were evaluated by non-linear regression analysis, the results of which were given in Table 1. Furthermore, the residual root mean square errors (RMSE) and the correlation coefficients (R2) of models were used to evaluate the quality of fit. RMSE can be expressed as: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u X n u 1 RMSE ¼ t ðq qi;cal Þ2 n 2 i¼1 i;exp

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the surface of the adsorbent form a saturated layer. The linear form of Langmuir isotherm is given by the following equation [42]: 1 1 1 1 ¼ þ qe qm kL qm C e

(6)

where qe (mg g1) is the amount adsorbed in equilibrium and Ce (mg L1) is equilibrium concentration of the adsorbate. qm (mg g1) and kL (L mg1) Langmuir constants are related to maximum sorption capacity and energy of sorption, respectively. One of the essential parameters of Langmuir equation is the equilibrium parameter or separation factor (RL) that indicates the type of isotherm to be favorable (0 < RL < 1) or unfavorable (RL > 1). RL is given by the following equation:

(5)

where qi,cal and qi,exp are the calculated and experimental values of the sorption capacity, respectively, and n is the number of data points. If the data obtained from the model are close to the experimental results, RMSE will be a small number. Table 1 shows that the coefficient of correlation (R2: 0.999) for the double exponential kinetic model is higher in comparison with the pseudo-first-order and pseudo-second-order kinetic models and the calculated values of RMSE for the double exponential kinetic model (RMSE: 0.074, 0.057 and 0.076 for Cd(II), Ni(II) and U(VI) sorption, respectively) are lower than those of the other kinetic models. It was found that the kinetic data were best fitted by double- exponential kinetic model. This model showed that the sorption process takes place in two steps: transport of the metal ion to the external surface of the adsorbent as a rapid phase and sorption of the metal ion on the interior surface of the adsorbent as a slow phase.

RL ¼

1 1 þ kL C 0

(7)

where C0 (mg L1) is the highest initial solute concentration. The Freundlich isotherm expresses adsorption at multilayer and on energetically heterogeneous surface. It is assumed that the stronger binding sites are occupied first which can be expressed by the following equation [43]: 1 (8) ln ðC e Þ Freundlich : ln ðqe Þ ¼ ln ðK F Þ þ n kF (mg g1)and 1/n are Freundlich constants related to sorption capacity and sorption intensity, respectively. Values of n greater than 1 show the favorable nature of adsorption [44]. The Dubinin– Radushkevich (D–R) equation is generally expressed as follows [45,46]:

3.9. Sorption isotherm

DubininRadushkevich : ln ðqe ¼ ln qDR BDR e2 Þ

The equilibrium sorption isotherm is fundamental in describing the interactive behavior between the adsorbent and sorbate, and is crucial in the design of the adsorption systems. In this study, the relationship between the metal biosorption capacity and the metal concentration in equilibrium has been described by three sorption isotherm models: Langmuir, Freundlich and Dubinin– Radushkevich (D–R). These models were analyzed at three different temperatures (25 8C, 35 8C and 45 8C). The Langmuir model assumes uniform energies of adsorption onto the surface of the adsorbent without any transmigration of sorbate on the plane surface. Furthermore, it assumes that the monolayer adsorption and maximum adsorption occur when the adsorbed molecules on

(9)

where e is the Polanyi potential related to the sorption energy (Eq. (10)). qDR (mmol g1) and BDR (mol2/J2) are (D–R) constants. 1 e ¼ RTln 1 þ (10) Ce where R is the gas universal constant (8.314 J mol1 K1) and T is the absolute temperature (K). The mean free energy of sorption (E (J mol1)) can be calculated according to the following equation: 1 E ¼ pffiffiffiffiffiffiffiffiffiffiffi 2BDR

(11)

Table 1 Kinetic parameters for Cd(II), Ni(II) and U(VI) sorption onto the PVA/TiO2/APTES nano hybrid adsorbent. qexp (mg g1)

Metal

Cd(II) Ni(II) U(VI)

12.91 4.63 8.76 qexp (mg g1)

Metal

Cd(II) Ni(II) U(VI) Metal

Cd(II) Ni(II) U(VI)

12.91 4.63 8.76

K1 (min1)

q (mg g1)

RMSE

R2

0.025 0.029 0.029

12.59 4.48 8.56

0.421 0.182 0.231

0.986 0.979 0.989

Pseudo-second-order model

12.91 4.63 8.76 qexp (mg g1)

Pseudo-first-order model

K2 (g mg1 min1)

q (mg g1)

RMSE

R2

0.002 0.008 0.004

14.31 5.05 9.59

0.251 0.149 0.192

0.995 0.986 0.993

Double-exponential kinetic model D1 (g L1)

KD1 (min1)

D2 (g L1)

KD2 (min1)

q (mg g1)

RMSE

R2

7.35 2.12 7.23

0.013 0.012 0.030

7.50 3.99 1.28

0.074 0.085 0.003

13.02 4.67 9.18

0.074 0.057 0.076

0.999 0.999 0.999


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Table 2 Langmuir, Freundlich and Dubinin–Radushkevich constants for Cd(II), Ni(II) and U(VI) sorption onto the PVA/TiO2/APTES nano hybrid adsorbent. Temperature

Metal

Cd(II)

U(VI)

Cd(II)

U(VI)

Ni(II)

U(VI)

KL (L mg1)

RL

R2

43.29 46.30 49.02 11.11 12.14 13.01 29.67 33.11 36.10

0.023 0.027 0.029 0.025 0.026 0.029 0.018 0.017 0.018

0.079 0.069 0.065 0.075 0.070 0.065 0.099 0.106 0.100

0.955 0.958 0.970 0.908 0.923 0.924 0.964 0.977 0.975

25 8C 35 8C 45 8C 25 8C 35 8C 45 8C 25 8C 35 8C 45 8C

Ni(II)

Cd(II)

qm (mg g1)

Temperature

Metal

Metal

Langmuir isotherm

25 8C 35 8C 45 8C 25 8C 35 8C 45 8C 25 8C 35 8C 45 8C

Ni(II)

Temperature

25 8C 35 8C 45 8C 25 8C 35 8C 45 8C 25 8C 35 8C 45 8C

sorption sites possess a heterogeneous nature. As can be noticed, the values of n are greater than 1 indicating that the adsorption is favorable. Table 2 indicates a comparison of maximum adsorption capacity of different heavy metal ions on the PVA/TiO2/APTES nanohybrid adsorbent. It can be seen that PVA/TiO2/APTES nanohybrid adsorbent has a relatively large adsorption capacity of 49.0, 13.1and 36.1 mg g1 for Cd(II), Ni(II) and U(VI) ions, respectively. The values of RL were between 0 and 1 indicating favorable sorption for Cd(II), Ni(II) and U(VI) ions. As can be seen, the values of the adsorption free energy are between 1 and 8 kJ mol1 indicating physical sorption for Cd(II), Ni(II) and U(VI) ions onto the PVA/TiO2/APTES nanohybrid adsorbent. The maximum sorption capacities of Cd(II), Ni(II) and U(VI) ions onto the PVA/TiO2/APTES nanohybrid adsorbent are compared with those of the other adsorbents reported in the literature in Table 3. As can be seen in Table 3, the maximum sorption capacities of Cd(II), Ni(II) and U(VI) ions onto the nanohybrid are in the range of maximum sorption capacities of these metal ions obtained by other researchers.

Freundlich isotherm KF(mg g1)

n

R2

4.21 5.34 6.07 1.49 1.76 2.09 2.47 2.51 2.91

2.50 2.69 2.78 2.96 3.08 3.25 2.40 2.33 2.37

0.982 0.980 0.985 0.995 0.998 0.998 0.996 0.995 0.997

3.10. Sorption thermodynamics The data obtained from the sorption experiments of the effect of temperature were used to estimate the values of thermodynamic parameters such as Gibbs energy changes (DG8), enthalpy changes (DH8), and entropy changes (DS8). The equilibrium adsorption constant (KC) depending on the temperature was used to determine the values of DG8, DH8 and DS8. The values of KC were obtained from the following equation [20]:

Dubinin–Radushkevich isotherm qDR (mmol g1)

BDR (mol2 J2) 108

E (KJ mol1)

R2

0.350 0.378 0.402 0.170 0.186 0.200 0.116 0.125 0.140

4.981 4.445 4.209 10.300 9.790 9.126 3.175 3.437 3.289

3.168 3.354 3.447 2.203 2.260 2.341 3.968 3.816 3.899

0.882 0.894 0.911 0.787 0.798 0.799 0.927 0.941 0.937

K C ¼ limCe ! 0

C es C el

(12)

where Cel (mg L1) is the equilibrium concentration of metal ions in liquid phase and Ces (mg L1) is the equilibrium concentration of metal ions in solid phases. The values of kC at different temperatures (25 8C, 35 8C and 45 8C) are calculated according to the plot of the experimental data of Ces/Cel versus Cel (figure is not presented). The adsorption standard free energy (DG8) is calculated from:

The value of E (KJ/mol) specifies the type of sorption mechanism. Physical sorption processes have sorption energy in the range of 1– 8 kJ mol1. If E value lies between 8 and 16 kJ mol1, the chemical sorption becomes a dominant mechanism. Parameters of these isotherm models at different temperatures and the results are presented in Table 2. Higher correlation coefficients show that Freundlich model fits the sorption data better than Langmuir and D–R models. The reason may be that Langmuir model assumes that the surface of the sorbent can accommodate only a monolayer of the Cd(II), Ni(II) and U(VI) ions without any interaction between the sorbed species. On the other hand, Freundlich isotherm model does not have any restriction on the sorption capacity of the sorbent, and is more appropriate in the situations where the

DG ¼ RTlnðK C Þ

(13)

where R (8.314 J mol1 K1) is the gas constant. The apparent thermodynamic parameters DH8 and DS8 for the sorption process are calculated from the slopes and intercepts of the linear variation of ln Kc vs. 1/T by the following equation [47]: lnðK C Þ ¼

DS0 R

DH0 R

1

(14)

t

The calculated values of the thermodynamic parameters are given in Table 4. The negative values of DG8 and increasing the negative

Table 3 Comparison of adsorption capacity (mg g1) of PVA/TiO2/APTES nano hybrid adsorbent for Cd(II), Ni(II) and U(VI) sorption with other adsorbents reported in the literature. Adsorbent

qm,Cd(II) (mg g1)

qm,Ni(II) (mg g1)

qm,U(VI) (mg g1)

References

Orange peel powder Gracillaria biomass Natural diatomite MnO2 loaded D301 resin Bagasse fly ash FA-treated N. zanardini Polyaryl ether ketone Polyacrylamide- zeolite Clinoptilolite Diatomite (Kieselguhr) Acid-activated kaolin PVA/TiO2/APTES

40.0 33.7 – 77.9 2.0 19.6 69.9 61.4 – – – 49.0

– 16.4 – – 1.7 16.2 68.0 10.3 16.6 – – 13.0

– – 25.6 –

[1] [6] [7] [34] [43] [45] [49] [50] [51] [52] [53] This study

– – – – 38.6 4.5 36.1


1663

Table 4 Thermodynamic parameters for Cd(II), Ni(II) and U(VI) sorption onto the PVA/TiO2/APTES nano hybrid adsorbent. Metal

Kc 25 8C

35 8C

45 8C

Cd(II) Ni(II) U(VI)

4.21 1.49 2.47

5.54 1.76 2.51

6.07 2.09 2.90

DH8 (J mol1)

DS8 (J mol1 K1)

10082.0 8924.0 3882.0

46.2 33.5 20.6

values with increasing temperature of all three Cd(II), Ni(II) and U(VI) ions confirm the spontaneous nature. Furthermore, the increase in DG8 values with an increase in temperature indicates that the adsorption is favorable at higher temperatures. The enthalpy of the adsorption, DH8, is a measure of the energy barrier that must be overcome by reacting molecules. The positive values of DH8 indicate the endothermic behavior of the adsorption reaction of metal ions and suggest that a large amount of heat is consumed to transfer the metal ions from the aqueous into the solid phase. The value of DS8 can be used to indicate whether the sorption reaction is ascribed to associative or dissociative mechanism. If DS8 becomes greater than 10 J mol1 K1, the dissociative mechanism becomes a dominant [48]. Before the adsorption, the heavy metal ions near the surface of the adsorbent will be more systematically configured than the subsequent adsorbed state and the ratio of free heavy metal ions to ions interacting with the adsorbent will be higher than the adsorbed state. The entropy changes in this investigation are positive in all metal ions implying that the dissociative mechanism is involved in the sorption processes. Also, the positive values of DS8 for all three metal ions indicate that the randomness at the solid–liquid interface during the adsorption process increases. 3.11. Regeneration of PVA/TiO2/APTES nano hybrid adsorbent Regeneration of the adsorbent for the repeated uses is of crucial importance in industrial practice for metal removal from wastewater. For desorption of Cd(II), Ni(II) and U(VI) ions from the PVA/TiO2/APTES nano hybrid adsorbent, 0.5 M HNO3/0.1 M HCl in equal ratio solution at a temperature of 25 8C, contact time of 5 h and initial metal ion concentration of 30 mg L1 was used as a desorbing agent. Under these conditions, the metal ions were now transferred from the sorbents to the desorbing agent until equilibrium was reached once more. The results of sorption– desorption investigation are shown in Fig. 8. For PVA/TiO2/APTES nano hybrid adsorbent, sorption capacity of Cd(II), Ni(II) and U(VI) ions decreased only from 12.91 to 11.51 (mg g1), 4.63 to 4.02

DG8 (J mol1) 25 8C

35 8C

45 8C

3563.0 983.6 2243.3

4290.4 1445.4 2352.6

4767.3 1945.9 2817.0

(mg g1) and 8.76 to 7.52(mg g1) after five cycles of sorptiondesorption, respectively. This shows that the PVA/TiO2/APTES nano hybrid adsorbent can be reused frequently without almost any significant loss in sorption performance and can be a promising material for removal and recovery of heavy metal ions from aqueous solutions. 4. Conclusion A novel cast PVA/TiO2/APTES nanohybrid adsorbent was used for sorption of Cd(II), Ni(II) and U(VI) ions from aqueous solutions. TiO2 content of 20 wt%, APTES content of 10 wt%, adsorbent dose of 1 g L1, contact time of 5 h, temperature of 45 8C and pHs 5.5, 5 and 4.5 for Cd(II), Ni(II) and U(VI) ions, respectively were optimum conditions for sorption process in the batch system. The SEM images of the nano hybrid adsorbents with 10 and 20 wt% TiO2 content showed that the TiO2 nanoparticles in these adsorbents were more uniformly dispersed in smaller size compared with the nanohybrid adsorbent with 30 wt% TiO2. FTIR spectra indicated that PVA/TiO2/APTES nanohybrid adsorbent was functionalized by amine groups. The pore diameter and pore volume of the nanohybrid adsorbent were 3.08 nm and 0.059 cm3 g1, respectively utilizing BJH method and the surface area of PVA/TiO2/APTES nanohybrid adsorbent was 35.98 m2 g1 utilizing BET method. The kinetic data were best fitted by double-exponential kinetic model with high regression coefficient (R2 > 0.99) and small RMSE. This model showed that sorption of Cd(II), Ni(II) and U(VI) ions consisted of two phases: the first phase was rapid and the second phase was slow. The sorption isotherms of all three heavy metal ions are exactly fitted by the Freundlich model in comparison with the Langmuir and D–R models. It showed that the sorption sites have a heterogeneous nature. The maximum sorption capacities of PVA/TiO2/APTES nanohybrid adsorbent were 49.0, 13.0 and 36.1 mg g1 for Cd(II), Ni(II) and U(VI) ions, respectively. By comparing the qm values obtained from the Langmuir model, it is observed that the functional groups on the surface of the nanohybrid adsorbent has a relatively stronger affinity for Cd(II) and U(VI) ions than Ni(II) ion. The affinity order for heavy metal ions is as follows: Cd(II) > U(VI) > Ni(II). The positive value of DH8 confirms the endothermic nature of the sorption process. The negative values of DG8 are due to the fact that the sorption process is spontaneous with the affinity of heavy metal ions onto the nanohybrid adsorbent. The increase in DG8 values with an increase in temperature indicates that the sorption is favorable at higher temperatures. Repeated sorption and desorption cycles showed the feasibility of these newly synthesized nanohybrid adsorbent for heavy metal removal. References [1] [2] [3] [4] [5]

Fig. 8. Five cycles of Cd(II), Ni(II) and U(VI) sorption–desorption with 0.5 M HNO3/ 0.1 M HCl in equal ratio solution.

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