Intensification of Co(II) adsorption from aqueous solution

Journal of Applied Research in Water and Wastewater 12 (2019) 144-149

Please cite this article as: T. Mansouri Jalilian, N. Azimi, S. Ahmadi, Intensification of Co(II) adsorption from aqueous solution onto Fe3O4/bentonite

nanocomposite by high frequency ultrasound waves, Journal of Applied Research in Water and Wastewater, 6 (2), 2019, 144-149.

Original paper

Intensification of Co(II) adsorption from aqueous solution onto Fe3O4/bentonite nanocomposite by high-frequency ultrasound waves Tahereh Mansouri Jalilian1, Neda Azimi*,1, Shahin Ahmadi2

1Department of Chemical Engineering, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran. 2Department of Chemistry, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran.

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Article history: Received 17 October 2019 Received in revised form 20 November 2019 Accepted 24 November 2019

The effect of ultrasound on cobalt adsorption from aqueous solution onto Fe3O4/Bentonite nanocomposite is investigated. Two layouts of using shaker and sono-separator equipped with ultrasound are considered. The effect of pH on Co(II) ions removal is investigated. Co(II) removal rate increased with increasing pH from 2 to 10, and it reduced after pH=10. For the shaker, the contact time (t) of 50 min is selected as the most effective case. However, for sono-separator the maximum value of Co(II) removal rate is 78% at t=10 min, and it decreased after 10 min. The effect of the adsorbent mass (AM) is investigated and Co(II) removal increased by increasing the specific surface area of the adsorbent. The highest Co(II) removal rates are 83.3% and 86% for the shaker and the sono-separator, respectively. No significant increase for Co(II) removal is observed for increasing AM more than 3 g. The effect of the transducer locations and initial concentration of Co(II) ions (C0) at pH=10 and AM =3 g are investigated. The results showed that the activation of all transducers had the best performance. Initially, with increasing C0 from 0.05 to 0.1 g/L, Co(II) removal rate increased from 84% to 86%, respectively, but with increasing C0 from 0.1 to 0.15 and 0.2 g/L, cobalt removal has been decreased. Finally, the experimental data are adopted with Langmuir and Freundlich isotherms. The comparison of these models showed that both models are well suited to experimental data and data compatibility with the Langmuir model is greater.

©2019 Razi University-All rights reserved.

Keywords: Adsorption Ultrasound Fe3O4 Bentonite Nanoparticles Sono-separator

1. Introduction

Cobalt is a most hazardous heavy metal in the industries effluents, and its combination is widely used in many industrial applications such as mining, electroplating, metallurgical, paints, and electronics (Saeed and Hadi 2017; Sayadi et al. 2014a). The presence of cobalt (Co(II))

ions in the environment can lead to many health problems, such as low blood pressure, vomiting, nausea, heart disease, vision problems, bleeding, diarrhea, bone defects and mutation (genetic changes) (Sayadi et al. 2014b) Therefore, finding the effective method to remove Co(II) ions from aqueous solutions for environmental protection is essential. Many technologies have been developed for the removal of

*Corresponding author Email: [email protected]

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Mansouri Jalilian et al. / J. App. Res. Wat. Wast. 12 (2019) 144-149



Co(II) ions from aqueous solutions such as chemical precipitation, oxidation, reverse osmosis, membrane electrolysis, coagulation, and absorption (Ghaleno et al. 2015, Mohan et al. 2007, Sayadi et al. 2009, Smedley et al. 2002). Among these available methods, the adsorption technique has been widely used, since it is simple, consistent, and economical and has a low cost. Many materials such as clay, oxide, activated carbon, zeolite, ion exchange resins, and cellulose have been used as adsorbents. These methods often have deficiencies and limitations such as low efficiency, high-energy requirements, the need for specific chemicals, producing large quantities of sludge and problems of sludge disposal with contaminants. Adsorption is an important technique of separation process in many natural, physical, biological, and chemical industries because of the high capability of solid substances to attract the molecules of gases or solutions. In recent years, the adsorption process has been used as one of the best methods for the removal of metal ions, which is preferred to other methods because of its easy to use and low costs (Jamshidi et al. 2016). However, adsorption processes have a low mass transfer rate, difficulty to the regeneration of adsorbent and limitations for development and application (Ji et al. 2006).

Recently, magnetite based nanocomposite absorbers have been used to adsorb heavy metals from aqueous effluents (Bhargavi et al. 2015, Nguyen et al. 2006, Wang et al. 2014). Magnetic nanoparticles of iron oxide (Fe3O4 or γ-Fe2O3) can be used as a suitable candidate for the preparation of magnetic nano-composites due to their specific characteristics. These features include magnetic properties such as high paramagnetic properties, suitable chemical properties such as low toxicity, biocompatibility, biodegradability, simple synthesis, and low cost (Wu et al. 2005). Besides, the magnetic adsorbent can be separated from the medium by a simple magnetic process such as a permanent magnet. Ultrasound energy used by many researchers to augment the chemical process because of inducing acoustic cavitation. Ultrasound is well known as the formation, growth, and collapse of very small bubbles that formed by the propagation of a pressure wave through a liquid. Ultrasound is a useful technique to intensify the mass transfer process and breaking the affinity between adsorbate and adsorbent. Shock waves have the potential of creating microscopic turbulence within interfacial films surrounding nearby solid particles (Fernandes et al. 2011, Hamdaoui et al. 2009). Acoustic streaming induced by the ultrasound is the movement of the liquid, which can be considered the conversion of sound to the kinetic energy. These phenomena increase the rate of mass transfer near the surface. Ultrasound, and its secondary effect, cavitation (nucleation, growth and transient collapse of tiny gas bubbles) improve the mass transfer through convection pathway that is emerged from physical phenomena such as micro-streaming, micro-turbulence, acoustic (or shock) waves and microjets without significant change in equilibrium characteristics of the adsorption/desorption system (Asfaram et al. 2015; Parvizian et al. 2015; Mondragon et al. 2012; Rahimi et al. 2013; Sayadi et al. 2014c; Zou et al. 2014).

In several researches (Jamshidi et al. 2016; Ji et al. 2006; Mehrabi et a. 2016; Schueller et al. 2001), it was illustrated that mass transfer rate and adsorption process were enhanced by the presence of ultrasound. It can be deduced that in most of these works, low frequency ultrasound is used in order to boost the adsorption process and application of the ultrasound waves with the frequency above 1 MHz in mass transfer processes are restricted in the literature. The ultrasound waves with frequency in the range of MHz are able to generate more intense acoustic streaming which it is responsible for mass transfer rate augmentation.

The novelty of this research is to investigate the effect of high frequency ultrasound waves on the adsorption of cobalt ions from aqueous solution using Fe3O4/ bentonite nanocomposite. A cubic container equipped with five 1.7 MHz ultrasound transducers was fabricated to introduce ultrasound waves into the aqueous solution. The results of Co(II) removal efficiency and mass transfer characteristics of using ultrasound were compared with those of shaking. The effect of five independent parameters such as pH, adsorbent mass (AM), initial concentration of Co(II), sonication time and the location of ultrasound transducers on the removal efficiency of Co(II) were investigated.

2. Experimental section 2.1. Experimental setup and apparatus

A real photograph of the cubic container equipped with five ultrasound transducers (namely sono-separator) used in the present work is depicted in Fig. 1. The main body of the sono-separator, which

is a cubic container, is fabricated of U-PVC plates with the height and wide of 12.6 cm. The volume of the sono-separator is about 2 liters. A Plexiglass cube with the dimension of 14cm×14cm×5cm as the base is located in the bottom of sono-separator. In this sono-separator, five piezoelectric transducers (PZT) with a diameter of 2.5 cm and a frequency of 1.7 MHz are used. So, four PZTs are placed on the body of each vertical face and one of them is located in the center of the bottom plate of the sono-separator. This arrangement causes to PZTs do not face to each other; the streams created by the PZTs do not collide together. Five PVC keepers known as Glend are used to place the PZTs on the body of the sono-separator.

Fig. 1. Real photograph of experimental setup used in the present work.

Fig. 2 (a) showed a real photograph of the piezoelectric transducer with the frequency of 1.7 MHz and the Glend used in the present work. Fig. 2 (b) depicted the real photograph of PZT actuator used in this study. The piezoelectric transducer inside the Glend is located between the two rubber gaskets to prevent fluid leakage. Because of the elasticity of the rubber gaskets, it does not prevent the vibration of piezoelectric transducer. In order to prevent fluid leakage, surrounding the Glends attached to the body of the sono-separator sealed with silicone glue. 2.2. Materials and solution preparation 2.2.1. Preparation of Fe3O4/bentonite magnetic composite

In all experiments, Deionized water (DI-water) is used to make the processes more efficient. The nanoparticles of Fe3O4 and bentonite manufacturing of Asia-Pacific Co. have been used to synthesis the adsorbent. Magnetic composite of Fe3O4/bentonite nanoparticles was prepared by adding 1 g of Fe3O4 nanoparticles and 10 g bentonite nanoparticles into a 400-mL DI-water at room temperature. The pH was slowly increased by adding NaOH (5 %) solution to about 10 and stirring continued for 30 min and stirring was then stopped. The suspension was heated to 95–110 °C for 2 h. After cooling, the prepared magnetic composite was repeatedly washed with DI-water. Through a simple magnetic procedure, the obtained materials were separated from water and dried in an oven at 110 °C.

2.2.2. Preparation of solution

A stock solution of Co(II) was prepared by dissolving cobalt nitrate in DI-water, and that solution was diluted to the desired concentration for actual use. Firstly, 100 mg of cobalt nitrate powder is dissolved in 1 liter of DI-water in a separate container (initial concentration of 100 mg/L). Then, using a shaker at 150 rpm, the desired solution at room temperature (25 ºC) is completely mixed. In order to determine the effect of pH, NaOH and HCL solutions with 0.1 molar concentration were used and the pH was measured using pH meter (model: Eutech pH 700, Singapore). In this study, the effect of adsorbent concentration (Fe3O4/bentonite magnetic composite) on the removal of Co(II) ions from the base solution is investigated. The amount of adsorbent was investigated as a variable with the domain of 0.5-4 g (0.5, 1, 2, 3 and 4) Fe3O4/bentonite nanoparticles. In order to make any solution, a certain amount of adsorbent is dissolved in a solution containing Co(II) ions. At this stage, each solution containing Co(II) ions and adsorbent at five different pH values of 2, 4, 7, 10 and 12 were investigated. By adding 0.1 M NaOH and 0.1 M HCL solutions, the solution can be base or acidic. It should be noted that the choice of the pH range with respect to the Eh-pH curves is related to the different modes of Co(II) ions.

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2.3. Experiment procedures

In this study, the experiments were carried out in two layouts: Firstly, a solution containing Co(II) ions and the sorbent was poured into a 2-liter beaker similar to the sono-separator and placed on a shaker at 400 rpm for a specified period. The solution was then passed through a Watmen filter with the size of 0.45 microns (made in Germany) to remove all magnetic bentonite nanoparticles. Then, the solution was analyzed by atomic adsorption decomposition to determine the recovery rate of Co(II) adsorption. In this research, an atomic adsorption device (Thermo model) was used to analyze the samples (Almasi et al. 2018); the calibration curve was determined for the Co(II) ions and then proceeded to the next steps of the experiments. Secondly, in order to investigate the effect of high frequency ultrasound waves on Co(II) adsorption, the previous experiments in the layout of using shaker, were repeated in the sono-separator. 2 liters of suspension containing Co(II) ions and adsorbent were poured into the sono-separator, the piezoelectric actuator switched on ultrasound transducers, and then ultrasound waves propagate into the solution. The solution was subjected to ultrasound waves for a specified period. In the presence of high-frequency ultrasonic waves, the need for mixing with a mechanical stirrer will be eliminated. After the prescribed time, the solution is removed from the sono-separator and finally, a permanent magnet used to separate Fe3O4/bentonite nanoparticles. So that, all Fe3O4/bentonite nanoparticles are taken with the magnet and the solution is free of adsorbent. Then, the solution was analyzed to determine the recovery rate of Co(II) adsorption. All experiments were carried out at room temperature and each experiment was repeated three times, and the average of the three obtained values was reported as the final amount of adsorbed Co(II) ions.

(a)

(b)

Fig. 2. Real photographs of (a) the glend and the piezoelectric transducer (PZT), (b) PZT actuator.

2.4. Experimental data processing

When the time of the adsorption is long enough, the adsorption system reaches to equilibrium state and thus the equilibrium amount adsorbed of Co(II) ions can be calculated by Eq. 1 (Mousavi et al. 2019; Nayeri et al. 2019).

0 ee

C Cq V

m

(1)

where, qe is the absorption capacity of Co(II) ions in equilibrium state,

C0 and Ce are the initial and final concentration of Co(II) ions in solution, respectively. V is the volume of solution, and m is the mass of adsorbent. The removal efficiency of Co(II) ions was calculated as follow (Nayeri et al. 2019; Mousavi et al. 2019).

0

0

Re efficiency 100eC Cmoval

C

(2)

3. Results and discussion 3.1. Effect of pH on the adsorption of Co(II) ions

pH is a key factor in the absorbing of heavy metal ions from solutions. Therefore, pH dependence was investigated for the removal of Co(II) ions at a constant contact time in both layouts of using the shaker and ultrasound waves. Fig. 3 shows the percentage removal of Co(II) ions from the solution using the shaker and the sono-separator at the temperature of 25 °C, adsorbent mass (AM) of 0.5 g, and the different contact times. At this step, all PZTs on the body of the sono-separator switched on and the ultrasound waves propagated into the sono-separator. As shown in the Fig. 3, for both layouts, a reduction of the efficiency of Co(II) removal increased from 2 to 10, and after pH=10 it decreased and it has the lowest amount of Co(II) removal at pH=12. In fact, pH is one of the most important parameters for the controlling of the adsorption process of metal ions. There are two main factors influencing the effect of pH on the adsorption of metal contaminants on the magnetic bentonite nanoparticles.

(a)

(b)

Fig. 3. Removal efficiency of Co(II) using shaker and sono-separator at T=25 °C and AM =1 g. (a) t=20 min, (b) t=40 min.

One of them is the metallic pollutant ion, and the other is the surface of adsorbent. In this section, the effect of pH on each of these factors investigated separately. The surface of the bentonite nanoparticles has a negative charge. In order to have a high adsorption capacity, the pollutant must have a positive charge. While the bentonite added to the magnetic nanoparticles of Fe3O4, at low acidic environments (i.e., pH less than 3), these nanoparticles converted into Fe (II) and (III). On the other hand, Fe3O4 nanoparticles can easily be converted to iron hydroxide (II) and (III). Thus, it should be noted that although bentonite is not influenced by pH, it easily loses its properties due to degradation of magnetite nanoparticles in acidic or base environments. On the other hand, it is necessary to study the properties of the metal contaminants to better understand the effect of pH. In order to investigate the effect

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of pH on the metal contaminants, it should be noted that the metal in the acidic environments is ionic and when they enter in the base environment, they lose their ionic state and react with the OH- groups in the environment. This process causes the metal to lose its positive charge and become neutral or negative (in the complexation between metal and OH- groups). Therefore, according to the mentioned factors, there is only a small range of acid-to-base pH, in which the amount of metal ion adsorption on the surface of absorbent is high. Optimization studies showed that the optimal pH value is about 10, which indicates that at this pH, the magnetic adsorbent and cobalt metal ions are completely active in the environment. According to Fig. 3, it can be seen that with increasing pH, the amount of Co(II) ion adsorption has increased, so that at pH=10 and pH=2, the maximum and the minimum values have been obtained, respectively. After pH=10 a decreasing trend can be seen. Therefore, all subsequent experiments were performed at pH=10. The comparison between Fig. 3 a and b shows that with increasing contact time from 20 min to 40 min, the removal rate of Co(II) ions in the shaker increased while for the use of the sonaractor it decreased. Therefore, the contact time is an important parameter and should be investigated further. In the next section, the effect of this parameter was evaluated. 3.2. Effect of contact time on the adsorption of Co(II) ions

Fig. 4 shows the time-dependent removal behavior of Co(II) ions from aqueous solution using magnetic bentonite adsorbent.

Fig. 4. Effect of contact time on the removal efficiency of Co(II) using shaker and sono-separator at T=25°C, AM =1 g and pH=10.

As shown, for both layouts, the amount of Co(II) removal increases with increasing the contact time and the equilibrium time from 2 min to 10 min. However, concerning the use of the shaker, the removal rate of Co(II) ions from the contact time between 10 min and 50 min was still increasing and in 50 min, the percentage of removal was equal. In fact, the percentage of Co(II) removal in the shaker has reached to its maximum value of 74% within 50 min. The initial adsorption rate is very fast to adsorb Co(II) ions due to a large number of sites on the nanoparticles. According to the results obtained for using the shaker, the contact time of 50 min was chosen as the most effective contact time to remove Co(II) ions. However, as shown in Fig. 4, in the case of the sono-separator, the removal rate of Co(II) is very fast in the first 2-10 minutes, and its maximum value at contact time of 10 min reached to 78 % and after that, the removal rate of Co(II) ions has been decreasing. In fact, the adsorbent surface and its sites initially increased by ultrasound propagation in the solution and hence, the adsorption rate of Co(II) was high while after 10 min, apparently due to the discharge of Co(II) ions from the adsorbent surface, the desorption process has occurred and Co(II) ions transferred from the surface of the absorbent to the aqueous solution. This means that high-frequency ultrasound waves can produce high percentages of removal of Co(II) from aqueous solution, due to the creation of micro-screams and acoustic cavitation phenomena. The application of ultrasound waves into the sono-separator increases the adsorption rate over a short time using a small amount of adsorbent material. Ultrasound through secondary activities such as cavitation (nucleation, growth, and temporary collapse of small gas bubbles) increases the mass transfer through the physical phenomena such as micro-streams, micro-turbulences, acoustic waves (or shock), and micro-jets. These phenomena occurred without significantly altering the equilibrium properties of the adsorption

/desorption system. According to the results obtained for using the sono-separator, the contact time of 10 min selected as the most effective contact time for the removal of Co(II) ions under the influence of high-frequency ultrasound waves. 3.3. Effect of adsorbent mass on the adsorption of Co(II) ions

In order to investigate the effect of adsorbent mass on the Co(II) ions adsorption process, four other values for adsorbent containing 1, 2, 3 and 4 g of the magnetic bentonite nanoparticles in 2 liters of Co(II) solution were tested. Fig. 5 shows the effect of the adsorbent mass on Co(II) ion removal from aqueous solution for both layouts of using shaker and the sono-separator. Based on this figure, the adsorption rate of Co(II) in the high amount of adsorbent increased due to increasing its surface area.

Fig. 5. Effect of absorbent mass (AM) on the removal efficiency of

Co(II) using shaker and sono-separator at T=25°C and pH=10.

Increasing the specific area and the availability of more active adsorbent sites in a higher amount of adsorbent is associated with an increase in adsorption rate. By using the shaker and the sono-separator, the highest removal percentage of Co(II) are 83.3% and 86%, respectively, with the contact times of 50 min and 10 min, respectively. In both cases, the efficient amount of adsorbent was 3 g and no significant increase was observed with increasing absorbent dose of more than 3 g for Co(II) removal. Considering that in all of the above experiments, the sono-separator with a shorter contact time compared to the shaker, performed better in removing Co(II) ions. So in the next experiments, the sono-separator was used. 3.4. Investigation the effect of PZTs locations on the adsorption of Co(II) ions

In this section, the effect of the position of PZTs on the removal of Co(II) from the aqueous solution at pH = 10 and AM = 3 g has investigated in five states: 1. Two piezoelectric transducers on the opposite faces of the sono-separator are activated (Case1). 2. Two piezoelectric transducers on the adjacent faces of the sono-separator are activated (Case 2). 3. All four piezoelectric transducers on the four faces of the sono-separator are activated (Case 3). 4. Only the piezoelectric transducer on the bottom plate of sono-separator is activated (Case 4). 5. All of the piezoelectric transducers on the body of the sono-separator are activated (Case 5). Table 1 shows the results of the study on the effect of the position of PZTs on the removal of Co(II) ions from the aqueous solution. The results show that the activation of all PZTs has had the best performance for removing Co(II) ions. Nevertheless, the results of the activation of the bottom PZT have also been close to the performance of the activation all PZTs. This is due to the ability of bottom PZT to create micro-streams, and therefore dispersion and throwing of absorbent nanoparticles in the aqueous solutions that prevent their sedimentation, and increase the special adsorbent surface due to acoustic cavitation and micro-jets. This effect has been greater in the case that all PZTs are switched on. According to this table, when two adjacent PZTs are active, the lowest percentage of Co(II) removal (71%) is observed, because the ultrasound waves induced from two adjacent PZTs have interfered and damped by each other. After case 4 and 5, case 3 states that the activation of all four PZTs on the vertical faces of sono-separator has a better performance than the cases of 1 and 2. In general, it can be said that high-frequency

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ultrasound streams have a high ability to disperse the nanoparticles inside the solution, and also the collision of fluid jets with them, causing more rotational forces and at a smaller scale near the nanoparticle surface, and therefore the mixing process and the adsorption of Co(II) removal on the adsorbent surface have intensified.

Table 1. Effect of PZT locations on the removal efficiency of Co(II) ions from aqueous solution (pH=10, m=3 g and T=25 °c).

Case of activated PZT Removal efficiency, %

Case 1 73.6 Case 2 71 Case 3 79.5 Case 4 82.8 Case 5 86

3.5. Effect of initial concentration of Co(II) ions on the adsorption process

The effect of initial cobalt concentration is another parameter that studied for the removal of Co(II) by magnetic bentonite nanoparticles. In all experiments of before sections, the initial concentration of Co(II) was 0.1 g/L. In this section, three concentrations including 0.05 g/L, 0.15 g/L and 0.2 g/L were investigated. Fig. 6 shows the effect of the initial concentration of Co(II) ions in the aqueous solution on its percentage removal at AM =3 g and pH=10 while all PZTs were activated.

Fig. 6. Effect of initial concentration of Co(II) on its removal efficiency using sono-separator at T=25°C, AM =3 g and pH=10.

The adsorption of Co(II) significantly affect by the initial concentration of Co(II) ions in the aqueous solution. In this section, the initial concentration of Co(II) varies from 50 to 200 mg/L, while the adsorbent mass is 3 g/L, pH=10, and the contact time is 10 min for use of the sono-separator. Experimental data were adapted to Langmuir and Freundlich isotherm models. Langmuir isotherm model is used to describe the chemical composition and coating of an absorber layer on the nanoparticles, and its linear form can be expressed by the following equation (Sayadi et al. 2016c).

1 1ee

e L m m

CC

q K q q (3)

where, qe is the amount of Co(II) adsorbed in equilibrium state in mg/g, Ce is equal to the concentration of Co(II) in the solution in mg/L, qmax and KL are the Langmuir constants, indicating the adsorbent saturation capacity and the energy term. By data fitting with the Langmuir model, as shown in Fig. 7, 1/qm is equal to 0.796, that means qm =1.25 g/L and KL = 2.96 L/g. The Freundlich isotherm model illustrates the distribution of active and energy sites and heterogeneous adsorbent surfaces by the following equation (Sayadi et al. 2016c).

1/n

e f eq K C (4)

where, Kf and 1/n are Freundlich constants related to absorption capacity and absorption intensity. From Fig. 6, it can be concluded that there is high dependence between the removal efficiency and the initial concentration of Co(II) ions. So that, by increasing the initial concentration, the available adsorbent sites are reduced and, as a

result, the removal efficiency decreases. Since in the low concentration, available sites to the adsorbent surface are greater. As shown in Fig. 6, by an increase in the concentration of Co(II) ions from 0.05 to 0.1 g/L, the percentage of Co(II) ions decreased from 84% to 86%, respectively. While by increasing Co from 0.1 to 0.15 and 0.2 g/L, the percentage of Co(II) ions has decreasing trend, which is due to the reduction of the adsorption capacity of Co(II) by nanoparticles and filling their capacity. By increasing the initial concentration of Co(II) ions in the solution, the removal efficiency is reduced. Increasing the initial concentration of Co(II) ions causes less adsorbed sites to adsorb more cobalt in the solution. Therefore, adsorbent in lower initial concentration has better adsorption properties. 3.6. Adsorption isotherms

As shown in Fig. 8, in the Freundlich model, the value of n is equal to 1.15 and Kf=1.992. The comparison between these two models shows that both models are well fitted with experimental data, and of course, the value of R2 is higher in the Langmuir model, and therefore the data compatibility with this model is greater.

Fig. 7. Data fitting by Langmuir adsorption model using sono-separator at T=25°C, AM =3 g and pH=10.

Fig. 8. Data fitting by Freundlich adsorption model using sono-

separator at T=25 °C, AM= g and pH=10.

4. Conclusions

Co(II) removal from aqueous solution by adsorption onto magnetic bentonite nanoparticles under the effect of high frequency ultrasound has been performed. The results of Co(II) removal efficiency and mass transfer characteristics of using ultrasound were compared with those of shaking. Results depicted that the optimal pH value is about 10, which indicates that at this pH, the magnetic adsorbent and cobalt metal ions are fully active in the environment. By increase in the adsorbent mass (AM), the adsorption rate of Co(II) increased because of the increase in the specific surface area of magnetic bentonite nanoparticles. In the identical conditions, sonication time for Co(II) adsorption was lower than the contact time by shaking to reach equal

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removal efficiency. Indeed, the highest removal efficiency of Co(II) for the shaker and the sono-separator were 83.3 % and 86 %, respectively, with the contact times of 50 minutes and 10 minutes, respectively. It concluded that acoustic and micro-streams generated by high frequency ultrasound have the high ability to induce mixing and strong mechanical effect inside the sono-separator. In addition, microjets induced by 1.7 MHz ultrasound collided with the surface of nanoparticles and so the adsorption of Co(II) on the adsorbent was increased. The comparison of Langmuir and Freundlich isotherms shows that both investigated models were well suited to experimental data and the data compatibility with Langmuir model was greater. From this study, it was found that it is possible to reach the high removal efficiency of Co(II) by magnetic bentonite nanoparticles and high frequency ultrasound in the little time compared to shaking.

Acknowledgment

The authors would like to thank Islamic Azad University, Kermanshah Branch for providing the support to carry out this work.

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Intensification of Co(II) adsorption from aqueous solution