Environmental pollution has became a serious problem that affects human health as well as all living organisms. This problem has emerged as a result of scientific progress in industrial and nuclear fields. This progress produces a large amount of pollutants of both a radioactive and non-radioactive nature, many of them are extremely hazardous to people and the environment especially radioactive pollutants which have high radiotoxicity and long half-lives. So, these pollutants must be treated and eliminated from wastewater to reduce the hazardous effects on the environment.
Cobalt is present in the wastewater of nuclear power plants with the radioactive isotope 60Co having a half-life of 5.3 years with γ-energy of 1.17 and 1.33 Mev and due to its long half-life and high γ-energy it must be eliminated from radioactive waste before the disposal process takes place. On the other hand, cobalt is produced from different industrial activities such as the production of jewelry, paints and pharmaceuticals. So, it must be removed from wastewater before discharge into aquatic streams. The permissible limits of cobalt in irrigation water and livestock wastewater are 0.05 and 1.0 mg L−1, respectively . The toxicity of cobalt causes serious effects on human health such as, asthma-like allergies, damage to the heart, causing heart failure, damage to the thyroid and liver and also may cause mutations (genetic changes) in living cells. In addition, an exposure to ionizing radiation leads to an increased risk of developing cancer .
Zinc is present in intermediate level radioactive waste as 65Zn having a half-life of 245 days and γ-energy of 1.115 Mev. In addition zinc is produced from sulfide ores and the manufacture of dyes. Zinc can be considered a very toxic element when it increases above permissible limits, which can cause damage to various systems in the human body , . The maximum allowable limit for zinc in drinking water was set by the Environmental Protection Agency to be 5 mg L−1.
Cadmium is one of the important nuclear elements and is considered a very toxic heavy metal, it is used as control rods in nuclear reactors to control the fission rate of uranium and plutonium. Cadmium may be found in wastewater discharge from industrial process such as the electroplating industry, the manufacture of nickel, cadmium batteries, solar battery production, fertilizers, pesticides, pigments and dyes and textile operations . It directly reaches water bodies through industrial effluent, causing a marked increase in its concentration. It has toxic effects and causes various types of acute and chronic disorders, and may cause nausea, salivation, muscular cramps and anemia, while extended exposure may also cause cancer . According to the World Health Organization guidelines, the permissible concentration of cadmium in drinking water is <0.005 mg L−1 .
Different methods were used for removal of heavy metals from wastewaters include ion-exchange, chemical precipitation, reverse osmosis, evaporation, membrane filtration, adsorption, and biosorption , , , , , , . However, there are some disadvantages in these methods such as incomplete metal removal, complex procedure, high cost and low efficiency. Thus, it is very important to remove these metals from water with high efficiency and low cost.
The adsorption process has proved to be an effective method for removal of radionuclides and heavy metal ions. Different materials were used as adsorbents in the adsorption technique such as activated carbons, natural materials, zeolite, clay, silica, modified silica, alumina and, etc. , , , , , , , , . However, the reports clarified low adsorption capacity of these adsorbents. So, new adsorbents with high adsorption capacities for radionuclides are required for practical applications.
Recent studies have used carbon nanotubes (CNTs), a member of the carbon family which can be thought of as cylindrical hollow micro-crystals of graphite. CNTs are considered as new adsorbents that have been proven to possess great potential for the removal of many kinds of pollutants such as heavy metals and radionuclides , , , , because of their unique physicochemical properties, like their surface functional groups which are generated on the surface of CNTs by the chemical oxidation method, they are highly porous, have a hollow structure and a large specific surface area.
Although CNTs have a good possibility for the adsorption of heavy metal ions from aqueous solutions, the removal efficiency is still limited. Therefore, the modification of carbon nanotubes is an important way to improve the removal efficiency of absorbent material for heavy metal ions and radionuclides.
Nanoparticles like nanosized ferric oxides, aluminum oxides, titanium oxides, manganese oxides, magnesium oxides and cerium oxides are likely to have an important role in water purification , , . Among them, MnO2 enjoys pride of place because of its lower cost and environmentally benign nature, high surface area and exchange performance , . All of these characteristics are important in enhancing the adsorption capacity. From the literature, it was found that Mn oxide coated activated carbon, zeolite and sand displayed higher adsorption capacity compared to Fe or Al oxide , , , , . So, synthesis of CNTs/MnO2 composite is important because it includes the properties of MnO2 and CNTs.
The present work aims to (1) synthesize carbon nanotubes (CNTs) by the chemical oxidation method and then synthesize CNTs/MnO2 composite by the co-precipitation method. (2) The synthesized CNTs/MnO2 composite was characterized by scanning electron microscope (SEM), Fourier transformed infrared spectroscope (FT-IR) and X-ray diffraction (XRD). (3) The synthesized CNTs/MnO2 composite was used as an adsorbent to investigate the adsorption potential of 60Co and 65Zn-radioisotopes in addition to Cd(II) ions from aqueous solutions under different experimental conditions such as pH, contact time and metal ion concentration. Equilibrium and kinetic studies were performed to describe the adsorption process. Different models were explained to determine the rates and mechanisms of the adsorption process.
2 Materials and methods
2.1 Synthesis of carbon nanotubes (CNTs)
CNTs were synthesized by the chemical oxidation method as previously reported elsewhere . Briefly, graphene oxide (GO) was synthesized according to a modified Hummers’ method . A mixture of concentrated H2SO4/H3PO4 (9:1) was added to 1.5 g of graphite at room temperature and stirred for 15 min. After that, 9.0 g of KMnO4 was then slowly added to keep the reaction temperature below 20°C. The solution was heated to 50°C and stirred for 12 h. After this it was cooled to room temperature and poured onto 200 mL of distilled water then 10 mL of 30% H2O2 was added. After that, it was filtered and centrifuged. The remaining solid material was washed with distilled water until the pH value reached 7 and then dried at 100°C for 24 h.
A specific amount of graphene oxide and a certain amount of HNO3 69% solution were sonicated in a bath sonicator at room temperature for 4 h (50/60 Hz). The obtained mixture was poured into 400 mL of distilled water. The solution was centrifuged for 1 h to remove excess of the acids. The remaining solid material was then washed with distilled water until the pH value reached 7 and then dried at 60°C for 12 h. The as-expected product was carbon nanotubes (CNTs).
2.2 Synthesis of manganese dioxide (MnO2)
A total of 8.4 g of manganese sulfate monohydrate (MnSO4·H2O) was dissolved in 15 mL of distilled water and 11.7 mL of 40% NaOH solution was added over 1 h with stirring, then 9.6 g of KMnO4 added at 80–90°C with stirring for another 1 h. The mixture was then filtered and washed with distilled water until the pH of the filtrate was neutral and was then dried at 100–120°C for 10 h.
2.3 Synthesis of CNTs/MnO2 composite
Specific amount of the synthesized CNTs and manganese sulfate monohydrate solution were mixed together. A solution 40% of NaOH was added with stirring for 1 h and KMnO4 was then added at 80–90°C with stirring for another 1 h. The product was filtered and washed with distilled water until the pH of the filtrate was neutral. Finally, drying was done at 100–120°C for 10 h. The as-expected product was CNTs/MnO2 composite.
2.4 The radioisotope preparation
The radioactive tracers of 60Co and 65Zn were obtained by neutron irradiation using the Egyptian Research Reactor ARE-RR-2 at Inshas, Egypt. An accurate weight of cobalt and zinc salts were separately wrapped in thin high purity aluminum foil and placed in thick aluminum irradiation capsule which was then subjected to pile neutrons in the reactor. The irradiated targets were left for a period of time in order to cool. A certain amount of the irradiated cobalt chloride and zinc sulfate were dissolved in the minimum amount of concentrated hydrochloric acid and re-dissolved in double distilled water to obtain a clear solution. The obtained isotopes were used as raids (tracers) in the equilibrium and kinetic measurements. The radioactivity of the obtained isotopes was γ-radiation counted using a single channel analyzer supplied with a well-type NaI(Tl) detector.
The morphologies of CNTs, MnO2 and CNTs/MnO2 composite were characterized by the XRD patterns obtained using a Rigaku X-ray diffractometer, the scanning was measured at values of 2θ in the range from 10 to 80°, using a CuKα radiation of an incident beam (λ=0.15406 nm) monochromator to determine the crystalline phase, the scanning speed was 8°/min. and the applied voltage and current were 45 kV and 250 mA, respectively, and SEM was conducted by an Hitachi S-300 N (Japan). FTIR analysis was conducted on a Nicolet Magna 560 spectrophotometer, using KBr pellets for sample preparation.
2.6 Adsorption experiments
The adsorption of 60Co and 65Zn radioisotopes in addition to Cd(II) ions by synthesized CNTs/MnO2 composite was carried out using a batch technique where 0.05 g of synthesized composite was mixed with 5 mL solution of sorbate, then shaken for 2 h in a flask shaker at room temperature and left for 24 h, the pH of the solutions were adjusted using solutions of sodium hydroxide and hydrochloric acid. After that, the radioactivities of the solutions were measured using a single channel analyzer supplied with a well-type NaI(Tl) detector (Spectech ST360 Radiation Counter, USA), three replicates were prepared in each case. In all cases, the activity was determined as a mean value after subtracting the background. While the concentration of Cd(II) ion was determined by an atomic absorption spectrophotometer (AAS-M5 Model from Thermo, UK).
The adsorption percentage (adsorption%) and the amount adsorbed (qe) of the metal ions were calculated according to the following equations:
For 60Co and 65Zn:
The adsorbed amount (qe, mg/g) of metal ions was calculated as follows:
where, Ai and Af are the initial and final specific activity of the aqueous solution before and after the adsorption process, respectively. C0 is the initial concentration of metal ions in the solution (mg/L), V is the volume of the solution (L) and m is the weight of CNTs/MnO2 composite (g).
For Cd(II) ion:
where, C0 and Ce are the initial and final metal ion concentrations (mg/L).
2.7 Desorption experiments
The desorption experiments were carried out using the synthesized sorbent loaded with 100 mg/L of either 60Co or 65Zn or Cd(II) ions for a certain time until equilibrium was achieved. Subsequently, the suspension was centrifuged and the supernatants were separated and assayed for each element. The solid residue was thoroughly treated with 5 mL of four different leaching solutions in four sealed glass flasks and shaken for 2 h and left for 24 h. The eluent solutions were 0.05 M HCl, H2O, 0.1 M NaOH, and 0.1 M MnCl2. Finally, the suspensions were centrifuged and then analyzed. The desorption percent (D%) was calculated by the equation:
3 Results and discussion
3.1 FT-IR analysis
The infrared spectrum of the synthesized CNTs, MnO2 and CNTs/MnO2 composites are recorded in Figure 1. The broad band around 3400 cm−1 corresponds to the O–H stretching vibration of the hydroxyl groups. This peak could be assigned to the hydroxyl group of the moisture, or the carboxylic groups on the surface of CNTs and CNTs/MnO2 composite. The bands around 2925–2850 cm−1 are due to asymmetric and symmetric C–H stretching vibrations in aliphatic –CH originating from the surface of CNTs , , as shown in Figure 1a and c. The peak around 1620 cm−1 is related to the O–H bending vibration. The peak at ≈1576 cm−1 is due to the stretching vibrations of the isolated C=C double bonds. The peaks at ≈1335 cm−1 and ≈1033 cm−1 might be due to the (OH) bending vibration of hydroxyl group and (–C–O) stretching vibration, respectively, as shown in Figure 1a and c. For CNTs (Figure 1a), the absorption band at 1703 cm−1 is due to the stretching vibration of (C=O) of the carboxylic groups (COOH). The bands in the range 698–416 cm−1 are due to C–H bending. According to the FTIR spectra of MnO2 (Figure 1b), the peaks at the 509 and 459 cm−1 regions refer to the Mn–O vibration. Concerning CNTs/MnO2 composite (Figure 1c), the peak at ≈1417 cm−1 is due to the deformation vibration of the hydroxyl groups (M–OH deformation vibration) . The broad peak at ≈514 cm−1 is attributed to the Mn–O and Mn–O–Mn vibrations , . The FT-IR results indicated the formation of the composite between CNTs and MnO2 and the major groups such as –OH, –COOH, C=C, –C–O and Mn–O are existed on the surface of CNTs/MnO2 composite.
3.2 X-ray diffraction
The XRD measurement of the synthesized MnO2 nanoparticles is shown in Figure 2a. This figure shows diffraction peaks around the 2θ angles of 28.8° (110), 37.5° (101), 43.1° (111), 56.8° (211) and 65.0° (002). All of the reflections of the XRD patterns can be readily indexed to the pure pyrolusite (β-MnO2) . This finding agrees well with the values reported in the standard spectrum (JCPDS Card. No. 24-0735) and such a presented pattern shows that the MnO2 is a highly crystalline material and is found as β-MnO2 .
Figure 2b show that the XRD pattern of CNTs is composed of a sharp peak at 2θ=25.9° (002) and a minor peak at 2θ=42.7° (100). On the other hand, for the CNTs/MnO2 composite, the XRD pattern indicated that the characteristic peaks of crystalline MnO2 are mostly absent and the diffraction peaks of the CNTs reduced sharply and do not appear clearly. This may be due to the coating of MnO2 on the surface of CNTs . This observation confirms that CNTs were coated with MnO2 nanoparticles successfully. This result agrees with that reported by Wang et al.  and Richter et al. . They reported that the MnO2 on CNTs and MnO2– coated zeolite, respectively is an amorphous phase.
3.3 Scanning electron microscopy
The morphology of CNTs, MnO2 and CNTs/MnO2 composite were investigated by SEM, as shown in Figure 3. From Figure 3a, the CNTs were observed to be twisted together, randomly aligned and a coil-like structure. While, the MnO2 structures are shown in Figure 3b in different heterogeneous shapes, mostly in the form of rods. SEM of CNTs/MnO2 composite (Figure 3c) displayed that manganese oxides are distributed in the CNTs’ matrix and covered uniformly on the surface of CNTs.
3.4 Sorption study
The adsorption of both 60Co, 65Zn and Cd(II) ions from aqueous solutions by the synthesized CNTs/MnO2 composite was carried out using the batch technique. The various parameters affecting the sorption of these elements were investigated individually to optimize their sorption on the synthesized CNTs/MnO2 composite. The results obtained are discussed in the following sections.
3.4.1 Effect of pH
The sorption of 60Co, 65Zn and Cd(II) ions on the synthesized CNTs/MnO2 composite was studied from aqueous solutions as shown in Figure 4. From this figure, the adsorption percentage increases with increasing pH till it reached the maximum values 73, 50 and 90% for 60Co, 65Zn and Cd(II), respectively, at pH 6. After this pH, metal ions starts to precipitate as a hydroxide. Also, it was observed that at low pH values, the sorption of 60Co, 65Zn and Cd(II) ions is low which is probably due to the protonation of the surface active sites and the increase of H3O+ ions in the aqueous solution. Thus, the positively charged surface sites caused the competition between H3O+ and 60Co, 65Zn and Cd(II) ions for the available binding surface active site that decreased 60Co, 65Zn and Cd(II) uptakes. With increasing in the initial pH values, the concentration of H3O+ ions decreased while the concentration of OH− ions increase which causes sorbent surface deprotonation, such observations mean that the surface of the CNTs/MnO2 composite tends to have a negative charge as shown in Eqs. (6, 7) . Hence, the attraction between the surface of the sorbent and the positively charge of metal ions in solution was enhanced (Eq. 8) . At pH value >6, the adsorption percentage increased slightly, this could be attributed to the precipitation of metal ions as hydroxide and ascribed to the speciation of 60Co, 65Zn and Cd(II) as shown in Figure 5.
In addition to, the presence of active groups on the composite surface, especially hydroxyl and carboxyl groups as mentioned in the FT-IR spectra. This leads to increase the adsorption process.
3.4.2 Effect of contact time
The effect of contact time on the sorption of 60Co, 65Zn and Cd(II) ions onto the surface of the synthesized CNTs/MnO2 composite was studied at different agitating times, 5 min–48 h, and the obtained data are represented in Figure 6. The data show that the sorption of 60Co, 65Zn and Cd(II) by the synthesized CNTs/MnO2 composite increases with time reaching equilibrium at ~4 h. The uptake percentages of 60Co, 65Zn and Cd(II) are 65.89, 35.46 and 86.68%, respectively. From the data depicted in Figure 6, it could be seen that further increase in the contact time beyond the 4 h for each ion resulted in a slight increase in the uptake percentage. An equilibration period of 24 h was selected for all further experiments to ensure that the sorption reaction achieves complete equilibrium.
3.4.3 Effect of initial concentration
The effect of initial metal ion concentrations on the uptake percent of 60Co, 65Zn and Cd(II) ions onto the synthesized CNTs/MnO2 composite is shown in Figure 7. The initial concentration is ranged from 50 to 200 mg/L for 60Co, 65Zn and Cd(II) ions.
Data in this figure show that, 60Co and Cd(II) uptake on the synthesized CNTs/MnO2 composite decreases from 90.31 and 74.63% to 42. 72 and 61.68%, respectively, as the initial concentrations of 60Co and Cd(II) increases from 50 to 200 mg/L. While the sorption of 65Zn decreases from 47.09 to 36.89% onto the same composite. This could actually be attributed to the sufficient adsorption sites available on the synthesized CNTs/MnO2 composite surface. However, at higher metal ion concentrations, the available adsorption sites are actually low compared with metal ion concentrations.
3.4.4 Adsorption isotherms
In this study, two different isotherm models namely the Langmuir  and Freundlich  isotherm models which are suitable models for describing the sorption of metal ions by solid surfaces are selected to fit the revealed experimental results of 60Co, 65Zn and Cd(II) sorption onto the synthesized CNTs/MnO2 composite that represented by the Langmuir and Freundlich equations (9, 10).
where qe (mg/g) is the amount adsorbed of metal ions at the equilibrium concentration Ce (mol/L), qm is the maximum amount adsorbed of 60Co, 65Zn and Cd(II) ions in mg/g and b is the Langmuir constant in L/mg related to the intensity of adsorption. K is the Freundlich isotherm constant (mg(1–1/n)L1/n/g) and 1/n is a Freundlich isotherm exponent.
The Langmuir model is based on the assumption of monolayer formation of adsorbate ions on the adsorbent surface. The plot between Ce/qe and Ce for the adsorption of 60Co, 65Zn and Cd(II) ions onto the synthesized CNTs/MnO2 composite is illustrated in Figure 8a,b. The constants of the Langmuir isotherm model were determined from the slope and intercept of the straight lines of the corresponding plots. These constants and the values of the correlation coefficients (R2) are listed in Table 1.
Langmuir and Freundlich constants.
|Sample||Langmuir isotherm||Freundlich isotherm|
In the Freundlich isotherm model, which is indicative of surface heterogeneity of the adsorbent, the relation between log qe and log Ce for the adsorption of 60Co, 65Zn and Cd(II) onto the synthesized CNTs/MnO2 composite is illustrated in Figure 8c,d. The values of Freundlich isotherm model parameters are determined from the slope and intercept of the straight lines of the corresponding plots. These values in addition to the values of the correlation coefficients (R2) are given in Table 1.
Comparing the calculated correlation coefficients (R2) listed in Table 1 for Freundlich and Langmuir models, it could be concluded that, the correlation coefficients (R2) of the Langmuir and Freundlich models for the adsorption of 60Co and Cd(II) ions on the synthesized CNTs/MnO2 composite are higher than 0.98. So, the adsorption process follows both the Langmuir and Freundlich models. This result indicates that the monolayer and multilayer adsorption of 60Co and Cd(II) ions onto CNTs/MnO2 composite is expected. While, for 65Zn, the correlation coefficient (R2) of Freundlich model (R2=0.99725) is higher than the Langmuir model (R2=0.86221). Therefore, the adsorption process follows the Freundlich model and multilayer adsorption is occurred.
3.4.5 Adsorption kinetic models
In this work, pseudo-first-order (Lagergren equation), pseudo-second-order and mass transport kinetic models are applied to study and analyze the obtained data from the sorption of 60Co, 65Zn and Cd(II) ions on the synthesized CNTs/MnO2 composite.
188.8.131.52 Pseudo-first-order model
A simple pseudo-first-order model was used to correlate the rates of reaction and expressed as follows :
where, qe and qt are the amounts of solute sorbed onto adsorbents (mg/g) at equilibrium and at time t, respectively, Kf is pseudo-first-order rate constant (1/min).
A plot of log (qe–qt) versus t for sorption of 60Co, 65Zn and Cd(II) ions onto the synthesized CNTs/MnO2 composite gives straight lines as shown in Figure 9a. The constants of the pseudo-first-order model, for the sorption of each metal ion, were estimated from the slope and the intercept of the straight lines of the corresponding plots and data is given in Table 2.
Kinetic parameters and correlation coefficients (R2) of pseudo-first-order and pseudo-second-order kinetic models.
184.108.40.206 Pseudo-second-order model
Pseudo-second-order model describes the kinetics of the sorption of ions onto sorbent materials and expressed as :
where, Ks is the pseudo-second-order rate constant (g/mg·min).
The kinetic plots of t/qt versus t for 60Co, 65Zn and Cd(II) removal are shown in Figure 9b. The pseudo-second-order constant (Ks) along with the correlation coefficient were determined and listed in Table 2.
These results suggest that a pseudo-second-order sorption is the predominant mechanism, as the correlation coefficient (R2) for 60Co, 65Zn and Cd(II) is extremely high and closer to unity and is higher compared to the results obtained from the pseudo-first-order kinetic model. This means the adsorption of 60Co, 65Zn and Cd(II) appears to be controlled by a chemisorption process.
220.127.116.11 Mass transport kinetics
18.104.22.168.1 Intra particle diffusion model
The intra-particle diffusion model was used to investigate the diffusion mechanism. The intra-particle diffusion (pore diffusion) is a transport process involving the movements of the species from the bulk solution to the solid phase. The intra-particle diffusion model, the Weber and Morris model, is presented in the Eq. (13) .
where, qt is the amount of metal ion adsorbed at time t (mg/g), Kid is the intra-particle diffusion rate constant (mg/g·min0.5). C is the intra-particle diffusion constant which is directly proportional to the boundary layer thickness.
The plot of qt versus t1/2 is given in Figure 10. For the adsorption of 60Co, 65Zn and Cd(II) ions on the synthesized CNTs/MnO2 composite, the values of the rate parameter (Kid) were calculated from the slope of the linear plots and values of the correlation coefficients (R2) are given in Table 3. The plots did not have a zero intercept as proposed by Eq. (13) indicating that intra-particle diffusion might not be the only controlling factor in determining the kinetics of the sorption process. The adsorption mechanism of these metals ions from aqueous solution is probably a combination of the boundary layer and intra particle diffusion which contributes to the rate-controlling step.
Kinetic parameters and correlation coefficients (R2) for mass transport kinetic models.
|Sample||Intra-particle diffusion||External (film) diffusion|
22.214.171.124.2 External (film) diffusion model
The external (film) diffusion model controls the adsorption rate during the initial adsorption period only. When the solute molecules transmitted from a liquid phase up to a solid phase; the boundary layer plays the most significant role in sorption. The external-film diffusion model may be applied using Eq. (14) as described by Boyd et al. :
where, F is the fractional attainment of equilibrium (F=qt/qe), Kfd is the film diffusion rate constant.
A linear plot of log (1–F) vs. time (t) with zero intercept would suggest that the kinetics of the sorption process is controlled by diffusion through the liquid film surrounding the solid sorbents. The relation between log (1–F) and t for the adsorption of 60Co, 65Zn and Cd(II) ions onto the synthesized CNTs/MnO2 composite is illustrated in Figure 11. The rate constant Kfd for the external film diffusion is determined from the slope of the straight lines of the corresponding plots. This constant and the values of the correlation coefficients (R2) are given in Table 3. From Figure 11 and Table 3, the plots give a straight line but not passing through the origin, in addition to the low values of correlation coefficients (0.953, 0.977 and 0.954) for 60Co, 65Zn and Cd(II) ions, respectively. This indicates that the external diffusion model will have only limited applicability in adsorption of 60Co, 65Zn and Cd(II) ions onto the CNTs/MnO2 composite.
3.4.6 Desorption study
The desorption of 60Co, 65Zn and Cd(II) ions from the loaded synthesized CNTs/MnO2 composite using different eluents H2O, NaOH, HCl and MnCl2 was studied and the results are shown in Figure 12. The results illustrate that the 60Co, 65Zn and Cd(II) ions are hardly eluted from the adsorbent surface by washing with both distilled water and 0.1 M NaOH. While it is easily desorbed by washing with 0.05 M HCl and 0.1 M MnCl2 solutions. The released percent of 60Co from loaded CNTs/MnO2 composite using 0.05 M HCl is 76.37%. But this value decreases to 64.67% using 0.1 M MnCl2 as an eluent. While the desorption percent of 65Zn from the loaded CNTs/MnO2 composite using 0.05 M HCl and 0.1 M MnCl2 is 99.86 and 85.96%, respectively. The released percent of Cd(II) from the loaded CNTs/MnO2 composite using 0.05 M HCl is 69.52%. While it decreases to 59.36% using 0.1 M MnCl2.
Hence, the efficiency of the used eluents to release 60Co and Cd(II) ions from synthesized CNTs/MnO2 composite follow the order: HCl>MnCl2>H2O>NaOH. While, for 65Zn it follow the order: HCl>MnCl2>NaOH>H2O.
3.5 Comparison with other adsorbents
The adsorption capacity of CNTs/MnO2 composite for the removal of Co(II), Zn(II) and Cd(II) ions from aqueous solutions was compared with other adsorbents reported in the literature and the values of the adsorption capacity was explained in Tables 4–6. From these tables, it indicates that the CNTs/MnO2 composite has a high adsorption capacity when compared with other adsorbents reported in the literature.
Comparison between Co(II) sorption capacity with published data.
|Saudi activated bentonite||7.3|||
Co(II)–IIP, Co(II)-imprinted polymers; NIP, non-imprinted polymers.
Comparison between Zn(II) sorption capacity with published data.
|Calotropis procera roots (CP)||9.69|||
|Chemically activated fruit of Kigelia Pinnata||7.042|||
|Rice husk ash||14.30|||
|Baggage fly ash||13.21|||
Comparison between Cd(II) sorption capacity with published data.
|CNTs oxidized by H2O2||2.600|||
|CNTs oxidized by HNO3||5.100|||
|CNTs oxidized by KMnO4||11.00|||
|Grafted cellulosic fabrics||13.69|||
|SDS – Fe3O4 nanoparticles||7.466|||
TS400, TiO2/sewage sludge that calcined at 400°C; SDS, sodium dodecyl sulfate.
The CNTs/MnO2 composite was synthesized via the co-precipitation method. It is used as an sorbent for the removal of some radionuclides (60Co and 65Zn-radioisotopes) and Cd(II) ions from aqueous solutions. The maximum adsorption capacities obtained from the Langmuir isotherm model were 33.234, 29.753 and 33.025 mg/g for 60Co, 65Zn and Cd(II) ions, respectively. The kinetic adsorption fits by the pseudo-second-order model very well. The removal process by CNTs/MnO2 composite is dependent on pH, contact time and initial ion concentration to reach to the highest adsorption value. According to the simple synthesis and high adsorption capacity of CNTs/MnO2 composite, it can be successfully used for removal of 60Co, 65Zn and Cd(II) ions from aqueous media.
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Cristian, P., Violeta, P., Anita-Laura, R., Raluca, I., Alexandrescu, E., Andrei, S., Daniela, I-E, Raluca, M. A., Cristina, M., Ioana, C. A.: Removal of zinc ions from model wastewater system using bicopolymer membranes with fumed silica. J. Water Proc. Eng. 8, 1 (2015).
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