Introduction

A serious worldwide environmental problem caused by toxic and radioactive heavy metals, such as Cu, Cd, As, Pb, and U, is water and soil pollution. These dangerous materials have significant impacts on the environment and consequently human health [1,2,3]. Uranium is a toxic and radioactive substance that usually exists in hexavalent oxidation state. It can naturally be found in subsurface environments and as industrial waste in the nuclear facilities and phosphate-based fertilizer companies [4, 5]. Living organisms are directly destroyed by uranium when it enters the body by mistake through any kinds of radioactive sources, such as dust in the air and particle in water and soil. Inside the body, uranium generates reactive radicals that consequently react with bio-molecules and causes several detrimental diseases, including liver and kidney damage, and even death [6, 7]. The maximum value of uranium in the drinking water is 2 µg/l, as designated by the World Health Organization (WHO) [8]. Lead is another heavy metal that is considered as the most harmful metal ion to the environment. It seriously influences the central nervous system. Similar to uranium, lead causes harm to the liver and reproductive system, kidney, basic cellular processes, and brain functions [9]. The upper limit of lead in the drinking water is 0.05 mg/l, as designated by WHO [10].

Several techniques, such as chemical precipitation, solvent extraction, ion exchange, electrolysis, membrane separation, activated sludge, reverse osmosis, flocculation (coagulation), and adsorption, are used for water treatment. They differ in their degree of success to remove heavy metals and treat wastewaters [11,12,13,14,15]. Adsorption method is considered as one of the most famous and widespread among the above mentioned treatment techniques due to its various advantages, such as environmental friendliness, wide adaptability, low cost, convenient operation, and high efficiency [16, 17]. Adsorbent properties, including specific surface area and particle sizes, are the essential factors that affect the adsorption efficiency. Therefore, synthesizing and developing an effective adsorbent is significant in the water treatment applications [18].

The magnetic nanoparticles due to several advantages such as the feasibility of preparation, easy separation (by an external magnetic field), high surface areas, and high chemical and mechanical stability attract increasing attention as potential adsorbents for heavy metal removal from aqueous solutions [19,20,21,22]. In the past decades, various magnetic materials, including a type of ferrites, MIIFe2O4 (M = Co, Zn, Mg, Cu, etc.), have been widely used in different technological applications [23]. Among the possible ferrite materials, CoFe2O4 is an interesting candidate for adsorption applications due to moderate saturation magnetization, exceptional chemical stability, and adequate mechanical hardness [24]. The nanostructured magnetic materials are synthesized by several methods, such as thermal decomposition of organometallic compounds, sol–gel method, co-precipitation, and hydrothermal synthesis [25,26,27,28,29]. Hydrothermal, which is also called solvothermal method, is the well-established method that is widely used to synthesize many nanoparticle materials. The kinetics of this method is typically slow due to relatively low temperatures of the synthesis process [30]. To enhance the specific surface area and adsorption capacity, the magnetic nanoparticles were coated by several materials or coated on various materials, including polyacrylamide [29], ethylenediamine tetraacetate (EDTA) [25], silica [31], graphene oxide [18], and carbon nanotubes (CNT) [22].

Recently, multi-wall carbon nanotubes (MWCNTs) have been widely used in various industries because of their unique structures and properties. MWCNTs present large surface areas, rich surface chemical functionalities, high aspect ratios, and nano-sized stability. Their usage has been widely spread in chemistry, catalysis, and nanoelectronic and magnetic devices [32]. Moreover, MWCNTs have been considered as a new adsorbent or as an additive to the adsorbents to increase total specific surface area and consequently adsorption capacity for immobilization of radiative and poisonous heavy metal and organic/inorganic contaminants [33,34,35]. However, MWCNTs are very hydrophobic. They easily aggregate in aqueous solution due to the high van der Waals interaction forces between MWCNTs, which cause effectively slowing down of adsorption behaviors and consequently reducing the adsorption capacity [36]. Therefore, combining the advantages of MWCNTs and magnetic nanoparticles with decoration of MWCNTs by CoFe2O4 can produce a potential novel adsorbent with enhanced specific surface area, desirable absorptivity, and effective magnetic separability [18].

Shen et al. synthesized the magnetite Fe3O4 nanoparticles with different average sizes by the co-precipitation method to remove different heavy metal ions, including Cd2+, Cr6+, Cu2+, and Ni2+ from wastewater; FeCl3 and FeCl2 were used as Fe sources to synthesize magnetic nanoparticles. They could achieve the different particle sizes of 8–35 nm magnetic Fe3O4 nanoparticles [37]. Habibi et al. synthesized the CoFe2O4 nanoparticles by wet chemical route. They used CoCl2 and FeCl3 as the magnetic element sources. The CoFe2O4 nanoparticles sizes of 47 nm were achieved [38]. Zhang et al. synthesized and characterized the bimetal oxide magnetic nanoparticles (MnFe2O4 and CoFe2O4) through the chemical co-precipitation method using FeCl3, Mn(NO3)2, and Co(NO3)2. They investigated the adsorption capacity of arsenic compounds on MnFe2O4 and CoFe2O4 nanoparticles and compared with Fe3O4. They observed that the maximum adsorption capacities for arsenite and arsenate were 94 and 90 mg g−1 and 100 and 74 mg g−1 for MnFe2O4 and CoFe2O4, respectively, which were higher than those for Fe3O4 [39]. Olmos et al. prepared different MFe2O4 nanoferrites (M = Co, Ni, and Zn) through the mechanochemical method and using FeCl3 and MCl2 to study the adsorption of Pb(II) from aqueous solution. They found a maximum adsorption capacity of 20.58 mg/g for CoFe2O4 [40]. Wei et al. investigated the adsorption capacity of CoFe2O4 to remove uranium from aqueous solutions. They reported a maximum adsorption capacity of 170.07 mg/g [41]. Fasfous et al. investigated uranium (VI) removal from aqueous solutions through MWCNTs. The maximum sorption capacity reported for uranium (VI) ions was 39.5 mg g−1 [42]. Liu et al. decorated the magnetic ferrite nanoparticles of MFe2O4 (M = Fe, Co, Ni) on CNTs through in situ high-temperature hydrolysis and polymerizing of metal chlorides, including FeCl3, CoCl2, and CNTs in polyol solution [43]. The synthesis of CoFe2O4/CNT magnetic nanocomposites was also reported by Jiang et al. They synthesized the CoFe2O4/CNT magnetic nanocomposites through a solvothermal treatment in ethylene glycol. They used a mixture of metal chlorides, including FeCl3 and CoCl2, sodium acetate, polyethylene glycol, and MWCNTs [44]. Tan et al. investigated the adsorption capacity of uranium (VI) on the synthesized CoFe2O4 and CoFe2O4/MWCNTs. They used a hydrothermal method using FeSO4·7H2O and CoCl2·4H2O to synthesize the magnetic nanoparticles and nanocomposites. The reported maximum adsorption capacity for uranium (VI) based on Langmuir model was 212.7 mg/g for CoFe2O4/MWCNTs, which indicated a high efficiency of uranium removal from aqueous solutions [22]. Zhu et al. synthesized functionalized magnetic mesoporous silica by solvothermal treatment to remove uranium from aqueous solutions. They decorated the magnetite Fe3O4 nanoparticles to reach higher uranium adsorption by synthesizing two new adsorbents of Fe3O4@nSiO2@mSiO2 and Fe3O4@nSiO2@mSiO2-DIM [53]. In another research, Guo et al. synthesized functionalized magnetic Fe3O4@SiO2 by simply embedding iron oxide into MCM-41. They reported large sorption capacity of 160 mg/g [54]. Table 1 summarizes the main parameters in the above-listed literature reviews including the synthesized adsorbents, the preparation method, the important chimerical materials which were used in the synthesis procedure, and their applications.

Table 1 The list of main parameters in synthesis and application of the investigated adsorbents in the literature reviews
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In the present study, magnetite CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites were synthesized by hydrothermal method. The synthesized samples were characterized by different characterization analyses, including Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) method. The adsorption properties of prepared CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites were also investigated by applying them as sorbents to sorb U(VI) and Pb(II) ions from wastewater. The batch sorption technique was used to assess the adsorbability of magnetite CoFe2O4 nanoparticles from the wastewater. Several parameters including pH, contact time, initial U(VI) concentration and solution volume, and adsorbent concentration, which influence the adsorption capacity, were also investigated.

Experimental section

Chemicals

The iron source, cobalt source, sodium source, and solvent that were used in this study are FeCl3·6H2O, CoCl2·6H2O, sodium carbonate (Na2CO3), and ethylene glycol. To investigate adsorption, 1000 ppm uranium standard (UO2(NO3)2) and lead standard (Pb(NO3)2) were also used to prepare sample solutions. All chemicals were purchased from Merck Co. The MWCNTs that were used in this work have the specifications including diameters of 10–20 nm and purity of 95–98%. To adjust the solution pH, negligible volumes of sodium hydroxide solution were used.

Preparation of CoFe2O4 nanoparticles

CoFe2O4 nanoparticles were prepared by a multistep hydrothermal method. At first, 3 g of FeCl3·6H2O and 1.32 g of CoCl2·6H2O were dissolved in 10 ml ethylene glycol. After that, 2.35 g of sodium carbonate (Na2CO3) was added to the mixture with stirring for about 30 min to 60 min. Then, the resulting solution was poured into a Teflon-lined autoclave with a stainless-steel cover and heated to 200 °C for 24 h. The final products were cooled to room temperature. The black precipitate was collected by a magnet, washed with distilled water and ethanol repeatedly to remove the impurities and dried at 70 °C in a vacuum oven for 6 h.

Preparation of CoFe2O4/MWCNTs nanocomposites

For the decoration of MWCNTs, an appropriate amount of MWCNTs was dispersed in 40 ml ethylene glycol by sonication for 30 min. Then, 3 g of FeCl3·6H2O, 1.32 g of CoCl2·6H2O, and 2.35 g of sodium carbonate (Na2CO3) were added to the prepared mixture with sonication for approximately 30 min to 60 min. Furthermore, similar reaction conditions that were used for the synthesis of CoFe2O4 nanoparticles were used to produce CoFe2O4/MWCNTs nanocomposites.

Characterization

The magnetic CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites were characterized by XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), FTIR spectroscopy, and Brunauer–Emmett–Teller (BET) method. XRD (Bruker D8 Advance) was used to collect XRD patterns at 30 kV and 20 mA and Cu Kα radiation (λ = 0.1540598 nm). A TENSOR 27 FTIR spectrometer was used to record FTIR spectra of synthesized samples. The morphology and particle sizes of synthesized MSs were measured using FE-SEM (ZEISS- SIGMA-VP-500). The BET surface area and pore size distributions were obtained from the nitrogen adsorption–desorption isotherms obtained by BELSORP-mini analyzers (BEL Japan, Inc.). The element concentrations in adsorption experiments were analyzed by SPECTRO Genesis simultaneous CCD-based radially viewed ICP-OES.

Adsorption tests

Adsorption experiments were performed by adding different amounts of CoFe2O4 or CoFe2O4/MWCNTs in uranium or lead solution at various conditions, including different solution concentrations, volumes and pH values, and contact times. After the treatment, solid phase was separated from the solution by using a magnet. The initial and equilibrium concentrations of uranium or lead were determined using ICP-OES. Standard and sample solutions were prepared from 1000 ppm uranium standard (UO2(NO3)2) and lead standard (Pb(NO3)2. Analytical calibration curves for U and Pb were obtained from 0.0, 1.0, 5.0, 10.0, 25.0, 50.0 and 100.0 ppm by diluting of 1000 ppm standard solutions. The measurement range for both samples were 0.8 ppm – 110 ppm with 0.9999 correlation coefficient for U and 0.9997 for Pb. High intensity emission and recommended lines of uranium and lead by ICP-OES apparatus were selected to determine the uranium concentration. The U(VI) or Pb(II) removal percentage (%) and the adsorption capacity (Qe, mg/g) were calculated using the following equations [1]:

$${\text{Adsorption }}\;{\text{capacity }}\;({\text{mg}}/{\text{g}}) = \frac{{\left( {C_{\text{i}} - C_{\text{f}} } \right) \times V}}{m}$$
(1)
$${\text{Removal}}\;(\% ) = \frac{{\left( {C_{\text{i}} - C_{\text{f}} } \right) \times 100}}{{C_{\text{i}} }}$$
(2)

where Ci is the initial concentration and Cf is the concentration at equilibrium after treatment; m (mg) and V (ml) are the mass of the adsorbent and the volume of the solution, respectively.

Results and discussion

The phase structures of the synthesized CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites are shown in Fig. 1. As shown in Fig. 1a, the XRD pattern of the synthesized magnetite CoFe2O4 nanoparticles indicates the presence of the characteristic peaks of (220), (311), (400), (422), (511), and (440), which were observed at the diffraction angles of 2θ = 30.23, 35.69, 43.26, 53.99, 57.29, and 62.83, respectively. These peaks are the main characteristic peaks of CoFe2O4, which confirm that the resultant nanoparticles are pure CoFe2O4. As can be observed, the position and intensities of nearly all of the peaks are well matched to those of the other experimental results and the standard PDF cards for CoFe2O4 nanoparticles (JCPDS 22-1086 and JCPDS 79-1744) [16, 45, 46]. Figure 1b shows the XRD pattern of the synthesized CoFe2O4/MWCNTs nanocomposites. The additional peak (002) that could be observed at 27.5° corresponds to graphite because of adding MWCNTs to the structure. As shown in this figure, the characteristic peaks related to the CoFe2O4 nanoparticles, which are decorated on the MWCNTs, are approximately same as before, and a few peaks related to the MWCNTs were added to the XRD pattern [16].

Fig. 1
figure 1

Powder X-ray diffraction (XRD) patterns of magnetite a CoFe2O4 nanoparticles, b CoFe2O4/MWCNTs nanocomposite

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The microstructures of the synthesized magnetic CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites were also characterized by SEM. The SEM images of synthesized magnetic nanoparticles, which are shown in Fig. 2a and b, indicate that the synthesized magnetic CoFe2O4 nanoparticles are composed of spherical nanoparticles that have the average diameter of approximately 40 nm. The SEM images of magnetic CoFe2O4/MWCNTs nanocomposites, which are shown in Fig. 2c and d, also indicate that the average diameter of CoFe2O4 nanoparticles decorated on MWCNTs is approximately 34 nm. The EDS spectra of the synthesized CoFe2O4 and CoFe2O4/MWCNTs nanostructures are shown in Fig. 3a and b. The elements of C, Fe, Co, and O can be observed in CoFe2O4 and CoFe2O4/MWCNTs.

Fig. 2
figure 2

The SEM images of the synthesized magnetic a, b CoFe2O4 nanoparticles, c, d CoFe2O4/MWCNTs nanocomposite

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Fig. 3
figure 3

EDS diagram of a CoFe2O4, b CoFe2O4/MWCNTs nanostructures for one point of their surfaces

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Figure 4 shows the FT-IR spectra of MWCNTs, synthesized magnetite CoFe2O4 nanoparticles, and CoFe2O4/MWCNTs nanocomposites. As shown in this figure, the IR spectra of different nanostructures include several main absorption bands. The absorption bands, which are located at about 580 and 410 cm−1, are related to the stretching vibrations of metal oxide in octahedral group complex Co(II)-O2− and Fe(III)-O2− tetrahedral group. These tetrahedral groups mainly confirm the existence of spinel ferrite and complex of the cobalt ferrite in the synthesized nanostructures [47, 48]. The absorption peak at around 1628 cm−1 and a broad absorption band near 3435 cm−1 are related to the deformation vibrations of absorbed H2O molecules and stretching vibrations of OH on the surface of particles [47]. The specific characteristic bands at 2890, 1560, and 1345 cm−1, which are displayed in MWCNT and CoFe2O4/MWCNTs spectra, are related to the symmetric and asymmetric stretching of C–H, C=C and COO− of MWCNT contents [7, 49].

Fig. 4
figure 4

Fourier transform infrared (FT-IR) spectra of MWCNTs, magnetite CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites

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The N2 sorption–desorption isotherm, which determines the specific surface area and pore size distributions, was measured at −196 °C in the interval of relative pressures from 10−6 to 0.995 using ultrahigh purity nitrogen by BELSORP mini analyzers. The samples were pretreated at 400 °C for 8 h. The surface specific area was calculated by BET method. Barrett–Joyner–Halenda (BJH) method was also used to calculate pore size distribution curve. The pore size was defined as the maximum of the pore size distribution curve. The specific surface areas and pore size distributions of the synthesized magnetic CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites are shown in Fig. 5. As shown in Fig. 5a and b, the adsorption–desorption isotherms indicate type III isotherms accompanied by type H3 hysteresis loop [50]. The specific surface areas were calculated as 113.4 and 186.1 m2 g−1 using BET equation for CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites, respectively, which indicated much higher values than those in previous reports (Table 2). Barrett–Joyner–Halenda (BJH) method was also used to calculate pore size distribution curve (inset, Fig. 5). The pore size was defined as the maximum of the pore size distribution curve. As shown inside Fig. 5a and b, synthesized magnetic CoFe2O4 nanoparticles showed a pore diameter and volume of 2.41 nm and 0.39 cm3 g−1, respectively. The corresponding values for synthesized magnetic CoFe2O4/MWCNTs nanocomposites were increased to 6.06 nm and 0.94 cm3 g−1, respectively. The considerable note is that the synthesized composites exhibit higher BET surface areas, pore volumes, and pore sizes, which were expected to have efficient influence on the enhancement of the heavy metal adsorption capacity from wastewater pollutants.

Fig. 5
figure 5

N2 adsorption–desorption isotherms measured 76 K of a CoFe2O4 and b CoFe2O4/MWCNTs magnetic material. Inset: pore size distributions from the adsorption branches through the BJH method

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Table 2 Summary of physical properties of CoFe2O4 and CoFe2O4/MWCNTs reported previously and those investigated in the present work
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Uranium and lead adsorption results of synthesized magnetic CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites in different solution pH are shown in Fig. 6a and b, respectively. The influence of the solution pH on the uranium and lead adsorption capacities was investigated using 0.01 g of CoFe2O4 or CoFe2O4/MWCNTs for a pH range of 2.0–12.0 and 60 min treatment time. The initial concentrations of uranium and lead solution were 50 ppm in 15 and 50 ml, respectively. As shown in these figures, the solution pH plays a significant role on the sorption behavior of metal ions on CoFe2O4 and CoFe2O4/MWCNTs. Results showed that the adsorption capacity increases with a pH rise from 2.0 to around 5.0 and decreases with further increase in pH from 5.0 to 12.0. The optimum pH for Pb(II) sorption onto both CoFe2O4 and CoFe2O4/MWCNTs was obtained around 5, whereas the optimum pH for U(VI) sorption onto CoFe2O4 nanoparticles was 6.0 and around 5 onto CoFe2O4/MWCNTs nanocomposites. In pH lower than optimum pH, the adsorption process is not favored, because the surface of adsorbent and the adsorbed metal ions are positively charged. Moreover, many adsorption sites were occupied by excess H+. By increasing pH higher than optimum pH, anionic species of metal ions become major carriers in the solution, which results in forming hydroxyl dissolved carbonate and bicarbonate anions, such as \({\text{UO}}_{2} \left( {{\text{CO}}_{3} } \right)_{3}^{4 - }\), \(\left( {{\text{UO}}_{2} } \right)_{2} {\text{CO}}_{3} \left( {\text{OH}} \right)_{3}^{ - }\), and \({\text{UO}}_{2} \left( {\text{OH}} \right)_{3}^{ - }\). These anion complexes lead to a slight decrease in adsorption capacity and removal efficiency. The magnetic property of the synthesized nanostructures adsorbents, which exhibits that they could easily separate from their dispersed solution using a normal magnet, is shown in Fig. 6c.

Fig. 6
figure 6

Effect of solution pH on a U(VI) adsorption, b Pb(II) adsorption by magnetite CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites, and c photographs of CoFe2O4 dispersion and magnetic separation

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The adsorption capacities of uranium and lead ions on CoFe2O4 and CoFe2O4/MWCNTs as a function of treatment time or contact time of 2–90 min for uranium and 2–180 min for lead are shown in Fig. 7a and b, respectively. The initial concentration of U(VI) in the aqueous solution was 35 ppm in 60 ml, and the quantity of synthesized adsorbents was considered to be 0.002 g. The values for Pb(II) were 70 ppm of initial concentration, 40 ml of solution volume, and 0.01 g of adsorbent amounts. As shown in these figures, the adsorption capacity of U(VI) or Pb(II) was rapidly increased, which can be due to the many accessibility of active sites on the adsorbents. Results indicated that the uranium sorption reaches equilibrium at its maximum value in the initial 5 min. These results indicated that the synthesized magnetic nanostructures exhibit considerably shorter time to reach its maximum value than in previous reports. The sorption process was not changed by a further increase in the contact time. The equilibriums for Pb(II) adsorption on CoFe2O4 and CoFe2O4/MWCNTs were observed at approximately 120 and 90 min, respectively.

Fig. 7
figure 7

Effect of contact time on a U(VI) adsorption, b Pb(II) adsorption by magnetite CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites

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One of the most important parameters used to compare adsorption capability of synthesized adsorbents is studying their sorption isotherms. The sorption isotherms are usually calculated by simulating experimental results based on one of the existing models, such as Langmuir and Freundlich models. Figures 8, 9, and 10 show the sorption isotherms based on adsorption percentage, Langmuir isotherms, and Freundlich isotherms for adsorption of uranium and lead ions onto magnetite CoFe2O4 and CoFe2O4/MWCNTs nanostructures, respectively. Langmuir model is expressed based on the following equation:

$$\frac{1}{{q_{\text{e}} }} = \frac{1}{{q_{\text{m}} }} + \frac{1}{{K_{\text{L}} q_{\text{m}} }} \cdot \frac{1}{{C_{\text{e}} }}$$
(3)

where qe is the equilibrium adsorption uranium or lead ions in mg/g, Ce is the equilibrium concentration ions in mg/l, qm is the maximum adsorption capacity in mg/g, and KL is the Langmuir constant, which is related to the energy of adsorption.

Fig. 8
figure 8

Adsorption percentages for a U(VI) and b Pb(II) ions onto magnetite CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites

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Fig. 9
figure 9

Langmuir isotherm for adsorption of a U(VI), b Pb(II) on CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites

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Fig. 10
figure 10

Freundlich isotherm for adsorption of a U(VI), b Pb(II) on CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites

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Freundlich model:

$$\ln \left( {q_{\text{e}} } \right) = \ln \left( {K_{\text{F}} } \right) + \left( {\frac{1}{n}} \right)\ln \left( {C_{\text{e}} } \right)$$
(4)

where qe is the amount of uranium or lead sorbed on the adsorbent in mg/g, Ce is the equilibrium concentration ions in mg/l, and KF (mg1−n ln/g) and n are empirical constants that indicate the relative adsorption capacity and the adsorption intensity, respectively. Table 3 summarizes the Langmuir and Freundlich adsorption constants and the calculated regression coefficients (R2). The Freundlich isotherm provided higher R2 values than the Langmuir isotherm for the adsorption of Pb and U ions onto either adsorbent, indicating its better fit with the experimental data.

Table 3 Parameters of adsorption isotherms for the adsorption of U(VI) and Pb(II) on CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites
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The maximum monolayer adsorption capacity (Qmax) of Pb(II) onto CoFe2O4 nanoparticles was calculated to be 109.89 mg g−1, and the corresponding value for CoFe2O4/MWCNTs nanocomposites was 217.39 mg g−1, which indicates the favorable effects of decorated MWCNT on the removal of Pb. The maximum adsorption capacities calculated from the Langmuir equation for U(VI) adsorption were 294.12 mg g−1 for CoFe2O4 nanoparticles and 500 mg g−1 for CoFe2O4/MWCNTs nanocomposites, which showed that the synthesized nanostructures present higher adsorption capacities than those of previous works (Table 4) [8].

Table 4 The maximum adsorption capacities Q (mg g−1) of U(VI) on CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites and other adsorbents
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Conclusions

In this research, magnetite CoFe2O4 nanoparticle and CoFe2O4/MWCNTs nanocomposite adsorbents with a high specific surface area, high adsorption capacity, and easily separated properties have been synthesized by the hydrothermal method for uranium and lead removal from aqueous solutions. The structural characteristics of synthesized nanostructures were confirmed by FT-IR, XRD, SEM, and BET analyses. The XRD pattern of synthesized magnetite CoFe2O4 nanoparticles indicates that the position and intensities of nearly all of the peaks are well matched to those of the other experimental results and the standard PDF cards for CoFe2O4 nanoparticles (JCPDS 22-1086 and JCPDS 79-1744). The SEM images indicate that the synthesized magnetic CoFe2O4 nanoparticles are composed of spherical nanoparticles, which have the average diameter of approximately 40 nm. Moreover, the average diameter of CoFe2O4 nanoparticles decorated on MWCNTs is approximately 34 nm. The estimated specific surface areas obtained by BET analysis were 113.4 and 186.1 m2 g−1 for CoFe2O4 and CoFe2O4/MWCNTs, respectively. The batch sorption technique was applied to investigate the adsorption properties of prepared nanocomposites. The adsorption capacities of uranium and lead onto CoFe2O4 nanoparticles and CoFe2O4/MWCNTs nanocomposites were obtained as 294.12, 500, 109.89 and 217.39 mg g−1, respectively. Several parameters, including pH, contact time, initial U(VI) concentration and solution volume, and adsorbent concentration, which influence the adsorption capacity were also investigated.