In this work the efficiency of mechanically prepared magnetic (x)ZnO(1 − x)Fe2O3 nanocrystallines for Ni(II) and Cd(II) ions removal was investigated. The produced nanoparticles were characterized using N2 adsorption, X-ray diffraction (XRD), and magnetization techniques. Batch mode experiments were performed to evaluate the parameters of the heavy metal ions adsorption on the nanoparticles. The concentration and temperature were found to be detrimental factors in the adsorption process as the amounts adsorbed were enhanced by their increase. While Cd(II) adsorption was found to comply with the Langmuir isotherm, the adsorption of Ni(II) ions fitted both Langmuir and Freundlich isotherms. The pseudo-second-order model was the kinetics model describing the adsorption process. The adsorption process was endothermic and spontaneous as indicated by the thermodynamic study results. The positive entropy obtained may suggest increased randomness at the solid–solution interface. A mechanism for the metal ions adsorption was proposed.

INTRODUCTION

Due to the great boom of the world population, the demand for clean healthy water is progressively increasing. Still, considerable amounts of lethal chemicals, such as dyes, pharmaceuticals, and metal ions are discharged into water systems. As a consequence of human, urban, and industrial activities, the problem is aggravated. Heavy metal ions usually enter into human bodies through food chains. Their accumulation may lead to serious diseases and/or damage, if ingested beyond their tolerance limits. Such harmful effects can extend to the environment also. To minimize their hazardous impact on both biota and environment, metal ions contaminated water treatment is of high priority (Babel & Kurniawan 2004).

The toxic heavy metal, cadmium, is discharged to ecosystems through processes such as fuel combustion, metal production, fertilizers’ application, metal finishing, and industry. Thus, the level of cadmium reaches a μg L−1 to mg L−1 range exceeding its 10 and 100 ng L−1 level in natural water as a result of such anthropogenic activities (Nordberg et al. 2014). Cadmium is a carcinogenic and teratogenic agent that affect adult and fetus internal organs (Boparai et al. 2011).

Similarly, nickel is a toxic metal found in the environment, which can cause respiratory tract carcinogenesis and allergic contact dermatitis once it enters the human body. Industries such as stainless steel and nickel electroplating are sources of Ni(II) in the environment (Meena et al. 2005).

Heavy metal ions are removed from water and wastewater by means of membrane filtration, liquid extraction, electro-dialysis, chemical precipitation, ion exchange (Fu & Wang 2011), along with other methods. For small-scale industries and domestic uses, these methods are not appropriate as they are costly with low feasibility (Li et al. 2007). On the other hand, adsorption is a more feasible technique that can be employed in industrial application using natural or synthetic materials. Thus, clays, zeolites, biomass, activated carbon, dried plant parts, saw dust, and biopolymers are used due to their abundance and low cost (Singh et al. 1998; Li et al. 2010).

Nowadays, nanomaterials are gaining significant importance in adsorption processes due to their great ability to remove pollutants from aqueous media. Their capacity to adsorb pollutants is due to the large surface to volume ratio along with the ease of tailoring of their surface properties by modifying their functionality and morphology (Roy et al. 2005; Ramana et al. 2013; Roy & Bhattacharya 2013; Arce et al. 2015).

Magnetic nanomaterials are becoming a focus of new research due to their being environmentally pleasant, naturally available, and easily recoverable. Due to its magnetic properties, iron oxide is extensively employed in water treatment for the efficient removal of contaminants (Tang & Lo 2013). Moreover, iron oxide nanoparticles can be functionalized to improve their adsorption and photocatalytic activity (Xu et al. 2012). Metal ferrites (MFe2O4) were obtained by incorporating oxides of cobalt (Ding et al. 2015; Nassar & Khatab 2016), manganese (Bhowmik et al. 2016), or calcium (Debnath et al. 2016) into iron oxide and were applied in the removal of dyes or heavy metal ions. Being non-toxic, environmentally harmless, cheap, and structurally stable, (ZnFe2O4) is a promising candidate for many applications (Reddy & Yun 2016). The co-presence of ferric and ferrous ion in its spinel structure provides it with wonderful properties that make it a focus of extensive research in different fields and applications (Kale et al. 2004; Thirupathi & Singh 2015).

In this work, (x)ZnO(1 − x)Fe2O3 has been fabricated and characterized. The effects of some experimental parameters such as initial concentration and temperature and on Cd(II) and Ni(II) removal were studied. Mathematical models were applied to describe the adsorption process and obtain the different parameters. Adsorption isotherms were modeled using the Langmuir and Freundlich equations. The kinetics of the process was investigated under the first- and second-order rate laws. Parameters such as rate constants, adsorption capacities, thermodynamic function variations (ΔHo, ΔSo, and ΔGo) were determined and their effect on the adsorption was scrutinized.

METHODOLOGY

Materials

Equimolar amounts of hematite (α-Fe2O3) and zinc oxide (ZnO) were mixed in a planetary milling (Fritsch-P7) and milled for 10 hours at 700 rpm and 10:1 ball to sample mass ratio. The crystallite structure of the nano-particles was investigated by X-ray powder diffraction (XRD) analysis using a diffractometer (D8) equipped with Cu-Kα radiation (λ= 1.5418 Å). The specific area and pore size were characterized by N2 adsorption–desorption carried out at 77 K in ASAP 2020 (Micromeritics) equipment. Prior to conducting the adsorption experiment, the sample was degassed with helium at 250 °C for 2.0 h to remove humidity and adsorbed impurities. The BET (Brunauer, Emmett, and Teller) equation and t-plot method of Lippens & De Boer (1965) were employed in the pores’ surface area calculations.

Nickel nitrate (Ni(NO3)2) and cadmium nitrate (Cd(NO3)2 salts were used to prepare aqueous (1,000 mg L−1) stock solution of Ni(II) and Cd(II), respectively. Desired concentrations were obtained by appropriate dilutions. NaOH and HNO3 solutions were used to adjust the pH. All the reagents were from Sigma-Aldrich and were used as received.

Experiments were performed in batch mode by mixing 10 ± 0.1 mg of (ZnFe2O4), to 25 mL of a known Ni(II) and Cd(II) solution concentration in a 50 mL Erlenmeyer flask. Adsorption studies were conducted at pH 7.0 and initial Ni(II) and Cd(II) concentrations in the range of 25–125 mg·L−1 to obtain equilibrium isotherms. A number of flasks were placed on a multi-position magnetic stirrer and stirred individually at 600 rpm. About a 15 mL portion of the solution was taken after 12 h contact time, centrifuged (centrifuge, Hettich Zentrifugen EBA 20), and then filtered. Residual nickel and cadmium ions content of filtrate were determined by atomic emission spectroscopy equipment (Genius, ICP-EOS, Germany).

Methods

The size of the crystalline magnetic particles was obtained from the XRD data using the Scherer equation: 
formula
1
where B is the full width at half maximum (FWHM) of the peak (in radians), the constant k has a value of ≈0.89, θ is diffraction angle, and λ for Cu-Kα is 1.5418 Å. The magnetic properties of the prepared nanopowder were measured at ambient temperature via a Lake Shore 4700 model magnetometer (VSM) with a magnet of 2 Tesla strength.
According to Equation (2), the mass (mg) of Ni(II) or Cd(II) removed by 1 g of adsorbent qe is: 
formula
2
At equilibrium, the adsorption is usually described by adsorption isotherms. The Langmuir and Freundlich equations are mostly adopted: 
formula
3
 
formula
4
where qe (mg·g−1) is the mass of solute adsorbed by a unit mass of nanopowder, Ce is the solute concentration at equilibrium (mg·L−1), qm is the mass of Ni(II) and Cd(II) sufficient to cover the site of adsorption forming a mono-layer (Weng & Huang 2004), and the constant KL is indicative of the adsorption free energy value. The slope and intercepts of the Ce/qe versus Ce graph (Equation (3)) can provide the qm and KL values, respectively. We can obtain the values of k and n from the intercept and slope of loqe versus log Ce graph (Equation (4)) (Gupta et al. 1998).
The prediction of the adsorption kinetics data is tested by either the pseudo-first- or second-order models. The pseudo-first-order kinetics is given by the following equation (Gupta et al. 1998): 
formula
5
where qt (in mg·g−1) is the mass of metal ions adsorbed by a gram of adsorbent at time t (minutes), and k1 is the constant for adsorption rate in reciprocal minutes. By plotting a graph of ln(qeqt) against t, qe and k1 can be respectively obtained from the intercept and slope. Sorption kinetics can be also represented by a pseudo-second-order rate law (Ho & McKay 1998): 
formula
6

Here, k2 represents the constant for adsorption rate (g.mg−1·min−1). Data satisfying this law will be linear if t/qt is plotted against t. The slope of such a graph gives qe, while k2 value is calculated from the intercept once qe is known.

The thermodynamic functions (ΔHo, ΔGo, and ΔSo) for the metal ions by zinc ferrite are calculated using the following formulae: 
formula
7
 
formula
8

Equation Ka = qm·KL can be employed to calculate the equilibrium constant. The ΔHo value can be obtained from the ln(Ka)) against (T−1) plot.

RESULTS AND DISCUSSION

Structural and magnetic characterization

XRD analysis

Figure 1 shows the diffraction patterns for (x)ZnO(1 − x)Fe2O3 mixture after 10 hours of milling. It can be seen that the XRD peaks are wider with relatively reduced intensity. Thus, mechanical milling brings about deformations leading to the creation of strained crystals of reduced size. All zinc oxides (ZnO), hematite (α-Fe2O3), and zinc ferrite ZnFe2O4 phases coexist in the XRD spectrum (Figure 1). This may be attributed to the high ratio of both ZnO and (α-Fe2O3) used to prepare the (x)ZnO(1 − x)Fe2O3 nanoparticles or to the lesser milling interval as obtaining only ZnFe2O4 requires a longer time (Chen et al. 2013). The coexistence of several phases in this sample usually enhances the adsorption efficiency of the obtained nanocrystallines. Guo et al. (2011) reported enhanced photocatalytic activity of BiFeO3 nanoparticles with parasitic γ-Fe2O3. This behavior was attributed to the formation of a heterojunction structure between the BFO and γ-Fe2O3 phases. The average size of the magnetic crystallite was found to be 110 nm by applying the Scherer formula to the most prominent peak (311).
Figure 1

X-ray diffraction patterns of ball-milled (x)ZnO(1 − x)Fe2O3 nanoparticles.

Figure 1

X-ray diffraction patterns of ball-milled (x)ZnO(1 − x)Fe2O3 nanoparticles.

Magnetic properties

Magnetic measurement performed at room temperature is shown in Figure 2. It can be observed that the sample is ferromagnetic with 7.5092 emu saturation (MS) and 9 Gauss coercivity field (Hc). The ferromagnetic behavior of the mixture due to the presence of Fe2O3 and zinc ferrite will improve its capacity to adsorb metal ions and pollutants. In an external magnetic field, a correlation between the size or number of atoms of a magnetic particle has been reported. Consequently, higher Ms values of large size particles are due to surface spin effects (Kodama et al. 1996). The higher Ms values for the samples under study compared to other data can be correlated to the larger particle size (110 nm) (Mozaffari et al. 2010).
Figure 2

Room temperature magnetic hysteresis loop of the ball-milled (x)ZnO(1 − x)Fe2O3.

Figure 2

Room temperature magnetic hysteresis loop of the ball-milled (x)ZnO(1 − x)Fe2O3.

BET surface area analysis

Figure 3 depicts the adsorption–desorption isotherm of adsorbent at the boiling point of N2. It can be clearly seen that the isotherm is of type II, as categorized by the IUPAC and Brunauer (Rouquerol et al. 2013). The isotherm is type H4 hysteresis loop, characteristic of aggregated particles with nonporous or macroporous adsorbents and unrestricted monolayer–multilayer adsorption (Rouquerol et al. 2013). The BET analysis revealed particles with SBET 4.51 m2·g−1 and pores with average volume of 0.0201 cm3/g. Similar values of specific surface area for ZnFe2O4 were obtained by Sakthivel et al. (2002). The relatively small specific area may be attributed to the large particle size (Zhang et al. 2010). The data obtained from the analysis are summarized in Table 1.
Table 1

N2 adsorption analysis data

Property Value 
t-plot external surface area 4.5294 m2/g 
BET surface area 4.5135 m2/g 
Pore volume 0.0201 cm3/g 
Pore size (from distribution plot) 36.0 A° 
BJH adsorption size 177.76 A° 
BJH desorption size 184.58 A° 
Property Value 
t-plot external surface area 4.5294 m2/g 
BET surface area 4.5135 m2/g 
Pore volume 0.0201 cm3/g 
Pore size (from distribution plot) 36.0 A° 
BJH adsorption size 177.76 A° 
BJH desorption size 184.58 A° 
Figure 3

(a) N2 adsorption–desorption curves of at 77 K for (x)ZnO(1 − x)Fe2O3 nanopowder. (b) Pore size distribution for (x)ZnO(1 − x)Fe2O3 nanopowder.

Figure 3

(a) N2 adsorption–desorption curves of at 77 K for (x)ZnO(1 − x)Fe2O3 nanopowder. (b) Pore size distribution for (x)ZnO(1 − x)Fe2O3 nanopowder.

Heavy metal ions adsorption study

Equilibrium study

The heavy metal ions were removed from the solutions at room temperature and pH 7.0. The pH was not extended beyond this value in order to avoid metal precipitation (Schiewer & Volesky 1995; Akar et al. 2009; Montazer-Rahmati et al. 2011), while at lower pH the H+ ions are preferentially adsorbed (Torab-Mostaedi et al. 2013). Figure 4 illustrates the data of Cd(II) and Ni(II) ions adsorption by the magnetic nanopowder. At high metal ions, the graphs show a plateau typical of type L of the Langmuir model.
Figure 4

Adsorption equilibrium isotherms of Cd(II) and Ni(II) at 25 °C.

Figure 4

Adsorption equilibrium isotherms of Cd(II) and Ni(II) at 25 °C.

The linearized Langmuir and Freundlich equations for the metal ions removal from solutions are shown (Figure 5) and their calculated constants (qm, KL, KF, and n) with correlation (r2) values are tabulated (Table 2). By looking at the Ni(II) ions removal data, we can see that r2 values are almost equal to unity, indicating that their adsorption fits Langmuir at 313 and 328 K while at 298 the isotherm fits better to the Freundlich model. Nevertheless, the Langmuir model fits better the adsorption equilibrium data Cd(II) ions.
Table 2

Langmuir and Freundlich isotherm parameters for the metal ions removal

Metal ion T(K) Langmuir constants
 
Freundlich constants
 
qm (mg·g−1KL (L·mg−1r2 n kf r2 
Ni (II) 298 57.14 0.0204 0.9906 2.099 4.21 0.9947 
313 71.94 0.0227 0.9926 1.9201 4.57 0.9592 
328 82.65 0.0236 0.9917 1.9231 5.39 0.9624 
Cd (II) 298 104.17 0.0264 0.9960 1.837 6.80 0.9706 
313 111.12 0.0281 0.9885 1.886 7.69 0.9800 
328 128.21 0.0294 0.9961 1.7717 7.97 0.9559 
Metal ion T(K) Langmuir constants
 
Freundlich constants
 
qm (mg·g−1KL (L·mg−1r2 n kf r2 
Ni (II) 298 57.14 0.0204 0.9906 2.099 4.21 0.9947 
313 71.94 0.0227 0.9926 1.9201 4.57 0.9592 
328 82.65 0.0236 0.9917 1.9231 5.39 0.9624 
Cd (II) 298 104.17 0.0264 0.9960 1.837 6.80 0.9706 
313 111.12 0.0281 0.9885 1.886 7.69 0.9800 
328 128.21 0.0294 0.9961 1.7717 7.97 0.9559 
Figure 5

(a) Langmuir and (b) Freundlich adsorption isotherms of Cd(II) and Ni(II) at different temperatures.

Figure 5

(a) Langmuir and (b) Freundlich adsorption isotherms of Cd(II) and Ni(II) at different temperatures.

The obtained data also indicate that the adsorption of both metal ions is favorable at higher temperature. Moreover, the maximum adsorption capacity of the nanoparticles towards Cd(II) is about 128.21 mg·g−1, which was remarkably higher in comparison to the 82.65 mg·g−1 value for Ni(II) at highest temperature.

Kinetic study

The kinetics data for Cd(II) and Ni(II) sorption as a function of time are shown in Figure 6 for the nanopowder at 298 K. It is evident that a sharp rise in the adsorption capacities qt takes place at the beginning and becomes steady after 100 minutes. Accordingly, this time was considered as the equilibrium time for both metal ions adsorption.
Figure 6

Adsorption of Cd(II) and Ni(II) on the (x)ZnO(1 − x)Fe2O3 nanopowder as a function of time.

Figure 6

Adsorption of Cd(II) and Ni(II) on the (x)ZnO(1 − x)Fe2O3 nanopowder as a function of time.

Figure 7(a) and 7(b) display the pseudo-first-order and the pseudo-second-order plots versus time sequentially. In Figure 7(a), ln (qeqt) versus t graph is represented. The plot shows linearity with regression coefficient 0.9615 and 0.9388 for cadmium and nickel ions adsorption, respectively. While the t/qt against t-plot describing the pseudo-second-order law (Figure 7(b)) represents a better fitting for the data (r2 > 0.99). On the other hand, the calculated qm values are in agreement with those obtained experimentally (Table 3). The obtained data clearly show the adsorption kinetics of both metal ions and suggests that kinetics in Cd(II) and Ni(II) adsorption follows the pseudo-second-order kinetic law. These findings agree well with previously reported data (Chen et al. 2008; Montazer-Rahmati et al. 2011).
Table 3

Rate constants for Cd(II) and Ni(II) ions adsorption on the adsorbent

Metal ions t1/2 (s) D× 1015 (cm2s−1qm(exp)a (mg·g−1First-order
 
  Second-order
 
  
k1 × 103 (min−1qm(cal)b (mg·g−1r2 k2 × 103 (g.(mg·min)−1qm(cal)b (mg·g−1r2 
Cd (II) 245.4 3.7 82.45 4.6 33.78 0.9615 3.01 81.3 0.9990 
Ni(II) 398.4 2.3 38.03 5.7 19.14 0.9388 3.99 37.74 0.9952 
Metal ions t1/2 (s) D× 1015 (cm2s−1qm(exp)a (mg·g−1First-order
 
  Second-order
 
  
k1 × 103 (min−1qm(cal)b (mg·g−1r2 k2 × 103 (g.(mg·min)−1qm(cal)b (mg·g−1r2 
Cd (II) 245.4 3.7 82.45 4.6 33.78 0.9615 3.01 81.3 0.9990 
Ni(II) 398.4 2.3 38.03 5.7 19.14 0.9388 3.99 37.74 0.9952 
Figure 7

Kinetics of Cd(II) and Ni(II) adsorption onto the (x)ZnO(1 − x)Fe2O3: (a) pseudo-first-order plot and (b) pseudo-second-order plot.

Figure 7

Kinetics of Cd(II) and Ni(II) adsorption onto the (x)ZnO(1 − x)Fe2O3: (a) pseudo-first-order plot and (b) pseudo-second-order plot.

Thermodynamic study

The variation of metal ions adsorption with temperature was tested at 298, 313, and 328 K. The quantity of Cd(II) and Ni(II) eliminated by the ferrite magnetic particles increases with the rise in temperature, as presented by Figure 8 of the Langmuir model.
Figure 8

Langmuir isotherms for adsorption of Cd(II) and Ni(II) onto nanopowder material at different temperatures.

Figure 8

Langmuir isotherms for adsorption of Cd(II) and Ni(II) onto nanopowder material at different temperatures.

Considering the results obtained in the section ‘Equilibrium study’, only the Langmuir isotherm is tested to model the experimentally obtained data. Langmuir equation constants are listed in Table 4.

Table 4

Thermodynamic parameters for Ni(II) and Cd(II) adsorption

Metal ions Temperature (K) Ka ΔGo (kJ·mol−1ΔSo (kJ·mol−1·K−1ΔHo (kJ·mol−1r2 
 298 1.168 −0.385 0.0482 13.98 0.9788 
Ni(II) 306 1.633 −1.276 0.0488 
 313 1.953 −1.825 0.0482 
 298 2.748 −2.505 0.0371 8.55 0.9808 
Cd(II) 306 3.122 −2.963 0.0368 
 313 3.772 −3.621 0.0371 
Metal ions Temperature (K) Ka ΔGo (kJ·mol−1ΔSo (kJ·mol−1·K−1ΔHo (kJ·mol−1r2 
 298 1.168 −0.385 0.0482 13.98 0.9788 
Ni(II) 306 1.633 −1.276 0.0488 
 313 1.953 −1.825 0.0482 
 298 2.748 −2.505 0.0371 8.55 0.9808 
Cd(II) 306 3.122 −2.963 0.0368 
 313 3.772 −3.621 0.0371 

The enthalpy (ΔHo) value is calculated from ln(Ka) versus T−1 plot of Figure 9. The values of (ΔGo) and (ΔSo) are obtained mathematically from Equations (7) and (8). In Table 4, the thermodynamic function of adsorption is summarized. The adsorption can be described as endothermic according to ΔH0 positive value. The entropy values are positive for metal ions adsorption indicating enhanced randomness at the solid–solution interface.
Figure 9

ln(Ka) versus the reciprocal temperature of cadmium ion and nickel adsorption.

Figure 9

ln(Ka) versus the reciprocal temperature of cadmium ion and nickel adsorption.

As can be seen from the tabulated data ΔGo < 0 indicates a feasible, spontaneous physisorption process. In addition, this indicates favorable adsorption at higher temperature in correlation with the decreasing ΔGo values as the solution is heated.

Mechanism of adsorption

The adsorbed species may also be transported from solutions to a solid phase through intra-particle diffusion or transport process. Intra-particular diffusion is the limiting step for the sorption–desorption phenomenon. The Weber Morris mode is the formula used to describe the mechanism by which intra-particles can diffuse (Weber & Morris 1963; Mckay et al. 1980; Mckay 1983): 
formula
9
where C is a constant and kdif is the rate constant for intra-particle diffusion. The kdif values for the tested adsorbent are obtained as slopes of the graphs (Figure 10) and reported in Table 5. The uptake of cadmium and nickel ions at the surface of the magnetic particles may be governed by the intra-particle diffusion kinetics, since the qt values are linear correlation with t1/2. The regression coefficient values are ≈1.0, indicating the applicability of this model. The intra-particle diffusion plots are shown in Figure 10, where the main parameters of this model are determined and gathered in Table 5. The values of C, obtained from the intercept of the graph, are the measure of the boundary layer effects or the extent of resistance to external mass diffusion. A greater value of C indicates larger thickness of the boundary layer.
Table 5

Intra-particle diffusion model parameters for Cd(II) and Ni(II) ions adsorption

Metal ions kdif1, mg/g·min1/2 C r2 kdif2, mg/g·min1/2 C r2 
Cd(II) 5.199 39.33 0.989 1.38 60.22 0.9714 
Ni(II) 5.621 6.585 0.9492 0.8434 23.952 0.9802 
Metal ions kdif1, mg/g·min1/2 C r2 kdif2, mg/g·min1/2 C r2 
Cd(II) 5.199 39.33 0.989 1.38 60.22 0.9714 
Ni(II) 5.621 6.585 0.9492 0.8434 23.952 0.9802 
Figure 10

qt versus t1/2 plot for the intraparticle diffusion.

Figure 10

qt versus t1/2 plot for the intraparticle diffusion.

According to Li et al. (2012), in a multistep diffusion graph, the first sharp section represents a fast adsorption process instantaneously taking place at the outer surface. The second step is a slow adsorption stage or the diffusion rate-determining step is attributed to intra-particle diffusion. The lines of the graph (Figure 10) deviate from the origin, indicating considerable boundary layer control. The graph clearly shows two steps, implying that the diffusion of the metal ions is controlled not only by intra-particle diffusion but other kinetics processes may be involved (Arami et al. 2008).

Using Equation (10), the diffusivity that greatly depends on the adsorbent's surface is calculated (Yadava et al. 1987): 
formula
10
where D is the mass diffusivity and r0 is the spherical-equivalent radius of the adsorbent particle (cm). Taking the particles’ radius from the XRD calculations as equal to 5.5 × 10−6 cm, the D values were found to be 3.7 × 10−9 and 2.3 × 10−9 cm2 s−1 for Cd+2 and Ni+2 diffusion, respectively. The diffusivity values clearly indicate the preferential tendency of adsorbent to remove cadmium rather than nickel ions.

COMPARISON OF ZNFE2O4 ADSORPTION CAPACITY WITH OTHER ADSORBENTS FOR CADMIUM AND NICKEL METAL IONS

The ability of the synthesized magnetic particles to eliminate the heavy metal ions under investigation was contrasted with other adsorbents reported in the literature, as listed in Table 6. The adsorbent employed in this study showed better adsorption performance than others, indicating that the tested nanopowder is a good candidate for cadmium and nickel ions removal from aqueous solutions.

Table 6

A comparison of (x)ZnO(1 − x)Fe2O3 adsorption capacity with other adsorbents for cadmium and nickel metal ions

Adsorbent Ni(II) (mg·g−1Cd(II) (mg·g−1Temp. (K) Reference 
Silica-gel-biomass 98.01 – 298 Akar et al. (2009)  
CuFe2O4 nano-particles – 17.54 298 Tu et al. (2012)  
NH2-MCM-41 12.36 18.25 298 Heidari et al. (2009)  
Magnetic graphene oxide – 91.29 298 Deng et al. (2013)  
Milled goethite – 125 298 Khezami et al. (2016)  
Milled goethite – 167 328 Khezami et al. (2016)  
Ni (15% wt)-doped α-Fe2O3 – 90.91 328 OuldM'hamed et al. (2015)  
Magnetic nanoparticles 11.53 – 298 Sharma & Srivastava (2010)  
Lemon peel 80.0 – 298 Thirumavalavan et al. (2011)  
Orange peel 81.3 – 298 Thirumavalavan et al. (2011)  
ZnFe2O4 57.1 104.2 298 Present work 
ZnFe2O4 83 128 328 Present work 
Adsorbent Ni(II) (mg·g−1Cd(II) (mg·g−1Temp. (K) Reference 
Silica-gel-biomass 98.01 – 298 Akar et al. (2009)  
CuFe2O4 nano-particles – 17.54 298 Tu et al. (2012)  
NH2-MCM-41 12.36 18.25 298 Heidari et al. (2009)  
Magnetic graphene oxide – 91.29 298 Deng et al. (2013)  
Milled goethite – 125 298 Khezami et al. (2016)  
Milled goethite – 167 328 Khezami et al. (2016)  
Ni (15% wt)-doped α-Fe2O3 – 90.91 328 OuldM'hamed et al. (2015)  
Magnetic nanoparticles 11.53 – 298 Sharma & Srivastava (2010)  
Lemon peel 80.0 – 298 Thirumavalavan et al. (2011)  
Orange peel 81.3 – 298 Thirumavalavan et al. (2011)  
ZnFe2O4 57.1 104.2 298 Present work 
ZnFe2O4 83 128 328 Present work 

CONCLUSION

Magnetic nanoparticles (x)ZnO(1 − x)Fe2O3 were prepared by mechanical milling of commercial ingredient samples. Their ability to eliminate nickel and cadmium ions was investigated under different experimental conditions. The adsorption data at equilibrium were found to comply with the Langmuir isotherm for Cd(II) and with both the Langmuir and the Freundlich for Ni(II). At all temperatures, the magnetic nanoparticles removed larger amounts of cadmium (128 mg·L−1) than nickel ions (83 mg·L−1) at 328 K. Furthermore, the kinetics follows the second-order rate law. The thermodynamic data revealed an endothermic, spontaneous, physisorption process. The suggested adsorption mechanism indicated that the process controlled intra-particle diffusion along with other kinetic models.

ACKNOWLEDGEMENTS

The authors would like to thank the National Plan for Sciences, Technology and Innovation (MAARIFAH), King Abdulaziz City for Sciences & Technology, Kingdom of Saudi Arabia.

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