(x)ZnO(1 − x)Fe₂O₃ nanocrystallines for the removal of cadmium(II) and nickel(II) from water: kinetic and adsorption studies

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 (ΔH^o, ΔS^o, and ΔG^o) were determined and their effect on the adsorption was scrutinized.

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).

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

Equation K_a = q_m·K_L can be employed to calculate the equilibrium constant. The ΔH^o value can be obtained from the ln(K_a)) against (T⁻¹) plot.

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.

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.

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

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).

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.

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.

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.