Magnetically synthesized MnFe₂O₄ nanoparticles as an effective adsorbent for lead ions removal from an aqueous solution

Natural lead is composed of a mixture of four stable isotopes: lead-204 (1.4%); lead-206 (24.1%); lead-207 (22.1%); and lead-208 (52.4%). Lead isotopes are the end products of each of the three naturally occurring radioactive elements: lead-206 for the uranium series, lead-207 for the actinium series, and lead-208 for the thorium series. There are 43 other isotopes of lead; all are radioactive, so that the removal of lead ions from the aqueous solutions is considered a critical task (Morton-Bermea et al. 2011). Water pollution is considered a global problem that affects millions of people rapidly, increasing in the fields such as industry and agriculture (Qiu 2010). The wastewater treatment is an important task because of the vicious spread of common diseases of wastewater such as cancer, kidney failure, typhoid fever, diarrhea, and liver hepatitis (Zou et al. 2016; Zare et al. 2018).

The wastewater treatment was taken from industrial effluent, wastewater, and sewage before being exhausted from water resources (Salem & Awwad 2014; Chen et al. 2019). Unfortunately, the presence of large amounts of heavy toxic metals in these effluents causes the biggest challenge in the purification of water, where it can cause death when reaching more than the allowable concentration limit in the human body (Afroze & Sen 2018; Wang et al. 2020). Lead is considered to be a long-term contaminant of these heavy metals. It is produced from the burning of fossil fuels and the mining operations in the atmosphere. It has many uses and applications, such as battery manufacturing, pesticide, ammunition, pigments, glassware, and metal products (Yao et al. 2016; Mohammad et al. 2017). Very low concentration of lead may prevent the synthesis of red blood cells, while above the lead upper limit causes damage to the nervous, reproductive, and cardiovascular systems (Long et al. 2014; Li et al. 2019b).

Consequently, it is essential to find effective treatments to reduce the lead ions in the wastewater and prevent lead toxicity. Various chemical separation techniques have been employed for the effective removal of the toxic heavy metals from the wastewater, such as coagulation, chemical precipitation, reduction, evaporation, cementation, reverse osmosis, ion exchange, and ultrafiltration (Demey et al. 2018; Son et al. 2018; Zhu et al. 2018; Li et al. 2019a; Brinza et al. 2020). Nonetheless, these procedures have numerous technical and economic limitations in the range of 1–100 mg/L to eliminate lead(II) ions from the dilute solutions (Vafajoo et al. 2018).

Various types of adsorbents, such as activated charcoal, zeolites, clays, apatite, and chitosan, have been used in this respect for the removal of lead(II) ions (Ziagova et al. 2007; Xu et al. 2019). Modified magnetic nanoparticles (MNPs) are considering efficient adsorbents for the removal of both the heavy metals and the organic pollutants because of their ease of separation from the aqueous solution under an external magnetic field, beside their advantages such as the nontoxicity, small diffusion resistance, and large surface area (Oliveira et al. 2003; Hu et al. 2007; Razmara et al. 2019). Combining the adsorption and magnetic properties enables these compounds to have an effective strategy for magnetic separation technology. The magnetic adsorption techniques are simple, quick, cheap, and automated (Donia et al. 2008; Zeraatkar Moghaddam et al. 2018). In recent years, this technique has received much more attention (Zhou et al. 2014; Zeraatkar Moghaddam et al. 2018). Recently, it is exciting to eliminate these metals using various magnetic ferrite nanoparticle adsorbents (Reddy & Yun 2016; Kefeni et al. 2017).

The novel contribution to the earlier publications is the facile synthesis of the MnFe₂O₄ by the co-precipitation method. Then, the characterizations of synthesized MNPs were carried out using the available spectroscopic tools such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The synthesized MNPs have been tested to remove lead(II) ions from an aqueous solution utilizing the magnetic separation technique. Various parameters affecting the adsorption of lead(II) ion process, such as the adsorbent weight, contact time, pH, and initial lead(II) ion concentration, have been examined to assess their impact on the adsorption efficiency, and desorption studies were also carried out to reuse the adsorbent.

Lead(II) nitrate, cesium(I) nitrate, and cobalt(II) nitrate were obtained from Sigma-Aldrich and were prepared by dissolving in deionized water. The effecting parameters, such as contact time, pH, temperature, and sorbent mass that influence the sorption process, have been investigated by dissolving 0.128 g of lead nitrate into 1,000 mL of deionized water to get the standard lead(II) solution (80 mg/L), whereas to study the effect of the initial metal concentration, the stock solution of lead nitrate (500 mg/L) was prepared. With suitable dilutions, different concentrations from lead(II) were obtained.

The magnetic MnFe₂O₄ nanoparticles MNPs were synthesized by the co-precipitation method according to Yakout et al. (2018). The process is to mix manganese nitrate and ferric nitrate with special weight ratios into 250 mL deionized until the temperature of the solution becomes 323 K for 2 h. Then the pH was adjusted up to pH ≈ 10 using 0.01 M NaOH solution. After cooling, deionized water was thoroughly washed with the obtained MNPs (MnFe₂O₄) until a pH value of 7 was reached.

The synthesized MnFe₂O₄ nanoparticles have been characterized using the available spectroscopic tools such as the SEM, XRD, and TEM in addition to FT-IR to demonstrate the morphological characteristics of the synthesized adsorbent and elemental composition of the materials, respectively.

The adsorption of MnFe₂O₄ has been investigated using the analytical grade standard solutions composed of lead nitrate, cesium nitrate, and cobalt nitrate (100 mg/L). The batch studies were performed by varying the adsorbent mass, pH, contact time, temperature, and initial metal ion concentration.

The main task in this section was to investigate the removal effectiveness of the synthesized MNPs MnFe₂O₄ toward different toxic ions such as cesium(I), cobalt(II), and lead(II) from aqueous solutions as promising potential adsorbents. In this concern, the removal efficiency, %R for cesium, cobalt, and lead ions were calculated, and the results are shown in Figure 1. MnFe₂O₄ nanoparticles removed lead(II) ions from the solution more than other metal ions.

To determine surface areas of MnFe₂O₄ sample, nitrogen adsorption/desorption isotherm was measured at 77 K on an automatic adsorption instrument (Quantachrome Instruments, Model Noval1000e series, USA) in relative pressure ranging from 10⁻⁶ to 0.999. Before the measurement, the sample was powdered to shorten the time required for reaching equilibrium in the isotherm study and degassed at 250 °C under nitrogen flow for 24 h. The cross-sectional area, i.e., the area occupied by an adsorbate molecule in a completed monolayer for the molecule at 77 K, was taken as 16.2 Ų.

The surface area of the adsorbent sample was obtained using standard methods, pore-volume, and pore size distribution, which were subsequently calculated from the N₂ adsorption data using NOVA Win 2.0 software.

The specific surface area of MnFe₂O₄ was 145 m²/g. Image of the prepared MNPs utilizing SEM was obtained, as appeared in Figure 2(a). The surface morphology study of MnFe₂O₄ showed that several ultra-fine particles with a diameter of about 10 nm are agglomerated. It is evident that the prepared material displays the uniform distribution of the crystals of MnFe₂O₄. It can be seen from the surface topography that there is grain between the particles. The particle size of the compound is around 10 nm.

The TEM micrograph of MnFe₂O₄ nanoparticles and the corresponding selected area electron diffraction pattern are shown in Figure 2(c). From the TEM image, the nanoparticles of specimens were spherical with the size ranging from 3 to 10 nm and the average particle of 8 nm that agrees well with that obtained from the XRD results given from Equation (7).

The FT-IR spectrum of MnFe₂O₄ is presented in Figure 2(d). The spectrum shows two major absorption bands under 1,000 cm⁻¹, a typical feature of ferrites. The absorption peaks at 420 and 580 cm⁻¹ may be attributed to the Fe–O and Mn–O stretching vibrations of MnFe₂O₄. The band at 3430 cm⁻¹ may be assigned to the stretching vibration of O–H surface water molecules or hydroxyl groups, and the crystalline phase formation is confirmed from XRD results (Huang et al. 2019).

Point zero charge (pHpzc) of MnFe₂O₄ nanoparticles was determined using the solid addition method by adding 1 g of the MnFe₂O₄ nanoparticles in 45 mL of 0.01 M of KNO₃ solutions in various locked conical flasks_. The pH value is varied between 2 and 10 using (0.1 M) HCl or (0.1 M) NaOH and kept overnight under stirring at a temperature of 25 °C. The final pH (pH_f) was measured and plotted versus the initial pH (pH₀). The pHpzc of MnFe₂O₄ nanoparticles was determined at which the pH_f equals pH₀ (Abbas et al. 2014). From Figure 3, the pHpzc of MnFe₂O₄ nanoparticles was found to be 6.5.

The effect of pH on lead(II) removal percentage (R%) at 25 °C is represented in Figure 4. The percentage lead(II) removal was increased, as the pH of the aqueous solution increased from 2 to 6.3 at adsorbent mass and lead(II) concentration of 1 g/L and 80 mg/L, respectively. The removal percentage of lead increased from 30 to 98.8% at pH values of 2 and 6.3, respectively. This phenomenon occurred due to the charge of the surface is neutral at pHpzc, which is 6.5 for manganese ferrite nanoparticles. Below the pHpzc, the surface of MnFe₂O₄ nanoparticles is positively charged, and lead(II) results in electrostatic repulsion. At pH higher than pHpzc, the surface of MnFe₂O₄ nanoparticles is negatively charged, increasing the adsorption with positively charged lead(II) species (Selambakkannu et al. 2018).

Moreover, it was clear that at low pH values, the adsorption of lead(II) ions onto MnFe₂O₄ nanoparticles was low due to the competition between lead(II) ions and hydronium ion (H₃O⁺). At high pH values, the sorption of lead(II) ions was increased because of a reduction in hydronium ion concentration (Zhang et al. 2013; Chen et al. 2020). Experiments were performed with a pH value of less than 6 because lead(II) ions would precipitate to form Pb(OH)₂ at a higher pH value of more than 6 (Kosa et al. 2012).

The effect of the MnFe₂O₄ nanoparticle mass on the percentage of lead(II) ions adsorbed from aqueous mediums was investigated in the adsorbent mass range (0.2–1.4 g/L), lead(II) concentrations of 80 mg/L, a pH value of 5.3, and a temperature of 25 °C (Figure 5). The figure shows that the percentage of lead(II) ions increased gradually with an increase in the adsorbent mass of MnFe₂O₄ nanoparticles and increasing the masses of MnFe₂O₄ nanoparticles from 0.2 to 1.4 g/L increased the percentage of adsorption of lead(II) ions from 35 to 100%. This increase in the percentage of adsorption of lead(II) ions may be attributed to the increase in the adsorption sites toward the metal ions and, consequently, the surface area of the adsorbent (Hassan Mohamed et al. 2020).

The adsorption of lead(II) ions by MnFe₂O₄ nanoparticles expanded with the expansion of time, and equilibrium was attained after 60 min (Figure 6(a)). Different kinetic sorption models were investigated to estimate the experimental data and understand the adsorption mechanism, including pseudo-first-order kinetic model, pseudo-second-order kinetic model, and intra-particle diffusion model.

The slope and intercept of the plot of log(Q_e−Q_t) against time were used to determine K₁ and Q_e, respectively (Figure 6(b)).

The results showed that the calculated adsorption capacity (Q_e,_cal) has a lower value than the amount of experimental adsorption capacity (Q_e,exp). Consequently, the kinetic behavior of lead(II) ions on MnFe₂O₄ nanoparticles could not be explained by the pseudo-first-order kinetic model (Hassan & Aly 2019). Other parameters from the linear regression plot are recorded in Table 1.

The fitting data of the intra-particle diffusion model gives a nonlinear relationship, and the value of I computed (I ≠ 0) implies that this model does not participate in the adsorption process, as presented in Figure 6(d). And consequently, the intra-particle diffusion model was not valid for the adsorption kinetics.

The adsorption capacities of MnFe₂O₄ nanoparticles were measured at a pH value of 5.3 with sorbent mass 1 g/L of MnFe₂O₄ and lead(II) ion concentration (40–280 mg/L). Different isotherm models (Langmuir, Freundlich, and Dubinin–Radushkevich (D–R)) have been developed for assessing the equilibrium adsorption data (Hajialigol & Masoum 2019). Figure 7 shows the nonlinear fitting of the sorption equilibrium data for lead(II) ions onto the MnFe₂O₄ nanoparticles at different temperatures (Figure 7(a), 7(b), and 7(c), respectively) such as Langmuir, Freundlich, and D–R adsorption isotherm models.

The linear plots presented in Figure 8(a) of C_e/Q_e against C_e designate the Langmuir isotherm applicability on the lead(II) ion sorption capacity of MnFe₂O₄ nanoparticles. The values of the Langmuir constants are given in Table 2. The adsorption capacity (Q_M) for MnFe₂O₄ nanoparticles is 75.75 (mg/g).

According to the model of Freundlich (Figure 8(b)), the adsorption data at different temperatures were investigated. The constants K_F and n are calculated and represented in Table 2. Values of n lying between 1 and 10 confirm that the adsorption was favorable (Selvi et al. 2001).

The adsorption isotherm data of lead(II) onto MnFe₂O₄ nanoparticles fitted well by the Langmuir model, with higher values of correlation coefficient R² than the Freundlich model.

The plots of ln Q_e versus ε² give straight lines as shown in Figure 8(c) with slopes and intercepts equal to β and ln Q_s, respectively. Because in physical adsorption, increasing the temperature reduces the amount of adsorption at 25, 40, and 55 °C. The computed values of means free energy, E, of the adsorption for lead(II) were found to be 6.196–7.940 kJ/mol (Table 2), when the value of the energy of adsorption below 8 kJ/mol the adsorption proceeds via chemisorption mechanism. (Selim et al. 2018; Wang et al. 2018).

The values of ΔG°, ΔH°, and ΔS° are reported in Table 3. The positive value of entropy of adsorption ΔS° reflects the affinity of the sorbent MnFe₂O₄ nanoparticles toward lead(II), and the positive value of enthalpy change ΔH° and the negative values of free energy change (ΔG°) at all temperatures confirmed that the adsorption process of lead(II) ions was endothermic and thermodynamically spontaneous (Foroutan et al. 2018).

The effect of natural organic matter (NOM) on the adsorption of lead(II) onto MnFe₂O₄ nanoparticles was discussed using the local canal's surface water. Figure 10 shows the isotherm tests of lead(II) adsorption onto MnFe₂O₄ nanoparticles using natural water compared to double distilled water. The Langmuir–Freundlich constants are listed in Table 4. As can be seen, the presence of the NOM increases lead(II) adsorption. NOM could adsorb onto the surfaces of the MnFe₂O₄ nanoparticles and then promote the adsorption of lead(II) due to its binding with lead(II) (Yang et al. 2011; Sun et al. 2012).

Desorption and sorbent recovery are the essential features of the sorbent in reasonable applications. As shown in Figure 11, the desorption of lead(II) from stacked adsorbent was examined using various eluents such as NaOH, Na₂CO₃, Na₃PO_4, and deionized water.

Table 5 includes a relative report about the adsorption capacity compared with other announced adsorbents. From Table 5, it is clear that the adsorption capacity of MnFe₂O₄ nanoparticles is more noteworthy than that of the most existing adsorbents. Therefore, MnFe₂O₄ nanoparticles could be a promising material used in lead(II) removal from leaden aqueous media. The cost, including the transport, preparation, and further recovery of the MnFe₂O₄ nanoparticles, is assessed under various conditions. While numerous researchers have stated as ‘low-cost sorbents for removal of lead’, although even several commercial adsorbents have been closely studied for the removal of lead(II), the actual costs are not stated in the literature. The cost of MnFe₂O₄ nanoparticles as $/g of lead(II) removed was found at approximately $1.3.

However, the exhausted MnFe₂O₄ nanoparticles loaded with lead(II) ions can be regenerated by 0.01 M NaOH. After the fourth regeneration time, it can be dried and used in construction, safe and economical disposal of the adsorbed lead(II).

This study reveals that MnFe₂O₄ nanoparticles are an effective adsorbent for removing lead(II) ions from laden aqueous media. The co-precipitation technique was used to synthesize the MnFe₂O₄ nanoparticles. The XRD analysis has demonstrated the development of spinel structures of the nanoparticles. The TEM analysis showed that the ferrite particles distributed have a truly uniform structure, and the particle size of the sample is around 8.7 nm.

From the experimental section, we concluded that the percentage removal of lead(II) steadily increases with increasing agitation time, temperature, adsorbate mass, and pH. The adsorption kinetics confirmed that the sorption followed pseudo-second-order kinetics. The lead(II) ion sorption onto MnFe₂O₄ nanoparticle result followed well with Langmuir and Freundlich isotherm models, but Langmuir isotherm is the best fitted than Freundlich isotherm. The positive value of ΔH° shows the endothermic nature of the process, and the negative value of ΔG° indicates that the sorption process was thermodynamically spontaneous. The presence of NOM increases lead(II) adsorption. Also, the desorption of lead-adsorbed MnFe₂O₄ nanoparticles was conducted using 0.01 M NaOH as eluent, and desorption efficiency was found to be more than 88% for four runs.

This study concluded that MnFe₂O₄ MNPs would be a promising, cost-effective, efficient, and renewable adsorbent for the treatment of lead(II) ions from contaminated wastewater.

The authors would like to thank the Nuclear Research Center, the Egyptian Atomic Energy Authority, to support this work.

All relevant data are included in the paper or its Supplementary Information.