Evaluation of the optical and magnetic properties of novel Nd0.9Zn0.1FeO3 perovskite nanoparticles and their adsorption of Pb2+ ions from water

Nd0.9Zn0.1FeO3 was prepared in a single-phase with an average crystallite size of 25.82 nm using a citrate combustion technique. The energy dispersive X-ray assures the chemical formula of the sample. The elemental mapping of Zn-doped NdFeO3 illustrates the good homogeneous distribution of the elements in the sample. Nd0.9Zn0.1FeO3 has antiferromagnetic properties with weak ferromagnetic components and has good UV absorbance. The values of the band gap for the direct and indirect transitions are 1.473 eV and 1.250 eV, respectively. The adsorption of nickel(II), cobalt(II), chrome(VI), cadmium(II), and lead(II) ions has been studied at pH 7. The highest removal efficiency (η = 73.72%) was observed for the lead ions from water. The current study has examined the kinetics, recoveries, and mechanisms of utilizing Nd0.90Zn0.10FeO3 to remove Pb2+ ions from water. The optimum conditions for the absorbing Pb2+ are pH 7 and a contact time of 60 min. The Freundlich isotherm model is the best model for the absorption of Pb2+ ions. While, the pseudo-second-order kinetic model describes the kinetic adsorption data. The sample has a good efficiency for removing Pb2+ ions from water several times.

are coordinated by six oxygen ions, forming a FeO 6 octahedron.There are many preparation techniques for prepared orthoferrites, such as solid-state reactions, coprecipitation, and the flash method.Among the prepared methods, the citrate combustion technique is inexpensive, effective, easy to prepare the nanoparticles, and fast 27 .
In the present work, Nd 0.9 Zn 0.1 FeO 3 was synthesized using a combustion method and was characterized by XRD, FESEM, EDX, and elemental mapping.The physical properties of the Zn-doped NdFeO 3 have been studied, such as their magnetic and optical properties.The ability of the sample to remove HMs from water was examined.Many parameters that affect the adsorption process of HM have been studied, such as the pH value, temperature, contact time, and regeneration of the material several times to remove the HM from water.The kinetic and isothermal models of adsorption have been studied.

Materials
Nd 0.9 Zn 0.1 FeO 3 was prepared using a citrate combustion technique.The chemicals used in the preparation techniques are Zn nitrate, Nd nitrate, citric acid and Fe nitrate with high purity (99.9%).All chemicals were purchased from Sigma Aldrich.All the chemicals are of analytical quality, and they are used just as received, requiring no further purification.

Preparation technique
Zn-doped NdFeO 3 nanoparticles were synthesized using a citrate combustion method 16,28 .The metal nitrates with a stoichiometric ratio were dissolved in 25 ml of distilled water.A citric acid was added to the metal solution, where the citric acid: metal nitrate ratio is 1:1.Ammonia was used to bring the solution's pH to 7. The resultant solution was stirred and heated on the magnetic stirrer for one hour at 100 °C at a rate of 220 rpm, then heated for four hours at 250 °C until the solution converted to ash powder.A gate mortar was used to grind the ash powder for different characterizations and measurements.

Characterization and measurements
XRD was used to study the crystallinity of the sample using a Bruker Advance D8 diffractometer.The morphology of the sample was investigated using FESEM (model Quanta 250), which was attached to EDX and elemental mapping.The magnetic hysteresis loop of the Zn-doped NdFeO 3 has been investigated via VSM (VSM; 9600-1 LDJ, USA).The optical properties of Nd 0.9 Zn 0.1 FeO 3 were investigated using a UV-visible spectrophotometer (Jasco V-630).

HMs removal from water
To determine η of HMs such as nickel(II), cobalt(II), chrome(VI), cadmium(II), and lead(II) ions from water.50 ppm standard solutions of HMs were prepared.Adding 10 ml of standard solutions in many beakers that contain 0.02 g of Zn-doped NdFeO 3 .Ammonia and diluted nitric acid were used to change the pH value.After shaking the solution on the electric shaker, 9 ml of the solution was withdrawn using a 0.2 µm syringe filter.Inductively coupled plasma spectrometry (ICP, Prodigy 7) was used to calculate the concentration of HMs.

Results and discussion
Figure 1 illustrates the XRD of Nd 0.9 Zn 0.1 FeO 3 nanoparticles.The zinc-doped NdFeO 3 has an orthorhombic structure.The main peak at 2θ = 32.578°for (121) is characterized for the Nd 0.9 Zn 0.1 FeO 3 perovskite.The noise in Fig. 1 indicates that the sample has a nanoscale.The average crystallite size was calculated using Scherer's Eq. (1).where λ refers to the wavelength of the X-ray (λ = 1.5406Å), θ is Bragg's angle, and β denotes the full width at half maximum.The average crystallite size (L) of Nd 0.9 Zn 0.1 FeO 3 is 25.817 nm.Equation (2) was used to calculate the lattice parameters 29 .
The lattice parameters (a, b, and c) were tabulated in Table 1.The values assure that Nd 0.9 Zn 0.1 FeO 3 has an orthorhombic structure.The unit cell volume was calculated from Eq. (3).
where M denotes the sample ' s molecular weight, Z = 4, and N refers to Avogadro's number.
The tolerance factor (t) refers to the stability of the perovskite structure and gives the relation between the ionic radii of the A (r A ), Fe (r Fe ), and oxygen ions (r O ), as shown in Eq. ( 5).
The r A value was estimated from Eq. ( 6).
The value of t is 0.8757, which indicates the orthorhombic structure of Nd 0.9 Zn 0.1 FeO 3 .FESEM of the Nd 0.9 Zn 0.1 FeO 3 nanoparticles, which have nanoscale Fig. 2 illustrates the agglomerated particles due to their magnetic properties and the preparation procedure 29 .The origin of the magnetic behavior of Nd 0.9 Zn 0.1 FeO 3 is the antiparallel spines of Fe 3+ ions.The FESEM image shows the particles have a porous and rough nature, which increased the surface area of Nd 0.9 Zn 0.1 FeO 3 and increased its ability to adsorb the HMs.The roughness of the surface of Zn-doped NdFeO 3 was studded using Gwyddion 2.50 software as illustrated from Fig. 3.The average roughness value and the maximum peak height are 5.09 nm and 3.30 nm, respectively.The presence of humps and grooves on the surface of the sample refers to the porous nature of the surface 30 . Figure 4 illustrates the EDX of the Nd 0.9 Zn 0.1 FeO 3 nanoparticles.The figure shows the presence of Fe, Zn, O, and (1)

Zn
Table 1.The lattice parameters, the unit cell volume, molecular weight, theoretical density, molecular weight, the tolerance factor, and the average crystallite size.www.nature.com/scientificreports/Nd elements without any impurities.The weight percent (wt%) and the atomic percent (wt%) of the elements were illustrated in the inset table, which assures the preparation of the sample in the same chemical formula: Nd 0.9 Zn 0.1 FeO 3 .Figure 5 illustrates the elemental mapping of Zn-doped NdFeO 3 .The good homogeneous distribution of the elements is shown in Fig. 5a.Each element in the sample was illustrated by a specific color in Fig. 5b-e.
Figure 6 illustrates the magnetization curve of the Nd 0.9 Zn 0.1 FeO 3 nanoparticles.The M-H loop shows the s-shape hysteresis loop without saturation, which indicates the antiferromagnetic properties of the Zn-doped NdFeO 3 nanoparticles.The magnetic interactions originate from the magnetic coupling between the Nd and Fe spins.The nanosized NdFeO 3 has weak ferromagnetic properties, as reported in many works 31,32 .Zn 2+ is a nonmagnetic ion because the electrons are aligned and paired in the valence orbitals.The magnetic behavior of Nd 0.9 Zn 0.1 FeO 3 originates from 31,33 : 1.The superexchange interaction between Fe 3+ ions via O 2− ions.2. The direct interaction between Fe 3+ ions.3. The magnetic interaction between Nd 3+ -O 2− -Nd 3+ and Nd 3+ -O 2− -Fe 3+ .These magnetic interactions lead to the antiferromagnetic properties of the Zn-doped NdFeO 3 .The values of M r , M s , and the coercive field (H c ) are reported in Table 2. M s illustrates the antiferromagnetic properties of the Zn-doped NdFeO 3 .The squareness ratio was calculated according to the following equation: The value of the SQR is 0.065, which indicates the magneto-static interactions between the particles.The shift of the hysteresis loop to the positive direction is due to the exchange bias (EB) field 34 .The EB originates from the coupling between the ferromagnetic spins and AFM spins in the sample.H EB was calculated from Eq. ( 8) and reported in Table 2.  where H 1 and H 2 are the intercepts of the M-H loop with the negative and positive x-axes, respectively.Based on the non-collinearity of the spins at the surface of the sample, magnetic anisotropy was present in the Zn-doped NdFeO 3 .Equation ( 9) was used to calculate the value of K. ( 8)  www.nature.com/scientificreports/ Figure 7 illustrates the dependence of the absorbance and the photon wavelength.The figure contains two regions, the first at the low wavelength (λ ≤ 370 nm) and the second at the high wavelength (λ > 370 nm).In the first region, λ is small and photon energy is high, which enables the electrons to transfer from the valence band (V.B.) to the conduction band (C.B.).In the second region, λ is high and the photon's energy is low, and they cannot jump the electrons from V.B. to C.B.
The optical extinction coefficient (k) is correlated with the electromagnetic energy loss in the sample.Equation (10) was used to calculate the values of k.
where α is the optical absorption coefficient and can be calculated using the following equation: where l refers to the spaceman's length and A is the absorbance.Figure 8 depicts how k depends on λ.The value of k increases with raising the wavelength of the photons, owing to the fact that by increasing the λ, the photon's energy (hν) decreases and they cannot jump the electrons from V.B. to C.B., so the electromagnetic energy dissipates and k increases.
The optical band gap (E g ) of Zn-doped NdFeO 3 can be determined using the Tauc plot.Equation ( 12) represents the Tauc equation 18,35 .where A is a constant.The type of the optical transition can be estimated from the value of (x), where x = 2 for the direct transition and x = 0.5 for the indirect transition.Figure 9 shows the direct transition Tauc plot for the Zn-doped NdFeO 3 nanoparticles, while Fig. 10 illustrates the indirect transition Tauc plot.The values of the direct and indirect energy gaps were reported in Table 3, which indicate that the sample can be used as a photocatalyst.
Figure 11 shows the relationship between η of the investigated sample and the type of heavy metals at pH 7. Equation ( 13) was used to calculate the value of the HM removal efficiency (η).
where C f denotes the final concentration while and C i refers to the initial concentration of the heavy metals.Nd 0.90 Zn 0.10 FeO 3 nanoparticles have a good ability to adsorb many HMs from aqueous solutions.The highest removal efficiency (η = 73.72%)was observed for the Pb 2+ ions from water.So studying the parameters that affect the absorption process will focus on the lead ions.

Effect of pH on the adsorption of Pb 2+ from water
Figure 12 illustrates the dependence of η on the pH value.At low pH values (acidic medium), the adsorption of the Pb 2+ ions is small due to the presence of excess H + ions in the solution, which competes with the adsorption of the Pb 2+ ions on the active cites.At high pH values (basic medium), the Pb 2+ ions precipitate as lead hydroxides, which is not favorable 36 .At pH = 7, the Pb 2+ ions are easily absorbed on the surface active sites of the investigated sample, so the optimum pH value for the adsorption of Pb 2+ is 7.

The influence of contact time
Figure 13 shows the dependence of the adsorption of Pb 2+ ions on the contact time.It is illustrated that as the contact time increased, the adsorption increased.At the start of the experiment, the active site number was large, which could adsorb Pb 2+ ions.While raising the contact time, the adsorption increases due to more lead ions being adsorbed on the investigated sample.From Fig. 13, the optimum contact time is observed at 60 min.

Adsorption isotherm models
The monolayer adsorption of Pb 2+ removal onto an Zn-doped NdFeO 3 surface is described by the Langmuir model.Since the surface has limited active sites and homogeneous adsorption energy, the Langmuir model states that once Pb 2+ ions bind to a binding site, no further adsorption can take place.Equation ( 14) describes the Langmuir isotherm model.( 14) where K L is the Langmuir constant, C e refers to the equilibrium metal concentration, q m denotes the highest adsorption capacity, and q e denotes the adsorption concentration.Figure 14 illustrates the Langmuir isotherm model for Zn-doped NdFeO 3 .The correlation coefficient (R 2 ) of the Langmuir model is listed in Table 4.The Freundlich isotherm, which is represented by Eq. ( 15), is used to study the multilayer adsorption of Pb 2+ ions on heterogeneous surfaces.

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where 1 n denotes the adsorption intensity, and K f refers to the Freundlich constant.By taking the logarithm for both sides of Eq. ( 15), the Freundlich model equation becomes Eq. ( 16).

The influence of temperature
Figure 16 illustrates the influence of temperature on the adsorption of Pb 2+ on the surface of Zn-doped NdFeO 3 .The temperature has a small effect on η in the temperature range of 300-340 K.The η of lead ions rose with increasing temperature, which indicates that the adsorption process is not only a physical adsorption but also a chemical adsorption.The active site number on the adsorbent surface increases with increasing temperature due to the bond rupture 37,38 .In the presence study, the adsorption is multilayer and applies the Freundlich isotherm.

Adsorption kinetic models
The kinetic models were used to study the type and rate of adsorption on Zn-doped NdFeO 3 .The pseudofirst-order, pseudo-second-order, and inter-particle diffusion kinetic models are represented by the following equations, respectively.( 17) ln(q e − q t ) = ln q e − k 1 2.303 t  www.nature.com/scientificreports/where q t is the Pb 2+ adsorbed at time t.The diffusion rate constants of pseudo-1st, 2nd, and inter-particles denote K 1 , K 2 , and K 3 , respectively.Figure 17 illustrates the pseudo-first order, which describes the weak and reversible physisorption mechanism.While Fig. 18 represents the pseudo-second order and refers to the presence of a chemisorption mechanism between the Pb 2+ and the surface active sites of Zn-doped NdFeO 3 , in the chemisorption mechanism, there are strong covalent bonds between the Pb 2+ ions and the adsorbent.Contrarily, Fig. 19 shows the intra-particle diffusion model, which is a quick and well-thought-out process.The value of the regression coefficient R 2 (> 0.98) is the maximum for the pseudo-second order, which means that the absorption is a chemisorption mechanism.

Regeneration test
To make the adsorption process more useful and less expensive, Zn-doped NdFeO 3 nanoparticles were used to remove Pb 2+ ions from water three times.Figure 20 illustrates the removal of Pb 2+ ions for three cycles with high removal efficiency.The values of η are 73%, 57%, and 52% for the first, second, and third cycles.

Conclusion
Nd 0.90 Zn 0.10 FeO 3 perovskite was synthesized using a citrate combustion method and was characterized by XRD, EDX, elemental mapping, and FESEM.XRD reveals that Nd 0.90 Zn 0.10 FeO 3 has a L of 25.817 nm.The FESEM image illustrates the porous nature of the prepared sample.Nd 0.9 Zn 0.1 FeO 3 has antiferromagnetic behavior with a M s of 2.5036 emu/g.Zn-doped NdFeO 3 has a good ability to absorb UV and visible light with a small optical band gap.The presence of HMs in water is a great environmental problem, and the investigated sample has a good ability to remove them from water.The highest removal efficiency of Pb 2+ ions from water at pH 7 is 73.72%.The pseudo-second-order kinetic model and the Freundlich isotherm model are the most fitting models for the experimental data.The absorption of Pb 2+ on the surface of Nd 0.9 Zn 0.1 FeO 3 is a chemisorption mechanism.The sample can be considered a good absorbing material for the removal of lead from water several times.In future work, the investigated sample will be used to prepare a nanocomposite with a metal oxide to increase the removal efficiency of heavy metals and dyes from water.

Figure 4 .
Figure 4. EDX of the Zn-doped NdFeO 3 .The inset table contains the weight and atomic percentages of the elements.

3 Figure 7 .Figure 8 .
Figure 7.The dependence of the absorbance on the photon wavelength.

Figure 15 Figure 13 .
Figure 15 illustrates the Freundlich model for the investigated sample.Table 4 contains the values of R 2 .

Figure 14 .
Figure 14.The relation between C e /q and C e for Zn-doped NdFeO 3 .

Figure 15 .Figure 16 .
Figure 15.The relation between the lnq e and the lnC e for Nd 0.90 Zn 0.10 FeO 3 .

Figure 17 .
Figure 17.The pseudo-first order model for adsorption of Pb 2+ from water.

Figure 18 .
Figure18.The pseudo-second-order model for adsorption of Pb 2+ from water.

Figure 19 .
Figure 19.The inter-particle diffusion model of lead absorption from water.

Figure 20 .
Figure 20.The regeneration of Nd 0.90 Zn 0.10 FeO 3 for removing the lead ions from water several times.

Table 2 .
The saturation magnetization (M s ), the remanence magnetization (M r ), the exchange bias field, and the magnetic anisotropic constant (K).

Table 3 .
The E g values for direct and indirect transitions.

Table 4 .
The Langmuir constant, R 2 , and the adsorption intensity for the Langmuir and Freundlich models.From Table4, the R 2 value of the Freundlich model is greater than that of the Langmuir model, which means that the Freundlich model is the most fit model.