Highly efficient lead removal from water by Nd0.90Ho0.10FeO3 nanoparticles and studying their optical and magnetic properties

Ho-doped NdFeO3 was synthesized using the citrate method. The X-ray diffraction (XRD) illustrated that Nd0.90Ho0.10FeO3 was crystalline at the nanoscale, with a crystallite size of 39.136 nm. The field emission scanning electron microscope (FESEM) illustrated the porous nature of Nd0.90Ho0.10FeO3, which increases the active sites to absorb the heavy metals on the sample surface. Energy-dispersive X-ray (EDX) data assures the prepared sample has the chemical formula Nd0.90Ho0.10FeO3. The magnetic properties of Nd0.90Ho0.10FeO3 were determined using the magnetization hysteresis loop and Faraday’s method. Many magnetic parameters of the sample have been discussed, such as the coercive field, the exchange bias (Hex), and the switching field distribution (SFD). Ho-doped NdFeO3 has an antiferromagnetic (AFM) character with an effective magnetic moment of 3.903 B.M. The UV–visible light absorbance of Nd0.90Ho0.10FeO3 is due to the transfer of electrons from the oxygen 2p state to the iron 3d state. Nd0.90Ho0.10FeO3 nanoparticles have an optical direct transition with an energy gap Eg = 1.106 eV. Ho-doped NdFeO3 can adsorb many heavy metals (Co2+, Ni2+, Pb2+, Cr6+, and Cd2+) from water. The removal efficiency is high for Pb2+ ions, which equals 72.39%. The Langmuir isotherm mode is the best-fit model for adsorbing the Pb2+ ions from water.

NdFeO 3 belongs to the ABO 3 orthoferrite materials with the space group Pbnm.The Fe 3+ ions form the FeO 6 octahedron.The magnetic properties of NdFeO 3 originate from the Dzyaloshinskii-Moriya exchange interaction of the antiparallel spins between Fe 3+ -O 2--Fe 3+ , Nd 3+ -O 2--Fe 3+ , and Nd 3+ -O 2--Nd 3+1,12 .The optical properties of NdFeO 3 originate from the transition of electrons from 2p to 3d orbitals 13,14 .P. T. H. Duyen et al. 15 studied the optical properties of Cd-doped NdFeO 3 , concluding that increasing the Cd concentration decreased the optical band gap.S. A. Mir et al. 16 prepared the Ni-doped NdFeO 3 and studied the dielectric properties of the samples.
Numerous photocatalysts, including CuS, TiO 2 , CuO, BaTiO 3 , ZnO, and others, are used today in dye degradation 17 .Their large bandgap energy, quick recombination of photoinduced charge carriers, and low visible light absorption, however, severely limit their practical applicability 18 .Therefore, it is essential to create photocatalysts with exceptional photocatalytic activity in the visible region and a small bandgap.Perovskite-oxidebased catalysts such as NdFeO 3 have lately piqued the interest of researchers due to their good photocatalytic activity, tunable bandgap, high stability, and quick photoinduced electron/hole mobility [19][20][21] .
The metallic elements that are characterized by their high atomic weight, specific gravity, and toxicity are called heavy metals (HMs), such as lead (Pb 2+ ), chromium (Cr 6+ ), nickel (Ni 2+ ), cadmium (Cd 2+ ), and copper (Co 2+ ).Cadmium, a heavy metal, damages the bones and kidneys 22 .Cr 6+ causes hemorrhage, severe diarrhea, and cancer in the digestive tract 23 .Increasing the concentration of lead in drinking water causes kidney malfunction, brain tissue damage, and anemia 24 .There are many techniques used to remove heavy metals from water, such as

OPEN
Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egypt.email: mmarmsci@ gmail.com;mmarmsci@cu.edu.egNd 0.90 Ho 0.10 FeO 3 characterizations and measurements The crystal structure of Nd 0.90 Ho 0.10 FeO 3 was studied using XRD (Bruker Advance D8 diffractometer, λ = 1.5406Å) with 2θ in the range of 20°-80°.The XRD data was indexed with the International Centre for Diffraction Data (ICDD) card number 01-089-6644.The morphology of Nd 0.90 Ho 0.10 FeO 3 was studied using FESEM (model Quanta 250) with EDX and elemental mapping.The magnetic properties of Nd 0.90 Ho 0.10 FeO 3 were studied by two techniques.The first is measuring the magnetization of the sample using the vibrating sample magnetometer (VSM; 9600-1 LDJ, USA), which uses a magnetic field up to 20 kOe at a temperature of 300 K.The second technique is Faraday's method, in which a small amount of Nd 0.90 Ho 0.10 FeO 3 was placed in a glass that was placed at the field gradient to measure the DC magnetic susceptibility with temperature 28 .The optical properties of Nd 0.90 Ho 0.10 FeO 3 were studied using a UV-visible spectrophotometer (Jasco (V-630)).

The heavy metals removal from water
The examination of the ability of Nd 0.90 Ho 0.10 FeO 3 to remove heavy metals such as Co 2+ , Ni 2+ , Pb 2+ , Cr 6+ , and Cd 2+ from water.The removal efficiency is represented by the following steps: 1.The standard solutions (50 ppm) of heavy metals were prepared.2. 10 mL of the standard solutions were added to a beaker with 0.02 g of the sample.3. The pH value of the solution was adjusted using the ammonia solution or diluted nitric acid.4. The beakers were stirred on the electric shaker for 1 h at 170 rpm. 5. Take 8 mL of the solutions using a syringe filter.www.nature.com/scientificreports/6. Inductively coupled plasma spectrometry (ICP, Prodigy 7) was used to determine the concentration of the heavy metals.

Results and discussion
Figure 2 illustrates the XRD of the Nd 0.90 Ho 0.10 FeO 3 nanoparticles.The most intense peak was observed at 2θ = 32.593ºwhich characterized the (121) plane.The sample has a single phase orthorhombic structure.The average lattice parameters were estimated from all peaks using the following equation 29 : The average lattice parameters are reported in Table 1.The crystallite unit cell volume was calculated according to Eq. (2).
The average crystallite size (L) of the sample was estimated from all peaks using the Scherer formula, which is represented by the following equation 30 : where λ denotes the X-ray wavelength, β is the full width at half maximum, and θ is the Bragg angle.The value of L is 39.136 nm, which indicates the sample was prepared at the nanoscale.
The tolerance factor relates to the symmetry of the crystal structure and was calculated using Eq. ( 4).
where r A , r Fe , and r O are the ionic radii of the A, Fe, and oxygen ions, respectively.The value of r A was calculated from Eq. ( 5).
The value of t is one for the ideal cubic structure, while t decreases to one for the orthorhombic structure, where the crystallite size distortion increases.For the investigated sample, the value of t is 0.8875, which indicates (1) www.nature.com/scientificreports/ the orthorhombic structure of the sample.The theoretical density (D x ) was calculated according to Eq. ( 6) and reported in Table 1.
where Z is the number of molecules in a unit cell (Z = 4), M refers to the molecular weight of the sample, and N is Avogadro's number.The substitution of Ho 3+ ions at the expense of the Nd 3+ ion led to an increase in the relative density of NdFeO 3 .According to M.M. Arman 10 , the relative density of NdFeO 3 is 6.33 g/cm 3 and its unit cell volume is 236.9 (Å) 3 , while Nd 0.90 Ho 0.10 FeO 3 has a relative density of 7.05 g/cm 3 and its unit cell volume is 235.6 (Å) 3 .This is due to the ionic radius of Ho 3+ ions (1.073 Å) being less than that of Nd 3+ ions (1.163 Å) 31 .Figure 3 illustrates the FESEM image of the Nd 0.90 Ho 0.10 FeO 3 nanoparticles.The agglomerated particles are a result of the synthesis procedure.The sample has a porous nature, which increases the surface area of Ho-doped NdFeO 3 .The presence of a lot of active sites of the Nd 0.90 Ho 0.10 FeO 3 increases the adsorption of HMs on the surface of the Nd 0.90 Ho 0.10 FeO 3 sample 3 .
Figure 4 shows the EDX of Ho-doped NdFeO 3 , which assures the presence of the elements Fe, Ho, Nd, and O in the Nd 0.90 Ho 0.10 FeO 3 .The table inset in Fig. 4 shows the atomic percentage (at%) and weight percentage (wt%) of the elements, which were calculated theoretically from the sample formula and experimentally from the EDX data.The values of at% and wt% of the theoretical and experimental are close to each other, which indicates that the sample was prepared in the same chemical formula, Nd 0.90 Ho 0.10 FeO 3 .The peak observed at 2.11 eV is due to the gold coating of the sample before scanning.In Fig. 4, the carbon ions were observed at 0.27 eV due to the carbon tap where the sample was put inside the FESEM.The slight difference in wt% and at% between experimental and theoretical values is caused by the oxygen deficiency.
Figure 5 shows the mapping of the elements in Ho-doped NdFeO 3 .Figure 5.a illustrates the homogeneous distribution of the elements in the sample.Figure 5b-e shows the distribution of each element in the sample by distinguished color.The wt% of the elements appearing in the elemental mapping was different from that obtained from EDX analysis due to the maps having been measured for too short a time.
The magnetic properties of Nd 0.90 Ho 0.10 FeO 3 were studied via the M-H hysteresis loop and Faraday's method.The magnetic behavior of Nd 0.90 Ho 0.10 FeO 3 originates from the magnetic coupling between the magnetic ions such as Fe 3+ , Nd 3+ , and Ho 3+ ions.

Table 2.
The values of remanence magnetization (M r ), the coercive field (H c ), the exchange bias (H ex ), the anisotropy constant (K), the switching field distribution (SFD), the rectangularity of the H-M loop (H a ), and the squareness ratio for the sample.Figure 6 illustrates the magnetization hysteresis loop of Nd 0.90 Ho 0.10 FeO 3 , which has AFM behavior with weak ferromagnetic (FM) components 32 .The values of the saturation magnetization (M s ) and coercive field (H c ) are reported in Table 2.The value of the squareness ratio (SQR) of the sample was calculated from Eq. ( 7).

Sample
The value of SQR was reported in Table 2 and indicates that the type of magnetic interaction is magneto-static interactions 33 .The exchange bias (H ex ) of the sample was calculated from the Eq.(8).
where H left and H right are the intercepts of the MH curve with the negative and positive x-axis, respectively.The presence of a shift in the MH loop around the origin originated from the presence of AFM ordering with (FM) spins in the sample.
The anisotropy constant (K) of Nd 0.90 Ho 0.10 FeO 3 was calculated from Eq. ( 9) 34 : where H c is the coercive field of the sample.The value of K was reported in Table 2.
Figure 7 shows the dependence of dM/dH on the magnetic field for Nd 0.90 Ho 0.10 FeO 3 nanoparticles.The switching field distribution (SFD) and the rectangularity of the H-M loop (H a ) of Nd 0.90 Ho 0.10 FeO 3 were calculated using the following equations 9 .
where ∆H represents the half width at the half maximum of the dM/dH peak.Table 2 contains the values of SFD and H a .
Many researchers have studied the preparation and properties of NdFeO 3 .M.M. Arman 10 prepared the NdFeO 3 nanoparticles using the citrate combustion method.The values of M s and M r of NdFeO 3 are 1.05 emu/g and 0.11 emu/g, respectively.T. Shalini et al. 35 studied the structure and magnetic behavior of NdFeO 3 , which has M s and M r equal to 0.521 emu/g and 0.098 emu/g, respectively.In the presence of work, the substitution of Ho 3+ ions instead of Nd 3+ ions increased the magnetic properties of the NdFeO 3 nanoparticles.Where the effective magnetic moment of Nd 3+ is 1.14 μ B while that of Ho 3+ is 10.6 μ B

36
. The presence of Ho 3+ in the NdFeO 3 increases the magnetic interactions between the magnetic ions such as Ho 3+ -O 2--Ho 3+ , Ho 3+ -O 2--Nd 3+ , and Ho 3+ -O 2-Fe 3+ ‫.‬ Figure 8 shows the dependence of the molar magnetic susceptibility (χ M ) on T(K).The behavior of χ M with temperature assures that the sample has AFM behavior.χ M decreases with raising the temperature up to the Neel temperature (T N ), after which Nd 0.90 Ho 0.10 FeO 3 has a paramagnetic behavior.The AFM properties originate from the magnetic interaction between the Fe 3+ ions.The relation between χ M and the applied field is inversely proportional according to the following equation: The relation between χ M -1 and the temperature is shown in Fig. 9, which applies the Curie-Weiss law.From the slope of the paramagnetic region in Fig. 9, the values of the effective magnetic moment (μ eff ), the Curie-Weiss constant (θ), and the Curie constant (C) were determined.The value of the μ eff was determined from the following equation: where C denotes the reciprocal of the slope in the paramagnetic part.The values of μ eff , θ, and C were reported in Table 3.The value of T N was determined from the differentiation of χ M (dχ M /dT) and listed in Table 3.
Figure 10 shows the dependence of the absorbance of the UV-visible light by the sample on the wavelength of the incident photons.In the low wavelength region (λ ≤ 460 nm), the absorbance increases rapidly with λ due to increasing the energy of the photons, which allows the electrons to be transferred from the 2p orbital oxygen valance band (V.B.) to the 3d orbital iron conduction band (C.B.).In the high wavelength region (λ > 460 nm), the energy of the photons is low and can't transfer the electrons from V.B. to C.B.
The optical absorption coefficient (α) is related to the quantity of UV-visible light absorption through the material.The value of α was determined using Eq. ( 14) 37 .where A is the absorbance and l denotes the length of the spacemen.Figure 11 shows the relationship between α and the wavelength.The values of α increased rapidly with increasing λ up to 460 nm, then α decreased slowly with increasing λ.The increasing of α is due to increasing the absorption of photons at low wavelengths and higher frequencies, while the decreasing of α is due to decreasing the absorption of photons at high wavelengths and lower frequencies.The optical extinction coefficient (k) denotes the losses of electromagnetic energy in Nd 0.90 Ho 0.10 FeO 3 nanoparticles.k was calculated using the Eq. ( 15). Figure 12 studies the dependence of k on wavelength of the photons.The increasing of k with increasing λ of photons is due to the fact that at high λ, the energy of the photons is small and doesn't absorb in the sample, increasing the energy losses, so k increases.
The Tauc plot was used to determine the optical band gap value (E g ) and the type of optical transition of Nd 0.90 Ho 0.10 FeO 3 .The Tauc equation was represented by Eq. ( 16) 37 .
where A and hν are the constant and photon energy, respectively.The (x) value was estimated to be the type of optical transition.For the direct transition, x equals 2, while for the indirect transition, x equals 1.The Tauc plot is represented in Fig. 13, which illustrates that the Nd 0.90 Ho 0.10 FeO 3 sample has a direct transition with E g = 1.106 www.nature.com/scientificreports/eV.J. S. Prabagar et al. 21prepared NdFeO 3 using the citrate sol-gel method with an optical bandgap of 2.48 eV and good photocatalytic activities.While introducing the Ho 3+ ions in the NdFeO 3 leads to a decrease in E g to 1.106 eV due to introducing orbitals and states of Ho 3+ ions in the NdFeO 3 system.The author recommended using Nd 0.90 Ho 0.10 FeO 3 as a photocatalyst for organic dye degradation in water.Figure 14 illustrates the dependence of the removal efficiency of the HMs at Ph = 7, which was calculated using Eq.(17).
where C f is the final concentration while and C i denotes the initial concentration of the HMs.The sample Nd 0.90 Ho 0.10 FeO 3 has the ability to remove a lot of HMs from water, which indicates that it has a large surface area.The adsorption of the HMs from an aqueous solution depends on many parameters, such as temperature, pH value of the solution, contact time, ionic radii of the HMs, initial concentration of the adsorbent, and molecular weight of the HMs.The highest efficiency of HMs from the water was observed for Pb 2+ ions with η = 72.39%due to the fact that Pb 2+ ions have a higher molecular weight than the other HMs, leading to easier adsorption.
The effect of the pH value of the solution on the removal efficiency (η) of Pb 2+ ions was studied and illustrated in Fig. 15.In the acidic region (pH ≤ 6), there are a lot of H + ions in the solution, which participate with the Pb 2+ ions in the adsorption on the active sites on the surface of Nd 0.90 Ho 0.10 FeO 3 , so η is low.The maximum adsorption of Pb 2+ ions was observed at pH = 7, which indicates the optimum pH condition for removal of Pb 2+ from aqueous solutions using Ho-doped NdFeO 3 .The FESEM images of the sample illustrate the porous nature of the surface of the sample, which increases the active sites that adsorb the HMs from the water.In the basic medium (pH = 8), the Pb 2+ ions can be precipitated as lead hydroxide, which is not favorable.The adsorption mechanism of the Pb 2+ ions from the water was studied using the adsorption isotherm models.In the present work, the Langmuir and Freundlich isotherm models were used to study the adsorption of Pb 2+ on the surface of the sample.
The Langmuir isotherm represents the adsorption of the Pb 2+ ions on the surface active sites as a single layer.The assumptions of the Langmuir isotherm are that the HMS is adsorbed on discrete surface active sites, each HM molecule adsorbs on one active site, the sample has a uniform adsorbing surface, and HM molecules don't interact with each other 38 .The following equation describes the Langmuir isotherm mode.
where C e is the equilibrium Pb 2+ ion concentration, q m , and K L denote the Langmuir constants.The equilibrium adsorption capacity (q e ) was calculated using Eq.(19).
where V and m are the volume of the Pb 2+ solution and the adsorbent mass, respectively.Figure 16 shows the fitting of the experimental data with the Langmuir isotherm model.
The Freundlich isotherm describes the mass transportation of the Pb 2+ ions from the aqueous solution to the active sites on the porous surface of Nd 0.90 Ho 0.10 FeO 3 .Equation (20) represents the Freundlich isotherm model.( 18)  where K f denotes the Freundlich constant.Figure 17 shows the fitting of the experimental data with the Freundlich isotherm.
From the inset tables in Figs.16 and 17, the values of R 2 are 0.9545 and 0.9399 for the Langmuir and Freundlich isotherm modes, respectively.The Langmuir isotherm mode is the best-fit model for adsorbing the Pb 2+ ions from water, and the HMs form monolayer adsorption.

Conclusion
Nd 0.90 Ho 0.10 FeO 3 was prepared in an orthorhombic structure with a crystallite size of 39.136 nm.FESEM illustrates the agglomerated grains due to the magnetic behavior of the sample.The EDX data shows that the elements Fe, Ho, Nd, and O are present in Ho-doped sample without any impurities.The antiferromagnetic properties of Nd 0.90 Ho 0.10 FeO 3 originate from the magnetic coupling of Fe 3+ -O 2--Fe 3+ , Nd 3+ -O 2--Fe 3+ , and Nd 3+ -O 2--Nd 3+ .The values of M s , M r , H c , H ex , K, and SDF are 2.42 emu/g, 0.25 emu/g, 200 Oe, -10.86 Oe, 514.14 erg/g, and 2.33, respectively.The sample has a direct optical transition with E g = 1.106 eV.Nd 0.90 Ho 0.10 FeO 3 is a good absorber of UV-visible light and can be used for photocatalysis of organic dye degradation in water.The Nd 0.90 Ho 0.10 FeO 3 nanoparticles have a high efficiency (72.39%) to remove the heavy metal Pb 2+ from water.The experimental data is more fitting for the Langmuir isotherm mode.

Data availability
The author declares that all the data supporting the findings of this study are available in the ICDD card number 01-089-6644.

Figure 7 .
Figure 7.The relation between dM/dH and the magnetic field for Nd 0.90 Ho 0.10 FeO 3 nanoparticles.

Figure 8 .Figure 9 .
Figure 8.The dependence of χ M on the temperature for Nd 0.90 Ho 0.10 FeO 3 .

3 Figure 10 . 3 Figure 11 .
Figure 10.The relation between the absorbance and the wavelength of the photons for Nd 0.90 Ho 0.10 FeO 3 .

Figure 12 .Figure 13 .
Figure 12.The relation between k and the wavelength for Nd 0.90 Ho 0.10 FeO 3 nanoparticles.

20 )Figure 14 .
Figure 14.The removal effectiveness of Nd 0.90 Ho 0.10 FeO 3 for the different heavy metals.

Figure 15 .
Figure 15.The dependence of the removal efficiency of Pb 2+ ions on the pH value of the solution.