Synthesis and Functional Properties of La₂FeCrO₆ Based Nanostructures

Abstract

Ordered double perovskite La₂FeCrO₆ nanoparticles (NPs) were synthesized via the citrate auto-combustion technique. The prepared sample was characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), and Raman spectroscopy, which confirmed the double perovskite structure of the studied sample. XRD illustrated that the investigated sample has an orthorhombic structure with an average crystallite size of 25.3 nm. La₂FeCrO₆ NPs exhibit a porous structure and spongy morphology, as determined through analyses using Brunauer–Emmett–Teller (BET) specific surface area and field emission scanning electron microscopy (FESEM). The studied sample exhibits anti-ferromagnetic (AFM) behavior with weak ferromagnetic (FM) components, as an example of d⁵(Fe³⁺)–d³(Cr³⁺) systems. The AFM behavior is caused by the super-exchange interaction between [Fe³⁺(d⁵)–O–Cr³⁺(d³)], according to the Kanamori–Goodenough (KG) rule. This behavior is induced by the pdπ hybridization between the e_g orbital of the transition metal and the pσ orbital of the oxygen, while the one induced by the pdσ hybridization is FM. The number of excited-state configurations mediated by the pdπ hybridization in the Fe–Cr pair is greater than that mediated by pdσ hybridization. Pb(II) heavy metal ions are used in adsorption studies. The electrostatic nature of the bonding between Pb(II) and the La₂FeCrO₆ nano ferrite sample is thought to be the main cause of the observed high sorption of La₂FeCrO₆ to a Pb(II) ion. La₂FeCrO₆ has a favorable morphology, which bodes well for its prospective applications in Li-ion batteries, water purification, and gas sensors.

1 Introduction

The double-perovskite (DPV) compound A₂B′B′′O₆ offers a versatile framework for engineering materials with untapped magnetic and electronic properties [1].

DPV obtained considerable attention and importance due to its potential applications: the magnetic memories [2, 3], the tunnel junctions [4], the spintronic field [5, 6], the microwave, and the high-power applications [7,8,9].

Furthermore, this class of materials displays numerous fascinating physical and chemical characteristics, including electronic structures that range from insulating to metallic, and half-metallic spin-polarized electrical conductivity [10, 11].

DPVs include perovskites with two different kinds of cations occupying the A or B sites, resulting in the formulae A₂B′B′′O₆ (A is a rare earth and/or an alkaline-earth metal, and B′ and B″ are d-block transition metals) that exhibit naturally well-ordered structures: B′O₆ and B′′O₆ octahedrons with FCC-NaCl sublattice arrangement [12].

DPVs are typically denoted as double B-site perovskites because A-site cations typically serve as electron donors to the [BO₆] framework. The ordering of various types of BO₆ octahedrons is influenced by the presence of a sizable difference in the ionic radii and formal valence between B′ and B′′ ions.

This ordering can be complete or partial, depending on the charge and the difference in the ionic radius of the two cations. The degree of ordering tends to increase with increasing size and charge differences.

On the other hand, it is challenging to construct ordered structures when the ionic radii of two types of transition metal elements are nearly identical. All these factors involving the crystal structure, lattice defect, exposed lattice plane, surface morphology, particle size, specific surface area, and pore structure highly affect the physicochemical characteristics of the DPV materials.

In the A₂B′B′′O₆ structures, the d-block elements are considered promising candidates for filling up the B-site cation position because of their multi-valence state and the unique 3d and 4d electronic configurations. This is the real cause of the perovskite-like structures found in transition metal oxides, and they frequently have exceptional physical properties [13]. Because of the large difference in atomic radius between B′ and B′′, it is simple to synthesize a well-ordered DPV containing the combination of B′ of a 3D element and B′′ of a 4D or 5D element [14,15,16,17]. Some DPV compounds in this category have a Curie Temperature T_c above the room temperature, such as Sr₂FeMoO₆ and Sr₂FeReO₆ [18]. According to K. Miura et al. [17], for La₂FeCrO₆, the majority-spin t_2g states of the Cr ions (HOMO) are below the Fermi level, while the minority-spin t_2g states of the Fe ions (LUMO) are above the Fermi level, which controls the constancy of the magnetic ordering.

In the present work, the La₂FeCrO₆ is prepared via the citrate auto-combustion technique, which is characterized by simplicity, low cost, and no waste. The structural, morphological, and magnetic properties of the sample are studied. The application of heavy metal (Pb²⁺) removal from waste water is given in the current work. The Langmuir and Freundlich isotherms for ad-sorption will be discussed.

2 Experimental Work

2.1 Materials

La(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O, Cr(NO₃)₃·9H₂O, and citric acid with high purity (99.999%) were employed as preliminary ingredients. All the utilized raw materials were provided by ACROS Organics Company in the UK.

2.2 Synthesis of La₂FeCrO₆ NPs

The preparation method is demonstrated through the schematic diagram shown in Fig. 1, as mentioned in the previous work [15].

Fig. 1

Schematic representation of the citrate auto-combustion method

2.3 Characterization Techniques

X-ray powder diffraction pattern was used to determine the XRD of the produced sample. Cu–Kα radiation having a wavelength of 0.1542 nm was produced using a Cu X-ray tube (line source of 12 × 0.04 mm²) (Netherlands-based X'Pert PRO Analytical). With the aid of High Score Plus software, it was run at 45 kV. By employing a SEM Model Quanta 250 FEG coupled with an EDAX unit, FESEM was used to examine the morphology and nanostructure of the materials. A VSM, Model Lake Shore 7410, was used to assess the sample's magnetic characteristics. The magnetization in emu/g was measured as a function of magnetic field intensity at a temperature of 300 K. The Raman measurements were performed with a Senterra-Bruker (Germany) instrument and a laser source (ND:YAG) at 532 nm. The BET technique [19] was used to calculate the specific surface area based on nitrogen ad-sorption and de-sorption isotherms at 77 K, which were acquired using a NOVA 2200, USA, together with an automated gas sorption system. The three-dimensional atomic force microscopy (AFM) pictures produced by a 5600 LS AFM provided complementary details regarding the surface microstructure of the sample under investigation.

2.4 Heavy Metal Removal

On the way to investigating the effectiveness of La₂FeCrO₆ in removing heavy metal ions, a solution of lead (Pb²⁺) ions of the necessary concentration (50 ppm) was prepared and placed in a beaker containing an amount of 0.10 g of La₂FeCrO₆. The optimal value of pH is adjusted well at pH 7. The concentration of Pb²⁺ present in the solution was determined in the filtrate by using inductively coupled plasma (ICP) spectrometry (Prodigy7).

The ad-sorption capacity at equilibrium (q_e) and the efficiency of metal ion removal were determined using the following equation [20]:

$$\eta = \frac{{C_{i} - C_{e} }}{{C_{i} }} \times 100$$

(1)

$$q_{e} = \frac{{(C_{i} - C_{e} )V}}{m}$$

(2)

The concentrations of a metal ion solution, expressed in mg/L, are represented by C_i and C_e, where C_i is the initial concentration and C_e is the final concentration. The mass of the adsorbent is represented by m, while the volume of the Pb(II) solution is represented by V.

3 Results and Discussion

3.1 XRD Analysis

The XRD of La₂CrFeO₆ presented in Fig. 2 demonstrates eight sharp diffraction peaks. The peak positions at 2θ of 22.76°, 32.39°, 39.98°, 46.44°, 52.3°, 57.74°, 67.78°, and 77.13° can be indexed to (110), (112), (022), (220), (222), (204), (224), and (332) diffraction planes of the orthorhombic La₂CrFeO₆ space group Fmm2. It coincides with card No. 00-056-0263. The pattern exhibits broad peaks with no evidence for other impurities. The Scherrer equation is applied, as stated in earlier studies [21, 22], to calculate the crystallite size of La₂CrFeO₆.

Fig. 2

a X-ray diffraction pattern; b the W–H plot of La₂CrFeO₆ sample; and c the crystal structure of double perovskite [23]

The crystallite size, as determined through calculation, is 23.3 nm, which confirms the nanoscale size of the investigated sample. The lattice parameters are calculated on the basis of the orthorhombic structure and are listed in Table 1.

Table 1 Lattice parameters (a, b and c), crystallite size from Scherrer equation (L), crystallite size (D) from W–H, micro strain (ε), tolerance factor (T), theoretical density (d_x), experimental density (D_exp.), dislocation density (δ), and degree of crystallinity (X_c)

It is very important to perform some additional calculations to get a closer look at the crystal structure of the La₂CrFeO₆ composite. The XRD diffraction pattern can reveal some information regarding, the tolerance factor (T) [24], the theoretical density (d_x) [25, 26], dislocation density (δ) [27, 28], the micro strain (ε) [29], and the degree of crystallinity (X_C). The experimental density (D_ex) of the sample is calculated according to the following equation:

$$D_{ex} = \frac{m}{V}$$

(3)

where m and V are the mass and volume of the pellet sample, respectively. The value of D_ex is reported in the Table. The difference between the theoretical and experimental densities is due to the porous nature of the sample. The porosity (P) of the investigated sample is calculated from the following equation [30] and equals 58.13%.

$$P\% = \left( {1 - \frac{{D_{exp.} }}{{d_{x} }}} \right) \times 100$$

(4)

It’s well known that the Scherrer equation omits the intrinsic strain that emerged in the nanocrystals as a result of point defects, grain boundaries, or any defect that generates an alternation of the sequence periodicity of the layers.

The Williamsons Hall (W–H) method is a straightforward technique for determining the size of crystal structures and estimating any deformation in the lattice structure using Eq. 5, as mentioned in the previous work [31].

$$\beta \cos \theta = \frac{K\lambda }{D} + 4\varepsilon \sin \theta$$

(5)

where the broadening of the XRD peak is caused by the size (β_size). This relation represents a simple straight line with good fitting, as shown in Fig. 2b. The crystallite size (D) can be obtained from the y-axis intercept, while ε is calculated from the slope of the line [32]. The calculated D and ε are tabulated in Table 1.

The degree of crystallinity (X_c) is estimated from the area underneath the crystalline peaks divided by the total area under the crystalline and non-crystalline peaks, as expressed by the following equation:

$$X_{C} = \frac{{X_{Cry} }}{{X_{all} }} \times 100$$

(6)

The degree of crystallinity is found to be 98.5%.

The calculation of the tolerance factor (T) is performed using the well-known Goldschmidt relation [33]. The value of the ‘T’ for the investigated sample is 0.959, which indicates an orthorhombic structure. However, the B′ and B″ elements in the compound are evenly balanced with transition metals, with an oxygen ion placed symmetrically between each pair as shown in Fig. 2c. If B′ and B″ differ from each other, the O²⁻ ions move somewhat in either charges or ionic radii, as in the example that is currently being discussed, while the B′O₆ and B″O₆ retain their octahedral symmetry. Along with the newly discovered structure, two non-equivalent O₂ atoms (O₁, O₂) are also present. Four O₂ atoms are placed on the XY plane, while the two O₁ atoms are situated on the Z axis. Contrarily, in a structure with differently bonded oxygen atoms, the angle between B′–O₁–B″ remains fixed at 180°, while the angle between B′–O₂–B″ may vary. The c/a ratio during structural optimization is extremely close to the value of ≈ √2. This situation gives rise to the anti-ferromagnetic characteristic, as will be discussed in detail later.

3.2 FE-SEM

The morphologies of the investigated sample, La₂FeCrO₆, are studied by FESEM, as illustrated in Fig. 3. The La₂FeCrO₆ NPS have a sponge-like morphology. As shown in the figure, the studied sample as they are formed, have pores and voids that depend on a lot of gases during the burning process. La₂FeCrO₆ has a favorable morphology, which bodes well for its prospective application in Li-ion batteries, water purification, and gas sensors.

Fig. 3

a and b FESEM images for La₂FeCrO₆

3.3 EDAX Mapping

The EDAX cation mapping of La₂FeCrO₆ is shown in Fig. 4a–e. The homogeneous distribution of the cations (La, Fe, Cr, and O) is observed in the mapping images with different colors. The mapping shows the uniformity of the distribution of the constituent elements throughout the sample matrix.

Fig. 4

a–e The elemental mapping images for La₂FeCrO₆

The pattern displayed in Fig. 5 is obtained through an EDAX analysis of the studied sample [34]. The figure displays the presence of the starting chemical composition without any impurities. The theoretical atomic-percentage (at%) and theoretical weight-percentage (wt%) of the elements (La, Cr, Fe and O) are calculated from the chemical composition La₂FeCrO₆. Table 2 displays both the atomic percentage (at%) and weight percentage (wt%) obtained through EDAX elemental analysis, as well as those calculated theoretically.

Fig. 5

Illustrates EDAX pattern for La₂FeCrO₆

Table 2 The expected and experimental values of the atomic percentage (at.%) and weight percentage (wt%) for the sample

3.4 Raman Spectroscopy (RS)

RS is extensively used for several lattice vibrational modes that are strongly related to the local crystal structure. Figure 6 displays the vibrational modes detected in the La₂FeCrO₆ NPs that originate from Fe/CrO₆ octahedra and La–O bonds, in addition to the presence of coupling between them. The Raman peaks are observed at 105, 154, 181, 275, 325, 376, 436, 498, 523, 693, and 1370 cm⁻¹. The peaks can be divided into four regimes based on their characteristics.

Fig. 6

Raman spectrum of La₂FeCrO₆

The lowest wave number modes below 200 cm⁻¹ can be assigned to the translational motion of the rare-earth element (La), which is coupled to the tilting vibrations of the Fe/CrO₆ octahedron and labelled as (A) [35,36,37].

In the frequency regime (200–350 cm⁻¹) [38], the peaks correspond to the A site and different tilt modes of the octahedral oxygen (T). The Raman peaks that arise in the regime (350–500 cm⁻¹) are due to Fe/CrO₆ octahedra bending modes and labelled as (B). The presence of stretching vibrations of oxygen can be identified in the Raman region above 500 cm⁻¹ (S) [36]. The strongest intense peak at 693 cm⁻¹ can be ascribed to the oxygen stretching modes in Fe/CrO₆ octahedra. It is improved by the photon induced transfer of an electron from a Fe³⁺ ion to a nearby Cr³⁺ ion, owing to the mixed tendency of the metal cations [37, 38].

In addition to the above modes, there is a high-frequency broad peak at 1370 cm⁻¹, which is positioned at an energy value slightly lower than twice that of the first order mode. This peak is considered an overtone mode of the fundamental stretching mode [39]. The Raman modes of the investigated sample, La₂FeCrO₆, are listed in Table 3.

Table 3 The Raman modes of LFCO sample

3.5 Atomic Force Microscopy (AFM)

The topographic features of the nanocrystalline DPV are characterized using the AFM. Characteristic AFM micrographs of the sample are displayed in Fig. 7a–c. The parameters of the surface profile can be divided into four categories: amplitude, spacing, hybrid, and functional.

Fig. 7

a–c Different AFM images of LaFCO: a 2D- AFM image. b 3D-AFM image. c AFM imaging topography

Firstly, the amplitude or height parameters, which are the principal parameters in characterizing the surface’s topography and include: R_p, the maximum profile peak height relative to the mean line, R_v, the lowest point of the deepest valley compared to the average line, R_T, the total roughness, R_a, the average roughness, R_q, the root mean square (RMS) of roughness.

Secondly, the functional or statistical parameters that provide information about the structure of the investigated surface are the skewness (R_Sk) and kurtosis (R_Ku) moments that determine the asymmetry and the sharpness, respectively. For bumpy surfaces, R_Ku < 3, while for spiky surfaces, R_Ku > 3. According to the value of R_Ku, the La₂FeCrO₆ sample has a bumpy surface (R_Ku = 2.69) that matches well with the porous structure seen in FESEM.

Thirdly, hybrid parameters that show quantitatively the spacing features of the surface as Δ_q, as well as the RMS of the mean profile slope that is taken between each two successive points of the profile. Finally, the spacing parameters like S_m, the average spacing of adjacent local peaks in the profile. Table 4 shows the measured values of the mentioned above parameters as provided by AFM.

Table 4 Maximum profile peak height (R_p), maximum profile valley depth (R_v), maximum height of the profile (R_T), average roughness(R_a), root mean square roughness (R_q), skewness of the line (R_sk), and kurtosis of the line (R_ku), root mean square of the mean profile slope Δ_q, the average spacing (S_m)

3.6 Brunauer–Emmett–Teller (BET) Technique

The N₂ ad-sorption-de-sorption isotherms of La₂FeCrO₆ are shown in Fig. 8. It is obvious that ad-sorption/de-sorption curves show a hysteresis feature characteristic of mesoporous structures [40, 41]. According to the International Union of Pure and Applied Chemistry (IUPAC) classifications [42], the (Ads/Des)-isotherm is classified as IV category.

Fig. 8

a, b Type of ad-sorption/de-sorption isotherms and pore size distribution of La₂FeCrO₆

The behavior of ad-sorption of N₂ gas molecules depends on the relative pressure, such that, in regions of lower pressure, the formation of a monolayer is followed by the formation of multilayers of the ad-sorbed molecules in the regions of higher pressure.

Materials with slit-shaped pores and no limiting ad-sorption at high P/P_o are classified as type H3. In the present case, the IV category is classified according to the form of the hysteresis loop and the texture (e.g., pore size distribution, pore geometry, and connectivity) as type H3.

Figure 8.b shows the distribution of pore radius against pore volume for the investigated sample. The maximum peak indicates the average value of the pore radius, which has a good impact on Pb²⁺ removal efficiency, as will be discussed later.

BET isotherms and BET multipoint plots are performed for the determination of the average values of specific surface area (S_BET), pore size, and total pore volume of La₂FeCrO₆ NPs, as reported in Table 5.

Table 5 The specific surface area (S_BET) (m²/g), total pore volume (TPV) (cc/g), and average pore size (TPS) (nm), for the investigated sample

3.7 Magnetic Properties

Figure 9a illustrates the magnetic hysteresis loop (MHL) for the La₂FeCrO₆ NPs. Because Fe³⁺ (r = 0.645 Å) and Cr³⁺ (r = 0.615 Å) have the same valence state and closer ionic radii, it is improbable for the investigated sample to exhibit long-range ordering of B-site cations. Therefore, it is anticipated that the B-site will contain a random mixture of Fe³⁺ and Cr³⁺ ions. Nevertheless, numerous attempts to create La₂FeCrO₆ with an ordered DPV structure have been made.

Fig. 9

a and b The hysteresis loop, and b The law of approach to saturation (LAS) plot for of La₂FeCrO₆ sample

The law of approach to saturation (LAS) is used to calculate the saturation magnetization (M_s) [43] according to the following equation:

$$M = M_{s} \left( {1 - \frac{A}{H} - \frac{B}{{H^{2} }}} \right) + \chi H$$

(7)

where M_s denotes the saturation magnetization of the domains per unit volume, “A” refers to a micro-stress-related constant, “B” denotes a constant that represents the contribution of magneto-crystalline anisotropy, and χH is the forced magnetization term. Figure 9b shows the law of approach to saturation (LAS) plot for the investigated sample. The M_s of the La₂FeCrO₆ NPs is determined by intercepting the straight line with the magnetization axis at 1/H² to approach zero.

The magnetic parameters such as coercivity (H_C), saturation magnetization (M_s), remnant magnetization (M_r), squareness (M_r/M_s), exchange bias (H_EB), and magnetic moment (n_B) are reported in Table 6.

Table 6 Saturation magnetization (M_s), remanent magnetization (M_r), coercivity (H_C), energy loss, squareness (M_r/M_s), anisotropy constant (K), and experimental magnetic moment n_B (exp)

The experimental magnetic moment per unit cell (n_B) is calculated from the M_s value using the following equation [44, 45]:

$$n_{B} = \frac{{M_{W} \times M_{S} }}{5585}$$

(8)

where M_w is the molecular weight of a particular ferrite composition.

The squareness ratio M_r/M_S can be used to determine the type of magnetic inter-grain exchange. It is less than 0.5, for the studied sample as reported in Table 5. Consequently, NPs interact magneto-statically [46, 47].

Based on the diamagnetic behavior of La³⁺ ions, the magnetic properties of perovskites are mainly originated from the magnetic interplay of spin states exhibited by the transition metal ions positioned in the B-site. In addition, the magnetic characteristics of LFCO sample depends not only on the magnetic coupling between Fe³⁺ and Cr³⁺ ions at the B-site, which can result in long-range ferromagnetic or antiferromagnetic order, but also on the way Fe³⁺ and Cr³⁺ ions are distributed at the B-site. This distribution can be fully ordered, partially ordered, or completely disordered, and is influenced by the differing d-orbitals and occupied states of the ions, including their t_2g or e_g orbitals and whether their orbital states are empty, half-filled, or fully filled. These factors also impact the ions' magnetic moments.

The La₂FeCrO₆ sample displays a combination of anti-ferromagnetic (AFM), and ferromagnetic (FM) behavior, as depicted in Fig. 9a. According to the Kanamori–Goodenough (KG) rule [48, 49], there are three indirect-exchange interactions performed via the non-magnetic anion O²⁻ or, in other words, through the overlap between the 2p orbital of oxygen and the d orbital of the transition metal ions Fe³⁺ (Cr³⁺).

These interactions can be detected as (Fe³⁺–O²⁻–Fe³⁺) or (Cr³⁺–O²⁻–Cr³⁺), in addition to (Fe³⁺–O²⁻–Cr³⁺). The irregular distribution of Cr and Fe ions at the B-sites leads to the occurrence of AFM behavior [50]. This feature is found to be predominant, as reported in the previous work [1,2,3,4,5,6]. The origin of the AFM behavior is the super-exchange interaction between Fe³⁺ and Cr³⁺ ions intermediated by O²⁻ [Fe³⁺(d⁵)–O–Cr³⁺(d³)]. The obtained behavior is caused by the pdπ hybridization between the e_g orbital of the transition metal and pσ orbital of the O₂, whereas FM is caused by pdσ hybridization.

Certainly, the pdσ hybridization is stronger than that of the pdπ-type. But the number of excited-state configurations mediated by the pdπ hybridization in the Fe–Cr pair is greater than that mediated by pdσ hybridization. Furthermore, among the pdπ hybridization types, the LUMO $\left( {\varepsilon_{{\pi^{ - } }}^{{{\text{Fe}}}} } \right)$ and HOMO $\left( {\varepsilon_{{\pi^{ + } }}^{{{\text{Cr}}}} } \right)$ have the lowest energy contribution [7].

By taking these contributions into account, the magnetic character of the double perovskite La₂FeCrO₆ is overwhelmed by the anti-ferromagnetic behavior accompanied by the existence of weak ferromagnetism. This trend was detected in other DPV oxides such as La₂MMnO₆ (M = Co, Ni) [51], and Ho₂MMnO₆ (M = Ni, Fe, Co) [52]. Figure 10 illustrates the microscopic mechanism of FM and AFM coupling between Fe and Cr.

Fig. 10

a–c The microscopic mechanism of FM and AFM coupling [7], and magnetic micro domains in La₂FeCrO₆ materials, where Fe³⁺ and Cr³⁺ are located in the B and B′ sites [53]

The competition between the FM and AFM states asserts itself through the raised H_EB phenomenon observed in the MHL. This symmetry breaking phenomenon can be considered as a unidirectional anisotropy type.

The horizontal shift of the center of magnetic hysteresis from the zero field position (H = 0) is illustrated in the inset of Fig. 9a. This shift is due to the H_EB, which can be calculated according to the following relationship [46]:

$${\text{H}}_{{{\text{EB}}}} = - {{\left( {{\text{H}}_{{1}} + {\text{ H}}_{{2}} } \right)} \mathord{\left/ {\vphantom {{\left( {{\text{H}}_{{1}} + {\text{ H}}_{{2}} } \right)} 2}} \right. \kern-0pt} 2}$$

(9)

The left and right coercivities are represented by H₁ and H₂ respectively. The H_EB is present in the sample as a result of the exchange coupling between the FM and the AFM components between Fe³⁺ and Cr³⁺ ions on B′ and B′′ sites [54].

The anisotropy constant (K) is calculated according to the Stoner-Wohlfarth relation as given by the following Eq. (10):

$$K = \frac{{H_{C} \times M_{S} }}{0.96}$$

(10)

where H_c is the coercivity. The value of anisotropy constant (K) is tabulated in Table 6.

Finally, the low magnetization is attributed to various factors, as previously mentioned, one of which is the disorder in the perovskite structure, which deviates from the ideal rock salt perovskite arrangement. The discrepant ionic radii of trivalent Cr and Fe pose a challenge to achieving an ordered phase, resulting in anti-site disorder where the regular Fe–Cr alternation is disrupted. Ateia et al. [36] prepared La₂Fe₂O₆, which has M_s and H_c equal to 1.060 emu/g and 304.63 Oe, respectively, while the values of M_s and H_c for the investigated sample La₂FeCrO₆ are 0.545 emu/g and 134.25 Oe, respectively. This decrease is due to the replacement of Cr³⁺ ions with a small magnetic moment (3.87 BM) instead of Fe³⁺ ions with a large magnetic moment (5.916 BM).

3.8 Heavy Metal Removal (Effect of Contact Time)

Ad-sorption is a chemical reaction that resembles a balanced reaction. When a solution comes into contact with a specific amount of ad-sorbent material, the concentration of the material adsorbed in the solution decreases until it reaches a state of equilibrium with the material on the ad-sorbent surface. When adsorption equilibrium is reached, the concentration of ad-sorbed material in the solution remains constant. Figure 11 shows the dependence of lead(II) ion removal on the contact time for La₂FeCrO₆ NPs at a pH of 7. The effectiveness of removing Pb²⁺ improves as the duration of contact with La₂FeCrO₆ increases, as a result of Pb²⁺ being adsorbed onto active sites on the surface of the La₂FeCrO₆. The maximum η of Pb²⁺ is 77% at 5 h of contact time.

Fig. 11

Effect of contact time on the ad-sorption efficiency of Pb²⁺ ions

The porous nature and nano size of La₂FeCrO₆ as illustrated in FESEM images increase the surface area, as well as the ad-sorption of Pb²⁺ on the ad-sorbent surface. According to the findings of this study, the DPV La₂FeCrO₆ is an effective adsorbent for Pb²⁺ removal. The Langmuir and Freundlich ad-sorption models [20, 55, 56] are used to scrutinize the ad-sorption isotherms, with the following equations, respectively.

$$\frac{{C_{e} }}{{q_{e} }} = \frac{1}{{q_{m} K_{L} }} + \frac{{C_{e} }}{{q_{m} }}$$

(11)

$$\ln q_{e} = \ln K_{f} + \frac{1}{n}\ln C_{e}$$

(12)

where K_L denotes the Langmuir constant, K_f refers to the Freundlich constant and q_m is the ad-sorption capacity (mg/g). Figure 12a and b illustrates the fitting of Pb(II) ad-sorption data onto La₂FeCrO₆ using the previous isotherm models.

Fig. 12

Linear fit of experimental data of the ad-sorption of Pb²⁺ onto La₂FeCrO₆

The correlation coefficient (R²) values for the Langmuir and Freundlich isotherms are 0.9898 and 0.9729, respectively, according to the inset table in Fig. 12.

Figure 12 shows that both models, along with the experimental data, are well fitted. The first model, considers the ad-sorption process of a monolayer of molecules on solid surfaces [57, 58]. Based on the data in Eq. 11 and Fig. 12a, the maximum ad-sorption capacity is calculated to be 17.36 mg per gram. The K_L, equals 0.21 L mg⁻¹ which designates high sorption energy between Pb(II) and La₂FeCrO₆. On the other side, in the Freundlich model, K_f and 1/n are determined from the intercept and slope of Fig. 12b.The low value of 1/n (0.511) confirms that Pb(II) ad-sorption on La₂FeCrO₆ is favorable. The electrostatic nature of the bonding between Pb(II) and the La₂FeCrO₆ nano ferrite sample is thought to be the main cause of the observed high sorption of La₂FeCrO₆ to the Pb(II) ion.

4 Conclusion

The double perovskite La₂FeCrO₆ was synthesized in nanophase by the citrate auto-combustion technique. The structure, morphology, and magnetic properties have been studied. XRD of the sample at 300 K shows that La₂FeCrO₆ is crystallized in the orthorhombic phase with a crystallite size of 22.6 nm. The sample has a sponge-like morphology with clear pores. EDAX mapping illustrated the uniform distribution of constituent elements in the sample. The sample La₂FeCrO₆ has antiferromagnetic behavior with a weak ferromagnetic contribution. The significant exchange bias found in the study of La₂FeCrO₆ provides strong support for the connection between its ferromagnetic (FM) and antiferromagnetic (AFM) components, which attributed to the presence of Fe³⁺ and Cr³⁺ ions in the B′ and B′′ sites. The maximum removal efficiency of Pb²⁺ is 77% at 5 h of contact time. The Langmuir and Freundlich isotherm models are the best descriptions of the ad-sorption isotherm of Pb²⁺ ions.