NZF Nanoscale Particles: Synthesis, Characterization and its Effective Adsorption of Bromophenol Blue

The ferrospinels NixZn1_xFe2O4 (x = 0.0 and 0.6) nanoparticles (NPs) were successfully prepared by a sol-gel method and analyzed by TGA/DTA, XRD, SEM-EDS, UV-Vis-DRS, and pHIEP. The adsorption potential of NZF NPs towards the Bromophenol blue (BPB) dye was investigated. The batch adsorption efficiency parameters were studied including contact time, pH, initial dye concentrations and catalyst dosage. Results indicated that NZF crystallized in single-phase and exhibited smaller crystallite size (49 nm vs. 59.24 nm) than that of the pristine (ZF). The SEM analysis showed that the materials are elongated-like shape. NZF catalyst showed a red-shift of absorption bands and a more narrowed band gap (2.30 eV vs. 1.65 eV) as compared to ZF. The adsorption process was found to be highly dependent to the pH of the solution, dye concentration and adsorbent dose. Under optimum conditions of 5 mg.L–1 BPB, 0.5 g.L–1 NZF catalyst, pH = 6, and 25 °C, up to ≈ 86.30% removal efficiency could be achieved after 60 min. Pseudo-second-order kinetic model gave the best fit with highest correlation coefficients (R2 ≥ 0.99). A high specific surface area, a stabilized dispersion state of NZF NPs and the electrostatic interaction between the BPB-2 anions and the NZF-H3O active sites on NZF surface were believed to be the main factors that can be responsible for the high adsorption efficiency. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Dye polluted water commonly originates from the industrial process, such as pharmaceutical, textile, dyeing, leather, synthetic detergent, plastic, and even house hold activities. The ment. Among these established techniques, the adsorption process has been found to be simple, efficient and economic for environmental remediation [2,3].
In this context, the use of the normal spinel Zinc ferrite or franklinite (ZnFe2O4) or modified spinel with appropriate properties can be benefit in the field of adsorption and catalysis [4]. Recently, ZnFe2O4, with a typical band gap Eg of about 1.9 eV [5], which enables it to absorb sunlight up to 653 nm or even larger, has potential application as the Fenton-like heterogeneous catalysts in the oxidation processes of refractory contaminant. However, due to their low valence band potential and poor quantum efficiency, ZnFe2O4 can not be used directly in the photocatalysis processes [6]. In the band gap engineering the solar light activated catalysts can be applied by transition metals doping for size-controlled functional nanomaterials [7][8][9][10][11] or for effective photoinduced electron-hole pair's separation, including semiconductor combination with matching band potentials [6,[11][12][13][14][15][16][17][18]. It is worthwhile to note that these catalysts based iron-containing compounds when used directly in solution, often give rise to particle agglomeration with severe loss of effective surface area due to the magnetic interactions. However, doping with transition metal, such as Ni +2 , Mn +2 , Co +2 , Cu +2 , etc. into the spinel ZnFe2O4 nanostructures, could be able to control the particle size, efficiently avoid particles from agglomeration, and hence improve the structural, morphological, magneto-optical, and heterogeneous Fenton-like activity.
Among the nanostructured spinel ferrite materials, the iron nickel zinc based ferrites (NixZn1-xFe2Os) have much interesting properties such as high saturation magnetization, high resistance and low eddy current losses [19]. This material has potential applications in the field of microelectronics, and magnetooptics, catalysts, absorbent, sensors, electronic devices and in water splitting photoelectrochemical process [20][21][22][23][24][25]. Up to now, various synthetic approach have been developed to synthesize nano-sized particles of ferrite materials, like sol-gel, hydrothermal, solvothermal, coprecipitation, microwave combustion and citrate precursor method [26][27][28][29][30][31][32]. Conversely, some disadvantages of the pre-cited methods like high-energy consuming, long reaction time in complicated equipment at high temperature, the use of a strong base.
Recently, much attention has been devoted to sol-gel method for the elaboration of nanoferrite functional materials [33][34][35]. The method gives inorganic materials with high purity under mild conditions [36]. In this study, Ni-doped ZnFe2O4 nano-catalyst was prepared by simple sol-gel route. The material is less explored in the field of sorption of organic pollutant including dyes and other organics. The assynthesized Ni0.6Zn0.4Fe2O4 (is denoted as NZF) catalysts was in detail characterized by Differential Thermal Analysis (DTA) -Thermo Gravimetric Analysis (TGA), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-EDS), pHIEP, and UV-vis Diffuse Reflectance (UV-Vis-DRS) techniques. The impact of Nicontent (x) on the crystallographic, morphology and optic properties of NZF was studied. The adsorption activity of NZF catalyst were investigated in the removal of Bromophenol blue dye (BPB), as a target contaminant, from simulated wastewater. We have also explored systematically the effects of the operational conditions such as reaction time, pH of the solution, the dye concentration and catalyst dosage on the efficiency of adsorption. The removal reactions as function of time were fitted with the pseudosecond-order kinetic model. Based on the studied properties of the material, a mechanism of the increased adsorption activity of NZF catalyst was elucidated.

Catalyst Characterizations
To determine the desired temperature for phase NZF formation, the sample was analyzed thermally by NETZSCH STA 449F3 STA449F3A-1085-M Instrument (DTA-TG 1000 °C) under a nitrogen atmosphere with a heating rate of 5 °C/min, from room temperature to 1000 °C. X-ray diffraction (Bruker D8 Advance) was performed to characterize the phase and structure of ZF and NZF nanoparticles using Cu-Kα radiation at angle ranging from 10 to 80° and a scanning rate of 10 °/min. The average crystallite size of the synthesized catalysts is calculated from the full-width-halfmaximum (FWHM) of the diffraction peaks using Scherrer Equation (1) [37]. (1) Where dXRD, λ, β, θ are the average crystallite size, the wavelength (1.5406 Å), the full-width at half maximum (FWHM) of the intense peak in radians and Bragg angle. The size and the morphologies of the sample were examined using Jeol JSM 6360LV. Scanning Electron Microscope coupled with EDS (SEM-EDS) analysis was carried out by using Jeol JSM-6410. UV-vis spectrophotometer (Perkin Elmer Lambda 650) for UV-vis diffuse reflection spectra from 900 to 200 nm. The band gap energy (Eg) of prepared nanoparticles can be found using Kubelka-Munk (K-M) model [38]. The absorption wavelength (λ) of a solid and its' band gap value can be given by the Equation (2).
The residual pollutant concentrations during adsorption process were monitored with UV Spectrophotometer (Shimadzu UV mini-1240) in the range 200 -800 nm, using 1 cm optical pathway cells. Polycrystalline spinel ferrite with formula Ni0.6Zn0.4Fe2O4 (denoted as NZF) was synthesized by the reaction of metal salts (M) including FeSO4.7H2O, Zn(COO)2.6H2O, and NiCl2.6H2O (molar ratio Ni 2+ : Zn 2+ : Fe 3+ = x : 1-x : 2 in distilled water with citric acid (CA) (M:CA of 1:1,1) with vigorous stirring. The obtained brown red solution was kept under stirring at room temperature for 24 h. The precipitated product was separated and thoroughly washed with distilled water and ethanol for three times and lastly dried in an oven at 80 °C for 24 h under normal pressure. At the end, samples were crushed and heated at 900 °C during 4 h. For comparison purpose, ZnFe2O4 (denoted as ZF) was also prepared in the similar route by calcination of solid Zinc ferrite particles at 1000 °C during 4 h.

Adsorption Experiment
Typically, 50 mg of adsorbent (i.e. ZF and NZF) was poured in 100 mL of BPB dye solution (10 mg/L). Before adsorption, the pH of each solution was adjusted in the range of 6-11. The adsorption was monitored at 25 °C in the dark for 60 min until equilibrium. After each 10 min, 5 mL of dye samples were centrifuged and analyzed by UV-vis spectrophotometer (Shimadzu UV mini-1240) at λmax (592 nm) of BPB solution. The adsorption capacity q (mg/g) was calculated by Equation (3).
The removal efficiency of the catalyst η (%) is expressed using Equation (4). (4) where q is the adsorption capacity of the adsorbate (mg/g), m is the weight of adsorbent (g), vis the volume of solution (L), vis the volume of solution (L), Ce and Ct are the concentrations of the dye at equilibrium and at time t, respectively, Ci is the initial dye concentration (mg/L), and Cf is the residual dye concentration after adsorption equilibrium (mg/L).

Characterization of the Powder
The DTA-TGA thermogram of the Ni0.6Zn0.4Fe2O4 (NZF) NPs is illustrated in Figure 1. The thermal decomposition of NZF proceeds with three main decomposition steps. The first weight loss occurred mainly in the range from 25 to 120 °C, corresponding to the exothermic peak in the TDA curve, namely at 120 °C.
The weight loss associated with this state (i.e. 18%) corresponding to the evaporation of adsorbed water molecules from NZF surface in  [39,40]. In the temperature range 120-425 °C, a second weight loss was observed, corresponding to the exothermic peaks in the TDA curve, namely at 250 °C and 425 °C, with respect to the output of the trapped water and the decomposition of the citrate network, respectively, representing weight loss of about 29%. The third weight loss (≈11%) occurred mainly from 445-750 °C and leveled off when the temperature approached 750 °C. The removal of organic impurities still remaining in the material after 800 °C. Therefore, sample was annealed from 900 °C to investigate the formation of the NZF phase.
Typical XRD patterns of spinel ZF and NZF samples are depicted in Figure 2. The diffraction peaks are the reflections corresponding to the hkl planes (111), (220), (311), (222), (400), (422), and (511). These diffraction peaks confirm the formation of cubic spinel structure of crystal (space group Fd-3m and JCPDS CARD 01-1108). There is no additional peak dealing with impurities. As shown in Figure 3 and Table 2, the decrease in the intensity of peaks as well as the shift in their positions to higher value of 2θ were observed in the XRD patterns of NZF as compared to ZF is an indicative of lattice cell parameter reduction for the cubic NZF sample with Ni-content (x=0.6) with low crystalline material. The average crystallite size was estimated by Scherrer's equation using the broadening of the (311) peak. The results are shown in Table 2. The undoped ZF exhibited a crystallite size of 59.24 nm and it is reduced to 49.00 nm with Ni doping (NZF).
The peak broadening in XRD pattern is an indicative of smaller nanoparticle sizes [41]. The lattice constants "a" of the samples calculated from their corresponding XRD pattern data are obtained by Fullprof program are displayed in Table 2. The results showed a decrease in lattice parameter values from 8.4404 Å (ZF) to 8.3857 Å (NZF) with Nickel content (x=0.6). This finding is in constancy with gradual substitution of Zn 2+ ions of larger ionic radius in ZF spinel by Ni 2+ ions with smaller ionic radius (0.83 Å vs 0.74 Å) in NZF nanoparticles [42][43][44][45][46]. The lattice parameter "a" is in good agreement with the standard JCPDS value of 8.441 Å (JCPDS No. 22-1012).
The theoretical density (Dt) and specific surface area (SS) of nano-catalysts were evaluated by Equations (5) and (6). where Z is equal to 8 chemical species in the unit cell of spinel lattice , Mc is the molar mass of the sample (g.mol −1 ), aexp is the lattice parameter of the ferrite (Å), and N is Avogadro's number (6.022×1023 mol −1 ). The theoretical density (Dt) is given in Table 2. The results showed an increase of Dt from 5.325 to 5.338 g/cm 3 as the Ni-content (x) increases from x=0 to x= 0.6. These should give specific surface areas of ~19.02 for ZF and 22.94 m 2 /g for NZF.
The morphologies and structures of the magnetically ferrospinel NZF NPs were investigated using SEM images (Figure 4 a-c). Typical low-resolution SEM images, as seen in Figure 4 a-b, clearly reveal that NZF consisted of large randomly shaped aggregates of smaller individual ultrafine NPs. Particles with size up to 500 nm constituted from ultrafine particles were observed. As the particles are very small, so they get aggregated to certain extent due to high surface-to-volume ratio as well as magnetic interactions.
The NZF nanoparticles were further characterized with EDX analysis. As illustrated in Figure 5c, the relative atomic abundance of O, Fe, Ni, and Zn elements present in NZF nanocatalyst are 64.33, 24.59, 6.99, and 4.09 %, respectively. Furthermore, the atomic ratio O/Fe (2.6) is slightly higher than 2 which is an indic-Copyright © 2020, BCREC, ISSN 1978-2993

Bulletin of Chemical Reaction Engineering & Catalysis, 15 (3), 2020, 730
ative of intrinsic defects in the crystalline stricture (as mentioned in XRD analysis). Basically, great number of available active sites is present in samples with intrinsic oxygen vacancy defects. This can be benefit for the heterogeneous Fenton-like reactions in terms of reactivity and selectivity.
The pH drift method was used to find the isoelectric point of NZF NPs catalyst. Typically, 150 mg of NZF NPs was added to a solution of NaCl (50 mL, 0.01 M) at room temperature and pH adjusted initially (pHi) from 4 to 10. The final pH (pHf) was determined after 48 h. The value of IEP can be found by plotting ΔpH versus initial pHi as shown in Figure 5. The pH of zero charge of NZF NPs was found to be 6.75. This is an indication that the surface of the material is negatively charged at pH values above 6.75. While at pH under 6.75, the surface is positively charged.
The optical absorption properties of the spinel ZF and NZF nano-catalysts were investigated by UV-Vis-DRS. As depicted in Figure 6, the pristine ZF show high absorption in 200-600 nm region which is typical to ferrite based nanosystems, which may have orbital contributions from 3d 4 -4s 1 , 3d 5 and 4S [47,48]. ZF and NZF nano-catalysts exhibit a well-defined absorbance edges in the visible region at around   These levels allow the promotion of electrons from VB to CB energy level. As shown in Figure 6, the two absorption edges in DRS spectrum of NZF spinel indicated the presence of two crystallite phases in NZF catalyst [49]. The optical properties of the ZF and NZF are collected in Table 3. The plot of the modified K-M function by Equation (7) permitted to find the band gap energy (Eg).
with α is the linear absorption coefficient, h is the Planck's constant, ν is the light frequency, Eg, and β are band gap energy and a constant involving properties of the bands respectively.
The Eg values were calculated using the Tauc plotting of (αhv) 2 vs. hν and (αhv) 1/2 vs. hν for indirect and direct transitions, respectively, by extrapolating the straight line of these plots to (αhv) 2 =0 and (αhv) 1/2 =0, as shown, respectively    Table 3 indicates the Eg values for ZF and NZF. Based on the above reported results in Table 3 and Figures 7 and 8, for indirect transitions the Eg value for ZF was estimated to be 2.30 eV, while for NZF was 1.65 eV (direct transition).

Effect of pH solution
Qualitative study of the evolution of absorption band maxima of BPB in aqueous solution at various pH: BPB is a redox indicator dye which shows color transition in the range from 3 to 6. The color of BPB solution is yellow at pH = 3, and exhibits bluish purple color at pH > 4.6. BPB exhibits two pKa values (pKa1 = 3 and pKa2 = 6) with absorption maxima at 440 nm and 592 nm in the visible region. Figure 9 depicts the dissociation equilibrium of the pH indicator BPB. Thus, BPB displays three forms: neutral form (BPB) at pH < 3 (pH < pKa1), first anionic form (BPBˉ) lays between pH 3 and 4.6 (pKa1 < pH < pKa2) and second anionic form (BPB -2 ) at pH > 4.6 (pKa1 < pKa2 < pH). Figure 10 illustrates the UV-visible spectra of BPB in aqueous solution at various pHs. At neutral pH (i.e. pH = 6.5 and C0 = 10 mg/L), BPB exhibited a main band at 592 nm and associated with two absorption bands in the ultraviolet region appeared at 308 and 380 nm. As can be seen in Figure 10, under neutral and alkali media (i.e. pH ≥ 6), BPB molecules exhibited constant absorption bands with same intensities while they are most sensitive to the pH of the solution under acidic media (i.e. pH = 2 and 4).
As the value of pH of solution decreases at 4, a new band in the region 440 nm appears in the spectrum attributed to the π→π* transition in the C=C-C=O and C=C-C=C links. These absorption bands correspond to the color changes of the Bromophenol blue from blue (BPB -2 ) to purple (BPB -1 ). One should keep in mind that the concentration of neutral BPB form is higher than the anionic form. Further decrease in pH beyond 4 (i.e. pH = 2), the main band of BPB blue shifted from 592 nm (anionic species BPB -2 ) to 440 nm (neutral and yellow form BPB). A such hypochromic shift (H-bands) compared to the monomeric species are attributed to the partially self-association of BPB monomers in solution and the change in environmental conditions such as polarity of the solvent will result in solvatochromism, which is due to the strong intermolecular van der Waals-like attractive forces between the BPB molecules. The isosbestic point, at 490 nm present in the absorption spectra confirms the equilibrium between acid (BPB or BPB -1 ) and basic (BPB -2 ) forms in the Bromophenol blue samples [50]. For that reason, the adsorption process was conducted at pH ≥ 6.
Quantitative study: the evolution of the degradation efficiency of BPB in aqueous solution at various pH: The pH of the wastewater is one of the vital parameters that determine the efficiency of adsorption process. Thus, the experiments were conducted over a range of pH values from 6 to 11 at a fixed dye concentration (5 mg.L -1 ), catalyst dosage (0.5 g/L of NZF) and 25 °C within 60 min of reaction. The plot of the adsorbent amount against time was illustrated in Figure 11. As shown in Table 4, the adsorption efficiency decreased from 86.30 to 4.27% as pH was increased from 6 to 11. The optimum pH of the BPB dye was 6. At this pH, the removal is maximum (up to 90.38 %) was achieved within 60 min. The link between the   [51]. In the 2-pK approach, we assume that most of the semiconductor oxides are amphoteric in nature, can associate or dissociate proton in aqueous solution. The IEP (the pH of zero point charge) of NZF nano-catalyst was about 10.2 [52]. The catalyst surface has a net positive charge at 6 ≤ pH < IEP, according to the following reaction (Equation (8)).
BPB is a redox indicator dye which has two functional groups (i.e. sulfonic acid group (-S(O 3 ) -) and oxyl group (O -)). Under weak acidic medium (i.e. pH=6 < pKa < IEP), BPB is having strong negative charges in water (BPB 2-). The surface of NZF with more positively charged (NZF-H3O + ) and concentration of -OH was less important to struggle with BPB 2-anions for vacant adsorption sites (Equation (10) The removal efficiency of NZF towards the BPB was high at pH = 6. However, when pH solution was increased up to 8 (pKa < pH < IEP), a slight decrease in the removal efficiency of BPB (up to 73.17%) was achieved within 60 min. At pH 8, the surface charge of NZF is less positively charged and the number of -OH was great than BPB 2-anions. So, there have been a competitive adsorption between anions which reduce the dye adsorption (Equations (12-13).

NZF-H3O + + -OH (great) → [NZF-H3O---OH]
By contrast, the adsorption efficiency of BPB was scarily low and depicts minimum effi-ciency (up to 4.27%) at pH 11. This might be explained on the basis of amphoteric behaviors of NZF catalyst under strong alkali media. At this pH, (pKa < IEP ≈ pH), both NZF surfaces and BPB dye were negatively charged along with very high concentration of -OH. Thus, -S(O3) − and/or (O -)) were unable to adsorb on the (NZFO -) surface due to the electrostatic forces of repulsion (Equations (14-16)). Moreover, at this pH too close to IEP and according to the principles of general colloid chemistry, an electrostatically stabilized dispersion system typically loses stability when the magnitude (i.e. absolute value) of the zeta potential decreases to less than approximately 30 mV. As a result, there will be some region surrounding the condition of zero zeta potential (i.e. the IEP) for which the system is not particularly stable. Within this unstable region, the particles may agglomerate, thereby increasing the particle size and reducing positive site of surface catalyst when transfer from neutral to basic medium. As a result, 86.30 % of dye was removed at pH = 6 within 60 min at mild conditions. Based on our results, additional research will be carried out at a pH of 6 [53].

Effect of time of the reaction on BPB dye removal
The effect of time on BPB removal at various equilibrium time (5-60 min) was investigated. The removal efficiency was plotted in Figure 12. A dramatically increase in the removal of BPB was observed in the initial stage (first 10 min). The removal of dye then gradually gets increased with increase of contact time up to 30 min in the second stage until equilibrium between the dye and the adsorbent. The equilibrium was reached during the optimum time of 60 min. Moreover, within 10 min the removal efficiency of 74.29 % with adsorbed amount of 14.24 mg/g of BPB were obtained. The presence of active sites on the NZF surface was believed to be responsible on the high rate of adsorption in the initial stage. However, the removal of the dye was insignificant after equilibrium. This can be explained by the slow pore diffusion or saturation of dye molecules [54].

Effect of the initial BPB concentration
The effect of initial dye concentrations on adsorption process at a fixed NZF dosage (1 g/L), pH 6 and 25 °C was studied using BPB concentrations ranging between 5-40 mg/L within 60 min. As shown in Figure 13 and summarized in Table 5, the adsorption kinetic of the BPB via NZF decreased with time and then achieved equilibrium after 60 min.
The removal efficiency is slightly decreased from 86.30 to 84.74% as the BPB concentrations ranged in the field 5-10 mg/L and then scarily declined to 28.39 % as the BPB concentrations varies to 20 mg/L. At higher BPB concentrations than 20 mg/L, the removal efficiency remained almost constant. Optimum concentration of BPB dye removal (up to ≈ 86.30%) was obtained at 5 mg/L. The low initial concentration (i.e. 5 mg/L), provides the necessary Van der Waal's driving force to overcome the mass transfer resistance of BPB dye to the active surface sites of the NZF adsorbent. Moreover, the number of collisions between dye molecules was improved which increase the adsorp-tion process. The active sites available on the surface of the NZF, are sufficient to immediately react with BPB molecules, and consequently result in an enhancement in the rate of removal of BPB (86.30 %). Further increase in dye concentration beyond 5-10 mg/L, obviously affect the adsorption yield. This will be attributed to the lesser driving force of concentration gradient between BPB and NZF catalyst, saturation of adsorption sites on the NZF surface or slow pore diffusion, and subsequently the probability of collision between BPB molecules and adsorption sites were reduced, resulting in an inadequate adsorption of BPB on the NZF surface.

Effect of catalyst dosage
The effect of catalyst dosage on the removal efficiency of BPB is shown in Figure 14 and the results are collected in Table 6. While keeping constant all the other operating parameters; initial BPB concentration of 5 mg/L, pH 6 and 25 °C during 60 min, the NZF dosage m/v was varied from 0.25 to 1.5 g/L. As shown in Figure   Figure 13. Effect of initial BPB concentration on the adsorbed amount of BPB  Therefore, the removal of BPB was highly influenced by the amount of catalyst. Therefore, the optimum of catalyst mass concentration was 0.5 g/L with 86.30% removal within 60 min. Scarily weak removal efficiency observed at a first catalyst dosage (i.e. 0.25 g/L) is due to the saturation of NZF catalyst surface by adsorbed matter. An increase in catalyst dosage up to 0.5 g/L would lead to more formation of the available active sites on the surface of the catalyst NZF for adsorption process, thus enhancing the removal efficiency of BPB dye. However, a decrease in the removal efficiency with further increase in catalyst dosage beyond 0.5 g/L.
According to general colloid chemistry principles, the discrepancy observed for the NZF NPs sample beyond 0.5 g/L could exclusively attributed to: (i) the electrostatically instabilized dispersion system; (ii) partially selfassociation of NZF NPs; (iii) low specific surface area; and (iv) less reactive sites for the adsorption of organic contaminants. The thermodynamic and mass transfer limitation and the tendency of agglomeration of NZF NPs are probably the primary factors responsible for the reduction of the removal yields. Thus, nearly complete adsorption degradation was achieved at the optimum catalyst dosage of 0.5 g/L for 60 min. Thus, 0.5 g/L of NZF was selected as the catalyst dosage for the subsequent experiments.

Effect of catalyst nature
The adsorption potential of ZF and NZF NPs toward BPB was investigated. The removal capacities of NZF compared with ZF spinel structures illustrated as NZF and ZF points in the legend of the Figure 15, are reported in Table 7. The results show also that ZF NPs depict low removal efficiency of BPB (up to 34.75%),

The kinetic of adsorption
Modeling with pseudo-first order: The model was developed by Lagergren as can be expressed by Equation (17) [55]. (17) where qe and qt are the dye adsorbed per unit weight of adsorbent at equilibrium and at time t, respectively. K1 is pseudo-first order rate constant (min -1 ). First order rate constant k1 was calculated from the slope value of the linear plot of log (qe-qt) vs. t. Correlation coefficient (R 2 ) was also calculated from the linear plots log (qe-qt) = f(t). The pseudo-first-order rate constants k2 and the correlation coefficients (R 2 ) at various operating keys parameters: pH of the solution, initial dye concentrations, adsorbent doses and nature of catalyst are respectively shown in Tables 4-7. The comparison between the experimental (qe,exp) data with the calculated values (qe,cal) does not agree well with the first-order kinetic model.
Modeling with pseudo second order: The pseudo second order kinetics equation developed by Ho and McKay, have the following linear form, Equation (18) [56]: where k2 is the pseudo-second-order constant (g/mg.min) and when time tends towards zero the initial adsorption rate (h (mg/g.min)) can be written as Equation (19): where qe is the adsorption capacity at equilibrium time, and k2 is the pseudo-second-order constant. These two parameters can be extracted directly from the slope and intercept of plots of t/qt vs. t, respectively at different conditions. The results are shown in Figures 16-19 and Tables 6-8. The correlation coefficient (R 2 ) values of the linear portions are very high, which indicates that the kinetic model of adsorption of BPB dye follows the pseudo-second-order. In this process the overall rate of adsorption was mainly controlled by chemisorption.

Comparison study
In comparison, the highest adsorbed amount at equilibrium was observed for Sorel's cement nanoparticles followed by NZF and ZF. This finding can be ascribed to the experimental conditions and properties of adsorbent such as specific surface area, pore size and the presence of functional groups on the external surface. The surface porosity was a factor that can be affected the uptake of the dye molecules since the BPB molecules have a large kinetic diameter in comparison with other organic molecules. Results showed that the spinel materials could be suitable for the adsorption of bromophenol blue dye. Table 8 summarizes the adsorption capacities of BPB on different adsorbents.

Potential adsorption mechanism
Scheme 1 shows the schematic diagram of the adsorption of BPB -2 anions on the NZF-H3O + active sites of NZF clay soil, as discussed in details in the Section 3.2.1.

Conclusions
In this study, ZnFe2O4 (ZF) and Ni0.6Zn0.4Fe2O4 (NZF) stable magnetically nano-ferrospinel particles were successfully synthesized by a facile sol-gel method for removal BPB dye from the industrial aqueous media. Single phase product formation was confirmed by XRD and SEM. From XRD and UV-vis-DRS results, it is highlighted that average crystallite size and band gap energy are decreased owing to nickel ion substitution. NZF exhibited high removal efficiency (up to ∼86.30%) to reduce BPB within 60 min under optimum conditions of 5 mg/L BPB, 0.5 g/L catalyst, pH = 6 and 25 °C, which was 3 times as much as that of pristine ZF. The adsorption kinetics were correlated with pseudo second order reaction. A high specific surface area, a stabilized dispersion state of NZFNPs and the electrostatic interaction between the BPB -2 anions and the NZF-H3O + active sites on NZF surface, were supposed to be was the key factors  Figure 17. Pseudo-second-order kinetics for adsorption of BPB at various initial BPB concentrations Figure 18. Pseudo-second-order kinetics for adsorption of BPB at various catalyst dosage of NZF Figure 19. Pseudo-second-order kinetics for adsorption of BPB on various catalyst in the increased of the adsorption efficiency of NZF nano-adsorbent towards BPB dye. In comparison with other adsorbents, NZF was found to be a promising material for the purification of water containing BPB dye. The dye adsorption conditions, surface area, porosity and the presence of function groups on the surface could be the raisons for the high adsorption capacity of any adsorbents. The studied material could be also effective in the area of catalysis due to its appropriate band gap which can be effectively used as photocatalyst. This study gives an introduction of our future research works on these kinds of materials in the field of advanced catalysis.