The characterization of the prepared nanomagnetite and its application in wastewater treatment are discussed in the following sections.X-ray diffraction (XRD)XRD was applied to determine the crystallinity of the tested sample. The peaks in the XRD pattern shown in Fig. 2 are between 30° and 62°, which correspond to standard magnetite26. These findings revealed that a single face-centered cubic (FCC) spinel structure of Fe3O4 was formed. (Device model SEIFERT XRD 3003 TT DIFRACTOR (GE, Germany) Equipped with a primary monochromator (CuK radiation, 2 ceta = 3 − 90°).Figure 2XRD of prepared magnetite-nanoparticles.Transmission electron microscopy (TEM)TEM is essential for material science and has many characteristics, such as particle size and morphology. As shown in Fig. 3, the particles are almost spherical in shape, and the average particle size is 29.2 nm, which is a reasonable size for obtaining a high surface area. (Device model: JEOL JEM-2100 (Origin. Japan). The BET surface area is 70.1 m2 g−1.Figure 3TEM image of the prepared magnetite nanoparticles.Zeta potentialThe zeta potential is the charge that appears at the interface between a solid surface and the surrounding liquid medium. Figure 4 shows the results obtained from the examined sample, which indicate that the surface of magnetite has a negative charge when a particle is dispersed in a liquid, and the functional groups on its surface will react with the positively charged ions in the surrounding medium (device model: MALVERN ZETASIZER, USA).Figure 4Zeta potential of the prepared magnetite nanoparticles.Fourier transform infrared (FTIR) spectroscopyThe FTIR transmittance spectrum analysis is shown in Fig. 5. A peak is observed at 556 cm−1, which corresponds to the stretching vibration of the Fe–O bonds in the sublattice of Fe3O4. The peak at 2930 cm−1 is attributed to –CH2 and chemical group stretching vibrations. The values at 1629 cm−1 and 3389.64 cm−1 indicate the stretching of C–O, C=C, and OH–, respectively27. (Device model (FTIR) JASCO, FTIR-300 E.Figure 5FTIR of prepared magnetite-nanoparticle.UV–visible spectroscopyThe UV–visible spectra of the prepared magnetite nanoparticles are displayed in Fig. 6. The UV–visible peak of the sample was obtained at 410 nm. The reported UV–visible peak was found to be at 407 nm by Suresh Kumar et al.28. The peak in the near-IR region confirmed the presence of magnetite nanoparticles.Figure 6UV–visible absorption spectroscopic analysis of the prepared magnetite nanoparticles indicating a peak at 410 nm.VSM of synthetic nanomagnetiteVibrating sample magnetometer analysis (VSM) was performed using a Lake shore model 7410 instrument to analyze the synthesized nanomagnetite, as shown in Fig. 7. As shown in the figure, the magnetization of the sample revealed that it was completely attracted to the magnet and had a magnetization of 60 (emu/g)29.Figure 7VSM of synthetic nanomagnetite.Figure 8 shows the pattern taken by the nanoparticles when subjected to a magnetic field, which confirms its magnetic properties.Figure 8Produced nanomagnetite (a) before being subjected to a magnetic field (b) after being subjected to a magnetic field.Adsorption of methylene blue dye using magnetite nanoparticlesThe results obtained in different runs are tabulated in Table 1. The highest removal efficiency (95.11%) was obtained by adding 50 mg of Fe3O4/L, 10 ppm dye conc and 6.5 pH.Table 1 Removal efficiency of methylene blue dye using nanomagnetite under different conditions.The reduced cubic model was the best model for representing the obtained data, with an R2 equal to 0.973 and an adjusted R2 of 0.94. Table 2 displays an ANOVA of the obtained model, which indicates that the model is significant with a p value of 0.0001. The suggested model that relates the studied parameters and removal efficiency according to the obtained results and statistical analysis was a reduced cubic model with R2 = 0.9697, adjusted R2 = 0.9395, and a p value lower than 0.0001, which confirmed that the model was significant. The ANOVA results of the models are displayed in Table 6. These results indicate that the model is significant and has a high confidence level, as the p value is lower than 0.05 and the F value is 32, which reveals the importance of the variance in each variable30.Table 2 ANOVA for the reduced cubic model.Dye removal equations by adsorptionThe removal efficiency (Y) equation was obtained from the statistical analysis of the coded values$$ \begin{aligned} {\text{Y}} = & {78}.{33}{-}{1}0.{\text{87 pH}} + {35}.{\text{84 Conc}}{-}{6}.{\text{73 dose}} + {12}.{\text{73 pH}}*{\text{Conc}}{-}{6}.{\text{43 pH}}*{\text{dose}} \\ & {-}\,{5}.{\text{43 pH}}^{{2}} {-}{24}.{\text{43 dose}}^{{2}} {-}{22}.{\text{27 pH}}^{{2}} *{\text{conc}} + {28}.0{\text{7 pH}}*{\text{dose}}^{{2}} \\ \end{aligned} $$
(7)
The removal efficiency (Y) equation was obtained from the statistical analysis of the actual values$$ \begin{aligned} {\text{Y}} = & {19}.{34}{-}{19}.{\text{957 pH}}{-}{16}.{\text{15 Conc}} + {1}.{\text{756 Dose}} + {6}.{\text{81 pH}}*{\text{Conc}} \\ & {-}\,0.{\text{194 pH}}*{\text{dose}} + {2}.{\text{283 pH}}^{{2}} {-}0.000{\text{49 dose}}^{{2}} {-}0.{\text{454 pH}}^{{2}} *{\text{Conc}} \\ & + \,0.000{\text{513 pH}}*{\text{dose}}^{{2}} \\ \end{aligned} $$
(8)
Interaction between the studied parametersA 2-D plot can be drawn for different variations in parameters, which exhibit a trend in which the response varies within the selected range of input parameters and the effect of each parameter over the other parameters. With the aid of statistical analysis, the interactions between the three studied parameters, namely, pH “A”, dye concentration “B” and dose “C”, can be studied using the obtained model graph contours.
a.
Effect of dye concentration and pH on dye removal
The effects of pH and concentration on removal efficiency are displayed in Fig. 9a–c. At a low magnetite dose, the removal efficiency increases as the pH and concentration increase simultaneously. At an average dose of magnetite, increasing the concentration of dye increases the removal efficiency even at a low pH. At a high dose of magnetite, the removal efficiency increases with increasing pH and concentration simultaneously, which confirms the interaction between the studied parameters.Figure 9Effect of dye concentration and pH on dye removal for nanomagnetite at (a) minimum, (b) medium, and (c) maximum concentrations.
b.
Effect of the magnetite dose and pH on dye removal
The effect of magnetite dose and pH on dye removal efficiency is illustrated in Fig. 10a–c. As the concentration of dye increases from the minimum to the maximum value, the removal efficiency increases even at low pH and magnetite doses.Figure 10Effect of pH and nanomagnetite concentration on the removal efficiency of Dye Conc. at (a) minimum, (b) medium, and (c) maximum values.
Adsorption isothermSynthetic wastewater solutions with concentrations of 10, 20, 40, 60, 80, and 100 ppm methylene blue dye were prepared. The dose of magnetite was 0.05 g/L for all the samples. All the samples were agitated on a mechanical shaker at a speed equal to 100 rpm. The concentrations of all the samples were measured over time until they reached equilibrium. The results are listed in Table 3.Table 3 Equilibrium concentrations for different samples.The adsorption capacity increased with increasing concentration up to 80 ppm and then decreased at 100 ppm, possibly due to the saturation of magnetite.Three models were applied: the Langmuir isotherm model, Freundlich isotherm model, and Dubinin–Radushkevich isotherm model.As shown in Figs. 11 and 12, the Langmuir isotherm was fitted with the experimental data, as it had an R2 of 0.9295; in contrast, the Freundlich isotherm had an R2 of 0.6787.Figure 11Figure 12Freundlich isotherm model.The maximum adsorption capacity was calculated from the line slope, which was qmax = 1/slope = 50 mg/g, and the Langmuir constant KL = 2 L/mg.The Langmuir model indicates that a monolayer is formed, the heat of adsorption Q is constant and independent of coverage, each adsorbate molecule occupies only one site, and the adsorption is localized (molecules remain at the site of adsorption until desorption).The Dubinin–Radushkevich isotherm model (D–R) can be applied to determine whether the adsorption process is physical or chemical and is expressed as31$$ {\text{ln qe}} = {\text{ln K}}_{{{\text{D}} – {\text{R}}}} – \upbeta \upvarepsilon^{{2}} $$
(9)
where KD−R (mg g−1) is the Dubinin–Radushkevich constant and the Polanyi potential is ε (mol2 J−2), which is equal to$$ \varepsilon = {\text{RT}}\,{\text{ln}}\left( {{1} + {1}/{\text{Ce}}} \right) $$
(10)
where T is the absolute temperature (K) and R is the universal gas constant (8.314 J K−1 mol−1). The constant β is related to E (kJ mol−1). The energy E is defined as the free energy change required to transfer 1 mol of ions from the solution to the solid:$$ E = \frac{1}{{sqrt\left( {2B} \right)}} $$
(11)
The linear relation of (ln qe) against ε2 is carried out as shown in Fig. 13, and the values β and KD−R are obtained from the slope and intercept of the line. The value of E represents the information adsorption mechanism; an E value less than 8 kJ mol−1 represents the physisorption process, and an E value within the range of 8–16 kJ mol−1 is assigned to the chemisorption process31. The calculated value of E is 2.23 kJ/mol, which indicates that the adsorption of M.B by the nanomagnetite physisorption process.Figure 13Dubinin–Radushkevich isotherm.Adsorption kineticsAdsorption kinetics were studied for a sample with an 80 ppm concentration, as it maintained its maximum adsorption capacity. Two kinetic models were applied: a pseudo-first-order model and a pseudo-second-order model. Figures 14 and 15 show that the pseudo first-order model fit the experimental data, as it had an R2 of 0.9269 versus an R2 of 0.0198 for the pseudo second-order model.Figure 14Pseudo-first-order model.Figure 15Pseudo-second order model.Thermodynamic studyA thermodynamic study was carried out by changing the temperature (20, 30, 40 °C) during the adsorption of methylene blue dye with nanomagnetite. Determination of the equilibrium concentration of different samples and thermodynamic parameters such as the free energy change (ΔG0), enthalpy change (ΔH0) and entropy change (ΔS0) were carried out using the following equations32:$$ \Delta {\text{G}}^{0} = \Delta {\text{H}}^{0} – {\text{T}}\Delta {\text{S}}^{0} $$
(12)
$$ \Delta {\text{G}}^{0} = – \,{\text{RT}}\,{\text{ln}}\left( {\text{k}} \right){\text{c}} $$
(13)
where T is the temperature in K, R is the universal gas constant (R = 8.314 J mol–1 K–1), and Kc is the equilibrium constant (kc = qe/Ce). A linear plot between ln (Kc) and 1/T is displayed in Fig. 16. The values of ΔS0 and ΔH0 were determined from the intercept and slope, respectively, as presented in Table 4. The value of ΔH0 is positive, which points to an endothermic reaction. The value of ΔS0 is positive, which indicates that the degrees of freedom increased at the solid–liquid interface during dye adsorption.Figure 16Linear plot of thermodynamic parameters for the adsorption of M.B.Table 4 Thermodynamic parameter values for the adsorption of M.B dye onto nanomagnetite.Application of advanced oxidation in removing methylene blue dye using nanomagnetite as a catalystThe parameters and removal efficiency are tabulated in Table 5. The suggested model that relates the studied parameters and removal efficiency according to the obtained results and statistical analysis was a reduced cubic model with R2 = 0.9697, adjusted R2 = 0.9395, and a p value lower than 0.0001, which confirmed that the model was significant. The ANOVA results of the models are displayed in Table 6. These results indicate that the model is significant and has a high confidence level, as the p value is lower than 0.05 and the F value is 32, which reveals the importance of the variance in each variable30. The maximum removal percentage reached 98.5% according to the tabulated values; this value is greater than that of most of the methods found in the literature33, and not only does this removal occur because the AOP does not produce any sludge.Table 5 Removal efficiency of methylene blue dye using magnetite and H2O2 under different conditions.Table 6 ANOVA for the reduced cubic model.Dye removal efficiency equations using the advanced oxidation approachThe removal efficiency (Y) equation obtained from the statistical analysis of the coded values is shown below:$$ \begin{aligned} {\text{Y}} = & 0{-}{1}.{\text{92 dose}} + {6}.{\text{17 Hydrogen peroxide}} + 0{13}.{\text{12 PH}} \\ & + \,{9}.{3}0{\text{ dose}}*{\text{Hydrogen peroxide}} + {38}.{\text{31 dose}}^{2} \\ & + \,{18}.{\text{59 Hydrogen peroxide}}^{2} + {51}.{\text{14PH}}^{2} \\ & + \,{22}.{\text{33 dose}}^{2} *{\text{Hydrogen peroxide}} \\ \end{aligned} $$
(14)
The actual removal efficiency is as follows:$$ \begin{aligned} {\text{Y}} = & {151}.{9}{-}0.{\text{223 dose}}{-}0.0{\text{45 Hydrogen peroxide}}{-}{29}.{\text{89 dose}} \\ & {-}\,0.000{\text{671 Hydrogen peroxide}} + 0.000{\text{135 dose}}^{{2}} \\ & + \,0.0000{\text{33 Hydrogen peroxide}}^{{2}} + {2}.{\text{525pH}}^{{2}} \\ & + \,{3}.{\text{675 dose}}^{{2}} *{\text{ Hydrogen peroxide}} \\ \end{aligned} $$
(15)
Effect of the catalyst dose and hydrogen peroxide on dye removal (interaction between study parameters)The catalyst dose “A” on the X-axis and hydrogen peroxide “B” on the Y-axis were studied while varying the pH.C. to its minimum, average, and maximum values to study its effect on dye removal. Figure 17a–c indicates that at a minimum pH, the removal efficiency increases as the peroxide dose increases; at the maximum pH, increasing the catalyst dose or peroxide dose increases the removal efficiency of the dye. Figure 18 shows photos of several treated samples after different runs.Figure 17Effect of the catalytic dose and hydrogen peroxide on the COD with respect to the pH at the (a) Minimum, (b) Medium, and (c) Maximum values.Figure 18Photographs of samples treated under different conditions.Figure 19a–c shows the separation of magnetite nanoparticles from treated water using a magnet. The solution was very clear after approximately 5 min, as shown in Fig. 20c. All the magnetite nanoparticles were attracted to the magnet below the beaker, which confirmed the magnetic characteristics of the produced nanomagnetite and the ease of separation from treated water, which distinguishes it as a catalyst for advanced oxidation in wastewater treatment.Figure 19Separation of nanomagnetite from treated water using a magnet: (a) initial sample, (b) after 2 min, and (c) after 5 min.Figure 20Kinetics of photocatalytic degradation of M.B dye using nanomagnetite.Effect of UV radiation on the removal efficiency of methylene blue solution using advanced oxidationTwo samples of methylene blue dye (100 ppm) were prepared, and the same conditions were applied for treatment via advanced oxidation under the optimum conditions, as determined from the abovementioned study. One of the samples was subjected to UV radiation at different time intervals—10, 20, 30, 40, 50, 60, and 70 min—and the other sample was not. The concentration was measured for both samples at different times, and the removal efficiency was calculated as listed in Table 7. The removal efficiency was improved by using UV radiation and reached a maximum value of approximately 99% after 50 min.Table 7 Effect of UV radiation on the oxidation process.Kinetics of the photocatalytic oxidation of M.B dye using nanomagnetiteTo investigate the photocatalytic degradation abilities of nanomagnetite, a quasifirst-order kinetic model was applied to analyze the kinetics of dye degradation, and the correlation equation is expressed as follows34,35:$$ {\text{Ln}}\left( {{\text{C}}/{\text{Co}}} \right) = {\text{kt}} $$
(16)
Co is the initial concentration of dye, C is the concentration of dye at any given time, and k is the rate constant. The linear relation between ln(C/Co) is represented by Fig. 20, and the rate constant k = 0.0316 min−1 is calculated from the slope of the line. The kinetic model is highly fitted to quasifirst-order kinetics, as R2 = 0.904.Assessment of the reuse of synthesized magnetite nanoparticlesA stock of 20 ppm methylene blue solution was prepared, and advanced oxidation was applied for the treatment of synthetic wastewater. Ten samples were prepared at the same concentration. The optimum conditions were applied after 24 h. The solution was withdrawn, and fresh synthetic wastewater was added to the same dose of magnetite. This process was repeated ten times to measure the efficiency of using reused magnetite as a catalyst. As shown in Table 8, the removal efficiency decreased from 100 to 90% after using magnetite ten times, which supports that the treatment process is economical, as shown in Fig. 21.Table 8 Magnetite reuse results in terms of removal efficiency.Figure 21Magnetite reuse after 10 cycles.Industrial samplesIndustrial waste samples were taken from the industrial zone at the tenth Ramadan textile factory, and several experiments were conducted. The initial and final COD concentrations were 1500 PPM and 195 PPM, respectively, which corresponds to 87.5% removal efficiency at this pH 2, 1250 peroxide dose and 200 ppm magnetite.