Iron-Loaded Pomegranate Peel as a Bio-Adsorbent for Phosphate Removal

: This study investigated the adsorption of phosphate from aqueous solutions using pomegranate peel (PP) as a bio-adsorbent. For this purpose, PP was activated via saponiﬁcation using sodium hydroxide (NaOH) followed by cationization using iron chloride (FeCl 3 ). The iron-loaded PP (IL-PP) was characterized using zeta potential measurement, scanning electron microscopy, and Fourier transform infrared analysis. The batch adsorption method was followed to determine the equilibrium time and effect of pH on the adsorption process. The full factorial design methodology was used to analyze the effects of inﬂuencing parameters and their interactions. The effective removal of phosphate up to 90% was achieved within 60 min, at pH 9 and 25 ◦ C temperature using a 150 mg dose of IL-PP. A non-linear method was used for the modeling of isotherm and kinetics. The results showed that the kinetics is best ﬁtted to the Elovich model (R 2 = 0.97), which assumes the dominance of the chemisorption mechanism, whereas the isotherm obeys both Langmuir (R 2 = 0.98) and Freundlich (R 2 = 0.94) models with a maximum phosphate uptake of 49.12 mg · g − 1 . Investigation of thermodynamic parameters indicated the spontaneity and endothermic nature of the process. These results introduce IL-PP as an efﬁcient bio-adsorbent of phosphate.


Introduction
Excess release of phosphorus is the main culprit for the eutrophication of freshwater and marine ecosystems [1]. Phosphorus is a nonrenewable and irreplaceable element for plant growth, and its role is crucial in agricultural production [2]. The accelerated growth in food demand has also increased the demand for phosphate fertilizers, which has placed stress on phosphate rock sources and is exhausting existing deposits [3]. The phosphate mining industry is also facing serious challenges regarding water availability for the mining process and a decrease in the quality of phosphate rocks [4]. Thus, the recovery of phosphate from wastewater is highly required to sustain the global food supply, preserve water resources, and protect the environment. Several biological, physical, and chemical methods exist for phosphate removal and recovery from aqueous solutions [5,6]. Among

The Stock Solution
The stock solution of phosphate (1000 mg·L −1 ) was prepared by dissolving Na 2 HPO 4 · 2H 2 O in deionized water, which was then diluted to the desired concentrations using distilled water. The adjustment of pH values of the phosphate solutions was done using hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. All chemicals used in this study were of analytical reagent grade.

Preparation and Activation of PP
PP was activated using an iron loading method similar to that of Nguyen et al. [43] for improving the PO 4 3− retention ability. First, PP was collected, cut into small pieces, and washed with distilled water several times until the washing solution became clear. It was oven-dried at 105 • C for 2 h and then ground to the desired particle size (<250 µm). The first step of the activation method was the base treatment or saponification, where 40 g of PP was stirred for 24 h with 1 L of a NaOH solution (0.05 M) at room temperature and then washed carefully with distilled water until the pH of washing solution became neutral. The saponification step aimed to improve the cationic exchange capacity of PP and promote the incorporation of iron ions (Fe 3+ ) on its surface. The second step was the iron loading, where the saponified PP was stirred with 500 mL of an iron chloride (FeCl 3 ) solution (0.25 M) at room temperature for 24 h. Finally, the iron-loaded PP (IL-PP) was carefully washed with distilled water again and oven-dried at 105 • C for 8 h, and then, it was mechanically milled with a planetary ball mill to the desired particle size (<250 µm) before use in the adsorption experiments.

Characterization of IL-PP
The zeta potentials of PP and IL-PP were measured using 10 mg suspensions mixed in bottles containing 30 mL of sodium chloride (NaCl) and disodium hydrogen phosphate (Na 2 HPO 4 ) solutions at different concentrations and pH values. After mixing, the equilibrium pH of the samples was measured and adjusted. Then, the zeta potential was measured with a Nano ZS apparatus (Malvern, Worcestershire, UK) using electrophoretic light scattering. All samples were prepared in triplicate, and the average of the measurements was used for data analysis.
Scanning electron microscopy (SEM) was also used to image the microstructures of PP and IL-PP and compare their surface morphologies. Samples were analyzed using a Hitachi S-4700 type II scanning electron microscope. A cold field emission gun and 10 kV acceleration voltage were applied to respectively produce and accelerate the electron beam. Micrographs were recorded by collecting secondary electrons with an Everhart-Thornley detector.
Fourier transform infrared (FTIR) spectra of PP and IL-PP were used to observe the functional groups present on their surface and assess the occurred changes after the activation of PP and the adsorption of phosphate (PO 4 3− ) by IL-PP. The spectra were recorded with a BIO-RAD Digilab Division FTS-65A/896 FTIR spectrophotometer having a 4 cm −1 resolution in the middle infrared range of 4000-400 cm −1 . Each spectrum was scanned 256 times. In addition to the spectra of each sample, single-reflection diamond attenuated total reflection accessory measurements were taken using the diffuse reflection technique and an angle of incidence of 45 • . The software Omnic 7.3 was used for FTIR data collection.

Batch Adsorption
The batch adsorption method was used to determine the equilibrium time and effect of pH on PO 4 3− adsorption by IL-PP. For this purpose, 50 mL of Na 2 HPO 4 solution (PO 4 -P concentration of 40 mg·L −1 ) was stirred at 150 rpm with different IL-PP doses (100 and 150 mg) doses and pH values (from 3 to 9) at a constant temperature of 25 • C. To identify the most important factors affecting the removal of PO 4 3− by IL-PP, 2 3 factorial design (three factors each, at two levels) with the Minitab 19 software was used. This technique allows the analysis of several factors simultaneously within a reduced total number of experiments [44]. The initial PO 4 -P concentration (40 mg·L −1 ), contact time (60 min), and stirring speed (150 rpm) were kept constant, and the three factors of the pH, adsorbent dose, and solution temperature were varied at two levels, as shown in Table 1.
where C i (mg·L −1 ) and C f (mg·L −1 ) are the initial and final PO 4 -P concentrations, respectively. The adsorbed amount of PO 4 -P was calculated using Equation (2): where C i (mg·L −1 ) and C e (mg·L −1 ) are the initial and equilibrium concentrations, respectively, of PO 4 -P in the solution; V (L) is the solution volume; and M (g) is the mass of the adsorbent. The isotherm of PO 4 3− adsorption by IL-PP was studied through a series of batch adsorption experiments at a stable temperature using different doses of IL-PP and a constant initial PO 4 -P concentration. Factorial design experiments were performed to identify and optimize the adsorption kinetics. To determine the isotherm and kinetic models that adequately describe PO 4 3− adsorption by IL-PP, isotherm and kinetics data were fitted to existing mathematical models by a nonlinear method using the Solver addin command in Microsoft Excel [45]. The best fitting kinetic and isotherm models were selected mainly based on the value of the nonlinear correlation coefficient (R 2 ). However, the chi-square (χ 2 ) statistics helped confirm this selection. A value of χ 2 close to 0 meant that the selected model fit the experimental data well, whereas a high value of χ 2 indicated that the model was inappropriate [46]. R 2 and χ 2 were calculated using Equations (3) and (4), respectively: where q e,exp (mg·g −1 ) is the amount of PO 4 -P uptake at equilibrium obtained from Equation (2), q e,cal (mg·g −1 ) is the amount of PO 4 -P uptake calculated from the model using the Solver add-in command, and q e,mean (mg·g −1 ) is the mean of the q e,exp values.
To study the thermodynamics of PO 4 3− adsorption by IL-PP, parameters such as the standard Gibbs free energy change (∆G), standard enthalpy change (∆H), and standard entropy change (∆S) were determined using Equations (5) and (6): where T is the absolute temperature in kelvins and R is the gas constant (8.314 J mol −1 K −1 ). K d is the distribution coefficient for the adsorption and was obtained by plotting ln (q e /C e ) against C e and extrapolating to zero Ce. Then, the obtained value was multiplied by 1000 as proposed by Milonjić [47].

Zeta Potential
Determining the zeta potential of the electric double layer surrounding the adsorbent surface at various solutions with different pH values and similar ionic strength (IS) is important because it provides insights into the adsorbent surface chemistry and possible interactions with the adsorbate [48]. Figure 1a shows that in the NaCl solution (IS = 10), IL-PP showed a positive zeta potential over the entire pH range considered in this study: from +5.8 mV at pH 3 to +16.1 mV at pH 9. By contrast, PP showed negative values: from −26.7 mV at pH 3 to −30.6 mV at pH 9. These results indicate that the PP surface became positively charged after the incorporation of Fe 3+ . The zeta potential of IL-PP in the Na 2 HPO 4 solution (IS = 10) decreased from +11.3 to −31.8 mV when the pH was increased from 3 to 9, and the isoelectric point can be interpolated at pH 5.4. This means that the IL-PP surface had an excess negative charge at pH > 5.4 and an excess positive charge at pH < 5.4. The decrease in surface charge is due to the neutralization of positive functional groups present on the IL-PP surface (mainly Fe 3+ ) by PO 4 3− . However, in the NaCl solution, Cl − could not neutralize the IL-PP surface, so the IL-PP surface had a high affinity toward PO 4 3− through a specific adsorption mechanism rather than a simple electrostatic attraction. Figure 1b shows that the zeta potential of IL-PP decreased when the Na 2 HPO 4 concentration was increased. The compression of the diffuse layer, which caused more PO 4 3− anions to attach to this layer could be the reason for this behavior [49].
Water 2021, 13, x FOR PEER REVIEW 5 of 20 charge at pH < 5.4. The decrease in surface charge is due to the neutralization of positive functional groups present on the IL-PP surface (mainly Fe 3+ ) by PO4 3− . However, in the NaCl solution, Cl − could not neutralize the IL-PP surface, so the IL-PP surface had a high affinity toward PO4 3− through a specific adsorption mechanism rather than a simple electrostatic attraction. Figure 1b shows that the zeta potential of IL-PP decreased when the Na2HPO4 concentration was increased. The compression of the diffuse layer, which caused more PO4 3− anions to attach to this layer could be the reason for this behavior [49].  Figure 2 shows SEM micrographs of PP (a) and IL-PP (b) at 50,000× magnification. The PP surface was relatively smooth and flat, but IL-PP had a much rougher surface with a coarser texture, which proves that Fe 3+ was incorporated. This modification of the morphology made the surface irregular and thus more suitable for PO4 3− uptake [50].  Figure 2 shows SEM micrographs of PP (a) and IL-PP (b) at 50,000× magnification. The PP surface was relatively smooth and flat, but IL-PP had a much rougher surface with a coarser texture, which proves that Fe 3+ was incorporated. This modification of the morphology made the surface irregular and thus more suitable for PO 4 3− uptake [50].  Figure 3 shows the FTIR spectra of PP and IL-PP before and after PO4 3− adsorption. Table 2 presents the results of the analysis carried out to identify the functional groups present on their surfaces and understand the possible interactions responsible for the incorporation of Fe 3+ onto the surface of PP and for the adsorption of PO4 3− by IL-PP. The observed bands in the PP surface agree with similar FTIR studies on functional groups present in PP [31,51]. However, the IL-PP spectra showed important changes characterized mainly by the appearance of a new peak at 801 cm −1 , which can be assigned to the Fe-OH band [52,53], and the disappearance of several bands at 1719, 1442, 1223, 876, and 747 cm −1 . These variations confirm the incorporation of Fe 3+ on the PP surface. The IL-PP spectra after PO4 3− adsorption revealed the appearance of a new peak at 1601 cm −1 , which can be attributed to the bending vibration of Fe-P and, therefore, confirms PO4 3− adsorption by IL-PP [54]. Table 2. FTIR analysis of PP and IL-PP before and after PO4 3− adsorption.  Figure 3 shows the FTIR spectra of PP and IL-PP before and after PO 4 3− adsorption. Table 2 presents the results of the analysis carried out to identify the functional groups present on their surfaces and understand the possible interactions responsible for the incorporation of Fe 3+ onto the surface of PP and for the adsorption of PO 4 3− by IL-PP. The observed bands in the PP surface agree with similar FTIR studies on functional groups present in PP [31,51]. However, the IL-PP spectra showed important changes characterized mainly by the appearance of a new peak at 801 cm −1 , which can be assigned to the Fe-OH band [52,53], and the disappearance of several bands at 1719, 1442, 1223, 876, and 747 cm −1 . These variations confirm the incorporation of Fe 3+ on the PP surface. The IL-PP spectra after PO 4 3− adsorption revealed the appearance of a new peak at 1601 cm −1 , which can be attributed to the bending vibration of Fe-P and, therefore, confirms PO 4 3− adsorption by IL-PP [54].

Effect of pH
The pH is a critical parameter in the adsorption process because it affects the chemistry of the solution and the stability of functional groups present on the adsorbent surface, which controls the adsorbent-adsorbate interaction [55]. Depending on the solution pH, PO4 3− can exist in four species: H3PO4 (pH~2.15), H2PO4 − (2.15 < pH < 7.20), HPO4 2− (7.20 < pH < 12.33), and PO4 3− (pH~12.33) [56]. Figure 4 shows that PO4 3− removal by 100 mg of IL-PP increased from 43.5% to 64.25% when the pH was increased from 3 to 9. This is because there is just one possible interaction between H2PO4 − and Fe 3+ , which is monodentate/mononuclear. However, there are three different possible interactions between HPO4 2− and Fe 3+ : monodentate/mononuclear, bidentate/mononuclear, and monodentate/binuclear [57]. This led to the high PO4 3− removal by IL-PP. These results are in agreement with the decrease in the zeta potential of the IL-PP surface in the Na2HPO4 solution when the pH was increased, which indicates that more PO4 3− ions were attached to this surface. For each sample, the equilibrium pH value was lower than the initial pH value, which indicates that large quantities of hydrogen ions were produced by Fe 3+ hydrolysis and reduced the equilibrium pH [58].

Effect of pH
The pH is a critical parameter in the adsorption process because it affects the chemistry of the solution and the stability of functional groups present on the adsorbent surface, which controls the adsorbent-adsorbate interaction [55]. Depending on the solution pH, PO 4 3  [56]. Figure 4 shows that PO 4 3− removal by 100 mg of IL-PP increased from 43.5% to 64.25% when the pH was increased from 3 to 9. This is because there is just one possible interaction between H 2 PO 4 − and Fe 3+ , which is monodentate/mononuclear. However, there are three different possible interactions between HPO 4 2− and Fe 3+ : monodentate/mononuclear, bidentate/mononuclear, and monodentate/binuclear [57]. This led to the high PO 4 3− removal by IL-PP. These results are in agreement with the decrease in the zeta potential of the IL-PP surface in the Na 2 HPO 4 solution when the pH was increased, which indicates that more PO 4 3− ions were attached to this surface. For each sample, the equilibrium pH value was lower than the initial pH value, which indicates that large quantities of hydrogen ions were produced by Fe 3+ hydrolysis and reduced the equilibrium pH [58]. Figure 5 shows the equilibrium time for PO 4 3− removal by IL-PP, which was studied using two different doses of IL-PP (100 and 150 mg) and fixed values for the PO 4 -P concentration (40 mg·L −1 ), pH (9), and temperature (25 • C). Within the first 2 min, rapid PO 4 3− uptake took place with removal rates of 51% and 76.5% for 100 and 150 mg, respectively, of IL-PP. This fast uptake is due to the presence of a large number of active sites to which a large amount of PO 4 3− anions could attach. Afterward, due to the saturation of available active sites, the removal rate decreased and equilibrium approached [13]. The equilibrium state for PO 4 3− removal was reached within 60 min. Removal rates of 64.25% and 90% were achieved with 100 and 150 mg, respectively, of IL-PP.   Figure 5 shows the equilibrium time for PO4 3− removal by IL-PP, which was studied using two different doses of IL-PP (100 and 150 mg) and fixed values for the PO4-P concentration (40 mg·L −1 ), pH (9), and temperature (25 °C). Within the first 2 min, rapid PO4 3− uptake took place with removal rates of 51% and 76.5% for 100 and 150 mg, respectively, of IL-PP. This fast uptake is due to the presence of a large number of active sites to which a large amount of PO4 3− anions could attach. Afterward, due to the saturation of available active sites, the removal rate decreased and equilibrium approached [13]. The equilibrium state for PO4 3− removal was reached within 60 min. Removal rates of 64.25% and 90% were achieved with 100 and 150 mg, respectively, of IL-PP.     Figure 5 shows the equilibrium time for PO4 3− removal by IL-PP, which was studied using two different doses of IL-PP (100 and 150 mg) and fixed values for the PO4-P concentration (40 mg·L −1 ), pH (9), and temperature (25 °C). Within the first 2 min, rapid PO4 3− uptake took place with removal rates of 51% and 76.5% for 100 and 150 mg, respectively, of IL-PP. This fast uptake is due to the presence of a large number of active sites to which a large amount of PO4 3− anions could attach. Afterward, due to the saturation of available active sites, the removal rate decreased and equilibrium approached [13]. The equilibrium state for PO4 3− removal was reached within 60 min. Removal rates of 64.25% and 90% were achieved with 100 and 150 mg, respectively, of IL-PP.

Factorial Design
The factorial design methodology was used to determine the importance of the three factors (pH, adsorbent dose, and temperature) and their interactions on PO 4 3− removal by IL-PP. Factorial design plots such as plots for the main effects and interactions, Pareto chart, and normal plot for the standardized effects describe the interactive relation between the factors and their levels [59]. This technique investigates all possible combinations and verifies the accuracy of the obtained mathematical model through the analysis of variance (ANOVA) to achieve optimum removal of PO 4 3− . Table 3 and Figure 6 present the results

Factorial Design
The factorial design methodology was used to determine the importance of the three factors (pH, adsorbent dose, and temperature) and their interactions on PO4 3− removal by IL-PP. Factorial design plots such as plots for the main effects and interactions, Pareto chart, and normal plot for the standardized effects describe the interactive relation between the factors and their levels [59]. This technique investigates all possible combinations and verifies the accuracy of the obtained mathematical model through the analysis of variance (ANOVA) to achieve optimum removal of PO4 3− . Table 3 and Figure 6 present the results of the factorial design experiments and average values for the response variable (PO4 3− removal rate) based on the high and low levels of the studied parameters.   Table 4 presents the main and interaction effects, model coefficients, standard deviation of each coefficient, standard errors, Fisher test value (F-value), and probability value (p-value). All of the main effects (pH, adsorbent dosage, temperature, and two-and three-way interactions) were significant at a 5% probability level (p < 0.05). Furthermore, the adjusted square correlation coefficient R 2 (adj) had a value of 99.99%, which indicates that the presented model perfectly fit the statistical model [44].  Table 4 presents the main and interaction effects, model coefficients, standard deviation of each coefficient, standard errors, Fisher test value (F-value), and probability value (pvalue). All of the main effects (pH, adsorbent dosage, temperature, and two-and three-way interactions) were significant at a 5% probability level (p < 0.05). Furthermore, the adjusted square correlation coefficient R 2 (adj) had a value of 99.99%, which indicates that the presented model perfectly fit the statistical model [44]. where A is the pH, B is the adsorbent dose, and C is the temperature; AB, AC, and BC represent the two-way interactions; and ABC represents the three-way interaction. Equation (7) describes how the experimental parameters and their interactions influence the response variable and thus can be used to predict responses for given levels of each parameter [60]. Positive values in the equation indicate that the PO 4 3− removal increases when this effect increases. By contrast, negative values indicate that the removal rate decreases when this effect increases [59]. An analysis of variance was performed to investigate the significance of parameters affecting PO 4 3− removal to ensure the accuracy of the model. Table 5 presents the sum of the squares used to estimate the effect of factors, the F-ratio (i.e., the ratio of individual mean square effects to the mean square error) and the p-value (i.e., the level of significance leading to the rejection of the null hypothesis). The results showed that the main effects of each factor, their two-way interactions, and the three-way interaction were statistically significant at p < 0.05.  Figure 7 shows the main effects of each parameter on PO 4 3− removal by IL-PP by giving the deviations between high and low levels of each parameter, which can help with identifying which parameters affect the response variable the most. A larger deviation is synonymous with a large effect [61]. The adsorbent dose appears to have the greatest effect on PO 4 3− removal by IL-PP, which is followed by pH and then temperature, which had an almost negligible effect. Figure 8 plots the interactions of the studied parameters. If the interaction lines are not parallel, this implies that the interaction has a strong effect, whereas parallel interaction lines indicate a weak effect [62]. The most important interaction for PO 4 3− removal by IL-PP appears to be pH*adsorbent dose, which is followed by adsorbent dose*temperature. The least important interaction was pH*temperature, which had almost parallel interaction lines.
which had an almost negligible effect. Figure 8 plots the interactions of the studied parameters. If the interaction lines are not parallel, this implies that the interaction has a strong effect, whereas parallel interaction lines indicate a weak effect [62]. The most important interaction for PO4 3− removal by IL-PP appears to be pH*adsorbent dose, which is followed by adsorbent dose*temperature. The least important interaction was pH*temperature, which had almost parallel interaction lines.   which had an almost negligible effect. Figure 8 plots the interactions of the studied parameters. If the interaction lines are not parallel, this implies that the interaction has a strong effect, whereas parallel interaction lines indicate a weak effect [62]. The most important interaction for PO4 3− removal by IL-PP appears to be pH*adsorbent dose, which is followed by adsorbent dose*temperature. The least important interaction was pH*temperature, which had almost parallel interaction lines.   A Pareto chart is helpful for observing the relative importance of the main effects of factors and their interactions. This chart can be used to evaluate the significance of effects on the basis of how much they exceed the reference line [63]. Figure 9 shows that all parameters and their interactions had a significant effect because their values exceeded that of the reference line (2.1, in red).
A Pareto chart is helpful for observing the relative importance of the main effects of factors and their interactions. This chart can be used to evaluate the significance of effects on the basis of how much they exceed the reference line [63]. Figure 9 shows that all parameters and their interactions had a significant effect because their values exceeded that of the reference line (2.1, in red).  Figure 10 shows a normal plot of the standardized effects, which was used to identify the "real" effects. Each point on this plot was attributed to an effect. Points far from the reference line likely represent the greatest effect and vice versa [63]. The adsorbent dose (B) had the greatest effect since its point was farthest from the reference line (in red),which followed by pH (A) and their interaction (AB). The adsorbent dose (B) and pH (A) had positive effects because their points are on the right side of the line, whereas their interaction (AB) had a negative effect because it is on the left side [44]. The significance of the effects of the parameters and their interactions can be ordered as follows: B > A > AB > C > BC > AC > ABC.  Figure 10 shows a normal plot of the standardized effects, which was used to identify the "real" effects. Each point on this plot was attributed to an effect. Points far from the reference line likely represent the greatest effect and vice versa [63]. The adsorbent dose (B) had the greatest effect since its point was farthest from the reference line (in red),which followed by pH (A) and their interaction (AB). The adsorbent dose (B) and pH (A) had positive effects because their points are on the right side of the line, whereas their interaction (AB) had a negative effect because it is on the left side [44]. The significance of the effects of the parameters and their interactions can be ordered as follows: B > A > AB > C > BC > AC > ABC.

Kinetics
Adsorption kinetics represents the progress of the adsorption process over time. Determining the adsorption kinetics helps with identifying the governing mass transfer mechanism and the characteristic mass transfer parameters [55]. To identify the mecha-

Kinetics
Adsorption kinetics represents the progress of the adsorption process over time. Determining the adsorption kinetics helps with identifying the governing mass transfer mechanism and the characteristic mass transfer parameters [55]. To identify the mechanisms and potential rate-controlling step for PO 4 3− adsorption by IL-PP, four kinetic models were examined: the pseudo-first-order, pseudo-second-order, Elovich equation, and intraparticle diffusion models. Equations (8)-(11) respectively present the nonlinear forms of these models: q t = q e 1 − e −k 1 t (8) q t = q e 2 k 2 t 1 + k 2 q e t (9) where q e and q t are the amounts of PO 4 -P adsorbed at equilibrium and at time t, respectively. k 1 (L·min −1 ), k 2 (g·mg −1 ·min −1 ), α (mg·g −1 ·min −1 ), and k 3 (mg·g −1 ·min −1 ) are constants of the pseudo-first-order, pseudo-second-order, Elovich equation, and intraparticle diffusion models, respectively. β (mg·g −1 ) is the desorption constant during any one experiment, and C is a constant describing the thickness of the boundary layer. Table 6 gives the adsorption constant of each model as well as the calculated and experimental values of q e (q e,cal and q e,exp , respectively), R 2 , and χ 2 . On the basis of the R 2 and χ 2 values and comparison between q e,cal and q e,exp , the PO 4 3− adsorption by IL-PP is best described by the Elovich equation (R 2 = 0.97, χ 2 = 0.007, q e,cal = 12.11). This kinetic model assumes that the process is controlled by chemisorption and suggests that the adsorbent surface is heterogeneous [64]. The Elovich kinetic model was also postulated by a similar study investigating PO 4 3− adsorption on iron hydroxideeggshell waste [65]. Figure 11 illustrates the experimental kinetics and Elovich fitting model for PO 4 3− adsorption by IL-PP.
On the basis of the R 2 and χ 2 values and comparison between qe,cal and qe,exp, the PO4 3− adsorption by IL-PP is best described by the Elovich equation (R 2 = 0.97, χ 2 = 0.007, qe,cal = 12.11). This kinetic model assumes that the process is controlled by chemisorption and suggests that the adsorbent surface is heterogeneous [64]. The Elovich kinetic model was also postulated by a similar study investigating PO4 3− adsorption on iron hydroxideeggshell waste [65]. Figure 11 illustrates the experimental kinetics and Elovich fitting model for PO4 3− adsorption by IL-PP. Figure 11. Experimental kinetics and Elovich fitting model for PO4 3− adsorption by IL-PP.

Isotherm
The isotherm is a graph relating qe to Ce at a constant temperature. Determining the adsorption isotherm helps to describe the adsorbent-adsorbate interaction and thus is indispensable for optimizing the adsorption mechanism pathways, expressing the adsorbent surface properties and capacity, and effectively designing the adsorption system [66]. The Langmuir and Freundlich models were tested to select the isotherm model that adequately describes PO4 3− adsorption by IL-PP. The nonlinear forms of these models are presented in Equations (12) and (13), respectively:

Isotherm
The isotherm is a graph relating q e to C e at a constant temperature. Determining the adsorption isotherm helps to describe the adsorbent-adsorbate interaction and thus is indispensable for optimizing the adsorption mechanism pathways, expressing the adsorbent surface properties and capacity, and effectively designing the adsorption system [66]. The Langmuir and Freundlich models were tested to select the isotherm model that adequately describes PO 4 3− adsorption by IL-PP. The nonlinear forms of these models are presented in Equations (12) and (13), respectively: q e = q max K L C e 1 + K L q e (12) q e = K F C e 1/n (13) where q e (mg·g −1 ) is the amount of PO 4 -P adsorbed at equilibrium and C e (mg·L −1 ) is the PO 4 -P concentration in the liquid phase at equilibrium. K L (L·mg −1 ) and q max (mg·g −1 ) are constants of the Langmuir isotherm and indicate the adsorption energy and adsorption density, respectively. K F and n (dimensionless) are constants of the Freundlich isotherm and indicate the total adsorption capacity and adsorption intensity, respectively. The dimensionless constant R L presents the separation factor and can be calculated using Equation (14): where K L is the Langmuir equilibrium constant and C i is the initial PO 4 -P concentration.
Similar to the kinetic model, the best fitting isotherm model was selected on the basis of the values of R 2 and χ 2 . Table 7 indicates that the PO 4 3− adsorption by IL-PP can be described by both the Langmuir (R 2 = 0.98, χ 2 = 0.78) and Freundlich (R 2 = 0.94, χ 2 = 2.62) isotherms, but the former fits better. The Langmuir model assumes that adsorption occurs on a homogenous surface through monolayer coverage. Conversely, the Freundlich model assumes that adsorption occurs on a heterogeneous surface through multilayer coverage and that the adsorbed amount increases with the equilibrium concentration [67]. The suitability of both Langmuir and Freundlich models for describing PO 4 3− adsorption by IL-PP suggests that active sites are homogeneously and heterogeneously distributed on the IL-PP surface, so more than one mechanism is involved in the adsorption process [68]. The Langmuir separation factor (R L = 0.21) is between 0 and 1, and the Freundlich adsorption affinity constant (n = 2.04) is between 1 and 10, which indicates favorable PO 4 3− adsorption by IL-PP [58]. Figure 12 shows the experimental isotherm and fitted Langmuir and Freundlich models for PO 4 3− adsorption by IL-PP.
Similar to the kinetic model, the best fitting isotherm model was selected on the basis of the values of R 2 and χ 2 . Table 7 indicates that the PO4 3− adsorption by IL-PP can be described by both the Langmuir (R 2 = 0.98, χ 2 = 0.78) and Freundlich (R 2 = 0.94, χ 2 = 2.62) isotherms, but the former fits better. The Langmuir model assumes that adsorption occurs on a homogenous surface through monolayer coverage. Conversely, the Freundlich model assumes that adsorption occurs on a heterogeneous surface through multilayer coverage and that the adsorbed amount increases with the equilibrium concentration [67]. The suitability of both Langmuir and Freundlich models for describing PO4 3− adsorption by IL-PP suggests that active sites are homogeneously and heterogeneously distributed on the IL-PP surface, so more than one mechanism is involved in the adsorption process [68]. The Langmuir separation factor (RL = 0.21) is between 0 and 1, and the Freundlich adsorption affinity constant (n = 2.04) is between 1 and 10, which indicates favorable PO4 3− adsorption by IL-PP [58]. Figure 12 shows the experimental isotherm and fitted Langmuir and Freundlich models for PO4 3− adsorption by IL-PP.

Thermodynamics
The thermodynamics was studied to determine whether the adsorption process is favorable, spontaneous, exothermic, or endothermic [69]. The change in the Gibbs free energy ∆G was calculated using Equation (5), while ∆H • and ∆S • were calculated from the slope and intercept of the plot of ln K d versus 1/T using Equation (6), as shown in Figure 13. adsorption by IL-PP was spontaneous and favorable. As the temperature was increased, the process became more spontaneous [70]. The positive ∆H • value (4044.59 J·mol −1 ) indicates that PO 4 3− adsorption by IL-PP is endothermic in nature [71]. The positive ∆S • value (80.04 J·K −1 ·mol −1 ) indicates increased randomness at the solid-solution interface and ion replacement during the adsorption process [72].

Thermodynamics
The thermodynamics was studied to determine whether the adsorption process is favorable, spontaneous, exothermic, or endothermic [69]. The change in the Gibbs free energy ∆G was calculated using Equation (5), while ∆H° and ∆S° were calculated from the slope and intercept of the plot of ln Kd versus 1/T using Equation (6), as shown in Figure 13.  J·mol −1 when the temperature was increased from 298 to 328 K, which indicates that PO4 3− adsorption by IL-PP was spontaneous and favorable. As the temperature was increased, the process became more spontaneous [70]. The positive ∆H° value (4044.59 J·mol −1 ) indicates that PO4 3− adsorption by IL-PP is endothermic in nature [71]. The positive ∆S° value (80.04 J·K −1 ·mol −1 ) indicates increased randomness at the solid-solution interface and ion replacement during the adsorption process [72].  Table 9 presents the maximum phosphate adsorption capacity of IL-PP and the most relevant iron-loaded bio-adsorbents. Generally, comparing the performance of bio-adsorbents is complicated because it should take in consideration the adsorption method followed (batch or fixed bed) and working parameters (pH, initial adsorbate concentration, contact time, temperature, adsorbent dose, interfering ions, etc。) [73]. Moreover, a cost-benefit analysis investigating the cost-effectivity, availability and the possibility of reuse is critical for a significant comparison.    Table 9 presents the maximum phosphate adsorption capacity of IL-PP and the most relevant iron-loaded bio-adsorbents. Generally, comparing the performance of bioadsorbents is complicated because it should take in consideration the adsorption method followed (batch or fixed bed) and working parameters (pH, initial adsorbate concentration, contact time, temperature, adsorbent dose, interfering ions, etc.) [73]. Moreover, a costbenefit analysis investigating the cost-effectivity, availability and the possibility of reuse is critical for a significant comparison. Table 9. Basic comparison of IL-PP with most relevant iron-loaded bio-adsorbents.

Conclusions
This study evaluated the efficiency of IL-PP at removing PO 4 3− from an aqueous solution. The results indicated that IL-PP is an efficient bio-adsorbent that can be optimized as a green technology for wastewater treatment, waste biomass management, and phosphate recovery. However, a more detailed study on the performance of IL-PP at removing PO 4 3− from real wastewater under real operating conditions is required to check the effect of interfering ions. The successful regeneration and application of phosphate-loaded IL-PP as a fertilizer must also be investigated to make this approach more sustainable and attractive especially in regions known by the huge cultivation of pomegranate fruit.