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

Abstract

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 (FeCl₃). 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 (R² = 0.97), which assumes the dominance of the chemisorption mechanism, whereas the isotherm obeys both Langmuir (R² = 0.98) and Freundlich (R² = 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.

1. 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 them, adsorption technologies are the most advantageous because of their simplicity, cost-effectiveness, and wide availability of existing and potential adsorbents that can potentially be directly applied to soil as fertilizer when loaded with phosphate [3,7].

Expanding agricultural activities in addition to irresponsible food production and consumption has resulted in more than 1.3 billion tons of agricultural and food waste deposited annually according to the Food and Agriculture Organization of the United Nations (FAO), which has had several environmental, financial, and social implications [8,9]. Although reducing this waste is one of the Sustainable Development Goals for 2030, efforts remain insufficient because more awareness and radical changes are needed in consumer attitudes and economic policies of countries. Different approaches have been considered for the revalorization of this biomass waste to gain environmental, financial, and social benefits, such as livestock feed, bioenergy production, bioactive compound recovery, food industry, and soil fertilization [10,11]. One approach has been to convert biomass waste into bio-adsorbents for pollutant removal and recovery [12,13,14,15,16]. However, the biomass waste must be activated to develop desirable physicochemical properties for phosphate uptake [17].

Pomegranate peel (PP) is a widely abundant biomass waste because of the huge production of the pomegranate fruit. Approximately 1.5 million tons of PP is estimated to be produced per year [16], which is a major environmental concern for producer countries (e.g., India, Iran, Turkey, the United States, China, Spain, and Morocco) [18,19]. To mitigate this problem, various revalorization methods have been introduced, such as the production of valuable compounds for essential oils [20,21], food additives [22], and medicinal products [23], as well as energy and value-added products such as bioethanol and biogas [24,25]. PP is increasingly being used as a bio-adsorbent for heavy metals, dyes, and other contaminants [9,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. The present study investigated the efficiency of activated PP at phosphate removal from an aqueous solution.

2. Materials and Methods

2.1. The Stock Solution

The stock solution of phosphate (1000 mg·L⁻¹) was prepared by dissolving Na₂HPO₄·2H₂O 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.

2.2. Preparation and Activation of PP

PP was activated using an iron loading method similar to that of Nguyen et al. [43] for improving the PO₄³⁻ 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 (Fe³⁺) on its surface. The second step was the iron loading, where the saponified PP was stirred with 500 mL of an iron chloride (FeCl₃) 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.

2.3. 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 (Na₂HPO₄) 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 (PO₄³⁻) 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.

2.4. Batch Adsorption

The batch adsorption method was used to determine the equilibrium time and effect of pH on PO₄³⁻ adsorption by IL-PP. For this purpose, 50 mL of Na₂HPO₄ solution (PO₄-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 PO₄³⁻ by IL-PP, 2³ 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 PO₄-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. Levels of parameters used in the factorial design for PO₄³⁻ removal by IL-PP.

Parameter	Coded Symbol	Low Level (−1)	High Level (+1)
pH	A	3	9
Adsorbent dose (mg)	B	100	150
Temperature (°C)	C	25	45

.

PO₄-P concentration was determined by Spectrophotometer Spectroquant Nova 60 (Merck, Germany) after filtering samples through 0.45 µm microporous membrane filters.

The PO₄³⁻ removal rate was calculated using Equation (1):

$$\mathrm{Removal}\%=\frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{f}}}{{\mathrm{C}}_{\mathrm{i}}}·100$$

(1)

where ${\mathrm{C}}_{\mathrm{i}}$ (mg·L⁻¹) and ${\mathrm{C}}_{\mathrm{f}}$ (mg·L⁻¹) are the initial and final PO₄-P concentrations, respectively.

The adsorbed amount of PO₄-P was calculated using Equation (2):

$${\mathrm{q}}_{\mathrm{e}}\left(\mathrm{mg}·{\mathrm{g}}^{-1}\right)=\left({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}\right)\frac{\mathrm{V}}{\mathrm{M}}$$

(2)

where C_i (mg·L⁻¹) and C_e (mg·L⁻¹) are the initial and equilibrium concentrations, respectively, of PO₄-P in the solution; V (L) is the solution volume; and M (g) is the mass of the adsorbent.

The isotherm of PO₄³⁻ 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 PO₄-P concentration. Factorial design experiments were performed to identify and optimize the adsorption kinetics. To determine the isotherm and kinetic models that adequately describe PO₄³⁻ adsorption by IL-PP, isotherm and kinetics data were fitted to existing mathematical models by a nonlinear method using the Solver add-in command in Microsoft Excel [45]. The best fitting kinetic and isotherm models were selected mainly based on the value of the nonlinear correlation coefficient (R²). However, the chi-square (χ²) statistics helped confirm this selection. A value of χ² close to 0 meant that the selected model fit the experimental data well, whereas a high value of χ² indicated that the model was inappropriate [46]. R² and χ² were calculated using Equations (3) and (4), respectively:

$${\mathrm{R}}^2=\frac{\sum \left({\mathrm{q}}_{\mathrm{e},\mathrm{cal}}-{\mathrm{q}}_{\mathrm{e},\mathrm{mean}}\right)²}{\sum {\left({\mathrm{q}}_{\mathrm{e},\mathrm{cal}}-{\mathrm{q}}_{\mathrm{e},\mathrm{mean}}\right)}^2+\sum \left({\mathrm{q}}_{\mathrm{e},\mathrm{cal}}-{\mathrm{q}}_{\mathrm{e},\exp }\right)²}$$

(3)

$${\mathsf{\chi }}^2=\sum \frac{\left({\mathrm{q}}_{\mathrm{e},\exp }-{\mathrm{q}}_{\mathrm{e},\mathrm{cal}}\right)²}{{\mathrm{q}}_{\mathrm{e},\mathrm{cal}}}$$

(4)

where q_e,exp (mg·g⁻¹) is the amount of PO₄-P uptake at equilibrium obtained from Equation (2), q_e,cal (mg·g⁻¹) is the amount of PO₄-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 PO₄³⁻ 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):

$$\Delta \mathrm{G}=-{\mathrm{R}\mathrm{T}\mathrm{lnK}}_{\mathrm{d}}$$

(5)

$${\mathrm{LnK}}_{\mathrm{d}}=\frac{\Delta \mathrm{H}°}{\mathrm{R}\mathrm{T}}+\frac{\Delta \mathrm{S}°}{\mathrm{R}}$$

(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].

3. Results and Discussion

3.1. Characterization Results

3.1.1. 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 Fe³⁺. The zeta potential of IL-PP in the Na₂HPO₄ 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 Fe³⁺) by PO₄³⁻. However, in the NaCl solution, Cl⁻ could not neutralize the IL-PP surface, so the IL-PP surface had a high affinity toward PO₄³⁻ 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 PO₄³⁻ anions to attach to this layer could be the reason for this behavior [49].

3.1.2. SEM Results

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 Fe³⁺ was incorporated. This modification of the morphology made the surface irregular and thus more suitable for PO₄³⁻ uptake [50].

3.1.3. FTIR Analysis

Figure 3 shows the FTIR spectra of PP and IL-PP before and after PO₄³⁻ adsorption.

Table 2. FTIR analysis of PP and IL-PP before and after PO₄³⁻ adsorption.

Adsorption Band (cm−1)			Assignment
PP	IL-PP	IL-PP after PO43− Adsorption
3323	3306	3327	–OH and N–H
2931	2918	2924	C–H, –CH3, or –CH2
1719	–	–	C=O and C–C
1615	1617	–	C=C, C=O, or N–H
–	–	1601	Fe-P
1442	–	–	–OH
1320	1313	1318	C–H, –CH3, or –CH2
1223	–	–	O–H
1031	1030	–	C-O and C-O–C
876	–	–	O–H, C=O, and O–H
–	801	–	Fe–OH
747	–	–	C–N

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 Fe³⁺ onto the surface of PP and for the adsorption of PO₄³⁻ 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 Fe³⁺ on the PP surface. The IL-PP spectra after PO₄³⁻ 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 PO₄³⁻ adsorption by IL-PP [54].

3.2. Batch Adsorption Results

3.2.1. 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, PO₄³⁻ can exist in four species: H₃PO₄ (pH~2.15), H₂PO₄⁻ (2.15 < pH < 7.20), HPO₄²⁻ (7.20 < pH < 12.33), and PO₄³⁻ (pH~12.33) [56]. Figure 4 shows that PO₄³⁻ 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 Fe³⁺, which is monodentate/mononuclear. However, there are three different possible interactions between HPO₄²⁻ and Fe³⁺: monodentate/mononuclear, bidentate/mononuclear, and monodentate/binuclear [57]. This led to the high PO₄³⁻ 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 PO₄³⁻ 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 Fe³⁺ hydrolysis and reduced the equilibrium pH [58].

3.2.2. Determination of the Equilibrium Time

Figure 5 shows the equilibrium time for PO₄³⁻ 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 PO₄³⁻ 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 PO₄³⁻ anions could attach. Afterward, due to the saturation of available active sites, the removal rate decreased and equilibrium approached [13]. The equilibrium state for PO₄³⁻ removal was reached within 60 min. Removal rates of 64.25% and 90% were achieved with 100 and 150 mg, respectively, of IL-PP.

3.2.3. 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 PO₄³⁻ 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 PO₄³⁻.

Table 3. Design matrix and results of the 2³ factorial design for PO₄³⁻ removal by IL-PP.

Run	pH (A)	Adsorbent Dose (B)	Temperature (°C)	Removal Rate (%)	Standard Deviation
1	1	−1	−1	61.08	2.74
2	−1	1	1	87.5	1.32
3	1	1	−1	90	2.5
4	−1	1	−1	81.66	2.93
5	−1	−1	−1	41.16	3.35
6	1	1	1	90.75	1.75
7	1	−1	1	68.25	2.61
8	−1	−1	1	49.16	2.56

and Figure 6 present the results of the factorial design experiments and average values for the response variable (PO₄³⁻ removal rate) based on the high and low levels of the studied parameters.

Table 4. Estimated effects and coefficients for PO₄³⁻ removal by IL-PP.

Term	Effect	Coef	SE Coef	T-Value	p-Value	VIF
Constant		71.1979	0.0276	2583.39	0.000
pH	12.6458	6.3229	0.0276	229.42	0.000	1.00
Adsorbent dose	32.5625	16.2813	0.0276	229.42	0.000	1.00
Temperature	5.4375	2.7188	0.0276	590.76	0.000	1.00
pH × Adsorbent dose	−6.8542	−3.4271	0.0276	−124.35	0.000	1.00
pH × Temperature	−1.4792	−0.7396	0.0276	−26.84	0.000	1.00
Adsorbent dose × Temperature	−2.1458	−1.0729	0.0276	−38.93	0.000	1.00
pH × Adsorbent dose × Temperature	−1.0625	−0.5312	0.0276	−19.28	0.000	1.00
S	0.135015
R2	100.00%
R2 (Adj)	99.99%
R2 (Pred)	99.99%

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 R² (adj) had a value of 99.99%, which indicates that the presented model perfectly fit the statistical model [44].

Model for PO₄³⁻ removal by IL-PP can be expressed using Equation (7):

$$\begin{array}{cc}\hfill {\mathrm{PO}}_4{}^{-3}\mathrm{removal}\%& =-72.00+5.583\mathrm{A}+0.92688\mathrm{B}+0.4250\mathrm{C}-0.0209\mathrm{A}·\mathrm{B}+0.06389\mathrm{A}·\mathrm{C}-0.000042\mathrm{B}·\mathrm{C}\hfill \\ & -0.000708\mathrm{A}·\mathrm{B}·\mathrm{C}\hfill \end{array}$$

(7)

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 PO₄³⁻ 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 PO₄³⁻ removal to ensure the accuracy of the model.

Table 5. Analysis of variance for PO₄³⁻ removal by IL-PP.

Source	DF	Adj SS	Adj MS	F-Value	p-Value
Model	7	7828.21	1118.32	61,347.57	0.000
Linear	3	7498.80	2499.60	137,120.90	0.000
pH	1	959.50	959.50	52,635.57	0.000
Adsorbent dose	1	6361.90	6361.90	348,995.57	0.000
Temperature	1	177.40	177.40	9731.57	0.000
Two-way interactions	3	322.63	107.54	5899.57	0.000
pH × Adsorbent dose	1	281.88	281.88	15,463.00	0.000
pH × temperature	1	13.13	13.13	720.14	0.000
Adsorbent dose × temperature	1	27.63	27.63	1515.57	0.000
Three-way interaction	1	6.77	6.77	371.57	0.000
pH × adsorbent dose × temperature	1	6.77	6.77	371.57	0.000
Error	16	0.29	0.02
Total	23	7828.50

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 PO₄³⁻ 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 PO₄³⁻ 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 PO₄³⁻ 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).

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.

3.3. Process Modeling

3.3.1. 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 PO₄³⁻ 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:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}}\right)$$

(8)

$${\mathrm{q}}_{\mathrm{t}}=\frac{{\mathrm{q}}_{\mathrm{e}}²{\mathrm{k}}_2\mathrm{t}}{1+{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}\mathrm{t}}$$

(9)

$${\mathrm{q}}_{\mathrm{t}}=\frac{1}{\mathsf{\beta }}\ln \left(1+\mathsf{\alpha }\mathsf{\beta }\mathrm{t}\right)$$

(10)

$${\mathrm{q}}_{\mathrm{t}}=\mathrm{k}₃\surd \mathrm{t}+\mathrm{C}$$

(11)

where q_e and q_t are the amounts of PO₄-P adsorbed at equilibrium and at time t, respectively. k₁ (L·min⁻¹), k₂ (g·mg⁻¹·min⁻¹), α (mg·g⁻¹·min⁻¹), and k₃ (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. Kinetic models and parameters of PO₄³⁻ adsorption by IL-PP.

Kinetic Model	qe,cal (mg·g−1)	Parameters	R2	χ2	qe,exp (mg·g−1)
Pseudo-first-order	11.44	k1 = 1.01 (L·min−1)	0.39	0.22	12
Pseudo-second-order	11.85	k2 = 0.19 (g·mg−1·min−1)	0.83	0.05
Elovich equation	12.11	α (×106) = 11.88 (mg·g−1·min−1) β = 1.72 (mg·g−1)	0.97	0.007
Intraparticle diffusion	12.22	k3 = 0.28 (mg·g−1·min−1) C = 10.04	0.91	0.03

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), R², and χ².

On the basis of the R² and χ² values and comparison between q_e,cal and q_e,exp, the PO₄³⁻ adsorption by IL-PP is best described by the Elovich equation (R² = 0.97, χ² = 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 PO₄³⁻ adsorption on iron hydroxide–eggshell waste [65]. Figure 11 illustrates the experimental kinetics and Elovich fitting model for PO₄³⁻ adsorption by IL-PP.

3.3.2. 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 PO₄³⁻ adsorption by IL-PP. The nonlinear forms of these models are presented in Equations (12) and (13), respectively:

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{\max }{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{q}}_{\mathrm{e}}}$$

(12)

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}{}^{1/\mathrm{n}}$$

(13)

where q_e (mg·g⁻¹) is the amount of PO₄-P adsorbed at equilibrium and C_e (mg·L⁻¹) is the PO₄-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):

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{c}}_{\mathrm{i}}}$$

(14)

where K_L is the Langmuir equilibrium constant and C_i is the initial PO₄-P concentration. Similar to the kinetic model, the best fitting isotherm model was selected on the basis of the values of R² and χ².

Table 7. Isotherm models and parameters of PO₄³⁻ adsorption by IL-PP.

Isotherm Model	Parameters	R2	χ2
Langmuir	KL = 0.09 (L·mg−1) qmax = 49.12 (mg·g−1) RL = 0.21	0.98	0.78
Freundlich	KF = 6.88 1/n = 0.49	0.94	2.62

indicates that the PO₄³⁻ adsorption by IL-PP can be described by both the Langmuir (R² = 0.98, χ² = 0.78) and Freundlich (R² = 0.94, χ² = 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 PO₄³⁻ 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 PO₄³⁻ adsorption by IL-PP [58]. Figure 12 shows the experimental isotherm and fitted Langmuir and Freundlich models for PO₄³⁻ adsorption by IL-PP.

3.3.3. 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.

Table 8. Thermodynamic parameters of PO₄³⁻ adsorption by IL-PP.

Temperature (K)	∆G (J·mol−1)	∆H (J·mol−1)	∆S (J·K−1·mol−1)
298	−19,836.35	4044.59	80.04
308	−20,573.93
318	−21,395.71
328	−22,235.05

indicates that the value of ∆G° decreased from −19,836.35 to −22,235.05 J·mol⁻¹ when the temperature was increased from 298 to 328 K, which indicates that PO₄³⁻ 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 PO₄³⁻ 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].

3.3.4. Comparison of IL-PP with Other Iron-Loaded Bio-Adsorbents

Table 9. Basic comparison of IL-PP with most relevant iron-loaded bio-adsorbents.

Bio-Adsorbent	qmax (mg PO4-P/g)	Reference
Fe(III)-loaded orange waste gel	42.72	[74]
Fe(II)-loaded wood particles	17.38	[75]
Fe(II)-loaded sugarcane bagasse	152	[54]
Fe(III)-loaded okara (ILO)	16.66	[43]
Fe(III)-loaded litchi seed waste	100	[76]
Fe(III) impregnated coir pith	70.92	[77]
IL-PP	49.12	This study

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.

4. Conclusions

This study evaluated the efficiency of IL-PP at removing PO₄³⁻ 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 PO₄³⁻ 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.