Adsorption of Cr(VI) by Mesoporous Pomegranate Peel Biowaste from Synthetic Wastewater under Dynamic Mode

Abstract

This study aims to eliminate hexavalent chromium Cr(VI) ions from water using pomegranate peel (PGP) powder. Dynamic measurements are carried out to examine the influence of the operating factors on the adsorption efficiency and kinetics. The analyzed PGP is found to be amorphous with relatively high stability, contains hydroxyl and carboxyl functional groups, a pH of zero charge of 3.9, and a specific surface-area of 40.38 m²/g. Adsorption tests indicate that PGP exhibits excellent removal effectiveness for Cr(VI) reaching 50.32 mg/g while the adsorption process obeys the Freundlich model. The thermodynamic study favors the exothermic physical adsorption process. The influence of operating parameters like the flow rate (1 to 3 mL/min), bed height (25 to 75 mm), concentration (10 to 30 mg/L), and temperature (298 to 318 K) on the adsorption process are investigated in column mode. To assess the performance characteristics of the column adsorption data, a non-linear regression has been used to fit and analyze four different kinetic and theoretical models, namely, Bohart-Adams, Thomas model, Clark, and Dose response. The obtained experimental results were found to obey the Dose Response model with a coefficient of regression R² greater than 0.977. This study proved the excellent efficiency in the treatment of chemical industry effluents by using cost-effect abundant biowaste sorbent. This research demonstrated great efficacy in the treatment of chemical industrial effluents by using an abundant, cost-effective biowaste sorbent, thereby achieving the UN SDGs (UN Sustainable Development Goals) primary objective.

1. Introduction

The contamination of wastewater by chromium Cr(VI) is a major problem due to its high toxicity and harmful effects on humans and animals [1]. The major effluents containing such toxic and hazardous element arises from industrial processes such as metallurgy, dyeing, and finishing industries [2]. Chromium exists in two major forms: Cr(III) and Cr(VI). The first form has low solubility and low reactivity resulting in low mobility in the environment and low toxicity in living organisms. However, Cr(VI) is a highly toxic [3] and carcinogenic [4] heavy metal when its concentration exceeds the standard limit fixed at 0.05 mg/L [5]. Its compounds include chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻) and chromic acid (H₂CrO₄). CrO₄²⁻ is predominant in basic solutions, H₂CrO₄ is predominant at pH < 1, while HCrO₄²⁻, and Cr₂O₇²⁻ are predominant at pH 2–6 [6].

Extensive studies were devoted toward the development of new cost-effective technologies for the removal of Cr(VI) metal ions from aquatic environments, like precipitation [7], ion exchange [8], electro-coagulation [9], and adsorption [10,11,12,13,14]. Nevertheless, most of the technique’s present drawbacks and disadvantages, primarily high energy consumption and cost. In contrast, adsorption is an eco-friendly, cost-effective, and widely used technique for heavy metals remediation from aqueous solutions, which meets the UN SDGs (UN Sustainable Development Goals) main aim [15,16,17].

In recent decades, green chemistry has emerged as a potential fabrication route and attracted considerable attention in scientific research. Specifically, fruit wastes are being investigated as potential natural sorbents for heavy metal ions, thanks to their biodegradability and cost-effectiveness, besides the presence of functional groups on adsorbent particles’ surface that are responsible for the reduction and elimination of contaminants and pollutants from aqueous media within a ‘sustainable development’ framework [18,19]. Valency, concentration, the ionic nature of substances in solution, pH of solution, and binding groups of sorbent material are key sorption parameters. The removal mechanism relies on the sorbate molecule’s size, concentration, affinity for the sorbent, bulk diffusion coefficient, and pore size distribution [20,21]. Sorbent’s physicochemical characteristics and sorption capacity determine the sorption’s efficacy.

Pomegranates (Punica granatum) are widely cultivated in north/tropical Africa, and the Middle East, the Mediterranean Basin, the Caucasus region, the Indian subcontinent, Central Asia, and the drier parts of Southeast Asia [22].

Punica granatum presents a great variety of therapeutic bioactive species such as ellagitannins, proanthocyanidin compounds, phenolics, and flavonoids [23], which are well-known to facilitate the adsorption process of metal ions. Several studies have been conducted to highlight the biological and functional characteristics of pomegranate peel as it contains large amounts of polyphenols such as ellagic acid, gallic acid, and tannin acid [24]. Also, it possesses two important hydroxycinnamic acids and derivatives of flavones [25]. It has been widely studied as an antioxidant [26], antimicrobial [27], preparation of nanoparticles [28], anticancer activities [24], and extraction of bioactive compounds [29]. Additionally, several works have proposed the use of low-cost wastes for water and effluent treatment [30], particularly for the removal and biosorption of dyes [31], uranium [32], and heavy toxic metal ions especially chromium [33,34].

Extensive studies were performed on the effectiveness of biomass and its regeneration for Cr(VI) ions removal using pomegranates peel. Salam and Narayanan (2019) [19] reported a maximum removal of 100% achieved in 3 min for a low concentration of 20 mg/L and an amount of biosorbent (RPP with 125 $\mathsf{\mu }\mathrm{m}$ size) of 0.5 g/L. Yi et al. (2017) [35] studied a modified Litchi peel as a biosorbent to eliminate Cr(VI), but the achieved capacity is relatively low reaching only 7.05 mg/g for a longer contact time of 100 min at 303 K for a concentration of 30 mg/g and an amount of biosorbent was 8 g/L. Rafiaee et al. (2020) [36] used pomegranate peel (PGP) adsorbent powder functionalized by polymeric coatings such as polypyrrole (PPy) and polyaniline (PANI) to reduce/remove Cr(VI)metal ions from wastewater. The highest achieved capacity was 59.47% in 90 min obtained in the batch system for 50 mg/L and biosorbent dose of 10 g/L. A new composite Anthracite/pomegranate peel synthesized by Seliem et al. exhibited a relatively high efficiency of 275 mg/g but for a longer time of 120 min at 25 °C and pH 3, the concentration of 70 mg/L and an amount of biosorbent 0.05 g.

In this study, pomegranate peel (PGP) adsorbent has been prepared by low-cost and eco-friendly physical and chemical processes. Structural, morphological, and physicochemical studies are investigated. This research aims to study the surface chemistry characteristics and evaluate the performance of this biomaterial for the elimination of toxic and hazardous heavy metal ions. Indeed, particular emphasis is devoted to investigate the adsorption of hexavalent chromium ions Cr(VI) from an aqueous solution under dynamic mode rather than batch. Several operating parameters are examined, including the flow rate, the solution pH, adsorbent dose, contact time, bed height, as well as the solution temperature. To confirm the experimental data, several models have been applied for both batch and dynamic measurements, subsequently a plausible mechanism is proposed.

2. Materials and Methods

The pomegranate (Punica granatum) peels (PGP) were collected from fresh juice vendors at a local market. To remove the impurities, the collected peels were thoroughly washed with tap water and rinsed using distilled water. Afterward, the obtained PGP product was dried at 100 °C for 24 h, grinded into a powdered sample then sieved to a particle size <500 µm.

The used chemicals were purchased from Merck Company. A stock solution of 1000 mg/L was prepared using potassium dichromate K₂Cr₂O₇, then different concentrations of chromium solution were obtained by consecutive dilution. Adsorption experiments in batch and dynamic modes were performed using a UV-vis spectrophotometer JENWAL 7315 (England) at maximum absorption of 540 nm after complexation with 1,5-diphenylcarbazide.

2.1. Characterization of the Powdered PGP

To evaluate the morphology and chemical composition of PGP powder, a Quanta 200 FEI scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) was used. Fourier-transform infrared (FTIR) spectroscopy was used to identify the functional groups of PGP. FTIR spectrometer (IR Affinity-1S) equipped with a single reflection ATR, operating in the wave number range 400–4000 cm⁻¹ acquired over 50 scans with a resolution of 4 cm⁻¹. The phase stability of PGP was checked by X-ray diffraction (XRD) using a Rigaku Ultima IV instrument equipped with a CPS detector and CuKα radiation (λ = 1.5418 Å). A Thermogravimetric analyzer TGA-2050 equipment was used to analyze the thermal degradation profile of the PGP adsorbent up to 600 °C with a heating rate of 10 °C/min. The PGP the Brunauer Emmett Teller (BET) specific surface area and the average pore size were computed by recording Nitrogen adsorption-desorption curves at 70 K using a Quanta chrome NOVA 2200 E BET analyzer.

2.2. Biosorption of Cr(VI) on PGP—Batch and Dynamic Experiments

2.2.1. Batch Studies

The adsorption isotherms were performed at varying Cr(IV) concentrations and the following operating conditions: m = 1.0 g/L, pH = 2 ± 0.2, T = 20 °C, stirring speed 50 rpm, contact time 2 h. After equilibrium, the adsorbent was separated from the suspension by a membrane filter (0.45 μm).

The concentration of PGP was determined by analyzing the supernatant. The amount of the adsorbate, q_e (mg/g), was estimated using Equation (1):

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{m}}\times \mathrm{V}$$

(1)

where C₀/C_e represents the initial/equilibrium concentrations of Cr (VI) (mg/L), V is the solution volume (L), and m is the PGP sorbent mass(g).

Langmuir (Equation (2)) and Freundlich (Equation (3)) models were used to fit the equilibrium experimental adsorption data by means of the following expressions:

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

(2)

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{f}}{\mathrm{C}}_{\mathrm{e}}{}^{\frac{1}{\mathrm{n}}}$$

(3)

where ${\mathrm{q}}_{\mathrm{e} }$is the amount of Cr(VI) adsorbed onto PGP at equilibrium (mg/g), ${\mathrm{C}}_{\mathrm{e}}$ is the Cr(VI) concentration in solution (mg/g), ${\mathrm{q}}_{\mathrm{m} }$ is the PGP capacity of the adsorbed Cr(VI) expressed per unit mass of adsorbent (mg/g), ${ \mathrm{K}}_{\mathrm{L} }$ is the Langmuir constant (L/mg), while ${\mathrm{K}}_{\mathrm{f}}$ and n correspond to Freundlich adsorption constant (mg ^1−1/n L^1/n) and intensity, respectively.

2.2.2. Dynamic Studies

The tests were conducted in dynamic mode. The performance of a fixed column was investigated using breakthrough curves, presented by the concentrations of pollutants vs. the time profile curve in a fixed bed column.

Packed-bed adsorption experiments were carried out in a cylindrical glass column (height = 35.0 cm, internal diameter = 11.0 cm) at 20 °C and an initial pH of 2 ± 0.2. Employing a peristaltic pump (ISMATEC A39494), the PGP dose (0.105, 0.210, and 0.315 g) corresponding to (25, 50, and 75 mm) bed heights and varying flow rates (1, 2, and 3 mL/min) were used for the removal of Cr(VI) (10, 20, and 30 mg/L). Using the breakthrough curves (breakthrough time—t_b and exhaustion time—t_e), it is possible to obtain various information about the system such as the treated effluent volume (V_eff in mL; Equation (4)), the total quantity of the adsorbed Cr(VI) (q_tot in mg; Equation (5)), the amount of adsorbate that passes through the glass column(W_total in mg; Equation (6)), the percentage of adsorbate uptake(R in%; Equation (7)), and the uptake (q_eq(exp) in mg/g; Equation (8)):

$${\mathrm{V}}_{\mathrm{eff}}={\mathrm{Ft}}_{\mathrm{e}}$$

(4)

$${\mathrm{q}}_{\mathrm{tot}}=\frac{\mathrm{FA}}{1000}=\frac{\mathrm{F}}{1000}{{\displaystyle \int }}_{\mathrm{t}=0}^{\mathrm{t}=\mathrm{total}}({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}})\mathrm{dt}$$

(5)

$${\mathrm{W}}_{\mathrm{tot}}=\frac{{\mathrm{C}}_0{\mathrm{Dt}}_{\mathrm{total}}}{1000}$$

(6)

$$\mathrm{R}\%=\frac{{\mathrm{q}}_{\mathrm{total}}}{{\mathrm{W}}_{\mathrm{total}}}\times 100$$

(7)

$${\mathrm{q}}_{\mathrm{eq}\left(\exp \right)}=\frac{{\mathrm{q}}_{\mathrm{total}}}{\mathrm{m}}$$

(8)

where F (mL/min) represents the flow rate, t_e (in min) the exhaustion time, C₀ (mg/L) the initial adsorbate concentration, C_t (mg/L) the column exit concentration of Cr(VI) at time t, and m (g) the biosorbent quantity.

The area under the breakthrough curve determined by integrating the ${\mathrm{C}}_{\mathrm{ad}}$ vs. t curve will be used to calculate the total amount of PGP adsorbed (maximum column capacity) [37]. The flow rate indicates the column’s empty bed contact time (EBCT), as given by Equation (9):

$$\mathrm{EBCT}=\frac{\mathrm{Bed} \mathrm{Volume} \left(\mathrm{mL}\right)}{\mathrm{Flow} \mathrm{Rate}\left(\mathrm{mL}/\min \right)}$$

(9)

Herein, it is important to mention that once the PGP sorbent becomes saturated, the fixed bed operation was stopped. The adsorption experiments were repeated at least in two replicates in order to estimate the error bars of the measurements.

2.3. Biosorption Modeling in Continuous Systems

The Bohart-Adams (BA), Clark, modified Dose response, and Thomas models were adopted to characterize the breakthrough curves obtained in the continuous biosorption studies. All related information is reported in

Table 1. The models used for modelling the adsorption data.

Model	Equation and Corresponding Plot		Parameters	References
Bohart-Adams	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}=\exp \left({\mathrm{K}}_{\mathrm{BA} }{\mathrm{C}}_0\mathrm{t}-{\mathrm{K}}_{\mathrm{BA} }{\mathrm{N}}_{0 }\frac{\mathrm{Z}}{\mathrm{U}}\right)$ $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ vs. t	(10)	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$:$\frac{\mathrm{final} \mathrm{adsorbent} \mathrm{concentration} }{\mathrm{initial} \mathrm{adsorbent} \mathrm{concentration} }$ KBA (L/mg.min): sorption rate coefficient N0 (mg/L): fixed bed sorption capacity per unit volume Z(mm): bed–depth in the column U (mm/min): linear velocity, t(min): contact time	[38]
Thomas	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}=\frac{1}{1+\exp \left(\frac{{\mathrm{K}}_{\mathrm{th}}{\mathrm{q}}_{\mathrm{th}}\mathrm{m}}{\mathrm{D}}-{\mathrm{k}}_{\mathrm{th}}{\mathrm{C}}_0\mathrm{t}\right)}$ $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ vs. t	(11)	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$: $\frac{\mathrm{final} \mathrm{adsorbent} \mathrm{concentration} }{\mathrm{initial} \mathrm{adsorbent} \mathrm{concentration} }$ m(g): adsorbent mass Kth (L/mg.min): kinetic constant of Thomas model qth (mg/g): adsorption capacity t (min): contact time	[39]
Modified Dose Response	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}=1-\frac{1}{1+{\left(\frac{{\mathrm{V}}_{\mathrm{eff}}}{\mathrm{b}}\right)}^a}$ ${\mathrm{q}}_0=\mathrm{b}\frac{{\mathrm{C}}_0}{\mathrm{m} }$ $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ vs. t	(12)	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$: $\frac{\mathrm{final} \mathrm{adsorbent} \mathrm{concentration} }{\mathrm{initial} \mathrm{adsorbent} \mathrm{concentration} }$ Veff (mL): volume of the effluent q0 (mg/g): adsorption capacity $\mathrm{a} \mathrm{and} \mathrm{b}\ne 0$: constant values in Dose Response model m (g): adsorbent mass	[40]
		(13)
Clarck	$\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}={\left(\frac{1}{1+{\mathrm{Ae}}^{-\mathrm{rt}}}\right)}^{\left(\frac{1}{\mathrm{n}-1}\right)}$ $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ vs. t	(14)	A and r (min−1): constant of the Clarck model. t(min): contact time n: constant value of Freundlich model	[41]

.

3. Results and Discussion

3.1. Characterizations

The SEM images of PGP powder at the magnification (×10,000), as shown in Figure 1a,b, manifest flat and smooth surface texture of the PGP particles before Cr(VI) uptake. The corresponding elemental composition as determined by EDX reveals the main constituent’s carbon and oxygen with minor amounts of Al, Si, Cl, P, K, Ca, Mg, Cu, and Fe, see Figure 1d. This indicates that the two major elements forming PGP are carbon (51.40%) and oxygen (46.21%).

The TGA analysis has been carried out to investigate the thermal properties of PGP, see Figure 2A. The mass loss reflects three main stages. The first stage (30–100 °C) corresponds to a weight loss of 10 wt.% owing to the evaporation of physisorbed water onto PGP. The second stage (150–250 °C) demonstrates a weight loss of 45 wt.% and is associated with the thermal decomposition of hemicellulose and pectin [25]. While the third stage (250–600 °C) manifests an exothermic peak with a considerable weight loss of 30 wt.%, most probably originating from the separation of lignin and decomposition of the cellulose material [42,43]. The total mass loss at 600 °C is extremely high reaching 85 wt.%, hence confirming the organic nature of the PGP.

The crystalline structure of PGP has been investigated by X−ray diffraction. As shown in Figure 2B, no characteristic pattern of crystalline phase(s) can be observed while a broad halo appears in the 2θ range (10−50°). This reflects the amorphous structure of PGP powder, in good agreement with the literature [44].

Figure 2C displays the FTIR spectra of PGP powder before and after Cr(VI) absorption. The bands in the range 2800–3420 cm⁻¹ are ascribed to −OH of carboxylic acid found in the benzene compound [28]. The bands located at 2850 and 2930 cm⁻¹ correspond to C−H aliphatic (CH₃) or (−CH₂), while the band at 1593 cm⁻¹ is assigned to the aromatic rings’ stretching of phenolic compounds (C−C and C=C aromatic vibrations). In addition, hydroxyl functional groups are defined by the absorption peak located at 1480 cm⁻¹ and attributed to the OH of alcohols or in-plane vibration of the OH bond in carboxylic groups. Moreover, the absorption band at 1050 cm⁻¹ indicates the C-O stretching of primary alcohol groups. Furthermore, after the adsorption of Cr(VI), a relatively significant shift of the main band from 3200 to 3400 cm⁻¹ is observed, hence confirming the occurrence of electrostatic interaction between PGP adsorbent and adsorbate. The band at 900 cm⁻¹ indicates that the Cr-sorbet PGP could be due to the formation of Cr(OH)₃, which suggests the reduction of Cr(VI) to Cr(III).

As shown in Figure 2D, the PGP N₂ adsorption-desorption isotherm exhibits a type IV characteristic, in accordance with the International Union of Pure and Applied Chemistry (IUPAC) classification. The characteristic BET parameters given in

Table 2. BET characteristics of the PGP powder.

	BET Surface Area (m2/g)	Total Pore Volume (cm3/g)	Average Pore Diameter (Å)	Particle Density (g/cm3)
PGP	40.38	2.77 × 10−3	32.13	0.25

, indicate a specific surface of 40.38 m²/g, an average pore diameter of 32.13 Å, and a pore volume of 0.00277 cm³/g. The distribution of pore diameter reveals that the adsorbent is mesoporous in nature, in good agreement with the literature [45].

The point of zero charge (pHpzc) of the PGP has been also determined. NaCl (0.01 N) solution is added to a series of flasks, and the initial pH is adjusted by the addition of NaOH (0.01 N, 0.1 N) and/or HCl (0.01 N, 0.1 N). After the pH adjustment, an amount of adsorbent (PGP) is mixed with aliquots of 15 mL. The containers have been placed and subjected to rigorous magnetic stirring for 48 h at 25 °C. The pHpzc value is then obtained by intersecting the (pH_final−pH_initial) vs. pH_initial curve with the bisector (Figure 3a). The pHpzc value of PGP powder is found to be 3.9. This value suggests the presence of acid functional groups on the PGP particles’ surface, which corroborates with FTIR analysis, and indicates that PGP can adsorb Cr(VI) by electrostatic interaction.

3.1.1. Effect of pH Value

The pH effect has been investigated between 2 and 12. It is observed that a marked removal rate of Cr(VI) ions is achieved at lower pH 2, then declines gradually with increasing the pH value. At acidic medium, the predominant species of chromium are dichromate (Cr₂O₇²⁻) and hydrogen chromate (HCrO₄⁻) meanwhile the surface of PGP particles becomes highly protonated, thereby favoring the adsorption and elimination of Cr(VI) metal ions. Under this condition, the phenolic groups (−OH) of the adsorbent easily exchange with the chromium species. Furthermore, the removal efficiency decreases significantly by increasing the pH to more than 4. The effect of pH value on the adsorption mechanism may be attributed to the presence of different functional carboxylic COO⁻ and hydroxylic OH⁻ groups onto the surface of PGP particles, hence favoring different possible interactions with Cr(VI) ions and other ionic species present in the solution. These results corroborate with zeta potential measurements which revealed a positive surface charge of PGP for pH smaller than 3.9 and a negative charge at pH greater to 3.9.

The rise in pH value causes a reduction in the positive surface of the adsorbent [46]. In fact, this decrease is directly associated with the electrostatic forces existing between PGP and chromium solution, as consequence a marked reduction in the sorption capacity is observed [6]. Furthermore, as the pH value increases, a competition between OH⁻ and CrO₄²⁻ as predominant ions occurs in the basic medium [47]. It was concluded that HCrO₄⁻ was adsorbed through electrostatic interaction to reduce to trivalent chromium Cr(III) [34]. The optimum pH of 2 will be utilized for the following experiments. The as-obtained results are found to be close to the results reported in a previous study using pomegranate peel for the elimination of hexavalent chromium [12]. The predominant form of Cr(VI) vs. pH (Figure 3a) is found to be HCrO₄⁻ at the pH of 3.0 and would change to CrO₄²⁻ and Cr₂O₇²⁻ when the pH > 6.5. Compared with CrO₄²⁻, HCrO₄⁻ is known to be more easily adsorbed on the surface active site owing to its low adsorption free energy [45].

3.1.2. Effect of Contact Time

Figure 4a shows the effect of contact time on Cr(VI) removal from aqueous solution onto PGP adsorbent. Based on the plot, the adsorption processes are almost accomplished during the first time and reach equilibrium within 2 h (removal percentage = 92%). After a certain period of time, the reaction slows down as the process reaching the steady state, because the number of vacant sites decreases due to the repulsive forces occurring between the solute molecules and the solid adsorbent phase [20].

3.2. Adsorption Isotherms

The non-linear Freundlich and Langmuir isotherms are shown in Figure 4b. It is noticed that the equilibrium data are well described by Freundlich isotherm compared to Langmuir isotherm. The model parameters, R² values and ${\chi }^2$ are presented in

Table 3. Langmuir and Freundlich isotherm models constants and determination coefficients for the adsorption of Cr(VI) by PGP powder.

Langmuir					Freundlich
${\mathrm{q}}_{\mathrm{m}\mathrm{(exp)}}$ (mg/g)	${\mathrm{q}}_{\mathrm{m}\mathrm{(theo)}}$ (mg/g)	${\mathrm{K}}_{\mathrm{L}}$ (L/mg)	${\mathrm{R}}^2$	${\mathrm{\chi }}^2$	$\mathrm{n}$	${\mathrm{K}}_{\mathrm{f}}$ (mg1−1/n.L1/n)	${\mathrm{R}}^2$	${\mathrm{\chi }}^2$
50.32	38.29 ± 2.58	0.08 ± 0.01	0.990	0.528	3.00 ± 0.07	4.49 ± 0.28	0.994	0.33

. The obtained results indicate that the regression coefficients for Cr(VI) metal ions adsorption with Freundlich isotherm are relatively high (0.994) compared to Langmuir (0.990) isotherm.

The value of 1/n for all adsorption curves is found to be less than unity, which reflects the favorable exchange characteristics between Cr(VI) and PGP. The presence of hydroxyls and carboxyls on the surface of PGP particles produces an irregularity in the distribution energy over the surface of the adsorbent and therefore confirms the suitability of the Freundlich isotherm.

Furthermore, the batch mode’s isotherm constants are significantly greater than the fixed bed mode. This may be due to the influence of the solution flow rate which is equal to zero in the batch mode, i.e., the contact time between Cr(VI) and PGP approximates the infinity [48].

3.3. Influence of Parameters on Fixed Bed

3.3.1. Flow Rate

The breakthrough curves have been recorded at varying flow rates (1, 2, and 3 mL/min), while the remaining operating parameters are kept constant, i.e., inlet Cr(VI) concentration 20 mg/Land bed-depth 50 mm, see Figure 5 and the computed parameters are summarized in

Table 4. Total removal percentage of Cr(VI), column uptake capacity, and column data parameters obtained at different bed heights, concentrations, and flow rates (T = 20 ± 2 °C, pH = 2 ± 0.2).

F (mL/min)	H (mm)	C0 (mg/L)	tb (min)	te (min)	Veff (L)	Area ${\mathrm{A}}^{\prime }$ (mg/min.L)	${\mathrm{q}}_{\mathrm{t}\mathrm{o}\mathrm{t}}$ (mg)	${\mathrm{W}}_{\mathrm{t}\mathrm{o}\mathrm{t}}$ (mg)	Total Removal (%)	${\mathrm{q}}_{\mathrm{e}\mathrm{q}\left(\exp \right)}$ (mg/g)
1	50	20	108	700	0.70	6087	6.08	14.00	43.47	28.67
2	50	20	45	320	0.64	2803	5.60	12.80	43.75	26.73
3	50	20	36	204	0.61	1746	5.23	12.24	42.79	24.85
2	25	20	24	201	0.40	1083	2.16	8.04	20.57	20.62
2	75	20	78	430	0.86	4187	8.37	17.20	48.66	26.62
2	50	10	72	385	0.77	1924	3.84	7.70	49.87	18.28
2	50	30	27	270	0.54	3151	6.30	16.20	38.88	30.00

. It is observed that the sorption rate is reduced with the increase in the flow rate. The empty bed contact time (EBCT) decreases significantly from 108 up to 38 min and the exhaust time (equivalent to 90% of the Cr (VI) concentration) also decreases markedly from 550 to 168 min. The flow rate is found to have a relatively moderate effect on the adsorption capacity; as the flow rate increases, the amount of adsorbed Cr(VI) (q_exp) decreases slightly from 28.67 to 24.85 mg/g. The observed decline in the adsorption capacity can be attributed to the shorter contact time for the adsorbent to interact with the solution, which is in agreement with the previous research findings [39].

3.3.2. Bed Height

The influence of varying the bed-height from 25 to 75 mm on the behavior of the breakthrough curves has been investigated at a fixed adsorbent flow rate of 2 mL/min and an inlet Cr(VI) concentration of 20 mg/L. The recorded breakthrough curves are displayed in Figure 5B and the computed values are given in Table 4. It is clearly observed that as the bed height increases, the Cr(VI) removal efficiency and ${\mathrm{q}}_{\mathrm{eq}\left(\exp \right)}$ increase simultaneously, from 20.57 to 48.66% and 20.62 to 26.62 mg/g. Consequently, while investigating the effect of PGP bed height on the adsorption of 20 mg/L Cr(VI) ions, the breakpoint value has been fixed at the point where $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ is equal to 0.076, 0.090, and 0.100 for a fixed bed height of 25, 50 and 75 mm respectively. It is found that with the increase of the bed height, the breakthrough curve slope decreases, resulting in a broader mass transfer [39]. For this reason, the specific surface area of the PGP biosorbent increases, consequently providing a large number of binding active sites thereby a higher adsorption rate of the column [49]. Also, it is observed that the breakthrough time increases significantly from 24 to 78 min with the increase in the bed height. This can be associated with the occurrence of rapid and larger mass transfer in the fixed bed mode [49].

3.3.3. Initial Concentration

Figure 5C illustrates the breakthrough curves of different inlet Cr(VI) concentrations onto PGP adsorbent obtained at a constant bed depth (50 mm) and fixed flowrate (2 mL/min), the corresponding data are given in Table 4. It can be observed that the value of $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ reaches 0.010, 0.039, and 0.067 min in the intervals 365, 300, and 230 min when the Cr(VI) inlet concentration varies from 10 to 30 mg/L. This indicates that the value of the breakthrough time (t_b) is slightly reduced with the rise in the inlet concentration of Cr(VI). The maximum adsorption capacity at 10, 20, and 30 mg/L Cr(VI) is found to be 18.32, 26.73 and 30.01 mg/g, respectively. In contrast, the adsorption efficiency is reduced by almost 22% (from 49.87 to 38.88%). Furthermore, it is noted that when the Cr(VI) concentration in the solution is high, the rate of Cr(VI) ions for the adsorptive sites has a more pronounced trend. Indeed, a higher concentration of Cr(VI) in the solution provides a random diffusion path [50] for sorbate ions onto the surface of PGP adsorbent. This may be explained by the enhanced driving force and the high mass-transfer flux from Cr(VI) solution toward PGP [51]. However, at lower inlet Cr(VI) concentration, the breakthrough curves become dispersed, and the binding sites tend to be slowly saturated. Indeed, similar results were reported for the biosorption of Cr(VI) by chemically surface-modified Lantana Camara sorbent [39].

3.3.4. Temperature

The influence of the medium temperature has been also investigated. The breakthrough curves recorded for different bed heights at various temperatures are shown in Figure 6, and the results of Dose Response Model are given in

Table 5. Dose response parameters for Cr(VI) adsorption by PGP in fixed bed (C₀ = 20 mg/L, pH = 2 ± 0.2, D = 2 mL/min).

T (K)	Z (mm)	a	b (mL)	q0 (mg/g)	qexp (mg/g)	Total Removal (%)	R2	$\mathit{\chi }\times {10}^4$	SD
303	25	2.24 ± 0.04	52.85 ± 0.51	10.06	10.15	38.00	0.997	2.76	0.01
	50	2.75 ± 0.07	138.32± 1.47	13.17	13.38	44.00	0.996	5.94	0.02
	75	2.92± 0.08	194.12 ± 2.12	13.98	13.93	44.12	0.996	5.20	0.02
313	25	1.28± 0.10	20.96 ± 1.67	3.99	3.89	30.06	0.927	38.50	0.06
	50	2.20 ± 0.14	51.85 ± 1.65	4.93	4.90	33.09	0.977	27.80	0.05
	75	3.00 ± 0.11	98.13 ± 1.38	6.23	6.20	34.10	0.994	9.17	0.03

. It is noted that with the rise of temperature, the adsorption rate of Cr(VI) ions decreases whereas the rate constant increases. Also, it is noticed that the adsorption rate is clearly variable at the same bed height. This is maybe due to the desorption process occurring following the rise in the thermal energy and the weakening of the interactions of the intermolecular bonds between solution ions and active sites of the PGP adsorbent. This signifies that the PGP particles’ surface is exothermic in nature and that the adsorption mechanism is favored at low temperatures. The above results are in accordance with the results obtained during the thermodynamic study in batch mode; the exothermic reaction of Cr(VI) ions adsorption onto PGP will be confirmed. This result has been previously reported by M. Zamouch et al. [21].

The adsorption Cr(VI) ions by PGP thermodynamic parameters in the temperature range 298–318 K, have been determined using Van’t Hoff equations [52]:

$$∆{\mathrm{G}}^{°}=-{\mathrm{RTLnK}}_{\mathrm{C} }$$

(15)

$$∆{\mathrm{G}}^{°}=∆{\mathrm{H}}^{°}-\mathrm{T}∆{\mathrm{S}}^{°}$$

(16)

$${\mathrm{K}}_{\mathrm{C}}=\frac{{\mathrm{q}}_{\mathrm{e}} }{{\mathrm{C}}_{\mathrm{e}}}$$

(17)

$${\mathrm{LnK}}_{\mathrm{C}}=\frac{∆{\mathrm{S}}^{°}}{\mathrm{R}}-\frac{∆{\mathrm{H}}^{°}}{\mathrm{RT}}$$

(18)

where $∆{\mathrm{G}}^{°}$ represents the Gibbs free energy (kJ/mol), R the gas constant (8.314 J/mol.K), T the absolute temperature (K), K_C the sorption distribution coefficient (L/g), ΔH° the enthalpy (kJ/mol.), and $∆{\mathrm{S}}^{°}$ the entropy (kJ/mol.K).

To better elucidate the adsorption process mechanism of Cr(VI) by PGP biosorbent, the respective thermodynamic plots are depicted in Figure 7 while the computed fitting parameters are given in

Table 6. Thermodynamic parameters calculated for the adsorption of Cr(VI) onto PGP.

Sample	$\mathbf{∆}{\mathbf{H}}^{°}$ (kJ/mol)	$\mathbf{∆}{\mathbf{S}}^{°}$ (J/mol.K)	$\mathbf{∆}{\mathbf{G}}^{°}$ (kJ/mol)
			298 K	308 K	318 K
PGP	−44.69	−139.28	−3.98	−2.30	−0.48

. The negative values of ΔH°, ΔS°, and ΔG° demonstrate that the adsorption process is spontaneous and exothermic, without the need for any external power source. Also, it signifies the randomness of adsorption of Cr(VI) onto PGP. The negative value of ∆S° corresponds to a decrease in the freedom of the adsorbed chemical species and describes the randomness of chemical species movement at the adsorbent-adsorbate interface during the sorption process. The negative values of ΔG° and high negative values of ∆S° with the rise of temperature manifest the favorable nature of the process. Whereas the observed decrease in ΔG° values with temperature signifies that the adsorption process occurs favorably at lower temperatures [53]. Similar results were also observed in the literature for Cr(VI) adsorption onto different adsorbents [6,54].

3.4. Modeling of Column Data

The fitted curves are shown in Figure 5 and the models’ corresponding calculated parameters are presented in

Table 7. Parameters of various models for Cr(VI) adsorption by PGP using non-linear regression. Experimental fixed conditions: T = 20 ± 2 °C, pH = 2 ± 0.2.

Parameters			Thomas Model						Dose Response Model
F (mL/min)	H (mm)	C0 (mg/L)	qecal (mg/g)	Total Removal (%)	KThX103 (L/mg)	R2	${\mathit{\chi }}^2$ $\times {10}^3$	±SD × 103	a	b (mL)	q0 (mg/g)	R2	${\mathit{\chi }}^2$ $\times {10}^3$	±SD $\times$103
1	50	20	25.69 ± 0.37	43.47	0.59 ± 0.02	0.976	3.16	56.22	2.73 ± 0.07	269.78 ± 2.98	27.79	0.988	1.47	38.38
2	50	20	21.59 ± 0.54	43.75	1.16± 0.07	0.953	5.85	76.50	2.46 ± 0.07	226.74 ± 2.73	23.26	0.988	1.45	38.05
3	50	20	21.52 ± 0.34	42.79	2.04 ± 0.09	0.976	3.11	55.80	2.94 ± 0.06	226.04± 1.80	22.86	0.994	0.71	26.73
2	25	20	16.63 ± 0.44	20.57	3.70 ± 0.28	0.970	4.50	67.05	3.07 ± 0.11	87.34± 1.12	17.57	0.993	1.02	31.89
2	75	20	22.77 ± 0.56	48.66	0.72 ± 0.03	0.956	5.72	75.63	2.49± 0.08	358.64 ± 5.95	26.02	0.979	2.71	52.09
2	50	10	15.75 ± 0.42	49.87	1.53 ±0.09	0.952	6.34	79.63	2.50 ± 0.09	330.88 ± 5.75	18.02	0.977	2.98	54.57
2	50	30	25.39 ± 0.45	38.88	0.95 ±0.04	0.972	3.19	56.48	2.35 ± 0.06	177.37 ± 1.84	27.22	0.991	0.99	31.53
			Clark model					Bohart-Adams model
D mL.min−1	H (mm)	C0 (mg/L)	$\mathrm{A}$	$\mathrm{r}\times {10}^3$ (min−1)	${\mathrm{R}}^2$	${\chi }^2$ $\times {10}^3$	±SD $\times$103	KBA$\times$103 (mL/mg.min)	N0 (mg/L)		R2		${\chi }^2$ $\times {10}^3$	±SD $\times$103
1	50	20	535.29 ± 139.02	17.31 ± 0.85	0.964	4.72	68.69	4.16 ± 0.36	6024.43 ± 88.76		0.900		0.26	16.32
2	50	20	184.20 ± 56.80	33.24 ± 2.64	0.933	8.44	91.87	4.24 ± 0.11	6689.07 ± 218.38		0.914		0.32	17.99
3	50	20	298.95 ± 89.87	56.11 ± 3.49	0.962	4.99	70.65	7.92 ±0.11	6839.09 ± 209.58		0.923		0.40	20.21
2	25	20	383.93 ± 188.25	103.12 ± 10.26	0.933	6.66	81.63	10.30± 1.52	6222.88 ± 213.18		0.946		0.37	19.39
2	75	20	206.66 ± 52.18	19.67 ± 1.16	0.941	7.76	88.12	3.30± 1.52	6721.11 ± 91.34		0.940		0.20	14.32
2	50	10	186.00 ± 42.24	20.76 ± 1.31	0.936	8.52	92.29	11.28 ± 1.16	4208.87 ± 73.41		0.923		0.27	16.49
2	50	30	154.18 ± 37.66	40.68 ± 2.51	0.957	4.91	70.10	3.28 ± 0.43	7191.59 ± 323.61		0.922		0.48	21.94

. The nonlinear plot $\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}$ vs. t has been fitted in the initial part of the curve (0–0.2) to compute the Bohart–Adams ${\mathrm{K}}_{\mathrm{BA}}$ and sorption capacity ${\mathrm{N}}_0$ parameters. It is found that the value of ${\mathrm{K}}_{\mathrm{BA}}$ increases from $4.16\times {10}^{-3}$ to $7.92\times {10}^{-3}$ with increasing the flow rate but decreases from $11.28\times {10}^{-3}$ to $3.28\times {10}^{-3}$ with the increase in the initial Cr(VI) concentration. Also, the value of ${\mathrm{N}}_0$ of the model is found to increase with increasing both Cr(VI) concentration and bed depth. This can be associated with the prevalence of external mass transfer activities occurring during the adsorption process within the column at the initial stage [55]. This corroborates with the results published by Han et al. [45] when employing green synthesized nanocrystalline chlorapatite for the sorption of Cr(VI).

The Thomas model parameters namely kinetic coefficient K_th and sorption capacity q_th, have been determined using a nonlinear regression for bed height, flow rate, and Cr(VI) concentration. From Table 7, it is noted that the value of K_th increases with increasing both bed height and flow rate. Also, the value of q_th increases markedly (from 15.75 to 25.39 mg/g) with the rise of Cr(VI) concentration (from 10 to 30 mg/L), which is attributed to the high driving force between Cr(VI) ions and PGP sorbent. However, the increase in the K_th value with the increase in the flow rate, signifies that the kinetics of the overall process is dominated by the external mass transfer mechanism [56]. This suggests that the Thomas model is more favorable for describing the adsorption processes, because the limiting steps do not proceed through external/internal diffusion [37]. Similar results are found in the literature when using different sorbate-sorbent systems [54].

According to the results given in Table 7, the rate of mass transfer (r) increases gradually with the rise of both flow rate and Cr(VI) concentration but decreases with the increase in bed height. Furthermore, the increase in the initial Cr(VI) concentration results in the enhancement of the driving force, favoring a better mass transfer of the adsorbate onto PGP particles’ surface. Nevertheless, with the rise in the bed height, an increase in the particles’ number alongside a reduction in the mass transfer rate occur concurrently [57].

The fitting results of the modified Dose Response parameters are also examined. The predicted breakthrough curves illustrated in Figure 5 evidenced a good agreement with the measurements. The obtained high value of the regression coefficient (R²) satisfies the validity of the model. The values of R² obtained for different operating conditions are higher than 0.977 and ${\chi }^2$ smaller than$2.98\times {10}^{-3}$, thus providing a better fitting for the Dose Response model compared to Thomas, Clarck, and Bohart-Adams models.

3.5. Adsorption Mechanism

The SEM image of PGP after the adsorption of Cr(VI) metal ions (Figure 1d) reveals that the particles’ surface becomes more rough and uneven. This confirms that the pores of the PGP sorbent are occupied by the ions of the adsorbate. Furthermore, the EDX spectrum (Figure 1e) of PGP after adsorption reveals the presence of Cr ions in the adsorbent, where its content reaches 3.64 wt.%. The presence of 1.5 wt.% K, maybe a residue belonging to the potassium dichromate (K₂Cr₂O₇) used during the preparation of Cr(VI) aqueous solutions.

Through the analytical results of EDX (Figure 1e) and FTIR (Figure 2C) spectra, as well as the data obtained from modeling of the equilibrium isotherms and kinetics, a plausible mechanism related to the adsorption of Cr(VI) onto PGP is illustrated in Figure 8. It is noted that the process is primarily controlled by electrostatic interaction between negatively charged functional groups present at the PGP particles’ surface and the positively charged Cr(VI) metal ions. Meanwhile, the surface adsorption via diffusion to the pores of PGP improves the mass transfer and provides an adsorption pathway besides an area responsible for the reduction of Cr(VI) metal ions to less toxic Cr(III).

3.6. Comparison with Other Sorbents in the Literature

To evaluate the sorption efficacy of PGP toward Cr(VI), a comprehensive comparison with other materials such as solvent impregnated resin (SIR), pomegranate peel powder (PGP) subjected to polyaniline (PANI), and polypyrrole (PPy) surface functionalization and modified Lantana Camara, is performed as illustrated in

Table 8. A comparison of adsorption capacity or removal percentage of Cr(VI) with different biosorbents.

Adsorbent	Qmax (mg/g)/R (%)	Operating Conditions	References
Pomegranate peel (Batch study)	22.87 mg/g	pH 2, dose 300 mg/L, contact time 30 min	[12]
Solvent impregnated resin (SIR)(Batch study)	28.2 0 mg/g	pH (0.3–2), dose 1 mg/L, contact time 30 min	[54]
Pomegranate peel (PGP) powder modified by polyaniline (PANI) and polypyrrole (PPy) (Batch study)	47.59%	pH 1, dose 10 mg/L, contact time 90 min	[36]
Nanocrystalline chlorapatite (ClAP) (Batch study)	63.47 mg/g	pH 3, dose 0.1 mg/L, contact time 25 min	[45]
Pomegranate peel (PGP) activated (PGP-1N H2SO4) (Batch study)	28.28 mg/g	pH 3, dose 4 mg/L, contact time 3 h	[6]
Modified Lantana Camara (Column study)	362.80 mg/g	pH 1.5, adsorbent bed height 40 mm, Breakthrough time 1250 min	[39]
Pomegranate peel (Column study) (Batch study)	30.00 mg/g 50.32 mg/g	pH 2, adsorbent bed height 50 mm, Breakthrough time 27 min pH 2, dose 1 mg/L, contact time 24 h	This study

. It can be observed that the proposed biosorbent exhibits a higher adsorption capacity than the previously reported sorbents; it is even higher than pomegranate peel activated with H₂SO₄. In addition, the obtained batch capacity is more significant than that of SIR and PGP for Cr(VI) removal. Nevertheless, the existence of some differences is more probably attributed to several factors, primarily the active sites and functional groups formed at the particles’ surfaces of the sorbents, besides the microstructure nature (surface area, porosity, agglomeration level) and the chemical formulation (intrinsic properties playing key role in terms of selectivity) of the prepared materials.

4. Conclusions

In the present study Pomegranate peel has been used as cost-effective biosorbent for the removal of hazardous Cr(VI). The adsorption mechanism of Cr(VI) adsorbate by PGP nanosorbent is determined and the physicochemical proprieties of PGP are presented. Both batch and dynamic studies are performed, and the isotherms and thermodynamic results are analyzed. It is found that the adsorption process occurs through an exothermic mechanism. Several operating parameters (pH, initial Cr(VI) concentration, temperature, bed depth, and flow rate) are found to significantly affect the Cr(VI) uptake through a fixed-bed column. The maximum uptake of 43.78 mg/g is achieved at pH 2 with a breakthrough time. The experimental data in dynamic mode has been fitted using four theoretical models namely Bohart-Adams, Clark, Dose Response, and Thomas. The Dose Response model successfully predicts the breakthrough curves for Cr(VI) removal under optimum operating conditions (flow rate, bed depth, Cr(VI) concentration) with R² > 0.97. The equilibrium data were found to obey Freundlich isotherm, thereby confirming the heterogeneous sorption of Cr(VI) onto PGP. The negative values of ΔH°, ΔG°, and ΔS° indicate an exothermic and spontaneous process.