Enhanced Chromium (VI) Adsorption onto Waste Pomegranate-Peel-Derived Biochar for Wastewater Treatment: Performance and Mechanism

Surface chemical modification allows for the rational construction of biochar with desirable structures and functionalities for environment purification. Fruit-peel-derived adsorbing material has been well studied in the adsorption of heavy-metal removal due to its abundance and non-toxicity, but its precise mechanism in removing chromium-containing pollutants remains unclear. Herein, we explored the potential application of engineered biochar prepared from fruit waste via chemical modification to remove chromium (Cr) from an aqueous solution. By synthesizing two types of agricultural residue-derived adsorbents, including pomegranate peel adsorbent (PG) and its modified product, pomegranate-peel-derived biochar (PG-B), via chemical and thermal decomposition methods, we elucidated the adsorption property of Cr(VI) on the studied materials and identified the cation retention mechanism of the adsorption process. Batch experiments and varied characterizations demonstrated that superior activity was exhibited in PG-B, which can contribute to the porous surfaces caused by pyrolysis and effective active sites resulting from alkalization. The highest Cr(VI) adsorption capacity is obtained at pH 4, a dosage of 6.25 g L−1, and a contact time of 30 min. The maximum adsorption efficiency of 90.50% in a short period (30 min) was obtained on PG-B, while PG reached a removal performance of 78.01% at 60 min. The results from kinetic and isotherm models suggested that monolayer chemisorption dominated the adsorption process. The Langmuir maximum adsorption capacity is 16.23 mg g−1. This study shortened the adsorption equilibrium time of pomegranate-based biosorbents and presents positive significance in designing and optimizing waste fruit-peel-derived adsorption materials for water purification.


Introduction
Chromium (Cr), one of the typical toxic heavy metal ions with the common forms of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) in the soil-liquid nexus, has induced increased pollution in water sources and severe neurotoxicity to humans, posing a prominent concern among varied environmental issues [1][2][3]. Much of the excess chromium accumulating in the natural environment comes from human industrial activities (such as leather, electroplating, printing, dyeing, etc.) and has been proven to be poisonous to vital organs and systems of the human body [4][5][6][7][8]. Furthermore, Cr(VI) has more substantial solubility than Cr(III) and can quickly enter human cells, cause damage to the liver, kidney, and other internal organs, and accumulate in the human body [9]. Worse yet, Cr(VI) is carcinogenic and may induce gene mutations [10]. The maximum concentration of Cr(VI) in surface water (0.1 mg L −1 ) has been restricted by the World Health Organization (WHO) [11,12]. Hence, numerous studies have been conducted to mitigate Cr(VI) poisoning and center the environmental remediation of polluted soil and water body, emphasizing the significant necessity for developing practical approaches to get rid of Cr(VI) from wastewater [13,14].

Materials
Sodium hydroxide (NaOH), nitrosylsulfuric acid (NOHSO 4 ), and sodium pyrosulfite (Na 2 S 2 O 5 ) were purchased from Chengdu Ke Long Chemical Industry Co., Ltd. (Chengdu, China). The pomegranate peels used as the starting materials were collected from Sichuan province, China (30 • 36 N, 103 • 59 E). All chemicals and reagents used in this study were analytical grade. Deionized water was used for all synthesis and batch experiments.

Preparation of PG and PG-B
The collected pomegranate peels were cleaned with deionized water and kept in the air blast drying oven at 80 • C for 15 h, followed by cooling to room temperature in a drying vessel. Subsequently, the obtained products were sieved to 0.3 mm followed by pouring them into 0.5 M of NaOH aqueous solution and keeping them for 5 h at room temperature. After that, the mixed solution was dried at 80 • C for 24 h. Finally, the PG adsorbent was obtained and kept in a drying vessel for further use after cooling down to room temperature.
To prepare PG-B adsorbents, the screened pomegranate peels were soaked in NOHSO 4 solution for 24 h followed by being filtered and cleaned with deionized water. Then, the obtained material was transferred to a muffle furnace with heating to 600 • C at a heating rate of 5 • C min −1 . After being calcined for 2 h and naturally cooled down to room temperature, the products were kept in 0.5 M of Na 2 S 2 O 5 aqueous solution for 24 h. After being filtered and washed with deionized water to remove excess surface ions, the obtained products were dried at 80 • C for 5 h.

Adsorption Experiments
Batch experiments were conducted to explore the adsorption properties of PG and PGB for Cr(VI)removal under a set of reaction parameters including different contact times (10,20,30,40,50,60,70,80,90, and 100 min), solution pH (4, 6, 7, 8, and 10), material doses (2.50, 3.75, 5.00, 6.25, and 7.50 g L −1 ), and initial Cr(VI) concentrations (10,20,30,40, and 50 mg L −1 ). For each experiment, the reaction mixture containing 20 mL of Cr(VI) aqueous solution with the desired concentration and adsorbents in specific dosages was equilibrated in a 50 mL polyethylene centrifuge tube and then put on the thermostatic oscillator operating at 150 rpm and 25 • C. After the reaction, the mixture was filtered through 0.45 µm filters, and the obtained supernatant was used to determine the Cr(VI) content following the 1,5-diphenylcarazide colorimetric method using a UV−Vis spectrophotometer (Thermo Fisher, Genesys180, Waltham, MA, USA). The Cr(VI) adsorption capacity (Q e ) and percentage removal efficiency were calculated as Equations (1) and (2), respectively.
Removal efficiency (%) = where C 0 and C e stand for the Cr(VI) concentration at the initial and adsorption equilibrium (mg L −1 ), respectively. V denotes the volume of the experimental solution (L) and m represents the weight of the adsorbents (g).

Kinetics and Isotherm Analysis
To elucidate the adsorption mechanism of Cr(VI) on the studied adsorbents (PG and PG-B), typical kinetic models and isotherm models were applied to simulate the obtained experimental data. The adsorption kinetics was described by the pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich models according to Equations (3)-(5), while the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models were used to fit the experimental data of equilibrium adsorption of Cr(VI) by PG and PG-B according to Equations (5)-(7), respectively.
Elovich model : Langmuir model : Freundlich model : Q e = K F C 1/n e (7) where C e (mg L −1 ) denotes the equilibrium concentration of adsorbates; Q t and Q e (mg g −1 ) stands for the amounts of Cr(VI) adsorbed at time t (min) and at equilibrium; Q 0 (mg g −1 ) is the maximum adsorption capacity of adsorbents; K F [(mg g −1 )(L mg −1 ) 1/n ] and K L (L mg −1 ) are the Freundlich and Langmuir equilibrium constants, respectively; and 1/n is the adsorption strength. k 1 (min −1 ) and k 2 (g mg −1 min −1 ) stand for the adsorption rate constants of the pseudo-first-order and pseudo-second-order models, respectively. a (mg (g h) −1 ) and b (g mg −1 ) denote the initial rate constant and desorption constant of the Elovich model. β represents the constant of the D-R model; ε stands for Polanyi adsorption potential; R is the ideal gas constant of 8.314 J (mol K) −1 ; T denotes the absolute temperature; E is the adsorption-free energy (J mol −1 ).

Characterization of Materials
The surface structure and element changes of PG and PG-B were analyzed by a scanning electron microscope equipped with an energy-dispersive spectrometer (SEM-EDS, Prox, Phenom, The Netherlands). The Fourier transform infrared (FT-IR) spectra were recorded in the 4000-400 cm −1 region by an infrared spectrophotometer (Thermo Nicolet, MA, Madison, WI, USA).

Characterization of PG and PG-B
The morphology and element analysis of the pomegranate peel adsorbent (PG) obtained by alkali treatment and pomegranate-peel-derived biochar (PG-B) prepared by pyrolysis and acid treatment has been described in Figure 1 via the SEM-EDS spectra, which indicated that chemical modification and pyrolysis treatment significantly affected surface structures of the pomegranate-peel-derived adsorption materials. As compared to PG adsorbents, more complicated microporous and loose structures were obtained on PG-B adsorbents, which may be attributed to the fact that high-temperature calcination and subsequent strong acid treatment increased the pore and specific surface area of the materials [20]. The surface of PG-B was heterogeneous, porous, and honeycombed, while the surface of the raw material was disordered, rough, and tightly bonded. The honeycomb structure formed by the planar arrangement of multiple layers of carbon increased the specific surface area of materials, leading to enhanced removal performance of Cr(VI) [36]. EDS analysis results showed that C and O, which were 64.9% and 25.99% in PG and 69.09% and 26.69% in PG-B, respectively, were the main elements of the two modified materials derived from pomegranate peels. After pyrolysis and chemical modification by NOHSO 4 and Na 2 S 2 O 5 , PG-B adsorbents showed markedly increased S and N content while P and K were the main characteristic element of PG adsorbents, which can be ascribed to the high temperature that affected surface properties of PG-B adsorbents. After the adsorption reaction, a much higher Cr content of 58.15% in PG-B than in PG adsorbents with a Cr content of 26.64% was observed, indicating that pomegranate-peel-derived biochar exhibited enhanced adsorption capacity. EDS analysis results showed that C and O, which were 64.9% and 25.99% in PG and 69.09% and 26.69% in PG-B, respectively, were the main elements of the two modified materials derived from pomegranate peels. After pyrolysis and chemical modification by NOHSO4 and Na2S2O5, PG-B adsorbents showed markedly increased S and N content while P and K were the main characteristic element of PG adsorbents, which can be ascribed to the high temperature that affected surface properties of PG-B adsorbents. After the adsorption reaction, a much higher Cr content of 58.15% in PG-B than in PG adsorbents with a Cr content of 26.64% was observed, indicating that pomegranate-peel-derived biochar exhibited enhanced adsorption capacity. Functional groups on PG and PG-B were elucidated by the FT-IR spectra ( Figure 2). The variations in the wave number of the main peaks associated with the studied materials before and after the adsorption reaction are presented in Table 1. The spectra of both PG and PG-B showed a strong and wide adsorption peak around 3000-3500 cm −1 , which can be assigned to the stretching vibrations of −OH existing in carboxylic acids, phenols, or alcohols [37]. The peaks that occurred near 2931 cm −1 of PG and 2914 cm −1 of PG-B were caused by the stretching vibration of the structural C−H of aliphatic methylene groups, where a significant diminution in intensity was observed in PG-B [38,39]. The ester−C=O band at 1731 cm −1 was observed in the spectrum of PG but was absent in PG-B. Moreover, the C−O at 1043 cm −1 moved to a high wavenumber at 1122 cm −1 . The above results implied that oxidation of the pomegranate peel was influenced markedly by thermal decomposition and chemical modification by NOHSO4 and Na2S2O5. After the adsorption of Cr(VI), redshift and attenuation of the peak intensity were observed on both PG and PG-B, for instance, the band at 1731 cm −1 in the spectrum of PG adsorbents moved to 1737 cm −1 as the peak intensity also decreased; for the spectrum of PG-B, the band at 1560 cm −1 moved to 1566 cm −1 and 1383 cm −1 moved to 1396 cm −1 with the peaks showing a decline in intensity, demonstrating that the surface functional groups on the material's surface, such as -−OH, C=O, and −NH2, were responsible for Cr(VI) adsorption. After the Cr(VI) adsorption on PG-B, the peak at 625 cm −1 resulting from the bending of N−H bonds showed a significant decline in intensity, indicating that superficial −NH2 was an important contributor to Cr(VI) removal [40,41]. These results indicated that both PG and PG-B were rich in oxygenated functional groups (−OH, C−H, C−N, −NH2, and C−C) while PG-B had more abundant functional groups, which had great potential for the adhesion of Cr(VI). Functional groups on PG and PG-B were elucidated by the FT-IR spectra ( Figure 2). The variations in the wave number of the main peaks associated with the studied materials before and after the adsorption reaction are presented in Table 1. The spectra of both PG and PG-B showed a strong and wide adsorption peak around 3000-3500 cm −1 , which can be assigned to the stretching vibrations of -OH existing in carboxylic acids, phenols, or alcohols [37]. The peaks that occurred near 2931 cm −1 of PG and 2914 cm −1 of PG-B were caused by the stretching vibration of the structural C-H of aliphatic methylene groups, where a significant diminution in intensity was observed in PG-B [38,39]. The ester-C=O band at 1731 cm −1 was observed in the spectrum of PG but was absent in PG-B. Moreover, the C−O at 1043 cm −1 moved to a high wavenumber at 1122 cm −1 . The above results implied that oxidation of the pomegranate peel was influenced markedly by thermal decomposition and chemical modification by NOHSO 4 and Na 2 S 2 O 5 . After the adsorption of Cr(VI), redshift and attenuation of the peak intensity were observed on both PG and PG-B, for instance, the band at 1731 cm −1 in the spectrum of PG adsorbents moved to 1737 cm −1 as the peak intensity also decreased; for the spectrum of PG-B, the band at 1560 cm −1 moved to 1566 cm −1 and 1383 cm −1 moved to 1396 cm −1 with the peaks showing a decline in intensity, demonstrating that the surface functional groups on the material's surface, such as -OH, C=O, and -NH 2 , were responsible for Cr(VI) adsorption. After the Cr(VI) adsorption on PG-B, the peak at 625 cm −1 resulting from the bending of N−H bonds showed a significant decline in intensity, indicating that superficial -NH 2 was an important contributor to Cr(VI) removal [40,41]. These results indicated that both PG and PG-B were rich in oxygenated functional groups (-OH, C-H, C-N, -NH 2 , and C-C) while PG-B had more abundant functional groups, which had great potential for the adhesion of Cr(VI).

Effect of pH on the Adsorption Process
The influence of solution pH on Cr(VI) adsorption by PG and PG-B was explored, and strong pH dependence is presented in Figure 3a,b. The solution pH value, as one of the essential parameters, can not only affect the charge and ion conditions of the material surface but also dominate the Cr(VI) form in solution, causing the difference in chemical behavior of Cr(VI) such as coordination, complexation, and electrostatic interaction in the system with a changed pH value [42,43]. Specifically, the hydrolysis of Cr(VI) primarily produced CrO4 2− , Cr2O7 2− , and HCrO4 2− , where the hydrolysis product was largely HCrO4 2− and Cr2O7 2− at pH 2~6 while CrO4 2− dominated when pH > 6 [44]. Similar responses to the pH variation and changes in the adsorption performance showing a slow increase in adsorption capacity followed by a rapid decrease with a rising pH were observed in the studied reaction system with PG and PG-B, indicating that the interaction between chromium and the surface of pomegranate-peel-derived adsorbents was strongly affected by the solution pH value. The maximum adsorption capacity of both PG and PG-B was reached at pH = 4, where the adsorption amount of Cr(VI) on PG-B and PG reached 4.34 mg g −1 and 3.75 mg g −1 with the corresponding removal efficiency of 90.41% and 78.17%, respectively, while a significant decline in removal efficiency was presented in other pH conditions. When pH > 4, the adsorption capacity of Cr(VI) on both P PG and PG-B decreased rapidly with the increase in pH value. For PG-B, the adsorption capacity decreased from 4.34 mg g −1 to 3.44 mg g −1 as the corresponding removal efficiency decreased from 90.41% to 71.91%, while the adsorption capacity decreased from 3.75 mg g −1 to 2.95 mg g −1 as the corresponding removal efficiency decreased from 78.17% to 61.46%. This may be attributed to the enhanced degree of protonation of the amine group and other surface functional groups on the adsorbents in favor of the electrostatic attraction to the chromium-containing anions in reaction systems, resulting in the superior performance of pomegranate-peel-derived biochar PG-B rich in -NH2 under low pH conditions. Moreover, another pathway of enhanced adsorption of Cr(VI) in the acid reaction system was

Effect of pH on the Adsorption Process
The influence of solution pH on Cr(VI) adsorption by PG and PG-B was explored, and strong pH dependence is presented in Figure 3a,b. The solution pH value, as one of the essential parameters, can not only affect the charge and ion conditions of the material surface but also dominate the Cr(VI) form in solution, causing the difference in chemical behavior of Cr(VI) such as coordination, complexation, and electrostatic interaction in the system with a changed pH value [42,43]. Specifically, the hydrolysis of Cr(VI) primarily produced CrO 4 2− , Cr 2 O 7 2− , and HCrO 4 2− , where the hydrolysis product was largely HCrO 4 2− and Cr 2 O 7 2− at pH 2~6 while CrO 4 2− dominated when pH > 6 [44]. Similar responses to the pH variation and changes in the adsorption performance showing a slow increase in adsorption capacity followed by a rapid decrease with a rising pH were observed in the studied reaction system with PG and PG-B, indicating that the interaction between chromium and the surface of pomegranate-peel-derived adsorbents was strongly affected by the solution pH value. The maximum adsorption capacity of both PG and PG-B was reached at pH = 4, where the adsorption amount of Cr(VI) on PG-B and PG reached 4.34 mg g −1 and 3.75 mg g −1 with the corresponding removal efficiency of 90.41% and 78.17%, respectively, while a significant decline in removal efficiency was presented in other pH conditions. When pH > 4, the adsorption capacity of Cr(VI) on both P PG and PG-B decreased rapidly with the increase in pH value. For PG-B, the adsorption capacity decreased from 4.34 mg g −1 to 3.44 mg g −1 as the corresponding removal efficiency decreased from 90.41% to 71.91%, while the adsorption capacity decreased from 3.75 mg g −1 to 2.95 mg g −1 as the corresponding removal efficiency decreased from 78.17% to 61.46%. This may be attributed to the enhanced degree of protonation of the amine group and other surface functional groups on the adsorbents in favor of the electrostatic attraction to the chromium-containing anions in reaction systems, resulting in the superior performance of pomegranate-peel-derived biochar PG-B rich in -NH 2 under low pH conditions. Moreover, another pathway of enhanced adsorption of Cr(VI) in the acid reaction system was the strong reducing action of electron donor groups (such as -OH and -COOH), causing the conversion of Cr(VI) to Cr(III) [45]. The superficial accumulation of negative charge resulting from the increased pH value and decreased Cr(VI) adsorption quantity caused the transformation of the main existence form of Cr(VI) into CrO 4 2− , which could be attributed to poor removal efficiency [46]. The above results elucidated that PG and PG-B were most effective for Cr(VI) removal at pH = 4, which agree with previous studies presenting similar tendencies that the maximum adsorption capacity was obtained under acidic conditions (pH = 3~5) [30,39]. the strong reducing action of electron donor groups (such as -OH and -COOH), causing the conversion of Cr(VI) to Cr(III) [45]. The superficial accumulation of negative charge resulting from the increased pH value and decreased Cr(VI) adsorption quantity caused the transformation of the main existence form of Cr(VI) into CrO4 2− , which could be attributed to poor removal efficiency [46]. The above results elucidated that PG and PG-B were most effective for Cr(VI) removal at pH = 4, which agree with previous studies presenting similar tendencies that the maximum adsorption capacity was obtained under acidic conditions (pH = 3~5) [30,39].

Effect of Material Dosage on the Adsorption Process
The role of adsorbent dosages on the Cr(VI) removal efficiency of PG and PG-B was investigated (Figure 3c,d). It was observed that the increased addition of adsorbents was beneficial to Cr(VI) adsorption efficiency on both PG and PG-B. Specifically, the removal efficiency of Cr(VI) on PG and PG-B increased from 63.13% to 81.35% and from 80.77% to 91.81%, respectively, while the corresponding adsorption capacity of Cr(VI) decreased from 7.58 mg g −1 to 3.25 mg g −1 and from 9.69 mg g −1 to 3.67 mg g −1 with an increase in the material dosage from 2.5 to 7.5 g. In addition, a slow rise in the removal efficiency followed by reaching stabilization at the adsorbent dosage of 6.25 g·L −1 was observed in the reaction system with PG-B while that of PG was on the constant rise though Cr(VI) adsorption efficiency of PG-B was always greater than that of PG, which indicated that increasing additions of adsorbents affected PG more than PG-B. The above-mentioned dosage-dominated efficiency promotion can be ascribed to the higher specific surface areas and more active sites, which was beneficial to Cr(VI) adsorption in the reaction system [47]. These results indicated that increasing the adsorbent addition can facilitate the removal of Cr(VI) by providing more active sites. In consideration of the removal efficiency and material cost, an addition of 6.25 g·L −1 was suitable as the optimal dosage in the latter experiments.

Effect of Material Dosage on the Adsorption Process
The role of adsorbent dosages on the Cr(VI) removal efficiency of PG and PG-B was investigated (Figure 3c,d). It was observed that the increased addition of adsorbents was beneficial to Cr(VI) adsorption efficiency on both PG and PG-B. Specifically, the removal efficiency of Cr(VI) on PG and PG-B increased from 63.13% to 81.35% and from 80.77% to 91.81%, respectively, while the corresponding adsorption capacity of Cr(VI) decreased from 7.58 mg g −1 to 3.25 mg g −1 and from 9.69 mg g −1 to 3.67 mg g −1 with an increase in the material dosage from 2.5 to 7.5 g. In addition, a slow rise in the removal efficiency followed by reaching stabilization at the adsorbent dosage of 6.25 g L −1 was observed in the reaction system with PG-B while that of PG was on the constant rise though Cr(VI) adsorption efficiency of PG-B was always greater than that of PG, which indicated that increasing additions of adsorbents affected PG more than PG-B. The above-mentioned dosage-dominated efficiency promotion can be ascribed to the higher specific surface areas and more active sites, which was beneficial to Cr(VI) adsorption in the reaction system [47]. These results indicated that increasing the adsorbent addition can facilitate the removal of Cr(VI) by providing more active sites. In consideration of the removal efficiency and material cost, an addition of 6.25 g L −1 was suitable as the optimal dosage in the latter experiments.

Effect of Contact Time
The removal efficiency of Cr(VI) in the studied reaction systems with PG and PG-B increased within 0 to 100 min (Figure 4b), where the adsorption equilibrium of both adsorbents was reached in a relatively short time, indicating the strong interaction between materials and Cr(VI). A significant increase in Cr(VI) adsorption efficiency on PG-B was presented at the initial 30 min with nearly 89% Cr(VI) removed, followed by reaching the stable state after 40 min of the reaction while the adsorption equilibrium of Cr(VI) on PG was reached at 60 min of reaction. The reaction solution treated with PG-B showed the highest adsorption capacity of 4.34 mg g −1 , presenting the highest removal efficiency of 90.41%. The rapid enhancement of the removal rate of Cr(VI) on PG and PG-B in the primary stage can be attributed to the abundant unused superficial adsorption sites. As the reaction progressed, the slowdown in the adsorption reaction was presented until the adsorption equilibrium was reached, which may be due to the fact that the binding sites were occupied by the oxygen anion containing chromium and interference between the excess adsorbents, therefore the active sites decreased [48]. We can conclude that the optimal reaction time for removing Cr(VI) by PG and PG-B was 60 min and 30 min, respectively. Similar diurnal variation trends of the removal rate were presented for Cr(VI) on pine cones gel beads nanocomposite modified by ferroferric oxide and on pomegranate-derived adsorbents [49,50].

Effect of Contact Time
The removal efficiency of Cr(VI) in the studied reaction systems with PG and PG-B increased within 0 to 100 min (Figure 4b), where the adsorption equilibrium of both adsorbents was reached in a relatively short time, indicating the strong interaction between materials and Cr(VI). A significant increase in Cr(VI) adsorption efficiency on PG-B was presented at the initial 30 min with nearly 89% Cr(VI) removed, followed by reaching the stable state after 40 min of the reaction while the adsorption equilibrium of Cr(VI) on PG was reached at 60 min of reaction. The reaction solution treated with PG-B showed the highest adsorption capacity of 4.34 mg g −1 , presenting the highest removal efficiency of 90.41%. The rapid enhancement of the removal rate of Cr(VI) on PG and PG-B in the primary stage can be attributed to the abundant unused superficial adsorption sites. As the reaction progressed, the slowdown in the adsorption reaction was presented until the adsorption equilibrium was reached, which may be due to the fact that the binding sites were occupied by the oxygen anion containing chromium and interference between the excess adsorbents, therefore the active sites decreased [48]. We can conclude that the optimal reaction time for removing Cr(VI) by PG and PG-B was 60 min and 30 min, respectively. Similar diurnal variation trends of the removal rate were presented for Cr(VI) on pine cones gel beads nanocomposite modified by ferroferric oxide and on pomegranatederived adsorbents [49,50].

Effect of Initial Concentration
The adsorption performance in varied initial Cr(VI) concentrations was investigated (Figure 4c,d). As expected, the adsorption capacity of Cr(VI) on PG and PG-B presented a significant promotion with an increasing Cr(VI) concentration of the starting solution, which was in accordance with previous studies reporting that high initial metal concentrations in the reaction system could conduct the rapid mass transfer of pollutants and

Effect of Initial Concentration
The adsorption performance in varied initial Cr(VI) concentrations was investigated (Figure 4c,d). As expected, the adsorption capacity of Cr(VI) on PG and PG-B presented a significant promotion with an increasing Cr(VI) concentration of the starting solution, which was in accordance with previous studies reporting that high initial metal concentrations in the reaction system could conduct the rapid mass transfer of pollutants and readily Toxics 2023, 11, 440 9 of 16 saturate binding sites on the surface of adsorbents [51][52][53]. Furthermore, when the initial Cr(VI) concentration went up to 50 mg L −1 , PG-B with a maximum adsorption capacity of 5.87 mg g −1 performed significantly better than PG, which reached 4.94 mg g −1 at the adsorption equilibrium. As compared to PG, enhancing the initial Cr(VI) concentration showed a greater influence on the adsorption performance of PG-B with the adsorption capacity maintaining a steady increase, indicating the slower saturation of adsorption sites on the PG-B surface. However, the adsorption efficiency of Cr(VI) on PG and PG-B decreased from 82.34% to 61.72% and 91.42% to 73.39%, respectively, upon increasing the Cr(VI) concentration from 10 mg g −1 to 50 mg g −1 , which may be attributed to the fact that the limitation in effective binding sites on the fixed number of PG and PG-B particles in the solution hindered the Cr(VI) removal. Considering the practical application and removal performance, 30 mg L −1 was the applicable concentration for further experiments.

Adsorption Isotherms
The adsorption isotherm played a crucial role in studying the adsorption process, indicating the strength of the interaction between adsorption materials and pollutants. The adsorption state of pollutants on the interface of adsorption materials and the structure of the adsorption layer can also be obtained by analyzing the shape and change of the adsorption isotherm. In this study, the Langmuir, Freundlich, and D-R models were used to fit the data obtained from the adsorption equilibrium ( Figure 5). According to the fitting parameters presented in Table 2, both the Freundlich and Langmuir models were appropriate for the Cr(VI) adsorption on PG and PG-B, while the latter presented a higher fitting degree, indicating that Cr(VI) adsorption on pomegranate-peel-derived adsorbents (PG and PG-B) was monolayer adsorption, where homogeneity adsorption sites and the binding process influenced by the chemical-physical interaction were involved [54,55]. According to the relevant results of the Langmuir model, the theoretical adsorption capacity of PG-B (16.229 mg g −1 ) was 26.9% higher than that of PG (12.793 mg g −1 ). The maximum adsorption capacity of the adsorbents in this study and of different bio-waste adsorbents used for the adsorption of water contaminants recently described in the literature was reported for various bio-waste adsorbents on pollutants and is summarized in Table 3. It is evident that PG-B has a stronger adsorption capacity for Cr(VI) of PG-B as it was considerably higher than the other bio-waste adsorbents, proving the superior performance of PG-B adsorbent due to its structural property. Compared with the previous pomegranate peel adsorbent, the PG-B adsorbent in this study achieved the adsorption equilibrium in a shorter time, showing great potential in the effective removal of adsorbents for Cr(VI) adsorption in wastewater purification. The adsorption equilibrium constants (R L ) of the two reaction systems obtained by further calculation (R L = 1/(1 + a × C 0 ) were located at 0~1, confirming beneficial adsorption. In addition, good linearity of the D-R model was found for Cr(VI) adsorption on both PG and PG-B with correlation coefficients (R 2 ) greater than 0.9 and average adsorption energy (E) between 8 kJ·mol −1 and 16 kJ·mol −1 , indicating that chemical adsorption played a leading role in the adsorption process of Cr(VI) on PG and PG-B.

Adsorption Kinetics
In consideration of the adsorption rate, one of the critical parameters determining adsorption performance, a slow adsorption process of Cr(VI) was observed in the reaction system treated with PG while the Cr(VI) removal by PG-B increased rapidly at first followed by slow adsorption. On the one hand, a large number of binding sites contributed to the rapid adsorption during the initial stage of the reaction. On the other hand, efficient mass transfer resulted from the concentration difference between the surface of the adsorbents and the solution, which facilitated the combination of Cr(VI) and unoccupied active sites. This faster adsorption rate has apparent advantages in practical applications. To gain further insight into the removal performance and adsorption behavior characteristics of Cr(VI) on pomegranate-peel-derived adsorbents-, several typical kinetic models, including the pseudo-first-order model, the pseudo-second-order model, and the Elovich model, were employed to evaluate the dynamics of the adsorption process of Cr(VI) on the studied adsorbents. The fitting results of the above kinetic models are shown in Figure 6, with the parameters of the kinetics equations exhibited in Table 2. For PG adsorbents, the equilibrium concentration calculated by the pseudo-first-order model differed greatly from the experimental value, and its R 2 was relatively small, indicating that the pseudo-first-order model was not suitable for the adsorption of Cr(VI) by PG. From the parameters listed in Table 4, a correlation coefficient higher than 0.9 suggested that the adsorption of Cr(VI) onto the PG adsorbent followed the Elovich model, indicating that PG has uniformly distributed surface adsorption energy during the entire adsorption process. As shown in Table 4, the pseudo-second-order model, which emphasized chemisorption as the rate-limiting step, can describe the kinetics of Cr(VI) by PG and PG-B with high correlation coefficient values (R 2 > 0.9). In addition, the theoretical adsorption capacities of Cr(VI) onto PG (3.813 mg g −1 ) and PG-B (4.489 mg g −1 ) were in better agreement with the experimental data (3.76 mg g −1 for PG and 4.34 mg g −1 for PG-B). A similar chemisorption-dominated removal mechanism was presented for Cr(VI) on wheat-residue-derived black carbon [65], sunflower waste carbonaceous adsorbents [66], agricultural waste, and timber industry waste carbons [67], in which reduction, ion exchange, and complexation were also involved [68,69]. In summary, the adsorption process of Cr(VI) onto PG and PG-B adsorbents can be ascribed to the result of a combination of multiple mechanisms due to the complex chemical composition of peel-derived materials rich in binding sites for heavy metals.    Based on the above isotherms and kinetics analysis, the Cr(VI) adsorption process on PG and PG-B was dominated by chemical adsorption and single-layer adsorption. The enhanced adsorption reaction at low pH could be induced by the electrostatic attraction between the protonated surface of materials and the oxygen-containing anion of chromium. Regarding data obtained from FT-IR analysis, the variation peaks after the adsorption reaction, which can be ascribed to oxygen-containing functional groups (such as phenolic hydroxyl, alkoxy, and carboxyl groups), were involved in PG and PG-B adsorbents, which were in accordance with the fact that pomegranate peel contained abundant flavonoids rich in C=C and C=O bonds [70], alkaloids with nitrogen-containing organic compounds [71], gallic acid, and phenols, which contained large amounts of −OH [72,73]. The above results showed that the bond energy interaction and electrostatic attraction between adsorbents and adsorbates could aptly describe the removal process of Cr(VI) on PG and PG-B, which can be summarized as the following three aspects: (i) The electrostatic interaction between Cr(VI) and protonated amine groups on adsorbents surface [74]; (ii) the reduction of hexavalent Cr(VI) to Cr(III) by substantial electron donors on the material surface, including C-C, C-H, -OH, and -COOH [33]; and (iii) the complexation between metal ions and oxygen-containing surface functional groups, such as -COOH, -OH, and -ROH [75].

Conclusions
In summary, the application of agricultural residue-derived adsorbents, including pomegranate peel adsorbent (PG) and its modified product, pomegranate-peel-derived biochar (PG-B), for removing heavy-metal pollutants has been demonstrated. Adsorption experiments, varied characterizations, and model analyses were conducted to reveal the key role that critical experiment parameters play in the removal performance of the studied adsorbents and clarify the adsorption mechanism of Cr(VI). For the pomegranatepeel-derived biochar treated with calcination and chemical modification by Na 2 S 2 O 5 , the Cr(VI) adsorption behavior was found to follow the second-order kinetic model. For the pomegranate peel adsorption material treated by alkali, the Elovich and Langmuir isotherm models presented good applicability in describing Cr(VI) adsorption. Chemisorption and electrostatic attraction dominated the adsorption process of Cr(VI) on the two materials studied. Our study can help understand the role of pomegranate peel-derived biochar in the remediation of heavy metals and open up a new strategy to obtain high-efficiency adsorbents for environment purification.