Removal of Acid Red 88 Using Activated Carbon Produced from Pomelo Peels by KOH Activation: Orthogonal Experiment, Isotherm, and Kinetic Studies

Activated carbon (PPAC) from pomelo peels was prepared by carbonization and KOH activation. The performance of PPAC was assessed by removing acid red 88 (AR88) in aqueous solution. The most suitable activation processes were found by orthogonal experiments, aimed to achieve the maximum of removal capacity of AR88. Moreover, the possible mechanisms of adsorption were studied through the results of characterization, isotherm ﬁtting, and kinetics simulation. Results showed the preparation parameter that mattered the most to AR88 removal eﬃciency was the activation temperature of PPAC, followed by impregnation ratio and activation time. The optimal preparation conditions of PPAC were at activation temperature 800 ° C, activation time 90 min, and impregnation ratio 2.5:1. The characterization results showed optimal PPAC had a microporous and amorphous carbon structure whose BETspeciﬁc area and total pore volume were 2504m 2 /g and 1.185cm 3 /g, respectively. The isotherm ﬁtting demonstrated that the sorption process followed the Langmuir model, and theoretical maximal sorption value was 1486mg/g. The kinetics simulation showed that the pseudo-second-order model described the sorption behavior better, suggesting chemisorption seemed to be the rate-limiting step in the adsorption process. This work presented that PPAC was a promising and eﬃcient adsorbent for AR88 from water.


Introduction
Dyes are important materials in paper, textile, leather, and other industries. After a dying process, a large amount of dye wastewater are generated and released into the environment. It is reported that approximately 1.6 million tons of dyes were produced annually [1]. Unfortunately, about 12% of synthetic dyes are lost during manufacturing and processing, and approximately 20% of them enter industrial wastewater [2]. Dye wastewater seriously threatened the ecological system and human health because many of the dye wastewater are difficult to treat and are very venenous or strongly oncogenic [3][4][5]. erefore, dyes must be removed during sewage discharge. As a common azo dye, acid red 88 (AR88, C 20 H 13 NaN 2 O 4 S) is mainly used for dyeing wool, silk, and nylon fabrics. AR88 is difficult to be degraded, and its biotransformed products are carcinogenic to humans [6].
us, it is of interest to take effective measures to treat dyecontaining wastewater.
Pomelo (Citrus maxima) is a widely cultivated Rutaceae arbor in South China and Southeast Asia. China is a large pomelo producer, and approximately 4.8 × 10 6 tons of pomelos were harvested in 2017, indicating 1.49 × 10 6 tons of pomelo peels were generated. However, most of the pomelo peels were thrown away, giving rise to wasting resources and potential contamination.
Previous reports found that pomelo peel is an attractive adsorbent precursor for adsorbing heavy metals [29], antibiotics [30,31], and organic pollutants [32,33]. In the interest of sustainability, this study selected pomelo peels to prepared AC (PPAC) for removing AR88. Potassium hydroxide (KOH) was used as a chemical activator. e influences of significant preparation parameters (activation temperature, activation time, and impregnation ratio) on AR88 removal were discussed by an orthogonal experiment. e possible mechanisms of adsorption were studied through the analysis of characterization (SEM, XRD, and BET), isotherm fitting (Langmuir, Freundlich, and Temkin), and kinetics simulation (pseudo-first-order model, pseudosecond-order model, and intraparticle diffusion model).

Materials and Regents.
Pomelo peels collected from a local fruit shop were the raw material. e raw precursor was first dried in the sun naturally and followed by drying in an oven at 110°C for 48 h. en, they were smashed with a grinder and passed through a sieve to get a particle size range of 80-200 CHN mesh in advance of the following experiment. Figure 1 shows the pomelo peels, pomelo peels meal, and carbonized pomelo peels. All chemicals used were of analytical grade. Distilled water was used for preparation of solutions. AR88 was chosen for adsorbate, and its structure is shown in Figure 2.

Adsorbent.
e preparation of PPAC primarily consisted of carbonization and activation. e brief preparation processes are illustrated in Figure 3. In brief, the dried precursor was put in a tubular stove for pyrolysis with N 2 flow at 600°C for 2 h. en, the carbonized material was impregnated with KOH at different mass ratios (1 : 1-2.5 : 1, chemical: sample), and the mixture was subjected to activation at different activation temperatures (650-800°C) for different activation times (90-180 min). After reaction, the resultant activated carbon (PPAC) was repeatedly rinsed with 0.1 M HCl solution and hot deionized water to remove superfluous alkali. Afterwards, PPAC was dried in an oven at 110°C for 24 h. Finally, PPAC was sieved into 80-200 CHN mesh powder for further experiments. e effect of activation conditions on adsorption efficiency of PPAC was evaluated on the basis of the amount of AR88 adsorbed at equilibrium q e (mg/g), which was figured out by the following equation: where C 0 and C e (mg/l) are the solution concentrations of AR88 at initial and at equilibrium, respectively; V (l) is the volume of AR88 solution; and W (g) is the mass of PPAC.
For the adsorption experiments, the procedures were conducted as follows: 0.1000 g PPAC was added to 300 ml of prepared AR88 solution without pH adjustment. e mixed suspension was stirred in a rotary shaker (25°C, 180 r/min). After reaction, the mixed suspension was filtered by 0.45 µm filtering membrane. e concentration of AR88 in solution was analyzed by an UV-Vis spectrophotometer (UV-2450, Shimadzu) at the wavelength of 503 nm. Each experiment was replicated, and average value was adopted in analysis.

Characterization of PPAC.
e SEM micrographs were analyzed using a scanning electron microscope (FEI Inspect F50). e X-ray diffraction (XRD) patterns of PPACs were recorded by an X-ray diffraction analyzer (Bruker D8 ADVANCE). Cu/Kα radiation was used, and the scan speed was 4°/min, operating at 40 kV and 30 mA. e textural characterization was manifested by N 2 adsorption-desorption at −196°C using an automated surface area and porosity analyzer (TriStar II 3020).

Orthogonal Experiment.
e orthogonal experiment was conducted to investigate the influence of preparation conditions on the dye removal. e levels of preparation conditions are illustrated in Table 1. e initial concentrations of AR88 solution were 500 mg/l. e aqueous samples for analysis were taken at equilibrium.

Adsorption Isotherms.
e adsorption mechanism can be partly deciphered via adsorption isotherm models. e initial concentrations of AR88 solution in isothermal adsorption experiments were 200, 300, 400, 500, and 600 mg/l. e adsorbents used were PPAC of Run 13 in the orthogonal experiment. e aqueous samples for analysis were taken at equilibrium. In this work, the isothermal experiment data were fitted using Langmuir, Freundlich, and Temkin isotherm models given by the following equations, respectively: where q m (mg/g) presents the theoretical maximal sorption value, K L (l/mg) presents the Langmuir coefficient, K F ((mg/g) × (l/mg) 1/n ) and n are the Freundlich coefficients, and B and K T (l/mg) are the Temkin coefficients.

Adsorption
Kinetics. e adsorption kinetics was used to study the adsorption rate and the latent rate-limiting steps. e initial concentrations of AR88 solution in kinetics adsorption experiments were 200, 400, and 600 mg/l. e adsorbents used were PPAC of Run 13 of the orthogonal experiment.
e concentrations of AR88 solution were analyzed at presupposed time intervals. e adsorbing capacity at time t, q t (mg/g), is given as follows: where C t (mg/l) is the concentration of AR88 in solutions at time t (min). e kinetics data were simulated by the pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model listed as follows, respectively: Pseudo − second − order model: Pomelo peels Pomelo peels meal Carbonization Activation

Rinsing with HCl solution
Rinsing with hot water Rinsing with hot water Drying Sieving PPAC powder   Journal of Chemistry where q e and q t (mg/g) have been defined in equations (1) and (5); k 1 (1/min), k 2 (g/(mg·min)), and k 3 (mg/(g·min 1/2 )) are the adsorption rate constants; and C (mg/g) is the constant of the intraparticle diffusion model.

Results and Discussion
3.1. Orthogonal Experiment. Table 2 illustrates the results of the orthogonal experiment. It was seen that the AR88 adsorption varied from 379.21 mg/g to 1473.59 mg/g. e range analysis was easy to show the influence of preparation parameters on AR88 adsorption. e average value of the experiment index was defined as k i (i � 1, 2, 3, 4), and the range is denoted by R in Table 3. Higher k i showed a bigger influence. Based on the range values R A > R C > R B , it indicated that the influence degree of the preparation conditions on AR88 adsorption was activation temperature > impregnation ratio > activation time. Meanwhile, since the k i was the highest at the combination A 4 B 1 C 4 , the best levels for AR88 adsorption are listed as follows: activation temperature was 800°C, activation time was 90 min, and impregnation ratio was 2.5 : 1. Figure 4 intuitively shows the influence of preparation conditions on AR88 adsorption. It presented that as the activation temperature raised from 650°C to 800°C, AR88 adsorption value increased. is might be because higher temperature was beneficial to higher reaction rate and the generation of more new pores. During the preparation, the development of porosity was followed by the reduction reactions as follows [34]: 6KOH + 2C ⟶ 2K + 3H 2 + 2K 2 CO 3 KOH + C ⟶ K + 1/2H 2 + CO e boiling point of element potassium is 770°C. At 800°C, the generated metallic potassium gas diffused into the internal structure of the char, expanding formed pores and producing fresh pores. ese further pores offered more tunnels for dye molecules to enter the porosities [35].
From Figure 4, it was also observed AR88 adsorption declined with increasing activation time. With a longer time, the carbon atoms on the skeleton could be burned, and some of original micropores enlarged and developed into mesopores.
ese caused the elimination of pore areas and volumes, which was futile for AR88 adsorption [36]. Figure 4 also exhibits that AR88 adsorption raised as the impregnation ratio increased. is might be because a small impregnation ratio meant less of activating agent, which induced insufficient contact between KOH and carbon from pomelo peels.
us, incomplete activation led to limited generation of pores. As the impregnation ratio increased, sufficient activation improved the development of porosity, which was favorable for the adsorption. Figure 5 shows the SEM micrographs of pomelo peel, carbonized pomelo peel, and PPAC (A 4 B 1 C 4 ). As shown in Figures 5(a) and 5(b), the morphology of pomelo peel was a smooth surface but with a few wrinkles. Comparatively, Figure 5(c) shows PPAC had a rougher surface with developed pores. ese pores might be created by that the organic matters were consumed by KOH and the thermal treatment, which suggested effective activation.   Figure 4: Effects of preparation conditions on AR88 adsorption of PPAC. Figure 6 shows the XRD patterns of PPAC. e results indicated that PPACs displayed two broad diffraction peaks at around 23.6°and 43.5°, which were corresponding to the (002) and (100) planes of graphitic carbon structure, respectively. e two peaks became weaker at a higher activation temperature, suggesting the graphite crystallites were damaged by heat. PPAC had an amorphous structure.

Activated Carbon Characterization.
e BET analysis results of PPAC (A 4 B 1 C 4 ) are tabulated in Table 4. It showed that the BET specific area and the total pore volume of PPAC were 2504 m 2 /g and 1.185 cm 3 /g, respectively. As for original pomelo peel, the BET specific area and the total pore volume were only 2.153 m 2 /g and 0.0019 cm 3 /g, respectively, which were much lower than PPAC. It also presented the average pore diameter of PPAC was 1.017 nm, which suggested that most of the pores were micropores. us, it indicated that PPAC might be an adsorbent with excellent performance.

Sorption Isotherms.
To ulteriorly understand the mechanism of AR88 adsorption onto PPAC, the isothermal adsorption data were fitted by Langmuir, Freundlich, and Temkin isotherm models, as illustrated in Figure 7. Table 5 lists the corresponding parameters and the correlation coefficient R 2 figured out from the plottings. Higher R 2 showed that the Langmuir model fitted more closely to the isothermal sorption data than the Freundlich model and Temkin model. is suggested the surface of PPAC was compatible physically with monolayer adsorption, where strong interactions had occurred between the dyes molecules and carbons' surface [37]. e basic features of the Langmuir model could be explained by another equilibrium parameter R L [38], which is as follows: where C 0 (mg/l) and K L (l/mg) have been defined in equations (1) and (2). e value (0.0027-0.0081) of R L <1 suggested that the adsorption of AR88 onto PPAC was a favorable process. From Table 5, it also indicated that the theoretical maximal sorption value acquired from the Langmuir isotherm model was 1486 mg/g. It was seen that PPAC could be viewed as a suitable adsorbent for removing AR88. Furthermore, the Freundlich coefficient n was 6.158 (>1), suggesting favorable adsorption [39].

Adsorption Kinetics.
Adsorption kinetics was applied for investigating the efficiency of adsorption at the solid-solution interface. Figure 8 presents that the sorption of AR88 onto PPAC demonstrated a rapid velocity at the beginning stage and 50% of the total adsorption occurred within 25 min.
is was probably because largest of adsorption active sites of PPAC were exposed to molecules of dye at the beginning. Figure 8 also presents that the adsorbed amount increased with increasing initial concentration of AR88. is was probably because a higher dye concentration provided more contact chances between molecules of dye and adsorption active sites.  Journal of Chemistry e kinetics data were simulated with three models, which are shown in Figures 9(a)-9(c). Table 6 lists the kinetics parameters.
Clearly, the pseudo-second-order model fitted kinetics data better (R 2 > 0.99) than the other two. Simultaneously, higher deviations between the experimental adsorption capacity q e,exp and the theoretical value q e,cal were observed according to the pseudo-first-order model and intraparticle diffusion model. us, the pseudo-second-order model gave a better compliance with the kinetics data due to a much higher R 2 and lower deviations between q e,exp and q e,cal . is indicated that pseudosecond-order model was more valid to interpret the sorption kinetics. ese results suggested that chemisorption seemed to be the rate-limiting step which was related to bond forces via electrons sharing or exchange between AR88 and PPAC [40].
Moreover, Table 6 shows the rate constant k 2 decreased as initial concentration of AR88 increased, which was probably because high concentrations generated intense competition for adsorption active sites, facilitating faster adsorption [41].

Conclusions
In this study, the pomelo peel was selected as a precursor to prepare activated carbon (PPAC) by using carbonization and KOH activation. As a cheap adsorbent, PPAC was used to remove AR88 dyes from water. e characterization results showed PPAC had a microporous and amorphous carbon structure. e BET specific area and the total pore volume of optimal PPAC were 2504 m 2 /g and 1.185 cm 3 /g, respectively. According to orthogonal experiments, the preparation parameter that mattered the most to AR88 removal efficiency was the activation temperature of PPAC, followed by impregnation ratio and activation time. e optimal preparation conditions of PPAC were at activation temperature 800°C, activation time 90 min, and impregnation ratio 2.5 : 1. Langmuir model fitted more closely to the isothermal sorption data than the Freundlich model and Temkin model. e theoretical maximal sorption value acquired from the Langmuir isotherm model was 1486 mg/g. e pseudo-second-order model was more valid to interpret the sorption process, which suggested that chemisorption seemed to be the rate-limiting step.
Data Availability e data are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest.