Chitosan/lemon residues activated carbon efficiently removal of acid red 18 from aqueous solutions: batch study, isotherm and kinetics

ABSTRACT In this research, chitosan-decorated activated carbon (AC-CS) was proposed. The AC was cross-linked with glutaraldehyde to prepare an adsorbent (AC-CS). The AC-CS has a rough surface. Adding the AC-CS directly to the dye solution can achieve simple and convenient removal of anionic azo dyes acid red 18 (AR-18). In the dye solution, the AC-CS was used as an adsorbent. The effects of pH, contact time, temperature, initial concentration of AR-18 and the AC-CS dosage on the adsorption efficiency were investigated. Full kinetic and isotherm analyses were also undertaken. In addition, the reusability of the AC-CS was evaluated, and the results showed that the removal rate of AR18 after regeneration remained relatively stable, above 90%. This experiment has shown that AC-CS is a promising anionic azo dye adsorbent. GRAPHICAL ABSTRACT


Introduction
In the past few decades, dyes are widely used in various industries. The textile, paper, leather and cosmetic industries consume more than 1 million tons of commercial dyes every year. Among them, the textile industry is the main dye utilization sector, accounting for nearly 60% of the world's dye consumption. Dye wastewater contains a large number of polar groups and chromophores, and direct discharge will have potential carcinogenic effects on organisms [1,2]. Dye wastewater has the characteristics of deep colour, high concentration, high organic and inorganic content, and it is very difficult to degrade. Dyes accumulate on sediments or water, and their non-biodegradable properties will hinder photosynthesis [3]. The presence of dyes can also affect biological characteristics and also cause harm to human health through food passages, drinking or skin absorption. Dye wastewater, increases chemical oxygen demand and biological oxygen demand, has high organic content and poor biodegradability [4]. AR-18 is a synthetic azo dye, widely used in textiles and food. It is highly water-soluble and will have a serious impact on water bodies after being discharged into the water [5]. At present, the adsorption method [6], the electrochemical reduction method [7], the photocatalytic degradation method, the membrane separation method [8], the electro-Fenton method [9] and other methods are mainly used to degrade and absorb AR-18. Among these methods, the adsorption method is the most widely used because of its low cost, high efficiency and easy operation [10].
Biochar is a kind of solid material rich in carbon, which is obtained from biomass raw material by pyrolysis under anaerobic conditions. In recent years, biochar has attracted more and more attention because of its high specific surface area, developed pore size and low cost. Agricultural waste is a promising adsorbent because of its natural richness, renewability and ecological friendliness. At present, tobacco residue [11], red oak [12], rice husk [1] and coconut husk [13] have been used as raw materials of biochar. Lemons rank third in the world citrus industry, and the most important countries for processing lemon products in the world are Argentina, Italy, Spain, Mexico and the United States [14]. China produces about 2.3 million tons of lemons a year. The lemon residue is the product of lemon after deep processing. Except for a small amount of it used to extract pectin and lemon essential oil, most of it is buried causing serious pollution to the environment. The lemon residue contains cellulose, gum, hemicellulose, chlorophyll, etc. It is an excellent raw material for preparing a water treatment agent. However, currently, there are few reports on the use of lemon residues [15]. Natural polysaccharides are widely used in wastewater treatment due to their richness, non-toxicity, harmlessness and good biocompatibility [16]. Among them, chitosan (CS) is a kind of high molecular product extracted from crustaceans, which is the only natural basic linear cationic polysaccharide in nature. It is mainly composed of a large number of protonated amine (-NH 3 + ) and deprotonated hydroxyl (-O-) functional groups. These functional groups provide highly available active sites and improve the adsorption performance. These functional groups can promote chemical grafting and the mutual attraction between positive and negative charges to form more complex structures [17]. However, chitosan also has certain limitations, such as poor chemical stability, low mechanical strength and easy dissolution under acidic conditions [16,18]. The physical and chemical properties of chitosan can be improved through methods such as nanomaterials, cross-linking, loading, etc., while also improving the adsorption properties [17,18]. Hence, in this study, CS was used to modify the lemon residue to prepare chitosan-lemon residue activated carbon adsorbent (AC-CS). The AC-CS was applied as an adsorbent for the removal of AR-18 azo dye. The effects of the amount of AC-CS, initial pH, initial temperature and reaction time of AR-18 on the adsorption were investigated. The equilibrium isotherm, adsorption kinetics and adsorption mechanism were also investigated. In comparison with other reported adsorbents, the AC-CS showed superior adsorption performance for AR-18 removal. The AC-CS has highly effective adsorbent properties.

Materials
The source of produced AC was lemon residues that were provided from a lemon-making factory (Anyue, Neijiang, China). Distilled water was used to wash it a few times and purify it. It was dried at 80°C overnight and then it was applied as a raw material. In this study, all the chemical reagents (AR-18, chitosan, glacial acetic acid (CH 3 COOH), glutaraldehyde (C 5 H 8 O 2 ), NaOH and HCl) used were purchased from Shanghai Yuanye Biochemical Technology Co., Ltd. (China) and Chengdu Jinshan Chemical Co., Ltd. (China). All chemicals and reagents in this experiment were analytical grade and utilized directly without any further purification.

Synthesis of chitosan/activated carbon composite (AC-CS)
The preparation process of AC-CS was shown in Figure 1. Lemon residues are wrapped in tin foil and placed in the muffle furnace to prepare activated carbon. The temperature of the muffle furnace could rise to 500°C and then maintained for 4 h. Then, it was grinded to make a fine powder and could be taken into use. Then, it was activated with 1M HCl to remove the inorganic salt in the AC, and then washed with deionization water to neutral dry. At 70°C, 2 g chitosan was dissolved in 100 mL 2% acetic acid solution and stirred for 2 h. The solution was labelled as solution A. Two grams of AC was added to solution A and stirred for 3 h, marking the solution as solution B. It was added dropwise to 100 mL of 2 M NaOH solution, left to stand for overnight, filtered, washed with water to neutrality and soaked in 2% glutaraldehyde solution for 4 h. Glutaraldehyde was used to cross-link to improve the physical and chemical properties of chitosan [17]. After cross-linking and curing, it was filtered, washed and dried in an oven at 80°C for 4 h to obtain the product AC-CS. Reusability and stability of adsorbent are critical criteria to determine the adsorbent in practical applications. To assess the stability of the AC-CS, the reusability of the adsorbent is researched under the same conditions by conducting the AR-18 removal efficiency repeatedly once. In this cycle experiment, the adsorbent can be used for the next cycle tests after centrifugation and drying. The regenerated adsorbent is named S-AC-CS.

Characterization of adsorbent
The morphology and microstructure of the prepared adsorbents were analysed by scanning electron microscopy (SEM; VEGA 3 SBH). The functional groups of the lemon residue and AC-CS were identified by the infrared spectrometer (WQF-510A). X-ray diffraction (XRD) was obtained from DX-2700 Dandong Haoyuan Instrument Co., Ltd. for 2θ values of 5-60°. The porosity and the specific surface area of the active carbons have been measured using N 2 adsorption, utilizing Better-size2000 Autosorb iQ2, the automatics adsorption volumetric system. The Brunauer-Emmett-Teller equation (BET) is the most applied procedure to investigate the properties of active carbon.

Batch sorption experiments
Adsorption experiments were carried out in a shaking incubator. The AR-18 stock solution with a concentration of 1 g/L was prepared and stored in the freezer at 4°C. Other concentrations of AR-18 solutions were obtained by diluting the stock solution with deionized water. The concentration of AR-18 in solution was detected by a visible spectrophotometer (723C UV-75*) at the wavelength of 507 nm [5]. The influence of parameters on adsorption including initial temperature, initial pH, reaction time and the AC-CS dosage were investigated to inspect the interaction between AC-CS and AR-18. The removal efficiency of AR-18 was calculated by the following equations [19][20][21]: where q e (mg/g) is the amount of AR-18 adsorbed per unit weight of AC-C, C 0 (mg/L) is the initial concentration of AR-18, C e (mg/L) is the final or equilibrium concentration of AR-18, M (g) is the dosage of AC-CS and V (L) is the volume of the tested solution.
Adsorption isotherm is a fundamental step in the study of the adsorption process. In the isotherm experiments, 0.1 g of AC-CS was added into solutions with a concentration (80 mg/L) of AR-18 at a constant temperature (40°C). Equilibrium isotherm models, including Langmuir and Freundlich models, were employed to fit the data by using the following equations [22,23]: ln where Q e (mg/g) is the adsorption capacity at equilibrium state, K L is the Langmuir constant, Q m (mg/g) is the maximum monolayer adsorption capacity of the adsorbent, C e (mg/L) is the concentration of the solution at equilibrium state, K F (L/mg) is the adsorption capacity and n is the intensity of the adsorption, all of which are the isotherm constants. Adsorption experiments of kinetics were carried out by investigating AR-18 uptake from the solution of 80 mg/L at different time intervals using 0.1/0.35 g of AC-CS at 40°C. Related data were analysed using pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. The three models are described as follows [24][25][26]: The pseudo-first-order kinetic model's equation: The pseudo-second-order kinetic model's equation: Thw Weber-Morris intra-particle diffusion model: where k 1 (min −1 ) and k 2 (g/mg min) are the kinetic model parameter of pseudo-first-order and pseudosecond-order models, respectively. q t and q e (mg/g) are the adsorption capacity at any time and at equilibrium, respectively.

Characterizations of AC-CS
The SEM image of raw AC and AC-CS are presented in Figure 2(a, b), respectively. Also, AC has a smooth surface, the porosity and functional groups of biochar are underdeveloped [27]. However, the irregular form was observed after modification. It is observed the surface of AC-CS is rough and coarser (Figure 2(b)), which is beneficial to provide considerably rough upon exposure to adsorbates in the solution. These surfaces can enhance the adsorption properties greatly [28]. AC, CS and AC-CS were characterized by the powder XRD and the result is shown in Figure 3. Compared with the XRD pattern of AC and AC-CS in Figure 3, the similar crystallinity of both materials, with a wide peak of 15-30°, indicates that both materials are amorphous and can be attributed to the (002) crystal face of amorphous carbon [29]. From the XRD pattern, the distinct diffraction peaks at 2θ = 20°for CS can be observed, which is a unique diffraction peak of CS. In the diffraction pattern of AC-CS, the broad peak at 15-30°is unchanged, but the diffraction peak of CS appears at 2θ = 21°. The peak of AC-CS is stronger than that of AC indicating that the diffractomicity of AC-CS is increased after modification [19]. All of these trends indicate AC-CS composites have been successfully constructed. Figure 4 shows the changes of functional groups before and after carbonization of lemon raw material. The lemon residues of the precursor have a strong peak C-H bending vibration and C-C frame vibration at 1060 cm −1 . C=C and C=N exist at 1651 cm −1 assigned to the stretching vibration of CH, CH 2 and CH 3 exist at 2379 cm −1 , strong peak exists at 3396 cm −1 and O-H exists in a wide range. However, most functional groups of lemon residues are destroyed after modification by heating. Figure 5 illustrates the adsorptiondesorption isotherms of as synthesized samples and the relevant calculated data such as pore volume and pore diameter. According to the IUPAC classification, all prepared samples display a type IV with an H 3 hysteresis loop, reflecting an obvious characteristic of a mesopores structure in the composites [30]. The pore size distribution of AC-CS is approximately 14-120 nm and the main pore size is 14.22 nm indicating mesopores are obtained in AC-CS. The BET surface area of AC-CS is 21.885 m 2 /g. It is worth noting that the BET surface area of AC-CS is small.

Adsorption of AR-18
The effects of adsorption time, adsorption temperature, pH, adsorbent dosage ratio and other parameters on adsorption were studied. The adsorption process was  shown in Figure 6. During the experiment, 100 mL of AR-18 solution with concentration of 80 mg/L was used.

Effect of AC-CS amount on AR-18 adsorption
The effect of the amount of AC-CS on the adsorption of AR-18 was shown in Figure 7(a), as the amount of AC-CS increases from 0.05 to 0.4 g. The AR-18 concentration was 80 mg/L, the temperature was 40°C, the pH was 3 and the contact time was 40 min. The removal rate of AR-18 increased from 44.81% to 98.96%. When the dosage of AC-CS was 0.05 g, the adsorption capacity was 158.4 mg/g, and when the dosage was 0.4 g, the adsorption capacity was 19.8 mg/g. As the dosage of AC-CS gradually increases, the removal rate of AR-18 gradually increases. The increase in the dosage of adsorbent is equivalent to increasing the active sites of adsorption and improving the adsorption performance [13,31].

Effect of initial pH on AR-18 removal
The influence of initial pH on AR-18 was shown in Figure  7(b). With the increase in pH from 1 to 9, the amount of AC-CS was 0.35 g, the concentration of AR-18 was 80 mg/L, the temperature was 40°C and the adsorption time was 40 min. The AR-18 removal rate decreases from 97.63% to 2.5%, and the adsorption capacity decreases from 22.32 to 0.57 mg/g. The change of pH will affect the change of surface charge of AC-CS [32]. AR-18 is an anionic dye, so there will be electrostatic adsorption between AC-CS and AR-18. At a lower pH, the amino and hydroxyl groups on the surface of AC-CS are protonated by H + to adsorb more AR-18. When the pH is neutral or even alkaline, the concentration of OHin the solution increases, and both OH − and AR-18 are negatively charged. The two compete for the adsorption sites of AC-CS at the same time leading to a decrease in the adsorption capacity of AC-CS for AR-18 and the reduction of its removal rate. pH still has a certain adsorption capacity under neutral or alkaline conditions, but the adsorption capacity is very weak, indicating that the electrostatic adsorption mechanism may play a dominant role in the adsorption process and other mechanisms such as van der Waals force, dispersion and other auxiliary adsorption [5,19,33,34].

Effect of temperature on AR-18 removal
The influence of AR-18 by temperature was shown in Figure 7(c). The transport of dye molecules on the surface of the adsorbent is greatly affected by the temperature. This experiment investigated the influence of temperature of 30-80°C on the adsorption performance. The amount of AC-CS was 0.35 g, the concentration of AR-18 was 80 mg/L, the pH was 5 and the adsorption time was 40 min. As shown in Figure 8(c), the adsorption rate was above 97.2% at 30°C, and reached the maximum of 99.68% at 40°C with the increase in temperature. However, as the temperature continued to rise, the removal range of the adsorption rate did not change significantly. This indicates that the adsorbent AC-CS is insensitive to the removal of AR-18 at high temperatures, and the appropriate test temperature can be selected during the experiment [35].

Effect of time on AR-18 removal
The adsorption capacity of AR-18 with different adsorption times was shown in Figure 7(d). The amount of AC-CS was 0.35 g, the concentration of AR-18 was 80 mg/L,  the pH was 5 and the adsorption temperature was 40°C.
With the increase in adsorption time, the adsorption rate increased rapidly in the first 20 min and then reached equilibrium slowly. This may be due to the high affinity and high surface energy of AC-CS in the first 20 min. The surface atoms of the adsorbent are not uniformly loaded, but because it cannot move freely, the adsorption energy of the AC-CS adsorbent is higher than that of liquid. Because of this electrostatic imbalance, the surface atoms on the AC-CS initially adsorb AR-18 from the solution reducing the surface energy and affinity. With the passage of time, the surface energy gradually decreases, the active site is occupied and the adsorption gradually reaches saturation to equilibrium [36].

Isotherm studies
To explore the adsorption capacity of AC-CS for dyes in wastewater, the adsorption isotherm of AC-CS was determined according to Langmuir and Freundlich models, and the maximum adsorption capacity was  calculated. The Langmuir model is used to analyse the adsorption isotherm of a single adsorbent. The model ignores the interaction forces between molecules. The Freundlich model is an empirical equation that assumes that the adsorption process occurs on a heterogeneous surface and is not limited to monolayer adsorption. The Freundlich model takes into account the different affinities of the binding sites on the adsorption surface under the interaction between adsorption molecules. The isotherm also believes that the sites with stronger affinity are first occupied [22,23,37,38]. As shown in Figure S1, AC-CS and S-AC-CS in the Langmuir model, where S-AC-CS refers to the adsorbent regenerated once. The surface of the adsorbent is homogeneous and adsorbed by monolayer. The linear relationship of the reaction is shown in Figure S1. As shown in Figure S2, the surface of AC-CS and S-AC-CS adsorbents in the Freundlich model is non-uniform and belongs to multi-molecular layer adsorption. AR-18 is adsorbed under the action of various forces. The linear relationship between the two models indicates that there are various adsorption mechanisms in the adsorption process.
The parameters of the adsorption isotherm are shown in Table S1. According to research, the AC can be reused 4-5 times by heating, NaOH and HCl activation and other methods after adsorption, and the adsorption rate is still above 80% [29]. The isotherm data are in good agreement with the experimental results. These findings reveal that the adsorption of AR-18 is controlled by the single-layer molecular adsorption mechanism, indicating that the prepared adsorbent is beneficial to the removal of AR-18. 1/n represents the strength of adsorption, and the smaller the value, the better the adsorption performance [39,40]. The correlation of the Langmuir model is better than that of the Freundlich model indicating that the adsorption of AR-18 is mainly monolayer adsorption. The Freundlich model cannot accurately describe the adsorption of AR-18 by the AC-CS adsorbent. It can be seen from Figure 8 that the Langmuir model can better describe the adsorption process of AC-CS to AR-18.

Kinetic studies
The kinetic model and mechanism schematic diagram are shown in Figure S3 and S4. The first-order kinetics is mainly physical adsorption, including intermolecular force, dispersion force, electrostatic interaction and other adsorption methods. The secondary kinetics is mainly chemisorption. There are functional groups on AC-CS, and the functional groups react with AR-18, which is conducive to the adsorption of AC-CS on AR-18. The correlation (R 2 ) of the kinetic equation fully explains the adsorption mode of AC-CS for AR-18.
It can be seen from Figure 9 that the rapid reaction of adsorption occurs 20 min before adsorption, and the data after 20 min is processed by the Weber-Morris intra-particle diffusion model. With increasing contact time at any concentration, the removal efficiency increased [21]. The same conditions refer to the same adsorption time and adsorbent dosage, the initial concentration of AR-18 is high and the adsorption rate is low because the amount of the adsorbent and the total specific surface area of the adsorbent are the same during the experiment. During the experiment, the volume of the AR-18 solution is 100 mL. With the increase in AR-18 concentration, the content of AR-18 is also more, but the amount of AR-18 that can be adsorbed by AC-CS is basically the same. Therefore, with the increase in AR-18 concentration, the adsorption efficiency of AC-CS decreases. The linear correlation curve of the kinetic model is shown in Figure 10. Table  S2 and Table S3 are the fitting equations for different  concentrations and the related linear R 2 . The secondorder kinetics describes the adsorption kinetics of AR-18 well, and its linear correlation R 2 > 0.99 is greater than the correlation of the first-order kinetic equation, so the adsorption is mainly chemical adsorption. The reason for the rapid adsorption at the beginning is that there are enough adsorption sites and a high driving force between AR-18 and AC-CS. The fitting line of the equation in Figure 10(c) does not pass through the origin indicating that the main speed control step is not the intra-particle diffusion [9]. The linear correlation curves of the kinetic models are shown in Figure 10. Table 1 summarizes some of the current methods for processing AR-18, such as adsorption, advanced  The SMCZ has low cost and high availability in nature but low adsorption capacity.

Comparison with other adsorbents
11.277 [47] PEI@PDA/MS The synthesis process was simple, reagent-saving, without use of toxic or expensive chemicals, and thus had the potential to be scalable. / [ 49] oxidation process (AOPs) and photocatalytic degradation. Table 1 compares the properties, advantages and adsorption capacity of the materials prepared by these methods.

Conclusion
The best conditions for AC-CS to adsorb AR-18 are pH 3, dosage of 0.35 g, temperature at 40°C and reaction time of 40 min. The lemon residue modified with chitosan has better adsorption performance for AR-18, with a maximum adsorption rate of 98.93%. After heating and regeneration, S-AC-CS still has good adsorption performance and the adsorption rate of AR-18 is above 85%.
Fitting the adsorption data of AR-18 the model conforms to the Langmuir model and the adsorption is dominated by monolayer adsorption. Its kinetics conforms to the second-order kinetic equation, indicating that the adsorption is mainly chemical adsorption.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Data availability statement
The data that support the findings of this study are available from the corresponding author, upon reasonable request.