Preparation of cotton fibers modified with aromatic heterocyclic compounds and study of Cr(VI) adsorption performance

In order to prepare low-cost, biodegradable and processable adsorbent materials for adsorption of heavy metal ion, two kinds of novel modified cottons (C-4-APD and C-2-APZ) were obtained by introducing 4-aminopyridin and 2-aminopyrazine into the surface of degreasing cotton, respectively, and used for the removal of Cr(VI) ions from aqueous solution. The two modified cottons were characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which confirmed the amino groups, pyridine groups and pyrazine groups grafted onto the surface of modified cottons. The maximum adsorption capacities of C-4-APD and C-2-APZ were 89.66 mg/g and 54.92 mg/g, respectively, at the optimum pH of 2 and an initial concentration of 300 mg/g. Kinetic, isotherm and thermodynamic studies were carried out to investigate the adsorption behavior of the modified cottons on Cr(VI) ions. The results showed that the adsorption of Cr(VI) ions by modified cottons followed a pseudo-second-order kinetic model, the equilibrium data were in good agreement with the Langmuir isotherm model, and the thermodynamic analysis indicates that the adsorption proceeds spontaneously. The recovery and reuse of modified cotton were achieved by washing with 2 wt% thiourea-hydrochloric acid solution (0.5 mol/L concentration of HCl), and the adsorption capacities of C-4-APD and C-2-APZ were maintained above 90% and 85%, respectively, after six cycles.


Introduction
The presence of chromium in the environment is generally Cr(III) and Cr(VI). Trace amounts of Cr(III) can reduce the blood glucose concentration in the human body to a certain extent, accelerate the metabolic process of sugar and substances required by the body, and the human insulin activity and stress response capacity can also be regulated by Cr(III) (Kooshki et al. 2021;Moradi et al. 2019). Cr(VI) has higher mobility, bioavailability and carcinogenic effect, and is 100 times more toxic than Cr(III), which can cause more serious harm to various organisms (Kimbrough et al. 1999;Singh and Chowdhuri 2017). At present, the main methods for treating chromiumcontaining wastewater are adsorption (Sun et al. 2021), chemical (Bao et al. 2020), ion exchange , electrochemical (Yao et al. 2020) and biological methods (Long et al. 2020), etc. The most industrial application is adsorption method, which is suitable for the deep purification of low concentration chromium-containing wastewater because of its simplicity and convenience (Zhao et al. 2015). However, most of the adsorbents reported in the literature for the removal of hexavalent chromium are prepared under complicated process conditions, raw materials are not easily available, costs are high, and they are also difficult to degrade in the natural environment, which may cause se condary pollution to the environment in practical applications.
Biomass materials are widely available, relatively low-priced, with large specific surface area, good stability and biodegradable, etc. At the same time, the surface of biomass materials is rich in hydroxyl groups and other reactive groups, which can be regulated and functionalized by chemical or physical methods, and are ideal choices for carrier materials , such as cellulose (Park et al. 2020;Sharma et al. 2018), lignin (Wu et al. 2008), hemicellulose (Wei et al. 2021), chitosan (Vakili et al. 2018;Eliodorio et al. 2017), etc. Highly efficient adsorbents were prepared by grafting the cellulose with functional groups (Zhang et al. 2019;Xie et al. 2017;Li et al. 2018), Liang et al. (2020) reported the maximum Cr(VI) uptake was 490.3 mg/g for quaternized cellulose grafted with polyethyleneimine in the presence of epichlorohydrin. The imidazole groups were successfully grafted onto the macroporous cellulose by a homogeneous method to fabricate 3D macroporous materials with synergistic advantages of cellulose and polyvinylimidazole, and the optimized adsorbent exhibited a fast adsorption rate (0.12 g/(mg/min)) and high adsorption capacity (134 mg/g) (Peng et al. 2020). However, for crude processed biomass materials such as cotton, hemp, and agricultural wastes, the corresponding reports are less available and have a low Cr(VI) adsorption capacity. For example, Rojas et al. (2018) reported the removal of Cr(VI) from water using cotton fibers coated with deacetylated chitosan, and the adsorption capacity for Cr(VI) was 7.5 mg/g at pH 2.0. The crude processed biomass material is more widely available and if it is utilized to improve and increase its adsorption capacity for Cr(VI), the processing steps for preparing biomassbased adsorbents can be more simplified and less costly.
Pyridines and pyrazines are important intermediate compounds that are widely used in the synthesis of medical (agricultural) drugs and in the preparation of dyes, and they can also be used as drugs and analytical reagents themselves. However, there are few reports on the use of pyridines and pyrazines for contaminant treatment. By electron-donating nitrogen, as well as electron-rich p-conjugated structures, and the nitrogen atoms in pyridine group and pyrazine group form hydrogen bonds, so the polymer containing pyridine group or pyrazine group has the ability of coordination chelation, which can be used for the removal of metal ions in solution and environmental pollution control. Geng et al. (2020) reported that two kinds of fluorescent conjugated microporous polymers containing pyrazine moieties were prepared by the polymerization reaction of 2,5-di-triphenylamine-ylpyrazine(DTPAPz) and N,N,N',N'-tetrapheny-2,5-(diazyl) pyrazine (TDPz), for adsorbing and fluorescent sensing of iodine. Bayramoglu et al. (2016) reported that aminopyridine modified Spirulina platensis biomass for Cr(VI) adsorption in aqueous solution. chemical modification of Spirulina platensis biomass was realized by sequential treatment of algal surface with epichlorohydrin and aminopyridine. The adsorption capacity at pH 3.0 was found to be 79.6 and 158.7 mg/g, for native and modified algal biomasses, respectively. Zheng et al. (2019) and Bayramoglu and Arica (2011) synthesized particles with high specific surface area and high adsorption capacity of 344.83 mg/g and 172.11 mg/g, respectively, for Cr(VI) using vinyl pyridine. It is clear from these studies that pyridines and pyrazines have good potential for the treatment of heavy metal ions and other contaminants.
Therefore, in order to prepare low-cost, biodegradable and processable adsorbent materials, 4-aminopyridine and 2-aminopyrazine were introduced to the surface of degreasing cotton using one-pot method in this study. The cotton-4-aminopyridine(C-4-APD) and the cotton-2-aminopyrazine(C-2-APZ) were obtained and used for the removal of Cr(VI) ions from aqueous medium. The morphology and structure property of the prepared samples were characterized by the SEM, FTIR, XPS and BET. Additionally, the adsorption behavior and thermodynamic of Cr(VI) on C-4-APD and C-2-APZ was investigated by Langmuir and Freundlich models, and the experimental data were fitted by two kinetic models, pseudo-first-order and pseudo-second-order (Simonin 2016), to analyze the mechanism of Cr(VI) adsorption, and its reusability was evaluated by repeated adsorption-elution, and the effect of ionic strength was also compared.
Preparation of cotton-4-aminopyridin and cotton-2-aminopyrazine 1 g of degreasing cotton was soaked in the solvent of 50 mL of N,N-dimethylformamide and 50 mL of epichlorohydrin to soak. In the first stage, the reaction temperature was controlled at 80°C, and the chloromethylated cotton was stirred slowly for 2 h with a magnetic stirrer to obtain chloromethylated degreasing cotton; In the second stage, 10 mmol of 4-aminopyridin or 2-aminopyrazine was accurately added to the above solution. The mixture continued to be stirred for 5 h at 90°C. The mass ratio of reactants to products, degreasing cotton: epichlorohydrin: 4-aminopyridine: 2-aminopyrazine: C-4-APD: C-2-APZ = 1: 59: 0.941: 0.951: 1.35 ± 0.05: 1.4 ± 0.05. The procedure for preparation of C-4-APD/C-2-APZ is illustrated in Fig. 1.

Characterization
Surface morphological characteristics of degreasing cotton, C-4-APD and C-2-APZ were examined by scanning electron microscopy (SEM) using a JSM-IT300A scanning electron microscope(JEOL Ltd.,Tokyo, Japan). Bonding structure and surface functionalization of samples were conducted on a VERTEX 70 FT-IR spectrometer by using potassium bromide pressed-disk technique, respectively. The scan range was 400 to 4000 cm -1 . The elemental composition of the samples was measured by XPS using Thermo Scientific K-Alpha (Thermo Fisher). The pore condition of the samples was measured by BET using ASAP 2460 Version 2.02 (Micromeritics).

Adsorption experiments
The effect of pH of the reaction system on the Cr(VI) adsorption capacity brought by C-4-APD and C-2APZ in Cr(VI) adsorption experiments, as well as their adsorption thermodynamics, adsorption kinetics, and recyclability were investigated by a series of adsorption experiments. All Cr(VI) adsorption experiments were performed in 150 mL beakers and shaken at 120 rpm in a mechanical shaker. To explore the effect of pH on the Cr(VI) adsorption capacity, 0.1 g of C-4-APD/C-2-APZ was placed into 100 ml of 100 mg/L Cr(VI) solution with different pH values. The pH was adjusted by concentrations of 0.5 M and 1 M hydrochloric acid. Cr(VI) concentrations were determined by ICP 2060 T inductively coupled plasma emission spectrometer (Jiangsu Tianrui Instruments Co., Ltd., Kunshan, China), and the adsorption capacity of modified cotton for Cr(VI) was calculated as follows: where C 0 and C e (mg/L) are the initial concentration and the equilibrium concentration of Cr(VI), V (L) is the volume of Cr(VI) solution, and m (mg) is the mass of the modified cotton.

Results and discussion
Characterization of the samples Figure 2 shows the scanning electron micrographs of degreasing cotton, C-4-APD and C-2-APZ. The surface of the degeasing cotton fiber was smooth and the diameter of the cotton fiber was about 15 um as seen in Fig. 2a. From Fig. 2b and Fig. 2c, it was observed that the surfaces of both C-4-APD and C-2-APZ become rough, indicating that organic monomers are grafted on the surface of cotton fibers. Figure 3 shows the FTIR spectra of degreasing cotton, C-4-APD and C-2-APZ. The FTIR profiles of degreasing cotton: the peaks at 3315 cm -1 are caused by stretching vibrations due to the presence of a large number of hydroxyl hydrogen bonds in the cotton fiber; the peaks at 1407 cm -1 and 1303 cm -1 are caused by the bending vibrations of CH 2 and CH present in large quantities in the cellulose backbone structure, respectively; the characteristic absorption bands at 1149 cm -1 and 1019 cm -1 are each The characteristic absorption bands at 1149 cm -1 and 1019 cm -1 are each attributed to the asymmetric stretching vibration of C-O-C and the stretching vibration of C-O in the cellulose six-membered ring structure . After attachment of 4-aminopyridine/2-aminopyrazine via epichlorohydrin coupling reaction, the spectrum of the modified degreasing cotton exhibits some changes. The peak formed at 3244 cm -1 was an N-H stretching vibration peak in the imine group (Bayramoglu et al. 2016).  Besides, the new characteristic peaks observed at 1643 cm -1 , 1638 cm -1 , and 1543 cm -1 are caused by C = N and C-N bonds in the aromatic heterocycles, confirming the successful grafting of pyridine and pyrazine groups on the degreasing cotton surface (Hu et al. 2014;Lu et al. 2020).
The mass fraction of elements of the degreasing cotton, C-4-APD and C-2-APZ were determined using XPS to study the amount of aromatic heterocyclic compounds introduced. As shown in Table 1, the degreasing cotton contains almost no elemental N. After grafting the 4-aminopyridine and 2-aminopyrazine onto the surface of the degreasing cotton, the N elemental content of C-4-APD and C-2-APZ increased significantly, which further proved the successful introduction of 4-aminopyridine and 2-aminopyrazine on the surface of degreasing cotton.
The pore condition of the samples was measured by BET. As shown in Table 2, There was no significant change in total area, indicating that the modified cotton did not increase the adsorption capacity by increasing the specific surface area.

Batch adsorption studies
Effect of pH Figure 4 shows the difference in the adsorption capacity of C-4-APD and C-2-APZ for Cr(VI) in the range of solution pH from 2 to 8, with an adsorption time of 4 h. In aqueous media, Cr(VI) ions exist mainly as soluble oxides, and the ionic species depend on the pH of the medium and the total concentration of Cr(VI) in the solution, when the solution pH is less than 2.0, the dominant species is H 2 CrO 4 . When the solution pH is greater than 6.4, only CrO 4 2is present, while the solution pH is between 2.0 and 6.4, HCrO 4and Cr 2 O 7 2dominate (Bayramoglu et al. 2016;Zheng et al. 2019). It can be seen from graphs that the adsorption of Cr(VI) by C-4-APD and C-2-APZ increases as the pH decreases, which mainly due to the N atoms in the pyridine, pyrazine and imine groups are prone to binding H ? under acidic conditions leading to protonation and gravitational binding between them and the negatively charged Cr(VI) anion, resulting in an increase in the removal efficiency of Cr(VI). As shown in Fig. 4, both adsorbents were highly dependent on the pH of the medium, and the adsorption capacity of the former was higher than that of the latter, which was due to the different electron cloud distribution of the N atoms between the 4-aminopyridin and 2-aminopyrazine. The N atoms of the pyridine group in 4-aminopyridin are in the opposite position of the amino group, which have stronger ability to bind H ? when protonated, while the N atoms of pyrazine group in 2-aminopyrazine are in the adjacent and interposition of amino group, respectively, of which the electron cloud distribution was more dispersed, and the ability of binding H ? was weaker when protonated. At the solution pH less than 2.0, the degreasing cotton skeleton started to be decomposed by corrosion, thus, it was determined that the adsorption capacity of C-4-APD and C-2-APZ for Cr(VI) reached the maximum at pH = 2.0, and the subsequent experiments were carried out at this pH value.  Adsorption kinetics Figure 5A shows the effect of adsorption time on the Cr(VI) ions adsorption capacity of C-4-APD and C-2-APZ. The adsorption rate of Cr(VI) was high within 30 min and approached the saturation level at about 360 min. In order to investigate a range of kinetic factors for the adsorption of Cr(VI) ions by C-4-APD and C-2-APZ, pseudo-first-order and pseudo-second-order kinetic models were used to fit the kinetic data: Table 2 The pore of degreasing cotton, C-4-APD and C-2-APZ

Materials
Average pore diameter/nm Total volume in pores/(cm 3 g -1 ) Total area in pores/(m 2 g -1 ) where Q e (mg/g) and Q t (mg/g) are the adsorption capacities of Cr(VI) ions at equilibrium and time t (min) respectively; k 1 (min -1 ) and k 2 ((g/mg)/min) are the pseudo-first-order rate constant and pseudo-second-order rate constant, respectively. Figure 5b and c are the fitted curves of the pseudofirst-order and pseudo-second-order kinetic models for the actual experimental data of C-4-APD and C-2-APZ, and a series of kinetic parameters calculated and fitted by the two adsorption equations are shown in Table 3. As can be seen from the table, the correlation coefficient R 2 of the fitted curves of the pseudo-firstorder model was higher than that of the fitted curves of the pseudo-second-order model, which were closer to 1. And the theoretical Q e values of the adsorbent in the case of pseudo-second-order kinetics are very close to the experimental Q e values, it was considered that C-4-APD and C-2-APZ for Cr(VI) ions follow the pseudo-second-order model, and This suggests that the rate-limiting step may be the chemical adsorption not the mass transport limitation (Bayramoglu and Yakuparica 2008;Bayramoglu et al. 2007). Figure 6a and d shows the adsorption isotherms of Cr(VI) ions by C-4-APD and C-2-APZ. It can be seen from the figure that the adsorption capacities of C-4-APD and C-2-APZ for Cr(VI) ions were almost the same at the initial concentration of Cr(VI) ions below 50 mg/L. However, as the initial concentration continued to increase, C-4-APD showed better adsorption behavior, which also indicated that C-4-APD had better proton binding ability. When the concentration of Cr(VI) ions solution increased to about 300 mg/L, the adsorption capacities of C-4-APD and C-2-APZ for Cr(VI) ions basically ceased to change at room temperature. The highest adsorption capacities of C-4-APD and C-2-APZ for Cr(VI) ions were about 89.66 mg/g and 54.92 mg/g, respectively. By comparing with the maximum adsorption capacities of other surface modified biomass materials (Table 4), we can find that the adsorption capacities of C-4-APD and C-2-APZ were better, moreover, C-4-APD and C-2-APZ are very easy to separate from solution, compared to high specific surface area of particulate or nanoparticle materials.

Adsorption isotherm and thermodynamic analysis
The Langmuir model predicts the formation of an adsorbed solute monolayer, with no side interactions between the adsorbed ions, which also assumes that the interactions take place by adsorption of one ion per binding sites and that the adsorbent surface is homogeneous and contains only one type of binding site, and the Freundlich model does not predict surface saturation, which considers the existence of a multilayered structure (Bayramoglu et al. 2007;Bayramoglu and Yakuparica 2008). Langmuir and Freundlich models were frequently used to evaluate the saturation binding data, the two equations are expressed as follows: where Q e (mg/g) is the amount of Cr(VI) ions bound to the C-4-APD and C-2-APZ at equilibrium, C e (mg/L) is the equilibrium concentration, and Q max (mg/g) is the apparent maximum adsorption capacity. K L is the Langmuir constant, K F and 1/n are the Freundlich constants. The fits of Langmuir and Freundlich isothermal adsorption models for Cr(VI) ions adsorbed by C-4-APD and C-2-APZ were shown in Fig. 6(b-f). The Table 3 Kinetic parameters for Cr(VI) ions uptake by C-4-APD and C-2-APZ

Sample
Pseudo-first-order Pseudo-second-order Q m (mg/g) Q e (mg/g) k 1 (min -1 ) R 2 Q e (mg/g) k 2 ((g/mg)/min) R 2 isotherm parameters of the two models were listed in Table 5. From the fitted curves, it can be seen that the correlation coefficients R 2 of the linear equations after fitting the Langmuir equation were closer to 1 than those of the Freundlich model, which proves that the Langmuir model can describe the experimental measured data more closely. In addition, the maximum adsorption capacities of C-4-APD and C-2-APZ were different from the experimental adsorption capacity. This was due to the formation of hydrogen bonds between the groups in C-4-APD or C-2-APZ, which occupy the adsorption sites and make the actual   (Fenti et al. 2020). These indicated a homogeny adsorption process, leading to monolayer binding.
The experimental data obtained at different temperatures were used to calculate the thermodynamic parameters of Cr(VI) on modified cotton. The thermodynamic parameters, namely, the Gibbs free energy change (DG), standard enthalpy change (DH), and standard entropy change (DS), were calculated according to the following equations to evaluate the adsorption thermodynamic behavior (Zheng et al. 2019;Shi et al. 2020): where K d (mL/g) is the thermodynamic equilibrium constant; and DH (KJ/mol) and DS (kJ/mol/K) can be determined from the slope and intercept of the linear plot of ln K d versus 1/T, respectively . As illustrated in Table 6, the negative DG and positive DH values implied spontaneous and endothermic nature, respectively. The positive DS value manifested the increased disorder and randomness at the solid-liquid interface during the adsorption of Cr(VI) on the surface of modified cottons (Song et al. 2016).

Effect of ionic strength
Dissolved salt ions can interfere with Cr(VI) adsorption by ion exchange, therefore, the effect of ionic strength on Cr(VI) removal by C-4-APD and C-2-APZ was investigated as shown in Fig. 7. In this experiment, the concentration of Cr(VI) was 400 ppm. As the concentration of NaCl or NaSO 4 increased from 0 M to 0.1 M, the inhibition of Cr(VI) removal by coexisting salt ions (NaCl or NaSO 4 ) was not significant, and the adsorption capacities remained above 80% and 88%, respectively. Therefore, the main mechanism of Cr(VI) removal by modified cotton is not ion exchange.

Reusability studies
Desorption and regeneration of adsorbent is one of the most important properties of adsorption separation technology. a thiourea-hydrochloric acid solution with a mass fraction of 2 wt% (the concentration of HCl was 0.5 mol/L) was chosen as the eluent. The mixed thiourea-hydrochloric acid solution can not only elute the Cr(VI) ions from the binding sites, but also thiourea can reduce Cr(VI) to the less toxic Cr(III) under acidic conditions. To investigate the reusability and stability of C-4-APD and C-2-APZ, we performed six adsorption-desorption processes using the same C-4-APD and C-2-APZ, and the adsorption amounts of the six experiments were in Fig. 8. As can be seen from the figure, the adsorption capacities of C-4-APD and C-2-APZ for Cr(VI) ions with multiple reuses did not change evidently and remained above 90 and 85%, which proved that C-4-APD and C-2-APZ have good reusability and stability. Which was due to the excellent flexibility of degreasing cotton as a substrate, the adsorption sites were not easily destroyed and deformed. Furthermore, the adsorption sites encapsulated by hydrogen bonds can replace the destroyed adsorption sites to some extent after multiple washes. Thus, the multiple adsorption-desorption process has less effect on the adsorption capacity of C-4-APD and C-2-APZ.

Conclusions
In summary, 4-aminopyridine and 2-aminopyrazine were successfully introduced to the surface of cotton fiber and used to remove Cr(VI) ions from aqueous solution in this study. SEM, FT-IR and XPS that a large number of amino, pyridine and pyrazine groups were introduced to the surface of cotton fiber after modification by 4-aminopyridine and 2-aminopyrazine. The Cr(VI) ions on the modified cotton fiber adsorption was mainly achieved by electrostatic interactions between the protonated functional groups on the adsorbent and the Cr(VI) anions. The maximum adsorption capacities of C-4-APD and C-2-APZ were 89.66 mg/g and 54.92 mg/g, respectively, at the optimum pH of 2 and an initial concentration of 300 mg/g. Kinetic studies showed that the adsorption process followed the pseudo-second-order kinetic model indicating that chemisorption as a rate-limiting  step that controls the adsorption process. The adsorption isotherm analysis showed that the Langmuir model was in good agreement with the adsorption equilibrium data and the adsorption was the monolayer adsorption process. The thermodynamic analysis indicates that the adsorption proceeds spontaneously. In addition, the adsorption capacities of adsorbents didn't significantly decrease after dissolving other salt ions or six times of repeated use. These results indicate the potential application of 4-aminopyridine and 2-aminopyrazine modified cotton fibers in the removal of Cr(VI) ions from aqueous solutions.