Facile Fabrication of Quaternized Sisal Fiber by Electron Beam Radiation and its Effective Adsorption for Indigo Carmine from Aqueous Solution


 The removal of Indigo carmine (IC) from the aquatic environment is necessary due to its high toxicity. In this study, methacryloxy ethyltrimethyl ammonium chloride (DMC) modified sisal fiber (SF-DMC) was prepared by a one-step process using radiation induced grafting polymerization. The adsorption performance of SF-DMC toward IC dye was investigated by batch adsorption experiments. The adsorption kinetic studies shows that adsorption equilibrium reached within 30 min, and it can be well described by pseudo-second-order model. The adsorption isotherms are well described by Langmuir model, and the theoretical maximum adsorption capacity are 709.22 to 892.86 mg/g at different temperature. The adsorption of IC onto SF-DMC is a spontaneous and exothermic reaction and low temperature is favorable for adsorption. Besides, SF-DMC has good selective adsorption for IC in the mixed anionic dyes and in high-salt solution. The dyes on SF-DMC can be desorbed by 2mol/L HCl solution. Therefore, the SF-DMC exhibits excellent adsorption performance, which is suitable for IC removal from high-salt wastewater.


Introduction
Indigo Carmine (IC) is a synthesized anionic dye that is widely used as a colorant in foods, cosmetics and dyeing of clothes (blue jeans) (Ketes et al., 2020). The inappropriate release of IC from large-scale use throughout dyeing process will cause environmental problem and severe harm on human health due to its high toxicity (Gopi et al., 2017;Ahmed et al., 2017). Thus, the removal of IC from the aquatic environment is considered to be necessary (Ahmad et al., 2021). Various methods are used to remove IC from e uents, including membranes (Gopi et al., 2017), chemical oxidation (Shu et al., 2016), photo-catalysis (Secula et al., 2020), electro-catalytic (Lei et al., 2021) and biological methods (Borba et al., 2020).
However, the application of the mentioned processes is limited because of the relatively high operational costs, low e ciency, greater energy consumption, and sludge generation.
Adsorption is considered as one of the most simple and attractive methods to purify polluted water (Chowdhury et al., 2020;Du et al., 2021). In recent years, the low cost adsorbents based on natural bers have been recognized as e cient, cost-effective, and environmental friendly for contamination removal purposes (Candido et al.,2021). Owing to small ber diameter and excellent osmotic stability, the plant proportion of cellulose accounts for 73% (Filho et al., 2020). Cellulose possesses a reactive surface which bears hydroxyl groups make it very suitable to be chemical modi ed and used as adsorbent.
The modi cation of ber matrix has attracted increasing attention, which are relevant strategies to improve the adsorption performance of plant bers. Various methods such as carbonization (Melike et  which need to be simpli ed. To our knowledge, if appropriate monomer and synthesis method were devised, facile fabrication of adsorbent will be obtained. In this paper, we attempted to synthesize a low cost and environmental friendly adsorbent by an one step process through grafting methacryloxy ethyltrimethyl ammonium chloride (DMC) onto SF. Considering the unique advantages of the high stability, ordered hydrophobic pore channels and densely accessible cationic sites, the adsorption performance of SF-DMC are expected to be good for anionic dyes. So, batch adsorption experiments of SF-DMC toward IC were conducted. The effects of solution pH, dosage, adsorption time, initial concentration, ion strength and the coexisted dyes on the adsorption of SF-DMC for IC were studied. The adsorbent may provide a useful alternative for industrial dye removal.

Materials
Sisal ber was obtained from commercial. DMC, HCl, NaOH and IC were obtained from a commercial supplier (Macklin Chemical Reagent Co., Ltd).

Preparation of SF-DMC
SF was boiling treated by 5% NaOH for 30 min, washed and dried at 40 o C. SF with mass of 2 g was vacuum sealed in polyethylene bags. 30 wt% DMC aqueous solution was nitrogen-bubbled to remove oxygen in solution. Then 50 mL DMC solution were extracted using a medical syringe and injected into the bags. The bags were irradiated by the electron beam by an accelerator (1 MeV, Wasik Associates Inc., USA) with the dose rate 10 kGy/pass. The total dose ranged from 0 to 60 kGy. After irradiation, the SF were ltered, washed and dried at 40 o C. Thus, SF was functionalized by DMC (SF-DMC).
The grafting yield (GY) was calculated using the mass increase using equation (1): where W 0 and W g are the mass of SF before and after grafting, respectively.

Characterization
Fourier transform infrared spectroscopy (FTIR) spectra was tested by spectrophotometer (Bruker Tensor 27). The morphologies were observed by scanning electron microscope (SEM) (Tescan Vega 3). Thermogravimetric analysis (TG) curves were recorded using a TGA 55 (TA instruments) with heating rate 10 ℃/min. Zeta potential was tested on a 90 Plus PALS system. The concentration of IC and MO was determined using a UV-vis spectrophotometer (UV-3600, Shimadzu) at 610 nm and 468 nm, respectively.

Batch Adsorption Experiments
In the pH effect studies, 0.03 g of dry SF-DMC was placed in a glass bottle containing 100 mL of 100 mg/L IC solution. Different pH values were adjusted by using 0.5 M HCl or NaOH. In the adsorption kinetic studies, 0.02 g SF-DMC was used and the adsorption times were 1, 3, 5, 10, 15, 20, 30, 60, 90, and 120 min. In the adsorption isotherm studies, 0.02 g and 100mL was used and the IC concentration ranged from 100 to 700 mg/L. The adsorption capacity (Q) and removal percent (R) of SF-DMC to IC was calculated using equation (2) and (3): where C 0 and C t are the IC concentrations before and after adsorption, respectively; V is the solution volume, and m is the weight of SF-DMC. The averages of triplicate measurements were used as the nal adsorption data.
3 Results And Discussion 3.1 Preparation Figure 1 shows the effect of radiation dose on the GY of DMC grafting onto SF. The GY increased with increasing radiation dose, and reached a maximum value of 73% at 60 kGy. This results can be explained by the decay mechanism of trapped radicals. The mechanism of RIGP is mainly a free radical reaction, and the grafting yield is determined by the total free radicals formed both in the monomer solution and the substrate (Zhang et al., 2012). The total free radicals increased with the increasing radiation dose. The grafting polymerization mainly occurs at the interface between monomer and polymers, so, GY nally reached at 60 kGy. Moreover, a higher radiation dose will result in the decomposition of cellulose content, thereafter lead the GY decreasing. Then, the SF-DMC with a GY of 73% was used for further characterization and adsorption studies.  and 380-580°C. After DMC grafting, the two weight loss zones located at 250 to 300°C and 300 to 450°C, which appeared at low temperature compared to SF, which corresponding to the decomposition of DMC and cellulose, respectively. Figure 3 shows the SEM images of SF and SF-DMC. The SF exhibited a ber bundle structure with diameters about 250 µm. The lignin and hemi-cellulose were removed by NaOH treatment and the ber shape is exposed. After DMC grafting, the ber bundle splitted into several thinner bers with diameter of 30-40 µm.

pH effect and Zeta potential
Solution pH plays a signi cant role in controlling the adsorption process. The IC adsorption capacity by SF-DMC was tested at various pH and shown in Figure 4 (a). The adsorption capacity was higher at all the pH range. The concentration of IC after equilibrium adsorption was nearly zero, which means that all the IC in aqueous solution was completely adsorbed. Zeta potential (mV) of SF-DMC was tested and shown in Figure 4 (b), which were positive at a wide pH range. So the positive adsorption site will generate strong electrostatic attraction to the anionic IC molecule at a wide pH range, thus resulted in the high adsorption capacity with pH independent.
3.3.2 Effect of dosage Figure 5 (a) displays the dosage effect of SF-DMC on IC adsorption. The IC removal e ciency increased signi cantly with increase of SF-DMC dosage. This is attributed to the more functional groups of SF-DMC were worked for IC adsorption. The removal e ciency reached 99.8% at 0.015 g/L. The adsorption capacity (Qe) was decreased with increase of the SF-DMC dosage and it was 500 mg/g at dose 0.2 g/L. In this study, the dosage (0.02 g SF-DMC in 100 mL IC) was conducted for further experiment. The initial adsorption rate h 0 (mg/g·min) (t→0) was expressed as equation (7)

Adsorption Kinetics: Effect of Contact Time
where Q t and Q e are the amounts of IC adsorbed per gram SF-DMC at time t and at the equilibrium time, respectively. Table 1 presents the values of the linear correlation coe cient (R 2 ), Q e , k 1 , and k 2 . Comparison of these values showed that the R 2 of the pseudo-second-order model was higher than that of the pseudo-rstorder model. The pseudo-second-order model described the kinetic data well (Figure 5c), suggesting that the IC adsorption onto the SF-DMC was a chemical adsorption process (Wu et al., 2021). The adsorption capacity was 500 mg/g, calculated using the pseudo-second-order model, which was in accordance with the experimental data.
The kinetic data is also described using the intra-particle diffusion model and the Weber-Morris plots are shown in Figure 5 (d). The plot is not straight, but presents multiply distinct regions. According to this model, the rst and second linear parts corresponded to surface diffusion and intra-particle diffusion and the third region mean equilibrium adsorption. The rst linear part did not pass through the origin, suggesting that intra-particle diffusion is not the sole rate determining step (Du et al., 2020).

Adsorption Isotherms: Effect of initial concentration
The adsorption isotherms were used to describe the distribution of target compounds between the solid and liquid phase at equilibrium. Figure 6 (a) shows the experimental data along with the tted isotherm model curves of IC adsorption onto SF-DMC conducted at 298, 308, and 328 K.
Modeling of the experimental data using appropriate isotherm model is often used for prediction of the adsorption mechanism. In this study, Langmuir, and Freundlich models were tted to the adsorption isotherms, which can be expressed as equation (8)  where Q e (mg/g) and C e (mg/l) are the equilibrium adsorption capacity and equilibrium concentration, Q m (mg/g) is the maximum adsorption capacity.
The isotherm constants obtained in this study are given in Table 2. Langmuir model have highest R 2 value (>0.99) at all temperatures, which suggested the adsorption of IC formed a monolayer on SF-DMC . The theoretical maximum adsorption capacity of IC onto SF-DMC was from 709.22 to 892.86 mg/g at temperature ranged from 298 to 328 K. In addition, it is observed that the adsorption capacity is negative correlated with temperature. According to Freundlich model, constant n gives an idea about the favor of the adsorption process. The n is greater than 1, indicating that IC was very favorable adsorbed by the SF-DMC (Hossain et al., 2021).
The maximum adsorption capacities of SF-DMC for IC were compared with other reported adsorbents given in Table 3. The SF-DMC had higher adsorption capacities than most of the other adsorbents. The small speci c gravity and ber diameter of SF combined with the advantage of RIGP, causing the large amounts of DMC monomer onto the surface of SF, which made SF-DMC very suitable for IC dye removal from aqueous solution.    ΔS and ΔG at different temperature were calculated from the slope and intercept by the linear plot and listed in Table 4. ΔG < 0, ΔH < 0 means that the IC adsorption by SF-DMC was a spontaneous and exothermic reaction. The ΔG values decreased as the adsorption temperature increasing, which means that the adsorption was more favorable at lower temperature (Ata et al, 2012). The result was consistent with the results of Langmuir model.

Effect of NaCl Concentration
The effect of NaCl on the adsorption performance is shown in Figure 7 (a). The initial concentration of the IC was 100 mg/L. C IC /C NaCl represents the molar concentration ratio of IC/NaCl. With increasing molar concentration of NaCl, the adsorption capacity of IC was little decreased. When the concentration of NaCl was 1000 times that of the IC, the adsorption capacity was 61.8% of the adsorption capacity without NaCl.

Regeneration and Reusability
The adsorption-desorption experiments were carried out for six cycles and shown in Figure 7 (b). After full adsorption (0.02 g SF-DMC added to 100 mL of 100 mg/L IC), the IC loaded SF-DMC was regenerated using HCl at different concentration. The desorption (%) by 1M HCl was 75%, and nearly 100% by 2 M HCl. So, 2 M HCl was selected to regenerate SF-DMC for further cycle adsorption. After 6 adsorptiondesorption cycles, the removal of IC still remained 95% of the rst use. The results suggested that 2 M HCl is a good elution solution and that the SF-DMC can be used repeatedly for IC removal.

Selective adsorption
It is essential to investigate the selective adsorption performance, thus to well understand the relationship between the adsorbent and dyes to guide the design and fabrication. The competitive adsorption of IC and methyl orange (MO) in binary mixture solution was investigated. Figure 8 ( Conclusion SF-DMC was synthesized by one-step grafting DMC onto SF using electron beam radiation. The results showed that SF was successfully quaternized. The maximum GY value obtained 73% at 60 kGy. The SF-DMC have good adsorption performance to IC such as pH independent, fast adsorption rate, high adsorption capacity, selective adsorption and repeated use. The adsorption kinetics and isotherms of SF-DMC were well obeyed the pseudo-second-order kinetics and Langmuir model respectively. The theoretical maximum adsorption capacity of IC onto SF-DMC was from 709.22 to 892.86 mg/g. The IC adsorption by SF-DMC was a spontaneous and exothermic, which was more favorable at lower temperature. SF-DMC have more preferential adsorption for IC than MO with a selectivity coe cient 72.55 at the molar ratio 1:1 of IC than MO. SF-DMC can be e ciently regenerated by 2 M HCl and repeated use at least for 6 times without the adsorption capacity obvious decrease.   Effect of pH for IC adsorption (a) and Zeta potential of SF (b) Figure 5 Effect of dosage on the adsorption (a); effect of adsorption time on adsorption IC (b); pseudo secondorder (c) and intra-particle model (d) Figure 6 Adsorption isotherms of IC onto SF-DMC; effect of initial concentration (a); Langmuir model (b)

Declarations
; Freundlich model (c) ; and Van't Hoff plots at different concentration (d)