Ethylenediamine and Pentaethylene Hexamine Modified Bamboo Sawdust by Radiation Grafting and Their Adsorption Behavior for Phosphate

Phosphate is an important component for the growth of plants and microorganisms; however, excess phosphate causes serious eutrophication in natural waters. New potential low-loss adsorbents from natural biomass for phosphate removal are desired. Bamboo is one of the most abundant renewable cellulose resources; however, the pure bamboo cellulose is poor to adsorb phosphate. To enhance the adsorption capacity, in this work, bamboo sawdust (BS) was chemically modified by two kinds of amines. First, glycidyl methacrylate (GMA) was grafted on BS using radiation induced graft polymerization. Then, the GMA-grafted BS was further modified by a ring-opening reaction with amines, including ethylenediamine (EDA) and pentaethylene hexamine (PEHA). The amine groups were then quaternized to prepare the BS-GMA-EDA-Q and BS-GMA-PEHA-Q adsorbents. The adsorbents were characterized by FTIR, SEM, TG, and XPS analysis. The adsorption performances of the adsorbents for phosphate were evaluated through batch experiments. The adsorption by BS-GMA-EDA-Q and BS-GMA-PEHA-Q both well obeyed the pseudo-second-order kinetic model and the Langmuir isotherm model, indicating that the adsorption process was chemical monomolecular layer adsorption. The maximum adsorption capacities for BS-GMA-EDA-Q and BS-GMA-PEHA-Q calculated by the Langmuir model were 85.25 and 152.21 mg/g, respectively. A total of 1 mol/L HCl was used to elute the saturated adsorbents. A negligible decrease in adsorption capacity was found after five adsorption–desorption cycles.


Introduction
As an eco-friendly material, bamboo can reach its full growth rapidly in a few months, which is one of the most abundant renewable cellulose resources [1]. Owning to its nontoxicity, cost-effectiveness, and environmentally friendly process, bamboo is widely used in construction and reinforcing fibers, paper, textiles, board, furniture, flooring tiles, food, transportation, and packaging industries, combustion, and other bioenergy applications. However, the overall processing efficiency is relatively low [2]. In recent years, progress has been made in the preparation of bamboo charcoal [3], pulp and cellulose nanofiber [4], bamboo-plastic composite materials [5], antioxidants and immunostimulants [6], and metal ion sensor [7] from bamboo processing residues. This progress shows good prospects for the development and application of bamboo waste [1].
Bamboo is a promising regeneration adsorbent for heavy metal adsorption from an aqueous solution [8]. Thus, bamboo-based bio-adsorbents for removing metal ions in an aqueous solution have been developed [9]. Bamboo charcoal and activated carbon are widely used for contaminated wastewater purification [10]. These reports show bamboo has good adsorption capacity for heavy metal ions. Some efforts have developed an efficient adsorbent by increasing the number of pores [11], or by increasing the number of accessible functional groups [12][13][14]. However, the selected precursor and synthetic conditions influenced the microstructure of bamboo charcoal and activated carbon, thus affecting target ions' removal efficiency [14]. Additionally, bamboo charcoal and activated carbon show poor selective adsorption. Chemically modified bamboo shows a higher adsorption capacity for heavy metals than for activated carbon [8]. The bamboo surface is rich in hydroxyl groups because the main components of bamboo are cellulose and hemicellulose, which account for about 50% of the dry matter [9]. Chemical modification of the hydroxyl group can prepare an adsorbent with higher binding capacity for target ions [8]. Functional groups, especially amino groups chemically grafted onto cellulose, can increase their adsorption performance [15,16]. Some papers focus on the chemical modification of bamboo cellulose and their adsorption performance to heavy metal ions [9,17,18], dyes [12]s and CO 2 [19]. However, few papers have investigated the adsorption of modified bamboo to anions.
The amine groups, which can be protonated or quaternized to form positive groups such as R-NH 3 + and R-NRH 2 + , are often used for anionic contaminants adsorption through electrostatic interaction [20][21][22]. R-NH 2 can be protonated at acidic pH and de-protonated at a high pH, so that the amine group can adsorb anions at an acidic pH. However, the quaternized amine can adsorb anions at a wide pH range even in alkaline conditions [21]. Adsorption commonly occurred at the interface between the adsorbent and the adsorbate in an aqueous solution. More functional groups onto the adsorbent surface can obtain high adsorption capacity and velocity. Radiation-induced graft polymerization (RIGP) is often used to increase functional groups and the capacity to bond anions. RIGP was widely used to modify cellulose biomass because RIGP generates radicals and thus form grafting, which are relevant for subsequent chemical modification and processing. The high grafting yield of RIGP can be achieved quickly and without heating [23]. The functional groups or the intermediate monomer were introduced onto the polymer's surface, so the adsorbents had a high adsorption velocity [24].
Phosphate is the only phosphorus available for plant uptake and is a key limiting nutrient in aquatic ecosystems [25]. The massive discharge of phosphate from human activities and agricultural and industrial production causes eutrophication of waterbodies. It is widely known that the control of phosphate in waterbodies can prohibit eutrophication [26]. Physical, chemical, and biological technologies have been used to remove phosphate from the wastewater. Most of these technologies are available for the removal of high concentration phosphate [27]. Water eutrophication may occur when phosphate in water is maintained at a very low concentration; thus, to protect against eutrophication, phosphate at lower concentrations should be removed.
In this work, two kinds of amines were grafted onto bamboo sawdust (BS) by RIGP technique and further modified to prepare quaternized BS. The phosphate adsorption performance of the two adsorbents before and after quaternization was investigated.

Preparation of BS-GMA-EDA-Q and BS-GMA-PHEA-Q Adsorbent
BS-GMA-EDA-Q and BS-GMA-PHEA-Q adsorbents were prepared by RIGP, and the synthesis process is illustrated in Figure 1.

EB Radiation Grafting
An emulsion solution containing 30% GMA and 3% Tween 20 was placed into a closed container, and oxygen was removed from the solution by nitrogen flow. Dry BS (2 g) was put into PE bags and then vacuum sealed and injected with 30 mL of GMA emulsion. The bags were placed on an electron beam irradiation device of a car and irradiated by an electron beam under an accelerator (Wasik Associates INC, Dracut, MA, USA) at 1MeV. The electron beam delivered a dose of 10 kGy/pass. Samples were irradiated with a dose of 30-70 kGy. After irradiation, the grafted BS was washed with deionized water, and dried in a vacuum at 50 • C.
The grafting yield (GY) was calculated using Equation (1): where W 0 and W g were the mass of BS before and after grafting, respectively.

Preparation of BS-GMA-EDA-Q and BS-GMA-PHEA-Q Adsorbent
The GMA-grafted bamboo powder (BS-GMA) was further functionalized by amines. BS-GMA (2.0 g) was immersed into 100 mL EDA or PEHA with DMF solution at 80 • C for 24 h. After the ring-opening reaction, the BS-GMA-EDA and BS-GMA-PEHA samples were washed and dried at 50 • C to a constant weight. The degree of amination (DA) was calculated by Equation (2): where W g is the weight of BS-GMA and W a is the weight of BS-GMA-EDA and BS-GMA-PEHA. M w a is the molecular weight of the amination agent, and the M w of GMA is 142.15.

Characterization
FTIR spectra were performed in attenuated total reflectance (ATR) mode on a Bruker Tensor 27 (Karlsruhe, Germany) at a wave number range of 4000-400 cm −1 . Surface mor-phologies were observed by a scanning electron microscope (SEM) (Tescan, Vega3) at a voltage of 10 kV. Thermogravimetric (TG) analysis was carried out on a thermogravimetric analyzer (TA instrument mode 600) in a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) analyses were conducted using an AXIS-Ultra instrument (Kratos Analytical). The total phosphate concentration (PO 4 3− , HPO 4 2− and H 2 PO 4 − ) were measured using an Ion chromatograph (MagIC Net 883, Metrohm, Switzerland).

Batch Adsorption Experiments
In the pH effect and effect of contact time studies, the adsorbents (0.05 g) were dispersed in phosphate aqueous solution in a beaker containing 50 mL of 25 mg/L phosphate solution. The beaker was shaken by a shaker at 120 rpm and 25 • C. Adsorption times varied from 5 to 300 min. In the effect of phosphate concentration study, the phosphate concentration ranged from 25 to 250 mg/L. Different pHs were adjusted using 0.5 M HCl or NaOH. The adsorption capacities of phosphate onto the BS-GMA-EDA-Q and BS-GMA-PHEA-Q at different time (Q t ) were calculated by Equation (3): where C 0 and C t were the phosphate concentration before and after adsorption, V was solution volume and m was the mass of adsorbent.

Synthesis of the BS-GMA-EDA-Q and BS-GMA-PHEA-Q
The RIGP technique is mainly dominated by free radical mechanism. The total amounts of free radicals generated in the substrates and the monomer is the main factor affecting the GY. Figure 2a shows the radiation dose effect on the GY. The GY increases with the dose, and it reached a maximum of 239.6% at 60 kGy. However, the GY of GMA decreased when the radiation dose increased above 60 kGy. These phenomena can be explained by the decay mechanism of the trapped radicals. The grafting polymerization was mainly controlled by the total amount of free radicals formed in the substrates. The high adsorbed dose will initiate more free radicals, thus introducing a high GY. Nevertheless, the grafting occurred only at the interface of the substrate and solution, so GY no longer increases with the dose increasing, the decomposition of cellulose molecules was also intensified [28]. The multiple influence factors result in the decrease of the GY. In this study, the grafted BS with the GY 239.6% at 60 kGy were selected for further experiment. The BS-GMA with the GY 239.3% is immersed into EDA or PEHA with the DMF solution for the epoxy ring-opening reaction. The effect of the monomer concentration on DA is shown in Figure 2b. DA increased with monomer concentration and reached 31% for both amine monomer at a monomer concentration 20 wt%, implying that BS-GMA is aminated with the same DA by EDA and PEHA. The BS-GMA -EDA and BS-GMA-PEHA at a monomer concentration of 20 wt% were selected for further experiment. Figure 3 shows the FT-IR spectra of BS based adsorbents. The typical absorption bands of cellulose were observed in the curve (a), including 3345, 2920, 1026, and 895 cm −1 due to O-H, C-H, C-O and C-O-C bonding, respectively [29]. The bands at 1724 cm −1 and 905 cm −1 corresponding to the stretching of the carbonyl and epoxy group of GMA confirmed the successful grafting of GMA on BS. After the ring-opening reaction, the bands at 1460 cm −1 are assigned to the N-H bands of EDA and PEHA [22]. The new bands appeared at 1650 cm −1 in curves e and f are the characteristic bands of quaternary nitrogen, confirming that the tertiary amines of EDA and PEHA were converted into the quaternary ammonium group [30,31].

SEM Photographs
SEM photographs of the spectra of BS-based adsorbents are shown in Figure 4. The surface structure of BS demonstrated ordered fibrils and no other substance attached because lignin was removed by NaOH treatment. After GMA grafting, the cellulose fiber's regular arrangement on the BS surface was destroyed (Figure 4b). After chemical modification by EDA and PEHA and further quaternization, the BS surface became rough due to the GMA graft polymerization and linkage of multiple amine chain attached to the BS surface (Figure 4c,d). After quaternization, the surface became rougher because of the immobilization of 1-bromohexane. The surface changes of BS before and after grafting, combined with the infrared spectra, confirmed that the chemical modification was successful.  Figure 5 shows the TG analysis of BS based adsorbents. BS shows high thermal stability up to 300 • C and decomposes rapidly above 300 • C, showing a one-step weight loss. The BS-GMA curve (b) shows a gradual weight decrease in the range of 230-400 • C. The weight loss was due to the complex thermal decomposition of the BS and the grafted GMA chains. However, for the aminated BS and quaternized BS, the thermal degradation behavior involved a two-stage process. The principal weight loss occurred between 220 and 450 • C, suggesting that these adsorbents could be used for phosphate adsorption.

XPS Analysis
XPS analysis was used to provide the element's chemical state on the adsorbent. Figure 6 shows the high-resolution N1s spectra of BS based adsorbents. The N1s spectra of BS-GMA-EDA were curve-fitted into two peaks located at 399.53 eV and 400.55 eV. N1s spectra of are BS-GMA-PEHA located at 398.77 eV and 400.33 eV [32]. After quaternization, the peak at 399.53 of BS-GMA-EDA shifted to 402.18 eV, and the peak at 398.77 eV shifted to 402.09 eV, showing that quaternization was successful [33][34][35].

XPS Analysis
XPS analysis was used to provide the element's chemical state on the adsorbent. Figure 6 shows the high-resolution N1s spectra of BS based adsorbents. The N1s spectra of BS-GMA-EDA were curve-fitted into two peaks located at 399.53 eV and 400.55 eV. N1s spectra of are BS-GMA-PEHA located at 398.77 eV and 400.33 eV [32]. After quaternization, the peak at 399.53 of BS-GMA-EDA shifted to 402.18 eV, and the peak at 398.77 eV shifted to 402.09 eV, showing that quaternization was successful [33][34][35].  Figure 7a shows the effect of pH on phosphate adsorption onto the BS-based adsorbents. Phosphate exists in different ionic species, including H 3 PO 4 , monovalent H 2 PO4 − , divalent HPO 4 2− , and trivalent PO 4 3− ions, depending on the pH of the solution [36]. At pHs greater than 2, phosphate mainly exists in the anionic forms of H 2 PO 4 − , HPO 4 2− , and PO 4 3− . BS has a negligible adsorption ability to phosphate at all pH ranges. Phosphate adsorption by the aminated BS (BS-GMA-EDA and BS-GMA-PEHA) was influenced by varying the pH. The adsorption capacity was higher at acidic conditions than the adsorption capacity at alkaline conditions. At lower pHs, the protonation of the amine makes the adsorbents positively charged, providing electrostatic attraction to the anionic phosphate molecule, thus resulting in a high adsorption performance. At a pH of 4, BS-GMA-EDA and BS-GMA-PEHA have the highest phosphate adsorption capacity. Below pH 4, the dominant species was transformed to H 2 PO 4 − and neutral H 3 PO 4 , causing the adsorption capacity decrease. At higher pHs, the adsorption capacity decreased due to the de-protonation of the amine and the competition of OH − .

Effect of pH
The zeta potential of BS-GMA-EDA and BS-GMA-PEHA explain the pH-dependent adsorption, as shown in Figure 7b. The zeta potential was higher in acidic conditions than those in alkaline conditions. Thus, the positive value in acidic conditions was favorable for phosphate adsorption. To increase the phosphate adsorption performance of the modified BS in a broader pH range, BS-GMA-EDA and BS-GMA-PEHA were quaternized.
Unlike BS-GMA-EDA and BS-GMA-PEHA, the phosphate adsorption capacity of BS-GMA-EDA-Q and BS-GMA-PEHA-Q was independent of pH. Moreover, the zeta potential was also very high in alkaline conditions. The electrostatic attraction strength between the quaternary ammonium salt group and the phosphate remained unchanged. Thus, the BS-GMA-EDA-Q and BS-GMA-PEHA-Q can absorb phosphate at a broad pH, which is more favorable for a wide range of applications.

Adsorption Kinetics
An essential feature of adsorbents is the adsorption kinetics, which allows the measurement of the adsorption rate. Figure 8a shows the phosphate adsorption capacity of the synthesized adsorbents at different contact times. The phosphate adsorption capacities of BS-GMA-EDA-Q and BS-GMA-PEHA-Q increased rapidly with increasing contact time and reached equilibrium at 180 min. Pseudo-first-order, pseudo-second-order, and Weber-Morris models were used to predict the adsorption mechanism and describe the phosphate adsorption rate of BS-GMA-EDA-Q and BS-GMA-PEHA-Q, expressed by Equations (4)-(6), respectively [37].
where q t and q e (mg/g) are the amounts of phosphate adsorbed on 1 g adsorbent at time t (min) and equilibrium, respectively. k 1 and k 2 are the rate constant for the respective model [36]. K in is the reaction rate constant (mg/g·min 1/2 ), and I is the intercept. The fitting parameters and the correlation coefficients (R 2 ) are listed in Table 1. Table 1. Kinetic parameters obtained from pseudo-first-order, pseudo-second-order kinetic and intra-particle diffusion model.

Model Parameters BS-GMA-EDA-Q BS-GMA-PEHA-Q
pseudo-first-order kinetics The correlation coefficients (R 2 ) of the pseudo-second-order kinetic model for BS-GMA-EDA-Q and BS-GMA-PEHA-Q were 0.9995 and 0.9995, respectively. The high R 2 revealed that the pseudo-second-order adsorption mechanism was predominant in the adsorption process. The pseudo-second-order kinetic model shows that there was a chemical adsorption mechanism between the phosphate and the two adsorbents.
The Weber-Morris model was used to investigate whether intra-particle diffusion was the only rate-controlling process in the adsorption process. The Weber-Morris model describe the ions' transport from aqueous solutions to the adsorbent. In this adsorption process, the plots present three distinct linear regions. The initial region represents the phosphate transport from the aqueous solution to the BS-GMA-EDA-Q and BS-GMA-PEHA-Q surfaces. The second region presents the gradual adsorption corresponding to intra-particle diffusion. The third plateau region indicates equilibrium adsorption. The first linear part did not pass through the origin, indicating that intra-particle diffusion was not the sole rate-determining step [38].

Adsorption Isotherms
The adsorption isotherm was used to describe the distribution of the adsorbate molecules on the adsorbents. BS-GMA-EDA-Q and BS-GMA-PEHA-Q adsorption isotherms were conducted at 25 • C with a phosphate concentration range of 25-250 mg/L.
The correlation between the equilibrium adsorption of q e (mg/g) and the equilibrium concentration C e (mg/L) in the solution is shown in Figure 9a. The adsorption capacity of BS-GMA-EDA-Q and BS-GMA-PEHA-Q increased with the increasing phosphate concentration, and finally reached a constant. Langmuir and Freundlich isotherm models were used to fit the adsorption data at various concentrations. The linear equation can be described by Equations (7) and (8) [39].
ln Q e = ln K F + 1 n ln C e (8) where C e referred the adsorbate concentration (mg/L) at equilibrium, Q e and Q m were the mass adsorbed on 1 g adsorbent and the maximum adsorption capacity (mg/g). Table 2 summarizes the constants of the Langmuir and Freundlich models. The Langmuir isotherm model assumes that the adsorbate has a monolayer adsorption on the adsorbent through a homogeneous surface. Once an adsorbate molecule was adsorbed on an active adsorption site, no more molecules can be adsorbed on the same site. The maximum adsorption capacity was achieved when all the adsorption sites of the adsorbent were saturated with the adsorbate. The Freundlich model was applied to describe adsorption on homogeneous and heterogeneous surfaces. The fitting curves of the Langmuir and Freundlich models are shown in Figure 9b,c, respectively. Comparatively, the Langmuir isotherm model can better describe the adsorption isotherms. The R 2 value (0.9972 and 0.9931) of the Langmuir model was very high, suggesting that the adsorbed phosphate forms a monolayer onto the surface of BS-GMA-EDA-Q and BS-GMA-PEHA-Q [40]. The obtained maximum adsorption capacity (Q max ) for phosphate by BS-GMA-EDA-Q and BS-GMA-PEHA-Q were 85.99 and 152.21 mg/g, respectively. The values were relatively higher compared with other similar adsorbents shown in Table 3.

Regeneration
To evaluate the reusability of BS-GMA-EDA-Q and BS-GMA-PEHA-Q, the adsorptiondesorption cycles were repeated five times using 1 M HCl as the elution reagent. Figure 10 shows that BS-GMA-EDA-Q and BS-GMA-PEHA-Q have a negligible decrease in adsorption capacity after five adsorption-desorption cycles, indicating that BS-GMA-EDA-Q and BS-GMA-PEHA-Q were stable and recyclable for phosphate removal.

Conclusions
Two BS-based adsorbents were successfully synthesized by grafting GMA on BS using the RIGP technique, followed by a ring-opening reaction and quaternization. The adsorption performance of BS-GMA-EDA-Q and BS-GMA-PEHA-Q adsorbents for phosphate removal was investigated.
(1) After quaternization, BS-GMA-EDA-Q and BS-GMA-PEHA-Q showed a high adsorption capacity for phosphate. The adsorption capacity was independent to pH. Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.