Accepted Manuscript. Polyethyleneimine-Bacterial Cellulose Bioadsorbent for Effective Removal of Copper and Lead Ions from Aqueous Solution

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1 Accepted Manuscript Polyethyleneimine-Bacterial Cellulose Bioadsorbent for Effective Removal of Copper and Lead Ions from Aqueous Solution Xuchen Jin, Zhouyang Xiang, Qingguo Liu, Yong Chen, Fachuang Lu PII: S (17) DOI: Reference: BITE To appear in: Bioresource Technology Received Date: 22 June 2017 Revised Date: 10 August 2017 Accepted Date: 13 August 2017 Please cite this article as: Jin, X., Xiang, Z., Liu, Q., Chen, Y., Lu, F., Polyethyleneimine-Bacterial Cellulose Bioadsorbent for Effective Removal of Copper and Lead Ions from Aqueous Solution, Bioresource Technology (2017), doi: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Polyethyleneimine-Bacterial Cellulose Bioadsorbent for Effective Removal of Copper and Lead Ions from Aqueous Solution Xuchen Jin, 1 Zhouyang Xiang, 1, * Qingguo Liu, 2 Yong Chen, 2 Fachuang Lu 1 1 State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou , China 2 Nanjing High Tech University Biological Technology Research Institute Co., Ltd., Nanjing , China *Corresponding author: fezyxiang@scut.edu.cn; Tel: ABSTRACT: Bacterial cellulose (BC) is a green biopolymer suitable for heavy metal ion removal from aqueous solution due to its nano-porous microstructure. Polyethyleneimine-bacterial cellulose (PEI-BC) was prepared by reductive amination of dialdehyde BC with polyethyleneimine. The capacity of PEI-BC in Cu(II) and Pb(II) adsorption from aqueous solution was investigated. The adsorption kinetics could be well expressed by pseudo-second-order model and the adsorption isotherm data were well fitted with Freundlish model. Adsorption processes of Cu(II) and Pb(II) by PEI-BC reached equilibrium very rapid in 30 and 60 min, respectively. The maximum adsorption capacity of PEI-BC on Cu(II) and Pb(II) was found to be 141 and 148 mg/g, respectively, which was higher than that of unmodified BC and other modified BC reported. PEI-BC also showed good reusability in the adsorption of Cu(II) and Pb(II). This study demonstrates that polyethyleneimine modification makes BC a potential bioadsorbent for heavy metal ion removal in waste water. Keywords: Adsorption; Bacterial Cellulose; Bioadsorbent; Polyethyleneimine; Heavy metal ion 1

3 1. Introduction The heavy metal pollution from industrial waste water is a severe problem. How to deal with and recycle the toxic heavy metal contaminated water has attracted extensive attention of researchers. Different methods of removing heavy metal ions from polluted wastewater have been developed, including chemical precipitation, ion exchange, flocculation, membrane filtration (Yuan and He, 2015), electrochemical treatment (Tran et al., 2017), adsorption (O Connell et al., 2008), etc. Among these methods, adsorption is considered as a very promising and effective method, since many of the natural materials that can be used for adsorption are green, renewable, and prone to chemical modifications. Cellulose and cellulose-based materials including some agro-industrial wastes such as crop straws/stalks, bagasse, crop/fruit/nut shells, etc. have been applied for heavy metal ion removal (Wang et al., 2017; Suhas et al., 2016; De Luna et al., 2015). The oxygen, nitrogen, and sulphur atoms in some functional groups such as amines, carboxylic acids and thiols can easily form strong chelation with metal ions (He et al., 2014). Therefore, the grafting of cellulose-based materials with those functional groups could enhance its adsorption ability and efficiency for heavy metal ions. For example, acrylic acid (Hajeeth et al., 2013) and maleic anhydride (Zhou et al., 2012) were used to modify cellulose with carboxylic acid groups; diethylenetriamine (Gurgel and Gil, 2009) and acrylonitrile N, N- methylenebisacrylamide (Zheng et al., 2010) were used to modify cellulose with amine groups; thioglycolic acid (Wu et al., 2012) was used to modify cellulose with thiol groups. These modifications produced cellulose or cellulose-based materials with enhanced adsorption ability for heavy metal ions. Bacterial cellulose (BC) is a type of extracellular cellulose mainly produced in vitro by 2

4 Acetobacter bacteria (Cheng et al., 2017). There is no essential difference in chemical structure between BC and cellulose from higher plant. They are both cellulose molecules made up of glucose molecules linked by β-1, 4 glycosidic bonds. However, compared to conventional plant based cellulosic materials, bacterial cellulose has a high degree of purity, near 100% of cellulose content and without other common components in plant fiber. Similar to cellulose from other primitive organisms such as algal cellulose, BC microfibril diameter is nm, about one tenth of the ordinary plant fiber filament (Xiang et al., 2016; Yan et al., 2008). The microfibrils of BC interlock with each other, forming a unique network structure (Xiang et al., 2017). Therefore, it has good tensile strength, water bonding ability and excellent shape maintenance ability (Xiang et al., 2017; Cacicedo et al., 2016). Because of the unique advantages of BC, the commercial applications of BC are becoming common, such as temporary skin tissue substitutes, separation membranes, low calorie foods such as coconut Nata, paper strengthening agent, etc. (Cacicedo et al., 2016). Due to its three-dimensional nano-reticular microstructure, BC may be better than regular cellulose in the adsorption of heavy metal ions (Chen et al., 2009a). In order to further enhance the absorption ability to heavy metal ions, different functional groups can also be applied to BC through its abundant hydroxyls. The studies about the application of functionalized BC to ion adsorption have often been reported (Chen et al., 2009a, 2009b; Lu et al., 2014; Oshima et al., 2008; Shen et al., 2009). For example, diethylenetriamine can be grafted onto BC to improve the metal ions adsorption because of the chelation of nitrogen element with metal ions (Shen et al., 2009). However, in previous reports, the ionic adsorption capacity of these modified materials needs to be further improved. Therefore, it is necessary to modify BC with new methods to increase its metal ion adsorption capacity. 3

5 Compared with the previous study of ion adsorption material from BC, this paper is devoted to the study of a novel polyethyleneimine modified BC (PEI-BC) adsorbent. Polyethyleneimine is a highly branched molecule containing a large amount of primary and secondary amine side groups, facilitating its adsorption to heavy metal ions. The capability of amines forming chelation with metal ions has an order of primary> secondary > tertiary amines, due to the steric hindrance effects (Boyd et al., 1947). Additionally, the large amount branched groups of polyethyleneimine may contribute to the formation of highly entangled structures with BC, so that heavy metal ions can be trapped more easily. Along with the three-dimensional nano-network structure of BC, PEI-BC is expected to demonstrate a great adsorption capacity for heavy metal ions. This article also studied the effects of ph, ion concentration, adsorption time and other factors on the adsorption of heavy metal ions by PEI-BC. The adsorption kinetics of the first- and second-order kinetics and Langmuir and Freundlich isotherms were used to evaluate the adsorption process. 2. Experimental 2.1 Materials Gluconacetobacterxylinus ATCC23767 was obtained from Nanjing High Tech University Biological Technology Research Institute Co., Ltd. (Nanjing, China) and used to produce the bacterial cellulose (BC) pellicles. BC pellicles were prepared through the static fermentation method according to Xiang et al., (2017). Polyethyleneimine (M w =~600), sodium periodate, sodium cyanoborohydride, sodium hydroxide, ethylenedinitrilo tetraacetic acid disodium salt (Na 2 EDTA), CuSO 4 5H 2 O and PbCl 2 were purchased from Shanghai Macklin Biochemical Co., Ltd. All other chemicals used are of analytical grade. 4

6 2.2 Preparation of PEI-BC Approximately 30 g of wet BC pellicles (moisture content = 98.5%) were cut into 1 cm 1 cm strips and then disintegrated by a lab blender (SKG 1246, China) at instant mode for 3 times. The disintegrated BC, 90mL distilled water and 0.21 g Na 2 IO 4 were mixed into a conical flask with magnetic stirring at 350 rpm. The reaction was carried out at room temperature and light absent condition for 2 days. After completion of the reaction, the dialdehyde BC were filtered and washed with distilled water several times until ph became neutral. With the dialdehyde BC, 5.6 g polyethyleneimine and 80 ml distilled water were mixed into a conical flask. As a catalyst, 0.21 g sodium cyanoborohydride was also added. The ph of the mixture was adjusted to with 0.1 M hydrochloric acid. The reaction was conducted with magnetic stirring at 350 rpm and room temperature for 6 hours. After reaction, the polyethyleneimine modified BC (PEI-BC) were filtered and washed with distilled water several times to remove impurities. 2.3 Characterization of PEI-BC Surface morphologies were evaluated on undisintegrated BC using a scanning electron microscope (SEM) (Evo-18, Carl Zeiss). The BC and PEI-BC were disintegrated by the lab blender for 2 min and dried on a glass plate in 105 C to form a film. The films were used for Fourier Transform Infrared (FT-IR) analysis. FT-IR spectra were recorded on a Vector 33 equipped with a MCT detector in the absorption mode with a resolution of 4 cm 1 in the range of cm 1. The nitrogen content of the PEI-BC was analyzed on the films through an Elemental Analyzer (Vario EL cube, Elementar, Germany). 2.4 Adsorption Experiments For the adsorption experiment, PEI-BC (with oven dried weight of 0.03 g) was added to 30 5

7 ml CuSO 4 or PbCl 2 solutions at room temperature. The metal ion initial concentrations, ph values and adsorption times varied according to different situations. To investigate the effects of ph on heavy metal ion adsorption, a series of CuSO 4 and PbCl 2 solutions with different ph values were configured by 0.1 M HCl and 0.1M NaOH. The initial concentration for both metal ion solutions was 100 mg/l. The ph values of CuSO 4 solutions were set to 2, 3, 3.5, 4, 4.5 and 5; the ph values of PbCl 2 solutions were set to 3, 3.5, 4, 4.5, 5, 5.5 and 6. The adsorption time was 3 h. For kinetic adsorption experiments the adsorption time was set to 3, 7.5, 15, 30, 60 and 90 min. The initial concentration for both metal ion solutions was 100 mg/l. The ph of CuSO 4 solution was controlled at 4.5, and the ph of PbCl 2 solution was controlled at 5.5. For adsorption isotherm experiments, the ph of CuSO 4 and PbCl 2 solutions were controlled at 4.5 and 5.5, respectively. The adsorption time was 3h. The initial concentrations of Cu(II) or Pb(II) were set to 100, 200, 400, 600, 800, 1000, 1200 and 1400 mg/l. The concentration of Cu(II) or Pb(II) ions in the aqueous solution was measured by an atomic absorption spectrophotometer (Z-2000, Hitachi, Japan). The amount of metal ions adsorbed from the solution was calculated by the following equation (Eq. 1): = (1) where C 0 and C e represent the initial metal ion concentration and adsorption equilibrium mental ion concentration (mg/l), respectively, while the V represents the volume of the solution (L) and m is the absolute dry mass of the PEI-BC used (g). 6

8 2.5 Regeneration of PEI-BC Desorption of metal ions from PEI-BC was examined in a 0.05 M Na 2 EDTA solution. PEI-BC from the adsorption experiments (initial ion concentration of 100 mg/l at ph 4.5 for Cu 2+ and ph 5.5 for Pb 2+ ) with metal ions adsorbed was added into 50 ml of the 0.05 M Na 2 EDTA solution. The mixtures were stirred on a magnetic stirrer at 350 rpm and room temperature for 1 h, and samples were taken from the solution to monitor the amount of metal ions desorbed into the solution. After the desorption test, the PEI-BC were separated and washed with0.05m NaOH solution followed by DI water, and reused in the next cycle of adsorption experiment. The adsorption desorption experiments were conducted for four cycles. 3. Results and Discussion 3.1 Characterization of PEI-BC Selective oxidation by sodium periodates on anhydroglucose units of BC opens the C2-C3 position to form dialdehyde cellulose (Coseria et al., 2013). In the Schiff base reaction mechanism, the amine groups on the polyethyleneimine can react with the aldehyde groups to form C=N structure. However, this first step reaction is reversible, the resulting C=N products can be hydrolyzed back to the original dialdehyde cellulose under the action of dilute acid or dilute alkali (Meng et al., 2014). In order to prevent the reaction from going to the opposite direction, the addition of NaBH 3CN can terminate the reversible reaction by converting C=N bonds into C-N bonds, forming polyethyleneimine modified BC (PEI-BC). The synthesis scheme of PEI-BC can be seen in the Supplementary Data. The BC modification was evaluated by FT-IR spectra, which can be found in the Supplementary Data. The peak at 3400 cm -1 peak corresponds to the abundant hydroxyl groups 7

9 on BC and PEI-BC. In PEI-BC spectra, the peak at 1760 cm -1 may be due to the C=O stretching, indicating that BC was successfully oxidized into dialdehyde BC. The peak at 875 cm -1 may represent the amine bending. The peak at 1630 cm -1 and the intensification of peak at 1430 cm -1 may be due to the C=N and C-N stretching, respectively, indicating that polyethyleneimines were successfully grafted onto BC. The SEM images demonstrate the fine nano-reticular microfibrillar structure of BC. After PEI modification, the microstructure of BC was mostly retained and the nano-porous architecture is very suitable for the adsorption of metal ions from aqueous solution. The nitrogen content of PEI-BC as determined by the Elemental Analyzer is 4.25%. This number may not be very high compared to other amine-based modification on BC such as that by diethylenetriamine (Shen et al., 2009). It may be the results that polyethyleneimine has many side chains and the strong steric hindrance effects lead to relatively low extent of reaction. However, due to the high primary amine quantities, the maximum adsorption capacity of Cu(II) and Pb(II) by PEI-BC may still be improved remarkably. 3.2 Effects of ph on Cu(II) and Pb(II) adsorption PH value is an important factor affecting metal ion adsorption. Precipitations were observed in CuSO 4 solution when the ph was higher than 5.5, so did the PbCl 2 solution when ph was greater than 6.3, which was probably due to the high hydroxyl ion concentration and the forming of metal hydroxide precipitates. To prevent precipitation from interfering with the adsorption experiments, the ph values of the Cu(II) and Pb(II) solutions were controlled between and , respectively. The effects of ph on the adsorption of Cu(II) and Pb(II) onto PEI-BC are shown in Fig. 1. The adsorption capacity of PEI-BC on Cu(II) and Pb(II) increases with the increasing of ph 8

10 values at low ph ranges, but Cu(II) adsorption reaches its maximum capacity at ph 4.5 while Pb(II) reaches its at ph 5.5. This may be attributed to that the adsorption of these ions on PEI-BC follows the principle of complexation adsorption. When ph is low, the large numbers of hydrogen ions compete with Cu(II) and Pb(II) for the adsorption site of NH 2. With the increase of ph, the amino groups are gradually deprotonated and their ability of chelation with Cu(II) and Pb(II) is enhanced, causing the increase of adsorption capacity (Niu et al., 2017; Huang and Chen 2009). Moreover, the abundant hydroxyls on BC ionize under high ph condition and may show a strong electrostatic attraction with metal ions in the solution. However, when ph value exceeds the optimum value, the adsorption capacity of PEI-BC decrease. The reason could be that the complexes formed by amino groups and metal ions are not stable at high ph, because both the surface charges of the adsorbents and the metal species may become negative at high hydroxide ion concentration (Shukla et al., 2002). Therefore, in the subsequent studies, ph was adjusted to 4.5 for the Cu(II) solution, and ph was adjusted to 5.5 for the Pb(II) solution. 3.3 Adsorption kinetic studies By studying the effects of contact time on the adsorption of metal ions onto PEI-BC, the equilibrium adsorption capacity can be obtained, which can be used to fit the adsorption kinetic model. Fig. 2 shows the effects of contact time on the amount of Cu(II) and Pb(II) adsorbed per gram of PEI-BC at ph 4.5 and ph 5.5, respectively. From the diagram, it can be seen that the PEI-BC adsorption of Cu(II) increased rapidly during the first 7.5 minutes, that was ~72.4% of the equilibrium adsorption capacity of Cu(II). The adsorption then gradually reached equilibrium at the 30th minute. As for Pb(II), the high adsorption rate had been maintained for the first 15 minutes, which accounted for 89.2% of the equilibrium adsorption capacity of Pb(II). The adsorption equilibrium reached at the 60th minute. There were a large number of functional 9

11 groups (-OH and -NH 2 ) for binding with metal ions by complexation reaction, ion exchange or electrostatic attraction. The fast adsorption rate at the beginning is attributed to the large numbers of available adsorption sites. The time to adsorption equilibrium was 30 min and 60 min for Cu(II) and Pb(II), respectively. Two common kinetic adsorption models are often used to conduct the fitting analysis of the experimental data, so as to determine the control mechanism of the adsorption process. One of them is the pseudo-first-order model dominated by physical mechanism (Lazaridis and Asouhidou 2003; Li et al., 2007), whose linear equation (Eq. 2) is listed below: ln q q = K t+lnq (2) where q e (mg/g) represents the amount of adsorption at adsorption equilibrium, and q t represents the amount of adsorption at time t. K 1 (min -1 ) stands for kinetic adsorption rate constant in the pseudo-first-order model. The first-order rate constant K 1 and q e are determined by linear fitting of ln(q e-q t) to t. The other model is the pseudo-second-order model dominated by chemical mechanism and its equation (Eq.3) is listed below: t/q =1/ K q +t/q (3) The meanings of q e and q t are the same as in Eq. 2. K 2 [g/(mg.min)] is the kinetic adsorption rate constant of the pseudo-second-order model. The second-order rate constant K 2 and q e were determined by linear fitting of t/q t to t. As shown in Table 1, because the time to Cu(II) adsorption equilibrium is short, causing that the number of data points fitted to the model is only two and the R 2 values for both models equal 1. However, for Cu(II) adsorption, the calculated q e is closer to the q e obtained by experiments 10

12 for pseudo-second-order model. For Pb(II), R 2 2 = is higher than R 2 1 = Therefore, the Cu(II) and Pb(II) adsorption onto the PEI-BC can be explained more properly by the second-order kinetic model and the adsorption is a chemical process. From the pseudo-second-order model, the K value for Cu(II) is much higher than that of Pb(II), which is also in consistent with the experimental results that the time to adsorption equilibrium for Cu(II) was faster than Pb(II). The rate constant K (pseudo-second-order model) is compared with those from previous studies of Cu(II) and Pb(II) adsorptions by various modified BC (Table 2). The K for Cu(II) adsorption by PEI-BC is much larger than most of the K from other studies. The K for Pb(II) adsorption by PEI-BC is comparable to that of other studies. It indicates that the adsorption of Cu(II) and Pb(II) by PEI-BC is a very rapid process. This may be due to that PEI has a highly branched structure and a large quantity of primary amines available for binding. The rapid adsorption indicates that the adsorption mainly occurs at the surface of PEI-BC (Niu et al., 2017). Therefore, PEI-BC may be used for a rapid control of accidental toxic metal pollution. 3.4 Adsorption isotherm studies Fig. 3 shows the effects of initial ion concentrations on the adsorption capacity of BC and PEI-BC to Cu(II) and Pb(II). The data can be used to fit the adsorption isotherm model. At initial ion concentration lower than 1000 mg/l, the saturated adsorption capacity increases almost linearly with the increment of initial ion concentration. However, when the initial ion concentration is higher than 1000 mg/l, the saturated adsorption capacity starts to decrease with an exception of Cu(II) adsorption on PEI-BC. The saturated amount of Cu(II) adsorption on PEI-BC still increases with the initial concentration of Cu(II), but the increase rate decreases. From the experiments, for unmodified BC, the maximum adsorption capacity of Cu(II) and Pb(II) 11

13 was 79 mg/g and 101 mg/g at 1000 mg/l initial ion concentration, respectively. For PEI-BC, the maximum adsorption capacity was 141 mg/g for Pb(II) at 1200 mg/l and 148 mg/g for Cu(II) at 1400 mg/l initial ion concentration. At initial ion concentration lower than 1000 mg/l, PEI-BC shows no advantage to unmodified BC. When the initial ion concentration is higher than 1000 mg/l, PEI-BC starts to show better saturated adsorption capacity than unmodified BC. PEI modification improves the saturated adsorption capacity of BC to Cu(II) by ~94% at 1200 mg/l and ~146% at 1400 mg/l, and the saturated adsorption capacity of BC to Pb(II) by ~107% at 1200 mg/l and ~154% at 1400 mg/l initial ion concentration (Fig. 3). These data show that PEI modification can greatly improve the adsorption capacity of BC at high initial metal ion concentration. This may be due to that, at high metal ion concentration, there are significant numbers of metal ions competing with other ions, such as hydrogen ions, to chelate or bind with the amino groups on PEI-BC. At high initial ion concentration, most of the BC and PEI-BC samples had saturated adsorption capacity even lower than those at lower initial ion concentration. The reason for this phenomenon may be the hindrance of excessive ions. On the one hand, the greater the concentration of electrolyte solution, the smaller the ionization degree is, leading to less binding; on the other hand, the hindrance between excess ions may lead to their failure to reach the appropriate adsorption sites. As for Cu(II), the saturated adsorption capacity of PEI-BC is still increasing at initial ion concentration larger than 1200 mg/l. This could be explained by the relatively stable complexes formed by the binding of Cu(II) to PEI groups (Jia et al., 2014). Langmuir isotherm and Freundlich isotherm models are usually used for the description of the adsorption processes of adsorbents. 12

14 The Langmuir isotherm has been well explained by the adsorption mechanism of many adsorbents and the linear equation (Eq. 4) is shown below (Ho et al., 2005): C /q =C /q +1/ bq (4) where C e (mg/l) represents the amount of metal ions in the solution at absorption equilibrium, q e (mg/g) stands for the equilibrium adsorption capacity to metal ions, q m (mg/g) is the maximum adsorption capacity to metal ions, and b (L/mg) is the Langmuir isotherm constant. Here, q m and b are determined by the linear fitting of C e/q e to C e. Frequently-used Freundlich isotherm is applied to heterogeneous adsorption systems and its linear equation (Eq. 5) is listed below (Karagoz et al., 2008; Shukla et al., 2006): logq =logk +logc /n (5) where q e and C e have the ssame definations as in Eq. 4, K f (L/g) is the constant defining adsorption capacity, and n is the constant describing the adsorption intensity. The value of n generally ranges from 2 to 3. Here, n > 1 means the adsorbent is favorable to adsorption, n = 1 predicts a linear adsorption, and n < 1 is unfavorable to adsorption. The K f and n can be determined from the linear plotting of log q e to log C e. Table 3 shows Langmuir and Freundlich isotherm constants and values of R 2. According to the fitting results, all the R 2 values from the Freundlish model are higher than those from the Langmiur model, so the Freundlish isotherm can better fit the experimental data than the Langmuir isotherm. In addition, most of the q m values predicted by the Langmuir model are deviated from the experimental data (Table 3). Therefore, adsorption process for both Cu(II) and Pb(II) ions on PEI-BC can be better interpreted by Freundlish isotherm model. The n values of Freundlish model for Cu(II) and Pb(II) adsorption onto PEI-BC are all larger than 1, showing that their adsorption processes are favorable. 13

15 The experimental maximum adsorption capacities of PEI-BC to Cu(II) and Pb(II) were 141 mg/g and 148 mg/g, respectively, which were higher than the values predicted by Langmuir isotherm model with 111 mg/g and 125 mg/g. In Table 2, these experimental data are thus used to compared with the maximum adsorption capacity from previous studies of Cu(II) and Pb(II) adsorptions by various modified BC. The maximum adsorption capacities for PEI-BC to Cu(II) and Pb(II) are larger than most of the values for other modified BC adsorbents. Better adsorption performance of PEI-BC for the removal of Cu(II) and Pb(II) from the aqueous solution may be due to the large quantities of active binding sites, which are resulted from the highly branched structure and abundant primary amines of polyethyleneimine groups. 3.5 Regeneration and reusability Desorption of metal ions from used adsorbents are generally carried out using acidic solutions (HCl and HNO3) or EDTA salts. Better desorption performance of EDTA salts than acidic solutions has been reported in previous studies (Liu and Bai, 2006). Therefore, Na 2 EDTA was used to regenerate the used PEI-BC in this study. The desorption percentages of PEI-BC desorption on Cu(II) and Pb(II) were recorded as 82.6 and 63.5%, respectively. These results may be explained by the strong chelating ability of PEI with metal ions, causing incomplete desorption (Jia et al., 2014). The re-adsorption capacity of PEI-BC on Pb(II) was comparable to the original adsorption with 27.9 mg/g, even though the re-adsorption capacity decreased slightly after each use. The re-adsorption capacity of PEI-BC on Cu(II) was lower than the original adsorption, but the re-adsorption capacity kept stable after each use. In general, PEI-BC showed good reusability in the removal of Cu(II) and Pb(II) from aqueous solutions. 14

16 4. Conclusions Polyethyleneimine-BC (PEI-BC) was synthesized and its capacity in Cu(II) and Pb(II) removals from aqueous solution was investigated. The adsorption kinetics could be well expressed by pseudo-second-order model and the adsorption isotherm data were well fitted with Freundlish model. PEI-BC showed much higher maximum adsorption capacity on Cu(II) and Pb(II) than unmodified BC. Compared with most of the reported modified BC adsorbents, PEI-BC has higher adsorption rate and adsorption capacity. PEI-BC could be reused four times with some decrease of adsorption capacity. This study demonstrates that polyethyleneimine modification makes BC a potential bioadsorbent for heavy metal ion removal in waste water. Acknowledgements This work was supported by National Natural Science Foundation of China ( ), Guangzhou Science and Technology Program (General Scientific Research Project ), Guangzhou Science and Technology Program (Key Scientific Research Project ), and China Postdoctoral Science Foundation (2016M590783). References Boyd, G.E., Adamson, A.W., Myers Jr., L.S., The exchange adsorption of ions from aqueous solutions by organic zeolites. II. Kinetics. J. Am. Chem. Soc. 69, Cacicedo, M.L., Castro, M.C., Servetas, I., Bosnea, L., Boura, K., Tsafrakidou, P., Progress in bacterial cellulose matrices for biotechnological applications. Bioresour. Technol. 213, Chen, S., Zou, Y., Yan, Z., Shen, W., Shi, S., Zhang, X., Wang, H., 2009a. Carboxymethylated-bacterial cellulose for copper and lead ion removal. J. Hazard. Mater. 15

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20 Figure Captions Fig. 1 The effects of ph on the adsorption of Cu(II) and Pb(II) onto PEI-BC (initial metal ion concentration 100 mg/l, 0.03g PEI-BC (oven dry weight) in 30 ml solution, contact time 3 h) Fig. 2 The effects of contact time on the adsorption capacity of PEI-BC for Cu(II) and Pb(II) (ph 4.5 for Cu(II), ph 5.5 for Pb(II), initial ion concentration 100 mg/l, 0.03 g PEI-BC (oven dry weight) in 30 ml solution) Fig. 3 The effects of Cu(II) and Pb(II) initial concentration on the adsorption capacity of BC and PEI-BC (ph 4.5 for Cu(II), ph 5.5 for Pb(II), contact time 3h, 0.03 g adsorbent (oven dry weight) in 30 ml solution) 19

21 Tables Table 1 The key kinetic parameters of first- and second-pseudo-order model. Metal ions C 0 (mg/g) q e,exp (mg/g) pseudo-first-order model K 1 (min -1 ) q e,cal (mg/g) R 1 2 K 2 [g/(mg. min)] pseudo-second-order model q e,cal (mg/g) R 2 2 Cu(II) Pb(II) Table 2 Comparison of rate constant from pseudo-second-order model and adsorption capacities of various adsorbents for the removal of Cu(II) and Pb(II) ions from the aqueous solution. Materials K, rate constant from pseudo-second-order model [g/(mg.min)] q m, maximum adsorption capacity (mg/g) Cu(II) Pb(II) Cu(II) Pb(II) References PEI-BC This study Diethylenetriamine-BC Carboxymethylated-BC Amino-BC Amidoximated-BC Shen et al. (2009) Chen et al. (2009a) Lu et al. (2014) Chen et al. (2009b) 20

22 Table 3 Langmuir and Freundlich isotherm constants and values of R 2 for the adsorption of Cu(II) and Pb(II) onto BC and PEI-BC. Metal ions Adsorbent Langmuir parameters Freundlish parameters b (L/mg) q m (mg/g) R 2 K f (L/g) n R 2 Cu(II) Pb(II) BC PEI-BC BC PEI-BC Table 4 Adsorption amounts of metal ions on PEI-BC from four adsorption desorption cycles. Cycle number Amounts of metal ions adsorbed (mg/g) Cu(II) Pb(II)

23 q e (mg/g) Cu(II) Pb(II) ph

24 q e (mg/g) Cu(II),pH=4.5 Pb(II),pH= t (min)

25 q e (mg/g) PEI-BC,Cu(II),pH=4.5 PEI-BC,Pb(II),pH=5.5 BC,Cu(II),pH=4.5 BC,Pb(II),pH= C 0 (mg/l)

26 Highlights 1. Bacterial cellulose (BC) was modified with polyethyleneimine (PEI) through amination. 2. PEI-BC adsorption for Cu and Pb ions reached equilibrium very fast in 30 and 60 min. 3. PEI-BC had much larger adsorption capacity for Cu(II) and Pb(II) than unmodified BC. 4. PEI-BC adsorption fitted pseudo-second-order rate model and the Freundlich isotherm. 22

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