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Improved solid-phase synthesis of the phosphorylated cellulose microsphere adsorbents for highly effective Pb removal from water: the batch and fixed-bed column performance and the adsorption mechanism 2+

Xiaogang Luo, Jun Yuan, Yingge Liu, Chao Liu, Xingrong Zhu, Xuehai Dai, Zhaocheng Ma, and Fen Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Sustainable Chemistry & Engineering

Improved solid-phase synthesis of the phosphorylated cellulose microsphere adsorbents for highly effective Pb2+ removal from water: the batch and fixed-bed column performance and the adsorption mechanism

Xiaogang Luo1,∗, Jun Yuan1, Yingge Liu1, Chao Liu1, Xingrong Zhu1, Xuehai Dai1, Zhaocheng Ma2,*, Fen Wang1,3,* 1

Key Laboratory for Green Chemical Process of Ministry of Education; Hubei Key Laboratory for

Novel Reactor and Green Chemistry Technology; School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, 693 Xiongchu Avenue, Wuhan 430073, China 2

Key Laboratory of Horticultural Plant Biology (Ministry of Education); College of Horticulture

and Forestry, Huazhong Agricultural University, Shizishan Street No.1, Wuhan 430070, China 3

School of Chemistry and Chemical Engineering, Sichuan University of Arts and Science, 400#Nanba Road, Dachuan District, Dazhou 635000, China

*

Corresponding author: Xiaogang Luo, Professor, Ph.D. School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, 693 Xiongchu

Avenue, Wuhan 430073, Hubei, China

Tel.: +86-139-86270668;

Email: [email protected]; [email protected] (X. Luo) Corresponding author: Dr. Zhaocheng Ma, Associate Professor. Key Laboratory of Horticultural Plant Biology (Ministry of Education), College of Horticulture and Forestry, Huazhong Agricultural University, Shizishan Street No.1, Wuhan 430070, PR China Email: [email protected] Corresponding author: Fen Wang, Ph.D. School of Chemistry and Chemical Engineering, Sichuan University of Arts and Science,400#Nanba Road, Dachuan District, Dazhou, Sichuan, China

Tel.: +86-158-82909291;

Email: [email protected]

ACS Paragon Plus Environment


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Abstract A highly effective adsorbent with phosphate groups bound to the cellulose microspheres was designed by an improved solid-phase modification method to adsorb the lead ions from water by a batch and fixed-bed column method. The phosphorylated cellulose microsphere adsorbents were prepared through esterification by which phosphate groups were introduced to the interface of regenerated cellulose microspheres which were previously prepared through a sol-gel transition process from a simple cellulose solution. Their morphological, structural and physicochemical properties were characterized by SEM, FTIR, XRD, and DSC, etc. Furthermore, EDX and XPS were used to confirm the chemical modification process and to investigate their phosphate adsorption mechanism. In the batch adsorption experiments, the equilibrium time and adsorption capacity were determined by both of equilibrium and kinetic adsorption experiments, which were also conducted to investigate the adsorption mechanism. In the dynamic adsorption experiments, multiple operation conditions such as flow rate, initial concentration, bed height and pH were evaluated and the experiment data were fitted to several dynamic adsorption models, such as Adams-Bohart, Thomas, Yoon-Nelson, Bed Depth Service Time (BDST), and Dose Response model, to study the performance of adsorption of Pb2+ onto the adsorbents. The results suggested that the chemical adsorption was the main controlled process during the adsorption process and that the adsorbents could highly effectively capture Pb2+ from water via chelation.


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Keywords: Phosphorylated cellulose microspheres; Lead ions; Batch adsorption; Fixed-bed column. Introduction Lead is released into the ecosystem and subsequently enters human body through food, drinking water or air. It causes encephalopathy, cognitive impairment, behavioral disturbances, kidney damage, anemia, and toxicity to the reproductive system of humans.1 All the countries on the earth are striving to reduce the level of elevated blood lead in children, which could seriously damage their intelligence development.2 Therefore, effective remove lead from water is essential for safe water supply, which will ensure the health and the life quality for millions of people under lead-contamination threats. Adsorption technology is one of the most attractive options in many of the processing technologies3, such as its convenience, ease of use and high efficiency. The key and difficult point of the adsorption technology is the design of adsorbent including the choice of adsorbent material and the design of the adsorbent structure. There are many choices for adsorbent material, such as activated carbon (AC), nature polymer, zeolite, and some other low-cost adsorbents.4 Among these materials, cellulose stands out due to its low-cost, wide availability, renewability, biodegradability and biocompatibility, etc.5 However, native cellulose does not have satisfactory adsorption capacity for heavy metal pollution with itself.6-8 Generally, the cellulose-based materials with chemical modification exhibit more high adsorption capacity for various aquatic pollutants.6 Various modified cellulose-based adsorbents



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have been fabricated to improve its adsorption capacity for lots of aquatic pollutants, such as toxic heavy metal ions9, inorganic anions10, or dyes11, etc. The chemical groups introduced in cellulose should be having a good affinity to Pb2+ in water. It is known that phosphate groups show excellent chelating properties.12 Thus, many phosphorylated materials were used for metal chelating. For example, Tatsuya Oshima and co-workers have prepared phosphorylated bacterial cellulose as the adsorbents for various lanthanide ions and transition metal ions removal.13 The heavy metal cations are Lewis acids, while the phosphate groups show Lewis base properties. Based on the Lewis acid−base theory, the heavy metal cations can interact with the phosphate groups via electrostatic interaction or/and chelation.14 Phosphorylation of cellulosic materials using phosphorylating agents such as phosphorus oxychloride (POCl3), phosphorus pentoxide (P2O5), phosphoric acid (H3PO4), diammonium hydrogen phosphate (NH4)2HPO4, and organophosphates have been studied for decades15 and the obtained materials showed excellent ability in chelating metals.12, 16 Thus, the presence of abundant phosphate groups at the surface of cellulose-based materials introduce more active sites for metal ion complexation, making it a potential solution for water treatment. Compared with the previous phosphorylation with H3PO4, the adsorbent fabricated by the solid phase synthesis method12 obviously possessed the advantages as follows: First, the adsorbents were chemical modified on the base of the regenerated cellulose microspheres. Therefore, they maintained the spherical three-dimensional structure of the regenerated cellulose microsphere, which was favorable for the application of fixed-bed column. Second,


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the recycling and reuse of the absorbents in this work is more convenient than the previous ones. In this study, phosphorylated cellulose microspheres were designed by introducing phosphate groups to the cellulose microspheres through an improved solid-phase modification method. The batch kinetic and equilibrium studies, and the influence of flow rate, fixed-bed height, initial phosphate solution concentration and pH at the fixed-bed column experiment were investigated for Pb2+ removal, and the adsorption mechanism was also summarized.

Materials and methods Reagents and materials Cotton linter pulp of ɑ-cellulose >95% was obtained from Hubei Chemical Fiber Group Ltd (Xiangfan, China), and its viscosity-average molecular weight (Mη) was measured by viscometry in cadoxen to be 12.5 × 104 Dalton.17 Urea (CH4N2O, ≥99.0%), dimethylformamide (DMF, ≥99.5%), phosphoric acid (H3PO4, ≥85.0%), sodium hydroxide (NaOH, ≥96.0%), ethanol (CH3CH2OH, ≥99.7%), and lead(II) nitrate (Pb(NO3)2, ≥99.0%) were purchased from Shanghai, China (Sinopharm Chemical Reagent Co., Ltd). The distilled water used in this work was obtained by using the Ultra-pure Water System (Heal Force, Shanghai, China). Fabrication of cellulose microspheres Cellulose microspheres (CM) were produced through a sol-gel transition process in accordance with the reported procedure.18 A solution containing 200 g of



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NaOH/urea/H2O with a weight ratio of 7/12/81 was pre-cooled to -12.5 oC, and then 6 g cellulose samples were dispersed into above solvent with vigorous stirring for 2 min to obtain a transparent cellulose solution. Six hundred (600) mL of liquid paraffin and 15 g of span80 were added into a 1000 mL three-neck flask and stirred at a constant speed for one hour, and then 150 mL cellulose solution was dispersed in the organic phase with continual stirring. After two hours, about 90 mL of hydrochloric acid solution (3.0 M) was dripped continuously via an injection pump until the mixture was adjusted to pH 7.0, and then the CM was formed. After one hour, the suspension was separated into two layers, the upper organic phase was recovered, and the lower layer was washed with deionized water, and then with ethanol three times to remove the residual liquid paraffin and span80. The obtained cellulose microspheres were further washed with deionized water to remove ethanol, and then kept in deionized water at 4 oC for further use. Fabrication of phosphorylated cellulose microspheres Phosphorylated cellulose microspheres (CM-P) were fabricated according to the reported method.13 CM kept in deionized water was washed gradually with a series of 300 mL DMF-water solutions with volume percent of 20%, 40%, 60%, 80% and 100%, so eventually CM was soaked in DMF solution. Then 4 g CM (dry weight) was soaked in 300 mL DMF along with 40 g urea. The above mixture was stirred at 100 o

C for one hour, and then 20.0 g 85% H3PO4 was added drop by drop. After dropwise,

the reaction was conducted to 135 oC and kept for eight hour. After cooling to 25 oC, 2 L deionized water was used to wash the mixture to obtain the CM-P.


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Pb2+ solution and detection The stock solution (1000 mg L-1) of Pb2+ was prepared by Pb(NO3)2 with 0.1 mL of 1M HNO3 to avoid precipitation of lead hydroxide. In this work, the working solutions were prepared by diluting the above stock solution with the deionized water, and the initial pH of each working solution was adjusted by addition of 0.1 mol L-1 NaOH or/and 0.1 mol L-1 HNO3. An atomic absorption spectroscope (AAS, SP3530AA, Shanghai Spectrum, china) was used to detect the Pb2+ concentration of the solution after adsorption Characterization The morphological and structural properties of the microspheres were observed with a field emission scanning electron microscopy (SEM, SIRION TMP, FEI). Elemental concentrations were measured by Energy Dispersive X-ray mappings (EDX). Specific surface area of the microspheres was determined via N2 adsorption and desorption test at 77K by Micromeritics ASAP-2020. Sample microspheres were ground into powders and dried in the vacuum oven at 333 K for 48 hours for other characterizations. Fourier-transform infrared (FT-IR) spectra of these samples were recorded on a FT-IR spectrometer (model 1600, Perkin-Elmer Co.) with KBr pellets. Wide-angle X-ray diffraction (XRD) analysis was carried out with an XRD diffractometer (D8-Advance, Bruker) to confirm the crystal structure property of these materials. The thermal analysis was carried out be the differential scanning calorimetry (DSC) (Seiko Instruments, DSC6220). The semi-quantitative chemical composition and surface chemical states of the samples were examined through the



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X-ray photoelectron Spectrometer (XPS) with an ESCALAB250 X-ray Photoelectron Spectrometer (Thermo Fisher, ESCALAB 250Xi). Batch adsorption experiments The experimental methods were the same as the previous works.2 In the experiment, the effects of dosage of the wet CM-P (0.01, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 g), temperature (25, 35 and 45 oC), and pH (1.0, 2.0, 3.0, 4.0, and 5.0) on the adsorption of the CM-P were studied. The wet CM-P samples were added to each conical flask (100 mL) which contained 30 mL Pb2+ solutions, and then the conical flasks were kept in the thermostatic shaker with shaking for 24 h at 250 rpm. The phosphorylated cellulose microspheres which adsorbed Pb2+ were coded as CM-P-Pb. Metal removal efficiencies and adsorption capacities were calculated by the equations of the following: ሺ஼బ ି஼೐ ሻ×ଵ଴଴

ܴ= ‫=ݍ‬

஼బ ሺ஼బ ି஼೐ ሻ×௏ ெ

%

(1) (2)

where q (mg g-1) is the adsorption capacity of the CM-P, C0 (mg L-1) is the initial concentration of Pb2+, R (%) is the metal removal efficiency of Pb2+, Ce (mg L-1) is the equilibrium concentration of Pb2+, V (L) is the volume of the Pb2+ solution, and M (dry weight: g) is the mass of the CM-P. Kinetics data were represented by nonlinear pseudo-first-order model and nonlinear pseudo-second-order model1, and that were given as followings: ‫ݍ‬௧ =

൫௘ ೖభ ೟ ିଵ൯ ௘ ೖభ ೟

‫ݍ‬௘

(3)

where ݇ଵ is the pseudo-first-order model rate constant (min-1);


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‫ݍ‬௧ =

௤೐ మ ௞మ ௧ ଵା௤೐ ௞మ ௧

(4)

where ݇ଶ is the pseudo-second-order model rate constant (g mg-1 min-1). Equilibrium adsorption data were analyzed by both of the Langmuir isotherm equation and the Freundlich isotherm equation, which were presented as follows: ௤ ௄ ஼೐ ಽ ஼೐ ሻ

೘ ಽ ‫ݍ‬௘ = ሺଵା௄

(5)

where ‫ܭ‬௅ is the Langmuir adsorption constant (L mg-1), and ‫ݍ‬௠ is the monolayer maximum adsorption capacity of the adsorbent (mg g-1); ଵ/௡

‫ݍ‬௘ = ‫ܭ‬௙ ‫ܥ‬௘

(6)

where n and ‫ܭ‬௙ (mg g-1) are the Freundlich adsorption constants. Fixed-bed adsorption process and modeling The experiments and the experimental models studies were conducted the same way as the previous works.19 The CM-P adsorbents of different amounts were loaded in a glass column, which internal diameter is 1.1 centimeter, and all experiments were maintained at 298 K. Before experiment, the column should be preconditioned to pH 5.0 by the eluting water adjusted by 0.1 M HNO3, meantime, the flow rate of the eluting water of the outlet of the column was measured in order to get the specified and stable flow condition. In order to get the breakthrough curves, the effluent samples were collected at specified time intervals. The effect of the experimental parameters, such as flow rate (1.0, 1.4 and 3.0 mL min-1), pH (3.0, 4.0 and 5.0), initial concentration (50, 100 and 150 mg L-1), and bed height (5.0, 7.5 and 10.0 cm) were investigated through the breakthrough curves. The data were applied to some models



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which were listed in Table S1, such as Adams-Bohart20, Thomas21, Yoon-Nelson22, BDST23, and Dose Response 24 models so on. The column adsorption capacity, qc (mg), for a given flow rate and inlet concentration equals the area under the curve of the adsorbed Pb2+ concentration Cads (Cads=C0-Ct), where C0 and Ct are the influent and effluent Pb2+ concentration respectively, versus time (min), which is calculated as following: ொ∙஺

ொ

௧ୀ௧

‫ݍ‬௖ = = ଵ଴଴଴ ‫׬‬௧ୀ଴ ‫ܥ‬௔ௗ௦ ݀‫ݐ‬ ଵ଴଴଴

(7)

where A is the area under the breakthrough curve, Q (mL min-1) is the flow rate and t (min) could be tb, ts, or tt that represent the breakthrough time, the saturation time and total time, respectively. The ݉ (mg), the amount of Pb2+ infused to the column at different time, is calculated as follows: ݉=

஼బ ∙ொ∙௧ ଵ଴଴଴

(8)

The metal removal, R (%), is calculated as following: ܴ=

௤೎ ௠

∙ 100

(9)

The weight of Pb2+ adsorbed per unit dry weight of the microspheres adsorbents: the adsorbing capacity (q (mg g-1)) can be calculated by: ‫=ݍ‬

௤೎ ெ

(10)

where M represents the quality of the CM-P in the experiment (g: dry weight). The empty contact time, EBCT (min), was obtained from dividing the bed volume by the flow rate as follows: EBCT =

୆ୣୢ ୴୭୪୳୫୬ ୊୪୭୵ ୰ୟ୲ୣ

(11)


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The mass transfer zone, MTZ (cm), was calculated as follows: ௧

MTZ = ‫ܪ‬ሺ1 − ௧್ ሻ ೞ

(12)

where H is the bed height (cm).

Results and discussion Characterization The CM-P (a, b) and CM-P-Pb (c, d) are the SEM testing images, which exhibited in Figure 1. The microspheres with good spherical shape (Figure 1a and 1c) have the average diameter of the microspheres in the range of 40-60 µm. The diameter distribution of microspheres is in line with Gaussian distribution with the sol-gel transition method.18 Thus, the CM-P microspheres were loaded in the fixed-bed column with the highest fractionation efficiency18, which was favorable for the dynamic adsorption. Surface structure of the microspheres was shown in Figure 1b and 1d, in which micro- and nanopore structure25 can be observed. The pore formation was the result of the H2O-induced phase separation during the sol-gel process, where the solvent–rich regions contributed to the pore formation.26 The porous structure not only allowed Pb2+ to penetrate into the microspheres, but also provided numerous adsorption sites to facilitate the Pb2+ adsorption. EDS patterns (Figure 1b and 1d insets) detect the element type and content on the surface of CM-P and CM-P-Pb. The presence of P on the CM-P indicated the introduction of the phosphate groups onto the CM and the presence of Pb on the CM-P-Pb demonstrated the adsorption performance of Pb2+.



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The physical properties of CM and CM-P were summarized in Table S2. The adsorbents, with high water content, could be used to handle pollutant in water, such as heavy metal ions. With the high specific surface area, numerous adsorption sites were provided during the adsorption process. High porosity, high mean pore volume, and high specific surface area of CM-P contibuited to its strong adsorption capacity. The FTIR spectra of all the samples were shown in Figure S1, which proved the success of phosphorylation and demonstrated that the phosphorylation of the CM played an important role during the adsorption process of Pb2+. The crystalline structure of cellulose, CM, CM-P and CM-P-Pb was studied with the X-ray diffraction as shown in Figure S2. The introduction of phosphate groups decreased the inter and intramolecular hydrogen bonds number and reduced the crystallinity of the CM-P with respect to the CM. 27, 28 The DSC thermo-grams of cellulose, CM, CM-P and CM-P-Pb were shown in Figure S3. In general, introduction of phosphate groups to the native microspheres obstructed chain folding, resulting in a decrease in the degradation temperature.29 Batch adsorption experiments The effects of dosage of the CM-P, temperature, and pH on the adsorption of the CM-P were shown in Figure S4. The dosage affects the amount of active adsorption sites.30 The pH affects the solubility of Pb2+ and has a strong impact on the ionization of phosphate groups on the surface of CM-P and charge of adsorption site.30 In terms of capacity and efficiency, operation condition of 0.3 g CM-P, pH of 5.0 and temperature of 25 oC was selected for subsequent experiment.


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Adsorption kinetics of Pb2+ on CM-P was examined, and the result was presented in Figure S5. The adsorption equilibrium was achieved within 30 minutes, and the CM-P had the highest adsorption capacity of 108.5 mg g-1. Kinetic data were regressed by two nonlinear models, and the kinetic parameters were listed in Table S3. It showed that higher correlation factor (r2) and a better agreement between the experimental

and

the

pseudo-second-order

calculated

model

fitted

qe the

values,

which

experiment

suggested

data

better

that

the

than

the

pseudo-first-order model. Thus the adsorption behavior was mainly controlled by chemical adsorption. Also, the adsorption capacity and adsorption equilibrium time of CM-P for Pb2+ were compared with previously reported cellulose-based adsorbents, shown in Table S4.2, 31-33 The adsorption isotherms of the Pb2+ adsorption onto the CM-P at 298, 308 and 318 K were displayed in Figure S6. The parameters of the two used models and their correlation coefficients were summarized in Table S5. It could be observed that the Pb2+ adsorption was better fitted for the Langmuir adsorption isotherm which suggested a monolayer sorption. Fixed-bed adsorption Flow rate (Q), bed height (H), initial concentration (C0) and pH are the important conditions in this experiment, these operation conditions have a great effect on followings, such as breakthrough and saturation time of the column, and on the dynamics of the column. The effects of these conditions above on the breakthrough curves of Pb2+ were also studied. The continuous Pb2+ adsorption process was



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continued through the saturation point (Ct/C0=0.9) of the column. From the breakthrough curves constructed from the experimental data, the performance could be evaluated from the breakthrough time (tb), volume of treated solution (Vb), adsorption capacity (qb), and metal removal efficiency (Rb) in the breakthrough point, saturation time (ts), volume of treated solution (Vs), adsorption capacity (qs), and metal removal efficiency (Rs) in the saturation point, adsorbent dosage in the column (M), equilibrium concentration of Pb2+ in the final raffinates flowing out from the adsorption bed (Ce), empty bed contact time (EBCT), and mass transfer zone (MTZ). These parameters of the breakthrough curves were summarized in Table 1. Effect of the flow rate In a fixed-bed column, because of the retention process, the flow rate was an important parameter affecting the adsorption of Pb2+.34 Three different flow rates of 1.0, 1.4 and 3.0 mL min-1 were used to investigate the effect of the flow rate on the breakthrough curves of Pb2+ when the inlet concentration was 100 mg L-1, the height of fixed-bed column was 7.5 cm, the pH was 5.0 and the temperature was 298K.The results of these experiments were shown in Figure 2 (a). With the flow rate increasing, the steepness of the breakthrough curves increased. Also, the breakthrough time as well as saturation occurred more quickly with a higher flow rate. The slower flow rate would provide longer residence time for Pb2+, which would transfer into the internal and external surface of the adsorbents, which allowed more adsorption sites to be occupied by Pb2+.35 Variation of the slope of breakthrough curves was related to the retention process. During using the higher flow rate, the


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mass transfer rate tended to increase and the amount of Pb2+ adsorbed onto unit bed height increased, leading to faster saturation with a steeper slope presented in the breakthrough curve. But the flow rate had no much more effect on the adsorbents’ adsorption capacity. Thus, selecting a faster flow rate was more important to improving the efficiency. Effect of bed height of the fixed-bed column Another important parameter, the bed height, affected the adsorption of Pb2+ in the fixed-bed column, for the axial dispersion would decrease in mass transfer and diffusion of Pb2+ into the adsorbents would increase with the bed height increasing.36 Three different bed heights of 5, 7.5 and 10 cm were used for the purpose of investigating the effect of the height of fixed-bed column height on the breakthrough curves of lead when the flow rate was 3 mL min-1, initial concertation was 100 mg L-1, pH was 5.0 and temperature was 298K. The results of these experiments were shown in Figure 2 (b), which showed that when with the fixed-bed height increased, both of the breakthrough time and saturation time increased. The appearance might be due to the longer fixed-bed height allowing a longer empty bed contact time. When the fixed-bed height increased from 5 cm to 7.5 cm, the slope of breakthrough curve became flat, which might be caused by the increasing fixed-bed height providing a broader mass transfer zone. The adsorption sites number became higher, increasing the adsorption area of the CM-P, when the bed height increased. In this situation, it had more time for the CM-P to reach adsorption equilibrium, with the increasing of adsorption capacity. This all



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may be due to the increasing amount of adsorbent with more adsorption sites by increasing the fixed-bed height. Effect of inlet concentration of Pb2+ The initial concentration of Pb2+ had a significant influence on the breakthrough curves, because stronger driving force was provided for Pb2+ adsorption by higher Pb2+ concentration during the adsorption process.34 Three different initial Pb2+ concentrations of 50, 100 and 150 mg L-1 were used to research the influence of initial concentration on the breakthrough curves of Pb2+ when the flow rate was 3 mL min-1, fixed-bed height was 7.5 cm, pH was 5.0 and temperature was 298 K. The results of these experiments were shown in Figure 2 (c). When the initial concentration of Pb2+ increased, the breakthrough time and saturation time decreased while the slope of breakthrough curve increased. An interesting phenomenon was found that the adsorption capacity decreased with the initial concentration increased at the breakthrough and saturation point. Higher initial concentration of Pb2+ can bring more Pb2+ into the internal and external surface of the adsorbent to bind with the adsorption sites, resulting in a steeper breakthrough curve and the achievement of earlier saturation of the column. It was demonstrated that the gradient concentration could influence the adsorption rate.37 Decreasing the initial concentration of Pb2+ increased the breakthrough and saturation time with the treated volume increasing. So the availability of adsorption sites in the internal and external surface of the adsorbent could capture Pb2+ with sufficient time,38 which may be the reason of decrease in adsorption capacity with


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initial concentration increasing. The same phenomenon also appeared in the work of Cruz-Olivares, J. 19 Effect of pH of solution Three different pH values of 3.0, 4.0 and 5.0 were applied for the purpose of investigating the influence of pH on the breakthrough curves when flow rate was 3 mL min-1, initial concentration was 100 mg L-1, fixed-bed height was 7.5 cm and temperature was 298 K. The results of these experiments were shown in Figure 2 (d). As pH decreased, breakthrough time, saturation time and the adsorption capacity decreased, while the slope of breakthrough curve increased. Under a low pH, the phosphate groups on the CM-P were protonated and the CM-P was positively charged, which would form a repulsive force and to hinder adsorb the lead in the solution.30 Excessive H+ were linked to phosphate groups of the CM-P, occupying the adsorption sites as a competitor of Pb2+. The result suggested that during the adsorption process the pH of the solution played a key role, which influenced on adsorption capacity and breakthrough curve, which were related with both of the characteristics of CM-P and Pb2+, and chemical properties of the aqueous solution.37 Modeling of the breakthrough curves Table 2 listed the calculated parameters of Adams-Bohart, Thomas, Yoon-Nelson, BDST, and Dose Response models at different operating conditions. These models were modeled using Polymath 6.0. The correlation coefficient values ranged from 0.981 to 0.998, indicating a good agreement between the experimental data and the



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column data generated from these models. By comparing the correlation coefficients (r2) obtained from all the dynamic models, the Dose Response model showed highest value among all the models, indicating that the Dose Response model might be closed to practical situation. Similar result was also reported in the published work39. The predicted and experiment breakthrough curves of Pb2+ adsorption by CM-P at different initial concentration were shown in Figure 3. For the Dose Response and Thomas models, the initial adsorption capacity, q0, decreased with the initial concentration of Pb2+ increased, which was similar with experimental data. The same influence was observed from the Adams-Bohart and BDST models, where with the initial concentration increased, the volumetric adsorption capacity decreased. The kYN, Yoon-Nelson proportionality constant, increased with increased Pb2+ concentration, which could be related to increased mass transfer driving force at a higher inlet concentration39. τ, the time required to reach 50% of the retention, significantly reduced with increasing initial concentration, due to faster saturation of column at higher concentration. The calculated τ values were similar to the experimental values. Column regeneration and reuse research Recovery of Pb2+ ions from CM-P-Pb is also an important step. The operating conditions, when the flow rate was 3 mL min-1, initial concentration was 100 mg L-1, fixe-bed height was 7.5 cm, pH was 5.0 and temperature was 298 K, were selected for this study, and these desorption profile and breakthrough curve for Pb2+ adsorption on the CM-P were shown in Figure S7. The result indicated that these materials could be


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repeatedly used for Pb2+ adsorption after treatment with 0.1 M HNO3, and the adsorption process for Pb2+ uptake was reversible no loss of binding efficiency. Mechanism analysis The XPS spectra of CM, CM-P, and CM-P-Pb were shown in Figure 4. In the survey of CM and CM-P (Figure 4a), in contrast to the spectra from CM, CM-P showed two new peaks at 134 and 1191 eV, which were assigned to P 2p, and P 2s species. The appearance of elemental phosphorus peaks strongly supported the assumption that phosphate groups had been successfully linked to the cellulose by chemical reaction. The O1s spectra of CM (Figure 4c) and CM-P (Figure 4d) showed a strong peak at 532.7 eV attributed to C-O bonds29 and a weak shoulder peak in CM-P spectra at around 531.5 eV for P=O groups15, respectively. The P 2p spectra of CM-P (Figure 4b) show combined doublets corresponding to P 2p1/2 at 134.5 eV and P 2p 3/2 at 133.6 eV.

15

The binding energies at 134.5 and 133.6 eV are usually assigned to P-O-C or

P-O3-2 groups when phosphoric acids react with cellulose.15 In the survey of CM-P-Pb and CM-P (Figure 4e), two new peaks occurred in CM-P-Pb spectra at around 140 eV were assigned to Pb 4f, possibly due to the adsorption of Pb2+. In the Pb 4f spectra (Figure 4f) of CM-P-Pb and Pb(NO3)2, the Pb 4f peaks in the spectrum from Pb(NO3)2 are centered at 144.1 eV for Pb 4f5/2 and 139.2 eV for Pb 4f7/2, while the Pb 4f peaks in the spectrum from CM-P-Pb are centered at 143.8 eV for Pb 4f5/2 and 138.9 eV for Pb 4f7/2, which suggests that Pb2+ may be loaded in the CM in the form of Pb-O-P.



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After phosphorylation, the phosphate groups linked to internal and external surface of the cellulose microspheres were verified by FTIR, EDS, and XPS. The presence of phosphate groups in CM-P was responsible for the Pb2+ adsorption which was an ion-exchange process.40 The adsorption and desorption phenomenon of Pb2+ could be explained using the follow equations. CM − POଷ Hଶ → CM − POଷ ଶି + 2H ା (13) PbሺNOଷ ሻଶ → Pbଶା + 2NOଷା

(14)

CM − POଷ ଶି + Pbଶା → CM − POଷ ሺPbሻ (15) CM − POଷ ሺPbሻ + HNOଷ → CM − POଷ ሺHሻଶ + PbሺNOଷ ሻଶ (16) In an acidic medium, lead exists in the form of Pb2+ in water. First, the –PO3H2 groups on the surface of CM-P were dissociated to the –PO32- and H+. Second, Pb2+ was uptaken by –PO32- due to the electrostatic interactions. The equilibrium of the adsorption process and the desorption process was established and occurred by Eq. (15) and Eq. (16), respectively. Phosphorylation mechanism and a reasonable adsorption mechanism can be illustrated distinctly. Phosphate groups are linked to cellulose via P-O-C aliphatic bonds through chemical reaction which has been confirmed by FTIR and XPS analysis. It’s assumed that Pb2+ is bound with phosphate groups through coordinated action as the adsorption mechanism. According to the Pearson HSAB theory, the large electronegativity differences between a moderately hard acid such as Pb2+ and hard bases (O-)/(OH) give rise to strong ionic interactions.41 The possible adsorption


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mechanism to explain interactions between Pb2+ and phosphate group is shown as a complex formation, which has been confirmed by FTIR, EDX, and XPS analysis. Table S6 summarized the relative surface concentration (atomic percent content) of C, O , P and Pb elements on CM-P and CM-Pb determined by XPS and EDS. It could be seen that P was found in CM-P, because of the phosphorylation and that Pb was found in CM-P-Pb beacause of the adsorption beheavior. The difference of value between XPS and EDS might be caused by two reasons. One is the preparation of the sample, which was cutted into powder for XPS, while intact microspheres for EDS. The other is that EDS only mesures a specific area on the sample and different area could yield different results. In view of all the results above, the schematic depiction of design of CM-P and the adsorption mechanism of Pb2+ onto CM-P were proposed and illustrated in Figure 5. New phosphorylated cellulose microsphere adsorbents have been developed for the removal of Pb2+ from water by using a solid-phase synthesis method from regenerated cellulose microspheres. The phosphate groups bonding to the adsorbent were the key to ensure the extremely high adsorption capacity for Pb2+. The adsorbents maintained the spherical three-dimensional structure after phosphorylation, which was favorable for the application of fixed-bed column. And the recycling and reuse of the used absorbents in this work is convenient.

Conclusions



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The phosphorylated cellulose microsphere adsorbents were successfully prepared by an improved solid-phase modification method, and were applied for effective Pb2+ removal from water. The adsorbents exhibited high water content and porosity which were favorable for the flux of the treated water. SEM images suggested that the particle size of the adsorbent showed normal distribution and the porous structure of the surface and internal of adsorbents, which were favorable for the adsorbents loading closely in fixed-bed column and Pb2+ entering into internal of microsphere adsorbents. In the batch adsorption experiment, the data of kinetic experiment were fitted by the pseudo-second-order model better, indicating that the adsorption of Pb2+ adsorption by CM-P was mainly controlled by chemical adsorption. It only needed 20 mins that the adsorption arrived at the equilibrium and the adsorption capacity was108.5 mg g-1. In the dynamic adsorption experiment, the operation conditions, such as flow rate, initial concentration, bed height and pH, and so on were evaluated and several dynamic adsorption models, such as Adams-Bohart, Thomas, Yoon-Nelson, Bed Depth Service Time (BDST) and Dose Response model were fitted well with the experimental date. The fixed-bed column displayed good capacity for treating water for a continuous time, which was highly desirable in environment engineering. The phosphate groups bound on the adsorbent play a key role in the Pb2+ removal from water. The chelation between the adsorbent and the adsorbate made the designed phosphorylated cellulose microspheres a promising candidate for the lead removal in water purification.


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Acknowledgments Support for this research provided by the National Natural Science Foundation of China (51303142), the Science and Technology Program of Guangdong Province (2015A010105018),

the

Natural

Science

Foundation

of

Hubei

Province

(2014CFB775), the Open Foundation of Collaborative Innovation Center of Wuhan Institute of Technology (P201109), and the Graduate Innovative Fund of Wuhan Institute of Technology (CX2015067).

Supporting Information Table S1-S6: Thomas, Adams-Bohart, Yoon-Nelson, BDST, and Dose Response model; The physical properties of the CM and CM-P; Kinetic parameters for Pb2+ uptake onto CM-P at 298 K; Comparison of adsorption capacities and adsorption equilibrium times of different cellulose-based adsorbents for Pb2+; Constant parameter and correlation coefficients calculated for various adsorption models at different temperatures for Pb2+ on CM-P; Relative surface concentration of C, O, P and Pb elements on CM-P and CM-Pb determined by XPS and EDS; Figure S1-S7: FTIR spectra of cellulose (a), CM (b), CM-P (c), CM-P-Pb (d); XRD spectra of cellulose (a), CM (b), CM-P (c), CM-P-Pb (d); DSC thermo-grams of cellulose (a), CM (b), CM-P (c), CM-P-Pb (d); Effect of: (a) dosage of CM-P; (b) pH; (c) temperature for Pb2+ adsorption; Pseudo-first-order kinetics and pseudo-second-order kinetics of Pb2+ onto CM-P; The adsorption equilibrium isotherms of Pb2+ onto CM-P at different



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temperatures; (a) Breakthrough curves for Pb2+ adsorption on the CM-P, (b) desorption profiles of Pb2+.

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(13) Oshima, T.; Kondo, K.; Ohto, K.; Inoue, K.; Baba, Y., Preparation of phosphorylated bacterial cellulose as an adsorbent for metal ions. React. Funct. Polym. 2008, 68 (1), 376-383. (14) Wang, Z. G.; Lv, N.; Bi, W. Z.; Zhang, J. L.; Ni, J. Z., Development of the affinity materials for phosphorylated proteins/peptides enrichment in phosphoproteomics analysis. ACS Appl. Mater. Interfaces 2015, 7 (16), 8377-8392. (15) Ghanadpour, M.; Carosio, F.; Larsson, P. T.; Wagberg, L., Phosphorylated Cellulose Nanofibrils: A Renewable Nanomaterial for the Preparation of Intrinsically Flame-Retardant Materials. Biomacromolecules 2015, 16 (10), 3399-3410. (16) Suflet, D. M.; Chitanu, G. C.; Popa, V. I., Phosphorylation of polysaccharides: New results on synthesis and characterisation of phosphorylated cellulose. React. Funct. Polym. 2006, 66 (11), 1240-1249. (17) Cai, J.; Liu, Y.; Zhang, L., Dilute solution properties of cellulose in LiOH/urea aqueous system. J. Polym. Sci. Part B: Polym. Phys. 2006, 44 (21), 3093-3101. (18) Luo, X.; Zhang, L., Creation of regenerated cellulose microspheres with diameter ranging from micron to millimeter for chromatography applications. J. Chromatogr., A 2010, 1217 (38), 5922-5929. (19) Cruz-Olivares, J.; Pérez-Alonso, C.; Barrera-Díaz, C.; Ureña-Nuñez, F.; Chaparro-Mercado, M. C.; Bilyeu, B., Modeling of lead (II) biosorption by residue of allspice in a fixed-bed column. Chem. Eng. J. 2013, 228, 21-27. (20) G. Bohart, E. Q. A., Some aspects of the behavior of charcoal with respect to chlorine. J. Am. Chem. Soc. 1920, 42 (3), 523-544. (21) Thomas, H. C., Heterogenrous ion exchange in a flowing systerm. J. Am. Chem. Soc. 1944, 66, 1664-1666. (22) Yoon, Y. H.; Nelson, J. H., Application of Gas Adsorption Kinetics I. A Theoretical Model for Respirator Cartridge Service Life. Am. Ind. Hyg. Assoc. J. 1984, 45 (8), 509-516. (23) Kumar, U.; Bandyopadhyay, M., Fixed bed column study for Cd(II) removal from wastewater using treated rice husk. J. Hazard. Mater. 2006, 129 (1-3), 253-259. (24) Calero, M.; Hernainz, F.; Blazquez, G.; Tenorio, G.; Martinlara, M. A., Study of Cr (III) biosorption in a fixed-bed column. J. Hazard. Mater. 2009, 171 (1), 886-893. (25) Luo, X.; Zeng, J.; Liu, S.; Zhang, L., An effective and recyclable adsorbent for the removal of heavy metal ions from aqueous system: Magnetic chitosan/cellulose microspheres. Bioresour. Technol. 2015, 194, 403-406. (26) Luo, X.; Lei, X.; Cai, N.; Xie, X.; Xue, Y.; Yu, F., Removal of Heavy Metal Ions from Water by Magnetic Cellulose-Based Beads with Embedded Chemically Modified Magnetite Nanoparticles and Activated Carbon. ACS Sustainable Chem. Eng. 2016, 4 (7), 3960-3969. (27) Bezerra, R.; Silva, M.; Morais, A.; Osajima, J.; Santos, M.; Airoldi, C.; Filho, E., Phosphated Cellulose as an Efficient Biomaterial for Aqueous Drug Ranitidine Removal. Materials 2014, 7 (12), 7907-7924.



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(28) Bezerra, R. D.; Morais, A. I.; Osajima, J. A.; Nunes, L. C.; Filho, E. C., Development of new phosphated cellulose for application as an efficient biomaterial for the incorporation/release of amitriptyline. Int. J. Biol. Macromol. 2016, 86, 326-375. (29) Božič, M.; Liu, P.; Mathew, A. P.; Kokol, V., Enzymatic phosphorylation of cellulose nanofibers to new highly-ions adsorbing, flame-retardant and hydroxyapatite-growth induced natural nanoparticles. Cellulose 2014, 21 (4), 2713-2726. (30) Ren, H.; Gao, Z.; Wu, D.; Jiang, J.; Sun, Y.; Luo, C., Efficient Pb(II) removal using sodium alginate–carboxymethyl cellulose gel beads: Preparation, characterization, and adsorption mechanism. Carbohydr. Polym. 2016, 137, 402-409. (31) Zhou, Y.; Wang, X.; Zhang, M.; Jin, Q.; Gao, B.; Ma, T., Removal of Pb(II) and malachite green from aqueous solution by modified cellulose. Cellulose 2014, 21 (4), 2797-2809. (32) Dong, C.; Zhang, H.; Pang, Z.; Liu, Y.; Zhang, F., Sulfonated modification of cotton linter and its application as adsorbent for high-efficiency removal of lead(II) in effluent. Bioresour. Technol. 2013, 146, 512-518. (33) Velazquez-Jimenez, L. H.; Pavlick, A.; Rangel-Mendez, J. R., Chemical characterization of raw and treated agave bagasse and its potential as adsorbent of metal cations from water. Ind. Crops Prod. 2013, 43, 200-206. (34) Bulgariu, D.; Bulgariu, L., Sorption of Pb(II) onto a mixture of algae waste biomass and anion exchanger resin in a packed-bed column. Bioresour. Technol. 2013, 129, 374-380. (35) Abdolali, A.; Ngo, H. H.; Guo, W.; Zhou, J. L.; Zhang, J.; Liang, S.; Chang, S. W.; Nguyen, D. D.; Liu, Y., Application of a breakthrough biosorbent for removing heavy metals from synthetic and real wastewaters in a lab-scale continuous fixed-bed column. Bioresour. Technol. 2017, 229, 78-87. (36) Hasan, S. H.; Ranjan, D.; Talat, M., Agro-industrial waste 'wheat bran' for the biosorptive remediation of selenium through continuous up-flow fixed-bed column. J. Hazard. Mater. 2010, 181 (1-3), 1134-1142. (37) Sotelo, J. L.; Ovejero, G.; Rodríguez, A.; Álvarez, S.; García, J., Analysis and modeling of fixed bed column operations on flumequine removal onto activated carbon: pH influence and desorption studies. Chem. Eng. J. 2013, 228, 102-113. (38) Singh, T. S.; Pant, K. K., Experimental and modelling studies on fixed bed adsorption of As(III) ions from aqueous solution. Sep. Purif. Technol. 2006, 48 (3), 288-296. (39) Calero, M.; Hernainz, F.; Blazquez, G.; Tenorio, G.; Martin-Lara, M. A., Study of Cr (III) biosorption in a fixed-bed column. J. Hazard. Mater. 2009, 171 (1-3), 886-893. (40) Srivastava, N.; Thakur, A. K.; Shahi, V. K., Phosphorylated cellulose triacetate-silica composite adsorbent for recovery of heavy metal ion. Carbohydr. Polym. 2016, 136, 1315-22.


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(41) Gunathilake, C.; Kadanapitiye, M. S.; Dudarko, O.; Huang, S. D.; Jaroniec, M., Adsorption of Lead Ions from Aqueous Phase on Mesoporous Silica with P-Containing Pendant Groups. ACS Appl. Mater. Interfaces 2015, 7 (41), 23144-52.



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Table captions Table 1. Breakthrough curves parameters for the adsorption of Pb2+onto CM-P Table 2. Adams-Bohart, Thomas, Yoon-Nelson, BDST, and Dose Response model parameters at different conducted conditions


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Table 1. Breakthrough curves parameters for the adsorption of Pb2+onto CM-P H

Q

C0 pH

Number

tb

-1

(mL min )

(cm)

(mg L )

1

1

7.5

100

2

1.4

7.5

3

3

4

Vb

qb

Rb -1

ts

Vs

qs

Rs -1

M

Ce -1

EBCT

MTZ

(min)

(cm)

(min)

(mL)

(mg g )

(%)

(min)

(mL)

(mg g )

(%)

(g)

(mg g )

5.0

480

480

93.52

98.36

570

570

96.84

85.77

0.450

99.06

7.12

1.18

100

5.0

360

495

101.55

99.51

486

680

109.75

78.84

0.440

93.05

5.08

1.25

7.5

100

5.0

150

450

93.02

99.27

250

750

111.40

71.33

0.450

93.55

2.37

3.00

3

5

100

5.0

90

270

76.11

98.08

150

450

99.00

68.61

0.300

94.74

1.58

2.00

5

3

10

100

5.0

220

660

101.75

98.72

270

810

106.83

84.45

0.600

94.02

3.16

1.85

6

3

7.5

50

5.0

260

780

106.54

99.43

490

1470

139.38

69.02

0.392

45.63

2.37

3.52

7

3

7.5

150

5.0

90

270

86.94

98.18

120

360

95.14

81,35

0.433

144.95

2.37

1,87

8

3

7.5

100

4.0

130

390

89.56

92.19

200

600

109.30

78.04

0.450

98.17

2.37

2.62

9

3

7.5

100

3.0

90

270

85.14

97.54

110

330

86.99

81.53

0.347

99.07

2.37

1.36

-1



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Table 2. Adams-Bohart, Thomas, Yoon-Nelson, BDST, and Dose Response model parameters at different conducted conditions Model

Parameters

Number 1

2

3

4

5

6

7

8

9

0.640

0.706

0.756

1.093

1.251

0.819

1.009

0.585

2.914

6.316

7.016

7.101

6.400

6.852

7.538

6.117

7.166

4.481

0.998

0.984

0.988

0.985

0.983

0.981

0.993

0.998

0.996

0.640

0.706

0.756

1.093

1.251

0.819

1.009

0.585

2.914

0.999

1.136

1.124

1.013

1.085

1.370

1.007

1.134

0.921

0.998

0.984

0.988

0.985

0.983

0.981

0.993

0.998

0.996

0.571

0.641

0.709

1.052

1.72

0.441

1.444

0.614

3.266

τcal (min)

504.3

393.6

180.0

105.3

231.6

332.4

101.5

162.0

94.9

τexp (min)

502.9

391.0

180.0

104.0

230.0

330.8

100.9

162.4

94.9

0.998

0.984

0.988

0.985

0.983

0.981

0.993

0.998

0.996

0.640

0.706

0.752

1.093

1.251

0.819

1.009

0.585

2.914

0.632

0.702

0.710

0.639

0.685

0.754

0.612

0.717

0.448

0.998

0.984

0.988

0.985

0.983

0.981

0.993

0.998

0.996

0.998

1.135

1.123

1.010

1.084

1.368

1.003

1.127

0.919

28.762

24.257

11.100

10.165

26.238

13.628

11.100

9.794

30.917

0.998

0.985

0.990

0.988

0.984

0.984

0.991

0.998

0.996

-1

kAB (L mg AdamsBohart

min-1)×10-3 N0 (mg L-1)× 103 r2 -1

kTh (L mg

min-1) ×10-3 Thomas

q0 (mg g-1)× 102 r2 kYN (min-1)

YoonNelson

×10

r

2 -1

kBDST (L mg

min-1) ×10-3 BDST

N0 (mg L-1) ×104 r2 -1

q0(mg g ) Dose

×102

Response

a r

2


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Figure captions Figure 1. SEM images of CM-P (a, b) and CM-P-Pb (c, d). (inset) EDS pattern of CM-P and CM-Pb. Figure 2. Breakthrough curves of Pb2+ removal by CM-P using fixed-bed columns of (a) different flow rates (C0=100 mg L-1, H=7.5 cm, pH=5.0，T=298 K), (b) different heights of bed (C0=100 mg L-1, Q=3 mL min-1, pH=5.0，T=298 K), (c) different initial concentrations. (Q=3 mL min-1, H=7.5 cm, pH=5.0，T=298 K), and (d) different pH (C0=100 mg L-1,Q=3 mL min-1, H=7.5 cm, T=298 K). Figure 3. Experimental and predicted breakthrough curves using the Adams-Bohart, Thomas, Yoon-Nelson, BDST, Dose Response and Mass Transfer model for the adsorption of Pb2+ by CM-P at different initial concentration: (a) 50 mg L-1; (b) 100 mg L-1; (c) 150 mg L-1. (Q=3.0 mL min-1, H=7.5 cm, pH=5.0) Figure 4. XPS spectra of CM ,CM-P, and CM-P-Pb: (a) survey of CM and CM-P; (b) P2p of CM-P; (c) O 1s of CM; (d) O1s of CM-P；(e) survey of CM-P and CM-P-Pb ; (f) Pb 4f of CM-P-Pb and Pb(NO3)2. Figure 5. Schematic depiction of preparation of the phosphorylated cellulose microsphere adsorbents and its Pb2+ adsorption mechanism.



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Figure 1. SEM images of CM-P (a, b) and CM-P-Pb (c, d). (inset) EDS pattern of CM-P and CM-Pb.


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Figure 2. Breakthrough curves of Pb2+ removal by CM-P using fixed-bed columns of (a) different flow rates (C0=100 mg L-1, H=7.5 cm, pH=5.0，T=298 K), (b) different heights of bed (C0=100 mg L-1, Q=3 mL min-1, pH=5,0，T=298 K), (c) different initial concentrations (Q=3 mL min-1, H=7.5 cm, pH=5,0，T=298 K), and (d) different pH (C0=100 mg L-1,Q=3 mL min-1, H=7.5 cm, T=298 K).

Figure 3. Experimental and predicted breakthrough curves using the Adams-Bohart, Thomas, Yoon-Nelson, BDST, Dose Response and Mass Transfer model for the adsorption of Pb2+ by CM-P at different initial concentration: (a) 50 mg L-1; (b) 100 mg L-1; (c) 150 mg L-1. (Q=3.0 mL min-1, H=7.5 cm, pH=5.0)



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Figure 4. XPS spectra of CM, CM-P, and CM-P-Pb: (a) survey of CM and CM-P; (b) P2p of CM-P; (c) O 1s of CM; (d) O1s of CM-P；(e) survey of CM-P and CM-P-Pb ; (f) Pb 4f of CM-P-Pb and Pb(NO3)2.


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Figure 5. Schematic depiction of preparation of the phosphorylated cellulose microsphere adsorbents and its Pb2+ adsorption mechanism.



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Table of content

Improved solid-phase synthesis of the phosphorylated cellulose microsphere adsorbents for highly effective Pb2+ removal from water: the batch and fixed-bed column performance and the adsorption mechanism Xiaogang Luo, Jun Yuan, Yingge Liu, Chao Liu, Xingrong Zhu, Xuehai Dai, Zhaocheng Ma, Fen Wang

In this study, phosphorylated cellulose microspheres were designed by introducing phosphate groups to the cellulose microspheres through an improved solid-phase modification method. The batch kinetic and equilibrium studies, the influence of flow rate, fixed-bed height, initial solution concentration and pH of the fixed bed column were investigated for Pb2+ removal, and the adsorption mechanism was also summarized.

Synopsis: phosphorylated cellulose microspheres were designed by introducing phosphate groups to the cellulose microspheres through an improved solid-phase modification method, with high adsorption efficiency and capacity of Pb2+ from water.


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Improved Solid-Phase Synthesis of ... - ACS Publications

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