Improved Solid-Phase Synthesis of Phosphorylated Cellulose

Research Article pubs.acs.org/journal/ascecg

Improved Solid-Phase Synthesis of Phosphorylated Cellulose Microsphere Adsorbents for Highly Effective Pb2+ Removal from Water: Batch and Fixed-Bed Column Performance and Adsorption Mechanism Xiaogang Luo,*,† Jun Yuan,† Yingge Liu,† Chao Liu,† Xingrong Zhu,† Xuehai Dai,† Zhaocheng Ma,*,‡ and Fen Wang*,†,§ †

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 ‡ Key Laboratory of Horticultural Plant Biology (Ministry of Education), College of Horticulture and Forestry, Huazhong Agricultural University, Shizishan Street No.1, Wuhan 430070, China § School of Chemistry and Chemical Engineering, Sichuan University of Arts and Science, #400 Nanba Road, Dachuan District, Dazhou 635000, China S Supporting Information *

ABSTRACT: A highly effective adsorbent with phosphate groups bound to cellulose microspheres was designed by an improved solid-phase modification method to adsorb lead ions from water by a batch and fixedbed 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 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, to study the performance of adsorption of Pb2+ onto the adsorbents. The results suggested that chemical adsorption was the main controlled process during the adsorption process and that the adsorbents could highly effectively capture Pb2+ from water via chelation. KEYWORDS: Phosphorylated cellulose microspheres, Lead ions, Batch adsorption, Fixed-bed column

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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 Earth are striving to reduce the level of elevated blood lead in children, which could seriously damage their intelligence development.2 Therefore, effective removal of lead from water is essential for a safe water supply, which will ensure the health and life quality for millions of people under lead-contamination threats. Adsorption technology is one of the most attractive options in many of the processing technologies,3 such as its convenience, ease of use, and high efficiency. The key and © 2017 American Chemical Society

difficult point of adsorption technology is the design of adsorbents 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), natural polymer, zeolite, and some other low-cost adsorbents.4 Among these materials, cellulose stands out due to its low cost, wide availability, renewability, biodegradability, biocompatibility, etc.5 However, native cellulose does not have satisfactory adsorption capacity for heavy metal pollution itself.6−8 Generally, the cellulose-based materials with chemical modReceived: February 14, 2017 Revised: April 28, 2017 Published: May 10, 2017 5108

DOI: 10.1021/acssuschemeng.7b00472 ACS Sustainable Chem. Eng. 2017, 5, 5108−5117

Research Article

ACS Sustainable Chemistry & Engineering

stirring. After 2 h, 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 1 h, 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 °C 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 percents of 20%, 40%, 60%, 80%, and 100%; so eventually CM was soaked in DMF solution. Then, 4 g of CM (dry weight) was soaked in 300 mL of DMF along with 40 g of urea. The above mixture was stirred at 100 °C for 1 h, and then 20.0 g of 85% H3PO4 was added drop by drop. After the dropwise addition, the reaction was conducted to 135 °C and kept for 8 h. After cooling to 25 °C, 2 L of deionized water was used to wash the mixture to obtain CM-P. Pb2+ Solution and Detection. A stock solution (1000 mg L−1) of Pb2+ was prepared by Pb(NO3)2 with 0.1 mL of 1 M 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 spectroscopy (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 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 an N2 adsorption and desorption test at 77 K by Micromeritics ASAP-2020. Sample microspheres were ground into powders and dried in a vacuum oven at 333 K for 48 h for other characterizations. The Fourier-transform infrared (FT-IR) spectra of these samples were recorded on an FT-IR spectrometer (model 1600, PerkinElmer Co.) with KBr pellets. Wideangle X-ray diffraction (XRD) analysis was carried out with an XRD diffractometer (D8-Advance, Bruker) to confirm the crystal structure property of these materials. Thermal analysis was carried with differential scanning calorimetry (DSC) (Seiko Instruments, DSC6220). The semiquantitative chemical composition and surface chemical states of the samples were examined with X-ray photoelectron spectrometery (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 °C), and pH (1.0, 2.0, 3.0, 4.0, and 5.0) on the adsorption of CM-P were studied. The wet CM-P samples were added to each conical flask (100 mL) which contained 30 mL of 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 following equations:

ification exhibit more high adsorption capacity for various aquatic pollutants.6 Various modified cellulose-based adsorbents have been fabricated to improve its adsorption capacity for lots of aquatic pollutants, such as toxic heavy metal ions,9 inorganic anions,10 dyes,11 etc. The chemical groups introduced in cellulose should have 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, Oshima and co-workers have prepared phosphorylated bacterial cellulose as 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. On the basis of Lewis acid−base theory, heavy metal cations can interact with 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 decades,15 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 introduces 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 advantages as follows: First, the adsorbents were chemically modified on the basis 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, the recycling and reuse of absorbents in this work are more convenient than the previous ones. In this study, phosphorylated cellulose microspheres were designed by introducing phosphate groups to cellulose microspheres through an improved solid-phase modification method. 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.

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MATERIALS AND METHODS

Reagents and Materials. Cotton linter pulp of α-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 Ultrapure 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 NaOH/urea/H2O with a weight ratio of 7/12/81 was precooled to −12.5 °C, and then 6 g of cellulose samples were dispersed into the above solvent with vigorous stirring for 2 min to obtain a transparent cellulose solution. Six hundred milliliters of liquid paraffin and 15 g of span80 were added into a 1000 mL three-necked flask and stirred at a constant speed for 1 h, and then 150 mL of cellulose solution was dispersed in the organic phase with continual

R=

(C0 − Ce) × 100 % C0

q=

(C0 − Ce) × V M −1

(1)

(2) −1

where q (mg g ) is the adsorption capacity of CM-P, C0 (mg L ) 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. 5109


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

Kinetics data were represented by a nonlinear pseudo-first-order model and nonlinear pseudo-second-order model,1 given as follows: qt =

(e k1t − 1) ek1t

qe

(3)

qc =

where k1 is the pseudo-first-order model rate constant (min−1) qt =

(4)

m=

(5)

C 0· Q · t 1000

(8)

(9)

2+

For the weight of Pb adsorbed per unit dry weight of the microspheres adsorbents, the adsorbing capacity (q (mg g−1)) can be calculated by q q= c (10) M

where KL is the Langmuir adsorption constant (L mg−1), and qm is the monolayer maximum adsorption capacity of the adsorbent (mg g−1) qe = KFCe1/ n

(7)

Metal removal, R (%), is calculated as follows: q R = c · 100 m

qmKLCe (1 + KLCe)

Cadsdt

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

where k2 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: qe =

t=t

∫t =0

−1

qe 2k 2t 1 + qek 2t

Q ·A Q = 1000 1000

(6) −1

where n and KF (mg g ) are the Freundlich adsorption constants. Fixed-Bed Adsorption Process and Modeling. The experiments and experimental model studies were conducted the same as the previous works.19 The CM-P adsorbents of different amounts were loaded in a glass column (internal diameter = 1.1 cm), and all experiments were maintained at 298 K. Before the experiments, the column should be preconditioned to pH 5.0 by the eluting water adjusted by 0.1 M HNO3. Meanwhile, 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 breakthrough curves. The data were applied to some models which are listed in Table S1, such as Adams−Bohart,20 Thomas,21 Yoon−Nelson,22 BDST,23 Dose Response,24 and 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

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 =

Bed volumn Flow rate

(11)

The mass transfer zone, MTZ (cm), was calculated as follows: ⎛ t ⎞ MTZ = H ⎜1 − b ⎟ ts ⎠ ⎝

(12)

where H is the bed height (cm).

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RESULTS AND DISCUSSION Characterization. CM-P (a, b) and CM−P-Pb (c, d) are the SEM testing images exhibited in Figure 1. The micro5110


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ACS Sustainable Chemistry & Engineering Table 1. Breakthrough Curves Parameters for Adsorption of Pb2+onto CM-P number

Q (mL min−1)

H (cm)

C0 (mg L−1)

pH

tb (min)

Vb (mL)

qb (mg g−1)

Rb (%)

ts (min)

Vs (mL)

qs (mg g−1)

Rs (%)

M (g)

Ce (mg g−1)

EBCT (min)

MTZ (cm)

1 2 3 4 5 6 7 8 9

1 1.4 3 3 3 3 3 3 3

7.5 7.5 7.5 5 10 7.5 7.5 7.5 7.5

100 100 100 100 100 50 150 100 100

5.0 5.0 5.0 5.0 5.0 5.0 5.0 4.0 3.0

480 360 150 90 220 260 90 130 90

480 495 450 270 660 780 270 390 270

93.52 101.55 93.02 76.11 101.75 106.54 86.94 89.56 85.14

98.36 99.51 99.27 98.08 98.72 99.43 98.18 92.19 97.54

570 486 250 150 270 490 120 200 110

570 680 750 450 810 1470 360 600 330

96.84 109.75 111.40 99.00 106.83 139.38 95.14 109.30 86.99

85.77 78.84 71.33 68.61 84.45 69.02 81,35 78.04 81.53

0.450 0.440 0.450 0.300 0.600 0.392 0.433 0.450 0.347

99.06 93.05 93.55 94.74 94.02 45.63 144.95 98.17 99.07

7.12 5.08 2.37 1.58 3.16 2.37 2.37 2.37 2.37

1.18 1.25 3.00 2.00 1.85 3.52 1,87 2.62 1.36

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 bed heights (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).

high mean pore volume, and high specific surface area of CM-P contributed to its strong adsorption capacity. The FTIR spectra of all the samples are shown in Figure S1, which proved the success of phosphorylation and demonstrated that phosphorylation of CM played an important role during the adsorption process of Pb2+. The crystalline structures of cellulose, CM, CM-P, and CMP-Pb were studied with X-ray diffraction as shown in Figure S2. The introduction of phosphate groups decreased the intermolecular and intramolecular hydrogen bond numbers and reduced the crystallinity of CM-P with respect to CM.27,28 The DSC thermograms of cellulose, CM, CM-P, and CM-PPb are shown in Figure S3. In general, introduction of phosphate groups to native microspheres obstructed chain folding, resulting in a decrease in the degradation temperature.29 Batch Adsorption Experiments. The effects of dosage of CM-P, temperature, and pH on the adsorption of CM-P are 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 the adsorption site.30 In terms of capacity and efficiency, operation conditions of 0.3 g of CM-P,

spheres with good spherical shape (Figure 1a and c) have average diameters 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 efficiency,18 which was favorable for dynamic adsorption. The surface structure of the microspheres is shown in Figure 1b and d, in which micropore and nanopore structures25 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 Pb2+ adsorption. EDS patterns (Figure 1b and d, insets) detect the element type and content on the surface of CM-P and CM-P-Pb. The presence of P on CM-P indicated the introduction of the phosphate groups onto CM, and the presence of Pb on CM-P-Pb demonstrated the adsorption performance of Pb2+. The physical properties of CM and CM-P are summarized in Table S2. The adsorbents, with high water content, could be used to handle pollutants in water, such as heavy metal ions. With a high specific surface area, numerous adsorption sites were provided during the adsorption process. High porosity, 5111


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selecting a faster flow rate was more important to improving the efficiency. Effect of Bed Height of 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 concentration was 100 mg L−1, pH was 5.0, and temperature was 298 K. The results of these experiments are shown in Figure 2(b), which showed that when with the fixed-bed height increased, both the breakthrough time and saturation time increased. The appearance might be due to a longer fixed-bed height allowing a longer empty bed contact time. When the fixed-bed height increased from 5 to 7.5 cm, the slope of the 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 CM-P, when the bed height increased. In this situation, it had more time for CM-P to reach adsorption equilibrium, with increasing the adsorption capacity. This all 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 a stronger driving force was provided for Pb2+ adsorption by a 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 are 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 when the initial concentration increased at the breakthrough and saturation points. A 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 surfaces of the adsorbent could capture Pb2+ with sufficient time,38 which may be the reason for a decrease in adsorption capacity with initial concentration increasing. The same phenomenon also appeared in the work of Cruz-Olivares.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 are shown in Figure 2(d). As pH decreased, breakthrough time, saturation time, and the adsorption capacity decreased, while the slope of the breakthrough curve increased.

pH of 5.0, and temperature of 25 °C were selected for subsequent experiment. Adsorption kinetics of Pb2+ on CM-P was examined, and the result is presented in Figure S5. The adsorption equilibrium was achieved within 30 min, and 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 are listed in Table S3. It showed a higher correlation factor (r2) and better agreement between the experimental and calculated qe values, which suggested that the pseudo-second-order model fitted the experimental data better 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 Pb2+ adsorption onto CM-P at 298, 308, and 318 K are displayed in Figure S6. The parameters of the two used models and their correlation coefficients are summarized in Table S5. It could be observed that 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 such factors such as breakthrough and saturation times of the column and on the dynamics of the column. The effects of these conditions on the breakthrough curves of Pb2+ were also studied. The continuous Pb2+ adsorption process was 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 for the breakthrough time (tb), volume of treated solution (Vb), adsorption capacity (qb), and metal removal efficiency (Rb) of the breakthrough point, saturation time (ts), volume of treated solution (Vs), adsorption capacity (qs), and metal removal efficiency (Rs) of 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 are summarized in Table 1. Effect of 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+ with an inlet concentration of 100 mg L−1, height of fixed-bed column of 7.5 cm, pH of 5.0, and temperature of 298 K.The results of these experiments are 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 times 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 in the slope of the breakthrough curves was related to the retention process. With using a higher flow rate, the 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 more effect on the adsorbents’ adsorption capacity. Thus, 5112


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Table 2. Adams−Bohart, Thomas, Yoon−Nelson, BDST, and Dose Response Model Parameters at Different Conducted Conditions number model

parameters

1

2

3

4

5

6

7

8

9

Adams−Bohart

kAB (L mg−1 min−1) × 10−3 N0 (mg L−1) × 103 r2

0.640 6.316 0.998

0.706 7.016 0.984

0.756 7.101 0.988

1.093 6.400 0.985

1.251 6.852 0.983

0.819 7.538 0.981

1.009 6.117 0.993

0.585 7.166 0.998

2.914 4.481 0.996

Thomas

kTh (L mg−1 min−1) × 10−3 q0 (mg g−1) × 102 r2

0.640 0.999 0.998

0.706 1.136 0.984

0.756 1.124 0.988

1.093 1.013 0.985

1.251 1.085 0.983

0.819 1.370 0.981

1.009 1.007 0.993

0.585 1.134 0.998

2.914 0.921 0.996

Yoon−Nelson

kYN (min−1) × 10 τcal (min) τexp (min) r2

0.571 504.3 502.9 0.998

0.641 393.6 391.0 0.984

0.709 180.0 180.0 0.988

1.052 105.3 104.0 0.985

1.720 231.6 230.0 0.983

0.441 332.4 330.8 0.981

1.444 101.5 100.9 0.993

0.614 162.0 162.4 0.998

3.266 94.9 94.9 0.996

BDST

kBDST (L mg−1 min−1) × 10−3 N0 (mg L−1) × 104 r2

0.640 0.632 0.998

0.706 0.702 0.984

0.752 0.710 0.988

1.093 0.639 0.985

1.251 0.685 0.983

0.819 0.754 0.981

1.009 0.612 0.993

0.585 0.717 0.998

2.914 0.448 0.996

Dose Response

q0 (mg g−1) × 102 a r2

0.998 28.762 0.998

1.135 24.257 0.985

1.123 11.100 0.990

1.010 10.165 0.988

1.084 26.238 0.984

1.368 13.628 0.984

1.003 11.100 0.991

1.127 9.794 0.998

0.919 30.917 0.996

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

Under a low pH, the phosphate groups on CM-P were protonated, and CM-P was positively charged, which would form a repulsive force and hinder adsorbtion of lead in the solution.30 Excessive H+ ions were linked to phosphate groups of 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 adsorption capacity and the 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 Breakthrough Curves. Table 2 lists 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 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 situations. A similar result was also reported in the published work.39 The predicted and experimental breakthrough curves of Pb2+

adsorption by CM-P at different initial concentrations are shown in Figure 3. For the Dose Response and Thomas models, the initial adsorption capacity, q0, decreased when the initial concentration of Pb2+ increased, which was similar to 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 an increased mass transfer driving force at a higher inlet concentration.39 Here, τ, the time required to reach 50% of the retention, significantly reduced with increasing initial concentration due to faster saturation of the column at higher concentrations. 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 of flow rate of 3 mL min−1, initial concentration of 100 mg L−1, fixed-bed height of 7.5 cm, pH of 5.0, and temperature of 298 K were selected for this study, and these desorption profile and breakthrough curves for Pb2+ adsorption on CM-P are shown in Figure S7. The result 5113


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

indicated that these materials could be repeatedly used for Pb2+ adsorption after treatment with 0.1 M HNO3, and the adsorption process for Pb2+ uptake was reversible with no loss of binding efficiency. Mechanism Analysis. The XPS spectra of CM, CM-P, and CM-P-Pb are shown in Figure 4. In the survey of CM and CMP (Figure 4(a)), 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 O 1s 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 groups.15 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 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. After phosphorylation, the phosphate groups linked to internal and external surfaces 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 − PO3H 2 → CM − PO32 − + 2H+

(13)

Pb(NO3)2 → Pb2 + + 2NO+3

(14)

CM − PO32 − + Pb2 + → CM − PO3(Pb)

(15)

CM − PO3(Pb) + HNO3 → CM − PO3(H)2 + Pb(NO3)2

(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 −PO32− and H+. Second, Pb2+ was taken up by −PO32− due to the electrostatic interactions. The equilibrium of the adsorption process and the desorption process was established and occurred by eqs 15 and 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 a 5114


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experiment, the data of the 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 min for the adsorption to arrive at equilibrium, and the adsorption capacity was 108.5 mg g−1. In the dynamic adsorption experiment, the operation conditions, such as flow rate, initial concentration, bed height, and pH were evaluated, and several dynamic adsorption models, such as Adams−Bohart, Thomas, Yoon− Nelson, Bed Depth Service Time (BDST), and Dose Response were fitted well with the experimental date. The fixed-bed column displayed good capacity for treating water for a continuous time, which is highly desirable in environmental engineering. The phosphate groups bound on the adsorbent play a key role in Pb2+ removal from water. The chelation between the adsorbent and the adsorbate made the designed phosphorylated cellulose microspheres a promising candidate for lead removal in water purification.

chemical reaction which has been confirmed by FTIR and XPS analysis. It is assumed that Pb2+ is bound with phosphate groups through coordinated action as the adsorption mechanism. According to 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 mechanism to explain interactions between Pb2+ and a phosphate group is shown as a complex formation, which has been confirmed by FTIR, EDX, and XPS analysis. Table S6 summarizes the relative surface concentration (atomic percent content) of C, O, P, and Pb elements on CMP and CM-Pb determined by XPS and EDS. It could be seen that P was found in CM-P because of phosphorylation and that Pb was found in CM-P-Pb because of the adsorption beheavior. The difference in value between XPS and EDS might be caused by two reasons. One is the preparation of the sample, which was cut into powder for XPS, while using intact microspheres for EDS. The other is that EDS only mesures a specific area on the sample, and different areas 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 are proposed and illustrated in Figure 5. New

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00472. Tables S1−S6: Adamas−Bohart, Thomas, Yoon−Nelson, BDST, and Dose Response model; physical properties of 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. Figures S1−S7: FTIR spectra of cellulose, CM, CM-P, and CM-P-Pb; XRD spectra of cellulose, CM, CM-P, and CM-P-Pb; DSC thermograms of cellulose, CM, CM-P, and CM-P-Pb; effect of dosage of CM-P, pH, and temperature for Pb2+ adsorption; pseudo-first-order kinetics and pseudo-second-order kinetics of Pb2+ onto CM-P; adsorption equilibrium isotherms of Pb2+ onto CM-P at different temperatures; breakthrough curves for Pb2+ adsorption on CM-P and desorption profiles of Pb2+. (PDF)

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

phosphorylated cellulose microsphere adsorbents have been developed for the removal of Pb2+ from water by using a solidphase 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. The recycling and reuse of the used absorbents in this work is convenient.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-139-86270668. E-mail: [email protected], [email protected] (X. Luo). *E-mail: [email protected] (Z.M.). *Tel.: +86-158-82909291. E-mail: [email protected] (F.W.).

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CONCLUSIONS 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

ORCID

Xiaogang Luo: 0000-0003-3493-2140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support for this research was provided by the National Natural Science Foundation of China (51303142), Science and Technology Program of Guangdong Province 5115


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(2015A010105018), Natural Science Foundation of Hubei Province (2014CFB775), Open Foundation of Collaborative Innovation Center of Wuhan Institute of Technology (P201109), and Graduate Innovative Fund of Wuhan Institute of Technology (CX2015067).

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Improved Solid-Phase Synthesis of Phosphorylated Cellulose

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