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Facile design of green engineered cellulose/metal hybrid macrogels for efficient trace phosphate removal Xiaojuan Lei, Xuehai Dai, Sihui Long, Ning Cai, Xiaogang Luo, and Zhaocheng Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00587 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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Industrial & Engineering Chemistry Research

Facile design of green engineered cellulose/metal hybrid macrogels for efficient trace phosphate removal Xiaojuan Lei,a Xuehai Dai,a Sihui Long,a Ning Cai, Zhaocheng Ma*b, Xiaogang Luo∗a

a

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, Wuhan 430073,Hubei, China b

Key Laboratory of Horticultural Plant Biology (Ministry of Education), College of Horticulture and Forestry, Huazhong Agricultural University, Shizishan Street No.1, Wuhan 430070, 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, China E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract Cellulose/metal hybrid macrogels were prepared by immobilizing uniformly dispersed thiolate-modified Fe2O3 nanoparticles into cellulose matrix. The structure and properties of the hybrid macrogels were characterized using SEM, EDS, water contact angle, XRD, FTIR, XPS, and adsorption tests. The hybrid macrogels exhibited good stability, convenient operation, high selectivity for trace phosphate removal, with a remaining phosphorus concentration of only 0.15 mg L-1 in 200 minutes (5 mg L-1

initial concentration). The influences of pH, ionic strength, and competitive

anions were also investigated. The hybrid macrogels could be recovered by simple and rapid magnetic separation, and regenerated in NaOH solution. During 5 cycles of adsorption-elution-regeneration stability test, the hybrid macrogels still retained 80% adsorption capacity of trace phosphate. In this work, utilization of natural polymer was combined with green and sustainable technology to develop economically sustainable, eco-friendly and cost-effective cellulose/metal hybrid macrogels for the application of trace phosphate removal from water. Keywords: Cellulose-based hybrid macrogels; Phosphate removal; Green technology. 1. Introduction Phosphorus is a salutary micronutrient for humans and ecosystems, while eutrophication has seriously harmed the public health and the survival of many aquatic species on a global scale. Specifically, the release of phosphorus into groundwater and surface water has caused serious problems to the drinking water


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supplies in China and elsewhere.1-2 The World Health Organization (WHO) has mandated a maximum discharge limit for phosphorus of 0.5-1.0 mg L-1. In China the emission standard for the urban sewage treatment plant was reduced to GB 1A (Total P is 0.5 mg L-1) from GB 1B (Total P is 1.0 mg L-1).3 Many chemical and biological techniques have been explored for the removal of phosphate from water,4-9 but only a few studies investigated the removal of trace phosphate from water to achieve the very low concentrations and guarantee the quality of potable water. As a result of its convenient operation, low cost, and non-secondary-pollution feature, adsorption has been considered as an attractive alternative for the removal of phosphate from surface water or groundwater. Iron oxides (goethite, hematite and ferrihydrite)10-11 have been extensively studied and used as potential adsorbents for aqueous phosphate because of their excellent adsorption capacity and relatively low operation cost. However, it is well recognized that the dispersion of iron oxide nanoparticle adsorbents into a groundwater system is limited by the following processes: mineral sorption, aggregation, microbiological activity and formation of voluminous corrosion products.12-19 To minimize the limitations outlined above, it would be highly advantageous to develop a remediation method that utilizes the reactivity of iron oxide nanoparticles whilst avoids the release of free nanoparticles into the environment. One feasible approach is to develop a ‘nanohybrid’,20-23 where generally the iron oxide nanoparticles are immobilized in a support material such as activated carbon, silica, and ion-exchangers. Recently, polymer has been demonstrated as a preferable alternative of supporter for the metal



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oxides

immobilization.

For

example,

Niti

et

al.

immobilized

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zirconium

sulfate-surfactant micelle mesostructure on Aramid-Polymer matrix, and the resultant hybrid adsorbent exhibited good capacity for the removal of phosphate.24 The immobilized nanoparticles could enhance the permeation and preconcentration of target phosphate within the matrix, making its removal more effective by the nanohybrid. However, the synthesis of commercial polymer uses a lot of toxic chemical reagents, such as nitrobenzene, poly acrylic acid, humic acid, and styrene. Additionally, the synthesis of polymer is fairly complicated, and usually accompanied by the production of toxic byproduct, resulting in serious secondary pollution to the environment. Recent research indicated that cellulose, the most abundant renewable natural polymer with many significant advantages, such as wide availability, non-toxic, low cost, carbon neutral and biodegradation, could be an attractive alternative support material for metal oxides immobilization and replace the commercial polymer. When dissolved in a “green solvent” (NaOH/ urea aqueous solution),25 cellulose could produce considerable macro-micro-nano structures arising from the H2O-induced phase separation during the preparation process, while the solvent-rich regions resulted in the formation of pores.26 Cellulose served as support materials is chemically inert and has little adsorption itself,27-29 while its rich interconnected 3D pores and large internal surface area could provide strong mechanical stability,30 improve the chemical stability of the hybrid adsorbents in water and facilitate the contact between nanoparticles and aqueous phosphate to maximize the capacity of


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adsorption. On the basis of above features, cellulose, which is eco-friendly, cost-effective, and widely available, could be a remarkable alternative as support material. The combination of thiolate modification of Fe2O3 nanoparticles and cellulose could produce attractive hybrid macrogels adsorbents, which possess good stability, convenient operation, and high selectivity for trace phosphate removal. Besides, the design of hybrid materials could immobilized and protected the modified Fe2O3 nanoparticles from releasing into the environment and causing secondary pollution. In this research, the eco-friendly magnetic porous cellulose/metal hybrid macrogels (CHM) were developed by immobilizing thiolate-modified Fe2O3 nanoparticles in a cellulose matrix. The synthetic hybrid macrogels were applied in the trace phosphate removal from water to achieve the very low concentrations and guarantee the quality of potable water. The thiolate modification of Fe2O3 nanoparticles in the hybrid macrogels could impart strong affinity for phosphate through the formation of an innersphere complex and thus facilitate the phosphate removal. The widely available natural polymer could be dissolved with a “green solvent” and cured into macrogels directly without cross-linking agents. The morphology and physicochemical properties of the hybrid macrogels were characterized by SEM, EDS, water contact angle, XRD analysis. The underlying adsorption mechanism was further explored with FTIR and X-ray photoelectron spectroscopy. Batch adsorption experiments indicated that CHM exhibits a capacity for phosphate removal similar to that of amorphous

zirconium

oxide

nanoparticles,

a


commercially

available


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phosphate-specific adsorbent,31 which could reduce the phosphorus concentration to ～0.15 mg L-1, meeting the discharge requirement of 0.5-1.0 mg L-1. 3 In addition to the influences of pH, ionic strength, and competitive anions on phosphate removal being investigated, the adsorption mechanism of CHM was particularly studied. Cyclic adsorption and regeneration experiments were also performed to evaluate the reusability of CHM for the practical application. 2. Materials and methods 2.1. Materials Cotton linter pulp (ɑ-cellulose>95%) was procured from Hubei Chemical Fiber Group Ltd. (Xiangfan, China). Its viscosity-average molecular weight (Mη) was determined to be 12.5×104 Dalton.32 The Fe2O3 nanoparticles powder (I1316044, 99.5%, 20-40nm) and 3-Mercaptopropionic acids (MPA) (M103036, 99%) were purchased from Aladdin industrial corporation (Shanghai, China). The stock solution of phosphate was prepared by dissolving a required amount of KH2PO4 (Sinopharm Chemical Reagent Co., Ltd, cn) in water. The stock solution was then diluted with distilled water to produce different initial concentrations. All the other chemicals were of analytical grades. 2.2. Preparation of CHM Modification of Fe2O3 with MPA was performed by mixing a known amount of Fe2O3 with 50 mL of MPA in toluene on a rotary shaker for 10 h. After separating with magnetic decantation, the particles were washed with ethanol three times to remove residual MPA.33


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CHM was prepared by an optimized extrusion dropping technology.34 First, a given amount of thiolate-modified Fe2O3 (3.0 g) was homogeneously dispersed into 162 mL of de-ionized water. Then urea (24.0 g) and NaOH (14.0 g) were added into the mixture. The mixture was ultrasonically treated for 30 min. After that it was pre-cooled to -12.5 ℃. Cellulose (8.0 g) was added immediately into the mixture with vigorous stirring for 5 min to obtain a cellulose nanohybrid solution. After degasification the cellulose nanohybrid solution was added dropwise into a sodium chloride solution (500 mL, 20 wt%) at a constant rate (3 mL min-1) using a syringe pump to obtain the porous hybrid macrogels (CHM). The resultant hybrid macrogels were washed several times with distilled water and kept in water for characterization and adsorption experiments. 2.3. Characterization of materials Photograph of the hybrid macrogels was taken with a digital camera (Canon A630). Powder X-ray diffraction measurements were performed using a Bruker D8-Advance diffractometer. FESEM scanning electron microscopy (SEM, SIRION TMP, FEI) were used in combination with an Energy Dispersive X-ray spectrometry. The samples were ground into powders and dried in air at room temperature prior to FTIR and XPS analysis. FTIR spectra were measured with a Perkin-Elmer FTIR spectrometer. XPS spectra were measured with an ESCALAB 250Xi X-ray photoelectron spectrometer. The Brunauer-Emmett-Teller (BET) surface area was determined by using a computer-controlled nitrogen gas adsorption analyzer



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(ASAP2010). The water content (ω), wet density (ρw), mean pore volume (Vp) , and porosity (Pr) of the wet CHM was measured with drainage method by a dilatometer.35-36 2.4. Phosphate adsorption and desorption experiments Adsorption studies were performed by exposing optimized dose of CHM (1 g) to 100 mL phosphate solution. The mixture was shaken at 300 rpm using a thermostatic shaker to allow CHM to be in full contact with phosphate in water. After 200 mins, the CHM was removed by a magnet and the remaining solution was analyzed for phosphorus concentration by SpectraMax M2 (Molecular Devices, USA). The process of adsorption includes the isothermal study and kinetic study. In isothermal study, the initial phosphorus concentration was 5, 10, 15, 20, 25 and 30 mg L-1, respectively. The temperature of adsorption process was controlled at 298 K, 303 K and 313 K, respectively. In kinetic study, the initial concentration of phosphorus was 5mg L-1, and the contact time ranged from 0 min to 200 min. HCl or NaOH (1.0 M) solutions were used for pH adjustment. A certain amount of KCl, K2SO4, K2CO3, KHCO3 or KNO3 were used to adjust the strength of competitive anionic. Desorption experiments were conducted using a single sample with a laboratory static aqueous method. Desorption and reuse experiments were performed in quintuplicate. Aqueous NaOH solution (0.1 M) was used as the desorption solution. The desorbed CHM were washed until it is neutral before they were reused for phosphate adsorption. The repeating experiments were carried out three times under the same conditions and calculating the average values to reduce the error of


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experimental results of phosphate adsorption. The standard deviations of experiments were found to be within ± 5.0% of average values. 3. Results and discussion 3.1. Structure and morphology analysis Fig. 1a shows the photograph of CHM in water. The CHM products are spherical and dark brown because of the presence of the thiolate-modified Fe2O3 nanoparticles. The mean diameter of CHM and the polydispersity factor are 2.1 mm and 0.06, respectively, showing a successful preparation of the spherical cellulose-based hybrid macrogels. The magnetic hybrid macrogels could be easily removed from water with a magnet, showing a sensitive magnetic response (Inset: Photograph of CHM attracted by a magnet). This property of the CHM is of great importance for its industrial application because it can limit the secondary pollution and lead to easy operations in the process of adsorption and desorption during the water treatment. The SEM images of CHM and phosphate-loaded CHM are shown in Fig. 1b and Fig. 1c, respectively. It could be observed that the porous structure of CHM was not destroyed after the adsorption of phosphate. Compared with CHM, the phosphate-loaded CHM also displays homogeneous and porous structure, showing a large surface area and good mechanical stability. Besides, Fig. S1 exhibited the photographs of CHM before and after extruding observed under a microscope, which showed the elasticity and compressibility of the hybrid macrogels.30 Usually, the cellulose regeneration in NaCl aqueous solution is a physical process, with no accompanying chemical reaction. When the urea hydrates on the surface of NaOH



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hydrogen-bonded cellulose complex are broken by coagulant, the cellulose chains aggregate rapidly to form hydrogels. Among a series of coagulants, NaCl aqueous solution could result in good porous structure and mechanical properties.37 The physical properties of the native cellulose macrogels and the CHM samples are shown in Table 1. The EDS analysis (Fig. 1b inset) performed on CHM after adsorption of phosphate shows that P is distributed throughout the CHM, confirming that the adsorption occurs on and in the hybrid macrogels. On the basis of this information, it is reasonable to suggest that not only the surface of CHM but also the reactive material (thiolate-modified Fe2O3 nanoparticles) inside the macrogels came into contact with water and the contaminants to maximize the capacity of adsorption. Water contact angle measurements indicate that CHM shows super-hydrophilicity with a contact angle of almost 0°for water in the air (Fig. 1c). As a consequence of the rich porous structure and the high hydrophilicity of the CHM, the phosphate ions could easily penetrate into the macrogels and interact with the thiolate-modified Fe2O3 nanoparticles by the formation of a complex to facilitate the phosphate adsorption onto CHM. 3.2. XRD and FTIR analysis The XRD patterns of Fe2O3, thiolate-modified Fe2O3, phosphate-loaded CHM, cellulose and CHM powder are shown in Fig. 2 (a). Fe2O3 and the thiolate-modified Fe2O3 had diffraction peaks at 30.45˚, 35.70˚, 43.26˚, 57.30˚ and 63.03˚, assigned to the corresponding diffraction planes of (220), (311), (400), (511) and (440).38 The peaks at 2θ = 11.49˚, 20.43˚ and 22.53˚ in cellulose, corresponding to the (110), (110)


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and (200) planes, suggest the cellulose II crystalline form.39 However, the presence of peaks at 2θ = 22.67˚ and 62.86˚ in the CHM indicate a good miscibility between the thiolate-modified Fe2O3 nanoparticles and the cellulose. If the thiolate-modified Fe2O3 and the cellulose are immiscible, each material would only show its characteristic peaks in the hybrid macrogels. Zhou et al. also reported the similar result in the published work, in which they proved a good miscibility between cellulose and chitin. 40

Considering that the adsorption primarily occurs on the amorphous regions40, the

phosphate would penetrate into the hybrid macrogels more easily because of the interaction between the thiolate-modified Fe2O3 and the cellulose. The FTIR spectra of Fe2O3, thiolate-modified Fe2O3, cellulose, CHM and phosphate-loaded CHM are displayed in Fig. 2 (b). For Fe2O3, the bands at 572 cm-1 and 696 cm-1 are ascribed to the Fe-O vibrations. For the thiolate-modified Fe2O3, new peaks at 1438 cm-1 and 1562 cm-1 correspond to the asymmetric and symmetric stretching vibrations of carboxylate group (COO-) from the bonding between the MPA COOH and Fe2O3. On the exposed side, new peaks at 1272 cm-1 and 1312 cm-1 might be attributed to the presence of sulfoxide (S=O) and sulfonate groups.33, 41 The spectrum of cellulose displays characteristic absorbance of cellulose II. The peaks at 3005-3712 cm-1 of CHM, corresponding to the stretching vibrations of the hydroxyl groups of cellulose, shift to lower wavenumbers as a result of the strong intermolecular hydrogen bonds between the sulfonic group of the thiolate-modified Fe2O3 and the hydroxyl group of cellulose.42 After adsorption of phosphate, the spectral region for structural diagnosis of the



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metal-orthophosphate complexes is from 1200 to 800 cm-1. A broad and weak peak at 1298 cm-1 may be ascribed to the P-O-H bending vibration. The peak at 1120 cm-l could be from the P=O stretching from the phosphate group with two OH ligands, indicating the strong interactions between phosphate and the sulfonic group of the thiolate-modified Fe2O3. The peak at 1032 cm-l may be ascribed to either the (FeO)2PO2 or to the (FeO)(OH)PO2 complexes. The peak at 893 cm-1 might be due to the MOPO3 complex. A set of peaks near 1120 cm-1 belongs to the bridging bidentate complexes. On the basis of this information, it is reasonable to suggest that the strong interactions between the thiolate-modified Fe2O3 and the phosphate result in the formation of innersphere complex.43 The FTIR study results indicate that both the modification of Fe2O3 and its interaction with the cellulose in the CHM played an important role in the adsorption of phosphate. 3.3. XPS analysis The XPS method is reasonably effective in investigating the ligand effect of transition metal complexes.44 Fig. 3 displays the XPS survey scan of CHM before and after the phosphate adsorption. The results indicate the obvious presence of phosphate on the surface of CHM after the adsorption of phosphate. A strong P 2p peak is observed for the phosphate-loaded CHM (Fig. 3a). In Fig. 3c, the typical Fe 2p spectra of CHM before and after phosphate adsorption are exhibited. Prior to the analyte adsorption, the Fe 2p spectrum contains a peak at a binding energy 724.2 eV. After adsorption of phosphate, however, the Fe 2p peak is observed at a higher binding energy 724.5 eV. The P 2p peak in the spectrum of KH2PO4 is located at


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133.5 eV. In the spectrum of phosphate-loaded CHM, the P 2p peak appears at a lower binding energy 133.2 eV (Fig. 3b). This indicates that some Fe atoms in CHM exist in a complex state after the adsorption of phosphate. In the complex formed in reaction, a lone pair of electrons in the thiolate-modified Fe2O3 is donated to the shared bond between the phosphate and the Fe. The electron cloud density of the Fe atom is reduced, leading to a higher binding energy peak. Herein, the XPS spectra provide the evidence of the phosphate binding to the Fe atoms, resulting in the formation of a phosphate-containing complex of the thiolate-modified Fe2O3. The high resolution XPS spectrum for the O 1s region is shown in Fig. 3d. Three peaks at 529.8 eV, 530.1 eV and 532.8 eV are observed for the phosphate-loaded CHM, corresponding to the O-Fe bonding, OH/O-P bonding, and the adsorbed water, respectively 45. The peaks at 530.1 eV could be attributed to the Fe-O-P bonding. The binding energy of 133.2 eV might be attribute to FePO4 and the phosphate adsorbed ferrihydrite,45 further confirming the formation of the innersphere complex. The XPS analyses provide convincing evidence for the phosphate adsorption onto the CHM by forming an innersphere complex of the Fe-O-P type, which is in good agreement with the conclusions from FTIR (Fig. 2(b)). 3.4. Removal mechanism of phosphate from water by CHM A schematic model is proposed to illustrate the mechanism of the adsorption of phosphate by CHM (Fig. 4). During the adsorption process, the high surface positive charges of the CHM facilitated the migration of the negative charged phosphate to the peripheral region of the hybrid macrogels by electrostatic attraction (a), and the



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peripheral phosphate could bind directly to the thiolate-modified Fe2O3 nanoparticles of CHM through ligand exchange to form an innersphere complex (b). The formation of innersphere complex has been demonstrated in FTIR and XPS study. Besides, the water contact angel and SEM characterizations revealed the high hydrophilicity and rich porous structure of CHM, respectively. As a consequence of its attractive structure, the dissolved phosphate species could easily penetrate into the hybrid macrogels and interact with the internal thiolate-modified Fe2O3 nanoparticles of CHM to maximize the capacity of adsorption (c). 3.5. Adsorption isotherms and kinetic studies on the phosphate removal by CHM The adsorption isotherms were studied to estimate the maximum loading capacity of the CHM. The experiments were conducted with the initial concentration of phosphorus of 5, 10, 15, 20, 25 and 30 mg L-1, respectively. Fig. 5a displays the adsorption isotherms of the adsorption of phosphate onto CHM. Table S1 compare the phosphate removal by different methods and Table S2 compare the phosphate adsorption by different composite gels. The obtained experimental data were fitted by the Langmuir and Freundlich models given in Eq. (1) and Eq. (2).

qe = q maxK LC e /(1 + K LC e )

(1)

1

q e = K F C en

(2)

where KL (L mg-1) is the Langmuir equilibrium constant related to the affinity of the binding sites with the adsorbate. qmax (mg g-1) is the monolayer capacity referred to


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the amount of phosphate required to occupy all the available sites per unit mass of sample. KF (mg g-1) is the Freundlich constant related to the adsorption capacity and 1/n is the heterogeneity factor. In general, the adsorption capacity of the adsorbent for the given adsorbate increases with the increase of KF. The surface heterogeneity could be considered to be less significant as the value of n is close to 1. In contrast, favorable adsorption would occur if n is greater than unity (ie. n>1). The values of the Langmuir and Freundlich parameters and the correlation coefficients (R2) are listed in Table 2. From Fig. 5a and the correlation coefficient (R2 > 0.99) in Table 2, it could be observed that the phosphate adsorption was better fitted for the Langmuir adsorption isotherm, showing that the adsorption of phosphate occurred in a monolayer fashion. The kinetics of the phosphate removal of CHM is depicted in Fig. 5b. The process of adsorption consists of two stages. The adsorption rate was very fast at the very beginning. About 91.6% phosphate was adsorbed onto CHM within 80 min. The residual phosphorus concentration decreased to about 0.15 mg L-1, meeting the discharge requirement of 0.5-1.0 mg L-13, 31. The fast adsorption rate was due to the good affinity between the CHM and the phosphate, which enhanced their contact efficiency in the aqueous solutions and facilitated the diffusion of phosphate ions from the bulk solution onto the active sites of CHM. Kinetic data could be fitted into various rate models to gain insight into the mechanism of phosphate adsorption.46 The pseudo-first-order model and pseudo-second-order model were given in Eq. (3) and Eq. (4), respectively. log(q e − qt ) = log q e −

k 1t 2.303


(3)


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1 t t = + 2 q qt k 2qe e

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(4)

where qe and qt are the amounts of uptake (mg L-1) at equilibrium and at time t, respectively, and k1 (h-1) and k2 (g mg-1 h-1) are the pseudo-first-order and pseudo-second-order

constants,

respectively.

The

fitting

curves

of

the

pseudo-second-order rate model are shown in Fig. 5b. (The inset shows the linear form of the pseudo-second-order rate model of the kinetic study). The kinetics parameters obtained are listed in Table 3. From the higher correlation coefficients (R2=0.998, R2=0.830) and the closer calculated qe values (3.65) with experimental ones (3.42), it could be concluded that the phosphate uptake onto CHM was better fitted by the pseudo-second-order model. Similar results were reported by Rodrigues et al. 47 and Tang et al. 48 on various phosphate adsorbents. 3.6. Effect of pH, ionic strength, and competing anions on phosphate removal by CHM The adsorption of anions at the solid-liquid interfaces is generally influenced by the pH of the solution. The effect of pH on phosphate adsorption onto CHM is shown in Fig. 6a. The uptake of phosphate increased as the pH of the phosphate solution decreased. Low pH was favorable for the protonation of the -SO3H groups on the surface of the thiolate-modified Fe2O3, which enhanced the electrostatic attraction between the positively charged surface of CHM and the phosphate anions thus facilitated the phosphate removal. The electrostatic interaction between the surface of CHM and the phosphate anions predominantly occurred via the following pathways; Fe − SOଷ H + ‫ܪ‬ା → Fe − SOଷ ‫ܪ‬ଶା


(5)

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Fe − Sܱଷ ‫ܪ‬ଶା + ‫ܪ‬ଶ ܱܲସି → (‫ ݁ܨ‬− ܱܵଷ ‫ܪ‬ଶ )ା (‫ܪ‬ଶ ܱܲସ )ି

(6)

2Fe − Sܱଷ ‫ܪ‬ଶା + ‫ܱܲܪ‬ସଶି → (‫ ݁ܨ‬− ܱܵଷ ‫ܪ‬ଶ )ଶା (‫ܱܲܪ‬ସ )ଶି

(7)

Yet the interaction between the CHM and the phosphate is a chemisorption dominated process. The electrostatic attraction is not a major factor during the process of adsorption. Herein the uptake of phosphate just decreased slightly when the pH increased from 2 to 7. As the solution pH changed from neutral to basic, the negative charge intensity of the surface of CHM increased. The uptake of phosphate by the CHM decreased as a consequence of the Coulomb repulsive interaction between the negatively charged CHM surface and the negatively charged phosphate species.31 The mechanism of the phosphate adsorption on CHM was also investigated by evaluating the effect of ionic strength on the adsorption performance. The ionic strength of the phosphate solution was adjusted with different concentrations of KNO3.31 As shown in Fig. 6a, the uptake of phosphate increased with the increase of the solution ionic strength. Similar results were also reported in other studies on the phosphate removal with various adsorbents.49 As suggested by McBride,50 the uptake of phosphate would decrease with the ionic strength increased if the outersphere complexes were formed by the phosphate species. In contrast, the uptake of phosphate would either increase or not change with the ionic strength increased if the innersphere complexes were formed by the phosphate species. Based on this information, it could be deduced that the uptake of phosphate on CHM followed the innersphere complex mechanism, in agreement with the conclusions of the FTIR and XPS analyses.



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As a consequence of the complex composition of natural water, the competition from other species would affect the uptake of phosphate by CHM to some extent.31 Other anions, including Cl-, SO42-，HCO3-, CO32- and NO3-, commonly co-exist with phosphate in natural water or wastewater, and would possibly compete with phosphate for the active sites of CHM. Shown in Fig. 6b the effects of these competing anions on the uptake of phosphate were tested at two concentrations (5 and 10 mM). The coexisting anions (Cl-, SO42-, HCO3-, CO32- and NO3-), even at very high concentrations compared with 5 mg L-1 phosphorus (100 times at 5 mM and 200 times at 10 mM), had no or even positive effect on the uptake of phosphate. As illustrated above, it could be concluded that CHM was capable of highly selectively removing phosphate effectively in the presence of exceptionally high concentrations of competing anions. Thus it is conducive to the industrial applications in natural water or wastewater treatment. 3.7. Desorption and reusable properties of CHM An important feature of CHM as a potential adsorbent is the reusability in the practical application.51 As the hydroxide is the hardest Lewis base among common inorganic anions,4 aqueous NaOH is an effective reagent for desorption.52 Phosphate-loaded CHM was quantitatively desorbed with 0.1M aqueous NaOH. The results demonstrated that the phosphate adsorbed onto CHM could be easily desorbed with 0.1 M NaOH solution. Shown in Fig. 7a the desorbed CHM were reused for phosphate removal, and the kinetics data of phosphate adsorption on the desorbed CHM could also be well fitted into the pseudo-second-order rate model (Fig. 7b). The


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phosphate adsorption and desorption capacities decrease slightly after the second cycle (Fig. 7a), showing that the CHM could retain its functionality over 5 cycles without significant loss of its original efficiency. Phosphate desorption might occur through an ion-exchange mechanism between the phosphate and hydroxide

51

. In

addition, as exhibited in the SEM characterization, CHM had rich porous structure, which could prevent the leakage of Fe2O3 NPs during the desorption process. Herein, the recovered CHM could be repeatedly utilized for phosphate removal from water, which successfully met the green and sustainable requirement in drinking-water purification. 4. Conclusions Economically sustainable, eco-friendly and cost-effective magnetic cellulose-based porous hybrid macrogels with excellent removal capacity toward trace phosphate have been developed via a simple and green technology. The cellulose-based hybrid macrogels are an effective means in preventing eutrophication caused by phosphate in a wide spectrum of water sources, and even the drinking water treatment systems. In the adsorption study, the hybrid macrogels were found to exhibit good stability, simple operation, high selectivity and excellent kinetic performance for trace phosphate removal. With a maximum adsorption capacity of 22.25 mg/g and a remaining phosphorus concentration of only ～0.15 mg L-1, the test samples meet the discharge requirement mandated by the WHO. Simultaneously the phosphate adsorption showed little pH dependence in the range from pH 2 to 6, while it decreased sharply with the pH increased above 7. The adsorption of phosphate



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increased with the increase of the solution ionic strength. Common coexisting anions showed minimal (or no) effect on the uptake of phosphate. The mechanisms for the adsorption of trace phosphate on the hybrid macrogels could be described as complexation between the phosphate and the thiolate-modified Fe2O3 nanoparticles, and mainly supplemented by the physical absorption process (electrostatic attraction). Regeneration through the treatment with NaOH solution allowed the hybrid macrogels reuse, and during many adsorption-elution-regeneration cycles, no measurable Fe2O3 nanoparticles were found in the effluents and the hybrid macrogels retained high adsorption capacity. Acknowledgments This work was supported by the National Natural Science Foundation of China (51303142), the Natural Science Foundation of Hubei Province (2014CFB775), and the Open Foundation of R&D Program of Low grade phosphate rock resources development and utilization Collaborative Innovation Center of Hubei Province (P201109).

References (1) Conley, D. J.; Paerl, H. W.; Howarth, R. W.; Boesch, D. F.; Seitzinger, S. P.; Havens, K. E.; Lancelot, C.; Likens, G. E., Controlling eutrophication: nitrogen and phosphorus. Sci. 2009, 323 (5917), 1014-1015. (2) Loganathan, P.; Vigneswaran, S.; Kandasamy, J.; Bolan, N. S., Removal and recovery of phosphate from water using sorption. Crit. Rev. Environ. Sci. Technol. 2014, 44 (8), 847-907. (3) Ren, Z.; Shao, L.; Zhang, G., Adsorption of phosphate from aqueous solution using an iron– zirconium binary oxide sorbent. Water, Air, Soil Pollut. 2012, 223 (7), 4221-4231. (4) Awual, M. R.; Jyo, A.; Ihara, T.; Seko, N.; Tamada, M.; Lim, K. T., Enhanced trace phosphate removal from water by zirconium (IV) loaded fibrous adsorbent. Water Res. 2011, 45 (15), 4592-4600. (5) Srivastava, S.; Srivastava, A. K., Biological phosphate removal by model based fed-batch cultivation of Acinetobacter calcoaceticus. Biochem. Eng. J. 2008, 40 (2), 227-232.


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(6) Yang, Y.; Lohwacharin, J.; Takizawa, S., Hybrid ferrihydrite-MF/UF membrane filtration for the simultaneous removal of dissolved organic matter and phosphate. Water Res. 2014, 65, 177-85. (7) Kuokkanen, V.; Kuokkanen, T.; Rämö, J.; Lassi, U.; Roininen, J., Removal of phosphate from wastewaters for further utilization using electrocoagulation with hybrid electrodes – Techno-economic studies. Journal of Water Process Engineering 2015, 8, e50-e57. (8) Uludag-Demirer, S.; Othman, M., Removal of ammonium and phosphate from the supernatant of anaerobically digested waste activated sludge by chemical precipitation. Bioresour. Technol. 2009, 100 (13), 3236-44. (9) Zheng, X.; Pan, J.; Zhang, F.; Liu, E.; Shi, W.; Yan, Y., Fabrication of free-standing bio-template mesoporous hybrid film for high and selective phosphate removal. Chem. Eng. J. 2016, 284, 879-887. (10) Liang, H.; Liu, K.; Ni, Y., Synthesis of mesoporous α-Fe2O3 using cellulose nanocrystals as template and its use for the removal of phosphate from wastewater. Journal of the Taiwan Institute of Chemical Engineers 2017, 71, 474-479. (11) Zhang, C.; Li, Y.; Wang, F.; Yu, Z.; Wei, J.; Yang, Z.; Ma, C.; Li, Z.; Xu, Z.; Zeng, G., Performance of magnetic zirconium-iron oxide nanoparticle in the removal of phosphate from aqueous solution. Appl. Surf. Sci. 2017, 396, 1783-1792. (12) Crane, R.; Scott, T., Nanoscale zero-valent iron: future prospects for an emerging water treatment technology. J. Hazard. Mater. 2012, 211, 112-125. (13) Nurmi, J. T.; Tratnyek, P. G.; Sarathy, V.; Baer, D. R.; Amonette, J. E.; Pecher, K.; Wang, C.; Linehan, J. C.; Matson, D. W.; Penn, R. L., Characterization and properties of metallic iron nanoparticles: spectroscopy, electrochemistry, and kinetics. Environ. Sci. Technol. 2005, 39 (5), 1221-1230. (14) Altavilla, C.; Ciliberto, E., Inorganic nanoparticles: synthesis, applications, and perspectives. CRC Press: 2010. (15) Elimelech, M.; Jia, X.; Gregory, J.; Williams, R., Particle Deposition & Aggregation: Measurement, Modelling and Simulation. Butterworth-Heinemann: 1998. (16) Phenrat, T.; Saleh, N.; Sirk, K.; Tilton, R. D.; Lowry, G. V., Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions. Environ. Sci. Technol. 2007, 41 (1), 284-290. (17) Schrick, B.; Hydutsky, B. W.; Blough, J. L.; Mallouk, T. E., Delivery vehicles for zerovalent metal nanoparticles in soil and groundwater. Chem. Mater. 2004, 16 (11), 2187-2193. (18) Tratnyek, P. G.; Johnson, R. L., Nanotechnologies for environmental cleanup. Nano today 2006, 1 (2), 44-48. (19) Zhang, W. x.; Elliott, D. W., Applications of iron nanoparticles for groundwater remediation. Remediation 2006, 16 (2), 7-21. (20) Ajayan, P. M.; Schadler, L. S.; Braun, P., Nanocomposite science and technology. 2003. WILEY-VCH Verlag GmbH & Co. Kga, Weinheim. (21) Nguyen, T. A.; Ngo, H. H.; Guo, W. S.; Zhang, J.; Liang, S.; Tung, K. L., Feasibility of iron loaded 'okara' for biosorption of phosphorous in aqueous solutions. Bioresour. Technol. 2013, 150, 42-9. (22) Mallampati, R.; Valiyaveettil, S., Apple Peels—A Versatile Biomass for Water Purification? ACS Applied Materials & Interfaces 2013, 5 (10), 4443-4449. (23) Biswas, B. K.; Inoue, K.; Ghimire, K. N.; Ohta, S.; Harada, H.; Ohto, K.; Kawakita, H., The adsorption of phosphate from an aquatic environment using metal-loaded orange waste. J. Colloid Interface Sci. 2007, 312 (2), 214-23. (24) Qiu, H.; Liang, C.; Zhang, X.; Chen, M.; Zhao, Y.; Tao, T.; Xu, Z.; Liu, G., Fabrication of a Biomass-Based Hydrous Zirconium Oxide Nanocomposite for Preferable Phosphate Removal and



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Recovery. ACS Appl. Mater. Interfaces 2015, 7 (37), 20835-44. (25) Cai, J.; Kimura, S.; Wada, M.; Kuga, S.; Zhang, L., Cellulose Aerogels from Aqueous Alkali Hydroxide–Urea Solution. Chemsuschem 2008, 1 (1-2), 149-54. (26) Jiang, L.; Liu, P., Novel magnetic fly ash/poly (acrylic acid) composite microgel for selective adsorption of Pb (II) ion: synthesis and evaluation. Ind. Eng. Chem. Res. 2014, 53 (8), 2924-2931. (27) Hokkanen, S.; Bhatnagar, A.; Sillanpää, M., A review on modification methods to cellulose-based adsorbents to improve adsorption capacity. Water Res. 2016, 91, 156-173. (28) O’Connell, D. W.; Birkinshaw, C.; O’Dwyer, T. F., Heavy metal adsorbents prepared from the modification of cellulose: A review. Bioresour. Technol. 2008, 99 (15), 6709-6724. (29) Suhas; Gupta, V. K.; Carrott, P. J.; Singh, R.; Chaudhary, M.; Kushwaha, S., Cellulose: A review as natural, modified and activated carbon adsorbent. Bioresour. Technol. 2016, 216, 1066-76. (30) Gericke, M.; Trygg, J.; Fardim, P., Functional Cellulose Beads: Preparation, Characterization, and Applications. Chem. Rev. 2013, 113 (7), 4812-4836. (31) Su, Y.; Cui, H.; Li, Q.; Gao, S.; Shang, J. K., Strong adsorption of phosphate by amorphous zirconium oxide nanoparticles. Water Res. 2013, 47 (14), 5018-5026. (32) 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. (33) Burks, T.; Avila, M.; Akhtar, F.; Göthelid, M.; Lansåker, P. C.; Toprak, M. S.; Muhammed, M.; Uheida, A., Studies on the adsorption of chromium(VI) onto 3-Mercaptopropionic acid coated superparamagnetic iron oxide nanoparticles. J. Colloid Interface Sci. 2014, 425, 36-43. (34) Luo, X.; Zhang, L., High effective adsorption of organic dyes on magnetic cellulose beads entrapping activated carbon. Journal of hazardous materials 2009, 171 (1-3), 340-7. (35) Luo, X.; Zhang, H.; Cao, Z.; Cai, N.; Xue, Y.; Yu, F., A simple route to develop transparent doxorubicin-loaded nanodiamonds/cellulose nanocomposite membranes as potential wound dressings. Carbohydr. Polym. 2016, 143, 231-8. (36) Luo, X.; Yuan, J.; Liu, Y.; Liu, C.; Zhu, X.; Dai, X.; Ma, Z.; Wang, F., 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. ACS Sustainable Chemistry & Engineering 2017, 5 (6), 5108-5117. (37) Wang, S.; Lu, A.; Zhang, L., Recent advances in regenerated cellulose materials. Prog. Polym. Sci. 2016, 53, 169-206. (38) Luo, X.; Liu, S.; Zhou, J.; Zhang, L., In situ synthesis of Fe3O4/cellulose microspheres with magnetic-induced protein delivery. J. Mater. Chem. 2009, 19 (21), 3538-3545. (39) Hartmann, M.; Kaplan, D., Biopolymers from renewable resources. Kaplan, DL, Ed 1998, 367. (40) Zhou, D.; Zhang, L.; Guo, S., Mechanisms of lead biosorption on cellulose/chitin beads. Water Res. 2005, 39 (16), 3755. (41) Adams, L.; Oki, A.; Grady, T.; McWhinney, H.; Luo, Z., Preparation and characterization of sulfonic acid-functionalized single-walled carbon nanotubes. Physica E: Low-dimensional Systems and Nanostructures 2009, 41 (4), 723-728. (42) Zhang, L.; Guo, J.; Du, Y., Morphology and properties of cellulose/chitin blends membranes from NaOH/thiourea aqueous solution. J. Appl. Polym. Sci. 2002, 86 (8), 2025-2032. (43) Tejedor-Tejedor, M. I.; Anderson, M. A., The protonation of phosphate on the surface of goethite as studied by CIR-FTIR and electrophoretic mobility. Langmuir 1990, 6 (3), 602-611. (44) Dambies, L.; Guimon, C.; Yiacoumi, S.; Guibal, E., Characterization of metal ion interactions with


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chitosan by X-ray photoelectron spectroscopy. Colloids Surf. Physicochem. Eng. Aspects 2001, 177 (2), 203-214. (45) Mallet, M.; Barthélémy, K.; Ruby, C.; Renard, A.; Naille, S., Investigation of phosphate adsorption onto ferrihydrite by X-ray photoelectron spectroscopy. J. Colloid Interface Sci. 2013, 407, 95-101. (46) Chen, C.; Li, X.; Zhao, D.; Tan, X.; Wang, X., Adsorption kinetic, thermodynamic and desorption studies of Th (IV) on oxidized multi-wall carbon nanotubes. Colloids Surf. Physicochem. Eng. Aspects 2007, 302 (1), 449-454. (47) Rodrigues, L. A.; da Silva, M. L. C. P., Adsorption kinetic, thermodynamic and desorption studies of phosphate onto hydrous niobium oxide prepared by reverse microemulsion method. Adsorption 2010, 16 (3), 173-181. (48) Tang, Y.; Zong, E.; Wan, H.; Xu, Z.; Zheng, S.; Zhu, D., Zirconia functionalized SBA-15 as effective adsorbent for phosphate removal. Microporous Mesoporous Mater. 2012, 155, 192-200. (49) Tanada, S.; Kabayama, M.; Kawasaki, N.; Sakiyama, T.; Nakamura, T.; Araki, M.; Tamura, T., Removal of phosphate by aluminum oxide hydroxide. J. Colloid Interface Sci. 2003, 257 (1), 135-140. (50) McBride, M. B., A critique of diffuse double layer models applied to colloid and surface chemistry. Clays Clay Miner. 1997, 45 (4), 598-608. (51) Pitakteeratham, N.; Hafuka, A.; Satoh, H.; Watanabe, Y., High efficiency removal of phosphate from water by zirconium sulfate-surfactant micelle mesostructure immobilized on polymer matrix. Water Res. 2013, 47 (11), 3583-3590. (52) Zhu, X.; Jyo, A., Column-mode phosphate removal by a novel highly selective adsorbent. Water Res. 2005, 39 (11), 2301-2308.

Table captions Table 1 Physical properties of the native cellulose macrogels and the CHM

Table 2 Constants and correlation coefficients calculated for various adsorption models for phosphate onto CHM (298 K and 5mg L-1)

Table 3 Comparison of the pseudo-first-order and pseudo-second-order models for



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the phosphate adsorption onto CHM (298 K and 5 mg L-1)

Table 1 Physical properties of the native cellulose macrogels and the CHM

Sample

Water

Wet density

Bulk

Pore volume

Porosity

BET surface

content ω

ρw

density ρg

Vp

Pr

area S

(%)

(g cm-3)

(g cm-3)

(cm3 g-1)

(%)

(m2 g-1)

91.75 ± 0.02

1.03 ± 0.01

1.55 ± 0.04

10.34 ± 0.02

91.16 ± 0.04

1.06 ± 0.01

1.95 ± 0.03

11.15 ± 0.01

native cellulose

94.52±0.10

33.27

macrogels

CHM

95.32 ± 0.20

46.53

Table 2 Constants and correlation coefficients calculated for various adsorption models for phosphate onto CHM (298 K and 5mg L-1) Langmuir

qmax (mg g-1)

KL (Lm g-1)

R2

CHM

22.25

1.090

0.993

Freundlich

KF (mg g-1)

1/n

R2

CHM

10.48

0.816

0.980

Table 3 Comparison of the pseudo-first-order and pseudo-second-order models for the phosphate adsorption onto CHM (298 K and 5 mg L-1) Pseudo-first-order

qe,exp

-1

qe,cal k1 (min-1)

R2

0.23

0.830

-1

kinetic model

(mg g )

(mg g )

CHM

3.42

2.86


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Pseudo-second-order

qe,exp

qe,cal

k2 (g mg-1 R2

-1

-1

-1

kinetic model

(mg g )

(mg g )

min )

CHM

3.42

3.65

1.84

0.998

Figure captions Fig.1. Photographs of CHM in water (a), (The inset shows CHM attracted by a magnet (0.01T)); SEM image of the cross-section of phosphate-loaded CHM (b), (The inset shows the EDS pattern of phosphate-loaded CHM); Water contact angle on CHM in air (c) Fig.2. XRD patterns of a, Fe2O3, b, thiolate-modified Fe2O3, c, CHM, d, phosphate-loaded CHM, e, cellulose (a); FTIR spectra of a, Fe2O3, b, thiolate-modified Fe2O3, c, cellulose, d, CHM, e, phosphate-loaded CHM, f, KH2PO4 (b) Fig.3. XPS analysis of CHM before and after the phosphate adsorption: (a) XPS survey scan of CHM before and after the phosphate adsorption; (b) P 2p spectra of KH2PO4 and the phosphate-loaded CHM; (c) Fe 2p spectra of CHM before and after the phosphate adsorption; (d) O 1s spectra of the phosphate-loaded CHM Fig.4. Schematic model illustration of the process of phosphate adsorption on CHM: electrostatic attraction (a), binding to the surface (b) and penetrating into the inside of CHM (c) Fig.5. Adsorption isotherms of phosphate onto CHM at pH 6 and 25˚C (a), kinetics of phosphate adsorption onto CHM at different temperatures: 25 ˚C, 30 ˚C and 40 ˚C (b)



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(The inset shows the linear form of the pseudo-second-order rate model of the kinetic study) Fig.6. Effect of pH and ionic strength on phosphate removal by CHM (a). Effect of competing anions on phosphate removal by CHM with initial phosphate concentration at 5 mg L-1, 25 ˚C and pH 6.0 (b) Fig.7. Cycle adsorption of batch experiments for CHM (initial phosphorus concentration at 5 mg L-1, 25 ˚C) (a). Adsorption rate curves of phosphate uptake for the desorbed CHM (b)

Fig.1. Photographs of CHM in water (a), (The inset shows CHM attracted by a magnet (0.01T)); SEM image of the cross-section of CHM (b) and phosphate-loaded CHM (c), (The inset shows the EDS pattern of phosphate-loaded CHM); Water contact angle on CHM in air (d)


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Fig.2. XRD patterns of a, Fe2O3, b, thiolate-modified Fe2O3, c, CHM, d, phosphate-loaded CHM, e, cellulose (a); FTIR spectra of a, Fe2O3, b, thiolate-modified Fe2O3, c, cellulose, d, CHM, e, phosphate-loaded CHM, f, KH2PO4 (b)

Fig.3. XPS analysis of CHM before and after the phosphate adsorption: (a) XPS survey scan of CHM before and after the phosphate adsorption; (b) P 2p spectra of



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KH2PO4 and the phosphate-loaded CHM; (c) Fe 2p spectra of CHM before and after the phosphate adsorption; (d) O 1s spectra of the phosphate-loaded CHM

Fig.4. Schematic model illustration of the process of phosphate adsorption on CHM: electrostatic attraction (a), binding to the surface (b) and penetrating into the inside of CHM (c)

Fig.5. Adsorption isotherms of phosphate onto CHM at pH 6 and 25 ˚C (a), kinetics of phosphate adsorption onto CHM at different temperatures: 25 ˚C, 30 ˚C and 40 ˚C (b) (The inset shows the linear form of the pseudo-second-order rate model of the kinetic study)


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Fig.6. Effect of pH and ionic strength on phosphate removal by CHM (a). Effect of competing anions on phosphate removal by CHM with initial phosphate concentration at 5 mg L-1, 25 ˚C and pH 6.0 (b)

Fig.7. Cycle adsorption of batch experiments for CHM (initial phosphorus concentration at 5 mg L-1, 25 ˚C) (a). Adsorption rate curves of phosphate uptake for the desorbed CHM (b)



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For Graphic Abstract Use Only

Facile design of green engineered cellulose/metal hybrid macrogels for efficient trace phosphate removal Xiaojuan Lei,a Xuehai Dai,a Sihui Long,a Ning Cai,a Zhaocheng Ma*b, Xiaogang Luo∗a

Utilization of natural polymer combined with green and sustainable technology in the removal of trace phosphate from drinking water.


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Facile Design of Green Engineered Cellulose ... - ACS Publications

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