Highly Efficient Phosphate Sequestration in Aqueous Solutions Using

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Highly efficient phosphate sequestration in aqueous solutions using nano-magnesium hydroxide modified polystyrene materials Qingrui Zhang, Zhaoxiang Zhang, Jie Teng, Haiming Huang, Qiuming Peng, Ti-Feng Jiao, Li Hou, and Bingbing Li Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 04 Mar 2015 Downloaded from http://pubs.acs.org on March 5, 2015

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Industrial & Engineering Chemistry Research

Highly efficient phosphate sequestration in aqueous solutions using nano-magnesium hydroxide modified polystyrene materials

Qingrui Zhang a, Zhaoxiang Zhang a, Jie Teng a, Haiming Huang* a, Qiuming Pengb, Tifeng Jiao* a , Li Hou a and Bingbing Li c a

Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering,

Yanshan University, Qinhuangdao 066004, PR China b

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, PR China C

Department of Chemistry and Biochemistry, Science of Advanced Materials, Central Michigan University, Mt Pleasant, Michigan 48858, USA

*To whom correspondence should be addressed (Tifeng Jiao and Haiming Huang) E-mail: [email protected] / [email protected] Tel: +86-335-8387-741 Fax: +86-335-8061-549

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ACS Paragon Plus Environment


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Abstract Phosphate removal is important for the control of eutrophication and adsorption may serve as a powerful supplement to biological phosphate sequestration. Here, we develop a new composite adsorbent (denoted HMO-PN) by encapsulating active nano-Mg(OH)2 onto macroporous polystyrene beads modified with fixed quaternary ammonium groups [CH2N+(CH2)3Cl]. The N+ tailored groups can accelerate the diffusion of target phosphate through electrostatic attractions. The performance of the as-prepared HMO-PN was found to depend on the pH value of aqueous medium. The HMO-PN also exhibits high sorption selectivity towards the target phosphate. Kinetic equilibrium of phosphate adsorption can be achieved within 100 mins, and the calculated maximum adsorption capacity is approximately 1.47 mmol/g (45.6 mg/g). Column experiments further show that the effluent concentration of phosphate can be reduced to below 0.5 mg/L (500 BV), suggesting highly efficient phosphate sequestration. Moreover, the exhausted HMO-PN can be readily regenerated using alkaline brine solution.

Keywords: Phosphate; Adsorption, Adsorbent, Mg(OH)2, Nanocomposite

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Introduction Phosphate (PO43-) pollutants in aqueous environments have drastically increased in recent years, due to the mass-production of industry wastewaters and domestic sewage. Excess phosphate can result in seriously hazardous algal blooms and eutrophication1, 2. To address these environmental issues, widespread works have been performed to enable the removal of phosphorus by using chemical precipitation3, adsorption4, 5 biological treatment6 and flocculation7. Among the available methods, the adsorption technique, a cost-effective solution for high efficient phosphate removal, has drawn particular attention due mainly to its potential for industrial-scale wastewater treatment. Nanomaterial-based adsorbents have been studied with their applications for trace phosphate sequestration

8-10

, due to their high surface area and abundant active adsorption sites. For

instance, A. Sarkar et.al reported a new Fe3O4@mZrO2 core/shell nanostructure that exhibits excellent adsorption capability for phosphate ions11. The well-known 2-D material graphene and its derivatives with large surface areas have also proven to be good adsorbents for phosphate removal and environmental remediation12-14. Considering their effective sorption behavior, such nanometer-sized structures with strong affinities between adsorbent and phosphate may be a desirable option for water purification15. However, the application of nanostructured materials for environmental remediation has been challenged by the difficulties of separation, high hydraulic resistance and potential nanosized agglomeration 16, 17. To resolve the above problems, magnetic hybrid materials were synthesized by doping ferric compounds (such as Fe3O4, γ-Fe2O3) onto nanoscale materials, so that the solid-liquid separation can be readily achieved by applying a strong magnetic field18,

19

. However, low oxidation stability, active chelating properties in

complexion water conditions, and acid corrosion defects significantly inhibit the use of 3



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ferromagnetic materials for phosphate treatments. An alternative solution is to develop engineered hybrid materials by encapsulating metallic nanoparticles (mainly metal oxides or hydroxides) onto conventional large size materials with abundant pore/ layered structures (e.g., biochar, fiber, silica, clay, graphene, layered double hydroxide (LDH) and polymers)20-26. Such strategy takes advantage of the large-scale material with built-in nanostructures to achieve highly effective solid-liquid separation, offering new opportunities for the purification of trace phosphorus and other pollutants. For example, diatomite incorporated with ferric hydroxide nanorods was found to exhibit efficient adsorption performance toward low concentrations of phosphates27 The nano-ZrO2 grafted graphene composite was also demonstrated to exhibit superb arsenic sorption capacities and powerful affinities28. Unfortunately, such nano-adsorbents are still constrained by several issues. (1) The anchorage of nanoparticles inevitably blocks some pore structures which inhibits ion transport, especially in high-speed packed columns or any plug-flow-type configuration, reducing the contact possibility between the embedded nanoparticles and target phosphorus. (2) The host material can only serve as a structural support and not assist in the phosphate sorption. Therefore, for environmental applications, it is desirable to develop a material device to that can optimize the functions of both the encapsulated nanoparticles and the host matrix. Nano-sized alkaline earth metal oxides have recently become popular in various adsorption applications as environment-friendly materials29-32. For example, nano-Mg(OH)2 has been widely used to remove dyes33 and soluble toxic ions from waters34, 35, due to its high surface areas, facile production from abundant natural minerals, and strong reactive activity / adsorption capacity; micro/nanorod-Mg(OH)2 prepared by bulk MgCO3 calcination demonstrates a nearly 4000-fold enrichment of the dilute dye wastewater36; hierarchically nanostructured MgO particles also 4


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exhibit excellent performance in As(III) and As(V) removal37. Such outstanding sorption behavior has been ascribed to the high zero potential (12.4) and the abundant surface positive charges37, which enable broad range of pH values, making them suitable for adsorption of anion pollutants through favorable electrostatic attractions. The nano-Mg(OH)2 material also exhibits efficient adsorption toward highly negatively charged PO43-. However, similar to other nano-adsorbents, it is difficult to apply Mg(OH)2 directly, owing to the high hydraulic resistance and separation bottlenecks. An additional concern is the loss of Mg(II) due to ionization dissolution. Here, we fabricated a new nanocomposite ( denoted HMO-PN) by anchoring nano- Mg(OH)2 particles onto macroporous polystyrene beads tailored with quaternary ammonium groups (-CH2N+(CH3)3Cl). The presence of positively charged N+ groups within polymeric backbones can greatly accelerate the target phosphate transport and enhance the ions accessibility against the troublesome applied potential. Moreover, the porous nanostructures of the cross-linked polystyrene matrix facilitate the growth of well-defined Mg(OH)2 nanostructures17 that inhibits the possible Mg(II) release by dissolution. Therefore, for the nanocomposite reported in this study, the synergetic effects of the enhanced diffusion by the N+-enriched matrix, and efficient sequestration of the encapsulated nano-Mg(OH)2 are expected to boost the performances of phosphate removal in water.

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Experimental Material Macroporous polystyrene beads with an average particles size of 0.6-0.8mm were obtained from Zhengguang Resin Co., Hangzhou, China. A phosphate stock solution (1000 mg/L) was prepared by dissolving KH2PO4 in double deionized water. The chemical reagents, including sodium hydroxide, sodium nitrate, magnesium nitrate, ethanol and trimethylamine, were purchased from Tianjin Chemical Engineering Co. All chemicals used were of analytical grade without further purification. Ultrapure water (18 MΩ cm) was produced by a Millipore-ELIX water purification system. Prior to the use of the polystyrene beads, extraction with ethanol in a Soxhlet apparatus was necessary to ensure the removal of residue impurities. Fabrication of the hybrid material (HMO-PN) The HMO-PN material was prepared by the following two steps (1) functionalization of quaternary ammonium groups onto the host matrix and (2) anchorage of nano-Mg(OH)2 onto the tailored polystyrene materials. Step I: Functionalization of quaternary ammonium groups onto the host matrix In this section, the polystyrene beads (PS) were used as a starting material for chemical modification and the simple reaction process is presented in scheme 1-step I. Briefly, 25 g of PS was immerged into 150 mL 1,2-dichloroethane solution for 24 h until swollen, the above beads were then added into 200 mL of chloromethyl ether in a round-bottom flasks, to which 20 g of anhydrous zinc chloride was added as a catalyst. The reagents were subjected to continuous stirring for 12 h at 313 K. Then, the resulting polymers were filtrated to obtain an intermediate -CH2Cl tailored product. Next, the ammonium reaction was conducted by adding 200 mL of trimethylamine solution (30 % mass) into the resultants polymer beads at 318 K to obtain the 6


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desired host material with quaternary ammonium groups (PN) after 24 h, Two hundred milliliters of ethanol and 1,000 mL of deionized water were used to rinse the PN beads to remove residual reactants. Step II Anchorage of nano-Mg(OH)2 particles As illustrated in scheme 1-stepII, the nano-Mg(OH)2 impregnation was performed using Mg(II) salts precursor via immersing diffusion and in situ co-precipitation methods. 20 g of the prepared PN particles were added into 150 mL of MgCl2-HCl- ethanol solution, which was stirred continuously for 8-10 h at 333K and subsequently evaporated to ensure Mg(II) ion transport into the inner pores of the polymeric supports. The Mg(II) loaded PN beads were then introduced into 1L of a 5 % NaOH solution to form nanosized Mg(OH)2 particles within the cross-linked pores of the polymeric host. Finally, the resultant materials were completely washed with ultrapure water until neutral pH conditions and heat-treated at 333 K for 6 h to immobilize the hybrid nanocomposite (HMO-PN). Characterization of the resulting HMO-PN sorbent The concentrations of the phosphate samples were assayed via the molybdenum blue spectrophotometric method. The amount of Mg(OH)2 loaded onto the host matrix was determined by digestion of the nanocomposite into a HNO3-HClO4 solution followed by ICP analysis (JA1100, U.S.). The morphology of the encapsulated Mg(OH)2 particles was recorded using field emission scanning electron microscopy (Hitachi S-4800) coupled with energy dispersive spectroscopy with an accelerating voltage of 5–15 kV. The embedded Mg(OH)2 nanoparticles deposited on copper grids were used for a transmission electron microscopy (TEM) study using a JEM 7700 instrument equipped with a Gatan CCD camera working at an accelerating voltage of 100 kV. The crystalline pattern of the loaded Mg(OH)2 was investigated by X-ray 7



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diffraction (XRD) using a Rigaku D/max 2550PC diffractometer (Rigaku Inc., Tokyo, Japan) with Cu Kα radiation. The pore structure parameters of the polystyrene before and after Mg(OH)2 implantation were performed by nitrogen sorption measurements using Micromeritics ASAP 2020 (U.S.), the surface areas were calculated using the multipoint BET equation and the pore volumes were obtained using the Barrett–Joyner–Halenda (BJH ) method. The AFM images were taken using a MultiMode 8 scanning probe microscopes (Veeco Instruments, Plainview, NY, USA) with silicon nitride cantilever probes, and an Fourier Transform Infrared (FT-IR) Spectroscopy analysis was conducted using a Nicolet iS/10 FT-IR spectrophotometer (Thermo Fisher Scientific, Inc.) by averaging 64 scans at a resolution of 4 cm−1. Experimental methods Batch sorption A series of batch sorption tests were performed to evaluate the phosphate sorption behavior of HMO-PN. To start the experiments, 0.05 g of prepared HMO-PN beads were added to 50 mL flasks containing phosphate solutions of known compositions. Common competing ions were also added when necessary, and the solution pH values were adjusted using 1% HNO3 or NaOH, before being transferred to an incubator shaker ( SZH-85 model, China) equipped with a thermostat and shaken under 200 rpm for 20 h at the desired temperatures. Finally, the phosphate content and corresponding pH at equilibrium were assayed. Kinetic experiments were performed by mixing 0.5 g of HMO-PN with 500 mL of 10 mg/L phosphate solution in a reaction vessel at 298 K. The solution was stirred, 0.5ml of solutions were taken at various time points, and the sample phosphate concentrations and sampling time were determined. Packed column experiments The column tests were conducted in a small plexiglass column (12 mm in diameter and 130 mm 8


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long) equipped with a water bath to maintain a constant temperature. 5mL of HMO-PN and its host material were packed in separated columns with the same operating conditions respectively. A peristaltic pump (LANGE-580，China) was used to ensure a constant flow rate. A synthetic aqueous solution containing phosphorus and common anions (SO42-,NO3- and Cl-) was prepared as feeding solutions for adsorption tests. The column adsorption run was performed under hydrodynamic conditions: the superficial liquid velocity (SLV) and the empty bed contact time (EBCT) were 0.75 m/h and 4 min, respectively.

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Results and discussion Characterization of the resulting nanocomposite HMO-PN The salient properties of HMO-PN are shown in Table S1. The as-obtained nanomaterials were characterized by SEM, TEM, XRD, and BET analysis. The spherical shapes of the beads were shown in Figure 1a. Figure 1b-c shows a comparison of the inner surface morphology before and after Mg(OH)2 loading. Serious pore and structure blockage implies the successful nanoparticle implantation onto the reticular structure of the matrix. The magnesium element area scanning analyses of the hemispherical profile by SEM-EDS further demonstrate that the Mg(OH)2 particles are uniformly impregnated in a ring-like region of the HMO-PN(Figure 1d), such an interesting distribution can be ascribed to the unique surface chemistry of the resultant nanomaterial, i.e., the presence of positively charged N+ groups will exert a strong repulsive force for Mg2+ ions precursor diffusion and inhibit Mg(OH)2 formation in the center region38. Atomic force microscopy (AFM) was also employed to obtain further insight into the morphological properties. The observed fine structure (Figure 1e and 1e’) and the highlighted regions (Figure 1f and 1f’ ) suggest the successful implantation of the Mg(OH)2 particles. Moreover, the outstanding vertical height variation before and after impregnation also demonstrates the potential pore blocking in the preparation procedures. BET analysis further reveals the detailed pore structure parameters ( Table S1). The Mg(OH)2 encapsulation results in apparent decreases in pore volume, size and surface area. TEM investigation (Figure 2a) reveals that the immobilized Mg(OH)2 exhibits a well-defined shape and monodisperse morphology with size approximately 5-10 nm. The bulk aggregations are mainly ascribed to the presence of polystyrene backgrounds39. Such distinguished morphology may be partially associated with the charged essentials of surface N+ groups bound to the cross-linked chains, which can exert an electrostatic field on the micro 10


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porous surroundings and further promote nanosized particle formation. The X-ray diffraction patterns (Figure 2b) imply the presence of crystalline Mg(OH)2 particles with diffraction angles at 19o, 31o, 38o, 51o and 59o, which corresponds with the standard XRD spectrum (PDF#44-1482, Figure S2).

The effects of solution pH on phosphate uptake The influence of pH on adsorption was determined and the results are shown in Figure 3a. Observably, the phosphate removal is clearly a pH-dependent process with the optimal pH range of approximately 6.0-12.0, representing wider sorption conditions than reported for other metal oxides. Such an attractive adsorption is mainly ascribed to the high zero potential (12.4) of the nano-Mg(OH)2 particles. Briefly, the species of Mg(OH)2 display different charge-carrying entity by solution pH variations. Low pH values (accurately solution pH 14, indicates the possible regeneration by alkaline solutions. In general, for ferric oxide, zirconium oxide and their derivatives, highly efficient removal was only observed within a narrow solution range ( in general pH=5-7), and washing the sorbent in water/ dilute acid after regeneration for reuse is a costly task. Therefore, the widened sorption pH range offers more opportunities and facilitates sorption/regeneration application for the engineered nanocomposite. Additionally, the gradually decreasing sorption trends between 3.0-6.0, may be attributed to the phosphorus species transformation from HPO4211



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/ H2PO4- to neutral H3PO4 (Figure 3b) and to the potential dissolution of Mg(OH)2 particles in acidic conditions. To examine the dissolution of encapsulated Mg(OH)2, the normalized Mg(II) release onto HMO-PN was evaluated with purified Mg(OH)2 as a reference. As illustrated in Figure 3c, low pH values lead to apparent Mg(OH)2 dissolution onto both sorbents, and trace dissolved Mg(II) can be detected at pH >10.0. Comparatively, the resultant HMO-PN still exhibits lower Mg(II) releases than common Mg(OH)2 particles, which indicates the potential elevation of the stability within HMO-PN. Such an important observation can be attributed to the unique composite structure, in which the closely cross-linked chains and nanopore structure within the host material inhibit OHdiffusion into the embedded nano-Mg(OH)2, and the fixed positively charged quaternary ammonium groups (N+) binding on the chains exert a OH- buffer effect via an ion-exchange reaction to form–N+(CH3)3OH-. Therefore, the specific polymeric structures results in a different solution chemistry between the interspace and outer solutions, and the increased chemical stability of HMO-PN. Competition with common anions Common anions, including SO42-, NO3- and Cl-, are ubiquitous in waters and wastewaters. Thus, competition influences should be determined from an engineering standpoint. Herein, the relevant experimental assessments were performed with the host PN used as a reference, and the results are illustrated in Figures4a-c. Both HMO-PN and its matrix PN exhibit efficient phosphate removal efficiencies without these common anions present, which indicates their potentially large sorption capacities. Nevertheless, high levels of common anions, including SO42-, NO3- and Clcause serious reverse-adsorption influences in both HMO-PN and PN. The removal efficiencies of phosphorus onto PN are near zero with above 16-folders additions of competitive anions, 12


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whereas HMO-PN still exhibits a relatively strong adsorption performance. Such favorable sorption behaviors suggest the possible presence of strong affinities and preferential sorption selectivity toward phosphate in aqueous solutions. This result can be ascribed to several aspects: (1) the immobilized nano-Mg(OH)2 within the PN matrix exhibits a large surface area and abundant active sites due to the nano-sized morphology; (2) Mg (OH)2 particles can enhance the phosphate uptake by electrostatic interaction with the positively charged MgOH2+ species and inner sphere chelation of the surface metal-OH reaction by PO43- /HPO42- substitution; and (3) the presence of N+(CH3)3Cl groups binding on the cross-linked chains can promote ion transport and accessibility, enabling the selective sequestration of phosphates by nano-Mg(OH)2. The adsorption process was illustrated in Figure 5. The bifunctional synergetic adsorption mechanisms are divided into the highly charged quaternary ammonium group enrichment, MgOH2+ species electrostatic adsorption and inner sphere complexation by surface hydroxyl groups. Next, the underlying sorption mechanism of Mg(OH)2 particles was elucidated by FT-IR spectra investigation (Figure S3). The fresh Mg(OH)2 sample and the samples loaded with various anions, including SO42-/Cl-/NO3-/PO43 –, which were selected as references provided valuable information for the adsorption process. The sharp and intense –OH stretching vibration peaks are detected at ~ 3698 cm-1

40

, and typical vibration bands at 3423–3426 cm–1 are related to the

stretching OH (νOH) of water molecules41. The symmetrical stretching vibration of PO43- appears at 1050 cm-1, and the peaks present at approximately 450 cm-1 can be assigned to the existence of Mg–O bonds42. The spectrum of the phosphate uptake sample exhibits a dramatic decrease in intensity in the surface hydroxyl peak as well as a significant PO43- band, which indicates the possible interaction between Mg-OH and the phosphate species. In addition, the distinct band shifts from 449 cm-1 to 453 cm-1 were observed in the phosphate-preloaded sample, which further 13



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demonstrates the formation of a strong Mg-O-P bond by inner-sphere complexation interactions,whereas for SO42-/Cl-/NO3-, negligible band variations of 449 cm-1/449 cm-1/450 cm-1 respectively, reveal the weak reactions.

.

To further investigate the strong sorption selectivity, the distribution ratio Kd (in l/g) of HMO-PN and PN towards phosphate uptake was determined by the following equation43 and the results are listed in Table S2: Kd =

(Co - Ce ) V x Ce m

(1)

where C0 (mg/L) and Ce (mg/L) are the target phosphate contents at the initial and equilibrium conditions, respectively. V(L) represents the solution volume, and m (g) is the mass of the sorbent. The substantially larger Kd values further prove the strong affinities and favorable selectivity between the composite HMO-PN and phosphates. Evaluation of the sorption kinetics Sorption kinetic tests onto HMO-PN and PN were conducted with the results shown in Figure 6a-c., Both the sorbents exhibit rapid adsorption process with equilibrium times of 80 min and 100 min for PN and HMO-PN, respectively. The slightly prolonged kinetic behavior observed in HMO-PN may be ascribed to the potential pore blocking from nano-Mg(OH)2 incorporation, as was detected by BET and SEM investigations. A 100 min equilibrium time is satisfactory for efficient adsorbent application. Such kinetic behaviors can also be associated with the unique matrix structure chemistry, i.e., the highly positively charged N+ groups binding on polymeric cross-linked chains can display preferential phosphorus diffusion enhancement by large electrostatic attractions and weaken the pore structure-dependent ion accessibilities. To further demonstrate the adsorption enhancement, SEM-EDS analysis was conducted to evaluate the

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rapid diffusion with the results in Figure S4，note that a new polystyrene composite (denoted HMO-PC) modified with neutral CH2Cl as matrix was also involved for a reference. Observably, the positively charged HMO-PN exhibits a fast phosphate sorption process with approaching sorption equilibrium in 60mins. Comparatively, phosphate permeation toward neutral HMO-PC displays a gradually process and low levels of phosphates can diffuse into the active sites for even 24 h reactions. Considering the similar polystyrene matrix, it is believed that the modified positively ammonium group of HMO-PN can significantly promote the target phosphate sorption diffusion . In addition, classical kinetic models are also employed to describe the above sorption data as follows44: The pseudo-first-order model: log(qe -qt ) = logqe -

k t 2.303

(2)

The pseudo-second-order model: t 1 t = + 2 q t k qe q e

(3)

The intraparticle diffusion model

q t = k p * t 0.5 + C

(4)

where qt and qe represent the amount of phosphate adsorbed (mg/g) at equilibrium and time t, respectively, and the k or kp values are the kinetic rate constants. The kinetic data (Table S3) can be accurately described by the pseudo-first-order model with a high correlation coefficient (R2>0.970), and the phosphate uptake is dependent on the intraparticle diffusion rate-control process.

Effects of the temperature and sorption isotherms Figure 6d illustrates the adsorption isotherms for phosphate uptake at three different 15



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temperatures. Higher temperatures favor phosphate removal onto HMO-PN within the defined ranges, indicating an endothermic process45, The classical Langmuir ,Freundlich and Temkin isotherm models can also be applied according to the following equations46. Langmuir model

Qe =

Qmax kLCe 1+ kLCe

(5)

Freundlich model

Qe = k F Ce1/ n

(6)

Temkin model

Qe =

RT Ln(kTCe) bT

(7)

where Ce is the concentration of phosphate at equilibrium, Qe represents the corresponding adsorption capacity, Qm is the maximum phosphate uptake per gram, and KL, KF and n are parameters as well as the kT , bT of Temkin constants to be determined, the detailed parameters are listed in Table S4. The larger R2 values of the sorption process can be well described by the Freundlich models. The maximum sorption capacity is approximately 1.47 mmol/g (45.6 mg/g) . Additionally, the adsorption capacity of HMO-PN for phosphate uptake is roughly compared to other adsorbents reported in the literatures (Table1) Table 1 Comparison of the phosphate sorption capacities of loaded composites. Adsorbent

Qmax(mg/g)

Optimal pH

Temperature (K)

Refs

lanthanum doped vesuvianite

6.70

6.0-8.0

298K

47

Iron oxide coated sand

1.50

5.0

298K

23

Activated carbon loaded with Fe(III)

16.58

2.0

303K

48

43.0

6.0-8.0

298K

16.45

2.0

298K

oxide Orange waste gel loaded with

49

zirconium Zirconia-functionalized graphite oxide

50

16


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3.74

2.0

298K

51

Ce(III)- -loaded orange waste

11.0

5.0-7.0

298K

52

Zirconium loaded okara (ZLO).

44.13

--

298K

53

Al-loaded skin split waste

21.65

--

298K

54

Carboxymethyl cellulose/Fe(II) treated

4.3

4.8

298K

55

Zr(IV) loaded apple peels

20.35

2.0-6.0

298K

56

Nano-Mg(OH)2 bifunctional materials

45.6

6.0-12.0

298K

Zirconium(IV)

loaded

fibrous

adsorbent

aspen wood fiber present study

Fixed-bed column Fixed-bed column tests were conducted to evaluate the potential application of HMO-PN. Figure 7 shows the effluent history of a column system to simulate phosphate wastewaters, with the PN matrix involved as a reference. The resultant HMO-PN exhibits a satisfactory sorption performance with a bed volume (BV) approaching 500, providing an effective working treatment before significant breakthrough by the discharge standards (0.5 mg/L) for wastewaters. Comparatively, the host PN matrix exhibits inefficient phosphate sequestration, and a rapid breakthrough (approximately 120BV) is observed due to its weak affinity and serious competitive sorption from Ca(II)/Mg(II)/Na(I) ions. Additionally, the exhausted HMO-PN can be readily regenerated and repeated use by 5 % sodium hydroxide and sodium chloride binary solutions within 12 BV, and the corresponding stripping rates for phosphate is around 90.4%. Such satisfactory behaviors are consistent with the results of the influence of solution pH, and is significant for its potential application. The laboratory-scale experimental results prove that the resultant HMO-PN is a promising sorbent for enhanced removal of phosphate in waters. Further study is still required to evaluate its feasibility for practical applications. Conclusion In the present study, a new nanomaterial was synthesized to enhance phosphate removal. 17



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Specifically, we encapsulated active nano-Mg(OH)2 into cross-linked polystyrene tailored with unique fixed quaternary ammonium groups (N+). The modified positively charged moieties can accelerate the PO43- ions transport into target sites within nanopore surroundings by strong electrostatic attraction. The embedded nanosized Mg(OH)2 particles with high selectivity, size-dependent properties, and wide applied pH ranges of application further boosted the efficient phosphate sequestration. The functional chemical properties and cross-linked structure of the matrix can also elevate the stability for Mg(II) releases. Such host charged functionalization design has industrial merit, which can resolve the nanoparticles encapsulation induced pore blocking and poor sorption diffusion bottlenecks. It can also be extended to other similar composite material fabrications. Supporting Information Salient properties of the polymeric host material, the XRD of Mg(OH)2, TEM of polystyrene, FT-IR analysis, sorption kinetic, isotherm data and phosphate diffusion analysis by SEM-EDS This information is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements We greatly acknowledge NSFC (21207112, 21473153), the NSF of Hebei Province (B2012203060, B2012203005, B2013203317), the China Postdoctoral Science Foundation (2012M510770, 2013T60265) and the Foundation for the Excellent Youth Scholars from Universities of Hebei Province (Y2011113, YQ2013026) and Key Laboratory of Reservoir Aquatic Environment ， Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science(Grant NO. RAE2014CE03B) and support program for the Top young Talents of Hebei Province. We also thank for the ACS ChemWorx English Editing Service for Premium Language Editing by Margot S. (Senior Editor). 18


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Figures Scheme 1

Fabrication procedures of the resultant HMO-PN

Figure 1. (a) Hemisphere profile of HMO-PN; (b) SEM of the host PN inner surfaces; (C) SEM of the HMO-PN inner surfaces; (d) cross-section Mg distribution of HMO-PN by SEM-EDS; (e) AFM line profile analysis of PN; (f) AFM line profile analysis of HMO-PN; (e’) AFM 3D analysis onto the inner surface of PN; (F’) AFM 3D analysis onto the inner surface of HMO-PN; (g) surface height variation comparison before and after Mg(OH)2 loading Figure 2 (a) TEM image of the resultant composites HMO-PN; (b) XRD analysis of HMO-PN and its host PN. Figure 3 (a) Effects of the solution pH on adsorption onto HMO-PN (b) distribution of the phosphate species; (b) Mg(II) release comparison of HMO-PN and Mg(OH)2 powders. Figure 4 Effect of competing anions on phosphate retention by HMO-PN and PN at 298 K (a) SO42- addition; (b) Cl- addition; (c) NO3- addition. (Initial phosphate 10 mg/L; S/L ratio 1.00 g/L pH =5.5-6.5 ). Figure 5 Sorption mechanism elucidation of the resulting nanocomposites HMO-PN. Figure 6 (a) Sorption kinetic curves of HMO-PN and PN at 298K (dose: 1 g/L, initial phosphate 10mg/L pH=6.2-6.8); (b) sorption isotherms of HMO-PN at different temperatures. Figure 7 Comparison of breakthrough curves of phosphate uptake onto HMO-PN and PN in fixed-bed columns. (adsorption: influent phosphate 2 mg/L, SO42- = 50 mg/L, NO3- =80 mg/L, Cl-= 100mg/L, pH = 5.5−6.2, SLV= 0.75 m/h, EBCT= 4 min). The encapsulated figure represents the desorption and cumulative stripping curves of HMO-PN .

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Scheme 1 Fabrication procedures of the resultant HMO-PN

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Figure 1. (a) Hemisphere profile of HMO-PN; (b) SEM of the host PN inner surfaces; (C) SEM of the HMO-PN inner surfaces; (d) cross-section Mg distribution of HMO-PN by SEM-EDS; (e) AFM line profile analysis of PN; (f) AFM line profile analysis of HMO-PN; (e’) AFM 3D analysis onto the inner surface of PN; (F’) AFM 3D analysis onto the inner surface of HMO-PN; (g) surface height variation comparison before and after Mg(OH)2 loading

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Figure 2. (a) TEM image of the resultant composite HMO-PN; (b) XRD analysis of HMO-PN and its host PN.

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Figure 3. (a) Effects of the solution pH on adsorption onto HMO-PN (conditions: 0.5g/L, 298k, initial Phosphate 10mg/L);(b) distribution of the phosphate species; (c) Mg(II) release comparison of HMO-PN and Mg(OH)2 powders.

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Figure 4 Effect of competing anions on phosphate retention by HMO-PN and PN at 298 K (a) SO42- addition; (b) Cl- addition; (c) NO3- addition. (Initial phosphate 10 mg/L; S/L ratio 1.00 g/L pH=5.5-6.5 ). 27



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Figure 5 Sorption mechanism elucidation of the resulting nanocomposite HMO-PN.

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Figure 6 (a-c) Sorption kinetic curves of HMO-PN and PN at 298K (dose: 1 g/L, initial phosphate 10 mg/L pH=6.2-6.8); (d) sorption isotherms of HMO-PN at different temperatures.

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Figure 7 Comparison of breakthrough curves of phosphate uptake onto PN, HMO-PN and the regenerated samples in fixed-bed columns. (adsorption: influent phosphate 2 mg/L, SO42- = 50mg/L, NO3- =80mg/L, Cl-=100mg/L, pH= 5.5−6.2, SLV= 0.75 m/h, EBCT=4 min). The encapsulated figure represents the desorption and cumulative stripping curves of HMO-PN .

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Graphic for manuscript 336x528mm (300 x 300 DPI)


Highly Efficient Phosphate Sequestration in Aqueous Solutions Using

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