Ammonium-Functionalized Hollow Polymer Particles As a pH

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Ammonium-functionalized hollow polymer particles as a pH-responsive adsorbent for selective removal of acid dye Yan Qin, Li Wang, Changwen Zhao, Dong Chen, Yuhong Ma, and Wantai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04199 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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ACS Applied Materials & Interfaces

Ammonium-functionalized hollow polymer particles as a pH-responsive adsorbent for selective removal of acid dye Yan Qin,† Li Wang,† Changwen Zhao,† Dong Chen,*, † Yuhong Ma,†, ‡ Wantai Yang*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, Beijing, China ‡

Key Laboratory of Carbon Fiber and Functional Polymers of the Ministry of Education, School

of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China KEYWORDS: Dye removal, ammonium-functionalized, adsorption, methyl blue, adsorption capacity

ABSTRACT: In this work, a novel type of ammonium-functionalized hollow polymer particles (HPP-NH3+) with a high density of ammonium groups in the shell has been specially designed and synthesized. Benefiting from both the high surface area and from the high density of positively charged ammonium groups, the as-prepared HPP-NH3+ can serve as a selective adsorbent for the removal of negatively charged acid dye (e.g. methyl blue a-MB). The equilibrium adsorption data of a-MB on the HPP-NH3+ were evaluated using Freundlich and

ACS Paragon Plus Environment

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Langmuir isotherm models, and Langmuir isotherm exhibited a better fit with a maximum adsorption capacity of 406 mg/g. Most importantly, due to the presence of dual functional groups (ammonium and carboxyl groups), the HPP-NH3+ showed a significant pH-dependent equilibrium adsorption capacity, which increased dramatically from 59 mg/g to 449 mg/g as the solution pH decreased from 9 to 2. This uniqueness makes the dye-adsorbed HPP-NH3+ can be facilely regenerated under mild condition (in weak alkaline solution, pH=10) to recover both aMB and the HPP-NH3+, while the recovery of conventional adsorbents is commonly performed under particularly severe conditions. The regenerated HPP-NH3+ can be reused for dye removal and the dye removal efficiency remained above 98% even after five adsorption-desorption cycles. Due to its high adsorption capacity, pH-sensitivity, easy regeneration and good reusability, the HPP-NH3+ has great potential for the application in the field of water treatment, controlled drug release and pH-responsive delivery.

1. INTRODUCTION Nowadays, the discharge of various organic dyes into water caused serious environmental problems all over the world since dyestuffs are toxic and carcinogenic to human beings and organisms.1-3 Acid dyes, typically water soluble, were widely used for textile, paper, leather, inkjet printing, and cosmetics.4, 5 The chemical structure of these dyes is usually very complex, which can mainly be classified into three types, namely anthraquinone, azo and triphenylmethane. Methyl blue (a-MB) is a well-known acid dye having structures related to triphenylmethane, which has been intensively used as coloring agent, disinfector in rubbers, pharmaceuticals, pesticides and so on.6 Due to their complex aromatic structures, acid dyes are


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stable to conventional chemical oxidation and biodegradation. In addition, many acid dyes will produce secondary pollutant when being oxidized or degraded.7, 8 In order to solve this environmental pollution, it is necessary to remove the dyes from effluents before their discharge. Various treatment processes such as coagulation/flocculation, photo degradation, membrane filtration, adsorption, chemical oxidation and biological degradation, have been widely used to eliminate dyes from wastewater. Among these method proposed, the adsorption technology is considered to be one of the most effective methods because of its simplicity, high efficiency, flexibility and insensitivity to toxic substance.9-16 Various kinds of adsorbents including activated carbon,17-21 silica gel,22-25 clay minerals,26,

27

fly ash28,

29

and

agricultural solid wastes30-32 have been employed as adsorbents for the removal of dyes from wastewater. Activated carbon is an excellent adsorbent due to its high specific surface area and strong affinity to organic compounds. But there are some disadvantages including high operation cost, long time consumption, and difficulties in regeneration. Agricultural byproducts, such as peanut hull, coir pith, and rice husk, can serve as economically attractive adsorbents without or with simple processing (drying, grinding). However, further improvement of adsorption capacities and mechanical strength is required for their practical applications. The natural clay minerals are low cost materials with high reserves, but suffer from low/limited adsorption capacity. As newly emerged adsorbents, macroporous polymeric adsorbents have drawn many attentions because of their perfect mechanical property and facile regeneration for repeated use.33, 34 In order to endow macroporous polymeric adsorbents with high selectivity and adsorption capacity, surface modification was performed to graft functional groups onto the surface of the macroporous polymeric adsorbent. However, the chemical modification process was often


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tedious, time-consuming and with low efficiency, and it was difficult to introduce high density of functional groups to the porous polymeric adsorbents. Additionally, mass diffusion of dyes in the polymeric adsorbents is severely influenced by the micropore structure, resulting in a slow adsorption rate and low adsorption capacity. Moreover, process for the regeneration of adsorbent is always carried out under particularly severe condition, for example in strong acid or alkaline solutions. From the above discussion, low-cost, high-performance and recyclable adsorbents are still lacking. Therefore, it is of great theoretical and practical significance to develop advanced adsorbents that are equally effective and easy to regenerate and dispose. As a high performance adsorbent, it should possess large surface area, perfect mechanical strength and adjustable surface chemistry and porous structure. To attain the above requirements, a novel type of hollow polymer particles (HPPs) with extremely high density of amino and carboxyl dual functional groups were specially designed and fabricated, and the chemical structure and synthetic route of these amino functionalized HPP (HPP-NH2) are shown in Scheme 1. AIBN, 75 oC

+ O

O IPA/n-heptane O excess

O

PMS

O H 2N

N H

O

NH 2

etched by acetone PMS@PDM

H2 N

NH

DVB/MAH AIBN, 75 oC

NH 2

HCl

OH

O

O

(HPP-NH 2 )

O

(HPP-NH 3 +)

HN

NH 2

O O

O

Na

N

S

S O Na O

NH

O

a-MB:

N H

H2 N O

pH=10 HO

N

O

NH 3 Cl

H2 N

High pH value desorption O

O

Cl

H 2N

Low pH value adsorption O O

H2 N

NH

O O (HPP)

HPP

O

H

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S

pH=2

O

Scheme 1. Synthetic route of HPP-NH3+ and schematic illustration for the adsorption/desorption of a-MB by the HPP-NH3+.


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The amino groups could be readily converted to ammonium ions through reaction with HCl. The as-prepared ammonium-functionalized HPP (HPP-NH3+) can serve as a pH-sensitive adsorbent for selective removal of acid dye from aqueous solution, and a-MB was selected as a model acid dye to evaluate the adsorption performance. Benefiting from the extremely high density of positively charged ammonium groups, the as-prepared particles show unique selective and high adsorption capacity for the removal of negatively charged acid dye. Most importantly, due to the presence of ammonium and carboxyl dual functional groups, the HPP-NH3+ showed a significant pH-dependent adsorption capacity. Furthermore, owing to this unique pH-sensitivity, the dye-adsorbed HPP-NH3+ can be facilely regenerated under mild condition (in weak alkaline solution, pH=10) to recover both a-MB and the HPP-NH3+, while the regeneration of conventional adsorbents is commonly performed under particularly severe conditions with low efficiency.11, 22, 24 The adsorption kinetics and isotherms were investigated in details. The effect of various experimental parameters, including solution pH, contact time, initial dye concentration were examined to provide more information about the adsorption characteristics of the HPPNH3+. Furthermore, the regeneration and reusability of the HPP-NH3+ were also evaluated.

2. EXPERIMENTAL SECTION 2.1. Materials. Maleic anhydride (MAH), styrene (St), 2,2′-azobisisobutyronitrile (AIBN), nheptane, isopentyl acetate, divinylbenzene (DVB, 55% mixture of isomers), diethylenetriamine (DETA), tetrahydrofuran (THF), acetone, ethanol, petroleum ether, sodium hydroxide (NaOH), hydrochloric acid (HCl) were all analytical grade reagent from Beijing Modern Eastern Fine Chemical Plants. All reagents were used as received. a-MB and methylene blue (b-MB) were obtained from Ciba and used without further purification, and the chemical structure of a-MB was shown in Scheme 1.


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2.2. Synthesis of HPPs. HPPs with unique structure were synthesized through precipitation copolymerization of DVB and MAH using in-situ formed poly(maleic anhydride-alt-styrene) (PMS) as template, followed by etching with acetone. The preparation of PMS particles was similar to that of poly(maleic anhydride-alt-vinyl acetate) particles.35, 36 In a typical experiment, the monomers MAH (2.45 g, 25 mmol) and St (1.30 g, 12.5 mmol), the initiator AIBN (0.0375 g, 1wt % of the monomers), the solvent isopentyl acetate (23 mL) and n-heptane (12 mL) were charged into a three-necked flask. After all the reagents were dissolved, the solution was purged with N2 for 20 min. The flask was put into an oil bath at 75°C, and then self-stable precipitation polymerization of MAH and St was allowed to proceed and yield the PMS particles (serve as the template cores in the next step). After 90 min, AIBN (0.0220 g) and a certain amount of DVB (1.10 g, the total vinyl groups was 13.0 mmol) were added into the reaction system and the reaction was continued for another 3 h with magnetic stirring at a speed of 200 rpm. The suspension were centrifuged at 8 000 rpm to collect the resultant PMS@poly(divinylbenzene-alt-maleic anhydride) (PMS@PDM) coreshell particles. The PMS@PDM core-shell particles were then etched by acetone to get the hollow nanoparticles which was designated as HPPs. The HPPs were washed with petroleum ether several times, dried in vacuum oven at 40°C to constant weight. Yield: 2.30g, 98%. 2.3. Preparation of HPP-NH3+. 1.0 g of HPP was dispersed in THF (20 mL), and 4 mL DETA was then added to this suspension. The reaction mixture was kept at 50°C for 24 h with stirring. The resultant hollow nanoparticles, named as HPP-NH2, were washed with deionized water and ethanol several times, dried in vacuum oven at room temperature to constant weight. Yield: 1.64g. Subsequently, HPP-NH2 was acidified with HCl in order to protonate the amino groups, leading to positively charged ammonium moieties.37 In a typical experiment, 1 g of HPP-


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NH2 was dispersed in a mixed solution of deionized water (10 mL) and ethanol (10 mL), and then 2 mL HCl (37 wt%) were added and the reaction system was stirred at 20°C for 24 h. The product was separated and washed with ethanol for several times, then dried in vacuum oven at room temperature to constant weight. The final product designated as HPP-NH3+. Yield: 1.20g. 2.4. Dye adsorption experiments. In a typical experiment, 20 mg of the HPP-NH3+ was dispersed in 20 mL a-MB solution with a predefined initial concentration, and then the tubes were shaken at RT for 24 h until equilibrium adsorption capacity (qe) was reached. At different time intervals, a certain amount of the suspension was withdrawn immediately from the system. The pH value of the solutions was adjusted by adding HCl and NaOH solutions. The HPP-NH3+ was removed by centrifugation at 4 000 rpm for 3 min and the pH values of the supernatant was adjusted to 7. The a-MB concentrations in the supernatant were estimated by measuring the absorbance at the wavelength corresponding to the maximum absorbance (λmax=310 nm) with UV-Vis spectrometer and calculated from the calibration curve. The adsorption capacity and the removal efficiency of a-MB were calculated using the following equation: qt =

R(%) =

(C o − C t ) * V m

(1)

(C o − C e ) × 100% (2) Co

where qt (mg/g) is the amount of a-MB adsorbed by the HPP-NH3+ at time t (min), C0 (mg/L) is the initial dye concentration in the solution, Ct (mg/L) is the dye concentrations in the liquid phase at time t (min), V (L) is the volume of the dye solution, m (g) is the mass of the HPP-NH3+ used, R(%) denotes the removal efficiency of a-MB, and Ce (mg/L) is the equilibrium dye concentration after adsorption.


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2.5. Dye desorption and HPP-NH3+ regeneration. The recyclability of the HPP-NH3+ was investigated by performing repeated adsorption and desorption cycles. During the adsorption step, 20 mg HPP-NH3+ was added into 20 mL a-MB aqueous solution (pH=2.5) with an initial concentration of 100 mg/L. After 24 h, the HPP-NH3+ was separated by centrifugation, poured into 20mL water (pH=10). After desorption in basic water 3 times, the recovered HPP-NH3+ was separated and treated with HCl to regenerate the adsorbent, which was reused for the dye adsorption in the next cycle.

2.6. Characterizations. SEM images of the PMS, PMS@PDM and HPPs were recorded on a Hitachi S4700 scanning electron microscopy. TEM images of the HPPs were observed on a Hitachi H800 transmission electron microscopy. FT-IR spectra of the HPP-NH2 and the HPPNH3+ particles before and after adsorption of a-MB were obtained on a Nicolet Nexus 670 spectrometer. Dye concentrations were determined by measuring the absorbance at 310 nm with HITACHI spectrophotometer U-3900H.

3. RESULTS AND DISCUSSION As shown in Scheme 1, the monodisperse PMS templates was obtained by self-stable precipitation polymerization.35, 36 DVB were then added into the reaction system to copolymerize with MAH, and the as-formed poly(divinylbenzene-alt-maleic anhydride) (PDM) crosslinked polymer chains were absorbed onto the surface of the PMS templates to form the PMS@PDM core-shell particles. After being etched with acetone the PMS template cores were removed and HPPs were successfully obtained. Through consecutive reaction with DETA and HCl, the anhydride groups in the crosslinked shell were facilely converted to ammonium groups. There was extremely high density of positively charged ammonium groups in the crosslinked shell of the HPP-NH3+, which can serve as effective binding sites for the adsorption of acid dye through


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electrostatic interaction. Additionally, both the high surface area and the mesopores in the shell can facilitate the rapid permeation and mass transport of the dye molecules, which facilitate the adsorption process and dramatically improve the adsorption rate. Therefore, the HPP-NH3+ showed excellent adsorption performance for anionic acid dye.

3.1. Characterization of the HPP-NH3+.

Figure 1. SEM images of (a) the PMS template, (b) the PMS@PDM core-shell particles, (c) HPPs, (d) the HPP-NH3+, (e) the a-MB adsorbed HPP-NH3+ and (f) the regenerated HPP-NH3+. The inset in (f) is TEM image of the regenerated HPP-NH3+. Scale bar: 1 µm.

3.1.1. Size and surface morphology of the HPP-NH3+. SEM and TEM were used to observe the morphological feature of PMS, PMS@PDM, HPPs, HPP-NH3+, a-MB adsorbed HPP-NH3+ and regenerated HPP-NH3+. As shown in Figure 1a, the number average diameter of the PMS templates is 560 nm with a narrow size distribution, and the surface of the polymer particles is very smooth. After DVB and MAH were copolymerized and absorbed by the PMS template, PMS@PDM core-shell particles with PDM crosslinked shell were formed. Figure 1b shows


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SEM image of the corresponding PMS@PDM core-shell particles with an average diameter in the range of 745-755 nm. Moreover, the surface of the particles became relatively rough, indicating the formation of PDM crosslinked shell. It is obvious from Figure 1c and d that the size and morphology of the particles remained unchanged after acetone extraction and successive reaction with DETA and HCl, indicating high strength of the crosslinked shell. It is noteworthy that the shell of the HPPs remained intact even after five adsorption-desorption cycles as shown in Figure 1e and f, demonstrating that the as-prepared HPP-NH3+ has high stability and recyclability. TEM images of the HPP-NH3+ were used to further confirm high strength of the crosslinked shell. It can be seen from Figure 1f and inset that HPPs with a diameter of 760 nm and a shell thickness of 90 nm were observed after 5 adsorption-desorption cycles. The specific surface area and pore structure of the HPP-NH3+ were determined by N2 adsorption/desorption, the N2 isotherms at 77 K were shown Figure S1. Based on the Barrett– Joyner–Halenda (BJH) model, the specific surface area and total pore volume of the HPP-NH3+ were 21 m2 g-1 and 0.325 cm3 g-1, respectively. Furthermore, the pore size distribution was calculated based on the BJH method and mesopores with diameters in the range of 5-48 nm was observed as shown in Figure S1 inset. The formation of these mesopores was mainly due to contraction of the 3D cross-linking networks resulted from cross-linking copolymerization of DVB and MAH.


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Figure 2. FT-IR spectra of (a) the HPP, (b) the HPP-NH2 and (c) the HPP-NH3+. 3.1.2. Chemical composition of the HPP-NH3+. In order to characterize the chemical composition of the HPP, HPP-NH2 and HPP-NH3+, FT-IR measurement were carried out and the FT-IR spectra are presented in Figure 2. As shown in Figure 2a, the strong absorption at 1820 cm-1 and 1778 cm-1 were attributed to C=O stretching vibration of anhydride groups, and the peaks at 1223 cm-1 and 1076 cm-1 were attributed to C-O and C–O–C stretching vibration of the cyclic anhydride, indicating the presence of anhydride groups in HPPs.38 After being treated with DETA, the typical band at (1820 cm-1 and 1778 cm-1) almost disappeared completely. Meanwhile, three new absorption bands appeared at 1643 cm-1 and 1558 cm-1, which was corresponding to amide I (νC=O) and carboxylate, respectively, confirming the formation of amide groups. These results indicated that anhydride moieties were thoroughly converted to amino groups, and this extremely high density of amino groups could further converted to ammonium groups.37 As can be seen clearly from Figure 2c, the absorption band of carboxylate disappeared (1558 cm-1 and 1400 cm-1), and meanwhile the peaks corresponding to C=O stretching vibration of carboxylic acid groups merged at 1709 cm-1. Besides, there was also a broad and strong peak in the range of 3200-2250 cm-1 ascribed to the characteristic absorption


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band of ammonium groups. These results provide direct evidence for the successful formation of positively charged ammonium groups, which can serve as pH-responsive binding sites for the adsorption of anionic dyes. The successful formation of the HPPs with PDM crosslinked shell and their following chemical modification were further confirmed by solid state

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C-NMR

spectroscopy, and the 13C CP/MAS NMR spectra of the functionalized HPPs are shown in Figure S2. The content of amino groups in the HPP-NH2 could be determined based on the N content from elemental analysis, and the elemental analysis data for the HPP-NH2 is shown in Table S1. The N content in the shell of the HPP-NH2 was about 11.5 wt%, which was smaller than the theoretical value calculated based on the initial feed ratio of MAH to DVB (14.5 wt%, assuming that each anhydride group consumes one DETA). This result indicated that for some DETA both primary amino groups might react with anhydride groups to form amide bonds (One DETA consumes two anhydride groups, as shown in Scheme S1). Consequently, the content of amino groups in the HPP-NH2 decreased dramatically, and the density of ammonium groups in the shell of the HPP-NH3+ would also decrease accordingly. In order to determine the accurate density of ammonium groups in the shell of the HPP-NH3+, titration of chloride ions was carried out with AgNO3, assuming the presence of one chloride counter ion per ammonium group. The titration process was described in the supporting information, and the experimental result shows that the density of ammonium groups in the HPP-NH3+ was about 2.72 mmol/g.

3.2. Adsorption behavior of the HPP-NH3+. In the present work, we chose two typical dyes b-MB (basic) and a-MB (acid), which are commonly presented in wastewaters, as the model dyes to evaluate the adsorption behavior of the as-prepared HPP-NH3+. The adsorption


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performance of the HPP-NH3+, including adsorption selectivity, adsorption capacity, pHdependence, regeneration and reusability has been investigated in details.

3.2.1. Adsorption selectivity. In order to investigate the selectivity of the HPP-NH3+, a series of batch adsorption experiment were carried out for a-MB and b-MB with an initial concentration of 200 mg/L, and the adsorption capacity at varying contact time was shown in Figure 3. As can be seen from Figure 3, the HPP-NH3+ showed a remarkable adsorption capacity for a-MB (199 mg/g), while extremely low adsorption capacity for b-MB (7. A similar trend was observed for the adsorption of acid orange II dye on chitosan/gelatine porous materials, which is also an ammonium functionalized adsorbent with ammonium and carboxyl dual functional groups.37

3.3. Dye desorption and adsorbent regeneration. As an advanced adsorbent, the HPP-NH3+ not only exhibited excellent adsorption performance but also showed excellent desorption property. The adsorbed a-MB can be readily desorbed under mild condition (pH=10). The desorption efficiency reached above 95% after desorption in basic water 3 times. The reason for this high desorption efficiency can be attributed to the pH-sensitivity of the HPP-NH3+. As described above, the adsorption capacity of a-MB on the HPP-NH3+ decreased dramatically from 449 mg/g to 59 mg/g when pH value of the solution increased from 2 to 9. That is to say, most of the a-MB adsorbed can be readily desorbed in alkaline solution (pH=10) to fully recover both the adsorbent and the adsorbed dye. The reusability is important for a high performance adsorbent for its practical application. To characterized the reusability of the HPP-NH3+, consecutive adsorption and desorption cycles were performed. Figure 7 shows the a-MB removal efficiency of the HPP-NH3+ after five adsorption-desorption cycles. It is clear that there was no obvious loss of dye removal efficiency, and the removal efficiency of a-MB was still as high as 98% even after five successive desorption-adsorption cycles, indicating the good reusability of the HPP-NH3+.


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Figure 7. Removal efficiency of a-MB by the regenerated HPP-NH3+ after five successive adsorption/desorption cycles. Initial dye concentration: 250 mg/L. The optical images of the HPP-NH3+ after dye adsorption and the regenerated adsorbent after five adsorption-desorption cycles are presented in Figure S3. Apparently, the deep blue color of the a-MB adsorbed HPP-NH3+ disappeared completely after dye desorption process, indicating that a-MB could be successfully desorbed. FT-IR spectra were performed to gain deep insights into the adsorption and desorption process. It can be seen from Figure 8b that four characteristic peaks of a-MB corresponding to sulfonate groups at 1169 cm−1, 1122 cm−1, 1034 cm−1, 1007 cm−1 appeared after the dye adsorption, indicating that a-MB has been successfully adsorbed onto the HPP-NH3+. Additionally, there are no other new peaks appearing in the FT-IR spectrum, indicating that the adsorption of a-MB onto the HPP-NH3+ is a physical adsorption. The absorption bands corresponding to sulfonate groups disappeared after the desorption process (Figure 8c), and the regenerated HPP-NH3+ showed the same adsorption peaks with the original HPP-NH3+. This result provides direct evidence for thorough desorption of a-MB and full recovery of the HPP-NH3+, which is crucial for the reuse of the regenerated HPP-NH3+.


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Figure 8. FITR spectra of (a) a-MB, (b) the a-MB adsorbed HPP-NH3+, (c) the regenerated HPP-NH3+ and (d) the original HPP-NH3+.

4. CONCLUSION In summary, a novel type of ammonium functionalized HPPs with unique multilevel structure has been specially designed and successfully synthesized through precipitation copolymerization of DVB and MAH using in-situ formed PMS as template, followed by etching with acetone and consecutive reaction with DETA and HCl. Due to the presence of an extremely high density of ammonium groups in the crosslinked shell (2.72 mmol/g), the as-prepared HPP-NH3+ can serve as a selective adsorbent for the removal of acid dye. The batch adsorption experiment demonstrated that initial a-MB concentrations, contact time and solution pH all showed important influence on the adsorption performance of a-MB on the HPP-NH3+. When the initial

a-MB concentration was 200 mg/L and 600 mg/L, the adsorption equilibrium was attained in 15 min and 24 h with adsorption capacities of 194 mg/g and 396 mg/g, respectively. Adsorption isotherm data indicated that Langmuir isotherm model showed a better fit with a maximum aMB adsorption capacity of 406 mg/g. Most importantly, due to the presence of ammonium and carboxyl dual functional groups, the HPP-NH3+ showed a significant pH-dependent adsorption


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capacity, which increased dramatically from 59 mg/g to 449 mg/g when the solution pH value decreased from 9 to 2. Furthermore, the dye-adsorbed HPP-NH3+ can be simply regenerated in basic solution (pH=10) to recover both a-MB and the HPP-NH3+ with high desorption efficiency (above 95%). The regenerated HPP-NH3+ can be reused for dye removal and the dye removal efficiency remained above 98% even after 5 absorption/desorption cycles. The high adsorption efficiency, pH-sensitivity and good reusability endow this novel type of adsorbent great potential for dye pollutant removal. ASSOCIATED CONTENT

Supporting Information N2 isotherms of the HPP-NH3+, 13C CP/MAS NMR spectra of the functionalized hollow polymer particles, the elemental analysis data for HPP-NH2, the titration process, schematic illustration for the side reaction between one DETA with two anhydride groups, Langmuir and Freundlich isotherm parameters and correlation coefficient of the HPP-NH3+, optical images of the a-MB adsorbed HPP-NH3+ and regenerated HPP-NH3+, the stability of the the HPP-NH3+ under severe condition. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *(W. Y.) Email: [email protected]; *(D. C.) Email: [email protected]

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge financial support from the National Natural Science Foundation of China (Grant 51221002, 51273012 and 51403011), and the Fundamental Research Funds for the Central Universities (No. ZY1405). REFERENCES (1) Crini, G. Non-conventional Low-cost Adsorbents for Dye Removal: a Review. Bioresour.

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Ammonium-Functionalized Hollow Polymer Particles As a pH

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