Preparation and characterization of polymeric ligand exchanger based on chitosan hydrogel for selective removal of phosphate

Reactive & Functional Polymers 85 (2014) 45–53

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Preparation and characterization of polymeric ligand exchanger based on chitosan hydrogel for selective removal of phosphate Byungryul An a, Ka-Young Jung a, Dongye Zhao b, Sang-Hyup Lee a,c, Jae-Woo Choi a,d,⇑ a

Center for Water Resource Cycle Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea Environmental Engineering Program, Department of Civil Engineering, 238 Harbert Engineering Center, Auburn University, Auburn, AL 36849, USA c Green School, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 136-701, Republic of Korea d Department of Energy and Environmental Engineering, University of Science and Technology (UST), Daejeon 305-350, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 18 July 2014 Received in revised form 22 September 2014 Accepted 11 October 2014 Available online 18 October 2014 Keywords: Phosphate Polymeric ligand exchanger Chitosan Sorption Stability

a b s t r a c t Polymeric ligand exchangers (PLEs) are typically prepared using commercial chelating resins which are often costly and less ‘‘green’’. In this work, we prepared a new PLE by immobilizing Cu(II) on a low-cost, natural biopolymer chitosan. It was confirmed that the Cu2+ ions were bonded to chitosan by complexing with the nitrogen and hydroxyl groups in the chitosan polymer chain, leading to a reduction in the size of the hydrogel and intensified density of the biopolymer. The chelating interaction between nitrogen and Cu2+ acts as a crosslinker that improves the physical and chemical stability of the PLE. The pH sorption tests confirmed a pKa of 7.0 for the biopolymer. The PLE reverses the affinity sequence of standard anionic resins, and displayed much greater affinity toward strong ligands such as phosphate than sulfate due to concurrent electrostatic and Lewis acid-base interactions between immobilized Cu2+ ions and phosphate regardless of solution pH. The maximum phosphate uptake was estimated to be 70 mg g1 and 35 mg g1 in single and binary-component systems, respectively. Fixed-bed column tests revealed that the PLE may be used for selective removal of phosphate of strong ligand characteristics over sulfate. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Eutrophication, caused by excess nutrients, has been frequently reported in reservoirs, ponds and rivers [1]. Although the phenomenon is not directly related to human health, it induces oxygen depletion and harmful algal blooms that can destroy aquatic life and complicate drinking water treatment. Both phosphate and nitrogen in water bodies has been considered a critical nutrient parameter of algal blooms [1]. However, researchers have demonstrated that phosphate is not only a core contaminant, but also a limiting factor for eutrophication in water bodies [2,3]. Moreover, even less than one part per million of phosphate in treated wastewater can be responsible for algal blooming and eutrophication [4]. Therefore, it is necessary to control the discharge of phosphate into water bodies using a new sorbent. Biopolymers such as alginate [5,6] and chitosan [7,8] have been investigated as adsorbents for treating industrial effluents including high concentrations of cationic or anionic contaminants [9]. ⇑ Corresponding author at: Center for Water Resource Cycle Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136791, Republic of Korea. Tel.: +82 2 958 5820; fax: +82 2 958 5839. E-mail address: [email protected] (J.-W. Choi). http://dx.doi.org/10.1016/j.reactfunctpolym.2014.10.003 1381-5148/Ó 2014 Elsevier B.V. All rights reserved.

Compared to conventional adsorbents such as ion exchange resins, the biopolymers offer some unique characteristics, such as natural abundance, non-toxicity, hydrophilicity, biocompatibility, biodegradability, and antibacterial properties [10]. Chitosan has been found to be an effective chelating resin for binding with transition metals (e.g., Cu2+, Ni2+ and Zn2+) because of the presence of free amine groups on the polymer chain. It has been known that nitrogen donor atoms (Lewis base) offer extremely high selectivity for transition metals (Lewis acid) via coordination bonding and over-competition with alkaline and alkaline-earth metal cations [11]. Chitosan derived by N-deacetylation from chitin which is one of the most abundant biopolymers is a polymer of b-(1 ? 4)-linked N-acetyl-D-glucosamine [12]. Chitosan has amino, hydroxyl, and acetamide functional groups. The metal sorption capacity of chitosan is determined by the following properties, the pKa value of chitosan, degree of deacetylation, and molecular weight, which can be changed during deacetylation. The affinity sequence of chitosan for heavy metals is generally Cu2+ > Ni2+ > and Zn2+, which is similar to that for the commercial chelating resins, such as DOW 3N and is in accordance with the classical Irving–Williams order [13]. Chitosan can be modified to form powders, nanoparticles, gel beads, membranes, and fibers

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B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53

for specific applications. The unique solubility properties lead to physical modification, e.g., chitosan flakes can be dissolved in organic or inorganic acid solutions such as acetic acid and hydrochloric acid [14], but not in sulfuric acid [15]. However, the forms of flakes, powders, or nanoparticles are not suitable for use in the fixed-bed column processes due to hydrodynamic limitations. Consequently, the granular form (bead) is often desired as a sorbent that can be applied with easy operation and simple management in water and wastewater treatment. Chitosan, as a weak-base hydrogel bead, has been used to remove arsenic [16] and dyes [8]. Such removal was accomplished by the electrostatic interaction between the fixed NH+3 groups and the target anions. The maximum uptake is accomplished at acidic pH, where the amino groups are protonated to NH+3. However, chitosan hydrogel beads usually begin dissolving in a moderately acidic solution, and the affinity for the target anions are often suppressed strongly by competing anions such as sulfate. To overcome these limitations, we hypothesized that the resin’s physical stability and adsorption affinity toward anionic ligands such as arsenate and phosphate can be greatly enhanced by converting chitosan to a new polymeric ligand exchanger (PLE). Various commercial chelating resins are summarized in Table 1. In general, the commercial chelating resins are rather expensive [17] and less environmentally friendly (either the resins themselves or the manufacturing processes involved). In contrast, chitosan is more cost-effective and much ‘‘greener’’. All referenced resins are of the granule type. The chitosan hydrogel beads (in this study) offered a copper uptake capacity of 0.87 mmol g1 (55 mg g1), which is comparable with some of the commercial resins. Therefore, chitosan hydrogel beads hold the potential to replace the commercial chelating resins as a new PLE template if the protonation of amine groups is properly controlled. The ligand exchanger concept was first introduced in 1962 by Helfferich [18]. Later, Zhu and SenGupta developed a copperloaded DOW 2N for selective removal of arsenate [19]. Much improved and selective removal of phosphate [4,20] and arsenate [21] were achieved by immobilizing copper ions onto a commercial chelating resin, known as DOW 3N. Typically, a PLE consists of two parts: (a) a matrix hosting a transition metal such as copper and iron, and (b) a transition metal as the fixed functional groups. The immobilized transition metal ions incur both Lewis acid-base (LAB) reactions and electrostatic interactions with a target ligand. As a result, the affinity of a PLE toward various contaminants can be determined by the strength of the LAB or the ligand strength. For example, in many cases, standard ion exchange resins often show greater affinity toward sulfate than phosphate or arsenate. As a result, the breakthrough for phosphate took place much earlier than that for sulfate. This is because the affinity of standard anion exchange resins is governed only by the electrostatic interactions. In contrast, the affinity for a PLE is often governed by the strength of LAB. For examples, An et al. [21] and Zhao and SenGupta [4] observed that copper-loaded DOW 3N removed arsenate and phosphate 10 and 3.5 times, respectively, more than commercial anion exchange resins; and the PLE offered 60 and 54 greater affinity (based on the separation factors) for arsenate and phosphate over strong competing ions, such as sulfate, nitrate, and chloride. However, the use of chitosan derived from natural

biopolymers as a PLE matrix has not been well explored and the metal immobilization mechanism and the effects of the degree of protonation are yet to be better understood. The aim of this present study was to develop and characterize the polymeric ligand exchanger based on chitosan and to test the PLE for phosphate removal. The specific objectives of this study were to: (1) prepare a new chitosan-based polymeric ligand exchanger, (2) elucidate the mechanism of metals sorption on the chitosan, (3) characterize the physical and chemical properties of the PLE, (4) investigate the pH effects on the phosphate removal, and (5) validate the selectivity enhancement for phosphate over sulfate. 2. Experimental 2.1. Materials Chitosan powder was obtained from Sigma–Aldrich (Reykjavik, Iceland) and used without further purification or treatment. It had an average molecular weight of 250,000 g mol1 and a degree of deacetylation (DD) of 75–85%. CuCl2  2H2O (ACS grade) was purchased from Sigma–Aldrich (USA). Other chemicals, including KH2PO4, Na2SO4, NaNO3, HCl, and NaOH (ACS grade), were obtained from Sigma–Aldrich (USA) and Ultrapure deionized water (DI, 18.2 MO) was used to prepare all solutions. 2.2. Preparation of chitosan hydrogel beads and copper-loaded chitosan (chitosan–Cu) The chitosan-based PLE was prepared in two steps: preparation of the chitosan hydrogel beads and loading metal ions onto the beads. Chitosan hydrogel beads were prepared following the commonly used approach as well documented by Guibal et al. [7]. To solidify the bead shape and increase the physical strength, a 2.5% (w/w) concentration of chitosan solution was used. Five grams of chitosan power was dissolved in 200 mL of 1% (v/v) HCl for 12 h at room temperature, during which the solution became viscous and translucent. The dissolved chitosan solution was added dropwise into 200 mL of 1 M NaOH using a separating funnel with mild stirring (100–120 rpm). White gel beads were immediately formed and then mildly mixed for more than 3 h to increase the gelation. The spherical bead shape was dependent on the travel distance of the drops of the chitosan solution. The prepared beads standing for CB were washed with DI until the solution pH of the mixture decreased to 7.0. The rinsed beads were then loaded with Cu2+ by equilibrating the beads with 400 mL of a Cu2+ solution at 100–5000 mg L1 for 24 h at pH 4–4.5 (noted as CB–Cu). After loading, they were rinsed with DI several times and then stored in DI before use. Note that for the CB–Cu beads used in the subsequent phosphate removal test, 5000 mg L1 Cu2+ was used during loading. 2.3. Characterization of chitosan beads and chitosan–Cu Compressive tests were used to gauge the physical strength. The tests were conducted via a Universal Testing Machine (Model:

Table 1 Comparison of copper uptake on the PS/DVB matrix chelating resins with different functional groups.

a

Name

Functionality

Matrix

Average particle size mm

Copper loading capacity (mmol g1)

Refs.

DOW 3N – CR20 CB

Di(2-picolyl)amine Iminodiacetate Polyethylenimine Amino group

PS/DVBa PS/DVB PS/DVB Chitosan polymer

0.35 – 0.5 31

1.5 0.9 0.8 0.8

Zhao and SenGupta [4] Chen and Chung [16] An et al. [5] This study

PS stands for polystyrene, DVB: divinylbenzene.

B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53

WL2111) (WITHLAB, Korea) operated at a 1.3 mm min1 velocity pressing on the 200 N loading cell. X-ray Diffraction (XRD) of chitosan powders and chitosan hydrogel before and after copper loading were obtained using a D8 advance with an LYNXEYE detector at 40 kV and 40 mA (1.6 kW) (Bruker, USA). Chitosan powder and chitosan hydrogel were prepared in dry and gel type forms, respectively. Fourier transform infrared spectroscopy (FTIR) spectra (model: Spotlight 200, PerkinElmer, USA) were used to determine the interactions between immobilized chitosan and Cu2+ and between Cu2+ and phosphate in a wavenumber range from 380 to 4000 cm1. The sample was first air dried and ground to particles, and a mixture with KBr at a sample-to-KBr ratio of 5:95 by weight with 95% KBr was pressed into a thin film at 50 MPa. 2.4. Phosphate adsorption isotherm tests A series of batch isotherm tests for CB–Cu were carried out with a fixed initial phosphate concentration of 50 mg L1 and under various amounts of CB–Cu ranging from 0 to 0.5 g in 55 mL glass tubes. The experiments were conducted in phosphate only systems as well as in the presence of various other competing anions (including sulfate of 50 mg L1). The adsorption was allowed to reach equilibrium by mixing the systems for 24 h. At equilibrium, the concentrations of the target anions in the aqueous phase were then measured. The uptake of phosphate was calculated using the following mass balance equation:

qe ¼

VðC o  C e Þ M

ð1Þ

where qe is the equilibrium mass uptake of phosphate (mg g1); V is the solution volume (L); Co and Ce are the initial and final concentrations of phosphate in solution, respectively (mg L1), and M is the mass of the sorbent added (g). 2.5. Effects of pH It is well known that chitosan can be dissolved in moderately acidic solutions. The amino group in chitosan, which is a key functional group for binding with heavy metals, undergoes protonation at acidic pH. Furthermore, phosphate species will also depend on solution pH (pKa = 2.1, 7.2, and 12.3) [22]. Therefore, batch sorption equilibrium tests were performed in a similar fashion to that in the isotherm tests to determine the effects of the solution pH on the phosphate removal efficiency. Phosphate solutions were prepared at 50 or 100 mg L1 PO3 4 at 55 mL glass vials, and the initial pH was adjusted from 3 to 9 using dilute HCl and NaOH. Each vial was rotated at 30 rpm for 24 h, and the copper, phosphate and TOC concentration in the supernatant was measured. 2.6. Fixed-bed column tests Fixed-bed column tests were performed to test the breakthrough behavior of phosphate as well as nitrate and sulfate for CB–Cu. The experiments used an acrylic column (30 mm in diameter and 50 mm in length) that was filled with approximately 35 mL of CB–Cu, and the flow rate was constantly maintained at 1 mL min1, which translates to an empty bed contact time (EBCT) of 35 min. Simulated contaminated water containing 4.9 mg L1 of phosphate, 50 mg L1 of sulfate, 50 mg L1 of nitrate, and 100 mg L1 of bicarbonate at an initial pH 7.8 was passed through in up-flow mode.

47

and phosphate, were analyzed with a Dionex Ion Chromatography instrument (Model: ICS-1000, USA) equipped with an AM 14 column. An ICP–OES (Prodigy, Reemanlabs, USA) was used to measure the aquatic copper concentration after filtration with 0.45 lm PTFE membranes (Millipore Corp., Bedford, MA). Total organic carbon (TOC), as a measure of the chitosan concentration in solution, was analyzed by a Shimadzu TOC-V analyzer equipped with an ASI-5000A auto sampler (Shimadzu, Japan). 3. Results and discussion 3.1. CB–Cu characterization To investigate the effects of Cu2+ concentration on the chemical and physical properties of CB–Cu, the copper loading was carried out at four initial concentrations of Cu2+ in the range of 100– 5000 mg L1, and their changes of shape and size are presented in Fig. 1 and Table 2. Fig. 1 reveals that the spherical shape of the beads is not distorted with varying concentration of Cu2+ and homogenous copper sorption occurred on the outer layer. The bead size is decreased with an increase in concentration of Cu2+ from 4.8 mm for CB to 3.1 mm for CB at 5000 mg L1 with a low standard deviation (SD) except for 100 mg L1 of Cu2+. At the 100 mg L1 Cu2+ loading solution, the bead looks swollen, but it becomes denser at elevated Cu2+ concentrations. In general, two models of chelating binding mechanisms have been proposed to interpret the uptake of transition metals to a chelating polymer: the bridge [23] and pendant [24,25] models. While both mechanisms are likely operative in our case, the present observation also suggests a role of water molecules as ligands and in formation of hydrogen bonds. Each chitosan polymer chain unit consists of one amino group and one OH group that directly chelate with transition metals, and water molecules are bound inter- or intramolecularly by hydrogen bonds (Fig. 2a). At a relatively low concentration of Cu2+ (100 mg L1), one copper molecule (Cu(H2O)2+ 4 ) only coordinates with one nitrogen to form a complex (Cu(H2O)2+ 3 –NH2) (Fig. 2b). As a result, no chelating interaction occurs between the copper ion and the same chain or other chains of the chitosan polymer. Therefore, some water molecules from the copper complex would possibly swell the outer layer, leading to the apparent size increase (Fig. 1). At a higher concentration of Cu2+, the hydrogel bead decreases in size, which is likely the result of the exclusion of water molecules from the chitosan via exchange of ligands from the copper complex, as shown in Fig. 2c. When the concentration of Cu2+ is increased, Cu2+ can coordinate with other nitrogen atoms in other chitosan units or chains by replacing H2O ligands, and by chelating with the hydroxyl group in chitosan with Cu2+, the water molecules can be excluded from chitosan. Both interactions result in compression of the hydrogel bead. Rhazi et al. postulated the following two copper coordination forms depending on the pH [26]: [Cu(–NH2)2+, 2OH, H2O] at pH in  5.3–5.8 and [Cu(–NH2)2+ 2 , 2OH , H2O] at pH P 5.8. Our work also suggests that the concentration of copper also can determine the

2.7. Chemical analyses The solution pH was measured with an ORION Star A211 (Thermo Scientific, USA). Four anions: chloride, nitrate, sulfate

Fig. 1. Photograph of a CB hydrogel bead loaded with a various concentrations of Cu2+ ranging from 0 to 5000 mg L1.

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B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53

Table 2 The change of size and amount of Cu2+ uptake after loading Cu2+ at various initial concentration of 100, 500, 10,000, and 5000 mg L1. Cu2+ loading concentration (mg L1)

0

100

500

1000

5000

Particle size (mm) Cu2+ uptake (mg g1)

4.8 ± 0.31 –

5.9 ± 0.59 4.8 ± 1.2

4.4 ± 0.41 27.7 ± 2.5

4.1 ± 0.25 31.9 ± 5.4

3.1 ± 0.19 53.1 ± 4.6

higher Cu2+ loading strongly enhances the inter/intra-molecular bonding of each polymer chain. Accordingly, highly compacted chitosan beads can be synthesized at elevated Cu2+, and the condensed bead is likely to show improved physical strength. 3.2. Physical properties Although the formation of chitosan hydrogel bead has elicited unique interest, there are still concerns and limitations for industrial applications. Two particular concerns have been the low mechanical strength and the high solubility of chitosan in acidic solutions. To overcome the chemical stability issue, crosslinking agents such as GLA can be introduced [27], as they result in more chemically stable chitosan beads despite the drawback of largely reduced active sites as the amino groups are occupied by the crosslinking agent. However, the problem of improving the mechanical strength remains. To evaluate mechanical strength, a compress load test was conducted using UTL for CB and CB–Cu prepared with the 5000 mg L1 of Cu2+ solution. Fig. 3 shows that both CB and CB– Cu hydrogel beads show a threshold of 0.90 and 10.9 kgf (dotted circles) with a SD of 0.17 and 0.38, respectively. The values indicate CB–Cu is 12 times stronger than CB under the load. The coordinating interaction between copper and nitrogen leads to be denser bead, (i. e., the density was increased from 0.036 mg/cm3 for CB to 0.11 mg/cm3) and appears to enhance the intensification and contraction of the inner chitosan network via inter/intra molecular interactions and the exclusion of water as described in Fig. 2c. As a result, a spherical shape with strong hardness was successfully prepared by decrease of porosity of CB–Cu due to immobilization of Cu2+ within chitosan hydrogel beads. The resulting enhanced mechanical strength provides another useful advantage. 3.3. Effect of chelating interaction on chemical stability In regards to chemical stability, TOC was also measured to estimate the degree of the re-dissolution of the chitosan beads at various pH ranging from 1 to 7 for CB and CB–Cu (Table 3). It was

Fig. 2. Proposed structure for CB (a), CB equilibrate with a low concentration of Cu2+ (b), CB equilibrate with a high concentration of Cu2+(c).

copper coordination form. The water content was determined to be 93% and 85% for CB and CB–Cu at 5000 mg L1, respectively, confirming that the chelation interaction between Cu2+ and the amino and hydroxyl groups indeed reduces the water molecules in the chitosan inner network. Table 2 gives the uptake of Cu2+ measured by desorption of 2+ Cu , and the obtained values were 4.8, 27.7, 31.9, and 53.3 mg g1 when equilibrated at 100, 500, 1000, and 5000 mg L1 of Cu2+, respectively. These results support Fig. 2c in the sense that the

Fig. 3. Compress load test for CB and CB–Cu.

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B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53

6000

5000

4000

Intensity

anticipated that CB–Cu, which is highly crosslinked by Cu2+, would resist pristine solubility in acidic conditions. For both CB and CB– Cu, TOC was less than 1 mg L1 at pH > 6, indicating the leachability of the polymer was negligible. For CB, the leaching was observed from pH 5 (7%), and complete dissolution takes place at pH 3, whereas the chitosan leaching for CB–Cu begins at pH 4, and the hydrogel bead is clearly dissolved at pH 2, revealing higher resistance in acidic solution. Table 3 gives the leachability of Cu2+ at various pH values which results from protonation of amino group in chitosan polymer. In the case of CB–Cu, the dissolution of chitosan coincides with the Cu2+ leaching in solution. Therefore, it would be concluded that Cu2+ is homogenously dispersed during loading and apparently enhances chemical stability against weak acid by crosslinking the chitosan chains of the inner network as they are firmly immobilized.

CP

3000 CP-Cu 2000 CB 1000

CB-CU

0 10

15

20

25

30

35

40

Degree 2-theta

3.4. XRD pattern

Fig. 4. XRD patterns of CP and CB before and after copper loading (5000 mg L1).

In light of the reduced size (Fig. 1) and the enhanced physical stability (Fig. 3), XRD patterns were analyzed to determine the possibility of structural change by chelating interactions. Fig. 4 shows the XRD patterns, which are characterized by scattering angles at 2h for CB (dissolved by HCl and precipitated by NaOH) and CB– Cu (dissolved, precipitated, and loaded with 5000 mg L1 of Cu2+). For comparison, two chitosan powders (CP) patterns were added before and after copper sorption [28]. The two peaks for CB occur with less intensity at the angles 2h = 20° and newly at 2h = 28°. The exact peak angle and shape have been reported for hydrogel [29]. The reduced intensity at 2h = 20° is due to both dissolution of chitosan powder with acid solution and precipitation by NaOH. However, after the chelating interaction between copper and nitrogen, the disappearance at 2h = 20° was observed for CP–Cu and CB–Cu, which indicates that the copper immobilization leads to the elimination of the fundamental crystal structure of chitosan. Ogawa et al. reported that chitosan consisted of up and down chains packed in an antiparallel fashion with hydrogen bonds [30], which formed a sheet structure, and they classified the chitosan structure as tendon (hydrated crystal) and annealed (anhydrous crystal) polymorphs. The authors concluded that an extended twofold helix (zigzag structure) is present in chitosan crystals, which are mainly generated by hydrogen bonding of each chain between the amino group and oxygen atom of water molecules. Therefore, the reduced or absent peak at 2h = 20° could represent some shift related to hydrogen bonding reducing the zigzag structure. Consequently, the suggested mechanism for chelation between Cu2+ and nitrogen resulting in a smaller hydrogel bead, as shown in Fig. 2a and b, can be reinforced by disappearance of hydrogen bond. The obtained peaks for CP–Cu at 15, 33, 35, and 37 indicate the copper crystal formed as either CuOH or CuO [31], whereas no peaks is detected for CB–Cu hydrogel bead, which suggests copper is present in molecular form. Therefore, the higher hardness (Fig. 3) is not the results of crystals but rather the higher density.

3.5. FTIR Fig. 5 shows the FTIR spectra of neat CB, neat CB–Cu, and CB–Cu with phosphate adsorbed. For CB, the broad peak at 3437 cm1 and the weak peak at 3292 cm1indicate the stretching vibrations of both the OH group and N–H bonds, respectively [29]. The peaks at 1656, 1599, and 1545 cm1 indicate, respectively, amide I, N–H bending vibrations of primary amines, and N–H bending vibrations in the amide group [32,33]. Upon copper loading (CB–Cu), the peak intensity at 3420 cm1 is significantly reduced, and a broad peak of 3284 cm1 moves toward a lower wavenumber of 3235 cm1. Such a shift can be attributed to an increased proportion of NH groups over hydroxyl group in the formation of chelating complexes [29]. Furthermore, the peak at 1599 cm1 disappears, and a new peak at 1545 cm1 becomes evident. Xie et al. observed that the N–H bending vibration at 1557 cm1 weakens with compositing of sodium cellulose sulfate, suggesting the formation of NH+3 from NH2 [34]. Conversely, the new peak at 1545 cm1 supports the formation from NH+3 to NH2 in this study. Upon uptake of phosphate, a new weak band is observed at 1120 cm1. According to Elzinga and Sparks, who observed three distinguishable bands at approximately 1120, 1010, and 970 cm1 for phosphate sorption on the surface of hematite, the peak at 1120 cm1 is assigned to phosphate complex with copper [35]. 3.6. Effects of pH The amino group in chitosan is a key functional group for binding with both cations and anions from solution. Depending on the prevailing pH, the functional group undergoes protonation and deprotonation (Eq. (2)), forming either NH2 or NH+3:

R-NH2 þ H3 Oþ $ R-NHþ3 þ H2 O

ð2Þ

Table 3 The amount of chitosan leased for CB and CB–Cu and concentration of Cu2+ from CB–Cu at various pH. Sample

Solution pH 1

2

3

4

5

6

7

Chitosan leaching amount (%)

CB CB–Cu

100 100

100 100

100 81.1

42 24

7 1.1

1.3 0.3

0.3 0.3

Cu leaching amount (%)

CB CB–Cu

– 100

– 99.5

– 84.7 (1.3)

– 28.3 (0.55)

– 12.1 (1.2)

– 5.0 (0.81)

– 1.3 (0.11)

(): Standard deviation.

B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53 CB-Cu-PO43-

30

1545

pka 6

pka 7

1.0

pka 8

a

25

0.8

-1

CB-Cu

Uptake, mg g

Transmittance, %

1120

CB

3235

1599

20 0.6 15 0.4 10 0.2

5

Degree of protonation, α

50

1656 3285

0

0.0 5

6

7

3420

pH 2500

2000

1500

1000

500

30

Wavenumber, cm-1

pka 6

-1

2+

Upon copper loading, however, the immobilized Cu ions become the predominant center functional group that can further bind with strong ligands such as phosphate from the solution phase, forming the stable CB–Cu-phosphate ternary complexes. Eqs. (3)–(6) illustrate the interactions between the amino group and anions/cations in solution for CB or CB–Cu. For CB at pH < pKa, the protonated amine takes up anions via standard ion pairing:  2 þ þ R-NHþ3 þ H2 PO4 ðSO2 4 Þ ! R-NH3 H2 PO4 =R-NH3 SO4

b

25

Uptake, mg g

Fig. 5. FTIR spectra of CB, fresh CB–Cu, and phosphate-loaded CB–Cu. CB–Cu was prepared at 5000 mg L1 of Cu2+ and phosphate was loaded at 10 mg L1.

0.8

20 SO42-

0.4 10 0.2

0

0.0 5

6

R-NH2 þ Cu2þ ! R-NH2 Cu2þ ðComplexÞ

ð5Þ

þ

HPO2 4

2þ

! ½R-NH2 Cu

HPO2 4



þ 2Cl

c

-1

80

60

60

40

40

20

20

0

0 5

6

7

ð6Þ

It should be noted that the dissociation constant (pKa) of CB is generally 6.5 ± 0.5, which is significantly affected by the degree of deacetylation (DD) and molecular weight (M.W.) [36]. Note that the Cu2+ binding capacity in chitin is only 1.5 mg-Cu (g-chitin)1 in our experiment, and thus, the sorption capacity of hydroxyl group is not considered in this study. Finally, the immobilized copper in chitosan chemically binds phosphate via both electrostatic and LAB interactions (Eq. (5)) [4]. To investigate the pH effect on phosphate uptake by the PLE, the phosphate uptake for CB–Cu was evaluated at various pH values (Fig. 6), and, for comparison, the phosphate uptake by CB is also provided. The Katchalsky equation [37], is used to give the degree of dissociation (a) as a function of pH at a given pKa. The calculated lines indicate that the chitosan protonation is very sensitive to the pH in the a range of 0.2–0.8. It is noteworthy that CB alone is able to adsorb phosphate. In the phosphate only system (Fig. 6a), CB 1 takes up to 25 mg g1 of PO3 4 at pH 6, which drops to 15 mg g at pH 7, and finally to 3 mg g1 at pH 9. Note that at pH 5 and 4, significant dissolution of CB hydrogel bead was detected, which is accordance with the TOC results (Table 2). Fig. 6a shows that the uptake of phosphate is in accord with the protonation of amino

HPO42-

80

8

9

pH 50

100

HPO42-

H2PO4-

d

PO43-

40

Uptake, mg g-1

½R-NH2 Cu

 Cl2

9

100 H2PO4-

Immobilized Cu in CB–Cu serves as the new central ion to facilitate ligand exchange, where stronger ligands (i.e., stronger donors of electron-lone pairs) replace weaker ligands: 2þ

8

pH

Uptake, mg g

Note that the Eq. (4) is usually impossible. Instead the free base form of the amine is able to bind with Cu2+ through surface complexation, resulting in the PLE (CB–Cu):

7

100

at pH > pKa, the deprotonated amine loses the ion exchange ability.

ð4Þ

0.6

PO43-

15

5

ð3Þ

2 2 2 R-NH2 þ HPO2 4 ðSO4 Þ ! R-NH2 HPO4 =R-NH2 SO4

1.0

pka 8

pka 7

Degree of protonation, α

3000

Fraction of phosphate species

3500

9

SO42-

80

30

60

20

40

10

20

0

Fraction of phosphate species

4000

8

0 5

6

7

8

9

pH Fig. 6. Phosphate equilibrium uptake in the absence (a and c) and the presence of competing ions (nitrate and sulfate) (b and d) as a function of pH for CB (a and b) and CB–Cu (c and d).

groups, approximately corresponding to the line of a of pKa 7.0 [36], suggesting that electrostatic forces (NH+3) govern the

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B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53

phosphate sorption capacity. By comparing the phosphate uptake with that of competing anions, such as sulfate (Fig. 6b), it is evident that sulfate uptake is greater than other anions, whereas the presence of the competing anions markedly decreased phosphate uptake from 25 mg g1 to 5 mg g1 at pH 6. These findings indicate that protonated chitosan offers much greater affinity toward sulfate than phosphate, which is generally true for standard ion exchange resins where only electrostatic interaction is involved in the sorption mechanism [4,20]. To test the hypothesis that the new PLE (CB–Cu) offers much improved selectivity for phosphate over sulfate, equilibrium uptake of phosphate by CB–Cu was measured at various pH levels and in the presence or absence of competing anions. As shown in Fig. 6c, the phosphate uptake reaches approximately 81 mg g1 at pH 5, which decreases to 54.6 mg g1 at pH 9. This trend is similar to that for copper-loaded DOW 3N as reported by Zhao and SenGuta [4]. The reduced phosphate uptake at elevated pH is attributed to increased competition from the hydroxyl ions. In the multicomponent systems (Fig. 6d), high uptakes of phosphate and sulfate uptakes are obtained at pH 5 and 6 (37 mg g1 PO3 4 and 36 mg g1 SO2 4 ). At pH 9, the phosphate uptake decreases to 28 mg g1, whereas the sulfate uptake dramatically drops after pH 6 and reaches 13 mg g1 at pH 9. The uptake of phosphate is less pH-dependent than that for sulfate with the optimal phosphate uptake occurring in the pH range of 5–7. The difference between the phosphate and sulfate uptakes enlarged at pH P 6 (Fig. 6b). Phosphate is considered a strong ligand that can complex with the transition metal cations through concurrent electrostatic and LAB interactions. In contrast, sulfate is a much weaker ligand that binds with copper predominantly through electrostatic interactions. Consequently, CB–Cu shows much higher selectivity toward phosphate over sulfate, which reverses the affinity sequence for standard anion exchange resins. In addition, the effect of pH on phosphate speciation should be taken into account as 2+ HPO2 much more strongly than H2PO 4 binds with Cu 4 . Usually, when PLE is prepared with the commercial chelating resins, e.g., DOW-3N [4,21] and CR20–Cu [20], the optimal pH for arsenic and phosphate was observed at around pH 7. However, in this work, the CB–Cu shows high phosphate uptake at pH as low as 5, where the less favorable phosphate species H2PO 4 is the predominant species. This unusual phenomenon may be explained based on the interactions between anions and CB–Cu. First, surface

accumulation of OH ions on CB–Cu causes an interfacial pH shift from the bulk solution pH. Based on the Donnan co-ion exclusion principle [38,39], the immobilized Cu2+ ions on the surface of chitosan can exclude H+ and attract OH ions. As a result, an excess of OH increases the pH at the solid-solution interface for additional influence. In addition, it is also possible that some residual hydroxyl groups remain in the inner and outer area of the chitosan chains during the preparation step to form a gelled bead under a high concentration of NaOH. Both effects tend to cause the interfacial pH to be higher that than in the bulk solution phase, and thus, 2 shift phosphate species from H2PO at the surface of 4 to HPO4 chitosan, as shown in Fig. 7. Consequently, the optimized pH is extended to include acidic conditions. Second, the free amino groups in the inner chitosan hydrogel bead can directly bind with phosphate via electrostatic interactions. As stated in Section 2.2, copper was loaded to chitosan at a solution pH of 4–4.5, which is able to force the amino groups to remain protonated regardless of the concentration of copper. The protonated amino groups are able to interact with anions such as phosphate and sulfate through Columbic interactions. 3.7. Isotherm tests Fig. 8 shows the phosphate sorption isotherm for CB–Cu at an initial pH of 7.5. The classical Langmuir isotherm model was employed to determine the maximum uptake of phosphate (Q) and the Langmuir affinity constant (b) by fitting the experiment data.

qe ¼

bQC e 1 þ bC e

ð7Þ

The maximum Q of phosphate is estimated to be 33.5 mg g1 for the multicomponent system and 84.5 mg g1 in the phosphate only system. However, in both cases, phosphate uptake is very favorable. The Langmuir affinity constant (b) was applied to predict whether an adsorption system is favorable or unfavorable. Weber and Chakravorti used a Langmuir isotherm expressed in terms of a dimensionless equilibrium parameter, RL [40].

RL ¼

1 1 þ bC 0

ð8Þ

where C0 is the initial phosphate or sulfate concentration (mg L1), and b is Langmuir constant calculated from Eq. (7). When the value

only PO43-

80

60

-1

Uptake, mg g in solid phase

100

with multi component

40

20

0 0

10

20

30

40

Ce, mg L-1 in solution phase Fig. 7. A schematic illustration of the interfacial pH shift (Donnan effect) and free amino group (NH+3) in chitosan hydrogel bead. Circle: interacted, non-circle: inactive amino groups remaining. Subscripts s and b stand for surface and bulk, respectively.

Fig. 8. Phosphate equilibrium uptake in the absence and presence of 100 mg L1 of nitrate and 100 mg L1 of sulfate (symbols: observed data; lines: Langmuir model fits).

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B. An et al. / Reactive & Functional Polymers 85 (2014) 45–53

Table 4 Langmuir isotherm parameters (maximum uptake (Q), Langmuir affinity (b)), equilibrium parameter (RL), and separation factor. Resin

Q (mg/g)

b (L/mg)

RL

Separation factor aP/S

Only phosphate Multi component

84.5 ± 2.9 34.8 ± 0.99

0.23 ± 0.026 0.57 ± 0.11

0.080 0.034

– 1.2

-1 NO3-

200

-

1.0 SO4

2-

150

0.8 0.6

Influent pH: 7.8 PO43- : 4.9 mg/L NO3- : 49.1 mg/L SO42- : 52.4 mg/L HCO3- : 100 mg/L EBCT: 35 min

PO43-

0.4 0.2

100

50

Cl-

0.0 0

50

100

150

200

250

0 300

B.V Fig. 9. Breakthrough behavior of phosphate and competing ions through a fixedbed column using a new PLE, CB–Cu.

of RL is higher than unity, the type of isotherm is unfavorable, and in the range from 0 to 1, the isotherm is favorable. The RL is obtained to be 0.080 and 0.034 for phosphate only and with competing ions, respectively which are lower than unity. Therefore it was observed that phosphate sorption type is favorable regardless of the presence of sulfate. The relative affinity can also be revealed in terms of the commonly used binary separation factor, aP/S, defined as:

aP=S ¼

qP  C S C P  qS



ð10Þ

Note that when a commercial standard anion exchanger was used to remove phosphate with competing anions, the following affinity sequence was generally obtained in previous studies [4,20]: Concentration of Cl , mg L

-1

1.2

Fraction, C C0

2  PO3 4 > SO4 > NO3 > Cl

250

1.4

i.e., the concentrations of nitrate and sulfate exceed the influent concentrations, indicating that CB–Cu offers greater affinity for phosphate over sulfate and nitrate. Therefore, the breakthrough sequence is as follows:

ð9Þ

where the subscripts P and S denote phosphate and sulfate, respectively; q is the uptake of a sorbate in the solid phase (mg g1), and C is the concentration in the aqueous phase (mg L1). In general, an aP/S value higher than unity indicates the sorbent prefers phosphate to sulfate. In the multicomponent system, the average separation factor was determined to be 1.2 (Table 4), indicating the PLE offers higher selectivity for phosphate over sulfate. It was reported that the typical aP/S value for standard strong base anion exchange resins is 0.2 and 0.1 for IRA 900 and IRA 958, respectively [21]. 3.8. Column breakthrough tests Fixed bed column tests were performed to demonstrate the applicability of the new sorbent for separation of various anions and to confirm the affinity toward various anions including nitrate, sulfate, and phosphate. Fig. 9 shows breakthrough curves phosphate and other anions. The chloride concentration shown in the right y-axis rises immediately and then decreases to zero. This change is due to the fact that the CB–Cu was prepared in the form of Cl as a counter ion, which is easily exchangeable by nitrate, phosphate and sulfate. Nitrate breaks through first and sharply reaches an its influent concentration at 50 BV. Sulfate breakthrough occurs at 24 BV and reaches full breakthrough at 120 BV. In contrast, the breakthrough of phosphate occurred later than sulfate and nitrate at 40 BV and gradually increased to reach the influent concentration after 250 BV. The chromatographic elution peaking phenomena were observed for nitrate and sulfate,

3  SO2 4 > PO4 > NO3 > Cl



ð11Þ

This sequence obtained from Fig. 9 is in accordance with batch tests in this study and some previous studies for phosphate selectivity using DOW 3N–Cu [4] and CR20–Cu [20]. 4. Conclusions The main findings and conclusions are summarized as follows:  A new PLE was successfully prepared based on a non-toxic natural biopolymer, chitosan, with no sophisticated procedures involved. The new PLE demonstrates much enhanced chemical and physical stability.  Increasing the Cu2+ concentration in solution during the PLE preparation induces a high degree of coordinating bonds, forcing hydrogen bonds from water molecules to be reduced and some of water molecular to be replaced by ligands. As result, the chitosan hydrogel shrank and became denser.  The batch experiment tests of Cu2+ immobilization at different concentrations suggested two types of chelating mechanisms between the chitosan polymer chain and the copper ions. First, at low Cu2+ concentrations, one nitrogen is attached directly to copper, and at elevated Cu2+ concentrations, copper is bonded to multiple nitrogen atoms.  The immobilized copper in CB–Cu hydrogel also acts as a crosslinking agent to enhance chemical stability. Namely, copper-loaded chitosan is much more resistant to acid dissolution than neat CB hydrogel.  CB–Cu demonstrated good selective phosphate removal even at pH as low as 5, and its optimal pH range is 5–7, which is lower than other reported PLEs. A shift of interface pH due to Donnan co-ion exclusion and uptake facilitated by protonated amino groups are responsible for the lowered pH range.  Both batch tests and column tests confirmed the high selectivity of the PLE for phosphate over sulfate, with a phosphate/sulfate binary separation value of 1.2.  Fixed bed column tests revealed a breakthrough sequence of: phosphate > sulfate > nitrate > chloride, which greatly differs for standard anion exchange resins. The breakthrough of phosphate, even at a 10 times lower concentration than sulfate, occurred later than sulfate with a gradual breakthrough profile. The new PLE offers an alternative ‘‘greener’’ material for selective removal of strong ligands, or for separation of anionic ligands based on ligand characteristics. Acknowledgements This work was supported by a grant from the Korea Institute of Science and Technology (KIST) Institutional Program (Project No. 2E24563).

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