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Aminoethanethiol-grafted porous organic polymer for Hg removal in aqueous solution 2+

Seenu Ravi, Pillaiyar Puthiaraj, Kyung Ho Row, Dong-Wha Park, and Wha-Seung Ahn Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02743 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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Industrial & Engineering Chemistry Research

Aminoethanethiol-grafted porous organic polymer for Hg2+ removal in aqueous solution Seenu Ravi, Pillaiyar Puthiaraj, Kyung Ho Row, Dong-Wha Park, and Wha-Seung Ahn* Department of Chemical Engineering, Inha University, Incheon, Korea *Corresponding author: [email protected]

Abstract A highly porous organic polymer, CBAP-1, was synthesized from terephthaloyl chloride and 1,3,5-triphenylbenzene via the Friedel-Crafts reaction, and functionalized with either ethylenediamine (EDA) or 2-aminoethanethiol (AET) for Hg2+ removal from water. Both materials were characterized by XRD, N2 adsorption-desorption isotherms, FT-IR, XPS, ICP, and elemental analysis, and the stability of the porous polymers under different pH and temperature conditions was examined. The adsorption experiments were carried out by varying contact time, Hg2+ concentration, and system pH to study the adsorption equilibrium and kinetics. The Hg2+ ion-adsorption capacities of CBAP-1(EDA) and CBAP-1(AET) were 181 and 232 mg/g, respectively, at room temperature and pH 5, and the observed adsorption isotherms could be fitted well to the Langmuir model (correlation factor R2 > 0.99). Under the optimum set of conditions, the adsorption equilibrium for CBAP-1(AET) was reached within a contact time of 10 min; CBAP-1(AET) exhibited an excellent distribution coefficient of greater than 2.41 × 107 mL/g. The adsorption kinetics could be satisfactorily described by a pseudo-second-order model. Hg2+ recovery in the presence of commonly co-existing metal ions like Na+, Ca2+, Mg2+, Pb2+, and Fe3+ was also investigated. CBAP-1(AET) showed high Hg2+ selectivity against other ions except Pb2+. CBAP-1(AET) was superior to CBAP-1(EDA) in terms of overall performance; it could efficiently remove > 96% of Hg2+ ions in 2 min from a 100 ppm Hg2+ solution. The material could be reused for 10 consecutive runs with 1

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negligible loss in adsorption capacity. Keywords: Hg2+ adsorption; porous organic polymer; 2-aminoethanethiol; ethylenediamine; water treatment.

1. Introduction Hg2+ is known as one of the most toxic, dangerous, and non-biodegradable inorganic pollutants, which can cause serious health problems due to its volatility and persistence.1-4 The

maximum contaminant level of Hg2+ in an aqueous environment is set at < 0.005 ppm. To

date, significant advances have been made in Hg2+ sequestration, including ion exchange,5, 6 membrane filtration,7, 8 chemical precipitation9, and adsorption.10 In particular, Hg2+ removal by adsorption has received significant attention because it is simple and economical. For this purpose, various adsorbent materials have been proposed including mesoporous silica,11-14 porous carbon,15-17 graphene,18 carbon nanotubes,19 magnetic porous particles,18, 20-25 metalorganic frameworks,26, 27 porous polymers,28 and biopolymers.29, 30 However, mesoporous silica and metal organic frameworks are often chemically unstable and costly to prepare, whereas organic functionalization remains a challenge for graphene and carbon nanotubes. Thus, the search for economically viable and chemically stable adsorbents that allow easy functionalization of diverse organic groups is an important area of contemporary research for removal of Hg2+ and other heavy metal ions from contaminated water. Recently, porous organic polymers (POPs) receives growing attention owing to their high surface area and uniform pores, diverse organic functionalization possible, easy synthesis, high chemical and hydrothermal stability for various applications

31-33

including

gas storage,34, 35 heterogeneous catalysis,33, 36 molecular separation,37, 38 and sensors.39 POPs 2


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have also been tested for the removal of pollutant species in a clean environment.40 Jinlong et al. reported an Fe3O4-incorporated porous melamine-based covalent organic framework for Hg2+ removal with an adsorption capacity of 96 mg/g.41 Ding et al. reported a covalent organic polymer with thiol receptors for sensing and adsorption of Hg2+ with an adsorption capacity of 236 mg/g.28 Baiyan et al. reported a porous aromatic framework nano-trap for Hg2+ with an adsorption capacity of 1000 mg/g.42 Ning et al. reported a stable covalent organic polymer for Hg2+ removal, which exhibited a capture capacity of 734 mg/g.43 Whilst these polymers show high capture capacities towards Hg2+, their demanding material synthetic protocols, which use expensive reagents, hinder practical implementation of these polymers. Recently, a porous aromatic polymer with incorporated carbonyl groups (CBAP-1) was prepared via a simple Friedel-Crafts reaction using commercially available terephthaloyl chloride and 1,3,5-triphenylbenzene.44 CBAP-1 can be prepared in large quantities using economic precursors and the carbonyl group incorporated in the highly porous framework renders the material attractive to diverse post-synthesis organic functionalization. In this work, 2-aminoethanethiol-functionalized CBAP-1 was prepared and proposed as an adsorbent for Hg2+ removal from waste water (Scheme 1); soft base sulfur (RS-) groups exhibit high affinity toward soft acid Hg2+ ions and thiol groups are frequently used as active candidates for Hg2+ ion separation.45 Hg2+ removal using ethylenediamine-functionalized CBAP-1(EDA) was also tested for comparison.

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Scheme 1. Synthesis of CBAP-1(EDA) and CBAP-1(AET).

2. Experimental section 2.1 Chemicals and reagents Terephthaloyl

chloride,

1,3,5-triphenylbenzene,

ethylenediamine

(EDA),

2-

aminoethanethiol (AET), anhydrous aluminum chloride, dichloromethane, methanol, mercury nitrate dihydrate, sodium borohydride, sodium hydroxide, nitric acid, and hydrochloric acid were obtained from Sigma Aldrich. All the reagents were of analytical grade and used as received. 2.2 Material synthesis CBAP-1 synthesis by the Friedel-Crafts reaction using an AlCl3 catalyst and postsynthesis EDA functionalization were reported previously.44 In this work, CBAP-1 was functionalized with 2-aminoethanethiol (AET) in a similar fashion. In a typical procedure, AET (2 mL) was dissolved in methanol (40 mL) and the freshly prepared CBAP-1 (1.00 g) was added to the solution, followed by reflux at 80 °C for 15 h under vigorous stirring. After 4


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reflux, the resulting mixture was cooled to room temperature (RT) and the resulting Schiffbase intermediate was reduced with excess NaBH4 (ca. 2.00 g). After vigorous stirring for 10 h at RT, the mixture was filtered and the solid was washed successively with methanol and water, and dried under vacuum at 130 °C for 5 h to yield CBAP-1(AET) as a dark brown powder. 2.3 Characterization The powder X-ray diffractogram (PXRD, Rigaku) of the adsorbents was obtained using CuKα (λ=1.54 Å) radiation at a scan rate of 0.5° min-1 over a scanning range of 2θ = 4– 60°. The N2 adsorption-desorption isotherms were measured in a BELsorp-Max (BEL, Japan) at 77 K. The specific surface areas of the samples were measured using the Brunauer– Emmett–Teller (BET) method, and pore size distributions (PSD) of the materials were estimated by nonlocal density functional theory (NLDFT) under assumption of cylindrical pore geometry. The samples were pre-treated at 130 °C for 12 h in a high vacuum. The Fourier-transform infrared (FT-IR) spectra were recorded on an OTSUKA IG-2000 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS, Thermo scientific, USA) was performed using a monochromatic Al Kα X-ray source and a hemispherical analyzer. The Hg2+ concentrations were measured by inductively coupled plasma atomic adsorption spectroscopy (ICP-AAS, PerkinElmer, AAnalyst400). 2.4 Adsorption isotherms Batch adsorption experiments of Hg2+ ions onto CBAP-1(EDA) and CBAP-1(AET) were carried out to study the adsorption under different experimental conditions; namely, pH of the solution, contact time and solution concentration. Hg2+ stock solutions of specified 5



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concentration were prepared. The pH of the solutions (in the range 2-10) was adjusted using either NaOH (0.01 M) or HNO3 (0.01 M) aqueous solution. All the adsorption runs were performed using a 10-mL aliquot of a suitably diluted stock solution. The adsorbent (10 mg) was added to the adsorbate vials and the solutions were equilibrated in a thermostatic waterbath shaker operated at 25 °C and 200 rpm for a preset period of 1 h. The amount of Hg2+ ions adsorbed onto the functionalized CBAP-1 materials was calculated according to Eq. (1).

qe = (Ci − Ce ) ×V m …………………...(1) where, qe is the amount of adsorbed metal ions at equilibrium, Ci is the initial metal ion concentration in aqueous solution (mg/L), Ce is the equilibrium metal ion concentration in aqueous solution (mg/L), V is the volume of the solution (L), and m is the mass of the adsorbent (g). The distribution coefficient values (Kd) of Hg2+ were also determined using the following equation:

K d = ((Ci − C f ) C f ) × (V m) ………………(2) where, Ci is the initial Hg2+ ion concentration in aqueous solution (mg/L), Cf is the final Hg2+ concentration in aqueous solution (mg/L), V is the volume of the solution (mL), and m is the mass of the adsorbent (g). The metal ion concentration of all the post-filtration aliquots was measured by ICP-AAS. 2.5 Adsorption kinetics The kinetic study of Hg2+ adsorption was carried out using 10 mL solutions of 50 6


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ppm Hg2+ concentration treated with 10 mg of sorbent, and the solutions were filtered and removed at 0, 2, 4, 6, 8, 10, 15, 20 and 30 min time intervals for Hg2 measurement. The adsorption kinetic data were fitted using either a pseudo-first order (PFO) or pseudo-second order (PSO) model. The PFO adsorption rate model is given as:46

log(qe − q) = logqe − (k1 2.303) t …………………(3) where qe and q are the amount of adsorbate adsorbed on adsorbent (mg/g) at equilibrium and time t, respectively, and k1 is the first-order adsorption rate constant (min-1). The slope and intercept of the plot of log (qe-q) vs. t were used to determine k1. Similarly, the PSO kinetic model is given as:47 2

t / qt = (1 k 2 qe ) + (t qe ) ……………….……. [4]

where k2 (g/mg.min) is the second-order adsorption rate constant and qe is the adsorption equilibrium capacity (mg/g). 2.6 Desorption studies Regeneration of a given adsorbent is an important factor for its use in the practical removal of Hg2+ ions from aqueous solutions. Desorption was carried out using CBAP1(AET) pre-loaded with Hg2+. A known weight of the loaded CBAP-1(AET) was subjected to acid desorption with 2 M HCl and 0.5 M thiourea for 15 min at RT. The slurry was filtered off to recover the adsorbent and the aqueous phase was analyzed to determine the Hg2+ concentration. The percentage of desorption (D) was calculated using the following equation:

D (%) = (Ce C0 ) × 100 ………………….(5)

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where Ce is the equilibrium aqueous metal concentration after desorption and Co is the initial metal ion concentration on the CBAP-1(AET) phase. After the desorption experiment, the filtered adsorbent was recovered and reused after drying at 50 °C for 4 h.

3. Results and discussion 3.1 Material characterization of CBAP-1, CBAP-1(EDA) and CBAP-1(AET) XRD patterns of CBAP-1, CBAP-1(EDA), and CBAP-1(AET) in Fig.S1 confirm the amorphous nature of the POP materials. The broad peaks of CBAP-1 at 2θ = 10.8 and 22.6° are due to the direct phenyl-phenyl ring interactions favored by the intrinsic flexibility of the amorphous

polymer network.44

Identical

diffraction

patterns

are

observed

after

functionalization with EDA and AET.

Fig. 1. N2-adsorption desorption isotherms of CBAP-1, CBAP-1(EDA) and CBAP-1(AET).

The N2 adsorption-desorption isotherms of the materials are shown in Fig. 1. All the 8


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materials exhibit type I isotherms with steep N2 gas uptakes in the relative pressure range of 0-0.05, which indicates that the polymers are microporous.48 CBAP-1 shows broad desorption hysteresis due to swelling of the porous network/irreversible gas uptake.49 The BET surface areas of CBAP-1, CBAP-1(EDA) and CBAP-1(AET) are 856, 450 and 422 m2 g−1, respectively, which are estimated in the 0.01 < P/P0 < 0.04 region of the adsorption isotherms (Table 1). The gradual decrease in surface area and total pore volume of the adsorbents from CBAP-1 to CBAP-1(EDA) and CBAP-1(AET) is a consequence of successive amine and thiol functionalization of the CBAP-1 backbone.50,

51

After

functionalization, minor mesopore regions in CBAP-1 disappear and the micropores with pore diameters in the range of 0.9 to 1.4 nm decrease to ca. 0.65 nm for both CBAP-1(EDA) and CBAP-1(AET). The approximate level of microporosity (the ratio of the micropore volume to the total pore volume (V0.1/Vtot)) of CBAP-1, CBAP-1(EDA), and CBAP-1(AET) is 73.6, 88.1, and 88.5%, respectively. The increases in microporosity level are due to the functionalization of EDA and AET into the CBAP-1 pore network.

Table 1. Textural properties of CBAP-1, CBAP-1(EDA) and CBAP-1(AET). 2

SABET (m /g)

a

d

V0.1 (cm g )

Vtot (cm g )

V0.1/Vtot (%)

Average micropore diameter (nm)

3 -1 b

3 -1 c

CBAP-1

856

0.340

0.460

73.6

0.9, 1.4, 2.2

CBAP-1(EDA)

450

0.156

0.177

88.1

0.65

CBAP-1(AET)

422

0.147

0.166

88.5

0.65

a

BET surface area (SABET); bpore volume (V0.1) at P/P0=0.10; ctotal pore volume (Vtot) at P/P0=0.99, and d Average pore size in microporous region .

9



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Table 2. Elemental analysis of CBAP-1, CBAP-1(EDA) and CBAP-1(AET). C

H a

a

N a

(wt.%)

(wt.%)

CBAP-1

86.21

4.02

CBAP-1(EDA)

79.81

7.21

CBAP-1(AET)

78.54

6.97

S a

(wt.%)

Al b

b

(wt.%)

(wt.%)

-

0.006

13.26

-

-

4.52

9.34

-

b

Elemental analysis and ICP-OES.

Fig. 2 shows the FT-IR spectra of CBAP-1, CBAP-1(EDA) and CBAP-1(AET). CBAP-1 shows a band at 1720 cm-1, which is ascribed to the C=O stretching frequency, and is completely absent in CBAP-1(EDA) and CBAP-1(AET), supporting successful grafting of the respective organic groups onto the CBAP-1. The peaks at 2930 and 2860 cm-1 are ascribed to the symmetric and asymmetric stretching vibrations of –CH bonds of the functionalized materials. The small peak at 1055 cm-1 is due to the C-N stretching frequency in CBAP-1(EDA), but in the case of CBAP-1(AET) the C-N stretching frequency is slightly shifted to 1030 cm-1 due to the presence of thiol groups. A broad peak at 610 cm-1 is ascribed to the C-S stretching frequency, signaling the presence of thiol functionality in CBAP1(AET). The observed results are in agreement with previous reports supporting successful functionalization of EDA and AET to the CBAP-1 network.44, 52

10


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Fig. 2. FT-IR spectra of CBAP-1, CBAP-1(EDA) and CBAP-1(AET).

The chemical states of CBAP-1, CBAP-1(EDA), and CBAP-1(AET) examined by XPS are shown in Fig. 3. The asymmetric N 1s XPS peaks (Fig. 3a) of CBAP-1(EDA) and CBAP-1(AET) are detected at 399.3 eV; the difference in peak intensities between these materials reflects their respective differences in nitrogen content. Fig. 3b (CBAP-1(AET)) shows a sole characteristic peak of S 2p1/2 at 163.2 eV, indicating successful functionalization of AET into the polymer matrix.26, 42 The elemental compositions were measured by EA and ICP-OES, and the corresponding values are given in Table 2.

11



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Fig. 3. XPS of CBAP-1(EDA) and CBAP-1(AET) over the spectral regions of (a) N1s and (b) S2p.

3.2 Chemical Stability To investigate the chemical stability of the prepared materials, CBAP-1(EDA) and CBAP-1(AET) were dispersed in aqueous solutions with pH values of 0.5 and 13.5 at 25 °C and in boiling water for 3 days. After the treatment, the POP samples were separated by filtration and rinsed with water and methanol. All the POP samples were then dried at 100 °C under vacuum for 24 h. Notably, the FT-IR spectra of all the treated samples exhibit the same set of characterized peaks as the pre-treated samples (Fig. 4a), indicating the high stability of the prepared POPs under these harsh conditions. Similarly, no obvious change in the BET surface area is noted (Fig. 4b).

12


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Fig. 4. Stability test of CBAP-1(EDA) and CBAP-1(AET) in HCl and NaOH aqueous solutions and boiling water: (a) FT-IR spectra and (b) surface area.

3.3 Adsorption studies 3.3.1 Effect of pH Fig. 5 shows the effect of pH on the Hg2+ removal efficiency of CBAP-1(EDA) and CBAP-1(AET). For both materials, the adsorption capacity of Hg2+ increases with increasing pH from 2 to 7. It is known that H+ ions protonate amine sites and generate a positive surface under low pH conditions, leading to low Hg2+ uptake.41 Thiol groups, however, remain active even under acidic conditions and show high Hg2+ adsorption capacity even at a pH value of 3. The maximum adsorption is achieved at pH 5 for CBAP-1(AET). At pH 6 and above, formation of insoluble Hg(OH)2 occurs. 53 Therefore, a pH of 5 is the optimal value for both materials to obtain maximum adsorption without any Hg(OH)2 precipitation.

13



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Fig. 5. Effect of pH on the removal efficiency of Hg2+ by CBAP-1(EDA) and CBAP-1(AET).

3.3.2 Adsorption isotherms The adsorption isotherms of Hg2+ ions obtained over CBAP-1(EDA) and CBAP1(AET) at room temperature are shown in Fig. 6a. As expected, the adsorption equilibrium values increase with increasing solution concentration of Hg2+ and approach saturation. The adsorption equilibrium values were fitted to both the Langmuir and Freundlich models to estimate the surface properties and adsorbent/adsorbate affinity. The Langmuir isotherm model assumes that adsorption takes place homogeneously on independent binding sites over the adsorbent monolayer surface, given by the following equation:54

Ce qe = (1 bqm ) + (Ce qm ) …………………..(6) where Ce is the equilibrium concentration of Hg2+ in the solution (mg/L), qe is the amount of Hg2+ captured by the adsorbent at equilibrium concentration (mg/g), qm is the adsorption 14


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capacity of the adsorbent (mg/g), and b represents the affinity of the adsorbent binding sites or the Langmuir constant (L/mg). Data fitting results for the Langmuir model are shown in Fig 6b.

Fig. 6. (a) Adsorption equilibrium capacity of Hg2+ and (b) Langmuir model data fitting for Hg2+ ions over CBAP-1(EDA) and CBAP-1(AET) at room temperature/pH = 5. The Freundlich model describes the adsorption of Hg2+ over a reversible heterogeneous surface of a given adsorbent, which is not restricted to the monolayer adsorption, and is given by:55

ln qe = ln K + ( 1n) ln Ce …………….. (7) where K and 1/n are constants, which are indicative of adsorption capacity and adsorption intensity, respectively. The calculated parameters for both models are summarized in Table 3. From the correlation coefficients (R2) given in Table 3, it is clear that the Langmuir isotherm model fits the isotherm data better than the Freundlich model (Fig. S2), which is similar to the previous studies on functionalized porous materials,13,

56

and the maximum binding

capacities of Hg2+ in CBAP-1(EDA) and CBAP-(AET) are estimated to be 181 and 232 mg g-1, respectively. 15



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Table 3.

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Adsorbent parameters for Hg2+ in equilibrium and kinetic models.

Langmuir adsorption isotherm

Parameters qm (mg/g)

CBAP-1(EDA) 181

CBAP-1(AET) 232

b

0.846

4.305

R n

0.997

0.998

4.13

6.13

K

75.8

142.4

0.793

0.912

2

Freundlich adsorption isotherm

R

2

pH

5

Kd (mL/g) Pseudo First order kinetics Pseudo second order kinetics

1.03×10 -1

-

K1 (min ) R

2 -1

K2 (g mg -1 min ) R

2

5 5

2.41×107 0.056

-

0.979

-

1.12

-

0.999

In Table 4, Hg2+ adsorption capacity values of representative adsorbents reported previously are compared. CBAP-1(EDA) and CBAP-1(AET) exhibit relatively high adsorption capacities compared with most other adsorbents. However, sulfur-functionalized mesoporous carbon and thiol-functionalized mesoporous silica show significantly higher adsorption capacities (entry 3 and 4). These high Hg2+ capacities are most likely owing to the mesoporosity of these adsorbents, which enables unhindered and easy diffusion of Hg2+ ions to the adsorption sites. However, adsorbents with high adsorption capacities often show problems due to the weak stability of either the host structure or functionalized chemical species.13 As shown in Table 1, CBAP-1 consists mostly of micropores with an average pore diameter of 1.4 nm, the size of which is further reduced after organic functionalization in CBAP-1(EDA) and CBAP-1(AET). Therefore, Hg2+ adsorption in these materials takes place 16


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through micropores with restricted diffusion, and only the near external surface of the porous particles is fully exposed to the adsorbate. Once these sites are saturated, the rest of the Hg2+ ions are blocked from internal diffusion toward the particle centers. This comparison implies that the Hg2+ adsorption capacities exhibited by CBAP-1(EDA) and CBAP-1(AET) can be improved further by making use of a POP structure with mesoporosity.40 This is confirmed by the low Hg2+ capacities exhibited by the microporous polymers (Table 4, entries 12 and 13) and the high adsorption capacities shown by the large pore volume and mesoporous polymers (Table 4, entries 14 and 15). Table 4. Comparison of the adsorption capacities of Hg2+ with various adsorbents. Capacity (mg/g)

Cycle number

Entry

adsorbents

1

amine-modified AC

119

-

15

2

AC cloth

70

-

16

3

sulfur-functionalized mesoporous carbon

435

-

17

4

thiol-functionalized mesoporous silica

401

6

13

5

Fe3O4@SiO2–SH

148.8

5

25

6

graphene oxide M NPs

16.6

-

18

7

SWCNT-SH

131

5

19

8

SH-MAC

37.6

-

22

9

thiol-functionalized CNT/Fe3O4

65.5

-

23

10

amino-functionalized magnetic graphene composites

168

5

24

11

ZIF-90-SH

22.4

-

27

12

COF-LZU8

236

-

28

13

Fe3O4-melamine based POP

96

5

41

17


Ref


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14

PAF-1

1000

3

42

15

TAPB-BMTTPA-COF

734

6

43

16

CBAP-1(EDA)

181

-

This work

17

CBAP-1(AET)

232

10

This work

3.3.3. Kinetic study The reaction kinetics of the adsorbent was examined using Hg(NO3)2 (100 ppm) and CBAP-1(AET) (10 mg) at pH = 5 by taking samples at time periods of 2, 4, 6, 8, 10, 15, and 30 min. Extremely rapid removal of Hg2+ ions was observed, in which > 98% of adsorbate ions were adsorbed within 5 min, probably because of Hg2+ adsorption by the AET groups available on the external surface of the material. The experimental values were fitted to both the PFO and PSO kinetic models (Fig. 7), and the corresponding parameters are given in Table 3. The R2 value for the PSO model of Hg2+ adsorption (0.998) is slightly better than the PFO model (0.964). The PSO model has been widely used to examine the scavenging ability of different contaminants from aqueous medium by adsorbents, which assumes chemisorption involving covalent forces or ion exchange.57, 58 The corresponding PSO rate constant (k2) of CBAP-1(AET) is ca. 1.12 g mg-1 min-1, which is significantly higher than those previously reported (< 0.5 g mg-1 min-1 ) for other materials.59

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Fig. 7. Kinetic studies of Hg2+ adsorption over CBAP-1(AET). (a) Adsorption percentage of Hg2+ versus contact time; (b) pseudo-first order kinetic model; (c) pseudo-second order kinetic model; and, (d) Hg2+ removal under an initial concentration of 10 ppm and pH = 5. The distribution coefficient of a given sorbent (Kd) is used to evaluate the affinity for a specific metal ion by a given adsorbent. The Kd value of CBAP-1(AET) at 25 °C was calculated from the kinetic study of a 10 mg/L Hg2+ solution (Fig. 7d) to be 2.41 × 107 mL/g, which stands as one of the highest distribution coefficients, competing well with those of benchmark adsorbents including ICMS (1.97 × 107 mL/g)13, commercial resin (5.1 × 105 mL/g)21, modified mesoporous carbon (6.82 × 105 mL/g)17, and TAPB-BMTTPA-COF (7.82 × 105 mL/g).43

19



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3.3.4. Adsorption mechanism The Hg2+ interactions with the thiol and amine units in CBAP-1(AET) were verified by XPS analysis after adsorption. The red shift of about 0.7 eV from the original value of the S2p peak suggests that the Hg2+ ions are adsorbed on the S sites in the polymer matrix (Fig. 3b). In addition, the broad N1s spectral peak of the amine is also red-shifted from its original value by ca. 1.0 eV (Fig. 3a), which indicates that the amine sites are also involved in binding Hg2+ ions. Based on the previous reports

12, 16, 19, 60

and binding tendency of Hg2+ ions with

amine and thiol groups mentioned above, a plausible adsorption mechanism can be proposed as shown in Scheme 2. Scheme 2a suggests the adsorption mechanism of Hg2+ by CBAP1(EDA), in which the lone pairs of nitrogen on each side of the ethyl chain are predominantly involved in capturing Hg2+ ions (NH2→Hg2+←NH2). Similarly, the thiol group in CBAP1(AET) (Scheme 2b) can capture Hg2+ ions together with the adjacent amine group, where interaction mostly involves through covalent bonding by thiol groups (R-S-Hg+←NH2) 60 via a soft acid-soft base interaction and coordination bonding by amines.

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Scheme 2. Host-guest interaction of Hg2+ and functionalized polymers (a) CBAP-1(EDA) and (b) CBAP-1(AET). 3.3.5. Adsorbent reusability Hg2+-captured CBAP-1(AET) could be easily regenerated upon treating with 2 M HCl and 0.5 M thiourea, resulting in 100% desorption of Hg2+ ions. Material recovered after the first cycle retained its previous capture capacity with practically no organic group loss during the acid-aided desorption process. The newly generated CBAP-1(AET) was subjected to a further 10 successive cycles and showed identical adsorption performances, without any noticeable loss in Hg2+ adsorption capacity, as shown in Fig. 8a. This performance result is significantly better than that for previously reported adsorbents such as mesoporous silica, metal-organic frameworks, sulfur reduced grapheme oxide which eventually lose their adsorption capacities upon repeated cycling.13, 26 In practice, the reusability of an adsorbent can also be affected by the presence of other co-existing metal ions in an aqueous system. Thus, the effect of Pb2+, Mg2+, Ca2+, Na+, and Fe3+ ions on the capturing performance was investigated using a solution mixture containing 10 ppm of each metal ion. CBAP-1(AET) reaches 100% adsorption of Hg2+ and Pb2+ within 5 min of contact time, as shown in Fig. 8b. CBAP-1(AET) is equally effective for adsorbing Pb2+, which is similar to the results of other Hg2+ adsorbents.43, 61 The coordination tendency for the other ions is found to be negligible within the short contact time in the presence of Hg2+ ions.

21



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Fig. 8. (a) Recycling test for Hg2+ removal in aqueous solution and (b) capture efficiency for the removal of coexisting metal ions.

4. Conclusions In summary, the porous organic polymer CBAP-1 functionalized with EDA or AET exhibited sufficiently high adsorption capacity towards Hg2+. The adsorption equilibrium data could be satisfactorily correlated with the Langmuir model and the maximum adsorption capacities for Hg2+ at the optimum pH value of 5 were 181 and 232 mg/g on CBAP-1(EDA) and CBAP-1(AET), respectively. The CBAP-1(AET) adsorbent showed a high distribution coefficient value of 2.41 × 107 mL/g for Hg2+ ions. In addition, adsorption equilibrium was attained within 5 min, following a pseudo-second order kinetic model. CBAP-1(AET) was recycled by treatment with 2 M HCl and 0.5 M thiourea, and the adsorption efficiency was maintained during the first 10 consecutive cycles without loss in adsorption capacity. The coordination tendency of the common metal ions was found to be negligible except for Pb2+, toward which CBAP-1(AET) showed high affinity. This work demonstrates that robust anchoring of organic functional groups to a porous polymer network offers excellent potential for capturing Hg2+ from waste water. The introduction of mesopores to the polymer 22


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Graphical Abstract 49x14mm (300 x 300 DPI)


Aminoethanethiol-Grafted Porous Organic Polymer ... - ACS Publications

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