PolycyclodextrinâClay Composites: Regenerable Dual-Site Sorbents

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Polycyclodextrin-Clay Composites: Regenerable Dual-Site Sorbents for Bisphenol A Removal from Treated Wastewater Itamar A Shabtai, and Yael G Mishael ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09715 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Materials & Interfaces

Polycyclodextrin-Clay Composites: Regenerable Dual-Site Sorbents for Bisphenol A Removal from Treated Wastewater

Itamar A. Shabtai and Yael G. Mishael*

Soil and Water Sci., The Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, 7610001, Israel Corresponding Author Yael G. Mishael Tel: 972-8-948-9171; Fax: 972-8-948-9856; E-mail: [email protected]

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The greatest challenge of wastewater treatment is the removal of trace concentrations of persistent micropollutants in the presence of the high concentration of effluent organic matter (EfOM). Micropollutant removal by sorbents is a common practice, but sorbent employment is often limited due to fouling induced by EfOM and challenging sorbent regeneration. We directly addressed these two issues by designing regenerable dual-site composite sorbents based on polymerized β-cyclodextrin, modified with a cationic group (pCD+) and adsorbed to montmorillonite (pCD+-MMT). This dual-site composite was tailored to simultaneously target an emerging micropollutant, bisphenol A (BPA), through inclusion in β-cyclodextrin cavities and target anionic EfOM compounds, through electrostatic interactions. The removal of BPA from treated wastewater by the composite was not compromised despite the high removal of EfOM. The composites outperformed many recently reported sorbents. Differences in composite performance was discussed in terms of their structures, as characterized with TGA, XRD, BET and SEM. The simultaneous filtration of BPA and EfOM from wastewater by pCD+-MMT columns was demonstrated. Furthermore, successful in-column regeneration was obtained by selectively eluting EfOM and BPA, with brine and alkaline solutions, respectively. Finally, the composites removed trace concentrations of numerous high priority micropollutants from treated wastewater more efficiently than commercial activated carbon. This study highlights the potential to design novel dualsite composites as selective and regenerable sorbents for advanced wastewater treatment.

Keywords bisphenol A; cyclodextrin; polymer-clay composites; wastewater treatment; filtration; regeneration

1. Introduction Treated wastewater effluent is often discharged and incorporated in water bodies used for potable water1,2 and agricultural irrigation.3 In the past few decades, numerous


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reports have indicated that organic micropollutants (e.g. pharmaceuticals, pesticides, etc.) are not fully removed by conventional wastewater treatment plants and are consequently discharged to the aquatic environment.4–6 A recent review has identified the removal of micropollutants as a major wastewater treatment challenge.7 Several EU countries have recently addressed this critical issue - with more countries likely to follow - and have begun implementing measures to significantly reduce micropollutant discharge.8,9 Micropollutant treatment can be carried out by adsorption to activated carbon4,10, but like many sorbents, it quickly becomes spent in the presence of effluent organic matter (EfOM) and its performance significantly declines.11 Thermal regeneration of spent activated carbon is costly and may lead to sorbent degradation.12,13 Furthermore, since EfOM also impedes downstream processes such as membrane filtration,14 its simultaneous removal is imperative. Clearly, there is a crucial need for regenerable sorbents, capable of simultaneous removal of micropollutants and EfOM. β-Cyclodextrin (βCD), a cyclic oligosaccharide composed of 7 α-(1,4)-D-glucopyranoses, shows potential as the basis of next-generation functional sorbents.15 Its unique structure forms a nano-sized hydrophobic cavity (0.78 nm aperture), inherently capable of size specific hydrophobic inclusion with various molecules.16 βCD is soluble and therefore often cross-linked or anchored to a solid support to obtain insoluble sorbents15,17, some of which have demonstrated superior adsorption of numerous micropollutants, compared with activated carbons.18–20 However, most studies do not report on the removal of micropollutants from “real” wastewater, do not test sorbents in column filtration (only in suspension) and do not demonstrate sorbent regeneration.21 We have recently designed polycation-clay composite sorbents for simultaneous filtration of anionic pharmaceuticals and EfOM.22,23 Here, our aim was to design a novel, dual-site composite sorbent, based on polymeric βCD and montmorillonite (MMT) for the simultaneous removal of non-ionic micropollutants and EfOM from treated wastewater. Bisphenol A (BPA), a potential endocrine disruptor24 commonly detected in



treated wastewater effluent,25 was chosen as a typical organic micropollutant. We cross-linked βCD with epichlorohydrin (EP) and incorporated cationic glycidyl trimethylammonium chloride (GTMAC) groups. It was hypothesized that BPA would be trapped in βCD cavities while EfOM would be targeted by cationic GTMAC groups. Both βCD and GTMAC adsorption sites can be regenerated by inducing favorable conditions for micropollutant desorption. BPA interactions with βCD polymers were studied with fluorescence spectroscopy. Polymeric βCD composites were prepared, characterized and their performance as sorbents for BPA and EfOM, individually and simultaneously, was investigated in equilibrium and time dependent experiments. The effect of composite structure on its adsorptive properties was discussed. Simultaneous column filtration of BPA and EfOM by composites was performed and selective in-column regeneration was demonstrated. Finally, we tested the composite’s ability to simultaneously remove an array of micropollutants from treated wastewater.

2. Experimental Section 2.1 Materials Wyoming Na-montmorillonite SWy-2 (MMT), cation exchange capacity of 76.4 meq/100 g, was obtained from the Source Clays Repository of the Clay Mineral Society (Columbia, MO). β-cyclodextrin (βCD), glycidyl trimethylammonium chloride (GTMAC), epichlorohydrin (EP) and bisphenol A (BPA) were purchased from Sigma Aldrich. Dialysis bags (Spectra/Por 6, molecular weight cut-off 8000 Da) were obtained from SpectrumLabs. Granular activated carbon was Hydraffin CC8×30 (ground and sieved 30 mV). 0.5

A

40

0.3 pCD+ - MMT 0.2

B

pCD0 - MMT

0.4

Zeta potential (mV)

Polymer loading (g/g composite)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*

pCD+ - MMT 20

0

-20

pCD0 - MMT

0.1 -40

βCD - MMT 0.0 0

5

10

15

20

Polymer added (g/L)

25

0.0

0.1

0.2

0.3

0.4


Figure 1. Adsorption of pCD0 and pCD+ (0.13 – 20 g/L) to MMT (2.5 g/L) (A) and zeta potentials of pCD0-MMT and pCD+-MMT as a function of polymer loading (B). Red asterisk signifies composite loading in X-ray diffractograms and SEM micrographs (Figure 2). Data points indicate individual results. We expected a structural difference between pCD+ and pCD0 composites, since highly charged polymers tend to adsorb in a planar conformation while neutral polymers tend to adsorb in an extended conformation.39 Composite structure and morphology (loading of 0.2 polymer/g composite) were characterized using X-ray diffraction (Figure 2A), BET surface analysis (Table S2) and scanning electron microscopy (Figures 2B-2D). The basal spacing of the dehydrated bare clay was 0.96 nm, as reported.40 The increase in basal spacing upon adsorption of pCD+ (1.92 nm) is only slightly larger than the height of a βCD cavity (0.8 nm) which indicates that the polymer is tightly intercalated between the clay layers. Upon adsorption of the high MW pCD0 no clear diffraction and increasing



intensity at low angles was observed, indicating exfoliation of the clay structure resulting from polymer adsorption in a very extended conformation. The effect of polymer adsorption on composite basal spacing was confirmed by remeasuring the diffraction after heating to 400 °C (Figure S5). A scheme of the different polymer conformations is shown in insets of the SEM micrographs (Figures 2B-2D). The different polymer conformations on the clay could affect βCD cavity accessibility for pollutants.

Figure 2. X-ray diffractograms and basal spacing (d=001) of oriented samples of MMT, pCD0-MMT and pCD+-MMT (loading of 0.22±0.02 g/g composite) at 105 ˚C (A) SEM micrographs of MMT (B), pCD+-MMT (C) and pCD0-MMT (D) and scheme of polymer nano-structure based on XRD and BET analyses (insets B, C and D, respectively).


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BET surface area decreased in the order MMT (29.36 m2/g) > pCD+-MMT (22.58 m2/g) > pCD0-MMT (2.56 m2/g) (Table S2) due to coating and blocking of the clay's porous structure by the adsorbed polymer. Compared with the clay, the composites exhibited lower pore volume and a larger average pore size (Table S2). The increase in average pore size (which reduces overall pore volume) reflects the increasing separation of the clay layers and corresponds to the observations of the XRD analysis (Figure 2A). Similar results were reported for surfactant-loaded clays.41 The more dramatic effect exerted by pCD0 compared with pCD+ is likely due to the larger hydrodynamic radius of the former, which led to grater pore blocking and clay exfoliation.

60

100

A

B

pCD0 - MMT

80

60

EfOM removal (%)

Bisphenol A removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pCD+ - MMT

40

pCD+ - MMT 40

20

20

0 0.0

pCD 0 - MMT 0.1

0.2

0.3


0.4

0 0.0

0.1

0.2

0.3

0.4


Figure 3. Removal of BPA (0.044 mM) (A) or EfOM (12 mg/L DOC) (B) by pCD0-MMT and pCD+-MMT (0.5 g/L). Data points indicate individual results. Differences between the bare clay and the composites at the nano-scale were also clearly observed at the micron scale using scanning electron microscopy (Figures 2B-2D). The bare clay (MMT) and pCD+-MMT were characterized by a dense, compact structure of closely packed clay tactoids. In comparison, the pCD0-MMT was characterized by an open, fluffy structure of loosely aggregated clay tactoids. The performance of the composites as sorbents was thoroughly investigated and discussed in the context of their contrasting structures.



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3.4. Removal of BPA and EfOM by pCD-MMT composites The removal of BPA (0.044 mM; 10 mg/L) or EfOM (12 mg/L DOC) by the composites, as a function of polymer loading (Figure 1), is presented in Figure 3. Pollutant removal by the βCD-MMT composites was not investigated due to the extremely low βCD loading. BPA removal increased with increasing polymer loading (Figure 3A). However, at higher pCD0 loading, a decrease in removal was observed, possibly due to entangling of the highly adsorbed polymer, decreasing accessibility to the βCD cavities. The trend in EfOM removal (Figure 3B) closely corresponded with composite zeta potential (i.e. GTMAC content), while EfOM removal by pCD0-MMT was very low, indicating an anion exchange mechanism, as reported.42 These results emphasize the instrumental role of each comonomer: βCD and GTMAC selectively target and remove BPA and EfOM, respectively, while EP facilitates cross-linking, which enhances loading on the clay. Table 2. Equilibrium and kinetic adsorption coefficients of bisphenol A to pCD0-MMT and pCD+-MMT, extracted from Langmuir and time-dependent Langmuir models.

Sorbent

Equilibrium

Kinetic

coefficients

coefficients

KL

Qmax (mol/g

R2

(L/mol)

C

D

KL

(L/mol·min)

(1/min)

(L/mol)

R2

βCD)

pCD0-MMT

4.6·104

9.8·10-4

0.98

3.3·102

7.6·10-3

4.3·104 0.94

pCD+-MMT

11.1·104

5.8·10-4

0.95

9.5·102

9.6·10-3

9.8·104 0.93

n.d.

n.d.

3.1·101

2.2·10-3

1.6·104 0.95

Activated carbon

The individual and simultaneous adsorption, in equilibrium (Figure 4) and time dependent (Figure 5) experiments, of BPA and of EfOM by pCD0-MMT (-25mV) and pCD+-MMT (35 mV) composites, was thoroughly studied. The pCD0-MMT and pCD+-


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MMT composites possessed similar polymer loading (0.22±0.02 g/g) and comparable βCD contents of 0.12 and 0.08 mmol βCD/g, respectively (Table S2). The removal of BPA from treated wastewater by pCD0-MMT and pCD+-MMT was as high as its removal from deionized water indicating that, under these conditions, BPA removal was not affected by the presence of EfOM (Figure 4B). The removal of EfOM by pCD+-MMT reached 9.2 mg/g while its removal by pCD0-MMT was 1.9 mg/g. These results support our main hypothesis that BPA specifically binds to βCD while EfOM binds to the charged GTMAC. This concept was reaffirmed by the simultaneous filtration of BPA and EfOM (see below).

BPA adsorbed (mmol/g βCD)

A

100

1.0

pCD0-MMT 75

0.5

pCD+-MMT

*

50

25

0.0 0.0

0.1

βCD cavities occupied (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

Composite

BPA removal

EfOM removal

(mmol/g βCD)

(mg/g composite)

DW

TWW

DW

TWW

pCD0-MMT 0.54±0.03 0.55±0.01

n.d.

1.9±0.5

pCD+-MMT 0.52±0.02 0.51±0.01

n.d.

9.2±0.2

0 0.3

0.2

BPA in equilibrium solution (mM)

Figure 4. BPA adsorption (0.002 – 0.26 mM) to pCD0-MMT or pCD+-MMT (0.33 g/L) (A) and BPA (0.044 mM) and EfOM (12 mg/L DOC) adsorption to pCD0-MMT or pCD+-MMT (0.33 g/L) from deionized water (DW) or treated wastewater (TWW) (B). Red asterisk points to the experimental conditions of results depicted in Figure 4B (0.044 mM BPA; 0.33 g/L composite). Data points in Fig A indicate individual results. The Langmuir equilibrium (Figure 4A) and kinetic (Figure 5A) coefficients of BPA adsorption from deionized water and treated wastewater, respectively, were extracted (Table 2). The Langmuir adsorption capacity (Qmax) of BPA from deionized water to pCD0MMT was significantly higher than to pCD+-MMT. Composite Qmax corresponded to 87% and 51% of the βCD cavities in pCD0-MMT and pCD+-MMT, respectively. As suggested, the planar conformation of pCD+ on MMT (Figure 2C) limits the accessibility to some



βCD cavities. In comparison, the larger, uncharged pCD0 would have less interaction with the clay surface and therefore a higher fraction of accessible βCD cavities (Figure 2D). An opposite trend was obtained for the affinity coefficient (KL), with lower affinity of BPA to pCD0-MMT (4.6·104 mol/L) and higher affinity to pCD+-MMT (11.1·104 mol/L) (Table 2). Thus, although pCD0-MMT contained a larger fraction of accessible βCD cavities than pCD+-MMT, the affinity to the latter appeared to be higher. The affinity coefficients calculated from the isotherms match those calculated from the kinetic experiments (carried out from treated wastewater), since the presence of EfOM did not affect BPA removal affinity. The affinity coefficients from the kinetic experiments, were calculated as the ratio between the adsorption (C) and desorption (D) coefficients = ⁄! (Table 2). Since the desorption coefficients of the two composites do not differ, the higher affinity of BPA to the pCD+ composite stems from its faster rate of adsorption (C). Indeed, the conformational differences between pCD+ and pCD0 in the adsorbed state (Figures 2C and 2D), a planar conformation vs. a more globular one, can explain the fast vs. slow adsorption rate, respectively. 0.10

100

A

GAC

0.08

EfOM removal (mg/g sorbent)

BPA adsorbed (mmol/g sorbent)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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pCD0 - MMT

0.06 +

pCD - MMT 0.04

0.08 0.06 0.04

0.02

0.02 0.00 0

0.00 0

500

50

100

1000

150

1500

200

B 80

60 GAC 40 pCD+-MMT 20 pCD0-MMT

250

2000

0 0

50

Time (min)

100

150

200

Time (min)

Figure 5. Adsorption kinetics of BPA (0.044 mM) (A) and EfOM (12 mg/L DOC) (B) from treated wastewater (TWW) to pCD0-MMT, pCD+-MMT or granular activated carbon


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(GAC) (0.5 g/L). BPA adsorption in the first 5 hours is shown in the inset. Data points indicate average of duplicates. The lower adsorption and desorption rate coefficients of activated carbon in comparison to polymer-clay composites were consistent with previous reports (Table 2). Furthermore, the adsorption rate coefficients of BPA to the pCD+ and pCD0 composites are equal or higher than coefficients reported for micropollutant adsorption by other polymer-clay composites.23,32,43 EfOM removal by pCD+-MMT was much higher than by pCD0-MMT, as expected, and similar to GAC. Overall, the results of the kinetic experiments imply that in filtration columns, in which performance is greatly impacted by adsorption kinetics, pCD+-MMT would have an advantage over both pCD0-MMT and activated carbon.23,32,44

3.5 Filtration of BPA and EfOM by pCD-MMT composite columns Simultaneous filtration of BPA (0.5 mg/L; 2.2·10-3 mM) and EfOM (12 mg/L DOC) from treated wastewater by columns packed with pCD0-MMT or pCD+-MMT, was tested for two cycles of filtration and regeneration (see section 2.3.4). The composites were mixed with quartz sand (1:100 w/w) to achieve a high flow rate and to reach sorbent exhaustion in a relatively brief time (Figure 6). 100

pCD 0-MMT 1 st cycle

60

40

20

100

200

Pore volumes

300

B

pCD 0-MMT 2 nd cycle +

pCD -MMT 1 cycle 0.08

pCD +- MMT 2 nd cycle

0.04

0.00 0

C

st

EfOM removal (%)

BPA adsorbed (mmol/g βCD)

80

0 0

60

0.12

A BPA removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

200

Pore volumes

300

40

20

0 0

100

200

300

Pore volumes

Figure 6. Simultaneous removal of BPA (0.002 mM) as percent removal (A) and cumulative adsorbed (B) and of EfOM (12 mg/L DOC) (C) presented as percent removal



from treated wastewater (TWW) by filtration columns packed with pCD0-MMT or pCD+MMT mixed with quartz sand (1:100 w/w). Data points indicate individual results. The filtration of BPA by both composites was initially very high (>90%) and gradually decreased to approximately 20-30% after 275 pore volumes (Figure 6A). However, the removal of BPA normalized to βCD cavity content in each column (mmol BPA/g βCD) was significantly higher by pCD+-MMT (Figure 6B). This is attributed to the more rapid adsorption kinetics of BPA to this composite (Table 2 and Figure 5A). Concomitantly, EfOM removal by the pCD+-MMT column ranged from 50 to 10%, while EfOM removal by the pCD0-MMT was

PolycyclodextrinâClay Composites: Regenerable Dual-Site Sorbents

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